Comprehensive Overview on the Computational, Experimental, Numerical, and Theoretical Assessments of Silica Aerogel Composites

Abstract

Silica aerogel (SiA) composites have gained importance due to their ability to overcome the challenges of pure SiA while retaining its superior properties. Their growing significance calls for a closer examination of its assessment methods and performance optimization strategies. Deeper understanding of various assessment methods is essential as it assists in the accurate prediction of the operational stability and environmental resilience of these composites. Addressing performance optimization also remains crucial for the mitigation of structural limitations in SiA composites. This review highlights the advancements and explores the strategies for evaluating the mechanical, thermal, flammability, and radiative properties of SiA composites. It offers an in-depth discussion, revealing not only their thermomechanical behavior, but also their remarkable resistance to fire and radiation. Additionally, this review also examines the development and refinement of theoretical and numerical models. Further, a systematic comparison of continuum mechanics-based simulations with nanoscale (molecular dynamics) simulations reveals critical insights into their accuracies, limitations, and applicability in modeling SiA composites. Exciting insights on the assessments and properties of SiA composites are explored across several experimental, theoretical, numerical, and computational studies. This review also provides an in-depth discussion of performance optimization strategies, limitations, and future prospects while briefly highlighting applications relevant to each assessment. Finally, it presents a distinctive comparative analysis of decade-long studies for each assessment, offering key insights to guide future studies.

1. Introduction

Silica aerogels are amorphous materials [1] that have aptly been hailed as the “miracle and dream material of the 21st century”, precisely because of their combination of peculiar properties, such as high transparency, exceptional thermal insulation, large specific surface area, and ultra-low density [2,3,4]. Over the past decade, silica aerogel composites have demonstrated extraordinary capabilities across a comprehensive array of domains, ranging from biodegradable packaging films [5], photocatalysis [6], building insulation applications [7], to its utilization in a Martian environment [8].

In 1931, Kistler [3] first prepared silica aerogels via the sol-gel reaction of sodium silicate with hydrochloric acid, and they dried the gels through supercritical drying (SCD). Conventionally, the application of silica aerogels is often restricted due to their poor machinability and their possession of brittle characteristics [9]. Over the past decade, several attempts have been recorded to improve the mechanical properties of silica aerogel composites through the addition of reinforcements and binders [5,10,11,12,13]. Despite the extensive research carried out on silica aerogels for nearly a century, scientific exploration still unveils possible solutions to overcome their negatives. For instance, Zhao S. et al. [14] first reported and established an additive manufacturing protocol for silica aerogels, thereby unlocking their miniaturization potential. Their study successfully presented a direct ink writing protocol, where the ink exhibited the required shear thinning behavior. Surprisingly, their work revealed that the printed silica aerogels possessed higher thermal stability than the silica aerogel powder starting material, consequently indicating their possible applications in areas where the local temperature gradients need to be restricted, such as those in wearable devices and in smartphones. Similarly, it is well known that the Si-O-Si bonds within the aerogels develop a network that lacks the sufficient cross-linking ability leading to structural fragility. Therefore, the dearth of bonding between the silica aerogel and the fibers results in the dust-drop phenomenon, which is caused by the shedding of silica aerogel in the composites [15]. This phenomenon hinders the mechanical properties of the composites and, therefore, limits its application. Shafi S. et al. [16] successfully prepared a dust-free composite, which showcased excellent mechanical properties, through acid-base catalyzed tetraethylorthosilicate sol-gel followed by supercritical CO₂ drying.

Despite the adversities of silica aerogel in space applications [17], Rocha H. et al. [8] have indicated encouraging outcomes using hydrophobic-reinforced silica aerogel (SiA) composites. Their study reported that their samples were subjected to gamma radiation exposure, outgassing, and thermal cycling tests to simulate the Martian environment. Recently, similar ground equivalent simulation tests have been carried out to evaluate the performance of polyimide (PI)-silica aerogel composite aerogels in operating space conditions [18]. The comprehensive tests reported valuable relations between structural integrity, fire resistance, and radiation. Scalable additive textile manufacturing strategies for the development of wearable textile applications in harsh environment have also been reported, where materials exhibited excellent thermal resistance [19]. Therefore, identical to the developments over mechanical properties, several studies have also advanced progress on the thermal conductivity, thermal insulation, and fire resistance of SiA composites over the past decade [7,10,16,20,21,22,23].

Over the years, numerous methods to record the mechanical characterization of SiA composites have been reported (as shown in Figure 1). Compression methods [10,14], tensile [13,24], flexural [25,26], hardness [13], and impact tests [13,27] are the commonly communicated techniques for the measurement of mechanical properties. A dynamical microindentation test [28] presents an added advantage of recording the continuous measurement of load–penetration curves, thereby allowing elastic and plastic behavior to be differentiated for better understanding. Similarly, thermal conductivity tests [23,24], thermogravimetric analysis (TGA) [5,6], differential scanning calorimetry (DSC) [29], and thermal cycling tests [8,18] are the usually described thermal characterization tests. Critical inferences on the flame retardant properties of SiA composites have also been established through oxygen index (OI) tests [30,31]. Meanwhile, surface morphology [7,19], XRD [20,25], porosity and pore size distribution [12,25], and FTIR spectroscopy [12,20] remain the recurrently mentioned tests that are used to determine microstructural integrity.

Conversely, the literature available on the mechanical modeling of SiA composites is relatively less exhaustive as compared to those solely on silica aerogels. An L. et al. in [19], prepared a finite element model of the composite in ABAQUS software (2017 release) to study the effect of the interfacial bonding between silica aerogels and fibers. Their study reported the discrepancies, owing to the linear elasticity assumptions for the aramid fibers, between the computational and experimental results in the initial stages of deformation. Over the past decades, molecular dynamics (MD) simulations have drawn significant attention from researchers. They enable the investigation of the mechanical behavior at the atomic scale, offering high spatial and temporal resolution. Recently, investigations have reported the utilization of MD simulations to study the thermal insulation performance [32] and mechanical properties [33] of SiA composites. In our previous work, the mechanical behavior and properties of glass fiber-reinforced SiA composites were investigated in an all-atom study [34], while the influence of the crack depth in different types of SiA nanocomposites was reported in tensile fracture simulations [35]. Both investigations utilized LAMMPS software (2018 release) [36] for reporting large-scale MD simulations while also demonstrating excellent agreement with experimental studies.

Over the years, numerous reviews [37,38,39,40,41,42] have extensively covered the synthesis of SiA composites, providing diverse insights into their fabrication for various applications. Notably, Linhares T. et al. [43] presented an extensive and thorough review on the impact of fibers on the final properties of SiA composites. Most of these reviews, along with the synthesis, also discussed the mechanical, chemical, microstructural, and thermal characteristics of SiA composites. There have been reviews that have also attempted to report theoretical models to determine the thermal conductivity of these composites [44]. No review has yet presented a detailed overview on the assessments of SiA composites. Fire resistance and radiation shielding are essential for space, building, and nuclear applications. Yet, only a few reviews [45,46,47,48] have briefly addressed advancements in the flammability and radiation resistance of these composites. Similarly, although a few reviews [49,50] have presented the computational models on SiA composites, no review has yet simultaneously discussed continuum mechanics-based finite element simulations with nanoscale simulations. This review, therefore, offers a comprehensive overview on the assessments of SiA composites, delving into their thermomechanical characterizations, computational methodologies, and theoretical frameworks. Ground test studies and laboratory-based simulations to determine the fire and radiation resistance of these composites have also been discussed for the first time in great detail. This review, therefore, presents and reports the recent progress in the assessments of SiA composites, offering a fresh perspective on their characterization, modeling, and performance. Our study, specifically, offers critical insights inferred during the characterization and analysis of these composites, consequently aiming to highlight patterns, pressing issues, and alternatives for future research. Limitations and future scope are critically discussed for each assessment. The applications linked to their respective composites are also briefly addressed. This review, therefore, unifies assessment methods and material properties, providing a holistic understanding of SiA composites. By integrating experimental, theoretical, numerical, and computational insights, it paves the way for future advancements of SiA composites in high-performance applications.

This review is organized as follows. Section 2 discusses the mechanical characterization on SiA composites. This section primarily examines the common mechanical properties and their temperature dependence, as well as the acoustic and sustainability aspects of the composites. Similarly, Section 3 presents the thermal characterization on SiA composites. This section provides a concise literature overview of the experimental studies conducted to evaluate the thermal conductivity and stability of these composites. Further, Section 4 explores the development and advancements of theoretical and numerical models reported for SiA composites. Subsequently, Section 5 presents a detailed analysis of flammability and radiation resistance assessments for the first time. This section comprehensively examines flammability through both small-scale and large-scale fire tests, while radiation resistance assessments are also examined due to encompassing space irradiation and electromagnetic radiation effects. Later, Section 6 provides a comparative analysis of advancements in continuum mechanics-based simulations and nanoscale simulations. Moreover, critical milestones are separately summarized for each assessment in each section. The tabular presentation for each assessment includes a brief comparison of several novel studies, which are aimed to give a comparative gist across different parameters. Further, each section mentioned above is sub-divided into two parts. The first subsection highlights the advancements within respective assessments, while the second subsection identifies several gaps and possible ideas ripe for future exploration. The conclusions are finally discussed in the last section.

The studies compiled in the tables were selected based on their methodological significance, benchmark contributions, and representation of diverse experimental, theoretical, and computational evaluation techniques for SiA composites. The chronological arrangement illustrates the progression of characterization strategies over the past decade. Furthermore, the presentation compares studies employing similar evaluation techniques and encompasses a wide range of aerogel compositions and fabrication routes.

2. Mechanical Characterization

2.1. Recent Advancements and Current Research Trends

The characteristic brittleness and low Young’s moduli of silica aerogels are a few of the primary reasons that hinders its industrial applicability. Figure 2 offers deeper insights into the key mechanical failure modes in SiA composites, while also highlighting their root causes and corresponding mitigation strategies. Although SiA composites have evolved gradually to address these challenges, it is essential to thoroughly assess their load-bearing and deformation recovery capabilities before practical application. Therefore, further research on lightweight SiA composites with superior mechanical properties is crucial.

Compressive, tensile, impact, and flexural tests are widely used to evaluate the mechanical strength, stiffness, deformation behavior, fracture resistance, and overall structural integrity of composites under different loading conditions. Building on this, the following studies demonstrate how these fundamental mechanical tests elucidate critical structure–property relationships, damage mechanisms, and performance trends specific to SiA composites. The investigations in [51] showed that the impact strength of epoxy/SiA composites initially decreased with an increase in silica aerogel mass fraction. This happened because, at lower filler concentrations, aerogel particles act as stress concentrators. However, as the filler content increases, the inherent porosity of the silica aerogel contributes to a higher overall porosity within the composite. This rise in porosity subsequently enhances the impact strength as porous structures facilitate the dispersion of shock waves traveling through the material. Consequently, increasing porosity can improve impact resistance by promoting shock wave dissipation. Their investigation, therefore, highlighted the dual role of silica aerogels in modulating the impact performance of epoxy composites. The results notably indicate that the composites showed yielding behavior in comparison to the brittle behavior of the neat epoxy.

Meanwhile, another work has also explored the use of natural attapulgite (crystalline hydrated magnesium aluminum silicate) nanofibers as a reinforcement in monolithic SiA composites to mitigate the brittleness and dry shrinkage of silica aerogels [52]. Their study remains particularly noteworthy as it employed attapulgite nanofibers with reduced diameters to minimize the size disparity with the aerogel matrix. Such scale-matched reinforcement minimizes strain mismatches and the interfacial stress concentrations typically observed with larger-diameter fibers, thereby enhancing monolith integrity and mechanical stability through more effective stress transfer and crack suppression across the composite structure. Similarly, Adhikary S. K. et al. [53] investigated the effectiveness of multi-wall carbon nanotubes (MWCNTs) in enhancing the compressive strength of lightweight aggregate concrete containing expanded glass and silica aerogel. Aimed at mitigating the interfacial separation between the hydrophobic aerogel particles and the cementitious matrix, their breakthrough study was the first to employ CNTs as nanoscale bridging agents to improve the mechanical load transfer in such composite systems. Their study demonstrated a notable 41.48% increase in the compressive strength of SiA composites with an optimal 0.6 wt% CNT loading.

Honeycomb structures have been widely adopted in several studies to enhance the mechanical properties of composites. Honeycomb materials are highly resilient and can quickly regain their original shape and size once the applied load is removed. Schwan M. et al. [54] developed a novel composite material by reinforcing chemically modified—fluffy and low-flexible—silica aerogels with aramid honeycombs. It is important to mention that small surface pores formed by air bubbles during sol-gel synthesis may weaken the composite’s structure. Their study showed that the compressive modulus depends on the cell dimension of honeycombs. Uniaxial compression tests further confirmed that the composite showed far better mechanical properties than pure SiA and empty honeycomb material.

Over the years, several investigations have led to the identification of patterns, trends, and correlations between the mechanical properties and synthesis parameters of SiA composites, enabling a deeper understanding of their performance optimization. Building on these insights, we present a systematically curated matrix in Figure 3, which captures the key optimization strategies aimed at improving the mechanical properties of SiA composites. The study in [55] revealed that the mechanical properties of SiA/carbon-fiber composites largely depend on the aerogel’s porosity. A higher fiber content leads to unbound fibers, creating micropores that weaken the composite’s structure. Therefore, a higher fiber content can also deteriorate the mechanical properties of SiA composites. Similarly, the studies in [56] performed compression–decompression cycles on reticulated polyurethane (PU) foam-reinforced SiA composites. Their study tested samples under high deformation conditions and revealed an interesting trend: the composites dried under supercritical conditions exhibit greater deformability than their APD counterparts. This is primarily due to their lower bulk density and the continuous solid aerogel network integrated into the PU foam during SCD. Notably, none of the tested composites fractured, demonstrating an enhanced compression endurance. Instead, the samples became significantly densified. Moreover, it was also observed that although APD is widely preferred for synthesis of SiA composites due to its economic benefits, it is typically prone to shrinkage and collapse of pores.

SiA composites have also been deployed in complicated high- and ultra-high-temperature service conditions. Therefore, it is also mandatory to investigate the temperature-dependent mechanical properties of SiA composites. Yang X. et al. [57] demonstrated that temperature significantly influences the compressive response of SiA composites, particularly at strains beyond 0.5, with the effect becoming more pronounced as deformation progresses. This behavior is of critical relevance for thermal protection systems (TPSs), where aerogel-based insulation not only endures prolonged compressive loading from aerodynamic pressure and pre-tightening forces, but it also operates under the elevated temperatures induced by aerodynamic heating. To meet the dual demands of insulation and mechanical support in TPS, there was a shift toward developing aerogel-based materials that can withstand prolonged compressive loads under extreme thermal conditions. Although their composites emerged as strong candidates for such multifunctional applications, their performance at high temperatures was hindered by excess stress relaxation. This limitation can be addressed by inhibiting nanoparticle agglomeration, enhancing fiber thermal resilience, and strengthening fiber–matrix interfaces, which is key to maintaining structural reliability under extreme thermo-mechanical conditions.

Similarly, the research in [58] analyzed compression tests on ceramic fiber-reinforced SiA composites, reporting their mechanical properties at both room and elevated temperatures. Their study concluded that as the thermal exposure temperature rises, the compression modulus and strength improves, while deformation recovery capability declines. This happens due to the combined impact of compressive mechanical load and high thermal load, which intensifies aerogel particle fusion and aggregation, leading to a higher unrecoverable proportion at elevated temperatures compared to room temperature. Moreover, once particle fusion occurs at an elevated temperature, the fused and viscous skeleton struggles to spring back after stress removal. As a result, when aerogel composites are subjected to high temperatures and mechanical loads, their ability to recover to their original size is significantly reduced. Similar to this, other materials like glass-fiber/SiA composite [59] have also shown that they can endure considerable compressive and flexural strain without structural destruction at high temperatures. Shafi S. et al. [60] also synthesized a glass-fiber/SiA composite with the addition of fumed silica, achieving excellent flexural strength and reduced bulk density. These enhanced properties were attributed to the unique dense mesoporous Si–O–Si network formed around the glass fibers by the silica aerogel. Additionally, it was observed that the fumed silica integrated into the matrix, with its macropores converting into mesopores, thereby enhancing and refining the overall aerogel pore structure.

SiA composites also offer exceptional acoustic insulation due to their ultra-low density, high porosity, and tuneable pore structure, making them ideal for noise reduction and sound absorption applications. Therefore, studies have also investigated the ability of SiA composites in reducing the sound transmission of glass [61]. Their study demonstrated that the SiA/polyester (PES) resin composite, synthesized via the sol-gel method with ultrasonic treatment (SA-U), exhibited a significantly higher average sound transmission loss (STL) compared to composites produced using the conventional sol-gel approach (SA-C). The SA-U sample features larger pore diameters compared to the SA-C sample. Another study, in [62], similarly concluded that SiA composites with larger particles enhances sound absorption by creating a bridging effect that connects air voids more effectively. Their study reported the utilization of the ultrasonic pulse velocity (UPV) test to determine the compactness and voids of SiA composites. Therefore, optimizing the silica aerogel particle size is crucial for maximizing sound absorption efficiency, reinforcing its effectiveness in acoustic applications.

In recent years, a new class of advanced SiA composites has emerged, demonstrating exceptional multi-functionality through novel design strategies, bio-inspiration, and advanced material integration. These rare systems transcend conventional thermal and structural roles, incorporating tailored chemistries, environmental responsiveness, and functional fillers to unlock specialized sustainable applications. Recently, a study took inspiration from the bio-silicification process in diatoms and explored a novel sustainable approach to develop green silica-polymer composite aerogels and cryogels [63]. The developed silk fibroin (SF)-based SiA composite was 1.4 times stronger with cyclic compressibility than its conventional version. This was possible through a modified approach, where oxidized silk fibroin with polyethyleneimine (SFO-PEI) was used to generate long-chain polyamines (LCPA) that mimic LCPA in diatoms. These polyamines acted as acid–base catalysts, promoting robust siloxane bond formation from TEOS through the silicification process. Moreover, despite the incorporation of the silicification process, the composite maintained its flexibility without any compromise. Interestingly, their study streamlined the synthesis of biomimetic SiA polymer composites by eliminating the need for surfactants and complex multi-step solvent exchange processes while also simplifying the washing procedure. These efforts reduce the environmental impact and are, therefore, identified as green SiA composites. Due to restricted silane precursor penetration in dense monolithic structures, their described method remains more appropriate for coating thin films or microparticles. By inducing nanoscale surface roughness without compromising mechanical integrity, these materials can significantly improve interactions with biological cells, thereby positioning them as strong candidates for biomedical implants, targeted drug delivery, and cell adhesion platforms. Owing to their distinctive structural, surface, and biocompatible characteristics, these biopolymeric aerogels remain highly promising for advanced biomedical applications.

Similarly, Renjith P. et al. [64] developed an advanced and novel magnetite (Fe₃O₄) nanoparticle (MNP)-doped SiA composite capable of remediating oil spills from water bodies. This scalable silica aerogel–melamine–formaldehyde foam composite possessed an extraordinary oil selection capacity, fast and repeatable sorption ability, and efficient performance under various adverse environmental conditions (acidic, alkaline, saline, and low/high temperature). While high oil sorption efficiency is essential, it alone does not ensure the practicality and reliability of the sorption technique. Therefore, their research utilized a promising approach to integrate external magnetic forces for sorbent retrieval. The nanoscale dimensions of MNPs facilitated the uniform dispersion while also crucially imparting magnetic properties. Vibrating sample magnetometer investigations revealed that the composite achieved a saturation magnetization of 12.45 emu/g. Also, as the composite absorbed and released oil through mechanical squeezing, it was required that it should withstand repeated cycles, necessitating sufficient structural integrity to prevent failure. Satisfactory cyclic compression tests confirmed the structural ruggedness of the foam composites. Meanwhile, a rapid and cost-effective solution for oil spill remediation is the need of the hour as such composites hold strong potential for practical deployment in mitigating harmful effects on marine and shoreline ecosystems.

Table 1 presents a timeline review of the notable research studies reporting various mechanical characterization assessments over the last decade. This presentation provides a concise comparative analysis of various characteristics examined through different tests, highlighting their relevance for specific applications with critical insights.

2.2. Future Directions

Recent research on SiA composites has spanned a wide range, from enhancing the mechanical properties of phase change materials [65] to addressing the trade-off between ionic conductivity and modulus in solid polymer electrolytes [66]. Although several studies have focused on improving the load-bearing and deformation recovery properties of SiA composites, further focus should be given to prevent the dust-release characteristic of these composites [59]. Dustiness typically occurs due to the weak interfacial bonding between the aerogel particles and fibers. A study, in [59], addressed this issue by vacuum sealing the SiA composite, effectively preventing the influence of the dust release from its material properties. Addressing this through strategies like vacuum sealing enhances the interfacial adhesion and minimizes particulate shedding. Future work should focus on advanced surface functionalization, chemical grafting, or hybrid encapsulation techniques to mitigate dust release, thereby improving composite reliability in sensitive operational environments.

Similarly, a lack of existing studies on the enhancement of interfacial bonding in SiA composites have also been reported [67]. The fiber–aerogel interface, which significantly influences the mechanical properties of SiA composites, has received limited attention despite its relatively weak bonding due to the aerogel’s ultra-high porosity and the fiber’s smooth surface. Therefore, further research should not only identify strategies to enhance the fiber–aerogel interface, but it should also mitigate crack propagation, thus preventing debonding and fiber detachment from the aerogel matrix. The study in [68] also sought potential modifications that could enhance the crushing resistance of aerogel. Meanwhile, Lyu S. et al. [58] also highlighted an urgent need for further research on the deformation recovery capabilities of fiber-reinforced SiA composites. Addressing deformation recovery is crucial for improving the durability and reusability of fiber-reinforced SiA composites, particularly in load-bearing and high-temperature applications, where structural integrity under cyclic stress is essential.

Extensive studies on the post-binding treatment of silica aerogels are also limited, mainly due to the risk of structural collapse caused by the infiltration of binding materials, which displaces the air within their porous network [69]. Therefore, further investigation should be carried out for developing a feasible and straightforward post-binding treatment method. This is essential to enhance the mechanical properties of the composite while preserving the aerogel’s key attributes, such as low density and excellent thermal insulation. Similarly, the use of polysaccharides in SiA composite research has received relatively less attention, despite previous studies demonstrating a significant enhancement in mechanical properties [70].

Despite the continuous emergence of energy-efficient materials and innovative technologies in the construction industry, the pursuit of more advanced solutions remains ongoing. Therefore, novel or improved materials, both should primarily satisfy key criteria, including practical applicability, functionality, cost-effectiveness, and environmental and social safety. While recent scientific advancements in SiA composites highlight their potential for mass application, research on the properties of cementitious composites incorporating silica aerogel remains limited [71]. Comprehensive data are still lacking for a thorough assessment of the feasibility and potential of silica aerogel as a heat-resistant additive in dry concrete mixtures. The effects of pre-treatment on the mechanical and thermal properties of SiA-based cementitious composites also remains uncertain. Additionally, the use of silica aerogel in thermally insulating concrete is still limited, likely due to its high production cost. Moreover, life-cycle studies on SiA composites can also be conducted by varying the amounts of silica aerogel and reinforcing materials. Therefore, overcoming these limitations in silica aerogel-based cementitious materials not only shapes future experimental and theoretical research, but it also contributes to optimizing cement formulations for enhanced performance.

Similarly, the influence of micro-sized silica aerogel on the mechanical properties of SiA composites remains largely unexplored [72]. Researchers should focus on this to understand the size-dependent reinforcement mechanisms, as well as to optimize the microstructure for enhanced mechanical performance. Very few studies have explored how the particle size of silica aerogel affects the insulation properties and sound absorption of lightweight materials [62]. Further research in these areas could enable SiA composites to be more widely used as supplementary sound absorbers in theaters alongside porous materials like acoustic panels, while also leveraging their excellent thermal insulation properties.

While numerous studies have enhanced the thermo-mechanical properties of SiA composites, several challenges remain in developing environmentally sustainable SiA composites. For instance, a study reported the development of environmentally friendly SiA composites, which were fabricated using silica aerogel granules with bio-based porcine gelatin as the binding agent [73]. Through several mechanical characterization tests, it was concluded that the developed composite samples were under-qualified for load-bearing applications and more suitable for light-weight hydrophobic applications. Therefore, further studies are required to improve the flexibility and reduce the brittleness of such composites. In a stride toward sustainable material innovation, the integration of reclaimed fibers into SiA composites has gained attention as a sustainable and cost-effective approach aligned with the principles of a circular economy. These fibers, derived exclusively through mechanical processing of textile wastes, such as cotton, polyester, and wool, serve as reinforcement matrices for SiA composites. Although their dimensions are typically reduced compared to virgin fibers, their incorporation not only diverts waste from landfills, but also adds mechanical integrity to the fragile aerogel matrix. The research in [74] reported the development of sustainable sound absorbers through the utilization of this strategy, thereby presenting a promising avenue for enhancing composite performance while addressing global sustainability goals through material reuse. More such scalable approaches should be investigated for the commercialization of such environmentally friendly and cost-effective approaches.

Table 1. Milestones in the mechanical characterization assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Composite Formulation	Processing Method	Standards	Application	Key Insights
Zhao C. et al. [2015] [31]	Compressive strength test	Particle-reinforced	Granular silica aerogel	One-shot free-rise polymer-foam process	GB/T 8813-2008 [75] (compressive strength)	Thermal insulation, fire prevention	Compressive strength of composites increased by 136%, and specific strength improved by 92.2% using polyethylene glycol (PEG) 600 as polyol.
Ji X. et al. [2019] [76]	V-notched shear test Uniaxial tension test Stress–strain behavior analysis	Matrix- and fiber-reinforced	Alumina plain-woven fiber and silica aerogel matrix	Impregnation, sintering, and ambient pressure drying (APD)	ASTM C1359-96 [77] (uniaxial tension test)	Integrated thermal protection system (ITPS)	The longitudinal tensile modulus and strength decrease with the off-axis angle. The fracture strain increases, leading to accelerated damage progression under biaxial tension and shear stress.
Zhu P. et al. [2019] [78]	Compressive strength test Bending strength test Freeze–thaw resistance test Water resistance test (softening coefficient test) Drying shrinkage test	Matrix- and particle-reinforced	Cement and silica fume (matrix) and silica aerogel granulates (particle)	Two-step mixing, dry and wet mixing	JGJ 70-2009 [79] (freeze thaw) JGJ 51·2002 [80] (water resistance and softening) JGJ 70-2009 [79] (drying shrinkage test) GB/T 17671-1999 [81] (compressive strength)	Non-load bearing thermal insulation layers of buildings	Silica aerogel enhances frost and water resistance, freeze-thaw performance, and dry shrinkage in composites due to its strong hydrophobicity.
Zolfaghari S. et al. [2019] [13]	Izod impact strength test (fracture energy) Hardness test Friction and wear test Tensile test	Matrix- and particle-reinforced	Polypropylene (PP) matrix and silica aerogel (SiA) particles	Melt mixing and compression molding	ASTM D 638 [82] (tensile test) ASTM D 2240 [83] (hardness) ASTM D 256 [84] (izod test)	Thermal barrier fields	Increasing the SiA content reduced tensile strength, elongation, and impact strength but enhanced hardness and friction coefficient, as well as improved the PP’s frictional properties.
Aragón-Gutierrez A. et al. [2020] [5]	Tensile test Stress–strain analysis	Matrix- and particle-reinforced	Silica aerogel powder, polylactic acid (PLA) matrix, and acetyl-tributyl citrate (ATBC) as plasticizer	Melt extrusion, compression molding, plasticization	NIL	Sustainable bionanocomposites for food packaging solutions	Low SiA content (0.5–1 wt.%) and slightly improved elongation at break, while 3 wt.% enhances the modulus but reduces elongation, maintaining sufficient flexibility for packaging.
Albooyeh A. et al. [2020] [85]	Tensile test Flexural test Compressive test Dynamic mechanical thermal test Hardness test Izod impact test Vibration test Operational modal testing Stress–strain behavior	Matrix- and particle-reinforced	SiA nanoparticles and epoxy matrix	Mixing, degassing, pouring, and curing	ASTM D638 [82] (tensile test) ASTM D790 [86] (flexural test) ASTM D695 [87] (compressive test) ASTM D4065 [88] (dynamic mechanical thermal test) ASTM D676 [89] (hardness test) ASTM D256 [84] (izod impact test) ASTM E756 [90] (vibration test)	Used in advanced composite materials for enhanced performance	Addition of 4 wt.% of SiA nanoparticles to the epoxy matrix significantly improves its mechanical properties, including the tensile, flexural, and compressive moduli, as well as its ductility, toughness, hardness, and vibration absorption.
Ismail F. I. et al. [2021] [91]	Compressive strength test Tensile strength test Flexural strength test	Matrix-, particle-, and fiber-reinforced	High-density polyethylene (HDPE) (matrix), silica aerogel (partilces), and kapok fiber	Hot pressing technique	ASTM D638-14 [82] (tensile strength test) ASTM D790-17 [86] (flexural strength test)	Recommended for energy-efficient building roof constructions	Incorporating 3 wt.% silica aerogel, along with HDPE and kapok fiber, enhances the mechanical properties of composite insulation boards, achieving improved tensile and flexural strength with excellent thermal insulation performance.
Aminoroaya A. et al. [2021] [92]	Three-point bending test (flexural modulus and flexural strength) Single edge V-notched beam (SEVNB) flexure test (fracture toughness)	Particle-reinforced	Silica aerogel modified with $\gamma$-trimethoxysilyl propyl methacrylate (SiA$\gamma$-MPS) and silica aerogel modified with trimethylchlorosilane (SiATMCS).	Two-step sol gel, surface modification, and ambient pressure drying (APD)	ISO 4049-2009 [93] (three-point bending test) ISO 6872-2015 [94] (SEVNB)	Dental composite formulation	Mesoporous silica aerogel fillers enhance filler–matrix interlocking, improve flexural properties, exhibit water resistance, and enable stable, high-performance dental composites with $\gamma$-MPS modification.
Adhikary S. K. et al. [2021] [53]	Compressive strength test	Matrix- and fiber-reinforced	Ordinary portland cement (matrix) polycarboxylate (superplasticise) multi-walled carbon nanotubes	Ultrasonication-assisted dispersion and manual mixing	BS EN196-1:2016 [95] (compressive strength)	High-strength lightweight concrete	The addition of 0.6 wt% CNTs to aerogel-based lightweight concrete improves compressive strength by 41% and effectively reduces separation gaps by filling voids.
Kim J. H. et al. [2021] [96]	Compressive test Stress–strain analysis	Matrix- and particle-reinforced	Polyurethane foam (matrix) and silica aerogel particles.	Homogenization, high-speed mixing, and free foaming	KS M ISO844 [97] (compression test)	Liquefied natural gas (LNG) insulation systems used in cryogenic environments	Polyurethane foam-silica aerogel (1 wt.%) enhances the compressive strength and elastic modulus, but higher aerogel content reduces uniformity, weakening mechanical performance due to pore formation on the surface.
Selver E. et al. [2021] [27]	Impact damage test Flexural test Stress–strain behavior	Matrix-, fiber-, and particle-reinforced	Woven glass fabric (satin weave) (primary) and silica aerogels (secondary) into the epoxy resin matrix.	Multi-step ultrasonic-assisted and vacuum-degassed mixing process	ASTM-D790 [86] (flexural)	Applicable for producing composites with enhanced mechanical properties and thermal insulation	Composites exhibited improved flexural strength, modulus, and energy absorption with ductility trade-offs.
An L. et al. [2021] [19]	Compressive stress–strain analysis	Fiber-reinforced	Aramid fibers and ceramic aerogel	In situ cross-linking reaction and ambient pressure drying (APD)	NIL	Low-cost wearable textile for harsh environments	The low density and high compressive strength of the composite depend on cross-linked interfacial interactions, ensuring performance in extreme temperature conditions.
Zhang Z. et al. [2022] [98]	Fluidity test Flexural strength test Compressive strength test Four-point bending test	Matrix-, particle-, and fiber-reinforced	Polyvinyl alcohol (PVA) fibers, SiO2 particles, and aerogel and cement matrix	Ultrasonic dispersion, planetary mixing, and molding	GB/T 2419 [99] (slurry fluidity test) GB/T 17671-1999 [81] (flexural strength and compressive strength)	Reduced thermal conductivity ductile cement-based composite (RTCDCC) material enables durable, insulated building materials	The incorporation of silane enhances the mechanical properties of cementitious composites by improving bonding and increasing both flexural compressive strength. However, it also delays early hydration and contributes to the formation of additional microcracks in the specimens.
Wang G. et al. [2023] [100]	Tensile strength test Tear strength test Dynamic mechanical analysis	Matrix-, particle-, and fiber-reinforced	SiO2, chopped glass fiber, modified glass fiber, and styrene–butadiene rubber (matrix)	Mechanical mixing, surface modification, and vulcanization	GB/T 528-2009 [101] (tensile and tear strength)	Thermal insulation	The addition of modified glass fiber improves tensile strength, cross-linking density, and interface bonding with the rubber matrix, enhancing mechanical properties and amplifying the strain behavior in composites.
Zhu Z. et al. [2024] [11]	Compressive properties test Stress–strain behavior	Matrix- and fiber-reinforced	Multi-walled carbon nanotubes (MWCNTs) in silica aerogel matrix	Sol-gel, freeze drying, and ultrasonic dispersion	NIL	Cost-effective CNTs/silica aerogel for industrial use	Uniform CNT dispersion enhances mechanical properties below 4 wt.%, but agglomeration above 6 wt.% reduces them.

Table 1. Milestones in the mechanical characterization assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Composite Formulation	Processing Method	Standards	Application	Key Insights
Zhao C. et al. [2015] [31]	Compressive strength test	Particle-reinforced	Granular silica aerogel	One-shot free-rise polymer-foam process	GB/T 8813-2008 [75] (compressive strength)	Thermal insulation, fire prevention	Compressive strength of composites increased by 136%, and specific strength improved by 92.2% using polyethylene glycol (PEG) 600 as polyol.
Ji X. et al. [2019] [76]	V-notched shear test Uniaxial tension test Stress–strain behavior analysis	Matrix- and fiber-reinforced	Alumina plain-woven fiber and silica aerogel matrix	Impregnation, sintering, and ambient pressure drying (APD)	ASTM C1359-96 [77] (uniaxial tension test)	Integrated thermal protection system (ITPS)	The longitudinal tensile modulus and strength decrease with the off-axis angle. The fracture strain increases, leading to accelerated damage progression under biaxial tension and shear stress.
Zhu P. et al. [2019] [78]	Compressive strength test Bending strength test Freeze–thaw resistance test Water resistance test (softening coefficient test) Drying shrinkage test	Matrix- and particle-reinforced	Cement and silica fume (matrix) and silica aerogel granulates (particle)	Two-step mixing, dry and wet mixing	JGJ 70-2009 [79] (freeze thaw) JGJ 51·2002 [80] (water resistance and softening) JGJ 70-2009 [79] (drying shrinkage test) GB/T 17671-1999 [81] (compressive strength)	Non-load bearing thermal insulation layers of buildings	Silica aerogel enhances frost and water resistance, freeze-thaw performance, and dry shrinkage in composites due to its strong hydrophobicity.
Zolfaghari S. et al. [2019] [13]	Izod impact strength test (fracture energy) Hardness test Friction and wear test Tensile test	Matrix- and particle-reinforced	Polypropylene (PP) matrix and silica aerogel (SiA) particles	Melt mixing and compression molding	ASTM D 638 [82] (tensile test) ASTM D 2240 [83] (hardness) ASTM D 256 [84] (izod test)	Thermal barrier fields	Increasing the SiA content reduced tensile strength, elongation, and impact strength but enhanced hardness and friction coefficient, as well as improved the PP’s frictional properties.
Aragón-Gutierrez A. et al. [2020] [5]	Tensile test Stress–strain analysis	Matrix- and particle-reinforced	Silica aerogel powder, polylactic acid (PLA) matrix, and acetyl-tributyl citrate (ATBC) as plasticizer	Melt extrusion, compression molding, plasticization	NIL	Sustainable bionanocomposites for food packaging solutions	Low SiA content (0.5–1 wt.%) and slightly improved elongation at break, while 3 wt.% enhances the modulus but reduces elongation, maintaining sufficient flexibility for packaging.
Albooyeh A. et al. [2020] [85]	Tensile test Flexural test Compressive test Dynamic mechanical thermal test Hardness test Izod impact test Vibration test Operational modal testing Stress–strain behavior	Matrix- and particle-reinforced	SiA nanoparticles and epoxy matrix	Mixing, degassing, pouring, and curing	ASTM D638 [82] (tensile test) ASTM D790 [86] (flexural test) ASTM D695 [87] (compressive test) ASTM D4065 [88] (dynamic mechanical thermal test) ASTM D676 [89] (hardness test) ASTM D256 [84] (izod impact test) ASTM E756 [90] (vibration test)	Used in advanced composite materials for enhanced performance	Addition of 4 wt.% of SiA nanoparticles to the epoxy matrix significantly improves its mechanical properties, including the tensile, flexural, and compressive moduli, as well as its ductility, toughness, hardness, and vibration absorption.
Ismail F. I. et al. [2021] [91]	Compressive strength test Tensile strength test Flexural strength test	Matrix-, particle-, and fiber-reinforced	High-density polyethylene (HDPE) (matrix), silica aerogel (partilces), and kapok fiber	Hot pressing technique	ASTM D638-14 [82] (tensile strength test) ASTM D790-17 [86] (flexural strength test)	Recommended for energy-efficient building roof constructions	Incorporating 3 wt.% silica aerogel, along with HDPE and kapok fiber, enhances the mechanical properties of composite insulation boards, achieving improved tensile and flexural strength with excellent thermal insulation performance.
Aminoroaya A. et al. [2021] [92]	Three-point bending test (flexural modulus and flexural strength) Single edge V-notched beam (SEVNB) flexure test (fracture toughness)	Particle-reinforced	Silica aerogel modified with $\gamma$-trimethoxysilyl propyl methacrylate (SiA$\gamma$-MPS) and silica aerogel modified with trimethylchlorosilane (SiATMCS).	Two-step sol gel, surface modification, and ambient pressure drying (APD)	ISO 4049-2009 [93] (three-point bending test) ISO 6872-2015 [94] (SEVNB)	Dental composite formulation	Mesoporous silica aerogel fillers enhance filler–matrix interlocking, improve flexural properties, exhibit water resistance, and enable stable, high-performance dental composites with $\gamma$-MPS modification.
Adhikary S. K. et al. [2021] [53]	Compressive strength test	Matrix- and fiber-reinforced	Ordinary portland cement (matrix) polycarboxylate (superplasticise) multi-walled carbon nanotubes	Ultrasonication-assisted dispersion and manual mixing	BS EN196-1:2016 [95] (compressive strength)	High-strength lightweight concrete	The addition of 0.6 wt% CNTs to aerogel-based lightweight concrete improves compressive strength by 41% and effectively reduces separation gaps by filling voids.
Kim J. H. et al. [2021] [96]	Compressive test Stress–strain analysis	Matrix- and particle-reinforced	Polyurethane foam (matrix) and silica aerogel particles.	Homogenization, high-speed mixing, and free foaming	KS M ISO844 [97] (compression test)	Liquefied natural gas (LNG) insulation systems used in cryogenic environments	Polyurethane foam-silica aerogel (1 wt.%) enhances the compressive strength and elastic modulus, but higher aerogel content reduces uniformity, weakening mechanical performance due to pore formation on the surface.
Selver E. et al. [2021] [27]	Impact damage test Flexural test Stress–strain behavior	Matrix-, fiber-, and particle-reinforced	Woven glass fabric (satin weave) (primary) and silica aerogels (secondary) into the epoxy resin matrix.	Multi-step ultrasonic-assisted and vacuum-degassed mixing process	ASTM-D790 [86] (flexural)	Applicable for producing composites with enhanced mechanical properties and thermal insulation	Composites exhibited improved flexural strength, modulus, and energy absorption with ductility trade-offs.
An L. et al. [2021] [19]	Compressive stress–strain analysis	Fiber-reinforced	Aramid fibers and ceramic aerogel	In situ cross-linking reaction and ambient pressure drying (APD)	NIL	Low-cost wearable textile for harsh environments	The low density and high compressive strength of the composite depend on cross-linked interfacial interactions, ensuring performance in extreme temperature conditions.
Zhang Z. et al. [2022] [98]	Fluidity test Flexural strength test Compressive strength test Four-point bending test	Matrix-, particle-, and fiber-reinforced	Polyvinyl alcohol (PVA) fibers, SiO2 particles, and aerogel and cement matrix	Ultrasonic dispersion, planetary mixing, and molding	GB/T 2419 [99] (slurry fluidity test) GB/T 17671-1999 [81] (flexural strength and compressive strength)	Reduced thermal conductivity ductile cement-based composite (RTCDCC) material enables durable, insulated building materials	The incorporation of silane enhances the mechanical properties of cementitious composites by improving bonding and increasing both flexural compressive strength. However, it also delays early hydration and contributes to the formation of additional microcracks in the specimens.
Wang G. et al. [2023] [100]	Tensile strength test Tear strength test Dynamic mechanical analysis	Matrix-, particle-, and fiber-reinforced	SiO2, chopped glass fiber, modified glass fiber, and styrene–butadiene rubber (matrix)	Mechanical mixing, surface modification, and vulcanization	GB/T 528-2009 [101] (tensile and tear strength)	Thermal insulation	The addition of modified glass fiber improves tensile strength, cross-linking density, and interface bonding with the rubber matrix, enhancing mechanical properties and amplifying the strain behavior in composites.
Zhu Z. et al. [2024] [11]	Compressive properties test Stress–strain behavior	Matrix- and fiber-reinforced	Multi-walled carbon nanotubes (MWCNTs) in silica aerogel matrix	Sol-gel, freeze drying, and ultrasonic dispersion	NIL	Cost-effective CNTs/silica aerogel for industrial use	Uniform CNT dispersion enhances mechanical properties below 4 wt.%, but agglomeration above 6 wt.% reduces them.

3. Thermal Characterization

3.1. Recent Advancements and Current Research Trends

Incorporating silica aerogel into refractory fibers can significantly improve their thermal insulation performance and extend their operational temperature range. The investigation in [102] highlighted the critical role of the fiber matrix in maintaining thermal insulation performance. Specifically, the incorporation of high-silica glass fibers (HSGFs) into the SiA matrix significantly enhanced the composite’s thermal stability at elevated temperatures. This improvement is attributed to the stable microstructure and phase composition of HSGF/SiA composites, which remain largely unchanged after high-temperature exposure—leading to minimal fluctuations in thermal conductivity post-heating. Additionally, glass-fiber mats/SiA composites have also displayed high thermal-insulating properties [103]. Similarly, the research in [104] introduced a delayed mixing technique, which proved to be a viable route for tailoring the density and thermal conductivity of epoxy-resin/SiA composites. Their findings underscored the importance of maintaining the aerogel’s porous architecture to achieve substantial reductions in the resin’s thermal conductivity.

A study has also reported efforts toward developing robust fibrous silica–bacterial cellulose (BC) composite aerogels, which demonstrate high thermal performance and are suitable for application in thermal insulation clothing fabrics [105]. These composites have also demonstrated superior thermal insulation across a wide temperature range (−72 °C to 210 °C), outperforming silk and cotton fabrics. In a comparative test at 80 °C, infrared imaging revealed that the composite layers retained significantly less heat than cotton, clearly confirming their better insulation efficiency. These advantages highlight their strong potential for use in wearable thermal insulation applications. Similarly, another study [106] incorporated sepiolite fibers into SiA composites, leveraging their unique hollow, brick-like fibrous structure to effectively disrupt heat transfer and enhance thermal insulation performance.

In recent years, SiA composites have gained significant attention for their role, driven by their outstanding insulation properties and lightweight nature, in energy and thermal management applications. For instance, due to rising global temperatures and growing energy demands, conventional insulation materials fall short in maintaining thermal comfort. To bridge this gap, researchers have integrated phase change energy storage into building insulation to enhance thermal regulation and energy efficiency. Phase change materials (PCMs) being the core of energy storage technology, are capable of storing or releasing significant heat at near-constant temperatures. Their high energy density, minimal volume change, and reliable performance make them ideal for enhancing building insulation. A study, in [107], developed a morphologically stable Na₂HPO₄·12H₂O SiA composite PCM (CPCM) with high enthalpy that demonstrated low thermal conductivity—effectively reducing heat transfer—and exhibited strong thermal inertia when exposed to high temperatures. The stabilized composite PCM was then fabricated into insulation boards, and its performance was assessed with a custom test setup that simulated building envelopes. The thermal performance evaluation revealed that CPCM panels significantly improve energy efficiency by increasing the thermal comfort duration at the center of the test unit by 43.38%. This demonstrated their strong potential for use in energy-saving building applications. Its excellent thermal reliability also allows it to maintain good working performance for extended durations in practical applications.

In the context of electric mobility, thermal runaway in power battery systems poses a pressing challenge, necessitating the development of high-performance thermal insulation materials capable of withstanding thermal-force coupling conditions. Addressing this challenge, the study in [108] developed a phenolic-reinforced SiA composite (RAs) that is capable of withstanding intense thermal shocks at 1000 °C. The composite demonstrated superior thermal insulation behavior under thermal-force coupling conditions, simulating real-world operational loads of 600 °C and up to 0.9 MPa. Instrumented thermal testing has revealed that increasing the silica content significantly reduces the thermal conductivity, thereby improving high-temperature thermal insulation and enhancing thermal shock resistance. These findings highlight the composite’s potential as a reliable, high-efficiency thermal barrier material for widespread use in thermal protection systems for new energy vehicle power batteries.

With growing demand for high-performance energy storage systems, enhancing battery safety and efficiency has also become a pressing concern. Among the critical components, battery separators play a decisive role not only in physically isolating electrodes, but also in influencing thermal stability and cycling durability. Traditional polyolefin separators, though commercially successful, suffer from low melting temperatures, raising serious safety concerns in lithium-ion batteries (LIBs). Therefore, the study in [109] presented a polypropylene/hydrophobic SiA composite separator that significantly enhanced thermal resilience, offering improved shape retention and minimizing the risk of internal short circuits. Their separator demonstrated a higher melting point compared to conventional separators, confirming its superior thermal stability. Such initiatives provide a new way to improve the safety and rate performance of power batteries.

Interestingly, another study [110] focused on the thermal insulation performance of an ambient pressure dried SiA composite, demonstrating its effectiveness, both at prototype and system levels, through the incorporation of realistic boundary conditions. Improving the energy performance of buildings fundamentally depends on reducing heat transfer through the building envelope. This can be effectively and feasibly achieved by integrating insulation materials with ultra-low thermal conductivity, such as aerogels (which provide superior thermal resistance even at reduced thicknesses). When integrated into building retrofitting activities, the composite maintains excellent thermal insulation under real-world conditions, closely matching its laboratory performance. These findings reinforce the potential of such cost-effective aerogel systems to significantly enhance building energy efficiency and aid in reducing CO₂ emissions.

Recently, significant attention has been directed toward the development of flexible and environment conscious thermal insulation materials. The study in [111] reported a utilization of temperature distribution curves to determine the thermal protective properties and thermal diffusivity of a polyethylene terephthalate (PET)/SiA nonwoven fiber composite. The thermal insulation performance of the composite stems from the synergy between the high porosity of nonwoven fibers ( 90%) and the nanostructured silica aerogel filling their micron-sized pores. The aerogel’s mesopores (<50 nm) significantly reduce gaseous thermal conductivity and thermal diffusivity, leading to a marked enhancement in the composite’s overall thermal protective properties compared to pure nonwoven fabrics.

Several studies have also employed thermogravimetric analysis (TGA) in SiA composites to evaluate their thermal stability, decomposition behavior, and residual mass at elevated temperatures. It typically helps to identify the temperature ranges where material degradation occurs, which is essential for assessing suitability in thermal insulation and high-temperature applications. For instance, an aramid nanofiber/SiA composite [112] and bioactive chitosan/SiA composite [113] underwent similar tests and showed successful results for their practical adoption. Similarly, Laskowski J. et al. [114] demonstrated that silica aerogels begin to lose hydrophobicity above 430 °C due to the oxidation of organic surface groups, resulting in a 22.3% weight loss under argon up to 1000 °C. The composite was then evaluated to determine its service temperature and was found suitable for low-temperature insulation (<200 °C). This is because RF aerogels begin to decompose above 250 °C, with noticeable weight loss starting even below 200 °C. However, when assessing thermal conductivity, tools like the TPS (transient-plane source) method have reported significant deviations—particularly for weakly conductive and heterogeneous materials—as seen in studies on low-density polyethylene foams, where results diverged by 14.8–52.5% compared to HFM (heat-flow meter) measurements. Therefore, the HFM method has proven to be more reliable than TPS in this case. Likewise, another study [115] systematically investigated the thermal insulation and heat transfer characteristics of aramid fiber-reinforced SiA composites through hot plate experiments, establishing a simple method to estimate thermal conductivity across various operating temperatures. Experimental measurement technology of thermal conductivity can be typically divided into two methods: the steady-state method and the transient-state method. Figure 4 further discusses and compares these two methods in detail.

Table 2 presents a timeline review of the notable research studies reported on the thermal conductivity and thermal performance of SiA composites over the last decade. This presentation offers a brief comparative description of the type of characteristics investigated, along with suitable standards, applications, material preparation methods, and critical insights.

3.2. Future Directions

SiA composites have great potential as insulation material. Despite their superior thermal insulation capabilities, organic insulation materials pose a significant fire hazard threat by their generation of intense heat and toxic smoke when ignited. In contrast, inorganic insulation materials, while offering better fire resistance, generally exhibit lower thermal insulation efficiency, resulting in increased structural weight when used extensively [116]. Consequently, there is a pressing need for the development of advanced insulation materials that overcome these limitations by reducing weight, enhancing fire resistance, and optimizing thermal insulation performance. While much of the previous research has concentrated on the synthesis and characterization of silica aerogels, there has been limited exploration into the high-temperature thermal insulation performance of silica aerogel-based composites [108]. Future studies should address this gap by investigating the behavior and efficiency of these composites under extreme conditions to better understand their potential for high-temperature applications. With the necessary optimizations, these lightweight engineered materials could also serve as effective thermal protection systems for future space applications [117].

Additionally, Martinez R. G. et al. [110] identified the key barriers limiting the large-scale adoption of super-insulating aerogel materials. Significant cost and energy savings can be unlocked by transitioning from traditional batch-based supercritical drying to more scalable continuous ambient pressure drying techniques. This shift, coupled with full solvent recovery and reuse, offers a more sustainable and economically viable path for large-scale aerogel production. They also emphasized that further research should focus on reducing production costs and developing scalable manufacturing technologies as these factors are crucial for advancing the use of SiA composites in building insulation applications.

The investigations in [104] explored how varying silica aerogel weight fractions and particle sizes affect the thermal conductivity of aerogel/epoxy composites. Initially, increasing aerogel content linearly reduced thermal conductivity due to enhanced scattering and fewer heat pathways. Smaller particles were more effective at low loadings because they introduced more barriers per weight. However, at higher loadings, resin infiltration into the aerogel pores, which is more pronounced in smaller particles due to their high surface area, diminishes their insulating effect. As a result, while smaller particles provide initial benefits, their advantage decreases at higher concentrations because of pore clogging. Further exploration into the impact of particle size and resin infiltration on thermal conductivity would help optimize the use of silica aerogels in composites, enhancing their insulation properties. Limited research has been conducted in this area, particularly regarding the balance between aerogel loading and pore clogging. Delving into this further could lead to more efficient composite formulations with improved thermal performance.

Similarly, the use of liquid epoxy as a binder in aerogel/epoxy composites has shown limited thermal insulation performance, likely due to the deep penetration of epoxy into the aerogel’s nanopores. However, this phenomenon remains insufficiently explored [118]. Further nanoscale investigation of binder–pore interactions will help improve interfacial compatibility, minimize pore clogging, and preserve the low thermal conductivity of silica aerogels, ultimately supporting the development of high-performance insulating composites.

Previous studies have relied on coaxial wet spinning techniques to fabricate hollow polymer fibers that serve as protective layers for aerogels. While effective, this method is both intricate and time consuming, limiting its scalability and industrial application. In contrast, recent research [105] has proposed novel non-spinning approaches to fabricate inorganic oxide-based aerogel fibers with enhanced thermal insulation and mechanical properties. Advancing such simplified fabrication techniques offers researchers the opportunity to develop high-strength, thermally efficient SiA fibers for use in extreme environments that are particularly valuable in aerospace, protective textiles, and advanced insulation systems.

Table 2. Milestones in the thermal characterization assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Reinforcing Material	Processing Method	Standards	Application	Key Insights
Wang X. et al. [2014] [119]	Thermal evolution process (differential thermal analysis, DTA)	Particle-reinforced	Titanium dioxide (TiO2)	Sol-gel and APD	NIL	Removal of organic pollutants	Increasing the heating rate is favorable for the crystallization of TiO2.
Eskandari N. et al. [2016] [120]	Thermal conductivity (TC) Vicat softening temperature	Matrix- and particle-reinforced	Silica aerogel (particles) and unplasticized polyvinyl chloride (UPVC) (matrix)	Two step sol-gel process, melt mixing, and hot pressing	ASTM D 1525 [121] (vicat softening temperature) ASTM C177 [122] (thermal conductivity)	Drainage pipes and window profiles in buildings	SiA significantly reduces UPVC’s TC, improving insulation for energy-efficient windows and buildings.
Li C. et al. [2016] [10]	Thermal conductivity (transient plane source) Flammability test Thermal stability analysis	Fiber-reinforced	Glass fiber (GF) film	Sol-gel and APD	ISO 5660 [123] (flammability)	Fire resistance and flexible composites	The fire hazards of the GF/aerogel composites decrease with an increase in the S value (where S is the H2O: TEOS molar ratio).
Li Z. et al. [2016] [15]	Thermal conductivity (hot disk method) Thermal stability analysis	Matrix- and fiber-reinforced	Aramid fiber (AF) (Kevlar-49) and silica aerogel (matrix)	Sol-gel and APD	NIL	Piping heat insulation	AF/aerogels exhibit ultra-low thermal conductivity and stability, making them ideal for heat insulation applications.
Lee K. J. et al. [2017] [22]	Thermal conductivity (heat flow meter method)	Fiber-reinforced	Polyethylene (PE) fiber blankets	Impregnation technique (using silica aerogel slurries) and APD	ASTM C518 [124] (thermal conductivity) ISO DIS 8301 [125] (thermal conductivity)	Super-insulation material	The composite’s thermal conductivity values were approximately 20% lower than those of bare PE and PE nonwoven fabrics without silica aerogel powders.
Ye X. et al. [2019] [20]	Thermal conductivity	Matrix- and particle-reinforced	Silicon carbide (SiC) coating-reinforced carbon foam (CF)	Sol-gel, pyrolysis, CVD, and APD	NIL	Thermal insulation	Obtained SiC/CF-aerogel possessed a superior high temperature insulation property than CF and SiC/CF.
Shang L. et al. [2019] [21]	Thermal conductivity (transient hot wire method) Thermal insulation performance	Fiber-reinforced	Anti-infrared radiation silica fibers	Sol-gel and supercritical CO2 drying	NIL	Aerospace insulation material (thermal protection structures)	Composite was stable under sustained durations in a high-temperature oxygen environment of 250 °C.
Liu Y. et al. [2020] [7]	Thermal conductivity (transient plane source) Moisture absorption Water vapor transmission	Fiber-reinforced	Polyester fiber felt (support) and silica aerogel (filling medium)	Sol-gel and APD	GB/T 5480-2017 [126] (moisture absorption) GB/T 12704.2-2009 [127] (water-vapor transmission)	Building insulation (even in humid areas)	Compounding fiber felt with aerogel reduces voids, lowering water vapor transmission and improving thermal insulation.
Krzemińska S. et al. [2020] [68]	Thermal conductivity Thermal stability analysis Resistance to thermal radiation (RHTI24) Thermal resistance	Fiber-reinforced	Aramid fibers (Kermel® meta-aramid fibers, 98% content) and conductive fiber (2% content)	Coating technique	EN ISO 6942:2002 [128] (resistance to thermal radiation) EN ISO 11612:2015 [129] (radiant heat transfer index (RHTI24))	Protective clothing fabric to improve its heat resistance	Aerogel coatings improved thermal resistance and reduced conductivity, but aerogel fragmentation during coating processing compromises insulation.
Yang H. and Ye F. [2022] [130]	Thermal conductivity (TC) (calculation-based approach) Specific heat capacity Thermal diffusivity	Particle-reinforced	Si3N4 (primary) and TiO2 (additional)	Sol-gel and APD	NIL	High-temperature wave permeable insulation material	At higher TiO2 contents, the thermal diffusivity and TC of the composites decrease, while an excessive increase is unsuitable for wave-transmitting applications.
Wang X. et al. [2023] [131]	Thermal conductivity (TC) (transient hot wire method) Thermal stability analysis Gross calorific value Combustion properties	Particle-reinforced	Montmorillonite (MMT)	Two-step acid-base catalyzed sol-gel process and APD	ISO 5660:2015 [123] (combustion properties)	Thermal insulation	MMT/SiA exhibit reduced TC, lower heat release rate, and improved thermal stability, enhancing thermal insulation and reducing fire hazards with increasing MMT content.
Pantaleo S. et al. [2024] [132]	Thermal conductivity (TC) (transient line source method) Thermal stability	Matrix- and particle-reinforced	Silica aerogel granulates (particle), acrylic copolymer emulsion, styrene-acrylic copolymer emulsion, vinyl acetate, ethylene, and acrylate emulsion (matrix)	Hand mixing and stirring	NIL	Housing refurbishing	The composites maintain low TC and exhibit minimal moisture absorption, optimizing energy efficiency in housing applications.
Yu D. et al. [2024] [133]	Thermal conductivity (guarded hot plate method) Thermal stability Thermal insulation performance Thermal shock resistance	Fiber- and particle-reinforced	Alkali-free glass fiber (GF) and silicon carbide (SiC) particles	Sol gel and super-critical drying	GB/T 10294-2008 [134] ISO 8302 [135] ASTM C177 [122] (all abv. thermal conductivity)	Architectural insulation	Composite exhibits excellent thermal insulation and improves thermal shock resistance with SiC.
Zhang T. et al. [2024] [136]	Thermal conductivity (heat flow meter method, guarded hot plate method, and water flow method) Thermal stability and insulation performance	Fiber-reinforced	Ceramic (aluminum silicate) fiber felt (CF)	Sol-gel method with supercritical CO2 drying	ASTM C518 [124] and ISO 8301 [125] (heat flow meter method) GB/T 10294-2008 [134], ISO 8302 [135] and ASTM C177 [122] (guarded hot plate method) YB/T4130-2005 [137] and ASTM C201-93 [138] (water flow method)	Thermal insulation in lithium-ion batteries	Flexible silica aerogel composites exhibit excellent thermal stability, low thermal conductivity, and strong thermal shock resistance, making them ideal for high-temperature insulation applications.
Liu M. at al. [2024] [139]	Thermal conductivity (water flow method, heat flow meter method, and plate heat flow meter method) Thermal stability Thermal insulation performance	Fiber- and particle-reinforced	Aerogel-supported carbon fiber and silicon carbide (SiC)	Single-step sol-gel impregnation and super critical drying CO2	YB/T 4130-2005 [137] and ASTM C201-93 [138] (water flow method) ASTM C518 [124] and ISO 8301 [125] (heat flow meter method) GB/T10295-2008 [140] and ASTM C518-04 [124] (plate heat flow meter method)	Thermal insulation (high temperature and thermal-force coupling conditions)	Composite exhibits excellent thermal insulation, maintains stability under extreme conditions, resists thermal stress, and effectively prevents thermal runaway propagation in high-energy systems.

Table 2. Milestones in the thermal characterization assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Reinforcing Material	Processing Method	Standards	Application	Key Insights
Wang X. et al. [2014] [119]	Thermal evolution process (differential thermal analysis, DTA)	Particle-reinforced	Titanium dioxide (TiO2)	Sol-gel and APD	NIL	Removal of organic pollutants	Increasing the heating rate is favorable for the crystallization of TiO2.
Eskandari N. et al. [2016] [120]	Thermal conductivity (TC) Vicat softening temperature	Matrix- and particle-reinforced	Silica aerogel (particles) and unplasticized polyvinyl chloride (UPVC) (matrix)	Two step sol-gel process, melt mixing, and hot pressing	ASTM D 1525 [121] (vicat softening temperature) ASTM C177 [122] (thermal conductivity)	Drainage pipes and window profiles in buildings	SiA significantly reduces UPVC’s TC, improving insulation for energy-efficient windows and buildings.
Li C. et al. [2016] [10]	Thermal conductivity (transient plane source) Flammability test Thermal stability analysis	Fiber-reinforced	Glass fiber (GF) film	Sol-gel and APD	ISO 5660 [123] (flammability)	Fire resistance and flexible composites	The fire hazards of the GF/aerogel composites decrease with an increase in the S value (where S is the H2O: TEOS molar ratio).
Li Z. et al. [2016] [15]	Thermal conductivity (hot disk method) Thermal stability analysis	Matrix- and fiber-reinforced	Aramid fiber (AF) (Kevlar-49) and silica aerogel (matrix)	Sol-gel and APD	NIL	Piping heat insulation	AF/aerogels exhibit ultra-low thermal conductivity and stability, making them ideal for heat insulation applications.
Lee K. J. et al. [2017] [22]	Thermal conductivity (heat flow meter method)	Fiber-reinforced	Polyethylene (PE) fiber blankets	Impregnation technique (using silica aerogel slurries) and APD	ASTM C518 [124] (thermal conductivity) ISO DIS 8301 [125] (thermal conductivity)	Super-insulation material	The composite’s thermal conductivity values were approximately 20% lower than those of bare PE and PE nonwoven fabrics without silica aerogel powders.
Ye X. et al. [2019] [20]	Thermal conductivity	Matrix- and particle-reinforced	Silicon carbide (SiC) coating-reinforced carbon foam (CF)	Sol-gel, pyrolysis, CVD, and APD	NIL	Thermal insulation	Obtained SiC/CF-aerogel possessed a superior high temperature insulation property than CF and SiC/CF.
Shang L. et al. [2019] [21]	Thermal conductivity (transient hot wire method) Thermal insulation performance	Fiber-reinforced	Anti-infrared radiation silica fibers	Sol-gel and supercritical CO2 drying	NIL	Aerospace insulation material (thermal protection structures)	Composite was stable under sustained durations in a high-temperature oxygen environment of 250 °C.
Liu Y. et al. [2020] [7]	Thermal conductivity (transient plane source) Moisture absorption Water vapor transmission	Fiber-reinforced	Polyester fiber felt (support) and silica aerogel (filling medium)	Sol-gel and APD	GB/T 5480-2017 [126] (moisture absorption) GB/T 12704.2-2009 [127] (water-vapor transmission)	Building insulation (even in humid areas)	Compounding fiber felt with aerogel reduces voids, lowering water vapor transmission and improving thermal insulation.
Krzemińska S. et al. [2020] [68]	Thermal conductivity Thermal stability analysis Resistance to thermal radiation (RHTI24) Thermal resistance	Fiber-reinforced	Aramid fibers (Kermel® meta-aramid fibers, 98% content) and conductive fiber (2% content)	Coating technique	EN ISO 6942:2002 [128] (resistance to thermal radiation) EN ISO 11612:2015 [129] (radiant heat transfer index (RHTI24))	Protective clothing fabric to improve its heat resistance	Aerogel coatings improved thermal resistance and reduced conductivity, but aerogel fragmentation during coating processing compromises insulation.
Yang H. and Ye F. [2022] [130]	Thermal conductivity (TC) (calculation-based approach) Specific heat capacity Thermal diffusivity	Particle-reinforced	Si3N4 (primary) and TiO2 (additional)	Sol-gel and APD	NIL	High-temperature wave permeable insulation material	At higher TiO2 contents, the thermal diffusivity and TC of the composites decrease, while an excessive increase is unsuitable for wave-transmitting applications.
Wang X. et al. [2023] [131]	Thermal conductivity (TC) (transient hot wire method) Thermal stability analysis Gross calorific value Combustion properties	Particle-reinforced	Montmorillonite (MMT)	Two-step acid-base catalyzed sol-gel process and APD	ISO 5660:2015 [123] (combustion properties)	Thermal insulation	MMT/SiA exhibit reduced TC, lower heat release rate, and improved thermal stability, enhancing thermal insulation and reducing fire hazards with increasing MMT content.
Pantaleo S. et al. [2024] [132]	Thermal conductivity (TC) (transient line source method) Thermal stability	Matrix- and particle-reinforced	Silica aerogel granulates (particle), acrylic copolymer emulsion, styrene-acrylic copolymer emulsion, vinyl acetate, ethylene, and acrylate emulsion (matrix)	Hand mixing and stirring	NIL	Housing refurbishing	The composites maintain low TC and exhibit minimal moisture absorption, optimizing energy efficiency in housing applications.
Yu D. et al. [2024] [133]	Thermal conductivity (guarded hot plate method) Thermal stability Thermal insulation performance Thermal shock resistance	Fiber- and particle-reinforced	Alkali-free glass fiber (GF) and silicon carbide (SiC) particles	Sol gel and super-critical drying	GB/T 10294-2008 [134] ISO 8302 [135] ASTM C177 [122] (all abv. thermal conductivity)	Architectural insulation	Composite exhibits excellent thermal insulation and improves thermal shock resistance with SiC.
Zhang T. et al. [2024] [136]	Thermal conductivity (heat flow meter method, guarded hot plate method, and water flow method) Thermal stability and insulation performance	Fiber-reinforced	Ceramic (aluminum silicate) fiber felt (CF)	Sol-gel method with supercritical CO2 drying	ASTM C518 [124] and ISO 8301 [125] (heat flow meter method) GB/T 10294-2008 [134], ISO 8302 [135] and ASTM C177 [122] (guarded hot plate method) YB/T4130-2005 [137] and ASTM C201-93 [138] (water flow method)	Thermal insulation in lithium-ion batteries	Flexible silica aerogel composites exhibit excellent thermal stability, low thermal conductivity, and strong thermal shock resistance, making them ideal for high-temperature insulation applications.
Liu M. at al. [2024] [139]	Thermal conductivity (water flow method, heat flow meter method, and plate heat flow meter method) Thermal stability Thermal insulation performance	Fiber- and particle-reinforced	Aerogel-supported carbon fiber and silicon carbide (SiC)	Single-step sol-gel impregnation and super critical drying CO2	YB/T 4130-2005 [137] and ASTM C201-93 [138] (water flow method) ASTM C518 [124] and ISO 8301 [125] (heat flow meter method) GB/T10295-2008 [140] and ASTM C518-04 [124] (plate heat flow meter method)	Thermal insulation (high temperature and thermal-force coupling conditions)	Composite exhibits excellent thermal insulation, maintains stability under extreme conditions, resists thermal stress, and effectively prevents thermal runaway propagation in high-energy systems.

4. Theoretical and Numerical-Based Frameworks

4.1. Recent Advancements and Current Research Trends

Silica aerogels are typical nanoporous materials known for their ultra-low thermal conductivity, which enhances its insulation properties. It is possible that the aerogel matrix, fibers, and opacifiers coexist in SiA composites. Solid conduction, gas conduction, and radiation are the primary heat transfer modes in aerogels. It is also widely known that the doping fibers and opacifiers increase the solid heat conduction and reduce the thermal radiation. The nanoporous silica aerogel skeleton creates an exceptionally long conduction path, significantly reducing solid-phase thermal conductivity. Similarly, gas-phase thermal conductivity is also effectively suppressed as a result of the ultra-fine pore size (~20 nm), which is smaller than the mean free path of gas molecules (~70 nm at atmospheric pressure). Therefore, effective thermal conductivity being the central focus, several theoretical, experimental, and numerical studies have been carried out to investigate insulation performance of SiA composites.

The models primarily utilized for SiA composites can be broadly divided into four categories, namely the superposition model, periodic regular unit cell model, finite volume method, and the direct experimental method. In the first model, the superposition model adds the solid, gas, and radiation thermal conductivity together. Similarly, another approach involves adopting a simplified unit cell model to analyze the overall solid–gas conductivity (λc), serving as an alternative to the conventional superposition model of solid and gas thermal conductivity. This introduces the second model based on the periodic regular unit cell simplified from the complex nanoporous structure. Studies, such as that in [141], have reported the utilization of the unit cell model to predict the effective thermal conductivity of SiA composites. Results obtained from the theoretical model showed great agreement with experimental results. Investigations have also proposed a fractal-intersecting sphere model, where the scale effect on gas conduction and solid-matrix conduction were both considered [142]. To account for the multiphase material’s radiative thermal conductivity and conductive thermal conductivity, Rosseland approximation and a mixing model were utilized, respectively. The model calculated the thermal conductivities of SiA with opacifiers and fibers, while further investigations reported the effects of doping concentration and particle size on thermal conductivity.

The third model, being a numerical method, is essentially based on the lattice Boltzmann method (LBM) [143,144] or finite volume method [145,146]. Similarly, the fourth model of direct experimentation, involves the adoption of the guarded hot plate, hot strip, or hot-wire method. By complementing numerical models that offer detailed thermal transport predictions with experimental validation, researchers ensure both computational accuracy and real-world applicability, as described in the following studies. For instance, it is well known that the microstructure influences the effective thermal conductivity of SiA composites; therefore, a proper reconstruction structure is required for the numerical method. The study in [144] adopted two reconstruction methods, namely the diffusion-limited cluster-cluster aggregation method (DCLA) and the open-cell method, to regenerate the microstructure of SiA. Discrete ordinate method (DOM) was adopted to solve the radiative transfer equation, while LBM was adopted to solve the energy transport equation with the volumetric radiative source term obtained by solving the radiative transfer equation. The Mie theory was then utilized to obtain the extinction coefficients of silica aerogel composites. The study then presented numerical predictions of the total effective thermal conductivity based on the two types of reconstruction methods adopted, and it then reported comparisons with the experimental data measured by the hot disk method. Numerical predictions based on the open-cell (netlike) structure showed better agreement with the experimental data compared to those based on the DLCA structure. In the DLCA method, the reconstruction process results in a single cluster, creating a lone, elongated heat flux path from top to bottom through the solid skeleton. This severely restricts thermal transport, causing the predicted effective thermal conductivity to approach zero. In contrast, the open-cell method generates a network of continuous solid skeleton chains along the heat transfer direction, facilitating multiple heat conduction pathways. As a result, numerical predictions based on the open-cell structure yield finite and more accurate thermal conductivity values. Similarly, the research in [146] performed structure reconstruction of the particle type and fibrous type multiphase material to investigate the combined conduction and radiation heat transfer of SiA material doped with opacifier particles and reinforced fibers. A key challenge in the heat transfer research of multiphase materials is to understand how their intricate structures influence thermal conductivity. The microstructures of real-world multiphase materials are also highly complex. The study then reported the utilization of the finite volume method with the discrete ordinate method within a numerical model to solve the energy equation and radiative transfer equation, respectively.

Interestingly, the researchers in [144] mentioned their preference of the Mie theory over Fourier-transform infrared spectroscopy. The Mie theory provides an exact, analytical solution to Maxwell’s equations for light scattering and absorption by spherical particles. This means it can precisely predict the spectral extinction coefficient—including both scattering and absorption contributions—based solely on particle size, refractive index, and wavelength. In contrast, FTIR spectroscopy is an experimental technique that measures absorption spectra and often cannot isolate the scattering component without additional modeling. Thus, when detailed theoretical insight and precise separation of scattering and absorption are required, the Mie theory is the preferred tool.

Recently, novel strategies to design lightweight super-insulating materials for high-temperature thermal insulation applications have also been reported [147]. Doped with hollow spherical opacifiers, their work investigated the influence of the outer diameter, inner diameter, and the mass fraction of doped hollow opacifiers. Meanwhile, there has been another study that has proposed theoretical frameworks for utilizing hollow fibers to reduce both the thermal radiation and heat conduction in SiA under high-temperature conditions [148]. These studies enhance the understanding of heat transfer mechanisms in porous materials, especially those with hollow structures, providing key insights for designing high-temperature thermal insulation systems. Interestingly, recent studies have also reported improvements in the accuracy of theoretical models over the much-used standard models over the years. For instance, the authors in [149] claimed higher model accuracy over Bjurström’s model [150] due to the latter’s failure in predicting the effective thermal conductivity of moist silica aerogel. Bjurström’s model held the assumption of homogeneous water film, which was not appropriate to describe the relationship of the solid, liquid, and gas phases. This highlights the need for more refined models that accurately capture the complex interactions between phases in SiA composites.

Table 3 presents a timeline review of the notable research studies reported on the theoretical and numerical frameworks of SiA composites over the last decade. This presentation offers a brief comparative description of the type of characteristics investigated through each model, along with the suitable temperature ranges, applications, and critical insights. It highlights the evolution of theoretical and numerical frameworks tailored to various SiA composites, incorporating several fibers and opacifiers.

4.2. Future Directions

While theoretical and numerical frameworks for thermal conductivity modeling have made significant strides, substantial gaps remain unaddressed as many existing models rely heavily on assumptions, leaving immense potential yet to be explored. Chen Y. et al. [149] have suggested the need for further studies to identify the influence of temperature and humidity on the insulating performance of moist silica aerogel composites. Particularly, the lack of quantitative agreement between existing theoretical results and experimental data that has been reported for moist-silica nanoporous materials with high-water content. Most importantly, their study acknowledged that incorporating the complex mechanisms of liquid heat conduction was beyond its scope. The lack of existing literature to establish empirical correlations for nanoscale thermal conductivity has remained the essential reason for its omittance. Further, the existing literature lacks comprehensive discussions on the moisture absorption characteristics and the service temperature limits of SiA composites [143]. Addressing moisture absorption issues can improve the long-term stability and reliability of SiA composites, reducing degradation over time. This would greatly reduce replacement costs and material wastage. Similarly, Zhao J. et al. [145] brought attention to the limited applicability of simple empirical models for analyzing different fiber distributions, as the non-uniform fiber distributions greatly affect the fiber conduction. Relatively rare simulation-based studies have explored the combined conductive and radiative heat transfer in thermal insulation, which closely model the actual fiber distributions.

Due to the inherently porous structure of aerogel blankets, defining their effective thermal conductivity is essential for accurately predicting their thermal resistance value across different operating conditions, which, henceforth, enhances their thermal efficiency in innovative designs [151]. There has been limited literature that has focused on modeling the thermal performance of aerogel blankets. Similarly, another study has also reported a lack of existing literature on the theoretical models of SiA composites, which are capable of predicting the complex refractive indexes of opacifiers [152]. Further research on theoretical models predicting refractive indices would provide insights into the influence of SiA-based insulation on transparency and infrared absorption, enabling the development of precise simulation tools for analyzing heat shielding and optical transparency in various environments, ultimately leading to more energy-efficient materials. Although several analytical methods (Rosseland approximation, the Mie scattering theory, the Hamilton model, and the Maxwell model) can be utilized in the investigation of heat transfer characteristics of SiA composites, some assumptions would lead to inevitable calculation errors [146]. Therefore, a more exact numerical model for the investigation of the combined effects of conduction and radiation heat transfer is necessary to specifically identify the influence of reinforced fibers and opacifier particles on the thermal insulation performance of SiA composites. Further insights on the common analytical methods are comparatively presented in Figure 5. The influence of doping concentration, particle type, and fiber distribution on the heat transfer behavior of SiA composite insulation materials has also not been fully examined. Similarly, very little theoretical work has been performed on opacifiers to improve its infrared extinction performance through surface microstructure alteration [153].

The intricate multi-scale system and complex spatially random structure of aerogels present significant challenges in the accurate and efficient prediction of their effective thermal conductivity. Clearly, experimental measurements and numerical simulations offer higher precision compared to other methods, but they are often labor-intensive, computationally expensive, and time consuming. Consequently, these approaches fail to meet the demand for the rapid evaluation of the effective thermal conductivity of aerogel composites in engineering applications. Therefore, similar to the work in [154], further intensive research should be carried out in the development of complete calculation algorithms for faster prediction of efficient thermal conductivity. The absence of such holistic models underscores the need for novel frameworks that bridge theoretical precision with computational feasibility, opening new frontiers in thermal conductivity research.

Table 3. Milestones in the theoretical- and numerical-based frameworks of SiA composites over the past decade.

Author and Year	Objective	Reinforcement	Characteristics Studied	Relations and Equations	Temperature	Application	Key Insights
Zhao J. J. et al. [2012] [145]	A numerical study that investigated the effective thermal conductivity of fiber-loaded composites employed a randomly parameterized 2D fiber distribution to simulate a realistic material structure, integrating heat conduction and radiation models.	Silica aerogel fiber-reinforced	Effective thermal conductivity Fiber extinction coefficient	Modified anomalous diffraction theory (fiber radiative properties) Two-flux radiation method (simulate radiative heat transfer) Finite volume method (solve heat transfer equations) Steady state energy equation (steady-state heat transfer)	High temperature	High-temperature thermal insulation applications	Effective thermal conductivity depends upon the fiber-length-to-diameter ratio, the inclination angle, and the fiber volume fraction.
Wang X. D. et al. [2013] [152]	Theoretical method with four sub-models to evaluate radiative characteristics and propose an optimized gradient design for temperature-dependent opacifier properties.	SiC-, TiO2-, ZrO-2, coal ash-, carbon black-, and Al2O3-particle-reinforced	Radiative heat conductivity Spectral extinction coefficient Rossland extinction coefficient	Fourier infrared spectral experiment (transmittance) Modified Kramers–Krönig (K–K) relation (complex refractive index) Spectral and Rossland extinction Radiative heat transfer model (radiative thermal conductivity) Beer–Lambert law (relates extinction to opacifier content) Mie theory (extinction efficiency)	High temperature (1300 K)	High-temperature application	SiC performs best at high temperatures; opacifier diameter affects radiative characteristics.
Hoseini A. et al. [2015] [151]	Comparison, accounting for conduction and radiation, of the theoretical and experimental thermal conductivity of composites, which were modeled using a unit cell with cylindrical fibers.	Cryogel Z- and thermal wrap fiber-reinforced	Effective thermal conductivity Porosity	Unit cell model Conduction heat transfer model (solid and gas phases) Zehner–Schlunder model (spherical packed bed’s thermal conductivity) Diffusion approximation method (radiative heat transfer)	−20 °C–80 °C	Thermal insulation systems	The effective thermal conductivity of Cryogel Z is less than thermal wrap; high porosity, small pore sizes, and large surface area are key to low thermal conductivity in aerogel blankets.
Dai Y. J. et al. [2017] [154]	To validate a theoretical model using a spherical hollow cube structure to predict and optimize SiA composites thermal conductivity under varying conditions.	SiO2 (fiber), SiC, TiO2 and C (opacifiers) are fiber- and particle-reinforced.	Effective thermal conductivity Radiative thermal conductivity Conductive thermal conductivity	Spherical hollow cube model (skeleton thermal conductivity of pure aerogels) Modified mixing model (prediction of the overall conduction) Mie theory (extinction efficiency and Rosseland mean extinction coefficient) Rosseland equation (predicts radiative thermal conductivity) Effective thermal conductivity equation Gas thermal conductivity equation	Room temperature–1000 K	Thermal insulation applications	SiC demonstrates the best overall performance, while a fiber diameter of approximately 4 μm and temperature-dependent optimal fiber concentrations effectively minimize thermal conductivity.
Liu H. et al. [2022] [155]	To study the effective thermal conductivity of fiber-reinforced composites through theoretical models, focusing on randomly distributed fibers and optimizing their diameter, inclination, and mass fraction for high-temperature insulation.	Silica aerogel fiber-reinforced	Radiative thermal conductivity Spectral extinction coefficient Effective thermal conductivity	Mie theory (radiative properties) Rosseland diffusion approximation (radiative heat transfer) Thermal conductivity model (combines conductive and radiative contributions) Series and parallel models (effective conductivity) Diffuse mismatch model (contact thermal resistance) Hamilton–Crosser semi-theoretical model (conductive thermal conductivity)	600–1400 K	High-temperature thermal insulation application	Effective thermal conductivity of fiber-reinforced composites depends on the fiber alignment (where a perpendicular angle minimizes), diameter, and mass fraction, optimizing thermal insulation across 600–1400 K.

Table 3. Milestones in the theoretical- and numerical-based frameworks of SiA composites over the past decade.

Author and Year	Objective	Reinforcement	Characteristics Studied	Relations and Equations	Temperature	Application	Key Insights
Zhao J. J. et al. [2012] [145]	A numerical study that investigated the effective thermal conductivity of fiber-loaded composites employed a randomly parameterized 2D fiber distribution to simulate a realistic material structure, integrating heat conduction and radiation models.	Silica aerogel fiber-reinforced	Effective thermal conductivity Fiber extinction coefficient	Modified anomalous diffraction theory (fiber radiative properties) Two-flux radiation method (simulate radiative heat transfer) Finite volume method (solve heat transfer equations) Steady state energy equation (steady-state heat transfer)	High temperature	High-temperature thermal insulation applications	Effective thermal conductivity depends upon the fiber-length-to-diameter ratio, the inclination angle, and the fiber volume fraction.
Wang X. D. et al. [2013] [152]	Theoretical method with four sub-models to evaluate radiative characteristics and propose an optimized gradient design for temperature-dependent opacifier properties.	SiC-, TiO2-, ZrO-2, coal ash-, carbon black-, and Al2O3-particle-reinforced	Radiative heat conductivity Spectral extinction coefficient Rossland extinction coefficient	Fourier infrared spectral experiment (transmittance) Modified Kramers–Krönig (K–K) relation (complex refractive index) Spectral and Rossland extinction Radiative heat transfer model (radiative thermal conductivity) Beer–Lambert law (relates extinction to opacifier content) Mie theory (extinction efficiency)	High temperature (1300 K)	High-temperature application	SiC performs best at high temperatures; opacifier diameter affects radiative characteristics.
Hoseini A. et al. [2015] [151]	Comparison, accounting for conduction and radiation, of the theoretical and experimental thermal conductivity of composites, which were modeled using a unit cell with cylindrical fibers.	Cryogel Z- and thermal wrap fiber-reinforced	Effective thermal conductivity Porosity	Unit cell model Conduction heat transfer model (solid and gas phases) Zehner–Schlunder model (spherical packed bed’s thermal conductivity) Diffusion approximation method (radiative heat transfer)	−20 °C–80 °C	Thermal insulation systems	The effective thermal conductivity of Cryogel Z is less than thermal wrap; high porosity, small pore sizes, and large surface area are key to low thermal conductivity in aerogel blankets.
Dai Y. J. et al. [2017] [154]	To validate a theoretical model using a spherical hollow cube structure to predict and optimize SiA composites thermal conductivity under varying conditions.	SiO2 (fiber), SiC, TiO2 and C (opacifiers) are fiber- and particle-reinforced.	Effective thermal conductivity Radiative thermal conductivity Conductive thermal conductivity	Spherical hollow cube model (skeleton thermal conductivity of pure aerogels) Modified mixing model (prediction of the overall conduction) Mie theory (extinction efficiency and Rosseland mean extinction coefficient) Rosseland equation (predicts radiative thermal conductivity) Effective thermal conductivity equation Gas thermal conductivity equation	Room temperature–1000 K	Thermal insulation applications	SiC demonstrates the best overall performance, while a fiber diameter of approximately 4 μm and temperature-dependent optimal fiber concentrations effectively minimize thermal conductivity.
Liu H. et al. [2022] [155]	To study the effective thermal conductivity of fiber-reinforced composites through theoretical models, focusing on randomly distributed fibers and optimizing their diameter, inclination, and mass fraction for high-temperature insulation.	Silica aerogel fiber-reinforced	Radiative thermal conductivity Spectral extinction coefficient Effective thermal conductivity	Mie theory (radiative properties) Rosseland diffusion approximation (radiative heat transfer) Thermal conductivity model (combines conductive and radiative contributions) Series and parallel models (effective conductivity) Diffuse mismatch model (contact thermal resistance) Hamilton–Crosser semi-theoretical model (conductive thermal conductivity)	600–1400 K	High-temperature thermal insulation application	Effective thermal conductivity of fiber-reinforced composites depends on the fiber alignment (where a perpendicular angle minimizes), diameter, and mass fraction, optimizing thermal insulation across 600–1400 K.

5. Radiation and Flammability Assessments

5.1. Recent Advancements and Current Research Trends

Radiation heat transfer and solid conduction are the possible ways of heat transfer mechanisms in vacuum conditions. It is crucial for materials to maintain their performance for sustained durations in space-environments. Therefore, to accurately measure aging and degradation due to environmental factors (high vacuum, thermal degassing, particle radiation, etc.), it is required to simulate operating space conditions through ground test activities or laboratory-based simulations. Typically, the thermal cycling test evaluates the material’s capability to survive thermal stresses within a space environment, while the outgassing test measures the material’s volatile content in a vacuum environment. Silica aerogels are known to demonstrate phenomenal properties in low-pressure environments at cryogenic temperatures. Rocha H. et al. [8] examined the properties of two silica aerogel composites that were reinforced with quartz and polyethylene terephthalate (PET) fibers, respectively. These composites displayed an inherent particle shedding tendency, presenting a risk of debris release during operations. Their study reported that the gamma radiation exposure led to an increase in storage modulus for both the fiber-reinforced aerogels, with a stronger effect observed in the PET fiber-reinforced variant. Despite the gamma radiation, the aerogels maintained their chemical structure and hydrophobicity, reporting contact angles well above 90°. Moreover, to mitigate contamination of other flight hardware, the authors recommended encapsulating these materials within a multi-layer insulation (MLI) or using other necessary protective solutions for full space qualification.

While the previous study provided critical insights on the interaction with gamma exposure, another study evaluated the damage on polyimide (PI) silica aerogel composites that is caused by space irradiation [18]. Their study observed the significant polymer degradation caused by proton irradiation, while UV radiation was found to primarily alter the molecular bonding state. Although both types of radiation influenced the composite’s mechanical properties, electron irradiation had minimal impact on its mechanical integrity and chemical stability. The study also reported that the PI-SiA composite with 40% silica aerogel powder showed no significant changes in chemical composition, microstructure, or key properties after thermal cycling, indicating strong resistance to long-term thermal stress. It remains largely unaffected by electron irradiation, with only slight degradation under proton and UV exposure. Further, necessary thermal degassing and fire resistance tests confirmed the material’s adaptability in both prior studies. Similar thermal cycling and outgassing tests have been reported on aramid fiber-reinforced SiA composites, which positively concluded their applicability for space-based applications [156].

Similar to the impact of cosmic rays on materials, the electromagnetic radiation and wave interference across different frequencies can induce adverse effects on both devices and biological systems if found within the range of an electromagnetic field. Emerging applications in satellite enclosures, wearable electronics, and aerospace insulation demands materials that simultaneously exhibit electromagnetic (EM) shielding, flame retardancy, and environmental resilience. These materials are frequently subjected to intense radiative heat in the above-mentioned applications. High-frequency EM waves, especially in dense circuitry or microwave regimes, can lead to localized heating, creating secondary ignition risks. Hence, EM shielding relates indirectly to thermal safety and fire mitigation. Ślosarczyk A. et al. [157] electroplated the carbon fiber with a nickel layer to develop a lightweight porous SiA composite capable of attenuating electromagnetic radiation. The developed material was tested in low-frequency microwave ranges and utilized absorption as the central mechanism to dampen electromagnetic radiation. The highly porous aerogel matrix played a dominant role in damping electromagnetic radiation. The aerogel structure, composed of fine silica particles, formed a porous network (10–12 nm pores filled with air). This caused electromagnetic waves to undergo repeated reflections, weakening their intensity. The study recorded that the composite achieved a remarkable electromagnetic radiation absorption capacity between 40 and 56 dB in a low-frequency range of 8–18 GHz. Despite deteriorating the thermal stability of the composite, the material overcame the primary negatives of metal-based shields, such as that of being heavy and corrosive in nature.

Recently, similar attempts to develop electromagnetic interference (EMI) shields for aerospace applications have also been recorded [158]: a study developed carbonized polyimide/silica aerogel (PIF/SiA) foams and reported a remarkable EMI shielding performance in the X band (specific EMI shielding effectiveness: 8541.9 dB/(g/cm²)) when compared with the original PIF/SiA composite foams. Again, absorption remained the key mechanism to attenuate EMI. The authors employed a microwave-assisted foaming technique followed by a thermal imidization process and pyrolysis for the composite’s preparation. Interestingly, a higher SiA content disrupted the anisotropic PIF/SiA structure, reducing pore connectivity and increasing pore size. Further, the carbonized PIF/SiA possessed high electrical conductivity, which reported a slight reduction with higher SiA content, indicating that the inorganic components hindered conductive network formation. Identical to its outstanding EMI shielding performance, the material also displayed an exceptional flame retardancy. PIF/SiA is a self-extinguishing material, with a burn time of less than 180 s. The fire resistance of the material was evaluated by simulating a real-life scenario through cone calorimetry. It featured a rich micro/nanoporous structure with strong interfacial interactions, effectively reducing heat conduction and restricting oxygen diffusion, thereby slowing the combustion reaction. Additionally, the high residual char and SiA formed a thermally stable barrier, restricting the transfer of combustion substances and oxygen to the burning interface while also blocking heat transfer, thereby slowing combustion and pyrolysis. The study also reported that the addition of SiA within the foam composite resulted in the formation of a large number of tortuous heat transfer paths, which are crucial for reduction in gas phase conduction. This phenomenon also allowed the material to exhibit excellent thermal insulation and infrared stealth performance.

Zhao C. et al. [31] have suggested a similar explanation for the flame-retardant properties of polyisocyanurate (PIR) rigid foam composites. Their study showed that the addition of a large quantity of inorganic materials improves the flame-retardant properties of foam composites. Silica aerogel, being one of the best insulation materials, offers exceptional thermal endurance to prevent rapid heat transfer. Therefore, this suggests that incorporating silica aerogel particles enhances the thermal stability of organic foams. This improvement stems from the aerogel’s high thermal performance, which serves as an effective barrier against rapid heat transfer and delays material degradation. Acting as an inorganic filler, silica aerogel also contributes to increased cross-link density in PIR foams by physically or chemically restricting the mobility of polyurethane chains. Furthermore, the integration of Si–O bonds into the PU C–C network enhances thermal resistance compared to conventional PIR foams. Further, the incorporation of silica aerogel dilutes the proportion of combustible material, thereby solidifying the flame-retardant properties within the foam composites.

Beyond combustion suppression, advanced strategies targeting radiative heat transfer mitigation have also been explored to enhance thermal stability in extreme environments. Silica nanofibrous aerogel with TiO₂ particles (SNFAT) have also demonstrated an excellent thermal insulation effect, which primarily occurs due to TiO₂’s strong infrared radiation suppression [159]: a study utilized an alcohol lamp for heating ceramic aerogels and, based on infrared transmittance results, indicated that the TiO₂ particles exhibited a much higher contribution than SiO₂ particles in blocking thermal radiation. A further numerical simulation revealed that incorporating TiO₂ as an opacifier would lead to a higher specific extinction coefficient, which effectively reduces the radiative thermal conductivity of samples. These findings underscore the critical role of nanoscale opacifiers in attenuating radiative heat flux, thereby advancing the thermal management capabilities of SiA composites under high-temperature oxidative conditions.

It is important to note that researchers have typically used cone calorimetry for big fire tests, thereby simulating its real combustion behavior, while limiting the use of the oxygen index (LOI), UL-94, and flame tests, which are classified under small fire tests. Therefore, Figure 6 provides a comparative overview of the methodologies employed in small-scale fire tests, offering insight into how each method is implemented. Figure 7 builds on this by critically evaluating their respective advantages, limitations, and application suitability. The research in [160] reported conducting similar small fire tests on a poly(dimethylsiloxane)/SiA composite. SiA composites have showed their utilization in unique building applications, such as exterior wall insulation [31], fire-smoke safety material [161], and also in composite cement paste [91]. They have proven to be promising insulation materials for space, transportation, industrial and building applications. Silica aerogel/glass fiber composites have also been reported being effectively utilized as fire shields to protect the frame structures of a building during an inferno [162]. Frame structures, typically composed of steel skeletons, are prone to softening and strength degradation, eventually leading to catastrophic structural failure. Generally, studies investigating fire resistance in laboratory setups utilize a turbo torch (butane flame) for heat exposure and an infrared thermometer for temperature measurement. Their study employed a laboratory setup to assess the fire resistance of the composites, and it revealed the influence of silica sol’s pH and aging time on fire resistance.

Similar flame penetration tests have been reported on room-temperature vulcanized silicone rubber (RTV-SiR) composites [163] and on quartz fiber felt-reinforced silica-polybenzoxazine aerogel composites [164] to determine their thermal and flame barrier performance. Rare reports have also described fire performance tests on SiA composites combined with paper-based honeycomb structures that have been subjected to burner flame testing exceeding 1000 °C, simulating aerospace safety standards [165]. The specimen was exposed for 300 s, during which no visible smoke or odor was detected. Remarkably, the composite retained its structural integrity, with only minor discoloration observed on the unexposed surface. These findings confirm that paper-reinforced SiA composites can maintain structural stability and thermal insulation under direct flame exposure, making them promising candidates for lightweight, fire-resistant applications in aerospace thermal protection systems. Studies have also highlighted the use of silica aerogels to enhance the heat resistance of protective clothing fabrics by mitigating thermal radiation exposure [68]. The composites demonstrated thermal radiation resistance between 13 and 17 s, placing all variants within the first protection class range of 7 to 20 s. Interestingly, exposure to a heat flux of 20 kW/m² did not cause any burning or melting of the aerogel-coated samples, and no surface degradation was observed. This signifies that the material meets high fire protection standards and can offer early-stage thermal shielding in environments with intense heat or radiant energy. Such equipment has proven to be critical for firefighters and workers exposed to flames and other high temperature factors, such as radiant, convective, and contact heat, which pose significant threat.

The results in [162] show that composites prepared at pH 4 exhibit higher fire resistance than those at pH 8 as acidic conditions promote uniform silica network growth and better aerogel coverage on glass fibers. In basic conditions, rapid condensation results in poor coverage due to trapped monomers. Aging time enhances the aerogel’s strength but reduces porosity, influencing fire resistance. Effective Si-OH group control is crucial for ensuring uniform aerogel distribution and optimal fiber coverage. Most importantly, it is important to note that the heat transfer within porous structures is strongly influenced by the solid skeleton, pore size, temperature, and chemical composition of the material. The fire resistance of the composites improves with increasing porosity of the silica aerogel as higher porosity corresponds to a reduced solid skeleton. Additionally, the thermal conductivity of the silica framework is inherently low due to the extended heat transfer paths within the three-dimensional network. Notably, the free path of air molecules is approximately 50 nm, while the synthesized silica aerogels in that study possessed mean pore diameters below this threshold. As a result, convective heat transfer was effectively suppressed across all samples, andthe variations in pore size no longer significantly influenced the overall thermal conductivity. The superior fire resistance of these composites arose from their finely tuned porous architecture, which effectively suppresses both solid-state conduction and convective heat transfer. This combination makes the composite an exceptional thermal barrier, enabling robust protection of the structural steel elements under extreme fire exposure. The study in [163] proved that the thermal insulation effect of silica aerogel particles coupled with surface char formation on RTV-SiR composite bolsters its thermal shielding property. Similarly, results have also been reported on glass fiber film composites [10], where the time-to-ignition (TTI) values increased with an increase in the H₂O:TEOS molar ratio, thereby strengthening its flame retardancy. As the molar ratio value increased, the substitution of surface Si-OH groups on nanoparticles with non-polar Si-(CH₃)₃ groups declined.

Table 4 presents a timeline review of the notable research studies carried on flammability and radiation assessments of SiA composites over the last decade. This presentation offers a brief comparative description of the type of tests and synthesis performed on these composites, along with suitable standards, applications, and critical insights. It highlights the evolution of flammability and radiation assessments, encompassing a broad range of SiA composites and advancements in their practical applications.

5.2. Future Directions

Despite abundant progress on the radiation and flammability properties on SiA composites, several key areas need urgent exploration. The current strategies utilized to manufacture various SiA-based composites are often complex and lengthy in nature. Hence, limiting their potential to produce at high speeds and scaled quantities. Therefore, it becomes essential to develop facile and effective techniques for the rapid production of high-end engineering applications, such as that for radiation shielding [158]. The aerogels in [8] underwent baking before thermal vacuum outgassing to ensure compliance with screening requirements. To eliminate the need for this additional post-processing step, improvements in the drying process of SiA composites are also necessary. Limited research has been reported on incorporating silica aerogel into roof tile materials or as part of composite insulation, despite its potential to significantly reduce heat transfer through the roof. This gap is particularly notable in tropical regions, where roof tiles are subjected to intense and prolonged solar radiation, as well as high annual rainfall [91]. Similarly, studies on the combination of monolithic silica aerogels and honeycombs are extremely rare [165]. Initiating further research on the topic would create possibilities of developing strong yet ultra-lightweight thermal protection systems for robust applications.

The most effective reduction in thermal conduction and radiative heat transfer was observed when the fibers were aligned perpendicularly to the incident light or temperature gradient. Therefore, when selecting fibers as the reinforcing phase, a layered structure should be preferred, ensuring that the fiber orientation remains as perpendicular as possible to the direction of radiant heat transfer, particularly in high-temperature applications. The inherent characteristics of fibers significantly influence mechanical properties and thermal insulation. However, studies on their impact on the performance of lightweight resin-based ablative thermal protection materials are nearly absent. Therefore, urgent research is needed to examine the influence of fiber properties on the structure and performance of aerogel composites [164].

A commonly overlooked issue with silica aerogels is the incorporation of non-hydrolyzable organic groups and residuals during surface modification to achieve hydrophobicity. These organic constituents introduce a potential fire hazard when exposed to heat radiation or fire conditions, posing a significant risk to thermal protection applications. Hence, further research should not just be restricted on providing deeper insights into the combustion characteristics of organic-modified aerogels, but also on focusing on identifying strategies in establishing safety guidelines and material standards for silica aerogel-based hydrophobic composites [10,131,166]. Expanding research on this topic would allow the possibility of optimizing performance of these composites for harsh environments, thereby making them viable for real-world applications. Similarly, another study has also reported a lack of existing literature on the flammability properties of thermoplastic composites [30]. While exploring halogen-free flame retardants can help develop environment-friendly, low-smoke emission thermoplastic composites, researching on recyclable and gradient-structured SiA-based thermoplastics could unlock new possibilities in the energy sector.

Table 4. Milestones in the flammability and radiation assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Reinforcing Material	Processing Method	Standards	Application	Key Insights
Motahari S. et al. [2015] [30]	Thermal conductivity (line source method) Thermal degradation stability Limiting oxygen index (LOI) test Combustion behavior	Matrix- and particle-reinforced	Polypropylene (PP) (matrix) and silica aerogel (particles)	Melt compounding method	ASTM D2863 [167] (LOI test) ASTM E1354 [168] (cone calorimeter)	Thermoplastic composites for automotive, aerospace, and building industries	Adding SiA to PP increased the thermal decomposition temperature by 24–34 °C, and this was attributed to the protective char layer and its physical cross-linking.
Rocha H. et al. [2019] [8]	Thermal conductivity (transient plane source) Gamma radiation exposure Thermal cycling test Vacuum outgassing test	Fiber-reinforced	Inorganic quartz fiber and inorganic polyethylene terephthalate (PET)-based fibers	Sol-gel and APD	ECSS-Q-ST-70-04C [169] (thermal cycling) ECSS-Q-ST-70-02C [170] (thermal vacuum outgassing tests)	Martian environment	Thermal insulation remained unaffected by thermal cycling or environmental exposure, demonstrating aerogel’s resilience to martian temperature variations.
Almeida C. M. et al. [2021] [156]	Thermal conductivity (transient plane source) Thermal stability Thermal cycling test Vacuum outgassing test	Fiber-reinforced	Kevlar pulp (KP), aramid felt (PAF), and aramid fibers	Sol-gel and APD	ECSS-Q-ST-70-04, ESA, 2008 [171] (thermal cycling simulation tests) ECSS-Q-ST-70-02C, ESA, 2008 [170] (outgassing test)	Suitable for shape adaption and vibration applications (such as launch pad rockets and thermal protection systems)	Elongated aramid fiber-reinforced SiA nanocomposites offer superior flexibility, vibration damping, and thermal stability, ideal for space applications.
Xi S. et al. [2023] [18]	Thermal conductivity (line source method) Thermal insulation test Thermal gravimetric analysis Fire resistance test Thermal cycling test Vacuum outgassing test Space irradiation tests (proton, UV, and electron irradiation tests)	Matrix- and particle-reinforced	Polyimide (PI) matrix, inorganic silica aerogel powders (ISAp), and polyvinylpolymethylsiloxane aerogel powders (PAp)	ISAp was created using sol-gel, while PAp was formed by radical polymerization. Both were dried through supercritical CO2 drying.	NIL	Space environment	PI-I40 shows resistance to electron irradiation, moderate sensitivity to proton and UV irradiation, and stability after thermal cycling. ISAp enhances thermal stability and fire resistance but reduces moisture resistance, while PAp improves moisture resistance but weakens thermal stability and fire resistance. Combined, they reduce thermal conductivity, ensuring strong insulation.
Ni L. et al. [2024] [158]	Electromagnetic interference (EMI) shielding performance Thermal conductivity (transient plane source) Thermal stability Thermal degradation process Limiting oxygen index (LOI) test Thermographic imaging Alcohol lamp combustion experiments	Particle-reinforced	Polyimide foam (PIF) and silica aerogel (SiA) powders	Microwave-assisted foaming and thermal imidization treatment	GB/T 2406.2-2009 [172] (LOI test) ISO 5660-1 [123] (cone calorimeter)	Aerospace, precision electronics, and transportation	Carbonized PIF/SiA foams demonstrate superior mechanical and EMI shielding performance over the original PIF/SiA composite foam. The latter shows exceptional infrared stealth performance, indicating potential for extended service life.

Table 4. Milestones in the flammability and radiation assessments of SiA composites over the past decade.

Author and Year	Tests	Reinforcement	Reinforcing Material	Processing Method	Standards	Application	Key Insights
Motahari S. et al. [2015] [30]	Thermal conductivity (line source method) Thermal degradation stability Limiting oxygen index (LOI) test Combustion behavior	Matrix- and particle-reinforced	Polypropylene (PP) (matrix) and silica aerogel (particles)	Melt compounding method	ASTM D2863 [167] (LOI test) ASTM E1354 [168] (cone calorimeter)	Thermoplastic composites for automotive, aerospace, and building industries	Adding SiA to PP increased the thermal decomposition temperature by 24–34 °C, and this was attributed to the protective char layer and its physical cross-linking.
Rocha H. et al. [2019] [8]	Thermal conductivity (transient plane source) Gamma radiation exposure Thermal cycling test Vacuum outgassing test	Fiber-reinforced	Inorganic quartz fiber and inorganic polyethylene terephthalate (PET)-based fibers	Sol-gel and APD	ECSS-Q-ST-70-04C [169] (thermal cycling) ECSS-Q-ST-70-02C [170] (thermal vacuum outgassing tests)	Martian environment	Thermal insulation remained unaffected by thermal cycling or environmental exposure, demonstrating aerogel’s resilience to martian temperature variations.
Almeida C. M. et al. [2021] [156]	Thermal conductivity (transient plane source) Thermal stability Thermal cycling test Vacuum outgassing test	Fiber-reinforced	Kevlar pulp (KP), aramid felt (PAF), and aramid fibers	Sol-gel and APD	ECSS-Q-ST-70-04, ESA, 2008 [171] (thermal cycling simulation tests) ECSS-Q-ST-70-02C, ESA, 2008 [170] (outgassing test)	Suitable for shape adaption and vibration applications (such as launch pad rockets and thermal protection systems)	Elongated aramid fiber-reinforced SiA nanocomposites offer superior flexibility, vibration damping, and thermal stability, ideal for space applications.
Xi S. et al. [2023] [18]	Thermal conductivity (line source method) Thermal insulation test Thermal gravimetric analysis Fire resistance test Thermal cycling test Vacuum outgassing test Space irradiation tests (proton, UV, and electron irradiation tests)	Matrix- and particle-reinforced	Polyimide (PI) matrix, inorganic silica aerogel powders (ISAp), and polyvinylpolymethylsiloxane aerogel powders (PAp)	ISAp was created using sol-gel, while PAp was formed by radical polymerization. Both were dried through supercritical CO2 drying.	NIL	Space environment	PI-I40 shows resistance to electron irradiation, moderate sensitivity to proton and UV irradiation, and stability after thermal cycling. ISAp enhances thermal stability and fire resistance but reduces moisture resistance, while PAp improves moisture resistance but weakens thermal stability and fire resistance. Combined, they reduce thermal conductivity, ensuring strong insulation.
Ni L. et al. [2024] [158]	Electromagnetic interference (EMI) shielding performance Thermal conductivity (transient plane source) Thermal stability Thermal degradation process Limiting oxygen index (LOI) test Thermographic imaging Alcohol lamp combustion experiments	Particle-reinforced	Polyimide foam (PIF) and silica aerogel (SiA) powders	Microwave-assisted foaming and thermal imidization treatment	GB/T 2406.2-2009 [172] (LOI test) ISO 5660-1 [123] (cone calorimeter)	Aerospace, precision electronics, and transportation	Carbonized PIF/SiA foams demonstrate superior mechanical and EMI shielding performance over the original PIF/SiA composite foam. The latter shows exceptional infrared stealth performance, indicating potential for extended service life.

6. Computational Studies

6.1. Recent Advancements and Current Research Trends

6.1.1. Continuum Mechanics and Nanoscale Simulations

SiA composites exhibit intricate thermal and mechanical behaviors, requiring advanced simulation techniques to achieve precise analysis and predictive modeling. Among the most widely used approaches are molecular dynamics (MD) simulations and finite element analysis (FEA). FEA is an effective macroscopic method that enables the assessment of the thermo-mechanical performance–stress distribution, heat transfer, effective thermal conductivity, and deformation under various loading conditions. In contrast, MD simulations operate at the atomic scale, capturing nanoscale interactions, phonon transport, and thermal conductivity variations, making them ideal for studying aerogels’ unique porous structures. It is a computational technique for analyzing the time-dependent behavior of molecular systems. It begins with an initial set of particle coordinates and velocities, and it then iteratively integrates the equations of motion to update their positions and velocities over time. Additionally, there have been few studies that have also reported attempts on hybrid approaches, such as coupling MD with FEA, and they have attempted to explore the gap between atomic-scale accuracy and large-scale structural predictions, thereby enhancing the reliability of simulation outcomes.

Simulations play a crucial role in complementing experimental and theoretical models by providing deeper insights into complex phenomena. Several studies have demonstrated their effectiveness in predicting material behavior, while others have integrated simulations with experimental data and have also validated challenging scenarios in theoretical models. For instance, opacifiers are generally spherical in shape, making the Mie theory the preferred approach for deriving the analytical solution of Maxwell’s equations for spherical particles, whereas numerical methods are used for non-spherical particles. Studies have thus reported utilization of FEA methods with theoretical models to solve Maxwell’s wave equation [153]. A consolidated roadmap (Figure 8) was developed to capture the foundational framework of each method, detailing their assumptions, governing laws, scope of application, drawbacks, and output interpretation. This integration provides a clearer understanding of how each approach contributes uniquely to the comprehensive evaluation of SiA composites. Similarly, despite conducting several physical assessments, Arshad A. et al. extended their advanced study on PEGylated SiA-based composites by employing COMSOL Multiphysics [173] to compare and understand the thermal comfort levels from the experimental results [174]. Originally aimed to enhance thermal comfort level through window integration, the synthesized composite displayed phenomenal capability to attenuate near-infrared light while retaining visible-light transparency. The researchers utilized ray optics and solid heat transfer models within finite element-based software, and they ascertained COMSOL’s aptness for application on novel smart composite materials. Similarly, studies have also employed ANSYS Fluent for transient heat transfer numerical simulation on a new type of phase change heat storage foam concrete [175]. Despite conducting physical analysis on the CaCl₂·6H₂O/SiA composite, the study still evaluated heat transfer performance through finite element-based simulations. Such initiatives facilitate the generation of experimentally validated synthetic data, which can serve as a foundational resource for future research. This, in turn, enables the development of more accurate and reliable predictive models, reducing the dependency on extensive physical experimentation. Such seamless incorporation of established databases into new research frameworks will strengthen the synergy between experimental, theoretical, and computational approaches, ultimately accelerating advancements in the field.

Building on the discussion of FEA simulations in thermal applications, similar successful attempts have been recorded using finite element-based simulations to evaluate the mechanical characteristics of SiA composites. Mei H. et al. developed lightweight thermal insulation integrated sandwich structures composed of SiC body-centered lattice core (BCLC) and quartz fiber-reinforced SiA composites [176]. Fabricated by the SLS technique combined with sol-gel method, the study reported successful compressive tests using an electronic universal testing machine. Still, extending their further research, the study also simulated the compression process of single- and double-layer structures using COMSOL Multiphysics, thereby elucidating the fracture behavior of the composite. FEA simulations have revealed that trusses in BCLCs efficiently transfer loads at non-contact points, whereas the load transfer is obstructed at contact points. Notably, stress concentration at these contact points is mitigated upon aerogel filling. Furthermore, the analysis indicates that the single-layer structure primarily fails due to compressive stress, while the double-layer structure exhibits tensile stress-dominated shear failure. This distinction has been largely attributed to the variations in the inclined angles of the trusses in the two BCLC configurations. Such initiatives enable simulations to validate experimental observations under controlled and repeatable conditions, thereby reducing uncertainties and lessening the reliance on extensive physical testing. Additionally, they enable the exploration of extreme or impractical scenarios that may be challenging to replicate experimentally.

While the discrepancies in the simulation of interfacial bonding of SiA composites [19] were previously discussed in the introduction, it is also possible that simulations can display inconsistent results due to improper assumptions, deviations from real-world conditions, or other influencing factors, eventually impacting the accuracy and reliability of results. Mishra et al. examined the thermal insulation properties of nonwoven samples embedded with SiA using finite element-based simulations, and they also compared them with experimental measurements [177]. Thermal analysis was necessary to determine the temperature distribution and related thermal quantities of the composite. The study reportedly deployed the Ansys workbench for thermal insulation predictions, while COMSOL Multiphysics was used for the simulation of heat flow. The nonwoven samples typically consisted of short cylindrical fibers, and these were primarily aligned along the machine direction. Given the cylindrical shape of the fibers, the solid–solid interfaces were limited to points or lines. As a result, the unit cell was modeled in its entirety without considering the detailed interactions at the solid–solid interfaces. It should be noted that fibrous structures are generally anisotropic, and they exhibit directionally non-homogeneous material geometry, with properties varying across different directions. Despite efforts to control them, the irregularities in fiber orientation can never be fully eliminated. Additionally, the stability and integrity of nonwoven materials are heavily reliant on the disorientation of binding or cross-laid fibers. Non-homogeneity is an inherent characteristic of fibrous materials that cannot be avoided. Hence, it is nearly impossible for nonwoven composites embedded with aerogel to produce absolute prediction results as compared to the experimental thermal performance. Still, the study successfully reported experimental and theoretical readings in close approximation (less than 10% error) with each other.

All the results presented above were derived using continuum mechanics, where the material was modeled as a continuous mass, disregarding its microscopic characteristics, such as chemical composition, grain size, crystal structure, and lattice spacing. However, capturing nanoscale phenomena, such as bond breaking and atomic rearrangements, is beyond the scope of such macroscopic approaches. Therefore, over the last few decades, MD simulations have received significant attention regarding the modeling of SiA composites, primarily due to advancements in computer science. Precisely due to their high temporal and spatial resolution, MD simulations are well suited for analyzing the various material behaviors of SiA composites, including fracture mechanics and phase transitions, with picosecond-level precision [50]. Figure 9 extensively compares the continuum mechanics-based FEM technique with MD simulations, offering a comprehensive multiscale perspective on SiA composites across macro and nano-scale regimes. Using Langevin dynamics, Gelb reported the utilization of coarse-grained simulations to investigate the structural properties of low-density SiA [178]. It was found that the obtained value of the bulk modulus was lower than the one obtained through experiments. The extension of this study successfully investigated the bulk modulus using fluctuation analysis and direct compression–expansion simulations [179]. Recently, utilization of MD simulations and hybrid Monte-Carlo methods to investigate the uniaxial tensile-compression response of SiA have been reported [180].

Therefore, with the development of such advanced computational methods, investigations on thermal decomposition can be carried out at atomic scale. Xiao J. et al. [181] investigated the pyrolysis behavior of a siliconoxycarbide (SiOC)-modified phenolic resin (PR) nanocomposite using reactive force field molecular dynamics simulation (ReaxFF MD). ReaxFF is typically used to study the thermal decomposition of polymers. Importantly, as this study focused on the effect of SiOC on the thermal decomposition of the SiOC-PR nanocomposite rather than its polymerization, details, such as reaction environments (alkaline or acidic), were not considered. The obtained simulation results were found to be in agreement with experimental studies. Similar to these studies on thermal simulations, several studies have reported mechanical properties of SiA composites using MD simulations. For instance, our past work reported the mechanical properties of three different SiA nanocomposites—reinforced with graphene sheets, glass fibers, and carbon nanotubes (CNTs) [35]. The reinforced materials were distributed randomly in silica aerogels to identify their influence on mechanical properties. Comparative results of tensile and compression tests of nanocomposites were reported along with their fracture behavior. Likewise, another study [182] utilized MD simulations to explore how silica primary particles interact physically with CNT surfaces at the nanoscale by considering different degrees of CNT oxidation. Recently, studies have also reported temperature-induced variations in the mechanical properties of silica aerogel/chitosan tricalcium phosphate nanocomposites [183]. Investigation of the temperature-dependent mechanical properties of nanocomposites using MD simulations is extremely rare, and promoting such novel research would reveal valuable insights into structure–property relationships. The aforementioned study obtained crucial insights on the changes in the Young’s modulus, stress–strain curve, ultimate strength, mean square displacement, and interaction energy at varied temperatures. Additionally, the study also obtained radial distribution function (RDF) values from MD simulations to validate the structural accuracy of a simulated nanocomposite system. This guarantees that the computational model accurately reflects the material’s atomic-scale structure.

Lu Z. et al. [184] proposed novel multi-scale modeling strategies for fiber-reinforced SiA composites, and they produced results that were in agreement with the experimental results. This multi-scale model integrated a two-level cluster structure model and a composite representative volume element (RVE) model. The cluster structure model determined the constitutive behavior of silica aerogels through a modified diffusion-limited cluster aggregation (DLCA) algorithm, leveraging discrete element method (DEM) simulations and theoretical derivations supported by molecular dynamics (MD) simulations. Meanwhile, the composite RVE model captured the fiber network structure using a modified Eshelby equivalent transformation (EET) model within a finite element (FE) framework. To establish a cohesive link between these models, the study developed a continuum damage constitutive model for SiA based on the homogenization method. This model was then integrated into finite element analysis (FEA) to predict the effective Young’s modulus and tensile strength of fiber-reinforced SiA composites by incorporating the constitutive relationships into both the aerogel matrix and the fiber–matrix interface.

Table 5 presents a timeline review of the notable research advances reported through the finite element and MD simulation of SiA composites over the last decade. This presentation offers a brief comparative description of the various characteristics studied in the composite, along with model details, temperature ranges, and critical insights. It highlights the evolution of computational assessments, covering a wide range of SiA composites and their advancements in the real-world applications.

6.1.2. Other Simulation Models

Although several continuum mechanics and nanoscale simulations have been reported alongside theoretical and numerical models, few studies have reported to examine properties of SiA composites through utilization of other modules and environments. For instance, a simulation study was conducted on a Building Information Model (BIM) in a tropical climate to evaluate the impact of cement render combined with an insulation board on the roof’s thermal energy efficiency [91]. Created in Integrated Environmental Solutions, the BIM replicated the tropical climate of Shah Alam, Malaysia, which is typically exposed to intense and prolonged solar radiation. The simulation was conducted on SiA-based pitched roofs with varied configurations to evaluate the thermal energy performance through annual cooling load and energy savings. After incorporating the necessary input values, the results reported to record the direct, diffused, and global solar radiation fluxes throughout the typical meteorological year of Malaysia. Such studies, therefore, not only allow one to determine the optimum roof configuration for extreme environments, but they also establish real-world reference data for future studies.

Similarly, although direct experimentation provides accurate insights, mathematical modeling has received limited attention due to the challenges in accurately representing complex physical phenomena. For instance, most mathematical models developed for heat exchangers are heavily based on several assumptions, often ignoring the flow field, temperature, and humidity distributions across different domains. Although these models provide results with acceptable accuracy, current studies cannot be used to improve the design of solid-desiccant-coated heat exchangers (DCHEs). Pure desiccant materials, such as silica gel, have been utilized in several experimental studies to evaluate the performance of DCHEs. Also, in order to improve the dehumidification capacity of DCHEs, several studies have focused on composite desiccants made by combining silica gel with different hygroscopic salts. On this note, the research in [185] developed a new mathematical platform to simultaneously simulate the heat and mass transfer phenomena that occur during the dehumidification/regeneration of DCHEs. The mathematical model, grounded in the fundamental conservation laws of mass, momentum, energy, and species, was developed to analyze the performance of the DCHE along a single fin-tube path. Importantly, only half of the domain was numerically modeled, leveraging the axis of symmetry involved in the flow. This model, therefore, not only considers the fluid flow, temperature, and moisture distributions across different domains, but it also incorporates the cross flow of air and water in a three-dimensional system. The proposed mathematical model was systematically validated against experimental results from two DCHEs and was further simulated using COMSOL Multiphysics. The model predictions deviated from experimental data by a maximum of ±14% for the outlet air humidity ratio and ±12% for temperature. Nevertheless, the model displayed its excellent applicability to design new DCHEs by comparing the dehumidification performance of four unique DCHEs configuration. Therefore, greater focus is needed to develop tailored simulation approaches that accurately replicate the real-world environment of SiA-based composites, extending beyond conventional continuum mechanics or nanoscale models to create more precise and application-specific predictions. Such customized simulation models enable researchers to minimize unwarranted assumptions, ensuring greater accuracy and applicability to real-world scenarios.

6.2. Future Directions

6.2.1. Continuum Mechanics

The investigations in [177] mentioned the literature scarcity to quantify, theoretically or experimentally, the thermal performance of aerogel-treated nonwoven blankets at sub-zero temperatures. This not only limits our understanding of their insulation efficiency in extreme conditions, but it also hinders the development of optimized material designs and accurate predictive models. The absence of such precise theoretical and experimental data makes it challenging to enhance simulation accuracy and ensure reliability in practical applications. Therefore, further research should focus on accurate FE simulation studies of SiA composite blankets, which will eventually contribute to a comprehensive reference database that can serve as high-quality synthetic data for future advancements. Similarly, despite extensive empirical and theoretical research on fiber-reinforced SiA, the use of flexible organic fibers, such as aramid fibers, as reinforcements remains largely unexplored [186]. Notably, most studies on aramid fiber-reinforced SiA composites have been predominantly experimental. Very few prior computational investigations have examined the impact of aramid fiber reinforcement on the thermal conductivity of silica aerogels. Studies similar to [153] can also be carried out using the finite element method to determine the influence of different opacifiers on the thermal insulation performance of SiA composites. Such initiatives hold immense potential to further develop numerical models to complement FEA simulations and show closer agreement with experimental results.

Interestingly, another study [174] also shed light on several possibilities to be unlocked in future studies. For instance, incorporating novel designs inspired by kirigami and utilizing energy-saving composites could enhance the durability and thermal performance of the material in future applications. Future research may focus on developing smart composite materials to assess the thermal comfort levels in buildings. Understanding heat transfer dynamics requires analyzing ray trajectory and heat transfer modeling. Further, integrating experimental data with theoretical building simulations provides an interdisciplinary approach to investigate how innovative smart composites in glass panes can regulate thermal comfort. Their research could pave the way for multifunctional smart windows and advanced spatiotemporal light control strategies.

Studies aiming to determine the load-bearing capabilities of a composite must first establish the relationship between its mechanical properties and microstructure, including the structures in nanometer and micrometer scale. Computational simulation has been widely employed for the purpose. Previously, atomistic models have also been simulated using MD simulations to investigate the nanoporous structure of SiA [187]. Although such models accurately report about the fractal dimension and the modulus–density relationship of SiA, they are insufficient to investigate the mechanical properties of SiA at a large scale, as the MD simulations are computationally expensive [184,188]. Although, several studies have previously reported the thermo-mechanical properties of SiA-based composites, there is relatively little literature that exists on the theoretical and numerical models investigating the structure properties of the composite. This happens because it is challenging to replicate the exact behavior of SiA composites in 3D numerical simulations. Specifically, the fibers should possess a high aspect ratio in order to reinforce the composites effectively. To address the challenge of modeling random fiber composites (RaFCs), a study [189] proposed a valuable approach to enhance the FE modeling of RaFCs by ignoring fiber intersections, where fibers were represented as 1D line elements. Similarly, another study [190] has suggested generating 3D models for RaFCs with a high fiber aspect ratio and a high fiber volume fraction. However, the aerogel matrix exhibits inferior mechanical properties compared to fibers, making conventional methods that couple 1D fiber elements with 3D matrix elements ineffective in accurately capturing local strain near the fibers. Therefore, similar to the work reported in [184], further novel work could be carried out to address the above discrepancies.

6.2.2. MD Simulations

Luo Z. et al. [191] mentioned that the selection and verification of dissipative particle dynamics (DPD) force parameters is very challenging. Typically, the common method to verify is to ensure that the obtained properties are consistent with experimental values. Hence, it is necessary that further research should be promoted to develop a practical multicomponent system. Luo Z. et al., therefore, employed the DPD method to simulate the mesoscopic formation of polyimide (PI) cross-linked silica aerogel dimers. By modeling the pre-cross-linking solution, the impact of the physical environment on material structure was analyzed. Therefore, extending and developing similar studies would allow DPD simulation to serve as a supplementary tool for experiments and to offer valuable insights to support them.

Some reactions within nanoscale simulations can lead to the formation/interaction of H-bonds with some chemical groups. Quantum mechanical calculations play a vital role in determining whether such reactions can occur from the H-bonded aggregates observed in the simulation trajectories. Therefore, a further investigation of the possibility such reactions can also be carried out using a hybrid quantum mechanics and molecular mechanics approach [192]. Interestingly, the study in [193] mentioned that MD studies confined to volumes smaller than 20³ nm³ result in smaller pore size distributions than those of experimental aerogels centered on 10 nm. Therefore, the simulated SiA in such MD studies are not representative enough for a detailed mechanical investigation. Figure 10 suggests a few ‘broad’ areas for future research within a multi-scale simulation domain, which would be potentially carried out using both continuum mechanics and nanoscale simulations.

Our previous studies have also identified promising possibilities for the further exploration of SiA composites using MD simulations. For instance, further research can be extended to explore the mechanical properties and nano mechanics of SiA composites during indentation tests. Concurrent nanoindentation experiments could also be conducted to validate simulation results, ensuring consistency between experimental observations and computational predictions [194]. Previously, experimental investigations carried on the fracture toughness of SiA have reported variable results, which has made it difficult to establish definite conclusions [195]. The high temporal and spatial resolution ability of MD simulations make them perfectly suited for such nanoscale investigations. Therefore, further research on the fracture behavior of SiA composites using MD simulations should be encouraged given the scarcity of existing literature in this area [196]. Similarly, MD simulations can also be utilized to study high-velocity shock compression and the inelastic behavior of SiA composites under cyclic loading [197,198]. Such studies reveal the operational limits while also assessing the fatigue behavior, thereby predicting its durability and failure mechanisms. Previously, several studies have investigated the mechanical properties of different SiA composites using MD simulations, though several important varieties still remain unreported. For instance, our study was the first to report an all-atom study on glass-fiber-reinforced SiA composites [34]. Such studies can be further extended to identify the influence of varying fiber reinforcements using MD simulations. Although the above-mentioned studies conducted by us focused on SiA, the application of MD simulations could be extended to SiA composites to explore their nanoscale mechanics. Such studies not only contribute to the development of multiscale frameworks, but they also serve as foundational groundwork for future studies intending to explore nanoscale behavior of SiA composites when using MD simulation.

Table 5. Milestones in the computational studies of SiA composites over the past decade.

Author and Year	Type	Model Objective	Model Details	Characteristics Studied	Relations and Equations	Temperature(s)	Key Insights
Patil S. P. et al. [2019] [34]	MD	This paper investigates the mechanical behavior of glass fiber-reinforced silica aerogel nanocomposites using MD simulations with LAMMPS and OVITO, where the focus is on the tensile strength, elastic modulus, and compressive properties.	Model construction Fiber length-to-diameter (L/D) ratios: 3, 6, and 9 Boundary conditions Periodic boundary conditions applied in three directions Density: 406 kg/m3 Strain rate: 0.004 ${\mathrm{ps}}^{-1}$ Ensemble used NVE (constant volume and energy) Six independent MD simulations performed for accuracy	Tensile strength Elastic modulus Compressive behavior Stress–strain behavior Structural deformation mechanisms	Vashishta interatomic potential (interactions between silica aerogel and glass fibers) Relationship between elastic modulus and density	7000 K (random velocity assignment) 7000–300 K ((5 K/ps, NVT) quenching) 300 K (final relaxation to form amorphous silica aerogel) 3000 K (second heating to and relaxation)	The simulation showed that the tensile strength increased from 0.2 GPa to 0.442 GPa. It established a linear modulus–density relationship and improved compressive resistance, confirming enhanced mechanical performance at 300 K.
Patil S. P. et al. [2020] [35]	MD	This study explored the mechanical properties of silica aerogel nanocomposites reinforced with glass fibers, graphene sheets, and CNTs through MD simulations using LAMMPS and OVITO.	Model construction Glass fibers and CNTs: aspect ratio ≈ 6.1 Graphene sheet: length-to-width ratio ≈ 1.9 Simulation box 320 Å × 320 Å × 320 Å Boundary conditions Periodic boundary conditions applied in three directions. Strain rate: 0.004 ${\mathrm{ps}}^{-1}$ Velocity–Verlet algorithm (0.5 fs) Ensemble used NVT (equilibration (100 ps, 300 K, 1 bar)) NVE (deformation simulations)	Mechanical properties	Vashishta potential (Si–O interactions) Brenner’s second-generation reactive empirical bond-order (or REBO) force field (C–C interactions) Van der Waals parameters (Si–C and O–C interactions)	Silica aerogel formation 7000 K (heating) 800–300 K (annealing) 300 K (quenching) 300 K (energy minimization) Nanocomposite formation 3000 K (50 ps)–300 K	Carbon-based nanocomposites improve tensile strength by 8–9× and elastic modulus by 9.5–11.5×. Crack penetration reduces fracture strength by 15–20%.
Mishra R. et al. [2021] [177]	FEA	This study investigated, comparing heat transfer with stagnant air under sub-zero conditions, the thermal insulation of aerogel-based nonwoven fabrics using FEM simulations (ANSYS and COMSOL) and experiments.	Modeling SOLIDWORKS ANSYS Steady and transient state thermal analysis Boundary conditions One face set at 283.15 K Other face at 293.15 K Convective coefficient: 27.07 W/(m2·K) COMSOL Conjugate heat transfer module Boundary conditions One face at 329.19 K Initial fabric temperature: 263.15 K Forced convection: 2.5 m/s wind Convection coefficient: 23.76 W/(m2·K) Without forced convection: open boundary at 263.15 K	Thermal properties	Maxwell and Hamilton model (effective thermal conductivity) Rosseland equation (radiative thermal conductivity)	248–258 K (ANSYS) 263.15–329.19 K (COMSOL)	Aerogel outperforms air as an insulator at all temperatures. Computational models predicted conductivity with <10% error in comparison to experimental results.
Xiao J. et al. [2023] [181]	MD	The objective is to investigate the pyrolysis behavior of siliconoxycarbide-modified phenolic resin (SiOC-PR) nanocomposites using reactive force field (ReaxFF) MD simulations via LAMMPS, as well as analyzing gaseous product evolution, reaction pathways, and thermal stability improvements.	Simulation box 63.56 Å × 63.56 Å × 175.58 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Time step: 0.1 fs Heating rate: 100 K/ps (cook-off simulations) Thermostat used Berendsen thermostat for temperature control Ensemble used NVT	Mechanical properties Thermal behavior and pyrolysis	Reaction kinetics equation Flynn–Wall–Ozawa (FWO) method Kissinger method Distributed activation energy model (DAEM) equation	300–5000 K (cook-off simulations) 2400–3400 K (isothermal simulations)	ReaxFF MD simulations revealed that SiOC-PR undergoes five-stage pyrolysis, where SiOC enhances oxidation resistance, regulates gaseous product evolution, and stabilizes phenolic fragments by Si-O-Si interactions.
Zhang W. et al. [2023] [33]	MD	This study investigates the effect of carbon doping (1–10%) on the mechanical properties of paraffin-reinforced silica aerogel (PRSA) when using MD simulations with LAMMPS, Avogadro, and Packmol.	Modeling Avogadro (paraffin structure modeling) Packmol (used to pack nanostructure) Simulation box 50 Å × 50 Å × 50 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Strain rate: 0.1 ${\mathrm{s}}^{-1}$ Number of atoms: 9000 Tensile testing performed along the x-axis. Velocity–Verlet algorithm for particle motion	Young’s modulus Stress–strain curve Ultimate strength	Lennard–Jones potential (interatomic interactions) Vashishta potential (silica structures) Newton’s second law (MD calculation)	300–7000 K ((1 ns, NVT) heating) 7000–300 K ((1 ns, NVT) cooling) 300 K ((1 ns, NPT) energy minimization) 0–3000 K ((1 ns) annealing) 300 K ((2 ns, NVT) final equilibration)	MD simulations show that carbon doping (3%) enhances PRSA’s mechanical properties, increasing the ultimate strength and Young’s modulus, while equilibrium is achieved at 300 K after 2 ns.
Maximiano P. and Simões P. N. [2023] [182]	MD	This study investigated, focusing on the aggregation, adsorption, oxidation effects, and structural organization at the nanoscale, silica aerogel–CNT interactions using GROMACS [199,200] MD simulations.	Boundary conditions Periodic boundary conditions to eliminate surface effects Non-bonded interactions cutoff: 1.2 nm Electrostatics: particle mesh ewald (PME) method Time step: 2.0 fs Production time: 1.0–2.2 µs Pressure: 1.0 bar Ensemble used NPT for equilibration (V-rescale thermostat, Berendsen barostat) NVT for production (V-rescale thermostat)	Silica particle aggregation Silica-CNT adsorption Siloxane bond orientation	OPLS-AA force field (bonded and non-bonded interactions) TIP4P (water model) Partial charges derived using CM5 method Gaussian16 and Multiwfn packages (electronic structure calculations)	298.15 K (equilibration and production)	Silica–CNT interactions vary by oxidation, affecting aggregation and adsorption. Van der Waals forces direct the structure, especially in octamer systems. H-bonds enhance ordered layers in oxidized CNTs.
Karimipour A. et al. [2024] [183]	MD	This study investigated, analyzing Young’s modulus, ultimate strength, stress–strain behavior, and atomic interactions, the temperature-dependent mechanical behavior of tricalcium phosphate/chitosan/silica aerogels nanocomposites (TCS-NCs) using MD simulations via LAMMPS, Avogadro, and Packmol.	Simulation box 90 Å × 90 Å × 90 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Time step: 0.1 fs Damping ratio: 0.01 Thermostat used Nose–Hoover thermostat Ensemble NVT (constant volume, constant temperature) NPT (constant pressure, constant temperature) Velocity–Verlet algorithm	Young’s modulus Stress–strain curve Ultimate strength Mean squared displacement (MSD) Interaction energy	Lennard–Jones potential (van der Waals interactions) Vashishta potential (silica aerogel interactions) Radial distribution function (RDF) Newton’s equation of Motion Electric potential energy	7000 K ((1 ns, NVT) initial heating) 7000–300 K ((1 ns, NVT) cooling) 300 K ((1 ns, NPT) energy minimization) 0–3000 K ((1 ns) annealing) 300 K ((2 ns, NVT) final equilibration) 297 K (initial temperature)	MD simulations revealed that increasing temperature influences TCS-NC’s mechanical properties, with the Young’s modulus peaking at 320 K and the ultimate strength decreasing as the temperature rises from 300 to 350 K.
Zhu C. Y. et al. [2024] [153]	FEM	This study investigated the impact of the opacifier protrusion shape, number, and volume on extinction properties, as well as the radiative thermal conductivity, when using COMSOL-based finite element method (FEM) simulations, thereby optimizing silica aerogel insulation for high-temperature applications.	Boundary conditions Perfectly matched layer (PML) (used to absorb outgoing waves and prevent reflections) Incident background electric field (defined by a plane wave propagating in the z-direction)	Extinction performance Radiative thermal conductivity Scattering and absorption properties Effect of protrusion shape, count, and volume	Maxwell’s wave equation (FEM simulation in COMSOL) Extinction, absorption, and scattering cross-sections Rosseland approximation (radiative thermal conductivity ) Spectral extinction coefficient	1300 K	The study used COMSOL to show that cylindrical protrusions (optimal at 12) significantly enhance the extinction properties, reducing the radiative thermal conductivity of silica aerogels by 22.6% at 1300 K.
Fu W. et al. [2024] [175]	FEA	This study numerically simulated the heat transfer on ${\mathrm{CaCl}}_2·6{\mathrm{H}}_2\mathrm{O}$ silica composite phase change material (CPCM) using ANSYS Fluent	Simulation model Transient heat transfer numerical simulation Boundary conditions Optimal mesh size: 257,413 All materials: homogeneous and isotropic Thermal properties: constant Algorithm: coupled Energy equation (second-order upwind scheme) Relaxation factors Pressure: 0.5 Momentum: 0.5 Density: 1.0 Body forces: 1.0 Energy: 0.75 Simulation duration: 10 days	Thermal and mechanical properties	Heat transfer equation Thermal conductivity equation for phase change Specific heat capacity equation Thermal resistance calculation Thermal inertia calculation Thermal storage coefficient calculation	293.15 K (initial temperature) 368.58 K (maximum temperature on external wall)	Numerical simulations showed that 20 wt% CPCM with an 80 mm thickness optimized thermal insulation, reducing peak temperature by 9.87 K with a delay time of 198,000 s and an attenuation factor of 6.6.
Zhang X. et al. [2024] [32]	MD	This study investigated the thermal conductivity of ${\mathrm{SiO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$ composite aerogels using non-equilibrium molecular dynamics (NEMD) simulations across varying ${\mathrm{Al}}_2{\mathrm{O}}_3$ content, temperature, and strains using LAMMPS and OVITO.	Boundary conditions Periodic boundary conditions were applied in all directions. Thermostat used Nose–Hoover thermostat Energy minimization (conjugate gradient method for 100 ps.) Velocity–Verlet algorithm (0.5 fs)	Thermal conductivity	EAM potential (Ai–O interactions) Extinction, absorption, and scattering cross-sections BKS potential ( Si-O interactions)	5000 K (randomized initialization) 300 K ((10 K/ps) quenching process)	MD simulations showed that adding ${\mathrm{Al}}_2{\mathrm{O}}_3$ makes the nanoporous ${\mathrm{SiO}}_2$ structure stronger, reducing thermal conductivity, while a slight increase occurs under elevated temperature and compression strain conditions.

Table 5. Milestones in the computational studies of SiA composites over the past decade.

Author and Year	Type	Model Objective	Model Details	Characteristics Studied	Relations and Equations	Temperature(s)	Key Insights
Patil S. P. et al. [2019] [34]	MD	This paper investigates the mechanical behavior of glass fiber-reinforced silica aerogel nanocomposites using MD simulations with LAMMPS and OVITO, where the focus is on the tensile strength, elastic modulus, and compressive properties.	Model construction Fiber length-to-diameter (L/D) ratios: 3, 6, and 9 Boundary conditions Periodic boundary conditions applied in three directions Density: 406 kg/m3 Strain rate: 0.004 ${\mathrm{ps}}^{-1}$ Ensemble used NVE (constant volume and energy) Six independent MD simulations performed for accuracy	Tensile strength Elastic modulus Compressive behavior Stress–strain behavior Structural deformation mechanisms	Vashishta interatomic potential (interactions between silica aerogel and glass fibers) Relationship between elastic modulus and density	7000 K (random velocity assignment) 7000–300 K ((5 K/ps, NVT) quenching) 300 K (final relaxation to form amorphous silica aerogel) 3000 K (second heating to and relaxation)	The simulation showed that the tensile strength increased from 0.2 GPa to 0.442 GPa. It established a linear modulus–density relationship and improved compressive resistance, confirming enhanced mechanical performance at 300 K.
Patil S. P. et al. [2020] [35]	MD	This study explored the mechanical properties of silica aerogel nanocomposites reinforced with glass fibers, graphene sheets, and CNTs through MD simulations using LAMMPS and OVITO.	Model construction Glass fibers and CNTs: aspect ratio ≈ 6.1 Graphene sheet: length-to-width ratio ≈ 1.9 Simulation box 320 Å × 320 Å × 320 Å Boundary conditions Periodic boundary conditions applied in three directions. Strain rate: 0.004 ${\mathrm{ps}}^{-1}$ Velocity–Verlet algorithm (0.5 fs) Ensemble used NVT (equilibration (100 ps, 300 K, 1 bar)) NVE (deformation simulations)	Mechanical properties	Vashishta potential (Si–O interactions) Brenner’s second-generation reactive empirical bond-order (or REBO) force field (C–C interactions) Van der Waals parameters (Si–C and O–C interactions)	Silica aerogel formation 7000 K (heating) 800–300 K (annealing) 300 K (quenching) 300 K (energy minimization) Nanocomposite formation 3000 K (50 ps)–300 K	Carbon-based nanocomposites improve tensile strength by 8–9× and elastic modulus by 9.5–11.5×. Crack penetration reduces fracture strength by 15–20%.
Mishra R. et al. [2021] [177]	FEA	This study investigated, comparing heat transfer with stagnant air under sub-zero conditions, the thermal insulation of aerogel-based nonwoven fabrics using FEM simulations (ANSYS and COMSOL) and experiments.	Modeling SOLIDWORKS ANSYS Steady and transient state thermal analysis Boundary conditions One face set at 283.15 K Other face at 293.15 K Convective coefficient: 27.07 W/(m2·K) COMSOL Conjugate heat transfer module Boundary conditions One face at 329.19 K Initial fabric temperature: 263.15 K Forced convection: 2.5 m/s wind Convection coefficient: 23.76 W/(m2·K) Without forced convection: open boundary at 263.15 K	Thermal properties	Maxwell and Hamilton model (effective thermal conductivity) Rosseland equation (radiative thermal conductivity)	248–258 K (ANSYS) 263.15–329.19 K (COMSOL)	Aerogel outperforms air as an insulator at all temperatures. Computational models predicted conductivity with <10% error in comparison to experimental results.
Xiao J. et al. [2023] [181]	MD	The objective is to investigate the pyrolysis behavior of siliconoxycarbide-modified phenolic resin (SiOC-PR) nanocomposites using reactive force field (ReaxFF) MD simulations via LAMMPS, as well as analyzing gaseous product evolution, reaction pathways, and thermal stability improvements.	Simulation box 63.56 Å × 63.56 Å × 175.58 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Time step: 0.1 fs Heating rate: 100 K/ps (cook-off simulations) Thermostat used Berendsen thermostat for temperature control Ensemble used NVT	Mechanical properties Thermal behavior and pyrolysis	Reaction kinetics equation Flynn–Wall–Ozawa (FWO) method Kissinger method Distributed activation energy model (DAEM) equation	300–5000 K (cook-off simulations) 2400–3400 K (isothermal simulations)	ReaxFF MD simulations revealed that SiOC-PR undergoes five-stage pyrolysis, where SiOC enhances oxidation resistance, regulates gaseous product evolution, and stabilizes phenolic fragments by Si-O-Si interactions.
Zhang W. et al. [2023] [33]	MD	This study investigates the effect of carbon doping (1–10%) on the mechanical properties of paraffin-reinforced silica aerogel (PRSA) when using MD simulations with LAMMPS, Avogadro, and Packmol.	Modeling Avogadro (paraffin structure modeling) Packmol (used to pack nanostructure) Simulation box 50 Å × 50 Å × 50 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Strain rate: 0.1 ${\mathrm{s}}^{-1}$ Number of atoms: 9000 Tensile testing performed along the x-axis. Velocity–Verlet algorithm for particle motion	Young’s modulus Stress–strain curve Ultimate strength	Lennard–Jones potential (interatomic interactions) Vashishta potential (silica structures) Newton’s second law (MD calculation)	300–7000 K ((1 ns, NVT) heating) 7000–300 K ((1 ns, NVT) cooling) 300 K ((1 ns, NPT) energy minimization) 0–3000 K ((1 ns) annealing) 300 K ((2 ns, NVT) final equilibration)	MD simulations show that carbon doping (3%) enhances PRSA’s mechanical properties, increasing the ultimate strength and Young’s modulus, while equilibrium is achieved at 300 K after 2 ns.
Maximiano P. and Simões P. N. [2023] [182]	MD	This study investigated, focusing on the aggregation, adsorption, oxidation effects, and structural organization at the nanoscale, silica aerogel–CNT interactions using GROMACS [199,200] MD simulations.	Boundary conditions Periodic boundary conditions to eliminate surface effects Non-bonded interactions cutoff: 1.2 nm Electrostatics: particle mesh ewald (PME) method Time step: 2.0 fs Production time: 1.0–2.2 µs Pressure: 1.0 bar Ensemble used NPT for equilibration (V-rescale thermostat, Berendsen barostat) NVT for production (V-rescale thermostat)	Silica particle aggregation Silica-CNT adsorption Siloxane bond orientation	OPLS-AA force field (bonded and non-bonded interactions) TIP4P (water model) Partial charges derived using CM5 method Gaussian16 and Multiwfn packages (electronic structure calculations)	298.15 K (equilibration and production)	Silica–CNT interactions vary by oxidation, affecting aggregation and adsorption. Van der Waals forces direct the structure, especially in octamer systems. H-bonds enhance ordered layers in oxidized CNTs.
Karimipour A. et al. [2024] [183]	MD	This study investigated, analyzing Young’s modulus, ultimate strength, stress–strain behavior, and atomic interactions, the temperature-dependent mechanical behavior of tricalcium phosphate/chitosan/silica aerogels nanocomposites (TCS-NCs) using MD simulations via LAMMPS, Avogadro, and Packmol.	Simulation box 90 Å × 90 Å × 90 Å Boundary conditions Periodic boundary conditions to eliminate surface effects Time step: 0.1 fs Damping ratio: 0.01 Thermostat used Nose–Hoover thermostat Ensemble NVT (constant volume, constant temperature) NPT (constant pressure, constant temperature) Velocity–Verlet algorithm	Young’s modulus Stress–strain curve Ultimate strength Mean squared displacement (MSD) Interaction energy	Lennard–Jones potential (van der Waals interactions) Vashishta potential (silica aerogel interactions) Radial distribution function (RDF) Newton’s equation of Motion Electric potential energy	7000 K ((1 ns, NVT) initial heating) 7000–300 K ((1 ns, NVT) cooling) 300 K ((1 ns, NPT) energy minimization) 0–3000 K ((1 ns) annealing) 300 K ((2 ns, NVT) final equilibration) 297 K (initial temperature)	MD simulations revealed that increasing temperature influences TCS-NC’s mechanical properties, with the Young’s modulus peaking at 320 K and the ultimate strength decreasing as the temperature rises from 300 to 350 K.
Zhu C. Y. et al. [2024] [153]	FEM	This study investigated the impact of the opacifier protrusion shape, number, and volume on extinction properties, as well as the radiative thermal conductivity, when using COMSOL-based finite element method (FEM) simulations, thereby optimizing silica aerogel insulation for high-temperature applications.	Boundary conditions Perfectly matched layer (PML) (used to absorb outgoing waves and prevent reflections) Incident background electric field (defined by a plane wave propagating in the z-direction)	Extinction performance Radiative thermal conductivity Scattering and absorption properties Effect of protrusion shape, count, and volume	Maxwell’s wave equation (FEM simulation in COMSOL) Extinction, absorption, and scattering cross-sections Rosseland approximation (radiative thermal conductivity ) Spectral extinction coefficient	1300 K	The study used COMSOL to show that cylindrical protrusions (optimal at 12) significantly enhance the extinction properties, reducing the radiative thermal conductivity of silica aerogels by 22.6% at 1300 K.
Fu W. et al. [2024] [175]	FEA	This study numerically simulated the heat transfer on ${\mathrm{CaCl}}_2·6{\mathrm{H}}_2\mathrm{O}$ silica composite phase change material (CPCM) using ANSYS Fluent	Simulation model Transient heat transfer numerical simulation Boundary conditions Optimal mesh size: 257,413 All materials: homogeneous and isotropic Thermal properties: constant Algorithm: coupled Energy equation (second-order upwind scheme) Relaxation factors Pressure: 0.5 Momentum: 0.5 Density: 1.0 Body forces: 1.0 Energy: 0.75 Simulation duration: 10 days	Thermal and mechanical properties	Heat transfer equation Thermal conductivity equation for phase change Specific heat capacity equation Thermal resistance calculation Thermal inertia calculation Thermal storage coefficient calculation	293.15 K (initial temperature) 368.58 K (maximum temperature on external wall)	Numerical simulations showed that 20 wt% CPCM with an 80 mm thickness optimized thermal insulation, reducing peak temperature by 9.87 K with a delay time of 198,000 s and an attenuation factor of 6.6.
Zhang X. et al. [2024] [32]	MD	This study investigated the thermal conductivity of ${\mathrm{SiO}}_2/{\mathrm{Al}}_2{\mathrm{O}}_3$ composite aerogels using non-equilibrium molecular dynamics (NEMD) simulations across varying ${\mathrm{Al}}_2{\mathrm{O}}_3$ content, temperature, and strains using LAMMPS and OVITO.	Boundary conditions Periodic boundary conditions were applied in all directions. Thermostat used Nose–Hoover thermostat Energy minimization (conjugate gradient method for 100 ps.) Velocity–Verlet algorithm (0.5 fs)	Thermal conductivity	EAM potential (Ai–O interactions) Extinction, absorption, and scattering cross-sections BKS potential ( Si-O interactions)	5000 K (randomized initialization) 300 K ((10 K/ps) quenching process)	MD simulations showed that adding ${\mathrm{Al}}_2{\mathrm{O}}_3$ makes the nanoporous ${\mathrm{SiO}}_2$ structure stronger, reducing thermal conductivity, while a slight increase occurs under elevated temperature and compression strain conditions.

7. Conclusions

Over the past decade, SiA composites have garnered significant attention, driving major advancements in their design, synthesis, and applications. Our comprehensive review systematically examined the fundamental experimental evaluations and material behaviors across the thermo-mechanical domain under varying environmental conditions. Special emphasis was placed on the acoustic damping, load-bearing capacities, deformation recovery mechanisms, and temperature-dependent mechanical properties of SiA composites. Furthermore, innovative approaches toward sustainable, energy-efficient materials were highlighted, spanning applications from building insulation and battery separators to marine thermal protection systems. We further examined a broad range of theoretical and numerical frameworks, critically identifying key gaps and also suggesting future research directions. These included the development of new models that more accurately replicate real-world conditions with fewer assumptions, as well as the advancement of complete calculation algorithms capable of rapidly and precisely predicting the effective thermal conductivity of SiA composites.

Despite notable experimental advances in ultra-lightweight insulating nanocomposites, a unified understanding of their fire and radiation resistance remains rather limited. This review critically examined the response of SiA composites to both cosmic and electromagnetic radiation, highlighting degradation mechanisms, structural stability under exposure, and the shielding efficiency at nanoscale interfaces. Ground-based testing methodologies, including accelerated aging and radiation simulation chambers, were also discussed to better characterize these interactions. Similarly, fire resistance evaluations through both small-scale and large-scale fire tests were critically analyzed, highlighting key mechanisms, such as char layer formation, prevention of pore structure collapse, and the development of effective thermal barriers, all of which are essential for safeguarding steel structures, as well as flexible wearables and textiles. Notable identified gaps included the need for scalable fabrication of radiation-shielding SiA composites, while also laying further emphasis on the halogen-free flame retardants and combustion characteristics of organic-modified SiA composites.

Similarly, our work uniquely provides a comparative analysis of continuum mechanics-based and nanoscale simulations, with an emphasis on multi-scale modeling strategies for SiA composites. The review critically examined the applications and limitations of the FE and MD simulations within the thermo-mechanical domain. Furthermore, this review also highlighted custom-built advanced simulation frameworks that integrated user-defined modules to replicate realistic service environments. Finally, future research directions were independently outlined for both FE and MD approaches, emphasizing the development of models that not only align closely with experimental results, but also optimize computational efficiency.

Overall, our review comprehensively offers a structured overview of the notable literature across multiple domains, enabling meaningful comparisons across several key factors. While remarkable advancements have been made over the past decade, this work emphasizes that the field now stands at a critical juncture, ready to move beyond isolated improvements toward sustainable and scalable solutions. By systematically highlighting critical gaps and challenges, we provided future researchers with targeted directions that could drive breakthroughs and unlock the full potential of SiA composites across advanced energy, aerospace, building, transportation, and thermal management applications.