Hot Bridge-Wire Ignition of Nanocomposite Aluminum Thermite Synthesized Using Sol-Gel-Derived Aerogel with Tailored Properties for Enhanced Reactivity and Reduced Sensitivity

Abstract

The development of nano-energetic materials has significantly advanced, leading to enhanced properties and novel applications in areas such as aerospace, defense, energy storage, and automobile. This research aims to engineer multi-dimensional nano-energetic material systems with precise control over energy release rates, spatial distribution, and temporal and pressure history. In this context, sol–gel processing has been explored for the manufacture of nanocomposite aluminum thermites using aerogels. The goal is to produce nano-thermites (Al/Fe₂O₃) with fast energy release rates that are insensitive to unintended initiation while demonstrating the potential of sol–gel-derived aerogels in terms of versatility, tailored properties, and compatibility. The findings provide insightful conclusions on the influence of factors such as secondary oxidizers (KClO₃) and dispersants (n-hexane and acetone) on the reaction kinetics and the sensitivity, playing crucial roles in determining reactivity and combustion performance. In tandem, ignition systems contribute significantly in terms of a high degree of reliability and speed. However, the advantages of using nano-thermites combined with hot bridge-wire systems in terms of ignition and combustion efficiency for potential, practical applications are not well-documented in the literature. Thus, this research also highlights the practicality along with safety and simplicity of use, making nano-Al/Fe₂O₃-KClO₃ in combination with hot bridge-wire ignition a suitable choice for experimental purposes and beyond.

1. Introduction

1.1. Potential Uses and Applications of Composite Energetic Materials

Composite energetic materials offer the prospect of improved reactive performance, concurrently facilitating safe handling contingent upon their sensitivity thresholds [1]. Their deployment spans military and commercial domains, with endeavors striving towards enhancing reliability and safety while concomitantly addressing the environmental impact [2]. Nonetheless, these materials are not exempt from limitations and challenges. For instance, the development of novel composite energetic materials necessitates an extensive and resource-intensive process, often relying on iterative trial-and-error methodologies rather than a well-defined optimization framework [3]. Furthermore, the industry is confronted with regulatory pressures to curtail or eliminate hazardous substances from the workflow while simultaneously striving for product improvements manifested as escalations in performance and safety metrics, coupled with the pursuit of more environmentally benign alternatives for product development [2].

Nano-energetic materials have evinced the potential of elevated heat release rates, tailorable burning rates, and reduced sensitivity, thereby occupying a pivotal role in defense applications [4], with widespread deployment in modern engineering [3], and routinely applied in other diverse fields encompassing automotive industry, aviation, aerospace, renewable energy, and even biomedical sectors, as illustrated in Figure 1. Nanocomposite energetic materials are characterized by the intimate intermixing of reactive components at the nanometer to atomic scale, engendering rapid combustion dynamics and efficient energy release mechanisms. A preceding investigation [5] elucidated the various preparatory methodologies and composite systems, underscoring their high energy density and release rate attributes, which have garnered significant research interest.

1.2. Ignition and Combustion of Metal and Metalloid Particles

Metal and metalloid particles possess and exhibit unique properties that render them attractive candidates for deployment across a diverse array of practical applications. However, each distinct metal or metalloid particle presents its own set of limitations and challenges in their deployment. Beryllium, for instance, is a highly energetic metal that finds limited utilization due to its extreme toxicity, scarcity, and prohibitive cost [6]. Boron, comparatively speaking, boasts the highest volumetric energy density among its counterparts, yet its ignition and combustion dynamics are significantly impeded by the presence of its oxide layer [7,8,9]. This delay in ignition and combustion has constrained its usage in practical applications. Nonetheless, researchers are actively exploring novel passivation materials and approaches to enhance its combustion characteristics. In contrast to beryllium and boron, aluminum, which is relatively safe to handle [10], exhibits extensive application prospects in the domains of thermites, pyrotechnics, explosives, and propellants [11] and can be deployed in space and underwater propulsion systems [10,12], as well as hydrogen generation applications [13].

The use of nano-aluminum in solid propellants has evinced the potential for enhancing combustion performance and reducing combustion times, thereby catalyzing improvements in jet momentum characteristics and mitigating slag formation propensities [14,15]. The synthesis of aluminum nanoparticles for fuel applications has been explored [16], underscoring their prospective integration into micro-electromechanical systems and as reactive nanomaterials across diverse application domains. Nano-sized aluminum particles have demonstrated significantly reduced ignition delay times in comparison to their micro-sized counterparts [14,17,18]. While nano-sized aluminum exhibited a relatively subdued effect on ignition delay time, micro-sized aluminum manifested an exponential influence on this parameter [14]. Furthermore, nano-sized aluminum exhibited more intense combustion dynamics and superior self-sustaining performance characteristics relative to their micro-sized counterparts [14]. Conversely, escalations in ambient temperature or oxygen content not only showed reductions in the ignition delay time of the particles but also exerted an influence on the combustion time [18,19]. Although aluminum nanoparticles exhibit lower ignition temperature thresholds in comparison to their micro-sized counterparts, they also exhibit a propensity for sintering and agglomeration due to solid-state diffusion and viscous flow phenomena [20]. The sintering and agglomeration of nanoparticles can facilitate the contact and condensation of fuel and oxidizer aggregates, thereby engendering reactive sintering and rapid melting and coalescence of the agglomerated particles. Recent endeavors [21,22] have indicated that reactive sintering constitutes a pivotal component of the combustion dynamics of nanocomposite energetic materials.

The sintering phenomenon in nanocomposites is a thermally activated process encompassing mass transport events manifesting at the atomic scale [23]. The sintering process is complex, subject to modulation by an array of factors, including chemical composition, thermodynamic parameters, pressure, time, temperatures, atmospheric conditions, degree of agglomeration, particle shape and size distribution, and the rates of heating and cooling [23]. The size distribution of the agglomerates is governed by the maximum entropy state of the agglomerate ensemble, calculated employing the Gibbs formula of entropy [24]. The small size of nanoparticles leads to high surface energy levels, thereby promoting different interactions with the surrounding environment and exerting an influence on their mobility characteristics, primarily dictated by Brownian motion [25].

Additionally, the presence of defects, cracks, and voids within a particle structure can exert a profound influence on its ignition dynamics, modulating its sensitivity and ignition threshold parameters. The spatial distribution of hotspot regions within the particle is contingent upon these defects, and as the reaction rate increases, the ignition mechanism transitions from a bulk-dominated regime to a surface-driven ignition. Thermal shock phenomena can also precipitate the formation of cracks within the particle, thereby exerting a further modulating influence on its ignition behavior [23,24,25]. Mesoscale simulations can be employed to explicitly account for defects and quantify their effects on shock sensitivity, enabling the prediction of ignition thresholds and probabilities for particles exhibiting varying degrees of initial grain cracking or interfacial debonding [26]. A spatially resolved rate theory approach can be utilized to investigate the coupling between point-defect diffusion/recombination dynamics and concentrated stress fields in the vicinity of crack tips, facilitating an understanding of void formation mechanisms near microstructural stressors [23]. Defects can also exert an influence on the melting temperatures [27], which are constant for void volumes below 1 nm³ but diminish with further increments in void volumes. For voids with volumes exceeding 8 nm³, the particle becomes unable to withstand the destabilizing forces associated with the void, culminating in an abrupt collapse of the crystal structure at 700 K [28].

Moreover, particle ignition is also associated with the melting of the oxide layer, wherein the corresponding melting point is considered to be the bulk melting temperature of the oxide layer, approximated at 2350 K [29,30]. The thickness of the oxide layer emerges as a crucial factor in this regard, with the melting temperature of the passivation layer playing a pivotal role in governing the ignition and combustion characteristics of metal-based energetic nanomaterials [28,31]. In addition, the combustion of metal particles is subject to profound influence by the boiling temperature. The boiling point of a substance is contingent upon intermolecular forces such as hydrogen bonding, ionic interactions, and Van der Waals forces [32,33]. These forces exert a governing influence in determining the boiling points of substances, which can subsequently impact the ignition behavior of thermites. The boiling temperatures of metal nanoparticles deviate from those of their bulk counterparts, a phenomenon that can exert a significant impact on the propensity for vapor-phase combustion. It is generally observed that the boiling temperatures of metals are lower than those of their respective oxides, with notable exceptions such as boron, silicon, and zirconium. When it comes to the ignition and combustion characteristics of nano-aluminum, these parameters have been extensively investigated across diverse environmental conditions [14,16,34,35,36,37,38].

The ignition temperature of aluminum particles is primarily contingent upon their size dimensions, albeit subject to modulation by factors such as heating rate profiles, oxide layer thickness, and oxidizer composition, among others. For aluminum particles exhibiting diameters in the vicinity of 10 µm, the ignition temperature regime spans 1000–2350 K, evincing a trend of diminishing ignition temperature thresholds with decreasing particle size. As an illustration, ignition can be initiated at temperatures as low as 1000 K, a value significantly lower than the bulk melting point of the oxide shell [39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54]. The ignition delay can be quantified through energy balance analysis [36]. In modeling particle oxidation dynamics, the gas temperature is typically set at 2500 K, employing Mott–Cabrera kinetics. The burning time of particles exceeding 20 µm exhibits a near-quadratic proportionality with respect to the particle size [39,55], while the burning time of smaller particles exhibits a weaker dependence on particle size. It is noteworthy that the ambient gas temperature exerts a profound influence on the combustion characteristics of nanoparticles [56]. Numerical simulations have also elucidated that increasing the initial temperature facilitates the ignition of nano-aluminum, with the two-stage ignition regime gradually dissipating with increments in the initial temperature [34]. The combustion process of nano-aluminum in air and CO₂ atmospheres exhibits no discernible difference under temperature-programmed conditions, with the oxygen concentration exerting a significant influence on the combustion dynamics [35].

1.3. Combustion Properties and Characteristics of Aluminum-Based Nano-Thermites

Aluminum-based thermites typically comprise aluminum as the reducing agent, coupled with various metal oxides serving as oxidizers [57,58,59]. Commonly employed oxidizers in aluminum-based thermites encompass CuO, Fe₂O₃, Fe₃O₄, MoO₃, and MnO₂ [57,58,59,60]. Nano-thermites not only exhibit elevated energy density, but their combustion rate kinetics can be readily tailored by modulating the fuel and oxidizer particle size, packing density, and compositional parameters [61].

Fundamental research endeavors and initiatives delving into the combustion dynamics of nano-thermites can be traced back to 1995 [62]. Flame propagation velocities of Al/MoO₃ nano-thermites with average particle dimensions spanning the 20–50 nm range were found to exhibit an increase of orders of magnitude greater than their conventional micro-sized counterparts. This phenomenon was ascribed to the reduced diffusion length scales arising from the intimate interfacial contact manifested at the nanoscale level. The study elucidated the superior reactivity of nano-thermites, marking the inception of extensive research efforts directed toward these materials.

Initial studies [63] suggested that the substitution of nano-aluminum particles for their micro-sized counterparts exerted a detrimental effect on the burning rate of thermites. This observation was attributed to the prolonged exposure of micro-thermites to the laser beam, resulting in significant preheating and, consequently, higher burning rates. Subsequent research initiatives, however, have unequivocally demonstrated that the addition of nanoparticles engenders enhancements in the burning rates. The burning rate is typically quantified by loosely packing the thermite within a burn tube, followed by ignition [64]. Photodetectors, pressure sensors, and high-speed cameras are commonly employed to acquire optical, pressure, and visual data, respectively, pertaining to the burning process. The packing density of the powder was maintained within the range of 5–10% of the theoretical maximum density. The results unveiled that the burning rate exhibited an escalation with diminishing particle size dimensions and manifested independence from the particle size below a threshold in the range of 80–110 nm.

Other studies have demonstrated that aluminum-based thermites exhibit elevated heat release characteristics, wherein the heat release of Al/MoO₃ is significantly higher in comparison to other thermite compositions [58,65]. The critical reaction temperature of super thermites undergoes a diminution of 200–400 °C upon the addition of aluminum nanoparticles, implying an enhanced reactivity [58]. The ignition temperatures of reactive aluminum-based thermites are consistent with the temperature of the polymorphic phase change of alumina, suggesting that the rate-limiting initiation step is the acceleration of the outward diffusion flux of aluminum [59].

The thermal characteristics and combustion dynamics of aluminum-based thermites are contingent upon their morphological attributes, wherein Al/ribbon-MoO₃ compositions evince a pronounced escalation in heat release and combustion performance in comparison to their Al/rod-MoO₃ counterparts [66]. The diffusion of Al and O atoms through the oxide shell of aluminum particles exerts an influence on the thermodynamic properties of Al₂O₃, a phenomenon that can subsequently impact the ignition process of the thermite. The doping of Al and O interstitial atoms within the Al₂O₃ matrix can engender modulations in its thermodynamic properties, rendering the oxide shell more ductile and potentially susceptible to spallation during the ignition phase [67].

Ensuring judicious handling protocols for aluminum-based thermites necessitates a comprehensive understanding of their thermal stability characteristics and the aging phenomena they undergo. As an illustration, thermal annealing at specific temperature regimes can precipitate unanticipated accelerated reactions and pressurization rates for certain thermites, underscoring the intricate nature of aging processes manifested in powdered nano-thermites [57].

1.4. The Crucial Role of Selecting the Appropriate Fuel and Oxidizer

Selecting the appropriate fuel and oxidizer plays a crucial role in determining the energy threshold requisite for initiating a chemical reaction, the heat generated during the reaction process, and the efficiency with which energy is harnessed. Recent studies [68] have elucidated the feasibility of increasing the energy output characteristics of aluminum-based energetic materials by employing oxidizer-activated aluminum fuel particles in combination with energetic polymers. This innovative approach evinces the potential to reduce the activation energy needed for the reaction, resulting in a more efficient energy release.

The addition of CuO to Al-Mg alloys in thermite reduces the activation energy of the reaction, with the best performance observed in Al_0.8Mg_0.2(MoO₃)_0.5CuO_0.5 [69]. The addition of CuO to Al-Mg alloys increases the exothermic heat in high-temperature regions of the thermite reaction [69], while the addition of KClO₄ to Al/Bi₂O₃ nano-thermite also reduces the activation energy, with Al/Bi₂O₃/10wt%-KClO₄ exhibiting the highest heat release among other samples [70], while the lowest activation energy was observed in Al/Bi₂O₃/30wt%-KClO₄ [70].

Additionally, the impact of doping La₂O₃ on the reactive characteristics of Al/CuO thermite altered the flame propagation mode, indicating a potential influence on the fraction of energy feedback [71]. The velocity of the luminous front in loosely packed Al/CuO thermites was influenced by the fill length and open porosity, impacting the burning rate [71].

The burning rate of thermites is influenced by thermodynamic and kinetic parameters such as mixture density, specific heat, thermal conductivity, activation energy, and pre-exponential factors [72]. Activation energy and specific heat emerge as the predominant contributing factors governing the burning velocity, reaction zone thickness, ignition delay, ignition temperature, and peak temperature [72]. The burning rate of thermite compositions under confinement is influenced by the pressure-driven flow, pore size, and fluid viscosity [73]. A study investigating the effects of thermodynamic and kinetic parameters on simulated Fe₂O₃-2Al thermite reaction propagation found that higher activation energy resulted in a slower and thicker reaction propagation wave [72]. Diluted Fe₂O₃-2Al thermite system with alumina was found to suppress burning rate, peak temperature, and expelled mass, signifying potential cost-effective and controlled applications in thermal processes [74]. Therefore, the key factors determining the activation energy, heat of reaction, and fraction of energy feedback in thermite include the choice of fuel and oxidizer and thermodynamic and kinetic parameters [69,70,72,75]. These factors interact synergistically to exert their influence on the thermite reaction propagation, heat release, and combustion performance [69,72].

1.5. Synthesis Methods of Composite Energetic Materials

The conventional approaches for the fabrication of composite energetic materials involve the physical intermixing of fuel and oxidizer particles. A major drawback of these methods is the formation of highly non-uniform and disorganized agglomerates [76]. In contrast, material synthesis techniques encompass the manufacturing of nascent metal particles, followed by a passivation process in a well-controlled oxidizing environment. This passivation step ensures the preservation of the active metal content within the particles while conferring stability to the system. When undertaking material synthesis, it is imperative to satisfy certain requisite criteria, including the capability to work with diverse material systems, produce particles spanning a wide and controllable size range, and mitigate the formation of impurities, spontaneous reactions, and agglomeration phenomena while concurrently maintaining elevated levels of productivity and economic viability and ensuring reproducibility of the resultant characteristics [76,77,78,79,80,81,82,83,84,85,86,87,88,89]. As shown in Figure 2, the synthesis techniques specifically tailored for thermite applications and their suitability are contingent upon the specific performance requirements and criteria, offering a multitude of advantages and trade-offs [76,77,78,79,80,81,82,83,84,85,86,87,88,89].

The sol–gel method facilitates the judicious tailoring of the chemical composition of the gel matrices by means of combining precursors, thereby enabling the deliberate modulation of the resultant material properties [90,91]. This attribute renders the sol–gel approach conducive to the fabrication of energetic nanocomposites exhibiting tunable performance characteristics [92]. The sol–gel technique is distinguished by its versatility, cost-effectiveness, and amenability for large-scale production at economically viable costs, rendering it suitable for a diverse array of material systems spanning metal, inorganic, organic, and hybrid compositions [93,94].

The sol–gel process constitutes a wet chemical synthesis route wherein a homogeneous colloidal suspension, termed a ‘sol’, is formed from precursors, subsequently undergoing gelation within a continuous liquid phase to engender the formation of a three-dimensional network structure, referred to as a ‘gel’, after which comes the subsequent drying, a sequence that can facilitate the production of nanoparticle materials with diverse chemical compositions [93]. The hydrolysis and condensation reaction pathways govern the sol–gel process dynamics, exerting a modulating influence on the formation of the final gel matrix and the sol-to-gel transition phenomena [95,96]. The controlled modification of sol–gel derived coatings and their respective applications can also contribute to enhancing reactivity at the molecular level [94]. The relative surface area of nanomaterials exhibits an augmentation in comparison to their bulk counterparts, contributing to enhanced reactivity [97].

Nonetheless, in-depth comprehension of the underlying chemical principles is imperative to attain optimal material property outlines and harness the myriad possibilities afforded by the sol–gel method, as the gel synthesis proceeds under the governance of kinetic control mechanisms, resulting in amorphous and metastable materials [90]. Additionally, conventional sol–gel approaches encounter limitations stemming from the disparate reactivities of the precursors and the high surface tension of water, thereby complicating the synthesis of hybrid compositions [98].

The judicious selection of the material construct, whether aerogels, xerogels, or cryogels, exerts a profound influence on the combustion dynamics and performance metrics of thermites, owing to their distinctive characteristics and properties such as surface area, thermal conductivity, and porosity [92,99,100,101]. The disparities manifested in the thermal stability and reactivity of thermites synthesized employing aerogels, xerogels, and cryogels are ascribed to the unique properties inherent to these materials [92,99,100,101,102].

Aerogels are distinguished by their unique properties, encompassing high surface areas, sharp pore size distributions, low thermal conductivities, and high sorption capacities, rendering them attractive candidates for a myriad of applications, including the synthesis of thermites [99,102]. Aerogels may enhance the combustion behavior and performance of thermites by providing efficient heat transfer and favorable reaction sites [99,102]. Aerogels can enhance the reactivity and energy release of thermites. However, their synthesis may necessitate the implementation of specific processing protocols and stringent synthetic control over the structural properties [92,102].

Xerogels are lightweight, high surface area mesoporous materials that can maintain their mesoporous structure at elevated temperatures, making them suitable for thermal superinsulation applications [100]. Xerogels may also contribute to improved combustion behavior and performance of thermites by facilitating rapid heat dissipation and reaction kinetics [100]. However, their limitations in terms of temperature stability and drying methods have to be considered [100].

Cryogels, particularly nitrogen-doped cryogels, evince outstanding specific and normalized capacitances, rendering them suitable for supercapacitor electrode applications [101]. Cryogels may offer distinctive advantages in enhancing the combustion behavior and performance of thermites, potentially influencing their reactivity and energy release during combustion [101]. However, the influence of nitrogen- and oxygen-based moieties has to be carefully considered [101].

1.6. Key Factors Affecting the Choice of Ignition System–Thermite Combinations

The efficiency of aluminum thermite reactions is impacted by the choice of the ignition system, wherein parameters such as ignition delay time, delay time to peak pressure, peak pressure, and decay after peak pressure influence reactivity and efficiency [103]. Indeed, different ignition systems manifest different performance implications in aluminum thermite reactions, resulting in certain thermites generating elevated pressure and shorter burning time in comparison to others, implying varying reactivity levels [59], while others exhibit a lower peak pressure, indicative of a higher reactivity [103,104].

The choice of ignition system in aluminum thermite reactions can include impact ignition, electric-spark initiation, and thermal trigger methods [103,105]. Impact ignition involves examining pressure build-up and decay in thermites upon ignition, which influences reactivity based on the holistic pressure history [103]. Electric-spark initiation has been studied in thermite mixtures based on nano-sized powders of aluminum and copper oxide, providing insights into the effect of the electric spark discharge energy on the ignition delay [105]. The hot bridge-wire, on the other hand, is classified as a thermal trigger method rather than an impact ignition method. This classification is based on the mechanism by which the detonator functions, involving the rapid heating and vaporization of the wire to create a shock wave and thermal energy that initiates the reaction, making it a thermal trigger method [106,107]. The type of hot bridge-wire that functions with specific current and resistance values is typically designed for applications where precise resistance values are crucial. This specific configuration allows the bridge-wire to handle a specific current while maintaining the corresponding resistance, ensuring reliable and consistent performance in systems requiring controlled resistance levels [106,107].

This initiation method must demonstrate a high degree of reliability and speed, providing a significant amount of energy for efficient ignition and combustion, along with safety and simplicity of use [106,107]. In contrast with bridge-wire ignition [108], laser systems require more precise alignment and control. They may also produce stray electromagnetic radiation that can interfere with other equipment. Additionally, they may require more maintenance and calibration. However, it is important to note that laser ignition systems have their own advantages, such as the ability to ignite materials in a non-contact manner and the ability to precisely control the ignition energy. The choice of ignition method will depend on the specific requirements of the application.

When selecting an ignition system for aluminum thermite reactions, key factors to consider include the particle size of aluminum, the species of oxidants, and the mass content of additives [58,104,109]. The particle size has been identified as a key factor affecting the reaction activity of thermites, with nano-sized particles exhibiting lower critical reaction temperatures and higher reaction activity compared to their micro-sized counterparts [58].

1.7. Controversial and Diverging Hypotheses

Some findings challenge some of the traditional assumptions about nano-thermite reactions and suggest the need for further investigation into the complex behavior of these energetic materials. The controversial hypotheses on nano-thermite ignition and combustion revolve around the reaction process, ignition delay time, temperature, peak pressure, oxygen providers, and the nature of the nano-thermite reactions.

For instance, it is deemed true that the addition of KClO₃ results in lower activation energy and lower ignition delay times and temperatures. However, in some cases, it is more likely to observe an increase instead of a decrease in activation energy and ignition delay times and temperatures.

In other instances, it is assumed that there is a direct and proportional relationship between the charging pressure and peak pressure. But once again, in some other cases, it was shown that the relationship is not always directly proportional but rather represented by a local maximum at which the ignition efficiency and combustion performance are at their maximum.

Also, the relationship between the maximum burning rate and purity was believed to be straightforward but appeared to be contrary to the expected trend, and higher purity does not always result in faster burning rates.

In another example, the optimum equivalence ratio was believed to be constant regardless of the oxidizer and based only on the chemistry involved with the reaction. However, the optimum equivalence ratio varies for each oxidizer synthesis technique. These variations may be attributed to the impurities present in the oxidizer.

In our research, a systematic and thorough approach is adopted to address each of the aforementioned key challenges and controversial and diverging hypotheses. The relevant factors are described in well-defined steps and carefully examined, and the data is analyzed in a methodical fashion. This step-by-step process provides comprehensive and well-supported conclusions.

2. Synthesis Method and Experimental Setup

Aerogels have attracted significant interest from various fields, including energy, environment, aerospace, and biomedical engineering, due to their unique properties [110]. Using Iron(III) nitrate nanohydrate, Fe(NO₃)₃·9H₂O, as a raw material, the sol–gel method combined with CO₂ supercritical fluid drying technology was used to prepare the aerogel oxidant ferric oxide, or Iron(III) oxide, nano-Fe₂O₃ gel.

It is noteworthy that the shape of the iron oxide nanoparticles can impact the combustion velocity of the nano-thermite reaction, with different contact configurations between nano-fuel and nano-oxidizer influencing the reaction mechanisms [111].

Nano-Al particles mixed with nano-Fe₂O₃ in nano-thermite compositions resulted in highly reactive composites with enhanced performance due to the higher surface area-to-volume ratio of nano-sized reactants [58,111,112,113,114,115,116,117,118,119]. Stabilization of colloidal nanoparticles in solvents offered intimate mixing between the oxidizer and the fuel, leading to improved dispersion and stability of the nano-thermite mixtures [58,112,113,117] and eliminating nanoparticle drying and re-dispersion issues [112].

For aluminum, the typical thickness of the oxide layer is around 24 nm [27]. The oxide layer thickness of Al-based nano-thermite is usually in the range of 3–5 nm. In our experiment, the oxide layer thickness of nano-Al was 4 nm, while the average diameter of nano-Al was 45 nm.

Additional oxygen is required to ensure that the conversion to aluminum oxide and iron takes place effectively. To facilitate this, potassium chlorate, KClO₃, was introduced into the reaction mixture in different proportions [120,121]. This helped to supply the necessary oxygen for the nano-thermite reaction to occur efficiently by supporting the exothermic redox reaction between the nano-Al and nano-Fe₂O₃ [70].

The synthesis method of the sol–gel-derived aerogel and the Al/Fe₂O₃-KClO₃ nanocomposite is described in Figure 3.

The synthesis and preparation of customized samples were followed by ignition and combustion to investigate the samples’ properties and characteristics. The experimental setup and the series of trials, along with the tests’ conditions, are described in

Table 1. Description of the experimental setup, the series of trials, and the tests’ conditions.

N	Description	Conditions
I	Constant volume combustion test	- Volume of closed explosive device: 6 mL - Bridge-wire current: 5 A - Charge: 100 mg - Charging pressures: 0; 60 kPa; 120 kPa
II	Ignition condition test	- The composition is the same as in Test I. - Electro-explosion tube with inner diameter: 6 mm - Bridge-wire resistance: 1 Ω - Charge: 100 mg - Charge density: 0.45 g/cm3 - Charging pressure: 60 kPa
III	Flame sensitivity test	- Test method: fuse method - Fuse Length: 5 cm - Experimental dosage: 20 mg
IV	Effect of equilibrium ratio on Al/Fe2O3 combustion performance	- Nano-Al/Fe2O3 with different balance ratios Φ - Quartz glass tube with inner diameter: 2 mm
V	Effect of Fe2O3 specific surface area on Al/Fe2O3 combustion performance	- Glass pipe diameter: 2 mm - Balance ratio Φ = 1.1 - Filling density: 0.16 g/cm3
VI	Effect of heat treatment temperature on Al/Fe2O3 combustion performance	- Glass pipe diameter: 2 mm - Balance ratio Φ = 1.0 - Charge density: 0.21 g/cm3

. Conducting a series of trials in scientific experiments is crucial for ensuring the accuracy, reliability, reproducibility, and validity of the findings, as well as improving the ability to control for confounding variables and perform robust analyses. Within the scope of this undertaking, the present study encompassed six distinct experimental trials carried out under tailored conditions and predetermined circumstances to achieve designated objectives and fulfill targeted aims relevant to our research.

While Table 1 describes the considered tests and their related conditions, Figure 4 illustrates the experimental framework covering aspects from the sol–gel-derived aerogel synthesis through the conducted series of trials, the manipulated independent variables, and their effect on the dependent variables to the obtained results.

As shown in Figure 4, the synthesis and preparation of the samples constituted a minor but crucial portion of the entire experimental process, as these samples were endowed with certain qualities, distinct features, and physicochemical characteristics that, at a later stage, influenced the results significantly. On the other hand, the experiments covered trial series, each of which manipulated more than one independent variable (e.g., KClO₃ content, dispersion agents, balance ratio) and measured their effects on dependent variables such as ignition delay times, peak pressures, and burning rates.

3. Results

The results are presented in sections. In each section, a descriptive table is provided to describe the experimental conditions, the controlled variables, the independent variables, and the dependent variables. Following each table, different types of graphs are used to represent the measured data.

In the majority of instances, line graphs with symbols are used to emphasize individual data points, as they are effective for highlighting key data while still showcasing the overall trend. In a few select cases, spline-connected graphs are preferred when smoother connections between data points are desired, as they provide a more flowing and visually continuous representation of the data.

Subsequent to each test description, separate plots are displayed as multiple panels, as this approach is advantageous when analyzing each dataset individually, in-depth, and without direct comparisons. Separate plots provide a detailed view of each dataset. Following the multiple panels approach, a subset of these plots is integrated into a unified graph, as this approach is beneficial in directly comparing trends or relationships between datasets visually and highlighting any correlations or discrepancies that may exist.

3.1. Constant Volume Combustion Test

The constant volume combustion test is a widely used method to analyze the ignition and combustion characteristics of thermite materials. This test provides a controlled environment to study the fuel itself, combustion behavior, and performance of thermite compositions.

Table 2. Constant volume combustion test: description and conditions.

N	Description	Conditions
I	Constant volume combustion test	- Volume of closed explosive device: 6 mL - Bridge-wire current: 5 A - Charge: 100 mg - Charging pressures: 0; 60 kPa; 120 kPa
I.1	Definition of variables
	Controlled variables	- Charging pressure: 60 kPa
	Independent variables	- KClO3 content: from 0 to 100 wt% - Al/Fe2O3 content: from 0 to 100 wt%
	Dependent variables	- Maximum pressure - Ignition delay time - Delay time to peak pressure
I.2	Definition of variables
	Controlled variables	- KClO3 content: 70 wt%
	Independent variables	- Charging pressure: 0, 60, 120 kPa - Filling density: 0.431, 0.456, and 0.485 g/cm3
	Dependent variables	- Maximum pressure - Ignition delay time - Delay time to peak pressure

provides a detailed description of the constant volume combustion test and its conditions and defines its controlled, independent, and dependent variables.

The pressure history of a thermite reaction is a crucial factor influencing its ignition time. The delay time to peak pressure, peak pressure, and decay after peak pressure are key pressure metrics that need consideration for thermite combustion [103].

Figure 5 and Figure 6 represent the measured data for the first part (I.1) of the constant volume combustion test and show the changes in Al/Fe₂O₃ content, the maximum (peak) pressure, the ignition delay time, the delay time to peak pressure, and the total delay time which represents the summation of ignition delay time and delay time to peak pressure, depending on the variation in KClO₃ content.

Figure 7 and Figure 8 represent the measured data for the second part (I.2) of the constant volume combustion test and show the changes in the maximum (peak) pressure, the ignition delay time, the delay time to peak pressure, and the total delay time, depending on the variation in the charging pressure and the filling density.

3.1.1. Constant Volume Combustion Test (Part I.1)

The data depicted in Figure 5a,b demonstrate that varying the KClO₃ content markedly influences the Al/Fe₂O₃ composition. From Figure 5a, an escalation in KClO₃ content diminishes the fuel proportion while augmenting the fraction by weight percentage of the oxygen providers within the mixture. Consequently, this escalates oxygen release while reducing the melting points. From Figure 5b, peak pressures attained during the reaction exhibit an upward trend with increasing KClO₃ levels, thereby enhancing reactivity and curtailing combustion durations (i.e., burning time). Conversely, reduced KClO₃ concentrations necessitate higher activation energies, instigating more sluggish and thicker reaction wavefronts [75].

Experimental evidence [103] indicates an inverse correlation between peak pressure and reactivity, wherein higher aluminum consumption of oxygen liberated from decomposing solid oxidizers diminishes maximum pressure levels. Conversely, adding KClO₃ into the mixture engenders an escalation in peak pressures, attributable to the influence of the multiphase changes and interface chemistry on the transient pressure response during the reaction [103]. The KClO₃ concentration exerts a profound influence on the reaction kinetics, with elevated concentrations potentially leading to increased reactivity and pressure development [103]. Furthermore, the pressure surge during the reaction is driven by oxygen generation and vaporization kinetics, which are modulated by the presence of KClO₃ in the mixture. Consequently, KClO₃ content directly impacts the maximum pressure attained during the reaction [103]. In essence, the mixture’s reactivity, which governs the reaction rate, can significantly influence the maximum combustion pressure [103,122,123,124,125].

The data illustrated in Figure 5c,d reveal that the quantity of aluminum in direct contact with Fe₂O₃ at lower KClO₃ concentrations emerged as a pivotal factor governing ignition and combustion characteristics, underscoring the criticality of oxidizer content in dictating these attributes. Consequently, reaffirming previous observations, the oxidizer proportion constitutes a paramount determinant that can profoundly sway the ignition delay time of nano-Al/Fe₂O₃-KClO₃ compositions, thereby exerting a substantial influence on their reactivity and combustion behavior [103,122,123,124,125].

By definition, the ignition delay time is the time interval between the application of an ignition stimulus and the initiation of a self-propagating reaction. Apart from oxidizer content, myriad factors can profoundly influence the ignition delay time, including the ignition source nature, particle size, and the thermite mixture’s composition [122,126,127,128,129]. On the other hand, the delay time to peak pressure is the duration required for the reaction to attain its maximum pressure subsequent to ignition, a juncture which, in our case, corresponds to the peak pressure apex. The significance of this delay lies in its role as a metric for quantifying the rate at which the reaction front propagates through the mixture, a crucial parameter for elucidating the reaction kinetics and its practical implications. Research has demonstrated that this delay can be employed to assess the dynamic particle size and macroscopic burning time in nano-Al/Fe₂O₃-KClO₃ reactions. Furthermore, variations in this delay can yield insights into the ignition and propagation characteristics essential for optimizing the performance of nano-Al/Fe₂O₃-KClO₃ materials across diverse applications [122,126,127,128,129]. A shorter delay time to peak pressure typically indicates a faster rate of reaction, while a longer delay time to peak pressure suggests a slower rate of reaction [122,126,127,128,129].

Figure 6 elucidates that an increase in KClO₃ content engenders a diminution in the Al/Fe₂O₃ proportion, conjointly exerting an influence on the activation energy, reactivity, reaction kinetics, pressure development, and reaction times of the mixture. The ignition time exhibited a slight and steady increase with KClO₃ additions from 30 wt% up to the 70–80 wt% range, followed by an abrupt surge upon further elevating the KClO₃ content from 80 to 100 wt%.

The addition of KClO₃ engenders a diminution in the activation energy and ignition temperature thresholds, consequently leading to a diminution in both the ignition delay time and the delay time peak pressure. However, the reduction in Al/Fe₂O₃ content arising from KClO₃ augmentation culminates in gradually reduced contact and interactions between the fuel and oxidizer. Consequently, the opposite outcome is observed in terms of ignition delay time, exhibiting an increase rather than a decrease. Nonetheless, with escalating KClO₃ concentrations, the delay time to peak pressure undergoes an abrupt plunge, attaining a local minimum at KClO₃ proportions corresponding to the 70–80 wt% range before exhibiting a slight resurgence. This behavior is indicative of a faster reaction rate, lower activation energy, and faster reaction waves, leading to high peak pressures upon KClO₃ addition. The underlying rationale for this phenomenon lies in the low Tammann temperature and melting temperature thresholds of KClO₃, facilitating its decomposition and permeation through the mixture’s lattice structure, rapidly establishing physical contact with Al/Fe₂O₃ particles and thereby promoting the interaction and initiating the reaction.

Additionally, the data evince a correlation between the diminution in Al/Fe₂O₃ content, the pressure build-up arising from gaseous product generation, the ignition delay time, the delay time to peak pressure, and the KClO₃ content. This interrelationship can be elucidated by the existence of an optimum for Al/Fe₂O₃ and KClO₃ concentrations within the mixture, conducive to optimized ignition and combustion performance [130,131,132]. This optimum resides within the 70–80 wt% range of KClO₃ content. The relationship between KClO₃ addition and the increase in ignition delay time and maximum pressure indicates that oxygen/gas release is not the sole determinant governing reaction initiation. This suggests that ignition preceding substantial oxygen release or occurring with minimal oxygen release is likely a consequence of direct interfacial contact between fuel and oxidizer, leading to condensed state mobility of reactive species [130,131,132], which confirms our previous statements. This phenomenon also implies facile decomposition of KClO₃ to form gas-phase products. The reaction between the generated oxidizing gases and aluminum particulates appeared to exert a governing influence on the burning rate [130,131,132].

3.1.2. Constant Volume Combustion Test (Part I.2)

The charging pressure, denoting the initial pressure at which the mixture is introduced into the explosion tube prior to initiation, emerges as a paramount factor that exerts influence on the characteristics of initiation and shock wave propagation within these tubes. Meticulous control and monitoring of this charging pressure are imperative to ensure consistency and precision in experimental setups involving explosion tube configurations [133,134,135,136,137].

With KClO₃ content maintained as a controlled variable at 70%, three distinct experimental sets were conducted under the following charging pressure conditions: 0, 60, and 120 kPa, corresponding to filling densities of 0.431, 0.456, and 0.485 g/cm³, respectively, as depicted in Figure 7a. The resultant data evinced a relationship between the charging pressure and the peak pressure, characterized by the existence of a local maximum, as illustrated in Figure 7b. Notably, this local maximum coincided with the optimum for both the ignition delay time and the delay time to peak pressure, manifested by the shortest delay times, indicative of low activation energy and ignition temperature thresholds and fast reaction rates, as elucidated in Figure 7c,d. This optimum was defined by a charging pressure of 60 kPa and a filling density of 0.458 g/cm³. Conversely, the highest charging pressure resulted in the lowest peak pressure and the longest delay times.

Numerous investigations have explored the influence of charging pressure on the maximum pressures manifested in explosion tubes. Empirical evidence and research findings [133,134,135,136,137] elucidate that the overpressure of the resultant shock wave exhibits a diminution with decreasing initial ambient pressure of the explosion. The peak pressure of the explosion waves also increases with increasing initial pressure within the vessel, indicative of a direct correlation between charging pressures and peak pressures. However, as demonstrated in our experiments in Figure 7 and Figure 8, this relationship is not invariably proportional but rather characterized by the existence of a local maximum at which the ignition efficacy and combustion performance attain their optimum. Another salient consideration is that both the charging pressure and filling density exert a governing influence on the reaction mechanism and heat transfer modes.

By definition, the filling density denotes the density at which the reactant particles are filled within the mixture. Consequently, the filling density exerts a profound influence on the reaction rate and behavior in thermite composites [133,134,135,136,137]. As elucidated previously, both the charging pressure and filling density bear practical implications on the ignition time of the reaction. Studies [133,134,135,136,137] have demonstrated that the filling density of rigid agglomerates is independent of scale and can modulate the burning rate in a thermite reaction. The oxygen density packing can play a crucial role in the reaction behavior and rate, where condensed phase mechanisms govern the reaction. This is why it is important to consider the filling density of reactant particles in order to understand and control the reaction behavior and rate in thermite nanocomposites. The reaction mechanism undergoes a transition from mass convection to heat diffusion as the filling density increases. The burning rate of nano-thermites is also affected by bulk density, with higher densities leading to faster rates.

3.2. Ignition Condition Test

Electro-explosive tube configurations have been employed as an alternative approach to investigate the ignition and combustion dynamics of nano-Al/Fe₂O₃-KClO₃ compositions. In this experimental setup, the nano-Al/Fe₂O₃-KClO₃ sample is loaded into a tubular enclosure, with ignition initiated at one extremity. Empirical evidence has demonstrated that parameters such as the inner diameter of the tube, the degree of confinement imposed, the charge mass and density, and the charging pressure can exert a profound influence on the ignition and combustion characteristics of the nano-Al/Fe₂O₃-KClO₃ mixture, leading to higher expansion rates of the chemical energy release area in certain instances.

Table 3. Ignition condition test: description and conditions.

N	Description	Conditions
II	Ignition condition Test	- The ingredients are the same as in Test I. - Electro-explosion tube with inner diameter: 6 mm - Bridge-wire resistance: 1 Ω - Charge: 100 mg - Charge density: 0.45 g/cm3 - Charging pressure: 60 kPa
II.1	Definition of variables
	Controlled variables	- Current: 5 A - Resistance: ~1 Ω - Fe2O3 specific surface area (m2/g): 187.7
	Independent variables	- KClO3 content: from 0 to 100 wt%
	Dependent variables	- Ignition delay time
II.2	Definition of variables
	Controlled variables	- Current: 5 A - Resistance: ~1 Ω - KClO3 content: 70 wt%
	Independent variables	- Fe2O3 specific surface area: 40.5, 103.2, 215.8 m2/g
	Dependent variables	- Ignition delay time

delineates the ignition condition test and its associated experimental parameters.

The ignition delay times of the thermite mixture were measured for a varying KClO₃ content in weight percentage, represented in part II.1 of the ignition condition test, and for a varying Fe₂O₃ specific surface area, represented in part II.2 of the same test.

Figure 9a shows the variation of the ignition delay time with KClO₃ content, while Figure 9b shows the variation of the ignition delay time with Fe₂O₃ specific surface area.

As shown in Figure 9a, under the experimental conditions of an induced current of 5 A, an average resistance of 1 Ω, and a controlled variable Fe₂O₃ specific surface area of 187.7 m²/g, ignition delay times increased proportionally with KClO₃ addition to the mixture. The main reason can be attributed to the lack of interfacial contact sites and localized hotspot regions between fuel and oxidizer particles, despite the propensity of KClO₃ to engender a diminution in the activation energy and ignition temperature thresholds.

On the other hand, as illustrated in Figure 9b, with KClO₃ content maintained as a controlled variable at 70 wt%, an escalation in the specific surface area of Fe₂O₃ can precipitate a transition in the ignition mechanism from bulk to surface, resulting in shorter ignition delay times attributable to faster reaction propagation [24]. Nanoparticles can increase the rate of chemical reaction on the surface of the reagents, leading to faster ignition times [138]. Indeed, the reactivity and combustion behavior of the mixture can be influenced and modulated by the specific area of Fe₂O₃. A greater specific area can facilitate an enhancement in the interfacial contact between fuel and oxidizer, potentially resulting in heightened reaction rates and improved efficiency. Thus, our findings corroborate the assertions of recent studies [117,130,131] that the ignition delay time exhibits a direct correlation with the particle size and specific area of the oxidizer.

3.3. Flame Sensitivity Test

The flame sensitivity test for nano-Al/Fe₂O₃-KClO₃ compositions serves to assess the ease of ignition initiation and to characterize the combustion dynamics of these energetic materials upon exposure to thermal stimuli. An increase in the 50% ignition distance metric during flame sensitivity testing implies that the nano-thermite is more difficult to ignite, and conversely, a diminution in this parameter signifies an enhanced susceptibility to ignition. Consequently, accurate evaluation of the flammability traits may necessitate the deployment of higher (or lower) energy ignition sources or tailored test conditions.

Table 4. Flame sensitivity test: description and conditions.

N	Description	Conditions
III	Flame sensitivity test	- Test method: fuse method - Fuse length: 5 cm - Experimental dosage: 20 mg
III.1	Definition of variables
	Controlled variables	- Fe2O3 specific surface area (m2/g): 215.8 - Dispersion agent: n-hexane
	Independent variables	- KClO3 content: from 0 to 100 wt%
	Dependent variables	- 50% ignition distance
III.2	Definition of variables
(a)	Controlled variables	- KClO3 content: 70 wt% - Dispersion agent: n-hexane
	Independent variables	- Fe2O3 specific surface area: 40.5, 103.2, 215.8 m2/g
	Dependent variables	- 50% ignition distance
(b)	Controlled variables	- Fe2O3 specific surface area (m2/g): 215.8 - Dispersion agent: n-hexane + acetone
	Independent variables	- KClO3 content: 30, 50, 70 wt%
	Dependent variables	- 50% ignition distance

delineates a comprehensive description of the flame sensitivity test protocol.

The 50% ignition distance metric for nano-Al/Fe₂O₃-KClO₃ compositions emerges as a critical parameter governing safety considerations and practical utility, as it delineates the spatial threshold at which ignition is initiated, thereby exerting an important impact on the controllability and predictability of the ensuing reaction [103,139]. This 50% ignition distance bears a direct impact on the reactivity and pressure development dynamics upon ignition, consequently shaping the potential applications and use cases [103]. Modulation and optimization of the 50% ignition distance for nano-Al/Fe₂O₃-KClO₃ compositions can be achieved through judicious manipulation of particle size, surface treatment, and the addition of tertiary components, enabling tailored tuning of this critical parameter [103,140,141].

The data illustrated in Figure 10 evince that the addition of KClO₃ into the mixture precipitates a diminution in the 50% ignition distance metric, indicative of a reduction in the activation energy and ignition temperature thresholds. Furthermore, upon undergoing melting and decomposition processes, KClO₃ can move faster through the lattice structure, facilitating enhanced interfacial contact and interactions with the fuel particles. This phenomenon consequently engenders an increased reactivity and faster rates of reaction.

The addition of n-hexane as a dispersive agent into nano-Al/Fe₂O₃-KClO₃ compositions engenders a reduction in the ignition delay time and an increase in reactivity [142]. However, it is also accompanied by an augmentation in the 50% ignition distance metric. Empirical data and numerical simulations [142] have elucidated that even small proportions of n-hexane (1–2%) can exhibit an enhancement in reactivity by approximately 40–60 ms at a compressed temperature of 700 K, with the ignition delay time decreasing rapidly with an increase in concentration. The sensitization effect of n-hexane is more pronounced in the temperature range of 700–900 K compared to higher temperatures, and the promoting effect of n-hexane is attributed to its dependence on specific reactions and temperature ranges [142].

The addition of acetone alongside n-hexane engenders a further increase in the 50% ignition distance metric. Both n-hexane and acetone are known for their exceptional solvating capabilities, rendering them well-suited for dissolving and dispersing the constituent components of nano-Al/Fe₂O₃-KClO₃ compositions [143,144]. This is crucial for the preparation of nano-Al/Fe₂O₃-KClO₃ as it allows for the uniform distribution of the components, leading to enhanced reactivity and energy release [143,144].

Figure 11 shows that under the controlled variable of Fe₂O₃ specific surface area and the concurrent presence of the dispersion agents n-hexane and acetone, the 50% ignition distance exhibits a diminution with escalating KClO₃ proportions within the mixture.

However, for the controlled variable of KClO₃ content and with the presence of n-hexane, an augmentation in the 50% ignition distance is observed, which implies that the dispersion agent facilitates an enhanced distribution and dispersion of the oxidizing components throughout the mixture matrix, thereby increasing the interfacial contact between the fuel and the oxidizer, followed by an increase in reactivity and sensitivity characteristics of the nano-Al/Fe₂O₃-KClO₃ compositions [145].

3.4. Effect of Equilibrium Ratio on Al/Fe₂O₃ Combustion Performance Test

The combustion performance and energy conversion efficiency of nano-Al/Fe₂O₃ are highly dependent on the ratio of the fuel to the oxidizer. This ratio is crucial for optimizing the nano-Al/Fe₂O₃ reaction. Usually, quartz glass tubular configurations are employed as the experimental setup to determine this ratio.

Table 5. Effect of the equilibrium ratio on the Al/Fe₂O₃ combustion performance test: description and conditions.

N	Description	Conditions
IV	Effect of equilibrium ratio on the Al/Fe2O3 combustion performance test	- Nano-Al/Fe2O3 with different equilibrium ratios Φ - Quartz glass tube with inner diameter: 2 mm - Filling density: 0.16 g/cm3
	Controlled variables	- Fe2O3 specific surface area: 215.8 m2/g
	Independent variables	- Balance ratio Φ: from 1.0 to 1.5
	Dependent variables	- Maximum burning rate

represents a detailed description of the effect of the equilibrium ratio on the Al/Fe₂O₃ combustion performance test, its conditions, and the related controlled, independent, and dependent variables.

The balance ratio and burning rate of nano-Al/Fe₂O₃ are influenced by factors such as particle size, additives, and composition [70,146,147]. These insights highlight the complex interplay of various components in determining the performance of nano-Al/Fe₂O₃.

The performance of nano-Al/Fe₂O₃ is also heavily influenced by its equilibrium composition ratio. This ratio refers to the stoichiometric balance between the metal fuel and metal oxide as components in the reaction. Research [140,148,149] has shown that the closer the composition ratio is to the equilibrium stoichiometry, the more enhanced the self-exothermic behavior of the nano-Al/Fe₂O₃ becomes, which also implies that a balanced equilibrium ratio can contribute to improved reactivity and overall performance of nano-Al/Fe₂O₃. Additionally, the combustion performance of nano-Al/Fe₂O₃ is influenced by the composition ratio, further underscoring the importance of maintaining the appropriate equilibrium ratio to achieve the desired combustion characteristics. Therefore, it is crucial to ensure that the equilibrium ratio of nano-Al/Fe₂O₃ is appropriately and judiciously balanced to optimize their reactivity and overall performance metrics [140,148,149].

In our experiments, as represented in Figure 12, the maximum burning rate exhibited a pronounced sensitivity to the stoichiometric balance ratio of the mixture composition.

The maximum burning rate rose to reach a local maximum at the stoichiometric balance ratio value of 1.1 before subsequently undergoing a diminution with deviations from this optimal ratio. This behavior suggests that an excess of fuel constituents engenders a deceleration in the burning rate kinetics, attributable to the reduced probability of interfacial contact between the fuel and oxidizer molecules. In other words, deviations from the stoichiometric balance ratio of 1.1, whether towards lower or higher values, also lead to increased activation energy, ignition delay times, and ignition temperatures.

3.5. Effect of Fe₂O₃ Specific Surface Area on Al/Fe₂O₃ Combustion Performance Test

The specific surface area test provides valuable insights into the reactivity and combustion properties of thermites. Attaining a comprehensive understanding of the specific surface area facilitates the optimization of thermite compositions, enabling the determination of ideal stoichiometric balances between fuel and oxidizer constituents, thereby enhancing combustion efficiency metrics. Furthermore, by establishing correlations between the specific surface area and the burning rate, thermite compositions can be judiciously tailored to achieve the desired combustion characteristics and targeted performance thresholds.

Table 6. Effect of Fe₂O₃ specific surface area on the Al/Fe₂O₃ combustion performance test: description and conditions.

N	Description	Conditions
V	Effect of Fe2O3 specific surface area on the Al/Fe2O3 combustion performance test	- Balance ratio Φ = 1.1 - Glass pipe diameter: 2 mm - Filling density: 0.16 g/cm3
	Controlled variables	- Balance ratio Φ = 1.1
	Independent variables	- Fe2O3 specific surface area
	Dependent variables	- Maximum burning rate - Pore volume - Fe2O3 purity: ~79 wt%

provides a detailed description of the effect of Fe₂O₃ specific surface area on the Al/Fe₂O₃ combustion performance test, encompassing the associated experimental conditions.

The data illustrated in Figure 13a show that the maximum burning rate exhibits a proportional and pronounced increase with Fe₂O₃ specific surface area reaching up to 100 m²/g, beyond which a transition is observed towards a more gradual and attenuated rate of increase within the specific surface area regime spanning 100 to 160 m²/g. Subsequently, the maximum burning rate undergoes a resurgence, exhibiting a sharp and proportional increase with the specific surface area. Recent research [148,149] has demonstrated that higher specific surface areas can engender a significant increase in the burning rate. Moreover, a salient correlation has been established between an expansive specific surface area and an increase in the mass burning rate, underscoring the pivotal role played by the specific surface area in governing the combustion dynamics of materials [148,149].

On the other hand, the data depicted in Figure 13b elucidate a logarithmic growth in pore volumes proportionally with an increasing Fe₂O₃ specific surface area. Researchers [148,149] have unveiled that the specific surface area and pore volumes exert a governing influence on the intrinsic burning rate of materials, with variations in temperature leading to changes in the burning rates of pores of different sizes. The physical processes manifesting at the burning surface exert a regulatory effect on the burning rate, which, in turn, is influenced by the material’s pore structure and surface area. Further studies have demonstrated that the pore structure and its impact on the propensity for spontaneous combustion are subject to modulation by factors such as pore size distribution and volume. These revelations underscore the crucial role played by pore characteristics in governing the combustion behavior [148,149].

Upon juxtaposing the trends exhibited by the burning rate, pore volume, and purity metrics in conjunction with the specific surface area of Fe₂O₃, as depicted in Figure 14, it can be deduced that for lower specific surface areas, the burning rate undergoes a proportional increase with pore volumes, evincing a similar logarithmic trajectory and momentum. However, upon surpassing a certain value of a specific surface area of 160 m²/g, the burning rate experiences a sharp increase with further increments in the specific surface area, exhibiting a heightened momentum compared to the pore volume trend. This phenomenon suggests the manifestation of a trend inversion and the emergence of a novel pattern. Such behavior can be elucidated by a modulation in the flame propagation dynamics, as well as variations in the heat transfer and diffusion mechanisms, exerting an influence on the intermixing of fuel and oxidizer particles.

Indeed, the presence of pores in Fe₂O₃ particles can enhance combustion performance. Studies [150] have shown that porous metal oxides (pMxOy) resulted in significantly increased reaction exergy and reduced activation energy of nano-Al/Fe₂O₃, leading to improved ignition and combustion performance. The combustion of porous Al/CuO nanolaminates shows a faster burning rate with the incorporation of pores while the flame temperature remains the same, indicating the influence of pore volume on combustion efficiency and suggesting a modification of the reaction chemistry and heat transfer mechanisms, respectively [151], which is in compliance with our results. Hence, the purity of Fe₂O₃ also has a significant impact on the burning rate of nano-Al/Fe₂O₃ [134].

3.6. Effect of Fe₂O₃ Heat Treatment Temperature on Al/Fe₂O₃ Combustion Performance Test

The purpose of the heat treatment temperature test for thermites is to evaluate the impact of heat treatment on the pore volume, purity, and specific surface area of these energetic materials. This protocol is crucial for attaining a comprehensive understanding of optimizing the properties of nano-Al/Fe₂O₃ and facilitating their deployment across diverse applications.

Table 7. Effect of Fe₂O₃ heat treatment temperature on the Al/Fe₂O₃ combustion performance test: description and conditions.

N	Description	Conditions
VI	Effect of Fe2O3 Heat Treatment temperature on the Al/Fe2O3 combustion performance test	- Glass pipe diameter: 2 mm - Balance ratio Φ = 1.0 - Charge density: 0.21 g/cm3
	Controlled variables	- Balance ratio Φ = 1.0 - Charge density: 0.21 g/cm3
	Independent variables	- Heat treatment temperature: from 250 to 400 °C
	Dependent variables	- Maximum burning rate - Fe2O3 specific surface area - Pore volume - Fe2O3 purity

represents a detailed description of the effect of Fe₂O₃ heat treatment temperature on the nano-Al/Fe₂O₃ combustion performance test, encompassing the associated experimental conditions.

Figure 15a shows a decrease in the maximum burning rate with the increase in heat treatment temperatures, with a changing momentum, especially between 250 and 300 °C. Figure 15b,c illustrate a reduction in specific surface area and pore volume of Fe₂O₃ with the increase in heat treatment temperatures. Figure 15d shows Fe₂O₃ purity increasing with the increase in heat treatment temperatures. This phenomenon suggests that heat treatment facilitates the elimination of impurities from the structural framework of the material.

Figure 16 illustrates the distinct points at which the burning rate, specific surface area, pore volume, and purity metrics undergo modulations in their respective momentum profiles, either exhibiting diminutions or escalations in response to variations in the heat treatment temperature regimes. For instance, the maximum burning rate exhibits momentum transitions at 250 °C and 300 °C during the decreasing phase, while the specific surface area undergoes a momentum shift at 300 °C. The pore volume experiences a subtle momentum modulation at 350 °C, whereas the purity metric manifests a momentum transition at 250 °C during its escalating phase. Hence, the interrelationships among these parameters are not straightforward, and contrary to the anticipated trend, high purity levels do not invariably translate to faster burning rates. Conversely, the presence of pores and voids facilitates enhanced intermixing and interfacial contact between the fuel and oxidizer, thereby facilitating combustion propagation dynamics. Furthermore, the pore volume and porosity characteristics can exert a governing influence on the reaction kinetics and heat transfer mechanisms within the thermite, impacting the overall burning rate performance. Increased porosity can enhance the intermixing and diffusion of reactants, as well as the transport of heat and combustion products, leading to faster burning rates. An analogous phenomenon is observed with respect to the specific surface area, wherein smaller particle sizes of the fuel and oxidizer constituents result in a higher specific surface area, thereby enabling improved interfacial contact and intermixing between the reactants. A higher specific surface area can enhance the intermixing, diffusion, and heat transfer processes, ultimately resulting in faster combustion propagation.

The decrease in specific surface area with increasing heat treatment temperature is often attributed to changes in the pore structure of the material. High-temperature heat treatment can cause pore expansion, pore coalescence, and the collapse of smaller pores, leading to a reduction in the overall specific surface area. The heat treatment process can also induce sintering and grain growth in the material, which can result in a decrease in the specific surface area.

Figure 17 presents a comparative analysis of the maximum burning rate, pore volume, and Fe₂O₃ purity metrics in relation to the Fe₂O₃ specific surface area, encompassing both scenarios: with and without the application of heat treatment. The study unveiled that elevated heat treatment temperature regimes precipitated a diminution in the specific surface area, accompanied by a reduction in pore volumes, as shown in Figure 17b. Conversely, an escalation in the purity levels was observed, as depicted in Figure 17c.

The research initiative delved into examining the effect of high-temperature heat treatment on purity. The findings indicated that at elevated temperatures, heat treatment can effectively purify the thermite mixture, provided that the treatment duration is optimized appropriately. However, this phenomenon led to a significant decrease in the maximum burning rate. This behavior is ascribed to the gradual diminution in the specific surface area while purity levels increased, suggesting that the presence of impurities led to a faster combustion with a higher burning rate [152,153,154].

4. Discussion

4.1. Thermal Run-Away and Heat Transfer

The ignition delay time is influenced by factors like oxidizer content, particle size, ignition source, and mixture composition. The delay time to peak pressure indicates the rate of reaction front propagation through the mixture. Increasing KClO₃ reduces activation energy and ignition temperature, decreasing ignition delay and delay to peak pressure. However, reduced Al/Fe₂O₃ contact due to less fuel offsets this, initially increasing ignition delay. An optimum Al/Fe₂O₃ and KClO₃ ratio around 70–80 wt% KClO₃ exists for enhanced ignition and combustion due to KClO₃’s low melting point allowing rapid permeation and contact with Al/Fe₂O₃. Oxygen release is not the sole driver of ignition–direct fuel/oxidizer interfacial contact and condensed mobility of reactive species also plays a role before substantial oxygen release occurs.

On the other hand, thermal runaway describes a process where an increase in temperature accelerates a reaction, which, in turn, releases more energy, further increasing the temperature in a positive feedback loop. The heat released from the thermite reaction can accelerate the reaction rate, leading to a runaway situation. Under some conditions, this may lead to an uncontrolled, rapid temperature rise and potential explosion. Thermites are prone to thermal runaway due to their highly exothermic nature. Factors like impurities, particle size, packing density, and composition can influence the sensitivity of thermites toward thermal runaway.

It is noteworthy that while the majority of pure chemical compounds undergo phase transition from solid to liquid at a well-defined melting point temperature; the Tammann temperature represents a characteristic threshold at which the absolute temperature of the material is halfway to its melting point. The Tammann temperature acquires significance due to the propensity of materials to exhibit an augmented susceptibility towards accidental ignition initiation at or beyond this temperature. Consequently, comprehending the Tammann temperature emerges as a pivotal consideration for elucidating the behavior of nano-Al/Fe₂O₃-KClO₃ compositions, particularly within the domains of thermochemistry and chemical kinetics [155], wherein the temperature-dependent reactivity profiles and reaction dynamics are of paramount importance.

The Tammann temperature plays a significant role in determining the thermal stability and reactivity of Al/Fe₂O₃-KClO₃ compositions, attributes that acquire paramount importance for their deployment across practical applications [155]. The Tammann temperature manifests a significant impact on the combustion behavior, wherein the heat transfer mechanisms driving the combustion process emerge as critical considerations within this context [156]. The temperature fields manifested during combustion are subject to modulation by factors such as gas generation phenomena, the heat of combustion, and the intrinsic reaction kinetics, all of which exhibit an interrelationship with the Tammann temperature [157]. The key determinants influencing the Tammann temperature of nano-thermites encompass the nature of the additives [158]. The size and volume loadings of these additives, in conjunction with their inherent thermal properties, play a crucial role in governing the Tammann temperature and the consequent combustion behavior of nano-thermites [158].

For a chemical reaction to be initiated, the constituent molecules must possess the requisite activation energy threshold. If one of the components undergoes a phase transition from solid to liquid, thereby enabling it to flow and permeate the surfaces of the other particles, a substantial augmentation in the number of atoms in direct interfacial contact is engendered. Consequently, the melting phenomenon can exert a profound influence on the propensity for ignition initiation. If the melting of one component precipitates an escalation in the percentage of atoms in physical contact, then a greater number of atoms possessing energies exceeding the activation energy will be in direct proximity, facilitating their interactions. This scenario implies that the reaction rate will undergo an escalation, concomitantly with an increase in the rate of thermal energy production. As a corollary, thermal runaway and ignition can be instigated at a lower temperature threshold compared to scenarios where melting does not occur. Therefore, if a composition approaches its ignition temperature and one of its constituent components undergoes melting, this phase transition can potentially culminate in ignition initiation at a lower temperature regime. The thermal runaway and ignition process is elucidated in Figure 18.

The transfer of energy from the exothermic reacting regions to the pre-reacting layers during the combustion process encompasses heat transfer modes such as conduction, convection, and radiation [156]. In the conduction mechanism, thermal energy is propagated from higher temperature domains to lower temperature domains through the vibrational motions of atoms and molecules. The compact structural configuration of metal fuel constituents facilitates an augmentation in conductive heat transfer. Convective heat transfer is engendered by the permeation of hot gaseous species through the interstices between granular entities within the composition. Uncompacted compositions and granulated or fractured structural morphologies engender an escalation in convective heat transfer phenomena. Thermal radiation, predominantly emanating from incandescent particles within the flame, is absorbed by the reacting composition. The abundant presence of solid and liquid particles within the flame and the intrinsically dark nature of thermite compositions maximize radiative heat transfer. However, while convective heat transfer has been traditionally regarded as the preeminent mechanism, recent investigations have postulated that the transport of condensed phase material exerts a pivotal role in governing heat transfer dynamics during the ignition of nano-thermites [156].

4.2. Degree of Consolidation

The degree of consolidation, alternatively referred to as the loading pressure or charging pressure, is a critical factor that influences the ignition and shock wave propagation characteristics. The degree of consolidation is intrinsically correlated with the degree of compaction manifested by the thermite composition when made into granular constructs or packed within a device configuration. The modulation of the charging pressure exerts a governing influence on the efficiency of energy feedback mechanisms. However, whether an escalation in the charging pressure engenders an augmentation or diminution in the burning rate is contingent upon the intrinsic nature of the thermite composition. If the thermite is predisposed towards gas generation and convective heat transfer emerges as a pivotal mode of energy feedback, then elevated charging pressures generally precipitate a decrease in the burning rate by attenuating gas permeability. This phenomenon can be elucidated by the presence of residual fire paths, even within tightly compacted compositions, which albeit exhibiting diminutive diameters and limited propagation distances, nonetheless facilitate the convective feedback of thermal energy. As the charging pressure undergoes an increase, these residual fire paths experience a constriction in their dimensions and a curtailment in their propagation lengths, thereby engendering a diminution in their efficacy as conduits for energy feedback, consequently precipitating a decrease in the burning rate [159,160,161,162,163,164].

Conversely, for thermite composition that exhibits a diminutive propensity towards gas generation during the combustion process, wherein conductive heat transfer mechanisms predominate, an escalation in the charging pressures generally engenders an augmentation in the burning rate. This phenomenon can be elucidated by the fact that for such thermites, an increase in the degree of compaction facilitates an enhancement in the thermal conductivity, thereby enhancing the efficiency of energy feedback mechanisms, consequently increasing the burning rate. However, the flame propagation velocity exhibits an escalatory trend with an increase in the loading density and a concomitant diminution in the particle size [159,160,161,162,163,164]. Indeed, both charging pressure and filling density influence the reaction mechanism and heat transfer modes. Filling density, which is the density of reactant particles in the mixture, significantly impacts the reaction rate and behavior in thermite composites. Higher filling densities can increase burning rates by transitioning from mass convection to heat diffusion reaction mechanisms, while higher confinement can lead to higher expansion rates of the chemical energy release area.

The degree of consolidation manifested by thermite compositions is subject to modulation by determinants such as the spatial distribution of nanoparticles, the coordination number quantifying the number of nearest neighbors, and the efficiency of mass transport phenomena. This consolidation exerts a profound influence on the performance metrics of nano-Al/Fe₂O₃-KClO₃ compositions, exerting a governing impact on their reaction rates and energy density. Attaining an optimal degree of consolidation poses challenges emanating from theoretical comprehension limitations and the non-uniform distribution of fuel and oxidizer constituents. The synthesis of core-shell structured nano-thermites has been postulated as a potential strategy to engender enhancements in energy density and combustion efficiency [159,160,161,162,163,164].

4.3. Balance Ratio, Degree of Mixing, and Dispersion Agents

The combustion performance and energy conversion efficiency of nano-Al/Fe₂O₃ thermites are highly dependent on the ratio of fuel to oxidizer. Optimizing this ratio is crucial for maximizing the reaction. The burning rate and fuel/oxidizer balance ratio are influenced by factors like particle size, additives, and composition, highlighting the complex interplay of components. Maintaining the appropriate equilibrium ratio is critical for achieving the desired combustion characteristics.

On the other hand, the ease of ignition initiation and combustion of nano-Al/Fe₂O₃-KClO₃ thermites, when exposed to thermal stimuli, is defined by the 50% ignition distance, which governs safety considerations and practical utility, as it defines the spatial threshold for ignition initiation, impacting reaction controllability and predictability. It directly affects reactivity, pressure development upon ignition, and potential applications. Tuning can be achieved by manipulating particle size, surface treatment, and adding tertiary components. Adding KClO₃ reduces the 50% ignition distance by lowering the activation energy and ignition temperature. Molten KClO₃ facilitates better fuel/oxidizer contact for increased reactivity. Adding the dispersant n-hexane reduces ignition delay but increases the 50% ignition distance, enhancing reactivity, especially at 700–900K temperatures. Adding acetone along with n-hexane further increases the 50% ignition distance. Both are good solvents for dispersing nano-Al/Fe₂O₃-KClO₃ components uniformly for better reactivity. With fixed Fe₂O₃ surface area, higher KClO₃ reduces 50% ignition distance. But with fixed KClO₃, adding n-hexane increases it by improving oxidizer dispersion for better fuel/oxidizer contact.

In other words, when thermite compositions exhibit inadequate intermixing, their burning rate kinetics undergo a decrease in comparison to compositions characterized by homogeneous mixing. This phenomenon arises due to the existence of numerous localized regions within the poorly mixed composition that manifest unfavorable fuel-to-oxidizer ratios, despite the overall volumetric composition adhering to the optimal stoichiometric balance. Consequently, each of these regions experiences a diminution in the burning rate. However, the heat of the reaction for the total amount of the composition does not undergo a significant reduction. The addition of dispersive agents engenders a substantial increase in the surface area. An augmentation in the porosity and surface area of the reactants enhances the convective mechanisms, which exert a crucial role in governing flame propagation dynamics, particularly within confined geometrical configurations. Additionally, the dispersion and distribution of the thermite components undergo an improvement, thereby facilitating an escalation in the physical contact between molecules, lowering the ignition temperature threshold, and consequently improving the burning rates.

4.4. Heat Treatment and Impurities

Initially, the prevailing notion was that the optimum equivalence ratio would remain invariant, irrespective of the oxidizer employed, and would be solely contingent upon the intrinsic chemistry governing the reaction. However, empirical evidence has elucidated that the optimum equivalence ratio exhibits variations contingent upon the specific synthesis technique utilized for the oxidizer. These deviations in the optimum equivalence ratio can be attributed to the quantity of impurities present within the oxidizer, such as carbon and hydroxyl groups. The elucidation of this behavior necessitates a comprehensive understanding of the chemistry associated with the impurities. A high concentration of carbon is likely to manifest in the form of a hydrocarbon. This hydrocarbon can function as a fuel, potentially competing with aluminum for reaction with Fe₂O₃ or the oxygen liberated from the oxygen provider, which, in this case, is KClO₃. This competing reaction consumes Fe₂O₃ that would otherwise be available for the aluminum/iron oxide reaction, thereby engendering a propensity towards a fuel-rich optimum burning rate.

The initial emphasis was placed on eliminating impurities from the oxidizers. One impurity that can impede flame propagation is bonded water in the form of hydroxyl hydrates. These hydrates can be removed by heating the mixture to temperatures of 300 °C or higher. The sol–gel materials exhibit significant levels of carbon and hydrogen impurities, typically up to 15% by mass. While these impurities have not been comprehensively characterized, they likely originate from a combination of the solvent, gelling agent, and side reactions involving the gelling agent. Upon heating, the sol–gel materials undergo physical transformations. The specific surface area of the sol–gel Fe₂O₃ materials decreases with heat treatment. Thermal treatment induces particle size growth, surface area reduction, and a transition from spherical amorphous particles to crystalline cubic and rhomboids. Based on reasoning, such changes were speculated to result in a decrease in the maximum burning rate for composites employing heat-treated sol–gel materials; however, the opposite trend was observed. This suggests that the increase may be attributable to a chemical change. Heat treatment of sol–gel materials leads to alterations in the elemental composition and crystalline phase. For instance, the non-heat-treated materials contain significant levels of organic contamination and water. Consequently, heat treatment yields pure Fe₂O₃ materials. The poorly crystalline hydrated ferrihydrite phase is transformed upon heating to the most thermodynamically stable iron oxide phase, α-Fe₂O₃. One possibility is that the organic and water contaminants present in the sol–gel material act as flame retardants, inhibiting or greatly reducing the rapid propagation of the thermite reaction. Contrary to expectations, the organic content was anticipated to function as a gas generator during the reaction, thereby enhancing the burning properties of the mixture. The results contradict this notion. It is evident that the factors favoring propagation are high heat of reaction, a relatively large fraction of energy feedback, and low activation energy.

Heat treatment reduces surface area due to pore structure changes like expansion, coalescence, and collapse of smaller pores. Higher heat treatment temperatures decrease surface area and pore volume but increase purity. While purity increases, the reduced surface area from heat treatment leads to a significant decrease in the maximum burning rate, suggesting impurities enable faster combustion.

5. Conclusions

In conclusion, an optimum fuel/oxidizer ratio is needed to balance interfacial contact effects for best performance. This optimum exists for a certain value of KClO₃ content at which reactivity is increased and melting points and activation energy are diminished. Charging pressure and filling density are also critical parameters that exhibit an optimum for achieving maximum reactivity, pressures, and minimum delay times in thermite reactions within explosion tubes by modulating reaction mechanisms and heat transfer. The electro-explosive tube setup allows the study of how parameters like composition and confinement affect the ignition delay and combustion behavior, with higher oxidizer surface areas promoting faster ignition. The 50% ignition distance is a critical parameter impacted by composition and dispersants, governing ignition ease, reactivity, and pressure development characteristics. Optimizing the fuel/oxidizer ratio close to the equilibrium stoichiometry around 1.1 is essential for maximizing the reactivity, combustion performance, and energy release of nano-Al/Fe₂O₃ thermites. Specific surface area is a crucial parameter that can be optimized, along with pore characteristics and purity, to control and enhance the reactivity, combustion efficiency, and performance of thermite compositions. Heat treatment temperature impacts critical parameters like purity, porosity, and surface area of thermites, which need to be optimized to achieve the desired reactivity and combustion performance.

The ignition characteristics and susceptibility towards accidental initiation of nano-Al/Fe₂O₃-KClO₃ compositions are subject to influence by a multitude of determinants, encompassing factors such as the propensity for ignition and the dynamics of flame propagation. These parameters are governed by the compositional makeup of thermites, the particle size, and packing density. The level of ignition sensitivity can exhibit variations contingent upon these factors, a consideration that acquires paramount significance for the safe handling and deployment of nano-Al/Fe₂O₃-KClO₃. The size of fuel and oxidizer particles exert a profound influence on the burning rate. Furthermore, thermites with nano-sized oxidizer particles can facilitate an expedited liberation of oxygen, consequently engendering an escalation in the pressurization and burning rates. Elevated burning rates are generally favored by any combination of low activation energy, high heat of reaction, and efficient energy feedback mechanisms. Conversely, diminished burning rates tend to manifest as a consequence of the antithetical scenario in each of these cases.

Through this research, a comprehensive understanding of optimizing the properties of nano-Al/Fe₂O₃ compositions was attained, pushing the boundaries towards innovative strategies to overcome the challenges and facilitate the potential widespread deployment and adoption across diverse applications, encompassing both military and civilian, such as microscale tactical applications, energy storage systems, micro-energetic systems, and micro-thrusters, as well as enhanced thermites, pyrotechnics, and propellants. This positions them as promising candidates for revolutionizing the field of nano-energetic materials and enabling a new generation of advanced applications, paving the way for a new era of on-demand energetic materials. Future research directions in this field should focus on advancing fundamental understanding, manufacturing and integration techniques, performance characterization, scalability, and exploring novel applications while addressing safety and environmental concerns.