Polymer Nanocomposite Ablatives—Part III

Abstract

Previous reviews by authors indicate the continuing development and improvement of thermal protective systems through the introduction of polymer nanocomposites into polymer matrix composites. These materials perform as thermal protective systems for a variety of aerospace applications, such as thermal protection systems (TPSs), solid rocket motor (SRM) nozzles, internal insulation of SRMs, leading edges of hypersonic vehicles, and missile launch structures. A summary of the most recent global technical research is presented. Polymeric resin systems continue to emphasize phenolic resins and other materials. New high-temperature organic resins based on phthalonitrile and polysiloxane are described and extend the increased temperature range of resin matrix systems. An important technical development relates to the transformation of the resin matrix, primarily phenolic resin, into an aerogel or a nanoporous material that penetrates uniformly within the reinforcing fiber configuration with a corresponding particle size of <100 nm. Furthermore, many of the current papers consider the use of low-density carbon fiber or quartz fiber in the use of low-density felts with high porosity to mimic NASA’s successful use of rigid low-density carbon/phenolic known as phenolic impregnated carbon ablator (PICA). The resulting aerogel composition with low-density non-wovens or felts possesses durability and low density and is extremely effective in providing insulation and preventing heat transfer with low thermal conductivity within the aerogel-modified thermal protective system, resulting in multiple features, such as low-density TPSs, increased thermal stability, improved mechanical properties, especially compressive strength, lower thermal conductivity, improved thermal insulation, reduced ablation recession rate and mass loss, and lower backside temperature. The utility of these TPS materials is being expanded by considering them for infrastructures and ballistics besides aerospace applications.

1. Introduction

Thermal protective systems char and erode during exposure to excessive heat to deliver protection to the underlying components and prevent catastrophic failure. Improvements in thermal insulation, ablation resistance, and lightweight characteristics are the stimuli for continued developments in TPSs [1,2]. Polymer nanocomposites (PNCs) and related nanotechnology techniques are prominent in providing distinctive features, such as TPS resin matrix modifiers, as well as surface treatment of TPS fiber reinforcement by supplying thermal resistance, lower density, and mechanical strength. The recent application of aerogel technology to TPSs has facilitated further utility of nanotechnology [3]. Aerogels are nanostructured ultra-lightweight nanoporous materials with skeletal frameworks that display a wide range of nano-morphologies. They are solid materials composed of a network of pores, which makes the material very lightweight and porous. The pores in aerogel act as insulators, preventing the transfer of heat through the material. Additionally, the low thermal conductivity of the gas trapped in the pores also helps to dissipate heat. This makes aerogel an ideal material for use in insulation and heat dissipation applications, both of which are critical to TPSs. These interrelated aerogel areas that apply to TPSs are discussed in this publication.

2. Resin Matrix and Reinforcement Systems

Phenolic resins continue to be the most desirable matrix resin in the development of TPSs, especially as this relates to the emergence of aerogel modification of the matrix resin. Boron-modified phenolic resins are considered, as well as closely related polybenzoxazine resins. New high-temperature resins consist of a polysiloxane resin and a phthalonitrile resin material. Reinforcement fibers, such as woven fibers based on carbon fiber and quartz fiber, are discussed in phenolic resin modification with nanomaterial additives. Low-density, high-porosity carbon fiber and/or quartz fiber needled felts or non-wovens are popular reinforcements used in aerogel description.

2.1. Phenolic Resins as Matrix Resin

Niu and co-workers [4] utilized modified boron phenolic resin with the in situ preparation of trisilanolheptaphenyl POSS (Figure 1) to obtain the heptylphenyl POSS-modified boron phenolic resin (POSSBPR).

Figure 2 illustrates the preparation of the modified boron phenolic resin with the heptaphenyl POSS, followed by impregnation of high silica fiber and curing under the temperature conditions shown in Figure 2. The resulting POSSBPR composition varied from 0 to 25%. POSS was used as an ethanol solution to impregnate silica fiber, followed by curing and molding. FTIR and NMR-²⁹Si confirmed the POSSBPR structure. Char yield at 1000 °C increased from 68.1% to 74.2% for POSSBPR containing 20% POSS. The linear ablation rate and mass ablation rate for POSSBPR4/silica fiber were 0.123 mm/s and 0.0602 g/s as compared to BPR/silica fiber control values of 0.130 mm/s and 0.0685 g/s, respectively, using oxy-acetylene test bed (OTB) conditions of 4186.8 kW/m² (418.68 W/cm²) for 20 s with sample dimensions of 30 mm diameter and 10 mm thickness. The authors attribute the improved reduced linear ablation rate and ablation mass rate to the formation of high graphitized carbon microstructure and high-temperature ceramics from the high-temperature treatment of POSSBPR, especially the POSS component.

Yang and co-workers [5] dispersed a constant amount of MoSi₂ to boron phenolic resin (0.57 to 0.43) and varied B₄C into boron-modified phenolic resin in ethanol and impregnated carbon fiber fabric with the resulting uniformly mixed “ceramizable” boron phenolic resin solution. Both MoSi₂ and B₄C were µm size. The prepregs were molded/cured into “ceramizable” composites. The overall process is shown in Figure 3.

Thermogravimetric analyses (TGA) of the “ceramizable” composites were carried out at 10°/min in air up to 1400 °C. The highest char yield of 32.9% was observed for sample BP-15, which contained 15% B₄C with a corresponding ratio of 0.57 to 0.43 of MoSi₂ to boron phenolic resin. The overall stoichiometry is 15 parts B₄C, 65 parts MoSi₂, and 50 parts of boron phenolic resin in 50 parts of ethanol. It is a ceramizable resin system with ~62% ceramic additives. The authors propose that the high char yield is attributable in part to B₄C reacting with oxygen at elevated temperatures to form B₂O_3, which increases char yield. The flexural strength of the ceramizable composites was determined at elevated temperatures of 800 °C/15 min, 1000 °C/15 min, 1200 °C/15 min, and 1400 °C/15 min. The best flexural strength properties during the range of temperatures were exhibited by BP-15, which contained 15% B₄C. The unexpected small reduction in the diameter size of carbon fiber in the ceramizable composites was shown by the authors to be responsible for observing continued high flexural strength, especially for the resulting diameter size of BP-15. The carbon fiber diameter of BP-15 was 7.118 µm (

Table 1. Diameter changes of carbon fibers treated at different temperatures (Data from [5]).

Treatment Temperature	$\overline{\mathit{d}}$/μm
	Carbon Fibers	BP-0	BP-15
RT	7.118	7.118	7.118
800 °C	6.034	6.178	6.892
1000 °C	5.393	5.487	6.296
1200 °C	4.999	5.097	5.592
1400 °C	4.863	6.042	6.681

) initially at RT and reduced to 6.681 µm after successive temperature treatments from 800 °C, 1000 °C, 1200 °C, and 1400 °C with 15 min intervals.

A control of carbon fiber exposed to similar temperature conditions up to 1400 °C in air with 15 min intervals showed that the diameter reduced from 7.118 µm to 4.863 µm and is indicative of thermal instability of carbon fiber in air at elevated temperature, as well as reaction with ceramic additives.

Regarding the reduction in carbon fiber diameter of other composites with less or no B₄C, such as BP-0, which contains solely MoSi₂ and boron phenolic resin, the carbon fiber diameter is 6.042 µm after multiple temperature increases and multiple 15 min intervals. Morphology and XRD analyses of composites suggest that MoSi₂-derived complex phase ceramics, such as MoB, MoB₂, Mo₂C, Mo_4.8Si₃C_0.6, etc., with high melting points were formed at high temperatures. The formation of B₂O₃ and the MoSi₂-derived complex phase ceramics participated in oxygen barriers, minimizing the oxidation or reduction in carbon fiber diameter and enhancing the performance of composites at elevated temperatures. Ablation data were reported for linear and mass ablation rates without indicating OTB test conditions. Nevertheless, the lowest linear ablation rate was 0.013 mm/s for 20% B₄C as compared to 0.0424 mm/s without B₄C particles. The lowest mass ablation rate of 0.0815 g/s was recorded for the composition containing 15% B₄C, while the 20% B₄C was higher, or 0.0840 g/s. Authors attribute this discrepancy to two factors: oxidation of B₄C is a weight gain reaction, while the volatilization of B₂O₃ is a weight loss process. A compromise between weight gaining and weight loss processes occurs with 15% B₄C particles as compared to 20% B₄C. The authors viewed BP-15, which contained carbon fibers, boron phenolic resin, MoSi_2, and B₄C particles and exhibited excellent high-temperature behavior.

An interesting publication by Li and co-workers [6] describes the Z-pinning effect on interlaminar mechanical and ablation performance of quartz fiber/phenolic composites. The objective of the study was the reinforcement of the composite interlaminar properties by Z pinning. Z pinning has been successful in significantly improving type I and type II fracture toughness and interlayer composite properties. Multi-ply quartz fiber prepreg was fabricated with Z pins made of quartz fiber and phenolic resin. Z pins were implanted into the prepregs by ultrasound methodology, followed by curing from 100 °C to 180 °C for multiple hours. The interlaminar effect of Z pins on composite depends on the bonding interface, which is mainly due to cured resin. The pyrolysis of the phenolic resin via TGA exhibited a weight loss rate of 14.38% at a temperature of 920 °C in an atmosphere of 20/80 for O₂ to N₂ for 60 s, indicating that the Z pin and laminate interface maintain the interface bridging bond after flame ablation. After 60 s of ablation in a flame of 920 °C, the interlaminar shear strength of the Z pin reinforced composites was 3.01 MPa, which is 84.7% stronger than that of the control. During the ablation process, the Z pins effectively suppressed the interlaminar delamination and the formation of large areas of bulges by enhancing the interlaminar strength and providing escape channels for the pyrolysis gas. Thus, it was demonstrated that the Z pin can effectively prevent mechanical erosion of the ablation surface and assist in improving composite ablation performance.

2.2. EPDM Rubber

Xi and co-workers [7] examined EPDM insulation for solid rocket motors and introduced carbon nanotubes (CNTs) to improve the strength of the composite insulation material since particle erosion occurs due to aluminized composite propellants striking the surface of the insulation material. CNTs are known to enhance the strength of the char layer. Other investigators have shown that the network formed by CNTs with EPDM enhances the mechanical properties of the char layer and reduces erosion caused by alumina particles. Furthermore, CNTs promote chemical vapor deposition (CVD) of pyrolysis gases in the char layer, which strengthens the char layer. CNTs improve the ablation resistance of composite insulation by strengthening the char layer, but at the same time, they produce a negative effect on the reduction of the thermal insulation performance of the composite insulation material due to the high thermal conductivity of CNTs. The use of CVD to coat CNTs based on earlier work on CVD of CNTs improved the compatibility of coated CNTs in EPDM. CVD coating of CNTs increased the diameter from 10–20 nm (uncoated) to 55–60 nm for coated CNTs. In all cases, mechanical properties (strength and elongation) of uncoated CNT formulations were highest, followed by coated CNTs, with the basic formulation (control, no added CNT) being the lowest.

TGA results shown in Figure 4 indicate that in the initial stage of heating when the temperature is lower than 450 °C, the weight loss of the basic formulation is more than that of the CNT formulation and coated CNT formulation (see the insert of Figure 4). This indicates that the thermal insulation material with added CNTs exhibits better thermal stability. When the decomposition temperature is reached, the residue amount of the basic formulation is 16.9%, while that of the CNT formulation and coated CNT formulation are 21.7% and 21.1%, respectively. Similarly, material conductivity, ablation rate, and backside temperature all followed a similar trend. The highest values were exhibited by uncoated CNT, followed by coated CNT, with the control showing the lowest. The char layer of the coated CNT showed the best compression resistance, followed by the CNT formulation. Control was quite inferior. The microstructure of the surface and cross-section of the char layer showed a compact structure with no pores on the coated CNT, while the CNT formulation is relatively compact with some small pores. The control exhibits a loose pore structure with a large pore size. Thus, the thermal insulation performance of coated CNTs provides greatly improved performance compared to uncoated CNTs. Further, the char layer of the coated CNT formulation not only improves thermal insulation but also provides better compressive resistance. These two characteristics indicate that the coated CNT not only improves thermal insulation performance by reducing the thermal conductivity of the formulation but also provides particle erosion resistance of the insulation materials by enhanced strength of the char layer.

A follow-up paper by the Guo group [8] involves a study related to the size and content of CNTs, providing particle erosion resistance of EPDM. Different CNT parameters of CNTs with different sizes are shown in

Table 2. Parameters of CNTs with different sizes (Data from [8]).

	Product Number	Inner Diameter (nm)	Outer Diameter (nm)	Length (µm)
Conventional CNTs	CNTs-010-0	3–5	8–15	3–12
Long CNTs	CNTs-010	3–5	8–15	30–50
Short CNTs	CNTs-007-1	3–5	8–15	0.5–2
Large diameter CNTs	CNTs-014	5–15	>50	10–15

.

A particle test motor was used in the study to investigate the particle erosion resistance of EPDM composites reinforced with different contents of CNTs with different sizes. The burning propellant produces molten Al₂O₃ particles, forming a dense particle stream that impacts the EPDM composite surface. Particle erosion resistance is determined by measuring thickness and mass values before and after the motor experiment. CNT content and size significantly influence the particle erosion resistance of the EPDM composites. The study showed that EPDM composites with a CNT content of 10 phr exhibited the best particle erosion resistance; the ablation rates of EPDM composites with lower (2 phr) or higher (20 phr) CNT contents were significantly higher. Further, long CNTs with excellent mechanical properties greatly improved the particle erosion resistance of the EPDM composites. The charring rate was 43.6% lower than that of conventional CNTs. Furthermore, EPDM composites containing large-diameter CNTs showed poor mechanical properties, high thermal conductivity, and low generation of pyrolytic carbon (carbon generated by CVD to strengthen the char layer), resulting in poor particle erosion performance.

2.3. New Thermosetting Resins

The heritage thermosetting resin since the inception of TPSs has been the phenolic (PF or Ph) resin. It has been modified structurally by inserting boron, silicon, and other inorganic elements or by using additives (ceramics and nanoparticles) and related phenolic resin systems, such as benzoxazines and cyanate esters. Notably, PF possesses unique features, such as a reasonable char yield of 62%, exceptional thermal stability on conversion to carbon/carbon composite at elevated temperatures, low viscosity/low MW reactive resin system, relative ease of transforming into desired aerogel (discussed later in this publication), and readily available at a reasonable cost. NASA and the US Department of Defense (DoD) branches generated a large database of phenolics depending on the utility of the resulting phenolic TPS application. Global efforts involving phenolic resins for TPSs include activities in Europe and Asia.

New thermosetting resins that are improvements over phenolic resin consist of polysiloxane and phthalonitrile resins. The authors have used these three resins in our research: legacy phenolic (SC1008), polysiloxane (ultra-high temperature resin, UHTR), and phthalonitrile (PN) (see more description of the UHTR and PN resins below). To better understand the thermal properties of these three resins, thermogravimetric analysis (TGA) tests were conducted on them. The phenolic (SC1008, Ph) was used as a control formulation. The aim of this test campaign was also to acquire the kinetic parameters to enable heat transfer modeling. In a char yield study conducted by Koo et al. [9], test conditions were developed for char yield determination with standard test procedures based on a NASA report [10]. Figure 5 reports the TGA patterns of the UHTR, SC1008, and PN high-temperature resin systems. The char yield and the mass residue (%) were measured using a TA Instruments Hi-Res TGA 2950. The tests were conducted in nitrogen at a heating rate of 20 °C/min from room temperature to 1000 °C.

UHTR has an average char yield of 89% compared with the SC1008 resin of 62% and PN resin of 75%, as shown in Figure 5 and

Table 3. Decomposition temperature at 10% mass loss and char yield values of the three high-temperature resins [9].

Material	Td @ 10% (°C)	Char Yield (%)
UHTR-6398	760	88.1
PN	636	74.9
SC1008	478	62.0

. There are three versions of polysiloxane resins manufactured by Techneglas, namely UHTR-F (solid flake), UHTR-6398 (high viscosity liquid), and UHTR-IPA (dispersed in isopropyl alcohol) resins, which have the highest char yield ranging from 87.8% to 88.9%. They also have the highest decomposition temperature at 10% mass loss of 740 °C to 820 °C compared with SC1008 resin (Table 3). New data show that UHTR-F has a T_g of 550 °C.

2.3.1. Polysiloxane

One of the new high-temperature thermosetting resin systems is a novel polysiloxane known as UHTR 6398-S, which is a solventless resin manufactured by Techneglas, Perrysburg, OH, USA [11,12]. UHTR is a colorless, semi-solid liquid that can be thermally cured at 350 °C/2 h or catalyzed with a small amount of base. Figure 6 shows the structural formula of UHTR, resin properties, and its comparison with other high-performance thermosetting resins [13]. As a carbon fiber UHTR composite, the CF/UHTR composite possesses a char yield of 93% (TGA) as compared to the CF/phenolic resin composite (MX4926, carbon fiber phenolic resin composite), exhibiting a value of 83% in nitrogen (Figure 7) [14].

Char yield is defined as the weight of material at 1000 °C divided by its weight after an isothermal period in a nitrogen environment using a heating rate of 20 °C/min. However, in air, the UHTR composite retains 25% char, while CF/phenolic is completely decomposed, further demonstrating the superior thermal stability of UHTR over phenolic resin by Hou et al. [14]. Using a hot melt process, CF/UHTR prepreg is cut into 1.27 × 1.27 cm squares (Figure 8) and molded into a cylindrical disc measuring 7.62 cm diameter and 1.27 cm thickness with post-curing of the disc at 350 °C/2 h. The resulting disc is subjected to OTB testing in the KAI laboratory.

Koo and his KAI team have been engaged in the development of many thermal protective ablative systems involving preparation, testing, and modeling for nearly two decades. These KAI activities have been summarized recently [15]. OTB results are presented as recession and mass loss percentages and surface and backside temperatures versus ablation parameters. The ablation parameter is defined as the product of heat flux X exposure time (heat load input).

Table 4. OTB test conditions (Data from [14]).

Simulated Reentry Conditions
Heat Flux (W/cm2)	Exposure Time (s)	Ablation Parameter (kJ/cm2)
500	60	30
1000	60	60
1500	60	90

shows typical OTB test conditions for the KAI team to evaluate TPS materials.

Recession percentages of the CF/UHTR MC and MX4926N MC materials over ablation parameters are shown in Figure 9.

The shaded areas represent the error range. The ablation parameter is defined as the heat flux multiplied by the time that the material is exposed to the OTB flame; this represents the heat load imposed on the material. Both materials present negative recession percentages at all tested conditions and imply the materials have swelled more than receded during the OTB aerothermal tests, as shown in Figure 9a by Y. Hou [14]. To separate the swell and material loss, the mass losses of both materials are plotted as ablation parameters, as shown in Figure 9b. Figure 9b shows that the mass losses of both materials increase as the ablation parameter increases. The shaded areas represent the error range. Mass losses of the CF/UHTR MC are approximately 1/8 of those of the MX4926N MC material in all test conditions, which indicates that the CF/UHTR MC has better ablation-resistant properties than the MX-4926N MC. Figure 10 shows the top surface temperatures and the backside heat-soaked temperatures of CF/UHTR MC and MX4926N MC in the OTB ablation tests. The temperatures of both materials are similar in all testing conditions. The surface temperatures of both materials have reached above 1800 °C, and the backside heat-soaked temperatures of both materials have remained below 250 °C. These results indicate that both materials have incredibly good thermal insulation properties.

Polysiloxane resin is known to be a precursor to silicon oxycarbide composition (SiOC) when pyrolyzed at temperatures from 1000 to 1500 °C (1832–2732 °F). Silicon oxycarbide is a hard glass structurally related to both silica and silicon carbide. It is of considerable research interest because of its high mechanical and dielectric properties, its interesting viscoelastic behavior at elevated temperatures, and its superior oxidation resistance in comparison to silicon carbide due to surface passivation with SiO₂. The use of silicon oxycarbide composition is mentioned in the aerogel discussion later.

A novel class of alumina-reinforced, pre-ceramic resin matrix ablative composites was developed, manufactured, characterized, and modeled for use as a TPS material by C. Yee [16]. High alumina paper and twill-woven mullite fiber were investigated as reinforcements for a Techneglas Ultra High-Temperature Resin (UHTR) [11,12] pre-ceramic polysiloxane matrix composite. These 2D laminate alumina/UHTR (A/U) composites minimize peak back face temperature primarily through rapid melt advection and low through-thickness thermal conductivity. A loading study was performed for boron carbide (B₄C) as an additive, which identified an optimal 7.5 wt% loading in UHTR, and the resulting changes to decomposition chemistry mechanisms were identified using analytical chemistry techniques. The bulk formation of networked Silicon Boron Oxycarbide (SiBOC) and precipitation of Highly Oriented Pyrolytic Graphite (HOPG) was identified in pyrolyzed and annealed UHTR with boron carbide. Four variants of A/U with and without boron carbide underwent aerothermal testing using the OTB, whose flow field was characterized using Schlieren imaging and a low-cost enthalpy probe. Alumina Paper/UHTR with 7.5 wt% B₄C (AP/UBC) was down-selected to be the primary focus of thermal characterization and ablation material response (MR) modeling efforts in CMA, FIAT, and ITRAC, three industry-standard one-dimensional ablation modeling codes. Ablative software outputs were compared against OTB experimental results to validate the AP/UBC material response model for use in preliminary design estimates for AP/UBC thicknesses of 0.100 inches or less.

All A/U composites were tested three times each at a cold wall heat flux of 1000 W/cm² for 30 s. This time was chosen after prototype testing to prevent excessive ablation crater formation, which results in straying from pseudo-1D heating conditions due to the change in surface geometry and can additionally extinguish the torch flame through turbulent backchanneling of exhaust. The test conditions also fall into standard time and cold wall heat flux test conditions for the OTB, permitting direct comparison of the A/U composite ablative performance against other materials tested on the OTB. The conditions are roughly comparable to the environment potentially experienced by a tactical scale solid rocket motor nozzle component or launch structure TPS element. Final modeling efforts were focused on AP/UBC based on its consistent performance, low back face peak heat soak temperatures, long time to peak back face temperature, and novel decomposition chemistry. To aid in the validation of the material model’s thermal properties, additional tests for AP/UBC were performed at 500 W/cm² for 30 s to provide different recovery temperatures and thermochemical environments.

A summary of average ablative testing metrics and their standard deviations are presented for all A/U composites in

Table 5. Averages and standard deviations of all 30 s A/U tests (Data from [16]).

Sample Type	Cold Wall Heat Flux	Time	Recession	Mass Loss	Peak Surface Temperature	Peak Back Face Heat Soak Temperature	Time to Peak Back Face Heat Soak
	W/cm2	s	mm	g	°C	°C	s
AF/U	1000	30	2.55 ± 0.87	1.05 ± 0.52	2181 ± 20	149 ± 13	200 ± 6
AF/UBC	1000	30	2.68 ± 0.78	0.57 ± 0.16	2162 ± 15	166 ± 26	199 ± 11
AP/U	1000	30	1.98 ± 0.13	0.58 ± 0.25	2093 ± 28	139 ± 18	241 ± 3
AP/UBC	1000	30	0.98 ± 0.24	0.72 ± 0.06	2118 ± 7	165 ± 6	220 ± 15
AP/UBC	500	30	−0.03 ± 0.11	0.40 ± 0.14	1779 ± 56	146 ± 13	213 ± 11

. The recession is based on post-test depth caliper measurements. Both woven mullite fiber reinforced composites exhibited similarly high recession levels due to advection behavior being dominated by the high fiber mass ratio. AP/U demonstrated the lowest peak back face heat soak temperature and longest time to heat soak conditions, while AP/UBC had the lowest recession of all A/U composites.

The 11 AP/UBC test models produced also achieved a consistent 88:12 resin/fiber post-cure weight ratio. Representative temperature profiles, as well as testing images, are shown in Figure 11. AP/UBC demonstrated slightly higher back face peak heat soak temperatures than AP/U but otherwise performed similarly during OTB testing. Despite the observed brittleness of SiBOC in alumina paper, charred AP/UBC samples exhibited decent toughness and did not delaminate or crumble when removed from the insulative alumina crucible.

AP/UBC was extensively characterized using both analytical chemistry and thermal characterization instruments to fully understand how the material decomposed and how that decomposition would affect the thermal properties during ablation. Analytical chemistry primarily focused on determining the pyrolysis gas and char atomic compositions for thermochemistry modeling, as well as positively identifying SiBOC as forming within the pyrolysis zone of the AP/UBC after 500 °C. TGA-FTIR and EDS were utilized to quantify the pyrolysis gas and char composition, while TGA-DTA, FTIR, Raman spectroscopy, and XPS were utilized to identify the SiBOC formation. Additionally, micro-CT was performed at Lawerence Berkeley National Laboratory (LBNL) at the Advanced Light Source (ALS) Beamline 8.3.2 [16,17], with results demonstrating the changes between virgin AP/UBC and pyrolyzed material, as shown in Figure 12.

Fully cured UBC is a networked polysiloxane containing distributed boron carbide crystals with limited boron oxide. During initial heating, boron carbide near the exposed surface oxidizes and eventually melts at approximately 450 °C. Starting at 525 °C, the UHTR decomposes into amorphous SiOC, which reacts with available boron carbide and boron oxide to produce amorphous SiBOC and precipitate a limited amount of HOP graphite. Water, CO, CO₂, butanol, benzene, methane, and residual-trapped IPA from the boron carbide dispersal process are emitted as a pyrolysis gas, with some of the mass loss mitigated by uptake of environmental oxygen and subsequent incorporation within the SiBOC matrix. Mass gain due to uptake of oxygen overcomes mass loss from pyrolysis at approximately 675 °C. In an environment lacking oxygen, nitridation of boron carbide will occur, leading to a lesser total mass gain. The SiBOC then further chemically bonds to the alumina reinforcement, forming AlSiBOC at the resin–reinforcement interface, greatly increasing the thermal and electrical conductivity of the pyrolyzing composite. AlSiBOC and SiBOC undergo phase separation and limited melting starting at 1400 °C, with the rate of phase transition to liquid for both the matrix and reinforcement peaking at approximately 2100 °C. A graphical timeline of the A/U composite decomposition process is presented in Figure 13.

Four alumina-reinforced, pre-ceramic polysiloxane resin matrix TPS composites with a density range of 1.04 to 1.34 g/cc were designed, fabricated, and tested under an aerothermal ablation environment to demonstrate their feasibility. Based on OTB aerothermal testing, the best-performing system was down-selected for further characterization and modeling. Alumina paper reinforcement, in combination with Techneglas UHTR, was identified as the best-performing ablative during OTB testing, both with and without added boron carbide flakes. Boron carbide was shown to be highly effective as an oxidative barrier and resulted in broad changes to overall decomposition chemistry, trading off a slightly higher peak back face heat soak temperature for better shape stability compared to a neat UHTR matrix. The resulting preliminary engineering designed models exhibited decent agreement with near-surface temperature response, mass loss, and recession, with FIAT demonstrating the best overall temperature profile predictions for both 1000 and 500 W/cm² conditions in the OTB. The AP/UBC model demonstrated good validity for preliminary design estimate purposes for thicknesses at or below 0.100 inch.

In conclusion, the author recommended that OTB flow field characterization can be improved through the use of designed gas analysis systems rather than utilizing COTS handheld sensors. This will also significantly improve the workflow of enthalpy probe operation due to the requirement of fewer tooling changes. A sensor designed to measure high NOx concentrations will be necessary to this end, and measuring the distribution of NOx within the flow will provide better data for surface thermochemistry inputs, as well as CFD modeling of the flow field. Alternatively, gas samples could be drawn from the enthalpy probe and shipped to a mixed gas analysis service provider. Varying oxygen and acetylene flow rates to characterize different flames using the enthalpy probe would be a worthwhile endeavor for CFD and combustion modeling validation data, as well as for studying NOx production and consumption rates in an open flame. To safely sample gas from the outer combustion zone and at stations above 500 W/cm² cold wall heat flux, it will be necessary to design and build a custom enthalpy probe capable of withstanding the extremely high temperature, with it being designed around preventing steam binding of the cooling lines either through use of a pressurizer design or high flow rate in a vertical orientation, with both options requiring significant expense to achieve and maintain. This upgraded enthalpy probe would be additionally useful for characterizing any oxy-fuel flames due to its ability to withstand the high temperatures of oxy-acetylene combustion.

2.3.2. Phthalonitrile

The other new thermosetting resin for TPSs is phthalonitrile (PN) resin. Early synthesis activity involved the efforts of Keller [18,19,20] of the US Navy Research Laboratory and Zhou et al. [21]. Recent phthalonitrile resin books [22,23] describe the extensive chemistry of phthalonitrile resins, composites, and applications.

Although PN resin possesses attractive thermal properties, there have been limited ablation data reported in the open literature thus far. Preliminary ablation studies conducted by Yang et al. [24] involve the use of butane flame with moderate heat flux. According to the authors, the butane flame technique is more convenient to adjust and more economical than OTB testing. The overall process involving the CF/PN composite laminate is shown in Figure 14. The PN structure, hot-pressing curve, and specimen lay-up illustrate the characteristics of the laminate.

PN with a carbon fiber (CF/PN) volume fraction of 41.7% was examined by the butane flame method [24]. The resin on the laminate surface significantly decomposes under one-sided flame heating. The degree of resin pyrolysis at deeper depth appears relatively small, while the remaining matrix forms a loose porous structure. The different morphologies at various positions along the thickness gradient emphasize the influence of different gas environments on pyrolysis behavior. A phenomenological mathematical model with high reliability is proposed based on FE simulations to quickly predict the rear surface temperature of CF/PN composite laminate under one-sided heating, which provides preliminary engineering guidance oriented to thermal protection/load-bearing integration structural design.

Blends of phenolic novolac phthalonitrile (NPN) with novolac cyanate ester (NCE) were examined recently using plasma arc jet ablation conditions [25]. The NPN was prepared by reacting phenolic novolac with 4-nitrophthalonitrile to form NPN. CE was obtained from Lonza Chemicals, USA. NPN and NCE were prepared in different weight ratios (1:3, 1:1, 3:1) and cured according to a schedule of 150°, 200 °C, and 250 °C for 1 h each, followed by 300 °C/2 h and 350 °C/3 h. Blends exhibited a two-phase morphology with PN domains distributed as different-sized spherical structures dispersed in a continuous cured CE matrix, as evidenced by SEM and AFM. Chopped silicon fiber composites with 3M micro-balloons and NPN/NCE blends were transformed into molded composites with densities of 0.50 +/− 0.02 g/cc. The cured resin blends and composites were pyrolyzed at 1000 °C to form polymer-derived ceramics. Thermal conductivity of the composite samples showed a marginal increase from 0.11 W/m·K to 0.14 W/m-K at 80 °C, with an increase in PN content in the blend. The flexural and compressive properties indicated that the composites exhibited good strength and modulus values. All the composites showed excellent thermal stability with extremely high char yields in the range of 75–81% at 900 °C. Percent retention of compressive properties with increasing PN concentration varied from 30 to 55% and agrees with char values. Exposure of the composites to very low re-entry heat flux conditions and extreme exposure time using radiant heating conditions (~55 W/cm² for 1200 s) and Plasma Arc jet (70 and 125 W/cm² for 50 s) resulted in excellent surface uniformity with minimal mass loss (4.77–5.77%) and excellent heat of ablation (8000–12,000 cal/g) with reduced back-wall temperature (230–260 °C).

3. Phenolic Resins–Transformation into Aerogel

It was previously mentioned in recent publications that the use of aerogels to further modify TPSs for improved performance has become a high-priority activity. Aerogels provide very efficient insulation due to their high porosity and lightweight pore structure in the nanometer range. They are proficient in insulation because the pores are so small that gas-phase heat conduction is extremely poor, resulting in limited heat transfer through the material. Aerogels appear to be an ideal modification material for TPSs due to their high porosity, light weight, low thermal conductivity, improved thermal stability, improved mechanical properties, and low dissipation of heat transfer. Recent aerogel reviews by Salimian [26] and Liu [27] and the more recent review by Jin [3], which discusses aerogels for TPSs, are available to provide an overview of aerogels.

As a nanoporous material with ultralow conductivity, aerogels have attracted considerable interest in applying the technique to TPSs for aerospace. Aerogels encompass a wide range of compositions, such as (1) inorganic oxide aerogels and composites for thermal protection, like SiO₂, Al₂O₃, ZrO₂, and multi-oxides like Al₂O₃/SiO₂ and ZrO₂·SiO₂, some of which have been examined in aerospace applications; (2) organic aerogels and composites for thermal protection–phenolic resin and polyimide; (3) carbon aerogels and composites for thermal protection; and (4) carbide (SiC and other carbides) aerogels and composites for thermal protection. However, only SiO₂, silicon oxycarbide, phenolic resin and/or silica hybrids, and carbon aerogels will be discussed as aerogel components.

As it will be described in this review, the mechanistic role of organic resin matrix aerogels as low- to medium-density ablative systems or TPS ablators allows the low- to medium-density ablative material to remove material from the spacecraft or space debris via heat, pressure, and velocity to minimize heat to the interior of the space vehicle. These low to medium systems (carbon fiber and/or quartz fiber needled felt systems) have significantly reduced density (0.2–0.9 g/cm³), providing dual functionality of thermal protection of the matrix resin combined with high-efficiency insulation of aerogels. Further, they feature low thermal conductivity, especially the quartz fiber felt system, high emissivity of the char layer surface, and excellent resistance to gas emission on the char surface.

This review will focus on organic aerogels for TPSs. As best as it can be determined, the first reported use of an organic aerogel in TPSs was conducted by Cheng and co-workers in 2017 [28]. PAN-based carbon fiber needled felt (NCF) with dimensions of 48 × 48 × 24 mm³ and density of 0.164 g/cc was immersed into a solution of phenolic resin (PR), 66.7% in isopropanol, hexamethylenetetramine (HMTA), and ethylene glycol (EG). Four solutions with different phenolic resin concentrations of weight ratio of PR to EG of 1:5 (sample PR1/5), 1:4 (PR1/4), 1:3 (PR1/3), and 1:2 (PR1/2) were prepared. The resulting systems were heated from 90 °C to 180 °C for multiple hours, leading to phase separation of the phenolic resin, followed by curing into phenolic aerogel. The resulting impregnated aerogel CF felt was immersed in ethanol at 60 °C for 48 h to remove EG. The resulting NCF-PR aerogel composites with densities of 0.27–0.37 g/cc were designated as NCF-PR1/2 through NCF-PR1/5.

Figure 15 illustrates the overall procedure. Photomicrographs [Figure 15b,c] of the composite show that the CF of NCF in the xy direction are distributed disorderly in the plane, while in the z direction, the main part of the fibers is uniformly distributed over the height of the composite. These CFs are tangled and restrained to form an integrated and quasi-layered structure, forming the NCF-PR aerogel composite anisotropic in these two directions. The high magnification images (Figure 15d,e) show the PR aerogels as uniform, and they completely fill the macropores between the CF and are coated with a thin layer on the fiber surface. The authors emphasize that the photomicrographs show a homogeneous PR aerogel matrix without fracture and indicate that agglomeration of pores is achieved. Further, a composite that is lightweight and superior in adhesion at the fiber/resin interface is obtained. The authors view the composite aerogel as a “bird’s nest structure.” The microstructure of the PR aerogels exhibits typical bi-continuous and percolating structures of aerogels prepared by “polymer-induced phase separation” (PIPS), where most of the volume consists of interconnected pores consisting of interconnected grape-like aggregates. When the content of PR resin increased from PR1/5 to PR1/2, aerogel particle size decreased from ~124 to 80 nm, with a narrow size distribution.

The NCF-PR aerogel composite possesses relatively high compressive strength (1.48 to 11.02 and 0.83 to 4.90 MPa in xy and z directions, respectively) and low thermal conductivity (0.131 to 0.230 and 0.093 to 0.180 W/(m-K) in the xy and z directions, respectively). More importantly, the NCF-PR aerogel composite exhibits good thermal insulation and ablation performance in an arc jet wind tunnel simulated environment (heat flux of 1.5 MW/m² (115 W/cm²) for 33 s) with a low linear ablation rate (sample NCF-PR1/2 exhibited a linear ablation rate value of 0.029 mm/s and the lowest of all samples), and the internal temperature peaks below 90 °C at 38 mm in-depth thermocouple position when the surface temperature exceeds 2000 °C. Thus, this early publication of a carbon fiber felt/phenolic resin composite aerogel suggested anticipated further publications with aerogel structural modification of the fiber-reinforced composite systems with expected improved ablation performance.

Poloni and co-workers in Europe [29,30] provided some technical and theoretical considerations for the use of aerogels in the design of newer and improved TPSs. Ablators that withstand intense thermal radiation conditions for distant planets require the development of porous carbon ablators with pore sizes that enhance heat-shielding performance by increased scattering of high-temperature thermal radiation. The authors’ focus related to pore size and the development of porous materials, such as aerogels. The use of aerogels is directed at NASA phenolic impregnated carbon ablators (PICAs), which are unusually lightweight ablators that consume part of the heat upon pyrolysis during re-entry into planetary atmospheres. PICA is prepared by using a carbon fiber felt to reinforce phenolic aerogels, resulting in a highly porous structure (85%) with advantages such as light weight, excellent thermal insulation, attractive mechanical properties, high thermal stability, and dimensional stability. As demonstrated by Cheng et al. [28] for aerogel-modified TPSs, Poloni and co-workers used the PIPS procedure on phenolic resins to create pores with sizes ranging from 15 nm to 3 µm. Poloni et al.’s conditions were somewhat different than Cheng et al.’s procedure. Polyvinyl pyrrolidone (PVP) was used as an additive, which is known to influence phase separation for the preparation of porous membranes. The formulation consisted of base-catalyzed phenolic resin (PR) dissolved in ethylene glycol (EG) at resin-to-EG volume ratios of 1:2 to 1:7.5. The polymers were pre-dissolved in the initial solution at 110 °C in PVP/PR ratios of up to 50%. The phase separation was induced by maintaining the polymer mixture at 150 °C for 12 h. A recent paper by Poloni et al. [30] describes the preparation of the High Enthalpy Flow Diagnostics Group (HEFDiG) material as HEFDiG Ablation-Research Laboratory Experiment Material (HARLEM) using PIPS technology with PR/PVP/EG technology, as shown in Figure 16.

A comparison of electron microscopy images of HARLEM with a propriety European phenolic ablator, ASTERM, and NASA PICA is shown in Figure 17. HARLEM is found to be reasonably similar to ASTERM and PICA based on photomicrographs.

The ablation performance of HARLEM was determined via arc jet testing. Cold wall heat flux of 5.4 MW/m² (540 W/cm²) for 30 s was used, and a recession rate of 48 µm/s was measured in situ by photogrammetry. The authors developed a relationship that considers the effective heat of ablation (h_eff) as it relates to the cold wall heat flux value divided by the product of the apparent density of HARLEM (0.27 g/cc) and the surface recession rate. An effective heat of ablation value of 417 MJ/kg was determined for HARLEM. Other effective heats of ablation for a variety of carbon–phenolic ablators tested in air at plasma wind tunnel facilities are tabulated by the authors and include ASTERM with values ranging from 65.7 to 182.6 MJ/kg, as well as PICA with a wide range of values from a low of 43.5 to a high of 382.7 MJ/kg. PICA surface recession rates were higher than surface recession rates for ASTERM and HARLEM despite the low densities of the carbon–phenolic ablators.

3.1. Phenolic Silica Hybrid Aerogels

Cheng and co-workers [31] modified their early phenolic aerogel study in 2017 [28] by co-curing phenolic resin with an aminosilane compound to develop phenolic silicon (PSi) hybrid aerogel. A lightweight carbon fiber–quartz fiber needled felt (C-QF, 1:1) reinforced by phenolic–silica (C-QF/PSi) aerogel nanocomposite was prepared by impregnating low-density C-QF hybrid needled felt with co-precursor solution of PSi hybrid aerogel, followed by copolymerization-induced nanoscale phase separation, solvent exchange, and ambient pressure drying (APD). The overall process is shown in Figure 18.

The hybrid needled felt is based on alternating T700 PAN-based carbon fiber stacked as weft piles and quartz fiber short-cut fiber webs with a volume ratio of 1:1 and a density of 0.352 g/cc with a high porosity of 81%. Synthesis of the PSi hybrid aerogels involved phenolic resin (PR), hexamethylenetetramine (HMTA), and ethylene glycol (EG) at a weight ratio of PR/HMTA/EG = 1:0.05:4 in solution, followed by the addition of (3-aminopropyl) triethoxysilane (APTES). The weight ratio of APTES/PR is 0.25, 0.5, 0.75, and 1.0, with the corresponding samples known as PSi25, PSi50, PSi75, and PSi100. Individual solutions were transferred into a sealed vessel containing C-QF felt, followed by vacuum impregnation of the felt and cured from 90 °C to 180 °C for multiple hours. C-QF/PSi with densities of 0.460 to 0.515 g/cc were obtained and known as C-QF/PSi x, with x = 25, 50, 75, and 100 according to their aerogel matrix. SEM images of the C-QF/PSi quasi-layered fibrous architecture suggest that the C-QF/PSi is anisotropic in the xy and z directions. Higher magnification of the SEM image revealed the full and uniform distribution of PSi in the 3D interconnected pores of C-QF.

The authors concur that a highly porous felt was impregnated uniformly with aerogel without fracture. A detailed description of the PSi branched aerogel network using SEM, FTIR, and solid-state ¹³C and ²⁹Si NMR analyses supported phenolic cured functional groups by ¹³C NMR, while ²⁹Si NMR suggests phenolic hydroxyl reacts with the triethoxy group to form Si–O–C bonds. FTIR spectra supported many of the proposed cured functional structures. TGA analyses of PR control exhibited a char yield of 55.45% versus a value of 60.79% for PSi100. The C-QF/PSi aerogel nanocomposite exhibited good mechanical properties, such as compressive strength that varied from 12.7 to 17.01 and 5.96 to 7.51 MPa in the xy and z directions, respectively. Thermal conductivity as low as 0.112 W/(m-K) was observed. More importantly, good thermal ablation and insulative properties were observed using an oxy-acetylene torch for 300 s at a surface temperature of 2000 °C for C-QF/PSi 75 (50:50 CF/QF). No heat flux measurements were reported. Linear ablation rates as low as 0.017 mm/s and mass loss of 0.011 g/s were recorded, while the value of CF/PR (100% CF/phenolic aerogel) is as high as 0.109 mm/s and 0.057 g/s, respectively. The authors claim that better performance is observed because the mass loss of quartz fiber is less than that of carbon fiber. Carbon fibers are oxidized at temperatures above 500 °C, as contrasted to quartz fibers that begin to melt above 1600 °C and undergo loss at higher temperatures. Furthermore, the char yield of PSi hybrid aerogels is remarkably higher than PR aerogels, and the P/Si hybrid forms a firmer and stronger char layer, along with the silica layer that covers the ablated surface and blocks the outside high temperature, oxidizing oxy-acetylene flame to effectively shield the underlying material. Additionally, lower thermal conductivity C-QF/PSi 75 favors reduced heat conduction to the interior of the material and hinders pyrolysis of the aerogel surface.

The remarkably low values of linear and mass loss ablation data for the 50:50 C-QF/PSi 75 were followed up by the Harbin Institute group in a later paper by Jin et al. [32], which involves the variation of carbon fiber mass fraction of 0%, 25%, 50%, 75%, and 100% of CF to QF felt reinforcement while maintaining matrix silicone phenolic aerogel constant. The overall preparation process is shown in Figure 19. The hybrid Q-CF reinforcements were designed as reinforcements, which were needle-punched from alternating stacks of laminates of hybrid Q-CF woven fabric and short-cut fiber mesh possessing lightweight density of 0.19 g/cc and porosity from 86 to 90%. Carbon fibers were based on Toray T700-12k, and quartz fibers of 85 tex were by Hubei Feilihua Quartz Glass Co., Jingzhou, China. The resulting hybrid felts with carbon fibers exhibiting mass fractions of 0%, 25%, 50%, 75%, and 100% were designated as QF, QCF-1, QCF-2, QCF-3, and CF, respectively. A different procedure and aerogel composition were used to prepare silicon phenolic aerogel (SPA). Firstly, it involved the preparation of individual aerogels with separate preparations of micron-scale silicon aerogel, followed by a nanoscale phenolic aerogel (Figure 19).

The silicone aerogel method involved the reaction of methyltrimethoxysilane (MTMS) with dimethyldiethoxysilane (DMDES) in water/ethanol and base within a glass reactor, followed by submerging the felt fabric into the ethanol solution. The fabric is fully vacuum-impregnated; the reactor is sealed and heated to 70 °C for 14 h so that the silicone components undergo gelation, crosslinking, and aging (Figure 19b, Q-CF preform). The resulting Q-CF preform is placed in a stainless steel container that contains a premixed 20% solution of phenolic resin in ethylene glycol (EG) with 2.2% hexamethylenetetramine (HMTA) to fully impregnate the Q-CF preform. The container is sealed and heated to 120 °C to 180 °C for multiple hours, followed by replacement of EG by ethanol at RT for 72 h and drying to constant weight at 25 °C. The resulting composites were identified as QF/SPA (100% QF), Q-CF/SPA-1 (25% QF/75% CF), Q-CF/SPA-2 (50% QF/50% CF), Q-CF/SPA-3 (75% QF/25% CF), and CF/SPA (100% CF). FTIR analyses supported the cure of the phenolic resin by HMTA and attached to the silicone aerogel. The use of XPS identified peaks of Si 2p centered between 102–106 eV that corresponded to Si-O-Si and Si-O-C bonds for silicon phenolic interpenetrating aerogel matrix bonds as Si-O-C and co-filled in the hybrid Q-CF fabric.

Mechanical performance characteristics, such as tensile/flexural strengths for Q-CF/SPA, were 16.9 MPa and 11.2 MPa versus QF/SPA values of 9.74 MPa and 8.18 MPa and values of 10.11 MPa and 8.81 MPa for CF/SPA and exhibited higher strengths for the aerogel hybrid composite than either of the pure carbon or quartz fibers. Similar trends were observed regarding modulus and compressive strength. Due to the complementary mechanical properties of QF and CF, the effect of the hybrid fabric appears to utilize the strength of the CF and the flexibility of the QF to achieve efficient synergy and maximize the toughening effect of the hybrid fabric on the composite. Oxidation behavior of the Q-CF/SPA by thermal gravimetrical analysis in air from RT to 1000 °C indicated that residual weights at 1000 °C of the hybrids were lower than QF/SPA by 67% but higher for CF/SPA at 21%. The reduced 67% in residue for Q-CF/SPA is due to the excellent oxidation resistance of QF and SiO₂ produced by the oxidation of silicone aerogels, limiting heat and weight transfer into the composite. Thermal conductivity values in the z direction were 0.044–0.060 W/(m-K) and 0.178–0.589 W/(m-K) in the xy direction. The high increase in the xy direction is attributable to increased CF content. Needle punching with the flexible quartz fibers allowed the quartz fibers to perform as the main body of z-directional fibers for lower, relatively constant thermal conductivity. Thus, the synergy of low thermal conductivity and strong mechanical properties of these hybrid fiber SPA interpenetrating aerogel nanocomposites identify them as unique materials with anticipated attractive ablation behavior by the authors.

Ablation oxy-acetylene torch conditions were 3.62 MW/m² (362 W/cm²) for 30 s with a constant surface temperature of 2500 °C. The backside temperature for Q-CF/SPA-2 peaked at 103 °C, which was 35 °C and 20 °C lower than QF/SPA and CF/SPA, respectively. The linear ablation rate was 0.058 mm/s, reduced by 50.9% and 33.2%, and the mass loss rate was 0.014 g/s, decreased by 51.7% and 67.6% compared to QF/SPA and CF/SPA. Various environmental processes, such as vaporization, sublimation, oxidation, carbonization, and complexity of the evolution of the internal composite structure and feedback to the aerodynamic thermal environment, were much greater than the pure thermal conductivity model. Consequently, carbon fiber and quartz fiber synergy play an important role in improving ablation performance and overcoming thermal conductivity drawbacks during the ablation process. Analyses of the Q-CF/SPA-2 top layer of continuous white accumulated material formed on the surface of Q-CF/SPA-2 showed it was denser and more uniform than the other ablative samples. The ablation disc was cut into 12 slices. The calculated densities were determined for each of the layers. The top surface layer possessed the highest density of 0.42 g/cc due to in situ solidification and accumulation of molten substances. Densities of slices 1 to 8 increased from 0.201 to 0.342 g/cc, showing that the degree of pyrolysis decreased (density value decreased from the high surface density value of 0.42 to 0.201–0.342) with increasing depth and implied excellent thermal insulation of Q-CF/SPA-2. The 12 slices were separated into four regions with density values for each region in brackets: slice 1 identified as the surface region (0.42 g/cc), the molten layer region consisted of slices 1 to 3 (0.201–0.225 g/cc), the pyrolysis zone consisted of slices 4 to 7 (0.260–0.290 g/cc), and the virgin materials were slices 8 to 12 (0.342–0.340 g/cc). The determination of the four regions was obtained by density values of the individual slices. FTIR analysis of these regions indicated strong peaks at 1035 and 785 cm⁻¹ for Si-O and indicated that only ablation occurred on the ablated surface, as well as the molten carbonized layer. The FTIR of the pyrolysis zone and the virgin material were well preserved with the characteristic peaks of PR due to the prevailing low-temperature state. XRD patterns of the surface layer identified cristobalite (SiO₂) and carbon. The white accumulation on the surface was SiO₂, which originated from the melting of quartz fiber and the oxidation of silicon aerogel. The rigorous analysis of oxy-acetylene flamed sample into regional layers provided considerable evidence of the pyrolysis and thermal penetration into the quartz–carbon hybrid fiber felt composite with silicone–phenolic aerogel. There have been many schematic descriptions of pyrolysis and thermal penetration into a variety of thermal protective systems in the literature, especially the heritage systems based on phenolic resin matrix with various reinforcing fibers, such as carbon fiber and quartz fiber, that have identified these different regions without any confirming scientific evidence related to the depth of penetration until these reported observations of the Harbin Institute.

A comparison of linear and mass ablation rates between the Harbin Institute’s earlier 50:50 quartz fiber/carbon fiber phenolic–silica aerogel nanocomposite [31] and this recent study is shown in

Table 6. Comparison of 50:50 QF-CF exposed to oxy-acetylene flame.

					Ablation Characteristics
Component	Aerogel	Density, g/cc	Surface Flame Temperature, °C	Time, s	Linear, mm/s	Mass, g/s	Backside Temperature, °C
Q-CF/PSi-75 A	PR C	0.46–0.515	2000	300	0.017	0.011	~100 E
Q-CF/SPA-2 B	Si/PR D	0.33	2500 G	30	0.058	0.014	103 F

A = Reference [31]. B = Reference [32]. C = Mainly PR aerogel with 0.75:1 ratio of (3-aminopropyl) triethoxysilane (APTES). D = 12.1: 30.3 mass ratio of silicone aerogel to PR aerogel. E = 80 mm thermocouple distance, 300 s. F = No thermocouple distance, 100 s. G = Heat flux, 3.62 KW/m² (362 W/cm²).

.

Several differences should be noted for Q-CF/SPA, which contained two different aerogels, such as initially silicon micron-scale aerogel, followed by phenolic nanoscale aerogel, while Q-CF/PSi-75 contained a PR aerogel that was crosslinked with varying amounts of APTES. One would expect better ablation performance from the combined micro and nanoscale aerogels of [32] as compared to a solely APTES crosslinked PR aerogel of [31]. Comparison data contained in Table 6 [31,32] provide support that Q-CF/SPA-2 exhibits better ablation performance than Q-CF/PSi-75, except for the linear ablation value of 0.058 mm/s compared with the lower value of 0.017 mm/s for Q-CF/PSi-75. The higher linear ablation value may be due to the more aggressive heat flux and higher temperature conditions [32].

It is evident that the 50/50 quartz–carbon fiber hybrid felt composite with silicone–phenolic aerogel system is a significant step forward in the development of novel TPSs based on hybrid quartz–carbon fiber felt, with silicon–phenolic aerogel exhibiting extremely attractive linear and mass ablation rates, as well as backside temperatures.

3.2. Phenolic Silicon Interpenetrating Aerogels

An unusual proposed interpenetrating aerogel nanocomposite based on silicon oxycarbide (SiOC) and phenolic resin was reported by the Zhang group [33]. The objective of the Jin et al. study was the improvement of the oxidation resistance of PICA-like ablators due to the 3D interconnection of the composite microstructure of those carbon-containing materials, such as carbon fiber and phenolic resin, which are susceptible to oxidation. A unique multiscale needled carbon fiber felt reinforced by silicon oxycarbide/phenolic interpenetrating aerogel nanocomposite was proposed as follows: the needled CF felt (NCF) was reinforced by SiOC by the sol–gel in situ method on the felt. Phenolic aerogel was prepared and introduced into the voids of the felt-SiOC by vacuum impregnation and sol–gel phase separation induced by high temperature. The overall process is shown in Figure 20.

The preparation of SiOC aerogels involved the addition of methyltrimethoxysilane (MTMS) to dimethyldiethoxysilane (DMDES) in ethanol with various proportions of MTMS to DMDES with distilled water and ammonia. Needled carbon fiber felt was impregnated with the SiOC aerogel solution, placed in a sealed vessel, and heated to 70 °C in an oven for 14 h for curing and crosslinking. After drying at 100 °C, the SiCF felt aerogel was obtained. The SiCF felt aerogel was immersed in a PF, HMTA, and EG solution. The weight ratio of PR, HMTA, and EG was 1:0.075:5. The sealed vessel with an intermittent vacuum between 0.10 and 0.01 MPa was applied three times by maintaining 60 min each time for satisfactory vacuum impregnation. Cure conditions were from 120 °C to 180 °C for multiple hours. The SiCF/PR wet gel was air-dried for 72 h at 25 °C. The authors claim that the SiCF/PR aerogel method is a robust multistage strategy based on sol–gel and polymerization-induced phase separation (PIPS) methods (Figure 20a–c). The attractive features of NCF as reinforcement were the low density of 0.20 g/cc, 86.7% high porosity, excellent heat resistance, and thermal insulation in the z direction due to the anisotropic structure of NCF (Figure 20a). The SiOC was prepared in situ in the NCF via impregnation of the precursor sol, crosslinking, curing, and drying (Figure 20b). The PR sol was vacuum impregnated with the prepared SiCF, followed by in situ sol–gel polymerization reaction, curing, solvent replacement, and drying for the formation of the phenolic aerogel that finally resulted in the successful preparation of SiCF/PF with a multiscale structure (Figure 20c,d). The multiscale architecture of the SiCF/PR can be seen in Figure 20f–h. The authors maintain the hierarchical SiOC-PR interpenetrating aerogels are uniformly inserted within the NCF and infiltrate the fiber surface due to multiple vacuum impregnation procedures and the relatively small particle size of the SiOC and PR ranging from 1 µm to 70 nm.

The SiOC aerogel possesses a microscale bi-continuous framework and grape-like microstructure composed of primary silicon microspheres. The resulting aerogel network consists of micron-sized silicon particles and nanoscale phenolic gel particles, filling the gap and covering the surface of the NCF by vacuum infiltration. TGA char analyses of the SiCF/PR conducted at a rate of 5 °C/min increased with an increased amount of silane and indicated that the SiOC segment improved the thermal stability of the composites. TGA char residue (5 °C/min to 1000 °C in argon) for phenolic aerogel NCF to SiOC-PR NCF interpenetrating aerogels increased from 75.96% to 80.59%, respectively. The multiscale nanocomposites exhibit excellent compression strength properties, with values approaching 5.83 and 4.57 MPa in xy and z directions, respectively, while maintaining 81% of the maximum stress after 100 cycles and a low thermal conductivity of 0.068 W/(m-K). OTB ablation conditions involved a heat flux of 1.5 MW/m² (150 W/cm²) for 300 s with the following results: the linear ablation rate dropped from 0.0282 to 0.0109 mm/s with the introduction of the SiOC aerogel, leading to a remarkable 61.35% reduction, whereas the mass loss rates exhibited less of a decrease of 0.0186 to 0.0157 g/s. It is apparent that the SiOC greatly enhances the oxidation ablation performance of the phenolic aerogel NCF.

The Zhang group of the Harbin Institute applied a similar multiscale nanocomposite-type procedure using needled quartz fiber felt [34] rather than needled carbon fiber felt [33]. The SiOC aerogel was introduced into needled quartz fiber felt (QF) with a density of 0.20 g/cc and high porosity of 90.9% using similar SiOC aerogel conditions, as in an earlier publication [34]. The SiOC aerogel was prepared using methyltrimethoxysilane and dimethyldiethoxysilane in ethanol, followed by distilled water and ammonia. The resulting SiOC aerogel mixed solution was vacuum-impregnated into the needled quartz fiber felt, transferred into a sealed container, and oven-heated at 70 °C for 14 h for crosslinking and curing. The SiQF aerogel was immersed into a mold containing phenolic resin composition, consisting of phenolic resin (PR), hexamethylenetetramine (HMTA), and ethylene glycol (EG). The mold was sealed, cyclically vacuumed from 0.100 to 0.010 MPa, and held for 30 min. Then, the mold was heated from 120 °C to 180 °C for multiple hours, yielding SiQF/PR aerogel with interpenetrating SiOC–phenolic aerogel nanocomposite.

The authors claim that the texture of the SiQF/PR nanocomposite is uniform, and the SiOC aerogel microspheres have uniformly filled the quartz fiber felt with a typical size of 1 µm and a final density of 0.30–0.35 g/cc. Further, the PR aerogel nanoparticles are uniformly distributed in the SiQF aerogel with a size of 70 nm that completely penetrates the surfaces of the fiber and the SiOC microspheres. Hence, the multiscale network of the SiQF/PR nanocomposite is homogeneous from the macroscopic scale to the nanoscale. SEM results indicate that the particle size increased from 0.289 to 1.442 µm as the SiOC composition was increased. The SiQF/PR exhibited satisfactory mechanical properties, such as compressive strength of 4.20 and 3.34 MPa in the xy and z directions, respectively. Thermal stability of SiQF/PR via TGA (RT to 1200 °C at 5 °C/min in argon and air, respectively) exhibited char residues of 82.7% in argon and 66.3% in air. Flame-retardant performance conditions using OTB flame with a heat flux of 1.8 MW/m² (180 W/cm²) for 120 s showed a surface temperature of 1954 °C and a backside temperature of 108 °C (30 mm thermocouple depth) for QF/PR (quartz felt phenolic aerogel solely), while SiQF/PR exhibited surface temperature of 1896 °C and a backside temperature of 56 °C (30 mm thermocouple depth). The authors mention that the linear ablation rate reduces from 0.029 mm/s for QF/PR to 0.023 mm/s SiQF/PR under these modified OTB conditions. These ablation temperatures and ablation rates do not compare as favorably with the recent OTB ablation data of SiCF/PR (needled carbon fiber felt with SiOC–phenolic interpenetrating aerogel nanocomposite) reported earlier by the Zhang group.

Wang and co-workers [35] treated needled quartz fiber felt (NQF) possessing a density of 0.20 g/cc with three kinds of ceramic particles, such as ZrB₂ (500–800 nm), SiO₂ (20–50 nm), and glass melt flux ultrasonically mixed in phenolic resin. The needle quartz fiber felt was surface-impregnated with the ceramic phenolic resin mixture to a depth of 3 mm of the felt and cured at 150 °C/3 h. The upper portion of the surface-treated NQF was infused with phenolic resin (PR), hexamethylenetetramine (HMTA), and ethylene glycol (EG) solution and placed in a Teflon mold. The mold was sealed and cured from 100 °C to 175 °C for multiple hours. The gel with NQF/CR fabric was heated to 70 °C for 72 h, followed by repeated washing with ethanol for several hours to remove EG. It was dried at RT to a constant dry weight. Figure 21 shows the overall process with a lower layer containing a dense layer of ceramic resin and an upper layer containing phenolic aerogel.

The NQF/CR/PR composites possessed densities that ranged from 0.62 g/cc to 0.70 g/cc for densified and graded structures. The phenolic aerogel network was uniformly embedded on the surface of the upper portion of the quartz fiber felt. The composite is proposed as possessing an anisotropic structure due to the direction of the interior fibers. Most fibers are randomly and disorderly distributed perpendicular to the thickness direction, with only a few needle-like fiber bundles being parallel to the thickness direction. The dense layer provided ablation resistance, while the lightweight layer with phenolic aerogel maintained reduced weight and attractive thermal insulation for the composite. SEM images of Figure 21b–e) provide microstructures of NQF, dense surface layer, internal layer, and phenolic aerogel. TGA (TGA conditions: 25 °C to 1000 °C at 5 °C/min in argon) for the dense layer was 71.35% residual char, while the internal lightweight layer char was 80.69%. The dual-layer graded structure contained a dense layer impregnated with ceramic particles to improve ablation resistance, while the lightweight layer impregnating the other areas of the felt with the aerogel precursor solution maintained the “lightweight-ness” and thermal insulation of the composite. The tensile and bending strengths for the dense layer are 39.2 MPa and 57.2 MPa, values attributable to the dense layer in the xy direction. OTB data for the dual layer system (1.5 MW/m² (150 W/cm²) for 90 s) with linear and mass rates were 0.010 mm/s and 0.020 g/s, respectively, at a high temperature exceeding 1700 °C. Backside temperature peaks at 52 °C within 3 min and 127 °C within 5 min when the surface temperature exceeds 1100 °C. The resulting dual-layer quartz exhibited excellent thermal insulative and ablation-resistant properties under OTB conditions.

Studies by Wang and co-workers [36] considered a similar multi-layer composite construction as the previous paper, whereby ablation-resistant ZrB₂ and radiation-resistant SiC particles are ultrasonically stirred in phenolic resin by preparing the following samples in

Table 7. Weight ratio of ceramic particles to PR.

QFPSx	Used Ceramic Resin
	CRx	ZrB2	SiC	PR
QFPS1	CR1	10	0	10
QFPS2	CR2	10	10	20
QFPS3	CR3	10	2	12

.

The mixed ceramic particles in phenolic resin (PR) were used to prepare a surface-densified layer by surface-impregnating quartz felt (QF) possessing a density of 0.14 g/cc and needled punched in the thickness direction. ZrB₂ (0.8–1 µm) and SiC (500–800 nm) were used to impregnate the QF felt to a depth of 3 mm and cure for 4 h at 120 °C. A phenolic formulation consisting of PR to ethylene glycol (EG) was 1:5, followed by added hexamethylenetetramine (HMTA) and (3-aminopropyl) triethoxysilane (APTES) to impregnate the upper layer felt by the phenolic formulation. The total resin system with quartz felt was sealed in a Teflon mold and cured at 100 °C to 175 °C for multiple hours. The resulting composite densities varied from 0.368 to 0.400 g/cc depending on the CRx used (Table 6). TGA data were as follows: TGA conditions: 25 °C to 1100 °C under argon atmosphere, but no heating rate was reported). The dual-layered aerogel composite QFPS with CR1 or QFPS1 exhibited a residual weight of 82.1% at 1100 °C. The tensile and bending strengths of the dual-layered composites were 11.2 and 16.2 MPa, respectively, with a volumetric rebound compressive strength of 0.48 MPa. The thermal conductivity of the internal lightweight layer was below 0.03W/(m-K) at 100 °C. OTB testing (1.5 MW/m² (150 W/cm²) for 90 s) resulted in a linear recession rate of 0.003 mm/s and a mass loss rate of 0.016 g/s. A backside temperature was below 70 °C via OTB conditions for 90 s. The values for linear recession and mass loss are much lower for the dual-layer quartz felt as compared to the previous paper by the Hong group. SiC instead of SiO₂ was used in the lower layer, as well as APTES silane for co-reaction with phenolic resin, to form a higher crosslinked aerogel. Both the use of SiC and silane formed a higher crosslinked aerogel and contributed to the excellent OTB results.

Pan et al. of the Harbin Institute group [37] proposed that although the mechanical properties and ablation resistance of PICA-like materials are improved by selective ceramic additives, the radiation resistance of PICA remains to be improved. The authors developed a nano-TiO₂-coated needled carbon fiber-reinforced phenolic resin (PR) aerogel composite with low density and excellent heat-insulating and infrared-radiation-shielding performance. Figure 22 shows the procedure used by the authors.

For the titanium-coated carbon fiber composition, Figure 22b, TiCF/PR nanocomposite was prepared by a two-step method, as shown in Figure 22a–c. For the needled carbon fiber felt, a fiber with a density of 0.24 g/cc and a high porosity of 90.3% is the reinforcing agent. To enhance infrared radiation resistance, nano-TiO₂ (via hydrolysis of tetrabutyl titanate, TBOT) was in situ situated on the surface of carbon fibers after undergoing hydrolysis, nucleation, and calcination, as shown in Figure 22b. Then, the PR aerogel generated from PR, hexamethylenetetramine (HMTA), ethylene glycol (EG), and γ-aminopropyltriethoxysilane (APTES, KH-550) was prepared by the sol–gel method within the TiCF preform, with KH-550 strengthening the PR chemical bonding network, Figure 22c. The lower portion of Figure 22 shows the chemical structures of the main components and products in each preparation step. Figure 22d represents the carbon fiber structure. Figure 22e shows the transformation of TBOT, deionized water (DI), and ethanol in solution to coat the CF. The TiCF preform was calcinated at 500 °C for 2 h, washed with DI, and heated at 120 °C to a constant weight. Figure 22f shows the formation of the phenolic aerogel. The macro/micro morphologies that correspond to the different preparation stages are illustrated in Figure 22g–j. Nano-TiO₂ adhered well to CF, Figure 22g. The small TiO₂ nanoparticles (~32 nm) uniformly covered the CF surface and formed a ceramic coating after being sintered, as shown in Figure 22h and the insert. The PR aerogel covered the TiCF and filled the bulk of the felt as a porous matrix, Figure 22i. The macrophotograph of TiCF/PR is shown in Figure 22j.

The microscopic morphology of TiCF/PR and TiCF shown in Figure 23a,b exhibits the TiO₂ coating completely enclosing the CF surface, along with PR aerogel nanoparticles in (a), while (b) distinguishes solely the TiO₂ coating on CF for TiCF composition. The nano-TiO₂ is formed by a compact accumulation of nanoparticles in the size of 35 nm (Figure 23c) during hydrolysis and sintering of TBOT. Figure 23d illustrates the molar ratios of C, O, and Ti as 80%, 14%, and 3%, respectively, via the insertion of EDS and the good interfacial contact between the nano-TiO₂ coating and the phenolic aerogel. It shows the phenolic aerogel grew directly on the coating surface and filled the coating fissures. Further, the PR aerogel consisted of nanoparticle aggregates and many tiny pores due to vacuum immersion and solvation/thermal methods. SEM images shown in Figure 23e,f indicate a pore size of 20 nm^–1 µm. The CF, TiO₂ coating, and porous PR aerogel synergistically constructed the ternary TiCF/PR IR radiation-resistant composite.

The resulting aerogel possessed a low density of 0.30–0.32 g/cc, low thermal conductivity of 0.034 and 0.312 W/(m-K) in the z and xy directions, and excellent thermal stability, with 13.9% residual weight at 1300 °C in air. As expected, the TiCF/PR composite exhibited excellent antioxidant ablation and IR radiation shielding performance in a high-temperature heated environment. OTB at 1.5 MW/m² (150 W/cm²) for 150 s resulted in a linear ablation rate of 26.2 µm/s, and the mass loss rate was 8.4 mg/s with a backside temperature of 179.1 °C. The novel TiCF/PR aerogel composite with low density and excellent heat insulation met the objective of providing IR radiation resistance for thermal protective material.

To further improve TiO₂-coated needled carbon fiber felt (density of 0.20 g/cc and 85% porosity), Wang and co-workers [38] used a co-gelation method by combining tetraethylorthosilicate (TEOS) and tetrabutyl titanate (TBOT) raw materials to prepare TiO₂-SiO₂ composite aerogels through co-gelation. The authors consider the co-gelation method the most promising way to strengthen the crosslinked structure and achieve microstructural modulation, overcoming the defect of weak chemical bonding of TiO₂. TiO₂-SiO₂ provides a low thermal conductivity of 0.024 W/m-K for acceptable thermal insulation performance. The insertion of Si-O bonds (bond energy of 452 kJ/mol) in the coupling network improved the mechanical strength of the material. The co-gel method allowed the homogeneous introduction of TiO₂, and its synergistic crosslinking strategy enabled the adoption of other stronger covalent bonds in the Ti–O framework to improve the structural strength and heat resistance of the matrix. As shown in Figure 24a,b, the fabrication of TiO₂-SiO₂ was supported by the co-gel methodology (d). The combination of TEOS, TBOT, and methyltrimethoxysilane (MTMS) in aqueous ethanol with a small amount of acetic acid was carried out. Acetic acid slows down individual gelation reactions and allows the desired slow mixing and co-gelation. The resulting solution is placed in a PTFE-coated vessel. The needled CF felt was inserted into the vessel and impregnated under a vacuum of 0.1 MPa for 30 min, followed by heating from 80 °C to 120 °C for multiple hours. After curing, the sample was dried at 80 °C and calcined in a muffle furnace at 400 °C for 2 h at a heating rate of 2 °C/min.

The resulting C_f/TS was identified as C-x-t, with its microscopic structure displayed in Figure 24g, where x denotes the molar ratio of the TEOS component (0.25); t denotes the heat treatment (calcination) temperature (400 °C). The TiO₂-SiO₂ aerogel was identified as TS-x-t or TiO₂-SiO₂-0.25-400. Next, the phenolic resin composition (phenolic resin (PR), hexamethylenetetramine (HMTA), and ethylene glycol (EG)) is introduced into the PTFE lined vessel containing needled carbon felt with TiO₂-SiO₂ (C_f/TS), vacuum impregnated for 30 min, sealed, and cured from 120 °C to 180 °C for multiple hours. Removal of EG and drying at 80 °C is shown in Figure 24c,e. The final needled carbon felt with TiO₂-SiO₂ impregnated with phenolic aerogel (C_f/TS-PR) was named CP-x-t, Figure 24f, and the microstructure of the synthesized C_f/TS-PR composite is shown in Figure 24h.

Microscopic morphology of different calcination temperatures indicated that to fully utilize the antioxidant and radiation resistance of TiO₂-SiO₂ coating, a temperature of 400 °C is adequate for a suitable thickness and defect-free coating. Characterization of the C_f/TS-PR composites begins with the structure of the TiO₂-SiO₂ coating surrounding the CF structure. It remains intact while the nanoscale PR aerogel fills in the voids of the fiber skeleton and encapsulates the TiO₂-SiO₂ coating surface, as well as most crack defects facilitated by a vacuum-assisted process. FTIR showed characteristic peaks for Ti-O and Ti–O–Si bonds, indicating the incorporation of TEOS to introduce the Si–O–Si bonds to the original TiO₂ structure. It indicates that the TiO₂-SiO₂ coating with Ti–O bonds as the main network with Si–O bonds as the minor component was accomplished through the sol–gel method.

Furthermore, the C_f/TS-PR exhibited a similar pattern compared to PR, indicating that the PR aerogel inside the C_f/TS maintained its original chemical structure. The C_f/TS-PR composites exhibited desirable mechanical properties, low thermal conductivity of 0.0756 W/(m-K), remarkable thermal stability, and outstanding ablation resistance. Linear ablation rates are as low as 0.004 and 0.003 mm/s at 1.0 MW/m² (100 W/cm²) and 1.5 MW/m² (150 W/cm²) for 240 s, respectively. Mass loss rates are similarly as low as 0.006 and 0.009 g/s for 1.0 1.0 MW/m² (100 W/cm²) and 1.5 MW/m² (150 W/cm²) for 240 s, respectively. A low backside temperature of 108 °C at 120 s for a heat flux of 1.5 MW/m² (150 W/cm²) was observed when the surface temperature was 1400 °C. These outstanding ablation resistance properties of the generic C_f/TS-PR are identified with CP-0.25-400 composite ratio of CF felt with 0.25 molar ratio of TEOS and 400 °C, temperature of calcination with 20 parts of PR in the CP-0.25-400 composite. This Ti-Si binary modified carbon felt/phenolic aerogel composition exhibited the best ablation resistance to date as compared to several lightweight ablative materials reported previously by several global investigators, but with a modest heat flux of 1.5 MW/m² (150 W/cm²).

3.3. Phenolic Aerogel Without Reinforcement

Xiong and Zhang, with their co-workers [39,40], decided to strengthen the benzoxazine (Bz) precursor aerogel by the addition of inorganic nanoparticles, such as SiO₂ to Bz aerogel to improve thermal stability, mechanical properties, and ablation resistance. Two types of polybenzoxazine (PBz) aerogels containing nano-silica were prepared by different routes. One method involved using tetraortho silicate (TEOS) in the PBz solution as a silica source, hydrolyzing, and copolymerizing to form polybenzoxazine/silica aerogels (PSAs) after an ambient drying process. The other is to add chitosan chains to the PBz/silica system and copolymerize to form polybenzoxazine/silica/chitosan aerogels (PSCAs). The two aerogels exhibited a 3D nanoporous network structure, lightweight characteristics, self-extinguishing properties, thermal stability, and excellent mechanical strength. The PSAs possess better thermal stability and mechanical strength but poor thermal insulation compared with PSCAs. The PSAs and PSCAs were prepared by the introduction of silica organic phases using different chemical routes by the sol–gel method and ambient pressure drying process.

Bz was dissolved in DMF; deacetylated chitosan in ethanol/water with 2% acetic acid (only for PCSAs) was added, followed by TEOS. Subsequently, the composition was mixed, transferred to a sealed vessel, and heated to 60 °C. Once the gel formed, the solvent was exchanged with ethanol (eight times) for DMF removal and dried to obtain PSA and PSCA aerogels. The density of PSAs was 0.40 g/cc, while PSCAs were lower in density or 0.26 g/cc. The pore size distribution of the two samples is mainly in the nanoscale range of 5–35 nm. The thermal conductivity of PSAs and PCSAs are 0.064 and 0.037 W/(m-K), respectively, at RT. The compressive strength of the PSAs is reported to be 3.62–7.74 MPa, depending on the % strain. PCSAs’ compressive strength is much lower. No OTB data were reported for the PBz/silica hybrid aerogels other than self-extinguishing fire resistance performance. The PBz/silica hybrid aerogels with CF or quartz felt reinforcement are suggested as the next steps in determining whether PBz/silica hybrid aerogel has merit in the TPS area.

An improved process for phenolic aerogel preparation using combined interfacial polymerization and sol–gel conditions was reported by Wu and co-workers [41]. These studies are conducted without any reinforcing fiber or felt material. The resulting aerogels, known as the “phenolic resin aerogel water-assisted method” (PRAW), exhibited porous structural features with a mesoporous diameter of 80 nm attributable to the stacking of the thick-connected nanoparticles. The authors claim that the overall process is facile, environmentally friendly, moderate in cost, and involves a relatively short cycle, resulting in PRAWs with superior mechanical performance with compressive strength of 18.33 MPa at 50% strain and attributable to the thick-connected nanoparticles construction of the PRAWs. The authors implied that previous phenolic aerogels were relatively thin in cellular structure and moderately fragile, resulting in aerogel fracture and, ultimately, disintegrating into powder. The overall method is shown in Figure 25.

The nanostructured composite occurs through water-assisted sol–gel polymerization for the PRAWs, which has superior mechanical properties and excellent thermal insulation. The PRAWs were prepared through a phase-interface reaction like interfacial polymerization with the reaction scheme shown in Figure 25. Sodium dodecyl sulfate (SDS) and deionized water (DIW), followed by γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH-560), were mixed in a reactor, followed by the addition of phenolic resin (PR). The resulting mixture was transferred to a Teflon-lined autoclave and heated from 80 °C to 180 °C for multiple hours. The final PRAW was washed with DIW and heated at 50 °C/5 h to remove SDS. Finally, the PRAW was obtained after drying at 110 °C for 12 h. Different PRAWs are obtained depending on the amount of KH-560 added. The lower portion of Figure 25 suggests the mechanism of preparing phenolic aerogel via phase separation polymerization involves the co-reaction of phenolic resin with KH-560 and the resulting proposed structure of the PR-epoxy network.

The authors state that curing with the epoxy KH-560 endows PR with a high crosslinking density and high molecular weight, resulting in the high strength of the PRAWs. SEM of the PRAWs reveals a porous framework consisting of spherical particles with a diameter of 80 nm, forming a continuous structure of both micropores and mesopores or a “bead string” structure. Phenolic particles are homogeneous, with an average size of 90 nm. Mechanical properties of the PRAWs displayed maximum compressive strength, varying from 2.4 MPa to 18.3 MPa, when compressed to disintegration. The corresponding compressive modulus for the PRAWs is from 23.1 MPa to 78.2 MPa. According to the authors, none of the samples exhibits brittle failure due to the deformation space inside the nanopore structure of the material. Compared with “thin-connected” traditional phenolic aerogels, the “thick-connected” phenolic aerogel PRAWs can reduce stress concentration during compression. Benchmarking the performance by considering specific compressive modulus versus specific compressive strength of the “thick-connected” of the PRAWs versus other “thin-connected” phenolic aerogels reported in the literature, PRAWs exceeded all other phenolic aerogels in mechanical performance.

The authors viewed the preparation of normal organic aerogels-linked necks occurs during the condensation of polymer particles during slow gelation, increasing to a specific size through the dissolution and re-precipitation of polymer agglomerates during aging. In the current work [41], the slow polymerization of PR with KH-560 in deionized water facilitated the formation of large polymer clusters and compacted particle connections to further increase the strength of the internal skeleton structure. Thus, the resulting material, PRAW, was endowed with outstanding mechanical properties, especially PRAW with an intermediate amount of KH-560 that can be compressed by 50% without catastrophic collapse due to the favored deformation space provided in this material. Remarkably, PRAW can be dried in ambient pressure due to a combined strong nanostructure configuration and the presence of macropores, which effectively resist capillary forces during ambient pressure drying.

The thermal stability of the PRAWs was studied by TGA, and a carbon residue of greater than 55% is observed for the PRAWs and is consistent with phenolic thermal performance for TPSs. Thermal conductivity of the PRAWs varied from 0.0617 to 0.0718 W/(m-K), with PRAW (intermediate amount of KH-560) exhibiting the lowest value of 0.0617 W/(m-K). These low thermal conductivity values are attributable to the inherent porous structures and low densities of the aerogels. The superior insulation properties are based on a balance between solid-phase heat transfer through the framework and gas-phase heat transfer through the porous structure. Without reinforcing fiber or felt, no OTB ablation data are reported, except PRAW exhibited excellent dynamic thermal insulation performance for a thickness of 20 mm, as well as high-temperature resistance (1200 °C), with a final backside temperature of 57.7 °C after 5 min. It will be interesting to determine whether these attractive ablation characteristics carry over when the same chemical transformation occurs when a carbon fiber felt reinforcing component is used.

4. Carbon Aerogel

Carbon aerogels possess extremely high thermal stability to 3000 °C, and excellent thermal insulation identifies them as the most promising candidates for lightweight aerospace materials. The apparent precursor to carbon aerogels is cured phenolic resin or the closely related modified phenolic resin, such as benzoxazine resin. The use of phenolic resins or benzoxazine has been examined successfully as precursors to carbon aerogel. Papers by Lorjai et al. [42] and Seraji et al. [43] report carbon aerogels without reinforcement. Later work by Li et al. [42] involved the preparation of reinforced carbon aerogels.

4.1. Carbon Aerogels–No Reinforcement

A paper by Lorjai and co-workers [42] illustrates a method to prepare benzoxazine (Bz) aerogels, allowing the use of a source of phenolic resins that avoids the generation of water formation during condensation of phenolic resins. The Bz was transformed into an organic aerogel and then carbonized into carbon aerogel. The method avoided solvent exchange and supercritical drying process, shortening the overall process considerably. The final carbon aerogel contained a mixture of micropore and mesopore structures. Depending on the concentration of Bz in the xylene solution, a 40% Bz solution yielded a higher density of the final carbon aerogel (0.83 g/cc) as compared to a 20% solution having a 0.30 g/cc density. No thermal conductivity, mechanical properties, or OTB data were reported. Nevertheless, the method developed by the authors is more convenient than the laborious method of resorcinol formaldehyde procedure carbon aerogel involving solvent exchange and a supercritical drying process.

Seraji and Arefazar [43] explored the use of a novolac/hexamethlenetetramine phenolic resin system as the organic precursor, impregnating resin in the sol–gel polymerization and vacuum impregnation process to obtain carbon aerogels (CAs). Different solvents were used in the process: 2-propanol as a solvent for novolac sol in the sol–gel step, while acetone was used as a solvent for novolac in the vacuum impregnation process. The two types of novolac sols were placed into a pressure vessel and heated at 120 °C/5 h. The resulting wet novolac gels were dried at 90 °C/24 h and 120 °C/12 h and then carbonized at 800 °C under argon for 2 h. The CAs were vacuum-impregnated with novolac solution and solvent-dried, followed by curing. The authors refer to the resulting products as carbon aerogel-based lightweight composite ablators (CALCAs) or carbon aerogel with a cured coating of phenolic resin. The CAs possessed densities of 0.142 and 0.189 g/cc, respectively. After being transformed into CALCAs, the densities increased from 0.356 to 0.756 g/cc and depended upon phenolic volume content from 13 to 70% by vacuum impregnation. Thermal conductivity increased from 0.10 to 0.30 W/m-K with increased density. Compressive strengths varied from 3.2 to 9 MPa with densities of 0.356 to 0.756 g/cc. The CALCAs are quite brittle. OTB data (2.5 MW/m² (250 W/cm²) for 45 s) for the CALCAs led to recession rates from 0.055 to 0.117 mm/s and mass loss rate of 0.029 to 0.031 g/s for CALCAs with densities of 0.36 to 0.76 g/cc. The recession and mass loss data varied with the increasing density of the CALCAs. Backside temperatures were 175 °C at 25 mm and 125 °C at 35 mm in-depth locations.

4.2. Carbon Aerogels–Reinforcement

Li and co-workers [44] were motivated by the favorable performance characteristics of carbon aerogels for the thermal protection of aerospace vehicles due to their excellent thermal stability and thermal insulation of CA. The Li and the Tang group used a 3D chopped phenolic fiber felt as a reinforcement precursor of phenolic resin, followed by crosslinking to improve the interfacial bonding strength through a crosslinking reaction during polymerization and simultaneous shrinkage during carbonization. According to the authors, the prepared carbon/carbon composites (C/C composites) possess a medium bulk density, low thermal conductivity, and good load-bearing capability and can be formed into large components. Their specific strength (133 MPa g⁻¹ cm⁻³) is considerably higher than recently reported CA monoliths or composites.

Figure 26 illustrates the preparation method for the C/C composites. It consists of impregnation, high-pressure-assisted polymerization/curing, ambient pressure drying (APD) without solution exchange, and carbonization. Phenolic resin (PR) and hexamethylenetetramine (HMTA) were combined with ethylene glycol (EG) and involved PIPS conditions. PR was used rather than resorcinol formaldehyde (RF) for lower chain branching and steric hindrance of the RF. The 3D organic phenolic fiber (PF) felt fabricated by needle-punched chopped PF nets was used as the reinforcement to form the composite. Most of the fibers are in the xy plane in Figure 26. The number of punched fibers in the z direction is small, according to X-ray tomography (XRT) images. High pressure-assisted polymerization resulted in a more uniform and robust structure and allowed APD without repeated solvent exchange of EG with ethanol. The authors claim that the process is “green” and reasonably efficient, requiring about a week as compared to 3 weeks for supercritical drying using a resorcinol formaldehyde resin system.

Shrinkage of matrix and C/C precursor (C/C-P) during carbonization indicated that the porous carbon precursor (PCP) and PF felt exhibit quite similar thermal trends in TGA and DSC curves during pyrolysis and imply similar carbonization shrinkage behavior. The 3D XRT images of the C/C-P and C/C composites indicated that no microcracks or large voids were observed due to a “remarkable assumed good shrinkage match during carbonization”. The SEM images of the C/C composites at different magnifications indicate no microcracks are observed. Further, SEM images show the carbon particles at the fiber/matrix interface adhere firmly to the fiber surface and imply that PF has reacted with the organic matric PR. The nanoparticles range from 30 to 120 nm, suggesting a meso-macroporous network.

The resulting C/C composites with a medium density of 0.6 g/cc possess a relatively high compressive strength of 80 MPa and an in-plane shear strength of 20 MPa. These properties are summarized in

Table 8. Summary of C/C composite properties before and after heating tests [44].

No.	Properties	Before	After
1	Bulk density (g cm−3)	0.60	0.58
2	Compressive strength (MPa)	80 ± 3	96 ± 5
3	Compressive modulus (MPa)	1557 ± 86	1707 ± 92
4	Specific strength (MPa g−1 cm−3)	133	155
5	Thermal conductivity (W m−1 K−1)	0.32	0.42
6	Interlaminar shear strength (MPa)	15 ± 2	17 ± 3
7	In-plane shear strength (MPa)	20 ± 4	23 ± 5

after OTB flame heating tests conducted at 1800 °C, with a heating time near 900 s. As the temperature approached 1800 °C, the backside temperature of several thicknesses of 7.5 mm, 10 mm, and 12 mm varied from 778 °C after 700 s, 735 °C after 740 s, and 685 °C after 820 s, respectively, for the three samples. These results demonstrate the excellent ultra-high temperature thermal insulation performance of these C/C composites. These C/C composites maintain structural performance characteristics even after undergoing OTB flame heating tests at 1800 °C for nearly 900 s (Table 8), as compared to most ablative TPS, which undergo relative degrees of disintegration under similar 1800 °C OTB flame conditions for 900 s. These C/C composites appear to be exceptional in performance and retained strength after stringent OTB test conditions.

Further, the authors compare PICAs (phenolic resin-impregnated carbon ablators) as a specific type of ablative insulation versus C/C composites in their paper. They indicate that PICA provides, in a sacrificial way, thermal insulation through a phase change, endothermic chemical reaction, and material decomposition, removing heat produced in the ablation process. It is accompanied by a high linear ablation rate (>20 µm/s) and mass ablation rate. In their paper [44], the authors mention the excellent performance of the C/C composites that benefit from their characteristics of aerogel-like structure and low crystallinity (TEM of C/C composite after heating tests, Table 8, indicates the amorphous carbon matrix becomes slightly crystalline with the interlayer decreasing in nm size and consistent with XRD analyses). The shrinkage and mass loss are, respectively, less than 0.3% and 6.8%, with a corresponding thickness loss rate of 0.03 µm/s, which can be mainly attributed to their mild oxidation under the relatively enclosed test environment. Moreover, the matrix and fibers have retained their initial morphologies (SEM images before and after heating), indicating their outstanding thermal stability at ultra-high temperatures. The authors tabulated detailed information on other reported aerogels/foams or their composites with similar bulk densities, compressive strengths, and specific strengths to convince the reader of the excellent strength and ablation performance of the authors’ C/C composites or CA.

5. New Applications

5.1. Infrastructures

Fiber-reinforced polymer composites are used globally to assist in the construction and repair of a wide variety of infrastructure areas, from buildings and bridges to roads and railways. These applications, in some instances, endure longer because they are high in strength, will not rust or corrode, and provide long-term resistance with little anticipated maintenance.

According to the authors of a recent paper [45], polymeric ablative materials for infrastructures are an important topic for consideration due to the potential of possible global terrorist threats. They claim that this subject is important based on the analysis of the effects of terrorist attacks in the US, such as Oklahoma City, and on the World Trade Center, as well as other parts of the world. Some of these infrastructure materials are beginning to be used in the protection of a few important public utilities. Despite many years of use of ablative materials, there are still many unexplored areas between the indicated composition and the relative merits of thermal protective properties of ablative composites for infrastructures. Stawarz and his team conducted research on the ablation properties of thermal protective hybrid epoxy nanocomposites by developing a program encompassing a design of experiments of epoxy components and nano-TiO₂, followed by an ablation study using high-temperature heat flux, erosive and ablative material wear, back surface of sample wall temperature, and related areas. Statistical analyses of the test results concluded that cured epoxy with 5% by volume of nano-TiO₂ showed the lowest abrasive ablative loss, the best thermal stability of the virgin material, and cohesion of the ablative layer, which improves the resistance to thermomechanical stresses of the composite and ensures the formation of a passive thermal protective layer. Epoxy composition with 5% by volume nano-TiO₂ was considered the best for an incredibly low, maximum temperature on the sample wall surface, as well as the lowest value of ablation lost weight. These preliminary studies are indicative of the potential value of considering TPS materials for critical buildings to mitigate terrorist threats.

Yu and co-workers [46] described the preparation of a novel nanostructured composite aerogel involving the reaction of phenol formaldehyde monomers and tetraethylorthosilicate (TEOS) as an inorganic precursor and crosslinker in the formation of the unique aerogel. The method of preparation of the aerogel is shown in Figure 27.

The use of phenolic silicon hybrid aerogel in infrastructure applications is novel and may generate more interest in the future. In the resulting phenol formaldehyde resin/silicon dioxide (PFR/SiO₂) aerogel known as PSi-x, where x corresponds to the content of SiO₂ in the composite aerogel, the PRF and the inorganic SiO₂ constituents formed an interpenetrating binary 3D network with domain sizes below 20 nm. The highly porous PFR/SiO₂ composite aerogel is mechanically resilient and can be compressed >60% without fracture, with thermal conductivity of 24–28 W/(m-K) and considerably lower in thermal conductivity than commercial insulation materials like expanded polystyrene and glass wool. The interpenetrating binary 3D network provides outstanding fire-retardant properties and can sustain a high-temperature flame (1300 °C) without disintegration. The composite aerogel prevents the temperature of the non-exposed side from increasing above 350 °C and suggests the material can provide extended protection against fire-induced collapse of reinforced concrete structures. The authors claim that composite aerogel can be integrated with architectural materials and has the potential for architectural, aerospace, and transport use.

5.2. Ballistics

Commercial ballistic composite components are based on high-performance fibers, such as aramid or S-2 glass, combined with a phenolic resin matrix, resulting in ballistic panels used extensively in courtrooms, police stations, government buildings, and banks globally. Other ballistic components that can be considered include personnel helmets and armor vests.

A recent paper by Toader and co-workers [47] describes using hybrid polyurea–polyurethane–MWCNT (PU/PURMWCNT) nanocomposite coatings for ballistic protection. Mechanical and thermal characterization (SEM, micro-CT, TGA, DSC, DMA, and tensile tests) of PU/PURMWCNT indicated that when 0.05 to 0.2 wt% MWCNT was introduced into the resinous system, the resulting dispersions were dispersed uniformly. The optimum quantity of MWCNT was 0.2 wt% for the successful utility of the PU/PURMWCNT for ballistic evaluation as a coating on aluminum. The tensile test results of the polyurea–polyurethane nanocomposite film containing 0.2% MWCNTs is the optimal composition for ballistic protection applications since it possesses the highest deformation energy. DMA analysis also demonstrated that the 0.2 wt% material exhibited a remarkable capacity for absorbing and dissipating energy. Experimental testing in a dynamic regime (laboratory method of ballistic testing using pressure impregnation on the coated substrate) of the polyurea-coated aluminum plates showed that the polymeric layer allows the metal plate to maintain its integrity at an acceleration pressure value that is almost three times higher than the one for the uncoated metallic specimen. Synthesized nanocomposites possess unique properties that one can recommend in the modernization of ballistic protection equipment and devices.

Other non-aerospace applications include in buildings, such as fire doors, for thermal insulation (ceramic and fiberglass) and can be used in walls and roofs to reduce heat transfer; vehicles, such as passenger aircraft and race cars; and industrial processes for metal and glass manufacturing, thermal interface materials for consumer electronics, and industrial electronic systems to prevent overloads of temperature.

6. Summary and Concluding Remarks

Technical efforts continue in the R&D of polymer nanocomposite ablatives (PNCAs). These activities are amplified in various areas that have significant improvements in ablative TPS performance. Nanotechnology plays an important role in improving ablation and insulation performance if the nanoparticles are dispersed well in the host resin. Several new nanoparticles were included in this publication, such as POSS, B₄C, SiC, SiO₂, TiO₂, and ZrB₂. In particular, specific nano-ceramic additives have provided enhanced ablation characteristics, thermal resistance, reduced density, and mechanical strength as these additives are incorporated into the host resin matrix or reinforcing agent of the final TPS.

An important technical development that has spurred vigorous research activity relates to the transformation of the resin matrix, primarily phenolic resin, into an aerogel or a nanoporous material that penetrates uniformly within the reinforcing fiber surface with a corresponding particle size of <100 nm of the resulting aerogel. Several benefits are identified with the use and performance of the aerogel surfacing phenomenon, which consists of reduced density, increased thermal stability, decreased thermal conductivity, improved mechanical strength, and enhanced ablation properties, such as reduced linear recession and mass loss, low backside temperature, and strengthened char.

Table 9. Ablation characteristics of several ablative composites.

Ablative Composites (Resin Is Bolded)	Linear Ablation Rate (LAR) [mm/s] or [%]	Mass Ablation Rate (MAR) [g/s] or [%]	Method of Ablation Test and Conditions	References
Boron-modified phenolic resin (PR) with heptaphenyl POSS impregnation of silica fiber (POSSBPR4/silica fiber)	0.123 (POSSBPR4/silica) and 0.130 (control BPR/silica) mm/s	0.0602 (POSSBPR4/silica) and 0.0685 (control BPR/silica) g/s	Oxy-acetylene test bed (OTB) at 418.68 W/cm2 for 20 s	Niu et al. [4]
A constant amount of MoSi2 to boron phenolic resin and varied B4C into boron-modified phenolic resin impregnation of carbon fiber fabric	0.013 (20% B4C) and 0.0424 (no B4C) mm/s	0.0815 (15% B4C) and 0.0840 (20% B4C) g/s	Unknown OTB test conditions	Yang et al. [5]
Z pinning effect on interlaminar mechanical and ablation performance of quartz fiber/phenolic composites	No data	No data	Ablation in a flame of 920 °C for 60 s of unknown heat flux	Li et al. [6]
Polysiloxane resin impregnation of carbon fabric	−5.8, −5.7, and −4.1% for 500, 1000, and 1500 W/cm2, respectively	1.1, 1.5, and 2% mass loss for 500, 1000, and 1500 W/cm2, respectively	OTB at 500, 1000, and 1500 W/cm2 for 60 s	Hou et al. [14]
Polysiloxane resin and boron carbide filler impregnation of alumina fabrics	−0.001 to 0.089 mm/s (Table 5 for more data)	0.013 to 0.035 g/s (Table 5 for more data)	OTB at 500 and 1000 W/cm2 for 30 s	Yee [16]
Phthalonitrile (PN) resin impregnation of carbon fiber (CF/PN)	No data	No data	Butane flame method	Yang et al. [24]
Phenolic novolac phthalonitrile (NPN) blends with novolac cyanate ester (NCE) impregnation of carbon fiber	Heat of ablation (8000–12,000 cal/g)	4.77–5.77%	Radiant heating conditions (~55 W/cm2 for 1200 s) and plasma arc jet (70 and 125 W/cm2 for 50 s)	Sreelal et al. [25]
Needle carbon fiber (NCF) felt in phenolic aerogel composite	0.029 mm/s	No data	OTB at 115 W/cm2 for 33 s	Cheng et al. [28]
Low-density (0.27 g/cc) HARLEM using PIPS technology with resole phenolic/ethylene glycol/poly(vinylypyrrolidone) (PR/EG/PVP) impregnation of carbon felt	48 µm/s	No data	Arc jet testing at 540 W/cm2 for 30 s	Poloni et al. [29,30]
Low-density carbon fiber–quartz fiber needled felt (C-QF) reinforced by phenolic–silica aerogel (C-QF/PSi) nanocomposite	0.017 mm/s	0.011 g/s	Surface temperature > 2000 °C using oxy-acetylene torch for 300 s; no heat flux measurement	Cheng et al. [31]
Low-density carbon fiber–quartz fiber needled felt (C-QF) reinforced by micron silicone aerogel and nano phenolic aerogel	0.058 mm/s	0.014 g/s	OTB at 362 W/cm2 for 30 s; surface temperature of 2500 °C	Jin et al. [32]
Needled CF felt (NCF) reinforced by SiOC using sol–gel method infiltrate with phenolic aerogel	0.0282 to 0.0109 mm/s	0.0186 to 0.0157 g/s	OTB at 150 W/cm2 for 300 s	Jin et al. [33]
Needled QF felt (NQF) reinforced by SiOC using sol–gel method on the felt infiltrate with phenolic aerogel (SiQF/PR)	0.023 mm/s	No data	OTB at 180 W/cm2 for 120 s	Jin et al. [34]
Low-density (0.20 g/cc) treated needled quartz fiber felt (NQF) with three ceramic particles: ZrB2 (500–800 nm), SiO2 (20–50 nm), and glass in phenolic resin	0.010 mm/s	0.020 g/s	OTB at 150 W/cm2 for 90 s at a high temperature > 1700 °C	Wang et al. [35]
Low-density (0.14 g/cc) ZrB2 (0.8–1 µm) and SiC (500–800 nm) in phenolic resin in impregnation of QF felt	0.003 mm/s	0.016 g/s	OTB testing 150 W/cm2 for 90 s	Wang et al. [36]
Low-density nano-TiO2-coated needled carbon fiber reinforced phenolic aerogel composite	26.2 µm/s	8.4 mg/s	OTB at 150 W/cm2 for 150 s	Pan et al. [37]
Low-density and low-porosity (0.20 g/cc and 85%) TiO2-coated needled carbon fiber felt with phenolic aerogel composites through co-gelation (TiO2-SiO2)	0.004 and 0.003 mm/s at 100 and 150 W/cm2 for 240 s, respectively	0.006 and 0.009 g/s for 100 and 150 W/cm2 for 240 s, respectively	OTB at 100 and 150 W/cm2 for 240 s	Wang et al. [38]
Medium-density C/C composites (0.6 g/cc) using 3D chopped phenolic fiber felt with phenolic resin	0.03 µm/s	0.3% and 6.8%	Oxy-acetylene flame heating tests were conducted at 1800 °C for 900 s, with no heat flux measurement	Li et al. [44]

provides a good and reasonable comparison of the many TPS material systems within the review by focusing on the ablator’s composition combined with ablation data and test methods, which are the critical parameters of the review article. Table 9 summarizes the linear ablation rate (LAR), mass ablation rate (MAR), ablation test methods, and test conditions of several PNCAs.

Table 10. Thermal, mechanical, and char yield characteristics of several ablative composites.

Ablative Composites (Resin Is Bolded)	Thermal Conductivity (k) [W/(m-K)]	Mechanical Properties [MPa]	Char Yield [%] “Different Definition Used by Each Group”	References
Boron-modified phenolic resin (PR) with heptaphenyl POSS impregnation of silica fiber (POSSBPR4/silica fiber)	No data	No data	Char yield at 1000 °C increased from 68.1% to 74.2% for POSSBPR containing 20% POSS	Niu et al. [4]
Constant amount of MoSi2 to boron phenolic and varied B4C into boron-modified phenolic resin impregnation of carbon fiber fabric	No data	Flexural strengths were determined at elevated temperatures of 800–1400 °C. The best flexural strength is from BP-15 (15% B4C) sample	Highest char yield of 32.9% for the BP-15 sample	Yang et al. [5]
Z pinning effect on interlaminar mechanical and ablation performance of quartz fiber/phenolic composites	No data	Interlaminar strength was 3.01 MPa, 84% stronger than that of the control	Char residue of 14.38% at 920 °C in 20/80 O2/N2 for 60 s	Li et al. [6]
Polysiloxane resin impregnation of carbon fabric	No data	Tensile/compressive/flexural strengths of CF/UHTR were 49.6/20.5/36.1 ksi	Char yield of CF/UHTR is 93% vs. 83% of CF/phenolic (the highest char yield in this review)	Hou et al. [14]
Polysiloxane resin with boron carbide impregnation of alumina fabric	AP/UBC was 0.2–0.5 W/(m-K)	No data	Char residues of boron carbide/UBC were 70–78% in air and 87% in nitrogen; AP/UBC was 66–67.5% in air and 67.7–72% in nitrogen	Yee [15]
Phthalonitrile (PN) resin impregnation of carbon fiber (CF/PN)	No data	No data	No data	Yang et al. [24]
Phenolic novolac phthalonitrile (NPN) blends with novolac cyanate ester (NCE) impregnation of carbon fiber	Increased k from 0.11 to 0.14 W/(m-K) at 80 °C with increased PN content and reduced back wall temperature (230–260 °C)	Compressive strength increased, with PN content varying from 30 to 55%	High char yields of 75–81% at 900 °C	Sreelal et al. [25]
Needle carbon fiber (NCF) felt in phenolic aerogel composite	Low k of 0.131 to 0.230 and 0.093 to 0.183 W/(m-K) in the xy and z directions, respectively	High compressive strength of 1.48 to 11.2 MPa and 0.83 to 4.90 MPa in xy and z directions, respectively	No data	Cheng et al. [28]
Low-density (0.27 g/cc) HARLEM using PIPS technology with resole phenolic/ethylene glycol/poly(vinylypyrrolidone) (PR/EG/PVP) impregnation of carbon felt	No data	Compressive strength varied from 0.1 to 0.25 MPa	No data	Poloni et al. [29,30]
Low-density carbon fiber–quartz fiber needled felt (C-QF) reinforced by phenolic–silica aerogel nanocomposite (C-QF/PSi)	Low k of 0.112 W/(m-K)	Compressive strength varied from 12.7 to 17.01 MPa and 5.96 to 7.51 MPa in the xy and z directions, respectively	Char yield of 55.45% for control PR vs. 60.79% for PSi100	Cheng et al. [31]
Low-density carbon fiber–quartz fiber needled felt (C-QF) reinforced by micron silicone aerogel and nano phenolic aerogel	Low k of 0.178–0.589 and 0.44–0.060 W/(m-K) in xy and z directions, respectively. The increase in the xy direction is attributable to increased CF content.	Tensile/flexural strengths of Q-CF/SPA were 16.9/11.2 MPa vs. QF/SPA of 9.74/8.18 MPa and values of 10.11/8.81 MPa for CF/SPA. Exhibited higher strengths for the aerogel hybrid composite than either of the pure carbon or quartz fibers	No data	Jin et al. [32]
Needled CF felt (NCF) reinforced by SiOC using sol-gel method infiltrate with phenolic aerogel	Low k of 0.068 W/(m-K)	Excellent compressive strength of 5.83 and 4.57 MPa in xy and z directions, respectively	Char yield residues for PR NCF to SiOC-PR increased from 75.96% to 80.59%	Jin et al. [33]
Needled QF felt (NQF) reinforced by SiOC using sol–gel method on the felt infiltrate with phenolic aerogel (SiQF/PR)	No data	Excellent compressive strength of 4.20 MPa and 3.34 MPa in the xy and z directions, respectively	Exhibited char residues at 1200 °C of 82.7% in argon and 66.3% in air	Jin et al. [34]
Low-density (0.20 g/cc) treated needled quartz fiber felt (NQF) with three ceramic particles: ZrB2 (500–800 nm), SiO2 (20–50 nm), and glass in phenolic resin	No data	Tensile/bending strength for the dense layer was 39.2/57.2 MPa, attributable to the dense layer in the xy direction	Char residues in argon were 71.35% for the dense layer and 80.69% for the internal lightweight char layer	Wang et al. [35,36]
Low-density (0.14 g/cc) ZrB2 (0.8–1 µm) and SiC (500–800 nm) in phenolic resin in impregnation of QF felt	Low k of 0.0756 W/(m-K)	Tensile/bending strengths were 11.2/16.2 MPa. Compressive strength was 0.48 MPa.	Char residue of 82.1%	Wang et al. [37]
Low-density (0.30–0.32 g/cc) nano-TiO2-coated needled carbon fiber-reinforced phenolic aerogel composite	Low k of 0.312 and 0.034 W/(m-K) in xy and z directions, respectively (lowest k in this review)	No data	Char residue of 13.9% at 1300 °C in air	Pan et al. [38]
Low-density and low-porosity (0.20 g/cc and 85%) TiO2-coated needled carbon fiber felt with phenolic aerogel composites through co-gelation (TiO2-SiO2)	Low k of 0.0756 W/(m-K)	No data	No data	Wang et al. [37]
Medium-density C/C composites (0.6 g/cc) using 3D chopped phenolic fiber felt with phenolic resin	Low k of 0.32 W/(m-K)	High compressive strength of 80 MPa, interlaminar strength of 15 MPa, and in-plane shear strength of 20 MPa (more data in Table 8)	No data	Li et al. [44]

summarizes the thermal conductivity, mechanical properties (tensile, compressive, flexural, or interlaminar), and char yield or char residues of these PNCAs. Table 10 provides a “reasonable comparison of thermal, mechanical, and char yield characteristics” of several PNCAs in this review. Low thermal conductivity is generally observed for this type of TPS material. In this review, the low-density (0.30–0.32 g/cc) nano-TiO₂-coated needled carbon fiber reinforced phenolic aerogel composite has the lowest k of 0.312 and 0.034 W/(m-K) in xy and z directions, respectively. Depending on the researcher, tensile, flexural, compressive, or interlaminar strengths were reported. The majority of the researchers reported higher compressive strength of their TPS materials, which are underlined in Table 10. In this review, the highest char yield of CF/UHTR is 93% vs. the 83% of CF/phenolic reported. These resins, prominently phenolic, polysiloxane, and phthalonitrile, were used in these PNCAs and are bolded in Table 9 and Table 10. These three resins were compared with their decomposition temperature at 10% mass loss and the char yield under the same TGA test conditions (Figure 5 and Table 3).

Related to the emergence of aerogels, many of the current papers consider the use of low-density carbon or quartz or carbon–quartz felts with high porosity to duplicate or improve the success of NASA’s PICA TPS, as shown in Table 9 and Table 10. The low-density ablator R&D seems to be an increasing research area that has attracted a great deal of activity from Chinese, US, and EU researchers. It is noticed that the ablation OTB test conditions selected in these studies are relatively low heat flux from 55 to 540 W/cm², with the majority of these ablative systems tested at 150 to 180 W/cm² for 20 to 240 s, with one study at 300 s and one study at 900 s (low-density C/C composite) for extended exposure time. Oxy-acetylene test bed (OTB) is the most common ablation method.

Carbon-containing carbon aerogels possess extremely high thermal stability to 3000 °C and exhibit excellent thermal insulation, identifying them as possibly the most promising candidate for lightweight (0.3 g/cc) aerospace materials. Carbon aerogels that are transformed into carbon/carbon composite aerogels maintain structural performance characteristics even after undergoing oxy-acetylene flame heating tests at 1800 °C for nearly 900 s. These highly thermally stable compositions possess a medium density of 0.6 g/cc and a relatively high compressive strength of 80 MPa with an in-plane shear strength of 20 MPa. These medium-density C/C composite aerogels are contrasted with PICAs and other aerogel-related TPSs to distinguish their superiority as a desirable lightweight aerospace composition.

New high-temperature organic resins based on phthalonitrile and polysiloxane are described and extend the increased temperature range of organic resin matrix systems used for medium-density (1.45 g/cc) ablative systems. The silica, carbon, and alumina fiber-reinforced polysiloxane ablative systems were tested and evaluated for 500 to 1500 W/cm² for 60 s and compared favorably to the traditional fiber-reinforced phenolic ablative systems. Additional R&D of low-density flexible ablators and medium-density using polysiloxane and phthalonitrile resins is in progress by the authors to replace traditional fiber-reinforced phenolic composite systems. Several ablative manufacturers, such as Toray Advanced Composites, Park Aerospace, Hexcel, and Syensqo, are in the process of scaling up this novel ablative technology.

Novel, unexpected applications for TPS materials identify infrastructure and ballistics as potential areas of the ablative performance characteristics for the newly designated uses. Whether these new applications merit consideration based on costly economics and modest scale of manufacture of TPS material remains to be determined.

The review authors are aware of environmental issues and sustainability in material science. We are unable to comment on the scale-up activities of NASA, Airbus, or German Aerospace, who prepare large-size TPS materials for space launch systems and whatever sustainable practices they may employ. Virtually all the reviewed papers involved newly developed aerogel-type resin matrix systems with multi-step processing to arrive at the reported structural composition. Small amounts of costly materials are employed, and multi-steps are required to arrive at final materials for mechanical, thermal, and ablation testing. It is estimated that due to expense and testing, less than 500 g of starting materials, such as resins and reactants, were used in arriving at these final materials. These reported novel aerogel TPSs are very preliminary and exploratory before deciding to scale up a suitable TPS material for large-scale preparation for space exploration. We anticipate that all researchers conducted their research in an environmentally sustainable and safe procedure to develop these materials.