Thermal Protection Materials Research Articles

Structure and Nanomechanics of PPTA-CNT Composite Fiber: A Molecular Dynamics Study

Poly phenylene terephthalamide (PPTA) fiber has both high mechanical properties and low thermal conductivities, making it ideal for the design of thermal protection material in hypersonic vehicles. In this paper, the impact of CNT additions on the nanostructure and mechanical performances of PPTA fibers is investigated by coarse-grained molecular dynamics (CGMD) simulation. It can be found that CNT addition performs as the skeleton of PPTA polymer and induces a higher degree of alignment of polymers under shear deformation during the fabrication process. Both strength and Young’s modulus of the PPTA fiber can be improved by the addition of CNTs. The interaction between CNTs and PPTA polymer in PPTA fiber is important to further improve the efficiency of force transfer and mechanical performance of PPTA-CNT composite fibers.

Design of Economical and Achievable Aluminum Carbon Composite Aerogel for Efficient Thermal Protection of Aerospace.

Insulation materials play an extremely important role in the thermal protection of aerospace vehicles. Here, aluminum carbon aerogels (AlCAs) are designed for the thermal protection of aerospace. Taking AlCA with a carbonization temperature of 800 °C (AlCA–800) as an example, scanning electron microscopy (SEM) images show an integrated three-dimensional porous frame structure in AlCA–800. In addition, the thermogravimetric test (TGA) reveals that the weight loss of AlCA–800 is only ca. 10%, confirming its desirable thermal stability. Moreover, the thermal conductivity of AlCA–800 ranges from 0.018 W m−1 K−1 to 0.041 W m−1 K−1, revealing an enormous potential for heat insulation applications. In addition, ANSYS numerical simulations are carried out on a composite structure to forecast the thermal protection ability of AlCA–800 acting as a thermal protection layer. The results uncover that the thermal protective performance of the AlCA–800 layer is outstanding, causing a 1185 K temperature drop of the structure surface that is exposed to a heat environment for ten minutes. Briefly, this work unveils a rational fabrication of the aluminum carbon composite aerogel and paves a new way for the efficient thermal protection materials of aerospace via the simple and economical design of the aluminum carbon aerogels under the guidance of ANSYS numerical simulation.

Multiscale, elastic, and low-density carbon fibre / siliconoxycarbide-phenolic interpenetrating aerogel nanocomposite for ablative thermal protection

Lightweight ablators are considered to be the most potential materials for ablative thermal protection. Herein, a novel needle carbon fibre enhanced siliconoxycarbide-phenolic interpenetrating aerogel (SiCF/PR) nanocomposite with a hierarchical multiscale structure was fabricated by a robust multistage constitutive method. The as-designed multiscale lightweight (0.30–0.35 g/cm3) nanocomposite perfectly inherited its porous nanostructure and captivating properties, including notable high compressive strength (4.57–5.83 MPa), great elasticity and heat-insulation (0.068 W/(m·K)). According to its outstanding oxidation and ablation resistance, the nanocomposite resisted flame blowing of 1000 °C by butane torch for 30 min without structural disintegration, Furthermore, the linear ablation rates show a great improvement from 0.0282 to 0.0109 mm/s with the increase of siliconoxycarbide contents under the oxyacetylene flame ablation of 1.5 MW/m2 for 300 s. These properties suggest that the SiCF/PR nanocomposite is a competitive candidate for the thermal protection of reentry spacecraft and hypersonic vehicles.

Design of the Thermal Restructured Carbon-Inorganic Composite Aerogel for Efficient Thermal Protection of Aero-Engines.

The heat insulation ability and thermal stability of thermal protection materials play extremely important role in the thermal protection of aero-engines under high temperature. Herein, we design the carbon-SiO2-Al2O3 (CSA) composite aerogel through thermochemical restructuring from the phenol-formaldehyde resin-SiO2-Al2O3 (PSA) composite aerogel. This thermochemical restructured aerogel not only shows better adhesion property under room temperature but also possesses higher thermal stability and desirable heat insulation ability under high temperature. Taking the PSA-0.5 composite aerogel as an example, the compressive strain-stress test unveils that it can be compressed by 66% without catastrophic collapse, which is beneficial for the adhesion with the metallic matrix. Meanwhile, the transmission electron microscopy and scanning electron microscopy images exhibit the unbroken three-dimensional structure for the CSA-0.5 composite aerogel, which confirmed the structural stability of the composite aerogel after thermochemical restructuring. The thermal cycle test indicates that the weight loss of the CSA-0.5 composite aerogel is only ca. 8%, firmly confirming its thermal stability. Importantly, the thermal conductivity of the CSA-0.5 composite aerogel ranges from 0.024 to 0.083 W m-1 K-1, indicating the superior performance of heat insulation. Moreover, the numerical simulation is carried out to validate the thermal protection effect of the CSA-0.5 composite aerogel as a thermal protection layer. Together with laminated cooling, it could enhance the surface cooling effectiveness of the metallic matrix to above 0.8. Briefly, this work paves a new pathway for efficient thermal protection materials of aero-engines via the rational design of the thermochemical restructured composite aerogel under the guidance of ANSYS numerical simulations.

Monte Carlo Sensitivities and Performance of Woven Deployable Entry Systems

A material response model is developed for the determination of erosion rates and thermal response of woven materials to enable accurate thermal protection material sizing and future missions to Venus and giant planets. The stagnation-point thermal response is evaluated using the model developed in Icarus, a material response finite volume solver. A Monte Carlo global sensitivity analysis is further performed using a stand-alone Python wrapper script developed to determine the interactions between key parameters in the material surface energy balance of the one-dimensional simulation, such as the convective blowing correction parameter and weave material properties. The time-accurate material response for the dual-weave erosion and surface temperatures show good agreement with stagnation flat face arcjet tests involving time-varying aeroheating–cooling cycles. The high-fidelity Monte Carlo sensitivity analysis technique was used to address challenges in thermal protection sizing for the Adaptive Deployable Entry and Placement Technology (ADEPT) system, a low ballistic coefficient hypersonic decelerator. The results provide new correlations for such weaves involving carbon–air equilibrium oxidation and wall enthalpy imposition through a B-prime formalism. Through the multivariate regression analyses and uncertainty rankings performed, only two properties were found to largely contribute to the transient thermal response of the gore, with emissivity contributing up to of the total output uncertainty at the front face and at the back face.

Polybenzoxazine/Silica Hybrid Aerogels for Thermally Insulating Ablation

Lightweight microablation thermal protection materials are one of the crucial factors contributing to the rapid development of next-generation aircrafts. However, the maintenance of a high mass residual and long-term antioxidant capacity of the matrix after bearing high-temperature aerobic environments remains a major challenge. Herein, we put forward a strategy for constructing polybenzoxazine/silica hybrid aerogels depending on the introduction into silica inorganic phases by different technologic preparation routes, possessing excellent thermal insulation, superior self-extinguishing properties, and outstanding thermal stability. In detail, the mass residual rate of polybenzoxazine/silica aerogels (PSAs) as-prepared could reach up to 40.19%, mainly due to the reinforced networking structure by the introduction of silica that would be preserved well due to the inorganic phase nature in the high-temperature oxidizing environment. The pore size of polybenzoxazine/silica/chitosan aerogels (PSCAs) is mainly distributed within 5–35 nm, which contributes to obtaining a low thermal conductivity (0.037 W m–1 K–1) due to the pore size being smaller than the mean free path of stationary air at normal temperature and pressure. PSAs and PSCAs both exhibit excellent self-extinguishing properties, attributed to the presence of a large number of aromatic ring structures and the introduction of the silica inorganic phase in polybenzoxazine itself. The microscopic morphology, crystalline shape, and Si-related chemical bonding of PSAs did not significantly change after muffle thermal treatment, including the three-dimensional network structure composed of polybenzoxazine and silica. This study provides a kind of approach for designing a thermal protection material matrix with high mass residual and excellent thermal insulation performance in the aerospace field.

Transport properties for neutral C, H, N, O, and Si-containing species and mixtures from the Gordon and McBride thermodynamic database

Accurate transport properties of non-ionized gas mixtures of C, H, O, N, and Si-containing species at temperatures up to 4000 K are essential in many scientific fields. Mixture transport properties are computed through the solution of linear transport systems, requiring collision integrals as functions of temperature for each binary collision pair in the mixture. Due to the dimensionality of the problem, no such database exists for all the 180 hydrocarbons and silicon species detailed in the nine-coefficient polynomial thermodynamic database of Gordon and McBride, widely used in many applications. This constraint was overcome by using a phenomenological inter-molecular potential energy surface suitable for transport properties, which describes the pair interaction approximated with two fundamental species physical properties, namely the dipole electric polarizability and the number of effective electrons participating in the interaction. These two parameters were calculated with ab initio quantum chemistry calculations, since they were not always available in literature. The studied methodology was verified and validated against other approaches at a species and collision integral level. Transport properties for a variety of equilibrium mixtures, including planetary atmospheres and chemical compositions of thermal protection materials relevant to aerospace applications, were calculated, assessing the predictive capabilities of this new database.

Sensitivity Analysis of the Catalysis Recombination Mechanism on Nanoscale Silica Surfaces.

A deep understanding of surface catalysis recombination characteristics is significant for accurately predicting the aeroheating between hypersonic non-equilibrium flow and thermal protection materials, while a de-coupling sensitivity analysis of various influential factors is still lacking. A gas–solid interface (GSI) model with a hyperthermal flux boundary was established to investigate the surface catalysis recombination mechanisms on nanoscale silica surfaces. Using the reactive molecular dynamics (RMD) simulation method, the effects of solid surface temperature, gas incident angle, and translational energy on the silica surface catalysis recombination were qualified under hyperthermal atomic oxygen (AO), atomic nitrogen (AN), and various AN/AO gas mixtures’ influence. It can be found that, though the Eley–Rideal (E–R) recombination mechanism plays a dominant role over the Langmuir–Hinshelwood (L–H) mechanism for all the sensitivity analyses, a non-linear increasing pattern of AO recombination coefficient γO2 with the increase in incident angle θin and translational energy Ek is observed. Compared with the surface catalysis under hyperthermal AO impact, the AN surface adsorption fraction shows an inverse trend with the increase in surface temperature, which suggests the potential inadequacy of the traditional proportional relationship assumptions between the surface adsorption concentration and the surface catalysis recombination coefficient for other species’ impact instead of AOs. For the incoming bi-component AO/AN gas mixtures, the corresponding surface catalysis coefficient is not the simple superposition of the effects of individual gases but is affected by both the intramolecular bond energies (e.g., O2, N2) and intermolecular energies (e.g., Si/N, Si/O).

Oxidation Behavior of Carbon Fibers in Ceramizable Phenolic Resin Matrix Composites at Elevated Temperatures.

Carbon fiber fabric-reinforced phenolic resin composites are widely used as thermal protection materials for thermal protection systems in hypersonic vehicles and capsules. In this work, carbon fiber fabric-reinforced boron phenolic resin composites modified with MoSi2 and B4C were prepared via a compression molding technique. The high-temperature performance of the composites as well as the oxidation behavior of the carbon fibers was studied. The results indicate that the incorporation of B4C improves the performance of composites at high temperatures. The residual weight rate of composites with 15 phr B4C (BP-15) sufficiently increased from 23.03% to 32.91% compared with the composites without B4C (BP-0). After being treated at 1400 °C for 15 min, the flexural strength of BP-15 increased by 17.79% compared with BP-0. Compared with BP-0, the line ablation rate and mass ablation rate of BP-15 were reduced by 53.96% and 1.56%, respectively. In addition, MoSi2 and B4C particles had a positive effect on the oxidation of carbon fibers in the composites. After treatment at 1400 °C, the diameter of the as-received carbon fiber was reduced by 31.68%, while the diameter of the carbon fiber in BP-0 and BP-15 decreased by 15.12% and 6.14%, respectively. At high temperatures, the liquid B2O3 from B4C and MoSi2-derived complex-phase ceramics (MoB, MoB2, Mo2C, Mo4.8Si3C0.6) acted as an oxygen barrier, effectively mitigating the oxidation degree of the carbon fibers.

Micro-fracture behaviors of needled short-chopped fiber reinforced phenolic aerogel composites based on in-situ X-ray micro-CT

Needled short-chopped fiber reinforced phenolic aerogel composites (PAC) are the most promising thermal protection materials for space applications. However, the complex microstructures deriving from the irregular needling yarns severely limits the revealing of their fracture mechanisms. Herein, for the first time, the micro-fracture behaviors of PAC are innovatively revealed using in-situ X-ray micro-CT under tensile loading. Furthermore, the high-precision finite element analysis of stress evolution is realized by establishing PAC model based on the micro-CT slices. The results indicate that the needling yarns will result in the enrichment of resin aerogel, leading to the stress concentration and micro-cracks initiation during loading. And the needling yarns can effectively hinder the separation and orientation of short-chopped fibers and retard the stress transmission between fibers and matrix, which are the main strengthening mechanisms of PAC. The present results will give references for revealing the fracture mechanisms of composites enhanced by needled fibers.

Thermal Conductivity Evolution of Carbon-Fiber Ablators Submitted to High Temperatures

Carbon-phenolic ablators are widely used as thermal protection materials for atmospheric entry capsules. The knowledge of their thermophysical properties is therefore of primary importance to correctly simulate their thermal response, eventually reducing design safety margins without the risk of compromising the integrity of the underlying vehicle structure and payload. In this work, we present the results of an experimental work that aimed to evaluate the effective thermal conductivity of the carbon-phenolic ablator ZURAM® in its charred state. The material thermal conductivity was obtained, up to 2790°C, using the available heat-capacity law for this material, ad hoc laser flash analysis thermal diffusivity measurements, and material density estimates. The results suggest a strong impact of the material heating history on the thermal conductivity. As a consequence of this hysteresis-like behavior, measurements taken during the heating or the cooling phases differ sensibly, with discrepancies far beyond the quantified experimental uncertainties. Heating-phase measurements follow a specific trend and could be reduced to a fitting polynomial law with associated error bounds. Measurements performed during the cooling phase seem to be affected by a modification process of the material intrinsic properties that is speculated to be related to the graphitization of the carbon fibers composing the material.

Multiscale Modeling of Gas–Solid Surface Interactions Under High-Temperature Gas Effect

The high-temperature gas effect is critical to the design of high-speed aircraft in terms of the thermal protection at their surfaces. However, studies on this topic are challenging for both experiments and numerical simulation. To cope with the breakdown of continuum methods and the extremely high cost of molecular methods, a multiscale model and simulation method for flow, diffusion, adsorption/desorption, reaction, and heat transfer is developed to understand the gas effect based on hard-sphere/pseudo-particle modeling (HS-PPM). The model was first used to describe the catalytic reactions of high-temperature gas without flow on the surface of copper, tungsten, platinum, nickel, silicon dioxide, etcetera. The recombination coefficients of oxygen and nitrogen atoms obtained at different temperatures are in good agreement with the existing simulation and experimental results, which verified the effectiveness and accuracy of the approach. The approach is also capable of simulating the complex physical and chemical processes in the gas flow near the silicon dioxide surface, with the critical flow velocity slip and temperature jump reasonably reproduced. These studies demonstrated that HS-PPM can be a comprehensive virtual experimental tool for the study of surface catalytic characteristics of thermal protection materials in a hypersonic environment and accurate prediction of the aerodynamic thermal behavior there.

Plasma Torch Testing of Thermal Protection Materials (TPMs)

This article presents an introduction to ground testing of thermal protection materials (TPMs) used to protect spacecraft and hypersonic vehicles. These test facilities must simulate the aerothermodynamic environments expected during flight. Often these tests have been done in arc jet facilities. However, inductively coupled plasma (ICP) torches can generate the relevant test flow field and have advantages. Guidance for using these facilities for thermal protection material performance verification and flight qualification is provided.

Novel strategy and multi-scale modelling of integrated multifunctional composite for thermal protection under extreme environment

In order to overcome the shortcomings of the heritage thermal protection materials with only a single function as well as the macroscopic evaluation method lacking the consideration of manufacture parameters, in this study, a novel strategy of integrated multifunctional composite with an overall 3-Dimensional reinforcement weave fabric and multifunctional zones was prepared, and a new multi-scale computational modelling for evaluating its thermal protection performance was established. The properties of constitute materials were characterized, as well as the thermal and ablative behavior of the integrated multifunctional composite were tested by oxyacetylene test. The new modelling took the detail weaving parameters as well as the physical and chemical mechanisms of the composite into consideration. The model was solved by our FORTRAN codes. The experimental and simulation results showed that this new integrated composite had high structural efficiency, thermal insulation and recession resistance. The multi-scale computational modelling can be applied on the evaluation of the performance of the integrated woven composite in a bottom-up manner, which gives a refined design method for thermal protection materials.

Inductively Coupled Plasma Facilities for Testing of Thermal Protection Materials

This is an overview of existing facilities for testing of thermal protection materials (TPM) for spacecraft based on the inductively plasma (ICP) sources with power over 100 kW. As a chief designer of the ICP plasma torch systems, the author provides the main parameters, describes design of the torches and power supplies, analyzes efficiency of plasma generation, and discusses trends in the ICP-based facilities development with power from 100 kW to 1 MW. The overview is based on data from the open sources and real parameters of APT-100 and −200 plasma systems currently in production by Applied Plasma Technologies, Corporation, Marshall, VA, USA.

Enhancing ductile removal of Cf/SiC composites during abrasive belt grinding using low-hardness rubber contact wheels

Cf/SiC composites are used as advanced thermal protection and friction materials. However, machining these materials is difficult because of their hard, brittle, anisotropic, and heterogeneous characteristics. This study investigated the removal behavior and surface integrity of Cf/SiC composites during abrasive belt grinding using rubber contact wheels of various hardness. Additionally, detailed analysis was performed on their thermal-mechanical coupling characteristics, surface integrity (that is, surface roughness, surface micro morphology, and subsurface damages), and the grinding chips produced. Results revealed that with decreasing hardness of the contact wheel, the surface roughness in all directions, grinding force, and temperature decreased significantly. Moreover, the surface removal morphology of the Cf/SiC composites changed from macro-fracture to micro-fracture, and the subsurface morphology changed from SiC matrix cracking and carbon fibers pull-out to matrix plastic flow and fiber micro-fracture, respectively. Furthermore, strip chips with plastically squeezed and cut surfaces were visible in the grinding chips obtained under the 40-HA contact wheel. Therefore, the ductile removal behavior of the Cf/SiC composites was enhanced, and the surface quality in abrasive belt grinding with low-hardness contact wheels was markedly improved.

Influences of gas permeation with adsorption effect on the transient thermal insulation performance of silica aerogel at pressure and temperature differences

Silica aerogel is a thermal protective material for an outer space launch under aerodynamic heating, significant temperature difference, and pressure gradient conditions. This paper proposes a gas permeation model by considering the gas adsorption effect for silica aerogel under pressure differences. The gas (N2) adsorption behavior of silica aerogel is simulated by the grand canonical Monte Carlo method. This simulation provides the concentration of the adsorbate gas layer and the average thickness of adsorbate gas in the nanopore. An unsteady-state heat transfer model within silica aerogel, coupling with the acquired gas diffusion effect, heat conduction, and thermal radiation, is developed to study the thermal insulation performance. The gas permeation with the gas adsorption inside silica aerogel exerts a prominent influence on the dynamic temperature response of the hot surface. At the temperature of 77–90 K, the gas adsorption has a remarkable impact on the gas permeation within silica aerogel, which would finally affect the energy migration. The order of the coefficient changes from 10−14 to 10−10 m2. In contrast, the adsorption effect is negligible at the temperature of 298–1300 K. When the gas diffusion and heat transfer directions are opposite, gas diffusion will impede heat transfer, and thus the thermal insulation performance will be improved up to 88.94% and 25.65% at unsteady- and steady-state, respectively.

Mechanical Properties and Microstructure of Hot-Pressed Silica Matrix Composites.

Silica is one of the most widely used ceramics due to its excellent chemical stability and dielectric property. However, its destructive brittle nature inhabits it from wider application as a functional ceramic. An improvement in toughness is a challenging topic for silica ceramic, as well as other ceramics. In the paper, silica ceramic with different types of boron nitride powders and alumina platelets was fabricated by hot-pressing. Introduction of the additives had great influence on the composites’ mechanical properties and microstructure. The silica matrix composite containing micro-sized boron nitride powders possessed the best mechanical properties, including the bending strength (134.5 MPa) and the fracture toughness (1.85 Mpa·m1/2). Meanwhile, the introduction of alumina platelets combined with boron nitride nanosheets achieved an effective enhancement of fracture toughness while maintaining the bending strength. Compared with the monolithic silica, the composite with simultaneous addition of alumina platelets and boron nitride nanosheets had a fracture toughness of 2.23 Mpa·m1/2, increased by approximately 27% (1.75 Mpa·m1/2). The crack deflection and platelet pullout were contributing to enhancement of the fracture toughness. The improved mechanical properties, combined with the intrinsic excellent dielectric and chemical properties, make the silica matrix composites promising wave transparent and thermal protection materials.

Comprehensive performance of multifunctional lightweight composite reinforced with integrated preform for thermal protection system exposed to extreme environment

The novel multifunctional lightweight composites (MLCs) reinforced with integrated preform were prepared for thermal protection system (TPS) of hypersonic vehicles in extreme aerothermodynamic environment. The high-temperature behavior including thermal physical properties decomposition performance, and the ablation resistance were tested by the oxy-acetylene torch system, as well as the micro-morphologies of MLCs from non-destructive testing were investigated. An integration factor was proposed to evaluate the comprehensive performance of MLCs. The results show that MLCs had high structural efficiency without delamination and defelting, excellent thermal insulation performance, and superior ablation resistance ability. The comprehensive performance of MLCs had obvious advantages to polymer-matrix composites (PMCs). The novel MLC reinforced with integrated preform can help for the development of thermal protection materials of hypersonic aircrafts.

Numerical study of heat and moisture transfer in thermal protective clothing against a coupled thermal hazardous environment

The objective of the study was to examine the effect of heat and moisture transfer on steam burn under a coupled thermal hazardous environment. A coupled model on the heat and moisture transfer between heat source, protective clothing and air gap was proposed. The developed model considers the non-transient equilibrium between three phases, the impinging jet flow between steam nozzle and fabric, the steam flow within the fabric induced by the pressure gradient, the dynamic moisture absorption, and the possible phase changes. The model was validated by a test apparatus of thermal protective performance. The results demonstrated that the developed model presented the potential for reasonably accurate prediction of the skin temperature and the steam burn in the coupled thermal environment. For the dry heat exposure, the radiant heat transfer absorbed by the skin was the main reason causing the skin burn. However, the existence of hot steam greatly reduced radiant and conductive heat transfer. The moisture phase change on the skin surface was the primary reason of the skin burn, indicating the occurrence of skin steam burn in the wet heat exposure. The research findings provided theoretical foundation and instruction for development of new thermal protective material and scientific selection of thermal protective equipment, which would be of great importance and necessity on safety improvement of fire-fighting operation.