Thermal Protection System Applications Research Articles

Flow and heat transfer characteristics of hydrocarbon fuel in lightweight C/SiC regenerative cooling combustor

To meet the requirements of higher thermal protection ability and lower structural weight of regenerative cooling structures in advanced hypersonic vehicles, carbon-fiber-reinforced silicon carbide (C/SiC) composites with high permittable service temperatures and low densities are promising for application in thermal protection systems. In this study, four cylindrical regenerative cooling configurations applying C/SiC composites for a designated scramjet engine combustor are proposed, and the thermal protection performance under the influence of the cooling equivalence ratio (0.4∼1.0) and anisotropic thermal conductivity is studied. The results show that the C/SiC composites can significantly improve the heat resistance of the entire regenerative cooling structure owing to their relatively low thermal conductivity, which reduces the total heat flux transferred to the inner structure. The thermal protection ability can be maintained, whereas the coolant flow rate is reduced by 50%. C/SiC composites with higher thermal conductivities in the circumferential direction are beneficial for uniform temperature distribution. A flow distribution deterioration phenomenon caused by the thermal cracking of the coolant and the increasing flow resistance in the cooling channels can be observed under certain conditions, leading to a mass flow rate difference of 2.5 times among the cooling channels. Moreover, introducing C/SiC composites into the regenerative cooling structure design can significantly reduce the total structural weight by over 50% compared to traditional superalloys.

High-purity rayon-based carbon fiber felt prepared by halogen gas purification for superior thermal insulation and oxidation resistance

High-purity rayon-based carbon fiber (CFRay) felt holds significant promise for applications in thermal protection systems and semiconductor crystal growth furnaces. This study employs halogen gas purification and high-temperature purification techniques in the fabrication of high-purity CFRay felt. Additionally, the influence of the purification process on the thermal insulation and oxidation resistance of CFRay felt is investigated. The glow discharge mass spectrometer analysis reveals that purification at a lower temperature of 2300 °C achieves a purity level of 6.9 ppm, surpassing both purified CFRay felt without halogen at 2300 °C (56.1 ppm) and high-temperature treated CFRay felt at 3000 °C (10.4 ppm). Thermal insulation and dynamic-static oxidation resistance tests show that the properties of CFRay felt are concurrently affected by the heat treatment temperature. Compared to high-temperature purification, halogen gas purification operates at lower temperatures, preventing the graphitization of CFRay, thereby ensuring high purity while enhancing thermal insulation and demonstrating relatively superior oxidation resistance. This study offers novel insights into the preparation and performance adjustment of high-purity CFRay felt.

Effects of thermal plasma on the microstructure and mechanical behavior of alumina-mullite based oxide/oxide ceramic matrix composites

Oxide/oxide ceramic matrix composites have potential applications in thermal protection systems due to their ability to withstand very high temperatures while maintaining their load bearing capability and oxidation resistance. Under hypersonic flight conditions vehicle components may be exposed to temperatures above the melting points of alumina and mullite, as well as to chemically active ionized gases. Therefore, it is necessary to understand the response of these CMCs to not just thermal loads, but also the added challenges of plasma environments. In this study, alumina-mullite based oxide/oxide CMCs were exposed to an inductively generated plasma jet for 30 min at a heat flux of 24 W/cm2 in near vacuum conditions. Resulting microstructures were characterized with XRD and SEM/EDS. Silicon rich, sub-micron globules were observed on the surface of the impinged region. Also, the first 5 μm below the surface were enriched with silicon. The back surface showed no silicon enrichment but did have a granulated surface texture in comparison to as manufactured samples. Short beam strength tests were performed as a measure of structural performance. Differences in the overall response and short beam strength were noted between the as manufactured and plasma exposed cases.

Pre-oxidized mesophase pitch modified phenolic composites with mosaic-structured char layers and excellent ablation resistance

Currently, phenolic resin (PR) is widely used as a matrix for the polymer-based ablative materials in aerospace industry. In this paper, pre-oxidized mesophase pitch (OMP) modified boron phenolic composites (OMPBPR) and their fiber-reinforced composites were prepared. PLM and SEM analyses showed that the OMP can co-carbonize with resin matrix to construct a mosaic-structured char layer incorporated pitch-based graphitic carbon during ablation. Furthermore, mercury intrusion porosimeter, XRD, Raman and TEM analyses showed that the inlaid graphitic carbon can effectively hinder the growth of defects in amorphous carbon (AC) framework, while improve the graphitization degree of char layer. The results demonstrated that the linear and mass ablation rates of OMPBPR were optimally reduced by 109% and 27.1% compared to the counterpart, and the linear ablation rate of fiber-reinforced composite was also optimally reduced by 65.5%. The above results indicate that the novel OMPBPR-based composites are promising materials for application in thermal protection systems.

Medium and high-entropy transition mental disilicides with improved infrared emissivity for thermal protection applications

• The medium and high-entropy transition mental disilicides present significantly reduced electrical conductivities. • The medium and high-entropy transition mental disilicides are promoting infrared emissivity materials for thermal protection applications. • The mechanism for the reduced thermal conductivity and high infrared emissivity are disclosed. Transition metal disilicides are widely used as heating elements and infrared emission coatings. However, the limited intrinsic infrared emissivity and high thermal conductivity are the main limitations to their applications as infrared emission coatings in the thermal protection system. To cope with these problems, four medium and high-entropy transition metal disilicides, i.e., (V 0.25 Ta 0.25 Mo 0.25 W 0.25 )Si 2 (ME-1), (Nb 0.25 Ta 0.25 Mo 0.25 W 0.25 )Si 2 (ME-2), (V 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2 )Si 2 (HE-1), and (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2 )Si 2 (HE-2), were designed and synthesized by spark plasma sintering method using transition metal binary disilicides as precursors. The introduction of multi-elements into transition metal disilicides not only improved the infrared emissivity but also reduced the electrical and thermal conductivity. Among them, (Cr 0.2 Nb 0.2 Ta 0.2 Mo 0.2 W 0.2 )Si 2 (HE-2) had the lowest electrical conductivity of 3789 S cm –1 , which is over one order of magnitude lower than that of MoSi 2 (50000 S cm –1 ), and total infrared emissivity of 0.42 at room temperature, which is nearly double of that of TaSi 2 . Benefiting from low electrical conductivity and phonon scattering due to lattice distortion, the medium and high-entropy transition metal disilicides also demonstrated a significant decline in thermal conductivity compared to their binary counterparts. Of all samples, HE-2 exhibited the lowest thermal conductivity of 6.4 W m − − 1 K − − 1 . The high-entropy transition metal disilicides also present excellent oxidation resistance at high temperatures. The improved infrared emissivity, reduced thermal conductivity, excellent oxidation resistance, and lower densities of these medium and high-entropy transition metal disilicides portend that they are promising as infrared emission coating materials for applications in thermal protection systems.

Oxidation and ablation behavior of a ceramizable resin matrix composite based on carbon fiber/phenolic resin

A ceramizable resin matrix composite with an alternating distribution of modifying fillers, namely borosilicate glass, Si, and ZrSi 2 , was designed based on carbon fiber/phenolic resin composite. Polycarbosilane was introduced as an interface layer. The oxidation and ablation behavior of the ceramizable resin matrix composite were studied. The results show that the strength of the composite after oxidation at 1200–1500 ℃ for 1800 s was around 100 MPa. The ablation resistance and thermal insulation of the composite are improved simultaneously due to the synergistic effect among borosilicate glass, Si, and ZrSi 2 . This result is attributed to the decrease in the discharge rate of pyrolysis gas through a two-step reaction and generation of ceramics with a high temperature and ablation resistance. These results indicate that the proposed composite is a promising low-cost material with a good strength and a good ablation resistance for application in thermal protection systems.

Quantification of complex protective surface oxide layer formed during plasma jet exposure of multicomponent ultra-high temperature carbides

Multi-component ultra-high temperature ceramics (MC-UHTCs) has garnered attention due to their improved thermo-mechanical and oxidation properties. MC-UHTC was progressively formed by adding a UHTC component at each step, producing binary, ternary and quaternary UHTC systems, processed by spark plasma sintering (SPS).We investigate the role of multi-elements and thermochemical stability of the surface oxide layer formed during plasma arc jet exposure (T > 2500 °C) of MC-UHTCs using core-level X-ray photoelectron spectroscopy (XPS) correlated with the structural investigation. The results indicate that adding a UHTC component improves oxidation resistance due to the assemblage of refractory phases forming complex mixed oxides via preferential oxidation. XPS spectra of Ta4f, Nb3d, Ti1p, and Hf4f showed that the nature of the chemical bonds switched from metal carbides to metal oxides (and metal oxy-carbides) in the binary system with retained metal carbides in ternary and quaternary UHTCs. The absence of a metal carbide peak in the high-resolution C1s scan of the binary system also supports this finding. The principal complex mixed oxide phases emanated in the exiguous oxidation of MC-UHTCs are quantified using Rietveld analysis. Overall, this study suggests that MC-UHTC is a promising material for possible application in thermal protection systems (TPS).

Lightweight, flexible, and heat‐insulated phenolic impregnated carbon ablator (PICA) with adjustable flexibility and high compressive resilience property

AbstractPhenolic impregnated carbon ablator (PICA) as a typical lightweight ceramic ablative material is widely used in thermal protection systems (TPS). However, the rigidity and poor mechanical property of the existing PICA limit its application in flexible TPS. For the purpose of solving these problems, a novel lightweight, flexible, and heat‐insulated PICA material is first successfully fabricated using resorcinol‐formaldehyde (RF) aerogel as matrix and flexible carbon fiber felt as reinforcement through vacuum impregnating and ambient pressure drying. The prepared composites have relatively low densities (0.20 ~ 0.27 g/cm3) and low thermal conductivities (~0.11 W/[m·K]) at room temperature. The flexible PICA has adjustable flexibility through changing the RF concentration of impregnated sols and exhibits excellent folding endurance, which can be folded more than 500 times without fracture. The material can recover to its original shape without fracture when the compressive strain is up to 60%, which shows high compressive resilience properties. The result of the back‐face temperature test indicates the prepared flexible PICA composite has improved thermal insulation property at high temperature by impregnating with RF aerogels. These results demonstrate the flexible PICA prepared in this work is a promising material that can be used in the flexible thermal protection field of aerospace.

Influence of high-enthalpy atmospheric plasma spraying power on oxidation resistance of ZrB2-LaF3 coatings

Abstract Good oxidation resistance of ZrB2-based coatings is highly desirable for successful high-temperature applications in thermal protection systems. Herein, ZrB2-LaF3 coatings with different microstructure and morphology were prepared by high-enthalpy atmospheric plasma spraying. Particular attention was paid to the effect of spraying power on the physical properties and oxidation resistance of as-sprayed coatings. According to a thorough analysis, spraying power exerts influence on the melting degree and travelling speed of ZrB2-LaF3 agglomerates in the flame during spraying process, thus affecting the microstructure, micro-topography and porosity/density of the coatings, and conclusively, leading to the difference of oxidation resistance. The coatings produced by the lower spraying powers (35 and 55 kW) revealed poor quality and the high degree of porosity with a large number of unmolten particles. Furthermore, these underwent severe oxidation due to the diffusion of oxygen through the pores. On the other hand, the use of excessive power (95 kW) deteriorated the quality of the coating due to drastic residual stress. In turn, the moderate spraying power (75 kW) was found to be optimum for achieving the coating with desirable characteristics, namely, fully molten particles at low porosity and residual stress, being thus beneficial for improving its oxidation resistance. This discovery is therefore beneficial for the practical applications of ZrB2-LaF3 coatings.

Low temperature densification mechanism and properties of Ta1-xHfxC solid solutions with decarbonization and phase transition of Cr3C2

As a novel member of the ultra-high temperature ceramic family, which have extremely high melting points and remarkable hardness, Ta1-xHfxC solid solution ceramics are promising for applications in thermal protection systems. Ta1-xHfxC (x = 0, 0.2, 0.5, 0.8, and 1.0) with 2 vol% Cr3C2, were densified up to 98.8% at 2000 °C using a two-step spark plasma sintering process. Effect of Cr3C2 on the linear shrinkage of Ta1-xHfxC was investigated. Possible ‘eutectic’ reaction within TaCCr3C2 ceramic was inferred to contribute to the shrinkage at 1462 °C. High-angle annular dark-field scanning transmission electron microscopy combined with energy-dispersive spectroscopy was employed to further confirm the mutual diffusion between rock-salt structured ‘CrCx’ and TaC. Flexural strength, fracture toughness and Vicker’s hardness of Ta1-xHfxC ceramics were in the range of 439–492 MPa, 4.0–5.8 MPa∙m1/2 and 14.9–19.9 GPa respectively. The coefficient of thermal expansion (in the temperature range of 25–1000 °C) and thermal conductivity (at 1000 °C) of Ta1-xHfxC varied from 7.17 to 7.51 × 10−6 K−1 and 31.9–42.9 W/m·K, respectively. The high-temperature strength of Ta0.5Hf0.5C decreased to 165 MPa up to 1600 °C, approximately 34% of room-temperature strength, and a ‘zig-zag’ load-displacement curve was observed.

Oxidation and ablation behaviors of carbon fiber/phenolic resin composites modified with borosilicate glass and polycarbosilane interface

In this work, modified carbon fiber-reinforced phenolic resin composites (CPR) were prepared. To improve the high temperature performance of CPR, borosilicate glass with different concentrations was incorporated and polycarbosilane (PCS) was added as an interface layer. The oxidation and ablation behaviors of the modified CPR composites were studied. It was found that the borosilicate glass filler changed the oxidation mechanism of composites at temperatures higher than 1000 °C, and caused the volatilization of borosilicate glass in addition to the oxidation of carbon. The strength of composites after oxidation was significantly improved via this modification. The thermal conductivities of composites decreased with the increase in content of fillers, leading to low back temperature during the ablation process and high heat-shielding performance. The ablation properties were improved via the formation of SiC with high melting point. The above results indicate that these modified composites are promising materials for application in thermal protection systems.

Mechanical model for the interfacial thermal stress in porous ceramic foam coatings bonded to a substrate

Coatings of porous ceramic foams have potential applications in thermal protection systems of space shuttles. This paper develops a mechanics model for evaluation of thermal stress of porous ceramic foam coating/substrate structures under thermal shock temperature variation. Numerical results show that interfacial stresses exhibit singularity at the edge of ceramic foam coating. Stress intensity factor will reduce when ceramic foam coating has a larger density or a larger thermal conductivity. Comparison between beam model and membrane model is made and it suggests that consideration of bending stiffness is essential for correct evaluation of the thermal stress in the ceramic foams.

Transformation Thermodynamics with Arbitrarily Shaped Non-Conformal Objects and Coatings

In this paper we apply transformation optics theory to thermodynamics and design thermal cloaks and concentrators with arbitrarily shaped non-conformal objects and coatings. Expressions of the required material parameters are derived analytically and then validated by numerical simulations. We apply this method to design a thermal cloak which can guide the heat flow around the inner domain without perturbation to external thermal fields. In this way, the object inside the inner domain is protected from the invasion of external heat fluxes. In contrast, a concentrator is designed to concentrate heat flows into a small region without disturbing outside temperature fields, which can considerably enhance the heat density in the designed domain. The proposed method extends the design flexibility in manipulating heat flux and will find wide applications in thermal protection systems, solar cells and so on.

Oxidation behavior of ZrB2/SiC laminates: Effect of composition on microstructure and mechanical strength

SiC–ZrB2 ceramic laminates with different SiC:ZrB2 ratios were prepared by tape casting, binder burnout and pressureless sintering at 2200°C in view of their application in thermal protection systems. Their oxidation behavior in air was investigated by TGA up to 1600°C and by long term oxidation tests at 1500 and 1600°C. Both microstructure and mechanical features of laminates were compared, before and after oxidation. The formation of a passive layer was always observed but the oxidation resistance worsened and the complexity of the oxide layer microstructure increased with the ZrB2 content. Composition also affected the residual strength and modulus after oxidation.

A lightweight, high compression strength ultra high temperature ceramic corrugated panel with potential for thermal protection system applications

Abstract ZrB 2 –SiC–graphite ultra high temperature ceramic (UHTC) corrugated panel was firstly proposed and fabricated with potential application for sandwich structured thermal protection system. The compression properties of the as-prepared ultra high temperature ceramic corrugated panel were evaluated at 1600 °C in air. The compression modulus and strength of this ultra high temperature ceramic corrugate panel were 312 MPa and 17 MPa respectively. The design of corrugated panel exhibited an excellent combination of lightweight and excellent compression properties. This study would provide a novel concept of ultra high temperature ceramic corrugated panel for the design of ultra high temperature thermal protection system applications.

Fabrication and properties of lightweight ZrB2and SiC-modified carbon bonded carbon fiber composites via polymeric precursor infiltration and pyrolysis

The lightweight UHTC-modified CBCF composites were prepared by PIP using B and Zr polymeric precursor. The mechanical properties, thermo properties and oxidation behavior were studied. They are important considerations for potential use in thermal protection system applications.

Ultra-High Temperature Ceramics (UHTCs) via Reactive Sintering

Current high temperature ceramics, such as ZrO2, Si3N4 and SiC, cannot be used at temperatures over 1600°C due to their low melting temperature or dissociation temperature. For ultrahigh temperature applications over 1800°C, materials with high melting points, high phase composition stability, high thermal conductivity, good thermal shock and oxidation resistance are needed. The transition metal diborides, mainly include ZrB2 and HfB2, have melting temperatures of above 3000°C, and can basically meet the above demands. However, the oxidation resistance of diboride monolithic ceramics at ultra-high temperatures need to be improved for the applications in thermal protection systems for future aerospace vehicles and jet engines. On the other hand, processing science for making high performance UHTCs is another hot topic in the UHTC field. Densification of UHTCs at mild temperatures through reactive sintering is an attracting way due to the chemically stable phase composition and microstructure as well as clean grain boundaries in the obtained materials. Moreover, the stability studies of the materials in phase composition and microstructures at ultra high application temperatures is also critical for materials manufactured at relatively low temperature. Furthermore, the oxidation resistance in simulated reentry environments instead of in static or flowing air of ambient pressure should be evaluated. Here we will report the concept, advantages and some recent progress on the reactive sintering of diboride–based composites at mild temperatures.

Response of Actively Cooled Metal Foam Sandwich Panels Exposed to Thermal Loading

Experimental and analytical results for the thermomechanical response of actively cooled metal foam sandwich panels are presented. With a favorable strength-to-weight ratio and a high specific internal surface area, sandwich panels with metal foam cores are proposed for actively cooled load-bearing components in aerospace thermal protection system applications. First, inconel foam sandwich panels are subjected, via experimental means, to through-the-thickness thermal gradients, defined by fixed temperature conditions on opposite face sheets. With clamped boundary conditions, provided through a test fixture designed to minimize the common conflict of thermal and mechanical boundary conditions in experimental apparatus, the metal foam sandwich panels exhibit out-of-plane bending deformation. The thermomechanical deformation within the panel is then mitigated through active cooling, achieved by pumping compressed air through the metal foam core. Subsequently, the experimental measurements are used to develop a sequentially coupled thermomechanical finite element model. Central to the numerical characterization of metal foam sandwich panels is an understanding of the response of metal foam under shear loading. The material model for the foam is taken from a series of experimental measurements of the shear response of metal foam, providing density-dependent relationships for material stiffness and strength. The numerical model provides a strain-temperature history for metal foam sandwich panels under through-the-thickness thermal gradients

Suitability of a Coal‐Derived Carbon‐Based Foam for use in Thermal Protection Systems of Common Aero Vehicles

Common Aero Vehicles (CAVs) are relatively small‐size, un‐powered, self‐maneuvering vehicles equipped with a variety of weapons and launched from space. One of the major obstacles hampering a full the realization of the CAV concept is a present lack of lightweight, high‐temperature insulation materials which can be used for construction of the CAV’s thermal protection system (TPS). A computational analysis is utilized in the present work to examine the suitability of a carbon‐based, coal‐derived foam for the TPS applications in the CAVs. Toward that end, a model is developed for the high‐temperature effective thermal conductivity of foam‐like materials. In addition, an insulation sizing procedure is devised to determine the minimum insulation thickness needed for thermal protection of the vehicle structure at different sections of a CAV. It is found that the carbon‐based foam material in question can be considered as a suitable TPS insulation material at the leeward side and at selected portions of the windward side of a CAV (specifically the portions which are further away from the vehicle nose).

Response of Carbon-Carbon Composites to Challenging Environments

ABSTRACTCarbon-carbon composites are an emerging class of composite materials having a unique combination of high-temperature properties and low densities. These properties are attractive for hot structure and thermal protection system applications in future aerospace vehicles. Aerospace service environments of particular interest are cyclic temperature, oxidizing environments to 3000°F. For carbon-carbon composites to serve as practical engineering materials in such challenging environments, their long-term mechanical and chemical stability is essential.Many aspects of proposed service environments for these composites pose significant challenges to their satisfactory performance. Among these aspects are the oxidizing nature of the environments (including both high and low oxygen partial pressures), high temperatures, moisture, cyclic temperature service, and foreign-object impact. This paper presents results from materials performance evaluations which cover each of these parameters. The focus is on oxidation-resistant carbon-carbon composites intended specifically for multi-use aerospace applications. Results are presented for the carbon-carbon material currently in use on Space Shuttle and for newer, more advanced structural forms of these composites.