Ultra‐High‐Temperature Ceramic HfB2‐SiC Coating for Oxidation Protection of SiC‐Coated Carbon/Carbon Composites

To protect the carbon/carbon (C/C) composites from oxidation, an outer ultra‐high‐temperature ceramics (UHTCs) HfB₂‐SiC coating was prepared on SiC‐coated C/C composites by in situ reaction method. The outer HfB₂‐SiC coating consists of HfB₂ and SiC, which are synchronously obtained. During the heat treatment process, the formed fluid silicon melt is responsible for the preparation of the outer HfB₂‐SiC coating. The HfB₂‐SiC/SiC coating could protect the C/C from oxidation for 265 h with only 0.41 × 10^?2 g/cm² weight loss at 1773 K in air. During the oxidation process, SiO₂ glass and HfO₂ are generated. SiO₂ glass has a self‐sealing ability, which can cover the defects in the coating, thus blocking the penetration of oxygen and providing an effective protection for the C/C substrate. In addition, SiO₂ glass can react with the formed HfO₂, thus forming the HfSiO₄ phase. Owing to the “pinning effect” of HfSiO₄ phase, crack deflecting and crack termination are occurred, which will prevent the spread of cracks and effectively improve the oxidation resistance of the coating.     

TEM Study of the High‐Temperature Oxidation Behavior of Hot‐Pressed ZrB2–SiC Composites

The oxidation behaviors of ZrB₂‐ 30 vol% SiC composites were investigated at 1500°C in air and under reducing conditions with oxygen partial pressures of 10⁴ and 10 ^{? 8} Pa, respectively. The oxidation of ZrB₂ and SiC were analyzed using transmission electron microscopy (TEM). Due to kinetic difference of oxidation behavior, the three layers (surface silica‐rich layer, oxide layer, and unreacted layer) were observed over a wide area of specimen in air, while the two layers (oxide layer, and unreacted layer) were observed over a narrow area in specimen under reducing condition. In oxide layer, the ZrB₂ was oxidized to ZrO₂ accompanied by division into small grains and the shape was also changed from faceted to round. This layer also consisted of amorphous SiO₂ with residual SiC and found dispersed in TEM. Based on TEM analysis of ZrB₂ – SiC composites tested under air and low oxygen partial pressure, the ZrB₂ begins to oxidize preferentially and the SiC remained without any changes at the interface between oxidized layer and unreacted layer.     

Oxidation Transitions for SiC Part I. Active‐to‐Passive Transitions

Oxidation of SiC can occur in a passive mode where a protective film is generated or in an active mode where a volatile suboxide is generated and can lead to rapid material consumption. The transition between these two modes of oxidation is a critical issue. Evidence indicates that this transition occurs via a different mechanism for the active‐to‐passive transition as compared with that of the passive‐to‐active transition. In Part I of this article, the former (active‐to‐passive mode) is explored. Three different types of SiC are examined: Si‐rich SiC, stoichiometric SiC, and C‐rich SiC. Evidence suggests that the SiO₂/SiC equilibrium requirements as well as formation of SiO(g) at the SiC surface and subsequent oxidation to SiO₂(s) are critical issues in the active‐to‐passive transition.     

Heat and Mass Transfer in Ceramic Lattices During High‐Temperature Oxidation

This work reports on the heat and mass transfer evolution of ceramic lattices during their oxidation at 1400°C and 1600°C in air. Si–SiC and Si–SiC–ZrB₂ systems were employed as skeleton material because they, previously produced as monolithic bars, showed promising oxidation behavior at high temperatures. Regular arrays of tetrakaidecahedra were first designed by CAD, then 3D printed and finally converted into ceramic by replica technique followed by reactive silicon infiltration. The surface area of each sample was calculated and specific weight variations were evaluated as a function of time. During oxidation, effective thermal conductivity and pressure drop of each sample were measured. Finally, results were correlated with the phenomena occurring during high‐temperature oxidation.     

High‐Temperature Isothermal Oxidation of Ultra‐High Temperature Ceramics Using Thermal Gravimetric Analysis

Oxidation of ZrB₂ + SiC composites is investigated using isothermal measurements to study the effects of temperature, time, and gas flow on oxidation behavior and microstructural evolution. A test method called dynamic nonequilibrium thermal gravimetric analysis (DNE‐TGA), which eliminates oxidation during the heating ramp, has been developed to monitor mass change from the onset of an isothermal hold period (15 min) as a function temperature (1000°C–1600°C) and gas flow (50 and 200 mL/min). In comparing isothermal to nonisothermal TGA measurements, the scale thicknesses from isothermal tests are up to 4 times greater, indicating that oxidation kinetics are faster for isothermal testing, where the oxide scale thickness is 110 μm after 15 min at 1600°C in air. Isothermal oxidation followed parabolic kinetics with a mass gain that is temperature dependent from 1000°C–1600°C. The mass gain increased from ~5 to 45 g/m² and parabolic rate constants increased from 0.037 to 2.2 g²/m⁴·s over this temperature range. The effect of flow velocity on oxidation is not significant under the given laminar flow environment where the gas boundary layer is calculated to be 4 mm. These values are consistent with diffusion of oxygen through the glass‐ceramic surface layer as rate limiting.     

Paralinear Oxidation of CVD SiC in Water Vapor

The oxidation kinetics of CVDSiC were monitored by thermogravimetric analysis (TGA) in a 50% H₂O/50% O₂ gas mixture flowing at 1.4 cm/s for temperatures between 1200" and 1400°C. Paralinear weight change kinetics were observed as the water vapor oxidized the SiC and simultaneously volatilized the silica scale. The long-term degradation rate of SiC is determined by the volatility of the silica scale. Rapid SiC surface recession rates were estimated from these data for actual aircraft engine combustor conditions.     

Paralinear Oxidation of CVD SiC in Simulated Fuel-Rich Combustion

The oxidation kinetics of CVD SiC were measured by thermogravimetric analysis (TGA) in a 4H₂·12H₂O·10CO·7CO₂·67N₂ gas mixture flowing at 0.44 cm/s at temperatures between 1300° and 1450°C in fused quartz furnace tubes at 1 atm total pressure. The SiC was oxidized to form solid SiO₂. At ≥1350°C, the SiO₂ was in turn volatilized. Volatilization kinetics were consistent with the thermodynamic predictions based on SiO formation. These two simultaneous reactions resulted in overall paralinear kinetics. A curve fitting technique was used to determine the linear and parabolic rate constants from the paralinear kinetic data. Volatilization of the protective SiO₂ scale resulted in accelerated consumption of SiC. Recession rates under conditions more representative of actual combustors were estimated from the furnace data.     

High‐Temperature Creep Behavior of SiOC Glass‐Ceramics: Influence of Network Carbon Versus Segregated Carbon

Three silicon oxycarbide samples with different carbon contents are analyzed in the present study with respect to their high‐temperature creep behavior. The tests were performed in compression at 1100°C, 1200°C, and 1300°C; in this temperature range the mechanism of creep relies on viscoelastic flow within the samples and has been modeled with the Jeffreys viscoelastic model. After the release of the applied mechanical stress, a viscoelastic recovery behavior was observed in all samples. The creep behavior of the investigated samples indicates two rheological contributions in SiOC: (i) a high viscous answer, coming from the silica‐rich network, and (ii) an elastic response from the segregated carbon phase within the samples. Furthermore, two distinct effects of the carbon phase on the HT creep behavior of SiOC were identified and are discussed in the present paper: the effect of the carbon presence within the SiOC network (the “carbidic” carbon), which induces a significant increase in the viscosity and a strong decrease in the activation energy for creep, as compared to vitreous silica; and the influence of the segregated carbon phase (the “free” carbon), which has been shown to affect the viscosity and the activation energy of creep and dominates the creep behavior in phase‐separated silicon oxycarbides.     

Oxidation resistance of SiC nanowires reinforced SiC coating prepared by a CVD process on SiC‐coated C/C composites

Oxidation protective SiC nanowires‐reinforced SiC (SiCNWs‐SiC) coating was prepared on pack cementation (PC) SiC‐coated carbon/carbon (C/C) composites by a simple chemical vapor deposition (CVD) process. This double‐layer SiCNWs‐SiC/PC SiC‐coating system on C/C composites not only has the advantages of SiC buffer layer but also has the toughening effects of SiCNWs. The microstructure and phase composition of the nanowires and the coatings were examined by SEM, TEM, and XRD. The single‐crystalline β‐SiC nanowires with twins and stacking faults were deposited uniformly and oriented randomly with diameter of 50‐200 nm and length ranging from several to tens micrometers. The dense SiCNWs‐SiC coating with some closed pores was obtained by SiC nanocrystals stacked tightly with each other on the surface of SiCNWs. After introducing SiCNWs in the coating system, the oxidation resistance is effectively improved. The oxidation test results showed that the weight loss of the SiCNWs‐SiC/PC SiC‐coated samples was 4.91% and 1.61% after oxidation at 1073 K for 8 hours and at 1473 K for 276 hours, respectively. No matter oxidation at which temperature, the SiCNWs‐SiC/PC SiC‐coating system has better anti‐oxidation property than the single‐layer PC SiC coating or the double‐layer CVD SiC/PC SiC coating without SiCNWs.     

Oxidation and defect control of CVD SiC coating on three-dimensional C/SiC composites

Two kinds of multi-layer CVD SiC coatings were prepared on a three-dimensional C/SiC composite. Oxidation behavior of the coating and the composite were studied and the effect of defects in the coating on its oxidation protection property were investigated. Above 1200 °C, thickness of the oxide film formed on the coating was related to oxidation time by the Fick’s first law X(t)²=Bt, the diffusion rate constant increased with oxidation temperature according to the Arrhenius’ relation ln B=−32?483/T+1.4048. Morphology of the interface between the CVD SiC and its oxide film was different after oxidation at temperatures from 1200 to 1500 °C. It was interpreted by consideration of the interfacial stress produced by thermal expansion mismatch and the CO gas pressure produced by interfacial reaction.     

Silicon Carbide Oxidation in High‐Pressure Steam

Silicon carbide is a candidate cladding for fission power reactors that can potentially provide better accident tolerance than zirconium alloys. SiC has also been discussed as a host matrix for nuclear fuel. Chemical vapor–deposited silicon carbide specimens were exposed in 0.34–2.07 MPa steam at low gas velocity (~50 cm/min) and temperatures from 1000°C to 1300°C for 2–48 h. As previously observed at lower steam pressure of 0.15 MPa, a two‐layer SiO₂ scale was formed during exposure to these conditions, composed of a porous cristobalite layer above a thin, dense amorphous SiO₂ surface layer. Growth of both layers depends on temperature, time, and steam pressure. A quantitative kinetics model is presented to describe the SiO₂ scale growth, whereby the amorphous layer is formed through a diffusion process and linearly consumed by an amorphous to crystalline phase transition process. Paralinear kinetics of SiC recession were observed after exposure in 0.34 MPa steam at 1200°C within 48 h. High‐pressure steam environments are seen to form very thick (10–100 μm) cristobalite SiO₂ layers on CVD SiC even after relatively short‐term exposures (several hours). The crystalline SiO₂ layer and SiC recession rate significantly depend on steam pressure. Another model is presented to describe the SiC recession rate in terms of steam pressure when a linear phase transition k_l governing the recession kinetics, whereby the reciprocal of recession rate is found to follow a negative unity steam pressure power law.     

High‐Temperature Oxidation of ZrB2–SiC–AlN Composites at 1600°C

The effect of AlN substitution on oxidation of ZrB₂–SiC was evaluated at 1600°C up to 5 h. Replacement of ZrB₂ by AlN, with 30 vol% SiC resulted in improved oxidation resistance with a thinner scale and reduced oxygen affected area. On the other hand, substitution of AlN for SiC resulted in a deterioration of the oxidation resistance with an abnormal scale and significant recession. The effect of SiC content was also studied, and was found to be consistent with the literature for the composites without AlN additions. A similar effect was observed when AlN was added, with the higher SiC content materials showing improved oxidation resistance. X‐ray photoelectron spectroscopy showed the presence of Al₂O₃ and SiO₂ on the surface, which could possibly lead to a modification in the viscosity of the glassy oxide scale. Possibly, the oxidation behavior of ZrB₂–SiC composites can be improved with controlled AlN additions by adjusting the Al:Si ratios.     

Surface Modification in Polycrystalline Y2O3 Under High‐Pressure/High‐Temperature Processing Conditions

A pressure‐induced phase transformation is used to refine the grain size of polycrystalline Y₂O₃, by a factor of 3000. A surface modification effect accompanies the observed grain refinement, which becomes more apparent with increasing holding time under high pressure. The surface‐modified layer exhibits lower hardness and lower oxygen content relative to the underlying material. Moreover, it possesses columnar‐grained structure with cubic symmetry, whereas the interior has a monoclinic structure.     

Synthesis of High‐Purity Ti3SiC2 by Microwave Sintering

High‐purity ternary laminated compound Ti₃SiC₂ was successfully synthesized by a microwave heating method in the flowing argon for the first time. The mixtures of titanium, silicon, and graphitic carbon (C_gc) or activated carbon (C_ac) with different molar ratios were used to investigate the reaction mechanisms. It was confirmed that Ti₃SiC₂ with high purity of 98 vol.% was achieved without the aids of Al. The optimum experimental parameters were determined as Ti/Si/C_gc having the molar ratio of 3/2.2/2, first holding at 1480°C for 30 min, and subsequent dwelling at 1300°C for 60 min.     

High‐Temperature Deformation Mechanisms in Monolithic 3YTZP and 3YTZP Containing Single‐Walled Carbon Nanotubes

Monolithic 3YTZP and 3YTZP containing 2.5 vol% of single‐walled carbon nanotubes (SWCNT) were fabricated by Spark Plasma Sintering (SPS) at 1250°C. Microstructural characterization of the as‐fabricated 3YTZP/SWCNTs composite shows a homogeneous CNTs dispersion throughout the ceramic matrix. The specimens have been crept at temperatures between 1100°C and 1200°C in order to investigate the influence of the SWCNTs addition on high‐temperature deformation mechanisms in zirconia. Slightly higher stress exponent values are found for 3YTZP/SWCNTs nanocomposites (n~2.5) compared to monolithic 3YTZP (n~2.0). However, the activation energy in 3YTZP (Q = 715 ± 60 kJ/mol) experiences a reduction of about 25% by the addition of 2.5 vol% of SWCNTs (Q = 540 ± 40 kJ/mol). Scanning electron microscopy studies indicate that there is no microstructural evolution in crept specimens, and Raman spectroscopy measurements show that SWCNTs preserved their integrity during the creep tests. All these results seem to indicate that the high‐temperature deformation mechanism is grain‐boundary sliding (GBS) accommodated by grain‐boundary diffusion, which is influenced by yttrium segregation and the presence of SWCNTs at the grain boundary.     

High‐Performance Carbon Nanotube‐Reinforced Bioplastic

The inherent properties of poly(lactide), a biocompatible and biodegradable polymer, are concurrently improved by the incorporation of a small amount of surface functionalized carbon nanotubes. A new method has been used to functionalize the CNTs' outer surface with hexadecylamine. A composite of PLA with functionalized CNTs has been prepared by melt‐extrusion. FT‐IR spectroscopy, Raman spectroscopy, DSC, and optical microscopy are used to investigate the thermal and mechanical property improvement mechanism in f‐CNTs containing PLA composite.

     

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al₂O₃ matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10^?4 s^?1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al₂O₃. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al₂O₃ matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.     

Oxidation Behavior of Hot Pressed ZrB2‐ZrC‐SiC Ceramic Composites

Dense ZrB₂‐SiC ceramics containing 40 vol% ZrC particles are fabricated via hot pressing method. Then the sintered ceramics are oxidized in air up to 1500°C, and the oxidation kinetics of the ceramic composites is deduced in combination with the reacted fraction curves. As indicated by the experimental results, the oxidation kinetics changes from reaction‐controlled process to diffusion‐controlled one with increasing of oxidation temperature. In addition, the oxidation kinetics parameters are obtained, which indicates that the oxidation resistance decays at elevated temperatures. Furthermore, the evolution of surface morphology and oxide scale during oxidation process is clarified.     

High‐frequency dielectric characterization for liquid crystalline polyimide/SiO2 nanocomposites

In this study, we investigated the influence of frequency, film thickness, and SiO₂ content on the dielectric constant (K) and loss tangent (tan δ) of liquid crystalline polyimide (LCPi) and liquid crystalline polyimide/SiO₂ (LCPi/SiO₂) nanocomposites in a high frequency environment. We tested the loss tangent of the LCPi and LCPi/SiO₂ nanocomposites within the high frequency 1 MHz to 3 GHz range, and determined its value to be between 0.01 and 0.001. In addition, we found a formant for frequencies ranging from 0.5 GHz to 1 GHz. We also inferred from the dielectric loss graphs of films with different thicknesses that the formants of the loss tangent shifted toward higher frequencies with increasing thicknesses. When measuring the dielectric constant at high frequencies, we found that the dielectric constant decreased markedly with increased SiO₂ contents. Using the dielectric constant of high‐frequency circuit board materials as the standard, the dielectric constant of the LCPi/SiO₂ nanocomposites at the frequency range from 1 MHz to 3 GHz was found to be as high as 2.2–3.4, thereby confirming the viability of LCPi/SiO₂ nanocomposites as candidate materials for high‐frequency circuit board. In addition, the volume resistivity (ρ_V) of the LCPi and LCPi/SiO₂ nanocomposites also increased with increased SiO₂ contents. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Glass Dielectrics in Extreme High‐Temperature Environment

Thin and flexible glass ribbons can be rolled into a film capacitor structures for power electronic circuits. Glass has excellent electrical properties and is a leading candidate to replace polymer films for high‐temperature applications. The dielectric properties of a low‐alkali aluminoborosilicate glass were characterized up to temperatures of 400°C. Low‐field permittivity values of 6 with dielectric loss below 0.01 were found for temperatures below 300°C. The dielectric breakdown strength exceeded 5 MV/cm for temperature of 400°C and high‐field polarization measurements showed that glass has over 95% energy efficiency at temperatures of 200°C, which is a target temperature for high‐temperature power electronic circuits driven by wide bandgap semiconductor devices.