Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO₂ was used to produce an oxygen partial pressure of ∼10⁻¹⁰ Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO₂ produced a non-protective oxide surface scale.     

Thermal Stability of Polycarbosilane-Derived Silicon Carbide Fibers under Reduced Pressures

Three types of polycarbosilane-derived SiC fibers—Nicalon, Hi-Nicalon, and Hi-Nicalon S—were exposed at temperatures of 1573–1773 K under a reduced pressure of 1.3 Pa. The thermal stability of the fibers was investigated through examinations of the gas evolution, grain growth, specific resistivity, fiber morphology, and tensile strength. The thermal decomposition of the silicon oxycarbide phase began at 1523 K; then, active oxidation of the β-SiC crystallites occurred at >1673 K. The active oxidation caused serious damage to the fiber structure, which resulted in significant degradation of the fiber strength. Hi-Nicalon had a tensile strength of ∼0.5 GPa after exposure at 1773 K, although Nicalon and Hi-Nicalon S fibers completely lost their strength, even after exposure at 1673 K. Hi-Nicalon fiber had relatively good thermal stability under reduced pressure.     

Effects of Oxygen Partial Pressure on the Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of oxygen. The diffusion rate constant was found to vary with the logarithm of the partial pressure of oxygen according to the theory of oxidation of thin films as proposed by Engell and Hauffe. An alternative explanation based on the change of free energy with surface coverage was also found to fit the data.     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen at 1823 to 1923 K

The oxidation of chemically vapor-deposited SiC in wet O₂ (water vapor partial pressure = 0.01 MPa, total pressure = 0.1 MPa) was examined using a thermogravimetric technique in the temperature range of 1823 to 1923 K. The oxidation kinetics follow a linear-parabolic relationship over the entire temperature range. The activation energies of linear and parabolic rate constants were 428 and 397 kJ · mol⁻¹, respectively. The results suggested that the rate-controlling step is a chemical reaction at an SiC/SiO₂ interface in the linear oxidation regime, and the rate-controlling step is an oxygen diffusion process through the oxide film (cristobalite) in the parabolic oxidation regime.     

掺杂钕的磷酸铈独居石固化体合成研究

采用共沉淀法合成了掺杂钕元素的独居石固化体。利用X射线衍射仪,扫描电子显微镜,电子探针等现代分析技术研究了固化体的化学组成和微观结构特征。结果表明,合成了单斜晶系掺杂钕元素的独居石;固化体结构紧密,孔隙较少,晶体发育良好;由于温度影响存在0.2~1.5μm的颗粒状晶体和较大的层片状晶体;合成的固化体钕元素的掺杂量为3wt%左右。制备的固化体具有良好的机械,化学稳定性,符合高放废物(high-level waste,简称HLW)地址处置固化体的基本要求。     

Oxidation of Hafnium Carbide and Titanium Carbide Single Crystals with the Formation of Carbon at High Temperatures and Low Oxygen Pressures

The isothermal oxidation of the 200 face of HfC and TiC single crystals was performed at temperatures of 700°—1500°C and at oxygen pressures of 0.08—80 kPa for 4 h. The weight gain by oxidation of the two crystals was followed using an electromicrobalance. A polished cross section of the oxidized crystals was observed using backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Hf, Ti, O, and C was performed by wavelength-dispersive X-ray microanalysis. The early-stage oxidation kinetics of HfC crystals were described by the contracting volume equation, followed by slowed reaction in the latter stage, whereas the same equation was applied to the oxidation of TiC over the entire oxidation time. The preferred {200} orientation of monoclinic HfO₂ occurred on the oxidized surface of the HfC crystal. The oxide scale on the HfC crystal consisted of a compact and pore-free black inner scale (zone 1) and a white/gray outer scale that contained many pores (zone 2). Zone 1 contained ∼25 at.% unoxidized carbon, and zone 2 contained 6—11 at.% carbon. The oxide scale of TiC was composed of an inner dense lamella subscale (zone 1) with a carbon content of 7—23 at.% and an outer region with laminations that was separated by pores and cracks (zone 2). The Ti₃O₅ phase, which exhibits a strong 020 line, was formed at depths of ≥40 μm in the scale obtained at 1500°C. Treatment with a concentrated HF solution allowed zone 1 to be separated from the HfC crystal in the form of carbon-containing films, which were characterized using Raman spectroscopy and transmission electron microscopy.     

Observations of Accelerated Silicon Carbide Recession by Oxidation at High Water-Vapor Pressures

A study of the exposure of SiC at 1200°C and high water-vapor pressures (1.5 atm) has shown SiC recession rates that exceed what is predicted based on parabolic oxidation at water-vapor pressures of less than or equal to ∼1 atm. After exposure to these conditions, distinct silica-scale structures are observed; thick, porous, nonprotective cristobalite scales form above a thin, dense silica layer. The porous cristobalite thickens with exposure time, while the thickness of the underlying dense layer remains constant. These observations suggest a moving-boundary phenomenon that is controlled by the rapid conversion of dense vitreous silica to a porous, nonprotective crystalline SiO₂.     

Thermodynamic Properties of Nonstoichiometric Yttria-Stabilized Zirconia at Low Oxygen Pressures

Thermodynamic properties of 8-mol%-yttria-stabilized zirconia have been determined in the 810° to 1040°C temperature range at low Po₂. A high-temperature solid-state coulometric titration method was used. The mass action constant, K_ma, can be represented at low Po₂ as K_ma=0.677 exp [(–3.98 ±0.03 eV)/kT].     

Kinetics of Self-Crack-Healing of Alumina/Silicon Carbide Composite Including Oxygen Partial Pressure Effect

Self-crack-healing behavior under a combustion gas atmosphere with a low oxygen partial pressure ,     , is important for actualizing ceramic gas turbines, but to date only self-crack-healing behavior in air has been investigated. In this study, we investigated crack-healing behaviors at 1273–1773 K under several levels of     . Crack-healing in atmospheres with     gave rise to the complete strength recovery of cracked specimens, resulting from passive oxidation. Based on the obtained results, the kinetics for strength recovery by self-crack-healing was expressed as a function of healing temperature, T _H (K), and     (Pa). The strength recovery rate for complete crack-healing, v _H (s⁻¹), could be expressed as Using this rate equation, one can evaluate the healing time for complete strength recovery under combustion gas in a gas turbine.     

High-Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide

The oxidation behavior of chemically vapor deposited (CVD) SiC at high temperature was investigated using a thermogravimetric technique in the temperatures range of 1823 to 1948 K. The specimens were prepared by chemical vapor deposition using SiCl₄, C₃H₈, and H₂ as source gases. The oxidation behavior of the CVD-SiC indicated "passive" oxidation and a two-step parabolic oxidation kinetics over the entire temperature range. The crystallization of the SiO₂ film formed may have caused this two-step parabolic behavior. The parabolic oxidation rate constant ( K _p) varied with the square root of the oxygen partial pressure ( P ^1/2_O2). The activation energy for the oxidation was determined to be 345 and 387 kJ · mol⁻¹. These values suggest that the diffusion process of the oxygen ion which passes through the SiO₂ film is rate-controlling.     

Reactions of Silicon Carbide and Silicon(IV) Oxide at Elevated Temperatures

The reaction between SiC and SiO₂ has been studied in the temperature range 1400–1600 K. A Knudsen cell in conjunction with a vacuum microbalance and a high-temperature mass spectrometer was used for this study. Two systems were studied—1:1 SiC (2 wt% excess carbon) and SiO₂; and 1:1:1 SiC, carbon, and SiO₂. In both cases the excess carbon forms additional SiC within the Knudsen cell and adjusts to the direct reaction of stoichiometric SiC and SiO₂ to form SiO( g ) and CO( g ) in approximately a 3:1 ratio. These results are interpreted in terms of the SiC-O stability diagram.     

High-Temperature Morphological Evolution of Lithographically Introduced Cavities in Silicon Carbide

Internal cavities of controlled geometry and crystallography were introduced in 6 H silicon carbide single crystals by combining lithographic methods, ion-beam etching, and solid-state diffusion bonding. The morphologic evolution of these internal cavities (negative crystals) in response to anneals of up to 128 h duration at 1900°C was examined using optical microscopy. Surface energy anisotropy and faceting had a strong influence on the geometric and kinetic characteristics of evolution. Decomposition of {12     10} cavity edges into {101     x } facets was observed after 16 h anneals, indicating that {12     10} faces are not components of the Wulff shape. The shape evolution kinetics of penny-shaped cavities were also investigated. Experimentally observed evolution rates decreased much more rapidly with those predicted by a model in which surface diffusion was assumed to be rate limiting. This suggested that the development of facets and the associated loss of ledges and terraces during the initial stages of evolution resulted in an evolution process limited by the nucleation rate of attachment/detachment sites (ledges) on the facets.     

Effect of Carbon and Boron on the High-Temperature Oxidation of Silicon Carbide

Silicon carbide has good oxidation resistance, due to the formation of a protective silica layer. Although amorphous silica is an excellent oxygen barrier, it is very sensitive to impurity elements, which affect its viscosity, oxygen diffusivity, and crystallization kinetics. This paper compares the oxidation rates of CVD SiC, sintered α-SiC, and CVD SiC- coated graphite in 1 atm oxygen at 1500^deg;C to determine the effects of small additions of boron and carbon. The formation of bubbles in the silica scale formed on sintered α-SiC in oxygen between 1230° and 1550^deg;C is also discussed.     

聚碳硅烷低温制备碳化硅泡沫陶瓷

采用具有连通气孔的聚氨酯海绵浸渍聚碳硅烷（polycarbosilane,PCS）、碳化硅微粉与四氢呋喃配制的浆料,挂浆素坯经热氧化不熔化处理后,在惰性气氛中于1 000℃热解制备碳化硅泡沫陶瓷。研究了固相含量和PCS含量对碳化硅泡沫陶瓷微观结构、体积密度、线收缩率和抗弯强度的影响,结果表明：固相含量为43.1%～69...     

Influence of Oxygen on the Formation of Aluminum Silicon Carbide

X-ray diffraction studies have been used to follow the formation of Al₄SiC₄ from Al₄C₃ and SiC and the role played by impurity oxygen. The phase Al₂OC forms in the early stages of reaction and reacts with SiC at ≈ 1700°C to produce Al₄SiC₄ plus a small amount of an aluminosilicate liquid. This liquid dissociates at higher temperatures, the resulting evolution of gases hindering complete densification. Higher densities are obtained on hot-pressing.     

High-Temperature Creep and Microstructural Evolution of Chemically Vapor-Deposited Silicon Carbide Fibers

The creep behavior of three types of silicon carbide fibers that have been fabricated via chemical vapor deposition is described. The fibers exhibit only primary creep over the range of conditions studied (1200°–1400°C, 190–500 MPa). A transmission electron microscopy study of the microstructural development that is induced by the creep deformation of SCS-6 silicon carbide fibers at 1400°C is presented. Significant grain growth occurs in all silicon carbide regions of the fiber during creep, in contrast to the reasonably stable microstructure that is observed after annealing at the same temperature and time.     

Strength of Nicalon Silicon Carbide Fibers Exposed to High-Temperature Gaseous Environments

The effects of exposures to high-temperature gaseous atmospheres on the strength of Nicalon SiC fibers were investigated. The exposure conditions were as follows: (1) H₂ with various P _H2O for 10 h at 1000° and 1200°C, and (2) air for 2 to 100 h at 800° to 1400°C. Individual fibers were tested in tension following each exposure. The strengths of the fibers were strongly influenced by the exposure atmosphere and temperature, but less affected by time at temperature. When exposed in air, a SiO₂ layer was formed on the surface, minimizing the degradation of strength. However, this beneficial effect was negated under conditions in which the SiO₂ layer became too thick. The most severe degradation resulted from exposure to a reducing atmosphere, presumably due to the reduction of SiO₂ inherent in the fibers.     

High-Temperature Behavior of Silicon Carbide Fibers in a Si-Al-Ca-O-N Ceramic Matrix

A Nicalon SiC fiber-reinforced Si-Al-Ca-O-N composite was fabricated by a slurry infiltration process followed by hot pressing at 1600°C. A carbon-rich interfacial layer (∼100 nm) as well as a crystalline silicon-rich layer (∼15 nm) was observed between the fiber and matrix. Based on this interfarcial phenomenology, the following behabior of SiC fibers in the matrix was proposed: fine SiC grains (diameter of ∼ 1.7 nm in as-received fibers) decomposed at fiber surfaces (SiC → C + Si), followed by silicon migration into the glasshy phase of the matrix. The glassy phase was interpreted to play a key role as a silicon consumer in fostering the formation of the carbon-rich layer. The presence of silicon implied that the oxygen activity in the matrix was low enough to avoid SiC oxidation.     

Silicon Carbide Whisker Stability During Processing of Silicon Nitride Matrix Composites

The effects of two different sources of SiC whiskers on the chemistry and microstructure of the SiC-whisker—Si₃N₄ composites were evaluated using scanning transmission electron microscopy. Analyses were performed after presintering in N₂ and after encapsulated hot isostatic pressing. Significant differences in the porosity, α- to β-Si₃N₄ conversion, and whisker degradation were observed after presintering. It was also noted that whiskers containing surface iron impurities were converted to Si₃N₄ during processing. Whiskers from the source having low surface iron exhibited little reaction. After hot isostatic pressing, some oxidation of the cleaner whiskers was observed.