High-Temperature Active Oxidation of Chemically Vapor-Deposited Silicon Carbide in an Ar─O2 Atmosphere

Active oxidation behavior of chemically vapor-deposited silicon carbide in an Ar─O₂ atmosphere at 0.1 MPa was examined in the temperature range between 1840 and 1923 K. The transition from active oxidation (mass loss) to passive oxidation (mass gain) was observed at certain distinct oxygen partial pressures ( P _O2^t). The values of P _O2^t increased with increasing temperature and with decreasing total gas flow rates. This behavior was well explained by Wagner's model and thermodynamic calculations. Active oxidation rates ( k _a) increased with increasing O₂ partial pressures and total gas flow rates. The rate-controlling step of the active oxidation was concluded to be O₂ diffusion through the gaseous boundary layer.     

Active-to-Passive Transition and Bubble Formation for High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Carbide in CO–CO2 Atmosphere

Oxidation behavior of chemically vapor-deposited SiC in CO─CO₂ atmospheres (0.1 MPa) was investigated using a thermogravimetric technique at temperatures from 1823 to 1923 K. Active or passive oxidation was observed depending on temperature and CO₂/CO partial pressure ratio ( P _co2/ P _co). The critical P _co2/ P _co value for the transition was 1O² times as large as a theoretical value calculated from the Wagner model. In the passive oxidation above 1873 K, SiO₂ bubbles were grown. The expansion and rupture of bubbles caused cyclic rapid mass gain and mass loss.     

High-Temperature Active Oxidation and Active-to-Passive Transition of Chemically Vapor-Deposited Silicon Nitride in N2–O2 and Ar–O2 Atmospheres

The oxidation behavior of chemically vapor-deposited silicon nitride in N₂–O₂ and Ar–O₂ atmospheres was studied using a thermogravimetric technique at temperatures 1823 to 1923 K. Active oxidation was observed at low oxygen partial pressures. The active oxidation rates increased with increasing oxygen partial pressure ( P _O2) up to a certain P _O2, and then passive oxidation occurred. The transition oxygen partial pressures from active to passive oxidation were determined. The rate-controlling step for the active oxidation could be oxygen diffusion through a gaseous boundary layer near the Si₃N₄ surface. Decomposition of Si₃N₄ does not seem to be associated with the mass loss behavior. The Wagner model was employed to explain the oxidation behavior.     

High-Temperature Oxidation of Chemically Vapor-Deposited Silicon Nitride in a Carbon Monoxide-Carbon Dioxide Atmosphere

Oxidation behavior of chemically vapor-deposited silicon nitride (CVD-Si₃N₄) in CO─CO₂ atmospheres between 1823 and 1923 K was investigated using a thermogravimetric technique. Mass loss of Si₃N₄ (active oxidation) was observed in a region of P _CO2/P_CO < 1, while mass gain (passive oxidation) was observed at around P _CO2 P _CO= 10. In the active oxidation region below P _CO2P_CO= 10 ^–4, carbon particles were formed on the Si³N⁴ surface as an oxidation product, and the mass-loss rates were independent of P _CO2/ P _CO In the active oxidation region above P _CO2/ P _CO= 10^–4 the mass-loss rates decreased with increasing P _CO2/ P co. The critical P _CO2/ P _CO value from the active to passive oxidation was 2 orders of magnitude larger than the calculated value predicted from the Wagner model.     

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.     

Oxidation Kinetics of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen

The oxidation kinetics of chemically vapor-deposited SiC in dry oxygen and wet oxygen ( P _H2O= 0.1 atm) at temperatures between 1200° and 1400°C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the Deal and Grove model for oxidation of silicon. It was found that in an environment containing even small amounts of impurities, such as high-purity Al₂O₃ reaction tubes containing 200 ppm Na, water vapor enhanced the transport of these impurities to the oxidation sample. Oxidation rates increased under these conditions presumably because of the formation of less protective sodium alumino-silicate scales.     

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.     

High-Temperature Mixed Oxidation of Nitride-Bonded Silicon Carbide in Oxidizing Gas Mixtures Containing 2% Cl2

Nitride-bonded silicon carbide ceramics have lower processing costs than many other SiC-based ceramics and adequate properties for use as high-temperature heat exchangers in oxidizing environments. Silicon nitride has much better resistance to attack by chlorine at temperatures above 900°C than silicon carbide. When nitride-bonded silicon carbide ceramics are exposed to gas mixtures containing 2% Cl₂ and small amounts of oxygen in this temperature range, the SiC is selectively chlorinated, leaving behind a porous matrix of silicon nitride. The rate of corrosion is controlled by a combination of interfacial kinetics at the surfaces of the SiC grains and transport of volatile species through the silicon nitride skeleton. In more oxidizing environments, the rate of chlorination is suppressed by the formation of a protective SiO₂ film. In highly oxidizing environments at temperatures in excess of 1200°C, the formation of volatile chloride reaction products at the interface between the SiC and the passivating SiO₂ layer causes bubbles to form in the SiO₂, which accelerates the oxidation.     

Devitrification and Delayed Crazing of SiO2 on Single-Crystal Silicon and Chemically Vapor-Deposited Silicon Nitride

The linear growth rate of cristobalite was measured in thin SiO₂ films on silicon and chemically vapor-deposited silicon nitride. The presence of trace impurities from alumina furnace tubes greatly increased the crystal growth rate. Under clean conditions, the growth rate was still 1 order of magnitude greater than that for internally nucleated crystals in bulk silica. Crystallized films cracked and lifted from the surface after exposure to atmospheric water vapor. The crystallization and subsequent crazing and lifting of protective SiO₂ films on silicon nitride should be considered in long-term applications.     

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.     

Thermal Analysis of a SiO2─Al2O3─CaO─CaF2 Glass

A SiO₂─Al₂O₃─CaO─CaF₂ ionomer glass was investigated using thermal analysis, X-ray diffraction, and scanning electron microscopy. The purpose of this investigation was to control the susceptibility of the glass to acid attack. The differential thermal analysis trace exhibited a sharp glass transition at about 645°C and two exotherms. The first exotherm corresponded to liquid–liquid phase separation followed by crystallization of fluorite. The second, much larger, exotherm was the result of crystallization of the remaining glass phase to form anorthite. Prolonged heat treatment below the glass-transition temperature demonstrated that crystallization of fluorite can occur without prior liquid–liquid phase separation.     

Spherulitic Growth of Apatite in a MgO─CaO─SiO2─P2O5 Glass

Differential thermal analysis, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and transmission electron microscopy were used to study the crystallization of a glass with a composition of 11.2 wt% MgO, 40.5 wt% CaO, 33.3 wt% SiO₂, and 15 wt% P₂O₅. A two-phase "composite," which was composed of apatite and an intermediate phase (H-phase), was formed under appropriate heat-treatment conditions. The spherulitic morphology of apatite phase transformed from "open sheaf" into ellipsoidal as samples were heated to a higher temperature. These phenomena were due to the intermediate H-phase becoming unstable at this temperature so that the retardation effect on the apatite dendritic growth disappeared.     

Grain-Size Distribution in Al2O3─ZrO2 Generated by High-Temperature Annealing

Grain-size distribution in various Al₂O₃─ZrO₂ (2.5 mol% Y₂O₃) ceramics during high-temperature annealing was examined. In alumina-rich alloys, the grain size of major and minor phases was very different, while grain size was almost uniform in zirconia-rich alloys. This difference in grainsize distribution was related to the difference in grain growth rate of the major phase and to the effectiveness of grain-boundary pinning by minor-phase grains.     

Bioactive Bone Cement Based on CaO─SiO2─P2O5 Glass

A CaO─SiO₂─P₂O₅─CaF₂ glass powder hardened within 4 min when mixed with an ammonium phosphate solution to form CaNH₄PO₄·H₂O. After it had soaked in a simulated body fluid for 3 d, forming hydroxyapatite, the cement showed a compressive strength of 80 MPa. Implanted into a rat tibia, the mixed paste formed a tight chemical bond to the living bone within 4 weeks. Such a bioactive cement could be useful not only for fixing various kinds of implants to the surrounding bones but also, by itself, as a bone filler.     

Subsolidus Phase Equilibria of the MgO─V2O5─SiO2 System

The subsolidus phase equilibria of the MgO─V₂O₅─SiO₂ system was studied by solid-state reaction and powder X-ray diffractometry. The resulting ternary is discussed with respect to corrosion of magnesia- and silica-containing refractories by vanadium-containing fuels.     

Zirconium–Hafnium Interdiffusion in Polycrystalline Fluorite-Cubic CeO2─ZrO2─HfO2 Solid Solution

Zr–Hf interdiffusion was studied in the temperature range of 1650° to 1850°C in air for polycrystalline fluorite-cubic systems of 90CeO₂·10(Zr_{1- x} Hf _x )O₂ and 60CeO₂·40(Zr_{1- x} Hf _x )O₂. Lattice and grain-boundary diffusion parameters were calculated from the Zr–Hf concentration distributions by using the grain-boundary diffusion equation of Oishi and Ichimura. The cation iattice diffusivity was close to that in the fluorite-cubic Y₂O₂-ZrO₂ solid solution.     

Thermodynamics of Nitrogen in B2O3, B2O3─SiO2, and B2O3─CaO Systems

The BN solubilities for B₂O₃, B₂O₃─SiO₂, and B₂O₃─CaO systems have been measured mainly at 1823 K using a graphite crucible. The capability of the systems for nitrogen dissolution is compared with that of silicate systems in terms of nitride capacity. The dependence of nitrogen solubility in molten CaO containing 15 mol% of B₂O₃ on oxygen and nitrogen partial pressures is also investigated. It has been found that there are two mechanisms for nitrogen dissolution, namely as chemically bonded nitrogen and as physically dissolved nitrogen gas.     

Li2O─SiO2─Al2O3─MeIIO Glass-Ceramic Systems for Tile Glaze Applications

In order to verify the possibility of using glass-ceramic materials as tile coatings, the devitrification processes of three industrial formulations belonging to the Li₂O─Al₂O₃─SiO₂ glass-ceramic system were investigated by differential thermal analysis, X-ray diffractometry, scanning electron microscopy, and IR spectroscopy. Compositional variations were made by addition of large amounts of MgO or CaO or PbO (ZnO) oxides as well as through smaller additions of other oxides. In these systems the surface crystallization contributes appreciably to the bulk crystallization mechanism. All the systems investigated show a high tendency toward crystallization even at very high heating rates, developing a very close network of interlocked crystals of synthetic β-spodumene-silica solid solutions (LiAlSi₄O₁₀). The results of this research are expected to establish the conditions under which these glass-ceramic systems can be practically used as tile glazes.