Highly textured chemically vapor‐deposited silicon carbide (CVD‐SiC) thick films were oxidized and compared to single‐crystal SiC and single‐crystal silicon. The oxidation rates of the (111) face of the cubic CVD‐SiC were the same as those of the (0001) face of the single‐crystal SiC. Similarly, the opposite faces of the two materials, () and (000), also oxidized at nominally the same rates. The () and (000) faces oxidized much faster than their opposite (lll)/(0001) faces. Ellipsometry measurements and kinetic results implied that differences existed between the oxides that grew on the opposite faces. A regression method was developed to analyze the oxide thickness versus time versus temperature behavior of the specimens simultaneously. This technique was compared to typical methods for analyzing temperature‐dependent processes and estimated temperature‐ dependent parameters (e.g., activation energy) and their errors more accurately.

The oxidation of single crystal SiC in dry oxygen (10-3-1 atm and 1200~176 followed parabolic kinetics. Two different apparent activation energies were calculated for oxidation of the (0001) C faces of SiC, approximately 120 k J/tool below 1350~ and 260 kJ/mol above 1350~ Two regimes were not apparent for oxidation of the (0001) Si faces, and apparent activation energies lay between 223 and 298 kJ/mol. Double oxidation experiments using 1~O2 and 1802 indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (< 1300~ with a substantial contribution from diffusion of ionic oxygen at higher temperatures. Epitaxially grown Si 13C films on alpha-Si 12C substrates via CVD were used to study carbon transport behavior during oxidation of SiC. Depth profiles for carbonaceous species using SIMS showed that carbon can transport quickly through the oxide layer, which eliminates the possibility that transport of carbonaceous species is rate controlling in the oxidation of SiC. Oxidation mechanisms of SiC are discussed on the basis of these results.

A modified associate species approach is used to model the liquid phase in oxide systems. The relatively simple technique treats oxide liquids as solutions of end‐member and associate species. The model is extended to representing glasses by treating them as undercooled liquids. Equilibrium calculations using the model allow the determination of species activities, phase separation, precipitation of crystalline phases, and volatilization. In support of nuclear waste glass development, a model of the Na2O–Al2O3–B2O3–SiO2 system has been developed that accurately reproduces its phase equilibria. The technique has been applied to the CaO–SiO2 system, which is used to demonstrate how two immiscible liquids can be treated.

15271. The diffusion model successfully predicted minimum A1 concentrations for protective A1203 scale formation during cyclic oxidation of Ni-Cr-AI(Zr) alloys containing more than 20% Cr.2. The success of the diffusion model in predicting N~ is strongly dependent on the ability to account for partial oxide spallation. The use of the oxide spalling model (16) provides this ability.3. It is not necessary to account for surface recession or the effect of the Cr concentration gradient on the diffusion of A1 when predicting minimum A1 concentrations for protective A1203 scale formation on Ni-Cr-AI(Zr) alloys.4. The decrease and subsequent recovery in the value of the solute concentration at the alloy surface is dependent on the amount of oxide which spalls each cycle and increases with an increasing number of thermal cycles. The increase in the amount of recovery with an increasing number of cycles is without a satisfactory explanation. ABSTRACTOxidation studies of CVD a-Si3N4 were performed in dry oxygen, oxygen-argon, and oxygen-nitrogen-argon gas mixtures of various oxygen and nitrogen partial pressures at a total pressure of I atm at 1100~176Parallel oxidation studies of single-crystal silicon were also conducted for direct comparison. It was observed that the oxidation of both Si3N4 and Si followed parabolic growth kinetics with activation energies of about 115 kcal/mol and about 30 kcal/mol, respectively. The formation of a single layer of SiO2 and evolution of N2 could not account for the much lower parabolic rate constants and much higher activation energy for Si3N4 than for Si during the oxidation. Detailed characterization of the oxidation scales using ellipsometry, step-by-step etching, SIMS, and XPS techniques indicated that a duplex oxidation scale consisting of SiO2 and SieN20 was formed when Si3N4 was oxidized. The intermediate Si2N20 scale was identified as a singlephase material, not a physical mixture of Si3N4 and SIO2. The low oxidation rate and high activation energy for SigN4 during the oxidation were attributed to the formation of Si2N20 and low oxygen diffusivity in this structurally dense phase. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.81.226.78 Downloaded on 2014-09-04 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.81.226.78 Downloaded on 2014-09-04 to IP ABSTRACTThe available thermodynamic and phase equilibria data for the binary systems Cd-Te, Cd-Se, and Cd-S have been analyzed using an associated solution model for the liquid phases and considering the CdTe, CdSe, and CdS phases as line compounds. The phase diagrams and the thermodynamic properties calculated from the model parameters agree well with the experimental data.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 129.81.226.78 Downloaded on 2014-09-04 to IP

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Homogeneous nucleation of diamond powder is reported. The experiments were performed in a low-pressure microwave-plasma reactor. The deposits were collected downstream of the reaction zone and subjected to wet oxidation to remove nondiamond carbons. The residues were analyzed by optical and electron microscopy, electron diffraction, and Raman spectroscopy. A variety of hydrocarbons diluted in argon, hydrogen, or oxygen gas mixtures were tested. In most cases only nondiamond materials, like graphite and carbyne, were obtained. Homogeneous nucleation of diamond was clearly observed in dichloromethane- and trichloroethylene-oxygen mixtures. The particles formed had crystalline shapes, mostly hexagonal. The largest particles were about 0.2 μm, although most of the particles were on the order of 50 nm in diameter. The powder was identified to be a mixture of polytypes of diamond.

An elementary-reaction mechanism of diamond growth by a vapor deposition process is proposed. The central postulate is that the main monomer growth species is acetylene. The mechanism basically consists of two alternating steps: surface activation by H abstraction of a hydrogen atom from a surface carbon and the addition of one or two acetylene molecules. During the addition reaction cycle a number of solid C–C bonds is formed and hydrogen atoms migrate from a lower to an upper surface layer. The mechanism is in general agreement with the macroscopic views of the Russian researchers and is consistent with the numerous experimental observations reported in the literature.

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