Kinetics of Crystallization of Stoichiometric SiO2 Glass in H2O Atmospheres

Crystallization rates were measured in vacuum, dry nitrogen, and water-saturated nitrogen atmospheres from 1300° to 1540°C. In all cases the observed rates were linear. Three reactions appeared to contribute to crystallization: the intrinsic crystallization, the impurity effect of H₂O vapor, and furnace contamination. Enhancement of crystallization by both water vapor and furnace contamination is attributed to the breaking of silicon—oxygen bonds of the glass structure. Competitive adsorption mechanisms were proposed to characterize the adsorption of water and impurity species. The activation energy for apparent intrinsic crystallization was 134 kcal/mole; the activation energy for crystallization in H₂O vapor was 77 kcal/mole.     

Kinetics of CeO2 Surface Area Reduction in a Mixture of HCI, H2O, and O2

Surface area reduction of a cerium dioxide powder is studied at 900 K in a HCI, H₂O, and O₂ atmosphere. The rate of surface area decrease is determined as a function of the surface chloride content and of the partial pressure of hydrogen chloride. It is shown that particle growth occurs due to the surface diffusion of cerium and hydroxyl ions, which results from hydrogen chloride adsorption. Adsorbed chloride ions react to give gaseous chlorine. The rate-determining step of the process of initial coarsening is hydrogen chloride fixation at the surface of cerium dioxide particles.     

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.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Kinetics of Anatase TiO2 Surface Area Reduction in a Mixture of HCl, H2O, and O2: l, Experimental Study

Changes in the surface area of anatase TiO₂ were investigated at 690 K as a function of the gas composition in mixtures of hydrogen chloride, water vapor, and oxygen. The chlorine content on the catalyst support was not dependent on the annealing time, which indicates that the adsorption equilibrium is reached very quickly. The kinetics of surface area were obtained as a function of partial pressure of HCI (from 0.1 to 40 kPa), water vapor (from 0.05 to 10 kPa), and oxygen (from 0.4 to 20 kPa). A mathematical expression for the rate of surface area loss was obtained which includes the partial pressure of water and the chlorine content at the surface of anatase.     

Kinetics of Anatase TiO2 Surface Area Reduction in a Mixture of HCI, H2O, and O2: II, Quantitative Modeling

From the experimental rate for surface area reduction of anatase TiO₂, expressed versus the partial pressure in H₂O and the surface content of chloride ion, a kinetic model is proposed in which two parallel mechanisms are involved. Two ways of material transport appear to contribute to the surface area loss: in the vapor phase and at the surface of the oxide. The related rate-determining steps are attributed to the formation of the volatile compound Ti(OH)₂Cl₂ in the first case, and to the diffusion of hydroxyl ions in the second one. At high partial pressure of water, the rate is enhanced by surface interactions between water molecules and chlorine-containing species.     

Equilibrium in the System CaSO4.2H2O-CaSO4.½H2O-H2O

Using a simple geometric model, the conditions of equilibrium for topotactical dehydration of gypsum to the hemihydrate in liquid H₂O were determined. The calculated equilibrium temperature agrees with the experimentally determined temperature at which the rate of topotactical dehydration of gypsum single crystals approaches zero.     

Phase Equilibria in the Systems NiO-Cr2,O3,-O2, MgO-Cr2O3−O2, and CdO-Cr2,O3,-O2, at High Oxygen Pressures

Phase equilibria were determined for the systems NiO-Cr₂O₃−O₂, MgO-Cr₂O₃,-O₂, and CdO-Cr₂O₃−O₂ from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO₄ structure) and the spinel NiCr₂O₄. The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO₄ and α-CdCrO₄ (both with the CrVO₄ structure), the high-temperature β-MgCrO₄ and β-CdCrO₄ (both with the α-MnMoO₄ structure), and the spinels MgCr₂O₄ and CdCr₂O₄. The cadmium system contains an additional phase, Cd₂CrO₅, which is primitive monoclinic.     

Stability of SrFeO3-Based Materials in H2O/CO2-Containing Atmospheres at High Temperatures and Pressures

The chemical stability of SrFeO₃-based perovskites in H₂O- and CO₂-containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO₂, p H₂O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)₂·H₂O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H₂O/CO₂ is important to understand in order to use these membrane materials for syngas production.     

Studies in Lithium Oxide Systems: XIII, Li,O.Al2O3.2SiO2-Li2O.Al2O3.2GeO2

Complete solid solubility was demonstrated to occur between LiAlGeO₄ and the low temperature form of Li AlSiO₂ (a-eucryptite). Hydrother-mal preparation was necessary for the silicate-rich compositions. Under atmospheric pressure, about 65 mole % LiAlGeO₄ entered the β-eucryβ-tite phase at 1150°C, but solid solutions containing more than 25 mole % LiAlGeO4 exsolved if held at lower temperatures. Directional thermal expansion data were obtained by X-ray diffraction methods on both α- and β-eucryptite and their solid solutions. Substitution of Ge⁴⁺ for Si⁴⁺ produced no significant difference in the thermal expansion coefficients in the α and β phases. An increase in the lattice parameters in the a and c directions took place as expected when Ge⁴⁺ (0.53 A) was substituted for Si⁴⁺ (0.39 A).     

Oxygen Fugacity Control in Nonflowing Atmospheres: I, Experimental Observations in CO/CO2 and O2/N2 Mixtures

The performance of a stabilized zirconia sensor and electrochemical oxygen pump combination in the isothermal control of oxygen fugacity in a nonflowing atmosphere was studied in a closed furnace system in the temperature range of 800° to 1100°C. Under certain conditions, large differences in oxygen pressure were found between sensors at the same constant temperature in close proximity to each other. This result reflected the existence of a stable steplike oxygen fugacity "front" in the furnace, separating regions which differed by several orders of magnitude in oxygen fugacity. This oxygen pressure profile in the furnace resulted from the steady-state transport of oxygen from oxygen leakage sources to the pump. The existence, magnitude, and position of the oxygen fugacity front were found to depend on the gas-phase composition, the relative locations of the leakage sources and the pump, the oxygen fugacity of the reference electrode, and the magnitude of the oxygen flux in the gas phase.     

Ractions of SiC with H2/H2O/Ar Mixtures at 1300°C

The reactions of a sintered α-SiC with 5% H₂/H₂O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O₂) > 10⁻²² atm) leads to a SiO₂ formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10⁻²² atm > P (O₂) > 10⁻²⁶ atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H₂O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O₂) < 10⁻²⁶ atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system.     

Electrical Conductivity of Y2O3 as a Function of Oxygen Partial Pressure in Wet and Dry Atmospheres

The electrical conductivity of polycrystalline Y₂O₃ has been studied as a function of the partial pressure of oxygen (10^–14 to 10⁵ Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P₂O₅. Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P _O2 and decreases with increasing P_H2O. At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P _O2 and increases with increasing P _H2O. In the Y₂O₃ samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P _H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P _O2 and low P _H2O, the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm². s^–1. V^–1     

Infrared Absorption Due to H2O and D2O in a Fluorozirconate Glass

A comparative study of infrared absorption due to H₂O and D₂O impurities in a fluorozirconate glass (53ZrF₄·20BaF₂·4LaF₃·3AlF₃·20NaF) was carried out. The H₂O and D₂O were introduced into the glass by reaction of the surface at 260°C with H₂O and D₂O vapor entrained in a stream of N₂. Reaction with H₂O produced IR absorption bands at 2.9 μm (O–H stretch) and 6.1 μm (H₂O bend). Reaction with D₂O produced bands at 3.9 μm (O–D stretch) and 8.3 μm (D₂O bend). The ratios of the corresponding D₂O/H₂O peak frequencies are 0.74 for both the stretching and bending vibrations, in good agreement with the value of 0.727 predicted from the difference in the OH and OD reduced masses.     

Surface Modification of Al2O3/TiC Cutting Ceramics

Cutting ceramics with a graded TiC-rich surface have been obtained by an in situ reaction. Al₂O₃/TiC ceramics, prepared by reaction sintering, have been subjected to various thermal posttreatments at atmospheric pressure. The temperature, dwelling time, and reaction atmosphere have been varied. It has been observed that a TiC-rich surface develops in CO-containing atmospheres. The microstructure of the bulk remains unaffected. Thermal treatment under a CO atmosphere at 1600 ° C for 1 h resulted in a dense TiC layer on the ceramic. A schematic model is proposed for this layer formation process.     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

The SiO2-SI3N4 Interface, Part II: O2 Permeation and Oxidation Reaction

Previous analyses of Si₃N₄ oxidation on the basis of diffusion control by a suboxide layer yielded impossibly high N₂ pressures. Those models assumed interfacial reactions as the oxidation mechanism. However, it is now thought that the oxidation process is in situ substitution of O for N in silicon oxynitride of graded composition rather than interfacial reaction. In this paper, diffusional and thermodynamic analyses appropriate to this mode of oxidation are developed for both the permeation and reaction aspects of oxidation; O₂ diffusivities are calculated from permeation energies; gas pressures in the oxide are derived from solution thermodynamics and found to be moderate.