Alumina Volatility in Water Vapor at Elevated Temperatures

The volatility of alumina in high-temperature water vapor was determined by a weight loss technique. Sapphire coupons were exposed at temperatures between 1250° and 1500°C, water partial pressures between 0.15 and 0.68 atm in oxygen, a total pressure of 1 atm, and flowing gas velocities of 4.4 cm/s. The water vapor pressure dependence of sapphire volatility was consistent with Al(OH)₃( g ) formation. The enthalpy of reaction to form Al(OH)₃( g ) from sapphire and water vapor was determined to be 210 ± 20 kJ/mol, comparing favorably to other studies. Microstructural examination of tested sapphire coupons revealed surface rearrangement consistent with a volatilization process.     

Variation of the Oxidation Rate of Silicon Carbide with Water-Vapor Pressure

Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000°-1400°C in H₂O/O₂ gas mixtures with compositions of 10-90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H₂O/argon mixtures at a temperature of 1100°C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water-vapor effects were observed. The oxidation kinetics increased as the water-vapor content increased. The parabolic oxidation rates in the range of 10-90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200°-1400°C. The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide-scale morphology with the partial pressure of water vapor and the temperature.     

Effect of Swelling on Thermal Conductivity and Postirradiation Densification of U3Si

The effect of swelling porosity on the thermal conductivity of irradiated U₃Si fuel was deduced from electrical conductivity measurements on samples with pore volume fractions of 0.01 to 0.27. The Maxwell-Eucken expression adequately represents the data, with β= 1.9±0.1. Postirradiation annealing of samples at 400° to 750°C produces densification. Calculations confirm that the pores in as-irradiated fuel are voids possibly containing a small amount of fission gas. Those retained after annealing are fission gas bubbles with internal pressure nearly in equilibrium with the surface tension of the surrounding solid.     

Diffusion of Water into Silica Glass at Low Temperature

Diffusion of water into silica glass was measured in the temperature range of 200° to 750° by treating the glass in air containing a constant water vapor pressure and analyzing the concentration profile using a Fourier transform infrared spectrometer. In the short-time diffusion heat treatments, the surface concentration was lower and the apparent diffusion coefficient was higher than the corresponding steady-state values. The temperature dependence of the steady-state diffusion coefficient showed two different activation energies. Above ∼550° the diffusion coefficients were similar to the published results with an activation energy of ∼80 kJ/mol, while below ∼550°, the diffusion coefficient was higher than the value obtained by extrapolation from higher temperatures, and the activation energy was ∼40 kJ/mol. Correspondingly, the water solubility–temperature relation showed a sudden change at around the same temperature: at temperatures above this temperature the solubility increased with decreasing temperature, while at lower temperatures the trend was reversed. It was suggested that this observed peculiarity was caused by the initial nonequilibrium reaction between water and SiO₂ glass and a change in enthalpy of the glass–water reaction.     

Use of Stabilized ZrO2 to Measure O2 Permeation

The rate of permeation of CaO-stabilized ZrO₂ (CSZ) by O₂ gas was measured from 640° to 1200°C with the CSZ tubing used simultaneously as the sample and the O₂ pressure detector. The apparent permeation rate depended significantly on the O₂ pressure at the low-pressure side. The rate measured by this method was orders of magnitude smaller than that measured under steady-state conditions, except when the O₂ partial pressure was high (>10⁻⁴ atm), in which case the agreement was good. The difference between steady-state permeability and non-steady-state permeability is related to the deviation in stoichiometry in a sample or detector. The transient response (measured under variable pressure difference) may be very different from steady-state permeation (measured under constant differential pressures across the membranes). To apply CSZ to typical O₂ gas permeability measurements, the O₂ pressure must be kept above ∼ 10^−3.5 atm. In this range, the permeability of CSZ may be regarded as a temperature-dependent material property which is governed by the electron-hole mobility. At lower O₂ pressures the permeation rate is a more complex function of the pressure difference and level.     

Electrical Conductivity and Defect Structure of Y2O3 as a Function of Water Vapor Pressure

The electrical conductivity of several sintered yttria ceramics with different levels of lower valent cation impurities was studied as a function of the partial pressure of water vapor (3 to 1600 Pa) at 500° to 1300°C in oxygen or air. At the higher temperatures yttria is a P -type conductor, but at the lowest temperatures and highest water vapor pressures, an ionic contribution becomes significant. The p -type conductivity decreases with increasing water vapor pressure. This is interpreted in terms of a model involving dissolution of hydrogen as interstitial protons and which counterbalance the effective charges of the lower valent impurities. At the higher water vapor pressures and lower impurity levels excess protons are probably compensated by interstitial hydroxide or oxygen ions. Indications of grain boundary segregation of impurities are reported.     

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

A high-pressure sampling mass spectrometer was used to detect the volatile species formed from SiO₂ at temperatures between 1200° and 1400°C in a flowing water vapor/oxygen gas mixture at 1 bar total pressure. The primary vapor species identified was Si(OH)₄. The fragment ion Si(OH)₃⁺was observed in quantities 3 to 5 times larger than the parent ion Si(OH)₄⁺. The Si(OH)₃⁺ intensity was found to have a small temperature dependence and to increase with the water vapor partial pressure as expected. In addition, SiO(OH)⁺, believed to be a fragment of SiO(OH)₂, was observed. These mass spectral results were compared to the behavior of silicon halides.     

Spontaneous Bubble Formation in Silicate Melts at High Temperatures

Heating previously melted glasses usually produces gas bubbles in the melt at constant pressure. Bubble formation generally occurs in a narrow temperature range; if experimental conditions are carefully controlled the temperature at which gas bubbles form and the characteristics of the bubbles can be reproduced with reasonable accuracy. Extensive measurements were made with binary silicates of lithia, soda, and potash. Significant bubble evolution occurred only when there was sulfur in the glass. Bubbles formed at quite high temperatures (1400° to 1500°C) in dry oxygen but did not form in reducing atmospheres and quite stable foams resulted. In dry reducing atmospheres some bubble formation was observed at lower temperatures (1200° to 1300°C). The behavior was considerably altered in atmospheres containing water vapor and by changes in base glass composition or sulfur content.     

Kinetics of Vapor-Phase Hydration of Magnesium Oxide

Magnesium oxide prepared by calcining the carbonate at 1000°C was sintered at 1800°C and ground under methanol. Well-sized 2 to 5 and 5 to 10 μ fractions were separated. After outgassing at 550°C in vacuum, the powders were hydrated at various temperatures to 98°C in a wide range of water vapor pressures, and the reaction was followed gravimetrically with a silica helix balance. At low temperatures and vapor pressures, where nucleation is exceedingly slow, the hydration of prenucleated samples was studied. The results are interpreted on the basis of a model in which an interface reaction controls the progress of the reaction. The reaction rate varies with the water vapor pressure p according to a function ( p / p * - 1), where p 0.3 p _sat. and appears to correspond to the equilibrium adsorption pressure. This suggests that the reaction depends on an adsorbed water vapor film which has free access to the reacting surface. The reaction rate constant varies with temperature according to an Arrhenius-type equation, with an activation energy 16,100 cal per mole.     

Effect of Water Vapor on Initial Sintering of Magnesia

The effects of water vapor on the initial sintering of magnesia powder compacts were studied from 800° to 1107°C. The magnesia was obtained by thermal decomposition of magnesium oxalate. Water vapor partial pressures from 8 × 10 ^–4 to 658 mm were used. Initial sintering occurred by a grain-boundary vacancy-diffusion mechanism with magnesium assumed to be the slow moving species. Increasing water vapor partial pressure increased sintering rates. A model based on the solubility of the hydroxide ion in MgO relates this increase to the increase in the cation vacancy concentration. The grain-boundary diffusion coefficient, D_G , varied with P _H2O. For partial pressures up to about 5 mm, n was approximately ½ and for partial pressures above 5 mm it was between 1.0 and 1.5. The activation energy for densification was 80 kcal/mole for partial pressures up to about 5 mm and 48 kcal/mole above this partial pressure.     

Effects of Complex Precursors on Alkoxide-Derived Cordierite Powder

Preparation of chemically homogeneous cordierite powders of submicrometer size was investigated using alkoxide precursors, synthesized from Mg metal, Al(OBu^s)₃, and partially prehydrolyzed Si(OEt)₄. The amount of partial prehydrolysis water in precursor synthesis was changed to study the effects on the hydrolysis products. In the hydrolysis of the diluted solution of the precursor, product morphology was quite sensitive to the precursor, and different morphologies such as gelation, gel precipitation, and powder precipitation were observed. A redispersable powder of submicrometer size was obtained only from the precursor synthesized at a H₂O/Si(OEt)₄ ratio of 1.2. Differences were also observed in minor crystalline phases among calcined hydrolysis products. The alkoxide precursors were characterized using Raman spectroscopy, gas chromatography, and NMR spectroscopy. The differences in hydrolysis products are most likely due to differences in hydrolysis and condensation of different chemical species in precursors.     

Role of Water Vapor in Crystallite Growth and Tetragonal-Monoclinic Phase Transformation of ZrO2

The role of water vapor in crystallite growth and the tetragonal-to-monoclinic phase transformation of ZrO₂ was studied using three specially prepared samples: an ultrafine powder of monoclinic ZrO₂ obtained by hydrolysis of ZrOCI₂, an aggregated powder of tetragonal ZrO₂ obtained by thermal decomposition of Zr(OH)₄ under reduced pressure, and an ultrafine powder of tetragonal ZrO₂ obtained by thermal decomposition of zirconyl acetate dispersed in caramel. The samples were heat-treated up to 1000°C in dry and wet atmospheres saturated with water vapor at 90°C. It was found that water vapor markedly accelerated crystallite growth for both monoclinic and tetragonal ZrO₂ and facilitated the tetragonal-to-monoclinic phase transformation. Water vapor increases surface diffusion and thus enhances crystallite growth and decreases surface energy, which leads to stabilization of the tetragonal phase.     

Oxidation of Chemically-Vapor-Deposited Silicon Nitride and Slnglem-Crystal Silicon

Oxidation of {111} single-crystal silicon and dense, chemically-vapor-deposited silicon nitride was done in clean silica tubes at temperatures of 1000° to woo°C. The oxidation rates of silicon nitride under various atmospheres (dry O₂, wet O₂, wet inert gas, and steam) were several orders of magnitude slower than those of silicon under the identical conditions. The activation energy for the oxidation of silicon nitride decreased from 330 to 259 kJ/mol in going from dry O₂ to steam while that for Si decreased from 120 to 94 kJ/mol. The parabolic rate constant for Si increased linearly as the water vapor pressure increased. However, the parabolic rate constant for silicon nitride showed nonlinear dependency on the water vapor pressure in the presence of oxygen. The oxidation kinetics of silicon nitride is explained by the formation of nitrogen compounds (NO and NH₃) at the reaction interface and the counterpermeation of these reaction products.     

Borate Volatility from SOFC Sealing Glasses

The volatility of borate species from glasses developed for solid oxide fuel cell seals was studied using thermodynamic calculations and compared with experimental results. Vapor pressure diagrams were used to identify the most volatile compounds under a range of expected operational conditions, e.g. oxidizing and reducing atmospheres with water vapor, at temperatures in the range of 700°–1000°C. The species with the highest vapor pressures were BO₂( g ) under dry conditions and B₃H₃O₆( g ) under wet, reducing conditions. The depletion of boron from glass surfaces to depths beyond 100 nm was characterized using Auger electron spectroscopy depth profile analysis. Weight loss experiments were conducted on several different glass compositions. The cumulative weight loss from a glass with 20 mol% B₂O₃ ("glass #59") was about 10 times greater than from a glass with 2 mol% B₂O₃ ("glass #27"), under the same conditions. The activation energy for volatilization from glass #59 was 371±86 kJ/mol and was 272±65 kJ/mol for "glass #27." The cumulative weight loss of each composition in forming gas with 30% water vapor was greater than in dry air at 800°C. Volatile species were collected in a water trap, and these results confirmed predictions about the effect of atmosphere and B₂O₃ content on volatilization behavior.     

Determination of Henry’s law constant and the diffusion and polytropic coefficients of air in aviation fuel

The volume change of air microbubbles on surface nucleation sites was studied experimentally and compared with predictions. Measurements were used to determine the polytropic constant, the diffusion coefficient, and the Henry’s law constant of air in distilled water, dodecane, and JP-8 aviation fuel. The liquids were exposed to sub-atmospheric pressures, but above their vapor pressures. In one type of experiment, bubble size reduction was recorded as the liquid’s ambient pressure was increased from a low pressure to atmospheric pressure though a series of step increases. The results were used to determine the polytropic constant. In another type of experiment, bubble growth was monitored in time following a sudden reduction in the liquid’s pressure from ambient. The Epstein-Plesset model of mass diffusion was coupled with a Lipschitzian optimization technique to determine the values of the diffusion coefficient and Henry’s law constant.     

On the dynamics and control of through-plane water distributions in PEM fuel cells

We provide a semi-analytic solution to simplify an experimentally validated numeric realization of a two-phase, reaction-diffusion, distributed parameter model of the through-plane water distributions as they evolve inside polymer electrolyte membrane (PEM) fuel cell gas diffusion layers. The semi-analytic solution is then analyzed for stability and to gain insight into the dynamics of the equilibrium (steady-state) water distributions. Candidate distributions for vapor and liquid water are then identified which allow maximum membrane hydration while simultaneously avoiding voltage degradation that results from anode liquid water accumulation (flooding). The desired anode water distributions could be maintained via control of the anode channel conditions (boundary value control) with the ultimate goal to maximize the hydrogen utilization and prolong fuel cell life.     

Mathematical analysis of transport properties of polymer films for food packaging. II. Generalized water vapor models

Deterioration or spoilage of dehydrated food products stored in flexible packaging materials depends on the partial pressure of water vapor in the environment of the stored food. Mathematical analysis of the diffusion of water vapor through semipermeable polymer films using Nernst-Planck equations is combined with non-liner water sorption isotherms on food to establish criteria and optimum conditions for storage stability of dehydrated food. Langmuir, Brunauer-Emmet-Teller (BET), Halsey, Oswin and Freundlich isotherms are used for various ranges of water activity. It is shown that a single parameter, the permeability-sorption constant, based on the physical properties of the polymer and the sorptive properties of the food, accounts for both diffusion and adostption and can be used to accurately determine maximum storage times and can be used to accurately determine maximum storage times and to optimize the selection of packaging films. The theory is extended to thermodynamically compatible solute-polymer systems, where the polymer film is swollen appreciably by the diffusing species.     

Kinetics of High-Temperature Hydrolysis of Magnesium Fluoride: I, Evaluation of Reaction Mechanism

The rates of hydrolysis of polycrystalline magnesium fluoride slabs were investigated by a gravimetric method as functions of temperature from 745° to 835°C and water-vapor pressure from 1 to 20 mm Hg. A linear rate law was found, and it was concluded that the rate-determining process was chemical reaction at the MgF₂-MgO interface. A three-step reaction mechanism is proposed: (1) the chemisorption of water vapor.

(2) formation of a hydroxyfluoride complex.

and (3) decomposition of the hydroxyfluoride complex to form products.

Assuming that the chemisorption step was in equilibrium and that the HF pressure was negligible, an expression was developed showing that the rate was proportional to the concentration of chemisorbed water vapor. The rate dependence on water-vapor pressure was expressed in terms of the Langmuir adsorption isotherm equation. Values of –16.9 kcal/mole and–4.2 eu were obtained for the enthalpy and entropy of chemisorption. The activation enthalpy and frequency factor for the decomposition of the chemisorbed intermediate, MgF₂.H₂O(chem), were 46.0 ± 0.5 kcal/mole and 1.75 × 1O¹⁰ sec⁻¹.     

Effects of Water Vapor on Oxidation of Silicon Carbide

The rate of oxidation of silicon carbide was studied at different partial pressures of water vapor. The diffusion-rate constant was found to vary with the logarithm of the partial pressure of water vapor according to the theory of thin-film oxidation as proposed by Engell and Hauffe. The products of oxidation were cristobalite and tridymite, depending on the temperature. The diffusing species appeared to be the same in the presence of partial pressures of water vapor and in the presence of partial pressures of oxygen.     

Competitive sorption of water vapor and CO2 in photocrosslinked PVCA film for a capacitive‐type humidity sensor

The sorption behavior of water vapor and CO₂ gas in photocrosslinked poly(vinyl cinnamate) (PVCA) film was examined at 30°C under atmospheric pressure. Both the water sorption isotherm and the CO₂ sorption isotherm obtained with quartz crystal microbalance (QCM) method obeyed the simple Langmuir's equation. Water vapor/CO₂ mixed‐gas sorption isotherms were also obtained. Total amount of sorbed mixed gases was clearly influenced by the partial pressure of water vapor (p_w) and CO₂ gas (p_c) in the atmosphere. A modified Langmuir's equation based on a dual‐site model was employed for predicting the competitive adsorption isotherm, and the isotherm was clearly described by the equation. The theoretically estimated amount of adsorbed water at the constant p_w decreased slightly with increasing p_c. The effect of this phenomenon on the sensitivity of the capacitive‐type relative humidity sensor was examined. As expected, the electrical capacitance of the sensor at the constant relative humidity decreased because of the coexistence of CO₂ gas. However, the influence was quite small in the CO₂ concentration range in the ordinary environment. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 401–407, 2002