R. E. Grace

Formation of point defects in strontium titanate

Abstract The electrical conductivity and thermoelectric power of single crystal strontium titanate were determined as a function of the P H 2 O P H 2 ratio over crystals at various temperatures. Electrical conductivity measurements were made in the temperature range from 900 to 1300°C; thermoelectric data were taken from 600 to 1300°C. A range of P H 2 O P H 2 ratios from 2 × 10−4 to 3 × 10−2 was investigated. Typical values of the electrical conductivity at a temperature of 1000°C and P H 2 O P H 2 ratios of 1× 10−3 and 1 × 10−2 were 1⋯2 Ω−1cm−1 and 0.53Ω−1cm−1, respectively; under the same conditions the thermoelectric power was found to be −0.600 mV/°K and −0.662 mV/°K. The thermoelectric measurements showed the crystals to be n-type. An oxygen vacancy defect model was found applicable to non-stoichiometric strontium titanate at elevated temperatures. In damp hydrogen the enthalpy of formation of an oxygen vacancy and two electrons in the conduction band was 86,500 ± 3000 cal mole .

Diffusion of point defects in strontium titanate

Abstract Single crystals of strontium titanate were equilibrated with damp hydrogen of fixed P H 2 O P H 2 ratio. Steps in the P H 2 O P H 2 ratio were imposed over the samples, and the tran electrical conductivity was measured until a new equilibrium was attained. By analysis of the transient electrical conductivity data the diffusion coefficient of oxygen vacancies was determined. The temperature dependence of the diffusion coefficient was obtained in the range from 900 to 1200°C. The enthalpy of motion for oxygen vacancies was found to be 6.0 kcal mole .

Formation of point defects in calcium titanate

Abstract The electrical conductivity and Seebeck coefficient of single crystal calcium titanate were measured as a function of temperature and P H 2 O ⧸ P H 2 . The electrical conductivity was determined at temperatures from 1100° to 1300°C. The P H 2 O ⧸ p H 2 ratio extended from 2 × 10 −4 to 8 × 10 −2 . At 1200°C the electrical conductivity was 3.70 Ω −1 cm −1 at P H 2 O ⧸ P H 2 = 10 −3 and 1.75 Ω −1 cm −1 at p H 2 0 ⧸ p H 2 = 10 −2 ; the Seebeck coefficients were − 500 and − 580μ v°⧸k under the same conditions. The electrical conductivity was proportional to the −1 3 power of P H 2 O P H 2 . The sign of the Seebeck coefficient indicated the majority carriers to be electrons. The results may be described by a majority defect model consisting of doubly ionized oxygen vacancies and electrons. The standard enthalpy of formation was found to be 93,000 ± 4000 cal⧸mol. of oxygen vacancies.

Intrinsic Diffusion and Vacancy Wind Effects in Ag-Cd Alloys

Diffusion in the Ag-Cd system was investigated at 600°C with vapor-solid and solid-solid diffusion couples in the α-phase for the determination of intrinsic diffusion coefficients, atomic mobilities and interdiffus ion coefficients. Intrinsic diffusion coefficients, DCd and DAg, determined experimentally, were compared with those predicted from the models of Darken, Manning, and Dayananda with the aid of tracer diffusion and thermodynamic data. The experimental coefficients for various alpha alloys were found to be higher than those evaluated from each of the three models. On the other hand, the experimental values of the ratio, DCd/DAg were found to be in good agreement with the models of Manning and Dayananda where the vacancy wind effects on intrinsic diffusion are taken into account. Darken’s relations are considered inadequate to interrelate tracer and intrinsic diffusion coefficients in the Ag-Cd system, since vacancy wind effects are found to be appreciable in this system.

Diffusion in ternary Ag-Zn-Cd solid solutions

Diffusion in the Ag-Zn-Cd system was investigated at 600°C with semi-infinite, vapor-solid couples in order to evaluate intrinsic diffusion coefficients and interdiffusion coefficients in theα andβ phases. Intrinsic diffusion coefficients were determined at several composition points in theα-phase region along concentration lines of 11 at. pct Zn, 18 at. pct Zn, 5 at. pct Cd, and 9 at. pct Cd. Interdiffusion coefficients in theα phase were determined at several points of intersection of various composition paths; these composition points of intersection were along lines of constant zinc-to-cadmium concentration ratios of 3.8, 1.8, and 1.2. All two-phase diffusion couples developed planar β/α interfaces. Intrinsic coefficients were computed at three composition points in the β region corresponding to a silver concentration of approximately 55 at. pct. Intrinsic coefficients for β alloys are two to three orders of magnitude larger than those for α alloys.

Diffusion of point defects in calcium titanate

Abstract Single crystal samples of calcium titanate were equilibrated with water-hydrogen atmospheres of a known P H 2 O ⧸ P H 2 , ratio at temperatures of 1100°, 1200° and 1300°C. After a step in the P H 2 O ⧸ P H 2 ratio over the sample, the electrical conductivity was measured continuously until a new equilibrium was attained. Diffusion coefficients were determined by analysis of the transient electrical conductivity data. The effective diffusion coefficient at 1200°C and P H 2 O ⧸ P H 2 = 10 −3 was 5.2 × 10 −6 cm 2 sec −1 ; the enthalpy of motion of oxygen vacancies was 13,000 cal⧸mol.

Diffusion studies in Ag-Zn alloys

Diffusion studies were made in the alpha and beta phases of the silver-zinc system at 600°C with vapor-solid and solid-solid diffusion couples in order to determine intrinsic diffusion coefficients and interdiffusion coefficients. Intrinsic diffusion coefficients, DZn and DAg, determined experimentally for bothα andβ alloys, were compared with those predicted from the models of Darken, Manning, and Dayananda with the aid of thermodynamic and single crystal tracer diffusion data. The experimental values of Dzn were close to those predicted by the models of Darken and Manning, while the experimental DAg values were higher than those determined by both models. The experimental values of the ratio, DZn/DAg, were found to be lower than those predicted by either Darken’s or Manning’s relations. Also, Dayananda’s model was found to be inapplicable with the available tracer diffusion data. This disagreement is considered to reflect the inadequacy of the single crystal tracer diffusivities in the analysis of intrinsic diffusion in polycrystalline Ag-Zn alloys.

Transition kinetics during linear to parabolic oxidation of chromium

Abstract The transition kinetics during linear to parabolic oxidation of chromium in H 2 O-H 2 gas mixtures have been measured thermogravimetrically. Based on a cation-deficit defect structure for Cr 2 O 3 , the transition follows a model first proposed by C. Wagner. Under conditions not far removed from the Cr, Cr 2 O 3 , H 2 O-H 2 equilibrium the linear region of oxidation is controlled by the adsorption and dissociation of H 2 O on Cr 2 O 3 ; the phase boundary reaction is independent of the concentration of cation vacancies at the Cr 2 O 3 /gas phase boundary.

Atomic mobilities and vacancy wind effects for diffusion in ternary Silver-zinc-cadmium solid solutions

Diffusion in the silver-zinc-cadmium system was investigated at 600°C with semiinfinite, vapor-solid couples in order to determine the atomic mobilities and contribution of the vacancy wind effect to the intrinsic diffusion fluxes of silver, zinc and cadmium in the alpha phase region. Atomic mobilities and vacancy wind parameters were computed at selected compositions in the alpha phase along isoconcentration lines of 11 at. pct Zn, 18 at. pct Zn, 5 at. pct Cd and 9 at. pct Cd. The existence of significant interactions between the diffusing species was indicated by the results of an earlier investigation; consequently, the vacancy wind effect was taken into consideration in the calculation of the mobilities. The mobilities of zinc and cadmium, determined by the method of Dayananda, were found to increase with increasing zinc and cadmium concentrations with the mobility of zinc being greater than that of cadmium. The results of the investigation show that the vacancy wind effect significantly affects the total intrinsic flux of each of the three species for diffusion in ternary silver-zinc-cadmium alloys. This investigation reports the experimental observation of the vacancy wind effect in a ternary system.

Diffusion-Limited Sulfidation of Wustite

The sulfidation of wustite in H2S−H2O−H2−Ar atmospheres has been studied at temperatures of 700, 800, and 900°C with thermogravimetric techniques. Polycrystalline wustite wafers were equilibrated in a flowing H2O−H2−Ar gas stream and then sulfidizedin situ. During an initial transient stage a protective layer of FeS formed on the sample, and an intermediate layer of Fe3O4 formed between the FeO and FeS layers. Subsequently, the reaction followed a parabolic rate law. The parabolic rate constant varied from 0.22×10−2 mg2 cm−4 min−1 at 700°C to 6.5×10−2 mg2 cm−4 min−1 at 900°C. The reaction rate was limited by the diffusion of iron through the intermediate Fe3O4 layer which grew concurrently with the FeS layer and at the expense of the FeO core. After the FeO core was completely converted to Fe3O4, the process entered a passive stage during which no further mass changes could be detected.