The oxidation kinetics were determined for single-crystal SrTi03 by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850" and 1460°C are represented b y D = 0.33 exp (-22.5 =t 5.0 kcal/RT) cmz/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 X exp (-26.1 + 5.0 kcal/R T ) cm2/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [Om], fixed by impurities in the fully oxidized crystal is calculated to b e 1.6 X lod5 by virtue of the relation between the oxygen self-diffusion coefficient, DOZ-, and the oxygen vacancy diffusion coefficient, Iloo; Do$-= [Oo]Don where the oxygen ion concentration [02-] is taken as unity.
The self-diffusion coefficient of aluminum has been determined in polycrystalline aluminum oxide over the range 1670° to 1905°C. Diffusion couples were employed using aluminum-26 in oxide form as a radioactive tracer. In the temperature range studied, experimental data can be represented by D=28 exp (-[(114 OOO±15 000)/ RTJI.
Phase Equilibria in the Ferrite Region of the System FeO-MgO-Fe203 183sulfuric acid solution. The sulfuric acid solution becomes more concentrated as evaporation takes place, and the acid attacks the alkali and alkaline-earth silicates. Upon complete evaporation, sulfates of these ions are deposited on the surface of the ware where evaporation is most rapid. These efflorescing salts are in addition to those which may migrate to the surface from sulfate compounds in the product. It was interesting to note that in the two crude clays tested, the efflorescing salts were the same whether the sulfate was derived from a sulfuric acid solution or from firing in a sulfurous atmosphere. This would indicate that the adsorption of sulfur trioxide groups was a rather important part of the efflorescence problem. V. Summary(1) Pure kaolin clay can adsorb enough sulfur trioxide during firing to cause efflorescence.(2) The adsorbed sulfate converts pure water migrating through the pore structure of the ceramic body into a sulfuric acid solution. The acid solution becomes concentrated by evaporation and attacks the silicate phases of the product.Efflorescence composed of sulfate salts is formed when the acidic solution evaporates from the surface of the ware.(3) Sulfate salts in the product can persist throughout the firing operation if time and temperature are not provided to complete their reactions with silicates.(4) Alkali and alkaline-earth impurities in clay contribute to the formation of sulfates when the clay is fired in a sulfurous atmosphere. The reluctance of these sulfates to react subsequently with the silicates accounts for the higher sulfate retention when these impurities are present in the clay.(5) Calcium sulfate is the most persistent sulfate in clay bodies of those studied. This is due to the higher reaction temperature required for calcium sulfate to react with silicates. AcknowledgmentThe writer is deeply grateful to the Structural Clay Products Research Foundation for its help and encouragement in this work. Thanks are extended to E.Phase equilibria in the femte region of the system Fe0-Mg0-Fe203 have been investigated up to 1300°C. over an oxygen-pressure range of 1.0 to 0.01 atmospheres. The five most important features of the equilibria in this system are as follows : (1) The spinel phase can exist with a deficiency of oxygen in the high-magnesium compositions, (2) Mg++ replaces Fe++ in Fer04 beyond the "stoichiometric MgFe204," corresponding to the formula (MgO),MgFe?O,, where x = 0.092 =t 0.004 and is independent of temperature, (3) the spinel field does not include a single-phase composition corresponding to MgFez04, (4) the lattice constant of spinels in this system depends on cation distribution, and the extent of distribution changes as a function of temperature in turn depends on the magnesium concentration in the spinel, and (5) the spinel field near its terminus has little or no width.
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