Extensive grain growth was observed by scanning electron microscopy in very porous AlzO,< compacts, even a t densities <40% of theoretical. After 3% shrinkage a t 17OO0C, the grain size increased from ~0 . 3to 0.51 pm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.

Magnetically aligned samples of sintered YBa&Cu307 z were used to test the effects of oxygendeficiency 5 (with 5 0.2) on the superconductive magnetization M, critical current density J" irreversibility field B;",upper critical field H, t, coherence length g, condensation energy F"penetration depth A, , and related properties as a function of temperature T and applied field H~~c. In selected cases, studies were also made with H~~ab. The 0 content was monitored in situ by thermogravimetric analysis. The open porosity and granularity of the material allowed rapid and homogeneous oxygenation. We found no significant enhancement of intragrain J, with chain-site 0 defects. With few exceptions, maximum J, occurred at full oxygenation. This implies that chain-site 0 defects are not strong or effective pinning centers over most of the field-temperature regime investigated. Except for T"which was practically independent of 5 within the interval 0 & 5~0. 11 (so called "90-K T, plateau" ), most properties such as J" F"H,z, B;", (T), A, , and g were strongly and continuously influenced by the 0 deficiency. The observed abnormal magnetization with H~~c was weak at low T but became more pronounced as T and fi increased. No abnormal magnetization was detected with H~~ab. As oxygen was removed, B;"(T)and H,~(T) separated, and both lines shifted to lower T and lower B. Moreover, B;,"was strongly correlated with J, at low temperature. Determination of the thermodynamic critical field H, yielded condensation energies F,(5) that were well correlated with J,(5). The results indicate that 0 defects weaken the energy barrier of the existing pinning sites and have a negative overall effect on the capacity of YBazCu307 to carry loss-free currents.

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The theory of grain growth in the presence of a liquid phase is examined using modifications of equations derived for coalescence of solid particles widely dispersed in a liquid. Although the grain diameter-time relation can still be represented by d3 = k t , the absolute growth rates are increased a s the amount of liquid is decreased. The grain growth kinetics in UOZ compacts containing 0.5 wtyo A1203 were studied for temperatures between 1960' and 2200°C. T h e interrelation of grain size, temperature, and time is in agreement with that predicted by the theory.