GRAIN-GROWTH KINETICS AND MICROSTRUCTURE IN THE HIGH-TC YBA2CU3O7-DELTA SUPERCONDUCTOR

The grain growth and microstructure development of YBa2Cu3O7-delta have been investigated utilizing two different starting particle size distributions (normal and bimodal). The grain growth exponent, n, was found to be about 0.21 for both normal and bimodal samples. An activation energy of 125 kJ/mole was calculated. The low value of n might be attributed to the high anisotropy of grain boundary energy in this system. Samples made from the bimodal powder were found to accelerate grain growth without introducing abnormal grain growth. Although most of the samples attained fractional densities greater than 0.95, the presence of various amounts of porosity (particularly in the case of the bimodal starting powder) did not affect the growth kinetics. The measured aspect ratio of grains did not significantly change during growth. A significant difference in aspect ratio was measured between samples made from the two different starting powders. Critical currents ranged from 10 to 120 A/cm2, but no concrete relationship with grain size was established. This implies that the grains produced by this experiment were in the size range where other factors, presumably microcracking, severely limited the current carrying capacity by the weak link effect.