THE EFFECT OF HOT ISOSTATIC PRESSING ON THE MICROSTRUCTURE AND CRITICAL-CURRENT DENSITY OF DIRECTIONAL SOLIDIFIED YBA2CU3OX SUPERCONDUCTORS

YBa2Cu3O(x) (123) high-temperature superconductors (HTSS) obtained by conventional directional solidification have been shown to be capable of carrying substantial critical currents (I(c)), and can be used in p ractical applications such as current leads. When these multidomained HTSS are subjected to thermal shock, however, the I(c) is found to be degraded. In order to ascertain the cause of this decrease, microscopi c studies have been performed. The results indicate that the decrease in I(c) can be linked to the generation and propagation of microcracks between the a-b plane grain boundaries. Hot isostatic pressing (HIPIN G) has been found to be effective in eliminating this degradation, and the I(c) of HIPED HTS samples is no longer sensitive to thermal cycli ng. Microstructural observations revealed that HIPING resulted in a re duction in the amount and the size of microcracks as compared with the rmally cycled HTSS. Moreover, large macrocracks are found to be bridge d by reaction products. The healing of microcracks and the strengtheni ng of a-b plane grain boundaries as well as the transformation of macr ocracks are believed to be responsible for the improved superconductin g and mechanical stability of the HIPED HTSS.