Microstructure of Nd-Ba-Cu-O superconductors prepared by the oxygen-controlled-melt-growth method

Microstructure. analyses by polarized light microscopy show that N4Ba2Cu2O10 (Nd422) particles in oxygen-controlled-melt-grow NdBaCuO bulk superconductors have similar influence on the NdBa2Cu3O7-x (Nd123) microstructure as Y211 particles in the Y123-211 system. The twin spacing decreases with shortening of mean distance between Nd422 particles. The a-b microcracking in the 123 matrix was ascribed to thermal stresses around Nd422 particles set up in the matrix during cooling due to the different thermal expansion of Nd123 and Nd422 phases. The relaxation of thermal stresses in the [100] directions by twin boundary motion was related to the observed rosette of detwinned regions or regions with predominance of one twin variant formed around Nd422 particles. The level of thermal stresses was estimated. The elongated c- and a-subgrains with rectangular cross section were shown to be a result of cellular growth, i.e. the subgrains in melt-processed REBaCuO (RE: rare earth) are growth related crystal defects.