Modified melt crystallization processes for improved RE-123 based bulk materials (RE = Y, Nd)

The considered modified melt texturing processes benefit by peculiarities of the quaternary phase diagram. Examples are discussed for YBaCuO by applying compositions which represent a mixture of Y123 and Y2O3 instead of Y2BaCuO5 or by processing with reduced oxygen partial pressures. The preferable orientation of 'single grains is initialized by seeding with Sm123. Large single grains (up to 2 in), high critical current densities (45 kA/cm(2)), high trapped magnetic fields (> 0.7 T at 77 K in 1 in discs, and 9.6 T in a minimagnet at 47 K) and effectiveness for levitation (1.8 N per 1 g YBaCuO and 84 N total for 35 mm discs) have been achieved in the modified processed YBaCuO materials. In contrast to Y123, nonstoichiometric metal-metal ratios are typical for the Nd123 superconductor and related solid solutions. The corresponding defects may enhance pinning but on the other hand they deteriorate T-c. The results of phase diagram investigations are used as a tool to control the cation stoichiometry and growth mechanism. This can be experimentally achieved by using an appropriate composition or by the oxygen potential controlled isothermal process. Critical current densities up to 75 kA/ cm(2) have been achieved in NdBaCuO materials. (C) 1998 Elsevier Science Ltd. All rights reserved.