Directional solidification of (Re = Y, Nd): microstructure and superconducting properties

Single-domain ceramics of (Re = Y, Nd) with high and high critical currents have been prepared in air by means of directional solidification and the leading microstructural factors controlling the pinning of vortices have been analysed. The interface pinning mechanism at secondary inclusions has been found to be more effective in than in in agreement with HRTEM pictures showing sharper interfaces in the first case. The observation of high and a vanishingly small fishtail effect in Nd123/422 composites suggest that the concentration of Nd - Ba antisite defects is minimized during the directional solidification process in air. Finally, it is demonstrated by TEM analysis that Nd123/422 ceramics show distinctive microstructural features compared with Y123/211 which could enhance the high-field critical currents and the irreversibility line. Frequency-dependent ac susceptibility studies provide evidence, however, that the thermal activation of vortices is enhanced in Nd123 as compared with Y123. It is suggested that incomplete oxygenation reduces the effective pinning energies.