Superconducting properties of sintered ex situ MgB2 tapes through ball milling process as a function of crystallite size in the as-milled and sintered states

Abstract

Ball milled MgB2 powders and ex situ processed tapes using those milled powders were sintered at various temperatures. Critical temperature (Tc) increases moderately with increasing sintering temperature (Tsin) up to 705 – 735 °C, at which best performance in transport critical current density (Jc) at 4.2 K in 10 T for the tape samples is obtained. In contrast, the Tc rapidly increases when sintered above that temperature. Although the transport Jc property is improved with increasing the Tsin up to this optimal Tsin, further increase of the Tsin degrades the Jc property in the high-field region. These behaviors of the superconducting properties are attributed to the crystallite growth. The reduced crystallite size brought about through milling process recovers by sintering, and this recovery is remarkable above the optimal Tsin. With increasing the crystallite size, the Tc increases, whereas both upper critical field (Hc2) and irreversibility field (Hirr) decrease. While MgB2 decomposition is promoted above the optimal Tsin, grain connectivity is improved with increasing the Tsin. Thus, high-temperature sintering not only improves the grain connectivity and hence the Jc property in the low-field region, but also brings about large Jc degradation in the high-field region due to the decrease in Hc2 and Hirr at 4.2 K. Since the crystallite size in the as-milled and sintering states are linked with one another, those superconducting properties of the optimally sintered tapes in which MgB2 decomposition is ignorable are easily deduced from the crystallite size of MgB2 in the filling powders.