The improvement of in-situ powder in tube MgB2 tapes by mixing MgB2 to the starting powder of MgH2 and B

The MgB2 tapes fabricated by the in-situ powder in tube (PIT) technique were investigated with respect to the relationship between the starting powder(s) and the critical current density, Jc. A powder of MgB2 or a mixture of Mg and B (Mg + 2B) powders was added to the mixture of MgH2 and B (MgH2 + 2B) powders in a ratio of x:(1 − x). The amount of x was controlled from 0 to 1, and the synthesis condition were 600 °C for 1 h in Ar gas. The Jc at 4.2 K and 10 T of the MgB2 tape systematically depend on x for both MgB2 and (Mg + 2B) additions, but the trends are not the same. The dependence of the Jc on x for MgB2 addition peaks at around x = 0.3 (Jc = 4.3 × 103 A/cm2), however the (Mg + 2B) addition monotonically degrades Jc. The cross-sectional image observed by scanning electron microscope shows that the MgB2 addition clearly densifies the tape core. On the other hand, the amplitude dependence of the AC magnetization curves shows that grain coupling of the tape core is unchanged or slightly improved by MgB2 addition, but degraded by (Mg + 2B) addition. Therefore, we conclude that the enhancement in Jc by an appropriate amount of added MgB2 is due to optimization of densification and degradation of the grain coupling between MgB2 grains in the tape core.