Improved superconducting properties of ex situ processed MgB2 tapes sintered at low temperatures under hydrogen atmosphere

Ex situ processed Fe-sheathed MgB2 tapes using milled powders with Te addition were sintered under Ar or H2 atmosphere. In comparison with Ar atmosphere, sintering under H2 improves grain connectivity in microstructure at a lower temperature. Concerning the superconducting properties, the sintering under H2 increases the critical temperature (Tc) of the tapes by up to 2 K, as observed for in situ processed conductors heat-treated under H2. However, the transport critical density (Jc) property shows different behavior. Whereas heat treatment under H2 just degrades the Jc for the in situ process, sintering under H2 improves the Jc at a lower temperature for the ex situ process. The optimal sintering temperature (Tsin) at which Jc at 4.2 K and 10 T shows best performance is lower for the sintering under H2 than that for Ar by more than 100 degrees C without Jc deterioration. This Jc enhancement at low Tsins is attributed to the improved grain connectivity and the increase in the Tc, upper critical, and irreversibility fields (Hc2 and Hirr).