The reduction of optimal heat treatment temperature and critical current density enhancement of ex situ processed MgB 2 tapes using ball milled filling powder

摘要

The optimal heat treatment temperature (Topt) at which best performance in the critical current density (Jc) property at 4.2?K is obtained is influenced by the quality or reactivity of the filling powder in ex situ processed MgB2tapes. Using a controlled fabrication process, theToptdecreases to 705–735?°C, which is lower than previously reported by more than 50?°C. TheToptdecrease is effective to suppress both the decomposition of MgB2and hence the formation of impurities such as MgB4, and the growth of crystallite size which decreases upper critical filed (Hc2). These bring about theJcimprovement and theJcvalue at 4.2?K and 10?T reaches 250?A/mm2. The milling process also decreases the critical temperature (Tc) below 30?K. The milled powder is easily contaminated in air and thus, theJcproperty of the contaminated tapes degrades severely. The contamination can raise theToptby more than 50?°C, which is probably due to the increased sintering temperature required against contaminated surface layer around the grains acting as a barrier.