Effects of heat treatment temperature on the formation of MgB 2 bulk superconductors prepared using MgB 4 and Mg powder

Abstract

2017) Abstract The effects of the heat treatment temperature (600 ℃ -1050 ℃ ) on the formation of MgB 2 and the superconducting properties have been examined. The self-synthesized MgB 4 and commercial Mg powders were used as raw materials for the formation of MgB 2 . The superconducting critical temperatures ( T c s) of MgB 2 bulk superconductors prepared at 600 ℃ -850 ℃ were as high as 37-38 K regardless of the heat treatment temperature. However, because MgB 4 is more stable than MgB 2 at above 850 ℃ , no superconducting signals were detected in the susceptibility-temperature curves of the samples prepared above 850 ℃ . As for the critical current density ( J c ), the sample heat-treated at a low temperature (600 ℃ ) for a prolonged period (40 h) showed a J c higher than those prepared at 650 ℃ -850 ℃ for a short period (1 h). The FWHM (full width at half maximum) result showed that the grain size of MgB 2 of the 600 ℃ sample was smaller than that of the other samples. The high J c of the 600 ℃ sample is attributed to the presence of large numbers of grain boundaries, which can act as flux pinning centers of MgB 2