Effects of heat treatment temperature on the formation of MgB2 bulk superconductors prepared using MgB4 and Mg powder

The effects of the heat treatment temperature (600 degrees C-1050 degrees C) on the formation of MgB2 and the superconducting properties have been examined. The self-synthesized MgB4 and commercial Mg powders were used as raw materials for the formation of MgB2. The superconducting critical temperatures (T(c)s) of MgB2 bulk superconductors prepared at 600 degrees C-850 degrees C were as high as 37-38 K regardless of the heat treatment temperature. However, because MgB4 is more stable than MgB2 at above 850 degrees C, no superconducting signals were detected in the susceptibility-temperature curves of the samples prepared above 850 degrees C. As for the critical current density (J(c)), the sample heat-treated at a low temperature (600 degrees C) for a prolonged period (40 h) showed a Jc higher than those prepared at 650 degrees C-850 degrees C for a short period (1 h). The FWHM (full width at half maximum) result showed that the grain size of MgB2 of the 600 degrees C sample was smaller than that of the other samples. The high J(c) of the 600 degrees C sample is attributed to the presence of large numbers of grain boundaries, which can act as flux pinning centers of MgB2.