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