The superconductor was synthesized in a flowing Ar atmosphere, using nm Mg powder as raw material, and was studied via in situ high temperature resistance measurements. The MgB2 phase was found to form at 430 to 490C; which was much lower than that for samples fabricated in the same gas environment by using μm-sized Mg powder. The sample density of the nano-MgB2 attained 1.7g/cm3, with a crystal porosity structure of less than 1μm, as deduced from scanning electron microscopic images. The μ-MgB2 had a much more porous structure, with a corresponding density of 1.0g/cm3. This indicated that the Mg raw particle size, beside the sintering temperature, was a crucial factor for the the formation of a high-density sample; even at temperatures which were much lower than the Mg melting point (about 650C). The X-ray diffraction patterns revealed a good MgB2 phase, with small amount of MgO and Mg and the transition temperature of nano-MgB2 was determined to be 39K by using temperature-dependent magnetization measurements.

Phase Formation of Polycrystalline MgB2 at Low Temperature using Nanometer Mg Powder. C.Chen, Z.Zhou, X.Li, J.Xu, Y.Wang, Z.Gao, Q.Feng: Solid State Communications, 2004, 131[3-4], 275-8