Stoichiometry dependence of superconductivity and microstructure in mechanically alloyed MgB2

Abstract

The correlations among stoichiometry variation, preparation route, and microstructure as well as superconducting properties were investigated in MgB2 bulk samples. We demonstrate that the different initial weight ratios of Mg and B between Mg0.9B2 and Mg1.1B2 are only slightly shifted towards Mg depletion during the mechanical alloying preparation at room temperature followed by a short annealing step. The samples with Mg surplus show the highest critical temperature Tc∼36K, highest critical current density Jc∼1.5×106A∕cm2 at 7.5 K in self-field, and a homogeneous microstructure. In contrast, the Mg-depleted samples exhibit a rather porous and multiphase microstructure with a broad grain size distribution. A direct correlation of stoichiometry variation and superconducting properties was not found, but rather a strong influence of the Mg content on lattice strain and grain connectivity.