Connectivity and Critical Currents in Polycrystalline MgB2

TL;DR

This study investigates how secondary phases like MgO affect the electrical connectivity and critical currents in polycrystalline MgB2, revealing that MgO reduces effective connectivity and alters the percolation threshold, impacting superconducting properties.

Contribution

It provides a systematic experimental analysis of MgO's impact on MgB2's transport properties and models the reduced connectivity due to secondary phases.

Findings

MgO increases the percolation threshold in MgB2.

Reduced MgB2 fraction lowers effective connectivity.

MgO presence decreases the irreversibility field.

Abstract

Current transport in polycrystalline magnesium diboride is highly non-uniform (percolative) due to the presence of secondary phases and also due to the intrinsic anisotropy of the material. The influence of secondary phases on the transport properties of MgB2 was investigated. Bulk samples were prepared from a mixture of MgB2 and MgO powders by the ex-situ technique in order to vary the MgO content systematically. The samples were characterized by resistive and magnetization measurements. The reduced MgB2 fraction is modeled by a reduced effective cross section (connectivity), which was assessed directly by the experiments. The presence of MgO also increases the percolation threshold, which reduces the zero resistivity (or irreversibility) field.