Enhanced critical current properties of in situ processed MgB2 wires using milled boron powder and low temperature solid-state reaction

Abstract

Abstract To get the fine grain structure of MgB 2 with high critical current properties, mechanical milling for as-received boron (B) powder and low temperature solid-state reaction of 550 or 600 °C were attempted to in situ powder-in-tube processed MgB 2 /Fe wires. The comparisons for the phase formation, crystallinity, grain size, microstructure and superconducting properties were performed for the wires using un-milled B powder or high temperature heat-treatment of 800 °C. It was found that the MgB 2 phase formation in the wires using the milled B powder was more activated when compared to the wires using the un-milled B powder under the same heat-treatment conditions, because of the improved reactivity of the B powder after milling. The upper critical fields and critical current densities of the MgB 2 wires using the milled B powder were also enhanced due to a smaller grain size and an increased volume of the superconducting phase. In addition, the low temperature heat-treatment of 550 and 600 °C also contributed to the enhanced superconducting properties by minimizing the grain growth for an increased grain boundary pinning when compared to a conventional high temperature heat-treatment of 800 °C. The solid-state reaction of a low temperature process for the samples using the milled B powder resulted in poorer crystallinity with a smaller grain size, which improved the superconducting properties.