Interfacial reactions and oxygen distribution in MgB₂wires in Fe, stainless steel and Nb sheaths

Abstract

Superconducting MgB2 wires have been made by powder-in-tube (PIT) processing of commercial MgB2 powder in Fe, stainless steel and composite metallic sheaths using annealing temperatures from 850 to 1100 °C. Technologically interesting Jc values around 106 A cm−2 in the self-field at 4.2 K have been achieved. However, as the annealing temperature is increased, substantial chemical reactions have been observed at the MgB2/sheath interface. We report on a detailed study using electron probe microanalysis of the reaction products in the interfacial region and the distribution of oxygen in the core. Iron and chromium borides are formed at the core/sheath interface in Fe and stainless steel sheaths respectively, with consequent depletion of B from the superconducting core. A similar effect is caused by the formation of a solid solution of B in Nb sheaths. Milling the starting powder before wire manufacture results in a very significant increase in the oxygen content of the core, which probably explains the relatively poor performance of these wires.