Structural characterization of multifilament heat treated<i>ex situ</i>MgB₂superconducting wires with Cu and Fe sheaths

Abstract

MgB2 powder produced by Alfa Aesar (98%) and Fe and Cu/Fe tubes have been used to make a set of 4-, 9- and 16-filament square wires. The MgB2/Fe, MgB2/Fe/Cu and MgB2/Fe/Cu/Fe wires were made by two-axial rolling and sintering at 950 °C /0.5 h. At 950 °C, MgB2 reacts with Fe forming an interfacial layer which is a mixture of different phases formed between Fe–O–Mg–B, such as Fe oxides, Fe borides (probably Fe2B, FeB), etc. The average layer thickness is 5–10% (10–15 µm) of the remaining MgB2 core. There is no direct observation of Fe diffusion into the MgB2 core. Similar interfacial layers are created in the case of Fe and Cu/Fe sheathed wires. Mg and traces of B were detected within the surrounding Fe matrix. When the inner Fe sheath is too thin and 'bridging' of Cu to MgB2 occurs, then 'catastrophic' diffusion of Cu takes place, leading to complete degradation of the superconducting properties of the MgB2 cores. Mg diffuses also into the Cu jacket. New phases of Mg with Cu (MgCu2) and probably other binary and ternary Cu–Mg–Fe and Fe–Cu–B alloys are created. (FeCu)2B of microhardness (1700 HV0.1) located in a softer eutectic mixture Cu–B (α+β) (600–700 HV0.1), 'pure' Fe borides, such as Fe2B (1800 HV0.1), FeB (>2000 HV), and Mg borides, such as MgB4 and higher, are likely to appear. Application of the outer (external) Cu sheath as the thermal stabilizer for MgB2 wires (in the presence of the inner Fe layer) must be designed especially carefully with respect to the predicted composite geometry (especially of the Fe layer thickness) obtained just before the final sintering.