Structural characterization of multifilament heat treated exsitu MgB2 superconducting wires with Cu and Fe sheaths

Abstract

MgB2 powder produced by Alfa Aesar (98%) and Fe and Cu/Fe tubes havebeen used to make a set of 4-, 9- and 16-filament square wires. TheMgB2/Fe,MgB2/Fe/Cu and MgB2/Fe/Cu/Fe wires were made by two-axial rolling and sintering at950 °C /0.5 h.At 950 °C, MgB2 reacts with Fe forming an interfacial layer which is a mixture of differentphases formed between Fe–O–Mg–B, such as Fe oxides, Fe borides (probablyFe2B, FeB), etc. The average layer thickness is 5–10%(10–15 µm) of theremaining MgB2 core. There is no direct observation of Fe diffusion into theMgB2 core. Similar interfacial layers are created in the case of Fe and Cu/Fesheathed wires. Mg and traces of B were detected within the surroundingFe matrix. When the inner Fe sheath is too thin and ‘bridging’ of Cu toMgB2 occurs, then ‘catastrophic’ diffusion of Cu takes place, leadingto complete degradation of the superconducting properties of theMgB2 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 asFe2B (1800 HV0.1),FeB (>2000 HV), and Mgborides, such as MgB4 and higher, are likely to appear. Application of the outer (external) Cu sheath as the thermal stabilizerfor MgB2 wires (in the presence of the inner Fe layer) must be designed especially carefully withrespect to the predicted composite geometry (especially of the Fe layer thickness) obtainedjust before the final sintering.