Powder (98% MgB2) and Fe and Cu/Fe tubes were 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 2-axis rolling and sintering (950C, 0.5h). At 950C, the MgB2 reacted with Fe to form an interfacial layer which was 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 was 5 to 10% (10 to 15µm) of the remaining MgB2 core. There was no direct observation of Fe diffusion into the MgB2 core. Similar interfacial layers were created in the case of Fe and Cu/Fe sheathed wires. Here, Mg and traces of B were detected within the surrounding Fe matrix. When the inner Fe sheath was too thin and bridging of Cu to MgB2 occurred, then so-called catastrophic diffusion of Cu took place, leading to complete degradation of the superconducting properties of the MgB2 cores. The Mg also diffused 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 were created. Also likely to appear were (FeCu)2B (microhardness of 1700HV0.1) located within a softer eutectic mixture of Cu–B (α+β) (600–700HV0.1), so-called pure Fe borides, such as Fe2B (1800HV0.1), FeB (>2000HV), and Mg borides, such as MgB4 and higher.

Structural Characterization of Multifilament Heat Treated ex situ MgB2 Superconducting Wires with Cu and Fe Sheaths. W.Pachla, A.Presz, P.Kovác, I.Husek, R.Diduszko: Superconductor Science and Technology, 2004, 17, 1289-94

Figure 5

Structure of Type-B Stacking-Fault, Seen along [001]