Superconducting Properties of Barrier-Free Ex Situ Processed MgB2 Tapes with Al and Al Alloys Sheaths

Ex situ processed MgB2 tapes using Al, Al alloy and Fe sheaths without barriers were heat treated under Ar and H2 atmospheres. Among these sheaths excluding Fe, pure Al sheath material shows the highest melting point of 650°C, allowing for the highest heat-treatment temperature. Additives to Al, that is, alloying of Al, lower the melting point to up to 575°C and impose limits on the heat-treatment temperature. Since high-temperature heat treatment is advantageous for improving the connectivity of MgB2 grains and hence the transport critical density (Jc) properties of the tapes, the reduction in the melting point of the sheath due to alloying is unfavorable for the Jc properties. Nevertheless, the Jc properties of tapes using Al alloy sheaths are superior to those using Al sheath. The Jc values of tapes using the Al and Al alloy sheaths correspond to about 5% and 30–50% of those of Fe-sheathed tapes, respectively. The transition width of dc magnetization curve of tape using Al sheath is clearly broader than that of tapes using Al alloy and Fe sheaths. The Vickers hardness of Fe, Al alloys and Al is about 280 HV0.5, 110 HV0.2, and 40 HV0.2, respectively, and since Al in the sheaths hardly diffuses into the MgB2 core layer of these tapes, the large Jc degradation and broad transition in the magnetization curve for pure Al-sheathed tape is just attributed to the reduced hardness of pure Al in comparison with the other sheaths.