Characterization of the thermal conductivity and mechanical properties of sheath alloy materials for Bi-2223 superconductor tapes

We evaluated the effects of adding alloying-element to the Ag sheath on the thermal conductivity and mechanical properties of Bi-2223 tapes. The thermal conductivity of Ag and Ag-alloys was evaluated by using the thermal integral method in the temperature range of 10 to 100 K, and correlated to the indirectly-measured values obtained from the density, specific heat, and thermal diffusivity. It was observed that the additions of Au, Pd, and Mg to the Ag sheath significantly decreased thermal conductivity at low temperatures, probably due to the presence of alloying-elements. Specifically, the thermal conductivity of the Ag_0.92Pd_0.06Mg_0.02 and Ag_0.973Au_0.025Mg_0.002 alloys at 30 K was 28.9 and 59.2 (W/(m·K)), respectively, which is about 17 to 35 times lower than that of Ag (997.2 (W/(m·K))). At the same time, these additions to the Ag sheath improved its mechanical strength. It is believed that this improvement is related to the presence of dispersed alloying-elements which leads to a smaller grain size.