Thermal conductivity of silver alloy stabilizers for high temperature superconductor current leads

In this rapid communication, we present thermal and electrical conductivity measurements of several Ag alloy sheaths for Bi-2223 tapes suitable for current lead applications. The main result is that sheaths based on AgAu alloys with a small addition of Mg and, mainly, AgMg tapes used as dispersion hardened sheaths for mechanical reinforcement show an unexpected low thermal conductivity. SEM analysis of these samples emphasizes a small grain size due to a partial oxidation of Mg, which causes a strong reduction of the phonon mean free path. Moreover, resistivity measurements show that the Mg atoms dispersed in the Ag matrix provide a significant reduction of the electron mean free path. Anyway, the AgAu alloy is preferable in order to preserve the performances of the superconducting filaments, therefore a compromise can be obtained by developing composite Bi-2223 tapes with the AgAu alloy around the superconducting filaments and an outer layer containing Mg around the filament bundle.