a t i o f,, w a s 0. 6 5 , h a d a t h e r m a l c o n d u c t i v i t y v a l u e a b o u t O.ZW/cmK a t 77K. This value is :as low as t h a t of Cu-Zn. I t w a s f o u n d t h a t thle t h e r m a l c o n d u c t i v i t y o f the t a p e was close to the calculated oiie based o n f s c and the independently measured thermal c o n d u c t i v i t i e s , o f t h e A g-A u a l l o y a n d t h e Bi-22 2 3 s u p e ir o n d u c t o r .Th e s u p ercon d u c t ing t a pes sheathed with the alloy were c o n f i r m e d to be s u i t a b l e f o r t h e a p p l i c a t i o n a s power c u r r e n t leads.

A superconducting oxide Y0.4Ba0.6CuO2.22 with T c higher than 88 K was synthesized. The crystal structure was found to be a derivative of the perovskite structure with orthorhombic unit cell dimensions of a=3.818 (1) Å, b=3.888 (1) Å, c=11.667 (4) Å, and V=173.19 (8) Å3. Basic unit cell of the cubic perovskite structure was tripled along the c axis, probably due to the ordering of Y3+ and Ba2+ ions and/or of oxygen vacancy. Apparent short bond length for Cu-O may partly be explained by oxygen vacancy amounting to a quarter, but the observed high T c suggests actual contraction of Cu-O bonds.

The influence of Ag-Au and Ag-Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) was studied. The critical current density (Jc) of Bi-2212/(Ag-Au) was not degraded by increasing the Au content of the Ag-Au alloy, while the Jc of Bi-2212/(Ag-Cu) was drastically degraded by increasing the Cu content of the Ag-Cu alloy. The microstructure of Bi-2212 on the Ag-Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi-2212 in contact with the Ag-Cu alloy was understood by the fact that Cu absorption of Bi-2212 from the Ag-Cu alloys changed the composition of the oxide layer. Ag-Au alloys are superior substrate materials for Bi-2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.

Oxide superconductor tape with Ag-Au sheath was studied for current leads. A 100-cm-long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag-11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross-section ratio in the tape is 0.35–0.7. Bi-Pb-Sr-Ca-Cu-O superconducting tape with Ag-11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall-Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

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