Improvement of Critical Current at Joint Between RE123-Coated Conductors Using YYb123 Intermedium

(Recently, our group proposed a technique for the formation of crystalline joints for use in RE123-coated conductors, based on using molten bulk Y123 or Yb123 as an intermedium. The melting temperature of Yb123 is approximately 1203 K in air, which is 70 K lower than that of Y123, and so using Yb123 allows the heat treatment temperature during the joint formation process to be reduced by about 70 K. This lower temperature is likely to limit degradation of the wire. However, it is challenging to produce a highly orientated Yb123 phase, and our previous experiments indicated that crystal growth does not occur in Yb123 at the joint boundary during a short time. For these reasons, increasing the critical current at the joint requires crystal growth of an intermedium at the boundary. The present study examined the partial substitution of Y for Yb in the bulk material. Y
_x
Yb
_1-
x
123 samples with
x
= 0, 0.1 or 0.2 were prepared and both crystal growth and the critical current at the joint boundary were assessed following heat treatment at 1203 K for 1 min under oxygen. X-ray diffraction patterns indicated that only the Y
_0.1
Yb
_0.9
123 phase contained highly oriented single crystals. The current-voltage characteristics of three joint samples were determined at 77 K and a current of 44.2 A was obtained from the sample with
x
= 0.1. This value of critical current was approximately 5 times higher than that for the
x
= 0 specimen (a Yb123 polycrystal), indicating a significant improvement in the critical current as a result of crystal growth in the intermedium.