Study on the heat treatment condition to improve coupling of grains in Bi 2− xPb xSr 2CaCu 2O y/Ag tapes

Abstract

We initially investigated the stable region of Ca 2PbO 4 impurity phase in Bi 2− x Pb x Sr 2CaCu 2O y ((Bi,Pb)-2212)/Ag tapes, in the oxygen partial pressure ( P O 2 )–temperature ( T) diagram using bulk (Bi,Pb)-2212 samples. With increasing temperature and decreasing P O 2 , the volume fraction of Ca 2PbO 4 decreased and Pb in Ca 2PbO 4 dissolved in Bi-2212 phase. These results indicate that rapid cooling from high temperature under low P O 2 is effective to avoid the segregation of Ca 2PbO 4. Taking this result into account, we prepared (Bi,Pb)-2212/Ag tapes applying rapid cooling. We found that rapid cooling is more effective to improve the coupling of grains and bring about high critical current density ( J c) values than slow cooling in (Bi,Pb)-2212/Ag tapes of x=0.2. This is due to the reduction of the segregation of Ca 2PbO 4 at the grain boundaries of (Bi,Pb)-2212 by the high cooling rate.