Formation and Thermal Stability of High-T_c Phase in Te-Doped Bi-Based Superconducting Glass-Ceramics

Abstract

The effect of the substitution of Te for Cu or Pb in nominal compositions on the formation of the high-T_c phase and on the thermal stability of the high-T_c phase under reannealing conditions at temperatures (500-800°C) lower than the formation temperatures such as 850°C in Bi-Pb-Sr-Ca-Cu-O (Bi-based system) superconducting glass-ceramics has been investigated. In the case of the substitution of Te for Cu, the formation of the high-T_c phase is largely enhanced, i.e., a large amount of the high-T_c phase is obtained by annealing glassy samples at 850°C for periods as shorter as 48h compared with the period of 250h required to obtain similar amount of the high-T_c phase in the absence of any Te addition. It is, however, found that the high-T_c phase decomposes by reannealing below 850°C and at the same time Pb-containing compounds of BiPb₂Sr_2.6CaCu_1.6O_x and Pb₂Sr_1.2Ca_1.8CuO_x are formed, meaning that the thermal stability of the high-T_c phase in the glass-ceramics obtained by the substitution of Te for Cu is low. In the case of the substitution of Te for Pb, it is found that not only the formation of the high-T_c phase is promoted but also the thermal stability is improved, because any definite decomposition of the high-T_c phase is not observed at temperatures lower than 830°C. It is concluded that the key factor to improve the thermal stability of the high-T_c phase is to depress the formation of Pb-containing compounds, and thus to prepare the high-T_c phase containing a proper amount of Pb.