Chemical and Mechanical Stability of the High-Tc Phase in the Bi-Pb-Sr-Ca-Cu-O System(II)

Effect of Annealing Atmosphare on Tc

Abstract

Effect of annealing atmosphere and temperature on the superconducting properties Tc have been discussed in the high-Tc phase in the Bi-Pb-Sr-Ca-Cu-O system. Special attention has been given to oxygen sorption/desorption and formation of impurity phases. The bulk samples without presence of amorphous phase formed due to mechanical grinding were used for compositions of Bi_0.9Pb_0.2Sr_1.0Ba_xCa_1.0Cu_1.6O_z (x=0 and 0.05). Annealing in O₂ and air above 700°C leads to marked weight increase and formation of (Bi, Pb)₃Sr₂Ca₂CuO_z. In contrast, almost monophasic samples of the high-Tc phase, absent from any impurity phases, were obtained on annealing in Ar. Furthermore, the weight of samples with x=0 and 0.05 decreses by Ar-annealing at temperatures of 350°C and 800-830°C. The weight decrease due to oxygen-desorption results in an elongation of the c-axis length of the high-Tc phase, accompanying an increase in Tc.