Effect of Oxygen Partial Pressure on Stability of High-Tc Phase in Sintered Bi-Pb-Sr-Ca-Cu-O Superconductos
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EFFECT OF OXYGEN PARTIAL PRESSURE ON STABILITY OF HIGH-TC PHASE IN SINTERED Bi-Pb-SrCa-Cu-O SUPERCONDUCTOS H.K. Lee, K.W. Lee, D.H. Ha and J.C. Park Korea Standards Research Institute, P.O.Box 3, 305-606, Korea
Taedok Science Town, Taejon,
ABSTRACT The effect of oxygen partial pressure on stability of high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system has been investigated using x-ray diffraction, resistivity and a.c. susceptibility measurements. It was found that part of the high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system was transformed into the low-Tc ghase ( 80 K) and insulating phases by heating in the temperature range 600 C to 880°C in 02 of one atmosphere. The original high-Tc phase was restored upon retreatment in a low oxygen partial pressure of 0.1 atmosphere. The reversible superconducting phase transformation is discussed in conjunction with the formation mechanism of the high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system.
INTRODUCTION 1
Since Maeda et al.( )discovered two new superconducting phases with Tc around 110 K (high-Tc) and 80 K (low-Tc), intensive studies on the formation of It was recently found that the single high-Tc phase have been carried out. doping of Pb in the Bi-Sr-Ca-Cu-O system markedly increased the fraction o f4) 2 3 the high-Tc phase compared to that of the undoped system( , ). Endo et al. reported that the low oxygen partial pressure of 1/13 atmosphere is effective However it was not clear in the formation of nearly single high-Tc phase. why the low oxygen partial pressure during sintering promotes the formation of the high-Tc phase in Bi-Pb-Sr-Ca-Cu-O system. In a previous report'5), we have shown that the high-Tc phase is transformed partly into the low-Tc phase and insulating phases by heating at 840°0 in a high oxygen partial pressure of one atmosphere. The inverse transformation was also observed by heating at the same temperature in a low oxygen partial pressure of 0.1 atmosphere. In this paper we report that the transformation from the high-Tc phase to the low-Tc phase occurs in the temperature range 600 - 880 C in 02 of one atmosphere. The inverse transformation in a low oxygen partial pressure of 0.1 atmosphere has been also investigated.
SAMPLE PREPARATION Samples were prepared from high purity starting materials of Bi 2 03, PbO, SrCO3, CaCO3 and CuO by solid state reaction method. After weighing out the starting materials to make nominal composition of Bi 2 Pbo.sSr2Ca2Cu3Ox, we first prepared nominal compositions of Bi2Sr2CalCu30x and Ca2PbOx separately by calcining at temperature below 800 C for 12 h. The calcined products were then mixed together and ground throughly. The mixed powder was pelletized with a diameter of I cm and sintered at 8300C for 2 h and 8400 for 80 h in a 10 % 02 - 90 % Ar atmosphere. After sintering, the samples were furnace 0 cooled down to 300 C.
RESULTS AND DISCUSSION Fig. 1 and Fig. 2 show the results of resistivity and ac susceptibility measurements(5), respectively,for samples heated at various temperatures, Tq, Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Mat
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