Characterization and phase transitions of (Bi,Pb)₂Sr₂Ca₂Cu₃O_x–Ag composite powder obtained by spray pyrolysis

Abstract

(Bi,Pb)₂Sr₂Ca₂Cu₃O_x–Ag (BPSCCO/Ag) composite precursor powder was prepared by spray pyrolysis. The as-prepared and thermally treated (750 °C (1 h) and 780 °C (3 h)) powders were characterized by transmission electron microscopy, ambient X-ray diffraction, In situ X-ray diffraction, magnetic susceptibility measurement and differential thermal analysis (DTA).

DTA curve exhibited two strong endothermic peaks above 790 °C, one very wide with the onset at 790 °C and the second at 840 °C. In situ X-ray diffraction measurements strongly supported the view that the first endothermic peak was due to Ca₂PbO₄ decomposition and a transient liquid formation through PbO–CuO–Ag eutectic. The second endothermic peak can be explained by onset of Bi-2212 peritectic melting that began at 840 °C with local formation of Bi-2201 intergrowths. The melting was complete at 875 °C.

Introduction

There are a few reports on metal/simple-oxide composite powders prepared by the spray pyrolysis route [1], [2], [3]. Majumdar et al. [2], for example, described the formation of Ag–CuO composite powder, while Matsumoto et al. [3] demonstrated viability of preparation of Ag–TiO₂ composite powder. Studies of metal/complex oxide composite powders, synthesized by spray pyrolysis, are even rarer. A study like this was performed by Tsudo et al. [4] on BPSCCO/Ag system. They studied the influences of the chemical composition of the precursor and the sintering conditions on the superconducting properties of (Bi,Pb)-2223 phase, such as T_c and J_c, and observed that the addition of Ag decreased both T_c and J_c.

Aerosol decomposition routes, such as spray pyrolysis, possess several advantages (chemical homogeneity of powder, uniform-sized particles, etc.) over conventional solid-state techniques [5]. The quality of the powders obtained by spray pyrolysis can be further improved by the addition of urea to the metal nitrates precursor solution [6]. The preparation of a BPSCCO/Ag composite powder containing chemically homogeneous, uniform-sized submicron BPSCCO particles, mixed with well dispersed silver could be useful in fusion-reformation process for obtaining Bi-2212 and (Bi,Pb)-2223 bulks, tapes or films. This is due to the important role that silver plays in the reduction of oxygen losses upon peritectic melting [7] of these two superconducting phases. The minimization of oxygen losses on melting should help the reformation of Bi-2212 and (Bi,Pb)-2223 during the solidification process [7], [8]. On the other hand, it has been reported [4], [9] that the addition of high contents of silver can decrease the critical temperature of oxide superconductors. Since Jin et al. [10] showed that the addition of 20% of silver did not affect the critical temperature of BSCCO phases, this weight percent of silver was chosen in the present work.

Presented here is a report on the characterization of BPSCCO/Ag composite powders as-synthesized by spray pyrolysis and after a rapid thermal treatment at 750 °C (1 h) and 780 °C (3 h). Afterwards, an in situ synchrotron X-ray diffraction study of the phase transitions that occur in the precursor powder was performed before the final conversion to (Bi,Pb)-2223 phase. The results of this study are supplemented by differential thermal analysis (DTA).