Phase evolution in silver-doped BiPbSrCaCuO(2223)/Ag superconducting composites
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Silver doping into (Bi,Pb)2Sr2Ca2Cu3O10 superconducting composite tapes was found to accelerate the formation process of high-r c (2223) phase owing to lowering the partial melting point of the samples. The differential thermal analysis (DTA) results showed that the partial melting temperature of the sample was lowered by about 10 °C from 850 °C to 840 °C by silver doping. However, with sufficient sintering both the silver-doped and undoped samples can reach a very high level of high-r c phase fraction, suggesting that the silver doping only speeds up the rate of high-r c phase formation, but does not change the final phase assemblage of the materials. The reaction kinetics was analyzed by using the Avrami equation, and the results indicated that the conversion process of low-r c (2212) phase to high-r c (2223) phase was a diffusion-controlled, two-dimensional reaction. The correlation of the phase evolution with electrical property inside the superconducting tape during the process of heat treatment was also discussed.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
Silver is the most suitable metal used in fabricating superconductor-metal composites such as wires, ribbons, and films because it is chemically inert to the superconducting oxide. Furthermore, silver mechanically supports the brittle ceramic superconductors and can be easily worked into different geometry. Especially, for the silver-sheathed (Bi, Pb)2Sr2Ca2Cu3O10 superconducting wires a critical current density exceeding 104 A/cm 2 at 77 K has been achieved,1^* which has already met the requirements for some practical applications. The silverdoping effect on the superconducting properties such as critical temperature (Tc), critical current density, and critical current density behavior in magnetic field has been investigated for the (Bi, Pb)2Sr2Ca2Cu3Oio system by several groups.5"12 Some authors have tried to use silver doping into the high-Tc superconductors as a technique to control the physical properties of the superconducting materials. However, there has been no detailed study of silver-doping effects on the phase assemblage and high-r c phase formation kinetics. Such a study is of obvious importance for understanding the phase chemistry involved in the process of superconductor materials fabrication and providing the means to achieve desirable microstructure, and hence, enhance critical current density of the materials. In this paper, we report the results of the study of silver doping effect on the phase evolution, high-7 c phase formation kinetics, and the correlation of the phase evolution with electrical property inside (Bi,Pb)2Sr2Ca2Cu3AgxOio (x = 0.0-3.5) tapes during the process of heat treatment. All the results were compared with those for the undoped samples.
Powders were prepared by thermal decomposition of metal nitrate solution having a cation ratio of Bi/Pb/Sr/Ca/Cu = 1.8/0.4/2.0/2.2/3.0 with continuous magnetic and manual stirring. The powders were calcined at 800 °C for 2 h, mixed with an appropriate amount of fine Ag 2 O powder
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