Preparation of High T c Bi-Pb-Sr-Ca-Cu-O Superconducting Fibers by Laser Heated Pedestal Growth Method
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PREPARATION OF HIGH Tc Bi-Pb-Sr-Ca-Cu-O SUPERCONDUCTING FIBERS BY LASER HEATED PEDESTAL GROWTH METHOD K.TOMOMATSU, A.KUME and H.TOMINAGA Fujikura Ltd., Kiba, Koto-ku, Tokyo 135, Japan ABSTRACT Highly oriented Bi-Pb-Sr-Ca-Cu-O superconducting fibers have been prepared by the laser heated pedestal growth method. The annealed fiber showed both the high Tc (•110K) phase's and the low Tc (•-80K) phase's superconducting transition. The fiber grown at 2mm/h had the Jc value as high as 1.Oxl0 3 A/cm 2 . The misalignment of the high Tc phase's a-b plane was measured by XRD less than + 20. It was found that the high Tc phase formed during the annealing was highly oriented as well as the low Tc phase. INTRODUCTION
Bi-Sr-Ca-Cu-O system involves at least three types of compounds: Bi2(Sr,Ca) 2 CUOy (TcZ10K, 10K phase)[1], Bi 2 (Sr,Ca) 3 ,Cu 2 Oy (Tc-•80K, low Tc phase)[2,3] and Bi 2 (Sr,Ca) 4 CU30 (Tcr11OK, high Tc phase)[2,3]. A large anisotropy in resistivity has Leen reported[4-6] for these materials. Because of this anisotropy, highly oriented material is expected to have the high critical current density. It has been found that the fibers with highly oriented low Tc phase grain structure can be prepared by the floating zone method[7]. Increasing the quantity of the the high Tc phase is another useful means for increasing critical current density. However, it has not been reported that the high Tc phase could be grown directly from the liquid phase. Many studies pointed out that the high Tc phase is formed from the low Tc phase in a partially molten condition[8-12]. In this study, Bi-PbSr-Ca-Cu-O superconducting fibers were prepared by the laser heated pedestal growth method and annealed in order to precipitate the high Tc phase. The relationship between fiber growth and post grown annealing conditions and the superconducting characteristics, and orientation of the high Tc phase are discussed. EXPERIMENTAL
Source rods of about 1.5mm in diameter were formed by cold isostatic pressing using mixed and heat-treated Bi 2 0 3 , PbO, SrC0 3 , CaCO 3 and CuO powders. A CO 2 laser beam was focused uniformly around the circumference of the top of the source rod by the parabolic mirror installed over the source rod. A small molten zone was formed on the top of the source rod. The seed rod, which was prepared by the same technique as the source rod, was dipped in the molten zone and pulled up. The growth rates were varied from 2mm/h to 100mm/h. The annealing conditions are listed in Table 1. Misalignment of the a-b plane of the high Tc phase was measured by Xray diffraction (XRD) analysis in order to estimate the orientation of the annealed fiber. The detector was fixed at the diffracted angle, 2 theta, where one of the (OOn) peaks of the high Tc phase can be detected as the fiber was rotated. The peak intensity change due to the angle between the fiber direction and the X-ray incident direction was measured. RESULTS AND DISCUSSION Before fiber growth, the binary section phase diagram of Bi-Pb-Sr-Ca-
Cu-O system near both the high Tc
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