Melt Processing of Bi-Ca-Sr-Cu-O Superconductors
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MELT PROCESSING OF Bi-Ca-Sr-Cu-O SUPERCONDUCTORS J.P. CRONIN, A. ARUCHAMY, S. ROSENFELDER, S.-J. KIM AND D.R. UHLMANN Arizona Materials Laboratories, The University of Arizona, Tucson, AZ 85712 O.H. EL-BAYOUMI AND R.J. ANDREWS Solid State Sciences Directorate, Rome Air Development Center, Hanscom Air Force Base, Bedford, MA 01731 ABSTRACT: Unidirectional crystallization of melt processed 2122 and 2223 composition in the BiCa-Sr-Cu-O system has been carried out. Crystallization was controlled by heating the glass or cooling the melt using a moving furnace and a stationary sample. Fairly large oriented crystalline specimens of 2122 phases were obtained by cooling from the melt in a shallow gradient furnace. Glassy samples processed under steep gradients showed better crystallinity than those processed under shallow gradients. INTRODUCTION: Melt processing of superconductors in the system Bi-Ca-Sr-Cu-O has received wide attention [1-7] since the initial discovery of these materials prepared via powder processing [8]. Bulk sintering of these superconductors usually results in standard shaped pieces which are porous and do not have the desired electrical characteristics. Melt processing of these materials, on the other hand, has many advantages over bulk processing for the fabrication of dense samples and complex geometries such as wires, films and odd shaped pieces. It can also provide flexibility for microstructural control and for enhancing properties through chemical homogeneity. The melt processing of these superconductors often involves isothermal crystallization or slow heating of the glass or slow cooling from the melt. Crystallization using either of these methods gives materials with good superconducting transition temperatures (Tc) but low critical current densities (Jc)- In order to obtain high Jc's in these layered materials, single crystals or polycrystalline samples with large grain size and good grain orientation are needed. This can be achieved by unidirectional crystallization. Such directional growth is also useful for processing these materials in the form of fibers and plates. The objective of the present work is to crystallize unidirectionally glasses and melts to produce oriented crystals. In this paper, we report the successful directional crystallization of the 2122 composition from melt and glassy precursors, as well as preliminary crystallization results on the 2223 composition. EXPERIMENTAL: Two compositions in the Bi-Ca-Sr-Cu-O system, namely 2122 and 2223, were melted in Pt crucibles in air at 1050 C for 2 hours. The starting materials were reagent grade Bi203, CuP, CaCO3 and SrCO3. The powders were ground together and calcined at 600 C for 8 hours and slowly heated to melting and held at 1050 C for 2 hours. The melts were splat quenched between two brass plates at room temperature, crushed, remelted and quenched. Crystallization of the glasses and melts was carried out in two furnaces having different temperature profiles, denoted A and B. The furnace with profile A had a temperature gradi
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