Melt processing of Ba 2 CuO 3 : Textured precursor for the fabrication of HgBa 2 CuO y superconductor
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A process for the fabrication of textured Ba2CuO3 material with a microstructure similar to that of melt-processed YBa2Cu3Oy (Y123) superconductor is discussed. The Ba2CuO3 samples were melt processed with the intent to use them as textured precursors for processing of HgBa2CuOy superconductor. The microstructure formation of the Ba2CuO3 phase was studied by observing the samples being quenched from intermediate stages of a melt growth schedule. The microstructure of melt-processed Ba2CuO3 reveals randomly oriented large-sized grains, similar to that of melt textured Y123. Other important microstructural features observed were finely distributed properitectic BaO particles, the absence of the platelet gaps within the domains, and the presence of a different kind of twin structure. The conversion of melt textured Ba2CuO3 into superconducting HgBa2CuOy phase by a two-step process is discussed.
I. INTRODUCTION
The main hurdle in obtaining high critical current densities (Jc) in oxide superconductors is their granularity. The current path is severely restricted by the randomly oriented grains and by intergrain weak links. An improvement in Jc has been observed by texturing the microstructure, and a further increase has been achieved by the introduction of flux pinning centers into the grains.1– 4 In recent years a lot of effort has been made in process innovations so as to achieve texture and to introduce flux pinning centers into the high Tc materials. The meltprocessing technique has evolved as a generic and leading technique to obtain textured materials, particularly with YBa2Cu3Oy (Y123) superconductors.1–3 In general, all the reported variations of the melt growth processes for Y123 involves slow cooling of an intimate mixture of the properitectic Y2BaCuO5 (211) phase and liquid phases through its peritectic temperature (Tp).4 The important microstructural features assigned for high Jc’s in the Y123 superconductors are the perfect alignment of grains within the domains and the introduction of properitectic phase (Y211) particles within the grains. For other superconducting materials with similar microstructural features one can also expect superior properties. In case of the Bi-family of superconducting materials the complex phase diagram limits the access to a similar microstructure. A similar situation exits with a)
Present address: ACCESS e.V., Materials Sciences, Intzerstrasse 5, Aachen, Germany. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 6, Jun 2001
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the other family of Tl-based superconductors. In the case of the Hg-based superconductors, processing of even the unaligned bulk material in a phase pure form has been found to be difficult. This is basically due to the decomposition of HgO and the evaporation of Hg at temperatures as low as 500 °C. This fact has limited the processing of Hg-based materials by conventional ceramic methods, and alternate methods have been proposed. Presently, a two-step process involving the prepara
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