X-ray diffraction study of texture in melt-processed Gd-123 as a function of 211 concentration
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X-ray diffraction study of texture in melt-processed Gd-123 as a function of 211 concentration E. Sudhakar Reddy, A. K. Singh, and T. Rajasekharan Defence Metallurgical Research Laboratory, Kanchanbagh P.O., Hyderabad-500058, India (Received 3 August 1998; accepted 14 December 1998)
The texture in the melt-processed GdBCO superconductor is studied as a function of Gd2 BaCuO5 additions, using a texture goniometer. A systematic variation observed in the background intensity is connected to the microstructural variations and the powder x-ray diffraction spectra.
I. INTRODUCTION
II. EXPERIMENTAL
Due to the anisotropy in the current-carrying capacity along different crystallographic directions in the RBa2 Cu3 O72d (R-123, R rare earths like Y, Gd, Sm, Nd, etc.) superconductor, the realization of optimal texture is important in the fabrication of samples supporting high critical current density (Jc ). A number of variations of the melt-processing technique first proposed by Jin et al.1 are now available in order to fabricate a highly textured material.2,3 The process essentially involves the slow cooling of an intimate mixture of R2 BaCuO5 (211) and liquid phases (BaCuO2 and CuO) through the peritectic formation temperature (Tp ) of 123. The microstructure of the melt-processed material, when fabricated in the absence of an externally imposed temperature gradient, essentially contains a few millimeter-sized domains which are divided into parallel platelets of the superconductor.4 Each platelet is separated from the next by a small gap. The parallel platelets have a common c-axis, but the domains themselves do not have a common orientation. By starting with compositions richer in 211, it is possible to leave fine 211 inclusions residual in the 123 matrix2–5 after melt processing. The effect of 211 addition on the microstructure is substantial: for instance, in YBCO, the platelet width decreases from ø15 mm in the stoichiometric composition to ø3 mm in a sample containing 40 mol% 211, and the interplatelet gaps similarly decrease from 3 to 0.1 mm. The critical current density of the samples increases with the 211 content due to the microstructural changes. An increase in the flux pinning due to the secondary defects5,6 comparable in size to the coherence length at the 211y123 interfaces contributes to this enhancement of Jc . In this paper, we have studied, by the x-ray diffraction technique, the samples of melt-processed Gd-123 by systematically increasing Gd-211 content. X-ray studies were done using a texture goniometer and powder x-ray diffraction. The aim was to correlate various features in the x-ray diffraction to the microstructural features which can be drastically altered by varying the 211 content.
Powders of Gd-123 and Gd-123 with 10, 20, 30, and 40 mol% extra 211 were prepared starting with Gd2 O3 , BaCo3 , and CuO of 99.9% purity or better. The mixtures with the required stoichiometry were first processed by a chemical route and the chemical precursors thus
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