Shape Forming Simultaneous with J c Enhancement in REBa 2 Cu 3 O 7 Superconductors
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Shape forming simultaneous with Jc enhancement in REBa2 Cu3 O7 superconductors E. Sudhakar Reddy and T. Rajasekharan Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500 058, India (Received 9 May 1997; accepted 5 September 1997)
An infiltration and growth (IG) process which enables the fabrication of three-dimensional (3D) components of REBa2 Cu3 O7 (RE Y, Gd, Sm, Nd, etc.) (RE-123) superconductors with a highly textured microstructure is described. The advantages of the process in comparison with conventional melt processing are discussed. The process has been demonstrated to yield highly favorable microstructures in the case Y-123 processed in air, as well as in the case of Gd-123 processed in reduced oxygen partial pressure. I. INTRODUCTION
The high temperature superconductors (HTS) having a general formula REBa2 Cu3 O7 (RE-123, RE rare earth Y, Gd, Sm, Nd, etc.) have their transition temperatures around 92 K. They are currently the most attractive from the point of view of bulk applications because they suffer to a much lesser extent from the problem of thermally activated flux creep in comparison with the compounds belonging to the other known groups of HTS. The only technique to obtain high critical current densities (Jc ’s) in bulk specimens of RE-123 is the melt-growth process introduced by Jin et al.1 In all its several variants,2–6 an intimate mixture of the properitectic RE2 BaCuO5yRE4 Ba2 Cu2 O8 (RE-211yRE422) phase and liquids (Ba2 Cu3 O5yBaCuO2 and CuO) are obtained by an incongruent melting of 123 at high temperature,1–6 or by mixing the constituent powders at room temperature.4 The mixture is then cooled very slowly through the peritectic formation temperature, Tp , of the RE-123. The imposition of a temperature gradient or seeding the material during the cooling process can control the formation of domains.7,8 The melt-processed material is highly textured and is free from weak links to a large extent. A major difficulty associated with the fabrication of large ceramic bodies stems from the shrinkage accompanying the sintering of green bodies. It can cause cracks and distortions in the final product, and the fabrication of very large components is impossible in many cases. However, there are instances in the literature where this problem has been overcome by the infiltration and reactive growth of liquids into shaped preforms. Some examples are processes for the fabrication of metal matrix and ceramic matrix composites, and that of reaction bonded silicon carbide.9,10 The processes enable the fabrication of large near-net shaped components due the avoidance of shrinkage during heat treatment. This paper reports a process that involves the infiltration of liquid phases into a preform and the growth 2472
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J. Mater. Res., Vol. 13, No. 9, Sep 1998
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of RE-123. We refer to that process as the infiltration and growth (IG) process. The samples obtained have highly textured, we
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