Microstructural studies of PrBa 2 Cu 3 O y during melt processing
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Microstructural studies of PrBa2 Cu3 Oy during melt processing E. Sudhakar Reddy and T. Rajasekharan Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad-500 058, India (Received 24 September 1997; accepted 27 December 1997)
The microstructures of PrBa2 Cu3 Oy during melt processing have been studied for samples quenched at different stages of the thermal cycle. The absence of the Pr2 BaCuO5 phase in the Pr–Ba–Cu–O phase diagram, analogous to the Y2 BaCuO5 (211) phase in the Y–Ba–Cu–O system, makes it interesting to study the microstructure of PrBa2 Cu3 Oy (Pr-123) during melt processing. The nature and morphology of the properitectic particles, the nucleation of Pr-123, and the microstructures resulting from melt processing are investigated and discussed as compared with those in YBa2 Cu3 Oy . At the properitectic stage the morphology of PrBaO3 (Pr-110) particles is angular. Pr-123 is found to nucleate on all Pr-110 particles with a needle-like morphology, whereas in other 123 systems no preferred morphology for the 123 during nucleation is observed. The melt textured microstructure of Pr-123 revealed similar features like domain formation with the trapped properitectic phase. The only difference is the absence of platelet gaps within domains as observed in other 123 systems. All these observations suggest that as compared to other 123 systems the microstructure of the Pr–Ba–Cu–O system behaves differently due to the absence of the analogous 211 phase.
I. INTRODUCTION
The microstructures of REBa2 Cu3 Oy (RE-123, RE Rare Earth element) high temperature superconductors have been observed to play an important role in obtaining bulk samples supporting high critical current densities. As compared to the material processed by conventional ceramic processing routes like sintering, HIP’ing, etc., the melt processing of RE-123 results in a better microstructure which can carry high critical current densities. Several variations of the melt processes are reported in the literature and are referred to as QMG, MPMG, SLMG, LPP, OCMG, Infiltration and Growth (IG), directional solidification, zone melting, etc.1–7 All these techniques involve either a slow translation or a slow cooling of a mixture of the properitectic RE2 BaCuO5 (RE-211, RE Y, Gd, Dy, etc.) phase and liquid phases (BaCuO2yBa2 Cu3 O5 1 CuO) through the peritectic formation temperature sTp d of the RE-123 phase, either in the presence or in the absence of a temperature gradient. When solidified through Tp , RE-123 is formed by a peritectic reaction between RE-211 and liquid phases. It has been observed that the peritectic reaction in these systems is not the conventional one wherein the peritectic phase nucleates and grows around each and every properitectic phase particle and proceeds further by a diffusion of ions through the peritectic phase.8 It is known that the peritectic formation of the RE-123 phase takes place by a peritectic reaction that is different from the above described peritect
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