Seeding Induced Aligned Microstructures (S.I.A.M.) in Yba 2 Cu 3 0 6+X
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SEEDING INDUCED ALIGNED MICROSTRUCTURES IN YBa 2Cu3 06+x
(S.I.A.M.)
LIJIE ZHANG, HELEN M. CHAN and MARTIN P. HARMER Dept. Materials Science & Engineering, Lehigh University, Bethlehem, PA 18015 ABSTRACT Annealing of YBa Cu O (123) above the peritectic temperature, foliowad b slow cooling at 0.5 C/min, was found to produce a microstructure containing regions (d - 200 microns) within which the grains were highly aligned. When these regions were used to "seed" 123 powder, an enhanced degree of texture was obtained, where the aligned regions were > imm in size. INTRODUCTION One of the major barriers to widespread commercial utilization of the YBa Cu +x (123) superconductor is the very low values of current anpity (Jc) which can be achieved in the bulk form ( < 1000 A/cm ). As elegantly demonstrated by Dimos et al. [1], this behavior is the result of "weak link" behavior exhibited by highly misoriented 123 grain boundaries. This problem can be overcome, however, if significant alignment of the 123 grains can be achieved. Thus Jin 2and his co-workers [2,3] observed Jc values of - 17 000 A/cm in 123 specimens consisting of highly aligned grains, which were obtained by solidification in the presence of a steep temperature gradient. More recently, Murakami et al. [4] have also reported a melt-derived process, in which cooling under a temperature gradient produces an aligned structure exhibiting high Jc values. In addition, Salama et al. [5] achieved highly textured grain structures in the absence of a temperature gradignt, but only through ultra-slow cooling (1°C/hr) from 1030 C to 980 C. One factor which all of these processing techniques have in common, is that the 123 is heated above its incongruent melting temperature (1002 C, [6]). This strongly suggests, therefore, that the anisotropy in 123 grain growth rates (which gives rise to the microstructural texture) is enhanced during the peritectic reaction: 211 + Liquid ---123. Furthermore, it should be feasible to increase the volume of individual aligned regions through control of the number density of 123 nuclei. This paper describes work which shows that the degree of alignment can be enhanced by "seeding" with clusters of aligned grains which we have termed domains. EXPERIMENTAL YBa Cu 3 0 6 + powder was prepared using the conventional mixed-oxide route from the constituent carbonates. Bar specimens (25 mm x 6 mm x 5 mm) were pressed at 25.8 ksi, and sintered in flowing oxygen. The heat-treatment schedules were as follows:-
Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society
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1. Sample heated at 8 °C/min to hold temperature (TH), and held for time (t ). The hold temperature was varied from 950 to 1100 C, and Ehe hold times varied from 0.5 to 20 hrs. 2. Samples were subsequently cooled at 0.5 °C/min to 800 °C, and furnace cooled to room temperature. 3. Oxygen annealing was carried out for 20 hrs at 500 flowing oxygen.
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RESULTS AND DISCUSSION Rhe results showed that no alignment was obtained for TH < 1000 C, thus confirming the import
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