Formation mechanism of Y 2 BaCuO 5 pattern in growing YBa 2 Cu 3 O x grains during melt-infiltration process
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Formation mechanism of Y2 BaCuO5 pattern in growing YBa2 Cu3 Ox grains during melt-infiltration process Young A. Jee and Suk-Joong L. Kanga) Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yousong-gu, Taejon 305-701, Korea
Hyungsik Chung Korea Institute of Machinery and Metals, 66, Sangnam-dong, Changwon 641-010, Korea (Received 4 November 1996; accepted 8 May 1997)
When a melt of BaO and CuO mixture was infiltrated into sintered Y2 BaCuO5 (211) compact to form YBa2 Cu3 Ox (123) superconductor, butterfly-like plane patterns of 211-free regions were observed to form within growing 123 grains. In a 123 grain, the 211-free region was found to be a pair of vertex-shared pyramids and 211 entrapped region to be the rest of the bulk of the grain. An observation of patterns and cracks formed within 123 grains revealed the base of the pyramids to be (001) plane. The difference in entrapment, which depends on crystallographic planes and results in the formation of the pattern, was explained by the dihedral angles between 123 and 211. The dihedral angle between a- (or b-) plane and 211, which is believed to be greater than zero degree, might cause the entrapment of 211 particles in a [100] (or [010]) direction. In contrast, the dihedral angle of most probably zero degree between c-plane and 211 inhibited the entrapment. The observed shape of 211 particles in front of a(or b-) and c-planes supports the above explanation of 211 entrapment to form the butterfly-like patterns.
I. INTRODUCTION
The improvement of critical current density has been one of the main issues in the development of bulk ceramic superconductors.1–4 For 123 superconductors, the most effective way for the improvement is the melt-texturing suggested first by Jin et al.5 ; the critical current density can be reached up to 105 Aycm2 .5–8 The microstructure obtained by this process is typified by crystallographically aligned 123 grains and 211 particles entrapped within them. The distribution of 211 particles, however, is usually nonuniform and sometimes forms particular patterns with apparently high and low density of 211 particles within 123 domains.9,10 Figure 1 is a schematic of a pattern often observed in melt-processed specimens. The 211 particles represented by open circles make “tracks” within a single crystal 123 grain. The cause of the formation of such a 211 pattern has been studied by several researchers.10–14 The explanations so far are based on the UCJ (Uhlmann, Chalmers, and Jackson) model15 which describes a trapping condition of second-phase particles during crystal growth in a melt. The model describes that, when the growth velocity of a crystal is low and the size of particles is small, the particles tend to be pushed by the growing crystal, but in an opposite case, the particles tend to be trapped within the crystal. Varanasi9,11 and a)
The author to whom all the correspondence should be addressed. J. Mater. Res., Vol. 13,
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