The relation between the undercooling and the growth rate of YBa 2 Cu 3 O 6+x superconductive oxide
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The relation between the undercooling and the growth rate of YBa2 Cu3 O61x superconductive oxide Y. Nakamura, A. Endo, and Y. Shiohara Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo, 135, Japan (Received 10 March 1995; accepted 5 October 1995)
To clarify the effect of undercooling on the crystal growth of Y-123, the growth rate was measured with different undercoolings. The growth rate of the h100j face shows a quadratic dependence of undercooling, while that of the h001j face shows a linear dependence in the sample with nominal 123 composition. In the case with 211-rich composition, the growth rate of each face was larger than that compared with nominal 123 composition since the mass flux from 211 particle for peritectic reaction becomes large. Addition of excess 211 alters the undercooling dependence of Ra from quadratic to linear. It is considered that the entrapment of 211 particles into 123 crystals supplies step sources beside screw dislocations. The growth rate of the h001j face is larger than that of the h100j face up to 26± of undercooling.
I. INTRODUCTION
Melt and growth processes for producing YBa2 Cu3 O61x (123) superconductive oxide1–3 are considered to be an effective process to obtain high critical current densities. To control the microstructure and alignment of the crystals, deep understanding of its growth mechanism is important. Many investigations4 –12 were performed to clarify the growth mechanism of 123 crystal from the partial molten state where Y2 BaCuO5 (211) and liquid phases coexist. Recently, it was found that the peritectic reaction of the 123 phase formation was proceeded by the solute diffusion between 211 particles dispersed in the liquid phase and the growing 123 interface.4 –7 Based on this idea, the growth models assuming the mass transfer limiting were suggested, and the growth rate was found to be affected by an interface undercooling as the driving force for solute diffusion.4 –6,8 On the other hand, a crystal showing faceted interfaces has a large growth anisotropy and evidently needs supersaturation or kinetic undercooling for its growth. Many 123 crystals show the faceted shapes, and this means the significant kinetic undercooling is needed for crystal growth; i.e., the experimentally measured undercooling acts as the driving force of interface kinetics as well as solute diffusion. Therefore, the undercooling is a principle parameter in controlling the growth of 123 crystals. About the growth anisotropy of 123 crystal, the growth rate of the h100j face has been believed to be fairly faster than that of the h001j face. However, only a few quantitative measurements of growth rate have been carried out as a function of undercooling.6,9,13,14 To clarify the effect of undercooling, the growth rate of the 123 crystal was evaluated using the melt and seeded growth process under the constant undercoolings, 1094
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J. Mater. Res., V
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