Peritectic solidification model for Y-system superconductive oxides
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Peritectic solidification model for Y-system superconductive oxides Y. Nakamuraa) and Y. Shiohara Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo, 135 Japan (Received 28 February 1996; accepted 13 May 1996)
A peritectic solidification growth model considering both solute diffusion in the liquid and interface kinetics is developed for the growth of YBa2 Cu3 O61x (123) superconductive oxide. The effect of interface kinetics on the growth rate and the rate control process is discussed using this model. The growth rate of the 123 crystal is evaluated as a function of the undercooling using the proper kinetic equation. The evaluated results by this model show good agreement with the experimental results. As a result of this evaluation, the growth of the 123 crystal is predicted to be limited by a mixed control process in general.
I. INTRODUCTION
II. SOLIDIFICATION MODEL
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. These melt-textured materials show highly aligned structures and are formed by a peritectic reaction between the Y2 BaCuO5 (211) and liquid phases. The benefits of these processes are to obtain larger grains which make fewer grain boundaries which act as weak links, and to introduce 211 particles as pinning centers into the grown 123 crystals. Many investigations4–13 were performed to clarify the growth mechanism of 123 crystals from the partial molten state where the 211 and liquid phases coexist in equilibrium. The peritectic reaction of the 123 phase formation was found to be proceeded by the solute diffusion between 211 particles dispersed in the liquid phase and the growing 123 interface.4–7 Based on this idea, growth models assuming limited mass transfer were suggested,4–6,8 and the growth rate was found to be affected by the size distribution of 211 particles, the temperature gradient at the interface, and the interface undercooling, which are the driving forces for solute diffusion. Furthermore, a crystal showing faceted interfaces has a large growth anisotropy and needs supersaturation or kinetic undercooling for growth. In this paper, a solidification growth model that considers both the solute diffusion through the liquid phase and the interface kinetics for crystal growth is suggested, and the effect of interface kinetics on the growth process of the 123 crystal is discussed.
A. General concept
In the Y–Ba –Cu–O system the yttrium solubility in the liquid phase on the line including the 123 and 211 phases is only 0.7 at. % at the peritectic temperature (1283 K) while barium and copper concentrations are high.14 Therefore, we assume that the important diffusion process in this system is the diffusion of yttrium solute. The necessary yttrium solute for 123 growth is considered to diffuse from 211 particles dispersed in the liquid phase to the growing 12
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