Diffusion solidification model on Y-system superconductors
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Diffusion solidification model on Y-system superconductors Teruo Izumi, Yuichi Nakamura, and Yuh Shiohara Superconductivity Research Laboratory, International Superconductivity Technology Center, Tokyo, Japan (Received 14 January 1992; accepted 10 March 1992)
The unidirectional solidification of the zone melt method was performed in order to clarify the growth mechanism on Y-system superconductors. A sharp faceted interface of YBa 2 Cu 3 0 > , (123) crystals was obtained in the sample grown at the low growth rate of 1 mm/h. The volume of the 211 phase changed drastically from liquid to 123 crystal. These results lead to the idea that the necessary solute for peritectic reaction is provided through a liquid. Based on this idea, we developed a simple solidification model that is in good agreement with the experimental results. I. INTRODUCTION Since high Tc oxide superconductors were discovered, much effort has been devoted to obtaining higher critical current densities (Jc). Recently, high Jc values have been reported in samples prepared by several solidification processes, including MTG (Melt Textured Growth),1 QMG (Quench and Melt Growth),2 and MPMG (Melt Powder Melt Growth)3 methods. Murakami et al. reported the highest Jc of 1.2 x 105 and 3.0 x 104 A/cm 2 at 77 K in 0 T and 1 T, respectively, in bulk 123 superconductor samples prepared by the MPMG method.3 The crystal growth mechanism of the superconducting 123 phase, however, has not yet been clarified, although it is important because alignment of the crystals and the fine dispersion of the nonsuperconducting Y 2 BaCu0 5 (211) phase particles are responsible for obtaining higher critical currents.
rods of 2 mm in diameter and 120 mm in length by a Cold Isostatic Press (CIP) using a rubber tube. The rod samples were suspended by a clip and inserted into the furnace at high speed, initially. Then, the samples were pulled upward at different growth rates (R), which were 1, 2, 3, 6, and 10 mm/h. The samples were dropped into a quench tank by releasing the clip during steady state solidification to observe the solidification interface morphology of the 123 crystals. The microstructures were investigated by optical microscopy and EPMA.
In this paper, the effects of growth rate (R) on the solidification behavior and the microstructures so produced were investigated. Furthermore, the details of the simple solidification model based on the experimental results are discussed.
Insulator
II. EXPERIMENTAL
7 mm
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Zone Heater
Supporting ^ Heater
A. Directional solidification The growth apparatus used in this study is shown in Fig. 1. The furnace consists of two parts: an outer heater coiled around a ceramic tube, and another filament loop located inside the outer heater. This inner heater mainly controls the partial melting zone. The partial melting zone, which is over the peritectic temperature, was kept about 10 mm in length and the maximum temperature in the zone was controlled at 1040 °C. The temperature gradient (G) at 1000 °C was about 180 °C/cm. The p
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