Growth Mechanism of RE-Ba-Cu-O Crystal by Liquid Phase Epitaxy Process
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Growth Mechanism of RE-Ba-Cu-O Crystal by Liquid Phase Epitaxy Process K. Nomura, S. Hoshi, X. Yao, Y. Nakamura, T. Izumi and Y. Shiohara Superconductivity Research Laboratory, International Superconductivity Technology Center 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan ABSTRACT Growth of YBa2Cu3Oy (YBCO) crystals both on the MgO and the YBCO substrates has been investigated in order to clarify the growth mechanism of the liquid phase epitaxy (LPE) process for the coated conductor. It was found that the slope angle of the growth grain varied with growing in the initial stage of the LPE growth. In the case of the MgO substrate, the slope angle increased with laterally growing the crystal. On the other hand, the slope angle decreased with growing the crystal in the case of the YBCO substrate. This phenomenon with the opposite tendency could be explained by considering the difference in the step-advancing rates between on the MgO and the YBCO surfaces. It was found that the deep and steep valley of the LPE grains due to the slow step-advancing rate on the MgO is the origin of the flux trapping inclusions. It is necessary that the seed films should cover the entire MgO surface without undesired orientation grains and impurity particles in order to obtain the high quality LPE layer.
INTRODUCTION There have been many studies to develop fabrication techniques for long wires or tapes of oxide superconductors. The wires and tapes are usually composites with a metal substrate or a metal sheath for both enough flexibility and a high mechanical strength. Recently, the coated conductor of REBa2Cu3Oy (REBCO, RE =Y, Nd, etc.) has been expected as a next generation conductor instead of the Bi-system conductors because of the better superconducting properties in the high magnetic fields at 77K. From the practical point of view, the engineering critical current density (Je) is also an important factor for the wire/tape conductor. In order to realize high Je, thin metal substrates with high mechanical strength and/or thick superconducting layer with high Jc (critical current density) are required. In the case of the REBCO coated conductors, the Ni based superalloys such as the Hastelloy can be used as the high mechanical strength substrates. Among the various fabrication methods, liquid phase epitaxy (LPE) process is suitable to fabricate a thick layer with high crystallinity at a higher growth rate than the conventional vapor deposition methods. However, the solvent of the Ba-Cu-O melt in the LPE process for the REBCO growth is very reactive with most of the metals due to both the high chemical reactivity of the solvent and the high temperature of the process. In our previous studies, we found that the MgO buffer layer on the Hastelloy substrate was effective to prevent the reaction between the Hastelloy substrate and the Ba-Cu-O melt as the solvent when the melt was saturated with the MgO [1]. And we have succeeded in growing the LPE layer on the Hastelloy substrate with the MgO buffer layer [2]. Through the above investigati
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