R&D of Coated Conductors in Japan
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R&D OF COATED CONDUCTORS IN JAPAN Yuh Shiohara and Toru Izumi Superconductivity Research Laboratory, ISTEC 1-10-13 Shiononome, Koto-ku, Tokyo, 135-0062 Japan
ABSTRACT The recent achievements of critical current densities in excess of 1MA/cm2 at 77K in YBCO films deposited over appropriate bi-axially textured buffer layers/metal substrates have stimulated interest in the potential future applications of coated conductors at liquid nitrogen temperature and high magnetic fields, which are called as the next/second generation tape conductors. Several different processes for obtaining the bi-axially textured buffer layers/metal substrates as well as thicker film deposition processes and non-vacuum processes are reviewed. This paper reviews the research goals of the project including the recent development of the related research by individual research group in Japan.
INTRODUCTION Taking advantage of YBCO, which has its high irreversibility fields at high temperatures, applications at the liquid nitrogen temperatures in preparing superconductive power transmission cables, coils for magnets or current limiter, etc. are highly expected, although its critical superconductive transition temperature is lower than those of BSCCO conductors. The powder in tube process when applied to YBCO does not provide the expected properties. The YBCO superconductive material has a characteristic critical current density of the order of 1MA/cm2 when deposited on single crystalline substrates. The critical current density of the film on a polycrystalline substrate is drastically reduced by the presence of high angle grain boundaries [1] and is termed as the weak link problem. Accordingly, for the tape applications it is strongly desired to grow the films with highly in-plane aligned crystalline structures, i.e. alignment along not only c-axis but a-axis. The intergranular critical current densities of the films so accomplished approach the film intragranular current densities. The YBCO coated conductors are basically constructed by the layers of buffer layers and superconducting layers on a metal substrate as shown in Fig. 1. Each layer including a metallic substrate has a specific role for the long length conductors, also as described in the figure. The candidate materials and the thickness of each layer are also presented in the figure. It is noted that a thin passivation layer for stabilization, insulation and encapsulation is excluded.
II1.1.1
Biaxially in-plane Alignment Homogeneity High Production Rate thickening
Superconducting Layer Y123, Nd123, Sm123, RE (mix ) 123 (Thickness; 1~10µm)
Alignment Control for SC Layer Prevention of Reaction Suppression of Crack formation Homogeneity, Flatness, High Production Rate
YSZ, MgO, CeO2 , NiO ZrO2 , BaZrO3 etc (Thickness; 1m/h Length; 1~10m Sub. Thickness ≦100µm SC Thickness ≧ 5µm J C ≧105~106A/cm 2 (77K) Production Rate; >1m/h
Textured metallic substrate type conductors Current research indicates that the substrate will be a composite comprised of a thin metallic substrate with a few
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