| Budget Amount *help |
¥2,300,000 (Direct Cost: ¥2,300,000)
Fiscal Year 1994: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 1993: ¥1,400,000 (Direct Cost: ¥1,400,000)
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| Research Abstract |
Two fabrication methods were investigated to develop the low resistivity joints of silver and Y-Ba-Cu-O superconducting oxide (YBCO) with the columnar crystal structure of 123 phase (YBa_2 Cu_3O_<7-X>) , which was obtained by unidirectional solidification from liquid+211 (Y_2BaCuO_5) phases. One of the methods was a solid state processing by the hot-press to make solid silver/YBCO joint, in which no detectable interaction layr was formed. The interface resistivity (rhoc) at 77K measured by the four-prove method decreased with the increase in joining temperature (T) or joining pressure (P) , and the minimum value : rhoc=2*10^<-12>OMEGA・m^2 was obtained when the joining conditions were P=32MPa, T=873K,t=3600s.Another method was a liquid state processing by using molten silver, in which the remarkable reaction layr was formed at the interface. Since the reaction was caused by the decomposition of 123 phase to liquid+211, the reaction layr was recovered to 123 phase by slow cooling and sol
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idification. In this case, rhoc remarkably decreased by the recovering treatment, and the minimum value was as low as about 10^<-12>OMEGA・m^2, Which satisfied the stability criteria for composite wire. Further, to study the structure controll of YBCO substrate, the growth mechanism of 123 columnar crystal was analyzed, which grew with a facet interface from liquid+211 through a peritectic reaction with solute diffusion in the liquid. The following relation was conducted between 123 growth rate (R) and undercooling (DELTATpc=Ti-Tp, Ti : interface temperature, Tp : peritectic temperature) by the analysis of kinetics for 123 columnar crystal growth : R=(Ca/eta)・{(ML・DELTATpc+DELTACrc)/(Tp/m_<L123>+Cf・Tp/m_<L211>)}^2, where Ca and Cf are constants, eta is the viscosity, ML=1/m_<L123>-1/m_<L211>, m_<L123> and m_<L211> are respectively the slope of liquidus of 123 and 211, DELTACrc is the change in equilibrium solute concentration caused by the surface energy of 211 particle at the 123 freezing front. This relation agreeed well with the experimental results. The transition of 123 crystals from columnar to equiaxd structure was successfully simulated by using the above R-DELTATpc relation and the experimental undercooling (DELTATp) for the nucleation of 123 crystals. Less
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