Junya Maeda

Reaction between YBCO/Ag superconductor and melted silver

In order to study the feasibility of applying liquid phase processing using melted silver to fabricate YBCO/silver contacts, the chemical reaction at the boundaries between the Y-Ba-Cu-O (YBCO) superconducting matrix and melted silver was observed. YBCO superconducting current leads prepared by the unidirectional solidification method were dipped into liquid silver melted by an electric furnace. Copper-poor layers were formed at the boundary between the YBCO matrix and melted silver, although the temperatures were lower than the YBCO/Ag melting temperature . It was considered that melted silver took up copper from the (Y123) crystal, and it is considered that the phase equilibrium at the boundary between the YBCO matrix and molten silver was changed from (Y211)-Y123 to another phase.

The effect of residual stress on crack propagation in a YBCO/Ag composite

Residual stress in a YBCO/Ag composite was estimated by studying the difference between the thermal expansion coefficients of the silver particles and the brittle Y1Ba2Cu3Ox (Y123) superconducting matrix using the finite element method. The influence of this residual stress on crack propagation around the silver particles dispersed in the Y123 matrix is discussed.

Joining of Melt-Processed RE-Ba-Cu-O Superconductor

The melt-processed Y-Ba-Cu-O superconducting bulks were jointed using a Yb-Ba-Cu-O superconducting solder having a lower peritectic temperature than that of the YBCO matrix. Generally, the Yb-Ba-Cu-O superconducting solders were sandwiched between Y-Ba-Cu-O superconducting bulks. The problems of this sandwich soldering were the segregation of RE211 particles and voids formed at the final solidified regions that might interrupt the current flow through the jointed region. In this paper, we discussed the solidification behavior in this sandwich-soldering region for understanding the formation of these non-superconducting phases at the final solidification regions.

Influence of oxygen partial pressure on growth morphologies in unidirectionally solidified YBCO-Ag superconducting composites

YBCO/Ag superconducting composites were fabricated by the unidirectional solidification method under various oxygen partial pressures. The effect of oxygen partial pressure on the growth interface stability and the morphology of (Y123)-Ag composite was investigated. The growth interface changed from planar to not planar with decreasing oxygen partial pressure. This phenomenon indicated that the interface stability was dominantly affected by the slope of liquidus, which is a significant function of the oxygen partial pressure. The morphology of the Y123-Ag composite was also strongly influenced by the oxygen partial pressure. In the case of 0.21 atm oxygen partial pressure , rod-like silver particles elongated along the growth direction were formed around the bottom of the facet crystal. On the other hand, spherical silver droplets were entrapped at the grain boundaries in the case of atm.

Fabrication Process of Low Resistivity Connection between YBCO Superconductor and Silver

YBCO superconducting rods prepared by the unidirectional solidification method were dipped into the molten silver in order to fabricate a metal contact on the surface of the rods. In the case of dipping into the pure molten silver, a copper poor reacted layer was formed at the interface between Y1Ba2Cu3Ox(Y123) and silver, although the temperature of the molten silver was controlled to be lower than the decomposition temperature of the Y123 phase. This reaction could be suppressed by means of addition of Y123 powder into the molten silver. This method successfully realized the contact with the low resistivity of 4.6x10−12Ωm2