Influence of Silver Infiltration on the Microstructure and Transport Properties of YbacuO
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INFLUENCE
OF SILVER INFILTRATION ON THE MICROSTRUCTURE AND TRANSPORT PROPERTIES OF YBaCuO.
L. RYELANDT, M. CASSART, A. VANDENBOSCH, F. DELANNAY and J-P. ISSI. Universit6 Catholique de Louvain, Unit6 de Physico-Chimie et de Physique des Mat6riaux, 1 Place Croix du Sud, B-1348 Louvain-laNeuve, Belgium ABSTRACT The results of measurements of the transport properties of high-Tc superconductors containing different amounts of silver are reported. The Tc and Jc values were found to be similar to that of the single phase YBaCuO compound whereas the electrical and thermal conductivities above Tc were drastically increased. A very low contact resistance was observed. INTRODUCTION The use of silver is now widely spreading in the fabrication of superconducting devices. For wires in a silver sheath, the heat treatments at high temperature lead to the infiltration of liquid silver into the pores of the ceramic core. In this work, we have investigated how the presence of silver affects the microstructure and the transport properties of the YBaCuO superconducting phase. SAMPLE PROCESSING The YBa2Cu3o7-a powders were synthesized starting from a mixture of precursors (Y 2 0 3 , BaC03 and CuO) using an usual heat treatment procedure. Wires of YBa2Cu307-a in a silver sheath were prepared by swaging followed by a heat treatment above 900 0 C under flowing 02 [1, 2]. In the samples treated at or above 940 0 C, one observes the infiltration of silver into the porosity of the ceramic core. In order to vary and adjust more precisely the silver content of the composites, pellet samples were also prepared by mixing YBaCuO powder with 5 to 25 wt % of Ag20 powder, then by pressing and sintering. The Ag/YBaCuO composite samples (wires and pellets) were heat-treated for 20h under flowing 02 at a temperature varying from 920 to 950 0 C. A reoxidation step at 450'C was performed before cooling down to room temperature. MICROSTRUCTURE X-ray diffraction (XRD) studies indicate that the samples are composed of orthorhombic YBa 2 Cu3O7-_ and metallic silver. Some CuO phase was only detected by electron probe microanalysis. No reaction layer was observed at the interface between silver and YBaCuO crystals. The grains of the 123 phase are rather irregular but compact and faceted (3-10 gm) . Although the
Mat. Res. Soc. Symp. Proc. Vol. 169. C1990 Materials Research Society
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infiltration of silver suggests spontaneous wetting of YBaCuO by liquid silver, the Ag content does not appear sufficient to fill all the open porosity which always amounts to at least 10 % in the final samples. TRANSPORT PROPERTIES All samples exhibited a sharp superconducting transition with a Tc value between 90 and 91 K. Figure 1 shows the variation of the room temperature resistivity in Ag-YBaCuO composites pellets as a function of the silver content. It may be seen that this resistivity can be reduced by a factor of 30 with respect to that of the pure YBaCuO ceramics prepared in the same conditions. This could be due to the filling of the porosity of the samples, and t
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