Additive enhanced nonaqueous electrodeposition of silver on Ba 2 YCu 3 O 7
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A nonaqueous plating bath for silver deposition on the Ba2YCu3Oy superconductor has been modified with thiourea to give enhanced coverage of the metal film at low thicknesses. For both porous and high density ceramic substrates, SEM and optical microscopy show nucleation of silver deposits is increased by the additive. The resulting silver-superconductor contact resistances are comparable to those achieved without additives. Adhesion of the deposits and ease of mechanical handling are improved with thiourea. The combination of highly reactive oxidized substrate, nonaqueous solvent, and strongly surface active agent forms a novel extension for electrodeposition science. I. INTRODUCTION
The reactivity of cuprate-based high temperature superconductors to ambient and aqueous environments1'2 requires means of protection for these substrates in their processing and applications. Accomplishing this isolation from the environment with stable metal layers simultaneously offers electrical conductivity parallel to that of the superconductor, as well as thermal sinking and physical support. Successful means to electrodeposition of metals such as silver, copper, lead, and tin on Ba2YCu3O7 have been described earlier by us. 3 Subsequent measurements showed that the plated metal-cuprate composite retains the magnetic and resistive behavior of the superconducting substrate.4 Moreover, contact resistances for silver electrodeposited on Ba2YCu3O7 are comparable to those of ultrasonically soldered indium-Ba2YCu3O7 junctions.4 The electrochemical route to these well-behaved silver-Ba2YCu3O7 interfaces is also applicable to the deposition of the more electronegative elements (Cu, Sn, and Pb) that are not readily plated in aqueous media and involves the use of nonaqueous solvents. Since water or protons participate in the half reactions for oxide reduction, the efficient deposition of metal species3 can proceed in these dry media against the otherwise difficult competition of surface reduction5"10 of the highly oxidized cuprate compounds which is suppressed in the absence of water. However, in order to achieve protection by the metal with minimum electroplated material, uniform coverage of the substrate through high density nucleation of the deposit must be sought. The microscopically rough, porous nature of most bulk ceramic forms of these cuprates exacerbates the difficulties of reaching thin, continuous films. Among the conventional means employed for aqueous media to improve coverage of electroplates are the addition of suitable surface active agents to the 1612
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 8, Aug 1990
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electrolyte which enhance nucleation through the mechanism of adsorption and change of electron transfer kinetics. Another route is a pulse deposition technique to increase the surface density of nuclei over that achieved in standard dc plating. The latter, more complex approach instrumentally, tends to promote the competitive path of substrate reduction during high current pul
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