THE CHEMICAL-STATE OF COPPER IN HIGH-T(C) CUPRATE SUPERCONDUCTORS ANDTHE CONSEQUENTIAL CORROSION OF ELECTRICAL CONTACTS

The inherent electrochemical reactivity of high-temperature ceramic su perconductors can be attributed to the presence of highly-oxidized spe cies. Consideration of the formal valency of copper in these materials appears to suggest a mixed Cu(III)/Cu(II) state. However, a mixed O(I )/O(II) state is equally valid. Measurements of the electrochemical po tentials of YBa2Cu3O7-delta and PbxBi2-xSr2Ca2Cu3O10 superconductors m ade in buffered solutions containing various concentrations of Cu2+ io ns suggested that the Cu(III)/Cu(II) state is unlikely. The open-circu it electrochemical potentials of the superconductors ranged between +4 00 and 1000 mV(SCE), indicating likely oxidizing behaviour with respec t to all contact metals. Galvanic coffosion tests were performed for b oth indium and gold in various aqueous solutions. In all cases, the su perconductors were observed to react cathodically, instigating coffosi on of the metals. The choice of contact material with the high-T(c) su perconductors is, therefore, Of great importance electrochemically, as is the need to maintain the superconductor/contact interface in a com pletely dry environment.