The Chemical State of Copper in High-Tc Cuprate Superconductors and the Consequential Corrosion of Electrical Contacts

The inherent electrochemical reactivity of high-temperature ceramic superconductors can be attributed to the presence of highly-oxidized species. Consideration of the formal valency of copper in these materials appears to suggest a mixed Cu(III)/Cu(II) state. However, a mixed O(1)/O(II) state is equally valid. Measurements of the electrochemical potentials of YBa2Cu3O7-6 and PbχBi2χSr2Cu2Cu3O10 superconductors made in buffered solutions containing various concentrations of Cu2+ ions suggested that the Cu(III)/Cu(II) state is unlikely. The open-circuit electrochemical potentials of the superconductors ranged between +400 and 1000 mV(SCE), indicating likely oxidizing behaviour with respect to all contact metals. Galvanic corrosion tests were performed for both indium and gold in various aqueous solutions. In all cases, the superconductors were observed to react cathodically, instigating corrosion of the metals. The choice of contact material with the high-Tc superconductors is, therefore, of great importance electrochemically, as is the need to maintain the superconductor/contact interface in a completely dry environment.