Anodic oxide films on tantalum: anomalies in steady–state and stepped field ionic conduction and incorporation of electrolyte species

Abstract

The high field ionic transport process by which these films grow has been investigated using a 1M solution of KCl, an electrolyte which has recently been shown not to be incorporated into the films, unlike those previously used. In the steady-state, the current density J ss at a field in the oxide E ss is given by J ss = J 0 exp{−(W 0 − 5qa 1 E ss )/kT }, where log 10 (J 0 /1 mA cm −2 ) = 12.4(5), W 0 = 1.67(7) eV, a 1 = 0.29( 7) nm and q is the proton charge. When the field is rapidly changed to a new value E from its initial steady-state value E ss , the current density J is given by J = J ss exp(5qa 2 (E − E ss )/kT ) where J ss is the initial value and a 2 = 0.12(4) nm. Thus, neither the previously reported curvature of the steady-state log J ss versus E ss line nor the dependence on the initial steady-state field of the slope of the stepped field log J versus E lines were present when this electrolyte was used instead of dilute sulphuric acid. It is concluded that these two effects, which had been considered by some to be fundamental attributes of the ionic conduction process, are due to the incorporation of electrolyte species in the outer part of the oxide, which grows by metal ion movement.