Transport numbers of metal and oxygen species in anodic tantala

Abstract The transport numbers of metal and oxygen species have been determined in amorphous anodic tantala films, using transmission electron microscopy to locate ion-implanted xenon marker layers within the films. The films were formed on sputtering-deposited tantalum at constant current density, in the range 0.01–10 mA cm−2, in 0.06 wt% H3PO4 solution at either 20 or 85 °C. The films grow by migration of metal and oxygen species through the film thickness, with formation of new film material at the film/electrolyte and metal/film interfaces respectively. The cation transport number, t+, increases due to either increase in current density or decrease in temperature: for current densities in the selected range, t+ increases from 0.18 to 0.32 at 20 °C, and from 0.14 to 0.29 at 85 °C. Low concentrations of phosphorus species, incorporated into an outer layer of the film, migrate inward during film growth. The migration rates are slower than those of oxygen species, by a factor in the range 0.2–0.3.