Evidence for an ideal transparent anodic oxide film on zirconium

The properties of a transparent oxide formed on zirconium by anodic oxidation in carbonate buffer are described. The transparent oxide has all the properties of an ideal valve-metal oxide. The film grows via a high field conduction mechanism. The potential and film thickness both increase linearly with time under constant-current growth conditions to well over 100 V with no sign of electronic leakage or breakdown. When the oxide is grown at a current density of 93 {mu}A/cm{sup 2}, the electric field in the oxide is 4.1 MV/cm, and the oxide is anisotropic with n{sub z} = 2.278 parallel and n{sub x} = 2.296 perpendicular to the field. The relative permittivity is 38.3 at the anodizing field, and the oxide shows electrostrictive effects similar to those exhibited by other valve-metal oxides with high relative permittivities. Both the refractive index and the relative permittivity increase when the field is removed, and there is a corresponding decrease in film thickness of just under 1%. Cathodic reduction inserts hydrogen into the oxide to a limited depth, and the outer layer thus formed is optically absorbing. Subsequent anodic oxidation removes the hydrogen and returns the film to its initial transparent state. The present study wasmore » carried out as part of an investigation comparing films formed anodically with films formed in steam on zirconium-based reactor tube materials.« less