Influence of film composition on the structure and dielectric properties of anodic films on Ti-W alloys

The influence of tungsten on an amorphous-to-crystalline transition, leading to formation of anatase, and dielectric properties is examined for anodic oxides on Ti-W alloys. The structural change, which occurs at voltages of ∼ 10 V on high-purity titanium, and provides sites for generation of high-pressure gas oxygen bubbles, is suppressed to progressively increased voltages by increasing tungsten content of the film. Phosphorus species, derived from the electrolyte, also hinder the change. The tungsten species are located in the inner ∼ 80% of the film thickness as result of the faster migration of Ti⁴⁺ ions relative to that of W⁶⁺ ions in the growing oxide. Although the reported permittivity of anodic tungsten oxide is higher than that of amorphous anodic titania, the permittivity of the films on alloys decreases gradually with increasing tungsten content.