Anodic films formed on a range of valve metals are classified into two types, i.e., barrier-type and porous-type. The barrier-type anodic titanium oxide is a potential dielectric material for electrolytic capacitors, while the porous-type anodic titanium oxide has potential applications in photocatalysis, photoelectrolysis, photovoltanics and as sensors.

The barrier-type anodic films are formed on high purity titanium in acidic and neutral aqueous electrolytes, but an amorphous-to-crystalline transition occurs at low voltages of less than 10 V. After the transition, the film growth proceeds, accompanying oxygen gas evolution on the crystalline oxide, which is located at the inner part of the anodic film. Thus, the films developed contain high density of flaws, and are not suitable for the capacitor application. In this paper we have demonstrated the effective suppression of the amorphous-to-crystalline transition of anodic titanium oxide by alloying of titanium. Magnetron sputtering is used to form homogeneous solid solutions exceeding the solubility limits of alloying elements at equilibrium

The most effective element in suppressing the amorphous-to-crystalline transition is silicon. By the addition of only 6 at% silicon to titanium, the amorphous film growth proceeds to more than 100 V at high current efficiency of ~100%. The contents of the alloying elements required to suppress the crystallization are strongly dependent upon the element. For instance, more than 25 at% aluminium is necessary to grow the anodic film without crystallization. It has been found that there is a good correlation between the content of the alloying elements for crystallization suppression and mobility of the alloying element species in growing anodic oxide; more slowly migration species are more effective in suppressing the crystallization.

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