Anodic Oxide Films Grown in Dehydrated Phosphate/Glycerol Electrolytes at High Temperatures

Traditional anodization was generally conducted in aqueous or water-containing solutions at room temperature or even lower temperatures. The two best known examples of the application of traditional anodization in the fabrication of nanostructured oxides are porous anodic alumina and titania nanotube films. In 1998, Melody et al. first observed that anodic oxide films on some valve metals such as tantalum can continue to grow at low anodization voltages in phosphate/glycerol solutions containing less than 0.1% water at elevated temperatures (>150 °C) in a non-thickness-limited (NTL) manner. Clearly, this type of anodization at high temperatures is conceptually different from traditional anodization. This Perspective provides a relatively thorough review of this topic. Particular attention is given to the main features for the NTL growth of anodic oxide films and the mechanism behind this behavior. The anodization behaviors for the NTL growth and typical microscopic morphologies of the resultant films on various metals are discussed. The main applications of the attained anodic films with different nanostructures and anodization in other dehydrated electrolyte systems such as the molten orthophosphoric acid are also involved. Finally, we highlight some unresolved issues for this type of anodization at high temperatures and the potential future directions of this field.