Formation of Al/(Ti, Nb, Ta)-composite oxide films on aluminum by pore filling

Aluminum specimens coated with porous anodic oxide films were dipped in solutions containing Ti⁴⁺, Nb⁵⁺, or Ta⁵⁺ ions and heated in air mainly at 400°C to deposit Ti, Nb or Ta oxide on the inner walls of pores. After 0 to 7 cycles of the dipping and heating process, the specimens were reanodized in a neutral borate solution to fill the pores with Al₂U₃. The incorporation of Ti, Nb, or Ta oxide in the anodic oxide films was examined by transmission electron microscopy, Rutherford backscattering spectroscopy, and impedance measurements. Incorporation of Ti, Nb, or Ta oxide was found only in the outermost part of the anodic oxide film after reanodizing up to 400 V. The Al/Ti composite oxide films consisted an outer Al/Ti-composite oxide layer, an intermediate crystalline alumina layer, and an inner amorphous alumina layer while the Al/Nb- and Al/Ta-composite oxide films consisted of an outer composite oxide layer and an inner amorphous alumina layer. The parallel electric capacitance of Al/Ti-composite oxide film was 40% higher than those of the other two due to its small thickness and high dielectric constant. The formation mechanisms of the composite oxide films during reanodizing are discussed in terms of the ion transport across the oxide film.