Aluminum specimens coated with niobium oxide by a sol-gel method were galvanostatically anodized in a neutral borate solution. The time-variations in anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide film were studied by TEM, EDX, RBS, and electrochemical impedance measurements.
It was found that the anode potential increases linearly with time after a rapid increase in potential at the initial period of anodizing. During anodizing, an anodic oxide film, which consisted of an inner Al2O3 layer and an outer Al-Nb composite oxide layer, grew at the interface between the Nb2O5 layer and the aluminum substrate. The interface between the outer and inner layers became more vaguer as the potential rose, and the concentrations of aluminum and niobium ions in the composite oxide layer also changed. The capacitance of the specimen after the sol-gel coating and anodizing was 150% as large as that without the sol-gel coating. This is due to the high dielectric constant of the composite oxide layer.
The mechanism of the anodic oxide film's growth on the specimen coated with Nb2O5 is discussed in terms of the pore distribution in the Nb2O5 layer and the transport of Nb-cations across the anodic oxide film during anodizing.
