Preparation of Nb2O5-Al2O3 Composite Anodic Oxide Film for an Aluminum Electrolytic Capacitor by Electrodeposition-Annealing and Anodization

Abstract

Nb was electrodeposited on etched aluminum foils for an Al electrolytic capacitor. After annealing at 500°C in air, the foils were anodized in H3BO4 solution at 530 V to form Nb2O5-Al2O3 composite anodic oxide film as a dielectric layer. The voltage–time variations during the anodization process were monitored. The structure, composition, and electrical properties of the anodized foils were investigated by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and electrochemical impedance spectroscopy. The obtained foils were assembled into aluminum electrolytic capacitors, and the capacitor performance was tested according to the Japanese Nichicon standard. It was found that after electrodeposition and annealing, the slope of the voltage–time curve of the aluminum foil became steeper during the anodization process. The composite anodic oxide film showed a triple-layer structure consisting of Nb2O5/Al-NbOx/Al2O3 layers. The specific capacitance (C and Cox) of the composite anodic film was about 14% greater than that of the aluminum anodic oxide film. However, the leakage current (I) of the composite film was increased and its specific resistance (Rox) and withstanding voltage (Uw) decreased relative to the aluminum anodic oxide film, probably due to the greater number of intrinsic defects. During the load life and shelf life test, the composite anodic oxide film demonstrated capacitor performance similar to that of the aluminum anodic film, and can thus be used as a dielectric layer for capacitors to enhance specific capacitance.