Formation of Composite Oxide Films on Aluminum by Sol-Gel Coating and Anodizing —For the Development of High Performance Aluminum Electrolytic Capacitors—

Abstract

Aluminum coated with ZrO2, SiO2, and BaTiO3 films by a sol-gel dip coating was anodized to examine the structure and dielectric properties of anodic oxide films. Anodizing leads to the formation of anodic oxide films, which consist of an outer Al- (Zr, Si, BaTiO3) composite oxide layer and an inner Al2O3 layer, at the interface between the coated-oxide layer and Al substrate. The composite oxide converted into Al2O3 at the interface between the outer and inner layers during anodizing for the ZrO2- and BaTiO3-coated specimens, whereas Al2O3 converted into Al-Si composite oxide for the SiO2-coated specimens. The capacitance of the anodic oxide films formed on ZrO2- and SiO2-coated specimens was about 20% larger than that of anodic oxide films on aluminum without coating, and the capacitance for BaTiO3-coated specimens was almost identical to that without coating. Film formation mechanisms are discussed in terms of inward transport of Si-bearing anions and outward transport of Zr- and Ti-bearing cations across the composite oxide layer.