Properties of aluminum solid electrolytic capacitors

The solid electrolytic capacitor structure (1) has been investigated for high purity aluminum utilizing both wire and porous anodes. The major problem with aluminum anodes is that a normal anodically formed aluminum oxide is extensively damaged during the high temperature decomposition (300-500°C) of aqueous solutions of manganese nitrate in the formation of manganese dioxide, the semiconductor cathode. A necessary process, to avoid this damage, is a preanodization water treatment of the cleaned aluminum surfaces which results in low reproducible dc leakages only if this treatment is carried out at temperatures above 70°C (Table I). The specificity of this process at these temperatures is in accord with the formation of a surface layer of a porous alumina monohydrate, boehmite (2,3). During the subsequent electrolytic oxidation of the aluminum, the dielectric anodic film is presumed to form at the metal surface underneath the boehmite layer. It is proposed the hydrated film protects the dielectric layer during pyrolysis by acting as a buffer between it and the deposited manganese dioxide with its associated pyrolysis products.