Effects of Surface Morphology on Pit Nucleation and Growth of High Purity Aluminum Foil in Electrolytic Capacitors Used for DC Etching

Abstract

The pit growth behavior of high purity aluminum foil in electrolytic capacitors used for DC etching in a hot HCl solution has been studied using grained foil. Hemispherical patterns with diameters of about 1∼3μm and cubic patterns with widths and lengths of 7∼15μm were formed by electrolytic grainings in HNO₃ and HNO₃/HCl solutions, respectively. By using cubic or hemispherical patterns, uniform distributions of pits were obtained for a surface that was untreated and one that was pre-treated during the early stage of DC etching. Tunnel pits were distributed most uniformly on hemispherical patterns grained at 1C/cm². Surface dissolution occurred as clusters of tunnel pits at the intersection of hemispherical patterns above a graining charge of 10C/cm². For cubic patterns grained at up to 30C/cm², the distribution of tunnel pits became uniform with an increase in the charge for graining. As a result, grainings affected the expansion of the surface area as measured by capacitance after DC etching. Cubic patterns are more effective for an increase in capacitance than hemispherical patterns.