The effect of indium impurity on the DC-etching behaviour of aluminum foil for electrolytic capacitor usage

Abstract

The effects of indium impurity on the etched morphology of high purity aluminium foils for electrolytic capacitor usage were investigated. The indium impurity was present either in as-received aluminium foils or deposited purposely on the foil surface through immersion-reduction reaction. The rolling line effect can be improved through introducing deposited indium on foil surfaces by immersion of the foil into In(NO3)(3) solution prior to DC-etching. Under a high immersion temperature and more concentrated Ir(NO3)(3) condition, surface etching competes with tunnel etching, and the foil thickness clearly decreases. Under low temperature and dilute In(NO3)(3) condition, surface etching is inhibited and tunnel etching prevails. The different DC-etching behaviour under the two In-deposition conditions is explained through the energy state difference between the two deposited-In and the activation energy difference between surface etching and tunnel etching. The modification of etchability by indium on aluminium surface is stronger than that of lead. This facilitates employing lower temperature and dilute In(NO3)(3) condition in the immersion-reduction process to optimize the etchability. (C) 1997 Elsevier Science Ltd.