1. Introduction

Electrolytic capacitors are characterized by the phase sequence electronic conductor / dielectric medium / electrolyte where the electronic conductor is a metal (typically Al or Ta) or an oxide (NbO) and the dielectric medium is an insulating oxide film (Al₂O₃, Ta₂O₅, SiO₂, and Nb₂O₅). The electrolyte is a viscous or solid. Electrolytes are usually defined as ionic conductors, but for electrolytic capacitors the electronic conductivity is predominant.