The dielectric properties of anodic aluminium oxide films

Different types of oxide films may be formed on aluminium by anodization depending upon the manufacturing process. In strong acids thick porous films are formed and these may be used to insulate aluminium conductors for windings. These same films if subsequently sealed serve to protect aluminium from corrosion. In weak acids, non-porous oxide films of very high electric strength are produced, but these have an upper thickness limit of about 1¿m. The latter films with an electrolyte as the cathode form the basis of industrial electrolytic capacitors. If, however, a self-healing evaporated layer of aluminium is used as the second electrode, greatly improved electrical properties are obtained, especially at low temperatures, and operation under a.c. stress is possible. For optimum properties a double anodizing process is used and extremely high purity of the metal and of the anodizing electrolytes are imperative. An experimental dry aluminium-oxide-film capacitor employing this principle is described.