Modelling the capacitance of d.c. etched aluminium electrolytic capacitor foil

A model for the capacitance of anode foil used in aluminium electrolytic capacitors is compared with experimental data for commercial foils from two different manufacturers. These foils are obtained by anodic electrochemical etching to produce a porous tunnel etched structure, followed by formation of a layer of dielectric aluminium oxide in the pores. Data for the density and size of tunnels is obtained by sectioning the foil parallel to its surface with an ultramicrotome to several depths. In this paper the internal structure is modelled as a spatially random collection of hollow dielectric cylinders. Comparison of the measured capacitance with that calculated from the dimensional data and the model are in good agreement. The model predicts optimum values for tunnel size and density as a function of oxide thickness.