The Equivalent Series Resistance of Anodically Formed Oxide Films on Aluminum

The equivalent series resistance of an electrolytic capacitor is the sum of several resistive elements, the major contributors being the electrolyte and the dielectric film on the anode. The frequency‐independent contribution of the electrolyte, , is determined by the resistivity of the solution and the geometry of the capacitor. The contribution of the dielectric film, , is the sum of the series resistance of the film itself and the resistance of the double layer, both of which are frequency‐dependent. may be determined by measuring the series resistance at frequencies sufficiently high so as to make negligible the contribution from other sources, or by extrapolating to . The resistance of anodic oxide films on aluminum is influenced by many factors and poses difficulties in measurement; a cell has been constructed which obviates some of these problems. A plot of vs. reciprocal frequency is nearly a straight line, the greatest deviation being shown at the higher voltages. In addition, (for a fixed frequency) is directly proportional to film thickness over the range studied. The intercept at zero formation voltage has been attributed to the double layer resistance. The linearity of the plot shows that the dissipation factor of the dielectric film is independent of film thickness, since capacitance is inversely proportional to film thickness, and amounts to about 0.6% at 100 cps.