Charge Storage Effects in Tantalum Oxide Films

Abstract

The phenomenon of the residual discharge current following an inverse time law, , was confirmed for tantalum electrolytic capacitors. It was shown that the coefficient was independent of charging fields up to 2/3 of the formation field and that the residual current increased with temperature. The activation energy was 2.5 kcal/mole. Furthermore, it was shown that, if the external resistor was removed and the cell was left with open terminals, the internal residual current continued to flow until the build‐up field modified the flow. If the oxide film were illuminated during discharging, two different photoeffects were recorded: the transient effects due to wavelengths of 320 mµ to about 600 µ and the stationary photocurrent mostly due to wavelengths shorter than 300 mµ.Several models are discussed which lead to an inverse time law. Preference was given to the idea that a net positive space charge arises within the oxide during charging; that this space charge causes two internal fields of opposite directions and that the residual discharge current is the difference of two currents flowing in the opposing fields, thus neutralizing the space charge. Irradiation during charging and discharging affected the positive transient effect and the space charge field.