The effect of moisture and metallic contact area has been investigated on the asymmetry properties of anodic tantalum oxide films, approximately 2400Aå thick. Moisture has been reversibly desorbed and adsorbed by vacuum baking followed by exposure at 50–85% RH. The cathodic currents are not affected significantly by this cycle, while the anodic leakage and charging current can be changed by factors of 106 and 102, respectively. The contact area experiment confirm previous results that the asymmetry is due to local inhomogeneities in the oxide film, since nonpolar characteristics, in the absence or presence of moisture, can be obtained with a contact area less than 0.01 mm2. This result indicates that the moisture effect is associated with defects in the oxide film. The oxide film can thus be considered as a normal dielectric with high symmetrical anodic and cathodic breakdown voltages. Distributed on the average 0.1 mm apart throughout the film are heterogeneities of an unknown nature which cause a symmetrical but lower breakdown than the rest of the oxide. The asymmetry results from an increase in the anodic breakdown at these distinct sites due to the presence of water.
The suitability of niobium as the anode for solid electrolytic capacitors has been investigated. Field crystallization of the amorphous niobium oxide film has been found to be a determining factor for the properties of these capacitors. Purity of the porous anodes as dependent on the sintering conditions, formation temperature and voltage, and formation electrolytes, parameters all known to control the initiation of crystallization, have been correlated with the initial and life test properties of these capacitors. The initial properties of niobium solid electrolytic capacitors prepared under optimum conditions compare favorably with equivalent tantalum components. However, with the current purity of niobium powder, only units with operating voltages up to 20 v survive a life test at 85°C, with the failures of capacitors rated at higher voltages attributed to the partial crystallization of the amorphous film during formation.
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