Low Voltage Specific Charge (CV/g) Loss in Tantalum Capacitors

The systematic decrease in charge per unit weight (CV/g) in electrolytic tantalum capacitors as a function of decreasing anodization voltage is explored from both experimental and theoretical viewpoints for a range of tantalum particle sizes. Analysis by transmission electron microscopy shows that a native thermal oxide about 3.3 nm in thickness is present on the tantalum surface before anodization and that the thickness is independent of tantalum particle size. Studies in which the initial oxide thickness is altered via a preanodization heat-treatment show that the anodic oxide thickness becomes independent of the initial thermal oxide thickness once its thickness is exceeded. A theoretical model based on a cylindrical geometry explains the loss in the CV/g with decreasing anodization voltage and attributes the behavior to the 3.3 nm oxide thickness corresponding to zero anodization voltage.