The properties of tantalum oxide in electrolytic capacitors

The tantalum electrolytic capacitor can be considered as a parallel plate capacitor in which tantalum metal is one electrode, a tantalum oxide film is the dielectric, and the tantalum oxide/electrolyte boundary is the other electrode. This model is useful for many purposes, although it is obviously oversimplified and does not take into account such phenomena as the assymetry of the system with regard to the effect of applied voltage, and neglects important parameters in the practical design of these capacitors. The tantalum electrode can take many physical forms, such as foil, wire, evaporated films, or, most commonly, a porous pellet made by pressing and sintering tantalum powder. The electrolyte can be solid or liquid, and must have a relatively high conductivity. Examples of liquid electrolytes used are sulphuric acid or lithium chloride solutions. The only solid electrolyte in commercial use is manganese dioxide formed on the surface of the tantalum oxide film by pyrolysis of manganese nitrate solutions. The dielectric is the vital part of the capacitor, and is a thin film of amorphous tantalum oxide, formed by anodic oxidation on the surface of the tantalum electrode. The desired characteristics of the capacitor are high capacitance per unit volume, low dissipation factor or equivalent series resistance (ESR), low leakage current (i.e. the current which flows when a specified direct voltage is applied across the capacitor), and high reliabilityunder conditions of use. The factors which determine reliability are extremely´ complex, and will not be further discussed, except inasmuch as there is probably a correlation between reliability and leakage, although this is by no means proved. The experimental results presented in this paper were obtained as part of a program to determine the factors affecting the capacitance, ESR, and leakage of the sintered powder type of tantalum anode, in liquid electrolyte systems. Most of the general trends observed are applicable to solid electrolytes, or to different forms of tantalum anode, such as foil or wire. The properties of capacitance, ESR, and leakage are in many ways interrelated, and in fact the design of a capacitor involves making a compromise between them. However, for convenience in presentation, they will first be considered separately, and then the interrelationships discussed.