Another type of electrolytic capacitor uses tantalum rather than aluminum. The tantalum can be foil, as is the aluminum in a conventional electrolytic capacitor. It can also take the form of a porous pellet, the irregular surface of which provides a large area in a small volume. An extremely thin oxide layer forms on the tantalum.
Tantalum capacitors have high reliability and excellent efficiency. They are often used in military applications because they almost never fail. They can be used in AF and digital circuits in place of aluminum electrolytics.