ANODES

Nickel

Because of the relatively low melting point of nickel, this anode material is used principally in tubes where the anode operating temperature is moderate. Although the thermal emissivity of nickel is not high, this material lends itself readily to a process called carbonizing. In this process, a well-adhering layer of amorphous carbon is deposited on the nickel anode to provide a thermal emissivity approaching that of a black body. Nickel is formed readily into the shapes desired for anodes. Care must be exercised in the design of the anode to avoid warping during exhaust. Like other metal anode materials, nickel has the advantage of light weight, so that elaborate supporting structures are not needed.

Tantalum

A metal which is finding increasing use as an anode material is tantalum. Although the properties of this material have been known for many years. it has been used commercially in transmitting tubes for a relatively short time. The appearance and many of the characteristics of tantalum are similar to those of molybdenum. Tantalum has the same metallic luster, a slightly higher melting point, a lower vapor pressure, and is more easily worked into various mechanical shapes. The principal advantage of tantalum is that it will clean up gases and. thus, is capable of helping to maintain a high vacuum in a tube during normal operation. Sudden tube overloads of short duration do not cause tantalum anodes to liberate appreciable gas.

Tantalum anodes are usually made with fins and with a rough surface to increase the effective heat-radiating area. Under conditions of maximum rated plate dissipation, tantalum anodes will show a red to orange-red color. They will normally show some color even when the tubes are lightly loaded. The color characteristic of tantalum anodes serves as a rough indication of the power being dissipated.