A cathode is the electrode of an electrical device in which the electrical current flows away from the device. A hot cathode can be a cathode that’s heated directly or indirectly. The cathode’s filament is the source of the electrons in the case of a directly heated cathode. The filament is electrically insulated from the cathode in the case of an indirectly heated cathode. A hot cathode emits more electrons than a cold cathode with the same surface area.
The first hot cathodes used direct heating. They were made of pure tungsten and heated to white incandescence. Later filaments were covered with a material that emits electrons more easily than tungsten, which reduces the operating temperature of a hot cathode. Further improvements to hot cathodes that use indirect heating include filaments made from sintered tungsten, thorium-tungsten alloys and tantalum. Some hot cathodes are also shaped like parabolic mirrors.
Hot cathodes that use indirect heating have an advantage when using alternating current, because this configuration produces less hum than a cathode that uses direct heating. The filament in a hot cathode that uses indirect heating is commonly called the heater. The heater is usually made of tungsten and surrounded by a tube made of nickel.
An oxide coating on a hot cathode’s filament can reduce the filament’s operating temperature. The first choice was pure barium oxide, and later formulations used a mixture of barium oxide, calcium oxide and strontium oxide. Hot cathodes coated with these compounds have a maximum operating temperature of 1,832 degrees Fahrenheit (1,000 degrees Celsius). They are subject to rapid degradation in high voltage conditions, and they are most often used in low-power vacuum tubes.
Hot cathodes that must handle high current commonly have coatings of hexaborides such as cerium hexaboride (CeB6) or lanthanum hexaboride (LaB6). Other types of hexaborides commonly used to coat hot cathodes include barium hexaboride, calcium hexaboride and strontium hexaboride. These types of hot cathodes burn more brightly and have longer lifetimes than tungsten hot cathodes, but they’re also more expensive.
Filaments that contain a small quantity of thorium are another option for hot cathodes. The filament is heated to white incandescence in an atmosphere containing hydrocarbons, which causes the thorium to migrate to the filament’s surface. Thoriated filaments can handle high voltage and have long lifetimes. The primary disadvantage of thoriated filaments is that thorium is radioactive. Alternatives to thorium in hot cathode filaments include cerium, lanthanum, yttrium and zirconium.