Question

Explain the term thermionic emission. Mention its one use.

Answer 1

The process of electron emission from a metal surface by supplying thermal energy to it is known as thermionic emission .
At ordinary temperatures, the energy possessed by free electrons in the metal is inadequate to cause them to escape from the surface. When heat is applied to the metal, some of heat energy is converted into kinetic energy, causing accelerated motion of free electrons. When the temperature rises sufficiently, these electrons acquire additional energy equal to the work function of the metal. Consequently, they overcome the restraining surface barrier and leave the metal surface.
Use: Metals with lower work function will require less additional energy and, therefore, will emit electrons at lower temperatures. The commonly used materials for electron emission are tungsten, thoriated tungsten and metallic oxides of barium and strontium . It may be added here that high temperatures are necessary to cause thermionic emission. For example, pure tungsten must be heated to about 2300ºC to get electron emission. However, oxide coated emitters need only 750ºC to cause thermionic emission.