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.
Metals with lower work function will require less additional energy and, therefore,
will emit electrons at lower temperatures The commonly used me
arterials for electron emission are tungsten, throated tungsten and metallic oxides of
barium and strontium.
It may be add here that high temperatures are necessary to
cause thermionic emission. For example, pure tungsten must be heat to about
2300 0 C to get electron emission. However, oxide coated emitters need only 750 0 C to
cause thermionic emission.
The amount of thermionic emission
increases rapidly as the emitter temperature is raise. The emission curent density
is give by Richardson-Dushman equation give below:
The following points may be noted from eqn. (ii) :
(i) The emission is markedly affect by temperature changes. Doubling the
temperature of an emitter may increase electron emission by more than 10 7 times.
For instance, emission from pure tungsten metal is about 10 -6 ampere per sq. cm.
at 1300 0 C but rises to enormous value of about 100 amperes when temperature is raise to 2900 0 C.
(ii) Small changes in the work function of the emitter can produce enormous effects
on emission. Halving the work function has exactly. the same effect as doubling
Since the work function of pure tungsten is 4.52 eV, the sample must be
contaminated. Thoriated tungsten has a work function ranging from 2.63 eV to
4.52 eV, depending upon the percentage of metallic thorium. Therefore, the sample
is most likely to be thoriate tungsten.