Electron Emission Introduction The reader is familiar with current conduction (i.e. flow of...
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Electron Emission Introduction The reader is familiar with current conduction (i.e. flow of electrons)through a conductor. Examples are: current through
Electron Emission Introduction The reader is familiar with
current conduction (i.e. flow of electrons)through a conductor.
Examples are: current through power lines, windings of transformers
and motors etc. Many electronic devices depend for what their
operation on the movement of electrons in an evacuated space. For
this purpose, the free electrons must be ejected from the surface
of metallic conductor by supplying sufficient energy from some
external source. This is known as electron emission. The emitted
electrons can be made to move in vacuum under the influence of an
electric field, thus constituting electric current in vacuum.
Slide 3
Metals are most suitable substances for electron emission
because they contain a large number of free electrons. However,
metal can not emit electrons from their surface under ordinary
conditions. This is due to the fact that the electrons in the metal
are free only to the extend that they may be transferred from one
atom to another within the metal but they cannot leave metal
surface to provide electron emission. However, if sufficient
external energy is given to free electrons, their kinetic energy is
increased and thus electrons will leave the metal surface. The
additional energy required to emit electrons from a metallic
surface is known as work function of the metal. The work function
of pure metals varies roughly from 2eV to 6eV. It may be noted that
it is desirable that metal used for electron emission should have
low work function so that a small amount of energy is required to
cause emission of electrons.
Slide 4
Types of Electron Emission. In order to liberate electrons from
a metallic surface, external energy equal to work function of the
metal must be supplied. The external energy supplied to the metal
may be in several forms such as heat energy, energy stored in
electric field, light energy or kinetic energy of the electric
charges bombarding the metal surface. Accordingly, the following
are the four principal methods of obtaining electron emission from
the surface of a metal: (1) Thermionic emission (2) Field emission
(3) Photo-electric emission (4) Secondary emission
Slide 5
The most commonly used type of emission is the thermionic
emission. In this case, the emission from the metallic surface is
caused by supplying thermal energy. The commonly used substances
for thermionic emission are tungsten, thoriated tungsten, oxide-
coated cathode ect. Although thermionic emitters should have the
desirable characteristic of low work function, high melting point
and high mechanical strength, yet their choice for a particular
situation would depend upon the service requirements.
Slide 6
For instance, tungsten has high melting point (3560C), high
mechanical strength, high operating temperature (2500C) and
comparatively high work function (4.52 eV) and would, therefore, be
suitable for applications involving high voltages (> 10 kV) e.g.
in X-ray tubes. On the other hand, oxide coated cathodes have low
work function (1.1 eV), low operating temperature (750C) and
comparatively low melting point and would, therefore, be suitable
for applications involving small voltages (
