for Radio imaging Technology and Physics Students.

1. Instructor:
Shabir Ahmad Usmani
Riphah International College Dargai Campus

2. Thermionic Emission
The phenomenon of thermionic emission was first observed by
Thomas A in 1883. He observed that electricity can supply from
a filament to a metal plate within an incandescent lamp.
In thermionic emission, electrons emission can be done
from heated materials which are broadly used in conventional
electron tubes as a source of electrons in the fields
of electronics & communication. The best example of this
emission is, electrons can emit from a hot cathode and enter
into a vacuum within a vacuum tube.

3. Thermionic emission definition is, when the heat energy is applied to metal then it
emits electrons from the surface of the metal and it is also known as
the thermionic emission effect. The term ‘Thermionic’ can be formed from the
two words namely Thermal (heat) & ions (charged particles). The thermionic
emission diagram is shown below.
What is Thermionic Emission?

4. Factors Affect Thermionic Emission
 There are three factors that affect this emission like metal surface
temperature, metal surface area & the function of the metal
The temperature of Metal Surface
 When the metal surface temperature is high then the emission rate of
electrons from the metal surface is higher.
Metal Surface Area
• When the metal surface area is larger, then the rate of electrons emitted
from the metal surface is high.

5. Continue…
Function of Metal
 The work function of the metal is low then the rate of electrons
emission from the metal surface is high.

6. Thermionic Emission Working
Thermionic emission mainly occurs in metals when they are heated at
very high temperatures. Metals generally work under two conditions
metals in normal temperature and metals in high temperature which
is discussed below.

7. Metals in Normal Temperature
Once the normal temperature is provided to the metal, then the
valence electrons can gain sufficient energy & break the connection
with the parent atom to become free, which is known as a free
electron.
In the metal, the free electrons will have some kinetic energy (K.E)
but they do not contain sufficient energy to get away from the metal.
So, the attractive force from the atomic nucleus will resist the free
electrons from the metal which tries to escape. The energy of the free
electrons within the metal is low as compared to the electrons within
a vacuum. Thus, free electrons need additional energy from the
external source to move into the vacuum

8. Metals under High Temperature
Once the high temperature is applied to the metal, then free electrons
will get sufficient energy & conquer the attractive force from the
atomic nucleus, which holds the free electrons within the metal.
The free electrons in the metal overcome the attractive force of the
atomic nuclei break the connection with the metal & move into the
vacuum. This emission mainly occurs in metals when they are
heated at very high temperatures. Once the heat energy is supplied
to the metal, then free electrons will escape from the metal surface
which is known as thermions. This emission process plays a key role
within the operation of the electronic device.

9. Advantages
The advantages of Thermionic Emission include the following.
 It plays a key role in both basic physics & digital electronic
technology.
• This emission discovery allows physicists to generate electrons
beams within a vacuum.
• Thermionic sources are not expensive, so one can easily operate in
fewer vacuum conditions & offer better brightness especially for
illumination of large-area than sources of field emission.

10. Applications
The applications of Thermionic Emission include the following.
 It is used in different applications like high-frequency-based vacuum
transistors used in electronics, power electronics, electron guns used
in scientific instrumentation, x-ray generation & energy converters
from solar energy & sources of high temperature.
 It is used in diode valves, vacuum tubes, cathode ray tubes (CRT),
electron microscopes, electron tubes, electrodynamics tethers, etc