Transcript Slide 1

PH0101 UNIT-5 LECTURE 4
Introduction
Thermionic effect
Richardson Law
Principle, construction and working of
Thermionic power generator
Advantage, disadvantage and applications
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1. Introduction
Thermionic power generator (TPG) is a device that
converts heat energy into electrical energy.
Thermionic emission is the basis for the working of this
system.
The thermionic emission is the emission of electrons from
metal surface due to heat.
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In 1873, the Britain professor Frederic Guthrie invented the
Thermionic phenomenon.
In 1883, Thomas A. Edison observed that the electrons are
emitted from a metal surface when it was heated. This effect is
called Edison effect.
Later in 1904, a British physicist John Ambrose Fleming
developed two-element vacuum tube known as diode.
After Fleming, Owen Willans Richardson worked with
thermionic emission and received a Nobel Prize in 1928 "for
his work on the thermionic phenomenon and especially for the
discovery of the law named after him
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• A thermionic energy converter (or) thermionic power
gnerator is a device consisting of two electrodes placed
near one another in a vacuum.
• One electrode is normally called the cathode, or emitter,
and the other is called the anode, or plate.
• Ordinarily, electrons in the cathode are prevented from
escaping from the surface by a potential-energy barrier.
• When an electron starts to move away from the surface, it
induces a corresponding positive charge in the
material,
which tends to pull it back into the surface.
• To escape, the electron must somehow acquire enough
energy to overcome this energy barrier.
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• At ordinary temperatures, almost none of the electrons
can acquire enough energy to escape.
• However, when the cathode is very hot, the electron
energies are greatly increased by thermal motion.
• At sufficiently high temperatures, a
number of electrons are able to escape.
considerable
• The liberation of electrons from a hot surface is
called thermionic emission
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The idea of electrons leaving the surface is shown in figure
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2. Thermionic effect
Thermionic effect is the ejection of electron from the heated
metal surface and forms as electron cloud at the cathode.
The number of electron emitted from the metal surface
depends on temperature and work function.
Work function(φ)
For Electrons to leave the surface of the metal, they
have to be supplied with enoughEnergy.
•
We call the minimum energy required to allow an
electron
to be liberated from a material as its WORK FUNCTION(φ)
•
Work Function differs from material to material – usually
few eV
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3. Richardson Law
Richardson law states that the emission current
density is exponentially depend on work function and
inversely depends upon the absolute temperature.
According to Richardson, the emission current density ‘J’ can
be expressed as,
J = At2 e (-/KT) Ampers/m3
Where
A-Emission constant (A/m3/K2)
 - Work function
T – absolute temperature (Kelvin)
K – Boltzman constant
J - emission current density.
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4.
Principle, Construction and working of
Thermionic power generator (TPG)
Principle
Thermionic power generator is based on the principles of
Thermionic effect that the electrons are emitted from a hot
metal surface and responsible for the production of electricity.
Construction
•
The TPG consist of tungsten metal, which is negatively
charged cathode acts as an emitter.
•
There is positively charged electrode is called collector.
It is collecting the ejected electrons. The emitter and collector
are kept in a vacuum quartz tube.
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Thermionic Power generator
Emitter
Quartz tube
Collector
Thermal energy
VL
RL
Electrical energy
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Working
•
The electrons within a metal can be treated as "electron
gas" in which individual outer most electrons are capable
of moving freely under the influence of a field.
•
This movement of electrons is responsible for the function
of electric circuits.
•
At the surface of a metal, a potential barrier exists which
prevents the electrons from escaping unless certain
conditions are met. This concept can be explained as
follows.
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Electrons escaped from cathode
C
Surface of cathode
Electrons Promoted
from EF

B
EF
A
• It is known from the free electron theory, at 0K, all the energy
levels up to EF (fermi energy) are completely filled and all the
energy level above the EF are completely empty.
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• The energy level from the surface of metallic cathode to
the level of EF (BC in figure) is the potential energy
barrier called work function (.).
• If any electron wants to escape from the surface of the
metallic cathode, they should cross this potential barrier.
• At 0K, all the electrons are bound within fermi energy level
and cannot escape from the surface of cathode (emitter).
• When the thermal energy is supplied on the emitter side,
some of the electrons are promoted to above the fermi
level.
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•
These activated electrons can cross the potential energy
barrier and escape
from the surface of cathode and
responsible for the current production.
•
As long as the temperature increased, the number of
electrons escapes from the surface of emitter increases.
Collector collects the emitted electrons and there is an
external circuit through which the current flows
•
The thermionic emission current density is determined by
the 'work function' of the material, which is basically the
magnitude of the potential energy barrier.
•
Good emitters should have low work functions.
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•
It is generally accepted that the thoriated tungsten is the
best cathode metal because of its lower value of work
function
•
The metal can be heated in two different ways.
•
One is direct heating and second is the indirect heating.
•
In the direct heating where the filament itself is the
cathode and the indirect heating where the cathode is
heated by a separate filament.
•
Pure tungsten is used as main metal in the case of direct
heating method
•
Nickel (or) Nickel alloys are used as main metal in the
case of indirect heating.
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5.
Advantage, disadvantage and applications
Advantages
• Higher efficiency and high power density
• Compact to use
Disadvantages
• There is a possibility of vaporization of emitter surface
• Thermal breaking is possible during operation
• The sealing is often gets failure
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Applications
• They are used in space power application for spacecraft
•They are used to power submarines and boats.
• They used in water pump for irrigation,
• They used in power plant for industry and domestic purpose
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