*
1.
Electron Emission
Electron emission is
actually the basic of tube working principal, it is defined as liberation
of free electron from a surface of a substance caused by the externak energy
transfered to the electrons.
Electron emission tends to occur on metal,
because metal is a substance with much free electron in between its molecul.
Nucleous attracting force does not strong enough to put the electron standstill.
Every time the free electrons move around from
one molecul to another but it can't leave out from metal
surface. In order to emit from the metal surface these free electrons
require additional external energy. The amount of outside energy
require by electron to emit from the metal surface is known as work
fuction. The work function ussually defined in elctron volt (eV) unit.
The additional external energy require by
the electron to emit from the metal surface could come from few sources
such as heat enery, energy stored in the electron field, light energy or
kinetic energy.
Accordingly there are following
four method of obtaining electron emission from the metal surface.
1.1 Thermionic emission
1.2 Field emission
1.3 Secondary emission
1.4 Photovoltaic emission
1.1 Thermionic Emission
In this method the additional
energy come to the electron in the from of heat energy, by the electrons
the energy trasnfered into kinetic energy. As the kinetic energy
of electron increase its movement become uncertain and then finally there
will be electrons that leave out from the metal surface.
The substance where the electrons
emit from, is known as emiter or cathode.
In case of vacum tube it is preferably to
call as cathode. And the substance that receive electron is known as anode
or plate.
With regard to thermionic emission there
are two type of cathode :
a) Direct Heated Cathode ( in short
DHC)
b) Indirect Heated Cathode ( is short IHC)
The simplified figure of DHC seen of Figure
2. In this type of cathode, both heater current and also emited electrons
come from it.
And the simplified structure of IHC seen
of Figure 3.
Heater current does not flow through the
cathode, but through the heater element that is known as filament. The
heat energy from filament conduct into cathode through insulator placed
between cathode and heater.
Material used for cathode must have the following
properties :
a. Low work function, so that the
electron emission could accur using only small
amount of energy
b. High melting point, as the thermionic
emission occurs at high temperature so the
substance used ascathode
must have high melting point
C. High mechanical strenght, Substances
used as cathode must have strong
mechanical strenght to
withstand the bombardment of positive ions.
In vacum tube no matter
how crefull the evacuation there
still always present
of gas molecul which
may be in the form of ion by impact with electron. Under
the influence
of electric field these positive ions will strike the cathode and if
the high voltage is being
used, the cathode is subject to considerable
bombardment and can be damaged.
Commonly used Thermionic Cathode
The high temperature
needed for satisfactory thermionic emission in vacum tubes limit the number
of suitable emitters to such substances as tungsteen, thoriated tungsten
and certain oxide coated metals
a. Tungsteen
It was the earliest material
used as cathode and has slightly higher work
function (4.52 eV). Theimportant
factors in its favor are : high melting point
(3650 degress K), greater mechanical
strenght and longer life. The
disadvantages are : high operating
temperature(2500 degrees K), high work
function and low emission efficiency.
Therefore, it is used in application
involving voltage exceeding
5kV, example XRay tubes.
b. Thoriated Tungsten
A mixture of two metals
may have lower work function than either of pure
metals alone. Thus a
tungsteen emitter with a small quantity of thorium has a
work function 2.63 eV
compared with 3.4 eV for thorium
and 4.52eV for
tungsteen. At the same time
thoriated tungsteen provides emission at
lower
temperature (1700 degress K)
and require smaller amount of heat energy.
c. Oxide-Coated Cathode
This type of cathode
consist of nickle ribbon coated with barium and strontium
oxide. The oxide
coated cathode has low work function (1.1 eV), operate at
comparatively low temperature
(750 degrees K) and also
has high emission
efficiency. However it
can't withstand high voltage,
therefore it is mostly
used in applicaation
where voltages involved do not exceed 1000 V.
In this type of emission additional
energy come in the form of electric field. When a conductor put in a place
very close to high voltage conductor, the electric field from the
conductor will exert attractive force on the free electron
in metal. If the positive field is big enough the free electron will succeed
in ovecoming restraining of the metal surface and it will emit from
the metal surface.
Very intense electrid field is required
to produce field emission. Ussually a voltage of the order of a million
volts percentimetre distance between the emitting surface and the positive
conductor is neccesarry to cause field emission. Field emisson can
be obtained at temperature much lower than required for thermionic emission
and therefore it is also sometimes called as cold cathode emission
or auto electronic emission.
Electron emission from a metalic surface by the bombardment of high speed electrons or other particles is known as secondary emission. When high speed electrons suddenly strike a metalic surface, they may give some or all of their kinetic energy to the free electrons in metal. If the energy of the striking electrons is sufficient, the free electron will escape from the metal surface and this phenomenom called as Secondary Emission. The principle of secondary emission describe on Figure 5. A glass envelope consisting electron source, cathode and plate. When electrons from electron source strike cathode they will knock out secondary electron from cathode which are attracted to plate by positive voltage from plate. The effect of secondary emission are very undesirable in many electron device for example in tetrode where secondary emission is responsible for negative resistance.
In this type of emission the additional energy
come to cathode by photons. When a beam of light strike the surface of
cathode the energy from photons will be transfered from the photons to
free electron within the cathode. If the energy from photons is greater
than the metal work function the free electron will knock out from the
cathode surface. The emitted electron called as photo electron. The amount
of photo electron depend of the light intensity.
We've just finished with electron
emission and hopefully you enjoy the discussion.
Now we're going to talk about
vacum tube and its working principle.
Vacuum Tube
An electric device which the flow of electron
is through a vacum is known as Vacum Tube.
There are several ways of classifying vacum
tubes according to the number of electrode. Vacum tube are classified as
under :
- Vacum Diode
- Vacum Triode
- Vacum Tetrode
- Vacum Pentode
