When a metal is heated sufficiently, the thermal energy supplied to the free electrons causes the emission of electrons from the metal surface. The electron emission originated by thermal energy is called thermionic emission. At average room temperature, the energy possessed by free electrons in a metal is insufficient to initiate thermionic emission.
When we supply external heat energy to the metal body, the heat energy increases the kinetic energy of the free electrons. The increase in kinetic energy causes increased motion of the free electrons. At sufficiently high temperature the kinetic energy of free electrons becomes so high that it can be more than the work function of the metal. As a result, the free electrons on the extreme metal surface, cross the surface barrier and get emitted into space.
The work function is a fundamental property of metal. It determines the minimum required extra energy for an electron to escape the surface barrier of metal. Surface barrier is the potential barrier offered by a metal to prevent escaping of free electrons from the metal surface. For ultimate escaping of the electron from metal i. e. to get emitted from the metal surface, a free electron has to overcome the backward pull by the surface barrier. The work function is not the same for all metals. The metals with lower work function cause more thermionic emission for same supplied heat.
The metals we usually use for thermionic emission, are tungsten, thoriated tungsten, metallic oxides of barium, and strontium. Not only the metals with low work function, but a thermionic emission also requires very high temperature. It is found that a pure tungsten filament must be heated to a temperature of 2300°C before it starts electron emission.
The metallic structure used to facilitate thermionic emission is called thermionic emitter. The emitter is also called cathode. The emitter or cathode is sufficiently heated in vacuum or evacuated space to initiate thermionic emission i.e. emission of electrons from the body of emitter or cathode. The metal or metallic substances used to construct a thermionic emitter should have three main features.
- It should have low work function. Low works function helps to emit electrons from the cathode surface in comparatively lower temperature.
- It should have a high melting point. The temperature required to emit an electron from cathode surface is quite high compared to the melting point of normal metals.Some of the common metals have low work function but till they are not suitable for constructing a thermionic emitter. This is because lower melting point causes vaporization of metals before they emit electrons. For example, copper has low work function but we can not use it as a thermionic emitter, because its melting point is only 810°C. So at thermionic emission temperature, the copper gets vaporized instead of emitting electrons from its solid surface.
- It should have high mechanical strength. Absolute vacuum cannot be created in space surrounding the cathode, so there may always be some gaseous molecules present in the space. After a collision with emitted electrons from the cathode, these gaseous molecules produce positive ions in the space. Due to
electrostatic farce, these positive ions strick the cathode. It sufficiently high electric field is applied, these bombardments may be significantly high to create damage on the cathode. To avoid the damage of the cathode due to ions collisions, the mechanical strength of the materials used for constructing cathode must be high enough. Considering the above-mentioned properties, we normally use, tungsten, thoriated tungsten, oxide coated metals for constructing cathode of thermionic emission.
- Work function = 4.52 ev
- Melting point = 3650°K
- Tensile strength = 100000 – 500000 psi @ room temperature
- The Thermionic emission temperature = 2327°C
- Emission efficiency 4 mA / watt
Tungsten was previously used as the materials for the thermionic emitter. It has high work function but still, it was used as the cathode because of its high melting point and the material is mechanically very strong. Due to work function, the operating temperature of tungsten cathode is high and at the same time, the emission efficiency is low since for maintaining the high temperature of the cathode the input energy to the system is high compared to emitted current from the cathode.
Sometimes the addition of one metal to other makes the work function of mixture lower. Thoriated Tungsten is the mixture of thorium and tungsten. Thorium has work function 3.4 ev and tungsten has work function 4.52 ev. When a small quantity of thorium is mixed with tungsten to make thoriated tungsten, the work function comes down to 2.63 ev. This causes the operating temperature of thermionic emission at 1700°C when the cathode is made of thoriated tungsten. So, the power input for heating the cathode elements is reduced hence, the emission efficiency is increased accordingly.
Oxide Coated Cathode
Here, the cathode for thermionic emission is made of nickel ribbon coated with barium and strontium oxide. The oxide coating reduces the work function of the system to a quite low value. It is about 1.1 ev. Low work function causes low operating temperature and high emission efficiency of the system. The operating temperature and thermionic emission efficiency of the system are 750°C and 200 mA/watt respectively.
Construction of Cathode for Thermionic Emission
The cathode or thermionic emitter is placed inside a vacuum container. So, only possible way to heat up the cathode is electrical heating. There are two types of electric heating used in thermionic emission, one is direct heating and other is indirect heating.
Directly Heated Cathode
In directly heated cathode, the cathode is made of in form of a filament. The filament is normally made of oxide coated nickel. When the current from the input source passes directly through the filament, it gets hot and emits electrons. A direct heating method is more efficient as the input current (input energy) directly heats the filament cathode to emit electrons. As the heating is quick, starting time of thermionic emission quick and at the same time, it is an efficient process. As the emitter is directly heated, any of fluctuation in input source will affect the emission. This is the main disadvantage of directly heated cathode thermionic emission.
Indirectly Heated Cathode
Here, the heating filament and emitting surface are separate and they are insulated to each other. The filament is surrounded by thin oxide coated metal sleeve. The input current possesses through the heating filament and hence it heats up the metallic sleeve from where electrons are emitted. Most modern thermionic emitters are indirectly heated cathode this is because of the following facts.
- The emission potential and heating potential are separate. The emitter can be connected to any required potential irrespective of heating potential.
- The fluctuations in input heating potential don’t affect the emission.
- Alternating current can also be used as a heating current of the system.