What is Thermionic Emission?
Thermionic emission is defined as the electron emission caused by a sufficiently high level of thermal energy. When a metal is heated sufficiently, the thermal energy supplied to the free electrons causes the emission of electrons from the metal surface. This occurs because the thermal energy given to the carrier overcomes the work function of the material. At average room temperature, the energy possessed by free electrons in a metal is insufficient to initiate thermionic emission.
All materials are composed of atoms which in turn consists of a nucleus, made of protons and neutrons, surrounded by electrons. These electrons are distributed at various levels around the nucleus and thus possess different levels of energy. Now, imagine that we start heating a particular material. The thermal energy so supplied increases the kinetic energy of the electrons within the material. This causes them to overcome the force of attraction which exists between them and the protons within the respective nuclei.
As a result, they get knocked-out from the material and will be liberated into space surrounding the material (Figure 1). As more heat is supplied, more are the number of electrons ejected. This phenomenon is known as thermionic emission i.e. emission of ions called thermions due to the thermal energy supplied. Thermionic emission was first observed by Thomas Alva Edison in 1883.
From the discussion presented, it might appear that the number of thermions emitted can be increased up to a large value just by increasing the temperature of the substance in-hand. However, this is not entirely true. The fact is that the number of thermions emitted is limited due to the effect of space charge – a phenomenon wherein the liberated thermions surround the electrode forming a shield, preventing the emission of further thermions.
Rate of Thermionic Emission
The number of thermions emitted per second from a substance is known as the rate of thermionic emission. This value depends on the:
- Nature of the Material
In general, every element can be characterized by its electronic configuration i.e. by the distribution of electrons surrounding its nucleus. When we speak of thermionic emission, our particular interest is in the valence electrons (electrons in the outermost shell). This is because these are the electrons which can be easily freed from the force of attraction so as to enable conduction. However, the energy which must be supplied differs from element to element and is regarded to be its threshold energy or work function.
- Surface Temperature
Higher is the temperature of the substance, greater is the rate of thermionic emission.
- Surface Area
If the surface area of the material considered is larger, then there will be a greater number of thermions emitted. This means that the rate of thermionic emission is directly proportional to the surface area of the material.
By analyzing these factors, it can be concluded that the substance chosen to be a thermionic emitter should have a low work function, larger surface area, and high melting point. A few examples of this kind are metals like tungsten, thoriated tungsten, tantalum, etc and coated metals like barium oxide, strontium oxide, etc.
The flow of thermions gives rise to the flow of current known as thermionic current. Mathematically the thermionic equation which gives the current density of electrons is expressed as:
- T is the absolute temperature,
- kB is the Boltzmann Constant,
- ΦW is the work function,
- e is the electron charge
- A is a constant.
Applications of Thermionic Emission
Thermionic emission forms the basic principle on which many of the devices used in the field of electronics and communication operates. Example applications of thermionic emission include vacuum tubes, diode valves, cathode ray tube, electron tubes, electron microscopes, X-ray tubes, thermionic converters, and electrodynamic tethers.
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 a low work function. A low work function helps to emit electrons from the cathode surface in a comparatively lower temperature.
- It should have a high melting point. The temperature required to emit an electron from the 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 the 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 it’s 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. The 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 others makes the work function of mixture lower. Thoriated Tungsten is a 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 a directly heated cathode, the cathode is made of in the 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 the 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.