What is Thermionic Emission?
Thermionic emission is a single term that describes an act of liberation of ions when thermal agitation is provided. The combination of the words thermionic and emission to form a single term is used to describe the release of subatomic particles (specifically electrons) by the action of heat. This is so because, in earlier times, the charged carriers were regarded as thermions.
It is also known as the Thermionic Effect. More simply, thermionic emission is the release of electrons from a metallic surface when a sufficient amount of thermal energy is provided to that material.
Origin: The concept of thermionic emission was proposed by Thomas Alva Edison in the year 1883. Thomas was working on one of his experiments where he was trying to find the cause for the breaking and uneven blackening of the filament within the bulb.
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Concept of Thermionic Emission
We all are aware of the fact that each individual entity in this universe is made of the smallest indivisible particle, regarded as an atom. The atom consists of three subatomic particles, namely electrons, protons and neutrons. The structure of an atom is such that the protons and neutrons comprise the nucleus of the atom. And the electron orbits the nucleus in the outer shells.
At room temperature, the electrons in the material do not have a sufficient amount of energy by which they can overcome the force that binds them to the nucleus of the atom. Thus, they get orbits around the nucleus; hence emissions do not take place.
Now, further, when some external heat is provided to the metal then, the action of thermal action raises the kinetic energy of the electrons within the metal. Due to this supply of energy, the nuclear force of attraction between the subatomic particles of the atom (i.e., electrons and protons) gets breakdown. Hence, the electrons get released into space.
Also, with the increase in the supply of thermal energy, i.e., when more heat is provided to the material, then a comparatively large number of electrons get freed in less amount of time.
This complete process of emission of electrons from the surface of the metal by the action of heat is thermionic emission.
This can be understood in terms of work function that at absolute 0, electrons have energy WF and the external energy provided to the electrons is WE. Then the difference in energy, i.e., WE – WF, is the required energy which releases the electrons, and this is known as the work function of the metal. The work function of the metal is denoted as φ.
The value of φ is different for different metals, and its unit is electron-volt. Generally, this lies between 1 to 6 eV.
So, after the liberation of the charges from the surface of the metal, then an equal number of charges with the same magnitude but opposite in polarity remains in the region from where the emission of electrons takes place.
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Factors Affecting Thermionic Emission
The factors that affect the thermionic effect are as follows:
- From the type of material, we are concerned about the work function possessed by that material. Basically, the major categorization of the metal is based on the presence of valence electrons in the outermost shell. This is due to the fact that these electrons will be getting the externally supplied thermal energy in order to get released into space. So, the minimum amount of energy that releases the electrons is its work function. The work function of the metal varies from one element to another.
- Thermionic emission and surface area of material exhibit directly proportional behaviour with respect to each other. Thus, for a greater area, the emission of the electrons will be more. Hence, we can say the electron emission will be more in metal with a large surface area in comparison to a metal with the same nature but a small surface area.
- The thermionic emission through a metal surface varies according to the temperature at its surface. Hence higher the temperature of the material more will be the rate of emission of electrons.
Thus, combining the three factors as a whole, we can say that to have better thermionic emission, the metal should possess a low work function along with a large surface area and a high value of the melting point.
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The emission of thermions depends on the nature of the material, along with its surface area and temperature.
Thus, mathematically it can be written as:
: T denotes the temperature of the surface,
φ denotes the work function of the metal,
k is the Boltzmann’s constant,
Js is the saturation current density and
A is the Plank’s constant
This is the Richardson-Dushman equation, and we could write it as:
The graph given below represents the Richardson-Dushman equation for various types of emitters:
You must note here that out of various pure metals, due to their high melting points, tungsten is widely used. Tungsten offers a temperature of 2500°K with work function φ = 4.4eV. This high value of the work function of tungsten is compensated by the high melting point offered by it.