Thermionic emission is the process whereby charged particles (usually electrons), known as thermions, are emitted from the surface of a heated material, usually a metal. The process is driven by the extra thermal energy that is added to the system. As a result, energised electrons can overcome the surface barrier of the material and escape into the surrounding space. This phenomenon has been used in various applications, including vacuum diodes, cathode ray tubes and, more recently, mass spectrometry.
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How does Thermionic Emission work?
Electrons & electron energy levels in a sodium atom
Atoms have electrons in various energy shells surrounding their atomic nuclei. If energy, such as heat, is applied to these atoms, the electrons absorb it, sometimes jumping to higher energy levels. If sufficient energy is absorbed, the electrons in the outermost shell of the atom may become energised enough to escape the atom altogether. The electrons that escape are known as free electrons. When this happens at the surface of a material, the electrons leave the material altogether becoming thermions in the process of thermionic emission.
The Work Function of a material affects Thermionic Emission
The atoms of different materials have different numbers of electrons and different electron shells. Consequently, the outermost electron shell, from which free electrons originate during thermionic emission, will also differ. That difference will be mainly in the number of electrons and the energy required to free those electrons. The minimum amount of energy (measured in electron-volts (eV)) required to free an electron from a particular atom type is called the 'Work Function' (represented by the symbol Phi (Φ)) of that material. Therefore, different materials will have different work function values. The lower the work function, the easier it will be for that material to produce free electrons through thermionic emission.
The Work Function of Different Materials
Material | Work Function (eV) |
|---|---|
Potassium | 2.29 |
Sodium | 2.36 |
Silver | 4.26 - 4.74 |
Copper | 4.53 - 5.10 |
Gold | 5.10 - 5.47 |
Platinum | 5.12 - 5.93 |
Tungsten | 4.32 - 4.55 |
Tungsten coated with Barium Oxide | 1.36 - 2.22 |
Tungsten is the typical metal that you will find in filaments used for thermionic emission. However, as can be seen from the table above, Tungsten has a relatively high Work Function of 4.32 - 4.55 eV. Consequently, this makes it difficult for it to release electrons from its surface. However, one way to lower Tungsten's Work Function is to coat it in the oxide of an alkaline earth metal like Barium or Strontium. As shown in the table, the Work Function of Tungsten coated in Barium Oxide can fall to as low as 1.36 eV, making the chemical combination a good thermionic emitter.
NB: Since Barium Oxide and Strontium Oxide lower the Work Function of Tungsten, you may ask why not just use the oxides themselves as the thermionic-emitting material. The answer is that they are not electrically-conductive. A material that conducts electricity is usually needed to efficiently heat up the filament in the first place. For this reason,Tungsten is ideally suited for this purpose as evidenced by its extensive use in incandescent bulb filaments of the past.
So what makes a good Thermionic Emitter?
In addition to a material's Work Function, a few other factors can affect the rate of thermionic emission. These are summarised below:
In short, the ideal thermionic emitter will be a material that has a relatively low Work Function, or is at least coated with something that induces one. Additionally, it will be in a form that maximises its surface area from which electrons can escape. It will also be conductive so that electricity can be used to heat the material efficiently, and it will have a low specific heat capacity so that it can reach a high temperature quickly. Finally, its melting point will be high so that it does not disintegrate when heated to the high temperatures needed for thermionic emission.
The Melting Point of Different Materials
Material | Melting Point (ºC) |
|---|---|
Potassium | 63 ºC |
Sodium | 98 ºC |
Tungsten | 3422 ºC |
Strontium Oxide | 2531 ºC |
Barium Oxide | 1923 ºC |
How to measure Thermionic Emission
Thermionic emission can be demonstrated with the following setup:
This configuration is actually the basis of a thermionic diode or vacuum tube, which were used extensively in electronic devices of the past. In fact, thermionic diodes are still used today as vacuum tube amplifiers in some specialised applications. As shown above, the low D.C. voltage circuit ('6V') is used to heat the tungsten filament to produce thermionic emission of electrons from its surface. At the same time, the filament is kept at a negative potential by the high voltage part of the circuit ('400V'). This acts as a cathode and repels the free electrons released from the metal. Then, on the other side of the chamber is the positive end of the high voltage circuit, acting as an anode and accelerating the electrons towards it. This creates a beam of electrons called a cathode ray. Importantly, the inside of the chamber is a vacuum so that the free electrons going to the anode do not collide with any air molecules. As a result of this flow of electrons, the high voltage part of the circuit is essentially closed. Consequently, a small amount of current flows, which can be measured with an ammeter ('mA') placed within the circuit.
Applications of Thermionic Emission
The principle of thermionic emission has been and continues to be used in various applications. In the past, it was the basis for thermionic diodes and cathode ray tubes that were used in all manner of electronics, particularly in TVs, oscilloscopes and radars. Presently, vacuum tube electronics are rarely used. However, thermionic emission still forms the basis of the magnetron in microwave ovens, electron ionisation in some forms of mass spectrometry, as well as in thermionic diodes of some specialised amplifiers.