What is Thermionic Emission?
Thermionic emission is a phenomenon in which electrons are released from a heated material due to the thermal energy overcoming the work function of the material. The work function is the minimum amount of energy required to remove an electron from the surface of a solid. Thermionic emission is also known as thermal electron emission or the Edison effect, after Thomas Edison, who first observed it in 1883.
Thermionic emission is the basis of many electronic devices, such as vacuum tubes, cathode-ray tubes, electron microscopes, X-ray tubes, and thermionic converters. In this article, we will explain the physics behind thermionic emission, the factors that affect its rate and current, the types of materials and structures used for thermionic emitters, and some of the applications and examples of thermionic emission in electronics and engineering.
What Causes Thermionic Emission?
To understand what causes thermionic emission, we need to recall some basic concepts of atomic structure and energy levels. All materials are composed of atoms, which consist of a nucleus (made of protons and neutrons) surrounded by electrons. The electrons are arranged in different shells or orbit around the nucleus, depending on their energy. The electrons in the outermost shell are called valence electrons, and they are responsible for most of the electrical and chemical properties of the material.
When a material is heated, some of the thermal energy is transferred to the electrons, increasing their kinetic energy. If the thermal energy is high enough, some of the valence electrons can overcome the attractive force between them and the nucleus, and escape from the surface of the material. This process is called thermionic emission, and the emitted electrons are called thermions (Figure 1).
Figure 1: Thermionic emission from a heated metal surface
The amount of thermal energy needed to emit an electron depends on the work function of the material, which is a measure of how tightly the electrons are bound to the material. Different materials have different work functions, ranging from less than 1 eV (electron volt) for some alkali metals to more than 5 eV for some transition metals. The lower the work function, the easier it is to emit electrons from the material.
The rate of thermionic emission also depends on the temperature and surface area of the material. The higher the temperature, the more thermal energy is available to excite the electrons. The larger the surface area, the more electrons can escape from the material. Therefore, to increase the rate of thermionic emission, we need to use materials with low work functions, high temperatures, and large surface areas.
How Do We Measure Thermionic Emission?
The number of electrons emitted per unit time from a material is called the rate of thermionic emission. This quantity can be expressed in terms of thermionic current, which is the electric current generated by the flow of thermions. The thermionic current can be measured by placing another electrode (called an anode) near the emitting electrode (called a cathode), and applying a positive voltage to attract the emitted electrons (Figure 2).
Figure 2: Measuring thermionic current with a cathode and an anode
The relationship between thermionic current and temperature can be described by the Richardson-Dushman equation, which is given by:
Where:
- J is the thermionic current density (in A/m<sup>2</sup>), which is the current per unit area of the cathode
- A is the Richardson constant (in A/m<sup>2</sup>K<sup>2</sup>), which depends on the type of material
- T is the absolute temperature (in K) of the cathode
- ϕ is the work function (in eV) of the cathode
- k is the Boltzmann constant (in eV/K), which is equal to 8.617 x 10<sup>-5</sup eV), and T is the absolute temperature (in K) of the cathode.
The Richardson-Dushman equation shows that the thermionic current density increases exponentially with temperature and decreases exponentially with the work function. The equation is valid only for ideal conditions, such as a perfectly flat and clean cathode surface, a uniform temperature distribution, and a negligible space charge effect. In reality, these conditions are rarely met, and the actual thermionic current density may deviate from the theoretical value.
What are the Types of Thermionic Emitters?
A thermionic emitter is a metallic structure that is used to facilitate thermionic emission. The emitter is also called a cathode, and it is usually placed inside a vacuum or an evacuated space to minimize the collisions of the emitted electrons with gas molecules. The emitter or cathode is heated by an electric current, either directly or indirectly, to reach the temperature required for thermionic emission.
The choice of material and structure for a thermionic emitter depends on several factors, such as the work function, the melting point, the mechanical strength, the surface area, and the emission efficiency. The emission efficiency is defined as the ratio of the thermionic current to the heating power input. A high emission efficiency means that less power is wasted in heating the cathode, and more power is converted into thermionic current.
There are three main types of thermionic emitters: tungsten, thoriated tungsten, and oxide-coated emitters. Each type has its own advantages and disadvantages, depending on the application and operating conditions.
Tungsten Emitters
Tungsten emitters are made of pure tungsten metal or tungsten wire. Tungsten has a high melting point (3650 K), a high mechanical strength (100000 – 500000 psi at room temperature), and a low vapor pressure. These properties make tungsten suitable for high-temperature and high-voltage applications, such as X-ray tubes and electron microscopes.
However, tungsten also has a high work function (4.52 eV), which means that it requires a high temperature (2327 °C) to emit electrons. This results in a low emission efficiency (4 mA/W) and a short lifetime due to evaporation and contamination of the cathode surface. Moreover, tungsten emitters have a low surface area, which limits the maximum thermionic current that can be obtained.
Thoriated Tungsten Emitters
Thoriated tungsten emitters are made of tungsten alloyed with a small amount of thorium (1-2%). Thorium is a radioactive element that has a lower work function (3.4 eV) than tungsten. When thorium is mixed with tungsten, it forms a thin layer of thorium oxide on the surface of the cathode, which reduces the effective work function to 2.63 eV. This allows thoriated tungsten emitters to operate at a lower temperature (1700 °C) than pure tungsten emitters, and to have a higher emission efficiency (20 mA/W).
Thoriated tungsten emitters also have a longer lifetime than pure tungsten emitters, because thorium oxide acts as a protective layer that prevents evaporation and contamination of the cathode surface. However, thoriated tungsten emitters still have a low surface area, which limits the maximum thermionic current that can be obtained. Moreover, thoriated tungsten emitters pose a health hazard due to their radioactivity and toxicity.
Oxide-Coated Emitters
Oxide-coated emitters are made of nickel ribbons or tubes coated with barium oxide and strontium oxide. These oxides have very low work functions (1.1 eV), which enable oxide-coated emitters to operate at a very low temperature (750 °C) compared to other types of emitters. This results in a very high emission efficiency (200 mA/W) and a long lifetime due to reduced evaporation and contamination of the cathode surface.
Oxide-coated emitters also have a high surface area, which allows them to produce high thermionic currents. However, oxide-coated emitters are fragile and sensitive to mechanical shocks and vibrations. They also require careful handling and storage to prevent damage or deterioration of the oxide coating.
How are Thermionic Emitters Constructed?
The construction of thermionic emitters depends on whether they are directly heated or indirectly heated by an electric current.
Directly Heated Emitters
Indirectly heated emitters, the cathode is made in the form of a filament or coil that carries the heating current directly through it. The filament is usually made of oxide-coated nickel, which has a low work function and a high emission efficiency. The advantage of directly heated emitters is that they have a quick response time and a simple construction. The disadvantage is that they are sensitive to fluctuations in the heating current and voltage, which can affect thermionic emissions.
Indirectly Heated Emitters
In indirectly heated emitters, the cathode and the heating element are separate and insulated from each other. The heating element is a filament or coil that surrounds a thin metal sleeve or tube that acts as the cathode. The heating element carries the heating current, while the cathode is connected to a different potential for thermionic emission. The advantage of indirectly heated emitters is that they are less affected by fluctuations in the heating current and voltage, and they can use alternating currents for heating. The disadvantage is that they have a slower response time and more complex construction.
What are the Applications of Thermionic Emission?
Thermionic emission has many applications in electronics and engineering, especially in devices that use vacuum tubes or electron beams. Some of examples of thermionic emission applications are:
- Vacuum tubes: Vacuum tubes are devices that use thermionic emission to control the flow of electric current in a vacuum. They consist of a cathode, an anode, and one or more electrodes called grids that control the current between the cathode and the anode. Vacuum tubes can be used as amplifiers, oscillators, switches, rectifiers, and modulators of electric signals. Vacuum tubes were widely used in radio, television, radar, computers, and other electronic devices before the invention of transistors.
- Diode valves: Diode valves are vacuum tubes that have only two electrodes: a cathode and an anode. They allow an electric current to flow only in one direction, from the cathode to the anode. They can be used as rectifiers to convert alternating current into direct current, or as detectors to demodulate radio signals.
- Cathode ray tubes: Cathode ray tubes are vacuum tubes that use thermionic emission to produce a beam of electrons that can be deflected by electric or magnetic fields. They can be used to create images on a phosphorescent screen, such as in television sets, computer monitors, oscilloscopes, and radar displays.
- Electron tubes: Electron tubes are vacuum tubes that use thermionic emission to generate or accelerate electron beams for various purposes. They can be used as electron guns for electron microscopes, X-ray tubes for X-ray generation, klystrons for microwave amplification, magnetrons for microwave generation, and traveling wave tubes for high-frequency amplification.
- Electron microscopes: Electron microscopes are devices that use electron beams to magnify and image objects at very high resolutions. They use thermionic emitters to produce electron beams that are focused by magnetic lenses and scanned over the sample. The electrons interact with the sample and produce signals that can be detected and displayed on a screen or recorded on a film. Electron microscopes can reveal details of the structure and composition of matter at nanometer scales.
- X-ray tubes: X-ray tubes are devices that use electron beams to generate X-rays for medical imaging, industrial inspection, security screening, and scientific research. They use thermionic emitters to produce electron beams that are accelerated by high voltages and directed at a metal target called an anode. The electrons collide with the atoms of the anode and produce X-rays by bremsstrahlung (braking radiation) or characteristic radiation. The X-rays can be filtered and collimated to form a beam that can penetrate matter and produce images of its internal structure.
- Thermionic converters: Thermionic converters are devices that use thermionic emissions to convert heat into electricity. They consist of two electrodes: a hot cathode and a cold anode separated by a vacuum or a gas-filled gap. The cathode emits electrons due to its high temperature, while the anode collects them due to its lower temperature. The difference in temperature creates a potential difference between the electrodes, which drives an electric current through an external circuit. Thermionic converters can be used as power sources for spacecraft, nuclear reactors, solar panels, and waste heat recovery systems.
- Electrodynamic tethers: Electrodynamic tethers are long conductive wires that can be used to generate electricity or propulsion in space. They use thermionic emitters at one end to emit electrons into space, creating a current along the tether. The tether interacts with the Earth’s magnetic field and produces a Lorentz force that can either generate electricity or change the orbit of a spacecraft. They use thermionic emitters at one end to emit electrons into space, creating a current along the tether. The tether interacts with the Earth’s magnetic field and produces a Lorentz force that can either generate electricity or change the orbit of a spacecraft.
Conclusion
Thermionic emission is a phenomenon in which electrons are released from a heated material due to the thermal energy overcoming the work function of the material. Thermionic emission is the basis of many electronic devices, such as vacuum tubes, cathode-ray tubes, electron microscopes, X-ray tubes, and thermionic converters. The rate and current of thermionic emission depend on the temperature, work function, and surface area of the material. The types of thermionic emitters include tungsten, thoriated tungsten, and oxide-coated emitters, each with its own advantages and disadvantages. Thermionic emission has many applications in electronics and engineering, especially in devices that use vacuum tubes or electron beams.
