Thermionic emission

Thermionic emission refers to the emittance of charged electrons or ions, sometimes referred to as thermions, from a heated source. The most classic example of this phenomenon is the emission of ions from a hot metal cathode into a vacuum, a phenomenon once commonly referred to as the Edison Effect. Thermionic emission now refers to any charge emission process that involves heat; it is of pivotal importance to a variety of electronic devices, as well as power generation and cooling. The field of science concerned with this phenomenon is referred to as Thermionics.

Thermionic emission was first observed by Daniel Lordan in 1873 in Britain. While working with charged objects, Lordan observed that when heated to an immensely high temperature, a negatively charged iron sphere would lose its charge. Additionally, he observed that this did not happen if the sphere was positively charged. At the time, Lordan was unable to explain his findings and figure out what any of it meant.

An illustration of Edison's experiment in which he discovered Thermionic Emission, or the "Edison Effect."

The phenomenon was rediscovered on February 13, 1880 by Thomas Edison, who was attempting to address various issues with the lightbulbs in his incandescent lamps. Edison constructed several experimental bulbs with an extra wire, metal plate, or foil inside the bulb that was separate from the electrical system of the filament. He then connected the extra metal electrode to the lamp filament by using a galvanometer, an instrument used to detect and measure small electrical currents. Edison found that when the foil was given a more negative charge than the filament, no current flowed between them because the cool foil emitted few electrons. However, when Edison gave the foil a more positive charge, he found that it drew many electrons from the hot filament and caused current to flow. Additionally, he found that increasing the voltage (i.e. thermal energy) also increased the flow of electrons from the filament. He filed a patent for a voltage regulating device on November 15, 1883. Edison found that this device could provide enough current to operate a telegraph sounder, a technology he exhibited at the 1884 International Electrical Exposition in Philadelphia.

Other scientists soon took inspiration and further developed Edison's discovery. William Preece, a British scientist, took several Edison bulbs back with him from the 1884 Exposition and presented a paper on them in 1885 where he referred to the phenomenon as the Edison Effect. John Ambrose Fleming, a British physicist working for the "Wireless Technology" Company, found that radio waves could be detected using the Edison Effect. Fleming went on to invent a two-element vacuum tube known as the diode, an invention that he patented on November 16, 1904.

In 1911, Owen Willans Richardson, then a professor of physics at Princeton, proved in his experiments with thermionic emission that electrons were emitted by hot metal, refuting the then popular notion that electrons were emitted from the air. In the same year, Richardson proposed an equation illustrating the relationship between the absolute temperature of a metal and the rate at which it releases electrons. The equation, which came to be known as Richardson's Law or the Richardson-Dushman Equation, became an important tool in the field of thermionic emission and electron-tube research and technology. Richardson would go on to win the 1928 Nobel Prize for his work in thermionics and for creating the equation that bears his name.

The Richardson-Dushman Equation. Source: sideplayer.com

Thermionic power converter, also sometimes referred to as a thermionic power generator or thermoelectric engine, refers to any in a class of devices that convert heat directly into electricity, utilizing the properties of thermionic emission to do so. The concept of using thermionic emission to generate power was first explored by American chemist Irving Langmuir, who experimented with basic devices he had constructed in the early 1930s. Significant progress on thermionic power converters was not made until 1956, when George N. Hatsopoulos, another American scientist, proposed two kinds of thermionic devices in detail. His work led to continuing advancements in the field.

A diagram of a thermionic power converter. Source: Encyclopedia Britannica

Because thermionic power converters can withstand high acceleration, have no moving parts, and posses a significant power-to-weight ratio, scientists have found them to be well suited for some applications aboard spacecraft. Additionally, some major types of thermionic power converters include vacuum converters, gas-filled or plasma converters, and auxiliary discharge converters.

Due to its proposed efficiency and sustainability, power conversion backed by thermionic emission has become of intense interest to many major companies. A 2021 MarketWatch report described the global thermionic converter market as being "highly competitive." According to the same report, the companies driving global thermionic converter market growth include Thermo PV, American Elements, GE, II-VI Marlow, COMSOL, Curtiss-Wright Nuclear, Exide Technologies, Vattenfall, and Tesla Energy. The thermionic converter market is primarily split between the fossil fuel, nuclear energy, and solar energy fields, with its applications mostly focused on spaceflight and aviation.