Thermionic emission is the liberation ofcharged particles from a hotelectrode whosethermal energy gives some particles enoughkinetic energy to escape the material's surface. The particles, sometimes calledthermions in early literature, are now known to beions orelectrons.Thermal electron emission specifically refers to emission of electrons and occurs when thermal energy overcomes the material'swork function.
After emission, an opposite charge of equal magnitude to the emitted charge is initially left behind in the emitting region. But if the emitter is connected to abattery, that remaining charge is neutralized by charge supplied by the battery as particles are emitted, so the emitter will have the same charge it had before emission. This facilitates additional emission to sustain anelectric current.Thomas Edison in 1880 while inventinghis light bulb noticed this current, so subsequent scientists referred to the current as theEdison effect, though it wasn't until after the1897 discovery of the electron that scientists understood that electrons were emitted and why.
Thermionic emission is crucial to the operation of a variety ofelectronic devices and can be used forelectricity generation (such asthermionic converters andelectrodynamic tethers) or cooling. Thermionicvacuum tubes emit electrons from ahot cathode into an enclosedvacuum and may steer those emitted electrons with appliedvoltage. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal orcarbides orborides oftransition metals. Vacuum emission frommetals tends to become significant only fortemperatures over 1,000 K (730 °C; 1,340 °F). Charge flow increases dramatically with temperature.
The termthermionic emission is now also used to refer to any thermally-excited charge emission process, even when the charge is emitted from onesolid-state region into another.
Because theelectron was not identified as a separate physical particle until the work ofJ. J. Thomson in 1897, the word "electron" was not used when discussing experiments that took place before this date.
The phenomenon was initially reported in 1853 byEdmond Becquerel.[1][2][3] It was observed again in 1873 byFrederick Guthrie in Britain.[4][5] While doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge (by somehow discharging it into air). He also found that this did not happen if the sphere had a positive charge.[6] Other early contributors includedJohann Wilhelm Hittorf (1869–1883),[7][8][9][10][11][12]Eugen Goldstein (1885),[13] andJulius Elster andHans Friedrich Geitel (1882–1889).[14][15][16][17][18]
Thermionic emission was observed again byThomas Edison in 1880 while his team was trying to discover the reason for breakage ofcarbonized bamboo filaments[19] and undesired blackening of the interior surface of the bulbs in hisincandescent lamps. This blackening wascarbon deposited from the filament and was darkest near the positive end of the filament loop, whichapparently cast a light shadow on the glass, as if negatively-charged carbon emanated from the negative end and was attracted towards and sometimes absorbed by the positive end of the filament loop. This projected carbon was deemed "electrical carrying" and initially ascribed to an effect inCrookes tubes where negatively-chargedcathode rays fromionized gas move from a negative to a positive electrode. To try to redirect the charged carbon particles to a separate electrode instead of the glass, Edison did a series of experiments (a first inconclusive one is in his notebook on 13 February 1880) such as the following successful one:[20]
This effect had many applications. Edison found that the current emitted by the hot filament increased rapidly withvoltage, and filed a patent for avoltage-regulating device using the effect on 15 November 1883,[21] notably the first US patent for an electronic device. He found that sufficient current would pass through the device to operate atelegraph sounder, which was exhibited at theInternational Electrical Exhibition of 1884 in Philadelphia. Visiting British scientistWilliam Preece received several bulbs from Edison to investigate. Preece's 1885 paper on them referred to the one-way current through the partial vacuum as theEdison effect,[22][23] although that term is occasionally used to refer to thermionic emission itself. British physicistJohn Ambrose Fleming, working for the BritishWireless Telegraphy Company, discovered that the Edison effect could be used to detectradio waves. Fleming went on to develop a two-elementthermionic vacuum tube diode called theFleming valve (patented 16 November 1904).[24][25][26] Thermionic diodes can also be configured to convert a heat difference to electric power directly without moving parts as a device called athermionic converter, a type ofheat engine.
FollowingJ. J. Thomson's identification of the electron in 1897, the British physicistOwen Willans Richardson began work on the topic that he later called "thermionic emission". He received aNobel Prize in Physics in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him".
Fromband theory, there are one or two electrons peratom in a solid that are free to move from atom to atom. This is sometimes collectively referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, and occasionally an electron will have enough velocity to exit the metal without being pulled back in. The minimum amount of energy needed for an electron to leave a surface is called thework function. The work function is characteristic of the material and for most metals is on the order of severalelectronvolts (eV). Thermionic currents can be increased by decreasing the work function. This often-desired goal can be achieved by applying various oxide coatings to the wire.
In 1901Richardson published the results of his experiments: the current from a heated wire seemed to depend exponentially on the temperature of the wire with a mathematical form similar to the modifiedArrhenius equation,.[27] Later, he proposed that the emission law should have the mathematical form[28]
whereJ is the emissioncurrent density,T is the temperature of the metal,W is thework function of the metal,k is theBoltzmann constant, andAG is a parameter discussed next.
In the period 1911 to 1930, as physical understanding of the behaviour of electrons in metals increased, various theoretical expressions (based on different physical assumptions) were put forward forAG, by Richardson,Saul Dushman,Ralph H. Fowler,Arnold Sommerfeld andLothar Wolfgang Nordheim. Over 60 years later, there is still no consensus among interested theoreticians as to the exact expression ofAG, but there is agreement thatAG must be written in the form:
whereλR is a material-specific correction factor that is typically of order 0.5, andA0 is a universal constant given by[29]
where and are the mass andcharge of an electron, respectively, and is thePlanck constant.
In fact, by about 1930 there was agreement that, due to the wave-like nature of electrons, some proportionrav of the outgoing electrons would be reflected as they reached the emitter surface, so the emission current density would be reduced, andλR would have the value1 −rav. Thus, one sometimes sees the thermionic emission equation written in the form:
However, a modern theoretical treatment by Modinos assumes that theband-structure of the emitting material must also be taken into account. This would introduce a second correction factorλB intoλR, giving. Experimental values for the "generalized" coefficientAG are generally of the order of magnitude ofA0, but do differ significantly as between different emitting materials, and can differ as between differentcrystallographic faces of the same material. At least qualitatively, these experimental differences can be explained as due to differences in the value ofλR.
Considerable confusion exists in the literature of this area because: (1) many sources do not distinguish betweenAG andA0, but just use the symbolA (and sometimes the name "Richardson constant") indiscriminately; (2) equations with and without the correction factor here denoted byλR are both given the same name; and (3) a variety of names exist for these equations, including "Richardson equation", "Dushman's equation", "Richardson–Dushman equation" and "Richardson–Laue–Dushman equation". In the literature, the elementary equation is sometimes given in circumstances where the generalized equation would be more appropriate, and this in itself can cause confusion. To avoid misunderstandings, the meaning of any "A-like" symbol should always be explicitly defined in terms of the more fundamental quantities involved.
Because of the exponential function, the current increases rapidly with temperature whenkT is less thanW.[further explanation needed] (For essentially every material, melting occurs well beforekT =W.)
The thermionic emission law has been recently revised for 2D materials in various models.[30][31][32]
In electron emission devices, especiallyelectron guns, the thermionic electron emitter will be biased negative relative to its surroundings. This creates an electric field of magnitudeE at the emitter surface. Without the field, the surface barrier seen by an escaping Fermi-level electron has heightW equal to the local work-function. The electric field lowers the surface barrier by an amount ΔW, and increases the emission current. This is known as theSchottky effect (named forWalter H. Schottky) or field enhanced thermionic emission. It can be modeled by a simple modification of the Richardson equation, by replacingW by (W − ΔW). This gives the equation[33][34]
whereε0 is the electric constant (also called thevacuum permittivity).
Electron emission that takes place in the field-and-temperature-regime where this modified equation applies is often calledSchottky emission. This equation is relatively accurate for electric field strengths lower than about108 V⋅m−1. For electric field strengths higher than108 V⋅m−1, so-calledFowler–Nordheim (FN) tunneling begins to contribute significant emission current. In this regime, the combined effects of field-enhanced thermionic and field emission can be modeled by the Murphy-Good equation for thermo-field (T-F) emission.[35] At even higher fields, FN tunneling becomes the dominant electron emission mechanism, and the emitter operates in the so-called"cold field electron emission (CFE)" regime.
Thermionic emission can also be enhanced by interaction with other forms of excitation such as light.[36] For example, excitedCesium (Cs) vapors in thermionic converters form clusters of Cs-Rydberg matter which yield a decrease of collector emitting work function from 1.5 eV to 1.0–0.7 eV. Due to long-lived nature ofRydberg matter this low work function remains low which essentially increases the low-temperature converter's efficiency.[37]
Photon-enhanced thermionic emission (PETE) is a process developed by scientists atStanford University that harnesses both the light and heat of the sun to generate electricity and increases the efficiency of solar power production by more than twice the current levels. The device developed for the process reaches peak efficiency above 200 °C, while most siliconsolar cells become inert after reaching 100 °C. Such devices work best inparabolic dish collectors, which reach temperatures up to 800 °C. Although the team used agallium nitride semiconductor in its proof-of-concept device, it claims that the use ofgallium arsenide can increase the device's efficiency to 55–60 percent, nearly triple that of existing systems,[38][39] and 12–17 percent more than existing 43 percent multi-junction solar cells.[40][41]
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