Thermionic emission is the flow of charge carriers from a surface or over some other kind of electrical potential barrier, caused by thermal vibrational energy overcoming the electrostatic forces restraining the charge carriers. In Physics, a charge carrier denotes a free (mobile unbound particle carrying an Electric charge. At a point in space the electric potential is the Potential energy per unit of charge that is associated with a static (time-invariant Electric field Electrostatics is the branch of Science that deals with the Phenomena arising from what seems to be stationary Electric charges Since Classical The charge carriers can be electrons or ions, and are sometimes referred to as "thermions". The total charge of the emitted carriers (either positive or negative) will be equal in magnitude and opposite in sign to the charge left in the emitting region. The most classical example of thermionic emission is the emission of electrons from a hot metal cathode into a vacuum (archaically known as the Edison effect), but the term is now used to refer to any thermally excited charge emission process, even when the charge is emitted from one solid-state region into another. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device In Language, an archaism is the use of a form of speech or writing that is no longer current This process is crucially important in the operation of a variety of electronic devices and can be used for power generation or cooling. A thermionic converter consists of a hot electrode which thermionically emits Electrons over a potential energy barrier to a cooler electrode producing a useful electric The magnitude of the charge flow increases dramatically with increasing temperature and for vacuum emission from metals tends to only become significant for temperatures over 1000 K. The science dealing with this phenomenon is thermionics. In Physics, thermionics is a branch of Electronics dealing with the emission of Electrons from Matter under the influence of Heat.
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History
The phenomenon was initially reported in 1873 by Daniel Lordan in Britain. Year 1873 ( MDCCCLXXIII) was a Common year starting on Wednesday (link will display the full calendar of the Gregorian calendar (or a Common While doing work on charged objects, Lordan discovered that a red-hot iron sphere with a negative charge would lose its charge (discharging electrons into vacuum). He also found that this did not happen if the sphere had a positive charge. He didn't understand what any of this meant. Other early contributors included Hittorf (1869–1883), Goldstein (1885), and Elster and Geitel (1882–1889).
The effect was rediscovered by Thomas Edison on February 13, 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening (darkest near one terminal of the filament) of the bulbs in his incandescent lamps. Events 1258 - Baghdad falls to the Mongols, and the Abbasid Caliphate is destroyed Year 1880 ( MDCCCLXXX) was a Leap year starting on Thursday (link will display the full calendar of the Gregorian calendar (or a Leap year The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general
Edison built several experiment bulbs, some with an extra wire, a metal plate, or foil inside the bulb which was electrically separate from the filament. He connected the extra metal electrode to the lamp filament through a galvanometer. A galvanometer is a type of Ammeter; an instrument for detecting and measuring Electric current. When the foil was given a more negative charge than the filament, no current flowed between the foil and the filament because the cool foil emitted few electrons. However, when the foil was given a more positive charge than the filament, the many electrons emitted from the hot filament were attracted to the foil, causing current to flow. This one-way flow of current was called the Edison effect (although the term is occasionally used to refer to thermionic emission itself). He found that the current emitted by the hot filament increased rapidly with increasing voltage, and filed a patent application for a voltage regulating device using the effect on November 15, 1883 (U. S. patent 307,031, the first US patent for an electronic device). He found that sufficient current would pass through the device to operate a telegraph sounder. This was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist took back with him several of the Edison Effect bulbs, and presented a paper on them in 1885, where he referred to thermionic emission as the "Edison Effect. Sir William Henry Preece ( February 15[[ 834]]- November 6 1913) was a Welsh Electrical engineer and Inventor. " [1] The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison Effect could be used to detect radio waves. Sir John Ambrose Fleming ( November 29, 1849 - April 18, 1945) was an English Electrical engineer and Physicist Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Dioden2jpg|thumb|right|150px|Figure 2 Various semiconductor diodes Events 534 - A second and final revision of the Codex Justinianus is published Year 1904 ( MCMIV) was a Leap year starting on Friday (link will display calendar of the Gregorian calendar (or a Leap year starting on
The thermionic diode can also be configured as a device that converts a heat difference to electric power directly without moving parts (a thermionic converter, a type of heat engine). A thermionic converter consists of a hot electrode which thermionically emits Electrons over a potential energy barrier to a cooler electrode producing a useful electric A heat engine is a physical or theoretical device that converts Thermal energy to mechanical output
Owen Willans Richardson worked with thermionic emission and received a Nobel prize in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him". Sir Owen Willans Richardson ( April 26, 1879 - February 15, 1959) was a British Physicist, professor at Princeton The Nobel Prize (Nobelpriset (Nobelprisen is a Swedish prize established in the 1895 will of Swedish chemist Alfred Nobel; it was first awarded in Peace, Literature
Richardson's Law
In any metal, there are one or two electrons per atom that are free to move from atom to atom. This is sometimes 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 the surface is called the work function. In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid The work function is characteristic of the material and for most metals is on the order of several electronvolts. 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 1901 Owen Willans Richardson 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 Arrhenius equation. Sir Owen Willans Richardson ( April 26, 1879 - February 15, 1959) was a British Physicist, professor at Princeton The Arrhenius equation is a simple but remarkably accurate formula for the temperature dependence of the Rate constant, and therefore rate of a chemical reaction The modern form of this law (demonstrated by Saul Dushman in 1923, and hence sometimes called the Richardson-Dushman equation) states that the emitted current density J (A/m2) is related to temperature T by the equation:
where T is the metal temperature in kelvin, W is the work function of the metal, k is the Boltzmann constant. Saul Dushman (1883-1954 was a Russian-American physical Chemist. Current density is a measure of the Density of flow of a conserved charge. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid Bridge from macroscopic to microscopic physics Boltzmann's constant k is a bridge between Macroscopic and microscopic physics The proportionality constant A, known as Richardson's constant, given by
where m and -e are the mass and charge of an electron, and h is Planck's constant. The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta.
Because of the exponential function, the current increases rapidly with temperature when kT is less than W. (For essentially every material, melting occurs well before kT=W. )
The thermionic emission equations are of fundamental importance in electronics, significantly affecting both older vacuum tube technology (e. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. g. CRT applications, like television picture tubes and computer monitors, as well as high end radio and microwave applications requiring the high power intrinsic to tube technology), and more modern semiconductor designs. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Television ( TV) is a widely used Telecommunication medium for sending ( Broadcasting) and receiving moving Images, either monochromatic A visual display unit, often called simply a monitor or display, is a piece of Electrical equipment which displays images generated from the Video Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 A semiconductor' is a Solid material that has Electrical conductivity in between a conductor and an insulator; it can vary over that
While A theoretically has a value of 1. 20 X 106 A m-2 K-2, in practice it strongly depends on material used. See work function for some practical values for A and W for some commonly used materials. In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid
Enhancement of thermionic emission
In most electronic devices, especially electron guns, the thermionic emitter will be given an electrical potential bias (or voltage), which creates an electric field and increases the emission current if it has the same sign as the thermion charge or decreases it if the signs are opposite. An electron gun (also called electron emitter) is an electrical component that produces an Electron beam that has a precise Kinetic energy and is most often The effect of the potential can usually be modeled by a simple modification of the Richardson-Dushman equation; the current emitted from the metal cathode into the vacuum depends on the metal's thermionic work function, and this function is lowered from its normal value by the presence of image forces and by the electric field at this cathode. This field enhancement is given by the field-enhanced thermionic emission (FEE) equation:
where Ec is the electric field strength at the cathode spot, ε0 is the vacuum permittivity. Permittivity is a Physical quantity that describes how an Electric field affects and is affected by a Dielectric medium and is determined by the ability
This equation is relatively accurate for electric field strengths lower than about 108 V m−1, in which range the enhancement is known as the Schottky effect. The Schottky diode (named after German physicist Walter H Schottky; also known as hot carrier diode) is a Semiconductor Diode with For electric field strengths higher than 108 V m−1, quantum tunneling begins to contribute some significant emission current, which is called field emission. In Quantum mechanics, quantum tunnelling is a nanoscopic phenomenon in which a particle violates the principles of Classical mechanics by penetrating a Field emission (FE is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields 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. [2] At even higher fields, field emission can become dominant and thermionic emission will no longer be significant.
Thermionic emission can also be enhanced by interaction with other forms of excitation such as light. [3]
See also
References
- ^ "Edison" by Matthew Josephson. The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or Space charge is a concept in which excess Electric charge is treated as being a Continuum of charge distributed over a region of space (either a volume or an area The thermoelectric effect is the direct conversion of temperature differences to electric Voltage and vice versa This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid An X-ray tube is a Vacuum tube that produces X-rays They are part of X-ray machines X-rays are part of the Electromagnetic spectrum, an McGraw Hill, New York, 1959, ISBN 07-033046-8
- ^ E. L. Murphy and R. H. Good, "Thermionic Emission, Field Emission, and the Transition Region", Phys. Rev. 102(6), pp. 1464-1473 (1956).
- ^ A. G. Mal'shukov1 and K. A. Chao, "Opto-Thermionic Refrigeration in Semiconductor Heterostructures," Phys. Rev. Lett. 86, pp. 5570-5573 (2001).
External links
- How vacuum tubes really work — Has a good section on thermionic emission, with equations
- Owen Richardson's Nobel lecture on thermionics, December 12, 1929. (PDF)
- Derivations of thermionic emission equations from an undergraduate lab
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