See Also

Thermionic emission

Thermionic emission is the flow of electron Electron

The electron is a fundamental [i] subatomic particle [i] that carries an electric charge [i] ... 

s from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The effect increases dramatically with increasing temperature . The science dealing with this phenomenon is thermionics. The charged particles are called thermions.

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Encyclopedia

Thermionic emission is the flow of electron Electron

The electron is a fundamental [i] subatomic particle [i] that carries an electric charge [i]... 

s from a metal or metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The effect increases dramatically with increasing temperature . The science dealing with this phenomenon is thermionics. The charged particles are called thermions.

History


The phenomenon was initially reported in 1873 by Frederick Guthrie in Britain. While doing work on charged objects, Professor Guthrie discovered that a red-hot iron sphere with a positive charge would lose its charge . He also found that this did not happen if the sphere had a negative charge. Other early contributors included Hittorf , Goldstein , and Elster and Geitel .



The effect was accidentally rediscovered by Thomas Edison Thomas Edison

Thomas Alva Edison was an American [i] inventor [i] and businessman [i] who developed man ... 

 on February 13, 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening of the bulbs in his incandescent lamp Incandescent light bulb

The incandescent light bulb or incandescent lamp is a source of artificial light [i] that works by ... 

s.

Edison built a bulb with the inside surface covered with metal foil. He connected the foil to the lamp filament through a galvanometer Galvanometer

A galvanometer is an instrument for detecting and measuring electric current [i]. ... 

. 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 . Edison saw no use for this effect, and although he patented it in 1883, he did not study it any further.

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. Fleming went on to develop the two-element vacuum tube Vacuum tube

In electronics [i], a vacuum tube or valve is a device generally used to amplify [i], ... 

 known as the diode Diode

In electronics [i], a diode is a component [i] that restricts the direction of mov... 

, which he patented on November 16, 1904.

The thermionic diode can also be configured as a device that converts a heat difference to electric power directly without moving parts .

Owen Willans Richardson worked with thermionic emission and received a Nobel prize Nobel Prize

The Nobel Prizes are prize [i]s instituted by the will [i] of Alfred Nobel [i], awarded to people... 

 in 1928 "for his work on the thermionic phenomenon and especially for the discovery of the law named after him".

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, and varies from metal to metal. A thin oxide coating is often applied to metal surfaces in vacuum tubes to give a lower work function, as it is easier for electrons to leave the surface of the oxide.

Richardson's Law, also called the Richardson-Dushmann equation, first published by Owen Willans Richardson in 1901, states that the emitted current density J is related to temperature T by the equation:

where T is the metal temperature in kelvin Kelvin

The Kelvin scale is a temperature [i] scale where absolute zero [i]—the coldest possible temperatu ... 

s, W is the work function of the metal, k is the Boltzmann constant Boltzmann constant

The Boltzmann constant is the physical constant [i] relating temperature [i] to energy [i].
... 

. The proportionality constant A, known as Richardson's constant, given by

A m?² K?²

where m and e are the mass and charge of an electron, and h is Planck's constant Planck's constant

Planck's constant is a physical constant [i] that is used to describe the sizes of quanta [i]. ... 

.

Because of the exponential function, the current increases rapidly with temperature when T is less than W. At higher temperatures the increase is quadratic in T.

The thermionic emission equations are of fundamental importance in electronics significantly affecting both older vacuum tube Vacuum tube

In electronics [i], a vacuum tube or valve is a device generally used to amplify [i], ... 

 technology and more modern semiconductor Semiconductor

A semiconductor is a material with electrical conductivity [i] that is intermediate between that of an ... 

 designs.

Field-enhanced thermionic emission


The Richardson-Dushman equation must be corrected for the Schottky Effect; the current emitted from the metal cathode into the vacuum depends on the metal's thermionic work function, and that this function is lowered from its normal value by the presence of image forces and by the electric field at this cathode. This enhancement is given by the Field-enhanced thermionic emission equation:

Where Ec is the electric field strength at the cathode spot, e0 is the vacuum permittivity Permittivity

Permittivity is a physical quantity [i] that describes how an electric field [i] affects and is affected ... 

.

This equation is relatively accurate for electric field strengths lower than about 108 V m-1. For electric field strengths higher than 108 V m-1 the use of the Murphy and Good equation for thermo-field emission is more appropriate.

See also


  • Cathode ray tube Cathode ray tube

    The cathode ray tube or CRT, invented by German [i] physicist [i] Karl Ferdinand Braun [i] ... 

  • Vacuum tube Vacuum tube

    In electronics [i], a vacuum tube or valve is a device generally used to amplify [i], ... 

  • Work function
  • X-ray tube X-ray tube

    An X-Ray tube is a vacuum tube designed to produce man made X-Ray [i] photon [i]s on demand. ... 

  • Space charge

External links


  • — Has a good section on thermionic emission, with equations
  • , December 12, 1929.
  • Equation Source for Field enhanced thermionic emission equation.