Thermionic emission
Thermionic emission is the flow of
electrons 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.
Encyclopedia
Thermionic emission is the flow of
electrons 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 on February 13, 1880, while trying to discover the reason for breakage of lamp filaments and uneven blackening of the bulbs in his
incandescent lamps.
Edison built a bulb with the inside surface covered with metal foil. He connected the foil to the lamp filament through a
galvanometer. 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 known as the
diode, 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 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
kelvins,
W is the work function of the metal,
k is the
Boltzmann constant. 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.
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 technology and more modern
semiconductor 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.
This equation is relatively accurate for electric field strengths lower than about 10
8 V m
-1. For electric field strengths higher than 10
8 V m
-1 the use of the Murphy and Good equation for thermo-field emission is more appropriate.
See also
External links
- — Has a good section on thermionic emission, with equations
- , December 12, 1929.
- Equation Source for Field enhanced thermionic emission equation.
