Thermionic emission

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Closeup of the filament on a low pressure mercury gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of barium, strontium and calcium oxides, the coating is sputtered away through normal use, often eventually resulting in lamp failure.
Closeup of the filament on a low pressure mercury gas discharge lamp showing white thermionic emission mix coating on the central portion of the coil. Typically made of a mixture of barium, strontium and calcium oxides, the coating is sputtered away through normal use, often eventually resulting in lamp failure.

Thermionic emission (archaically known as the Edison effect) is the flow of charged particles called thermions from a charged metal or a charged metal oxide surface, caused by thermal vibrational energy overcoming the electrostatic forces holding electrons to the surface. The charge of the thermions (either positive or negative) will be the same as the charge of the metal/metal oxide. The effect increases dramatically with increasing temperature (1000–3000 K). The science dealing with this phenomenon is thermionics.

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[edit] 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 negative charge would lose its charge (discharging ions into vacuum). He also found that this did not happen if the sphere had a positive charge.[citation needed] Other early contributors included Hittorf (1869–1883), Goldstein (1885), and Elster and Geitel (1882–1889).

The Edison effect in a diode tube. A diode tube is connected in two configurations, one has a flow of electrons and the other does not. Note that the arrows represent electron current, not conventional current.
The Edison effect in a diode tube. A diode tube is connected in two configurations, one has a flow of electrons and the other does not. Note that the arrows represent electron current, not conventional current.

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.

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. 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 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." [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. 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 (a thermionic converter, a type of heat engine).

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".

[edit] 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. The work function is characteristic of the material and for most metals is on the order of several electronvolt. 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. 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:

J = A T^2 e^{-W \over k T}

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 = {4 \pi m k^2 e \over h^3} = 1.20173 \times 10^6 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 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.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.

[edit] 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 (FEE) equation:

J (E_s,T,W) = A T^2 e^{ - (W - \Delta W) \over k T}
\Delta W = \left[{e^3 E_c \over (4 \pi \epsilon_0)}\right]^{1/2}

Where Ec is the electric field strength at the cathode spot, ε0 is the vacuum permittivity.

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 (T-F) emission is more appropriate.


[edit] References

  1. ^ "Edison" by Matthew Josephson. McGraw Hill, New York, 1959, ISBN 07-033046-8

[edit] See also

[edit] External links

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