Thermionic emission

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Thermionic emission (archaically known as the Edison effect) 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 (1000–3000 K). The science dealing with this phenomenon is thermionics. The charged particles are called thermions.

1 History
2 Richardson's Law
3 Field-enhanced thermionic emission
4 Application
5 See also
6 External links

[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 positive charge would lose its charge (discharging ions into vacuum). He also found that this did not happen if the sphere had a negative charge. 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 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 (darkest near one terminal of the filament) 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 (although the term is occasionally used to refer to thermionic emission itself). 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 (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, 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 (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 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 (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 E_c 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] Application

A company called Eneco has begun to use this effect, substituting a vacuum with a thermoelectric semi-conductor. They are working on producing a product that can be used as a small scale thermal conversion device. It will be able to convert excess heat into electricity, or when an electric current is applied it acts as a small cooling device.

Update Nov. 2006

Dell and Apple computers are in talks with Eneco in possible applications in new portable devices.