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Concept
Thermionic emission
Summary
Thermionic emission (also known as thermal electron emission or the Edison effect) is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.
The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over .
This process is crucially important in the operation of a variety of electronic devices and can be used for electricity generation (such as thermionic converters and electrodynamic tethers) or cooling. The magnitude of the charge flow increases dramatically with increasing temperature.
The term 'thermionic emission' is now also used to refer to any thermally-excited charge emission process, even when the charge is emitted from one solid-state region into another.
Because the electron was not identified as a separate physical particle until the work of J. J. Thomson in 1897, the word "electron" was not used when discussing experiments that took place before this date.
The phenomenon was initially reported in 1853 by Edmond Becquerel. It was rediscovered in 1873 by Frederick Guthrie in Britain.
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