Editing Thermionic emission (section)

{{short description|Thermally induced flow of charge carriers from a surface}}
{{redirect-distinguish|Thermion|fermion}}
[[File:Thermionic filament.jpg|thumb|right|250px|Closeup of the [[electrical filament|filament]] in a [[mercury-vapor lamp|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]] [[oxide]]s, the coating is [[sputtering|sputtered]] away through normal use, eventually resulting in lamp failure.]]'''Thermionic emission''' is the liberation of [[charged particles]] from a hot [[electrode]] whose [[thermal energy]] gives some particles enough [[kinetic energy]] to escape the material's surface. The particles, sometimes called ''thermions'' in early literature, are now known to be [[ion source|ions]] or [[electrons]]. '''Thermal electron emission''' specifically refers to emission of electrons and occurs when thermal energy overcomes the material's [[work function]].

After emission, an opposite charge of equal magnitude to the emitted charge is initially left behind in the emitting region. But if the emitter is connected to a [[Electric battery|battery]], that remaining charge is neutralized by charge supplied by the battery as particles are emitted, so the emitter will have the same charge it had before emission. This facilitates additional emission to sustain an [[electric current]]. [[Thomas Edison]] in 1880 while inventing [[Edison light bulb|his light bulb]] noticed this current, so subsequent scientists referred to the current as the '''Edison effect''', though it wasn't until after the [[J. J. Thomson#Discovery of the electron|1897 discovery of the electron]] that scientists understood that electrons were emitted and why.

Thermionic emission is crucial to the operation of a variety of [[Electronics|electronic]] devices and can be used for [[electricity generation]] (such as [[thermionic converter]]s and [[electrodynamic tether]]s) or cooling. Thermionic [[vacuum tubes]] emit electrons from a [[hot cathode]] into an enclosed [[vacuum]] and may steer those emitted electrons with applied [[voltage]]. 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 {{convert|1000|K|C F|lk=on}}. Charge flow increases dramatically with 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 physics|solid-state]] region into another.