Thermionic emission

Thermionic emission is the release of electrons from a material’s surface when heating gives them enough energy to overcome the surface work function.

Work function: The minimum energy needed to remove an electron from a solid’s surface. It sets the energy barrier that heating must overcome for electrons to escape.
Cathode-ray tube: A vacuum device that forms images by steering an electron beam onto a phosphor screen. Its electron gun uses a heated cathode as an electron source.
Frederick Guthrie: A Scottish physicist who reported electron emission from heated metals in the nineteenth century. His experiments helped establish heat-driven charge emission as a distinct phenomenon.
Photoelectric effect: The emission of electrons from a material after it absorbs light. Light, rather than heating, supplies the energy for electron escape.
Richardson–Dushman equation: An equation relating thermionic current density to temperature and a material’s work function. It predicts how strongly emission rises with temperature.
Electron gun: A device that produces and shapes a beam of electrons. Thermionic cathodes are a common source for its beam.
Owen Willans Richardson: A British physicist who developed a quantitative theory of electron emission from heated bodies. His work yielded the temperature dependence captured by the Richardson–Dushman equation.
Field emission: Electron emission caused by a strong electric field that enables tunneling through a surface barrier. It can release electrons without heating them over the work-function barrier.
Space-charge-limited current: Electric current limited by the charge cloud that accumulated carriers create near an emitter. Emitted electrons can repel later electrons and limit current.
X-ray tube: A vacuum device that produces X-rays when accelerated electrons strike a target. Its cathode emits the electrons that generate X-rays at the anode.

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