In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. (Cf. cold cathodes, where field emission is used and which do not require heating.) The heating element is usually an electrical filament. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface. In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... Diagram of a copper cathode in a Daniells cell. ... Properties The electron is a fundamental subatomic particle that carries a negative electric charge. ... 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. ... Note: Principles are mostly the same for cold cathode ion sources as in particle accelerators to create electrons. ... Also known as Fowler-Nordheim tunneling, field emission is a form of quantum tunneling in which electrons pass through a barrier in the presence of a high electric field. ... The Electric Filament is a thread of metal which is used to convert heat into electricity for the incandescent light bulb made in 1878 by Joseph Wilson Swan ...

Hot cathodes may be either directly heated, where the filament is itself the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode itself. The filament is most often made of tungsten. In the indirectly heated cathodes, the filament is usually called heater instead; this configuration minimizes the introduction of hum when the filament was energized with alternating current. The cathode for indirectly heating is usually realized as a nickel tube coated with the emissive layer. General Name, Symbol, Number tungsten, W, 74 Chemical series transition metals Group, Period, Block 6, 6, d Appearance grayish white, lustrous Atomic mass 183. ... city lights viewed in a motion blurred exposure. ...

The cathode is typically covered with an emissive layer, made of a material with lower work function, which emits electrons more easily than bare tungsten metal, reducing the necessary temperature and lowering the emission of metal ions. The work function is the minimum energy (usually measured in electron volts) needed to remove an electron from the Fermi level in a metal to a point at infinite distance away outside the surface. ...

A common approach is a oxide-coated cathode. The earliest material used was barium oxide; it forms a monoatomic layer of barium with an extremely low value of the work function. More modern formulations utilize a mixture of barium oxide, strontium oxide and calcium oxide. Other standard formulation is barium oxide, calcium oxide, and aluminium oxide in 5:3:2 ratio. Thorium oxide is used as well. Oxide-coated cathodes operate at about 800-1000 °C, orange-hot. They are used in most of small glass tubes. They are rarely used in high-power tubes, as they are sensitive to high voltage and oxygen ions and undergo rapid degradation under such conditions. [1] ... General Name, Symbol, Number barium, Ba, 56 Chemical series alkaline earth metals Group, Period, Block 2, 6, s Appearance silvery white Atomic mass 137. ... Strontium Oxide SrO is formed when strontium reacts with oxygen. ... Calcium oxide (CaO), commonly known as lime, quicklime or burnt lime, is a widely used chemical compound. ... Aluminium oxide or aluminum oxide is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. ... Thorium dioxide (ThO2), also called thorium(IV) oxide (IUPAC) is a white, crystalline powder. ...

For the manufacturing convenience, the oxide-coated cathodes are usually coated with carbonates, which are then converted to oxides by heating, and the metal monolayer has to be formed, in a process called electrode activation. The activation may be achieved by microwave heating, direct electric current heating, or electron bombardment while the tube is on the exhausting machine, until the production of gases ceases. The purity of cathode materials is crucial for the tube lifetime. [2] Carbonate is an anion with a charge of -2 and an empirical formula of CO32-. For an aqueous solution, carbonate exists in three forms. ...

Thoriated filaments are another option. A small amount of thorium is added to the tungsten of the filament. The filament is heated white-hot, at about 2400 °C, thorium atoms migrate to the surface of the filament, and form the emissive layer. Thoriated filaments can last for very long time and are resistant to high voltages. They are used in nearly all big high-power vacuum tubes for radio transmitters, and in some tubes for hi-fi amplifiers. Their lifetimes tend to be longer than of oxide cathodes. [3] General Name, Symbol, Number thorium, Th, 90 Chemical series Actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass 232. ...

The emissive layers degrade slowly with time, and much quicker when the cathode is overloaded with too high current. The result is weakened emission and diminished power of the tubes, or brightness of the CRTs, affected.

The activated electrodes can be destroyed by contact with oxygen or other chemicals (eg. aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements. This results in diminished emissivity. This process is known as cathode poisoning. High-reliability tubes for the early computer Whirlwind had to be developed, with filaments free of traces of silicon. General Name, Symbol, Number oxygen, O, 8 Chemical series Chalcogens Group, Period, Block 16, 2, p Appearance colorless Atomic mass 15. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ... In chemistry, a silicate is a compound consisting of silicon and oxygen (SixOy), one or more metals, and possibly hydrogen. ... The Whirlwind computer was developed at the Massachusetts Institute of Technology. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance dark gray, bluish tinge Atomic mass 28. ...

Slow degradation of the emission layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes.

Hot cathodes are the main source of electrons in electron guns in cathode ray tubes, electron microscopes, vacuum tubes, and in some fluorescent lamps. Electron gun from a cathode ray tube An electron gun is a component that produces an electron stream that has a precise kinetic energy, being used in all TVs and monitors which use cathode ray tube technology, and in other instruments, eg. ... Cathode ray tube employing electromagnetic focus and deflection Cutaway rendering of a color CRT The cathode ray tube or CRT, invented by Karl Ferdinand Braun, is the display device that was traditionally used in most computer displays, video monitors, televisions and oscilloscopes. ... It has been suggested that Selected area diffraction be merged into this article or section. ... In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... A compact fluorescent lamp with an integrated electronic ballast A fluorescent lamp is a type of lamp that uses electricity to excite mercury vapor in argon or neon gas, resulting in a plasma that produces short-wave ultraviolet light. ...