In vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy (Cf. cold cathodes, where field emission is used and which do not require heating. A cold cathode is an element used within some Nixie tubes Gas discharge lamps Gas filled tubes and Vacuum tubes Cold cathodes do not Field emission (FE is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields ) The heating element is usually an electrical filament. An electrical filament is a thread of Metal, usually Tungsten, which is used to convert Electricity into light in Incandescent light bulbs (as developed Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area.
Hot cathodes are the main source of electrons in electron guns in cathode ray tubes, electron microscopes, vacuum tubes, and in some fluorescent lamps. An electron gun (also called electron emitter) is an electrical component that produces an Electron beam that has a precise Kinetic energy and is most often The cathode ray tube (CRT is a Vacuum tube containing an Electron gun (a source of electrons and a Fluorescent screen with internal or An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor.
Hot cathodes may be either directly heated, where the filament itself is the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode; this configuration minimizes the introduction of hum when the filament is energized with alternating current. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant The filament is most often made of tungsten. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 With indirectly heated cathodes, the filament is usually called the heater instead. The cathode for indirectly heating is usually realized as a nickel tube which surrounds the heater.
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. In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid Cathodes can be made of pure sintered tungsten as well; tungsten cathodes in the shape of a parabolic lens are used in electron beam furnaces. Sintering is a method for making objects from powder, by heating the material (below its Melting point - solid state sintering until its particles adhere In Mathematics, the parabola (pəˈræbələ from the Greek παραβολή) is a Conic section, the intersection of a right circular A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging Thorium can be added to tungsten to increase its emissivity, due to its lower work function. Thorium (ˈθɔːriəm is a Chemical element with the symbol Th and Atomic number 90 Some cathodes are made of tantalum. Tantalum (ˈtæntələm (formerly tantalium /tænˈtæliəm/ is a Chemical element with the symbol Ta and Atomic number 73
A common type is an oxide-coated cathode. The earliest material used was barium oxide; it forms a monoatomic layer of barium with an extremely low work function. Barium oxide, BaO is a white Hygroscopic compound formed by the burning of Barium in Oxygen, although it is often formed through the decomposition Barium (ˈbɛəriəm is a Chemical element. It has the symbol Ba, and Atomic number 56 More modern formulations utilize a mixture of barium oxide, strontium oxide and calcium oxide. Strontium oxide or strontia, SrO is formed when Strontium reacts with Oxygen. Calcium oxide ( CaO) commonly known as burnt lime, lime or quicklime, is a widely used Chemical compound. Another standard formulation is barium oxide, calcium oxide, and aluminium oxide in 5:3:2 ratio. Thorium oxide is used as well. Thorium dioxide (ThO2 also called thorium(IV oxide (IUPAC is a white crystalline powder Oxide-coated cathodes operate at about 800-1000 °C, orange-hot. They are used in most small glass vacuum 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. 
For manufacturing convenience, the oxide-coated cathodes are usually coated with carbonates, which are then converted to oxides by heating, and then the metal monolayer is formed in a process called electrode activation. In Chemistry, a carbonate is a salt or Ester of Carbonic acid. 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 tube lifetime. 
Due to concerns about thorium radioactivity and toxicity, there are efforts to find alternatives. One of them is zirconiated tungsten, where zirconium dioxide is used instead of thorium dioxide. Zirconia redirects here For the Sailor Moon character see Dead Moon Circus. Other replacement materials are lanthanum(III) oxide, yttrium(III) oxide, cerium(IV) oxide, and their mixtures. Lanthanum(III oxide is La2O3 a chemical compound containing the Rare earth element Lanthanum and Oxygen. Yttrium oxide is Y 2 O 3 It is an air-stable white substance Yttrium oxide is used as a common starting material for both Cerium(IV oxide, ceric oxide, ceria, or sometimes simply cerium oxide or cerium dioxide, is a pale yellow-white powder CeO2 
Lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) are used as the coating of some high-current cathodes. Lanthanum hexaboride ( La[[Boron B]]6, also called lanthanum boride and LaB) is an inorganic chemical a Boride of Lanthanum Cerium hexaboride ( Ce[[Boron B]]6, also called cerium boride, CeBix, CEBIX, and (incorrectly CeB) is an inorganic Hexaborides show low work function, around 2. 5 eV. They are also resistant to poisoning. Cerium boride cathodes show lower evaporation rate at 1700 K than lanthanum boride, but it becomes equal at 1850 K and higher. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic Cerium boride cathodes have one and a half times the lifetime of lanthanum boride, due to its higher resistance to carbon contamination. Boride cathodes are about ten times as "bright" as the tungsten ones and have 10-15 times longer lifetime. They are used eg. in electron microscopes, microwave tubes, electron lithography, electron beam welding, X-Ray tubes, and free electron lasers. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 Electron beam lithography (often abbreviated as e-beam lithography) is the practice of scanning a beam of Electrons in a patterned fashion across a surface covered Electron beam welding (EBW is a fusion Welding process in which a beam of high-velocity Electrons is applied to the materials being joined An X-ray tube is a Vacuum tube that produces X-rays They are part of X-ray machines X-rays are part of the Electromagnetic spectrum, an A free-electron laser, or FEL is a Laser that shares the same optical properties as conventional lasers such as emitting a Beam consisting of coherent  However these materials tend to be expensive.
Other hexaborides can be employed as well; examples are calcium hexaboride, strontium hexaboride, barium hexaboride, yttrium hexaboride, gadolinium hexaboride, samarium hexaboride, and thorium hexaboride. Calcium hexaboride (CaB6 is a compound of Calcium and Boron in which the coordination number of the calcium is 18
Thoriated filaments are another option. A small amount of thorium is added to the tungsten of the filament. Thorium (ˈθɔːriəm is a Chemical element with the symbol Th and Atomic number 90 The filament is heated white-hot, at about 2400 °C, and thorium atoms migrate to the surface of the filament and form the emissive layer. Thoriated filaments can have very long lifetimes 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 those of oxide cathodes. 
In addition to the listed oxides and borides, other materials can be used as well. Some examples are carbides and borides of transition metals, e. For the Software development tool targeting the Symbian OS, see Carbide In chemistry a boride is a chemical compound between boron and a less electronegative element In Chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings It commonly refers to any element in g. zirconium carbide, hafnium carbide, tantalum carbide, hafnium diboride, and their mixtures. Zirconium carbide ( Zr[[Carbon C]] is an extremely hard refractory Ceramic material commercially used in Tool bits for cutting Hafnium carbide ( Hf[[Carbon C]] is a Chemical compound. It is the most refractory Binary compound known with a melting point over Tantalum carbide ( Ta[[Carbon C]] is an extremely hard ( Mohs hardess 9-10 refractory Ceramic material commercially used in Hafnium diboride is an ultra-high temperature Ceramic composed of Hafnium and Boron. Metals from groups IIIB (scandium, yttrium, and some lanthanides, often gadolinium and samarium) and IVB (hafnium, zirconium, titanium) are usually chosen. Scandium (ˈskændiəm is a Chemical element that has the symbol Sc and Atomic number 21 Yttrium (ˈɪtriəm is a Chemical element with symbol Y and Atomic number 39 Terminology The Trivial name " Rare earths " is sometimes used to describe all the lanthanoids together with Scandium and Yttrium Gadolinium (ˌgædəˈlɪniəm is a Chemical element that has the symbol Gd and Atomic number 64 Samarium (səˈmɛəriəm is a Chemical element with the symbol Sm and Atomic number 62 Hafnium (ˈhæfniəm is a Chemical element that has the symbol Hf and Atomic number 72 Zirconium (zɚˈkoʊniəm /ˌzɝˈkoʊniəm/ is a Chemical element with the symbol Zr and Atomic number 40 Titanium (taɪˈteɪniəm is a Chemical element with the symbol Ti and Atomic number 22 
In addition to tungsten, other refractory metals and alloys can be used, e. Refractory metals are a class of Metals that are extraordinarily resistant to Heat and Wear. g. tantalum, molybdenum and rhenium and their alloys. Tantalum (ˈtæntələm (formerly tantalium /tænˈtæliəm/ is a Chemical element with the symbol Ta and Atomic number 73 Molybdenum (məˈlɪbdənəm from the Greek word for the metal " Lead " is a Group 6 Chemical element with the symbol Mo Rhenium (ˈriːniəm is a Chemical element with the symbol Re and Atomic number 75
A barrier layer of other material can be placed between the base metal and the emission layer, to inhibit chemical reaction between these. A diffusion barrier is a thin layer (usually micrometres thick of Metal usually placed between two other metals The material has to be resistant to high temperatures, have high melting point and very low vapor pressure, and be electrically conductive. Materials used can be e. g. tantalum diboride, titanium diboride, zirconium diboride, niobium diboride, tantalum carbide, zirconium carbide, tantalum nitride, and zirconium nitride. Titanium Diboride (chemical formula TiB2 is an extremely hard Ceramic compound (33 GPa composed of Titanium and Boron that has very good Tantalum carbide ( Ta[[Carbon C]] is an extremely hard ( Mohs hardess 9-10 refractory Ceramic material commercially used in Zirconium carbide ( Zr[[Carbon C]] is an extremely hard refractory Ceramic material commercially used in Tool bits for cutting Zirconium nitride, ZrN is a Nitride of Zirconium. Zirconium Nitride comes in various forms 
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. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the aluminium, or silicates), either present as residual gases, entering the tube via leaks, or released by outgassing or migration from the construction elements. WikipediaNaming For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI Outgassing (sometimes called offgassing, particularly when in reference to indoor Air quality is the slow release of a Gas that was trapped This results in diminished emissivity. This process is known as cathode poisoning. High-reliability tubes had to be developed for the early Whirlwind computer, with filaments free of traces of silicon. The Whirlwind Computer was developed at the Massachusetts Institute of Technology. Silicon (ˈsɪlɪkən or /ˈsɪlɪkɒn/ silicium is the Chemical element that has the symbol Si and Atomic number 14
Slow degradation of the emissive layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes. Failure causes are defects in design process quality or part application which are the underlying cause of the failure or which initiate a process which leads to failure