In vacuum tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. 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 area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment and do not require heating.
From the 1920s to the 1960s, virtually every electronic device used hot cathode vacuum tubes. Today, hot cathodes are used as the source of electrons in fluorescent lamps, vacuum tubes, and electron guns in cathode ray tubes and laboratory equipment such as electron microscopes,
A cathode electrode in a vacuum tube or other vacuum system is a metal surface which emits electrons into the evacuated space of the tube. Since the negatively charged electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal, and require energy to leave it. This energy is called the work function of the metal. In a hot cathode, the cathode surface is induced to emit electrons by heating it with a filament, a thin wire of refractory metal like tungsten with current flowing through it. The increased thermal motion of the metal atoms knocks electrons out of the surface; this process is called thermionic emission.
There are two types of hot cathodes:
- Directly heated cathode: In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes. Today they are used in fluorescent tubes and most high power transmitting vacuum tubes.
- Indirectly heated cathode: In this type, the filament is not the cathode but rather heats a separate cathode which then emits electrons. Indirectly heated cathodes are used in most low power vacuum tubes. For example, in most vacuum tubes the cathode is a nickel tube heated by a tungsten filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons. The filament of an indirectly-heated cathode is usually called the heater. The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament. Many vacuum tubes use alternating current to heat the filament. In a tube in which the filament itself was the cathode, the alternating electric field from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.
In order to improve electron emission, cathodes are usually treated with chemicals, compounds of metals with a low work function. These form a metal layer on the surface which emits more electrons. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current. The untreated tungsten filaments used in early vacuum tubes (called "bright emitters") had to be heated to 2500°F (1400°C), white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature so they only have to be heated to 800-1100°F (425-600°C)
The most common type of indirectly-heated cathode is the oxide-coated cathode, in which the nickel cathode surface has a coating of alkaline earth metal oxide to increase emission. The earliest material used was barium oxide; it forms a monatomic layer of barium with an extremely low work function. More modern formulations utilize a mixture of barium oxide, strontium oxide and calcium oxide. Another standard formulation is barium oxide, calcium oxide, and aluminium oxide in a 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 small glass vacuum tubes, but are rarely used in high-power tubes because the coating is degraded by positive ions that bombard the cathode, accelerated by the high voltage on the tube.
For manufacturing convenience, the oxide-coated cathodes are usually coated with carbonates, which are then converted to oxides by heating. 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. The Ba content significantly increases on the surface layers of oxide cathodes down to several tens of nanometers in depth, after the cathode activation process. The lifetime of oxide cathodes can be evaluated with a stretched exponential function. The survivability of electron emission sources is significantly improved by high doping of high‐speed activator.
Lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6) are used as the coating of some high-current cathodes. 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. 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 e.g. in electron microscopes, microwave tubes, electron lithography, electron beam welding, X-Ray tubes, and free electron lasers. 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.
Thoriated filaments are another option, discovered in 1914 and made practical by Irving Langmuir in 1923. A small amount of thorium is added to the tungsten of the filament. 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. Heating the filament in a hydrocarbon atmosphere carburizes the surface and stabilizes the emissive layer. Thoriated filaments can have very long lifetimes and are resistant to high voltages, because fresh thorium continually diffuses to the surface, renewing the layer. They are used in nearly all 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.
Due to concerns about thorium radioactivity and toxicity, efforts have been made to find alternatives. One of them is zirconiated tungsten, where zirconium dioxide is used instead of thorium dioxide. Other replacement materials are lanthanum(III) oxide, yttrium(III) oxide, cerium(IV) oxide, and their mixtures.
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.g. zirconium carbide, hafnium carbide, tantalum carbide, hafnium diboride, and their mixtures. Metals from groups IIIB (scandium, yttrium, and some lanthanides, often gadolinium and samarium) and IVB (hafnium, zirconium, titanium) are usually chosen.
A barrier layer of other material can be placed between the base metal and the emission layer, to inhibit chemical reaction between these. 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.
A cathode heater is a heated wire filament used to heat the cathode in a vacuum tube or cathode ray tube. The cathode element had to achieve the required temperature in order for these tubes to function properly. This is why older electronics often needed some time to "warm up" after being powered on; this phenomenon can still be observed in the cathode ray tubes of some modern televisions and computer monitors. The cathode heats to a temperature that causes electrons to be 'boiled out' of its surface into the evacuated space in the tube, a process called thermionic emission. The temperature required for modern oxide-coated cathodes is around 800–1000 °C (1472–1832 °F)
The cathode is usually in the form of a long narrow sheet metal cylinder at the center of the tube. The heater consists of a fine wire or ribbon, made of a high resistance metal alloy like nichrome, similar to the heating element in a toaster but finer. It runs through the center of the cathode, often being coiled on tiny insulating supports or bent into hairpin-like shapes to give enough surface area to produce the required heat. The ends of the wire are electrically connected to two pins protruding from the end of the tube. When current passes through the wire it becomes red hot, and the radiated heat strikes the inside surface of the cathode, heating it. The red or orange glow seen coming from operating vacuum tubes is produced by the heater.
There is not much room in the cathode, and the cathode is often built with the heater wire touching it. The inside of the cathode is insulated by a coating of alumina (aluminum oxide). This is not a very good insulator at high temperatures, therefore tubes have a rating for maximum voltage between cathode and heater, usually only 200 - 300 V.
Heaters require a low voltage, high current source of power. Miniature receiving tubes for line-operated equipment used on the order of 0.5 to 4 watts for heater power; high power tubes such as rectifiers or output tubes would have used on the order of 10 to 20 watts, and broadcast transmitter tubes might need a kilowatt or more to heat the cathode.  The voltage required was usually 5 or 6 volts AC. This was supplied by a separate 'heater winding' on the device's power supply transformer that also supplied the higher voltages required by the tubes' plates and other electrodes. A more common approach used in transformerless line-operated radio and television receivers such as the All American Five was to connect all the tube heaters in series across the supply line. Since all the heaters were rated at the same current, they would share voltage according to their heater ratings. Battery-operated radio sets used direct-current power for the heaters (commonly known as filaments), and tubes intended for battery sets were designed to use as little filament power as necessary, to economize on battery replacement. The final models of tube-equipped radio receivers were built with subminiature tubes using less than 50 mA for the heaters, but these types were developed at about the same time as transistors which replaced them. Where leakage or stray fields from the heater circuit could potentially be coupled to the cathode, direct current was sometimes used for heater power. This would eliminate a source of noise in sensitive audio or instrumentation circuits.
The emissive layers on coated cathodes degrade slowly with time, and much more quickly when the cathode is overloaded with too high current. The result is weakened emission and diminished power of the tubes, or in CRTs diminished brightness.
The activated electrodes can be destroyed by contact with oxygen or other chemicals (e.g. 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 had to be developed for the early Whirlwind computer, with filaments free of traces of silicon.
Slow degradation of the emissive layer and sudden burning and interruption of the filament are two main failure modes of vacuum tubes.
Transmitting tube hot cathode characteristics
|Material||Operating temperature||Emission efficacy||Specific emission|
|Tungsten||2500 K||5 mA/W||500 mA/cm2|
|Thoriated tungsten||2000 K||100 mA/W||5 A/cm2|
|Oxide coated||1100 K||500 mA/W||10 A/cm2|
|Barium aluminate||1300 K||400 mA/W||4 A/cm2|
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