- Hot cathode is also a name for a hot filament ionization gauge, a vacuum measuring device.
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 area.
Principles and Variants
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. The filament is most often made of tungsten. 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. 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. Thorium can be added to tungsten to increase its emissivity, due to its lower work function. Some cathodes are made of tantalum.
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. 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 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. 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. 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. Other replacement materials are lanthanum(III) oxide, yttrium(III) oxide, cerium(IV) oxide, and their mixtures. 
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 eg. 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. 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. 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.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. 
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 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.