Types of Thermionic Emitter or Cathode with Respect to Material– As we know metal surface wherefrom the emission of electrons occurs, are called cathode or emitters. These days, the following three types of materials, are being used for making cathodes or emitters of various sorts of tubes.
Types of Thermionic Emitter
- Thoriated Tungsten
- Oxide Coated Emitters
Due to its efficiency regarding emission of electrons in empty places/ vacuum in a better manner, tungsten is considered as one of the best cathode materials to be used in vacuum tubes. It is grey in color and despite being hard/ tough in nature no brittles are created on its surface even at high temperatures. Tungsten is a metal with an operating temperature of around 2500⁰Kelvin or approx. 2200⁰C. Due to high thermal stability, tungsten can work better on high temperatures. Its melting point is too high i.e. approx. 3300⁰C. The leaner or narrower a tungsten metal wire, the greater will be its tensile strength or ability to withstand tension. This metal gets oxidize in presence of oxygen too quickly. Tungsten can work fairly well up to a temperature of 2000⁰C in air-tight tubes or vacuum tubes, nitrogen or organ-filled gas tubes.
It can work for long hours during rough and tough weather conditions, however, emission of vapors gradually commences from it, due to which process of electron emission becomes too weak. Tungsten cathodes are mostly used in tubes consuming too much applied voltage (e.g. x-ray tubes, high voltage rectifier tubes and some types of high-power transmission tubes). A tungsten cathode provides 500mA/Cm2 current at 2500⁰Kelvin. The work function of tungsten is too high that’s why it is operated on high temperatures. Due to the requirement of high heat power, its performance gets dwindled. At heating power, emission currents up to 4 to 20-milli ampere per watt from tungsten can be obtained from tungsten.
If one to 3 per cent of thorium oxide is amalgamated with tungsten, the mixture of these two materials is called thoriated tungsten. In other words, Thoriated tungsten material is composed as a result of mixture of pure tungsten and very low quantity of thorium oxide that’s it consists of tungsten and thorium oxide. Through a proper heat treatment of this material, thorium chloride converts into thorium. Thorium atoms come up on surface of the tube filament where they at a temperature of about 1900⁰Kadopt a shape of a very thin layer to facilitate the process of emission of electrons. (That’sa very fine layer of oxide builds up on tungsten through a very meticulous heat treatment). As a result of presence of oxide, very little quantity of energy is required for electrons emitting surface. Thus, at a uniform temperature, more electrons are emitted from thoriated tungsten as compared to pure tungsten (emission mechanism greatly improves due to the mingling of thorium oxide with tungsten however, it’s thermal or heat stability is comparatively lower). Emission currents up to 50-100-milli ampere are generated when power equivalent to one watt of heating is supplied on thoriated tungsten. Therefore, thoriated tungsten cathode is a better emitter than pure tungsten itself.
As the layer of thorium may be damaged or become utterly useless due to bombardment with gas ions, therefore thoriated tungsten emitter is used only in such tubes which have an excellent vacuum or empty space. Such emitters are used in medium voltage tubes instead of high voltage tubes. It is worth remembering here that cathode made of thoriated tungsten normally functions on 1600-1900⁰K and at this temperature, minimum of 50 and maximum 100-mile ampere per square centimeter electron current can be produced at per watt heating power on the surface. Mostly these kinds of cathodes are used in a majority of transmission tubes ranging from 50 watts to several kilo watts. The expected age of tube filaments manufactured from such material is about 1000-5000 hours. Thoriated tungsten filaments are used in low and medium power transmitting tubes.
Oxide Coated Emitters
Oxide coated emitters or cathodes are manufactured through coating an equal uniform kind of barium, strontium or calcium layer on some base metals (e.g. nickel, cobalt, titanium etc.). Usually, this layer is overlapped on nickel. As the work function of oxide coated material is too low (1.0 eV), therefore cathodes or emitters made from this metal are more efficient than those manufactured from tungsten or thoriated tungsten (that’s why it result in comparatively large electron emission). These emitters act at a very low temperature (800⁰C) and if they are used in places where operating voltage is too high or where ion bombardment is too large, their age gets less markedly. Thermal stability of such emitters is also too low. As the work function of oxide coated emitters is minimum i.e. one electron volt (1.0 eV) (low work function means emission of electron in excessive quantity), therefore, they can start working only at a temperature of 1000⁰K and at this temperature, they can generate 100-milli amperes per square centimeter current (100mA/Cm2) electron currents. Due to functioning on low temperatures, oxide coated emitters consume low power that’s one can save power by using them as these are mostly used in small tubes. The application of this cathode is too common due to its efficiency with respect to heating power in almost all kind of receiving tubes and small-sized transmitting tubes. Table number 1.1 contains information about work function of different cathode materials, operating temperatures per watt, mile amperes and their common uses.
|Material Name||Work Function||Operating
|Milliamperes Per Watt||General Use||Current/Cm2|
|Tungsten||4.5||2200 – 3000||4 – 20||Above 1Kw, 5000V||500mA/2500k|
|1627 – 1650||50 – 100||To 1Kw, 5000V||50mA/1600k|
|Oxide coated||1 – 2||700 – 800||250 – 1000||To 100W, 75V||100mA/1000k|
Types of Cathodes with respect to Heating
We know that for emission of electrons, every thermionic cathode has to be heated up to a certain temperature with respect to its material. The objective of electron emission can be achieved by using any one of the above four mentioned methods. In case of thermal emission, heat energy can be acquired by using following two types of cathodes.
- Directly Heated Cathode
- Indirectly Heated Cathode
In the afore-mentioned two methods, a low voltage source is used in order toprovide supply to filament current or cathode circuit. In case of direct heating, cathode filament is prepared in ribbon form and its resistance is too high. In the direct heating method, electric current is supplied directly to filament or cathode, which functions as an emitter. When this cathode is connected with a direct electric energy source, current passes through it, resulting ultimately in the production of heat (or I2R loss occurs), through which desired temperature is achieved. In case of AC, fluctuation in emission exists, due to which pulsating emission current is generated. The directly heated cathode work better on DC. Filament is fixed with a support or support wire. These types of filaments are mostly made from tungsten material which can work better on high temperatures. All emitters (tungsten, thoriated tungsten & oxide coated) are used for direct heated filaments. Directly heated cathode is solidly built and it is commonly used in power rectifier and transmitting tubes. The filament used in common incandescent bulbs is a good example of directly heated filament. However, it has to be kept in mind that the filament used in an electron tube does not heat as much as the filament of a light bulb, because the objective of an electron tube is not providing light, but to emit electrons. Some shapes of directly heated cathode have been displayed in figure 1.
An indirectly heated cathode consists of a filament which heats another or a separate electron emitting surface. In such a configuration, cathode and source of filament power both are separated. (i.e. in this type, there is no heater tube filament, rather a separate power supply is provided on heater). Moreover, in spite of usage of a high efficiency electron surface, filament is being operated at a relatively low temperature. In indirectly heated type, emitter consists of two parts i.e. a heater and a sleeve. As heater has to work on a higher temperature compared to an emitter, therefore, it is made of tungsten or its alloy, whereas sleeve usually is made from nickel, on which a layer of some electron emitting material (e.g. barium oxide or strontium) is being coated. In figure 2 indirectly heated cathode and its symbols/ shapes have been displayed.
In indirect heat method, electric current is provided to a separate heated element, which is located inside a cylindrical sleeve or cathode. There is a layer of emitting material on the cathode. In this way, the cathode heats up indirectly through acquiring energy from the heater element. Remember that oxide-coated emitters are always used indirectly as heated cathodes. They get heat through radiation from within the nickel-made cylindrical-shaped tungsten insulated wire-made heater. (Heat produced through the movement of airwaves is referred to as radiation). Such cathodes can work on AC & DC with equal efficiency. The indirectly heated cathode is commonly used in all small tubes.
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