What is Thermionic emission: Definition and 16 Discussions

Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.
The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K (730 °C; 1,340 °F).
The term 'thermionic emission' is now also used to refer to any thermally-excited charge emission process, even when the charge is emitted from one solid-state region into another. This process is crucially important in the operation of a variety of electronic devices and can be used for electricity generation (such as thermionic converters and electrodynamic tethers) or cooling. The magnitude of the charge flow increases dramatically with increasing temperature.

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  1. Abimbola1987

    Measurement of vacuum diode electron flow

    Dear Sirs, After my theoretical post on vacuum diode electron flow, I decided to try and measure it physically. I didn't have a vacuum tube, so I improvised with some tungsten wire, aluminum sheets and some screws and the result was usable. Obviously the tungsten wire fails after some time due...
  2. Abimbola1987

    Thermionic emission and current density

    Dear Sirs, Maybe this is general knowledge, but I couldn't find the answer where I looked, so please bear with me. Consider a circuit consisting of a mechanical generator (some spinning magnets and coils) and a wire across the generators output. At some point the wire gets hot and starts a...
  3. H

    How does thermionic emission work?

    I’ve been curious about understanding the mechanism behind themionic emission from what I have read I found that themionic emission happen when the energy from added temperature excess the work function of the material. I also readed when temperature excesses 1000k themionic emission happens but...
  4. W

    B Related to - Thermionic emission

    Hey currently I am doing O levels and in my textbook I read that when a metallic wire (in CRO - the cathode) is heated up, it starts emitting its free electrons (thermionic emission). And also its a fact that electrons are responsible for electrical conductivity. A question just popped up in my...
  5. G

    I Filament for thermionic emission in open air

    I’m looking for material capable to act as filament of thermionic emitter in open air. Classical tungsten filament is not capable, because it readily oxidizes. We experimented with filaments made from SiC and ZrO2 (Yttria stabilized). At 1900K SiC provides certain degree of emission, but it is...
  6. G

    What do electric field lines look like for two metal plates?

    Hello. I'm now taking accelerator physics class and the lectures said in the thermionic electron gun, anode metal mesh should be grounded in order to prevent electrons which traveled beyond the mesh from coming back due to the electric field lines. Without grounding of the anode, I believe...
  7. 1

    Thermionic Emission Effects on Surfaces

    If there is continue emission of electrons from surface then what will be its effect on that surface? Some of my friends told me that there are billions billions electrons. Nothing happen. But i think there is also emission of billions of electrons within small time. Can this process make a...
  8. Nemika

    Cathode ray tube- thermionic emission

    I have just learned about the cathode ray tube in the class room. There is a part called electron gun in it which emits electrons and these electrons finally strike the fluorescent screen.(I hope its correct till here.) But I want to ask that won't there be a loss of electrons in this process...
  9. C

    Thermionic Emission: Questions Explained

    Hi, in the latest lesson my professor explained the thermionic emission; I guess it is a simplified approach, since I am not a Physicist. Anyway, there are some things not clear to me; I'll show you the approach and I hope someone of you could help me. As usual, W= work function and E_{f}=...
  10. 3

    A heated cathode inducing thermionic emission of electrons.

    Hello all. I have been looking at this problem: I wrote three equations there, one for each part, which I think will help me solve each part; is my approach to the problem using those equations that you see there correct? I am just looking for some advice on where and how to start this problem...
  11. G

    Question on thermionic emission

    The electron needs greater energy than the work function in order to escape the material. But when it does escape, does it subtract the work function from it's kinetic energy or not?
  12. G

    Thermionic emission at same temperature

    As it is known, the thermionic emission (emission of electrons at high temperature) current density in vacuum depends on the temperature and the work function of the material. The work function is a material specific constant. Let's compare thermionic emission with thermal radiation. With...
  13. B

    Percent change in thermionic emission

    % change in thermionic emission Q1. Determine the % change in thermionic emission for an oxide-coated filament of work function of 1.3eV if the temperature is decreased by 1.00% at a temperature of 2300K I'm uncertain but i used dJ/J=dT/T ( 2 + ((1160x1.3)/1000) to get a 15% change in...
  14. B

    Child's Law and Thermionic Emission

    I have a question regarding Child's Law with thermionic emission. I understand that due to space-charge effects, that the emission current from a cathode reaches a saturation current at a certain temperature, explained mathematically by Richardson's Law. Utilizing an accelerating positive...
  15. C

    Thermionic emission and diffusion theory

    What is the main physical difference behind these two theories? 1) I understand that the thermionic emission theory is applied in metal-semiconductor contacts and heterostructures where the energy band off-sets are large. Whereas the diffusion theory is applied in a simple homojunction, of...
  16. C

    Thermionic Emission: What Causes Electrons to be Emitted?

    Thermionic Emission... Why exactly does thermionic emission happen? ie - Why does heating a metal cause electrons to be emitted? I know that it must be that the heat is giving the electrons enough energy to escape from their atoms but in what form is this? ie - I know that electrons can be...