Filament for thermionic emission in open air

In summary, the conversation revolved around finding a material that can act as a filament for a thermionic emitter in open air. It was mentioned that classical tungsten filaments are not suitable due to oxidation, and experiments with SiC and ZrO2 filaments were conducted. However, SiC quickly forms a layer of SiO2 on its surface, blocking emission, and ZrO2 does not emit at all. Other materials such as Ni-chrom and Kanthal heating wires were also tried with no success. Suggestions were made to try using Halfnium electrodes, which can handle exposure to oxygen well, and to possibly try building an electric arc lighter or reverse engineering a plasma cutter.
  • #1
Gideon Y
23
6
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 rapidly blocked by formation of SiO2 layer on filament surface.
ZrO2 doesn’t emits at all, despite low work function and good electrical conductivity when hot. We heated it up to 2200K. We guess that it simply has no free electrons available for emission, as its conducts by circulation of oxygen ions.

Can somebody recommend material which can be used ?
Does anybody have experience with LaCrO3 for such application ?
 
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  • #2
When I was ten years old, I tried to obtain a thermionic current from a 1kW electric fire, using a plate as an anode, held positive with a battery! And using a sensitive pair of earphones as my detector, I did detect slight emission. So maybe a lot of filament power is needed.
 
  • #3
Actually in past we tried filaments made from Ni-chrom and Kanthal heating wires. No emission.
This makes sense, as at their maximum operation temperature versus work functions of their alloys they are not supposed to emit anything measurable.
 
  • #4
Tungsten with argon shield .
 
  • #5
Nidum said:
Tungsten with argon shield .
Sure, this is what we do now. I'm trying to get rid from argon :-)
 
  • #6
If you're still searching for an electrode material to use, you may consider using Halfnium. Halfnium electrodes are used in plasma cutters, can be found rather cheaply, and from what i understand, handle exposure to oxygen quite well. High frequency start modules for welding machines also may be worth looking into, along with focusing UV light onto your electrode, although you may have to build your hf circuit from scratch as I don't believe they are still being manufactured. If you don't mind a few Xrays (and low current), you could try building a lenard tube, which allows electrons from a crt to escape into open air.
 
  • #7
Thank you for very useful information regarding hafnium and its applications. Yesterday I researched a little-bit about hafnium electrodes and plasma torch, and got a good feeling that this might be the right direction.

We considered in past solutions based on UV (photoelectric effect) and on lenard tube but found them inferior, due to low efficiency and X-ray radiation problem.

Thank you for your help !
 
  • #8
Glad I could help, I hope it works out. If not, you might try reverse engineering an electric arc lighter, I think they use a high voltage pilot arc to ionize a channel and then run apply a low voltage high current on top of that.
 

1. What is thermionic emission and how does it work?

Thermionic emission is a phenomenon in which electrons are emitted from a heated surface. This is due to the thermal energy causing the electrons to overcome the potential barrier on the surface and escape into the surrounding environment.

2. Why is a filament used for thermionic emission in open air?

A filament is used because it can reach high temperatures, typically around 2000-3000 degrees Celsius, which is necessary for thermionic emission to occur. In open air, the filament is exposed to oxygen which can oxidize and degrade the filament, limiting its lifespan.

3. What materials are commonly used for filaments in thermionic emission?

Some common materials used for filaments in thermionic emission include tungsten, thoriated tungsten, lanthanum hexaboride, and tantalum. These materials have high melting points and are able to withstand the high temperatures required for thermionic emission.

4. How is the filament heated in thermionic emission?

The filament is typically heated using an electric current. The filament is made of a material with high resistance, so when an electric current flows through it, the resistance causes the filament to heat up. This heat is what allows thermionic emission to occur.

5. What factors affect the efficiency of thermionic emission from a filament in open air?

The efficiency of thermionic emission can be affected by several factors, including the temperature of the filament, the material of the filament, the presence of impurities on the surface of the filament, and the surrounding gas environment. Higher temperatures and cleaner surfaces generally lead to more efficient thermionic emission.

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