Filament for thermionic emission in open air

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Discussion Overview

The discussion focuses on identifying suitable materials for thermionic emitters that can operate in open air, addressing challenges related to oxidation and emission efficiency. Participants share their experiences with various materials and propose alternatives, exploring both theoretical and practical aspects of thermionic emission.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that classical tungsten filaments oxidize readily and are therefore unsuitable for open air thermionic emission.
  • Experiments with SiC show some emission at 1900K, but a SiO2 layer forms quickly, blocking further emission.
  • Another participant mentions that ZrO2 does not emit at all, despite its low work function and good conductivity when heated, suggesting it lacks free electrons for emission.
  • A participant shares a personal anecdote about attempting to generate thermionic current using a high-power electric fire, indicating that significant filament power may be necessary.
  • Previous attempts with Ni-chrom and Kanthal wires resulted in no measurable emission, aligning with expectations based on their work functions.
  • Suggestions are made to consider hafnium as a potential electrode material, highlighting its use in plasma cutters and resistance to oxygen exposure.
  • One participant expresses interest in hafnium after researching its applications, while also noting past considerations of UV solutions and lenard tubes as inferior due to efficiency and radiation concerns.
  • A suggestion is made to explore reverse engineering an electric arc lighter, which utilizes a high voltage pilot arc to ionize a channel.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific material for thermionic emission in open air, with multiple competing views and experiences shared regarding the effectiveness of various materials.

Contextual Notes

Limitations include unresolved questions about the efficiency of suggested materials, dependencies on specific conditions for thermionic emission, and the potential hazards associated with certain methods.

Gideon Y
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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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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.
 
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.
 
Tungsten with argon shield .
 
Nidum said:
Tungsten with argon shield .
Sure, this is what we do now. I'm trying to get rid from argon :-)
 
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.
 
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 !
 
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.
 

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