A question on heated filaments?

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Sven Andersson
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A question on heated filaments?

Post by Sven Andersson »

A heated filament is often made of Tungsten. However, to increase the efficiency the filament is often treated with substances that lower the work function. Barium, Strontium etc. Electrons pop off easier from them then.

Now, where can I buy or scavenge (from new or old equipment) a treated Tungsten filament (with a low work function) that can survive in a (low pressure) hydrogen atmosphere? Would it be possible to scavenge from some arc discharge lamp?

Another question; the life time of this filament is not important, so what about using some other metal than Tungsten? Even if it lasts only 10 minutes that may be enough to do measurements and observe what happens.
Dan Knapp
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Re: A question on heated filaments?

Post by Dan Knapp »

A good source for small quantities of filament material is: https://electrontubestore.com
They supply components to the hobbyist group who make their own vacuum tubes.
Jeroen Vriesman
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Re: A question on heated filaments?

Post by Jeroen Vriesman »

Dan,

none of the oxide coated filaments survive atmospheric conditions, they react with the water in the air, forming hydroxides.

Usually such a "indirectly heated" oxide-coated filament is stored with a carbonate coating, and then activated in vacuum by heating it, the carbonate releases CO2 and becomes an oxide.

You can use a normal light-bulb tungsten wire for 10 minutes, but you have to make it white-hot to get some emission, and it will vaporize, and there is also the tungsten-water cycle if you have just a little bit of water in your chamber.

Next step is thoriated tungsten wire, I can sell you some, or you can buy it at the electron tube store mentioned in the previous post.

This is tungsten wire with about 2% thorium oxide. To make it work, you need to "activate" the wire.

The idea is to have a monoatomic layer of thorium on the surface, where the vaporization of the thorium is the same as the amount of thorium diffusing to the surface, this happens at about 1800 deg. C. Emission is then very efficient (about 1000 times more than pure tungsten for the same temperature).

But first you need to create some thorium in the filament, this is done by heating it to about 2800 deg. C for about one minute, some of the thorium oxide reduces to pure thorium and some tungsten oxide is created.
Al the thorium at the surface is then vaporized, so you need to wait for a while at a lower temperature to see the emission getting better.

This activated thoriated-tungsten filament can handle H2 (H2 actually reduces more ThO), it can handle some exposure to atmospheric conditions, but it is still very sensitive to the tungsten-water cycle.
But for the short-term this filament should be ok.

Next step would be carburazation, I'm working on that, it needs a H2 + CH4 atmosphere, temperature control etc. is a bit difficult.
Roberto Ferrari
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Re: A question on heated filaments?

Post by Roberto Ferrari »

Jeroen

I carburized successfully just with a mechanical pump pulling benzene vapours (old times...) through the filaments.
Measuring with precision voltage and current you can stop the operation as soon as you get the resistance change.
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Richard Hull
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Re: A question on heated filaments?

Post by Richard Hull »

The tungsten-water cycle is a big deal for would-be tungsten filament makers. I read about it 20 some years ago and took great note of it. Anyone wanting to do work in this area needs to read up on this vicious cycle. Casual vacuum systems are not up to eliminanting the amount of water that must be removed when using tungsten filaments.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Sven Andersson
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Re: A question on heated filaments?

Post by Sven Andersson »

Richard Hull wrote:The tungsten-water cycle is a big deal for would-be tungsten filament makers. I read about it 20 some years ago and took great note of it. Anyone wanting to do work in this area needs to read up on this vicious cycle. Casual vacuum systems are not up to eliminanting the amount of water that must be removed when using tungsten filaments.

Richard Hull
I just wonder; must the water vapor level be lower than in a Fusor? When you operate a Tungsten filament?
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Richard Hull
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Re: A question on heated filaments?

Post by Richard Hull »

Depends on how deep you pump the fusor to begin with and how dry your deuterium is.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Richard Bonomo
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Re: A question on heated filaments?

Post by Richard Bonomo »

Jeroen Vriesman wrote: Thu Sep 01, 2016 10:44 am Dan,

none of the oxide coated filaments survive atmospheric conditions, they react with the water in the air, forming hydroxides.

Usually such a "indirectly heated" oxide-coated filament is stored with a carbonate coating, and then activated in vacuum by heating it, the carbonate releases CO2 and becomes an oxide.

You can use a normal light-bulb tungsten wire for 10 minutes, but you have to make it white-hot to get some emission, and it will vaporize, and there is also the tungsten-water cycle if you have just a little bit of water in your chamber.

Next step is thoriated tungsten wire, I can sell you some, or you can buy it at the electron tube store mentioned in the previous post.

This is tungsten wire with about 2% thorium oxide. To make it work, you need to "activate" the wire.

The idea is to have a monoatomic layer of thorium on the surface, where the vaporization of the thorium is the same as the amount of thorium diffusing to the surface, this happens at about 1800 deg. C. Emission is then very efficient (about 1000 times more than pure tungsten for the same temperature).

But first you need to create some thorium in the filament, this is done by heating it to about 2800 deg. C for about one minute, some of the thorium oxide reduces to pure thorium and some tungsten oxide is created.
Al the thorium at the surface is then vaporized, so you need to wait for a while at a lower temperature to see the emission getting better.

This activated thoriated-tungsten filament can handle H2 (H2 actually reduces more ThO), it can handle some exposure to atmospheric conditions, but it is still very sensitive to the tungsten-water cycle.
But for the short-term this filament should be ok.

Next step would be carburazation, I'm working on that, it needs a H2 + CH4 atmosphere, temperature control etc. is a bit difficult.
Hello! I realize that this was posted some time ago, but I just now found it when I was looking for some information on the activation of thoriated tungsten filaments used as electron emitters.

Most literature I've seen talks about heating the filament to 2800 K ("flashing") for some seconds, rather that 2800 C. Which is correct?

-- Rich
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