Metallic - Non Metallic Grid?

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Tom McCarthy
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Metallic - Non Metallic Grid?

Post by Tom McCarthy »

From what I've seen it's only metallic grids that have been used for the inner/negative grid. Is there any reason for this other than most metals having better conductivity or something else? Also, sources of metal grids would be helpful as I'm having trouble finding them on the net.

Thanks,
Tom
Anson Tsang
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Re: Metallic - Non Metallic Grid?

Post by Anson Tsang »

Use shapeways for the inner cathode , you just have to draw a cad file, send it to them and they will build it to you at a reasonable price. Alternatively, just build your own with metal wires.
Anson Tsang, 13years old Elsa High School student in HK
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Re: Metallic - Non Metallic Grid?

Post by Tom McCarthy »

Sound, I had seen you talking about getting one from Shapeways before. Just didn't know you could request.

Cheers!
Tom
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

All grids must be 100% metal. Tungsten is number one with Tantalum, Hafnium and Molybdenum being OK, too. Many use Stainless steel in demo fusors, but they melt real easy if not carefully contolled during powered runs in a real fusor.

What would have led anyone to imagine a non-metallic grid would work at all?

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Metallic - Non Metallic Grid?

Post by Tom McCarthy »

I thought that the...Effectiveness of a grid was based on its electrical conductivity or should I say thermal conductivity? From there, that lead me to think that silver would be best followed by copper, aluminium etc.

Dad has a TIG welder so would that be better over a MIG? For welding a Tungsten grid if I was to get one. Also, what are the parameters or attributes that make one material better than the other for the grid? I've seen very little mention of this in the FAQs that I've read.

Tom
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

The worst, most resistive and least conductive of all pure metals would perform no better or worse as a grid that the finest highest electrical and thermally conducting pure metal grid.

Needed and demanded

1. Pure metal grid
2. Pure metal grid that can take some intense heating and not melt
3. Electrical and thermal conductivity of the pure metal used is a non-issue and is never considered in any design criteria.
4. Only the melting point of the pure metal is a key issue and the weak point in any metal grid.

Pure silver or gold are among the worst choices for grid material

Tungsten #1 choice. Molybdenum and Tantalum tied for #2, Hafnium #3 and maybe Titanium #4 as suitable for a real fusor.

Richard Hull
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Chris Bradley
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Re: Metallic - Non Metallic Grid?

Post by Chris Bradley »

(I'm glad Richard tried to handle this question!!! 'Where do you start!?')
Tom McCarthy wrote:I thought that the...Effectiveness of a grid was based on its electrical conductivity
It might sound, from this response, that your original question might, rather, turn on what you think/thought 'metallic' meant?

(In point of fact, there are non-metallic conductors. Confusingly, there are also a few rare examples of metallic 'non-metals'. But this is a different topic!!)

The grid needs to resist thermal loads and, optimally, will need to survive ion bombardment which usually leads to embrittlement and an accelerating propensity for the grid to snap due to residual or thermal cycling stresses.

Fact is, nothing much, in thin wire form, survives ion bombardment that well and you should have a plan as to how to 'replace', rather than depend on survival of, the grid. That being said, as far as I read it there is scant empirical evidence of one grid material being 'better' than another, so 'built to last' seems to be just about the only worthwhile metric. In this regard, tungsten is the material of choice for incandescent bulbs, so that's why it is generally considered one to go for. But a good stainless steel serves a purposive function, is cheap and is more ductile than tungsten which you might well find is such a stiff material that it compromises being able to make the right shape (and it also means that it is more readily prone to springing apart once any welds fail, or bends 'un-bend').
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Re: Metallic - Non Metallic Grid?

Post by Tom McCarthy »

Between the two answers, I think that clears up the question. From the information ye've provided me with I gather there's only one thing to consider with the metal used for the grid - Melting point. Would i be correct in saying that or should "ion bombardment resistance" be included as a secondary parameter?

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Tom
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

Ion bombardment and forced electron emission is what causes the heating! Heating and ion bombardment issues are, effectively identical. Just focus on a pure metal grid that resists melting and you are there.

Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by David Swan »

A quick observation / question.

I've done something really daft in buying some 1mm dia tungsten wire. It is 99.99% from Sigma-Aldrich, 1 metre (15g) cost me over £50 (Not so much daft as stupid you'll say, and I'd agree).

Unfortunately it's so brittle that any attempt to bend it to any radius small enough to be useful just snaps it.

So, is it worth going to a smaller dia (0.25mm?) or else how the heck do you anneal tungsten? (Yeah, I know, you can't).

ATM, this is only for a demo fusor so I could get away with the 0.8mm SS welding wire that I have miles of, but I like to do things properly, and you never know I may try to go the whole way.
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

There is a tempered special tungsten wire that is quite flexible, but it has to be specified.

Stainless steel wire is fine for a demo fusor probably 95% or more of the demo fusors use common MIG stainless steel welding wire. I mentioned SS wire was often used in demo fusors in my first reply post above.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Metallic - Non Metallic Grid?

Post by Tom McCarthy »

David Swan wrote:
1mm dia tungsten wire...So, is it worth going to a smaller dia (0.25mm?)...I could get away with the 0.8mm SS welding wire
Forgive me if I'm wrong, but isn't anything 1mm or smaller to thin to weld? That's what Da told me anyways...Though you did say "welding wire" that's not specially treated or something for welding by any chance?

Tom
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

MIG SS welding wire is on spools and is usually pure, bright metal with no flux of any sort. Stick welding rod is almost always flux coated.

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
Julian Kang
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Re: Metallic - Non Metallic Grid?

Post by Julian Kang »

Is spot welding the only way to form an inner grid? I was thinking of getting an inner grid from shapeways or welding one together with tungsten wire and using a home-made spot welder using a tutorial I found on youtube, although I'm not sure whether it will work. http://www.youtube.com/watch?v=vrlvqib94xQ
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Ed Meserve
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Re: Metallic - Non Metallic Grid?

Post by Ed Meserve »

Would .5mm Tungsten/Rhenium be strong enough for an inner grid? It is being sold by the foot on Ebay:

http://www.ebay.com/itm/ONE-FOOT-0-020- ... vi-content
tungsten-rheniumwire.jpg
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Rich Feldman
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Re: Metallic - Non Metallic Grid?

Post by Rich Feldman »

Julian et al,
Fusor grids don't need to be welded. We've seen grids that are formed from a single length of flexible wire (e.g. Meiro spiral ball grid, see Nick's new thread), or wire hoops overlapping without any bond, or tied at the joints with thinner wire. Demo fusors are particularly undemanding. People who report material failures have run fusors at very high powers and/or for very long times.

Experts:
Anyone know how carbon holds up in fusor grid construction? Doug Coulter talked about it a while back.
What I'm thinking of is carbon-carbon composite, e.g in the most intensely heated surfaces of Space Shuttle orbiters.
Not easy or cheap to fabricate at home, but I bet these guys could make a dandy grid. http://www.carboncompositesinc.com/carb ... osites.asp

The electrical resistivity is 35 micro ohm meters. That's around 1000 times higher than metals considered to be "poor" conductors,
but run the numbers. Rods the thickness of mechanical pencil leads can conduct tens of mA with negligible voltage drop or ohmic heating, in a fusor environment.
All models are wrong; some models are useful. -- George Box
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Chris Bradley
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Re: Metallic - Non Metallic Grid?

Post by Chris Bradley »

Depends what you mean by 'composite'. A ceramic matrix would be fine, but engineering composites would likely contain substances not at all suitable to interfacing with a plasma.
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Re: Metallic - Non Metallic Grid?

Post by Julian Kang »

Thanks for the advice Rich, I will still probably spot weld the hoops together then.

Julian
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Richard Hull
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Re: Metallic - Non Metallic Grid?

Post by Richard Hull »

A hand carved or machined hollow graphite sphere made up from a solid block might be an interesting trial balloon. Anyone got the guts, working fusor and know how to do this? Even a crude attempt would be insightful. I would be worried you would get it about done and break it in the final stages.

I still think thick tungsten wire is the way to go though for ease of construction and reliable operation.

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
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