Sciencemadness Discussion Board - Need conductor non-corroding in air @ 2800*C

Need conductor non-corroding in air @ 2800*C

Quince - 14-1-2006 at 22:36

I've got some single-crystal magnesia for insulator, but I can't find an appropriate conductor.
Tungsten remains solid, but burns in air. Platinum melts, and plating onto Tungsten probably won't help. What can I use?

FPMAGEL - 15-1-2006 at 02:42

Are the electrode used for heating or are they in a mixture that is that tempture, as maybe a high temp steel cyl(tube) with active cooling liquard pumped through it(say LN2) might keep it intake for a couple of hours.

This might sound weird but old forum member had a site about magtnic cooling, it used a substance(hydrated compound IIRC) that removed heat. They use it to get close to kevin zero. Maybe the current going throught the electrode might make the heat go into the air.

That temp is close to plasma, if a reaction isn't based on a specfic tempture, will you bealbe to raise the tempture up anought 1000C, then a current, and liquard N2 should work(don't quato me)

There is a substance, and now i think about it, it might come in handy,2d.

[Edited on 15-1-2006 by FPMAGEL]

[Edited on 15-1-2006 by FPMAGEL]

[Edited on 15-1-2006 by FPMAGEL]

Quince - 15-1-2006 at 03:05

The point is that the plasma is air plasma. So the electrodes which of course contact the plasma in order to generate it (DC discharge, not RF induced one) could reach that temperature in the discharge emitting areas. So what do I make them out of?

FPMAGEL - 15-1-2006 at 03:52

"http://www.freepatentsonline.com/4999336.html" in this area tungsten (VI) oxide, yeah two days. yeah do you know about ip logging or that cheap iis etc crap, as it might speed up finding time.

[Edited on 15-1-2006 by FPMAGEL]

Quince - 15-1-2006 at 04:52

I don't see how to get a PDF on that site, but here they have it:
http://free.patentfetcher.com/Patent-Fetcher.php?submit=Fetc...

In any case, I don't think I could produce the described material. I doubt anyone's selling it. Also, the procedure describes AVOIDING high temperatures as the material would sinter, which is what the patent says NOT to do. So I don't think this is a high temperature material.

[Edited on 15-1-2006 by Quince]

jpsmith123 - 15-1-2006 at 05:55

I've never heard of any kind of conductor that would hold up at anywhere near that temperature in air.

Unless it only has to last a short time, in which case maybe tantalum or rhenium would last a few minutes until it burns up.

Even your magnesia insulator...would that still be an insulator at that high temperature?

I'm wondering, what are you trying to do that you need something like that?

Quince - 15-1-2006 at 06:06

Well, the area that reaches this temperature is concentrated at a very small surface, so even if the surface is not normally an insulator but is thin enough to maintain breakdown, it would be good. Like maybe a very thin layer of magnesia ceramic, but how to keep it on the surface?

What about conductive ceramics? Any that withstands the temperature?

12AX7 - 15-1-2006 at 06:53

Come to think of it, there used to be a fellow who invented a magnesia lamp. Wrap it with a bit of platinum for heater, once it gets glowing, ions (or electrons) start migrating through the ceramic and it becomes conductive. Add a ballast, and you've got yourself a light bulb that'll operate open in free air.

Tim

Marvin - 15-1-2006 at 08:10

Cheap solutions tend to be specific to an application, so the less you tell us about it, the more expensive our general answers are likley to be.

Thoughts...

If you have a ceramic that will stand the temperature does the electrode really have to stay solid?

Why arnt you using an inert blanket gas so you can use something like carbon?

Could you use something like acetylene/oxygen to generate the temperature instead?

Other more general solitions might be useful if the material conducts electricity, an induction furnace could be built to heat the material with RF power, or a loop of wire embedded in the material could heat it without having to form an arc.

Whatever you do the plasma is going to eat away the electrodes somewhat. Arcs arnt that well behaved.

You could cool the electrodes, restrict the flow of the arc with ceramic, or concentrate it with magnets so the air itself carries the current into the zone the electrodes cannot work in.

I have the feeling whatever you are trying to do there is probably a better way of doing it that doesnt require conductor or insulator to operate at those temperatures.

jpsmith123, Smart question. I wouldn't place money on anything insulating well under those conditions.

12AX7 - 15-1-2006 at 10:55

As I recall, Quince has been occupying the last approx. six months (longer?) of his hobby life attempting to build a cool, variable, atmospheric plasma (what's the term, MHCD, micro-hollow cathode discharge?). Glow discharge, not arc. The intensity is varied at audio frequencies, thus producing acoustic energy. See also plasma tweeter.

Come to think of it, I wonder if anything on the periodic table besides vacuum (which isn't on the table anyway

) is insulating at that temperature. A quick check shows MgO has a bandgap of 7.8eV. At 2500K, there will be about 2 electrons out of every 10^16 in the conduction band. That's more than silicon at room temperature, and I'd be willing to bet impurities will drop that a lot, since MgO isn't commonly available as pure as undoped silicon (although your MgO may happen to be for semiconductor puposes, in which case purity might be pretty good, though that doesn't say anything of conductivity at 2500K).

If you need a restriction, I like the idea of a magnetic field. It may need to be intense though, and you'll have to finagle the field past the cathodes so the current can find its way to the anode for one!

Tim

[Edited on 1-15-2006 by 12AX7]

Quince - 15-1-2006 at 11:08

Actually, I've been occupying my months working on my thesis, occasionally finding time to play around with the plasma setup. I'm using sapphire right now, and I didn't notice any problem, but I wanted to go to higher temperature materials because there is still a tad of erosion inside the microhollow after some use. The magnesia I'm certain is very pure, as it's single crystal stuff, not ceramic.

Alternatively I'll try bigger microhollows. I didn't try it yet as I never found larger microhollows used in the literature and assumed the MHCD effect doesn't work. Or maybe turning off MHCD current once the main discharge ignites, though that's supposed to make the latter unstable. In this case, temperature is probably not going to be an issue as most of the heat is not on the conductor but deeper in the plasma, so with good heatsinking of the conductor it should be fine.

I had plated the microhollows of the conductor (tungsten) with platinum. It didn't last at all. I haven't tried the diffusion patent yet, but probably the diffusion would continue at this temperature and the platinum won't stay near the surface protecting the tungsten, and end up in the atmosphere again.

[Edited on 15-1-2006 by Quince]

Oxydro - 15-1-2006 at 11:49

Halfnium is used in plasma torche electrodes, I believe - it must have some good properties for the purpose.

FPMAGEL - 15-1-2006 at 16:20

Oxydro was on the write track with the plasma tourch.
http://en.wikipedia.org/wiki/Tungsten_carbide melting piont 2870 °C its a yellow color, $40 for the tip of the gun.

[Edited on 16-1-2006 by FPMAGEL]

Quince - 15-1-2006 at 18:42

According to http://www.philacarbide.com/Properties.htm, it only has oxidation resistance to 1000*F. Another reference cites "infiltration brazed tungsten carbide cladding (1,922º F)".
Halfnium melts at 2230*C, which is not bad, but is it oxidation resistant at that temperature?

I'm having trouble finding any info on what's the maximum temperature that conductive ceramics remain oxidation resistant.

What about graphite, it has a high melting point, but what temperature is it oxidation resistant to? Though I guess heat dissipation would be a problem...

[Edited on 16-1-2006 by Quince]

neutrino - 15-1-2006 at 19:41

I doubt you need these special exotic materials for working with these temperatures. With proper cooling, you should be able to keep your electrodes cool enough while still maintaining a plasma. After all, Hf is used in plasma cutters even though it would oxidize and melt at plasma temperatures. Discharge terminals for electrostatic generators, Tesla coil terminals and Van de Graff spheres all run fine as well.

Quince - 15-1-2006 at 20:00

That may be the case. I was able to vaporize a thin tungsten wire in the center of the plasma while using my platinum plated tungsten electrodes, so the discharge area is not the hottest. However, there's definitely erosion after a couple of hours.

Can anyone suggest a way to block UV that would not affect sound at all? There's definitely UV because I was seeing spots after looking away, even though the visible brightness of the plasma is not that much, it's actually translucent.

Rosco Bodine - 15-1-2006 at 21:36

A tantalum substrate coated with an iridium - aluminum alloy might do the trick . At high temperature the iridium
aluminum alloy * might * convert to an iridium - alumina - tantalum fusion layer coating . Just a guess / idea on this , never tried it and never heard of it either .

Quince - 15-1-2006 at 22:07

Sounds similar to this:
http://free.patentfetcher.com/Patent-Fetcher.php?submit=Fetc...

Is tungsten a reasonable substitution for tantalum (I already have it but don't have tantalum in the necessary shape), and magnesium instead of aluminum (the oxide withstands higher temperature)?

The patent references two others for plating the iridium onto the substrate; both use iridium chloride. The biggest problem for the DIYer seems to be depositing the aluminum over the iridium. There's no way a DIYer can do vapor deposition as suggested in the patent.

[Edited on 16-1-2006 by Quince]

FPMAGEL - 16-1-2006 at 01:32

"According to http://www.philacarbide.com/Properties.htm, it only has oxidation resistance to 1000*F. Another reference cites "infiltration brazed tungsten carbide cladding (1,922º F)".
Halfnium melts at 2230*C, which is not bad, but is it oxidation resistant at that temperature?"
It does but, with underground power lines in corrsive earth, a current is aplied to reverse the effects.

"I'm having trouble finding any info on what's the maximum temperature that conductive ceramics remain oxidation resistant."
There are alot of ceramics that can't be oxidised they are already Oxidised, but they aren't conductive(well 80%), but with different designs say TiC with cermics on the out side it might work. Some one before mentioned that at a high tempture some compounds are. 1D

"What about graphite, it has a high melting point, but what temperature is it oxidation resistant to? Though I guess heat dissipation would be a problem..."
I'am guessing hear,but it should be around 2000C, the ring bonds are strong but you are talking plasma, the electrons are striped away, stuffing up orbitly strength and stuff.

"I doubt you need these special exotic materials for working with these temperatures. With proper cooling, you should be able to keep your electrodes cool enough while still maintaining a plasma."
Yep but that will invole a comprosed tank of gas, say argon at say 100atm.

"Can anyone suggest a way to block UV that would not affect sound at all? "
You looking to reenter the atomsphere or something, if not maybe O3 will do the trick.

[Edited on 16-1-2006 by FPMAGEL]

12AX7 - 16-1-2006 at 08:00

Oh, and look up the spelling, it is hafnium, no English "half".

Quote:

Originally posted by FPMAGEL
<Hafnium>
It does but, with underground power lines in corrsive earth, a current is aplied to reverse the effects.

What the? Do you suggest galvanic protection? How the hell are you going to pull that off without an...what the fuck!?

Quote:

I'am guessing hear,but it should be around 2000C, the ring bonds are strong but you are talking plasma, the electrons are striped away, stuffing up orbitly strength and stuff.

Dude. Graphite is listed as melting at 3527°C. I don't even know what you're going on about with "orbitly".

But in any case, he already knows graphite is conductive and high-melting. What he doesn't know is, will it vaporize or oxidize? The answer is yes, it will. I've done plenty of arcing with graphite rods on my welder to know that a rod more than yellow hot will emit a light blue flame (carbon monoxide burning in air). After some time at white heat, you can easily tell the diameter has evaporated away.

Quote:

<electrode cooling>
Yep but that will invole a comprosed tank of gas, say argon at say 100atm.

Well, yeah, if you want to use a standard tank of gas, no shit... in fact it's more like 200atm. But that would be expensive, which is why he wants to avoid any shielding gas.

Which has fuck all to do with cooling the electrodes!

Tim

Twospoons - 16-1-2006 at 13:08

Quote:

Originally posted by Quince

Can anyone suggest a way to block UV that would not affect sound at all? There's definitely UV because I was seeing spots after looking away, even though the visible brightness of the plasma is not that much, it's actually translucent.

Given that I've seen a ref to MHCD used as a highly efficient, short UV source, maybe you should be a little more careful? Like this -> :cool:

A powerful 350nm source would be practically invisible to the human eye, but capable of much damage.

You might get away with a folded horn (or similar), painted 'black' to UV. UV is absorbed, sound bounces out of the horn.

[Edited on 16-1-2006 by Twospoons]

zoomer - 16-1-2006 at 17:17

As I understand it, you only need a very small conductor. What about conducting diamond (i.e. boron-doped [BDCD]) film? Polycrystaline BDCD doesn't sublimate until ~3500C, the only question would be it's reactivity at those conditions, but I'm guessing diamond remains inert even at those temps. Using the hot filament method of CVD you might manage the conductors even in a DIY project.

http://www.nedo.go.jp/english/archives/170117/pdf/H-12Y_E.pd...

Z

FPMAGEL - 16-1-2006 at 18:05

"But in any case, he already knows graphite is conductive and high-melting. What he doesn't know is, will it vaporize or oxidize? The answer is yes, it will. I've done plenty of arcing with graphite rods on my welder to know that a rod more than yellow hot will emit a light blue flame (carbon monoxide burning in air). After some time at white heat, you can easily tell the diameter has evaporated away."
You know that graphite forms layers of carbon in a ring(I think like benzene), and its a strong bond, so MY theory is that the high tempture cause those bonds to break, which causes the carbon to oxidize. If you pump electricty through it, the carbon will be Negitively charged so the oxygen want be attracted to it.

"Do you suggest galvanic protection? "
Yes, you don't need much a micro wind turbine does the trick.
I know this piss people of , but google.com

"Which has fuck all to do with cooling the electrodes!" argon is enort, the plasma torch uses comprosed air to cool the TiC. But the tank has 10,000 psi, that would help more than say 1000psi(haven't got the fomula infrount of me), ie drawing away the heat from the electrode. No wondering you don't understand my posts, sorry about that.....

[Edited on 17-1-2006 by FPMAGEL]

[Edited on 17-1-2006 by FPMAGEL]

lordmagnus - 16-1-2006 at 23:57

Hummm,

Maybe tantalum, rhenium, or a high order of tungsten. I'd get into the periodic table, and start googleing the various rare metals for their melting points, Maybe OSMIUM?

Quince - 17-1-2006 at 01:06

lordmagnus, these oxidize in air at this heat.

FPMAGEL, I don't understand what a micro wind turbine has to do with galvanic protection.

FPMAGEL - 17-1-2006 at 21:15

"FPMAGEL, I don't understand what a micro wind turbine has to do with galvanic protection. "
You only need about 200watts, for a coulpe of Ks(they didn't mention the pipe size), to protect it.

In general, a current is sent through a water/gas pipe, this stops the pipe reacting with the surounding dirt, this related to thread by the fact it might stop oyxgen reacting with the graphite.

[Edited on 18-1-2006 by FPMAGEL]

Quince - 17-1-2006 at 22:01

Well, as these are the electrodes, the plasma current is already flowing through them. So would they be protected when serving as the cathodes, or anodes? In either case, one side still can't be protected.

FPMAGEL - 18-1-2006 at 00:54

AC current and high hertz should protect both. I don't know about what type you would need, high voltage or amps or both

What are you making?

[Edited on 18-1-2006 by FPMAGEL]

neutrino - 18-1-2006 at 14:52

Would the positive and negative cycles of the AC have that net effect in this case? I know that AC in HCl solution will dissolve most metals including Pt. I doubt this is too different.

Quince - 18-1-2006 at 15:21

There's always DC bias in my case that exceeds the AC, so there's never a zero crossing. I see no way to implement galvanic protection; the answer's in materials and geometry I guess. I'll try using larger microhollows, but I have to go to a jeweller as all my diamond bits have worn out from drilling the sapphire (I was told to drill under water or the heat unbonds the diamond particles, but too late). Also, I don't know if larger microhollows will work at all. All I've seen in the literature are sub-millimeter, and mine are already about a millimeter.