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Author: Subject: Electrode material that won't oxidize?
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[*] posted on 1-5-2005 at 10:32


Quote:

Typical applications include furnace components, chemical and food processing, nuclear engineering and sparking electrodes.


Interesting. If you could get the inconel to protect your copper...




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[*] posted on 1-5-2005 at 23:42


These conductive ceramics 12AX7 mentioned, how do you bond them to metal? Can they be made at home?



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[*] posted on 2-5-2005 at 05:37


can they be made at home? and can they be fused to metal (Cu esp).

I say yes. But even I would have difficulty in doing so, even if I used all the resources at my job. (which could be replicated for home use...)

How about a mechanical fastening?

Other note on semi-conductive ceramics or semi-conductors in general. In my experiences, it is difficult to establish plasma from a semi-conductor at any distance. Ussualy you have to strike the electrodes on eachother, or make the semi-conductor molten 1st. (not all semics are the same of course though)

melting Si is a perfect example. its your basic semic, it doesnt conduct much at room temp. It esp. wont establish plasma. However when arc melting it, a pull rod may be used(a copper pole that will accept plasma, that is positioned such that the plasma may be driven over the materials intended to except the plasma. like creating an imposed plasma. The small amounts of heat and radiation eventualy get the surface of the poor thermal conductive materials near its plastic phase and then a plasma can be established.)
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[*] posted on 4-5-2005 at 00:16


The talk of desktop fusion using lithium tantalate set me thinking. Ta is stable at high temps and forms a non-stoichiometric oxide TaO (2 and a bit)which is a conductor. It might do the job.
Unfortunately, it's not cheap.
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[*] posted on 5-5-2005 at 18:35


Would gold plated onto copper work? I know that both melt below the target temp, but with creative heat management you might get away with it. You have to flash the copper with nickel first, or the gold will migrate into the copper with use, but I don't think that would interfere with your objective. Au over Cu would be a lot cheaper than Pt over titanium, and a lot easier. Both of those are difficult in plating apps.

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[*] posted on 5-5-2005 at 18:54


Now that I think of it, you can probably plate a layer of silver or copper (could even make it thick so it conducts heat better :) ) onto a semiconductor.

Tantalum oxide conductivity sounds nice, but nonetheless, Ta is listed as 480C max in air. :(

Tim
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[*] posted on 5-5-2005 at 21:59


Tungsten oxidizes above 400*C, but it would provide a good substrate for plating as it won't melt. Now just to figure out how to platinum-plate it.



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[*] posted on 6-6-2005 at 22:55


Well, I've got tungsten of a good thickness, 0.8 mm, but how the hell do I cut and drill this? Carbide bits don't last very long (and when trying to drill the sapphire I'm using as insulator, they barely scratched it). How do I handle this? The tungsten also breaks easily. What a crappy metal.

[Edited on 7-6-2005 by Quince]




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[*] posted on 7-6-2005 at 00:30


Try heating it above the brittle-ductile transition temperature. One website lists it as 450-900°F so a torch will do well.

Tim
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[*] posted on 7-6-2005 at 00:42


Great idea!

What about the sapphire? I got some diamond-coated drill bits for the holes, but I don't know how to cut it. Chemical etching would be too slow.

BTW, looking at this mutual characteristics graph for the 4X150A:
http://www.webace.com.au/~electron/tubes/FIG23.jpg
Now, I'm guessing for lower Va the curves would be similar. And it looks like it's only linear for Vg1 > 0...
What ways to linearize? Modulating Vg2 as in ultralinear wouldn't work because it takes the tube away from the current sink mode I intend. Maybe the diode tube predistortion as in the long lost linear gain stage thread at diyaudio can work here? Can Hawksford EC be applied to a transconductance output stage?

[Edited on 7-6-2005 by Quince]




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[*] posted on 7-6-2005 at 05:31


Just do plain old SE...

If you still refuse to get that Al2O3 ultrasonic drilled, just etch the damn thing.

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[*] posted on 7-6-2005 at 15:17


Well it is single ended, in that there's a single tube driving the output. But the load is in series with the tube, not parallel to it. This current drive is needed because of the nature of the plasma as a load (constant voltage or possibly negative resistance), and for safety (if the plasma turns to an arc, that's like a short circuit and things will fry, but current drive prevents that).

I didn't find any local jeweller with the ultrasonic thingamajig to cut the crystal.




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[*] posted on 7-6-2005 at 21:20


Quote:
Originally posted by Quince
Well it is single ended, in that there's a single tube driving the output. But the load is in series with the tube, not parallel to it.


And this is different from a resistively loaded SE stage how?

If you must, add NFB, CC or CV, whichever works best.

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[*] posted on 7-6-2005 at 21:46


In SE the load is in parallel with the output stage-ground connection. Here the load is between the B+ rail and output stage, so the output stage acts as pass device to ground.

In voltage output class A, the output device is usually loaded by a constant current source. Here that doesn't make sense, since it's current output, so it needs a constant voltage source. So the B+ must be very well regulated, given that it goes in one side of the load. It's gotta be very low output impedance, the way ground is in the voltage output case. One thing I'm not clear is if it might be better NOT to have a capacitor after the regulator. I remember for example one discussion at tubecad where Broskie noticed an improvement after removing the post regulator capacitance in a particular application. Hmm.

[Edited on 8-6-2005 by Quince]




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[*] posted on 7-6-2005 at 22:23


Regarding platinum plating tungsten:

I read that after dissolving in aqua regia, repeated evaporation and addition of more HCL will result in most of the HNO3 evaporating, leaving chloroplatinic acid, which can be used for plating.

I came across this patent:
http://free.patentfetcher.com/Patent-Fetcher.php?submit=Fetc...
This patent is for plating tantalum, but I'm thinking it might work here. Instead of purely plating, a heating step is involved which diffuses platinum into the surface of the metal being plated, forming an abrasion resistant layer. What do you guys think? How would I modify the procedure for tungsten (or would it work at all)? What can I substitute for the lead acetate in his plating solution (I don't have any source of that material)? One thing I notice is that the heating step is done in a vacuum. Would air be a problem? The only thing I have that can reach that temperature is a propane torch...

[Edited on 8-6-2005 by Quince]

[Edited on 8-6-2005 by Quince]




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[*] posted on 8-6-2005 at 05:29


A local lapidary club could help with cutting the sapphire, probably sooner and much cheaper than most jewelers.

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[*] posted on 11-6-2005 at 06:44


The tungsten only stable in vacuum ;) but resistant against temperatures.


[Edited on 11-6-2005 by Madandcrazy]
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[*] posted on 30-7-2006 at 18:49


Hey, I managed to machine the tungsten and sapphire. It's a lot of work, but if you use diamond bits under water it's doable.



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[*] posted on 30-7-2006 at 20:21


Answers to what you seek could likely be found at this

http://ecsmeet2.peerx-press.org/jsp/mas/reportTechProg.jsp?M...

where I noticed a paper to be presented there, that is on the very material I would have suggested

The oxidation of electroformed iridium and iridium-rhodium alloys in stagnant air at 1300 and 1400ºC is investigated and described.
http://ecsmeet2.peerx-press.org/ms_files/ecsmeet2/2006/04/27...


WSi2 an MoSi2 could possibly work, they could be formed as surface layers on the base metal.

Any chance you could use N2 instead of air?
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[*] posted on 30-7-2006 at 20:23


No, the plasma has to be open to the air without any barrier.



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[*] posted on 30-7-2006 at 23:48


Makes it real tugh then There are various metal + Si/B/N compounds that are conductive enough, but won't take an oxidising environment at that temperature. There are oxides that are conductive enough, but not when they are cold. And the size restrictions make it difficult to cool the electrodes enough that you could get away with exotics just at the active region (it's fairly common to cool electrodes enought that more common materials con be used, with just a limit amount of exotics at the areas really stressed).

Tungsten base with a coating of MoSi2 and/or WSi2, or platinum or platinum/iridium or similar alloys are pretty much it. The tungsten will need the coating else it will oxidise away in a few hours.

There are things that might work, except that I suspect the size restrictions might prevent using the. Nickel oxide with a few atom-percent of lithium is a semiconductor; I've made it with a room temperature conductivity around that of Nichrome, just by firing a mix of nickel and lithium carbonates to cone 12 (which well below your 1600 C), A mix of aluminum and chromium oxides, with a few to 30 atom-percent of Cr, are also conductive. But making and fabbing those to your size needs might be difficult.

[Edited on 31-7-2006 by not_important]
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[*] posted on 30-7-2006 at 23:55


My plan remains the platinum/plating+bake to diffuse method from the patent.

Though I'm using tungsten as the substrate instead of tantalum, so I'm not sure if it will work as well.

[Edited on 31-7-2006 by Quince]




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[*] posted on 31-7-2006 at 08:39


How well it will work will depend on several things. The first is how good is your vacuum, as the cooking starts as platinum black which will give no protection until diffused into a uniform layer. If there's much O2 around it will start oxidising the tungsten long before you reach a temperature where the platinum starts to diffuse. You might be able to get away with an argon atmosphere.

Another issue regards the differences in tantalum and tungsten alloys with platinum, something I can't answer. If the thermal expansion of the alloy is greatly different than the base metal, adhesion problems may arise.

Ta2O5 tends to stay put, losing oxygen around 1500 C to give a lower oxide, while WO3 tends to sublimate which could lead to surface erosion. The Ta-Pt surface may end up as Ta2O5-Pt, protecting the underlaying metal, while W-Pt would turn into a network of Pt as the W eroded away until the Pt detachs.

You might be able to get around potential problems by repeateding the plate and diffuse cycle, starting with a very long diffusion time and shortening it in each following cycle. The idea is to get a graded concentration of Pt, hopefully avoiding too great a differences in thermal properties, and ending up with a nearly 100% Pt surface.
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[*] posted on 10-2-2007 at 19:41


I'm not too happy with the platinum plating, in that it's not very hard (though the baking process wasn't carried out optimally due to limitations in my equipment). I'm considering rhodium plating (and a bonus is that rhodium can take 200 degrees more heat), but since rhodium is over twice the price of platinum, I don't want to do it if it won't work well. I've come across the following:
Quote:
Except in extremely clean conditions and then only with very special proprietary electrolytes, all rhodium is deposited with cracks. Copper and Silver migrate right through the cracks and show up as brown (copper) or gray to black (silver). Put nickel under the rhodium as a diffusion barrier.

Now, my substrate being tungsten, will this still be a problem? Is a baking procedure as in the patent still useful or even possible here? I'm worried that thermal expansion will make the plating break off from the substrate if there's no diffusion. On the other hand, if continued diffusion takes place during operation, eventually more and more tungsten will migrate and the plating will lose effectiveness as an oxygen barrier.




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