Homebrew cold plasma torch

[dmendesf]

For some really tight seals (when epoxy needs to glue to the skin of PTFE wires) either a chemical treatment with acids or a cold plasma torch is used to make the PTFE porous and don't let the pressure escape between the epoxi and the PTFE cables. So, questions:
What's the difference between a cold and hot plasma?
From some homebrew projetcs I've seen a high voltage source (how many volts? DC or.AC? No idea) and some gas.(sometimes a specific gas like H2, sometimes just air) flowing out of a "pen type" structure made of glass.
 None of the projects I found was very detailed (I have a lot of experience with.electronics but not chemistry, much less with plasma). Can someone point good references about this topic? I know you can buy these cold plasma  pens already assembled but they're in the kilodollar range (too expensive for homebrew applications) and I wold learn nothing.


Thanks

Daniel

[LaserSteve]

Cold plasma as used for surface treatment is starved for current and runs at, near, or greater pressures then atmospheric.  Usually has a very high excitation field to sustain it and is confined to a gas jet that ionizes easier than nitrogen. I've used Argon, Oxygen, Helium, and Krypton.  At such high pressures the mean free path of ions is very low, but for excited molecules stripped of one atom it can be quite long.

In my case I used a 15 KV 10 mA DC power supply with a pulsed high frequency AC riding on top of it from an oddball photocopier power supply that I had laying around at home.  I worked at a university at the time. (Drum charge neutralizer power supply)

I've used both transverse excitation where the easily ionized  gas is blown though an arc discharge and looks like a stretched one dimensional candle flame, and the glass capillary method.
Transverse eats gas but is easier.  Basically you set up a stable arc in air and blow the gas through it in a confined jet.  The arc stretches into the gas flow.

The capillary version is much easier to control, but requires much R&D.   There is a photo somewhere of my index finger in a 5 mA argon arc, where I had good isolation of the PSU ensuring that all the current returned to the electrodes, so I didn't get shocked and barely charged up, wearing insulated shoes/standing on a mat  of course.   No damage at all as humans have a thick layer of dead skin, that I was very, very, very slowly decomposing.  Not recommended, but I worked in a lab using large amounts of high voltage DC and that plasma was very tame by my standards. During one of the demos a terrified chemist tackled me thinking I was getting shocked, even though I was perfectly safe.  That hurt.  The plasma can be that gentle, but you really have to know what you're doing and run tests.

Not easy.  Beautiful and has wonderful results in the right situation, but not easy.

You can do this with just oxygen or nitrogen, but having even a tiny percentage of an easily ionized gas in the mixture helps tremendously.

A better name is "athermal plasma".

find:

Review
Flexible Cold Atmospheric Plasma Jet Sources
Carles Corbella *, Sabine Portal and Michael Keidar

See this video for a very tame version without much plasma length.



The process is best described as "Fiddly" and situation dependent.  The situation being what you have laying around that may or may not work as an exciter.  Also eats rare gas tank contents like crazy if you're not careful.

I don't recommend feeding this process with low frequency long arcs at 50-60 hz from devices like neon transformers.  For various reasons that is just NOT safe at all.

Very much have to be an adult and evaluate your setup's safety.

We were flipping surfaces of a nanofiber coated plastic heart catheter to hydrophobic without damaging the nanofibers.  Physicists and chemists were coming from all over campus to see it run.

I later had a version at another university that impinged a small plasma spot onto an optically flat water jet.   

Steve

[Alexander_Pashenko]

Steve, it may be a bit off topic, but you imply that plasma surface treatment would not work at low pressure (10-100 mTorr)? Is that really so? And, I always thought that cold plasma = no current by ions, only by electrons. So, DC sources and DC currents are prohibited. Only RF excitation and electrodes that are isolated by dielectric to fully block DC, but still let high-frequency RF to go through (like metal-glass-plasma-glass-metal). Because two even slightly asymmetrical electrodes could somewhat rectify RF current and produce DC current in plasma. Or maybe that's more relevant to low pressure plasma and not for normal pressure cold plasma.

[LaserSteve]

Plasma treatments work fine at low pressure!

From spectroscopy in air, in an argon seeded Cold Plasma near STP, we found:

Some N2+, O2+, O3, O-, OH-, N-, NO2 and NO3, H2O3+, OH, etc...

Yes, the electrons have a longer free path, forgive me if I mis-spoke, that was 15 years ago.

If water, Co, Co2 or hydrocarbons are present, it gets complex fast. 

There are other effects as well, the discharge is rich in far IR, Mid IR, and a 750-950 nm band. some 350-420 nm UV, and the 600 nm nitrogen orange band.  Very little visible light compared to the IR and  UV.

Steve

[LaserSteve]

I should also mention that commercial grade, flyback-based, Ozone generator power supplies work well for this method. They generally can take abuse a lab grade unit cannot.

Steve

[Phil1977]

You may be inspired by commercial systems like this:

https://www.relyon-plasma.com/plasmabrush-pb3/

They work with inert gases as with standard pressurized air. I think it may be possible to DIY such a thing with a quartz tube, and copper rod and copper foil as electrodes. A power supply from an ozone generator sounds like a good idea.

[Marco]

How does the plume of reactive oxygen species relate in size to the visible light emitting plume? The light show is nice, but doesn't add much functionality.

[dmendesf]

Thank you all for your insights. I´m going for the simplest route as this is a homebrew project with a testable objective in ming> make PTFE coating porous to seal wires with epoxi in high pressure vessels.

[LaserSteve]

Marco,

 The pretty light show is an excellent diagnostic.  Actual effects can extend beyond the visible plasma, especially if you have a large electrode array made using semiconductor style techniques.  Probably at least a centimeter from the plasma tip in the video.  A good, optimized, system will emit very little visible light. Most non-academics can't afford the non-contact diagnostics, so the "light show" is a bonus.

He chose helium in the video because it looks better than the dull plasmas of Argon or Oxygen on a color camera.  If there was no IR blocking filter on the camera you might have seen quite a bit more plasma length. In air, the emissions are mainly in the UV and IR.   After all that's clickbait, not optimized. and made from over-the-counter parts.  Still a good video for a beginner to watch.

If you want a good example of this look at a propane plumbing torch on a thermal imager, the plume stays coherent and active for almost half a meter from the visible blue flame.

Personal opinion. Modern cell phone cameras with the fancy UV and IR blocking filters suck for plasma science and Near Ir such as lasers and Leds.. End Personal Opinion.

Steve

[Phil1977]

If you need more diagnostics you can also just treat clear adhesive tape with the plasma and check if a sharpy draws a solid line or little droplets of ink are forming. Seen this test a lot of times at professional stations.

[LaserSteve]

Found this...

https://www.mdpi.com/2079-6412/15/6/644

Let us know how this works please, Teflon is a favorite material of mine, and the ability to fully  bond Hysol 1C White  (Torr Seal precursor, minus an additive or two} to it would be awesome for vacuum work.

https://www.microtonano.com/Loctite-Hysol-1C-Torr-Seal-High-Vacuum-Epoxy-Resin-Sealant.php

EA 1C sticks to it fairly well to begin with if contained in a hole in the Teflon.

Steve