Decorative plasma display tubes / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Decorative plasma display tubes

first previous 2 3 4 5 next last

Move Thread

LAN_403

Carl A. Willis

Thu Feb 14 2013, 03:33PM

Registered Member #9640 Joined: Wed Jan 16 2013, 07:53AM
Location: Albuquerque, New Mexico
Posts: 26

zrg wrote ...

Are you sure that green color of Xe is produced because of air traces? None of my experiments proved that; however I got intense green color of different tints when mixed xenon with organic compounds. Did you wash the flasks before evacuating and did you pre-heat them under vacuum?
How do you exactly add air to the greenish xenon flasks? Can it come through the vacuum grease often used to cover o-rings and similar viton/rubber items in the system?

At this point I am very certain that air is responsible for the green color (and have strong evidence that nitrogen specifically is responsible).

My cleaning procedure involves washing with sulfuric acid and hydrogen peroxide, followed by DI water, followed by alcohol. The glass is dried at 150 C immediately before connection to the pumps, and then the tubes are exhausted to a non-striking vacuum with gentle heating from a Meker burner.

I add air via a pair of valves (needle valve backed by a quarter-turn shutoff valve).

I discovered the importance of air to this effect when I found a significant leak in the metal seal on a valve that I have since replaced. The green disappeared after the replacement. Adding air in a controlled manner to <1 torr returned the green color. At that point, I began an effort to tie the color to a particular component of air. Pure oxygen, argon, CO2, water, butane, and perchloroethylene are among the things I have added in this attempt, and none of them produces the emerald-green color (although perchloroethylene produces a greenish-yellow color). I do not have pure nitrogen in a cylinder yet, but one has been ordered and should be here next Tuesday.

I do use a very small amount of Dow Corning High Vacuum Grease on the tubulation adapter's o-ring. But it's not a convincing explanation for the green color since it is always there and the green color is definitively linked to the act of opening a valve and letting air enter the system.

In your own experiments on YouTube, you show a vial of xenon with some air at low pressure that you have cooled with liquid nitrogen and then hold near a Tesla coil. The xenon condenses in the vial, as does the air. As the vial warms, you hold it near a Tesla coil. At the low pressures, early in the evaporation of the contents, a brilliant green color is emitted. I suggest that this is due to nitrogen and xenon evaporating into the discharge together.

When I have my pure nitrogen tank, I will do some more work and report back. Air is a complicated mixture, and while I am reasonably certain that I have narrowed down the culprit to nitrogen, there may still be surprises.

Best regards,
Carl

klugesmith

Thu Feb 14 2013, 08:28PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Carl and zrg, I have a question about baking out vessels under vacuum, before filling them. The step that's traditionally done with a bombarding transformer, on tubes that have an electrode at each end. Is that something you use the kiln for? Or does aggressive chemical cleaning allow you to skip it?

zrg

Thu Feb 14 2013, 11:08PM

Registered Member #4762 Joined: Sun May 06 2012, 05:59PM
Location: Russia
Posts: 93

klugesmith wrote ...

Carl and zrg, I have a question about baking out vessels under vacuum, before filling them. The step that's traditionally done with a bombarding transformer, on tubes that have an electrode at each end. Is that something you use the kiln for? Or does aggressive chemical cleaning allow you to skip it?

For most of the plasmaflasks (Xe, Ar, Kr ones) any baking or cleaning is not required at all, as gas pressures are much greater than any leftovers pressures, which therefore do not affect the visual qualities of the plasma. However it won't harm, surely. For neon at lower pressures (and other neon sign mixes like K4 mix, or "electric blue" Ar-Hg mix) baking is always required to evacuate the gases and pollutions from the glass. The easiest way, if you don't have a bombarder or a kiln, is just to heat the outer surface of the glass with vacuum inside, manually with a properly adjusted blowtorch. That requires some caution to not to soften it, but it works reliably after getting some skill in it.

BTW, here's an overview of my, ehm, vacuum station. Beware of the extreme trash levels :)

The first one shows a piece of fore pump, a diff and "high vacuum" part (ending with a glass tube near HFSSTC to measure vacuum levels). It achieved vacuum levels up to 1*10^-5, as I measured later with a proper vacuum gauge which shows Torr with digits, not the discharge color :)

Second photo shows the process of Xe-I flask filling with a nitrogen trap to protect the diff from iodine vapors, HFSSTC as a vacuum gauge, the flask, the needle valve between Xe tank and flask vacuum line, and some other items. The designations make it hard to see the pieces, but I've lost the original photo, and this one describes the overall process better that any other shots I made.

I'm rebuiding the system now with some better equipment (Alcatel 2010 forepump, MKS MicroPirani 925C as vacuum gauge, etc), but all my flasks shown anywhere on the internet were made on this very system shown (or it's previous, more primitive editions. The very first one included neon maker's glass diff pump, the one with a series of glass "mushrooms", you know).

macona

Fri Feb 15 2013, 01:13AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Other than baking out the bombarder transformer does one other thing, it heats up and activates the getter materials in the electrodes. The getter materials attach to the reactive elements still in the tubes like oxygen and nitrogen that would reduce the tube's life. There are slip on inductive heaters that are used now to activate the getters without using a massive bombarder transformer.

Adding any energy to a vacuum will help knock the water molecules off the walls, that is the primary purpose of baking. You can do this several ways, the simplest is heat. I have these flexible silicone strip heaters that you can wrap around stuff to get them hot. On my mass spec system I will bake it at around 300F for a day or so at full vacuum. It is all stainless and conflat so it can handle the temps. You can also use IR lamps.

Another way is with UV light. But this means you need to have the lamp internally or the material needs to transmit the UV. You can use the fluorescent lamps intended for anti-bbacterial use for this.

Another method is to evacuate and back fill with argon and apply HF at low pressures to ionize it. It will burn off anything organic inside the chamber and knock loose those water molecules. Argon pumps fast so it is quick to evacuate for the final fill.

Carl A. Willis

Fri Feb 15 2013, 04:25AM

Registered Member #9640 Joined: Wed Jan 16 2013, 07:53AM
Location: Albuquerque, New Mexico
Posts: 26

Zrg, you have quite a capable high vacuum system. Your workbench looks a lot like mine when I am really having fun.

I agree that there seems to be no compelling reason to "bombard" these kinds of things in the sense of the process used with neon tubing. Neon electrodes contain some mixture of alkaline-earth carbonates that are activated as chemisorption media when they are bombarded, and for those experimenters who use electrodes, they probably should be heated under vacuum just because of the coating. If it's not calcined, the coating is probably a sponge loaded with water vapor and CO2. I have used some borosilicate neon electrodes on a few occasions (one is depicted in my YouTube channel). Sometimes they can be heated up by bringing the surrounding glass toward its annealing temperature. On a few occasions I have used a makeshift induction heater. But my procedure for the electrodeless volumes is just to warm the glass, in essence, and exhaust to a non-striking vacuum. Pumpout is usually complete in about 10 minutes.

-Carl

Conundrum

Sun Feb 17 2013, 08:15AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Re. UV light.
Perhaps a NUV laser such as one from a Bluray burner would be OK for this?
would allow selective "etching" of the getter as long as you have good OD6+ safety goggles.

macona

Mon Feb 18 2013, 01:41AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Conundrum wrote ...

Re. UV light.
Perhaps a NUV laser such as one from a Bluray burner would be OK for this?
would allow selective "etching" of the getter as long as you have good OD6+ safety goggles.

Not really, you need light to hit all surfaces, with a laser you would be there forever. And power wise you would be better off with a fluorescent or tungsten lamp in the chamber. But for something like these display tubes just ionizing them or heating them is the way to go.

Carl A. Willis

Wed Feb 27 2013, 08:02AM

Registered Member #9640 Joined: Wed Jan 16 2013, 07:53AM
Location: Albuquerque, New Mexico
Posts: 26

I've finally tracked down the source of the green plasma color in xenon-air mixtures. It's a phenomenon that is particular to the presence of nitrogen, and is intensified by using pure nitrogen rather than air. No other purified components of air (oxygen, argon, CO2, water, or hydrocarbons) produce this emerald-green color; nitrogen is both a necessary and sufficient additive to xenon to produce the color. Interestingly, N2 does NOT produce this color in combination with Kr or Ar.

I upgraded my setup to allow better experiments to be done and to track down the best conditions for the emerald-green Xe-N2 discharge. One upgrade was the addition of a second Baratron capacitance manometer. Now I have a high range unit (1000 torr) and a low-range unit (1.000 torr) so I can record pressures accurately over these wide ranges. I also obtained purified nitrogen in a cylinder, as I mentioned in earlier plans. My experiments so far have examined the influence of nitrogen between partial pressures of about 0.100 torr and 3-4 torr, and Xe between about 50 and 100 torr. Low N2 pressures produce a hint of green while the structure of the discharge remains that of pure xenon. At about 1 torr, the green color is intense and the discharge form (a whitish filament) still is dominated by xenon influences. At 3-4 torr, however, the filament becomes more diffuse and takes on a pinkish color, which I find objectionable to the aesthetics. If I had to recommend "best" conditions for this Xe-N2 mixture, it would be in the range of 1.0 torr N2 and 50-70 torr Xe. However, beautiful diffuse glows are possible with Xe around 20-30 torr and N2 around 0.5-1 torr.

My YouTube video showing the 100-torr Xe-1.00 torr N2 mixture may be seen here.

A final observation is that the Xe-N2 mix produces an "afterglow" following removal of the energizing current. Nitrogen is well-known for the Rayleigh afterglow in N2-O2 mixtures, which is a sort of yellowish color. The afterglow in Xe-N2 has a shorter duration and lower intensity, and has the same green color as the plasma.

On the basis of all of the above observations, I suggest that the green color is due to excited states of the nitrogen molecule (or molecular ions of nitrogen), and that the electric discharge in xenon efficiently produces light quanta or otherwise acts as a driver for the formation of these states. A careful visible spectral analysis could in principle identify the relevant nitrogen species and states.

-Carl

Steve Conner

Wed Feb 27 2013, 11:32AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Thanks for the info Carl, I think that explains this effect that I saw with my old plasma tube. It wouldn't light any more from the built-in driver, so I started abusing it with a Tesla coil, and got discharges that were half pink and half green.

Plasma4

Carl A. Willis

Wed Feb 27 2013, 03:37PM

Registered Member #9640 Joined: Wed Jan 16 2013, 07:53AM
Location: Albuquerque, New Mexico
Posts: 26

Hi Steve,

That's a pretty plasma tube. It looks to have a pretty high partial pressure of neon in it too. Do you still have that tube and does it still behave the same way?

-Carl

first previous 2 3 4 5 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob