The microwave plasma mystery

If silica has such a great thermal expansion, how come it is used in experiments involving gases passing over a hot bed catalyst (such as, IIRC ethanol vapour being dehydrated by hot alumina powder to ethylene) in a setup where the lower part of the horizontal tube, and only that part, is heated with a Bensen burner? Just curious. Also for NI3, paint the bullseye of a target with it and try playing darts!

Mono Keras

I think other metals could get excited too, but most of them would emit in UV+. I was looking here: https://www.physics.nist.gov/PhysRefData/Handbook/Tables/magnesiumtable3.htm at the lines for neutral magnesium, and all 3s2 transitions to a mix 3s3p state are under 300 nm. The visible lines correspond to transitions between excited states, and that would involve exciting the 3s electron with enough energy for it to leap two levels above then stepping back to its fundamental level piecemeal. I should be able to compute the transition probability with a bit of quantum mechanics, but I'm not sure it would account for a very bright emission anyway.

Mono Keras

You should post this Red Nile on youtube it was a cool Project on Plasma and high voltages. I never thought you would study it with a spectrometer and the chemistry and Physics behind it.

Daniel Blake Shoemaker

Interesting! Maybe it's using a different power supply circuit then. Thanks for checking!

John Hiesey

As for how sensitive NI3 is, you can often set it off just by looking at it angrily.

Silviu T

Silica has quite a large thermal expansion coefficient; if the heating is uneven then some parts of the cuvette expanded significantly more than other parts leading to high internal stresses and cracks.

Silviu T

I went back and checked my home microwave, and it runs at 60hz. However, the plasma pulses at 120hz

Nile Red

Oops! My bad. thanks for responding!

Nik

I actually referenced that video in mine!

Nile Red

I think they arent able to handle temperature change as well and they also had really sharp corners. Something rounded would definitely work a lot better. for the pickle, thats really cool. I actually attached some grapes to my transformer at one point to get a cleaner spectrum from it. It is interesting though, that with most things, its only the potassium and sodium that really gets ionized. Im not sure if its just because alkali metals and their salts have a low ionization energy or something.

Nile Red

hmm interesting. Yeah, Im also not 100% sure about it.

Nile Red

I've seen that strategy of punching a hole in your microwave to film stuff more clearly, but I think an easier and more flexible approach is to film through the mesh, just make sure you're focused on the contents inside and have your aperture as open as possible, so the mesh disappears out of focus. Edit: Oh yeah here's a good video I saw on the subject: https://www.youtube.com/watch?v=8bXhsUs-ohw

Nik

Fair enough for plasma. I tend to use plasma only to refer to what you find in Tokamaks reactors and the like, but you're right in saying that having a plasma does not require high temperature, as long as most of your atoms are ionised. I'm surprised your quartz "cuvettes" blew up. What could have caused this? I also discovered this: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/sodium.html. It seems you can make pickles glow like sodium lamps just by connecting them to the mains! For nitrogen triiodide, it is said it is so sensitive than you can set it off just with alpha particles. Maybe you could have a go? Cheers!

Mono Keras

As far as I know, its a weird cutoff where when a certain amount of it is fully ionized, then its considered plasma. I was stuck in a weird position where if i called it plasma i could be wrong, but if i said it wasnt plasma, i could also be wrong. Ive never heard of that sodium nitrate experiment before I tried using quartz, but all i had were some cuvettes and they just blew up. I also had some tubes, but the plasma seems to work better in larger containers. I was actually thinking about using it as some sort of poor man spec. With just a beaker though, the plasma is a bit too unreliable and tends to start and stop a lot. What's weird is that I couldnt actually find much easy to access info about why the microwave hums at 120hz. Your explanation is likely the answer though. Also, I really like in depth commentary like this, so thank you for taking the time to write all of that up. For nitrogen triioidide, maybe I could make it. I have been wanting to for a while, but I feel like if i do, id want to do something slightly different or creative with it. Right now, tho, i cant think of anything

Nile Red

First off, let's get nitpicky: is it really a plasma? Here what you get are sodium or other atoms thermally excited, but do they get permanently ionised? No. What you get is more or less a sodium lamp, or the same thing you get when you sparkle a flame with salts. It's not free electrons floating around with nuclei, it's "merely" 3p to 3s transition, so that doesn't qualify as "plasma", Ok, I'm pedantic, and I apologise. Frankly, I'm amazed you didn't figure it out sooner :) Do you know about the experiment in which, starting with molten sodium nitrate and a vacuum bulb, you collect highly pure sodium inside the bulb by driving the ions through the heated glass of the bulb? It's interesting to do. (http://www.sciencemadness.org/talk/viewthread.php?tid=148098) Anyways, the spectrometre was a fine idea, but it doesn't show you the absorption bands, and that could be useful too. Having both an emission and an absorption spectrum, using some sort of microprism lens or grating, you could detect the surrounding gas composition? Also did you think about using a quartz vessel instead of glass? Also interesting to note that you didn't get any green light coming from boron ions (I suppose you use borosilicate glass). So that means that while the sodium ions are mobile, the boron atoms are stuck into the silicon dioxide amorphous complex. Finally, potassium ions are bigger than sodium ones, and seem to be unable to migrate through the glass, so exchanging sodium for potassium might not work the way you think. Finally, given that you can get rid of the sodium pollution, this could be a nice way to analyse unknown compounds. Poor man's spectrometer? Finally, 120 Hz. I'm not sure how the magnetron is fed, probably with a roughly generated continuous current. That doesn't matter much, since spectral purity is not really what you're looking for here. Most probably you end up with a microwave carrier AM modulated by the mains 60 Hz signal, which gives you two peaks of power (max/min) and therefore an expansion/contraction cycle at 120 Hz. But well done Nile! Postscript: suggestion for a further physics experiment: is it possible to set off nitrogen triiiodide using a run-of-the-mill laser pointer? :p

Mono Keras

f'n awesome! showing the whole process really useful. In hindsight something (like the yellow glow) might seem obvious, but a priori it really isn't. Very Dunning Kruger. I thought it was florescence in the camera lens for longer than I should have.

Jeremy

It's interesting that you read the frequency at 120 Hz. I would expect 60 Hz, since microwaves typically use a voltage doubler that takes energy from the transformer to charge a capacitor on one half cycle (60 times per second), and then takes energy from the transformer and the capacitor on the other half cycle (also 60 times per second) to power the magnetron. Ben at Applied Science also has a video on this: https://www.youtube.com/watch?v=I2k2g00onL0 I would guess you were picking up the hum of the transformer at 120 Hz when the microwave was empty, and picking up the second harmonic of the 60 Hz the magnetron is actually running at when the plasma was going.

John Hiesey

Yeah, the cuvette was just what I had, but a rounded one would probably work a lot better.

Nile Red

The human eye is much more sensitive towards the middle of the visible spectrum (yellow-green) than the ends of it (red-orange and blue-violet respectively). That is why the yellow line of sodium usually masks other emission lines. That happens even if the ratio sodium:other is small. For instance, a plasma containing 90% potassium ions and only 10% sodium ions will still appear yellow to the eye (but you will see the difference in real relative intensity with a spectrophotometer).

Silviu T

Sodium-free suggestion: a container made of fused silica. Transparent and no sodium. (edit) Oh you tried. Something smoother and with less right angles should work better.

Silviu T

LOL! About halfway through the video I started thinking "there is something else common to all the experiments he's running, that contains a lot of sodium. The walls of the beaker." Nice confirmation later on. :)

Silviu T

Sodium color is coming from the powdered soda glass in the match head, which is added to increase the friction/heat from striking it. Ah, darn it, you figured it out lol.

Larry B

Can you get your claws on some very pure/clear Al2O3 beakers? Do such things exist? Maybe some of that transparent aluminium (AlONT)? At this point I just want to get a few junk microwaves and start nuking various metals and what not. Maybe throw some magnets into the mix for fun. The one I'd really like to see ( but wouldnt try for obvious reasons ) is mercury...

Clifton Ballad

Very cool! Also I don’t think stopping it when it started to heat the metal of the oven red hot is “wimping out”! I would call it “ self-preservation” 😉

André Forcier

He works in the LED lighting industry

Nile Red

https://youtu.be/qhXbAoqWCwY?t=260 (time: 4:20)

Very cool! (Why does your dad have a visible light spectrometer?)

Michael Aichlmayr

Oh it was AC.

Daniel Blake Shoemaker

Wow, I used to work at Home with High voltage Plasma. I had NaCl, CaCl2, KCl solutions I placed them on copper pipes and added 12,000 volts, 30 mA Jacob ladder Reg Iron state transformer but the PLASMA was always orange. I could not SEE any difference in those salts strange eh Red Nile?

Daniel Blake Shoemaker

Hype!

Äonothem

NICE!

ARPLATINUM