Experiments with old tubes, Scanning electron microscope

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Experiments with old tubes, Scanning electron microscope

Postby segelfam » Fri Oct 08, 2010 3:45 am

Hi,
I made some experiments with old crt, picture- and vidicontubes. With the crt or picture tubes you can make magnified images of the cathode layer. With vidicon or plumbicon tubes you can build a simpe scanning electron microscope for low magnification (~ 50..100). Sorry the pages are in german language, but I think the images and schematics are self explaining

Thomas

http://www.rapp-instruments.de/NewsPage/CRT/crt.htm
http://www.rapp-instruments.de/NewsPage/Vidikon/vidikon.htm
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Re: Experiments with old tubes, Scanning electron microscope

Postby Doug Coulter » Fri Oct 08, 2010 1:38 pm

Cool stuff! I take it you re-figured focus voltages on the CRT to get that image of the cathode?

I've seen the trick of using a known mesh to calibrate electron microscopes, but never seen one made starting with a tube! Did you use a vidicon because of smaller spot size than most crts?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Experiments with old tubes, Scanning electron microscope

Postby segelfam » Sat Oct 09, 2010 4:59 am

Hi Doug
You're right. For looking the cathode You have to change the voltages. For example for the DG 7-74 ( 3 ARP 1)
Uf 6,3 V > 2 V
Ug1 ~ -50 V > 0V
Ua1 500 V > 1500V
Ua2 1000 V > 1500V
Ug3 ~100V > 0V
Because there is no negative bias at the wehnelt g1 You have to reduce the heater voltage to reduce the emission. Otherwise the image will be blured by space charge effects.
Yes, I use vidicons because of much smaller spot size. I also tried with a crt gun, the difference is considerably.
Another nice image I got from the small parts of an old watch.

Thomas
Attachments
clock102 Kopie.jpg
clock102 Kopie.jpg (28.88 KiB) Viewed 15187 times
eguns.jpg
eguns.jpg (10.93 KiB) Viewed 15187 times
heater.jpg
heater.jpg (10.46 KiB) Viewed 15187 times
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Re: Experiments with old tubes, Scanning electron microscope

Postby johnf » Sat Oct 09, 2010 5:09 am

Thomas
I'm impressed I never thought of using a vidicon I might use this at work.
Funny I was asked if I could put a SEM in our IBA chamber yesterday. My reply was that it was possible but maybe not in my life time ---how things change

Excellent work and ideas
John
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Re: Experiments with old tubes, Scanning electron microscope

Postby segelfam » Sat Oct 09, 2010 5:41 am

Hi John,
my experiment is nice for demonstration but I'm not shure if its good enough for professional work. The resolution will be mostly 10 µm, not to compare with the real worlds SEMs having arround 2 to 10 nm.
Another drawback is the vidicons cathode made from sensible barium layers which are destroyed by extended air contact. For this I flood the vacuum system with ballon gas (Helium) to change the sample. So the vidicon tube is filled with helium and I can change the sample for until now arround 50 times with no change of cathode properties.
After flooding the camber with air you have to increase the heater current after flooding to get the same emission and after around 10 cycles all the barium has gone.
But I found a paper where they use a carbon fiber field emission cathode with a very simple electron optic and reached resolution around 30 nm.
I think this would be the right way to get a excellent device.I will try realize this approach in the future

Thomas
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Field emission characteristics of a graphite nanoneedle cathode and its application to scanning electron microscopy.pdf
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Re: Experiments with old tubes, Scanning electron microscope

Postby chrismb » Sat Oct 09, 2010 7:04 am

One thing to note; I've used balloon gas recently and it appears to have oxygen in it. I don't know how much, but it appears to have affected my spectrometry work. I presume it is likely that oxygen is added to reduce asphyxiation risks.
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Re: Experiments with old tubes, Scanning electron microscope

Postby Doug Coulter » Sat Oct 09, 2010 1:44 pm

I'll have to put some balloon gas into my system with the mass spec. Could be O2, but might just be plain air, probably for cheapness as much as for any other reason. I know the welding grade stuff I get is pretty pure, having checked that. All of it is very dry, because the way they make it is to liquefy air, then distill off the gases one by one -- and water is poison for welding, one of the major reasons you use the special gases (hydrogen embrittlement). I do know that "real" helium gives more lift than the party store cheap tanks do.

I've done some fooling around with cathodes and materials for emission. The tri-alkali mix is indeed messed up by air, fairly badly, you can usually only get a couple exposures before emission falls so much the thing is useless. From the books, you start with a mix of carbonates, and during activation lose CO2 and wind up with oxides and some reduced metal. Air by itself doesn't ruin this, but any water does, by making hydroxides which won't re-activate again. So the balloon gas might be fine if it's dry and dry N2 (cheapest at the welding store) should also be fine.

After reading some more, I discovered that Y2O3 (yttria) works a lot better in these applications. It doesn't seem to mind air exposure at all. You only get perhaps half the emission of the other mix, but it is stable and "forever". We put it on via electrophoresis, thin, and it reduces the required cathode temperature (tungsten filament from a small 12w halogen) for a given emission from more or less white hot to orange (hotter than most vacuum tubes, but still reasonable). For example, in this case we were going for 10 ma emission off a 6v, 2a filament. To get that we needed more or less full voltage without a coating, but only 3.4v with one in a little triode/ion gage I made. We got half a pound of this to fool with, a multi lifetime supply as only a few milligrams makes the slurry that will coat many filaments/cathodes.

I hear the really serious electron microscopes made by Crewe at U of Chicago (the first ones to resolve atoms, just before scanning tunneling microscopes came along and stole all the thunder) used a single crystal tip field emission source of tungsten, cold. Not much current, but a really good point source, effectively about one atom in size. I have messed with making these too, and it's not really hard (selective electrolytic etching) if you can deal with the low current you get, and keep back-accelerated ions from heating and sputtering the tip off. It sometimes takes a few tries to get a good one, you have to get lucky with the crystal orientation in your rod stock, but welding rods seem to be a good source of mostly correct orientation.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Experiments with old tubes, Scanning electron microscope

Postby segelfam » Sat Oct 09, 2010 9:00 pm

Hi,
I used ballongas because of it light weight. Because the vidicon is mounted vertical, I supose the helium readly fills the tube, preventing the air from reached the uppermost cathode. Other clean and cheap gases like nitrogen or argon are to heavy. People from Linde corporation ( the biggest supplier of technical gases and kryogenic fluids) told me that ballon gas is just dirty helium. From leaks in helium cooling systems the gas is polluted with air and it is to expensive to clean but can't be used anymore because the air would close the fine tubes in the liquifier at low temperatures. So they sell it as ballon gas.
Of course it would be fine to give the cathode a new emission layer, perhaps from yttria. But I'm afraid it's very difficult to remove the cathode from the wehnelt cylinder without destroying the isolation and the fine heater wires.
I know about the works of Crewe, Welter et al. and the excellent microscopes. The problem with the tungsten tip is that for a stable emission you need a very good vacuum of 10-10 to 10-11 mbar, very difficult to reach, with amateur vacuum devices, even unpossible with epoxid or soldered tubes.
I have build a field emission microscope and a field ion microscope, the worked fine but normaly after some minutes the tips were destoyed by ions resulting from the bad vakuum.
[url]http:/www.rapp-instruments.de/diverse/FEM/fem.htm[/url]
[url]http:/www.rapp-instruments.de/diverse/FIM/fim.htm[/url]
The field emitter with the carbon multitips can work at much higher pressure from 10-6 to 10-5 mbar

Thomas
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Re: Experiments with old tubes, Scanning electron microscope

Postby segelfam » Sat Oct 09, 2010 9:03 pm

Sorry, in my last post I set the wrong links
Field emission microscope
Field ion microscope
Last edited by segelfam on Sat Oct 16, 2010 9:35 am, edited 1 time in total.
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Re: Experiments with old tubes, Scanning electron microscope

Postby Doug Coulter » Sun Oct 10, 2010 2:28 pm

URLs are a bit of a trick here, here's how to do them nice looking.
I put the following inside code tags so the board wouldn't translate them so you can see how it looks while editing.

Code: Select all

[url=http://www.youtube.com/watch?v=BLKiMbC6s2k ]Pink Floyd[/url]



Which looks like this if it's not inside code tags:

Pink Floyd


So you put the actual url inside the first set of brackets after adding an = sign, the thing you want to show blue as words between the url tags.

If you just want to drop in a URL plain looking, you just put it in the text on its own line and it will become a link automagically.

You can also just edit your posts -- when logged in you should see an edit button on each of your posts (if not, let me know, I find that really useful while working on a long one).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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