chief
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self-made high-vacuum diffusion pump
Has anyone tried to make such a pump ?
==> It needs an pre-vacuum (0.1-1 Torr, hopefully a refrigerator-thing might work)
In effect it is nothing more than similar to a water-pump, only that a vapor-stream of oil does the job, and because of the low vapor-pressure of the
oil (which is condensed in another part of the pump, then refluxed) the vacuum gets very high ...
The basic design of such a pump would be possible of hardware-store waterpipe-parts (copper, to be soldered); also the galvanoplastical way comes into
mind.
Such a device would allow thin-film deposition-experimenting (solar cells, eg.) ...
Besides another idea for a high vacuum: The water-pump,but not with water, instead with some oil (since the H2O-vapor-pressure limits the available
vacuum), closed-circuit, re-used medium.
The high viscosity of the material could be come by usind 2 pumps: 1 ordinary 5 m^3/h from the hardware-store, and another from a cheapo-high-pressure
cleaner.
Because Oil would run within these, there would be a much higher lifetime of the mechanical parts, also.
[Edited on 18-11-2008 by chief]
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Tacho
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I once tried to lower the vapour pressure of water in a water-jet pump by adding ethylene glycol. The result was a bucket full of foam.
Do you know any cheap and widely available oil that has very low vapour pressure?
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watson.fawkes
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| Quote: | Originally posted by chief
Has anyone tried to make such a pump ?
==> It needs an pre-vacuum (0.1-1 Torr, hopefully a refrigerator-thing might work)
[...]
The basic design of such a pump would be possible of hardware-store waterpipe-parts (copper, to be soldered); also the galvanoplastical way comes into
mind.
[...]
Besides another idea for a high vacuum: The water-pump,but not with water, instead with some oil
[...]
The high viscosity of the material could be come by usind 2 pumps: 1 ordinary 5 m^3/h from the hardware-store, and another from a cheapo-high-pressure
cleaner. |
A similar-purpose vacuum rig is on my current design table, and I've been reading up on how to do
this.
From what I've read, modern refrigerator compressors do not make an adequate backing pump. They're piston-based, and don't have tolerances adequate
for getting those pressures. Evidently air-conditioner compressors still use a rotary-vane mechanism and can be pressed into service. On the
small-amount-of-money front, though, Harbor Freight is now carrying some Chinese-manufactured refrigeration service pumps that are adequate.
The suggestion in another thread to make a mineral-oil ejection pump is something I'm seriously considering giving a try. It might work just fine, but
I'm not sure what the ultimate pressure would be. I know I don't yet have a vacuum gauge sufficient to measure a possible success. As far as pumps for
such an ejector, two stages are surely not necessary for a small unit. Given the viscosity, it just means using a larger pump. Temperature control
might also become an issue.
With either of these backing pump systems, you'll also need a cold trap at the roughing pump on it's way to the diffusion pump. This is to capture
backstreaming oil vapor that would kill the high-vacuum section as well as contaminate the diffusion pump fluid.
As for building a diffusion pump, it's certainly possible, but probably not out of completely off-the-shelf plumbing. Some metal working is going to
be necessary, but it shouldn't be more than some cutting and bending/rolling. Brazing is a better construction method than solder, since the diffusion
fluid operates at relatively high temperature. 2% silver brazing rod should be adequate; it's a standard material in the refrigeration service trade.
For further information, I recommend perusing the web site of The Bell Jar, a newsletter for amateur vacuum enthusiasts.
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Tacho
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I wonder what would happen to residual gasses if you heated (melted) a chunk of sodium or calcium in a compartment conected to the main vacuum
chamber. I imagine all non noble gases would be trabped as non volatile compounds.
More: After heating the sodium/calcium, you could use methanol+ice (-30ºC easily) to cool another compartment with previously heated activated
carbon. Adsorption could trap what is left. This is done usually with liquid nitrogen, but who knows how much gas would be trapped at -40ºC...
Opinions?
[Edited on 19-11-2008 by Tacho]
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chief
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This is called a "getter"-pump, and was used eg. for evacuating glassware etc.: It also works well with CO2 and CaO and other such combinations: First
the to-be evacuated thing is flooded with the absorbable gas, then its sealed, then maybe the special part of the glasstube (or apparatud is glowed);
_very_ high vacuum is possible this way.
But the disadvantage is: Can't be used for molecular-beam- or other plasma- experimenting, because there constantly a low amount of gas has to be fed
in, ionized, accelerated etc., and non-reacted gaseous stuff has to be sucked away to sustain the vacuum.
The cooling you mentioned: This goes into the direction of cryo-pumps, which just basically cool the gas below its boiling point; such cryo-pumps are
the most powerful ones today available, up to 1000 000 m^3/s !
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watson.fawkes
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The main old-school getters for vacuum tubes were barium and titanium, but all sorts of stuff was used, including Mg, Al, rare earths, and more.
Heating the getter to deposit it, called flashing, was typically done with an RF induction coil. This is typically done under vacuum, not with an
intentional gas fill. If the seal in a tube is good, vacuum improves over time as the getter scavenges more and more of the free ballistic-moving
atoms in the chamber.
The standard renewable getter pump uses titanium wire. Heating the titanium sputters fresh ions. As long as you've got more titanium, you can take out
more gas. This only works well at high vacuum, obviously. The molar efficiency of a titanium getter is said to be > 50% (atoms of gas per atom of
titanium). Wind fine titanium wire around a tungsten heater wire or, if you've got better electrical control, heat a titanium wire directly.
The second kind of pump is a sorption pump, which works by either/both absorption and adsorption. Depending on the material used, they can be renewed
with a bake-out under vacuum. Linde makes a number of special-purpose sorption materials.
As for getting started, consider a low-atmosphere gas discharge tube. That's just a pair of electrodes, a sign transformer, a glass container, and a
vacuum rig. You'll need to get down into the 1-10 torr range to make this work, but it's pretty minimal in terms of equipment. It's sort of like a
"hello, world" construction project.
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chief
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I saw that bell-jar page: They just _sell_ things ..., bad for the budget ...
A good design has to be simple and cheap: Standard cooking-oil (canola ?) should have a low enough vapor-pressure to be used as medium in a
water-pipe-pump. Since the outflow would be re-used, by the driving-pump (water-pump from the hardware-store), _this_ could be used in parallel (maybe
5 water-pumps), no waste-material considering necessary. 5-10 liter should be sufficient.
Maybe this vacuum could even reach into the mTorr-range ...
From there on the selfmade diffusion-pump for extra-super vacuum; only expensive part: The water-pump; but it would run with oil, therefore: Should be
of metal only (no cheap plastique), and would last quite long. Available for maybe 70 $/EUR (I have one, continuously running at 80 V instead of 220,
but with extra-cooling from a 10-$ vent, since the built-in fan is less efficient at the low 80-V RPM)
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Tacho
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Chief, let me see if I get this right: you are saying that some sort of getter pump would result in a vacuum that is too high? Interesting. I think
that is an easy to solve problem.
In other words, a refrigerator pump and some sodium would render me a vacuum high enough for a clean sputtering?
About adsorption pumps: I know they work, there is a lot of info about them. But would they give useful results at -30 or -40ºC, instead of cryogenic
temperatures? And using activated carbon instead of some exotic adsorption material?
About triglicerydes as vacuum oil, it sounds promising, wouldn't there be too many impurities with higher vapour pressure?
One more thing, since discharge tubes were mentioned, wouldn't a discharge be a decent vacuum gauge? Set a flyback transformer(+fluorescent electronic
ballast) to give a 10KV discharge inside your chamber. At the point you can no longer see any glow, wouldn't it be a pretty decent vacuum for most
purposes? Putting some fluorescent material near it could extend the range.
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chief
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@Tacho: No, not right: A getter-pump just enables maybe 1 experiment/day, a good electrical-driven thing as I proposed possibly more than 10/day. Also
the getter-material doesn't work continuously; also it wouldn't be very good at catching Argon, would it ?
The simpler getter-materials require the vessel to be flooded with a special gas before (eg. CO2); that requires a 100 % leak-free apparatus, or a
CO2(eg.)-atmosphere aroung the apparatus eg. ...
Getter-pumps maybe the best choice for evacuating a sealed glass-tube or something, but not a apparatus which is repeatedly to be re-opened, or where
a constant material-stream (to-be-ionized gas) is to be fed into ...
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Tacho
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I see.
Anyway, considering the simplicity of it, I would be interested in reading some more about CO2 +CaO to obtain vacuum. Do you know any on-line
information about it? My google results were not very specific. Is there a book that gives more details?
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watson.fawkes
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If there's one thing I've learned about vacuum systems, it's that each level of vacuum is significant more expensive (however measured) than the last.
It behooves anybody to considering this to pick an application, learn what its vacuum requirements are, and then build just to that level. On top of
that, it's generally wise not to pick a complicated first project.
The Bell Jar is unfortunately too commercial for its own good, I'm afraid. Nevertheless, their public articles are certainly useful for the beginner.
Vegetable oil of any kind is a bad choice for an ejector pump fluid, since it oxidizes over time and in doing so make (more or less) varnish. If
you're looking for something to buy retail, use light mineral oil or a light lubricating oil.
Adsorption pumps do not require low temperatures, because they operate on a different principle than a cryogenic pump. These are both, strictly
speaking, traps. A cryogenic pump cools off the atom, more or less putting it into solid phase; it will come off when it's heated up. An adsorption
trap, on the other hand, holds its particles by a change in surface tension energy. You have to heat them up in order to supply energy to overcome
that binding. If this seems too similar, a cryogenic trap will capture noble gasses; adsorption traps don't.
Glow discharge doesn't make a very good gauge for quantitative measurement because of the high variation in what gas might be in the gauge tube. As a
sort of idiot light (idiot dark?) it might be useful.
As for gauges, I can't imagine trying to debug a vacuum system without some kind of quantitative gauge. Ask yourself when facing a leaky system: what
kind of information to want to have about how fast the leak is?
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chief
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@Tacho: There is a book: "Anorganisch-chemische Experimentierkunst"; I read it a long time ago, from a library where I don't have any access to any
more.
But every gas that somehow reacts (completely) with a solid or liquid to form a solid with low vapor-pressure can be used. I recall to have been very
impressed by the good-vacuum obtainable (although I cannot remember the numbers any more); after my knowledge back then it was fully possible to make
electronic vacuum-tubes that way.
The rule of thumb when using (CO2 + X ==> carbonate) would be: The higher the decomposition-temp of the resulting carbonate, the lower the
vapor-pressure ...
[Edited on 19-11-2008 by chief]
[Edited on 19-11-2008 by chief]
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chemrox
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Try searching Scientific American: The Amateur Scientist .. seems like there would be something in there on it
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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ducatidragon916
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use of mineral oil diff. pump
Does anyone know if a person could use common Mineral oil for a diffusion pump fluid? Or does the mineral oil requires to be super distilled or
something like that!??? I am trying to find an oil that is cheep and doesn't contaminate like silicone does...I am not very familiar with the chemical
composition of diffusion pump oils and would like some guidance on this. anyone??
Thanks!!!!
George
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entropy51
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I do not think that mineral oil will cut it.
I have used dibutyl phthalate in the past. Now I use 8-oil, which I think is dioctyl sebacate.
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Panache
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Quote: Originally posted by entropy51  | I do not think that mineral oil will cut it.
I have used dibutyl phthalate in the past. Now I use 8-oil, which I think is dioctyl sebacate. |
How does this oil perform, diocyl phlathate makes a good bubbler material for adsorbing organics and water
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Merciless
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| Quote: Originally posted by chief |
But the disadvantage is: Can't be used for molecular-beam- or other plasma- experimenting, because there constantly a low amount of gas has to be fed
in, ionized, accelerated etc., and non-reacted gaseous stuff has to be sucked away to sustain the vacuum.
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I have run both an Ion Beam Sputtering tank and a Magnatron Sputtering tank that both used diffusion pumps as the high vacuum pump.
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jock88
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Homemade Pirani gauge here
http://www.drkfs.net/pirani.htm
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Dr.Bob
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You might be able to find a used diffusion pump on ebay or other science site. They are not trivial, and normally take a fair amount of power. I
remember using one in college, and it was a pain to stop and start it, so I always thought they were best left on for a period of time, not easily
switched on and off. And they required a substantial roughing pump, I really like the big old Welch pumps, and those are often available used at
surplus stores, auctions and online. But I think you will have a hard time using a small vacuum pump for it.
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crystal
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Why not just buy vacuum pump oil? They're not expensive and easy to find. Just look for one which meet the viscosity you need, they are usually graded
in ISO viscosity grade.
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