I vaguely remember from somewhere that there are laboratories that have exotic equipment for producing hard vacuums.If so what what areas of research are involved.What do they hope to glean?
Is it possible to produce a vacuum that is greater than atmospheric pressure for instance in lifting equipment.
Surely zero psi is as low as you can get?
I don’t know about pure science, but some steps in semiconductor manufacturing that are done in close to a complete vacuum. For example, if you’re using an electron gun to implant arsenic ions into a wafer, you don’t want the beam picking up nitrogen and oxygen ions from normal air.
The vaccum process basically consists of a blower (usually a roots blower, like what you see on drag racers) or a series of blowers that pump the processing chamber down to a near vacuum, along with cryogenic or electrostatic “pumps”, that remove the last bit of ambient gas. The cryogenic pumps trap air molecules by freezing them to the filter elements; electrostatic pumps use charged plates to attract the air molecules.
I’m not aware of any research being done to try to learn things from hard vacuums, but that doesn’t mean it’s not happening. However, there are many relatively mundane practical uses for hard vacuums in experimental physics. As you might suspect, they’re used when having air around would interfere with what you’re trying to do.
Some examples I can think of off the top of my head:
[list]
[li]Scanning tunneling microscopy–you don’t want air molecules getting between the tip and the sample. (probably atomic force microscopy as well)[/li][li]Cryostats seem to usually be under a pretty hard vacuum.[/li][li]Particle accelerators I’m sure operate under quite a hard vacuum.[/li][li]High-resolution spectroscopy seems to often be done under vacuum.[/li]
I’m not sure I understand your question about “greater than atmospheric pressure.” But yes, zero psi (or whatever units you wish to use) is as low as you can get.
I could have SWORN this would be a spirited discussion on the merits of a Kirby v.s. a ShopVac 
Cartooniverse
It surely is. Vacuum is a relative state; it is merely an area of lower pressure than the surroundings. It is infact quite impossible to have pressure lower than zero, for the same reason you can’t have any physical measurement be lower than zero. What is negative pressure? negative time? negative distance? negative area?
Lifting equipment merely needs to produce enough pressure differential so that the force of the air pressure is great enough to counteract the force of gravity.
Having worked with high vacuum equipment for a while (I was a Mass Spectrometer Technician a few years ago) I can tell you that high vacuum work is very important in that field. It’s also very tricky. Not only do you need a high vacuum, you need a reproducably high vacuum. To get very accurate low pressures, a standard “mechanical type” or “rough” vacuum pump just won’t do… Quite simply, there are diminishing returns for getting really low vacuums, plus mechanical pumps are highly irregular. When you have equipment that requires a vacuum of a few millionths of a torr, and is sensitive to changes of a few billionths of a torr (1 torr=1/760 of average atmospheric pressure; thus this equipment has sensitivities down to about 1/1,000,000,000,000 of atmospheric pressure) mechanical pumps just don’t do. Thus, special types of pumps known as “diffusion” pumps or other fine pumps are used. At standard pressure, diffusion pumps are fairly useless… worse, near atmospheric pressure can damage them. Diffusion pumps, the second stage in getting very low pressures, operate on a really neat principle. A low volatility oil is sprayed over the inside walls of the pump, and in this fine mist, molecules of air will diffuse into the oil. Then the oil is pumped out of the chamber, taking some of the air with it. The oil is then heated, driving off the diffused air. Since rates of diffusion are easily predictable using fairly simple mathematical equations, and not the highly unpredictable mechanical means, very accurate and low pressures can be obtained.
Besides mass spectrometry, certain kinds of electron microscopy and other types spectroscopy require scrupulously low vacuums, though mass spec I’m fairly certain is the most common procedure requiring vacuums this low. There are some mass spec techniques that require much less sensitive vacuum methods, but standard electron ionization mass spectometry, the oldest and most common type, still requires amazingly low pressures. An standard E.I. mass spectrometer will take up most of a room, about 15 feet long and with metal pipes with the outer width of your thigh and the inner bore of your wrist. Keeping one of these babys running requires intimate knowledge of electronics, plumbing and cooling, high voltage electricity, high vacuum plumbing, electronic lenses as well as fairly advanced phyisical chemistry knowledge. Our mass spectrometrist in college was quite amazing at all the stuff he could do.
Actually, STM is done in room pressure… that’s the beauty of it. Standard SEM, or scanning electron microscopy, and TEM or transmission electron microscopy, use an electron beam shot across space; air molecules would definitely get in the way.
STM operates on a very different procedure. The sample must either be conducting, or covered with a very thin conducting
coating (gold is often used). STM uses a very peculiar quantum mechanical technique known as tunnelling. It’s akin to you walking up to a solid wall and passing through it without damage to either you or the wall. It seems that electrons have the ability to “tunnel” through potentials that standard physics would otherwise say is impossible. How it works is easily explainable, but not in a short space, and this post is long enough already. Anyways, tunnelling requires very close distances, so that the probe tip is almost touching the sample, but not so close as to allow for a regular current, since tunneling currents are on the range of a few microamperes, well below the noise range of a standard current. The tip is then scanned across the surface. Since the tunneling current changes with distance from the sample, what the STM detects is height levels of the sample on the order of a few angstroms… well capable of resolving individual atoms. Since air is not that conductive, STM can occur at atmospheric pressure. It may even require atmospheric pressure, since the dielectric of a vacuum may be too low to occur… But I’m not sure about that… the point is if you don’t need a vacuum, why use it?
Sorry for the hijack, but I felt that it was impotant to set the info straight… no hard feelings?
Oops, I stand corrected. I was a little hesitant about using that example, but I remember hearing something recently about the difficulties of doing vibrational isolation on a system under vacuum, and I could have sworn the person had given STM as an example. I figured that must have been the usual case. Oh well.
There are many fields requiring various degrees of vacuum. If you just need to get rid of most of the air - say, for testing spacecraft components or for handling extreme UV light or soft X-rays (which get absorbed by air), you don’t need a very good vacuum. I’d imagine 0.1 torr (1/10,000 atmosphere) is good enough. However, when you want to use high voltage in such a system, things get more hairy - there is just enough air to get ionized and try to carry electrical current, but not enough so that their motion is inhibited, so they succeed in carrying a lot of current. This is how fluorescent lights and neon signs work, but you usually don’t want that happening in your experiment. So you need an even better vacuum, so there are very few molecules that try to carry current. You need about 10^-6 torr (1/100,000,000 atmosphere). Things that fall in this category are include testing of many spacecraft components and experiments related to soft X-rays and extreme UV.
Another use for hard vaccum is when even trace amounts of air can interfere with something in your experiment, usually a beam of particles or light. There are many methods for making very thin films by turning the coating material into gas and letting them settle (evaporation, sputtering, etc). They are used for optical coatings and semiconductor manufacturing (I think), and require 10^-5 torr or better. Some beams are even more sensitive because they need to travel farther, or because of the high accuracy required. Particle accelerators can be many miles long, so need 10^-10 torr or so. Same with gravitational wave experiments such as LIGO, where laser beam travels many miles and are analyzed for minute fluctuations.
The oil diffusion pump mentioned by jayron works well, but many people don’t like them because they use oil which can potentially contaminate the experiment. Turbomolecular pumps are fairly popular. These look just like the turbines on a jet engine, and physically sweep air molecules out of the vacuum chamber. You can get 10^-7 torr with one of these. Cryogenic pumps are popular for even harder vacuum. These work by exposing a very cold surface to the vacuum chamber so that any remaining air condenses onto the surface. Both of these only start working at “soft” vacuum, so need separate roughing pumps. I think gravitational wave detectors and particle accelerators often use ion pumps. Finding out how they work is left as an excercise to the reader. (i.e. I don’t know, sorry.)
The hardest vacuum achieved as far as I know is the Wake Shield experiment, where they held out a physical shield in front of the space shuttle (ok, in front of some instruements on a Shuttle) to sweep off residual air. They got about 10^-14 torr. That’s ten quadrillionth of an atmosphere.
Ugh… diffusion pumps… I once had to clean a 20 and a 14 inch diffusion pump by hand; the smell of Santovac-5 still remains fresh within my nostrils to date. Yuck!
Anyway, an ion pump is a combination of an ionization gauge and a gutter pump. It acts by initiating electric discharges between a stainless steel anode and a titanium cathode. Gas molecules that are ionized at the anode will fly toward the cathode at such high speeds that they get buried within the titanium there, thus permanently removed them from the vacuum system. Titanium sputtered off the cathode will also absorb gas molecules and deposit them on the anode. Ion pumps have a high purchase and maintenance cost, much like cyropumps, so turbomolecular pumps are preferred whenever possible. I remember turbomolecular pumps can go down to 10E-9 torrs given ample time and a good baking.
As for the OP, seems like most areas of research have been mentioned already. Does this make my a hijacker?
p.s. I believe only mass spectrometers in the time-of-flight category need to be anywhere close to 15 feet long. 5 feet is already a pretty large beast, and a reflectron setup can really cut the length down too.
The Lab Rat’s Bible - Building Scientific Apparatus
jayron writes:
Negative voltage? Negative temperature?
Sorry, I agree with your point, just not your generalization: “you can’t have any physical measurement be lower than zero”
casdave,
Referring to the thread that made you post this topic and in reference to your last question:
I used to work in a chemical laboratory where they were experimenting with a number of high tech rubber, foam, and plactics. They had some pretty cool stuff. In particular they had developed a sort of high density rubber that they were using for a number of things. The rubber was very rigid, yet still felt tacky like silicone. One use for this material was in a super-suction cup. The construction of this cup was such that there was a steel cone, for increased rigidity. Inside the cone was a rubber suction cup. The suction cup assembly included a screw, such that when the screw was all the way in, the inside of the suction cup was compressed (flattened, sort of). They placed the suction cup against a polished steel plate and backed the screw out creating a very tight suction bond with the polished steel plate. Both the suction cup and the steel plate were attached to an instrument that could measure the seperation force required to break the bond. These super-suction cups were capable of withstanding greater than 200 psi seperation force. The cups I saw were approximately 4 inches in diameter and surely could have been used to lift some reasonably heavy objects, though I have no idea what they were used for (or even if they were ever used). I know the rubber was used for some high pressure O-rings. This was about 17 years ago, so I’m sure there have been some improvements.
I was pretty impressed with this, but in talking to one of the researchers there, he claimed that much greater ‘suctions’ had been done with milled, highly planar, highly polished steel blocks. Apparently, two such blocks with a thin layer of oil and considerable pressure to evacuate the trapped air molecules are capable of much greater ‘suction’ strength.
Um… Any temperature scale that has negative numbers is an arbitrary scale. Temperature is a measure of the average molecular kinetic energy. Any scale (Farenheit, Celsius) which does not have zero temperature at zero energy does not measure a physical quantity. I could just as easily devise a weight scale called “kerblooeys” and say that 40 pounds is equal to 0 “kerblooeys”. Thus, we could have “negative weight.” It wouldn’t really be “less than no weight” however, merely a trick of the numbers. Since we want a temperature scale which has reasonably meaningful numbers in the range of everyday experience, it becomes useful to offset the scale by a certain amount, but it still doesn’t mean that we can have a negative amount of energy.
WRT voltage, voltage is often considered a vector quantity. Voltage has a value (numerical) and a direction. Since electricity flows two ways (lets say forward along a wire, or backward along the same wire) we express directionality using a convenient bidimensional quantity, the +/- signs. Once again, while voltage is expressed as a positive or negative number, the physical quantity being measured, cannot be negative. Voltage is a measure of the potential energy of an electron placed in an electric field and subjected to the given voltage. If the electron moves against the field, the voltage is said to be positive, while if it moves with the field, it is said to be negative. The assignment of positive or negative in this case is arbitrary and does not represent “negative energy,” merely a bookeeping necessity.
Any other physical quantities that you know of that could be negative?
Actually, temperature can be negative. Temperature is defined as dE/dS where E is energy and S is entropy. Therefore if you can arrange a system where entropy decreases for increased energy, the temperature of the system is negative. There are better explanations on the web, such as here and here. This is not a mathematical trick, such systems have been constructed.
Another scaler (not vector) quantity that can be negative is electrical charge. I wouldn’t call voltage a vector quantity either - potential has no direction involved. If I rub a balloon on my sweater and charge it to 100 volts, which direction is this 100-volt long vector?
However, I do agree that there is no negative pressure. If you have no air molecules at all, that’s zero pressure, and you can’t have fewer than zero molecules. The suction cup mentioned by JoeyBlades uses forces other than pressure - friction or intermolecular forces. When two perfectly clean and flat surfaces come into contact, they bond together pretty well. It’s especially a problem in vacuum - there isn’t even air to prevent the bonding, so moving parts easily get stuck.
You are correct, STM imaging can be done in air. it makes life a little more difficult as acoustic vibrations can dedgrade your imaging, but you just have to be quiet.
A sizable percentage of STM’d are in fact designed to be used in a vacuum, not for the STM’s sake but for the sample’s. If you are scanning gold or graphite then operating in air is just fine. But most surfaces that you might be interested in looking at will oxidize in air.
Take for example Silicon. one of the first useful things done with an STM (way back in 1982) was to look at a Silicon <111> surface to determine how the atoms reconstruct and confirm the best guess at the time. But to look at silicon, you have to be in a serious vacuum. It is a very reactive surface and will be covered with all kinds of crap if you expose it to the air (it is still posible to take STM images of the crapped-up surface, but boy are they ugly). To keep a clean surface (and more inportantly, get a clean surface to begin with), you need to keep your chamber preasure below 10^-10 torr.
10^-10 torr is a f***ing hard vacuum. have to bake the entire chamber, you can’t leave a fingerprint anywhere inside the chamber, and you have to be very carefull about what materials you use to build things (a lot of materials have vapor preasures high enough to cause trouble if they get warm). It’s a big pain in the butt, but it’s the only way to go if you want clean samples.
-luckie
Having cleaned a few mass spec sources in my day, I quite agree with the cleanliness problems working at pressures under 10^-10 torr. After all of the carbon deposits have been removed with sharkskin, a complex series of solvents are used to progressively clean the lenses. Basicly, you use toluene to clean off the fingerprints, acetone to clean off the toluene, ethyl acetate to clean off the acetone, then diethyl ether to clean off the EA. At THAT high of a vacuum, nearly everything has a vapor pressure, so you go the other direction: if you have something with a REALLY high vapor pressure, it will all be removed when the air is pumped out.
Thanks folks for the responses
I believe this was one of the primary problems with Robert Boyle’s experiments with vacuums. There was only so much force that could be applied to the evacuation pump, even with strong assistants - some of his hypotheses about the properties of a vacuum were counterindicated in the crude partial vacuums he could achieve. – although suffocating small animals was easy enough.