Chapter 17: Phases and Phase Changes
Applications

Vacuum

A Physicists vacuum chamber

Another thing that physics is good for is... nothing. That is, physics is good for producing regions with nothing in them: vacuums. Of course, no vacuum is perfect, all we can really produce is a volume with very little in it. As you might guess, the vacuum that a physicist talks about bears little resemblance to the vacuum cleaner you have at home. As one example, a physicist's vacuum can cost anywhere from a few thousand to over $100,000 (over a $1,000,000 with all the accessories). However, the two do have some points in common.

Why would anyone want to produce a region with nothing in it, especially if it is going to cost a lot of money? The essential reason is cleanliness. A simple example of this is a light bulb. If the bulb is open to the atmosphere, the filament will burn out immediately. In this case, the dirt (defined as any unwanted material) is oxygen. Connecting the bulb to a vacuum pump and removing the air before sealing the bulb is necessary to make the bulb do its job (a pump that costs $1000 will do in this case). There are many more examples. Other household objects that contain a vacuum include fluorescent light tubes, television and computer monitors, barometers, cars, and microwave ovens.

There are also a lot of important devices that require vacuum while they are being made. Computer components are a good example. The central part of a hard disk drive, the actual "disk" is made by depositing a complex series of metal layers on a polished aluminum or glass disk. Some of the layers are less than 100 nm thick, and their properties depend on how clean the material is. A tiny speck of dust could ruin the entire system, or a tiny amount of oxygen could degrade the properties of the metal, rendering it useless. Similarly, producing microprocessors and other silicon chips must be done under clean conditions. The process of producing a chip is quite involved, and several of the steps require varying levels of vacuum.

Vacuum level	Pressure range (Torr)
Rough (or low) Vacuum	1 < P < 10^-3
Medium Vacuum	10^-3 < P <10^-5
High Vacuum (HV)	10^-6 < P < 10^-8
Ultrahigh Vacuum (UHV)	P < 10^-9

How good a vacuum is a vacuum? Technically speaking, any container that contains lower pressure than the surrounding atmosphere can be considered a vacuum. You can make a weak vacuum by sticking a glass over your mouth and sucking. Unfortunately, there are two ways of describing levels of vacuum. One way is to measure the pressure remaining in the system; the lower the pressure the better the vacuum. This is the method most scientists and engineers use, and it is the only sensible system if the vacuum is very good. The scale shown in the table at the right describes various levels of vacuum and the names that scientists use to categorize them. This scale is a bit informal, and some scientists use a slightly different scale.

The other method of describing vacuum is to measure the difference between the pressure outside the system and inside. On this scale, low numbers mean poor vacuum, and the maximum possible vacuum is whatever the pressure is that day. This method has become traditional among manufacturers of low vacuum systems, because, when selling something, it is always easier to charge more for a device that gives a "bigger" result rather than a smaller one.

Metal and rubber o-rings

How is a vacuum produced? Each vacuum system has two essential elements: a chamber and a pump. The pump captures the gas molecules inside the chamber and either holds onto them permanently or moves them out of the system. How good the ultimate vacuum inside the chamber is depends on several things, such as how well the chamber is sealed, how clean the inside is before pumping begins, and how strong the pump is.

For anything but ultrahigh vacuum, the most common seals are o-rings made of synthetic rubber (often called elastomers). These o-rings are cheap, easy to use, and provide an

A rough pump

adequate seal. For UHV systems, metal o-rings are used (usually copper, indium, or gold is the sealing material). These seals are more expensive and more difficult to use, however, they provide two huge advantages. They seal exceptionally well, and they allow the entire system to be "baked." When a system is baked, the entire system is usually placed under blankets and heated up to above 100 °C while the pump is working. This drives water vapor and other volatile materials off the walls of the chamber, allowing them to be captured by the pump. If a chamber is not baked, the ultimate pressure is limited by the vapor pressure of the impurities adsorbed on the walls of the chamber.

Most high vacuum or ultrahigh vacuum systems actually have more than one pump. The pumps that can reach the best vacuum cannot be turned on at atmospheric pressure, so the system has a "rough pump" that is turned on first, and which brings the chamber down to some moderate pressure. After the rough pumping, the main pump is turned on and the system is baked. Only after the bake is turned off and the chamber cooled down does it reach its ultimate pressure. For a good UHV system this process can take several days. You can read more about different kinds of vacuum pumps in the links at the end of this essay.

At this point, you may be wondering if there is any connection at all between "physics vacuum" and your vacuum cleaner at home. The answer is yes, of course. Think for a moment about the way your vacuum cleaner works. The motor turns a fan that blows air out of the region where the dirt bag is located. Since air can flow through the walls of the bag, the air that is dragged in through the nozzle passes into the bag, bringing dirt with it. The motor and fan are really a vacuum pump, and the dirt bag is the chamber. However, in this case, the goal is not to maintain cleanliness inside the chamber, but cleanliness outside the chamber. There is a vacuum cleaner site that contains a great deal of useful information about the cleaners themselves, and includes a wonderful glossary that defines many physics terms (power, amperage, pressure, etc.) as they apply to vacuum cleaners.

Further study links:

Vacuum history 1.

Home-made vacuum 1. 2.

Vacuum Technology 1. 2. 3. 4.

Vacuum in space 1.

Further Study Questions:

In the chart above, pressures are stated in units of Torr. How is a Torr related to other units of pressure?

Who was Torr?

Describe at least three types of pumps which can be used to reach high vacuum

Another piece of crucial vacuum technology is the gauge that allows vacuum to be measured. What types of gauges are common? in what ranges of vacuum do they work?

Another common vacuum most of us have heard of is outer space. How good is the vacuum where the shuttle orbits? Way out in interstellar space?