Cryogenics

There are essentially only four physical processes that are used to produce cryogenic temperatures and cryogenic environments: heat conduction, evaporative cooling, cooling by rapid expansion (the Joule-Thompson effect), and adiabatic demagnetization. The first two are well known in terms of everyday experience. The third is less well known but is commonly used in ordinary refrigeration and air conditioning units, as well as cryogenic applications. The fourth process is used primarily in cryogenic applications and provides a means of approaching absolute zero.

Heat conduction is familiar to everyone. When two bodies are in contact, heat flows from the higher temperature body to a lower temperature body. Conduction can occur between any and all forms of matter, whether gas, liquid, or solid, and is essential in the production of cryogenic temperatures and environments. For example, samples may be cooled to cryogenic temperatures by immersing them directly in a cryogenic liquid or by placing them in an atmosphere cooled by cryogenic refrigeration. In either case, the sample cools by conduction of heat to its colder surroundings.

The second physical process with cryogenic applications is evaporative cooling, which occurs because atoms or molecules have less energy when they are in the liquid state than when they are in the vapor, or gaseous, state. When a liquid evaporates, atoms or molecules at the surface acquire enough energy from the surrounding liquid to enter the gaseous state. The remaining liquid has relatively less energy, so its temperature drops. Thus, the temperature of a liquid can be lowered by encouraging the process of evaporation. The process is used in cryogenics to reduce the temperature of liquids by continuously pumping away the atoms or molecules as they leave the liquid, allowing the evaporation process to cool the remaining liquid to the desired temperature. Once the desired temperature is reached, pumping continues at a reduced level in order to maintain the lower temperature. This method can be used to reduce the temperature of any liquid. For example, it can be used to reduce the temperature of liquid nitrogen to its freezing point, or to lower the temperature of liquid helium to approximately 1K (-458°F [-272°C]).

The third process makes use of the Joule-Thompson effect, and provides a method for cooling gases. The Joule-Thompson effect involves cooling a pressurized gas by rapidly expanding its volume, or, equivalently, creating a sudden drop in pressure. The effect was discovered in 1852 by James P. Joule and William Thompson, and was crucial to the successful liquefaction of hydrogen and helium.

A valve with a small orifice (called a Joule-Thompson valve) is often used to produce the effect. High pressure gas on one side of the valve drops very suddenly, to a much lower pressure and temperature, as it passes through the orifice. In practice, the Joule-Thompson effect is used in conjunction with the process of heat conduction. For example, when Kamerlingh Onnes first liquefied helium, he did so by cooling the gas through conduction to successively lower temperatures, bringing it into contact with three successively colder liquids: oxygen, nitrogen, and hydrogen. Finally, he used a Joule-Thompson valve to expand the cold gas, and produce a mixture of gas and liquid droplets.

Today, the two effects together comprise the common refrigeration process. First, a gas is pressurized and cooled to an intermediate temperature by contact with a colder gas or liquid. Then, the gas is expanded, and its temperature drops still further. Ordinary household refrigerators and air conditioners work on this principle, using freon, which has a relatively high boiling point. Cryogenic refrigerators work on the same principle but use cryogenic gases such as helium, and repeat the process in stages, each stage having a successively colder gas until the desired temperature is reached.

The fourth process, adiabatic demagnetization, involves the use of paramagnetic salts to absorb heat. This phenomenon has been used to reduce the temperature of liquid helium to less than a thousandth of a degree above absolute zero in the following way. A paramagnetic salt is much like an enormous collection of very tiny magnets called magnetic moments. Normally, these tiny magnets are randomly aligned so the collection as a whole is not magnetic. However, when the salt is placed in a magnetic field by turning on a nearby electromagnet, the north poles of each magnetic moment are repelled by the north pole of the applied magnetic field, so many of the moments align the same way, that is, opposite to the applied field. This process decreases the entropy of the system.

Entropy is a measure of randomness in a collection; high entropy is associated with randomness, zero entropy is associated with perfect alignment. In this case, randomness in the alignment of magnetic moments has been reduced, resulting in a decrease in entropy. In the branch of physics called thermodynamics, it is shown that every collection will naturally tend to increase in entropy if left alone. Thus, when the electromagnet is switched off, the magnetic moments of the salt will tend to return to more random orientations. This requires energy, though, which the salt absorbs from the surrounding liquid, leaving the liquid at a lower temperature. Scientists know that it is not possible to achieve a temperature of absolute zero, however, in their attempts to get ever closer, a similar process called nuclear demagnetization has been used to reach temperatures just one millionth of a degree above absolute zero.