Cryogenics

Cryogenics is the science of producing and studying low-temperature environments. The word cryogenics comes from the Greek word "kryos," meaning cold; combined with a shortened form of the English verb "to generate," it has come to mean the generation of temperatures well below those of normal human experience.

More specifically, a low-temperature environment is termed a cryogenic environment when the temperature range is below the point at which permanent gases begin to liquefy. Among others, they include oxygen, nitrogen, hydrogen, and helium. The origin of cryogenics as a scientific discipline coincided with the discovery by nineteenth century scientists, that the permanent gases can be liquefied at exceedingly low temperatures. Consequently, the term cryogenic applies to temperatures from approximately -148°F (-100°C) down to absolute zero.

The temperature of a sample, whether it be a gas, liquid, or solid, is a measure of the energy it contains, energy that is present in the form of vibrating atoms and moving molecules. Absolute zero represents the lowest attainable temperature and is associated with the complete absence of atomic and molecular motion. The existence of absolute zero was first pointed out in 1848 by William Thompson (later to become Lord Kelvin), and is now known to be -459°F (-273°C). It is the basis of an absolute temperature scale, called the Kelvin scale, whose unit, called a Kelvin rather than a degree, is the same size as the Celsius degree. Thus, -459°F corresponds to -273°C corresponds to 0K (note that by convention the degree symbol is omitted, so that 0K is read "zero Kelvin"). Cryogenics, then, deals with producing and maintaining environments at temperatures below about 173K (-148°F [-100°C]).

In addition to studying methods for producing and maintaining cold environments, the field of cryogenics has also come to include studying the properties of materials at cryogenic temperatures. The mechanical and electrical properties of many materials change very dramatically when cooled to 100K or lower. For example, rubber, most plastics, and some metals become exceedingly brittle, and nearly all materials contract. In addition, many metals and ceramics lose all resistance to the flow of electricity, a phenomenon called superconductivity. Very near absolute zero (2.2K) liquid helium undergoes a transition to a state of superfluidity, in which it can flow through exceedingly narrow passages with no friction.