Cryogenics, the production of very low temperatures and the study of phenomena at those temperatures. The range of temperatures with which cryogenics is concerned is not precisely defined, but it generally includes temperatures from -238° F. (- 150° C.) down to absolute zero (- 459.67° F., or -273.15° C.). The coldest temperatures that have been obtained are within a fraction of a degree above absolute zero.
At very low temperatures, certain materials take on peculiar properties. Liquid helium acts as though it has no viscosity (resistance to flowing), and its thermal conductivity is several hundred times that of copper or silver at room temperature. Some metals become superconductive-they lose all resistance to electric current, and current will continue to flow through them even after the power is cut off. These and other unique properties created by cold give cryogenics its practical importance.
A major application of cryogenics is the fractional distillation of air to produce oxygen, nitrogen, and other gases. This process requires cooling the air to low temperatures to liquefy the gases in it. Natural gas, oxygen, nitrogen, and other gases are often liquefied for storage and transport because they occupy much less space in liquid rather than gaseous form.
Liquefied gases are used as coolants in various industrial processes. In the food industry, liquid nitrogen is used to freeze foods quickly. In medicine, it is used to chill a needlelike instrument called a cryoprobe. Doctors use the cryoprobe to freeze and destroy certain kinds of tissue, including warts, skin tumors, and eye tumors. Cryogenic temperatures are also used to preserve certain biological substances, including bull semen (for breeding purposes), whole blood, and bone marrow.
One way to produce ultracold temperatures is through magnetism. Certain materials will become warm when magnetized and cool when demagnetized. By carefully controlling this process, magnetic refrigerators can be made that will reach extremely low temperatures. Another method of producing ultracold is to compress a gas, remove the heat by means of ordinary refrigeration, and then allow the gas to expand.