I

INTRODUCTION

Cryogenics , study and use of materials at very low temperatures. The upper limit of cryogenic temperatures has not been agreed on, but the National Institute of Standards and Technology has suggested that the term cryogenics be applied to all temperatures below -150° C (-238° F or 123° above absolute zero on the Kelvin scale). Some scientists regard the normal boiling point of oxygen (-183° C or -297° F), as the upper limit. Cryogenic temperatures are achieved either by the rapid evaporation of volatile liquids or by the expansion of gases confined initially at pressures of 150 to 200 atmospheres. The expansion may be simple, that is, through a valve to a region of lower pressure, or it may occur in the cylinder of a reciprocating engine, with the gas driving the piston of the engine. The second method is more efficient but is also more difficult to apply. See Heat.

II

DEVELOPMENT

Pioneering work in low-temperature physics by the British chemists Sir Humphry Davy and Michael Faraday, between 1823 and 1845, prepared the way for the development of cryogenics. Davy and Faraday generated gases by heating an appropriate mixture at one end of a sealed tube shaped like an inverted V. The other end was chilled in a salt-ice mixture. The combination of reduced temperature and increased pressure caused the evolved gas to liquefy. When the tube was opened, the liquid evaporated rapidly and cooled to its normal boiling point. By evaporating solid carbon dioxide mixed with ether, at low pressure, Faraday finally succeeded in reaching a temperature of about 163 K (about -110° C/-166° F).

If a gas initially at a moderate temperature is expanded through a valve, its temperature increases. But if its initial temperature is below the inversion temperature, the expansion will cause a temperature reduction as the result of what is called the Joule-Thomson effect. The inversion temperatures of hydrogen and helium, two primary cryogenic gases, are extremely low, and to achieve a temperature reduction through expansion, these gases must first be precooled below their inversion temperatures, the hydrogen by liquid air and the helium by liquid hydrogen. This method is generally not able to bring about liquefaction in one step, but by cascading the effects, the French physicist Louis Paul Cailletet and the Swiss scientist Raoul Pierre Pictet were able in 1877 to produce droplets of liquid oxygen. The success of these experimenters marked the end of the idea of permanent gases and established the possibility of liquefying any gas by moderate compression at temperatures below the critical temperature.

The Dutch physicist Heike Kamerlingh Onnes set up the first liquid-air plant in 1894, using the cascade principle. Investigators in Britain, France, Germany, and Russia developed various improvements in the process during the following 40 years. The British chemist Sir James Dewar first liquefied hydrogen in 1898 and Kamerlingh Onnes liquefied helium, the most difficult of the gases to liquefy, in 1908. Since then increased attention has been given to studying phenomena at low temperatures. The increased efficiency of having a refrigerant gas operate in a reciprocating engine or in a turbine has continued to be a challenge. The work of the Soviet physicist Peter Leonidovich Kapitza and the American mechanical engineer Samuel Collins has been noteworthy. A helium-liquefier based on Collins's design has provided the opportunity for many nonspecialist laboratories to conduct experiments at the normal boiling point of helium, 4.2 K (-268.9° C/-452.0° F).

III

ADIABATIC DEMAGNETIZATION

The evaporation of liquid helium at reduced pressures produces temperatures as low as 0.7 K (-272.44° C/-458.4° F). Still lower temperatures can be attained by adiabatic demagnetization. This procedure requires that a magnetic field be established around a paramagnetic substance, that is, a substance made of paramagnetic ions, while the substance is cooled in liquid helium. The field aligns the ionic magnets and later, when the field is removed, the tiny magnets resume their random alignments, reducing the thermal energy of the whole sample in the process. The temperature, therefore, falls to levels as low as 0.002 K (-273.15° C/-459.67° F). Similar alignment of atomic nuclei that have periods of magnetization followed by removal of the magnetic field have produced temperatures close to 0.00001 K.

For storing liquids at cryogenic temperatures, Dewar flasks have proved useful. Such vessels consist of two flasks, one within the other, separated by an evacuated space. The outside of the inner flask and the inside of the outer flask are both silvered to prevent radiant heat from passing across the vacuum. Substances colder than liquid air cannot be handled in open Dewar flasks because air would condense in the sample or form a solid plug to prevent escape of released vapors; the accumulated vapors would eventually rupture the container.

Measurement of temperatures in the cryogenic range presents problems. One procedure is to measure the pressure of a known quantity of hydrogen or helium, but this procedure fails at the lowest temperatures. The vapor pressure of helium-4, that is, helium of atomic mass 4, or of helium-3 (atomic mass 3) supplements the preceding method. Determinations of the electrical resistance of metals or semiconductors and their magnetic measurements extend the range still further.

IV

CHANGE IN PROPERTIES

At cryogenic temperatures many materials behave in ways unfamiliar under ordinary conditions. Mercury solidifies and rubber becomes as brittle as glass. The specific heats of gases and solids decrease in a way that confirms the predictions of quantum theory. The electrical resistance of many, but not all, metals and metalloids decreases abruptly to zero at temperatures of a few degrees Kelvin. If an electric current is introduced into a ring of metal that has been cooled to the superconductive state, it will continue to travel around the ring and may be detected hours later. The ability of a superconductive material to retain current has led to experiments for constructing computer memory modules that would operate at these low temperatures. As of the late 1980s, however, supercooled computers had not yet proved practical, even with the discovery of materials that exhibit superconductivity at somewhat higher than liquid-helium temperatures. See Superconductivity.

The behavior of helium-4 at low temperatures is remarkable in two ways. First, it remains liquid even after the most extreme cooling. To solidify helium-4 it is necessary to subject the liquid to a pressure in excess of 25 atmospheres. Liquid helium-4 changes, furthermore, to a superfluid state at temperatures below 2.18 K (-270.97° C/-455.75° F). In this state its viscosity appears to be nearly zero. It forms thick films on the surface of the containers, and helium flows through the film without resistance. Theory still fails to account fully for this behavior. Helium-3 does not exhibit superfluidity.

V

APPLICATIONS

Among the many important industrial applications of cryogenics are the large-scale production of oxygen and nitrogen from air. The oxygen can be used in a variety of ways, for example, in rocket engines, for cutting and welding torches, for supporting life in space and deep-sea vehicles, and for blast furnace operations. The nitrogen goes into the making of ammonia for fertilizers, and it is used to prepare frozen foods by cooling them rapidly enough to prevent destruction of cell tissues. It can also serve as a refrigerant and for transporting frozen foods.

Cryogenics has also made possible the commercial transportation of liquefied natural gas. Without cryogenics, nuclear research would lack liquid hydrogen and helium for use in particle detectors and for the powerful electromagnets needed in large particle accelerators. Such magnets are also being used in nuclear fusion research. Infrared devices, masers, and lasers can employ cryogenic temperatures as well.

Cryogenic surgery, or cryosurgery, is being used for the treatment of Parkinson disease, the technique being based on the selective destruction of tissue by freezing it with a small cryogenic probe. A similar technique has also been employed to destroy brain tumors and to arrest cervical cancer.