I.

Introduction

Cryogenics, study and use of materials at very low temperatures. The National Institute of Standards and Technology (NIST) 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 can approach, but can never reach, absolute zero, the theoretical absence of all thermal energy (heat) at zero degrees on the Kelvin scale (0 K), equivalent to -273.15°C (-459.67°F). The term cryogenics comes from a combination of Greek words meaning “frost producing.”

Cryogenic temperatures are achieved by a number of methods under laboratory or industrial conditions, often applied in a sequence. The earliest cooling methods used either the rapid evaporation of volatile liquids or the expansion of gases confined initially at pressures of 150 to 200 atmospheres. (One atmosphere is a unit of pressure based on normal atmospheric pressure measured at sea level.) Devices used to chill materials to cryogenic temperatures are called cryogenic refrigerators or “cryocoolers.” Other techniques employed for specialized scientific research include using laser beams or radio waves to remove more and more thermal energy from a substance, often when it is confined in a magnetic field. Scientists have cooled substances to temperatures as low as a few hundred billionths of a degree Kelvin above absolute zero.

II.

Development of Cryogenics

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).

The temperature of a gas that is allowed to expand can increase or decrease depending on the initial temperature of the gas. The special temperature at which a particular gas will cool down instead of heat up when it expands is called the inversion temperature. 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.

The increased efficiency of having a refrigerant gas operate in a reciprocating engine or in a turbine continued to be a challenge. The work of the Soviet physicist Peter Leonidovich Kapitza and the American mechanical engineer Samuel Collins led to better ways to study phenomena at low temperatures. A helium-liquefier based on Collins’s design 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.

Cryogenic Technologies

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 (see Magnetism) 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.

To reach temperatures closer to absolute zero, techniques using magnetic fields, lasers, and radio waves can be used. In one approach, gases of atoms are confined by a magnetic field. Multiple laser beams are used to cool the atoms in the gas by first exciting the electrons, which then emit photons that carry energy away from the atoms. After the gas has been placed in a much stronger magnetic trap, radio waves can then be used to selectively remove the highest-energy atoms, leaving only atoms at the lowest energy state. Another technique is to trap atoms or molecules in a “box” of laser light. Two additional lasers can be used to create an optical wall that confines the atoms or molecules on one side of the laser light box. The space that holds the atoms and molecules can be expanded and contracted with the lasers, lowering the temperature with no addition of heat.

Different types of cryogenic refrigeration devices (commonly called cryocoolers) have been developed for use in industry, in military and space technology, and in scientific research. Most of these devices use the expansion of gases or fluids to draw away heat. Among the most widely used cryocoolers are Stirling cryocoolers, which work with the aid of a compressor. However, moving mechanical parts can cause vibrations and wear. Another design called a pulse tube cryocooler eliminates most moving parts that cause friction and wear, and uses acoustic power in an oscillating gas system. Research is under way to develop more efficient and compact pulse tube cryocoolers.

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. Devices used to maintain substances or objects at cryogenic temperatures are call cryostats.

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. Available devices include cryogenic thermometers that use semiconductor film materials and diode temperature microsensors.

IV.

Changes in Properties at Cryogenic Temperatures

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, metalloids, and some metal alloys decreases abruptly to zero at temperatures below 23 K, a property called superconductivity. 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. Since the discovery of the first so-called high-temperature superconductor in 1986, researchers have identified a number of ceramic compounds containing copper-oxide that become superconducting at temperatures as high as 125 K. 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.

The behavior of helium at low temperatures is remarkable in a number of ways. The two stable isotopes of helium, helium-4 (2 protons + 2 neutrons) and helium-3 (2 protons + 1 neutron), show unusual properties at different temperatures. Both isotopes remain 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 unless its temperature is reduced even further, to less than 0.00093 K (-273.15°C/-459.67°F).

One of the most dramatic achievements in cryogenic research has been the creation of Bose-Einstein condensates. When a gas of atoms that are composite bosons (atoms with even numbers of protons and neutrons in the nucleus) is confined in a magnetic field and cooled to extremely low temperatures using lasers and radio waves, some of the atoms in the gas can take on the same quantum state and behave together like a single giant particle. This special state of matter was predicted by the physicists Albert Einstein and S. N. Bose. Researchers have also created a related low-temperature quantum phenomenon call a fermionic condensate. Atoms in a gas that are composite fermions (atoms with an odd number of protons and neutrons in the nucleus) form pairs and behave like bosons. Like the atoms in a Bose-Einstein condensate, the atoms in a fermionic condensate can then “condense” into the same quantum state.

V.

Applications of Cryogenics

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 cooling is often used in space telescopes that observe objects in infrared and microwave wavelengths. More efficient and compact cryocoolers allow cryogenic temperatures to be used in an increasing variety of military, medical, scientific, civilian, and commercial applications, including infrared sensors, superconducting electronics, and magnetic levitation trains.

Bose-Einstein condensates and fermionic condensates are useful for scientific research into quantum phenomena such as superfluidity and superconductivity. Such unusual states of matter may also lead to quantum computing and devices such as atomic lasers. Chemical reactions and other properties of molecules can also be studied at cryogenic temperatures.

Cryogenic temperatures are also used in cryobiology—the study of life and life processes at very low temperatures. Cryobiology includes cold temperatures used in medicine and surgery, as well as the cryogenic preservation of biological and medical materials.