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Cryogenics is the science of producing and studying low-temperature conditions. The word cryogenics comes from the Greek word cryos, 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. Permanent gases are elements that normally exist in the gaseous state and were once believed impossible 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 100°C (148°F) down to absolute zero (the coldest point a material could reach).

The temperature of any materialsolid, liquid, or gasis a measure of the energy it contains. That energy is due to various forms of motion among the atoms or molecules of which the material is made. A gas that consists of very rapidly moving molecules, for example, has a higher temperature than one with molecules that are moving more slowly.

In 1848, English physicist William Thomson (later known as Lord Kelvin; 18241907) pointed out the possibility of having a material in which particles had ceased all forms of motion. The absence of all forms of motion would result in a complete absence of heat and temperature. Thomson defined that condition as absolute zero.

Words to Know

Absolute zero: The lowest temperature possible at which all molecular motion ceases. It is equal to 273°C (459°F).

Kelvin temperature scale: A temperature scale based on absolute zero with a unit, called the kelvin, having the same size as a Celsius degree.

Superconductivity: The ability of a material to conduct electricity without resistance. An electrical current in a superconductive ring will flow indefinitely if a low temperature (about 260°C) is maintained.

Thomson's discovery became the basis of a temperature scale based on absolute zero as the lowest possible point. That scale has units the same size as the Celsius temperature scale but called kelvin units (abbreviation K). Absolute zero is represented as 0 K, where the term degree is omitted and is read as zero kelvin. The Celsius equivalent of 0 K is 273°C, and the Fahrenheit equivalent is 459°F. One can convert between Celsius and Kelvin scales by one of the following equations:

°C = K 273 or K = °C + 273

Cryogenics, then, deals with producing and maintaining environments at temperatures below about 173 K.

One aspect of cryogenics involves the development of methods for producing and maintaining very low temperatures. Another aspect includes the study of the properties of materials at cryogenic temperatures. The mechanical and electrical properties of many materials change very dramatically when cooled to 100 K or lower. For example, rubber, most plastics, and some metals become exceedingly brittle. Also many metals and ceramics lose all resistance to the flow of electricity, a phenomenon called superconductivity. In addition, helium that is cooled to very nearly absolute zero (2.2 K) changes to a state known as superfluidity. In this state, helium can flow through exceedingly narrow passages with no friction.


Cryogenics developed in the nineteenth century as a result of efforts by scientists to liquefy the permanent gases. One of the most successful of these scientists was English physicist Michael Faraday (17911867). By 1845, Faraday had managed to liquefy most permanent gases then known to exist. His procedure consisted of cooling the gas by immersion in a bath of ether and dry ice and then pressurizing the gas until it liquefied.

Six gases, however, resisted every attempt at liquefaction and were known at the time as permanent gases. They were oxygen, hydrogen, nitrogen, carbon monoxide, methane, and nitric oxide. The noble gaseshelium, neon, argon, krypton, and xenonwere yet to be discovered. Of the known permanent gases, oxygen and nitrogen (the primary constituents of air), received the most attention.

For many years investigators labored to liquefy air. Finally, in 1877, Louis Cailletet (18321913) in France and Raoul Pictet (18461929) in Switzerland succeeded in producing the first droplets of liquid air. Then, in 1883, the first measurable quantity of liquid oxygen was produced by S. F. von Wroblewski (18451888) at the University of Krakow. Oxygen was found to liquefy at 90 K, and nitrogen at 77 K.

Following the liquefaction of air, a race to liquefy hydrogen ensued. James Dewar (18421923), a Scottish chemist, succeeded in 1898. He found the boiling point of hydrogen to be a frosty 20 K. In the same year, Dewar succeeded in freezing hydrogen, thus reaching the lowest temperature achieved to that time, 14 K. Along the way, argon was discovered (1894) as an impurity in liquid nitrogen. Somewhat later, krypton and xenon were discovered (1898) during the fractional distillation of liquid argon. (Fractional distillation is accomplished by liquefying a mixture of gases, each of which has a different boiling point. When the mixture is evaporated, the gas with the highest boiling point evaporates first, followed by the gas with the second highest boiling point, and so on.)

Each of the newly discovered gases condensed at temperatures higher than the boiling point of hydrogen but lower than 173 K. The last element to be liquefied was helium gas. First discovered in 1868 in the spectrum of the Sun and later on Earth (1885), helium has the lowest boiling point of any known substance. In 1908, Dutch physicist Heike Kamerlingh Onnes (18531926) finally succeeded in liquefying helium at a temperature of 4.2 K.

Methods of producing cryogenic temperatures

Cryogenic conditions are produced by one of four basic techniques: heat conduction, evaporative cooling, cooling by rapid expansion (the Joule-Thomson 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 in cryogenic applications. The fourth process is used primarily in cryogenic applications and provides a means of approaching absolute zero.

Heat conduction is a relatively simple concept to understand. When two bodies are in contact, heat flows from the body with the higher temperature to the body with a lower temperature. Conduction can occur between any and all forms of matter, whether gas, liquid, or solid. It 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 (or transfer) of heat to its colder surroundings.

The second process for producing cryogenic conditions is evaporative cooling. Humans are familiar with this process because it is a mechanism by which our bodies lose heat. Atoms and molecules in the gaseous state are moving faster than atoms and molecules in the liquid state. Add heat energy to the particles in a liquid and they will become gaseous. Liquid perspiration on human skin behaves in this way. Perspiration absorbs body heat, becomes a gas, and evaporates from the skin. As a result of that heat loss, the body cools down.

Cryogens and Their Boiling Points

Boiling Point
Cryogen °F °C K
Oxygen 297 183 90
Nitrogen 320 196 77
Hydrogen 423 253 20
Helium 452 269 4.2
Neon 411 246 27
Argon 302 186 87
Krypton 242 153 120
Xenon 161 107 166

In cryogenics, a container of liquid is allowed to evaporate. Heat from within the liquid is used to convert particles at the surface of the liquid to gas. The gas is then pumped away. More heat from the liquid converts another surface layer of particles to the gaseous state, which is also pumped away. The longer this process continues, the more heat is removed from the liquid and the lower its temperature drops. Once some given temperature is reached, pumping continues at a reduced level in order to maintain the lower temperature. This method can be used to reduced 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 1 K.

A third process makes use of the Joule-Thomson effect, which was discovered by English physicist James Prescott Joule (18181889) and William Thomson, Lord Kelvin, in 1852. The Joule-Thomson effect depends on the relationship of volume (bulk or mass), pressure, and temperature in a gas. Change any one of these three variables, and at least one of the other two (or both) will also change. Joule and Thomson found, for example, that allowing a gas to expand very rapidly causes its temperature to drop dramatically. Reducing the pressure on a gas accomplishes the same effect.

To cool a gas using the Joule-Thomson effect, the gas is first pumped into a container under high pressure. The container is fitted with a valve with a very small opening. When the valve is opened, the gas escapes from the container and expands quickly. At the same time, its temperature drops. The first great success for the Joule-Thomson effect in cryogenics was achieved by Kamerlingh Onnes in 1908 when he liquefied helium.

The Joule-Thomson effect is an important part of our lives today, even though we may not be aware of it. Ordinary household refrigerators and air conditioners operate on this principle. 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. The heat needed to keep this cycle operating comes from the inside of the refrigerator or the interior of a room, producing the desired cooling effect.

The fourth process for producing cryogenic temperatures uses a phenomenon known as adiabatic demagnetization. Adiabatic demagnetization makes use of special substances known as paramagnetic salts. A paramagnetic salt consists of a very large collection of particles that act like tiny (atom-sized) magnets. Normally these magnetic particles are spread out in all possible directions. As a result, the salt itself is not magnetic. That condition changes when the salt is placed between the poles of a magnet. The magnetic field of the magnet causes all the tiny magnetic particles in the salt to line up in the same direction. The salt becomes magnetic, too.

At this exact moment, however, suppose that the external magnet is taken away and the paramagnetic salt is placed within a liquid. Almost immediately, the tiny magnetic particles within the salt return to their random, every-which-way condition. To make this change, however, the particles require an input of energy. In this example, the energy is taken from the liquid into which the salt was placed. As the liquid gives up energy to the paramagnetic salt, its temperature drops.

Adiabatic demagnetization has been used to produce some of the coldest temperatures ever observedwithin a few thousandths of a degree kelvin of absolute zero. A related process involving the magnetization and demagnetization of atomic nuclei is known as nuclear demagnetization. With nuclear demagnetization, temperatures within a few millionths of a degree of absolute zero have been reached.


Following his successful liquefaction of helium in 1908, Kamerlingh Onnes turned his attention to the study of properties of other materials at very low temperatures. The first property he investigated was the electrical resistance of metals. Electrical resistance is the tendency of a substance to prevent the flow of an electrical current through it. Scientists had long known that electrical resistance tends to decrease with decreasing temperature. They assumed that resistance would completely disappear at absolute zero.

Research in this area had great practical importance. All electrical appliances (ovens, toasters, television sets, and radios, for example) operate with low efficiency because so much energy is wasted in overcoming electrical resistance. An appliance with no electrical resistance could operate at much less cost than existing appliances.

What Onnes discovered, however, was that for some metals, electrical resistance drops to zero very suddenly at temperatures above absolute zero. The effect is called superconductivity and has some very important applications in today's world. For example, superconductors are used to make magnets for particle accelerators (devices used, among other things, to study subatomic particles such as electrons and protons) and for magnetic resonance imaging (MRI) systems (a diagnostic tool used in many hospitals).

The discovery of superconductivity led other scientists to study a variety of material properties at cryogenic temperatures. Today, physicists, chemists, material scientists, and biologists study the properties of metals, as well as the properties of insulators, semiconductors, plastics, composites, and living tissue. Over the years, this research has resulted in the identification of a number of useful properties. One such property common to most materials that are subjected to extremely low temperatures is brittleness. The recycling industry takes advantage of this by immersing recyclables in liquid nitrogen, after which they are easily pulverized and separated for reprocessing.

Still another cryogenic material property that is sometimes useful is that of thermal contraction. Materials shrink when cooled. To a point (about the temperature of liquid nitrogen), the colder a material gets the more it shrinks. An example is the use of liquid nitrogen in the assembly of some automobile engines. In order to get extremely tight fits when installing valve seats, for instance, the seats are cooled to liquid nitrogen temperatures, whereupon they contract and are easily inserted in the engine head. When they warm up, a perfect fit results.

Cryogenic liquids are also used in the space program. For example, cryogenic materials are used to propel rockets into space. A tank of liquid hydrogen provides the fuel to be burned and a second tank of liquid oxygen is provided for combustion.

Another space application of cryogenics is the use of liquid helium to cool orbiting infrared telescopes. Infrared telescopes detect objects in space not from the light they give off but from the infrared radiation (heat) they emit. However, the operation of the telescope itself also gives off heat. What can be done to prevent the instrument from being blinded by its own heat to the infrared radiation from stars? The answer is to cool parts of the telescope with liquid helium. At the temperature of liquid helium (1.8 K) the telescope can easily pick up infrared radiation of the stars, whose temperature is about 3 K.

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Cryogenics is the science that addresses the production and effects of very low temperatures. The word originates from the Greek words kryos meaning "frost" and genic meaning "to produce." Under such a definition, it could be used to include all temperatures below the freezing point of water (0°C). However, Professor Heike Kamerlingh Onnes of the University of Leiden in the Netherlands first used the word in 1894 to describe the art and science of producing much lower temperatures. He used the word in reference to the liquefaction of permanent gases such as oxygen, nitrogen, hydrogen, and helium. Oxygen had been liquefied at 183°C a few years earlier (in 1887), and a race was in progress to liquefy the remaining permanent gases at even lower temperatures. The techniques employed in producing such low temperatures were quite different from those used somewhat earlier in the production of artificial ice. In particular, efficient heat exchangers are required to reach very low temperatures. Over the years the term "cryogenics" has generally been used to refer to temperatures below approximately 150°C.

According to the laws of thermodynamics, there exists a limit to the lowest temperature that can be achieved, which is known as absolute zero. Molecules are in their lowest, but finite, energy state at absolute zero. Such a temperature is impossible to reach because the input power required approaches infinity. However, temperatures within a few billionths of a degree above absolute zero have been achieved. Absolute zero is the zero of the absolute or thermodynamic temperature scale. It is equal to 273.15°C or 459.67°F. The metric or SI (International System) absolute scale is known as the Kelvin scale whose unit is the kelvin (not Kelvin), which has the same magnitude as the degree Celsius. The symbol for the Kelvin scale is K, as adopted by the Thirteenth General Council on Weights and Measures (CGPM) in 1968, and not °K. Thus, 0°C equals 273.15 K. The English absolute scale, known as the Rankine scale, uses the symbol °R and has an increment the same as that of the Fahrenheit scale. In terms of the Kelvin scale, the cryogenic region is often considered to be that below approximately 120 K (153°C). The common permanent gases referred to earlier change from gas to liquid at atmospheric pressure at the temperatures shown in Table 1, called the normal boiling point (NBP). Such liquids are known as cryogenic liquids or cryogens. When liquid helium is cooled further to 2.17 K or below, it becomes a superfluid with very unusual properties associated with being in the quantum mechanical ground state. For example, it has zero viscosity and produces a film that can creep up and over the walls of an open container, such as a beaker, and drip off the bottom as long as the temperature of the container remains below 2.17 K.

The measurement of cryogenic temperatures requires methods that may not be so familiar to the general public. Normal mercury or alcohol thermometers freeze at such low temperatures and become useless. The platinum resistance thermometer has a well-defined behavior of electrical resistance versus temperature and is commonly used to measure temperatures accurately, including cryogenic temperatures down to about 20 K. Certain semiconducting materials, such as doped germanium, are also useful as electrical resistance thermometers for temperatures down to 1 K and below, as long as they are calibrated over the range they are to be used. Such

Cryogen (K) (°C) (°R) (°F)
Methane 111.7 161.5 201.1 258.6
Oxygen 90.2 183.0 162.4 297.3
Nitrogen 77.4 195.8 139.3 320.4
Hydrogen 20.3 252.9 36.5 423.2
Helium 4.2 269.0 7.6 452.1
Absolute zero 0 273.15 0 459.67

secondary thermometers are calibrated against primary thermometers that utilize fundamental laws of physics in which a physical variable changes in a well-known theoretical way with temperature.

The production of cryogenic temperatures almost always utilizes the compression and expansion of gases. In a typical air liquefaction process the air is compressed, causing it to heat, and allowed to cool back to room temperature while still pressurized. The compressed air is further cooled in a heat exchanger before it is allowed to expand back to atmospheric pressure. The expansion causes the air to cool and a portion of it to liquefy. The remaining cooled gaseous portion is returned through the other side of the heat exchanger where it precools the incoming high-pressure air before returning to the compressor. The liquid portion is usually distilled to produce liquid oxygen, liquid nitrogen, and liquid argon. Other gases, such as helium, are used in a similar process to produce even lower temperatures, but several stages of expansion are necessary.

Cryogenics has many applications. Cryogenic liquids like oxygen, nitrogen, and argon are often used in industrial and medical applications. The electrical resistance of most metals decreases as temperature decreases. Certain metals lose all electrical resistance below some transition temperature and become superconductors. An electromagnet wound with a wire of such a metal can produce extremely high magnetic fields with no generation of heat and no consumption of electric power once the field is established and the metal remains cold. These metals, typically niobium alloys cooled to 4.2 K, are used for the magnets of magnetic resonance imaging (MRI) systems in most hospitals. Superconductivity in some metals was first discovered in 1911 by Kamerlingh Onnes, but since 1986, another class of materials, known as high temperature superconductors, have been found to be superconducting at much higher temperatures, currently up to about 145 K. They are a type of ceramic, and because of their brittle nature, they are more difficult to fabricate into wires for magnets.

Other applications of cryogenics include fast freezing of some foods and the preservation of some biological materials such as livestock semen as well as human blood, tissue, and embryos. The practice of freezing an entire human body after death in the hope of later restoring life is known as cryonics, but it is not an accepted scientific application of cryogenics. The freezing of portions of the body to destroy unwanted or malfunctioning tissue is known as cryosurgery. It is used to treat cancers and abnormalities of the skin, cervix, uterus, prostate gland, and liver.

see also Superconductors; Temperature; Thermodynamics.

Ray Radebaugh


Barron, Randall (1985). Cryogenic Systems. Oxford: Oxford Press.

Flynn, Thomas (1997). Cryogenic Engineering. New York: Marcel Dekker.

Scurlock, Ralph G., ed. (1993). History and Origins of Cryogenics. Oxford: Clarendon Press.

Seeber, Bernd, ed. (1998). Handbook of Applied Superconductivity. Bristol: Institute of Physics Publishing.

Shachtman, Tom (1999). Absolute Zero and the Conquest of Cold. Boston: Houghton Mifflin Company.

Weisend, John G., II, ed. (1998). Handbook of Cryogenic Engineering. Philadelphia: Taylor and Francis.

Internet Resources

"Cold Facts Newsletter." Available from Cryogenic Society of America, <>.

Cryogenic Information Center. More information available from <>.

Cryogenic Technologies Group, National Institute of Standards and Technology. More information available from <>.

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Cryogenics is the study and use of materials at extremely low temperatures. Such low temperatures cause changes in the physical properties of materials that allow them to be used in unusual engineering, industrial, and medical applications. For example, in the cryogenic temperature range, air becomes a liquidor even a solidand living tissue freezes instantly. Matter behaves strangely at the lowest temperatures of the cryogenic range. Electric currents never stop flowing, liquids run uphill, and rubber becomes as brittle as glass. In medicine, cryogenic cooling is used in some diagnostic techniques, such as magnetic resonance imaging (MRI). Cryosurgery uses liquid nitrogen to kill unhealthy tissue by freezing it. Cryogenics is expected to play an important role in the development of better procedures for preserving human organs for transplant.

Early Research

British chemists Michael Faraday (1791-1867) and Sir Humphry Davy (1778-1829) did pioneering work in low-temperature physics that led to the development of cryogenics. In the early to middle 1800s they were able to produce gases by heating mixtures at one end of a sealed tube in the shape of an inverted "V." A salt and ice mixture was used to cool the other end of the tube. This combination of reduced temperature and increased pressure caused the gas that was produced to liquefy (turn to a liguid). When they opened the tube, the liquid quickly evaporated and cooled to its normal boiling point.

In 1877, French mining engineer Louis Paul Cailletet announced that he had liquefied oxygen and nitrogen. Cailletet was able to produce only a few droplets of these liquefied gases, however. In his research with oxygen, Cailletet collected the oxygen in a sturdy container and cooled it by evaporating (drying up) sulphur dioxide in contact with the container. He then compressed the oxygen as much as possible with his equipment. Next he reduced the pressure suddenly, causing the oxygen to expand. The sudden cooling that resulted caused a few drops of liquid oxygen to form.

The need for a way to store liquefied gases led to another important development in cryogenics. In 1891 Scottish chemist James Dewar (1842-1923) inroduced the container known today as the "Dewar flask." The Dewar flask is actually two flasks, one within the other, separated by an evacuated space (a vacuum). The inside of the outer flask and the outside of the inner flask are coated with silver. The vacuum and the silvered sides of the container are designed to prevent heat passage.

Dewar was also the first person to liquefy hydrogen in 1898. In 1908 Dutch physicist Heike Kamerlingh Onnes (1853-1926; 1913 Nobel Prize winner for physics) liquefied the most difficult gas of all, helium. He liquefied it at the lowest temperature ever achieved in a laboratory to that date, 4.2 kelvins (the kelvin measurement is a scale of temperatures measured in degrees Celcius from absolute zero). This marked a significant milestone in the history of cryogenics. Since that achievement, increased attention has been devoted to the study of physical phenomena of substances at very low temperatures.

The Cooling Process

A substance is normally cooled by placing it next to something colder. To make the substance supercold, however, heat must also be removed and the substance must be insulated (encased). An important method of cryogenic supercooling involves liquefying gases and using these gases to cool other substances. One technique is to convert to liquid form a gas that can be liquefied by pressure alone. Then a gas requiring a lower temperature to become a liquid is placed in a container and immersed (dipped) in the first. The gas that is already liquefied cools the second and converts it to a liquid. After several repetitions of this process, the targeted gas is liquefied. A Dewar flask is normally used to store such very low temperature liquefied gases.

Since the 1930s, cryogenic cooling has most often been achieved through magnetic means. In 1926, Canadian chemist William Francis Giauque theorized that if the disorderly spin of electrons in a substance could be slowed down, then the substance would cool down. His experiments proved this theory. In cooling by demagnetization, a strong magnetic force is used to give the outside energy required to line up the molecules of a paramagnetic substance (one made up of paramagnetic ions). This also raises the temperature. At the same time the substance is cooled in liquid helium. When the substance cools down to its starting temperature, the magnetic field is removed. This causes the ions to resume their disorderly alignment (order). The energy the ions use to move comes from the heat energy of the substance, which causes the temperature of the substance to drop.

Liquid nitrogen is one of the safest cooling agents available. In medicine it is used to kill unhealthy tissues by freezing them. Cryogenic processes are also used to supply "banks" storing eye corneas, blood, and sperm for future surgical procedures. Some embryos have also been frozen and stored for later implantation (surgical placement) in women.


In 1961 American surgeon Irving S. Cooper introduced a freezing technique called cryosurgery. Cryosurgery is relatively bloodless because the low temperatures used constrict the blood vessels, stemming the flow. Special instruments are used that have freezing tips to kill the damaged tissue and shields to protect surrounding tissue. Cooper used cryosurgery to freeze and destroy damaged tissue in the brains of patients with Parkinson's disease (a degenerative illness). Since then, cryosurgery has found many applications. It is used to repair detached retinas and to remove cataracts. It is also used to treat liver cancer and prostate cancer.

Cryosurgery is also widely used in the fields of dermatology, gynecology, plastic surgery, orthopedics, and podiatry. Cryosurgery has also been used successfully for more than 30 years in veterinary medicine.

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cryogenics Branch of physics that studies materials and effects at temperatures approaching absolute zero. Some materials exhibit highly unusual properties such as superconductivity or superfluidity at such temperatures. The Joule-Thomson effect can be used to reduce temperatures to 0.3K. To cool gases even further requires magnetic process such as the adiabatic process. Cryogenics has been used to freeze human bodies in the uncertain hope that future technology may be able to revive the subjects.

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cry·o·gen·ics / ˌkrīəˈjeniks/ • pl. n. [treated as sing.] the branch of physics dealing with the production and effects of very low temperatures. ∎ another term for cryonics. DERIVATIVES: cry·o·gen·ic adj. cry·o·gen·i·cal·ly / -ik(ə)lē/ adv.

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cryogenics: see low-temperature physics.

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