Nitrogen is a liquid under -195.8 degrees Celsius (77 K).

This is a diagram of an infrared space telescope, that needs a cold mirror and instruments. One instrument needs to be even colder, and it has a cryocooler. The instrument is in region 1 and its cryocooler is in region 3 in a warmer region of the spacecraft (see MIRI (Mid-Infrared Instrument) or James Webb Space Telescope).

A medium-sized dewar is being filled with liquid nitrogen by a larger cryogenic storage tank.

In physics, cryogenics is the production and behaviour of materials at very low temperatures.

The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of "cryogenics" and "cryogenic" by accepting a threshold of 120 K (or –153 °C) to distinguish these terms from the conventional refrigeration.^[1][2][3] This is a logical dividing line, since the normal boiling points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and normal air) lie below 120 K, while the Freon refrigerants, hydrocarbons, and other common refrigerants have boiling points above 120 K.^[4][5] The U.S. National Institute of Standards and Technology considers the field of cryogenics as that involving temperatures below -153 °C (120 K; -243.4 Fahrenheit) ^[6]

Discovery of superconducting materials with critical temperatures significantly above the boiling point of nitrogen has provided new interest in reliable, low cost methods of producing high temperature cryogenic refrigeration. The term "high temperature cryogenic" describes temperatures ranging from above the boiling point of liquid nitrogen, −195.79 °C (77.36 K; −320.42 °F), up to −50 °C (223 K; −58 °F).^[7] The discovery of superconductive properties is first attributed to Heike Kamerlingh Onnes on July 10, 1908. The discovery came after the ability to reach a temperature of 2 K. These first superconductive properties were observed in mercury at a temperature of 4.2 K.^[8]

Cryogenicists use the Kelvin or Rankine temperature scale, both of which measure from absolute zero, rather than more usual scales such as Celsius which measures from the freezing point of water at sea level^[9][10] or Fahrenheit which measures from the freezing point of a particular brine solution at sea level.^[11][12]

Definitions and distinctions

Cryogenics

The branches of engineering that involve the study of very low temperatures (ultra low temperature i.e. below 123 K), how to produce them, and how materials behave at those temperatures.

Cryobiology

The branch of biology involving the study of the effects of low temperatures on organisms (most often for the purpose of achieving cryopreservation). Other applications include Lyophilization (freeze-drying) of pharmaceutical^[13] components and medicine.

Cryoconservation of animal genetic resources

The conservation of genetic material with the intention of conserving a breed. The conservation of genetic material is not limited to non-humans. Many services provide genetic storage or the preservation of stem cells at birth. They may be used to study the generation of cell lines or for stem-cell therapy.^[14]

Cryosurgery

The branch of surgery applying cryogenic temperatures to destroy and kill tissue, e.g. cancer cells. Commonly referred to as Cryoablation.^[15]

Cryoelectronics

The study of electronic phenomena at cryogenic temperatures. Examples include superconductivity and variable-range hopping.

Cryonics

Cryopreserving humans and animals with the intention of future revival. "Cryogenics" is sometimes erroneously used to mean "Cryonics" in popular culture and the press.^[16]

Etymology

The word cryogenics stems from Greek κρύος (cryos) – "cold" + γενής (genis) – "generating".

Cryogenic fluids

Cryogenic fluids with their boiling point in Kelvin^[17] and degree Celsius.

Industrial applications

Catalogue image of a cryogenic valve

Cryogenic valves in situ, with condensed atmospheric humidity

Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally purchasable around the world. Liquid helium is also commonly used and allows for the lowest attainable temperatures to be reached.

These liquids may be stored in Dewar flasks, which are double-walled containers with a high vacuum between the walls to reduce heat transfer into the liquid. Typical laboratory Dewar flasks are spherical, made of glass and protected in a metal outer container. Dewar flasks for extremely cold liquids such as liquid helium have another double-walled container filled with liquid nitrogen. Dewar flasks are named after their inventor, James Dewar, the man who first liquefied hydrogen. Thermos bottles are smaller vacuum flasks fitted in a protective casing.

Cryogenic barcode labels are used to mark Dewar flasks containing these liquids, and will not frost over down to −195 degrees Celsius.^[18]

Cryogenic transfer pumps are the pumps used on LNG piers to transfer liquefied natural gas from LNG carriers to LNG storage tanks, as are cryogenic valves.

Other applications

Astronomical instruments on the Very Large Telescope are equipped with continuous-flow cooling systems.^[21]

Some applications of cryogenics:

• Nuclear magnetic resonance (NMR) is one of the most common methods to determine the physical and chemical properties of atoms by detecting the radio frequency absorbed and subsequent relaxation of nuclei in a magnetic field. This is one of the most commonly used characterization techniques and has applications in numerous fields. Primarily, the strong magnetic fields are generated by supercooling electromagnets, although there are spectrometers that do not require cryogens. In traditional superconducting solenoids, liquid helium is used to cool the inner coils because it has a boiling point of around 4 K at ambient pressure. Cheap metallic superconductors can be used for the coil wiring. So-called high-temperature superconducting compounds can be made to super conduct with the use of liquid nitrogen, which boils at around 77 K.
• Magnetic resonance imaging (MRI) is a complex application of NMR where the geometry of the resonances is deconvoluted and used to image objects by detecting the relaxation of protons that have been perturbed by a radio-frequency pulse in the strong magnetic field. This is most commonly used in health applications.
• In large cities, it is difficult to transmit power by overhead cables, so underground cables are used. But underground cables get heated and the resistance of the wire increases, leading to waste of power. Superconductors could be used to increase power throughput, although they would require cryogenic liquids such as nitrogen or helium to cool special alloy-containing cables to increase power transmission. Several feasibility studies have been performed and the field is the subject of an agreement within the International Energy Agency.

Cryogenic gases delivery truck at a supermarket, Ypsilanti, Michigan

• Cryogenic gases are used in transportation and storage of large masses of frozen food. When very large quantities of food must be transported to regions like war zones, earthquake hit regions, etc., they must be stored for a long time, so cryogenic food freezing is used. Cryogenic food freezing is also helpful for large scale food processing industries.
• Many infrared (forward looking infrared) cameras require their detectors to be cryogenically cooled.
• Certain rare blood groups are stored at low temperatures, such as −165 °C, at blood banks.
• Cryogenics technology using liquid nitrogen and CO₂ has been built into nightclub effect systems to create a chilling effect and white fog that can be illuminated with colored lights.
• Cryogenic cooling is used to cool the tool tip at the time of machining in manufacturing process. It increases the tool life. Oxygen is used to perform several important functions in the steel manufacturing process.
• Many rockets use cryogenic gases as propellants. These include liquid oxygen, liquid hydrogen, and liquid methane.
• By freezing the automobile or truck tire in liquid nitrogen, the rubber is made brittle and can be crushed into small particles. These particles can be used again for other items.
• Experimental research on certain physics phenomena, such as spintronics and magnetotransport properties, requires cryogenic temperatures for the effects to be observed.
• Certain vaccines must be stored at cryogenic temperatures. For example, the Pfizer–BioNTech COVID-19 vaccine must be stored at temperatures of −90 to −60 °C (−130 to −76 °F). (See cold chain.)^[22]

Production

Cryogenic cooling of devices and material is usually achieved via the use of liquid nitrogen, liquid helium, or a mechanical cryocooler (which uses high-pressure helium lines). Gifford-McMahon cryocoolers, pulse tube cryocoolers and Stirling cryocoolers are in wide use with selection based on required base temperature and cooling capacity. The most recent development in cryogenics is the use of magnets as regenerators as well as refrigerators. These devices work on the principle known as the magnetocaloric effect.

Detectors

There are various cryogenic detectors which are used to detect particles.

For cryogenic temperature measurement down to 30 K, Pt100 sensors, a resistance temperature detector (RTD), are used. For temperatures lower than 30 K, it is necessary to use a silicon diode for accuracy.

See also

• Absolute zero
• Coldest temperature recorded on Earth
• Cryogenic grinding

References

Further reading

• Haselden, G. G. (1971), Cryogenic fundamentals, Academic Press, New York, ISBN 0-12-330550-0.