Understanding the Concept of Cryogenics

What Cryogenics Is and How It's Used

Liquid nitrogen is a good example of a cryogenic fluid.
Liquid nitrogen is a good example of a cryogenic fluid. Science Photo Library / Getty Images

Cryogenics is defined as the scientific study of materials and their behavior at extremely low temperatures. The word comes from the Greek cryo, which means "cold", and genic, which means "producing". The term is usually encountered in the context of physics, materials science, and medicine. A scientists who studies cryogenics is called a cryogenicist. A cryogenic material may be termed a cryogen.

Although cold temperatures may be reported using any temperature scale, the Kelvin and Rankine scales are most common because they are absolute scales that have positive numbers.

Exactly how cold a substance has to be to be considered "cryogenic" is a matter of some debate by the scientific community. The U.S. National Institute of Standards and Technology (NIST) considers cryogenics to include temperatures below −180 °C (93.15 K; −292.00 °F), which is a temperature above which common refrigerants (e.g., hydrogen sulfide, freon) are gases and below which "permanent gases" (e.g., air, nitrogen, oxygen, neon, hydrogen, helium) are liquids. There is also a field of study called "high temperature cryogenics", which involves temperatures above the boiling point of liquid nitrogen at ordinary pressure (−195.79 °C (77.36 K; −320.42 °F), up to −50 °C (223.15 K; −58.00 °F).

Measuring the temperature of cryogens requires special sensors.

Resistance temperature detectors (RTDs) are used to take temperature measurements as low as 30 K. Below 30 K, silicon diodes are often used. Cryogenic particle detectors are sensors that operate a few degrees above absolute zero and are used to detect photons and elementary particles.

Cryogenic liquids are typically stored in devices called Dewar flasks.

These are double-walled containers that have a vacuum between the walls for insulation. Dewar flasks intended for use with extremely cold liquids (e.g., liquid helium) have an additional insulating container filled with liquid nitrogen. Dewar flasks are named for their inventor, James Dewar. The flasks allow gas to escape the container to prevent pressure buildup from boiling that could lead to an explosion.

Cryogenic Fluids

The following fluids are most often used in cryogenics:

FluidBoiling Point (K)

Uses of Cryogenics

There are several applications of cryogenics. It is used to produce cryogenic fuels for rockets, including liquid hydrogen and liquid oxygen (LOX). The strong electromagnetic fields needed for nuclear magnetic resonance (NMR) are usually produced by supercooling electromagnets with cryogens. Magnetic resonance imaging (MRI) is an application of NMR that uses liquid helium. Infrared cameras frequently require cryogenic cooling. Cryogenic freezing of food is used to transport or store large quantities of food. Liquid nitrogen is used to produce fog for special effects and even specialty cocktails and food.

Freezing materials using cryogens can make them brittle enough to be broken into small pieces for recycling. Cryogenic temperatures are used to store tissue and blood specimens and to preserve experimental samples. Cryogenic cooling of superconductors may be used to increase electric power transmission for big cities. Cryogenic processing is used as part of some alloy treatments and to facilitate low temperature chemical reactions (e.g., to make statin drugs). Cryomilling is used to mill materials that may be too soft or elastic to be milled at ordinary temperatures. Cooling of molecules (down to hundreds of nano Kelvins) may be used to form exotic states of matter. The Cold Atom Laboratory (CAL) is an instrument designed for use in microgravity to form Bose Einstein condensates (around 1 pico Kelvin temperature) and test laws of quantum mechanics and other physics principles.

Cryogenic Disciplines

Cryogenics is a broad field that encompasses several disciplines, including:

Cryonics - Cryonics is the cryopreservation of animals and humans with the goal of reviving them in the future.

Cryosurgery - This is a branch of surgery in which cryogenic temperatures are used to kill unwanted or malignant tissues, such as cancer cells or moles.

Cryoelectronics - This is the study of superconductivity, variable-range hopping, and other electronic phenomena at low temperature. The practical application of cryoelectronics is called cryotronics.

Cryobiology - This is the study of the effects of low temperatures on organisms, including the preservation of organisms, tissue, and genetic material using cryopreservation.

Cryogenics Fun Fact

While cryogenics usually involves temperature below the freezing point of liquid nitrogen yet above that of absolute zero, researchers have achieved temperatures below absolute zero (so-called negative Kelvin temperatures). In 2013 Ulrich Schneider at the University of Munich (Germany) cooled gas below absolute zero, which reportedly made it hotter instead of colder!


S. Braun, J. P. Ronzheimer, M. Schreiber, S. S. Hodgman, T. Rom, I. Bloch, U. Schneider. "Negative Absolute Temperature for Motional Degrees of Freedom" Science 339, 52–55 (2013).