Cryogenics

Cryogenics is the branch of physics and engineering concerned with the production, study, and practical use of materials at extremely low temperatures. The field encompasses scientific, industrial, and medical applications, and has contributed significantly to developments in superconductivity, aerospace engineering, food preservation, and biological sciences. The term generally refers to temperatures below approximately 120 K, a threshold recognised for distinguishing cryogenic temperatures from those used in conventional refrigeration.

Definitions, Temperature Scales, and Scope

Cryogenics is commonly defined as the study and application of phenomena occurring at temperatures lower than about 120 K. This temperature conveniently separates the boiling points of permanent gases—such as helium, hydrogen, neon, nitrogen, and oxygen—from those of common refrigerants like hydrocarbons and Freon compounds, which boil above 120 K. Cryogenic research relies on temperature scales measured from absolute zero, using Kelvin or Rankine units, rather than Celsius or Fahrenheit scales.
The term high-temperature cryogenic describes temperatures above the boiling point of liquid nitrogen but still significantly below ambient conditions, particularly in contexts such as emerging superconducting technologies. The discovery of superconductive behaviour in mercury at 4.2 K by Heike Kamerlingh Onnes in 1908 marked the beginning of modern cryogenic research and highlighted the transformative potential of low-temperature phenomena.

Distinctions Within the Field

Several specialised subfields fall under the broader discipline:

• Cryogenics: Concerned with generating and maintaining ultra-low temperatures and studying the behaviour of materials under such conditions.
• Cryobiology: Focuses on the biological effects of low temperatures, including cryopreservation, freeze-drying of pharmaceuticals, and conservation of genetic resources.
• Cryosurgery: Uses cryogenic temperatures to destroy diseased tissues, particularly in cancer treatment.
• Cryoelectronics: Examines electronic properties at cryogenic temperatures, including superconductivity and associated phenomena.
• Cryonics: Involves cryopreservation of humans or animals with the expectation of future revival; distinct from and often confused with scientific cryogenics in popular culture.

The word cryogenics derives from the Greek cryos (cold) and genis (generating), reflecting the field’s origins in the production and control of extreme cold.

Cryogenic Fluids and Storage Technologies

Cryogenic applications commonly use liquefied gases with low boiling points, especially liquid nitrogen and liquid helium. These fluids require specialised storage in Dewar flasks, double-walled vacuum containers devised to minimise heat transfer. Glass Dewars housed within protective metal casings are typical in laboratories, while advanced systems incorporate an inner liquid-nitrogen jacket to insulate liquid helium.
Industrial cryogenic systems include transfer pumps and valves used to move liquefied natural gas (LNG) and other cryogenic fluids. Labelling and handling equipment, such as cryogenic-resistant barcodes, are designed to withstand temperatures approaching −195 °C without degradation.

Cryogenic Processing and Material Behaviour

During the Second World War it was observed that metals exposed to cryogenic temperatures often displayed improved wear resistance. These findings evolved into modern cryogenic processing techniques, introduced commercially in the 1960s. Cryogenic tempering extends traditional heat-treatment cycles by further cooling alloys below ambient temperature and subsequently tempering them. This method can enhance durability and structural stability, depending on alloy composition and processing requirements.
Cryogenic techniques also support specialist cooling for chemical reactions requiring low temperatures, such as those used in synthesising components of pharmaceutical drugs. In food and biotechnology industries, cryogenic blast freezers or immersion freezers preserve perishable materials rapidly and safely. Some materials that are too elastic or soft for milling at room temperature can be ground effectively at cryogenic temperatures through a process known as cryomilling.

Cryogenic Fuels and Aerospace Applications

Cryogenic fuels play a vital role in modern rocketry. Liquid hydrogen, often paired with liquid oxygen as an oxidiser, has been routinely used for propulsion in systems such as NASA’s Space Shuttle. Liquid methane is emerging as an alternative, offering operational benefits in storage and performance. Cryogenic oxidisers remain central to many launch systems, including those originally developed in the Soviet space programme.
Cryogenic methods have also been applied in aviation. The Tupolev Tu-155 became the first aircraft to fly on liquefied natural gas in 1989, demonstrating the feasibility of cryogenic fuel systems outside spaceflight.

Scientific and Medical Applications

Cryogenics provides critical support for a wide range of scientific instruments and facilities:

• Nuclear Magnetic Resonance (NMR): Strong magnetic fields for NMR spectroscopy are often generated using superconducting solenoids cooled by liquid helium or, in some high-temperature superconducting systems, by liquid nitrogen.
• Magnetic Resonance Imaging (MRI): A direct extension of NMR, MRI relies on superconducting magnets to create detailed images of biological tissues through proton relaxation.
• Cryo-Electron Microscopy (cryo-EM): Biological samples are rapidly plunge-frozen to preserve native structure, enabling high-resolution imaging of proteins and other cellular components.

These technologies rely heavily on the unique thermal and electrical properties achievable only under cryogenic conditions.

Industrial, Chemical, and Structural Uses

Cryogenic engineering underpins numerous industrial processes. LNG facilities use cryogenic pumps and transfer systems to manage liquefied gases at large scale. Chemical plants employ cryogenic reactors to maintain controlled, low-temperature environments for sensitive reactions. In material sciences, cryogenic testing helps characterise toughness, brittleness, and thermal expansion, providing essential data for aerospace, infrastructure, and energy applications.