Cryogenic seals provide a mechanical containment mechanism for materials held at cryogenic temperatures, such as cryogenic fluids. Various techniques, including

soldering Soldering (; ) is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involv ...

and

welding Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as bra ...

are available for creating seals; however, specialized materials and processes are necessary to hermetically entrap cryogenic constituents under vacuum-tight conditions. Most commonly used are

liquid helium Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures. Liquid helium may show superfluidity. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temp ...

and liquid nitrogen, which boil at very low temperatures, below −153 °C (120 K), as well as

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or e ...

with low freezing points and refrigerating mixtures.Indium Wire and Cryogenic Sealing
/ref>European Organization for Nuclear Research - An Introduction to Cryogenics
/ref> Pure

indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts ...

wire or solder preform washers are accepted as the most reliable low temperature sealing materials. When correctly formed, indium will afford leak rates of less than 4.0x10 -9 mbar- liter/sec.Design, Development and Testing of Vacuum Compatible Seals at Cryogenic Temperatures
Alternative cryogenic seal materials include silicone grease conical seals, and Pb/Sn (lead-tin) wire seals.

History

Fundamental cryogenic processing began in the 1940s, albeit primitive. Steel cutting tools were immersed in liquid nitrogen to enhance their service life. Mechanical processes utilizing cryogenics were documented well in the 1950s and by the 1980s cryogenic fluids began to be considered for storage and use in modern devices. Today, cryogenic seals are a necessity in high-tech commercial, medical, and military applications to encapsulate the cryogenic fluids critical for device resolution and function.

Applications

Applications which utilize cryogenic seals include: * Magnetic resonance imaging (MRI) *

Chromatography In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it through a system ( ...

*Dilution Refrigeration Units *Cooled Detectors *Optical Windows *Infrared Detectors * Centrifugal Cryogenic Pumps *Unmanned Aerial Vehicle Systems *

Missile In military terminology, a missile is a guided airborne ranged weapon capable of self-propelled flight usually by a jet engine or rocket motor. Missiles are thus also called guided missiles or guided rockets (when a previously unguided rocke ...

Warning Receivers *

Satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioi ...

Tracking Systems *Infrared Telescopes

Indium seals

Advantages

Advantages of indium cryogenic seals: *Established/proven design techniques for indium seal assemblyVacuum Service for a Gemini 200 NMR

/ref> *Option for disassembly and re-assembly *Indium can be reformed into useful seals after use *Soft and pliable at room temperature, due to the low melting temperature of indium, so it fills imperfections. This creates an impervious bond between the mating surfaces, crafting a

hermetic seal A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...

which remains

malleable Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

at cryogenic temperatures *Seal integrity remains following

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

from room temperature to immersion in cryogenic bathCompact indium seal for cryogenic optical windows
/ref> *Seal quality is independent of the mating surface composition, for instance

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

, germanium,

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

, or

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

. *Indium forms a self-passivating oxide layer, 80-100Ǻ thick. This layer is easy to remove with an acid etch, and the underlying, exposed

metal can be compressed to form a tight, hermetic bond.Indium for Sealing
/ref>

Disadvantages

*Bulky mechanical structure required to compress indium between the flanges. *Pulsating loads cause creep of indium seals, which loosens the bolt tension, thereby reducing the quality of the seal.

Process information for indium seals

*Mating surfaces should be kept as clean as possible, and may be cleaned using

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscib ...

. *Clean, oxide-free indium will cold weld to itself. The mating ends of a wire seal will weld together under

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

.Indium Cold Welding
/ref> * A more reliable alternative to a seal made from indium wire is a seal that uses an indium washer. Washers minimize the risk of seal degradation and cryogenic leaks by eliminating the interface between connected butt ends of wire. Washers are manufactured as a continuous ring with no breaks. *As many fasteners as possible should be used to clamp the indium seal. *Indium material used must be ultra-pure (99.9 minimum purity) to prevent hardening of the material at sub-zero temperatures, as well as to restrict impurities of elements with low vapor-pressure. *Material used for indium cryogenic seals should be manufactured from vacuum-cast material to prevent

outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...

after fixturing in the assembly.

Reliability testing

*Helium leak tests *Cryogenic temperature shock testing

Types

*Compact indium seal *Compressible hermetic seal *Compression seal *Cryogenic vacuum seal *Demountable cryogenic seal *Indium cryogenic vacuum seal *Indium o-ring flange seals *Indium seal *Indium wire o-ring *Indium wire seal *Low profile indium seal *Low temperature seal *Reusable cryogenic vacuum seal *Reusable indium wire seal *Reusable, low-profile, cryogenic wire seal *Soft metal seal *Vacuum compatible seal *Vacuum compatible seal at cryogenic temperature *Corner joint seal *Face joint seal

Manufacturers

Indium CorporationCMR-DirectGarlock HelicoflexAIMESPI MetalsSealwiseKapton®Mylar®Indium Wire Extrusion

External links

Physical Constants of Pure Indium

A reusable, low-profile, cryogenic wire seal

Corrosion of Indium-Base Solders

Indium Compressed into Hermetic Seals

HelioxTL System Operator's Handbook - Generic System Manual

Cryogenic Society of America, Inc.

VIDEO showing how to make an indium seal on a cryogenic system

References

{{Reflist Cryogenics