Materials for use in vacuum
Encyclopedia
Materials for use in vacuum are materials showing very low rate of outgassing
in vacuum
, and, where applicable, tolerant to the bake-out
temperatures. The requirements grow increasingly stringent with the desired degree of vacuum achievable in the vacuum chamber
.
The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed
on the material surface (therefore materials with low affinity to water have to be chosen, which eliminates many plastics). Materials may sublimate in vacuum (this excludes some metals and their alloys, most notably cadmium and zinc). Or the gases can be released from porous
materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.
The gases liberated from the materials not only lower the vacuum quality, but also can be readsorbed on other surfaces, creating deposits and contaminating the chamber.
Yet another problem is diffusion of gases through the materials themselves. Atmospheric helium
can diffuse even through Pyrex
glass, even if slowly; this however is usually not an issue.
In addition to the gas-related issues, the materials have to maintain adequate strength through the entire required temperature range (sometimes reaching cryogenic temperatures), maintain their properties (elasticity, plasticity, electrical and thermal conductivity or lack of it, etc.), be possible to machine, and if possible not being overly expensive. Yet another concern is the thermal expansion coefficient match of adjanced parts.
The most common sources of trouble in vacuum systems are:
s have inacceptable outgassing rates, others (e.g. graphite
) lose lubricating properties.
applications have to cope with radiation damage
and high-intensity ultraviolet radiation, thermal loads from solar radiation, radiation cooling of the vehicle in other directions, and heat produced within the spacecraft's systems. Another concern, for orbits closer to Earth, is the presence of atomic oxygen, leading to corrosion
of exposed surfaces; aluminium is an especially sensitive material.
Corrosion-sensitive surfaces can be protected by a suitable plating
, most often with gold
; a silica layer is also possible. However the coating layer is subject to erosion by micrometeorites.
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, and, where applicable, tolerant to the bake-out
Bake-out
Bake-out is a term used in several areas of technology, fabrication, and in building construction, referring to the act of using heat and sometimes a vacuum to release volatile compounds from something as a form of cleaning or processing...
temperatures. The requirements grow increasingly stringent with the desired degree of vacuum achievable in the vacuum chamber
Vacuum chamber
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space...
.
The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
on the material surface (therefore materials with low affinity to water have to be chosen, which eliminates many plastics). Materials may sublimate in vacuum (this excludes some metals and their alloys, most notably cadmium and zinc). Or the gases can be released from porous
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.
The gases liberated from the materials not only lower the vacuum quality, but also can be readsorbed on other surfaces, creating deposits and contaminating the chamber.
Yet another problem is diffusion of gases through the materials themselves. Atmospheric helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
can diffuse even through Pyrex
Pyrex
Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...
glass, even if slowly; this however is usually not an issue.
In addition to the gas-related issues, the materials have to maintain adequate strength through the entire required temperature range (sometimes reaching cryogenic temperatures), maintain their properties (elasticity, plasticity, electrical and thermal conductivity or lack of it, etc.), be possible to machine, and if possible not being overly expensive. Yet another concern is the thermal expansion coefficient match of adjanced parts.
Materials to avoid
Materials outgas by two mechanisms: release of adsorbed and absorbed gases, and evaporation of the material itself. The former can be reduced by a bakeout, the latter is an intrinsic property of the material.The most common sources of trouble in vacuum systems are:
- CadmiumCadmiumCadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
, often present in the form of cadmium plating, or in some solderingSolderingSoldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
and brazingBrazingBrazing is a metal-joining process whereby a filler metal is heated above and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux...
alloys - ZincZincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, problematic for high vacuum and higher temperatures, present in some construction alloys, e.g. brassBrassBrass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin... - MagnesiumMagnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
- PVCPVCPolyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
, usually in the form of wireWireA wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
insulation - PaintPaintPaint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
s - Many plastics, namely many plastic tapes
Metals
- Austenitic stainless steelAustenitic stainless steelAustenitic steels have austenite as their primary phase . These are alloys containing chromium and nickel , structured around the Type 302 composition of iron, 18% chromium, and 8% nickel. Austenitic steels are not hardenable by heat treatment...
s are the most common choice for high vacuumHigh VacuumHigh Vacuum is a science fiction novel by Charles Eric Maine. It was first published in 1957 by Ballantine Books.-Synopsis:The first manned moon ship, Alpha, runs out of fuel just before landing in the Mare Imbrium, and crashes, killing one of the four-man crew and marooning the rest...
and ultra-high vacuum systems. Not all alloys are suitable; e.g. the free-machining 303 steel contains sulfurSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
, which tends to outgas. Alloys with good weldability under argon arc welding are usually chosen.- 304 stainless steel is a common choice of a stainless steel.
- 304L stainless steel, a low-carbon variant of 304 steel, is used for ultra-high vacuum systems.
- 347 stainless steel does not accept high polish.
- 321 stainless steel is chosen when low magnetic permeability is needed.
- Mild steel can be used for moderate vacuums above 10-6 torr. Outgassing can be lowered with suitable (e.g. nickel) platingPlatingPlating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology...
. It has high permeability to hydrogen and tendency to rust. - AluminiumAluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
and aluminium alloys are another class of frequently used materials. They are well-machinable and have low outgassing, unless the alloys contain higher proportion of zincZincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. The parts must not be anodized, as the oxide layer traps (and outgases) water vapor. Aluminium and its alloys have low strength at high temperatures, distort when being welded, and the copper-containing ones are poorly weldable. Aluminium wire rings can be used as cheap gaskets in demountable seals. Aluminium has high thermal conductivity, good corrosion resistance, and low solubility of hydrogen. Loss of strength at high temperatures limits its use in bakeable applications, but aluminium is advantageous for large-size systems due to its lower weight and lower cost than stainless steel. Use of aluminium is limited by difficulties in its welding and brazing. It can be used for x-ray windows. - Aluminium bronzeAluminium bronzeAluminium bronze is a type of bronze in which aluminium is the main alloying metal added to copper, in contrast to standard bronze or brass...
is a material looking and machining similar to brassBrassBrass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
. It is not susceptible to gallingGallingGalling usually refers to the adhesive wear and transfer of material between metallic surfaces in relative converging contact during sheet metal forming and other industrial operations....
, which makes it suitable for sliding fits against stainless steel. - NickelNickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
is widely used in vacuum technology, e.g. as mechanical parts in vacuum tubeVacuum tubeIn electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s. It is relatively low-cost, can be spot welded, can be easily machined, has high melting point and is resistant to many corrosive fluids and atmospheres. Its potential drawback is its ferromagnetismFerromagnetismFerromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
, which restricts applications that would be influenced by magnetic fields. - BerylliumBerylliumBeryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
is used primarily for x-ray windows. - Oxygen-free copperOxygen-free copperOxygen-free copper or Oxygen-free high thermal conductivity copper generally refers to a group of wrought high conductivity copper alloys that have been electrolytically refined to reduce the level of oxygen to .001% or below....
is widely used. It is easily machined and has good corrosion resistance. It is unsuitable for bakeable vacuum envelopes due to its tendency to oxidize and create scales. Copper rings are used in demountable seals. Normal copperCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
is unsuitable for high vacuum as it is difficult to outgas completely. Copper is insensitive to hydrogen and impermeable to hydrogen and helium, has low sensitivity to water vapor, but is attacked by mercury. Its strength falls sharply above 200 °C. Its vapor pressure becomes significant at above 500 °C. - BrassBrassBrass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...
is suitable for some applications. It has good corrosion resistance. Its zinc content may cause problems; zinc outgassing can be reduced by nickel-plating. - IndiumIndiumIndium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
wire is used as a gasket in demountable seals. - GoldGoldGold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
wire is used as a gasket in demountable seals for ultra-high vacuum. - ZirconiumZirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
is corrosion-resistant. It has low production of secondary electronsSecondary electronsSecondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
, so it is used as a coating of areas where reducing their production is important. It is used for neutronNeutronThe neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
windows. It is costly and scarce, its uses are therefore limited. Zirconium and zirconium hydride are used for gettering.
Plastics
- Some fluoropolymerFluoropolymerA fluoropolymer is a fluorocarbon based polymer with multiple strong carbon–fluorine bonds. It is characterized by a high resistance to solvents, acids, and bases.-History:Fluoropolymers were accidentally discovered in 1938 by Dr. Roy J...
s, e.g. polyvinylidene fluoridePolyvinylidene fluoridePolyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
, are suitable for use in vacuum. They have low outgassing and tolerant to higher temperatures.- PolytetrafluoroethylenePolytetrafluoroethylenePolytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....
is commonly used inside of vacuum systems. It is self-lubricating, a good electrical insulator, tolerant to fairly high temperatures, and has low outgassing. It is not suitable for barrier between vacuum and atmosphere, as it is somewhat permeable for gases.
- Polytetrafluoroethylene
- PolyethylenePolyethylenePolyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
is usable but requires thorough outgassing. NalgeneNalgeneThermo Scientific Nalgene is a brand of polyethylene plastic developed and manufactured by Nalge Nunc International originally developed for laboratory storage containers. It is now used in the manufacture of water containers, vials, bottles, bags and carboys for packaging, shipping, and storage...
can be used as a cheaper alternative for Bell jarBell jarA bell jar is a piece of laboratory equipment used for creating vacuums.http://www.belljar.net/about.htm It can be similar in shape to a bell, and can be manufactured out of a variety of materials . A bell jar is placed on a base which is vented to a hose fitting, which can be connected via a hose...
s. - Vespel polyimidePolyimidePolyimide is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955...
is very expensive, but machines well, has good electrical insulator properties and is compatible with ultra-high vacuum. - PVCPVCPolyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
, despite its high outgassing rate, can be used in limited applications for rough vacuum lines. - NylonNylonNylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
is self-lubricating but has high outgassing rate and high affinity to water. - Acrylics have high outgassing rate and high affinity to water.
- PolycarbonatePolycarbonatePolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
s and polystyrenePolystyrenePolystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
are good electrical insulators with moderate outgassing. - Some elastomerElastomerAn elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
s have sufficient vacuum properties:- Nitrile rubberNitrile rubberNitrile rubber, also known as Buna-N, Perbunan, or NBR, is a synthetic rubber copolymer of acrylonitrile and butadiene. Trade names include Nipol, Krynac and Europrene....
is used for demountable vacuum seals. - VitonVitonViton is a brand of synthetic rubber and fluoropolymer elastomer commonly used in O-rings and other molded or extruded goods. The name is a registered trademark of DuPont Performance Elastomers L.L.C.....
is used for demountable vacuum seals. It is better for lower pressures than nitrile rubber. It is bakeable to 200 °C.
- Nitrile rubber
Glasses and ceramics
- Borosilicate glassBorosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
is often used for smaller assemblies and for viewports. It can be machined and joined well. Glasses can be joined with metalsGlass-to-metal sealGlass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic...
. - PorcelainPorcelainPorcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
and alumina ceramics, when fully vitrified and therefore non-porous, are excellent insulators usable to 1500 °C. Some ceramics can be machined. Ceramics can be joined with metals.
Lubricants
Lubrication of moving parts is a problem for vacuum. Many lubricantLubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
s have inacceptable outgassing rates, others (e.g. graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
) lose lubricating properties.
- Vacuum greaseVacuum greaseVacuum grease is a lubricant with low volatility and is used for applications in low pressure environments. Lubricants with higher volatility would evaporate, causing two problems:* They would not be present to provide lubrication....
s are greaseGreaseGrease may refer to:* Grease , a type of industrial lubricant* Yellow grease, in rendering, used frying oils, or lower-quality grades of tallow...
s with low outgassing.- KrytoxKrytoxKrytox is a trademarked name of a family of high-performance synthetic lubricants with a variety of applications. Invented by researchers at DuPont, it is a colourless polymer containing ether functionality...
is a fluorether-based vacuum grease, useful from -75 to over 350 °C, not flammable even in liquid oxygen, and highly resistant to ionizing radiationIonizing radiationIonizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
. - Polyphenyl etherPolyphenyl etherPhenyl ether polymers are a class of polymers that contain a phenoxy and/or a thiophenoxy group as the repeating group in ether linkages. Commercial phenyl ether polymers belong to two chemical classes: polyphenyl ethers and Polyphenylene oxides...
greases - Torrlube
- Krytox
- Molybdenum disulfideMolybdenum disulfideMolybdenum disulfide is the inorganic compound with the formula MoS2. This black crystalline sulfide of molybdenum occurs as the mineral molybdenite. It is the principal ore from which molybdenum metal is extracted. The natural amorphous form is known as the rarer mineral jordisite. MoS2 is less...
is a dry lubricantDry lubricantDry lubricants or solid lubricants are materials which despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid media....
usable in vacuum. - Hexagonal boron nitride is a graphite-like dry lubricant used in space vehicles.
Materials for use in space
In addition to the concerns above, materials for use in spacecraftSpacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
applications have to cope with radiation damage
Radiation damage
Radiation damage is a term associated with ionizing radiation.-Causes:This radiation may take several forms:*Cosmic rays and subsequent energetic particles caused by their collision with the atmosphere and other materials....
and high-intensity ultraviolet radiation, thermal loads from solar radiation, radiation cooling of the vehicle in other directions, and heat produced within the spacecraft's systems. Another concern, for orbits closer to Earth, is the presence of atomic oxygen, leading to corrosion
Corrosion in space
Corrosion in space is the corrosion of materials occurring in outer space. Instead of moisture and oxygen acting as the primary corrosion causes, the materials exposed to outer space are subjected to vacuum, bombardment by ultraviolet light and x-rays, high-energy charged particles...
of exposed surfaces; aluminium is an especially sensitive material.
Corrosion-sensitive surfaces can be protected by a suitable plating
Plating
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology...
, most often with gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
; a silica layer is also possible. However the coating layer is subject to erosion by micrometeorites.