View source for Chemistry:Materials for use in vacuum

[[File:Long Duration Exposure Facility after deployment.jpg|thumb|The [[Organization:Long Duration Exposure Facility|Long Duration Exposure Facility]] was used to test various materials in vacuum.]]
'''Materials for use in vacuum''' are materials that show very low rates of [[Physics:Outgassing|outgassing]] in [[Physics:Vacuum|vacuum]] and, where applicable, are tolerant to [[Biology:Bake-out|bake-out]] temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the [[Engineering:Vacuum chamber|vacuum chamber]].
The materials can produce gas by several mechanisms. Molecules of gases and water can be [[Chemistry:Adsorption|adsorbed]] on the material surface (therefore materials with low affinity to water have to be chosen, which eliminates many plastics). Materials may [[Sublimation (phase transition)|sublimate]] in vacuum (this includes some metals and their alloys, most notably cadmium and zinc). Or the gases can be released from [[Earth:Porosity|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 reabsorbed on other surfaces, creating deposits and contaminating the chamber.

Yet another problem is diffusion of gases through the materials themselves. Atmospheric [[Chemistry:Helium|helium]] can diffuse even through [[Chemistry:Pyrex|Pyrex]] glass, even if slowly (and elevated temperatures above room temperature are generally needed);<ref name="taylor_diffusion_1938">{{cite journal |last1=Taylor |first1=Nelson W. |last2=Rast |first2=William |date=1938 |title=The Diffusion of Helium and of Hydrogen Through Pyrex Chemically Resistant Glass |url=https://aip.scitation.org/doi/10.1063/1.1750133 |journal=The Journal of Chemical Physics |volume=6 |issue=10 |pages=612–619 |doi=10.1063/1.1750133 |bibcode=1938JChPh...6..612T |access-date=2021-08-28}}</ref> this however is usually not an issue. Some materials might also expand or increase in size causing problems in delicate equipment.

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 [[Physics:Thermal conductivity|thermal conductivity]] or lack of it, etc.), be machinable, and if possible not be overly expensive. Yet another concern is the thermal expansion coefficient match of adjacent parts.

==Materials to avoid==
Materials outgas by three mechanisms: release of [[Chemistry:Absorption|absorbed]] gases (de[[Physics:Sorption|sorption]] from the bulk of the material), release of [[Chemistry:Adsorption|adsorbed]] gases ([[Physics:Desorption|desorption]] from the surface only), and evaporation of the material itself. The former can be reduced by a bakeout, the latter is an intrinsic property of the material.<ref name="vacphytec">{{cite book|title=Vacuum Physics and Technology|author=Meurant, G.|date=1980|publisher=Elsevier Science|isbn=9780080859958|url=https://books.google.com/books?id=tfLWfAx1ZWQC&pg=PA346|page=346|accessdate=2015-09-08}}</ref> Some outgassed materials can deposit on other surfaces, contaminate the vacuum system and be difficult to get rid of.

The most common sources of trouble (out-gassing) in vacuum systems are:
* [[Chemistry:Cadmium|Cadmium]], often present in the form of cadmium plating, or in some [[Physics:Soldering|soldering]] and [[Engineering:Brazing|brazing]] alloys
* [[Chemistry:Zinc|Zinc]], problematic for high vacuum and higher temperatures, present in some construction alloys, e.g. [[Chemistry:Brass|brass]] and some brazing alloys. Tends to poison [[Physics:Hot cathode|hot cathode]]s and form conductive deposits on surfaces.<ref name="fermi"/> Any materials that have been zinc-coated by [[Chemistry:Galvanization|galvanization]] should be avoided, or they have the coating removed first.
* [[Chemistry:Magnesium|Magnesium]]
* [[Engineering:Paint|Paint]]s
* [[Chemistry:Lead|Lead]] and [[Chemistry:Antimony|antimony]] used in some soft [[Chemistry:Solder|solder]]s due to outgassing at higher temperatures.<ref name="fermi">{{cite journal|url=https://digital.library.unt.edu/ark:/67531/metadc1185924/m1/1/|publisher=Fermi National Accelerator Laboratory|title= TM-1615: Materials for Ultra-High Vacuum|author= G. Lee | date=August 15, 1989 |doi=10.2172/6985168 |accessdate=2015-09-08|doi-access=free}}</ref>
* Many plastics, namely many plastic tapes (special attention should be paid to adhesives). Fiberglass composites, e.g. [[Chemistry:Micarta|Micarta]] (G-10) and G-30, should be avoided. Even [[Chemistry:Kapton|Kapton]] and [[Engineering:Teflon|Teflon]] are sometimes advised against. See below for further discussion of plastics.<ref name="fermi"/>
** PVC, usually in the form of [[Engineering:Wire|wire]] insulation (also a source of leaks)
* Various residues, e.g. [[Engineering:Flux (metallurgy)|flux]] from soldering and brazing, and lubricants from machining making thorough cleaning imperative. Getting the outgassable residues from tight crevices can be challenging; a good mechanical design that avoids such features can help.

There are also additional physical issues which come with vacuum, including the growth of [[Engineering:Whisker (metallurgy)|whiskers]] from materials such as Tin or Zinc, which can cause physical issues or electrical shorts <ref name="nasa">{{cite web|url=https://nepp.nasa.gov/whisker/background/index.htm |title=Basic Info on Tin Whiskers }}</ref>

==Review of materials and issues to consider==

===Metals===
* Austenitic stainless steels are the most common choice for [[Physics:Vacuum|high vacuum]] and [[Physics:Ultra-high vacuum|ultra-high vacuum]] systems. Not all alloys are suitable; e.g. the free-machining 303 steel contains [[Chemistry:Sulfur|sulfur]], which tends to outgas. Alloys with good weldability under argon [[Physics:Arc welding|arc welding]] are usually chosen.
** '''304 [[Chemistry:Stainless steel#Comparison of standardized steels|stainless steel]]''' is a common choice of a stainless steel.
** '''304L [[Chemistry:Stainless steel#Comparison of standardized steels|stainless steel]]''', a low-carbon variant of 304 steel, is used for ultra-high vacuum systems.
** '''316L [[Chemistry:Stainless steel#Comparison of standardized steels|stainless steel]]''' a low-carbon and low-magnetic stainless steel, used in accelerator technologies.
** '''347 [[Chemistry:Stainless steel#Comparison of standardized steels|stainless steel]]''' does not accept high polish.
** '''321 [[Chemistry:Stainless steel#Comparison of standardized steels|stainless steel]]''' is chosen when low magnetic permeability is needed.
* Mild steel can be used for moderate vacuums above {{convert|1e-6|torr}}. Outgassing can be lowered with suitable (e.g. nickel) [[Physics:Plating|plating]]. It has high permeability to hydrogen and tendency to rust. For use it should be thoroughly degassed in vacuum.
* [[Chemistry:Aluminium|Aluminium]] and aluminium alloys are another class of frequently used materials. They are well-machinable and have low outgassing, unless the alloys contain higher proportions of [[Chemistry:Zinc|zinc]]. The parts must not be anodized, as the oxide layer traps (and then outgasses) water vapor. Anodizing also makes the surface non-conducting, so that its surface will charge up in electrostatic systems. The best treatment is [[Engineering:Chromate conversion coating|Alochroming]], which seals the surface, makes it hard and conductive. Its outgassing rate is considerably less than non-treated aluminium. 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.<ref name="vacphytec"/>
* [[Chemistry:Aluminium bronze|Aluminium bronze]] is a material that looks and machines similar to [[Chemistry:Brass|brass]]. It is not susceptible to [[Physics:Galling|galling]], which makes it suitable for sliding fits against stainless steel.
* [[Chemistry:Nickel|Nickel]] is widely used in vacuum technology, e.g. as mechanical parts in [[Physics:Vacuum tube|vacuum tube]]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 [[Physics:Ferromagnetism|ferromagnetism]], which restricts applications that would be influenced by magnetic fields.<ref name="vacphytec"/>
* Nickel alloys, e.g. [[Chemistry:Cupronickel|cupronickel]]<ref name="fermi"/>
* [[Chemistry:Beryllium|Beryllium]] is used primarily for x-ray windows.
* [[Chemistry:Oxygen-free copper|Oxygen-free copper]] 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 [[Chemistry:Copper|copper]] 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 {{convert|200|C}}. Its vapor pressure becomes significant at above {{convert|500|C}}.<ref name="vacphytec"/>
* [[Chemistry:Brass|Brass]] is suitable for some applications. It has good corrosion resistance. Its zinc content may cause problems; zinc outgassing can be reduced by nickel-plating.
* [[Chemistry:Indium wire|Indium wire]] is used as a gasket in demountable seals.
* [[Chemistry:Gold|Gold]] wire is used as a gasket in demountable seals for ultra-high vacuum, as well as an alternative to lead-tin solder for making electrical connections.
* [[Chemistry:Platinum|Platinum]] is a highly chemically inert material with high cost and low outgassing.
* [[Chemistry:Zirconium|Zirconium]] is corrosion-resistant. It has low production of [[Physics:Secondary electrons|secondary electrons]], so it is used as a coating of areas where reducing their production is important. It is used for [[Physics:Neutron|neutron]] windows. It is costly and scarce, its uses are therefore limited. Zirconium and zirconium hydride are used for gettering.
* [[Chemistry:Tungsten|Tungsten]] is often used in high temperature applications as well as for filaments in electron/ion optics. It becomes brittle from [[Engineering:Work hardening|work hardening]] when mechanically deformed, or subjected to very high temperatures.
* [[Chemistry:Molybdenum|Molybdenum]] and [[Chemistry:Tantalum|tantalum]] are useful for high temperature applications.<ref name="fermi"/>
* [[Chemistry:Titanium|Titanium]] and [[Chemistry:Niobium|niobium]] are good materials.
* [[Chemistry:Solder|Solder]]s are sometimes unavoidable for soft-soldered joints. Tin-lead solders (Sn50Pb50, Sn60Pb40, Sn63Pb37) can be conditionally used when the apparatus is not to be baked and operating temperatures aren't elevated (lead tends to outgas). A better choice for vacuum systems is the tin-silver eutectic, Sn95Ag5 (Sn-Ag eutectic is actually 96.5-3.5); its melting point of {{convert|230|C}} allows bakeout up to {{convert|200|C}}. A similar 95-5 alloy, Sn95Sb5, is unsuitable as antimony has similar vapor pressure as lead. Take care to remove [[Engineering:Flux (metallurgy)|flux]] residues.
* [[Chemistry:List of brazing alloys|Brazing alloys]] are used for joining materials by [[Engineering:Brazing|brazing]]. Care has to be taken while choosing the alloys, as some elements tend to outgas. Cadmium and zinc are the worst common offenders. Silver, a common component of brazing alloys, can be problematic at higher temperatures and lower pressures. A silver-copper eutectic, named e.g. Cusil, is recommended. A superior alternative is a copper-silver-tin alloy called Cusiltin. Copper-silver-phosphorus alloys, e.g. Sil-Fos, are also suitable.<ref name="fermi"/>

===Plastics===
* Some [[Chemistry:Fluoropolymer|fluoropolymer]]s, e.g. [[Chemistry:Polyvinylidene fluoride|polyvinylidene fluoride]], are suitable for use in vacuum. They have low outgassing and are tolerant to higher temperatures.
** [[Chemistry:Polytetrafluoroethylene|Polytetrafluoroethylene]] (PTFE or Teflon) is commonly used inside of vacuum systems. It is self-lubricating, a good electrical insulator, tolerant to fairly high temperatures, and has low out-gassing. It is not suitable for barrier between vacuum and atmosphere, as it is somewhat permeable for gases. Ceramics is a superior choice, however.<ref name="fermi"/>
* [[Chemistry:Polyethylene|Polyethylene]] is usable but requires thorough out-gassing. Nalgene can be used as a cheaper alternative for [[Organization:Bell jar|bell jar]]s.
* [[Chemistry:Vespel|Vespel]] [[Chemistry:Polyimide|polyimide]] is very expensive, but machines well, has good electrical insulator properties and is compatible with ultra-high vacuum.
* PVC, despite its high outgassing rate, can be used in limited applications for rough vacuum lines.
* [[Chemistry:Nylon|Nylon]] is self-lubricating but has high outgassing rate and high affinity to water.
* [[Physics:Acrylate polymer|Acrylics]] have high outgassing rate and high affinity to water.
* [[Physics:Polycarbonate|Polycarbonate]]s and [[Chemistry:Polystyrene|polystyrene]] are good electrical insulators with moderate outgassing.
* PEEK (polyetheretherketone) has relatively low out-gassing values (0.31% TML, 0.00% CVCM, 0.06% WVR).
* [[Chemistry:Kapton|Kapton]] is a type of polyimide film, has very low outgassing. Kapton is discouraged if a ceramic alternative can be used.<ref name="fermi"/>
* Some [[Physics:Elastomer|elastomer]]s have sufficient vacuum properties to be employed in vacuum o-rings:
** [[Chemistry:Nitrile rubber|NBRs]], ([[Chemistry:Nitrile rubber|nitrile rubber]]), commonly used for demountable vacuum seals (bakeable only up to 100&nbsp;°C).
** [[Physics:FKM|FKMs (FPMs)]], ([[Chemistry:Viton|Viton]]) is used for demountable vacuum seals. It is better for lower pressures than [[Chemistry:Nitrile rubber|nitrile rubber]] and chemically much more [[Chemistry:Chemically inert|inert]]. It is bakeable to 200&nbsp;°C.
** [[Physics:FFKM|FFKM]]s (FFPMs) very low out-gassing similar to PTFE and withstands baking temperatures up to 300&nbsp;°C, while chemically one of the most [[Chemistry:Chemically inert|inert]] sealing elastomers.

===Glasses and ceramics===
* [[Chemistry:Borosilicate glass|Borosilicate glass]] is often used for smaller assemblies and for viewports. It can be machined and joined well. Glasses can be [[Engineering:Glass-to-metal seal|joined with metals]].
* [[Chemistry:Porcelain|Porcelain]] and alumina ceramics, when fully vitrified and therefore non-porous, are excellent insulators usable up to 1500&nbsp;°C. Some ceramics can be machined. Ceramics can be [[Engineering:Glass-ceramic-to-metal seals|joined with metals]].
* Macor is a machinable ceramic that is an excellent alternative to alumina, as the firing process of alumina can change the dimensions and tolerances.

===Lubricants===
Lubrication of moving parts is a problem for vacuum. Many [[Physics:Lubricant|lubricant]]s have unacceptable outgassing rates,<ref name="Carre, Bertrand, 1999" >{{Cite journal
  |title=Analysis of Hubble Space Telescope Reaction Wheel Lubricant
  |year=1999
  |journal=Journal of Spacecraft and Rockets
  |last1=Carré  |first1=D. J. 
  |last2=Bertrand  |first2=P. A. 
  |volume=36
  |issue=1
  |doi=10.2514/2.3422 
  |url=http://arc.aiaa.org/doi/abs/10.2514/2.3422
  |pages=109–113
  |bibcode=1999JSpRo..36..109C
}}</ref> others (e.g. [[Chemistry:Graphite|graphite]]) lose lubricating properties.
* [[Chemistry:Vacuum grease|Vacuum grease]]s are [[Physics:Grease (lubricant)|grease]]s with low outgassing.
** [[Chemistry:Ramsay grease|Ramsay grease]] is an old composition of paraffin wax, vaseline and natural rubber, usable up to about 25&nbsp;°C, for low vacuums to about 1&nbsp;Pa.
** Krytox is a fluorether-based vacuum grease, useful from −75 to over 350&nbsp;°C, not flammable even in [[Physics:Liquid oxygen|liquid oxygen]], and highly resistant to [[Physics:Ionizing radiation|ionizing radiation]].
** [[Chemistry:Polyphenyl ether|Polyphenyl ether]] greases
** Torrlube, a brand encompassing a range of lubricating oils based on [[Chemistry:Perfluoropolyether|perfluoropolyether]]s.<ref name="torrlube">{{cite web|url=http://www.torrlube.com/|title=TorrLube.com &#124; The Unrivaled Leader in High Vacuum Lubrication|publisher=torrlube.com|accessdate=2015-09-08}}</ref>
* [[Physics:Dry lubricant|Dry lubricant]]s, can be incorporated in plastics as fillers, as a component of sintered metals, or deposited on metal, ceramic and plastic surfaces.
** [[Chemistry:Molybdenum disulfide|Molybdenum disulfide]] is a dry lubricant usable in vacuum.
** [[Chemistry:Tungsten disulfide|Tungsten disulfide]] is another dry lubricant usable in vacuum. It can be used at higher temperatures than MoS<sub>2</sub>. Tungsten disulfide used to be significantly more expensive, but molybdenum disulfide prices have risen to a comparable range.<ref name="lowerfriction">{{cite web|url=http://www.lowerfriction.com/pdf/8.pdf|date=2 December 2008|title=Comparison between Molybdenum Disulfide & Tungsten Disulfide|author=Ketan|accessdate=2015-09-08}}</ref> Usable from −188 to +1316&nbsp;°C in vacuum, from −273 to +650&nbsp;°C in normal atmosphere.<ref name="appliedtungstenite">{{cite web|url=http://www.appliedtungstenite.com/html/aerospace.html|title=Applied Tungstenite: Aerospace: Tungsten Disulfide WS2 dry film Lubricant and Citric Acid Passivation|publisher=appliedtungstenite.com|accessdate=2015-09-08}}</ref>
** [[Chemistry:Boron nitride#Hexagonal BN|Hexagonal boron nitride]] is a graphite-like dry lubricant used in space vehicles.
[[File:4-inch-cyclotron.jpg|thumb|This is a picture of Ernest Lawrence's first 4-inch cyclotron. This is a D-shaped cyclotron and contains glass-to-metal vacuum joints made from Faraday wax (the red joints).]]

=== Adhesives ===
* Torr-Seal, or its generic equivalent Hysol-1C (US brand name) or Loctite 9492 (EU brand name), is an epoxy with resin and hardener for use in vacuum environments. It will begin to degrade at high temperatures but otherwise is very stable with very little outgassing. Other vacuum-rated epoxies are also available. For mounting or joining thin metal foils, grids, or other small pieces that are not expected to undergo stress, silver or gold paste may be used as an adhesive. After fixing the material(s) with silver paste, the piece must be baked (to >200&nbsp;°C) in air for >24 hours to remove volatiles prior to insertion into vacuum.
* Faraday wax is perhaps a cheaper alternative to Torr-Seal, but just as effective. Indeed some of its physical properties make it more favourable than epoxies. Faraday Wax is a dark red wax with a low melting point. It is reasonably malleable, but when moved quickly is snaps cleanly. Joints can be made with Faraday Wax very easily, simply heat the two surfaces then press the wax against the heated surface similar to soldering. These joints are suitable down to 10{{sup|−7}}&nbsp;mbar<ref>{{Citation |title=Faraday Wax - DIY Historic High Vacuum Sealant |url=https://www.youtube.com/watch?v=iJW7-27zRfU |language=en |access-date=2022-06-22}}</ref> and can be made between glass and metal. It was firstly described by [[Biography:Michael Faraday|Michael Faraday]] in "Chemical Manipulation" 1827.<ref>{{Cite book |last=Faraday |first=Michael |url=https://books.google.com/books?id=g7M5AAAAcAAJ&pg=PA479 |title=Chemical Manipulation: Being Instructions to Students in Chemistry, on the Methods of Performing Experiments of Demonstration Or of Research, with Accuracy and Success |date=1827 |publisher=W. Phillips |pages=479 |language=en}}</ref> By weight:
** 1 part [[Chemistry:Beeswax|beeswax]]
** 5 parts colophony (rosin)
** 1 part [[Venetian red]]
* A modern wax intended for high-vacuum use is Apiezon Wax W.

==Materials for use in space==
In addition to the concerns above, materials for use in [[Engineering:Spacecraft|spacecraft]] applications have to cope with [[Physics:Radiation damage|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 [[Chemistry:Corrosion in space|corrosion]] of exposed surfaces; aluminium is an especially sensitive material{{citation needed|date=November 2015}}. Silver, often used for surface-deposited interconnects, forms layer of silver oxide that flakes off and may erode up to a total failure.

Corrosion-sensitive surfaces can be protected by a suitable [[Physics:Plating|plating]], most often with [[Chemistry:Gold|gold]]; a [[Chemistry:Silica|silica]] layer is also possible. However the coating layer is subject to erosion by micrometeoroids.
==See also==
*[[Engineering:Vacuum engineering|Vacuum engineering]]

==References==
{{reflist}}

[[Category:Vacuum systems]]
[[Category:Materials]]

{{Sourceattribution|Materials for use in vacuum}}