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Basics 3:An Introduction to Materials for Use in Vacuum

Last revision November 22, 2000

1. Selection Criteria

"The selection of materials for use in vacuum systems is a very important part of the design and should be considered in consultation with the user. Not only must the material be capable of being fabricated into the required components but it must stand up to the environmental conditions of temperature,pressure,and chemical attack etc. imposed on it by the vacuum processes,without limiting the attainable pressure that is required. " Quoted from Halliday

2. Material Property Requirements

1. Mechanical Properties
The material must be capable of being machined and fabricated.
It must have adequate strength at maximum and minimum temperatures to be encountered,and must retain it's elastic,plastic,and/or fluid properties over the expected temperature range.
2. Thermal Properties
The material's vapour pressure must remain low at the highest temperature.
Thermal expansion of adjacent materials must be taken into account, especially at joints.

3. Gas Loading
Materials must not be pourous.
Materials must be free of cracks and crevices which can trap cleaning solvents and become a source of virtual leaks later on.
Surface and bulk desorption rates must be acceptable at extremes of temperature and radiation.

3. Commonly Used Materials

          1. Metals for the Vacuum Envelope
                    Austenitic Stainless Steel is the most commonly used metal for high and ultra-high vacuum systems,since it fulfills all of the requirements above. U.S. 321 , 347, and 304 are chosen most frequently for satisfactory argon-arc welding. 321 is used when low magnetic permeability is required. U.S. 347 will not accept a high polish.

                    Stainless steel is relatively economical,has acceptable outgassing rates,and can be fabricated easily. The main disadvantage is that components may require 'truing up' due to distortion caused by welding operations.

                    Aluminum and Aluminum Alloys are very cheap,easy to machine, and have a low outgassing rate as long as the alloy does not have a high zinc content. They have the disadvantage of low strength at high temperatures and high distortion when welding. Alloys with copper content also present welding problems.

                    " A material which we were glad to learn about is "aluminum bronze". It is an aluminum copper alloy (which you mention in passing) and looks and machines a lot like brass. However, it seems to be a very satisfactory vacuum material and its anti-galling properties make it useful when you need something which will work in a sliding fit with stainless."-( E-mail from Fred Schamber of RJ Lee Instruments Limited).

                    Aluminum that will be exposed to vacuum should never be anodized due to serious outgassing problems. There are also some potentially violent chemical reactions that can develop when cleaning freshly machined Aluminum with Trichloroethane or Trichloroethylene based vapour degreasers.

                    References:
                    I recently had a call from someone who has used Anodized Aluminum as an insulator down into the ultra high vacuum region. Admittedly the pieces used were very small and initial outgassing was a problem but low costs were a major advantage.

                    Metal coating has gone high tech, there may be some information available at WWW.Anodizing.Org

                    The Trichloroethane, etc. chemical hazards are documented MSDS sheets. Try Vermont Safety Information Resources,Inc. :
                    1,1,1-TRICHLOROETHANE Chemical Safety Card

                    Mild Steel may be used down to about 10E-3 mbar or lower if plated. High permeability to hydrogen and possible rusting make this material unsuitable for lower pressure vacuum envelopes.

                    Oxygen Free High Conductivity (OFHC)Copper is easily machined with good corrosion resistance,and is widely used for vacuum applications. It is not generally used for vacuum envelopes that require baking due to possible heavy oxidation,scaling,and the difficulty of brazing in a hydrogen atmsophere.

                    Brass has good corrosion resistance and may be suitable for some applications. Brass components to be used in vacuum are usually Nickel plated to reduce outgassing due to the Zinc content in Brass.

                    Glass to Metal & Glass to Ceramic Seals

                    None of the Above are suitable for glass to metal seals,which are usually made of Inconel or Kovar . These materials are very difficult to machine,very expensive,and best bought as parts of components from vacuum equipment suppliers.

                    e-mail:
                    "To machine kovar use a sharp tungsten carbide tool and 50/50 mix of methanol with oil as lubricant. Although the mixture is flamable, (caution) the methanol seems to greatly improve machinibility." Mike Lines mlines@moxtek.com

                    More data on glass & ceramic to metal seals can be found in:Donald, I.W., "Preparation, properties and chemistry of glass- and glass-ceramic-to-metal seals and coatings", Journal of Materials Science, 28, 1993, pp. 2841-2886.,Section 6, "Applications for seals and coatings", and Table II, "Commercially available sealing glasses".

          2. Metals Used in Demountable Seals
                    Copper Rings are commonly used for high and ultra-high vacuum applications. Varian Associates' , Conflat t.m. flanges use a copper ring compressed between two knife edges, are bakeable to 450 Degrees C ,and are widely used.

                    Aluminum Wire Rings are very cheap and bakeable to 200 Degrees C..

                    Indium Wire can be used between flat flanges. It is very soft and continues to flow after initial tightening.

                    Gold wire is often used for Ultra-High Vacuum seals between flat surfaces,and can be baked to 450 DEgrees C.. Gold is somewhat easier to recycle than Indium,offsetting it's high initial cost.

                    Swagelok,Tylok,and other Proprietary Brand Seals are now widely used. The manufacturer's directions for each brand of seal must be followed to the letter. Never mix one brand of seal with another-mating surfaces will most likely be cut at different angles and they will not be compatible. Mating components of the various seals should also be of the same metal, e.g. don't use brass ferrules on stainless steel fittings and vice versa.

          3. Plastics
                    Generally,the use of plastics should be kept to a minimum due to their high gas permeability and high desorption rates compared with metals,glass and ceramics. In spite of this,plastics are often used in vacuum systems because of their insulating properties,elasticity,and price. Regal Plastics web page has some information on a number of different types of plastic.
                    PTFE has self-lubricating properties,a relatively low outgassing rate, is a good electrical insulator, and can be used at higher temperatures than other plastics. High permeability makes PTFE unsuitable as part of the vacuum envelope.
                    Nylon has self lubricating properties but a high outgassing rate and a high adsorption rate for water.
                    Acrylics have the same undesirable vacuum properties as nylon.
                    Polycarbonates and Polystyrene have moderate outgassing rates and water adsorption characteristics and are good electrical insulators.
                    PVC has a high outgassing rate but does find application for rough vacuum lines and temporary connections such as leak detectors.
                    Polyethylene may be usable if well outgassed.
                    Nalgene t.m. bell jars are available from Fisher Scientific and in some cases may be a cheaper substitute for glass bell jars if some discretion is used.
                    Vespel ®Polyimide is ultra-high vacuum compatible, easily machined,and an excellent insulator from DuPont. Guide pamphlets on Vespel are available from DuPont at 1-800 222 VESP . (Wilmington,DE 19898,U.S.A.) Vespel tends to be very expensive.
                    G10 Glass Epoxy is available in blocks,difficult to machine, and has a high initial outgassing rate. I know of at least one Boulder,Colorado company that uses G10 Glass epoxy as a substitute for Vespel and tolerates the long initial outgassing time as a cost saving measure.
                    Fluoroplastics Include Kel-F (tm),PVDF (Kynar (tm)),TFE,etc. Kel F has a fairly tolerable outgassing rate, is not subject to cold flow, and while expensive is much cheaper than vespel for some applications.

          4. Elastomers
                    Nitrile Rubber a.k.a. Buna N t.m. is widely used in demountable seals,i.e. "O" rings.
                    Viton ® is bakeable to 200 Degrees C. and more suitable at lower pressures. Viton® does have a tendency to compression set.O Ring Chart

          5. Ceramics
                    Fully vitrified electrical porclean and vitrified alumina are excellent insulator that have a low outgassing rate,low gas permeability, and can be used to 1500 Degrees C. There are also some machinable ceramics available. All ceramics are brittle and must be handled with care.

          6. Glass
                    Borosilicate Glass ,a.k.a. Pyrex , is often used for small systems and viewing windows. Glass can be obtained as components from stock,is easy to fabricate into components,and has high corrosion resistance.

          7. Pumping Fluids
                    More comprehensive information on various types of pumping fluids is available from manufacturers. One good source is Inland Vacuum Products catalog. (In N.A. 1-800 962 8099). Inland also operate a reclamation service.
Rotary Pumps
                    High Quality,Low Vapour Pressure Mineral Oils are used for most rotary pump applications.
                    Fomblin (t.m. Montedison Sp.A.) , a perfluoro polyether derived fluid, is used for pumping oxygen and other aggressive or corrosive gases or vapours.

Diffusion Pumps
                    Mineral Oils prepared by molecular distillation from mineral stock are used down to 10E-07 mbarr..Admission of air when hot will produce carbonaceous compounds with high vapour pressures,degrading pump performance. Deposits on surfaces in systems which are under bombardment with charged particles will produce conducting layers.
                    Silicon Fluids produce ultimate pressures down to 10E-09 mbarr. and are stable when hot. They find wide application in frequently cycled industrial plants.Deposits on surfaces in systems which are under bombardment with charged particles will produce electrically insulating layers,making them unsuitable for electronic applications such as mass spectrometers and leak detectors. Silicon fluids are also poor lubricants.

                    Polyphenyl Ethers are thermally stable and have excptionally low vapour pressures,and are commonly used in ultra-high vacuum and clean systems. Backstreaming rates are low and chemical stability is good. Deposits on surfaces in systems which are under bombardment with charged particles will produce conducting layers.Lubricating properties are good. Polyphenyl ethers are the fluids of choice for surface studies,electron microscopes,mass spectrometry,and similar applications. Their only real drawback is high price.

                    "Santovac Fluids, Inc. acquired the Polyphenyl Ether business, which includes SANTOIVAC 5 and 5P Ultra fluids and grease, from the Monsanto Company a few years ago. However, I should point out that we assisted Monsanto in the development of this unique technology and we and our affiliated company, Findett Corporation, have been the sole producer of this familiy of products since they first introduced nearly 25 years ago.".. e-mail from Manny Joaquim,President/CEO,SANTOVAC FLUIDS, INC.

                    PPE's are also used as coatings and/or lubricants for electrical and electronic connectors and switches. Visit the Santovac web page for more.

                    SANTOVAC FLUIDS, INC.
                    8 Governor Drive
                    St. Charles, MO 63301

                    Tel: 314-723-0240, or 800-370-2740
                    Fax: 314-723-4210
                    E-Mail: mjoaquim@compuserve.com

                    Perfluoro Polyether (a.k.a. Fomblin t.m.)is chemically stable, has high oxidation resistance, and is inert to many aggressive chemicals such as oxygen. Fomblin is very expensive but is used in electron microscopes,ion implanters,and wafer fabs.

          8. High Vacuum Compatible Lubricants
                    Krytox A perfluorinated grease made by Dupont
                    Aerospace Lubricants also make several vacuum rated greases, including one similar to Krytox 240AC. Aerospace is at 800-441-9160.
                    Torrlube : A syringe applicable,vacuum compatible lubricant for moving parts in vacuum. Recommended.
                    E-mail: sales@torrlube.com
                    Polyphenyl Ethers were originally developed as lubricants for the aerospace industry.

Vacuum Compatible Epoxies and Leak Sealers
                    Glue is no substitute for having done the job right in the first place, but sometimes you may have no choice. Here are a few that we have tried here.

                    Varian Torr Seal is a solvent free epoxy resin that can be used at pressures of 10e-09 mbar and below at temperatures from -45 Degrees C. to bake temperatures of 120 Degrees C. Torr seal dries to a hard,white consistency, and tends to be a bit pricey.

                    Kurt J. Lesker KL-325K ,a solvent free epoxy packaged in a divider pouch. KL-320K is useful in the 10e-05 to 10e-07 torr range. Lesker also sell a conductive epoxy ( KL-325K ) but it's not recommended for pressures below 10e-03 torr.

Vacseal
                    Vacseal is a low vapour pressure silicon resin available in aerosol spray cans and brush-on applicator bottles. The manufacturer claims it's good for applications from liquid helium temperatures to 450 Degrees C.

                    Vacseal has saved the day for many an experimental system around here with problems like tiny virtual leaks in bad welds. This material should be used very sparingly,if at all on pyrex as it seems to penetrate the glass making glassblowing operations impossible. On the other hand,I once coated the inside of a tiny leak in an ultra high vacuum chamber with Vacseal and,after curing and baking, was able to reduce ultimate pressure from 5X10E-09 mbar to 6X10e-10 mbar.

          10. Vacuum Compatible Tapes
                    3M 850 Polyester Film Tape has a low outgassing rate and is sometimes used to attach low power sputtering targets, in lieu of the expensive Silver bearing epoxy techniques used at higher power levels.

4. Materials That Should Not Be Used In Vacuum

"From ghosties and ghoulies and long legged beasties and things that go bump in the night Good Lord please deliver us."_ _ _ Scottish prayer

This list should include just about everything not listed above,the general rule being if in doubt don't use it. Sometimes you just have to improvise and experiment a bit. Some common nasties:

Cadmium Plating,often used for small screws, must be avoided for vacuum applications due to some really nasty outgassing problems. Small screws used in the chamber can be nickel plated brass or plain copper, and should be drilled or relieved (I.e. file off one side of the threaded area.) to prevent virtual leaks.

PVC Insulated wire generally has a high outgassing rate, and should be replaced with teflon insulated wire.

Paint is a common source of outgassing and virtual leaks.

Yellow Transformer Tape outgasses like mad, can be replaced with ordinary (plumbers) teflon tape.

5. Conclusion

The preceding article is by no means complete,but it's a start. E-mail Roy Schmaus if you have suggestions or comments.

Quotations and the basis for a good bit of this article are from the following reference:
B.S. Halliday "An Introduction to Materials for Use in Vacuum
Vacuum
/Volume 37/Numbers 8/9/pages 583 to 585/1987
0042-207X/87$3.00 Pergamon Press

Copy from Basic Vacuum Technology Resources

 

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