"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
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.
" 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
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".
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.
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.
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.
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.
"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
Aerospace Lubricants also make several vacuum rated greases, including one similar to Krytox 240AC. Aerospace is at 800-441-9160.
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.
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.
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:
The preceding article is by no means complete,but it's a start. E-mail 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
0042-207X/87$3.00 Pergamon Press