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Chapter 2 Materials used in vacuum system

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Contents

1. Gas Load in the vacuum system

2. Material Property Requirements

3. Common used materials

4. Materials can not be used in vacuum system

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1. High pressure/rough vacuum: volume gas

2. Low pressure: outgassing, evaporation, permeation, backstreaming, leak;

Gas load in a vacuum system

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Outgassing Rate in Torr Liters per Square Cm.

Material Condition 1 Hour 10 Hours 100 Hours Aluminum Cleaned _ 8X10e-09 _ Aluminum Anodized _ 1X10e-07 _ Aluminum Anodized _ 1X10e-07 _ Aluminum degassed 1.7X10e-07 2.7X10e-08 4.6X10e-09 Aluminum none 1.3X10e-06 _ _ Aluminum 6061-T6 none _ 2.5X10e-09 _ Aluminum 6061-T6 @ 200 deg. C hot _ 4.5X10e-09 _

Aluminum 6061-T6 Bake 13.5 hr. @ 200 deg. C _ 3.7X10e-10 _

Aluminum 6061-T6 @ 300 deg. C hot _ 1.4X10e-08 _

Aluminum 6061-T6 Bake 15 hr. @ 300 deg. C _ 1.6X10e-10 _

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Outgassing Rate in Torr Liters per Square Cm.

Material Condition 1 Hour 10 Hours 100 Hours Neoprene _ 3X10e-05 1.5X10e-05 _ PVC _ _ 8X10e-07 1.3X10e-07 Mylar outgassed 2X10e-07 _ _ Neoprene As received 2X10e-04 _ _ Silicone rubber As received 3X10e-05 _ _ Teflon As received 5X10e-06 _ _ PVC As received 9X10e-07 _ _ Textolite As received 7X10e-06 _ _ Mylar As received 3X10e-06 _ _ Zirconium _ 1.3X10e-06 _ _ Butyl rubber _ 1.5X10e-06 _ _

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Outgassing Rate in Torr Liters per Square Cm.

Material Condition 1 Hour 10 Hours 100 Hours Kel F _ 4X10e-08 _ _ Plexiglass Outgassed 1X10e-06 _ _ Polyethylene _ 2.6X10e-07 _ _ Nylon _ 1.2X10e-05 _ _ Porcelain Glazed 6.5X10e-07 _ _ Steatite _ 9eX10e-08 _ _ Epon 828 degassed 6.7X10e-07 5.9X10e-08 9.4X10e-09 Teflon degassed 4.6eX10e-07 2.1X10e-07 9X10e-09

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5. Backstreaming• The flow of gas and vapour in a vacuum system is away from the chamber, through the pump, and out to the atmosphere.

• In molecular flow where molecules behave as individuals with some of them going against the main flow direction.

• Undesirable things downstream of the chamber (like pump oil)

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1. Mechanical Properties Be capable of being machined and fabricated. Adequate strength at maximum and minimum temperatureRetain it's elastic,plastic,and/or fluid properties over the expected temperature range.

2. Thermal Properties Vapor pressure. Thermal expansion of adjacent materials

Material Property Requirements

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4. How to choose a material for vacuum system

3. Gas Loading Materials must not be porous. Materials must be free of cracks and crevices. Surface and bulk desorption rates.

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Commonly Used Materials

Metals for the Vacuum Envelope

1. Austenitic Stainless Steel• Used in high and ultra-high vacuum systems,• Satisfactory argon-arc welding. U.S. 321 , 347, and 304 • Low magnetic permeability. • Economical.• Acceptable outgassing rates• Can be fabricated easily.

Disadvantage • Components may require 'truing up' due to distortion caused by welding operations.

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2. Aluminum and Aluminum Alloys• Cheap

• Easy to machine

• Low outgassing rate as long as the alloy does not have a high zinc content.

Disadvantages

• Low strength at high temperatures

• High distortion when welding.

• Alloys with copper content also present welding problems.

• Should never be anodized due to serious outgassingproblems.

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

4.Oxygen Free High Conductivity (OFHC)Copper• easily machined• good corrosion resistance• not generally used for vacuum envelopes that require baking • possible heavy oxidation,scaling in high temperature• difficulty of brazing in a hydrogen atmosphere.

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5.Brass• good corrosion resistance • used in vacuum are usually Nickel plated to reduce outgassing due to the Zinc content in Brass.

6.Glass to Metal & Glass to Ceramic Seals • Inconel or Kovar . • Difficult to machine• very expensive• best bought as parts of components from vacuum equipment suppliers.

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Metals Used in Demountable Seals

1.Copper Rings are commonly used for high and ultra-high vacuum applications. Flanges use a copper ring compressed between two knife edges, are bakeable to 450 Degrees C, and are widely used.

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

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

4.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.

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Plastics

• high gas permeability

• high desorption rates

• kept to a minimum use in vacuum

• high insulating properties

• Elasticity

• price.

General properties

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1. PTFE (聚四氟乙烯)• self-lubricating properties• a relatively low outgassing rate, • good electrical insulator, • can be used at higher temperatures than other plastics. • High permeability

2. Nylon(尼龙)• self lubricating properties• a high outgassing rate • a high adsorption rate for water.

3. Acrylics (丙烯酸树脂)have the same undesirable vacuum properties as nylon.

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4. Polycarbonates(聚碳酸脂)and Polystyrene (聚苯乙烯)• moderate outgassing rates and water adsorption • good electrical insulators.

5. PVC (聚氯乙烯 )• high outgassing rate • application for rough vacuum lines • temporary connections such as leak detectors

6. Polyethylene (聚乙烯)may be usable if well outgassed

7 Polyimide(聚酰亚胺)• ultra-high vacuum compatible• easily machined,• excellent insulator• very expensive.

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8. Glass Epoxy （玻璃环氧片）

• difficult to machine

• a high initial outgassing rate.

• long initial outgassing time

9.Fluoroplastics (氟塑料）

• fairly tolerable outgassing rate

• not subject to cold flow

• No very expensive.

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ElastomersNitrile Rubber is widely used in demountable seals, i.e. "O" rings.

bakeable to 200 Degree

suitable at lower pressures.

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Ceramics

Fully vitrified electrical porcelain and vitrified alumina • excellent insulator • low outgassing rate• low gas permeability, • can be used to 1500 Degrees C. • some machinable ceramics available. • brittle and must be handled with care.

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Glass

Borosilicate（硼硅酸盐）Glass , i.e.. Pyrex , is often used for small systems and viewing windows.

• easy to fabricate into components,

• high corrosion resistance.

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Pumping Fluids High Quality, Low Vapour Pressure Mineral Oils;

Mineral Oils prepared by molecular distillation from mineral stock are used down to 10-7 mbar;

Silicon Fluids produce ultimate pressures down to 10-9 mbar. and are stable when hot;

Polyphenyl Ethers （聚苯乙醚）are thermally stable and have exceptionally low vapor pressures,and are commonly used in ultra-high vacuum and clean systems. Backstreaming rates are low and chemical stability is good;

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Materials That Should Not Be Used In Vacuum

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

2 Paint is a common source of outgassing and virtual leaks.

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