Vacuum Fundamentals

High-Vacuum Technology Course

Week 5

Paul Nash

HE Subject Leader (Engineering)

Vacuum Fundamentals

• Recap on last session

• Progress on assignments to date

Vacuum Technology

Vacuum Fundamentals

• To be able to describe the properties of a variety of materials in a vacuum environment

• To be able to select materials for use in a vacuum environment

Learning Objectives

Vacuum Fundamentals

Vacuum Pumps

Work from previous session ……..

Vacuum Fundamentals

By stating the flow in terms of pressure times volume flow rate, the variation in gas density with pressure is allowed for

This gives a flow unit that relates directly to the actual quantity of gas in the flow:

Throughput

time

volumepressureThroughput

Vacuum Fundamentals

Vacuum Pump Types

Low and medium vacuumRotary VaneMechanical Booster

High and Ultra-High vacuumDiffusion pumpTurbomolecular pumpCryogenic pumpIon pump

Vacuum Fundamentals

Vacuum Fundamentals

Foreline TrapsFor prevention of pump oil vapour

migrating back into the system at low pressures from rotary pumps

Vacuum Pump Accesories

Vacuum Fundamentals

Dust FiltersFor prevention of dust being carried

from the process into the pump. Increases pump life by reducing wear, but reduces pumping speed by 10 – 25%.

Chemical FiltersFor prevention of aggressive chemical

vapour dust being carried from the process into the pump. Usually activated charcoal.

Vacuum Pump Accesories

Vacuum Fundamentals

Dust FiltersFor prevention of dust being

carried from the process into the pump. Increases pump life by reducing wear, but reduces pumping speed by 10 – 25%.

Vacuum Pump Accesories

Vacuum Fundamentals

Oil Mist FiltersFor trapping pump oil carried out

in exhaust gases from rotary pumps.

Vacuum Pump Accesories

Vacuum Fundamentals

High to Ultra-high Pump Comparisons

Vacuum Fundamentals

High to Ultra-high Pump Comparisons

Vacuum Fundamentals

High to Ultra-high Pump Comparisons

Vacuum Fundamentals

High to Ultra-high Pump Comparisons

Vacuum Fundamentals

Materials

Vacuum Fundamentals

General requirements:

Very low rates of outgassing

Tolerant to bakeout temperatures (can be up to 450oC)

Requirements are more stringent as we head to higher vacuum levels

Materials for use in a vacuum

Vacuum Fundamentals

Specific risks:

Materials may sublimate in vacuum(transition from solid to gas)

Gases can be released from porous materials or cracks/crevices

Traces of lubricants and residues from machining may be present on surfaces

Materials for use in a vacuum

Vacuum Fundamentals

Materials to avoid

• Cadmium– Often present in the form of plating (fasteners

etc.) or in some brazing alloys

• Zinc– A problem in high vacuum and high

temperatures. Present in some alloys like brass (some electrical fittings)

Vacuum Fundamentals

Materials to avoid

• Magnesium– Low melting point (650oC at atmosphere).

Contains free hydrogen gas

• PVC– Often found in wire insulation, dust caps etc.

Vacuum Fundamentals

Materials to avoid

• Paints

• Polymers– Many have an affinity to water– Especially plastic tapes. Mould release

residue can be an issue too. Polymers may generate a static charge attracting dust

– Nylon has a high outgassing rate

Vacuum Fundamentals

Materials to use - Metals

• Stainless Steel– Most common choice in HV and UHV systems– 304 - common– 304L – Low carbon variant of 304 especially

in UHV systems– 321 – for when low magnetic permeability is

required

• BUT…. Avoid 303 grade – contains sulphur and tends to outgas

Vacuum Fundamentals

Materials to use - Metals

• Mild Steel– Can be used above 10-6 Torr– Can be nickel plated to reduce outgassing

• BUT…. Can pass hydrogen and has a strong tendency to rust

Vacuum Fundamentals

Materials to use - Metals

• Aluminium & Al Alloys– Low outgassing– Easy to machine– Low weight and lower cost than S/S

• BUT…– Some alloys contain a high proportion of zinc– Must NOT be anodised– Poor strength at high temperatures– Not easy to weld

Vacuum Fundamentals

Materials to use - Metals

• Nickel– Relatively low cost– Easy to machine– Can be spot-welded– High melting point (1455oC)– Resistant to corrosive fluids and atmospheres

• BUT…– Ferromagnetism restricts applications that

would be affected by magnetic fields

Vacuum Fundamentals

Materials to use - Metals

• Copper (Oxygen-free) C10100 & C10200– ‘Oxygen-free’ type is widely used– Easy to machine– Impermeable to hydrogen and helium– Low sensitivity to water vapour

• BUT…– Normal copper is difficult to outgas fully– Tends to oxidise and form scale– Strength falls above 200oC and vapour pressure is an

issue over 500oC

Vacuum Fundamentals

Materials to use - Metals

• Tungsten– Can be used at high temperatures – Can be used for filaments

• BUT…– Becomes brittle when work-hardened

Vacuum Fundamentals

Materials to use - Metals

• Indium– Can be used in wire form for demoutable

seals

• Gold– Can be used in wire form for demoutable

seals in UHV systems

Vacuum Fundamentals

Materials to use - Metals

• Beryllium– Can be used for X-Ray windows– High resistance to vacuum– BUT…Toxic in dust form

• Zirconium– Low production of secondary electrons– Can be used for Neutron windows– Can be used as a molecular capture medium– BUT….Expensive and relatively rare

Vacuum Fundamentals

Materials to use - Polymers

• PTFE– Good electrical insulator– Tolerant to high temperatures– Low outgassing

• BUT…– Cannot be used as a barrier between vacuum

and atmosphere as it is permeable to gases

Vacuum Fundamentals

Materials to use - Polymers

• Kapton– Good electrical insulator– Tolerant to high temperatures– Very low outgassing– Available in tape and film form

Vacuum Fundamentals

Materials to use - Polymers

• PEEK – Polyether ether ketone– Excellent mechanical & chemical resistance– Suitable for UHV applications– Very low outgassing

• BUT…– Has a melting point of 343oC

Vacuum Fundamentals

Materials to use - Polymers

• Nitrile rubber– Used for demountable seals (‘O’ rings etc.)– Good electrical insulator

• BUT…– Poor chemical resistance– Can dry out and crack

Vacuum Fundamentals

Materials to use - Polymers

• Viton– Used for demountable seals (‘O’ rings etc.)– Can also be used as a seating face in valves– Good electrical insulator– Good chemical resistance– Bakeable to 200oC

Vacuum Fundamentals

Materials to use - Ceramics

• Borosilicate glass– Used for viewports– Can be machined and joined with metals– Low coefficient of thermal expansion –

resistant to thermal shock

Vacuum Fundamentals

Materials to use - Ceramics

• Porcelain and Alumina– Excellent electrical insulation– Non-porous if fully vitrified– Low coefficient of thermal expansion – usable

to 1500oC

• BUT…– Alumina can be dimensionally unstable during

vitrification

Vacuum Fundamentals

Materials to use – Lubricants & adhesives

• Vacuum Greases– Low outgassing– http://www.2spi.com/catalog/vac/

• Adhesives for use in vacuum– Low outgassing, stable at high temperature– ‘Torr Seal’– ‘Vacseal’

• Vacuum Greases– Low outgassing– http://www.2spi.com/catalog/vac/

• Adhesives for use in vacuum– Low outgassing, stable at high temperature– ‘Torr Seal’– ‘Vacseal’