High-Vacuum Technology
Course
Week 5
Paul Nash
HE Subject Leader (Engineering)
Vacuum Fundamentals
Vacuum Technology
• Recap on last session
• Progress on assignments to date
Vacuum Fundamentals
Learning Objectives
• 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
Vacuum Fundamentals
Vacuum Pumps
Work from previous session ……..
Vacuum Fundamentals
Throughput
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:
pressure  volume
Throughput 
time
Vacuum Fundamentals
Vacuum Pump Types
Low and medium vacuum
Rotary Vane
Mechanical Booster
High and Ultra-High vacuum
Diffusion pump
Turbomolecular pump
Cryogenic pump
Ion pump
Vacuum Fundamentals
Vacuum Fundamentals
Vacuum Pump Accesories
Foreline Traps
For prevention of pump oil vapour
migrating back into the system at low
pressures from rotary pumps
Vacuum Fundamentals
Vacuum Pump Accesories
Dust Filters
For 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 Filters
For prevention of aggressive chemical
vapour dust being carried from the process into
the pump. Usually activated charcoal.
Vacuum Fundamentals
Vacuum Pump Accesories
Dust Filters
For 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 Fundamentals
Vacuum Pump Accesories
Oil Mist Filters
For trapping pump oil carried out
in exhaust gases from rotary pumps.
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
Materials for use in a vacuum
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
Vacuum Fundamentals
Materials for use in a vacuum
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
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 Fundamentals