non-evaporable getters · = 100 l/s duration of ... 5 10 1.4 5 3 h 2 sorption regeneration cycle ....
TRANSCRIPT
Non-Evaporable Getters
Dr. Oleg B. Malyshev
Senior Vacuum Scientist
ASTeC Vacuum Science Group,
STFC Daresbury Laboratory, UK
1 VS-2, 18-19 October 2011, Ricoh Arena , Coventry
Sorption and diffusion
Gas atoms or molecules
(adsorbate)
Solid surface (adsorbent)
Sticking probability s1
Physisorption (dipole or
van der Waals forces)
Chemisorption (covalent
linkage)
Binding energy
Reflecting probability (1s)
Handbook of vacuum technology. Ed.
K. Jousten, Weley-VCH, Weinheim,
2008, Chapters 6,11
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Solid
adsorbent
Gas,
adsorbate
absorbed molecules diffusion
surface diffusion adsorbate desorption
Classification of sorption pumps
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Sorption pumps
Adsorption pumps (physi- and cryosorption )
Getter pumps (physi- and chemisorption)
With ionisation Without ionisation
Orbitron
pumps
Ion getter
pump
Evaporation
pumps Bulk getter
pumps
NEG coating
NEG
pumps
Getters
Evaporation pumps
Adsorption mainly
Chemical binding at surface
Covering with a fresh material
after saturation
Bulk (non-evaporable) getter pumps
Bulk getter not only adsorb gases at the
surface
but also employs an effect of gas
diffusion into a getter material
Re-activation by heating to an activation
temperature
Gas accumulation in getter materials due to:
Adsorption at the surface
Absorption (i.e. solvation) in the bulk
Due to diffusion, reversible process
Chemical binding
Irreversible process
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Complications in studying and using NEG
Complexity of getter material and processes:
Oxide layers
Surface roughness
Bulk structure and morphology
Temperature (activation and operation)
History of using the getter
Gas-solid combinations
Optimum condition of operation depends on
applications
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NEG structure
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Metal substrate
NEG powder
• Zr, V, Ti as active metals mixed with Fe, Al
• Developed surface area and boundaries between
grains
• Protective oxide layer before activation
How adsorption works
Sticking coefficient
Local parameter, depends
on incident angle and
surface orientation
Sticking probability
Statistically averaged value
for the surface
Capture probability β
Statistical value for a pump
with a given geometry
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Cannel with sorbing walls L/d=10
α
α
α β
Q
1 104
1 103
0.01 0.1 1
1 104
1 103
0.01
0.1
1
α
β
Types of NEGs (SAES getters)
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St707 Strip
The thickness of
the getter layer is
about 70
micrometers on
each side of the
strip
Getter Wafer
Modules (based on
St101 or St707
getter strips) are
specifically designed
to maximize
hydrogen pumping
speed.
Cartridge
pumps can
be mounted
on CF
flanges
Compressed
getter pills
and washers
for various
applications
Pumping
What can be pumped by NEG
M + O2 MO
M + N2 MN
M + CO2 CO + MO
MC + MO
M + CO MC + MO
M + H2O H + MO
MO + H (bulk)
M + H2 M + H (bulk)
M + CxHy MC + H (bulk)
at T > 500C
What can’t be pumped
Hydrocarbons, CxHy, etc.
at T < 500C
He, Ne, Ar, Kr, Xe (inert
gases)
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Activation
First activation consiredations:
If capacity Q=5000 mbarl
Activation pressure Pa= 10-4
mbar
Pumping speed during
activation Seff = 100 l/s
Duration of activation t:
Pa= 10-2 mbar and Seff = 100 l/s
Pa= 10-3 mbar and Seff = 10 l/s
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dayshrst
hrsSP
Qt
effact
6140105
4.1105
5
3
H2 sorption
regeneration cycle
Activation temperature and duration (SAES Getters)
Short (tens minutes)
activation used for
independent activation
T 300C allows
activation during vacuum
system bakeout (without
special arrangement)
NEG partially activated
during bakeout
at T 250C
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Problems and limitations
Limited capacity for pumping
Aging (limited number of
activation cycles before full
saturation)
Hydrogen
enbrittlement (2 x
pumping capacity)
Thermal fatigue
(number of activation
cycles before initial
peel-off)
Storage (in vacuum,
N2 or inert gas
atmosphere)
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Operation at high temperature
Below activation
temperature
High temperature
increases the diffusion
Continues re-activation
Operation at activation
temperature for high
pumping speed and
capacity.
Requires another pump
for H2 pumping
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Use of NEGs
High gas load
UHV/XHV application
In combination with a pump for noble gases and
hydrocarbons, ex. such as TMP and SIP
Purification of noble gases
The only gases that can’t be pumped by NEG
Low power consumption
Power used for activation only
Hydrogen pressure regulator in UHV system
By changing the NEG temperature
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NEG application in accelerators
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Usual accelerator vacuum chamber
Long tube with length L >> a, where a - transversal dimension
Average pressure depends on vacuum conductance u(L,a) of
the beam vacuum chamber
16 WS-63, 14-19 September 2010, Ávila, Spain
1
12 2B
eff
LP qL k T
u S
P
z
L
Vacuum chamber cross sections
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Beam pipe
Circular or elliptical
4 mm d, a, b 200 mm
Vacuum chamber with an antechamber
for larger vacuum conductance, U
Distributed pumping In dipole magnetic field With NEG strips
(LEP in CERN)
d a
b
Two concepts of the ideal vacuum chamber
Traditional:
surface which outgasses as little as
possible (‘nil’ ideally)
surface which does not pump
otherwise that surface is
contaminated over time
Results in
Surface cleaning, conditioning,
coatings
Vacuum firing, ex-situ baling
Baking in-situ to up to 300C
Separate pumps
18/40
New (C. Benvenuti, CERN, ~1998):
surface which outgasses as little as
possible (‘nil’ ideally)
a surface which does pump,
however, will not be contaminated
due to a very low outgassing rate
Results in
NEG coated surface
There should be no un-coated parts
Activating (baking) in-situ at 150-
180C
Small pumps for CxHy and noble
gases
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Stainless steel vs NEG coated vacuum chamber under SR
19/40 EVC-11, 20-24 September 2010, Salamanca, Spain
V.V. Anashin et al. Vacuum 75 (2004), p. 155.
NEG coating for accelerators
First used in the ESRF (France);
ELETTRA (Italy), Diamond LS (UK);
Soleil (France) – first fully NEG coated;
LHC (Switzerland) – longest NEG coated vacuum chamber;
SIS-18 (Germany) and many others.
NEG film capacity for CO and CO2 is ~1ML:
If P = 10-9 mbar then 1 ML can be sorbed just in ~103 -104 s;
Lab measurements of different NEG coatings often don’t repeat
CERN’s data on sticking probability and capacity;
However, NEG coated parts of accelerators work well.
20/40 EVC-11, 20-24 September 2010, Salamanca, Spain
NEG coating for accelerators (2)
What is required:
Input data for accelerator design:
(D,E,Ta), (M, Ta), pumping capacity;
Better understanding:
what and why;
practical ‘do’s and ‘don’t’s;
Further development of this coating:
lower , Ta, SEY;
higher (M), pumping capacity;
optimising for an application.
21/40 EVC-11, 20-24 September 2010, Salamanca, Spain
What NEG coating does
Reduces gas desorption:
A pure metal film ~1-m thick
without contaminants.
A barrier for molecules from
the bulk of vacuum chamber.
Increases distributed
pumping speed, S:
A sorbing surface on whole
vacuum chamber surface
S = Av/4;
where – sticking probability,
A – surface area,
v – mean molecular velocity
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Vacuum NEG Subsurface Bulk
Coating Layers
Deposition method
Planar magnetron deposition
Cylindrical magnetron deposition
23/40 EVC-11, 20-24 September 2010, Salamanca, Spain
Details are in the talk A3.TM.OR.13 by Dr. R. Valizadeh
ASTeC activation procedure
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O.B. Malyshev, K.J. Middleman, J.S. Colligon and R. Valizadeh. J. Vac. Sci. Technol. A 27 (2009), p. 321.
NEG pumping properties
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Pressure ratio P1/P2 measured during
gas injection is used to estimate:
initial sticking probability and sorption capacity
Films deposited on Si test sample from a single metal wire
Ti film
Average grain size
100 – 150 nm.
26 WS-63, 14-19 September 2010, Ávila, Spain
V film
Average grain size
100 – 150 nm.
Hexagonal lattice structure
Zr film.
Average grain size
100 – 150 nm. Hexagonal
lattice structure
27/40 WS-63, 14-19 September 2010, Ávila, Spain
Single metal pumping properties
140 160 180 200 220 240 260 280 300 3201 10
3
0.01
0.1
1
Ti
Zr
V
Hf
CO
stick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3201 10
4
1 103
0.01
0.1
Ti
Zr
V
Hf
Activation temperature [ C]
H2 s
tick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3201 10
3
0.01
0.1
1
10
Ti
Zr
V
Hf
CO
pum
pin
g c
apac
ity [
ML
] Zr is best:
Lowest activation Temp. and
highest capacity
Hf
Ti
V has highest activation
temperature 27
Binary films deposited on Si test sample from twisted wires.
Ti-V film.
Average grain size
50 – 100 nm.
Hexagonal lattice
structure.
WS-63, 14-19 September 2010, Ávila, Spain 28
Ti-Zr film.
Average grain size
50 – 100 nm.
Hexagonal lattice
structure.
Zr-V
Average grain size
10 – 20 nm
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Binary alloy pumping properties
140 160 180 200 220 240 260 280 300 3201 10
3
0.01
0.1
1
Ti-Zr
Ti-V
Zr-V
CO
stick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3201 10
4
1 103
0.01
0.1
Ti-Zr
Ti-V
Zr-V
Activation temperature [ C]
H2
stick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3201 10
3
0.01
0.1
1
10
Ti-Zr
Ti-V
Zr-V
CO
pu
mp
ing c
apac
ity
[M
L]
Zr-V is best
Ti-Zr activation temperature
is lower than for Ti-V
Zr-Hf was not studied
Ternary NEG film deposited on Si test sample from
twisted Ti, V, Zr, and Hf wires and TiZrV alloy wire
Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.
Average grain size 5 nm. Hexagonal lattice structure.
Ti-Hf-Zr twisted wire V-Hf-Zr twisted wire
Ti-Zr-V alloy wire Ti-Zr-V twisted wire
30
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Ternary alloy pumping properties
140 160 180 200 220 240 260 280 300 3201 10
3
0.01
0.1
1
CO
stick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3201 10
4
1 103
0.01
0.1
Ti-Zr-V
Hf-Zr-V
Ti-Zr-Hf
Ti-Hf-V
Activation temperature [ C]
H2
stick
ing p
robab
ility
140 160 180 200 220 240 260 280 300 3200.01
0.1
1
10
CO
pu
mp
ing c
apac
ity
Hf-Zr-V, Ti-Zr-Hf and
Ti-Hf-V are comparable
Ti-Zr-V has the highest
activation temperature
Twisted wires vs. alloy target: reducing Ta
1.E-02
1.E-01
1.E+00
140 160 180 200 220 240 260 280 300 320
Activation temperature, oC
CO
sti
ck
ing
pro
ba
bil
ity
TiZrV(twisted wires)
TiZrV (alloy wire)
TiZrV (alloy wire)
32 WS-63, 14-19 September 2010, Ávila, Spain
R. Valizadeh, O.B. Malyshev, J.S. Colligon, V. Vishnyakov. Accepted by J. Vac. Sci. Technol. Aug. 2010.
Quaternary NEG alloy film deposited on Si test sample from twisted Ti, V, Zr, and Hf wires.
Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.
Very glassy structure.
33
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Quaternary alloy pumping properties
140 160 180 200 220 240 260 280 300 3200.01
0.1
1
CO
stic
king
pro
babi
lity
140 160 180 200 220 240 260 280 300 3201 10
4
1 103
0.01
0.1
Ti-Zr-Hf-V
Hf-Zr-V
Ti-Zr-Hf
Ti-Hf-V
Ti-Zr-V
Ti-Zr
Zr-V
Zr
Activation temperature [ C]
H2
stic
king
pro
babi
lity
140 160 180 200 220 240 260 280 300 3200.01
0.1
1
10
CO
pum
ping
cap
acity
Ti-Zr-Hf-V is the best
Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and
Zr are comparable
Ti-Zr-V is lower
Zr-V (best binary alloy) has the
lowest activation temperature
Pressure in the accelerator vacuum chamber
Improving pumping
properties is limited:
1.
0.005 < H2 < 0.01
0.1 < CO < 0.5
0.4 < CO2 < 0.6
Reducing the
desorption yields
. in orders of magnitude
is a realistic task
WS-63, 14-19 September 2010, Ávila, Spain 35
P
where
- desorption yield
- sticking probability
Reducing the gas desorption from the NEG coatings
Main gases in the NEG coated vacuum
chamber are H2 and CH4
Only H2 can diffuse through the NEG
film under bombardment or heat
CH4 is most likely created on the NEG
surface from diffused H2 and C
(originally from sorbed CO and CO2)
Therefore the H2 diffusion must be
suppressed
Where H2 come from?
36 WS-63, 14-19 September 2010, Ávila, Spain
Reducing the gas desorption from the NEG coatings
Gas molecules are contained
on the NEG coating surface
after exposure to air
inside the NEG coating
trapped during deposition
in subsurface substrate layer
in the substrate bulk
Vacuum
NEG
Coating
Subsurface
Layers
Bulk
37 WS-63, 14-19 September 2010, Ávila, Spain
Reducing the gas desorption from the NEG coatings
Gas molecules are contained
on the NEG coating surface
after exposure to air
minimise exposure to air
inside the NEG coating
trapped during deposition
purity of discharge gas
background pressure
in subsurface substrate layer
substrate bakeout before NEG deposition
in the substrate bulk
vacuum firing
Vacuum
NEG
Coating
Subsurface
Layers
Bulk
38 WS-63, 14-19 September 2010, Ávila, Spain
WS-63, 14-19 September 2010, Ávila, Spain 39
SEM images of films (film morphology )
columnar dense
Best for pumping A first candidate for a barrier
O.B. Malyshev, R. Valizadeh, J.S. Colligon et al. J. Vac. Sci. Technol. A 27 (2009), p. 521.
Modified NEG pumping properties
evaluation rig:
To measure sticking probability
To measure electron stimulated gas
desorption as a function of
Electron energy
Dose
Wall temperature (20-100C)
Activation/bakeout temperature
Can be used for samples with:
NEG coating
Low desorption coating
No coatings
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Electron stimulated desorption
Electron Stimulated Desorption (ESD) studies programme
ESD as a function of
Activation/bakeout temperature
Electron energy
Electron dose
Coating density, morphology and structure
Deposition conditions
Substrate
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ESD: stainless steel vs activated NEG coated vacuum chamber
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1 1020
1 1021
1 1022
1 1023
1 108
1 107
1 106
1 105
1 104
1 103
0.01
0.1
H2
CH4
H2O
CO
O2
Ar
CO2
dose [electrons/m2]
yie
ld [
mole
cule
s/e
lectr
on]
1 1020
1 1021
1 1022
1 1023
1 108
1 107
1 106
1 105
1 104
1 103
0.01
0.1
H2
CH4
CO
dose [electrons/m2]
yie
ld [
mole
cule
s/e
lectr
on]
O.B. Malyshev, A. Smith, R. Valizadeh, A. Hannah.
Accepted by J. Vac. Sci. Technol., Aug. 2010.
Baked to 250°C for 24 hrs Activated to 180°C for 24 hrs
1 103
0.01 0.1 1
1
10
100
1 103
G1
G4
Surface coverage [monolayers ]
Rat
io [
P1
/P2]
Electron
bombardment 1
Electron
bombardment 2
Electron stimulated NEG activation
43 WS-63, 14-19 September 2010, Ávila, Spain
Non-activated NEG
The electron stimulated NEG activation efficiency
estimated as 7.9×10-4 < 1 < 2.4×10-3 [CO/e-] 1
COCO
e D
Activated at 180°C P1
P2
(Ee-) for different gases for NEG coating
44 EVC-11, 20-24 September 2010, Salamanca, Spain
NEG cartridges vs films
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Bulk NEG NEG film
Thickness 50 m – 5 mm 0.5 – 3
Materials Zr-V-Fe (St 707)
Zr-Al (St 101)
Ti-Zr-V (CERN)
Ti-Zr-Hf-V (ASTeC)
Activation T 300 – 700 C 150 – 300 C
Pumping capacity Large 1 ML for CO, CO2
No. activation cycles
before full staturations
A few hundreds < 100
Working range HV, UHV, XHV UHV, XHV
Activation by photon or
electron bombamdment
n/a possible
Main purpose pumping Barrier coating to reduce
outgassing + pumping
Conclusions
NEGs pay and increasingly important role in vacuum
technology including UHV/XHV
They have wide range of applications, including particle accelerators
NEG can be activated in vacuum by heat,
NEG films can also be activated by photon or electron bombardment
Ones activated no power or controllers required for
operation
No noise and cooling channels
In some applications no cables, feedthroughs
To use them some special knowledge of NEG installation,
activation operation are essential
46/40 VS-2, 18-19 October 2011, Ricoh Arena , Coventry