A novel class of Non Evaporable Getter (NEG) alloys with lower

A novel class of Non Evaporable Getter (NEG)
alloys with lower hydrogen equilibrium pressure
and reduced outgassing properties
P. Manini, Fabrizio Siviero, A. Gallitognotta, L. Caruso,
T. Porcelli,G. Bongiorno, E. Maccallini,
SAES Getters, Lainate – Italy
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Outline
General features of NEGs
Review of some outgassing properties of St 172 alloy
Outgassing of NEG pumps activated in isolated conditions
A novel family of alloy (ZAO®) for improved NEG pumps
Preliminary ZAO® outgassing measurements
Perspectives and conclusions
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General features of NEGs
NEG pumps are used in UHV applications including particle accelerators
and high energy physics machines.
NEG pumps provide :
very large pumping speed in a compact and light package,
Great trapping efficiency for H2 ,
powerless operation,
vibration and maintenance free operation
negligible magnetic interference (µ <1.005 for St 172 ).
P. Manini OLAV IV, Hsinchu, 2014
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General features of NEGs
NEG pumps are used in UHV applications including particle accelerators
and high energy physics machines.
NEG pumps provide :
very large pumping speed in a compact and light package,
Great trapping efficiency for H2 ,
powerless operation,
vibration and maintenance free operation
negligible magnetic interference (µ <1.005 for St 172 ).
Main NEG limitations :
1. The large gas load emitted during the activation
2. The inability to pump noble gases
3. The possibility to release particles, a sensitive issue in specific areas
In this presentation we report on the activity carried out at SAES to
investigate and mitigate gas emission during NEG activation.
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Gas release during the activation
During the activation, the following mechanisms take place :
Desorption of the physisorbed species (water, CO, CO2…)
Decomposition of the surface compounds (mainly carbides and oxides)
which diffuse into the getter volume. However, Carbon, Oxygen and
Hydrogen can also recombine on the surface and desorb.
Diffusion of the atomic hydrogen from the bulk of the getter to the surface
with surface recombination and desorption.
Catalytic reactions, leading to the generation and desorption of methane
and longer chain hydrocarbons.
In some situations the pressure increase is not acceptable or
may require time to be recovered, making the use of NEG less
advantageous.
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Outgassing test set up
Test was carried out on a UHV bench equipped with a QMS
EXT
QMS
Main
Volume
CapaciTorr D100
V1
TMP
DIAPHRAGM
VACUUM PUMP
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1° test : outgassing from NEG pump during activation
Outgassing tests have been carried out on a Capacitorr ® D 100 pump
(100l/sH2).
The getter material is St 172 (St 707® + Zr) in the shape of highly porous
sintered disks. The high porosity ensures large pumping speed and
perfomance reducing the amount of getter material.
Capacitorr D 200
Capacitorr D 100
Capacitorr D 50
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1° test : outgassing from NEG pump during activation
confronto
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1° test : outgassing from NEG pump during activation
H2
CO
CH4
H2O
O2
CO2
CnHm
Heavier HC
Ptot
1,E-05
1,E-06
Pressure (Torr)
1,E-07
1,E-08
H2O
CO2
CO
1,E-09
CH4
HC
1,E-10
550°C
1,E-11
350°C
1,E-12
15
P. Manini OLAV IV, Hsinchu, 2014
20
25
9
30
35
40
Time (min)
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Conclusions from the 1° test
Two main emission peaks were identified at 350°C and 550°C.
350°C : the outgassing rate is relatively high but very short ( a few minutes)
and quickly decreasing. This seems to be associated to the released of
physisorbed molecules.
550°C : H2 is the main contribution (out diffusion from the getter volume).
All the other gases being much lower
At the beginning of the 550°C step, hydrocarbons are generated. This is
likely due to catalytic reaction between residual C and H being released.
These HC are burned away as the getter stays hot.
It is advisable a pre-conditioning step at 200-350°C before the full
activation to remove physisorbed species and reduce the amount of
released gases.
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2° test : outgassing of NEG activated w/o auxiliary pumps
Auxiliary pumping is generally required during getter activation (450 – 550 °C):
• Remove H2 desorbed according to the equilibrium pressure law
• Remove physisorbed species leaving the getter surface
However, in some applications, auxiliary pumping is available only during the
initial system pumpdown.
This means that, in case of need to reactivate the getter, the process should be
carried out in closed-off conditions.
Experiments were carried out to study:
Efficiency of NEG reactivation without auxiliary pumping
Pressure evolution and composition during activation
A NEXTorr® D 100-5 was used in this experiment
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Experimental set up and procedures
Experimental setup:
• UHV system with QMS
• NEXTorr ®D100-5 mounted on a nipple with an
elbow (≈ 50% drop of S)
Procedure:
1.
2.
3.
4.
5.
Bakeout of the system at 180°C for 10 hours
1st NEG activation under TMP pumping
H2 pumping speed measurement
2nd activation in closed-off condition
H2 pumping speed measurement
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Activation under pumping & reactivation in isolated conditions
H2
1st Act
2nd Act
H2
H2O
H2O
CO
CO2
CH4
CO
CO2
CH4
Notwithstanding closed-off conditions we have limited outgassing, much lower than the 1st
activation.
The surface is “clean”: absence of the initial peak due to desorption of physisorbed species
and carbon contamination
NEG reactivation w/o external pumping is possible and provides good UHV conditions
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Further improvements : ZAO®
A novel family of alloy ( Zr-V-Ti-Al) called ZAO has been developed which has
several specific advantages compared to St 172 (currently used in the
Capacitorr and NEXTorr product line)
Lower equilibrium isotherm ( even at 200°C the equilibrium pressure of
hydrogen is @1x10-10 mbar or lower)
Larger capacity for all active gases
Ability to withstand more reactivation cycles without loosing significant
performances
Better mechanical properties : disks are intrinsically more robust, less
prone to generate particles, higher embrittlement limit.
This getter alloy will replace St 172 in Capacitorr® /NEXTorr ®in those
applications where lower outgassing, less particle emission and higher
capacity are required.
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Hydrogen equilibrium isotherm
HYDROGEN EQUILIBRIUM ISOTHERMS
1,E+01
St172
ZAO1
Hydrogen equilibrium pressure (Torr)
1,E+00
600°
1,E-01
500°
1,E-02
400°
1,E-03
200°
1,E-04
300°
1,E-05
0,1
1
10
Hydrogen concentration (Torr.l/g)
100
1000
Lower equilibrium pressure and larger H2 storage capacity for each given temperature.
Higher embrittlement limit.
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3° test : outgassing from ZAO® NEG pump
confronto
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Comparison between St 172 and ZAO® tests
P H2 (Torr)
1,E-06
1,E-06
1,E-06
8,E-07
172
6,E-07
ZAO
4,E-07
2,E-07
1,E-10
0
20
40
60
80
100
Time (min)
The H2 evolution during activation is about 5 times higher for
Capacitorr ®D 100 (St 172)
The total amount of hydrogen released is also a factor 2 larger
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What about pumping performances?
Capacitorr CT D100 based on ZAO
sorption test (according to ASTM F798-97)
Pumping Speed [l/s]
100
10
20 reactivation
cycles
Activation: 550°C x 240' @ 9,9 V
(55 W)
Sorption @ 1 E-5 Torr CO
Sorption temperature: 200 °C
1
0,0001
0,001
0,01
0,1
1
10
100
Sorbed Quantity [Torr*l]
The sorbed quantity in each cycle corresponds to 1 year operation @ 3x10-8 mbar CO.
After 20 cycles of reactivations, CO speed decreases marginally vs the initial value.
This material can be used not only in XHV-UHV but also in HV applications ( 10-8 and
higher). This may be of interest in machines with tight space issues and high gas loads.
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Perspectives
The ZAO® alloy is promising in many aspects, as it opens
the opportunity to improve getter pumps in term of
degassing, larger gas capacity, particle emission and
mechanical properties.
A new pump design which incorporates these improved
features is being developed (STELLAtorrTM) which can work
either @ RT (UHV-XHV) or at 150-200°C @ 10-8 -10-7 Torr.
This significantly extend the operational pressure range
for NEGs from UHV-XHV to HV
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Example : Capacitorr D 3500 design
Nominal Speed (nude configuration) is 3500 l/S H2 which is reduced
to 2200 l/s once a connecting nipple is used
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Capacitorr vs STELLAtorr design
Similar performances in
a more compact and
user friendly design
CF 200
S ≈ 2200 l/S(H2)
CF 150
S ≈ 2200 l/S(H2)
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Conclusions
The outgassing from the getter surface during the activation is a complex
phenomenon involving several mechanisms. To minimize the gas load it is advisable
to condition the NEG at temperature in the 200-350°C range before activating it.
The subsequent activation step is mainly responsible for the emission of H2 from the
getter bulk (be careful, the more massive is the getter the larger the emitted H2
amount !).
NEG pump reactivation in closed off condition is doable and well UHV compatible
A novel class of alloys (ZAO®) has been developed with lower equilibrium pressure for
hydrogen and better outgassing properties. Data are preliminary but promising.
ZAO® has a significantly larger total capacity than St 172 and can be used at 150200°C in high vacuum application ( < 1x10-7 mbar) where tight space requirements
are coupled to high gas loads.
The pumps will come in a very compact design (STELLAtorrTM).
P. Manini OLAV IV, Hsinchu, 2014
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Thank you for your attention
w w w. s a e s g r o u p . c o m

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