WHITE PAPER
AluVaC®: All-Aluminum CF Components and Chambers
Part 2 – EN
Outgassing Rates of Aluminum
compared to Stainless Steel
 Extremely low outgassing rates:
Down to 1 · 10-14 mbar · l / s / cm2
 Simple treatments:
No vacuum-firing required
 Energy-efficient procedures:
Bake-out at 120 °C for 24 hours
2016-06, #WP00002 Rev. A
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum
compared to Stainless Steel
Low material outgassing is crucial for achieving and preserving extremely low pressures in the
ultrahigh vacuum (UHV, p < 1 · 10-07 mbar) and extreme high vacuum (XHV, p < 1 · 10-11 mbar)
region. This document presents the results of different comparative measurements on the
outgassing of aluminum and stainless steel products. Small CF components for raw material
studies as well as welded and fully equipped UHV-chambers have been investigated.
Aluminum as a Vacuum Material
Due to low its density (2.7 g/cm3) as well as
excellent machinability, aluminum is known
as a very attractive construction material.
Therefore, the use of aluminum materials in
general mechanical engineering, automotive
industries and building technologies has
dramatically increased over the last decades.
For use in vacuum technologies, aluminum
materials offer additional attractive properties, e.g. a very low magnetic permeability
(µr < 1,00005) and a low material activation
under radiation. Moreover, in very low pressure ranges of ultrahigh (UHV) and extreme high
vacuum (XHV) the outgassing rate of in-situ
baked materials becomes the decisive factor
for vacuum suitability and energy efficiency.
Multiple research activities on measuring and
comparing the outgassing properties of aluminum and stainless steels have been carried
out over the last decades [1]. Yet, due to diverse test parameters such as alloys, treatments and measuring methods, the published
results allow no direct comparison of the two
materials. Furthermore, up to now, no all-aluminum CF components and chambers were
2016-06
available to investigate their outgassing rates
under realistic vacuum conditions.
Material Requirements for
UHV / XHV applications
Depending on their final application, there exist
distinct demands on chemical composition,
gas-permeation and outgassing characteristics
of materials for UHV applications. Specific
data can be found in corresponding guidelines
or company standards [2,3,4].
Outgassing rates are basically differentiated
between those of in-situ baked and non insitu baked systems. In-situ baked systems
are not exposed to atmosphere after baking.
Hence, the surface adsorbates such as water
and hydrocarbons have already been removed. Outgassing is then determined by diffusion and emission of atoms from inside the
bulk material. For stainless steel outgassing is
dominated by hydrogen.
Outgassing Values in Literature
Different alloys, surface treatments and manufacturing techniques have been analyzed
with varying measuring methods and set ups.
#WP00002 Rev. A
1
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
In case of metals, which are in-situ baked
at temperatures up to 450 °C for several hours, surface outgassing rates of about
10-12…10-14 mbar · l / s / cm2 [5] were recorded. A
review of the most important published results
by Wong [1] provides an overview of the outgassing rates obtained in numerous experiments for different materials.
For the purpose of comparing aluminum
and stainless steel, the most relevant measurement was conducted by Young [6]. The
outgassing rate was measured for two comparable sample tanks. The stainless steel
tank was pre-polished and intensively baked
(250 °C for 30 h plus 450 °C for 17 h), whereas the aluminum tank was simply cleaned and
baked at much lower temperature (250 °C)
and a much shorter amount of time (15 h). For
both, a specific surface outgassing of 4 · 10-13
Torr · l / s / cm2 (i. e. 5 · 10-13 mbar ·l / s / cm2) was
obtained. For stainless steel, lower outgassing
rates can only be reached by vacuum firing at
around 950 °C in order to reduce hydrogen
content in the bulk material. The lowest outgassing rate listed in [1] for stainless steel is
1.6 · 10-14 mbar · l / s / cm2 after vacuum firing at
1000 °C for 3 h and an additional in-situ bake
at 360 °C for 25 h.
It can be noted, that for identical materials
outgassing rates over several orders of magnitude have been obtained. Consequently,
outgassing values depend on sample type,
pre-treatments, measuring methods and
practical setups. The listed specific material
outgassing rates can only give an orientation,
but should not be used for precise calculation
in other systems. Instead, an individual determination should be conducted for each setup.
2016-06
In case of working with gas-dependent pressure gauges such as Bayard-Alpert-sensors,
attention also has to be paid to the composition of the compared residual gases.
Methods of Measuring Outgassing
Rates in the UHV
To determine the outgassing rates of aluminum
versus commonly used stainless steel, identically shaped samples of the two materials
have been tested in three different methods.
All parts were put through company standard cleaning and baking procedures under
cleanroom conditions prior to measurement.
For investigation of the outgassing behavior before in-situ bake-out, the results of residual gas analysis (RGA) measurements via
(1) throughput method at room temperature
have been used. The methods of (2) accumulation and (3) rate-of-rise have been applied to
obtain material outgassing rates after in-situ
bake-out. Values determined by the methods
(1) and (2) give basic information on process
quality and material properties. The rate-of-rise method (3) was performed on ready-to-use
cylindrical CF chambers equipped with blank
flanges. With a particularly developed experimental setup, total outgassing as well as residual gas composition could be determined.
QMS
Ion gauge
Test chamber
Pump
system
Figure 1: Setup of Throughput Measurement Method
#WP00002 Rev. A
2
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
(1) Throughput Method
The gas throughput method uses the principal
of vacuum induced desorption of molecules
from the material surface. For several hours,
the samples are placed in a vacuum test
chamber, which is persistently evacuated
with a constant pumping speed S. The setup
is illustrated in Figure 1. The system is held
at room temperature. Sample outgassing is
determined by measuring the total pressure
P in the test chamber. With known pumping
speed S and sample area A, the total
outgassing rate qA per sample area follows
from
S
qA = Ptotal –
A
In order to get information on the gas
composition and partial pressures, mass
spectra are recorded periodically. To eliminate
outgassing contributions originating from
the test system, a pre-recorded blank run
(often called background measurement) of
the vessel is subtracted from the sample
measurement. Outgassing in the throughput
measurements is dominated by surface
adsorbates like water and hydrocarbons.
The mass-spectral contributions reflect pretreatment and surrounding conditions of
Sample
the samples. The parts measured, were put
through standardized ultrasonic cleaning and
baking procedures.
(2) Accumulation Method
Pressure changes resulting from outgassing
of small samples can fall below detection limits
of commonly used vacuum pressure gauges.
In order to determine such low outgassing
rates, the method of accumulation is used. The
measurement setup (as schematically depicted in
Fig. 2, left) consists of a measurement chamber,
where a total pressure gauge (BARION®
extended), a quadrupole mass spectrometer and
a pump group with known pumping speed S are
attached. Via valve, the sample under test can be
connected to the chamber after accumulation time
tac. There, a detectable pressure rise [as shown
in Fig. 2, right] is observed until the accumulated
gas amount depleted. Eventually, the pressure in
the test chamber will return to the level present
before opening the valve (p < 5 · 10-11 mbar). By
integration of the pressure over the gas releasing
time (tend-ti), the surface specific outgassing rate of
the sample can be determined from:
qA =
S
tac · A
tend
p(t)dt
ti
Pressure rise after opening valve
BARION® extended
chamber with
p < 5E-11
QMS
Pump
group
Pressure [mbar]
1E-08
1E-09
1E-10
Sample signal
1E-11
Background
1E-12
-20
0
20
40
60
Time [s]
80
100
120
140
Figure 2: Setup of Accumulation Measurement (left) and exemplary pressure curve after opening the valve between sample and test chamber (right)
2016-06
#WP00002 Rev. A
3
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
Small tube-like chambers, drilled from
full metal, with an inner surface area A of
130 cm2 and a volume V of 0.1 l were used
as samples in the presented setup. Identical
samples made of aluminum (6xxx series alloy)
and stainless steel (alloy 316L) with different
surface treatments were investigated. In-situ
bake-out was performed for 24 h at 120 °C
and 200 °C for aluminum and stainless steel,
respectively.
detail, two identical CF chambers (Figure 3),
one all-aluminum (Fig. 3, left) the other made
of stainless steel (Fig. 3, right), where in-situ
baked at 120 °C for 24 hours. As reference
to typically performed baking procedures, the
steel chamber was subsequently baked at
200 °C for 24 h.
Results of Outgassing Measurement
(1) Throughput Method (without in-situ bake)
(3) Rate-of-Rise
In the rate-of-rise method, the total outgassing
rate Q of a closed vessel with a volume V can
be determined by measuring the change in
pressure ∆p in a defined time lapse ∆t:
Q=
With the inner vessel area A, the surface
specific outgassing rate qA of the outgassing
material can be determined by:
qA =
·
Prior to pressure recording, the system is insitu baked for several hours, cooled down
to room temperature and then closed up. In
Figure: CF chambers made of all-aluminum (left) and stainless steel
(right) used for comparable outgassing measurement with rateof-rise method.
2016-06
The results of residual gas analysis of a
number of analyzed samples were evaluated.
An exemplary mass spectrum is shown in
figure 4. It has to be noted, that water (peak
at amu 18) is generally dominating the spectra
by more than one order of magnitude. The
contribution of all masses m/e > 45 (i.e. nonatmospheric components) is lower than 1 %
of total pressure, indicating an extremely
high surface purity of the tested sample. This
demonstrates a very high level of cleaning
quality.
The results of total outgassing determination
show typical minor differences between
stainless steel and aluminum parts. Results
rather depend on cleaning procedures
and process conditions. Total outgassing
rates of < 2 · 10-9 mbar · l / s / cm2 (according
to VACOM® Vacuum Class HV1) and
< 5 · 10-10 mbar · l / s / cm2 (according to VACOM®
Vacuum Class HV2) can be achieved by using
standardized VACOM cleaning procedures
(without additional surface treatments). Note:
Complete VACOM® Vacuum Classes are
presented in the corresponding product data
sheet.
#WP00002 Rev. A
4
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
Mass spectrum sample at 10 h and Background at t1 10 h
Pressure [mbar]
1,00E-06
1,00E-07
Outgassing of the sample after 10 h
1,00E-08
Background after 10 h
1,00E-09
1,00E-10
1,00E-11
1,00E-12
1,00E-13
1,00E-14
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
Mass M/e
Figure 4: E
xemplary mass spectrum recorded on stainless steel by throughput method after 10 hours measurement time of sample (purple)
and blank run of test chamber (grey), showing pressure over mass m/e.
(2) Accumulation Method (with in-situ bake)
Multiple samples made of aluminum and
stainless steel have been investigated. Figure
5 shows representative measurements of
stainless steel (316L) and aluminum (from
6xxx series).
The obtained values confirmed the listed
outgassing rates of stainless steels. They lie below
1 · 10-12 mbar · l / s / cm2 for non vacuumfired
samples and below 1 · 10-13 mbar · l / s / cm2 after
vacuum firing. In exception of vacuum-firing, no
significant decrease in outgassing of stainless
steel was achieved for different surface
treatments.
All aluminum samples tested showed outgassing
rates well below 2 · 10-12 mbar · l / s / cm2. A
procedure was found, which resulted in
extremely low outgassing values. All samples
treated with this procedure showed outgassing
rates below 6 · 10-15 mbar · l / s / cm2, the lowest
even in the order of 1 · 10-15 mbar · l / s / cm2.
It can be stated, that aluminum samples
undergoing this defined procedure possess
comparable or even lower outgassing rates
than vacuum fired stainless steel samples.
(3) Rate-of-Rise Method (with in-situ bake)
Figure 6 shows a plot of the measured pressure
Stainless steel (316L)
Aluminum (6xxx)
1E-09
Pressure [mbar]
Pressure [mbar]
1E-09
1E-10
1E-11
1E-10
1E-11
1E-12
1E-12
0
10
20
30
40
50
Time [s]
60
70
80
90
100
0
qA = 7.1 · 10-13 mbar · l / s / cm2
10
20
30
40
50
Time [s]
60
70
80
90
100
qA = 1.2 · 10-13 mbar · l / s / cm2
Figure 5: R
epresentative pressure curves of accumulation samples of stainless steel (left) and aluminum (right) with determined outgassing
rates of 7.1 · 10-13 mbar · l / s / cm² and 1.2 · 10-13 mbar · l / s / cm², respectively.
2016-06
#WP00002 Rev. A
5
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
curves (multiplied by volume) over time. Each
curve represents an individual measurement.
The slope of the curves illustrates the degree
of outgassing – the flatter the line the lower
the outgassing of the system. Resultant
outgassing values determined from pressure
rise for each measurement are listed in Table
1.
Stainless - 120 °C
Pressure p · volume V [mbar·l]
6E-05
Stainless - 200 °C
Al - 120 °C
4E-05
Al - treated - 120 °C
2E-05
In addition to the total outgassing rates,
spectral measures using a quadrupole mass
spectrometer have been taken on the residual
gas composition in the vessels. As assumed,
the spectra of stainless steel were dominated
by hydrogen. However, the mass spectra of
aluminum materials also showed significant
peaks for masses 12, 16, 28 and 44 besides
hydrogen.
Conclusion
1E-09
0
1
2
3
4
5
Time [h]
Figure 6: P
ressure rise in stainless steel (upper curves) and
aluminum (lower curves) test chambers. The slope
represents the degree of material outgassing
In-situ
bake-out
Outgassing rate qA
[mbar · l / (s · cm2)]
Stainless
steel
24 h at 120 °C
1.2 · 10-12
24 h at 200 °C
1.0 · 10-12
Aluminum
24 h at 120 °C
8.6 · 10-14
Aluminum
treated
24 h at 120 °C
1.5 · 10-14
Chamber
Table 1: D
etermined outgassing rates from in-situ baked out
CF chambers at room temperature
The test chambers made of aluminum show
much lower outgassing rates than identical
chambers made of stainless steel. Even
raising the baking temperature from 120 °C
up to 200 °C for the stainless steel chamber
only lead to a slight decrease in outgassing
– the achieved value still lies more than one
magnitude higher than that of the aluminum
reference.
The best result of 1.5 · 10-14 mbar · l / s / cm2
2016-06
was obtained with a surface treated aluminum
chamber. This outgassing rate even falls
below lowest rate for vacuum fired stainless
steel published in the review by Wong [1].
Outgassing rates of identical aluminum and
stainless steel samples with different surface
treatments were determined using three
different measurement methods. The results
confirm and proof the existing impression of
aluminum being an extremely low outgassing
material. In direct comparison to stainless
steel, the results of two different measurement
methods
revealed
remarkably
lower
outgassing rates for AluVaC® components
(accumulation method) as well as for an
AluVaC® chamber (rate-of-rise method). The
values obtained were more than one order of
magnitude smaller, than those of the stainless
steel references.
For the first time, the extraordinary suitability of
an aluminum chamber for use in the ultrahigh
vacuum (UHV) and extreme high vacuum
(XHV) region was demonstrated by a real and
ready-to-use AluVaC® chamber.
The exceptional low outgassing rate of
components and chambers manufactured
#WP00002 Rev. A
6
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com
Outgassing Rates of Aluminum compared to Stainless Steel
with the AluVaC® technology has favorable
effects for users: Pump down times are
dramatically decreased and preserving
ultrahigh vacuum conditions becomes
significantly less time and energy consuming.
Additionally, the heating energy for baking
procedures is lowered significantly due to the
reduction of in-situ bake-out temperature to
120 °C and the elimination of the vacuum firing
processes required for reaching comparably
low outgassing rates with stainless steels. To
sum up, a great amount of time, energy and
money can be saved using AluVaC® chambers
and components in UHV systems.
NOTE:
With the expertise gained from presented
investigation and the great experiences
gathered by developing, manufacturing and
testing of numerous samples of different
metals, VACOM standardized the outgassing
rates of the in-house manufactured CF
components. Using the VACOM® Vacuum
Classes the user can select the perfectly
fitting component to his particular vacuum
requirements.
References
[1] Wong, http://home.fnal.gov/~mlwong/outgas_rev.htm (2002)
[2] http://photon-science.desy.de/sites/site_photonscience/content/e58/e176720/e177229/e177918/e265554/
e265560/Vakuum_005_DESY_UHV_Richtlinien_1-5_final_stamp_eng.pdf
[3] http://indico.gsi.de/getFile.py/access?resId=20&materialId=0&confId=1420_ Technical Guideline_Testing the
Cleanliness of Cryostat Insulation Vacuum Components (2011)
[4] http://cds.cern.ch/record/1047073/files/p321.pdf_ M. Taborelli_CERN_Cleaning and surface properties
(2007)
[5] Jousten, K.: Wutz Handbuch Vakuumtechnologie, 10. überarb. Aufl. Wiesbaden: Vieweg u. Teubner, 2010
[6] J.R. Young: Outgassing characteristics of stainless steel and aluminum with different surface treatments,
J Vac Sci Tech 6(3), 1969, pp. 398-400
„All-aluminum CF components and chambers“
Are you interested in further information on this topic?
Please send an email with the subject “White Paper, WP00002“ to
info@vacom.de or visit our website www.vacom.de to find all available white papers in
our “downloads“ section.
2016-06
#WP00002 Rev. A
7
VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany
Tel. +49 3641 4275-0  Fax +49 3641 4275-82  info@vacom-vacuum.com  www.vacom-vacuum.com  www.vacom-shop.com