Outgassing Rates of Aluminum Compared to Stainless Steel

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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

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 available to investigate their outgassing rates under realistic vacuum conditions. Due to low its density (2.7 g/cm3) as well as excellent machinability, aluminum is known Material Requirements for as a very attractive construction material. UHV / XHV applications Therefore, the use of aluminum materials in general mechanical engineering, automotive Depending on their final application, there exist industries and building technologies has distinct demands on chemical composition, dramatically increased over the last decades. gas-permeation and outgassing characteristics of materials for UHV applications. Specific For use in vacuum technologies, aluminum data can be found in corresponding guidelines materials offer additional attractive proper- or company standards [2,3,4]. ties, e.g. a very low magnetic permeability

(µr < 1,00005) and a low material activation Outgassing rates are basically differentiated under radiation. Moreover, in very low pressu- between those of in-situ baked and non in- re ranges of ultrahigh (UHV) and extreme high situ baked systems. In-situ baked systems vacuum (XHV) the outgassing rate of in-situ are not exposed to atmosphere after baking. baked materials becomes the decisive factor Hence, the surface adsorbates such as water for vacuum suitability and energy efficiency. and hydrocarbons have already been remo- ved. Outgassing is then determined by diffu- Multiple research activities on measuring and sion and emission of atoms from inside the comparing the outgassing properties of alu- bulk material. For stainless steel outgassing is minum and stainless steels have been carried dominated by hydrogen. out over the last decades [1]. Yet, due to di- verse test parameters such as alloys, treat- Outgassing Values in Literature ments and measuring methods, the published results allow no direct comparison of the two Different alloys, surface treatments and ma- materials. Furthermore, up to now, no all-alu- nufacturing techniques have been analyzed minum CF components and chambers were with varying measuring methods and set ups.

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In case of metals, which are in-situ baked In case of working with gas-dependent pres- at temperatures up to 450 °C for sever- sure gauges such as Bayard-Alpert-sensors, al hours, surface outgassing rates of about attention also has to be paid to the compositi- 10-12…10-14 mbar · l / s / cm2 [5] were recorded. A on of the compared residual gases. review of the most important published results by Wong [1] provides an overview of the out- Methods of Measuring Outgassing gassing rates obtained in numerous experi- Rates in the UHV ments for different materials. To determine the outgassing rates of aluminum For the purpose of comparing aluminum versus commonly used stainless steel, iden- and stainless steel, the most relevant mea- tically shaped samples of the two materials surement was conducted by Young [6]. The have been tested in three different methods. outgassing rate was measured for two com- All parts were put through company stan- parable sample tanks. The stainless steel dard cleaning and baking procedures under tank was pre-polished and intensively baked cleanroom conditions prior to measurement. (250 °C for 30 h plus 450 °C for 17 h), where- For investigation of the outgassing behavi- as the aluminum tank was simply cleaned and or before in-situ bake-out, the results of re- baked at much lower temperature (250 °C) sidual gas analysis (RGA) measurements via and a much shorter amount of time (15 h). For (1) throughput method at room temperature both, a specific surface outgassing of 4· 10-13 have been used. The methods of (2) accumu- Torr · l / s / cm2 (i. e. 5 · 10-13 mbar ·l / s / cm2) was lation and (3) rate-of-rise have been applied to obtained. For stainless steel, lower outgassing obtain material outgassing rates after in-situ rates can only be reached by vacuum firing at bake-out. Values determined by the methods around 950 °C in order to reduce hydrogen (1) and (2) give basic information on process content in the bulk material. The lowest out- quality and material properties. The rate-of-ri- gassing rate listed in [1] for stainless steel is se method (3) was performed on ready-to-use 1.6 · 10-14 mbar · l / s / cm2 after vacuum firing at cylindrical CF chambers equipped with blank 1000 °C for 3 h and an additional in-situ bake flanges. With a particularly developed experi- at 360 °C for 25 h. mental setup, total outgassing as well as residual gas composition could be determined. It can be noted, that for identical materials outgassing rates over several orders of ma- QMS Ion gauge gnitude 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, Test chamber but should not be used for precise calculation Pump in other systems. Instead, an individual deter- system mination should be conducted for each setup. Figure 1: Setup of Throughput Measurement Method

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(1) Throughput Method the samples. The parts measured, were put through standardized ultrasonic cleaning and The gas throughput method uses the principal baking procedures. of vacuum induced desorption of molecules from the material surface. For several hours, (2) Accumulation Method the samples are placed in a vacuum test chamber, which is persistently evacuated Pressure changes resulting from outgassing with a constant pumping speed S. The setup of small samples can fall below detection limits is illustrated in Figure 1. The system is held of commonly used vacuum pressure gauges. at room temperature. Sample outgassing is In order to determine such low outgassing determined by measuring the total pressure rates, the method of accumulation is used. The P in the test chamber. With known pumping measurement setup (as schematically depicted in speed S and sample area A, the total Fig. 2, left) consists of a measurement chamber, ® outgassing rate qA per sample area follows where a total pressure gauge (BARION from S extended), a quadrupole mass spectrometer and q = P – a pump group with known pumping speed S are A total A attached. Via valve, the sample under test can be In order to get information on the gas connected to the chamber after accumulation time composition and partial pressures, mass tac. There, a detectable pressure rise [as shown spectra are recorded periodically. To eliminate in Fig. 2, right] is observed until the accumulated outgassing contributions originating from gas amount depleted. Eventually, the pressure in the test system, a pre-recorded blank run the test chamber will return to the level present (often called background measurement) of before opening the valve (p < 5 · 10-11 mbar). By the vessel is subtracted from the sample integration of the pressure over the gas releasing measurement. Outgassing in the throughput time (tend-ti), the surface specific outgassing rate of measurements is dominated by surface the sample can be determined from: adsorbates like water and hydrocarbons. S tend The mass-spectral contributions reflect pre- q = p(t)dt A t · A t treatment and surrounding conditions of ac i

Sample BARION® extended Pressure rise after opening valve 1E-08 1E-09 chamber with e [mbar ] 1E-10 Sample signal p < 5E-11 1E-11 essur Background

QMS Pr 1E-12 Pump -20 0 20 40 60 80 100 120 140 group Time [s]

Figure 2: Setup of Accumulation Measurement (left) and exemplary pressure curve after opening the valve between sample and test chamber (right)

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Small tube-like chambers, drilled from detail, two identical CF chambers (Figure 3), full metal, with an inner surface area A of one all-aluminum (Fig. 3, left) the other made 130 cm2 and a volume V of 0.1 l were used of stainless steel (Fig. 3, right), where in-situ as samples in the presented setup. Identical baked at 120 °C for 24 hours. As reference samples made of aluminum (6xxx series alloy) to typically performed baking procedures, the and stainless steel (alloy 316L) with different steel chamber was subsequently baked at surface treatments were investigated. In-situ 200 °C for 24 h. bake-out was performed for 24 h at 120 °C and 200 °C for aluminum and stainless steel, Results of Outgassing Measurement respectively. (1) Throughput Method (without in-situ bake) (3) Rate-of-Rise The results of residual gas analysis of a In the rate-of-rise method, the total outgassing number of analyzed samples were evaluated. rate Q of a closed vessel with a volume V can An exemplary mass spectrum is shown in be determined by measuring the change in figure 4. It has to be noted, that water (peak pressure ∆p in a defined time lapse ∆t: at amu 18) is generally dominating the spectra by more than one order of magnitude. The

Q = contribution of all masses m/e > 45 (i.e. non- atmospheric components) is lower than 1 % With the inner vessel area A, the surface of total pressure, indicating an extremely specific outgassing rate qA of the outgassing high surface purity of the tested sample. This material can be determined by: demonstrates a very high level of cleaning quality. · qA = The results of total outgassing determination Prior to pressure recording, the system is in- show typical minor differences between situ baked for several hours, cooled down stainless steel and aluminum parts. Results to room temperature and then closed up. In 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

Figure: CF chambers made of all-aluminum (left) and stainless steel sheet. (right) used for comparable outgassing measurement with rate- of-rise method.

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Mass spectrum sample at 10 h and Background at t1 10 h

1,00E-06

1,00E-07 Outgassing of the sample after 10 h

1,00E-08 Background after 10 h

bar] 1,00E-09 [ m

e 1,00E-10 u r ss 1,00E-11 r e P 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: Exemplary 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) All aluminum samples tested showed outgassing rates well below 2 · 10-12 mbar · l / s / cm2. A Multiple samples made of aluminum and procedure was found, which resulted in stainless steel have been investigated. Figure extremely low outgassing values. All samples 5 shows representative measurements of treated with this procedure showed outgassing stainless steel (316L) and aluminum (from rates below 6 · 10-15 mbar · l / s / cm2, the lowest 6xxx series). even in the order of 1 · 10-15 mbar · l / s / cm2. The obtained values confirmed the listed It can be stated, that aluminum samples outgassing rates of stainless steels. They lie below undergoing this defined procedure possess 1 · 10-12 mbar · l / s / cm2 for non vacuumfired comparable or even lower outgassing rates samples and below 1 · 10-13 mbar · l / s / cm2 after than vacuum fired stainless steel samples. vacuum firing. In exception of vacuum-firing, no significant decrease in outgassing of stainless (3) Rate-of-Rise Method (with in-situ bake) steel was achieved for different surface treatments. Figure 6 shows a plot of the measured pressure

Stainless steel (316L) Aluminum (6xxx)

1E-09 1E-09

r] 1E-10 ar] 1E-10 mb [ [mb a

r e r e u u s s 1E-11 1E-11 r e r e P P

1E-12 1E-12 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 Time [ s] Time [ s]

-13 2 -13 2 qA = 7.1 · 10 mbar · l / s / cm qA = 1.2 · 10 mbar · l / s / cm

Figure 5: Representative 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.

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VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany Tel. +49 3641 4275-0  Fax +49 3641 4275-82  [email protected]  www.vacom-vacuum.com  www.vacom-shop.com Outgassing Rates of Aluminum compared to Stainless Steel curves (multiplied by volume) over time. Each was obtained with a surface treated aluminum curve represents an individual measurement. chamber. This outgassing rate even falls The slope of the curves illustrates the degree below lowest rate for vacuum fired stainless of outgassing – the flatter the line the lower steel published in the review by Wong [1]. the outgassing of the system. Resultant outgassing values determined from pressure In addition to the total outgassing rates, rise for each measurement are listed in Table spectral measures using a quadrupole mass 1. spectrometer have been taken on the residual gas composition in the vessels. As assumed, Stainless - 120 °C 6E-05 the spectra of stainless steel were dominated

]

l Stainless - 200 °C ·

ar by hydrogen. However, the mass spectra of Al - 120 °C [m b V

aluminum materials also showed significant 4E-05 Al - treated - 120 °C m e

ol u peaks for masses 12, 16, 28 and 44 besides v

· p

r hydrogen. 2E-05 ss u e P r Conclusion 1E-09 0 1 2 3 4 5 Time [h] Outgassing rates of identical aluminum and Figure 6: Pressure rise in stainless steel (upper curves) and aluminum (lower curves) test chambers. The slope stainless steel samples with different surface represents the degree of material outgassing treatments were determined using three different measurement methods. The results In-situ Outgassing rate q confirm and proof the existing impression of Chamber A bake-out [mbar · l / (s · cm2)] aluminum being an extremely low outgassing -12 Stainless 24 h at 120 °C 1.2 · 10 material. In direct comparison to stainless steel -12 24 h at 200 °C 1.0 · 10 steel, the results of two different measurement -14 Aluminum 24 h at 120 °C 8.6 · 10 methods revealed remarkably lower Aluminum 24 h at 120 °C 1.5 · 10-14 ® treated outgassing rates for AluVaC components Table 1: Determined outgassing rates from in-situ baked out (accumulation method) as well as for an CF chambers at room temperature AluVaC® chamber (rate-of-rise method). The values obtained were more than one order of The test chambers made of aluminum show magnitude smaller, than those of the stainless much lower outgassing rates than identical steel references. chambers made of stainless steel. Even For the first time, the extraordinary suitability of raising the baking temperature from 120 °C an aluminum chamber for use in the ultrahigh up to 200 °C for the stainless steel chamber vacuum (UHV) and extreme high vacuum only lead to a slight decrease in outgassing (XHV) region was demonstrated by a real and – the achieved value still lies more than one ready-to-use AluVaC® chamber. magnitude higher than that of the aluminum reference. The exceptional low outgassing rate of The best result of 1.5 · 10-14 mbar · l / s / cm2 components and chambers manufactured

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VACOM Vakuum Komponenten & Messtechnik GmbH  In den Brückenäckern 3  07751 Großlöbichau  Germany Tel. +49 3641 4275-0  Fax +49 3641 4275-82  [email protected]  www.vacom-vacuum.com  www.vacom-shop.com Outgassing Rates of Aluminum compared to Stainless Steel with the AluVaC® technology has favorable NOTE: effects for users: Pump down times are With the expertise gained from presented dramatically decreased and preserving investigation and the great experiences ultrahigh vacuum conditions becomes gathered by developing, manufacturing and significantly less time and energy consuming. testing of numerous samples of different Additionally, the heating energy for baking metals, VACOM standardized the outgassing procedures is lowered significantly due to the rates of the in-house manufactured CF reduction of in-situ bake-out temperature to components. Using the VACOM® Vacuum 120 °C and the elimination of the vacuum firing Classes the user can select the perfectly processes required for reaching comparably fitting component to his particular vacuum low outgassing rates with stainless steels. To requirements. sum up, a great amount of time, energy and money can be saved using AluVaC® chambers and components in UHV systems.

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

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