A novel class of Non Evaporable Getter (NEG) alloys with lower hydrogen equilibrium pressure and reduced outgassing properties

Transcription

1 A novel class of Non Evaporable Getter (NEG) alloys with lower hydrogen equilibrium pressure and reduced outgassing properties P. Manini, FabrizioSiviero, A. Gallitognotta, L. Caruso, T. Porcelli,G. Bongiorno, E. Maccallini, SAES Getters, Lainate Italy making innovation happen, together

2 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 2

3 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 ). 3

4 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. 4

5 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. 5

6 Outgassing test set up Test was carried out on a UHV bench equipped with a QMS Main Volume EXT QMS CapaciTorr D100 V1 TMP DIAPHRAGM VACUUM PUMP

7 1 test : outgassing from NEG pump during activation Outgassing tests have been carried out on a Capacitorr D 100pump (100l/sH2). The getter material is St 172 (St 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 100 Capacitorr D 200 Capacitorr D 50 7

8 1 test : outgassing from NEG pump during activation confronto 8

9 1 test : outgassing from NEG pump during activation H2 CO CH4 H2O O2 CO2 CnHm Heavier HC Ptot 1,E-05 1,E-06 1,E-07 Pressure (Torr) 1,E-08 1,E-09 H2O CH4 CO2 CO 1,E C HC 1,E-11 1,E C Time (min)

10 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 C before the full activation to remove physisorbedspecies and reduce the amount of released gases. 10

11 2 test : outgassing of NEG activated w/o auxiliary pumps Auxiliary pumping is generally required during getter activation ( C): Remove H 2 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 was used in this experiment 11

12 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. Bakeoutof the system at 180 C for 10 hours 2. 1 st NEG activation under TMP pumping 3. H 2 pumping speed measurement 4. 2 nd activation in closed-off condition 5. H 2 pumping speed measurement 12

13 Activation under pumping & reactivation in isolated conditions 1 st Act H 2 2 nd Act H 2 H 2 O CO CO 2 CH 4 H 2 O CO CO 2 CH 4 Notwithstanding closed-off conditions we have limited outgassing, much lower than the 1 st activation. The surface is clean : absence of the initial peak due to desorption of physisorbedspecies and carbon contamination NEG reactivation w/o external pumping is possible and provides good UHV conditions 13

14 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 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. 14

15 Hydrogen equilibrium isotherm HYDROGEN EQUILIBRIUM ISOTHERMS 1,E+01 Hydrogen equilibrium pressure (Torr) 1,E+00 1,E-01 1,E-02 1,E-03 1,E-04 St172 ZAO1 1,E-05 0, Hydrogen concentration (Torr.l/g) Lower equilibrium pressure and larger H2 storage capacity for each given temperature. Higher embrittlement limit

16 3 test : outgassingfrom ZAO NEG pump confronto 16

17 Comparison between St 172 and ZAO tests P H2 (Torr) 1,E-06 1,E-06 1,E-06 8,E-07 6,E-07 4,E ZAO 2,E-07 1,E 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 17

18 What about pumping performances? 100 Capacitorr CT D100 based on ZAO sorption test (according to ASTM F798-97) Pumping Speed [l/s] 10 Activation: 550 C x 9,9 V (55 W) 1 E-5 Torr CO Sorption temperature: 200 C 20 reactivation cycles 1 0,0001 0,001 0,01 0, Sorbed Quantity [Torr*l] The sorbedquantity in each cycle corresponds to 1 year 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. 18

19 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 (STELLAtorr TM ) which can work RT (UHV-XHV) or at Torr. This significantly extend the operational pressure range for NEGs from UHV-XHV to HV 19

20 Example: CapacitorrD 3500 design Nominal Speed (nude configuration) is 3500 l/s H2 which is reduced to 2200 l/s once a connectingnippleisused 20

21 Capacitorrvs STELLAtorrdesign Similarperformances in a more compact and user friendly design CF 200 S 2200 l/s(h2) CF 150 S 2200 l/s(h2) 21

22 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 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 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 (STELLAtorr TM ). 22

23 Thank you for your attention