sorb ac general.pdf - SAES Getters

SORB-AC ® Cartridge Pumps MK5 Serieswe support your innovation

SORB-AC ® Cartridge Pumps - MK5 SeriesGENERAL INFORMATIONSORB-AC MK5 Series Cartridge Pumps, a type of vacuum pump manufactured by SAES Getters S.p.A.,sorb active gases with a nonevaporable getter (NEG) material. They are used alone or in combinationwith other vacuum pumps in several applications in high and ultrahigh vacuum (HV and UHV).WORKING PRINCIPLESThe pumping process sorbs by chemical reaction active gas molecules impinging on the clean metalsurface of each particle of the getter material contained in the pump cartridge. A clean metal surfacemeans a metal surface not covered by oxide or carbide layers which result when the getter materialis exposed to air during manufacturing, installation or air venting steps.The method used to obtain this clean metal surface distinguishes non-evaporable getter pumps fromothers: the getter material does not need to be evaporated, as does titanium in a titanium sublimationpump. It only needs to be heated, under dynamic vacuum, to diffuse the passivation layer coveringthe surface of each getter particle into the bulk of the getter material. The passivation layer consistsmainly of oxides and carbides. This heat treatment, called activation, enhances the kinetics of thisdiffusion process, which is negligible at room temperature.The getter material maintains its original configuration of metal particles coated on a substrate. Theactivation process is illustrated schematically in Figure 1, in which a single getter particle is considered.H 2OH 2 OpassivatinglayerH 2InactiveNEGparticleH 2O 2N 2 COHEATVacuumorInert GasActiveNEGparticleO 2CON 2Figure 1 - Schematic representation of the activation process for NEG.GETTER MATERIALSThe getter material used in the SORB-AC cartridge pumpsMK5 series is a Zr-V-Fe alloy with the trade name St 707,manufactured by SAES Getters S.p.A. 1This alloy pumps all the active gases and is especiallyefficient in pumping hydrogen and its isotopes. Theactive gases are permanently sorbed. Hydrogen and itsisotopes form solid solutions in the alloy. Hydrogen andits isotopes can be removed from the alloy in accordancewith Sieverts' Law.St 707 alloy is a getter material with superior diffusivitycharacteristics. This allows St 707 to be fully activatedat temperatures in the range of 350-450°C. Figure 2illustrates the surface composition as a function of theheating treatment (about 10 minutes) on the alloy. Thelow activation temperature often makes St 707 thepreferred alloy by most users.1 . The Zr-Al alloy (trade name St 101), is available only for the GP 50 andGP 100 pump models. If an St 101 cartridge is required for the GP 200or GP 500 pump models it is necessary to order the correspondingpumps of the MK4 series.Surface Composition (AT %)100908070605040302010Temperature (°C)Zr - V - FeCVO0 200 400 600 800 1000Figure 2 - NEG surface composition vsactivation temperature (Ref. K. Ichimura, et al. J.Vac. Sci. Technol. A5 (2) (1987) 220)ZrWe support your innovation1

PUMPING CHARACTERISTICSBehavior of the getter materialsIt is possible to identify four main families of gases with respect to their interaction with gettermaterials. The first family consists of hydrogen and its isotopes, which are sorbed reversibly. Thesecond family consists of other active gases such as CO, CO 2, O 2 and N 2, which are sorbed irreversibly.The third family consists of gases which are sorbed in a combination of the two above described wayssuch as water and hydrocarbons. The fourth family consists of rare gases, which are not sorbed bythe getter materials.HydrogenHydrogen does not form a stable compound with the getter alloy but diffuses rapidly into the bulkof the active material, where it is stored as a solid solution. A given concentration of hydrogen insidethe getter alloy corresponds to an equilibrium pressure of hydrogen, which is strongly dependent ontemperature. This dependence is described by Sieverts' Law, given by the following expression:Where:Log P = A +2 Log q - B Tq = is the concentration, in liter•Torr/gram of alloyP = is the equilibrium pressure, in TorrT = is the getter temperature, in KA = 4.8 for St 707 alloyB = 6116 for St 707 alloySORB-AC pumps can be used as heat controlled reversible pumps, which pump hydrogen at lowtemperature and release it at high temperature. The equilibrium curves for the St 707 alloy are shownin Figures 3.10 0St 707Hydrogen Equilibrium Pressure (Torr)10 -210 -410 -610 -8500°C400°C300°C200°C600°CEmbrittmement Lilmit (20 Torr I/g)10 -10 10 -2 10 -1 10 0 10 1Hydrogen Concentration (Torr I/g)Figure 3 - Hydrogen equilibrium pressure for the St 707 alloyOther Active GasesOther active gases such as carbon monoxide, carbon dioxide, nitrogen and oxygen are chemisorbedirreversibly by the getter material. The chemical bonds of the gas molecule are first broken on thesurface of the getter and then the constituent elements are sorbed as atoms, forming oxides, carbidesand nitrides. The strength of the chemical bonds between the getter and these elements is so strongthat even if the getter material is heated to temperatures of the order of 1000°C, no gases are releasedinto the vacuum environment. On the contrary, high temperature treatments promote the diffusionof the gas species into the bulk of the getter material, cleaning the surface for further sorption.We support your innovation2

Water Vapor and HydrocarbonsWater vapor and hydrocarbon chemical bonds are cracked on the surface of the getter material.Hydrogen, oxygen and carbon are then sorbed as explained above. However, sorption efficiency ofhydrocarbons at temperatures under 500°C is very low.Rare GasesGetter materials do not sorb rare gases. SORB-AC non-evaporable pumps can therefore be used toremove impurities of active gases in rare-gas filled devices.Pumping PerformancesDue to the working principle of getter materials and their typical behavior toward different gas families,the pumping performance (pumping speed and gettering capacity) of SORB-AC pumps depends onthe type of gas sorbed and the temperature of the getter cartridge. The pumping performance of SORB-AC pumps are measured according to the ASTM standard F798-82, which specifically refers tononevaporable getter devices. The procedure is an application of the dynamic-flow method, also usedfor measurements in other standards. The test is carried out at constant pressure over the getterdevice. It is important to notice that the testing pressure does not affect the pumping speed of thepump when the pressure is below 10 -5 Torr.10000Pumping Speed (I/s)1000H 2 - 25°CH 2 O - 280°CH 2 - 280°CCO - 280°C100H 2 O - 25°CCO - 25°C100.01 0.1 1 10 100 1000Sorbed Quantity (Torr I)Figure 4 - Typical pumping speed curves of a SORB-AC cartridge pumpIn Figure 4, the pumping speed curves of a getter pump for H 2, CO and H 2O are shown. The pumpingspeed value is plotted versus the quantity of gas sorbed during the test. As mentioned above, it wouldbe meaningless and misleading to plot it versus the pressure as in sputter ion pumps. The differentsorption mechanisms of the getter material towards H 2 (high diffusion rate both at high and roomtemperature) and CO (low diffusion rate at room temperature, high diffusion rate at high temperature)are also evident in the curves.The effect of the diffusion phenomena for the sorption of CO is interesting to note. When diffusionis not present, i.e. operating at room temperature, only the surface of the getter is available for thesorption of the gas. Therefore the capacity is limited and the pumping speed drops to low valueswhen the surface becomes saturated. In this case, many reactivations will be necessary to use thetotal capacity of the getter cartridge. When diffusion is present, (i.e. operating at high temperature),the bulk of the getter material is available for gas sorption. The capacity for a single activation processis much larger and an almost constant value of the pumping speed is maintained for a much longertime. There is not a big difference in sorption performance of H 2 at different operating temperaturesdue to the high diffusivity of this gas even at room temperature, and unlike CO, the capacity is reducedwhen operating at high temperature due to the high equilibrium pressure.The sorption of water vapor is preceded by the dissociation of the water molecule on the getter surface,and is limited by the capacity of the alloy for O 2, while H 2 diffuses quickly into the bulk of the gettermaterial.Oxygen and nitrogen in the bulk diffusion regime are sorbed with pumping speeds of approximately:65% (O 2) and 15% N 2) of the sorption speed of H 2.Hydrocarbons are sorbed only at elevated temperatures and with very low efficiency. A pumpingspeed for CH 4 of about 0.5% of the speed for CO is observed at 500°C.We support your innovation3

Activation of SORB-AC Cartridge PumpsAs previously mentioned, the nonevaporable getter material used in the getter cartridges developsits pumping characteristics after an activation process, i.e., after being heated to a suitably hightemperature under dynamic vacuum for an appropriate time. The heat treatment diffuses the thinprotective layer that is formed on the surface of the getter particles by air exposure during manufacture.Diffusion of this layer into the bulk makes the surface of the getter clean and able to sorb the gasmolecules of the vacuum system in which it is operated. The efficiency of this activation process isrelated to the diffusion coefficient of the specific getter material, which in turn depends on an exponentialfunction of the temperature:D = D 0exp (- E/T). It is also related to the square root of time, as with all the diffusion processes.In first approximation, the activation efficiency, considered related to the quantity of passive speciesremoved from the surface of the getter particles, can be expressed by the following formula:Activation efficiencyFigure 5 gives the suggested time/temperature combinations for the best (100%) activation for St 707(450°C for 45 minutes) alloy based cartridges. The curves show that a shorter activation must becompensated by a higher temperature activation in order to obtain the same final efficiency and viceversa. Both these parameters are limited by practical and physical constraints to the range of valuesindicated in the figure.∞Dt800Melting Area of the Constantan Strip700Activation Temperature (°C)600500400300200★100%60%St 707 ®100★ Standard Activation Conditions10 100 1000Activation Time (Minutes)Figure 5 - Activation efficiency for St 101 and St 707 alloysHowever, in practical cases it is not always possible to use the "ideal" time/temperature combinations.Suitable operation modes can be chosen where only a "partial" activation is considered sufficient.This results in a condition where the getter surface develops less than full pumping speed. Activationconditions giving 60% of the optimum pumping speed are also shown in Figure 5. The activationprocess should be carried out after a pump down of the vacuum system to a pressure of 10 -4 Torr orless. After having reached this pressure, the heater can be energized to reach the desired activationtemperature.During the heating phase gases desorb from the getter cartridge. This is due to the physisorbed gaseswhich form the external monolayers covering the surface of the getter material, while the internalchemisorbed layers are diffused into the bulk of the getter material.Desorbed gases initially include H 2, H 2O, CO, CO 2, CH 4, and eventually mainly H 2, due to the behaviorof the getter material toward this gas. Baking the system with the getter cartridge maintained at atemperature relatively higher than the other components of the system has been found to be effectiveway to minimize this gas evolution. This procedure minimizes the migration of the gases desorbedfrom the wall of the system toward the getter cartridge, which has a real surface area much largerthan the system walls themselves.We support your innovation4

During activation it is advisable not to exceed pressures in the 10 -3 Torr range to avoid phenomenaof corrosion of the heater wire, RF discharges between the heater and other pump elements andcontamination of the gettering material due to the sorption of active gases during the activationprocess.For this purpose, and depending on the pumping speed of the backing pump, it may be advisable toactivate the getter cartridge by successive steps, not applying the full power to the heater at thebeginning of the activation process. In this case, reaching the activation temperature may take a fewhours compared to 30 minutes usually needed if full power is applied at the beginning.Operation of SORB-AC Cartridge PumpsWhen activated, the SORB-AC cartridge pump can be operated at various temperatures according tothe load of active gases, taking into account two main facts. First, the higher the temperature, thehigher the diffusivity of the sorbed gases into the bulk of the getter material. Second, the higher thetemperature, the higher the H 2 equilibrium pressure. High temperature operation is preferred whenhigh gas loads are present. In this way it is possible to maintain a high diffusion rate and consequentlya constant pumping speed for all the active gases.Temperatures in the range of 280°C are typically suggested for St707, but different values can beadopted. High getter temperature is usually not compatible with UHV operation due to the desorptionof hydrogen from the getter material. For example the use of an St 707 cartridge at 280°C will causean initial hydrogen equilibrium pressure on the getter material in the range of 10 -7 - 10 -8 Torr due tothe residual H 2 content of the alloy and depending on the presence of additional pumping devices.Room temperature operation is indicated when the gas load is low, and for the reason mentionedabove, it is mandatory when the operating pressure is below 10 -8 Torr. When pumping high purityhydrogen, operation at room temperature is also possible with high gas loads due to the high diffusivityof H 2 at that temperature. In this latter case, a careful evaluation of the gas load and of the quantityof gas sorbed is necessary to avoid possible embrittlement.Note that in the curves in Figure 3, the actual concentration of H 2 in the alloy (and therefore the residualcapacity) to be known before embrittlement occurs can be determined by a simple measurement ofthe pressure in static vacuum at a given operating temperature.Reactivation of SORB-AC Cartridge PumpsThe reactivation of the getter cartridge of a SORB-AC pump is necessary when the cartridge is exposedto air or when it’s pumping speed falls below acceptable limits. In both cases the surface of the gettermaterial becomes covered by a passivation layer of mainly carbides and oxides. If reactivation ispreceded by an air exposure (and a successive pump down cycle), it should follow the same procedureand has the same characteristics of the first one. If the reactivation follows normal in-vacuum operationwithout air venting, it may be shorter and carried out at a lower temperature (indicatively 25% lower).Moreover during a reactivation which follows normal in-vacuum operation the only gas released ishydrogen. Through successive reactivations it is possible to use the entire capacity of the gettermaterial. When the pumping speed no longer recovers sufficiently after reactivation, the cartridgemust be replaced.Regeneration of SORB-AC Cartridge PumpsHydrogen and hydrogen isotopes sorbed by the cartridge can be released from the getter materialthrough a regeneration treatment. Regeneration is necessary when:••the pumping speed for hydrogen or hydrogen isotopes has fallen below acceptable limits becausethe equilibrium pressure has been approached.equilibrium is far from being reached but the hydrogen or hydrogen isotopes quantity pumped isapproaching the embrittlement limit of 20 liter Torr/g.Embrittlement of the material takes place when the quantity of hydrogen sorbed in the getter materialis high enough to modify the mechanical characteristics of the alloy causing it to flake into smallparticles from the substrate. The process of hydrogen sorption and successive desorption (regeneration),can be visualized on the Sieverts’ plots of the specific alloy (see Figure 6).The temperature increase of the getter alloy establishes a high hydrogen equilibrium pressure, allowinghydrogen removal by means of a standard backing pump. From the plot in Figure 5 can see that theregeneration process will be more efficacious at higher temperature.We support your innovation5

P (Torr)DesorptionT 2T 1▼)(The time necessary for the regenerationof a cartridge is given by the expression:t =M F(1 1q fq i10A B T)▼Sorptionq f q iT 1= sorption temperatureT 2= regeneration temperatureq (Torr•l/g)Figure 6 - Regeneration CycleWhere:t = is the regeneration time, in secondsM = is the mass of the getter material, in gramsF = is the pumping speed of the backing pumps in, liter/sq f = is the final H 2concentration, in liter Torr/gq i = is the initial H 2concentration, in liter Torr/gA = 4.8 for St 707 alloy cartridgesB = 6116 for St 707 alloy cartridgesT = is the regeneration temperature in KDue to the exponential shape of the regeneration curve, a significant amount of time is saved whenthe regeneration is not programmed to be complete (where 100% of the sorbed hydrogen is released)but only partial (for example 90%). Thus if a given amount of hydrogen must be sorbed in each cycle(for instance 10 liter Torr/g) it is much better to operate using q f= 2; q i= 12 than using q f= 0; q i= 10.SPECIAL INSTRUCTIONSThermal Fatigue1000The reactivation and regeneration processescan be performed many, but not an infinitenumber of times. Due to thermal fatigue inthe getter-coated strips, peel-off of thegettering powder eventually occurs, whichmainly depends on the temperature and onthe duration of the heating process. Figure 7shows the number of permissible thermalcycles, at 700°C, 600°C and 500°C as a functionof the hold time before peel-off starts to occur.Cycles Before Initial Peel-Off900800700600500400300200700° C600°C500° C100Figure 7 - Thermal Fatigue Effects on NEG Strip00 15 30 45 60 75 90 105 120Hold Time (Minutes)We support your innovation6

Air VentingOpening the vacuum chamber where MK5 SORB-AC pumps are mounted should be performed onlywhen the getter material is at room temperature or at least below 50°C. After each air exposure, anew reactivation of the getter cartridge is required.A progressive reduction of pumping speed for hydrogen and active gases is observed after successiveexposures to air. After about 30 air exposures at room temperature followed by reactivation, St 707cartridges at 280°C still have a pumping speed for hydrogen of about 40% of the original pumpingspeed.If dry nitrogen is used instead of air, the pumping speed reduction after the same number of exposures,appears to be lower (residual pumping speed still about 80% of the initial value, see Figure 8). Thisis because, for the first cycles, the effect of the reactivation which follows a nitrogen exposure isgreater than the new contamination caused by exposure to nitrogen. Further improvement is obtainedwhen pure argon is used as a protective gas during maintenance operations.120Percentage of Initial Pumping Speed10080604020NitrogenAir00 5 10 15 20 25 30 35Number of ExposuresFigure 8 - effects of air and N 2 exposuresVacuum Failure During Activation or RegenerationDuring activation or regeneration of SORB-AC cartridge pumps, air must not be allowed to suddenlyenter the vacuum system. Such an occurrence could cause burning, i.e. a quick oxidation, of thecoated strip. This happens if the temperature of the cartridge at the moment of vacuum failure isabove 200°C in the case of St 707 cartridges. The burning is slow and progressive, not explosive.Should a serious vacuum failure take place when the temperature of the cartridge is high, althoughbelow the above-indicated values, permanent damage of varying degrees will occur according totemperature, but not burning. In this case, pumping characteristics of the getter material would beaffected to a greater or lesser extent depending on temperature and time. Also, in these cases, a moreefficient recovery of gettering efficiency can be obtained by using a reactivation procedure at atemperature higher than the normally adopted value.Excessive Heating of the CartridgeThe St 707 cartridges use constantan as a substrate material, and they should not be heated totemperature over 750°C to avoid possible copper or manganese evaporation from the material. Attemperatures over 850°C, it is possible to have the formation of eutectic compounds between copperand zirconium, which could cause melting of the cartridge.Mechanical ShocksDue to the mechanical characteristics of the insulating elements of the heater (ceramic spacers),particular care must be observed in handling during assembly (and removal) of the pumping system.Accidental dropping and similar mechanical shock could result in breakage of the insulating elementswith possible short circuits of the electrical path.We support your innovation7

E.VS.03.02The SAES Getters Group manufacturing companies are ISO9001 certified, the Asian and Italian companies are ISO14001certified also. Full information about our certifications for each company of the Group is available on our website at:www.saesgetters.com© 2002 SAES Getters. Printed in Italy. All rights reserved. SAES ® is registered trademark of SAES Getters S.p.A., its subsidiaries and affiliates.SAES Getters reserves the right to change or modify product specifications at any time without notice.SAES Getters Groupwww.saesgetters.comneg_technology@saes-group.com