sorb ac general.pdf - SAES Getters

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