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CHAPTER 15GETTER PUMPGigin Ginanjar01312003##Content15.1 General15.2 Flash getter15.3 Titanium Sublimation Getter Pumps15.4 Non-Evaporable Getters#15.1 General

The general Idea is that when molecule strikes the gettering surface, it forms chemical compounds and is removed from gas phaseGettering surface can be generated in several way including sputtering deposition, evaporation and sublimationIn most cases the surface saturation and no longer effective, it is regenerated.Some material call bulk getter. This material the active surface regenerated by heat to cause the gas to diffuse of the material .

For several different type molecules H2,H2O,CO2,N2,O2 interacting in the surface.If n is the number of molecules strikes unit area per secondN is number of molecules per unit volume, is average molecular velocityExample if the molecules stick to surface and do not return to gas phase, for nitrogen at room temperature. It represent pumping speed 11.6 liters/sec for square centimeter of surface area.

N2#15.2 Flash getter

Common application flash getter is sealed cathode ray tube and other form of electron tubeFlash getter place within chamber or glass envelope with electron component and pump down , sealed off and getter is activated.Flash getter usually chemically reactive metal which can readily vaporized. Alloy Barium are common used. BaAl4 alloy will decompose at 1000 oC giving Ba vaporized and liquid Al. The getter can heated by RF induction or by joule heating

#Flash getter videohttp://www.youtube.com/watch?v=gQBmt-z0w9g

To vacuum pump( sealed off)HeaterFlash getter#515.3 Titanium Sublimation Getter Pumps

This is often used in conjunction with an ion pump. Titanium can readily deposited by sublimation from a heated source called Titanium Sublimation Pump (TSP)Wire type is made from four to six filament on flange that heated by passing 20 to 50 Ampere of current.Filament made by of molybdenum-titanium alloy which has characteristic that when titanium sublimed leaving only molybdenum, the resistivity lower in that location reduce temperature. To prevent catastrophic failure allow more titanium to be sublimed.

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Titanium Sublimation Getter PumpsTitanium sources normally operated only intermittently.Chemically reactive gas like O2,N2,CO are pumped very well, gas more stable H2O,CH4 are not pump .Rare gas like He, Ar, Ne are not pumpedWith ion/TSP pump receiving surface must far enough form the filament so it does not heat up and out gas. Is often made of copper and some time can be cooled by water or liquid nitrogenFor prolonged us, Ti flakes begin form can be electrostatically attract to ion pump anode. And it make anode short to ground and the pump not working. Is necessary to clean chemically.#TSP Video

http://www.youtube.com/watch?v=3_PSGE5XhQk#

15.4 Non-Evaporable Getters

Non Evaporable Getter (NEG) or bulk getter consist very chemically active usually zirconium or a combination Zr and another material (Fe ,V, Ni , Graphite)The material are ground into powder using manufacturing method. They compress in to solid form which very porous (large surface area)After fabrication NEG material first exposed to air, form thin protecting film which prohibit reaction with atmospheric gases After installation NEG must be activated. By heating 400-900oC for 1-30 minutesDuring operation NEGs gas phase molecule strike and react with the surface. Although this material has large surface area. Heating also molecules diffuse into bulk material. Most this material heat at 200-400oC

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Non-Evaporable Getters (1)Pumping characteristic of these material depends on its history.Total pumping speed and total quantity sorbed would depend on the area of material usedInteraction of NEG material with N2,O2,CO,CO2 and H2O cause of nitrides, oxides, carbides Although than oxygen, carbon, nitrogen will not released when material heated. #Non-Evaporable Getters (2)H2 Hydrogen does not react chemically with the getter alloy. Dissociation occurs and individually forming solid solution.The sorption of hydrogen is reversible. The amount its depends on the temperature and partial pressure of H2H2O molecule can absorb on the surface . For diffusing into bulk material. Resulting hydrogen and oxygen move to bulk. The pumping limited by rate of dissosiation of H2O moleculesNoble Gas, not pump by gettering. It possible operate getter pumps in noble gas atmosphere in order to improve the purity of the gas. NEG material can be purchase in various form including ring shaped container. They available as array parallel fins. The same as flash getter usually operate in situation pressure reduced below atmospheric pressure

#Non-Evaporable Getters (3)Relative pumping speed of several gas compared to N2It depends on how much surface area is available.To achieve larger pumping speed one simple use more material.For situation getter in side chamber the constriction of tubing my become limiting factorFor nitrogen at room temperature this about 11.6 liters/sec per cm2.

#Non-Evaporable Getters Video

http://www.youtube.com/watch?v=wTRPAUoFJqQ#Thank You#