Fundamentals of Vacuum Technology (1248463), страница 20
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2.59 Cross section of an adsorption pump2.1.8 Sorption pumps2.1.8.1Adsorption pumpsAdsorption pumps (see Fig. 2.59) work according to the principle of thephysical adsorption of gases at the surface of molecular sieves or otheradsorption materials (e.g. activated Al2O3). Zeolite 13X is frequently usedas an adsorption material. This alkali aluminosilicate possesses for a massof the material an extraordinarily large surface area, about 1000 m2/g ofsolid substance. Correspondingly, its ability to take up gas is considerable.The pore diameter of zeolite 13X is about 13 •, which is within the order ofsize of water vapor, oil vapor, and larger gas molecules (about 10 •).Assuming that the mean molecular diameter is half this value, 5 · 10-8 cm,about 5 á 1018 molecules are adsorbed in a monolayer on a surface of1 m2 .
For nitrogen molecules with a relative molecular mass Mr = 28 thatcorresponds to about 2 · 10-4 g or 0.20 mbar · l (see also Section 1.1).Therefore, an adsorption surface of 1000 m2 is capable of adsorbing amonomolecular layer in which more than 133 mbar á l of gas is bound.Hydrogen and light noble gases, such as helium and neon, have arelatively small particle diameter compared with the pore size of 13 • forzeolite 13X. These gases are, therefore, very poorly adsorbed.The adsorption of gases at surfaces is dependent not only on thetemperature, but more importantly on the pressure above the adsorptionsurface. The dependence is represented graphically for a few gases by theadsorption isotherms given in Fig.
2.60. In practice, adsorption pumps arePressure [Torr]Adsorbed quantity of gas per quantity of adsorbent [mbar ál á gÐ1]The term Òsorption pumpsÓ includes all arrangements for the removal ofgases and vapors from a space by sorption means. The pumped gasparticles are thereby bound at the surfaces or in the interior of theseagents, by either physical temperature-dependent adsorption forces (vander Waals forces), chemisorption, absorption, or by becoming embeddedduring the course of the continuous formation of new sorbing surfaces.
Bycomparing their operating principles, we can distinguish betweenadsorption pumps, in which the sorption of gases takes place simply bytemperature-controlled adsorption processes, and getter pumps, in whichthe sorption and retention of gases are essentially caused by the formationof chemical compounds. Gettering is the bonding of gases to pure, mostlymetallic surfaces, which are not covered by oxide or carbide layers. Suchsurfaces always form during manufacture, installation or while venting thesystem. The mostly metallic highest purity getter surfaces are continuouslygenerated either directly in the vacuum by evaporation (evaporatorpumps) or by sputtering (sputter pumps) or the passivating surface layerof the getter (metal) is removed by degassing the vacuum, so that the purematerial is exposed to the vacuum.
This step is called activation (NEGpumps NEG = Non Evaporable Getter).Pressure [mbar]Fig. 2.60 Adsorption isotherms of zeolite 13X for nitrogen at Ð195 ¡C and 20 ¡C, as well as forhelium and neon at Ð195 ¡C50HomeVacuum generationconnected through a valve to the vessel to be evacuated. It is on immersingthe body of the pump in liquid nitrogen that the sorption effect is madetechnically useful. Because of the different adsorption properties, thepumping speed and ultimate pressure of an adsorption pump are differentfor the various gas molecules: the best values are achieved for nitrogen,carbon dioxide, water vapor, and hydrocarbon vapors.
Light noble gasesare hardly pumped at all because the diameter of the particles is smallcompared to the pores of the zeolite. As the sorption effect decreases withincreased coverage of the zeolite surfaces, the pumping speed falls off withan increasing number of the particles already adsorbed. The pumpingspeed of an adsorption pump is, therefore, dependent on the quantity ofgas already pumped and so is not constant with time.The ultimate pressure attainable with adsorption pumps is determined in thefirst instance by those gases that prevail in the vessel at the beginning of thepumping process and are poorly or not at all adsorbed (e.g.
neon or helium)at the zeolite surface. In atmospheric air, a few parts per million of thesegases are present. Therefore, pressures < 10-2 mbar can be obtained.If pressures below 10-3 mbar exclusively are to be produced with adsorptionpumps, as far as possible no neon or helium should be present in the gasmixture.After a pumping process, the pump must be warmed only to roomtemperature for the adsorbed gases to be given off and the zeolite isregenerated for reuse. If air (or damp gas) containing a great deal of watervapor has been pumped, it is recommended to bake out the pumpcompletely dry for a few hours at 200 ¡C or above.To pump out larger vessels, several adsorption pumps are used in parallelor in series. First, the pressure is reduced from atmospheric pressure to afew millibars by the first stage in order to ÒcaptureÓ many noble gasmolecules of helium and neon.
After the pumps of this stage have beensaturated, the valves to these pumps are closed and a previously closedvalve to a further adsorption pump still containing clean adsorbent isopened so that this pump may pump down the vacuum chamber to the nextlower pressure level. This procedure can be continued until the ultimatepressure cannot be further improved by adding further clean adsorptionpumps.2.1.8.22.1.8.3Sputter-ion pumpsThe pumping action of sputter-ion pumps is based on sorption processesthat are initiated by ionized gas particles in a Penning discharge (coldcathode discharge). By means of Òparalleling many individual Penning cellsÓthe sputter ion pump attains a sufficiently high pumping speed for theindividual gases.Operation of sputter-ion pumpsThe ions impinge upon the cathode of the cold cathode discharge electrodesystem and sputter the cathode material (titanium).
The titanium depositedat other locations acts as a getter film and adsorbs reactive gas particles(e.g., nitrogen, oxygen, hydrogen). The energy of the ionized gas particlesis not only high enough to sputter the cathode material but also to let theimpinging ions penetrate deeply into the cathode material (ion implantation).This sorption process ÒpumpsÓ ions of all types, including ions of gaseswhich do not chemically react with the sputtered titanium film, i.e. mainlynoble gases.The following arrangement is used to produce the ions: stainless-steel,cylindrical anodes are closely arranged between, with their axesperpendicular to, two parallel cathodes (see Fig.
2.61). The cathodes are atnegative potential (a few kilovolts) against the anode. The entire electrodesystem is maintained in a strong, homogeneous magnetic field of a fluxdensity of B = 0.1 T, (T = Tesla = 104 Gauss) produced by a permanentmagnet attached to the outside of the pumpÕs casing. The gas dischargeproduced by the high tension contains electrons and ions. Under theinfluence of the magnetic field the electrons travel along long spiral tracks(see Fig. 2.61) until they impinge on the anode cylinder of thecorresponding cell. The long track increases ion yield, which even at lowgas densities (pressures) is sufficient to maintain a self-sustained gasdischarge. A supply of electrons from a hot cathode is not required.Because of their great mass, the movement of the ions is unaffected by themagnetic field of the given order of magnitude; they flow off along theSublimation pumpsSublimation pumps are sorption pumps in which a getter material isevaporated and deposited on a cold inner wall as a getter film.
On thesurface of such a getter film the gas molecules form stable compounds,which have an immeasurably low vapor pressure. The active getter film isrenewed by subsequent evaporations. Generally titanium is used insublimation pumps as the getter. The titanium is evaporated from a wiremade of a special alloy of a high titanium content which is heated by anelectric current. Although the optimum sorption capacity (about one nitrogenatom for each evaporated titanium atom) can scarcely be obtained inpractice, titanium sublimation pumps have an extraordinarily high pumpingspeed for active gases, which, particularly on starting processes or on thesudden evolution of greater quantities of gas, can be rapidly pumped away.As sublimation pumps function as auxiliary pumps (boosters) to sputter-ionpumps and turbomolecular pumps, their installation is often indispensable(like the ÒboostersÓ in vapor ejector pumps; see Section 2.1.6.2).PZ← ⊕Direction of motion of the ionized gasmolecules• → Direction of motion of the sputteredtitanium-ÐÐSpiral tracks of the electronsPZ Penning cellsFig.
2.61 Operating principle of a sputter-ion pump51HomeVacuum generationTitanium atoms•Ö Gasmoleculesü⊕ IonsElectronsB Magnetic fieldFig. 2.62 Electrode configuration in a diode sputter-ion pumpshortest path and bombard the cathode.The discharge current i is proportional to the number density of neutralparticles n0, the electron density n-, and the length l of the total dischargepath:i = n0 · nÐ · σ · l(2.25)The effective cross section s for ionizing collisions depends on the type ofgas. According to (2.25), the discharge current i is a function of the numberparticle density n0, as in a Penning gauge, and it can be used as ameasure of the pressure in the range from 10-4 to 10-8 mbar.
At lowerpressures the measurements are not reproducible due to interferencesfrom field emission effects.In diode-type, sputter-ion pumps, with an electrode system configurationas shown in Fig. 2.62, the getter films are formed on the anode surfacesand between the sputtering regions of the opposite cathode. The ions areburied in the cathode surfaces.
As cathode sputtering proceeds, the buriedgas particles are set free again. Therefore, the pumping action for noblegases that can be pumped only by ion burial will vanish after some timeand a Òmemory effectÓ will occur.Unlike diode-type pumps, triode sputter-ion pumps exhibit excellentstability in their pumping speed for noble gases because sputtering and filmforming surfaces are separated. Fig.
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