The term “sorption pumps” includes all arrangements for the removal of gases and vapors from a space by sorption means. The pumped gas particles are thereby bound at the surfaces or in the interior of these agents, by either physical temperature-dependent adsorption forces (van der Waals forces), chemisorption, absorption, or by becoming embedded during the course of the continuous formation of new sorbing surfaces. By comparing their operating principles, we can distinguish between adsorption pumps, in which the sorption of gases takes place simply by temperature-controlled adsorption processes, and getter pumps, in which the sorption and retention of gases are essentially caused by the formation of chemical compounds. Gettering is the bonding of gases to pure, mostly metallic surfaces, which are not covered by oxide or carbide layers. Such surfaces always form during manufacture, installation or while venting the system. The mostly metallic highest purity getter surfaces are continuously generated either directly in the vacuum by evaporation (evaporator pumps) or by sputtering (sputter pumps) or the passivating surface layer of the getter (metal) is removed by degassing the vacuum, so that the pure material is exposed to the vacuum. This step is called activation (NEG pumps NEG = Non Evaporable Getter).
Adsorption pumps (see Fig. 2.59) work according to the principle of the physical adsorption of gases at the surface of molecular sieves or other adsorption materials (e.g. activated Al2O3). Zeolite 13X is frequently used as an adsorption material. This alkali aluminosilicate possesses for a mass of the material an extraordinarily large surface area, about 1000 m2/g of solid substance. Correspondingly, its ability to take up gas is considerable.
The pore diameter of zeolite 13X is about 13 Å, which is within the order of size of water vapor, oil vapor, and larger gas molecules (about 10 Å). Assuming that the mean molecular diameter is half this value, 5 · 10-8 cm, about 5 · 1018 molecules are adsorbed in a monolayer on a surface of 1 m2 . For nitrogen molecules with a relative molecular mass Mr = 28 that corresponds to about 2 · 10-4g or 0.20 mbar · l . Therefore, an adsorption surface of 1000 m2 is capable of adsorbing a monomolecular layer in which more than 133 mbar · l of gas is bound.
Hydrogen and light noble gases, such as helium and neon, have a relatively small particle diameter compared with the pore size of 13 Å for zeolite 13X. These gases are, therefore, very poorly adsorbed.
The adsorption of gases at surfaces is dependent not only on the temperature, but more importantly on the pressure above the adsorption surface. The dependence is represented graphically for a few gases by the adsorption isotherms given in Fig. 2.60. In practice, adsorption pumps are connected through a valve to the vessel to be evacuated. It is on immersing the body of the pump in liquid nitrogen that the sorption effect is made technically useful. Because of the different adsorption properties, the pumping speed and ultimate pressure of an adsorption pump are different for the various gas molecules: the best values are achieved for nitrogen, carbon dioxide, water vapor, and hydrocarbon vapors. Light noble gases are hardly pumped at all because the diameter of the particles is small compared to the pores of the zeolite. As the sorption effect decreases with increased coverage of the zeolite surfaces, the pumping speed falls off with an increasing number of the particles already adsorbed. The pumping speed of an adsorption pump is, therefore, dependent on the quantity of gas already pumped and so is not constant with time.
The ultimate pressure attainable with adsorption pumps is determined in the first instance by those gases that prevail in the vessel at the beginning of the pumping 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 these gases are present. Therefore, pressures < 10-2 mbar can be obtained.
If pressures below 10-3 mbar exclusively are to be produced with adsorption pumps, as far as possible no neon or helium should be present in the gas mixture.
After a pumping process, the pump must be warmed only to room temperature for the adsorbed gases to be given off and the zeolite is regenerated for reuse. If air (or damp gas) containing a great deal of water vapor has been pumped, it is recommended to bake out the pump completely dry for a few hours at 392°F (200°C) or above.
To pump out larger vessels, several adsorption pumps are used in parallel or in series. First, the pressure is reduced from atmospheric pressure to a few millibars by the first stage in order to “capture” many noble gas molecules of helium and neon. After the pumps of this stage have been saturated, the valves to these pumps are closed and a previously closed valve to a further adsorption pump still containing clean adsorbent is opened so that this pump may pump down the vacuum chamber to the next lower pressure level. This procedure can be continued until the ultimate pressure cannot be further improved by adding further clean adsorption pumps.
Sublimation pumps are sorption pumps in which a getter material is evaporated and deposited on a cold inner wall as a getter film. On the surface of such a getter film the gas molecules form stable compounds, which have an immeasurably low vapor pressure. The active getter film is renewed by subsequent evaporations. Generally titanium is used in sublimation pumps as the getter. The titanium is evaporated from a wire made of a special alloy of a high titanium content which is heated by an electric current. Although the optimum sorption capacity (about one nitrogen atom for each evaporated titanium atom) can scarcely be obtained in practice, titanium sublimation pumps have an extraordinarily high pumping speed for active gases, which, particularly on starting processes or on the sudden evolution of greater quantities of gas, can be rapidly pumped away. As sublimation pumps function as auxiliary pumps (boosters) to sputter-ion pumps and turbomolecular pumps, their installation is often indispensable (like the “boosters” in vapor ejector pumps; see the page on oil diffusion pumps for more information).