Sorption pump

In a sorption pump, the gas is trapped within the adsorbing material (zeolites or active charcoal) called molecular sieve. Zeolites are porous aluminium silicates which adsorb large amount of gas when cooled to low temperature (usually 77K). The pump is filled with zeolite and put in a bucket containing liquid nitrogen (see Fig. 1.11). 

Gases that are condensable at 77K are trapped by cryocondensation . ( Cryosorption is instead the trapping of gas with a lower melting temperature inside the pores of the molecular sieve that has a huge surface/volume ratio, typically about 700m2/cm3.) 

If gases like H2, He and Ne are to be trapped, the pump must be cooled to 4.2 K. Detailed information on cryopumping can be found in ref. [10]. Ultimate pressure of the order of l(U5torr can be achieved. 

When cooled with liquid He, this type of pump can reach pressure below 10-8 torr. 

The sorption pumps are clean but are one shot , that is, two pumps in parallel and connected by valves alternatively are needed for a continuous pumping. When the first pump is saturated, the second pump is started, while the first is regenerated removing the liquid nitrogen, the trapped gas is expelled through the blow-off valve. The pump (with zeolite) is heated to 200-300°C to remove water vapour. Charcoal pumps are heated to about 100°C. 

---

Each test electrode was transferred to the EC chamber and subjected to electrochemical stabilization. The EC chamber was then evacuated rapidly by two sorption pumps and a cryopump to transfer the electrode to the XPS chamber again. 

Fig. 1.11. Scheme of a sorption pump with liquid nitrogen refrigeration. 

Pump-down of the antechamber, following solution experiments, involved sorption pumping and cryo-pumping resulting in the pressure decreasing from ambient to 10 Torr in 5 minutes. The resulting sample surfaces were subsequently examined by both LEFT) and AES after transfer back to the analysis-chamber. 

Pumps which bond or incorporate gases by adsorption or absorption to surfaces which are substantially free of gases (sorption pumps). 

The pumping system is the most important part of the vacuum line, it is certainly the most expensive, and it should therefore be chosen with care. Because the pumping system on a chemist s vacuum line must be robust and capable of removing large quantities of gases, often in repeated cycles, sorption pumps, getter ion pumps, and sublimation pumps are generally unsuitable and are therefore not discussed in this book. 

Other vacuum pumps include the sorption type based on the high gas take-up of charcoal or moleculai sieve material at liquid nitrogen temperatures. Sorption pumps may be used in place of rotary pumps, with a desirable freedom from rotary pump oil vapor, especially in systems where the amount of gas to be handled is limited. 

The experimental set-up normally used is shown schematically in Fig. 8. The gases, various chlorides and oxygen, are supplied with the aid of flow controllers. Typical values of the gas flows Qi are 30 < Qo2 < 500 seem, 2 < QSici4 < 140 seem, and some tens of seem for the other chlorides in use (seem = standard (STP) cubic centimeter per minute). The reactor consists of a microwave cavity and a furnace capable of heating the substrate between room temperature and 1200 °C. The cavity is connected to a 2.45 GHz generator with a maximum power of 200 W. A sorption pump is used to maintain a clean atmosphere within the tube during deposition. The pressure in a typical run is selected between 1.3 10 3 and 2.7 10 2 bar. 

As in high vacuum generation in macroscopic scale, the micropump is divided into two parts. As a backing pump for the pressure range from a few 102 Pa to atmospheric pressure a micro-sorption pump is under investigation, which relies on surface adsorption effects, as well as a scroll pump, which ranks with its displacement principle among the classic backing pumps. 

Stopcocks 15 and 1 can be opened and using the cold from the liquid nitrogen as a sorption pump, transfer the mixed compound into the holding trap (you may want to lightly heat the original compound to facilitate the transfer). 

A.M. Russel, Use of Water Aspirator in Conjunction with Sorption Pumping on an... 

A water pump can reach pressures of 1 Torr. An oil vacuum pump can reach 20 mTorr. A turbomolecular pump can reach pressures of 10 10 Torr (10-8 Pa). A sorption pump can reach pressures of 10-2 Torr by exposing the system to a porous zeolite cooled to liquid nitrogen temperature with a Dewar flask placed on the outside. 

The ion pump needs no forepump, but it is necessary to reduce the pressure initially to 10 or 10 ° Torr before turning on the pump. With care, this may be accomplished with a rotaiy oil pump, but it is often done instead with a sorption pump such as the Varian Vac-Sorb pump, in which the air in the system is adsorbed on a molecular sieve (synthetic zeolite, Linde type 5A) chilled with liquid nitrogen. This type of pump has the advantage of presenting no danger of contaminating the system with oil. 

The stainless steel gas/vacuum handling system used for this work facilitates a base pressure of 5x10 Torr (1 Torr = 133.3 Pa). It is equipped with a f-Nj cooled zeolite sorption pump, a 30 L/s ion pump, a BARATRON capacitance manometer (0.001-1000 Torr), and a model MIOOM DYCOR quadrupole mass spectrometer. 

The exhaust pressure is the pressure at the outlet of a pump. If the maximum pressure of a pump is lower than the atmospheric pressure (e.g. roots pump and oil diffusion pump), it must be backed with a mechanical rotaiy pump. In addition, some types of pumps have no outlet, such as ionisation and sorption pumps. 

Vacuum pumps are generally divided into 13 categories according to the working principle, as listed in Table 2.5. They include water jet pump, water ring pump, steam ejector, oil-sealed rotaiy pump, Roots pump, vacuum diffusion pump, oil vapom booster pump, sputtering-ion pump, radial field pump, titanium sublimation pump, sorption pump, molecular pump and cryopump [9],... 

FIGURE 10.6 UETV, electrochemical device C, electrochemical cell, with sample SC, auxiliary and reference electrodes E, electrolyte AI, argon inlet V, valve for the separation between the electrochemical pre-chamher and the main UHV chamber SM, sample manipulator SP, sorption pump P, the turbomolecular pumps M, mass spectrometer S, sputter gun SC, sample of single crystals R, x-ray emission tube L, low-energy electron diffraction system H, heat lamp X, x-ray photoelectronic spectrometer and T, transfer rod with sample holder. 

---