Diffusion Pump Oils

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

HREELS experiments [66] were performed in a UHV chamber. The chamber was pre-evacuated by polyphenylether-oil diffusion pump the base pressure reached 2 x 10 Torr. The HREELS spectrometer consisted of a double-pass electrostatic cylindrical-deflector-type monochromator and the same type of analyzer. The energy resolution of the spectrometer is 4-6 meV (32-48 cm ). A sample was transferred from the ICP growth chamber to the HREELS chamber in the atmosphere. It was clipped by a small tantalum plate, which was suspended by tantalum wires. The sample was radia-tively heated in vacuum by a tungsten filament placed at the rear. The sample temperature was measured by an infrared (A = 2.0 yum) optical pyrometer. All HREELS measurements were taken at room temperature. The electron incident and detection angles were each 72° to the surface normal. The primary electron energy was 15 eV. 

A drawback of oil diffusion pumps is the so-called back-steaming. It is the flow of a small quantity of oil vapour towards the inlet of the pump. A water-cooled baffle like that shown in Fig. 1.13 can be put above the inlet. Baffles are made up of arrays of optically dense fins cooled by a continuous water flow. A baffle always reduces the pumping speed. 

The vacuum chambers were pumped down by means of an oil diffusion pump backed by a rotary vane vacuum pump. The base pressure achieved was 1 x 10 5 Torr (1.33 x 10 Pa). High-purity argon gas was bled into the chamber, the high-vacuum valve throttled, and the chamber pressure maintained as close as possible to 2 x 10 2 Torr (2.66Pa). For some of the experiments, the dc self-bias on the magnetron electrode was also measured. 

The various oil diffusion pumps manufactured by LEYBOLD differ in the following design features (see Fig. 2.45 a and b). 

For the smaller air-cooled, oil diffusion pumps, plate baffles are used. The air-cooled arrangement consists of a copper plate with copper webs to the housing wall. The temperature of the plate baffle remains nearly ambient during the operation of the diffusion pump. 

If extreme demands are made on freedom from oil vapor with vacuum produced by oil diffusion pumps, cold traps should be used that are cooled with liquid nitrogen so that they are maintained at a temperature of-196 °C. 

Mercury diffusion and vapor-jet pumps are less sensitive to air ingress than oil diffusion pumps. The oxidation of the hot mercury caused by the air ingress is negligible in regard to the operating characteristics of the pump when compared with the mercury loss in the forepump line. 

Substances are present in the vacuum vessel which have a higher vapor pressure than the driving medium being used among these are, for example, mercury, which is particularly hazardous because the mercury vapors will form amalgams with the nonferrous metals in the oil diffusion pump and thus make it impossible to achieve perfect vacuums. 

More modern diffusion pumps are operated with a range of synthetic oils having room-temperature vapour pressures of between 10 and 10 Torr. Although many of the oils previously used in oil diffusion pumps were not particularly stable to chemical attack at the normal working temperature of the diffusion pump, the oils available today (based on a variety of materials such as naphthalene, poly(phenyl ether), or silicones) are generally stable to oxidation at their normal working temperatures and many are particularly suitable when contact with more aggressive materials cannot be ruled out. 

The choice of vacuum pumps for the sample preparation line will depend on the size of its volume and the budget. In general it is advantageous to have somewhat higher pumping speeds available for the vacuum line, but this can well be provided by a conventional oil diffusion pump, which is less expensive than a turbomolecular device, backed again by a rotary pump. 

A vacuum system composed of a combination of a rotary pump and an oil diffusion pump is frequently used. Although use of a rotary pump alone can reach low pressures of 1—10—2 Torr, use of both rotary and oil diffusion pumps is desirable because less gas remains in the reaction chamber. 

Fig. 1. Photo and illustration of the HRTEM allowing acquisition of images of catalysts under working conditions (4). The microscope is equipped with an FEG, a quadrupole mass spectrometer (QMS), a Gatan image filter (GIF), and a Tietz F144 CCD for data acquisition. The differential pumping system consists of IGPs, turbo molecular pump units (TMP, MDP), and an oil diffusion pump (ODP). The differential pumping stages are set up by apertures inside the TEM column (denoted by black bars) at the objective lens (OL), the first (Cl) condenser aperture, the second (C2) condenser aperture, and the selected area aperture (SA).

The subject of vacuum-line technique is approached here from the perspective of the novice user who may wish to construct a system in collaboration with a glassblower. The literature on vacuum technology [3-11] and glassblowing [12-17] is vast and there are many commercial firms that specialize in it. Within the past decade, newer components such as Teflon-glass needle valves, O-ring seals, and oil diffusion pumps have been introduced into vacuum systems. 

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