Vacuum technique

A further reason to employ UHV techniques is the high ionization and fragmentation cross sections for hydrocarbons H, C, CH, etc ion peaks can be particularly troublesome at the higher photon energies. One should therefore use well-trapped turbomolecular pumps or turbomolecular pumps with magnetic levitation, backed by a molecular drag pump, which itself is backed by an oil-free diaphragm pump. 

The term ract/t/m refers to the condition of an enclosed space that is devoid of all gases or other material content. It is not experimentally feasible to achieve a perfect vacuum, although one can approach this condition quite closely. It is possible routinely to obtain a vacuum of 10 Torr and with more sophisticated techniques 10 ° Torr (1.3 X 10 ° bar) it is even possible by special techniques to obtain a vacuum of 10 ° Torr, or about 30 molecules per cubic centimeter. One Torr, the conventional unit of pressure in vacuum work, is the pressure equivalent of a manometer reading of 1 mm of liquid mercury  

For Dewar flasks, metal evaporation apparatus, and most research apparatus, a vacuum of 10 ° to 10 ° Torr is sufficient this is in the high vacuum range, while 10 ° Torr would be termed ultrahigh vacuum. However, for many routine purposes a utility vacuum or forepump vacuum of about 10 ° Torr will suffice, and for vacuum distillations only a partial vacuum of the order of 1 to 50 Torr is needed. 

The principal types of vacuum pumps will be discussed in this section. The water aspirator is a crude but useful pump for many routine operations. Rotary oil pumps are used for pumping on refrigerant baths and as the forepump for backing low-pressure pumps. For most high-vacuum work, diffusion pumps are utilized to achieve pressures 

Water Aspirator. The water aspirator produces only a partial vacuum but one that is entirely adequate for a large number of simple chemical operations, such as filtration by suction, distillation under reduced pressure, and evacuation of desiccators. 

This device operates through Bernoulli s principle. Water enters the aspirator from a pipe at mains pressure (about 4 atm) and leaves against 1 atm external pressure from a nozzle of lesser diameter. In an internal constriction of still smaller diameter, the pressure drops nearly to zero. The air from the system being evacuated enters at this constriction and is carried along with the water. 

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Surface defects (Section VII-4C) are also influenced by the history of the sample. Such imperfections may to some extent be reversibly affected by processes such as adsorption so that it is not safe to regard even a refractory solid as having fixed surface actions. Finally, solid surfaces are very easily contaminated detection of contamination is aided by ultra-high-vacuum techniques and associated cleaning protocols [24]. 

Ultrahigh vacuum techniques have become common, especially in connection with surface spectroscopic and diffraction studies, but also in adsorption on very clean surfaces. The techniques have become rather specialized and the reader is referred to Ref. 8 and citations therein. 

Soriaga M P 1992 Ultra-high vacuum techniques in the study of single-crystal electrode surfaces Prog. Surf. Sc/. 39 325... 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

Some elements, such as the rare eartlrs and the refractory metals, have a high afflnity for oxygen, so vaporization of tlrese elements in a irormaT vacuum of about 10 " Pa, would lead to the formation of at least a surface layer of oxide on a deposited flhrr. The evaporation of these elements therefore requires the use of ultra-high vacuum techniques, which can produce a pressure of 10 Pa. 

The development of the STEM is relatively recent compared to the TEM and the SEM. Attempts were made to build a STEM instrument within 15 years after the invention of the electron microscope in 1932. However the modern STEM, which had to await the development of modern electronics and vacuum techniques, was developed by Albert Crewe and his coworkers at the University of Chicago. ... 

The scientific study of surfaces, and the full recognition of how much a surface differs from a bulk structure, awaited a drastic improvement in vacuum technique. The next Section is devoted to a brief account of the history of vacuum. 

Steinherz and Redhead (1962), while advances in vacuum techniques from a specifically chemical viewpoint were discussed by Roberts (1960). 

Roberts, M.W. (1960, August) High-vacuum techniques, J. Roy. Inst. Chem. 275. 

S. Deshman Principals of Vacuum Technique, Mir, Moscow, 1964 (translation to Russian). 

Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

On his return to Princeton after the war, Hugh Taylor organized catalytic research at the Frick Chemical Laboratory. He applied high vacuum technique, liquid air cryoscopy to the study of adsorptive characteristics of catalysts, correlating rates of catalytic reactions and rates of adsorption. He introduced the concept of activated adsorption and defended it against all comers. ... 

Griffiths. H. Trans. Inst. Chem. Eng. 23 i, 1945) 1)3. Some problems of vacuum technique from a chemical engineering standpoint. 

Reimann, a. L., 1952, Vacuum Technique London, Chapman Hall. 

Yarwood, j., 1955, High Vacuum Technique-, London, Chapman Hall. 

J. A. Belk, Vacuum Techniques in Metallurgy, Pergamon, London, 1963. 

Impurity levels at this molecular weight become extremely critical and careful vacuum techniques must be used where appropriate to exclude all contaminations. 

S Dushman, JM Lafferty. Scientific Foundations of Vacuum Technique. 2nd ed. New York Wiley, 1962. 

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