Vacuum Work

The checkers used a stirrer for vacuum work (Teflon bearing, Fisher Scientific Company). The submitters used a mercury-sealed stirrer. 

Silicone greases do, however, have uses in extrusion for coating dies etc., to facilitate stripping down. Greases have also found uses in the laboratory for lubricating stop cocks and for high-vacuum work. 

Silver-copper-palladium alloys with liquidus temperatures of 800-1 000°C have very low vapour pressures combined with good wetting and flow characteristics and are widely employed in vacuum work. They exhibit a lower tendency to stress corrosion than silver-copper, and do not form brittle alloys with other metals. 

If the seals are to be used for vacuum work it is advisable to tip the ends of the drawn wire with nickel in order to seal up any fine channels that may open between the copper layer and the central alloy core. 

For high vacuum work, where it is desirable to shorten the pumping path to a minimum, a useful built-in cold trap is shown in Figure 52, V. The outer jacket is prepared from a round-bottomed flask to which the side arms are attached, as shown in Figure 52, IV. The inner tube is shaped with a flare at its upper end and its heated... 

Generally you ll grease joints on two occasions. One, when doing vacuum work to make a tight seal that can be undone the other, doing reactions with... 

Leak hunting is a craft which can only be learnt with practice and it is bedevilled more than most aspects of vacuum work by Murphy s Law (see Chapter 1). Pinholes and cracks usually appear in the most inaccessible places, and for good reason. Just because a region is inaccessible, e.g. the back of a tube-to-tube joint, near a wall or metal support, the glass-blowing will have been difficult and its quality and finish less than perfect. 

Fig. 3.20. Common conductivity probe for vacuum work. A couplings to shielded cables from conductivity meter 5 a B. 19 or B.24 socket fitting onto cone on observation/reaction vessel C graded-seal soda glass to borosilicate glass D shielded leads spot-welded to thick Pt wires E sealed through the soda glass probe and held together by the lead glass bead F.

There were no substantial differences between the magnitudes of the photovoltages on the different substrate electrodes despite the 1,4-V range in their vacuum work functions (Fig. 7). The slight decrease in Voc on Pt substrates was caused by the enhanced rate of recombination at this highly catalytic electrode... 

DC sputtering Metals, some semiconductors some insulators (reactive sputtering) Vacuum, works best with conductors, better than vapor evaporation for high-melting species, better adhesion than vapor evaporation, but rougher surface reactive sputtering of metals in presence of oxygen or other reactive gas can deposit some insulators... 

The method employed was similar to that of Ref. 35, but with several improvements. ab initio, norm-conserving, nonlocal pseudopotential were used to represent the metal ions. This capability enables reliably realistic representation of the metal s electronic structure. Thus the cadmium pseudopotential was able, for example, to reproduce the experimental cadmium-vacuum work function using no adjustable parameters (unlike the procedure followed in Ref. 35). Pseudopotentials of the Troullier and Martins form [53] were used with the Kleinman-Bylander [54] separable form, and a real space... 

While the chemical resistance varies somewhat, stainless steel is fairly resistant to most acids and bases, is not amalgamated by mercury, and is generally resistant to oxidizing agents. While it can be used in fluorine handling, Monel and nickel are much better for this purpose. The resistance of stainless steel to atmospheric corrosion is an advantage in vacuum work because a corroded surface tends to outgas. 

Vacuum-jacketed glass apparatus should be handled with extreme care to prevent implosion. Equipment such as Dewar flasks should be taped or shielded. Only glassware designed for vacuum work should be used for that purpose. 

In many labs it is standard to use red heavy-wall rubber tubing to connect mechanical pumps to vacuum systems. However, there is no technical reason for using red (colored) tubing. Any tubing that meets the qualifications for vacuum work (or your specific vacuum work) should suffice, regardless of color. 

Standard Teflon stopcocks cannot be used for any vacuum work. A completely different design from those discussed thus far is required for a Teflon vacuum stopcock. This design is shown in Fig. 3.22 and is called a rotary Teflon valve. It is called a valve instead of a stopcock because its operation and design is similar to those of a metal designed valve. 

A sealant, such as Apiezon W, is a good example of a material that was designed for an extremely limited application. Apiezon W is a hard black wax that needs to be heated to 80°-90°C before it is soft enough to apply to the members you wish to join. At room temperatures it is hard and has no lubrication abilities whatsoever and is therefore not usable for a stopcock. It has a relatively high vapor pressure (10 3 torr), so it cannot be used for most vacuum work. However, at temperatures of about 100° to 150°C, it becomes very thin like hot honey and can easily be applied to joint members. Therefore, if you have a standard taper joint that will not be in a high-heat or ultrahigh-vacuum environment, there is no need to separate often, and you don t want it to accidentally separate, Apiezon W is the type of sealant you need. 

There is one other substitute for stopcock grease on standard taper joints the Teflon sleeve. These sleeves are like socks for your joints. Because they are made of Teflon, they are not attacked by solvents, alkalines, and most other chemicals. Thus, they are wonderful for items like solvent flasks which are under constant fume or chemical contact. However, they are not capable of maintaining a static vacuum and should not be used for vacuum work. Thin Teflon sleeves are less expensive, but cannot take physical abuse. Heavier Teflon sleeves have a greater initial cost, but can be used over and over. 

Surprisingly, many assumed defects of a joint or stopcock are caused by using an item improperly. By trying to make do with what you have, you either may jeopardize the quality of your work or may subject yourself and others around you to potential dangers. Thus, adding extra grease onto a standard stopcock to that it can be used for vacuum work will be a losing battle. 

The third form of attachment is with some type of ground joint. It would be wonderful if this attachment could be made with a full-length standard taper joint (see Sec. 3.1.1), but alignment of several joints that allow for easy assembly and disassembly of several parts is seldom possible. Ball-and-socket joints would seem like an alternative, but they are not really acceptable for high-vacuum work and should not be considered. The use of ball-and-socket joints is not really impossible, but because their reliability is always in doubt, the use of these joints for vacuum line assembly should never be your first choice. 

The beauty of liquid traps is that once in place they require no further oversight, care, or maintenance. Once you have seen the damages caused by a manometer or a McLeod gauge that has burped, you understand the value of liquid traps. However, the value of liquid traps can be overemphasized, and they should not be used as panaceas for clumsy vacuum work. They will not stop all the mercury (or other fluids) that are being battered around within a system, so do not depend on liquid traps to make up for carelessness. 

Ball-and-socket joints should not be used for high-vacuum systems because they are not intended for vacuum work. It is possible to obtain specially made ball joints with O-rings that are acceptable for some vacuum work. Some manufacturers supply sockets that have not been ground, and when used in tandem with ball joints with O-rings, they can achieve satisfactory vacuum performance. 

If you require metal components on your vacuum system, select metals with low gas permeation, such as 300 series stainless steel. It is nonmagnetic and, like glass, is a poor conductor of heat and electricity. Stainless steel, also like glass, is relatively nonreactive, and therefore is less likely to rust or be affected by chemicals. If welding the stainless steel is required, select 304L stainless steel, which is low in carbon. Otherwise, at welding temperatures the carbon will combine with the chromium (within the stainless steel) to form chromium carbide and the corrosion protection of the chromium will be lost. Type 303 stainless steel should not be used for vacuum work because it contains selenium, which has a high vapor pressure. 

Raymond s new vacuum worked beautifully on the interior of his new car, he still was not able to get the pine needles from last year s Christmas tree out of the side pockets. 

---