Making a High-Resolution Standard for SEM

John Heckman
1997 Microscopy Today  
The most often-mentioned method to flatten 0-rings is boiling. This method works well with viton. The advantages of boiling are that the method is easy to do reproducibly, and it drives out a lot of 'glup,' to use the technical term. The disadvantage is that the water must then be removed. For viton, this is pretty easy, since that elastomer can be baked out at about 200° C. Another method is heating to 50° C in an oven. This is not quite high enough for viton, which will not lie flat after an
more » ... vernight heating at that temperature. Heating at a somewhat higher temperature should work. The advantages of the oven method are that one need not remove water, and it is the best-controlled method for those elastomers, such as buna-N and fluorocarbons, that cannot be heated above 70° C. Following the heating process, the 0-ring can be cooled either slowly or rapidly. Slow cooling works, but there may be situations where shock cooling would be advantageous. In particular, to shape an 0-ring to a particular non-flat or non-round configuration, it might be good to heat, shape, then shock-cool. Another idea is to put the 0-ring around a beaker and freeze it in the round state. This method can work if condensation is avoided, but it depends on where the 0-ring goes. If it has to fit on the bottom of a part, such as a column section, the reassembly of the column takes so long that the 0-ring would thaw and fall out. A caution about silicone 0-rings is that they are very permeable to He. As a result, leak-checking can give false positives for several days. Both viton and silicone 0-rings can be baked out at about 200° C, and that may be a good idea for a standard practice, since it will drive off volatiles in the 0-rings. Buna-N and fluorocarbon cannot be heated above 70° C, and at that for only a few hours. Buna-N just melts, but fluorocarbon decomposes. Ethylene-propylene is the most radiation-resistant of the common elastomers. It is used for seals that are close to the beam, and it remains relatively flexible under circumstances where either viton or neoprene harden. The Parker 0-Ring Handbook* is a useful source of information about many properties and applications of various available elastomers.
doi:10.1017/s1551929500061484 fatcat:w7ndsyhklfe2zby5h4cg4zx6jq