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I have typed up a summary of important information about sorbs:

Here is a summary of key points:

  • The bonding agent does indeed have a very significant effect on the pumping abilities of a sorb. Data shows that poor thermal conductors like SS or ZrO2 cements have little or no pumping speed, while Ag-Sn braze bonds and copper cements have the best pumping speed. This would suggest that using Stycast as a bonding agent is not ideal, since it has very poor thermal conductivity (~1/5 that of 316 SS at 4K).
  • Baking out sorbs is always a good idea, but heating them too much can be bad. Experiments have shown that charcoal heated to very high temperatures (~900 Celcius) in preparation had a reduced pumping speed. However, heating to moderate temperatures (<250 Celsius) seems to be fine.
  • Contamination of sorbs by pump oil is significant enough that after a few weeks under vacuum, pumping speed can drop by almost an order of magnitude. Sorbs should be baked out and cooled in the presence of a cold surface onto which contaminants can condense, for example a clean metal surface at or below LN2 temperatures.

This is a first draft. There is more that I would like to add to it, but there is enough in it for now. The references are listed below. -- Nick Hutzler, 28 October 2008

Some other good papers that did not yet make it into the summary: