Material Limitations
Materials which are not allowed due to high vapour pressure:
- majority of organic compounds cannot be used:
- plastics other than teflon and PEEK: gaskets are made of copper, and are single-use; plastics in other uses are replaced with ceramics or metals
- glues: special glues for high vacuum must be used
- common steel: due to oxidizing, which greatly increases adsorption area, only stainless steel is used
- lead: soldering is performed using lead-free solder
- indium: Indium is commonly used as a deformable gasket material for vacuum seals, especially in cryogenic apparatus, but its low melting point prevents use in baked systems.
- zinc, cadmium: High vapor pressures during system bake-out.
Technical limitations:
- screws: threads have a high surface area and tend to "trap" gases, therefore are avoided
- welding: standard welding cannot be used due to high surface area and introduction of gas chambers, which would collect gas at atmospheric pressure, and release it slowly during evacuation (removal of gas).
Read more about this topic: Ultra-high Vacuum
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