Materials for use in vacuum are materials showing very low rate of
outgassing in
vacuum, and, where applicable, tolerant to the
bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum achievable in the
vacuum chamber. The materials can produce gas by several mechanisms. Molecules of gases and water can be
adsorbed on the material surface (therefore materials with low affinity to water have to be chosen, which eliminates many plastics). Materials may
sublimate in vacuum (this excludes some metals and their alloys, most notably cadmium and zinc). Or the gases can be released from
porous materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.