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The Long Duration Exposure Facility was used to test various materials in vacuum.
Materials for use in vacuum are materials that show very low rates of outgassing in vacuum and, where applicable, are tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved 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 includes 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.
The gases liberated from the materials not only lower the vacuum quality, but also can be reabsorbed on other surfaces, creating deposits and contaminating the chamber.
Yet another problem is diffusion of gases through the materials themselves. Atmospheric helium can diffuse even through Pyrex glass, even if slowly (and elevated temperatures above room temperature are generally needed);[1] this however is usually not an issue. Some materials might also expand or increase in size causing problems in delicate equipment.
In addition to the gas-related issues, the materials have to maintain adequate strength through the entire required temperature range (sometimes reaching cryogenic temperatures), maintain their properties (elasticity, plasticity, electrical and thermal conductivity or lack of it, etc.), be machinable, and if possible not be overly expensive. Yet another concern is the thermal expansion coefficient match of adjacent parts.
^Taylor, Nelson W.; Rast, William (1938). "The Diffusion of Helium and of Hydrogen Through Pyrex Chemically Resistant Glass". The Journal of Chemical Physics. 6 (10): 612–619. Bibcode:1938JChPh...6..612T. doi:10.1063/1.1750133. Retrieved 2021-08-28.
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