Materials for use in vacuum need to show very low rates of outgassing in vacuum and, where applicable, be tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the vacuum chamber. Materials 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); 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.
Materials to avoid Materials outgas by three mechanisms: release of absorbed gases (desorption from the bulk of the material), release of adsorbed gases (desorption from the surface only), and evaporation of the material itself. The former can be reduced by a bakeout, the latter is an intrinsic property of the material. Some outgassed materials can deposit on other surfaces, contaminate the vacuum system and be difficult to get rid of. The most common sources of trouble (out-gassing) in vacuum systems are:
Cadmium, often present in the form of cadmium plating, or in some soldering and brazing alloys Zinc, problematic for high vacuum and higher temperatures, present in some construction alloys, e.g. brass and some brazing alloys. Tends to poison hot cathodes and form conductive deposits on surfaces. Any materials that have been zinc-coated by galvanization should be avoided, or they have the coating removed first. Magnesium Paints Lead and antimony used in some soft solders due to outgassing at higher temperatures. Many plastics, namely many plastic tapes (special attention should be paid to adhesives). Fiberglass composites, e.g. Micarta (G-10) and G-30, should be avoided. Even Kapton and Teflon are sometimes advised against. See below for further discussion of plastics. PVC, usually in the form of wire insulation (also a source of leaks) Various residues, e.g. flux from soldering and brazing, and lubricants from machining making thorough cleaning imperative. Getting the outgassable residues from tight crevices can be challenging; a good mechanical design that avoids such features can help. There are also additional physical issues which come with vacuum, including the growth of whiskers from materials such as Tin or Zinc, which can cause physical issues or electrical shorts
Review of materials and issues to consider
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