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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. 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; 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 possible to machine, and if possible not being overly expensive. Yet another concern is the thermal expansion coefficient match of adjacent parts. ==Materials to avoid== Materials outgas by two mechanisms: release of absorbed and adsorbed gases, 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.〔 * Magnesium * PVC, usually in the form of wire insulation (source of virtual leaks too) * Paints * Lead and antimony used in some soft solders and outgassing at higher temperatures * Many plastics, namely many plastic tapes (beware especially of the adhesives). Fiberglass composites, e.g. Micarta (G-10) and G-30, should be avoided. Even kapton and teflon are sometimes advised against.〔 * Various residues, e.g. flux from soldering and brazing, and lubricants from machining. Thorough cleaning of the parts is important. Getting the outgassable residues from tight crevices can be challenging; a good mechanical design that avoids such features can simplify one's life. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Materials for use in vacuum」の詳細全文を読む スポンサード リンク
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