Materials in a vacuum give off gas

    In the vacuum system, there are special requirements for the vacuum performance of the material, among which two important parameters are the air permeability and outgassing of the material.

    The vacuum system has air permeability. If there is a pressure difference between the two sides of the vacuum chamber, any material will always permeate some gas more or less. The permeability is related to the type of gas and the type of material, and the difference is very large. For example, iron and nickel have higher permeability to hydrogen.

    The surface of the substance adsorbs (or freezes) organic compounds such as water and oil. When the substance is in a vacuum environment, the adsorbed substance on the surface will slowly volatilize or sublime into a gas and gradually release it. Especially for materials with a large surface area such as porous, the gas released will greatly affect the degree of vacuum. To achieve a higher degree of vacuum, materials with less gas must be selected. If no gas is introduced from the outside, the pressure that the vacuum chamber can reach after a sufficient time is determined by the exhaust speed and the amount of gas released.

    The outgassing of materials is a very complicated process. First of all, the gas emitted from the surface of the object may be adsorbed by other surfaces, where it will become a new source of gas emission. Secondly, there is continuous gas diffusion inside the object to the surface. The amount of gas emitted will continue to decrease over time, but it is impossible to reach zero. Therefore, according to the required pressure, until the gas release is negligible, it must be vented continuously, and to maintain the gas pressure, it must be continuously vented.

    The use of brass materials should be avoided as far as possible in the ultra-high vacuum, because brass contains a large amount of Zn, and the vapor pressure of Zn is high. If the temperature of brass in vacuum rises, a large amount of Zn will be released, which will pollute the vacuum system.

    If the amount of gas released is proportional to the amount of adsorption, the outgassing rate should decrease exponentially with time, but in fact the outgassing rate decreases in inverse proportion with time. This phenomenon also proves that there are many complicated intermediate states in the process of gas adsorption and release. In general, the outgassing rate of metal and glass is smaller, while resin and rubber are larger. Therefore, in the field of high vacuum, try to avoid resin and rubber.

    Regarding the gas emission, a brief summary is as follows:

    (1) To achieve high vacuum, materials with lower outgassing rate must be selected;

    (2) The amount of gas emitted decreases with time, but it cannot reach zero;

    (3) Generally, the outgassing rate decreases in the order of rubber, resin, ceramic, metal, and glass;

    (4) The main component of the emitted gas is water vapor;

    (5) The outgassing rate is related to the environment and surface condition of the material, and it is difficult to accurately predict.

(huiputech)