Overview of high temperature venting

    Except in individual cases, material outgassing is the main source of gas for high vacuum and ultra-high vacuum systems and sealed electric vacuum devices. With the outgassing of materials, there will be secondary processes such as surface chemical reactions. The research on the outgassing of vacuum materials mainly focuses on high-temperature outgassing. The reasons are as follows:

    1) For quasi-static vacuum devices (such as electron tubes, ion sources, etc.), the outgassing rate increases exponentially with temperature, so it is difficult to detect without high temperature.

    2) High temperature gas can release the total amount of gas in the material in a short period of time. Therefore, it is convenient to obtain the data of the gas concentration, select materials based on this, and put forward the specifications of the material pretreatment process.

    3) Some parts of high-power electric vacuum devices that are most afraid of outgassing are in high-temperature working conditions. However, some structural materials such as electrodes, targets, evaporation sources, heating devices and other devices are often in a high temperature state during the process of the vacuum system.

    It is generally believed that high-temperature outgassing is mainly determined by the diffusion process in the body, and the amount of gas desorbed on the surface only accounts for a small part of the total outgassing. glass. The high-temperature degassing of ceramics and mica has no essential difference with the normal temperature degassing except for the acceleration of the diffusion process. The gas diffused by the high temperature body of metal is different. Because the gas dissolved in the metal is in the atomic state, the molecular gas released in the vacuum is often formed through surface reaction. The general types of metal outgassing are H2, CO, CO2, H2O, N2 and O2, and most of them are H2, CO, CO2, and H2O. Among them, H2 and N2 first diffuse and escape in the atomic state, and then combine in the molecular state on the surface. CO and CO2 are formed by the reaction of C that diffuses to the surface with metal oxides on the surface or O2 and H2O in the gas phase. There are also some metals (such as Ni, Fe) that are mainly controlled by the diffusion of oxygen in the body. Therefore, decarburization of metals can reduce the outgassing of CO and CO2. Some H2O comes directly from the surface oxide layer, while others are synthesized by the reaction of hydrogen diffused in the body and surface oxides.

    The surface layer of glass and metal is also an important source of high temperature outgassing. To this end, various surface treatment processes, such as chemical cleaning, organic vapor degreasing, polishing, corrosion, atmospheric baking and oxidation, etc., can greatly reduce the outgassing of materials.

(Huiputech, huiputech)