Permanently sealed by vacuum

    Vacuum sealing is the basic technology to maintain the vacuum state, which can be divided into two types: motion sealing and fixed sealing. The motion seal mainly refers to the seal that conveys movement from the atmosphere to the vacuum, and the conveyed movement includes linear, rotating, etc. Fixed sealing can be divided into two ways: permanent sealing and non-permanent sealing.

    The most common permanent seals are between metal, glass and metal, ceramic and metal.

    The most commonly used vacuum system is the sealing between metals. The material is mainly stainless steel (SUS304\SUS316), and the sealing method is mostly argon arc welding. When welding, blow a large amount of inert gas argon into the welding part to avoid oxidation of the welding part.

    Metal materials that cannot be welded by argon arc welding (copper, etc.) or the sealing between different metal materials (stainless steel and copper) are usually vacuum brazed. Fill the metal joint with silver wax and place it in a vacuum furnace to gradually increase the temperature. The silver wax melts and penetrates into the metal joint, and then cools down and solidifies to complete the vacuum brazing. The difficulty of vacuum brazing lies in the need for special fixtures, and the advantage is that there is no deformation and no air bubbles in the welding part.

    When hard glass (borosilicate glass) and metal are joined, the joint is required to have a small coefficient of thermal expansion between room temperature and hundreds of degrees Celsius. The metal used for sealing is mainly cobalt alloy (Fe:Ni:Co=54:29:17). Cobalt alloy and other metals (stainless steel) are sealed by argon arc welding. The thermal expansion coefficient between cobalt alloy and cobalt glass is the same, and it can be sealed directly by glass processing technology. Various vacuum observation windows are made using this technique.

    Ceramic is an insulating material, and it is inseparable from the sealing of ceramic and metal when electrical signals need to be imported or measured. A more common method is to first coat Mo or Mn powder on the surface of the ceramic. After heat treatment in a mixed gas of H2 and N2, a thin Mo or Mn layer can be formed on the surface of the ceramic. A firm Ni or Cr layer is plated on it, and the electrode is sealed by vacuum brazing on the Ni or Cr layer. The main purpose of using Co alloy is to eliminate the deformation caused by thermal expansion.

(huiputech)