Making a vacuum chamber

    The vacuum chamber is a container that can keep the inside in a vacuum state. The volume, material and shape of the vacuum chamber should be considered in the production of the vacuum chamber.

    Stainless steel is currently the main structural material of ultra-high vacuum systems. Among them, 300 series stainless steel is a low carbon steel containing 10% to 20% Cr. It has excellent corrosion resistance, low outgassing rate, non-magnetic, good weldability, low electrical and thermal conductivity, and can be at -270℃~900 The advantages of working in the temperature range of ℃, etc., are widely used in high vacuum and ultra-high vacuum systems.

    In recent years, in order to reduce the manufacturing cost of the vacuum cavity, the use of cast aluminum alloy to make the cavity has gradually become popular. In addition, titanium alloys are used to make vacuum chambers for special purposes.

    In order to reduce the surface area of the inner wall of the cavity, sandblasting or electrolytic polishing are usually used to obtain a flat surface. The cavity of the ultra-high vacuum system is more surface treated by electrolytic polishing.

    Welding is one of the most important links in the manufacture of vacuum chambers. In order to avoid the chemical reaction between the molten metal and oxygen in the atmosphere and thus affect the welding quality, argon arc welding is usually used to complete the welding. Argon arc welding refers to spraying the shielding gas argon around the tungsten electrode during the welding process to prevent the oxidation reaction of the molten high-temperature metal.

    In principle, the argon arc welding of the ultra-high vacuum chamber must be internal welding, that is, the welding surface is on the vacuum side, so as to avoid the existence of dead angles and virtual leakage. The vacuum chamber does not allow double welding and double sealing inside and outside.

    The surface of the inner wall of the vacuum chamber adsorbs a large number of gas molecules or other organic substances, which becomes a gas release source that affects the degree of vacuum. In order to achieve ultra-high vacuum, the cavity should be baked at a high temperature of 150~250℃ to promote the release of gas on the surface and inside of the material as soon as possible. Baking methods include winding a heating tape on the outer wall of the cavity, fixing an armored heating wire on the outer wall of the cavity, or directly placing the cavity in a baking tent. A more economical and simple baking method is to use a heating belt, and the outside of the heating belt is wrapped with aluminum foil to prevent heat loss and also make the cavity uniformly heated.

    When the newly completed cavity is baked for the first time, it generally takes a week, and the individual baking time can be appropriately reduced after repeated baking. In order to measure the vacuum degree more accurately, the vacuum gauge should also be degassed after stopping the baking. If the vacuum pump capacity is sufficient and the baking time is sufficient, the vacuum degree can be increased by several orders of magnitude after baking.

    An atmospheric pressure produces a pressure of about 1kgf on an area of 1cm2, which is a pressure of 1T for a flange with a diameter of 20cm. For the cylindrical or spherical cavity, the pressure is dispersed due to the particularity of the structure, and the wall thickness of the cavity is 2~4mm without deformation. However, for a square cavity, the flat plate on the side has to withstand a ton of pressure, and it is necessary to increase the wall thickness or install stiffeners to prevent deformation. The square cavity is generally heavier than the cylindrical and spherical cavity, and the price is higher.

(huiputech)