Important technical indicators of vacuum material performance

    The most important indicator of the vacuum performance of the components used in the ultra-high vacuum system is the outgassing rate of the material.

    Almost all materials that have been exposed to the atmosphere have adsorbed one or more layers of gas molecules on the surface. Experiments have proved that polarized molecules like water can condense up to hundreds of molecular layers. A monomolecular layer is adsorbed on the surface of 1cm², and there are about 5×10¹⁴ molecules. These molecules can form a pressure of about 1Pa in a volume of 1cm³ at room temperature.

    The desorption (outgassing) rate of the adsorbed gas in vacuum is related to the space gas pressure and the wall temperature. The continuously desorbed surface is continuously contaminated by the space gas molecules. Therefore, the desorption rate decreases exponentially with the pumping time. During pumping, only 0.1%~0.01% of surface molecules are desorbed into the space every second. It is not difficult to understand why the pumping curve starts to slow down after 10^-3Pa.

    Baking is the main method to reduce the outgassing rate of materials. Therefore, the materials selected for ultra-high vacuum components must be able to withstand high-temperature baking to meet the requirements of ultra-high vacuum systems for outgassing rates.

    Secondly, the saturated vapor pressure of the materials used in the parts should be low. The sealing materials usually used for high vacuum, such as ordinary rubber, sealing grease, and metal materials such as zinc and brass are not suitable for ultra-high vacuum.

    Another important vacuum index of UHV system components is air tightness. The biggest difference between the leakage rate (referred to as the leakage rate) and the outflow rate of a component is that the leakage rate is a constant air source and has nothing to do with the pumping time. The air outlet rate gradually decays with the pumping time.

    The air leakage rate is an important factor that affects the acquisition of ultra-high vacuum. The equilibrium pressure formed by the air leakage should be far less than the equilibrium pressure of the material out of the air. For dynamic ultra-high vacuum systems, the total air leakage rate of the system is usually controlled after baking. The air leakage rate of all materials in the vacuum chamber is 1/10~1/100. The air leakage rate of movable parts (flanges or valves) requires that the sealing part can still maintain a reliable seal after repeated disassembly (or long-term work). One point is still difficult to achieve.

(Huiputech)