For general vacuum equipment, material release is the main gas source in the vacuum system. Therefore, the purpose of vacuum degassing is to remove as many impurities as possible and reduce the gas content in the material.
The solubility of gases in metals is a function of ambient pressure and temperature. There are two types of changes in solubility with temperature: those that exhibit endothermic effects during dissolution, and those that increase with temperature; It releases heat during dissolution, and its solubility decreases with increasing temperature.
The dissolution of gases in metals is a reversible process. When metal is exposed to a vacuum environment, its original dynamic equilibrium state is disrupted, and the gas tends to dissolve. The process of dissolved gas is also determined by the diffusion rate of impurities. Due to the low diffusion rate of impurities and the thickness of the metal, it can be treated approximately as the diffusion gas of an infinitely thick solid.
For cases where solubility increases with temperature, increasing the degassing temperature has little effect on the degassing efficiency. In fact, with the increase of temperature, although the concentration of gas in the material increases, the increase in concentration is very small under high vacuum conditions, and at the same time, the diffusion of gas accelerates, which can also reach equilibrium with external pressure in a short period of time. Therefore, the key to vacuum degassing is to increase the working vacuum degree of the degassing equipment, which generally requires the working vacuum degree of the material during degassing to be above 10-3Pa.
Because the release rate is temperature dependent, it is necessary to use actual temperature data when designing a vacuum system. If there is no such data, estimation can be made based on the values at two different temperatures. The exhaust rate varies exponentially, so the exhaust volume is a slow changing function of time (i.e., as time extends by an order of magnitude, the exhaust rate decreases slowly). After being exposed to the atmosphere for a long time, materials that have already been degassed can be reabsorbed and restored to their original state. If a frequently operated vacuum system is exposed to the atmosphere for a short period of time (such as within 1 hour) between two runs, it can be equivalent to a 10 hour exhaust time in vacuum. Therefore, for a vacuum system, in order to reduce the air release rate and shorten the evacuation time, it should be kept in a vacuum state frequently.
In addition, the gas release rate of materials is not only related to the properties of the material and the duration of gas release, but also to the manufacturing process, storage environment, and surface pretreatment methods (such as cleaning, baking, gas discharge bombardment, surface treatment, etc.) of the material. For example, for clean surfaces, the higher the smoothness, the less water vapor is adsorbed; Baking in dry nitrogen or air can form a dense pale yellow oxide film on the surface of stainless steel, reduce gas release, and oxidize surface pollutants into gas or burn them off; When using organic solvents to remove grease, the surface monolayer contamination cannot be removed and can only be removed by baking under vacuum. For example, baking in a vacuum environment with a temperature above 200 ℃ can effectively remove water vapor, but to effectively remove hydrogen, vacuum baking must be carried out at a temperature above 400 ℃. Based on research on the amount of gas released from materials, the following consensus has been reached:
(1) The different varieties, manufacturing and pretreatment processes of similar materials have a significant impact on the gas content;
(2) Among various pretreatment methods, the best degassing effect is achieved by burning dry hydrogen and vacuum treatment (baking, annealing, melting). Proper surface plating and surface corrosion are also beneficial. The effect of chemical cleaning on reducing gas emissions is not very significant, but the preliminary chemical treatment of materials and parts cannot be ignored to avoid contamination of the hydrogen furnace and vacuum container during further hydrogen burning or vacuum degassing, which may lead to re contamination of other materials processed in the future.
(3) The material that has already been degassed cannot be touched directly by hand, otherwise it will restore the total amount of deflation.
(4) The thicker the material, the lower the temperature, and the slower the decay of the gas release rate. This situation conforms to Fick's diffusion law.
