Vacuum degassing of materials

    For general vacuum equipment, the venting of materials is the most important source of gas in the vacuum system. Therefore, the purpose of vacuum degassing is to remove as much of these impurities as possible and reduce the gas content in the material.

    The solubility of a gas in a metal is a function of ambient pressure and temperature. There are two types of changes in solubility with temperature. When there is an endothermic effect when dissolving, the solubility increases with the increase of temperature; when the dissolution is exothermic, the solubility decreases with increasing temperature.

    The dissolution of gas in metal is a reversible process. When the metal is exposed to a vacuum environment, the original dynamic equilibrium state is destroyed, and the gas tends to dissolve. The process of de-dissolving gas is also determined by the diffusion rate of impurities. Because the diffusion rate of impurities is very small, and the metal has a certain thickness, it can be treated as the diffusion of an infinitely thick solid.

    For the case that the solubility increases with the increase of temperature, increasing the degassing temperature has little effect on the degassing effect. In fact, as the temperature rises, although the concentration of gas in the material increases, under high vacuum conditions, the increase in concentration is small, and at the same time the diffusion of gas is accelerated, and it can also reach equilibrium with the external pressure in a short time. Therefore, the key to vacuum degassing is to increase the working vacuum of the degassing equipment. Generally, the working vacuum of the material is required to be above 10^-3Pa when degassing.

    Because the degassing rate is related to the temperature, when designing the vacuum system, the temperature data in actual use must be selected. If there is no such data, it can be estimated based on the values at two different temperatures. The outgassing rate changes exponentially, so the outgassing volume is a slow changing function of time (that is, when the time is extended by an order of magnitude, the outgassing rate decreases more slowly). The material that has been outgassed (after degassing) can be re-inhaled and restored to its original state after being exposed to the atmosphere for a long time. If a vacuum system that operates frequently is exposed to the atmosphere for a short period of time (for example, within 1h) between two operations, it can be equivalent to a 10h outgassing time in a vacuum. Therefore, for the vacuum system, in order to reduce the outgassing rate and shorten the evacuation time, it should always be kept in a vacuum state.

    In addition, the outgassing rate of the material is not only related to the nature of the material and the outgassing time experienced, but also related to the material's manufacturing process, material storage environment and surface pretreatment methods (such as cleaning, baking, gas discharge bombardment, surface treatment). Etc.) have a lot to do. For example, for a clean surface, the higher the finish, the less water vapor will be absorbed; baking in dry nitrogen or air can form a dense light yellow oxide film on the surface of stainless steel, which can also reduce outgassing, and The contaminants on the surface can be oxidized into gas or burned off; when using organic solvents to degrease, the monolayer contamination on the surface cannot be removed, and can only be removed by baking under vacuum. For example, baking in a vacuum environment with a temperature above 200°C can effectively remove moisture, but to effectively remove hydrogen, vacuum baking must be performed at a temperature above 400°C. Based on the research on the outgassing of materials, people have reached the following consensus:

    (1) Different varieties of similar materials, different manufacturing and pretreatment processes have a great influence on the gas content;

    (2) Among the various pretreatment methods, hydrogen burning and vacuum treatment (baking, annealing, smelting) have the best degassing effect. Correct surface plating and surface corrosion are also beneficial. The effect of chemical cleaning on reducing outgassing is not significant, but the initial chemical treatment of materials and parts should not be ignored, so as to avoid the pollution of the hydrogen furnace and vacuum container during further hydrogen burning or vacuum degassing, resulting in the re-processing of other materials in the future Pollution.

    (3) The material that has been degassed cannot be touched directly by hand, otherwise the total degassing will be restored.

    (4) The thicker the material, the lower the temperature, and the slower the outgassing rate decays. This situation complies with Fick's law of diffusion.

(Huiputech)