Inhalation and deaeration of solids

    In the atmospheric state or in the low vacuum area, when the solid is in contact with the gas, air will be sucked on the surface and inside of the solid. In the high vacuum area, the absorbed or adsorbed gas will be partially or completely released gradually. In vacuum technology, this phenomenon of suction and degassing is not only related to the acquisition and maintenance of vacuum, but also affects the accuracy and reliability of vacuum measurement results. Therefore, it is very important to understand and master this phenomenon. There are two situations for solid gettering: one is getting on the solid surface, which is called adsorption, and the other is getting inside of the solid, which is called absorption. The combined effect of adsorption and absorption is collectively referred to as absorption. The reverse process of solid deflation and absorption is called desorption.

    (1) Adsorption. As we all know, there is a relatively strong binding force between the internal molecules of a solid. At room temperature, these molecules are combined in a certain arrangement to form a solid structure with a certain shape. Inside the solid, each molecule is in equilibrium under the action of binding forces from all directions. However, the force of the molecules on the solid surface in one direction outward is not saturated. Therefore, the entire surface is covered with outwardly extending suction, that is, the residual force field, whose acting distance is about one molecular diameter. Therefore, when gas molecules collide with the solid surface, they enter the scope of the residual force field, causing the gas molecules to form adsorption on the solid surface. Adsorption is mainly related to the following factors:

    ①The larger the solid surface area, the rougher the surface condition, the stronger the adsorption capacity, especially the material with a large number of pores or powder has a high adsorption capacity. Such as activated carbon, silica gel, activated alumina, molecular sieves, phosphorus pentoxide, etc.;

    ② The adsorption force decreases rapidly as the distance between gas molecules and the solid surface increases. Therefore, generally the adsorption layer is only a single molecular layer. When the temperature is lower than the critical temperature of the gas, a gas film composed of multiple layers of gas molecules will appear. In addition, materials with a large number of micropores can also produce multi-molecular layer adsorption under low pressure;

    ③When the temperature is constant and the pressure is low, the adsorption effect increases with the increase of the pressure. When the pressure increases to a certain value, the adsorption performance of the solid tends to be saturated;

    ④ When the pressure is constant, the lower the temperature, the greater the adsorption capacity;

    ⑤The lower the boiling point of the gas, the smaller the adsorption capacity;

    ⑥At a certain temperature, the adsorption time under low pressure is inversely proportional to the pressure.

    (2) Absorption. Gas molecules penetrate into the solid to form a solid solution or produce a chemical reaction to form a new solid compound. This phenomenon is called absorption. Absorption is also related to gas pressure and temperature, but it does not have the general regularity like adsorption phenomenon.

    (3) Desorption. The gas molecules absorbed by the solid will leave the solid and be released again under certain conditions, which is desorption. The difficulty of desorption is related to the type of gas, solid material and surface state. But the general rule is that under a certain pressure, as the temperature increases, the desorption speed increases. This is because each adsorbed gas molecule must be desorbed from the solid surface, and a certain amount of energy must be obtained to overcome the binding force between it and the solid surface molecule. This energy is called heat of desorption. Therefore, when the temperature of the solid material is increased and the energy obtained by the adsorbed gas molecules is greater than the required heat of desorption, the gas molecules are desorbed. Heating at a lower pressure can accelerate the desorption.

(Huiputech)