The process of gas infiltration, diffusion, passage and overflow of the solid barrier from the denser side to the less dense side becomes permeation. The steady-state flow rate in this case is called permeability. Permeability is related to the type of gas and material. For metals, some metals (such as stainless steel, copper, aluminum, molybdenum, etc.) have a small gas permeability coefficient, which can be ignored in most practical applications. But for some metals (such as iron, nickel, etc.), hydrogen has a higher permeability to them. The permeability of hydrogen to steel increases with the increase of carbon content, so it is better to choose low-carbon steel as the material of the vacuum chamber; in addition, some metals have selective permeability to gas, such as hydrogen can easily permeate through palladium, and oxygen is easy to cast. Over silver and so on. This property can be used for gas purification and vacuum leak detection. The penetration of gas into glass, ceramics, etc. is generally carried out in the form of a molecular state. The permeation process is related to the diameter of gas molecules and the size of the micropores inside the material. The pore diameter of quartz glass containing pure silica is about 0.4nm. Other glasses are filled with alkali metal ions (potassium, sodium, barium, etc.) to make their effective pore diameter smaller. Quartz glass has a large permeability, while its permeability to other glasses is small. Since the diameter of helium molecules is the smallest among various molecules, the penetration of helium into quartz glass is the largest among gas-solid partners.
The permeation process of gas into organic materials (such as rubber and plastic) is generally carried out in a molecular state. Because the micropores of organic materials are relatively large, the permeability of gas to organic materials is much greater than that of glass and metals.
Outgassing performance of materials
Any solid material can dissolve and absorb some gas during the manufacturing process and when stored in the atmosphere. When the material is placed in a vacuum, the original dynamic balance is destroyed, and the material will outgas due to dissolution and desorption. The commonly used outgassing rate unit is Pa*L/(s*cm2). The outgassing rate is usually positive with the gas content and temperature in the material. The unit of the total outgassing volume: Pa*L/cm2< br> (1) Venting at room temperature. The main component of most organic materials outgassing is water vapor, which is characterized by a higher rate of abandonment and a slower decay over time. Therefore, such materials are generally not suitable for use as internal parts of vacuum vessels. The outgassing rate of metals, glass, and ceramics is low, and the decay rate is also faster with time. The room temperature outgassing of glass and ceramics mainly comes from the surface, the main outgassing component is water vapor, followed by CO and CO2. After the glass is baked and heated, the water vapor in the surface oxide film can be basically removed, so that its room temperature outgassing rate is significantly reduced. The outgassing process after the gas adsorbed on the surface is removed is determined by the diffusion in the body. Generally, the outgassing components in the body are H2, N2, CnHn, CO, CO2, and O2, most of which are H2.
(2) High temperature venting. Certain structural materials, such as electrodes, targets, evaporation sources, heating devices, and other equipment, are often at a high temperature during the process of the vacuum system. It is generally believed that the high-temperature outgassing of materials is mainly determined by the diffusion process in the body, and the amount of gas desorbed on the surface only accounts for a small part of the total outgassing. Glass, ceramics. Except for the acceleration of the diffusion process, the high-temperature deflation of mica is not essentially different from the normal-temperature deflation. The diffusion of gas from high-temperature metal bodies is different. Since the gas dissolved in the metal is in an atomic state, the molecular gas emitted in a vacuum is often formed through surface reactions. Generally, the types of metal outgassing are H2, CO, CO2 and N2, O2, and most of the previous four. Among them, H2 and N2 first diffuse and escape in the atomic state, and then combine in the molecular state on the surface. CO and CO2 are formed by the reaction of C that diffuses to the surface with metal oxides on the surface or O2 and H2O in the gas phase. There are also some metals (such as Ni, Fe) that are mainly controlled by the diffusion of oxygen in the body. Therefore, decarburization of metals can reduce the outgassing of CO and CO2. Some H2O comes directly from the surface oxide layer, while others are synthesized by the reaction of hydrogen diffused in the body and oxides.
The surface layer of glass and metal is also an important source of high temperature outgassing. To this end, various surface treatment processes, such as chemical cleaning, are used. Organic vapor degreasing, polishing, corrosion, atmospheric baking and oxidation, etc., can greatly reduce material outgassing. In addition, the outgassing rate of the material is not only related to the experienced outgassing time, but also has a lot to do with the surface pretreatment method and surface condition of the material. For example: For a clean surface, the higher the surface finish, the less water vapor will be absorbed; for example, when the surface is cleaned and degreasing with organic solvents, the monomolecular layer pollution on the surface cannot be removed, only Rely on baking under vacuum to get rid of. For example, baking in a vacuum environment with a temperature above 200°C can effectively remove water vapor, but to effectively remove hydrogen, vacuum baking must be carried out at a temperature above 400°C. For the design of the vacuum system, it is not enough to have only the data on the outgassing rate of the material, because there are many vacuum valves with selective air extraction capabilities, so if you can further know the various gas components in the outgassing of the material Ratio, you can select a suitable vacuum pump in a targeted manner, and get a more reasonable design
The vapor pressure and evaporation (sublimation) rate of the material
At a certain temperature, in a closed vacuum space, as a result of liquid (or solid) vaporization, the vapor density of the space gradually increases. When a certain vapor pressure is reached, the surface of the liquid (or solid) will be separated from the liquid (or solid) within a unit of time. The number of molecules is equal to the number of condensing molecules returning from the space to the surface of the liquid (or solid), that is, the evaporation (or sublimation) rate and the condensation rate are dynamically balanced. At this time, the vaporization can be considered to stop, and the vapor pressure at this time is called the temperature Below, the saturated vapor pressure of the liquid (or solid).
There is the following relationship between vapor pressure Pv and evaporation (sublimation) rate W:
W=0.058Pv√(M/T) where W; evaporation (sublimation) rate, g/(cm2·s)Pv; Saturated vapor pressure at temperature T PaM; molecular weight, g/mol In vacuum technology, the vapor pressure and evaporation (sublimation) rate of materials are parameters that need attention. Such as: vacuum grease, the saturated vapor pressure of the hot filament of the vacuum gauge can be the origin of the ultimate vacuum; the sublimation rate of the vacuum coating material and getter is a parameter that needs to be considered when designing vacuum coating equipment and getter pumps ; The saturated vapor pressure of the cryogenic liquefied gas is a parameter related to the limit pressure of the cryogenic condensate pump. Obviously, materials with high vapor pressure within the working temperature range of the vacuum system cannot be used. Within the working temperature range, the saturated vapor pressure of all materials facing vacuum should be low enough, and the vacuum system should not fail to reach the required working vacuum due to its own vapor pressure or outgassing characteristics. Although the vapor pressure of some materials at room temperature is very low and sometimes not noticeable, as the temperature increases, the vapor pressure can eventually rise to the measured value. For example, some insoluble metals need to be raised above 1500°C to measure their vapor pressure. However, the vapor pressure of some metals (such as zinc, cadmium, lead, etc.) at 300 to 500°C is very high, exceeding the pressure required by the high vacuum system. For example, the vapor pressure of cadmium at 300°C is 10Pa, so these metals (or their alloys) cannot be used in baked high vacuum systems or ultra-high vacuum systems. Some other materials, such as certain plastics or rubbers, cannot be used in an ultra-high-altitude environment because they cannot be heated and baked and the steam pressure is too high.