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What is gas adsorption?

Author: BOWAH



The phenomenon that the gas or vapor is floated by the solid surface and attached to the surface to form a single or multi-layer gas molecular layer is called adsorption. The solid that can trap the gas is called the adsorbent, and the gas component that is adsorbed is called the adsorbate. Adsorption occurs because of the presence of a force field on the surface of the adsorbent.

  According to the different adsorption forces, gas adsorption can be divided into physical adsorption and chemical adsorption. Physical adsorption is when gas molecules are attached to the surface of the adsorbent by the attraction of van der Waals force. It is similar to the liquefaction process of gas. However, it is generally non-selective to the adsorbed gas. The lower the temperature, the greater the adsorption capacity, which can form multi-layer adsorption. The inhalation effect of molecular sieve adsorption pump and cryopump belongs to physical adsorption. Chemical adsorption is achieved by the formation of adsorption chemical bonds between solid surface atoms and gas molecules. It is similar to chemical reactions. Compared with physical adsorption, chemical adsorption is characterized by strong adsorption, large adsorption heat, stable and difficult desorption, and selective adsorption. When the temperature is high, the number of gas molecules chemically adsorbed increases, and it can only form a single layer of adsorption against the surface (physical adsorption can also be formed on the chemically adsorbed molecules), and the inhalation of the getter in the sputtering ion pump and the electron tube Effects include chemical adsorption.

  The reverse process of gas adsorption, where the adsorbed gas or vapor is released from the surface and re-entered into space, is called desorption or desorption. Desorption phenomena can be naturally occurring or artificially accelerated. There are two cases of natural desorption. One is from a macroscopic average point of view. After each adsorbed gas molecule stays on the surface for a period of time, it will desorb and fly back to space. At this time, other gas molecules will also undergo new adsorption. Under the condition of constant gas temperature and pressure, the adsorption rate is equal to the desorption rate, and the amount of gas adsorption on the surface remains constant; in another case, in the process of vacuuming, the space gas pressure decreases continuously, and the desorption rate on the surface is large. With the dry adsorption rate, the amount of gas adsorption gradually decreases, and the gas is slowly released from the surface. This phenomenon is called the outgassing or outgassing of the material in a vacuum. The most concerning issues in engineering are the total amount of gas adsorption on the surface and the outgassing rate during evacuation. However, there is no very accurate and general calculation method. It can only be concluded from practical experience: in the low vacuum stage, the surface adsorption and Compared with the space gas, the surface outgassing is very small, and its influence can be ignored; in the medium vacuum stage, the surface outgassing amount is close to the space gas amount, and the two should be paid equal attention to; in the high vacuum and even ultra-high vacuum stage, Surface outgassing (excluding system air leakage) has become the main gas load, and the speed of outgassing directly affects the evacuation time.

  The occurrence of gas desorption is consciously promoted by artificial means, which is called degassing or degassing in vacuum technology. Manual degassing can shorten the time for the system to reach the ultimate vacuum; a clean surface can be obtained that is not covered by gas molecules. The heating and baking degassing method promote desorption by increasing the temperature of the suction surface, increasing the thermal motion energy of molecules, and exhausting while heating. It is often used in the degassing of the inner surface and internal components of the ultra-high vacuum system container and the filament in the vacuum electronic device. The degassing of internal metal components; the degassing method of ion bombardment is generally to form a gas discharge in the space to generate an ion region, so that high-energy ions bombard the solid surface to be cleaned, generate gas sputtering, and desorb the adsorbed gas, which is A very effective, simple and rapid degassing method, widely used in thin film technology, surface science, and other equipment with gas discharge conditions or ion sources.

What is gas adsorption?
​The phenomenon that the gas or vapor is floated by the solid surface and attached to the surface to form a single or multi-layer gas molecular layer is called adsorption. The solid that can trap the
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