Diffusion pump

- Apr 09, 2020-

Diffusion pump

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  As we all know, oil diffusion pump is a kind of pump often used in high vacuum system. So, the question is, what are the methods? The methods are as follows: first, the choice of good performance of the diffusion pump oil on the oil diffusion pump limit pressure has a great impact. If no. 3 diffusion pump oil is used, the ultimate pressure can only reach 10-5pa; When 275 silicone oil is used, the ultimate pressure can reach 10-7pa, and the ultimate pressure can be increased by 2 orders of magnitude. Secondly, the partial pressure of the pumped gas also changes with the difference of the pump oil, which is the influence of the composition of the pump oil. Therefore, the oil diffusion pump with good performance must be selected to obtain ultra-high vacuum. Since the diffusion pump oil is so important, how should we choose? The requirements of the diffusion pump oil is: the pump oil relative molecular weight to be large, room temperature vapor pressure to be low, thermal stability and oxidation resistance to be good, freezing point and low temperature viscosity to be low, non-toxic, corrosion resistance, cost to be low. High vacuum oil diffusion pump generally USES no. 3 diffusion pump oil, ultra-high vacuum USES silicone oil, requires the ultra-high vacuum system without hydrocarbons, the diffusion pump USES fluorine oil. Second, the use of reasonable baking technology oil diffusion pump to obtain ultra-high vacuum system should be carried out high temperature baking, the purpose is to reduce the rate of the material. Materials in the atmosphere can absorb and absorb some gas, when in a vacuum, it will produce desorption, gas slowly released, in order to accelerate desorption, using baking.                        

Bake? Sounds simple, doesn't it? No, no, no, no, no, no, no, no, no, no. The principle is higher temperature is better, can speed up the air, but also to consider whether the material can withstand the high temperature. Bake at the right temperature. Usually metal materials, such as stainless steel, baking temperature of 250- 450C; The baking temperature of nonmetallic materials is 80-- 100. Baking time above 4h - 8h. The baking time of extremely high vacuum is more than 24h. Performance of typical metal oil diffusion pump system (after baking) and residual gas components in ultra-high vacuum system of metal oil diffusion pump. Third, add a cold trap device to prevent the backflow of pump oil and transfer the pump oil through the pump inlet flow to the pumped container phenomenon is called backflow. The phenomenon of oil molecules moving along the wall into the pumped container is called reverse migration. Both backflow and reverse migration affect the ultimate pressure of the vacuum system. Cold trap is widely used in ultra-high vacuum system, it is installed between the pump inlet and the vacuum chamber, not only can effectively capture the back flow of the diffusion pump steam and partial split solution, but also can remove the steam from the vacuum chamber. Ultra high vacuum system with various traps. Ultra-high vacuum of 10-8pa ~ l0-10pa (or higher) can be obtained after thorough high temperature baking and degassing. The commonly used cold trap is the liquid nitrogen cold trap, because at the liquid nitrogen temperature, the pump oil vapor pressure is much lower than the ultra-high vacuum requirements. The design of liquid nitrogen cold trap must ensure that the molecules passing through the trap will meet the cold surface, and the structure must be reasonable. After installing the liquid nitrogen cold trap, the pumping speed of oil diffusion pump is reduced no more than 40%.

Reducing the reverse diffusion of pumped gas is one of the factors that restrict the diffusion pump to obtain ultra-high vacuum. The anti-diffusion of gas is related to the molecular mass. The smaller the mass, the easier the anti-diffusion. There are two methods to prevent the reverse diffusion of hydrogen in the diffusion pump: first, reduce the exhaust pressure at the outlet of the diffusion pump, using two diffusion pumps in series to achieve; The second is to install a liquid-nitrogen-cooled titanium sublimation trap on the inlet pipe of the diffusion pump to catch the anti-diffusion hydrogen.


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