Oil diffusion pump is a kind of pump often used in high vacuum system, and some technical measures can be taken to obtain ultra-high vacuum, the specific methods are as follows:
1、Select diffusion pump oil with good performance
The pump oil has great influence on the ultimate pressure of oil diffusion pump. Such as the use of No. 3 diffusion pump oil, the ultimate pressure can only reach 10-5Pa; and change to 275 (705) silicone oil, the ultimate pressure can reach 10-7Pa, the ultimate pressure increased by 2 orders of magnitude. Secondly, the sub-pressure of the pumped gas also changes with the different pump oils, which is the influence of the pump oil composition. In order to obtain ultra-high vacuum with oil diffusion pump, it is necessary to choose diffusion pump oil with good performance: the relative molecular weight of the pump oil should be large, the vapor pressure at room temperature should be low, the thermal stability and oxidation resistance should be good, the freezing point and low temperature viscosity should be low, it should be non-toxic, corrosion-resistant, and the cost should be low.
2、Adopt reasonable baking technology
Use oil diffusion pump to obtain ultra-high vacuum system are to be baked at high temperature, the purpose is to reduce the outgassing rate of the material. Materials in the atmosphere can absorb and adsorb some gases, when in a vacuum, it produces desorption, so that the gas is slowly released, in order to accelerate the desorption, the use of baking.
3, increase the cold trap device to prevent the pump oil return and migration
Pump oil through the pump inlet to the pumped container is called the phenomenon of reflux. Oil molecules move along the wall into the phenomenon of being pumped container, called anti-migration. Both reflux and anti-migration affect the ultimate pressure of the vacuum system. Cold traps are widely used in ultra-high vacuum systems, which are installed between the pump inlet and the vacuum chamber, and can not only effectively trap the reflux vapors of the diffusion pump and part of the cracked material, but also remove the vapors from the vacuum chamber. Ultra-high vacuum system equipped with various types of traps. After thorough high-temperature baking and degassing, an ultrahigh vacuum of 10- 8Pa to l0-10Pa (or higher) can be obtained. A commonly used cold trap is the liquid nitrogen cold trap, because at liquid nitrogen temperatures, the vapor pressure of the pump oil is considerably lower than that required for UHV. The liquid nitrogen cold trap must be designed so that the molecules passing through the trap will all touch the cold surface, and at the same time be structurally sound. The pumping speed of the oil diffusion pump is reduced by no more than 40% when a liquid nitrogen cold trap is fitted.
4, reduce the pumped gas counter-proliferation
The counter-diffusion of pumped gas is one of the factors limiting the diffusion pump to obtain ultra-high vacuum. Gas counter-proliferation and molecular quality, the smaller the quality, the more likely to counter-proliferation. Diffusion pump pumping hydrogen is easy to counter-diffusion, to prevent hydrogen counter-diffusion method has two kinds: one, reduce the diffusion pump outlet exhaust pressure, using two diffusion pumps in series to realize; the second, in the diffusion pump inlet pipeline installation of liquid nitrogen cooled titanium sublimation trap to capture the counter-diffusion of hydrogen.

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