A Method of Obtaining Ultra-High Vacuum by Oil Diffusion Pump
Oil diffusion pump is a kind of pump commonly used in high vacuum system. Ultra-high vacuum can be obtained by adopting some technical measures.
1. Selection of Diffusion Pump Oil with Good Performance
Pump oil has a great influence on the limit pressure of oil diffusion pump. If the No. 3 diffuse pump oil is used, its limit pressure can only reach 10 ^ - 5Pa, but when 275 silicone oil is used, its limit pressure can reach 10 ^ - 7Pa, and the limit pressure can be increased by two orders of magnitude. Secondly, the partial pressure of the pumped gas varies with the difference of the pump oil, which is the influence of the composition of the pump oil. In order to obtain ultra-high vacuum with oil diffusion pump, it is necessary to select diffusion pump oil with good performance. The requirements for diffusion pump oil are: high relative molecular weight, low vapor pressure at room temperature, good thermal stability and oxidation resistance, low solidification point and low temperature viscosity, non-toxic, corrosion resistance and low cost.
At present, the high vacuum oil diffusion pump generally uses No. 3 diffusion pump oil, and the ultra-high vacuum uses silicone oil, which requires the ultra-high vacuum system without hydrocarbons, and the diffusion pump uses fluorine oil.
2. Adopting Reasonable Baking Technology
Ultra-high vacuum system obtained by oil diffusion pump must be baked at high temperature in order to reduce the material outgassing rate. Material can absorb and absorb some gases in the atmosphere. When it is in vacuum, it will desorb and release the gases slowly. In order to accelerate desorption, baking is used.
The choice of baking temperature varies with the material. The principle is that the higher the temperature is, the better it can speed up the outgassing, but it is also necessary to consider whether the material can withstand the high temperature. The appropriate baking temperature should be chosen. Usually, the baking temperature of metal materials, such as stainless steel, is 250 - 450C, and that of non-metallic materials is 80 - 100 C. The baking time is more than 4h-8h. The baking time in extremely high vacuum is more than 24 hours. Performance of typical metal oil diffusion pump system (after baking) and residual gas composition in ultra-high vacuum system of metal oil diffusion pump.
3. Increase cold trap device to prevent pump oil reflux and migration
The phenomenon of pumping oil flowing through the inlet of the pump to the pumped container is called reflux. The phenomenon of oil molecules moving along the wall into the pumped container is called reverse migration. Both reflux and reverse migration affect the limit pressure of vacuum system. Cold traps are widely used in ultra-high vacuum systems. They are installed between the inlet of the pump and the vacuum chamber. They can not only effectively capture the back-flow vapor and some decomposition products of the diffusion pump, but also extract the vapor from the vacuum chamber. Ultra-high vacuum system with various traps. After thorough high temperature baking degassing, the ultra-high vacuum of 10 ^ - 8Pa to l0 ^ - 10Pa (or higher) can be obtained. The common cold trap is liquid nitrogen cold trap, because at liquid nitrogen temperature, the vapor pressure of pump oil is much lower than the requirement of ultra-high vacuum. The design of liquid nitrogen cold trap must ensure that the molecules passing through the trap will encounter the cold surface, and the structure should be reasonable. When the liquid nitrogen cold trap is installed, the pumping speed of the oil diffusion pump decreases by no more than 40%.
4. Reducing Backdiffusion of Pumped Gases
The reverse diffusion of pumped gas is one of the factors that restrict the diffusion pump to obtain ultra-high vacuum. Gas reverse diffusion is related to molecular mass. The smaller the mass, the easier it is to reverse diffusion. Diffusion pump pumping hydrogen is easy to reverse diffusion, there are two methods to prevent hydrogen reverse diffusion: one is to reduce the exhaust pressure at the outlet of diffusion pump, which is realized by two diffusion pumps in series; the other is to install liquid nitrogen cooled titanium sublimation trap on the intake pipeline of diffusion pump to capture hydrogen reverse diffusion.