As we all know, oil diffusion pump is a kind of pump often used in high vacuum system. Taking some technical measures, ultra-high vacuum can be obtained. So, the question is, what are the specific methods?

Its methods are:

1. Select diffusion pump oil with good performance

The pump oil has a great influence on the ultimate pressure of the oil diffusion pump. If the No. 3 diffusion pump oil is used, its ultimate pressure can only reach 10-5Pa; and if 275 (705) silicone oil is used instead, its ultimate pressure can reach 10-7Pa, and the ultimate pressure is increased by 2 orders of magnitude. Secondly, the partial pressure of the pumped gas also changes with the pump oil, which is the influence of the pump oil composition. To obtain ultra-high vacuum with oil diffusion pump, it is necessary to choose diffusion pump oil with good performance.

Since diffusion pump oil is so important, how should we choose it?

The requirements for diffusion pump oil are: 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. .

The high vacuum oil diffusion pump generally uses No. 3 diffusion pump oil, and the ultra-high vacuum adopts silicone oil, which requires an ultra-high vacuum system that does not contain hydrocarbons, and the diffusion pump adopts fluorine oil.

2. the use of reasonable baking technology

High-temperature baking is required to obtain ultra-high vacuum systems with oil diffusion pumps, in order to reduce the outgassing rate of materials. The material can absorb and adsorb some gases in the atmosphere. When it is in a vacuum, it will desorb and release the gas slowly. In order to accelerate the desorption, baking is used.

bake? Sounds easy, doesn't it? No, no, baking is also important.

Different materials have different baking temperatures. The principle is that it is better to have a higher temperature, which can accelerate the outgassing, but also consider whether the material can withstand high temperature. The baking temperature should be selected appropriately. Usually metal materials, such as stainless steel, the baking temperature is 250 ° C - 450 ° C; non-metallic materials baking temperature is 80 ° C - 100 ° C. The baking time is more than 4h-8h. The baking time of extremely high vacuum is more than 24h. Performance of a typical metal oil diffusion pump system (after baking) and residual gas components in a metal oil diffusion pump ultra-high vacuum system.

3. Increase the cold trap device to prevent the backflow and migration of pump oil

The phenomenon that the pump oil flows through the inlet of the pump to the pumped container is called backflow. The phenomenon of oil molecules moving along the wall into the pumped container is called back migration. Regardless of reflux or reverse migration, it affects 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. It can not only effectively capture the reflux vapor and part of the cracked product of the diffusion pump, but also can extract the vapor from the vacuum chamber. Ultra-high vacuum systems equipped with various types of traps. After thorough high temperature baking and degassing, the ultra-high vacuum degree of 10-8Pa ~ 10-10Pa (or higher) can be obtained. The commonly used cold trap is the liquid nitrogen cold trap, because at the temperature of liquid nitrogen, the vapor pressure of the pump oil is much lower than the requirement of ultra-high vacuum. The design of the 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 the liquid nitrogen cold trap is installed, the pumping speed of the oil diffusion pump is reduced by no more than 40%.

4.reduce the back diffusion of the pumped gas

The back-diffusion of the pumped gas is one of the factors that limit the ultra-high vacuum obtained by the diffusion pump. Gas anti-diffusion is related to molecular mass, the smaller the mass, the easier the anti-diffusion. Diffusion pump is easy to reverse diffusion of hydrogen, and there are two methods to prevent hydrogen reverse diffusion: one is to reduce the exhaust pressure at the outlet of the diffusion pump and use two diffusion pumps in series to achieve; the other is to install it on the inlet pipeline of the diffusion pump Liquid nitrogen-cooled titanium sublimation traps to trap back-diffused hydrogen.