Oil diffusion pump to obtain ultra-high vacuum method
Oil diffusion pump is a kind of vacuum pump commonly used in the process of obtaining ultra-high vacuum. However, in order to obtain the highest vacuum in use, there are certain requirements. Let's know what oil diffusion pump needs to do in order to achieve ultra-high altitude.
The first is about diffusion pump oil. Diffusion pump oil has a great influence on the limit pressure of oil diffusion pump. In the case of using ordinary diffusion pump oil, the limit vacuum of oil diffusion pump may be 10 ^ - 5pa, but if 275 silicone oil is used, its limit pressure can reach 10 ^ - 7 PA. And the partial pressure of the extracted gas will also change with the pump oil. Therefore, in order to obtain the highest vacuum degree, the diffusion pump oil with good performance is essential. In the selection process, the relative molecular weight of pump oil should be as large as possible, the vapor pressure at room temperature should be low, in addition, the thermal stability and oxidation resistance should be good, the freezing point and low temperature year should also be low, and non-toxic and corrosion-resistant. Special requirements do not contain hydrocarbons in the vacuum system, can use fluorine oil.
The second is baking. Baking is also a necessary means for oil diffusion pump to obtain ultra-high vacuum. Because the pump material in the atmosphere will certainly absorb some gases, and in the vacuum, these gases will be desorbed and slowly released. Therefore, in order to reduce the outgassing rate of materials, it is necessary to desorb these gases in advance by baking. Different materials, baking time and temperature will be different.
The third is to prevent pump oil backflow and migration. These two, pump oil through the inlet into the container is called backflow, and oil molecules through the wall of the device into the container is called reverse migration. In order to prevent the backflow and migration of pump oil, it is necessary to add a cold trap device. Generally, in the oil diffusion pump, the liquid nitrogen cold trap is usually installed between the pump inlet and the vacuum chamber, which can effectively capture the backflow steam, part of the pyrolysis products and the steam from the vacuum chamber during the operation of the oil diffusion pump. After baking, the system of oil diffusion pump combined with cold trap can obtain ultra-high vacuum of 10 ^ - 8 PA ～ 10 ^ - 10 Pa or even higher.
The fourth is to reduce the reverse diffusion of the extracted gas. The reverse diffusion of the pumped gas is also one of the factors limiting the ultra-high vacuum of the oil diffusion pump. The gas reverse diffusion is related to the molecular weight. The smaller the mass, the easier the reverse diffusion. Therefore, reverse diffusion is easy to occur when the oil diffusion pump is pumping hydrogen. To reduce this phenomenon, one is to install a titanium sublimation trap cooled by liquid nitrogen on the inlet pipe of the oil diffusion pump to capture the reverse diffusion hydrogen, and the other is to connect two diffusion pumps in series to reduce the exhaust pressure at the outlet of the diffusion pump.