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What Are the Factors That Affect the Ultimate Vacuum of the Vacuum Chamber
1. Residual pressure of the original gas in the container
Before the ultra-high vacuum system starts to evacuate, a certain amount of gas is stored in the container, pipelines, cold traps and other components. If a pump with a certain pumping speed is used to remove the original gas in the container, its pressure decreases exponentially with the pumping time.
If the system has no other gas source and only the original gas, a pump with a small pumping speed for this gas can quickly pump out the gas molecules in the container. As the pumping time increases, the pressure in the container continues to decrease and can reach a very low pressure, so it is not a factor that limits the system’s ultimate pressure. It is important to pay attention to maintaining a certain pumping speed for each component of the original gas when selecting a vacuum unit.
The ultra-high vacuum pump has a strong selectivity for gas, so the gas source must be analyzed individually, that is, not only the amount of gas released, but also the composition of gas released must be known. At the same time, the pump should also be selected according to the amount of gas released and the composition of gas released, which is a problem that should be paid special attention to when selecting the main pump for the ultra-high vacuum system. It is not enough to choose a single means of pumping. It must be considered comprehensively and matched comprehensively to achieve this goal. In order to solve this problem, the method of flushing the original gas in the system can also be used, that is, repeatedly flushing the system with a gas that is easily discharged by the unit to replace the gas that is difficult to discharge by the unit, which also helps to reduce the ultimate pressure. However, due to leakage, penetration, degassing and chemical reactions of the system itself, this gas may continue to be produced. The method of flushing the system can only be used when the system starts. If the unit does not have a certain pumping speed for this gas, the ultimate pressure of the system will also be affected by the residual pressure of this gas.
2.System Leakage
Leakage is an important factor in limiting the ultimate pressure. Once a considerable amount of leakage occurs in the system, the ultimate pressure in the system will be limited. When the system pumping speed is constant, reducing the leakage rate can reduce the ultimate pressure of the system.
Leaks mainly come from pores and defects in raw materials, poor welding of welds, or cracking of welds due to excessive force due to improper weld design, poor sealing and “cold leakage”. In the selection of materials for ultra-high vacuum systems, vacuum smelted materials have less gas content, and cold-rolled materials have fewer pores and defects than hot-rolled materials. In terms of technology, melting welding should be used uniformly, and silver welding, copper welding and other processes should be avoided. Silver welding and copper welding belong to brazing, that is, the parent metal is not melted, and the two metals are bonded together with flux. After being subjected to cold and hot shocks and stress, it is easy to detach at the place where the bonding strength is low to produce leaks. Therefore, this welding method is not used in the process of ultra-high vacuum systems. At present, 1Cr18Ni9Ti or 0Cr18Ni9Ti stainless steel materials are mostly used in ultra-high vacuum systems because they have excellent high and low temperature performance, vacuum performance, welding performance, corrosion resistance and mechanical processing performance. However, stainless steel should pay special attention to the following points in the process of argon arc welding:
- In the process of argon arc welding, try to reduce the number of arcing and arc extinguishing. When starting the arc for the second time, be sure to melt the arc extinguishing point before moving forward. Practice has shown that leaks often occur at the arc extinguishing or arc starting point, which is often caused by insufficient overlap between the arc starting point and the previous arc extinguishing point or moving forward without melting.
- Try to avoid long-term melting with high current, otherwise the alloy elements will be burned too much during the welding process. For example, nickel is reduced due to volatilization after welding, and the metallographic structure is no longer a stable austenite structure, but is transformed into martensite. At the same time, excessive welding current and long duration will also make the grains in the molten pool area coarse, resulting in a large heat-affected zone, high stress, poor mechanical strength, and poor corrosion resistance. After being stressed during use, these welds are easily torn. For parts that have to be welded with high current specifications, it is best to perform vacuum annealing at 900~1000℃ after welding to refine the grains in the molten pool area and eliminate stress in the weld. Using small current standard welding, the molten pool area is small, the heat-affected zone is small, and the alloy elements are less volatilized. After welding, the weld is still in a stable austenite structure. After repeated impacts from room temperature to low temperature (about 100K), it is not easy to leak. Therefore, stainless steel should not be repeatedly welded during welding. Care should also be taken when repairing welds after a weld leaks. The more times the welds are welded, the greater the changes in the metallographic structure and the composition of the alloy elements, which is harmful. The extremely high vacuum sealing connection generally adopts a gold wire ring sealing structure. The surface roughness of the metal contact surface is less than 0.2μm, and the matching clearance of the concave and convex flanges is δ≤0.05mm. As long as it is carefully assembled, there will be no leakage after sealing. When checking for leaks, use a highly sensitive leak detector to carefully check the parts for leaks. In order to be safe and reliable, a double-layer protective vacuum structure can be used in the structure.
3.Degassing
The degassing sources of vacuum devices include: desorption of surface adsorbed gas, release of gas dissolved in the material through the diffusion surface, evaporation, decomposition, dissociation of materials, gas generated by chemical reaction between gas and solid surface, etc. In ultra-high vacuum systems, the selection of materials is very important. Generally, stainless steel, copper, oxygen-free copper, tungsten, molybdenum, tantalum, gold, silver, borosilicate glass, etc. are used. They have certain strength, stable chemical properties, low vapor pressure and decomposition pressure. Rubber, grease, ordinary plastic, brass (containing zinc with high vapor pressure), low-temperature alloy (containing tin, lead alloy), etc. are not suitable for use.
The following analysis discusses the relationship between the above-mentioned various degassing sources and materials, the factors affecting degassing, the degree of influence on the ultimate pressure, and how to reduce the degassing rate.
❶ Desorption of surface adsorbed gas
In ultra-high vacuum systems, the amount of gas desorbed from the surface, the gas composition, and the experimental method of desorption are very important. To remove the surface adsorbed gas, proper baking is the most effective way. Since the reasonableness of baking temperature and uniformity can make the amount of gas desorption differ by several orders of magnitude, the selection of baking temperature and the guarantee of baking temperature uniformity are very important. The gas adsorbed on the solid surface can also be removed by glow discharge of inert gas at 1~10 Pa, or by bombarding the material with electrons and ions to release the adsorbed gas. There are also methods to desorb the gas adsorbed on the solid surface by light irradiation and ultrasonic vibration. After baking, discharge or bombardment, the water vapor released from the surface is significantly reduced. In the stainless steel system, water vapor accounts for 90% of the gas released before baking. After the baking and degassing is thorough, hydrogen is the main component of the degassing, and the remaining gases are N2, O2, CO, CO2, CH4, etc. Hydrogen is released by the hydrogen dissolved in the metal during the smelting process and diffuses to the vacuum side of the wall. CO, CO2, and CH4 are generated by complex chemical reactions between the solid surface and the gas. At high temperatures, the carbon dissolved in the metal diffuses to the solid surface and reacts with the oxygen, hydrogen and water vapor on the metal surface to generate CO, CO2, and CH4.
In addition to baking, freezing is also a major means of reducing water vapor. It can not only freeze the water vapor to be desorbed on the surface and reduce the amount of degassing, but also produce a certain pumping speed for water vapor and reduce the water vapor gas molecules in the space. At the same time, on the solid surface at a lower temperature, the probability of chemical adsorption of carbon, hydrogen and oxygen will also become smaller. If the system is exposed to the atmosphere for a long time, in order to avoid the adsorption of water vapor, it is better to introduce dry nitrogen before opening the container. After doing so, the exhaust time can be shortened to a few tenths in the room temperature exhaust device. Before the system is opened, fill it with dry nitrogen to a pressure of several hundred Pa and maintain it for several minutes. After the surface is fully adsorbed with dry nitrogen to a saturated state, it can be filled with atmosphere. At this time, since the container wall has fully adsorbed dry nitrogen, water vapor in the air is rarely adsorbed on the surface of the wall. Even if adsorbed, the binding is very weak and it is easier to desorb.
❷ Desorption of dissolved gas
Solid materials often dissolve some gases during smelting or casting. Solid materials that have been placed in the atmosphere for a long time will also dissolve part of the atmosphere due to diffusion. These gases diffuse in the solid as impurity atoms in the solid. If the system is baked at 450℃ for 10h and then cooled to room temperature, the partial pressure of hydrogen in the system becomes 1×10-10Pa. At 1000℃, it only needs to be baked for 4h. Since desorption releases gas, mainly hydrogen, it is difficult to obtain a very low pressure in a stainless steel device. To solve the problem of hydrogen partial pressure, freezing is a desirable method. Because the diffusion system of hydrogen at low temperature is greatly reduced compared to room temperature.
In addition, the choice of materials is also very important. Some people suggest using aluminum alloy to make vacuum containers. Since aluminum alloy is a non-ferromagnetic alloy and has a low degassing rate, it is suitable for manufacturing devices such as accelerators. As a vacuum container and pipeline material, it is widely used abroad, especially in Japan. However, it is very common to use stainless steel as a material for vacuum systems. This is because the surface of stainless steel is covered with a very strong thin layer of chromium oxide, which is a stabilizer and has less surface degassing.
Stainless steel also has good processing and welding properties and has excellent properties as a vacuum material. The main component of degassing after baking is hydrogen. Before processing, the stainless steel raw materials should be placed in a vacuum annealing furnace and vacuum degassed at 700℃ for 10 hours, which can greatly reduce the outgassing of hydrogen, which is very necessary for the manufacture of ultra-high vacuum containers. In order to reduce the total outgassing volume of the system by 1000 times, the unbaked surface area of the entire system should not exceed 1/1000 of the total area of the system. The baking temperature does not need to be too high, and low-temperature baking can completely remove the gas adsorbed on the surface.
❸ Evaporation and decomposition of materials
The selection of materials for ultra-high vacuum systems must first consider the low vapor pressure of the selected materials, otherwise it will cause large gas loads. For example, brass contains zinc with high vapor pressure, and low-melting alloys contain tin, lead, etc. Grease, plastic, rubber, etc. are even more unsuitable.
Secondly, the thermal stability of the material should be considered. Polymer compounds have poor thermal stability and are easily oxidized. For example, grease is pyrolyzed at high temperatures to release hydrogen and hydrocarbons. Stainless steel is the best metal material for ultra-high vacuum systems. Copper and copper alloys should not be used as much as possible, because copper and copper alloys exposed to the atmosphere oxidize quickly at high temperatures. When copper must be used in a vacuum system, it is best to use vacuum-smelted oxygen-free copper and avoid using electrolytic copper. When copper pipes are used as water-cooling pipes or as pipes for transmitting low-temperature liquids, oxidation caused by repeated baking can easily cause failures. Tungsten, molybdenum, and tantalum are also best made by vacuum smelting, with small outgassing.
Other materials should also be vacuum pre-vented before use. For the same reason, copper welding and silver welding are best not used during welding, because some flux with higher vapor pressure is used in these welding processes.
Is there a material more suitable for ultra-high vacuum systems than stainless steel? Aluminum alloys have been used to manufacture large vacuum devices such as accelerators. However, due to the disadvantages of aluminum alloys, such as their porosity, containing more gas, low high-temperature strength, and difficulty in welding, the use of aluminum alloys to make vacuum containers has great limitations. However, the permeability of aluminum alloy to hydrogen at room temperature is about 10-7 times that of 300 series stainless steel. Therefore, vapor deposition of a 10μm thick aluminum film on stainless steel can reduce the amount of hydrogen outgassing by 105 times. Aluminum composites are used as electron tube electrode materials on stainless steel. As long as sufficient attention is paid to smelting and forging, aluminum alloys have the potential to become materials for ultra-high vacuum systems.
❹ Gases generated by chemical reactions between gases and solid surfaces
In ultra-high vacuum systems, gases generated by chemical reactions between gases and solid surfaces and between gases dissolved inside solids and solid surfaces are an important source of gas.
Carbon in stainless steel diffuses to the metal surface and reacts chemically with oxygen to generate carbon monoxide. In a vacuum system, after heating the metal filament, the partial pressures of water vapor, carbon monoxide and methane increase. The increase in these gases is related to the presence of hydrogen. After reducing the partial pressure of hydrogen, the partial pressures of these gases also decrease. Since hydrogen is decomposed into atoms and diffuses into the metal, it is chemically active and easily reacts chemically inside and on the surface of the metal.
In a vacuum system, multiple chemical reactions can be carried out simultaneously on metal and glass walls. The gases generated by chemical reactions are different depending on the history and usage conditions of various materials. In extremely high vacuum conditions, gases other than H2 have a certain relationship with the presence of H2, so reducing the partial pressure of H2 is still the main issue.
4.Leakage
When a solid material is placed in a gas, the surrounding gas molecules will dissolve in the solid surface layer. It is different from the gas originally dissolved inside the solid. The gas pressure on both sides of the vacuum container wall is different, and the concentration of the dissolved gas molecules is also different. When the concentration on both sides of the wall is different, the gas molecules diffuse from the side with high concentration to the side with low concentration, and finally diffuse to the inner wall of the vacuum container and release. This process is called gas penetration.
The non-metallic materials used in the vacuum system, such as glass and organic materials, have a dissociation degree n=1, and the permeation rate of dissolved gas molecules is proportional to the pressure difference. Helium has a large permeability through glass, which directly affects the acquisition of extremely high vacuum. Therefore, it is not suitable to use glass or organic materials as the wall of the extremely high vacuum system. Rare gases such as helium and neon do not dissolve in metal materials, which is beneficial to the acquisition of extremely high vacuum. Diatomic gas molecules dissolve only after dissociation into atoms. The main component of the gas released from stainless steel is hydrogen. In particular, after good degassing, 99% of the residual gas is hydrogen. Therefore, hydrogen penetration is one of the difficulties in obtaining an extremely high vacuum.
5.Reflux
The phenomenon that the gas or vapor in the vacuum pump body flows back to the vacuum chamber is called reflux. In an extremely high vacuum system, since the pressure of the vacuum chamber is lower than the ultimate pressure of the vacuum pump, the influence of reflux on the ultimate pressure is particularly significant.
For an extremely high vacuum system, all vacuum pumps are gas sources. In order to reduce the reflux of the pump to the vacuum chamber, a trap needs to be connected between the vacuum pump and the vacuum chamber to block the reflux of the gas by utilizing the vacuum pump’s exhaust capacity.
Since the ultimate pressure of the current vacuum pump is relatively high, the design of the trap is extremely important in an extremely high vacuum system. The focus of the design is to improve the capture coefficient of the trap. In a vacuum system using a diffusion pump, there is also a problem of reverse diffusion. In a diffusion pump, the gas flow not only occurs in the exhaust direction, but also a small amount of gas molecules flow in the opposite direction of the vapor flow, and diffusion occurs from the low vacuum end to the high vacuum end. This phenomenon is called reverse diffusion. The degree of back diffusion is related to the compression ratio of the diffusion pump. The greater the compression ratio, the smaller the back diffusion. The compression ratio is related to the gas mass. The compression ratio of light gas is much smaller than that of heavy gas.
For high vacuum systems, the impact of back diffusion is not important, but for ultra-high vacuum systems, the limitation of back diffusion on the ultimate vacuum must be considered. If a diffusion pump is used to obtain an extremely high vacuum, two diffusion pumps need to be connected in series so that the front-stage diffusion pump reduces the outlet pressure of the main diffusion pump, thereby reducing the back diffusion of the main pump. Experiments have shown that this method can improve the ultimate vacuum of an extremely high vacuum system.
