1, The residual pressure of the gas inside the container
Before the extremely high vacuum system starts pumping, a certain amount of gas is pre stored in the containers, pipelines, cold traps, and other components. If a pump with a certain pumping speed is used to remove the existing gas in the container, the pressure decreases exponentially with pumping time.
If the system has no other gas source and only the original stored gas, a small pumping speed can quickly remove the gas molecules in the container. As the pumping time increases, the pressure inside the container continuously decreases and can reach very low pressures, so it is not a limiting factor for 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 vacuum pump with extremely high vacuum has strong selectivity for gases, therefore, individual analysis of the gas source is necessary, that is, not only to know the amount of gas released, but also to know the composition of the gas released. At the same time, the selection of pumps should also be based on the amount and composition of air release, which is a special consideration when choosing the main pump for extremely high vacuum systems. Choosing a single exhaust method is not feasible. It must be comprehensively considered and combined in order to achieve this goal. To solve this problem, the method of flushing the existing gas in the system can also be used, that is, repeatedly flushing the system with a gas that is easy to be eliminated by the unit to replace the gas that is difficult to eliminate by the unit, which also helps to reduce the ultimate pressure. However, due to the leakage, permeation, deflation, and chemical reactions of the system itself, it is still possible to continue producing this gas. The method of flushing the system can only be used when the system starts up. If the unit does not have a certain pumping speed for this gas, the residual pressure of this gas will also affect the ultimate pressure of the system.
2, System leakage
Leakage is an important factor in limiting the ultimate pressure, and once a considerable amount of leakage occurs in the system, the ultimate pressure inside the system will be limited. When the pumping speed of the system is constant, reducing the leakage rate is necessary to lower the maximum pressure of the system.
Leakage mainly comes from porosity and defects in raw materials, poor welding of welds, or excessive stress on welds due to improper weld design, poor sealing, and "cold leakage". In the selection of materials for extremely high vacuum systems, materials produced by vacuum smelting have lower gas content, and cold-rolled materials have fewer pores and defects than hot-rolled materials. Melting welding should be uniformly used in the process, avoiding the use of silver welding, copper welding and other processes. Silver welding and copper welding belong to brazing, which means that the parent metal does not melt, and the two metals are bonded together with flux. After being subjected to cold, hot shock and stress, it is easy to detach in areas with low bonding strength, resulting in leakage holes. Therefore, this welding method is not used in the process of extremely high vacuum systems. At present, 1Cr18Ni9Ti or 0Cr18Ni9Ti stainless steel materials are commonly used in extremely high vacuum systems due to their excellent high and low temperature performance, vacuum performance, welding performance, corrosion resistance, and mechanical processing performance. However, special attention should be paid to the following points in the argon arc welding process for stainless steel:
① During argon arc welding, try to minimize the number of arc initiation and extinguishing. When starting the second arc, be sure to melt the extinguishing point before moving forward. Practice has proven that leaks often occur at the arc extinguishing or starting point, often caused by insufficient overlap or forward movement without melting between the starting and previous arc extinguishing points.
② Try to avoid prolonged melting with high current, otherwise the alloy elements will be burned too much during the welding process. For example, nickel decreases after welding due to volatilization, and the metallographic structure is no longer a stable austenite structure, but transforms into martensite. At the same time, excessive welding current and prolonged duration can also cause coarse grains in the molten pool area, resulting in a large heat affected zone, high stress, poor mechanical strength, and poor corrosion resistance. After being subjected to force during use, these welds are prone to tearing. For parts that must be welded with high current specifications, it is best to perform vacuum annealing treatment at 900-1000 ℃ after welding to refine the grain size in the molten pool area and eliminate internal stress in the weld seam. Using low current standard welding, the molten pool area is small, the heat affected zone is small, and the alloy elements evaporate less. After welding, the weld remains in a stable austenitic structure, and after repeated impacts from room temperature to low temperature (about 100K), it is not easy to leak gas. Therefore, stainless steel should not be repeatedly welded multiple times during the welding process. Attention should also be paid to the repair welding after the weld seam leaks. The more times the welding is performed, the greater the changes in the metallographic structure and alloy element composition, which can be harmful. Ultra high vacuum sealed connections generally use a gold wire ring sealing structure, with a surface roughness of less than 0.2 μ m on the metal contact surface and a fitting gap of no more than 0.05mm between the concave and convex flanges. As long as they are carefully assembled, there will be no air leakage after sealing. During leak detection, a highly sensitive leak detector should be used to carefully and meticulously inspect the components. For stability and reliability, a double-layer vacuum protection structure can be adopted in the structure.
3, Release air
The sources of gas release for vacuum devices include desorption of adsorbed gases on the surface, release of gases dissolved inside the material through diffusion surfaces, evaporation, decomposition, and dissociation of materials, and gases generated by chemical reactions between gases and solid surfaces. The selection of materials is crucial in extremely high vacuum systems. Generally, stainless steel, copper, oxygen free copper, tungsten, molybdenum, tantalum, gold, silver, borosilicate glass, etc. are used. They have a certain strength, stable chemical properties, and low vapor pressure and decomposition pressure. Rubber, grease, ordinary plastics, brass (containing high vapor pressure zinc), low-temperature alloys (containing tin and lead alloys), etc. are not suitable for use.
Below is an analysis and discussion of the relationship between the various sources and materials of gas release mentioned above, the factors that affect gas release, the degree to which the ultimate pressure is affected, and how to reduce the gas release rate.
① The desorption of surface adsorbed gases is crucial in extremely high vacuum systems, where the amount, composition, and experimental methods of the gas desorbed from the surface are crucial. Removing surface adsorbed gases and baking appropriately is the most effective method. Due to the fact that the reasonable baking temperature and uniformity can lead to a difference of several orders of magnitude in the amount of gas desorption, 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 1-10Pa, or by electron and ion bombardment of the material to release the adsorbed gas. Light irradiation and ultrasonic vibration can also be used to desorb gases adsorbed on solid surfaces. After baking, discharging, or bombardment, the amount of water vapor released from the surface is significantly reduced. The stainless steel system releases 90% water vapor in the gas before baking. After thorough baking and degassing, hydrogen is the main component of the released gas, and the remaining gases include N2, O2, CO, CO2, CH4, etc. Hydrogen gas is released by the diffusion of hydrogen dissolved in metals during the smelting process towards the vacuum side of the wall. CO, CO2, and CH4 are generated through complex chemical reactions between solid surfaces and gases. At high temperatures, carbon dissolved in metals diffuses to the solid surface and reacts with oxygen, hydrogen, and water vapor on the metal surface to produce CO, CO2, and CH4. In addition to baking, freezing is also the main means of reducing water vapor. It can not only freeze the water vapor to be desorbed on the surface, reducing the amount of gas released, but also generate a certain pumping speed for water vapor, reducing the amount of water vapor gas molecules in the space. At lower temperatures, the probability of chemical adsorption of carbon with hydrogen and oxygen on solid surfaces will also decrease. If the system is exposed to the atmosphere for a long time, it is better to introduce dry nitrogen gas before opening the container to avoid the adsorption of water vapor. After doing so, the exhaust time in the room temperature exhaust device can be reduced to one tenth. Before opening the system, fill it with dry nitrogen gas to a pressure of several hundred pascals and maintain it for a few minutes to fully adsorb the dry nitrogen gas on the surface until it reaches saturation, then it can be filled into the atmosphere. At this point, as the container wall has fully adsorbed dry nitrogen gas, water vapor in the air is rarely adsorbed onto the surface of the container wall. Even if adsorbed, the binding is weak and relatively easy to desorb.
② Desorption solid materials that dissolve gases often require the dissolution of some gases during smelting or casting processes. Solid materials that are left in the atmosphere for a long time will also dissolve a portion of the atmosphere due to diffusion. These gases diffuse as impurity atoms in the solid. If the system is baked at 450 ℃ for 10 hours and then lowered to room temperature, the partial pressure of hydrogen in the system becomes 1 × 10-10Pa. And it only needs to be baked for 4 hours at 1000 ℃. Due to the release of gas, mainly hydrogen, during desorption, it is quite difficult to achieve very low pressure in stainless steel equipment. Freezing is a viable solution to address the issue of hydrogen partial pressure. Because the diffusion system of hydrogen is greatly reduced at low temperatures compared to room temperature.
In addition, the selection of materials is also very important. Someone suggested using aluminum alloy to manufacture vacuum containers. Due to its non ferromagnetic nature and low gas release rate, aluminum alloy is suitable for manufacturing accelerators and other devices. As a material for vacuum containers and pipelines, it is widely used abroad, especially in Japan. However, it is common to use stainless steel as the material for vacuum systems. This is because the surface of stainless steel is covered with a very strong layer of chromium oxide, which is a stabilizer and has less surface gas release. The processing and welding properties of stainless steel are also very good, and it has excellent performance as a vacuum material. The main component that releases gas after baking is hydrogen. Before processing, stainless steel raw materials should be placed in a vacuum annealing furnace and subjected to 10 hours of vacuum degassing treatment at 700 ℃, which can greatly reduce the release of hydrogen gas. This is very necessary for the manufacture of extremely high vacuum containers. To reduce the total gas release of the system by 10 times, the un baked surface area of the entire system should not exceed 1/1000 of the total system area. The baking temperature does not need to be too high, low-temperature baking can completely remove the adsorbed gases on the surface.
③ The evaporation and decomposition of materials in extremely high vacuum systems require consideration of low vapor pressure when selecting materials, otherwise it will cause significant air load.
For example, brass contains zinc with high vapor pressure, while low melting alloys contain tin, lead, etc. Oils, plastics, rubber, etc. are not suitable for use.
Secondly, the thermal stability of the material should be considered. Polymer compounds have poor thermal stability and are prone to oxidation. If oil is pyrolyzed at high temperatures, it releases hydrogen and hydrocarbons. It is best to use stainless steel as the metal material for extremely high vacuum systems, and avoid copper and copper alloys 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 oxygen free copper produced by vacuum smelting and avoid using electrolytic copper. When using copper pipes as water cooling pipes or low-temperature liquid transmission pipes, oxidation caused by repeated baking can easily lead to malfunctions. Tungsten, molybdenum, and tantalum are also best smelted in vacuum with low gas release.
Other materials should also undergo vacuum degassing treatment before use. For the same reason, it is best not to use brazing or silver welding during welding, as these welding processes require the use of some high vapor pressure welding agents.
Is there a material more suitable for extremely high vacuum systems than stainless steel? Aluminum alloy has been used to manufacture large vacuum devices such as accelerators. However, aluminum alloy materials have significant limitations in making vacuum vessels due to their porous nature, high gas content, low high-temperature strength, and difficulty in welding. However, the hydrogen permeability of aluminum alloy at room temperature is about 10-7 times that of 300 series stainless steel. Therefore, depositing a 10 μ m thick aluminum film on stainless steel can reduce the amount of hydrogen released by 105 times. Aluminum composite is used as an electrode material for electronic tubes on stainless steel. As long as sufficient attention is paid to smelting and forging, aluminum alloy has the potential to become a material for extremely high vacuum systems.
④ The gas generated by the chemical reaction between gas and solid surface in an extremely high vacuum system, as well as the gas generated by the chemical reaction between the dissolved gas inside the solid and the solid surface, is an important gas source.
Carbon in stainless steel diffuses to the metal surface and reacts with oxygen to produce carbon monoxide. In a vacuum system, heating the metal filament increases the partial pressures of water vapor, carbon monoxide, and methane. The increase of these gases is related to the presence of hydrogen. After reducing the partial pressure of hydrogen, the partial pressure of these gases also decreases. Due to the fact that hydrogen decomposes into atomic states and diffuses into the interior of metals, it is chemically active and prone to chemical reactions both inside and on the surface of metals.
In a vacuum system, multiple chemical reactions can occur simultaneously on both metal and glass walls. The history and usage conditions of various materials are different, and the gases generated by chemical reactions are also different. In extremely high vacuum conditions, there is a certain relationship between gases other than H2 and the presence of H2, therefore, reducing the partial pressure of H2 remains the main approach.
4, Leakage
When solid materials are placed in gas, surrounding gas molecules will dissolve in the surface layer of the solid. 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 dissolved gas molecules is also different. When the concentration on both sides of the container wall is different, gas molecules diffuse from the side with higher concentration to the side with lower concentration, and finally diffuse to the inner wall of the vacuum container and release. This process is called gas permeation.
The non-metallic materials used in vacuum systems, such as glass and organic materials, have a dissociation degree of n=1, and the permeation rate of dissolved gas molecules is proportional to the pressure difference. Helium has a high permeability through glass, which directly affects the acquisition of extremely high vacuum. Therefore, glass or organic materials should not be used as the wall of the extremely high vacuum system. Not dissolving rare gases such as helium, neon, etc. in metal materials is advantageous for obtaining extremely high vacuum. Diatomic gas molecules dissolve only after dissociating into atoms. The main component of the gas released from stainless steel is hydrogen. Especially after good degassing, 99% of the residual gas is hydrogen. Therefore, hydrogen permeation is one of the difficulties in obtaining extremely high vacuum.
5, Reflux
The phenomenon of gas or vapor flowing back into the vacuum chamber inside a vacuum pump is called reflux. In extremely high vacuum systems, due to the pressure in the vacuum chamber being lower than the limit pressure of the vacuum pump, the impact of reflux on the limit pressure is particularly significant.
For extremely high vacuum systems, all vacuum pumps are gas sources. In order to reduce the backflow of the pump to the vacuum chamber, a trap needs to be connected between the vacuum pump and the vacuum chamber to block the backflow of gas, utilizing the pumping capacity of the vacuum pump.
Due to the current high limit pressure of vacuum pumps, the design of traps is extremely important in extremely high vacuum systems. The focus of the design is to improve the capture coefficient of the trap. In vacuum systems using diffusion pumps, there is also an issue of anti diffusion. In a diffusion pump, the airflow not only occurs in the pumping direction, but also a small amount of gas molecules flow in the opposite direction of the vapor flow, causing diffusion from the low vacuum end to the high vacuum end. This phenomenon is called counter diffusion. The degree of anti diffusion is related to the compression ratio of the diffusion pump. The larger the compression ratio, the smaller the anti diffusion, and the compression ratio is also related to the gas quality. The compression ratio of light gases is much smaller than that of heavy gases. For high vacuum systems, the effect of anti diffusion is not significant, but for ultra-high vacuum systems, the limitation of anti diffusion on the ultimate vacuum must be considered. If a diffusion pump is used to obtain extremely high vacuum, two diffusion pumps need to be connected in series to lower the outlet pressure of the main diffusion pump, thereby reducing the anti diffusion of the main pump. Experimental results have shown that this method can improve the ultimate vacuum of the extremely high vacuum system.
