1. The residual pressure of the original gas in the container
Before the very high vacuum system starts pumping, a certain amount of gas is stored in the container, pipes, cold traps and other components in advance.
If a pump with a certain pumping speed is used to remove the original gas in the container, the pressure decreases exponentially with the pumping time. If the system has no other gas source but the original gas, a small pumping velocity of the gas can quickly remove the gas molecules from the container. As the pumping time increases, the pressure in the vessel decreases and can reach very low pressures, so it is not a limiting factor for the ultimate pressure of the system.
It is important to take care to maintain a certain pumping speed for each of the original gas components when selecting the vacuum unit. Very high vacuum pumps have a strong selectivity for gases, so the gas source must be analysed individually, i.e. not only the amount of gas deflated, but also the composition of the gas. At the same time, the selection of the pump should also be selected according to the amount of air released and the composition of the air release, which is a problem that should be paid special attention to when selecting the main pump for the very high vacuum system. It is not possible to choose a single pumping method, it must be comprehensively considered and comprehensively matched to achieve this purpose. 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 removed by the unit to replace the gas that is difficult to remove by the unit, which also helps to reduce the ultimate pressure. However, if the system itself is likely to continue to produce such gas due to leakage, infiltration, outgassing, and chemical reactions, the method of flushing the system can only be used when the system starts to start. 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.
Air leakage is an important factor in limiting the ultimate pressure, and once a significant amount of leakage occurs in the system, the ultimate pressure in the system will be limited. When the pumping speed of the system is constant, the ultimate pressure of the system can be reduced by reducing the leakage rate.
Leakage mainly comes from porosity and defects of raw materials, poor weld welding or cracking due to excessive force on the weld due to improper weld design, poor sealing and "cold leakage". In the selection of materials for extremely high vacuum systems, the materials smelted by vacuum have less gas content, and the cold-rolled materials have fewer pores and fewer defects than the hot-rolled materials. In the process, fusion welding should be used, and silver welding, brazing and other processes should be avoided. Silver soldering and brazing belong to brazing, that is, the parent metal is not melted, and the two metals are glued together with flux, and after being subjected to cold and thermal shock and stress, it is easy to disconnect and leak holes in the place where the bonding strength is small, so the extremely high vacuum system does not use this welding method in the process.
At present, 1Cr18Ni9Ti or 0Cr18Ni9Ti stainless steel materials are mostly used for extremely high vacuum systems, because it has excellent high and low temperature performance, vacuum performance, welding performance, corrosion resistance and machining performance. However, special attention should be paid to the following points in the process of argon arc welding of stainless steel:
In the process of argon arc welding, minimize the number of arc initiation and arc extinguishing. When starting the arc for the second time, be sure to melt the arc extinguishing place and then move forward. Practice has proved that leakage often occurs at the arc extinguishing or arc starting, which is often caused by insufficient overlap between the arc starting and the previous arc extinguishing or moving forward without melting.
Try to avoid long-term melting with high current, otherwise the alloying 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 changes to martensitic. At the same time, the welding current is too large and the duration is too long, which 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. These welds are susceptible to tearing after being stressed during use. For parts that have to be welded with high current specifications, it is best to carry out 900 1000 vacuum annealing treatment after welding, so that the grain in the molten pool area can be refined and the internal stress of the weld can be eliminated. The welding is standardized with small current, the molten pool area is small, the heat affected zone is small, the volatilization of alloying elements is less, and the weld is still in a stable austenite structure after welding, and it is not easy to leak air after repeated impact from room temperature to low temperature (about 100K). Therefore, stainless steel should not be welded repeatedly during the welding process. It should also be noted that the more times the weld is welded, the greater the change in the composition of the metallographic structure and alloying elements, which is harmful.
The extremely high vacuum sealed connection generally adopts the gold wire ring sealing structure, and the surface roughness of the metal contact surface is less than 02 m, the mating clearance of the concave-convex flange δ 005mm, as long as it is carefully assembled, there will be no air leakage after sealing. In the case of leak detection, the parts should be carefully and carefully tested with a high-sensitivity leak detector. In order to be safe and reliable, a double-layer vacuum protection structure can be used in the structure.
3. Deflat
The venting sources of the vacuum device include: the desorption of the adsorption gas on the surface, the release of the gas dissolved in the material through the diffusion surface, the evaporation, decomposition and dissociation of the material, and the gas generated by the chemical reaction between the gas and the solid surface. In very high vacuum systems, the choice of material is very important. Generally, stainless steel, copper, oxygen-free copper, tungsten, molybdenum, tantalum, gold, silver, borosilicate glass, etc. They have a certain strength, are chemically stable, and have low vapor pressure and decomposition pressure. Rubber, grease, ordinary plastics, brass (containing zinc with high vapor pressure), low-temperature alloys (tin-containing and lead alloys) are not suitable for use.
The following analysis discusses the relationship between the above various venting sources and materials, the factors that affect the degassing, the degree of influence on the ultimate pressure, and how to reduce the degassing rate.
Desorption of surface adsorbed gases
In very high vacuum systems, the amount of gas desorbed on the surface, the composition of the gas, and the experimental method of desorption are very important. Removing adsorbed gases from the surface and baking properly is the most effective method. Because the baking temperature and uniformity are reasonable or not, the amount of gas desorption can be several orders of magnitude different, so 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 the glow discharge of the inert gas at 1 10Pa, and the adsorbed gas can also be released by bombarding the material with electrons and ions. There are also gases adsorbed on solid surfaces by light irradiation and ultrasonic vibration.
After baking, discharging or bombardment, the water vapor emitted from the surface is significantly reduced. Stainless steel system, 90% water vapor is released before baking. After the baking and degassing are complete, hydrogen is the main component of the gas, and the remaining gases are N2, O2, CO, CO2, CH4, etc. Hydrogen is released by the diffusion of hydrogen dissolved by the metal during the smelting process towards the vacuum side of the wall. CO, CO2, and CH4 are produced by complex chemical reactions between solid surfaces and gases. At high temperatures, the dissolved carbon in the metal diffuses to the solid surface and reacts with oxygen, hydrogen and water vapor on the metal surface to form 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 and reduce the amount of gassing, but also produce a certain pumping rate of water vapor and reduce the water vapor gas molecules in the space. At the same time, the probability of chemical adsorption of carbon, hydrogen and oxygen on the solid surface at lower temperatures will also be reduced. If the system is exposed to the atmosphere for a long time, it is better to introduce dry nitrogen before opening the container in order to avoid water vapor adsorption. By doing so, the exhaust time can be reduced to a few tenths in a room-temperature exhaust unit. Before the system is turned on, dry nitrogen is filled to a pressure of several hundred pascals for several minutes, so that the surface fully adsorbs the dry nitrogen to a saturated state, and then it can be filled into the atmosphere. At this time, since the container wall has fully adsorbed the dry nitrogen, the water vapor in the air is rarely adsorbed to the surface of the vessel wall. Even if adsorbed, the binding is very weak and relatively easy to desorption.
Desorption of dissolved gases
Solid materials often have to dissolve some gases during the smelting or casting process. Solid materials that have been placed in the atmosphere for a long time will also dissolve a part of the atmosphere due to diffusion. These gases diffuse in solids as impurity atoms in solids. 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. At 1000, it only needs to be baked for 4 hours. Since desorption requires the release of gases, mainly hydrogen, it is difficult to obtain very low pressures in stainless steel units. In order to solve the problem of partial pressure of hydrogen, freezing is a desirable solution. This is because at low temperatures, the diffusion system of hydrogen is much smaller than at room temperature. In addition, the choice of materials is also very important. It was proposed to make vacuum vessels from aluminum alloys.
Since aluminum alloy is a non-ferromagnetic alloy and has a small outgassing rate, it is suitable for the manufacture of accelerators and other devices, and is used more in foreign countries, especially in Japan, as a vacuum vessel and pipeline material. However, it is common to use stainless steel as the material for vacuum systems. This is due to the fact that the surface of stainless steel is covered with a very strong thin layer of chromium oxide, which is a stabilizer, and the surface is less outgassing. Stainless steel also has good processing and welding properties, and has excellent performance as a vacuum material. The main ingredient that is deflated after baking is hydrogen. Before processing, the stainless steel raw materials should be placed in a vacuum annealing furnace and vacuum degassed for 10h at 700 degrees, which can greatly reduce the outgassing of hydrogen, which is very necessary for the manufacture of extremely high vacuum containers. In order to reduce the total outgassing of the system by a factor of 10, the unbaked surface area of the entire system should not exceed 1 1000 of the total area of the system. The temperature of baking does not need to be too high, and low-temperature baking can completely remove the gas adsorbed on the surface.
Evaporation and decomposition of materials
The selection of materials for very high vacuum systems should first consider the low vapor pressure of the selected materials, otherwise it will cause large air loads. For example, brass contains zinc with high vapor pressure, and low-melt alloys contain tin, lead, etc. Grease, plastic, rubber, etc. should not be used. Secondly, the thermal stability of the material should be considered. Polymer compounds have poor thermal stability and are easy to oxidize. For example, grease is pyrolyzed at high temperature, releasing hydrogen and hydrocarbons. It is best to use stainless steel as a metal material for very high vacuum systems, and try not to use copper and copper alloys, 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 smelted in vacuum and avoid electrolytic copper. When copper pipes are used as water cooling pipes or as low-temperature liquid pipes, oxidation due to repeated baking is easy to cause failure. Tungsten, molybdenum, and tantalum are best smelted in vacuum, and the amount of outgassing is small. Other materials should also be pre-vented by vacuum before use. For the same reason, it is best not to use brazing or silver welding when welding, because some fluxes with high vapor pressure are used in these welding processes.
Is there a better material for an extremely high vacuum system than stainless steel? Aluminum alloys have been used to make large vacuum devices such as accelerators. However, due to its porability, aluminum alloy contains more gas, low high temperature strength, and is more difficult to weld, which makes the limitation of making vacuum containers with aluminum alloy great. However, the permeability of aluminum alloy to hydrogen at room temperature is about 10-7 times that of 300 series stainless steel, and the evaporation of 10 m thick aluminum film on stainless steel can reduce the amount of hydrogen outgassing by 105 times. Aluminum composite is used on stainless steel as an electrode material for electron tubes. With sufficient attention paid to smelting and forging, aluminum alloys have the potential to become materials for very high vacuum systems.
A gas produced by the chemical reaction surface of a gas and a solid surface
In very high vacuum systems, the gas produced by the interaction between the gas and the solid surface, as well as the chemical reaction between the dissolved gas inside the solid and the solid surface, is an important gas source. Carbon monoxide is formed by the carbon in stainless steel diffusing to the metal surface and reacting with oxygen. In a vacuum system, the partial pressure of water vapor, carbon monoxide and methane increases when the metal filament is heated. The increase in these gases is associated with the presence of hydrogen. When the partial pressure of hydrogen is reduced, the partial pressure of these gases also decreases. Because hydrogen is decomposed into atoms and diffuses into the interior of the metal, it is chemically active and prone to chemical reactions inside and on the surface of the metal. In a vacuum system, a variety of chemical reactions can be carried out simultaneously on both metal and glass walls. The history and conditions of use of various materials are different, and the gases produced by chemical reactions are also different. In the case of very high vacuum, gases other than H2 have a certain relationship with the presence of H2, so reducing the partial pressure of H2 is still the main thing.
When a solid material is placed in a gas, the surrounding gas molecules dissolve in the solid surface layer. It is different from the gas that is originally dissolved inside the solid. The concentration of dissolved gas molecules varies depending on the gas pressure on both sides of the vacuum vessel wall.
When the concentrations on both sides of the vessel wall are different, the gas molecules will diffuse from the side with a large concentration to the side with a small concentration, and finally diffuse to the inner wall of the vacuum vessel and be released, a process called gas penetration.
The dissociation degree of non-metallic materials such as glass and organic materials used in vacuum systems is n=1, and the permeation rate of dissolved gas molecules is directly proportional to the pressure difference. Helium has a large permeability through glass, which directly affects the acquisition of extremely high vacuum, so it is not suitable to use glass or organic materials as the wall of the very high vacuum system.
Noble gases such as helium, neon, etc., are not dissolved in metal materials, which is advantageous for obtaining extremely high vacuum. Diatomic gas molecules are dissolved only after they are dissociated into atoms. The main component of the gas emitted from stainless steel is hydrogen. In particular, 99% of the residual gas is hydrogen. Therefore, the permeation of hydrogen is one of the difficulties in obtaining extremely high vacuum.
The phenomenon of returning the gas or vapor flow from the vacuum pump body to the vacuum chamber is called regurgitation. In very high vacuum systems, the effect of reflux on the ultimate pressure is particularly significant because the pressure in the vacuum chamber is lower than the ultimate pressure of the extraction pump.
For very high vacuum systems, all vacuum pumps are air 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 by using the pumping capacity of the vacuum pump. Due to the high ultimate pressures of vacuum pumps, the design of the well is extremely important in very high vacuum systems. The focus of the design is to increase the trap capture factor.
In vacuum systems where diffusion pumps are used, there is also the issue of anti-diffusion. In the diffusion pump, the air flow not only occurs in the pumping direction, but also a small number of gas molecules flow in the opposite direction of the vapor flow, and the diffusion from the low vacuum end to the high vacuum end occurs, which is called reverse 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 related to the mass of the gas, and the compression ratio of light gas is much smaller than that of heavy gas. For high vacuum systems, the effect of anti-diffusion is not important, but for ultra-high vacuum systems, the limitation of anti-diffusion on ultimate vacuum must be considered. If a diffusion pump is used to obtain a very high vacuum, it is necessary to connect the two diffusion pumps in series, so that the fore diffusion pump can reduce the outlet pressure of the main diffusion pump, thus reducing the reverse diffusion of the main pump, and the experiment proves that the ultimate vacuum of the very high vacuum system can be improved by this method.