If the electronic and microelectronic information devices are cooled from 77K to 4.2K, the noise can be reduced by several orders of magnitude; below the liquid helium temperature, the matter is less affected by external disturbances, the thermoelectromotive force is eliminated, and ion conduction is stopped, so that it can be found New phenomena of matter, etc. Therefore, it is still meaningful to further study the liquefaction technology.
The liquefaction of hydrazine is generally achieved by means such as a hydrazine liquefier. Although its cooling capacity is provided by the expander, the temperature reduction of the compressed helium is carried out by a multi-stage heat exchanger, which plays an important role in the liquefaction of helium, so this type of equipment is called helium. Liquefier. The liquid helium machine plays an important role in the liquefaction process of the crucible is a low-temperature refrigerator, and the high-purity helium gas directly enters the cold head of the refrigerator to be liquefied. Therefore, the machine is called a liquid helium machine.
The use of chillers for liquefaction of hydrazine is a dream that scientists and technicians all over the world dream of. At present, the liquefaction of hydrazine using a liquefier is quite mature regardless of its machine, equipment, manufacturing technology and process flow of each unit. It can manufacture large-scale hydrazine liquefaction plants with a production scale of over 5000L/h, and small-scale production of < 5L/h liquid helium equipment. However, due to the complicated process, large equipment, large floor space and large investment, the wide application of small-capacity liquid helium equipment (< 5L/ h) is limited, such as nuclear magnetic resonance instrument, superconducting magnetometer, high-speed superconducting magnetic levitation. Train and other aspects. There is limited space on the superconducting high-speed maglev train. It is impossible to install the liquefier or to supply liquid helium at the station. Otherwise, the operation time will be affected. The practical method is to re-liquefy the steam on the vehicle using a cryogenic refrigerator.
In the 1960s and 1970s, the application of chillers and throttling valves for liquefaction was mainly applied. The refrigerator has GM, V-M, ST, SV, etc. Countries with developed industries such as the United States and Britain have done a lot of work. For example, the Cryodyne small sputum liquefier developed by ADL Company of the United States has two main and two compressors. The cooling capacity is provided by a three-stage GM cryogenic refrigerator with external liquid nitrogen and an expansion valve (the valve is installed). In the external liquid helium container). The liquid helium production is 0. 5L / h. Japan has studied the use of Stirling refrigerators and throttle valves since 1970 to reliquefy the cooled superconducting coils on the maglev train. The Stirling refrigerator, which was later developed to a temperature lower than 4.2 K, directly liquefies the helium vapor. Toshiba uses a GM machine plus a throttle valve, the G-M / JT experimental device. At 4. 2K, the cooling capacity is 5W. When the second-stage regenerator packing of the refrigerator is changed from Pb to Er 3 Ni, the working pressure of the GM machine is changed by changing the helium pressure and flow of the JT circuit.至1. 02M Pa. Entering the throttling. The pressure is 0. 5~0. 6M Pa. The throttling system uses a diaphragm compressor to provide 2M Pa pressure helium gas, the low pressure is 0. 01～0. 02M Pa. Before the valve is compressed, the helium gas enters the purification equipment first, removes trace impurities, and finally enters the throttle valve to reach the liquid helium temperature. The cooling capacity of the two-stage cold head of the GM machine is 8W/70K, 2W/ 16. 9K. In order to achieve a cooling capacity of more than 1W for 4. 4K, the cooling capacity of the first and second cold heads of the GM machine is 15W/70K. And 3W / 15K, for this purpose, the GM machine is modified, and the first-stage cylinder plus liquid nitrogen cooling method is applied to reduce the shuttle loss of the refrigerator. After the improvement, the cooling capacity of the first-stage cold head of the GM machine is 15. 3W/ 70. 15K, and the second-stage cold head is 3. 05W/ 14. 3K. Prior to this, the Dutch company Philips applied two PE H-100s. The chiller and the expansion ejector of the type chiller and the throttle valve are liquefied, and the PL He-210 type hydrazine is developed. The two refrigerators provide four temperature levels, which are respectively 87. 0K, 20. 1K, 62.5K, 15.1K. The working pressure is 2000kPa. The 2000kPa medium pressure helium gas from the helium cylinder is dried by the series. Purification, heat exchange. The heat exchanger E 5 and the compressed compressor from the compressor are collected, and then expanded to 110 kPa through the throttle valve. After the throttling, the gas-liquid mixture is generated into the mobile container, and the liquid helium is stored in the container, and the steam is exchanged through the stages. The heater returns to the compressor. The production capacity of the device is 5 L / h liquid helium.
The above two types of sputum liquefaction devices for application chillers, although larger improvements than sputum liquefiers, still require compressors and gas storage devices.
Therefore, the entire device is still relatively complicated. Moreover, the former has less production.
The latter process is complicated, and the maintenance of two cryogenic Stirling refrigerators is very troublesome, which greatly limits their promotion and application.
In recent years, due to the wide application of magnetic cold storage materials (such as Er 3 Ni) in the refrigerator, the performance of the refrigerator has been greatly improved, so that the minimum temperature of the refrigerator has been further reduced, and its cold head minimum temperature. From 10K to below 4. 2K, even lower than the super-flowing temperature; in the cooling capacity of 4.2K from several hundred mW to more than 1W, the research of liquid helium-grade (4.2K) refrigerators flourished and became low temperature. A hot topic in engineering research.
Below, we discuss the performance of magnetic cold storage materials and describe the development of liquid helium temperature refrigerators.
The scientific and technological workers engaged in refrigerator work know that the regenerator is the heart of the refrigerator and is a key component. Its efficiency plays a decisive role in the performance of the refrigerator. For example, the domestic KL N-20Y four-cylinder liquid nitrogen refrigerator, liquid Nitrogen production was 20 L/h. When the efficiency of the regenerator decreased by 1%, the liquid nitrogen production decreased by 2 L/h. In addition to the process, the efficiency of the regenerator is determined by the performance of the packing material. At present, the copper mesh and the stainless steel mesh are applied at a temperature of 300 K to 77 K, and the mesh diameter is from 0.015 to 0.04 mm, and is woven into a 200-400 mesh laminated filler. 77～20K generally use 200-400 mesh stainless steel mesh and 0. 08～0. 20mm lead pill heap filling; 20K or less application of lead pellets, barium sulfide and other materials, however, lead pellets decrease with temperature, especially below 15K, The specific heat capacity decreases sharply with the decrease of the thermal resonance of the crystal lattice, and the density of the working medium increases, which leads to the increase of the heat capacity and the negative effect of heat capacity. However, the materials such as barium sulfide, antimony and antimony alloy are expensive and scarce. And the minimum temperature of the refrigerator can not reach 4. 2K or less, losing the practical application value. Finding low-temperature cold storage materials below 15K is the key to improving the efficiency and performance of small refrigerators.
The lattice specific heat of the material and the specific heat of the electron become less with the decrease of the temperature, and the abnormality of the cooperative process related to the degree of the transition state and the degree of the excited state is higher than that of the thermal, electron spin and spin interaction. The superconducting specific heat of the nuclear spin and the 4 f electron interaction causing the specific heat of the core to change into the superconducting state has a relatively large specific heat capacity at ultra-low temperature. It is ideal to use these specific heat materials as the filler of the regenerator. of. In the 1970s and 1980s, foreign scientists used rare earth compounds as thermal storage materials to conduct research and experiments to obtain better results.
In the thermodynamic system, the relationship between specific heat capacity and entropy is: S = ∫c T dT (1) The relation (1) is also applicable to the magnetic phase transition process for the magnetic spin system. When a magnetic phase transition occurs, a large entropy change occurs in a narrow temperature range, and the temperature of the above formula can be approximated as a constant. Then, as can be seen from the above equation, the more the entropy value changes during the phase change Large, its specific heat capacity c is also greater. When changing from a fully ordered state to a completely disordered state, the magnetic entropy relationship is:
SJ = NK p In ( 2J + 1) ( 2) where J is the full-angle momentum quantum number of the spin, which means that the larger the number of spins, the more spin J is, and the magnetic phase change occurs. Large entropy variable. The Q value of the 4 f rare earth ion such as Er is greater than 7, and has the highest value. Therefore, the compound of Er may have a large specific heat capacity during the magnetic phase transition. The experiment proves that, below 10K, the specific heat capacity of the compound (E) is several times larger than that of lead, and its heat capacity value is larger at a temperature of > 15K, which has a large specific heat capacity in a wide temperature range. It is advantageous to use as a cryogenic refrigerator, and it provides favorable conditions for the development of liquid helium-class refrigerators.
Unit Volume and Specific Heat Ratio of Rare Earth Metal Compound Er (Ni 1-x Co x ) The cylinder and piston of a cryogenic refrigerator manufactured by the US Bureau of Standards Engineering Plastics achieve liquid helium temperature on a single-stage Stirling refrigerator. The cooling capacity at 4. 2K is 4 mW, and the lowest temperature is 3. 1 K, which is used for cooling of the superconducting quantum interferometer. Japan's Matsubara Yang first used a three-stage vascular machine to reach a temperature below 4. 2K. 5K温度。 The minimum temperature of 2. 9K; and the use of a two-stage GM machine plus a single-stage pulse tube refrigerator to achieve 3. 5K temperature.
Japan's Tokyo Institute of Technology and Toshiba's research team and Mitsubishi research team in the second-class GM refrigerator (US CVI company's GM machine), using 295g Er 3 Ni diameter of 0. 2 ~ 0. 3mm spherical particles instead of lead pellets as cold storage The lowest pressure of the second stage cold head when the operating speed is 60r/min and 24r/min under the condition of 0M Pa and the exhaust pressure is 0. 2~2. For the 4.96K and 3.22K.4. 2K, the measured cooling capacity is 0. 28W. Japan's Mitsubishi Electric Corporation uses the same GM machine, the first and second, except for the cylinder, the overall refrigeration system and the University of Tokyo. The diameter of the crushed one is 0.13 mm. 0 mm. 0以上。 When the ventilator speed is 20r / min, The chilling capacity is 0. 8W at the time of 4. 2K. Zhejiang University and Hangzhou Oxygen Machine Research Institute used the rare earth compound Er 3 N i instead of the lead pellet on the second-stage regenerator to obtain the liquid helium temperature on the Z 1He - 051253 pneumatic secondary Solvent refrigerator. The self-made secondary pulse tube refrigerator of Zhejiang University reached 3. 1K temperature, and the cooling capacity reached 4.40K at 300mW. The proportion was 40%, 27% and 33%. The compressor used commercial 6kW G-M compressor. Helium is supplied by a reduced pressure from a 501 helium gas tank. It first heats through the first-stage vascular cold-head coil, cools to about 50K, then enters the outer-stage heat exchanger of the second-stage regenerator, and then enters the second-stage cold head for liquefaction. The liquefaction amount of the device is 0. 13L / h liquid helium.
The application of the magnetic material in the refrigerator, the lowering of the cold head temperature of the cryogenic refrigerator and the increase of the cooling capacity at 4.2 K provide important conditions for the development of the small liquid helium machine. Of course, there are still some difficulties in exploring the development of small liquid helium machines. The prominent contradiction is that the cooling capacity is insufficient at 4. 2K. If the production of the liquid helium machine is 3 L / h, according to the German data, the cooling capacity required for the refrigerator at 4.2 K is about 8. 5 W. If the working temperature of the liquid nitrogen cooling refrigerator is used, the helium gas used for liquefaction is used together. Its cooling capacity is used as a cooling screen to reduce heat leakage from the outside of the system. A cold head of a liquid helium machine with a production rate of 3 L/h requires a cooling capacity of about 5 W (4.2 K). According to the current level of chiller development, the literature also reported that the chiller can easily reach a cooling capacity of more than 1 W (4.2 K), so that each chiller provides 1. 25 W of cooling capacity and 4 sets of low temperature. In combination with a refrigerator, it is entirely possible to supply 5W of cooling at 4. 2K.
A cryogenic refrigerator for a liquid helium machine is preferably a two-stage Stirling refrigerator. Because of its compact structure, the compression and expansion process of the working medium is carried out on the same machine, while other refrigerators require a separate compressor, and in the 1970s a second-stage Stirling refrigerator used its second-stage regenerator for lead pellets. The conditions can produce 4 to 5 L of liquid hydrogen per hour. If a rare earth composite cold accumulating material is used, its performance will be further improved. Of course, the application of magnetic materials to Stirling refrigerators should also solve the problem of powdering and heat conduction of rare earth magnetic materials. According to the current state of the art, these two problems are not difficult to solve.
From the above discussion, it can be known that the development of a liquid helium machine with a yield of 3 to 4 L/h is conditional and completely possible. Although there are difficulties, it can be overcome through research.
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