Recently, mainland scientists have discovered a refrigerant substance that does not require liquid helium, and the extremely low temperature of minus 273.056 °C has been reached by adiabatic demagnetization.
Unlike heat, which has almost no upper limit, the extreme temperature value for absolute zero is minus 273.15 degrees, but this is only a theoretical number that scientists have calculated using instruments. In superconductivity research and applications, we need to achieve ultra-low temperature (generally -150°C~-273°C). Extremely low temperatures, on the other hand, are closer to absolute zero (-273.15°C) and are generally measured in milliCalvin (mK).
The study of various strange physical properties of matter at extremely low temperatures is an important direction of fundamental physics research. This is closely related to national security and strategic high-tech fields such as large scientific installations, condensed matter physics, deep space exploration, materials science, particle physics, astronomical exploration, and quantum computing.
At present, the means to achieve low temperature mainly include adsorption refrigeration, adiabatic demagnetization refrigeration and dilution refrigeration.
01
Dilution of refrigerators and scarce helium
Dilution refrigeration technology is currently the most popular refrigeration method. Helium-3 and helium-4 are two stable isotopes of helium, and when the temperature is above 0.87K, the two solutions are able to miscible in any proportion. When the temperature is lower than 0.86K, the mixture will be separated into two phases, of which the phase containing more helium-3 is called the concentrated phase, and the phase containing less helium-3 is called the diluted phase. When helium-3 is continuously withdrawn from the diluted phase, in order to maintain equilibrium, the helium-3 of the concentrated phase will run into the diluted phase and endothermically reduce the temperature of the whole system, which can maintain an extremely low temperature of 1.8 mK. In a superconducting quantum computer, the spiral pipe is the dilution refrigerator that maintains low temperatures.
Schematic diagram of a helium dilution refrigerator. The circulating medium is helium-3 (light gray) and helium-4 (dark gray) does not circulate
It is a pity that helium is a bottleneck resource for the mainland. Helium is present in very low levels in the air, and helium in nature is usually produced by the decay of radioactive elements such as uranium ore and is enriched in associated natural gas deposits. This kind of natural gas ore associated with uranium ore is very rare in China, and the mainland imports 95% of helium.
Quantum computing is the main battlefield of scientific and technological competition in various countries around the world, and the dilution refrigerator provides the necessary extremely low temperature environment for the normal operation of quantum computers, and is an irreplaceable key equipment in quantum computing research. At present, such instruments in mainland China are completely dependent on imports, and they will no longer be able to be imported at the beginning of 2023, which is a core technology that needs to be broken urgently. Therefore, the development of ultra-low temperature refrigeration technology without helium is one of the key core technologies that need to be overcome in the field of scientific research in mainland China. The development of domestic liquid helium-free dilution refrigerator or the development of adiabatic demagnetization refrigerator are feasible technical routes.
The dilution refrigerator provides extremely low temperatures for quantum computers, with a temperature of about 10 mK at the bottom
02
Adiabatic demagnetization refrigeration technology
At extremely low temperatures (<1K), all gases, including helium, have been converted into liquids or solids, and it is difficult to achieve lower temperatures by gas expansion refrigeration alone. At the beginning of the 20th century, P. Langevin observed the magnetocaloric effect of excitation heat release and demagnetization and heat absorption in paramagnetic materials, and then P. Debye and W.F. Giauque carried out a study on the acquisition of low temperature based on the transition process from magnetic disorder to order in paramagnetic salts, and obtained a minimum temperature of 250 mK by using Gd2(SO4)3.8H2O paramagnetic salt, and won the Nobel Prize in Chemistry in 1949.
Entropy is a measure of the disorder of the system, the larger the disorder, the higher the entropy, and the entropy of the system will be reduced by temperature and magnetization. A paramagnetic substance can produce an additional magnetic field in the same direction as the original magnetic field in the magnetic field, which is called the excitation process. The entropy of the spin system in the paramagnetic salt needs to increase during the demagnetization process, but the entropy of the whole system can only flow from the lattice system into the spin system due to the constant adiabatic total entropy. Since the entropy of lattice systems is usually very small, a significant cooling is required for the spin system to remain essentially the same. This is known as the magnetocaloric effect.
Magnetic refrigeration is a solid-state refrigeration technology developed based on the magnetocaloric effect of magnetic materials releasing heat in the excitation state and absorbing heat in the demagnetization state, and its essence is the change of the order degree of the magnetic moment inside the magnetic material. Materials with this effect are called magnetocaloric materials, also known as magnetothermal materials.
Magnetic refrigeration is a solid-state refrigeration technology, which has the advantages of wide range of operating temperature zones, high intrinsic refrigeration efficiency and wide range of material resources, and is one of the most potential refrigeration technologies in the field of low temperature and refrigeration. In recent years, a large number of magnetocaloric materials in different temperature zones have been discovered and synthesized, and magnetic refrigeration has gradually developed into a refrigeration and cryogenic technology that can cover the whole temperature zone. Especially in the field of ultra-low temperature, ultra-low temperature magnetic refrigeration (also known as adiabatic demagnetization refrigeration) is not affected by gravity and does not rely on the scarce working fluid helium-3, and has gradually become a hot cryogenic technology in cutting-edge science such as space exploration and quantum computing.
The principle of refrigeration of magnetic refrigeration materials
| name | Technical features: | Typical research institutes |
| Extremely low temperature magnetic refrigeration (≤1 K) | Magnetic refrigeration Carnot cycles are constructed by using magnetic materials such as paramagnetic salt hydrates, and heat transfer is strengthened by using structures such as thermal buses in magnetocaloric modules to obtain cooling capacity | Institute of Physics and Chemistry of Chinese Academy of Sciences, Beijing Institute of Technology, Tsinghua University, Ganjiang Institute of Chinese Academy of Sciences, NASA, European Space Agency, etc |
| 低温磁制冷（4~20 K） | Using the magnetic materials represented by RFeSi (R is rare earth) base, the active magnetic refrigeration or composite refrigeration cycle is constructed, and the heat transfer medium is usually helium | Institute of Physics and Chemistry of Chinese Academy of Sciences, Beijing Institute of Technology, Institute of Physics of Chinese Academy of Sciences, Ganjiang Institute of Chinese Academy of Sciences, National Institute of Materials Research of Japan, Pacific Northwest Laboratory of the United States, etc |
| Magnetic refrigeration at room temperature | Magnetic materials such as Gd-based and laFeSi-based are used to construct an active magnetic refrigeration cycle, and the heat transfer medium is usually water and other mediums | Institute of Physics and Chemistry of Chinese Academy of Sciences, Baotou Rare Earth Research Institute, Institute of Physics of Chinese Academy of Sciences, Technical University of Denmark, University of Victoria, Canada, etc |
Characteristics of magnetic refrigeration technology in different temperature zones and typical research institutions
Magnetic refrigeration system
03
New ultra-solid substances are used for refrigeration
A supersolid state is a novel quantum state of matter that appears near absolute zero, in which atoms in matter appear in a regular arrangement while flowing "viscous-free" between them, combining the seemingly contradictory characteristics of solid and superfluid. Since the ultra-solid state was put forward as a theoretical speculation in the 70s of the 20th century, scientists from all over the world have not found reliable experimental evidence for the existence of ultra-solid matter in solid matter.
Recently, a joint research team from Su Gang's team at the Chinese Academy of Sciences and Beijing University of Aeronautics and Astronautics has discovered for the first time a novel state of matter called "spin supersolid" in a quantum magnetic material called "spin supersolid" in a cobalt-based frustration triangular lattice material, sodium barium cobalt salt (Na2BaCo(PO4)2), and obtained experimental evidence of its existence. Subsequently, the researchers observed that the magnetic material exhibited a huge magnetocaloric effect associated with spin supersolids during the demagnetization and cooling process, which gave the new material excellent potential as a high-efficiency magnetic refrigeration material.
Cobalt-based frustration triangular lattice materials
In the subsequent research, the researchers overcame many technical problems such as heat leakage and temperature measurement at extremely low temperatures, and developed a new type of cryogenic measurement device, which used the material to obtain an extremely low temperature of 94mK, that is, minus 273.056 degrees Celsius, through a quasi-adiabatic demagnetization process, to achieve ultra-low temperature refrigeration without liquid helium, and named this newly discovered effect "spin ultra-solid giant magnetic card effect".
Due to the huge magnetic card effect of spin ultrasolid materials, the development of new magnetic refrigerators using this effect will provide an extremely low temperature environment and sufficient cooling capacity for deep space exploration or quantum computing. It can also solve the problem of lack of helium and high-end dilution refrigerators in the mainland.
The target material is the pink crystal