Application of KYb(WO4)2 single crystal in ultra-low temperature refrigeration

Abstract

The application provides application of KYb(WO4)2 single crystal in extremely low temperature refrigeration. Due to the influence of rare earth ion crystal field, the spin direction of the magnetic ion of the KYb(WO4)2 single crystal is limited to a specific angle, so that the interaction of the magnetic dipole moment thereof is only in the order of millidegrees, and the extremely low temperature refrigeration in the millidegree level can be realized under a very small magnetic field. Meanwhile, the crystal is easy to grow and does not contain crystal water, and the physical properties, mechanical and thermal properties thereof are extremely stable compared with traditional paramagnetic salts. Therefore, in application, the complex noble metal-based thermal bus process can be avoided, the cost of the extremely low temperature adiabatic demagnetization refrigeration is greatly reduced under the premise of keeping the same refrigeration effect, and the stability and reliability of a prototype in an extreme environment such as outer space are improved.

Description

Technical Field

[0001] This invention belongs to the field of cryogenic refrigeration technology, specifically relating to the application of KYb(WO4)2 single crystals in cryogenic refrigeration. Background Technology

[0002] With the continuous development of condensed matter physics, space observation and quantum technology, the demand for cryogenic refrigeration is increasing.

[0003] Currently, the mainstream cryogenic refrigeration technologies include adiabatic demagnetization refrigeration (ADR) and dilution refrigeration (DR). DR utilizes... 3 Cooling is achieved by the endothermic reaction of He atoms flowing from the dense phase to the dilute phase at extremely low temperatures. It can achieve continuous cooling and generate a large cooling capacity, but DR also requires scarce and expensive gaseous working fluids, and relies on gravity phase separation, oil-free mechanical pump sets and complex gas circuits, which greatly limits its application in special occasions such as space.

[0004] ADR (Alternating Demagnetizing) is a solid-state cooling method that utilizes the magnetocaloric effect of magnetocaloric materials to achieve extremely low temperatures. Compared to DR (Dry Cooling), ADR offers advantages such as high efficiency, independence from gravity, and precise temperature control. Furthermore, its compact structure gives it a natural advantage in deep space exploration, defense, and aerospace applications, making it a core technology internationally. Several space cooling projects have already adopted adiabatic demagnetizing refrigeration technology to achieve extremely low temperatures below 100 mK: internationally, there are satellite systems such as ASRTO-E, ASRO-H, and SPICA; domestically, there are planned projects such as the next-generation primordial gravitational wave telescope. ADR's unique advantages, such as not requiring expensive... 3 He gas as the working fluid, compact design, and low price make it well-suited to meet the above requirements.

[0005] However, suitable materials are still lacking for the development of cryogenic systems. Summary of the Invention

[0006] The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. To this end, the present invention provides the application of KYb(WO4)2 single crystals in cryogenic refrigeration. KYb(WO4)2 single crystals possess stable crystal properties and can achieve cryogenic refrigeration capabilities down to extremely low temperatures.

[0007] The first aspect of the present invention provides the application of KYb(WO4)2 single crystals in cryogenic refrigeration.

[0008] Entropy is a measure of the disorder of a system: the greater the disorder, the higher the entropy. When a system undergoes an adiabatic process, the entropy becomes zero. It's easy to understand that the main factors affecting the entropy change of a magnetic medium system are lattice entropy and magnetic entropy. For a magnetic medium molecular system with N spins, the increase or decrease in lattice entropy is closely related to the thermal motion of the magnetic medium molecules; while the change in magnetic entropy is inseparable from the changes in the system's temperature and magnetic field. In the experiment, without applying a magnetic field, the system temperature is increased until the occupancy of each medium molecule's 2S+1 states is the same. If we take G, it represents the total number of ways to arrange the N spins into the 2S+1 occupied states, i.e., G = (2S+1). N Then the magnetic entropy σ of the system s =k B lnG=Nk B ln(2S+1), at which point the spins of the system are completely disordered. Then, while keeping the temperature constant, the magnetic field is slowly increased to separate the energies of these 2S+1 states. If the population (i.e., the number of particles) in the low-energy states increases, the magnetic entropy of the system decreases. At this point, the spin orientation tends to align with the direction of the external field. This is the isothermal magnetization process of the magnetic medium.

[0009] For a specific magnetic medium system, as the quasi-static magnetic field slowly increases, the magnetic moments of the molecules within the medium tend to align parallel to the direction of the external magnetic field, reducing spin disorder (i.e., decreasing the magnetic entropy and increasing the lattice entropy). This indicates an increase in the intensity of molecular thermal motion and a rise in system temperature. At this point, the system temperature can be kept constant through heat exchange with the external environment. Once the system is fully polarized, quasi-statically reducing the magnetic field to zero achieves adiabatic demagnetization. The spin orientation of the molecules in the medium returns to disorder, increasing magnetic entropy and decreasing lattice entropy. This reduces the intensity of molecular thermal motion, lowering the temperature of the magnetic medium and achieving millikan-level cryogenic cooling. At extremely low temperatures, because the interactions between molecules in the magnetic medium are not negligible, the magnetic entropy of the system will increase slightly, while the lattice entropy will decrease slightly until a new equilibrium is reached.

[0010] Therefore, the ideal material for achieving adiabatic demagnetization refrigeration needs to meet the following conditions: low lattice entropy, high magnetic ion density, multiple ground-state energy levels, high and low thermal conductivity, and low temperature-time interaction. Currently widely used paramagnetic salts, such as ferric ammonium sulfate (FAA: FeNH4(SO4)2·12H2O) and potassium chromium alum (CPA: CrK(SO4)2·12H2O), contain a large amount of water of crystallization, resulting in relatively small interactions between magnetic ions, which brings many benefits to adiabatic demagnetization refrigeration. However, in the experimental environment of extremely low temperature and high vacuum, these paramagnetic salts easily lose their water of crystallization, altering their crystal structure, which also poses a considerable challenge to their growth and packaging processes.

[0011] In addition, the thermal conductivity of the paramagnetic salt working medium containing crystal water is very low at extremely low temperature, in order to increase the axial heat conduction and overcome the Kapitza thermal resistance at low temperature, the solution that can be used is the metal heat bus method. The person needs to penetrate thousands of pure gold wires and high-purity copper wires in the FAA and CPA crystals to greatly increase the axial heat conduction capacity and radial heat transfer area. At present, the price of gold per gram is about 450 yuan, and the cost of thousands of gold wires of about 500 grams is about 220,000 yuan, or the second choice is to choose high-purity oxygen-free copper wires, and the cost is also considerable.

[0012] Further, gadolinium gallium garnet (GGG: Gd3GaO 15 ) and other spin frustration materials can also be used as adiabatic demagnetization refrigeration materials. From the crystal structure, the rare earth magnetic Gd 3+ ion in GGG is just at the vertex of a perfect non-distorted equilateral triangle, and different triangles are connected into a two-dimensional spin frustration cage network (2D Hyper-Kagome lattice) by sharing the vertex, which greatly makes up for the defect that the distance between the nearest neighbor Gd 3+ ions is small, and the strong quantum spin fluctuation caused by the spin frustration of the cage network greatly reduces the temperature at which the magnetic order appears, thereby greatly enhancing the ability of adiabatic demagnetization refrigeration at extremely low temperature. However, the lowest application temperature of GGG can only reach 500 mK, and refrigeration at 100 mK cannot be realized.

[0013] Further, although some powder polycrystalline samples of paramagnetic salts without crystal water can be used to realize extremely low temperature refrigeration, compared with high-quality single crystal samples, the powder samples at least have the following problems: (1) the thermal conductivity of the powder sample is low at extremely low temperature, which is not conducive to the timely exchange of heat and cold; (2) the powder sample has no anisotropy, and the low-temperature refrigeration of the circulating rotation type cannot be realized.

[0014] The present application relates to a technical solution of the application of KYb(WO4)2 single crystal in extremely low temperature refrigeration, which has at least the following beneficial effects:

[0015] The application of KYb(WO4)2 single crystal in extremely low temperature refrigeration can synthesize large single crystals and new quantum magnetic material systems with practical application potential, which has important scientific research and practical dual values. Specifically:

[0016] Firstly, the existing magnetic refrigeration working medium that can reach 100 mK and below is mostly paramagnetic salt containing crystal water. Its main defects are: low thermal conductivity at extremely low temperature, easy loss of crystal water leading to structure collapse, and then affecting the low-temperature refrigeration capacity. Although the current heat bus technology can solve this problem to some extent, it also needs very expensive and complex packaging and growth process.

[0017] In the second aspect, the existing paramagnetic salt without crystal water, such as GGG, etc., although it overcomes the difficulty of encapsulation and low thermal conductivity, but its minimum refrigeration temperature can only reach about 500 mK.

[0018] In the third aspect, the existing research on KYb(WO4)2crystal is mostly focused on optical properties and its application in laser. Even if the magnetism of KYb(WO4)2single crystal is studied, the minimum temperature is at most 5K, and the research in the extremely low temperature region, especially in the 1K temperature region, is lacking. According to the existing research, it cannot be inferred whether KYb(WO4)2single crystal has refrigeration capacity below 5K, and it is impossible to determine whether it can be applied to refrigeration in the 100mK temperature region.

[0019] In the fourth aspect, the present application discovers the magnetic and thermodynamic behavior of KYb(WO4)2in the extremely low temperature region. According to the specific heat and magnetic entropy data in the application of the present application, it is conservatively estimated that the minimum refrigeration temperature of KYb(WO4)2can reach the 80mK temperature region, and it has the potential to be lower to the 30mK temperature region.

[0020] In summary, KYb(WO4)2has the same level of refrigeration capacity in the 100mK temperature region as traditional paramagnetic salts, while maintaining stable crystal properties. The application of KYb(WO4)2single crystal does not require special encapsulation process. In addition, compared with the existing inorganic oxide paramagnetic salt GGG, the stability of KYb(WO4)2crystal is similar, but its refrigeration capacity can reach the extremely low temperature that GGG cannot reach. Therefore, the application of KYb(WO4)2single crystal in the extremely low temperature refrigeration has the advantages of the above two systems, but avoids their shortcomings, which can greatly improve the existing technology.

[0021] According to some embodiments of the present application, the extremely low temperature is ≤100mK.

[0022] According to some embodiments of the present application, the extremely low temperature is ≤80mK.

[0023] According to some embodiments of the present application, the extremely low temperature is ≤30mK.

[0024] According to some embodiments of the present application, the application includes superconductor material preparation, biological sample preservation and space science.

[0025] According to some embodiments of the present application, the application in the extremely low temperature refrigeration also includes satellite detection, quantum computing and frontier detection of condensed matter physics.

[0026] According to some embodiments of the present application, the preparation method of KYb(WO4)2single crystal comprises the following steps:

[0027] S1: grinding and mixing K2CO3, WO3, Yb2O3 and a cosolvent in a solvent;

[0028] S2: sintering the product of step S1 to obtain KYb(WO4)2 single crystal.

[0029] According to some embodiments of the present application, in step S1, the molar ratio of K2CO3, WO3 and Yb2O3 is 1:3-5:1.

[0030] According to some embodiments of the present application, in step S1, the molar ratio of K2CO3, WO3 and Yb2O3 is 1:1:4.

[0031] According to some embodiments of the present application, in step S1, the molar ratio of the sum of K2CO3, WO3 and Yb2O3 to the cosolvent is 1:5-10.

[0032] According to some embodiments of the present application, in step S1, the molar ratio of the sum of K2CO3, WO3 and Yb2O3 to the cosolvent is 1:5-10.

[0033] According to some embodiments of the present application, in step S1, the cosolvent comprises K2W2O7.

[0034] According to some embodiments of the present application, in step S1, the solvent comprises ethanol.

[0035] According to some embodiments of the present application, in step S2, the sintering procedure is: increasing the temperature to 920℃ within 5h, keeping the temperature for 16-24h, decreasing the temperature to 870℃ after the reactants melt, decreasing to room temperature after the crystal crystallizes.

[0036] According to some embodiments of the present application, in step S2, the time for decreasing the temperature to 870℃ after the reactants melt is 80h-120h.

[0037] According to some embodiments of the present application, in step S2, the time for decreasing to room temperature after the crystal crystallizes is ≥5h. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a graph of the specific heat of KYb(WO4)2 single crystal versus temperature under different magnetic fields.

[0039] Figure 2 is a graph of the entropy of KYb(WO4)2 single crystal versus temperature under different magnetic fields.

[0040] Figure 3 is a graph of the energy gap in the two-energy-level system of KYb(WO4)2 single crystal versus magnetic field.

[0041] Figure 4is the plot of the magnetic field intensity of KYb(WO4)2 single crystal as a function of magnetic field at different temperatures.

[0042] Figure 5 is the plot of the magnetic field intensity of KYb(WO4)2 single crystal as a function of B / T at different temperatures and the fitting plot of the Brillouin function of paramagnetic system.

[0043] Figure 6 is the plot of the magnetic specific heat of KYb(WO4)2 single crystal as a function of magnetic field at different temperatures. DETAILED DESCRIPTION

[0044] The following are specific embodiments of the present application, and the technical solutions of the present application are further described in combination with the embodiments, but the present application is not limited to these embodiments.

[0045] In some embodiments of the present application, the present application provides the application of KYb(WO4)2 single crystal in ultra-low temperature refrigeration.

[0046] It should be noted that entropy is a measure of the degree of disorder of a system, and the greater the degree of disorder, the higher the entropy. When the system undergoes an adiabatic process, the entropy change is zero. It is easy to understand that the main factors affecting the entropy change of the magnetic medium system are the lattice entropy and the magnetic entropy. For a magnetic medium molecular system with N spins of S, the increase or decrease of the lattice entropy is closely related to the thermal motion of the magnetic medium molecules; while the change of the magnetic entropy is inseparable from the change of the temperature and the magnetic field of the system. When the temperature of the system is increased without applying a magnetic field, the occupation of each medium molecule is the same as 2S+1 state, if G is the total number of all ways of arranging N spins in 2S+1 occupied states, i.e. G=(2s+1) N , then the magnetic entropy σ s of the system is B lnG=Nk B ln(2S+1), at this time the spins are completely disordered. Then the temperature is kept constant and the magnetic field is slowly increased to separate the energy of the 2S+1 states, if the population (i.e. the number of particles) in the low-energy state increases, the magnetic entropy of the system decreases, at this time the orientation of the spin tends to the direction of the external field, which is the isothermal magnetization process of the magnetic medium.

[0047] For a specific magnetic medium system, as the quasi-static magnetic field slowly increases, the magnetic moment of the molecules in the medium tends to be parallel to the direction of the external magnetic field, the disorder degree of the spin decreases, i.e., the magnetic entropy of the system decreases, and the lattice entropy increases. This indicates that the intensity of the thermal motion of the molecules of the magnetic medium increases, and the temperature of the system increases. At this time, the temperature of the system can be kept constant by heat exchange between the system and the outside world. After the system is completely polarized, the magnetic field is quasi-statically reduced to zero to realize adiabatic demagnetization, the spin orientation of the molecules in the medium recovers disorder, the magnetic entropy increases, the lattice entropy decreases, the intensity of the thermal motion of the molecules decreases, and the temperature of the magnetic medium decreases, i.e., the low-temperature refrigeration of the order of millikelvin can be realized. However, at extremely low temperatures, the interaction between the molecules in the magnetic medium cannot be ignored, so the magnetic entropy of the system slightly increases, and the lattice entropy slightly decreases until a new equilibrium is reached.

[0048] Therefore, the ideal material for realizing adiabatic demagnetization refrigeration needs to meet the following conditions: small lattice entropy, high magnetic ion density, multiple ground state energy levels, high thermal conductivity, and small interaction at low temperatures. The paramagnetic salt currently widely used, such as ferric ammonium sulfate (FAA: FeNH4(SO4)2·12H2O) and chrome potassium alum (CPA: CrK(SO4)2·12H2O), has relatively small interaction between magnetic ions due to the presence of a large amount of crystal water, which brings many benefits to adiabatic demagnetization refrigeration. However, in an extremely low-temperature high-vacuum experimental environment, these paramagnetic salts easily lose crystal water and change their crystal structure, which poses a considerable challenge to their growth and packaging processes.

[0049] In addition, the thermal conductivity of the paramagnetic salt working medium containing crystal water at extremely low temperatures is very low. In order to increase the axial thermal conductivity and overcome the Kapitza thermal resistance at low temperatures, the metal thermal bus method can be used. The personnel need to axially insert thousands of pure gold wires and high-purity copper wires into the FAA and CPA crystals to significantly increase their axial thermal conductivity and radial heat transfer area. The current price of gold per gram is around 450 yuan, and thousands of gold wires with a weight of about 500 grams cost about 220,000 yuan. Alternatively, even if high-purity oxygen-free copper wires are selected, the cost is also considerable.

[0050] Further, gadolinium gallium garnet (GGG: Gd3GaO 15 ) and other spin frustration materials can also be used as adiabatic demagnetization refrigeration materials. From the crystal structure, the rare earth magnetic Gd 3+ ions in GGG are located at the vertices of a perfect undistorted equilateral triangle, and different triangles are connected into a two-dimensional spin frustration cage superlattice network (2D Hyper-Kagome lattice) by sharing vertices, which largely compensates for the nearest neighbor Gd 3+The defects with small distance between ions, strong quantum spin fluctuation caused by spin frustration characteristic of cage lattice, greatly reduce the temperature of magnetic order, thus greatly enhancing the ability of adiabatic demagnetization refrigeration at extremely low temperature. However, the lowest application temperature of GGG can only reach 500 mK temperature range, and cannot achieve refrigeration at 100 mK temperature range.

[0051] Further, although some powder polycrystalline samples of paramagnetic salt without crystal water can be used to achieve extremely low temperature refrigeration, compared with high-quality single crystal samples, the powder samples at least have the following problems: (1) the thermal conductivity of the powder sample is low in the extremely low temperature range, which is not conducive to the timely exchange of heat and cold; (2) the powder sample has no anisotropy, and cannot realize the low-temperature refrigeration of the circulating rotation type.

[0052] It can be understood that the application of KYb(WO4)2 single crystal in extremely low temperature refrigeration can synthesize large single crystals and new quantum magnetic material systems with practical application potential, which has important scientific research and practical dual values. Specifically:

[0053] Firstly, the existing magnetic refrigeration working medium that can reach 100 mK and below temperature range is mostly paramagnetic salt containing crystal water. Its main defects are: low thermal conductivity at extremely low temperature, easy loss of crystal water leading to structure collapse, and thus affecting the low-temperature refrigeration capacity. The current thermal bus technology can solve this problem to some extent, but it also requires very expensive and complex packaging and growth process.

[0054] Secondly, the existing paramagnetic salt without crystal water, such as GGG, although it overcomes the difficulties of packaging and low thermal conductivity, its lowest refrigeration temperature can only reach about 500 mK.

[0055] Thirdly, the existing research on KYb(WO4)2 crystal is mostly focused on optical properties and its application in lasers. Even if the magnetism of KYb(WO4)2 single crystal is studied, the lowest temperature is at most 5K, and there is a lack of research at extremely low temperature, especially in the 1K temperature range. According to the existing research, it cannot be inferred whether KYb(WO4)2 single crystal has refrigeration capacity below 5K temperature range, and it is impossible to determine whether it can be applied to refrigeration at 100 mK temperature range.

[0056] Fourthly, the present application discovers the magnetic and thermodynamic behaviors of KYb(WO4)2 at extremely low temperature. According to the specific heat and magnetic entropy data in the application of the present application, it is conservatively estimated that the lowest refrigeration temperature of KYb(WO4)2 can reach 80 mK temperature range, and it has the potential to be lower to 30 mK temperature range.

[0057] In summary, KYb(WO4)2 has the same level of refrigeration capacity as traditional paramagnetic salts in the 100 mK temperature range, while maintaining stable crystal properties. The application of KYb(WO4)2 single crystals does not require special packaging processes. In addition, compared with existing inorganic oxide paramagnetic salt GGG, KYb(WO4)2 has similar stability of crystal properties, but its refrigeration capacity can reach an extremely low temperature that GGG cannot reach. Therefore, the application of KYb(WO4)2 single crystals in extremely low temperature refrigeration combines the advantages of the above two systems, while avoiding their disadvantages, which can greatly improve the existing technology.

[0058] In some embodiments of the present application, the extremely low temperature is ≤ 100 mK.

[0059] In some embodiments of the present application, the extremely low temperature is ≤ 80 mK.

[0060] In some embodiments of the present application, the extremely low temperature is ≤ 30 mK.

[0061] In some embodiments of the present application, the applications include superconductor material preparation, biological sample preservation, and space science.

[0062] It can be understood that, because the superconducting properties of superconductors usually require extremely low temperatures to be realized, there is a close relationship between extremely low temperature refrigeration and superconductor materials. The relationship between extremely low temperature refrigeration and superconductor materials includes:

[0063] Superconductivity: Superconductivity is a property of a material that loses electrical resistance at low temperatures, i.e. electric current can flow through it without losing energy. This property is very rare at room temperature and usually requires very low temperatures to be realized.

[0064] Critical temperature (Tc): Each superconductor material has a specific critical temperature below which the material exhibits superconductivity. For most superconductors, this critical temperature is in the extremely low temperature range, usually close to absolute zero (-273 degrees Celsius or 0 Kelvin).

[0065] Refrigeration requirements: Since superconductor materials need to be at very low temperatures to exhibit superconductivity, refrigeration is key to realizing this property. Common methods of extremely low temperature refrigeration include using liquid helium (-269°C) or liquid nitrogen (-196°C) to cool the sample.

[0066] The superconducting properties of superconductor materials have important applications in many fields, including magnetic resonance imaging (MRI), particle accelerators, maglev trains, power transmission, etc. These applications usually require operation at low temperatures, so special low-temperature cooling systems are required.

[0067] In summary, cryogenic refrigeration is essential for realizing the superconducting properties of superconductor materials, which have wide applications in scientific and engineering fields, particularly in areas requiring high current density and low energy consumption.

[0068] It is understood that cryogenic refrigeration has wide applications in biological sample preservation, mainly to maintain the quality, integrity and long-term preservation of biological samples. Specific applications include at least:

[0069] Cell preservation: Cryogenic refrigeration is used to preserve cell lines, cell cultures and cell samples. Freezing cell samples can prevent cell death and cytoplasm degradation, helping to maintain their biological activity for future research and application, such as drug screening, tissue engineering and regenerative medicine.

[0070] Tissue samples: In biomedical research, medical diagnosis and organ transplantation, tissue samples such as liver, kidney, heart, etc. need to be preserved. Cryogenic refrigeration ensures that these tissue samples are not damaged in the frozen state, so as to carry out pathological research and clinical analysis.

[0071] Genetic material: DNA and RNA samples need to be preserved at low temperature to maintain their stability. These samples are commonly used in molecular biology research, genetics research, criminology and clinical genetics analysis.

[0072] Preservation of infectious disease viruses: Cryogenic preservation is used to preserve infectious disease viruses for vaccine development, drug research and epidemiological research. This is crucial for long-term preservation of viruses and future research.

[0073] Stem cell preservation: Stem cells are cells that have the potential to differentiate into various cell types, used in tissue engineering, regenerative medicine and drug screening. They need to be preserved at low temperature to maintain their stemness and biological activity.

[0074] Vaccine preservation: Some vaccines need to be preserved at extremely low temperatures to maintain their effectiveness. For example, the viral mRNA in the COVID-19 vaccine needs to be preserved at extremely low temperatures to ensure that the vaccine does not lose its effectiveness during distribution and vaccination.

[0075] Biological repository: Research institutions, medical institutions and biomedical companies usually maintain biological sample libraries, which require cryogenic refrigeration facilities to store a large number of biological samples for future research and application.

[0076] In summary, cryogenic refrigeration plays a key role in biological sample preservation, ensuring the long-term preservation of biological samples and maintaining their quality and integrity for future research and medical applications. This is very important for medical research, drug development, genetics research and clinical diagnosis.

[0077] It is understood that cryogenic refrigeration also has a variety of applications in space science, such as:

[0078] Space telescopes: Space telescopes such as the Hubble Space Telescope and the James Webb Space Telescope require cryogenic refrigeration to cool their infrared and ultraviolet detectors. This can reduce noise and improve observation performance for deep space observations and research into celestial phenomena in the universe.

[0079] Space electronics: Electronic devices and sensors in space need to operate in a low-temperature environment to ensure their stability and performance. This includes satellite communication equipment, detectors, satellite navigation systems, etc.

[0080] Space experiments: Some scientific experiments need to be conducted at extremely low temperatures to simulate extreme conditions in space. These experiments may involve cryogenic atomic physics, ultra-cold molecular research, and low-temperature materials science.

[0081] Superconducting magnets: Some tasks in space science require strong magnetic fields, such as ion propulsion, particle detection, and magnetic science research. Superconducting magnets usually need to operate at extremely low temperatures to maintain superconducting properties, thereby generating stronger magnetic fields.

[0082] Planetary exploration: In planetary exploration missions, some instruments and equipment need to operate in extreme temperature conditions to adapt to the environment of the target planet or satellite. Cryogenic refrigeration is used to maintain the performance of these devices.

[0083] Remote sensing detectors: Remote sensing detectors in space, such as radar, infrared imaging instruments, and spectrometers, require low-temperature cooling to obtain high-resolution data for Earth observation, resource exploration, and Earth science research.

[0084] It is easy to understand that cryogenic refrigeration plays a key role in space science, supporting various space missions and scientific research. It helps improve the performance of instruments and equipment, expand the field of scientific research, and better understand the universe and the Earth.

[0085] In some embodiments of the present application, the method for preparing KYb(WO4)2 single crystals comprises the following steps:

[0086] S1: grinding and mixing K2CO3, WO3, Yb2O3 and a cosolvent in a solvent;

[0087] S2: sintering the product of step S1 to obtain the KYb(WO4)2 single crystal of the present application.

[0088] In some embodiments of the present application, in step S1, the molar ratio of K2CO3, WO3 and Yb2O3 is 1:3-5:1.

[0089] In some embodiments of the present application, in step S1, the molar ratio of K2CO3, WO3 and Yb2O3 is 1:4:1.

[0090] In some embodiments of the present application, in step S1, the molar ratio of the sum of K2CO3, WO3 and Yb2O3 to the cosolvent is 1:5-10.

[0091] In some embodiments of the present application, in step S1, the molar ratio of the sum of K2CO3, WO3 and Yb2O3 to the cosolvent is 1:6-9.

[0092] In some embodiments of the present application, in step S1, the solvent comprises ethanol.

[0093] In some embodiments of the present application, in step S2, the sintering procedure is: heating to 920℃ within 5h, keeping for 16-24h, after the reactants melt, cooling to 870℃, after the crystal crystallizes, cooling to room temperature.

[0094] In some embodiments of the present application, in step S2, after the reactants melt, the time for cooling to 870℃ is 80h-120h.

[0095] In some embodiments of the present application, in step S2, after the crystal crystallizes, the time for cooling to room temperature is ≥5h.

[0096] The technical solutions of the present application will be better understood in combination with the specific embodiments below.

[0097] Embodiment

[0098] This embodiment first prepares KYb(WO4)2 single crystal, specifically comprising the following steps:

[0099] S1: grinding and mixing K2CO3, WO3, Yb2O3 and cosolvent in solvent;

[0100] S2: sintering the product of step S1 to obtain the KYb(WO4)2 single crystal of the present application.

[0101] In step S1:

[0102] The molar ratio of K2CO3, WO3 and Yb2O3 is 1:4:1.

[0103] The molar ratio of K2CO3, WO3, Yb2O3 and cosolvent is 11.5:88.5, the cosolvent is K2W2O7, the mortar is agate mortar, and the solvent is ethanol.

[0104] Among them, K2CO3 is superior pure, WO3 purity is 99%, and Yb2O3 purity is 99%.

[0105] Grind thoroughly to ensure the reactants are mixed evenly. Wait until the ethanol has completely evaporated and the reactants are dry and free of particles, then gently transfer them into a crucible using weighing paper for later use.

[0106] Then, the crucible is placed in a box furnace to grow KYb(WO4)2 single crystals. The temperature curve is set as follows: the temperature is raised to 920℃ after 5 hours and held at this temperature for 20 hours to allow the reactants to fully melt. Then, the temperature is slowly lowered to 870℃ after 100 hours. After the crystal crystallizes, it is lowered to room temperature after 5 hours to obtain colorless and transparent irregular blocky single crystals.

[0107] Single-crystal XRD diffraction confirmed that the crystal prepared in this experiment was indeed a KYb(WO4)2 single crystal.

[0108] Figure 1 This is a graph showing the change of specific heat with temperature. The graph indicates that the specific heat decreases almost exponentially with temperature under different magnetic fields, while at higher temperatures it decreases with temperature (T). 2 The specific heat exhibits an inverse proportional relationship, and its extreme value remains constant at 3.6 J / K / mol under different magnetic fields. Furthermore, as the magnetic field decreases, the extreme value of the specific heat shifts to lower temperatures, with the temperature corresponding to the extreme value varying with the magnetic field (see reference). Figure 3 As shown, the system exhibits a linear dependence, which is the well-known Schottky specific heat effect of a two-level system. At zero field, due to measurement limitations, the maximum specific heat could not be measured even at the measurement limit of 0.1 K. The experimental values ​​can be fitted using the Schottky equation for a two-level system. The fitting results show that, within the allowable range of experimental error, the temperature corresponding to the maximum specific heat is around 80 mK, meaning the interaction of the system is only on the order of tens of milliks. That is, when the isothermally magnetized magnetic medium system is adiabatically demagnetized, the magnetic medium molecules, which are identically distributed on the two levels, redistribute themselves across different energy levels of the system's 2S+1 state. This increases the magnetic entropy and decreases the lattice entropy, thus achieving extremely low-temperature cooling of 80 mK.

[0109] Figure 2 This indicates that under zero field conditions, the entropy of the system quickly reaches the saturation value Rln2 as the temperature increases. At this point, if the temperature of the system is kept at 2K and the magnetic field is increased to 2T (e.g., ...), the system will be in a quasi-static state. Figure 2 (As shown by the vertical arrow in the diagram), the system undergoes an isothermal magnetization process. After the system stabilizes, it is adiabatic demagnetized, as shown in the diagram. Figure 2 As shown by the horizontal arrow, the system undergoes an isentropic adiabatic process, with the temperature dropping to the millikan level.

[0110] Furthermore, the magnetic field strength, specific heat, and magnetocaloric properties of the prepared KYb(WO4)2 single crystal were further characterized using a magnetic measurement system (MPMS) and a physical property measurement system (PPMS).

[0111] The lowest temperature used for magnetic susceptibility measurement is 1.8K (e.g.,Figure 4 The data of the field sweep at different temperatures (as shown in Fig. 2) show that the required field to fully magnetize the system decreases as the temperature decreases.

[0112] The measured data were then fitted with the Brillouin function of paramagnetic system (as shown in Fig. 3, the grey dashed line is the fitted curve of the Brillouin function), and the fitting results fully demonstrate the paramagnetic properties of the KYb(WO4)2 single crystal. Figure 5 Figure 5 In the figure, the grey dashed line is the fitted curve of the Brillouin function), and the fitting results fully demonstrate the paramagnetic properties of the KYb(WO4)2 single crystal.

[0113] In addition, the specific heat data of the KYb(WO4)2 single crystal is as low as 0.25 K (as shown in Fig. 4), and no abnormal peak of the specific heat can be observed in the temperature measurement range of the instrument, which indicates that no long-range ordered phase is established in the sub-temperature region. That is, the data of the magnetic field strength and the magnetic specific heat show that the KYb(WO4)2 single crystal does not have a long-range ordered phase transition even in the sub-temperature region, and it is a very good paramagnetic material. Figure 6

[0114] Therefore, at least the following conclusions can be drawn:

[0115] (1) The KYb(WO4)2 single crystal does not contain crystalline water, has stable physical properties, and has high thermal conductivity, which overcomes the difficulties of low thermal conductivity of some powder samples and unstable properties of some paramagnetic salts containing crystalline water, and does not require complex preparation process and high preparation cost;

[0116] (2) Compared with the lowest refrigeration temperature region of 500 mK reported by GGG, the lowest refrigeration temperature region of the KYb(WO4)2 single crystal is 80 mK, and its refrigeration capacity has achieved a breakthrough of one order of magnitude, which is of great significance to the low-temperature refrigeration technology;

[0117] (3) Due to the clever balance of magnetic dipole interaction and exchange interaction, KYb(WO4)2 is a paramagnetic material with small interaction;

[0118] (4) The KYb(WO4)2 single crystal can be grown into a large single crystal by the Kyropoulos method, and the low-temperature refrigeration of the circulating rotation type can be realized. Therefore, the application of KYb(WO4)2 crystal to achieve adiabatic demagnetization refrigeration is of great significance to the further development of the low-temperature technology.

[0119] ​​The spin direction of the magnetic ion of the KYb(WO4)2 single crystal is limited to a specific angle due to the influence of the rare earth ion crystal field, so that the interaction of the magnetic dipole moment thereof is only in the order of millidegrees, which can realize extremely low temperature refrigeration in the order of millidegrees under a very small magnetic field. Meanwhile, the crystal is easy to grow and does not contain crystal water, and the physical properties, mechanical and thermal properties thereof are extremely stable relative to traditional paramagnetic salts. Therefore, in application, the complex noble metal-based thermal bus process can be avoided, the cost of the extremely low temperature adiabatic demagnetization refrigeration is greatly reduced under the premise of maintaining the same refrigeration effect, and the stability and reliability of the prototype in extreme environments such as outer space are improved at the same time.

[0120] The application has been described in detail above in combination with the embodiments, but the application is not limited to the above embodiments, and various changes can be made within the knowledge range of the ordinary skill in the art without departing from the purpose of the application.

Claims

1. The application of KYb(WO4)2 single crystal in cryogenic refrigeration, characterized in that, The extremely low temperature is ≤100mK, and the KYb(WO4)2 single crystal is prepared by the following steps: S1: Grind and mix K2CO3, WO3, Yb2O3 and co-solvent in a solvent; S2: The product of sintering step S1, to obtain the KYb(WO4)2 single crystal; In step S1, the molar ratio of K2CO3, WO3 and Yb2O3 is 1:3 to 5:1; In step S2, the sintering procedure is as follows: heat up to 920℃ within 5 hours, hold for 16~24 hours, after the reactants melt, cool down to 870℃ for 80~120 hours, and after the crystals crystallize, cool down to room temperature.

2. The application according to claim 1, characterized in that, The applications include superconductor material preparation, biological sample preservation, and space science.

3. The application according to claim 1, characterized in that, In step S1, the molar ratio of the sum of K2CO3, WO3 and Yb2O3 to the cosolvent is 1:5~10.

4. The application according to claim 1, characterized in that, In step S1, the solvent includes ethanol.

5. The application according to claim 1, characterized in that, In step S2, after the crystals crystallize, the time required to cool to room temperature is ≥5 hours.

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