Application of heat-insulating and demagnetizing refrigeration material

Abstract

The invention relates to the technical field of crystal growth and extremely low temperature region magnetic refrigeration materials, and particularly discloses an application of a heat-insulating demagnetizing refrigeration material in preparation of refrigeration equipment, the heat-insulating demagnetizing refrigeration material is a K3GdSi2O7 crystal, and a refrigeration temperature region of the refrigeration equipment is a sub-Kelvin temperature region. According to the K3GdSi2O7, through the unique crystal structure and magnetic ion arrangement of the K3GdSi2O7, the large magnetic entropy capacity, the extremely low working temperature zone and the low critical field requirement are combined, and the K3GdSi2O7 shows excellent comprehensive performance and application prospects in the extremely low temperature (especially sub-Kelvin) heat insulation demagnetization refrigeration field.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of crystal growth and magnetic refrigeration materials in extremely low temperature region, and particularly relates to application of adiabatic demagnetization refrigeration material. BACKGROUND

[0002] The history of magnetic refrigeration technology can be traced back to 1917 when Weiss and Piccard first observed the magnetic hysteresis effect while studying the magnetization behavior of nickel. In 1933, Giauque and MacDougall used this effect to practice adiabatic demagnetization refrigeration, successfully cooling to 0.25 K using this principle, laying the foundation for low-temperature physics research. In the 1970s, Brown achieved a larger cooling effect at room temperature using gadolinium (Gd), marking the advancement of magnetic refrigeration technology towards practicality. In 1997, Pecharsky and Gschneidner discovered the Giant Magnetocaloric Effect in Gd5Si2Ge2-based compounds, greatly promoting the research of magnetic refrigeration materials and making the commercialization of magnetic refrigeration technology possible.

[0003] Rare earth elements, particularly gadolinium (Gd), play an important role in this field due to their unique electronic structure. Gd 3+ ions have 7 unpaired 4f electrons, according to Hund's rule, with a spin quantum number as high as S = 7 / 2, corresponding to a magnetic entropy of Rln(2S+1) = Rln(8) J·mol -1 ·K -1 , which makes it have excellent performance potential in the magnetic refrigeration process. Therefore, gadolinium-containing compounds have always been the focus of magnetic refrigeration material research. Early low-temperature magnetic refrigerants mainly include paramagnetic salts such as Gd2(SO4)3·8H2O, and some oxides such as garnet-structured Gd3Ga5O 12 (GGG). However, some existing magnetic refrigeration materials, especially oxides, often require high magnetic fields (e.g., ≥5 T) to exhibit significant magnetocaloric effects, and their performance decreases significantly at lower magnetic fields (e.g., ≤1 T), which limits their practical application in compact, low-cost magnet systems. For example, materials such as DyVO3 and DyBO3 have small magnetic entropy changes under a 0-1 T magnetic field change. Therefore, developing magnetic refrigeration materials that are still efficient at low magnetic fields and can work in extremely low temperature regions remains a challenge. SUMMARY

[0004] In view of the deficiencies of the prior art described above, the present application provides an application of adiabatic demagnetization refrigeration material to solve the problem of low efficiency or insufficient low working temperature range of existing adiabatic demagnetization refrigeration materials at low magnetic fields.

[0005] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: In a first aspect, the present invention provides an application of a thermally insulating and demagnetizing refrigeration material in the preparation of a refrigeration device, wherein the thermally insulating and demagnetizing refrigeration material is K3GdSi2O7 crystal, and the refrigeration temperature range of the refrigeration device is the sub-Kelvin temperature range.

[0006] Preferably, the K3GdSi2O7 crystal belongs to the hexagonal crystal system with space group P63 / mcm.

[0007] Preferably, the unit cell parameters of the K3GdSi2O7 crystal are a = b = 9.9395(3) Å, c = 14.4282(8) Å, and the unit cell volume is 1234.44(10) Å. 3 .

[0008] Preferably, the Gd in the K3GdSi2O7 crystal is... 3+ Ions occupy two types of unequal lattice sites in the crystal structure: the first type of site is the 4d Wyckoff site; the second type of site is the 2a Wyckoff site.

[0009] Preferably, Gd is located at the 4d Wyckoff position. 3+ The ion has a distorted octahedral coordination environment, with Gd located at the 2a Wyckoff site. 3+ The ions have a triangular prism coordination environment.

[0010] Preferably, within the ab plane of the K3GdSi2O7 crystal, Gd 3+ The ions are arranged in a honeycomb lattice structure, with the nearest neighbor Gd-Gd distance being 5.74 Å.

[0011] Preferably, the absolute value of the magnetic entropy change generated by the K3GdSi2O7 crystal under the conditions of a temperature of 0.3 K and a magnetic field changing from 0 T to 1.5 T is not less than 14 J / mol·K.

[0012] Preferably, the relative difference between the absolute values ​​of the magnetic entropy change measured along the direction parallel to the c-axis and perpendicular to the c-axis of the K3GdSi2O7 crystal under the same temperature and magnetic field variation conditions does not exceed 10%.

[0013] Preferably, the K3GdSi2O7 crystal is prepared by a flux method.

[0014] Preferably, the potassium source, gadolinium source, and silicon source in the preparation method are each independently selected from: (a) inorganic precursors that can provide the corresponding element in a high-temperature flux; or (b) organometallic compounds or organic salts that can be thermally decomposed at high temperatures (e.g., above 400°C) and ultimately provide oxides or halides of the corresponding element.

[0015] Preferably, the potassium source in the inorganic precursor of type (a) is selected from at least one of potassium oxide (K2O), potassium hydroxide (KOH), potassium carbonate (K2CO3), potassium nitrate (KNO3), potassium fluoride (KF), potassium chloride (KCl), potassium bromide (KBr), potassium iodide (KI), potassium sulfide (K2S), and potassium sulfate (K2SO4).

[0016] Preferably, the gadolinium source in the type (a) inorganic precursor is selected from at least one of the following: gadolinium oxide (Gd2O3), gadolinium hydroxide (Gd(OH)3), gadolinium carbonate (Gd2(CO3)3), gadolinium nitrate (Gd(NO3)3), gadolinium fluoride (GdF3), gadolinium chloride (GdCl3), gadolinium bromide (GdBr3), gadolinium iodide (GdI3), gadolinium sulfide (Gd2S3), and gadolinium sulfate (Gd2(SO4)3).

[0017] Preferably, the silicon source in the type (a) inorganic precursor is selected from at least one of silicon dioxide (SiO2), silicic acid, silicates, silicon tetrachloride (SiCl4), silicon tetrafluoride (SiF4), and silicon nitride (Si3N4).

[0018] Preferably, examples of the type (b) organometallic compounds or organosalts include, but are not limited to: acetates (e.g., potassium acetate K(CH3COO) as a potassium source or gadolinium acetate Gd(CH3COO)3 as a gadolinium source), oxalates (e.g., potassium oxalate K2C2O4 as a potassium source or gadolinium oxalate Gd2(C2O4)3 as a gadolinium source), citrates, or organosilicon compounds (e.g., tetraethyl orthosilicate Si(OC2H5)4 as a silicon source).

[0019] Preferably, the preparation method uses a flux, which is selected from at least one of the following: LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, KBr, KI, CsBr, CsI, K2CO3, Rb2CO3, Cs2CO3, PbF2, PbCl2, PbBr2, B2O3, PbO, Bi2O3, or a mixture thereof.

[0020] Preferably, in the preparation method, the molar ratio of the flux to the total molar amount of all reactants (potassium source, gadolinium source, silicon source) is (5 ~ 30):1.

[0021] Preferably, in the preparation method, the maximum holding temperature for heating is 900~1300℃, and the cooling rate is 0.5~15℃ / h.

[0022] Beneficial effects: This invention discloses the application of an insulating and demagnetizing refrigeration material in the preparation of refrigeration equipment. The insulating and demagnetizing refrigeration material is K3GdSi2O7 crystal, and the refrigeration temperature range of the refrigeration equipment is sub-Kelvin. K3GdSi2O7 crystal has the following advantages: 1. Extremely low refrigeration temperature range and high efficiency: Due to Gd... 3+ The ions possess high magnetic moments and form a unique honeycomb lattice in the crystal structure, suppressing the magnetic phase transition temperature of this material in an extremely low sub-Kelvin region (experiments have confirmed it to be below 0.125 K). This characteristic effectively delays its long-range magnetic order, thus concentrating the release of enormous magnetic entropy in the sub-Kelvin temperature range. The magnetic entropy change achieved through the Schottsky effect enables lower and more efficient cryogenic cooling compared to traditional antiferromagnetic salts that undergo magnetic phase transitions at higher temperatures (e.g., 1-4 K). 2. Large magnetic entropy capacity: This material is based on Gd... 3+ The octet degenerate ground state of the ion (S=7 / 2) has a theoretical saturation magnetic entropy as high as Rln(8) J·mol⁻¹. -1 ·K -1 It possesses the inherent potential and high cooling capacity of a high-performance magnetic refrigerant. 3. Significantly reduced technical barriers and costs: A key advantage of this invention is the smaller magnetic field required to induce a significant magnetic entropy change. This means that in practical applications, permanent magnets or conventional electromagnet systems with lower field strength, smaller size, and more economical manufacturing costs can be used to drive the refrigeration cycle, greatly reducing the implementation threshold and application cost of AKelvin refrigeration technology, which is conducive to the promotion and popularization of this technology.

[0023] In summary, the K3GdSi2O7 provided by this invention combines large magnetic entropy capacity, extremely low operating temperature range and low critical field requirements through its unique crystal structure and magnetic ion arrangement, demonstrating excellent comprehensive performance and application prospects in the field of ultra-low temperature (especially sub-Kelvin) adiabatic demagnetization refrigeration. Attached Figure Description

[0024] Figure 1 X-ray diffraction (P-XRD) pattern of powder obtained after grinding K3GdSi2O7 single crystal and its Rietveld refinement results; Figure 2 This is a schematic diagram of the crystal structure of K3GdSi2O7. Figure 3 An optical photograph of a K3GdSi2O7 single crystal obtained in an embodiment of the present invention shows that it is a colorless and transparent hexagonal prism crystal. Figure 4 Curie-Weiss fitting plot of data measured under two magnetic field directions in K3GdSi2O7; Figure 5 The isothermal magnetization curves (MH curves) of K3GdSi2O7 single crystal at different temperatures. Figure 6 The magnetic entropy change of K3GdSi2O7 single crystal under different external magnetic field directions (B∥c and B⊥c) and different magnetic field variations is calculated. The curve of ΔS as a function of temperature; Figure 7 The curves showing the specific heat (Cp) of K3GdSi2O7 as a function of temperature, measured under zero magnetic field and different applied magnetic fields; Figure 8 The curve showing the change of magnetic entropy of K3GdSi2O7 with temperature, calculated from specific heat data, indicates that it saturates to Rln(8) J·mol at high temperatures. -1 ·K -1 . Detailed Implementation

[0025] This invention provides an application of a thermally insulating and demagnetizing refrigeration material. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0026] Extremely low temperature (especially in the sub-Kelvin temperature range) refrigeration technology plays a crucial role in fields such as quantum computing, cutting-edge exploration of condensed matter physics, and space exploration. Adiabatic demagnetization refrigeration is one of the most important technical means to achieve this temperature range, and its core lies in finding a paramagnetic refrigerant with high magnetic entropy change, low magnetic order temperature, and stable thermodynamic properties.

[0027] The K3RESi2O7 (RE = rare earth element) series of silicate compounds have attracted attention due to their diverse crystal structures. Existing studies have shown that the structure of this series of compounds changes with the radius of the rare earth ions. For example, the study by Vidican et al. (2003) indicated that when the radius of the rare earth ions is small (such as Lu), the magnetic ions may exhibit a triangular lattice arrangement in the crystal lattice; while when the radius of the rare earth ions increases to Gd... 3+ At this time, the crystal structure transforms into a unique two-dimensional layered structure, in which Gd 3+Ions form a honeycomb-like arrangement within the ab plane. This unique geometry is thought to introduce a significant magnetoresistance effect. Magnetoresistance refers to the competitive interaction of magnetic interactions within a specific crystal lattice structure, preventing the system from forming a single lowest-energy state (long-range magnetic order), thus resulting in a highly degenerate ground state. This effect effectively suppresses long-range magnetic order in materials, lowering their magnetic phase transition temperature to extremely low levels. This property is highly advantageous for adiabatic demagnetizing refrigeration because it means the material can maintain a paramagnetic state over a wider range of low temperatures, thereby preserving a large amount of magnetic entropy for release in the target refrigeration temperature range.

[0028] In the K3RESi2O7 series, K3GdSi2O7, as a gadolinium-based silicate, is valued for its high Gd content. 3+ The in-plane density of ions and the unique two-dimensional honeycomb structure mentioned above theoretically have the potential to achieve significant magnetic entropy changes at liquid helium temperatures and even in the sub-Kelvin temperature range. Furthermore, due to its structural characteristics, it may have a relatively low critical magnetic field, which helps to reduce the difficulty and cost of implementing refrigeration systems.

[0029] However, current research on K3GdSi2O7 still has significant shortcomings. Detailed experimental reports on its magnetocaloric properties in the sub-Kelvin temperature range (such as precise magnetic ordering temperature and the relationship between magnetic entropy change and temperature and magnetic field) are lacking, and its feasibility and effectiveness as an ultra-low temperature thermal insulation and demagnetizing refrigeration material lack sufficient data support and verification.

[0030] Based on this, this embodiment of the invention provides an application of a thermally insulating and demagnetizing refrigeration material in the preparation of refrigeration equipment. The thermally insulating and demagnetizing refrigeration material is K3GdSi2O7 crystal, and the refrigeration temperature range of the refrigeration equipment is the sub-Kelvin temperature range.

[0031] In some embodiments, the unit cell parameters of the K3GdSi2O7 crystal are a = b = 9.9395(3) Å, c = 14.4282(8) Å, and the unit cell volume is 1234.44(10) Å. 3 .

[0032] In some embodiments, the Gd in the K3GdSi2O7 crystal 3+ Ions occupy two types of unequal lattice sites in the crystal structure: the first type of site is the 4d Wyckoff site; the second type of site is the 2a Wyckoff site.

[0033] In some implementations, Gd located at the 4d Wyckoff position 3+ The ion has a distorted octahedral coordination environment, with Gd located at the 2a Wyckoff site. 3+The ions have a triangular prism coordination environment.

[0034] In some embodiments, Gd is located within the ab plane of the K3GdSi2O7 crystal. 3+ The ions are arranged in a honeycomb lattice structure, with the nearest neighbor Gd-Gd distance being 5.74 Å.

[0035] In some embodiments, the absolute value of the magnetic entropy change of the K3GdSi2O7 crystal under the conditions of a temperature of 0.3 K and a magnetic field changing from 0 T to 1.5 T is 14.6 J / mol·K.

[0036] In some embodiments, the relative difference between the absolute values ​​of the magnetic entropy change measured along the direction parallel to the c-axis and perpendicular to the c-axis of the K3GdSi2O7 crystal under the same temperature and magnetic field variation conditions is 3.5%.

[0037] The K3GdSi2O7 crystal provided in this application embodiment has the following advantages: 1. Excellent sub-Kelvin magnetocaloric properties: due to Gd 3+ The high magnetic moments of the ions and the strong magnetoresistance effect caused by the unique honeycomb lattice formed in the crystal suppress the magnetic phase transition temperature of the K3GdSi2O7 crystal to an extremely low sub-Kelvin temperature range (experiments confirmed to be below 0.125 K). This characteristic effectively delays its long-range magnetic order, thereby concentrating the release of the huge magnetic entropy change in the sub-Kelvin region through the Schottky mechanism. Compared with traditional paramagnetic or antiferromagnetic salts that undergo magnetic phase transitions at higher temperatures (such as 1-4 K), this invention achieves a lower and more efficient effective cooling temperature range. At the same time, the material excites a magnetic entropy that accounts for a significant portion of its theoretical saturation (Rln(8) J·mol⁻¹). -1 ·K -11. **High-proportion (not less than 80%) significant magnetic entropy change requires a smaller magnetic field. This means that smaller and more economical magnet systems can be used to drive the refrigeration cycle in practical applications, significantly lowering the threshold and application cost of AKelvin refrigeration technology.** 2. **Good magnetic isotropy:** Magnetic characterization shows that the magnetic susceptibility and magnetocaloric effect of the K3GdSi2O7 crystal exhibit minimal differences in different crystallographic directions, demonstrating near-isotropy. Compared to materials with strong magnetic anisotropy, this characteristic offers significant advantages in constructing magnetic refrigerators: strict orientation alignment of the refrigerant crystals is unnecessary, greatly simplifying the fabrication process and structural design of the refrigeration unit, reducing manufacturing complexity and cost, and facilitating the large-scale application of this material. 3. **Excellent chemical and physical stability:** The K3GdSi2O7 crystal belongs to the silicate family, has a stable structure, is expected to have good chemical inertness, is not easily deliquescent, can be stably stored in air for a long time, and is easy to cut, polish, and perform other mechanical processing. This excellent stability ensures the reliability and durability of the material's performance in refrigeration equipment, simplifies storage and maintenance processes, and provides an important guarantee for its integration and application in actual devices.

[0038] This invention investigated the magnetocaloric properties of K3GdSi2O7 through systematic measurements of magnetic susceptibility, isothermal magnetization, and specific heat. The results show that this material exhibits: 1. Excellent paramagnetic properties and high magnetic entropy change capability: The K3GdSi2O7 crystal displays typical paramagnetic behavior over a wide temperature range, and its effective magnetic moment is similar to that of Gd... 3+ The ion values ​​are consistent with theoretical values, and no magnetic hysteresis is observed, indicating that the material exhibits a highly reversible magnetocaloric response with extremely low energy loss. Specifically, under conditions of an initial temperature of 2 K and an external magnetic field variation of 0 to 9 T, its maximum magnetic entropy change can reach approximately 37-39 J·kg. -1 ·K -1 1. It exhibits extremely high refrigeration efficiency per unit mass. 2. Extremely low operating temperature range and unique entropy release mechanism: Specific heat and low temperature magnetization measurements confirm that no magnetically ordered phase transition was observed in the material above 0.125 K. This key feature, combined with its magnetocaloric data, indicates that its huge magnetic entropy is mainly released in the sub-Kelvin temperature range through the Schottky effect. Compared with traditional working fluids that undergo magnetically ordered phase transition at higher temperatures, this invention can effectively extend the high-efficiency refrigeration range to a more extreme low-temperature field, meeting the urgent needs of modern physics research for sub-Kelvin temperature refrigeration. 3. Low critical field advantage, which is conducive to practical application: Experiments have confirmed that K3GdSi2O7 can reach its theoretical saturation magnetic entropy (Rln(8) J·mol). -1 ·K -1A significant magnetic entropy change with a high proportion (not less than 80%) requires a smaller magnetic field. This characteristic means that in practical adiabatic demagnetizing refrigeration devices, a magnet system with lower field strength, smaller size, and more economical cost can be used to drive the complete refrigeration cycle. This significantly reduces the equipment cost, energy consumption, and technical threshold of AKelvin refrigeration technology, laying a solid foundation for its large-scale promotion and application.

[0039] In some embodiments, the K3GdSi2O7 crystals are prepared by a flux method.

[0040] In some embodiments, the potassium source, gadolinium source, and silicon source in the preparation method are each independently selected from: (a) inorganic precursors that can provide the corresponding element in a high-temperature flux; or (b) organometallic compounds or organic salts that can be thermally decomposed at high temperatures (e.g., above 400°C) and ultimately provide oxides or halides of the corresponding element.

[0041] In some embodiments, the potassium source in the (a) type inorganic precursor is selected from at least one of potassium oxide (K2O), potassium hydroxide (KOH), potassium carbonate (K2CO3), potassium nitrate (KNO3), potassium fluoride (KF), potassium chloride (KCl), potassium bromide (KBr), potassium iodide (KI), potassium sulfide (K2S), and potassium sulfate (K2SO4).

[0042] The gadolinium source in the inorganic precursor of type (a) is selected from at least one of the following: gadolinium oxide (Gd2O3), gadolinium hydroxide (Gd(OH)3), gadolinium carbonate (Gd2(CO3)3), gadolinium nitrate (Gd(NO3)3), gadolinium fluoride (GdF3), gadolinium chloride (GdCl3), gadolinium bromide (GdBr3), gadolinium iodide (GdI3), gadolinium sulfide (Gd2S3), and gadolinium sulfate (Gd2(SO4)3).

[0043] The silicon source in the inorganic precursor of type (a) is selected from at least one of silicon dioxide (SiO2), silicic acid, silicates, silicon tetrachloride (SiCl4), silicon tetrafluoride (SiF4), and silicon nitride (Si3N4).

[0044] Examples of the organometallic compounds or organosalts of class (b) include, but are not limited to: acetates (e.g., potassium acetate K(CH3COO) as a potassium source or gadolinium acetate Gd(CH3COO)3 as a gadolinium source), oxalates (e.g., potassium oxalate K2C2O4 as a potassium source or gadolinium oxalate Gd2(C2O4)3 as a gadolinium source), citrates, or organosilicon compounds (e.g., tetraethyl orthosilicate Si(OC2H5)4 as a silicon source).

[0045] In some embodiments, a flux is used in the preparation method, and the flux is selected from at least one or a mixture of the following: LiF, NaF, KF, RbF, CsF, LiCl, NaCl, KCl, RbCl, CsCl, KBr, KI, CsBr, CsI, K2CO3, Rb2CO3, Cs2CO3, PbF2, PbCl2, PbBr2, B2O3, PbO, and Bi2O3.

[0046] In some embodiments, in the preparation method, the molar ratio of the flux to the total molar amount of all reactants (potassium source, gadolinium source, silicon source) is (5 ~ 30):1.

[0047] In some embodiments, the maximum holding temperature during heating in the preparation method is 900~1300℃, and the cooling rate is 0.5~15℃ / h.

[0048] The flux-based crystal growth process employed in this invention is characterized by its simplicity, readily available raw materials, and controllable cost, enabling the stable preparation of high-quality K3GdSi2O7 single crystals at the millimeter or even near-centimeter scale. This method effectively suppresses polycrystalline formation and defect introduction by precisely controlling the temperature field and stoichiometry during nucleation and growth, thereby obtaining single crystal samples with complete structure and uniform composition.

[0049] In the study of physical properties, obtaining high-quality single crystals is of great significance for the accurate characterization of the intrinsic properties of materials. Compared with polycrystalline samples, single-crystal materials can effectively avoid the scattering effects of grain boundaries and defects on phonon transport, thus more realistically reflecting key physical parameters such as thermal conductivity. Especially in magnetic refrigeration applications, the excellent thermal conductivity of single crystals helps to achieve more efficient heat exchange processes, thereby improving refrigeration efficiency and cycle rate.

[0050] The preparation method of this invention solves the problems of inaccurate physical property measurement and limited application development caused by sample size limitations or poor quality in previous studies, and provides a reliable sample basis for further basic research on this type of compound and its application in functional devices.

[0051] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are merely some embodiments of the present invention, not all embodiments, and are intended only to illustrate the present invention and not to limit it. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Example The preparation of K3GdSi2O7 crystals includes the following steps: Gadolinium oxide (from McLean, 99.9%) and silicon dioxide (from McLean, 99.9%) were mixed in stoichiometric proportions, and ten molar amounts of potassium fluoride (from McLean, 99.9%) were added. The mixture was thoroughly ground in an agate mortar and then placed in a platinum crucible, which was covered with a platinum sheet to reduce volatilization. The crucible containing the raw materials was placed in a muffle furnace under normal air atmosphere and heated to 1100°C at a rate of 10°C / min and held for 10 hours. The temperature was then slowly reduced to 800°C at a rate of 3°C / h, and finally allowed to cool naturally to room temperature. After removing the sample from the crucible, it was immersed in deionized water at 40°C. The flux was removed by utilizing the property that KF is readily soluble in water while the target product is insoluble in water. Residual raw materials adhering to the crystal surface were then separated by ultrasonic vibration to obtain K3GdSi2O7 crystals.

[0053] Performance testing The K3GdSi2O7 crystals obtained in the examples were tested using a Rigaku SmartLab 9 KW powder X-ray diffractometer (P-XRD). The obtained XRD data were imported into FullProf Suite (FPS) for refinement. This software uses known crystal structure data to calculate the theoretical powder XRD diffraction pattern, and also provides a comparative analysis function to analyze the degree of fit between theoretical calculations and measured data. Figure 1 The refined results are shown, with black circles representing experimental data and red circles representing theoretical simulation values. The two are in high agreement, and the blue circles also show relatively small errors. In terms of goodness of fit, Rwp = 12.4, χ² = 12.4. 2 = 4.92. Here, a smaller Rwp (weighted residual error) indicates a smaller deviation between the experimental data and the theoretical fit; generally, an acceptable range is Rwp less than 15, χ² 2 Less than 5. Powder XRD analysis concluded that the sample obtained in the experiment had high purity, meeting the requirements for analytical testing.

[0054] The prepared K3GdSi2O7 crystal was structurally characterized. The testing conditions were as follows: Bruker D8VENTURE single-crystal diffractometer, Mo target, Kα radiation source (λ = 0.71073 Å), and a testing temperature of 301.0 K. The structure was analyzed using Olex2 software, and the obtained crystallographic data are shown in Table 1. The crystal structure is referenced... Figure 2 Optical photographs Figure 3 .

[0055] Table 1

[0056] Magnetic susceptibility measurement: Using a Power Proof-of-Material Measurement System (PPMS), the relationship between the magnetic susceptibility of the material and temperature was measured under an external magnetic field of 2000 Oe, as shown below. Figure 4 As shown, within the temperature range of 2–200 K, the reciprocal of the material's magnetic susceptibility exhibits a highly linear relationship with temperature, consistent with typical Curie-Weiss paramagnetism. The effective magnetic moment μ calculated based on the fitting... eff The values ​​are 7.85 / 8.03 μB, compared to the theoretical value of 7.94 μB for Gd. B The close proximity also demonstrates the high quality of the crystals prepared in this embodiment and the accuracy of the structural model. The fitted Curie-Western temperature θ... CW The value is close to 0, indicating weak magnetic interactions between Gd atoms within the crystal. Further comparison of data under different external magnetic field directions revealed no significant anisotropy. This is advantageous for magnetic applications requiring the cooling material to function effectively in various directions. Isothermal magnetization curves of the material under different magnetic fields were also plotted, visually demonstrating its magnetization behavior. Figure 5 As shown, the magnetization of the sample increases linearly with increasing magnetic field strength at low magnetic fields and gradually approaches saturation, reflecting the process by which the magnetic moments within the crystal gradually align with the direction of the external magnetic field. Figure 5 In the hysteresis study, the crystal did not exhibit any hysteresis effect, and the cyclic variation curves of magnetization intensity with the magnetic field were symmetrical and coincident. This phenomenon indicates that no magnetic residue or hysteresis appears during the magnetization process, and the interaction between magnetic ions Gd is weak, making it impossible to form spontaneous magnetization or magnetic domain structures, which is consistent with a typical paramagnetic model. To measure the data required for calculating the magnetic entropy, isothermal magnetization curves (MH) were scanned in the magnetic field range of 0–9 T and the temperature range of 2–30 K, and –ΔS was calculated using the trapezoidal integral method. A larger value of –ΔS indicates that the material can absorb or release more heat during the magnetic field change process. The final results are as follows: Figure 6 As shown, –ΔS reaches 37.3 J / kg·K (B∥c) and 38.6 J / kg·K (B⊥c) under magnetic field differences of 2 K and 9 T, respectively, indicating its potential application in the field of magnetic refrigeration. Furthermore, the magnetocaloric effect under different external magnetic field directions shows that the –ΔS values ​​are almost equal in both directions, which is consistent with the previous discussion on the magnetic isotropy of this material.

[0057] Specific heat measurement: The specific heat of the samples from 0.1 to 2 K was measured using a PPMS equipped with a dilution cooling module. This data was then calibrated using specific heat data from 1.8 to 50 K obtained with a conventional module, yielding specific heat data for the samples from 0.1 to 50 K. Figure 7As shown. Under a 1.5 T magnetic field, data from 20–50 K were used for double Debye fitting. The magnetic specific heat (Cp, mag) was obtained by subtracting the fitted phonon specific heat (Cp, ph) from the measured specific heat data (Cp). The relationship between the magnetic entropy of the sample and temperature until saturation could then be obtained through area integration. Figure 8 It can be seen that even at an extremely low temperature of 0.3 K, the material can still generate a considerable magnetic entropy change that can be utilized, which is 14.6 J / mol·K. Moreover, the magnetic field required to excite a significant magnetic entropy change that accounts for a high proportion (not less than 80%) of its theoretical saturation magnetic entropy (Rln(8) J·mol-1·K-1) is relatively small, which is 1.5 T. This is very advantageous for cryogenic refrigeration.

[0058] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. The application of a thermally insulating and demagnetizing refrigeration material in the preparation of refrigeration equipment, wherein the thermally insulating and demagnetizing refrigeration material is K3GdSi2O7 crystal, and the refrigeration temperature range of the refrigeration equipment is the sub-Kelvin temperature range.

2. The application according to claim 1, characterized in that, The K3GdSi2O7 crystal belongs to the hexagonal crystal system with space group P63 / mcm.

3. The application according to claim 1, characterized in that, The unit cell parameters of the K3GdSi2O7 crystal are a = b = 9.9395(3) Å, c = 14.4282(8) Å, and the unit cell volume is 1234.44(10) Å. 3 .

4. The application according to claim 1, characterized in that, The Gd in the K3GdSi2O7 crystal 3+ Ions occupy two types of unequal lattice sites in the crystal structure: the first type of site is the 4d Wyckoff site; the second type of site is the 2a Wyckoff site.

5. The application according to claim 4, characterized in that, Gd located at the 4d Wyckoff position 3+ The ion has a distorted octahedral coordination environment, with Gd located at the 2a Wyckoff site. 3+ The ions have a triangular prism coordination environment.

6. The application according to claim 1, characterized in that, Within the ab plane of the K3GdSi2O7 crystal, Gd 3+ The ions are arranged in a honeycomb lattice structure, with the nearest neighbor Gd-Gd distance being 5.74 Å.

7. The application according to claim 1, characterized in that, The absolute value of the magnetic entropy change of the K3GdSi2O7 crystal under the conditions of a temperature of 0.3 K and a magnetic field changing from 0 T to 1.5 T is not less than 14 J / mol·K.

8. The application according to claim 1, characterized in that, Under the same temperature and magnetic field conditions, the relative difference between the absolute values ​​of the magnetic entropy change measured along the direction parallel to the c-axis and perpendicular to the c-axis for the K3GdSi2O7 crystal does not exceed 10%.

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