Low-temperature magnetocaloric material based on divalent europium and application of low-temperature magnetocaloric material

Abstract

The invention provides a low-temperature magnetocaloric material based on divalent europium and application of the low-temperature magnetocaloric material, the chemical formula of the low-temperature magnetocaloric material is EuX2, and X is F, Cl, Br or I. The magnetic order temperature of the material is lower than 1.8 K, and the magnetic entropy change value of the material under 0-5T magnetic field change exceeds 30J kg <-1 > K <-1 >. Typically, according to the EuCl2 material provided by the invention, due to the single ion behavior and the free spinning characteristic of Eu < 2 + > ions, the material shows a huge magnetocaloric effect. The maximum magnetic entropy change value of EuCl2 reaches 74.6 J kg <-1 > K <-1 > under 0-5T magnetic field change, and is 96% of the theoretical limit; and the magnetic field change reaches 36.8 Jkg <-1 > K <-1 > under 0-1T low magnetic field change, which is far better than that of reported materials. In quasi-adiabatic demagnetization measurement, the EuX2 material can reach the lowest temperature of 428mK at the initial temperature of 2K and the magnetic field of 4T, and the holding time at the temperature is as long as 1h, which is far beyond that of all reported other materials under the working condition at present. The results show that the EuX2 material has huge application value in adiabatic demagnetization refrigeration and thermal management.

Description

Technical Field

[0001] This invention relates to a low-temperature magnetocaloric material based on divalent europium and its application in adiabatic demagnetization. Background Technology

[0002] Cryogenic refrigeration technology (0.005-40K) plays a crucial role in fields such as hydrogen and helium liquefaction, quantum computing, high-energy physics, superconductivity, and modern space technology. Liquid helium, the lowest known liquid, is an excellent cryogenic refrigerant with a boiling point of only 4.2K (-268.93℃), providing extreme low-temperature environments (below 0.6K) under reduced pressure. However, facing the challenges of helium scarcity and rising costs, developing helium-free cryogenic refrigeration technology has become a pressing problem for the scientific community. Against this backdrop, adiabatic demagnetization (ADR) refrigeration technology stands out with its unique advantages, as it does not rely on the scarce helium... 3 He- 4 He gas can achieve sub-Kelvin-level ultra-low temperatures, opening up new avenues for the application and research of low-temperature science.

[0003] The core of ADR technology lies in the application of the magnetocaloric effect (MCE) of materials, in which the magnetic entropy change (ΔS) M As a crucial parameter for evaluating the strength of magnetic entropy change (MCE), it directly determines the cooling effect. Therefore, finding and developing materials with large magnetic entropy changes in the cryogenic field has become key to improving the performance of ADR technology.

[0004] For a long time, researchers have focused on Gd-based magnetocaloric materials, which rely on Gd... 3 The unique large ground-state spin (J = S = 7 / 2, L = 0) and magnetic isotropy of the + ion exhibit a significant magnetocaloric effect in the low-temperature region. However, despite extensive research on various Gd-based materials, including molecular clusters, inorganic salts, inorganic metal oxides, coordination polymers, and metal-organic frameworks (MOFs), their actual magnetic entropy changes remain far below their theoretical limit ΔS. M =nRln(2S+1) / M W .

[0005] To achieve a magnetic entropy change close to the theoretical value, the selected material typically needs to meet four conditions: first, a large total angular momentum (J) to achieve a large saturation magnetic moment; second, weak magnetic anisotropy, especially in the L=0 state, particularly when the orbital angular momentum L=0, making the material easier to magnetize in an external magnetic field; third, a magnetic density M... R / M w (where M) R M represents the weight of a magnetic element. wThe high total weight of the compound ensures efficient utilization of magnetic elements; fourthly, weak magnetic interactions and near-single-ion behavior allow each magnetic ion to respond independently to changes in the magnetic field. Based on these conditions, rare earth 4f ions, especially R with a large J value, are highly effective. 3+ Ions, with their unique magnetic characteristics, have become ideal candidates for developing high-performance magnetocaloric materials. Furthermore, the atomic coordination environment plays a crucial role in influencing magnetic density, magnetic anisotropy, magnetic interaction strength, and crystal field effects. These factors significantly affect the magnetic ground state and actual magnetic entropy change of rare-earth-based compounds.

[0006] It is noteworthy that in the research of high-efficiency low-temperature magnetic refrigeration materials, Eu was found to... 3+ Ions are not magnetic, but Eu 2 + However, the ions have the same properties as Gd 3+ Ions share the same magnetic properties: J = S = 7 / 2, L = 0, and are magnetically isotropic.

[0007] Despite Eu 2+ Magnetocaloric materials have shown potential applications in low-temperature physics, but research on their magnetocaloric effects under extreme low-temperature conditions (especially below 2K) remains limited. Two borate compounds, EuB4O7 and EuB2O4, have been reported to exhibit significant magnetocaloric effects at low temperatures. However, their magnetic entropy changes in practical applications fall far short of their theoretically predicted upper limits. Specifically, within a magnetic field range of 0 to 5T, the magnetic entropy changes of these two materials are only 47.6 J / kg, respectively. -1 K -1 and 50.8J kg -1 K -1 Their theoretical limits are as high as 56.3 J kg. -1 K -1 and 72.6 J kg -1 K -1 This significant difference is presumably due primarily to Eu... 2+ The complex interaction mechanisms between ions and their coordination environment constrain magnetic properties, resulting in relatively low magnetic density, which in turn limits the actual performance of the magnetocaloric effect.

[0008] Therefore, developing magnetocaloric materials with high magnetic entropy changes and close to the theoretical limit, especially in the extremely low temperature range, has become a current research focus. Ideal materials should possess large saturation magnetic moments, weak magnetic anisotropy, high magnetic density, and nearly independent magnetic ion behavior, which are crucial for improving the magnetocaloric effect and cooling efficiency. Therefore, in addition to selecting Eu materials with large J values ​​and weak magnetic anisotropy... 2+ In addition to ions, compounds with suitable ligands and coordination environments must also be selected. Summary of the Invention

[0009] Therefore, the object of this invention is to provide an Eu with a high magnetic entropy change approaching the theoretical limit. 2+ Basic magnetocaloric materials.

[0010] In exploring Eu 2+ During the low-temperature magnetocaloric effect of the base compound, the inventors of this invention noted that EuX2 (X = F, Cl, Br, or I) materials, due to their simple chemical structure and high magnetic ion density (M... Eu / M w The halogen concentrations (80%, 68%, 49%, and 37%, respectively) exhibit high theoretical magnetic entropy changes, demonstrating the potential to achieve significant magnetocaloric effects at low temperatures. The inventors discovered that the physical properties of these materials are closely related to the type and crystal structure of the halogen elements. Specifically, EuF₂ has a cubic structure, EuBr₂ has a tetragonal structure, and EuI₂ has a monoclinic structure; all three materials exhibit an antiferromagnetic ground state. EuCl₂, however, has an orthorhombic structure and exhibits magnetic characteristics characteristic of a ferromagnetic ground state.

[0011] The inventors of this invention, through first-principles calculations of powder X-ray diffraction (PXRD), electron localization function (ELF), and density of states (DOS), ultra-low temperature magnetic measurements, and Brillouin function fitting, unexpectedly discovered the ferromagnetic ground state and EuCl2 material of EuCl2. 2+ The ions exhibit single-ion behavior and near-free spin characteristics. Furthermore, through magnetic measurements of EuCl2 with an orthorhombic structure and a ferromagnetic ground state, the inventors discovered that under a magnetic field variation of 0-5T, its -ΔS M It can reach an astonishing 74.6 J kg -1 K -1 (1.8K). This material not only has the highest efficiency under this magnetic field variation among currently reported low-temperature magnetocaloric materials, but it is also very close to its theoretical limit (77.5 J kg). -1 K -1 The content reached 96%. More importantly, the easy saturation of EuCl2 also allows it to achieve a high concentration of 36.8 J / kg under low magnetic field variations of 0-1T. -1 K -1 The magnetic entropy change of EuCl2 is a record high, surpassing previous measurements for low-temperature magnetocaloric materials under the same conditions. Thanks to EuCl2's large magnetic entropy change, through direct quasi-adiabatic demagnetization measurements, the material's retention time at sub-Kelvin temperatures (~428 mK) exceeds that of all previously reported materials under the same operating conditions. This superior magnetocaloric property makes EuCl2 a highly promising low-temperature refrigerant for adiabatic demagnetization refrigeration applications.

[0012] In this invention, the term "low temperature" generally refers to a temperature not higher than 2K.

[0013] The first aspect of the present invention provides a low-temperature magnetocaloric material based on divalent europium, wherein the chemical formula of the low-temperature magnetocaloric material is EuX2, wherein X = F, Cl, Br or I, wherein EuF2, EuBr2 and EuI2 have antiferromagnetic ground state magnetic characteristics and have cubic, tetragonal and monoclinic structures respectively; EuCl2 has ferromagnetic ground state magnetic characteristics and has an orthogonal structure.

[0014] According to the low-temperature magnetocaloric material based on divalent europium provided by the present invention, the phase transition temperature of the low-temperature magnetocaloric material is below 1.8K. That is, as the temperature increases, the low-temperature magnetocaloric material undergoes a transition from a magnetically ordered (antiferromagnetic or ferromagnetic) phase to a paramagnetic phase near a temperature below 1.8K.

[0015] The low-temperature magnetocaloric material based on divalent europium provided by the present invention has a magnetic entropy change value higher than 30 J / kg under a magnetic field variation of 0-5T. -1 K -1 Typically, the magnetic entropy change of EuCl2 is as high as 74.6 J kg. -1 K -1 It reaches 96% of its theoretical limit, making it the component with the best magnetocaloric performance in this material system.

[0016] The low-temperature magnetocaloric material based on divalent europium provided by the present invention is preferably composed of EuCl2, and its magnetic entropy change value under a magnetic field change of 0-1T is 36.8 J kg. -1 K -1 .

[0017] The low-temperature magnetocaloric material based on divalent europium provided by the present invention, wherein the space group of the EuCl2 is Pnma.

[0018] According to the low-temperature magnetocaloric material based on divalent europium provided by the present invention, the shortest distance between Eu-Eu atoms in the low-temperature magnetocaloric material is [missing information]. Specifically, in the (010) plane, the shortest distance between Eu-Eu atoms is Between the (010) planes, the shortest distance between Eu-Eu atoms is Eu in the low-temperature magnetocaloric material provided by this invention 2+ The shortest distance between ions is greater than that of other divalent Eu ions previously reported. 2+ Based on magnetocaloric compounds. It is believed that Eu... 2+ A larger distance between ions helps to weaken the Heisenberg interaction and promotes single-ion behavior.

[0019] The low-temperature magnetocaloric material based on divalent europium provided by the present invention, wherein, in the component EuCl2 with optimal magnetocaloric performance, Eu... 2+ The ions exhibit near-single-ion behavior and free spin properties.

[0020] The second aspect of the present invention provides the application of the above-mentioned low-temperature magnetocaloric material based on divalent europium in adiabatic demagnetization refrigeration.

[0021] According to the application of the low-temperature magnetocaloric material provided by the present invention, in the quasi-adiabatic demagnetization measurement, the lowest adiabatic demagnetization temperature that the low-temperature magnetocaloric material can reach is 428 mK at an initial temperature and magnetic field of 2 K and 4 T; and the lowest adiabatic demagnetization temperature that the low-temperature magnetocaloric material can reach is 346 mK at an initial temperature and magnetic field of 2 K and 5 T.

[0022] According to the application of the low-temperature magnetocaloric material provided by the present invention, during the adiabatic demagnetization cooling process, after reaching the minimum temperature, the low-temperature magnetocaloric material is maintained at the minimum temperature for at least 1 hour. Specifically, in the quasi-adiabatic demagnetization measurement conducted by the present invention, under an initial temperature of 2K and a magnetic field of 4T, the low-temperature magnetocaloric material maintains the minimum temperature of 428mK for up to 1 hour after reaching this temperature.

[0023] Compared with existing materials, the low-temperature magnetocaloric materials of this invention, especially EuCl2 materials, have significant advantages. The EuCl2 material provided by this invention exhibits a huge magnetocaloric effect. Under a magnetic field change of 0-5T, the magnetic entropy change of EuCl2 is higher than 30 J / kg. -1 K -1 Among them, the magnetic entropy change of component EuCl2 reaches as high as 74.6 J kg under a magnetic field variation of 0-5 T. -1 K -1 It is 96% of the theoretical limit; under a low magnetic field change of 0-1T, the magnetic entropy change reaches 36.8 J kg. -1 K -1 Both values ​​are significantly higher than the magnetic entropy changes of all previously reported materials under the same magnetic field. In adiabatic demagnetization refrigeration applications, two parameters for evaluating material performance are the minimum achievable temperature and the holding time at low temperatures. A large magnetic entropy change is beneficial for a long holding time at low temperatures. Quasi-adiabatic demagnetization measurements show that EuX2 material can hold for up to 1 hour after reaching the minimum temperature, far exceeding all other materials reported under these operating conditions. Therefore, EuX2 material has significant application value in adiabatic demagnetization refrigeration. Attached Figure Description

[0024] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0025] Figure 1 The values ​​represent the room-temperature X-ray diffraction patterns of the EuCl2 material in Example 1, where the horizontal axis represents the diffraction angle and the vertical axis represents the diffraction intensity.

[0026] Figure 2 The X-ray diffraction pattern of EuCl2 material in Example 1 at 5K is shown, where the horizontal axis represents the diffraction angle and the vertical axis represents the diffraction intensity.

[0027] Figure 3 Figure 1 shows a schematic diagram of the crystal structure of EuCl2 material in Example 1. Figure 1(a) shows the crystal structure of EuCl2 in three-dimensional space, Figure 2(b) shows the nearest neighbor environment of Eu, Cl1 and Cl2, and Figure 2(c) shows the two (010) crystal planes of EuCl2 along the b-axis.

[0028] Figure 4 The figure shows the magnetization-temperature (MT) curve of the EuCl2 material in Example 1 under a 0.01T magnetic field. The inset shows a magnified view of the low-temperature region, where the horizontal axis represents temperature and the vertical axis represents magnetization. The arrows indicate the direction of heating and cooling.

[0029] Figure 5 The 1 / magnetic susceptibility-temperature (χ) of the EuCl2 material in Example 1 under an applied magnetic field of 100 Oe is... -1 The -T) curve is fitted with the corresponding Curiewes law, where the horizontal axis is temperature and the vertical axis is the reciprocal of magnetic susceptibility, 1 / χ.

[0030] Figure 6 The four magnetic configurations of EuCl2 material in Example 1 are shown in Figures (a), (b), (c), and (d), which are linear ferromagnetic, type A antiferromagnetic, type C antiferromagnetic, and type G antiferromagnetic configurations, respectively. The arrows indicate the direction of the magnetic moment.

[0031] Figure 7 The graph shows the density of states (DOS) curve of the EuCl2 material in Example 1, where the horizontal axis represents energy and the vertical axis represents the electronic density of states.

[0032] Figure 8 The image shows the electron localization function (ELF) of the EuCl2 material in Example 1. (a) The ELF of the (010) crystal plane occupied by Eu and Cl atoms, where the color scale on the left indicates the ELF value from 0 to 0.9, and the corresponding color changes from blue to green and finally to red. (b) The curve of the ELF value between Eu and the nearest neighbor atom of Cl1 or Cl2 as a function of distance, where the horizontal axis is the distance between Eu and Cl1 or Cl2 atoms, and the vertical axis represents the ELF value.

[0033] Figure 9The image shows the magnetization curve of the EuCl2 material in Example 1 at a temperature of 0.4K. The dashed line represents the Brillouin function at 0.4K, where the horizontal axis represents the magnetic field strength and the vertical axis represents the magnetization strength.

[0034] Figure 10 The graph shows the isothermal magnetization curves of the EuCl2 material in Example 1 within the temperature range of 0.4-20K, where the horizontal axis represents the magnetic field strength and the vertical axis represents the magnetization intensity.

[0035] Figure 11 The magnetic entropy change (-ΔS) of the EuCl2 material in Example 1 when the magnetic field changes to 0-1T, 0-2T, 0-3T, 0-4T, and 0-5T is given. M The curve shows the change of temperature with temperature, where the horizontal axis represents temperature and the vertical axis represents -ΔS. M .

[0036] Figure 12 The image shows the adiabatic demagnetization cooling curves of EuCl2 material in Example 1 under different initial temperatures and magnetic fields ((T0,H0)=(2K,1T), (2K,2T), (2K,4T), (2K,5T), (4K,5T)). The horizontal axis represents the magnetic field strength, and the vertical axis represents the temperature.

[0037] Figure 13 The figure shows the holding time curves of EuCl2 material in Example 1 under quasi-adiabatic demagnetization measurements, at the same initial temperature of 2K and magnetic field of 4T, compared with Na2BaCo(PO4)2 (NBCP), CrK(SO4)2·12H2O (CPA), and Fe(SO4)2(NH4)2·6H2O (FAA) materials at the lowest achievable temperature. The inset shows the holding time curves of EuCl2 material under other initial conditions ((T0,H0)=(4K,5T), (6K,5T), (6K,4T), (2K,2T), (2K,1T)), where the horizontal axis is time and the vertical axis is temperature.

[0038] Figure 14 The magnetic entropy change (-ΔS) of EuF2, EuBr2, and EuI2 materials in Examples 2-4 when the magnetic field changes from 0 to 5 T is shown. M The curve shows the change of temperature with temperature, where the horizontal axis represents temperature and the vertical axis represents -ΔS. M . Detailed Implementation

[0039] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0040] The raw materials and equipment used in the embodiments of this invention are described below:

[0041] (1) The low-temperature magnetocaloric materials EuF2 (CAS: 14077-39-5) and EuCl2 (CAS: 13769-20-5) used in the examples are white powders, EuBr2 powder (CAS: 13780-48-8) is light gray, and EuI2 (CAS: 22015-35-6) is light yellow. All of them have a purity of 99.99% and an average particle size of 50 micrometers. They were purchased from Aladdin Company.

[0042] (2) The X-ray diffractometers used were a room-temperature Rigaku D / MAX-2400 and a variable-temperature X-ray diffractometer manufactured by Rigaku Corporation, Japan. The wavelength of the Cu target used was [wavelength missing]. The test temperature range of the variable-temperature X-ray diffractometer is 5-300K.

[0043] (3) The instrument used for basic magnetic measurements is the Magnetic Property Measurement System (MPMS-3) from Quantum Design, USA; in the extremely low temperature range of 0.4-2K, magnetic measurements are performed using the MPMS-3 system equipped with... 3 He inserted the pole.

[0044] (4) Quasi-Adiabatic Demagnetization Measurements use the apparatus provided with the Comprehensive Physical Property Measurement (PPMS) system.

[0045] Example 1

[0046] Performance characterization of low-temperature magnetocaloric material EuCl2

[0047] (1) Crystal structure measurement

[0048] The crystal structure of the low-temperature magnetocaloric material EuCl2 was determined using powder diffraction.

[0049] Figure 1 and Figure 2 Powder diffraction patterns of EuCl2 at room temperature (300 K) and 5 K are presented. Rietveld structure refinement reveals that EuCl2 exhibits an orthorhombic structure with space group Pnma, and no structural transformation occurs from 5 K to 300 K.

[0050] Figure 3 A schematic diagram of the crystal structure of EuCl2 material is given, showing that each Eu atom is coordinated to nine Cl atoms, and the spacing between Eu and Cl atoms is... The variation within the range results in a slightly twisted structure resembling a three-sided triangular prism. Cl atoms occupy two different positions: at the Cl1 position, each Cl atom coordinates with five Eu atoms, and at the Cl2 position, each Cl atom coordinates with four Eu atoms, forming a ClEu4 tetrahedron with shared corners. In the (010) plane, Eu atoms form one-dimensional Eu-Eu chains, where the shortest Eu-Eu distance is... Between the (010) planes, the Eu-Eu distance is It is worth noting that EuCl2 contains Eu 2+ The shortest distance between ions is greater than previously reported for divalent Eu. 2+ Basic magnetocaloric compound. Eu 2+ A larger distance between ions helps to weaken the Heisenberg interaction and promotes single-ion behavior.

[0051] (2) Determination of the magnetic ground state

[0052] The magnetic properties of the low-temperature magnetocaloric material EuCl2 were measured using the MPMS-3 system. Ultra-low temperature measurements, from 1.8 K to 0.4 K, were performed using the MPMS-3 system's accompanying... 3 He inserts the pole to perform the operation.

[0053] Figure 4 The magnetization-transition (MT) curves of EuCl2 material under a 0.01T magnetic field in the range of 0.4K to 300K are presented. It can be seen that there is no significant difference between the heating and cooling curves in the high-temperature region. As the temperature decreases, the magnetization increases sharply, undergoing a magnetic phase transition around 1.6K. By performing a first-order differential on the MT curve, the phase transition temperature can be determined to be 1.69K.

[0054] Figure 5 The temperature-dependent reciprocal of magnetic susceptibility (1 / χ = H / M) curve is given. This is obtained using the Curie-Weiss law (χ... -1 =3K B (T-θ CW ) / N(μ B μ eff ) 2 By fitting the data, the effective magnetic moment μ is obtained. eff Paramagnetic Curie temperature θ CW They are 7.61μ B And 1.32K. Based on The freedom of calculation Eu 2+ The theoretical effective magnetic moment of the ion (J = S = 7 / 2, L = 0) is 7.94 μ. B Landes factor g J The value is 2. It can be observed that the experimental effective magnetic moment value of the EuCl2 material is 7.61 μ. BThe results are close to the theoretical value, and the positive paramagnetic Curie temperature (1.32K) indicates that ferromagnetic interactions dominate in EuCl2 materials.

[0055] To further determine the ground-state magnetic configuration of EuCl2, first-principles calculations based on density functional theory (DFT) were performed. The experimentally measured cell parameters and atomic positions of EuCl2 at 5 K were used as the initial model in the calculations.

[0056] Consider four different spin configurations: linear ferromagnetic, A-type, C-type, and G-type antiferromagnetic, as illustrated in the diagrams below. Figure 6 As shown, the calculated total energies are -85.733528 eV / fu, -85.730356 eV / fu, -85.730425 eV / fu, and -85.729547 eV / fu, respectively. The results indicate that the linear ferromagnetic configuration has the lowest energy, confirming the ferromagnetic ground state of EuCl2 material.

[0057] Furthermore, the electronic structure of EuCl2 material was analyzed by calculating its density of electronic states (DOS). Figure 7 The total density of states (DSO) of EuCl2 and the local DSO of the Eu-4f and Cl bands are presented. The significant band gap of 3.033 eV demonstrates the insulating properties of EuCl2. For the Eu-4f band, the spin-up states lie below the Fermi level, exhibiting a sharp peak near -0.35 eV and high localization, while the spin-down band lies above the Fermi level. On the other hand, the Cl band is almost uniformly distributed, with no significant hybridization between it and the Eu-4f band, indicating that the contribution of Cl atoms to the total DSO and total magnetic moment is negligible. The calculated magnetic moment contributed by Eu-4f is 6.936 μ. B Very close to a single Eu 2+ The classical theoretical magnetic moment of ions is 7μ. B (μ Jz =g J Jμ B (J = S = 7 / 2). These results reveal the ferromagnetic properties of EuCl2 materials, and the negligible coupling between Eu and Cl suggests that Eu... 2+ The single-ion behavior of ions and weak magnetic interactions.

[0058] (3) Eu 2+ Single-ion behavior of ions

[0059] The electronic localization function (ELF) is a powerful tool for quantitatively characterizing the localization of electrons in space, and it is crucial for understanding electron distribution and exchange interactions in solids. Based on the crystal structure and ferromagnetic ground state of EuCl2, the ELF was further calculated to reveal the influence of the coordination environment on the electronic and magnetic properties of the material.

[0060] Figure 8 Figure (a) shows the electron localization function diagram on the (010) plane, which is the only crystal plane occupied by both Eu and Cl atoms. Figure 8 Figure (b) plots the electron localization function values ​​between the nearest-neighbor Eu-Cl (Eu-Cl1 and Eu-Cl2) extracted from it. Typically, the electron localization function value varies from 0 to 1, corresponding to a change from complete delocalization to complete localization of the electrons. Figure 8 As can be seen, the electron localization function values ​​around Eu and Cl are greater than 0.8, indicating that electrons are highly localized around these atoms. From... Figure 8 Figure (b) also reveals that although Cl1 and Cl2 are slightly different in position, their electron localization function values ​​are almost identical, and due to the high electronegativity of Cl ions, their electron localization function values ​​are slightly higher than those of Eu. In contrast, in the region between Eu and Cl, the electron localization function value is close to zero (<0.1), indicating almost no electron sharing, further demonstrating the presence of Eu in EuCl2 materials. 2+ Single-ion magnetic behavior of ions.

[0061] To further verify the presence of Eu in EuCl2 materials 2+ The near-single-ion and free-spin behavior of the ions was simulated using the free Heisenberg spin model, and the results were compared with experimental data. The free Heisenberg spin model provides a theoretical basis for describing magnetization under applied magnetic field H and temperature T. The expression for magnetization is: M = M0B J (x), where the Brillouin function B J (x) is defined as: Here, M0 = Jg J μ B x = Jg J μ B μ0H / k B T, for Eu 2+ Ions, J = S = 7 / 2, g J =2.

[0062] Figure 9 The spin-free Eu at 0.4 K is given. 2+The Brillouin function of the ions was calculated and compared with the magnetization curve of EuCl2 material at 0.4 K. It can be seen that the magnetization of EuCl2 material increases sharply with increasing magnetic field, reaching 97.4% of the saturation magnetic moment at a low field of 1 T, indicating the easy magnetization and saturation of this material. The good match between the experimental curves and the Brillouin function strongly supports the magnetization of EuCl2. 2+ The free spin behavior of ions.

[0063] (4) Measurement of magnetocaloric effect

[0064] Figure 10 Isothermal magnetization curves are given in the temperature range of 0.4–20 K, based on Maxwell's relations. The magnetic entropy change curve can be calculated, such as Figure 11 As shown.

[0065] from Figure 11 As can be seen, under a magnetic field change of 0-5T, the magnetic entropy change of EuCl2 material reaches 74.6 J kg. -1 K -1 The magnetic entropy change of EuCl2 reaches 96% of the theoretical limit, making it the largest reported to date. Under a low magnetic field variation of 0-1T, the magnetic entropy change of EuCl2 reaches 36.8 J / kg. -1 K -1 It is also far higher than the magnetic entropy change of all previously reported materials under the corresponding magnetic field.

[0066] (5) Quasi-adiabatic demagnetization measurement

[0067] Quasi-adiabatic demagnetization measurements were performed using a Power Property Measurement System (PPMS) to evaluate the performance of EuCl2 materials in adiabatic demagnetization refrigeration applications. The lowest achievable temperature and the holding time at low temperatures are key parameters for evaluating the performance of adiabatic demagnetization refrigeration materials. The holding time parameter indicates how long the system can operate effectively without restarting the cooling cycle, which is crucial for refrigeration applications requiring continuous or long-term operation.

[0068] Figure 12 The adiabatic demagnetizing cooling curves of EuCl2 material under different initial temperatures and magnetic fields are presented. It can be seen that as the magnetic field decreases, the temperature initially decreases, gradually stabilizes, and finally reaches its lowest temperature near zero magnetic field. At initial temperatures and magnetic fields of 2 K and 4 T, the lowest temperature reached by EuCl2 material is 428 mK; at initial temperatures and magnetic fields of 2 K and 5 T, the lowest temperature reached by EuCl2 material is 346 mK.

[0069] Figure 13The holding time of EuCl2 material was compared with that of other cryogenic refrigerants Na2BaCo(PO4)2 (NBCP), CrK(SO4)2·12H2O (CPA), and Fe(SO4)2(NH4)2·6H2O (FAA). Despite the presence of heat leakage, the holding time of EuCl2 material exceeded one hour under any initial conditions, significantly longer than that of other materials under the same operating conditions.

[0070] Examples 2-4

[0071] Performance characterization of low-temperature magnetocaloric materials EuF2, EuBr2 and EuI2

[0072] The EuF2, EuBr2, and EuI2 materials in Examples 2-4 exhibit different crystal structures. EuF2 exhibits a cubic structure with a space group of Fm-3m; EuBr2 exhibits a tetragonal structure with a space group of P4 / n; while EuI2 material has the lowest symmetry, exhibiting a monoclinic structure with a space group of P121 / c1.

[0073] Magnetic measurements of the EuF2, EuBr2, and EuI2 materials from Examples 2-4 were performed using MPMS-3. The results showed that the ground-state magnetism of all three materials exhibited antiferromagnetic characteristics. Based on Maxwell's relation... The magnetic entropy change curve of the material can be calculated, such as... Figure 14 The results showed that the magnetic entropy change of EuF2 material at 1.8 K under a magnetic field variation of 0-5 T was 39.8 J kg. -1 K -1 The magnetic entropy change of EuBr2 material at 1.8 K under a magnetic field variation of 0-5 T is 38.7 J / kg. -1 K -1 The magnetic entropy change of EuI2 material at 1.8 K under a magnetic field variation of 0-5 T is 31.6 J kg. -1 K -1 .

[0074] Comparative Example 1: EuO

[0075] EuO was prepared by the thermal reduction reaction of commercially available Eu₂O₃ (white powder, 99.9% purity, CAS 1308-96-9, purchased from Aladdin) and Eu (lumpy, 99.9% purity, CAS 7440-53-1, purchased from Innoca). The process was carried out in a glove box. Eu₂O₃ and Eu were placed in a crucible and vacuum-sealed under a helium atmosphere. The mixture was then placed in an induction furnace and reacted at 1800°C for 30 minutes, followed by natural cooling to obtain EuO powder with a cubic structure and space group Fm-3m.

[0076] Magnetic measurements of the EuO material in Comparative Example 1 were performed using MPMS-3. The results showed that the phase transition temperature of the EuO material is around 67 K, exhibiting a ferromagnetic ground state. The magnetic entropy change of the EuO material at 69 K under a magnetic field variation of 0–5 T was 18 J / kg. - 1 K -1 It is not suitable for use in low-temperature regions (below 2K).

[0077] Comparative Example 2: EuS

[0078] EuS is a black powder with a purity of 99.99% and an average particle size of 50 micrometers. Its CAS number is 12020-65-4, and it was purchased from Aladdin Company. EuS has a cubic structure and a space group of Fm-3m.

[0079] Magnetic measurements of the EuS material in Comparative Example 2 were conducted using MPMS-3. The results showed that the phase transition temperature of the EuS material is around 17 K, exhibiting a ferromagnetic ground state. The magnetic entropy change of the EuS material at temperatures as low as 5 K under a magnetic field variation of 0–5 T is 10 J / kg. -1 K -1 Its performance is far lower than that of EuX2 (X = F, Cl, Br, I) materials, and it is not suitable for applications in the low-temperature region (less than 2K).

[0080] Comparative Example 3: EuSe

[0081] EuSe is a brown powder with a purity of 99.9% and an average particle size of 50 micrometers. Its CAS number is 12020-66-5, and it was purchased from Aladdin Company. EuSe has a cubic structure and a space group of Fm-3m.

[0082] Magnetic measurements of the EuSe material in Comparative Example 3 were conducted using MPMS-3. The results showed that the phase transition temperature of EuSe is around 4K, exhibiting a ferromagnetic ground state. The magnetic entropy change of EuSe material at temperatures as low as 2K under a magnetic field variation of 0–5T is 13 J / kg. -1 K -1 Its performance is far lower than that of EuX2 (X = F, Cl, Br, I) materials, and it is not suitable for applications in the low-temperature region (less than 2K).

Claims

1. A low-temperature magnetocaloric material based on divalent europium, wherein the chemical formula of the low-temperature magnetocaloric material is EuX2, where X = F, Cl, Br or I, and wherein, EuF2, EuBr2, and EuI2 exhibit antiferromagnetic ground-state magnetic characteristics and, in turn, cubic, tetragonal, and monoclinic structures; EuCl2 exhibits ferromagnetic ground-state magnetic characteristics and has an orthogonal structure.

2. The low-temperature magnetocaloric material according to claim 1, wherein, The phase transition temperature of the low-temperature magnetocaloric material is below 1.8K.

3. The low-temperature magnetocaloric material according to claim 1, wherein, The magnetic entropy change of the low-temperature magnetocaloric material under a magnetic field change of 0-5T is higher than 30J / kg. -1 K -1 .

4. The low-temperature magnetocaloric material according to claim 1, wherein, The composition of the low-temperature magnetocaloric material is EuCl2.

5. The low-temperature magnetocaloric material according to claim 4, wherein, The space group of EuCl2 is Pnma.

6. The low-temperature magnetocaloric material according to claim 4 or 5, wherein, In the (010) plane, the shortest distance between Eu-Eu atoms is Between the (010) planes, the shortest distance between Eu-Eu atoms is 7. The low-temperature magnetocaloric material according to claims 4-6, wherein, Eu 2+ The ions exhibit near-single-ion behavior and free spin properties.

8. The application of the low-temperature magnetocaloric material according to any one of claims 1 to 7 in adiabatic demagnetization refrigeration.

9. The application of the low-temperature magnetocaloric material according to claim 8, wherein, In the quasi-adiabatic demagnetization measurement, the lowest adiabatic demagnetization temperature reached by the low-temperature magnetocaloric material was 428 mK at an initial temperature of 2 K and a magnetic field of 4 T; and the lowest adiabatic demagnetization temperature reached by the low-temperature magnetocaloric material was 346 mK at an initial temperature of 2 K and a magnetic field of 5 T.

10. The application of the low-temperature magnetocaloric material according to claim 8, wherein, During the adiabatic demagnetization cooling process, after reaching the minimum temperature, the low-temperature magnetocaloric material is maintained at the minimum temperature for at least 1 hour.

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