A compact gadolinium-based fluorocarbonate magnetic refrigeration material, a preparation method and application thereof

Abstract

The application belongs to the technical field of magnetic functional materials, and discloses a dense gadolinium-based fluorocarbonate magnetic refrigeration material and a preparation method and application thereof. The chemical formula of the gadolinium-based fluorocarbonate magnetic refrigeration material is NaGdCO3F2. The material is constructed by anti-magnetic sodium ions and fluorine ions and carbonic acid root mixed bridging ligands to form a dense gadolinium-based three-dimensional honeycomb framework structure. The specific spatial arrangement effectively suppresses the magnetic exchange interaction while maintaining high magnetic density, induces short-range magnetic correlation near 0.5K, suppresses the magnetic order temperature to 0.22K, and widens the effective refrigeration temperature range by generating a Schottky abnormal specific heat wide peak. Under 1T, 2T and 7T magnetic fields, the maximum magnetic entropy change reaches 23.1, 38.9 and 59.1 J·kg ‑1 ·K ‑1 respectively. As an excellent low-temperature magnetic refrigerant, the material has the advantages of good thermal stability, simple preparation process and low cost, and has a wide industrial application prospect in the field of adiabatic demagnetization refrigeration materials.

Description

Technical Field

[0001] This invention belongs to the field of magnetic functional materials, specifically relating to a dense gadolinium-based fluorocarbonate magnetic refrigeration material, as well as the preparation method and application of the material. Background Technology

[0002] With the rapid iteration of superconducting quantum computing hardware, deep space astrophysical exploration, and cutting-edge research in condensed matter physics, how to efficiently obtain and maintain sub-Kelvin (<1K) cryogenic environments has become a key technological bottleneck restricting high-end instruments. Among existing methods for obtaining cryogenic temperatures, solid-state adiabatic demagnetizing refrigeration (ADR) technology stands out due to its independence from scarce resources. 3 With its significant advantages such as abundant resources, compact system, and ability to adapt to weightless environments, it is widely recognized as an ideal solution for achieving extremely low temperature environments.

[0003] The core of an adiabatic demagnetizing refrigeration system is the magnetic refrigerant. To obtain sufficient cooling capacity, the magnetic refrigeration material must possess a large magnetic entropy change, which typically requires a high magnetic ion density. However, there is an inherent contradiction in material design: highly dense magnetic ions significantly enhance dipole-magnetic exchange interactions within the system, inevitably leading to an increase in the material's long-range magnetic order temperature. If the material undergoes a long-range magnetic order transition prematurely, its magnetic entropy will be significantly depleted, severely weakening its effective cooling capacity in the sub-Kelvin temperature range.

[0004] In summary, finding a way to broaden the effective cooling temperature range while maintaining a large magnetic entropy change, breaking down the barrier between large magnetic entropy change and low magnetic order temperature, and developing a new generation of ultra-low temperature magnetic refrigerant that combines good structural stability, low magnetic order temperature, and large magnetic entropy change is a key technical challenge that urgently needs to be overcome in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a dense gadolinium-based fluorocarbonate magnetic refrigeration material, its preparation method and application, and to solve the technical problem of how to design and prepare a high magnetic density gadolinium-based magnetic refrigeration material with both large magnetic entropy change and low magnetic order temperature.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a dense gadolinium-based fluorocarbonate magnetic refrigeration material, wherein the chemical formula of the magnetic refrigeration material is NaGdCO3F2.

[0008] Furthermore, the cell information of the magnetic refrigeration material is as follows: it belongs to an orthorhombic crystal system with space group Pnma; the cell parameters of the crystal are... , , , , , , Unit cell volume .

[0009] Furthermore, the crystal structure of the magnetic refrigeration material consists of [GdO4F4] polyhedra with a twisted tetragonal antiprism configuration. These polyhedra are connected by zigzag chains along the b-axis via fluorine ion bridging. Adjacent zigzag chains are connected along the c-axis via carbonate bridging to construct a three-dimensional honeycomb lattice framework. The material also possesses diamagnetic Na... + The cations are located within the structural channels of the three-dimensional honeycomb lattice framework.

[0010] A second aspect of the present invention provides a method for preparing a dense gadolinium-based fluorocarbonate magnetic refrigeration material, comprising the following steps:

[0011] S1. Measure the gadolinium source, fluorine source, sodium source and carbonate source according to the preset molar ratio and add them to the solvent. Stir thoroughly to form a homogeneous precursor solution.

[0012] S2. The precursor solution is transferred to a high-pressure reactor with a polytetrafluoroethylene liner, sealed, and subjected to constant temperature heating reaction under set conditions.

[0013] S3. After the reaction is complete, the temperature is lowered to room temperature. The resulting product is washed and dried to obtain the dense gadolinium-based fluorocarbonate magnetic refrigeration material.

[0014] Further, in step S1, the molar ratio of the gadolinium source, fluorine source, sodium source, and carbonate source is controlled at 1:(1~6):(0.5~4):(0.5~4); the gadolinium source is selected from at least one of gadolinium nitrate hexahydrate, gadolinium acetate tetrahydrate, or gadolinium chloride hexahydrate; the fluorine source is selected from at least one of hydrofluoric acid, sodium fluoride, or ammonium fluoride; the sodium source is selected from at least one of sodium hydroxide, sodium carbonate, sodium nitrate, or sodium fluoride; and the carbonate source is selected from at least one of sodium carbonate, sodium bicarbonate, or ammonium carbonate.

[0015] Furthermore, in step S2, the set temperature of the isothermal heating reaction is 150~250℃, and the isothermal time is 24~72h.

[0016] A third aspect of the present invention provides an application of the above-mentioned dense gadolinium-based fluorocarbonate magnetic refrigeration material in adiabatic demagnetization refrigeration. As a preferred embodiment of the present invention, under test conditions with applied magnetic fields of 1T, 2T, and 7T, the maximum magnetic entropy change of the magnetic refrigeration material reaches 23.1, 38.9, and 59.1 J·kg, respectively. -1 ·K -1 Furthermore, the magnetic order temperature of the magnetic refrigeration material is suppressed to 0.22K.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] (1) Overcoming the physical constraints of high magnetic moment density and low magnetic order temperature: This invention introduces diamagnetic Na + A unique three-dimensional honeycomb lattice framework was constructed using cation and fluoride / carbonate mixed bridging ligands. This specific spatial configuration ensures extremely high Gd content within the framework. 3+ While increasing spin density, the magnetic exchange interaction is effectively suppressed, allowing short-range magnetic correlation of magnetic dipole interactions in this specific structure. Unlike the λ-type phase transition of most magnetic refrigeration materials, this material exhibits a broad Schottky anomaly-type specific heat peak at 0.5 K. The former rapidly releases magnetic entropy before and after the phase transition point, while the latter extends the magnetic entropy to a lower temperature range, effectively broadening the refrigeration range. Under test conditions with applied magnetic fields of 1 T, 2 T, and 7 T, the maximum magnetic entropy change of the magnetic refrigeration material reached 23.1, 38.9, and 59.1 J·kg, respectively. -1 ·K -1 Under a 2T magnetic field, this material can react with the commercially available magnetic refrigerant gadolinium gallium garnet (Gd3Ga5O). 12 (GGG) 38.4 J·kg can only be achieved at a high field of 7T. -1 ·K -1 Quite similar. In addition, the magnetic order temperature is 0.22K, which also has a magnetic entropy change far exceeding that of GGG, thus solving the problem mentioned above that it is difficult to have both a large magnetic entropy change and a low magnetic order temperature.

[0019] (2) This invention fundamentally solves the problem of instability in traditional hydrated magnetic refrigerants: The magnetic refrigeration material provided by this invention has a completely water-free, highly dense framework structure. Compared with traditional paramagnetic salt refrigerants, this material does not undergo any dehydration or structural collapse at room temperature or in high vacuum, exhibiting extremely excellent physicochemical and thermodynamic stability.

[0020] (3) It has extremely high process feasibility and cost advantages: The preparation method adopted in this invention is a conventional solvothermal method with mild reaction conditions and inexpensive and readily available precursor reagents. This process does not require extreme high-temperature sintering or a harsh oxygen-free environment, is simple to operate, produces high-purity products with good reproducibility, and is extremely suitable for large-scale industrial production. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the single crystal structure and coordination environment of NaGdCO3F2 prepared in Example 1 of the present invention;

[0022] Figure 2 This is a comparison diagram of the X-ray diffraction test curve and the theoretical simulation curve of the NaGdCO3F2 powder prepared in Example 1 of the present invention;

[0023] Figure 3 Thermogravimetric curve of NaGdCO3F2 prepared in Example 1 of this invention at 30-800℃;

[0024] Figure 4 The magnetization intensity curve of NaGdCO3F2 prepared in Example 1 of the present invention at 2-8K under a magnetic field variation of 0-7T;

[0025] Figure 5 The graph shows the temperature-dependent magnetic susceptibility of NaGdCO3F2 prepared in Example 1 of this invention under an external magnetic field of 0.1T.

[0026] Figure 6 The graph shows the change of magnetic entropy of NaGdCO3F2 prepared in Example 1 of this invention under different magnetic fields as a function of temperature.

[0027] Figure 7 The specific heat / (T·R) and specific heat map of NaGdCO3F2 prepared in Example 1 of the present invention under a 0T magnetic field.

[0028] Figure 8 The graph shows the specific heat of NaGdCO3F2 prepared in Example 1 of this invention as a function of temperature under different magnetic fields.

[0029] Figure 9 The magnetic entropy change of NaGdCO3F2 prepared in Example 1 of this invention, obtained by fitting the specific heat and magnetization intensity curves under different magnetic fields, is compared with GGG curves. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0031] Example 1

[0032] This embodiment provides a method for preparing the magnetic refrigeration material NaGdCO3F2:

[0033] Gd(NO3)3·6H2O, Na2CO3, and NaF were added to 10 mL of water in a molar ratio of 1:2.5:2.5 and stirred for half an hour. The mixture was then transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene and heated at 220°C for 48 hours. After cooling to room temperature, the mixture was washed and dried to obtain colorless, transparent crystals.

[0034] Example 2

[0035] Single-crystal structure characterization of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0036] (1) Single-crystal X-ray diffraction data of the complex were collected using a Bruker D8 VENTURE diffractometer (Mo Kα radiation, λ=0.071073nm). The structure was analyzed using ShelXT software. Detailed crystallographic data are shown in Table 1.

[0037] Table 1. Crystallographic data of the coordination compounds involved in this invention.

[0038]

[0039] R1 = Σ (|F0|–|F C |) / Σ |F0|; wR2 = [Σ w (|F0|–|F C |) 2 / Σ w F0 2 ] 1 / 2

[0040] (2) Combination Figure 1 As can be seen from the crystal structure diagram, Gd inside the framework 3+ The ions are in an octet coordination state, forming twisted tetragonal antiprism [GdO4F4] polyhedra. These polyhedra are connected by fluoride ion bridges along the b-axis in zigzag chains, while adjacent chains are connected along the c-axis by carbonate ions, ultimately constructing a three-dimensional honeycomb-like dense lattice framework with abundant channels. Diamagnetic Na+ cations are located within these channels, serving to support the framework and spatially isolate the magnetic centers.

[0041] Example 3

[0042] X-ray powder diffraction characterization of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0043] The powder samples obtained in Example 1 were characterized using a Rigaku Smartlab 9 kW X-ray powder diffractometer (Cu-Kα radiation, λ=0.15418 nm). The results are shown in [Figure number missing]. Figure 2 The positions and relative intensities of the diffraction peaks in the XRD pattern obtained by the experiment are highly consistent with the standard pattern obtained by simulation based on single crystal data, and the baseline is smooth with no impurity peaks, which confirms that the product obtained by the preparation process of the present invention has extremely high phase purity.

[0044] Example 4

[0045] Thermogravimetric analysis of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0046] Thermodynamic stability tests were performed on the material using a TGA_600 thermogravimetric analyzer. Throughout the process of uniform heating from room temperature to 300°C, the TGA curve of the sample remained horizontal, and no significant mass decay was observed. This fully demonstrates that there is no water of crystallization or free solvent molecules within the crystal framework, and that it can maintain absolute structural integrity even under extreme temperature cycling, completely overcoming the defects of traditional hydrated paramagnetic salt refrigerants that are prone to efflorescence and deterioration.

[0047] Example 5

[0048] Characterization of the basic magnetic properties of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0049] (1) The magnetic properties of the material obtained in Example 1 were tested using a Power Property Measurement System (PPMS). The isothermal magnetization of the material at different temperatures (2-8K) increased rapidly with the increase of the applied magnetic field. Due to the extremely weak internal antiferromagnetic coupling, the magnetization of the material rapidly saturated under a strong field, reaching 7.02 Nμm at 2K and 7T. B This value is related to Gd 3 + Theoretical saturation magnetization of ions (7 Nμ) B (Highly consistent, see) Figure 4 .

[0050] (2) The measured values ​​at room temperature (300K) under an applied magnetic field of 0.1T. The value is 7.79cm 3 ·K·mol -1 This is related to Gd 3+ The theoretical pure spin value of the ion is in high agreement (7.88 cm⁻¹). 3 ·K·mol -1 ). For the reciprocal of magnetic susceptibility ( The data were fitted using the Curie-Weiss law, and the extracted Weiss constant θ was −0.66K. (See...) Figure 5 This negative value explicitly indicates the Gd within the three-dimensional frame. 3+ There is a weak antiferromagnetic coupling interaction between the ions.

[0051] Example 6

[0052] Characterization of the magnetocaloric effect of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0053] Magnetic entropy change is the most crucial indicator for measuring the cooling capacity of magnetic refrigerants. This was determined by analyzing the isothermal magnetization data measured at various temperatures in Example 3. Figure 4Thermodynamic integration was performed using Maxwell's relations to calculate the magnetic entropy change of the material under different magnetic field variations. The results are shown in [Figure number missing]. Figure 6 Thanks to the weak magnetic interactions and the high magnetic density resulting from the dense framework, this material releases an astonishing magnetic entropy in the extremely low-temperature region. Specifically, the maximum magnetic entropy change of NaGdCO3F2 material reaches 59.1 J·kg⁻¹ at a temperature of 2 K and a magnetic field of 7 T. -1 ·K -1 The magnetic entropy change is 95.1% of the theoretical value, significantly better than the 38.4 J·kg⁻¹ of the existing commercial magnetic refrigeration material GGG under the same conditions. -1 ·K -1 .

[0054] Example 7

[0055] Specific heat characterization of the magnetic refrigeration material NaGdCO3F2 in this embodiment:

[0056] To determine the magnetic ordering temperature of NaGdCO3F2, an ultra-low temperature zero-field specific heat test was performed on the material, and the results are as follows: Figure 7 As shown, the specific heat peak appears at 0.5 K as the temperature decreases, but unlike the λ-type phase transition peak of most other magnetic refrigeration materials, this peak is broad. Furthermore, the specific heat / (T·R) also indicates that the true magnetic order temperature is not at 0.5 K, but at 0.22 K. This is due to the Schottky anomaly formed by short-range magnetic correlation caused by the special structure of this material. Such a broad peak retains the magnetic entropy to a lower temperature range, thus maintaining a large magnetic entropy at lower temperatures and widening the refrigeration range.

[0057] Example 8

[0058] To determine the magnetic entropy change of NaGdCO3F2 below 2K, specific heat tests were performed on the material under different magnetic fields at extremely low temperatures. The results are as follows: Figure 8 As shown. Using The calculated magnetic entropy change of the material. Figure 9 The results show that the magnetic entropy changes obtained by fitting the magnetization curve are quite close, ensuring the accuracy of the material's magnetic entropy change value. Since commercial permanent magnets can provide magnetic fields of approximately 2T or higher, if the magnetic refrigeration material possesses a large magnetic entropy change even in magnetic fields below 2T, the application of bulky and expensive superconducting magnets can be avoided. Therefore, the focus is on the magnetic entropy under 1T and 2T magnetic fields. Figure 9 It can be seen that the maximum magnetic entropy of this material at 1T and 2T reached 23.1 and 38.9 J·kg, respectively. -1 ·K -1The magnetic entropy at 1T is greater than that of commercial magnetic refrigerant GGG at 2T, and the maximum magnetic entropy at 2T is also greater than that of GGG at a high field of 7T, which is 38.4 J·kg. -1 ·K -1 This means that the material can completely replace GGG, which uses superconducting magnets, under the low field provided by the permanent magnet. This result fully demonstrates that the material described in this invention has extremely high application value in low-power, miniaturized cryogenic adiabatic demagnetizing refrigeration equipment.

[0059] The embodiments described above are merely preferred embodiments of the present invention, and not all feasible embodiments of the present invention. Any obvious modifications made by those skilled in the art without departing from the principles and spirit of the present invention should be considered to be included within the scope of protection of the claims of the present invention.

Claims

1. A dense gadolinium-based fluorocarbonate magnetic refrigeration material, characterized in that, The chemical formula of the magnetic refrigeration material is NaGdCO3F2.

2. The dense gadolinium-based fluorocarbonate magnetic refrigeration material according to claim 1, characterized in that, The magnetic refrigeration material belongs to the orthorhombic crystal system, with space group Pnma and cell parameters of [missing information]. , , , , , , Unit cell volume .

3. The dense gadolinium-based fluorocarbonate magnetic refrigeration material according to claim 1, characterized in that, The crystal structure of the magnetic refrigeration material consists of [GdO4F4] polyhedra with a twisted tetragonal antiprism configuration. These polyhedra are connected by zigzag chains along the b-axis via fluorine ion bridging. Adjacent zigzag chains are connected along the c-axis by carbonate ion bridging to form a three-dimensional honeycomb lattice framework. Furthermore, the material contains diamagnetic Na... + The cations are located in the structural channels of the three-dimensional honeycomb lattice framework.

4. The dense gadolinium-based fluorocarbonate magnetic refrigeration material according to claim 1, characterized in that, The magnetic cooling material induced short-range magnetic correlation at 0.5±0.1K and suppressed the magnetic order temperature to 0.22±0.04K.

5. A method for preparing a dense gadolinium-based fluorocarbonate magnetic refrigeration material as described in any one of claims 1 to 4, characterized in that, The process includes the following steps: adding gadolinium source, fluorine source, sodium source and carbonate source to a solvent in a set molar ratio and stirring until homogeneous to form a precursor solution; transferring the precursor solution to a reaction vessel with a polytetrafluoroethylene liner and carrying out a constant-temperature solvothermal reaction under sealed conditions; after the reaction is completed, naturally cooling to room temperature and obtaining the magnetic refrigeration material by separation and washing.

6. The method for preparing the dense gadolinium-based fluorocarbonate magnetic refrigeration material according to claim 5, characterized in that, The molar ratio of the gadolinium source, fluorine source, sodium source, and carbonate source is 1:(1~6):(0.5~4):(0.5~4); the gadolinium source is at least one of gadolinium nitrate hexahydrate, gadolinium acetate tetrahydrate, or gadolinium chloride hexahydrate; the fluorine source is at least one of hydrofluoric acid, sodium fluoride, or ammonium fluoride; the sodium source is at least one of sodium hydroxide, sodium carbonate, sodium nitrate, or sodium fluoride; and the carbonate source is at least one of sodium carbonate, sodium bicarbonate, or ammonium carbonate.

7. The method for preparing the dense gadolinium-based fluorocarbonate magnetic refrigeration material according to claim 5, characterized in that, The isothermal solvothermal reaction is carried out at a temperature of 150~250℃ for 24~72h.

8. The application of a dense gadolinium-based fluorocarbonate magnetic refrigeration material as described in any one of claims 1 to 4 in ultra-low temperature magnetic refrigeration.

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