Low-temperature magnetic refrigeration fluoride material, preparation method and application thereof

Abstract

The application discloses a low-temperature magnetic refrigeration fluoride material and a preparation method and application thereof, in particular to an application of KGd2F7 in preparation of an adiabatic demagnetization refrigeration material. The KGd2F7 is applied in the adiabatic demagnetization refrigeration material, and compared with a currently commercially used hydrated paramagnetic salt, the KGd2F7 has a large magnetic entropy density and can provide a higher refrigeration capacity, in addition, the KGd2F7 is a stable compound, cannot be dehydrated and weathered, has no corrosiveness, and is convenient to apply.

Description

Technical Field

[0001] This invention belongs to the field of magnetic refrigeration materials technology, specifically relating to a low-temperature magnetic refrigeration fluoride material, its preparation method, and its application. Background Technology

[0002] Cryogenic refrigeration, especially ultra-low temperature refrigeration (<1 K), has important applications in condensed matter physics, quantum technology, and space exploration. Currently, the main ultra-low temperature refrigeration technologies include adsorption refrigeration, dilution refrigeration, and adiabatic demagnetization refrigeration. Among these, adiabatic demagnetization refrigeration does not require the use of scarce resources. 3 He refrigerant, which does not rely on gravity to operate, can achieve a low lower limit of cooling temperature. Therefore, this technology has broad application prospects in the field of cryogenic refrigeration.

[0003] Adiabatic demagnetizing refrigeration is based on the magnetocaloric effect (MCE), which is the effect of a magnetic refrigeration medium material producing a significant temperature change when the external magnetic field is altered. An ideal low-temperature magnetic refrigeration medium material needs to possess a low magnetic ordering temperature and a large magnetic entropy to achieve high-capacity refrigeration at low temperatures. Currently, commercially available low-temperature magnetic refrigeration materials mainly include gadolinium gallium garnet (Gd3Ga5O4). 12 GGG and hydrated paramagnetic salts. Although GGG exhibits extremely low long-range ordering temperatures under zero magnetic field, it develops short-range ordering below 0.8 K, which limits its cooling effect at even lower temperatures. 12 (R = Tb, Gd, Nd, Dy), PHYSICAL REVIEW APPLIED 19, 014038(2023)). Although hydrated paramagnetic salts can be used for refrigeration in the mK temperature range, their internal magnetic ion distribution is relatively sparse, resulting in a low magnetic entropy change density, making it difficult to meet the needs of high-capacity refrigeration. Furthermore, hydrated paramagnetic salts are generally unstable and are prone to dehydration, weathering, or structural degradation under conditions such as high vacuum or extremely low temperatures, resulting in poor long-term stability (“Ultra-low temperature adiabatic demagnetizing refrigerator for sub-open temperature range,” Acta Physica Sinica, Vol. 70, No. 9 (2021) 090702). Therefore, developing stable magnetic refrigeration materials with high magnetic entropy change density that can be used for ultra-low temperature refrigeration is of great significance.

[0004] Rare earth fluorides, as an important class of inorganic functional materials, possess excellent physicochemical stability and show broad application prospects in fields such as catalysis and optical materials. Recently, the literature reported the magnetic refrigeration rare earth fluoride material KYb3F. 10(Xu QF, Liu XY, Wu RT, et al. Temperature Below 30 mK Achieved by Adiabatic Demagnetization Refrigeration[J]. Journal of the American Chemical Society, 2025, 147(30): 27089-27094), with its large magnetic entropy change and low refrigeration temperature, it is an ultra-low temperature refrigerant with high application value. KYb3F 10 The preparation method involves adding hydrofluoric acid dropwise to a mixed aqueous solution of YbCl3·6H2O and KCl under stirring, and reacting the mixture in a hydrothermal reactor at 180°C for 3 days. After washing and drying, the product is KYb3F. 10 .

[0005] The rare earth fluoride KYb3F 10 There are two problems with the preparation process. First, hydrofluoric acid is used directly, but hydrofluoric acid is highly toxic and may pose a safety hazard to production. Second, the yield is low (calculated based on rare earth element Yb, KYb3F). 10 With a yield of only about 71%, rare earth elements are expensive, and low yields result in huge waste. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides an application of KGd2F7 in the preparation of thermally insulating and demagnetizing refrigeration materials. Preferably, it is used in the preparation of low-temperature thermally insulating and demagnetizing refrigeration materials.

[0007] According to an embodiment of the present invention, the low temperature is ≤4K, preferably ≤2K, more preferably ≤1K, and even more preferably 0.1-1K or 1-2K, for example 0.1K, 0.2K, 0.3K, 0.4K, 0.5K, 0.6K, 0.7K, 0.8K, 0.9K or 1K.

[0008] According to an embodiment of the present invention, the KGd2F7 is a polycrystalline material. Specifically, its crystal structure belongs to the cubic crystal system, and its space group is [insert space group here]. (or written as Fm-3m), cell parameters a = 5.750(1) Å.

[0009] According to an embodiment of the present invention, in the crystal structure of the KGd2F7, K + and Gd 3+ Cations co-occupy 4 a Wyckoff sites form a face-centered cubic lattice, and K + With Gd 3+The molar ratio is 1:2.

[0010] According to an embodiment of the present invention, in the crystal structure of the KGd2F7, the cation (K + and Gd 3+ ) was F - Ion separation. Due to the magnetic cation Gd 3+ By non-magnetic F - The ions are fully separated and further reacted with K. + This dispersion makes Gd 3+ The extremely weak magnetic interaction between ions effectively suppresses the magnetic order of KGd2F7, causing the release of magnetic entropy change to occur in the low-temperature region.

[0011] In the crystal structure of KGd2F7 of this invention, rare earth Gd 3+ The ion has 7 unpaired 4f electrons, and its ground-state spin quantum number is as high as S = 7 / 2, corresponding to a theoretical saturation magnetic entropy of Rln(8) J. mol -1 K -1 Therefore, gadolinium-containing compounds have a larger potential magnetic entropy and higher cooling capacity.

[0012] According to an embodiment of the present invention, the absolute value of the magnetic entropy change generated by KGd2F7 under the conditions of a temperature of 1 K and a magnetic field change from 0 T to 2 T is not less than 18 J·mol(Gd). 3+ ) —1 ·K —1 .

[0013] The present invention also provides a method for preparing the KGd2F7, which includes the following steps: KGd2F7 was prepared by mixing a gadolinium-containing compound, a potassium-containing compound, hydrochloric acid, and water, and then heating the mixture. Furthermore, in the gadolinium-containing compound or potassium-containing compound, at least one compound contains fluorine.

[0014] According to an embodiment of the present invention, the molar ratio of fluorine in the fluorine-containing compound to gadolinium in the gadolinium-containing compound is 3.5-10:1, preferably 4:1.

[0015] According to embodiments of the present invention, the gadolinium-containing compound is one or more of the following compounds or their hydrates: gadolinium oxide (Gd₂O₃), gadolinium hydroxide (Gd(OH)₃), gadolinium carbonate (Gd₂(CO₃)₃), gadolinium fluoride (GdF₃), gadolinium chloride (GdCl₃), gadolinium bromide (GdBr₃), gadolinium sulfate (Gd₂(SO₄)₃), gadolinium nitrate (Gd(NO₃)₃), and gadolinium acetate (Gd(CH₃COO)₃). For example, GdCl₃·6H₂O.

[0016] According to embodiments of the present invention, the potassium-containing compound is one or more of the following compounds or their hydrates: potassium hydroxide (KOH), potassium carbonate (K₂CO₃), potassium sulfate (K₂SO₄), potassium nitrate (KNO₃), potassium fluoride (KF), potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). Preferably, it is a mixture of KF and KCl, for example, in a molar ratio of 1:1.

[0017] According to an embodiment of the present invention, in the gadolinium-containing compound and the potassium-containing compound, the molar ratio of potassium to gadolinium is 6-10:1, preferably 8:1.

[0018] According to an embodiment of the present invention, the method is carried out in a hydrothermal reactor.

[0019] According to an embodiment of the present invention, the temperature of the heating reaction is 180-220 °C, and the heating reaction time is 16-48 h.

[0020] According to embodiments of the present invention, the water content is not particularly limited, as long as it is sufficient to partially or completely dissolve the raw materials. Preferably, the pH value of the solution after mixing the gadolinium-containing compound, potassium-containing compound, hydrochloric acid, and water is between 4 and 7, more preferably between 5 and 6.

[0021] According to an embodiment of the present invention, the preparation method includes the following steps: (1) After mixing the potassium-containing compound with water, it is then mixed with hydrochloric acid to obtain the first solution; (2) The gadolinium-containing compound was mixed with water to obtain a second solution; (3) Add the second solution to the first solution and heat the reaction to prepare the KGd2F7.

[0022] Preferably, in step (3), the second solution is added dropwise to the first solution.

[0023] Preferably, the method further includes a post-processing step: after the reactants are cooled to room temperature, the resulting solid product is washed with deionized water and dried to obtain the target product; the target product is KGd2F7 polycrystalline powder.

[0024] The beneficial effects of this invention are: (1) The KGd2F7 of this invention is used in thermal insulation and demagnetizing refrigeration materials. Compared with the currently commercially available hydrated paramagnetic salts, it has a higher magnetic entropy density and can provide higher refrigeration capacity. In addition, KGd2F7 is a stable compound that will not dehydrate and weather, is non-corrosive, and is convenient to use.

[0025] (2) The present invention uses a hydrothermal method to prepare KGd2F7. The reaction conditions of the method are milder, the product has high purity and high yield. The yield of the target product is calculated based on the amount of Gd input, and the yield reaches 95% or more. At the same time, it does not require the use of highly toxic hydrofluoric acid, and is highly safe. Attached Figure Description

[0026] Figure 1 XRD diffraction pattern of KGd2F7 prepared in Example 1; Figure 2 This is the face-centered cubic lattice diagram of the cations in KGd2F7 of this invention; Figure 3 Isothermal magnetization curve of KGd2F7 prepared in Example 1; Figure 4 (a) The specific heat of KGd2F7 prepared in Example 1 as a function of temperature is measured under zero magnetic field and under an applied magnetic field of 2 T. Figure 4 (b) is a graph showing the relationship between the magnetic entropy change of KGd2F7 and temperature, calculated from the specific heat data. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0028] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0029] Example 1 Preparation of KGd2F7 Weigh 0.9304 g (16 mmol) of KF and 1.1928 g (16 mmol) of KCl, dissolve them in 40 ml of deionized water, add 19.4 μL of concentrated hydrochloric acid (36-38 wt%), stir well, and label it as solution 1.

[0030] Weigh 1.4868 g (4 mmol) of GdCl3·6H2O, dissolve it in 10 mL of deionized water, stir well, and label it as solution 2.

[0031] While magnetically stirring, solution 2 was added dropwise to solution 1. After stirring for 30 minutes, the mixture was transferred to a 100 mL hydrothermal reactor (Teflon liner, stainless steel reactor) and placed in an oven at 180°C for 36 hours. The resulting product was then washed with deionized water, centrifuged at high speed, and dried at 80°C for 3 hours to obtain KGd2F7 polycrystalline powder. The product mass was 0.9737 g, and the yield was 100% based on the amount of Gd added.

[0032] Example 2 Scale-up preparation of KGd2F7 Weigh 9.2955 g (0.16 mol) of KF and 11.9282 g (0.16 mol) of KCl, dissolve them in 400 mL of deionized water, add 0.194 mL of concentrated hydrochloric acid (36-38 wt%), stir well, and label it as solution 3.

[0033] Weigh 14.8680 g (40 mmol) of GdCl3·6H2O, dissolve it in 100 mL of deionized water, stir well, and label it as solution 4.

[0034] While magnetically stirring, solution 4 was added dropwise to solution 3. After stirring for 60 minutes, the mixture was transferred to a 1000 mL hydrothermal reactor (Teflon liner, stainless steel reactor) and placed in an oven at 180°C for 48 hours. The resulting product was then washed with deionized water, centrifuged at high speed, and dried at 80°C for 3 hours to obtain KGd₂F₇ polycrystalline powder. The product mass was 9.530 g, and the yield was 97% based on the amount of Gd added.

[0035] Crystal structure testing of KGd2F7 The polycrystalline powder obtained in Example 1 was tested using a powder X-ray diffractometer (model: Bruker D8 ADVANCE) (Cu target, K). source, = 1.5404 Å). (Appendix) Figure 1 The XRD pattern of KGd2F7 indicates that it has a cubic crystal system and a space group of 1. Cell parameters a = 5.750(1) Å. Figure 2 The face-centered cubic lattice diagram of KGd2F7 (showing only cations) shows K + and Gd 3+ Cations co-occupy 4 a Wyckoff sites form a face-centered cubic lattice, and K + With Gd 3+ The molar ratio is 1:2.

[0036] Isothermal magnetization curve measurement Weigh 18.1 mg of KGd2F7 powder sample using a precision balance and place it into a clean capsule. To prevent the powder from moving during measurement, secure it with a small amount of non-magnetic tape, then attach the capsule to the end of the sample rod of the superconducting quantum interference device (SQUID). Test the powder at 2K, 4K, 6K, and 8K temperatures on the system. Isothermal magnetization curves within a magnetic field range of 2T to 2T, the test results are as follows: Figure 3 As shown.

[0037] According to Maxwell's relations The magnetic entropy change of the sample was calculated. The absolute value of the magnetic entropy change of the KGd₂F₇ material in the 2-4 K temperature range, under the condition of a magnetic field changing from 0 T to 2 T, is approximately 23 J·mol⁻¹. —1 ·K —1 This indicates that it has good application prospects in the field of magnetic refrigeration.

[0038] Specific heat measurement Specific heat measurement was performed using a Power Physical Measurement System (PPMS) equipped with a dilution cooling module. The specific steps included: pressing the powder sample into tablets using a tablet press, holding the tablets at 400 °C for 2 hours in a box furnace, and fixing the resulting sample (with a bottom surface area less than 3 × 3 mm) onto a puck. The sample amount for specific heat measurements above 2 K was approximately 10 mg, while the sample amount for specific heat measurements below 2 K was approximately 1 mg. The curves showing the change in specific heat as a function of temperature at zero field and magnetic field strength of 2 T are shown in Figure C. P (T), data as follows Figure 4 As shown in (a). The magnetic specific heat value is obtained by subtracting the fitted phonon specific heat data from the measured specific heat data, and then according to... The relationship between magnetic entropy change and temperature was calculated (e.g.) Figure 4 (b) shows that the absolute value of the magnetic entropy change of the KGd2F7 material under the conditions of a temperature of 1 K and a magnetic field change from 0 T to 2 T is 18.6 J·mol (Gd). 3+ ) —1 ·K —1 It is evident that this material can be used for refrigeration below 1 K, and the magnetic field required to excite the magnetic entropy change is relatively small, at 2 T, which is very advantageous for cryogenic refrigeration applications.

[0039] The embodiments of the present invention have been described above by way of example. However, the scope of protection of the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An application of KGd2F7 in the preparation of thermally insulating and demagnetizing refrigeration materials.

2. The application according to claim 1, characterized in that, Application in the preparation of low-temperature insulating and demagnetizing refrigeration materials; preferably, the low temperature is ≤4K, more preferably, the low temperature is ≤1K.

3. The application according to claim 1, characterized in that, The KGd2F7 is a polycrystalline material. Preferably, its crystal structure belongs to the cubic crystal system, and its space group is . Cell parameters a = 5.750(1) Å; Preferably, in the crystal structure of KGd2F7, K + and Gd 3+ Cations co-occupy 4 a Wyckoff sites form a face-centered cubic lattice, and K + With Gd 3+ The molar ratio is 1:2; Preferably, in the crystal structure of KGd2F7, the cations are subjected to F - Ion separation.

4. The application according to claim 1, characterized in that, The absolute value of the magnetic entropy change produced by KGd2F7 under the conditions of a temperature of 1 K and a magnetic field change from 0 T to 2 T is not less than 18 J·mol (Gd 3+ ) —1 ·K —1 .

5. The method for preparing KGd2F7 according to any one of claims 1-4, characterized in that, The method includes the following steps: KGd2F7 was prepared by mixing a gadolinium-containing compound, a potassium-containing compound, hydrochloric acid, and water, and then heating the mixture. Furthermore, in the gadolinium-containing compound or potassium-containing compound, at least one compound contains fluorine.

6. The method according to claim 5, characterized in that, The molar ratio of fluorine in fluorine-containing compounds to gadolinium in gadolinium-containing compounds is 3.5-10:

1.

7. The method according to claim 5, characterized in that, The gadolinium-containing compound is one or more of the following compounds or their hydrates: gadolinium oxide, gadolinium hydroxide, gadolinium carbonate, gadolinium fluoride, gadolinium chloride, gadolinium bromide, gadolinium sulfate, gadolinium nitrate, and gadolinium acetate.

8. The method according to claim 5, characterized in that, The potassium-containing compound is one or more of the following compounds or their hydrates: potassium hydroxide, potassium carbonate, potassium sulfate, potassium nitrate, potassium fluoride, potassium chloride, potassium bromide, and potassium iodide.

9. The method according to claim 5, characterized in that, In the gadolinium-containing compounds and potassium-containing compounds, the molar ratio of potassium to gadolinium is 6-10:

1.

10. The method according to claim 5, characterized in that, The method is carried out in a hydrothermal reactor. Preferably, the heating reaction temperature is 180-220 °C, and the heating reaction time is 16-48 h.

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