Gadolinium potassium fluoride single crystal as well as preparation method and application thereof

Abstract

The invention relates to the technical field of heat insulation, demagnetization and refrigeration, and discloses a gadolinium potassium fluoride single crystal as well as a preparation method and application thereof. The preparation method of the gadolinium potassium fluoride single crystal comprises the following steps: putting a gadolinium precursor, potassium fluoride and water into a gold lining pipe according to a certain proportion, sealing the gold lining pipe, and putting the gold lining pipe into a kettle body of a high-pressure kettle; adding water into an interlayer between the gold lining pipe and the kettle body, sealing, heating, growing at constant temperature, cooling to room temperature, and washing to obtain the gadolinium potassium fluoride single crystal. The gadolinium potassium fluoride single crystal is prepared by utilizing a high-pressure hydrothermal method, the used raw materials are low in cost, non-toxic and harmless, the gadolinium potassium fluoride single crystal with a relatively large size is successfully prepared by adjusting the proportion of the initial raw materials, and the gadolinium potassium fluoride single crystal has a distorted triangular lattice structure, so that a blocking effect and a magnetothermal effect between magnetic ions are caused, and a temperature interval generated by the magnetothermal effect is relatively low; complex and diverse magnetic ground states are brought, a strong magnetocaloric effect is shown, and the material has great advantages as a potential heat insulation and demagnetization refrigeration material.

Description

Technical Field

[0001] The present invention relates to the technical field of adiabatic demagnetization refrigeration materials, and in particular to a potassium gadolinium fluoride single crystal and a preparation method and application thereof. Background Art

[0002] When a magnetic material is magnetized or demagnetized in a magnetic field, its temperature changes, a phenomenon known as the magnetocaloric effect. This effect can be described as a temperature change caused by a change in magnetic entropy. For example, when a paramagnetic material is placed in an external magnetic field, its magnetic moment aligns with the magnetic field. This process is exothermic, so the temperature of the material rises. On the contrary, when the magnetic field is removed, the magnetic moment is randomly distributed, the material absorbs heat, and the temperature of the system drops. Magnetic refrigeration technology is a refrigeration method based on the magnetocaloric effect. It uses the negative magnetic entropy difference of specific magnetic materials for refrigeration. The development of magnetic refrigeration technology depends on the research and development progress of magnetic refrigeration materials with large magnetocaloric effect. After a long period of work accumulation, the exploration and research of many new magnetic refrigeration materials have greatly promoted the progress of magnetic refrigeration technology.

[0003] At present, the magnetic refrigeration technology in the low-temperature range is still mainly based on dilution refrigerators, but the He3 in dilution refrigerators is expensive and relies on imports. In order to solve this problem, a new solid-state refrigeration technology based on magnetocaloric effect-magnetic refrigeration technology has been proposed. A very critical part of magnetic refrigeration technology is the development of magnetic refrigeration materials. However, among the existing low-temperature magnetic refrigeration materials, some materials only show effective magnetocaloric effect within a very narrow temperature range, and some materials have weak magnetocaloric effect in the low-temperature range required for practical applications. For many of the above problems, it is urgent to explore new material systems and technical strategies.

[0004] Rare earth elements exhibit complex electronic structures and magnetism due to their unfilled 4f electron shells. The 4f electron clouds of these elements are relatively concentrated and shielded by 5s and 5p electrons, which makes them relatively isolated from external influences, forming unique electronic structures and magnetic properties. The total angular quantum number of rare earth elements is large, which is conducive to obtaining a large isothermal magnetic entropy change. For 4f rare earth ions, due to the complex electronic structure and strong spin-orbit coupling of their 4f electron shells, the energy levels will experience significant splitting (i.e., zero-field splitting) under zero magnetic field conditions. In most cases, this zero-field splitting is very large, which means that in the absence of an external magnetic field, the energy difference between energy levels is sufficient to suppress thermally excited transitions at room temperature and near room temperature conditions. Therefore, the magnetocaloric effect of most 4f rare earth ions at room temperature is not obvious, because the reorientation of the magnetic moment requires crossing a large energy barrier, which is thermodynamically unfavorable. Trivalent gadolinium (Gd(III)) is a special case because its 4f shell has seven unpaired electrons, the spin quantum number S = 7 / 2, and the electronic structure of gadolinium is almost isotropic, which means that its magnetic response is similar in all directions. Moreover, because its inner electrons are shielded by the outer electrons, the exchange between Gd-Gd is very weak, and the molar magnetic entropy tends to approach the limit value Rln(8) or 2.08R. This characteristic of gadolinium leads to its relatively small zero-field splitting, making thermally excited transitions possible at room temperature. Therefore, gadolinium-based materials can show obvious changes in magnetic entropy when the magnetic field changes, thereby having a large magnetocaloric effect, which makes gadolinium and its alloys have high practical value in magnetic refrigeration technology.

[0005] When designing Gd(III)-based magnetic refrigeration materials, reducing the influence of antimagnetic components is one of the keys to improving the magnetocaloric effect of the material. The presence of antimagnetic components will weaken the overall magnetism of the material and reduce the isothermal magnetic entropy change. Therefore, reducing the proportion of such components by chemical synthesis can significantly enhance the magnetic refrigeration performance of the material. Selecting small and light bridging ligands, such as carboxyl (-COOH-), hydroxyl (-OH), -F, -CO3, -BO3 and -NO3, is an effective strategy to reduce the influence of antimagnetic components. These small molecule ligands not only help to form a stable coordination structure, but also avoid excessively strong magnetic exchange interactions, thereby maintaining good single-ion magnetism. In addition, small ligands help to build a denser coordination network, reduce the proportion of non-magnetic components, and further improve the magnetic ion density and magnetocaloric effect of the material.

[0006] Recently, frustrated antiferromagnets have attracted wide attention for their applications in magnetic refrigeration at very low temperatures, as they tend to form a highly degenerate magnetic ground state under competitive interactions. However, it remains a multifaceted scientific challenge to find the best balance between the chemical composition, structural design, and magnetic properties of the material and design a new type of high-performance Gd-based magnetic refrigeration material that has both high magnetic entropy change and stable operation within the target temperature range.

[0007] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0008] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a potassium gadolinium fluoride single crystal and a preparation method and application thereof, aiming to solve the problem of how to design and prepare a new type of high-performance Gd-based magnetic refrigeration material that has both high magnetic entropy change and can work stably within the target temperature range.

[0009] The technical solution of the present invention is as follows:

[0010] The first aspect of the present invention is to provide a method for preparing potassium gadolinium fluoride (KGdF4) single crystal, comprising the steps of:

[0011] Putting a gadolinium precursor, potassium fluoride and water into a gold bushing tube, and then sealing the gold bushing tube and placing it in a body of an autoclave;

[0012] Water is added into the interlayer between the gold bushing tube and the autoclave body, the autoclave is sealed and then heated up, and after constant temperature growth and programmed cooling, the autoclave is cooled to room temperature, the product is taken out and washed to obtain the potassium gadolinium fluoride single crystal.

[0013] Optionally, the gadolinium precursor is selected from one or more of gadolinium chloride, gadolinium fluoride, gadolinium nitrate, gadolinium carbonate and gadolinium hydroxide.

[0014] Optionally, the water is deionized water or distilled water.

[0015] Optionally, the molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-100) mmol: (0.5-5) ml.

[0016] Preferably, the molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-20) mmol: (0.5-2) ml.

[0017] Optionally, the temperature to which the temperature is increased is 500-650°C.

[0018] Optionally, the constant temperature growth time is 1 to 3 days.

[0019] Optionally, the programmed cooling is to reduce the temperature to 80-120° C. at a cooling rate of 1-18° C. / h.

[0020] Optionally, the cooling to room temperature is natural cooling from 80-120°C to 10-30°C.

[0021] The second aspect of the present invention is to provide a potassium gadolinium fluoride single crystal, wherein the potassium gadolinium fluoride single crystal belongs to the orthorhombic system, has a space group of Pnma, has a centrosymmetric structure, and has a unit cell parameter of α=β=γ=90°, Z=4, the unit cell volume is

[0022] Alternatively, the potassium gadolinium fluoride single crystal is prepared by the method for preparing the potassium gadolinium fluoride single crystal.

[0023] The third aspect of the present invention is to provide an application of the potassium gadolinium fluoride single crystal in adiabatic demagnetization refrigeration.

[0024] Beneficial effects:

[0025] The present invention utilizes a high-pressure hydrothermal method to prepare potassium gadolinium fluoride single crystals. The raw materials used are low in cost and non-toxic. By adjusting the ratio of the initial raw materials and the mineralizer, potassium gadolinium fluoride single crystals of larger size are successfully obtained. The potassium gadolinium fluoride single crystal belongs to the orthorhombic crystal system, the space group is Pnma, and it is a central symmetric structure. The magnetic ions form a distorted triangular lattice along the c direction. The KGdF4 crystal has a distorted triangular lattice structure, which will lead to a frustration effect between the magnetic ions, so that the temperature range of the magnetocaloric effect of the KGdF4 crystal is lower than that of traditional paramagnetic or antiferromagnetic salts, and it brings about a complex and diverse magnetic ground state, causing the material to exhibit a strong magnetocaloric effect. Therefore, the larger-sized potassium gadolinium fluoride single crystals prepared by the present invention have great advantages as potential adiabatic demagnetization refrigeration materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram and macroscopic photograph of the crystal structure of the KGdF4 single crystal prepared in Example 1 of the present invention.

[0027] Figure 2 This is a curve showing the change of the isothermal magnetic moment (M) of the KGdF4 crystal prepared in Example 1 of the present invention with the magnetic field (B) when the magnetic field is tested along the b direction and c direction of the crystal.

[0028] Figure 3 This is a curve of the change of the negative magnetic entropy difference calculated along the c direction of the crystal in the magnetic field for the KGdF4 crystal prepared in Example 1 of the present invention with temperature under different magnetic field changes. DETAILED DESCRIPTION

[0029] The present invention provides a potassium gadolinium fluoride single crystal and a preparation method and application thereof. To make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0030] Unless otherwise defined, all technical terms and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0031] The embodiment of the present invention provides a method for preparing a potassium gadolinium fluoride single crystal, comprising the steps of:

[0032] Putting gadolinium precursor, potassium fluoride and water into a gold bushing according to a certain ratio, sealing the gold bushing and placing it in a body of an autoclave;

[0033] Water is added into the interlayer between the gold bushing tube and the autoclave body, the autoclave is sealed and then heated up, and after constant temperature growth and programmed cooling, the autoclave is cooled to room temperature, the product is taken out and washed to obtain the potassium gadolinium fluoride single crystal.

[0034] The present invention uses a high-pressure hydrothermal method to prepare potassium gadolinium fluoride single crystals. The raw materials used are low in cost and non-toxic. By adjusting the ratio of the initial raw materials and the mineralizer, a larger-sized potassium gadolinium fluoride single crystal is successfully obtained, which provides a basis for its subsequent application as an adiabatic demagnetization refrigeration material. On the one hand, the magnetic property test based on large-sized potassium gadolinium fluoride single crystals is more reliable than the test based on polycrystalline powder samples; on the other hand, the potassium gadolinium fluoride crystals prepared by the embodiment of the present invention have the advantages of stable physicochemical properties, good mechanical properties, not easy to deliquesce and easy to store. These stable properties bring great advantages to the processing and application of potassium gadolinium fluoride crystals in the field of adiabatic demagnetization refrigeration.

[0035] In some embodiments, the gadolinium precursor is selected from one or more of gadolinium chloride, gadolinium fluoride, gadolinium nitrate, gadolinium carbonate and gadolinium hydroxide, but is not limited thereto.

[0036] In some embodiments, the water is deionized water or distilled water.

[0037] In some embodiments, the molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-100) mmol: (0.5-5) ml.

[0038] Specifically, the following feeding ratios of gadolinium precursor, potassium fluoride and water can be preferably selected: 0.1mmol:10mmol:0.5ml, 0.1mmol:10mmol:1ml, 0.1mmol:10mmol:2ml, 0.1mmol:10mmol:3ml, 0.1mmol:10mmol:4ml, 0.1mmol:10mmol:5ml, 0.3mmol:50mmol:0.5ml, 0.3mmol:50mmol:1ml, 0.3mmol:50mmol: The invention can be one of the following: ol:2ml, 0.3mmol:50mmol:3ml, 0.3mmol:50mmol:4ml, 0.3mmol:50mmol:5ml, 0.5mmol:100mmol:0.5ml, 0.5mmol:100mmol:1ml, 0.5mmol:100mmol:2ml, 0.5mmol:100mmol:3ml, 0.5mmol:100mmol:4ml or 0.5mmol:100mmol:5ml, but is not limited to these.

[0039] The present invention can successfully obtain a larger-sized KGdF4 single crystal by adjusting the ratio of the initial raw material and the mineralizer, thus providing a basis for subsequent application as an adiabatic demagnetization refrigeration material.

[0040] In some preferred embodiments, the molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-20) mmol: (0.5-2) ml.

[0041] In some embodiments, the temperature increased is 500-650°C.

[0042] In some embodiments, the isothermal growth period is 1 to 3 days.

[0043] In some embodiments, the programmed temperature reduction is to reduce the temperature to 80-120° C. at a cooling rate of 1-18° C. / h.

[0044] In some embodiments, the cooling to room temperature is natural cooling from 80-120°C to 10-30°C.

[0045] The embodiment of the present invention provides a potassium gadolinium fluoride single crystal, which belongs to the orthorhombic crystal system, has a space group of Pnma, a centrosymmetric structure, and a unit cell parameter of α=β=γ=90°, Z=4, the unit cell volume is

[0046] The potassium gadolinium fluoride belongs to the orthorhombic system, the space group is Pnma, and it is a centrosymmetric structure. The magnetic ions form a distorted triangular lattice along the c direction. The magnetic ions in the triangular lattice layer are connected by sharing edges, and the distance between the magnetic ions is Because the distance within the triangular lattice layer is relatively close, it may bring about a stronger magnetic exchange interaction. The distorted triangular lattice structure of KGdF4 crystal will lead to the frustration effect between magnetic ions, making the temperature range of the magnetocaloric effect of KGdF4 crystal lower than that of traditional paramagnetic or antiferromagnetic salts, and bringing about complex and diverse magnetic ground states, causing the material to exhibit a strong magnetocaloric effect.

[0047] In some specific embodiments, the potassium gadolinium fluoride single crystal is prepared by the method for preparing the potassium gadolinium fluoride single crystal.

[0048] The embodiment of the present invention also provides an application of the potassium gadolinium fluoride single crystal in adiabatic demagnetization refrigeration.

[0049] The KGdF4 crystal provided by the present invention is a Gd-based fluoride with a new structural system, and has a distorted triangular lattice structure that leads to a frustration effect between magnetic ions, thereby bringing about a complex and diverse magnetic ground state, causing the material to exhibit a strong magnetocaloric effect, and has the potential to be used as an extremely low temperature adiabatic demagnetization refrigeration material. At the same time, the KGdF4 crystal prepared by the present invention has a large size, and the magnetic property test based on a large-size single crystal is more reliable than the test based on a polycrystalline powder sample. In addition, the KGdF4 crystal also has the characteristics of stable physicochemical properties, good mechanical properties, not easy to deliquesce, and easy to preserve, which brings great advantages to its processing and application in the field of adiabatic demagnetization refrigeration.

[0050] The following describes it in detail through specific embodiments.

[0051] Example 1

[0052] Preparation of KGdF4 single crystal

[0053] The specific steps are as follows: using GdF3, KF and H2O as the initial raw materials, 0.1725g of GdF3 and 1.394g of KF raw materials are mixed and placed in a gold bushing tube, 2ml of H2O is added, and the gold bushing tube is sealed. The sealed gold bushing tube is placed in the autoclave body, water is added to the interlayer between the gold bushing tube and the autoclave body, the autoclave is sealed, and then the sealed autoclave is placed in a resistance furnace, the furnace body temperature is set to 600°C, and after 3 days of constant temperature growth, a cooling program is set, and the temperature is slowly reduced to 120°C, and the cooling rate is 15°C / h. Then naturally cool to room temperature, open the autoclave, take out the gold bushing tube, and wash the product with deionized water and alcohol to obtain the KGdF4 single crystal.

[0054] Characterization and testing

[0055] The structure of the obtained KGdF4 single crystal was characterized. The test conditions were: the test instrument was Bruker D8VENTURE single crystal diffractometer, molybdenum target, Kα radiation source (λ=0.07107nm), the test temperature was 300K, and the structure was analyzed by ShelXL software. The relevant crystallographic data can be found in Table 1.

[0056] Table 1

[0057]

[0058]

[0059] The crystal structure of KGdF4 single crystal is as follows Figure 1 As shown in A, the crystal structure is composed of KF7 and GdF9. 3+ -Gd 3+ The FFs are connected by the same edge. + -K + They are connected in a coplanar manner. When the magnetic ion Gd 3+ A distorted triangular lattice is formed.

[0060] Macroscopic photo of KGdF4 single crystal Figure 1 As shown in B, the obtained crystal is a block-like transparent crystal.

[0061] The KGdF4 single crystal was further tested to evaluate its application prospects in the field of adiabatic demagnetization refrigeration.

[0062] Magnetization data measurement: Magnetization data were measured using a magnetic measurement system (MPMS) equipped with the VSM option. The single crystal sample was glued to a quartz rod using GE varnish to obtain anisotropic magnetization data. In a changing magnetic field, the isothermal magnetic moment (M) curves along the single crystal sample B / / b (in-plane) and B / / c (out-of-plane) between 0T and 7T were measured as a function of the magnetic field (B), as shown in Figure 2. Figure 2 shown.

[0063] Figure 2 The magnetic susceptibility of KGdF4 in two field directions is shown, and the magnetic field exhibits nonlinear M versus B behavior both in-plane and out-of-plane, showing very small anisotropy.

[0064] In order to measure the specific size of the magnetocaloric effect, the negative magnetic entropy difference under different magnetic fields is calculated. The larger the negative magnetic entropy difference, the stronger the magnetic cooling effect.

[0065] like Figure 3As shown in Figure 2, it is calculated that when the initial magnetic field is 7T and the final magnetic field is 0T, the maximum negative magnetic entropy difference is 57.4J kg -1 K -1 .

[0066] It can be seen that the KGdF4 single crystal prepared in the embodiment of the present invention has a large magnetocaloric effect, has certain competitiveness in magnetic refrigeration in the milliKelvin range, and has the potential to be used as an extremely low temperature adiabatic demagnetization refrigeration material.

[0067] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A method for preparing potassium gadolinium fluoride single crystal, characterized in that: Includes steps: Putting a gadolinium precursor, potassium fluoride and water into a gold bushing tube, and then sealing the gold bushing tube and placing it in a body of an autoclave; Water is added into the interlayer between the gold bushing tube and the autoclave body, the autoclave is sealed and then heated up, and after constant temperature growth and programmed temperature reduction, the autoclave is cooled to room temperature, the product is taken out and washed to obtain the potassium gadolinium fluoride single crystal.

2. The method for preparing potassium gadolinium fluoride single crystal according to claim 1, characterized in that: The gadolinium precursor is selected from one or more of gadolinium chloride, gadolinium fluoride, gadolinium nitrate, gadolinium carbonate and gadolinium hydroxide; the water is deionized water or distilled water.

3. The method for preparing potassium gadolinium fluoride single crystal according to claim 1, characterized in that: The molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-100) mmol: (0.5-5) ml.

4. The method for preparing potassium gadolinium fluoride single crystal according to claim 3, characterized in that: The molar volume feed ratio of the gadolinium precursor, potassium fluoride and water is (0.1-0.5) mmol: (10-20) mmol: (0.5-2) ml.

5. The method for preparing potassium gadolinium fluoride single crystal according to claim 1, characterized in that: The temperature reached by the heating is 500-650°C.

6. The method for preparing potassium gadolinium fluoride single crystal according to claim 1, characterized in that: The constant temperature growth time is 1 to 3 days.

7. The method for preparing potassium gadolinium fluoride single crystal according to any one of claim 1, characterized in that: The programmed temperature reduction is to reduce the temperature to 80-120° C. at a temperature reduction rate of 1-18° C. / h.

8. The method for preparing potassium gadolinium fluoride single crystal according to any one of claims 1 to 7, characterized in that: The cooling to room temperature is natural cooling from 80-120°C to 10-30°C.

9. A potassium gadolinium fluoride single crystal, characterized in that: The potassium gadolinium fluoride single crystal belongs to the orthorhombic system, the space group is Pnma, and it is a centrosymmetric structure. The unit cell parameters are α=β=γ=90°, Z=4, the unit cell volume is And / or, prepared by the method for preparing potassium gadolinium fluoride single crystal according to any one of claims 1 to 8.

10. Use of the potassium gadolinium fluoride single crystal as claimed in claim 9 in adiabatic demagnetization refrigeration.

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