Rare earth gadolinium-based borate monocrystal and preparation method and magnetic refrigeration application thereof

Abstract

The chemical formula of the rare earth gadolinium-based borate monocrystal is K3Gd3 (BO3) 4, the rare earth gadolinium-based borate monocrystal belongs to a monoclinic system, the space group of the rare earth gadolinium-based borate monocrystal is P21 / c, and the cell parameters of the rare earth gadolinium-based borate monocrystal are alpha = 90 degrees, beta = 110.4705 (34), gamma = 90 degrees and Z = 16. The rare earth gadolinium-based borate single crystal shows excellent magnetic refrigeration performance in a low-temperature area, and compared with an existing commercial material gadolinium gallium garnet (GGG) crystal, the rare earth gadolinium-based borate single crystal has more excellent magnetic entropy change value and refrigeration efficiency and is a magnetic refrigeration material with excellent performance.

Description

Technical fields:

[0001] This invention relates to the field of rare earth materials, specifically to a rare earth gadolinium borate single crystal, its preparation method, and its application in magnetic refrigeration. Background technology:

[0002] Magnetic refrigeration is a refrigeration technology based on the magnetocaloric effect (MCE). It benefits from gadolinium ions (Gd... 3+ With a large spin ground state (S = 7 / 2), the autoparamagnetic salt Gd₂(SO₄)₃·8H₂O has successfully achieved cryogenic cooling at 0.25 K, and magnetic refrigeration has attracted much attention in aerospace exploration and low-temperature physics. Although the paramagnetic properties of these salts enable them to reach cooling temperatures below Kelvin, these hydrated paramagnetic salts are chemically unstable and prone to dehydration, and the crystal water significantly reduces their magnetocaloric effect. Gd₃Ga₅O₅, as a commercially available magnetic refrigeration material... 12 There are significant limitations; the gadolinium ion mass fraction decreases due to the dilution effect of gallium, which restricts the material's ability to generate a large magnetocaloric effect (-ΔS). m =38.4 J / (kg·K) @ T=2K, H=7T). To obtain magnetic refrigeration materials with significant magnetocaloric effects, the unique bonding chemistry of borate compounds is crucial for forming diverse crystal structures. To date, some gadolinium-based borate materials with significant magnetocaloric effects have been successfully obtained, such as Gd3TeBO9 and Gd... 17.33 (BO3)4(B2O5)2O 16 Gd5Si2BO 13 K3Li3Gd7(BO3)9. Currently, most developed magnetic refrigeration materials contain Gd... 3+ -Gd 3+ The interactions remain strong, requiring higher temperatures to achieve an ordered state, thus resulting in a higher magnetic refrigeration temperature range. Therefore, exploring novel gadolinium-based compounds in borate systems is of great significance for overcoming the shortcomings of existing materials in the field of magnetic refrigeration. Summary of the Invention:

[0003] The purpose of this invention is to provide a rare earth gadolinium borate single crystal, its preparation method, and its application in magnetic refrigeration.

[0004] This invention is achieved through the following technical solutions:

[0005] A rare-earth gadolinium borate single crystal, characterized by having the chemical formula K3Gd3(BO3)4, belonging to the monoclinic crystal system, space group P21 / c, and cell parameters as follows: α=90°, β=110.4705(34), γ=90°, Z=16.

[0006] The Gd3(BO3)4 crystal structure contains 12 crystallographically independent Gd atoms, 12 independent K atoms, 16 independent B atoms, and 48 independent O atoms, all occupying crystallographic Wyckoff 4e sites. Of the 12 Gd atoms, 8 are heptagonally coordinated GdO7 and the other 4 are octagonally coordinated GdO8. All 16 B atoms and O atoms form trigonal planar BO3 groups. Its dense three-dimensional structure is mainly composed of GdO7 and GdO8 polyhedra connected by common points, edges, and planes, with the BO3 groups providing auxiliary connections in three dimensions. + Ions fill the gaps in the three-dimensional framework, playing a role in charge balance.

[0007] This invention also protects the preparation method of the above-mentioned rare earth gadolinium borate single crystal, characterized in that it is prepared by spontaneous crystallization, which specifically includes the following steps: uniformly mixing and grinding a potassium-containing compound, a gadolinium-containing compound and a boron-containing compound in a K:Gd:B molar ratio of 6-8:1:8-12, placing it in a metal crucible, heating it to 920-950℃ at a rate of 60-100℃ / h to fully melt it, holding it at a constant temperature for 12-48h, and then slowly cooling it to room temperature at a rate of 5-10℃ / h. After washing with hot water, K3Gd3(BO3)4 crystal is finally obtained.

[0008] Among them, the potassium-containing compound is any one of K2CO3, KHCO3, K2C2O4, KNO3, KOH, K2B2O4·3H2O, and K2B4O7·4H2O; the gadolinium-containing compound is any one of Gd2O3, Gd(NO3)3·6H2O, and Gd(OH)3; and the boron-containing compound is H3BO3, B2O3, K2B4O7·4H2O, or K2B2O4·3H2O.

[0009] The method for preparing rare earth gadolinium borate single crystals can also be carried out by a flux-induced spontaneous crystallization method, wherein the flux is any one of KF, KCl, and KBr. This method specifically includes the following steps:

[0010] A potassium-containing compound, a gadolinium-containing compound, a boron-containing compound, and a flux were uniformly mixed and ground in a K:Gd:B:flux molar ratio of 4-7:1:4-8:1-5. The mixture was placed in a metal crucible and heated to 800-950℃ at a rate of 20-100℃ / h to fully melt it. The mixture was then held at this temperature for 12-48h and then slowly cooled to room temperature at a rate of 1-20℃ / h. After washing with hot water, K3Gd3(BO3)4 crystals were finally obtained. Among them, the potassium-containing compound is any one of K2CO3, KHCO3, K2C2O4, KNO3, KOH, K2B2O4·3H2O, and K2B4O7·4H2O; the gadolinium-containing compound is any one of Gd2O3, Gd(NO3)3·6H2O, and Gd(OH)3; and the boron-containing compound is H3BO3, B2O3, K2B4O7·4H2O, or K2B2O4·3H2O.

[0011] This invention also protects the use of rare-earth gadolinium borate single crystals, which are used as paramagnetic magnetic refrigeration materials for magnetic refrigeration processes in low-temperature environments ranging from 4 to 20 K. At 4 K and a 9 T magnetic field, its magnetic entropy change can reach 45.4 J / kg·K. Compared with existing commercially available gadolinium gallium garnet (GGG) crystals, this crystal exhibits superior magnetic entropy change and refrigeration efficiency.

[0012] The beneficial effects of this invention are as follows: This invention provides a novel type of gadolinium-based borate single crystal with simple composition, stable physicochemical properties, and inexpensive raw materials, as well as its preparation method and applications. The aforementioned rare-earth gadolinium-based borate single crystal exhibits excellent magnetic refrigeration performance in the low-temperature region. Compared with existing commercially available gadolinium gallium garnet (GGG) crystals, it has superior magnetic entropy change and refrigeration efficiency, making it a high-performance magnetic refrigeration material. Attached image description:

[0013] Figure 1 This is the single-crystal diffraction pattern of K3Gd3(BO3)4 prepared in Example 1 of this invention;

[0014] Figure 2 This is a crystal structure diagram of K3Gd3(BO3)4 prepared in Example 1 of this invention;

[0015] Figure 3 This is the powder XRD diffraction pattern of K3Gd3(BO3)4 single crystal after grinding prepared in Example 2;

[0016] Figure 4 This is the temperature-varying magnetic susceptibility curve of the K3Gd3(BO3)4 crystal prepared in Example 3 at 500oe;

[0017] Figure 5The 0-9T isothermal magnetic moment loop at 4-20K for the K3Gd3(BO3)4 crystal prepared in Example 4;

[0018] Figure 6 This is the magnetic entropy change diagram of the K3Gd3(BO3)4 crystal prepared in Example 4. Detailed implementation method:

[0019] The following is a further description of the invention, but not a limitation thereof.

[0020] Example 1: Preparation of K3Gd3(BO3)4 single crystals by spontaneous crystallization

[0021] Raw materials used (analytical grade): 0.16 mol KHCO3, 0.01 mol Gd2O3, and 0.2 mol H3BO3. After accurate weighing, the raw materials were thoroughly mixed and ground, and then transferred to a φ50mm×60mm gold crucible. The crucible was placed in a molten salt furnace, and the temperature was increased to 920℃ at a rate of 60℃ / h to completely melt the raw materials, and held at this temperature for 12 hours. Subsequently, the temperature was slowly lowered to room temperature at a rate of 5℃ / h. After washing with hot water, K3Gd3(BO3)4 single crystals were obtained.

[0022] The K3Gd3(BO3)4 single crystal obtained in Example 1 was subjected to single-crystal X-ray diffraction testing using a Rigaku XtaLAB Synergy single-crystal diffractometer. The results are as follows: Figure 1 As shown, characteristic diffraction points of a single crystal appeared, indicating that the obtained sample is a single crystal. Through indexing, data reconstruction, and crystal structure analysis, it was determined that the K3Gd3(BO3)4 single crystal belongs to the monoclinic crystal system, with space group P21 / c and cell parameters as follows: α=90°, β=110.4705(34), γ=90°, Z=16, and its three-dimensional crystal structure model is shown in Figure 2.

[0023] Example 2: Preparation of K3Gd3(BO3)4 crystals by spontaneous crystallization

[0024] Raw materials used (analytical grade): 0.12 mol K₂B₄O₇·4H₂O, 0.02 mol Gd₂O₃. The raw materials were accurately weighed, thoroughly mixed and ground, and transferred to a φ50mm×70mm platinum crucible. The crucible was placed in a molten salt furnace, and the temperature was increased to 950℃ at a rate of 100℃ / h to completely melt the raw materials, and held at this temperature for 24 h. Subsequently, the temperature was slowly lowered to room temperature at a rate of 10℃ / h. After washing with hot water, K₃Gd₃(BO₃)₄ single crystals were obtained.

[0025] The K3Gd3(BO3)4 single crystal obtained in Example 2 was ground into powder and tested using a Bruker D8 advance powder X-ray diffractometer. The obtained diffraction pattern is shown below. Figure 3 As shown, Rietveld's refined analysis confirms that the K3Gd3(BO3)4 single crystal belongs to the monoclinic crystal system, with space group P21 / c and cell parameters as follows: α=90°, β=110.4705(34), γ=90°, Z=16.

[0026] Example 3: Preparation of K3Gd3(BO3)4 crystals by spontaneous crystallization using KCl flux

[0027] Raw materials used (analytical grade): 0.025 mol K₂CO₃, 0.01 mol Gd(OH)₃, 0.04 mol B₂O₃, and 0.05 mol KCl. After accurate weighing, the raw materials were thoroughly mixed and ground, and transferred to a φ40mm×30mm platinum crucible. The crucible was placed in a molten salt furnace, and the temperature was increased to 900℃ at a rate of 100℃ / h to completely melt the raw materials, and held at this temperature for 24 hours. Subsequently, the temperature was slowly lowered to room temperature at a rate of 1.5℃ / h. After washing with hot water, K₃Gd₃(BO₃)₄ single crystals were obtained.

[0028] The K3Gd3(BO3)4 single crystal obtained in Example 3 was placed in a Quantum Design PPMS-9 integrated property system for magnetic testing. Under an applied magnetic field of 500 Oe and a temperature range of 4K-300K, its molar magnetic susceptibility as a function of temperature is shown in the curve below. Figure 4 As shown, the magnetic susceptibility of K3Gd3(BO3)4 increases monotonically with decreasing temperature. By fitting the χ-T curve to the Curie-Weiss equation, the magnetic susceptibility of Gd3Gd3(BO3)4 is obtained. 3+ The effective magnetic moment of the ion is 8.20 μ. B =7.94μ B The high degree of agreement confirms that Gd in the K3Gd3(BO3)4 crystal is present. 3+ With its free ionic state and negligible magnetic interaction, the reciprocal of its magnetic susceptibility exhibits a good linear relationship with temperature, indicating that K3Gd3(BO3)4 single crystal is a stable rare-earth paramagnetic material.

[0029] Example 4: Preparation of K3Gd3(BO3)4 crystals by spontaneous crystallization using KF flux

[0030] Raw materials used (analytical grade): 0.05 mol KHCO3, 0.005 mol Gd2O3, 0.045 mol H3BO3, and 0.02 mol KF. After accurate weighing, the raw materials were thoroughly mixed and ground, and then transferred to a φ50mm×60mm gold crucible. The crucible was placed in a molten salt furnace, and the temperature was increased to 880℃ at a rate of 20℃ / h to completely melt the raw materials. This temperature was maintained for 48 hours. Subsequently, the temperature was slowly lowered to room temperature at a rate of 3℃ / h. After washing with hot water, K3Gd3(BO3)4 single crystals were obtained.

[0031] Example 5: Study on the magnetic refrigeration performance of K3Gd3(BO3)4 single crystal

[0032] The K3Gd3(BO3)4 crystal obtained in Example 4 was subjected to magnetic refrigeration performance testing using the Quantum Design PPMS-9 integrated property system. The isothermal magnetization of the crystal was measured in a temperature range of 4K-20K and a magnetic field range of 0-9T. Figure 5 As shown, the magnetization increases monotonically with increasing magnetic field and decreasing temperature, exhibiting typical Brillouin function characteristics. It increases rapidly in the low-field region, while the rate of increase slows down and gradually approaches saturation in the high-field region. At 4K and 9T, the saturation magnetization reaches 6.81 μm. Β Approaching its theoretical value of 7μ Β This indicates that Gd in K3Gd3(BO3)4 crystal 3+ It exists in a free ion state and its magnetic interaction is negligible.

[0033] Based on Maxwell's relations, the magnetic entropy change of K3Gd3(BO3)4 crystal was calculated using magnetization data under varying temperature and field conditions. The results are as follows: Figure 6 As shown: Maximum magnetic entropy change: -ΔS at 4K temperature and 7T m The concentration reached 39.56 J / kg·K; it further increased to 45.4 J / kg·K at 4K and 9T. All of these values ​​are higher than those of commercially available Gd3Ga5O. 12 The magnetic entropy change of (GGG) crystal (38.4 J / kg·K, T=2K, H=7T) confirms that K3Gd3(BO3)4 has superior low-temperature magnetic refrigeration performance.

Claims

1. A rare earth gadolinium borate single crystal, characterized in that, Its chemical formula is K3Gd3(BO3)4, it belongs to the monoclinic crystal system, space group P21 / c, and its unit cell parameters are: α=90°, β=110.4705(34), γ=90°, Z=16.

2. A method for preparing a rare earth gadolinium borate single crystal according to claim 1, characterized in that, The K3Gd3(BO3)4 crystal was prepared by spontaneous crystallization. The method specifically includes the following steps: potassium-containing compound, gadolinium-containing compound and boron-containing compound are uniformly mixed and ground in a K:Gd:B molar ratio of 6-8:1:8-12. The mixture is placed in a metal crucible and heated to 920-950℃ at a rate of 60-100℃ / h to fully melt it. The mixture is kept at a constant temperature for 12-48h, and then slowly cooled to room temperature at a rate of 5-10℃ / h. After washing with hot water, K3Gd3(BO3)4 crystal is finally obtained.

3. The preparation method according to claim 2, characterized in that, The potassium-containing compounds are any one of K2CO3, KHCO3, K2C2O4, KNO3, KOH, K2B2O4·3H2O, and K2B4O7·4H2O; the gadolinium-containing compounds are any one of Gd2O3, Gd(NO3)3·6H2O, and Gd(OH)3; and the boron-containing compounds are H3BO3, B2O3, K2B4O7·4H2O, or K2B2O4·3H2O.

4. A method for preparing a rare earth gadolinium borate single crystal according to claim 1, characterized in that, The K3Gd3(BO3)4 crystals are prepared by spontaneous crystallization using a flux, wherein the flux is any one of KF, KCl, and KBr. The specific steps of the method are as follows: potassium-containing compounds, gadolinium-containing compounds, boron-containing compounds, and flux are uniformly mixed and ground in a K:Gd:B:flux molar ratio of 4-7:1:4-8:1-5. The mixture is placed in a metal crucible and heated to 800-950℃ at a rate of 20-100℃ / h to fully melt it. The temperature is maintained for 12-48h, and then slowly cooled to room temperature at a rate of 1-20℃ / h. After washing with hot water, K3Gd3(BO3)4 crystals are finally obtained.

5. The preparation method according to claim 4, characterized in that, The potassium-containing compounds are any one of K2CO3, KHCO3, K2C2O4, KNO3, KOH, K2B2O4·3H2O, and K2B4O7·4H2O; the gadolinium-containing compounds are any one of Gd2O3, Gd(NO3)3·6H2O, and Gd(OH)3; and the boron-containing compounds are H3BO3, B2O3, K2B4O7·4H2O, or K2B2O4·3H2O.

6. The use of the rare earth gadolinium borate single crystal according to claim 1, characterized in that, The rare earth gadolinium borate single crystal is used as a paramagnetic magnetic refrigeration material for magnetic refrigeration processes in low-temperature environments of 4-20K.

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