A pyrochlore-structured ultra-low temperature magnetic refrigeration material and its preparation method and application

Abstract

The present invention provides a pyrochlore-structured, ultra-low-temperature magnetic refrigeration material with the chemical formula Ce2Sn2O7, a cubic crystal system, and the space group Fd-3m. Compared to traditional magnetic refrigeration materials (paramagnetic salt hydrates), this novel pyrochlore-structured, ultra-low-temperature magnetic refrigeration material Ce2Sn2O7 exhibits superior magnetocaloric properties, achieving ultra-low-temperature refrigeration at the 100mK level during adiabatic demagnetization measurements. It also boasts a compact structure, high stability, high magnetic entropy density, and ease of preparation.

Description

Technical Field

[0001] The invention relates to a pyrochlore structured ultra-low temperature magnetic refrigeration material and a preparation method and application thereof. Background Art

[0002] Ultra-low temperature refrigeration technology is widely used in national security and strategic high-tech fields, including large scientific facilities, deep space exploration, materials science, and quantum computing. However, ultra-low temperature refrigeration has always relied on the scarce element helium, particularly given the global helium shortage. Scientists have discovered that adiabatic demagnetization refrigeration does not require helium, making this cooling technology increasingly important in a variety of applications.

[0003] The magnetocaloric effect refers to the phenomenon of controlling the temperature of a magnetic material using an external magnetic field under adiabatic conditions. When the external magnetic field increases, the spin disorder of the magnetic material decreases, resulting in a decrease in magnetic entropy and a corresponding increase in lattice entropy, ultimately manifesting as an increase in the material's temperature. Conversely, when the magnetic field decreases, the temperature of the magnetic material decreases, achieving a cooling effect.

[0004] Utilizing the magnetocaloric effect, a Carnot cycle with isothermal magnetization and adiabatic demagnetization has been developed to achieve cooling. This technology, unaffected by gravity and independent of helium resources, boasts high cooling efficiency, low vibration, and high reliability, making it widely used in deep space exploration and other fields.

[0005] In adiabatic demagnetization refrigeration technology, commonly used magnetic materials are paramagnetic salts, such as ammonium ferric sulfate (FeNH4(SO4)2·12H2O, FAA), potassium chromium sulfate (KCr(SO4)2·12H2O, CPA) and cerium magnesium nitrate (Ce2Mg3(NO3) 12 24H2O, CMN, etc. These paramagnetic salt materials are characterized by the near-independence of their magnetic ions, resulting in magnetic ordering only at very low temperatures. This property enables paramagnetic salt materials to retain entropy down to very low temperatures, resulting in excellent cryogenic cooling performance.

[0006] In 1933, physicist Giauque experimentally used the hydrated paramagnetic salt material Gd2(SO4)3·8H2O as a refrigerant for the first time, achieving an extremely low temperature of 250mK, for which he was awarded the Nobel Prize.

[0007] However, after in-depth literature research and a large number of experimental explorations, the inventors of the present invention found that existing paramagnetic salts have many limitations when used as refrigerants in practical applications. First, traditional paramagnetic salts are mostly hydrates, and their magnetic ion density is relatively low, which directly limits the improvement of refrigeration power per unit volume. At the same time, their chemical properties are quite unstable and easily subject to deliquescence and decomposition, which undoubtedly increases the difficulty and complexity in practical applications. Secondly, the exchange interaction within the paramagnetic salt is weak, making it difficult to effectively utilize the magnetically excited thermal conductivity, and it can only rely on the lattice heat conduction mechanism, resulting in low refrigeration cold extraction efficiency. Finally, the growth cycle of hydrated paramagnetic salts is long, the processing process is difficult, and it is often accompanied by a certain degree of corrosiveness. These unfavorable factors have greatly restricted its practical application.

[0008] On this basis, the inventor's research team proposed for the first time that the emerging quantum frustrated magnetic material is expected to solve the above problems to a certain extent. This material not only has the characteristics of compact structure and high chemical stability, but also has a large spin density, which provides the possibility of improving refrigeration efficiency. More importantly, the quantum frustration effect allows the material to maintain spin fluctuations even at extremely low temperatures, thereby ensuring significant magnetic entropy. In addition, due to the large exchange interaction inside it and the large number of low-energy magnetic excitations gathered near the quantum phase transition point, these characteristics together give the quantum frustrated magnetic material excellent thermal conductivity. Therefore, this emerging material is expected to solve the limitations of traditional paramagnetic salts in practical applications to a certain extent, and open up new avenues for the development of refrigeration technology. Summary of the Invention

[0009] Therefore, the purpose of the present invention is to provide a quantum frustrated magnetic material with a compact structure, stable chemical properties, high spin density, simple preparation conditions, and the ability to achieve ultra-low temperatures below 100 mK, as an excellent working medium for adiabatic demagnetization refrigeration.

[0010] During the research and development process, the inventors of the present invention discovered that the "two-in, two-out" rule of the tetrahedral dipole moment gives rare earth pyrochlore materials a unique quantum spin ice state; however, the higher multi-level degrees of freedom possessed by f electrons often lead to the formation of "hidden" topological order. Taking Ce2Sn2O7 as an example, its ground state is proposed to be a magnetic octapole quantum spin liquid; related neutron scattering experimental results show that compared with magnetic dipoles, Ce2Sn2O7 exhibits a more complex magnetization density distribution. Therefore, the inventors of the present invention realized that the continuous excitation of its spin and the potential "hidden" topological order will produce a significant magnetocaloric effect, making it promising to become an excellent magnetic refrigeration material.

[0011] In the present invention, the term "extremely low temperature" generally refers to a temperature not higher than 1K.

[0012] A first aspect of the present invention provides a pyrochlore structured ultra-low temperature magnetic refrigeration material, the chemical formula of which is Ce2Sn2O7, belonging to the cubic crystal system, and the space group is Fd-3m.

[0013] According to the ultra-low temperature magnetic refrigeration material provided by the present invention, the lattice constant of the material is

[0014]

[0015] Figure 1 Schematic diagram of the crystal structure of pyrochlore Ce2Sn2O7 provided by the present invention. Specifically, Figure 1 The image above shows the positions of all atoms in the Ce2Sn2O7 lattice, where the green atom with the largest diameter represents the magnetic ion Ce. 3+ ; For simplicity, Figure 1 The figure below shows only the 3+ and O 2+ The tetrahedral structure is formed. Figure 1 It can be seen that the magnetic ion Ce 3+ The spin pointing satisfies the "two in, two out" rule.

[0016] According to the ultra-low temperature magnetic refrigeration material provided by the present invention, the Curie-Weiss temperature of the ultra-low temperature magnetic refrigeration material is -0.124K.

[0017] According to the present invention, the ultra-low temperature magnetic refrigeration material has a magnetization intensity that increases with increasing external magnetic field strength. At a temperature of 1.8K, the magnetization of the ultra-low temperature magnetic refrigeration material reaches saturation at a magnetic field strength of approximately 7T.

[0018] According to the ultra-low temperature magnetic refrigeration material provided by the present invention, wherein, at a temperature of 2.4K, the maximum magnetic entropy change of the ultra-low temperature magnetic refrigeration material under a 0-7T magnetic field change is ≤4.677J / kg·K -1 .

[0019] According to the ultra-low temperature magnetic refrigeration material provided by the present invention, when demagnetized from an 8T magnetic field at an ambient temperature of 1.8K and the magnetic field drops to zero, the temperature of the ultra-low temperature magnetic refrigeration material reaches 91.3mK;

[0020] At an ambient temperature of 1.8K, when demagnetization is performed from a 6T magnetic field and the magnetic field drops to zero, the temperature of the ultra-low temperature magnetic refrigeration material can reach 106mK.

[0021] A second aspect of the present invention provides a method for preparing the above-mentioned pyrochlore-structured ultra-low temperature magnetic refrigeration material, the preparation method comprising:

[0022] (1) Prepare the raw materials CeO2, SnO2 and Sn in a molar ratio of 4:3:1 and grind them thoroughly;

[0023] (2) The ground raw materials were pressed into tablets and vacuum-sealed in a quartz tube to synthesize Ce2Sn2O7 with a pyrochlore structure using a solid-phase reaction method.

[0024] According to the preparation method provided by the present invention, the solid phase reaction method comprises: reacting the reaction raw materials at 920-980°C for 10-48 hours, taking them out and fully grinding them, and then sintering them at 950-1050°C for 10-48 hours.

[0025] In some preferred embodiments of the present invention, the solid phase reaction method comprises: reacting the reaction raw materials at 940-960° C. for 30-40 hours, taking them out and grinding them thoroughly, and then sintering them at 980-1020° C. for 10-48 hours.

[0026] The third aspect of the present invention provides the use of the ultra-low temperature magnetic refrigeration material described in the first aspect of the present invention or the ultra-low temperature magnetic refrigeration material prepared according to the preparation method of the second aspect of the present invention as an adiabatic demagnetization refrigeration medium.

[0027] The new pyrochlore-structured ultra-low temperature magnetic refrigeration material Ce2Sn2O7 provided by the present invention has good magnetocaloric properties compared to traditional magnetic refrigeration materials (paramagnetic salt hydrates). It can achieve ultra-low temperature refrigeration of 100mK level during the adiabatic demagnetization measurement process. At the same time, it has the advantages of compact structure, high stability, high magnetic entropy density and easy preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which:

[0029] Figure 1 Schematic diagram of the crystal structure of Ce2Sn2O7 with pyrochlore structure of the present invention;

[0030] Figure 2 This is a comparison chart of the X-ray diffraction pattern of the Ce2Sn2O7 material prepared in Example 1 of the present invention and a standard card;

[0031] Figure 3 The magnetic susceptibility temperature curve and the magnetic susceptibility inverse curve of the Ce2Sn2O7 material prepared in Example 1;

[0032] Figure 4 Isothermal magnetization curves of the Ce2Sn2O7 material prepared in Example 1 at different temperatures of 1.8 to 10 K;

[0033] Figure 5 The isothermal magnetic entropy change curve of the Ce2Sn2O7 material prepared in Example 1 during the isothermal magnetization process is as a function of the magnetic field;

[0034] Figure 6 Figure 2 shows the temperature change curves of the Ce2Sn2O7 material of the present invention when demagnetized at an ambient temperature of 1.8K starting from initial magnetic fields of 2T, 4T, 6T, and 8T, respectively; and the temperature change curve of the spin ice material Er2Ti2O7 as a control after demagnetization at an initial magnetic field of 4T at 2K;

[0035] Figure 7 1 is a curve showing the temperature change over time of the Ce2Sn2O7 material of the present invention during the field reduction process and after the magnetic field drops to zero. DETAILED DESCRIPTION

[0036] The present invention will be further described in detail below in conjunction with specific embodiments. The examples given are only for illustrating the present invention, not for limiting the scope of the present invention.

[0037] Example 1

[0038] Preparation of pyrochlore-structured Ce2Sn2O7

[0039] (1) CeO2, SnO2, and Sn (all with a purity of 99.99%) were used as raw materials, and the raw materials were prepared according to the stoichiometric molar ratio of 4:3:1. The raw materials were mixed evenly and ground thoroughly;

[0040] (2) The tablets were pressed using a tablet press to allow for a more complete reaction, and then vacuum-sealed in a quartz tube (to prevent Sn from reacting with O2 in the air). The tablets were reacted at 950°C for 35 hours, removed and fully ground, and then sintered at 1000°C for 35 hours. Pure phase powder was synthesized using a solid-phase reaction method.

[0041] Figure 2 This is a comparison chart of the X-ray diffraction pattern of the Ce2Sn2O7 material prepared in this example and the standard card.

[0042] Magnetic property characterization

[0043] The magnetic properties of Ce2Sn2O7 material prepared in Example 1 were characterized by using Quantum Design MPMS test system. The magnetic susceptibility and magnetization intensity data are shown in Figure 2. Figure 3-5 shown.

[0044] Figure 3 The magnetic susceptibility temperature curve and the magnetic susceptibility inverse curve of the Ce2Sn2O7 material obtained in Example 1 are shown. Figure 3 Figure a) shows the magnetic susceptibility temperature curve and the inverse magnetic susceptibility curve from 1.8 to 15 K, as well as the Curie-Weiss fitting curve from 1.8 to 6 K; Figure 3Figure b) shows the magnetic susceptibility temperature curve and the inverse magnetic susceptibility curve from 1.8 to 300 K, where the fluctuations in the box are caused by crystal field splitting (the inverse magnetic susceptibility tends to a constant).

[0045] Figure 3 The results show that Ce2Sn2O7 satisfies the Curie-Weiss law at very low temperatures, and the Curie-Weiss temperature is fitted to be -0.124K. 3+ The ions are affected by the crystal field, and the ground state is a doublet state. At very low temperatures, it shows an effective spin of -1 / 2 and has a weak antiferromagnetic interaction. As the temperature rises, the crystal field splits the first excited doublet state and begins to contribute to magnetism, which is manifested as Figure 3 b) Fluctuations within the box in the figure.

[0046] The magnetization intensity-magnetic field change data of the Ce2Sn2O7 material prepared in Example 1 was tested. Figure 4 Isothermal magnetization curves of the Ce2Sn2O7 material prepared in Example 1 at different temperatures of 1.8 to 10 K; Figure 5 The calculated curve of isothermal magnetic entropy change versus magnetic field during the isothermal magnetization process.

[0047] Figure 4 The results show that the magnetization intensity of Ce2Sn2O7 increases monotonically with the increase of external magnetic field. At 1.8K, it is close to saturation at 7T magnetic field. Based on this and the Maxwell relationship, the entropy change in the isothermal magnetization process at different temperatures is obtained, as shown in the following figure: Figure 5 As shown in the figure, at a temperature of 2.4 K, the magnetic entropy can change by 2.945 J / (mol·K) under a 7 T magnetic field. These experimental results show that the pyrochlore-structured Ce2Sn2O7 material can achieve refrigeration under magnetic field regulation.

[0048] Magnetic refrigeration performance characterization

[0049] The Ce2Sn2O7 material prepared in Example 1 was mixed with an equal mass of silver powder to enhance the thermal conductivity of the sample. The mixture was then pressed into two cylinders with a diameter of 8 mm and thermometers placed therein. Adiabatic demagnetization tests were performed using a commercial Quantum Design PPMS testing system.

[0050] The temperature change curves of the Ce2Sn2O7 material of the present invention when demagnetized at an ambient temperature of 1.8K starting from an initial magnetic field of 2T, 4T, 6T and 8T respectively; and the temperature change curve of the spin ice material Er2Ti2O7 as a control after demagnetization at an initial magnetic field of 4T at 2K, as shown in FIG. Figure 6 shown.

[0051] Figure 6It shows that when the magnetic field drops to zero, the temperature of the Ce2Sn2O7 material of the present invention reaches 91.3mK and 106mK when the initial magnetic field is 8T and 6T, respectively. Compared with the traditional spin ice material Er2Ti2O7, under the same conditions, when the magnetic field drops to zero, the temperature of Ce2Sn2O7 continues to drop to 75mK and 103mK, and remains near the lowest temperature for a long time. This comparison result shows that Ce2Sn2O7, as a material with the same pyrochlore structure, is a candidate material for quantum spin liquid. It does not show order until extremely low temperatures, has stronger quantum fluctuations, and has a large number of degenerate states in the ground state, thereby carrying entropy. Therefore, during the adiabatic demagnetization process, very low temperatures can be reached.

[0052] Figure 7 The figure shows the temperature variation over time of the Ce2Sn2O7 material of the present invention during the field reduction process and after the magnetic field is reduced to zero. The inset is a magnified view of the temperature change of the sample after the magnetic field is returned to zero. It can be seen that the temperature of the Ce2Sn2O7 material of the present invention continues to decrease after the magnetic field is reduced to zero, and remains near the minimum temperature for a long time.

[0053] From the above performance characterization, it can be seen that the pyrochlore-structured Ce2Sn2O7 material has good magnetocaloric properties and can achieve ultra-low temperature refrigeration of 100mK during the adiabatic demagnetization measurement process. It also has the advantages of compact structure, high stability, high magnetic entropy density and simple preparation.

[0054] The above embodiments are preferred implementation modes of the present invention, but the present invention is not limited to the above embodiments. Any other modifications, equivalent replacements and improvements that do not deviate from the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A pyrochlore-structured, ultra-low temperature magnetic refrigeration material having the chemical formula Ce2Sn2O7, a cubic crystal system, and a space group of Fd-3m, wherein: At 2.4 K, the maximum magnetic entropy change of the ultra-low temperature magnetic refrigeration material under a 0-7T magnetic field change is ≤4.677J / kg·K -1 At an ambient temperature of 1.8 K, when demagnetized from an 8 T magnetic field and the magnetic field drops to zero, the temperature of the ultra-low temperature magnetic refrigeration material reaches 91.3 mK; at an ambient temperature of 1.8 K, when demagnetized from a 6 T magnetic field and the magnetic field drops to zero, the temperature of the ultra-low temperature magnetic refrigeration material reaches 106 mK. The preparation method of the ultra-low temperature magnetic refrigeration material includes: (1) Prepare the raw materials CeO2, SnO2 and Sn in a molar ratio of 4:3:1 and grind them thoroughly; (2) The ground raw materials are pressed into tablets and vacuum-sealed in a quartz tube to synthesize Ce2Sn2O7 with pyrochlore structure by solid-phase reaction method. The solid phase reaction method includes: reacting the reaction raw materials at 920-980°C for 10-48 hours, taking them out and fully grinding them, and then sintering them at 950-1050°C for 10-48 hours.

2. The ultra-low temperature magnetic refrigeration material according to claim 1, wherein: The lattice constant of the material is a = b = c = 10.65(2) Å.

3. The ultra-low temperature magnetic refrigeration material according to claim 1, wherein: The Curie-Weiss temperature of ultra-low temperature magnetic refrigeration materials is -0.124 K.

4. The ultra-low temperature magnetic refrigeration material according to claim 1, wherein: The magnetization intensity of the ultra-low temperature magnetic refrigeration material increases with the increase of the external magnetic field intensity; at a temperature of 1.8 K, the magnetization of the ultra-low temperature magnetic refrigeration material reaches saturation at a magnetic field intensity of 7 T.

5. The ultra-low temperature magnetic refrigeration material according to claim 1, wherein: The solid phase reaction method includes: reacting the reaction raw materials at 940-960° C. for 30-40 hours, taking them out and fully grinding them, and then sintering them at 980-1020° C. for 10-48 hours.

6. Use of the ultra-low temperature magnetic refrigeration material according to any one of claims 1 to 5 as an adiabatic demagnetization refrigeration medium.

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