Europium-based silicate as well as preparation method and application thereof

Abstract

The invention belongs to the field of magnetic materials, and particularly discloses europium-based silicate as well as a preparation method and application thereof. The general chemical formula of the europium-based silicate is Eu2SiO4, and the europium-based silicate is subjected to secondary magnetic phase change from a paramagnetic state to a ferromagnetic state at the magnetic phase change temperature of 5.8 K; and at the temperature of 5.5 K, when the magnetic field change is 0-2T, the maximum magnetic entropy change is greater than 10J. Kg <-1 >. K <-1 >. The europium-based silicate disclosed by the invention generates paramagnetic to ferromagnetic phase change near the liquid helium temperature, is accompanied by a low-magnetic-field giant magnetothermal effect near the magnetic phase change temperature, shows large magnetic entropy change and refrigerating capacity near the liquid helium temperature, and has a wide application prospect in the field of magnetic refrigeration in a liquid helium temperature region.

Description

Technical Field

[0001] The invention belongs to the field of magnetic materials, and in particular relates to europium-based silicate and a preparation method and application thereof. Background Art

[0002] The application of cryogenic refrigeration technology has penetrated into many fields such as human life, scientific research, data center thermal management, biomedicine, etc. Liquid helium temperature zone refrigeration is an indispensable and important part of cryogenic refrigeration, and plays an important technical support role in the fields of low-temperature physics, low-temperature superconductivity, space exploration, aerospace, nuclear magnetic resonance, magnetic levitation, military industry, etc. Magnetic refrigeration is a new type of solid-state refrigeration technology that uses the magnetocaloric effect (MCE) of magnetic materials to achieve refrigeration. The magnetocaloric effect is an intrinsic property of the magnetic material itself, which means that when the external magnetic field changes, the magnetic entropy of the magnetic material changes, causing the material itself to absorb or release heat from the outside world, thereby achieving a refrigeration effect. Magnetic refrigeration technology has many advantages such as high efficiency and energy saving, green environmental protection, miniaturization, stability and reliability, and is expected to become a new generation of refrigeration technology to replace traditional gas compression refrigeration.

[0003] High-performance magnetocaloric materials play a key role in magnetic refrigeration technology. In particular, refrigerants with a large magnetocaloric effect (MCE) under low external magnetic fields (≤2T) are very desirable because they can simplify the design by using permanent magnets and greatly reduce the cost of magnetic refrigerators. At present, magnetic refrigeration materials mainly include intermetallic compounds and oxides. Among them, rare earth-based oxides have large magnetic moments and low phase transition temperatures, and are very promising low-temperature magnetic refrigeration materials. Moreover, the preparation process of rare earth-based oxides is simple, the cycle is short, and the stability is good. It has gradually become a new research hotspot in the field of magnetic refrigeration materials in the liquid helium temperature zone. However, the magnetocaloric effect of rare earth metal oxide materials is still relatively small compared with alloy materials. Therefore, it has always been a challenge to develop rare earth metal oxide materials with large magnetocaloric effect under low magnetic fields that can be obtained by permanent magnets.

[0004] A variety of magnetic refrigeration materials are disclosed in the prior art. For example, CN111403137A discloses a rare earth oxide magnetic refrigeration material and a preparation method thereof. The material chemical formula is RE2ZnMnO6 (RE = any one or any several elements among Gd, Tb, Dy, Ho, Er and Tm). It is prepared by sol-gel method. The phase transition temperature is 2.2-6.8K. Under the change of magnetic field of 0-7T, the isothermal magnetic entropy change is only between 11.5 and 25.3J·kg -1 ·K -1 CN105836755A discloses a gadolinium borate material and its preparation method and application, which relates to gadolinium borate. At a temperature of 3K, when the magnetic field changes from 0 to 7T, the magnetic entropy change reaches a maximum value of 47.3 J·kg -1 ·K-1 , but its maximum magnetic entropy change under 0-2T magnetic field change is less than 10J·kg -1 ·K -1 CN117448963A discloses a rare earth alkali metal silicate polycrystalline compound and its synthesis method. The material chemical formula is LiR9(SiO4)6O2 (R = Gd, Tb, Dy, Ho or Er). At 2.6K, when the magnetic field changes from 0 to 7T, the maximum magnetic entropy change reaches 57.6J·kg -1 ·K -1 However, its maximum magnetic entropy change under 0-1T magnetic field change is only about 7.0 J·kg -1 ·K -1 , the maximum magnetic entropy change is also less than 10 J·kg -1 ·K -1 .

[0005] Therefore, developing a magnetocaloric material with low raw material price and excellent magnetocaloric performance under low magnetic field is not only based on energy saving considerations, but also contains very high economic and social benefits. It has great scientific value and strategic significance for the early terminal application of magnetic refrigeration technology, supporting low-temperature scientific research and low-temperature applications in the field of nuclear magnetic resonance. Summary of the invention

[0006] In view of the problem that the magnetic refrigeration materials involved in the above-mentioned prior art have poor magnetocaloric performance under low magnetic field drive, the present invention will provide a europium-based silicate and a preparation method and application thereof.

[0007] To achieve the above purpose, the following technical solutions are specifically included:

[0008] On the one hand, the present invention provides a europium-based silicate, whose chemical formula is Eu2SiO4, which undergoes a secondary magnetic phase transition from a paramagnetic state to a ferromagnetic state at a magnetic phase transition temperature of 5.8K; at a temperature of 5.5K, when the magnetic field changes from 0 to 2T, its maximum magnetic entropy change is greater than 10J·kg -1 ·K -1 .

[0009] The europium-based silicate of the present invention has a low-temperature magnetic phase transition temperature, and a PM-FM secondary magnetic phase transition from a paramagnetic (PM) state to a ferromagnetic state (FM) occurs near the magnetic phase transition temperature, accompanied by a low-magnetic field giant magnetocaloric effect, and can exhibit a large magnetic entropy change and refrigeration capacity near the temperature of liquid helium.

[0010] The present invention can use Maxwell's relation to calculate the magnetic entropy change under different magnetic field strength changes according to the isothermal magnetization curve of europium silicate, and can obtain the maximum magnetic entropy change under the change of the magnetic field strength. For example, at a temperature of 5.5K, the isothermal magnetization curve of europium silicate is tested, and the magnetic entropy change when the magnetic field is 0-2T is calculated using Maxwell's relation, and the value of the maximum magnetic entropy change when the magnetic field is 0-2T can be obtained.

[0011] In some embodiments, at a temperature of 5.5 K, when the magnetic field changes from 0 to 0.5 T, the maximum magnetic entropy change of the europium-based silicate is greater than 10 J·kg -1 ·K -1 , preferably 10-13 J·kg -1 ·K -1 , more preferably 12.2 J·kg -1 ·K -1 .

[0012] In some embodiments, at a temperature of 5.5K, when the magnetic field changes from 0 to 1T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 20 J·kg -1 ·K -1 , preferably 20-22 J·kg -1 ·K -1 , and more preferably 21.6 J·kg -1 ·K -1 .

[0013] In some embodiments, at a temperature of 5.5 K, when the magnetic field changes from 0 to 1.5 T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 25 J·kg -1 ·K -1 , preferably 27-29 J·kg -1 ·K -1 , and more preferably 28.4 J·kg -1 ·K -1 .

[0014] In some embodiments, at a temperature of 5.5K, when the magnetic field changes from 0 to 2T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 30 J·kg -1 ·K -1 , preferably 30-35 J·kg -1 ·K -1 , and more preferably 33.7 J·kg -1 ·K -1 .

[0015] When the external magnetic field increases from 0 to 0.5T or 1T or 1.5T or 2T, the greater the magnetic entropy change of the europium-based silicate, the better the magnetic refrigeration effect. And the smaller the change of the external magnetic field, the smaller the weight and volume of the required magnet group, which is more conducive to the application. The europium-based silicate of the present invention exhibits a large magnetic entropy change near the temperature of liquid helium, especially when the magnetic field changes from 0 to 0.5T, the maximum magnetic entropy change of the europium-based silicate is ≥10J·kg -1 ·K -1 , has a large low-magnetic field magnetocaloric effect and is more suitable for application in magnetic refrigeration.

[0016] In some embodiments, the europium-based silicate comprises a monoclinic system, a space group of P21 / c, and a lattice constant of:

[0017] In some embodiments, the europium-based silicate comprises an orthorhombic crystal system, a space group of Pnma, and a lattice constant of:

[0018] The europium-based silicate of the present invention has two types of crystal phases: monoclinic system and orthorhombic system.

[0019] In some embodiments, at 5.5K, the thermal conductivity of the europium-based silicate is 1-3 W·K -1 ·m -1 ; At 4.2K, the thermal conductivity of the europium-based silicate is 0.5-2W·K -1 ·m -1 .

[0020] In some embodiments, at 5.5K, the thermal conductivity of the europium-based silicate is 2-3 W·K -1 ·m -1 ; At 4.2K, the thermal conductivity of the europium-based silicate is 1-2W·K -1 ·m -1 .

[0021] The europium-based silicate of the present invention not only exhibits a large magnetic entropy change near the temperature of liquid helium, but also has excellent thermal conductivity and can be applied to the field of magnetic refrigeration technology in the liquid helium temperature range, which helps to promote the practical development of green, energy-saving and environmentally friendly magnetic refrigeration technology, and has great scientific value and strategic significance for the construction of scientific research platforms in the liquid helium temperature range and low-temperature applications in the field of nuclear magnetic resonance.

[0022] In another aspect, the present invention provides a method for preparing the europium-based silicate, comprising the following steps:

[0023] S1: Eu2O3, SiO2, a solid non-metallic reducing agent and a solvent are sequentially mixed, ball-milled and dried to obtain a premix;

[0024] S2: sintering the premix in an inert gas atmosphere and / or a reducing gas atmosphere to obtain the europium-based silicate, wherein the sintering temperature is greater than 1350° C. and the sintering time is greater than 8 hours.

[0025] In the preparation method of europium-based silicate of the present invention, each raw material is subjected to solid phase reaction by sintering to obtain europium-based silicate. The preparation method of the present invention is simple, does not require the use of acid or alkali and other substances that seriously pollute the environment, has significant green environmental protection advantages, has low energy consumption, and is suitable for large-scale industrial production.

[0026] In some embodiments, in step S1, the solid non-metallic reducing agent includes at least one of silicon powder, coke, graphite, and graphene.

[0027] In some embodiments, in step S1, the solvent includes ethanol.

[0028] In some embodiments, the ratio of the amounts of Eu2O3, SiO2 and the solid non-metallic reducing agent substance is (1-3):1:(1-3).

[0029] In some embodiments, in step S2, the sintering temperature is 1400-1600° C., and the sintering time is 10-18 hours.

[0030] Preferably, the sintering temperature can be 1400°C, 1450°C, 1500°C, 1550°C, 1600°C, etc., and the sintering time can be 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, etc., as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0031] In some embodiments, in step S2, the sintering further includes heat treatment and quenching, the heat treatment temperature is 1025-1050° C., the heat treatment time is 6-9 days, and the quenching medium is liquid nitrogen or an ice-water mixture.

[0032] Preferably, the temperature of the heat treatment can be 1025°C, 1030°C, 1035°C, 1040°C, 1045°C, 1050°C, etc., and the sintering time can be 6 days, 7 days, 8 days, 9 days, etc., as well as specific point values ​​between the above-mentioned point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0033] After quenching and heat treatment, the high temperature state of europium silicate can be maintained, which is generally called high temperature phase europium silicate. On the contrary, europium silicate that has not undergone quenching and heat treatment maintains the state at room temperature because the cooling process is conventional natural cooling, which is generally called room temperature phase europium silicate. Room temperature phase and high temperature phase europium silicate can be mixed and used, and both can obtain higher magnetocaloric properties, which is beneficial to practical applications.

[0034] In some embodiments, in step S2, the reducing gas includes hydrogen.

[0035] In some embodiments, in step S2, when the sintering atmosphere is a mixed atmosphere of an inert gas and a reducing gas, the mixed atmosphere may be a mixed atmosphere containing 10 vol% H2 and 90 vol% Ar, or a mixed atmosphere containing 5 vol% H2 and 95 vol% Ar.

[0036] In addition, the present invention also provides an application of the europium silicate in the preparation of a magnetic refrigeration device. The europium silicate of the present invention has an obvious magnetocaloric effect at low temperatures, and is very suitable for the preparation of magnetic refrigeration devices. The magnetic refrigeration device made of europium silicate is particularly suitable for refrigeration in the liquid helium temperature zone, and is suitable for the fields of low-temperature physics, low-temperature superconductivity, magnetic resonance medical imaging, and magnetic levitation rail transportation.

[0037] In some embodiments, when the europium silicate is used in the preparation of a magnetic refrigeration device, the europium silicate includes room temperature phase europium silicate and high temperature phase europium silicate.

[0038] Compared with the prior art, the present invention has the following beneficial effects: the europium-based silicate of the present invention has a low-temperature magnetic phase transition temperature, and a PM-FM secondary magnetic phase transition from a paramagnetic (PM) state to a ferromagnetic state (FM) occurs near the magnetic phase transition temperature, accompanied by a low-magnetic field giant magnetocaloric effect, and exhibits a large magnetic entropy change and refrigeration capacity near the liquid helium temperature, which is very suitable for refrigeration in the liquid helium temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a diagram of the crystal structure of Eu2SiO4 of the present invention, wherein illustrations a and b represent the crystal structures of monoclinic and orthorhombic Eu2SiO4, respectively.

[0040] Figure 2 It is a comparison chart of the X-ray diffraction spectrum of Eu2SiO4 prepared in Example 1, Example 2, Example 4, Example 6 and Comparative Example 1 of the present invention and the theoretical spectrum.

[0041] Figure 3 It is a comparison chart of the X-ray diffraction spectrum of Eu2SiO4 prepared in Example 3, Example 5, Example 7 and Comparative Example 2 of the present invention and the theoretical spectrum.

[0042] Figure 4 The zero-field cooling (ZFC) and field cooling (FC) thermomagnetic curves of Eu2SiO4 prepared in Example 4 of the present invention under a 0.01T magnetic field.

[0043] Figure 5 This is the isothermal magnetization curve of Eu2SiO4 prepared in Example 4 of the present invention at 2-30K under a 0-2T magnetic field change.

[0044] Figure 6 This is a relationship diagram between the magnetic entropy change and temperature of Eu2SiO4 prepared in Example 4 of the present invention under different magnetic field changes.

[0045] Figure 7 The thermal conductivity comparison curves of the room temperature phase Eu2SiO4 prepared in Example 4 of the present invention, the high temperature phase Eu2SiO4 prepared in Example 8 and the HoCu2 material under the temperature change of 5-10K. DETAILED DESCRIPTION

[0046] The technical solution of the present invention is further described below in conjunction with the embodiments and the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. The methods or operations used in the embodiments, unless otherwise specified, are conventional methods or conventional operations in the art.

[0047] Unless otherwise specified, the experimental methods used in the examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, can be obtained from commercial channels.

[0048] Example 1

[0049] A method for preparing europium-based silicate comprises the following steps:

[0050] S1. Preparation of mixed material: According to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix; wherein, in order to better achieve the mixing of the raw materials, the amount of anhydrous ethanol added can be determined based on the general viscosity of the raw materials after mixing, the ball milling speed is 100-1000rpm, and the ball milling time is 10-60min. The following embodiments and comparative examples have similar situations;

[0051] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tube furnace, and sintered at 1600° C. in an argon atmosphere for 15 hours to obtain Eu2SiO4 at room temperature after cooling.

[0052] Example 2

[0053] A method for preparing europium-based silicate comprises the following steps:

[0054] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0055] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 15 hours at 1600° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0056] Example 3

[0057] A method for preparing europium-based silicate comprises the following steps:

[0058] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and coke being 1:1:2, weigh corresponding amounts of Eu2O3, SiO2 and coke, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0059] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered at 1600° C. in an argon atmosphere for 15 hours, and room temperature phase Eu2SiO4 is obtained after cooling.

[0060] Example 4

[0061] A method for preparing europium-based silicate comprises the following steps:

[0062] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0063] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 15 hours at 1500° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0064] Example 5

[0065] A method for preparing europium-based silicate comprises the following steps:

[0066] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and graphene powder of 1:1:2, weigh corresponding amounts of Eu2O3, SiO2 and graphene powder, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry to obtain a premix;

[0067] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 15 hours at 1400° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0068] Example 6

[0069] A method for preparing europium-based silicate comprises the following steps:

[0070] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0071] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 13 hours at 1400° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0072] Example 7

[0073] A method for preparing europium-based silicate comprises the following steps:

[0074] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and graphite being 1:1:2, weigh corresponding amounts of Eu2O3, SiO2 and graphite, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry and obtain a premix;

[0075] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 12 hours at 1400° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0076] Example 8

[0077] A method for preparing europium-based silicate comprises the following steps:

[0078] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0079] S2, reduction sintering: the premixed material in step S1 is placed in a graphite crucible, and placed in a tube furnace, and sintered at 1600° C. in an argon atmosphere for 15 h, and after cooling, room temperature phase Eu2SiO4 is obtained;

[0080] S3. Insulation heat treatment and quenching: The room temperature phase Eu2SiO4 is sealed in a quartz tube, placed in a muffle furnace (air), heated from room temperature to 1050°C and kept for 8 days, then quenched with liquid nitrogen as the medium to obtain the high temperature phase Eu2SiO4.

[0081] Comparative Example 1

[0082] The only difference between this comparative example and Example 6 is that in step S2, the temperature of reduction sintering is 1300° C., and specifically includes the following steps:

[0083] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and Si being 2:1:1, weigh corresponding amounts of Eu2O3, SiO2 and Si, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry, and obtain a premix;

[0084] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 13 hours at 1300° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0085] Comparative Example 2

[0086] Compared with Example 7, the only difference between this comparative example and Example 7 is that in step S2, the reduction sintering time is 6 hours, and specifically includes the following steps:

[0087] S1. Preparation of mixed material: according to the molar ratio of Eu2O3, SiO2 and graphite being 1:1:2, weigh corresponding amounts of Eu2O3, SiO2 and graphite, add anhydrous ethanol, and then fully ball mill with a planetary ball mill, dry and obtain a premix;

[0088] S2. Reduction sintering: The premixed material in step S1 is placed in a graphite crucible, and placed in a tubular furnace, and sintered for 6 hours at 1400° C. in an atmosphere of 10 vol% hydrogen and 90 vol% argon, and room temperature phase Eu2SiO4 is obtained after cooling.

[0089] Performance Testing:

[0090] (1) X-ray powder diffraction (XRD): The Eu2SiO4 samples prepared in each embodiment and comparative example were subjected to X-ray powder diffraction phase analysis using a D8A A25 X-ray diffractometer from Brucker Company. The results are shown in the accompanying drawings. Figure 2-3 shown.

[0091] (2) Magnetothermal performance test: The VSM module of Quantum Design comprehensive physical property measurement system (PPMS) was used to test and analyze the magnetothermal performance of Eu2SiO4 samples prepared in Example 4 and Example 8. The magnetic entropy change under different magnetic field changes can be calculated based on the isothermal magnetization curves at different temperatures using Maxwell's relationship. The test and analysis results are shown in the attached figure. Figure 4-6 shown.

[0092] (3) Thermal conductivity test: The Quantum Design Physical Property Measurement System (PPMS) was used to test the change of thermal conductivity of the sample with temperature using the thermal transport analysis (TTO) module. The results are shown in the attached figure. Figure 7 shown.

[0093] From the XRD spectra of Examples 1-7 and Comparative Examples 1-2, it can be seen that the Eu2SiO4 prepared in Examples 1-7 and Comparative Examples 1-2 is in room temperature phase, which belongs to the monoclinic system, has a space group of P21 / c, and a lattice constant of: β=118.131°.

[0094] In Example 8, a high temperature phase Eu2SiO4 was obtained. The high temperature phase europium-based silicate belongs to the orthorhombic system, the space group is Pmn, and the lattice constant is:

[0095] Attached photos Figure 4 The ZFC and FC thermomagnetic curves and dM / dT curves of the Eu2SiO4 sample prepared in Example 4 of the present invention under a 0.01T magnetic field. Figure 4 It can be seen that the sample exhibits obvious ferromagnetic order. The dM / dT transformation curve of the ZFC curve shows that the magnetic phase transition temperature of the sample is 5.8K from the extreme point of dM / dT, and a PM-FM secondary magnetic phase transition from paramagnetic (PM) state to ferromagnetic state (FM) occurs near the magnetic phase transition temperature.

[0096] Attached photos Figure 5 This is the isothermal magnetization curve of the Eu2SiO4 sample prepared in Example 4 of the present invention at 2-30K under a 0-5T magnetic field. Figure 5 It can be seen that the magnetization intensity of the material increases rapidly with the increase of the external magnetic field under a 1T external magnetic field, and tends to saturation when the magnetic field is 5T.

[0097] Attached photos Figure 6The relationship between magnetic entropy change and temperature of the Eu2SiO4 sample prepared in Example 4 of the present invention under different magnetic field changes. Figure 6 It can be seen that the maximum magnetic entropy change of the material -ΔS M Appears at 5.5K. Figure 6 It can be seen that when the magnetic field changes to 0.5T, 1T, 1.5T and 2T, the maximum magnetic entropy change (value) is 12.2J·kg -1 ·K -1 , 21.6 J·kg -1 ·K -1 , 28.4 J·kg -1 ·K -1 and 33.7 J·kg -1 ·K -1 The magnetocaloric effect of Eu2SiO4 in low magnetic field is significantly greater than that of magnetic refrigeration materials in the prior art (conventionally less than 10 J·kg -1 ·K -1 ), is a magnetic refrigeration material with excellent performance in the liquid helium temperature range.

[0098] Table 1

[0099]

[0100] Table 1 summarizes the phase transition temperatures and magnetic entropy changes under different magnetic fields of the room temperature phase Eu2SiO4 prepared in Example 4 and the high temperature phase Eu2SiO4 prepared in Example 8. The phase transition temperatures of Eu2SiO4 prepared in Example 4 and Example 8 are consistent, and the magnetic entropy change values ​​of the high temperature phase Eu2SiO4 prepared in Example 8 under different magnetic fields are very close to the room temperature phase Eu2SiO4 prepared in Example 4, which is very beneficial to the industrial preparation and application of the material. At the same time, the room temperature phase and the high temperature phase Eu2SiO4 can be mixed and applied as a magnetic refrigeration material.

[0101] Figure 7 The thermal conductivity of the room temperature phase Eu2SiO4 prepared in Example 4, the high temperature phase Eu2SiO4 prepared in Example 8 and the commercial GM refrigerator cold storage material HoCu2 is compared. Above 7K, the thermal conductivity of the room temperature phase Eu2SiO4 prepared in Example 4 is significantly higher than that of HoCu2, indicating that it has excellent thermal conductivity in the liquid helium temperature range. Among them, the thermal conductivity of the room temperature phase Eu2SiO4 prepared in Example 4 at 5.5K reaches 2.7W·K -1 ·m -1 ; At liquid helium temperature (4.2K), its thermal conductivity is 1.5W·K -1 ·m -1 .

[0102] From the attached picture Figure 1It can be seen that the Eu2SiO4 prepared in Examples 1 and 8 of the present invention has monoclinic and orthorhombic phases, and has a high magnetocaloric effect; however, the products prepared in Comparative Examples 1-2 have other Eu2SiO4 phases in addition to monoclinic and orthorhombic phases. 3+ Mixed phase, Eu 3+ The impurity phase will significantly reduce the magnetocaloric effect of the material, resulting in a decrease in the magnetic refrigeration capacity of the material. Therefore, when preparing the Eu2SiO4 of the present invention, the sintering temperature needs to be greater than 1350°C and the sintering time needs to be greater than 8 hours to obtain Eu2SiO4 with a higher magnetocaloric effect.

[0103] In summary, the present invention uses europium oxide, silicon oxide and a solid non-metallic reducing agent as raw materials and adopts solid phase reaction sintering to obtain a new type of liquid helium temperature range magnetic refrigeration material. The material has excellent magnetocaloric properties in the liquid helium temperature range and has broad application prospects in the field of low-temperature magnetic refrigeration.

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A europium silicate, characterized in that: Its chemical formula is Eu2SiO4. It undergoes a secondary magnetic phase transition from paramagnetic state to ferromagnetic state at a magnetic phase transition temperature of 5.8K. At a temperature of 5.5K, when the magnetic field changes from 0 to 2T, its maximum magnetic entropy change is greater than 10J·kg -1 ·K -1 .

2. The europium silicate according to claim 1, characterized in that Include at least one of the following: Ⅰ. At 5.5K, when the magnetic field changes from 0 to 0.5T, the maximum magnetic entropy change of the europium-based silicate is greater than 10 J·kg -1 ·K -1 ; II. At a temperature of 5.5K, when the magnetic field changes from 0 to 1T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 20 J·kg -1 ·K -1 ; III. At a temperature of 5.5K, when the magnetic field changes from 0 to 1.5T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 25 J·kg -1 ·K -1 ; IV. At a temperature of 5.5K, when the magnetic field changes from 0 to 2T, the maximum magnetic entropy change of the europium-based silicate is greater than or equal to 30 J·kg -1 ·K -1 .

3. The europium silicate according to claim 1, characterized in that Include at least one of the following: A. The europium-based silicate includes a monoclinic system, a space group of P21 / c, and a lattice constant of: β=118.131°; B. The europium-based silicate includes an orthorhombic system, a space group of Pnma, and a lattice constant of:

4. The europium silicate according to claim 1, characterized in that At 5.5K, the thermal conductivity of the europium-based silicate is 1-3 W·K -1 ·m -1 ; At 4.2K, the thermal conductivity of the europium-based silicate is 0.5-2W·K -1 ·m -1 .

5. A method for preparing the europium-based silicate according to any one of claims 1 to 4, characterized in that: The steps include: S1: Eu2O3, SiO2, a solid non-metallic reducing agent and a solvent are sequentially mixed, ball-milled and dried to obtain a premix; S2: sintering the premix in an inert gas atmosphere and / or a reducing gas atmosphere to obtain the europium-based silicate, wherein the sintering temperature is greater than 1350° C. and the sintering time is greater than 8 hours.

6. The method for preparing europium silicate according to claim 5, characterized in that: In step S2, the sintering temperature is 1400-1600° C., and the sintering time is 10-18 hours.

7. The method for preparing europium silicate according to claim 5, characterized in that: In step S2, after the sintering, heat treatment and quenching are also performed. The temperature of the heat treatment is 1025-1050° C., the time of the heat treatment is 6-9 days, and the quenching medium is liquid nitrogen or an ice-water mixture.

8. The method for preparing europium silicate according to claim 5, characterized in that: In step S1, the solid non-metallic reducing agent includes at least one of silicon powder, coke, graphite, and graphene.

9. The method for preparing europium silicate according to claim 5, characterized in that: In step S1, the solvent includes ethanol; the ratio of the amounts of Eu2O3, SiO2 and the solid non-metallic reducing agent substance is (1-3):1:(1-3).

10. Use of the europium silicate according to any one of claims 1 to 4 in preparing a magnetic refrigeration device.

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