Application of Ca9Gd(VO4)7 in the field of ultra-low temperature magnetic refrigeration

Abstract

The application discloses application of Ca9Gd(VO4)7 in the field of magnetic refrigeration at extremely low temperature, and relates to the technical field of magnetic refrigeration materials. The extremely low temperature magnetic refrigeration material Ca9Gd(VO4)7 provided by the application can exhibit excellent magnetic heat performance and stability under the condition of extremely low temperature (≤200 mK). The Gd 3+ The nearest neighbor distance of the ions is large, the interaction between the magnetic ions is extremely weak, no magnetic order phase transition appears in the temperature range as low as 50 mK, so that the magnetic entropy can be kept large in the extremely low temperature range, and excellent extremely low temperature refrigeration performance is exhibited under different initial temperatures and magnetic field conditions; the material does not contain crystal water, the structural stability and thermal stability are high, and the Gd 3+ The ions are almost not affected by the crystal field, the material is nearly isotropic in magnetism, the single crystal does not need to be strictly oriented, and the material is suitable for single-stage or multi-stage adiabatic demagnetization refrigeration systems, and provides a high-efficiency and stable magnetic refrigeration material for the extremely low temperature field.

Description

Technical Field

[0001] This invention relates to the field of magnetic refrigeration materials technology, and in particular to the application of Ca9Gd(VO4)7 in the field of ultra-low temperature magnetic refrigeration. Background Technology

[0002] In high-end technology fields such as quantum computing, condensed matter physics research, deep space exploration, and precision detection instruments, ultra-low temperature environments play a crucial role. Many key devices and experimental processes must operate stably under ultra-low temperature conditions. Therefore, how to obtain and maintain a reliable ultra-low temperature environment over a long period has become one of the core issues restricting the development of related technologies. Currently, the main technical routes for obtaining ultra-low temperature environments below 1 K include dilution refrigeration and adiabatic demagnetization refrigeration. While dilution refrigeration can achieve continuous cooling and reach extremely low operating temperatures, its system structure is complex, it is highly dependent on scarce resources such as helium-3, has high operating and maintenance costs, and its use is limited in space applications or special environments. In contrast, adiabatic demagnetization refrigeration has a relatively simple structure, reliable operation, less dependence on external mechanical systems, and does not require the consumption of large amounts of scarce refrigerants. Therefore, it shows good application prospects in portable cryogenic devices, space payloads, and special environment refrigeration systems. The core of adiabatic demagnetization refrigeration technology lies in the performance of magnetic refrigeration materials. The efficiency and reliability of magnetic refrigeration systems also largely depend on the magnetocaloric properties of magnetic refrigeration materials under low temperature and low magnetic field conditions. Ideal magnetic refrigeration materials should still have a large magnetic entropy change and good thermal stability in the milliKelvin (mK) or even lower temperature range.

[0003] In related technologies, materials used in cryogenic adiabatic demagnetization refrigeration are mainly paramagnetic salts, such as ferric ammonium sulfate and potassium chromium sulfate. While these materials exhibit good magnetocaloric properties within a certain temperature range, they generally contain a large amount of water of crystallization, making them prone to water loss under high vacuum or long-term operating conditions. This leads to changes in the material structure and magnetocaloric properties, severely impacting refrigeration efficiency and system stability. Furthermore, the presence of water of crystallization results in low thermal conductivity at extremely low temperatures. Improving thermal conductivity typically requires the introduction of auxiliary structures such as metal wires, which not only increases manufacturing difficulty but also significantly raises operating costs. On the other hand, paramagnetic salts also have limitations in mechanical strength, chemical stability, and processing performance, making it difficult to meet the comprehensive requirements of next-generation cryogenic refrigeration devices for long-term stable operation, structural reliability, and environmental adaptability. To address these issues, researchers have developed anhydrous inorganic magnetic compounds, multi-component rare earth compounds, and novel structural magnetic material systems, hoping to develop high-performance magnetic refrigeration materials by optimizing crystal structure and magnetic ion arrangement. For example, Chinese patent CN118343824A discloses a gadolinium-based low-temperature magnetic refrigeration material with the chemical formula NaGdSiO4. This material exhibits a maximum magnetic entropy change of 46.3 J kg when the magnetic field changes by 7 T. -1 K -1 (2.6K), maximum cooling capacity is 293.8J kg -1 Chinese patent CN108840364A discloses an inorganic gadolinium-based complex crystal, Gd(OH)SO4, which exhibits a maximum magnetic entropy change of 53.49 J / kg under a magnetic field variation of 0-7 T at 2 K. -1 K -1 The maximum magnetic entropy change under a magnetic field variation of 0-5T is approximately 49 J kg. -1 K -1 However, the aforementioned materials generally suffer from problems such as insufficient magnetic entropy retention in ultra-low temperature regions (e.g., ≤200mK), poor material stability, and inconvenience in engineering applications. Therefore, developing magnetic refrigeration materials that are free of crystallization water, structurally stable, and still possess excellent magnetocaloric properties under extremely low temperature conditions is of great practical significance for promoting the development of related high-end technologies. Summary of the Invention

[0004] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes the application of Ca9Gd(VO4)7 in the field of ultra-low temperature magnetic refrigeration, aiming to solve the problems of insufficient magnetic entropy retention, poor material stability, and inconvenience in engineering application of current magnetic refrigeration materials under ultra-low temperature conditions (≤200mK), and to develop a magnetic refrigeration material that is free of crystallization water, structurally stable, high-performance, stable and reliable, and still has excellent magnetocaloric properties under ultra-low temperature conditions.

[0005] An embodiment of the first aspect of the present invention provides an ultra-low temperature magnetic refrigeration material, wherein the chemical formula of the ultra-low temperature magnetic refrigeration material is Ca9Gd(VO4)7, and the ultra-low temperature is ≤200mK.

[0006] The ultra-low temperature magnetic refrigeration material according to the first aspect of the present invention has at least the following beneficial effects: The ultra-low temperature magnetic refrigeration material provided by the present invention has the chemical formula Ca9Gd(VO4)7, and can exhibit excellent magnetocaloric properties and stability under ultra-low temperature (≤200mK) conditions, providing a new material option for ultra-low temperature magnetic refrigeration technology. The material has the following beneficial effects: (1) In the crystal structure of the material, Gd 3+ The nearest neighbor distance between ions is about 0.892 nm, which makes the exchange interaction between magnetic ions extremely weak, thus effectively suppressing magnetic ordering behavior at low temperatures. The material did not undergo magnetic phase transition or long-range magnetic ordering in the temperature range down to 50 mK, indicating that its magnetic interaction energy scale is extremely low and it can retain a large magnetic entropy in a lower temperature range. The material can be maintained in a lower temperature range, providing favorable conditions and material basis for efficient magnetic refrigeration processes in the sub-Kelvin or even lower temperature ranges, and has extremely high practical application value. (2) The specific heat test of the material shows that its magnetic specific heat peak value is extremely low, indicating that Ca9Gd(VO4)7 can release magnetic entropy in a concentrated manner in the extremely low temperature range, thereby achieving a deeper cooling effect in the process of adiabatic demagnetization, making the temperature change with the magnetic field more significant in the process of adiabatic demagnetization, improving the refrigeration effect brought about by the unit magnetic field change, improving the overall efficiency of the magnetic refrigeration system, and having excellent potential for extremely low temperature magnetic refrigeration. (3) Gd 3+ The ions have a large spin number (S = 7 / 2) and are almost unaffected by the anisotropy of the crystal field, making Ca9Gd(VO4)7 exhibit near-isotropic magnetic properties. Therefore, in practical magnetic refrigeration applications, this material does not require strict orientation treatment of the single crystal and can be placed in any direction in the magnetic field, which significantly reduces the complexity of the single crystal orientation and assembly process, reduces the operational difficulty and cost in engineering applications, and improves the convenience of engineering applications. (4) The Ca9Gd(VO4)7 crystal structure does not contain water of crystallization, and has higher structural stability and thermal stability. This material is not prone to structural degradation under high vacuum, extremely low temperature and relatively high preparation or use temperature conditions, avoiding the problem of magnetic refrigeration performance decay or even failure due to the removal of water of crystallization, thus making it more suitable for long-term stable operation of extremely low temperature magnetic refrigeration systems.

[0007] In summary, the ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 provided by this invention exhibits significant advantages in the field of ultra-low temperature magnetic refrigeration due to its ultra-weak magnetic interaction, low-temperature non-magnetic ordered characteristics, concentrated release behavior of magnetic entropy in the ultra-low temperature region, near-isotropic magnetic characteristics, and excellent structural stability. It can achieve efficient, stable, and convenient magnetic refrigeration applications in the sub-Kelvin and even lower temperature regions, and has broad engineering application prospects.

[0008] In some embodiments of the present invention, the extremely low temperature is ≤200mK. Exemplarily, it can be 200mK, 195mK, 100mK, 70mK, 60mK, 50mK, or within the range of any two of the above values.

[0009] In some embodiments of the present invention, the extremely low temperature is ≤195mK.

[0010] In some embodiments of the present invention, the extremely low temperature is ≤100mK.

[0011] In some embodiments of the present invention, the extremely low temperature is ≤70mK.

[0012] In some embodiments of the present invention, the extremely low temperature is ≤60mK.

[0013] In some embodiments of the present invention, the extremely low temperature is ≤50mK.

[0014] In some embodiments of the present invention, the ultra-low temperature magnetic refrigeration material is hexagonal. Hexagonal crystal is a crystal system in crystallography, possessing sixfold rotation axes or sixfold inversion axes, with unit cell parameters a=b≠c, α=β=90°, and γ=120°. The hexagonal crystal material Ca9Gd(VO4)7 proposed in this invention exhibits excellent magnetocaloric properties and stability under ultra-low temperature conditions, providing a new material option for ultra-low temperature magnetic refrigeration technology.

[0015] In some embodiments of the present invention, the magnetic specific heat peak value of the ultra-low temperature magnetic refrigeration material is 216~222 mK, for example, it can be 216 mK, 217 mK, 218 mK, 219 mK, 220 mK, 221 mK, 222 mK, preferably 218 mK. Under conditions of a temperature range of 0.05–5 K and an applied magnetic field range of 0–5 T, the specific heat of the material was tested. The results showed that the peak value of its magnetic specific heat appeared around 218 mK (the specific heat test may have errors, so the peak value of the magnetic specific heat is an approximate range around 218 mK, such as 216–222 mK). This temperature range is extremely rare in Gd-based magnetic refrigeration material systems, indicating that Ca9Gd(VO4)7 can release magnetic entropy in a lower temperature range, thereby achieving a deeper cooling effect during the adiabatic demagnetization process. This makes the temperature change with the magnetic field more significant during the adiabatic demagnetization process, improves the cooling effect brought about by the unit magnetic field change, and improves the overall efficiency of the magnetic refrigeration system, demonstrating excellent potential for ultra-low temperature magnetic refrigeration.

[0016] In some embodiments of the present invention, the ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 does not undergo a phase transition or long-range magnetic order at zero magnetic field and temperatures as low as 50 mK. This is because the Gd content in the crystal structure of Ca9Gd(VO4)7... 3+ The material exhibits a large nearest neighbor distance between ions, resulting in an extremely low magnetic interaction energy scale. No significant magnetic ordering phase transition was observed under zero or low magnetic field conditions. Specific heat testing revealed that the material remained free of magnetic phase transition or long-range magnetic ordering even at temperatures as low as 50 mK, indicating an extremely low magnetic interaction energy scale. This allows it to retain a large magnetic entropy at even lower temperatures, providing favorable conditions and a material basis for efficient magnetic refrigeration processes in sub-Kelvin and even lower temperature ranges. Furthermore, it offers ample available magnetic entropy resources for adiabatic demagnetization refrigeration processes, demonstrating significant practical application value.

[0017] In some embodiments of the present invention, under the conditions of an initial temperature of 2.0 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤50 mK.

[0018] In some embodiments of the present invention, under the conditions of an initial temperature of 2.5 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤60 mK.

[0019] In some embodiments of the present invention, under the conditions of an initial temperature of 3.0 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤70 mK.

[0020] In some embodiments of the present invention, under the conditions of an initial temperature of 4.0 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤100 mK.

[0021] In some embodiments of the present invention, under the conditions of an initial temperature of 6.5 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤195 mK.

[0022] In some embodiments of the present invention, a magnetic entropy phase diagram is constructed based on the magnetic entropy distribution relationship calculated from the specific heat test data, and the corresponding isentropic change curve is obtained from the magnetic entropy phase diagram.

[0023] In some embodiments of the present invention, the isentropic change curve in the magnetic entropy phase diagram corresponds to the ideal cooling path in the adiabatic demagnetization refrigeration process, which describes the temperature evolution relationship followed by the material as the magnetic field changes under adiabatic conditions.

[0024] This invention constructs a magnetic entropy phase diagram based on the magnetic entropy distribution relationship calculated from specific heat data, and extracts isentropic change curves from it. The isentropic change curves show that, within a low applied magnetic field range, the material can achieve a sustained and significant cooling effect when demagnetized along the isentropic path. Since the system is not limited by magnetic phase transitions throughout the entire ultra-low temperature region, its adiabatic demagnetization process is not hindered by phase transitions, and the temperature decreases smoothly as the magnetic field decreases, which is beneficial for obtaining a lower final cooling temperature. This invention uses an applied magnetic field of 5 T as the initial magnetic field condition to perform adiabatic demagnetization treatment on the aforementioned ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 at different initial temperatures. Demagnetization along an isentropic path yields the following cooling effects: when the initial temperature is 2.0 K, the minimum cooling temperature reaches approximately 50 mK; when the initial temperature is 2.5 K, the minimum cooling temperature reaches approximately 60 mK; when the initial temperature is 3.0 K, the minimum cooling temperature reaches approximately 70 mK; when the initial temperature is 4.0 K, the minimum cooling temperature reaches approximately 100 mK; and when the initial temperature is 6.5 K, the minimum cooling temperature reaches approximately 195 mK. These results demonstrate that, over a wide range of initial temperatures, the aforementioned magnetic refrigeration material can achieve significant cooling during adiabatic demagnetization and stably achieve temperatures in the hundreds of millikelvin range or even lower, fully proving that this material is suitable for adiabatic demagnetization refrigeration applications in ultra-low temperature regions.

[0025] Within the aforementioned operating magnetic field range, the system's magnetic entropy is highly sensitive to changes in the magnetic field, resulting in a large temperature change caused by a unit change in the magnetic field during the adiabatic demagnetization process, thereby effectively improving refrigeration efficiency. Therefore, the aforementioned ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 is not only suitable for single-stage adiabatic demagnetization refrigeration systems, but can also be used as a pre-stage magnetic refrigeration material in multi-stage magnetic refrigeration systems, providing stable and reliable initial low-temperature conditions for subsequent lower-temperature refrigeration processes.

[0026] In addition to optimizing crystal structure and magnetic ion arrangement to obtain materials with large magnetic entropy changes in the milliKelvin temperature range, the material synthesis process, cost control, and environmental friendliness have also become important factors that must be comprehensively considered in the research and development of new magnetic refrigeration materials. How to prepare ultra-low temperature magnetic refrigeration materials with simple processes, short cycles, and suitability for industrial production has become an urgent problem to be solved.

[0027] Based on this, an embodiment of the second aspect of the present invention provides a method for preparing the above-mentioned ultra-low temperature magnetic refrigeration material, comprising the steps of: S100. Weigh calcium carbonate, vanadium pentoxide and gadolinium trioxide according to the stoichiometric ratio, mix them evenly to obtain a mixed powder; S200: Press the mixed powder into blocks to obtain initial raw material blocks; S300. The initial raw material block is pre-fired, cooled, ground into fine powder, and then pressed into block shape again to obtain pre-fired block. S400. The pre-fired block is sintered once to obtain sintered material; S500. After grinding the sintered material into fine powder, it is subjected to isostatic pressing to obtain the initial raw material rod. S600. The initial raw material bar is sintered a second time to obtain a shaped raw material bar; S700. The molded raw material rod is placed in an inert atmosphere for single crystal growth to obtain the single crystal rod of the ultra-low temperature magnetic refrigeration material.

[0028] This invention uses calcium carbonate (CaCO3), vanadium pentoxide (V2O5), and gadolinium trioxide (Gd2O3) as raw materials to obtain Ca9Gd(VO4)7 through high-temperature sintering. The preparation method is simple, has a short preparation cycle, does not require the use of polluting acids or alkalis, meets environmental protection requirements, satisfies the needs of sustainable development, and is suitable for large-scale industrial production. It has broad application prospects in high-end technology fields such as low-temperature physics, space exploration, and aerospace.

[0029] In some embodiments of the present invention, the molar ratio of calcium carbonate, vanadium pentoxide, and gadolinium trioxide is 18:7:1. Other ratios will result in impurities in the synthesized compound, thereby affecting the growth of single crystals and consequently the magnetic refrigeration performance.

[0030] In some embodiments of the present invention, in step S100, calcium carbonate, vanadium pentoxide and gadolinium trioxide are weighed according to stoichiometric ratio, and then thoroughly ground and mixed to obtain a mixed powder.

[0031] In some embodiments of the present invention, the pressing conditions in step S200 are not limited, as long as the sheet can be pressed into a sheet and placed in a muffle furnace for sintering. For example, it can be pressed at 18 t for 5 minutes.

[0032] In some embodiments of the present invention, in step S300, the temperature of the pre-firing treatment is 700~800°C. Exemplarily, it can be 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, or a range consisting of any two of the above values.

[0033] In some embodiments of the present invention, in step S300, the pre-burning treatment time is 8 to 12 hours. Exemplarily, it can be 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, or within any two of the above values.

[0034] In some embodiments of the present invention, the pressing conditions in step S300 are not limited, as long as the sheet can be pressed into a sheet and placed in a muffle furnace for sintering. For example, it can be pressed at 18 t for 5 minutes.

[0035] In some embodiments of the present invention, in step S400, the temperature of the first sintering is 900~1000℃. Exemplarily, it can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, or within any two of the above values.

[0036] In some embodiments of the present invention, in step S400, the sintering time is 20 to 24 hours. Exemplarily, it can be 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, or within any two of the above values.

[0037] In some embodiments of the present invention, in step S500, the pressure processed by the isostatic pressing bar is 150~200MPa. Exemplarily, it can be 150MPa, 160MPa, 170MPa, 180MPa, 190MPa, 200MPa, or within the range of any two of the above values.

[0038] In some embodiments of the present invention, in step S600, the temperature of the secondary sintering is 900~1000℃. Exemplarily, it can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, 960℃, 970℃, 980℃, 990℃, 1000℃, or within any two of the above values.

[0039] In some embodiments of the present invention, in step S600, the secondary sintering time is 20 to 24 hours. Exemplarily, it can be 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, or within any two of the above values.

[0040] This invention, through two sintering processes, ensures that the intermediate products react fully at the given reduction sintering temperature and time, while avoiding excessively high temperatures that could cause the final product to volatilize and affect its structure and properties. Furthermore, the sintering temperature and time provided by this invention help reduce energy consumption.

[0041] In some embodiments of the present invention, sintering is carried out in a sintering furnace.

[0042] Specifically, the sintering furnace includes a muffle furnace, a tube furnace, or a vacuum furnace.

[0043] In some embodiments of the present invention, in step S700, the inert atmosphere includes argon and / or nitrogen.

[0044] In some embodiments of the present invention, in step S700, the forming raw material rod is placed in an optical floating zone furnace and single crystal growth is performed under an argon atmosphere, with a gas flow rate of 1.5~2.5 L / min. Exemplarily, the gas flow rate can be 1.5 L / min, 1.6 L / min, 1.7 L / min, 1.8 L / min, 1.9 L / min, 2.0 L / min, 2.1 L / min, 2.2 L / min, 2.3 L / min, 2.4 L / min, 2.5 L / min, or within any range of two of the above values.

[0045] An embodiment of the third aspect of the present invention provides an application of the above-mentioned ultra-low temperature magnetic refrigeration material or the above-mentioned method for preparing ultra-low temperature magnetic refrigeration material in the field of ultra-low temperature magnetic refrigeration.

[0046] This invention provides the application of hexagonal material Ca9Gd(VO4)7 in the field of ultra-low temperature magnetic refrigeration. It is applicable to technical fields that require the realization of its function, performance, or stable operation under ultra-low temperature or ultra-low temperature conditions. Its beneficial effects include: (1) This invention uses the hexagonal crystal system material Ca9Gd(VO4)7 as the ultra-low temperature magnetic refrigeration material. Due to the presence of Gd in the crystal structure, 3+ The nearest neighbor distance between the ions reaches approximately 0.892 nm, and the interaction between the magnetic ions is extremely weak. No magnetic order phase transition occurs even in the temperature range as low as 50 mK, which enables the material to maintain a large magnetic entropy in the lower temperature range, providing a material basis for efficient magnetic refrigeration in the sub-Kelvin and even lower temperature ranges.

[0047] (2) The Ca9Gd(VO4)7 material used in this invention exhibits a significant broad peak value of magnetic specific heat around 218 mK, which can concentrate the release of magnetic entropy in the extremely low temperature region, making the temperature change with the magnetic field more significant during the adiabatic demagnetization process, thereby improving the cooling effect brought about by the unit magnetic field change and improving the overall efficiency of the magnetic refrigeration system.

[0048] (3) Gd in the Ca9Gd(VO4)7 material of the present invention 3+ Ions are almost unaffected by the anisotropy of the crystal field, and the material has near-isotropic magnetism. In practical applications, it can be placed and used in any direction without strict orientation of the single crystal, which significantly reduces the technical difficulty of material assembly and use and improves the convenience of engineering applications.

[0049] (4) The Ca9Gd(VO4)7 material used in this invention does not contain water of crystallization and has high structural and thermal stability. Compared with traditional paramagnetic salt magnetic refrigeration materials, it avoids the problem of performance degradation or failure caused by the removal of water of crystallization. It is suitable for long-term stable operation under high vacuum, extremely low temperature and high temperature conditions.

[0050] In some embodiments of the present invention, the applications include applications in the fields of cryogenic physics, deep space exploration, superconductivity, or aerospace and space science.

[0051] In some embodiments of the present invention, the application includes use in a single-stage adiabatic demagnetizing refrigeration system or as a pre-stage magnetic refrigeration material in a multi-stage magnetic refrigeration system.

[0052] The ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 of this invention exhibits high sensitivity of its magnetic entropy to changes in the magnetic field within the aforementioned working magnetic field range. This results in a larger temperature change per unit magnetic field change during the adiabatic demagnetization process, thereby effectively improving refrigeration efficiency. Therefore, the aforementioned ultra-low temperature magnetic refrigeration material Ca9Gd(VO4)7 is not only suitable for single-stage adiabatic demagnetization refrigeration systems but can also be used as a pre-stage magnetic refrigeration material in multi-stage magnetic refrigeration systems, providing stable and reliable initial low-temperature conditions for subsequent lower-temperature refrigeration processes.

[0053] An embodiment of the fourth aspect of the present invention provides an ultra-low temperature magnetic refrigeration device, comprising a substrate and the ultra-low temperature magnetic refrigeration material described above or the ultra-low temperature magnetic refrigeration material prepared by the above preparation method.

[0054] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims, and drawings. Attached Figure Description

[0055] Figure 1 The curve of specific heat versus temperature of the Ca9Gd(VO4)7 material prepared in Example 1 of the present invention under zero magnetic field conditions; Figure 2 The temperature-magnetic field phase diagram of the specific heat of the Ca9Gd(VO4)7 material prepared in Example 1 of this invention under zero magnetic field conditions; Figure 3 Temperature-magnetic field phase diagram of magnetic entropy of Ca9Gd(VO4)7 material prepared in Example 1 of this invention under different magnetic field conditions; Figure 4 The isentropic lines are extracted from the magnetic entropy temperature-magnetic field phase diagram of the Ca9Gd(VO4)7 material prepared in Example 1 of this invention. Detailed Implementation

[0056] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0057] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0058] In the description of this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are both real numbers. Unless otherwise stated, the various reaction or operation steps may be performed sequentially or not. Preferably, the reaction methods in this invention are performed sequentially.

[0059] Experimental methods in the following examples, unless otherwise specified, are generally performed under standard conditions or as recommended by the manufacturer. Unless otherwise specified, the materials and reagents used in these examples are commercially available.

[0060] Example 1 Preparation of single crystals of hexagonal Ca9Gd(VO4)7 (1) Raw material preparation: Take analytical grade calcium carbonate (CaCO3), vanadium pentoxide (V2O5) and gadolinium trioxide (Gd2O3) and weigh them in a molar ratio of 18:7:1.

[0061] (2) Mixing and pressing: The above raw materials are placed in an agate mortar and ground until they are mixed evenly to obtain a mixed powder; the mixed powder is pressed into blocks to obtain the initial raw material blocks.

[0062] (3) Pre-firing: The initial raw material blocks are placed in a high-temperature furnace and pre-firing at 750°C in an air atmosphere for 10 hours; after cooling to room temperature, they are taken out, ground and pressed into blocks again to obtain pre-firing blocks.

[0063] (4) Sintering: The pre-fired blocks are placed in a high-temperature furnace and sintered at 900°C for 24 hours in an air atmosphere to obtain sintered material.

[0064] (5) Isostatic pressing into rods: The sintered material is ground into fine powder and then put into a mold. It is pressed into rod-shaped raw material under isostatic pressing conditions of 150~200 MPa to obtain raw material rods.

[0065] (6) Secondary sintering: The raw material rod is sintered again at 1000°C for 20 hours to densify it and form a shaped raw material rod suitable for single crystal growth.

[0066] (7) Single crystal growth in optical floating zone furnace: The shaped raw material rod is used as a feed rod and placed in an optical floating zone furnace. Single crystal growth is carried out in Ar atmosphere. The gas flow rate is controlled at 2.0 L / min. Directional growth is achieved through a stable melting zone, and finally a hexagonal Ca9Gd(VO4)7 single crystal rod is obtained.

[0067] Example 2 The difference between this embodiment and embodiment 1 is as follows: in step (3), pre-firing is performed at 700°C and held for 12 hours; in step (4), sintering is performed at 950°C for 22 hours; in step (6), sintering is performed again at 950°C for 22 hours; in step (7), the gas flow rate is controlled at 1.5 L / min. Everything else is the same as in Example 1, and will not be repeated here.

[0068] Example 3 The difference between this embodiment and embodiment 1 is as follows: in step (3), pre-firing is performed at 800°C and held for 8 hours; in step (4), sintering is performed at 100°C for 20 hours; in step (6), sintering is performed again at 900°C for 24 hours; in step (7), the gas flow rate is controlled at 2.5 L / min. Everything else is the same as in Example 1, and will not be repeated here.

[0069] Test case (1) XRD test The Ca9Gd(VO4)7 materials prepared in Examples 1-3 were analyzed by single-crystal X-ray diffraction (XRD) technology, confirming that the prepared compounds were Ca9Gd(VO4)7 single crystals.

[0070] (2) Specific heat test The specific heat properties of the hexagonal magnetic refrigeration material Ca9Gd(VO4)7 prepared in Example 1 were tested using the PPMS DynaCool integrated physical property measurement system manufactured by Quantum Design, combined with a dilution refrigeration (DR) device.

[0071] Under conditions of a temperature range of 0.05-5 K and an applied magnetic field range of 0-5 T, the specific heat behavior of the Ca9Gd(VO4)7 single crystal obtained in Example 1 was systematically measured, and the specific heat variation curve of the material with temperature under zero magnetic field conditions was obtained. Figure 1 ) and specific heat temperature-magnetic field phase diagram ( Figure 2Test results show that, under zero magnetic field conditions, the material does not exhibit magnetic phase transition or long-range magnetic order in the temperature range down to 50 mK, indicating that its magnetic interaction is extremely weak; at the same time, a significant broad peak of magnetic specific heat appears near about 218 mK, reflecting that the material has significant magnetic entropy release behavior in the extremely low temperature region.

[0072] By integrating the curves of specific heat versus temperature under different magnetic field conditions, the relationship between the magnetic entropy of Ca9Gd(VO4)7 and temperature and magnetic field was obtained, and a model was constructed as follows. Figure 3 The magnetic entropy temperature-magnetic field phase diagram is shown. The results show that the material can still maintain a large magnetic entropy in the extremely low temperature region, exhibiting excellent magnetocaloric properties, and the magnetic entropy can be continuously extended to below 50 mK, providing sufficient magnetic entropy reserves for adiabatic demagnetization refrigeration.

[0073] Furthermore, based on the isentropic change curve extracted from the magnetic entropy temperature-magnetic field phase diagram, the demagnetization and cooling path of Ca9Gd(VO4)7 under ideal adiabatic conditions was obtained, as follows: Figure 4 As shown, the isentropic curve describes the evolution trajectory of the material temperature as the applied magnetic field is gradually reduced under adiabatic conditions. It represents the demagnetizing and cooling path followed by the material as the magnetic field changes under ideal adiabatic conditions, and is used to characterize the intrinsic cooling capacity of the material in the adiabatic demagnetizing refrigeration process.

[0074] Using an applied magnetic field of 5 T as the initial magnetic field condition, adiabatic demagnetization of the hexagonal material Ca9Gd(VO4)7 was performed at different initial temperatures. Demagnetization along an isentropic path yielded the following cooling effects: at an initial temperature of 2.0 K, the lowest cooling temperature reached approximately 50 mK; at 2.5 K, approximately 60 mK; at 3.0 K, approximately 70 mK; at 4.0 K, approximately 100 mK; and at 6.5 K, approximately 195 mK. These results demonstrate that, over a wide range of initial temperatures, the Ca9Gd(VO4)7 magnetic refrigeration material can achieve significant cooling during adiabatic demagnetization, stably reaching temperatures in the hundreds of millikrvin range or even lower, fully proving its suitability for adiabatic demagnetization refrigeration applications in the ultra-low temperature region.

[0075] The above is a detailed description of the preferred embodiments of this application. However, this application is not limited to the above embodiments. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of this application. All such equivalent modifications or substitutions are included within the scope defined by the claims of this application.

Claims

1. A cryogenic magnetic refrigeration material, characterized in that, The chemical formula of the ultra-low temperature magnetic refrigeration material is Ca9Gd(VO4)7, and the ultra-low temperature is ≤200mK.

2. The ultra-low temperature magnetic refrigeration material according to claim 1, characterized in that, The ultra-low temperature magnetic refrigeration material is hexagonal; And / or, the ultra-low temperature magnetic refrigeration material is a single crystal material.

3. The ultra-low temperature magnetic refrigeration material according to claim 1, characterized in that, The magnetic specific heat peak value of the ultra-low temperature magnetic refrigeration material is 216~222 mK.

4. The ultra-low temperature magnetic refrigeration material according to claim 1, characterized in that, Including at least one of (a1) to (a5): (a1) Under the conditions of an initial temperature of 2.0 K, an external magnetic field of 5 T and adiabatic demagnetization, the minimum refrigeration temperature of the ultra-low temperature magnetic refrigeration material is ≤50 mK; (a2) Under the conditions of an initial temperature of 2.5 K, an external magnetic field of 5 T, and adiabatic demagnetization, the minimum refrigeration temperature of the ultra-low temperature magnetic refrigeration material is ≤60 mK; (a3) Under the conditions of an initial temperature of 3.0 K, an external magnetic field of 5 T and adiabatic demagnetization, the minimum refrigeration temperature of the ultra-low temperature magnetic refrigeration material is ≤70 mK; (a4) Under the conditions of an initial temperature of 4.0 K, an external magnetic field of 5 T and adiabatic demagnetization, the minimum refrigeration temperature of the ultra-low temperature magnetic refrigeration material is ≤100 mK; (a5) Under the conditions of an initial temperature of 6.5 K, an external magnetic field of 5 T and adiabatic demagnetization, the minimum cooling temperature of the ultra-low temperature magnetic refrigeration material is ≤195 mK.

5. A method for preparing an ultra-low temperature magnetic refrigeration material as described in any one of claims 1-4, characterized in that, Including the following steps: Calcium carbonate, vanadium pentoxide and gadolinium trioxide were weighed according to stoichiometric ratio and mixed evenly to obtain a mixed powder. The mixed powder is pressed into blocks to obtain the initial raw material blocks; The initial raw material block is pre-fired, cooled, ground into fine powder, and then pressed into blocks again to obtain pre-fired blocks; The pre-fired block is sintered once to obtain sintered material; After grinding the sintered material into fine powder, it is subjected to isostatic pressing to obtain the initial raw material rod. The initial raw material bar is sintered a second time to obtain a shaped raw material bar; The shaped raw material rod is placed in an inert atmosphere for single crystal growth to obtain the single crystal rod of the ultra-low temperature magnetic refrigeration material.

6. The method for preparing the ultra-low temperature magnetic refrigeration material according to claim 5, characterized in that, The molar ratio of calcium carbonate, vanadium pentoxide, and gadolinium trioxide is 18:7:

1.

7. The method for preparing the ultra-low temperature magnetic refrigeration material according to claim 5, characterized in that, Including at least one of (b1) to (b8): (b1) The temperature of the pre-firing treatment is 700~800℃; (b2) The pre-firing treatment time is 8 to 12 hours; (b3) The temperature of the first sintering is 900~1000℃; (b4) The sintering time for the first sintering is 20-24 hours; (b5) The pressure processed by the isostatic pressing bar is 150~200MPa; (b6) The temperature of the secondary sintering is 900~1000℃; (b7) The secondary sintering time is 20-24 hours; (b8) The shaped raw material rod is placed in an optical floating zone furnace and single crystal growth is carried out under an argon atmosphere with a gas flow rate of 1.5~2.5 L / min.

8. The application of an ultra-low temperature magnetic refrigeration material as described in any one of claims 1-4 or a method for preparing an ultra-low temperature magnetic refrigeration material as described in any one of claims 5-7 in the field of ultra-low temperature magnetic refrigeration.

9. The application according to claim 8, characterized in that, The applications include those in the fields of low-temperature physics, deep space exploration, superconductivity, or aerospace and space science.

10. A cryogenic magnetic refrigeration device, characterized in that, It includes a matrix, and an ultra-low temperature magnetic refrigeration material as described in any one of claims 1-4 or an ultra-low temperature magnetic refrigeration material prepared by the preparation method as described in any one of claims 5-7.

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