Method for sol-gel preparation of perovskite rare-earth manganite magnetocaloric materials

Abstract

The application discloses a sol-gel preparation method of a perovskite rare earth manganese oxide magnetocaloric material, and belongs to the technical field of functional material preparation. The method comprises the following steps in sequence: preparing a metal ion precursor solution, adding citric acid and ethylene glycol to perform complexation and esterification reaction to form a wet gel, vacuum drying to obtain a dry gel, pre-burning to remove organic matters, and finally high-temperature calcination and crystallization. The molar ratio of the metal ion, the citric acid and the ethylene glycol is controlled in the range of 1:1.5-4:3-6, and the temperature and time of the drying, pre-burning and calcination stages are optimized. The calcination temperature can be adjusted in the range of 900-1100 DEG C according to the target magnetocaloric temperature zone. Compared with the solid phase reaction method, the calcination temperature of the method is lower, and the process condition is mild. The prepared product has high phase purity, the particles are nanometer or submicron, and the particle size distribution is uniform. The magnetic heat performance can be controlled by changing the calcination temperature, and the method is suitable for room temperature or high-temperature magnetocaloric scenes.

Description

Technical Field

[0001] This application relates to functional material preparation technology, and more specifically, provides a method for synthesizing perovskite rare earth manganese oxide magnetocaloric materials using a sol-gel process. Background Technology

[0002] Magnetothermology, based on the magnetocaloric effect, offers advantages in environmental friendliness, energy efficiency, and operational stability, and is expected to replace traditional gas compression refrigeration technology, becoming an important development direction. Among various magnetothermology materials, rare earth manganese oxides (generally represented as RE) with a perovskite crystal structure are promising candidates. 1-x A x MnO3 (where RE represents rare earth elements and A represents alkaline earth metals) has received widespread attention in recent years due to its significant magnetocaloric effect near the Curie temperature.

[0003] The magnetocaloric properties of these doped perovskite manganese oxides are closely related to their microstructure, specifically depending on parameters such as particle size, uniformity of distribution, and degree of crystallinity. Traditional preparation methods mainly rely on solid-state reaction methods, which, although relatively simple to operate, typically require multiple calcinations and grindings at temperatures above 1200℃. This process easily introduces impurity phases, leading to compositional deviations, coarse particles, and uneven particle sizes, making it difficult to obtain high-quality nano- or submicron-sized powders, thus limiting further improvements in magnetocaloric properties.

[0004] The sol-gel method, as a wet chemical synthesis route, can achieve uniform mixing of components at the molecular scale, and is often used to prepare high-performance perovskite oxides due to its relatively low synthesis temperature, high product purity, and fine particle size with narrow distribution. However, existing sol-gel processes still have room for optimization in the selection of key parameters (such as the ratio of metal ions to complexing agents and heat treatment procedures), especially in how to precisely control the phase composition, microstructure, and corresponding magnetocaloric properties of the product by adjusting process conditions to meet the needs of magnetocaloric applications in different temperature ranges. Currently, a systematic solution is lacking. Based on this, this application proposes an improved preparation method. Summary of the Invention

[0005] This application addresses the problems of uneven particle size, insufficient purity, and poor process adaptability in the preparation of existing perovskite rare earth manganese oxide magnetocaloric materials by providing a sol-gel preparation method to obtain powder materials with particles at the nano or submicron scale and excellent magnetocaloric properties.

[0006] To achieve the above objectives, the technical solution adopted in this application includes the following steps: Ingredients and dissolution: Based on the stoichiometric composition of the target perovskite rare earth manganese oxide, accurately weigh the corresponding metal salt raw materials, add them to deionized water, and stir continuously under heating conditions until completely dissolved to obtain a clear mixed solution.

[0007] The metal salt raw materials used can be nitrates, acetates, or sulfates. These raw materials have good water solubility, which is conducive to the full dispersion of metal ions, and they are easily decomposed during subsequent heat treatment without introducing additional impurities.

[0008] Complexation and gelation: Citric acid (as a complexing agent) and ethylene glycol (as a crosslinking agent) are added sequentially to the above mixed solution, controlling the molar ratio of metal ions, citric acid, and ethylene glycol in the system to be 1:(1.5~4):(3~6). The mixture is continuously stirred at 60~80℃ to allow the system to undergo sufficient complexation and esterification reactions, ultimately forming a uniform, transparent wet gel.

[0009] As an optional formulation, when the molar ratio of metal ions, citric acid, and ethylene glycol is 1:2:4, the coordination effect between the complexing agent and the metal ions is optimal, the crosslinking agent effectively promotes the formation of the gel network, and the resulting wet gel exhibits ideal homogeneity without layering or clumping. Furthermore, the preferred heating and stirring temperature is 75°C, and the stirring time is controlled at 5-6 hours. This condition ensures that the complexation and esterification reactions proceed fully while avoiding the problem of excessively high temperatures leading to rapid solvent evaporation and incomplete reaction.

[0010] Drying: The obtained wet gel is vacuum dried at 100~120℃ to obtain the dry gel precursor. The vacuum drying time is generally set to 10~15 hours. This can effectively remove the solvent and free water in the wet gel, prevent the gel from cracking or pulverizing during the drying process, and thus ensure the structural integrity of the dry gel precursor.

[0011] Pre-calcination treatment: The dry gel precursor is placed in a muffle furnace and heat-treated at a temperature range of 280~350℃ for 2~4 hours to remove organic matter and residual moisture. After heat treatment, it is naturally cooled to room temperature and then ground to obtain pre-calcined powder.

[0012] A preferred pre-calcination condition is: temperature 300℃, holding time 210 minutes, and a heating rate of 5℃ / min in the muffle furnace. Using a slower heating rate can avoid powder agglomeration caused by the rapid decomposition of organic matter producing a large amount of gas, while precise temperature and time settings help ensure complete decomposition of organic matter and prevent premature crystallization of precursors.

[0013] High-temperature calcination: The pre-calcined powder is placed in a muffle furnace and calcined at 900~1100℃ for 8~12 hours to allow the product to fully crystallize. After calcination, the powder is cooled in the furnace and then ground to obtain perovskite rare earth manganese oxide powder. The heating rate during the calcination stage is also preferably 5°C / min.

[0014] The calcination temperature can be adjusted according to the target magnetic transition temperature. For example, when the calcination temperature is set to 950℃, the resulting product is a nano-sized powder with a Curie temperature in the room temperature range, which is suitable for room temperature magnetocooling; when the calcination temperature is set to 1050℃, the product has higher crystallinity, the particle size reaches the submicron level, and the Curie temperature is higher, which is suitable for high temperature magnetocooling.

[0015] This method is applicable to target perovskite rare earth manganese oxides, such as La. 0.8-x Gd x Sr 0.2 MnO3 (x = 0.05, 0.10, or 0.15) or La 0.7 Sr 0.3 MnO3. When preparing the first type of material, the high-temperature calcination temperature is 950°C. La was prepared... 0.7 Sr 0.3 For MnO3, the high-temperature calcination temperature is 1050°C.

[0016] Compared with traditional preparation techniques, this application has the following advantages: 1. This method adopts the sol-gel route, and the entire synthesis process is completed in the liquid phase. It does not require complicated equipment, and the calcination temperature (900~1100℃) is much lower than that of the traditional solid-phase reaction method (usually higher than 1200℃), which is beneficial to reduce energy consumption. 2. By strictly controlling the molar ratio of metal ions to citric acid and ethylene glycol (especially when the ratio is 1:2:4), uniform distribution of metal ions at the molecular scale is achieved, effectively avoiding component segregation. The resulting powder has high purity, a single phase, particle size in the nanometer or submicron range, and a narrow particle size distribution. 3. By adjusting key process parameters (such as calcination temperature), the grain size, microstrain, and oxygen content of the product can be changed, thereby controlling the magnetocaloric properties of the material (including Curie temperature, magnetic entropy change, etc.) to adapt it to the magnetocaloric application requirements of different temperature ranges, which has good industrialization and promotion value. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The X-ray diffraction (XRD) pattern of the perovskite rare earth manganese oxide prepared in Example 1 of this invention; Figure 2 This is a scanning electron microscope (SEM) image of the perovskite rare earth manganese oxide prepared in Example 1 of the present invention. Figure 3 The curves showing the magnetization variation of the product obtained in Example 1 of this invention at different temperatures are shown. Figure 4 This is a scanning electron microscope (SEM) image of the perovskite rare earth manganese oxide prepared in Example 2 of the present invention; Figure 5 The curves showing the magnetization intensity changes of the product obtained in Example 2 of this invention at different temperatures are shown. Detailed Implementation

[0019] The technical solution of this application is described below with reference to the accompanying drawings. The described embodiments are only some examples and do not represent all possible implementation methods.

[0020] Example 1: This example is used to prepare La suitable for room temperature magnetocooling applications. 0.8-x Gd x Sr 0.2 MnO3 (x=0.05, 0.10, 0.15) perovskite rare earth manganese oxide powder, the specific steps are as follows: Ingredients and dissolution: According to La 0.8-x Gd x Sr 0.2 To determine the stoichiometric ratio of MnO3, weigh out La(NO3)3·6H2O, Gd(NO3)3·6H2O, Sr(NO3)2, and Mn(NO3)2 (50%) solutions respectively. Add all the weighed raw materials to a 50mL beaker containing 40mL of deionized water, and stir on a 75℃ constant temperature magnetic stirrer until completely dissolved to form a clear mixed solution.

[0021] Complexation and gelation: Citric acid and ethylene glycol were added sequentially to the above mixed solution to achieve a total molar ratio of metal ions to citric acid to ethylene glycol of 1:2:4. The mixture was stirred at 75°C for 5.5 hours, and the system gradually transformed into a viscous, homogeneous, and transparent wet gel.

[0022] Drying: The obtained wet gel was placed in a vacuum drying oven at 120℃ and kept at that temperature for 12 hours to remove solvent and moisture, resulting in a fluffy dry gel precursor.

[0023] Pre-calcination treatment: The dry gel precursor was placed in a muffle furnace and heated to 300°C at a heating rate of 5°C / min, and held at that temperature for 210 minutes to completely decompose the organic matter. After naturally cooling to room temperature, the sample was removed and ground to obtain a fine pre-calcined powder.

[0024] High-temperature calcination: The pre-calcined powder is placed in a muffle furnace and heated to 950°C at a rate of 5°C / min, and held at that temperature for 10 hours to allow the product to fully crystallize. After natural cooling to room temperature in the furnace, the product is removed and ground to obtain black La. 0.8- x Gd x Sr 0.2 MnO3 perovskite rare earth manganese oxide powder.

[0025] Product characterization: X-ray diffraction (XRD) test results ( Figure 1 The results showed that the product had a single-phase perovskite structure and no impurity phases were detected; scanning electron microscopy (SEM) observation showed that... Figure 2 The results indicate that the particle size distribution is uniform, with a particle size of approximately 177-249 nm, belonging to the submicron-scale powder category; the magnetization intensity of the sample as a function of temperature was measured using a magnetic property testing system under an applied magnetic field of 100 Oe. Figure 3 The results showed that when x=0.05, 0.10, and 0.15, the corresponding Curie temperatures of the products were 306 K, 276 K, and 259 K, respectively, all of which were within the room temperature magnetocaloric operating range, and exhibited a significant magnetocaloric effect near the Curie temperature.

[0026] Example 2: This example is used to prepare La suitable for high-temperature magnetocooling applications. 0.7 Sr 0.3 The MnO3 perovskite rare earth manganese oxide powder process is basically the same as that in Example 1, with the only difference being: Ingredients and Dissolving: According to La 0.7 Sr 0.3 The stoichiometric ratio of MnO3 is determined by weighing out a solution of La(NO3)3·6H2O, Sr(NO3)2, and Mn(NO3)2 (50%), without adding Gd(NO3)3·6H2O.

[0027] High-temperature calcination: The pre-calcined powder was heated to 1050°C at a heating rate of 5°C / min and held for 10 hours. The remaining conditions were the same as in Example 1.

[0028] Product characterization: XRD pattern showed that the product was a single-phase perovskite structure with no impurities. SEM observation ( Figure 4 The particle size is approximately 400-700 nm, indicating submicron-sized powder, and the crystallinity is significantly higher than that of the product in Example 1. The magnetization intensity as a function of temperature, measured under an applied magnetic field of 100 Oe, is shown in the figure. Figure 5 The results indicate that the Curie temperature of the product is approximately 367 K, making it suitable for high-temperature magnetocaloric cooling, and it exhibits a significant magnetocaloric effect near the Curie temperature.

[0029] Performance Testing and Analysis The products obtained in Examples 1 and 2 were subjected to systematic magnetic property tests. The results showed that the perovskite rare earth manganese oxide powders prepared by this method were all single perovskite mineral phases with high purity and uniform particle size distribution. By changing the calcination temperature (950℃ and 1050℃), products with different grain sizes and Curie temperatures could be obtained, thereby enabling the directional preparation of materials suitable for room temperature or high temperature magnetocaloric applications. All products exhibited excellent magnetocaloric effects near their Curie temperatures, meeting the requirements for magnetocaloric materials.

[0030] In summary, This method comprises five stages: ingredient preparation and dissolution, complexation and gelation, drying, pre-calcination, and high-temperature calcination. The molar ratio of metal ions, citric acid, and ethylene glycol (1:1.5~4:3~6) is precisely controlled, and the process parameters for drying, pre-calcination, and calcination are optimized. Simultaneously, the high-temperature calcination temperature is adjusted according to the target magnetocaloric temperature range. This preparation process is simple to operate, with mild reaction conditions, and the calcination temperature is significantly lower than that of traditional solid-state reaction methods. The obtained product has high phase purity, with particles at the nanometer or submicron scale and a narrow particle size distribution. Furthermore, by adjusting the process parameters, the magnetocaloric properties of the material can be effectively modified to match the magnetocaloric application requirements of different temperature ranges (such as room temperature and high temperature), demonstrating good prospects for industrialization.

[0031] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A sol-gel preparation method for a perovskite rare earth manganese oxide magnetocaloric material, characterized in that, Includes the following steps: S1 precursor solution preparation: Based on the stoichiometric ratio of the target perovskite rare earth manganese oxide, accurately weigh the corresponding metal salt raw materials, dissolve them in deionized water, and continuously stir under heating conditions until a transparent and clear mixed solution of metal ions is formed. S2 gelation transformation: Citric acid as a chelating agent and ethylene glycol as an esterification crosslinking agent are added sequentially to the mixed solution obtained in S1. The molar ratio of metal ions, citric acid and ethylene glycol is controlled. The system is stirred at a constant temperature of 60~80℃ to allow the system to undergo chelation and polyesterification reactions, and gradually transform into a homogeneous, transparent wet gel. S3 Drying treatment: The wet gel obtained in S2 is vacuum dried to remove free water and some bound water, and a fluffy dry gel precursor is obtained. S4 Pre-calcination to remove organic matter: The dry gel precursor is placed in a muffle furnace for low-temperature pre-calcination to decompose and remove residual organic components. After cooling, it is ground to obtain pre-calcined powder. S5 High-temperature crystallization: The pre-calcined powder is placed in a muffle furnace and calcined at 900~1100℃ to fully crystallize the product into a pure phase perovskite structure. After cooling in the furnace, it is ground to obtain the perovskite rare earth manganese oxide powder.

2. The preparation method according to claim 1, characterized in that: In step S1, the raw material used to provide metal ions is selected from one or more of the following salts: metal nitrates, metal acetates, and metal sulfates.

3. The preparation method according to claim 1, characterized in that: In step S2, the molar ratio of citric acid, ethylene glycol, and metal ions is controlled at (1.5~4):(3~6):

1.

4. The preparation method according to claim 3, characterized in that: In step S2, the molar ratio of citric acid, ethylene glycol, and metal ions is specifically set to 2:4:

1.

5. The preparation method according to claim 3, characterized in that: The heating and stirring process in step S2 is carried out under a constant temperature of 75°C, and the stirring time is 5 to 6 hours.

6. The preparation method according to claim 1, characterized in that: The vacuum drying conditions in step S3 are as follows: the drying temperature is set between 100°C and 120°C, and the drying time is controlled between 10 and 15 hours.

7. The preparation method according to claim 1, characterized in that: In step S4, the dry gel precursor is placed in a muffle furnace (heating rate of 5°C / min) for pre-calcination to remove organic matter. The process conditions are: the treatment temperature is controlled in the range of 280°C to 350°C and maintained at this temperature for 2 to 4 hours. As a further preferred embodiment, the heat treatment temperature is 300°C and the duration is 3.5 hours.

8. The preparation method according to claim 1, characterized in that: The high-temperature calcination process in step S5 is carried out as follows: the muffle furnace is heated from room temperature to the target temperature range of 900°C to 1100°C at a heating rate of 5°C / min, and then held at this temperature range for 8 to 12 hours to allow the product to fully crystallize.

9. The preparation method according to claim 8, characterized in that: The calcination temperature in step S5 is selectively set based on the target magnetic transition temperature range; Among them, the powder prepared by calcining at 950℃ is suitable as a magnetorheological material in the room temperature range. When calcined at 1050℃, the resulting powder is suitable as a magnetorheological material in the high-temperature zone.

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