Preparation method of iron-based soft magnetic composite material

Abstract

The invention belongs to the field of material preparation, and particularly relates to a preparation method of an iron-based soft magnetic composite material, which mainly comprises the following steps: coating the surface of iron powder by using an improved Pechini sol-gel method, and then preparing a two-stage coating layer with authigenic Fe3O4 as a core and nanoscale MgO as a shell layer on the surface of the iron-based powder by drying and staged sintering, and the resistivity of the powder is greatly improved. The soft magnetic composite powder core is prepared through pressing and sintering processes, and the resistivity of the coated powder core is 3 orders of magnitude higher than that of the uncoated powder core. According to the soft magnetic composite material prepared through the method, the powder core resistivity can be guaranteed, the saturation magnetization intensity of the powder core can be guaranteed, the soft magnetic composite magnetic core with the high resistivity can reduce magnetic loss generated under medium-high frequency, excellent magnetic performance is achieved, development of the new energy industry is greatly promoted, the motor performance is improved, and energy loss is reduced.

Description

Technical Field

[0001] This invention belongs to the field of materials technology, specifically relating to a method for preparing an iron-based soft magnetic composite material. Background Technology

[0002] Electric motors are indispensable energy-using machines in daily life, ubiquitous in industrial production, and are among the most power-consuming devices. With the rise of new energy vehicles in recent years, electric motors, as the "heart" of these vehicles, provide power, making the selection of their materials particularly important. Currently, the main material used in electric motors is traditional silicon steel sheets. However, due to the poor three-dimensional formability of silicon steel sheets, processing difficulties, easy material waste, low resistivity, and high eddy current losses, they are difficult to apply in high-speed motors.

[0003] Soft magnetic composite materials (SMC), also known as "insulating coated iron powder," are widely used in the manufacture of soft magnetic cores for motors and other electronic devices due to their excellent soft magnetic properties, such as high saturation magnetization, high resistivity, and low losses at medium to high frequencies. With the development of high-speed motors and increasing motor speeds, power electronic devices are gradually moving towards higher frequencies, smaller sizes, and higher conversion efficiency. To meet these development needs, higher requirements are being placed on the soft magnetic properties of SMC. Currently, a key technical challenge for SMC is the high eddy current losses generated at medium to high frequencies. Summary of the Invention

[0004] In view of this, the present invention discloses a method for preparing an iron-based soft magnetic composite material, which uses a highly insulating coating material and a suitable coating process to reduce eddy current losses in medium- and high-frequency applications. The method specifically includes the following steps:

[0005] S1. Preparation of iron powder;

[0006] S2. Weigh magnesium nitrate and dissolve it in deionized water. Stir with a strong magnetic stirrer for 30 minutes until the magnesium nitrate is completely dissolved to form a magnesium nitrate solution.

[0007] Tartaric acid was then dissolved in deionized water and stirred for 1 hour to obtain a tartaric acid solution. The molar ratio of tartaric acid in the tartaric acid solution to magnesium nitrate in the magnesium nitrate solution was 1:1. When the molar ratio was less than 1:1, the reaction between the metal ions and the complexing agent was incomplete, resulting in poor sol uniformity. When the molar ratio was greater than 1:1, it would lead to an increase in the introduced carbon content, which would increase the decomposition of organic matter in subsequent heat treatment and affect the surface quality of the film.

[0008] Magnesium nitrate solution and tartaric acid solution were mixed and stirred for 0.5 h to obtain a sol; then glycerol was added to the above sol and stirred vigorously in a paraffin oil bath for 1 h at a temperature of 110~120℃ to obtain the sol system.

[0009] The purpose of this study is to allow the carboxyl groups in tartaric acid to undergo an esterification reaction with the hydroxyl groups in glycerol, thereby fixing the metal ions at the atomic level within the cross-linked polymer network and further stabilizing the sol system.

[0010] Compared to solid-phase mixing, the traditional sol-gel method has better uniformity and improves the coating effect of iron powder. However, it has certain limitations. Although the mixing uniformity is better than that of the solid-phase method, there are technical problems of microscopic inhomogeneity between colloidal particles. The stability of the sol decreases, resulting in dry gels, films, block materials, etc., which are prone to cracking during the drying process (supercritical drying is required to obtain aerogels).

[0011] This patent employs a sol-gel method that utilizes metal ion complexation combined with an organic polymer network. By using an esterification polymerization reaction between α-hydroxy acid (tartaric acid) and polyol (propionic acid alcohol), metal ions are atomically immobilized within the cross-linked polymer network. This method, with its superior uniformity, solves the problems encountered by traditional methods in preparing highly uniform oxide powders.

[0012] S3. Add the surfactant solution to the stable sol system obtained in step S2, and place it in a water bath and stir for 0.5 h at a water bath temperature of 90~95℃ to obtain the final sol system.

[0013] S4. Mix the iron powder with the final sol system to coat the surface of the iron powder with the sol system, and then put the coated iron powder into a vacuum drying oven to dry at a temperature of 90~100℃ for 6~7 hours to obtain precursor dry gel coated iron powder.

[0014] The formation of a dry gel requires solvent evaporation, followed by shrinkage and cross-linking of the gel network. Drying at temperatures below 90°C will result in an incomplete process, leading to a loose gel structure with low strength. Drying at temperatures above 100°C will damage the coating structure, causing the solvent (especially water) to vaporize violently and generate enormous vapor pressure. This pressure will rupture the fragile gel network from the inside, causing cracks and pores in the coating layer.

[0015] S5. The precursor dry gel-coated iron powder obtained in step S4 is placed in a tube furnace for sintering. A two-stage sintering process is used. First, the temperature is raised from room temperature to 350-450°C at a heating rate of 2-3.5°C / min and held for 1-1.5 hours to prepare an Fe3O4 thin film. Then, the temperature is raised to 550-570°C at a heating rate of 1-5°C / min and held for 1.5-2 hours to prepare nanoscale MgO thin film crystals, ultimately preparing a bi-stage coated soft magnetic composite powder. The bi-stage coated soft magnetic composite powder is then washed three times with deionized water and ethanol, followed by drying.

[0016] Precursor dry gels typically contain a large amount of water of crystallization, hydroxyl groups, and undecomposed organic matter / nitrates. Excessive heating during the first-stage sintering process can cause these substances to decompose rapidly in a short time, generating large amounts of water vapor, CO2, and NO. x When gases rapidly escape from the surface of iron powder, they can cause an "explosion," shattering the forming solid framework and resulting in numerous cracks and pores within the surface. This can even cause the film itself to shatter, leading to a decrease in powder resistivity and a poorer coating effect. Appropriately increasing the temperature allows sufficient time for the gases produced by decomposition to escape, forming a dense film structure. A slower rate and longer heating time significantly reduce production efficiency and increase energy costs. If the first-stage holding temperature is too low or the time is too short, a continuous film cannot be formed; if the first-stage holding temperature is too high or the time is too long, Fe2O3 with even lower resistivity and weaker magnetic properties will be generated, or even lead to complete particle oxidation and a sharp decline in performance. Higher temperatures result in faster oxidation rates, but also more complex Fe products.

[0017] Before the first stage of heating reaches the temperature range of 350℃~450℃, the Mg transformation process of the final sol system is as follows: Final sol system → Dry gel film ([Mg(TA)) 2- Complex) → Low-temperature annealing at around 250°C (amorphous basic magnesium carbonate + carbon) → Medium-temperature annealing at around 350°C, below the crystallization temperature, amorphous MgO / Mg(OH)2 mixture, Mg mainly exists in the form of amorphous magnesium oxide and / or magnesium hydroxide mixture.

[0018] The excessively rapid heating rate in the second-stage sintering process leads to the formation of numerous small, disordered grains. These grains compete with each other for growth, ultimately resulting in a microstructure with uneven grain size and numerous grain boundaries. Too many grain boundaries become accumulation points for defects and impurities, reducing the overall quality of the film. Furthermore, the rapid temperature increase causes organic matter to decompose drastically within a very narrow temperature range, generating a large amount of gas. Sacrificing some time in the heating process can result in a significant improvement in film quality.

[0019] The temperature is controlled within the range of 550℃ to 570℃. This ensures the smooth formation of the MgO film and prevents the Fe reactant Fe3O4 from decomposing.

[0020] Through the above process, the final structure formed is a core-shell nanoparticle or core-shell composite film with Fe3O4+MgO (Fe3O4 as the core and nano-sized oxygen MgO as the shell).

[0021] A stable sol environment means that conditions such as pH and ion concentration are controllable, which can promote the uniform and dense growth of the initial oxide film, rather than forming a loose and irregular rust layer. The preparation of a stable sol system is not only a key prerequisite for forming a high-quality Fe3O4 layer, but also a decisive factor in the successful construction of the entire Fe3O4 / MgO bilayer film structure.

[0022] Fe3O4 itself is a ferrite with high resistivity, but it is still a magnetic material. The first layer of Fe3O4 can be formed on the surface of iron powder through a mild oxidation process. This oxide layer can effectively isolate direct contact between metal particles. While the Fe3O4 layer is magnetic, it participates in magnetic conduction, reducing the proportion of non-functional volume and improving the overall saturation magnetization and permeability of the composite material.

[0023] The pre-formed Fe3O4 crystal nuclei provide excellent heterogeneous nucleation sites for MgO crystallization. This greatly promotes the crystallization process of MgO, enabling it to crystallize at lower practical temperatures and at a faster rate, forming a denser shell. The dense MgO shell acts as an excellent physical and chemical barrier. It prevents the internal Fe3O4 nuclei from being further oxidized to Fe2O3 (such as γ-Fe2O3 or α-Fe2O3), thus stabilizing the ferrimagnetism of Fe3O4. Amorphous MgO tends to undergo heterogeneous nucleation on the surface of existing, well-crystallized Fe3O4 nuclei. This is because the energy barrier (nucleation energy) required for nucleation on an existing solid surface is much lower than that for spontaneous nucleation in solution (homogeneous nucleation). The newly formed MgO grains use the Fe3O4 nuclei as "seeds" or "templates," growing, connecting, and eventually crystallizing on their surface to form a complete MgO shell enveloping the Fe3O4 nuclei.

[0024] During the second heating step, the existing amorphous MgO matrix "confins" the Fe3O4 cores within a localized space. This effectively inhibits the Ostwald ripening and aggregation of Fe3O4 nanoparticles at high temperatures. Without this protective layer, Fe3O4 nanoparticles easily grow at high temperatures, leading to changes in magnetism and a decrease in specific surface area.

[0025] S6. Then, the double-coated soft magnetic composite powder is pressed and sintered to obtain the iron-based soft magnetic composite material.

[0026] This invention cleverly balances the seemingly contradictory requirements of "insulation" and "magnetic permeability" through its Fe3O4 and MgO dual-layer coating strategy. The inner Fe3O4 layer acts as a magnetic transition layer, reducing magnetic dilution and ensuring high permeability, while the outer MgO layer, as the top-level insulator, provides extremely high resistivity and thermal / mechanical stability. During powder pressing, the rigid inorganic MgO layer withstands pressure better than the soft organic layer, making it less prone to cracking under high pressure, thus ensuring the integrity of the internal insulation network of the pressed magnetic core. The Fe3O4 layer, as the inner layer, also provides good adhesion and a certain buffering effect. The dual-layer insulation barrier (Fe3O4 + MgO) together constitutes a thicker, denser, and more stable insulation layer, perfectly isolating the metal particles from each other. This significantly improves the bulk resistivity of the entire composite material, thereby minimizing eddy current losses in alternating magnetic field applications (such as AC motors).

[0027] MgO has a melting point as high as ~2852°C, far exceeding the Curie temperature and operating temperature of most soft magnetic alloys. This means that the coating layer will not decompose, melt, or fail during subsequent powder heat treatment (such as stress annealing) and device operation, ensuring stable and reliable performance. MgO is chemically stable at room temperature and does not readily react with internal magnetic particles or the external environment, effectively preventing oxidation and corrosion of the magnetic powder and improving the long-term reliability of the material. Because eddy current losses are effectively suppressed, soft magnetic composite materials can operate at higher frequencies without overheating. This makes nano-MgO-coated SMCs highly suitable for high-frequency power electronic devices, such as high-frequency transformers, electromagnetic interference (EMI) filters, power inductors in switching power supplies, and motors for new energy vehicles.

[0028] The nano-MgO coating can act as a lubricant, reducing friction and wear of the powder during compression molding (a key step in SMC manufacturing). The smoother particle surface helps to achieve higher green density and compact strength during compression, reducing the formation of microcracks and thus producing more robust and complex core shapes.

[0029] In an alternating magnetic field, eddy currents are generated in the magnetic powder core, resulting in both skin effect and increased magnetic loss. Eddy current loss is expressed by equation (1):

[0030] = (1)

[0031] In the formula: Here, C is the eddy current loss, C is the proportionality constant, B is the magnetic flux density, f is the frequency, ρ is the resistivity, and d is the effective diameter of the powder particles. The insulating coating of the powder particles can reduce the effective particle size d and significantly increase the resistivity ρ of the magnetic material, making it an effective measure to reduce eddy current losses.

[0032] As a supplement to the technical solution of this invention, step S1, the preparation of iron powder specifically includes the following steps: First, iron powder with an average particle size of 40~50μm is prepared using the atomization method in rapid solidification technology. Then, the iron powder is placed in a box-type resistance furnace and heated from room temperature to 450~500℃ at a heating rate of 5℃ / min, and held at that temperature for 1 hour to anneal the iron powder. Annealing can decompose or volatilize organic contaminants (such as grease, adsorbed hydrocarbons) on the surface of the iron powder. These contaminants usually reduce surface energy and hinder wetting. After removal, the surface energy may increase, thereby improving wettability. This results in a thinner and more uniformly distributed liquid film forming on the surface of the iron powder coated by the sol-gel method, which, after sintering, forms a more uniform and dense insulating layer and improves its bonding performance with the iron powder.

[0033] Secondly, iron powder may experience lattice distortion and internal stress during atomization, leading to work hardening. Annealing softens the powder through recrystallization, improving its plasticity and facilitating higher green body density and strength during subsequent pressing. It can also alleviate residual stress within the powder particles, preventing cracks or deformation caused by uneven stress during pressing, which would affect the density of the iron powder core.

[0034] As a supplement to the technical solution of the present invention, in step S2, the mass fraction of magnesium nitrate in the magnesium nitrate solution is greater than 0 wt% and less than or equal to 3 wt%.

[0035] As a supplement to the technical solution of this invention, in step S2, the volume ratio of glycerol to sol is 1~1.5:10. After mixing glycerol and sol, an appropriate amount of ammonia is added to the sol to make the pH value neutral or weakly alkaline (pH 7-8). This ensures that the Fe3O4 layer prepared subsequently is neutral or weakly alkaline, reducing chemical corrosion and obtaining a stable sol system. A ratio of propionic acid alcohol to sol lower than 1~1.5:10 will result in incomplete esterification of propionic acid alcohol with tartrate, leading to decreased stability; a ratio higher than 1~1.5:10 will increase the organic content, affecting the surface quality of the coating.

[0036] As a supplement to the technical solution of the present invention, the method for preparing the surfactant in step S3 is as follows: polyvinylpyrrolidone (PVP, model K30) and deionized water are prepared into a surfactant solution at a mass-volume ratio of (1~3):(97~99), wherein the unit of measurement of polyvinylpyrrolidone is g, the unit of measurement of deionized water is ml, and the ratio means that 1~3g of polyvinylpyrrolidone corresponds to 97~99ml of deionized water, and the two are directly mixed to form a solution.

[0037] The surfactant can also be prepared by mixing hydroxyethyl cellulose powder (HEC, model HEC-100) with deionized water at a mass-volume ratio of (1~3):(97~99) to form a surfactant solution. The unit of measurement for polyvinylpyrrolidone is g, and the unit of measurement for deionized water is ml. The ratio means that 1~3g of polyvinylpyrrolidone corresponds to 97~99ml of deionized water. The two are directly mixed to form a solution.

[0038] The optimal surfactant concentration is the critical micelle concentration. Below this concentration, surfactant cannot fully adsorb onto the surface of the sol particles, leading to van der Waals forces dominating the particle structure. This results in uneven coating or agglomeration of the iron powder surface. Excessive surfactant forms micelles, which may disrupt the stability of the sol particles through micellar dissolution (e.g., micelle encapsulation of particles causing flocculation). This causes some surfactant to precipitate and agglomerate, significantly affecting the uniformity of the sol. Therefore, properly controlling the surfactant concentration is crucial for the sol system.

[0039] As a supplement to the technical solution of the present invention, in step S3, the volume ratio of the stable sol system to the surfactant solution is 6~10:1.

[0040] An excessively high ratio of stable sol-gel system to surfactant can lead to insufficient surfactant molecules in the sol, preventing the formation of a complete coating layer on the surface of newly formed nanoparticles. This results in high surface tension in the sol, affecting wetting performance. Conversely, an excessively low ratio can cause the surfactant to self-assemble into various complex liquid crystal phases (such as layered phases, hexagonal phases, etc.) instead of simple spherical micelles or microemulsions. This also leads to a complex reaction environment, difficult purification, and increased costs.

[0041] As a supplement to the technical solution of this invention, in step S4, the mass-to-volume ratio of iron powder to the final sol system is 20:7, where the unit of measurement for iron powder is g, and the unit of measurement for the stable sol system is ml. This ratio ensures that the iron powder can be completely mixed with the final sol system.

[0042] As a supplement to the technical solution of this invention, the pressing process of the double-coated soft magnetic composite powder in step S6 specifically involves placing 1% aluminum dihydrogen phosphate as a lubricant into a powder mixer along with the double-coated soft magnetic composite powder, and mixing for 5 hours until fully homogeneous. Subsequently, the mixed double-coated soft magnetic composite powder is loaded into a ring-shaped pressing mold and pressed to form a composite block. Before pressing, a lubricant—a mixture of 3% (mass fraction) polyvinyl alcohol, 2% (mass fraction) graphite, and gasoline—is applied to the surface of the mold to reduce friction during pressing and improve demolding convenience.

[0043] As a supplement to the technical solution of the present invention, the sintering process in step S6 is specifically as follows: the pressed composite block is placed in a tube furnace for sintering. The sintering process is divided into two stages. First, the temperature is raised to 350~400℃ at a heating rate of 5℃ / min and held for 1~1.5h to remove gasoline and some polyvinyl alcohol from the surface. Then, the temperature is raised to 540~560℃ at a heating rate of 5℃ / min and held for 1~1.5h to prepare the final soft magnetic composite iron material.

[0044] Beneficial effects: This invention, through the Pechini sol-gel method combined with a two-stage coating process, precise control of key parameters, and multi-stage process optimization, effectively solves the core problems in the preparation of traditional soft magnetic materials, such as "difficulty in balancing insulation and magnetic permeability" and "poor material uniformity." It achieves significant improvements in material performance, process feasibility, and economy, as detailed below:

[0045] 1. The double-layer coated soft magnetic composite powder prepared in this invention uses Fe3O4 as the inner magnetic transition layer, which reduces the magnetic dilution effect and fully guarantees the high magnetic permeability of the material, avoiding the magnetic weakening caused by a single insulating layer. The outer nano-sized MgO layer acts as a dense insulating barrier, which can efficiently isolate metal particles, significantly improve the bulk resistivity of the material, and minimize eddy current losses in an alternating magnetic field. This solves the contradiction between "high magnetic permeability" and "high insulation" that traditional single-layer coating cannot achieve simultaneously. At the same time, the dense MgO shell can physically isolate air, preventing the inner Fe3O4 from further oxidizing into Fe2O3 (such as γ-Fe2O3, α-Fe2O3), which has weaker magnetic properties. Meanwhile, the Fe3O4 crystal nuclei provide heterogeneous nucleation sites for MgO, reducing the nucleation energy barrier of MgO and enabling it to crystallize rapidly at lower temperatures to form a complete shell, further enhancing insulation and magnetic stability.

[0046] 2. In the final sol-gel preparation process, the metal ions are atomically fixed in the cross-linked polymer network through the esterification polymerization reaction of tartaric acid and glycerol. This completely solves the problems of "microscopic inhomogeneity of colloidal particles", "poor sol stability" and "easy cracking during drying" that exist in the traditional sol-gel method. Uniform and stable sol can be prepared without relying on complex supercritical drying process. While simplifying the process, it ensures that the subsequent coating layer has uniform thickness and is defect-free.

[0047] The Pechini process achieves the crucial role of "uniform dispersion of precursors"—it utilizes the esterification polymerization reaction of tartaric acid and glycerol to disperse Mg... 2+Uniform dispersion and fixation within the cross-linked polymer network with atomic-level precision completely avoids localized enrichment or vacancies of Mg elements, providing a direct precursor basis for the uniform formation of the subsequent amorphous MgO matrix. This "uniformity" imparted by the sol is directly inherited in subsequent steps: after drying to form a precursor dry gel in step S4 and sintering to transform it into an amorphous MgO matrix in the first stage of step S5, MgO can completely encapsulate the Fe3O4 core surface in a continuous and uniform spatial network structure.

[0048] Therefore, in the second-stage sintering, the amorphous MgO matrix derived from the uniformity of the sol can "confine" the Fe3O4 cores in a local space, effectively inhibiting the Ostwald ripening and agglomeration of Fe3O4 nanoparticles at high temperatures, avoiding magnetic attenuation and decrease in specific surface area caused by particle growth, and ensuring the uniformity of the material's microstructure. Attached Figure Description

[0049] Figure 1 XRD pattern of the bi-stage coated soft magnetic composite powder prepared in Example 2

[0050] Figure 2 (a) Low-magnification SEM image of the bi-stage coated soft magnetic composite powder prepared in Example 2.

[0051] Figure 2 (b) High-magnification SEM image of the bi-stage coated soft magnetic composite powder prepared in Example 2.

[0052] Figure 3 (a) SEM image of the graded coated soft magnetic composite powder prepared in Example 2.

[0053] Figure 3 (b) Fe element distribution in the SEM image of the bi-level coated soft magnetic composite powder prepared in Example 2.

[0054] Figure 3 (c) Mg element distribution in the SEM image of the bi-stage coated soft magnetic composite powder prepared in Example 2.

[0055] Figure 3 (d) The distribution of O element in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared in Example 2.

[0056] Figure 4 (a) Low-magnification SEM image of the fracture surface of the iron-based soft magnetic composite material prepared in Example 2.

[0057] Figure 4 (b) High-magnification SEM image of the fracture surface of the iron-based soft magnetic composite material prepared in Example 2.

[0058] Figure 5The resistivity saturation magnetization of iron-based soft magnetic composite materials with different MgO contents is shown as a line graph.

[0059] Figure 6 Line graph showing the density of iron-based soft magnetic composite materials under different compression pressures.

[0060] Figure 7 Line graphs showing the resistivity and saturation magnetization of iron-based soft magnetic composite materials under different pressing pressures.

[0061] Figure 8 (a) SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 3.

[0062] Figure 8 (b) Fe element distribution in the SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 3;

[0063] Figure 8 (c) The distribution of Mg element in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 3;

[0064] Figure 8 (d) shows the distribution of O element in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 3;

[0065] Figure 9 SEM images of the bi-level coated soft magnetic composite powder prepared in Comparative Example 4, obtained from surface scanning analysis.

[0066] Figure 10 (a) SEM image of the bi-level coated soft magnetic composite powder prepared in Comparative Example 8;

[0067] Figure 10 (b) Fe element distribution in the SEM image of the bi-level coated soft magnetic composite powder prepared in Comparative Example 8;

[0068] Figure 10 (c) Mg element distribution in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared for Comparative Example 8.

[0069] Figure 11 SEM images of the bi-level coated soft magnetic composite powder prepared in Comparative Example 6, obtained from surface scanning analysis.

[0070] Figure 12 SEM images of the bi-level coated soft magnetic composite powder prepared in Comparative Example 8, obtained from surface scanning analysis.

[0071] Figure 13 (a) SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 1;

[0072] Figure 13 (b) Fe element distribution in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 1;

[0073] Figure 13 (c) The distribution of Mg element in the surface scan SEM image of the bi-stage coated soft magnetic composite powder prepared in Comparative Example 1;

[0074] Figure 13 (d) shows the distribution of O element in the surface scan SEM image of the bi-level coated soft magnetic composite powder prepared in Comparative Example 1. Detailed Implementation

[0075] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0076] Example 1: A method for preparing an iron-based soft magnetic composite material, comprising the following steps:

[0077] S1. First, iron powder with an average particle size of 50μm is prepared by atomization in rapid solidification technology. Then, the iron powder is placed in a box-type resistance furnace and heated from room temperature to 480℃ at a heating rate of 5℃ / min and held for 1 hour to perform annealing treatment on the iron powder.

[0078] S2. Tartaric acid solution and magnesium nitrate solution are mixed, with magnesium nitrate solution having a mass fraction of 0.5 wt%, to prepare a sol. The molar ratio of tartaric acid to magnesium nitrate is 1:1. Then, glycerol is added to the sol and stirred vigorously in a paraffin oil bath at 110°C for 1 hour. The volume ratio of glycerol to the sol is 1:10. A sol system is obtained. After mixing glycerol and the sol, ammonia is added to the sol to make the pH neutral.

[0079] S3. Weigh 1g of hydroxyethyl cellulose powder (HEC, model HEC-100) or 1g of polyvinylpyrrolidone (PVP, model K30), mix it with 49ml of deionized water, and stir electromagnetically for 12h at a stirring speed of 80r / min to prepare a surfactant solution. Add the surfactant solution to the sol system obtained in step S2, and stir in a water bath at 92℃ for 0.5h to obtain the final sol system. The volume ratio of the sol system to the surfactant solution is 6:1.

[0080] S4. Mix the iron powder with the final sol system at a mass-to-volume ratio of 20:7. In this ratio, the unit of measurement for the iron powder is g, and the unit of measurement for the stable sol system is ml. Coat the surface of the iron powder with the sol system, then place the coated iron powder in a vacuum drying oven at 95℃ for 6 hours to obtain precursor dry gel-coated iron powder.

[0081] S5. The precursor dry gel-coated iron powder is placed in a tube furnace for sintering. The sintering adopts a two-stage sintering process. First, it is heated from room temperature to 370℃ at a heating rate of 2℃ / min and held for 1 hour. Then, it is heated to 550℃ at a heating rate of 2℃ / min and held for 2 hours. Finally, a soft magnetic composite powder with dual-stage coating is prepared. Then, the soft magnetic composite powder with dual-stage coating is washed three times with deionized water and ethanol and then dried.

[0082] S6. Iron-based soft magnetic composite material is obtained by pressing and sintering the double-coated soft magnetic composite powder.

[0083] The pressing process involves placing 1% aluminum dihydrogen phosphate as a lubricant into a powder mixer along with the double-coated soft magnetic composite powder and mixing for 5 hours until fully homogeneous. Subsequently, the mixed double-coated soft magnetic composite powder is loaded into a ring-shaped pressing mold and pressed to form a composite block with a pressing force of 1000 MPa.

[0084] The sintering process is as follows: first, the temperature is raised to 370℃ at a heating rate of 5℃ / min and held for 1.2h; then, the temperature is raised to 550℃ at a heating rate of 5℃ / min and held for 1h to prepare the final soft magnetic composite iron material.

[0085] Example 2: A method for preparing an iron-based soft magnetic composite material, comprising the following steps:

[0086] S1. First, iron powder with an average particle size of 50μm is prepared by atomization in rapid solidification technology. Then, the iron powder is placed in a box-type resistance furnace and heated from room temperature to 490℃ at a heating rate of 5℃ / min and held for 1 hour to perform annealing treatment on the iron powder.

[0087] S2. Tartaric acid solution and magnesium nitrate solution are mixed, with the magnesium nitrate solution containing 2 wt% magnesium nitrate, to prepare a sol. The molar ratio of tartaric acid to magnesium nitrate is 1:1. Glycerol is then added to the sol and vigorously stirred in a paraffin oil bath at 115°C for 1 hour. The volume ratio of glycerol to the sol is 1.2:10. A sol system is obtained. After mixing glycerol and the sol, ammonia is added to the sol to neutralize the pH.

[0088] S3. Weigh 1.2g of hydroxyethyl cellulose powder (HEC, model HEC-100) or 1.2g of polyvinylpyrrolidone (PVP, model K30), mix it with 48.8ml of deionized water, and stir electromagnetically for 12h at a stirring speed of 90r / min to prepare a surfactant solution. Add the surfactant solution to the sol system obtained in step S2, and stir in a water bath at 93℃ for 0.5h to obtain the final sol system. The volume ratio of the sol system to the surfactant solution is 8:1.

[0089] S4. Mix the iron powder with the final sol system at a mass-to-volume ratio of 20:7. In this ratio, the unit of measurement for iron powder is g, and the unit of measurement for the stable sol system is ml. Coat the surface of the iron powder with the sol system, then place the coated iron powder in a vacuum drying oven at 95℃ for 7 hours to obtain precursor dry gel-coated iron powder.

[0090] S5. The precursor dry gel-coated iron powder is placed in a tube furnace for sintering. The sintering adopts a two-stage sintering process. First, it is heated from room temperature to 400℃ at a heating rate of 2℃ / min and held for 1.3h. Then, it is heated to 560℃ at a heating rate of 3℃ / min and held for 1.5h. Finally, a soft magnetic composite powder with dual-stage coating is prepared. Then, the soft magnetic composite powder with dual-stage coating is washed three times with deionized water and ethanol and then dried.

[0091] S6. Iron-based soft magnetic composite material is obtained by pressing and sintering the double-coated soft magnetic composite powder.

[0092] The pressing process involves placing 1% aluminum dihydrogen phosphate as a lubricant into a powder mixer along with the double-coated soft magnetic composite powder and mixing for 5 hours until fully homogeneous. Subsequently, the mixed double-coated soft magnetic composite powder is loaded into a ring-shaped pressing mold and pressed to form a composite block with a pressing force of 1100 MPa.

[0093] The sintering process is as follows: first, the temperature is raised to 380℃ at a heating rate of 5℃ / min and held for 1.3h; then, the temperature is raised to 545℃ at a heating rate of 5℃ / min and held for 1.3h to prepare the final soft magnetic composite iron material.

[0094] like Figure 1 The image shown is the XRD pattern of the bi-stage coated soft magnetic composite powder prepared in step S5 of this embodiment. The phases are mainly composed of Fe, MgO, and Fe3O4.

[0095] like Figure 2 The image shown is the SEM image of the powder after double-stage coating in this embodiment. Figure 2 (a) is a low-magnification SEM image. Figure 2 (b) is a high-magnification SEM image. (From...) Figure 2 It can be seen that the powder surface is surrounded by fine nano-MgO particles, and there is a Fe3O4 layer between the iron powder and MgO.

[0096] like Figure 3 As shown in (a), this is the SEM image of the dual-stage coated soft magnetic composite powder prepared in step S5 of this embodiment. Figure 3 (b) Fe element distribution in the SEM image of the double-coated soft magnetic composite powder obtained by surface scanning analysis. Figure 3 (c) SEM image of the double-coated soft magnetic composite powder, showing the distribution of Mg element. Figure 3 (d) shows the distribution of O element in the SEM image of the double-coated soft magnetic composite powder. Combined with EDS surface scanning, it can be seen that the main components of the powder are Fe, Mg, and O. It can be seen that Mg and O elements are uniformly distributed on the surface of Fe powder without obvious separating interfaces, further verifying the existence of Fe3O4 and MgO coatings.

[0097] like Figure 4 The image shown is a fracture surface morphology diagram of the iron-based soft magnetic composite material after sintering in step S6 of this embodiment. Figure 4 (a) is a low-magnification SEM image. Figure 4 (b) is a high-magnification SEM image, showing nanoscale MgO particles at the fracture surface. The MgO coating on the surface remains intact after pressing and sintering, without any excessive damage. The nanoscale MgO particles are mainly due to the modified Pechini sol-gel technology, a nanotechnology that can prepare nanoscale coatings at low temperatures and in a short time. The double-coated soft magnetic composite powder core has higher resistivity.

[0098] Example 3: A method for preparing an iron-based soft magnetic composite material, comprising the following steps:

[0099] S1. First, iron powder with an average particle size of 50μm is prepared by atomization in rapid solidification technology. Then, the iron powder is placed in a box-type resistance furnace and heated from room temperature to 500℃ at a heating rate of 5℃ / min and held for 1 hour to perform annealing treatment on the iron powder.

[0100] S2. Tartaric acid solution and magnesium nitrate solution are mixed, with the magnesium nitrate solution having a magnesium nitrate mass fraction of 3 wt%, to obtain a sol. The molar ratio of tartaric acid to magnesium nitrate is 1:1. Then, glycerol is added to the above sol and stirred vigorously in a paraffin oil bath at 120°C for 1 hour to obtain the sol system. The volume ratio of glycerol to the above sol is 1.5:10. Ammonia is added to the sol to make the pH neutral.

[0101] S3. Weigh 1.5g of hydroxyethyl cellulose powder (HEC, model HEC-100) or 1.5g of polyvinylpyrrolidone (PVP, model K30), mix it with 48.5ml of deionized water, and stir electromagnetically for 12h at a stirring speed of 120r / min to prepare a surfactant solution. Add the surfactant solution to the sol system obtained in step S2, and stir in a water bath at 95℃ for 0.5h to obtain the final sol system. The volume ratio of the sol system to the surfactant solution is 10:1.

[0102] S4. Mix the iron powder with the final sol system at a mass-to-volume ratio of 20:7. In this ratio, the unit of measurement for iron powder is g, and the unit of measurement for the stable sol system is ml. Coat the iron powder surface with the sol system, then place the coated iron powder in a vacuum drying oven at 100℃ for 6.5 hours to obtain precursor dry gel-coated iron powder.

[0103] S5. The precursor dry gel-coated iron powder is placed in a tube furnace for sintering. The sintering adopts a two-stage sintering process. First, it is heated from room temperature to 450℃ at a heating rate of 3.5℃ / min and held for 1.5h. Then, it is heated to 570℃ at a heating rate of 4℃ / min and held for 2h. Finally, a soft magnetic composite powder with dual-stage coating is prepared. Then, the soft magnetic composite powder with dual-stage coating is washed three times with deionized water and ethanol and then dried.

[0104] S6. Iron-based soft magnetic composite material is obtained by pressing and sintering the double-coated soft magnetic composite powder.

[0105] The pressing process involves placing 1% aluminum dihydrogen phosphate as a lubricant into a powder mixer along with the double-coated soft magnetic composite powder and mixing for 5 hours until fully homogeneous. Subsequently, the mixed double-coated soft magnetic composite powder is loaded into a ring-shaped pressing mold and pressed to form a composite block with a pressing force of 1200 MPa.

[0106] The sintering process is as follows: first, the temperature is raised to 400℃ at a heating rate of 5℃ / min and held for 1.5h; then, the temperature is raised to 560℃ at a heating rate of 5℃ / min and held for 1.5h to prepare the final soft magnetic composite iron material.

[0107] Example 4

[0108] The difference between this embodiment and Embodiment 2 is that in step S2, the mass fraction of magnesium nitrate in the magnesium nitrate solution is 1.5 wt%.

[0109] Example 5

[0110] The difference between this embodiment and Embodiment 2 is that in step S2, the mass fraction of magnesium nitrate in the magnesium nitrate solution is 1.0 wt%.

[0111] Example 6

[0112] The difference between this embodiment and embodiment 2 is that in step S6, the pressing force is 1000MPa during the pressing process.

[0113] Example 7

[0114] The difference between this embodiment and embodiment 2 is that in step S6, the pressing force is 1200MPa during the pressing process.

[0115] Comparative Example 1

[0116] The difference between this comparative example and Example 2 is that in step S2, the mass fraction of magnesium nitrate in the magnesium nitrate solution is 3.5 wt%. When the magnesium nitrate concentration increases by more than 2 wt%, although the resistivity increases, the magnetic properties decrease, and the saturation magnetization is 150 emu / g, which is less than the optimal value (214 emu / g).

[0117] Figure 13 (a) is the SEM image of the bi-stage coated soft magnetic composite powder prepared in this comparative example; Figure 13 (b) Fe element distribution in the SEM image of the double-coated soft magnetic composite powder by surface scanning analysis; Figure 13 (c) The distribution of Mg element in the surface scan SEM image of the double-coated soft magnetic composite powder; Figure 13 (d) SEM image of the double-coated soft magnetic composite powder, showing the distribution of O element. With increasing MgO mass fraction, the Fe powder surface is completely coated with MgO, resulting in a significant decrease in Fe element strength. It is evident that Fe3O4 exists only in the core, while the exterior is coated with MgO. However, increasing MgO content leads to a decrease in the density of the powder core (6.2 g / cm³). 3 This leads to a decrease in its saturation magnetization (150 emu / g).

[0118] Comparative Example 2

[0119] The difference between this comparative example and Example 2 is that in step S2, the magnesium nitrate in the magnesium nitrate solution has a magnesium nitrate mass fraction of 0 wt%, and the resistivity of the uncoated powder core is 0.032 mΩ·mm.

[0120] like Figure 5The figure shows line graphs of resistivity and saturation magnetization of the iron-based soft magnetic composite materials prepared in Examples 1-5 and Comparative Examples 1-2. When the MgO concentration is 1%, the resistivity is 81 mΩ·mm, which is 2500 times that of uncoated pure iron powder (0.032 mΩ·mm). The resistivity gradually increases with the increase of MgO concentration. When the MgO concentration is 0.5 wt%, although it has excellent saturation magnetization, the resistivity still needs to be improved. When the MgO concentration is 1 wt%, the iron-based soft magnetic composite material still maintains a good magnetization of 213 emu / g, which is close to 210 emu / g of uncoated pure iron powder. However, with the increase of MgO concentration to 2.5 wt%, the saturation magnetization of the soft magnetic composite powder core gradually decreases.

[0121] Comparative Example 3

[0122] The difference between this comparative example and Example 2 is in S2:

[0123] S2. A sol is prepared by mixing tartaric acid solution and magnesium nitrate solution, wherein the magnesium nitrate solution contains 2 wt% magnesium nitrate. The molar ratio of tartaric acid to magnesium nitrate is 1:1. The mixture is then magnetically stirred to ensure homogeneity. Glycerol is not added to the sol.

[0124] like Figure 8 As shown in (a), this is the SEM image of the bi-stage coated soft magnetic composite powder prepared in this comparative example. Figure 8 (b) Fe element distribution in the SEM image of the two-stage coated soft magnetic composite powder prepared without the addition of glycerol; Figure 8 (c) Mg element distribution in the SEM image of the two-stage coated soft magnetic composite powder prepared without the addition of glycerol; Figure 8 (d) SEM image of the O element distribution in the bi-stage coated soft magnetic composite powder prepared without glycerol. Comparative Example 3 uses the traditional sol-gel method, which is less stable than the sol-gel method proposed in this patent, and the uniformity of the prepared components is as follows: Figure 8 As shown, Mg exhibits a certain degree of segregation.

[0125] Comparative Example 4

[0126] The difference between this comparative example and Example 2 is in S2:

[0127] S2. A sol is prepared by mixing tartaric acid solution and magnesium nitrate solution, wherein the magnesium nitrate solution contains 2 wt% magnesium nitrate. The molar ratio of tartaric acid to magnesium nitrate is 1:1. Then, glycerol is added to the sol and vigorously stirred in a paraffin oil bath at 110°C for 1 hour. The volume ratio of glycerol to the sol is 2:10. Figure 9The image shown is a surface scanning electron microscope (SEM) image of the bi-stage coated soft magnetic composite powder prepared in this comparative example. The excessive use of crosslinking agent in this comparative example disrupted this kinetic stability, leading to premature instability of the sol, precipitation, or localized gelation, thus affecting the surface quality of the coating film.

[0128] Comparative Example 5

[0129] The difference between this comparative example and Example 2 is that the volume ratio of the stable sol system to the surfactant solution in step S3 is 11:1.

[0130] like Figure 10 As shown in (a), this is the SEM image of the bi-level coated soft magnetic composite powder prepared in this comparative example. Figure 10 (b) Fe element distribution in the SEM image of the double-coated soft magnetic composite powder by surface scanning analysis; Figure 10 (c) SEM image of the two-stage coated soft magnetic composite powder, showing the distribution of Mg. Insufficient surfactant leads to high surface tension in the sol, reducing its wetting and spreading ability on the substrate, resulting in a decreased coating effect and incomplete coating.

[0131] Comparative Example 6

[0132] The difference between this comparative example and Example 2 is that the volume ratio of the stable sol system to the surfactant solution in step S3 is 4:1.

[0133] like Figure 11 The image shown is a SEM image of the bilevel coated soft magnetic composite powder prepared in this comparative example. When the surfactant content is too high, and the surfactant concentration is much higher than its critical micelle concentration (CMC), a large number of micelles with varying morphologies will form in the solution. These micelles become heterogeneous centers for the sol-gel reaction, leading to an uneven gel network structure, which in turn reduces the uniformity of the coated film and degrades the surface quality due to excessive organic matter decomposition.

[0134] Comparative Example 7

[0135] The difference between this comparative example and Example 2 lies in step S4: iron powder is mixed with the final sol system at a mass-to-volume ratio of 20:7, where the unit of measurement for iron powder is g and the unit of measurement for the stable sol system is ml. The iron powder surface is coated with the sol system, and then the coated iron powder is dried in a vacuum drying oven at 80°C for 6-7 hours to obtain a precursor dry gel coated with iron powder. If the precursor curing temperature is too low, a large number of undecomposed organic groups and hydroxyl groups remain in the gel network. These substances almost inevitably lead to porous, rough, cracked, and non-dense films after sintering, and even poor bonding with the matrix. The resistivity of the sample prepared in this comparative example is 62 mΩ·mm, and the saturation magnetization is 190 emu / g, both lower than the optimal values.

[0136] Comparative Example 8

[0137] The difference between this comparative example and Example 2 lies in step S4: iron powder is mixed with the final sol system at a mass-to-volume ratio of 20:7, where the unit of measurement for iron powder is g and the unit of measurement for the stable sol system is ml. The iron powder surface is coated with the sol system, and then the coated iron powder is dried in a vacuum drying oven at 110°C for 7 hours to obtain precursor dry gel-coated iron powder.

[0138] like Figure 12 The image shown is a surface scanning electron microscope (SEM) image of the bi-stage coated soft magnetic composite powder prepared in this comparative example. Due to excessively high drying temperature, water evaporates rapidly; high temperatures cause the solvent and moisture on the film surface to evaporate quickly. The surface liquid disappears rapidly, forming a dry "shell." This uneven shrinkage generates enormous tensile stress within the film. When this stress exceeds the mechanical strength of the gel skeleton, the film cracks. If the stress exceeds the adhesion between the film and the substrate, it leads to peeling. This significantly reduces the coating effect, resulting in a resistivity of 12 mΩ·mm, far below the optimal value. Furthermore, the poor coating effect results in a density of only 6.0 g / cm³. 3 The saturation magnetization is 120 emu / g, which is less than the optimal value, and the coating effect is poor, resulting in poor surface quality.

[0139] Comparative Example 9

[0140] The difference between this comparative example and Example 2 is that the heating rate in the first stage of the sintering process in step S5 is 5°C / min. If the heating rate is too fast, pores and microcracks in the film will form conductive channels. Numerous grain boundaries and defects (especially oxygen vacancies) will become traps and transport paths for charge carriers, leading to a sharp decrease in resistivity and poor insulation performance. The resistivity is 55 mΩ·mm, which is lower than the optimal value, and the saturation magnetization is 170 emu / g.

[0141] Comparative Example 10

[0142] The difference between this comparative example and Example 2 is that the heating rate in the first stage of the sintering process in step S5 is 6°C / min. Fe3O4 and MgO have different coefficients of thermal expansion. Rapid heating will generate huge, instantaneous thermal stress between the two layers. Due to the excessively rapid heating, the material does not have time to relax this stress through mechanisms such as creep. When the stress exceeds either layer, it will cause the entire bilayer film system to crack, the resistivity to drop to 75 mΩ·mm, which is less than the optimal value, and the saturation magnetization to 175 emu / g.

[0143] Comparative Example 11

[0144] The difference between this comparative example and Example 2 is that the pressing force in step S6 is 1300 MPa. The resistivity of the obtained iron-based soft magnetic composite material is 78 mΩ·mm, and the saturation magnetization is 190 emu / g.

[0145] Comparative Example 12

[0146] The difference between this comparative example and Example 2 is that the pressing force in step S6 is 900 MPa. The resistivity of the obtained iron-based soft magnetic composite material is 122 mΩ·mm, and the saturation magnetization is 196 emu / g.

[0147] Comparative Example 13

[0148] The difference between this comparative example and Example 2 is that the pressing force in step S6 is 800 MPa. The resistivity of the obtained iron-based soft magnetic composite material is 128 mΩ·mm, and the saturation magnetization is 180 emu / g.

[0149] like Figure 6 The figures show the densities of the iron-based soft magnetic composite materials in Examples 2, 6, 7, and Comparative Examples 11 to 13 under different compressive forces. The density of the iron-based soft magnetic composite materials tends to stabilize at approximately 7.1 g / cm³ when the compressive force is between 1000 MPa and 1200 MPa. 3 The lower the density of the prepared iron-based soft magnetic composite material, the lower its saturation magnetization. With an MgO content of 2wt%, its density can reach 7.1 g / cm³. 3 7.7 g / cm³ compared to pure iron 3 The density decreased only slightly, thus effectively preserving the core properties of the material.

[0150] like Figure 7The figure shows line graphs of resistivity and saturation magnetization of iron-based soft magnetic composite materials under different compressive forces in Examples 2, 11, and 13. When the pressure is too high (1300 MPa), the hard iron powder particles undergo plastic deformation and mutual compression and friction under enormous stress. This causes the brittle MgO insulating layer covering the particle surface to be scratched, cracked, or even peeled off, resulting in a significant decrease in resistivity of 50 mΩ·mm and a saturation magnetization of 190 emu / g, both lower than the optimal values. When the pressure is too low, even with subsequent sintering, the low green density leads to insufficient sintering driving force, making it difficult to fully fill the pores through mass migration. The final sintered body still has low density and compaction, which will cause a decrease in the magnetic properties of the sample. When the compressive force is 900 MPa, the saturation magnetization is 196 emu / g, lower than the optimal value.

[0151] The resistivity of the iron-based soft magnetic composite materials prepared in the examples and comparisons was detected by the four-probe method, and the saturation magnetization was measured by a comprehensive physical testing system. The results are shown in the table below:

[0152] Statistical table of resistivity and saturation magnetization of iron-based soft magnetic composite materials

[0153]

[0154] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing an iron-based soft magnetic composite material, characterized in that, Includes the following steps: S1. Preparation of iron powder; S2. Tartaric acid solution and magnesium nitrate solution are mixed to prepare sol, wherein the molar ratio of tartaric acid to magnesium nitrate is 1:1; then glycerol is added to the above sol and the mixture is vigorously stirred in a paraffin oil bath for 1 hour at a temperature of 110~120℃ to obtain the sol system. S3. Add the surfactant solution to the sol system obtained in step S2, and place it in a water bath and stir for 0.5 h at a water bath temperature of 90~95℃ to obtain the final sol system. S4. Mix the iron powder with the final sol system so that the surface of the iron powder is coated with the sol system. Then put the coated iron powder into a vacuum drying oven and dry it at a temperature of 90~100℃ for 6~7 hours to obtain precursor dry gel coated iron powder. S5. The precursor dry gel-coated iron powder is placed in a tube furnace for sintering. The sintering adopts a two-stage sintering process. First, it is heated from room temperature to 350-450℃ at a heating rate of 2-3.5℃ / min and held for 1-1.5h. Then, it is heated to 550-570℃ at a heating rate of 1-5℃ / min and held for 1.5-2h to obtain a soft magnetic composite powder with dual-stage coating. Then, the soft magnetic composite powder with dual-stage coating is washed three times with deionized water and ethanol and then dried. S6. Iron-based soft magnetic composite material is obtained by pressing and sintering the double-coated soft magnetic composite powder.

2. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, In step S1, the preparation of iron powder specifically includes the following steps: First, using the atomization method in rapid solidification technology, an average particle size of [missing information] is prepared. The iron powder is then placed in a box-type resistance furnace and heated from room temperature to 450~500℃ at a heating rate of 5℃ / min, and held at that temperature for 1 hour to anneal the iron powder.

3. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, In step S2, the magnesium nitrate mass fraction in the magnesium nitrate solution is greater than 0 wt% and less than or equal to 3 wt%.

4. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, In step S2, the volume ratio of glycerol to sol is 1~1.5:

10. After the glycerol and sol are mixed, ammonia is added to the sol to make the pH value neutral or weakly alkaline.

5. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, The surfactant in step S3 is prepared by mixing polyvinylpyrrolidone and deionized water in a mass-volume ratio of (1~3):(97~99) to form a surfactant solution, wherein the unit of measurement for polyvinylpyrrolidone is g and the unit of measurement for deionized water is ml. Alternatively, a surfactant solution can be prepared by mixing hydroxyethyl cellulose powder with deionized water at a mass-to-volume ratio of (1~3):(97~99), wherein the unit of measurement for polyvinylpyrrolidone is g and the unit of measurement for deionized water is ml.

6. The method for preparing an iron-based soft magnetic composite material according to claim 5, characterized in that, In step S3, the volume ratio of the stable sol system to the surfactant solution is 6~10:

1.

7. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, In step S4, the mass-to-volume ratio of iron powder to the final sol system is 20:7, where the unit of measurement for iron powder is g and the unit of measurement for the stable sol system is ml.

8. The method for preparing an iron-based soft magnetic composite material according to claim 1, characterized in that, In step S6, the pressing process of the double-coated soft magnetic composite powder is as follows: 1% aluminum dihydrogen phosphate is used as a lubricant and is placed together with the double-coated soft magnetic composite powder in a powder mixer and mixed for 5 hours until fully uniform; then the mixed double-coated soft magnetic composite powder is loaded into a ring pressing mold and pressed to form a composite block with a pressing force of 1000~1200MPa.

9. The method for preparing an iron-based soft magnetic composite material according to claim 8, characterized in that, The sintering process in step S6 specifically involves heating the pressed composite block to 350-400℃ at a heating rate of 5℃ / min and holding it at that temperature for 1-1.5 hours, followed by heating it to 540-560℃ at a heating rate of 5℃ / min and holding it at that temperature for 1-1.5 hours to prepare the final soft magnetic composite iron material.

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