A method for preparing soft-hard magnetic nanometer dual-phase particles by self-propagating combustion without external ignition

Abstract

The application discloses a method for preparing soft-hard magnetic nanometer dual-phase particles by a self-propagating combustion method without external ignition, and comprises the following steps: preparation of soft magnetic phase nanoparticles, formation of a hard magnetic phase surface enrichment layer, self-propagating combustion without external ignition, and heat treatment. By moving the shell layer construction to the precursor stage, the application can reduce free nucleation and macroscopic phase separation, improve interface continuity, and provide a more direct process for the regulation of shell thickness and coverage, thereby providing a new technical path for obtaining soft-hard magnetic nanometer dual-phase particles with a soft magnetic core-hard magnetic shell.

Description

Technical Field

[0001] This invention relates to the field of nanocomposite magnetic material preparation technology, and in particular to a method for preparing soft and hard magnetic nanoparticles by a self-propagating combustion method without external ignition. Background Technology

[0002] The research focus of soft / hard magnetic dual-phase composite materials lies in integrating the high saturation magnetization of the soft magnetic phase with the large coercivity of the hard magnetic phase into the same composite system, thereby maintaining sufficient demagnetization stability while preserving magnetization capability. For such materials, the key to determining the overall magnetic properties is not merely whether the two phases coexist, but more importantly, whether the scale of the soft magnetic phase, the spacing between the soft and hard magnetic phases, and the interface connection method are suitable for forming effective exchange coupling. Existing patent literature indicates that when the hard and soft magnetic phases are simply mixed, they often exhibit independent responses, making it difficult to simultaneously obtain high coercivity and high saturation flux density. This demonstrates that the interface configuration and distribution uniformity have a decisive influence on the soft-hard magnetic composite system.

[0003] Currently, the preparation of soft and hard magnetic composite materials mainly employs methods such as separate preparation followed by composite, mechanical mixing followed by heat treatment, and alloy reduction or sintering. For example, patent CN103449807B discloses a method for separately preparing SrFe using hydrothermal methods. 12 O 19 and Ni (x) Zn (1-x) A method for preparing biphase composite hard magnetic ferrites by pressing Fe2O4 nanopowder into tablets in a specific ratio and calcining at 700℃; patent CN104973859A discloses a method for hydrothermal preparation of SrFe 12 O 19 The method involves mixing CoFe2O4 powder with the powder in a specific ratio, followed by grinding to obtain exchange-coupled composite ferrite powder. While this approach can achieve composite formation of both soft and hard magnetic phases, the publicly available process shows that the two phases are primarily generated independently before contacting and combining. Interface formation largely depends on subsequent pressing, grinding, or sintering processes, making it difficult to simultaneously achieve uniform coating, continuous interface, and precise adjustment of shell thickness at the single-particle scale.

[0004] For example, patent CN101299370B discloses a type of magnet synthesized from hard and soft magnetic phases and its preparation method. The technical route includes alloy melting, hydrogen explosion, ball milling, soft magnetic phase coating or mixing, magnetic field pressing, sintering, and magnetic field heat treatment, which is a typical approach of powder metallurgy and high-temperature densification. This method can construct a composite structure in which the hard magnetic phase is dispersed in the soft magnetic phase. However, it mainly targets the forming process of micron-sized hard magnetic particles and bulk magnets, resulting in a long process chain. Furthermore, the focus of structural control is on the phase distribution and interphase distance in the sintered magnet, rather than on in-situ shell construction at the nanoparticle level. For applications where uniform coating of the soft magnetic core / hard magnetic shell is desired at the single-particle scale, and further control of shell continuity and thickness is required, this approach still suffers from coarse particle size control.

[0005] For example, patent CN111292910B discloses a rapid preparation method for Co / SmCo composite magnetic materials with a special structure. This method obtains Co / SmCo samples with core-shell characteristics through a short-term incomplete calcium reduction reaction combined with a specific precursor structure. This scheme demonstrates that metallic soft / hard magnetic systems can also form core-shell composite structures through special reduction routes. However, its reaction depends on the metal reduction process and specific precursor design, making the system more inclined towards metallic permanent magnet materials. This differs from the liquid-phase precursor coating-shell formation approach for oxide-type soft and hard magnetic nanoparticles. From a process adaptability perspective, directly transferring this type of method to ferrite or other hard magnetic oxide shell systems is not straightforward.

[0006] In the direction of combustion synthesis, related patents also indicate that this route has advantages such as high powdering efficiency, uniform precursors, and fine particle size. For example, patent CN1308104C discloses a method for preparing barium ferrite micropowder using a nitrate / citric acid system through evaporation, gelation, and self-combustion; patent CN101481241A discloses a route for preparing hard magnetic phase and soft magnetic phase powders by sol-gel self-propagating combustion, followed by mixing and annealing to obtain multiphase nanocrystalline permanent magnet ferrite powder. The aforementioned patents show that self-combustion or self-propagating combustion is indeed beneficial for shortening the diffusion distance, reducing the phase formation temperature, and obtaining finer powders; however, judging from the disclosed content, CN1308104C mainly focuses on the preparation of single-phase hard magnetic powders, and although CN101481241A involves a two-phase system, it still involves first preparing hard and soft magnetic powders separately, and then mixing and annealing them, without solving the problems of directional enrichment of hard magnetic phase precursors on the surface of pre-made soft magnetic cores, in-situ shell formation, and continuous adjustment of shell thickness.

[0007] Furthermore, patent CN103890869A discloses core-shell nanoparticles with a hard-soft magnetic heterostructure and their preparation method. Its background section explicitly points out that simply mixing hard magnetic nanoparticles and soft magnetic nanoparticles often fails to simultaneously achieve ideal coercivity and saturation flux density. Therefore, a core-shell structure is needed to improve the interfacial connection between the two types of magnetic materials. This document illustrates from another perspective that the key to hard and soft magnetic phases lies not in "whether they are composite," but in "how they are composited." However, the disclosed scheme mainly focuses on the hard magnetic core / soft magnetic shell heterostructure and employs steps such as slurry preparation, gelation, drying, and subsequent processing to achieve particle construction. This differs from the technical route that uses pre-fabricated soft magnetic nanoparticles as the core, preferentially enriches the surface of hard magnetic phase precursors, and then forms a hard magnetic shell through combustion-heat treatment.

[0008] Therefore, although existing soft and hard magnetic dual-phase material preparation technologies have been able to achieve coexistence of two phases, exchange coupling, or local core-shell construction, they still generally have the following shortcomings: First, many schemes still adopt the approach of separately preparing hard magnetic phases and soft magnetic phases and then mixing them, with interface construction lagging behind the formation process of the two phases, which easily leads to random composite interfaces and insufficient coating; Second, some routes rely on multi-step processes such as pressing, sintering, ball milling, and reduction, which is not conducive to simplifying the process and improving the consistency of nanoscale structures; Third, for the formation location, coverage degree, and thickness distribution of hard magnetic shells, existing public technologies still lack a universal method that can achieve "surface enrichment first, then in-situ shell formation" in the liquid phase precursor stage. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of existing technologies by providing a technical solution for preparing soft and hard magnetic dual-phase nanoparticles using a self-propagating combustion method without external ignition. This method uses pre-prepared and dispersed soft magnetic phase nanoparticles as the core. In a solution containing a hard magnetic phase metal salt precursor, the hard magnetic phase precursor is preferentially enriched on the surface of the soft magnetic core through pH control and complexation / deposition behavior. Subsequently, a self-propagating combustion process without external ignition rapidly transforms the surface enrichment layer into a shell precursor, which is then subjected to subsequent heat treatment to form a continuous or semi-continuous hard magnetic shell. By advancing shell construction to the precursor stage, it is expected to reduce free nucleation and macroscopic phase separation, improve interfacial continuity, and provide a more direct process for controlling shell thickness and coverage, thus providing a new technical path for obtaining soft magnetic core-hard magnetic shell type soft and hard magnetic dual-phase nanoparticles.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0011] A method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition, characterized by comprising the following steps:

[0012] S1. Prepare and disperse soft magnetic phase nanoparticles to obtain a soft magnetic phase nanoparticle dispersion system.

[0013] S2. Add a hard magnetic phase metal salt precursor, citric acid and deionized water to the soft magnetic phase nanoparticle dispersion system to prepare a hard magnetic phase metal salt precursor solution containing soft magnetic phase nanoparticles. Adjust the pH of the solution by adding ammonia water so that the hard magnetic phase precursor forms a surface enrichment layer on the surface of the soft magnetic phase nanoparticles through adsorption or precipitation.

[0014] S3. The solution of the hard magnetic phase precursor containing the surface enrichment layer is evaporated and concentrated, and then it undergoes self-propagating combustion reaction without external ignition to obtain combustion products. The surface enrichment layer in the combustion products is converted into nano-oxide shell precursor in situ.

[0015] S4. Perform heat treatment on the combustion products to meet the requirements of hard magnetic phase formation, so that the hard magnetic phase crystallizes on the surface of the soft magnetic phase nanoparticles to form a continuous or semi-continuous shell, thereby obtaining soft magnetic core-hard magnetic shell type soft and hard magnetic nanoparticles.

[0016] The present invention, by adopting the above-described technical solution, has the following beneficial effects:

[0017] This invention uses pre-prepared and dispersed soft magnetic phase nanoparticles as the core. In a solution containing a hard magnetic phase metal salt precursor, the hard magnetic phase precursor is preferentially enriched on the surface of the soft magnetic core through pH control and complexation / deposition behavior. Subsequently, a self-propagating combustion process without external ignition is used to rapidly transform the surface enrichment layer into a shell precursor, which is then subjected to subsequent heat treatment to form a continuous or semi-continuous hard magnetic shell. By advancing the shell construction to the precursor stage, it is expected to reduce free nucleation and macroscopic phase separation, improve interfacial continuity, and provide a more direct process for controlling the shell thickness and coverage, thus providing a new technical route for obtaining soft magnetic core-hard magnetic shell type soft and hard magnetic dual-phase nanoparticles. Attached Figure Description

[0018] The present invention will be further described below with reference to the accompanying drawings:

[0019] Figure 1 This is a flowchart of a method for preparing soft and hard magnetic nanoparticles using a self-propagating combustion method without external ignition, according to the present invention.

[0020] Figure 2 This is a schematic diagram illustrating the specific steps of the preparation method in this invention;

[0021] Figure 3 This is a scanning electron microscope (SEM) image from the present invention;

[0022] Figure 4 The image shows the XRD pattern of the soft and hard magnetic nanoparticles obtained in this invention. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0025] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion.

[0026] like Figures 1 to 2 As shown, this invention provides a method for preparing soft and hard magnetic dual-phase nanoparticles using a self-propagating combustion method without external ignition, comprising the following steps:

[0027] S1. Prepare and disperse soft magnetic phase nanoparticles to obtain a soft magnetic phase nanoparticle dispersion system.

[0028] The soft magnetic phase nanoparticles were prepared by at least one of the following methods: co-precipitation, hydrothermal method, or sol-gel method.

[0029] The preparation of soft magnetic phase nanoparticles using the co-precipitation method specifically includes the following steps:

[0030] S1.1 The metal salt precursor that forms a soft magnetic phase is dissolved in deionized water to obtain a metal salt solution with a total concentration of 0.05 to 1.0 mol / L, preferably 0.1 to 0.5 mol / L;

[0031] S1.2 Add an alkalizing agent under stirring conditions to adjust the pH of the reaction system to 9-11, preferably to 9.5-10.5, and react at 60-95℃ for 0.5-4h, preferably 1-2h, to precipitate the soft magnetic phase.

[0032] After the S1.3 reaction is completed, the resulting precipitate is washed, centrifuged and dried to obtain soft magnetic phase nanoparticles.

[0033] The alkalizing agent can be selected from at least one of ammonia or sodium hydroxide. When using ammonia, the dropping rate should be controlled within a slow and uniform range to avoid localized pH spikes that could lead to particle agglomeration. When using sodium hydroxide, the alkali solution can be prepared in advance and then added dropwise to improve the controllability of the reaction.

[0034] The dispersion of soft magnetic phase nanoparticles can be achieved through ultrasonic dispersion, mechanical stirring, or a combination thereof. Dispersants or surfactants can be added to inhibit agglomeration and improve system homogeneity, ensuring the effective enrichment of subsequent hard magnetic phase precursors on the surface of the soft magnetic core.

[0035] The dispersant or surfactant can be selected from one or more of polyethylene glycol, polyvinylpyrrolidone, sodium citrate, and sodium dodecyl sulfate. The dispersion time is generally controlled within 5 to 60 minutes, preferably 10 to 30 minutes.

[0036] The soft magnetic phase nanoparticles are preferably ferrite-based soft magnetic materials, including Fe3O4, γ-Fe2O3, NiFe2O4, and Ni. 0.5 Zn 0.5 At least one of Fe2O4.

[0037] The average particle size of the soft magnetic phase nanoparticles is 5–100 nm. If the particle size is too large, it is not conducive to the formation of a uniform shell; if the particle size is too small, significant agglomeration and surface energy-driven growth may occur during subsequent dispersion and heat treatment.

[0038] S2. Add a hard magnetic phase metal salt precursor, citric acid and deionized water to the soft magnetic phase nanoparticle dispersion system to prepare a hard magnetic phase metal salt precursor solution containing soft magnetic phase nanoparticles. Adjust the pH of the solution by adding ammonia water so that the hard magnetic phase precursor forms a surface enrichment layer on the surface of the soft magnetic phase nanoparticles through adsorption or precipitation.

[0039] Hard magnetic phases include at least one of M-type hexagonal ferrite, W-type hexagonal ferrite, and hard magnetic oxide systems.

[0040] Metal salt precursors include nitrates corresponding to the metal elements constituting the hard magnetic phase. When the hard magnetic phase is an M-type hexagonal ferrite, the nitrates include iron nitrates, strontium nitrates, or barium nitrates.

[0041] To further adjust the magnetocrystalline anisotropy, crystallization behavior, and phase formation temperature of the hard magnetic shell, the nitrate includes nitrates corresponding to doped elements. Nitrates corresponding to doped elements include at least one of cobalt nitrate, zinc nitrate, aluminum nitrate, and rare earth metal nitrates. Rare earth metal nitrates may include nitrates corresponding to rare earth elements such as lanthanum, neodymium, and samarium.

[0042] The total concentration of hard magnetic phase metal ions can be adjusted according to the target shell thickness and the rheological properties of the system. It is typically controlled between 0.03 and 1.0 mol / L, preferably between 0.05 and 0.5 mol / L. If the concentration is too low, there will be insufficient hard magnetic phase precursors available for shell formation per unit volume of solution, which is not conducive to the formation of a continuous shell; if the concentration is too high, the system viscosity will increase significantly, the tendency for local precipitation will be enhanced, and free nucleation or aggregation will be easily induced.

[0043] Citric acid serves a dual role as both a complexing agent and a fuel in this invention. On one hand, citric acid can form stable complexes with metal ions, improving solution homogeneity and inhibiting localized hydrolysis. On the other hand, citric acid and nitrate ions form a fuel-oxidant system during subsequent concentration, providing a reaction basis for self-propagating combustion without external ignition. Preferably, the equivalence ratio of metal nitrate to citric acid satisfies the fuel-oxidant equilibrium condition, or is in a condition biased towards fuel and oxidant. More preferably, the molar ratio of total metal ions to citric acid can be controlled within the range of 1:(0.5–2.0), more preferably 1:(0.8–1.5).

[0044] After adding hard magnetic phase metal nitrates and citric acid to the soft magnetic phase nanoparticle dispersion system, the mixture is stirred for 10–120 min, preferably 20–60 min, at room temperature to 80°C to promote the full complexation and uniform distribution of metal ions in the system. Ammonia is then added to adjust the pH of the solution to 7.5–12.5, preferably 8.5–11.0. The purpose of pH adjustment is to enhance the preferential adsorption or deposition behavior of the hard magnetic phase precursor on the surface of the soft magnetic core, thereby forming a surface enrichment layer. This surface enrichment layer can be a metal-citric acid complex layer, a metal hydroxide layer, a basic salt layer, or a composite enrichment layer composed of the above components.

[0045] The ratio of soft magnetic phase nanoparticles to hard magnetic phase precursors can be designed according to the target soft-hard magnetic ratio and shell thickness. It is typically controlled by the final molar or mass ratio of the soft magnetic phase to the hard magnetic phase. Preferably, the molar ratio of the soft magnetic phase to the hard magnetic phase is 1:0.2 to 1:5, more preferably 1:0.5 to 1:3. As the amount of hard magnetic phase precursor added increases, the thickness of the formed shell usually increases; as the concentration of the soft magnetic core increases, the amount of shell precursor that can be distributed per unit core surface decreases, and the shell tends to become thinner.

[0046] S3. The solution of the hard magnetic phase precursor containing the surface enrichment layer is evaporated and concentrated, so that it undergoes self-propagating combustion reaction without external ignition to obtain combustion products. The surface enrichment layer in the combustion products is converted in situ into nano-oxide shell precursor.

[0047] The hard magnetic phase precursor solution obtained in step S2 is evaporated and concentrated under heating and stirring conditions. The evaporation temperature is generally controlled at 60–120°C, preferably 70–100°C. As the solvent gradually evaporates, the solute concentration in the system continuously increases, the enriched layer on the surface of the soft magnetic core gradually densifies, and the overall system viscosity increases synchronously. When the solid content of the system increases to 10–70 wt%, preferably 20–55 wt%, the system reaches a state conducive to self-sustaining reaction.

[0048] In some embodiments, the system can be pre-dried or gelled during the evaporation and concentration process or before combustion. The pre-drying temperature can be 80–150°C, and the time can be 0.5–6 h; the gelling treatment can be carried out at 60–120°C for 0.5–10 h. Through appropriate pre-drying or gelling treatment, the hard magnetic precursor can be more fully fixed on the surface of the soft magnetic core, reducing the migration and independent nucleation probability of free precursors in the bulk solution phase, thereby improving the shell coverage and uniformity.

[0049] When the system concentration reaches a certain threshold, the fuel-oxidant system composed of citric acid and nitrate nitrate ions can undergo a self-propagating combustion reaction without external ignition under the action of heat accumulation. This process is accompanied by rapid exothermic reaction and gas release, which may include CO2, H2O vapor, and NO. x Since the surface enrichment layer is pre-formed on the surface of the soft magnetic core, during combustion, the shell precursor preferentially transforms in situ around the soft magnetic core, forming a nano-oxide shell precursor coating the surface of the soft magnetic core. The resulting combustion products typically exhibit porous and loose morphology. This porous and loose structure is beneficial for the rapid formation of the hard magnetic phase during subsequent heat treatment and, to some extent, reduces the tendency for abnormal sintering and growth of particles.

[0050] In step S3, the stability of the self-propagating combustion process and the continuity of the shell precursor are closely related to factors such as the ratio of metal nitrate and citric acid, the pH value of the system, the solid content, the concentration ratio, and the solution viscosity. Generally speaking, under conditions of near-equilibrium of fuel and oxidant or a slight fuel bias, it is more conducive to the formation of combustion behavior with moderate exothermicity and uniform gas release, thereby obtaining a more continuous and uniformly distributed shell precursor. If the system is excessively fuel biased, the organic residue will increase, and the burden of subsequent heat treatment will increase. If the system is excessively oxidant biased, the combustion process may be too intense, and the integrity of the local shell may be affected.

[0051] S4. Perform heat treatment on the combustion products to meet the requirements of hard magnetic phase formation, so that the hard magnetic phase crystallizes on the surface of the soft magnetic phase nanoparticles to form a continuous or semi-continuous shell, thereby obtaining soft magnetic core-hard magnetic shell type soft and hard magnetic nanoparticles.

[0052] The combustion products obtained in step S3 are subjected to heat treatment to achieve phase formation and crystallization of the hard magnetic phase shell. The heat treatment regime includes the heating rate, peak temperature, holding time, and atmospheric conditions. The heating rate is generally 1–20 °C / min, preferably 3–10 °C / min; the holding time is generally 0.1–10 h, preferably 0.5–4 h. The heat treatment atmosphere can be selected from air, oxygen, inert atmosphere, or a mixture thereof, depending on the target hard magnetic phase system and interface diffusion control requirements.

[0053] The peak heat treatment temperature can be adjusted appropriately for different hard magnetic phase systems. When the hard magnetic phase is M-type hexagonal ferrite, the preferred heat treatment temperature is 700–1100℃, more preferably 800–900℃; when the hard magnetic phase is W-type hexagonal ferrite, the heat treatment temperature can be further increased to 850–1200℃. If the heat treatment temperature is too low, the hard magnetic shell will not crystallize sufficiently, and the magnetic anisotropy will not be established adequately; if the temperature is too high or the holding time is too long, it may cause intensified interdiffusion at the soft and hard phase interface, shell coarsening, or even the formation of non-target phases.

[0054] In this invention, the shell morphology can be either a continuous shell or a semi-continuous shell. A semi-continuous shell can be understood as a structural state where the hard magnetic shell covers 40%–95% of the soft magnetic core surface, while a continuous shell covers 100% of the soft magnetic core surface. By controlling one or more of the following: soft magnetic core feeding concentration, total molar amount of metal nitrate, ratio of metal nitrate to citric acid, pH value, solid content, concentration ratio, and heat treatment regime, the shell thickness and coverage can be jointly adjusted. Generally, the shell thickness can be controlled within the range of 2–100 nm, preferably 5–50 nm. A thinner shell is beneficial for enhancing the exchange coupling between the hard and soft phases, but if the shell is too thin, a stable coating may not be formed; a thicker shell, while beneficial for improving coverage and the integrity of the hard magnetic phase, may increase the diffusion distance and weaken the contribution of the soft magnetic phase to the overall saturation magnetization.

[0055] To suppress interfacial interdiffusion and the formation of non-target phases, the heat treatment process can be optimized by lowering the peak temperature, shortening the high-temperature holding time, increasing the heating rate, and adjusting the oxygen partial pressure. If necessary, a staged heat treatment approach can also be adopted. For example, the first stage removes residual organic matter and stabilizes the shell precursor structure at a lower temperature, while the second stage completes the crystallization of the hard magnetic phase at a higher temperature. This type of heat treatment generally helps improve shell continuity and interface stability.

[0056] Example 1

[0057] Weigh out 1.08 g of FeCl3·6H2O and 0.40 g of FeCl2·4H2O, and mix them together. 3+ / Fe 2+The metal salt solution was dissolved in 40 mL of deionized water at a molar ratio of 2:1. Under nitrogen protection, the system was heated to 80 °C with stirring at 600 rpm, followed by slow dropwise addition of 25 wt% NH3·H2O to adjust the pH to 10.0–10.5. The reaction was continued at 80 °C for 1 h to ensure complete precipitation of the Fe3O4 precursor. The resulting black precipitate was washed three times with deionized water, centrifuged at 8000 rpm for 5 min, and then vacuum dried at 60 °C for 6 h to obtain soft magnetic phase Fe3O4 nanoparticles.

[0058] Weigh 0.100 g of the Fe3O4 nanoparticles prepared above, add them to 15 mL of deionized water, treat them under ultrasonic conditions for 20 min, and then mechanically stir for 20 min to obtain a uniformly dispersed soft magnetic core suspension.

[0059] With SrFe 12 O 19 As a hard magnetic phase, hard magnetic phase precursor solutions were prepared at a molar ratio of soft magnetic phase to hard magnetic phase of 1:1. The raw material masses used for each group of samples are shown in Table 1. This precursor solution was then slowly added to the above Fe3O4 soft magnetic core suspension, and stirring was continued for 30 min. Subsequently, 25 wt% NH3·H2O was slowly added dropwise to adjust the pH of the system to 9.0–9.5, and stirring was continued for 30 min. This allowed the hard magnetic phase precursor to preferentially adsorb or deposit on the surface of Fe3O4 particles, forming a surface enrichment layer. The resulting suspension was the precursor solution used for subsequent self-propagating combustion.

[0060] The precursor solution was placed in an evaporating dish and concentrated in a water bath at 85°C until the system gradually changed from a flowing liquid to a viscous gel. It was then transferred to a heating platform at 250–300°C for further heating. The system underwent a self-propagating combustion reaction without the need for an external open flame, yielding a fluffy and porous combustion product powder.

[0061] The powder obtained from combustion was gently ground and then subjected to a two-stage heat treatment: the first stage involved heating to 350℃ at a rate of 3℃ / min and holding for 1 hour to remove residual organic matter and stabilize the shell precursor structure; the second stage involved heating to 850℃ at a rate of 5℃ / min and holding for 1 hour to allow the hard magnetic phase to crystallize and form a shell on the surface of the soft magnetic core. After the heat treatment, the furnace was cooled to obtain Fe3O4@SrFe with different soft magnetic phase / hard magnetic phase ratios. 12 O 19 Core-shell type soft and hard magnetic dual-phase nanoparticles. The sample with a molar ratio of soft magnetic phase to hard magnetic phase of 1:1 is designated as sample 1.

[0062] Example 2

[0063] This embodiment is basically the same as embodiment 1, except that:

[0064] The hard magnetic phase precursor solution was prepared with a soft magnetic phase to hard magnetic phase molar ratio of 1:2. The mass of the raw materials used is shown in Table 1 for the corresponding 1:2 ratio.

[0065] After undergoing the same suspension preparation, surface deposition, self-propagating combustion, and two-stage heat treatment, Fe3O4@SrFe with a soft magnetic phase to hard magnetic phase molar ratio of 1:2 was obtained. 12 O 19 The core-shell type soft and hard magnetic dual-phase nanoparticles are designated as sample 2.

[0066] Example 3

[0067] This embodiment is basically the same as embodiment 1, except that:

[0068] The hard magnetic phase precursor solution was prepared with a soft magnetic phase to hard magnetic phase molar ratio of 1:3. The mass of the raw materials used is shown in Table 1 corresponding to the 1:3 ratio.

[0069] After undergoing the same suspension preparation, surface deposition, self-propagating combustion, and two-stage heat treatment, Fe3O4@SrFe with a soft magnetic phase to hard magnetic phase molar ratio of 1:3 was obtained. 12 O 19 The core-shell type soft and hard magnetic dual-phase nanoparticles are designated as sample 3.

[0070] Example 4

[0071] This embodiment is basically the same as Example 2. The molar ratio of the soft magnetic phase to the hard magnetic phase is still 1:2, and the mass of the raw materials used in the hard magnetic phase precursor solution is the same as in Example 2, as shown in the data corresponding to sample 2 in Table 1. The difference is:

[0072] After adding Fe3O4 nanoparticles to 15 mL of deionized water, 0.010 g of polyvinylpyrrolidone (PVP) was added as a dispersant to improve the dispersibility of the soft magnetic core suspension and inhibit particle aggregation.

[0073] After subsequent surface deposition, self-propagating combustion, and two-stage heat treatment, Fe3O4@SrFe was obtained. 12 O 19 The core-shell type soft and hard magnetic dual-phase nanoparticles are designated as sample 4.

[0074] Example 5

[0075] This embodiment is basically the same as Embodiment 2. The molar ratio of the soft magnetic phase to the hard magnetic phase is still 1:2, and the mass of raw materials used for the hard magnetic phase precursor is the same as in Embodiment 2, as shown in the data corresponding to Sample 2 in Table 1. The difference is:

[0076] In the two-stage heat treatment, the first stage still raises the temperature to 350℃ at a heating rate of 3℃ / min and holds it for 1 hour; the second stage raises the temperature to 850℃ at a heating rate of 5℃ / min, but the holding time is extended to 2 hours to further promote the crystallization of the hard magnetic phase and the formation of the shell structure.

[0077] The remaining steps are the same as in Example 2, and Fe3O4@SrFe is finally obtained. 12 O 19 The core-shell type soft and hard magnetic dual-phase nanoparticles are designated as sample 5.

[0078] Table 1

[0079] Table 2 shows the magnetic property parameters of the composite magnetic powders obtained in Examples 1-5.

[0080] Table 2 shows the magnetic properties of composite magnetic powders obtained under different soft magnetic phase / hard magnetic phase ratios and different process control conditions. A comparison of samples 1-3 shows that the saturation magnetization gradually increases with the increase of the soft magnetic phase ratio, while the intrinsic coercivity increases with the increase of the hard magnetic phase ratio. Comprehensive comparison reveals that when the molar ratio of soft magnetic phase to hard magnetic phase is 1:2, the sample exhibits higher remanent magnetization and maximum energy product, indicating that this ratio achieves a better synergistic effect and overall magnetic property matching between the soft and hard magnetic phases.

[0081] The addition of an appropriate amount of PVP improves the dispersibility of the Fe3O4 soft magnetic core in the liquid system and suppresses particle agglomeration, thus facilitating the uniform adsorption and deposition of the hard magnetic phase precursor on the surface of the soft magnetic core and promoting the formation of a more uniform shell structure. This structure helps improve the interfacial contact quality and synergistic effect between the soft and hard magnetic phases, mitigating the adverse effects of uneven local component distribution or soft magnetic interconnections. Therefore, sample 4 exhibits superior overall magnetic properties, with improved remanent magnetization and maximum energy product, indicating that the introduction of the dispersant is beneficial for constructing a more stable and uniform core-shell soft-hard magnetic composite structure.

[0082] While extending the second heat treatment time is beneficial for further crystallization of the hard magnetic phase and the improvement of the shell structure, excessively long holding times can also lead to particle growth, intensified local sintering, and enhanced diffusion at the interface between the hard and soft magnetic phases, thereby weakening the effective synergistic effect between the two phases at the nanoscale. Simultaneously, increased particle coarsening and agglomeration may also reduce the coupling advantage during magnetization reversal, hindering the attainment of superior remanent magnetization and maximum energy product. Therefore, the overall magnetic properties of sample 5 did not show further improvement, indicating that under the process conditions of this invention, a moderate heat treatment time is more conducive to balancing the phase formation of the hard magnetic phase and the coupling effect between the hard and soft magnetic phases.

[0083] The scanning electron microscope (SEM) images of the sample (soft magnetic phase: hard magnetic phase = 1:2) prepared according to the method of Example 1, after self-propagating combustion, slight grinding, and annealing treatment, are shown below. Figure 3 As shown in the figure, SEM results indicate that the sample retained some loose aggregate morphology after annealing. The aggregates were generally loose with a rough surface, and irregular packing of voids could be observed in some areas. This suggests that the porous and loose characteristics formed by rapid heat release and gas release during self-propagating combustion did not completely disappear during the subsequent annealing process. Meanwhile, because the sample was slightly ground before characterization, some large aggregates were broken up, resulting in a final observed morphology of particle aggregation retaining certain loose aggregate characteristics.

[0084] It should be noted that, Figure 3 The SEM images shown are mainly used to characterize the typical particle aggregation morphology of the composite magnetic powder obtained by the method of this invention after self-propagating combustion and annealing treatment. Examples 1 to 5 all adopted the same basic process route of soft magnetic core dispersion, hard magnetic phase precursor surface enrichment, self-propagating combustion, and annealing phase formation. The differences mainly lie in the ratio of soft magnetic phase to hard magnetic phase, the addition of dispersant, and process parameters such as heat treatment holding time. These parameters mainly affect the relative content, crystallinity, interfacial contact state, and magnetic properties of the hard magnetic phase shell, while having a relatively small impact on the overall morphological characteristics of the self-propagating combustion products—a loose, porous structure that forms particle aggregates after slight grinding. Therefore, Figure 3 As a representative morphology diagram, it can reflect the typical morphological characteristics of the soft and hard magnetic nano-phase particles obtained by the method of the present invention, and is not intended to limit the present invention to only this single embodiment or single ratio condition.

[0085] Figure 4 This is the XRD pattern of the soft and hard magnetic nanoparticles obtained in this invention. Figure 4 It is evident that after self-propagating combustion and subsequent annealing, the sample can form the target ferrite phase, and the diffraction peaks show a similarity to the hard magnetic phase SrFe. 12 O 19 Corresponding characteristic peaks are observed, while diffraction information related to the soft magnetic phase ferrite is retained. However, due to the low content and small grain size of the soft magnetic phase, the diffraction signal related to the soft magnetic phase is relatively weak. Nevertheless, considering the raw material design, preparation process, and magnetic property results, it can be concluded that the obtained product is a soft-hard magnetic dual-phase composite magnetic powder. Figure 3 The SEM results show that the present invention, through the process route of "soft magnetic core dispersion - hard magnetic phase precursor surface enrichment - self-propagating combustion - annealing phase formation", can not only obtain composite magnetic powder morphology with loose agglomeration characteristics, but also promote the crystallization of hard magnetic phase during heat treatment, thereby forming soft and hard magnetic dual-phase composite particles.

[0086] The above are merely specific embodiments of the present invention, but the technical features of the present invention are not limited thereto. Any simple changes, equivalent substitutions, or modifications made based on the present invention to achieve substantially the same technical effect are all covered within the protection scope of the present invention.

Claims

1. A method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition, characterized in that... Includes the following steps: S1. Prepare and disperse soft magnetic phase nanoparticles to obtain a soft magnetic phase nanoparticle dispersion system. S2. Add a hard magnetic phase metal salt precursor, citric acid and deionized water to the soft magnetic phase nanoparticle dispersion system to prepare a hard magnetic phase metal salt precursor solution containing soft magnetic phase nanoparticles. Adjust the pH of the solution by adding ammonia water so that the hard magnetic phase precursor forms a surface enrichment layer on the surface of the soft magnetic phase nanoparticles through adsorption or precipitation. S3. The solution of the hard magnetic phase precursor containing the surface enrichment layer is evaporated and concentrated to allow it to undergo self-propagating combustion reaction without external ignition, thereby obtaining combustion products. The surface enrichment layer in the combustion products is in situ converted into a nano-oxide shell precursor. S4. Perform heat treatment on the combustion products to meet the requirements of hard magnetic phase formation, so that the hard magnetic phase crystallizes on the surface of the soft magnetic phase nanoparticles to form a continuous or semi-continuous shell, thereby obtaining soft magnetic core-hard magnetic shell type soft and hard magnetic nanoparticles.

2. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: The soft magnetic phase nanoparticles in step S1 are prepared by at least one of the following methods: co-precipitation, hydrothermal method, or sol-gel method.

3. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 2, characterized in that: The soft magnetic phase nanoparticles are prepared by a co-precipitation method, specifically including the following steps: S1.1 The metal salt precursor that forms a soft magnetic phase is dissolved in deionized water to obtain a metal salt solution with a total concentration of 0.05 to 1.0 mol / L; S1.2 Add an alkalizing agent under stirring to adjust the pH of the reaction system to 9-11, and react at 60-95℃ for 0.5-4h to precipitate the soft magnetic phase; S1.3 The obtained precipitate was washed, centrifuged and dried to obtain soft magnetic phase nanoparticles.

4. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: When the soft magnetic phase nanoparticles in step S1 are dispersed, they are dispersed by ultrasonic dispersion, mechanical stirring or a combination thereof, and a dispersant or surfactant may be added to inhibit agglomeration.

5. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: The soft magnetic phase nanoparticles in step S1 include Fe3O4, γ-Fe2O3, NiFe2O4, and Ni 0.5 Zn 0.5 At least one of Fe2O4.

6. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: The average particle size of the soft magnetic phase nanoparticles in step S1 is 5–100 nm.

7. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: The hard magnetic phase in step S2 includes at least one of M-type hexagonal ferrite, W-type hexagonal ferrite, and hard magnetic oxide system.

8. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 7, characterized in that: The metal salt precursor in step S2 includes nitrates corresponding to the metal elements constituting the hard magnetic phase, and the nitrates include nitrates corresponding to the doping elements.

9. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 8, characterized in that: When the hard magnetic phase is M-type hexagonal ferrite, the nitrate includes iron nitrate, strontium nitrate, or barium nitrate.

10. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 8, characterized in that: The nitrates corresponding to the doping elements include at least one of cobalt nitrate, zinc nitrate, aluminum nitrate, and rare earth metal nitrate.

11. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: In step S2, ammonia is added to adjust the pH of the solution to 7.5–12.

5.

12. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: In step S2, the equivalence ratio of the metal salt precursor of the hard magnetic phase to the citric acid satisfies the fuel-oxidant balance or fuel / oxidant bias conditions, and the solid content of the system after evaporation and concentration is 10-70 wt%.

13. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: In step S3, pre-drying or gelation treatment is performed during the evaporation and concentration process or before combustion.

14. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 8, characterized in that: In step S3, the self-propagating combustion is driven by a fuel-oxidant system composed of citric acid and nitrate ions, and the resulting combustion products have a porous and loose structure.

15. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: The heat treatment in step S4 includes: heating rate of 1 to 20 °C / min, holding at that temperature for 0.1 to 10 h, and being carried out in air, oxygen, inert atmosphere or a mixture thereof.

16. The method for preparing soft and hard magnetic dual-phase nanoparticles by self-propagating combustion without external ignition as described in claim 1, characterized in that: In step S4, the continuous or semi-continuous shell has a coverage rate of 40% to 95% on the surface of the soft magnetic core, while the continuous shell has a coverage rate of 100% on the surface of the soft magnetic core.

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