Preparation method of copper-plated soft magnetic composite material, soft magnetic composite material and application thereof

Abstract

This invention discloses a method for preparing copper-plated soft magnetic composite material, the soft magnetic composite material and its application. The preparation method includes the following steps: (1) preparing iron-based soft magnetic powder and activating it; (2) preparing a copper plating solution containing a copper source; (3) chemical plating; (4) drying; (5) low-temperature pre-crystallization treatment; (6) annealing: (6.1) heating to 500-600℃ at 3-8℃ / min and holding for 10-30min; (6.2) heating to 750-850℃ at 8-12℃ / min and holding for 15-30min; (6.3) cooling to 300-400℃ at 1-3℃ / min and holding for 60-90min; (6.4) cooling to 200-250℃ at 1-2℃ / min and holding for 90-120min, thereby achieving the dual continuous function of "Cu-rich phase insulation at grain boundaries and thermal and magnetic conduction of iron grains".

Description

Technical Field

[0001] This invention relates to the field of soft magnetic materials technology, specifically to a method for preparing copper-plated soft magnetic composite materials, the soft magnetic composite materials themselves, and their applications. Background Technology

[0002] As power electronic devices develop towards higher power density and miniaturization, the contradiction between eddy current loss and heat dissipation in soft magnetic cores is becoming increasingly prominent. High power density scenarios place core demands on soft magnetic composite materials: they must possess high resistivity to suppress high-frequency eddy current loss, while maintaining high thermal conductivity to quickly dissipate operating heat and avoid thermal runaway leading to performance degradation.

[0003] Copper is an ideal modifying element with both insulating and thermal conductivity potential due to its excellent thermal conductivity (approximately 400 W / m·K) and extremely low solid solubility in α-Fe (≤2 at%).

[0004] Existing patents related to copper-plated soft magnetic materials have not resolved the core contradiction between high resistance and high thermal conductivity, and have obvious technical defects: one type involves chemically plating copper followed by high-temperature sintering to form a continuous coating layer, such as the Chinese invention patent with patent application number CN201811267678.0 (publication number CN109175354B) which discloses a method for preparing diamond / W-Cu composite material. Although this patent can achieve insulation through copper layer isolation, the continuous plating layer blocks the heat conduction between iron grains, resulting in a significant decrease in thermal conductivity (usually <25W / m·K), falling into the technical paradox of "insulation is heat insulation".

[0005] Another type is the preparation of Fe-Cu pre-alloyed powder by gas atomization, such as the Cu-Fe in-situ alloy foil and its preparation method disclosed in Chinese invention patent application number CN201911251036.6 (publication number CN110923694B). Copper is uniformly dissolved in the iron matrix, which increases the resistivity while destroying the integrity of the crystal lattice, and the thermal conductivity drops to below 35W / m·K. Moreover, the process has high cost and poor formability.

[0006] Another type involves simple annealing modification after chemical copper plating. For example, Chinese invention patent CN201911196153.7 (publication number CN110899692B) discloses a method for preparing iron-based alloy powder. Annealing is only used to eliminate interfacial stress, without precise control over the solid solution properties of copper. Copper either diffuses disorderly or partially dissolves, failing to form an effective insulating structure and retain its high thermal conductivity. The increase in resistivity is limited (typically <5×10⁻⁶). 4 μΩ·cm).

[0007] Existing technologies have not utilized the limited solid solution properties of copper in α-Fe to guide the directional segregation of copper atoms at grain boundaries through precise process control, forming a unique structure of "grain boundary insulation and grain thermal conductivity." Therefore, developing a process-controllable method for preparing Fe-Cu soft magnetic composite materials to achieve synergistic optimization of high resistivity and high thermal conductivity is of great significance for overcoming the heat dissipation bottleneck of high power density devices and can fill the performance gaps of existing copper-containing soft magnetic materials. Summary of the Invention

[0008] The first technical problem to be solved by the present invention is to provide a method for preparing copper-plated soft magnetic composite materials that can take into account both high resistivity and high thermal conductivity, in light of the current state of the prior art.

[0009] The second technical problem to be solved by the present invention is to provide a soft magnetic composite material prepared by the above-mentioned preparation method, in view of the current state of the prior art.

[0010] The third technical problem to be solved by the present invention is to provide an application of the above-mentioned soft magnetic composite material in light of the current state of the prior art.

[0011] The technical solution adopted by the present invention to solve the first technical problem mentioned above is as follows: A method for preparing a copper-plated soft magnetic composite material includes the following steps: (1) Prepare iron-based soft magnetic powder and activate it, wherein the iron-based soft magnetic powder contains iron element; (2) Prepare a copper plating solution containing a copper source; the order of steps (1) and (2) above is not important; (3) Chemical plating: The activated iron-based soft magnetic powder in step (1) is mixed with the copper plating solution in step (2) at a mass ratio of 1:5 to 1:15, and chemical plating is carried out at a temperature of 70 to 90°C. After the reaction, the reaction solution is separated into solid and liquid. The solid material is rinsed with deionized water until the cleaning solution is neutral to obtain copper plating powder. (4) Drying: Dry the copper plating powder from step (3) until the moisture content is ≤0.5%; (5) Low-temperature pre-crystallization treatment: Place the dried powder from step (4) in a vacuum or inert atmosphere, heat it to 200-250°C at 2-5°C / min, keep it at that temperature for 30-60 minutes, and then let it cool naturally to room temperature. (6) Annealing: The powder processed in step (5) is subjected to four stages of annealing in an annealing furnace. A flowing reducing atmosphere and nitrogen are continuously introduced throughout the process to replace the atmosphere in the annealing furnace in real time. (6.1) Directional pre-diffusion: The temperature is increased to 500-600℃ at a rate of 3-8℃ / min and held for 10-30min to allow copper atoms to diffuse directionally toward the grain boundaries; (6.2) Segregation in two-phase region: The temperature is increased to 750-850℃ at 8-12℃ / min and held for 15-30min. The limited solid solution of copper in α-Fe is utilized to enrich and segregate excess copper along the grain boundaries. (6.3) Low-temperature nanocrystallization: slowly cool down to 300-400℃ at 1-3℃ / min and hold for 60-90min to allow the copper-rich layer at the grain boundary to precipitate out as nanoscale particles; (6.4) Grain boundary phase stabilization: slowly cool down to 200-250℃ at 1-2℃ / min, hold for 90-120min, and then cool down to below 150℃ with the furnace.

[0012] Preferably, the preparation method further includes step (7) plasma modification: the annealed powder is placed in a plasma furnace, and a mixture of argon and methane is introduced, wherein the volume fraction of methane in the mixture is 5-10 vol%, the plasma power is 100-150 W, the mixture is treated at room temperature for 10-15 min, and the agglomerated particles are removed by passing it through a 100-mesh sieve, thereby obtaining the copper-plated soft magnetic composite material.

[0013] A mixed gas of argon and methane is used for room temperature plasma treatment, which forms an amorphous carbon protective layer of 2-5 nm only on the surface of the copper nanophase at the grain boundary. This not only prevents the copper nanophase from oxidizing, coarsening and changing its phase structure during subsequent pressing and service, but also slightly increases the resistivity of the material without reducing the thermal conductivity or affecting the thermal and magnetic conduction of the iron grains. The salt spray resistance of the material is more than twice that of iron-based soft magnetic powder, and the structural and performance stability under high power density conditions is greatly improved.

[0014] Preferably, in step (1), the iron-based soft magnetic powder is at least one of pure iron powder, iron-silicon alloy powder, iron-silicon-aluminum alloy powder, iron-aluminum alloy powder, iron-chromium alloy powder, iron-cobalt alloy powder, and iron-silicon-chromium alloy powder, and the iron-based soft magnetic powder has D50 < 200 μm and D99 < 350 μm.

[0015] Preferably, in step (2), the copper plating solution contains, by concentration, 20-35 g / L of copper source, 15-35 g / L of reducing agent, 20-35 g / L of complexing agent, 5-10 g / L of accelerator, 0.05-2 g / L of surfactant, 0.01-1 g / L of stabilizer, and pH adjuster, with the remainder being deionized water. The pH adjuster maintains the pH value of the copper plating solution at 7-9.

[0016] The copper source is at least one of copper sulfate, copper chloride, and basic copper carbonate; the reducing agent is at least one of sodium hypophosphite and sodium borohydride; the complexing agent is at least one of citric acid, sodium citrate, tartaric acid, potassium sodium tartrate, ethylenediaminetetraacetic acid, or sodium ethylenediaminetetraacetic acid; the accelerator is at least one of potassium ferrocyanide, 2,2'-bipyridine, and o-phenanthroline; the surfactant is at least one of polysorbate-80, sorbitan monostearate, polyethylene glycol, and sodium dodecyl sulfate; the stabilizer is at least two of potassium iodate, potassium nitrate, fumaric acid, or thiourea; and the pH adjuster is at least one of sodium hydroxide, ammonia, and sodium carbonate.

[0017] Preferably, in step (2), the mass ratio of the complexing agent to the copper source is 1:1 to 1.2:1. By strictly controlling the mass ratio of the complexing agent to the copper source, the coating is made into a thin, easily diffused structure, avoiding the obstruction of subsequent copper atom diffusion caused by a dense, thick coating, thus laying the compositional basis for grain boundary segregation.

[0018] Preferably, the inert atmosphere in step (5) is at least one of nitrogen or argon.

[0019] In step (6), the reducing atmosphere is 70-80 vol% hydrogen and 20-30 vol% nitrogen, with an overall atmosphere flow rate of 0.5-1 L / min.

[0020] Preferably, in step (1), the activation method for the iron-based soft magnetic powder is as follows: the iron-based soft magnetic powder is soaked in a 5-8 vol% dilute acid solution until the surface oxide layer is removed, and then the iron-based soft magnetic powder soaked in dilute acid is rinsed with deionized water until the cleaning solution is neutral. Then, it is placed in a 30-50 ppm activation solution and soaked at room temperature for 2-3 minutes. Finally, the activation solution is washed with deionized water to obtain the activated iron-based soft magnetic powder.

[0021] The oxide layer is removed using conventional dilute acid, and the surface catalytic sites are formed by activation with an activating solution (such as palladium chloride). The process is simple and easy to implement, providing a foundation for the uniform deposition of subsequent electroless copper plating.

[0022] Preferably, in step (3), under the stirring condition of 200-300W ultrasonic power, the reaction is carried out at 70-90℃ in a step-controlled temperature for 20-40 minutes. Specifically, the temperature is first kept at 70℃ for 5-15 minutes, then kept at 80℃ for 10-20 minutes, and finally kept at 90℃ for 5-15 minutes. During the temperature holding process, the stirring rate increases with the temperature and the stirring rate is between 200r / min and 300r / min. Finally, the copper plating thickness of the copper plating powder is 0.3-1μm.

[0023] The process employs a combination of stepped temperature control, variable speed stirring, and a fixed ratio of iron-based soft magnetic powder and copper plating solution. This allows copper atoms to be uniformly deposited at catalytic points on the surface of the iron-based soft magnetic powder, forming a thin copper plating layer with a thickness of 0.3–1 μm. The plating layer thickness is precisely controllable, ensuring sufficient copper source for subsequent grain boundary segregation while avoiding excessive plating layer that could lead to copper phase coarsening.

[0024] Preferably, in step (4), a staged heating drying method under vacuum conditions is adopted: the copper plating powder from step (3) is placed under a vacuum of ≤-0.09MPa, first kept at 35-45℃ for 0.5-1.5h, then kept at 55-65℃ for 1.5-2.5h, and finally kept at 75-85℃ for 0.5-1.5h, until the powder moisture content is ≤0.5%. This invention replaces the traditional single-temperature drying method, adopts a staged heating drying method with low, medium, and high temperatures, and controls the vacuum degree to ≤-0.09MPa, effectively avoiding agglomeration, plating cracking, and internal stress caused by rapid heating of the powder, ensuring the bonding between the plating and the iron-based soft magnetic powder, and at the same time keeping the powder moisture content ≤0.5%, avoiding oxidation and porosity during subsequent annealing.

[0025] The technical solution adopted by this invention to solve the second technical problem mentioned above is as follows: a soft magnetic composite material prepared by the above preparation method, wherein the soft magnetic composite material is composed of iron-based soft magnetic powder grains and copper nanoscale discontinuous segregated phases distributed at α-Fe grain boundaries; the copper nanoscale discontinuous segregated phases are in a discontinuous network and uniformly distributed at the grain boundaries, the copper content inside the grains is less than 0.05wt%, and there is no copper solid solution; the volume resistivity of the soft magnetic composite material is ≥3×10 5 μΩ·cm, thermal conductivity ≥70W / m·K, effective magnetic permeability ≥110 at 100kHz.

[0026] The technical solution adopted by the present invention to solve the third technical problem mentioned above is: an application of the above-mentioned soft magnetic composite material, which is suitable for power electronic devices with high power density and which need to take into account both heat dissipation and electromagnetic performance.

[0027] Compared with the prior art, the advantages of the present invention are as follows:

[0028] 1. Process Innovation: This invention differs fundamentally from the existing "continuous coating" and "uniform alloying" processes by not simply coating a continuous copper layer onto the surface of iron-based soft magnetic powder. Instead, it utilizes the thermodynamic properties of the Fe-Cu binary system to achieve precise control over the entire copper process—from surface deposition to directional diffusion, grain boundary segregation, and nanocrystalline precipitation—through a four-stage annealing process. This upgrades the traditional single-stage annealing process to a four-stage mild annealing. Each step is designed around the limited solid solution characteristics of copper in α-Fe, and a continuously flowing reducing atmosphere is maintained throughout the process to keep the annealing furnace under slight positive pressure. The reducing atmosphere and nitrogen are constantly replaced within the furnace to prevent copper atom oxidation. (1) Directional pre-diffusion stage: 500~600℃ is the single-phase region of α-Fe supersaturated solid solution with corresponding composition. Copper atoms diffuse slowly under thermal drive and preferentially agglomerate and enrich towards the grain boundary to achieve directional pre-distribution of Cu elements; (2) Two-phase segregation stage: 750~850℃ is the two-phase region of copper-rich phase and α-Fe. Based on the limited solid solubility of Cu in α-Fe, the solid solubility of Cu pre-enriched at the grain boundary in α-Fe decreases significantly during the heating process. Excess Cu cannot be dissolved in the grain and preferentially enriches and segregates along the grain boundary to form a copper-rich layer at the grain boundary. (3) Low temperature nanocrystallization stage: slowly cool down to 300-400℃ and keep warm to cause the continuous copper-rich layer at the grain boundary to spheroidize and break, forming 10-30nm discontinuously distributed nanoparticles, inhibiting further coarsening of the Cu-rich phase; (4) Grain boundary phase stabilization stage: Further slow cooling and heat preservation to stabilize the structure of the copper nanophase at the grain boundary and avoid phase structure changes during subsequent molding and service.

[0029] After annealing using the above method, copper elements can be observed under a microscope to form a nanoscale discontinuous segregation phase with a thickness of 10-30 nm at the grain boundaries. The copper content inside the grains is less than 0.05 wt%, and no copper solid solution is formed.

[0030] This invention employs a conventional chemical plating and four-stage graded annealing process, which has a high parameter tolerance rate, is suitable for large-scale industrial production, and can effectively reduce production costs.

[0031] 2. Performance Synergistic Breakthrough: Through full-process process control, excess Cu preferentially precipitates as a 10-30nm nanoscale discontinuous segregated phase at the α-Fe grain boundaries, with a grain boundary segregation efficiency of ≥90%, forming a unique microstructure of "no Cu solid solution inside the grains and Cu nanoscale phase isolation at the grain boundaries", achieving the dual continuous function of "Cu-rich phase insulation at grain boundaries and thermal and magnetic conductivity of iron grains". The prepared copper-plated soft magnetic composite material exhibits a synergistic effect of high resistivity and high thermal conductivity, with a volume resistivity ≥3×10⁻⁶. 5μΩ·cm, thermal conductivity ≥70W / m·K, and effective magnetic permeability ≥110 at 100kHz, which solves the core contradiction of existing technology that "insulation is heat insulation".

[0032] 3. Precise application scenarios: The soft magnetic composite material prepared by this invention is suitable for high power density power electronic devices such as new energy vehicles and photovoltaic inverters, which require both heat dissipation and electromagnetic performance. It fills the performance gap of existing copper-containing soft magnetic materials and can be widely used in high-end power electronic equipment, with broad market application prospects. Attached Figure Description

[0033] Figure 1 This is a SEM image of the original surface of the iron-based soft magnetic powder (pure iron powder) before activation in Example 1 of the present invention. Figure 2 This is a SEM image of the surface morphology of the powder particles after chemical copper plating in Example 1 of the present invention (i.e. after step (3)). Figure 3 for Figure 2 EDS surface distribution of copper on the surface of the powder particles in the box; Figure 4 for Figure 2 EDS surface distribution of iron on the surface of the powder particles in the box; Figure 5 This is a SEM image of the cross-section of the powder particles after annealing (i.e. after step (6)) in Embodiment 1 of the present invention; Figure 6 This is an EDS surface scan of iron element spectrum of the powder particles after annealing (i.e. after step (6)) in Embodiment 1 of the present invention; Figure 7 This is an EDS line scan copper element spectrum of the powder particles after annealing (i.e. after step (6)) in Embodiment 1 of the present invention; Figure 8 This is an EDS element line scan distribution spectrum of the cross section of the powder particles after annealing (i.e. after step (6)) in Embodiment 1 of the present invention. Detailed Implementation

[0034] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Unless otherwise specified, the reagents, materials, equipment, etc. used in the embodiments can be obtained commercially. The embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0035] Example 1

[0036] The preparation method of the copper-plated soft magnetic composite material in this embodiment includes the following steps: (1) Preparation and activation of iron-based soft magnetic powder: The iron-based soft magnetic powder in this embodiment is pure iron powder prepared by water atomization, purchased from Shanxi Xinsheng New Material Co., Ltd., with a loose density of 3.5 g / cm³, D50=146 μm, and D99=293 μm. The SEM morphology of the pure iron powder is shown in the figure. Figure 1 As shown, pure iron powder was soaked in 6 vol% hydrochloric acid solution for 4 min to remove the surface oxide layer. Then, the pure iron powder soaked in dilute acid was rinsed with deionized water until the cleaning solution was neutral. After that, it was placed in 40 ppm palladium chloride activation solution and soaked at room temperature for 2.5 min. After rinsing with deionized water, activated iron powder was obtained. (2) Preparation of copper plating solution: According to concentration, copper sulfate (CuSO4·5H2O) is used as copper source to prepare a 30g / L solution. Sodium hypophosphite (reducing agent) is added to make its concentration 25g / L, sodium citrate (complexing agent) to make its concentration 30g / L, potassium ferrocyanide (accelerator) to make its concentration 8g / L, polyethylene glycol (surfactant) to make its concentration 1g / L, potassium iodate and thiourea (stabilizer) to make its total concentration 0.5g / L (potassium iodate and thiourea mass ratio is 1:1), and the remainder is deionized water. The pH value of the copper plating solution is adjusted to 8 with ammonia water; the mass ratio of complexing agent to copper source is 1:1; the order of the above steps (1) and (2) is not important. (3) Chemical plating: Under ultrasonic stirring conditions with an ultrasonic power of 250W, activated iron powder and copper plating solution are mixed at a mass ratio of 1:10. The reaction is carried out in a step-controlled temperature for 30 minutes. First, the temperature is kept at 70℃ for 10 minutes with a stirring rate of 200r / min. Then, the temperature is kept at 80℃ for 15 minutes with a stirring rate of 250r / min. Then, the temperature is kept at 90℃ for 5 minutes with a stirring rate of 300r / min. After the reaction, solid and liquid are separated (the solid and liquid separation method is the existing one, such as centrifugation, vacuum filtration, etc.). The separated solid material is rinsed with deionized water until the cleaning solution is neutral to obtain copper plating powder. The copper plating layer thickness of the copper plating powder is about 0.6μm. SEM images of the surface morphology of the powder particles after chemical copper plating are shown below. Figure 2 As shown, a thin copper plating layer is uniformly covered on the surface of the powder; the EDS surface distribution spectrum of copper on the surface of the powder particles after electroless copper plating is shown in the figure. Figure 3 As shown, copper is uniformly distributed on the powder surface; the EDS surface distribution pattern of iron on the powder particle surface after electroless copper plating is shown below. Figure 4 As shown; (4) Gradient vacuum drying: The copper-plated powder is placed in a vacuum drying oven with a vacuum degree ≤ -0.09MPa, first kept at 40℃ for 1h, then kept at 60℃ for 2h, and finally kept at 80℃ for 1h. This graded heating and drying method ensures that the final powder moisture content is ≤0.5%. (5) Low-temperature pre-crystallization treatment: The dried powder is placed in an argon atmosphere and heated to 220°C at 3°C / min. After holding at the temperature for 45 min, it is naturally cooled to room temperature. (6) Four-stage grain boundary precise control annealing: The powder treated in step (5) is placed in a tube annealing furnace and a mixed reducing atmosphere of 75 vol% hydrogen and 25 vol% nitrogen is continuously introduced to maintain a slight positive pressure in the annealing furnace. The reducing atmosphere in the annealing furnace is replaced in real time. The atmosphere flow rate is 0.8 L / min. Four-stage annealing is carried out: (6.1) The temperature is increased to 550℃ at 5℃ / min and held for 20 min; (6.2) The temperature is increased to 800℃ at 10℃ / min and held for 20 min; (6.3) The temperature is decreased to 350℃ at 2℃ / min and held for 75 min; (6.4) The temperature is decreased to 220℃ at 1℃ / min and held for 100 min. Then, the temperature is cooled to below 150℃ with the furnace. SEM images of the cross-section of the powder particles after annealing are shown below. Figure 5 As shown; the cross-sectional EDS scan iron element spectrum of the annealed powder particles is as follows. Figure 6 As shown, iron is the main matrix element of the powder; the cross-sectional EDS line scan copper element spectrum of the annealed powder particles is shown below. Figure 7 As shown, scanning along the particle boundary to the particle interior reveals a sharp increase in copper signal at the boundary, while the copper signal inside the particle is below the detection limit; the EDS elemental line scan distribution of the cross-section of the annealed powder particles is shown below. Figure 8 As shown, from the grain boundary to the core, the copper element signal gradually decreases with increasing scanning depth, while the iron element signal gradually increases with increasing scanning depth, indicating that copper element is enriched at the grain boundary and has a very low copper element content inside the grain. (7) Plasma modification: The annealed powder is placed in a plasma furnace and a mixed gas (argon and 8 vol% methane) is introduced. The plasma power is 120 W, and the treatment is carried out at room temperature for 12 min. The agglomerated particles are removed by passing through a 100-mesh sieve, and finally copper-plated soft magnetic composite material is obtained.

[0037] Example 2

[0038] The difference between this embodiment and Example 1 is as follows: In step (2), the copper source is copper chloride with a concentration of 25 g / L, the complexing agent is disodium ethylenediaminetetraacetate with a concentration of 25 g / L, and the mass ratio of the complexing agent to the copper source is 1.2:1; in step (6), the segregation temperature of the two-phase region is 750℃, and the temperature is maintained for 30 min. The remaining steps and parameters are the same as in Example 1.

[0039] Example 3

[0040] The difference between this embodiment and Embodiment 1 is that: in step (1), the iron-based soft magnetic powder is iron-silicon alloy powder (Si mass fraction is 3.5%); in step (6), the segregation temperature of the two-phase region is 850℃, and the holding time is 15min. The remaining steps and parameters are the same as in Embodiment 1.

[0041] Example 4

[0042] The difference between this embodiment and Embodiment 1 is that step (7) plasma modification is not performed. After step (6) annealing is completed, the furnace is cooled to room temperature and the agglomerated particles are removed by passing through a 100-mesh sieve to obtain the copper-plated soft magnetic composite material. The remaining steps and parameters are the same as in Embodiment 1.

[0043] Comparative Example 1

[0044] The difference between this comparative example and Example 1 is that step (6) uses a single low-temperature annealing, that is, directly heating to 350°C at 10°C / min, holding for 120 min and then cooling with the furnace, without four-stage grain boundary control. The remaining steps and parameters are the same as in Example 1.

[0045] Comparative Example 2

[0046] The difference between this comparative example and Example 1 is that step (6) uses high-temperature homogenization annealing, that is, heating to 1050°C at 10°C / min, holding for 2 hours and then cooling with the furnace, without four-stage grain boundary control. The remaining steps and parameters are the same as in Example 1.

[0047] Comparative Example 3

[0048] The difference between this comparative example and Example 1 is that steps (2), (3), and (4) are different, while the rest are the same as in Example 1.

[0049] In this embodiment: (2) Preparation of phosphating solution: The phosphating solution includes phosphoric acid, silane coupling agent and epoxy resin. The mass percentage of phosphoric acid relative to iron-based soft magnetic powder (activated pure iron powder) is 0.3 wt%, the mass percentage of silane coupling agent relative to iron-based soft magnetic powder (activated pure iron powder) is 1.0 wt%, and the mass percentage of epoxy resin relative to iron-based soft magnetic powder (activated pure iron powder) is 0.5 wt%. (3) Chemical plating: Under ultrasonic stirring conditions of 250W, activated iron powder and phosphating solution were stirred and reacted at 50°C for 30 minutes, and then rinsed with acetone until the cleaning solution was neutral. (4) Drying: Dry in an oven at 100°C until the moisture content is ≤0.5%.

[0050] The copper-plated soft magnetic composite materials obtained in each embodiment and comparative example were pressed into toroidal cores with a diameter of φ25mm×φ15mm×5mm under 1000MPa, and then subjected to performance testing after heat treatment in a nitrogen atmosphere at 650℃ for 1h.

[0051] Table 1: Sample performance test results of each embodiment and comparative example

[0052] As can be seen from Table 1:

[0053] (1) In Examples 1-4, through four-stage grain boundary precise control annealing, efficient segregation of copper at the α-Fe grain boundary was achieved, forming a 15-30 nm nanoscale discontinuous segregated phase, ultimately obtaining the synergistic performance of "high resistance and high thermal conductivity", with a volume resistivity of not less than 3×10 5 μΩ·cm, thermal conductivity is not less than 70W / m·K, and magnetic properties are excellent, which perfectly solves the core contradiction between high resistance and high thermal conductivity in the prior art; among them, Example 4 has no plasma modification step, thermal conductivity is slightly improved and resistivity is slightly decreased, still meeting the core performance indicators, verifying that step (7) is an optional optimization step and does not affect the realization of the core performance of the present invention.

[0054] (2) In Comparative Example 1, due to the lack of grain boundary control technology, copper did not undergo effective segregation, and the resistivity was only 4.5 × 10⁻⁶. 4 μΩ·cm, which is far lower than in the various embodiments, and cannot meet the insulation requirements of high-power scenarios.

[0055] (3) Comparative Example 2 uses existing alloying homogenization annealing, copper is completely dissolved in the iron matrix. Although the magnetic properties are still acceptable, the resistivity and thermal conductivity are significantly degraded, and performance synergy cannot be achieved.

[0056] (4) Comparative Example 3 uses a traditional phosphate insulation layer. Although the resistivity is close to that of the Example, the thermal conductivity is only 15 W / m·K, which has a thermal barrier problem and cannot meet the heat dissipation requirements of high power density devices. At the same time, the non-magnetic phosphate layer destroys the continuity of the magnetic circuit, the effective permeability is significantly lower than that of Example 1, and the coercivity (125 A / m) is significantly increased, highlighting the inherent defects of the traditional insulation coating technology.

[0057] The key to achieving the above-mentioned performance in this invention lies in the formation of a bicontinuous structure of "grain boundary insulation and grain thermal conductivity". When current attempts to pass through iron-based soft magnetic powder particles, it must pass through the high-resistivity grain boundary segregation layer, thus being effectively blocked; while heat is mainly conducted through the low-thermal-resistance iron grains, avoiding the thermal barrier effect of the continuous insulating layer. This structure is a thermodynamically inevitable result of the Fe-Cu system during annealing in the two-phase region: at the annealing temperature, the equilibrium solid solubility of copper in α-Fe is only about 2%, and the excess copper in the coating (the total copper content is usually greater than 5%) cannot be completely dissolved and is forced to segregate to the lower-energy grain boundary positions, forming nanoscale precipitates after cooling.

[0058] This invention uses iron-based soft magnetic powder as a substrate. After removing the oxide layer with dilute acid and activating with palladium chloride, a thin copper coating of 0.3–1 μm is prepared by step-temperature controlled chemical plating. Following gradient drying and low-temperature pre-crystallization, a four-stage grain boundary controlled annealing process is employed. Utilizing the limited solid solution of copper in α-Fe (no more than 2 at%), copper preferentially precipitates as a 10–30 nm discontinuous phase at the grain boundaries. Finally, plasma modification is performed to obtain the final product. The composite material prepared by this invention has a volume resistivity of not less than 3 × 10⁻⁶. 5 With a resistance of μΩ·cm and a thermal conductivity of not less than 70W / m·K, it achieves synergistic optimization of high resistance and high thermal conductivity, and is suitable for high power density power electronic inductors such as new energy vehicles and photovoltaic inverters.

[0059] The testing standards for the above performance metrics are as follows: (1) Density test is based on GB / T5163-2006 "Determination of density, open porosity and total porosity of permeable sintered metal materials (excluding cemented carbide)"; (2) Magnetic properties (saturation magnetic induction and coercivity) were tested in accordance with GB / T13012-2008 "Measurement Method of DC Magnetic Properties of Soft Magnetic Materials"; (3) The effective permeability test at 100kHz was conducted in accordance with GB / T28869.3-2023 "Measuring methods for magnetic cores made of soft magnetic materials - Part 3: Magnetic properties under high excitation levels"; (4) Volume resistivity was tested in accordance with GB / T351-2019 "Methods for measuring resistivity of metallic materials", using a four-probe tester and a test pressure of 5 MPa; (5) Thermal conductivity test is performed according to GB / T22588-2008 "Measuring thermal diffusivity or thermal conductivity by flash method".

Claims

1. A method for preparing a copper-plated soft magnetic composite material, characterized in that, Includes the following steps: (1) Prepare and activate iron-based soft magnetic powder; (2) Prepare a copper plating solution containing a copper source; the order of steps (1) and (2) above is not important; (3) Chemical plating: The activated iron-based soft magnetic powder in step (1) is mixed with the copper plating solution in step (2) at a mass ratio of 1:5 to 1:15, and chemical plating is carried out at 70 to 90°C. After the reaction, the reaction solution is separated into solid and liquid. The solid material is rinsed with deionized water until the cleaning solution is neutral to obtain copper plating powder. (4) Drying: Dry the copper plating powder from step (3) until the moisture content is ≤0.5%; (5) Low-temperature pre-crystallization treatment: Place the dried powder from step (4) in a vacuum or inert atmosphere, heat it to 200-250°C at 2-5°C / min, keep it at that temperature for 30-60 min, and then cool it to room temperature; (6) Annealing: The powder treated in step (5) is subjected to four stages of annealing, with a continuous flow of reducing atmosphere and nitrogen throughout the process: (6.1) Increase the temperature to 500-600℃ at a rate of 3-8℃ / min and hold for 10-30 min; (6.2) Increase the temperature to 750-850℃ at a rate of 8-12℃ / min and hold for 15-30 min; (6.3) Cool down to 300-400℃ at a rate of 1-3℃ / min and hold for 60-90min; (6.4) Cool down to 200-250℃ at 1-2℃ / min, hold for 90-120min, and then cool down to below 150℃.

2. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that, It also includes step (7) plasma modification: the annealed powder is placed in a plasma furnace, and a mixture of argon and methane is introduced, wherein the volume fraction of methane in the mixture is 5-10 vol%, the plasma power is 100-150 W, and the mixture is treated at room temperature for 10-15 min to obtain the copper-plated soft magnetic composite material.

3. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that: In step (1), the iron-based soft magnetic powder is at least one of pure iron powder, iron-silicon alloy powder, iron-silicon-aluminum alloy powder, iron-aluminum alloy powder, iron-chromium alloy powder, iron-cobalt alloy powder, and iron-silicon-chromium alloy powder.

4. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that: In step (1), the activation method for iron-based soft magnetic powder is as follows: the iron-based soft magnetic powder is soaked in dilute acid solution until the surface oxide layer is removed, and then the iron-based soft magnetic powder soaked in dilute acid is rinsed with deionized water until the cleaning solution is neutral. Then, it is placed in the activation solution and soaked at room temperature for 2-3 minutes. Finally, the activation solution is washed with deionized water to obtain activated iron-based soft magnetic powder.

5. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that: In step (2), the copper plating solution contains, by concentration, 20-35 g / L of copper source, 15-35 g / L of reducing agent, 20-35 g / L of complexing agent, 5-10 g / L of accelerator, 0.05-2 g / L of surfactant, 0.01-1 g / L of stabilizer, and pH adjuster, with the remainder being deionized water. The pH adjuster maintains the pH value of the copper plating solution at 7-9. The copper source is at least one of copper sulfate, copper chloride, and basic copper carbonate; the reducing agent is at least one of sodium hypophosphite and sodium borohydride; the complexing agent is at least one of citric acid, sodium citrate, tartaric acid, potassium sodium tartrate, ethylenediaminetetraacetic acid, or sodium ethylenediaminetetraacetic acid; the accelerator is at least one of potassium ferrocyanide, 2,2'-bipyridine, and o-phenanthroline; the surfactant is at least one of polysorbate-80, sorbitan monostearate, polyethylene glycol, and sodium dodecyl sulfate; the stabilizer is at least two of potassium iodate, potassium nitrate, fumaric acid, or thiourea; and the pH adjuster is at least one of sodium hydroxide, ammonia, and sodium carbonate.

6. The method for preparing the copper-plated soft magnetic composite material according to claim 5, characterized in that: In step (2), the mass ratio of the complexing agent to the copper source is 1:1 to 1.2:

1.

7. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that: In step (3), under the stirring condition of 200-300W ultrasonic power, the reaction is carried out at 70-90℃ in a step-controlled temperature for 20-40 minutes. Specifically, the temperature is first kept at 70℃ for 5-15 minutes, then kept at 80℃ for 10-20 minutes, and finally kept at 90℃ for 5-15 minutes. During the temperature holding process, the stirring rate increases between 200r / min and 300r / min as the temperature increases.

8. The method for preparing the copper-plated soft magnetic composite material according to claim 1, characterized in that, In step (4), a drying method of graded heating under vacuum conditions is adopted: the copper plating powder from step (3) is placed under a vacuum of ≤-0.09MPa, first kept at 35-45℃ for 0.5-1.5h, then kept at 55-65℃ for 1.5-2.5h, and finally kept at 75-85℃ for 0.5-1.5h, until the moisture content of the powder is ≤0.5%.

9. A soft magnetic composite material prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The soft magnetic composite material is composed of iron-based soft magnetic powder grains and copper nanoscale discontinuous segregation phases distributed at α-Fe grain boundaries; the copper nanoscale discontinuous segregation phases are in a discontinuous network and uniformly distributed at the grain boundaries, with a copper content of less than 0.05 wt% and no copper solid solution inside the grains; the volume resistivity of the soft magnetic composite material is ≥3×10⁻⁶. 5 μΩ·cm, thermal conductivity ≥70W / m·K, effective magnetic permeability ≥110 at 100kHz.

10. An application of the soft magnetic composite material according to claim 9, characterized in that, This soft magnetic composite material is suitable for power electronic devices with high power density that require consideration of both heat dissipation and electromagnetic performance.

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