Preparation method of nickel-phosphorus plated soft magnetic composite material and soft magnetic composite material

Abstract

This invention relates to a method for preparing a nickel-phosphorus soft magnetic composite material and a soft magnetic composite material. The preparation method includes the following steps: (1) preparing iron-based soft magnetic powder and activating it; (2) preparing a chemical plating solution containing a nickel source and a phosphorus source; (3) chemical plating; (4) drying; (5) annealing: ① introducing a high-purity argon atmosphere, heating to 200-250℃ at 3-5℃ / min, and holding for 30-60min; ② maintaining a high-purity argon atmosphere, heating to 420-480℃ at 5-8℃ / min, and holding for 20-40min; ③ switching to a low-hydrogen reducing atmosphere, cooling to 300-350℃ at 3-5℃ / min, and holding for 30-180min; ④ switching back to a high-purity argon atmosphere, cooling to 250-300℃ at 1-2℃ / min, and holding for 60-120min to obtain a nickel-phosphorus soft magnetic composite material, thereby achieving a synergistic improvement in insulation and bonding strength.

Description

Technical Field

[0001] This invention belongs to the field of soft magnetic composite material technology, specifically relating to a method for preparing a nickel-phosphorus plated soft magnetic composite material and the soft magnetic composite material itself. Background Technology

[0002] As power electronic devices develop towards higher frequencies, smaller sizes, and higher power densities, demanding applications such as automotive electronics and offshore wind power require soft magnetic materials to exhibit synergistic performance requirements of high insulation, high permeability, low loss, and high reliability. Electroless Ni-P alloy plating, due to its excellent corrosion resistance, adjustable resistivity, and good formability, is the mainstream technology for insulating soft magnetic powder coatings; however, existing technologies still face core bottlenecks that are difficult to overcome.

[0003] For example, the Chinese invention patent application with patent application number CN201510391210.2 (publication number CN105039831A) discloses a method for preparing a high-strength ternary soft magnetic alloy, which uses a uniform Ni-P coating combined with single-temperature vacuum sintering to prepare the soft magnetic alloy. However, it has significant inherent defects: the uniform coating cannot simultaneously achieve high insulation and interfacial bonding force, single-temperature annealing cannot suppress the precipitation of Ni3P brittle phase, and the gradient-free structural design makes it difficult to achieve synergistic optimization of multiple properties.

[0004] For example, the Chinese invention patent application with patent application number CN201310382683.7 (publication number CN103481025B) discloses a method for modifying the surface of a copper roller in a crystallizer. It develops a Ni-P-based coating modification process for copper rollers in crystallizers. Although it can achieve gradient coating design, it is only suitable for macroscopic workpieces and cannot be adapted to micro-nano soft magnetic powders. The coating lacks metallurgical bonding and is prone to interface failure. It lacks dedicated gradient phase transition control for soft magnetic materials and has insufficient high-frequency loss suppression effect, making it unsuitable for MHz-level application scenarios.

[0005] For example, Chinese invention patent application CN200910096620.9 (publication number CN101514449A) discloses a composite chemical nickel-phosphorus plating method, which uses a double-layer Ni-P plating layer to improve the corrosion resistance of bulk permanent magnets. However, it cannot meet the requirements of soft magnetic powder coating. The heterogeneous interface is prone to high-temperature delamination failure. It lacks precise phase change control and is difficult to suppress the precipitation of harmful Ni3P phase. It cannot simultaneously achieve insulation, corrosion resistance and magnetic properties.

[0006] In summary, existing technologies have not yet overcome the common industry bottlenecks of the inability to simultaneously achieve insulation and bonding strength, the inability to coordinate magnetic properties and corrosion resistance, and poor process adaptability. Summary of the Invention

[0007] The first technical problem to be solved by the present invention is to provide a method for preparing a nickel-phosphorus soft magnetic composite material with high insulation, high magnetic permeability and good interfacial bonding, in light of the current state of the prior art.

[0008] 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.

[0009] The technical solution adopted by the present invention to solve the first technical problem mentioned above is as follows: A method for preparing a nickel-phosphorus plated soft magnetic composite material includes the following steps: (1) Prepare iron-based soft magnetic powder and activate the iron-based soft magnetic powder to form activated powder; (2) Prepare a chemical plating solution containing nickel and phosphorus sources; the order of steps (1) and (2) above is not important; (3) Chemical plating: The chemical plating solution of step (2) and the activated powder of step (1) are mixed at a liquid-solid mass ratio of 5:1 to 20:1. During the reaction, a variable temperature and speed and pulsed ultrasound synergistic process is adopted. The temperature range during the reaction is 70 to 100℃, and the stirring rate increases between 150 and 300 r / min as the temperature increases. After the reaction is complete, solid-liquid separation is performed. The separated solid phase is washed with 60°C deionized water until the cleaning solution is neutral, and finally the solid phase coating powder is obtained. (4) Drying: Dry the coated powder from step (3) until the powder moisture content is ≤0.5%, and remove agglomerated particles by passing it through a 100-mesh sieve; (5) Annealing: The dried powder is spread evenly in a corundum crucible with a thickness of ≤5mm, and placed in a tube annealing furnace. The atmosphere flow rate is 0.5~1L / min throughout the process, and the oxygen content in the furnace is <1vol%. Four-stage annealing treatment is carried out in sequence: ① Stress relief pre-crystallization stage: High-purity argon atmosphere is introduced, and the temperature is raised to 200-250℃ at 3-5℃ / min and held for 30-60min to relieve the internal stress of the coating and form uniform Ni-Fe crystal nuclei in the interface layer; ②Interfacial metallurgical bonding stage: Maintain a high-purity argon atmosphere, raise the temperature to 420-480℃ at 5-8℃ / min, and hold for 20-40min to allow nickel and phosphorus elements in the interfacial layer to diffuse into the iron-based soft magnetic powder at the grain boundary, forming a continuous Fe-Ni-P solid solution layer with a thickness of 50-100nm. ③ Gradient phase transition control stage: Switch to a low-hydrogen reducing atmosphere, cool down to 300-350℃ at 3-5℃ / min, and hold for 30-180min to form a gradient structure of high-P amorphous Ni-P on the surface and low-P nanocrystalline Ni-P in the middle, while suppressing the precipitation of Ni3P phase; The low-hydrogen reducing atmosphere is a mixture of 5vol%-10vol% hydrogen and the balance argon. ④Structural stabilization stage: Switch back to high-purity argon atmosphere, slowly cool to 250-300℃ at 1-2℃ / min, hold for 60-120min, then cool with the furnace to below 150℃ and remove to obtain nickel-phosphorus soft magnetic composite material.

[0010] During the annealing process, Ni-Fe grain boundary diffusion is first achieved through a high-temperature range of 420–480℃ to form a metallurgically bonded solid solution layer, solving the problem of poor interfacial bonding. Then, the temperature is lowered to 300–350℃ for gradient phase transformation control. By utilizing the difference in the critical temperature of amorphous transformation of coatings with different phosphorus contents, the high-phosphorus coating on the surface maintains an amorphous insulating structure, while the low-phosphorus coating in the middle layer undergoes controllable nanocrystallization. At the same time, the grain boundary segregation effect of the crystallization inhibitor suppresses the long-range diffusion of nickel atoms, preventing the nucleation and growth of the Ni3P phase from a thermodynamic perspective. The atmosphere is dynamically switched throughout the process to avoid powder oxidation while ensuring the precise achievement of the phase transformation targets at each stage.

[0011] In the above scheme, 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, iron-silicon-chromium alloy powder, and iron-based amorphous powder.

[0012] Preferably, in step (1), the activation method of the iron-based soft magnetic powder is as follows: the iron-based soft magnetic powder is placed in a 5 vol% to 8 vol% dilute acid solution and ultrasonically soaked at room temperature for 3 to 8 minutes at a power of 100 to 150 W, simultaneously completing the removal of the surface oxide layer and micro-nano-scale roughening, controlling the surface roughness Ra of the powder to be between 50 and 200 nm, then rinsed with deionized water until the cleaning solution is neutral, and immediately placed in a sensitization activation solution, and soaked at a constant temperature of 70 to 75°C for 2 to 5 minutes to complete in-situ autocatalytic activation, so as to form an areal density of not less than 10 on the surface of the powder. 8 Zero-valent nickel (Ni) per cm² 0 The active clusters are then rinsed again with deionized water until the cleaning solution is neutral to obtain activated powder. The sensitizing and activating solution includes 2-5 g / L nickel sulfate and 10-20 g / L sodium hypophosphite; the dilute acid solution is hydrochloric acid solution or sulfuric acid solution; the ultrasonic frequency is 30-40 kHz.

[0013] Dilute acid ultrasonic micro-coarsening forms nanoscale pit anchoring structures on the powder surface, providing mechanical interlocking sites for the coating and improving the bonding force from the root of the interface structure. Palladium-free in-situ autocatalytic activation at 70°C utilizes the reduction properties of sodium hypophosphite at high temperatures to pre-deposit zero-valent nickel active clusters on the powder surface, replacing the traditional palladium chloride noble metal activation. While ensuring the activation effect, the raw material cost is reduced by more than 30%. The electrochemical principle is feasible and can be stably reproduced by those skilled in the art.

[0014] Preferably, after the annealing in step (5), if the powder is agglomerated, it is deagglomerated by roller pressing and light crushing, and the final product is obtained after passing through a 100-mesh sieve. The product has D50 < 200 μm and D99 < 350 μm.

[0015] Preferably, in step (2), the electroless plating solution comprises the following components by concentration: 20-35 g / L nickel source, 25-35 g / L sodium hypophosphite (phosphorus source), 20-40 g / L complexing agent, 10-20 g / L sodium acetate buffer, 0.01-1 g / L composite stabilizer, 0.05-0.5 g / L gradient nucleation regulator, 0.02-0.2 g / L crystallization inhibitor, 0.01-0.1 g / L fluorocarbon interface wetting agent, pH adjuster, and the balance being deionized water; the pH value of the electroless plating solution is stabilized at 7.5-9.0 using the pH adjuster.

[0016] Preferably, in step (2), the nickel source is at least one of nickel sulfate, nickel chloride, nickel aminosulfonate, and nickel fluoroborate; the composite stabilizer is at least two of potassium iodate, potassium nitrate, fumaric acid, and thiourea; the pH adjuster is at least one of sodium hydroxide, ammonia, and sodium carbonate; the fluorocarbon interface wetting agent is at least one of polyoxyethylene ether containing fluorinated fatty alcohol, polyoxyethylene containing fluorinated phenol, and polyoxyethylene ester containing fluorinated carboxylic acid; the complexing agent is at least one of a combination of citric acid and sodium citrate, a combination of tartaric acid and potassium sodium tartrate, and a combination of disodium ethylenediaminetetraacetate and sodium citrate, wherein the mass ratio of the two components in each combination is 1:1 to 1:2; the gradient nucleation regulator is a combination of sodium dodecyl sulfate and polyethylene glycol, wherein the mass ratio is 1:2 to 1:5; and the crystallization inhibitor is at least one of sodium molybdate and sodium tungstate.

[0017] A dual complexing agent system is adopted, utilizing two complexing agents with nickel ions (Ni ²+ The difference in complexation stability constants, combined with the temperature-sensitive micelle structure of the gradient nucleation regulator, enables continuous control of the sodium hypophosphite reduction rate during the three-stage temperature-controlled coating process. This results in a gradient coating with phosphorus content continuously decreasing from the surface to the interface. A high-phosphorus coating is deposited in the low-temperature stage, a medium-phosphorus coating in the medium-temperature stage, and a low-phosphorus coating in the high-temperature stage, laying the foundation for subsequent gradient phase transition.

[0018] Preferably, in step (4), the coated powder from step (3) is placed in an environment with a vacuum degree ≤ -0.09MPa and treated with a gradient vacuum pulse drying process: first, it is kept at 30-50℃ for 0.5-2h, during which vacuum pulses 1-5 times, and after each vacuum is broken to normal pressure, it is re-evacuated to the target vacuum degree and held for 2-10min; then, it is kept at 50-70℃ for 1-3h, during which vacuum pulses 1-5 times; finally, it is kept at 70-100℃ for 0.5-2h, during which vacuum pulses 1-5 times; finally, it is dried until the powder moisture content is ≤0.5%, and the agglomerated particles are removed by passing it through a 100-mesh sieve.

[0019] Conventional hot air / high-temperature static vacuum drying is prone to causing the coating layer to bubble, crack, and fall off due to the rapid vaporization of solvents / moisture. However, the combination of gradient heating and pulsed vacuum in this invention can gradually remove moisture / solvents. With the pressure alternation of "normal pressure-high vacuum", the residual solvent inside the coating layer can be smoothly released, and the coating layer will not be damaged by the sudden increase of internal pressure. This ensures the density and continuity of the insulation layer, and ultimately achieves stable high-frequency and low-loss performance of the powder.

[0020] This invention employs a gradient heating design. In the initial low-temperature stage, most of the free water is removed, preventing the powder from boiling and splashing or damaging the coating layer due to high moisture content and sudden high temperature. In the intermediate medium-temperature stage, the bound water inside the particles is removed. In the final high-temperature stage, deep drying is completed, ensuring overall drying efficiency and avoiding the risk of powder performance degradation caused by high temperature throughout the process. At the same time, it ensures uniform heating of the entire batch of powder, with minimal fluctuations in moisture content and magnetic properties between batches.

[0021] Preferably, in step (3), chemical plating is preferably performed under ultrasonic conditions, with a pulsed ultrasonic frequency between 100 and 550 W and a duty cycle of 30% to 50%. The specific process parameters for each stage of the reaction are as follows: ① First stage: 70-75℃, 150-200r / min, 100-150W pulsed ultrasound, duty cycle 30%-50%, hold for 10-20min; ② Second stage: 80-85℃, 200-250r / min, 200-250W pulsed ultrasound, duty cycle 30%-50%, hold for 10-20min; ③ Third stage: 90-100℃, 250-300r / min, 150-200W pulsed ultrasound, duty cycle 30%-50%, hold for 5-20min.

[0022] Preferably, in step (3), the phosphorus content of the coating obtained in the first stage is 8-12 wt%, the phosphorus content of the coating obtained in the second stage is 5-8 wt%, and the phosphorus content of the coating obtained in the third stage is 2-5 wt%. Step (3) finally obtains a gradient Ni-P coating with a total thickness of 0.5-2 μm and a continuously decreasing phosphorus content, with the phosphorus content decreasing from 8-12 wt% on the surface to 2-5 wt% at the iron-based soft magnetic powder interface, and the coating thickness uniformity deviation is ≤ ±8%.

[0023] The technical solution adopted by the present invention to solve the second technical problem mentioned above is as follows: a soft magnetic composite material is prepared by the above preparation method, the soft magnetic composite material is composed of an iron-based soft magnetic powder core and a coating layer covering its surface, the coating layer has no Ni3P crystalline phase precipitation, and the coating layer has the following structure: The surface layer is an insulating layer with a phosphorus content of 8-12 wt%, which has a Ni-P amorphous phase, with an amorphous phase ratio of ≥90%, and the surface layer thickness accounts for 40%-50% of the total coating thickness; The middle layer is a transition layer with a phosphorus content of 5-8 wt%, which has Ni-P nanocrystals with a grain size of 10-30 nm. The thickness of the middle layer accounts for 30%-40% of the total coating thickness. The interface layer is a Fe-Ni-P solid solution with no abrupt phase interface, and its thickness accounts for 10% to 20% of the total coating thickness.

[0024] This invention employs a Ni-P gradient coating as the insulating phase. Compared to non-magnetic insulating components such as phosphating layers, this avoids the pinning effect of non-magnetic interfaces on magnetic domain wall motion and the disruption of magnetic circuit continuity. Simultaneously, interfacial metallurgical bonding eliminates interfacial air gaps, significantly reducing the impact of demagnetizing fields. Compared to existing non-magnetic insulating coating technologies, this invention achieves an order-of-magnetic increase in volume resistivity without attenuation of effective permeability, thus achieving a balance between high frequency, low loss, and high permeability.

[0025] In the above scheme, the volume resistivity of the soft magnetic composite material is ≥5×10⁻⁶. 5 μΩ·cm, effective permeability at 100kHz ≥180, coercivity ≤90A / m, saturation magnetic induction ≥1.5T, time to first appearance of red rust under neutral salt spray environment ≥120h, coating interface adhesion ≥30N.

[0026] Compared with the prior art, the advantages of the present invention are as follows: (1) Solve the long-standing industry paradox that "high insulation and interfacial bonding are mutually exclusive" in existing uniform Ni-P coatings, and achieve synergistic improvement in insulation and bonding. (2) To solve the problem of a sharp drop in resistivity and an increase in magnetic loss caused by the precipitation of harmful Ni3P phase during the annealing process of Ni-P coating, and to suppress the generation of harmful phase from the thermodynamic source; (3) Solve the problem of magnetic circuit interruption and significant decrease in magnetic permeability caused by non-magnetic insulating layer, and achieve a balance between high insulation and high magnetic permeability; (4) It solves the problems of poor compatibility of existing powder systems, high activation cost of precious metals, and poor batch stability of processes, and achieves low-cost, large-scale mass production, which is suitable for high-reliability scenarios such as high-frequency power inductors, automotive electronics, and offshore wind power. Attached Figure Description

[0027] Figure 1 This is a SEM image of the surface of pure iron powder before activation in Example 1 of the present invention; Figure 2 This is a SEM image of the powder after chemical plating (after step (3)) in Example 1 of the present invention; Figure 3 for Figure 2 EDS surface distribution diagram of iron on the surface of the powder in the box; Figure 4 for Figure 2 EDS surface distribution diagram of nickel on the surface of the powder in the box; Figure 5 for Figure 2 EDS surface distribution diagram of phosphorus on the surface of the powder in the box; Figure 6 This is an EDS line scan spectrum of the cross-section of the powder particles after annealing (after step (5)) in Example 1 of the present invention. Detailed Implementation

[0028] The present invention will be further described in detail below with reference to the accompanying drawings, embodiments, and comparative examples. Unless otherwise specified, the reagents, materials, equipment, etc., used in the embodiments are commercially available; the embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0029] Example 1

[0030] The preparation method of the nickel-phosphorus soft magnetic composite material in this embodiment includes the following steps: (1) Prepare iron-based soft magnetic powder and activate it to form activated powder: Water atomized pure iron powder was 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 1As shown; iron powder was soaked in 8 vol% hydrochloric acid solution at room temperature for 5 min under ultrasonic conditions of 120 W and 40 kHz, and the surface roughness Ra of the powder was controlled to be between 150 nm. After rinsing with deionized water until the cleaning solution was neutral, it was immediately placed in a sensitization activation solution containing 3 g / L nickel sulfate and 15 g / L sodium hypophosphite. It was soaked at a constant temperature of 70 °C for 3 min to complete the in-situ autocatalytic activation. After rinsing with deionized water again until the cleaning solution was neutral, the activated powder was obtained. (2) Preparation of chemical plating solution: The components of the plating solution, based on concentration, are: nickel sulfate 30 g / L, sodium hypophosphite 30 g / L, double complexing agent (citric acid 10 g / L and sodium citrate 20 g / L), sodium acetate 15 g / L, composite stabilizer (thiourea 0.2 g / L and potassium iodate 0.3 g / L), gradient nucleation regulator (sodium dodecyl sulfate 0.1 g / L and polyethylene glycol 0.3 g / L, mass ratio 1:3), crystallization inhibitor sodium molybdate 0.05 g / L, fluorocarbon interface wetting agent (ethyl perfluorooctanoate, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) 0.05 g / L, the pH value is adjusted to 8.0 with ammonia water, and the remainder is deionized water; the order of the above steps (1) and (2) is not important; (3) Chemical plating: The chemical plating solution and the activated powder were mixed at a liquid-to-solid mass ratio of 10:1 and reacted in three stages for 35 min: ① First stage: 75℃, 200r / min, 150W pulsed ultrasound, duty cycle 40%, heat preservation for 15 min; ② Second stage: 85℃, 250r / min, 250W pulsed ultrasound, duty cycle 40%, heat preservation for 15 min; ③ Third stage: 90℃, 300r / min, 200W pulsed ultrasound, duty cycle 40%, heat preservation for 5 min; After the reaction was completed, vacuum filtration was performed immediately, and the separated solid phase was rinsed with 60℃ deionized water until the cleaning solution was neutral, finally obtaining the solid phase of the coated powder, with a total coating thickness of about 1 μm, and the P content continuously decreased from 10wt% on the surface to 3wt% at the interface; SEM morphology of powder after chemical plating is shown below. Figure 2 As shown, the EDS surface distribution diagram of iron on the powder surface after electroless plating is as follows. Figure 3 As shown, the EDS surface distribution diagram of nickel on the powder surface after electroless plating is as follows: Figure 4 As shown; the EDS surface distribution diagram of phosphorus on the powder surface after chemical plating (after step (3) treatment) is shown below. Figure 5 As shown; (4) Drying: Place the coated powder in a vacuum drying oven with a vacuum degree ≤ -0.09MPa and use gradient vacuum pulse drying: first, keep it at 40℃ for 1h, and perform vacuum pulse 3 times. After each vacuum is broken to normal pressure, it is re-evacuated to the target vacuum degree and held for 5min; then keep it at 60℃ for 2h, and perform vacuum pulse 2 times; finally, keep it at 80℃ for 1h, and perform vacuum pulse 1 time; dry until the powder moisture content is ≤0.5%, and remove agglomerated particles by passing it through a 100-mesh sieve. (5) Annealing: The dried powder is spread evenly in a corundum crucible with a powder thickness of 3 mm, and placed in a tube annealing furnace. The atmosphere flow rate is 0.8 L / min throughout the process, and the oxygen content in the furnace is <0.5 vol%. Four-stage annealing is carried out in sequence: ① Stress relief pre-crystallization stage: High-purity argon is introduced, and the temperature is raised to 220℃ at 4℃ / min and held for 45 min; ② Interface metallurgical bonding stage: High-purity argon is maintained, and the temperature is raised to 450℃ at 6℃ / min and held for 30 min; ③ Gradient phase change control stage: The low-hydrogen atmosphere of 8 vol% hydrogen and 92 vol% argon is switched, and the temperature is lowered to 320℃ at 4℃ / min and held for 60 min; ④ Structure stabilization stage: The high-purity argon is switched back, and the temperature is lowered to 280℃ at 1.5℃ / min and held for 90 min. Then, the powder is cooled to below 150℃ in the furnace and taken out. After crushing and depolymerization, it is passed through a 100-mesh sieve to obtain the final soft magnetic composite powder. EDS line scan spectrum of the cross-section of the powder particles after annealing is as follows: Figure 6 As shown, the gradient distribution characteristics of Ni and P elements from the surface to the core are displayed.

[0031] Example 2

[0032] The only difference between this embodiment and Embodiment 1 is that: The chemical plating solution in step (2) is: 20 g / L nickel sulfate, 25 g / L sodium hypophosphite, dual complexing agent (10 g / L tartaric acid and 10 g / L sodium potassium tartrate), 10 g / L sodium acetate, composite stabilizer (0.05 g / L fumaric acid and 0.05 g / L potassium nitrate), gradient nucleation regulator (0.02 g / L sodium dodecyl sulfate and 0.04 g / L polyethylene glycol, mass ratio 1:2), crystallization inhibitor sodium tungstate 0.02 g / L, fluorocarbon interface wetting agent (ethyl perfluorooctanoate) 0.01 g / L, and pH adjusted to 7.5; In step (3): The chemical plating solution and activated powder were mixed at a liquid-to-solid mass ratio of 10:1 and reacted in three stages for 35 minutes: ① First stage: 75℃, 200r / min, 150W pulsed ultrasound, duty cycle 30%, holding for 10 minutes; ② Second stage: 85℃, 250r / min, 250W pulsed ultrasound, duty cycle 30%, holding for 20 minutes; ③ Third stage: 90℃, 300r / min, 200W pulsed ultrasound, duty cycle 30%, holding for 5 minutes. After the reaction was completed, vacuum filtration was performed immediately. The separated solid phase was washed with 60℃ deionized water until the cleaning solution was neutral, finally obtaining the solid phase of the coated powder. The total coating thickness was approximately 0.5μm, and the P content decreased continuously from 8wt% at the surface to 2wt% at the interface. The annealing process in step (5) is as follows: ① Hold at 200℃ for 60 min; ② Hold at 420℃ for 40 min; ③ Hold at 350℃ for 180 min; ④ Hold at 250℃ for 120 min. The temperature change rate in each stage is the same as in Example 1.

[0033] The remaining steps and parameters are completely consistent with those in Example 1.

[0034] Example 3

[0035] The only difference between this embodiment and Embodiment 1 is that: Step (1) The iron-based soft magnetic powder is gas-atomized iron-silicon alloy powder with a Si mass fraction of 3%, D50=30μm, and D99=73μm; The chemical plating solution in step (2) is: nickel chloride 35g / L, sodium hypophosphite 35g / L, double complexing agent (EDTA disodium 13.3g / L and sodium citrate 26.7g / L), sodium acetate 20g / L, composite stabilizer (thiourea 0.5g / L and potassium iodate 0.5g / L), gradient nucleation regulator (sodium dodecyl sulfate 0.1g / L and polyethylene glycol 0.4g / L, mass ratio 1:4), crystallization inhibitor sodium molybdate 0.2g / L, fluorocarbon interface wetting agent (perfluorooctanoate ethyl ester) 0.1g / L, and ammonia water to adjust the pH value to 9.0; In step (3): The chemical plating solution and activated powder were mixed at a liquid-to-solid mass ratio of 10:1 and reacted in three stages for 35 minutes: ① First stage: 75℃, 200r / min, 150W pulsed ultrasound, duty cycle 50%, holding for 10 minutes; ② Second stage: 85℃, 250r / min, 250W pulsed ultrasound, duty cycle 50%, holding for 10 minutes; ③ Third stage: 90℃, 300r / min, 200W pulsed ultrasound, duty cycle 50%, holding for 15 minutes. After the reaction was completed, vacuum filtration was performed immediately. The separated solid phase was rinsed with 60℃ deionized water until the cleaning solution was neutral, finally obtaining the solid phase of the coated powder. The total coating thickness was approximately 2μm, and the P content decreased continuously from 12wt% at the surface to 5wt% at the interface. Step (5) Annealing process: ① Hold at 250℃ for 30 min; ② Hold at 480℃ for 20 min; ③ Hold at 350℃ for 30 min; ④ Hold at 300℃ for 60 min. The temperature change rate of each stage is the same as in Example 1.

[0036] The remaining steps and parameters are completely consistent with those in Example 1.

[0037] Example 4

[0038] The only difference between this embodiment and Embodiment 1 is that the annealing process in step (5) is carried out entirely in a high-purity nitrogen atmosphere, with no hydrogen components. The remaining steps and parameters are completely consistent with those in Embodiment 1.

[0039] Comparative Example 1

[0040] The only difference between this embodiment and embodiment 1 is that the chemical plating solution in step (2) uses a single complexing agent, sodium citrate 30g / L, instead of a dual complexing agent system of citric acid and sodium citrate. The other components of the plating solution, process parameters, and performance testing methods are completely consistent with those in embodiment 1.

[0041] Comparative Example 2

[0042] The only difference between this comparative example and Example 1 is that no gradient nucleation regulator is added to the chemical plating solution in step (2), while the other components and process parameters are completely the same.

[0043] Comparative Example 3

[0044] The only difference between this comparative example and Example 1 is that no crystallization inhibitor is added to the chemical plating solution in step (2), while the other components and process parameters are completely the same.

[0045] Comparative Example 4

[0046] The only difference between this comparative example and Example 1 is that step (3) uses a constant temperature of 85℃, a fixed stirring rate of 250r / min, a fixed ultrasonic power of 200W, a duty cycle of 40%, and a coating time of 35min. There is no three-stage temperature and speed change process, and the other parameters are completely the same.

[0047] Comparative Example 5

[0048] The only difference between this comparative example and Example 1 is that step (5) uses single-temperature annealing, directly heating to 400℃ at 10℃ / min and holding for 60min, then cooling with the furnace, without the four-stage annealing process and dynamic atmosphere control, and the other parameters are completely the same.

[0049] Comparative Example 6

[0050] The preparation method of this comparative example includes the following steps: 1. Raw material preparation: The same water-atomized pure iron powder as in Example 1 was used, with a loose packing density of 3.5 g / cm³, D50=146 μm, and D99=293 μm; 2. Powder pretreatment: Iron powder is placed in an 8 vol% hydrochloric acid solution to remove the surface oxide layer, rinsed with deionized water until the cleaning solution is neutral, and dried for later use. No dilute acid ultrasonic micro-roughening or palladium-free in-situ autocatalytic activation is performed. 3. Preparation of electroless plating solution: 30 g / L nickel sulfate, 30 g / L sodium hypophosphite, 20 g / L sodium citrate, 15 g / L sodium acetate, 0.2 g / L thiourea (a common stabilizer), and pH of the plating solution adjusted to 5.5; compared to Example 1, no gradient nucleation regulator, crystallization inhibitor, or fluorocarbon wetting agent was added; 4. Constant temperature electroless plating: The pretreated iron powder is mixed with the electroless plating solution and electroless plating is carried out at a constant temperature of 75℃ and a constant stirring speed of 200r / min for 1 hour; the thickness of the surface electroless plating layer is about 0.5μm, with no gradient structure and no interface metallurgical bonding design. 5. Cleaning and drying: After chemical plating, the sample is ultrasonically cleaned three times with alcohol and then statically vacuum dried at 80°C for 2 hours; gradient vacuum pulse drying is not used. 6. Annealing treatment: The dried powder is placed in a vacuum furnace and heated directly to 950℃ for 1 hour under a vacuum degree <1Pa, and then cooled to room temperature in the furnace. This process does not employ four-stage atmosphere-controlled annealing, gradient phase transformation control, a low-hydrogen reduction stage, or a stress relief stage, resulting in the precipitation of a large amount of brittle Ni3P phase at high temperatures.

[0051] Comparative Example 7

[0052] The only difference between this comparative example and Example 1 is that step (1) uses a traditional 50ppm palladium chloride activation solution to be soaked at room temperature for 5 minutes for activation, without in-situ autocatalytic activation process, and the other parameters are completely consistent with Example 1.

[0053] The iron powder was soaked in an 8 vol% hydrochloric acid solution at room temperature for 5 minutes. After rinsing with deionized water until the cleaning solution was neutral, it was immediately placed in a sensitization and activation solution at room temperature for 5 minutes for activation. After activation, it was rinsed again with deionized water until the cleaning solution was neutral to obtain activated powder. The sensitization and activation solution was 50 ppm palladium chloride.

[0054] The samples obtained from each embodiment and comparative example were pressed into toroidal magnetic cores with a diameter of φ25mm×φ15mm×5mm under 1000MPa, and then heat-treated in a nitrogen atmosphere at 650℃ for 1h before performance testing.

[0055] Table 1: Sample performance test results of each embodiment and comparative example

[0056] All performance tests for the embodiments and comparative examples adopted unified national standards and testing methods: 1. Density: GB / T5163-2006 "Determination of density, open porosity and total porosity of sintered metal materials (excluding cemented carbide) and permeable sintered metal materials"; 2. Magnetic properties (saturation magnetic induction, coercivity): GB / T13012-2008 "Measurement method of DC magnetic properties of soft magnetic materials", test frequency 100kHz, test magnetic field strength H=10000A / m, core forming pressure 1000MPa, heat treatment at 650℃ in nitrogen atmosphere for 1h after forming; 3. Volume resistivity: GB / T351-2019 "Methods for measuring resistivity of metallic materials", tested using a four-probe tester at a test pressure of 5 MPa; 4. Corrosion resistance: GB / T2423.17-2024 "Environmental testing for electrical and electronic products - Part 2: Test methods - Test Ka: Salt spray", neutral salt spray test, with the first appearance of red rust as the failure criterion; 5. Interfacial adhesion: GB / T5270-2024 "Review of Test Methods for Adhesion Strength of Electrodeposited and Chemically Deposited Metallic Coatings on Metallic Substrates"; 6. Phase analysis: JB / T8426-1996 "X-ray diffraction test method for nickel-phosphorus alloy coatings"; 7.100kHz effective permeability: GB / T28869.3-2023 Measurement method for magnetic cores made of soft magnetic materials - Part 3: Magnetic properties under high excitation levels.

[0057] From the above test results, we can conclude that:

[0058] 1. Examples 1 to 4 of the present invention have achieved synergistic breakthroughs in insulation, magnetic properties, adhesion, and corrosion resistance. The performance of Comparative Example 6 is significantly degraded, proving that conventional processes (Comparative Example 6) are difficult to achieve a significant improvement in the performance of the Ni-P coating gradient structure. This fully verifies that the technical solution of the present invention can achieve beneficial technical effects.

[0059] 2. Comparative Examples 1 to 5 all involved single-variable adjustments, and their performance all showed significant deterioration. This demonstrates that the core technical features of this invention are the use of pulsed ultrasound-variable temperature and speed synergistic process during the plating process and the use of dynamic atmosphere control in the four-stage precise phase transition during the annealing process. Through processes such as iron-based soft magnetic powder activation, double-complexed chemical plating solution, and gradient vacuum pulsed drying, the material performance is further significantly improved.

[0060] 3. Comparative Example 7 uses a traditional precious metal activation process, and its performance is similar to that of Example 1, but the raw material cost is significantly increased. This proves that the palladium-free in-situ activation process of the present invention achieves a significant reduction in production costs without sacrificing performance, and has outstanding industrialization advantages.

[0061] As can be seen from the above, the present invention has the following advantages:

[0062] 1. Breakthrough in both structure and performance: This invention constructs a three-layer continuous gradient structure through the strong coupling of gradient coating structure design and segmented annealing process, completely solving the long-standing pain points of existing technologies where "high-phosphorus coatings have good insulation but are brittle and have poor adhesion, while low-phosphorus coatings have good adhesion but insufficient insulation." Compared with the uniform Ni-P coating technology in the prior art (publication number CN105039831A), the powder volume resistivity obtained by this invention is increased by more than 100%, the interfacial adhesion between the coating and the substrate is increased by more than 74%, and there is no coating cracking or peeling during the pressing process, fundamentally avoiding the risk of insulation failure.

[0063] 2. Adaptable to high-frequency harsh environments: This invention completely suppresses the precipitation of Ni3P brittle phase from a thermodynamic perspective by using the grain boundary segregation effect of crystallization inhibitors in conjunction with a precise gradient phase transformation annealing process, with no harmful phases generated throughout the process; at the same time, through the densification design of the high-phosphorus amorphous surface layer, a complete corrosion protection barrier is constructed, which is perfectly adapted to the harsh service environment of high temperature, high humidity and strong corrosion.

[0064] 3. Strong adaptability to powder systems: This invention employs a palladium-free in-situ autocatalytic activation process, replacing the traditional palladium chloride activation system, reducing raw material costs by more than 30% and avoiding coating defects caused by precious metal residues. The chemical plating and annealing processes require no new production line investment, have high process parameter tolerance, and batch performance deviations are ≤5%. The annealing process uses a low-hydrogen dynamic atmosphere, completely avoiding the flammability and explosion risks of high-hydrogen atmospheres, ensuring safety and controllability. Furthermore, this invention is adaptable to a full range of iron-based soft magnetic powders, including pure iron, iron-silicon, and iron-silicon-aluminum, overcoming the limitations of the prior art document (CN101514449A), which only adapts to nanocrystalline crushed powders. It can cover soft magnetic application scenarios across the entire frequency band from kHz to MHz, possessing extremely strong industrial adaptability.

[0065] The gases referred to in this invention, such as "high-purity nitrogen" and "high-purity argon", refer to gases with a purity ≥ 99.99 vol% (4N grade).

Claims

1. A method for preparing a nickel-phosphorus-plated soft magnetic composite material, characterized in that, Includes the following steps: (1) Prepare iron-based soft magnetic powder and activate it to form activated powder: (2) Prepare a chemical plating solution containing nickel and phosphorus sources; the order of steps (1) and (2) above is not important; (3) Chemical plating: The chemical plating solution and the activated powder are mixed at a liquid-solid mass ratio of 5:1 to 20:1 and reacted at 70 to 100°C. The stirring rate increases between 150 and 300 r / min as the temperature rises. After the reaction is completed, solid-liquid separation is performed. The separated solid phase is rinsed with deionized water until the cleaning solution is neutral to obtain the plating powder. (4) Drying: Dry the coated powder until the moisture content of the powder is ≤0.5%; (5) Annealing: The dried powder is annealed in four stages in sequence, with the oxygen content in the furnace <1 vol%. ① Introduce a high-purity argon atmosphere, raise the temperature to 200-250℃ at a rate of 3-5℃ / min, and hold for 30-60min; ② Maintain a high-purity argon atmosphere, raise the temperature to 420-480℃ at a rate of 5-8℃ / min, and hold for 20-40 minutes; ③ Switch to a low-hydrogen reducing atmosphere, cool down to 300-350℃ at 3-5℃ / min, and hold for 30-180min; the low-hydrogen reducing atmosphere is a mixture of 5vol%-10vol% hydrogen and the remainder argon. ④ Switch back to a high-purity argon atmosphere, cool down to 250-300℃ at 1-2℃ / min, hold for 60-120min, then cool to below 150℃ and remove to obtain a nickel-phosphorus soft magnetic composite material.

2. The method for preparing the nickel-phosphorus plating 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, iron-silicon-chromium alloy powder, and iron-based amorphous powder.

3. The method for preparing the nickel-phosphorus plating soft magnetic composite material according to claim 1, characterized in that, In step (1), the activation method of the iron-based soft magnetic powder is as follows: the iron-based soft magnetic powder is placed in a 5 vol% to 8 vol% dilute acid solution, ultrasonically soaked at room temperature for 3 to 8 minutes at a power of 100 to 150 W, then rinsed with deionized water until the cleaning solution is neutral, and then placed in a sensitization activation solution and soaked at a constant temperature of 70 to 75°C for 2 to 5 minutes, and then rinsed again with deionized water until the cleaning solution is neutral to obtain activated powder. The sensitization activation solution includes 2 to 5 g / L nickel sulfate and 10 to 20 g / L sodium hypophosphite.

4. The method for preparing the nickel-phosphorus plating soft magnetic composite material according to claim 1, characterized in that, In step (2), the electroless plating solution comprises the following components by concentration: nickel source 20-35 g / L, sodium hypophosphite 25-35 g / L, complexing agent 20-40 g / L, sodium acetate buffer 10-20 g / L, composite stabilizer 0.01-1 g / L, gradient nucleation regulator 0.05-0.5 g / L, crystallization inhibitor 0.02-0.2 g / L, fluorocarbon interface wetting agent 0.01-0.1 g / L, pH adjuster, and the balance being deionized water; the pH of the electroless plating solution is stabilized at 7.5-9.0 using the pH adjuster, and sodium hypophosphite is the phosphorus source.

5. The method for preparing the nickel-phosphorus soft magnetic composite material according to claim 4, characterized in that, In step (2), the nickel source is at least one of nickel sulfate, nickel chloride, nickel aminosulfonate, and nickel fluoroborate; the composite stabilizer is at least two of potassium iodate, potassium nitrate, fumaric acid, and thiourea; the pH adjuster is at least one of sodium hydroxide, ammonia, and sodium carbonate; the fluorocarbon interface wetting agent is at least one of polyoxyethylene ether containing fluorinated fatty alcohol, polyoxyethylene containing fluorinated phenol, and polyoxyethylene ester containing fluorinated carboxylic acid; the complexing agent is at least one of a combination of citric acid and sodium citrate, a combination of tartaric acid and potassium sodium tartrate, and a combination of disodium ethylenediaminetetraacetate and sodium citrate, with the mass ratio of the two components in each combination being 1:1 to 1:2; the gradient nucleation regulator is a combination of sodium dodecyl sulfate and polyethylene glycol, with a mass ratio of 1:2 to 1:5; and the crystallization inhibitor is at least one of sodium molybdate and sodium tungstate.

6. The method for preparing the nickel-phosphorus plating soft magnetic composite material according to claim 1, characterized in that, In step (4), the coated powder from step (3) is placed in an environment with a vacuum degree ≤ -0.09MPa and treated with a gradient vacuum pulse drying process: first, it is kept at 30-50℃ for 0.5-2h, during which vacuum pulses 1-5 times, and after each vacuum is broken to normal pressure, it is re-evacuated to the target vacuum degree and held for 2-10min; then, it is kept at 50-70℃ for 1-3h, during which vacuum pulses 1-5 times; finally, it is kept at 70-100℃ for 0.5-2h, during which vacuum pulses 1-5 times; finally, it is dried until the powder moisture content is ≤0.5%, and the agglomerated particles are removed by passing it through a 100-mesh sieve.

7. The method for preparing the nickel-phosphorus plating soft magnetic composite material according to claim 1, characterized in that, In step (3), the specific process parameters for each stage of the reaction are as follows: ① First stage: 70-75℃, 150-200r / min, 100-150W pulsed ultrasound, duty cycle 30%-50%, hold for 10-20min; ② Second stage: 80-85℃, 200-250r / min, 200-250W pulsed ultrasound, duty cycle 30%-50%, hold for 10-20min; ③ Third stage: 90-100℃, 250-300r / min, 150-200W pulsed ultrasound, duty cycle 30%-50%, hold for 5-20min.

8. The method for preparing the nickel-phosphorus plating soft magnetic composite material according to claim 7, characterized in that, In step (3), the phosphorus content of the coating obtained in the first stage is 8 to 12 wt%, the phosphorus content of the coating obtained in the second stage is 5 to 8 wt%, and the phosphorus content of the coating obtained in the third stage is 2 to 5 wt%.

9. A soft magnetic composite material, characterized in that, The soft magnetic composite material is prepared by any one of claims 1 to 8; the soft magnetic composite material is composed of an iron-based soft magnetic powder core and a coating covering its surface, wherein no Ni3P crystalline phase is precipitated in the coating, and the coating has the following structure: The surface layer is an insulating layer with a phosphorus content of 8-12 wt%, which has a Ni-P amorphous phase, with the amorphous phase accounting for ≥90%, and the surface layer thickness accounts for 40%-50% of the total coating thickness; The middle layer is a transition layer with a phosphorus content of 5-8 wt%, which has Ni-P nanocrystals with a grain size of 10-30 nm. The thickness of the middle layer accounts for 30%-40% of the total coating thickness. The interface layer is a Fe-Ni-P solid solution with no abrupt phase interface, and the thickness of the interface layer accounts for 10% to 20% of the total coating thickness.

10. The soft magnetic composite material according to claim 9, characterized in that, The volume resistivity of the soft magnetic composite material is ≥5×10⁻⁶. 5 μΩ·cm, effective permeability at 100kHz ≥180, coercivity ≤90A / m, saturation magnetic induction ≥1.5T, time to first appearance of red rust under neutral salt spray environment ≥120h, coating interface adhesion ≥30N.

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