A surface passivation method for nanocrystalline soft magnetic powder and its application

Abstract

The application provides a surface passivation method for nanocrystalline soft magnetic powder, and belongs to the technical field of surface treatment of magnetic materials.The surface passivation method comprises the following steps: preparing a water-based passivation solution, dispersing nanocrystalline soft magnetic powder in the water-based passivation solution, regulating the free acid concentration in the water-based passivation solution to be 0.03-0.05 mol / L, making the nanocrystalline soft magnetic powder undergo a passivation reaction, and obtaining the passivated nanocrystalline soft magnetic powder after dehydration.The surface passivation of the nanocrystalline soft magnetic powder is realized by one-step method, the process is simple, the corrosion-resistant component of the nanocrystalline powder is used as a basis, a specific passivation agent system is selected, the reaction conditions with the water-based medium are accurately matched, a balanced process window in which the passivation agent can effectively react to form a film and the oxidation of water can be inhibited to the minimum is constructed, and finally a thin and dense high-quality passivation layer with strong adhesion is formed on the surface of the nanocrystalline soft magnetic powder.

Description

Technical Field

[0001] This invention belongs to the field of magnetic material surface treatment technology, specifically relating to a surface passivation method for nanocrystalline soft magnetic powder and its application. Background Technology

[0002] The emergence of various smart devices has driven technological advancements in related electronic components. Among these, magnetic components are widely used as step-up / step-down, filtering, and power factor correction inductors in various smart electronic devices, such as metal powder cores, molded inductors, multilayer inductors, and electronic transformers.

[0003] Metallic magnetic powder cores are a type of soft magnetic material characterized by distributed air gaps. These cores consist of soft magnetic metal powder, a powder surface insulator, and a binder. The particles are insulated from each other by the surface insulator, while the powder particles maintain a certain strength through the binder. The air gaps between the particles become the primary energy storage sites. Common magnetic powder cores include iron powder cores, iron-silicon powder cores, iron-silicon-aluminum powder cores, permalloy powder cores, and amorphous magnetic powder cores. Their manufacturing methods generally include powder surface passivation, passivation powder coating and granulation, powder pressing, core annealing, and core surface spraying. Surface passivation typically employs surface phosphating or oxide coating methods, but these methods present two major technical challenges: Organic solvent dependence: Surface phosphating and other methods widely use passivating agents such as phosphoric acid and phosphates dissolved in organic solvents such as acetone for treatment. This process is heavily dependent on organic solvents such as acetone and ethanol, posing a significant risk of flammability and explosion, and also generating volatile organic compound emissions, high solvent costs, and complex recycling and treatment, resulting in poor safety and environmental performance.

[0004] Complex process: Oxide coating and other methods have disadvantages such as complex process flow and long process time, which further increases the production cost. They also require organic solvents such as ethanol as solvents.

[0005] Therefore, to address the aforementioned technical challenges, a novel passivation process for soft magnetic metal powder has been proposed. For example, invention patent CN113426994A discloses a passivation process for soft magnetic metal powder used in inductor forming, specifically including the following steps: S1, passivating the soft magnetic powder with phosphate as a passivating agent, and obtaining a preliminary passivated material after drying; S2, reacting the preliminary passivated material obtained in S1 in CO gas to obtain a preliminary product; S3, mixing phosphate and nano-adsorbent at a weight ratio of 1:0.1-0.5 to obtain a mixture, and using the mixture as a passivating agent to passivate the preliminary product obtained in S2 again, and obtaining the finished powder after drying. However, this multi-step process is quite complex.

[0006] Therefore, there is an urgent need for a passivation method that can completely eliminate organic solvents, effectively suppress powder oxidation under mild conditions, obtain a high-performance insulating layer, and has a simple process and low cost. Summary of the Invention

[0007] To address the aforementioned technical problems in the prior art, this invention provides a surface passivation method for nanocrystalline soft magnetic powder and its application. Utilizing the corrosion-resistant components of the nanocrystalline powder itself as a foundation, and through precise control of the "balanced process window" comprised of free acid concentration, reaction temperature, and reaction time, a specific passivating agent system is constructed to achieve a surface passivation method for nanocrystalline soft magnetic materials that "replaces alcohol with water."

[0008] This invention provides a surface passivation method for nanocrystalline soft magnetic powder, comprising: preparing an aqueous passivation solution, dispersing the nanocrystalline soft magnetic powder in the aqueous passivation solution, adjusting the free acid concentration in the aqueous passivation solution to 0.03-0.05 mol / L, causing the nanocrystalline soft magnetic powder to undergo a passivation reaction, and obtaining the passivated nanocrystalline soft magnetic powder after dehydration.

[0009] The surface passivation method provided by this invention limits the free acid concentration in the water-based passivation solution, placing the nanocrystalline soft magnetic powder at the transitional edge between the active dissolution zone and the passivation zone at a thermodynamic level. This creates a delicate balance: the passivation solution has a certain degree of corrosivity, sufficient to slightly etch the powder surface, exposing fresh active sites and promoting the adsorption and reaction of the passivating agent; however, at the same time, the acidity is insufficient to trigger a violent and uncontrollable dissolution of iron (as in a strong acid environment). Kinetically, under these conditions, the redox potential of dissolved oxygen in the water is greatly suppressed, significantly slowing down the rate of oxidative corrosion of iron by oxygen as a depolarizing agent. Simultaneously, it is also the most suitable environment for the formation and stable existence of organic acid passivation films.

[0010] Preferably, the nanocrystalline soft magnetic powder is an Fe-Si-B-Nb-Cu alloy.

[0011] The typical nanocrystalline powder processed in this invention is an Fe-Si-B-Nb-Cu alloy. Si and Nb are key corrosion-resistant elements. Silicon (Si): Exists in the alloy as a solid solution, significantly increasing the electrode potential of the iron matrix and preferentially forming a dense oxide layer containing Si (such as SiO2 or its hydrate) on the surface. This initial oxide layer has good chemical stability, providing initial protection in aqueous environments and slowing down the rapid corrosion of the iron matrix. Niobium (Nb): Refines the grain size, improves the thermal stability of the alloy, and tends to segregate at grain boundaries, enhancing the corrosion resistance of the grain boundary regions. Simultaneously, the addition of Nb improves the amorphous forming ability of the alloy, resulting in a uniform, grain-boundary-free nanocrystalline / amorphous composite structure in the heat-treated powder. This reduces the active channels for electrochemical corrosion, making it more resistant to uniform corrosion than ordinary iron powder.

[0012] By limiting the concentration of free acid in the water-based passivation solution, iron is positioned at the transitional edge between the active dissolution zone and the passivation zone.

[0013] Preferably, the preparation process of the water-based passivation solution includes: dissolving the passivating agent in deionized water, wherein the passivating agent includes tannic acid and citric acid; The mass ratio of tannic acid to citric acid in the passivating agent is 3-2:1.

[0014] The tannic acid-citric acid composite system embodies a synergistic mechanism of "first activation and cleaning, then strong chelation." Citric acid, as a mild organic acid, first contacts the powder surface. Its carboxyl groups can coordinate with Fe atoms, assisting in the dissolution of trace amounts of high-valence iron oxides on the surface, adjusting the local pH value, and cleaning the surface, providing an ideal interface for the tannic acid reaction. Simultaneously, citric acid has a certain degree of weak reducing property, which can, to some extent, reduce Fe... 3+ Reduced to more reactive Fe 2+ Tannic acid is a polyphenol compound containing a large number of catechol and pyrogallol groups, making it a very strong metal chelating agent. It can react with Fe... 2+ / Fe 3+ This process forms a highly stable, dense, and water-insoluble multi-molecular complex membrane. This organic-inorganic hybrid membrane not only provides excellent physical shielding, but also allows the numerous phenolic hydroxyl groups in its molecules to form stable coordination bonds with the matrix, achieving a strong chemical bond. The membrane is dense and uniform, with strong adhesion and excellent insulation properties.

[0015] More preferably, the dissolution time is at least 10 minutes.

[0016] More preferably, the resistivity of the deionized water is ≥0.2 MΩ·cm.

[0017] More preferably, when dissolving the passivating agent in deionized water, nitrogen gas should be passed through the water for 10-30 minutes to remove dissolved oxygen.

[0018] Preferably, the mass ratio of nanocrystalline soft magnetic powder to water-based passivation solution is 1:0.5-1.5.

[0019] Preferably, the passivation reaction temperature is 50℃-80℃, and the passivation reaction time is 15-45 min.

[0020] This invention simultaneously optimizes the reaction temperature (50℃-80℃), thereby improving reaction kinetics. It promotes the adsorption and diffusion of passivating agent molecules on the powder surface, as well as their chelation / coordination reactions with Fe atoms, facilitating the rapid formation of a complete and dense coating layer. This "protective layer" is applied to the powder surface before significant oxidation occurs due to water molecules and residual oxygen. The upper temperature limit (80°C) is set to avoid excessively high water temperatures leading to an increase in the ion product of water and enhanced solution conductivity, which could potentially exacerbate electrochemical corrosion; it also prevents the possible decomposition of organic acid passivating agents.

[0021] Preferably, the dehydration is carried out by vacuum filtration drying, wherein the drying is vacuum drying at a temperature of 80°C-120°C for 1-2 hours.

[0022] The present invention also provides a passivated nanocrystalline soft magnetic powder prepared by the above-described surface passivation method.

[0023] The present invention also provides a nanocrystalline magnetic powder core prepared from the passivated nanocrystalline soft magnetic powder.

[0024] Preferably, under the condition of 1 M 20 mT, the loss of the nanocrystalline magnetic powder core is less than 470 mWcm. 3 Furthermore, the insulation resistance at 100 V is not less than 60 GΩ.

[0025] The present invention also provides the application of the nanocrystalline magnetic powder core in magnetic components or electronic components.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention achieves surface passivation of nanocrystalline soft magnetic powder in one step. The process is simple. By carefully selecting a specific passivating agent system and accurately matching the reaction conditions with the aqueous medium, a balanced process window is constructed that can effectively react the passivating agent to form a film and suppress the negative effects of water (oxidation) to the minimum. Finally, a thin, dense, and high-quality passivation layer with strong adhesion is formed on the surface of the nanocrystalline soft magnetic powder.

[0027] (2) This invention uses deionized water as the sole solvent to prepare the passivation liquid, replacing flammable and explosive organic solvents. This eliminates the risk of fire and explosion and VOC emissions from the source. The production process is inherently safe. When further dissolving organic acids such as tannic acid (derived from grains) as passivating agents, the entire system is non-toxic and biodegradable, realizing the transformation from "high environmental impact chemical raw materials" to "renewable natural raw materials". The waste liquid generated is a simple acidic aqueous solution containing organic acid salts, which does not contain heavy metals or difficult-to-degrade organic matter. It can be treated by simple neutralization and precipitation (such as with lime milk), which is extremely low in cost and has great value for large-scale industrial application. Detailed Implementation

[0028] To further illustrate the technical means and effects adopted by the present invention in order to achieve the intended purpose, the following detailed description is provided in conjunction with embodiments and comparative examples.

[0029] All raw materials were purchased from the market.

[0030] Example 1: Water-based tannic acid-citric acid composite passivated nanocrystalline powder 1 A. Take 100 grams of Fe-Si-B-Nb-Cu nanocrystalline gas atomization powder with a D50 of 15 μm.

[0031] B. Preparation of passivation solution: Dissolve 0.2 g tannic acid and 0.1 g citric acid in 100 mL deionized water (after purging with nitrogen for 20 min to remove oxygen) and stir until dissolved. The concentration of free acid at this time is measured to be 0.035 mol / L.

[0032] C. Add 100 g of the weighed nanocrystalline gas-atomized powder to the above passivation solution (solid-liquid mass ratio 1:1), and stir at 30 rpm for 30 min at 50°C. During the reaction, the free acid concentration needs to be tested every 10 min. When the free acid concentration is lower than 0.03 mol / L, passivating agent needs to be added to make the free acid concentration between 0.03 and 0.05 mol / L. After the reaction is completed, vacuum filter to separate the solid and liquid, and wash twice with deionized water.

[0033] D. Place the separated powder in a vacuum drying oven at 100°C and dry for 2 hours to obtain passivated powder A.

[0034] E. Passivation powder A was mixed with 2 wt% epoxy resin and pressed into a 20.3×12.7 magnetic ring under 600 MPa. After curing at 200°C for 1 h, the ring was taken out for testing.

[0035] F. Magnetic performance test: The permeability was measured to be 17.9 at 100 kHz, 84.2% under DC bias of 100 Oe, and the loss was 448 mW / cm at 1 MHz and 20 mT. 3 Insulation resistance at 100 V is ≥60 GΩ (Table 1).

[0036] Example 2: Water-based tannic acid-citric acid composite passivated nanocrystalline powder 2 The preparation method of Example 2 is the same as that of Example 1, except that: The passivation solution was prepared by dissolving 0.3 g of tannic acid and 0.1 g of citric acid in 100 mL of deionized water. The free acid concentration at this time was measured to be 0.048 mol / L. Other treatments were the same as in Example 1, and passivation powder B and magnetic ring were obtained.

[0037] Magnetic performance testing: The permeability was measured to be 17.2 at 100 kHz, with a DC bias of 84.9% at 100 Oe, and a loss of 439 mW / cm² at 1 MHz and 20 mT. 3 Insulation resistance at 100 V is ≥60 GΩ (Table 1).

[0038] Comparative Example 1: Traditional Organic Solvent Method Except for replacing the solvent deionized water in Example 1 with an equal amount of anhydrous ethanol and reacting under the same process parameters, the other steps were exactly the same, resulting in comparative powder C and magnetic powder core.

[0039] Magnetic performance testing: The permeability was measured to be 17.5 at 100 kHz, 84.5% at DC bias of 100 Oe, and the loss was 468 mW / cm at 1 MHz and 20 mT. 3 The insulation resistance at 100 V is 15 GΩ (Table 1).

[0040] Comparative Example 2: Outside the Process Window of Water-Based Passivated Nanocrystalline Powder In Example 1, the amount of tannic acid and citric acid was adjusted to 0.1 g, and the free acid concentration was 0.025 mol / L. Except for stirring the reaction at 40°C for 10 min, the other steps were exactly the same, and comparative powder D and magnetic powder core were obtained.

[0041] Magnetic performance testing: The permeability was measured to be 18.2 at 100 kHz, with a DC bias of 82.7% at 100 Oe, and a loss of 519 mW / cm at 1 MHz and 20 mT. 3 The insulation resistance at 100V is 38 GΩ (Table 1).

[0042] Comparative Example 3: Water-based tannic acid-citric acid composite passivated iron-silicon-chromium powder Except for replacing the nanocrystalline powder in Example 1 with an equal amount of iron-silicon-chromium powder with a D50 of 8 μm, the other steps were exactly the same, resulting in comparative powder E and a magnetic powder core. Magnetic performance testing: The permeability was measured to be 28.7 at 100 kHz, 82.5% at a DC bias of 100 Oe, and 937 mW / cm at 1 MHz and 20 mT. 3 The insulation resistance at 100 V is 1.2 GΩ. The appearance is yellowish, and obvious oxidation and rust are observed under a magnifying glass (Table 1).

[0043] Comparative Example 4: Water-based tannic acid-citric acid composite passivated amorphous powder Except for replacing the nanocrystalline powder in Example 2 with an equal amount of FeSiB amorphous powder with D50=15 μm, the other steps were exactly the same, resulting in comparative powder F and magnetic powder core.

[0044] Magnetic performance testing: The permeability was measured to be 16.5 at 100 kHz, 93.5% at DC bias of 100 Oe, and the loss was 1082 mW / cm at 1 MHz and 20 mT. 3 The insulation resistance at 100V is 4.7 GΩ. The appearance is yellowish, and obvious oxidation and rust are observed under a magnifying glass (Table 1).

[0045] Table 1. Performance Test Comparison Table of Examples and Comparative Examples As shown in Table 1, the water-based passivation method provided by this invention, without using any organic solvents, utilizes the corrosion-resistant components of the nanocrystalline powder itself as a foundation. By precisely controlling the "balanced process window" formed by the free acid concentration, reaction temperature, and reaction time, a specific passivating agent system is constructed, achieving a surface passivation method for nanocrystalline soft magnetic materials using water instead of alcohol. The prepared nanocrystalline magnetic powder core exhibits lower magnetic loss than traditional organic solvent processes, while maintaining comparable or even better magnetic permeability and superior insulation resistance. This demonstrates the dual advantages of this invention's process in terms of both environmental friendliness and performance.

[0046] Meanwhile, the cost advantages in raw material procurement, equipment investment, and hazardous waste treatment are also obvious.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A surface passivation method for nanocrystalline soft magnetic powder, characterized in that, include: A water-based passivation solution was prepared, and the nanocrystalline soft magnetic powder was dispersed in the water-based passivation solution. The concentration of free acid in the water-based passivation solution was adjusted to 0.03-0.05 mol / L to cause the nanocrystalline soft magnetic powder to undergo a passivation reaction. After dehydration, the passivated nanocrystalline soft magnetic powder was obtained.

2. The surface passivation method according to claim 1, characterized in that, The nanocrystalline soft magnetic powder is an Fe-Si-B-Nb-Cu alloy.

3. The surface passivation method according to claim 1, characterized in that, The preparation process of the water-based passivation solution includes: dissolving the passivating agent in deionized water, wherein the passivating agent includes tannic acid and citric acid; The mass ratio of tannic acid to citric acid in the passivating agent is 3-2:

1.

4. The surface passivation method according to claim 1, characterized in that, The mass ratio of nanocrystalline soft magnetic powder to water-based passivation solution is 1:0.5-1.

5.

5. The surface passivation method according to claim 1, characterized in that, The passivation reaction temperature is 50℃-80℃, and the passivation reaction time is 15-45 min.

6. The surface passivation method according to claim 1, characterized in that, Dehydration is achieved by vacuum filtration and drying, with the drying process conducted at a temperature of 80°C-120°C.

7. A passivated nanocrystalline soft magnetic powder prepared by the surface passivation method according to any one of claims 1-6.

8. A nanocrystalline magnetic powder core prepared from the passivated nanocrystalline soft magnetic powder according to claim 7.

9. The nanocrystalline magnetic powder core according to claim 8, characterized in that, Under the condition of 1 M 20 mT, the loss of the nanocrystalline magnetic powder core is less than 470 mWcm. 3 Furthermore, the insulation resistance at 100 V is not less than 60 GΩ.

10. The application of the nanocrystalline magnetic powder core according to claim 8 in magnetic components or electronic components.

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