A low-loss, high-overlap sendust magnetic powder core and its preparation method

Abstract

The present invention belongs to the field of soft magnetic functional materials, and specifically relates to a low-loss, high-overlap sendust magnetic powder core and a preparation method thereof, comprising the following steps: preparing a coating agent, passivating the coated powder, bonding and lubricating, and pressing and annealing. This method is based on the hydrolysis characteristics of metasilicic acid and the hydrolysis-promoting effect of an alkaline catalyst to prepare nanoporous silica as a coating agent. At the same time, the etching effect of alkaline substances on silica can effectively increase the specific surface area of nanoporous silica and enhance the bonding strength with sendust powder; the vacuum calcination treatment not only removes residual reagents, but also promotes the full thermal decomposition of metasilicic acid into silica for in-situ coating, and stabilizes its lattice structure. The coating layer of the low-loss, high-overlap sendust magnetic powder core of the present invention is firmly bonded, thin and uniform in thickness, and has a stable and dense structure, which can effectively reduce the hysteresis loss of the sendust magnetic powder core, improve DC bias performance, and avoid the increase of eddy current loss.

Description

Technical Field

[0001] The present invention belongs to the field of soft magnetic functional materials, and in particular relates to a low-loss, high-overlay sendust magnetic powder core and a preparation method thereof. Background Art

[0002] Since its successful development in the 1980s, Sendust powder cores have been widely used, capturing a significant market share in the mid- and low-end markets. They are widely used in energy storage inductors, filter inductors, switching power supplies, and other applications. However, Sendust powder cores have a significant drawback: low saturation magnetic induction, which results in poor DC bias performance. In the 5G era, magnetic powder cores are commonly used in high-power environments. While the powder cores are electromagnetically induced by high-frequency AC currents, they are also affected by the superimposed DC component. Under high DC bias currents, the powder cores easily saturate, resulting in a decrease in overall performance. Furthermore, the powder cores generate greater eddy current losses in high-frequency environments. Therefore, ensuring low high-frequency losses while improving DC bias performance in Sendust powder cores is a current research hotspot.

[0003] At present, the insulation coating process of sendust magnetic powder core can be divided into chemical coating, physical coating and physical and chemical mixed coating. Chemical coating forms a coating film on the surface through the chemical reaction of acidic or alkaline auxiliary materials with sendust powder. The coating quality is closely related to the concentration and content of acid and alkali agents, which can easily cause uneven coating and acid and alkali residue problems, ultimately leading to the deterioration of the electromagnetic properties of the magnetic powder core. In physical coating, the coating agent does not react with sendust powder. Inorganic oxides or organic resins are directly coated on the powder surface. However, the bonding strength between inorganic oxides and soft magnetic powders is poor and they are easy to fall off during mixing and pressing. Organic resins are easy to decompose during high-temperature annealing, which exposes the powder surface and aggravates eddy current loss. In physical and chemical mixed coating, sendust powder not only needs to undergo a passivation reaction, but also needs to undergo secondary coating, which is a relatively complicated process. In addition, the coating layers obtained by the three insulating coating methods currently have different thicknesses, making it difficult to form a uniform surface coating. The DC bias performance and loss performance of the resulting Sendust powder core cannot simultaneously meet the requirements of a high-performance powder core.

[0004] Chinese patent publication number CN111192735A discloses an insulating coated metal soft magnetic powder and its preparation method. It mainly uses organic resin for direct coating. However, the organic resin coating is easily destroyed after high-temperature sintering, and part of the magnetic powder core surface is exposed, which greatly increases eddy current loss. Chinese patent publication number CN107464650A discloses a method for preparing a metal soft magnetic powder core that is resistant to high-temperature heat treatment. By improving the process of silicone resin, the high-temperature resistance of the silicone resin coating layer is improved. However, the reaction still decomposes at high temperatures, and the silicon oxide formed after decomposition is not absolutely densely coated around the magnetic powder core, which increases losses. Chen Yuanxing et al. mentioned in "The Effect of Fe-6.5Si Alloy Powder Morphology Characteristics on the Performance of Magnetic Powder Cores" that only metasilicic acid is used as a binder to facilitate the pressing and molding of iron-silicon powder. The hydrolysis effect of metasilicic acid is not reflected, and the silicon dioxide generated by hydrolysis is small, the coating effect is poor, and therefore there is no superiority in performance.

[0005] Therefore, a new insulation coating method is urgently needed to improve the performance of metal soft magnetic powder cores. Summary of the Invention

[0006] The present invention provides a low-loss and high-overlap sendust magnetic powder core and a preparation method thereof, aiming to solve at least one of the technical problems existing in the above-mentioned prior art.

[0007] The low-loss and high-overlapping sendust magnetic powder core preparation method of the present invention is based on the hydrolysis characteristics of metasilicic acid and utilizes the hydrolysis-promoting effect of an alkaline catalyst to prepare nanoporous silica as a coating agent for sendust powder; at the same time, the etching effect of alkaline substances on silica can effectively increase the specific surface area of nanoporous silica and enhance the bonding strength with sendust powder; the vacuum calcination treatment promotes the full thermal decomposition of metasilicic acid into silica for in-situ coating while removing residual reagents. The low-loss, high-overlap sendust magnetic powder core of the present invention: 1. By coating with nanoporous silica with strong adsorption effect, the bonding strength between the coating layer and the sendust powder is enhanced. The formed coating layer is uniform and thin (the thickness is only 10~50nm), which can effectively reduce the hysteresis loss of the magnetic powder core and improve the DC bias performance; 2. Through vacuum calcination treatment, the residual alkaline catalyst is removed, and the residual metasilicic acid is promoted to be fully thermally decomposed into silica for in-situ coating, while stabilizing the nanoporous silica lattice structure, improving the density and stability of the coating layer, and effectively avoiding the increase of eddy current loss.

[0008] A method for preparing a low-loss, high-overlap sendust magnetic powder core comprises the following steps:

[0009] S1: preparing a coating agent by dispersing metasilicic acid in anhydrous ethanol, adding a mixed solution of an alkaline catalyst and deionized water to adjust the pH while stirring, and continuing to stir to form a sol to obtain a nanoporous silica coating solution;

[0010] S2: Passivation coating powder: the nanoporous silica coating solution obtained in step S1 is mixed with sendust powder, stirred evenly, baked and dried, and then placed in a tube furnace for vacuum calcination to obtain nanoporous silica-coated sendust powder;

[0011] S3: bonding and lubrication, mixing the nanoporous silica-coated sendust powder obtained in step S2 with a composite water-soluble binder, stirring evenly and drying, then adding a lubricant, mixing evenly and sieving to obtain a composite sendust powder;

[0012] S4: pressing and annealing, pressing the composite sendust powder obtained in step S3, and annealing it at high temperature in a nitrogen atmosphere after pressing to obtain a low-loss and high-overlap sendust magnetic powder core.

[0013] Preferably, in step S1, the mass ratio of metasilicic acid to ethanol is (3-5):10;

[0014] Preferably, in step S1, the alkaline catalyst is at least one of concentrated ammonia, urea, hydrazine, ammonium bicarbonate, and sodium bicarbonate;

[0015] Furthermore, in step S1, the alkaline catalyst is more preferably concentrated aqueous ammonia;

[0016] Preferably, in step S1, the mass ratio of the alkaline catalyst to deionized water is (1-4):20, and the pH is adjusted to 8-10;

[0017] Preferably, in step S1, the stirring speed is 200-400 rpm, more preferably 300 rpm, and the stirring time after adjusting the pH to 8-10 is continued for 30-90 min.

[0018] Preferably, in step S2, the particle size of the sendust powder is 200 mesh to 800 mesh, and the mass ratio of the sendust powder to the nanoporous silica coating solution in step S1 is 10:(1-3);

[0019] Preferably, in step S2, the stirring speed is 200-400 rpm, and the stirring time is 30-60 min;

[0020] Preferably, in step S2, the baking temperature is 80-120°C and the baking time is 60-90 minutes;

[0021] Preferably, in step S2, the vacuum calcination temperature is 350-550°C, the heating rate is 2-6°C / min, and the holding time is 1-3h.

[0022] Preferably, in step S3, the composite water-soluble adhesive includes an aqueous polyurethane emulsion and an acrylic acid ester water-based adhesive, and based on the nanoporous silica-coated sendust powder in step S2, includes 0.3-0.6 wt% of the aqueous polyurethane emulsion, 0.2-0.5 wt% of the acrylic acid ester water-based adhesive and 4-8 wt% of deionized water.

[0023] Preferably, in step S3, the drying temperature is 80-120° C., and the drying time is 60-120 min.

[0024] Preferably, in step S3, the amount of lubricant added is 0.3-0.6 wt % based on the nanoporous silica-coated sendust powder in S2, and the lubricant is at least one of aluminum stearate, magnesium stearate, and stearamide.

[0025] Preferably, in step S4, the pressing pressure is 18-20 t / cm 2 , the annealing temperature is 700~750℃, and the annealing time is 1~3h.

[0026] A low-loss and high-overlay sendust magnetic powder core is prepared by the above method.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] 1. In the present invention, the silicon dioxide prepared by hydrolysis of metasilicic acid is nanometer-grade, with a particle size of 10 to 50 nm. The coating layer formed on the surface of the sendust particles is relatively thin. On the one hand, during the pressing process of the magnetic powder core, more distributed air gaps are provided per unit volume, and the DC bias performance is better. On the other hand, the thinner coating layer is conducive to reducing the hysteresis loss of the magnetic powder core.

[0029] 2. In the present invention, at least one of concentrated ammonia, urea, hydrazine, ammonium bicarbonate, and sodium bicarbonate is used as a catalyst for the hydrolysis of metasilicic acid. On the one hand, the pH of the system can be adjusted to promote the hydrolysis of metasilicic acid. On the other hand, the etching effect of the alkaline catalyst on the hydrolysis product, nanoporous silica, increases its specific surface area, enhances its adsorption effect on the sendust powder particles, and effectively improves the bonding strength between the coating layer and the sendust powder. The resulting coating layer is denser and less susceptible to damage, effectively avoiding the increase of eddy current loss.

[0030] 3. In the present invention, by optimizing the calcination process in vacuum, on the one hand, the residual alkaline catalyst and impurities in the sendust powder can be removed, and on the other hand, the unhydrolyzed metasilicic acid is decomposed into nano-silica, which is used to in-situ coat the sendust particles, while making the generated nano-porous silica lattice more stable, thereby making the coating more stable. DETAILED DESCRIPTION

[0031] The technical solution of the present invention is further described in detail below through specific embodiments. Based on the embodiments of the present invention, all other embodiments obtained by other technicians without creative work are within the scope of protection of the present invention.

[0032] Example 1.

[0033] S1: Prepare a coating agent by dispersing 40 g of metasilicic acid in 100 g of anhydrous ethanol. Add an appropriate amount of ammonia solution (prepared from 4 g of concentrated ammonia water and 20 g of deionized water) while stirring at 300 rpm to adjust the pH to 9. Continue stirring at 300 rpm for 1 hour to form a sol to obtain a nanoporous silica coating solution.

[0034] S2: Passivation coating powder: 143g of the nanoporous silica coating solution obtained in step S1 was mixed evenly with 1000g of 200-mesh sendust powder, stirred at 200rpm for 60min, and placed in an oven at 100°C for 70min to obtain a dry powder; the dried powder was placed in a tube furnace, evacuated to vacuum with a vacuum pump, and then heated to 450°C at a rate of 5°C / min, kept at this temperature for 2h, and cooled to room temperature to obtain nanoporous silica-coated sendust powder;

[0035] S3: bonding and lubrication: the nanoporous silica-coated sendust powder obtained in step S2 is mixed with 0.4% aqueous polyurethane emulsion, 0.3% acrylate water-based adhesive, and 5% deionized water based on the mass of the powder, stirred for 20 minutes, and then dried in an oven at 100°C for 90 minutes. After cooling to room temperature, 0.4wt% stearic acid amide is added, and the mixture is stirred and mixed uniformly to obtain a composite sendust powder;

[0036] S4: Pressing and annealing, the composite sendust powder obtained in step S3 was pressed at 20t / cm 2 After pressing, annealing is carried out at a high temperature of 730°C for 3 hours to obtain a low-loss and high-overlap sendust magnetic powder core.

[0037] Example 2

[0038] S1: Prepare a coating agent by dispersing 30 g of metasilicic acid in 100 g of anhydrous ethanol, adding an appropriate amount of urea solution (prepared from 2 g of urea and 20 g of deionized water) while stirring at 200 rpm to adjust the pH to 9, and then continue stirring at 200 rpm for 90 minutes to form a sol to obtain a nanoporous silica coating solution;

[0039] S2: Passivation coating powder: 125g of the nanoporous silica coating solution obtained in step S1 was mixed evenly with 1000g of 400-mesh sendust powder, stirred at 300rpm for 40min, and placed in an 80°C oven for 90min to obtain a dry powder; the dried powder was placed in a tube furnace, evacuated to vacuum with a vacuum pump, and then heated to 400°C at a rate of 4°C / min, and kept at this temperature for 1.5h. After cooling to room temperature, the nanoporous silica-coated sendust powder was obtained;

[0040] S3: bonding and lubrication: the nanoporous silica-coated sendust powder obtained in step S2 is mixed with 0.3% aqueous polyurethane emulsion, 0.3% acrylate water-based adhesive, and 4% deionized water based on the mass of the powder, stirred for 20 minutes, and then dried in an oven at 80°C for 60 minutes. After cooling to room temperature, 0.4 wt% stearic acid amide is added, and the mixture is stirred and mixed uniformly to obtain a composite sendust powder;

[0041] S4: Pressing and annealing, the composite sendust powder obtained in step S3 was heated to 18t / cm 2 After pressing, annealing is carried out at a high temperature of 700°C for 3 hours to obtain a low-loss and high-overlap sendust magnetic powder core.

[0042] Example 3

[0043] S1: Prepare a coating agent by dispersing 30 g of metasilicic acid in 100 g of anhydrous ethanol, adding an appropriate amount of hydrazine solution (prepared from 1 g of hydrazine and 20 g of deionized water) while stirring at 400 rpm to adjust the pH to 8, and then continue stirring at 400 rpm for 30 minutes to form a sol to obtain a nanoporous silica coating solution;

[0044] S2: Passivation coating powder: 100 g of the nanoporous silica coating solution obtained in step S1 was mixed evenly with 1000 g of 200-mesh sendust powder, stirred at 300 rpm for 40 min, and placed in an oven at 100° C. for 70 min to obtain a dry powder; the dried powder was placed in a tube furnace, evacuated to a vacuum using a vacuum pump, and then heated to 550° C. at a rate of 6° C. / min, maintained at this temperature for 1 h, and cooled to room temperature to obtain nanoporous silica-coated sendust powder;

[0045] S3: bonding and lubrication: the nanoporous silica-coated sendust powder obtained in step S2 is mixed with 0.6% aqueous polyurethane emulsion, 0.2% acrylate water-based adhesive, and 8% deionized water based on the mass of the powder, stirred for 20 minutes, and then dried in an oven at 100°C for 90 minutes. After cooling to room temperature, 0.4wt% stearic acid amide is added, and the mixture is stirred and mixed uniformly to obtain a composite sendust powder;

[0046] S4: Pressing and annealing, the composite sendust powder obtained in step S3 was heated to 18t / cm 2 After pressing, annealing is carried out at a high temperature of 700°C for 2 hours to obtain a low-loss and high-overlap sendust magnetic powder core.

[0047] Example 4

[0048] S1: Prepare a coating agent by dispersing 50 g of metasilicic acid in 100 g of anhydrous ethanol, adding an appropriate amount of ammonium bicarbonate aqueous solution (prepared from 2 g of ammonium bicarbonate and 20 g of deionized water) while stirring at 400 rpm to adjust the pH to 8, and then continue stirring at 400 rpm for 1 hour to form a sol to obtain a nanoporous silica coating solution;

[0049] S2: Passivation coating powder: 132g of the nanoporous silica coating solution obtained in step S1 was mixed evenly with 1000g of 200-mesh sendust powder, stirred at 400rpm for 30min, and placed in a 120°C oven for 60min to obtain a dry powder; the dried powder was placed in a tube furnace, evacuated to vacuum with a vacuum pump, and then heated to 350°C at a rate of 3°C / min, kept at this temperature for 3h, and cooled to room temperature to obtain nanoporous silica-coated sendust powder;

[0050] S3: bonding and lubrication: the nanoporous silica-coated sendust powder obtained in step S2 is mixed evenly with 0.3% aqueous polyurethane emulsion, 0.5% acrylate water-based adhesive, and 5% deionized water based on the mass of the powder, stirred for 20 minutes, and then dried in an oven at 120°C for 60 minutes. After cooling to room temperature, 0.4wt% stearic acid amide is added, and the mixture is stirred and mixed to obtain a composite sendust powder;

[0051] S4: Pressing and annealing, the composite sendust powder obtained in step S3 was heated to 18t / cm 2 After pressing, the powder core was annealed at 750°C for 1 hour to obtain a low-loss and high-overlap sendust magnetic powder core.

[0052] Comparative Example 1.

[0053] Compared with Example 1, Comparative Example 1 does not add ammonia solution, and other steps are the same.

[0054] Comparative Example 2

[0055] Compared with Example 1, Comparative Example 2 only does not undergo vacuum calcination treatment, and the other steps are the same.

[0056] Comparative Example 3

[0057] Compared with Example 2, Comparative Example 3 only does not add urea solution, and the other steps are the same.

[0058] Comparative Example 4

[0059] Compared with Example 2, Comparative Example 4 does not undergo vacuum calcination treatment, and other steps are the same.

[0060] Comparative Example 5

[0061] For Example 1, in Comparative Example 5, the purchased nano-silica solution was used for coating, 100 g of the nano-silica solution was mixed with 1000 g of 200-mesh sendust powder, and the S3 bonding and lubrication step and the S4 pressing and annealing step were the same.

[0062] The composite sendust powder obtained in Examples 1-4 and Comparative Examples 1-5 was pressed into a magnetic ring with an outer diameter of 27.00 mm, an inner diameter of 14.70 mm, and a height of 11.10 ± 0.10 mm. After heat treatment, the magnetic ring was made into a coil. The inductance value of the magnetic ring was tested to calculate the magnetic permeability and DC bias performance; and the power loss of the magnetic ring was tested.

[0063] The performance test results of Examples 1-4 and Comparative Examples 1-5 are shown in Table 1:

[0064] Table 1 Magnetic properties of materials in Examples and Comparative Examples

[0065]

[0066] The test results show that compared with Comparative Examples 1 and 2, Examples 1, 3 and 4 have better DC bias performance and lower power loss; compared with Comparative Examples 3 and 4, Example 2 has better DC bias performance and lower power loss; compared with Comparative Example 5, Examples 1-4 have obvious advantages and better comprehensive electromagnetic performance. This is because the sendust magnetic powder cores of Examples 1-4 are uniformly coated with a thin and dense nanoporous silica layer. The nanoporous silica is prepared by promoting the hydrolysis of metasilicic acid with an alkaline substance. The alkaline substance not only acts as a catalyst to ionize or hydrolyze hydroxide ions (OH -) promotes the hydrolysis of orthosilicic acid, and at the same time acts as an etchant to etch the surface of silica, increasing the specific surface area and improving the bonding strength between nanoporous silica and sendust powder; the vacuum calcination treatment effectively improves the crystallinity of silica, forming a dense and thin silica coating on the surface of sendust particles. On the one hand, during the pressing process of the magnetic powder core, more distributed air gaps are created per unit volume, and the DC bias performance is better. On the other hand, the thinner coating is beneficial to reducing the hysteresis loss of the magnetic powder core, so the resulting sendust powder has excellent performance.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a low-loss, high-overlay sendust magnetic powder core, characterized in that: The following steps are involved: S1: preparing a coating agent by dispersing metasilicic acid in anhydrous ethanol, adding an aqueous alkaline catalyst solution while stirring to adjust the pH, and continuing to stir to form a sol to obtain a nanoporous silica coating solution; S2: Passivation coating powder: the nanoporous silica coating solution obtained in step S1 is mixed with sendust powder, stirred evenly, baked and dried, and then placed in a tube furnace for vacuum calcination to obtain nanoporous silica-coated sendust powder; S3: bonding and lubrication, mixing the nanoporous silica-coated sendust powder obtained in step S2 with a composite water-soluble binder, stirring evenly and drying, then adding a lubricant and mixing, mixing evenly and sieving to obtain a composite sendust powder; S4: pressing and annealing, pressing the composite sendust powder obtained in step S3, and then annealing it at high temperature in a nitrogen atmosphere to obtain a low-loss and high-overlap sendust magnetic powder core.

2. The preparation method according to claim 1, wherein In step S1, the mass ratio of metasilicic acid to anhydrous ethanol is (3-5):

10.

3. The preparation method according to claim 1, wherein In step S1, the alkaline catalyst is at least one of concentrated ammonia water, urea, hydrazine, ammonium bicarbonate, and sodium bicarbonate.

4. The preparation method according to claim 1, wherein In step S1, the mass ratio of the alkaline catalyst to deionized water in the alkaline catalyst aqueous solution is (1-4):20, the pH is adjusted to 8-10, the stirring speed is 200-400 rpm, and the stirring time after adjusting the pH to 8-10 is continued for 30-90 minutes.

5. The preparation method according to claim 1, wherein In the step S2, the particle size of the sendust powder is 200 mesh to 800 mesh, and the mass ratio of the sendust powder to the nanoporous silica coating solution in the step S1 is 100:(10-15).

6. The preparation method according to claim 1, wherein In step S2, the stirring speed is 200-400 rpm, the stirring time is 30-60 min, the baking temperature is 80-120° C., and the baking time is 60-90 min; the vacuum calcination temperature is 350-550° C., the heating rate is 2-6° C. / min, and the holding time is 1-3 h.

7. The preparation method according to claim 1, wherein In step S3, the nanoporous silica-coated sendust powder is used as the calculation basis, the composite water-soluble binder includes 0.3-0.6 wt% aqueous polyurethane emulsion, 0.2-0.5 wt% acrylic acid ester water-based adhesive and 4-8 wt% deionized water; the drying temperature is 80-120° C., and the drying time is 60-120 min.

8. The preparation method according to claim 1, wherein The lubricant in step S3 is at least one of aluminum stearate, magnesium stearate, and stearamide.

9. The preparation method according to claim 1, wherein In step S4, the pressing pressure is 18-20 t / cm 2 , the annealing temperature is 700~750℃, and the annealing time is 1~3h.

10. A low-loss, high-overlap sendust magnetic powder core, characterized in that: The product is prepared by the preparation method according to any one of claims 1 to 9.

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