A method for preparing a metal soft magnetic powder core that is easy to shape and has high magnetic properties

Abstract

The invention discloses a method for preparing a metal soft magnetic powder core that is easy to form and has high magnetic properties, comprising the following steps: (1) sieving iron silicon powder to obtain coarse-grained oversize powder and fine-grained undersize powder; (2) putting the coarse-grained oversize powder into an aluminizing agent for mixing, and then reacting at high temperature to obtain aluminized iron silicon powder, and separating the aluminized coarse-grained iron silicon powder by magnetic separation; (3) washing and drying the fine-grained undersize powder, putting it into aluminizing slurry for mixing, and then reacting at high temperature to obtain aluminized iron silicon powder, and separating the aluminized fine-grained iron silicon powder by magnetic separation; (4) insulating the aluminized coarse-grained iron silicon powder and the aluminized fine-grained iron silicon powder; after the insulation is completed, mixing with resin powder and a release agent, pressing and molding, and annealing to obtain a metal soft magnetic powder core.

Description

Technical Field

[0001] The invention relates to a method for preparing a metal soft magnetic powder core which is easy to shape and has high magnetic performance, and belongs to the technical field of magnetic materials. Background Art

[0002] Metal soft magnetic powder core is a composite magnetic material made of fine metal powder, which is widely used in power electronics, communications, new energy, automotive electronics and other fields. Sendust powder core and Sendust aluminum powder core are two kinds of soft magnetic powder cores that are widely used in metal soft magnetic powder cores. Due to the difference in the alloy composition of magnetic powder, they show completely different performance characteristics. Sendust powder core has good DC bias performance, but when working in an alternating frequency environment, due to its low resistivity, it is easy to form large eddy currents between magnetic powder and magnetic powder and inside magnetic powder, resulting in higher losses; Sendust aluminum powder core has lower losses due to its higher resistivity, but the lower proportion of iron in its alloy composition also causes its DC bias performance to decline.

[0003] In order to integrate the advantages of the above two metal soft magnetic powder cores, the industry usually physically mixes iron silicon powder and sendust powder in a certain proportion to prepare a composite metal soft magnetic powder core. Due to the differences in physical properties such as morphology and particle size of the two magnetic powders, the performance of the composite metal soft magnetic powder core prepared by the existing technical method is often not ideal. Summary of the invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method for preparing a composite metal soft magnetic powder core that can effectively combine the performance of an iron silicon powder core and an iron silicon aluminum powder core. The metal soft magnetic powder core prepared by this method has both the high DC bias performance of an iron silicon soft magnetic powder core and the low loss characteristics of an iron silicon aluminum soft magnetic powder core, and exhibits excellent comprehensive magnetic properties.

[0005] Technical solution: The method for preparing the metal soft magnetic powder core of the present invention comprises the following steps:

[0006] (1) Screening the iron silicon powder to obtain coarser-sized oversize powder and finer-sized undersize powder;

[0007] (2) Adding the coarse-grained powder to the aluminizing agent and mixing them; reacting the aluminizing agent mixed with the iron silicon powder at a high temperature so that the aluminum atoms in the aluminizing agent gradually diffuse into the iron silicon powder to obtain aluminized iron silicon powder, and separating the coarse iron silicon powder after aluminization by magnetic separation;

[0008] (3) washing and drying the finer sieve powder and then mixing it in aluminizing slurry; reacting the aluminizing slurry mixed with iron silicon powder at high temperature so that the aluminum atoms in the aluminizing agent gradually diffuse into the iron silicon powder to obtain aluminized iron silicon powder, and separating the fine iron silicon powder after aluminizing by magnetic separation;

[0009] (4) insulating the coarse-grained iron silicon powder after the aluminum infiltration treatment in step (2) and the fine-grained iron silicon powder after the aluminum infiltration treatment in step (3); after the insulation is completed, adding 0.2-0.6% of resin powder and 0.2-0.6% of a release agent according to the total mass of the magnetic powder, and performing compression molding and annealing to obtain a metal soft magnetic powder core.

[0010] Wherein, in step (1), the mesh size of the sieve is 250-700 meshes.

[0011] Wherein, in step (2), the amount of aluminizing agent added is 20-35% of the mass of the magnetic powder, and the aluminizing agent is composed of aluminum powder with a particle size range of D50=20-180 μm, an inert filler, a catalyst and an auxiliary agent; wherein, in the aluminizing agent, the mass percentage of aluminum powder is 71-82%, the mass percentage of the inert filler is 16-28%, the mass percentage of the catalyst is 0.5-2%, and the mass percentage of the auxiliary agent is 0.5-1%.

[0012] Wherein, the inert filler is aluminum oxide; the catalyst is one or a combination of two of ammonium chloride, sodium fluoride or aluminum chloride; the auxiliary agent is one or a combination of two of chromium powder, nickel powder, yttrium oxide or hafnium oxide.

[0013] Wherein, in step (2), the reaction temperature is 650-1050°C and the reaction time is 2-8h.

[0014] Wherein, in step (3), the amount of aluminizing slurry added is 30-50% of the mass of the magnetic powder; the aluminizing slurry is composed of aluminum powder with a particle size range of D50=10-900nm, a binder and a solvent; the aluminizing slurry is prepared by adding 200-250g of aluminum powder and 1-3g of binder to 80-180mL of solvent.

[0015] Wherein, the binder is one or a combination of polyvinyl acetal, polyvinyl butyral or polyurethane resin; and the solvent is one or a combination of ethanol, ethyl acetate, butyl acetate or n-butanol.

[0016] Wherein, in step (3), the reaction temperature is 500-900°C and the reaction time is 2-9h.

[0017] Wherein, in step (4), the mixing mass ratio of the coarse-grained iron-silicon powder after aluminizing treatment and the fine-grained iron-silicon powder after aluminizing treatment is 1-3:1.

[0018] Wherein, in step (4), the insulation treatment method is: mixing the mixed powder, phosphoric acid, aluminum oxide and ethanol, and insulating the magnetic powder, and the insulation treatment time is 15 to 35 minutes; obtaining an Al2O3 insulating layer on the surface of the powder to achieve insulation of the mixed powder; wherein phosphoric acid is added in an amount of 2.5 to 11% of the mass of the mixed powder, aluminum oxide powder is added in an amount of 3 to 6‰ of the mass of the mixed powder, and ethanol is added in an amount of 8 to 14% of the mass of the mixed powder.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant effects: on the one hand, the method of the present invention can plate a layer of iron silicon aluminum alloy on the surface of the iron silicon particles through aluminizing treatment, thereby increasing the resistivity of the magnetic powder, reducing eddy currents between the magnetic powders and reducing losses, while the interior of the magnetic powder particles is still iron silicon alloy, so that the metal soft magnetic powder core has good DC bias performance; on the other hand, the powder used in the method of the present invention is conventional iron silicon powder, and the particle size distribution of the powder does not change significantly after aluminizing treatment. At the same time, the aluminizing treatment of coarse particles will also increase the surface roughness of the particles, thereby promoting the improvement of formability. After the formability is improved, the magnetic core density and the green strength of the magnetic core can be effectively improved; therefore, the metal soft magnetic powder core prepared by the method of the present invention is easy to form, has excellent DC bias performance and low loss, and has good comprehensive magnetic properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a transmission electron microscope photograph of the cross-section near the surface of the magnetic powder after aluminizing treatment. DETAILED DESCRIPTION Example 1

[0021] The method for preparing the metal soft magnetic powder core of the present invention comprises the following steps:

[0022] (1) Screening the iron silicon powder with a mesh size of 700 to obtain a coarser-sized powder on the sieve and a finer-sized powder under the sieve;

[0023] (2) 150 g of the coarse-grained powder on the sieve is added to 50 g of an aluminizing agent and mixed; the aluminizing agent is composed of aluminum powder with a particle size range of D50 = 20 μm, an inert filler of aluminum oxide, a catalyst of ammonium chloride, and an auxiliary agent of chromium powder; wherein the aluminizing agent comprises aluminum powder, aluminum oxide, ammonium chloride, and chromium powder in a mass ratio of 82:16:1:1;

[0024] (3) the aluminizing agent mixed into the iron silicon powder in step (2) is kept at 1050° C. for 8 h, then the powder is cooled at a cooling rate of 45° C. / min, and finally separated by magnetic separation to obtain aluminized coarse iron silicon powder;

[0025] (4) 150 g of the finer-sized undersize powder is washed and dried, and then put into 50 g of aluminizing slurry for mixing; the aluminizing slurry is composed of aluminum powder with a particle size range of D50=10 nm, a binder of polyvinyl acetal, and a mixed solvent formed by ethanol and ethyl acetate; wherein the aluminizing slurry is prepared by adding 250 g of aluminum powder and 1 g of polyvinyl acetal to 100 mL of the mixed solvent; wherein the volume ratio of ethanol to ethyl acetate in the mixed solvent is 1:1;

[0026] (5) The aluminized slurry mixed with the iron silicon powder in step (4) is kept at 500° C. for 2 hours. After the end of the heat preservation, the magnetic powder is cleaned and separated by magnetic separation to obtain fine iron silicon powder after the aluminized treatment;

[0027] (6) The coarse iron silicon powder after aluminizing in step (3) and the fine iron silicon powder after aluminizing in step (5) are mixed in a mass ratio of 1:1, and then 3% phosphoric acid, 3‰ alumina powder and 14% ethanol are added according to the mass percentage of the mixed powder and stirred evenly for 15 minutes to achieve insulation of the mixed powder;

[0028] (7) After the insulation is completed, the powder is dried and then mixed with resin powder and a release agent. After mixing, the mixture is molded at a pressure of 2015 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added is 0.2% of the total mass of the magnetic powder (dried powder), and the amount of release agent added is 0.2% of the total mass of the magnetic powder.

[0029] After testing, the density of the magnetic ring prepared in Example 1 is 6.81 g / cm 3 , the green strength is 152N. After testing, its magnetic permeability is 78 under 100kHz 1V, the magnetic permeability percentage is 55% under 100Oe DC bias, and the loss is 280mW / cm under 50kHz 100mT. 3 . Example 2

[0030] The method for preparing the metal soft magnetic powder core of the present invention comprises the following steps:

[0031] (1) Screening the iron silicon powder with a mesh size of 500 to obtain a coarser-sized powder on the sieve and a finer-sized powder under the sieve;

[0032] (2) 150 g of the coarse-grained powder on the sieve is added to 30 g of an aluminizing agent and mixed; the aluminizing agent is composed of aluminum powder with a particle size range of D50 = 95 μm, an inert filler aluminum oxide, a composite catalyst of sodium fluoride and ammonium chloride, and a composite auxiliary agent of yttrium oxide and hafnium oxide; wherein the aluminizing agent comprises aluminum powder, aluminum oxide, a catalyst (sodium fluoride, ammonium chloride), and an auxiliary agent (yttrium oxide, hafnium oxide) in a mass ratio of 71:28:0.5:0.5;

[0033] (3) the aluminizing agent mixed into the iron silicon powder in step (2) is kept at 650° C. for 6 hours, then the powder is cooled at a cooling rate of 30° C. / min, and finally separated by magnetic separation to obtain aluminized coarse iron silicon powder;

[0034] (4) 150 g of the finer sieved powder is washed and dried, and then mixed with 75 g of aluminizing slurry; the aluminizing slurry is composed of aluminum powder with a particle size range of D50 = 900 nm, a composite binder of polyvinyl butyral and polyurethane resin, and a mixed solvent of butyl acetate and n-butanol; wherein the aluminizing slurry is prepared by adding 200 g of aluminum powder, 1.5 g of polyvinyl butyral, and 1.5 g of polyurethane resin into 180 mL of the mixed solvent; wherein the volume ratio of butyl acetate to n-butanol in the mixed solvent is 1:1;

[0035] (5) The aluminized slurry mixed with the iron silicon powder in step (4) is kept at 780° C. for 4 hours. After the end of the heat preservation, the magnetic powder is cleaned and separated by magnetic separation to obtain fine iron silicon powder after the aluminized treatment;

[0036] (6) The coarse iron silicon powder after aluminizing in step (3) and the fine iron silicon powder after aluminizing in step (5) are mixed in a mass ratio of 3:1, and then 7% phosphoric acid, 4‰ alumina powder and 12% ethanol are added according to the mass percentage of the mixed powder and stirred evenly for 25 minutes to achieve insulation of the mixed powder;

[0037] (7) After the insulation is completed, the powder is dried and then mixed with resin powder and a release agent. After mixing, it is molded at a pressure of 1860 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added is 0.4% of the total mass of the magnetic powder, and the amount of release agent added is 0.4% of the total mass of the magnetic powder.

[0038] After testing, the density of the magnetic ring prepared in Example 2 is 6.75g / cm 3 , the green strength is 138N. After testing, its magnetic permeability is 58 under 100kHz 1V, the magnetic permeability percentage is 74% under 100Oe DC bias, and the loss is 300mW / cm under 50kHz 100mT. 3 . Example 3

[0039] The method for preparing the metal soft magnetic powder core of the present invention comprises the following steps:

[0040] (1) Screening the iron silicon powder with a mesh size of 250 to obtain a coarser-sized powder on the sieve and a finer-sized powder under the sieve;

[0041] (2) 150 g of the coarse-grained powder on the sieve is placed in 45 g of an aluminizing agent and mixed; the aluminizing agent is composed of aluminum powder with a particle size range of D50 = 180 μm, an inert filler of aluminum oxide, a composite catalyst of aluminum chloride and ammonium chloride, and an auxiliary agent; wherein the aluminizing agent is composed of aluminum powder, aluminum oxide, a catalyst (sodium fluoride, ammonium chloride), and an auxiliary agent (yttrium oxide, hafnium oxide) in a mass ratio of 78:19.5:2:0.5;

[0042] (3) the aluminizing agent mixed into the iron silicon powder in step (2) is kept at 800° C. for 2 h, then the powder is cooled at a cooling rate of 15° C. / min, and finally separated by magnetic separation to obtain aluminized coarse iron silicon powder;

[0043] (4) 150 g of the finer sieved powder was washed and dried, and then mixed with 60 g of aluminizing slurry; the aluminizing slurry consisted of aluminum powder with a particle size range of D50 = 500 nm, a binder polyurethane resin, and a butyl acetate solvent; wherein the aluminizing slurry was prepared by adding 230 g of aluminum powder and 2 g of polyurethane resin into 130 mL of butyl acetate;

[0044] (5) The aluminized slurry mixed with the iron silicon powder in step (4) is kept at 900° C. for 9 hours. After the end of the heat preservation, the magnetic powder is cleaned and separated by magnetic separation to obtain fine iron silicon powder after the aluminized treatment;

[0045] (6) The coarse iron silicon powder after aluminizing in step (3) and the fine iron silicon powder after aluminizing in step (5) are mixed in a mass ratio of 2:1, and then 11% phosphoric acid, 6‰ alumina powder and 9% ethanol are added according to the mass percentage of the mixed powder and stirred evenly for 35 minutes to achieve insulation of the mixed powder;

[0046] (7) After the insulation is completed, the powder is dried and then mixed with resin powder and a release agent. After mixing, it is molded at a pressure of 1705 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added is 0.6% of the total mass of the magnetic powder, and the amount of release agent added is 0.6% of the total mass of the magnetic powder.

[0047] After testing, the density of the magnetic ring prepared in Example 3 is 6.67g / cm 3, the green strength is 133N. After testing, its magnetic permeability is 44 under 100kHz 1V, the magnetic permeability percentage is 84% ​​under 100Oe DC bias, and the loss is 260mW / cm under 50kHz 100mT. 3 .

[0048] Comparative Example 1

[0049] The iron silicon powder and the sendust aluminum powder were mixed in a mass ratio of 1:1, and then mixed with the resin powder and the release agent, and then molded at a pressure of 2015 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of the resin powder added was 0.2% of the total mass of the magnetic powder, and the amount of the release agent added was 0.2% of the total mass of the magnetic powder.

[0050] After testing, the density of the magnetic ring prepared in Comparative Example 1 is 6.43 g / cm 3 , the green strength is 101N. After testing, its magnetic permeability is 91 under 100kHz 1V, the magnetic permeability percentage is 24% under 100Oe DC bias, and the loss is 451mW / cm under 50kHz 100mT. 3 .

[0051] Comparative Example 2

[0052] In Comparative Example 2, the iron silicon powder was not screened, and the iron silicon powder was directly treated according to steps (2) to (3) in Example 1. Subsequently, 3% phosphoric acid, 3‰ aluminum oxide powder and 14% ethanol were added according to the mass percentage of the mixed powder and uniformly stirred for 15 minutes to achieve insulation of the mixed powder. After the insulation was completed, the powder was dried and then mixed with resin powder and a release agent. After mixing, the powder was molded at a pressure of 2015 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added was 0.2% of the total mass of the magnetic powder, and the amount of release agent added was 0.2% of the total mass of the magnetic powder.

[0053] After testing, the density of the magnetic ring prepared in Comparative Example 2 is 6.51 g / cm 3 , the green strength is 113N. After testing, its magnetic permeability is 73 under 100kHz 1V, the magnetic permeability percentage is 56% under 100Oe DC bias, and the loss is 347mW / cm under 50kHz 100mT. 3 .

[0054] Comparative Example 3

[0055] In Comparative Example 3, the iron silicon powder was not screened, and the iron silicon powder was directly treated according to steps (4) to (5) in Example 1. Subsequently, 3% phosphoric acid, 3‰ alumina powder and 14% ethanol were added according to the mass percentage of the mixed powder and uniformly stirred for 15 minutes to achieve insulation of the mixed powder. After the insulation was completed, the powder was dried and then mixed with resin powder and a release agent. After mixing, the powder was molded at a pressure of 2015 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added was 0.2% of the total mass of the magnetic powder, and the amount of release agent added was 0.2% of the total mass of the magnetic powder.

[0056] After testing, the density of the magnetic ring prepared in Comparative Example 3 is 6.58 g / cm 3 The green strength is 116N. After testing, its magnetic permeability is 75 under 100kHz 1V, the magnetic permeability percentage is 55% under 100Oe DC bias, and the loss is 311mW / cm under 50kHz 100mT. 3 .

[0057] Comparative Example 4

[0058] A method for preparing a metal soft magnetic powder core comprises the following steps:

[0059] (1) Screening the iron silicon powder with a mesh size of 700 to obtain a coarser-sized powder on the sieve and a finer-sized powder under the sieve;

[0060] (2) 150 g of the coarse-grained sieve powder was added into 50 g of an existing commercial aluminizing agent and mixed;

[0061] (3) the aluminizing agent mixed into the iron silicon powder in step (2) is kept at 1050° C. for 8 h, then the powder is cooled at a cooling rate of 45° C. / min, and finally separated by magnetic separation to obtain aluminized coarse iron silicon powder;

[0062] (4) 150 g of the finer sieved powder was washed and dried, and then added into 50 g of the existing commercial aluminizing slurry for mixing;

[0063] (5) The aluminized slurry mixed with the iron silicon powder in step (4) is kept at 500° C. for 2 hours. After the end of the heat preservation, the magnetic powder is cleaned and separated by magnetic separation to obtain fine iron silicon powder after the aluminized treatment;

[0064] (6) The coarse iron silicon powder after aluminizing in step (3) and the fine iron silicon powder after aluminizing in step (5) are mixed in a mass ratio of 1:1, and then 3% phosphoric acid, 3‰ alumina powder and 14% ethanol are added according to the mass percentage of the mixed powder and stirred evenly for 15 minutes to achieve insulation of the mixed powder;

[0065] (7) After the insulation is completed, the powder is dried and then mixed with resin powder and a release agent. After mixing, it is molded at a pressure of 2015 MPa to obtain a 1.06-inch standard magnetic ring; wherein the amount of resin powder added is 0.2% of the total mass of the magnetic powder, and the amount of release agent added is 0.2% of the total mass of the magnetic powder.

[0066] After testing, the density of the magnetic ring prepared in Comparative Example 4 is 6.74 g / cm 3 The green strength is 115N. After testing, its magnetic permeability is 59 under 100kHz 1V, the magnetic permeability percentage is 71% under 100Oe DC bias, and the loss is 342mW / cm under 50kHz 100mT. 3 .

[0067] The method of the present invention forms an iron-silicon-aluminum layer on the surface of the iron-silicon magnetic powder particles by aluminizing the iron-silicon powder. Figure 1 As shown, it can not only increase the resistivity of iron silicon powder and reduce loss, but also improve its formability by improving the powder morphology. The prepared composite metal soft magnetic powder core has the advantages of high DC bias performance, low loss and easy forming.

Claims

1. A method for preparing a metal soft magnetic powder core which is easy to shape and has high magnetic properties, characterized in that: The steps include: (1) Screening the iron silicon powder to obtain coarser-sized powder on the sieve and finer-sized powder under the sieve; the mesh number of the sieve is 250-700 mesh; (2) The coarse-sized powder on the sieve is placed in an aluminizing agent and mixed; the aluminizing agent mixed with the iron silicon powder is reacted at a high temperature so that the aluminum atoms in the aluminizing agent gradually diffuse into the iron silicon powder to obtain aluminized iron silicon powder, and the coarse iron silicon powder after aluminizing treatment is separated by magnetic separation; the aluminizing agent includes the following components in percentage by mass: 71-82% aluminum powder, 16-28% inert filler, 0.5-2% catalyst and 0.5-1% auxiliary agent; wherein the particle size range of the aluminum powder is D50 = 20-180 μm; the coarse iron silicon powder after aluminizing treatment is: a layer of iron silicon aluminum alloy is coated on the surface of the iron silicon particles, and the interior of the magnetic powder particles is still iron silicon alloy; the amount of aluminizing agent added is 20-35% of the mass of the magnetic powder; (3) The finer sieved powder is washed and dried, and then put into the aluminizing slurry for mixing; the aluminizing slurry mixed with the iron silicon powder is reacted at high temperature to make the aluminum atoms in the aluminizing agent gradually diffuse into the iron silicon powder to obtain aluminized iron silicon powder, and the fine iron silicon powder after aluminizing treatment is separated by magnetic separation; the fine iron silicon powder after aluminizing treatment is: a layer of iron silicon aluminum alloy is coated on the surface of the iron silicon particles, and the inside of the magnetic powder particles is still iron silicon alloy; the amount of aluminizing slurry added is 30-50% of the mass of the magnetic powder; (4) insulating the coarse-grained iron silicon powder after aluminizing in step (2) and the fine-grained iron silicon powder after aluminizing in step (3); mixing the powders with resin powder and a mold release agent after the insulation is completed, and performing compression molding and annealing to obtain a metal soft magnetic powder core; wherein the mixing mass ratio of the coarse-grained iron silicon powder after aluminizing to the fine-grained iron silicon powder after aluminizing is 1 to 3:

1.

2. The method for preparing a metal soft magnetic powder core according to claim 1, characterized in that: In step (2), the inert filler is aluminum oxide; the catalyst is one or a combination of ammonium chloride, sodium fluoride or aluminum chloride; and the auxiliary agent is one or a combination of chromium powder, nickel powder, yttrium oxide or hafnium oxide.

3. The method for preparing a metal soft magnetic powder core according to claim 1, characterized in that: In step (2), the reaction temperature is 650-1050°C and the reaction time is 2-8h.

4. The method for preparing a metal soft magnetic powder core according to claim 1, characterized in that: In step (3), the aluminizing slurry is composed of aluminum powder with a particle size range of D50=10~900nm, a binder and a solvent; the aluminizing slurry is prepared by adding 200~250g of aluminum powder and 1~3g of binder to 80~180mL of solvent.

5. The method for preparing a metal soft magnetic powder core according to claim 4, characterized in that: The binder is one or a combination of polyvinyl acetal, polyvinyl butyral or polyurethane resin; the solvent is one or a combination of ethanol, ethyl acetate, butyl acetate or n-butanol.

6. The method for preparing a metal soft magnetic powder core according to claim 1, characterized in that: In step (3), the reaction temperature is 500-900°C and the reaction time is 2-9h.

7. The method for preparing a metal soft magnetic powder core according to claim 1, characterized in that: In step (4), the insulation treatment method is: mixing the mixed powder, phosphoric acid, aluminum oxide and ethanol, insulating the magnetic powder, the insulation treatment time is 15 to 35 minutes, and an Al2O3 insulation layer is obtained on the surface of the powder; wherein phosphoric acid is added in an amount of 2.5 to 11% of the mass of the mixed powder, aluminum oxide powder is added in an amount of 3 to 6‰ of the mass of the mixed powder, and ethanol is added in an amount of 8 to 14% of the mass of the mixed powder.

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