AU2020103177A4

AU2020103177A4 - Method For Preparing FeSiBCr/SiO2 Nanocrystalline Soft Magnetic Composite Iron Core

Info

Publication number: AU2020103177A4
Application number: AU2020103177A
Authority: AU
Inventors: Xi’an FAN; Zhenyi HUANG; Li Liu; Shenghua Wu; Zhaoyang WU; Chen XIAN
Original assignee: Anhui University of Technology AHUT
Current assignee: Anhui University of Technology AHUT
Priority date: 2019-11-06
Filing date: 2020-11-02
Publication date: 2021-01-14
Anticipated expiration: 2028-11-02
Also published as: CN110767441A; CN110767441B

Abstract

The disclosure belongs to the technical field of powder metallurgy and soft magnetic materials, and provides a method for preparing a FeSiBCr/SiO2 nanocrystalline soft magnetic composite (SMC) iron core. The method is implemented by using a FeSiBCr amorphous powder with a purity of 99.0 wt% or more as a raw material, surface modifying FeSiBCr amorphous powder particles with a silane coupling agent to improve surface activities of FeSiBCr amorphous particles, coating FeSiBCr magnetic particles with a SiO2 insulating shell layer by chemical liquid phase in-situ deposition, hot pressing and subjecting to stress relief annealing to obtain a FeSiBCr/SiO2 nanocrystalline SMC iron core with insulation between particles. The FeSiBCr/SiO2 nanocrystalline SMC iron core of the disclosure shows excellent electromagnetic features such as high magnetic induction and resistivity, desired frequency stability, and low coercivity and iron loss, achieving performance that is not seen in existing SMC materials. 1/Z DRAWINGS 150 0* Omi 6ml 2 -v- lOmI Y~50 - 12m1 0. ------ 1000 -100 0 -100 01000 00 Magneic Fild'. e -2000 1000 100 0 200 14m 6 600 44 j2 100 1000 10000 100000 Frequency(Hz) FIG. 2

Description

1/Z

DRAWINGS

150 Omi 0*

6ml

2 -v- lOmI Y~50 - 12m1

0. ------

1000

-100 0 -100 01000 00 Magneic Fild'. e

-2000 1000 100 0 200

14m

6 600

44

j2

100 1000 10000 100000 Frequency(Hz) FIG. 2

METHOD FOR PREPARING FeSiBCr/SiO2 NANOCRYSTALLINE SOFT MAGNETIC COMPOSITE IRON CORE TECHNICAL FIELD The disclosure relates to the technical field of powder metallurgy and soft magnetic materials, in particular to a method for preparing a FeSiBCr/SiO2 nanocrystalline soft magnetic composite (SMC) iron core. BACKGROUND In the fields of functional materials and structural materials, iron-based nanocrystalline SMC materials have excellent soft magnetic properties such as high saturation induction, high magnetic permeability, low coercivity and low remanence, as well as excellent mechanical properties. The SMC materials have a low cost and a broad range of promising applications. At present, commercial iron-based SMC materials are usually prepared by pressing SMC powders with a core-shell structure (with ferromagnetic powder as the core and insulating coating agent as the shell). Therefore, the performance of the SMC powders has an important effect on performance of the iron-based SMC materials. The type and amount of the insulating coating agent are key factors for adjustment of the performance of the SMC powders. Therefore, insulating coating is a most important process in producing a nanocrystalline SMC iron core. A basis for optimizing the performance of iron-based SMC materials and magnetic core elements is to ensure uniformity, integrity and compactness of the core-shell structure in the SMC materials. However, most SMC powder preparation methods today have defects such as difficulty in controlling a reaction process and an uneven insulating coating. Moreover, a relatively large content of SiO 2 as a non magnetic phase is easy to be introduced, which greatly reduces electromagnetic properties of the SMC powders and materials. In order to further meet requirements of future motor frequency conversion control technology, it is necessary to carry out research and development of nanocrystalline SMC iron cores which can withstand high energy density input with low iron loss. For example, Xu et al. (Xu L, Yan B. Fe-6.5%Si/SiO2 powder cores prepared by spark plasma sintering: Magnetic

properties and sintering mechanism [J]. International Journal of Modern Physics B, 2017, 31(16 19): 17440111-17440116.) prepared Fe-Si/SiO2 SMC powders by a simple mechanical ball milling process with electrical resistivity reaching 1.7x10-5 Qm and excellent soft magnetic properties. S. Wu (Wu S, Sun A Z, Lu Z W, Chen C, Gao X X. Magnetic properties of iron-based soft magnetic composites with Si02 coating obtained by reverse microemulsion method [J], Journal of Magnetism and Magnetic Materials, 2015, 381: 451-456.) prepared Fe/Si02 soft magnetic composite powders coated with an amorphous Si02 layer where an iron core loss at 150 kHz was only 10% of an uncoated sample. However, most SMC powder preparation methods today have defects such as difficulty in controlling a reaction process and an uneven insulating coating. Moreover, a relatively large content of Si2 as a non-magnetic phase is easy to be introduced, which greatly reduces electromagnetic properties of the SMC powders and materials. It is to be understood that any acknowledgement of prior art in this specification is not to be taken as an admission that this prior art forms part of the common general knowledge in Australia or elsewhere. SUMMARY An objective of the disclosure is to overcome shortcomings of the above prior art, and provide a method for preparing a FeSiBCr/Si02 nanocrystalline SMC iron core. The method of the disclosure can enable control of thickness of a Si02 coating layer of composite particles in the FeSiBCr/Si02 nanocrystalline SMC iron core, achieving relatively high resistivity and frequency stability. Obtained Fe-Si alloy composite powders have relatively low iron loss and relatively desired heat stability. To achieve the above purpose, the disclosure provides the following technical solutions. A method for preparing a FeSiBCr/Si02 nanocrystalline soft magnetic composite (SMC) iron core includes the following steps: step (1): placing a FeSiBCr amorphous powder with a purity of 99.0 wt% or more as a raw material and absolute ethanol in a reaction flask, dispersing under mechanical stirring, adding an ethanol/water solution, further dispersing, heating to 40-60°C, adding a silane coupling agent/ethanol solution and surface modifying the FeSiBCr amorphous powder to obtain a FeSiBCr amorphous powder solution; step (2): adding a tetraethyl orthosilicate (TEOS)/ethanol solution and an ammonia/water/ethanol solution to the FeSiBCr amorphous powder solution dropwise under stirring at the same time, mechanical stirring, adding ethanol, further mechanical stirring to allow a complete reaction, washing with absolute ethanol repeatedly and vacuum drying to obtain a FeSiBCr/Si02 amorphous composite powder; step (3): weighing the FeSiBCr/Si02 amorphous composite powder, placing in a specific graphite mold for hot pressing, cooling to room temperature in a furnace after the hot pressing is completed, cutting to obtain an iron core ring, and subjecting the iron core ring to stress relief annealing in a protective gas to eliminate residual stress. Preferably, in step (1), the FeSiBCr amorphous powder includes 86-87 wt% of Fe, 7-8 wt% of Si, 2-3 wt% of B and 2-3 wt% of Cr. Preferably, in step (1), the FeSiBCr amorphous powder has a particle size of 300-400 mesh.

Preferably, in step (2), a mass ratio of the FeSiBCr amorphous powder to the silane coupling agent is (13-16):1. Preferably, in step (2), preparation of the FeSiBCr/SiO2 amorphous composite powder is

carried out in a thermostat water bath at 40-60°C. Preferably, in step (2), a volume ratio of the TEOS/ethanol solution to the ammonia/water/ethanol solution is 1:1.

Preferably, in step (2), the TEOS/ethanol solution and the ammonia/water/ethanol solution are added at a rate of 5 mL/h, and the mechanical stirring is carried out at 700-800 r/min. Preferably, in step (2), the vacuum drying is carried out at 70°C for 4 h.

Preferably, in step (3), the FeSiBCr/SiO2 amorphous composite powder is sintered at 580 680°C and 50-70 MPa with a heating rate of 30-60°C/min and a temperature holding time of 10 min; and the protective gas is nitrogen or argon or a mixture of nitrogen and argon.

BRIEF DESCRIPTION OF DRAWINGS To describe the technical solutions in examples of the disclosure more clearly, the following text briefly describes the accompanying drawings required for describing the examples The

accompanying drawings in the following description only show some examples of the disclosure, and a person of ordinary skill in the art may still derive other drawings from these accompanying drawings without creative efforts.

FIG. 1 shows hysteresis loops of FeSiBCr/SiO2 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS in examples. FIG. 2 shows curves of relative permeability ([r) of FeSiBCr/SiO2 SMC iron cores prepared

with different addition amounts of TEOS in examples with test frequency. FIG. 3 shows magnetic hysteresis loops and relative permeability of FeSiBCr nanocrystalline soft magnetic core (FeSiBCr soft magnetic iron core) and FeSiBCr/SiO2 nanocrystalline SMC

core (FeSiBCr/SiO2 composite iron core) in an example with test frequency. DETAILED DESCRIPTION In order to make the objectives, technical solutions and advantages of examples of the

disclosure clearer, the following text clearly and completely describes the technical solutions in the examples of the disclosure with reference to the examples of the disclosure. Apparently, the described examples are some rather than all of the examples. All other examples obtained by a

person of ordinary skill in the art based on the examples of the disclosure without creative efforts shall fall within the protection scope of the disclosure. In the following examples, the FeSiBCr amorphous powder used was purchased from Yahao

New Material Technology Co., Ltd., Qinhuangdao, Hebei. The FeSiBCr amorphous powder was water atomized alloy powder with excellent sphericity, purity of 99.90wt.% or more, and particle size of 300-400 mesh. Based on mass fraction, the FeSiBCr amorphous powder included about 86.50 wt% of Fe, 7.53 wt% of Si, 2.48 wt% of B, 2.57 wt% of Cr, and impurity elements as balance, where the impurity elements was relatively low in content. Examples 1-6: A method for preparing a FeSiBCr/Si02 nanocrystalline SMC iron core was carried out with the following steps: Step (1): 50 g of FeSiBCr amorphous powder and 300 mL of absolute ethanol were added into a three-necked flask, and dispersed under mechanical stirring (750 r/min) at room temperature for 5 min. 50 mL of ethanol/water solution (40 mL of ethanol and deionized water as balance) was added and further dispersed for 15 min. The solution was heated to 40°C in a thermostat water bath. 3.4 g of silane coupling agent (3-Aminopropyl)triethoxysilane (APTES) was dissolved in absolute ethanol to prepare 50 mL of solution which was then added to the solution in the three-necked flask. The FeSiBCr amorphous powder was surface modified to obtain a FeSiBCr amorphous powder solution. Step (2): A TEOS/ethanol solution and an ammonia/water/ethanol solution were prepared and drawn into 50 mL syringes A and B respectively (syringe A: X mL of TEOS and absolute ethanol as balance; syringe B: 2.4 mL of ammonia, 6.4 mL of deionized water, and 41.2 mL of absolute ethanol). The TEOS/ethanol solution and the ammonia/water/ethanol solution were added dropwise to the FeSiBCr amorphous powder solution at the same time at a rate of 5 mL/h at a constant temperature of 40°C under stirring. After addition was completed, mechanical stirring (750 r/min) was carried out for 1 h. 100 mL of ethanol was added and mechanically stirred (750 r/min) to allow a complete reaction. Washing was carried out with absolute ethanol repeatedly. Vacuum drying was carried out at 70°C for 4 h to obtain a FeSiBCr/Si02 amorphous composite powder. Step (3): The prepared FeSiBCr/Si02 amorphous composite powder was weighed and placed in a specific graphite mold for hot pressing. A longitudinal pressure of 50 MPa was applied to the graphite mold. A heating rate was 50°C/min and a sintering temperature was 630°C which was held for 10 min. After sintering, a sample was cooled to room temperature in a furnace. A cylindrical sample was cut into a ring shape with an outer diameter of 30 mm, an inner diameter of 20 mm, and a height of 6 mm. An iron core ring was subjected to stress relief annealing in a protective gas to eliminate residual stress to obtain a FeSiBCr/Si2 nanocrystalline SMC iron core.

In Examples 1-6, the addition amount X of TEOS was 0, 6, 8, 10, 12, and 15 mL in sequence respectively, and the FeSiBCr/Si02 amorphous composite powders prepared with different addition amounts of TEOS were tested for saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (He) with specific results shown in Table 1. The FeSiBCr/Si02 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS were tested for saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (He) with specific results shown in Table 2. When X was 0 mL, the prepared product was FeSiBCr soft magnetic iron core. Table 1 Saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (He) of FeSiBCr/Si02 amorphous composite powders prepared with different addition amounts of TEOS Addition amount of TEOS (mL) 0 6 8 10 12 15 FeSiBCr/Si02 Ms (emu/g) 165.7 162.8 160.6 157.9 151.6 147.8 amorphous Mr (emu/g) 0.51 0.53 0.57 0.60 0.75 0.85 composite powder He (Oe) 7.5 7.8 8.1 8.5 9.7 10.6 Table 2 Saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (He) of FeSiBCr/SiO2 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS Addition amount of TEOS (mL) 0 6 8 10 12 15 FeSiBCr/SiO2 Ms (emu/g) 164.2 160.3 158.4 155.7 143.7 140.9 nanocrystalline Mr (emu/g) 0.52 0.54 0.59 0.69 0.76 0.78 SMC iron core He (A/m) 13.9 14.2 14.9 15.6 16.3 16.4 Hysteresis loops of the FeSiBCr/SiO2 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS were shown in FIG. 1. All the prepared iron core samples reached saturation of magnetic induction when applied magnetic field strength reached 8,000 Oe, and had high M, low He and low Mr. Examples 7-12: A method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core was carried out with the following steps: Step (1): 50 g of FeSiBCr amorphous powder and 300 mL of absolute ethanol were added into a three-necked flask, and dispersed under mechanical stirring (750 r/min) at room temperature for 5 min. 50 mL of ethanol/water solution (40 mL of ethanol and deionized water as balance) was added and further dispersed for 15 min. The solution was heated to 50°C in a thermostat water bath. 3.4 g of silane coupling agent APTES was dissolved in absolute ethanol to prepare 50 mL of solution which was then added to the solution in the three-necked flask. The

FeSiBCr amorphous powder was surface modified to obtain a FeSiBCr amorphous powder solution. Step (2): A TEOS/ethanol solution and an ammonia/water/ethanol solution were prepared and drawn into 50 mL syringes A and B respectively (syringe A: X mL of TEOS and absolute ethanol as balance; syringe B: 1.3 mL of ammonia, 6.4 mL of deionized water, and 42.3 mL of absolute ethanol). The TEOS/ethanol solution and the ammonia/water/ethanol solution were added dropwise to the FeSiBCr amorphous powder solution at the same time at a rate of 5 mL/h at a constant temperature of 60°C under stirring. After addition was completed, mechanical stirring (750 r/min) was carried out for 1 h. 100 mL of ethanol was added and mechanically stirred (750 r/min) to allow a complete reaction. Washing was carried out with absolute ethanol repeatedly. Vacuum drying was carried out at 70°C for 4 h to obtain a FeSiBCr/SiO2 amorphous composite powder. Step (3): The prepared FeSiBCr/SiO2 amorphous composite powder was weighed and placed in a specific graphite mold for hot pressing. A longitudinal pressure of 50 MPa was applied to the graphite mold. A heating rate was 50°C/min and a sintering temperature was 630°C which was held for 10 min. After sintering, a sample was cooled to room temperature in a furnace. A cylindrical sample was cut into a ring shape with an outer diameter of 30 mm, an inner diameter of 20 mm, and a height of 6 mm. An iron core ring was subjected to stress relief annealing in a protective gas to eliminate residual stress to obtain a FeSiBCr/SiO2 nanocrystalline SMC iron core. In Examples 7-12, the addition amount X of TEOS was 0, 6, 8, 10, 12, and 15 mL in sequence respectively, and the FeSiBCr/SiO2 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS were tested for resistivity and iron loss at different frequencies with specific results shown in Table 3. When X was 0 mL, the prepared product corresponded to FeSiBCr soft magnetic iron core. Table 3 Resistivity and iron loss at different frequencies of FeSiBCr/SiO2 nanocrystalline SMC iron cores prepared with different addition amounts of TEOS

Addition amount of Thickness of shell Resistivity Iron loss (w-kg-1 )

Classification TEOS (mL) (nm) (Q-m) W1/2 Wus5 W/io W1/20 W1/so

0 0 6.15x10- 7 0.09 0.38 1.15 3.42 6.23

FeSiBCr/SiO 2 6 50 1.02x10 4 0.04 0.23 0.47 1.29 2.51

SMC iron core 8 90 1.39x10-4 0.03 0.12 0.36 1.08 1.98

10 160 1.83x10- 4 0.02 0.09 0.21 0.92 1.39

12 370 1.81x10- 4 0.02 0.09 0.22 1.07 1.71

15 153 - 0.07 0.32 0.74 2.03

Curves of relative permeability (r) of FeSiBCr/SiO2 nanocrystalline SMC iron cores

prepared with different addition amounts of TEOS with test frequency were shown in FIG. 2.

Initial pr values of all iron core samples were relatively large. With increase of the addition amount of TEOS, the initial pr values first decreased and then increased. As the addition amount

of TEOS increased from 6 mL to 12 mL, the FeSiBCr/SiO2 SMC iron cores had an increased

thickness of SiO2 insulating layer and a decreased pr value. When the addition amount of TEOS continued to increase to 15 mL, the thickness of SiO 2 insulating layer between FeSiBCr magnetic

particles was reduced while the pr values of the samples increased due to accumulation of free

SiO2 and uneven coating. At the same time, with increase of test frequency, the pr of all samples decreased drastically and then stabilized.

Example 13: A method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core was carried out with the following steps:

Step (1): 50 g of FeSiBCr amorphous powder and 300 mL of absolute ethanol were added

into a three-necked flask, and dispersed under mechanical stirring (750 r/min) at room temperature for 5 min. 50 mL of ethanol/water solution (40 mL of ethanol and deionized water as

balance) was added and further dispersed for 15 min. The solution was heated to 50°C in a

thermostat water bath. 3.4 g of silane coupling agent APTES was dissolved in absolute ethanol to prepare 50 mL of solution which was then added to the solution in the three-necked flask. The

FeSiBCr amorphous powder was surface modified to obtain a FeSiBCr amorphous powder

solution. Step (2): A TEOS/ethanol solution and an ammonia/water/ethanol solution were prepared

and drawn into 50 mL syringes A and B respectively (syringe A: 10 mL of TEOS and 40 mL of

absolute ethanol; syringe B: 1.8 mL of ammonia, 6.4 mL of deionized water, and 41.8 mL of absolute ethanol). The TEOS/ethanol solution and the ammonia/water/ethanol solution were

added dropwise to the FeSiBCr amorphous powder solution at the same time at a rate of 5 mL/h

at a constant temperature of 50°C under stirring. After addition was completed, mechanical stirring (750 r/min) was carried out for 1 h. 100 mL of ethanol was added and mechanically stirred (750 r/min) to allow a complete reaction. Washing was carried out with absolute ethanol repeatedly. Vacuum drying was carried out at 70°C for 4 h to obtain a FeSiBCr/SiO2 amorphous composite powder.

Step (3): The prepared FeSiBCr/SiO2 amorphous composite powder was weighed and placed in a specific graphite mold for hot pressing. A longitudinal pressure of 50 MPa was applied to the graphite mold. A heating rate was 50°C/min and a sintering temperature was 630°C which was

held for 10 min. After sintering, a sample was cooled to room temperature in a furnace. A cylindrical sample was cut into a ring shape with an outer diameter of 30 mm, an inner diameter of 20 mm, and a height of 6 mm. An iron core ring was subjected to stress relief annealing in a

protective gas to eliminate residual stress to obtain a FeSiBCr/SiO2 nanocrystalline SMC iron core. In Example 13, when addition amount of TEOS was 0 mL, a FeSiBCr soft magnetic iron

core was obtained. The FeSiBCr/SiO2 nanocrystalline SMC iron core and corresponding FeSiBCr soft magnetic iron core in Example 13 were tested for magnetic hysteresis loops with results shown in FIG. 3 (a) and relative permeability with test frequency with results shown in FIG. 3 (b).

In FIG. 3 (a), both soft magnetic iron cores reached saturation of magnetic induction when applied magnetic field strength reached 8,000 Oe, and had high saturation induction M, low coercivity He and low remanence Mr. Compared with the FeSiBCr/SiO2 nanocrystalline SMC

iron core, the FeSiBCr nanocrystalline soft magnetic iron core had Ms reduced from 164.2 emu/g to 155.3 emu/g; He increased slightly from 13.9 A/m to 19.8 A/ m, and remanence increased from 0.9 emu/g to 1.4 emu/g. In FIG. 3(b), after coating with a non-magnetic phase SiO 2 with an

insulating effect, the iron core had a reduced relative content of magnetic phase, and at the same time an increased distance between the magnetic particles FeSiBCr, which weakened interactive coupling of the magnetic particles. This resulted in that, the FeSiBCr soft magnetic iron core had

a larger relative permeability compared with the FeSiBCr/SiO2 SMC iron core at a relatively low frequency (<3 kHz). Effective magnetic permeability of the FeSiBCr/SiO2 SMC iron core dropped only when the test frequency exceeded 50 kHz, and dropped in a relatively slow rate.

However, effective permeability of the FeSiBCr nanocrystalline soft magnetic iron core dropped sharply when the frequency exceeded 1 KHz. This indicated that, frequency stability of the FeSiBCr/SiO2 SMC iron core was much better than that of the FeSiBCr nanocrystalline soft

magnetic iron core without a SiO 2 coating. Example 14: A method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core was carried out with

the following steps:

Step (1): 50 g of FeSiBCr amorphous powder and 300 mL of absolute ethanol were added into a three-necked flask, and dispersed under mechanical stirring (750 r/min) at room temperature for 5 min. 50 mL of ethanol/water solution (40 mL of ethanol and deionized water as

balance) was added and further dispersed for 15 min. The solution was heated to 50°C in a thermostat water bath. 3.4 g of silane coupling agent APTES was dissolved in absolute ethanol to prepare 50 mL of solution which was then added to the solution in the three-necked flask. The

FeSiBCr amorphous powder was surface modified to obtain a FeSiBCr amorphous powder solution. Step (2): A TEOS/ethanol solution and an ammonia/water/ethanol solution were prepared

and drawn into 50 mL syringes A and B respectively (syringe A: 6 mL of TEOS and 44 mL of absolute ethanol; syringe B: 6.4 mL of ammonia, 6.4 mL of deionized water, and 37.2 mL of absolute ethanol). The TEOS/ethanol solution and the ammonia/water/ethanol solution were

added dropwise to the FeSiBCr amorphous powder solution at the same time at a rate of 5 mL/h at a constant temperature of 50°C under stirring. After addition was completed, mechanical stirring (750 r/min) was carried out for 1 h. 100 mL of ethanol was added and mechanically

stirred (750 r/min) to allow a complete reaction. Washing was carried out with absolute ethanol repeatedly. Vacuum drying was carried out at 70°C for 4 h to obtain a FeSiBCr/SiO2 amorphous composite powder.

Step (3): The prepared FeSiBCr/SiO2 amorphous composite powder was weighed and placed in a specific graphite mold for hot pressing. A longitudinal pressure of 65 MPa was applied to the graphite mold. A heating rate was 60°C/min and a sintering temperature was 680°C which was

protective gas to eliminate residual stress to obtain a FeSiBCr/SiO2 nanocrystalline SMC iron core. Example 15: A method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core was carried out with the following steps: Step (1): 50 g of FeSiBCr amorphous powder and 300 mL of absolute ethanol were added

into a three-necked flask, and dispersed under mechanical stirring (750 r/min) at room temperature for 5 min. 50 mL of ethanol/water solution (40 mL of ethanol and deionized water as balance) was added and further dispersed for 15 min. The solution was heated to 50°C in a

thermostat water bath. 3.4 g of silane coupling agent APTES was dissolved in absolute ethanol to prepare 50 mL of solution which was then added to the solution in the three-necked flask. The FeSiBCr amorphous powder was surface modified to obtain a FeSiBCr amorphous powder solution. Step (2): A TEOS/ethanol solution and an ammonia/water/ethanol solution were prepared and drawn into 50 mL syringes A and B respectively (syringe A: 8 mL of TEOS and 42 mL of absolute ethanol; syringe B: 6.4 mL of ammonia, 6.4 mL of deionized water, and 37.2 mL of absolute ethanol). The TEOS/ethanol solution and the ammonia/water/ethanol solution were added dropwise to the FeSiBCr amorphous powder solution at the same time at a rate of 5 mL/h at a constant temperature of 50°C under stirring. After addition was completed, mechanical stirring (750 r/min) was carried out for 1 h. 100 mL of ethanol was added and mechanically stirred (750 r/min) to allow a complete reaction. Washing was carried out with absolute ethanol repeatedly. Vacuum drying was carried out at 70°C for 4 h to obtain a FeSiBCr/Si02 amorphous composite powder. Step (3): The prepared FeSiBCr/SiO2 amorphous composite powder was weighed and placed in a specific graphite mold for hot pressing. A longitudinal pressure of 70 MPa was applied to the graphite mold. A heating rate was 30°C/min and a sintering temperature was 580°C which was held for 10 min. After sintering, a sample was cooled to room temperature in a furnace. A cylindrical sample was cut into a ring shape with an outer diameter of 30 mm, an inner diameter of 20 mm, and a height of 6 mm. An iron core ring was subjected to stress relief annealing in a protective gas to eliminate residual stress to obtain a FeSiBCr/SiO2 nanocrystalline SMC iron core. It can be seen from Examples 1-15 that, in the prepared FeSiBCr nanocrystalline SMC iron cores, the FeSiBCr magnetic particles were uniformly, densely and continuously coated by the SiO2 insulating shell. Moreover, parameters of the chemical liquid phase in-situ deposition can be used to control thickness, uniformity, and continuity of the SiO 2 insulating shell. Furthermore, the iron cores of the disclosure showed excellent electromagnetic properties such as high resistivity, desired frequency stability, low coercivity, and low iron loss, which had obvious advantages compared with similar SMC iron cores. Fe-Si alloy composite powders prepared by the method of the disclosure had high resistivity, low loss and desired thermal stability with a simple preparation process and a low cost of raw materials, improving production efficiency and showing no pollution to the environment. At the same time, FeSiBCr nanocrystalline SMC iron cores obtained by the method of the disclosure had excellent frequency stability and low coercivity, which met demands of development of electromagnetic conversion equipment in applications with high frequency. Therefore, the disclosure was suitable for popularization and application. Compared with the prior art, the technical solutions provided by the disclosure have the following advantages: (1) The disclosure successfully synthesizes a FeSiBCr/SiO2 amorphous composite powder by chemical liquid phase in-situ deposition, and successfully prepares a FeSiBCr/SiO2 nanocrystalline SMC iron core by further hot pressing. In a prepared soft magnetic iron core, FeSiBCr magnetic particles are uniformly, densely and continuously coated by a SiO 2 insulating shell, which realizes a structure of an SMC iron core with insulation between magnetic particles. (2) The FeSiBCr/SiO2 nanocrystalline SMC iron core of the disclosure shows excellent electromagnetic features such as high magnetic induction, high resistivity, desired frequency stability, low coercivity, and low iron loss. Compared with a FeSiBCr soft magnetic iron core without a SiO 2 insulating layer, the iron core of the disclosure has significantly increased resistivity and greatly reduced iron loss. As test frequency increases, the iron loss shows a relatively large decrease. When the test frequency exceeds 20 KHz, advantage of the FeSiBCr/SiO2 nanocrystalline SMC iron core in iron loss is especially prominent. (3) In the disclosure, parameters such as addition amount of silicon source, reaction temperature, water content, and ammonia content in the chemical liquid phase in-situ deposition can be used to control hydrolysis and polycondensation reaction rates with tetraethyl orthosilicate (TEOS), so as to control uniformity, continuity and shell thickness of the SiO 2 insulating shell. (4) The simple method of the disclosure has improved production efficiency, no pollution to the environment and a relatively low cost of raw materials, which is suitable for popularization and application. The foregoing examples are only used to explain the technical solutions of the disclosure, and are not intended to limit the same. Although the disclosure is described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions on some technical features therein. These modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the examples of the disclosure.

Claims

What is claimed is: 1. A method for preparing a FeSiBCr/SiO2 nanocrystalline soft magnetic composite (SMC) iron core, comprising the following steps:

step (1): placing a FeSiBCr amorphous powder with a purity of 99.0 wt% or more as a raw material and absolute ethanol in a reaction flask, dispersing under mechanical stirring, adding an ethanol/water solution, further dispersing, heating to 40-60°C, adding a silane coupling

agent/ethanol solution and surface modifying the FeSiBCr amorphous powder to obtain a FeSiBCr amorphous powder solution; step (2): adding a tetraethyl orthosilicate (TEOS)/ethanol solution and an

ammonia/water/ethanol solution to the FeSiBCr amorphous powder solution dropwise under stirring at the same time, mechanical stirring, adding ethanol, further mechanical stirring to allow a complete reaction, washing with absolute ethanol repeatedly and vacuum drying to obtain a

FeSiBCr/SiO2 amorphous composite powder; step (3): weighing the FeSiBCr/SiO2 amorphous composite powder, placing in a specific graphite mold for hot pressing, cooling to room temperature in a furnace after the hot pressing is

completed, cutting to obtain an iron core ring, and subjecting the iron core ring to stress relief annealing in a protective gas to eliminate residual stress.
2. The method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core according to

claim 1, wherein in step (1), the FeSiBCr amorphous powder comprises 86-87 wt% of Fe, 7-8 wt% of Si, 2-3 wt% of B and 2-3 wt% of Cr.
3. The method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core according to

claim 1, wherein in step (1), the FeSiBCr amorphous powder has a particle size of 300-400 mesh.
4. The method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core according to claim 1, wherein in step (2), a mass ratio of the FeSiBCr amorphous powder to the silane

coupling agent is (13-16):1; wherein in step (2), preparation of the FeSiBCr/SiO2 amorphous composite powder is carried out in a thermostat water bath at 40-60°C;

wherein in step (2), a volume ratio of the TEOS/ethanol solution to the ammonia/water/ethanol solution is 1:1; wherein in step (2), the TEOS/ethanol solution and the ammonia/water/ethanol solution are

added at a rate of 5 mL/h, and the mechanical stirring is carried out at 700-800 r/min; wherein in step (2), the vacuum drying is carried out at 70°C for 4 h.
5. The method for preparing a FeSiBCr/SiO2 nanocrystalline SMC iron core according to

claim 1, wherein in step (3), the FeSiBCr/SiO2 amorphous composite powder is sintered at 580-

680°C and 50-70 MPa with a heating rate of 30-60°C/min and a temperature holding time of 10 min; and the protective gas is nitrogen or argon or a mixture of nitrogen and argon.