Core-Shell Structured Composite Powder Electromagnetic Wave Absorber Formed by Coating Fe-Based Nanocrystalline Alloy with Carbon, and Preparation Method Thereof

Abstract

Disclosed is a core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon and a preparation method thereof. The core-shell structured composite powder includes a core of an Fe-based nanocrystalline alloy, and a shell of an amorphous carbon layer, the shell accounting for 5-25 wt % of the core-shell structured composite powder electromagnetic wave absorber, wherein the core-shell structured composite powder electromagnetic wave absorber has a particle size of 3-10 μm; the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Si.sub.aB.sub.b, where atomic percentage contents of Si and B are 3-15 respectively, and a balance is the atomic percentage content of Fe.

Claims

1. A core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon, comprising a core of an Fe-based nanocrystalline alloy, and a shell of an amorphous carbon layer, the shell accounting for 5-25 wt % of the core-shell structured composite powder electromagnetic wave absorber, wherein the core-shell structured composite powder electromagnetic wave absorber has a spherical-like core-shell structure, and a particle size of 3-10 μm; the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Si.sub.aB.sub.b, where a and b represent an atomic percentage content of a corresponding element respectively, and meet requirements of
3≤a≤15,
3≤b≤15, and a balance being an atomic percentage content of Fe; the Fe-based nanocrystalline alloy has an amorphous/α-Fe dual-phase structure, wherein the α-Fe has a grain size of 10-30 nm; and the amorphous carbon layer has an average thickness of 0.3-1 μm.

2. The core-shell structured composite powder electromagnetic wave absorber as claimed in claim 1, wherein the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Co.sub.xNi.sub.ySi.sub.aB.sub.bC.sub.cCu.sub.dTM.sub.e, where TM represents at least one selected from the group consisting of Nb, Mo, Cr, and Mn; where x, y, a, b, c, d, and e represent an atomic percentage content of a corresponding element respectively, and meet requirements of
0≤x≤15,
0≤y≤15,
0≤x+y≤20,
0≤a≤15,
0≤b≤15,
0≤c≤15,
b 6≤a+b+c≤30,
0≤d≤2,
0≤e≤4, and a balance being an atomic percentage content of Fe.

3. The core-shell structured composite powder electromagnetic wave absorber as claimed in claim 1, wherein an electromagnetic wave absorber coating is formed from a mixture of the core-shell structured composite powder electromagnetic wave absorber and a wave-transparent matrix in a mass ratio of 3:2, and under a condition that the electromagnetic wave absorber coating has a thickness of 1.5-2.5 mm, the electromagnetic wave absorber coating exhibits a reflection loss lower than −10 dB within a frequency of 8-18 GHz, and a minimum reflection loss of −54 dB.

4. The core-shell structured composite powder electromagnetic wave absorber as claimed in claim 2, wherein an electromagnetic wave absorber coating is formed from a mixture of the core-shell structured composite powder electromagnetic wave absorber and a wave-transparent matrix in a mass ratio of 3:2, and under a condition that the electromagnetic wave absorber coating has a thickness of 1.5-2.5 mm, the electromagnetic wave absorber coating exhibits a reflection loss lower than −10 dB within a frequency of 8-18 GHz, and a minimum reflection loss of −54 dB.

5. A method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon as claimed in claim 1, comprising i) preparing a Fe-based nanocrystalline alloy powder by a) providing raw materials according to a nominal composition formula of the Fe-based nanocrystalline alloy, each of the raw materials having a purity of not less than 99 wt %; b) mixing the raw materials, and melting a resulting mixed material in an induction melting furnace or a non-consumable-electrode arc furnace in an argon atmosphere, to obtain a chemically uniform master alloy ingot; c) crushing the master alloy ingot and screening, to obtain an alloy powder with a particle size of less than 300 μm; and d) placing the alloy powder in a stainless steel ball mill tank in a ball-to-powder mass ratio of 20:1; vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing a sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 50-85 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the Fe-based nanocrystalline alloy powder with a particle size of 2-8 μm; ii) using a commercial carbon powder or preparing a carbon powder by steps of a) mechanically crushing graphite and screening, to obtain a graphite powder with a particle size of less than 300 μm; and b) placing the graphite powder in a stainless steel ball mill tank in a ball-to-powder mass ratio of 20:1, vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing a sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 30 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the carbon powder with a particle size of 1-3 μm; and iii) preparing a core-shell structured composite powder electromagnetic wave absorber by a) mixing the Fe-based nanocrystalline alloy powder obtained in step 1 and the carbon powder obtained in step 2 in a preset ratio, and placing a resulting mixture in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1 or 30:1, vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing a sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 6-10 h, at a rotation speed of 200 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the core-shell structured composite powder electromagnetic wave absorber with a particle size of 3-10 μm.

6. The method as claimed in claim 5, wherein the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Co.sub.xNi.sub.ySi.sub.aB.sub.bC.sub.cCu.sub.dTM.sub.e, where TM represents at least one selected from the group consisting of Nb, Mo, Cr, and Mn; x, y, a, b, c, d, and e represent an atomic percentage content of a corresponding element respectively, and meet requirements of
0≤x≤15,
0≤y≤15,
0≤x+y≤20,
0≤a≤15,
0≤b≤15,
0≤c≤15,
6≤a+b+c≤30,
0≤d≤2,
0≤e≤4, and a balance being an atomic percentage content of Fe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 shows the X-ray diffraction (XRD) pattern of the composite powder as prepared in Example 1.

[0035] FIG. 2 is a scanning electron microscope (SEM) image of the composite powder as prepared in Example 1.

[0036] FIG. 3 is a graph showing a hysteresis loop of the composite powder as prepared in Example 1.

[0037] FIG. 4 is a graph showing the variation of the complex permeability and complex permittivity of the composite powder/paraffin coating as prepared in Example 1 with a thickness of 2 mm within a frequency range of 2-18 GHz.

[0038] FIG. 5 is a graph showing the variation of the RL of the composite powder/paraffin coating as prepared in Example 1 with different thicknesses within a frequency range of 2-18 GHz.

[0039] FIG. 6A is a transmission electron microscopy (TEM) image of the composite powder as prepared in Example 1.

[0040] FIG. 6B is a higher resolution transmission electron microscopy (TEM) image of the composite powder as prepared in Example 1.

[0041] FIG. 6C is a corresponding selected area electron diffraction (SAED) pattern of FIG. 6A.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENT(S)

[0042] In order to explain the embodiments of the present disclosure or the technical solutions in the prior art more clearly, drawings that are needed in the descriptions of embodiments or the prior art are briefly described below. Obviously, the drawings described below are some embodiments of the present disclosure. For those of ordinary skill in the art, other drawings could be obtained on the basis of these drawings without creative labor.

[0043] It should be noted that the embodiments of the present disclosure and the features in the embodiments could be combined with each other if there is no conflict. Hereinafter, the technical solutions of the present disclosure will be described in detail in conjunction with the drawings and the examples.

[0044] To make the object, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be described clearly and completely in conjunction with the accompanying drawings in the embodiments of the present disclosure. Obviously, the described examples are only part of the examples of the present disclosure, rather than all the examples. The following description of at least one exemplary example is actually only illustrative, and in no way serves as any limitation to the present disclosure and its application or use. On the basis of the examples of the present disclosure, all other examples obtained by those of ordinary skill in the art without creative work shall fall within the scope of the present disclosure.

[0045] The following non-limiting examples may enable those of ordinary skill in the art to more fully understand technical solutions of the present disclosure, but do not limit the present disclosure in any way.

[0046] Unless otherwise specified, the test methods described in the following examples were conventional methods. Unless otherwise specified, the reagents and materials were commercially available.

[0047] The term “optimum matching thickness (dm)” used therein refers to a thickness of which the composite powder/paraffin composite coating sample could exhibit the lowest RL among the thicknesses.

[0048] The term “optimum matching frequency (fm)” used therein refers to a frequency at which the composite powder/paraffin composite coating sample could exhibit the lowest RL.

[0049] The present disclosure provides a core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon, which comprises a core of an Fe-based nanocrystalline alloy, and a shell of an amorphous carbon layer, the shell accounting for 5-25 wt % of the core-shell structured composite powder electromagnetic wave absorber, wherein

[0050] the core-shell structured composite powder electromagnetic wave absorber has a spherical-like core-shell structure, and a particle size of 3-10 μm;

[0051] the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Si.sub.aB.sub.b, where a and b represent an atomic percentage content of a corresponding element respectively, and meet requirements of

3≤a≤15,

3≤b≤15, and

[0052] a balance being an atomic percentage content of Fe;

[0053] the Fe-based nanocrystalline alloy has an amorphous/α-Fe dual-phase structure, wherein the α-Fe has a grain size of 10-30 nm; and

[0054] the amorphous carbon layer has an average thickness of 0.3-1 μm.

[0055] In some embodiments, the Fe-based nanocrystalline alloy has a composition formula of Fe.sub.bal.Co.sub.xNi.sub.ySi.sub.aB.sub.bC.sub.cCu.sub.dTM.sub.e,

[0056] where TM represents at least one selected from the group consisting of Nb, Mo, Cr, and Mn; x, y, a, b, c, d, and e represent an atomic percentage content of a corresponding element respectively, and meet requirements of

0≤x≤15,

0≤y≤15,

0≤x+y≤20,

0≤a≤15,

0≤b≤15,

0≤c≤15,

6≤a+b+c≤30,

0≤d≤2,

0≤e≤4, and

[0057] a balance being an atomic percentage content of Fe.

[0058] In some embodiments, an electromagnetic wave absorber coating is formed from a mixture of the above-mentioned core-shell structured composite powder electromagnetic wave absorber and a wave-transparent matrix in a mass ratio of 3:2, and under a condition that the electromagnetic wave absorber coating has a thickness of 1.5-2.5 mm, the electromagnetic wave absorber coating exhibits a Δf.sub.RL<−10 dB of 8-18 GHz, and a minimum reflection loss of −54 dB.

[0059] The present disclosure also provides a method for preparing the above-mentioned core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon, comprising

[0060] step 1, preparing a Fe-based nanocrystalline alloy powder by

[0061] a. providing raw materials according to a nominal composition formula of the Fe-based nanocrystalline alloy, each of the raw materials having a purity of not less than 99 wt %;

[0062] b. mixing the raw materials, and melting a resulting mixed material in an induction melting furnace or a non-consumable-electrode arc furnace in an argon atmosphere, to obtain a chemically uniform master alloy ingot;

[0063] c. crushing the master alloy ingot and screening, to obtain an alloy powder with a particle size of less than 300 μm; and

[0064] d. placing the alloy powder in a stainless steel ball mill tank in a ball-to-powder mass ratio of 20:1; vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing the sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 50-85 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the Fe-based nanocrystalline alloy powder with a particle size of 2-8 μm;

[0065] step 2, using a commercial carbon powder or preparing a carbon powder by steps of

[0066] a. mechanically crushing graphite and screening, to obtain a graphite powder with a particle size of less than 300 μm; and

[0067] b. placing the graphite powder in a stainless steel ball mill tank in a ball-to-powder mass ratio of 20:1, vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing the sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 30 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the carbon powder with a particle size of 1-3 μm; and

[0068] step 3, preparing a core-shell structured composite powder electromagnetic wave absorber by

[0069] mixing the Fe-based nanocrystalline alloy powder obtained in step 1 and the carbon powder obtained in step 2 in a preset ratio, and placing a resulting mixture in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1 or 30:1, vacuumizing the stainless steel ball mill tank and charging with argon gas, sealing the stainless steel ball mill tank and placing the sealed stainless steel ball mill tank in a planetary ball mill, and ball milling for 6-10 h, at a rotation speed of 200 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling; cooling for 0.5 h and taking out, to obtain the core-shell structured composite powder electromagnetic wave absorber with a particle size of 3-10 μm.

EXAMPLE 1

Fe-Based Nanocrystalline Alloy of Fe.SUB.90.Si.SUB.7.B.SUB.3 .as the Core of the Composite Powder

[0070] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0071] Step 1, Preparing a Fe-Based Nanocrystalline Alloy Powder

[0072] a. raw materials of Fe, Si, and B (each with a purity of not less than 99 wt %) were weighed according to the nominal composition of Fe.sub.90Si.sub.7B.sub.3, and mixed;

[0073] b. the resulting mixed material was smelted repeatedly for four times in a non-consumable-electrode arc furnace in an argon atmosphere, obtaining a chemically uniform master alloy ingot;

[0074] c. the master alloy ingot was mechanically crushed and screened, obtaining an alloy powder with a particle size of less than 300 μm; and

[0075] d. the alloy powder was placed in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1. The stainless steel ball milling tank was then vacuumized, charged with argon gas as a protective gas, and sealed. The sealed stainless steel ball milling tank was placed in a planetary ball mill for ball milling. The ball milling was performed for 85 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling. After a cooling of 0.5 h, a product was taken out, obtaining the Fe-based nanocrystalline alloy powder with a particle size of 2.8 μm.

[0076] Step 2, Preparing a Carbon Powder

[0077] a. the commercial graphite was mechanically crushed and screened, obtaining a graphite powder with a particle size of less than 300 μm; and

[0078] b. the graphite powder was placed in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1. The stainless steel ball milling tank was then vacuumized, charged with argon gas as a protective gas, and sealed. The sealed stainless steel ball milling tank was placed in a planetary ball mill for ball milling. The ball milling was performed for 30 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling. After a cooling of 0.5 h, a product was taken out, obtaining the carbon powder with a particle size of 1-3 μm.

[0079] Step 3, Preparing a Composite Powder Formed by Coating Fe-Based Nanocrystalline Alloy with Carbon

[0080] the Fe-based nanocrystalline alloy powder obtained in step 1 and the carbon powder obtained in step 2 were mixed in a weight ratio of 92:8, and placed in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1. The stainless steel ball milling tank was vacuumized, charged with argon gas as a protection gas, and sealed. The sealed stainless steel ball milling tank was placed in a planetary ball mill for ball milling. The balling milling was performed for 10 h, at a rotation speed of 200 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling. After a cooling of 0.5 h, a product was taken out, obtaining the composite powder with a particle size of 3-10 μm.

[0081] Step 4, Structure Characterization, Morphology Observation and Performance Test of the Composite Powder

[0082] Microstructure of the composite powder was characterized by XRD. As shown in FIG. 1 and FIGS. 6A-6C, the composite powder has a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell, and the Fe-based nanocrystalline alloy has an average grain size of about 7 nm. The morphology of the composite powder was observed by SEM. As shown in FIG. 2, the composite powder has an irregular spherical morphology, and has an average particle size of 3.4 μm. The magnetic properties of the composite powder were measured by a vibrating sample magnetometer (VSM). FIG. 3 shows the magnetic properties of the composite powder. The composite powder exhibits typical soft magnetic properties, and a saturation magnetization (Ms) of 157.6 emu/g. The composite powder and paraffin were mixed to be uniform in a weight ratio of 3:2, and then pressed into a ring composite coating sample with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm. The complex permeability μ=μ′−jμ″ and complex permittivity ε=ε′−jε″ of the ring composite coating sample within a frequency of 2-18 GHz were measured by using vector network analyzer. As shown in FIG. 4, the permittivity of the composite coating sample prepared from the composite powder increases significantly. The RL curve of the composite coating sample prepared from the composite powder electromagnetic wave absorber was calculated according to the transmission line principle in combination with measured electromagnetic parameters, to evaluate electromagnetic wave absorbing performance of the composite coating sample. The simulated curve of RL with different thicknesses as a function of frequency is shown in FIG. 5. As shown in FIG. 5, the composite powder/paraffin composite coating sample with an optimum matching thickness (d.sub.m) of 1.9 mm exhibits great electromagnetic wave absorbing ability within a frequency of 10.0-16.5 GHz, a RL.sub.min of −54.8 dB at an optimum matching frequency (f.sub.m) of 12.7 GHz, and Δf.sub.RL<−10 dB of 6.5 GHz. In addition, when the thickness was 1.7 mm, the Δf.sub.RL<−10 dB is 7.3 GHz, covering most of the X-band (8-12 GHz) and the entire Ku-band (12-18 GHz).

EXAMPLE 2

Fe-Based Nanocrystalline Alloy of Fe.SUB.90.Si.SUB.7.B.SUB.3 .as the Core of the Composite Powder

[0083] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0084] Steps 1 to 4 were the same as described in Example 1, except that in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 85:15.

[0085] The composite powder had a spherical-like morphology, and an average particle size of 3.7 μm. The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell, and exhibits a M.sub.s of 145.7 emu/g, with a typical soft magnetic properties. The composite powder/paraffin composite coating sample with an optimum matching thickness of 1.6 mm exhibited a RL.sub.min of −23.2 dB at a frequency of 17.5 GHz, and a Δf.sub.RL<−10 dB of 4.2 GHz.

EXAMPLE 3

Fe-Based Nanocrystalline Alloy of Fe.SUB.87.Si.SUB.3.B.SUB.10 .as the Core of the Composite Powder

[0086] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0087] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.81Si.sub.3B.sub.10, and the ball milling was performed for 70 h; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 95:5. The Fe-based nanocrystalline alloy powder had an average particle size of 7.4 μm

[0088] The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder had a spherical-like morphology, and an average particle size of 8 μm. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 163.4 emu/g. The permittivity of the composite powder/paraffin composite coating sample increased, and the permeability decreased slightly. The composite coating sample with an optimum matching thickness of 2.3 mm exhibited a RL.sub.min of −17.5 dB, and a Δf.sub.RL<−10 dB of 5.1 GHz.

EXAMPLE 4

Fe-Based Nanocrystalline Alloy of Fe.SUB.82.Si.SUB.15.B.SUB.3 .as the Core of the Composite Powder

[0089] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0090] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.82Si.sub.15B.sub.3, and the ball milling was performed for 50 h; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 75:25, and the ball-to-powder mass ratio was adjusted to 30:1. The alloy particles (as the core) had an average particle size of 8 μm.

[0091] The composite powder had an irregular spherical morphology, and an average particle size of 10 μm. The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell, and exhibited typical soft magnetic properties and a M.sub.s of 145.3 emu/g. The composite powder/paraffin composite coating sample with a thickness of 2.5 mm exhibited great electromagnetic wave absorbing ability within a frequency of 4.0-6.0 GHz, a RL.sub.min of −29.0 dB, and an optimal reflection loss peak at a frequency of 4.9 GHz, which could be used as a low-frequency wave absorber.

EXAMPLE 5

Fe-Based Nanocrystalline Alloy of Fe.SUB.80.Si.SUB.10.B.SUB.10 .as the Core of the Composite Powder

[0092] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0093] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.80Si.sub.10B.sub.10, and the ball milling was performed for 50 h; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 95:5, and the ball milling was performed for 8 h.

[0094] The composite powder had an irregular spherical morphology. The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell, and exhibited typical soft magnetic properties, and a M.sub.s of 167.8 emu/g. The composite powder/paraffin composite coating sample with a thickness of 1.9 mm exhibited a RL.sub.min of −39.4 dB at a frequency of 6.4 GHz, and a Δf.sub.RL<−10 dB of 4.1 GHz, which could have a better application prospect in the low frequency range. When the thickness of the composite coating sample was 1.3 mm, the Δf.sub.RL<−10 dB reached 7.5 GHz.

EXAMPLE 6

Fe-Based Nanocrystalline Alloy of Fe.SUB.75.Si.SUB.12.B.SUB.13 .as the Core of the Composite Powder

[0095] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0096] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.75Si.sub.12B.sub.13; in step 3, the ball milling was performed for 8 h.

[0097] The composite powder also had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 143.9 emu/g. The composite powder was irregularly spherical shaped. The composite powder/paraffin composite coating sample with a thickness of 2.0 mm exhibited a RL.sub.min of −39.8 dB at a frequency of 6.0 GHz. When the thickness of the composite coating sample was 1.3 mm, the Δf.sub.RL<−10 dB reached 7.6 GHz.

EXAMPLE 7

Fe-Based Nanocrystalline Alloy of Fe.SUB.70.Si.SUB.15.B.SUB.15 .as the Core of the Composite Powder

[0098] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0099] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.70Si.sub.15B.sub.15; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 75:25, the mixing ball was performed for 8 h, and the ball-to-powder mass ratio was adjusted to 30:1.

[0100] The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 135.7 emu/g. The composite powder had an irregular spherical morphology. The composite powder/paraffin composite coating sample with a matching thickness of 2.2 mm exhibited a RL.sub.min of −39.9 dB at a frequency of 5.1 GHz, and a Δf.sub.RL<−10 dB of 3.1 GHz. When the thickness of the composite coating sample was 1.3 mm, the Δf.sub.RL<−10 dB reached 6.6 GHz.

EXAMPLE 8

Fe-Based Nanocrystalline Alloy of Fe.SUB.67.Ni.SUB.15.Si.SUB.3.B.SUB.15 .as the Core of the Composite Powder

[0101] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0102] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.67Ni.sub.15Si.sub.3B.sub.15, and the ball milling was performed for 50 h; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 95:5, and the ball milling was performed for 6 h.

[0103] The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 157.8 emu/g. The composite powder had irregularly spherical morphology. The composite powder/paraffin composite coating sample with a thickness of 2.1 mm exhibited a RL.sub.min of −25.7 dB at a frequency of 10.8 GHz, and a Δf.sub.RL<−10 dB of 3.0 GHz. When the thickness of the composite coating sample was 1.1 mm, the Δf.sub.RL<−10 dB reached 5.9 GHz.

EXAMPLE 9

Fe-Based Nanocrystalline Alloy of Fe.SUB.76.Co.SUB.4.Ni.SUB.2.Si.SUB.3.B.SUB.15 .as the Core of the Composite Powder

[0104] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0105] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.76Co.sub.4Ni.sub.2Si.sub.3B.sub.15, and the ball milling was performed for 50 h; in step 3, the ball milling was performed for 6 h.

[0106] The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 155.7 emu/g. The composite powder had spherical-like morphology. The composite powder/paraffin composite coating sample with a thickness of 1.5 mm exhibited a RL.sub.min of −32.1 dB at a frequency of 11.2 GHz, and a Δf.sub.RL<−10 dB of 3.5 GHz.

EXAMPLE 10

Fe-Based Nanocrystalline Alloy of Fe.SUB.71.Co.SUB.4.Ni.SUB.2.Si.SUB.15.B.SUB.3.Nb.SUB.3.C.SUB.2 .as the Core of the Composite Powder

[0107] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0108] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.71Co.sub.4Ni.sub.2Si.sub.15B.sub.3Nb.sub.3C.sub.2; in step 3, the ball milling was performed for 6 h.

[0109] The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 150.2 emu/g. The composite powder had a spherical-flaky-mixed morphology. The composite coating sample exhibited a RL.sub.min of −36.2 dB, an f.sub.m of 12.3 GHz, a d.sub.m of 1.4 mm, and a Δf.sub.RL<−10 dB of 3.1 GHz, with a reduced thickness, which was more suitable for the application of wave absorbing coatings.

EXAMPLE 11

Fe-Based Nanocrystalline Alloy of Fe.SUB.67.Co.SUB.8.Ni.SUB.2.Si.SUB.8.B.SUB.8.C.SUB.4.Cu.SUB.1.Mo.SUB.2 .as the Core of the Composite Powder

[0110] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0111] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.67Co.sub.8Ni.sub.2Si.sub.8B.sub.8C.sub.4Cu.sub.1Mo.sub.2; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 85:15, and the ball milling was performed for 6 h.

[0112] The composite powder exhibited a Ms of 148.3 emu/g. The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder had an irregular spherical morphology. The composite powder possessed a smaller matching thickness, which indicated its better wave absorbing performance. The composite coating sample with a matching thickness of only 1.2 mm exhibited a minimum RL of −20.2 dB at a frequency of 11.9 GHz, and a Δf.sub.RL<−10 dB of 2.5 GHz.

EXAMPLE 12

Fe-Based Nanocrystalline Alloy of Fe.SUB.52.Co.SUB.15.Ni.SUB.2.Si.SUB.15.B.SUB.3.C.SUB.8.Cu.SUB.2.Cr.SUB.1.Mn.SUB.2 .as the Core of the Composite Powder

[0113] The method for preparing the core-shell structured composite powder electromagnetic wave absorber formed by coating Fe-based nanocrystalline alloy with carbon was as follows:

[0114] Steps 1 to 4 were the same as described in Example 1, except that: in step 1, the composition of the Fe-based nanocrystalline alloy was adjusted to Fe.sub.52Co.sub.15Ni.sub.2Si.sub.15B.sub.3C.sub.8Cu.sub.2Cr.sub.1Mn.sub.2; in step 3, the Fe-based nanocrystalline alloy powder and the carbon powder were mixed in a weight ratio of 75:25, and the ball milling was performed for 6 h.

[0115] The composite powder had an irregular spherical morphology. The composite powder had a multiphase microstructure including nanocrystalline α-Fe/amorphous alloy core and amorphous carbon shell. The composite powder exhibited typical soft magnetic properties, and a M.sub.s of 135.8 emu/g. The composite powder/paraffin composite coating sample with a thickness of 1.0 mm, exhibited a RL.sub.min of −15.3 dB at a frequency of 15.7 GHz, and a Δf.sub.RL<−10 dB of 3.3 GHz.

Comparative Example 1

[0116] Fe.sub.90Si.sub.7B.sub.3 was used as the wave absorber.

[0117] The method for preparing the wave absorber was as follows:

[0118] Step 1: Preparing a Fe-Based Nanocrystalline Alloy Powder

[0119] a. raw materials of Fe, Si, and B (each with a purity of not less than 99 wt %) were weighed according to the nominal composition of Fe.sub.90Si.sub.7B.sub.3, and mixed;

[0120] b. the resulting mixed material was smelted repeatedly for four times in a non-consumable-electrode arc furnace in an argon atmosphere, obtaining a chemically uniform master alloy ingot;

[0121] c. the master alloy ingot was mechanically crushed and screened, obtaining an alloy powder with a particle size of less than 300 μm;

[0122] d. the alloy powder was placed in a stainless steel ball milling tank in a ball-to-powder mass ratio of 20:1. The stainless steel ball milling tank was then vacuumized, charged with argon gas as a protective gas, and sealed. The sealed stainless steel ball milling tank was placed in a planetary ball mill for ball milling. The ball milling was performed for 85 h, at a rotation speed of 350 rpm, with a shut down of 5 minutes for every 30 minutes of milling to cool, in a forward and reverse operation mode to ensure a uniform ball milling. After a cooling of 0.5 h, a product was taken out, obtaining a Fe-based nanocrystalline alloy powder with a particle size of 2.8 μm.

[0123] Step 2: Structure Characterization, Morphology Observation and Performance Test of the Fe-Based Nanocrystalline Alloy Powder

[0124] This step was the same as the step 4 in Example 1.

[0125] The Fe-based nanocrystalline alloy powder had a spherical-like morphology, a nanocrystalline α-Fe/amorphous dual-phase structure. The Fe-based nanocrystalline alloy powder exhibited typical soft magnetic properties, and a Ms of 196.3 emu/g. The alloy powder/paraffin composite sample with a thickness of 2.4 mm exhibited a RL.sub.min of −16.7 dB, a Δf.sub.RL<−10 dB of 5.2 GHz, and an optimal reflection loss peak at a frequency of 13.8 GHz. Compared with Comparative Example 1, the composite coating samples of examples of the present disclosure has a smaller thickness, a lower RL, and a larger Δf.sub.RL<−10 dB, indicating a greater electromagnetic wave absorbing ability.

Comparative Example 2

[0126] Comparative Example 2 was performed according to CHUAI. The composite sample prepared from the electromagnetic wave absorber (with a thickness of 2.0 mm) exhibited a RL.sub.min of −45.3 dB, but a Δf.sub.RL<−10 dB of only 5.4 GHz. Furthermore, the electromagnetic wave absorber had a complicated and higher-cost preparation process. That is to say, the gas atomization method combined with a wet ball milling had a longer period, and was more difficult to control. Compared with Comparative Example 2, the wave absorbers of examples of the present disclosure has the advantages of a simpler preparation process, a larger Δf.sub.RL<−10 dB, and a smaller sample thickness.

Comparative Example 3

[0127] Comparative Example 3 was performed according to the reference DUAN et al., Graphene to Tune Microwave Absorption Frequencies and Enhance Absorption Properties of Carbonyl Iron/Polyurethane Coating, Progress in Organic Coatings, Vol. 125 (2018) pages 89-98 “DUAN”. DUAN prepared a carbonyl iron/graphene/polyurethane composite wave absorbing coating by a ultrasonic mixing-rolling-curing method. When the coating had a thickness of 1 mm, the wave absorbing coating exhibited a RL.sub.min of −27.0 dB, and a Δf.sub.RL<−10 dB of 6.5 GHz. In contrast, the method for preparing the electromagnetic wave absorber according to the present disclosure is simple and effective, allows for controllable ball milling process conditions, and thus is suitable for industrial production. In terms of performance, the electromagnetic wave absorber coating according to the present disclosure exhibited greater wave absorbing ability, and a RL.sub.min of −54.7 dB, and allowed for a Δf.sub.RL<−10 dB of 7.3 GHz when the thickness was 1.7 mm Compared with Comparative Example 3, the electromagnetic wave absorbers of examples of the present disclosure had the advantages of a simpler preparation process, greater wave absorbing ability, and larger Δf.sub.RL<−10 dB.

Comparative Example 4

[0128] Comparative Example 4 was performed according to the reference XIONG et al., Carbon Coated Core-Shell FeSiCr/Fe3C Embedded in Carbon Nanosheets Network Nanocomposites for Improving Microwave Absorption Performance, Nano, Vol. 15 (2020) 2050094 “XIONG”. XIONG synthesized FeSiCr/Fe.sub.3C@C/C nanocomposite powder by an arc melting method combined with an arc discharge plasma. The composite sample prepared from the wave absorber with a thickness of 2.4 mm exhibited a RL.sub.min of −42.3 dB, and a Δf.sub.RL<−10 dB of only 3.7 GHz. In contrast, the wave absorber prepared in the present disclosure exhibited better wave absorbing performance. The composite coating sample prepared from the wave absorber exhibited a RL.sub.min of −54.7 dB, and a Δf.sub.RL<−10 dB of 6.5 GHz when the thickness of the composite coating sample was 1.9 mm, which met the comprehensive performance requirements “thinner, lighter, and broader and greater”. In terms of the process preparation, the method according to the present disclosure is effective, reliable and convenient. Compared with Comparative Example 4, the wave absorber of examples of the present disclosure has the advantages of a simpler preparation process and more excellent wave absorbing performance.

[0129] Detailed data of Examples 1-12 and Comparative Examples 1-4 are shown in Table 1 and Table 2.

TABLE-US-00001 TABLE 1 Composition of the core alloy, content of the shell carbon layer, time for ball milling and electromagnetic wave absorbing performance of the composite powder of Examples 1-12 Composition Content of Time for ball- of the core alloy carbon milling (atom %) (wt %) a + b/h M.sub.s/emu/g RL.sub.min/dB f.sub.m/GHz Δf.sub.RL<−10 dB/GHz d.sub.m/mm Example 1 Fe.sub.90Si.sub.7B.sub.3 8  85 + 10 157.6 −54.8 12.7 6.5 1.9 Example 2 Fe.sub.90Si.sub.7B.sub.3 15  85 + 10 145.7 −23.2 17.5 4.2 1.6 Example 3 Fe.sub.87Si.sub.3B.sub.10 5  70 + 10 163.4 −17.5 13.0 5.1 2.3 Example 4 Fe.sub.82Si.sub.15B.sub.3 25  50 + 10 145.3 −29.0 4.9 2.0 2.5 Example 5 Fe.sub.80Si.sub.10B.sub.10 5 50 + 8 167.8 −39.4 6.4 4.1 1.9 Example 6 Fe.sub.75Si.sub.12B.sub.13 8 85 + 8 143.9 −39.8 6.0 3.6 2.0 Example 7 Fe.sub.70Si.sub.15B.sub.15 25 85 + 8 135.7 −39.9 5.1 3.1 2.2 Example 8 Fe.sub.67Ni.sub.15Si.sub.3B.sub.15 5 50 + 6 157.8 −25.7 10.8 3.0 2.1 Example 9 Fe.sub.76Co.sub.4Ni.sub.2Si.sub.3B.sub.15 8 50 + 6 155.7 −32.1 11.2 3.5 1.5 Example 10 Fe.sub.71Co.sub.4Ni.sub.2Si.sub.15B.sub.3Nb.sub.3C.sub.2 8 85 + 6 150.2 −36.2 12.3 3.1 1.4 Example 11 Fe.sub.67Co.sub.8Ni.sub.2Si.sub.8B.sub.8C.sub.4Cu.sub.1Mo.sub.2 15 85 + 6 148.3 −20.2 11.9 2.5 1.2 Example 12 Fe.sub.52Co.sub.15Ni.sub.2Si.sub.15B.sub.3C.sub.8Cu.sub.2Cr.sub.1Mn.sub.2 25 85 + 6 135.8 −15.3 15.7 3.3 1.0

[0130] In Table 1, time for ball milling a+b: a represents time for ball milling during the preparation of the alloy powder, and b represents time for ball milling after mixing; M.sub.s represents saturation magnetization; RL.sub.min represents the minimum reflection loss; f.sub.m represents optimum matching frequency; Δf.sub.RL<−10 dB represents effective absorption bandwidth; and d.sub.m represents optimum matching thickness.

TABLE-US-00002 TABLE 2 Electromagnetic wave absorbing performance of the composite sample prepared from the powder of Comparative Examples 1 to 4 Composition of the absorber Preparation Items (atom %) M.sub.s/emu/g RL.sub.min/dB f.sub.m/GHz Δf.sub.RL<−10 dB/GHz d.sub.m/mm process Comparative Fe.sub.90Si.sub.7B.sub.3 196.3 −16. 7 13.8 5.2 2.4 Ball milling Example 1 Comparative Fe.sub.0.2P.sub.0.05C.sub.0.45B.sub.0.3/graphene 148.1 −45.3 12.6 5.4 2.0 Gas atomization combined Example 2 with ball milling Comparative CIP/graphene/polyurethane — −27.0 12.9 6.5 1.0 Ultrasonic mixing, Example 3 rolling, and curing Comparative Fe.sub.83.36Si.sub.14.55Cr.sub.2.09/Fe.sub.3C@C 123.7 −42.3 11.5 3.7 2.4 Plasma arc method Example 4

[0131] In Table 2, M.sub.s represents saturation magnetization; RL.sub.min represents minimum reflection loss; f.sub.m represents optimum matching frequency; Δf.sub.RL<−10 dB represents effective absorption bandwidth; and d.sub.m represents optimum matching thickness.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present disclosure, not to limit them. Although the technical solutions of present disclosure has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that the technical solutions recited in the foregoing embodiments could still be modified, or some or all of the technical features could be replaced with equivalents; these modifications or replacements shall not render the corresponding technical solutions out of the scope of technical solutions of the present disclosure.