A lightweight iron-nickel alloy-based magnetic composite wave-absorbing material and its preparation method

Abstract

The present invention relates to a lightweight iron-nickel alloy-based magnetic composite absorbing material and a preparation method, belonging to the technical field of microwave absorbing materials. The present invention uses a spherical nickel-ferrite complex as a precursor and utilizes a polyvinyl pyrrolidone-derived carbothermal reduction method to prepare the lightweight iron-nickel alloy-based magnetic composite absorbing material. By adjusting the polyvinyl pyrrolidone content, the degree of carbothermal reduction can be controlled, thereby affecting the composition, structure, and electromagnetic properties of the product. Compositionally, a higher degree of carbothermal reduction leads to an increase in the iron-nickel and iron content. Iron-nickel and iron have excellent electrical and magnetic conductivity, which facilitates a lower fill factor, while carbon materials help improve impedance matching characteristics. Structurally, the multiphase interface structure induces significant dielectric relaxation, which facilitates enhanced dielectric loss. The composite absorbing material of the present invention has an effective absorption bandwidth of up to 3.48 GHz at a fill factor of 50 wt% and a thickness of 1.7 mm. The process of the present invention does not involve highly toxic chemicals, requires inexpensive equipment, and consumes low energy.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave absorbing materials, and in particular relates to a lightweight iron-nickel alloy-based magnetic composite microwave absorbing material and a preparation method thereof. Background Art

[0002] Absorbing materials can efficiently absorb incident microwave energy through a variety of loss mechanisms, thereby weakening or completely eliminating reflected echoes, and achieving electromagnetic stealth of internal targets. According to the electromagnetic absorption model of a single-layer absorber, magnetic loss-type metal-based absorbing materials have been widely used because of their high complex dielectric constant and complex magnetic permeability, which are more conducive to the absorption of broadband microwaves at low matching thickness. Common magnetic loss-type metal-based absorbing materials include carbonyl iron, iron silicon aluminum, iron nickel, iron cobalt alloy, etc., but the filling degree of the above materials is generally higher than 70 wt%. For example, in the oriented flaky carbonyl iron powder / epoxy resin absorbing material developed by Min et al., the filling degree of carbonyl iron powder is 75 wt% ( Journal of Materials Science , 2017,52). The filling degree of FeSiAl@Al2O3@SiO2 core-shell composite absorber developed by Guo et al. is 80 wt% ( Chemical Engineering Journal , 2020, 384). Cheng et al. designed and synthesized FeCo alloy absorbers with varying Fe / Co molar ratios, achieving a filler density of 70 wt% (Journal of Alloys and Compounds, 2017, 704). A higher filler density means a higher usage volume, which translates to a higher cost. Furthermore, the production process for traditional magnetic absorbers is costly. For example, the production of carbonyl iron powder often involves high temperatures and high pressures, and also involves the use of highly toxic chemicals. Commonly used atomization powder production equipment is expensive, consumes a lot of energy, and struggles to produce specialty magnetic metal powders, including nanopowders. Mechanical crushing and ball milling also face challenges such as high energy consumption, high noise levels, and difficulty producing specialty magnetic metal powders. Therefore, finding a low-cost production process to produce high-efficiency, magnetically lossy metal-based absorbers that can operate at lower fillers has become an urgent issue. Summary of the Invention

[0003] In order to reduce the use cost of magnetic loss type metal-based absorbing materials, the present invention provides a lightweight iron-nickel alloy-based magnetic composite absorbing material and also provides a preparation method of the lightweight iron-nickel alloy-based magnetic composite absorbing material.

[0004] A lightweight iron-nickel alloy-based magnetic composite absorbing material is a gray-black magnetic powder with uniformly distributed iron, nickel, carbon, and oxygen elements. The mass fraction of iron is 20%-70%, and the mass fraction of nickel is 5%-20%. The powder particles are nearly spherical irregular polyhedrons with a certain degree of adhesion. The particle size is between 100-700 nm.

[0005] The lightweight iron-nickel alloy-based magnetic composite absorbing material has a maximum effective absorption bandwidth of 3.48 GHz at a filling degree of 50 wt% and a thickness of 1.7 mm; within a thickness range of 1-5 mm, the effective absorption frequency range is 3.4-18 GHz.

[0006] The preparation steps of a lightweight iron-nickel alloy-based magnetic composite absorbing material are as follows:

[0007] (1) Preparation of nickel ferrite composite

[0008] Dissolve 0.2737 g of nickel acetate tetrahydrate in 30 mL of ethylene glycol to obtain a nickel salt solution; dissolve 0.5947 g of ferric chloride hexahydrate in 30 mL of ethylene glycol to obtain an iron salt solution;

[0009] Pour the nickel salt solution into the iron salt solution, add 0.5 g of ammonium acetate, and continue stirring for 45 min to obtain a mixed solution;

[0010] The mixed solution was transferred into a reactor, and the reactor was placed in an oven, and the reaction was carried out at a constant temperature of 180°C for 30 hours, and then naturally cooled to room temperature; centrifuged, and thoroughly washed with deionized water and anhydrous ethanol, dried and ground to obtain a nickel ferrite precursor powder; in a muffle furnace, the temperature was increased to 350°C at a rate of 2°C / min and kept warm for 1 hour; the temperature was further increased to 500°C at a rate of 5°C / min, kept warm for 1 hour, and naturally cooled to room temperature, washed and dried to obtain a nickel ferrite composite; the nickel ferrite composite is a magnetic orange-red powder composed of nickel ferrite, nickel, and ferric oxide phases, wherein the mass fraction of iron is 59-60%, the mass fraction of nickel is 10-11%, the morphology is regular spherical, and the particle size is between 100-600 nm;

[0011] (2) Preparation of composite absorbing materials

[0012] 1 g of nickel-ferrite composite and 0.3-0.6 g of polyvinyl pyrrolidone were added to 25 mL of anhydrous ethanol and stirred for more than 1 hour to obtain a mixed solution. The mixed solution was vacuum-dried at 40°C, taken out and ground to obtain a composite precursor powder. The composite precursor powder was placed in a tube furnace and heated to 650°C at a rate of 5°C / min in a nitrogen atmosphere. The temperature was kept at this temperature for 2 hours, naturally cooled, and ground to obtain a lightweight iron-nickel alloy-based magnetic composite absorbing material.

[0013] The beneficial technical effects of the present invention are embodied in the following aspects:

[0014] 1. The present invention prepares a nickel-ferrite composite by first solvent thermal treatment followed by air heat treatment, and then prepares a lightweight iron-nickel alloy-based magnetic composite absorbing material by first compounding with polyvinyl pyrrolidone and then carbon thermal reduction in an inert atmosphere. By adjusting the amount of polyvinyl pyrrolidone compounded, the degree of carbon thermal reduction of the nickel-ferrite composite can be regulated. The degree of reduction affects the composition and structure of the product, thereby determining the electromagnetic properties. From a compositional perspective, a higher degree of reduction results in a product primarily composed of iron-nickel, iron, and carbon, while a lower degree of reduction results in a product composed primarily of iron-nickel, iron, carbon, and ferroferric oxide. Iron-nickel alloys and iron have excellent electrical and magnetic conductivity, which facilitates reducing the fill factor while ensuring high complex dielectric constant and complex magnetic permeability, while carbon materials with controllable resistivity contribute to improved impedance matching characteristics. From a structural perspective, the presence of multiphase interfaces and crystal defects induces significant dielectric relaxation, which contributes to enhanced dielectric loss, while smaller particle size also effectively suppresses eddy current effects. Therefore, the maximum effective absorption bandwidth of the lightweight iron-nickel alloy-based magnetic composite absorbing material is 3.48 GHz at a filling degree of 50 wt% and a thickness of 1.7 mm, and the effective absorption frequency range is 3.4-18 GHz within a thickness range of 1-5 mm.

[0015] 2. The process of the present invention does not involve highly toxic chemicals or ultra-high temperature and high pressure processes. The required equipment is inexpensive and the energy consumption is relatively low. In addition, the product has a certain adjustment space for electromagnetic properties and has high application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is the XRD spectrum of the prepared nickel ferrite composite.

[0017] Figure 2 is the SEM image of the prepared nickel ferrite composite.

[0018] Figure 3 This is the XRD spectrum of the iron-nickel alloy-based absorbing material CR-0.3 prepared in Example 1.

[0019] Figure 4 This is a SEM photograph of the iron-nickel alloy-based absorbing material CR-0.3 prepared in Example 1.

[0020] Figure 5 This is the electromagnetic parameter spectrum of the iron-nickel alloy-based absorbing material CR-0.3 prepared in Example 1.

[0021] Figure 6 This is the XRD spectrum of the iron-nickel alloy-based absorbing material CR-0.5 prepared in Example 2.

[0022] Figure 7 This is a SEM photograph of the iron-nickel alloy-based absorbing material CR-0.5 prepared in Example 2.

[0023] Figure 8 This is the electromagnetic parameter spectrum of the iron-nickel alloy-based absorbing material CR-0.5 prepared in Example 2.

[0024] Figure 9 This is a reflection loss curve of the iron-nickel alloy-based absorbing material CR-0.5 prepared in Example 2.

[0025] Figure 10 This is the XRD spectrum of the iron-nickel alloy-based absorbing material CR-0.6 prepared in Example 3.

[0026] Figure 11 This is a SEM photograph of the iron-nickel alloy-based absorbing material CR-0.6 prepared in Example 3.

[0027] Figure 12 This is the electromagnetic parameter spectrum of the iron-nickel alloy-based absorbing material CR-0.6 prepared in Example 3. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below with reference to the accompanying drawings.

[0029] Example 1

[0030] The preparation steps of a lightweight iron-nickel alloy-based magnetic composite absorbing material are as follows:

[0031] (1) Preparation of nickel ferrite composite

[0032] Dissolve 0.2737 g of nickel acetate tetrahydrate in 30 mL of ethylene glycol to obtain a nickel salt solution; dissolve 0.5947 g of ferric chloride hexahydrate in 30 mL of ethylene glycol to obtain an iron salt solution;

[0033] Pour the nickel salt solution into the iron salt solution, add 0.5 g of ammonium acetate, and continue stirring for 45 min to obtain a mixed solution;

[0034] The mixed solution was transferred into a reactor, and the reactor was placed in an oven, and the reaction was carried out at a constant temperature of 180°C for 30 hours, and then naturally cooled to room temperature; centrifuged, and thoroughly washed with deionized water and anhydrous ethanol, dried, and ground to obtain a nickel ferrite precursor powder; in a muffle furnace, the temperature was increased to 350°C at a rate of 2°C / min and kept warm for 1 hour; the temperature was further increased to 500°C at a rate of 5°C / min, kept warm for 1 hour, and naturally cooled to room temperature, washed, and dried to obtain a nickel ferrite composite; the nickel ferrite composite is a magnetic orange-red powder composed of nickel ferrite, nickel, and ferric oxide phases, wherein the mass fraction of iron is 59%, the mass fraction of nickel is 11%, the morphology is regular spherical, and the particle size is between 100-600 nm;

[0035] (2) Preparation of composite absorbing materials

[0036] 1 g of nickel-ferrite composite and 0.3 g of polyvinyl pyrrolidone were added to 25 mL of anhydrous ethanol and stirred for more than 1 h to obtain a mixed solution. The mixture was vacuum-dried at 40°C, taken out and ground to obtain a composite precursor powder. The composite precursor powder was placed in a tube furnace and heated to 650°C at a rate of 5°C / min in a nitrogen atmosphere. The temperature was kept at this temperature for 2 h, naturally cooled, and ground to obtain a lightweight iron-nickel alloy-based magnetic composite absorbing material. The composite absorbing material is a gray-black magnetic powder with evenly distributed iron, nickel, carbon, and oxygen elements. The mass fraction of iron is 23.18% and the mass fraction of nickel is 8.13%. The powder particles are nearly spherical irregular polyhedrons with a certain degree of adhesion. The particle size is 100-700 nm. The lightweight iron-nickel alloy-based magnetic composite absorbing material has an effective absorption bandwidth of 1.68 GHz at a filling degree of 50 wt% and a thickness of 1.7 mm. Within the thickness range of 1-5 mm, the effective absorption frequency ranges are 4.96-8.84 GHz, 11.64-12.96 GHz, and 13.76-18 GHz.

[0037] See also Figure 1 , XRD spectrum of the composite absorbing material prepared in Example 1. Figure 1 Characteristic diffraction peaks attributable to nickel ferrite, ferric oxide, and nickel can be seen. Residual organic matter on the powder surface after the solvothermal reaction may act as a carbothermal reducing agent for the primary nickel ferrite phase, leading to the formation of nickel and ferric oxide phases after heat treatment. EDS data indicate that the mass fraction of iron in the nickel ferrite composite is 59% and the mass fraction of nickel is 11%.

[0038] See also Figure 2 , SEM photo of the composite absorbing material prepared in Example 1. Figure 2It can be seen that the prepared nickel ferrite composite particles are regular spherical, with a relatively concentrated particle size distribution and a particle size between 100-600 nm.

[0039] See also Figure 3 The XRD spectrum of the composite absorbing material CR-0.3 prepared in Example 1 is as follows: Figure 3 Multiple distinct diffraction peaks can be seen, coinciding well with the characteristic peaks of iron-nickel, iron, and ferroferric oxide, with no other impurities. The differing reduction difficulties of iron and nickel ions, as well as the distribution of the carbothermal reducing agent, lead to the appearance of different phases in the carbothermal reduction product. EDS results also indicate that the mass fraction of iron in CR-0.3 is 23.18%, and the mass fraction of nickel is approximately 8.13%. Furthermore, the iron, nickel, carbon, and oxygen elements are evenly distributed. Overall, CR-0.3 is a complex of iron-nickel, iron, carbon, and ferroferric oxide.

[0040] See also Figure 4 , SEM photo of the composite absorbing material CR-0.3 prepared in Example 1. Figure 4 It can be seen that after carbon thermal reduction, the original regular spherical structure has been transformed into an irregular ellipsoid, and there is a more obvious adhesion growth phenomenon, and the particle size is about 100-700 nm.

[0041] See also Figure 5 The iron-nickel alloy-based magnetic composite absorbing material CR-0.3 prepared in Example 1 was melted and mixed with 50 wt% paraffin wax to form a ring sample with an outer diameter of 7 mm and an inner diameter of 3.04 mm. The complex dielectric constant and complex permeability spectra of the sample in the range of 2-18 GHz were obtained using the standard coaxial line method. Figure 5From the perspective of the complex permittivity of the CR-0.3 sample, the complex permittivity changes steadily from 8.46 at 2 GHz to 8.15 at 12.76 GHz, then rapidly increases to 9.26 at 14.08 GHz, before rapidly decreasing to 5.95 at 18 GHz. The corresponding imaginary part of the complex permittivity changes from 0.8 at 2 GHz to 1.6 at 18 GHz. The lower imaginary part indicates that the CR-0.3 sample has poor electrical conductivity at this frequency, which may be related to the presence of Fe3O4 and amorphous carbon materials with high resistivity. Furthermore, a more pronounced dielectric relaxation phenomenon occurs in the 12-18 GHz range, possibly related to the presence of a multiphase interface structure, which promotes enhanced dielectric loss in this frequency band. The real part of the complex magnetic permeability of the CR-0.3 sample varies within a narrow range, from 1.17 at 2 GHz to 1.16 at 18 GHz, while the imaginary part fluctuates only around 0.1. In general, the lower polyvinylpyrrolidone content leads to a lower degree of reduction of CR-0.3, that is, it leads to the formation of insufficiently reduced product ferrosoferric oxide, which in turn results in a lower complex dielectric constant and complex magnetic permeability, and thus poor wave absorption properties.

[0042] Based on the electromagnetic parameters of the ring-shaped sample obtained in Example 1, a single-layer uniform absorbing coating model was used to simulate and calculate its reflection loss characteristics. The lightweight iron-nickel alloy-based magnetic composite absorbing material exhibited an effective absorption bandwidth of 1.68 GHz at a 50 wt% filler and a thickness of 1.7 mm. Within a thickness range of 1-5 mm, the effective absorption frequency ranges were 4.96-8.84 GHz, 11.64-12.96 GHz, and 13.76-18 GHz.

[0043] Example 2

[0044] The preparation steps of a lightweight iron-nickel alloy-based magnetic composite absorbing material are as follows:

[0045] (1) Preparation of nickel ferrite composite

[0046] Dissolve 0.2737 g of nickel acetate tetrahydrate in 30 mL of ethylene glycol to obtain a nickel salt solution; dissolve 0.5947 g of ferric chloride hexahydrate in 30 mL of ethylene glycol to obtain an iron salt solution;

[0047] Pour the nickel salt solution into the iron salt solution, add 0.5 g of ammonium acetate, and continue stirring for 45 min to obtain a mixed solution;

[0048] The mixed solution was transferred into a reactor, and the reactor was placed in an oven, and the reaction was carried out at a constant temperature of 180°C for 30 hours, and then naturally cooled to room temperature; centrifuged, and thoroughly washed with deionized water and anhydrous ethanol, dried, and ground to obtain a nickel ferrite precursor powder; in a muffle furnace, the temperature was increased to 350°C at a rate of 2°C / min and kept warm for 1 hour; the temperature was further increased to 500°C at a rate of 5°C / min, kept warm for 1 hour, and naturally cooled to room temperature, washed, and dried to obtain a nickel ferrite composite; the nickel ferrite composite was a magnetic orange-red powder composed of nickel ferrite, nickel, and ferric oxide phases, wherein the mass fraction of iron element was 59%, the mass fraction of nickel element was 11%, the morphology was regular spherical, and the particle size was between 100-600 nm;

[0049] (2) Preparation of composite absorbing materials

[0050] 1 g of nickel-ferrite composite and 0.5 g of polyvinyl pyrrolidone were added to 25 mL of anhydrous ethanol and stirred for more than 1 h to obtain a mixed solution. The mixture was vacuum-dried at 40°C, taken out and ground to obtain a composite precursor powder. The composite precursor powder was placed in a tube furnace and heated to 650°C at a rate of 5°C / min in a nitrogen atmosphere. The temperature was kept at this temperature for 2 h, naturally cooled, and ground to obtain a lightweight iron-nickel alloy-based magnetic composite absorbing material. The composite absorbing material is a gray-black magnetic powder with evenly distributed iron, nickel, and carbon elements. The mass fraction of iron is 66.43% and the mass fraction of nickel is 14.54%. The powder particles are nearly spherical irregular polyhedrons with a certain degree of adhesion. The particle size is approximately between 200 and 700 nm. The lightweight iron-nickel alloy-based magnetic composite absorbing material has an effective absorption bandwidth of 3.48 GHz at a filling degree of 50 wt% and a thickness of 1.7 mm. Within the thickness range of 1-5 mm, the effective absorption frequency range is 3.4-18 GHz.

[0051] See also Figure 6 , XRD spectrum of the iron-nickel alloy-based magnetic composite absorbing material CR-0.5 prepared in Example 2. Figure 6 Six distinct diffraction peaks can be seen, corresponding to the characteristic diffraction peaks of iron, nickel, and iron. It can be inferred that the high polyvinylpyrrolidone content significantly enhances the degree of carbothermal reduction, resulting in the absence of incompletely reduced oxides within CR-0.5. Furthermore, EDS results show that the mass fraction of iron in CR-0.5 is 66.43%, and the mass fraction of nickel is 14.54%. The iron, nickel, and carbon elements are evenly distributed. Therefore, CR-0.5 is a composite of iron, nickel, iron, and carbon.

[0052] See also Figure 7SEM photograph of the iron-nickel alloy-based magnetic composite absorber CR-0.5 produced in Example 2. With increasing reduction, the adhesion growth phenomenon becomes increasingly pronounced, with numerous particles interconnected to form micron-sized agglomerates. The particle morphology has transformed into a nearly spherical polyhedron, with a particle size between 200 and 700 nm.

[0053] See also Figure 8 The iron-nickel alloy-based magnetic composite absorbing material CR-0.5 prepared in Example 2 was melt-mixed with 50 wt% paraffin wax to form a ring sample with an outer diameter of 7 mm and an inner diameter of 3.04 mm. The complex dielectric constant and complex permeability spectra of the ring sample in the range of 2-18 GHz were obtained using the standard coaxial line method. The real part of the complex dielectric constant of the sample steadily decreased from 15.88 at 2 GHz to 9.26 at 18 GHz, and the imaginary part of the complex dielectric constant fluctuated from 0.37 at 2 GHz to 6.26 at 18 GHz. Compared with CR-0.3, the increase in the degree of reduction brought about a significant increase in conductivity, that is, the imaginary part of the complex dielectric constant increased significantly. In addition, dielectric relaxation still exists in the range of 7-11 GHz, which contributes to the enhancement of dielectric loss in this frequency range. Thanks to the increase in the degree of reduction, the content of the strong ferromagnetic phase increases accordingly, and the complex permeability also increases. The real part of the complex permeability of the CR-0.5 sample slowly decreases from 1.16 at 2 GHz to 0.76 at 18 GHz. The imaginary part of the complex permeability remains around 0.25 in the 2-10 GHz range and gradually decreases in the 10-18 GHz range. Compared to the CR-0.3 sample, the real and imaginary parts of the complex permeability of the CR-0.5 sample are improved, which not only improves impedance matching but also contributes to the enhancement of magnetic loss.

[0054] See also Figure 9 Based on the electromagnetic parameters of the ring-shaped sample obtained in Example 2, a single-layer uniform absorbing coating model was used to simulate and calculate the reflection loss curve of the iron-nickel alloy-based magnetic composite absorbing material CR-0.5. It can be seen that the CR-0.5 sample has excellent wave absorption properties. Its effective absorption frequency range at 1.7 mm is 14.36-17.84 GHz, and its effective absorption bandwidth reaches 3.48 GHz, covering 58% of the Ku band. Furthermore, within the thickness range of 1-5 mm, the effective absorption frequency range is 3.4-18 GHz. Overall, increasing the amount of polyvinyl pyrrolidone added promotes an increase in the degree of reduction, leading to an increase in the content of the conductive and magnetic phases (iron-nickel and iron), which in turn leads to an increase in the complex dielectric constant and complex magnetic permeability, improving the reflection loss characteristics.

[0055] Example 3

[0056] The preparation steps of a lightweight iron-nickel alloy-based magnetic composite absorbing material are as follows:

[0057] (1) Preparation of nickel ferrite composite

[0058] Dissolve 0.2737 g of nickel acetate tetrahydrate in 30 mL of ethylene glycol to obtain a nickel salt solution; dissolve 0.5947 g of ferric chloride hexahydrate in 30 mL of ethylene glycol to obtain an iron salt solution;

[0059] Pour the nickel salt solution into the iron salt solution, add 0.5 g of ammonium acetate, and continue stirring for 45 min to obtain a mixed solution;

[0060] The mixed solution was transferred into a reactor, and the reactor was placed in an oven, and the reaction was carried out at a constant temperature of 180°C for 30 hours, and then naturally cooled to room temperature; centrifuged, and thoroughly washed with deionized water and anhydrous ethanol, dried, and ground to obtain a nickel ferrite precursor powder; in a muffle furnace, the temperature was increased to 350°C at a rate of 2°C / min and kept warm for 1 hour; the temperature was further increased to 500°C at a rate of 5°C / min, kept warm for 1 hour, and naturally cooled to room temperature, washed, and dried to obtain a nickel ferrite composite; the nickel ferrite composite is a magnetic orange-red powder composed of nickel ferrite, nickel, and ferric oxide phases, wherein the mass fraction of iron is 59%, the mass fraction of nickel is 11%, the morphology is regular spherical, and the particle size is between 100-600 nm;

[0061] (2) Preparation of composite absorbing materials

[0062] 1 g of nickel-ferrite composite and 0.6 g of polyvinyl pyrrolidone were added to 25 mL of anhydrous ethanol and stirred for more than 1 h to obtain a mixed solution. The mixture was vacuum-dried at 40°C, taken out and ground to obtain a composite precursor powder. The composite precursor powder was placed in a tube furnace and heated to 650°C at a rate of 5°C / min in a nitrogen atmosphere. The temperature was kept at this temperature for 2 h, naturally cooled, and ground to obtain a lightweight iron-nickel alloy-based magnetic composite absorbing material. The composite absorbing material is a gray-black magnetic powder with evenly distributed iron, nickel, carbon, and oxygen elements. The mass fraction of iron is 25.71% and the mass fraction of nickel is 6.18%. The powder particles are nearly spherical irregular polyhedrons with a certain degree of adhesion. The particle size is approximately between 200 and 500 nm. The lightweight iron-nickel alloy-based magnetic composite absorbing material has an effective absorption frequency range of 7.72-13.12 GHz, 14.28-15.72 GHz, and 16.32-18 GHz when the filling degree is 50 wt% and the thickness is within the range of 1-5 mm.

[0063] See also Figure 10, the XRD spectrum of the iron-nickel alloy-based magnetic composite absorbing material CR-0.6 prepared in Example 3. From the figure, we can see the characteristic diffraction peaks attributed to iron-nickel, iron and ferroferric oxide. Compared with CR-0.5, although the amount of polyvinyl pyrrolidone added is further increased, the inhomogeneity of the carbon thermal reduction reaction is also enhanced, resulting in insufficient reduction in some areas, that is, the formation of ferroferric oxide phase. According to the EDS results, the mass fraction of iron element in CR-0.6 is 25.71%, and the mass fraction of nickel element is 6.18%, and the iron, nickel, carbon and oxygen elements are evenly distributed. Compared with CR-0.5, the high content of carbon element also directly indicates the reduction in the degree of carbon thermal reduction, and indirectly reflects the inhomogeneity of carbon thermal reduction.

[0064] See also Figure 11 The following is an SEM photograph of the iron-nickel alloy-based magnetic composite absorbing material CR-0.6 prepared in Example 3. The photograph shows that the adhesion growth phenomenon has been reduced, and the particles are generally nearly spherical polyhedrons with a particle size between 200-500 nm.

[0065] See also Figure 12The iron-nickel alloy-based magnetic composite absorber CR-0.6 prepared in Example 3 was melt-mixed with 50 wt% paraffin wax to form a ring sample with an outer diameter of 7 mm and an inner diameter of 3.04 mm. The complex permittivity and complex permeability spectra of the ring sample were obtained in the 2-18 GHz range using a standard coaxial line method. The real part of the complex permittivity varied steadily from 7.05 at 2 GHz to 7.25 at 9.4 GHz, then decreased to 5.43 at 12.48 GHz, increased to 6.69 at 14.4 GHz, and finally decreased to 5.8 at 18 GHz. The imaginary part of the complex permittivity remained around 0.5 from 2 to 8 GHz, then exhibited two relaxation peaks, with peak values ​​of 1.93 and 1.76 at 10.4 GHz and 15 GHz, respectively. Compared with the CR-0.5 sample, the lower complex dielectric constant of the CR-0.6 sample is mainly due to the lower degree of reduction, that is, the appearance of the ferroferric oxide phase. In addition, the more complex dielectric relaxation process is also closely related to the uneven carbon thermal reduction process. The real part of the complex magnetic permeability slowly decreases from 1.33 at 2 GHz to 1.1 at 18 GHz, while the imaginary part of the complex magnetic permeability fluctuates around 0.2. Compared with the CR-0.5 sample, the real part of the complex magnetic permeability is slightly increased and the imaginary part is slightly decreased, indicating a reduction in magnetic loss. Based on the electromagnetic parameters of the annular sample obtained in Example 3, a single-layer uniform absorbing coating model was used to simulate and calculate the reflection loss characteristics of the iron-nickel alloy-based magnetic composite absorbing material CR-0.6. The effective absorption bandwidth of the sample is 0 GHz at 1.7 mm. In the thickness range of 1-5 mm, the effective absorption frequency range is 7.72-13.12 GHz, 14.28-15.72 GHz and 16.32-18 GHz. In general, the complex dielectric constant and complex magnetic permeability are also maintained at a low level, resulting in poor wave absorption properties.

[0066] It will be easily understood by those skilled in the art that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A lightweight iron-nickel alloy-based magnetic composite absorbing material, characterized by: The composite absorbing material is a gray-black magnetic powder in which iron, nickel, carbon, and oxygen elements are evenly distributed. The powder particles are in the form of irregular, nearly spherical polyhedrons with a certain degree of adhesion. The particle size is 100-700 nm. The mass fraction of the iron element and the mass fraction of the nickel element are divided into three cases: the mass fraction of the iron element is 23.18% and the mass fraction of the nickel element is 8.13%, the mass fraction of the iron element is 66.43% and the mass fraction of the nickel element is 14.54%, or the mass fraction of the iron element is 25.71% and the mass fraction of the nickel element is 6.18%. The lightweight iron-nickel alloy-based magnetic composite absorbing material has a maximum effective absorption bandwidth of 3.48 GHz at a filling degree of 50 wt% and a thickness of 1.7 mm; within a thickness range of 1-5 mm, the effective absorption frequency range is 3.4-18 GHz; The preparation steps of the composite absorbing material are as follows: (1) Preparation of nickel ferrite composite Dissolve 0.2737 g of nickel acetate tetrahydrate in 30 mL of ethylene glycol to obtain a nickel salt solution; dissolve 0.5947 g of ferric chloride hexahydrate in 30 mL of ethylene glycol to obtain an iron salt solution; Pour the nickel salt solution into the iron salt solution, add 0.5 g of ammonium acetate, and continue stirring for 45 min to obtain a mixed solution; The mixed solution was transferred into a reactor, which was placed in an oven and reacted at 180°C for 30 h, and then naturally cooled to room temperature. The nickel ferrite precursor powder was centrifuged, washed thoroughly with deionized water and anhydrous ethanol, dried and ground to obtain a nickel ferrite precursor powder; the temperature was raised to 350°C in a muffle furnace at a heating rate of 2°C / min and kept warm for 1 hour; the temperature was further raised to 500°C at a heating rate of 5°C / min and kept warm for 1 hour, and naturally cooled to room temperature, washed and dried to obtain a nickel ferrite composite; the nickel ferrite composite was a magnetic orange-red powder composed of nickel ferrite, nickel, and ferric oxide phases, wherein the mass fraction of iron was 59% and the mass fraction of nickel was 11%, the morphology was regular spherical, and the particle size was between 100-600 nm; (2) Preparation of composite absorbing materials 1 g of nickel-ferrite composite and 0.3-0.6 g of polyvinyl pyrrolidone were added to 25 mL of anhydrous ethanol and stirred for more than 1 hour to obtain a mixed solution. The mixed solution was vacuum-dried at 40°C, taken out and ground to obtain a composite precursor powder. The composite precursor powder was placed in a tube furnace and heated at a rate of 5°C / min to 650°C in a nitrogen atmosphere. The temperature was kept at this temperature for 2 hours, naturally cooled, and ground to obtain a lightweight iron-nickel alloy-based magnetic composite absorbing material.

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