A flaky porous structure wave-absorbing composite material and a preparation method thereof

Abstract

The application discloses a kind of sheet porous structure wave-absorbing composite material and preparation method thereof.The application comprises the following steps: (1) preparing Fe2O3 and pore-forming agent interpenetrating spherical particles; (2) the obtained Fe2O3 and pore-forming agent interpenetrating spherical particles are hydrogen reduced, then ball milling is carried out, to obtain sheet structure; (3) removing pore-forming agent, to obtain porous sheet structure; (4) hydrogen reduction again and carrying out multiple nitriding, to obtain composite material of sheet porous structure with Fe or Fe2N, Fe3N, Fe4N, Fe 16 Iron-based nitride such as Fe and N2, two or more kinds of structure composition.The material prepared by the application has excellent wave-absorbing performance under high frequency, and is suitable for high-frequency electromagnetic shielding and stealth material fields.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing composite materials, specifically to sheet-like porous microwave absorbing composite materials and their preparation methods. Background Technology

[0002] In recent years, with the rapid development of 5G communication, the Internet of Things, radar detection, and electronic equipment, electromagnetic radiation and interference problems have become increasingly prominent. In the military field, the stealth penetration capability of weapons and equipment also places stringent requirements on electromagnetic wave absorbing materials. The design goal of absorbing materials has been upgraded from a single high absorption rate to a comprehensive performance index of "thin, light, wide, and strong"—that is, thin thickness, low density, wide effective absorption bandwidth, and high absorption intensity. Traditional materials such as ferrites and magnetic metal powders are limited by bottlenecks such as low resistivity, low high-frequency permeability, or excessive density, and are gradually becoming unable to meet the requirements. Iron-based nitrides, especially γ'-Fe4N, have become a research hotspot for the next generation of absorbing materials due to their combination of high saturation magnetization, excellent corrosion resistance, high resistivity, and good high-frequency stability. More importantly, by controlling the microstructure (such as constructing sheet-like or porous structures) and phase composition (designing multiphase interfaces), the complex permittivity and complex permeability can be effectively adjusted without sacrificing magnetism, thereby stimulating loss mechanisms such as interfacial polarization and multiple reflections, thus breaking through the performance ceiling of traditional absorbing materials.

[0003] To improve the performance of microwave absorbing materials, the material structure is often controlled, including the construction of porous, sheet-like and multiphase interfaces to optimize the material's microwave absorption performance.

[0004] 1. Porous morphology: The introduction of a large number of micro and nano channels brings three major advantages: First, it significantly reduces the material density, which is conducive to achieving "lightweighting"; second, the air inside the pores forms a large number of solid-gas interfaces with the pore walls, and electromagnetic waves undergo multiple reflections and scatterings in the pores, which greatly increases the equivalent propagation path and improves the dissipation probability; third, by adjusting the porosity, the real part of the complex permittivity can be effectively reduced, the impedance matching can be improved, and more electromagnetic waves can enter the interior of the material rather than be reflected on the surface.

[0005] 2. Sheet-like morphology: Fabricating magnetic particles into two-dimensional sheet-like structures can overcome the Snoek limit of spherical particles, utilizing shape anisotropy to push the natural resonant frequency to higher frequencies, thus maintaining high complex permeability in the GHz range. Simultaneously, the high specific surface area of ​​the sheet-like structure enhances surface polarization, and the microcapacitor network formed between the sheets also contributes to dielectric loss, which is beneficial for broadening the effective absorption bandwidth.

[0006] 3. Multiphase Interface: Heterogeneous interfaces where different phases coexist form strong interfacial polarization. Due to the differences in conductivity and dielectric constant among the phases, charge accumulates and relaxes at the interface, resulting in significant dielectric loss. Multiphase interfaces can also induce changes in space charge polarization, dipole polarization, and magnetocrystalline anisotropy, synergistically enhancing electromagnetic loss and optimizing impedance matching, resulting in broad and strong absorption peaks.

[0007] Currently, there are no reports on microwave absorbing composite materials based on iron-nitrogen compounds that have a sheet-like porous structure and a multiphase heterogeneous interface at the microscopic level. Summary of the Invention

[0008] The purpose of this invention is to address the performance deficiencies of existing high-frequency absorbing materials by providing a microwave absorbing composite material with a sheet-like porous structure and a multiphase heterogeneous interface at the microscopic level, as well as its preparation method.

[0009] During the formation of iron-nitrogen compounds, iron will form Fe₂N, Fe₃N, Fe₄N, and Fe. 16 A series of iron-based nitrides, including N2. Among them, Fe 16 N2 is a metastable phase at low temperatures. At low temperatures, the diffusion of nitrogen atoms is limited, and long-range ordering is insufficient, resulting in a high free energy. At high temperatures, it easily transforms into the more stable Fe4N or Fe3N through phase transition. Fe4N has a face-centered cubic structure, with nitrogen atoms occupying octahedral interstitial sites in an ordered manner, forming a low and flat Gibbs free energy surface. This makes it the most thermodynamically stable phase over a wide temperature range (e.g., 300~550 ℃), and its structural stability is significantly better than other iron nitrides. Fe3N has a hexagonal close-packed structure, and its formation free energy is higher than that of Fe4N. It is a metastable phase thermodynamically. During long-term annealing or reduction nitriding, Fe3N will gradually transform into the more stable Fe4N through the ordered rearrangement of nitrogen atoms and lattice reconstruction. Fe2N, on the other hand, requires an extremely high nitrogen chemical potential environment to form. Due to its excessively high nitrogen content, the formation free energy conditions are harsh. Under prolonged reducing nitriding conditions, the system continuously relaxes towards thermodynamic steady state, nitrogen atoms diffuse sufficiently, the highly stable Fe4N phase preferentially grows and becomes the dominant phase, while metastable phases such as Fe3N gradually recede. This constructs a high-density, structurally stable multiphase heterogeneous interface with Fe4N as the matrix and coexisting with other residual phases. The stable existence of Fe4N itself is the key to maintaining the continuous enhancement of this interface polarization—it avoids interface decay caused by the decomposition of metastable phases, thereby continuously enhancing the synergistic effect of interface polarization and electromagnetic loss.

[0010] This invention, through precise control of nitriding temperature, time, and nitrogen potential, enables the in-situ construction of a multiphase core-shell structure based on iron-based nitrides on powder particles, thus creating a multiphase interface. Combined with a lamellar porous structure, the prepared microwave absorbing composite material exhibits excellent electromagnetic wave absorption performance at high frequencies. This preparation method is easy to implement and simple to operate, making it suitable for fields such as high-frequency electromagnetic shielding and stealth materials.

[0011] The specific technical solution adopted in this invention is as follows: A method for preparing a sheet-like porous microwave absorbing composite material specifically includes the following steps: (1) Prepare spherical particles with an average particle size of less than 2 micrometers in which Fe2O3 and pore-forming agent are interlocked; (2) The obtained Fe2O3 and pore-forming agent interlocked spherical particles were hydrogen-reduced and then ball-milled to obtain a plate-like structure; (3) Wash with water to remove the pore-forming agent and obtain a porous sheet-like structure; (4) Hydrogen reduction again (to prevent oxidation of particles caused by ball milling) and the first high-temperature rapid nitriding to nitrid the surface of the structure; The first high-temperature rapid nitriding process is as follows: After reduction, the surface is rapidly nitrided again in an Ar protective atmosphere in a furnace at 10℃ / min to 500~600℃ for a first time (less than 1.0h). In a high nitrogen potential mixed atmosphere with an NH3 to H2 volume ratio greater than or equal to 4:1, the active nitrogen atoms [N] from ammonia decomposition diffuse into the powder surface only, forming a dense γ′-Fe4N shell, while the core remains α-Fe, resulting in a core-shell structure Fe4N@Fe. The high nitrogen potential causes the surface of the already formed γ′-Fe4N shell to continue reacting with active nitrogen, transforming into ε-Fe3N with a higher nitrogen content, while the inner layer remains Fe4N, ultimately forming a three-layer core-shell structure Fe3N@Fe4N@Fe.

[0012] Reaction equation: 4Fe + [N] → γ′-Fe4N Reaction equation: 3γ′-Fe4N + [N] → 4ε-Fe3N (surface transformation) After the first nitriding is completed, the furnace is switched to an Ar atmosphere and cooled to room temperature in preparation for the second nitriding.

[0013] (5) A second low-temperature long-term nitriding process is carried out to fully nitrid the interior and obtain a sheet-like porous composite material. The second low-temperature, long-term nitriding process specifically involved: Under an Ar protective atmosphere, the temperature was increased at 10℃ / min to 150~250℃, and a second long-term stable nitriding was performed for 4~20h in a mixed atmosphere of NH3 and H2 with a volume ratio of 1~4:1. Due to the low temperature, the surface nitriding reaction basically did not proceed. The surface layer ε-Fe3N and the sublayer Fe4N were relatively stable at low temperature and did not decompose, while the core underwent a phase transition; the core, due to α″-Fe 16 N2 has a low formation temperature range, and metastable α″-Fe will precipitate in an ordered manner within the α-Fe lattice. 16 N2, generating Fe3N@Fe4N@Fe 16 N2@Fe multilayer core-shell structure. After nitriding, the furnace was switched to an Ar atmosphere and cooled to room temperature.

[0014] Reaction equation: 16Fe + 2[N] → α″-Fe 16 N2 (nuclear conversion) With further extension of nitridation time (or appropriate increase in temperature), metastable Fe... 16 Under a continuous nitrogen supply, N2 absorbs active nitrogen atoms and gradually transforms into the more stable γ′-Fe4N, thereby growing a new Fe4N layer inside the original Fe4N layer, ultimately forming Fe3N@Fe4N@Fe 16 N2@Fe4N@Fe double γ′ shell structure.

[0015] Reaction equation: α″-Fe 16 N2 + 2[N] → 4γ′-Fe4N (nuclear transformation) These multiphase core-shell particles, directly "tailored" by the nitriding process, retain the impedance matching advantages brought by the plate-like, porous secondary molding, while also additionally embedding abundant Fe4N@Fe, Fe3N@Fe4N, and Fe... 16 Heterogeneous interfaces such as N2@Fe4N can amplify interface polarization and multiple relaxation losses, providing a new basic structure for the design of "thin, light, wide, and strong" absorbing materials.

[0016] In a preferred embodiment, the method for preparing spherical particles in which Fe2O3 and a pore-forming agent are interlocked includes spray pyrolysis and hydrothermal methods.

[0017] In a preferred embodiment, a first high-temperature rapid nitriding is performed to nitrid the structural surface; the conditions for the first high-temperature rapid nitriding are: nitriding for 0.5-1.0 h in a high nitrogen potential mixed atmosphere with a volume ratio of NH3 to H2 of 4~7:1, and a nitriding temperature of 500~600℃. In a preferred embodiment, the method for preparing the Fe2O3 and pore-forming agent interlocked spherical particles is a spray pyrolysis method, specifically including the following steps: (1) Dissolve water-soluble iron salt and pore-forming agent in deionized water in a certain proportion, and diffuse them by ultrasonic vibration to form a uniform precursor solution with an iron salt concentration of 0.2 mol / L; (2) The precursor solution is atomized by ultrasonic spraying and then undergoes dehydration and pyrolysis oxidation reactions in a tube furnace under the influence of airflow to obtain spherical particles in which Fe2O3 and pore-forming agent are interlocked.

[0018] In a preferred embodiment, water-soluble iron salt and pore-forming agent are dissolved in deionized water at a ratio of 1:0.5~5, and a uniform precursor solution is formed by ultrasonic vibration diffusion. The precursor solution is atomized by ultrasonic spraying and, driven by air flow, is passed through a tube furnace at a flow rate of 5~10L / min to undergo dehydration reaction at 400℃ and pyrolysis oxidation reaction at 800℃ to obtain spherical particles in which Fe2O3 and pore-forming agent are interlocked. The particles obtained by spray pyrolysis were reduced with hydrogen in a tube furnace to pure Fe (Fe is easier to ball mill into flakes than Fe2O3) and spherical particles interlocked with a pore-forming agent. The reducing atmosphere was H2:Ar = 3:1 by volume, the reduction temperature was 300~500℃, the reduction time was 0.5~5h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature.

[0019] In a preferred embodiment, the fabrication process of the sheet-like structure is as follows: (1) The particles obtained by spray pyrolysis are reduced with hydrogen in a tube furnace to spherical particles in which pure Fe and pore-forming agent are interlocked; (2) Place the spherical particles, grinding beads, and grinding organic additives into a grinding jar at a ratio of 1:20 to 30:20, and grind the spherical particles into a sheet-like structure with a diameter-to-thickness ratio greater than 45 by high-energy grinding.

[0020] In a preferred embodiment, specifically: Spherical particles and grinding beads are mixed at a ratio of 1:20~30 with a ball milling organic additive and placed into a ball milling jar. The ball milling time is 3~48 hours. The spherical particles are ball milled into a sheet-like structure by high-energy ball milling. In a preferred embodiment, the process of washing away the pore-forming agent to obtain a porous sheet structure involves dissolving the ball-milled sheet powder in deionized water, dissolving the pore-forming agent in the water by magnetic stirring, centrifuging, adding deionized water again after centrifugation, repeating the process three to four times, and drying to obtain a powder with a porous sheet structure.

[0021] In a preferred embodiment, the method for further hydrogen reduction is as follows: The obtained powder was reduced again with hydrogen in a tube furnace (to prevent oxidation of the particles caused by ball milling). The reducing atmosphere was H2:Ar=3:1, the reduction temperature was 300~500℃, the reduction time was 0.5~5h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature.

[0022] In a preferred embodiment, the method further includes: taking a certain mass of powder and pouring it into a centrifuge bottle, adding 5-50% of the powder mass of paraffin wax, heating to allow the powder and paraffin wax to fully mix and become uniform, and pressing it into shape under a pressure of 100-300 MPa.

[0023] The beneficial effects of this invention are mainly reflected in: (1) The raw materials are readily available, the process is simple, the cost is low, and it is suitable for industrial production; (2) By introducing and removing pore-forming agents and adjusting their ratio, a large number of pores are introduced to reduce density, enhance multiple reflections of electromagnetic waves and improve impedance matching, thereby achieving lightweight and efficient absorption; (3) By using the sheet structure, the shape anisotropy is used to break through the Snoek limit to improve the high-frequency permeability, while the surface polarization and microcapacitance effect are used to broaden the effective absorption band.

[0024] (4) Control the nitriding process to obtain multiphase compounds, and use the difference in conductivity and dielectric between heterogeneous phases to induce strong interfacial polarization and multiple relaxation, thereby synergistically enhancing electromagnetic loss and making the absorption peak broad and strong. Attached Figure Description

[0025] Figure 1 This is a flowchart of the preparation process for sheet-like porous composite materials; Figure 2 This is the XRD characterization diagram of the porous composite material; Detailed Implementation

[0026] To better illustrate the present invention, the following embodiments are provided to further explain the content of the present invention. However, the content of the present invention is not limited to the embodiments described below, and the methods and technologies involved in the solutions should not be construed as limitations on the present invention.

[0027] Example 1: Preparation of sheet-like porous composite phase absorbing soft magnetic material A method for preparing a sheet-like porous microwave absorbing composite material, such as... Figure 1 As shown, it includes the following steps: (1) Constructing a sheet-like porous structure Water-soluble iron salt and pore-forming agent were dissolved in deionized water at a ratio of 1:4, and a uniform precursor solution with a concentration of 0.2 mol / L was formed by ultrasonic vibration diffusion. The precursor solution is atomized by ultrasonic spraying and, driven by air flow, is passed through a three-stage tube furnace at a flow rate of 5L / min to undergo dehydration reaction at 400℃ and pyrolysis oxidation reaction at 800℃ to obtain spherical particles with an average particle size of less than 2 micrometers that are interwoven with Fe2O3 and pore-forming agent. The particles obtained by spray pyrolysis were reduced with hydrogen in a tube furnace to pure Fe (Fe is easier to ball mill into flakes than Fe2O3) and spherical particles interlocked with a pore-forming agent. The reducing atmosphere was H2:Ar=3:1, the reduction temperature was 400℃, the reduction time was 4h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature. Then, the spherical particles and grinding beads were mixed in a mass ratio of 1:30 and placed into a grinding jar with a grinding organic additive. The grinding time was 48 hours. The spherical particles were then ground into a sheet-like structure with a diameter-to-thickness ratio of 50 by high-energy grinding. The ball-milled flake powder is dissolved in deionized water, and the pore-forming agent is dissolved in the water by magnetic stirring. Then, it is centrifuged, and after centrifugation, deionized water is added again and magnetic stirring is repeated three to four times. After drying, a porous flake structure powder is obtained.

[0028] (2) Preparation of composite materials The obtained powder was reduced again with hydrogen in a tube furnace (to prevent oxidation of particles caused by ball milling). The reducing atmosphere was H2:Ar=3:1, the reducing temperature was 400℃, the reducing time was 2h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to Ar atmosphere and cooled to room temperature. After reduction, the surface was rapidly nitrided again for 0.5 h at 500 °C at a rate of 10 °C / min under an Ar protective atmosphere in the furnace. In the high nitrogen potential mixed atmosphere of NH3 and H2 (volume ratio 5:1), the active nitrogen atoms [N] from ammonia decomposition diffused into the powder surface, forming a dense γ′-Fe4N shell. The surface of the formed γ′-Fe4N shell continued to react with active nitrogen, transforming into ε-Fe3N with a higher nitrogen content, while the core remained α-Fe, resulting in a core-shell structure Fe3N@Fe4N@Fe. After the first nitriding, the furnace was switched to an Ar atmosphere and cooled to room temperature in preparation for a second nitriding.

[0029] Under an Ar protective atmosphere, the temperature was increased to 180 °C at a rate of 10 °C / min, followed by a second long-term stable nitriding process in a 3:1 mixture of NH3 and H2 for 10 h. Due to the low temperature, the surface nitriding reaction was essentially inactive; the surface ε-Fe3N and the sublayer Fe4N remained relatively stable at low temperatures without decomposition, while a phase transition occurred in the core. The core, due to the α″-Fe... 16N2 has a low formation temperature range, and metastable α″-Fe will precipitate in an ordered manner within the α-Fe lattice. 16 N2, generating Fe3N@Fe4N@Fe 16 N2@Fe multilayer core-shell structure. After nitriding, the furnace was cooled to room temperature in an Ar atmosphere. XRD phase analysis of this multilayer core-shell structure was performed, and the results are as follows. Figure 2 As can be seen from the figure, Fe, Fe3N, Fe4N, and Fe are present simultaneously in the composite material. 16 N2 has four phases.

[0030] (3) Preparation of microwave absorbing materials A certain mass of powder is poured into a centrifuge bottle, and 25% of the powder mass of paraffin is added. The mixture is heated to allow the powder and paraffin to fully blend and mix evenly. The mixture is then pressed into shape under a pressure of 250 MPa.

[0031] Comparative Example 1: Preparation of porous composite phase absorbing soft magnetic materials This embodiment uses the same method as Embodiment 1, except that the ball milling step is omitted to remove the sheet-like structure and only porous powder is prepared. A porous composite phase absorbing soft magnetic material is obtained.

[0032] Comparative Example 2: Preparation of sheet-like porous single-phase absorbing soft magnetic material This embodiment uses the same method as Example 1, except that the second nitriding step is omitted to remove the composite phase and prepare only single-phase powder. A sheet-like porous single-phase absorbing soft magnetic material is obtained.

[0033] Comparative Example 3: Preparation of sheet-like composite phase microwave absorbing soft magnetic materials This embodiment uses the same method as Example 1, but omits the addition of the pore-forming agent to remove the porous structure, preparing only solid powder. A sheet-like composite phase absorbing soft magnetic material is obtained.

[0034] Table 1: Effects of porous, sheet-like, and two-step nitriding processes on the microwave absorption performance of magnetic powder. Note: Two-step nitriding means first performing rapid nitriding at 500~600 ℃, then cooling to room temperature and then heating to 150~250 ℃ for long-term stable nitriding; one-step nitriding means performing nitriding only at 500~600 ℃.

[0035] Table 1 shows the impact of porous, sheet-like, and two-step nitriding steps on the microwave absorption performance of magnetic powder. Constructing porous and sheet-like structures, along with the subsequent two-step nitriding treatment, significantly improves the absorption performance. Specifically, this manifests as an increased effective absorption bandwidth, reduced thickness, and a larger absolute value of reflection loss, meaning the material can achieve stronger absorption performance at a thinner thickness. Therefore, these three steps play a crucial role in improving the absorption performance of sheet-like porous microwave absorbing composite materials. This invention, by precisely controlling the nitriding temperature, time, and nitrogen potential, can in-situ construct a multiphase core-shell structure based on iron-based nitrides on powder particles, thus creating a multiphase interface. Combined with the sheet-like porous structure, the prepared microwave absorbing composite material exhibits excellent electromagnetic wave absorption performance at high frequencies. This preparation method is easy to implement and simple to operate, making it suitable for fields such as high-frequency electromagnetic shielding and stealth materials.

[0036] Example 2: Preparation of sheet-like porous composite phase wave-absorbing soft magnetic material A method for preparing a sheet-like porous microwave absorbing composite material, such as... Figure 1 As shown, it includes the following steps: (1) Constructing a sheet-like porous structure Water-soluble iron salt and pore-forming agent were dissolved in deionized water at a ratio of 1:0.5, and a uniform precursor solution with a concentration of 0.2 mol / L was formed by ultrasonic vibration diffusion. The precursor solution is atomized by ultrasonic spraying and, driven by air flow, is passed through a three-stage tube furnace at a flow rate of 10 L / min to undergo dehydration reaction at 400℃ and pyrolysis oxidation reaction at 800℃ to obtain spherical particles with an average particle size of less than 2 micrometers that are interwoven with Fe2O3 and pore-forming agent. The particles obtained by spray pyrolysis were reduced with hydrogen in a tube furnace to pure Fe (Fe is easier to ball mill into flakes than Fe2O3) and spherical particles interlocked with a pore-forming agent. The reducing atmosphere was H2:Ar=3:1, the reducing temperature was 300℃, the reducing time was 5h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature. Then, the spherical particles and grinding beads were mixed in a mass ratio of 1:20 and placed into a grinding jar with a grinding organic additive. The grinding time was 48 hours. The spherical particles were then ground into a sheet-like structure with a diameter-to-thickness ratio of 50 by high-energy grinding. The ball-milled flake powder is dissolved in deionized water, and the pore-forming agent is dissolved in the water by magnetic stirring. Then, it is centrifuged, and after centrifugation, deionized water is added again and magnetic stirring is repeated three to four times. After drying, a porous flake structure powder is obtained.

[0037] (2) Preparation of composite materials The obtained powder was reduced again with hydrogen in a tube furnace (to prevent oxidation of particles caused by ball milling). The reducing atmosphere was H2:Ar=3:1, the reducing temperature was 500℃, the reducing time was 0.5h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature. After reduction, the surface was rapidly nitrided again for 1 hour at 550°C under an Ar protective atmosphere in the furnace, with the temperature increased at 10°C / min. In the high nitrogen potential mixed atmosphere of NH3 and H2 (volume ratio 7:1), the active nitrogen atoms [N] from ammonia decomposition diffused into the powder surface, forming a dense γ′-Fe4N shell. The surface of the formed γ′-Fe4N shell continued to react with active nitrogen, transforming into ε-Fe3N with a higher nitrogen content, while the core remained α-Fe, resulting in a core-shell structure Fe3N@Fe4N@Fe. After the first nitriding, the furnace was switched to an Ar atmosphere and cooled to room temperature in preparation for a second nitriding.

[0038] Under an Ar protective atmosphere, the temperature was increased to 150 °C at a rate of 10 °C / min, followed by a second long-term stable nitriding process in a 1:1 mixture of NH3 and H2 for 20 h. Due to the low temperature, the surface nitriding reaction was essentially inactive; the surface ε-Fe3N and the sublayer Fe4N remained relatively stable at low temperatures without decomposition, while a phase transition occurred in the core. The core, due to the α″-Fe... 16 N2 has a low formation temperature range, and metastable α″-Fe will precipitate in an ordered manner within the α-Fe lattice. 16 N2, generating Fe3N@Fe4N@Fe 16 The N2@Fe multilayer core-shell structure was used. After nitriding, the furnace was cooled to room temperature in an Ar atmosphere. XRD phase analysis of the multilayer core-shell structure revealed the simultaneous presence of Fe, Fe3N, Fe4N, and Fe3N in the composite material. 16 N2 has four phases.

[0039] (3) Preparation of microwave absorbing materials A certain mass of powder was poured into a centrifuge bottle, and 5% of the powder's mass of paraffin wax was added. The mixture was heated to allow the powder and paraffin wax to fully blend and become homogeneous. The mixture was then pressed into shape under a pressure of 300 MPa. Testing showed that the prepared microwave absorbing composite material exhibited excellent electromagnetic wave absorption performance at high frequencies.

[0040] Preparation of sheet-like porous composite phase wave-absorbing soft magnetic materials Example 3: Preparation of sheet-like porous composite phase wave-absorbing soft magnetic materials A method for preparing a sheet-like porous microwave absorbing composite material, such as... Figure 1 As shown, it includes the following steps: (1) Constructing a sheet-like porous structure Water-soluble iron salt and pore-forming agent were dissolved in deionized water at a ratio of 1:5, and a uniform precursor solution with a concentration of 0.2 mol / L was formed by ultrasonic vibration diffusion. The precursor solution is atomized by ultrasonic spraying and, driven by air flow, is passed through a three-stage tube furnace at a flow rate of 10 L / min to undergo dehydration reaction at 400℃ and pyrolysis oxidation reaction at 800℃ to obtain spherical particles with an average particle size of less than 2 micrometers that are interwoven with Fe2O3 and pore-forming agent. The particles obtained by spray pyrolysis were reduced with hydrogen in a tube furnace to pure Fe (Fe is easier to ball mill into flakes than Fe2O3) and spherical particles interlocked with a pore-forming agent. The reducing atmosphere was H2:Ar=3:1, the reducing temperature was 350℃, the reducing time was 3h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to an Ar atmosphere and cooled to room temperature. Then, the spherical particles and grinding beads were mixed in a mass ratio of 1:25 and placed into a grinding jar with a grinding organic additive. The grinding time was 48 hours. The spherical particles were then ground into a sheet-like structure with a diameter-to-thickness ratio of 60 by high-energy grinding. The ball-milled flake powder is dissolved in deionized water, and the pore-forming agent is dissolved in the water by magnetic stirring. Then, it is centrifuged, and after centrifugation, deionized water is added again and magnetic stirring is repeated three to four times. After drying, a porous flake structure powder is obtained.

[0041] (2) Preparation of composite materials The obtained powder was reduced again in a tube furnace with hydrogen (to prevent oxidation of particles caused by ball milling). The reducing atmosphere was H2:Ar=3:1, the reducing temperature was 450℃, the reducing time was 1h, the heating rate was 5℃ / min, and the total flow rate of the reducing atmosphere was controlled at 400ml / min. After the reduction was completed, the furnace was switched to Ar atmosphere and cooled to room temperature. After reduction, the surface was rapidly nitrided again for 0.2 h at 600 °C at a rate of 10 °C / min under an Ar protective atmosphere in the furnace. In the high nitrogen potential mixed atmosphere of NH3 and H2 (volume ratio 7:1), the active nitrogen atoms [N] from ammonia decomposition diffused into the powder surface, forming a dense γ′-Fe4N shell. The surface of the formed γ′-Fe4N shell continued to react with active nitrogen, transforming into ε-Fe3N with a higher nitrogen content, while the core remained α-Fe, resulting in a core-shell structure Fe3N@Fe4N@Fe. After the first nitriding, the furnace was switched to an Ar atmosphere and cooled to room temperature in preparation for a second nitriding.

[0042] Under an Ar protective atmosphere, the temperature was increased to 250 °C at a rate of 10 °C / min, followed by a second long-term stable nitriding process in a 4:1 mixture of NH3 and H2 for 4 hours. Due to the low temperature, the surface nitriding reaction was essentially inactive; the surface ε-Fe3N and the sublayer Fe4N remained relatively stable at low temperatures without decomposition, while a phase transition occurred in the core. The core, due to the α″-Fe... 16 N2 has a low formation temperature range, and metastable α″-Fe will precipitate in an ordered manner within the α-Fe lattice. 16 N2, generating Fe3N@Fe4N@Fe 16 The N2@Fe multilayer core-shell structure was used. After nitriding, the furnace was cooled to room temperature in an Ar atmosphere. XRD phase analysis of the multilayer core-shell structure revealed the simultaneous presence of Fe, Fe3N, Fe4N, and Fe3N in the composite material. 16 N2 has four phases.

[0043] (3) Preparation of microwave absorbing materials A certain mass of powder was poured into a centrifuge bottle, and 50% of the powder's mass of paraffin wax was added. The mixture was heated to allow the powder and paraffin wax to fully blend and become homogeneous. The mixture was then pressed into shape under a pressure of 100 MPa. Testing showed that the prepared microwave absorbing composite material exhibited excellent electromagnetic wave absorption performance at high frequencies.

[0044] The above embodiments are used to illustrate and explain the present invention, and are not intended to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims fall within the protection scope of the present invention.

Claims

1. A method for preparing a sheet-like porous microwave absorbing composite material, characterized in that, The preparation method includes the following steps: (1) Prepare spherical particles with an average particle size of less than 2 micrometers in which Fe2O3 and pore-forming agent are interlocked; (2) The obtained Fe2O3 and pore-forming agent interlocked spherical particles were hydrogen-reduced and then ball-milled to obtain a plate-like structure; (3) Wash with water to remove the pore-forming agent and obtain a porous sheet-like structure; (4) Reduce hydrogen again and perform the first high-temperature rapid nitriding to nitrid the surface of the structure; the conditions for the first high-temperature rapid nitriding are: nitriding in a high nitrogen potential mixed atmosphere with a volume ratio of NH3 and H2 greater than or equal to 4:1 for less than 1.0 h, and nitriding temperature of 500~600℃. (5) A second low-temperature long-time nitriding is carried out to allow the interior to be fully nitrided, resulting in a sheet-like porous composite material. The conditions for the second low-temperature long-time nitriding are: nitriding for 4 to 20 hours in a mixed atmosphere of NH3 and H2 with a volume ratio of 1 to 4:1, and nitriding temperature of 150 to 250°C.

2. The preparation method according to claim 1, characterized in that, The methods for preparing spherical particles in which Fe2O3 and pore-forming agents are interlocked include spray pyrolysis and hydrothermal methods.

3. The preparation method according to claim 1, characterized in that, The preparation method of the Fe2O3 and pore-forming agent interlocking spherical particles is a spray pyrolysis method, which specifically includes the following steps: (1) Dissolve water-soluble iron salt and pore-forming agent in deionized water in a certain proportion, and diffuse them by ultrasonic vibration to form a uniform precursor solution with a concentration of 0.2 mol / L; (2) The precursor solution is atomized by ultrasonic spraying and then undergoes dehydration and pyrolysis oxidation reactions in a tube furnace under the influence of airflow to obtain spherical particles in which Fe2O3 and pore-forming agent are interlocked.

4. The preparation method according to claim 3, characterized in that, The water-soluble iron salt includes one of ferric nitrate and ferric chloride; the pore-forming agent includes one of sodium chloride, potassium nitrate, and potassium chloride; the mass ratio of water-soluble iron salt to pore-forming agent is 1:0.5~5; the carrier gas flow is air with a flow rate of 5~10 L / min; the dehydration reaction occurs at 400℃, and the pyrolysis oxidation reaction occurs at 800℃.

5. The preparation method according to claim 1, characterized in that, The preparation process of the sheet-like structure is as follows: (1) The particles obtained by spray pyrolysis are reduced with hydrogen in a tube furnace to spherical particles in which pure Fe and pore-forming agent are interlocked; (2) Place the spherical particles, grinding beads, and grinding organic additives into a grinding jar at a ratio of 1:20 to 30:20, and grind the spherical particles into a sheet-like structure with a diameter-to-thickness ratio greater than 45 by high-energy grinding.

6. The preparation method according to claim 4, characterized in that, The ball milling organic aid includes one or more of anhydrous ethanol, pump oil, oleic acid, and stearic acid, mixed in any proportion.

7. The preparation method according to claim 1, characterized in that, The hydrogen reduction atmosphere is an H2:Ar volume ratio of 3:1, a reduction temperature of 500~600℃, a reduction time of 0.5~2h, a heating rate of 5℃ / min, and a total flow rate of the reducing atmosphere controlled at 400ml / min.

8. The preparation method according to claim 1, characterized in that, Also includes: Take a certain mass of the prepared sheet-like porous composite material powder, add 5-50% of the powder mass of paraffin, heat to allow the powder and paraffin to fully blend and mix evenly, and press into shape under a pressure of 100-300 MPa.

9. A sheet-like porous structure microwave absorbing composite material prepared by a method according to any one of claims 1-8, characterized in that, The composite material is Fe or Fe2N, Fe3N, Fe4N, Fe 16 A combination of two or more N2 iron-based nitrides.

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