A method for preparing submicron-scale sheet-like Fe4N magnetic microwave absorbing material

Abstract

The application discloses a preparation method of submicron flaky Fe4N magnetic wave-absorbing material and belongs to the technical field of magnetic wave-absorbing material preparation. The method comprises the following steps: firstly, obtaining submicron flaky iron oxide powder by wet grinding of super-pure iron concentrate; secondly, carrying out low-temperature one-step reduction and nitriding reaction on the powder in an ammonia and hydrogen mixed atmosphere to obtain submicron flaky Fe4N powder; and thirdly, carrying out surface coating treatment on the submicron flaky Fe4N powder to obtain the submicron flaky Fe4N magnetic wave-absorbing material with Fe4N phase purity of greater than or equal to 90wt%, average particle size of 50-500nm and diameter-thickness ratio of (4-10):1. The method has the advantages of low raw material cost, simple process flow, mild reaction condition, continuous mass production, good electromagnetic loss absorption performance of the obtained magnetic wave-absorbing material and wide application prospect.

Description

Technical Field

[0001] This invention relates to a method for preparing Fe4N magnetic microwave absorbing material, specifically a method for preparing submicron-sized sheet-like Fe4N magnetic microwave absorbing material, belonging to the field of magnetic microwave absorbing material preparation technology. Background Technology

[0002] With the rapid development of modern information and communication technology and stealth technology, while electromagnetic technology brings convenience to human life, electromagnetic pollution is inevitably harming human health. There is an urgent need for microwave absorbing materials that combine thinness, wide frequency band, light weight, and strong absorption. Magnetic metals and their compounds are considered irreplaceable microwave absorbing materials due to their high saturation magnetization and magnetic loss capacity. Existing technologies have extensively studied traditional magnetic microwave absorbing materials such as carbonyl iron, ferrites, and metal alloys. However, traditional irregular or spherical metallic magnetic powders suffer from problems such as eddy current effects at high frequencies, leading to decreased permeability and limited impedance matching.

[0003] Fe4N, a soft magnetic material with high saturation magnetization, good chemical stability, and moderate resistivity, shows potential for application in microwave absorption. Compared to spherical or amorphous particles, sheet-like magnetic particles exhibit anisotropic shape and high specific surface area. Impedance matching can be optimized by adjusting the aspect ratio, enhancing interfacial polarization loss. Furthermore, it can effectively construct three-dimensional conductive network structures or multilayer reflective structures in composite materials, which is beneficial for multiple scattering and interference loss of electromagnetic waves, significantly improving microwave absorption performance.

[0004] In existing technologies, the preparation of sheet-like iron nitride mostly employs complex solution chemistry methods to obtain sheet-like precursors, followed by a traditional segmented method of "high-temperature reduction followed by long-term nitriding" to obtain Fe4N products. For example, patent document (CN114920217B) discloses an iron-based nitride microwave absorbing material prepared by hydrothermal synthesis combined with reduction nitriding treatment. This material exhibits good microwave absorption performance, achieving an effective absorption width of 4–7 GHz (reflectivity <-10 dB) at an ultrathin thickness of 1–2 mm, with a maximum absorption intensity between -40 dB and -80 dB. However, this material involves a hydrothermal reaction, making large-scale production difficult and costly. Furthermore, the resulting material consists of near-spherical porous particles lacking shape anisotropy, making it impossible to utilize the multi-scattering network constructed from the structure to suppress surface eddies. Furthermore, the two-step preparation of iron nitride generates intermediates with extremely high surface energy, which are prone to violent thermal agglomeration and grain sintering. This not only causes the collapse of the plate-like microstructure but also closes the defect diffusion channels inside the particles, making it difficult for subsequent nitrogen atoms to penetrate and resulting in low product purity. Currently, there are also reports of methods to obtain iron nitride microwave absorbing materials by "nitriding first and then ball milling". For example, patent document (CN110408849B) discloses a method for preparing nano-iron nitride microwave absorbing materials with multi-scale grains. This method involves mixing raw materials, nitriding the iron alloy powder to be nitrided, and then ball milling with high energy to obtain nano-iron nitride microwave absorbing materials with multi-scale grains. This material contains grains of various sizes, which increases the number of scatterings of electromagnetic waves during transmission inside the particles, thereby improving the material's absorption capacity. At the same time, it has a wide absorption bandwidth and a thin absorption matching thickness. However, this method sacrifices the controllability and dispersion of the morphology of iron nitride absorbing material, fails to solve the problem of nanoparticle aggregation, and is prone to impedance mismatch and narrowing of effective bandwidth, thereby degrading the absorption performance.

[0005] Overall, these existing technologies suffer from problems such as complex precursor preparation processes, high costs, high nitriding temperatures (typically >500℃) which easily lead to particle sintering and growth, morphological damage, poor product dispersibility, and easy oxidation. Summary of the Invention

[0006] To address the shortcomings of existing methods for preparing high-purity sheet-like Fe4N, the present invention aims to provide a method for preparing submicron-sized sheet-like Fe4N magnetic absorbing materials. This method allows for controllable morphology and phase composition of Fe4N, enabling the production of high-purity, average particle size between 50 and 500 nm, aspect ratio of (4 to 10):1, and excellent electromagnetic parameter matching of submicron-sized sheet-like Fe4N magnetic absorbing materials. Furthermore, this method utilizes low-cost raw materials, employs a simple process flow, and uses mild reaction conditions, allowing for continuous large-scale production. It effectively solves the key technical problems of existing methods for preparing Fe4N magnetic absorbing materials, such as complex processes, high costs, low product purity, and difficulty in controlling morphology.

[0007] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing submicron-scale sheet-like Fe4N magnetic microwave absorbing material, which includes the following steps:

[0008] (1) The mixture of ultrapure iron concentrate, dispersant and dispersion medium is ground in a nano-sand mill, and then subjected to solid-liquid separation, washing and drying to obtain submicron-sized flake iron oxide powder.

[0009] The grinding process uses coarse grinding beads with a diameter ranging from 0.2 mm to 0.5 mm and fine grinding beads with a diameter ranging from 0.05 mm to 0.15 mm. The mass ratio of coarse grinding beads to fine grinding beads is (0.5~2.0):1, the diameter ratio of coarse grinding beads to fine grinding beads is (2~5):1, the grinding speed is 2000 rpm to 3000 rpm, and the grinding time is 5~15 h.

[0010] (2) Submicron-sized sheet-like iron oxide powder was placed in a mixed atmosphere of ammonia and hydrogen and subjected to a one-step reduction nitridation reaction at a temperature below 500°C to obtain submicron-sized sheet-like Fe4N powder;

[0011] (3) The submicron-sized sheet-like Fe4N powder is surface coated to obtain the submicron-sized sheet-like Fe4N magnetic absorbing material.

[0012] The key to obtaining submicron-sized sheet-like Fe4N magnetic absorbing materials in this invention lies in: using ultrapure iron concentrate as raw material, and efficiently obtaining submicron-sized sheet-like iron oxide powder through physical methods. This powder has a micro-nano morphology and exhibits high reactivity, thus enabling efficient reduction and nitriding at low temperatures. This not only avoids the sintering or collapse of the sheet-like structure caused by subsequent high-temperature reduction and nitriding processes, ensuring the average particle size and aspect ratio of the submicron-sized sheet-like Fe4N magnetic absorbing material, but also yields a high-purity Fe4N phase, endowing the Fe4N magnetic absorbing material with excellent electromagnetic loss absorption performance. More specifically, this invention uses readily available and inexpensive ultrapure iron concentrate as the direct raw material, and obtains submicron-sized flake-shaped iron oxide powder through "nano-grinding". The key to the nano-grinding process lies in two aspects: firstly, by utilizing the buffering effect of the liquid phase and the dispersing effect of the dispersant, the cold welding and agglomeration phenomena during high-speed grinding are synergistically suppressed, which is conducive to achieving efficient flake formation; secondly, by optimizing the grinding media, especially by strictly controlling parameters such as the particle size distribution, mass ratio, and particle size ratio of the grinding beads, the energy during the grinding process is precisely divided and assigned, which not only achieves high-speed shearing impact and crushing and refining of the iron concentrate, but also enables selective peeling and flattening of the iron concentrate, allowing the iron oxide crystals to be processed into nano-thick flake-shaped iron oxide powder along the cleavage plane in a highly efficient and controllable manner. This micro-nano-sized flake-shaped iron oxide powder is used in subsequent reduction and nitriding steps, laying the geometric foundation for obtaining submicron-sized flake-shaped Fe4N with anisotropic shape, which is difficult to achieve by traditional chemical precipitation or direct ball milling methods. Based on the acquisition of submicron-sized lamellar iron oxide powder, and due to its high reactivity, low-temperature reduction and nitriding can be achieved. This invention employs a "low-temperature one-step reduction nitriding" method, using an ammonia-hydrogen mixture as the reaction medium. At a relatively mild temperature (below 500°C), the hydrogen reduction and nitriding processes of iron are simultaneously completed. Hydrogen in the ammonia-hydrogen mixture acts as a reducing agent, accelerating oxygen removal, while ammonia decomposition provides active nitrogen atoms for nitriding, effectively suppressing the formation of α-Fe impurity phases and promoting the preferential nucleation and stable growth of the Fe4N phase. Under these low-temperature conditions, grain sintering, growth, and lamellar structure collapse at high temperatures are effectively suppressed, thus efficiently converting Fe3O4 into the target Fe4N phase while completely preserving the lamellar structure of the precursor. Furthermore, to improve the stability of submicron-sized flake-like Fe4N powder, this invention further employs a coating strategy. Organic amorphous polymers are used to coat the surface, creating a physical barrier to isolate oxygen and water and modulating the surface dielectric properties. Subsequent high-intensity ultrasonic treatment not only ensures the uniformity of the coating layer and the monodispersity of the powder, but also optimizes the spatial distribution of the powder and coating agent, promoting the repair of surface defects, reducing surface energy, further stabilizing the metastable Fe4N phase, and improving the overall microwave absorption performance of the material.

[0013] The selection of grinding beads and grinding conditions in the grinding process of this invention are optimized. The grinding media consists of coarse zirconia grinding beads with a diameter of 0.2 mm to 0.5 mm and fine zirconia grinding beads with a diameter of 0.05 mm to 0.15 mm, mixed at a mass ratio of (0.5 to 2):1 and a diameter ratio of (2 to 5):1, with a filling rate of 40 to 80%. The spindle speed is not less than 2000 rpm, and the grinding time is 5 to 15 hours. Following the energy distribution principle of "coarse beads shearing and fine beads peeling," coarse grinding beads are selected to provide macroscopic shearing force, which dominates the primary lamellarization of particles; fine grinding beads are embedded in the interlayer gaps, using their high specific surface area to perform nanoscale fine peeling of the micro-flake edges. The selective mixing forms a dynamic and synergistic hierarchical energy field, at which point the kinetic energy of a single bead is moderate, which can effectively peel off the lamellar layers while avoiding lamellar fracture caused by excessive impact. A further preferred grinding speed range is 2000 to 2500 rpm, which is sufficient to allow the iron oxide lattice to slide along the corresponding cleavage plane. When the rotational speed exceeds 3000 rpm, the centrifugal adhesion effect of the grinding beads intensifies, the effective grinding zone volume shrinks, and the slurry temperature rises rapidly. The thermoplastic deformation of the particles may lead to spheroidization. Since iron concentrate is highly susceptible to over-grinding (i.e., lamellar fatigue fracture and aspect ratio reduction) at high rotational speeds in nano-grind mills, the grinding time must be controlled within 10 hours, with a preferred range of 5–10 hours.

[0014] The submicron-scale sheet-like Fe4N magnetic microwave absorbing material of the present invention has a Fe4N phase purity ≥90wt%, exhibits a sheet-like morphology, a planar particle size D50 of 50~500nm, an aspect ratio of (4~10):1, a saturation magnetization of 150~200emu / g, and a coercivity of 120~210Oe. It has electromagnetic wave absorption performance in the frequency range of 2~18GHz, with significant absorption effect in the S-band (2~4GHz), and a maximum absorption intensity between -25dB and -45dB.

[0015] As a preferred embodiment, the ultrapure iron concentrate is predominantly Fe3O4 with a purity greater than 99.5 wt%. The ultrapure iron concentrate of this invention has a total iron content of over 72%, and the total content of impurity oxides such as silicon, aluminum, calcium, and magnesium should be less than 0.3 wt%. High purity is fundamental to ensuring the magnetic properties and phase purity of the final product. The ultrapure iron concentrate of this invention is a product easily obtained from ordinary iron concentrate using existing mineral processing methods, and is a commercially viable product with a relatively low price compared to other iron-containing raw materials.

[0016] As a preferred embodiment, the dispersion medium includes water. Water has good wetting properties on the surface of iron concentrate and is inexpensive, making it a preferred dispersion medium.

[0017] As a preferred embodiment, the dispersant includes at least one of PVP, polyethylene glycol 400, and oleic acid. The preferred dispersant is rich in polar oxygen or nitrogen, which can act well on the surface of the iron concentrate, thus improving the dispersibility of the iron concentrate.

[0018] As a preferred embodiment, the mass content of ultrapure iron concentrate in the mixed slurry is 10%~30%, and the mass of dispersant is 0.5%~5% of the mass of ultrapure iron concentrate. At the preferred concentrations of ultrapure iron concentrate and dispersant, it is possible to ensure that the submicron-sized flake-like iron oxide precursors after stripping are highly dispersed, avoiding agglomeration.

[0019] As a preferred embodiment, the pH value of the mixed slurry is adjusted to 9-11. Adjusting the pH value of the grinding slurry to 9-11 provides a slightly alkaline environment that helps stabilize the flaky particles through surface charge repulsion, thereby reducing viscosity and preventing disordered agglomeration during the grinding process.

[0020] As a preferred embodiment, the drying process employs vacuum low-temperature drying or low-temperature drying under a protective atmosphere, with a temperature of 40-60°C and a time of 4-12 hours. More preferably, low-temperature vacuum drying is used, with a vacuum drying pressure of 90-100 kPa. This effectively prevents particle agglomeration, and simultaneously, the escape path of internal moisture is more intense and dispersed, leaving abundant pores and channels in the accumulation of flaky particles, making the gas-solid reaction faster and more thorough.

[0021] As a preferred embodiment, the volume ratio of ammonia to hydrogen in the ammonia-hydrogen mixed atmosphere is (1~5):1. A further preferred embodiment employs a continuously flowing atmosphere for the ammonia-hydrogen mixed atmosphere, which not only ensures the stability of the atmosphere but also removes generated water vapor. The volume ratio of ammonia to hydrogen is further preferably (2~4):1. At this ratio, the reduction and nitriding rates are well matched, and the reaction is stable and controllable. When the volume ratio of ammonia to hydrogen is less than 2:1, there is excessive H2, the reduction reaction dominates, and the partial pressure of NH3 is too low to provide sufficient active nitrogen atoms, resulting in a very weak nitriding reaction and difficulty in obtaining the target product. When the volume ratio of ammonia to hydrogen is greater than 4:1, there is excessive NH3, the concentration of active nitrogen atoms produced by decomposition is too high, and the reaction tends to generate a more stable nitrogen-rich phase. Simultaneously, excessive ammonia weakens the reducing ability, the overall reaction rate is too slow, and the holding time required to achieve complete conversion is significantly prolonged.

[0022] As a preferred embodiment, the conditions for the reduction nitriding reaction are: a temperature of 350~450℃, a heating rate of 2~10℃ / min, and a holding time of 4~8h. Under these preferred conditions, hydrogen preferentially reduces Fe3O4 to a more reactive intermediate state, followed by the rapid infiltration of active nitrogen atoms generated from the decomposition of ammonia, directly generating the target phase Fe4N. This "low-temperature one-step method" effectively avoids the abnormal growth of iron nitride grains and the fusion of lamellar structures at high temperatures (>500℃), ensuring that the product has high phase purity (Fe4N content > 90%) and a complete lamellar geometry. However, the reduction nitriding reaction time should not be too long, as this would increase the risk of lamellar particles coarsening and agglomerating through surface diffusion.

[0023] As a preferred embodiment, the surface coating process is as follows: the flake-shaped Fe4N powder is placed in a solution containing a coating agent, subjected to ultrasonic treatment, and then sequentially subjected to solid-liquid separation, washing, and drying.

[0024] As a preferred embodiment, the solvent in the solution containing the coating agent is ethanol. Using ethanol as a solvent provides good solubility for the coating agent and good surface wettability for submicron-sized flake-like Fe4N powder, which is beneficial for coating modification.

[0025] As a preferred embodiment, the concentration of the coating agent in the solution containing the coating agent is 1~10 wt%;

[0026] As a preferred embodiment, the coating agent includes at least one of polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, polyaniline, and polydopamine; the preferred coating agent can enhance material properties by providing steric hindrance, forming a coating film, and introducing dielectric or polarization losses.

[0027] As a preferred embodiment, the ultrasonic treatment conditions are: ultrasonic frequency of 25~40kHz, ultrasonic power of 100~300W, and ultrasonic time of 10~30min. The ultrasonic treatment operates in pulse mode, which can prevent localized overheating that could lead to the Fe4N phase transformation. This process not only breaks up soft agglomerates generated during the coating process, ensuring uniform coating of individual powder particles, but also makes the coating layer denser through cavitation, further optimizing the interfacial compatibility and dispersibility of the powder.

[0028] Compared with existing technologies, the beneficial technical effects of the present invention are as follows:

[0029] 1) In the preparation process of the submicron-scale sheet-like Fe4N magnetic microwave absorbing material of the present invention, for the first time, ultrapure iron concentrate is processed into a submicron-scale sheet-like structure by using the energy distribution principle of "coarse bead shearing and fine bead peeling" through nano-sand milling process. This breaks through the technical bottleneck of existing technology that makes it difficult to achieve efficient sheet-like formation of iron concentrate. The cost is reduced by more than 70% compared with iron salt solution, and it is suitable for large-scale production. Furthermore, the sheet thickness and aspect ratio can be precisely controlled by adjusting the milling parameters to meet the requirements of low frequency band and strong microwave absorption performance.

[0030] 2) In the preparation process of the submicron-scale sheet-like Fe4N magnetic absorbing material of the present invention, the preparation of the submicron-scale sheet-like iron oxide precursor can be carried out by a low-temperature one-step reduction nitriding method, which shortens the traditional reduction nitriding process cycle from 12~24h to 4~8h and reduces energy consumption by more than 40%. Moreover, the reduction and nitriding processes are completed simultaneously in an ammonia-hydrogen mixed atmosphere, which avoids the collapse of sheet-like morphology and coarsening of particles caused by high-temperature sintering, and precisely controls the reaction path to generate high-purity (>90%) Fe4N phase. This solves the technical problems of the traditional two-step process being cumbersome, the intermediate product being easily oxidized, and the high-temperature treatment being prone to introducing impurities.

[0031] 3) In the preparation process of the submicron-scale sheet-like Fe4N magnetic microwave absorbing material of the present invention, ultrasonic-assisted coating treatment is used to prevent the agglomeration of submicron-scale materials, while promoting the formation of a continuous, uniform, and dense coating layer on the particle surface by the coating agent, thereby achieving a synergistic improvement in the material's antioxidant stability, magnetic property retention rate, and microwave absorption performance.

[0032] In summary, the submicron-sized sheet-like Fe4N material prepared by this invention maintains a complete sheet-like morphology and a high-purity Fe4N phase, with a uniform and dense surface coating. It exhibits excellent microwave absorption performance in the 2~18GHz frequency band, especially in the S-band (2~4GHz). At the same time, it has good oxidation resistance and process repeatability, realizing a full-chain technological innovation from mineral raw materials to high-performance microwave absorbing materials, and providing a feasible path for the engineering application of a new generation of lightweight, low-frequency, and strongly absorbing magnetic microwave absorbing materials. Attached Figure Description

[0033] Figure 1 The X-ray diffraction pattern of the submicron-sized sheet-like Fe4N prepared in Example 1 is shown in the figure. The main phase is γ'-Fe4N (JCPDS No. 04-004-9107), with almost no other impurities.

[0034] Figure 2 The image shows a scanning electron microscope (SEM) image of the submicron-sized sheet-like Fe4N prepared in Example 1; its two-dimensional sheet-like morphology is clearly visible in the image.

[0035] Figure 3The reflection loss curves of the submicron-sized sheet-like Fe4N prepared in Example 1 after being coated with paraffin in the frequency range of 2~18 GHz.

[0036] Figure 4 Hysteresis loop diagram of saturation magnetization-coercivity of submicron-scale sheet-like Fe4N prepared in Example 1.

[0037] Figure 5 The image shows a scanning electron microscope (SEM) image of the iron oxide powder prepared under dry ball milling conditions in Comparative Example 1. The image shows that the powder has an irregular or spherical structure and no obvious or complete lamellar structure.

[0038] Figure 6 The image shows a scanning electron microscope (SEM) image of the iron oxide powder prepared under the condition of using a single-size grinding medium, as shown in Comparative Example 2. The image shows that the powder contains a large number of irregular nanofragments and coarse particles that have not been fully exfoliated.

[0039] Figure 7 The X-ray diffraction pattern of the submicron-sized sheet-like Fe4N prepared under high temperature conditions in Comparative Example 3 is shown in the figure. It can be seen from the figure that the α-Fe impurity peak is obvious, indicating that the high temperature disrupted the ammonia-hydrogen balance and the purity of the prepared submicron-sized sheet-like Fe4N product was low.

[0040] Figure 8 The X-ray diffraction pattern of submicron-sized sheet-like Fe4N prepared under high ammonia-to-hydrogen ratio conditions is shown in Example 4. The characteristic diffraction peaks of γ'-Fe4N are significantly reduced, while diffraction peaks corresponding to ε-Fe3N appear, indicating that the main phase of the product begins to transform into the more stable high-nitrogen potential iron-nitrogen compound Fe3N. Detailed Implementation

[0041] To better understand this invention, specific embodiments are now used to more clearly illustrate and analyze its content. However, the claims of this invention are not limited to the following examples and conditions. Other examples obtained by those skilled in the art without creative effort are all within the protection scope of this invention.

[0042] Example 1

[0043] (1) Flake treatment: Take 5000g of ultrapure iron concentrate powder with a TFe content of 72.1wt%, add 20L of deionized water, prepare a slurry with a solid content of 20wt%, add dispersant polyethylene glycol 400 at 1.0% of the powder mass, grind with a horizontal nano-sand mill, the grinding media is 0.1mm:0.3mm=1:1 zirconium oxide beads, the filling rate is 50%, the spindle speed is 2500rpm, the cooling water temperature is controlled at 40℃, and the grinding time is 12h to obtain submicron-sized flake iron oxide precursor.

[0044] (2) Low-temperature one-step reduction nitriding: The precursor was dried in a vacuum drying oven (below 100 kPa) at 60°C for 6 hours. 100 g of the dried powder was placed in a tube furnace and heated to 400°C at 5°C / min. A mixed gas of NH3 / H2 with a volume ratio of 2:1 was introduced and reacted for 6 hours. After the reaction was completed, the powder was cooled to room temperature under N2 protection to obtain submicron-sized flake-like Fe4N powder.

[0045] (3) Coating and ultrasonic passivation: In an argon glove box, 10g of Fe4N powder was mixed with 95mL of coating agent solution (1.5% polyvinylpyrrolidone, the remainder ethanol), and treated for 10min under ultrasonic frequency of 30kHz and power density of 300W. The ultrasonic mode was pulsed. After treatment, the powder was washed with ethanol 3 times and vacuum dried at 50℃ (90KPa) for 10h to obtain submicron-sized flake Fe4N product.

[0046] For characterization and performance testing, see [link / reference]. Figures 1-4 XRD analysis (see Figure 1 The product was shown to be a single Fe4N phase with a purity of 98.8%; SEM observation (see [link to SEM image]). Figure 2 The planar particle size D50 is 120 nm, and the aspect ratio is approximately 7:1; the VSM test saturation magnetization is 185 emu / g, and the coercivity is 161 Oe (see [reference]). Figure 4 The sample (submicron-sized flake-like Fe4N) was mixed with a paraffin matrix at a mass ratio of 7:3 to form a coaxial ring. Its electromagnetic parameters in the frequency range of 2–18 GHz were tested using a vector network analyzer, and the reflection loss was calculated. The results show that when the matching thickness is 3.7 mm, the maximum reflection loss of -39 dB is reached at 3.5 GHz (see [link to relevant documentation]). Figure 3 ).

[0047] Example 2

[0048] (1) Flake treatment: Take 5000g of ultrapure iron concentrate powder with a TFe content of 72.1wt%, add 20L of deionized water, prepare a slurry with a solid content of 20wt%, add dispersant polyethylene glycol 400 at 1.0% of the powder mass, grind with a horizontal nano-sand mill, the grinding media is 0.1mm:0.3mm=1:1 zirconia beads, the filling rate is 50%, the spindle speed is 2800rpm, the cooling water temperature is controlled at 40℃, and the grinding time is 15h to obtain submicron-sized flake iron oxide precursor.

[0049] (2) Low-temperature one-step reduction nitriding: The precursor was dried in a vacuum drying oven at 60°C for 6 hours. 100g of the dried powder was placed in a tube furnace and heated to 350°C at 5°C / min. A mixed gas of NH3 / H2 with a volume ratio of 2:1 was introduced and reacted for 8 hours. After the reaction was completed, the powder was cooled to room temperature under N2 protection to obtain submicron-sized flake-like Fe4N powder.

[0050] (3) Coating and ultrasonic passivation: In an argon glove box, 10g of Fe4N powder was mixed with 95mL of coating agent solution (1.5% polyvinylpyrrolidone, the remainder ethanol), and treated for 10min under ultrasonic frequency of 30 kHz and power density of 200W. The ultrasonic mode was pulsed. After treatment, the powder was washed with ethanol 3 times and vacuum dried at 50℃ for 10h to obtain submicron-sized flake Fe4N product.

[0051] Characterization and Performance Testing: XRD analysis showed the product to be a single Fe4N phase with a purity of 98%; SEM observation revealed a planar particle size D50 of 100 nm and an aspect ratio of approximately 8:1; VSM testing showed a saturation magnetization of 155 emu / g and a coercivity of 172 Oe; the sample (submicron-sized flake-like Fe4N) was mixed with a paraffin matrix at a mass ratio of 7:3 to form a coaxial ring. The electromagnetic parameters of this ring in the 2–18 GHz frequency range were tested using a vector network analyzer, and the reflection loss was calculated. The results showed that when the matching thickness was 4.0 mm, the maximum reflection loss of -41 dB was reached at 2.8 GHz.

[0052] Example 3

[0053] (1) Flake treatment: Take 5000g of ultrapure iron concentrate powder with a TFe content of 72.1wt%, add 15L of deionized water, prepare a slurry with a solid content of 25wt%, add dispersant polyethylene glycol 400 at 0.5% of the powder mass, grind with a horizontal nano-sand mill, the grinding media is 0.1mm:0.3mm=1:1 zirconia beads, the filling rate is 50%, the spindle speed is 2500rpm, the cooling water temperature is controlled at 40℃, and the grinding time is 12h to obtain submicron-sized flake iron oxide precursor.

[0054] (2) Low-temperature one-step reduction nitriding: The precursor was dried in a vacuum drying oven at 60°C for 6 hours. 100g of the dried powder was placed in a tube furnace and heated to 400°C at 5°C / min. A mixed gas of NH3 / H2 with a volume ratio of 3:1 was introduced and reacted for 4 hours. After the reaction was completed, the powder was cooled to room temperature under N2 protection to obtain submicron-sized flake-like Fe4N powder.

[0055] (3) Coating and ultrasonic passivation: In an argon glove box, 10g of Fe4N powder was mixed with 95mL of coating agent solution (1.5% polyvinylpyrrolidone, the remainder ethanol), and treated for 10min under ultrasonic frequency of 30kHz and power density of 300W. The ultrasonic mode was pulsed. After treatment, the powder was washed with ethanol 3 times and vacuum dried at 50℃ for 10h to obtain submicron-sized flake Fe4N product.

[0056] Characterization and performance testing: XRD analysis showed that the product was a single Fe4N phase with a purity of 98.5%; SEM observation showed a planar particle size D50 of 210 nm and an aspect ratio of approximately 5:1; VSM testing showed a saturation magnetization of 145 emu / g and a coercivity of 150 Oe; the sample (submicron-sized flake-like Fe4N) was mixed with a paraffin matrix at a mass ratio of 7:3 to form a coaxial ring. The electromagnetic parameters of this ring in the frequency range of 2–18 GHz were tested using a vector network analyzer, and the reflection loss was calculated. The results showed that when the matching thickness was 3.5 mm, the maximum reflection loss of -36 dB was reached at 3.1 GHz.

[0057] Compare with Example 1

[0058] This comparative example is mainly used to illustrate that it is difficult to achieve effective platying of iron concentrate using conventional dry ball milling.

[0059] 5000g of the same ultrapure iron concentrate powder as in Example 1 was weighed and ground in a dry mill with zirconium oxide grinding media of the same material and particle size, without the addition of any liquid solvent or dispersant. The ultrapure iron concentrate was ground for 12 hours at a speed of 500 rpm and a ball-to-powder ratio of 10:1. Other steps and operations were the same as in Example 1. The results showed that the iron oxide powder obtained by dry ball milling had low flake formation efficiency and uneven thickness.

[0060] SEM characterization of the obtained iron oxide powder revealed (e.g.) Figure 5 As shown in the figure, its sheet formation efficiency is low, the thickness is uneven, and it contains a large number of fine particles generated by strong impact and extrusion. The particle surface is rough, no obvious and complete sheet structure is observed, and severe agglomeration occurs between particles. Laser particle size analysis shows that although its particle size distribution range is widened, it is mainly due to the increase of fine particles caused by breakage, rather than the formation of anisotropic sheet particles, resulting in the microwave absorption performance of the final product being significantly worse than that of Example 1.

[0061] Compare with Example 2

[0062] This comparative example is mainly used to illustrate that in the nano-grinding process, it is impossible to achieve the efficient sheet formation described in this invention by using only a single-size grinding media, thus confirming the necessity of the "energy distribution principle of coarse bead shearing and fine bead peeling".

[0063] The only difference compared to Example 1 is that the grinding media used only 0.3mm zirconia beads of a single size, and its total mass remained consistent with the total mass of the mixed grinding beads in Example 1. SEM characterization of the resulting powder revealed (e.g.) Figure 6As shown): Compared with the product of Example 1, the regularity and uniformity of its sheet morphology are significantly reduced. There is a large number of irregular nanofragments and coarse particles that have not been fully peeled off in the product. The projected area (sheet diameter) of the particles is wider, but the average diameter-to-thickness ratio is low. The edges are mostly broken rather than smooth sheet-like.

[0064] Compare with Example 3

[0065] This comparative example is mainly used to illustrate that the traditional high-temperature nitriding method is difficult to obtain high-purity Fe4N phase and maintain its lamellar morphology due to thermodynamic phase transformation and particle sintering behavior.

[0066] The only difference compared to Example 1 is that the one-step reduction nitriding temperature was set to 500°C, and the reaction was carried out for 6 hours in a 2:1 volume ratio NH3 / H2 mixed gas. After reduction nitriding under these conditions, the iron nitride content in the resulting submicron-sized flake-like Fe4N product was analyzed as follows: Figure 7 As shown in the figure, the Fe4N phase in the product has low purity, and there is severe sintering and agglomeration between particles, passivation and curling of the lamellar edges, increased surface roughness, and destruction of morphological integrity. The saturation magnetization is 165 emu / g. The microwave absorption performance test shows that the maximum reflection loss is only -12.5dB at a thickness of 3.5mm.

[0067] Compare with Example 4

[0068] This comparative example is mainly used to illustrate that the volume ratio of nitrogen-hydrogen mixed gas is a key process parameter for controlling the reaction path and product phase. Deviating from the reasonable range will lead to phase transformation and significant deterioration of microwave absorption performance.

[0069] The only difference compared to Example 1 is that the volume ratio of NH3 to H2 in the one-step reduction nitriding reaction is 6:1, and the reduction nitriding reaction is carried out at 400°C for 6 hours. After reduction nitriding under these conditions, the iron nitride content in the resulting submicron-sized flake-like Fe4N product was analyzed as follows: Figure 8 As shown in the figure, a distinct Fe3N phase appears in the product, while the Fe4N content is extremely low, indicating that the excessively high nitrogen potential causes the reaction to tend towards the formation of a more stable nitrogen-rich phase. At this point, the saturation magnetization is only 132 emu / g, the coercivity is 113 Oe, and no effective absorption bandwidth is observed in the S-band (2~4 GHz) (RL < -10 dB).

Claims

1. A method for preparing a submicron-sized sheet-like Fe4N magnetic microwave absorbing material, characterized in that: Includes the following steps: (1) The mixture of ultrapure iron concentrate, dispersant and dispersion medium is ground in a nano-sand mill, and then subjected to solid-liquid separation, washing and drying to obtain submicron-sized flake iron oxide powder. The grinding process uses coarse grinding beads with a diameter ranging from 0.2 mm to 0.5 mm and fine grinding beads with a diameter ranging from 0.05 mm to 0.15 mm. The mass ratio of coarse grinding beads to fine grinding beads is (0.5~2.0):1, the diameter ratio of coarse grinding beads to fine grinding beads is (2~5):1, the grinding speed is 2000 rpm to 3000 rpm, and the grinding time is 5~15 h. (2) Submicron-sized sheet-like iron oxide powder is placed in a mixed atmosphere of ammonia and hydrogen and subjected to a one-step reduction nitridation reaction at a temperature of 350~450℃ to obtain submicron-sized sheet-like Fe4N powder; (3) The submicron-sized sheet-like Fe4N powder is surface coated to obtain the submicron-sized sheet-like Fe4N magnetic absorbing material.

2. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, characterized in that: The main phase of the ultrapure iron concentrate is Fe3O4, with a purity greater than 99.5 wt%.

3. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, characterized in that: The dispersion medium includes water; The dispersant includes at least one of PVP, polyethylene glycol 400, and oleic acid.

4. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, 2, or 3, characterized in that: The mass content of ultrapure iron concentrate in the mixed slurry is 10% to 30%, and the mass of dispersant is 0.5% to 5% of the mass of ultrapure iron concentrate.

5. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 4, characterized in that: The pH value of the mixed slurry is adjusted to 9-11.

6. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, characterized in that: The drying process employs vacuum low-temperature drying or low-temperature drying under a protective atmosphere, with a temperature of 40~60℃ and a time of 4~12h.

7. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, characterized in that: The volume ratio of ammonia to hydrogen in the ammonia-hydrogen mixture atmosphere is (1~5):

1.

8. A method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1 or 7, characterized in that: The conditions for the reduction nitriding reaction are: temperature of 350~450℃, heating rate of 2~10℃ / min, and holding time of 4~8h.

9. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 1, characterized in that: The surface coating process is as follows: the flaky Fe4N powder is placed in a solution containing a coating agent, subjected to ultrasonic treatment, and then sequentially undergoes solid-liquid separation, washing, and drying.

10. The method for preparing a submicron-scale sheet-like Fe4N magnetic absorbing material according to claim 9, characterized in that: The solvent in the solution containing the coating agent is ethanol; The concentration of the coating agent in the solution containing the coating agent is 1~10 wt%; The coating agent includes at least one of polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, polyaniline, and polydopamine; The conditions for ultrasonic treatment are: ultrasonic frequency of 25~40 kHz, ultrasonic power of 100~300 W, and ultrasonic time of 10~30 min.

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