Fe@fe / sio2 gradient impedance electromagnetic wave absorbing material and preparation method thereof

Abstract

This invention relates to the field of electromagnetic wave absorbing materials, specifically to a Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material and its preparation method. The specific technical solution involves coating the surface of iron oxide nanoparticles with a SiO2 shell and performing high-temperature reduction heat treatment to transform the iron oxide core into a Fe magnetic core. Simultaneously, Fe single atoms, iron oxide, and Fe nanoparticles are loaded onto the inner wall of the SiO2 shell. This structure utilizes the SiO2 shell to block the continuous conductive network and suppress the skin effect, while the gradient impedance structure optimizes impedance matching to facilitate the entry of electromagnetic waves into the particle interior. The magnetic losses of the Fe magnetic core and the Fe magnetic particles on the inner wall of the SiO2 shell, as well as the dielectric losses of the Fe single atoms, iron oxide, and Fe magnetic nanoparticles, synergistically and efficiently dissipate electromagnetic waves. Furthermore, after reflection from the Fe magnetic core, the electromagnetic wave undergoes continuous reflection within the hollow cavity between the yolk and shell due to the gradient impedance structure, achieving efficient attenuation of electromagnetic wave energy.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave absorbing materials technology, specifically to a Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material and its preparation method. Background Technology

[0002] As modern electronic components become increasingly integrated and thinner, the internal electromagnetic environment of these devices is becoming more complex, leading to more prominent electromagnetic interference problems. While electromagnetic shielding materials can block interference, their reflected waves can easily cause secondary electromagnetic pollution. Electromagnetic wave absorbing materials, by converting electromagnetic energy into heat or other forms of energy, offer an effective solution. Therefore, developing thin, wide-bandwidth electromagnetic wave absorbing materials is not only crucial for ensuring the stable operation of electronic components but also for enhancing the anti-interference capabilities of equipment in complex electromagnetic environments.

[0003] The attenuation of electromagnetic waves depends on the impedance matching characteristics of the material surface and its dielectric and magnetic loss capabilities, but a single material often struggles to simultaneously achieve both of these factors. Research indicates that by constructing multilayered heterogeneous interfaces and microporous structures, the reflection and scattering of electromagnetic waves within the material can be increased, significantly improving energy dissipation efficiency while extending the propagation path. Among various structures, the yolk-shell structure serves as a typical example. Its unique internal cavity not only introduces continuous electromagnetic wave reflection, but the abundant heterogeneous interfaces also induce strong interfacial polarization. Furthermore, this structure combines the advantages of adjustable thickness, light weight, and high absorption efficiency, demonstrating great application potential in the development of broadband, high-efficiency microwave absorbing materials.

[0004] However, limitations remain in the composition control and spatial design of yolk-shell structure microwave absorbing materials, restricting further improvements in their absorption performance. Under alternating electromagnetic fields, significant polarization losses occur at the interface of yolk-shell structure microwave absorbing materials. The magnetic loss of the magnetic core improves impedance matching while synergistically enhancing electromagnetic wave absorption capacity. However, the significant differences in electromagnetic parameters between the shell and the core inevitably lead to significant reflection of incident electromagnetic waves at the material surface and interface, limiting the broadening of the electromagnetic wave absorption frequency band and the reduction of the thickness of the absorbing material.

[0005] Therefore, how to allow more electromagnetic waves to enter the interior of the material and synergistically enhance magnetic-dielectric loss under the influence of magnetism and electricity is a key technical problem facing yolk-shell structure microwave absorbing materials. Current technology lacks a material structure and preparation method that can simultaneously achieve impedance matching optimization and synergistic magnetic-dielectric loss enhancement through composition gradient design. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with an egg yolk-shell structure and its preparation method. By precisely controlling the material composition and structure, this invention constructs a material with an egg yolk-shell heterostructure. This material consists of an Fe magnetic core, controllable voids, and a SiO2 shell loaded with Fe single atoms, Fe nanoparticles, and iron oxide particles, from the inside out. This successfully constructs a three-layer Fe@Fe / SiO2 impedance gradient electromagnetic wave absorbing material with an egg yolk-shell structure.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material. The method involves coating the surface of iron oxide nanoparticles with a SiO2 shell and performing high-temperature reduction heat treatment to transform the iron oxide core into a Fe magnetic core. Simultaneously, Fe single atoms, iron oxide, and Fe nanoparticles are loaded onto the inner wall of the SiO2 shell.

[0008] Specifically, the following steps are included: Step 1: Dissolve sodium hydroxide and ferric chloride separately in deionized water, stir well, and obtain sodium hydroxide solution and ferric chloride solution respectively; Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a uniform rate and stir at medium and low temperature. After the addition is complete, continue stirring at the same temperature to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction. After the reaction, the product was centrifuged, washed and dried to obtain ferric oxide nanoparticles. The ferric oxide particles had a cubic structure. The particle size was controlled by adjusting the dropping rate of sodium hydroxide solution, and finally, uniform cubic ferric oxide particles with a structure of 100nm-1500nm were obtained.

[0009] Step 4: Disperse iron oxide nanoparticles in dilute hydrochloric acid solution for etching, and then obtain etched iron oxide particles by centrifugation and washing. Step 5: The etched iron oxide particles were ultrasonically dispersed in ethanol, and deionized water, concentrated ammonia and tetraethyl orthosilicate were added. The mixture was stirred at room temperature, then centrifuged, washed and dried to obtain Fe2O3@SiO2 core-shell structured particles. Step 6: The Fe2O3@SiO2 core-shell structured particles are subjected to high-temperature reduction heat treatment in a mixed atmosphere of hydrogen and inert gas to obtain Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with an egg yolk-shell structure.

[0010] Preferably, in step 1, the molar ratio of sodium hydroxide to ferric chloride is 1:1-5:1; in step 2, the ferric chloride solution is stirred at a temperature of 75℃-100℃ under medium and low temperature conditions; the rate of adding sodium hydroxide solution is 3-15 ml / min; and the stirring time after the addition is completed is 3 min-15 min; in step 3, the hydrothermal reaction temperature is 80℃-150℃, and the time is 1d-7d.

[0011] Preferably, in step 4, the concentration of the dilute hydrochloric acid is 0.01M-1M, the mass-to-volume ratio of the iron oxide nanoparticles to the hydrochloric acid is 50-500mg:50mL, and the ultrasonic time is 5min-50min.

[0012] Preferably, in step 5, the mass fraction of the concentrated ammonia is 28%-30%, and the volume ratio of the ethanol, deionized water, concentrated ammonia and tetraethyl orthosilicate is 60-100:15-25:0.8-1.2:0.05-5; the stirring time at room temperature is 3h-72h.

[0013] Preferably, in step 6, the volume concentration of hydrogen in the mixed atmosphere of hydrogen and inert gas is 20%-80%; the reduction heat treatment temperature is 400℃-800℃; the holding time is 1h-72h; and the gas flow rate of the mixed atmosphere of hydrogen and inert gas is 10mL / min-100mL / min.

[0014] Correspondingly, a Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material is provided, wherein the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material has a yolk-shell hollow structure, specifically, there is a gap between the Fe core and SiO2, and Fe single atoms, iron oxide and Fe nanoparticles are attached to the surface of the SiO2 shell.

[0015] Preferably, the innermost Fe core of the electromagnetic wave absorbing material has a particle size of 100nm-1400nm, the middle hollow layer has a particle size of 20nm-500nm, the outer SiO2 shell has a thickness of 5nm-100nm, and the outer SiO2 shell is loaded with Fe single atoms, iron oxide and Fe nanoparticles with a particle size of 3nm-30nm.

[0016] Accordingly, an electromagnetic wave absorber includes a matrix and the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material of claim 8 dispersed in the matrix; the matrix is ​​at least one of paraffin wax, epoxy resin, silicone rubber, polyurethane, polyvinylidene fluoride or polyimide; the mass fraction of the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material in the electromagnetic wave absorber is 40-95 wt%.

[0017] Preferably, the electromagnetic wave absorber has electromagnetic wave absorption performance in the 2-18GHz frequency band; and the electromagnetic wave absorber has an effective absorption bandwidth of greater than 5.0GHz when it is 0.8-2.0mm thick.

[0018] Correspondingly, an Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material or electromagnetic wave absorber is used in electromagnetic interference protection of electronic devices, microwave absorbing coatings, radar wave absorbing structural components, electromagnetic compatibility materials for communication equipment, or low detectability functional composite materials.

[0019] The present invention has the following beneficial effects: 1. This invention regulates the volume shrinkage of the Fe core during the reduction process, constructing a controllable void between the Fe core and the SiO2 shell. Simultaneously, Fe single atoms, iron oxide, and Fe nanoparticles are attached to the surface of the SiO2 shell, forming a unique yolk-shell structure with gradient impedance. This yolk-shell structure extends the propagation path of electromagnetic waves within the material and significantly enhances multiple reflections and scattering of electromagnetic waves within the particles, improving energy dissipation efficiency and thus broadening the effective absorption bandwidth.

[0020] 2. By precisely controlling the reduction temperature and reduction time, this invention successfully loads Fe single atoms, Fe nanoparticles, and iron oxide particles onto the SiO2 shell, overcoming the shortcomings of the traditional yolk-shell structure that relies on a single magnetic core to provide magnetic loss. It introduces multiple magnetic losses and significantly improves the overall magnetic response intensity of the material.

[0021] 3. This invention utilizes a multi-heterogeneous structure of Fe core-void-Fe particles (referring to Fe single atoms, iron oxide, and Fe nanoparticles)-SiO2 shell to successfully construct an impedance gradient at the micro-nano scale of a single particle, achieving excellent surface impedance matching. Simultaneously, abundant interfacial polarization and dipole polarization sites are constructed at the heterogeneous interface, optimizing the synergistic effect of dielectric and magnetic losses, and achieving efficient electromagnetic wave absorption in a wide frequency band of 2-18 GHz.

[0022] 4. The preparation process of this invention achieves the construction of a yolk-shell heterostructure, the control of cavity size, and the loading of Fe nanoparticles into the outer shell layer by changing the reduction conditions. The entire process is simple, highly reproducible, and uses low-cost, environmentally friendly raw materials, making it suitable for large-scale industrial production.

[0023] 5. The Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with yolk-shell structure described in this invention exhibits strong reflection loss and wide effective absorption bandwidth at relatively thin thicknesses, and has broad application prospects in electromagnetic protection fields such as 5G / 6G communications and national defense.

[0024] 6. The material preparation process of this invention is controllable, achieving strong absorption and wide-band absorption performance at an ultra-thin thickness, showing broad application potential in the fields of modern complex electromagnetic compatibility and high-performance stealth technology. Attached Figure Description

[0025] Figure 1 X-ray diffraction patterns of Fe@Fe / SiO2 prepared in Example 1 and Fe@Fe / SiO2 prepared in Example 2; Figure 2 Scanning electron microscope image of Fe@Fe / SiO2 prepared in Example 1; Figure 3 Scanning electron microscope image of Fe@Fe / SiO2 prepared in Example 2; Figure 4 Scanning electron microscope image of Fe@Fe / SiO2 prepared in Example 3; Figure 5 Transmission electron microscope image of Fe@Fe / SiO2 prepared in Example 3; Figure 6 The electromagnetic wave absorption performance of Fe@Fe / SiO22-18GHz prepared in Example 1; Figure 7 The electromagnetic wave absorption performance of Fe@Fe / SiO22-18GHz prepared in Example 2; Figure 8 The electromagnetic wave absorption performance of Fe@Fe / SiO22-18GHz prepared in Example 3; Figure 9 The electromagnetic wave absorption performance of Fe@Fe / SiO22-18GHz prepared in Example 4; Figure 10 The electromagnetic wave absorption performance of Fe@Fe / SiO22-18GHz prepared in Example 5. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Unless otherwise specified, the technical means used in the implementation examples are conventional means well known to those skilled in the art.

[0028] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0029] Example 1: Preparation of 200nm Fe@Fe / SiO2 (210nm-250nm) electromagnetic wave absorbing material Step 1: Weigh 10.8g of sodium hydroxide and 16.2g of anhydrous ferric chloride granules and dissolve them separately in 50ml of deionized water. After stirring evenly, sodium hydroxide solution and ferric chloride solution are obtained respectively. Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a rate of 12.5 ml / min and stir at 75°C. After the addition is complete, continue stirring at the same temperature for 5 min to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction at 100°C for 2 days. After the reaction was completed and cooled to room temperature, the product was centrifuged, washed and dried to obtain uniform cubic iron oxide nanoparticles with an average particle size of approximately 200 nm. Step 4: Weigh 0.1g of iron oxide nanoparticles and disperse them in 50mL of 0.1M dilute hydrochloric acid solution. Sonicate for 10min, then centrifuge and wash to obtain etched iron oxide particles. Step 5: The etched iron oxide particles were redispersed in 80 mL of ethanol, and 20 mL of deionized water, 1 mL of concentrated ammonia and 0.1 g of tetraethyl orthosilicate (TEOS) were added in sequence. The mixture was stirred at room temperature for 6 h, then centrifuged, washed and dried to obtain Fe2O3@SiO2 particles. The SiO2 shell coating thickness of the particles was characterized to be about 10 nm. Step 6: Place the Fe2O3@SiO2 powder in a 30%H2 / 70%Ar hydrogen-argon mixed atmosphere for high-temperature reduction heat treatment and keep it at 550℃ for 18h to obtain Fe@Fe / SiO2 gradient impedance absorbing material.

[0030] The Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with an egg yolk-shell structure prepared in the embodiments of the present invention was subjected to XRD pattern testing. Based on the X-ray diffraction results, 2 θ Fe characteristic diffraction peaks are present at 44.6°, 65.0°, and 82.3°, corresponding to the (110), (200), and (211) crystal planes of α-Fe. No SiO2 characteristic peaks are observed, indicating that SiO2 exists in an amorphous form. (See...) Figure 1 As shown.

[0031] Scanning electron microscopy was used to observe the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with a yolk-shell structure prepared in the embodiments of the present invention. The obtained sample showed a uniform cubic particle morphology. The SiO2 shell coating suppressed the agglomeration of magnetic Fe particles during the reduction process. The overall particle size was between 210 nm and 250 nm. Figure 2 As shown.

[0032] The obtained absorbing material was mixed with paraffin at a filling ratio of 80 wt%, and pressed into a coaxial ring sample. The complex permittivity and complex permeability were measured using a vector network analyzer, and the reflection loss was calculated based on transmission line theory. Test results show that the material achieves excellent effective absorption bandwidth at an ultrathin thickness. The material achieves an effective absorption bandwidth of 6.08 GHz at a thickness of approximately 1.4 mm, with a minimum reflection loss of -32.35 dB. (See [link to relevant documentation]). Figure 6 As shown.

[0033] Example 2: Preparation of 500nm Fe@Fe / SiO2 (510nm-530nm) electromagnetic wave absorbing material Step 1: Weigh 10.8g of sodium hydroxide and 27.03g of ferric chloride hexahydrate granules and dissolve them separately in 50ml of deionized water. After stirring evenly, we obtain sodium hydroxide solution and ferric chloride solution. Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a rate of 10 ml / min and stir at 100°C. After the addition is complete, continue stirring at the same temperature for 5 min to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction at 100°C for 4 days. After the reaction was completed and cooled to room temperature, the resulting reaction product was centrifuged, washed, and dried to obtain uniform cubic iron oxide nanoparticles with an average particle size of approximately 500 nm. Step 4: Weigh 0.1g of iron oxide nanoparticles and disperse them in 50mL of 0.1M dilute hydrochloric acid solution. Sonicate for 10min, then centrifuge and wash to obtain etched iron oxide particles. Step 5: The etched iron oxide particles were redispersed in 80 mL of ethanol, and 20 mL of deionized water, 1 mL of concentrated ammonia and 0.1 g of tetraethyl orthosilicate (TEOS) were added in sequence. The mixture was stirred at room temperature for 6 h, then centrifuged, washed and dried to obtain Fe2O3@SiO2. The SiO2 shell coating thickness of the particles was characterized to be about 10 nm. Step 6: Fe2O3@SiO2 powder was subjected to high-temperature reduction heat treatment in a 40%H2 / 60%Ar hydrogen-argon mixed atmosphere for 18 hours at 550℃ to obtain Fe@Fe / SiO2 gradient impedance absorbing material. The overall particle size was between 510nm and 530nm, and the SiO2 shell coating thickness was approximately 10nm. See [link to relevant documentation]. Figure 3 As shown.

[0034] The Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with an egg yolk-shell structure prepared in this embodiment of the invention was subjected to XRD spectrum analysis. The test results were similar to those of Example 1. θ Fe characteristic diffraction peaks are present at 44.6°, 65.0°, and 82.3°, corresponding to the (110), (200), and (211) crystal planes of α-Fe. No SiO2 characteristic peaks are observed, indicating that SiO2 exists in an amorphous form. (See...) Figure 1 As shown.

[0035] The obtained material was composited with paraffin at a filling ratio of 80 wt% and then subjected to electromagnetic parameter testing. The test results show that the material achieves excellent effective absorption bandwidth at an ultrathin thickness. The material achieves an effective absorption bandwidth of 7.2 GHz at a thickness of approximately 1.4 mm, with a minimum reflection loss of -24.7 dB. (See [link to relevant documentation]). Figure 7 As shown.

[0036] Example 3: Preparation of 1μm Fe@Fe / SiO2 (1020nm-1050nm) electromagnetic wave absorbing material Step 1: Weigh 5.4g of sodium hydroxide and 13.515g of ferric chloride hexahydrate granules respectively and dissolve them in 25mL of deionized water. After stirring evenly, a sodium hydroxide solution and a ferric chloride solution without precipitate particles are obtained. Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a rate of 6 ml / min and stir at 75°C. After the addition is complete, continue stirring at the same temperature for 5 min to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction at 100°C for 4 days. After the reaction was completed and cooled to room temperature, the resulting reaction product was centrifuged, washed, and dried to obtain uniform cubic iron oxide nanoparticles with an average particle size of approximately 1 μm. Step 4: Weigh 0.1g of iron oxide nanoparticles and disperse them in 50mL of 0.1M dilute hydrochloric acid solution. Sonicate for 10min, then centrifuge and wash to obtain etched iron oxide particles. Step 5: The etched iron oxide particles were redispersed in 80 mL of ethanol, and 20 mL of deionized water, 1 mL of concentrated ammonia and 0.1 g of tetraethyl orthosilicate (TEOS) were added in sequence. The mixture was stirred at room temperature for 6 h, then centrifuged, washed and dried to obtain Fe2O3@SiO2. The SiO2 shell coating thickness of the particles was characterized to be about 10 nm. Step 6: Fe2O3@SiO2 powder was subjected to high-temperature reduction heat treatment in a 40%H2 / 60%Ar hydrogen-argon mixed atmosphere for 18 hours at 550℃ to obtain Fe@Fe / SiO2 gradient impedance absorbing material. The overall particle size was between 1020nm and 1050nm, and the SiO2 shell coating thickness was approximately 10nm. See [link to relevant documentation]. Figure 4 As shown.

[0037] The Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with an egg yolk-shell structure prepared in the embodiments of the present invention was observed by transmission electron microscopy, and its TEM image is shown below. Figure 5 As shown. The cubic particles are approximately 1 μm in size, with a SiO2 shell coating thickness of approximately 10 nm. The interior consists of irregularly shaped spherical Fe cores, surrounded by a SiO2 shell. A distinct cavity is formed between the Fe cores and the SiO2 shell. Fe particles (referring to Fe single atoms, iron oxide, and Fe nanoparticles) are uniformly loaded on the inner surface of the SiO2 shell, with a size of approximately 20 nm-25 nm. This ultimately forms a three-layer gradient structure of Fe@Fe / SiO2, consisting of Fe cores, voids, Fe particles, and a SiO2 shell, exhibiting a yolk-shell structure. (See...) Figure 5 As shown.

[0038] The obtained material was composited with paraffin at a filling ratio of 80 wt% and then subjected to electromagnetic parameter testing. The test results show that the material has a thinner matching thickness corresponding to the maximum effective absorption bandwidth. The material achieves an effective absorption bandwidth of 7.12 GHz at a thickness of approximately 1.2 mm, and a minimum reflection loss of -21.4 dB. (See [link to relevant documentation]). Figure 8 As shown.

[0039] Example 4: Preparation of 500nm Fe@Fe / SiO2 (550nm-610nm) electromagnetic wave absorbing material Step 1: Weigh 10.8g of sodium hydroxide and 27.03g of ferric chloride hexahydrate granules respectively and dissolve them in 50ml of deionized water. After stirring evenly, a sodium hydroxide solution and a ferric chloride solution without precipitate particles are obtained. Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a rate of 10 ml / min and stir at 100°C. After the addition is complete, continue stirring at the same temperature for 5 min to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction at 100°C for 4 days. After the reaction was completed and cooled to room temperature, the resulting reaction product was centrifuged, washed, and dried to obtain uniform cubic iron oxide nanoparticles with an average particle size of approximately 500 nm. Step 4: Weigh 0.1g of iron oxide nanoparticles and disperse them in 50mL of 0.1M dilute hydrochloric acid solution. Sonicate for 10min, then centrifuge and wash to obtain etched iron oxide particles. Step 5: The etched iron oxide particles were redispersed in 80 mL of ethanol, and 20 mL of deionized water, 1 mL of concentrated ammonia and 0.8 g of tetraethyl orthosilicate (TEOS) were added in sequence. The mixture was stirred at room temperature for 36 h, then centrifuged, washed and dried to obtain Fe2O3@SiO2. The SiO2 shell coating thickness of the particles was characterized to be about 50 nm. Step 6: Place Fe2O3@SiO2 powder in a 40%H2 / 60%Ar hydrogen-argon mixed atmosphere for high-temperature reduction heat treatment and hold at 550℃ for 18h to obtain Fe@Fe / SiO2 gradient impedance microwave absorbing material, wherein the overall particle size is between 550nm and 610nm and the SiO2 shell coating thickness is about 50nm.

[0040] The obtained material was composited with paraffin at a filling ratio of 80 wt% and then subjected to electromagnetic parameter testing. The test results showed that, compared with Example 2, the effective absorption bandwidth decreased with increasing SiO2 shell thickness. The material achieved an effective absorption bandwidth of 5.04 GHz and a minimum reflection loss of -44.2 dB at a thickness of approximately 1.5 mm. Figure 9 As shown.

[0041] Example 5: 1μm Fe@Fe / SiO2 (1050nm-1070nm) electromagnetic wave absorbing material Step 1: Weigh 5.4g of sodium hydroxide and 13.515g of ferric chloride hexahydrate granules respectively and dissolve them in 25mL of deionized water. After stirring evenly, a sodium hydroxide solution and a ferric chloride solution without precipitate particles are obtained. Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a rate of 6 ml / min and stir at 75°C. After the addition is complete, continue stirring at the same temperature for 5 min to obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was transferred to a high-pressure reactor for hydrothermal reaction at 100°C for 4 days. After the reaction was completed and cooled to room temperature, the resulting reaction product was centrifuged, washed, and dried to obtain uniform cubic iron oxide nanoparticles with an average particle size of approximately 1 μm. Step 4: Weigh 0.1g of iron oxide nanoparticles and disperse them in 50mL of 0.1M dilute hydrochloric acid solution. Sonicate for 10min, then centrifuge and wash to obtain etched iron oxide particles. Step 5: The etched iron oxide particles were redispersed in 80 mL of ethanol, and 20 mL of deionized water, 1 mL of concentrated ammonia and 0.1 g of tetraethyl orthosilicate (TEOS) were added in sequence. The mixture was stirred at room temperature for 18 h, then centrifuged, washed and dried to obtain Fe2O3@SiO2. The SiO2 shell coating thickness of the particles was characterized to be about 20 nm. Step 6: Place Fe2O3@SiO2 powder in a 40%H2 / 60%Ar hydrogen-argon mixed atmosphere for high-temperature reduction heat treatment and hold at 550℃ for 48h to obtain 1μm Fe@Fe / SiO2 gradient impedance absorbing material, wherein the overall particle size is between 1050nm and 1070nm and the SiO2 shell coating thickness is about 20nm.

[0042] The obtained material was composited with paraffin at a filling ratio of 80 wt% and then subjected to electromagnetic parameter testing. The test results showed that, compared with Example 3, the effective absorption bandwidth slightly decreased after prolonged heat preservation time. The material achieved an effective absorption bandwidth of 7.08 GHz with a thickness of approximately 1.3 mm and a minimum reflection loss of -37.5 dB. (See [link to relevant documentation]). Figure 10 As shown.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material, characterized in that: By coating the surface of iron oxide nanoparticles with a SiO2 shell and subjecting them to high-temperature reduction heat treatment, the iron oxide core is transformed into an Fe magnetic core. At the same time, Fe single atoms, iron oxide, and Fe magnetic nanoparticles are loaded on the inner wall of the SiO2 shell.

2. The preparation method according to claim 1, characterized in that: Includes the following steps: (1) Iron oxide nanoparticles were dispersed in dilute hydrochloric acid solution for etching, and then the etched iron oxide particles were obtained by centrifugation and washing. (2) The etched iron oxide particles were dispersed in ethanol, and deionized water, concentrated ammonia and tetraethyl orthosilicate were added. The mixture was stirred at room temperature, then centrifuged, washed and dried to obtain Fe2O3@SiO2 core-shell structured particles. (3) Fe2O3@SiO2 core-shell structured particles are subjected to high-temperature reduction heat treatment in a mixed atmosphere of hydrogen and inert gas to obtain Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material with egg yolk-shell structure.

3. The preparation method according to claim 2, characterized in that: In step (1), the preparation of the iron oxide nanoparticles includes the following steps: Step 1: Dissolve sodium hydroxide and ferric chloride separately in deionized water, stir well, and obtain sodium hydroxide solution and ferric chloride solution respectively; Step 2: Add sodium hydroxide solution dropwise to ferric chloride solution at a uniform rate and stir at medium and low temperature. After the addition is complete, continue stirring to react and obtain a reddish-brown ferric hydroxide gel precursor. Step 3: The ferric hydroxide gel precursor was subjected to a hydrothermal reaction. After the reaction was completed, the product was centrifuged, washed and dried to obtain ferric oxide nanoparticles.

4. The preparation method according to claim 3, characterized in that: In step 1, the molar ratio of sodium hydroxide to ferric chloride is 1:1-5:1; in step 2, the ferric chloride solution is stirred at a temperature of 75℃-100℃ under medium and low temperature conditions; the sodium hydroxide solution is added dropwise at a rate of 3-15 ml / min; and the stirring time continues for 3-15 min after the addition is completed; in step 3, the hydrothermal reaction temperature is 80℃-150℃, and the time is 1-7 days.

5. The preparation method according to claim 2, characterized in that: In step (1), the mass-to-volume ratio of the iron oxide nanoparticles to hydrochloric acid is 50-500 mg: 50 mL; dispersion is performed by ultrasound for 5-50 min.

6. The preparation method according to claim 2 or 5, characterized in that: In step (2), the mass fraction of the concentrated ammonia is 28%-30%, and the volume ratio of the ethanol, deionized water, concentrated ammonia and tetraethyl orthosilicate is 60-100:15-25:0.8-1.2:0.05-5; the stirring time at room temperature is 3h-72h.

7. The preparation method according to claim 6, characterized in that: In step (3), the volume concentration of hydrogen in the mixed atmosphere of hydrogen and inert gas is 20%-80%; the heat treatment temperature for reduction is 400℃-800℃; the holding time is 1h-72h; and the gas flow rate of the mixed atmosphere of hydrogen and inert gas is 10mL / min-100mL / min.

8. A Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material prepared by the preparation method according to claims 1-7, characterized in that: The Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material has a hollow structure of egg yolk and shell, specifically, there is a gap between the Fe core and SiO2, and Fe single atoms, iron oxide and Fe nanoparticles are attached to the surface of the SiO2 shell.

9. An electromagnetic wave absorber, characterized in that, The material comprises a matrix and the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material of claim 8 dispersed in the matrix; the matrix is ​​at least one of paraffin, epoxy resin, silicone rubber, polyurethane, polyvinylidene fluoride or polyimide; the mass fraction of the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material in the electromagnetic wave absorber is 40-95 wt%.

10. The application of the Fe@Fe / SiO2 gradient impedance electromagnetic wave absorbing material of claim 8 or the electromagnetic wave absorber of claim 9 in electromagnetic interference protection of electronic devices, microwave absorbing coatings, radar wave absorbing structures, electromagnetic compatibility materials for communication equipment, or low detectability functional composite materials.

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