Lignin carbon-coated neodymium iron oxide wave-absorbing material, preparation method and application thereof

Abstract

The application discloses a lignin carbon-coated neodymium-iron oxide wave-absorbing material and a preparation method and application thereof, and the preparation method comprises the following steps: mixing neodymium acetylacetone, iron acetylacetone, a solvent, a precipitating agent and a surfactant to prepare a neodymium-iron precursor mixed solution, and after reaction, a neodymium-iron oxide magnetic material is obtained; the neodymium-iron oxide magnetic material is dissolved in anhydrous ethanol together with lignin, and after dialysis in water, a lignin-coated neodymium-iron oxide nanosphere aqueous solution is obtained; and after freeze-drying and heat treatment, the lignin carbon-coated neodymium-iron oxide wave-absorbing material is obtained. The wave-absorbing material prepared by the application not only can maintain excellent magnetism, but also can improve impedance matching, and the microwave absorption performance is further enhanced by coupling with the magnetic performance; the wave-absorbing material can be applied to the field of microwave absorption, and the wave-absorbing material has acid and alkali resistance, oxidation resistance and corrosion resistance, and can be used for electromagnetic wave absorption in a harsh environment.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorption technology, specifically relating to a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, its preparation method, and its application. Background Technology

[0002] Microwave absorbing materials are electromagnetic pollution control materials that can effectively absorb electromagnetic radiation without causing secondary pollution. Currently, existing microwave absorbing materials mainly employ a composite design of dielectric and magnetic losses. By synergistically utilizing multiple loss mechanisms such as dielectric polarization, conductivity loss, hysteresis loss, and natural resonance, the absorption intensity and effective absorption bandwidth coverage of the materials are significantly improved. Among these, carbon materials and ferrite materials such as Fe3O4 have been extensively studied due to their excellent dielectric and magnetic properties. However, traditional magnetic materials such as Fe3O4 suffer from drawbacks such as easy agglomeration, poor thermal stability, poor impedance matching, narrow magnetic loss band, and poor chemical properties, making them unsuitable for microwave absorption in complex environments.

[0003] Biochar, with its abundant sources and low cost, has become an ideal carbon source for microwave absorption in recent years. However, existing composites of biochar and magnetic materials still face significant bottlenecks, severely restricting the performance improvement and practical application of microwave absorbing materials. For example, the morphology of biochar is often difficult to control, making it hard to meet the needs of multi-band microwave absorbing materials. Composites of other non-biochar materials (such as graphene and carbon nanotubes) with magnetic materials suffer from high preparation costs and difficulties in large-scale production. Regarding the composite process of biochar and magnetic materials, especially in terms of interface structure control, simple preparation processes often only achieve the macroscopic superposition of the two components, making it difficult to optimize the interface and structure. Complex preparation processes, such as template methods, require a demolding process to remove the template, resulting in complex and costly methods that negate the natural advantages of the abundant sources and low cost of biochar materials. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a lignin-carbon-coated neodymium iron oxide microwave absorbing material that can maintain excellent magnetism, improve impedance matching, have strong microwave absorption performance, and can absorb electromagnetic waves in harsh environments, as well as its preparation method and application.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0006] A method for preparing a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material includes the following steps:

[0007] (1) NdFeO, acetylacetone, solvent, precipitant and surfactant are mixed and stirred to obtain NdFeO precursor mixed solution. The NdFeO precursor mixed solution is reacted at 220℃~280℃. After the reaction, the supernatant is removed by centrifugation, and after washing and drying, NdFeO oxide magnetic material is obtained. The NdFeO oxide magnetic material is NdFeO3·Fe3O4 magnetic powder.

[0008] (2) The obtained neodymium iron oxide magnetic material and lignin were added to anhydrous ethanol to obtain a neodymium iron oxide-lignin mixture. The obtained neodymium iron oxide-lignin mixture was dialyzed in water to encapsulate neodymium iron oxide to form nanospheres, and an aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres was obtained.

[0009] (3) The aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres obtained above is first freeze-dried, and then heated to 650℃~950℃ under an inert atmosphere or nitrogen atmosphere for heat treatment to carbonize the lignin and obtain lignin carbon-encapsulated neodymium iron oxide microwave absorbing material.

[0010] In the preferred embodiment of the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, in step (1), the molar ratio of neodymium acetylacetonate to iron acetylacetonate is 2:14-17, the amount of solvent used is such that the total concentration of neodymium ions and iron ions is 0.1 mol / L-0.4 mol / L, the amount of precipitant used is 1.5 times to 2 times the total molar amount of neodymium ions and iron ions, and the amount of surfactant used is 20% to 40% of the total mass of neodymium acetylacetonate and iron acetylacetonate.

[0011] In the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, preferably, in step (1), the solvent is one or both of ethylene glycol and propylene glycol, the precipitant is one or both of urea and hexamethylenetetramine, and the surfactant is one or both of polyethylene glycol and polyvinylpyrrolidone.

[0012] In the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, preferably, in step (1), the reaction time is 8h to 12h, the centrifugation speed is 8000rpm to 10000rpm, the centrifugation time is 10min to 15min, the cleaning is performed by alternating cleaning with anhydrous ethanol and water, the drying is vacuum drying, the vacuum drying temperature is 60℃ to 80℃, and the vacuum drying time is 12h to 18h.

[0013] In the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, preferably, in step (2), the lignin is one or more of sulfate lignin, enzymatically hydrolyzed lignin, alkali lignin, or organic solvent lignin; the mass ratio of the neodymium iron oxide magnetic material to the lignin is 0.5 to 2:1, and the mass-volume ratio of the lignin to the anhydrous ethanol is 1 mg to 4 mg: 1 mL.

[0014] In the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, preferably, in step (2), the neodymium iron oxide magnetic material and lignin are added to anhydrous ethanol and then subjected to ultrasound until the lignin is completely dissolved in anhydrous ethanol. The dialysis time is 24h to 48h, and the dialysis is carried out using a dialysis bag with a molecular weight of 8kDa to 14kDa.

[0015] In the above-mentioned method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, preferably, in step (3), the freeze-drying temperature is -50℃ to -60℃, the freeze-drying time is 24h to 48h, the heat treatment time is 2h to 4h, and the heating rate is 2℃ / min to 5℃ / min.

[0016] As a general technical concept, the present invention also provides a lignin-carbon-coated neodymium iron oxide microwave absorbing material prepared by the above-mentioned method for preparing lignin-carbon-coated neodymium iron oxide microwave absorbing material.

[0017] Preferably, the average particle size of the lignin-carbon-coated neodymium iron oxide microwave absorbing material is 200 nm to 800 nm.

[0018] As a general technical concept, the present invention also provides an application of the above-mentioned lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material in the field of microwave absorption.

[0019] Compared with the prior art, the advantages of the present invention are as follows:

[0020] (1) This invention provides a method for preparing a lignin-carbon-coated neodymium iron oxide (NdFeO) microwave absorbing material. This method uses a two-step process of solvothermal and thermal decomposition to prepare lignin-carbon-coated NdFeO nanoparticles. By coating the NdFeO nanoparticles (magnetic particles) with lignin-carbon, not only can excellent magnetism be maintained, but also defects are introduced due to the difference in surface charge density between lignin and magnetic particles, which adjusts the electromagnetic parameters, improves impedance matching, and further enhances the microwave absorption performance of the absorbing material by coupling dielectric properties with magnetic properties. This provides a variety of loss mechanisms for microwave absorption, such as dielectric loss and conductivity loss of lignin-carbon and magnetic loss of magnetic particles. The composite of lignin-carbon and NdFeO gives the material a rich heterogeneous interface, which enhances the interfacial polarization loss. The defects caused by the difference in charge density between the two can act as polarization centers, inducing dipole polarization, leading to an increase in polarization loss and enhancing dielectric loss.

[0021] (2) In the preparation method of this invention, lignin is not only used to encapsulate NdFeO nanoparticles, but also as a reducing agent in the high-temperature reduction process of NdFeO to reduce iron and NdFeO ions, forming a carbon coating on the surface of NdFeO. This not only reduces the steps required for removing non-magnetic phase impurities after using other reducing agents, but the lignin carbon can also serve as a protective layer for the NdFeO nanoparticles, reducing their contact with air and increasing the acid and alkali resistance, oxidation resistance, and corrosion resistance of the magnetic particles. Therefore, the lignin carbon-encapsulated NdFeO microwave absorbing material prepared by this invention can be applied to microwave absorbing coatings in harsh environments. Furthermore, this invention opens up new areas for the application of lignin. This method has a simple preparation process, uses environmentally friendly solvents, is low in cost, and has good repeatability.

[0022] (3) In this invention, when the neodymium iron oxide-lignin mixture is dialyzed in water (preferably deionized water), water and anhydrous ethanol slowly replace each other. As the water content of the system increases, lignin forms nanospheres through π-π stacking under the amphiphilic and hydrophobic effects. This invention can control the size of the nanospheres by adjusting the lignin concentration and the heat treatment temperature. Generally, the higher the lignin concentration, the more lignin forms nanospheres, and the larger the size of the prepared nanospheres. The higher the heat treatment temperature, the more intense the condensation and pyrolysis reactions of lignin molecules, and the smaller the size of the nanospheres. Microwave absorption performance is greatly affected by the intrinsic electromagnetic parameters of the material. Therefore, this invention enhances microwave absorption performance by controlling the changes in the physical and chemical properties of the material through controlling the morphology of the material.

[0023] (4) The lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material of the present invention can be widely used in the field of microwave absorption, such as communication, medical devices, transportation and other fields. At the same time, the present invention also opens up new fields for the utilization of lignin. Attached Figure Description

[0024] Figure 1 The image shows the X-ray diffraction (XRD) pattern of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1 of this invention.

[0025] Figure 2 The X-ray photoelectron spectra (a), C 1s spectrum (b), O 1s spectrum (c), Fe 2p spectrum (d), and Nd 3d spectrum (e) of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1 of this invention are shown.

[0026] Figure 3 Scanning electron microscope (a) and transmission electron microscope (b) images of lignin-encapsulated neodymium iron oxide nanospheres prepared in Example 1 of this invention.

[0027] Figure 4 The images shown are: scanning electron microscope (a), transmission electron microscope (b), high-angle annular dark field image (c), C elemental mapping (d), Fe elemental mapping (e), and Nd elemental mapping (f) of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1 of this invention.

[0028] Figure 5 The magnetic hysteresis loop diagrams (a) and (b) of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1 and Comparative Example 1 are shown.

[0029] Figure 6 The images show the 2D reflection loss diagram (a) and 3D reflection loss diagram (b) of the neodymium iron oxide microwave absorbing material prepared in Comparative Example 1, and the 2D reflection loss diagram (c) and 3D reflection loss diagram (d) of the lignin carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1.

[0030] Figure 7 The reflection loss diagrams of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material prepared in Example 1 of this invention at different thicknesses are shown. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. The materials and instruments used in the following embodiments are all commercially available. Specifically, neodymium acetylacetone was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., iron acetylacetone from Jiangsu Aikon Biomedical R&D Co., Ltd., and lignin sulfate from Yueyang Forest & Paper Co., Ltd.

[0032] Example 1

[0033] A method for preparing a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to the present invention includes the following steps:

[0034] (1) Weigh 2 mmol (0.88 g) of neodymium acetylacetone and 14 mmol (4.94 g) of iron acetylacetone and dissolve them in 60 mL of ethylene glycol. Under magnetic stirring, add 1.5 g (25 mmol) of urea and 2 g of polyethylene glycol 10000 (PEG-10000). After stirring evenly, a neodymium-iron precursor mixed solution is obtained. Pour the neodymium-iron precursor mixed solution into a polytetrafluoroethylene-lined reactor and react at 280 °C for 12 h. After the reaction is complete, pour the solution into a centrifuge tube and centrifuge at 8000 rpm for 15 min. Remove the supernatant and collect the precipitate. Wash the collected precipitate three times alternately with anhydrous ethanol and deionized water. Specifically, the precipitate is first ultrasonically dispersed with anhydrous ethanol, washed, and centrifuged to collect the precipitate. Then, the precipitate is ultrasonically dispersed with deionized water, washed, and centrifuged to collect the precipitate. Repeat this operation three times. The washed precipitate was redispersed in anhydrous ethanol and vacuum dried at 60°C for 12 hours to obtain neodymium iron oxide magnetic material, which is NdFeO3·Fe3O4 magnetic powder.

[0035] (2) Dissolve 20 mg of lignin sulfate in 10 mL of anhydrous ethanol, then add 20 mg of neodymium iron oxide magnetic material, and sonicate for 30 min to completely dissolve the lignin sulfate in the anhydrous ethanol, obtaining a neodymium iron oxide-lignin mixture. Pour the neodymium iron oxide-lignin mixture into a 14 kDa dialysis bag, place it in deionized water, and place it on a magnetic stirrer to continuously rotate it in the deionized water to increase the replacement rate of anhydrous ethanol and deionized water. Change the deionized water periodically and dialyze for 24 h to allow the lignin to encapsulate the neodymium iron oxide to form nanospheres, obtaining an aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres.

[0036] (3) The aqueous solution of lignin-encapsulated NdFeO nanospheres was freeze-dried at -52℃ for 24h, and then placed in a tube furnace and heated to 650℃ for 2h in a nitrogen atmosphere at a rate of 5℃ / min to carbonize the lignin. The material was then naturally cooled to room temperature to obtain lignin-carbon-encapsulated NdFeO microwave absorbing material, denoted as NdFeO@C-650.

[0037] Example 2

[0038] A method for preparing a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to the present invention includes the following steps:

[0039] (1) Weigh 2 mmol of neodymium acetylacetone and 14 mmol of iron acetylacetone and dissolve them in 60 mL of ethylene glycol. Add 1.5 g of urea and 2 g of polyethylene glycol 10000 under magnetic stirring. After stirring evenly, a neodymium-iron precursor mixed solution is obtained. Pour the neodymium-iron precursor mixed solution into a reaction vessel lined with polytetrafluoroethylene and react at 280 °C for 12 h. After the reaction is completed, pour the solution into a centrifuge tube and centrifuge at 8000 rpm for 15 min. Remove the supernatant and collect the precipitate. Wash the collected precipitate three times alternately with anhydrous ethanol and deionized water. Specifically, the precipitate is first ultrasonically dispersed with anhydrous ethanol, washed, and centrifuged to collect the precipitate. Then, the precipitate is ultrasonically dispersed with deionized water, washed, and centrifuged to collect the precipitate. Repeat this operation three times. The washed precipitate was redispersed in anhydrous ethanol and vacuum dried at 60°C for 12 hours to obtain neodymium iron oxide magnetic material, which is NdFeO3·Fe3O4 magnetic powder.

[0040] (2) Dissolve 20 mg of lignin sulfate in 10 mL of anhydrous ethanol, then add 20 mg of neodymium iron oxide magnetic material, and sonicate for 30 min to completely dissolve the lignin sulfate in the anhydrous ethanol, obtaining a neodymium iron oxide-lignin mixture. Pour the neodymium iron oxide-lignin mixture into a 14 kDa dialysis bag, place it in deionized water, and place it on a magnetic stirrer to continuously rotate it in the deionized water to increase the replacement rate of anhydrous ethanol and deionized water. Change the deionized water periodically and dialyze for 24 h to allow the lignin to encapsulate the neodymium iron oxide to form nanospheres, obtaining an aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres.

[0041] (3) The aqueous solution of lignin-encapsulated NdFeO nanospheres was freeze-dried at -52℃ for 24h, and then placed in a tube furnace and heated to 700℃ for 2h in a nitrogen atmosphere at a rate of 5℃ / min to carbonize the lignin. The solution was then cooled to room temperature to obtain lignin-carbon-encapsulated NdFeO microwave absorbing material, denoted as NdFeO@C-700.

[0042] Example 3

[0043] A method for preparing a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to the present invention includes the following steps:

[0044] (1) Weigh 2 mmol of neodymium acetylacetone and 14 mmol of iron acetylacetone and dissolve them in 60 mL of ethylene glycol. Add 1.5 g of urea and 2 g of polyethylene glycol 10000 under magnetic stirring. After stirring evenly, a neodymium-iron precursor mixed solution is obtained. Pour the neodymium-iron precursor mixed solution into a reaction vessel lined with polytetrafluoroethylene and react at 280 °C for 12 h. After the reaction is completed, pour the solution into a centrifuge tube and centrifuge at 8000 rpm for 15 min. Remove the supernatant and collect the precipitate. Wash the collected precipitate three times alternately with anhydrous ethanol and deionized water. Specifically, the precipitate is first ultrasonically dispersed with anhydrous ethanol, washed, and centrifuged to collect the precipitate. Then, the precipitate is ultrasonically dispersed with deionized water, washed, and centrifuged to collect the precipitate. Repeat this operation three times. The washed precipitate was redispersed in anhydrous ethanol and vacuum dried at 60°C for 12 hours to obtain neodymium iron oxide magnetic material, which is NdFeO3·Fe3O4 magnetic powder.

[0045] (2) Dissolve 20 mg of lignin sulfate in 10 mL of anhydrous ethanol, then add 20 mg of neodymium iron oxide magnetic material, and sonicate for 30 min to completely dissolve the lignin sulfate in the anhydrous ethanol, obtaining a neodymium iron oxide-lignin mixture. Pour the neodymium iron oxide-lignin mixture into a 14 kDa dialysis bag, place it in deionized water, and place it on a magnetic stirrer to continuously rotate it in the deionized water to increase the replacement rate of anhydrous ethanol and deionized water. Change the deionized water periodically and dialyze for 24 h to allow the lignin to encapsulate the neodymium iron oxide to form nanospheres, obtaining an aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres.

[0046] (3) The aqueous solution of lignin-encapsulated NdFeO nanospheres was freeze-dried at -52℃ for 24h, and then placed in a tube furnace and heated to 750℃ for 2h in a nitrogen atmosphere at a rate of 5℃ / min to carbonize the lignin. The solution was then cooled to room temperature to obtain lignin-carbon-encapsulated NdFeO microwave absorbing material, denoted as NdFeO@C-750.

[0047] Comparative Example 1

[0048] A method for preparing a neodymium iron oxide microwave absorbing material is basically the same as that in Example 1, except that step (2) is not performed.

[0049] Characterization analysis:

[0050] (1) X-ray diffraction characterization analysis of lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material

[0051] The NdFeO@C-650 prepared in Example 1 was subjected to X-ray diffraction analysis (XRD, Rigaku SmartLabSE, Japan, 10°-90°, 2° / min), and the results are as follows: Figure 1As shown. By Figure 1 It can be seen that the microwave absorbing material is a multiphase composite material composed of C, NdFeO3, Fe2O3 and Fe3O4. There is a diffraction peak at 22°, corresponding to the (120) crystal plane of C. There are 13 diffraction peaks at 26°, 32°, 39°, 40°, 46°, 48°, 53°, 58°, 68°, 74°, 78°, 83° and 87°, corresponding to the (111), (121), (220), (022) and (202) crystal planes of NdFeO3. The spectrum shows six diffraction peaks at 30°, 35°, 43°, 50°, 57°, and 63°, corresponding to the (220), (311), (400), (421), (511), and (440) crystal planes of Fe2O3, and two diffraction peaks at 37° and 46°, corresponding to the (311) and (400) crystal planes of Fe3O4. The peaks in the spectrum are relatively sharp, indicating that the overall crystallinity of the absorbing material is good.

[0052] (2) X-ray photoelectron spectroscopy characterization and analysis of lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material

[0053] The elemental composition and chemical valence state of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material obtained in Example 1 were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA). The C 1s peak at 284.8 eV was used as an internal standard to ensure the accuracy and reliability of the chemical bond state determination. The results are as follows: Figure 2 As shown.

[0054] Depend on Figure 2 (a) shows that the main characteristic peaks confirm the presence of C 1s, O 1s, Fe 2p, and Nd 3d. For example... Figure 2 As shown in (b), this absorbing material mainly contains C=C single bonds, CO single bonds, and C=O double bonds. Figure 2 As shown in (c), the O 1s spectrum peak at 533.75 eV corresponds to adsorbed oxygen, the peak at 532.32 eV corresponds to CO, and the peak at 530.29 eV corresponds to Nd-O / Fe-O. For example... Figure 2 As shown in (d), the peaks of Fe 2p splitting correspond to Fe 2+ and Fe 3+ Fe 2p 1 / 2 and Fe 2p 3 / 2 .like Figure 2 As shown in (e), the Nd 3d spectrum exhibits two irregular main peaks, which can be attributed to Nd. 3+ Nd 3d 3 / 2 and Nd 3d 5 / 2It can also be noted that some Nd atoms are in a metallic state, i.e., Nd 0 (3d) 5 / 2 Optoelectronics 979eV, 3d 3 / 2 The photoelectric effect (1001 eV) indicates the successful preparation of lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material.

[0055] (3) Characterization and analysis of lignin-encapsulated neodymium iron oxide nanospheres by scanning electron microscopy and transmission electron microscopy

[0056] The aqueous solution of lignin-coated NdFeB nanospheres obtained in step (2) of Example 1 was first dried, and then the morphology and microstructure of the lignin-coated NdFeB nanospheres were characterized by scanning electron microscopy (SEM, German ZEISS Sigma 300) and transmission electron microscopy (TEM, American FEI Talos F200S). The results are as follows. Figure 3 As shown. Figure 3 (a) is a SEM image of lignin-encapsulated neodymium iron oxide nanospheres. Figure 3 (b) is a TEM image of lignin-encapsulated neodymium iron oxide nanospheres.

[0057] from Figure 3 As can be seen from (a) and 3(b), the lignin-encapsulated neodymium iron oxide nanospheres prepared in this invention are mainly spherical with a relatively uniform size distribution and an average particle size of 678.63 nm. Furthermore, it shows that the neodymium iron oxide magnetic particles are uniformly coated with lignin on the outer layer, forming a core-shell structure.

[0058] (4) Characterization and analysis of lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material by scanning electron microscopy, transmission electron microscopy and X-ray energy dispersive spectroscopy

[0059] The morphology, microstructure, and elemental distribution of the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material obtained in Example 1 were characterized using scanning electron microscopy (SEM, ZEISS Sigma 300, Germany) and transmission electron microscopy (TEM, FEI Talos 200X, USA). The results are as follows: Figure 4 As shown. Figure 4 (a) is a SEM image of the absorbing material obtained in Example 1. Figure 4 (b) is a TEM image of the absorbing material obtained in Example 1. Figure 4 (c) is the high-angle annular dark field (HAADF) diagram of the absorbing material obtained in Example 1. Figure 4 (d) Figure 4 (e) Figure 4 (f) is its corresponding EDS mapping diagram.

[0060] like Figure 4As shown in (a), the NdFeO@C-650 microwave absorbing material prepared in Example 1 is mainly spherical. From Figure 4 (b) It can be seen that the surface of neodymium iron oxide is coated with carbon, and the lattice stripes of NdFeO3 can also be observed, specifically 0.29 nm, corresponding to the (200) crystal plane. Figure 4 (c) is the HAADF image of the NdFeO@C-650 absorbing material, as shown in the image. Figure 4 As shown in (d), (e), and (f), the corresponding elemental mapping images reveal that C and Fe are uniformly distributed within the image area, while Nd is mainly concentrated inside. This confirms that the absorbing material has a core-shell structure with lignin carbon as the outer shell and NdFeO as the core. The outer carbon layer can protect the magnetic particles from harsh environments, making it suitable for more diverse application scenarios.

[0061] (5) Magnetic property analysis of microwave absorbing materials

[0062] The magnetic properties of the lignin-carbon-coated NdFeO@C-650 microwave absorbing material from Example 1 and the NdFeO microwave absorbing material from Comparative Example 1 were analyzed using a hysteresis loop test (VSM, LakeShore 8604, USA). The results are as follows: Figure 5 As shown.

[0063] from Figure 5 In (a) and (b), it can be observed that both NdFeO@C-650 and NdFeO possess a certain degree of coercivity, and their hysteresis loops exhibit an overall "S" shape, indicating that both are ferromagnetic. The saturation magnetization of NdFeO and NdFeO@C-650 are 44 emu / g and 24 emu / g, respectively, and their coercivity is 214 Oe and 163 Oe, respectively. Their magnetism is mainly provided by the ferromagnetic Fe3O4 and the paramagnetic NdFeO3. The difference in coercivity and magnetization between NdFeO and NdFeO@C-650 can be attributed to the composite of non-magnetic lignin carbon, which leads to a decrease in the magnetic properties of NdFeO@C-650. However, the composite of C enriches the microwave loss mechanism, provides a large number of heterogeneous interfaces, and enhances the dielectric loss of the material. This characteristic is beneficial for improving impedance matching characteristics and significantly improving microwave absorption performance. Therefore, the lignin carbon-encapsulated NdFeO oxide microwave absorbing material of the present invention not only possesses excellent magnetism but also improves impedance matching and enhances microwave absorption performance.

[0064] (6) Microwave absorption performance test of lignin-carbon-coated neodymium iron oxide microwave absorbing material

[0065] The materials obtained from Comparative Example 1 (NdFeO) and Example 1 (NdFeO@C-650) were respectively mixed with paraffin wax in a ratio of 3:7. The mixtures were pressed into concentric rings with an outer diameter of 7 mm and an inner diameter of 3 mm. Electromagnetic parameters were tested on a vector network analyzer in a frequency range of 1–18 GHz. The resulting 2D and 3D reflection loss contour maps are shown below. Figure 6 As shown. Figure 6 (a) is a 2D contour map of reflection loss for Comparative Example 1. Figure 6 (b) is a 3D contour map of reflection loss for Comparative Example 1; Figure 6 (c) is a 2D reflection loss contour plot of Example 1. Figure 6 (d) is a 3D reflection loss contour map of Example 1. Figure 7 The diagram shows the reflection loss of the absorbing material of Example 1 with different thicknesses.

[0066] from Figure 6 (a) and (b) show that the maximum reflection loss RL of NdFeO at 2.7 mm and 14.96 GHz is achievable. min It is -4.49 dB. From Figure 6 (c) and (d) show that the maximum reflection loss RL of NdFeO@C-650 at 5 mm and 15.12 GHz can be observed. min It has a gain of -14.39 dB and a maximum absorption bandwidth (EAB) of 3.23 GHz (14.2 GHz to 17.43 GHz) at a thickness of 4.9 mm, and exhibits an absorption band ratio (RL) at 15.76 GHz. min The value is -13.7 dB. Therefore, it can be concluded that the composite of lignin-charcoal optimizes impedance matching and effectively improves the microwave absorption performance of the material.

[0067] from Figure 7 It can be observed that NdFeO@C-650 has good microwave absorption performance at a thickness of 4.5mm to 5mm, and its reflection loss is <-10dB in the high-frequency region. It has an effective absorption bandwidth and can absorb 90% of microwaves.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for preparing a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material, characterized in that, Includes the following steps: (1) NdFeO, acetylacetone, solvent, precipitant and surfactant are mixed and stirred to obtain NdFeO precursor mixed solution. The NdFeO precursor mixed solution is reacted at 220℃~280℃. After the reaction, the supernatant is removed by centrifugation, and after washing and drying, NdFeO oxide magnetic material is obtained. The NdFeO oxide magnetic material is NdFeO3·Fe3O4 magnetic powder. (2) The obtained neodymium iron oxide magnetic material and lignin were added to anhydrous ethanol to obtain a neodymium iron oxide-lignin mixture. The obtained neodymium iron oxide-lignin mixture was dialyzed in water to encapsulate neodymium iron oxide to form nanospheres, and an aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres was obtained. (3) The aqueous solution of lignin-encapsulated neodymium iron oxide nanospheres obtained above is first freeze-dried, and then heated to 650℃~950℃ under an inert atmosphere or nitrogen atmosphere for heat treatment to carbonize the lignin and obtain lignin carbon-encapsulated neodymium iron oxide microwave absorbing material.

2. The method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to claim 1, characterized in that, In step (1), the molar ratio of neodymium acetylacetonate to iron acetylacetonate is 2:14 to 17, the amount of solvent used is such that the total concentration of neodymium ions and iron ions is 0.1 mol / L to 0.4 mol / L, the amount of precipitant used is 1.5 to 2 times the total molar amount of neodymium ions and iron ions, and the amount of surfactant used is 20% to 40% of the total mass of neodymium acetylacetonate and iron acetylacetonate.

3. The method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to claim 1, characterized in that, In step (1), the solvent is one or both of ethylene glycol and propylene glycol, the precipitant is one or both of urea and hexamethylenetetramine, and the surfactant is one or both of polyethylene glycol and polyvinylpyrrolidone.

4. The method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to claim 1, characterized in that, In step (1), the reaction time is 8h to 12h, the centrifugation speed is 8000rpm to 10000rpm, the centrifugation time is 10min to 15min, the washing is done by alternating washing with anhydrous ethanol and water, the drying is vacuum drying, the vacuum drying temperature is 60℃ to 80℃, and the vacuum drying time is 12h to 18h.

5. The method for preparing the lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to any one of claims 1 to 4, characterized in that, In step (2), the lignin is one or more of sulfate lignin, enzymatically hydrolyzed lignin, alkali lignin, and organic solvent lignin; the mass ratio of the neodymium iron oxide magnetic material to the lignin is 0.5 to 2:1, and the mass-volume ratio of the lignin to the anhydrous ethanol is 1 mg to 4 mg: 1 mL.

6. The method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to any one of claims 1 to 4, characterized in that, In step (2), the neodymium iron oxide magnetic material and lignin are added to anhydrous ethanol and then sonicated until the lignin is completely dissolved in the anhydrous ethanol. The dialysis time is 24h to 48h. The dialysis is carried out using a dialysis bag with a molecular weight of 8kDa to 14kDa.

7. The method for preparing lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material according to any one of claims 1 to 4, characterized in that, In step (3), the freeze-drying temperature is -50℃ to -60℃, the freeze-drying time is 24h to 48h, the heat treatment time is 2h to 4h, and the heating rate is 2℃ / min to 5℃ / min.

8. A lignin-carbon-coated neodymium iron oxide microwave absorbing material prepared by any one of claims 1 to 7.

9. The lignin-carbon-coated neodymium iron oxide microwave absorbing material according to claim 8, characterized in that, The average particle size of the lignin-carbon-coated neodymium iron oxide microwave absorbing material is 200 nm to 800 nm.

10. The application of a lignin-carbon-encapsulated neodymium iron oxide microwave absorbing material as described in claim 8 or 9 in the field of microwave absorption.

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