A core-shell structure hollow urchin-like Fe3O4@C porous composite wave-absorbing material and a preparation method thereof

Abstract

The application discloses a kind of core-shell structure hollow urchin-like Fe3O4@C porous composite wave-absorbing materials and preparation method thereof, belong to electromagnetic wave absorbing material field.The material with urchin-like a-FeOOH prepared by hydrothermal method as pre-phase, then adjust the mass ratio of pre-phase and glucose, prepare different carbon content a-FeOOH@carbon hydrate as intermediate phase by hydrothermal method.Finally, under argon atmosphere, urchin-like Fe3O4@C is prepared by annealing.The application prepares carbon layer wrapped hollow urchin-like Fe3O4 particles, not only get porous urchin-like strong magnetic material, and external carbon layer wrapped structure also overcomes the defect that bare Fe3O4 material is easily oxidized by external environment.In addition, C layer and Fe3O4 magnetic core are combined, improve electromagnetic loss and impedance matching.The wave-absorbing material prepared by the application has unique structure, controllable, excellent microwave absorption performance, and is a kind of light weight high efficiency wave-absorbing material.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic wave absorbing materials, specifically relating to a core-shell structured hollow sea urchin-shaped Fe3O4@C porous composite wave absorbing material and its preparation method. Background Technology

[0002] In recent years, electromagnetic technology has been widely applied in civilian and military fields, such as high-frequency heating, radio communication technology, radar, and positioning and navigation. While bringing great convenience to people, it has also posed significant hazards to human health, the environment, and electronic equipment. To address this issue, various electromagnetic wave absorbing materials have been extensively studied. Dielectric loss and magnetic loss are the two main microwave loss modes in absorbers. Single magnetic or dielectric absorbing materials are insufficient to meet the requirements of thinness, light weight, wide bandwidth, and strong absorption. Therefore, using dielectric and magnetic composite materials is an effective method for preparing high-performance absorbers.

[0003] In magnetic materials, Fe3O4 is a common material with strong magnetic loss, possessing advantages such as a large constant magnetic moment, good electrical conductivity, high dielectric constant, and high Curie temperature. Approximately 40% of research in the field of ferrite-based absorbers focuses on Fe3O4 and its composites. However, due to the high density of Fe3O4, greater thicknesses are typically required to achieve effective wave absorption. Furthermore, Fe3O4 simultaneously possesses Fe... 2+ and Fe 3+ This inherent instability and susceptibility to oxidation limit its practical applications. To address these issues, the most effective method is to combine it with other dielectric materials to create composite absorbers with balanced dielectric and magnetic losses. Simple mixing easily leads to magnetic materials attracting each other, resulting in uneven distribution. The magnetic core and dielectric shell structure of the absorber can solve this problem. Carbon materials are low-density, high-dielectric-loss, and inexpensive. If used as a shell, it is not only lightweight and widely applicable but also protects the Fe3O4 magnetic core from environmental corrosion and oxidation. The microstructure of the absorbing agent also significantly affects microwave absorption; hollow porous structures exhibit high interfacial effects, making it possible to realize lightweight absorbing materials. Summary of the Invention

[0004] The purpose of this invention is to provide a lightweight, core-shell structured, hollow, urchin-like Fe3O4@C porous composite microwave absorbing material with strong absorption and good impedance matching. Furthermore, by adjusting the amount of glucose used, the electromagnetic parameters and absorption characteristics of the absorbing agent are controlled, resulting in a lightweight absorbing agent with high absorption performance and impedance matching.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A method for preparing a core-shell structured hollow sea urchin-like Fe3O4@C porous composite microwave absorbing material includes the following steps:

[0007] FeSO4·7H2O was mixed evenly with glycerol and deionized water. The mixture was placed in a hydrothermal reactor for reaction. After the reaction was completed, the product was centrifuged and dried to obtain sea urchin-shaped α-FeOOH powder.

[0008] α-FeOOH and anhydrous glucose were dissolved in deionized water and stirred until homogeneous. The mixture was then placed in a hydrothermal reactor for reaction. After the reaction was completed, the product was centrifuged and dried to obtain α-FeOOH@hydrated carbon powder.

[0009] The a-FeOOH@hydrated carbon powder was annealed in a high-purity argon atmosphere and then cooled in the furnace to obtain a core-shell structured hollow sea urchin-shaped Fe3O4@C porous composite microwave absorbing material.

[0010] Furthermore, the heating rate during annealing is 4℃ / min, the maximum temperature is 550℃, and the holding time is 6h.

[0011] Furthermore, the centrifugal drying step is as follows: centrifuge with deionized water 2 to 5 times, then centrifuge with anhydrous ethanol 2 to 5 times, and finally dry in a vacuum drying oven.

[0012] Furthermore, the reaction conditions of the mixture of FeSO4·7H2O, glycerol and deionized water in the hydrothermal reactor were 120℃ and the reaction time was 24h. After the reaction was completed, the product was centrifuged and vacuum dried at a speed of 3500-7500 rpm for 2-10 min. The drying temperature was room temperature to 60℃ and the drying time was 5-12h.

[0013] Furthermore, the reaction conditions of the mixture of α-FeOOH, anhydrous glucose, and deionized water in the hydrothermal reactor were 180℃ and the reaction time was 12h. After the reaction was completed, the product was centrifuged and vacuum dried at a speed of 5500-8500 rpm for 2-10 min. The drying temperature was 40-80℃ and the drying time was 5-15h.

[0014] Furthermore, FeSO4·7H2O was mixed with glycerol and deionized water by first stirring together and then sonicating for 15 minutes to form a homogeneous solution; the stirring speed was 600-1000 rpm and the stirring time was 4-10 minutes.

[0015] Furthermore, the volume ratio of glycerol to deionized water is 1:7.

[0016] Furthermore, the mass ratio of α-FeOOH powder to anhydrous glucose is 1:0.5–10.

[0017] Furthermore, during annealing, a carbon embedding method is used to prevent the oxygen in the high-purity argon gas from oxidizing the products. Specifically, the reactants are placed in a small crucible with a lid, and then placed in a large crucible containing graphite powder. The lid of the large crucible has a small gap.

[0018] Furthermore, the method includes the following steps:

[0019] Step (1) Preparation of urchin-like α-FeOOH precursor phase:

[0020] 1) Mix FeSO4·7H2O with a certain volume of glycerol and deionized water. The mixing method is to stir together and then sonicate for 15 minutes to ensure the formation of a homogeneous solution. The ratio of FeSO4·7H2O to glycerol is 1-2 g: 5 mL.

[0021] 2) Place the homogeneous mixture in a hydrothermal reactor with a 50 mL PTFE liner and react at 120 °C for 24 h.

[0022] 3) After the reaction is complete, centrifuge with deionized water 2 to 5 times, then centrifuge with anhydrous ethanol 2 to 5 times, and finally dry in a vacuum drying oven to obtain sea urchin-shaped α-FeOOH powder.

[0023] Step (2) Preparation of α-FeOOH@hydrated carbon powder:

[0024] 1) Dissolve a certain mass ratio of α-FeOOH powder and anhydrous glucose in deionized water and stir for 30-60 minutes until homogeneous;

[0025] 2) Place the homogeneous mixture in a hydrothermal reactor with a 50 mL PTFE liner and react at 180 °C for 12 h.

[0026] 3) After the reaction is complete, centrifuge with deionized water 2 to 5 times, then centrifuge with anhydrous ethanol 2 to 5 times, and finally dry in a vacuum drying oven to obtain α-FeOOH@hydrated carbon powder.

[0027] Step (3) Preparation of sea urchin-like Fe3O4@C:

[0028] 1) The a-FeOOH@hydrated carbon powder was annealed in a high-purity argon atmosphere. The heating rate during annealing was 4℃ / min, the maximum temperature was 550℃, and the holding time was 6h. After furnace cooling, a core-shell structured hollow sea urchin-shaped Fe3O4@C porous composite microwave absorbing material was obtained.

[0029] Furthermore, in step (1), the ratio of FeSO4·7H2O to glycerol is 0.111g:5mL.

[0030] Furthermore, in step (1), the volume ratio of the certain volume of glycerol to deionized water is 1:7, wherein the volume of glycerol is 5 mL and the volume of deionized water is 35 mL.

[0031] Furthermore, in step (1), the common stirring speed is 600-1000 rpm and the stirring time is 4-10 min.

[0032] Furthermore, in step (1), the centrifugation speed is 3500-7500 rpm and the centrifugation time is 2-10 min.

[0033] Furthermore, in step (1), the vacuum drying temperature is RT (room temperature) to 60°C, and the drying time is 5 to 12 hours.

[0034] Furthermore, in step (2), the mass ratio of the α-FeOOH powder to anhydrous glucose is 1:0.5 to 10, preferably 1:0.5, 1:1, 1:2, 1:4, 1:6, 1:8, and 1:10, and even more preferably 1:1. (Actually, samples with all seven ratios were tested, but the 1:1 ratio was better for subsequent performance testing.)

[0035] Furthermore, in step (2), the stirring speed is 600-1000 rpm and the stirring time is 30-60 min.

[0036] Furthermore, in step (2), the centrifugation speed is 5500-8500 rpm and the centrifugation time is 2-10 min.

[0037] Furthermore, in step (2), the vacuum drying temperature is 40-80°C and the drying time is 5-15 hours.

[0038] Furthermore, in step (3), the carbon embedding method is used during annealing to prevent the oxygen in the high-purity argon gas from oxidizing the products. Specifically, the reactant is placed in a small crucible with a lid, and then placed in a large crucible with graphite powder inside, leaving a small gap in the lid of the large crucible.

[0039] This invention also provides a method for preparing a core-shell structured hollow sea urchin-shaped Fe3O4@C porous composite microwave absorbing material, which is prepared by the above method.

[0040] Beneficial effects

[0041] This invention provides a core-shell structured hollow urchin-shaped Fe3O4@C porous composite microwave absorbing material and its preparation method. The process is simple and low-cost. The electromagnetic parameters and microwave absorption characteristics can be adjusted to obtain a lightweight and efficient microwave absorbing composite material with good coordination of magnetic loss and electrical loss and impedance matching.

[0042] The core-shell structure of Fe3O4 core and C shell prepared in this invention, by changing the amount of carbon source added, yields a microwave absorbing material with good impedance matching characteristics and coordinated magnetic and electrical losses. The porous urchin-like structure microwave absorbing agent prepared in this invention provides an interface for multiple reflections and scattering of electromagnetic waves, enhancing interface polarization and the electromagnetic wave propagation path, thereby enhancing the electromagnetic wave absorption effect. The urchin-like Fe3O4@C described in this invention has a minimum reflection loss of -73.5 dB, and an absorption bandwidth of 4.78 GHz with a reflection loss below -10 dB (i.e., electromagnetic wave absorption rate exceeding 90%), which has important reference value for the development of novel lightweight core-shell structure microwave absorbing materials. Attached Figure Description

[0043] Figure 1 The images show the XRD patterns of the precursor α-FeOOH prepared according to the methods of the present invention and the Fe3O4@C prepared according to Examples 1, 2 and 6.

[0044] Figure 2 The images show SEM images of the precursor α-FeOOH prepared according to Example 1 and the Fe3O4@C prepared according to Examples 1, 2, 3, 4, 5, 6 and 7, respectively. Figure 2 a, b, c, d, e, f, g, and h).

[0045] Figure 3 TEM images of Fe3O4@C prepared in Example 6 of the present invention. Figure 3 b is Figure 3 A high-magnification magnified image of the "sea urchin needle tip".

[0046] Figure 4 The three-dimensional reflection loss diagrams of Fe3O4@C as a microwave absorber obtained according to Examples 1, 2, 3, 4, 5, 6, and 7 of the present invention are shown, respectively corresponding to... Figure 4 a, b, c, d, e, f, and g.

[0047] Figure 5 This is a two-dimensional minimum reflection loss diagram of Fe3O4@C as a microwave absorber obtained according to Examples 1, 2, 3, 4, 5, 6 and 7 of the present invention.

[0048] Figure 6 The minimum RL value and maximum effective absorption bandwidth (EAB) of Fe3O4@C as a microwave absorber obtained by the method of the present invention according to Examples 1, 2, 3, 4, 5, 6 and 7 are given. Detailed Implementation

[0049] The present invention will be further explained in detail below with reference to the accompanying drawings and specific embodiments, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0050] Example 1

[0051] A core-shell structured hollow urchin-like Fe3O4@C porous composite microwave absorbing material and its preparation method, comprising the following steps:

[0052] I. Preparation of sea urchin-like α-FeOOH precursor phase:

[0053] (1) Mix 0.111g FeSO4·7H2O, 5mL glycerol and 35mL deionized water. The mixing method is to stir together at 750rpm for 5min and then sonicate for 15min to ensure that a homogeneous solution is formed.

[0054] (2) The homogeneous mixture was placed in a hydrothermal reactor with a capacity of 50 mL polytetrafluoroethylene liner and reacted at 120 °C for 24 h.

[0055] (3) After the reaction was completed, the mixture was centrifuged three times with deionized water at a speed of 5500 rpm for 4 min. Then it was centrifuged three times with anhydrous ethanol, and the precipitate was collected. Finally, it was dried in a vacuum drying oven at 40℃ for 6 h to obtain sea urchin-shaped α-FeOOH powder.

[0056] II. Preparation of α-FeOOH@hydrated carbon mesophase:

[0057] (1) Dissolve 0.05 g of α-FeOOH and anhydrous glucose in 40 mL of deionized water at a mass ratio of 1:0.5 and stir magnetically at 700 rpm for 50 min until homogeneous.

[0058] (2) The homogeneous mixture was placed in a hydrothermal reactor with a 50 mL PVC liner and reacted at 180 °C for 12 h.

[0059] (3) After the reaction was completed, the mixture was centrifuged three times with deionized water at a speed of 7500 rpm for 5 min. Then it was centrifuged three times with anhydrous ethanol, and the precipitate was collected. Finally, it was dried in a vacuum drying oven at 60℃ for 10 h to obtain α-FeOOH@hydrated carbon powder.

[0060] III. Preparation of sea urchin-like Fe3O4@C:

[0061] (1) The intermediate product was annealed in a high-purity argon atmosphere and the carbon embedding method was used for auxiliary preparation. The heating rate during annealing was 4℃ / min, the maximum temperature was 550℃, the holding time was 6h, and the sea urchin-shaped Fe3O4@C powder was obtained after furnace cooling.

[0062] Example 2

[0063] The difference between this embodiment and Example 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:1, while the remaining steps are the same as in Example 1. The amount of Fe3O4 and C generated is controlled by adjusting the proportion of materials added.

[0064] Example 3

[0065] The difference between this embodiment and Embodiment 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:2, while the remaining operation steps are the same as in Embodiment 1.

[0066] Example 4

[0067] The difference between this embodiment and Embodiment 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:4, while the remaining operation steps are the same as in Embodiment 1.

[0068] Example 5

[0069] The difference between this embodiment and Embodiment 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:6, while the remaining operation steps are the same as in Embodiment 1.

[0070] Example 6

[0071] The difference between this embodiment and Embodiment 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:8, while the remaining operation steps are the same as in Embodiment 1.

[0072] Example 7

[0073] The difference between this embodiment and Embodiment 1 is that in step two, the mass ratio of α-FeOOH (0.05g) to anhydrous glucose is 1:10, while the other operation steps are the same as in Embodiment 1.

[0074] Comparative Example 1

[0075] This comparative example provides a sea urchin-like Fe3O4@C composite material. The preparation method differs from Example 7 in that: the hydrothermal reaction in step two is carried out at 200℃ for 12 hours; the annealing process in step three involves heating at 400℃ under argon for 2 hours at a heating rate of 1℃ / min, followed by further treatment at 600℃ for 2 hours at a heating rate of 5℃ / min. Finally, a sea urchin-like Fe3O4@C composite material is obtained.

[0076] Comparative Example 2

[0077] This comparative example provides a sea urchin-like Fe3O4@C preparation method that differs from Example 1 in the following ways: In step one, FeSO4·7H2O (1.39 g) and H3COONa·4H2O (1.36 g) are dissolved in 50 mL of deionized water. After vigorous stirring at 60 °C for a period of time, the yellow slurry is centrifuged and washed several times with distilled water. In step two, the obtained FeOOH precursor is ultrasonically dispersed in 40 mL (20 mg / mL) of glucose solution to form a suspension. The hydrothermal treatment and subsequent procedures are the same. In step three, the atmosphere is argon, and direct annealing is performed (without using the carbon embedding method). The annealing temperature and time are the same.

[0078] Comparative Example 3

[0079] This comparative example provides a sea urchin-like Fe3O4, the preparation method of which differs from that of Example 1 in that step two is omitted. In step three, the sea urchin-like FeOOH precursor is first calcined at 300℃ and 5℃ / min for 1 hour to obtain sea urchin-like Fe2O3. Then, the sea urchin-like Fe2O3 is subjected to a mixture of H2 / Ar (1:4 volume ratio) at 400℃ for 2 hours, followed by natural cooling to room temperature under nitrogen protection.

[0080] Figure 1 The XRD patterns of the precursor α-FeOOH prepared according to Example 1 and Fe3O4@C prepared according to Examples 1, 2 and 6 show that the prepared samples are well matched with orthogonal α-FeOOH (JCPDS81-0462) and face-centered cubic Fe3O4 (JCPDS 88-0866) and have no other impurities, indicating that the present invention has successfully prepared the target precursor α-FeOOH and the final product Fe3O4@C.

[0081] Figure 2 SEM images of the precursor α-FeOOH prepared according to Example 1 and Fe3O4@C prepared according to Examples 1, 2, 3, 4, 5, 6 and 7 (respectively). Figure 2 (a, b, c, d, e, f, g, and h). As shown in the figure, α-FeOOH is spherical in shape, with a diameter of approximately 1 μm. Examples 1, 2, 3, 4, 5, 6, and 7 are also spherical in shape, the difference being that the sea urchin needles in Example 1 are blunted, making the overall shape more like a massage ball. When the carbon source is more abundant, obvious carbon spheres appear in Examples 5, 6, and 7. This is likely because the increased amount of glucose leads to the formation of new nucleation centers for carbon sphere growth. The hydrothermal conditions at this time (180°C, 12 h) are also the conditions for preparing carbon spheres using glucose alone.

[0082] Figure 3 TEM images of Fe3O4@C obtained in Example 6, from... Figure 3As can be seen from a magnified view, the prepared Fe3O4@C has a hollow structure. Figure 3 b shows that the shell thickness is approximately 2.5 nm and exhibits nanoscale pores. Through... Figure 3 It can be demonstrated that the Fe3O4@C obtained in Example 6 is a core-shell structured hollow sea urchin-shaped Fe3O4@C porous composite microwave absorbing material.

[0083] Figure 4 The three-dimensional reflection loss diagrams of Fe3O4@C obtained in Examples 1, 2, 3, 4, 5, 6, and 7 are shown, respectively corresponding to... Figure 4 a, b, c, d, e, f, and g. The test sample was prepared by mixing the test powder with paraffin wax at a 1:1 mass ratio, and then pressing it into a ring (outer diameter 7.0 mm, inner diameter 3.0 mm). An Agilent E5071C vector network analyzer was used, and the electromagnetic parameters of the absorbing material in the 1-18 GHz frequency range were tested using the coaxial method. The figure shows that Fe3O4@C (Example 2) with a mass ratio of 1:1 has the lowest reflection loss (RL), which is -73.5 dB at a thickness of 3.23 mm. With increasing carbon source content, the minimum reflection loss of the absorbing agent first increases and then decreases.

[0084] Figure 5 The figures show the two-dimensional minimum reflection loss diagrams for Fe3O4@C obtained in Examples 1, 2, 3, 4, 5, 6, and 7. It can be seen from the figures that the RL of Example 1... min = -69.2dB, corresponding to a thickness of 4.67mm; RL of Example 2 min = -73.5dB, corresponding to a thickness of 3.23mm; RL of Example 3 min = -56.0dB, corresponding to a thickness of 2.71mm; RL of Example 4 min = -63.7dB, corresponding to a thickness of 3.31mm; RL of Example 5 min = -23.8dB, corresponding to a thickness of 5.00mm; RL of Example 6 min = -14.8dB, corresponding to a thickness of 5.00mm; RL of Example 1 min = -8.9dB, corresponding to a thickness of 5.00mm. Among them, Example 2 has the smallest reflection loss value, while Example 7 is not suitable for practical applications because its minimum RL value is greater than -10dB.

[0085] Figure 6 The figures show the minimum RL value and maximum effective absorption bandwidth of Fe3O4@C obtained in Examples 1, 2, 3, 4, 5, 6, and 7. As can be seen from the figures, Example 2 has the minimum RL value of -73.5 dB and the maximum EAB value of 4.78 GHz, exhibiting the best microwave absorption capability.

[0086] Table 1

[0087]

[0088]

[0089] Table 1 shows the typical absorption parameters for Examples 1, 2, 3, 4, 5, 6, 7 and Comparative Example 3, which are the absorber mass percentage, minimum RL value, and maximum effective absorption bandwidth, respectively. It can be seen that, under the same absorber mass percentage, the absorption performance of Examples 1, 2, 3, and 4 is almost always better than that of Comparative Example 3.

[0090] The above are merely preferred embodiments of the present invention, and the present invention is not limited to the contents of these embodiments. For those skilled in the art, various changes and modifications can be made within the scope of the technical solutions of the present invention, and any changes and modifications made are within the protection scope of the present invention.

Claims

1. A method for preparing a core-shell structure hollow urchin-like Fe3O4@C porous composite wave-absorbing material, characterized in that, The method comprises the following steps: FeSO4·7H2O is mixed with glycerol and deionized water, and the mixture is placed in a hydrothermal reactor for reaction, with a reaction temperature of 120℃ and a reaction time of 24h; after the reaction, the product is centrifuged and vacuum dried, with a centrifugal speed of 3500-7500rpm, a centrifugal time of 2-10min, a drying temperature of room temperature-60℃, and a drying time of 5-12h; and a-FeOOH powder in the shape of sea urchin is obtained; a-FeOOH and anhydrous glucose are dissolved in deionized water, and the mixture is stirred uniformly and then placed in a hydrothermal reactor for reaction, with a reaction temperature of 180℃ and a reaction time of 12h; after the reaction, the product is centrifuged and vacuum dried, with a centrifugal speed of 5500-8500rpm, a centrifugal time of 2-10min, a drying temperature of 40-80℃, and a drying time of 5-15h; and a-FeOOH@hydrated carbon powder is obtained after centrifugal drying of the product; The a-FeOOH@hydrated carbon powder is annealed under a high-purity argon atmosphere, and a core-shell structure of hollow sea urchin-shaped Fe3O4@C porous composite wave-absorbing material is obtained after furnace cooling.

2. The preparation method of the core-shell structure hollow urchin-like Fe3O4@C porous composite wave-absorbing material according to claim 1, characterized in that, The annealing temperature is raised at a rate of 4℃ / min, the highest temperature is 550℃, and the holding time is 6h.

3. The method for preparing the core-shell structured hollow urchin-shaped Fe3O4@C porous composite microwave absorbing material according to claim 1, characterized in that, The centrifugal drying step is: centrifuging 2-5 times with deionized water, centrifuging 2-5 times with anhydrous ethanol, and finally drying in a vacuum drying oven.

4. The preparation method of the core-shell structured hollow urchin-shaped Fe3O4@C porous composite microwave absorbing material according to claim 1, characterized in that, FeSO4·7H2O, glycerol and deionized water are mixed by stirring together and then ultrasonicating for 15min to form a uniform solution, with a stirring speed of 600-1000rpm and a stirring time of 4-10min.

5. The preparation method of the core-shell structure hollow urchin-like Fe3O4@C porous composite wave-absorbing material according to claim 1, characterized in that, The volume ratio of glycerol to deionized water is 1:

7. 6.The method of claim 1, wherein the method is characterized by, The mass ratio of a-FeOOH powder to anhydrous glucose is 1:0.5-10.

7. The method of claim 1, wherein the method comprises the steps of: preparing a Fe304@C core-shell structure by a chemical co-precipitation method; and preparing a porous composite wave-absorbing material by a solvothermal method. During annealing, the carbon embedding method is used to prevent the product from being oxidized by oxygen in the high-purity argon, specifically, the reaction material is placed in a small crucible with a cover, and then the whole is placed in a large crucible with graphite powder inside, with a small gap left in the cover of the large crucible.

8. A core-shell structure hollow urchin-like Fe3C@C porous composite wave-absorbing material, characterized in that, Prepared by the method of any one of claims 1-7.

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