N-O doped flower-like porous carbon microspheres and preparation method and application thereof

Abstract

The invention provides N-O doped flower-like porous carbon microspheres as well as a preparation method and application thereof, and belongs to the technical field of electromagnetic wave absorbing materials. According to the invention, microspheres are coated by using a simple soap-free emulsion polymerization method, a complex multi-layer heterogeneous interface structure is realized, N and O atoms are introduced, and heteroatom doping is realized so as to improve the wave-absorbing performance; a unique flower-shaped structure is grown on the surface of the microsphere through a self-assembly process, and an exquisite flower-shaped porous carbon microsphere can be obtained through carbonization, so that an ingenious structure beneficial to multiple reflection and scattering generation is realized. Under the conditions that the mass ratio of the flower-like porous carbon microspheres to the paraffin is 1: 5 and the matching thickness is 2.5-3mm, the minimum reflection loss of the flower-like porous carbon microspheres is-40.8 to-48.0 dB, the optimal effective absorption bandwidth is 4.40 GHz, and the flower-like porous carbon microspheres have excellent electromagnetic wave absorption performance.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic wave absorbing materials, and particularly relates to an N-O doped flower-like porous carbon microsphere, a preparation method thereof, and an application thereof. Background Art

[0002] With the pursuit of a convenient life by people, wireless wearable devices have developed and become popular rapidly. However, the widespread use of mobile communication devices has also caused serious electromagnetic pollution. Electromagnetic pollution can interfere with the normal operation of precision equipment, disrupt the human micro-magnetic field, and induce physical diseases. Therefore, it is crucial to develop efficient electromagnetic wave absorbing materials. Electromagnetic wave absorbing materials can absorb or weaken the electromagnetic waves projected onto their surfaces through the form of energy conversion, and usually require the characteristics of light weight, thinness, strong absorption ability, and wide effective bandwidth. At present, the widely studied electromagnetic wave absorbing materials are mainly divided into two categories: magnetic materials and carbon materials. Among them, magnetic materials have the disadvantages of high density, easy corrosion, and low Curie temperature, which limit their development. In contrast, carbon materials have been widely concerned due to their light weight, stable properties, high compatibility, highly adjustable dielectric constant, and easily controllable surface microstructure. Among them, carbon microspheres have attracted attention due to their low density, high loading, and easily controllable microstructure. However, the high conductivity of carbon microspheres themselves affects their impedance matching with electromagnetic waves, and often has disadvantages such as low absorption intensity and narrow absorption bandwidth, which limits their practical applications.

[0003] Previous studies have shown that the structural design and heteroatom doping of carbon microspheres can effectively improve their electromagnetic wave absorption performance. Different from ordinary spherical shapes, the complex inner and surface structures can not only provide more sites for heteroatom doping, promoting dipole polarization, but also offer a larger specific surface area and a more abundant pore structure, which is conducive to the incidence of electromagnetic waves and the processes of multiple reflection and scattering, enhancing electromagnetic wave loss. For example, in the paper titled "Doped Porous Carbon Spheres with Controllable Vesicle Structure: Preparation and the Effects of Pore Size on Electromagnetic Wave Absorption Properties" by Z. Liu et al. in *Small* (2024) 2402000, heteroatom-doped carbon spheres with a unique vesicle structure were prepared using SiO₂ spheres as hard templates. First, the SiO₂ template was prepared by the hydrolysis method, and then porous carbon microspheres were obtained through in-situ polymerization, carbonization, and alkali etching processes in sequence. By controlling the particle size of the SiO₂ microspheres, the regulation of the vesicle pore size of the carbon microspheres was achieved. When the vesicle pore size was 327 nm, an effective absorption bandwidth (EAB) of 6.32 GHz was achieved, and the minimum reflection loss (RLmin) was -36.10 dB. In the paper "Enhanced electromagnetic microwave absorption performance of lightweight bowl-like carbon nanoparticles" by J. Fu et al. in *Industrial & Engineering Chemistry Research* 56 (2017) 11460 - 11466, bowl-like carbon nanoparticles (BLCNs) were obtained by calcining the prepared bowl-like polydopamine, and the unique role of the carbon sphere microstructure in EMA performance was explored with the help of the unique structure. It was proved that the unique bowl-like structure not only covered all the advantages of the hollow structure, including rich interfaces, enhanced reflection, etc., but also greatly strengthened the interfacial polarization. An RLmin of -45.3 dB was achieved, and the maximum effective bandwidth reached 4.2 GHz. Clever inner and surface microstructure design can improve impedance matching and increase dielectric loss, which is crucial for the electromagnetic wave absorption performance of materials. However, the commonly used morphology control methods currently usually use template assistance or acid-base solvent etching, etc., with complex processes, high costs, difficult large-scale preparation, and unnecessary environmental hazards. Therefore, developing a preparation method with cheap raw materials and simple processes to achieve the design of the structure and morphology control of carbon-based microwave absorption materials is still a challenging topic. Summary of the Invention

[0004] The object of the present invention is to provide an N-O doped flower-like porous carbon microsphere, its preparation method and application, and to prepare an N-O doped flower-like porous carbon microsphere with excellent electromagnetic wave absorption performance by means of a simple and green synthesis process.

[0005] In order to achieve the above object of the invention, the present invention provides the following technical solutions:

[0006] The present invention provides a preparation method of an N-O doped flower-like porous carbon microsphere, comprising the following steps:

[0007] Mix acrylonitrile, an olefinic reaction monomer, a first initiator and water, and carry out a first polymerization reaction to obtain polymer seeds;

[0008] Mix the polymer seeds with a first coating monomer and carry out a first coating to obtain a first coating product;

[0009] Mix the first coating product with a second coating monomer and carry out a second coating to obtain a second coating product;

[0010] Mix the second coating product with a third coating monomer and carry out a third coating to obtain a multi-layer coated polymer microsphere;

[0011] Mix the multi-layer coated polymer microsphere, acrylonitrile, a second initiator and an organic solvent, and carry out a second polymerization reaction to obtain a polymer flower ball;

[0012] Carry out carbonization on the polymer flower ball to obtain an N-O doped flower-like porous carbon microsphere.

[0013] Preferably, the olefinic reaction monomer includes one or more of styrene (St), methyl methacrylate (MMA), divinylbenzene (DVB) and diallyl maleate (DAM); the first initiator and the second initiator independently include azobisisobutyronitrile, azobis (2-amidinopropane) hydrochloride, potassium persulfate, ammonium persulfate or sodium persulfate.

[0014] Preferably, in the step of the first polymerization reaction, the mass ratio of acrylonitrile to the olefinic reaction monomer is 2-5:1; the mass of the first initiator is 0.8-2.0% of the total mass of acrylonitrile and the olefinic reaction monomer; the temperature of the first polymerization reaction is 70-75 °C and the time is 2-4 h.

[0015] Preferably, the first coating monomer, the second coating monomer and the third coating monomer independently include one or more of styrene (St), methyl methacrylate (MMA), divinylbenzene (DVB), acrylonitrile (AN) and diallyl maleate (DAM); the masses of the first coating monomer, the second coating monomer and the third coating monomer are independently 20-50% of the total mass of acrylonitrile and the olefinic reaction monomer in the first polymerization reaction.

[0016] Preferably, the temperatures of the first coating, the second coating, and the third coating are each independently 70 to 75°C, and the time is 2 to 4 h.

[0017] Preferably, in the step of the second polymerization reaction, the mass ratio of the multi-layer coated polymer microspheres to acrylonitrile is 1:3 to 6, and the mass of the second initiator is 0.8 to 1.5% of the total mass of the multi-layer coated polymer microspheres and acrylonitrile; the temperature of the second polymerization reaction is 70 to 75°C, and the time is 12 to 24 h.

[0018] Preferably, the particle size of the polymer flower spheres is 1.2 to 2.0 μm.

[0019] Preferably, the temperature of the carbonization is 700 to 900°C, the time is 2 to 3 h, and the atmosphere is nitrogen or argon; the heating rate to the carbonization temperature is 1 to 3°C / min.

[0020] The present invention provides N-O doped flower-like porous carbon microspheres prepared by the preparation method described in the above technical solution. The N-O doped flower-like porous carbon microspheres contain C, N, and O elements, have a multi-layer structure and a flower-like surface morphology, and the particle size is 0.8 to 1.2 μm.

[0021] The present invention provides the application of the N-O doped flower-like porous carbon microspheres described in the above technical solution as a microwave absorbing material in the field of electromagnetic waves.

[0022] The present invention provides a preparation method of N-O doped flower-like porous carbon microspheres. The preparation method includes the synthesis of multi-layer coated polymer microspheres, the synthesis of polymer flower spheres, and the carbonization of polymer flower spheres. The present invention uses a simple soap-free emulsion polymerization method to coat the microspheres, realizing a complex multi-layer heterogeneous interface structure, and simultaneously introducing N and O atoms to achieve heteroatom doping to improve the microwave absorbing performance; a unique flower-like structure is grown on the surface of the microspheres through a layer-by-layer coating self-assembly process, and delicate flower-like porous carbon microspheres can be obtained through carbonization, realizing a clever structure conducive to multiple reflection and scattering.

[0023] By changing the monomer types of the polymer seeds and the coating monomer types in the present invention, the core of the multi-layer coated polymer microspheres of the flower-like carbon microspheres can be adjusted; by adjusting the ratio of the multi-layer coated polymer microspheres to the outer layer of acrylonitrile petals, the surface morphology of the material can be regulated, and then the dielectric loss ability and impedance matching characteristics can be adjusted, enabling the material to meet the requirements of the new type of wave-absorbing material of "thin, light, wide, and strong". Under the conditions that the mass ratio of the N-O doped flower-like porous carbon microspheres to paraffin is 1:5 and the matching thickness is 2.5 - 3 mm, its minimum reflection loss is -40.8 - -48.0 dB, and the optimal effective absorption bandwidth is 4.40 GHz, having excellent electromagnetic wave absorption performance, and making up for the deficiencies of carbon materials such as low absorption intensity, poor impedance matching, and narrow absorption bandwidth.

[0024] The N-O doped flower-like porous carbon microsphere material prepared in the present invention has a unique multi-layer coating, a morphology of overlapping and interpenetrating flower flakes, and a pore structure, which is conducive to the incidence, multiple reflections, and scattering of electromagnetic waves (between the coating layer and the core, between microspheres, and between the surface flower flakes and pores). After carbonization, heterogeneous interfaces will be formed between multi-layer coating layers of different materials, significantly improving the multi-polar relaxation process, and the interpenetrating structure of the flower flakes can form more conductive networks, which is conducive to the transfer and transfer of electrons, thereby generating microcurrents and causing conductive losses. Secondly, there are abundant N and O atom doping and defects in the material, which will act as polarization centers, resulting in an asymmetric charge distribution, promoting the occurrence of dipole polarization, and increasing the polarization loss. Therefore, while preparing a unique flower-like microsphere morphology with polymer microspheres as seeds in the present invention, N and O heteroatoms can be directly introduced, so that this flower-like porous carbon microsphere has more excellent electromagnetic wave absorption performance than traditional carbon materials.

[0025] The preparation process of the N-O doped flower-like porous carbon microspheres prepared in the present invention is simple in operation, low in cost, and green and environmentally friendly. Description of the Drawings

[0026] Figure 1 SEM images of the microspheres formed in each step of preparing the multi-layer coated polymer microspheres in step A of Example 1. Among them, a is the uncoated acrylonitrile and styrene polymerized microspheres, b is the microspheres after the first layer is coated with the coating monomer DVB, c is the microspheres after the second layer is coated with the coating monomer St, and d is the microspheres after the third layer is coated with the coating monomer AN;

[0027] Figure 2 SEM image of the multi-layer coated polymer microspheres@acrylonitrile flower balls prepared in Example 1;

[0028] Figure 3 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 1;

[0029] Figure 4TEM images of the N-O doped flower-like porous carbon microspheres prepared in Example 1, where a is the TEM image of the multi-layer flower-like porous carbon microspheres, and b is the partially enlarged TEM image of a;

[0030] Figure 5 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 2;

[0031] Figure 6 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 3;

[0032] Figure 7 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 4;

[0033] Figure 8 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 1 and their EDS maps of C, N, and O elements;

[0034] Figure 9 Reflection loss diagram of the N-O doped flower-like porous carbon microspheres prepared in Example 1;

[0035] Figure 10 Reflection loss diagram of the N-O doped flower-like porous carbon microspheres prepared in Example 2;

[0036] Figure 11 Reflection loss diagram of the N-O doped flower-like porous carbon microspheres prepared in Example 3;

[0037] Figure 12 Reflection loss diagram of the N-O doped flower-like porous carbon microspheres prepared in Example 4. Detailed implementation mode

[0038] In the present invention, unless otherwise specified, the raw materials or reagents required for preparation are all commercially available products well-known to those skilled in the art.

[0039] The present invention provides a preparation method of N-O doped flower-like porous carbon microspheres, comprising the following steps:

[0040] Mix acrylonitrile, an olefinic reaction monomer, a first initiator, and water, and carry out a first polymerization reaction to obtain polymer seeds;

[0041] Mix the polymer seeds with a first coating monomer and carry out a first coating to obtain a first coating product;

[0042] Mix the first coating product with a second coating monomer and carry out a second coating to obtain a second coating product;

[0043] Mix the second coating product with a third coating monomer and carry out a third coating to obtain multi-layer coated polymer microspheres;

[0044] Mix the multi-layer coated polymer microspheres, acrylonitrile, a second initiator, and an organic solvent, and carry out a second polymerization reaction to obtain polymer flower balls;

[0045] Carry out carbonization on the polymer flower balls to obtain N-O doped flower-like porous carbon microspheres.

[0046] In the present invention, preferably, acrylonitrile and an olefinic reaction monomer are added to a reactor, a first initiator and an ultrapure water solvent are added, and after being stirred evenly in an oil bath, a first polymerization reaction is carried out by stirring to obtain polymer seeds; a first coating monomer is added dropwise to carry out a first coating reaction; then a second coating monomer is added dropwise to carry out a second coating reaction; then a third coating monomer is added dropwise to carry out a third coating reaction.

[0047] In the present invention, the olefinic reaction monomer preferably includes one or more of styrene (St), methyl methacrylate (MMA), divinylbenzene (DVB), and diallyl maleate (DAM), more preferably styrene St or divinylbenzene DVB, and further preferably styrene St. The olefinic reaction monomers in the present invention are all commercially available products. For example, styrene sold by Shanghai Aladdin Biochemical Technology Co., Ltd. under the trade name styrene, and methyl methacrylate sold by Sinopharm Chemical Reagent Co., Ltd. under the trade name methyl methacrylate.

[0048] In the step of the first polymerization reaction in the present invention, the mass ratio of acrylonitrile to the olefinic reaction monomer is preferably 2-5:1, more preferably 3-4:1; in this step, acrylonitrile and the olefinic reaction monomer are in a mass ratio of 2-5:1. If the proportion of acrylonitrile is less than 2, the microspheres obtained by polymerization agglomerate severely; if the proportion of acrylonitrile is higher than 5, it will affect the electromagnetic wave absorption performance of the flower-like porous carbon microspheres prepared subsequently.

[0049] In the step of the first polymerization reaction, the main functions of acrylonitrile and the olefinic reaction monomer are to polymerize under the initiation of the initiator to form seeds of multi-layer coated polymer microspheres, and in addition, N atoms can be introduced in this process.

[0050] In the present invention, the first initiator preferably includes azobisisobutyronitrile, azobisisobutylamidine hydrochloride, potassium persulfate, ammonium persulfate or sodium persulfate, more preferably azobisisobutylamidine hydrochloride and azobisisobutyronitrile; the mass of the first initiator is preferably 0.8-2.0% of the total mass of acrylonitrile and the olefinic reaction monomer, preferably 1.0-1.8%, more preferably 1.5-1.6%. The first initiators of the present invention are all commercially available products, such as azobisisobutylamidine hydrochloride sold by Guangzhou Yuanda New Materials Co., Ltd. under the trade name azobisisobutylamidine hydrochloride. In this step, if the dosage of the first initiator is less than 0.8%, the polymerization reaction will not proceed sufficiently; if the dosage of the initiator is higher than 2.0%, agglomeration will occur.

[0051] In the present invention, the water is preferably ultrapure water; the mass of the water is preferably 10-20 times the total mass of acrylonitrile and the olefinic reaction monomer, more preferably 15-20 times; in this step, if the amount of ultrapure water is less than 10 times the weight of the monomer mixture, serious agglomeration will occur; if the amount of ultrapure water is higher than 20 times, the yield will be too low and resource waste will be caused.

[0052] In the present invention, the temperature of the first polymerization reaction is preferably 70-75 °C, more preferably 72-75 °C, and the time is preferably 2-4 h, more preferably 3 h. In this step, the reaction temperature is 70-75 °C and the reaction time is 3 h. If the reaction temperature is too low or the time is too short, the polymerization reaction will be difficult to occur or will not proceed sufficiently; if the reaction temperature is too high, microsphere agglomeration will occur, and if the time is too long, resource waste will be caused.

[0053] In the present invention, the first coating monomer, the second coating monomer and the third coating monomer preferably independently include one or more of styrene (St), methyl methacrylate (MMA), divinylbenzene (DVB), acrylonitrile (AN) and diallyl maleate (DAM), more preferably DVB, AN or St; the coating monomers are all commercially available products, such as styrene St sold by Jiangsu Bost Chemical Technology Co., Ltd. under the trade name styrene.

[0054] In this step, the addition of the coating monomer is to continue to initiate the coating reaction on the surface of the formed polymer seeds to form a multi-layer coating structure. Generally, the effect is more obvious when there are more than 2 coating layers; the coating layer can effectively enhance the heterogeneous interface structure of the polymer microspheres, thereby improving the electromagnetic wave absorption performance of the subsequent carbon microspheres.

[0055] In the present invention, the mass of the first coating monomer, the second coating monomer and the third coating monomer is preferably independently 20-50% of the total mass of acrylonitrile and the olefinic reaction monomer in the first polymerization reaction, more preferably 25-30%. If there are too many monomers, the reaction time will be too long and the coating layer will also be too thick.

[0056] In the present invention, the temperatures of the first coating, the second coating, and the third coating are preferably independently 70 - 75°C, more preferably 72°C, and the time is 2 - 4 h, more preferably 3 h. In this step, the temperature of each coating process is independently 70 - 75°C, and the reaction time is 3 h. If the reaction temperature is too low or the time is too short, the coating process will be difficult to occur or incomplete; if the reaction temperature is too high, agglomeration of the seeds or self-polymerization of the coating monomers will occur, and if the time is too long, it will cause waste of resources.

[0057] After completing the third coating, the present invention preferably filters the obtained product by suction, washes the obtained solid successively with ethanol and deionized water to remove unreacted monomers and initiators, and then dries it in an oven at 40 - 60°C for 12 - 24 h to obtain multi-layer coated polymer microspheres. Among them, the purpose of drying is to remove ethanol and water to ensure the dryness of the product. The heterogeneous structure formed by multi-layer coating in the present invention is beneficial to improving the electromagnetic wave absorption performance.

[0058] After obtaining the multi-layer coated polymer microspheres, the present invention adds the multi-layer coated polymer microspheres and acrylonitrile into a reaction kettle, then adds an organic solvent and a second initiator, stirs evenly, ultrasonically disperses for 10 - 20 min, seals the reaction kettle, and carries out the second polymerization reaction.

[0059] In the present invention, in the step of the second polymerization reaction, the mass ratio of the multi-layer coated polymer microspheres to acrylonitrile is preferably 1:3 - 6, more preferably 1:4 - 5. In this step, the function of the outer layer of acrylonitrile is to shape the outer layer flower-like structure, and acrylonitrile grows on the surface of the multi-layer coated polymer microspheres in the form of thin flakes. This is because in this process, acrylonitrile in the system continuously polymerizes on the surface of the microsphere seeds, but when the molecular weight of the polyacrylonitrile polymerized on the microsphere surface reaches the critical value, it begins to precipitate out on the microsphere surface, thus forming sheet-like inserts, and the interpenetrating growth of the inserts leads to the formation of a flower-like morphology. For the ratio of the multi-layer coated polymer microspheres to acrylonitrile, if the proportion of acrylonitrile is less than 3, the formed outer layer is incomplete, and if it is too much, it is easy to cause serious agglomeration of the microspheres.

[0060] In the present invention, the second initiator preferably includes azobisisobutyronitrile, azobis(isobutyramidine) hydrochloride, potassium persulfate, ammonium persulfate, or sodium persulfate, more preferably azobis(isobutyramidine) hydrochloride and azobisisobutyronitrile; the mass of the second initiator is preferably 0.8 - 1.5% of the total mass of the multi-layer coated polymer microspheres and acrylonitrile, more preferably 1.0 - 1.2%. In this step, it is not advisable for the proportion of the second initiator to exceed the range. If the dosage of the initiator is less than 0.8%, the reaction will be incomplete; if the dosage of the initiator is higher than 1.5%, agglomeration will occur.

[0061] In the present invention, the organic solvent is preferably acetone; the mass of the organic solvent is preferably 125-175%, more preferably 150-160%, of the total mass of the multi-layer coated polymer microspheres and acrylonitrile in the second polymerization reaction. In this step, if the mass of acetone added is too low, microsphere aggregation will occur, and if it is too high, the thickness of the outer cauliflower-like spheres will be relatively thin. In the present invention, acetone mainly has two functions. One is to act as a dispersant to make the prepared multi-layer coated polymer microspheres disperse uniformly; the other is to act as a solvent to dissolve the polyacrylonitrile molecules on the microsphere surface when they are of low molecular weight. When the molecular weight reaches the critical value, precipitation starts on the microsphere surface, forming sheet-like inserts. The interpenetrating growth of the inserts leads to the formation of a flower-like morphology.

[0062] In the above step, the ultrasonic dispersion is for 10-20 min. If the ultrasonic dispersion time is too short, the microspheres will not disperse evenly in acetone, resulting in an uneven flower-like morphology; if the ultrasonic time is too long, it will cause waste of resources.

[0063] In the present invention, the temperature of the second polymerization reaction is preferably 70-75 °C, more preferably 75 °C, and the time is preferably 12-24 h, more preferably 12 h. In this step, the reaction temperature is 70-75 °C and the reaction time is 12-24 h. If the reaction temperature is too low or the time is too short, the polymerization reaction will be difficult to occur or will be insufficient; if the reaction temperature is too high, microsphere aggregation will occur, and if the time is too long, it will cause waste of resources.

[0064] After completing the second polymerization reaction, in the present invention, the obtained product is preferably cooled and then washed successively with ethanol and deionized water, and then dried in an oven at 40-60 °C for 12-24 h to obtain polymer cauliflower-like spheres. In this step, washing with ethanol and deionized water is to remove unreacted monomers and initiators; the filtered product needs to be dried in an oven at 40-60 °C for 12-24 h. The main purpose is to remove ethanol and water to ensure the dryness of the product. Drying conditions exceeding the above range will cause waste of energy.

[0065] In the present invention, the particle size of the polymer cauliflower-like spheres (i.e., multi-layer coated polymer microspheres@acrylonitrile cauliflower-like spheres) is preferably 1.2-2.0 μm.

[0066] After obtaining the polymer cauliflower-like spheres, in the present invention, the polymer cauliflower-like spheres are preferably placed in an alumina crucible and heated from room temperature to the carbonization temperature in a tubular furnace under a protective gas condition for carbonization, and then naturally cooled to room temperature to obtain N-O doped flower-like porous carbon microspheres.

[0067] In the present invention, the temperature of carbonization is preferably 700 - 900 °C, more preferably 800 - 900 °C, the time is preferably 2 - 3 h, and the atmosphere is preferably nitrogen or argon; the heating rate for heating to the temperature of carbonization is preferably 1 - 3 °C / min, more preferably 2 - 2.5 °C / min.

[0068] In the step of carbonization, the copolymer is protected by gas to generate carbon without being oxidized, and the multi-layer coated polymer microsphere@acrylonitrile flower ball is carbonized, and N - O doped multi-layer flower-like porous carbon microspheres are formed through the processes of pyrolysis and graphitization. Controlling the heating rate within 1.0 - 3.0 °C / min can maintain the stability of the structure and ensure sufficient pyrolysis and carbon reduction of the material.

[0069] The present invention provides N - O doped flower-like porous carbon microspheres prepared by the preparation method described in the above technical solution. The N - O doped flower-like porous carbon microspheres contain C, N, and O elements, have a multi-layer structure and a flower-like surface morphology, and the particle size is 0.8 - 1.2 μm.

[0070] The present invention provides the application of the N - O doped flower-like porous carbon microspheres described in the above technical solution as microwave absorbing materials in the field of electromagnetic waves. The present invention has no special limitation on the method of this application, and it can be applied according to the methods well-known in the art.

[0071] The following describes the specific embodiments of the present invention in detail, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention. The experimental methods described in the embodiments of the present invention are all conventional methods unless otherwise specified.

[0072] The following experimental methods and detection methods are all conventional methods unless otherwise specified; the following reagents and raw materials are all commercially available unless otherwise specified.

[0073] Example 1

[0074] A. Preparation of multi-layer coated polymer microspheres:

[0075] Acrylonitrile and styrene were added to a three-necked flask in a mass ratio of 3:1. An initiator, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, was added in an amount of 1.5% of the mass of the acrylonitrile and styrene monomer mixture, and ultrapure water solvent was added in an amount 15 times the mass of the acrylonitrile and styrene monomer mixture. The resulting mixture was stirred evenly in an oil bath and then stirred and reacted at a temperature of 72 °C for 3 h to obtain uncoated acrylonitrile and styrene polymer microspheres. Then, the coating monomer DVB was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and styrene monomer mixture, and the reaction was carried out for 3 h. After that, the coating monomer St was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and styrene monomer mixture, and the reaction was carried out for 3 h. Then, the coating monomer AN was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and styrene monomer mixture, and the reaction was carried out for 3 h. Filtration was carried out, and the solid was washed successively with ethanol and deionized water, and then dried in an oven at 45 °C for 12 h to obtain multi-layer coated polymer microspheres.

[0076] B. Synthesis of multi-layer coated polymer microsphere@acrylonitrile flower ball

[0077] The multi-layer coated polymer microspheres prepared in step A and acrylonitrile were added to a reaction kettle in a mass ratio of 1:5. Then, acetone accounting for 160% of the total mass of the multi-layer coated polymer microspheres and acrylonitrile and 1.0% of the initiator 2,2'-azobisisobutyronitrile AIBN were added, stirred evenly, and ultrasonically dispersed for 15 min. After that, the reaction kettle was sealed and reacted at 75 °C for 12 h. After cooling, it was washed successively with ethanol and deionized water, and then dried in an oven at 45 °C for 12 h to obtain multi-layer coated polymer microsphere@acrylonitrile flower balls.

[0078] C. Carbonization of polymer flower balls

[0079] The multi-layer coated polymer microsphere@acrylonitrile flower balls obtained in step B were placed in an alumina crucible and heated from room temperature to 800 °C in a tubular furnace under the protection of nitrogen gas with a heating rate of 2 °C / min, calcined for 2 h, and then naturally cooled to room temperature to obtain N-O doped flower-like porous carbon microspheres.

[0080] Example 2

[0081] A. Preparation of multi-layer coated polymer microspheres

[0082] Add acrylonitrile and styrene into a three-necked flask according to a mass ratio of 5:1. Add initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride at 1.8% of the mass of the acrylonitrile and styrene monomer mixture and add ultrapure water solvent at 20 times the mass of the acrylonitrile and styrene monomer mixture. After stirring the obtained mixture evenly in an oil bath, stir and react at 75 °C for 3 h. Dropwise add coating monomer DVB, with a mass of 30% of the mass of the above acrylonitrile and styrene monomer mixture, and react for 3 h. Then dropwise add coating monomer St, with a mass of 30% of the mass of the above acrylonitrile and styrene monomer mixture, and react for 3 h. Then dropwise add coating monomer AN, with a mass of 30% of the mass of the above acrylonitrile and styrene monomer mixture, and react for 3 h. Filter by suction, wash the solid successively with ethanol and deionized water, and then dry in an oven at 60 °C for 12 h to obtain multi-layer coated polymer microspheres.

[0083] B. Synthesis of multi-layer coated polymer microsphere@acrylonitrile flower balls

[0084] Add the multi-layer coated polymer microspheres prepared in step A and acrylonitrile into a reaction kettle according to a mass ratio of 1:5. Then add acetone at 150% of the total mass of the multi-layer coated polymer microspheres and acrylonitrile and 1.0% initiator 2,2'-azobis(2-methylpropionitrile) AIBN, stir evenly, ultrasonically disperse for 20 min, then seal the reaction kettle, react at 75 °C for 12 h, cool, wash successively with ethanol and deionized water, and then dry in an oven at 60 °C for 12 h to obtain multi-layer coated polymer microsphere@acrylonitrile flower balls.

[0085] C. Carbonization of polymer flower balls

[0086] Place the multi-layer coated polymer microsphere@acrylonitrile flower balls obtained in step B in an alumina crucible, heat from room temperature to 900 °C in a tubular furnace under the protection of nitrogen gas and a heating rate of 2.5 °C / min, calcine for 2 h, and then naturally cool to room temperature to obtain N-O doped flower-like porous carbon microspheres.

[0087] Example 3

[0088] A. Preparation of multi-layer coated polymer microspheres

[0089] Add acrylonitrile and divinylbenzene into a three-necked flask according to a mass ratio of 4:1. Add initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride according to 1.5% of the mass of the acrylonitrile and divinylbenzene monomer mixture, and add ultrapure water solvent according to 20 times the mass of the acrylonitrile and divinylbenzene monomer mixture. After stirring the obtained mixture evenly in an oil bath, stir and react at 72 °C for 3 h. Dropwise add coating monomer AN, with a mass of 25% of the mass of the above acrylonitrile and divinylbenzene monomer mixture, and react for 3 h. Then dropwise add coating monomer St, with a mass of 25% of the mass of the above acrylonitrile and divinylbenzene monomer mixture, and react for 3 h. Then dropwise add coating monomer AN, with a mass of 25% of the mass of the above acrylonitrile and divinylbenzene monomer mixture, and react for 3 h. Filter by suction, wash the solid with ethanol and deionized water in sequence, and then dry in an oven at 45 °C for 20 h to obtain multilayer-coated polymer microspheres.

[0090] B. Synthesis of Multilayer-Coated Polymer Microsphere@Acrylonitrile Flower Balls

[0091] Add the multilayer-coated polymer microspheres prepared in step A and acrylonitrile into a reaction kettle according to a mass ratio of 1:4. Then add acetone accounting for 150% of the total mass of the multilayer-coated polymer microspheres and acrylonitrile and 1.2% of initiator 2,2'-azobis(2-methylpropionitrile) AIBN, stir evenly, disperse ultrasonically for 20 min, then seal the reaction kettle, react at 75 °C for 12 h, cool, wash with ethanol and deionized water in sequence, and then dry in an oven at 45 °C for 12 h to obtain multilayer-coated polymer microsphere@acrylonitrile flower balls.

[0092] C. Carbonization of Polymer Flower Balls

[0093] Place the multilayer-coated polymer microsphere@acrylonitrile flower balls obtained in step B in an alumina crucible, heat from room temperature to 800 °C in a tube furnace under the protection of nitrogen gas and a heating rate of 2 °C / min, calcine for 2 h, and then naturally cool to room temperature to obtain N-O doped flower-like porous carbon microspheres.

[0094] Example 4

[0095] A. Preparation of Multilayer-Coated Polymer Microspheres

[0096] Acrylonitrile and methyl methacrylate were added into a three-necked flask in a mass ratio of 5:1. An initiator azobisisobutyronitrile was added in an amount of 1.6% of the mass of the acrylonitrile and methyl methacrylate monomer mixture, and ultrapure water solvent was added in an amount 15 times the mass of the acrylonitrile and methyl methacrylate monomer mixture. After the obtained mixture was stirred evenly in an oil bath, it was stirred and reacted at 75 °C for 3 h. Then, diallyl maleate DAM as the coating monomer was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and methyl methacrylate monomer mixture, and the reaction was carried out for 3 h. After that, styrene St as the coating monomer was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and methyl methacrylate monomer mixture, and the reaction was carried out for 3 h. Then, acrylonitrile AN as the coating monomer was added dropwise, with a mass of 25% of the mass of the above acrylonitrile and methyl methacrylate monomer mixture, and the reaction was carried out for 3 h. The mixture was filtered by suction, and the solid was washed with ethanol and deionized water, and then dried in an oven at 45 °C for 12 h to obtain multilayer-coated polymer microspheres.

[0097] B. Synthesis of Multilayer-Coated Polymer Microspheres@Acrylonitrile Flower Balls

[0098] The multilayer-coated polymer microspheres prepared in step A and acrylonitrile were added into a reaction kettle in a mass ratio of 1:6. Then, acetone with a mass of 175% of the total mass of the multilayer-coated polymer microspheres and acrylonitrile and 1.2% of the initiator azobisisobutyronitrile AIBN were added. After stirring evenly and ultrasonic dispersing for 15 min, the reaction kettle was sealed and reacted at 70 °C for 24 h. After cooling, it was washed successively with ethanol and deionized water, and then dried in an oven at 45 °C for 12 h to obtain multilayer-coated polymer microspheres@acrylonitrile flower balls.

[0099] C. Carbonization of Polymer Flower Balls

[0100] The multilayer-coated polymer microspheres@acrylonitrile flower balls obtained in step B were placed in an alumina crucible and heated from room temperature to 900 °C in a tube furnace under the protection of nitrogen gas with a heating rate of 3 °C / min, calcined for 2 h, and then naturally cooled to room temperature to obtain N-O doped flower-like porous carbon microspheres.

[0101] Characterization and Performance Testing

[0102] The multilayer-coated polymer microspheres, polymer microspheres@acrylonitrile flower ball products and carbonized products prepared in Example 1 were characterized by electron microscopy analysis using a scanning electron microscope sold by Hitachi, Ltd. under the trade name S-4800 under conventional conditions. The results are shown in Figures 1 - 4 .

[0103] Figure 1SEM images of the microspheres formed in each step of preparing the multi-layer coated polymer microspheres in step A of Example 1. Among them, a is the uncoated acrylonitrile and styrene polymerized microspheres, b is the microspheres after the first layer is coated with the monomer DVB, c is the microspheres after the second layer is coated with the monomer St, and d is the microspheres after the third layer is coated with the monomer AN; from Figure 1 it can be seen that the morphology of the polymer microspheres is uniform before and after coating, and they always have good dispersibility; from the comparison of a-d, it can be seen that the particle size of the microspheres increases after each coating with the monomer, and the particle size of the microspheres increases significantly after three coatings.

[0104] Figure 2 SEM image of the multi-layer coated polymer microsphere @ acrylonitrile flower sphere prepared in Example 1;

[0105] Figure 3 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 1; Figure 4 TEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 1, where a is the TEM image of the flower-like porous carbon microspheres, and b is the partially magnified TEM image of a; from Figures 2 - 4 it can be seen that the polymer microspheres in Example 1 exhibit a regular flower-like morphology, and the particle size is in the range of 1.2 - 2.0 μm 。 And it can be seen that after carbonization, the particle size of the microspheres decreases, being 0.8 - 1.2 μm, but still maintains the flower-like morphology, Figure 4 The TEM image further shows the clear morphology of the flower-like microspheres after carbonization.

[0106] Figure 5 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 2; Figure 6 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 3; Figure 7 SEM image of the N-O doped flower-like porous carbon microspheres prepared in Example 4; from Figures 5 - 7 the results, it can be seen that the product is a multi-layer coated flower-like porous carbon microsphere with a particle size of 0.8 - 1.2 μm.

[0107] Figure 8 SEM image of the N-O doped flower-like porous carbon microspheres of Example 1 and the corresponding EDS maps of C, N, and O elements. From Figure 8 it can be seen that the flower-like porous carbon microspheres are composed of C, N, and O elements, proving the successful doping of N and O atoms.

[0108] Application Test Example 1

[0109] According to the method described by D. Wang et al. in the article "Preparation and Electromagnetic-wave-absorption properties of N,O-doped PMMA&DVB&AN carbon microspheres with porous hollow structures" in *Chemical Engineering Journal*, 456(2023)140987. The N-O doped flower-like porous carbon microspheres prepared in Example 1 were ground into powder state with an agate mortar. The obtained multi-layer flower-like porous carbon microsphere powder was mixed with paraffin according to a mass ratio of 1:5 (sample: paraffin). It was heated to 80 °C in a water bath of an ultrasonic cleaner to melt the paraffin and mix and stir it evenly with the sample, and then poured into a mold and pressed into a hollow coaxial circular ring sample (inner diameter is 3.04 mm, outer diameter is 7.00 mm). A vector network analyzer of model PNA N5224A produced by Agilent Technologies was used to measure the electromagnetic parameters of the sample at 2-18 GHz by the coaxial method. The reflection loss diagram of the sample was calculated through the transmission line theory (reference: Z. Liu et al., "Doped Porous Carbon Spheres with Controllable Vesicle Structure: Preparation and the Effects of Pore Size on Electromagnetic Wave Absorption Properties", *Small*, (2024)2402000), and the results are listed in Figure 9 , as Figure 9 can be seen, for this multi-layer flower-like porous carbon microsphere, under the conditions of a filling rate of 16.7% and a matching thickness of 2.50 mm, its minimum reflection loss can reach -48.0 dB (12.56 GHz), and it has an optimal effective absorption bandwidth of 4.30 GHz (8.65 - 12.95 GHz) at a thickness of 3.00 mm.

[0110] Application Test Example 2

[0111] The reflection loss diagram of the N-O doped flower-like porous carbon microspheres prepared in Example 2 was measured according to the method of Application Test Example 1 above, and the results are listed in Figure 10 , as Figure 10 can be seen, for this multi-layer flower-like porous carbon microsphere, under the conditions of a filling rate of 16.7% and a matching thickness of 2.50 mm, its minimum reflection loss can reach -42.8 dB (11.92 GHz), and it has an optimal effective absorption bandwidth of 4.15 GHz (13.17 - 17.32 GHz).

[0112] Application Test Example 3

[0113] Test the reflection loss graph of the N-O doped flower-like porous carbon microspheres prepared in Example 3 according to the method of Application Test Example 1 above, and the results are listed in Figure 11 , from Figure 11 It can be seen that for this multi-layer flower-like porous carbon microsphere, under the conditions of a filling ratio of 16.7% and a matching thickness of 2.50 mm, its minimum reflection loss can reach -40.8 dB (12.24 GHz), and it has an optimal effective absorption bandwidth of 4.40 GHz (13.39 - 17.79 GHz).

[0114] Application Test Example 4

[0115] Test the reflection loss graph of the N-O doped flower-like porous carbon microspheres prepared in Example 4 according to the method of Application Test Example 1 above, and the results are listed in Figure 12 , from Figure 12 It can be seen that for this multi-layer flower-like porous carbon microsphere, under the conditions of a filling ratio of 16.7% and a matching thickness of 2.50 mm, its minimum reflection loss can reach -42.7 dB (12.88 GHz), and it has an optimal effective absorption bandwidth of 4.07 GHz (9.15 - 13.22 GHz).

[0116] From the above test results, it can be seen that for the multi-layer flower-like porous carbon microspheres of the present invention, under the conditions of a mass ratio of 1:5 with paraffin and a matching thickness of 2.5 - 3 mm, its minimum reflection loss is -40.8 to -48.0 dB, and the optimal effective absorption bandwidth is 4.40 GHz.

[0117] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of N-O doped flower-like porous carbon microspheres, characterized in that, It includes the following steps: Mix acrylonitrile, vinyl reaction monomer, first initiator and water, and carry out the first polymerization reaction to obtain polymer seeds; Mix the polymer seeds with the first coating monomer and carry out the first coating to obtain the first coated product; Mix the first coated product with the second coating monomer and carry out the second coating to obtain the second coated product; Mix the second coated product with the third coating monomer and carry out the third coating to obtain multi-layer coated polymer microspheres; Mix the multi-layer coated polymer microspheres, acrylonitrile, second initiator and organic solvent, and carry out the second polymerization reaction to obtain polymer flower balls; Carry out carbonization on the polymer flower balls to obtain N-O doped flower-like porous carbon microspheres.

2. The preparation method according to claim 1, wherein The vinyl reaction monomer includes one or more of styrene, methyl methacrylate, divinylbenzene and diallyl maleate; the first initiator and the second initiator independently include azobisisobutyronitrile, azobis (2-amidinopropane) hydrochloride, potassium persulfate, ammonium persulfate or sodium persulfate.

3. The preparation method according to claim 1 or 2, characterized in that In the step of the first polymerization reaction, the mass ratio of acrylonitrile to vinyl reaction monomer is 2-5:1; the mass of the first initiator is 0.8-2.0% of the total mass of acrylonitrile and vinyl reaction monomer; the temperature of the first polymerization reaction is 70-75 °C, and the time is 2-4 h.

4. The preparation method according to claim 1, wherein, The first coating monomer, the second coating monomer and the third coating monomer independently include one or more of styrene, methyl methacrylate, divinylbenzene, acrylonitrile and diallyl maleate; the masses of the first coating monomer, the second coating monomer and the third coating monomer are independently 20-50% of the total mass of acrylonitrile and vinyl reaction monomer in the first polymerization reaction.

5. The preparation method according to claim 1 or 4, characterized in that, The temperatures of the first coating, the second coating and the third coating are independently 70-75 °C, and the time is 2-4 h.

6. The preparation method according to claim 1, characterized in that, In the step of the second polymerization reaction, the mass ratio of the multi-layer coated polymer microspheres to acrylonitrile is 1:3-6, the mass of the second initiator is 0.8-1.5% of the total mass of the multi-layer coated polymer microspheres and acrylonitrile; the temperature of the second polymerization reaction is 70-75 °C, and the time is 12-24 h.

7. The preparation method according to claim 1 or 6, characterized in that, The particle size of the polymer flower balls is 1.2-2.0 μm.

8. The preparation method according to claim 1, characterized in that, The temperature of the carbonization is 700-900 °C, the time is 2-3 h, and the atmosphere is nitrogen or argon; the heating rate to the temperature of the carbonization is 1-3 °C / min.

9. The N-O doped flower-like porous carbon microspheres prepared by the preparation method according to any one of claims 1 to 8, characterized in that, The N-O doped flower-like porous carbon microspheres contain C, N and O elements, have a multi-layer structure and a flower-like surface morphology, and the particle size is 0.8-1.2 μm.

10. Application of the N-O doped flower-like porous carbon microspheres according to claim 9 as a wave-absorbing material in the field of electromagnetic waves.

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