Three-dimensional helical carbon nanofiber, preparation method thereof and wave absorption application

Abstract

The application belongs to the technical field of electromagnetic wave absorbing materials, and particularly relates to a three-dimensional spiral carbon nanofiber, a preparation method thereof and wave absorbing application. The preparation method comprises the following steps: (a) preparing a composite fiber with a three-dimensional spiral structure by using an electrostatic spinning technology, wherein the composite fiber comprises a carbon source polymer, polyurethane, a pore-forming agent and a magnetic transition metal salt; (b) removing the polyurethane in the composite fiber and carbonizing the carbon source polymer in the composite fiber; wherein the carbonization process comprises three holding stages, the first holding stage is a pre-oxidation stage, the second holding stage is a primary carbonization stage, and the third holding stage is a deep carbonization stage; and the prepared fiber has the advantages of light weight, wide absorption frequency band, high absorption intensity, good chemical stability and the like, and can be used as a high-efficiency wave absorbing material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electromagnetic wave absorbing materials, and particularly relates to a three-dimensional helical carbon nanofiber, a preparation method thereof and wave-absorbing application. BACKGROUND

[0002] The three-dimensional helical carbon nanofiber has great application potential in the fields of electromagnetic shielding, drug release, supercapacitors and other frontier fields due to its unique spatial helical structure. It not only has excellent mechanical properties and unique electrical properties, but also has a large specific surface area, and has become a core material for promoting the cross-development of material science, chemistry, physics and other disciplines, and has attracted high attention from the scientific research and industrial communities.

[0003] At present, the preparation technology of three-dimensional helical carbon nanofiber shows a diversified trend, and chemical vapor deposition method, biomass template method and the like are widely used in the field.

[0004] For example, the document (In situ construction of hierarchical core-shell Fe3O4@C nanoparticles-helical carbon nanotool hybrid composites for highly efficient electromagnetic wave absorption. Carbon, 171 (2021): 395-408.) utilizes the chemical vapor deposition method to prepare helical carbon nanocoils, however, the method has strict requirements on equipment and reaction conditions, the reaction process is complex, the product needs to be separated from the catalyst Fe / Sn and Al2O3 matrix, and the like, and is obtained through high-efficiency purification technology, resulting in high production cost.

[0005] The document (Hierarchical porous structure for superior microwave absorption in biomass-derived carbon microcoils. Ceramics International, 49 (2023): 35885-35897) uses purple leaf plum leaves as a carbon source, treats the leaves with a boiling NaOH / Na2SO3 solution for 90-120 min, washes and dries them, then ultrasonically treats them with a 30% volume fraction H2SO4 solution for 30 min, washes and freeze-dries them to obtain helical vessels in the leaves, and finally carbonizes them at high temperature to obtain helical carbon nanocoils. Although the method has abundant raw material sources and low cost, the preparation process is complicated, and high-temperature carbonization is prone to cause structure collapse.

[0006] Therefore, the existing preparation technology has disadvantages, and a new method needs to be developed to meet the actual application requirements.

[0007] Electromagnetic wave absorbing materials (absorbing materials) can effectively deal with electromagnetic radiation and interference problems, and help military equipment to achieve stealth effect, and become a research hotspot in the field of scientific research. A large number of studies have confirmed that impedance matching characteristics and loss attenuation characteristics are two key factors affecting the electromagnetic wave absorption capacity.

[0008] Traditional absorbing materials include ferrite, ceramic, carbon material and conductive polymer. Among them, carbon nanofiber material has outstanding advantages in developing "thin, light, wide and strong" high-performance absorbing materials due to its low density, strong chemical stability, good electrical conductivity and flexible structure design. However, it cannot be ignored that the high electrical conductivity of carbon nanofiber gives it a high dielectric constant, which will cause impedance mismatch, resulting in a large amount of reflection of incident electromagnetic waves on the material surface; and the single loss mechanism of pure carbon nanofiber makes it difficult to achieve strong absorption and wide frequency performance requirements.

[0009] In view of the above problems existing in carbon nanofiber absorbing materials, researchers usually adopt two strategies: one is to design the structure of carbon nanofiber, such as building core-shell, porous structure, etc., to give it multiple heterogeneous interfaces, and then produce multiple interface polarization loss; the second is to composite carbon nanofiber with magnetic loss material, which can adjust the impedance matching characteristics and introduce additional magnetic loss mechanism. Although these strategies have achieved certain results, the carbon nanofiber absorbing materials prepared by material compounding and structure design still face many challenges in achieving wide frequency strong absorption performance at a low doping ratio.

[0010] If three-dimensional spiral carbon nanofiber is applied to absorbing materials, its unique three-dimensional spiral structure may become the key to breaking through the above bottleneck. SUMMARY

[0011] The purpose of the present application is to solve the problems existing in the prior art, and to provide a three-dimensional spiral carbon nanofiber, a preparation method thereof and an absorbing application.

[0012] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0013] A preparation method of a three-dimensional spiral carbon nanofiber, comprising the following steps:

[0014] (a) preparing a composite fiber with a three-dimensional spiral structure by using electrospinning technology, the composite fiber comprising a carbon source polymer and polyurethane;

[0015] (b) removing the polyurethane in the composite fiber, and carbonizing the carbon source polymer in the composite fiber;

[0016] The carbonization process includes three holding stages, the first holding stage is a pre-oxidation stage, in which intramolecular cyclization reaction of the carbon source polymer occurs, linear macromolecular chains are cyclized and crosslinked to form a ring ladder structure, so that the fiber has higher thermal stability, preventing the three-dimensional spiral structure from collapsing or being broken due to the melting of the molecular chain at a high temperature in the subsequent carbonization process;

[0017] The second holding stage is a primary carbonization stage, in which the molecular chains of the carbon source polymer gradually form a conjugated carbon ring structure through dehydrogenation and denitrification reactions, and release nitrogen-containing gas (such as NH3, HCN, etc.) and oxygen-containing gas (such as CO2); the ring ladder structure formed in the pre-oxidation stage is further crosslinked to form a stable carbon network structure, the internal pores of the fiber are gradually closed, the surface roughness is reduced, and a preliminary turbostratic graphite microcrystalline structure is formed;

[0018] The third holding stage is a deep carbonization stage, in which, due to the gradual increase in temperature, the carbon microcrystals are further ordered at a high temperature, the interlayer spacing of the graphite is reduced, a graphite domain with higher crystallinity is formed, and the residual nitrogen, oxygen and other impurity elements are completely removed, and the carbon purity is improved; in addition, due to the transformation of the turbostratic structure to a graphite-like structure, the electrical conductivity and thermal stability of the fiber are enhanced; the internal stress of the fiber is gradually released at a high temperature, and the gradient heating avoids the collapse or breakage of the three-dimensional spiral structure, and finally a stable three-dimensional spiral carbon nanofiber is formed.

[0019] The present application adopts electrospinning technology and gradient carbonization process to prepare three-dimensional spiral carbon nanofiber, compared with the existing preparation method, the technology is simple to operate and is easier to realize large-scale production.

[0020] As a preferred technical solution:

[0021] The preparation method of the three-dimensional spiral carbon nanofiber as described above, in step (a), the carbon source polymer is polyacrylonitrile, polyvinylpyrrolidone, polyvinyl alcohol or polyimide, the mass ratio of the carbon source polymer to the polyurethane is 1:1.2-3; the electrospinning technology uses a coaxial, eccentric or parallel spinning nozzle; the diameter of the composite fiber is 400-1000 nm.

[0022] The preparation method of the three-dimensional spiral carbon nanofiber as described above, in step (b), the temperature of the first holding stage is 200-300℃, the gas atmosphere is air, and the holding time is 1-6h; the temperature of the second holding stage is 700-900℃, the gas atmosphere is nitrogen or inert gas, and the holding time is 1-6h; the temperature of the third holding stage is 1000-1500℃, the gas atmosphere is nitrogen or inert gas, and the holding time is 0.5-8h.

[0023] The preparation method of the three-dimensional spiral carbon nanofiber as described above, in step (b), the polyurethane in the composite fiber is removed by treating the composite fiber with a dissolving solution to dissolve the polyurethane and leave the carbon source polymer, or by pyrolysis during the carbonization process.

[0024] The preparation method of the three-dimensional spiral carbon nanofiber as described above, in step (a), the composite fiber further comprises a pore-forming agent (polymethyl methacrylate, polystyrene, polytetrafluoroethylene particles, etc.), which is dispersed in the carbon source polymer, and the mass ratio of the carbon source polymer to the pore-forming agent is 1:0.05-0.5; in step (b), the pore-forming agent in the composite fiber is removed by pyrolysis during the carbonization process.

[0025] The preparation method of the three-dimensional spiral carbon nanofiber as described above, in step (a), the composite fiber further comprises a magnetic transition metal salt, which is dispersed in the carbon source polymer;

[0026] The magnetic transition metal salt is Fe salt, Co salt or Ni salt (such as iron nitrate nonahydrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, etc.), and the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.05-0.5;

[0027] In step (b), after carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles, metal cobalt nanoparticles or metal nickel nanoparticles), which can be used as catalysts for growing carbon nanotubes on the surface of the fiber in the subsequent process.

[0028] In the prior art, when three-dimensional spiral carbon nanofibers are prepared by chemical vapor deposition or plasma technology, the catalyst needs to be removed, which may result in poor uniformity of the product, while in the present application, the magnetic metal nanoparticles serving as catalysts are part of the fiber, thus effectively avoiding this defect.

[0029] The preparation method of the three-dimensional spiral carbon nanofiber as described above further comprises step (c), that is, carbon nanotubes are modified on the surface of the product of step (b) by chemical vapor deposition or hydrothermal synthesis, and the final three-dimensional spiral carbon nanofiber is a three-dimensional spiral magnetic carbon nanofiber modified with carbon nanotubes;

[0030] In the chemical vapor deposition, the reaction temperature is 500-1000℃, the reaction time is 1-10h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (b), and the carbon source of the carbon nanotubes is pyrrole, acetylene, benzene, n-hexane, methane, ethylene or propylene, etc., and the gas atmosphere is nitrogen or inert gas;

[0031] In the hydrothermal synthesis, the reaction temperature is 200 DEG C, the reaction time is 20h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (b), and the carbon source of the carbon nanotube is pyrrole.

[0032] The application further provides a three-dimensional spiral carbon nanofiber A prepared by the method for preparing a three-dimensional spiral carbon nanofiber without step (c) according to any one of the above.

[0033] The application further provides a three-dimensional spiral carbon nanofiber B prepared by the method for preparing a three-dimensional spiral carbon nanofiber with step (c) according to the above.

[0034] The application further provides a wave-absorbing application of the three-dimensional spiral carbon nanofiber B according to the above, wherein the three-dimensional spiral carbon nanofiber B is used as a wave-absorbing agent to absorb electromagnetic waves; in general, when a standard circular ring is pressed by mixing with wax for testing by using a coaxial method, when the matching thickness (the thickness corresponding to the minimum reflection loss) is 1-3mm and the wave-absorbing agent incorporation ratio (namely, the mass ratio of the three-dimensional spiral carbon nanofiber B in the test circular ring sample) is 5%-15%, the minimum reflection loss is-89.2dB--36.7dB, and the maximum effective absorption bandwidth is 5.6-10.3GHz.

[0035] The application adopts the electrospinning technology to utilize two polymers with different physical properties to form a chiral spiral structure of nanofiber by solidification shrinkage and curling in the whipping process of the charged jet flow, and successfully prepares the three-dimensional spiral carbon nanofiber by combining a high-temperature carbonization process. On this basis, the magnetic metal nanoparticles in the carbon nanofiber are used as a catalyst, and pyrrole, acetylene, benzene, n-hexane, methane, ethylene, propylene and the like are selected as carbon sources to grow carbon nanotubes in situ on the surface of the carbon nanofiber, and then the three-dimensional spiral magnetic carbon nanofiber modified by the carbon nanotubes is obtained, which has the advantages of light weight, wide absorption frequency band, large absorption intensity, good chemical stability and the like, and can be used as a high-efficiency wave-absorbing material.

[0036] The three-dimensional spiral structure of the three-dimensional spiral carbon nanofiber of the application not only provides dielectric loss, but also enhances the cross-polarization effect of the electric field and the magnetic field; the carbon nanotube modification on the surface of the fiber endows the material with a multi-level nanostructure, which not only optimizes the impedance matching characteristics, but also can induce multiple scattering; the magnetic nanoparticles in the fiber construct a multi-level heterogeneous interface, introduce more polarization sites, and at the same time, enhance the magnetic loss effect. These characteristics enable the material to realize the microwave absorption performance of thin matching thickness, light weight, strong absorption and wide frequency band in the 2-18GHz frequency band, which can meet the stringent electromagnetic protection requirements of the civil field such as electromagnetic interference protection of electronic equipment, and the military field such as stealth equipment, and has a wide application prospect and great economic value.

[0037] Advantages:

[0038] (1) In view of the defects that the preparation process is complicated and the spiral structure is prone to collapse due to internal thermal stress in high-temperature carbonization in the biomass template method, the two polymers are solidified and shrunk to form stable spiral composite fibers in electrospinning, and then the thermal stability is enhanced through a pre-oxidation stage, and the stress is released through gradient heating carbonization, so that the spiral structure collapse is effectively avoided, and three-dimensional spiral carbon nanofibers with ideal morphology can be obtained.

[0039] (2) Compared with the chemical vapor deposition method and the plasma technology, the magnetic metal nanoparticles as catalysts in the present application are part of the fibers, and do not need to be removed subsequently, thereby avoiding the problem of poor product uniformity and simplifying the preparation process.

[0040] (3) The three-dimensional spiral magnetic carbon nanofibers prepared by the present application can realize strong absorption and wide frequency band with a low doping ratio in wave absorption applications, meet the requirements of high-performance wave absorption materials such as thinness, lightness, wide frequency band and strong absorption, and overcome the problems of impedance mismatch and single loss mechanism of traditional carbon nanofiber wave absorption materials. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 SEM image of the composite fiber with a three-dimensional spiral structure of Example 1;

[0042] Figure 2 Raman spectrum of the three-dimensional spiral carbon nanofiber of Example 1;

[0043] Figure 3 Reflection loss curve of the torus sample of Example 1 under 10 thicknesses (specific values are shown in the legend, unit: mm);

[0044] Figure 4 TEM image of the composite fiber after carbonization in Example 2;

[0045] Figure 5 Reflection loss curve of the torus sample of Example 2 under 10 thicknesses (specific values are shown in the legend, unit: mm). DETAILED DESCRIPTION

[0046] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. In addition, it should be understood that those skilled in the art can make various modifications or changes to the present application after reading the content taught by the present application, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

[0047] The following are the test methods of the relevant performance indicators in each embodiment:

[0048] Minimum reflection loss, maximum effective absorption bandwidth: after the test material is pressed into a circular ring sample with paraffin wax (manufacturer: China Pharmaceutical Group Co., Ltd., melting point: 60-62℃), the electromagnetic parameters of the sample (test frequency: 2-18GHz) are tested and the minimum reflection loss and the maximum effective absorption bandwidth are calculated according to the coaxial method test requirements; simulation is performed on the same sample to obtain the reflection loss curves (i.e. 10 curves) under 10 thicknesses, and the minimum reflection loss is the minimum value of the reflection loss in the 10 curves; the maximum effective absorption bandwidth is obtained by analyzing each curve to meet the frequency interval of "reflection loss ≤-10dB" (corresponding to electromagnetic wave absorption rate ≥90%) according to the 10 curves, and calculating the frequency width (i.e. effective absorption bandwidth, if the frequency interval is 6.5-18GHz, the effective absorption bandwidth is 11.5GHz), and selecting the maximum value from the frequency width of the 10 curves.

[0049] Example 1

[0050] A preparation method of a three-dimensional spiral carbon nanofiber, the specific steps are as follows:

[0051] (1) Preparation of raw materials;

[0052] Carbon source polymer: polyacrylonitrile, weight average molecular weight: 150000g / mol;

[0053] Solvent: N,N-dimethylformamide;

[0054] Magnetic transition metal salt: iron nitrate nonahydrate;

[0055] Polyurethane: manufacturer: Covestro, model: 9370AU;

[0056] Carbon source: methane;

[0057] Dissolution solution: tetrahydrofuran;

[0058] (2) Preparation of spinning solution a and spinning solution b;

[0059] The preparation process of the spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added to the solvent and stirred uniformly to obtain the spinning solution a; wherein, the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.2, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0060] The preparation process of the spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein, the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0061] (3) Preparation of a three-dimensional spiral carbon nanofiber;

[0062] (3.1) Injecting the spinning solution a and the spinning solution b into injectors respectively, using a eccentric spinning nozzle (composed of an inner tube and an outer tube, the center distance of the inner tube and the outer tube is 20% of the diameter of the outer tube, the spinning solution b goes through the outer tube, and the spinning solution a goes through the inner tube), and using electrospinning technology to prepare the composite fiber with three-dimensional spiral structure; wherein the mass ratio of the carbon source polymer and the polyurethane is 1:1.2; the spinning process parameters are: voltage 15kV, the push injection rate of the spinning solution a is 1mm / min, the push injection rate of the spinning solution b is 1mm / min, the distance between the spinning nozzle and the receiving drum is 16cm, the rotating speed of the receiving drum is 30rpm, the temperature is 21.5℃, and the humidity is 47.9%;

[0063] The diameter of the prepared composite fiber is 835nm, and the SEM is as shown in Figure 1 ;

[0064] (3.2) After drying the composite fiber, placing it in a proper amount of dissolving solution (with ultrasonic, the ultrasonic power is 180W), dissolving the polyurethane and retaining the carbon source polymer, taking out and drying, and then carbonizing the carbon source polymer in the composite fiber;

[0065] The carbonization process is: first, in an air atmosphere, heating from room temperature to 250℃ at a heating rate of 1℃ / min, and then keeping the temperature for 3h, then adjusting to a nitrogen or inert gas atmosphere, heating to 800℃ at a heating rate of 3℃ / min, and then keeping the temperature for 4h, and then keeping the atmosphere unchanged, heating to 1200℃ at a heating rate of 5℃ / min, and then keeping the temperature for 6h;

[0066] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles);

[0067] (3.3) Under the protection of nitrogen or inert gas, placing the product of step (3.2) in a tube furnace, and introducing a carbon source (flow rate is 60mL / min), and reacting at 800℃ for 3h, and the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), to obtain the three-dimensional spiral carbon nanofiber modified by carbon nanotubes (the Raman spectrum is as shown in Figure 2 ).

[0068] The three-dimensional spiral carbon nanofiber prepared in this embodiment is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 6%, and the matching thickness is 2.29mm (the actual test uses a circular ring sample with a specification of: outer diameter 7mm, inner diameter 3.04mm, and thickness 2.07mm), the minimum reflection loss is-59.7dB (the reflection loss curve is as shown in Figure 3 ), and the maximum effective absorption bandwidth is 8.2GHz.

[0069] Example 2

[0070] A method for preparing a three-dimensional spiral carbon nanofiber, the specific steps are as follows:

[0071] (1) Preparation of raw materials;

[0072] Carbon source polymer: polyvinylpyrrolidone, weight average molecular weight is 1300000 g / mol;

[0073] Solvent: N, N-dimethylformamide;

[0074] Magnetic transition metal salt: cobalt nitrate hexahydrate;

[0075] Polyurethane: the manufacturer is the company, the brand is 9370AU;

[0076] Dissolution solution: tetrahydrofuran;

[0077] Carbon source: pyrrole;

[0078] Mixed solution: composed of deionized water and ethanol with a volume ratio of 4:1;

[0079] (2) Preparation of spinning solution a and spinning solution b;

[0080] The preparation process of spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added to the solvent and stirred uniformly to obtain the spinning solution a; wherein, the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.05, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0081] The preparation process of spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein, the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0082] (3) Preparation of three-dimensional spiral carbon nanofiber;

[0083] (3.1) The spinning solution a and the spinning solution b are respectively injected into the syringe, and the parallel spinning nozzle (composed of two parallel and identical diameter circular tubes, the two circular tube parts are partially overlapped, and the center distance of the two circular tubes is 60% of the diameter of the circular tube) is used to prepare the composite fiber (diameter is 992 nm) with three-dimensional spiral structure by using electrospinning technology; wherein, the mass ratio of the carbon source polymer to the polyurethane is 1:1.2; the spinning process parameters are: voltage 17kV, the push injection rate of the spinning solution a is 1mm / min, the push injection rate of the spinning solution b is 1mm / min, the distance between the spinning nozzle and the receiving drum is 16cm, the rotating speed of the receiving drum is 30rpm, the temperature is 19.0℃, and the humidity is 40.5%;

[0084] (3.2) After the composite fiber is dried, it is placed in an appropriate amount of dissolution solution (with ultrasonic, the ultrasonic power is 180W), the polyurethane is dissolved, the carbon source polymer is retained, and then the carbon source polymer in the composite fiber is carbonized;

[0085] The carbonization process is: first, in an air atmosphere, from room temperature, at a heating rate of 1℃ / min, to 250℃, and then holding for 3h, then adjusting to a nitrogen or inert gas atmosphere, at a heating rate of 3℃ / min, to 800℃, and then holding for 4h, then keeping the atmosphere unchanged, at a heating rate of 5℃ / min, to 1200℃, and then holding for 6h;

[0086] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal cobalt nanoparticles), and the TEM is as shown in Figure 4 ;

[0087] (3.3) The carbon source is dispersed in the mixed solution, the product of step (3.2) is soaked therein for 1h, and then poured into a reaction kettle, and the reaction is carried out at 200℃ for 20h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), after the reaction is completed, the reaction system is naturally cooled, the product is taken out, and then sequentially washed with deionized water and anhydrous ethanol, and then dried, and then placed in a tube furnace, and then annealed at 600℃ for 2h under the protection of nitrogen or inert gas, to obtain a three-dimensional spiral carbon nanofiber modified by carbon nanotubes; wherein, the volume ratio of the carbon source to the mixed solution is 1:12, and the mass-volume ratio of the product of step (3.2) to the carbon source is 100mg:1mL.

[0088] The three-dimensional spiral carbon nanofiber prepared in this embodiment is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 6% and the matching thickness is 2.15mm (the actual test uses a circular ring sample with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 1.98mm), the minimum reflection loss is -72.0dB (the reflection loss curve is as shown in Figure 5 ), and the maximum effective absorption bandwidth is 8.2GHz.

[0089] Example 3

[0090] A preparation method of a three-dimensional spiral carbon nanofiber, and the specific steps are as follows:

[0091] (1) Preparation of raw materials;

[0092] Carbon source polymer: polyvinyl alcohol, weight average molecular weight is 127000g / mol;

[0093] Solvent: N,N-dimethylformamide;

[0094] Magnetic transition metal salt: nickel nitrate hexahydrate;

[0095] Polyurethane: the manufacturer is the company, the brand is 9370AU;

[0096] Carbon source: pyrrole;

[0097] Dissolution solution: tetrahydrofuran;

[0098] (2) Preparation of spinning solution a and spinning solution b;

[0099] The preparation process of the spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain the spinning solution a; wherein the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.1, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0100] The preparation process of the spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0101] (3) Preparation of three-dimensional spiral carbon nanofiber;

[0102] (3.1) The spinning solution a and the spinning solution b are respectively injected into the syringe, and a coaxial spinning nozzle (composed of an inner pipe and an outer pipe coaxially, the spinning solution b passes through the outer pipe, and the spinning solution a passes through the inner pipe) is used to prepare a composite fiber (with a diameter of 761 nm) with a three-dimensional spiral structure by using the electrospinning technology; wherein the mass ratio of the carbon source polymer to the polyurethane is 1:1.3; the spinning process parameters are: voltage 17 kV, the push injection rate of the spinning solution a is 1 mm / min, the push injection rate of the spinning solution b is 1 mm / min, the distance between the spinning nozzle and the receiving drum is 16 cm, the rotating speed of the receiving drum is 30 rpm, the temperature is 23.2℃, and the humidity is 46.0%;

[0103] (3.2) After the composite fiber is dried, it is placed in an appropriate amount of dissolution solution (accompanied by ultrasonic, the ultrasonic power is 180 W), the polyurethane is dissolved away, and the carbon source polymer is retained, and then the composite fiber is taken out and dried, and then the carbon source polymer in the composite fiber is carbonized;

[0104] The carbonization process is: first, in an air atmosphere, the temperature is raised from room temperature to 200℃ at a rate of 1℃ / min, and then kept for 3h, then adjusted to a nitrogen or inert gas atmosphere, and then raised to 800℃ at a rate of 3℃ / min, and then kept for 4h, and then kept the atmosphere unchanged, and then raised to 1200℃ at a rate of 5℃ / min, and then kept for 6h;

[0105] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal nickel nanoparticles);

[0106] (3.3) Under the protection of nitrogen or inert gas, the product of step (3.2) is placed in a tube furnace, and a carbon source (flow rate is 60 mL / min) is introduced, and the reaction is carried out at 800℃ for 4h, and the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), to obtain a three-dimensional spiral carbon nanofiber modified by carbon nanotubes.

[0107] The three-dimensional spiral carbon nanofiber prepared in the embodiment is used as a wave-absorbing agent for absorbing electromagnetic waves. When the incorporation ratio of the wave-absorbing agent is 6% and the matching thickness is 1.41 mm (the actual test uses a circular ring sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.3 mm), the minimum reflection loss is -67.2 dB, and the maximum effective absorption bandwidth is 8 GHz.

[0108] Example 4

[0109] A preparation method of a three-dimensional spiral carbon nanofiber, and the specific steps are as follows:

[0110] (1) Preparation of raw materials;

[0111] Carbon source polymer: polyimide, weight average molecular weight is 110000 g / mol;

[0112] Solvent: N,N-dimethylformamide;

[0113] Magnetic transition metal salt: ferric chloride hexahydrate;

[0114] Polyurethane: manufacturer is Covestro, brand is 9370AU;

[0115] Carbon source: acetylene;

[0116] Dissolution solution: tetrahydrofuran;

[0117] (2) Preparation of spinning solution a and spinning solution b;

[0118] The preparation process of the spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain the spinning solution a; wherein, the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.2, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0119] The preparation process of the spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein, the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0120] (3) Preparation of three-dimensional spiral carbon nanofiber;

[0121] (3.1) inject the spinning solution a and the spinning solution b into the syringes respectively, use coaxial spinning nozzle (consist of coaxial inner tube and outer tube, the spinning solution b goes through the outer tube, the spinning solution a goes through the inner tube), use electrospinning technology to prepare the composite fiber (diameter is 720 nm) with three-dimensional spiral structure; wherein the mass ratio of the carbon source polymer and the polyurethane is 1:1.4; the spinning process parameters are: voltage 18 kV, the push injection rate of the spinning solution a is 1 mm / min, the push injection rate of the spinning solution b is 1.1 mm / min, the distance between the spinning nozzle and the receiving drum is 16 cm, the rotating speed of the receiving drum is 30 rpm, the temperature is 22.6℃, and the humidity is 44.9%;

[0122] (3.2) after drying the composite fiber, place it in a proper amount of dissolving solution (with ultrasonic, the ultrasonic power is 180 W), dissolve the polyurethane and reserve the carbon source polymer, take out and dry, then carbonize the carbon source polymer in the composite fiber;

[0123] The carbonization process is: first, in air atmosphere, increase the temperature from room temperature to 250℃ at a rate of 1℃ / min, then keep the temperature for 3h, then adjust to nitrogen or inert gas atmosphere, increase the temperature to 800℃ at a rate of 3℃ / min, then keep the temperature for 2h, then keep the atmosphere unchanged, increase the temperature to 1100℃ at a rate of 5℃ / min, then keep the temperature for 6h;

[0124] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles);

[0125] (3.3) under the protection of nitrogen or inert gas, place the product of step (3.2) in a tube furnace, introduce carbon source (flow rate is 60 mL / min), react at 900℃ for 2h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), to obtain carbon nanotube modified three-dimensional spiral carbon nanofiber.

[0126] The three-dimensional spiral carbon nanofiber prepared in this embodiment is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 6% and the matching thickness is 1.70 mm (the actual test uses a circular ring sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.14 mm), the minimum reflection loss is-53 dB, and the maximum effective absorption bandwidth is 6.7 GHz.

[0127] Example 5

[0128] A preparation method of a three-dimensional spiral carbon nanofiber, the specific steps are as follows:

[0129] (1) preparation of raw materials;

[0130] Carbon source polymer: polyacrylonitrile, weight average molecular weight is 150000 g / mol;

[0131] Solvent: N,N-dimethylformamide;

[0132] Magnetic transition metal salt: nickel chloride hexahydrate;

[0133] Polyurethane: 9370AU, a product of Covestro Co., Ltd.

[0134] Carbon source: benzene;

[0135] Dissolution liquid: tetrahydrofuran;

[0136] (2) Preparation of spinning solution a and spinning solution b;

[0137] The preparation process of the spinning solution a is as follows: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain the spinning solution a; wherein the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.3, and the mass ratio of the carbon source polymer to the solvent is 1:11;

[0138] The preparation process of the spinning solution b is as follows: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein the mass ratio of the polyurethane to the solvent is 1.1:8.9;

[0139] (3) Preparation of three-dimensional spiral carbon nanofiber;

[0140] (3.1) The spinning solution a and the spinning solution b are respectively injected into injectors, and a composite fiber with a three-dimensional spiral structure (the diameter is 413 nm) is prepared by using an electrospinning technology with a core-spun spinning nozzle (composed of an inner circular tube and an outer circular tube, the center distance between the inner circular tube and the outer circular tube is 20% of the diameter of the outer circular tube, the spinning solution b passes through the outer circular tube, and the spinning solution a passes through the inner circular tube); wherein the mass ratio of the carbon source polymer to the polyurethane is 1:1.5; the spinning process parameters are as follows: the voltage is 20 kV, the push injection rate of the spinning solution a is 1 mm / min, the push injection rate of the spinning solution b is 1.2 mm / min, the distance between the spinning nozzle and the receiving drum is 16 cm, the rotating speed of the receiving drum is 30 rpm, the temperature is 24℃, and the humidity is 48.3%;

[0141] (3.2) After the composite fiber is dried, it is placed in an appropriate amount of dissolution liquid (accompanied by ultrasonic, the ultrasonic power is 180 W), the polyurethane is dissolved away, and the carbon source polymer is retained, and then the composite fiber is taken out and dried, and then the carbon source polymer in the composite fiber is carbonized;

[0142] The carbonization process is as follows: first, in an air atmosphere, the temperature is increased from room temperature to 250℃ at a rate of 1℃ / min, and then kept for 1h, then adjusted to a nitrogen or inert gas atmosphere, and then increased to 700℃ at a rate of 3℃ / min, and then kept for 6h, and then kept the atmosphere unchanged and increased to 1000℃ at a rate of 5℃ / min, and then kept for 8h;

[0143] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal nickel nanoparticles);

[0144] (3.3) Under the protection of nitrogen or inert gas, the product of step (3.2) is placed in a tube furnace, a carbon source (flow rate of 60 mL / min) is introduced, and the reaction is carried out at 500℃ for 10h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), and carbon nanotube modified three-dimensional spiral carbon nanofibers are obtained.

[0145] The three-dimensional spiral carbon nanofibers prepared in this example are used as wave absorbers to absorb electromagnetic waves; when the matching thickness is 1.51mm (the actual test uses a circular ring sample with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 2.05mm), the minimum reflection loss is -60.1dB, and the maximum effective absorption bandwidth is 7.3GHz.

[0146] Example 6

[0147] A preparation method of three-dimensional spiral carbon nanofibers, the specific steps are as follows:

[0148] (1) Preparation of raw materials;

[0149] Carbon source polymer: polyacrylonitrile, weight average molecular weight of 150000g / mol;

[0150] Solvent: N,N-dimethylformamide;

[0151] Magnetic transition metal salt: iron acetate;

[0152] Polyurethane: manufacturer is Covestro Company, brand is 9370AU;

[0153] Carbon source: n-hexane;

[0154] Dissolution solution: tetrahydrofuran;

[0155] (2) Preparation of spinning solution a and spinning solution b;

[0156] The preparation process of spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain the spinning solution a; wherein, the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.4, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0157] The preparation process of spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein, the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0158] (3) Preparation of three-dimensional spiral carbon nanofibers;

[0159] (3.1) Injecting the spinning solution a and the spinning solution b into the injectors respectively, using the parallel spinning nozzle (consisting of two parallel and same diameter circular tubes, the two circular tubes partially overlap, the center distance of the two circular tubes is 60% of the diameter of the circular tube), using the electrospinning technology to prepare the composite fiber (885nm in diameter) with three-dimensional spiral structure; wherein the mass ratio of the carbon source polymer and the polyurethane is 1:2; the spinning process parameters are: voltage 17kV, the push injection rate of the spinning solution a is 1mm / min, the push injection rate of the spinning solution b is 1.3mm / min, the distance between the spinning nozzle and the receiving drum is 16cm, the rotating speed of the receiving drum is 30rpm, the temperature is 21.7℃, and the humidity is 53.2%;

[0160] (3.2) After drying the composite fiber, placing it in a proper amount of dissolving solution (with ultrasonic, the ultrasonic power is 180W), dissolving the polyurethane and retaining the carbon source polymer, taking out and drying, and then carbonizing the carbon source polymer in the composite fiber;

[0161] The carbonization process is: first, in an air atmosphere, heating from room temperature to 300℃ at a heating rate of 1℃ / min, and then keeping the temperature for 4h, then adjusting to a nitrogen or inert gas atmosphere, heating to 900℃ at a heating rate of 3℃ / min, and then keeping the temperature for 1h, and then keeping the atmosphere unchanged, heating to 1500℃ at a heating rate of 5℃ / min, and then keeping the temperature for 0.5h;

[0162] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles);

[0163] (3.3) Under the protection of nitrogen or inert gas, placing the product of step (3.2) in a tube furnace, introducing a carbon source (flow rate is 60mL / min), and reacting at 600℃ for 8h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), to obtain the three-dimensional spiral carbon nanofiber modified by carbon nanotubes.

[0164] The three-dimensional spiral carbon nanofiber prepared in this embodiment is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 12% and the matching thickness is 1.60mm (the actual test uses a circular ring sample with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 2.22mm), the minimum reflection loss is-55.2dB, and the maximum effective absorption bandwidth is 7GHz.

[0165] Example 7

[0166] A preparation method of a three-dimensional spiral carbon nanofiber, the specific steps are as follows:

[0167] (1) Preparation of raw materials;

[0168] Carbon source polymer: polyacrylonitrile, weight average molecular weight is 150000g / mol;

[0169] Solvent: N,N-dimethylformamide;

[0170] Magnetic transition metal salt: mixture of iron nitrate nonahydrate and cobalt nitrate hexahydrate with a mass ratio of 1:1;

[0171] Polyurethane: 9370AU from Covestro;

[0172] Carbon source: ethylene;

[0173] Dissolution liquid: tetrahydrofuran;

[0174] (2) Preparation of spinning solution a and spinning solution b;

[0175] The preparation process of spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain spinning solution a; wherein the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.5, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0176] The preparation process of spinning solution b is: the polyurethane is dissolved in the solvent to obtain spinning solution b; wherein the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0177] (3) Preparation of three-dimensional spiral carbon nanofiber;

[0178] (3.1) Spinning solution a and spinning solution b are respectively injected into the syringe, and a core-spun nozzle (composed of an inner tube and an outer tube, the center distance between the inner tube and the outer tube is 20% of the diameter of the outer tube, spinning solution b passes through the outer tube, and spinning solution a passes through the inner tube) is used to prepare a composite fiber (with a diameter of 849 nm) with a three-dimensional spiral structure by electrospinning technology; wherein the mass ratio of the carbon source polymer to the polyurethane is 1:2.5; the spinning process parameters are: voltage 16 kV, the push injection rate of spinning solution a is 1 mm / min, the push injection rate of spinning solution b is 1.4 mm / min, the distance between the spinning nozzle and the receiving drum is 16 cm, the rotating speed of the receiving drum is 30 rpm, the temperature is 22.9℃, and the humidity is 43.2%;

[0179] (3.2) After the composite fiber is dried, it is placed in an appropriate amount of dissolution liquid (with ultrasonic, the ultrasonic power is 180 W), the polyurethane is dissolved away, and the carbon source polymer is retained, then it is taken out and dried, and then the carbon source polymer in the composite fiber is carbonized;

[0180] The carbonization process is: first, in an air atmosphere, the temperature is raised from room temperature to 250℃ at a rate of 1℃ / min, and then kept for 5h, then adjusted to a nitrogen or inert gas atmosphere, the temperature is raised to 800℃ at a rate of 3℃ / min, and then kept for 3h, and then the atmosphere is kept unchanged, the temperature is raised to 1300℃ at a rate of 5℃ / min, and then kept for 4h.

[0181] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles and metal cobalt nanoparticles);

[0182] (3.3) Under the protection of nitrogen or inert gas, the product of step (3.2) is placed in a tube furnace, a carbon source (flow rate of 60 mL / min) is introduced, and the reaction is carried out at 700°C for 6h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), and a carbon nanotube modified three-dimensional spiral carbon nanofiber is obtained.

[0183] The three-dimensional spiral carbon nanofiber prepared in this embodiment is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 9% and the matching thickness is 1.98mm (the actual test uses a circular ring sample with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 2.37mm), the minimum reflection loss is -75.6dB, and the maximum effective absorption bandwidth is 8.9GHz.

[0184] Example 8

[0185] A preparation method of a three-dimensional spiral carbon nanofiber, the specific steps are as follows:

[0186] (1) Preparation of raw materials;

[0187] Carbon source polymer: polyacrylonitrile, weight average molecular weight of 150000g / mol;

[0188] Solvent: N,N-dimethylformamide;

[0189] Magnetic transition metal salt: mixture of iron nitrate nonahydrate, cobalt nitrate hexahydrate and nickel nitrate hexahydrate with a mass ratio of 1:1:1;

[0190] Polyurethane: manufacturer is Koso, model is 9370AU;

[0191] Carbon source: propylene;

[0192] Dissolution solution: tetrahydrofuran;

[0193] (2) Preparation of spinning solution a and spinning solution b;

[0194] The preparation process of the spinning solution a is: the carbon source polymer and the magnetic transition metal salt are added into the solvent and stirred uniformly to obtain the spinning solution a; wherein, the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.2, and the mass ratio of the carbon source polymer to the solvent is 1:9;

[0195] The preparation process of the spinning solution b is: the polyurethane is dissolved in the solvent to obtain the spinning solution b; wherein, the mass ratio of the polyurethane to the solvent is 1.2:8.8;

[0196] (3) preparing a three-dimensional helical carbon nanofiber;

[0197] (3.1) injecting the spinning solution a and the spinning solution b into injectors respectively, using a eccentric spinning nozzle (composed of an inner tube and an outer tube, the center distance of the inner tube and the outer tube is 20% of the diameter of the outer tube, the spinning solution b goes through the outer tube, and the spinning solution a goes through the inner tube), and using electrospinning technology to prepare a composite fiber (with a diameter of 896 nm) with a three-dimensional helical structure; wherein the mass ratio of the carbon source polymer to the polyurethane is 1:3; the spinning process parameters are: voltage 16 kV, the push injection rate of the spinning solution a is 1 mm / min, the push injection rate of the spinning solution b is 1.2 mm / min, the distance between the spinning nozzle and the receiving drum is 16 cm, the rotating speed of the receiving drum is 30 rpm, the temperature is 21.8℃, and the humidity is 42.7%;

[0198] (3.2) after drying the composite fiber, placing it in an appropriate amount of dissolving solution (with ultrasonic, the ultrasonic power is 180 W), dissolving the polyurethane and retaining the carbon source polymer, taking out and drying, and then carbonizing the carbon source polymer in the composite fiber;

[0199] The carbonization process is: first, in an air atmosphere, heating from room temperature to 250℃ at a heating rate of 1℃ / min, and then keeping the temperature for 6h, then adjusting to a nitrogen or inert gas atmosphere, heating to 800℃ at a heating rate of 3℃ / min, and then keeping the temperature for 5h, and then keeping the atmosphere unchanged, heating to 1400℃ at a heating rate of 5℃ / min, and then keeping the temperature for 2h;

[0200] After carbonization, the magnetic transition metal salt in the composite fiber is converted into magnetic metal nanoparticles (metal iron nanoparticles, metal cobalt nanoparticles and metal nickel nanoparticles);

[0201] (3.3) under the protection of nitrogen or inert gas, placing the product of step (3.2) in a tube furnace, and introducing a carbon source (flow rate is 60 mL / min), and reacting at 1000℃ for 1h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (3.2), to obtain a three-dimensional helical carbon nanofiber modified by carbon nanotubes.

[0202] The three-dimensional helical carbon nanofiber prepared in this example is used as a wave absorber to absorb electromagnetic waves; when the wave absorber incorporation ratio is 5% and the matching thickness is 1.60 mm (the actual test uses a circular ring sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.09 mm), the minimum reflection loss is -89.2 dB, and the maximum effective absorption bandwidth is 10.3 GHz.

[0203] Example 9

[0204] A method for preparing a three-dimensional helical carbon nanofiber, which is different from that of Example 1 only in that no magnetic transition metal salt is added in step (2), and no step (3.3) is performed, and the product of step (3.2) is directly used as the three-dimensional helical carbon nanofiber.

[0205] The three-dimensional helical carbon nanofiber prepared in this example is used as a wave-absorbing agent to absorb electromagnetic waves. When the matching thickness is 2.05 mm (the actual test sample is a circular ring with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.07 mm), the minimum reflection loss is -36.7 dB, and the maximum effective absorption bandwidth is 5.6 GHz.

[0206] Compared with Example 1, the minimum reflection loss of the sample of Example 9 is increased, and the maximum effective absorption bandwidth is decreased. This is because no magnetic transition metal salt is added in Example 9, and no carbon nanotube modification step is performed. There is no magnetic metal nanoparticle in the fiber, and the magnetic loss mechanism cannot be introduced. Only the dielectric loss of the three-dimensional helical structure is relied on, the loss mechanism is single, which leads to the weakening of the absorption intensity. At the same time, the lack of carbon nanotube modification cannot form a multi-level nanostructure, which is difficult to optimize the impedance matching characteristics, and cannot trigger multiple scattering, so that the attenuation path of electromagnetic waves in the material is reduced, and the wave-absorbing performance is weakened.

[0207] Comparative Example 1

[0208] A method for preparing a carbon nanofiber, which is basically the same as Example 9, and is different only in that the carbonization process of step (3.2) is: first heated from room temperature to 800℃ at a heating rate of 3℃ / min in a nitrogen or inert gas atmosphere, and then heated to 1200℃ at a heating rate of 5℃ / min while maintaining the atmosphere, and then maintained for 6h, that is, the pre-oxidation stage is omitted.

[0209] The carbon nanofiber prepared in this comparative example is used as a wave-absorbing agent to absorb electromagnetic waves. When the matching thickness is 2.11 mm (the actual test sample is a circular ring with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.07 mm), the minimum reflection loss is -20.3 dB, and the maximum effective absorption bandwidth is 3.7 GHz. (Comparative Example 1 and Comparative Example 2 remain the same as Example 9, and the incorporation ratio is changed to 5%)

[0210] Comparative Example 2

[0211] A method for preparing carbon nanofibers, substantially the same as example 9, the only difference is that the carbonization process of step (3.2) is: first heated from room temperature to 250℃ at a rate of 1℃ / min in air atmosphere, then adjusted to nitrogen or inert gas atmosphere, heated to 1200℃ at a rate of 5℃ / min, and kept for 6h, that is, the primary carbonization stage is omitted.

[0212] The carbon nanofibers prepared in this comparative example are used as wave-absorbing agents to absorb electromagnetic waves; when the matching thickness is 2.29mm (the actual test sample is a circular ring with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 2.07mm), the minimum reflection loss is -35.0dB, and the maximum effective absorption bandwidth is 5.2GHz.

[0213] Compared with example 9, the minimum reflection loss of the samples of comparative example 1 and comparative example 2 increases, and the maximum effective absorption bandwidth decreases, because the gradient carbonization containing the pre-oxidation stage, the primary carbonization stage, and the deep carbonization stage of example 9 is beneficial to maintaining the integrity of the three-dimensional spiral structure of the carbon nanofibers, the higher the integrity of the three-dimensional spiral structure, the better the cross-polarization is provided, and the better the wave-absorbing performance of the sample is. The three-dimensional spiral structure of the carbon nanofibers is destroyed due to the lack of one of the stages in comparative example 1 and comparative example 2, which in turn leads to a decrease in the wave-absorbing performance of the sample.

[0214] Example 10

[0215] A method for preparing three-dimensional spiral carbon nanofibers, and the only difference from example 1 is that in step (2), when preparing the spinning solution a, a pore-forming agent (polymethyl methacrylate, weight average molecular weight 120000g / mol) is also added, and the mass ratio of the carbon source polymer to the pore-forming agent is 1:0.05, and the pore-forming agent in the composite fiber is removed by pyrolysis in the subsequent carbonization process of step (3.2).

[0216] The three-dimensional spiral carbon nanofibers prepared in this example are used as wave-absorbing agents to absorb electromagnetic waves; when the matching thickness is 1.52mm (the actual test sample is a circular ring with an outer diameter of 7mm, an inner diameter of 3.04mm, and a thickness of 2.07mm), the minimum reflection loss is -73.2dB, and the maximum effective absorption bandwidth is 8.9GHz.

[0217] Compared with example 1, the minimum reflection loss of the sample of example 10 decreases, and the maximum effective absorption bandwidth increases, because the carbon nanofibers with a porous structure can reduce electromagnetic wave reflection, optimize impedance matching, and enhance polarization loss, while the electromagnetic wave is reflected / scattered multiple times in the pores, enhancing the wave-absorbing performance of the material.

[0218] Example 11

[0219] A method for preparing a three-dimensional helical carbon nanofiber, and the difference from Example 1 is only that in step (3.2), the composite fiber is not treated with a dissolving solution, and the carbon source polymer in the composite fiber is directly carbonized, and the polyurethane in the composite fiber is removed by pyrolysis during the carbonization process.

[0220] The three-dimensional helical carbon nanofiber prepared in this example is used as a wave-absorbing agent for absorbing electromagnetic waves; when the incorporation ratio of the wave-absorbing agent is 6% and the matching thickness is 1.98 mm (the actual test uses a circular ring sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 2.07 mm), the minimum reflection loss is -53.3 dB, and the maximum effective absorption bandwidth is 7.6 GHz.

[0221] Compared with Example 1, the minimum reflection loss of the sample in Example 11 increases, and the maximum effective absorption bandwidth decreases, which may be because when the polyurethane is directly carbonized and pyrolyzed, although micropores are formed at the interface and local heteroatom doping (such as nitrogen and oxygen) is formed during the removal process, the micropore distribution is uneven and the amount of heteroatom doping does not reach the optimal proportion: both can improve the impedance matching by adjusting the electromagnetic parameters and optimizing the loss mechanism, and theoretically can improve the wave-absorbing performance, but in fact, the pyrolysis of polyurethane interferes with the formation of ordered graphite microcrystalline structures of carbon nanofibers, destroys the overall conductive network structure, and weakens the conductive loss, ultimately causing the wave-absorbing performance to decrease slightly.

[0222] Example 12

[0223] A method for preparing a three-dimensional helical carbon nanofiber, and the difference from Example 2 is only that in step (2), when preparing the spinning solution a, a pore-forming agent (polystyrene with a weight average molecular weight of 350000 g / mol) is also added, and the mass ratio of the carbon source polymer to the pore-forming agent is 1:0.5, and the pore-forming agent in the composite fiber is removed by pyrolysis during the carbonization process of step (3.2).

[0224] The three-dimensional helical carbon nanofiber prepared in this example is used as a wave-absorbing agent for absorbing electromagnetic waves; when the incorporation ratio of the wave-absorbing agent is 6% and the matching thickness is 2.19 mm (the actual test uses a circular ring sample with an outer diameter of 7 mm, an inner diameter of 3.04 mm, and a thickness of 1.98 mm), the minimum reflection loss is -80.2 dB, and the maximum effective absorption bandwidth is 8.5 GHz.

[0225] Compared with Example 2, the minimum reflection loss of the sample in Example 12 decreases, and the maximum effective absorption bandwidth increases, which is because the carbon nanofiber with a porous structure can reduce electromagnetic wave reflection, optimize impedance matching, and enhance polarization loss, and at the same time, the electromagnetic wave undergoes multiple reflections / scattering in the pores, enhancing the wave-absorbing performance of the material.

Claims

1. A method for preparing three-dimensional helical carbon nanofibers, characterized in that, Includes the following steps: (a) Using electrospinning technology to prepare composite fibers with a three-dimensional helical structure, the composite fibers including carbon source polymer and polyurethane; (b) Remove the polyurethane from the composite fiber and carbonize the carbon source polymer in the composite fiber; The carbonization process includes three heat preservation stages: the first heat preservation stage is the pre-oxidation stage, the second heat preservation stage is the primary carbonization stage, and the third heat preservation stage is the deep carbonization stage. (c) Carbon nanotubes are modified on the surface of the product in step (b) by chemical vapor deposition or hydrothermal synthesis. In step (a), the carbon source polymer is polyacrylonitrile, polyvinylpyrrolidone, polyvinyl alcohol or polyimide, and the mass ratio of the carbon source polymer to polyurethane is 1:1.2~3; the electrospinning technology uses coaxial, eccentric or parallel spinning nozzles; the diameter of the composite fiber is 400~1000nm; In step (a), the composite fiber also includes a magnetic transition metal salt, which is dispersed in the carbon source polymer; In step (b), the temperature of the first heat preservation stage is 200~300℃, the gas atmosphere is air, and the heat preservation time is 1~6h; the temperature of the second heat preservation stage is 700~900℃, the gas atmosphere is nitrogen or inert gas, and the heat preservation time is 1~6h; the temperature of the third heat preservation stage is 1000~1500℃, the gas atmosphere is nitrogen or inert gas, and the heat preservation time is 0.5~8h.

2. The method for preparing three-dimensional helical carbon nanofibers according to claim 1, characterized in that, In step (b), removing polyurethane from the composite fiber means treating the composite fiber with a dissolving solution before carbonization to dissolve the polyurethane and retain the carbon source polymer, or it means removing the polyurethane from the composite fiber by pyrolysis during the carbonization process.

3. The method for preparing three-dimensional helical carbon nanofibers according to claim 1, characterized in that, In step (a), the composite fiber also includes a pore-forming agent, which is dispersed in the carbon source polymer, and the mass ratio of the carbon source polymer to the pore-forming agent is 1:0.05~0.5; in step (b), the pore-forming agent in the composite fiber is removed by pyrolysis during the carbonization process.

4. The method for preparing three-dimensional helical carbon nanofibers according to claim 1, characterized in that, In step (a), the magnetic transition metal salt is an Fe salt, a Co salt, or a Ni salt; the mass ratio of the carbon source polymer to the magnetic transition metal salt is 1:0.05~0.5; In step (b), after carbonization, the magnetic transition metal salt in the composite fiber is transformed into magnetic metal nanoparticles.

5. The method for preparing three-dimensional helical carbon nanofibers according to claim 1, characterized in that, In step (c), during chemical vapor deposition, the reaction temperature is 500~1000℃, the reaction time is 1~10h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (b), the carbon source of the carbon nanotubes is pyrrole, acetylene, benzene, n-hexane, methane, ethylene or propylene, and the gas atmosphere is nitrogen or inert gas. During hydrothermal synthesis, the reaction temperature is 200℃, the reaction time is 20h, the reaction catalyst is the magnetic metal nanoparticles in the product of step (b), and the carbon source of the carbon nanotubes is pyrrole.

6. A three-dimensional helical carbon nanofiber, characterized in that, The three-dimensional helical carbon nanofibers were prepared using the method described in any one of claims 1 to 5.

7. The microwave absorption application of a three-dimensional helical carbon nanofiber as described in claim 6, characterized in that, The three-dimensional helical carbon nanofibers are used as microwave absorbers to absorb electromagnetic waves. When the microwave absorber has a mass ratio of 5% to 15% and a matching thickness of 1 to 3 mm, the minimum reflection loss is -89.2 dB to -36.7 dB and the maximum effective absorption bandwidth is 5.6 to 10.3 GHz.

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