Carbon-coated ferrite hollow composite microspheres, and preparation method and application thereof

Abstract

The application provides a carbon-coated ferrite hollow composite microsphere and a preparation method and application thereof, and belongs to the technical field of electromagnetic shielding materials. The ferrite, a first silane coupling agent and a dispersing agent are surface-activated and modified, carbon nanotubes and a second silane coupling agent are activated, the obtained surface-activated and modified ferrite powder and the silane coupling agent modified carbon nanotubes are mixed with a template agent, one-time granulation is performed on the obtained mixed slurry to obtain primary microspheres; the primary microspheres, a resin solution and a curing accelerator are mixed and then subjected to secondary granulation, and the obtained core-shell intermediate is heat-treated in a protective atmosphere to obtain the carbon-coated ferrite hollow composite microsphere. Through the route of "three kinds of spinel ferrite + carbon nanotube -> template agent one-time granulation -> resin secondary coating -> heat treatment", the Ku full-waveband high-efficiency wave absorption, low dielectric loss impedance matching, absorption-dominant shielding, lightweight, uniform structure and environmentally-friendly process are realized for the first time.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding materials technology, and in particular to a carbon-coated ferrite hollow composite microsphere, its preparation method, and its application. Background Technology

[0002] With the rapid popularization of Ku-band (12~18GHz) radar detection, satellite communication, and 5G smart electronic devices, problems such as electromagnetic radiation interference, electromagnetic leakage of equipment, and radar detection of military targets are becoming increasingly prominent. The industry's demand for multifunctional electromagnetic materials that combine broadband absorption, high-efficiency electromagnetic shielding, lightweight and low density, and structural stability is becoming increasingly urgent. Ku-band electromagnetic functional materials have become a core research direction in the fields of civilian electromagnetic protection and military stealth. Ferrite materials, with their excellent magnetic loss characteristics, are the mainstream absorbing substrate in the Ku band. However, single ferrites have inherent technical defects: First, due to the Snoek limit, the resonant frequency band of a single ferrite is narrow and cannot fully cover the entire 12~18GHz Ku band; Second, ferrites themselves have high density and unadjustable dielectric parameters, resulting in poor impedance matching with free space, severe electromagnetic wave surface reflection, and low absorption efficiency.

[0003] To overcome the aforementioned shortcomings, existing technologies generally employ composite modification schemes using carbon materials and ferrites. For example, core-shell composite materials with phenolic resin carbonized and coated nickel ferrite are mainly used in lithium-ion battery anode materials. However, these materials use only a single nickel ferrite raw material, lacking a multi-ferrite synergistic design, thus failing to achieve Ku-band wideband absorption. Furthermore, the materials have high dielectric constants and poor impedance matching, lacking integrated absorption-shielding functionality. Similarly, magnetic hollow carbon spheres with phenolic resin coated with magnetite (Fe3O4) use only a single magnetite magnetic material, resulting in a single ferrite component, limited frequency band coverage, and excessively high dielectric loss in the Ku band. Poor adaptability; multi-shell hollow carbon-coated nanoparticle composite materials, with carbon-coated hollow inorganic particles as the core, cannot achieve both broadband absorption and electromagnetic shielding effects; porous microsphere / ferrite / conductive layer multilayer absorbing materials, which combine ferrite and conductive layers through physical composite methods, have poor component uniformity, lack hollow cavity structure, high material density, insufficient lightweight, and have not achieved stable absorption across the Ku band; multi-core multi-shell hollow carbon-coated materials, which mainly prepare carbon shells through dopamine carbonization, have uncontrollable carbon layer conductivity, high dielectric loss, and cannot meet the impedance matching requirements of the Ku band.

[0004] In summary, existing technologies generally suffer from three major pain points: first, the ferrite composition is singular and cannot cover the entire Ku band; second, there is no controllable hollow structure, resulting in high dielectric parameters and poor impedance matching; and third, the carbon shell structure is uncontrollable, making it difficult to simultaneously achieve efficient microwave absorption and electromagnetic shielding. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a carbon-coated ferrite hollow composite microsphere, its preparation method, and its application. The carbon-coated ferrite hollow composite microsphere prepared by this invention achieves multiple effects for the first time, including high-efficiency absorption across the entire Ku band, low dielectric loss, impedance matching, absorption-dominant shielding, and controllable structure.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing carbon-coated ferrite hollow composite microspheres, comprising the following steps: The composite ferrite, the first silane coupling agent, and the dispersant are surface-activated to obtain surface-activated ferrite powder, wherein the composite ferrite includes CoFe2O4, NiFe2O4, and CuFe2O4. Carbon nanotubes and a second silane coupling agent are activated to obtain silane coupling agent modified carbon nanotubes. The surface-activated modified ferrite powder, silane coupling agent modified carbon nanotubes, and template agent are mixed to obtain a mixed slurry; The mixed slurry is granulated in one step to obtain microspheres; The primary microspheres, resin solution and curing accelerator are mixed and then granulated a second time to obtain a core-shell intermediate. The core-shell intermediate was heat-treated in a protective atmosphere to obtain the carbon-coated ferrite hollow composite microspheres. The mass ratio of CoFe2O4, NiFe2O4 and CuFe2O4 is (30~50):(20~40):(20~40); The mass of the carbon nanotubes is 5-20% of the mass of the composite ferrite; The mass ratio of the composite ferrite to the template agent is (7:3) to (5:5); The template agent includes a hydrocarbon sacrificial agent; the hydrocarbon sacrificial agent is polyethylene wax and / or microcrystalline wax; The heat treatment includes a pyrolysis stage and a carbonization stage performed sequentially. The temperature of the pyrolysis stage is 300~400℃ and the holding time is 1~3h. The temperature of the carbonization stage is 600~800℃ and the holding time is 2~4h.

[0007] Preferably, the mass of the first silane coupling agent is 4 to 7% of the mass of the composite ferrite.

[0008] Preferably, the amount of the second silane coupling agent is 5-15% of the mass of the carbon nanotubes.

[0009] Preferably, the mass ratio of resin to primary microspheres in the resin solution is 1 to 4:1.

[0010] Preferably, the resin in the resin solution includes thermosetting phenolic resin and / or resorcinol-formaldehyde resin.

[0011] Preferably, the curing accelerator comprises propylene carbonate.

[0012] Preferably, the mass of the curing accelerator is 1 to 2% of the mass of the resin in the resin solution.

[0013] The present invention also provides carbon-coated ferrite hollow composite microspheres prepared by the preparation method described above, comprising a carbon shell, a hollow cavity, ferrite nanoparticles and carbon nanotubes, wherein the carbon nanotubes and ferrite nanoparticles are dispersed in the hollow cavity, and the ferrite nanoparticles comprise CoFe2O4, NiFe2O4 and CuFe2O4.

[0014] This invention also provides the application of the carbon-coated ferrite hollow composite microspheres described above as a Ku-band electromagnetic wave shielding agent in the field of electromagnetic shielding.

[0015] This invention provides a method for preparing carbon-coated ferrite hollow composite microspheres, comprising the following steps: surface-activating and modifying composite ferrite, a first silane coupling agent, and a dispersant to obtain surface-activated modified ferrite powder, wherein the composite ferrite includes CoFe2O4, NiFe2O4, and CuFe2O4; activating carbon nanotubes and a second silane coupling agent to obtain silane coupling agent-modified carbon nanotubes; mixing the surface-activated modified ferrite powder, the silane coupling agent-modified carbon nanotubes, and a template agent to obtain a mixed slurry; granulating the mixed slurry once to obtain primary microspheres; mixing the primary microspheres, a resin solution, and a curing accelerator and then granulating again to obtain a core-shell intermediate; and further granulating the core-shell intermediate in… The carbon-coated ferrite hollow composite microspheres are obtained by heat treatment in a protective atmosphere; the mass ratio of CoFe2O4, NiFe2O4 and CuFe2O4 is (30~50):(20~40):(20~40); the mass of the carbon nanotubes is 5~20% of the mass of the composite ferrite; the mass ratio of the composite ferrite to the template agent is (7:3)~(5:5); the template agent includes a hydrocarbon sacrificial agent; the hydrocarbon sacrificial agent is polyethylene wax and / or microcrystalline wax; the heat treatment includes a pyrolysis stage and a carbonization stage performed sequentially, the temperature of the pyrolysis stage is 300~400℃, and the holding time is 1~3h; the temperature of the carbonization stage is 600~800℃, and the holding time is 2~4h.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention selects three spinel ferrites to achieve a stepped complementary coverage of magnetic resonance frequencies: CoFe2O4 has a high magnetocrystalline anisotropy constant, and its natural resonance frequency covers the mid-to-high frequency band of 14~18GHz, serving as the main absorption component in the high-frequency band of the Ku band; NiFe2O4 is a typical soft magnetic spinel ferrite, and its natural resonance frequency covers the mid-to-low frequency band of 8~12GHz, serving as an auxiliary absorption component in the low-frequency band of the Ku band; CuFe2O4, due to its unique Jahn-Teller effect, can exist in both cubic and tetragonal crystal phases simultaneously, and its structural distortion is conducive to enhancing interface polarization and dipole polarization, serving as a component for bandwidth broadening and interface polarization enhancement. Through a multi-polarization loss mechanism, it compensates for the insufficient bandwidth of a single magnetic loss. After the three are synergistically combined, the reflection loss (RL) is less than -10dB (absorption rate ≥90%) in the entire 12~18GHz band, reaching as low as -48.2dB, with an effective bandwidth ≥6GHz, completely covering the entire Ku band; This invention employs a composite structure of "hollow carbon shell + carbon nanotube conductive network + multiferrite" to simultaneously achieve synergistic effects on dielectric loss, magnetic loss, and conduction loss: the carbon shell and carbon nanotubes form a three-dimensional conductive network, providing dielectric and conduction losses; after the template agent is decomposed, a hollow cavity is formed, introducing a large amount of air (ε≈1), controlling the real part (ε') of the complex permittivity to 5.0~9.0 (conventional carbon-based materials ε>20), and the imaginary part (ε") to 1.0~2.8, achieving extremely low dielectric loss and excellent impedance matching. The real part μ' of the complex permeability is between 1.3 and 1.4, and the imaginary part μ'' is between 0.45 and 0.53. A significant magnetic loss resonance peak (μ'' peak value) appears near 15 GHz, which is consistent with CoFe2O4, The natural resonance characteristics of NiFe2O4, CuFe2O4, and other spinel ferrites in the Ku band are consistent, and the complementary superposition of their resonant frequencies is the core mechanism for achieving broadband absorption across the entire Ku band. Simultaneously, the difference between the μ' / ε' ratio (μ' / ε'≈0.16~0.28) and free space (μ0 / ε0≈1) reflects the impedance matching characteristics of the material. The low dielectric constant combined with moderate permeability allows electromagnetic waves to efficiently penetrate the material and be absorbed by multiple loss mechanisms. The shielding effectiveness (SE) reaches 15~29dB (absorption ratio >90%, reflection loss ≤1.8dB), with absorption as the dominant factor, avoiding secondary electromagnetic pollution associated with traditional metal shielding. Furthermore, the hollow cavity reduces the apparent density to 1.3~2.1g / cm³. 3 The concentration is significantly lower than that of traditional ferrites (>4.5g / cm³). 3 ) and metals (>7g / cm 3 ), to meet the requirements of lightweight design; This invention employs a two-step granulation process: the first granulation rapidly solidifies the mixed slurry into uniform primary microspheres; the second granulation rapidly solidifies the resin under the promotion of a curing accelerator, precisely controlling the coating layer thickness to form a core-shell intermediate with controllable particle size. The template agent is directly pyrolyzed and released during heat treatment, forming a hollow cavity in one step. The resin carbonizes during heat treatment to form a dense carbon shell. Furthermore, the preparation process of this invention is simple to operate, easy to scale up, and has high batch repeatability.

[0017] In summary, this invention, through the route of "three spinel ferrites + carbon nanotubes → primary granulation with template agent → secondary resin coating → heat treatment," solves the long-standing technical problems in the prior art, such as insufficient bandwidth of single ferrites, high dielectric constant due to lack of hollow structure, uncontrollable conductivity of carbon layers, and poor physical composite uniformity. For the first time, it achieves multiple effects such as high-efficiency absorption across the entire Ku band, low dielectric loss impedance matching, absorption-dominant shielding, lightweight, uniform structure, and environmentally friendly process.

[0018] This invention also provides carbon-coated ferrite hollow composite microspheres prepared by the preparation method described above. The particle size of the carbon-coated ferrite hollow composite microspheres of this invention exhibits a normal distribution (D0). 50 The carbon shell thickness is 1.5~3.0μm, and the hollow cavity diameter is 2~4.5μm (which can be controlled by the ratio of composite ferrite to template agent). The ferrite nanoparticles are dispersed in the hollow cavity. The three types of ferrite provide complementary magnetic losses. The carbon nanotubes and carbon shell form a continuous conductive network. Multiple mechanisms work together to achieve the dual function of "mainly absorbing waves and secondarily shielding". Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the carbon-coated ferrite hollow composite microspheres described in this invention; Figure 2 This is a flowchart illustrating the preparation method of carbon-coated ferrite hollow composite microspheres in Example 1. Figure 3 The image shows the XRD pattern of the magnetic components in the carbon-coated ferrite hollow composite microspheres prepared in Example 1. Figure 4 The reflection loss curves of Example 1 under different matching thicknesses; Figure 5 The images shown are SEM images of the carbon-coated ferrite hollow composite microspheres prepared in Example 1, where (a) is the overall morphology and (b) is the internal structure. Detailed Implementation

[0020] This invention provides a method for preparing carbon-coated ferrite hollow composite microspheres, comprising the following steps: The composite ferrite, the first silane coupling agent, and the dispersant are surface-activated to obtain surface-activated ferrite powder, wherein the composite ferrite includes CoFe2O4, NiFe2O4, and CuFe2O4. Carbon nanotubes and a second silane coupling agent are activated to obtain silane coupling agent modified carbon nanotubes. The surface-activated modified ferrite powder, silane coupling agent modified carbon nanotubes, and template agent are mixed to obtain a mixed slurry; The mixed slurry is granulated in one step to obtain microspheres; The primary microspheres, resin solution and curing accelerator are mixed and then granulated a second time to obtain a core-shell intermediate. The core-shell intermediate was heat-treated in a protective atmosphere to obtain the carbon-coated ferrite hollow composite microspheres. The mass ratio of CoFe2O4, NiFe2O4 and CuFe2O4 is (30~50):(20~40):(20~40); The mass of the carbon nanotubes is 5-20% of the mass of the composite ferrite; The mass ratio of the composite ferrite to the template agent is (7:3) to (5:5); The template agent includes a hydrocarbon sacrificial agent; the hydrocarbon sacrificial agent is polyethylene wax and / or microcrystalline wax; The heat treatment includes a pyrolysis stage and a carbonization stage performed sequentially. The temperature of the pyrolysis stage is 300~400℃ and the holding time is 1~3h. The temperature of the carbonization stage is 600~800℃ and the holding time is 2~4h.

[0021] Unless otherwise specified, all raw materials used in this invention are commercially available products in the field.

[0022] This invention involves surface-activating and modifying a composite ferrite, a first silane coupling agent, and a dispersant to obtain surface-activated modified ferrite powder. The composite ferrite includes CoFe2O4, NiFe2O4, and CuFe2O4.

[0023] In this invention, the dispersant preferably comprises ethanol and water, and the volume ratio of ethanol to water is preferably 9:1.

[0024] In this invention, the mass ratio of CoFe2O4, NiFe2O4, and CuFe2O4 is (30~50):(20~40):(20~40), specifically 40:30:30, 50:25:25, 35:35:30, 45:30:25, 40:30:30, 50:30:20, or 40:40:20. Limiting the mass ratio of CoFe2O4, NiFe2O4, and CuFe2O4 within the above range enables a stepped complementary coverage of the magnetic resonance frequencies of the three spinel ferrites, thereby forming a continuous broadband magnetic loss response in the Ku band (12~18GHz). This ensures that the composite microspheres have sufficient magnetic loss capability throughout the entire band. If the proportion of a certain ferrite component is too high or too low, its contribution to magnetic loss in that frequency band will be insufficient, resulting in a narrowing of the effective absorption bandwidth and making it impossible to achieve full band coverage of 12~18GHz.

[0025] In this invention, the mass of the first silane coupling agent is preferably 4 to 7% of the mass of the composite ferrite, specifically 4%, 5%, 6% or 7%.

[0026] In this invention, the first silane coupling agent preferably includes KH-550.

[0027] In this invention, the surface activation modification temperature is preferably room temperature, and the time is preferably 2 hours.

[0028] After the surface activation modification is completed, the present invention preferably filters, washes and spray-dries the obtained system in sequence to obtain the surface-activated modified ferrite powder.

[0029] In this invention, the spray drying temperature is preferably 120~180℃.

[0030] In this invention, CoFe2O4, NiFe2O4 and CuFe2O4 are mixed in a certain ratio, added to an ethanol-water dispersant and stirred for 30 min, then silane coupling agent KH-550 is added, and the mixture is ultrasonicated and stirred at a constant temperature for 2 h. After filtration, washing and spray drying, the surface-activated modified ferrite powder is obtained.

[0031] This invention activates carbon nanotubes and a second silane coupling agent to obtain silane coupling agent modified carbon nanotubes.

[0032] In this invention, the amount of the second silane coupling agent is preferably 5 to 15% of the mass of the carbon nanotubes, specifically 5%, 10% or 15%.

[0033] In this invention, the diameter of the carbon nanotube is preferably 10 nm, and the length is preferably 500 nm.

[0034] In this invention, the second silane coupling agent preferably includes KH-550.

[0035] In this invention, the activation temperature is preferably 60°C and the activation time is preferably 4 hours.

[0036] After activation, the present invention preferably centrifuges, washes and dries the resulting system sequentially to obtain the silane coupling agent modified carbon nanotubes.

[0037] In this invention, a carbon nanotube ethanol dispersion is preferably prepared, and a silane coupling agent KH-550 is added. The mixture is ultrasonically dispersed for 30 minutes, stirred at 60°C for 4 hours to activate it, and then centrifuged, washed, and dried at 60°C for 12 hours to obtain the silane coupling agent modified carbon nanotube.

[0038] After obtaining surface-activated modified ferrite powder and silane coupling agent modified carbon nanotubes, the present invention mixes the surface-activated modified ferrite powder, silane coupling agent modified carbon nanotubes and template agent to obtain a mixed slurry.

[0039] In this invention, the mass of the carbon nanotube is 5-20% of the mass of the composite ferrite, specifically 5%, 6%, 8%, 10%, 12%, 14%, 15%, 18% or 20%. Limiting the mass of the carbon nanotube to the above range enables it to form a three-dimensional conductive network with the carbon shell formed later, thereby reducing dielectric loss and conduction loss.

[0040] In this invention, the mass ratio of the composite ferrite to the template agent is (7:3) to (5:5), specifically 7:3, 6.5:3.5, 6:4, 5.5:4.5, 7:3, 5.5:4.5 or 6:4. Limiting the mass ratio of the composite ferrite to the template agent within the above range can control the diameter of the hollow cavity.

[0041] In this invention, the template agent is preferably a high-melting-point microcrystalline wax, and the melting point of the high-melting-point microcrystalline wax is preferably 110°C.

[0042] After obtaining the mixed slurry, the present invention granulates the mixed slurry in one step to obtain microspheres.

[0043] In this invention, the primary granulation is preferably spray granulation or fluidized bed granulation.

[0044] In this invention, the temperature of the primary granulation is preferably 115~120℃.

[0045] In this invention, the particle size of the primary microspheres is preferably 5 μm.

[0046] After obtaining the primary microspheres, the present invention mixes the primary microspheres, resin solution and curing accelerator and then granulates them a second time to obtain a core-shell intermediate.

[0047] In this invention, the mass ratio (coating amount) of resin to primary microspheres in the resin solution is preferably 1 to 4:1, specifically 1:1, 1.2:1, 1.8:1, 2.5:1, 2.8:1, 3:1, 3.5:1 or 4:1.

[0048] In this invention, the resin preferably includes thermosetting phenolic resin and / or resorcinol-formaldehyde resin (RF resin); the thermosetting phenolic resin and resorcinol-formaldehyde resin can form a homogeneous amorphous carbon shell during heat treatment.

[0049] In this invention, the solid content of the resin is preferably ≥98%.

[0050] In this invention, the solvent of the resin solution preferably includes an aqueous ethanol solution, and the concentration of the resin solution is preferably 10-20 wt%, specifically 10 wt%, 12 wt%, 15 wt%, or 20 wt%.

[0051] In this invention, the curing accelerator preferably includes propylene carbonate.

[0052] In this invention, the mass of the curing accelerator is preferably 1 to 2% of the mass of the resin.

[0053] In this invention, the secondary granulation is preferably spray granulation or fluidized bed granulation, more preferably centrifugal spray granulation, and the rotation speed of the centrifugal spray granulation is preferably 20,000~24,000 rpm, more preferably 20,000, 22,000 or 24,000 rpm.

[0054] In this invention, the temperature of the secondary granulation is preferably 100~120℃.

[0055] In this invention, the primary microspheres are preferably dispersed in a resin solution, a curing accelerator is added, the mixture is stirred and dissolved, and then sonicated and stirred at room temperature for 30 minutes to uniformly coat the surface of the primary microspheres with resin. Finally, the mixture is centrifuged and spray-dried to obtain secondary microspheres, which are the core-shell intermediates.

[0056] After obtaining the core-shell intermediate, the present invention heat-treats the core-shell intermediate in a protective atmosphere to obtain the carbon-coated ferrite hollow composite microspheres.

[0057] In this invention, the protective atmosphere preferably includes nitrogen.

[0058] In this invention, the heat treatment includes a pyrolysis stage and a carbonization stage performed sequentially. The temperature of the pyrolysis stage is 300~400℃, specifically 300, 310, 320, 350, 380, or 400℃, and the holding time is 1~3h, specifically 1, 1.5, 2, 2.5, or 3h. The heating rate from room temperature to the temperature of the pyrolysis stage is preferably 1~5℃ / min, specifically 1, 2, 3, 4, or 5℃ / min. The pyrolysis stage completes the pyrolysis and release of the template agent.

[0059] In this invention, the temperature of the carbonization stage is 600~800℃, specifically 600, 620, 50, 700, 720, 750 or 800℃, and the holding time is 2~4h, specifically 2, 2.5, 3, 3.5 or 4h. The heating rate from the temperature of the pyrolysis stage to the temperature of the carbonization stage is preferably 2~5℃ / min, specifically 2, 3, 4 or 5℃ / min; the carbonization stage is resin carbonization.

[0060] In this invention, the heat treatment is preferably carried out in a tubular furnace.

[0061] After the heat treatment is completed, it is preferable to allow it to cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0062] The present invention also provides carbon-coated ferrite hollow composite microspheres prepared by the preparation method described above, comprising a carbon shell, a hollow cavity, ferrite nanoparticles and carbon nanotubes, wherein the carbon nanotubes and ferrite nanoparticles are dispersed in the hollow cavity, and the ferrite nanoparticles comprise CoFe2O4, NiFe2O4 and CuFe2O4. Figure 1 This is a schematic diagram of the structure of the carbon-coated ferrite hollow composite microspheres described in this invention.

[0063] In this invention, the ferrite nanoparticles serve as magnetic components, and all magnetic components have a spinel structure.

[0064] In this invention, the particle size of the ferrite nanoparticles is preferably 20~50nm.

[0065] In this invention, the particle size of the carbon-coated ferrite hollow composite microspheres exhibits a normal distribution, D 50 The preferred size is 5.5~7.4μm.

[0066] In this invention, the thickness of the carbon shell is preferably 1.5~3.0 μm, specifically 1.5, 1.55, 1.7, 1.75, 1.9, 1.95, 2.15 or 3.0 μm; the diameter of the hollow cavity is preferably 2~4.5 μm, specifically 2, 3, 3.2, 3.4, 3.5 or 4 μm.

[0067] In this invention, the density of the carbon-coated ferrite hollow composite microspheres is preferably 1.5~2.1 g / cm³. 3 Specifically, it can be 1.5, 1.55, 1.62, 1.7, 1.77, 1.95, 2.05, or 2.1 g / cm³. 3 .

[0068] This invention utilizes a strategy combining "removable templates" and "resin-coated carbonization" to construct a composite microsphere structure consisting of "hollow carbon shells encapsulating multi-component ferrite nanoparticles and carbon nanotubes." Three spinel ferrites with complementary magnetic resonance frequencies—NiFe2O4 (nickel ferrite, covering the low-to-mid frequency range), CoFe2O4 (cobalt ferrite, covering the mid-to-high frequency range), and CuFe2O4 (copper ferrite, enhancing interfacial polarization and eddy current loss)—are carefully selected. Their magnetic resonance frequencies form a continuous coverage in the Ku band, achieving broadband absorption across the entire 12–18 GHz band through a synergistic effect. Simultaneously, carbon nanotubes are introduced, forming a three-dimensional conductive network with the resin-formed carbon shell, further enhancing dielectric and conduction losses. The hollow structure introduces a large amount of air (ε≈1), significantly reducing the effective dielectric constant of the composite material. By controlling the carbonization temperature at 600–800℃, the graphitization degree of the carbon shell is limited, further suppressing dielectric losses, thus perfectly achieving impedance matching with free space. The carbon shell provides conductive loss, the hollow structure provides multiple reflection losses, the three types of ferrite provide complementary magnetic losses, and the carbon nanotubes and carbon shell form a continuous conductive network. Multiple mechanisms work together to achieve the dual function of "primarily absorbing waves and secondarily shielding".

[0069] This invention also provides the application of the carbon-coated ferrite hollow composite microspheres described above as a Ku-band electromagnetic wave shielding agent in the field of electromagnetic shielding.

[0070] The present invention does not impose any special limitation on the specific method of application, and any method known to those skilled in the art can be used.

[0071] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0072] In the examples and comparative examples, the particle size of CoFe2O4 nanoparticles, NiFe2O4 nanoparticles, and CuFe2O4 nanoparticles is 20~50 nm.

[0073] Example 1 (Composite ferrite:microcrystalline wax mass ratio = 7:3, coating amount 2.5 times): The preparation method of carbon-coated ferrite hollow composite microspheres, the flowchart is as follows: Figure 2 This includes the following steps: (1) Take 2.0 g of CoFe2O4 nanoparticles, 1.5 g of NiFe2O4 nanoparticles, and 1.5 g of CuFe2O4 nanoparticles (mass ratio 40:30:30), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.25 g of silane coupling agent KH-550 (5% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 180 ℃ to obtain surface-activated modified ferrite composite powder.

[0074] (2) Take 0.5 g of carbon nanotubes (10% of the mass of composite ferrite) in ethanol dispersion, add 0.075 g KH-550, ultrasonically disperse for 30 min, stir at 60 ℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60 ℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0075] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 7:3. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0076] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 115°C, and collect the microspheres.

[0077] (5) Prepare a phenolic resin ethanol solution with a mass fraction of 15% by mixing thermosetting phenolic resin (17.5 g, 2.5 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.35 g of propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0078] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain a resin-based slurry.

[0079] (7) The resin-based slurry from step (6) is centrifuged and spray-dried at a speed of 20,000 rpm and a drying temperature of 100 ℃. Secondary microspheres are collected.

[0080] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 350℃ at 2℃ / min and hold for 2h; then heat up to 700℃ at 4℃ / min and hold for 3h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres. Figure 5 The images shown are SEM images of the carbon-coated ferrite hollow composite microspheres prepared in Example 1, where (a) is the overall morphology and (b) is the internal structure.

[0081] Example 2 (composite ferrite:microcrystalline wax mass ratio = 6.5:3.5, coating amount 1.2 times): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 2.5 g of CoFe2O4 nanoparticles, 1.25 g of NiFe2O4 nanoparticles, and 1.25 g of CuFe2O4 nanoparticles (mass ratio 50:25:25), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.30 g of silane coupling agent KH-550 (6% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0082] (2) Take 0.4 g of carbon nanotubes (8% of the mass of composite ferrite) in ethanol dispersion, add 0.06 g KH-550, ultrasonically disperse for 30 min, stir at 60 ℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60 ℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0083] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 6.5: 3.5. Add the surface-activated modified ferrite composite powder from step (1) and the silane coupling agent modified carbon nanotube from step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0084] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 115°C, and collect the microspheres.

[0085] (5) Prepare a phenolic resin ethanol solution with a mass fraction of 12% by mixing thermosetting phenolic resin (9.0 g, 1.2 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.18 g of propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0086] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0087] (7) The resin-based mixed slurry from step (6) was centrifuged and spray-dried at a speed of 22,000 rpm and a drying temperature of 100 ℃ to obtain secondary microspheres.

[0088] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 320°C at 3°C / min and hold for 2.5h; then heat up to 650°C at 5°C / min and hold for 3h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0089] Example 3 (Composite ferrite:microcrystalline wax mass ratio = 6:4, coating amount 3.5 times): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 1.75 g of CoFe2O4 nanoparticles, 1.75 g of NiFe2O4 nanoparticles, and 1.5 g of CuFe2O4 nanoparticles (mass ratio 35:35:30), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.2 g of silane coupling agent KH-550 (4% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0090] (2) Take 0.9 g of carbon nanotubes (18% of the mass of composite ferrite) in ethanol dispersion, add 0.135 g KH-550, ultrasonically disperse for 30 min, stir at 60 ℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60 ℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0091] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 6:4. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0092] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 120°C, and collect the microspheres.

[0093] (5) Prepare a phenolic resin ethanol solution with a mass fraction of 18% by mixing thermosetting phenolic resin (30 g, 3.5 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.60 g of propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0094] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0095] (7) The resin-based mixed slurry from step (6) was centrifuged and spray-dried at a speed of 24,000 rpm and a drying temperature of 100 ℃, and secondary microspheres were collected.

[0096] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 380℃ at 1℃ / min and hold for 1.5h; then heat up to 750℃ at 3℃ / min and hold for 2.5h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0097] Example 4 (composite ferrite:microcrystalline wax mass ratio = 5.5:4.5, coating amount 1.0 times): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 2.25 g of CoFe2O4 nanoparticles, 1.5 g of NiFe2O4 nanoparticles, and 1.25 g of CuFe2O4 nanoparticles (mass ratio 45:30:25), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.35 g of silane coupling agent KH-550 (7% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0098] (2) Take 0.3 g of carbon nanotubes (6% of the mass of composite ferrite) in ethanol dispersion, add 0.045 g KH-550, ultrasonically disperse for 30 min, stir at 60℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0099] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 5.5: 4.5. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0100] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 115°C, and collect the microspheres.

[0101] (5) Prepare a 10% phenolic resin ethanol solution by mixing thermosetting phenolic resin (9.0g, 1.0 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.18g propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir and dissolve to obtain a mixed slurry.

[0102] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0103] (7) The resin-based mixed slurry from step (6) is centrifuged and spray-dried at a speed of 20,000 rpm and a drying temperature of 100 ℃ to obtain secondary microspheres.

[0104] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 300 ℃ at 5 ℃ / min and hold for 3 h; then heat up to 600 ℃ at 2 ℃ / min and hold for 4 h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0105] Example 5 (composite ferrite:microcrystalline wax mass ratio = 7:3, coating amount 3.0 times, high carbonization temperature): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 2.0 g of CoFe2O4 nanoparticles, 1.5 g of NiFe2O4 nanoparticles, and 1.5 g of CuFe2O4 nanoparticles (mass ratio 40:30:30), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.25 g of silane coupling agent KH-550 (5% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0106] (2) Take 0.6 g of carbon nanotubes (12% of the mass of composite ferrite) in ethanol dispersion, add 0.09 g of KH-550, ultrasonically disperse for 30 min, stir at 60℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0107] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 7:3. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0108] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 115°C, and collect the microspheres.

[0109] (5) Prepare a phenolic resin ethanol solution with a mass fraction of 15% by mixing thermosetting phenolic resin (21.6 g, 3.0 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.43 g of propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0110] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0111] (7) The resin-based mixed slurry from step (6) was centrifuged and spray-dried at a speed of 24,000 rpm and a drying temperature of 100 ℃, and secondary microspheres were collected.

[0112] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 350 ℃ at 2 ℃ / min and hold for 2 h; then heat up to 800 ℃ at 4 ℃ / min and hold for 2 h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0113] Example 6 (composite ferrite:microcrystalline wax mass ratio = 5.5:4.5, coating amount 2.8 times, medium carbonization temperature): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 2.5 g of CoFe2O4 nanoparticles, 1.5 g of NiFe2O4 nanoparticles, and 1.0 g of CuFe2O4 nanoparticles (mass ratio 50:30:20), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.25 g of silane coupling agent KH-550 (5% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0114] (2) Take 0.7 g of carbon nanotubes (14% of the mass of composite ferrite) in ethanol dispersion, add 0.035 g KH-550, ultrasonically disperse for 30 min, stir at 60℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0115] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 5.5: 4.5. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0116] (4) Spray granulation of the microcrystalline wax-based mixed slurry from step (3) at 115°C, and collect the primary microspheres. (5) Prepare a 15% phenolic resin ethanol solution by mixing thermosetting phenolic resin (19.6 g, 2.8 times the mass of the primary microspheres) and anhydrous ethanol, add 0.39 g propylene carbonate (2% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0117] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0118] (7) The resin-based mixed slurry from step (6) was centrifuged and spray-dried at a speed of 22,000 rpm and a drying temperature of 100 ℃ to obtain secondary microspheres.

[0119] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 350 ℃ at 2 ℃ / min and hold for 2 h; then heat up to 720 ℃ at 4 ℃ / min and hold for 2.5 h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0120] Example 7 (Composite ferrite:microcrystalline wax mass ratio = 6:4, coating amount 1.8 times, lower carbonization temperature): The preparation method of carbon-coated ferrite hollow composite microspheres includes the following steps: (1) Take 2.0 g of CoFe2O4 nanoparticles, 2.0 g of NiFe2O4 nanoparticles, and 1 g of CuFe2O4 nanoparticles (mass ratio 40:40:20), mix them, and add them to 25 mL of ethanol / water mixed solution (ethanol to water volume ratio 9:1). Stir and disperse for 30 min. Add 0.3 g of silane coupling agent KH-550 (6% of the composite ferrite mass), ultrasonically disperse for 30 min, and stir and react at 60 ℃ for 2 h. After the reaction is complete, filter, wash three times with anhydrous ethanol, and spray dry at 120 ℃ to obtain surface-activated modified ferrite composite powder.

[0121] (2) Take 0.5 g of carbon nanotubes (10% of the mass of composite ferrite) in ethanol dispersion, add 0.05 g KH-550, ultrasonically disperse for 30 min, stir at 60℃ for 4 h, centrifuge, wash twice with anhydrous ethanol, and vacuum dry at 60℃ for 12 h to obtain silane coupling agent modified carbon nanotubes.

[0122] (3) Heat the high melting point microcrystalline wax to 110 °C to melt it according to the mass ratio of composite ferrite: microcrystalline wax = 6:4. Add the surface-activated modified ferrite composite powder in step (1) and the silane coupling agent modified carbon nanotube in step (2). Stir at 500 rpm for 15 min to form a uniform microcrystalline wax-based mixed slurry.

[0123] (4) Spray granulation of the microcrystalline wax-based mixed slurry in step (3) at 120°C, and collect the microspheres.

[0124] (5) Prepare a phenolic resin ethanol solution with thermosetting phenolic resin (12.6 g, 1.8 times the mass of the primary microspheres) and anhydrous ethanol. Add 0.19 g of propylene carbonate (1.5% of the mass of the thermosetting phenolic resin) as a curing accelerator, stir to dissolve, and obtain a mixed slurry.

[0125] (6) Take the microspheres obtained in step (4) and disperse them in the above mixed slurry. Disperse them ultrasonically at room temperature for 30 min while stirring to obtain the resin-based mixed slurry.

[0126] (7) The resin-based mixed slurry from step (6) is centrifuged and spray-dried at a speed of 20,000 rpm and a drying temperature of 100 ℃ to obtain secondary microspheres.

[0127] (8) Place the secondary microspheres from step (7) in a corundum boat and put it into a tubular atmosphere furnace. Introduce N2 atmosphere. Heating program: heat up to 310 ℃ at 4 ℃ / min and hold for 2.5 h; then heat up to 620 ℃ at 3 ℃ / min and hold for 3.5 h, and finally cool naturally to room temperature to obtain the carbon-coated ferrite hollow composite microspheres.

[0128] Comparative Example 1 (physical mixing only, no hollow structure, no carbon coating) 2.0 g of CoFe2O4 nanoparticles, 1.5 g of NiFe2O4 nanoparticles, and 1.5 g of CuFe2O4 nanoparticles (mass ratio 40:30:30) were mixed with 0.5 g of carbon nanotubes (10% of the total mass of the composite ferrite). 50 mL of anhydrous ethanol was added, and the mixture was ball-milled for 2 h. The mixture was then filtered and dried to obtain a physically mixed powder. No microcrystalline wax template pore-forming, phenolic resin coating, or carbonization treatment was performed.

[0129] Comparative Example 2 (using only CoFe2O4, without multiferrite synergy) The only difference from Example 1 is that in step (1), only 5.0 g of CoFe2O4 nanoparticles are used, and NiFe2O4 and CuFe2O4 are not used.

[0130] Comparative Example 3 (using only NiFe2O4, without multiferrite synergy) The only difference from Example 1 is that in step (1), only 5.0 g of NiFe2O4 nanoparticles are used, and CoFe2O4 and CuFe2O4 are not used.

[0131] Comparative Example 4 (using only CuFe2O4, without multiferrite synergy) The only difference from Example 1 is that in step (1), only 5.0 g of CuFe2O4 nanoparticles are used, and CoFe2O4 and NiFe2O4 are not used.

[0132] Comparative Example 5 (without carbon nanotubes, lacking carbon nanotube conductive network) The only difference from Example 1 is that carbon nanotubes are not added in step (2), and only surface-activated modified ferrite composite powder is mixed with microcrystalline wax in step (3).

[0133] Comparative Example 6 (microcrystalline wax ratio too low, hollow cavity too small) The only difference from Example 1 is that the amount of high melting point microcrystalline wax in step (3) is changed to 1.25 g (composite ferrite: microcrystalline wax mass ratio = 8:2), which is lower than the lower limit of the range (7:3)~(5:5) defined by the present invention.

[0134] Comparative Example 7 (excessive microcrystalline wax ratio, excessively large hollow cavity, and excessively thin carbon shell) The only difference from Example 1 is that the amount of high melting point microcrystalline wax in step (3) is changed to 7.5 g (composite ferrite: microcrystalline wax mass ratio = 4:6), which is higher than the upper limit of the range (7:3)~(5:5) defined by the present invention.

[0135] Figure 3 The image shows the XRD pattern of the magnetic components in the carbon-coated ferrite hollow composite microspheres prepared in Example 1. It can be seen that the phase composition of the magnetic components includes CoFe2O4, NiFe2O4 and CuFe2O4.

[0136] Performance testing: 1. Apparent density measurement Materials prepared from the examples and comparative examples with a certain mass m were placed in a graduated cylinder and gently tapped until the volume remained constant. The volume V was recorded. The formula for calculating the apparent density is shown in Equation 1, and the results are shown in Table 1. Formula 1.

[0137] 2. The properties of the materials prepared in the examples and comparative examples were tested, and the results are shown in Table 1.

[0138] Table 1. Structural test results of materials prepared in the Examples and Comparative Examples

[0139] 3. Electromagnetic parameter testing (coaxial method) The electromagnetic properties of the product prepared above were tested. The test method was as follows: composite microspheres were mixed with paraffin wax, and the volume fraction of composite microspheres in the resulting mixture was 45%. A coaxial ring was prepared with an inner diameter of 3 mm, an outer diameter of 7 mm, and a thickness (i.e., height) of 3 mm. The electromagnetic parameters were then tested using a vector grid analyzer to analyze its electromagnetic properties. Electromagnetic parameters in the Ku band (12~18 GHz) were compared and analyzed using 15 GHz as a representative frequency point. The test results are shown in Table 2. It can be seen that: From the perspective of dielectric constant, the real part ε' of the complex dielectric constant of Examples 1-7 is between 5.0 and 9.0, and the imaginary part ε'' is between 1.0 and 2.8. Compared with conventional carbon-based microwave absorbing materials (ε' is usually >20), the dielectric constant of the present invention is significantly reduced. This is mainly attributed to two aspects: firstly, the hollow cavity formed after carbonization of the microcrystalline wax template introduces a large amount of air (ε≈1), effectively reducing the overall dielectric constant; secondly, the carbonization temperature is controlled at 600~800℃, resulting in a lower degree of graphitization of the carbon shell formed by the carbonization of phenolic resin, limiting conductivity and avoiding the dielectric constant spike caused by highly conductive carbon materials. The lower dielectric constant is beneficial for achieving impedance matching with free space and reducing the reflection of electromagnetic waves on the material surface; From the perspective of permeability, the real part μ' of the complex permeability in Examples 1-7 is between 1.3 and 1.7, and the imaginary part μ'' is between 0.45 and 1.35. It is noteworthy that all examples exhibit a significant magnetic loss resonance peak (μ'' peak value) near 15 GHz, which is consistent with the natural resonance characteristics of the three spinel ferrites, CoFe2O4, NiFe2O4, and CuFe2O4, in the Ku band. The complementary superposition of the magnetic resonance frequencies of these three ferrites is the core mechanism by which this invention achieves broadband absorption across the entire Ku band. Comparing Examples 1-7 reveals that as the carbonization temperature increases from 600℃ to 800℃, the dielectric constant shows an upward trend (ε' increases from 5.0 to 9.0). This is because the high temperature promotes the graphitization of the carbon shell, increasing its contribution to conductive losses. Simultaneously, the magnetic permeability remains relatively stable, indicating that the ferrite exhibits good thermal stability within the 600-800℃ range. Increasing the amount of carbon nanotubes also leads to an increase in the dielectric constant, but it is beneficial for improving shielding effectiveness. When the amount of microcrystalline wax relative to composite ferrite increases from 43% (ferrite:microcrystalline wax = 7:3) to 100% (ferrite:microcrystalline wax = 5:5), the hollow cavity diameter increases (2.5-4.5 μm), the dielectric constant further decreases, and the absorption ratio increases to over 94%. Considering both absorption performance and shielding effectiveness, Example 1 (carbonization temperature 700℃, carbon nanotubes 10 wt%, composite ferrite: microcrystalline wax = 7:3) achieved optimal impedance matching (ε' = 6.5, μ' = 1.4, ε' / μ' ≈ 4.6). With a thickness of 2.8 mm, its RLmin reached -45.6 dB and covered the entire Ku band, while achieving a shielding effectiveness of 20~24 dB and an absorption rate exceeding 93%. The above results show that the present invention, through the structural design of "multi-ferrite synergy + hollow carbon shell + carbon nanotube conductive network", successfully controls the dielectric constant of the composite microspheres at a low level while retaining sufficient magnetic loss capability, laying a material foundation for achieving efficient Ku-band wave absorption and good electromagnetic shielding. Although Example 3 has lower reflection loss and larger SE compared to Example 1, the process conditions of Example 1 are more economical and energy-saving, the amount of carbon nanotubes added in Example 3 is higher than that in Example 1, and the energy consumption of the pyrolysis and carbonization process stages in Example 3 is higher, which is not conducive to actual production.

[0140] Table 2. Physical properties and Ku-band electromagnetic shielding performance of carbon-coated ferrite hollow composite microspheres in Examples 1-7

[0141] Figure 4The reflection loss curves for Example 1 under different matching thicknesses show that the best absorption performance for 12~18GHz is achieved with a matching thickness of 2.8mm, covering the entire Ku band, with a minimum reflection loss of -45.6dB and a resonant frequency of 15.2GHz.

[0142] Table 3 compares the electromagnetic shielding performance results of the comparative examples with those of Example 1. Compared with Example 1, Comparative Example 1 suffers from poor impedance matching due to the lack of a hollow cavity to reduce the dielectric constant and the absence of a carbon shell to form a conductive network, resulting in most electromagnetic waves being reflected by the surface. Comparative Examples 2-4 show that the magnetic loss band of a single spinel ferrite cannot independently cover 12-18 GHz. Comparative Example 5 suffers from even worse impedance matching and higher reflection loss due to the lack of carbon nanotubes, resulting in poor overall absorption performance despite a high absorption ratio. In Comparative Example 6, insufficient microcrystalline wax leads to an excessively small cavity, which cannot effectively reduce the overall dielectric constant, worsens impedance matching, narrows the absorption bandwidth, and significantly reduces the absorption ratio. In Comparative Example 7, excessive microcrystalline wax results in an excessively large cavity and an excessively thin carbon shell, which damages the structural integrity. Some microspheres experience shell collapse during carbonization, leading to an excessive reduction in dielectric constant and insufficient dielectric loss. As can be seen from the above, the four elements of the present invention—hollow carbon coating structure, multi-ferrite synergy, carbon nanotube conductive network, and microcrystalline wax dosage—are indispensable. The absence or deviation of any one of these elements will lead to failure of Ku full-band coverage or a significant decrease in core performance (effective absorption bandwidth, absorption ratio, and shielding effectiveness).

[0143] Table 3 Comparison of electromagnetic shielding performance results between Comparative Example and Example 1

[0144] Note: The absorption percentages in Tables 2 and 3 reflect the percentage of material absorbed after entering the material. The calculation process is as follows: The reflection coefficient S was measured using a vector network analyzer. 11 and transmission coefficient S 21 Calculate the complex permittivity and complex permeability; The shielding effectiveness (SE) is calculated as shown in Equation 2: Overall shielding effectiveness: Formula 2; The reflection coefficient (power) is calculated as shown in Equation 3: Formula 3; The transmission coefficient (power) is calculated as shown in Equation 4: Equation 4; The absorption coefficient is calculated as shown in Equation 5: Formula 5; The reflection loss is calculated as shown in Equation 6: Formula 6; The absorption loss is calculated as shown in Equation 7: Formula 7; verify (Ignoring multiple reflections) The absorption ratio is calculated as shown in Equation 8: Formula 8.

[0145] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing carbon-coated ferrite hollow composite microspheres, characterized in that, Includes the following steps: The composite ferrite, the first silane coupling agent, and the dispersant are surface-activated to obtain surface-activated ferrite powder, wherein the composite ferrite includes CoFe2O4, NiFe2O4, and CuFe2O4. Carbon nanotubes and a second silane coupling agent are activated to obtain silane coupling agent modified carbon nanotubes. The surface-activated modified ferrite powder, silane coupling agent modified carbon nanotubes, and template agent are mixed to obtain a mixed slurry; The mixed slurry is granulated in one step to obtain microspheres; The primary microspheres, resin solution and curing accelerator are mixed and then granulated a second time to obtain a core-shell intermediate. The core-shell intermediate was heat-treated in a protective atmosphere to obtain the carbon-coated ferrite hollow composite microspheres. The mass ratio of CoFe2O4, NiFe2O4 and CuFe2O4 is (30~50):(20~40):(20~40); The mass of the carbon nanotubes is 5-20% of the mass of the composite ferrite; The mass ratio of the composite ferrite to the template agent is (7:3) to (5:5); The template agent includes a hydrocarbon sacrificial agent; the hydrocarbon sacrificial agent is polyethylene wax and / or microcrystalline wax; The heat treatment includes a pyrolysis stage and a carbonization stage performed sequentially. The temperature of the pyrolysis stage is 300~400℃ and the holding time is 1~3h. The temperature of the carbonization stage is 600~800℃ and the holding time is 2~4h.

2. The preparation method according to claim 1, characterized in that, The mass of the first silane coupling agent is 4 to 7% of the mass of the composite ferrite.

3. The preparation method according to claim 1, characterized in that, The amount of the second silane coupling agent is 5-15% of the mass of the carbon nanotubes.

4. The preparation method according to claim 1, characterized in that, The mass ratio of resin to primary microspheres in the resin solution is 1~4:

1.

5. The preparation method according to claim 1 or 4, characterized in that, The resin in the resin solution includes thermosetting phenolic resin and / or resorcinol-formaldehyde resin.

6. The preparation method according to claim 1, characterized in that, The curing accelerator includes propylene carbonate.

7. The preparation method according to claim 1 or 6, characterized in that, The mass of the curing accelerator is 1 to 2% of the mass of the resin in the resin solution.

8. The carbon-coated ferrite hollow composite microspheres prepared by the preparation method according to any one of claims 1 to 7, characterized in that, It includes a carbon shell, a hollow cavity, ferrite nanoparticles and carbon nanotubes, wherein the carbon nanotubes and ferrite nanoparticles are dispersed in the hollow cavity, and the ferrite nanoparticles include CoFe2O4, NiFe2O4 and CuFe2O4.

9. The application of the carbon-coated ferrite hollow composite microspheres as described in claim 8 as a Ku-band electromagnetic wave shielding agent in the field of electromagnetic shielding.

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