Heterogeneous composite hollow microsphere absorber with hierarchical cavity structure and preparation method thereof

Abstract

The present invention discloses a heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure, comprising an internal hollow cavity and a composite shell structure; wherein the hollow cavity contains air; and the shell structure is a composite structure with silicon dioxide as the continuous phase and a carbon-cavity-metal composite material as the dispersed phase. This multi-level cavity structure can introduce a rich interface, achieving multiple scattering and dissipation of incident electromagnetic waves; the large internal cavity can reduce the overall density of the hollow microspheres; the small cavity within the spherical shell further reduces the density of the hollow microspheres and dissipates electromagnetic waves through various loss mechanisms of metal and carbon materials, thereby improving the absorbing performance of the heterogeneous hierarchical hollow structure absorber. The present invention also provides a method for preparing the heterogeneous hierarchical cavity structure absorber, which adopts a spray drying method combined with heat treatment molding and has the characteristics of rapidity and efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of functional materials, and more specifically to a heterogeneous composite hollow microsphere absorber having a hierarchical cavity structure and a preparation method thereof. Background Art

[0002] With the rapid development of electronic technology, electromagnetic waves have been widely used in numerous areas of production and life, such as industrial positioning and flaw detection, radar detection and ranging, satellite navigation, television broadcasting, medical diagnosis, and precision instrumentation. While the widespread use of electronic devices has brought great convenience to people's lives and production, it also inevitably generates electromagnetic waves, which can seriously interfere with the normal operation of precision instruments and human health. Furthermore, the rapid development of radar detection technology that uses electromagnetic echo location and ranging has also posed a severe challenge to the anti-detection and stealth technologies of various types of equipment. Therefore, high-performance electromagnetic wave absorbing materials play an irreplaceable role in both the operation and stealth design of high-end equipment and human health protection. To date, researchers have developed a variety of absorbing materials, and the most critical functional components in absorbing materials are absorbers such as ferrites, metal powders, conductive polymers, and functional ceramics. These absorbers convert the electromagnetic wave energy incident on their surfaces into heat or other forms of energy, dissipating it, thereby avoiding secondary electromagnetic contamination and achieving electromagnetic stealth. As demand continues to rise, the requirements for lightweight and enhanced functionalization of absorbing materials are becoming increasingly stringent. Since the absorber accounts for most of the volume and plays a key role in the absorbing material, lightweighting of the absorber is the premise and key to achieving overall lightweighting of the absorbing material.

[0003] Chemical or physical foaming of absorbent composites is an effective method for reducing their density. However, conventional foaming processes are difficult to control and can significantly reduce the mechanical stability of the material. In contrast, introducing hollow structures into absorbers is a key approach to lightweighting absorbers and, ultimately, absorbent composites. Extensive research has been conducted on the construction of hollow structures in micro- and nano-absorbers, and the chemical composition and structure of the materials can be extensively manipulated. However, current challenges in forming and regulating the properties of hollow structures include complex and time-consuming preparation processes, the inability to efficiently produce them in large quantities, and difficulties in hierarchical design and coordinated control of structure and properties. These challenges hinder the mass production and application development of lightweight absorbers. Furthermore, in addition to lightweighting, material functionality is increasingly demanding, and the functional and strength deficiencies of hollow absorbers with simple compositions and structures are becoming increasingly prominent. Summary of the Invention

[0004] To address the challenges of current hollow absorbers, which lack the composite components and hierarchical cavity design, the first objective of the present invention is to provide a heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure. This absorber consists of a single large internal cavity and numerous smaller cavities within the shell. Its chemical composition is metal, silica, and carbon. The hierarchical cavity structure and the silica-carbon-magnetic particle heterogeneous composite shell enable reduced density and controllable electromagnetic wave loss.

[0005] Another object of the present invention is to provide a method for preparing a heterogeneous composite hollow microsphere absorber having a hierarchical cavity structure.

[0006] In order to achieve the above first object, the present invention adopts the following technical solutions:

[0007] A heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure, comprising an internal hollow cavity and a composite shell structure;

[0008] The hollow cavity contains air; the shell structure is a composite structure with silicon dioxide as a continuous phase and a carbon-cavity-metal composite material as a dispersed phase.

[0009] In the absorber of the present invention, the multi-level cavity structure can introduce abundant interfaces, realizing multiple scattering and dissipation of incident electromagnetic waves; the large internal cavities can reduce the overall density of the hollow microspheres; the small cavities within the spherical shells further reduce the density of the hollow microspheres and dissipate electromagnetic waves through the interface between metal and carbon materials, dielectric, leakage conduction, magnetic loss and other mechanisms, thereby improving the absorbing performance of the heterogeneous hierarchical hollow structure absorber.

[0010] Furthermore, the particle size of the absorber is 2-30 μm and the density is 1.4-2.2 g / cm 3 .

[0011] Furthermore, the diameter of the hollow cavity is 1-15 μm.

[0012] Furthermore, the structure of the carbon-cavity-metal composite material includes a cavity, a carbon shell covering the cavity, and a metal located in the cavity.

[0013] Furthermore, the diameter of the cavity in the carbon-cavity-metal composite material is 10-60 nm.

[0014] Furthermore, in the absorber, the mass percentage of the carbon-cavity-metal composite material is 10-40%.

[0015] Furthermore, in the carbon-cavity-metal composite material, the cavity component is air, and the molar ratio of carbon to metal is 15:1-35:1.

[0016] Furthermore, the metal in the carbon-cavity-metal composite material is a single substance, an alloy or a mixture of different metals.

[0017] Furthermore, the metal is selected from one or more of iron, copper, nickel, silver, cobalt, and aluminum, or an alloy formed by two or more of the aforementioned metals.

[0018] Furthermore, the metal component in the carbon-cavity-metal composite material is obtained from initially added metal nanoparticles, or from initially added metal oxide nanoparticles through reduction conversion.

[0019] In order to achieve the above second purpose, the present invention adopts the following technical solutions:

[0020] A method for preparing a heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure comprises the following steps:

[0021] (1) Dispersing metal or metal oxide nanoparticles in a concentrated ammonia solution, and then adding ethyl orthosilicate; stirring and reacting at room temperature to obtain silica-coated nanoparticles;

[0022] (2) dispersing the silica-coated nanoparticles in a mixture of resorcinol, formaldehyde, and concentrated ammonia, heating and stirring to react, thereby obtaining nanoparticles doubly coated with silica and phenolic resin;

[0023] (3) heat-treating the nanoparticles doubly coated with silica and phenolic resin in an inert or reducing atmosphere to obtain nanoparticles doubly coated with silica and carbon;

[0024] (4) soaking the silicon dioxide and carbon double-coated nanoparticles in an alkaline solution to obtain carbon-cavity-metal composite particles;

[0025] (5) mixing the silica sol and the carbon-cavity-metal composite particles, stirring them uniformly to obtain a mixed slurry, and atomizing and drying the mixed slurry to obtain a spherical intermediate;

[0026] (6) heat-treating the intermediate in a reducing or inert atmosphere to obtain the heterogeneous composite hollow microsphere absorber having a hierarchical cavity structure.

[0027] Furthermore, in step (1), the ratio of the metal or metal oxide nanoparticles to the reaction solution A and ethyl orthosilicate is 2 g:2000 mL:10 mL.

[0028] Furthermore, in step (1), the stirring reaction time is 4-10 hours.

[0029] Furthermore, in step (1), the preparation of the concentrated ammonia solution comprises the following steps:

[0030] Anhydrous ethanol and deionized water were mixed uniformly in a mass ratio of 5:1, and then 2% by mass (equivalent to the mass percentage of the mixed solution) of concentrated ammonia water (25 wt.%) was added to the mixed solution, and the mixture was stirred and mixed uniformly to obtain a reaction solution A.

[0031] Furthermore, in step (1), the metal or metal oxide nanoparticles have a particle size of 10-50 nm, and the components include but are not limited to iron, copper, nickel, silver, cobalt, aluminum and their oxides, or composite oxides formed by two or more of the above metals.

[0032] Furthermore, in step (2), the ratio of the silica-coated nanoparticles to the reaction solution B is 1 g:1000 mL.

[0033] Furthermore, in step (2), the preparation of the mixture of resorcinol, formaldehyde and concentrated ammonia water comprises the following steps:

[0034] Anhydrous ethanol and deionized water were mixed in a ratio of 3:1, and then 0.5% by mass (equivalent to the percentage by mass of the mixed solution) of resorcinol, 2% by mass (equivalent to the percentage by mass of the mixed solution) of formaldehyde solution (37 wt.%), and 3.5% by mass (equivalent to the percentage by mass of the mixed solution) of concentrated ammonia water (25 wt.%) were added to the mixed solution, and the mixture was stirred and mixed to obtain reaction solution B.

[0035] Furthermore, in step (2), the temperature of the heating and stirring reaction is 35° C., and the time is 12-30 hours.

[0036] Furthermore, in step (3), when the nanoparticles are heat-treated in an inert atmosphere, the heat treatment temperature is 350°C and the time is 3 hours; when the nanoparticles are heat-treated in an inert atmosphere, the heat treatment temperature is 750°C and the time is 3 hours.

[0037] Furthermore, in step (4), the alkali solution is an aqueous solution of sodium hydroxide with a concentration of 3.6 M; and the treatment time is 30 h.

[0038] Furthermore, the silica sol has a solid content of 20-30 wt %, and the nanoparticles therein have a particle size of 10-50 nm.

[0039] Furthermore, in step (5), the conditions for the atomization drying are: an inlet temperature of 200-400°C, and a drying atmosphere of air.

[0040] Furthermore, in step (6), the heat treatment temperature is 600-1200° C., and the holding time is 2-12 hours.

[0041] The beneficial effects of the present invention are as follows:

[0042] The heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure provided by the present invention ensures structural stability while reducing density through the support of an internal continuous silica phase. This absorber simultaneously achieves the assembly of microscopic nanoparticles and the reduction of macroscopic size, preventing nanoparticle agglomeration and facilitating the absorber's use. Furthermore, the gauge absorber features a rich heterogeneous interface and cavity structures of varying sizes, effectively enhancing its electromagnetic wave absorption capacity. Furthermore, the density and electromagnetic performance of the hollow microsphere absorber can be adjusted by controlling the ratio of the components and the preparation conditions. Finally, the preparation method for this heterogeneous hierarchical hollow structure absorber utilizes a spray drying method combined with heat treatment molding, resulting in rapid and efficient production. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0044] Figure 1 Schematic diagram showing the preparation process of heterogeneous composite hollow microsphere absorbers with hierarchical cavity structure.

[0045] Figure 2 The X-ray diffraction (XRD) pattern of the core-shell structured composite microsphere absorber prepared in Example 2 (Ni) is shown.

[0046] Figure 3 The scanning electron microscope (SEM) image of the core-shell structure composite microsphere absorber prepared in Example 2 (Ni) is shown.

[0047] Figure 4 The scanning electron microscope (SEM) image of the core-shell structure composite microsphere absorber prepared in Example 2 (Ni) is shown.

[0048] Figure 5 The transmission electron microscope (TEM) image of the core-shell structure composite microsphere absorber prepared in Example 2 (Ni) is shown.

[0049] Figure 6 The electromagnetic wave absorption curve of the core-shell structure composite microsphere absorber prepared in Example 2 (Ni) is shown. DETAILED DESCRIPTION

[0050] In order to more clearly illustrate the present invention, the present invention is further described below in conjunction with preferred embodiments and accompanying drawings. Similar components in the accompanying drawings are represented by the same reference numerals. It should be understood by those skilled in the art that the following detailed description is illustrative rather than restrictive and should not be used to limit the scope of protection of the present invention.

[0051] Example 1

[0052] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of ferric oxide nanoparticles (25 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of ethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated ferric oxide nanoparticles. Add 1 g of the silica-coated ferric oxide nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 15 hours to obtain silica- and phenolic resin-coated ferric oxide nanoparticles. The above process was repeated to obtain more Fe2O3 nanoparticles doubly coated with silica and phenolic resin. The Fe2O3 nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0053] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 11g of the aforementioned carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 250°C in an air atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 750°C for 3h under a nitrogen atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 9.0μm, a density of 1.56g / cm³, a hollow cavity diameter of 5.8μm, and a small cavity diameter of 27nm within the shell. The molar ratio of carbon to metal was 16.5:1.

[0054] Example 2

[0055] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of nickel nanoparticles (30 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated nickel nanoparticles. Add 1 g of the aforementioned silica-coated nickel nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 20 hours to obtain nickel nanoparticles doubly coated with silica and phenolic resin. Repeat the above process to obtain more nickel nanoparticles doubly coated with silica and phenolic resin. The silicon dioxide and carbon doubly coated nickel nanoparticles were heat treated at 900°C for 6 hours and then immersed in a 3.6M (mol per liter) sodium hydroxide aqueous solution at room temperature for 30 hours to separate and obtain carbon-small-size cavity-metal composite particles.

[0056] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 13g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 260°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 850°C for 3h under a nitrogen atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 9.5μm, a density of 1.62g / cm³, a hollow cavity diameter of 6μm, and a small cavity diameter of 33nm within the shell. The molar ratio of carbon to metal was 15:1.

[0057] Example 3

[0058] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of cobalt nanoparticles (40 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated cobalt nanoparticles. Add 1 g of the aforementioned silica-coated cobalt nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 30 hours to obtain cobalt nanoparticles doubly coated with silica and phenolic resin. Repeat the above process to obtain more cobalt nanoparticles doubly coated with silica and phenolic resin. The cobalt nanoparticles doubly coated with silica and carbon were heat treated at 900°C for 6 hours and then immersed in a 3.6M (mol per liter) sodium hydroxide aqueous solution at room temperature for 30 hours to separate and obtain carbon-small-size cavity-metal composite particles.

[0059] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 18g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 280°C in an air atmosphere to obtain a spherical intermediate. The spherical intermediate was then heat-treated at 700°C for 5h under a nitrogen atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 11μm, a density of 1.42g / cm³, a hollow cavity diameter of 7.3μm, and a small cavity diameter within the shell of 43.6nm. The molar ratio of carbon to metal was 21.5:1.

[0060] Example 4

[0061] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of nickel oxide nanoparticles (35 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated nickel oxide nanoparticles. Add 1 g of the silica-coated nickel oxide nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 22 hours to obtain nickel oxide nanoparticles doubly coated with silica and phenolic resin. The above process was repeated to obtain more nickel oxide nanoparticles doubly coated with silica and phenolic resin. The nickel oxide nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0062] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 14g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 210°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 1100°C for 6h under a nitrogen atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 8μm, a density of 1.92g / cm³, a hollow cavity diameter of 4.7μm, and a small cavity diameter within the shell of 38.2nm. The molar ratio of carbon to metal was 21:1.

[0063] Example 5

[0064] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of nickel cobalt oxide nanoparticles (25 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated nickel cobalt oxide nanoparticles. Add 1 g of the silica-coated nickel cobalt oxide nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 17 hours to obtain nickel cobalt oxide nanoparticles doubly coated with silica and phenolic resin. The above process was repeated to obtain more nickel cobalt oxide nanoparticles doubly coated with silica and phenolic resin. The nickel cobalt oxide nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0065] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 12.2g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 190°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 900°C for 8h under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 7μm, a density of 2.12g / cm³, a hollow cavity diameter of 3.6μm, and a small cavity diameter of 27.3nm within the spherical shell. The molar ratio of carbon to metal was 18:1.

[0066] Example 6

[0067] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of nickel ferrite nanoparticles (20 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated nickel ferrite nanoparticles. Add 1 g of the silica-coated nickel ferrite nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 19 hours to obtain nickel ferrite nanoparticles doubly coated with silica and phenolic resin. The above process was repeated to obtain more nickel ferrite nanoparticles doubly coated with silica and phenolic resin. The nickel ferrite nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0068] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 13.6g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 240°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 1150°C for 10 hours under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 10.7μm, a density of 1.84g / cm³, a hollow cavity diameter of 4.6μm, and a small cavity diameter of 21.9nm within the spherical shell. The molar ratio of carbon to metal was 20.9:1.

[0069] Example 7

[0070] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of cobalt oxide nanoparticles (15 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated cobalt oxide nanoparticles. Add 1 g of the aforementioned silica-coated cobalt oxide nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 25 hours to obtain cobalt oxide nanoparticles doubly coated with silica and phenolic resin. The above process was repeated to obtain more cobalt oxide nanoparticles doubly coated with silica and phenolic resin. The cobalt oxide nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0071] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 15.1g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 330°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 900°C for 5h under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 14.2μm, a density of 1.51g / cm³, a hollow cavity diameter of 7.6μm, and a small cavity diameter within the shell of 16.1nm. The molar ratio of carbon to metal was 22.8:1.

[0072] Example 8

[0073] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of iron nanoparticles (20 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated iron nanoparticles. Add 1 g of the aforementioned silica-coated iron nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 27 hours to obtain iron nanoparticles doubly coated with silica and phenolic resin. Repeat the above process to obtain more iron nanoparticles doubly coated with silica and phenolic resin. The silicon dioxide and carbon doubly coated iron nanoparticles were heat treated at 900°C for 6 hours and then immersed in a 3.6M (mol per liter) sodium hydroxide aqueous solution at room temperature for 30 hours to separate and obtain carbon-small-size cavity-metal composite particles.

[0074] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 16.8g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 380°C in an air-based drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 800°C for 5h under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 12.7μm, a density of 1.69g / cm³, a hollow cavity diameter of 6.3μm, and a small cavity diameter within the shell of 21.7nm. The molar ratio of carbon to metal was 20:1.

[0075] Example 9

[0076] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of ferroferric oxide nanoparticles (50 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of ethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated ferroferric oxide nanoparticles. Add 1 g of the silica-coated ferroferric oxide nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 30 hours to obtain silica- and phenolic resin-coated ferroferric oxide nanoparticles. The above process was repeated to obtain more Fe3O4 nanoparticles doubly coated with silica and phenolic resin. The Fe3O4 nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0077] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 18g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 350°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 700°C for 9h under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 11.2μm, a density of 1.75g / cm³, a hollow cavity diameter of 5.7μm, and a small cavity diameter of 54.6nm within the shell. The molar ratio of carbon to metal was 31:1.

[0078] Example 10

[0079] Mix 5000 mL of anhydrous ethanol and 1000 mL of deionized water, add 99 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution A. Separately, mix 3000 mL of anhydrous ethanol and 1000 mL of deionized water, add 16.9 g of resorcinol, 68 g of formaldehyde solution (37 wt.%), and 118 g of concentrated aqueous ammonia (25 wt.%), and stir to obtain reaction solution B. Add 2 g of cobalt ferrite nanoparticles (40 nm in diameter) to 2000 mL of reaction solution A, and then add 10 mL of tetraethyl orthosilicate while stirring. Stir and react at room temperature for 5 hours to obtain silica-coated cobalt ferrite nanoparticles. Add 1 g of the silica-coated cobalt ferrite nanoparticles to 1000 mL of reaction solution B, and stir at 35°C for 24 hours to obtain cobalt ferrite nanoparticles doubly coated with silica and phenolic resin. The above process was repeated to obtain more cobalt ferrite nanoparticles doubly coated with silica and phenolic resin. The cobalt ferrite nanoparticles doubly coated with silica and carbon were heat-treated at 900°C for 6 hours and then immersed in a 3.6M (mol / L) sodium hydroxide solution at room temperature for 30 hours to separate the carbon-small-size cavity-metal composite particles.

[0080] 50g of 25% solids acidic silica sol (sol particle size 20nm) was placed in a glass beaker, and 14.9g of carbon-small-cavity-metal composite particles were added. The mixture was magnetically stirred at room temperature to obtain a mixed slurry. The mixed slurry was spray-dried at an inlet temperature of 270°C in an air drying atmosphere to obtain a spherical intermediate. The resulting spherical intermediate was then heat-treated at 1000°C for 4h under an argon atmosphere to obtain a core-shell composite microsphere absorber. The composite microspheres had an average particle size of 8.2μm, a density of 1.83g / cm³, a hollow cavity diameter of 3.5μm, and a small cavity diameter within the shell of 44.2nm. The molar ratio of carbon to metal was 23.4:1.

[0081] Performance testing:

[0082] The electromagnetic properties of the products prepared in the above examples were tested. The testing method involved mixing the hollow composite microspheres (absorbing materials) prepared in each example with paraffin wax at a mass ratio of 30%-60% to form coaxial rings with an inner diameter of 3 mm, an outer diameter of 7 mm, and a thickness of 2 mm. The electromagnetic parameters and performance of the rings were then measured using a vector network analyzer. The results are shown in Table 1.

[0083] Table 1 Performance parameters of samples in various embodiments

[0084]

[0085] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A heterogeneous composite hollow microsphere absorber with a hierarchical cavity structure, characterized in that: including an internal hollow cavity and a composite shell structure; The hollow cavity contains air; the composite shell structure is a composite structure with silicon dioxide as the continuous phase and carbon-cavity-metal composite particles as the dispersed phase; The structure of the carbon-cavity-metal composite particle comprises a cavity, a carbon shell covering the cavity, and a metal located in the cavity; The metal is selected from one or more of iron, copper, nickel, silver, cobalt, and aluminum, or an alloy formed from two or more of the foregoing metals; The diameter of the cavity in the carbon-cavity-metal composite particle is 10-60 nm; In the absorber, the mass percentage of carbon-cavity-metal composite particles is 10-40%; The particle size of the absorber is 2-30 μm and the density is 1.4-2.2 g / cm 3 ; The absorber is composed of a single large-sized cavity inside and a plurality of small-sized cavities inside the spherical shell; The preparation method of the heterogeneous composite hollow microsphere absorber comprises the following steps: (1) Dispersing metal or metal oxide nanoparticles in a concentrated ammonia solution, and then adding ethyl orthosilicate; stirring and reacting at room temperature to obtain silica-coated nanoparticles; (2) dispersing the silica-coated nanoparticles in a mixture of resorcinol, formaldehyde, and concentrated ammonia, heating and stirring to react, thereby obtaining nanoparticles doubly coated with silica and phenolic resin; (3) heat-treating the nanoparticles doubly coated with silica and phenolic resin in an inert or reducing atmosphere to obtain nanoparticles doubly coated with silica and carbon; (4) soaking the silicon dioxide and carbon double-coated nanoparticles in an alkaline solution to obtain carbon-cavity-metal composite particles; (5) mixing the silica sol and the carbon-cavity-metal composite particles, stirring them uniformly to obtain a mixed slurry, and atomizing and drying the mixed slurry to obtain a spherical intermediate; (6) heat-treating the intermediate in a reducing or inert atmosphere to obtain the heterogeneous composite hollow microsphere absorber having a hierarchical cavity structure; In step (1), the metal or metal oxide nanoparticles have a particle size of 10-50 nm and are composed of iron, copper, nickel, silver, cobalt, aluminum and their oxides, or composite oxides formed by two or more of the above metals; In step (6), the heat treatment temperature is 600-1200°C and the holding time is 2-12h.

2. The wave absorbing agent according to claim 1, characterized in that: The diameter of the hollow cavity is 1-15 μm.

3. The wave absorbing agent according to claim 1, characterized in that: In the carbon-cavity-metal composite particles, the cavity component is air, and the molar ratio of carbon to metal is 15:1-35:

1.

4. The wave absorbing agent according to claim 1, characterized in that The metal in the carbon-cavity-metal composite particles is a single substance, an alloy or a mixture of different metals.

5. The wave absorbing agent according to claim 1, characterized in that: The silica sol has a solid content of 20-30 wt %, and the nanoparticles therein have a particle size of 10-50 nm.

6. The wave absorbing agent according to claim 1, characterized in that: The conditions for the atomization drying are: an inlet temperature of 200-400° C., and an air drying atmosphere.

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