A method for preparing core-shell structured microwave-transparent microspheres, its products and applications

Abstract

The application discloses a preparation method of a shell-core structure wave-transparent microsphere, which comprises the following steps: (1) coating a silicon source and a nitrogen source on a core body through a sol-gel method, drying, and then performing high-temperature conversion in a protective gas atmosphere to obtain an intermediate microsphere with a sacrifice layer; and (2) coating a carbon source on the intermediate microsphere, high-temperature carbonizing to form a carbon layer, and then high-temperature calcining to convert the carbon layer and part of the sacrifice layer; through reasonable design of a high-temperature process, high-quality and controllable conversion and sintering of each layer structure can be realized, and each layer structure is combined closely; the preparation method is beneficial to controlling the shell wall thickness and reducing the dielectric constant; meanwhile, the sacrifice layer is prepared on the surface of the core body, a protection effect on the core body is formed, the core body can also be well maintained in a high-temperature environment, and loss affecting performance is avoided; and the application further discloses a shell-core structure wave-transparent microsphere and application thereof.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials, specifically relating to a method for preparing core-shell structured microwave-transparent microspheres, as well as their products and applications. Background Technology

[0002] Thermally conductive ceramic particles, fibers, whiskers, and other materials typically have higher thermal conductivity and dielectric constant than polymers, with thermal conductivity values ​​sometimes exceeding those of polymers by 2 to 3 orders of magnitude. However, their dielectric loss is lower. Therefore, adding thermally conductive ceramic particles to polymers can significantly reduce the dielectric loss of composite materials, but it will increase the dielectric constant to some extent.

[0003] With the miniaturization and high-frequency development of electronic products, the requirements for signal transmission speed and reliability are also increasing. Although high thermal conductivity ceramic particles have high thermal conductivity, their intrinsic dielectric constant is high. Simply adding thermally conductive ceramic particles can no longer meet the development needs of packaging materials for today's electronic products. Therefore, improving the thermal conductivity and reducing the dielectric constant of electronic packaging composite materials has become a technological research and development trend in this field.

[0004] To achieve the above objectives, existing technologies generally employ two approaches:

[0005] I. Modification and coating of glass microspheres or gel-sol coating methods are used. Coating materials include alumina, AlN, BN, ferrite, titanium dioxide, etc. However, the finished products generally have low bonding strength between the glass microspheres and the coating materials. Moreover, the preparation temperature of this type of method is low, resulting in low density of the formed coating shell, poor material crystal state, large phonon scattering, and difficulty in effectively improving thermal conductivity.

[0006] Second, by modifying and coating ceramic materials, such as silicon carbide, AlN, BN, etc., and then sintering at low temperature to prepare a ceramic fused layer, the density is relatively high. However, due to the use of low temperature sintering, there are still many crystal interface defects in the entire coating shell, which cannot further reduce the phonon scattering probability.

[0007] To address the aforementioned issues, application number 202311203866.8 discloses a Si3N4@SiO2 core-shell structured foam ceramic material and its preparation method. The Si3N4@SiO2 core-shell structured foam ceramic material has a honeycomb structure in its microstructure, with the pore walls composed of Si3N4 and SiO2. The inner side of the pore wall is a Si3N4 layer, and the outer side is a SiO2 layer. The SiO2 between adjacent pore walls is sintered together.

[0008] Although the aforementioned patented technology has produced a high-temperature resistant, heat-insulating, and wave-transparent integrated material, this material is prepared by pressure to create a green embryo, thereby completing the preparation of the irregular structure. The main body of the structure is mainly bonded by SiO2, resulting in low compressive strength and easy breakage. At the same time, the excessively low thermal conductivity causes heat to easily accumulate inside the material, which cannot meet the heat dissipation requirements of electronic products. Summary of the Invention

[0009] In view of the problems in related technologies, this invention proposes a method for preparing core-shell structured microwave-transparent microspheres, as well as their products and applications, to overcome the aforementioned technical problems existing in the existing related technologies.

[0010] The technical solution of this invention is implemented as follows:

[0011] A method for preparing a core-shell structured microwave-transparent microsphere includes the following steps:

[0012] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under a protective gas atmosphere to obtain intermediate microspheres with sacrificial layer;

[0013] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0014] This invention enables high-quality, controllable transformation and sintering of each layer structure through a rationally designed high-temperature process, resulting in a tight bond between the layers. This preparation method is beneficial for controlling the shell wall thickness and reducing the dielectric constant. On the other hand, the introduction of a sacrificial layer on the surface of the core provides protection for the core, allowing it to maintain its shape and structure well even under high-temperature conditions, thus avoiding performance losses. At the same time, the sacrificial layer also serves as the silicon source for the transformation of the carbon layer into silicon carbide.

[0015] Preferably, the molecular structure of the silicon source contains an alkoxysilyl group.

[0016] More preferably, the silicon source is one or more of methyltrimethoxysilane, dimethyldimethoxysilane, triaminopropyltriethoxysilane, butadienyltriethoxysilane, and triaminopropyltriethoxysilane mixed in any proportion.

[0017] Preferably, step (1) includes the following steps:

[0018] (1-A) The silicon source and hydrolysant are mixed in a molar ratio of 0.1 to 0.3:10 and stirred uniformly at a temperature of 25 to 35°C for 0.8 to 1.2 h to obtain a partially hydrolyzed organic-silica binary sol system;

[0019] (1-B) Prepare the nitrogen source and the core. The amount of ammonia source added is prepared according to the molar ratio of ammonia source to hydrolysate of 0.8 to 1.2 to 10. The amount of core added is prepared according to 30 to 50 wt% of the organic-silica binary sol system.

[0020] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 25–35°C for 1.8–2.2 h.

[0021] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0022] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1150-1250℃ for 0.8-1.2h, and a disordered Si3N4 layer is formed on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The introduction of the sacrificial layer provides high-temperature protection for the core, allowing the preparation process to continue heating to 1350-1450℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres. The SiO2@β-Si3N4 microspheres are the intermediate microspheres.

[0023] Preferably, in step (1-A), the silicon source comprises two or more raw materials mixed in equal proportions, and the degree of hydrolysis is 30-50%.

[0024] Preferably, the nitrogen source is one or a mixture of two of ammonia and urea in any proportion.

[0025] More preferably, the ammonia or urea is prepared at high temperature in a nitrogen atmosphere.

[0026] The main components of the nucleus may be one or more of silicon oxide, aluminum oxide, silicates, aluminates, and aluminosilicate minerals.

[0027] Preferably, the melting temperature of the main component of the core is ≥1800°.

[0028] Preferably, the silicon dioxide is amorphous silicon dioxide microspheres with a purity ≥99.9% and a particle size of 10–100 μm.

[0029] The disordered arrangement of amorphous silica microspheres is conducive to the preparation of polycrystalline Si3N4 sacrificial layers, which in turn facilitates the growth of polycrystalline SiC layers in step (2), ultimately resulting in a shell wall with fewer interface defects. The resulting shell wall retains a dense layer and a protrusion structure, which can form an interface phase with the polymer matrix (such as resin) in the subsequent wave-transparent thermal conductive coating, further improving the thermal conductivity.

[0030] Preferably, the silica microspheres are solid or hollow.

[0031] More preferably, the wall thickness of the hollow silica microspheres is ≥1 / 5 of the sphere radius.

[0032] Preferably, the alumina is amorphous alumina microspheres with a particle size of 10–100 μm.

[0033] Preferably, the aluminosilicate mineral is mullite microspheres with a particle size of 10–100 μm.

[0034] Preferably, step (2) includes the following steps:

[0035] (2-A) The intermediate microspheres obtained in step (1) are coated with a carbon source, and the intermediate microspheres are added to the aqueous solution of the carbon source and mixed evenly.

[0036] The mass ratio of carbon source to intermediate microspheres is 1–20:1, and the mass concentration of the aqueous solution of carbon source is 1–20 wt%.

[0037] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at a temperature of 120-160°C for 5-10 hours. After the hydrothermal reaction, the mixture was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0038] (2-C) Under a nitrogen protective atmosphere, the temperature is gradually increased to 900-1100℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor is carbonized for 1.5-2.5h to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0039] (2-D) Maintaining a nitrogen protective atmosphere, the temperature is further increased to 1650–1750 °C to calcine the SiO2@β-Si3N4@C microspheres. This process transforms the carbon layer and part of the sacrificial layer. The high temperature facilitates the decomposition of β-Si3N4, supplying liquid silicon and nitrogen. The liquid silicon reacts with the carbon source to form silicon carbide, which in turn forms a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining a core-shell structured transparent microsphere.

[0040] Preferably, in step (2-A), the carbon source is one or more of glucose, fructose and sucrose mixed in any proportion.

[0041] Preferably, in step (2-D), the high-temperature calcination time is 8 to 12 minutes, thereby controlling the degree of loss of the sacrificial layer by controlling the calcination time.

[0042] The present invention also discloses a core-shell structured microwave-transparent microsphere, comprising a core.

[0043] The main components of the core are one or more of silicon dioxide, aluminum oxide, silicates, aluminates and aluminosilicate minerals;

[0044] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0045] First, the crystal-like SiC-SiN intermediate phase layer and silicon carbide layer specifically designed in this invention form a heat conduction channel, and the structure of each layer is tightly bonded. Compared with the prior art, this invention effectively improves the thermal conductivity of the raw materials and meets the heat dissipation requirements of the wave-transparent device. At the same time, the SiC-SiN intermediate phase layer and silicon carbide layer also serve as a high-temperature protective layer for the core.

[0046] Secondly, the microwave-transparent microspheres prepared by this invention have excellent pressure resistance and wear resistance. Even when filled with high content in polymer materials, they will not cause problems such as extrusion, cracking, or collapse. Applying them to the preparation of microwave-transparent devices can effectively improve the mechanical properties of microwave-transparent devices.

[0047] Finally, the transparent microspheres of the present invention have a low dielectric constant and good wave transmission properties, which meet the signal transmission requirements of transparent devices such as electronic products and antenna equipment.

[0048] Preferably, the melting temperature of the main component of the core is ≥1800°.

[0049] Preferably, the silicon dioxide is amorphous silicon dioxide microspheres with a purity ≥99.9% and a particle size of 10–100 μm. Since the silicon dioxide microspheres are randomly arranged amorphous, a polycrystalline sacrificial layer is prepared using SiN, followed by the growth of a polycrystalline silicon carbide layer, resulting in a shell with fewer interface defects. This shell retains a dense layer and a protrusion structure, which forms an interface phase with the polymer matrix, further improving thermal conductivity.

[0050] Preferably, the silica microspheres are solid or hollow.

[0051] More preferably, the wall thickness of the hollow silica microspheres is ≥1 / 5 of the sphere radius.

[0052] Preferably, the alumina is amorphous alumina microspheres with a particle size of 10–100 μm;

[0053] Preferably, the aluminosilicate mineral is mullite microspheres with a particle size of 10–100 μm.

[0054] Preferably, the silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. Preferably, the thickness of the silicon carbide layer is 50-300 nm, and the thickness of the SiC-SiN intermediate phase layer is 100-300 nm.

[0055] The present invention also discloses the application of the above-mentioned core-shell structured microwave-transparent microspheres in microwave-transparent thermally conductive substrates.

[0056] Specifically, the microwave-transparent thermal conductive substrate includes a microwave-transparent thermal conductive coating, which contains the following raw materials in parts by weight: 45-55 parts of polymer matrix, 5-70 parts of microwave-transparent microspheres, 0.1-0.5 parts of dispersant and 0.1-0.5 parts of KH550 silane coupling agent;

[0057] The polymer matrix is ​​one of epoxy resin, phenolic resin, polyurethane, and polyacrylic acid resin.

[0058] The wave-transparent thermally conductive coating is coated on an insulating substrate with a thickness of 0.5 to 3 mm to obtain a wave-transparent thermally conductive substrate, or the wave-transparent thermally conductive coating is directly injection molded and heated to obtain a wave-transparent thermally conductive substrate. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of the fabrication process of the microwave-transparent microspheres of the present invention;

[0060] Figure 2 One of the electron microscope images of the microwave-transparent microspheres of the present invention;

[0061] Figure 3 The second electron microscope image of the wave-transparent microspheres of the present invention;

[0062] Figure 4 Here is an electron microscope image of Example 4;

[0063] Figure 5 This is an electron microscope image of Comparative Example 3.

[0064] Figure Labels

[0065] 1. Nucleus;

[0066] 2. Sacrificial layer;

[0067] 3. SiO2@β-Si3N4@carbon source microsphere precursor;

[0068] 4. SiO2@β-Si3N4@C microspheres;

[0069] 5. SiC-SiN intermediate phase layer;

[0070] 6. Silicon carbide layer. Detailed Implementation

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

[0072] Example 1

[0073] Reference Figure 1 A method for preparing core-shell structured microwave-transparent microspheres includes the following steps:

[0074] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under protective gas atmosphere to obtain intermediate microspheres with sacrificial layer.

[0075] (1-A) The silicon source is triaminopropyltriethoxysilane and butadienyltriethoxysilane.

[0076] The triaminopropyltriethoxysilane, butadienyltriethoxysilane, and hydrolyzing agent (ethanol) were mixed in a molar ratio of 0.05:0.1:10 and stirred uniformly at 25°C for 0.8 h to obtain a partially hydrolyzed organo-silica binary sol system with a degree of hydrolysis of 30%.

[0077] (1-B) Preparation of nitrogen source and core: The nitrogen source is ammonia water prepared at high temperature in a nitrogen atmosphere. The core is amorphous silica microspheres with a purity ≥99.9%, a melting temperature ≥1800°, a hollow structure, a sphere radius of 30μm, and a sphere wall thickness of 6μm.

[0078] The amount of ammonia source added is prepared with a molar ratio of ammonia source to hydrolysate of 0.8:10, and the amount of core body added is prepared as 30 wt% of the organic-silica binary sol system.

[0079] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 25°C for 1.8 hours.

[0080] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0081] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1150℃ for 0.8h, and a disordered Si3N4 layer is formed on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The temperature is further increased to 1350℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres.

[0082] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0083] (2-A) Coat the intermediate microspheres obtained in step (1) with a carbon source (glucose) and add the intermediate microspheres to the aqueous solution of the carbon source and mix evenly.

[0084] The mass ratio of carbon source to intermediate microspheres is 5:1, and the mass concentration of the aqueous solution of carbon source is 5 wt%.

[0085] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at 120°C for 5 hours. After the hydrothermal reaction, it was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0086] (2-C) Under a nitrogen protective atmosphere, the temperature is gradually increased to 900℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor is carbonized for 1.5h to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0087] (2-D) Maintaining a nitrogen protective atmosphere, continue heating to 1650℃ and calcine the SiO2@β-Si3N4@C microspheres for 8 minutes. This process transforms the carbon layer and part of the sacrificial layer, yielding a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining core-shell structured transparent microspheres.

[0088] The core-shell structured microwave-transparent microspheres prepared in this embodiment include a core;

[0089] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0090] The silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. The thickness of the silicon carbide layer is 50 nm, and the thickness of the SiC-SiN intermediate phase layer is 100 nm.

[0091] Prepare the following weights of raw materials:

[0092] 45 kg of polymer matrix (epoxy resin), 6.75 kg of the above-prepared microwave-transparent microspheres, 0.1 kg of DH5038 dispersant and 0.1 kg of KH550 silane coupling agent were mixed and stirred to obtain a microwave-transparent thermally conductive coating.

[0093] The wave-transparent thermally conductive coating is coated onto an insulating substrate with a thickness of 0.5 mm to obtain a wave-transparent thermally conductive substrate.

[0094] Example 2

[0095] Reference Figure 1 A method for preparing core-shell structured microwave-transparent microspheres includes the following steps:

[0096] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under protective gas atmosphere to obtain intermediate microspheres with sacrificial layer.

[0097] (1-A) The silicon source is triaminopropyltriethoxysilane.

[0098] The triaminopropyltriethoxysilane and the hydrolysing agent (ethanol) were mixed at a molar ratio of 0.2:10 and stirred uniformly at 30°C for 0.9 h to obtain a partially hydrolyzed organo-silica binary sol system with a degree of hydrolysis of 35%.

[0099] (1-B) Prepare a nitrogen source and a core body. The nitrogen source is urea prepared at high temperature in a nitrogen atmosphere. The core body is amorphous silica microspheres with a purity ≥99.9%, a melting temperature ≥1800°, a hollow structure, a sphere radius of 45μm, and a sphere wall thickness of 10μm.

[0100] The amount of ammonia source added is prepared with a molar ratio of ammonia source to hydrolysate of 0.9:10, and the amount of core body added is prepared as 35 wt% of the organic-silica binary sol system.

[0101] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 28°C for 1.9 hours.

[0102] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0103] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1180℃ for 0.9h to form a disordered Si3N4 layer on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The temperature is then raised to 1380℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres.

[0104] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0105] (2-A) Coat the intermediate microspheres obtained in step (1) with a carbon source (fructose) and add the intermediate microspheres to the aqueous solution of the carbon source and mix evenly.

[0106] The mass ratio of carbon source to intermediate microspheres is 8:1, and the mass concentration of the aqueous solution of carbon source is 8 wt%.

[0107] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at 130°C for 6.5 h. After the hydrothermal reaction, it was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0108] (2-C) Under a nitrogen protective atmosphere, the temperature was gradually increased to 950℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor was carbonized for 1.8h to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0109] (2-D) Maintaining a nitrogen protective atmosphere, continue heating to 1680℃ and calcine the SiO2@β-Si3N4@C microspheres for 9 minutes. This process transforms the carbon layer and part of the sacrificial layer, yielding a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining core-shell structured transparent microspheres.

[0110] The core-shell structured microwave-transparent microspheres prepared in this embodiment include a core;

[0111] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0112] The silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. The thickness of the silicon carbide layer is 100 nm, and the thickness of the SiC-SiN intermediate phase layer is 150 nm.

[0113] Prepare the following weights of raw materials:

[0114] A microwave-transparent thermal conductive coating was prepared by mixing and stirring 48 kg of polymer matrix (phenolic resin), 16.8 kg of microwave-transparent microspheres, 0.2 kg of DH5038 dispersant and 0.2 kg of KH550 silane coupling agent.

[0115] The wave-transparent thermally conductive coating is coated onto an insulating substrate at a thickness of 1 mm to obtain a wave-transparent thermally conductive substrate.

[0116] Example 3

[0117] Reference Figure 1 A method for preparing core-shell structured microwave-transparent microspheres includes the following steps:

[0118] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under protective gas atmosphere to obtain intermediate microspheres with sacrificial layer.

[0119] (1-A) The silicon source is methyltrimethoxysilane and dimethyldimethoxysilane.

[0120] The methyltrimethoxysilane, dimethyldimethoxysilane and hydrolyzing agent (ethanol) are mixed in a molar ratio of 0.1:0.1:10 and stirred uniformly at 30°C for 1 hour to obtain a partially hydrolyzed organo-silica binary sol system with a degree of hydrolysis of 40%.

[0121] The nitrogen source is one or a mixture of two of ammonia and urea in any proportion, and the ammonia or urea is prepared at high temperature in a nitrogen atmosphere.

[0122] (1-B) Prepare a nitrogen source and a core body. The nitrogen source is ammonia water prepared at high temperature in a nitrogen atmosphere. The core body is amorphous silica microspheres with a purity ≥99.9%, a melting temperature ≥1800°, a solid structure, and a particle size of 20μm.

[0123] The amount of ammonia source added is prepared with a molar ratio of ammonia source to hydrolysate of 1:10, and the amount of core body added is prepared as 40 wt% of the organic-silica binary sol system.

[0124] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 30°C for 2 hours.

[0125] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0126] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1200℃ for 1 hour to form a disordered Si3N4 layer on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The temperature is then raised to 1400℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres.

[0127] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0128] (2-A) Coat the intermediate microspheres obtained in step (1) with carbon source (sucrose) and add the intermediate microspheres to the aqueous solution of carbon source and mix evenly.

[0129] The mass ratio of carbon source to intermediate microspheres is 10:1, and the mass concentration of the aqueous solution of carbon source is 10 wt%.

[0130] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at 140°C for 7.5 h. After the hydrothermal reaction, it was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0131] (2-C) Under a nitrogen protective atmosphere, the temperature is gradually increased to 1000℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor is carbonized for 2 hours to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0132] (2-D) Maintaining a nitrogen protective atmosphere, continue heating to 1700℃ and calcine the SiO2@β-Si3N4@C microspheres for 10 min. This process transforms the carbon layer and part of the sacrificial layer, yielding a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining core-shell structured transparent microspheres.

[0133] Reference Figure 2 and Figure 3 The core-shell structured transparent microspheres prepared in this embodiment include a core.

[0134] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0135] The silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. The thickness of the silicon carbide layer is 150 nm, and the thickness of the SiC-SiN intermediate phase layer is 200 nm.

[0136] Prepare the following weights of raw materials:

[0137] 50 kg of polymer matrix (polyurethane), 25 kg of microwave-transparent microspheres, 0.3 kg of DH5038 dispersant and 0.3 kg of KH550 silane coupling agent were mixed and stirred to obtain a microwave-transparent thermally conductive coating.

[0138] The wave-transparent thermal conductive coating is coated onto an insulating substrate with a thickness of 1.5 mm to obtain a wave-transparent thermal conductive substrate.

[0139] Example 4

[0140] Reference Figure 1A method for preparing core-shell structured microwave-transparent microspheres includes the following steps:

[0141] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under protective gas atmosphere to obtain intermediate microspheres with sacrificial layer.

[0142] (1-A) The silicon source is methyltrimethoxysilane and dimethyldimethoxysilane.

[0143] The methyltrimethoxysilane, dimethyldimethoxysilane, and hydrolyzing agent (ethanol) were mixed in a molar ratio of 0.1:0.1:10 and stirred uniformly at 30°C for 1 hour to obtain a partially hydrolyzed organo-silica binary sol system with a degree of hydrolysis of 45%.

[0144] (1-B) Prepare a nitrogen source and a core body. The nitrogen source is urea prepared at high temperature in a nitrogen atmosphere. The core body is amorphous silica microspheres with a purity ≥99.9%, a melting temperature ≥1800°, a solid structure, and a particle size of 50μm.

[0145] The amount of ammonia source added is prepared with a molar ratio of ammonia source to hydrolysate of 1:10, and the amount of core body added is prepared as 45 wt% of the organic-silica binary sol system.

[0146] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 30°C for 2 hours.

[0147] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0148] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1200℃ for 1 hour to form a disordered Si3N4 layer on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The temperature is then raised to 1400℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres.

[0149] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0150] (2-A) Coat the intermediate microspheres obtained in step (1) with a carbon source (glucose) and add the intermediate microspheres to the aqueous solution of the carbon source and mix evenly.

[0151] The mass ratio of carbon source to intermediate microspheres is 15:1, and the mass concentration of the aqueous solution of carbon source is 15 wt%.

[0152] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at 150°C for 8.5 h. After the hydrothermal reaction, it was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0153] (2-C) Under a nitrogen protective atmosphere, the temperature is gradually increased to 1000℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor is carbonized for 2 hours to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0154] (2-D) Maintaining a nitrogen protective atmosphere, continue heating to 1700℃ and calcine the SiO2@β-Si3N4@C microspheres for 10 min. This process transforms the carbon layer and part of the sacrificial layer, yielding a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining core-shell structured transparent microspheres.

[0155] Reference Figure 2 and Figure 3 The core-shell structured transparent microspheres prepared in this embodiment include a core.

[0156] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0157] The silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. The thickness of the silicon carbide layer is 250 nm, and the thickness of the SiC-SiN intermediate phase layer is 250 nm.

[0158] Prepare the following weights of raw materials:

[0159] 52 kg of polymer matrix (polyacrylic acid resin), 31.2 kg of microwave-transparent microspheres, 0.4 kg of DH5038 dispersant and 0.4 kg of KH550 silane coupling agent were mixed and stirred to obtain a microwave-transparent thermally conductive coating.

[0160] The wave-transparent thermal conductive coating is coated onto an insulating substrate with a thickness of 2 mm to obtain a wave-transparent thermal conductive substrate.

[0161] Example 5

[0162] Reference Figure 1 A method for preparing core-shell structured microwave-transparent microspheres includes the following steps:

[0163] (1) Silicon source and nitrogen source are coated onto the core by sol-gel method, dried, and then converted at high temperature under protective gas atmosphere to obtain intermediate microspheres with sacrificial layer.

[0164] (1-A) The silicon source is methyltrimethoxysilane.

[0165] The methyltrimethoxysilane and hydrolyzing agent (ethanol) were mixed at a molar ratio of 0.3:10 and stirred uniformly at 35°C for 1.2 h to obtain a partially hydrolyzed organo-silica binary sol system with a degree of hydrolysis of 50%.

[0166] (1-B) Preparation of nitrogen source and core: The nitrogen source is ammonia water prepared at high temperature in a nitrogen atmosphere, and the core is amorphous silica microspheres with a purity ≥99.9%, a melting temperature ≥1800°, a solid structure, and a particle size of 70μm.

[0167] The amount of ammonia source added is prepared with a molar ratio of ammonia source to hydrolysate of 1.2:10, and the amount of core body added is prepared as 50 wt% of the organic-silica binary sol system.

[0168] The ammonia source and the nucleus were added to the organic-silica binary sol system, and the mixture was stirred at 35°C for 2.2 hours.

[0169] (1-C) is poured into a mold to react and form a gel, and then dried to obtain microspheres with siloxane dry gel coating the core.

[0170] (1-D) Under a nitrogen protective atmosphere, the temperature is maintained at 1250℃ for 1.2h to form a disordered Si3N4 layer on the surface of the microspheres. The Si3N4 layer serves as the sacrificial layer. The temperature is further increased to 1450℃ to sinter the Si3N4 into high-quality β-Si3N4, thus obtaining SiO2@β-Si3N4 microspheres.

[0171] (2) The intermediate microspheres are coated with carbon source, and carbonized at high temperature in a protective gas atmosphere to form a carbon layer. Then, the carbon layer and part of the sacrificial layer are converted by high temperature calcination to obtain a microwave-transparent microsphere with a core-shell structure.

[0172] (2-A) Coat the intermediate microspheres obtained in step (1) with a carbon source (fructose) and add the intermediate microspheres to the aqueous solution of the carbon source and mix evenly.

[0173] The mass ratio of carbon source to intermediate microspheres is 12:1, and the mass concentration of the aqueous solution of carbon source is 12 wt%.

[0174] (2-B) The mixture was transferred to a hydrothermal reactor for hydrothermal reaction at 160°C for 10 hours. After the hydrothermal reaction, it was washed and centrifuged, and then vacuum dried to obtain SiO2@β-Si3N4@carbon source microsphere precursor.

[0175] (2-C) Under a nitrogen protective atmosphere, the temperature was gradually increased to 1100℃, and the SiO2@β-Si3N4@ carbon source microsphere precursor was carbonized for 2.5h to form a carbon layer, thus obtaining SiO2@β-Si3N4@C microspheres;

[0176] (2-D) Maintaining a nitrogen protective atmosphere, continue heating to 1750℃ and calcine the SiO2@β-Si3N4@C microspheres for 12 minutes. This process transforms the carbon layer and part of the sacrificial layer, yielding a β-SiC shell with low phonon scattering, thus producing SiO2@β-SiC. x N y Mesophase@β-SiC microspheres are prepared by transforming the β-SiCxNy layer into a SiC-SiN layer at high temperature, thus obtaining core-shell structured transparent microspheres.

[0177] Reference Figure 2 and Figure 3 The core-shell structured transparent microspheres prepared in this embodiment include a core.

[0178] The surface of the core is sequentially bonded with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

[0179] The silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as shell walls. The thickness of the silicon carbide layer is 200 nm, and the thickness of the SiC-SiN intermediate phase layer is 300 nm.

[0180] Prepare the following weights of raw materials:

[0181] 55 kg of polymer matrix (polyacrylic acid resin), 35.75 kg of microwave-transparent microspheres, 0.5 kg of DH5038 dispersant and 0.5 kg of KH550 silane coupling agent were mixed and stirred to obtain a microwave-transparent thermally conductive coating.

[0182] The wave-transparent thermally conductive coating is coated onto an insulating substrate with a thickness of 3 mm to obtain a wave-transparent thermally conductive substrate.

[0183] Comparative Example 1

[0184] The commercially available hollow silica microspheres are specifically the S38HS product manufactured by 3M Company of the United States.

[0185] Prepare the following weights of raw materials:

[0186] The coating was prepared by mixing and stirring 48 kg of polymer matrix (phenolic resin), 16.8 kg of the above microspheres, 0.2 kg of DH5038 dispersant and 0.2 kg of KH550 silane coupling agent.

[0187] The coating is applied to an insulating substrate in a 1 mm thickness to obtain a substrate.

[0188] Comparative Example 2

[0189] Low-temperature silicon carbide (SIC) solid-phase coated SiO2 microspheres.

[0190] Prepare the following weights of raw materials:

[0191] The following mixtures were prepared: 52 kg of polymer matrix (polyurethane), 26 kg of the above microspheres, 0.3 kg of DH5038 dispersant and 0.3 kg of KH550 silane coupling agent; the mixture was stirred to obtain the coating.

[0192] The coating is applied to an insulating substrate with a thickness of 1.5 mm to obtain a substrate.

[0193] Comparative Example 3

[0194] Commercially available hollow mullite microspheres.

[0195] Prepare the following weights of raw materials:

[0196] The following mixtures were prepared: 52 kg of polymer matrix (polyacrylic resin), 31.2 kg of the above microspheres, 0.4 kg of DH5038 dispersant and 0.4 kg of KH550 silane coupling agent; the mixtures were stirred to obtain the coating.

[0197] The coating is applied to an insulating substrate with a thickness of 2 mm to obtain a substrate.

[0198] Comparative Example 4

[0199] Commercially available solid silica microspheres.

[0200] Prepare the following weights of raw materials:

[0201] The following mixtures were prepared: 55 kg of polymer matrix (polyacrylic acid resin), 35.75 kg of the above microspheres, 0.4 kg of DH5038 dispersant and 0.4 kg of KH550 silane coupling agent; the mixtures were stirred to obtain the coating.

[0202] The coating is applied to an insulating substrate with a thickness of 2 mm to obtain a substrate.

[0203] Performance testing

[0204] Thermal conductivity is determined using the transient plane heat source method, following the standard ISO 22007-2, and is used to measure the thermal conductivity and thermal diffusivity of plastic products.

[0205] The dielectric constant is determined using a resonant cavity test method. This method uses a network analyzer to measure the resonant frequency and the Q value of the resonant cavity fixture. The test begins with a blank setup, followed by loading the sample under test. When the sample volume and other parameters of the resonant cavity are known, the dielectric constant is calculated using these measurements.

[0206] Mechanical properties were tested using a universal testing machine.

[0207] (1) The microwave-transparent microspheres of the present invention were compared with other commercially available materials. The D50 of all of them was 20μm. The results are shown in the table below.

[0208]

[0209] (2) The wave-transparent thermal conductive substrates prepared in Examples 1 to 5 and the substrates prepared in Comparative Examples 1 to 4 were tested, and the results are shown in the table below.

[0210]

[0211] (3) Electron microscopy analysis

[0212] Figure 4 The electron microscope image of Example 4 shows that the morphology of the microwave-transparent microspheres remains intact, without any problems such as compression, breakage, or collapse. Figure 5 The image shown is an electron microscope image of Comparative Example 3, which shows that the hollow mullite microspheres undergo compression and collapse, resulting in an increase in dielectric constant.

[0213] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the present invention should also fall within the protection scope of the claims of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A method for preparing a shell-core structure wave-transparent microsphere, characterized in that, The method comprises the following steps: (1) coating a silicon source and a nitrogen source on a core body by a sol-gel method, drying, and then performing high-temperature conversion in a protective gas atmosphere to obtain an intermediate microsphere with a sacrifice layer; the main components of the core body are one or more than one of silica, alumina, silicate, aluminate and aluminosilicate minerals; (2) coating a carbon source on the intermediate microsphere, carbonizing the intermediate microsphere in a protective gas atmosphere at high temperature to form a carbon layer, and then calcining the carbon layer and part of the sacrifice layer at high temperature to obtain a wave-transparent microsphere with a shell-core structure.

2. The production method according to claim 1, characterized by, the silica is amorphous silica microspheres; in step (1), the following steps are included: (1-A) mixing the silicon source and a hydrolysis agent according to a molar ratio of 0.1-0.3:10, uniformly stirring at a temperature of 25-35°C for 0.8-1.2h to obtain a partially hydrolyzed organic-silica binary sol system; (1-B) preparing a nitrogen source and a core body, the amount of the ammonia source is prepared according to a molar ratio of ammonia source:hydrolysis agent of 0.8-1.2:10, and the amount of the core body is prepared according to 30-50wt% of the organic-silica binary sol system, adding the ammonia source and the core body into the organic-silica binary sol system and continuing to stir at 25-35°C for 1.8-2.2h; (1-C) pouring into a mold to react into a gel, and then performing drying treatment to obtain a microsphere of a siloxane xerogel coated core body; (1-D) forming a disordered Si3N4 layer on the surface of the microsphere in a nitrogen protective atmosphere at 1150-1250°C for 0.8-1.2h, the Si3N4 layer serving as the sacrifice layer, and then continuing to heat to 1350-1450°C to sinter the Si3N4 into β-Si3N4 to obtain SiO2@β-Si3N4 microspheres, which are the intermediate microspheres.

3. The preparation method according to claim 2, characterized in that, in step (1-A), the silicon source contains alkoxysilane groups in the molecular structure, and the silicon source comprises two or more than two raw materials mixed at equal proportions; the nitrogen source is one or a mixture of two of ammonia water and urea at any ratio; the degree of hydrolysis of the partially hydrolyzed hydrolysis is 30-50%.

4. The production method according to claim 2 or 3, characterized by, the silica microspheres are of solid structure or hollow structure; the wall thickness of the silica microspheres of hollow structure is ≥1 / 5 of the radius of the sphere.

5. The production process according to claim 1 or 2, characterized in that, in step (2), the following steps are included: (2-A) coating a carbon source on the intermediate microspheres obtained in step (1), adding the intermediate microspheres into a carbon source aqueous solution and mixing uniformly, the mass ratio of the carbon source to the intermediate microspheres is 1-20:1, and the mass concentration of the carbon source aqueous solution is 1-20wt%; (2-B) transferring into an autoclave for hydrothermal reaction at a temperature of 120-160°C for 5-10h, and then performing cleaning, centrifugation and vacuum drying to obtain SiO2@β-Si3N4@carbon source microsphere precursors; (2-C) gradually heating to 900-1100°C in a nitrogen protective atmosphere, carbonizing the SiO2@β-Si3N4@carbon source microsphere precursors for 1.5-2.5h to form a carbon layer, and obtaining SiO2@β-Si3N4@C microspheres. (2-D) keeping nitrogen protection atmosphere continues to warm up to 1650~1750℃, high temperature calcination of SiO2@β-Si3N4@C microspheres, carbon layer and part of the sacrificial layer conversion, form a small phonon scattering β-SiC shell layer, prepared SiO2@β-SiC x N y Interphase@β-SiC microspheres, β-SiCxNy layer is converted into SiC-SiN layer at high temperature, that is, a kind of shell-core structure of wave-transparent microspheres is prepared.

6. The production method according to claim 5, wherein In step (2-A), the carbon source is one or more than one of glucose, fructose and sucrose in any proportion; In step (2-D), the high-temperature calcination time is 8-12 min.

7. A shell-core structure wave-transparent microsphere, characterized in that, The wave-transparent microspheres are prepared by the preparation method of any one of claims 1-6, and comprise a core, The main component of the core is one or more than one of silicon oxide, aluminum oxide, silicate, aluminate and aluminosilicate mineral; The surface of the core is sequentially combined with a SiC-SiN intermediate phase layer and a silicon carbide layer, and the SiC-SiN intermediate phase layer completely covers the core.

8. The wave-transparent microspheres according to claim 7, wherein the silicon carbide layer completely covers the SiC-SiN intermediate phase layer, and the SiC-SiN intermediate phase layer and the silicon carbide layer serve as a shell wall. The thickness of the silicon carbide layer is 50-300 nm, and the thickness of the SiC-SiN intermediate phase layer is 100-300 nm.

9. Use of the wave-transparent microspheres according to any one of claims 7-8 in a wave-transparent substrate.

10. Use according to claim 9, characterized in that, The wave-transparent heat-conductive substrate comprises a wave-transparent heat-conductive coating; The wave-transparent heat-conductive coating comprises the following raw materials by weight: 45-55 parts of a polymer matrix, 5-70 parts of wave-transparent microspheres, 0.1-0.5 parts of a dispersing agent and 0.1-0.5 parts of KH550 silane coupling agent. The polymer matrix is one of epoxy resin, phenolic resin, polyurethane and polyacrylic resin. The wave-transparent heat-conductive coating is coated on an insulating substrate at a thickness of 0.5-3 mm to obtain a wave-transparent heat-conductive substrate, or the wave-transparent heat-conductive coating is directly injection molded and heated to form a wave-transparent heat-conductive substrate.

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