Boron nitride thermal insulation material and preparation method thereof

Abstract

The invention provides a boron nitride thermal insulation material and a preparation method thereof.The preparation method comprises the steps that silane monomers and metal oxide particles are added into boron nitride powder, then blending is conducted at the room temperature, a first precursor is obtained, and the surface of the first precursor is coated with a metal silicon oxygen hybrid layer; the boron nitride powder is mixed with a carbon source and a silicon source, then the mixture is placed in a mixed atmosphere of inert gas and acetylene gas, a carbon thermal reduction reaction is carried out, a second precursor is obtained, and the surface of the second precursor is coated with a silicon carbide layer; flaky boron nitride, the first precursor and the second precursor are added into a mixed solvent, after uniform mixing, directional freezing is performed at the temperature of-80 DEG C to-60 DEG C, then the temperature is increased to 250-400 DEG C, a reaction is performed in an inert atmosphere for 3-4 hours, the boron nitride thermal insulation material is obtained, and the boron nitride thermal insulation material is obtained, specifically, flaky boron nitride, the first precursor and the second precursor are mixed uniformly and then are subjected to directional freezing at the temperature of-80 DEG C to-60 DEG C; therefore, the heat insulation effect is effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat insulation, and particularly relates to a boron nitride heat insulation material and a preparation method thereof. Background Art

[0002] During the operation of a water heater, a large amount of heat needs to be transferred to the water body. If the insulation of the equipment shell or pipeline is poor, heat will be dissipated to the surrounding environment, resulting in increased consumption of gas, electricity or steam. An excellent heat insulation layer can significantly reduce heat loss, shorten the preheating time, reduce the operating load of the circulation pump, and thus significantly improve the overall energy efficiency of the system. Good heat insulation can also reduce the temperature of the outer surface of the equipment, prevent scalding, delay the aging of the shell material, protect the surrounding flammable and explosive environment, and improve the safety of use. During intermittent or long-cycle operation, the heat insulation layer can help maintain the water temperature or the temperature of the heat medium, reduce the energy consumption fluctuation caused by frequent start-up heating, and extend the service life of the equipment.

[0003] In related technologies, boron nitride materials are used as heat insulation materials. Boron nitride can withstand high temperatures above 800 °C in air and even up to 2000 °C in an inert atmosphere, making it very suitable for long-term use in high-temperature heat exchange environments. Although boron nitride has a low thermal conductivity in the thickness direction, its thermal conductivity in the plane direction can be as high as 200 - 400 W / m·K. When using non-oriented powders or blocks, heat bridges often form in the plane, reducing the heat insulation effect. Traditional boron nitride powder-filled or sintered materials mostly have a random porous structure, making it difficult to form a boron nitride heat conduction path, resulting in poor heat insulation effect. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a boron nitride heat insulation material and a preparation method thereof, aiming to solve the problem of poor heat insulation effect.

[0005] To solve the above technical problem, the present invention is implemented as follows. The present invention provides a preparation method of a boron nitride heat insulation material, and the steps include: S1. Add a silane monomer and metal oxide particles to boron nitride powder, and then blend them at room temperature to obtain a first precursor, and a metal silicon-oxygen hybrid layer is coated on the surface of the first precursor; S2. Mix boron nitride powder with a carbon source and a silicon source, and then place it in a mixed atmosphere of an inert gas and acetylene gas to carry out a carbothermal reduction reaction to obtain a second precursor, and a silicon carbide layer is coated on the surface of the second precursor; S3. Add flaky boron nitride, the first precursor, and the second precursor to a mixed solvent, mix them evenly, carry out directional freezing at a temperature of -80 to -60 °C, and then raise the temperature to 250 to 400 °C and react in an inert atmosphere for 3 to 4 hours to obtain a boron nitride heat insulation material, wherein the mixed solvent includes ethanol and tetrahydrofuran.

[0006] In some embodiments of the present invention, in step S1, the silane monomer includes at least one of methyltriethoxysilane, allyltriethoxysilane, and (3-mercaptopropyl)trimethoxysilane, the metal oxide particles include at least one of germanium oxide, titanium oxide, and aluminum oxide, the particle size of the boron nitride powder is 80-120 nm, and the chemical general formula of the boron nitride powder is (boron nitride) n , where n = 2-10.

[0007] In some embodiments of the present invention, step S1 includes: S1.1. Add the boron nitride powder and metal oxide particles to the mixed solution, perform ultrasonic dispersion for 30 minutes, and then perform magnetic stirring at a stirring speed of 200-300 rpm. The mixed solution includes anhydrous ethanol and deionized water; S1.2. Maintain the rotation speed, then dropwise add the silane monomer and stir for 1-2 hours, then add tris(hydroxymethyl)aminomethane and stir for 2-3 hours, keeping the pH = 6.5-7.2 to obtain the first precursor solution; S1.3. Increase the rotation speed to 300-400 rpm, continue to stir for 1-2 hours, intermittently stop during the stirring process, add an ethanol-aqueous solution to adjust the viscosity of the first precursor solution to reach a preset viscosity, and perform vacuum cold air drying for 12-18 hours after stirring is completed to obtain the first precursor.

[0008] In some embodiments of the present invention, in step S2, the carbon source includes at least one of glucose, dopamine, sucrose, and polyvinyl alcohol, and the silicon source includes at least one of tetraethyl orthosilicate, methyltriethoxysilane, and hydroxy-terminated polydimethylsiloxane.

[0009] In some embodiments of the present invention, step S2 includes: S2.1. Add the boron nitride powder, carbon source, and silicon source to the mixed solution in sequence, adjust the temperature to less than 30 °C, perform ultrasonic dispersion for 30 minutes, then add tris(hydroxymethyl)aminomethane and stir, add dilute acetic acid to adjust the pH value to 4.5-5.5 to obtain a composite dispersion. The mixed solution includes anhydrous ethanol and deionized water; S2.2. Pour the composite dispersion into a flat-bottomed ceramic evaporating dish, place the flat-bottomed ceramic evaporating dish in a blast drying oven at 70 °C, let it stand for 12 hours, scrape the surface thin layer of dry powder after drying is completed, and grind it with a mortar to obtain the composite precursor powder; S2.3. Uniformly spread the composite precursor powder on a quartz boat, then place it in a tube furnace, fill it with high-purity inert gas for 30 minutes of pre-cleaning, then switch to a mixed atmosphere of inert gas and acetylene and continue to ventilate, set the heating rate to 5 °C / min, heat to 1200 °C and keep it warm for 2 hours, and obtain the second precursor after cooling.

[0010] In some embodiments of the present invention, step S3 includes: S3.1. Add the flaky boron nitride, the first precursor, and the second precursor into a mixed solvent, stir for 30 minutes, and adjust the pH to 6.5 - 7.0 to obtain a multiphase composite dispersion; S3.2. Pour the multiphase composite dispersion into a rectangular silicone mold, place the rectangular silicone mold on a freezing device equipped with a freezing platform. The temperature of the cold source below the freezing device is -80~-60°C, the temperature of the heat source above the freezing device is 30~50°C. The freezing direction of the freezing device is perpendicular to the ground where it is placed, the temperature gradient in the freezing direction is greater than 80°C / cm, and the freezing time is 12~24 hours; S3.3. After freezing is completed, transfer the rectangular silicone mold to a vacuum freeze dryer. The drying temperature is -50°C, the vacuum degree is ≤20 Pa, and the drying time is 24 hours. After freeze drying is completed, place it in a tubular furnace, introduce high-purity inert gas to purge for 30 minutes, set the heating rate to 3~5°C / min, raise the temperature to 200~250°C and hold for 1 hour, then raise the temperature to 300~400°C and hold for 3 hours. After the holding is completed, naturally cool to room temperature to obtain the boron nitride thermal insulation material.

[0011] In some embodiments of the present invention, calculated by mass ratio, in the first precursor, the boron nitride powder: the silane monomer: the metal oxide particles = 20:4~6:1~4; in the second precursor, the boron nitride powder: the carbon source: the silicon source = 10:2~4:2~5.

[0012] The present invention provides a boron nitride thermal insulation material, which is made by the preparation method of a boron nitride thermal insulation material as described above. The boron nitride thermal insulation material includes a first precursor, a second precursor, and flaky boron nitride; wherein, The surface of the first precursor is coated with a metal silicon-oxygen hybrid layer for realizing chemical connection and curing between the flaky boron nitrides; The surface of the second precursor is coated with a silicon carbide layer for providing a structural skeleton support and a thermal shielding function between the flaky boron nitrides; The flaky boron nitride has an oriented arrangement along the freezing direction for constructing a thermal resistance path as a core skeleton material.

[0013] Compared with the prior art, the beneficial effects of a boron nitride thermal insulation material and its preparation method in the present invention are as follows: Flaky boron nitride is pushed by ice crystals into a highly vertically aligned lamellar array during directional freezing, and then the structure is retained by directional freeze-drying to form a multi-level vertical pore structure. Heat must cross multiple vertical lamellae and intermediate pores to be transferred to the opposite side, greatly extending the heat transfer path and reducing the effective thermal conductivity. The metal-silicon-oxygen hybrid layer of the first precursor is distributed in the gaps between the flaky boron nitride, and has the characteristics of flexible chain-breaking and recombination, which can scatter and dissipate high-frequency phonons and reduce lattice heat conduction. The silicon carbide coating layer on the surface of the second precursor is both a rigid barrier with high thermal stability and has good reflection ability in the mid- and far-infrared bands, inhibiting the penetration of radiative heat and further enhancing heat insulation. This network can not only maintain the structural integrity at high temperatures, but also absorb thermal stress to prevent thermal shock cracking and ensure the long-term stability of heat insulation performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a flowchart of the preparation method of the boron nitride thermal insulation material in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0016] Please refer to Figure 1 , the present invention provides a preparation method of a boron nitride thermal insulation material, and the steps include: S1. Add silane monomers and metal oxide particles to the boron nitride powder, and then blend them at room temperature to obtain a first precursor, and the surface of the first precursor is coated with a metal-silicon-oxygen hybrid layer. Calculated by mass ratio, in the first precursor, boron nitride powder: silane monomer: metal oxide particle = 20: 4-6: 1-4.

[0017] In step S1, the silane monomer includes at least one of methyltriethoxysilane, allyltriethoxysilane, and (3-mercaptopropyl) trimethoxysilane, the metal oxide particle includes at least one of germanium oxide, titanium oxide, and aluminum oxide, the particle size of the boron nitride powder is 80-120 nm, and the chemical general formula of the boron nitride powder is (BN) n , where n = 2-10.

[0018] Step S1 includes: S1.1. Add the boron nitride powder and the metal oxide particles to the mixed solution, perform ultrasonic dispersion for 30 minutes, and then perform magnetic stirring, and the stirring speed is 200-300 rpm, where the mixed solution includes absolute ethanol and deionized water.

[0019] The ultrasonic power is 200 - 300 W. The ultrasonic wave breaks the boron nitride aggregates, forming a stable nano - level dispersion system. There is weak adsorption between the metal oxide particles and the surface of boron nitride. The polarity of the solution system is controlled to enhance the uniformity of the distribution of metal oxides and prevent agglomeration. The initial adsorption capacity of the metal oxide particles on the surface of boron nitride is improved, and a uniform reaction system is established, providing a contact template for the subsequent selective condensation of silane, realizing the precursor - level physical fusion of multi - phase components, improving the efficiency of the hybridization reaction, controlling the particle size distribution, and contributing to the uniformity of the particle size of the coating layer.

[0020] S1.2. Maintain the rotation speed, then dropwise add the silane monomer and stir for 1 - 2 hours, then add tris(hydroxymethyl)aminomethane and stir for 2 - 3 hours, keeping the pH = 6.5 - 7.2 to obtain the first precursor solution.

[0021] Adding tris(hydroxymethyl)aminomethane provides a buffer environment and maintains the pH at 6.5 - 7.2. The three hydroxyl groups and the amino structure can form hydrogen bonds or complex structures with silicon - hydroxyl bonds, metal - hydroxyl bonds, etc., playing a dual role as a cross - linking auxiliary site and a complexing stabilizer. Inducing the formation of a three - dimensional metal - silicon - oxygen network structure on the surface of boron nitride, realizing the hybrid connection between silane and metal oxides, avoiding the agglomeration of single silicon - oxygen bonds. Through the auxiliary sites provided by tris(hydroxymethyl)aminomethane, the stability and elasticity of the network are enhanced, forming a flexible rubber - like shell structure, improving the binding force of the coating layer to the surface of boron nitride, and providing cross - linking active sites for subsequent heat treatment.

[0022] S1.3. Increase the rotation speed to 300 - 400 rpm, continue to stir for 1 - 2 hours, intermittently stop during the stirring process, add an ethanol - aqueous solution to adjust the viscosity of the first precursor solution to the preset viscosity, and perform vacuum cold - air drying for 12 - 18 hours after the stirring is completed to obtain the first precursor.

[0023] The increase in rotational speed promotes the collision between particles, enhances the condensation rate. Intermittent stirring helps the particles to settle and resuspend, preventing agglomeration. Adding an ethanol - aqueous solution to adjust the viscosity of the system to the preset range can avoid the formation of a cracked structure during drying. Vacuum cold - air drying slowly removes the solvent at a low temperature, enabling the coating layer to solidify and take shape on the surface of boron nitride. Controlling the rheological properties of the system can avoid the shrinkage and cracking of the silicon - oxygen shell layer or the structural collapse, forming a hybrid shell layer with a controllable thickness, continuous surface, and good interfacial adhesion on the particle surface. The drying process forms low - temperature setting precursor particles with a thermally active structure - bridging function. The first precursor exhibits a hybrid connection phenomenon of one or more metal - silicon - oxygen bonds, providing an elastic and dense composite interface layer, and offering the potential for cross - linkable activation under subsequent heat treatment conditions. The preset viscosity is 20 - 150 mPa·s, which is used for subsequent mold - pouring operations. It has good fluidity and the particles are easy to orient. The introduction of water is not only a solvent but also a reactant. After increasing the proportion of water, the hydrolysis of silane is faster, resulting in more silicon - hydroxyl bonds, and the condensation reaction is intensified, the bonding structure increases, and the viscosity rapidly rises. At a higher water proportion, the reaction liquid quickly gels and thickens, and the viscosity increases. Ethanol can form hydrogen - bond complexes with silicon - hydroxyl bonds and metal - hydroxyl bonds, hindering their condensation, interfering with the hydrolysis reaction rate, slowing down cross - linking, delaying the increase in the system viscosity, and at the same time having a low surface tension and fast volatilization, which helps to reduce the overall viscosity of the system. In S1.3, while stirring, slowly add the ethanol - water mixture drop by drop, adding 2 - 3 mL each time. After stirring for 5 minutes, measure the viscosity and control the viscosity within the target range.

[0024] S2. Mix boron nitride powder with a carbon source and a silicon source, and then place them in a mixed atmosphere of inert gas and acetylene gas to carry out a carbothermal reduction reaction to obtain a second precursor, and a silicon carbide layer is coated on the surface of the second precursor. Calculated by mass ratio, in the second precursor, boron nitride powder: carbon source: silicon source = 10: 2 - 4: 2 - 5.

[0025] In step S2, the carbon source includes at least one of glucose, dopamine, sucrose, and polyvinyl alcohol, and the silicon source includes at least one of tetraethyl orthosilicate and methyltriethoxysilane.

[0026] Step S2 includes: S2.1. Add boron nitride powder, carbon source, and silicon source to the mixed solution in sequence, adjust the temperature to less than 30 °C, ultrasonically disperse for 30 minutes, then add tris (hydroxymethyl) aminomethane and stir. Add dilute acetic acid to adjust the pH value to 4.5 - 5.5 to obtain a composite dispersion liquid, where the mixed solution includes anhydrous ethanol and deionized water.

[0027] Glucose, sucrose, etc. remain basically stable at lower temperatures. They will be pyrolyzed into carbon under subsequent high-temperature conditions, providing a reducing agent and carbon donor for carbon thermal reduction. Dopamine can self-polymerize under certain conditions to form a polydopamine film, improving the surface adhesion of the particles. Polyvinyl alcohol, due to its good film-forming properties, can be used as a binder to improve the dispersion state of the precursor particles. Ethyl orthosilicate hydrolyzes to form silicon hydroxide under acidic conditions. Methyltriethoxysilane is similar in that its hydrolysis product has organic side chains, which reduces the reaction rate and improves the toughness of the subsequent silicon carbide coating. Tris(hydroxymethyl)aminomethane acts as a buffer and complexing agent. Its hydroxymethyl group forms hydrogen bonds or weak chemical bonds with the surface groups of boron nitride and metal oxides. Dilute acetic acid adjusts the pH of the system to 4.5-5.5, promoting the hydrolysis of the silicon source to form silicon hydroxide, providing sufficient silicon-based precursors for subsequent condensation and reduction reactions. A uniformly dispersed composite dispersion is obtained, which lays a good foundation for subsequent drying and high-temperature reduction reactions. The initial hydrolysis product is pre-coated on the surface of the boron nitride particles to ensure the formation of a continuous silicon carbide coating layer during subsequent carbon thermal reduction. Controlling the pH helps to regulate the hydrolysis rate of the silicon source, avoid agglomeration caused by excessively rapid gelation, and improve the controllability of the reaction.

[0028] S2.2. Pour the composite dispersion into a flat-bottomed ceramic evaporating dish, place the flat-bottomed ceramic evaporating dish in a forced air drying oven at 70°C, and let it stand for 12 hours. After drying, scrape a thin layer of dry powder from the surface and grind it in a mortar to obtain a composite precursor powder.

[0029] During the drying process, the gradual evaporation of the solvent causes the hydrolyzed silicon and carbon sources in the solution to partially condense, forming a preliminary silicon-oxygen network and a colloidal carbon layer. This process forms a continuous silicon-oxygen precursor film on the surface of the boron nitride particles, while the carbon source is dispersed on the particle surface or filled between the particles. Slight grinding helps break up any lumps that may form, ensuring uniform particle size and facilitating subsequent high-temperature reduction reactions. This forms a solid, uniform composite precursor powder, providing a uniform reaction substrate for the carbothermal reduction reaction. The partial silicon-oxygen network formed in the precursor powder provides an initial structural template for the subsequent conversion to a silicon carbide layer. The drying and grinding process ensures that the system is free of excess residual solvent, avoiding bubbles or localized uneven reactions during high-temperature reactions.

[0030] S2.3. Spread the composite precursor powder evenly in a quartz boat, then place it in a tube furnace, fill it with high-purity inert gas for pre-cleaning for 30 minutes, then switch to a mixed atmosphere of inert gas and acetylene for continuous ventilation, set the heating rate to 5°C / min, heat to 1200°C and keep warm for 2 hours, and obtain the second precursor after cooling.

[0031] At high temperatures (1200 °C), a reduction reaction occurs between the silicon-oxygen network and the carbon source to form a silicon carbide layer; the addition of acetylene helps provide sufficient carbon and may form a thin carbon film on the reaction surface, improving the continuity and uniformity of the silicon carbide coating. The formed silicon carbide coating tightly covers the surface of the boron nitride particles, forming a boron nitride@silicon carbide core-shell structure, which not only improves the mechanical strength of the particles but also has a positive impact on the heat insulation performance through the infrared reflection and low thermal conductivity characteristics of silicon carbide. The obtained second precursor has a uniform silicon carbide coating, significantly enhancing the high-temperature resistance, thermal shock resistance, and infrared shielding performance of the boron nitride particles; the silicon carbide layer generated during the carbothermal reduction reaction can act as a rigid skeleton, providing structural reinforcement in the subsequent overall material; this reaction step ensures the full reaction of the silicon source and the carbon source in the composite precursor, forming a continuous silicon carbide phase, effectively blocking heat conduction, and improving the heat insulation effect.

[0032] In S2.1, through ultrasonic dispersion, pH regulation, and temperature control, it is ensured that boron nitride, the carbon source, and the silicon source are uniformly dispersed in the ethanol-water system, and partial hydrolysis of the silicon source is promoted to form a precursor solution. The technical effect is to pre-construct the reaction interface, laying a uniform foundation for the subsequent carbothermal reduction. In S2.2, drying is carried out at 70 °C with forced air, causing the composite dispersion to slowly volatilize to form a solid precursor powder, promoting the gelling of the silicon-oxygen network and the carbon source on the surface of boron nitride. The technical effect of this step is to form a stable, uniform, and reactive precursor powder. In S2.3, through a carbothermal reduction reaction in a tubular furnace under a mixed atmosphere of inert gas and acetylene with the temperature raised to 1200 °C, the silicon source and the carbon source are reduced to form a continuous silicon carbide coating, thereby obtaining a second precursor with high thermal shock resistance and infrared shielding performance.

[0033] S3. Add flaky boron nitride, the first precursor, and the second precursor to a mixed solvent, mix evenly, conduct directional freezing at a temperature of -80 to -60 °C, and then raise the temperature to 250 to 400 °C and react in an inert atmosphere for 3 to 4 hours to obtain a boron nitride thermal insulation material, where the mixed solvent includes ethanol and tetrahydrofuran.

[0034] Step S3 includes: S3.1. Add flaky boron nitride, the first precursor, and the second precursor to the mixed solvent, stir for 30 minutes, and adjust the pH to 6.5 - 7.0 to obtain a multiphase composite dispersion.

[0035] After the components in the dispersion are uniformly mixed, a multi-phase stable suspension is formed, ensuring sufficient raw material supply and uniformity for the directional arrangement during the freezing process. The pH adjustment ensures the stable state of the hydrolysis products of silane and the surface functional groups of the precursors (such as silicon-hydroxyl bonds, metal-hydroxyl bonds, etc.), laying the foundation for subsequent cross-linking reactions. This step ensures the full contact and uniform distribution of the flaky boron nitride and the precursors, laying a good foundation for constructing a highly oriented composite network in the subsequent directional freezing and thermal curing stages, thus facilitating the formation of a material structure with low thermal conductivity and high heat insulation performance.

[0036] The adjustment method is as follows: If the initial pH > 7.0 (alkaline), use a pipette or burette to take dilute acetic acid solution and drop it into the multi-phase composite dispersion while stirring. Add 0.1 - 0.2 mL each time and measure again after the pH drops; if the initial pH < 6.5 (acidic), similarly use a pipette or burette to take dilute ammonia water solution and drop it into the multi-phase composite dispersion, adding 0.1 - 0.2 mL each time and measuring again after the pH rises.

[0037] S3.2. Pour the multi-phase composite dispersion into a rectangular silicone mold, place the rectangular silicone mold on a freezing device equipped with a freezing platform. The temperature of the cold source below the freezing device is -80~-60°C, the temperature of the heat source above the freezing device is 30~50°C. The freezing direction of the freezing device is perpendicular to the ground, the temperature gradient in the freezing direction is greater than 80°C / cm, and the freezing time is 12 - 24 hours.

[0038] Under the action of an obvious temperature gradient, ice crystals grow rapidly from the cold source side, pushing the solid particles in the solution into the ice crystal gaps along the vertical direction, making them show a highly oriented arrangement. The ice crystals play a template role during the freezing process, and their morphology determines the porous oriented structure left after subsequent drying. The vertically layered structure formed by the oriented arrangement can extend the heat conduction path, significantly reduce the overall thermal conductivity of the material, and enhance the infrared reflection ability. At the same time, the formed multi-level thermal resistance network is beneficial to improving the thermal shock resistance and overall mechanical stability. The structures and usage methods of the freezing device and the rectangular silicone mold used are known to those skilled in the art and will not be elaborated here.

[0039] S3.3. After freezing, transfer the rectangular silicone mold to a vacuum freeze dryer. The drying temperature is -50°C, the vacuum degree is ≤20 Pa, and the drying time is 24 hours. After the vacuum freeze drying is completed, place it in a tubular furnace, introduce high-purity inert gas to purge for 30 minutes, set the heating rate to 3 - 5°C / min, raise the temperature to 200 - 250°C and hold for 1 hour, then raise the temperature to 300 - 400°C and hold for 3 hours. After the holding is completed, naturally cool to room temperature to obtain the boron nitride thermal insulation material.

[0040] During the vacuum freeze-drying process, ice crystals directly sublime, which not only avoids the structural collapse that may be caused by liquid-phase drying but also perfectly preserves the porous and ordered structure formed by directional freezing. During the heat treatment process at 200-400 °C, functional groups such as silicon-oxygen, metal-oxygen, and carbon sources in the precursor undergo cross-linking condensation reactions to transform into a stable solid network structure. Among them, the metal-silicon-oxygen hybrid layer in the first precursor and the silicon carbide coating layer in the second precursor form a firmly bonded interface complex through thermal curing. This step not only fixes the directionally induced structure but also forms a strong chemical bonding network between the composite components, significantly improving the mechanical strength, heat resistance, and infrared insulation performance of the material. Through continuous thermal cross-linking, the finally obtained boron nitride thermal insulation material exhibits excellent performance in terms of thermal shock resistance, low thermal conductivity, and long-term thermal insulation.

[0041] S3.1 realizes the uniform mixing of flaky boron nitride, the first precursor, and the second precursor, ensuring sufficient contact between the components. By adjusting the pH to 6.5-7.0, the surface functional groups of the components are stabilized, laying a good foundation for subsequent cross-linking curing. S3.2 uses a strictly controlled temperature gradient (lower cold source -80~-60 °C, upper heat source 30~50 °C) to induce the directional growth of ice crystals, enabling the components to be arranged orderly in the vertical direction. The formed directional porous structure greatly extends the heat conduction path, effectively reducing the overall thermal conductivity and providing a geometric template for subsequent cross-linking. S3.3 Vacuum freeze-drying preserves the directional structure through ice crystal sublimation, avoiding solvent residues and structural collapse. The thermal cross-linking process is carried out in an inert atmosphere, promoting the cross-linking curing of the active functional groups in the precursor to generate a stable and firmly bonded interface composite network structure, thereby improving the mechanical strength, thermal stability, and infrared insulation performance of the material.

[0042] In one embodiment, in step S3.1, the mixed solvent further includes a self-healing polymer precursor, and the self-healing polymer precursor includes a polymer main chain component, a dynamic cross-linking monomer component, and an auxiliary cross-linking component. Among them, the polymer main chain component includes at least one of hydroxy-terminated polydimethylsiloxane, hydroxy-terminated polyethylene glycol, and hydroxy-terminated polyacrylate; the dynamic cross-linking monomer component includes at least one of 1,3-bis(hydroxymethyl)urea, 1,3-bis(hydroxyethyl)urea, and N,N'-bis(hydroxyethyl)thiourea; and the auxiliary cross-linking component includes at least one of 1,3,5-tris(aminomethyl)benzene and tris(hydroxymethyl)benzene.

[0043] The polymer main-chain component has a long-chain structure and sufficient flexibility. Taking hydroxyl-terminated polydimethylsiloxane as an example, its silicon-oxygen backbone has an extremely low glass transition temperature and good flexibility, and can remain soft without embrittlement at low temperatures; hydroxyl-terminated polyethylene glycol has good hydrophilicity and fluidity, which can improve the interfacial compatibility between the polymer and the inorganic precursor. Although hydroxyl-terminated polyacrylate is relatively hard, its terminal hydroxyl groups can participate in subsequent cross-linking to form a certain degree of network. By contacting with boron nitride, the first precursor, and the second precursor, the polymer main-chain component can act as a flexible binder to fill and compensate for the microscopic voids between different components, enhance the synergistic effect of the overall structure, improve the interfacial bonding, and reduce delamination or cracking caused by thermal expansion differences. In the composite system, the polymer main-chain component provides flexible support for the final thermal insulation material, alleviates the mechanical stress caused by temperature differences, enables the material to exhibit excellent crack resistance when damaged, and creates favorable conditions for subsequent self-healing.

[0044] The dynamic cross-linking monomer component contains urea bonds or thiourea bonds, which are characterized by being able to cross-link through hydrogen bonds or reversible covalent bonds under mild conditions. When subjected to external stress or temperature fluctuations, these bonds can break and reform to achieve the self-healing effect. For example, dynamic urea bonds can be formed between the hydroxymethyl groups in 1,3-bis(hydroxymethyl)urea and the terminal hydroxyl groups of the main chain, and this cross-linking has the ability to self-repair after fracture. As a small-molecule cross-linking agent, the dynamic cross-linking monomer component reacts with the polymer main chain and other functional groups to form a three-dimensional dynamic cross-linking network, enabling the entire composite structure to quickly recover when damaged and maintaining the integrity of the interface and structure. The dynamic cross-linking network can automatically repair through reversible bond recombination when microcracks or local damage occur on the material surface, thereby extending the service life and reliability of the thermal insulation material. In a high-temperature or thermal cycling environment, the fracture and recombination process of the dynamic cross-linking monomer component can absorb part of the thermal stress, reduce the accumulation of interfacial stress, and prevent crack propagation.

[0045] The auxiliary cross-linking component contains multiple reaction sites (for example, 1,3,5-tris(aminomethyl)benzene has three amino groups), and can undergo multi-point reactions with the polymer main chain and the dynamic cross-linking monomer to form a network with a high cross-linking density. Such compounds can not only participate in the cross-linking reaction but also act as an interfacial coupling agent to enhance the chemical / physical bonding between the self-healing polymer precursor and the inorganic precursor (such as boron nitride materials and their coatings), and improve the coordination of the overall composite structure. The auxiliary cross-linking component promotes the formation of a more uniform and dense cross-linking network, aiming to enhance the overall stability of the composite material during the thermal cross-linking curing process. The increase in cross-linking density directly improves the mechanical strength and thermal shock resistance of the material, and at the same time improves the durability of the thermal insulation material during long-term use.

[0046] To protect the dynamic cross-linking ability of the self-healing polymer, the maximum temperature and heat preservation time should be controlled during the thermal curing process in step S3.3 to prevent the self-healing network from being completely inactivated. For example, preliminary cross-linking can be carried out when the temperature rises to 200 - 250 °C. After maintaining for 1 hour, the temperature is gradually increased to no more than 350 °C to retain some reversible bond structures.

[0047] In one embodiment, in step S3.1, the mixed solvent further includes a self-healing polymer precursor and an auxiliary component, where the auxiliary component includes at least one of dimethylformamide and N,N-dimethylacetamide.

[0048] In step S3.1, electrospinning is performed on the self-healing polymer precursor, auxiliary component, first precursor, and second precursor in the mixed solvent. After electrospinning is completed, ethanol and tetrahydrofuran containing flaky boron nitride are added. The steps of electrospinning are as follows: Using a coaxial electrospinning device, the inner layer solution includes the auxiliary component, self-healing polymerization precursor, first precursor, and second precursor, and the outer layer solution includes a polyurethane solution. The ambient temperature is controlled at 20 - 25 °C, and the relative humidity is controlled at 30 - 50%. The voltage is 15 - 20 kV, the flow rate of the inner layer solution is 1 - 2 mL / h, the flow rate of the outer layer solution is 0.5 - 1 mL / h, and the distance from the nozzle to the collector is 15 - 20 cm. The collector is a high-speed rotating cylinder (rotation speed of 1000 - 2000 rpm) to obtain a directionally aligned electrospinning product.

[0049] The mixed solution is respectively injected into the coaxial nozzle, and then the high-voltage power supply is turned on for electrospinning. The fibers are stretched under the action of the electric field to form an electrospinning product with a scale from nano to micron. The fiber membrane collected on the collector shows a continuously directionally aligned structure, where the core part contains a mixed system of the first precursor, second precursor, and self-healing polymerization precursor, and is coated with a polyurethane protective shell layer. By using a rotating collector, the fibers are directionally aligned on the collector, and the layered arrangement of the electrospinning product is achieved by combining the electrostatic field force and the mechanical movement of the collector.

[0050] After chopping the electrospinning product (or dispersing it in a suitable granular form), it is mixed with flaky boron nitride into the mixed solvent (ethanol and tetrahydrofuran mixture), and evenly dispersed in a magnetic stirrer for 30 minutes. At the same time, the pH is adjusted to 6.5 - 7.0, and then step S3.2 is carried out.

[0051] In addition to ethanol and tetrahydrofuran, a self-healing polymerization precursor and auxiliary components (including dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc)) are added in S3.1 of this embodiment. These components can help improve the solution stability and spinnability: the auxiliary components can adjust the viscosity and conductivity of the solution to ensure uniform and continuous fiber formation during the electrospinning process. In the mixed solution, the self-healing polymerization precursor coexists with the first and second precursors, providing an ideal basis for subsequent component curing and interfacial cross-linking. The inner layer solution contains auxiliary components, a self-healing polymerization precursor, the first precursor, and the second precursor, and a polyurethane solution is used for the outer layer to form a protective shell layer. The collector is a high-speed rotating cylinder. Using the electrostatic field force and mechanical shear, the fibers are oriented and arranged in a layered structure on the collector. The inner core of the oriented fiber membrane contains inorganic and self-healing components, forming a continuous "core-shell" structure, and the outer polyurethane protective layer not only protects the internal structure but also makes the overall fiber easier to integrate subsequently. By adjusting the voltage, flow rate, and nozzle distance, the uniformity of the fiber diameter and morphology is achieved, thus laying a uniform microstructural foundation for the subsequent overall directional freeze-curing of the material.

[0052] After electrospinning, the fiber membrane is first chopped or dispersed in a suitable granular form, and then mixed with flaky boron nitride into a mixed solvent of ethanol and tetrahydrofuran, and uniformly dispersed in a magnetic stirrer for 30 minutes while adjusting the pH to 6.5 - 7.0. This enables good contact between the self-healing polymer precursor, the inorganic precursor, and the original flaky boron nitride, ensuring the overall coordinated arrangement of each component during the subsequent directional freezing stage. The presence of the self-healing polymer can enhance the interfacial bonding between boron nitride and the precursor, reducing interfacial detachment and thermal bridge effects caused by temperature changes. The multiphase composite dispersion liquid in the mold is directionally frozen under the strong temperature difference environment of the lower cold source (-80 to -60 °C) and the upper heat source (30 to 50 °C). With the aid of ice crystals as templates, each component is arranged orderly in the vertical direction. After vacuum freeze-drying, it is then heated in a tube furnace under an inert atmosphere to 250 °C at a rate of 3 - 5 °C / min and held for 1 hour, and then raised to 350 °C and held for 3 hours to complete curing. The oriented flaky boron nitride and the electrospun product embedded therein form a large number of interfaces, and these interfaces significantly increase the heat conduction path, thereby greatly reducing the overall thermal conductivity. The self-healing polymer precursor forms a partially reversible cross-linked network during the thermal curing process. When microcracks occur, self-repair can be achieved to ensure the long-term stability of the thermal insulation performance. After overall directional freezing and thermal cross-linking curing, a regular and ordered porous layered structure is formed, which not only improves the heat insulation effect but also has excellent performance in terms of thermal shock resistance and mechanical load resistance.

[0053] By using coaxial electrospinning technology to prepare a core-shell fiber membrane with oriented arrangement from a self-healing polymer precursor, an auxiliary component, a first precursor, and a second precursor in S3.1, and then mixing it with flaky boron nitride to participate in subsequent directional freezing and thermal cross-linking curing. As a nanofiber membrane, the electrospinning product can synergistically form a multi-level and layered thermal barrier with the oriented flaky boron nitride, extend the heat transfer path, and effectively reduce the overall thermal conductivity. The self-healing polymer precursor and the auxiliary cross-linking component form a dynamic cross-linking network in the fiber core, increase the adhesion between components, achieve self-repair of microcracks, and ensure long-term thermal insulation stability. Coaxial electrospinning and rotational collection achieve a highly oriented arrangement of fibers, and this ordered structure is further fixed by the directional freezing process, ensuring a macroscopic layered and oriented arrangement of the entire composite material, thereby obtaining excellent thermal insulation and thermal shock resistance. By using an auxiliary solvent to regulate the physical parameters of the spinning solution, the entire process from fiber generation, composite assembly to directional freezing and curing has high reproducibility and controllability, facilitating large-scale industrial application.

[0054] In one embodiment, the diameter of the flaky boron nitride can reach 1–2 μm, the thickness is <100 nm, its length and width are much larger than the thickness, and it has an obvious two-dimensional morphology. When the flaky boron nitride is vertically arranged, the heat conduction path in the thickness direction is minimized, forming a lamellar thermal resistance wall, significantly reducing the thermal conductivity of the material in the thickness direction, while maintaining a certain thermal conductivity in the plane, which is conducive to heat dispersion in the plane and avoids local overheating. In two-way thermal gradient freezing, the flaky particles will be pushed by ice crystals into a vertical arrangement; while spherical or amorphous powders are difficult to form a continuous layered channel. The gaps left between the lamellae are uniform and ordered, and become multi-level oriented pores after freeze-drying, which helps to further block heat convection and thermal radiation. The flaky boron nitrides can form a laminated structure with each other, and with the help of the hybrid shell layer and silicon carbide shell layer in the first and second precursors, a three-dimensional network combining rigidity and flexibility is riveted between the layers, improving the compressive strength and thermal shock resistance. The surface functional groups of the large-sized flaky boron nitride are more likely to undergo physical and chemical bonding with the silicon-oxygen and silicon carbide layers, forming a firm interface; while the contact area of the nanospheres and amorphous boron nitride is large, but it is difficult to form a continuous layered network. The laminated structure has a multiple scattering and reflection effect on the radiation in the mid- and far-infrared (6–25 μm) band, further enhancing the passive radiative cooling effect. By adjusting the lamellar thickness and the interlayer filler (the first and second precursors), the optical and thermal radiation properties can be precisely controlled.

[0055] The present invention provides a boron nitride thermal insulation material, which is made by a preparation method of a boron nitride thermal insulation material. The boron nitride thermal insulation material includes a first precursor, a second precursor, and flaky boron nitride; wherein, The surface of the first precursor is coated with a metal-silicon-oxygen hybrid layer for realizing chemical connection and curing between the flaky boron nitrides; The second precursor is coated with a silicon carbide layer, which is used to provide structural skeleton support and thermal shielding function between the flaky boron nitrides; The flaky boron nitrides are arranged in an oriented manner along the freezing direction and are used as the core skeleton material to construct a thermal resistance path.

[0056] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A preparation method of a boron nitride heat insulation material, characterized in that the steps Including: S1. Add silane monomers and metal oxide particles to boron nitride powder, and then blend them at room temperature to obtain a first precursor, the surface of which is coated with a metal silicon oxide hybrid layer; S2. Mix boron nitride powder with a carbon source and a silicon source, and then place it in a mixed atmosphere of inert gas and acetylene gas to carry out a carbothermal reduction reaction to obtain a second precursor, the surface of which is coated with a silicon carbide layer; S3. Add flaky boron nitride, the first precursor, and the second precursor to a mixed solvent, mix them evenly, carry out directional freezing at a temperature of -80 to -60 °C, and then raise the temperature to 250 to 400 °C and react in an inert atmosphere for 3 to 4 hours to obtain a boron nitride thermal insulation material, wherein the mixed solvent includes ethanol and tetrahydrofuran.

2. The preparation method of a boron nitride heat insulation material according to claim 1, characterized in that, In step S1, the silane monomer includes at least one of methyltriethoxysilane, allyltriethoxysilane, and (3-mercaptopropyl)trimethoxysilane, the metal oxide particles include at least one of germanium oxide, titanium oxide, and aluminum oxide, the particle size of the boron nitride powder is 80 to 120 nm, and the chemical general formula of the boron nitride powder is (boron nitride) n , where n = 2 to 10.

3. The preparation method of a boron nitride heat insulation material according to claim 1 or 2, characterized in that, Step S1 includes: S1.

1. Add boron nitride powder and metal oxide particles to a mixed solution, carry out ultrasonic dispersion for 30 minutes, and then carry out magnetic stirring, with the stirring speed being 200 to 300 rpm, wherein the mixed solution includes absolute ethanol and deionized water; S1.

2. Maintain the rotation speed, then dropwise add silane monomers and stir for 1 to 2 hours, and then add tris(hydroxymethyl)aminomethane and stir for 2 to 3 hours, keeping the pH = 6.5 to 7.2 to obtain a first precursor solution; S1.

3. Raise the rotation speed to 300 to 400 rpm, continue to stir for 1 to 2 hours, intermittently stop during the stirring process, add an ethanol-aqueous solution to adjust the viscosity of the first precursor solution to reach a preset viscosity, and carry out vacuum cold air drying for 12 to 18 hours after stirring is completed to obtain a first precursor.

4. The preparation method of a boron nitride heat insulation material according to claim 1, characterized in that In step S2, the carbon source includes at least one of glucose, dopamine, sucrose, and polyvinyl alcohol, and the silicon source includes at least one of tetraethyl orthosilicate and methyltriethoxysilane.

5. The preparation method of a boron nitride heat insulation material according to claim 1 or 4, characterized in that, Step S2 includes: S2.

1. Add boron nitride powder, a carbon source, and a silicon source to a mixed solution in sequence, adjust the temperature to less than 30 °C, carry out ultrasonic dispersion for 30 minutes, then add tris(hydroxymethyl)aminomethane and stir, and add dilute acetic acid to adjust the pH value to 4.5 to 5.5 to obtain a composite dispersion liquid, wherein the mixed solution includes absolute ethanol and deionized water; S2.

2. Pour the composite dispersion liquid into a flat-bottomed ceramic evaporating dish, place the flat-bottomed ceramic evaporating dish in a blast drying oven at 70 °C, let it stand for 12 hours, scrape the surface thin-layer dry powder after drying is completed, and grind it with a mortar to obtain a composite precursor powder; S2.

3. Uniformly spread the composite precursor powder in a quartz boat, then place it in a tube furnace, charge high-purity inert gas for 30 minutes of pre-cleaning, then switch to a mixed atmosphere of inert gas and acetylene and continue to ventilate, set the heating rate to 5 °C / min, heat up to 1200 °C and keep it warm for 2 hours, and obtain a second precursor after cooling.

6. The preparation method of a boron nitride heat insulation material according to claim 1, characterized in that, Step S3 includes: S3.

1. Add flaky boron nitride, the first precursor, and the second precursor to a mixed solvent, stir for 30 minutes, and adjust the pH to 6.5 to 7.0 to obtain a multiphase composite dispersion liquid; S3.

2. Pour the multi-phase composite dispersion liquid into a rectangular silicone mold, place the rectangular silicone mold on a freezing device equipped with a freezing platform. The temperature of the cold source below the freezing device is -80~-60°C, the temperature of the heat source above the freezing device is 30~50°C. The freezing direction of the freezing device is perpendicular to the ground, the temperature gradient in the freezing direction is greater than 80°C / cm, and the freezing time is 12~24 hours; S3.

3. After freezing, transfer the rectangular silicone mold to a vacuum freeze dryer. The drying temperature is -50°C, the vacuum degree is ≤20 Pa, and the drying time is 24 hours. After freeze drying, place it in a tubular furnace, purge with high-purity inert gas for 30 minutes, set the heating rate to 3~5°C / min, heat up to 200~250°C and hold for 1 hour, then heat up to 300~400°C and hold for 3 hours. After the heat preservation is completed, naturally cool to room temperature to obtain a boron nitride thermal insulation material.

7. The preparation method of a boron nitride heat insulation material according to claim 1, characterized in that, Calculated by mass ratio, in the first precursor, the boron nitride powder: the silane monomer: the metal oxide particles = 20:4~6:1~4; In the second precursor, the boron nitride powder: the carbon source: the silicon source = 10:2~4:2~5.

8. A boron nitride heat-insulating material, characterized in that, Prepared by the preparation method of a boron nitride thermal insulation material according to any one of claims 1-7, the boron nitride thermal insulation material includes a first precursor, a second precursor, and flaky boron nitride; wherein, The surface of the first precursor is coated with a metal silicon-oxygen hybrid layer for realizing chemical connection and curing between flaky boron nitrides; The surface of the second precursor is coated with a silicon carbide layer for providing a structural skeleton support and a thermal shielding function between flaky boron nitrides; The flaky boron nitride has an oriented arrangement along the freezing direction for constructing a thermal resistance path as a core skeleton material.

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