SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity and preparation method thereof

Abstract

The application discloses SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity and a preparation method thereof. The ceramic fiber aerogel has the characteristics of a highly directional structure, that is, layers are aligned in the same direction, and each individual layer has a highly directional one-dimensional fiber structure. Benefiting from the highly directional structure, the aerogel exhibits anisotropic thermal performance. In addition, the ceramic fiber aerogel has excellent thermal stability and exhibits a radial recoverable strain of up to 60%. The preparation method of the ceramic fiber aerogel comprises the following steps: preparing highly oriented SiC nanofibers through electrospinning, forming an amorphous SiO2 thermal protection layer on the surface of the nanofibers through heat treatment, and preparing the ceramic fiber aerogel through a freeze-drying technology. The highly directional structure and the characteristics of high anisotropic thermal conductivity of the ceramic fiber aerogel make it have great application potential in the field of extreme thermal insulation.

Description

Technical Field

[0001] The present invention relates to the field of ceramic fiber aerogel thermal insulation under extreme conditions, and in particular to a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity and a preparation method thereof. Background Art

[0002] Silicon carbide (SiC) aerogels, due to their low density, highly porous structure, and excellent high-temperature thermal and chemical stability, hold great promise for applications in aerospace, military, and other fields. However, conventional SiC aerogels typically exhibit isotropic thermal conductivity and are unable to achieve high anisotropic thermal conductivity, limiting their application in directional thermal insulation under extreme conditions. Many natural biomaterials possess ordered microstructures, many of which are anisotropic and exhibit exceptionally high, directionally dependent anisotropic thermal conductivity. While freeze-drying is currently widely used to prepare SiC aerogels with controllable macrostructures, microstructural control remains challenging. Furthermore, chemical vapor deposition (CVD) is commonly used to prepare SiC nanowires, but its disordered growth and short fiber lengths limit its application in constructing ordered aerogels. Therefore, there is an urgent need to prepare SiC@SiO2 ceramic fiber aerogels with high anisotropic thermal conductivity and to develop a simple and efficient method for controlling the growth of highly oriented, ordered silicon carbide fibers. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity and a preparation method thereof. The aerogel exhibits anisotropic thermal properties with an anisotropy coefficient ratio of up to 5.08 and has excellent thermal insulation properties under extreme conditions.

[0004] Research has found that electrospinning can produce highly oriented and continuous SiC nanofibers, which can be used as building blocks to self-assemble into aerogels through freeze-drying. By manipulating the microstructure of the fibers, an anisotropic structure from the microscopic to the macroscopic scale can be constructed, significantly reducing the thermal conductivity of the aerogel.

[0005] This project aims to prepare highly oriented SiC fibers through electrospinning, introduce a SiO2 shell onto the fiber surface, and combine this with freeze-drying to create SiC@SiO2 fiber aerogels with high anisotropic thermal conductivity, achieving ultra-low thermal conductivity and excellent thermal insulation properties. This strategy not only improves the thermal insulation performance of the aerogel but also provides new insights into the design of novel anisotropic thermal insulation materials. The research focuses on how to control the microstructure, interfacial properties, and macrostructure to achieve more efficient thermal management.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] One aspect of the present invention provides a method for preparing SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, comprising the following steps:

[0008] (1) Dissolving a silicon source in a solvent to obtain a silicon source solution; uniformly mixing the silicon source solution and a polyvinyl pyrrolidone solution to obtain a silicon carbide precursor spinning solution; preferably, the silicon source is a mixture of solid polycarbosilane (PCS) and liquid polycarbosilane (LPCS) in a mass ratio of 1:1.5 to 2:1. PCS has a high molecular weight and a high ceramic conversion rate, but it results in a high solution viscosity. If PCS is used alone for electrospinning, a large amount of solvent is usually required to achieve spinnability, which may lead to problems such as spinning difficulties, uneven fiber diameter, and excessive residual solvent during final carbonization. LPCS is in a liquid state, has a low molecular weight, and has a low viscosity. The fluidity of LPCS helps the fiber to be better formed, reduces defects in the fiber, and may form a smoother fiber surface. The present invention mixes LPCS with PCS to effectively reduce the overall viscosity of the solution, making it easier to perform electrospinning. In addition, the synergistic effect of PCS and LPCS can produce fibers with high strength, good uniformity, and a smooth surface. The present invention can achieve a balance between spinnability and ceramic yield by mixing PCS with LPCS. The PCS provides the skeleton of the final ceramic material, while the LPCS helps form a good fiber shape. The present invention optimizes the yield and performance of the final SiC fiber by adjusting the ratio of the two materials. Tetrahydrofuran is used as the solvent; the polyvinyl pyrrolidone solution is obtained by dissolving polyvinyl pyrrolidone in anhydrous ethanol.

[0009] (2) using directional electrospinning technology to prepare a silicon carbide precursor spinning solution into a directional SiC fiber film precursor; the receiver used in the directional electrospinning technology is a high-speed orientation receiver;

[0010] (3) In order to obtain the desired structure during the high-temperature pyrolysis process, the SiC fiber film precursor is heated to achieve pre-oxidation, and then the pre-oxidized SiC fiber film precursor is placed in an inert gas atmosphere such as argon for high-temperature pyrolysis to obtain a SiC fiber film with a high degree of orientation. Preferably, the pre-oxidation temperature is 180-210°C and the time is 1-3h; the high-temperature pyrolysis temperature is 1400°C-1550°C and the time is 1-3h. The pre-oxidation process is to remove the organic solvent that has not been completely volatilized in the spinning membrane. Secondly, pre-oxidation at an appropriate temperature (180-210°C) can solidify the fiber so that it maintains the structural morphology (oriented structure) of the fiber during the subsequent high-temperature pyrolysis (1400°C-1550°C).

[0011] (4) Oxidizing the SiC fiber membrane to form a SiO2 shell on its surface. Introducing a thin amorphous SiO2 shell on the surface of the SiC nanofiber can effectively reduce the interfacial thermal conductivity, form a phonon barrier, and further improve the thermal insulation performance of the aerogel to obtain a SiC@SiO2 fiber membrane; preferably, the oxidation temperature is 800-1200°C and the time is 0.5-2h.

[0012] (5) The SiC@SiO2 fiber membrane is immersed in silica sol and stacked layer by layer, and then freeze-dried to obtain a highly anisotropic SiC@SiO2 ceramic fiber aerogel. Preferably, the silica sol is borosilicate aluminum silica sol.

[0013] Another aspect of the present invention is to provide a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, which is prepared according to the preparation method provided by the first aspect of the present invention. The ceramic fiber aerogel has a highly oriented structure characterized by layers aligned in the same direction, and each individual layer has a highly oriented one-dimensional fiber structure. Thanks to the highly oriented structure, the aerogel exhibits anisotropic thermal properties, with an extremely low radial (perpendicular to the SiC@SiO2 fibers) thermal conductivity of only 0.018 W / m -1 K -1 The axial (parallel SiC@SiO2 fiber) thermal conductivity is about 5 times higher, at 0.0914W / m -1 K -1 , and its anisotropy coefficient ratio is as high as 5.08. In addition, SiC@SiO2 ceramic fiber aerogel has excellent thermal stability (-196℃~1300℃) and exhibits a radially recoverable strain of up to 60%.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] The highly anisotropic SiC@SiO2 ceramic fiber aerogel of this invention is prepared by combining electrospinning with freeze-drying technology. This highly anisotropic SiC@SiO2 ceramic fiber aerogel is successfully produced. The aerogel's anisotropic structure features aligned layers stacked in the same direction, with highly oriented one-dimensional fibers within each layer. Its preparation is simple, and the raw materials are readily available, allowing it to be produced in a typical chemical laboratory. This makes it readily available for widespread use.

[0016] 2. The SiC@SiO2 ceramic fiber aerogel in the present invention has highly anisotropic thermal properties, with an anisotropy coefficient ratio of up to 5.08. It can be used in low-temperature environments of -196°C and high-temperature environments of 1300°C, and has excellent high-temperature thermal insulation properties; and has good radial compressible deformation, which makes it have broad application prospects in extreme thermal environments such as aerospace and high-temperature industries, providing a new approach for efficient thermal management. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is an SEM image of the silicon carbide fiber membrane prepared in Example 1;

[0018] Figure 2 This is the SEM image of the SiC@SiO2 fiber membrane prepared in Example 1;

[0019] Figure 3 TEM image of the SiC@SiO2 fiber membrane prepared in Example 1;

[0020] Figure 4 This is an optical photograph of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity prepared in Example 1:

[0021] Figure 5 This is a comparison of the thermal conductivities of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity prepared in Example 1 in the axial and radial directions at room temperature;

[0022] Figure 6 This is an optical photograph of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity prepared in Example 1 treated in a butane blowtorch and liquid nitrogen;

[0023] Figure 7 This is an SEM image of the layered multi-arch structure of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity prepared in Example 1;

[0024] Figure 8 Graphs showing the stress-strain curves of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity prepared in Example 1 in the axial and radial directions, respectively. DETAILED DESCRIPTION

[0025] In conjunction with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention are further described in detail. The specific embodiments described below are only used to explain the present invention and are not intended to limit the present invention. The reagents and raw materials used in the following examples are all commercially available products and are commercially available.

[0026] Example 1

[0027] A SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity comprises the following steps:

[0028] 0.3 g of solid polycarbosilane and 0.2 g of liquid polycarbosilane were dissolved in 6 ml of tetrahydrofuran and stirred at room temperature for 30 minutes. Similarly, 0.5 g of polyvinyl pyrrolidone (PVP) was dissolved in 4 ml of anhydrous ethanol at room temperature and stirred for 30 minutes, and then the two solutions were mixed and stirred for 8 hours to prepare a spinning precursor. The resulting clear and transparent solution was transferred to a 10 ml plastic syringe equipped with a 25G spinning needle. Finally, an electrospinning device was used for spinning with a needle tip voltage of 18 kV (high-speed directional receiver, a speed of 2800 r / min, a constant feed rate (0.17 mm / min), and a receiving distance of 15 cm. The fiber membrane was then pre-oxidized in an oven at 190 ° C for 2 hours to solidify the fiber. Finally, the pre-oxidized fiber was kept at 1450 ° C for 2 hours at a heating rate of 2 ° C / min in an argon environment to obtain the desired highly oriented silicon carbide fiber membrane.

[0029] The SiC fiber membrane was annealed at 1000°C in a muffle furnace for 30 minutes to introduce a thin amorphous SiO2 shell layer on the surface of the SiC fiber to obtain the SiC@SiO2 fiber membrane.

[0030] Aluminum borosilicate silica sol was prepared by stirring 1.95g of aluminum chloride hexahydrate, 0.1g of boric acid, 7.6g of tetraethyl orthosilicate, and 100ml of deionized water at room temperature for 30 minutes. Finally, highly oriented SiC@SiO2 fiber membranes of a specified size were fully immersed in the prepared silica sol and stacked layer by layer in the same direction. The membranes were rapidly cooled with liquid nitrogen and then freeze-dried for 48 hours to produce SiC@SiO2 ceramic fiber aerogels with high anisotropic thermal conductivity.

[0031] Figure 1 This is the SEM image of the silicon carbide fiber membrane prepared in Example 1. Figure 1 It can be seen that the silicon carbide fiber membrane has a smooth surface and a highly oriented structure. Figure 2 This is the SEM image of the SiC@SiO2 fiber membrane prepared in step (2) of Example 1. Figure 2 It can be seen that the fibers are closely arranged after being coated with amorphous silica. The transmission electron microscope image shows that the thickness of the amorphous silica shell is about 50nm ( Figure 3 ), the prepared SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity has the characteristics of light weight ( Figure 4 ), after testing, the radial direction (vertical SiC@SiO2 fiber) has an ultra-low thermal conductivity of 0.018W / m -1 K -1 The axial thermal conductivity (parallel to SiC@SiO2 fibers) is about 5 times higher, at 0.0914 W / m -1 K -1, the anisotropy coefficient is as high as 5.08. At the same time, it can be observed that the aerogel has excellent thermal insulation properties and structural stability under butane torch (~1300℃) and liquid nitrogen ( Figure 6 ), indicating the importance of structural design of high anisotropic thermal conductivity SiC@SiO2 ceramic fiber to the improvement of thermal insulation performance, and it has excellent thermal insulation performance. Due to the high anisotropic structure and layered multi-arch structure ( Figure 7 ), the aerogel has 60% recoverable elastic deformation in the radial direction and excellent axial stiffness performance ( Figure 8 ), indicating that the high anisotropic thermal conductivity SiC@SiO2 ceramic fiber aerogel prepared by the present invention has excellent mechanical properties and broad application prospects.

Claims

1. A method for preparing SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, characterized by: The following steps are involved: (1) dissolving a silicon source in a solvent to obtain a silicon source solution; The silicon source solution and the polyvinyl pyrrolidone solution are uniformly mixed to obtain a silicon carbide precursor spinning solution; (2) Using directional electrospinning technology to prepare silicon carbide precursor spinning solution into oriented SiC fiber film precursor; (3) heating the SiC fiber film precursor to achieve pre-oxidation, and then placing the pre-oxidized SiC fiber film precursor in an inert gas atmosphere for high-temperature pyrolysis to obtain a SiC fiber film; (4) Oxidizing the SiC fiber membrane to form a SiO2 shell layer on its surface to obtain a SiC@SiO2 fiber membrane; (5) The SiC@SiO2 fiber membrane was immersed in silica sol and stacked layer by layer, and then freeze-dried to obtain SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity.

2. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (1), the silicon source includes at least one of solid polycarbosilane and liquid polycarbosilane.

3. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 2, characterized in that: The silicon source is a mixture of solid polycarbosilane and liquid polycarbosilane in a mass ratio of 1:1.5 to 2:

1.

4. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (2), the receiver used in the oriented electrospinning technology is a high-speed orientation receiver.

5. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (3), the pre-oxidation temperature is 180-210 o C, time is 1-3h.

6. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (3), the temperature of the high temperature pyrolysis is 1400℃-1550℃ o C, time is 1-3h.

7. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (4), the oxidation temperature is 800-1200 o C, time is 0.5-2h.

8. The method for preparing a SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity according to claim 1, characterized in that: In step (5), the silica sol is borosilicate aluminum silica sol.

9. A SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, characterized by: The invention relates to a novel crystalline silicon nitrate-containing slurry prepared by the preparation method according to any one of claims 1 to 8.

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