SiC-coated SiO ceramic fiber aerogel with high anisotropy and thermal conductivity and preparation method of SiC-coated SiO ceramic fiber aerogel

Abstract

The invention discloses SiC-coated SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity and a preparation method of the SiC-coated SiO2 ceramic fiber aerogel. The ceramic fiber aerogel has a highly-oriented structure, and is characterized in that layers are aligned and arranged along the same direction, and a highly-oriented one-dimensional fiber structure is arranged in each single layer. The aerogel exhibits anisotropic thermal properties thanks to a highly oriented structure. In addition, the ceramic fiber aerogel has excellent thermal stability, and shows radial recoverable strain as high as 60%. The preparation method of the ceramic fiber aerogel comprises the following steps: preparing high-orientation SiC nanofibers through electrostatic spinning, forming an amorphous SiO2 thermal protection layer on the surfaces of the high-orientation SiC nanofibers through thermal treatment, and performing layer-by-layer stacking through a freeze-drying technology to obtain the ceramic fiber aerogel, the ceramic fiber aerogel has huge application potential in the field of extreme heat insulation due to the characteristics of a highly oriented structure and high anisotropic heat conductivity.

Description

Technical Field

[0001] The 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 have great application prospects in aerospace, military and other fields due to their low density, highly porous structure, excellent high-temperature thermal stability and chemical stability. However, traditional SiC aerogels usually exhibit isotropic thermal conductivity and cannot achieve high anisotropic thermal conductivity, which limits their directional thermal insulation applications under extreme conditions. Many natural biomaterials have ordered microstructures. Many of these special microstructures are anisotropic and exhibit special high anisotropic thermal conductivity related to direction. However, freeze-drying technology is currently widely used to prepare SiC aerogels with controllable macrostructures, but microstructure control is still challenging; secondly, chemical vapor deposition (CVD) is often used to prepare SiC nanowires, but its disordered growth and short fiber problems limit its application in constructing ordered aerogels. Therefore, it is urgent to prepare SiC@SiO2 ceramic fiber aerogels with high anisotropic thermal conductivity and develop a simple and efficient preparation method for controlling the growth of highly oriented and ordered silicon carbide fibers. Summary of the invention

[0003] In view of the deficiencies in the prior art, 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 as high as 5.08 and has excellent thermal insulation properties under extreme conditions.

[0004] The study found that electrospinning technology can prepare SiC nanofibers with high orientation and continuity, and use them as building blocks to self-assemble into aerogels through freeze drying. By regulating the microstructure of the fiber, an anisotropic structure from micro to macro can be constructed, which significantly reduces the thermal conductivity of the aerogel.

[0005] This project is dedicated to preparing highly oriented SiC fibers by electrospinning, introducing a SiO2 shell layer on the fiber surface, and combining freeze-drying technology to construct SiC@SiO2 fiber aerogels with high anisotropic thermal conductivity to achieve ultra-low thermal conductivity and excellent thermal insulation performance. This strategy not only improves the thermal insulation performance of aerogels, but also provides new ideas for the design of new anisotropic thermal insulation materials. The focus of this study is on how to control the microstructure, interface characteristics 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 the 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 leads to a large 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. In the present invention, mixing LPCS with PCS can 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 smooth surface. By mixing PCS with LPCS, the present invention can achieve a balance between spinnability and ceramic yield. PCS is responsible for providing the skeleton of the final ceramic material, while LPCS helps to 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. The solvent is tetrahydrofuran; the polyvinyl pyrrolidone solution is obtained by dissolving polyvinyl pyrrolidone in anhydrous ethanol;

[0009] (2) using oriented electrospinning technology to prepare a silicon carbide precursor spinning solution into a oriented SiC fiber film precursor; the receiver used in the oriented 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 highly oriented characteristic. 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 layer on its surface. Introducing a thin amorphous SiO2 shell layer 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, and the radial (perpendicular to the SiC@SiO2 fiber) thermal conductivity is extremely low, only 0.018W / m -1 K -1 , the axial (parallel SiC@SiO2 fiber) thermal conductivity is about 5 times higher, 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 radial recoverable strain of up to 60%.

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

[0015] 1. The preparation method of the SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity in the present invention is to successfully produce a highly anisotropic SiC@SiO2 ceramic fiber aerogel by combining electrospinning with freeze-drying technology. The anisotropic structure of the aerogel has arranged layers stacked in the same direction and highly oriented one-dimensional fibers in each independent layer. The preparation is simple, the raw materials are easily available, and it can be prepared in general chemical laboratories. It is easy to promote.

[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, and can be used in low-temperature environments of -196°C and high-temperature environments of 1300°C, with excellent high-temperature thermal insulation properties; and has good radial compressible deformation, so that it has broad application prospects in extreme thermal environments such as aerospace and high-temperature industries, providing a new way for efficient thermal management. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a 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 diagram of the thermal conductivity 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 a 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 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 scheme in the embodiments of the present invention is 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] 0.3g solid polycarbosilane and 0.2g liquid polycarbosilane were dissolved in 6ml tetrahydrofuran and stirred at room temperature for 30 minutes. Similarly, 0.5g polyvinyl pyrrolidone (PVP) was dissolved in 4ml 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 18kV (high-speed directional receiver, a rotation speed of 2800r / min, a constant feed rate (0.17mm / min), and a receiving distance of 15cm. 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 for 30 minutes in a muffle furnace to introduce a thin amorphous SiO2 shell layer on the surface of the SiC fiber to obtain the SiC@SiO2 fiber membrane.

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

[0031] Figure 1 is a 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 is a 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 SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity has the characteristics of light weight ( Figure 4 ), after testing, the radial (vertical SiC@SiO2 fiber) has an ultra-low thermal conductivity of 0.018W / m -1 K -1 The axial (parallel SiC@SiO2 fiber) thermal conductivity is about 5 times higher, which is 0.0914W / 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 spray gun (~1300℃) and liquid nitrogen ( Figure 6 ), indicating the importance of structural design of high anisotropic thermal conductivity SiC@SiO2 ceramic fiber to improve 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 has broad application prospects.

Claims

1. A method for preparing SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, characterized in that: 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 a silicon carbide precursor spinning solution into a directional 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 is 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 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 oriented 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° C. and the time is 1-3 hours.

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 high temperature pyrolysis temperature is 1400°C-1550°C, and the 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° C. and the time is 0.5-2 h.

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

9. A SiC@SiO2 ceramic fiber aerogel with high anisotropic thermal conductivity, characterized in that: The invention discloses a novel novel nanostructured carbon foam prepared by the preparation method described in any one of claims 1 to 8.

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