Ceramic aerogel with multi-scale heterogeneous structure and method for preparing the same

Abstract

The application relates to a ceramic aerogel with a multi-scale heterogeneous structure and a preparation method thereof, and comprises the following steps: carbon fibers are immersed in a reduced graphene oxide aqueous solution, and a Cf / RGO composite material is obtained after drying; methyltriethoxysilane and dimethyldimethoxysilane are used as silicon sources, deionized water is used as a hydrolysis agent, and tetramethylammonium hydroxide is used as a catalyst, and after mixing, a siloxane gel is obtained, and a siloxane dry aerogel is obtained after drying; the layer-by-layer stacked Cf / RGO is placed on the siloxane dry aerogel to carry out high-temperature gas phase silicon infiltration, and a SiC / C composite fiber material is obtained, and then air atmosphere heat treatment is carried out, and a layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel is obtained. The aerogel has the characteristics of light weight, high temperature resistance, strong mechanical properties and the like, realizes the synergistic optimization of "mechanics-thermal-electromagnetic properties", and provides a new idea for cross-scale design of stealth materials under high-temperature multi-physical field coupling conditions.

Description

Technical Field

[0001] This invention relates to the field of new materials technology, and in particular to a ceramic aerogel with a multi-scale heterogeneous structure and its preparation method. Background Technology

[0002] Traditional stealth materials (such as stealth coatings and organic materials) often face problems such as oxidation failure, sharp decline in mechanical properties, and weakened stealth performance at high temperatures, making it difficult for them to meet the stringent requirements of extreme thermo-mechanical-electromagnetic multi-field coupling conditions. Therefore, the development of novel high-temperature structural stealth materials urgently needs to overcome the technical bottleneck of synergistic optimization of "mechanical-thermal-electromagnetic properties" and construct a multifunctional integrated system that combines lightweight, high-temperature resistance, and infrared / radar compatible stealth characteristics.

[0003] Ceramic-based materials are a typical type of high-temperature stealth material. However, traditional ceramic-based high-temperature stealth materials often face bottlenecks such as high density, high brittleness, high thermal conductivity, and limited stealth frequency bands, making it difficult to meet the demands for lightweight, wide-band compatibility, and multifunctional integration. In recent years, ceramic aerogels have shown promising application prospects in the field of high-temperature stealth due to their ultra-low density, high porosity, and excellent heat resistance. However, single-component and single-structure aerogels suffer from insufficient mechanical strength at high temperatures, limited infrared thermal radiation modulation capabilities, and a lack of multi-scale attenuation mechanisms for electromagnetic waves, severely restricting their application in stealth scenarios. To overcome these limitations, multi-level structural design has been extensively studied. For example, introducing nanowire networks can improve the mechanical toughness of ceramic aerogels; layered interfaces can extend the scattering path of electromagnetic waves; hollow fibers can not only effectively enhance the dissipation of electromagnetic waves but also hinder heat transfer to suppress thermal radiation; and so on. However, how to achieve precise integration of the above-mentioned multi-scale structures through controllable synthesis technology remains a current research challenge.

[0004] In addition, combining rational component design with structural control is a more effective technical means to construct a multifunctional system that combines lightweight, high-temperature resistance, load-bearing capacity, wide-band electromagnetic loss, and infrared suppression characteristics. For example, high-temperature infrared suppression and optimized impedance matching can be achieved through the layered combination of ceramic aerogels. Furthermore, the oxidation of the ceramic aerogel surface forms an interface layer, optimizing thermo-electromagnetic synergistic stealth performance through multiple reflection / absorption mechanisms between different materials. The core challenge of current research lies in how to synergistically solve the technical difficulties of lightweighting, mechanical load-bearing capacity, high-temperature stability, and wide-temperature-range stealth through multi-scale structural design and component optimization, and achieve dynamic compatibility and long-term maintenance of cross-frequency band stealth functions. Novel high-temperature structural stealth materials are a key area of ​​current stealth materials; however, their development is limited by key issues such as structural failure, mechanical property degradation, and multi-band stealth performance mismatch of traditional materials under thermo-mechanical coupling.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a ceramic aerogel with a multi-scale heterogeneous structure and its preparation method. This method can prepare layered hollow fiber framework-based SiC@SiO2 nanowire aerogels, achieving synergistic optimization of "mechanical-thermal-electromagnetic properties".

[0007] In a first aspect, the present invention provides a method for preparing ceramic aerogels with multi-scale heterogeneous structures, comprising the following steps:

[0008] S1. Add ascorbic acid to the aqueous solution of graphene oxide to obtain an aqueous solution of reduced graphene oxide. Impregnate carbon fibers in the aqueous solution of reduced graphene oxide and dry to obtain Cf / RGO composite material.

[0009] S2. Using methyltriethoxysilane and dimethyldimethoxysilane as silicon sources, deionized water as hydrolyzing agent, and tetramethylammonium hydroxide as catalyst, they are mixed and stirred evenly to obtain siloxane gel. The siloxane gel is dried to obtain siloxane dry aerogel.

[0010] S3. The stacked Cf / RGO composite material is placed on a siloxane dry aerogel and subjected to high-temperature vapor-phase silicon infiltration to obtain SiC / C composite fiber material. The SiC / C composite fiber material is then subjected to air atmosphere heat treatment to obtain layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel.

[0011] Preferably, step S1 includes:

[0012] S11. Dissolve graphene oxide in deionized water to obtain an aqueous solution of graphene oxide;

[0013] S12. Add ascorbic acid to the aqueous solution of graphene oxide and stir thoroughly to obtain a reduced aqueous solution of graphene oxide.

[0014] S13. The carbon fiber is impregnated in a reduced graphene oxide aqueous solution and dried in an oven at 50~70℃ (preferably 60℃) to obtain the Cf / RGO composite material.

[0015] Preferably, the concentration of the graphene oxide aqueous solution is 10~20 mg / ml, more preferably 15 mg / ml.

[0016] Preferably, the mass ratio of ascorbic acid to the aqueous graphene oxide solution is (1.5~2.5):1, more preferably 2:1.

[0017] Preferably, step S2 includes:

[0018] S21. Mix methyltriethoxysilane, dimethyldimethoxysilane, deionized water, and tetramethylammonium hydroxide and stir until homogeneous to obtain siloxane gel.

[0019] S22. The siloxane gel is dried in an oven at 50~70℃ (preferably 60℃) to obtain siloxane dry aerogel.

[0020] Preferably, the mass ratio of methyltriethoxysilane, dimethyldimethoxysilane, deionized water and tetramethylammonium hydroxide is (1~3):(0.5~1.5):(1.5~4.5):(0~0.05), more preferably 2:1:3:0.02.

[0021] Preferably, step S3 includes:

[0022] S31. Place the stacked Cf / RGO composite material on top of the siloxane dry aerogel and place it in a crucible. Place the crucible in a high-temperature atmosphere furnace and perform high-temperature vapor phase silicon infiltration to obtain SiC / C composite fiber material.

[0023] S32. The SiC / C composite fiber material is placed in a muffle furnace for air atmosphere heat treatment to obtain layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel.

[0024] Preferably, the high-temperature vapor-phase silicon infiltration is carried out in an inert gas atmosphere at a temperature of 1500~1600℃ for 2~3 hours; more preferably, the temperature is 1550℃ and the holding time is 2.5 hours.

[0025] Preferably, the inert gas is argon.

[0026] Preferably, the air atmosphere heat treatment involves heating to 800-1000°C at a rate of 1-3°C / min and holding for 2-4 hours; more preferably, the temperature is increased to 950°C at a rate of 2°C / min and held for 3 hours.

[0027] In a second aspect, the present invention provides a ceramic aerogel with a multi-scale heterogeneous structure, prepared by the above-described preparation method. The ceramic aerogel is a layered hollow fiber framework-based SiC@SiO2 nanowire aerogel with an average spacing of 350-450 μm between each layer and an average diameter of 200-250 nm for the nanowires. The nanowires exhibit a core-shell structure, and an amorphous SiO2 layer is formed on the SiC surface with a thickness of 5-10 nm.

[0028] A third aspect of the present invention provides the application of the above-described ceramic aerogel having a multi-scale heterogeneous structure.

[0029] Specifically, the applications include, but are not limited to:

[0030] A. Used as a structural thermal protection material for aerospace. In space exploration, the surface of spacecraft may face high temperatures and mechanical impacts. This material is lightweight, highly elastic, and resistant to high temperatures, and is expected to be used in aerospace structural thermal protection materials.

[0031] B. Used as heat insulation / stealth material in engine parts of ships / aircraft and other equipment. This part requires materials with good high temperature resistance, fire resistance and load-bearing capacity, and this material has these properties. In addition, military ships and military aircraft also have certain requirements for their high temperature stealth performance, and this material also has good infrared stealth performance at high temperatures;

[0032] C. Used as a refractory material for steam turbine furnaces. The furnace wall temperature in a typical steam turbine is 1100~1200℃, and it also has a certain load-bearing capacity. This material has good fireproof and heat insulation properties, and also has a certain load-bearing capacity, making it a promising candidate for use as a refractory material in furnaces.

[0033] The present invention has at least the following beneficial effects:

[0034] (1) This invention uses carbon fiber and siloxane dry gel as raw materials and combines the in-situ transformation and attachment growth mechanism in SiC fiber synthesis to successfully prepare a multi-scale heterogeneous structure material—layered hollow fiber framework-based SiC@SiO2 nanowire aerogel (LHSNA). The aerogel structure uses hollow SiC fibers as a layered framework, and a large number of SiC nanowires are grown on its surface with the assistance of hollow SiC fibers. The hollow structure and the heterogeneous interface are constructed by heat treatment. This cross-scale (nanowire-hollow microfiber-millimeter-scale layered) microstructure and the synergistic design of the two heavy components (SiC, SiO2) realize the functional integration of electromagnetic loss, infrared stealth and mechanical load-bearing, that is, the synergistic optimization of "mechanical-thermal-electromagnetic properties".

[0035] (2) The aerogel prepared by this invention has light weight (17.92 mg / cm³). 3 It features high temperature resistance (1200 ℃) and strong mechanical properties. At room temperature, this material has the advantages of low matching thickness (1.8 mm), wide absorption bandwidth (5.46 GHz), and high reflection loss (-67.62 dB). Even at 800℃, it still has the characteristics of low matching thickness (2.5 mm) and high efficiency absorption (3.2 GHz) in the X-band (8-12 GHz).

[0036] (3) The aerogel prepared by the present invention has achieved a thermal gradient of more than 1000 ℃ by optimizing the structure and composition.

[0037] (4) The preparation process of the present invention is simple and has low energy consumption. It realizes the simplified preparation of SiC-based fiber aerogels with complex structures and two components, and provides a new idea for the cross-scale design of stealth materials under high temperature and multi-physics coupling conditions. Attached Figure Description

[0038] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0039] Figure 1 An optical photograph of the stacked Cf / RGO layers provided by this invention.

[0040] Figure 2 Photos of Cf before and after immersion in RGO aqueous solution provided by this invention.

[0041] Figure 3 This is a SEM image of Cf vapor-phase silicon infiltration of Comparative Example 2 of the present invention without RGO impregnation.

[0042] Figure 4 The images show the results of vapor-phase silica infiltration of siloxane dry gel provided by the present invention; wherein, (a) is an optical image of siloxane dry gel before and after the vapor-phase silica infiltration reaction, and (b) is the XRD pattern of the product after the reaction.

[0043] Figure 5 LHSNA photographs of different shapes and sizes provided for this invention.

[0044] Figure 6 The image shows a load-bearing demonstration of the LHSNA provided by this invention.

[0045] Figure 7 SEM images of the hollow skeleton provided by this invention at different stages of preparation.

[0046] Figure 8 The EDS elemental distribution diagram of the SiC@SiO2 nanowires provided by this invention.

[0047] Figure 9 The σ-ε curve of LHSNA provided for this invention under 40% deformation and 50 compressions perpendicular to the lamellar plane (at 800°C).

[0048] Figure 10 The following diagrams illustrate the preparation and material composition characterization of the sample provided by this invention: (a) is a schematic diagram of sample preparation; (b) demonstrates the lightweighting of the sample; and (c) shows the XRD patterns of the products at different stages.

[0049] Figure 11 SEM and TEM images of the samples provided for this invention; wherein, (a) is a SEM image of the sample; (b)-(c) are SEM images of hollow fibers; (d)-(e) are SEM images of nanowires; (f) is the diameter distribution of nanowires; (g1)-(g2) are TEM images of SiC@SiO2 nanowires; (h1)-(h2) are TEM images of hollow SiC@SiO2 fibers; and (i1)-(i2) are TEM images of sheet-like SiC@SiO2.

[0050] Figure 12 The mechanical properties of LHSNA provided by this invention are shown in the following figures: (a) is the σ−ε curve of LHSNA under different compression deformations perpendicular to the lamellar plane; (b) is a comparison of the compressive modulus of LHSNA with other elastic ceramic aerogels; (c) is the σ−ε curve of LHSNA under 100 compressions with 40% deformation perpendicular to the lamellar plane; (d) is the stress and modulus variation curve of LHSNA under 100 compressions with 40% deformation perpendicular to the lamellar plane; (e) is the stress variation curve of LHSNA under 50 compressions with 40% deformation perpendicular to the lamellar plane (at 800℃); (f) is the σ−ε curve of LHSNA under compression parallel to the lamellar plane; (g) is an optical photograph of the compression rebound of LHSNA on the surface of a butane torch flame (1000℃); and (h) is an optical photograph of the compression rebound of LHSNA on the surface of liquid nitrogen (-196℃).

[0051] Figure 13 The diagram shows the radar stealth performance of the LHSNA provided by this invention; (a)-(c) are the microwave reflection loss curves of the LHSNA; (d) is a comparison of the microwave absorption performance of the LHSNA with SiC-based materials of different structures; (e) is the 3D radar wave scattering pattern of the LHSNA; (f) is the RCS curve of the LHSNA in the range of -90° to 90°; (g)-(h) are the microwave reflection loss curves of the LHSNA in the range of 8-12 GHz at 800 ℃; and (i) is the effective absorption bandwidth of the LHSNA in the range of 8-12 GHz at 200 ℃, 400 ℃, 600 ℃, and 800 ℃.

[0052] Figure 14 The following figures illustrate the infrared stealth performance of the LHSNA provided by this invention: (a) shows the infrared emissivity of the LHSNA at different temperatures; (b) shows the thermal conductivity of the LHSNA at different temperatures; (c) shows the temperature curve of the LHSNA at approximately 1100 °C; (d) shows the thermal imaging of the LHSNA at 100 °C to 300 °C; (e) shows the thermal imaging of the LHSNA at 1100 °C for different times; and (f) shows the thermogravimetric curve of the LHSNA.

[0053] Figure 15 This is a microstructure diagram of the aerogel obtained in Comparative Example 1 of the present invention.

[0054] Figure 16 This is a microstructure diagram of the aerogel obtained in Comparative Example 4 of the present invention.

[0055] Figure 17 This is an overall appearance diagram of the aerogel obtained in Comparative Example 4 of the present invention. Detailed Implementation

[0056] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0057] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms “comprising,” “including,” “having,” or “containing” are used in this specification, indicating the presence of features, steps, operations, devices, components, and / or combinations thereof, they are open-ended terms, meaning including but not limited to. Additionally, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included within the scope of this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0058] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0059] This embodiment provides a method for preparing ceramic aerogels with multi-scale heterogeneous structures, including the following steps:

[0060] S1. Preparation of carbon source: Commercial carbon fiber (Cf), industrial-grade graphene oxide (GO), L-ascorbic acid, and deionized water were used as raw materials. First, a GO aqueous solution with a concentration of 15 mg / ml was prepared. A certain amount of ascorbic acid (mLAA:mGO = 2:1) was added to the GO aqueous solution and stirred thoroughly for 6 h to obtain a reduced graphene oxide (RGO) aqueous solution. The commercial carbon fiber was impregnated in the RGO aqueous solution and then dried in an oven at 60 °C for 48 hours to obtain the Cf / RGO composite material.

[0061] S2. Preparation of silicon source (siloxane dry gel): Methyltriethoxysilane (MTES) and dimethyldimethoxysilane (DMES) were used as silicon sources, deionized water was used as hydrolysis agent, and tetramethylammonium hydroxide (TMAOH) was used as catalyst. They were mixed in a mass ratio of 2:1:3:0.02 and stirred evenly to obtain siloxane gel. The gel was placed in an oven at 60 °C and dried for 48 hours to obtain siloxane dry aerogel.

[0062] S3, stack the Cf / RGO layers (such as...) Figure 1 The sample (as shown) was placed on top of a siloxane dry aerogel and then placed in a crucible. The crucible was placed in a high-temperature atmosphere furnace for high-temperature vapor-phase silicon infiltration at 1550 °C for 2.5 h in an Ar atmosphere, resulting in a SiC / C composite fiber material. The composite fiber material obtained in the previous step was then placed in a muffle furnace and heated to 900 °C at a rate of 2 °C / min for 3 h, ultimately yielding a layered hollow fiber framework-based SiC@SiO2 (ceramic) nanowire aerogel.

[0063] The layered hollow fiber framework-based SiC@SiO2 nanowire aerogel (LHSNA) prepared in this embodiment is formed by the in-situ conversion of carbon fibers (Cf) impregnated with reduced graphene oxide (RGO) layer by layer. A schematic diagram of its preparation is shown below. Figure 10 As shown in (a) of the figure. The preparation process mainly includes the following three stages: impregnation with RGO, vapor phase silicon infiltration, and air atmosphere heat treatment. Impregnation of Cf surface with RGO is an ingenious step of this invention (Cf before and after impregnation with RGO is as shown in the figure). Figure 2As shown in the figure, the RGO on the Cf surface first reacts with SiO and CO2, which plays an important role in maintaining the hollow fiber matrix skeleton in the later stage. In high-temperature vapor-phase silicon infiltration, siloxane aerogel pyrolyzes and releases SiO, CO2 and CO gas. RGO and Cf react with SiO as carbon sources to generate SiC, and the reaction formula is shown in formula (1). Siloxane aerogel acts as both a carbon source and a silicon source. The SiO and CO gas generated by the decomposition of siloxane aerogel undergo a continuous reaction to grow into ultra-long SiC nanowires, and the reaction formula is shown in formula (2). In addition, the CO2 gas from the pyrolysis of siloxane aerogel can also react with Cf and RGO, and the reaction formula is shown in formula (3). On the one hand, it provides a large amount of CO for reaction formula (2), and on the other hand, this reaction leads to the consumption of carbon reaction sites in Cf and RGO. This invention also discovered that powdered silica dry gel can not only serve as a silicon source, but also forms bulk SiOC ceramics after vapor-phase silicon infiltration (optical images of the silica dry gel before and after the vapor-phase silicon infiltration reaction are shown in the figure). Figure 4 As shown in (a), the XRD pattern of the product after the reaction is as follows. Figure 4 (As shown in (b)). The transformation from powdered siloxane dry gel to bulk SiOC ceramics is the result of the synergistic effect of pyrolysis chemical reaction and thermal stress-induced physical densification. This enables the preparation of multiple different products from a single reaction, greatly improving resource utilization. During the heat treatment process, not only can unreacted carbon inside the carbon fibers be removed to achieve a hollow structure, but a SiO2 layer can also be formed on the SiC surface to form a core-shell structure, achieving multiple benefits in one step.

[0064] SiO (g) + 2C (s) → SiC (s) + CO (g) Formula (1);

[0065] SiO (g) + 3CO (g) → SiC (s) + 2CO2 (g) Equation (2);

[0066] C (s) + CO2 (g) → 2CO (g) Equation (3);

[0067] like Figure 10 As shown in (b), the density of LHSNA is 17.92 mg / cm³. 3 It can be placed on peach blossoms. LHSNA can achieve the preparation of larger sizes and has the characteristics of easy processing (such as...). Figure 5 (As shown). Furthermore, LHSNA can withstand more than 1500 times its own weight without deformation; this demonstrates its lightweight and high-strength characteristics (e.g., Figure 6 (As shown). The preparation process mainly includes the following three stages: impregnation with RGO, vapor phase silicate diffusion, and heat treatment. The material composition of each stage is analyzed below. Figure 10Figure (c) shows the XRD patterns at different stages. As can be seen from the figure, after impregnation with RGO, the intensity of the amorphous peak of C near 26.5° increases significantly. After CVI, the product shows strong and sharp diffraction peaks at 35.8°, 41.6°, 60.2°, 71.9°, and 75.6°, which correspond to the (111), (200), (220), (311), and (222) crystal planes of β-SiC, respectively. The intensity of the main diffraction peaks from strong to weak is (111), (220), (311), (200), and (222), which is consistent with JCPDS 29-1129. At the same time, a diffraction peak was also observed at 33.8°, which can be attributed to the stacking faults generated on the (111) crystal plane of SiC. No diffraction peak of C was observed. After heat treatment in air atmosphere, the product showed a broad peak at 22°, indicating the formation of amorphous silicon dioxide oxide.

[0068] Microstructure analysis of the sample:

[0069] The microstructure of the sample is as follows Figure 11 As shown in (a)-(e) of the figure, the overall morphology of the sample exhibits a nanowire-filled layered structure, with an average spacing of approximately 400 μm (0.4 mm) between layers. The layered structure was first analyzed, and its microstructure is shown in the figure below. Figure 11 As shown in (b) and (c), the layered structure is composed of a hollow fiber skeleton. The hollow fibers are transformed from C fibers, with a diameter of approximately 7.3 μm and a wall thickness of approximately 1.2 μm. The hollow fibers are composed of a large number of slender fibers, and there are numerous pore structures between the slender fibers, which may be due to the growth of SiC on the surface of the C fibers. A small number of lamellar network structures exist around the hollow fibers, which are transformed from RGO. SEM images of the hollow skeleton at different stages of preparation are shown below. Figure 7 As shown, combined with the preceding material composition analysis, its morphological transformation pathway can be obtained as "solid C fibers — RGO-coated C fibers — C@SiC core-shell fibers — hollow SiC@SiO2 fibers". The nanowires in the interlayered gaps are as follows: Figure 11 As shown in (d) and (e) of the figure, the nanowires exhibit a long-range and continuous structure. Furthermore, these nanowires demonstrate excellent flexibility, exhibiting a curved shape, which is due to the bending of the ultralong single-crystal silicon carbide fibers under the influence of gravity and external forces. In addition, due to the heat treatment in an air atmosphere, the SiO2 layer formed on the surface of the SiC nanowires creates numerous connection points, transforming the virtual overlap into a solid connection. These structural features contribute to the improvement of their electromagnetic, thermal, and mechanical properties. The diameter distribution of the nanowires is shown in the figure. Figure 11As shown in (f), most of the nanowires have diameters between 200 and 250 nm, with an average diameter of 226.53 nm. To further clarify the composition of the sample, TEM analysis was performed. In the final product, SiC@SiO2 exists in three forms. First, there are the SiC@SiO2 nanowires generated by the gas-phase reaction, whose TEM and HRTEM results are shown in... Figure 11 As shown in (g1) and (g2), the EDS element distribution is as follows: Figure 8 As shown in the figure, the nanowires exhibit a significant core-shell structure, with an amorphous SiO2 layer forming on the SiC surface. The thickness of the SiO2 layer ranges from 5 to 10 nm. Secondly, hollow SiC@SiO2 fibers formed by the transformation of C fibers are also observed, and their TEM and HRTEM results are shown below. Figure 11 As shown in (h1) and (h2) in the figure, the hollow fiber is composed of a large number of long, thin fibers with pores between them. The fibers are also composed of SiC and SiO2 formed by surface oxidation, with the SiO2 layer thickness ranging from 4 to 10 nm. Finally, a layered network structure is formed by RGO conversion, and its TEM and HRTEM are shown in the figure. Figure 11 As shown in (i1) and (i2), the layered distribution and thickness of SiO2 are consistent with the two forms mentioned above. This structure is mainly due to the gas-solid growth mechanism and adhesion growth mechanism of SiC. Notably, different degrees and numbers of black shadowed regions appeared in the TEM images of both the RGO-derived SiC@SiO2 sheet structure and the SiC@SiO2 nanowires synthesized by gas-phase reaction. This is likely due to the high-density growth of SiC grains during the gas-phase silicon infiltration process. In summary, the final sample is a SiC@SiO2 nanowire aerogel with a layered hollow fiber framework (LHSNA). In conclusion, the ingenious carbon fiber template conversion method can achieve efficient structural control of SiC fiber-based aerogels. Its preparation process is simple, energy-efficient, and simplifies the preparation of complex structures and two-component SiC-based fiber aerogels.

[0070] Mechanical property analysis of LHSNA:

[0071] Thanks to its microstructure, LHSNA exhibits excellent mechanical properties. For example... Figure 12 As shown in (a), LHSNA exhibits good resilience perpendicular to the lamellar plane, achieving 100% resilience at 60% compressive strain. Figure 12(a) shows that the compressive stress-strain (σ-ε) curve of LHSNA is a closed-loop curve, which can be divided into three stages. The first stage is the initial linear stage, i.e., under compressive strain below 40%. The second stage is the transition region, where the compressive stress and stress growth rate gradually increase between 40% and 60% strain. In this stage, the nanowires in the interlayer gaps are compressed and come into close contact, leading to an increase in the stress growth rate. The third stage is the densification stage (ε>60%), where the σ value shows a sharp increase. This is due to the gradual reduction of the interlayer gaps by external force, resulting in the densification of the pore structure. Simultaneously, fluctuations in the σ-ε curve are observed when the compressive strain is 80%, which may be due to the rupture of the hollow fiber skeleton under high compression. The compressive modulus of LHSNA along the layered direction, calculated from the linear elastic system, is 49.2±3.1 Kpa, which is higher than the compressive modulus of elastic ceramic aerogels. Figure 12 As shown in (b) in the diagram. Figure 12 As shown in (c) and (d), NLSA exhibits good fatigue resistance in LHSNA perpendicular to the lamellar plane. Under 100 cycles of 40% compressive deformation, its stress and modulus can recover to 83% of their initial values. The nanowire-filled lamellar hollow fiber skeleton structure design allows the nanowires to undergo flexible deformation in the interlayer gaps when the aerogel is subjected to external forces, thereby effectively dispersing and absorbing external energy. At the same time, the gaps in the lamellar structure provide sufficient space for the nanowires to maintain structural stability during deformation, further improving the energy dissipation efficiency of the material. Therefore, this unique structural design not only endows the aerogel with good elasticity, enabling it to maintain stable performance after multiple cyclic loading, but also significantly improves its fatigue resistance, allowing it to maintain excellent mechanical properties even under extreme conditions. LHSNA also exhibits good compressive properties under high temperature conditions. Under 50 cycles of compression at 800℃ with 40% deformation, its stress value can recover to 85% of its initial value. Its σ-ε curve and stress change curve are shown in Figures 1-2. Figure 9 and Figure 12 As shown in (e) in the figure. The anisotropic structure gives LHSNA excellent mechanical properties along the lamellar plane, as shown in its σ-ε curve. Figure 12 As shown in (f) of the figure, when ε is 1.7%, the stress reaches 119.1 kPa and the modulus reaches 7005.9 kPa. This is because the nanowires and hollow fiber layers form a continuous reinforcing network, and their high intrinsic modulus directly improves the in-plane load-bearing capacity; secondly, the layered confinement of the hollow fiber skeleton effectively restricts the spatial degree of freedom of the nanowires, suppresses in-plane buckling instability, and achieves efficient stress transfer through the cross-linking of hollow fibers and nanowires. This results in the LHSNA having extremely high stiffness along the layered plane. 32When ε is 1.7%, the outer edge of the LHSNA begins to bend, and the stress growth rate decreases. When ε is 7.8%, the LHSNA fractures. To further investigate the mechanical properties of the LHSNA under harsh conditions, compression tests were conducted on the NLSA at 50% deformation on the outer flame surface of a butane torch (1000℃) and on a liquid nitrogen surface (-196℃). The experimental results show that after removing the external stress, the LHSNA can recover to its original geometry, as shown in the figure. Figure 12 As shown in (g) and (h) in the figure. In summary, LHSNA exhibits excellent mechanical properties in different dimensions and under different environments.

[0072] Electromagnetic wave absorption performance of LHSNA:

[0073] LHSNA's electromagnetic wave absorption performance perpendicular to the layered direction, such as Figure 13 As shown in (a)-(c) of the figure, when the matching thickness is 1.55 mm, the LHSNA achieves a minimum reflection loss (RLmin) of -67.62 dB at 16.12 GHz. When the matching thickness is 1.8 mm, its maximum effective absorption bandwidth (EABmax) is 5.46 GHz. The LHSNA prepared in this invention is compared with other SiC-based materials with different structures in terms of matching thickness, effective absorption bandwidth, and minimum reflection loss, as shown in the figure. Figure 13 As shown in (d) of the figure, the present invention has significant comprehensive advantages, demonstrating that the LHSNA exhibits strong absorption loss (-67.62 dB) and wideband absorption (5.46 GHz) at low fill ratio (5%) and low matching thickness (1.8 mm). To further demonstrate the practical application value of the LHSNA prepared in this study, Radar Cross Section (RCS) simulations were performed on the sample at 16.12 GHz using CST Studio Suite 2023 software to obtain its true electromagnetic wave absorption performance (the added sample thickness was 1.55 mm). The results are shown in the figure. Figure 13 As shown in (e)-(f) of the figure, the addition of an LHSNA with a thickness of only 1.55 mm can significantly attenuate the RCS signal. Compared with the RCS value of a pure PEC conductive layer, the RCS value after adding the LHSNA can be reduced by 20 dB·m². It is noteworthy that the average RCS value after adding the LHSNA can be completely reduced to below -10 dB·m² in the range of θ from -60° to 60°, indicating that the LHSNA has excellent radar wave stealth capability. Microwave absorption performance at high temperatures has always been a key and challenging research topic. The microwave absorption performance of the LHSNA at 800 °C is shown in the figure. Figure 13As shown in (g)-(h), when the matching thickness is 2.5 mm, the HFSNA achieves an EABmax of 3.2 GHz within the 8-12 GHz range at 800 ℃. Figure 13 As shown in (i), the LHSNA achieves an effective absorption bandwidth of at least 3 GHz in the 8-12 GHz range at temperatures of 200℃, 400℃, 600℃, and 800℃, and the effective absorption bandwidth shifts to lower frequencies with increasing temperature. These results indicate that the LHSNA is temperature insensitive and exhibits excellent microwave absorption performance over a wide temperature range.

[0074] LHSNA's infrared stealth capabilities:

[0075] The synergistic design of multi-scale, multi-component structures and dual-component structures (SiC, SiO2) not only contributes to the material's excellent mechanical and microwave absorption properties but also enhances its infrared stealth performance. The infrared stealth performance of the material is related to its temperature and infrared emissivity, requiring both a reduction in infrared emissivity and an improvement in its thermal insulation properties. First, its infrared emissivity at different temperatures was examined, such as... Figure 14 As shown in (a) of the figure, the infrared emissivity of LHSNA increases with increasing temperature. This may be because the increased thermal motion of free electrons in the material at higher temperatures may increase the interaction between electrons and electromagnetic waves, thereby enhancing the absorption and emission of infrared light. Thermal conductivity is an important indicator for measuring the thermal insulation performance of materials. Figure 14 Figure (b) shows the thermal conductivity of LHSNA in the vertical layering direction at different temperatures (room temperature is air atmosphere, high temperature is argon atmosphere). As can be seen from the figure, the solid-state thermal conductivity of LHSNA in the vertical layering direction is 20 mW•m⁻¹•k⁻¹, which is lower than the thermal conductivity of air (26 mW•m⁻¹•k⁻¹), and its thermal conductivity at 1200 ℃ is only 101 mW•m⁻¹•k⁻¹. In order to further investigate its thermal protection performance under extreme conditions, the ultra-high temperature infrared stealth performance under ultra-high temperature conditions was investigated simultaneously using a high-temperature thermocouple and an infrared thermal imager. Figure 14 Figure (c) shows the temperature distribution curves on both sides of the sample. As can be seen, when a high temperature of approximately 1100 °C is applied to the side perpendicular to the layered structure plane for 10 min, with a sample thickness of 2.5 cm, the temperature on the other side using LHSNA is only about 75 °C, resulting in a temperature gradient greater than 1000 °C. Infrared thermal imaging of LHSNA at 100–300 °C perpendicular to the layered direction is shown below. Figure 14As shown in (d) (sample thickness 1 cm), it exhibits excellent infrared stealth performance in the range of 100–300 °C. At 300 °C, the surface radiation temperature is only 106 °C, reducing radiation to one-third of the ambient temperature. LHSNA applied infrared thermal imaging at approximately 1100 °C for different times perpendicular to the layered direction, as shown in [image / data]. Figure 14 As shown in (e), when the time is 10 min, the infrared radiation temperature of the heated surface is approximately 1120 ℃, and the temperature of the back surface is approximately 55 ℃. This has some error compared to the results of the thermocouple test mentioned earlier. This is because the thermocouple test measures the surface temperature of the material, while the infrared radiation temperature of the material is also related to its emissivity, which helps to reduce the infrared radiation temperature. Excellent thermal stability is also an essential condition for the material to be used in high-temperature environments. Figure 14 Figure (f) shows the thermogravimetric curve of LHSNA in air atmosphere. As can be seen from the figure, its weight gain is only 3% at 1200 °C. To further investigate the high-temperature resistance of LHSNA, ultra-high temperature thermal ablation experiments were conducted on it, such as... Figure 14 As shown in (f) in the figure, it can be seen that under the conditions of approximately 1200 °C and single-sided ablation for 60 min, no significant changes were found in its appearance and size, and the sample weight gain and linear shrinkage rate were both less than 0.5%.

[0076] In summary, this invention successfully developed a layered hollow fiber framework-based SiC@SiO2 nanowire aerogel (LHSNA) with high-temperature resistance, mechanical strength, and infrared-radar compatible stealth properties through a multi-scale heterogeneous structure design strategy. A simple and efficient preparation of the multi-component structure (layered + hollow fiber + nanowire + core-shell structure) and dual-component (SiC + SiO2) was achieved by combining a simple high-temperature vapor-phase silicon infiltration technique with a heat treatment process. This resulted in a lightweight material system (density 17.92 mg / cm³) with excellent mechanical properties (excellent compression resilience and fatigue resistance) and a high operating temperature (1200 ℃). Through core-shell heterogeneous interface polarization loss and loss enhancement of the multi-component structure, LHSNA achieved an effective absorption bandwidth of 5.46 GHz at room temperature with a thickness of 1.8 mm; at high temperatures (800 ℃), it still maintained an effective absorption bandwidth of 3.2 GHz in the X-band (thickness 2.5 mm). Through the heat loss of the multi-component structure and the phonon scattering effect of the heterogeneous interface, LHSNA exhibits excellent infrared stealth performance at high temperatures, achieving a thermal gradient exceeding 1000℃ at 1100℃ (with a thickness of 2.5 cm). The "mechanical-thermal-electromagnetic" synergistic optimization strategy proposed in this work is of great significance for the development of stealth protection materials for hot-end components in aerospace, and contributes to the development of novel lightweight, high-performance structurally compatible stealth materials.

[0077] Comparative Example 1

[0078] S1. Preparation of carbon source: Commercial carbon fiber (Cf), industrial-grade graphene oxide (GO), L-ascorbic acid, and deionized water were used as raw materials. First, a GO aqueous solution with a concentration of 15 mg / ml was prepared. A certain amount of ascorbic acid (mLAA:mGO = 2:1) was added to the GO aqueous solution and stirred thoroughly for 6 h to obtain a reduced graphene oxide (RGO) aqueous solution. The commercial carbon fiber was impregnated in the RGO aqueous solution and then dried in an oven at 60 °C for 48 hours to obtain the Cf / RGO composite material.

[0079] S2. Preparation of silicon source (siloxane dry gel): Methyltriethoxysilane (MTES) and dimethyldimethoxysilane (DMES) were used as silicon sources, deionized water was used as hydrolysis agent, and tetramethylammonium hydroxide (TMAOH) was used as catalyst. They were mixed in a mass ratio of 2:1:3:0.02 and stirred evenly to obtain siloxane gel. The gel was placed in an oven at 60 °C and dried for 48 hours to obtain siloxane dry aerogel.

[0080] S3. Place the stacked Cf / RGO layers onto a siloxane dry aerogel and then place it in a crucible. Place the crucible in a high-temperature atmosphere furnace for high-temperature vapor-phase silicon infiltration at 1550 °C for 2.5 h in an Ar atmosphere to obtain SiC / C composite fiber material. Place the composite fiber material obtained in the previous step in a muffle furnace and heat it to 600 °C at a rate of 2 °C / min for 3 h.

[0081] In this comparative example, the heat treatment temperature in the air atmosphere was changed to 600℃, resulting in a layered hollow fiber framework-based SiC nanowire aerogel, the microstructure of which is as follows: Figure 15 As shown.

[0082] Comparative Example 2

[0083] S1. Preparation of carbon source: Commercial carbon fiber (Cf) is used as carbon source.

[0084] S2. Preparation of silicon source (siloxane dry gel): Methyltriethoxysilane (MTES) and dimethyldimethoxysilane (DMES) were used as silicon sources, deionized water was used as hydrolysis agent, and tetramethylammonium hydroxide (TMAOH) was used as catalyst. They were mixed in a mass ratio of 2:1:3:0.02 and stirred evenly to obtain siloxane gel. The gel was placed in an oven at 60 °C and dried for 48 hours to obtain siloxane dry aerogel.

[0085] S3. Place the stacked Cf / RGO layers onto a siloxane dry aerogel and then place it in a crucible. Place the crucible in a high-temperature atmosphere furnace for high-temperature vapor-phase silicon infiltration at 1550 °C for 2.5 h in an Ar atmosphere, thereby obtaining a SiC / C composite fiber material. Place the composite fiber material obtained in the previous step into a muffle furnace and heat it to 900 °C at a rate of 2 °C / min for 3 h.

[0086] The results of this comparative example are as follows: Figure 3 As shown in the figure, the carbon source is directly commercial carbon fiber (Cf) without impregnation with RGO. Although nanowire structures can be formed, the fiber structure is fragile. After vapor-phase silicon infiltration, the product will have difficulty maintaining its continuous and complete fiber structure.

[0087] Comparative Example 3

[0088] S1. Preparation of carbon source: Commercial carbon fiber (Cf), industrial-grade graphene oxide (GO), L-ascorbic acid, and deionized water were used as raw materials. First, a GO aqueous solution with a concentration of 15 mg / ml was prepared. A certain amount of ascorbic acid (mLAA:mGO = 2:1) was added to the GO aqueous solution and stirred thoroughly for 6 h to obtain a reduced graphene oxide (RGO) aqueous solution. The commercial carbon fiber was impregnated in the RGO aqueous solution and then dried in an oven at 60 °C for 48 hours to obtain the Cf / RGO composite material.

[0089] S2. Preparation of silicon source: Silicon powder is used as the silicon source.

[0090] S3. Place the stacked Cf / RGO layers onto a siloxane dry aerogel and then place it in a crucible. Place the crucible in a high-temperature atmosphere furnace for high-temperature vapor-phase silicon infiltration at 1550 °C for 2.5 h in an Ar atmosphere, thereby obtaining a SiC / C composite fiber material. Place the composite fiber material obtained in the previous step into a muffle furnace and heat it to 900 °C at a rate of 2 °C / min for 3 h.

[0091] In this comparative example, silicon powder was used directly as the silicon source, and the results showed that only a small amount of nanowires were formed.

[0092] Comparative Example 4

[0093] S1. Preparation of carbon source: Commercial carbon fiber (Cf) is used as carbon source.

[0094] S2. Preparation of silicon source: Silicon powder is used as the silicon source.

[0095] S3. Place the stacked Cf / RGO layers onto a siloxane dry aerogel and then place it in a crucible. Place the crucible in a high-temperature atmosphere furnace for high-temperature vapor-phase silicon infiltration at 1550 °C for 2.5 h in an Ar atmosphere, thereby obtaining a SiC / C composite fiber material. Place the composite fiber material obtained in the previous step into a muffle furnace and heat it to 900 °C at a rate of 2 °C / min for 3 h.

[0096] In this comparative example, commercially available carbon fiber (Cf) was used as the carbon source, and silicon powder was used as the silicon source. The results showed that only a very small amount of agglomerated nanowires were formed, and their microstructure was as follows: Figure 16 As shown, the overall appearance of the sample is as follows: Figure 17 As shown.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing ceramic aerogels with multi-scale heterogeneous structures, characterized in that, Includes the following steps: S1. Add ascorbic acid to the aqueous solution of graphene oxide to obtain an aqueous solution of reduced graphene oxide. Impregnate carbon fibers in the aqueous solution of reduced graphene oxide and dry to obtain Cf / RGO composite material. S2. Using methyltriethoxysilane and dimethyldimethoxysilane as silicon sources, deionized water as a hydrolyzing agent, and tetramethylammonium hydroxide as a catalyst, the mixture is stirred evenly to obtain a siloxane gel. The siloxane gel is then dried to obtain a siloxane dry aerogel. The mass ratio of methyltriethoxysilane, dimethyldimethoxysilane, deionized water, and tetramethylammonium hydroxide is (1~3):(0.5~1.5):(1.5~4.5):(0~0.05). S3. The stacked Cf / RGO composite material is placed on a siloxane dry aerogel and subjected to high-temperature vapor-phase silicon infiltration. The high-temperature vapor-phase silicon infiltration is carried out in an inert gas atmosphere at a temperature of 1500~1600℃ for 2~3h to obtain SiC / C composite fiber material. The SiC / C composite fiber material is then subjected to air atmosphere heat treatment. The air atmosphere heat treatment is performed by heating to 800~1000℃ at a rate of 1~3℃ / min for 2~4h to obtain layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel.

2. The preparation method according to claim 1, characterized in that, Step S1 includes: S11. Dissolve graphene oxide in deionized water to obtain an aqueous solution of graphene oxide; S12. Add ascorbic acid to the aqueous solution of graphene oxide and stir thoroughly to obtain a reduced aqueous solution of graphene oxide. S13. The carbon fiber is impregnated in a reduced graphene oxide aqueous solution and dried in an oven at 50~70℃ to obtain the Cf / RGO composite material.

3. The preparation method according to claim 1, characterized in that, The concentration of the graphene oxide aqueous solution is 10~20 mg / ml; the mass ratio of ascorbic acid to the graphene oxide aqueous solution is (1.5~2.5):

1.

4. The preparation method according to claim 1, characterized in that, Step S2 includes: S21. Mix methyltriethoxysilane, dimethyldimethoxysilane, deionized water, and tetramethylammonium hydroxide and stir until homogeneous to obtain siloxane gel. S22. Place the siloxane gel in an oven at 50~70℃ to dry it, and obtain siloxane dry aerogel.

5. The preparation method according to claim 1, characterized in that, Step S3 includes: S31. Place the stacked Cf / RGO composite material on top of the siloxane dry aerogel and place it in a crucible. Place the crucible in a high-temperature atmosphere furnace and perform high-temperature vapor phase silicon infiltration to obtain SiC / C composite fiber material. S32. The SiC / C composite fiber material is placed in a muffle furnace for air atmosphere heat treatment to obtain layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel.

6. A ceramic aerogel with a multi-scale heterogeneous structure, characterized in that, The ceramic aerogel is prepared by the preparation method according to any one of claims 1-5. The ceramic aerogel is a layered hollow fiber skeleton-based SiC@SiO2 nanowire aerogel with an average spacing of 350~450μm between each layer and an average diameter of 200~250 nm for the nanowires. The nanowires exhibit a core-shell structure, and an amorphous SiO2 layer is formed on the SiC surface with a thickness of 5~10 nm.

7. The application of the ceramic aerogel with multi-scale heterogeneous structure as described in claim 6 in the manufacture of spacecraft, ships, and steam turbines.

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