Bamboo-like silicon carbide nanowire based on freeze-dried vegetables / fruits as well as preparation method and application of bamboo-like silicon carbide nanowire

Abstract

The invention belongs to the technical field of preparation of high-temperature heat-insulating and electromagnetic wave-absorbing materials, and relates to a bamboo-like silicon carbide nanowire based on freeze-dried vegetables / fruits and a preparation method and application of the bamboo-like silicon carbide nanowire based on the freeze-dried vegetables / fruits, and the preparation method comprises the following steps: putting the vegetables / fruits in a freeze dryer to obtain the freeze-dried vegetables / fruits; putting the freeze-dried vegetables / fruits into a high-temperature tubular furnace for carbonization to obtain a carbon template; putting the carbon template and a silicon source into a graphite crucible, putting the graphite crucible into a high-temperature tubular furnace, and calcining to obtain the bamboo-like silicon carbide nanowire based on the freeze-dried vegetables / fruits. The bamboo-like silicon carbide nanowire prepared by the method has excellent high-temperature heat insulation property and remarkable broadband electromagnetic wave absorption property, so that the enhancement of double functions of heat insulation and wave absorption is synergistically realized on the basis of a single material. The preparation process is simple and stable in flow, good in repeatability, green in raw material and extremely low in cost, and the problems of collapse and atrophy of the material structure caused by high temperature are solved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high-temperature thermal insulation and electromagnetic wave absorbing material preparation, and particularly relates to a bamboo-shaped silicon carbide nanowire based on freeze-dried vegetables / fruits and a preparation method and application thereof. BACKGROUND

[0002] With the continuous exploration of extreme fields, the demand for high-temperature thermal insulation materials is increasing. In the fields of aerospace, national defense and military industry, the demand for high-temperature thermal insulation materials in extreme high-temperature places is becoming more and more demanding. With the development of 5G communication technology, the use of intelligent devices, and the development of wireless technology, electromagnetic pollution is increasing. Solving the problem of electromagnetic pollution and the important application of electromagnetic absorbing materials in the field of national defense and military requires the research of high-performance electromagnetic absorbing materials. Therefore, the research of high-temperature thermal insulation and electromagnetic absorbing materials with excellent performance has received widespread attention.

[0003] Silicon carbide nanowire materials have low density, low thermal conductivity, good high-temperature stability, chemical stability, corrosion resistance, oxidation resistance, and excellent dielectric properties, which stand out among ceramic materials. Traditional preparation methods of silicon carbide nanowires result in material structure collapse and shrinkage due to high temperature, and the high cost limits their large-scale preparation and application. In order to reduce the manufacturing cost, Chinese patent CN114249323A discloses a method for preparing SiC nanowires by using food waste (such as cereals and beans) as a carbon source through high-temperature carbon thermal reduction reaction. Although this route has significant advantages in raw material cost and environmental protection, and although it improves the problem of material structure collapse and shrinkage caused by high temperature to some extent, this method can only obtain conventional smooth linear morphology, and cannot form special structures with higher interface effect and defect density, making it difficult to meet the stringent demands of high-temperature insulation performance and wave absorption performance in extreme high-temperature places.

[0004] Therefore, how to improve such biomass template method process, while maintaining its green and low-cost core advantages, through process innovation to effectively regulate the micro-morphology of silicon carbide nanowires, so as to prepare silicon carbide nanowire materials with complete structure, regular morphology, excellent high-temperature thermal insulation performance and wide-band high-efficiency electromagnetic wave absorption performance, has become an urgent and challenging topic in the field. SUMMARY

[0005] In order to overcome the shortcomings of the prior art, the application provides a bamboo joint-shaped silicon carbide nanowire based on freeze-dried vegetables / fruits and a preparation method and application thereof, wherein natural and sustainable biomass vegetables / fruits are used as raw materials, the natural porous structure is completely retained as a template through freeze-drying, an innovative silicon source system is introduced, and the bamboo joint-shaped silicon carbide nanowire with excellent high-temperature heat insulation and electromagnetic wave absorption performance is prepared, the joint of the bamboo joint-shaped structure is used as a mechanical support point to enhance the stability of the three-dimensional network structure and avoid the problem of porosity reduction caused by structure collapse at high temperature. The method is high in cost-effectiveness, simple in operation, friendly to the environment and resource-saving.

[0006] In order to achieve the above-mentioned application purposes, the application provides the following technical solutions: The application provides a preparation method of a bamboo joint-shaped silicon carbide nanowire based on freeze-dried vegetables / fruits, which comprises the following steps: (1) freeze-drying vegetables / fruits in a freeze-drying machine to obtain freeze-dried vegetables / fruits; (2) carbonizing the freeze-dried vegetables / fruits in a high-temperature tube furnace to obtain a carbon template; (3) mixing tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol according to a mass ratio of 80-200:5-20:5-10:10-50:10-30, stirring and drying to obtain a silicon source; the stirring time is 24-72 hours, and the drying time is 12-24 hours.

[0007] (4) mixing the carbon template and the silicon source according to a mass ratio of 1-5:1-30, uniformly mixing in a graphite crucible, and then calcining in a high-temperature tube furnace at 1100-1500 DEG C for 3-8 hours, and cooling to room temperature to obtain the bamboo joint-shaped silicon carbide nanowire based on freeze-dried vegetables / fruits.

[0008] Preferably, the vegetables / fruits are one or more than two of eggplant, shiitake mushroom, mango, strawberry, purple sweet potato, carrot, fig, apple, juicy peach and pumpkin. Some common vegetables / fruits with low cost, environmental friendliness and sustainable regeneration, such as eggplant, shiitake mushroom, mango, strawberry, purple sweet potato, carrot, fig, apple, juicy peach and pumpkin, are selected as carbon sources, a large amount of free water and bound water in the substances is removed through a freeze-drying machine, the density of the substances is reduced, the frame structure of the substances is maintained without collapse, Preferably, the calcination environment is carried out in an inert atmosphere, the heating rate is 1-10 DEG C / min, and the cooling rate is 1-10 DEG C / min. The inert atmosphere is a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

[0009] Preferably, the mass ratio of the tetraethoxysilane, the silicon dioxide, the expandable graphite, the deionized water and the anhydrous ethanol is 100-200:5-15:5-9:30-50:20-30.

[0010] Preferably, the carbonization temperature is 550-800℃, the carbonization time is 1-2 hours, the temperature rising rate is 1-10℃ / min, and the calcination environment is in an inert atmosphere.

[0011] Preferably, the freeze-drying time is 24-72 hours, and the freeze-drying temperature is-50--80℃.

[0012] The second aspect of the present application provides the freeze-dried vegetable / fruit-based bamboo-shaped silicon carbide nanowires prepared by the above preparation method.

[0013] The third aspect of the present application provides the application of the freeze-dried vegetable / fruit-based bamboo-shaped silicon carbide nanowires in the preparation of materials with high-temperature thermal insulation performance and wide-frequency high-efficiency electromagnetic wave absorption performance.

[0014] The application provides a preparation method of a bamboo-shaped silicon carbide nanowire by selecting a vegetable / fruit and silicon source system with natural porous network and surface heterogeneity. The natural porous network and surface heterogeneity of the selected vegetables / fruits (eggplant, shiitake mushroom, mango, strawberry, purple sweet potato, carrot, fig, apple, juicy peach, pumpkin) provide spatial constraints and nucleation sites for the bamboo-shaped structure. The micrometer-sized hollow channels of the carbonized vegetables / fruits can guide the directional diffusion of gaseous intermediates such as SiO and CO along the channels, so that the silicon carbide nanowire preferentially grows along the channel axis; and the unevenness of the inner wall of the channel can cause local airflow disturbance, so that the silicon carbide nanowire appears "kink" during the growth process, forming a bamboo-shaped morphology. During pyrolysis, a large number of defects (such as carbon vacancies and edge dangling bonds) are generated on the surface, and these defects act as high-energy nucleation sites, which can make the silicon carbide nanowire preferentially precipitate at the defects; when the carbon source in a region is consumed, the gaseous intermediates continue to diffuse and nucleate at adjacent defects, forming a bamboo structure with alternating nodes and sections. The carbon source formed by mixing, stirring and drying of tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol generates gaseous intermediates (SiO and CO) during the reaction, and the concentration fluctuation of the gaseous intermediates is the chemical driving force for the formation of the bamboo-shaped structure. The reaction between the silicon source and the carbon template is not a continuous and stable process, but a dynamic balance between generation and consumption. For example, during the reaction of silicon dioxide to generate SiO, local temperature fluctuations can cause the partial pressure of SiO to rise periodically, and when the partial pressure reaches supersaturation, the silicon carbide nanowire rapidly nucleates and grows to form a node; then, the partial pressure of SiO decreases due to consumption, and the growth rate slows down to form the inter-nodal thin part, and finally the bamboo-shaped structure is formed. CO is a byproduct of carbon thermal reduction, and when the concentration of CO is high, the reaction rate increases, and the silicon carbide easily accumulates at the top of the nanowire to form a node; when the concentration of CO decreases, the reaction rate decreases, and the nanowire grows slowly along the axis to form the inter-nodal part, further enhancing the bamboo-shaped characteristics. Finally, the bamboo-shaped silicon carbide nanowire is generated.

[0015] In the three-dimensional network formed by the interweaving of bamboo-like silicon carbide nanowires, micro-nano multi-level pores are formed due to the size difference between the nodes and the inter-nodes: the size of these pores is much smaller than the average free path of air molecules, and the collision of air molecules is significantly inhibited, and the gas heat conduction is greatly reduced. The large micropores inherited by the freeze-dried vegetable / fruit-based carbon template and the nanoscale pores formed by the bamboo-like nanowires synergistically extend the heat conduction path, making it difficult for heat to be quickly transmitted through gas. The node region of the bamboo-like structure has a large number of defects and heterojunction interfaces, which is the key to inhibiting solid heat conduction. The node region of the bamboo-like silicon carbide nanowire is the main accumulation area of stacking defects, which can destroy the integrity of the crystal lattice and cause multiple scattering of phonons during propagation. The surface of the bamboo-like silicon carbide nanowire is usually wrapped in 2-5 nm of amorphous SiO2, and in the node region, the thickness of the SiO2 layer increases due to the rapid accumulation of silicon carbide nanowires, forming a SiC / SiO2 heterojunction interface; due to the large difference in phonon spectrum between SiC and SiO2, phonons will be strongly reflected and scattered at the interface, further inhibiting solid heat conduction. The network structure formed by the interweaving of bamboo-like silicon carbide nanowires has high porosity and low packing density: low packing density means that the proportion of solid skeleton in the material is low, and the path of heat conduction through the solid is reduced; at the same time, high porosity makes the air content in the material high, and the thermal conductivity of air is much lower than that of SiC, further reducing the total thermal conductivity. The node of the bamboo-like structure acts as a mechanical support point, enhancing the stability of the network structure and avoiding the decrease in porosity due to the collapse of the structure at high temperatures, thereby maintaining long-term thermal insulation performance. Therefore, the bamboo-like silicon carbide nanowires prepared by this method have excellent high-temperature thermal insulation performance.

[0016] The difference between the node and the inter-node of the bamboo-shaped structure of the silicon carbide nanowire and the defects form a large number of polarization centers, which are the core enhancement mechanism of the wave absorption performance. There are differences in morphology and dielectric constant between the node (thick diameter) and the inter-node (thin diameter) of the bamboo-shaped silicon carbide nanowire, forming a large number of heterogeneous interfaces; at the same time, the SiO2 layer on the surface of the node and the SiC core layer form the SiC / SiO2 interface, which will cause the accumulation of electric charges at the interface, resulting in strong interface polarization. The stacking defects of the node of the bamboo-shaped silicon carbide nanowire will destroy the charge distribution balance of SiC, forming a large number of inherent dipoles (such as Si vacancies and C dangling bonds); under the action of electromagnetic waves, these dipoles undergo orientation polarization, consuming electromagnetic wave energy. The porous network formed by the interweaving of the bamboo-shaped silicon carbide nanowire can adjust the dielectric constant of the material and improve the impedance matching, so that more electromagnetic waves enter the interior of the material. The porous structure can reduce the effective dielectric constant of the material and reduce the reflection of electromagnetic waves on the surface of the material. The multi-level pores (micropores-nanopores) in the network of the bamboo-shaped silicon carbide nanowire provide multiple reflection-scattering paths for electromagnetic waves: after entering the material, electromagnetic waves undergo multiple reflections in micropores and are scattered by the bamboo-shaped silicon carbide nanowire in nanopores, prolonging the propagation path of electromagnetic waves in the material and allowing the energy to be fully absorbed. The network structure and crystal characteristics of the bamboo-shaped silicon carbide nanowire can synergistically enhance the conductive loss while ensuring the stability of the wave absorption at high temperatures. The inter-node part of the bamboo-shaped silicon carbide nanowire is a continuous crystal with good electrical conductivity, which can form a conductive network; under the action of electromagnetic waves, free electrons move along the network to generate Joule heat, consuming electromagnetic wave energy. The high crystallinity and SiO2 protective layer of the bamboo-shaped silicon carbide nanowire make it remain stable in structure and dielectric performance at high temperatures. Therefore, the bamboo-shaped silicon carbide nanowire significantly improves the electromagnetic wave absorption performance by enhancing polarization loss, optimizing impedance matching and prolonging the electromagnetic wave path.

[0017] The advantages and beneficial effects of the present application are: (1) The bamboo-shaped silicon carbide nanowire prepared by the present application has excellent high-temperature heat insulation property (back surface temperature < 260℃ under 1300℃ burning) and significant broadband electromagnetic wave absorption performance (the highest effective absorption bandwidth reaches 6.99GHz), thereby synergistically realizing the dual functions of heat insulation and wave absorption on a single material. It can be applied to strategic fields such as aerospace, national defense and military industry, which have extremely strict requirements for material performance.

[0018] (2) The raw materials are green and the cost is extremely low, the reaction temperature is significantly reduced, the problem of material structure collapse and shrinkage caused by high temperature is solved, and the process flow is simple, stable and good in repeatability. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 XRD pattern of SiCnws-1; Figure 2Infrared power plot for SiCnws-1; Figure 3 SEM plot of SiCnws-1 at 1000x magnification; Figure 4 SEM plot of SiCnws-1 at 10000x magnification; Figure 5 Infrared image of SiCnws-1; Figure 6 Reflection loss plot of SiCnws-1 at 2-18 GHz frequency; Figure 7 XRD plot of SiCnws-2; Figure 8 Infrared power plot for SiCnws-2; Figure 9 SEM plot of SiCnws-2 at 1000x magnification; Figure 10 SEM plot of SiCnws-2 at 10000x magnification; Figure 11 Infrared image of SiCnws-2; Figure 12 Reflection loss plot of SiCnws-2 at 2-18 GHz frequency; Figure 13 XRD plot of SiCnws-3; Figure 14 Infrared power plot for SiCnws-3; Figure 15 SEM plot of SiCnws-3 at 1000x magnification; Figure 16 SEM plot of SiCnws-3 at 10000x magnification; Figure 17 Infrared image of SiCnws-3; Figure 18 Reflection loss plot of SiCnws-3 at 2-18 GHz frequency; Figure 19 XRD plot of SiCnws-4; Figure 20 Infrared power plot for SiCnws-4; Figure 21 SEM plot of SiCnws-4 at 1000x magnification; Figure 22 SEM plot of SiCnws-4 at 10000x magnification; Figure 23 Infrared image of SiCnws-4; Figure 24Reflection loss plot of SiCnws-4 at 2-18 GHz frequency; Figure 25 SEM plot of SiCnws-5 amplified 10000 times; Figure 26 Infrared power plot of SiCnws-5; Figure 27 SEM plot of SiCnws-6 amplified 10000 times; Figure 28 Infrared power plot of SiCnws-6; Figure 29 SEM plot of SiCnws-7 amplified 10000 times; Figure 30 Infrared power plot of SiCnws-7; Figure 31 SEM plot of SiCnws-8 amplified 10000 times; Figure 32 Infrared power plot of SiCnws-8. DETAILED DESCRIPTION

[0020] The present application provides a preparation method of bamboo joint-like silicon carbide nanowires based on freeze-dried vegetables / fruits, comprising the following steps: (1) freeze-drying vegetables / fruits in a freeze dryer to obtain freeze-dried vegetables / fruits; (2) carbonizing the freeze-dried vegetables / fruits in a high-temperature tube furnace to obtain a carbon template; (3) putting the carbon template and silicon source with a mass ratio of 1-5:1-30 into a graphite crucible and mixing uniformly, and then putting into a high-temperature tube furnace for calcination at 1100-1500 ℃ for 3-8 hours to obtain bamboo joint-like silicon carbide nanowires based on freeze-dried vegetables / fruits; In the present application, all the preparation raw materials are commercially available products well known to those skilled in the art, unless otherwise specified.

[0021] In the present application, the vegetables / fruits preferably include eggplant, shiitake mushroom, mango, strawberry, purple sweet potato, carrot, fig, apple, juicy peach, and pumpkin. In the embodiments of the present application, the vegetables are eggplant and purple sweet potato, and the fruits are fig and juicy peach.

[0022] In the present application, the freeze-drying time of the vegetable / fruit is preferably 24-72 hours, more preferably 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 44 hours, 48 hours, 52 hours, 56 hours, 60 hours, 64 hours, 68 hours or 72 hours. The freeze-drying temperature is preferably -50--80℃, more preferably -50℃, -55℃, -60℃, -65℃, -70℃, -75℃ or -80℃. It is further preferred that the freeze-drying time is 48 hours and the freeze-drying temperature is -60℃.

[0023] In the present application, the carbonization temperature is 550-800℃, the holding time is 1-2 hours, the heating rate is 1-10℃ / min, and the calcination environment is preferably in an inert atmosphere, such as a nitrogen atmosphere, an argon atmosphere or a helium atmosphere. Preferably, the temperature is 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, preferably the holding time is 1 hour, 1.1 hour, 1.2 hour, 1.3 hour, 1.4 hour, 1.5 hour, 1.6 hour, 1.7 hour, 1.8 hour, 1.9 hour or 2 hours, preferably the heating rate is 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min, and preferably the calcination environment is a nitrogen atmosphere, an argon atmosphere or a helium atmosphere. In the embodiments of the present application, the freeze-dried vegetable / fruit is placed in a high-temperature tube furnace for carbonization at a temperature of 800℃ for freeze-dried eggplant, 600℃ for freeze-dried purple sweet potato, 600℃ for freeze-dried figs, and 700℃ for freeze-dried juicy peaches, with a holding time of 1 hour, a heating rate of 5℃ / min, and a calcination environment of an argon atmosphere.

[0024] In the present application, the mass ratio of the carbon template and the silicon source is (1-5):(1-30), more preferably 1:1, 1:5, 1:10, 1:15, 1:20, 1:25, 1:30, 2:1, 2:5, 2:15, 2:25, 3:1, 3:5, 1:10, 3:20, 3:25, 4:1, 4:5, 4:10, 4:15, 4:25, 5:1, 5:10, 5:15, 5:20 or 5:30. In the embodiments of the present application, the mass ratio of the carbon template and the silicon source is 1:10.

[0025] In this invention, the silicon source is obtained by stirring and drying tetraethoxysilane, silicon dioxide, expandable graphite, deionized water, and anhydrous ethanol. The stirring time is preferably 24-48 hours, more preferably 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 44 hours, or 48 hours; the drying time is preferably 12-24 hours, more preferably 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, or 24 hours. In an embodiment of this invention, the silicon source is obtained by stirring and drying tetraethoxysilane, silicon dioxide, expandable graphite, deionized water, and anhydrous ethanol. The stirring time is 24 hours, and the drying time is 12 hours.

[0026] In this invention, the mass ratio of tetraethoxysilane, silicon dioxide, expandable graphite, deionized water, and anhydrous ethanol to form the silicon source is (80-200):(5-20):(5-10):(10-50):(10-30), preferably (100-200):(5-15):(5-9):(30-50):(20-30). In an embodiment of this invention, the mass ratio of tetraethoxysilane, silicon dioxide, expandable graphite, deionized water, and anhydrous ethanol to form the silicon source can be 150:10:6:45:30.

[0027] In this invention, the calcination is carried out in an inert atmosphere, more preferably in a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere. In an embodiment of this invention, the calcination environment is an argon atmosphere.

[0028] In this invention, the heating rate is 1-10℃ / min, preferably 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min; the cooling rate is 1-10℃ / min, preferably 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, or 10℃ / min. In an embodiment of this invention, the heating rate is 2℃ / min, and the cooling rate is 5℃ / min. In this invention, the calcination temperature is 1100-1500℃, preferably 1100℃, 1150℃, 1200℃, 1250℃, 1300℃, 1350℃, 1400℃, 1450℃, or 1500℃; the holding time is 3-8 hours, preferably 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, or 8 hours. In an embodiment of this invention, the calcination temperature is 1500℃, and the holding time is 6 hours.

[0029] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0030] Example 1 A method for preparing bamboo-like silicon carbide nanowires based on freeze-dried eggplant includes the following steps: (1) Place the eggplant in a freeze dryer at -60℃ for 48 hours to freeze dry the eggplant to obtain freeze-dried eggplant; (2) Freeze-dried eggplant was placed in a high-temperature tube furnace at 800℃ for 1 hour to obtain freeze-dried eggplant carbon template; the heating rate was 5℃ / min, and the calcination was carried out in an argon atmosphere. (4) Tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol are mixed in a mass ratio of 150:10:6:45:30, stirred for 24 hours and dried for 12 hours to obtain a silicon source. (5) Place the freeze-dried eggplant carbon template and silicon source in a graphite crucible with a mass ratio of 1:10, and calcine them in a high-temperature tube furnace under an argon atmosphere at 1500°C for 6 hours. The heating rate is 2°C / min and the cooling rate is 5°C / min. After cooling to room temperature, silicon carbide nanowires based on freeze-dried eggplant are obtained, denoted as SiCnws-1.

[0031] Figure 1 XRD pattern, SEM images at magnification of 1000 and 10000 times for SiCnws-1 prepared in Example 1 ( Figure 3 and Figure 4 The XRD pattern shows that the diffraction peaks match the standard diffraction peaks of silicon carbide, confirming the successful growth of silicon carbide nanowires. SEM images reveal that the material consists of a three-dimensional network structure composed of abundant silicon carbide nanowires. The nanowires are extremely thin and long, and exhibit a bamboo-like structure.

[0032] The thermal insulation of SiCnws-1 was tested using a 1300℃ butane spray gun, and the infrared power graph is shown below. Figure 2 As shown, the infrared power measured 2 cm behind the material using a laser power energy meter reached a maximum of 8.73 mW within 0-250 seconds; the infrared image behind the material was obtained using an infrared thermal imager. Figure 5 The highest temperature reached was 196℃ within 0-250 seconds. Reflection loss diagram at frequencies of 2-18 GHz ( Figure 6 As can be seen, SiCnws-1 exhibits a minimum reflection loss of -39.81dB and an effective bandwidth of 3.96GHz at a thickness of 3 mm and a GHz of 13.15GHz.

[0033] Example 2 A method for preparing bamboo-like silicon carbide nanowires based on freeze-dried purple sweet potato includes the following steps: (1) Freeze-dry purple sweet potatoes by placing them in a freeze dryer at -60℃ for 48 hours to obtain freeze-dried purple sweet potatoes; (2) The freeze-dried purple sweet potato was placed in a high-temperature tube furnace and carbonized at 600°C for 1 hour in an argon atmosphere. The heating rate was 5°C / min to obtain the freeze-dried purple sweet potato carbon template. (2) Tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol are mixed in a mass ratio of 150:10:6:45:30, stirred for 24 hours and dried for 12 hours to obtain a silicon source; (3) The freeze-dried purple sweet potato carbon template and silicon source with a mass ratio of 1:10 were placed into a graphite crucible and placed in a high-temperature tube furnace and calcined at 1500℃ for 6 hours in an argon atmosphere. The heating rate was 2℃ / min and the cooling rate was 5℃ / min. After cooling to room temperature, silicon carbide nanowires based on freeze-dried purple sweet potato were obtained, denoted as SiCnws-2.

[0034] Figure 7 The XRD pattern of SiCnws-2 shows that the diffraction peak positions match the standard diffraction peaks of silicon carbide, confirming the successful growth of silicon carbide nanowires. SEM images at 1000x and 10000x magnification are shown in Figures 9-10, revealing a three-dimensional network structure composed of abundant silicon carbide nanowires. The nanowires are extremely fine and long, resembling bamboo joints. Thermal insulation tests were conducted on SiCnws-2 using a 1300℃ butane spray gun. Figure 8 The infrared power graph shows that the highest infrared power measured 2 cm behind the material using a laser power energy meter was 10.41 mW within 0-250 seconds; the infrared image behind the material was obtained using an infrared thermal imager. Figure 11 The highest temperature within 0-250 seconds is 252.8℃. The reflection loss at frequencies of 2-18 GHz is as follows: Figure 12 As shown, SiCnws-2 exhibits a minimum reflection loss of -33.96 dB and an effective bandwidth of 4.38 GHz at a thickness of 2.5 mm and a GHz speed of 10.95 GHz.

[0035] Example 3 A method for preparing bamboo-like silicon carbide nanowires based on freeze-dried figs includes the following steps: (1) Freeze-dry figs by placing them in a freeze dryer at -60℃ for 48 hours to obtain freeze-dried figs; (2) Place the freeze-dried figs in a high-temperature tube furnace under an argon atmosphere at 600°C for 1 hour, with a heating rate of 5°C / min, to obtain a freeze-dried fig carbon template. (3) Tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol are mixed in a mass ratio of 150:10:6:45:30, stirred for 24 hours and dried for 12 hours to obtain a silicon source; (4) The freeze-dried fig carbon template and silicon source were mixed in a mass ratio of 1:10 in a graphite crucible and calcined in a high-temperature tube furnace under an argon atmosphere at 1500℃ for 6 hours. The heating rate was 2℃ / min and the cooling rate was 5℃ / min. After cooling to room temperature, silicon carbide nanowires based on freeze-dried figs were obtained, denoted as SiCnws-3.

[0036] Figure 13 The XRD pattern of SiCnws-3 shows that the diffraction peak positions match the standard diffraction peaks of silicon carbide, confirming the successful growth of silicon carbide nanowires. SEM images at magnifications of 1000 and 10000 are shown below. Figure 15-16 As shown, the material consists of a three-dimensional network structure composed of abundant silicon carbide nanowires. The nanowires are extremely long and thin, resembling bamboo joints. Thermal insulation tests were conducted on SiCnws-3 using a 1300℃ butane spray gun. Figure 14 The infrared power graph shows that the highest infrared power measured 2 cm behind the material using a laser power energy meter is 9.19 mW within 0-250 seconds; the infrared image behind the material measured using an infrared thermal imager (…) Figure 17 The highest temperature within 0-250 seconds is 224.6℃. The reflection loss at frequencies of 2-18 GHz is as follows: Figure 18 As shown, SiCnws-3 exhibits a minimum reflection loss of -30.55dB and an effective bandwidth of 6.99GHz at a thickness of 3 mm and a GHz speed of 13.24 GHz.

[0037] Example 4 A method for preparing bamboo-like silicon carbide nanowires based on freeze-dried peaches includes the following steps: (1) Place the peaches in a freeze dryer at -60℃ for 48 hours to freeze dry them to obtain freeze-dried peaches; (2) The freeze-dried peaches were placed in a high-temperature tube furnace under an argon atmosphere and carbonized at 700°C for 1 hour. The heating rate was 5°C / min to obtain the freeze-dried peach carbon template. (3) Tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol are mixed in a mass ratio of 150:10:6:45:30, stirred for 24 hours and dried for 12 hours to obtain a silicon source; (4) The freeze-dried peach carbon template and silicon source with a mass ratio of 1:10 were placed into a graphite crucible and calcined at 1500℃ for 6 hours in an argon atmosphere in a high-temperature tube furnace. The heating rate was 2℃ / min and the cooling rate was 5℃ / min. After cooling to room temperature, silicon carbide nanowires based on freeze-dried peach were obtained, denoted as SiCnws-4.

[0038] Figure 19 The XRD pattern of SiCnws-4 shows that the diffraction peak positions match the standard diffraction peaks of silicon carbide, confirming the successful growth of silicon carbide nanowires. SEM images at magnifications of 1000 and 10000 (21-22) reveal that the material consists of a three-dimensional network structure composed of abundant silicon carbide nanowires. The nanowires are extremely long and thin, resembling bamboo joints.

[0039] The thermal insulation of SiCnws-4 was tested using a butane spray gun at 1300℃. Figure 20 The infrared power map was obtained using a laser power energy meter, showing a maximum infrared power of 9.14 mW at 2 cm behind the material within 0-250 seconds; the infrared image behind the material was obtained using an infrared thermal imager. Figure 23 The highest temperature during the 0-250 second period is 213℃. The reflection loss diagram for frequencies from 2-18 GHz is shown below. Figure 24 As shown, SiCnws-4 exhibits a minimum reflection loss of -40.43dB and an effective bandwidth of 5.92GHz at a thickness of 2.6 mm and a GHz speed of 11.59 GHz.

[0040] Comparative Example 1 The only difference from Example 1 is that expandable graphite is not added in step (4). Instead, tetraethoxysilane, silicon dioxide, deionized water, and anhydrous ethanol are mixed, stirred, and dried in a mass ratio of 150:10:45:30 to obtain a silicon source. The resulting silicon carbide nanowires based on freeze-dried eggplant are designated as SiCnws-5.

[0041] Figure 25 The SEM image of SiCnws-5 at 10,000x magnification shows that the generated silicon carbide nanowires are slender and curved.

[0042] The thermal insulation of SiCnws-5 was tested using a 1300℃ butane spray gun. The infrared power graph is shown below. Figure 26As shown, the infrared power measured 2 cm behind the material using a laser power energy meter reached a maximum of 16.13 mW within 0-250 seconds; the highest temperature measured behind the material using an infrared thermal imager within 0-250 seconds was 372.6℃. SiCnws-5 exhibited a minimum reflection loss of -22.23 dB and an effective bandwidth of 2.85 GHz at a thickness of 3 mm and a GHz frequency of 10.28 GHz. This is because the absence of expandable graphite in the silicon source disrupts the regulation of nucleation sites and the stability of the reaction environment for the formation of the bamboo-like structure, leading to abnormal nanowire morphology and failure of the core enhancement mechanism. After removing the expandable graphite, the nucleation sites rely solely on surface defects of the carbon template, resulting in insufficient numbers and uneven distribution. This fails to induce alternating growth between nodes and internodes, causing excessively rapid diffusion of gaseous intermediates and stable local concentrations without fluctuations. The key conditions for the formation of the bamboo-like structure are lost, ultimately resulting in the formation of slender, curved nanowires. The curved linear nanowires lack a bamboo-like structure, resulting in uniformly large pore sizes, some exceeding the mean free path of air molecules (70 nm). This intensifies air molecule collisions and significantly increases gas thermal conductivity. The absence of bamboo-like segment stacking defects and the SiC / SiO2 thick-shell heterostructure reduces phonon scattering paths, further enhancing solid-state thermal conductivity. While the sheet-like structure of expandable graphite could enhance the bonding between the silicon source and the carbon template, its absence reduces adhesion between the nanowires and the carbon template, making them prone to slippage and adhesion at high temperatures. This increases the risk of pore collapse, further weakening insulation capabilities. The curved linear structure also lacks segment-intersegment interfaces, resulting in… The scarcity of trapped dipoles weakens interfacial and dipole polarization, and insufficient nucleation sites lead to uneven crystallinity and loose distribution of silicon carbide nanowires. The effective dielectric constant deviates from the optimal range, resulting in poor impedance matching with air and increased electromagnetic wave surface reflectivity. The lack of a bamboo-like multi-level porous structure prevents electromagnetic waves from achieving multiple reflections and scatterings, allowing energy to penetrate the material without being fully absorbed. The conductivity of expandable graphite, which could have helped build a continuous conductive network, is missing, resulting in an incomplete nanowire conductive network, hindered free electron movement, reduced Joule heating, and weakened conductivity loss. Consequently, the high-temperature insulation and electromagnetic wave absorption performance are poor.

[0043] Comparative Example 2 The only difference from Example 1 is that tetraethoxysilane is not added in step (4). Instead, silicon dioxide, expandable graphite, deionized water, and anhydrous ethanol are mixed, stirred, and dried in a mass ratio of 10:6:45:30 to obtain a silicon source. The resulting silicon carbide nanowires based on freeze-dried eggplant are denoted as SiCnws-6.

[0044] Figure 27 The SEM image of SiCnws-6 at 10,000x magnification shows that the generated silicon carbide nanowires are thin, elongated, and curved. Thermal insulation testing of SiCnws-6 was performed using a 1300℃ butane spray gun; the infrared power spectrum is shown below. Figure 28As shown, the infrared power measured 2 cm behind the material using a laser power energy meter reached a maximum of 16.51 mW within 0-250 seconds; the highest temperature measured behind the material using an infrared thermal imager within 0-250 seconds was 394.2℃. SiCnws-6 exhibited a minimum reflection loss of -20.34 dB and an effective bandwidth of 3.04 GHz at a thickness of 3 mm and a GHz frequency of 9.15 GHz. This is because the absence of tetraethoxysilane in the silicon source disrupts the silicon source supply balance and reactivity matching for the formation of the bamboo-like structure, leading to abnormal nanowire morphology and incomplete reaction. The lack of rapidly supplied SiO during the initial nucleation stage results in low nucleation density and uneven growth of the silicon carbide nanowires. A single silicon source cannot form the periodic fluctuations of SiO partial pressure, thus losing the key chemical driving force for the formation of the bamboo-like structure, ultimately resulting in the formation of slender, curved nanowires. Without the bamboo-like segment defects at the interface with the thick SiC / SiO2 shell, phonon scattering is significantly reduced, and solid thermal conductivity increases. The interwoven pores have a more uniform size and lack multi-level pores. Some pores exceed the mean free path of air molecules, intensifying air molecule collisions and increasing gas thermal conductivity. The carbothermic reduction reaction efficiency of single silica is low, and a small amount of unreacted carbon may remain, further increasing the overall thermal conductivity. Without bamboo-like segment support points, nanowires are prone to adhesion and pore collapse at high temperatures, making the thermal insulation structure prone to failure. Interface polarization and dipole polarization are weakened. The single silicon source leads to uneven crystallinity of silicon carbide nanowires, causing the effective dielectric constant to deviate from the optimal range, resulting in poorer impedance matching with air, increased electromagnetic wave surface reflectivity, and a shorter electromagnetic wave absorption path. Low nucleation density leads to discontinuous conductive networks, hindering the movement of free electrons, reducing Joule heating, and weakening conductive losses, resulting in poor high-temperature thermal insulation and electromagnetic wave absorption performance.

[0045] Comparative Example 3 The only difference from Example 1 is that silicon dioxide is not added in step (4). Tetraethoxysilane, expandable graphite, deionized water, and anhydrous ethanol are mixed, stirred, and dried in a mass ratio of 150:6:45:30 to obtain a silicon source. The resulting silicon carbide nanowires based on freeze-dried eggplant are designated as SiCnws-7.

[0046] Figure 29 The SEM image of SiCnws-7 at 10,000x magnification shows that the generated silicon carbide nanowires are slender and curved. Thermal insulation tests were performed on SiCnws-7 using a 1300℃ butane spray gun; the infrared power spectrum is shown below. Figure 30As shown, the infrared power measured 2 cm behind the material using a laser power energy meter reached a maximum of 16.31 mW within 0-250 seconds; the highest temperature measured behind the material using an infrared thermal imager within 0-250 seconds was 386℃. SiCnws-7 exhibited a minimum reflection loss of -18.55 dB and an effective bandwidth of 2.54 GHz at a thickness of 3 mm and a GHz speed. This is because the absence of silica disrupts the chemical driving force for the formation of the bamboo-like structure, leading to abnormal nanowire morphology and reaction process. After removing silica, relying solely on tetraethoxysilane to provide SiO, the reaction process is more stable, and the SiO / CO concentration does not fluctuate periodically. This prevents the induction of rapid nucleation at the nodes and slow growth between the nodes, resulting in a bamboo-like structure that ultimately forms slender, curved nanowires. Without the bamboo-like segment defects at the interface of the SiC / SiO2 thick shell, phonon scattering is greatly reduced, the solid heat conduction path is smoother, the interwoven pore size is more uniform, lacking multi-level pores, and some pores exceed the mean free path of air molecules, air molecule collisions are intensified, gas heat conduction increases, polarization loss is reduced, impedance matching is weakened, and ultimately the material's high-temperature thermal insulation and electromagnetic wave absorption performance is poor.

[0047] Comparative Example 4 The only difference from Example 1 is that okra was replaced with eggplant. The resulting silicon carbide nanowires based on freeze-dried okra are denoted as SiCnws-8.

[0048] Figure 31 The SEM image of SiCnws-8 at 10,000x magnification shows that the generated silicon carbide nanowires are relatively thick and long, and exhibit a curved shape. Thermal insulation testing of SiCnws-8 was performed using a 1300℃ butane spray gun; the infrared power spectrum is shown below. Figure 32 As shown, the infrared power measured 2 cm behind the material using a laser power energy meter reached a maximum of 13.99 mW within 0-250 seconds; the highest temperature measured behind the material using an infrared thermal imager within 0-250 seconds was 340℃. SiCnws-8 exhibited a minimum reflection loss of -15.56 dB and an effective bandwidth of 2.94 GHz at a thickness of 3 mm and a GHz speed. This is because the natural structure of okra is more inclined towards coarser tubular channels and relatively smooth inner walls, with fewer surface defects, making it impossible to form bamboo-like silicon carbide nanowires. Phonon scattering is weakened, solid thermal conductivity increases, and there are no bamboo-like segment-intersegment differences and defects forming a large number of polarization centers. Interface polarization and dipole polarization are significantly reduced, and the lack of bamboo-like multi-level pores reduces electromagnetic wave multiple reflection and scattering, resulting in insufficient energy absorption and poor high-temperature insulation and electromagnetic wave absorption performance.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a freeze-dried vegetable / fruit based, bamboo-like silicon carbide nanowire, characterized by, The method comprises the following steps: (1) freeze-drying the vegetables / fruits in a freeze dryer to obtain freeze-dried vegetables / fruits; (2) carbonizing the freeze-dried vegetables / fruits in a high-temperature tube furnace to obtain carbon templates; (3) mixing tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol according to a mass ratio of 80-200:5-20:5-10:10-50:10-30, and stirring and drying to obtain a silicon source; (4) mixing the carbon templates and the silicon source according to a mass ratio of 1-5:1-30, uniformly mixing in a graphite crucible, and calcining at 1100-1500°C for 3-8 hours in a high-temperature tube furnace to obtain bamboo-shaped silicon carbide nanowires based on freeze-dried vegetables / fruits.

2. The production method according to claim 1, characterized by, The vegetables / fruits are one or more than two of eggplant, shiitake mushroom, mango, strawberry, purple sweet potato, carrot, fig, apple, juicy peach and pumpkin.

3. The preparation method according to claim 1, characterized in that, The calcining is performed in an inert atmosphere, the heating rate is 1-10°C / min, and the cooling rate is 1-10°C / min.

4. The method of claim 1, wherein, The mass ratio of the tetraethoxysilane, silicon dioxide, expandable graphite, deionized water and anhydrous ethanol is 100-200:5-15:5-9:30-50:20-30.

5. The preparation method according to claim 1, characterized in that, The stirring time in step (3) is 24-72 hours, and the drying time is 12-24 hours.

6. The method of claim 1, wherein, The carbonization temperature is 550-800°C, the carbonization time is 1-2 hours, and the heating rate is 1-10°C / min.

7. The preparation method according to claim 1, characterized in that, The freeze-drying time is 24-72 hours, and the temperature is -50 to -80°C.

8. Bamboo-shaped silicon carbide nanowires based on freeze-dried vegetables / fruits prepared by the preparation method of any one of claims 1-7.

9. Application of the bamboo-shaped silicon carbide nanowires based on freeze-dried vegetables / fruits in the preparation of materials with high-temperature heat insulation performance and wide-frequency high-efficiency electromagnetic wave absorption performance.

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