Preparation method of cable-like structure SiO2@SiC composite absorbing material

Abstract

The present invention relates to a method for preparing a SiO2@SiC composite absorbing material with a cable-like structure. The present invention uses biomass in nature to obtain fluffy and porous biochar through drying and carbonization. The silicon source is a mixed silicon source composed of high-purity silicon powder and silicon dioxide in a specific proportion. A small amount of CaCO3 powder is placed at the bottom of a crucible, and then the silicon source, corundum ring and carbon source are added in sequence to react to obtain a nanowire product containing silicon carbide; then, by decarbonization and controlled oxidation, SiO2@SiC with a cable-like structure is obtained. The present invention utilizes a simple chemical vapor deposition method to prepare SiO2@SiC with a cable-like structure, and uses low-cost waste biomass to obtain SiO2@SiC nanowires with high-efficiency absorbing performance, with an optimal reflection loss of less than ‑50dB and a maximum effective absorption bandwidth greater than 4.52GHz. The process of the present invention is simple and controllable, and is convenient for industrial application.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon carbide absorbing materials, and in particular relates to a method for preparing a SiO2@SiC composite absorbing material with a cable-like structure. Background Art

[0002] Absorbing material is a material that can absorb or weaken incident electromagnetic waves. It can convert the electromagnetic wave energy absorbed from the outside into heat energy and dissipate it. It is a vital material in fields such as radar monitoring.

[0003] The one-dimensional structure of silicon carbide nanowires, due to its unique nanoscale fiber structure, also has the characteristics of wide band gap, high dielectric loss, high specific surface area and corrosion resistance. It is often used as a reinforcing material to make up for the defects in composite materials. Therefore, it is considered to be a potential material for preparing high-performance electromagnetic wave absorption materials.

[0004] Traditionally, silicon carbide nanowires are manufactured from silicon carbide powder through gel casting, foaming, and freezing. These processes are complex and require additional additives. Therefore, the development of porous silicon carbide materials with high-efficiency microwave absorption properties through low-carbon, simple, and economical methods has attracted significant interest. Patent 1, "CN 118387866A," utilizes natural microcrystalline graphite as a carbon source to prepare SiC / C composite absorbers at 1350-1600°C. The materials exhibit an impedance matching coefficient |Zin / Z0| ≈ 1, a reflection loss ≤ -20dB, and an effective absorption bandwidth ≥ 2GHz. Patent 2, "CN 115504799A," utilizes calcium aluminate-coated carbon nanofiber cement as a binder, and uses granular silicon carbide, activated alumina, silica fume, and metallic silicon as raw materials. The materials are prepared through a mixing, vibration molding, and sintering process. Patent 3 "CN202410624750.X" A silicon carbide crucible for aluminum alloy smelting and its preparation method, which relates to "A preparation method for a silicon carbide crucible for aluminum alloy smelting, characterized in that the specific steps of the preparation method are: the first step, 20-30wt% of silicon carbide particles A with a particle size of 0.5-1.0mm, 5-15wt% of silicon carbide particles B with a particle size of 0.1-0.5mm, 30-40wt% of silicon carbide fine powder A, 20-30wt% of silicon carbide fine powder B and 3-7wt% of xylitol are mixed to obtain a mixture A; the mixture A is placed in a silica sol and the vacuum degree is - The method comprises the following steps: impregnating the mixture B at a pressure of 0.08 to -0.10 MPa for 20 to 40 minutes to obtain a mixture B; drying the mixture B at 90 to 110°C for 8 to 10 hours, and heat treating the mixture B at 400 to 600°C for 2 to 4 hours to obtain silicon dioxide-coated silicon carbide; the silicon carbide particles A, the silicon carbide particles B, the silicon carbide fine powder A, and the silicon carbide fine powder B have the same chemical composition: a SiC content greater than 97.0 wt% and an FeO content less than 0.5 wt%. Therefore, there are few reports in the prior art on the preparation of SiO2@SiC nanowires using industrial silicon and biochar as raw materials through chemical vapor reaction. Summary of the Invention

[0005] This invention addresses the shortcomings of the prior art and proposes, for the first time, a method for preparing a SiO2@SiC composite absorber with a cable-like structure. The carbon source in this invention is waste biomass found in nature, such as coffee hulls, peanut shells, rice husks, tobacco stalks, and cotton stalks. These biomasses are mostly byproducts of agricultural and forestry production and are often discarded as waste. These waste biomass raw materials not only have high reactivity but also possess a highly porous structure. Direct carbonization yields fluffy, porous biochar. The high alkali metal content of these biomass materials enhances their reactivity during the carbothermal reduction process, making them ideal as a carbon source for producing silicon carbide nanowires. The silicon source is a mixed powder of silicon powder and silicon dioxide powder. A suitable carbon source and silicon source are placed in a crucible and reacted at high temperature through a chemical vapor reaction to produce SiO2@SiC nanowires with a cable-like structure.

[0006] The present invention provides a method for preparing a cable-shaped SiO2@SiC composite absorbing material, and the specific steps are as follows:

[0007] (1) Carbon source pretreatment: Grind the clean biomass into powder to obtain pretreated material;

[0008] (2) Carbonization: The pretreated material obtained in step (1) is placed in a mold to be formed, and then placed in a furnace with a protective gas to be carbonized to obtain fluffy and porous biochar;

[0009] (3) Silicon source pretreatment: Silicon powder and silicon dioxide are fully mixed by ball milling to obtain a mixed silicon source containing silicon and silicon dioxide; the mass ratio of silicon powder to silicon dioxide is 1-2:2-1;

[0010] (4) High-temperature carbon thermal reduction: a layer of calcium carbonate powder is laid on the bottom of the crucible, and then the mixed silicon source obtained in step (3) is evenly spread on the surface of the CaCO3 powder, and then a corundum ring is placed as a support, and the fluffy porous biochar obtained in step (2) is placed on the upper end of the corundum ring; protective gas is first introduced, and then the temperature is raised to the reaction temperature, the reaction temperature is 1200-1600°C, and the reaction time is greater than 2 hours;

[0011] (5) Carbon removal and controlled oxidation: The product obtained in step (4) is directly placed in a muffle furnace for heating to remove residual carbon and controllably oxidize to obtain SiO2@SiC nanowires; the temperature for carbon removal and controlled oxidation is 890-910°C, and the time is greater than or equal to 100 minutes.

[0012] The present invention provides a method for preparing a SiO2@SiC composite absorbing material with a cable-like structure. The biomass is selected from at least one of coffee hulls, peanut shells, rice husks, tobacco stalks, and cotton stalks. After being ground into a powder, the particle size is 50-100 mesh. For industrial applications, a powder with the desired particle size can be obtained using a vibration mill.

[0013] Preferably, the present invention provides a method for preparing a SiO2@SiC composite absorber with a cable-like structure. Powdered biomass is placed in a mold and vacuum-dried to form the material. The vacuum drying temperature is -50°C to -40°C, and the drying time is 12 to 24 hours. Other drying methods are also applicable to the present invention. Freeze-drying is preferred because it preserves the biomass's original pore structure, resulting in a more fluffy and porous biochar during the carbonization process.

[0014] The present invention provides a method for preparing a SiO2@SiC composite absorbing material with a cable-like structure. The method comprises placing the pre-material obtained in step (1) in a tubular furnace filled with argon gas for carbonization to obtain fluffy and porous biochar.

[0015] Preferably, the present invention provides a method for preparing a cable-like structured SiO2@SiC composite absorbing material.

[0016] During carbonization, the argon flow rate is 8-25 ml / min, the carbonization temperature is 500-800°C, and the time is greater than or equal to 100 minutes. In actual application, the carbonization time can be appropriately extended according to the amount of material and the degree of carbonization.

[0017] Preferably, in the method for preparing a SiO2@SiC composite absorbing material with a cable-like structure according to the present invention, in step (3), the purity of the silicon and silicon dioxide powders used is >99wt%, and the mesh size is -100 mesh.

[0018] As a preferred method for preparing a cable-like structure SiO2@SiC composite absorbing material of the present invention, in step (3), the silicon powder and silicon dioxide used are prepared in a mass ratio of 1:2, and the optimal reflection loss (RL) can be obtained by combining with the subsequent process. min ) is less than -50dB, the maximum effective absorption bandwidth (EAB max ) is a high-quality product with a frequency greater than 4.5GHz.

[0019] During the high-temperature carbon thermal reduction of the present invention, the atmosphere in the crucible is almost composed of a CO / CO2+SiOx atmosphere system, which is slightly different from the atmosphere outside the crucible, which is a protective gas. The reason for controlling the protective gas outside the crucible to move at an extremely low flow rate is to prevent the reaction gas in the crucible from being excessively fluctuated by the atmosphere, thereby causing abnormal reactions. At the same time, the air pressure inside and outside the crucible is basically the same. The crucible cover is combined with the low-flow rate of the protective gas outside the crucible to ensure that the reaction in the crucible is as little affected by the outside world as possible. Therefore, as a preferred embodiment, the present invention also needs to control the flow rate of the protective gas (such as argon) outside the crucible to 8-20 ml / min.

[0020] Preferably, in the method for preparing a cable-like structured SiO2@SiC composite absorbing material of the present invention, in step (4), the reaction temperature is 1400-1600°C, more preferably 1580-1600°C. A higher reaction temperature, combined with the mixing of silicon powder and silicon dioxide in a silicon source in a mass ratio of 1:2 and other conditions, can produce a product with extremely excellent performance. SiC is an electromagnetic wave absorbing material, and SiO2 is a transparent material for electromagnetic waves. When electromagnetic waves are incident on the SiO2@SiC material, the outermost SiO2 coating can prevent the electromagnetic waves from being directly reflected. Instead, the electromagnetic waves first pass through the SiO2 outer layer, then pass through the SiO2 / SiC heterogeneous layer, and reach the interior of the SiC core, thereby converting the electromagnetic energy into heat energy for dissipation. The SiO2 coating allows the incident electromagnetic wave to reach the SiC core more smoothly. After SiC absorbs some of the energy, some of the electromagnetic wave energy will be reflected. The accumulated charge and interface polarization of the SiO2 / SiC heterogeneous layer will cause the reflected electromagnetic wave to be reflected back to the SiC core. Therefore, the multi-layer and multi-site reflection and interface polarization brought by SiO2 will improve the material's absorption and energy absorption. At the same time, the reaction temperature of the present invention is increased. According to the reaction mechanism, when the silicon carbide fiber is generated, there are still trace amounts of silicon oxides inside. These trace amounts of silicon oxides are beneficial to improving the electromagnetic performance of the product. Therefore, in practical applications, in order to pursue lower reflection loss and wider absorption bandwidth, a reaction temperature of 1580 to 1600 ° C can be adopted.

[0021] Preferably, in the method for preparing a cable-like structured SiO2@SiC composite absorbing material of the present invention, in step (4), after maintaining the reaction temperature, the temperature is lowered at a rate of 4-6°C / min.

[0022] Preferably, in a method for preparing a cable-like SiO2@SiC composite absorbing material, in step (4), the mass ratio of calcium carbonate to silicon source is 1:1.1-1.5, preferably 1:1.2-1.5. The purpose of using calcium carbonate in the present invention is to make CO2 the dominant gas in the system, inhibit reaction (4), and allow the reaction to proceed to reaction (5). The purpose of controlling the mass ratio of calcium carbonate to silicon source to 1:1.2-1.5 is to control the relative content of CO2 and SiO gas in the system, and the CaO generated during the reaction consumes part of the silicon source. It was also found that the diameter distribution of the product obtained by the present invention is narrower than that of a system without calcium carbonate.

[0023] In step (4), the mass ratio of the fluffy porous biochar to the silicon source is preferably fluffy porous biochar: silicon source = 3.0-6.5:7.5.

[0024] Preferably, the present invention provides a method for preparing a SiO2@SiC composite absorbing material having a cable-like structure, wherein the product obtained in step (4) is directly placed in a muffle furnace and heated to 890-910°C at a heating rate of 8-12°C / min, then kept at this temperature for 110-130 minutes to remove residual carbon and perform controlled oxidation to obtain SiO2@SiC nanowires. If the temperature is too low, too little silicon dioxide will be generated in a short period of time, or even no continuous silicon dioxide layer will appear. In the most extreme case, only the outer layer of the silicon carbide fiber will not contain silicon dioxide. If the temperature is too high, the silicon dioxide coating will be incomplete or the coating quality will be too poor (mainly uneven).

[0025] In (4),

[0026] As the reaction temperature gradually rises, the CaCO3 powder at the bottom layer will be decomposed and release a large amount of CO2 gas after being heated. CO2 gas and carbon source will generate CO gas at high temperature; the mixed silicon source composed of Si and SiO2 will undergo a disproportionation reaction at high temperature to generate a large amount of SiO gas. In a closed environment, a SiC substrate will first be formed through reaction (4) at high temperature. During the cooling stage, in an environment where CO2 gas is the dominant gas, the reaction (5) will be suppressed, and the two gases of CO gas and SiO gas will proceed towards reaction (6). When the temperature starts to cool down in the later stage of the reaction, since the melting point of silicon oxide is much lower than that of SiC, starting to cool down at a higher temperature and controlling the rate will form a relatively uniform silicon oxide film on the outer layer of the obtained SiC. The specific reaction mechanism is as follows:

[0027] CaCO3(s)=CaO(s)+CO2(g)#(1)

[0028] SiO2(s)+Si(s)=2SiO(g)#(2)

[0029] CO2(g)+C(s)=2CO(g)#(3)

[0030] SiO(g)+2C(s)=SiC(s)+CO(g)#(4)

[0031] SiO(g)+3CO(g)=SiC(s)+2CO2(g)#(5)

[0032] 3SiO(g)+CO(g)=SiC(s)+2SiO2(s)#(6);

[0033] The outer layer of the SiO2@SiC nanowire obtained by the present invention is silicon dioxide, and the inner layer is silicon carbide; or the outer layer of the SiO2@SiC nanowire obtained by the present invention is silicon dioxide, and the inner layer is silicon carbide and a trace amount of silicon oxide.

[0034] The beneficial effects of the present invention are:

[0035] (1) The present invention uses waste biomass as raw material, which has low raw material cost and a simple and fast process. After vacuum drying, fluffy and porous biochar can be obtained, providing a high-value recycling method for waste biomass.

[0036] (2) The large amount of alkali metals contained in the biomass of the present invention makes it have good reaction activity in the carbon thermal reduction process.

[0037] (3) In the present invention, more CO2 gas is generated by adding CaCO3 powder, making the subsequent reaction more complete.

[0038] (4) The present invention utilizes a simple chemical vapor reaction method to prepare SiO2@SiC nanowires with high-efficiency wave absorption performance, and the best reflection loss (RL min ) is -50.40dB, the maximum effective absorption bandwidth (EAB max ) is 4.52GHz. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Attachment Figure 1 This is a flow chart of the preparation process of SiO2@SiC nanowires in Example 1;

[0040] Attachment Figure 2 Schematic diagram of the specific reaction apparatus in Example 1;

[0041] Attachment Figure 3 This is the microscopic morphology prepared using coffee shells as the carbon source in Example 1;

[0042] Attachment Figure 4 This is a transmission electron micrograph of the product prepared using coffee shells as a carbon source in Example 1;

[0043] Attachment Figure 5 This is an energy spectrum characterization diagram of the product prepared using coffee shells as the carbon source in Example 1;

[0044] Attachment Figure 6 This is a test of the microwave absorption performance of the product prepared using coffee shells as the carbon source in Example 1;

[0045] Attachment Figure 7 This is the microscopic morphology prepared using coffee shells as the carbon source in Comparative Example 1;

[0046] Attachment Figure 8 This is the energy spectrum characterization diagram of the product prepared using coffee shell as the carbon source in Comparative Example 1.

[0047] from Figure 1 The basic process flow of the present invention can be seen.

[0048] from Figure 2 It can be seen that the basic composition and structure of the reaction device used in the present invention.

[0049] from Figure 3 It can be seen that a large number of nanowires are interwoven to form nanovoids.

[0050] from Figure 4 It can be seen that the nanowire has a diameter of 28nm and has a 2nm thick shell.

[0051] from Figure 5 It can be seen that the color distribution of O and Si overlaps, the distribution color is lighter, and there is a layer of amorphous SiO2.

[0052] from Figure 6 It can be seen that the optimal dielectric loss is -50.40dB, which is equivalent to being able to dissipate 99.999% of the electromagnetic wave energy.

[0053] from Figure 7 It can be seen that the nanowires are intertwined and entangled.

[0054] from Figure 8 It can be seen that the O content is very small and the color does not overlap with Si. DETAILED DESCRIPTION

[0055] The present invention will be further described in detail below in conjunction with specific embodiments, but the protection scope of the present invention is not limited to the contents described above.

[0056] Example 1: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0057] (1) Grinding coffee husks into powder and placing them in a circular mold, then vacuum drying the powder at -45°C to form a mold;

[0058] (2) The coffee husks (10 g) vacuum-dried in step (1) were placed in a horizontal tube furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 600°C, a heating rate of 10°C / min, and a holding time of 2 h to obtain 3.85 g of coffee husk biochar;

[0059] (3) ball milling silicon powder and silicon dioxide, both with a purity greater than 99 wt%, in a mass ratio of 1:2 in a ball mill for 5 h to obtain a mixed silicon source composed of Si and SiO2;

[0060] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (3.85 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1600°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 4.13 g of product was obtained after cooling to room temperature;

[0061] (5) The product obtained in step (4) was directly placed in a muffle furnace, heated to 900°C at a heating rate of 10°C / min, kept warm for 2 h, and cooled to obtain 3.15 g of SiO2@SiC.

[0062] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -50.40dB, the maximum effective absorption bandwidth (EAB max ) is 4.52GHz. (Its diameter ranges from 28-63nm)

[0063] Example 2: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0064] (1) Grinding coffee husks into powder and placing them in a circular mold, then vacuum drying the powder at -40°C to form a mold;

[0065] (2) The coffee shells (10 g) vacuum-dried in step (1) were placed in a horizontal tube furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 500°C, a heating rate of 10°C / min, and a holding time of 2 h to obtain 6.24 g of coffee shell biochar;

[0066] (3) ball milling silicon powder and silicon dioxide with a purity of >99% in a mass ratio of 1:1.5 for 5 h in a ball mill to obtain a mixed silicon source composed of Si and SiO2;

[0067] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (6.24 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1500°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 6.53 g of product was obtained after cooling to room temperature;

[0068] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 1.94g of SiO2@SiC (with a diameter distribution of 26-59nm).

[0069] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -12.18dB, the maximum effective absorption bandwidth (EAB max ) is 3.32GHz.

[0070] Example 3: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0071] (1) Grinding coffee husks into powder and placing them in a circular mold, then vacuum drying the powder at -50°C to form a mold;

[0072] (2) The coffee husks (10 g) vacuum-dried in step (1) were placed in a horizontal tube furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 700°C, a heating rate of 10°C / min, and a holding time of 2 h to obtain 3.57 g of coffee husk biochar;

[0073] (3) ball milling silicon powder and silicon dioxide with a purity of >99% in a mass ratio of 1.5:1 for 5 h in a ball mill to obtain a mixed silicon source composed of Si and SiO2;

[0074] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (3.57 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1400°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 4.58 g of product was obtained after cooling to room temperature;

[0075] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 1.37g of SiO2@SiC (with a diameter distribution of 25-53nm).

[0076] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -6.42dB. Example 4: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0077] (1) Grinding coffee husks into powder and placing them in a circular mold, then vacuum drying the powder at -45°C to form a mold;

[0078] (2) The coffee husks (10 g) vacuum-dried in step (1) were placed in a horizontal tube furnace filled with argon at a flow rate of 10 ml / min, a carbonization temperature of 800°C, a heating rate of 10°C / min, and a holding time of 2 h to obtain 3.49 g of coffee husk biochar;

[0079] (3) Milling silicon powder and silicon dioxide, both with a purity greater than 99%, in a ball mill at a mass ratio of 2:1 for 5 h to obtain a mixed silicon source composed of Si and SiO2;

[0080] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (3.49 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1300 ° C, the heating rate was 10 ° C / min, the holding time was 5 h, the cooling rate was 5 ° C / min, and 3.82 g of product was obtained after cooling to room temperature;

[0081] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 0.43g of SiO2@SiC (with a diameter distribution of 26-49nm).

[0082] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -4.80dB. Example 5: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0083] (1) Grinding tobacco straw into powder and placing it in a circular mold, then vacuum drying the powder at -50°C to form a mold;

[0084] (2) placing the tobacco straw (10 g) vacuum-dried in step (1) into a horizontal tubular furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 800° C., a heating rate of 10° C. / min, and a holding time of 2 h to obtain 3.35 g of tobacco straw biochar;

[0085] (3) ball milling silicon powder and silicon dioxide with a purity of >99% in a mass ratio of 1:2 in a ball mill for 5 h to obtain a mixed silicon source composed of Si and SiO2;

[0086] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (3.35 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1600°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 5.22 g of product was obtained after cooling to room temperature;

[0087] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 2.71g of SiO2@SiC (with a diameter distribution of 57-90nm).

[0088] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -43.68dB, the maximum effective absorption bandwidth (EAB max ) is 2.84GHz.

[0089] Example 6: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0090] (1) Grinding tobacco straw into powder and placing it in a circular mold, then vacuum drying the powder at -40°C to form a mold;

[0091] (2) placing the vacuum-dried tobacco straw (10 g) formed in step (1) into a horizontal tubular furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 500° C., a heating rate of 10° C. / min, and a holding time of 2 h to obtain 5.78 g of tobacco straw biochar;

[0092] (3) Milling silicon powder and silicon dioxide, both with a purity greater than 99%, in a ball mill at a mass ratio of 2:1 for 5 h to obtain a mixed silicon source composed of Si and SiO2;

[0093] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (5.78 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1300 ° C, the heating rate was 10 ° C / min, the holding time was 5 h, the cooling rate was 5 ° C / min, and 6.89 g of product was obtained after cooling to room temperature;

[0094] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 0.62g of SiO2@SiC (with a diameter distribution of 47-76nm).

[0095] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -5.78dB. Example 7: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0096] (1) Grinding tobacco straw into powder and placing it in a circular mold, then vacuum drying the powder at -40°C to form a mold;

[0097] (2) placing the tobacco straw (10 g) vacuum-dried in step (1) into a horizontal tubular furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 600° C., a heating rate of 10° C. / min, and a holding time of 2 h to obtain 4.07 g of tobacco straw biochar;

[0098] (3) ball milling silicon powder and silicon dioxide with a purity of >99% in a mass ratio of 1:1.5 for 5 h in a ball mill to obtain a mixed silicon source composed of Si and SiO2;

[0099] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (4.07 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1400°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 5.28 g of product was obtained after cooling to room temperature;

[0100] (5) The product obtained in step (4) was directly placed in a muffle furnace and heated to 900°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and cooled to obtain 1.23g of SiO2@SiC (with a diameter distribution of 54-87nm).

[0101] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -7.56dB. Example 8: A method for preparing a SiO2@SiC composite absorbing material with a cable-like structure, the specific steps are as follows:

[0102] (1) Grinding tobacco straw into powder and placing it in a circular mold, then vacuum drying the powder at -45°C to form a mold;

[0103] (2) placing the vacuum-dried tobacco straw (10 g) formed in step (1) into a horizontal tubular furnace filled with argon at an argon flow rate of 10 ml / min, a carbonization temperature of 700°C, a heating rate of 10°C / min, and a holding time of 2 h to obtain 3.67 g of tobacco straw biochar;

[0104] (3) ball milling silicon powder and silicon dioxide with a purity of >99% in a mass ratio of 1.5:1 for 5 h in a ball mill to obtain a mixed silicon source composed of Si and SiO2;

[0105] (4) 5 g of CaCO3 powder was placed at the bottom of the crucible to generate CO2 gas, and then the carbon source (3.67 g) and silicon source (7.5 g) obtained in step (2) and step (3) were placed in a corundum crucible. The crucible was covered with a lid and placed in a muffle furnace filled with argon for reaction. The argon flow rate was 10 ml / min, the maximum reaction temperature was 1500°C, the heating rate was 10°C / min, the holding time was 5 h, the cooling rate was 5°C / min, and 4.76 g of product was obtained after cooling to room temperature;

[0106] (5) The product obtained in step (4) was directly placed in a muffle furnace, heated to 900°C at a heating rate of 10°C / min, kept warm for 2 hours, and cooled to obtain 2.13g of SiO2@SiC (with a diameter distribution of 53-88nm).

[0107] (6) The product obtained in step (5) is subjected to a wave absorbing performance test, and the optimal reflection loss (RL min ) is -16.44dB, the maximum effective absorption bandwidth (EAB max ) is 2.06GHz.

[0108] Comparative Example 1

[0109] Other conditions are the same as those in Example 1, except that: calcium carbonate is not added in step (4); in step (5), the temperature is heated to 800°C at a heating rate of 10°C / min, kept at this temperature for 2 hours, and the product is cooled to obtain the optimal reflection loss (RL min ) is -10.25dB, the maximum effective absorption bandwidth (EABmax The product diameter distribution is 115~236nm.

Claims

1. A method for preparing a cable-like structured SiO2@SiC composite absorbing material, characterized in that: The steps include: (1) Carbon source pretreatment: Grind the clean biomass into powder to obtain pretreated material; (2) Carbonization: The pretreated material obtained in step (1) is placed in a mold to be formed, and then placed in a furnace with a protective gas to be carbonized to obtain fluffy and porous biochar; (3) Silicon source pretreatment: Silicon powder and silicon dioxide are fully mixed by ball milling to obtain a mixed silicon source containing silicon and silicon dioxide; the mass ratio of silicon powder to silicon dioxide is 1-2:2-1; (4) High-temperature carbon thermal reduction: a layer of calcium carbonate powder is laid on the bottom of the crucible, and then the mixed silicon source obtained in step (3) is evenly spread on the surface of the CaCO3 powder, and then a corundum ring is placed as a support, and the fluffy porous biochar obtained in step (2) is placed on the upper end of the corundum ring; first, a protective gas is introduced, and then the temperature is raised to the reaction temperature, the reaction temperature is 1200-1600°C, and the reaction time is greater than 2 hours; (5) Carbon removal and controlled oxidation: The product obtained in step (4) is directly placed in a muffle furnace for heating to remove residual carbon and controllably oxidize to obtain SiO2@SiC nanowires; the temperature for carbon removal and controlled oxidation is 890-910°C, and the time is greater than or equal to 100 min.

2. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: The biomass is selected from at least one of coffee shells, peanut shells, rice husks, tobacco straws and cotton straws; after being ground into powder, the particle size thereof is 50-100 meshes.

3. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: The powdered biomass is placed in a mold and vacuum dried to form the mold; the vacuum drying temperature is -50°C to -40°C, and the drying time is 12 to 24 hours.

4. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: The pretreated material obtained in step (1) is placed in a tubular furnace filled with argon gas for carbonization to obtain fluffy and porous biochar.

5. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 4, characterized in that: During carbonization, the argon flow rate is 8-25 ml / min, the carbonization temperature is 500-800°C, and the time is greater than or equal to 100 minutes.

6. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: In step (3), the purity of the silicon and silicon dioxide powder used is >99 wt %, and the mesh size is -100 mesh.

7. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: In step (3), the silicon powder and silicon dioxide are prepared in a mass ratio of 1:

2.

8. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: During high-temperature carbothermal reduction, the flow rate of the protective gas outside the crucible needs to be controlled at 8~20 ml / min.

9. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: In step (4), the reaction temperature is 1400-1600°C; In step (4), after maintaining the reaction temperature, the temperature is lowered at a rate of 4-6°C / min; In step (4), the mass ratio of calcium carbonate to silicon source is 1:1.1-1.

5.

10. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: In step (4), the reaction temperature is 1580-1600°C; In step (4), the mass ratio of calcium carbonate to silicon source is 1:1.2~1.

5.

11. The method for preparing a cable-like SiO2@SiC composite absorbing material according to claim 1, characterized in that: The product obtained in step (4) is directly placed in a muffle furnace and heated to 890-910°C at a heating rate of 8-12°C / min, and then kept warm for 110-130 minutes to remove residual carbon and perform controllable oxidation to obtain SiO2@SiC nanowires.

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