Method for preparing nano-silicon carbide precursor by laser-induced rapid thermal shock

Abstract

The application discloses a kind of laser-induced rapid thermal shock preparation nanometer silicon carbide precursor method, specifically related to nanometer silicon carbide precursor preparation technical field, including the following steps: step one, carbon source raw materials and silicon source raw materials are mixed according to set ratio, after ball milling, sieving and drying treatment, obtain silicon-carbon composite precursor raw material;Step two, the silicon-carbon composite precursor raw material after drying is laid in the double-layer porous material carrier of high thermal conductivity, low reflection characteristic, forms the even layer to be reacted of set thickness, and the double-layer porous material carrier is supported on the heat insulating material of low thermal conductivity coefficient.The application effectively inhibits the carbon deposition generated by secondary pyrolysis by using extremely fast thermal shock, ensures the uniformity and stability of pyrolysis process by closed-loop dynamic power regulation, prevents grain coarsening by in-situ rapid cooling, and the prepared nanometer silicon carbide precursor has good microstructure uniformity and pore structure characteristics.

Description

Technical Field

[0001] This invention relates to the field of precursor preparation technology for nano-silicon carbide, and more specifically, to a method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock. Background Technology

[0002] Silicon carbide (SiC), as a third-generation wide-bandgap semiconductor material, holds an irreplaceable position in power electronics, aerospace, and extreme environment devices due to its high breakdown electric field, high thermal conductivity, and excellent chemical stability. Currently, one of the mainstream technologies for the industrial preparation of high-performance nano-silicon carbide is the precursor impregnation pyrolysis method. This process typically involves impregnating a polymer precursor (such as polysilane) into a reinforcing matrix, followed by crosslinking and thermal pyrolysis in a high-temperature electric furnace, ultimately transforming it into nano-silicon carbide products.

[0003] However, traditional PIP processes rely on macroscopic external heat sources such as resistance furnaces or roasting furnaces for overall heating, which presents insurmountable physical and chemical bottlenecks: Firstly, the thermodynamic lag and kinetic mismatch mean that the heating rate of the electric resistance furnace is extremely slow (usually only 10-20℃ / min), resulting in a pyrolysis reaction cycle of several hours. This slow heating causes gaseous hydrocarbons (such as methane and ethylene) produced by precursor cracking to remain in the micropores of the material for too long, which can easily trigger a serious "secondary cracking reaction". This produces microcrystalline carbon (coke) that blocks the primary pores, which not only reduces the specific surface area of ​​the product, but also causes uncontrolled coarsening of nanocrystals under long-term high-temperature conditions, ultimately destroying the microscopic uniformity of the material. Secondly, uneven thermal field distribution and delayed thermal response, the pyrolysis method that relies on heat conduction will inevitably lead to significant temperature differences between material layers, which can easily cause incomplete local carbonization or overburning, and cannot guarantee the stability of the product in terms of crystal structure and physicochemical properties. Third, the energy utilization efficiency is low. The overall heating mode has a large amount of furnace heat storage loss, resulting in extremely high energy consumption. Moreover, it is difficult to achieve instantaneous and precise power control for the complex kinetic evolution stages during pyrolysis (such as dehydroxylation, crosslinking, carbothermic reduction, etc.).

[0004] In recent years, laser heating technology has been highly anticipated for its application in the field of rapid pyrolysis due to its advantages such as high energy density, extremely rapid heating, and non-contact radiation. However, in practical applications, existing laser pyrolysis platforms still face two major technical challenges: First, the lack of precise positioning between the laser main beam and the reaction layer makes it easy for focusing deviations to occur during operation, resulting in hot or cold spots and causing local overheating or underheating. Second, traditional single-layer flat plate carriers cannot cope with the enormous exhaust pressure brought about by rapid heating. The large amount of volatile oil and gas generated by pyrolysis is difficult to expel instantly, and it is easy to accumulate locally inside the material layer, generating a high-pressure "boiling" effect, which seriously damages the flatness and microstructure integrity of the material layer. Summary of the Invention

[0005] To overcome the above-mentioned defects of the prior art, the present invention provides a method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock, comprising the following steps: Step 1: Mix carbon source raw materials and silicon source raw materials in a set ratio, and then obtain silicon-carbon composite precursor raw materials after ball milling, sieving and drying. Step 2: The dried silicon-carbon composite precursor raw material is laid in a double-layer porous material carrier with high thermal conductivity and low reflectivity to form a flat reaction layer of a set thickness, and the double-layer porous material carrier is supported on a heat insulation material with low thermal conductivity. Step 3: Place the double-layer porous material carrier inside the sealed box, and use a dual-beam auxiliary infrared correction light source to adjust the spatial position of the carrier so that the dual beams converge and overlap on the surface of the layer to be reacted, thereby completing the precise focusing and positioning of the main laser beam. Step 4: Close the sealed chamber and continuously introduce inert gas until the oxygen concentration inside the chamber drops below the preset threshold. Simultaneously turn on the dual-point focusing far-infrared thermometer and the flue gas analyzer to monitor the pyrolysis process online in real time. Step 5: Start the semiconductor laser heating device so that the laser spot radiates onto the surface of the layer to be reacted. According to the preset pyrolysis final temperature and heating rate, the output power of the semiconductor laser is dynamically adjusted through the platform data acquisition terminal combined with the real-time feedback data of the temperature measuring instrument and the flue gas analyzer to realize the closed-loop temperature-controlled pyrolysis reaction. Step 6: After pyrolysis, turn off the semiconductor laser heating device and use continuously introduced inert gas to cool the product in situ. After cooling to room temperature, take out the material and sieve it to obtain the nano silicon carbide precursor.

[0007] As a further improvement to the technical solution of the present invention, the carbon source material in step one is one or more of biomass powder, phenolic resin, asphalt, glucose or organic polymer materials; the silicon source material is one or more of polycarbosilane, polysiloxane, silica sol, nano-SiO2, tetraethoxysilane or organosilicon resin; and the mass ratio of the carbon source material to the silicon source material is 1:0.5 to 1:3.

[0008] As a further improvement to the technical solution of the present invention, the conditions for ball milling in step one are: ball milling speed 40-90 rpm, ball milling time 6-18 h, ball diameter 12-45 mm, and ball-to-material mass ratio 3:1-8:1; the sieving process is to pass through a 100-200 mesh sieve; the drying process is to dry at a temperature of 90-105℃ for 8-11 h.

[0009] As a further improvement to the technical solution of the present invention, the thickness of the reaction layer in step two is 0.2mm to 1.5mm; the double-layer porous material carrier is composed of two metal mesh structures arranged alternately in the upper and lower layers, forming a through exhaust space between the two layers. The thermal conductivity of the carrier at room temperature is higher than 15W / (m·K), and it has low reflection absorption characteristics for semiconductor lasers in the 830-950nm band.

[0010] As a further improvement to the technical solution of the present invention, in step two, the heat insulation material is at least one of quartz fiber mesh, silicon carbide fiber mesh, or foam ceramic board with through pores, and the thermal conductivity of the heat insulation material at room temperature is less than 0.2 W / (m·K); in step three, the dual-beam auxiliary infrared correction light source is set at a specific angle, and the two auxiliary beams are made to converge at a point on the surface of the layer to be reacted by adjusting the position of the carrier, thereby determining the radiation center of the main laser beam.

[0011] As a further improvement to the technical solution of the present invention, in step five, the power of the semiconductor laser heating device is 400W to 10kW, the wavelength is 830 to 950nm, and the power density acting on the surface of the layer to be reacted is 10 to 500W / cm². 2 The diameter of the laser spot is 5mm to 60mm, and the spot completely covers the preset radiation area of ​​the layer to be reacted; the heating rate of the pyrolysis process is 10 to 1000℃ / s, the final pyrolysis temperature is 400 to 2000℃, and the pyrolysis reaction time is controlled within 3 to 50s.

[0012] As a further improvement to the technical solution of the present invention, the response time of the dual-point focusing far-infrared thermometer in steps four and five is less than 10ms, which is used to monitor the temperature difference between the center and the edge of the light spot in real time; the platform data acquisition terminal combines the temperature difference feedback of the thermometer and the gas concentration feedback of the flue gas analyzer to construct a millisecond-level closed-loop adjustment logic, which is used to realize the real-time dynamic optimization of the semiconductor laser output power to eliminate the thermal hysteresis effect.

[0013] As a further improvement to the technical solution of the present invention, the specific method of in-situ cooling in step six is ​​as follows: after turning off the semiconductor laser heating device, the flow rate of the inert gas is increased from 50-200 sccm to 200-500 sccm, and the product is forcibly cooled by the inert gas flow and convection heat transfer until the temperature measuring instrument shows that the product temperature has dropped below 40°C before the sealed box is opened.

[0014] As a further improvement to the technical solution of the present invention, the sampling frequency of the flue gas analyzer in step four is not less than 1Hz, which is used to monitor the concentration of gas components such as CO, CO2, CH4 and H2 in the pyrolysis tail gas in real time, and to help determine the evolution stage of the pyrolysis reaction based on the abrupt change nodes of the gas components, and to feed back the determination signal to the platform data acquisition terminal to coordinately adjust the laser heating power.

[0015] This invention proposes a method for preparing nano-silicon carbide precursors that integrates rapid thermal shock, precise dual-beam positioning, dynamic closed-loop power adjustment, and rapid porous exhaust. Compared with traditional resistance furnace pyrolysis and existing laser pyrolysis methods, it has the following advantages: 1. This invention utilizes the extremely high power density of semiconductor lasers to rapidly heat the precursor. This instantaneous and intense thermal shock effect triggers a "micro-explosion" of gaseous phase precipitation within the material, drastically reducing the physical residence time of volatile components within the micropores from tens of minutes in traditional processes to milliseconds. This instantaneous expulsion process fundamentally disrupts the thermodynamic equilibrium of the secondary pyrolysis reaction, significantly inhibiting the deposition of pyrolysis carbon deposits within the micropores. This maximizes the preservation of the initial pore structure of the precursor, and the specific surface area of ​​the resulting product can be stably controlled between 20 and 295 m². 2 Within the range of / g, the pore structure is uniformly developed; 2. This invention does not adopt the traditional open-loop heating mode of "set-and-hold" heating. Instead, it constructs a two-dimensional closed-loop control logic based on temperature field gradient and gas component evolution by fusing real-time data from a dual-point focusing far-infrared thermometer and a flue gas analyzer. When the flue gas analyzer detects a "concentration mutation" peak in a specific component in the pyrolysis gas, it indicates that the reaction has entered a violent chemical stage. The platform's data acquisition terminal can instantly adjust the output power of the semiconductor laser. The millisecond-level closed-loop response overcomes the defects of traditional resistance furnaces, such as large thermal inertia and temperature control lag, and achieves precise control of the pyrolysis reaction process, ensuring the uniformity and stability of batch product quality.

[0016] 3. The double-layer porous material carrier of the present invention provides a "one-way valve"-like rapid exhaust path for the oil and gas generated by rapid pyrolysis through the through-flow channel formed between the upper and lower layers. This solves the problem of poor exhaust of single-layer carriers leading to local boiling and structural disturbance of the material layer. At the same time, the high thermal conductivity and low reflectivity of the metal mesh carrier not only facilitates the uniform transfer of laser energy, but also achieves in-situ forced convection cooling of the high-temperature products after the reaction by rapidly increasing the gradient of the inert gas flow rate. This rapid cooling method, similar to "physical quenching", greatly shortens the residence time of the products at high temperature, effectively suppresses the uncontrolled growth and coarsening of nanocrystals, and ensures the consistency of product morphology and microstructure.

[0017] 4. This invention significantly reduces the traditional precursor pyrolysis process, which takes several hours or even tens of hours, to within tens of seconds, and eliminates the need for complex post-processing, greatly improving production efficiency and reducing overall energy consumption. The resulting nano-silicon carbide precursor has excellent hierarchical porous structure and electrical conductivity. In addition to its high application value in the traditional silicon carbide ceramics and ceramic matrix composites fields, its high specific surface area also shows great application potential in the fields of carbon-coated precursors for silicon-based anode materials in lithium-ion batteries and high-temperature gas separation membranes. Attached Figure Description

[0018] Figure 1 A flowchart illustrating the rapid preparation method of nano-silicon carbide precursor for implementation purposes; Figure 2 Schematic diagram of the apparatus for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock; Figure 3 The image shows a scanning electron microscope (SEM) image of the nano-silicon carbide precursor obtained in Example 1. Figure 4 The figures show the pore area and pore volume distribution characteristics of the nano-silicon carbide precursor obtained in Example 1. In the figures, (a) represents the pore area; (b) represents the pore area distribution; (c) represents the pore volume; and (d) represents the pore volume distribution. Figure 5 Here is a scanning electron microscope (SEM) image of the nano-silicon carbide precursor obtained in Example 2; Figure 6 The figures show the pore area and pore volume distribution characteristics of the nano-silicon carbide precursor obtained in Example 2. In the figures, (a) represents the pore area; (b) represents the pore area distribution; (c) represents the pore volume; and (d) represents the pore volume distribution. Figure 7 The image shows a scanning electron microscope (SEM) image of the nano-silicon carbide precursor obtained in Example 3. Figure 8 The figures show the pore area and pore volume distribution characteristics of the nano-silicon carbide precursor obtained in Example 3. In the figures, (a) is the pore area; (b) is the pore area distribution; (c) is the pore volume; and (d) is the pore volume distribution.

[0019] The attached diagram is labeled as follows: 1. Sealed enclosure; 2. Laser emitter; 3. Laser lens; 4. Dual-point focusing far-infrared thermometer; 5. Double-layer porous material carrier; 6. Thermal insulation material; 7. Spring damping mechanism; 8. High-precision weighing sensor; 9. Air inlet; 10. Exhaust outlet; 11. Flue gas analyzer; 12. External semiconductor laser emitter; 13. Water-cooled plate; 14. Water-cooled pump; 15. Circulating water tank; 16. Nitrogen cylinder; 17. Platform data acquisition terminal. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The technical solutions of the present invention are not limited to the following specific embodiments. Those skilled in the art can make equivalent transformations and improvements to the parameters of the present invention based on the actual material characteristics without departing from the spirit and substance of the invention, and these all fall within the protection scope of the present invention.

[0021] Reference Figure 1 and Figure 2 The present invention provides an apparatus and process route for implementing the above method. The apparatus mainly includes a sealed box 1, a laser emitter 2, a laser lens 3, a dual-point focusing far-infrared thermometer 4, a double-layer porous material carrier 5, a heat insulation material 6, a spring damping mechanism 7, a high-precision weighing sensor 8, an air inlet 9, an exhaust outlet 10, a flue gas analyzer 11, an external semiconductor laser emission source 12, a water-cooled plate 13, a water-cooled pump 14, a circulating water tank 15, a nitrogen cylinder 16, and a platform data acquisition terminal 17.

[0022] Its core working logic is as follows: by using dual-beam auxiliary infrared correction light sources set at a specific angle on both sides of the sealed box 1, a converging light spot is formed on the surface of the material, thereby completing the precise spatial positioning of the main laser beam and the layer to be reacted. During the reaction, the dual-point focusing far-infrared thermometer 4 captures the temperature gradient between the center and edge of the light spot in real time with a millisecond-level response time. Together with the flue gas analyzer 11, it tracks the concentration of components such as CO, CO2, CH4 and H2 in the pyrolysis tail gas in real time, forming a two-dimensional monitoring signal. This monitoring signal is fed back to the platform data acquisition terminal 17. The microprocessor in the terminal uses a preset PID algorithm to generate millisecond-level control commands and dynamically adjust the output current of the external semiconductor laser emission source 12, ultimately achieving precise dynamic matching between the laser output power and the pyrolysis reaction process.

[0023] The preparation method of the present invention specifically includes the following steps: S1. Precursor pretreatment: Carbon source materials (such as biomass powder, phenolic resin, asphalt, glucose, or organic polymer materials, etc.) are mixed with silicon source materials (such as polycarbosilane, polysiloxane, silica sol, nano-SiO2, tetraethoxysilane, or organosilicon resin, etc.) at a mass ratio of 1:0.5 to 1:3, and then placed in a ball mill for wet or dry grinding. The ball milling process parameters are set as follows: ball milling speed 40 to 90 rpm, ball milling time 6 to 18 h, grinding ball diameter 12 to 45 mm, and ball-to-material mass ratio 3:1 to 8:1. After ball milling, the slurry or powder is passed through a 100 to 200 mesh sieve and placed in an oven to dry at a temperature of 90 to 105 °C for 8 to 11 h, finally obtaining silicon-carbon composite precursor materials with uniform particle size.

[0024] S2. Material Spreading and Focusing: The dried precursor raw material is evenly spread on a metal mesh double-layer porous material carrier 5 with a thermal conductivity higher than 15 W / (m·K) and low reflectivity to semiconductor lasers with wavelengths of 830–950 nm, forming a flat reaction layer with a thickness of 0.2 mm to 1.5 mm. A through-venting space is formed between the upper and lower layers of the double-layer carrier, which can provide a smooth exhaust channel for gaseous products during rapid heating and reaction, avoiding the "boiling" phenomenon caused by local high pressure. Then, the support platform of the loaded material carrier is placed on a heat-insulating material 6 with a thermal conductivity lower than 0.2 W / (m·K) (such as quartz fiber mesh, silicon carbide fiber mesh, or foam ceramic board). The device is pushed into the sealed box, the dual-beam auxiliary infrared correction light source is turned on, and the spatial position of the carrier inside the sealed box is finely adjusted so that the two auxiliary infrared beams converge at a point on the surface of the reaction layer, thereby completing the precise focusing and positioning of the laser main beam.

[0025] S3. Construction of an inert atmosphere environment: Close the sealed chamber 1, turn on the dual-point focusing far-infrared thermometer 4 and the flue gas analyzer 11, and continuously introduce inert gases such as nitrogen, helium or argon through the symmetrical air inlets 9 on both sides of the chamber; set the gas flow rate to 50-200 sccm to purge the oxygen inside the sealed chamber and form a stable airflow covering layer above the reaction layer. Once the oxygen concentration inside the chamber drops below 1%, the reaction preparation state begins.

[0026] S4. Laser Thermal Shock and Closed-Loop Temperature Control: Activate the semiconductor laser heating device. The laser wavelength is 830–950 nm, the power is adjustable from 400 W to 10 kW, and the laser spot diameter is 5 mm–60 mm, ensuring complete coverage of the preset radiation area of ​​the layer to be reacted. The power density applied to the material surface ranges from 10 to 500 W / cm². 2 .

[0027] During the pyrolysis process, the platform data acquisition terminal 17 analyzes the monitoring data fed back by the dual-point focusing far-infrared thermometer 4 and the flue gas analyzer 11 in real time. When a "sudden spike" is detected in the CO2 or CH4 concentration in the flue gas, the terminal determines that the reaction has entered a violent cracking stage. By dynamically controlling the laser output power, closed-loop temperature control is achieved, so that the heating rate of the material is precisely maintained between 10 and 1000℃ / s. The entire pyrolysis reaction can be completed within 3 to 50 seconds, and the final pyrolysis temperature is set between 400 and 2000℃.

[0028] S5. In-situ forced cooling and collection: After the reaction reaches the preset pyrolysis final temperature and is maintained for a set time, the semiconductor laser emission source is immediately turned off. At this time, the flow rate of the inert gas is instantaneously increased from the initial 50-200 sccm to 200-500 sccm. The strong convective heat transfer effect of the large flow rate is used to rapidly and forcibly cool the product, which greatly shortens the relaxation time of the product under high temperature environment and effectively avoids uncontrolled coarsening of the grains. After the reading of the far-infrared thermometer drops below 40°C, the sealed box is opened, the material carrier is taken out and the black particulate product inside is collected. After being sieved through a 50-100 mesh screen, the nano silicon carbide precursor is finally obtained.

[0029] Example 1 (Heating rate: 10℃ / s) This embodiment uses a relatively mild heating rate to mainly investigate the initial development process of the pore structure of nano-silicon carbide precursors in the medium and low temperature region.

[0030] Steps and parameters: S1. Take 10g of poplar wood powder as carbon source material, mix it with silicon dioxide as silicon source material in a set ratio, grind it in a ball mill, pass it through a 120-mesh sieve, and dry it in an oven at 105℃ for 10h. S2. Divide the dried silicon-carbon composite precursor raw material into four equal parts and spread them evenly on a double-layer porous metal mesh carrier with a thermal conductivity of 15W / (m·K). The thickness of the spread material is 1mm. Place a heat insulation material with a thermal conductivity of 0.15W / (m·K) under the carrier and turn on the dual-beam auxiliary infrared correction light source to adjust the convergence point. S3. Introduce nitrogen gas at a flow rate of 150 sccm. Once the oxygen content drops below 1%, start a semiconductor laser with a wavelength of 838 nm and an output power of 800 W. S4. Set the heating rate to 10℃ / s, and set the pyrolysis final temperature to 400℃, 500℃, 600℃ and 700℃ for the four groups of samples respectively. Hold the final temperature for 10s after reaching it. S5. Turn off the laser, use nitrogen gas flow to assist cooling, and remove the product when the temperature gauge shows that it has dropped below 40°C. Pass the product through a 100-mesh sieve to obtain the final product.

[0031] Analysis of the effect of this embodiment: The scanning electron microscope (SEM) image of the nano-silicon carbide precursor obtained in this embodiment is shown below. Figure 3 As shown in the figure, at a heating rate of 10℃ / s, as the final pyrolysis temperature increases from 400℃ to 700℃, the surface of the precursor gradually transitions from a relatively dense structure to a uniformly developed porous structure, with pore sizes concentrated in the range of 5–80 nm.

[0032] Combination Figure 4 BET pore structure analysis of (a)-(d) shows that this temperature range belongs to the initial pyrolysis stage of the silicon-carbon precursor. At 400℃~500℃, the pore volume is low, mainly due to surface dehydroxylation and the volatilization of a small amount of small molecules; while when the final pyrolysis temperature reaches 600℃, the pore area increases dramatically (reaching 26.76m²). 2 / g), with a micropore volume of approximately 0.0136 cm³. 3 / g, accounting for 56.20% of the total pore volume, indicating that the carbothermic reduction reaction began to dominate the development of the pore structure at this point, and the overall pore volume distribution was concentrated in the range of 0.1–0.5 cm. 3 / g.

[0033] Example 2 (Heating rate: 50℃ / s) This embodiment increases the heating rate to observe the nonlinear relationship between the development of material pore structure and heating rate when the thermal shock intensity increases.

[0034] Steps and parameters: The steps of raw material pretreatment, material thickness, auxiliary infrared focusing, nitrogen flow rate and final temperature setting are the same as in Example 1. The difference is that in this example, a semiconductor laser device with an output power of 3kW and a wavelength of 915nm is activated to increase the heating rate to 50℃ / s. The final pyrolysis temperature is set to 400℃, 500℃, 600℃ and 700℃ for the four groups of samples, respectively. After reaching the final temperature, it is held for 10s, then the laser is turned off, the temperature is cooled down, and the finished product is obtained by passing it through a 100-mesh sieve.

[0035] Analysis of the effect of this embodiment: Its scanning electron microscope (SEM) image is as follows Figure 5 As shown, compared with Example 1, at the same pyrolysis final temperature of 700℃, the heating rate of 50℃ / s significantly accelerated the formation of cracks and pores on the precursor surface, made the structure more rough, and the lamellar peeling morphology gradually became more prominent.

[0036] Combination Figure 6 As shown in the pore area and pore volume distribution diagrams (a)-(d), after increasing the heating rate, the total pore volume of the product is generally distributed between 0.05 and 0.2 cm³. 3 / g range; It is worth noting that at a final temperature of 700℃, the ratio of micropores (0.35~2nm) to mesopores (10~50nm) changed, and the development of large mesopores was better than that in Example 1. This indicates that accelerating the heating rate can enhance the "micro-explosion effect" inside the material, which is conducive to the rapid overflow of gaseous products and thus promotes the formation of large-diameter pores. Since the high heating rate leads to a very short pyrolysis time of the product, it avoids secondary cracking and carbon deposition that clogs the micropores. Therefore, even at a lower final temperature, it can still maintain a high pore volume stability.

[0037] Example 3 (Heating rate: 100℃ / s) This embodiment further increases the heating rate, aiming to demonstrate the evolution limit of the product microstructure under extremely rapid thermal shock conditions.

[0038] Steps and parameters: The steps of raw material pretreatment, material thickness, auxiliary infrared focusing, nitrogen flow rate and final temperature setting are the same as in Example 1. The difference is that in this example, a semiconductor laser device with an output power of 4.5kW and a wavelength of 940nm is activated to increase the heating rate to 100℃ / s. The final pyrolysis temperature is set to 400℃, 500℃, 600℃ and 700℃ for the four groups of samples, respectively. After reaching the final temperature, it is held for 10s, then the laser is turned off, the temperature is cooled down, and the finished product is obtained by passing it through a 100-mesh sieve.

[0039] Analysis of the effect of this embodiment: Scanning electron microscope (SEM) image as shown Figure 7 As shown, under this process condition, even at the initial pyrolysis temperature of 400℃, the material surface already exhibits a clear porous structure. With the thermal shock of increasing temperature, the crystal skeleton is accelerated to rearrange, forming a complex three-dimensional porous network of interwoven cracks and lamellae.

[0040] from Figure 8 The pore area and pore volume distribution characteristics in (a)-(d) show that, under the extremely rapid heating condition of 100℃ / s, the pore volume at the final pyrolysis temperature of 700℃ reaches 0.0252 cm³. 3 Compared to Examples 1 and 2, the pore volume distribution in Example 3 is significantly concentrated in the mesopore and macropore regions. This indicates that a microstructure with a multi-level pore distribution can be directionally prepared by combining a high heating rate with a high final temperature, providing richer active sites and reaction interfaces for subsequent silicon carbide ceramics and composite materials.

[0041] In summary, the preparation method of the present invention has significant inherent regularity: (1) The heating rate has a decisive influence on the orientation of the precursor pore structure. When the target is to prepare a precursor with a high micropore ratio, a heating rate of 10 to 50 °C / s combined with a pyrolysis final temperature of 400 to 600 °C is preferred. When the target is to prepare a precursor with a high proportion of mesopores and macropores and a coexistence of multi-level pores, a heating rate of 100 °C / s or higher combined with a pyrolysis final temperature of 700 °C or higher is preferred. (2) The dual-beam assisted infrared correction light source used in this invention solves the problem of "cold spot" or "overburning" caused by optical path deviation in traditional laser heating. At the same time, the platform data acquisition terminal integrates the millisecond-level temperature difference feedback of the dual-point focusing far-infrared thermometer and the component mutation feedback of the flue gas analyzer to construct a closed-loop control logic. When the pyrolysis reaction enters the violent cracking period (the concentration of CH4 in the flue gas suddenly increases), the terminal can reduce the laser output power in time to suppress excessive thermal shock damage. When it enters the carbothermic reduction reaction period (the concentration of CO in the flue gas increases), the power is appropriately increased to accelerate the deep reaction. This dynamic control strategy is one of the core technical advantages of this invention that distinguishes it from the traditional fixed power pyrolysis method. (3) After the reaction is completed, the flow rate of inert gas is immediately increased to 200-500 sccm, which is to "freeze" and protect the micro-nano structure of the high-temperature product. Compared with the slow cooling of the traditional electric furnace, which causes the nanocrystals to stay at high temperature for a long time and thus undergo uncontrolled coarsening, the rapid cooling of the present invention can keep the product grain size and pore volume distribution at the same level as the final state of the reaction, ensuring the consistency and stability of the nano silicon carbide precursor structure.

[0042] In summary, this invention utilizes the advantages of high energy density and fast response of semiconductor lasers, combined with the rapid exhaust characteristics of a double-layer porous carrier, dual-beam infrared-assisted positioning, millisecond-level dynamic closed-loop temperature control, and in-situ rapid cooling process, to successfully achieve efficient, uniform, and controllable preparation of nano-silicon carbide precursors.

[0043] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing nano-sized silicon carbide precursors by laser-induced rapid thermal shock, characterized in that, Includes the following steps: Step 1: Mix carbon source raw materials and silicon source raw materials in a set ratio, and then obtain silicon-carbon composite precursor raw materials after ball milling, sieving and drying. Step 2: The dried silicon-carbon composite precursor raw material is laid in a double-layer porous material carrier with high thermal conductivity and low reflectivity to form a flat reaction layer of a set thickness, and the double-layer porous material carrier is supported on a heat insulation material with low thermal conductivity. Step 3: Place the double-layer porous material carrier inside the sealed box, and use a dual-beam auxiliary infrared correction light source to adjust the spatial position of the carrier so that the dual beams converge and overlap on the surface of the layer to be reacted, thereby completing the precise focusing and positioning of the main laser beam. Step 4: Close the sealed chamber and continuously introduce inert gas until the oxygen concentration inside the chamber drops below the preset threshold. Simultaneously turn on the dual-point focusing far-infrared thermometer and the flue gas analyzer to monitor the pyrolysis process online in real time. Step 5: Start the semiconductor laser heating device so that the laser spot radiates onto the surface of the layer to be reacted. According to the preset pyrolysis final temperature and heating rate, the output power of the semiconductor laser is dynamically adjusted through the platform data acquisition terminal combined with the real-time feedback data of the temperature measuring instrument and the flue gas analyzer to realize the closed-loop temperature-controlled pyrolysis reaction. Step 6: After pyrolysis, turn off the semiconductor laser heating device and use continuously introduced inert gas to cool the product in situ. After cooling to room temperature, take out the material and sieve it to obtain the nano silicon carbide precursor.

2. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In step one, the carbon source material is one or more of biomass powder, phenolic resin, asphalt, glucose, or organic polymer materials; the silicon source material is one or more of polycarbosilane, polysiloxane, silica sol, nano-SiO2, tetraethoxysilane, or organosilicon resin; and the mass ratio of the carbon source material to the silicon source material is 1:0.5 to 1:

3.

3. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: The conditions for ball milling in step one are as follows: ball milling speed 40-90 rpm, ball milling time 6-18 h, ball diameter 12-45 mm, and ball-to-material mass ratio 3:1-8:1; the sieving process is to pass through a 100-200 mesh sieve; the drying process is to dry at a temperature of 90-105℃ for 8-11 h.

4. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In step two, the thickness of the reaction layer is 0.2 mm to 1.5 mm. The double-layer porous material carrier is composed of two metal mesh structures that are spaced apart from each other, forming a through-venting space between the two layers. The thermal conductivity of this carrier at room temperature is higher than 15 W / (m·K), and it has low reflection absorption characteristics for semiconductor lasers in the 830-950 nm band.

5. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In step two, the heat insulation material is at least one of quartz fiber mesh, silicon carbide fiber mesh, or foam ceramic board with through pores, and the thermal conductivity of the heat insulation material at room temperature is less than 0.2 W / (m·K); in step three, the dual-beam auxiliary infrared correction light source is set at a specific angle, and the two auxiliary beams are made to converge at a point on the surface of the layer to be reacted by adjusting the position of the carrier, thereby determining the radiation center of the main laser beam.

6. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In step five, the semiconductor laser heating device has a power of 400W to 10kW, a wavelength of 830 to 950nm, and a power density applied to the surface of the layer to be reacted of 10 to 500W / cm². 2 The diameter of the laser spot is 5mm to 60mm, and the spot completely covers the preset radiation area of ​​the layer to be reacted; the heating rate of the pyrolysis process is 10 to 1000℃ / s, the final pyrolysis temperature is 400 to 2000℃, and the pyrolysis reaction time is controlled within 3 to 50s.

7. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In steps four and five, the response time of the dual-point focusing far-infrared thermometer is less than 10ms, which is used to monitor the temperature difference between the center and edge of the light spot in real time. The platform data acquisition terminal combines the temperature difference feedback of the thermometer and the gas concentration feedback of the flue gas analyzer to construct a millisecond-level closed-loop adjustment logic, which is used to realize the real-time dynamic optimization of the semiconductor laser output power to eliminate the thermal hysteresis effect.

8. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: The specific method of in-situ cooling in step six is ​​as follows: after turning off the semiconductor laser heating device, the flow rate of the inert gas is increased from 50-200 sccm to 200-500 sccm. The product is forced to cool down by the inert gas flow and convection heat transfer until the temperature measuring instrument shows that the product temperature has dropped below 40°C before the sealed box is opened.

9. The method for preparing nano-silicon carbide precursors by laser-induced rapid thermal shock according to claim 1, characterized in that: In step four, the sampling frequency of the flue gas analyzer is not less than 1Hz, which is used to monitor the concentration of gas components such as CO, CO2, CH4 and H2 in the pyrolysis tail gas in real time. The analysis of the abrupt change nodes of the gas components helps to determine the evolution stage of the pyrolysis reaction, and the determination signal is fed back to the platform data acquisition terminal to coordinate the adjustment of the laser heating power.

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