A densification process based on fly ash for extracting SiC

Abstract

This disclosure proposes a densification process for SiC extracted from fly ash, belonging to the field of inorganic non-metallic materials technology. The process includes: pretreating fly ash and preparing β-SiC powder or SiC particles from the pretreated fly ash via carbothermal reduction reaction or ultra-high-speed gas flow method; using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, depositing SiC fiber reinforcement on the surface of the mandrel via CVD reaction using carbon or tungsten wire as the mandrel; mixing TaC powder and CNTs in a predetermined mass ratio and bonding them with epoxy resin to form a slurry, which is then subjected to static settling and vacuum filtration to obtain a composite additive; mixing the β-SiC powder or SiC particles, the composite additive, and the SiC fiber reinforcement to form a sintering raw material; using zirconium oxide as a phase transformation toughening agent and boron nitride as a lubricant and pore regulator; and subjecting the sintering raw material to multi-stage spark plasma sintering treatment to form densified SiC fibers.

Description

Technical Field

[0001] This disclosure belongs to the field of inorganic non-metallic materials technology, specifically relating to a densification process based on SiC extracted from fly ash. Background Technology

[0002] In existing technologies, the utilization of fly ash is usually limited by its inherent structural defects and impurity content, which directly affects the performance and density of the final product.

[0003] For example, one of the existing technologies, Chinese patent application CN114935531A, discloses a method for restoring the paleopore structure of tight sandstone reservoirs on steep slopes of rift basins. Although this method can accurately depict diagenesis and pore structure, it does not solve the problem of the chain reaction of temperature gradient and structural defects during densification, nor does it involve the utilization of fly ash.

[0004] For example, one of the existing technologies, Chinese patent application CN119932909A, discloses an inorganic fiber modifier and its application in the preparation of high-strength corrosion-resistant inorganic fibers. It improves the performance of fibers through processes such as slag ball gradient removal and structural strengthening. However, it also fails to solve the temperature gradient problem in the densification process and does not utilize SiC extracted from fly ash as an additive to further improve the material performance.

[0005] While these technical solutions have improved the material's performance to some extent, they have failed to effectively address the temperature gradient problem during densification and the resulting chain reaction of structural defects, which limits the material's final performance and application range.

[0006] Furthermore, the utilization of fly ash in existing technologies is relatively limited, failing to fully realize its potential value.

[0007] To address the aforementioned technical shortcomings, this disclosure proposes a densification process based on SiC extraction from fly ash, aiming to eliminate the temperature gradient problem during densification, suppress structural defects caused by fly ash impurities, and block the chain reaction of defects on performance. Summary of the Invention

[0008] This disclosure aims to at least solve one of the technical problems existing in the prior art, and to provide a densification process based on SiC extraction from fly ash.

[0009] This disclosure provides a densification process based on SiC extraction from fly ash, the process comprising: β-SiC powder or SiC particles are prepared by pretreating fly ash and then preparing the pretreated fly ash by carbothermal reduction reaction or hypersonic gas flow method. Using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, SiC fiber reinforcements were prepared by deposition on the surface of carbon or tungsten wires via CVD reaction. TaC powder and CNTs were mixed in a predetermined mass ratio and bonded with epoxy resin to form a slurry. After standing and vacuum filtration, a composite additive was obtained. The β-SiC powder or SiC particles, the composite additive, and the SiC fiber reinforcement are mixed to form a sintering raw material. Zirconia is used as a phase transformation toughening agent, and boron nitride is used as a lubricant and pore regulator. The sintering raw material is subjected to multi-stage spark plasma sintering treatment to form dense SiC fibers. Optionally, the carbothermic reduction reaction is carried out at a temperature of 1800~2200°C for 2-10 hours.

[0010] Optionally, the particle size of the β-SiC powder is 0.1 to 5 μm, and the particle size of the SiC particles is 10-60 nm.

[0011] Optionally, the mass ratio of TaC powder, CNTs and epoxy resin is 1:(1-10):2.

[0012] Optionally, the slurry can be left to stand for 24-36 hours.

[0013] Optionally, the content of the composite additive is 0.5-1.5% of the mass of the fly ash.

[0014] Optionally, the CVD reaction on the mandrel surface is carried out at a temperature of 1200~1400°C for 30-90 hours.

[0015] Optionally, the discharge plasma multi-stage sintering process includes the following steps: First stage: rapidly heat to 800-1200°C at a rate of 5-15 °C / s, with a pressure of less than 20 MPa and a holding time of 1-10 min; Second stage: Heat to 1500-1800°C at a rate of 8-30 °C / s, apply a medium to isostatic pressure of 30-50 MPa, and hold for 5-15 minutes; The third stage involves heating the temperature at a rate of 6-20 °C / s to the final sintering temperature of 1900-2200 °C, while simultaneously increasing the isostatic pressure to 80-200 MPa, and holding the temperature for 10-30 minutes.

[0016] Optionally, the pretreatment of fly ash includes: Use a 50-120 mesh sieve to sieve the fly ash and wash it with water. After stirring, let it stand, pour off the top layer of clear water, repeat the washing process several times, and then dry it.

[0017] Optionally, during the multi-stage discharge plasma sintering treatment of the SiC fiber reinforcement, the densification process is monitored by an online X-ray diffractometer and an infrared thermal imager. If a local high-temperature area exceeds the warning temperature, the pressure is adjusted, and the process continues until densification is completed.

[0018] This disclosure proposes a densification process for SiC extracted from fly ash. The process includes: pretreating fly ash and preparing β-SiC powder or SiC particles from the pretreated fly ash via carbothermal reduction reaction or hypersonic gas flow method; using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, and using carbon filament or tungsten filament as the core shaft, depositing SiC fiber reinforcement on the core shaft surface via CVD reaction; mixing TaC powder and CNTs in a predetermined mass ratio and bonding them with epoxy resin to form a slurry, which is then subjected to static settling and vacuum filtration to obtain a composite additive; mixing the β-SiC powder or SiC particles, the composite additive, and the SiC fiber reinforcement to form a sintering raw material; using zirconium oxide as a phase transformation toughening agent and boron nitride as a lubricant and pore regulator, subjecting the sintering raw material to multi-stage spark plasma sintering treatment to form densified SiC fibers. The densification process based on SiC extraction from fly ash proposed in this disclosure can eliminate the temperature gradient problem in the densification process, and improve the densification degree and material properties by introducing fly ash-based nanocomposite additives to suppress defects caused by fly ash particles. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the densification process based on SiC extraction from fly ash, as described in a specific embodiment of this disclosure. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this disclosure, the disclosure will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain this disclosure and represent a part of the embodiments of this disclosure, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the protection scope of this disclosure.

[0021] As shown in Figure 1, this disclosure provides a densification process S100 based on SiC extraction from fly ash, specifically including the following steps S110~S140: S110. Pre-treat fly ash and mix the pre-treated fly ash with multi-walled carbon nanotubes. Carbothermic reduction reaction is carried out under an inert atmosphere to obtain β-SiC powder. In addition, the pre-treated fly ash can also be used to prepare SiC particles by ultra-high speed airflow method.

[0022] In step S110, the pretreatment of fly ash is carried out as follows: industrial waste fly ash is collected, sieved through a 50-120 mesh sieve, washed with water, stirred and allowed to stand, the upper layer of clear water is poured off, the washing process is repeated several times, and then dried to remove metal impurities from the fly ash and obtain pure fly ash.

[0023] In step S110, the pretreated fly ash can be converted into β-coal SiC powder by carbothermic reduction reaction. The specific process is as follows: The pretreated fly ash and CNTs mixture is placed in a high-temperature pyrolysis furnace under an inert atmosphere and heated to 1800~2200°C. The silica in the fly ash is converted into silicon carbide through carbothermic reduction reaction (the main reaction is: SiO2 + 3C → SiC + 2CO). Subsequently, the reaction product is acid washed, filtered and dried to obtain high-purity β-SiC powder with a particle size of 0.1 to 5 μm.

[0024] It should be understood that in the above steps, carbon nanotubes serve as the main carbon source, methylchlorosilane (MTS) as the silicon source, hydrogen (H₂) as the carrier gas and surfactant, and oxygen (O) as the oxidant. Through a series of chemical reactions, the amorphous silica remaining in fly ash is converted into high-purity silicon nitride and silicon oxide, which are then further refined to obtain SiC.

[0025] In step S110, the pretreated fly ash can be used to form SiC particles using a supersonic airflow method. The specific process is as follows: the fly ash can be poured into a high-pressure autoclave and subjected to pressure filtration, washing, and drying in sequence to obtain clean silicon oxide powder. An appropriate amount of the obtained silicon oxide powder is taken and formed into nano-sized SiC particles under the action of supersonic airflow. The particle size of the SiC particles is 10-60 nm.

[0026] It should be noted that the carbothermal reduction reaction and the hypersonic gas flow method described above are two parallel and feasible technical routes for preparing SiC raw materials. Both methods are for obtaining SiC raw materials from fly ash that can be used for subsequent densification sintering. The products obtained by the above two methods can be used as SiC fiber reinforcements, composite additives, etc. in subsequent processes for mixing and sintering.

[0027] S120. Mix TaC powder and CNTs in a predetermined mass ratio and bind them with epoxy resin to form a slurry. After standing and vacuum filtration, a composite additive is obtained.

[0028] In some preferred embodiments, the mass ratio of TaC powder, CNTs, and epoxy resin is 1:(1-10):2. It should be noted that the epoxy resin is cured under heat treatment at 80°C for 2 hours.

[0029] In some preferred embodiments, the slurry is allowed to stand for 24-36 hours.

[0030] S130: Using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, SiC fiber reinforcement is prepared by deposition on the surface of the mandrel through CVD reaction using carbon or tungsten wire as the mandrel.

[0031] In some preferred embodiments, when SiC fibers are prepared by CVD, the vaporization temperature of MTS is controlled at 30~50°C, the deposition zone temperature is controlled at 1200~1400°C, and the time is 30~90h.

[0032] It should be understood that the preparation of SiC fibers using chemical vapor deposition (CVD) equipment involves using SiC powder as raw material, acetylene (EA) as carbon source gas, Ar as dilution gas, NH as hydrogen carrier, and O as oxidant. The precursor of silicon carbide fibers is generated at high temperature through combustion synthesis.

[0033] It should be noted that the SiC fiber reinforcement obtained in step S130, the SiC powder obtained in step S110, and the composite additive obtained in step S120 are all used as in-situ repair materials in the subsequent sintering process.

[0034] S140. The β-SiC powder or SiC particles obtained in step S110, the composite additive obtained in step S120, and the SiC fiber reinforcement obtained in step S130 are mixed to form a sintering raw material. At the same time, zirconium oxide is used as a phase transformation toughening agent, and boron nitride is used as a lubricant and pore control agent. Under these conditions, the sintering raw material is subjected to multi-stage spark plasma sintering treatment to form dense SiC fibers.

[0035] In some preferred embodiments, the content of the composite additive is 0.5-1.5% of the mass of the fly ash.

[0036] In some preferred embodiments, the discharge plasma multi-stage sintering process includes the following steps: The first stage involves rapidly heating to 800-1200°C at a rate of 5-15 °C / s, with a pressure less than 20 MPa, and holding for 1-10 minutes. No pressure or a very low pressure (e.g., 5-20 MPa) may be applied at this stage. The aim is to quickly bypass the activation temperature of the oxide layer on the powder surface while avoiding cracking caused by thermal shock.

[0037] Second stage: The temperature is increased to 1500-1800°C at a rate of 8-30 °C / s, and a medium to isostatic pressure of 30-50 MPa is applied. The holding time is 5-15 min. During this stage, the powder begins to diffuse rapidly and densify.

[0038] The third stage involves heating at a rate of 6-20 °C / s to the final sintering temperature of 1900-2200 °C, while simultaneously increasing the isostatic pressure to 80-200 MPa and holding for 10-30 minutes to achieve complete densification and grain growth.

[0039] It should be noted that the above-mentioned multi-stage sintering process is carried out under vacuum or inert atmosphere protection, and the second stage sintering process is performed after the first stage, followed by the third stage sintering process. In-situ repair is simultaneously achieved during the discharge plasma sintering processes of the second and third stages.

[0040] In this embodiment, through the above-mentioned multi-stage adaptive control, combined with the addition of zirconium oxide (ZrO2) as a phase transformation toughening agent and boron nitride (BN) as a lubricant and pore regulator, the final porosity, microstructure and mechanical strength of the sintered body are synergistically regulated.

[0041] It should also be noted that during the multi-stage spark plasma sintering treatment of the SiC fiber reinforcement, the densification process was monitored using an online X-ray diffractometer and infrared thermal imager. If a localized high-temperature region exceeded the warning temperature, the pressure was adjusted, and this process continued until densification was complete. It is important to note that the pressure adjustment here needs to be set according to the warning temperature. For example, at a warning temperature of 80°C, the pressure is adjusted to 40 MPa. As another example, at a warning temperature of 100°C, the pressure is adjusted to 70 MPa.

[0042] This embodiment employs a multi-stage adaptive SPS process, combined with in-situ monitoring and AI optimization systems, to dynamically adjust temperature and pressure parameters, avoid temperature gradients, and introduce fly ash-based nanocomposite additives to repair pores and cracks in situ during sintering, thereby significantly improving the density and performance of the material.

[0043] The densification process based on SiC extracted from fly ash will be further explained below with reference to specific embodiments: Example 1 This example presents a densification process for SiC extracted from fly ash, including the following steps: S1. Fly ash screening and washing: Use a 50-mesh sieve to screen the fly ash to remove large particles; add water to wash the fly ash, stir and let it stand for 12 hours, pour off the upper layer of clear water, and repeat 4 times. S2. Fly ash drying and treatment: Dry the fly ash obtained in the previous step; pour the fly ash into a high-pressure autoclave, and perform pressure filtration, washing and drying treatment in sequence to obtain clean silica powder. Take an appropriate amount of the obtained silica powder and form nano-sized SiC particles under the action of supersonic airflow. S3. Using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, carbon wire or tungsten wire is used as the core shaft. The SiC fiber reinforcement is prepared by CVD reaction at a temperature of 1300℃ for 60h. S4. Preparation of composite additive: CNTs and TaC are mixed in mass ratio and bonded with epoxy resin to form a slurry. After standing for 24 hours, the composite additive is obtained by vacuum filtration. S5. In-situ modification of fly ash: Nanoscale SiC particles, SiC fiber reinforcement and composite additives are mixed to form sintering raw materials. The amount of composite additives added is 0.5% of the mass of fly ash (the melting points of SiC and CNTs are 2150°C and 3000°C, respectively). The sintering raw materials are sintered by multi-stage adaptive SPS.

[0044] During the above-mentioned discharge plasma sintering process, the densification process is monitored by an online X-ray diffractometer and an infrared thermal imager. If a local high-temperature area exceeds the warning temperature of 80°C, the pressure is adjusted to 40 MPa. This process continues until densification is completed and in-situ defect repair is achieved.

[0045] In step S2, the obtained nanoscale SiC has a particle size of 10-60 nm. Nanoscale SiC particles in this size range can be used as highly active raw materials for subsequent carbothermal reduction reactions or directly as part of the sintering raw materials.

[0046] In step S3, the mass ratio of CNTs, TaC, and epoxy resin is 1:1:2.

[0047] In step S4, the SPS sintering furnace used is a high-temperature vacuum sintering furnace of model KL-SPS-73 from Yantai Jinlin Surface Thermal Technology Co., Ltd.

[0048] In step S4, the specific operations are as follows: The material is placed in a graphite crucible, and under an argon protective atmosphere, the following steps are performed: First stage: The temperature is rapidly increased to 1000°C at a rate of 5°C / s, a relatively low pressure of 10 MPa is applied, and the temperature is held for 5 min to remove impurities and initially activate the powder. Second stage: The temperature is increased to 1650°C at a rate of 10°C / s, the isostatic pressure is increased to 40 MPa, and the temperature is held for 10 min to promote rapid densification of the powder. Third stage: The temperature is increased to the final sintering temperature of 2050°C at a rate of 15°C / s, while the isostatic pressure is simultaneously increased to 150 MPa, and the temperature is held for 20 min to achieve complete densification and grain optimization.

[0049] Example 2 This example presents a densification process and apparatus based on SiC extraction from fly ash, including the following steps: S1. Fly ash screening and washing: Use a 120-mesh sieve to screen the fly ash to remove large particles; add water to wash the fly ash, stir and let it stand for 12 hours, pour off the top layer of clear water, and repeat 8 times. S2. Fly ash drying and treatment: Dry the fly ash obtained in the previous step; pour the fly ash into a high-pressure autoclave, and perform pressure filtration, washing and drying treatment in sequence to obtain clean silica powder. Take an appropriate amount of the obtained silica powder and form nano-sized SiC particles under the action of supersonic airflow. S3. Using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, carbon wire or tungsten wire is used as the core shaft. The SiC fiber reinforcement is prepared by CVD reaction at a temperature of 1300℃ for 60h. S4. Preparation of composite additive: CNTs and TaC are mixed in mass ratio and bonded with epoxy resin to form a slurry. After standing for 36 hours, the composite additive is obtained by vacuum filtration. S5. In-situ modification of fly ash: Nanoscale SiC particles, SiC fiber reinforcement and composite additives are mixed to form sintering raw materials, and multi-stage adaptive SPS sintering is adopted.

[0050] During the above-mentioned discharge plasma sintering process, the densification process is monitored by an online X-ray diffractometer and an infrared thermal imager. If a local high-temperature area exceeds the warning temperature of 100°C, the pressure is adjusted to 70MPa. This process continues until densification is completed and in-situ defect repair is achieved.

[0051] In step S3, the mass ratio of CNTs, TaC, and epoxy resin is 1:1:2.

[0052] In step S3, the particle size of the nanoscale SiC obtained is 10-60 nm.

[0053] In step S4, the SPS sintering furnace used is a high-temperature vacuum sintering furnace of model KL-SPS-73 from Yantai Jinlin Surface Thermal Technology Co., Ltd.

[0054] In step S4, the specific operation is as follows: The material is placed in a graphite crucible, and the atmosphere and vacuum are as follows: High-purity argon is used as the protective gas, and the vacuum pump is started to reduce the furnace pressure to 5 × 10^ . -2 Pa below and remain below.

[0055] First stage: Heat to 1100°C at a rate of 10 °C / s, pressure 15 MPa, and hold for 5 min.

[0056] Second stage: Heat to 1700°C at a rate of 20 °C / s, increase pressure to 45 MPa, and hold for 10 min.

[0057] The third stage: the temperature is increased to the final temperature of 2100°C at a rate of 15 °C / s, the pressure is increased to 120 MPa simultaneously, and the temperature is held for 20 min.

[0058] This disclosure proposes a densification process based on SiC extracted from fly ash, which has the following advantages over the prior art: This disclosure provides a densification process based on SiC extracted from fly ash, which can eliminate the temperature gradient problem in the densification process, and suppress the defects caused by fly ash particles by introducing fly ash-based nanocomposite additives, thereby improving the densification degree and material properties.

[0059] It is understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of this disclosure, and this disclosure is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and substance of this disclosure, and these modifications and improvements are also considered to be within the scope of protection of this disclosure.

Claims

1. A densification process based on SiC extraction from fly ash, characterized in that, The process includes: β-SiC powder or SiC particles are prepared by pretreating fly ash and then preparing the pretreated fly ash by carbothermal reduction reaction or hypersonic gas flow method. Using methylchlorosilane as the reaction gas source, hydrogen as the carrier gas and reducing agent, and argon as the diluent gas, SiC fiber reinforcements were prepared by deposition on the surface of carbon or tungsten wires via CVD reaction. TaC powder and CNTs were mixed in a predetermined mass ratio and bonded with epoxy resin to form a slurry. After standing and vacuum filtration, a composite additive was obtained. The β-SiC powder or SiC particles, the composite additive, and the SiC fiber reinforcement are mixed to form a sintering raw material. Zirconia is used as a phase transformation toughening agent, and boron nitride is used as a lubricant and pore regulator. The sintering raw material is subjected to multi-stage spark plasma sintering treatment to form dense SiC fibers.

2. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The carbothermic reduction reaction is carried out at a temperature of 1800~2200°C for 2-10 hours.

3. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The particle size of the β-SiC powder is 0.1 to 5 μm, or the particle size of the SiC particles is 10-60 nm.

4. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The mass ratio of TaC powder, CNTs and epoxy resin is 1:(1-10):

2.

5. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The slurry should be left to stand for 24-36 hours.

6. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The content of the composite additive is 0.5-1.5% of the mass of the fly ash.

7. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The CVD reaction deposition temperature on the mandrel surface is 1200~1400°C, and the time is 30-90h.

8. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The multi-stage spark plasma sintering process includes the following steps: First stage: rapidly heat to 800-1200°C at a rate of 5-15 °C / s, with a pressure of less than 20 MPa and a holding time of 1-10 min; Second stage: Heat to 1500-1800°C at a rate of 8-30 °C / s, apply a medium to isostatic pressure of 30-50 MPa, and hold for 5-15 minutes; The third stage involves heating the temperature at a rate of 6-20 °C / s to the final sintering temperature of 1900-2200 °C, while simultaneously increasing the isostatic pressure to 80-200 MPa, and holding the temperature for 10-30 minutes.

9. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The pretreatment of fly ash includes: Use a 50-120 mesh sieve to sieve the fly ash and wash it with water. After stirring, let it stand, pour off the top layer of clear water, repeat the washing process several times, and then dry it.

10. The densification process based on SiC extraction from fly ash according to claim 1, characterized in that, The SiC fiber reinforcement was subjected to multi-stage discharge plasma sintering treatment. The densification process was also monitored by an online X-ray diffractometer and an infrared thermal imager. If a local high-temperature area exceeded the warning temperature, the pressure was adjusted and the process was continued until densification was completed.

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