Method for microwave synthesis of silicon carbide whiskers from porous precursor

Abstract

The invention belongs to the technical field of inorganic non-metallic materials, and discloses a method for microwave synthesis of silicon carbide whiskers from a porous precursor, which comprises the following steps: mechanically stirring and mixing a silicon source and a carbon source according to a carbon-silicon molar ratio of (3-25): 1, and obtaining the porous precursor by adopting a certain forming process and a certain pore-forming process; the porous precursor is wrapped with quartz powder and placed in a microwave sintering furnace to be sintered, a sample is subjected to heat preservation for 15-25 min at the temperature of 900-1350 DEG C, and the three-dimensional silicon carbide whiskers are prepared. A silicon source and a carbon source are mixed and molded in the form of a porous precursor and then are subjected to microwave heating, in the microwave heating process, excited microwave plasmas enter pores in a three-dimensional multidirectional form, gas in the pores is quickly accumulated under the action of buried powder, the air pressure is quickly increased, the plasmas are promoted to generate an instantaneous gas-phase reaction, and the pore-forming effect is improved. And the three-dimensional silicon carbide crystal whiskers with good crystal development are formed by virtue of sufficient space in the holes.

Description

Technical Field

[0001] The invention belongs to the technical field of inorganic non-metallic materials, and in particular relates to a method for synthesizing silicon carbide whiskers using porous precursor microwaves. Background Art

[0002] Silicon carbide whiskers have broad application prospects in related fields such as mechanics, electricity and heat due to their excellent characteristics of high melting point, low density, high strength, acid and alkali corrosion resistance, oxidation resistance and ablation resistance. Silicon carbide whiskers are currently the most important reinforcing material for ceramic matrix composites and have been widely used in aerospace and other fields.

[0003] At present, there are four main methods for preparing silicon carbide fibers: precursor conversion method, chemical vapor deposition method, ultrafine powder high temperature sintering method and activated carbon fiber conversion method. The above methods all use traditional heating methods and require a closed environment, high temperature, a certain pressure, inactive gas filling, etc. The technology is complex, the sintering environment and raw materials are demanding, the production efficiency is low, and large-scale preparation is difficult. In addition, due to the lack of sufficient growth space, the growth of silicon carbide whiskers is limited, the number is small, and the growth is slow.

[0004] CN114436674 "A method for preparing a grid-like silicon carbide fiber" uses a certain molding method and pore-making process to prepare a porous precursor; the porous precursor is placed in a microwave sintering furnace and microwave sintered at 600℃ to 1800℃ to obtain a network-like silicon carbide fiber. Since the porous precursor pore structure is a connected hole, the gas released from the pore diameter will be released in a disorderly manner, and the gas pressure is relatively low, thus forming a network-like silicon carbide fiber. The formed silicon carbide fiber has the defects of easy surface oxidation, relatively low tensile strength and hardness.

[0005] CN117658497 "A method for preparing SiC nano whiskers composited with γ-C2S clinker mineral" records that a precursor is prepared by using calcium raw materials, silicon powder, and organic silicon source as raw materials; the prepared precursor is pre-pressed into a blank, buried in quartz sand, and microwave heating is used to synthesize SiC nano whiskers composite silicate cement clinker in situ. The precursor is not a porous precursor, and the synthesized silicon carbide whiskers are few, short, and irregular.

[0006] Based on the prior art, the present invention innovatively adopts a combination of a porous precursor and a powder embedding process to provide sufficient growth space for silicon carbide whiskers, thereby greatly increasing the number and length of silicon carbide whiskers. Summary of the invention

[0007] The present invention provides a method for synthesizing silicon carbide whiskers using microwaves from a porous precursor. The method innovatively adopts a combination of a porous precursor and a powder embedding process. A silicon source and a carbon source are mixed and formed in the form of a porous precursor and then microwave heated. During the microwave heating process, excited microwave plasma enters the pores in a three-dimensional multi-directional form, a gas phase reaction occurs, and a three-dimensional silicon carbide whisker is formed.

[0008] The present invention first prepares a porous precursor containing a certain proportion of carbon source and silicon source, and uses three-dimensional microwave plasma to generate a gas phase reaction in the pores to form a three-dimensional silicon carbide whisker. Due to its unique three-dimensional structure, it can be directly used to prepare silicon carbide whisker toughened ceramics, catalyst carriers, high-performance absorbing materials, etc. The present invention adopts microwave heating, the porous precursor is heated evenly, the reaction efficiency is high, the rate is fast, the preparation time is short, and the energy utilization efficiency is high.

[0009] The technical solution of the present invention is: Step 1: Mix the silicon source and the carbon source by mechanical stirring, the carbon-silicon molar ratio is (3-25):1, and a certain molding process and pore-forming process are used to obtain a porous precursor; Step 2: Wrap the porous precursor with quartz powder and place it in a microwave sintering furnace for sintering. Keep the sample at 900-1350° C. for 15-25 minutes to obtain three-dimensional silicon carbide whiskers.

[0010] The carbon source is a mixture of one or more of carbon powder, activated carbon, petroleum coke, asphalt, coal powder, biomass carbon, starch, and saturated glucose solution.

[0011] The silicon source is a mixture of one or more of silicon powder, alkaline silica sol with a mass fraction of 30%, ethyl orthosilicate, quartz powder, white carbon black, polysiloxane, and biomass silicon.

[0012] The particle size of the quartz powder is 40-600 meshes, and the added amount accounts for 260-290% of the mass of the porous precursor.

[0013] The sintering temperature in the microwave sintering furnace is 900-1350° C., and the sintering method is sintering at a power of 0.8-1.2 kW for 5 minutes and then sintering at a power of 4-6 kW for 1-60 minutes.

[0014] The molding methods include but are not limited to mechanical compression molding, isostatic compression molding, colloidal molding, extrusion molding, plastic molding, and casting molding; The pore-forming process includes, but is not limited to, foaming method, solid pore-forming agent method, emulsion method, organic foam impregnation method, and freeze-drying method.

[0015] The emulsion method in the pore-forming process is as follows: the silicon source and the carbon source are mixed evenly to form a mixed powder, the carbon-silicon molar ratio is (3-25):1, water accounting for 10-200% of the mass of the mixed powder is used as the dispersion medium, liquid alkane accounting for 0.1-6 times the volume of water is used as the organic dispersion phase, Tween 80 accounting for 5-50% of the mass of the organic dispersion phase is used as the emulsifier, and tetramethylammonium hydroxide is used as the dispersant, and the added amount accounts for 0.1-2% of the mass of the powder; the above materials are placed in a ball mill for ball milling, the ball-milled slurry is placed in a stirring barrel for stirring, and ammonium chloride is added as an initiator at the same time, and the amount of ammonium chloride added is 0.2%-2% of the mass of the alkaline silica sol. After being stirred evenly, it is injected into the mold, and the slurry is quickly solidified and dried to obtain a porous precursor.

[0016] The freeze-drying method in the pore-forming process is as follows: a silicon source and a carbon source are uniformly mixed to form a mixed powder, the carbon-silicon molar ratio is (3-25):1, water accounting for 10-200% of the mass of the mixed powder is used as a dispersion medium, a binder accounting for 1-20% of the mass of the powder is added, and after stirring for 25-35 minutes, it is placed in a freeze dryer for freeze molding, and a porous precursor is obtained after reduced pressure curing and drying.

[0017] The binder includes but is not limited to sodium carboxymethyl cellulose, polyvinyl alcohol, dextrin, pulp waste liquid and the like.

[0018] The foaming method in the pore-forming process is as follows: a silicon source and a carbon source are mixed by mechanical stirring, and the carbon-silicon molar ratio is (3-25):1. After the carbon powder is evenly dispersed, a foaming agent is added, and stirring is continued for 25-35 minutes. After the slurry is fully foamed, a curing agent is added, and the slurry is poured into a mold after stirring for 5-15 minutes; a porous precursor is obtained after drying and demolding.

[0019] The foaming agent is sodium dodecyl sulfate or triethanolamine dodecyl sulfate, and the added amount accounts for 3-10% of the mass of the carbon powder.

[0020] The curing agent is a 20% by mass ammonium chloride aqueous solution, and the added amount accounts for 0.2-2% of the mass of the carbon powder.

[0021] The curing and drying temperature is 40-70°C, and the curing and drying time is 20-90h.

[0022] Compared with the prior art, the present invention has the following beneficial effects: The quartz powder in the present invention melts at high temperature and wraps the porous precursor to form a closed space. In the closed space, C on the pore size reacts with SiO2 to generate CO and SiO gas, which are ionized under the action of microwaves and react rapidly to generate SiC. Since the porous precursor contains a large number of pores, the gas is released from the pore walls into the pores. Under the action of the quartz powder wrapping, the gas in the pores accumulates rapidly, the gas pressure increases rapidly, and the plasma is promoted to undergo an instantaneous gas phase reaction. With the help of the sufficient space inside the pores, three-dimensional silicon carbide whiskers with well-developed crystals are formed.

[0023] Compared with the dense green body in the prior art, the porous green body can provide sufficient space for the whisker growth.

[0024] In addition, the quartz powder forms a dense fused quartz shell outside the porous precursor, which can prevent the oxidation of SiC whiskers and form well-developed pure SiC whiskers.

[0025] This pure silicon carbide whisker is a single crystal with highly ordered atomic arrangement and almost no defects, so its strength is close to the theoretical value of a complete crystal. Therefore, silicon carbide whiskers have high tensile strength, hardness and high temperature resistance and can withstand temperatures up to 2000°C.

[0026] The present invention mixes silicon source and carbon source in the form of porous precursor and then performs microwave heating. During the microwave heating process, excited microwave plasma enters the pores in a three-dimensional multi-directional form, undergoes gas phase reaction, and forms three-dimensional silicon carbide whiskers.

[0027] The present invention adopts a microwave heating method, the porous precursor is heated evenly, the reaction efficiency is high, the rate is fast, the preparation time is short, and the energy utilization efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 1.

[0029] Figure 2 This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 2.

[0030] Figure 3 This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 3.

[0031] Figure 4 This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 4.

[0032] Figure 5 This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 5.

[0033] Figure 6This is a microscopic photograph of the three-dimensional silicon carbide whisker prepared in Example 6. DETAILED DESCRIPTION

[0034] In order to better understand the content of the present invention, the present invention will be further described below in conjunction with specific embodiments and drawings. The following embodiments are implemented based on the technology of the present invention, and detailed implementation methods and operating steps are given, but the protection scope of the present invention is not limited to the following embodiments.

[0035] Embodiment 1: Step 1: Mix the silicon source and the carbon source by mechanical stirring, with a carbon-silicon molar ratio of 3:1. After the carbon powder is evenly dispersed, add the foaming agent and continue stirring for 30 minutes. After the slurry is fully foamed, add the curing agent and stir for 10 minutes before pouring the slurry into a mold. After curing, drying and demolding, a porous precursor is obtained. The curing and drying temperature is 40°C, the curing and drying time is 20 hours, and the mass of the porous precursor is 30g.

[0036] The carbon source is starch, the silicon source is silicon powder, the foaming agent is sodium dodecyl sulfate, and the addition amount accounts for 3% of the mass of the carbon source; the curing agent is 20% ammonium chloride aqueous solution, and the addition amount accounts for 0.2% of the mass of the carbon source.

[0037] Step 2: Wrap the porous precursor with quartz powder with a particle size of 40 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 260% of the mass of the porous precursor. The sintering temperature is 900°C. The sintering method is sintering at 0.8kw power for 5 minutes, and then sintering at 4kw power for 1 minute. The sample is heated to 900°C and kept warm for 15 minutes to obtain three-dimensional silicon carbide whiskers.

[0038] Embodiment 2: Step 1: Mix the silicon source and the carbon source evenly to form a mixed powder, with a carbon-silicon molar ratio of 7:1, using water accounting for 150% of the mass of the mixed powder as the dispersion medium, using liquid alkane accounting for 4 times the volume of water as the organic dispersed phase, using Tween 80 accounting for 30% of the mass of the organic dispersed phase as the emulsifier, and using tetramethylammonium hydroxide as the dispersant, the addition amount of which accounts for 1% of the mass of the powder; place the above materials in a ball mill for ball milling, place the ball-milled slurry in a stirring barrel for stirring, and add ammonium chloride as an initiator at the same time, and the addition amount of ammonium chloride is 1% of the mass of the silicon source. After stirring evenly, inject into the mold, and the slurry is quickly cured and dried to form, the curing and drying temperature is 70°C, and the curing and drying time is 90h, to obtain a porous precursor with a mass of 50g; The silicon source is alkaline silica sol with a mass fraction of 30%, and the carbon source is carbon powder.

[0039] Step 2: Wrap the porous precursor with quartz powder with a particle size of 50 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 270% of the mass of the porous precursor. The sintering temperature is 1000°C. The sintering method is sintering at 1.0kw power for 5 minutes, and then sintering at 4.5kw power for 20 minutes. The sample is heated to 1000°C and kept warm for 18 minutes to obtain three-dimensional silicon carbide whiskers.

[0040] Embodiment 3: Step 1: Mix the silicon source and the carbon source evenly to form a mixed powder with a carbon-silicon molar ratio of 10:1. Use water accounting for 130% of the mass of the mixed powder as the dispersion medium, add a binder accounting for 10% of the mass of the powder, stir for 30 minutes, place in a freeze dryer for freeze molding, and obtain a porous precursor after reduced pressure curing and drying. The curing and drying temperature is 60°C, the curing and drying time is 70 hours, and the mass of the porous precursor is 40g.

[0041] The silicon source is quartz powder, the carbon source is saturated glucose solution, and the binder is polyvinyl alcohol.

[0042] Step 2: Wrap the porous precursor with quartz powder with a particle size of 60 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 280% of the mass of the porous precursor. The sintering temperature is 1100°C. The sintering method is sintering at 1.0 kW power for 5 minutes, then sintering at 4.8 kW power for 10 minutes. The sample is heated to 1100°C and kept warm for 19 minutes to obtain three-dimensional silicon carbide whiskers.

[0043] Embodiment 4: Step 1: Mix the silicon source and the carbon source by mechanical stirring, with a carbon-silicon molar ratio of 6:1. After the carbon powder is evenly dispersed, add the foaming agent and continue stirring for 25 minutes. After the slurry is fully foamed, add the curing agent, stir for 8 minutes, and then pour the slurry into a mold; after curing, drying and demolding, a porous precursor is obtained. The curing and drying temperature is 50°C, the curing and drying time is 60 hours, and the mass of the porous precursor is 35g.

[0044] The carbon source is activated carbon, the silicon source is silicon powder, the foaming agent is triethanolamine dodecyl sulfate, and the added amount accounts for 7% of the mass of the carbon source; the curing agent is 20% ammonium chloride aqueous solution, and the added amount accounts for 1% of the mass of the carbon source.

[0045] Step 2: Wrap the porous precursor with quartz powder with a particle size of 70 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 290% of the mass of the porous precursor. The sintering temperature is 1200°C. The sintering method is sintering at 1.0kw power for 5 minutes, and then sintering at 5kw power for 30 minutes. The sample is heated to 1200°C and kept warm for 20 minutes to obtain three-dimensional silicon carbide whiskers.

[0046] Embodiment 5: Step 1: Mix the silicon source and the carbon source evenly to form a mixed powder, with a carbon-silicon molar ratio of 12:1, and use water accounting for 60% of the mass of the mixed powder as the dispersion medium, n-octane accounting for 2 times the volume of water as the organic dispersed phase, Tween 80 accounting for 10% of the mass of the organic dispersed phase as the emulsifier, and tetramethylammonium hydroxide as the dispersant, the addition amount of which accounts for 0.5% of the mass of the powder; place the above materials in a ball mill for ball milling, place the ball-milled slurry in a stirring barrel for stirring, and add ammonium chloride as an initiator at the same time, and the addition amount of ammonium chloride is 1% of the mass of the alkaline silica sol. After stirring evenly, inject into the mold, and the slurry is quickly cured and dried to form, the curing and drying temperature is 58°C, the curing and drying time is 78h, and the mass of the porous precursor is 35g, so that the porous precursor is obtained by colloidal molding and emulsion method.

[0047] The silicon source is tetraethyl orthosilicate, the carbon source is carbon powder, and the binder is sodium carboxymethyl cellulose.

[0048] Step 2: Wrap the porous precursor with quartz powder with a particle size of 80 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 280% of the mass of the porous precursor. The sintering temperature is 1300°C. The sintering method is sintering at 1.0kw power for 5 minutes, and then sintering at 5kw power for 50 minutes. The sample is heated to 1300°C and kept warm for 20 minutes to obtain three-dimensional silicon carbide whiskers.

[0049] Embodiment 6: Step 1: Evenly mix the silicon source and the carbon source to form a mixed powder with a carbon-silicon molar ratio of 25:1. Use water accounting for 50% of the mass of the mixed powder as the dispersion medium, add polyvinyl alcohol accounting for 5% of the mass of the powder, place in a freeze dryer for freeze molding, and after reduced pressure curing and drying, the curing and drying temperature is 68°C, and the curing and drying time is 75h to obtain a porous precursor with a mass of 45g. The porous precursor is prepared by colloidal molding and freeze drying.

[0050] The silicon source is biomass silicon in terms of mass fraction, and the carbon source is biomass carbon.

[0051] Step 2: Wrap the porous precursor with quartz powder with a particle size of 80 mesh and place it in a microwave sintering furnace for sintering. The amount of quartz powder added accounts for 260% of the mass of the porous precursor. The sintering temperature is 1350°C. The sintering method is sintering at 1.2kw power for 5 minutes, and then sintering at 6kw power for 60 minutes. The sample is heated to 1350°C and kept warm for 25 minutes to obtain three-dimensional silicon carbide whiskers.

[0052] The present invention can have other forms of embodiments according to the above preparation method, which are not listed one by one. Therefore, any simple modification, equivalent change and modification made to the above embodiments by any technician familiar with the profession according to the technical essence of the present invention without departing from the scope of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A method for synthesizing silicon carbide whiskers using microwaves from porous precursors, characterized in that: Step 1: Mix the silicon source and the carbon source by mechanical stirring, and the molar ratio of carbon to silicon is (3-25):1, and a porous precursor is obtained by a molding process and a pore-forming process; Step 2: Wrap the porous precursor with quartz powder and place it in a microwave sintering furnace for sintering, and keep the sample at 900-1350°C for 15-25 minutes. The quartz powder melts at high temperature and wraps the porous precursor. The quartz powder forms a dense molten quartz shell outside the porous precursor to form a closed space. In the closed space, C on the pore diameter reacts with SiO2 to generate CO and SiO gas, which are ionized under the action of microwaves and react rapidly to generate SiC. The porous precursor contains a large number of pores, and the gas is released from the pore wall into the pore. Under the action of the quartz powder wrapping, the gas in the pore accumulates rapidly, and the gas pressure increases rapidly, which promotes the instantaneous gas phase reaction of the plasma, and with the help of the sufficient space inside the pore, a three-dimensional silicon carbide whisker with well-developed crystals is formed.

2. The method for synthesizing silicon carbide whiskers from porous precursors by microwaves according to claim 1, characterized in that: The carbon source is a mixture of one or more of carbon powder, activated carbon, petroleum coke, asphalt, coal powder, biomass carbon, starch, and saturated glucose solution.

3. The method for synthesizing silicon carbide whiskers from porous precursors by microwave according to claim 1, characterized in that: The silicon source is a mixture of one or more of silicon powder, alkaline silica sol with a mass fraction of 30%, ethyl orthosilicate, quartz powder, white carbon black, polysiloxane, and biomass silicon.

4. The method for synthesizing silicon carbide whiskers from porous precursors by microwaves according to claim 1, characterized in that: The mass of the porous precursor is 30-50g.

5. The method for synthesizing silicon carbide whiskers from porous precursors by microwave according to claim 1, characterized in that: The particle size of the quartz powder is 40-80 mesh, and the added amount accounts for 260-290% of the mass of the porous precursor.

6. The method for synthesizing silicon carbide whiskers from porous precursors by microwaves according to claim 1, characterized in that: The sintering temperature in the microwave sintering furnace is 900-1350°C, and the sintering method is sintering at a power of 0.8-1.2kw for 5 minutes and then sintering at a power of 4-6kw for 1 minute.

7. The method for synthesizing silicon carbide whiskers from porous precursors by microwaves according to claim 1, characterized in that: The pore-forming process is one of the following: foaming method, solid pore-forming agent method, emulsion method, organic foam impregnation method, and freeze-drying method.

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