A method for preparing hollow spherical silicon carbide powder

Abstract

The present invention relates to a method for preparing hollow spherical silicon carbide powder. A green body is prepared by mixing silica sol and plant carbon source. After partial carbonization treatment of the green body, it is placed in a microwave sintering furnace for microwave sintering at 700 - 1000 °C to obtain the hollow spherical silicon carbide powder. The present invention utilizes the forming characteristics of silica sol gel to fully mix the plant carbon source with nano-scale silica sol; the plant carbon source is prone to release a large amount of gas when heated. By using rapid microwave heating, the organic matter in the plant carbon source volatilizes rapidly, generating instantaneous microwave plasma. The plant carbon source quickly becomes a molten state, and the gas expands rapidly, forming a cavity, and then a ceramic microsphere with a hollow spherical structure is prepared. The present invention can be directly used for preparing porous silicon carbide ceramics, porous silicon carbide coatings, catalyst carriers, high-performance microwave absorbing materials, heat insulation materials, etc., and has extremely high application prospects and development space.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic non-metallic materials, and particularly relates to a method for preparing hollow spherical silicon carbide powder. Background Art

[0002] In recent years, the rapid development of advanced electronic communication devices has brought great convenience to people's social networks. However, the use of these electronic devices generates a large amount of electromagnetic radiation, which not only harms human health but also endangers the natural environment. Currently, electromagnetic radiation has become the fourth largest source of pollution after water, air, and noise pollution. As a material that can effectively reduce the reflection of incident electromagnetic waves and absorb electromagnetic waves, electromagnetic wave absorbing materials can solve the problem of electromagnetic pollution and thus become one of the current research hotspots. Among many electromagnetic wave absorbing materials, SiC has become a promising electromagnetic wave absorbing material due to its high hardness, chemical corrosion resistance, high temperature resistance, and adjustable dielectric properties.

[0003] Hollow spherical SiC has good wave absorption effect, which can not only reduce electromagnetic pollution but also be used in national defense construction. Some stealth aircraft, armored vehicles, etc. often use it as a manufacturing material to avoid radar and satellite signals. At the same time, its unique hollow spherical structure also increases its heat insulation performance, showing extremely high application prospects. Summary of the Invention

[0004] The present invention provides an energy-saving and environmentally friendly preparation method for in-situ forming hollow spherical silicon carbide powder using natural plant carbon sources. The present invention utilizes the forming characteristics of silica sol-gel to fully mix the plant carbon source with nano-scale silica sol; when the plant carbon source is heated, a large amount of gas is released. By using rapid microwave heating, the organic matter in the plant carbon source volatilizes rapidly, generating instantaneous microwave plasma, and the plant carbon source quickly becomes a molten state. The gas expands rapidly, forming a cavity, and then ceramic microspheres with a hollow spherical structure are prepared.

[0005] The present invention can be directly used to prepare porous silicon carbide ceramics, porous silicon carbide coatings, catalyst carriers, high-performance wave absorbing materials, heat insulation materials, etc., showing extremely high application prospects and development space.

[0006] In order to achieve the above object, the technical solution adopted by a method for preparing hollow spherical silicon carbide powder of the present invention is as follows:

[0007] Step 1: Stir and mix a plant carbon source and silica sol in a mass ratio of (0.1 - 10):1 for 25 - 35 min. Add a curing agent, and then stir for 1 - 3 min before pouring into a mold and naturally drying in air for 24 h to obtain a green body.

[0008] Step 2: Place the green body obtained in Step 1 in a vacuum tube furnace, introduce argon protective gas for pyrolytic carbonization, keep the temperature below 1000 °C, and hold for 10 - 60 min to partially carbonize the green body.

[0009] Step 3: Heat the partially carbonized green body in a microwave cavity, raise the temperature to 700 - 1000 °C and hold for 1 - 30 min. After natural cooling, hollow spherical silicon carbide powder is formed in-situ.

[0010] Among them, the silica sol used is acidic or alkaline, and the solid content is 20% or 30%.

[0011] The main reasons for using silica sol are as follows:

[0012] (1) In silica sol, nano-scale silica is uniformly dispersed in water and can be uniformly attached to the plant carbon source after being mixed with the plant carbon source.

[0013] (2) Silica sol belongs to metastable substances and can gel after adding electrolytes. This can be achieved by adding electrolytes while stirring and mixing with the plant carbon source to solidify it, avoiding the sedimentation of the plant carbon source and realizing the preparation of a green body of plant carbon source uniformly loaded with nano-silica.

[0014] Among them, the plant carbon source used includes but is not limited to starch, sucrose, glucose, wood, walnut shell, coconut shell, etc.

[0015] Among them, the starch used is one or more of potato starch, bean starch, and cereal starch.

[0016] Among them, the curing agent used is an ammonium electrolyte salt.

[0017] Among them, the microwave frequency is 2450 ± 50 MHz.

[0018] Among them, the microwave heating power regime is 0.05 - 2 KW / min.

[0019] Among them, the green body formed after natural cooling in Step 3 is mainly in an amorphous phase, and it becomes β-phase silicon carbide after microwave calcination. The silicon carbide powder is in a hollow spherical shape.

[0020] Among them, in Step 2, place the green body obtained in Step 1 in a vacuum tube furnace, introduce argon protective gas for pyrolytic carbonization, and control the temperature below 1000 °C so that the green body can be partially carbonized.

[0021] Among them, step two of the present invention is to place it in a vacuum tube furnace and introduce argon protective gas for pyrolysis and carbonization, rather than direct microwave heating. This is because: the plant carbon source does not absorb electromagnetic waves. As a carbon source, the plant carbon source cannot be directly heated under microwave conditions. It is necessary to introduce argon protective gas for partial carbonization. Only after carbonization can the plant carbon source absorb electromagnetic waves.

[0022] For example, starch does not absorb electromagnetic waves mainly because of its molecular structure and chemical properties. Starch is a polysaccharide compound composed of a large number of glucose molecules connected by α-1,4- and α-1,6-glycosidic bonds. Starch molecules usually present a fibrous or granular form, which makes starch have a certain degree of transparency and reflectivity; the absorption of electromagnetic waves depends on the ability of a substance to absorb electromagnetic wave energy. For starch, due to the particularity of its molecular structure, there are certain gaps and intervals between its molecules, which allows electromagnetic waves to propagate freely between starch molecules without being absorbed by the molecules. In addition, the chemical bonds (glycosidic bonds) in starch molecules have a weak absorption capacity for electromagnetic waves; therefore, starch has a low absorption capacity for electromagnetic waves, and most electromagnetic waves will pass through starch without being absorbed, but reflected or transmitted. This is also one of the reasons why when observing starch products, such as flour, rice, etc., they usually appear white or transparent.

[0023] The reason why carbon can absorb electromagnetic waves is related to its electronic structure and molecular vibration. First of all, the electronic structure of carbon atoms determines its energy level distribution. Carbon atoms have 6 electrons, distributed in the orbits of 1s² 2s² 2p². Among them, the two electrons in the 2p orbit can form a conjugated system, which enables carbon atoms to absorb and emit electromagnetic waves; secondly, the vibration of carbon atoms in molecules can also lead to the absorption of electromagnetic waves. Carbon atoms can form covalent bonds with other atoms to form different molecular structures. When carbon atoms in molecules vibrate, they absorb electromagnetic waves of specific frequencies, depending on the mode of vibration and the difference in energy levels; in general, the electronic structure and molecular vibration of carbon atoms enable carbon to absorb electromagnetic waves. This is also why carbon materials exhibit absorption characteristics in the infrared spectrum and other electromagnetic wave ranges.

[0024] In addition, the plant carbon source is only partially carbonized, and the organic matter cannot be completely decomposed. During microwave heating, part of the organic matter is decomposed to produce gas. The organic matter in the plant carbon source volatilizes rapidly, generating instant microwave plasma, and the plant carbon source quickly becomes molten. The gas expands rapidly to form a cavity, thereby preparing a ceramic microsphere with a hollow spherical structure. Therefore, the present invention limits the introduction of argon protective gas for pyrolysis carbonization, the temperature is lower than 1000 degrees Celsius, and the insulation time is 10-60 minutes, so that the green body can be partially carbonized.

[0025] The present invention in-situ forms hollow spherical silicon carbide materials. During the forming stage, nano-silica has been uniformly loaded on the surface of the plant carbon source. After heat treatment, with the aid of microwave heating, while releasing gas to form a hollow structure, the reaction between carbon and silica occurs in-situ to generate silicon carbide, which is conducive to the formation of hollow spherical powders with uniformly distributed silicon carbide. In-situ formation is crucial for improving the use performance, such as playing a key role in wave absorption, heat insulation, wear resistance, erosion resistance, etc.

[0026] The present invention innovates from two aspects of raw materials and heating methods, utilizes the forming characteristics of plant carbon source (i.e., silica sol-gel), and fully mixes it with nano-scale silica sol. Starch is easy to release a large amount of gas when heated. By using rapid microwave heating, the organic matter in the starch quickly volatilizes, generating instantaneous microwave plasma, the starch quickly becomes molten, the gas expands rapidly, forming a cavity, and then ceramic microspheres with a hollow spherical structure are prepared. This material can be directly used to prepare porous silicon carbide ceramics, porous silicon carbide coatings, catalyst carriers, high-performance wave-absorbing materials, heat-insulating materials, etc.

[0027] In current industrial production, chemical carbon sources such as methanol and sodium acetate are often used. Not only are they costly, but chemical carbon sources themselves also have certain toxicity and will cause environmental pollution after use. However, the plant carbon source selected in the present invention is easier to obtain, has a lower usage cost, better actual effects, and is more environmentally friendly, which is conducive to enterprises to build a sustainable development model for industrial production. Description of the Drawings

[0028] Figure 1 XRD pattern of the hollow spherical silicon carbide powder prepared in Example 2;

[0029] Figure 2 Micrograph of the hollow spherical silicon carbide powder prepared in Example 2. Detailed Description of the Invention

[0030] Example 1

[0031] Step 1: Stir and mix glucose and 20% solid content alkaline silica sol in a mass ratio of 2:3. After stirring for 30 min, add ammonium persulfate accounting for 1% of the mass of the silica sol, stir for 2 min, and then pour it into a mold and leave it in the air for 24 h to obtain a green body;

[0032] Step 2: Place the green body in a vacuum tube furnace, introduce the protective gas argon for carbonization, the carbonization temperature is 1000 °C, and keep it warm for 10 min.

[0033] Step 3: Place the partially carbonized embryo in the microwave cavity and heat it at a power rate of 0.2 KW / min. Raise the temperature to 1000 °C and keep it warm for 10 min. After natural cooling, hollow spherical silicon carbide powder is obtained.

[0034] Example 2

[0035] Step 1: Stir and mix potato starch and alkaline silica sol with a solid content of 30% in a mass ratio of 5:4. After stirring for 30 min, add ammonium chloride accounting for 1% of the mass of the silica sol, stir for 2 min, then pour it into a mold and leave it in the air for 24 h to obtain a green body.

[0036] Step 2: Place the green body in a vacuum tube furnace and introduce the protective gas argon for carbonization. The carbonization temperature is 600 °C and keep it warm for 30 min.

[0037] Step 3: Place the partially carbonized embryo in the microwave cavity and heat it at a power rate of 0.5 KW / min. Raise the temperature to 900 °C and keep it warm for 20 min. After natural cooling, hollow spherical silicon carbide powder is obtained.

[0038] Example 3 Step 1: Stir and mix walnut shells (D50 = 5 μm) and acidic silica sol with a solid content of 20% in a mass ratio of 1:4. After stirring for 30 min, add ammonium chloride accounting for 2% of the mass of the silica sol, stir for 2 min, then pour it into a mold and leave it in the air for 24 h to obtain a green body.

[0039] Step 2: Place the green body in a vacuum tube furnace and introduce the protective gas argon for carbonization. The carbonization temperature is 700 °C and keep it warm for 10 min.

[0040] Step 3: Place the partially carbonized embryo in the microwave cavity and heat it at a power rate of 0.6 KW / min. Raise the temperature to 800 °C and keep it warm for 10 min. After natural cooling, hollow spherical silicon carbide powder is obtained.

[0041] Example 4

[0042] Step 1: Stir and mix sweet potato starch and acidic silica sol with a solid content of 20% in a mass ratio of 5:1. After stirring for 30 min, add ammonium chloride accounting for 2% of the mass of the silica sol, stir for 2 min, then pour it into a mold and leave it in the air for 24 h to obtain a green body.

[0043] Step 2: Place the green body in a vacuum tube furnace and introduce the protective gas argon for carbonization. The carbonization temperature is 400 °C and keep it warm for 60 min.

[0044] Step 3: Place the partially carbonized embryo in the microwave cavity and heat it at a power rate of 1 KW / min. Raise the temperature to 900 °C for insulation, with an insulation time of 20 min. After natural cooling, hollow spherical silicon carbide powder is obtained.

[0045] Example 5

[0046] Step 1: Stir and mix potato starch and alkaline silica sol with a solid content of 30% at a mass ratio of 2:1. After stirring for 30 min, add ammonium chloride accounting for 0.5% of the mass of the silica sol to the mixed slurry. After stirring for 2 min, pour it into a mold and let it stand in the air for 24 h to obtain a green body.

[0047] Step 2: Place the green body in a vacuum tube furnace and introduce the protective gas argon for carbonization. The carbonization temperature is 500 °C and the insulation time is 10 min.

[0048] Step 3: Place the partially carbonized embryo in the microwave cavity and heat it at a power rate of 0.2 KW / min. Raise the temperature to 800 °C for insulation, with an insulation time of 20 min. After natural cooling, hollow spherical silicon carbide powder is obtained.

[0049] The present invention may have other forms of embodiments according to the above preparation method, which will not be listed one by one. Therefore, any person skilled in the art, without departing from the scope of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing hollow spherical silicon carbide powder, characterized in that: Step 1: Stir and mix a plant carbon source and silica sol at a mass ratio of (0.1 - 10):1 for 25 - 35 min, add a curing agent, stir for another 1 - 3 min, then pour the mixture into a mold and let it stand in air for 24 h to obtain a green body; Step 2: Place the green body obtained in Step 1 in a vacuum tube furnace, introduce argon as a protective gas for pyrolytic carbonization, and partially carbonize the green body; Step 3: Heat the partially carbonized green body in a microwave cavity, raise the temperature to 700 - 1000 °C, hold for 1 - 30 min, and after natural cooling, in-situ form hollow spherical silicon carbide powder.

2. The preparation method of the hollow spherical silicon carbide powder according to claim 1, wherein: The silica sol is acidic or alkaline, and the solid content is 20% or 30%.

3. The preparation method of the hollow spherical silicon carbide powder according to claim 1, characterized in that: The plant carbon source used includes starch, sucrose, glucose, wood, walnut shells, and coconut shells.

4. The preparation method of the hollow spherical silicon carbide powder according to claim 3, characterized in that: The starch used is one or more of tuber starches, legume starches, and cereal starches.

5. The preparation method of the hollow spherical silicon carbide powder according to claim 1, characterized in that: The curing agent is an ammonium electrolyte salt.

6. The preparation method of the hollow spherical silicon carbide powder according to claim 1, wherein: Step 2: Place the green body obtained in Step 1 in a vacuum tube furnace, introduce argon as a protective gas for pyrolytic carbonization, the temperature is lower than 1000 degrees Celsius, and the holding time is 10 - 60 min, and the green body is partially carbonized.

7. The preparation method of the hollow spherical silicon carbide powder according to claim 1, wherein: The microwave frequency is 2450 ± 50 MHz.

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