CN110791810B - Preparation method of silicon carbide crystal - Google Patents

Abstract

The invention relates to a preparation method of silicon carbide crystal, belonging to the technical field of silicon carbide preparation. The preparation method of silicon carbide crystal of the present invention includes the following steps: providing a mixed system composed of amorphous silica-wrapped carbon source precursor and water; in the mixed system, the amorphous silica-wrapped carbon source precursor The mass ratio to water is 100:8 to 11; the mixed system is pressed and formed into a green body; the green body is embedded in the quartz sand for microwave sintering and synthesis. In the preparation method of silicon carbide crystal of the present invention, by burying a body containing a certain amount of moisture in quartz for microwave sintering, due to the thermal effect and athermal effect in the microwave field, the activity of local molecules is improved, thereby reducing the initial generation of silicon carbide. temperature, shorten the microwave sintering time, and reduce the energy consumption of microwave sintering.

Description

Preparation method of silicon carbide crystal

Technical Field

The invention relates to a preparation method of silicon carbide crystals, belonging to the technical field of preparation of silicon carbide.

Background

The SiC has the characteristics of high strength, high hardness, high elastic modulus, good chemical stability and the like, and can be widely used in the traditional industrial fields of abrasive materials, refractory materials, metallurgy, high-temperature structural ceramics and the like. In recent years, the material has attracted much attention because of its wide band gap, and is useful as a high-power, high-frequency microelectronic device, particularly, in the high-temperature field. The traditional method for producing SiC is invented by Acheson in 1893, is still the only practical method for industrial production, but has long heating period and high energy consumption, and causes serious environmental pollution problem worldwide.

In recent years, many new methods have been tested, wherein the carbothermal reaction method is more common for preparing silicon carbide crystals, and the microwave sintering method is a more new method for preparing SiC materials in recent decades. Compared with the traditional sintering, the method has the advantages of rapidness, energy conservation, environmental protection, low synthesis temperature, easy operation and the like. Different raw materials and different heating methods are adopted, and SiC crystals can be prepared at different low temperature sections. SiC whiskers were synthesized at 1400 ℃ by Kahar et al at university of Malaysia Perlis using a mixture of graphite and silica. SiO for Wang et al, university of Tibet₂And graphite powder at 1450 deg.c to prepare well crystallized 3C-SiC crystal grain and nanometer line. Multi-walled carbon nanotubes and SiO for use by Tony et al, university of Malaysia Perlis₂SiC nanotubes were prepared at 1350 ℃. Wang et al, Wuhan science and technology university, used Co as a catalyst, and kept the temperature at 1150 ℃ for 30min to prepare the beta-SiC crystal. The synthesis temperature for producing SiC crystals by microwave heating is mostly much higher than 1000 ℃, and most processes require vacuum or catalysts as auxiliary conditions. To date, Zhang et al, at the university of nanjing forestry, reported that the minimum temperature for the formation of SiC crystals was 700 ℃, but with the use of a Mg catalyst and precursor template and the necessity that the reaction must be carried out in a vacuum environment.

In the prior art, a chinese patent with an authorization publication number of CN104328478B discloses a method for preparing SiC whiskers, which comprises the following steps: 1) preparing a precursor of amorphous silicon dioxide wrapped carbon source by taking a carbon source and a silicon source; the carbon source is coal; 2) and pressing the obtained precursor into a tablet, embedding the tablet into quartz sand, and performing microwave sintering synthesis reaction to obtain the product. The method can overcome the defect that the silicon carbide synthesis process in the prior art needs to be carried out in a vacuum environment, and has lower microwave sintering temperature, but the energy consumption is higher due to higher temperature for initially generating the silicon carbide.

Disclosure of Invention

The invention aims to provide a preparation method of silicon carbide crystals, which can reduce the energy consumption of microwave sintering by reducing the temperature for starting to generate silicon carbide.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:

a preparation method of silicon carbide crystals comprises the following steps:

providing a mixed system consisting of amorphous silicon dioxide coated carbon source precursor and water; the mass ratio of the amorphous silicon dioxide wrapped carbon source precursor to water in the mixed system is 100: 8-11;

pressing and molding the mixed system to prepare a blank;

and embedding the green body into quartz sand to be synthesized by microwave sintering.

According to the method for synthesizing the silicon carbide crystal, the blank body containing certain moisture is embedded into quartz for microwave sintering, the thermal effect and the non-thermal effect in a microwave field are utilized, the activity of local molecules is improved, and the temperature for starting to generate the silicon carbide is reduced.

Preferably, the mass ratio of the amorphous silicon dioxide wrapped carbon source precursor to water in the mixed system is 100: 10-11.

Preferably, the apparent density of the blank is 2.09-2.28 g/cm³。

The pressing is dry pressing. The dry-pressing molding adopts a uniaxial tablet press.

Preferably, the pressure adopted by the compression molding is 4-5 MPa; and the pressure maintaining time of the compression molding is 50-60 s.

The blank is a cylindrical blank.

Preferably, the molar ratio of silicon dioxide to carbon in the carbon source in the amorphous silicon dioxide coated carbon source precursor is (0.9-1.1): 2.9-3.1. Further preferably, the molar ratio of the silica to the carbon in the carbon source is 1:3.

The proper primary carbon source particle size is a prerequisite for electric field enhancement and microwave plasma effect, and can promote crystal nucleation and growth. Preferably, the average grain diameter of the carbon source is 0.06-0.20 mm. Further preferably, the carbon source has an average particle size of 0.10 mm.

Preferably, the carbon source is obtained by continuous screening.

Preferably, the frequency of the microwave used for microwave sintering synthesis is 2.425-2.475 GHz. Preferably, the maximum input current of the microwave sintering furnace adopted for the microwave sintering is 240 mA.

Preferably, the microwave sintering synthesis is to heat the reaction system to the sintering temperature in a microwave field, and then preserve heat and cool.

The sintering temperature is 600-1100 ℃.

Preferably, the heat preservation time is 0 min.

Preferably, the time for heating the reaction system to 600 ℃ is 28-35 min.

Optionally, when the sintering temperature is higher than 600 ℃, the heating rate of heating the reaction system from 600 ℃ to the sintering temperature is 10-180 ℃/min. When the sintering temperature is higher than 900 ℃, the temperature of the reaction system is increased from 600 ℃ to 900 ℃ at the speed of 75-180 ℃/min, and then the temperature is increased to the sintering temperature at the speed of 10-40 ℃/min. After the temperature is increased to 900 ℃, gas is reduced, the plasma effect is weakened, and residual carbon is subjected to coupling reaction, so the temperature increase rate is slower.

The blank is embedded into quartz sand to be synthesized by microwave sintering, namely, the cylindrical blank is placed into an alumina crucible and covered by quartz sand powder, and then the crucible is placed into an insulating structure containing mullite walls and alumina fibers to be embedded and sintered. The adopted insulation structure has a good heat insulation structure design. The purpose of the burying is to prevent the carbon source from being oxidized too fast in the microwave sintering furnace in the air atmosphere.

Preferably, the carbon source is coal.

Preferably, the preparation method of the amorphous silicon dioxide coated carbon source precursor comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and water, adding citric acid, adjusting the pH to 3-4, and stirring for 2-2.5 hours to obtain a mixture A; the volume ratio of the ethyl orthosilicate to the ethanol to the water is 23:35: 80;

2) respectively adding carbon sources into the mixture A, and stirring for 2.5-3 hours to obtain a mixture B;

3) and adding ammonia water into the mixture B, adjusting the pH value to 8-9, stirring for 1-2 h, drying and grinding to obtain the catalyst.

Preferably, in the step 1), heating by using a water bath in the process of mixing ethyl orthosilicate, ethanol and water; the temperature of the water bath heating is 30-40 ℃. And in the process of heating in a water bath, continuously stirring the system.

Preferably, the ammonia water used in the step 3) is obtained by mixing concentrated ammonia water and water in a volume ratio of 1: 3-5. The concentration of the strong ammonia water is 25-28%.

Preferably, the drying temperature is 75-80 ℃, and the drying time is 20-24 h.

Drawings

FIG. 1 is an XRD pattern of silicon carbide prepared in example 1;

FIG. 2 is a Raman diagram of the silicon carbide prepared in example 1;

FIG. 3 is an XRD pattern of silicon carbide prepared in example 2;

FIG. 4 is an SEM photograph of the silicon carbide prepared in example 2;

FIG. 5 is an XRD pattern of silicon carbide prepared in example 3;

FIG. 6 is an SEM photograph of the silicon carbide prepared in example 3;

FIG. 7 is an XRD pattern of silicon carbide prepared in example 4;

FIG. 8 is an SEM photograph of the silicon carbide prepared in example 4;

FIG. 9 is an XRD pattern of silicon carbide prepared in example 5;

FIG. 10 is an SEM photograph of the silicon carbide prepared in example 5;

FIG. 11 is an XRD pattern of silicon carbide prepared in example 6;

fig. 12 is an SEM image of silicon carbide prepared in example 6.

Detailed Description

The technical solution of the present invention will be further described with reference to the following embodiments.

The diluted ammonia water adopted in the specific embodiment is prepared by mixing concentrated ammonia water with the concentration of 26% and water according to the volume ratio of 1: 3; the adopted coal particles are obtained by continuous screening; the model of the adopted microwave sintering furnace is SL-10kW, and the manufacturer is Nanjing Sanle microwave technology development Co.

Due to the special thermal effect and non-thermal effect in the microwave field, the temperature rise process of the blank body during microwave sintering is in a tendency of first-speed and second-speed, and in the preparation method of the silicon carbide crystal, the time for the reaction system to rise to 600 ℃ is not more than 35 min.

Example 1

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a volume ratio of 23:35:80, adding citric acid to adjust the pH value to 3, and putting the system into a water bath at 30 ℃ to heat and stir for 2 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.1mm into the mixture A, and stirring for 3 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 1: 3;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH to 9, and stirring for 2 hours to obtain a mixture C;

4) carrying out forced air drying on the mixture C at 80 ℃ for 24h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.6g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 60s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.1 mm;

6) putting the obtained cylindrical blank into an alumina crucible, covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, then putting the crucible into a microwave sintering furnace, adjusting the input current to 240mA, carrying out embedding burning by adopting microwaves with the frequency of 2.45GHz, heating the reaction system to 600 ℃ from room temperature within 30min in the embedding burning process, preserving the heat for 0min, and cooling the reaction system along with the furnace to obtain the mullite-alumina ceramic.

XRD test and Raman spectrum test are respectively carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and Raman spectrum patterns obtained by the test are respectively shown in figure 1 and figure 2; as can be seen from FIGS. 1 and 2, with coal having an average particle size of 0.10mm, a small amount of silicon carbide crystals were formed when microwave sintering was carried out at 600 ℃ and it was shown that silicon carbide crystals could be synthesized by the preparation method of this example at 600 ℃.

Example 2

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a volume ratio of 23:35:80, adding citric acid to adjust the pH value to 3, and putting the system into a water bath at 30 ℃ to heat and stir for 2 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.15mm into the mixture A, and stirring for 3 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 1: 3;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH to 9, and stirring for 2 hours to obtain a mixture C;

4) carrying out forced air drying on the mixture C at 80 ℃ for 24h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.6g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 60s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.3 mm;

6) putting the obtained cylindrical blank into an alumina crucible, covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, then putting the crucible into a microwave sintering furnace, adjusting the input current to 240mA, and carrying out embedding burning by adopting microwaves with the frequency of 2.45GHz, wherein in the embedding burning process, the temperature of a reaction system is increased from room temperature to 600 ℃ within 31min, the temperature is increased from 600 ℃ to 900 ℃ at 180 ℃/min, the temperature is maintained for 0min, and the aluminum oxide/aluminum oxide composite material is obtained after furnace cooling.

XRD test and scanning electron microscope test are carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and SEM patterns obtained by the test are respectively shown in fig. 3 and fig. 4; as is clear from FIGS. 3 and 4, in the present example, coal having an average particle size of 0.15mm was used, and when microwave sintering was carried out at 900 ℃, a small amount of silicon carbide crystals were formed, and the degree of crystallinity was good, and a small amount of silicon carbide whiskers were formed.

Example 3

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a volume ratio of 23:35:80, adding citric acid to adjust the pH value to 3, and putting the system into a water bath at 30 ℃ to heat and stir for 2 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.1mm into the mixture A, and stirring for 3 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 1: 3;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH to 9, and stirring for 2 hours to obtain a mixture C;

4) carrying out forced air drying on the mixture C at 80 ℃ for 24h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.6g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 60s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.1 mm;

6) putting the obtained cylindrical blank into an alumina crucible, covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, then putting the crucible into a microwave sintering furnace, adjusting the input current to 240mA, and carrying out embedding burning by adopting microwaves with the frequency of 2.45GHz, wherein in the embedding burning process, the temperature of a reaction system is increased from room temperature to 600 ℃ within 30min, the temperature is increased from 600 ℃ to 900 ℃ at 180 ℃/min, the temperature is maintained for 0min, and the aluminum oxide/aluminum oxide composite material is obtained after furnace cooling.

XRD test and scanning electron microscope test are carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and SEM patterns obtained by the test are respectively shown in fig. 5 and fig. 6; as can be seen from FIGS. 5 and 6, in the present example, coal having an average particle size of 0.10mm was used, and when microwave sintering was carried out at 900 ℃, silicon carbide crystals were formed in a large amount, and crystallinity was good, and a large amount of silicon carbide whiskers were formed.

Example 4

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a volume ratio of 23:35:80, adding citric acid to adjust the pH value to 3, and putting the system into a water bath at 30 ℃ to heat and stir for 2 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.1mm into the mixture A, and stirring for 3 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 1: 3;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH to 9, and stirring for 2 hours to obtain a mixture C;

4) carrying out forced air drying on the mixture C at 80 ℃ for 24h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.6g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 60s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.1 mm;

6) putting the obtained cylindrical blank into an alumina crucible and covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, then putting the crucible into a microwave sintering furnace, adjusting the input current to 240mA, and carrying out burial burning by adopting microwaves with the frequency of 2.45G Hz, wherein in the burial burning process, the temperature of a reaction system is increased from room temperature to 600 ℃ within 30min, the temperature is increased to 900 ℃ from 600 ℃ at the heating rate of 180 ℃/min, the input current is controlled to be stabilized at 240mA, the temperature is increased to 1100 ℃ at the heating rate of 40 ℃/min, the temperature is kept for 0min, and the mullite-silica-based ceramic material is obtained after furnace cooling.

XRD test and scanning electron microscope test are carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and SEM patterns obtained by the test are respectively shown in fig. 7 and fig. 8; as can be seen from FIGS. 7 and 8, in the present example, coal having an average particle size of 0.10mm was used, and when microwave sintering was carried out at 1100 ℃, silicon carbide crystals were formed in a large amount, and crystallinity was good, and a large amount of silicon carbide whiskers were formed.

Example 5

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a volume ratio of 23:35:80, adding citric acid to adjust the pH value to 4, and putting the system into a water bath at 30 ℃ to heat and stir for 2.5 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.2mm into the mixture A, and stirring for 3 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 0.9: 2.9;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH to 9, and stirring for 2 hours to obtain a mixture C;

4) carrying out forced air drying on the mixture C at the temperature of 80 ℃ for 20h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.64g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 50s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.5 mm;

6) putting the obtained cylindrical blank into an alumina crucible and covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, adjusting the input current to 240mA, and carrying out embedding burning by adopting microwaves with the frequency of 2.45GHz, wherein in the embedding burning process, the temperature of a reaction system is increased from room temperature to 600 ℃ within 35min, the temperature is increased to 900 ℃ from 600 ℃ at the heating rate of 75 ℃/min, the input current is controlled to be stable at 240mA, then the temperature is increased to 1100 ℃ at the heating rate of 33 ℃/min, the temperature is kept for 10min, and the mullite-containing silicon carbide is obtained after furnace cooling.

XRD test and scanning electron microscope test are carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and SEM patterns obtained by the test are respectively shown in fig. 9 and fig. 10; as can be seen from fig. 9 and 10, in this example, more silicon carbide crystals are generated when the microwave sintering is performed to 1100 ℃, and the crystallinity is better, and a large amount of silicon carbide whiskers are generated, but compared with example 4, the heat preservation is performed at 1100 ℃ for a longer time, and the energy consumption is higher.

Example 6

The preparation method of the silicon carbide crystal of the embodiment comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and distilled water according to a certain volume ratio of 23:35:80, adding citric acid to adjust the pH value to 3, and then putting the system into a water bath at 40 ℃ to heat and stir for 2 hours to obtain a mixture A;

2) adding coal particles with the average particle size of 0.06mm into the mixture A, and stirring for 2.5 hours to obtain a mixture B; the molar ratio of Si in the mixture A to C in the coal is 1.1: 3.1;

3) dropwise adding diluted ammonia water into the mixture B to adjust the pH value to 8, and stirring for 1h to obtain a mixture C;

4) carrying out forced air drying on the mixture C at 75 ℃ for 24h, and grinding to obtain an amorphous silicon dioxide coated carbon source;

5) taking 6g of prepared amorphous silicon dioxide to wrap a carbon source, adding 0.06g of distilled water, uniformly stirring, and performing dry pressing molding by using a single-shaft tablet press, wherein the pressure of the pressing molding is 4MPa, and the pressure maintaining time is 60s, so as to obtain a cylindrical blank body with the phi of 30 multiplied by 4.1 mm;

6) putting the obtained cylindrical blank into an alumina crucible and covering the alumina crucible with quartz sand powder, putting the crucible into an insulating structure containing mullite walls and alumina fibers, adjusting the input current to 240mA, and carrying out embedding burning by adopting microwaves with the frequency of 2.45GHz, wherein in the embedding burning process, the temperature of a reaction system is increased from room temperature to 600 ℃ within 29min, the temperature is increased to 900 ℃ from 600 ℃ at the heating rate of 75 ℃/min, the input current is controlled to be stable at 240mA, the temperature is increased to 1100 ℃ at the heating rate of 10 ℃/min, the temperature is maintained for 10min, and the mullite-containing silicon carbide is obtained after furnace cooling.

XRD test and scanning electron microscope test are carried out on the silicon carbide crystal prepared by the preparation method of the embodiment, and XRD diffraction patterns and SEM patterns obtained by the test are respectively shown in fig. 11 and fig. 12; as can be seen from fig. 11 and 12, in this example, more silicon carbide crystals are generated when the microwave sintering is performed to 1100 ℃, and the crystallinity is better, and a large amount of silicon carbide whiskers are generated, but compared with example 4, the heat preservation is performed at 1100 ℃ for a longer time, and the energy consumption is higher.

Claims (4)

1. A preparation method of silicon carbide crystals is characterized by comprising the following steps: the method comprises the following steps:

providing a mixed system consisting of amorphous silicon dioxide coated carbon source precursor and water; the mass ratio of the amorphous silicon dioxide wrapped carbon source precursor to water in the mixed system is 100: 8-11;

pressing and molding the mixed system to prepare a blank;

embedding the green body into quartz sand to be synthesized by microwave sintering;

the average grain diameter of the carbon source is 0.06-0.20 mm;

the frequency of the microwave adopted for microwave sintering synthesis is 2.425-2.475 GHz;

the microwave sintering synthesis is that the reaction system is heated to the sintering temperature in a microwave field, and then the temperature is preserved and cooled; the sintering temperature is 600-1100 ℃; the heat preservation time is 0 min; heating the reaction system to 600 ℃ for 28-35 min; when the sintering temperature is higher than 600 ℃, the heating rate of heating the reaction system from 600 ℃ to the sintering temperature is 10-180 ℃/min;

the molar ratio of silicon dioxide to carbon in the carbon source in the amorphous silicon dioxide coated carbon source precursor is (0.9-1.1): 2.9-3.1.

2. The method for producing a silicon carbide crystal according to claim 1, wherein: the pressure adopted by the compression molding is 4-5 MPa; and the pressure maintaining time of the compression molding is 50-60 s.

3. The method for producing a silicon carbide crystal according to claim 1, wherein: the carbon source is coal.

4. The method for producing a silicon carbide crystal according to claim 1, wherein: the preparation method of the amorphous silicon dioxide coated carbon source precursor comprises the following steps:

1) mixing ethyl orthosilicate, ethanol and water, adding citric acid, adjusting the pH to 3-4, and stirring for 2-2.5 hours to obtain a mixture A; the volume ratio of the ethyl orthosilicate to the ethanol to the water is 23:35: 80;

2) respectively adding carbon sources into the mixture A, and stirring for 2.5-3 hours to obtain a mixture B;

3) and adding ammonia water into the mixture B, adjusting the pH value to 8-9, stirring for 1-2 h, drying and grinding to obtain the catalyst.

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