RU2791964C1 - Method for producing silicon carbide powder - Google Patents

Abstract

FIELD: production of silicon carbide powder.

SUBSTANCE: production of silicon carbide powder used as a source for growing single crystals of silicon carbide. To obtain silicon carbide powder, silicon dioxide and carbon are mixed, the resulting mixture is placed in a vacuum furnace, the furnace is filled with an inert gas, and the mixture is subjected to heat treatment in an inert gas atmosphere, followed by annealing of excess carbon in air. The initial components are mixed in the ratio SiO₂:C = 1:(3.2-4.0) (mol.). The thickness of the initial mixture layer is proportional to the square of the average grain diameter of the carbon powder and should not exceed 15-18 cm for an average grain diameter of 120 mcm at a bulk density of 0.6-0.8 g/cm³. Heat treatment in a vacuum furnace is carried out in two stages: first, for 4-5 hours at an inert gas pressure of 0.02-0.03 MPa and a temperature of 1600-1700°C, then reheat and hold at a temperature of 1900-2000°C for 1-2 hours at a pressure of 0.05-0.06 MPa.

EFFECT: invention makes it possible to increase the yield of alpha phase silicon carbide powder.

1 cl, 1 dwg, 1 tbl

Description

The invention relates to a technology for producing single-crystal silicon carbide (SiC) - a wide-gap semiconductor material used in power electronics and for creating integrated circuits based on it.

Ingots of single-crystal SiC are usually grown by the sublimation-condensation method (the so-called modified Lely method). In accordance with this method, a growth crucible is placed opposite each other - on top of the plate of the seed single crystal SiC and below the source of silicon carbide (SiC powder) [Tairov Yu.M. Growth of bulk SiC // Materials Science and Engineering: B. 1995. Vol. 29. N1-3. P. 83-89]. For reproducible production of high-quality single-crystal SiC ingots, the SiC powder used in the sublimation-condensation method must meet a number of requirements, namely: contain a single phase in its composition - have, if possible, the same particle size distribution, sufficiently high bulk density and chemical purity.

The method for obtaining SiC powder should be efficient, that is, the most complete use of the original silicon and carbon (in elemental form or in the form of available chemical compounds). The cost of carrying out the method should be minimized.

A known method for producing high-purity silicon carbide powder from silicon and carbon [CN 113120909 (A), Preparation method of high-purity semi-insulating silicon carbide powder, C01B 32/984, 2021]. According to this method, high-purity silicon and carbon powders are mixed in a ratio of 1:1.1 and heated in a furnace at a pressure of high-purity argon or hydrogen of 0-100 kPa until the silicon is completely evaporated, obtaining cubic silicon carbide powder (the so-called low-temperature phase of silicon carbide or beta- modification or β - SiC). Further, the obtained powder of the beta modification of silicon carbide is heated to a temperature of 1800-2500°C to obtain hexagonal polytypes of silicon carbide (the so-called alpha modification or α-SiC). The resulting SiC powder is subjected to annealing in air to remove excess carbon, crushing of the formed silicon carbide cakes, and screening. The disadvantage of this method is the low yield of the product, due to the fact that the formation of silicon carbide occurs due to the diffusion of carbon into the spreading liquid silicon. Additional losses are introduced by mechanical processing operations (crushing, screening), which, in addition, lead to a decrease in the purity and contamination of the product. The method makes it possible to obtain silicon carbide powder consisting of a mixture of α-SiC hexagonal polytypes.

SiC fine powder can be prepared by collecting the dispersed phase of a mixed aerosol containing a decomposable silicon compound (eg, SiCl ₄ , CH ₃ SiCl _{3 ,} etc.) and a carbon compound (eg, oil, N-hexane) in a hot gas [ JPS5983922 (A), Preparation of silicon carbide powder, C01B31/36, (IPC1-7): C01B31/36, 1984]. After collecting the condensate and heat treating it, a SiC powder with a bulk density of 0.15 g/cm ³ or more is obtained. The method does not provide a high yield of the product due to large losses in the collection of the dispersed phase.

For the synthesis of SiC powder, the most common method in industry is the carbothermal reduction of silicon dioxide SiO ₂ with carbon C. The total chemical reaction that occurs during the carbothermal reduction process can be written as follows:

and the process flow mechanism is due to the reduction of silicon dioxide SiO ₂ to gaseous SiO monoxide, followed by diffusion of SiO monoxide deep into the carbon grains, for the final reaction with the formation of silicon carbide [B. Abolpour, R. Shamsoddini Mechanism of reaction of silica and carbon for producing silicon carbide, Progress in Reaction Kinetics and Mechanism. 2019 Vol. 45. P. 1-14.].

A known method of obtaining SiC powder by reacting silicon dioxide SiO ₂ and carbon in a furnace at a temperature of 2200-2300°C [Kaynarsky I.S., Degtyareva E.V. Carborundum refractories, Kharkov: State scientific and technical publishing house of literature on ferrous and non-ferrous metallurgy, 1963, 252 pp.], followed by grinding the resulting silicon carbide cakes and screening the SiC powder. Silicon monoxide SiO at such a high temperature and with such an intense release of CO does not have time to completely react with carbon and is carried away from the furnace space. The silicon yield of the product is low. In addition, high temperatures lead to the fact that the powder is inhomogeneous in phase and polytype composition, that is, it is a mixture of carbon, silicon, and various silicon carbide polytypes.

To synthesize SiC powder, it is proposed to heat a mixture of initial powders of silicon dioxide SiO ₂ and carbon C under an argon pressure of 1-200 MPa to a temperature of SiC powder synthesis, that is, 1400-1800 ° C, followed by annealing excess carbon in air and grinding pieces of silicon carbide [KR 20120052787 (A), Silicon carbide and method for manufacturing the same, C01B 31/36, C04B 35/565, 2012]. The disadvantage of this method is the low yield of the process, due to the excess pressure of the inert gas, which prevents the chemical reaction of the interaction of silicon dioxide SiO ₂ and carbon.

A known method for producing SiC powder by high-temperature heating of silicon and carbon-containing raw materials in a nitrogen atmosphere at a pressure of 0.049-0.13 MPa or in a stream of nitrogen at a rate of 0.5-3.3 l/h to a temperature of 1600-1900°C [RF Patent 2240979, Method for producing silicon carbide, SW 31/36, 2004]. The use of subatmospheric pressures leads to losses of silicon monoxide SiO due to its removal from the furnace space. Carbon monoxide liberated by reaction (1) captures carbon particles, which are also carried away from the furnace space. All of the above leads to a decrease in the efficiency of the synthesis process and the yield of the process. In addition, the silicon/carbon ratio in the initial mixture changes, and changes occur in the polytype and phase composition of the synthesized SiC powder.

To obtain high-purity SiC powder (alpha modification of silicon carbide) with a large crystallite size, it was proposed to obtain and use a gel in which a carbon-containing compound (sucrose, fructose, maltose, etc.) is dispersed in a network structure of silicon dioxide [US 2021163301 (A1) , Method for producing large granular alpha-phase silicon carbide powders with a high purity, C01B 32/97, C01B 32/977, 2021]. The gel is subjected to heat treatment to decompose the carbon-containing compound and obtain a silicon dioxide/carbon composite at a temperature of 1100-1250°C. During repeated heat treatment at a higher temperature (2000-2100°C), the powder coarsens due to the growth of crystallites. As a result of coarsening of crystallites due to the processes of nonstoichiometric sublimation and condensation of silicon carbide, an uncontrolled change in the polytype and phase composition of the powder occurs (the appearance of atomic carbon, various hexagonal polytypes of silicon carbide). The process is very expensive and complicated, it does not allow to obtain a high yield of the product due to the uncontrolled behavior of the composite at high temperatures.

A method is also known, including mixing a silicon source (tetraethoxysilane, oxysilane polymer or high purity silicon dioxide SiO ₂ ) with a carbon source (high purity organic compound having oxygen in its molecule and giving residual carbon after heating), the stage of forming silicon carbide by calcining in non-oxidizing environment and the stage of subsequent processing of the SiC powder by heating to 2000-2100°C to increase the average grain diameter of the SiC powder, for 5-20 minutes at least once [GB2301349 (A), Process for producing high purity silicon carbide powder for preparation of a silicon carbide single crystal, C01B 31/36, C30B 23/00, (IPC1-7): C01B31/36, C30B29/36, 1996]. To improve the purity of the synthesized SiC powder, a non-oxidizing gas diluted with hydrogen halide (1-3% vol.) is passed through the furnace space. The method makes it possible to synthesize high-purity SiC powder with a grain size of 10 to 500 µm. The method has low efficiency due to large losses of silicon monoxide SiO, carried out of the furnace space together with carbon monoxide CO. The removal of silicon monoxide SiO from the reaction space leads to a violation of the silicon/carbon ratio and, consequently, to a violation of the phase and polytype homogeneity of the SiC powder. The output of the process is reduced.

Closest to the claimed is a method for producing silicon carbide, including: (a) mixing a source of silicon (colloidal silicon dioxide, silica gel, silicon dioxide Sol, etc.) and carbon (carbon black, fullerene, etc.) in a ratio of 1:1 to 4:1 (mass.) in the mixer and (b) heating in a sealed crucible at a pressure of 0.03-0.5 Torr to a temperature of 1300-1900°C for the synthesis of SiC powder [KR 20110021530 (A), High purity silicon carbide manufacturing method and system, C01B 31/36; C04B 35/565, 2011]. When using an organic carbon source between stages (a) and (b) is preheating the mixture to a temperature of from 700 to 1200°C for carbonization of the carbon source. The process was carried out in a crucible in a gaseous atmosphere of an inert gas (argon).

The use of vacuum leads to the loss of silicon monoxide SiO, which is sublimated together with carbon monoxide CO, and consequently to a decrease in the efficiency of the process of obtaining SiC powder. The silicon/carbon ratio is violated, which also leads to a change in the phase and polytype composition. The use of colloidal powders in the initial mixture, in combination with the constant pumping out of a sealed crucible, will always lead to material losses due to the ease of capture of small particles of carbon by the flow of gas released as a result of the reaction of carbothermal reduction - carbon monoxide CO, which will also lead to a decrease in the yield of the process.

The objective of the claimed invention is to create a method for obtaining a powder of alpha modification of silicon carbide with a high yield of the process in terms of the main initial reagents (silicon dioxide and carbon).

The technical result of the invention is to increase the yield of SiC powder.

The essence of the proposed method lies in the fact that silicon dioxide and carbon are mixed, the resulting mixture is placed in a vacuum furnace, the furnace is filled with an inert gas and the mixture is subjected to heat treatment in an inert gas atmosphere, followed by annealing excess carbon in air, characterized in that the initial components are mixed in the ratio of SiO ₂ :C = 1:(3.2-4.0) (mol.), and the thickness of the layer of the initial mixture is proportional to the square of the average grain diameter of the carbon powder and should not exceed 15-18 cm for an average grain diameter of 120 μm, at bulk density 0.6-0.8 g/cm ³ . Heat treatment is carried out in two stages, in a vacuum furnace filled with an inert gas: first, for 4-5 hours at an inert gas pressure of 0.02-0.03 MPa and a temperature of 1600-1700 ° C, then re-heating and holding at a temperature of 1900 -2000°C for 2-3 hours at a pressure of 0.05-0.06 MPa.

The proposed method has differences that make it possible to achieve a technical result, which consists in increasing the yield of alpha silicon carbide powder, namely:

As is known, the process of carbothermal reduction leads to the release of large volumes of carbon monoxide CO according to reaction (1), which, passing through the thickness of the reaction mixture of silicon dioxide SiO ₂ , carbon C and synthesized silicon carbide SiC, capture small particles of carbon or silicon dioxide and carry them out from the furnace space (the so-called Stokes force or friction force or drag force). Limiting the thickness of the initial reaction mixture layer used at a given pressure and temperature in the synthesis process makes it possible to limit the value of the Stokes force and prevent the removal of fine particles of the reaction mixture powders from the furnace space, which leads to an increase in the yield of the SiC powder production process for silicon Si and carbon C. A necessary condition for the implementation of a highly efficient process for the synthesis of SiC powder is to minimize the capture of powder particles of the reaction mixture by the CO flow and their removal from the reaction cell, since this leads to a decrease in the product yield. The trapping effect is usually most pronounced for carbon particles - as the lightest of the composition of the reaction mixture. A particle of carbon C, silicon dioxide SiO ₂ or silicon carbide SiC at the surface of the powder is affected by gravity and drag force (Stokes force), due to the presence of an upward flow of carbon monoxide CO. The thicker the layer H of the reaction mixture, the greater the volume of carbon monoxide CO exits per unit area of the surface and the higher the Stokes force acting on the particles of the reaction mixture on the surface. At a certain value of the flow rate, numerically equal to the sedimentation rate, known in physics [Khodakov G.S., Yudkin Yu.P. Sedimentation analysis of highly dispersed systems M.: Chemistry, 1981. 192 p.], a particle can be captured by an ascending flow of carbon monoxide CO. The authors of the proposed method found that for a carbon powder with an average grain diameter of 120 μm (the so-called median of the powder), the effect of particle capture into the flow is not yet observed if the layer thickness of the reaction mixture does not exceed 15–18 cm, with a standard bulk density of the initial mixture of 0, 6 - 0.8 g/cm ³ . For the rate of sedimentation it is known:

where R is the radius of a round particle, g is the acceleration of gravity, ρ is the density of the particle captured by the gas flow, η is the dynamic viscosity of the gas.

An accurate calculation of the limiting velocity V _S is impossible due to the presence of complicating factors (unknown parameters of the mixture of gases - carbon monoxide and argon, the spread of carbon particles in size and shape), but it is possible to roughly estimate the maximum value of the layer thickness H of the initial mixture for a powder of any grain size, at which the effect particle entrainment by the flow has not yet been observed, from the proportion, using the found value for carbon powder with a median of 120 µm, presented above:

where H ₁ is the maximum thickness of the powder layer of particles of size R ₁ with density ρ ₁ , H ₂ is the same for particles of size R ₂ with density ρ ₂ .

Powders of silicon dioxide SiO ₂ and carbon C are mixed in a ratio that practically corresponds to the stoichiometry of reaction (1), that is, M _{SiO 2} : M _C = 1: (3.2-4.0) (mol.). A slight excess of carbon C makes it possible to minimize the amount of unreacted particles of silicon monoxide SiO leaving the reaction mixture in the process of carbothermal reduction, as well as to exclude the processes of agglomeration of the resulting SiC powder grains. An increase in the ratio of components in the reaction mixture above 4.0 is undesirable, as it will lead to a decrease in the efficiency of the process in terms of carbon.

The first stage of heat treatment (synthesis of the beta modification of SiC) is carried out at a sufficiently high pressure of an inert gas, which prevents the loss of silicon monoxide SiO due to its leaving together with carbon monoxide CO from the furnace space.

The second stage of heat treatment (conversion of the beta modification to the alpha modification) is carried out at a sufficiently high pressure to prevent dissociative sublimation of the SiC powder. As is known, during sublimation, the solid phase - SiC powder - is enriched in carbon, and the gas phase is enriched in silicon Si vapor, which easily leaves the reaction cell, as a result of which the yield of the SiC powder production process decreases. In the proposed method, silicon vapor is practically not formed, and the yield of the SiC powder production process increases.

Additionally, an increase in the service life of the furnace is achieved due to the absence of aggressive silicon vapors or gaseous derivatives of silicon carbide in the gas phase, which actively interact with the graphite fittings of the reaction cell, which leads to the need for premature replacement of expensive parts of the graphite fittings.

Thus, the optimal choice of the values of technological parameters of the SiC powder synthesis process (the ratio of components in the initial mixture, the layer thickness of the initial mixture of silicon dioxide SiO ₂ and carbon C in the reaction cell, the temperature and pressure in the furnace space, the duration of the heat treatment stages) makes it possible to implement a highly efficient process synthesis of silicon carbide powder. In addition, the absence of gaseous silicon in the process of high-temperature heat treatment makes it possible to increase the service life of expensive graphite fittings, that is, to reduce the cost of the method.

The method is illustrated in the drawings.

Fig. 1 is a diagram of a vacuum furnace for implementing the proposed method;

Fig. 2 - diagram of the capture of a solid particle of the reaction mixture by a flow of carbon monoxide CO: Fg - gravity, Fc - Stokes force;

Fig. 3 - data of X-ray phase analysis of the reaction mixture after exposure for 4 hours at an argon pressure of 0.02 MPa: (a) T = 1300°C; (b) Т = 1600°С; (c) T = 1700°С. Reflections from the beta modification of silicon carbide are indicated by the letter “K”;

Fig. 4 - the content of the beta modification of silicon carbide (β-SiC) in the reaction mixture after exposure for 4 hours at different temperatures in an argon atmosphere, p = 0.02 MPa;

Fig. 5 - data of X-ray phase analysis of the reaction mixture after the second stage of heat treatment for 2 hours: (a) T = 2000°C, p = 0.02 MPa, the main phases are the alpha modification of silicon carbide and carbon (2H-C); (b) T = 2000°C, p = 150 Pa, the main phases are a mixture of silicon carbide alpha modification polytypes (6H-SiC, 15R-SiC, 4H-SiC) and carbon (3R-C).

As a technical means for implementing the proposed method for producing SiC powder, a cylindrical vacuum furnace 1 (Fig. 1) is used, to which a vacuum system and a gas purge system are connected - control and pressure maintenance. High-purity argon with a purity of at least 6N or any other inert or non-oxidizing gas is used as the atmosphere during powder synthesis. Inside the vacuum furnace 1 there is a heat-insulating screen 2 and a cylindrical resistive or induction heater 3. A reaction cell 4 is installed in the cavity of the heater 3, equipped with a gas-permeable cover 5 fixed on the edges of the reaction cell and made of a homogeneous highly porous material, for example, high-purity graphite felt. The initial mixture of reagents 6 - silicon dioxide SiO ₂ and carbon C - is placed inside the reaction cell 4, at the bottom, in the form of a mixture of powders of silicon dioxide and carbon - a layer of thickness H. The reaction cell 4 is made of dense structural graphite. In the process of SiC powder synthesis, the removal of large volumes of carbon monoxide CO formed from the reaction cell 4 and further from the furnace space is carried out through the pores of the highly porous cover 5 of the reaction cell.

When carrying out the method, the mixing of the reagents and the creation of the initial mixture is carried out in a clean mixer outside the reaction cell, until a homogeneous mixture is formed. As initial reagents, the following are most often used: silicon dioxide powder of OSCh 12-4 qualification TU 6-09-3379-79 and graphite powder OSCh 8-4 GOST 23463-79 with amend. 12.

Powders of silicon dioxide SiO ₂ and carbon C are mixed in the ratio M _{SiO 2} : M _C = 1: (3.2-4.0) (mol.).

The prepared initial mixture is placed in the reaction cell, and that, in turn, in the space of the vacuum furnace.

The reaction cell 4 used for the implementation of the method cannot be airtight, since in the process of carbothermal reduction of silicon dioxide SiO ₂ a large amount of carbon monoxide CO is released, which must be removed through the top cover 5, namely, through the pores of highly porous graphite felt (Fig. . 1).

Formula (3) gives the values of the thickness of the layer of the reaction mixture, for which the effect of the capture of solid particles of the powder by the flow of carbon monoxide CO is not observed or is insignificant (Fig. 2). For particle sizes less than 12 μm, the maximum thicknesses of the reaction mixture layer obtained by formula (3) will be on the order of 0.15-0.18 cm and below. Such small values of H will lead to a decrease in the productivity of the method, due to the limitation of the size of a single load of the initial mixture by weight. When using for these particle sizes thicknesses greater than those determined by formula (3), the efficiency of the method decreases due to the partial departure of carbon powder from the reaction mixture due to the capture of carbon monoxide CO by the flow. The removal of carbon particles from the furnace space in this case is limited only by using the porous top cover 5 of the reaction cell 4.

An important issue is the choice of technological parameters for the first stage of heat treatment (carbothermal reduction of silicon dioxide SiO ₂ with carbon to the beta modification of silicon carbide). The components for the preparation of the initial mixture are powder of ground quartz glass or polycrystalline quartz SiO ₂ and crystalline powder of carbon C. In the process of chemical interaction in a wide range of technological parameters, the main phases in the reaction mixture are cristobalite SiO ₂ and low-temperature beta modification of silicon carbide (β - SiC), and with increasing temperature and duration of the carbothermal reduction process, the percentage of beta-modification of silicon carbide in the powder continuously increases, and cristobalite decreases (Fig. 3). On FIG. 4 shows the percentage of beta-modification of silicon carbide in the powder obtained at different temperatures, with a 4-hour exposure at an argon pressure of 0.02 MPa. At a temperature of 1700°C, which is selected for carrying out the carbothermal reduction of silicon dioxide SiO ₂ within the framework of the method, there is an almost complete transformation of the powder into a low-temperature beta modification of silicon carbide. A further increase in temperature can lead to the formation of a mixture of hexagonal silicon carbide polytypes and is undesirable. A slight decrease in the temperature of carbothermal reduction or an increase in the pressure of the inert gas is possible, although it will lead to a slight decrease in the content of the beta modification of silicon carbide β - SiC in favor of cristobalite in the reaction mixture. Accordingly, the exposure can be extended up to 5 hours to complete the reaction at 1600°C.

The second stage of heat treatment of the reaction mixture, which is a mixture of powder of low-temperature beta modification of silicon carbide with excess carbon, is carried out at 1900-2000°C and maintained at this temperature for 2-3 hours in an atmosphere of inert gas - high purity argon, at pressure 0.05-0.06 MPa.

At the specified pressure and temperature, a solid-state reaction proceeds to convert the low-temperature beta modification of silicon carbide (β-SiC) into a high-temperature alpha modification of silicon carbide (for example, 6H-SiC). Sublimation of the low-temperature beta-modification of silicon carbide at the indicated pressure and temperature is difficult, mainly solid-state transformation takes place. Note that sublimation will inevitably lead to the appearance in the reaction mixture of a set of hexagonal silicon carbide polytypes corresponding to the alpha modification of SiC (Fig. 5), as well as to the appearance of an additional amount of carbon in the reaction mixture and a decrease in the yield of SiC powder.

Obviously, the temperature of the second stage of heat treatment can be increased, but - to suppress sublimation - with a simultaneous increase in the pressure of the inert gas (the equilibrium pressure corresponding to the evaporation of silicon carbide increases with increasing temperature). A decrease in the annealing temperature below 1900°C may lead to incomplete transformation of the beta modification of silicon carbide into the alpha modification of silicon carbide or will require an increase in the holding time, since, according to our data, solid-state transformation is difficult at temperatures below 1900°C.

After the second stage of heat treatment at a temperature of 1900-2000°C, the reaction mixture is a mixture of the alpha modification of silicon carbide and excess carbon. Excess carbon can be removed by annealing the reaction mixture in air at a temperature of 800-900°C. The required duration of the annealing stage in air is 5-6 hours and also depends on the thickness of the reaction mixture layer in the annealing vessel (limited by air access to the lower layers of the reaction mixture).

The resulting SiC powder is a single-phase silicon carbide alpha modification powder.

The method is carried out as follows.

For experimental verification of the method, a reaction cell 4 is used, made of MPG-7 dense structural graphite, subjected to vacuum annealing at a temperature of 2200°C, with an inner diameter of 170 mm and a side wall height of 200 mm. The top cover 5 is made of high-purity felt from Karbotek and also pre-cleaned by high-temperature vacuum annealing.

Powders of silicon dioxide SiO ₂ and carbon C are weighed in the required amount on a laboratory balance, after which they are loaded into a laboratory powder mixer or mixed manually with a laboratory spoon. The amounts of powders used (with a molar ratio of silicon dioxide SiO ₂ to carbon C 1: (3.2-4) (mol.)): silicon dioxide SiO ₂ 120-1200 g, carbon C, respectively, 80-960 g.

After obtaining a homogeneous mixture, the initial mixture is poured into reaction cell 4, on which a cover made of porous graphite felt 5 is placed on top. Reaction cell 4 with cover 5 is placed inside a vacuum furnace, after which pumping from the atmosphere to vacuum (less 10 ^-2 Pa).

After pumping out, the space of the vacuum furnace is filled with inert gas argon to a pressure of 0.02-0.03 MPa, then the specified pressure is maintained. The pressure maintenance is usually carried out in dynamic mode, that is, with a small flow of inert gas through the space of the vacuum furnace (1-10 l/h). The reaction cell is heated to a temperature of 1600-1700°C. The heating rate is 10-20°C/min. After holding at a temperature of 1600-1700°C for 4-5 hours, the reaction cell is heated to a temperature of 1900-2000°C for 0.5-1 hour, while increasing the pressure to 0.05-0.06 MPa, and exposure at the specified temperature for 2-3 hours. Heating is carried out using a resistive graphite heater made of structural annealed A-2 graphite, located in the space of a vacuum furnace.

Next, the reaction cell is cooled to room temperature. After cooling, the vacuum furnace is depressurized by filling it with argon to a pressure of 0.1 MPa. The reaction cell is removed, its contents are poured into quartz containers, which are installed inside an atmospheric muffle furnace with silicate heaters. The reacted mixture is annealed for at least 5-6 hours at a temperature of 800-900°C in air. After cooling, the synthesized SiC powder is poured into plastic containers for storage.

The polytype and phase composition, as well as the ratio of the main phases in the SiC powder, is determined by X-ray phase analysis, the bulk density is determined using a measuring cup and laboratory scales. The granulometric composition of the initial powders is determined by laser diffraction granulometry. The output of the process is determined in terms of silicon, as a percentage of the original amount.

The effect of capturing particles of carbon and silicon dioxide by an upward flow of carbon monoxide is visually observed by the deposition of fine particles on the inner surfaces of the top cover 5 of the reaction cell 4.

The data of 12-fold tests of the method are presented in the Table.

Table. Results of the 12-fold trial of the method NN Ref. substances
SiO ₂ (Mass, g; Median, µm); C (Mass, g; Median, µm); H cm Carbotherm.
restored
T, °C;
p, MPa;
τ, hour High-temp. annealing
T, °C;
p, MPa;
τ, hour Annealing in air
T, °C;
τ, hour Phase composition* Bulk density. g/cm ³ Yield, % (by silicon) 1 600, 120
420, 42
7 1500
0.005
6 -
- 800
5 3C-SiC, SiO ₂ 0.25 50 ** 2 600, 100
420, 42
6.5 1500
0.005
5 -
- 800
2 3С-SiC, SiO ₂ , C 0.3 47** 3 300, 120
200, 42
2.5 1700
0.02
5 -
- 900
4 3С-SiC 0.3 85 4 400, 120
260, 42
4 1850
0.03
4 -
- 900
5 3С-SiC, 6Н-SiC 0.35 70** 5 600, 120
410, 42
6 1700
0.03
5 1900
0.00015
2 800
6 6H-SiC, 15R-SiC, 4H-SiC 1.1 63** 6 600, 100
420, 120
6.5 1600
0.02
4 2000
0.05
3 900
6 6H-SiC 0.85 91 7 860, 100
590, 120
10 1600
0.02
5 2000
0.06
2 900
5 6H-SiC 0.9 95 8 1200, 120
800, 42
15 1700
0.02
4 1900
0.05
2 800
6 6H-SiC 0.8 35** 9 600, 120
420, 42
7 1600
0.03
5 1900
0.05
1 900
6 6H-SiC 0.9 70** 10 600, 100
420, 42
7 1700
0.03
4 2000
0.05
2 900
6 6H-SiC 0.85 76** eleven 200, 120
120, 42
2.5 1700
0.02
4 2000
0.06
2 900
6 6H-SiC 0.71 85 12 200, 120
140, 42
6 1700
0.02
4 2000
0.06
2 900
5 6H-SiC 0.82 79** Note. * - symbols of the following phases: 3С-SiC - beta modification of silicon carbide, 6H, 15R, 4H - various polytypes corresponding to alpha modification of silicon carbide, SiO ₂ - cristobalite, C - carbon; ** - the removal of carbon particles from the reaction mixture to the inner surfaces of the reaction cell and into the furnace space was observed.

Based on the test data, a number of conclusions can be drawn, namely:

1) At the first stage of heat treatment, almost complete carbothermal reduction of silicon dioxide SiO ₂ to low-temperature beta modification of silicon carbide (β-SiC) occurs at a temperature of at least 1600-1700 ° C, with an inert gas pressure of 0.02-0.03 MPa. At higher temperatures, the powder obtained as a result of the first stage of heat treatment consists of two phases - a low-temperature beta modification of silicon carbide (β - SiC) and a high-temperature alpha modification (α - SiC), and the product yield decreases.

2) To anneal the excess carbon that remains in the silicon carbide powder after the first or second stage of heat treatment, temperatures of 800-900°C are required, the annealing time in air should be 5-6 hours, depending on the amount of silicon carbide powder.

3) The silicon carbide powder obtained as a result of the first stage of heat treatment has an extremely low bulk density (0.2-0.4 g/cm ³ ) and cannot be directly used as a source for growing SiC single crystals, but must be subjected to the second stage of heat treatment .

4) The second stage of heat treatment makes it possible to obtain a single-phase powder of silicon carbide alpha modification at a temperature of 1900-2000°C and an inert gas pressure of 0.05-0.06 MPa. An increase in temperature or a decrease in pressure leads to sublimation of silicon carbide, as a result, to a decrease in yield and the production of a powder, which is a mixture of polytypes of the alpha modification of silicon carbide.

5) The results of the implementation of the method depend on the particle size distribution of the initial powders, primarily carbon powder. Increasing the thickness of layer H of the reaction mixture above 15-18 cm for carbon powder (with a median of 120 μm) leads to a decrease in the yield of the process, due to the capture of carbon particles by the upward flow of carbon monoxide CO. For carbon particles with a median of 42 μm, the trapping effect is not observed only when the layer thickness of the reaction mixture is H ≤ 2 cm, in accordance with formula (3).

6) As a result of successive stages of heat treatment and annealing in air under optimal conditions, it is possible to synthesize alpha-modified silicon carbide powder with a bulk density of 0.7-1.0 g/cm ³ .

In general, in comparison with the prototype, the proposed method allows to increase the process yield up to 80 - 95% (silicon) and allows to increase the service life of the furnace due to the absence of an aggressive effect on the furnace structural elements.

Claims (1)

A method for producing silicon carbide powder, in which silicon dioxide and carbon are mixed, the resulting mixture is placed in a vacuum furnace, the furnace is filled with an inert gas, and the mixture is subjected to heat treatment in an inert gas atmosphere, followed by annealing of excess carbon in air, characterized in that the initial components are mixed in the ratio SiO ₂ :C = 1:(3.2-4.0) (mol.), and the thickness of the layer of the initial mixture is proportional to the square of the average grain diameter of the carbon powder and should not exceed 15-18 cm for an average grain diameter of 120 μm at a bulk density 0.6-0.8 g/cm ³ , and the heat treatment is carried out in two stages in a vacuum furnace filled with an inert gas: first for 4-5 hours at an inert gas pressure of 0.02-0.03 MPa and a temperature of 1600-1700 °C, then reheat and hold at a temperature of 1900-2000°C for 1-2 hours at a pressure of 0.05-0.06 MPa.

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