JP2024094924A - Method for producing molded product with high silicon carbide content, and method for producing silicon carbide single crystal using molded product with high silicon carbide content - Google Patents

Abstract

[Problem] To provide a method for producing a high-SiC molded product that is used as a C source and a Si source for a SiC single crystal in the production of a SiC single crystal by a solution process, the method for producing a high-SiC molded product being capable of obtaining a dense high-SiC molded product in a desired shape with reduced voids even while applying a reactive impregnation method.
[Solution] A method for producing a molded product with a high silicon carbide content, which is used as a carbon source and a silicon source in the production of silicon carbide single crystals by a solution process, comprising: pressurizing a granulated material containing silicon carbide powder and a thermosetting resin; impregnating the carbonized product produced by a curing reaction into a molded product with molten silicon, thereby reacting carbon derived from the thermosetting resin with the molten silicon to produce silicon carbide.
[Selected Figure] Figure 1

Description

The present invention relates to a method for producing a molded product with a high silicon carbide content that is suitable as a carbon source and a silicon source in the production of silicon carbide single crystals. The present invention also relates to a method for producing silicon carbide single crystals using a molded product with a high silicon carbide content as a carbon source and a silicon source.

Silicon carbide (SiC) is expected to be a next-generation power semiconductor material. For example, the SiC power device market is expected to expand at an average annual growth rate of 30% worldwide, and in particular in the automotive field, such as motor drives for electric vehicles (EVs), an annual growth rate of over 100% is expected. However, SiC substrates are more expensive than Si substrates, and current technology has limitations in reducing defects in SiC single crystal substrates. Therefore, improving the quality of SiC single crystal substrates is an issue.

In the case of Si single crystal substrates, since the melting point of Si is 1414°C under normal pressure, high-quality, large-diameter single crystals can be obtained from Si melt by the Czochralski method (CZ method) or the floating zone melting method (FZ method). In contrast, SiC does not melt under normal pressure, and when heated, it sublimes at a temperature of about 2000°C, so the CZ method and FZ method cannot be applied. Therefore, SiC single crystals are mainly manufactured by the sublimation method. At present, the sublimation method is the only method for mass-producing SiC single crystals, and 6-inch SiC single crystal substrates manufactured by this method are commercially available. In addition, development is underway to mass-produce 8-inch SiC single crystal substrates. However, the sublimation method has a slow crystal growth rate and precise temperature control of the crystal growth environment is difficult, making it difficult to further reduce defects. Furthermore, while it is theoretically possible to achieve higher quality by using the Repeated A-Face (RAF) method, this creates cost constraints.

As an alternative to the sublimation method, a solution method (TSSG: Top Seeded Solution Growth) has been proposed as a crystal growth technique for SiC single crystals. In the production of SiC single crystals by the solution method, a Si melt is placed in a graphite crucible, C is dissolved from the crucible into the Si melt, a SiC seed crystal is brought into contact with this Si-C solution, and a SiC single crystal is obtained by solution growth on the SiC seed crystal. In the solution method, the crystal growth of SiC progresses in a state close to thermal equilibrium, and a high-quality SiC single crystal with fewer defects can be obtained compared to the sublimation method. Since the solubility of C in the above-mentioned Si melt is extremely low at about 1 at%, transition metals are generally added to the Si melt to increase the amount of C dissolved. Examples of added elements include transition metals such as Ti, Cr, Ni, and Fe.

The solution method is considered promising as a manufacturing technology for high-quality SiC single crystals, but some problems have been pointed out. As the SiC single crystal grows, the Si component is gradually lost from the Si-C solution, while C is continuously supplied from the crucible. In addition, the amount of added elements such as transition metals contained in the Si-C solution does not change, so the proportion of Si in the solution decreases. This increases the solubility of C in the Si-C solution, and over time, C is dissolved in excess, causing the Si/C composition ratio in the solution to change. As a result, it becomes difficult to continue the stable growth of SiC single crystals for a long period of time. In addition, excessive dissolution of C into the Si-C solution produces fine SiC polycrystals on the inner wall of the crucible, and it has been pointed out that these SiC polycrystals float in the Si-C solution and hinder single crystal growth.

In the solution method, a technique has been proposed to suppress the compositional fluctuation of the Si-C solution as described above and to suppress the occurrence of polycrystals precipitated on the inner wall of the crucible. For example, Patent Document 1 describes a method for producing a SiC single crystal in which a crucible containing SiC as the main component and having an oxygen content of 100 ppm or less is used as a container for the Si-C solution, the crucible is heated, and both Si and C originating from the SiC, the main component of the crucible, are dissolved into the Si-C solution from the high-temperature region of the crucible surface that contacts the Si-C solution, and a SiC seed crystal is brought into contact with the Si-C solution from the top of the crucible to grow a SiC single crystal on the SiC seed crystal. According to the technique described in Patent Document 1, the crucible, which is the container for the Si-C solution, becomes a supply source of both Si and C elements, the Si/C composition ratio in the solution is stabilized, and a SiC single crystal can be stably grown for a long period of time. In addition, since the oxygen content of the crucible is 100 ppm or less, gas generation in the Si-C solution is suppressed, making it possible to stably produce high-quality SiC single crystals with few defects over a long period of time. Patent Document 1 describes that a crucible mainly composed of SiC may be subjected to a heat treatment to impregnate it with Si if necessary, but does not describe reacting the impregnated Si with a carbon (C) raw material present in the crucible to produce SiC.

As a technique for manufacturing molded products whose main component is SiC, a method (reactive impregnation method) is known in which a molded body containing a C raw material is impregnated with a Si melt to produce SiC. For example, Non-Patent Document 1 describes a method of impregnating a mixed compact of SiC powder and C powder with a Si melt to obtain a structure with a high SiC content. In addition, Non-Patent Document 2 describes a method of impregnating a carbon preform obtained by mixing crystalline cellulose and phenolic resin and carbonizing the mixture with a Si melt to obtain a structure with a high SiC content. In both techniques, structures with a SiC content of 80% or more are obtained. In these techniques, the majority of the remainder excluding SiC is composed of Si, and in some cases, a small amount of unreacted C remains.

JP 2017-31036 A

Suyama et al., Diamond and Related Materials, 2003, Vol. 12, pp. 1201-1204 Margiotta et al., J. Mater. Res. 2008, Vol. 23, pp. 1237-1248

It is assumed that graphite is used as the C powder described in Non-Patent Document 1, but the inventors have found through their investigations that when graphite reacts with molten Si to produce SiC, volume expansion occurs, making it difficult to maintain the shape of the green compact.
Furthermore, as the inventors furthered their studies on the technology described in Non-Patent Document 2, they found that since the organic raw material is carbonized to produce the C raw material, the bulk density of the C raw material can be reduced and volume expansion is unlikely to occur when it reacts with Si to produce SiC, but on the other hand, due to the influence of moisture (water vapor) generated during the curing reaction (dehydration condensation) of the phenolic resin, numerous voids are generated in the obtained molded product, making it difficult to maintain the desired shape with high precision.

The present invention aims to provide a method for producing a high-SiC content molded product that is used as both a C source and a Si source for the SiC single crystal produced by a solution process, and that can obtain a dense high-SiC content molded product with a desired shape and reduced voids even while applying a reactive impregnation method. The present invention also aims to provide a method for producing a SiC single crystal that can stably continue the growth of a SiC single crystal by a solution process for a longer period of time.

The present inventors have conducted extensive research in view of the above problems. As a result, they have found that a SiC powder and a thermosetting resin are mixed (kneaded) to obtain a homogeneous composition, the mixture is granulated to obtain a granulated product, the granulated product is pressure molded, and the molded product is cured and carbonized, and the carbide is impregnated with molten Si, thereby producing SiC by the reaction between the C derived from the thermosetting resin and the molten Si, and the molded product obtained can substantially maintain the shape of the pressure molded product before being impregnated with molten Si, and gas such as water vapor generated during the curing reaction can be released to the outside through fine voids between the particles constituting the molded product, so that voids are unlikely to be generated, and as a result, a dense SiC-rich molded product with a desired shape can be obtained.
The present invention has been completed through further investigation based on these findings.

The above-mentioned problems of the present invention have been solved by the following means.
[1]
A method for producing a silicon carbide-rich molded product used as a carbon source and a silicon source in the production of silicon carbide single crystals by a solution process, comprising the steps of:
The manufacturing method is a method for manufacturing a molded product with a high silicon carbide content, which includes pressure molding a granule containing silicon carbide powder and a thermosetting resin, and impregnating the carbonized product obtained by a curing reaction with molten silicon to react carbon derived from the thermosetting resin with the molten silicon to produce silicon carbide.
[2]
The method for producing a silicon carbide-rich molded product according to [1], wherein the thermosetting resin contains at least one of the following (a) to (f):
(a) thermosetting phenolic resin, (b) thermosetting epoxy resin, (c) thermosetting urethane resin, (d) thermosetting urea resin, (e) thermosetting acrylic resin,
(f) Thermosetting alkyd resin [3]
The method for producing a silicon carbide-rich molded product according to [1] or [2], wherein the thermosetting resin is composed of carbon, hydrogen and oxygen elements.
[4]
The method for producing a silicon carbide-rich molded product according to any one of [1] to [3], wherein the content of the silicon carbide powder in the granules is 90 mass% or less.
[5]
The method for producing a silicon carbide-rich molded product according to any one of [1] to [4], wherein the carbonization treatment product has a residual carbon rate of 15 mass% or more.
[6]
The method for producing a silicon carbide-rich molded product according to any one of [1] to [5], wherein the silicon carbide-rich molded product is used as a crucible for containing a solution in the production of a silicon carbide single crystal by the solution method.
[7]
A method for producing a silicon carbide single crystal, comprising using a high silicon carbide content molded product obtained by the method for producing a high silicon carbide content molded product according to any one of [1] to [6] as a crucible for containing a solution, thereby allowing the crucible to function as a carbon source and a silicon source for the silicon carbide single crystal.

According to the manufacturing method of high SiC content molded products of the present invention, it is possible to obtain dense high SiC content molded products with the desired shape and reduced voids, even while applying the reactive impregnation method. In other words, it is possible to obtain high SiC content molded products that are suitable as a C source and a Si source in the production of SiC single crystals by the solution method. Furthermore, according to the manufacturing method of SiC single crystals of the present invention, it is possible to stably continue the growth of SiC single crystals by the solution method for a longer period of time, and it is possible to stably obtain SiC single crystals with reduced defects.

FIG. 1 is a flow chart showing the chronological steps of a preferred embodiment of the method for producing a high silicon carbide content molded product of the present invention. FIG. 2 is an explanatory diagram showing the state immediately after impregnation of the carbonized molded product obtained by pressure-molding a granulated material containing silicon carbide powder and a thermosetting resin and subjecting the pressure-molded product to a curing reaction in the manufacturing method of a molded product with a high silicon carbide content of the present invention (left diagram), and the state after that in which Si and C have reacted to produce SiC (right diagram). 3 is an explanatory diagram that shows a schematic diagram of a method for producing a SiC single crystal by a solution method. The left diagram shows a state in which a seed crystal is in contact with a Si-C solution, and the right diagram shows a state in which a SiC single crystal is grown from the seed crystal by solution growth. FIG. 4 is an optical microscope photograph of a cross section of the high SiC content molded article of Example 1. FIG. 5 is an electron microscope photograph of a cross section of the high SiC content molded article of Example 1. FIG. 6 is an electron microscope photograph of a cross section of the high SiC content molded article of Example 2. FIG. 7 is an electron microscope photograph of a cross section of the high SiC content molded article of Example 3. FIG. 8 is a photograph showing the appearance of the SiC-containing molded articles of Comparative Example 1 and Comparative Example 2. FIG. 9 is an electron microscope photograph of a cross section of the SiC-containing molded articles of Comparative Example 1 and Comparative Example 2. FIG. 10 is a photograph showing the appearance of the SiC-containing molded article of Comparative Example 3.

A preferred embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment except as specified by the present invention.

[Method for producing molded products with high silicon carbide content]
The method for producing a molded product with a high SiC content of the present invention (hereinafter also referred to as the "method for producing a molded product of the present invention") includes pressure molding a granulated product containing SiC powder and a thermosetting resin, and impregnating the carbonized product of the molded product obtained by a curing reaction with molten Si. In one embodiment, carbonization can be advanced in a temperature increase stage for impregnation with molten Si without performing a carbonization process for obtaining a carbonized product separately. In the present invention, the "molded product with a high SiC content" refers to a molded product in which the ratio of the area of the SiC portion to the total area (30,000 μm ² ) of the areas of the three non-overlapping areas (within a rectangular frame of 10,000 μm ² ) of the areas of the respective areas is 75% or more (preferably 80% or more, more preferably 85% or more) when three non-overlapping areas of the cross section of the molded product are observed.
The high SiC content molded product obtained by the molded product manufacturing method of the present invention is suitable as a C source and a Si source in the production of SiC single crystals by the solution method. That is, the high SiC content molded product can be used as a crucible for containing a Si-C solution. The high SiC content molded product can also be immersed in a Si-C solution and used as a C source and a Si source. In the present invention, the form in which the high SiC content molded product is used as a crucible includes both a form in which the crucible is composed only of the high SiC content molded product and a form in which the high SiC content molded product is arranged along the inner wall of a crucible other than the high SiC content molded product (for example, a graphite crucible) (use as a liner).

<Preparation of granules containing silicon carbide powder and thermosetting resin>
Granules containing SiC powder and thermosetting resin can be obtained by mixing (kneading) SiC powder and thermosetting resin to form a uniform composition, and subjecting this mixture (composition) to a granulation process. When the thermoplastic resin is in a solution state dissolved in a solvent, it is preferable to mix it with the SiC powder and then dry it to remove the solvent. The drying process depends on the type of solvent and the type of thermosetting resin, but it can be heated and dried at, for example, 120°C or less so as not to cause a curing reaction, and it is more preferable to heat and dry at 100°C or less. The drying time is not particularly limited, and it is sufficient to dry it until it reaches the desired dry state.
The mixing ratio of the SiC powder and the thermosetting resin (excluding the solvent) is preferably SiC powder/thermosetting resin (mass ratio) of 10/1 to 1/5, more preferably 8/1 to 1/3, even more preferably 6/1 to 1/1, and particularly preferably 5/1 to 2/1. The content of the SiC powder in the granulated product obtained by the granulation process is preferably 90 mass% or less. In other words, it is preferable to mix a relatively large amount of the thermosetting resin as a C source.
The granulation process is not particularly limited, and various methods capable of pulverizing the mixture can be adopted. For example, methods using a mortar, a stone mill, a stamp mill, an autogenous grinder, a cutter mill, a mixer mill, a cryomill, a spray dryer, etc. can be mentioned. It is preferable to classify the granulated material obtained by this granulation process using a sieve, and to subject those with a desired particle size to the next step of pressure molding (press molding). The particle size of the granulated material to be subjected to the next step of pressure molding (the particle size of the secondary particles formed by granulation) is preferably adjusted to 600 μm or less by a sieve (using a granulated material that has passed through a sieve with an opening of 600 μm), more preferably adjusted to 500 μm or less, and even more preferably adjusted to 400 μm or less. This particle size is usually 0.5 μm or more, preferably 1 μm or more, and more preferably 5 μm or more. The particle size of the granulated product is preferably in the range of 0.5 to 600 μm, more preferably 1 to 500 μm, even more preferably 5 to 500 μm, and even more preferably 5 to 400 μm. It is also preferable to use a sieve with an opening size of 10 to 100 μm and use granulated product that does not pass through this sieve to control the lower limit of the particle size of the granulated product.

(Silicon carbide powder)
The SiC powder used as the raw material is not particularly limited as long as it is powdered SiC. For example, SiC powder having a volume-based median diameter (d50) of 20 μm or less can be used. From the viewpoint of suppressing the generation of excessive voids in the molded body, it is preferable to use SiC powder of 15 μm or less, more preferably to use SiC powder of 10 μm or less, and even more preferably to use SiC powder of 6 μm or less. The d50 of the SiC powder is usually 0.2 μm or more, preferably 0.3 μm or more, more preferably 0.5 μm or more, and also preferably 1 μm or more. Such SiC powder can be obtained from the market. The preferred range of d50 of the SiC powder is preferably 0.2 to 20 μm, more preferably 0.3 to 20 μm, more preferably 0.5 to 15 μm, even more preferably 0.5 to 10 μm, and even more preferably 1 to 6 μm.

(Thermosetting resin)
Thermosetting resin means a resin that undergoes a curing reaction by heat to produce a cured product with a three-dimensional network structure. When simply referred to as "thermosetting resin", it means a thermosetting resin in a state before the curing reaction, or in a state in which the curing reaction has occurred to a certain extent but has not yet completely cured. Examples of thermosetting resins include a combination of a prepolymer and a curing agent (crosslinking agent, curing catalyst, thermal polymerization initiator, etc.), a combination of a prepolymer, a monomer, and a curing agent, and a prepolymer having a reactive group.
The thermosetting resin functions as a binder during the granulation process and pressure molding, contributing to the shape stability of the secondary particles or molded body, and is carbonized before the SiC production reaction, serving as a C source in the SiC production reaction.
The type of thermosetting resin is not particularly limited, and a wide variety of general thermosetting resins can be used. For example, at least one of the following (a) to (f) can be used as the thermosetting resin.

(a) thermosetting phenolic resin (b) thermosetting epoxy resin (c) thermosetting urethane resin (d) thermosetting urea resin (e) thermosetting acrylic resin (f) thermosetting alkyd resin

(a) Thermosetting phenolic resin is a resin that forms a three-dimensional network structure by a thermosetting reaction. It is usually made of a combination of a prepolymer and a curing agent, and may contain a phenolic compound as a monomer. Phenol resin is widely known as a thermosetting resin, and commercially available products can be widely used.
(b) Thermosetting epoxy resins form a three-dimensional network structure by a ring-opening polymerization reaction of epoxy groups due to a thermosetting reaction. For example, a combination of an epoxy compound and a curing agent can be used. Epoxy resins are widely known as thermosetting resins, and a wide variety of commercially available products can be used.
(c) Thermosetting urethane resin is a urethane resin that forms a three-dimensional network structure by a thermosetting reaction. It is generally a combination of a polyisocyanate compound and a polyol. This polyisocyanate compound may be a prepolymer. The polyol may be a prepolymer such as a polyether polyol or a polyester polyol. Urethane resin is widely known as a thermosetting resin, and commercially available products can be widely used.
(d) Thermosetting urea resin is a urea resin that forms a three-dimensional network structure by a thermosetting reaction. It is generally a combination of a polyisocyanate compound and a polyamine. This polyisocyanate compound may be a prepolymer. The polyamine may be a prepolymer such as polyether polyamine or polyester polyamine. Urea resin is widely known as a thermosetting resin, and commercially available products can be widely used.
(e) Thermosetting acrylic resin is an acrylic resin that forms a three-dimensional network structure by a thermosetting reaction. For example, it is composed of a combination of an acrylic monomer and a thermal polymerization initiator, or a combination of an acrylic prepolymer and a hardener. Acrylic resin is widely known as a thermosetting resin, and a wide variety of commercially available products can be used.
(f) Thermosetting alkyd resins are those that form alkyd resins with a three-dimensional network structure by a thermosetting reaction. Thermosetting alkyd resins are widely known as thermosetting resins, and commercially available products can be widely used.
Among them, the thermosetting resin used in the method for producing a molded article of the present invention is preferably composed of carbon, hydrogen, and oxygen elements from the viewpoint of efficiently leaving only carbon while removing elements other than carbon by carbonization treatment. From this viewpoint, a thermosetting phenolic resin is suitable as the thermosetting resin used in the method for producing a molded article of the present invention.

<Pressure molding>
The granulated material is molded into a desired shape by the above-mentioned pressure molding. The pressure in the pressure molding is not particularly limited as long as the granulated material is bonded to each other to obtain a desired molded product. For example, the pressure can be about 10 to 60 MPa. Pressure molding itself is a widely known molding method, and a general pressure molding device can be used as appropriate. Since fine granulated material is pressure molded, the molded product obtained has many fine pores, which allow gas generated by the thermosetting reaction to escape to the outside.

<Curing reaction>
The above-mentioned curing reaction allows the thermosetting resin to form a tertiary network structure, and a strong molded body can be obtained. The temperature and time of the curing reaction can be appropriately set according to the type of thermosetting resin, etc. For example, in the case of a phenolic resin, the curing reaction can be sufficiently promoted by holding the resin at about 200°C for about 30 minutes. The atmosphere of the curing reaction is not particularly limited. The curing reaction can usually be carried out in the air. In addition, by heating the resin while pressing it, it is also possible to perform pressure molding while the curing reaction is occurring.

<Carbonization treatment>
In the method for producing a molded product of the present invention, the molded body after the curing reaction is subjected to a carbonization treatment before being impregnated with molten Si. This carbonization treatment can be performed by heating to about 500 to 600 ° C. in an inert gas, a reducing gas, or in a vacuum. The heating time may be set to about 1 to 10 hours, for example, as long as the carbonization proceeds sufficiently. This carbonization treatment carbonizes the thermosetting resin to carbon atoms, which become a C source in the SiC generation reaction by Si impregnation in the subsequent process. This carbonization treatment preferably makes the carbon residue rate of the molded body 10% by mass or more, more preferably 13% by mass or more, even more preferably 15% by mass or more, and even more preferably 17% by mass or more. The method for calculating the carbon residue rate will be described in the examples below.
Incidentally, even if the carbonization treatment for obtaining the carbonized product is not carried out separately, the carbonization can be advanced during the temperature increase stage (high temperature) for the impregnation with molten Si.

<Generation of silicon carbide by silicon impregnation>
By impregnating the carbonized product with molten Si, the C derived from the thermosetting resin reacts with the molten Si to generate SiC inside or on the surface of the molded product, and a dense SiC molded product can be obtained.
Since the melting point of Si is 1414°C, the temperature of the molten Si is higher than that. For example, the carbonized material is impregnated with molten Si at about 1500°C, and the SiC generating reaction can be caused for several minutes to several hours (for example, 10 minutes to 5 hours, preferably 20 minutes to 3 hours). This reaction can be carried out in an inert gas, a reducing gas, or in a vacuum.
The carbonized material to be impregnated with molten Si has fine voids because it is made up of fine granules bound together before carbonization, and fine pores are also generated by carbonization of the cured thermosetting resin. Therefore, the molten Si spreads uniformly throughout the carbonized material quickly by capillary action, and a more uniform and dense SiC molded product can be obtained with high efficiency.

Figure 1 is a flow diagram showing the chronological order of each step in a preferred embodiment of the method for producing the molded product of the present invention described above. Also, Figure 2 is an explanatory diagram showing the state immediately after the carbonization treatment product is impregnated with Si, and the state after that in which Si and C react to produce SiC.

[Method for producing silicon carbide single crystal]
The method for producing a SiC single crystal of the present invention includes using a high SiC content molded product obtained by the method for producing a molded product of the present invention as a crucible for containing a Si-C solution in the production of a SiC single crystal by a solution method, thereby allowing the crucible to function as a C source and a Si source for the SiC single crystal. The method for producing a SiC single crystal by a solution method is itself publicly known, and for example, JP 2017-31036 A and the like can be appropriately referred to. The method for producing a SiC single crystal of the present invention has a feature not found in the prior art in that a high SiC content molded product obtained by the method for producing a molded product of the present invention is used as a crucible for containing a Si melt.

Figure 3 is an explanatory diagram showing a schematic diagram of one embodiment of the method for producing a SiC single crystal of the present invention. In the embodiment shown in Figure 3, a high SiC content molded product 1 (SiC crucible 1) is arranged as a liner along the inner wall of a graphite crucible 2. A Si melt 3 to which transition metals and the like have been added is contained in this high SiC content molded product 1, and both Si and C from the SiC crucible are dissolved in the Si melt 3 to obtain a Si-C solution 3. A SiC seed crystal 4 held by a holder 5 is brought into contact with this Si-C solution 3, and a SiC single crystal 6 is obtained by solution growth on the SiC seed crystal. The solution growth (growth) conditions can be, for example, in an inert gas at 1800 to 2200°C for 1 to 48 hours with a pulling speed of 0.1 to 0.5 mm/hour.

The present invention will be described in more detail with reference to the following examples. The present invention should not be construed as being limited to the following examples except as provided for in the present invention.

[Preparation of silicon carbide-containing molded articles]
<Example>
A homogeneous mixture was obtained by mixing SiC raw material powder (manufactured by Pacific Random Corporation, product name: GMF, particle size (d50): 2.3 μm) with a phenolic resin solution (manufactured by Gun-ei Chemical Industry Co., Ltd., product name: PL2211, thermosetting resin solution). The mixing ratio of the SiC raw material powder and the phenolic resin solution in the mixture was SiC:phenolic resin solution = 3:2 in mass ratio. At this time, SiC:phenolic resin (solid content of the phenolic resin solution) = 7:3 (mass ratio).
Next, the mixture was dried in air at 80°C for 30 minutes to remove the solvent, and then granulated using a mortar to obtain a granulated product. The granulated product was then classified into particle sizes of 212 to 75 μm (Example 1, passed through a sieve with a mesh size of 212 μm but not through a sieve with a mesh size of 75 μm), 75 to 53 μm (Example 2, passed through a sieve with a mesh size of 75 μm but not through a sieve with a mesh size of 53 μm), and 53 to 25 μm (Example 3, passed through a sieve with a mesh size of 53 μm but not through a sieve with a mesh size of 25 μm), and the classified granulated product was uniaxially pressed at a pressure of 40 MPa to obtain the molded bodies (diameter: 8 mm, height: 3 mm) of Examples 1 to 3.
Next, each of the molded bodies was held at 90°C for 1 minute in air, then heated to 200°C at a heating rate of 1°C/min, and held at 200°C for 30 minutes to harden the phenolic resin, thereby obtaining a hardened body of each molded body. Next, these hardened bodies were heated at 600°C for 2 hours in an argon gas atmosphere to carbonize the hardened phenolic resin, thereby obtaining a preform made of SiC and C. The ratio of the mass (A2) of carbon to the total mass (A1) of these preforms (residual carbon ratio, (A2/A1) x 100) was 19 mass% in all cases.
Next, a sufficient amount of Si chunks was placed on top of each preform to react with the C in each preform to generate SiC, and the preforms were held in a vacuum at 1450°C for 10 minutes. As a result, the preforms were impregnated with molten Si, and the Si reacted with the C in the preforms to generate SiC. Next, the preforms were naturally cooled while maintaining the vacuum, and the SiC-rich molded products of Examples 1 to 3 were obtained.

Comparative Example
A homogeneous mixture was obtained by mixing SiC raw material powder (manufactured by Pacific Random Corporation, trade name: GMF, particle size (d50): 2.3 μm), graphite powder (manufactured by Ito Graphite Industries, trade name: GE-1, particle size (d50): 2.7 μm), and polycarbosilane (manufactured by Nippon Carbon Co., Ltd., trade name: NIPSI-HU) as a binder. At this time, three types of mixtures were prepared in which the ratio ((D2/D1)×100) of the mass (D2) of the graphite powder to the total weight (D1) of the SiC raw material powder and the graphite powder was 6 mass% (Comparative Example 1), 13 mass% (Comparative Example 2), and 23 mass% (Comparative Example 3). The content of polycarbosilane in each mixture was 3 mass%.
Next, each mixture was dried in air at 80°C for 30 minutes, and then uniaxially pressed at a pressure of 40 MPa to obtain a molded body (diameter: 8 mm, height: 3 mm). Next, these molded bodies were heated in an argon gas atmosphere at 1000°C for 2 hours to convert polycarbosilane to SiC, and a preform made of SiC and carbon was obtained. The residual carbon ratios of these preforms were 6% by mass (Comparative Example 1), 13% by mass (Comparative Example 2), and 23% by mass (Comparative Example 3), respectively.
Next, a sufficient amount of Si chunks was placed on top of each preform to react with the C in each preform to generate SiC, and the preform was held in a vacuum at 1450°C for 10 minutes. As a result, the preform was impregnated with molten Si, and this Si reacted with the C in the preform to generate SiC. Next, the preform was naturally cooled while maintaining the vacuum, and the SiC-containing molded products of Comparative Examples 1 to 3 were obtained.

4 and 5 are respectively an optical microscope photograph of a cross section of the SiC-rich molded product of Example 1 and an electron microscope photograph of the cross section enlarged. The SiC-rich molded product is composed of SiC and residual Si, and when evaluated by image analysis, the area ratio of SiC in the observed area of the cross section (total of three non-overlapping areas (area of each area: within a rectangular frame of 10,000 μm ² ): 30,000 μm ² ) was 88%, indicating that the molded product has a high SiC content.
6 is an electron microscope photograph of a cross section of a molded article with a high SiC content in Example 2, and Fig. 7 is an electron microscope photograph of a cross section of a molded article with a high SiC content in Example 3. Both are composed of SiC and residual Si, and the area ratio of SiC to the observed area of the cross section (total of three non-overlapping areas (area of each area: within a rectangular frame of ^10,000 µm2): 30,000 ^µm2 ) was 91% in Example 2 and 90% in Example 3, indicating that both were molded articles with a high SiC content.

8 and 9 are external and cross-sectional electron microscope photographs of the SiC-containing molded articles of Comparative Example 1 and Comparative Example 2, respectively. Although the impregnation of molten Si progressed, the area ratio of SiC in the observed area of the cross section (total of three non-overlapping areas (area of each area: within a rectangular frame of 10,000 μm ² ): 30,000 μm ² ) was low, being 59% in Comparative Example 1 and 66% in Comparative Example 2. The reason why the area ratio of SiC increased slightly in Comparative Example 2 is considered to be that more of the voids in the preform were filled with the generated SiC as the graphite addition ratio increased.
Considering the bulk density of graphite (approximately 1.7 to 1.9 g/ ^cm3 for isotropic graphite), the generation of SiC (density 3.2 g/ ^cm3 ) through reaction with Si is accompanied by volume expansion. Figure 10 is a photograph of the appearance of the SiC-containing molded product of Comparative Example 3. It is believed that the shape of the molded product could not be maintained as a result of localized excessive volume expansion occurring during the generation of SiC through molten Si infiltration.
Thus, when graphite powder is used as the C raw material to obtain a SiC-containing molded article, even if an attempt is made to increase the SiC content while maintaining the desired shape, it is found that the increase in the SiC content in the molded article is limited.

As described above, it was shown that the SiC-rich molded products of Examples 1 to 3 could maintain the desired shape and achieve a high SiC content. As a result of analyzing the preforms obtained by the carbonization treatment using mercury porosimetry, the bulk density of the carbonized part of the resin in each of Examples 1 to 3 was 1.0 g/cm ^3. This is a bulk density that can generate SiC (density 3.2 g/cm ³ ) with almost no volume change when reacting with Si. In addition, in Examples 1 to 3, volume shrinkage occurred while maintaining the shape during the curing and carbonization processes, and it is considered that the voids generated during molding were appropriately reduced due to the effect of the resin. As a result, it is considered that the porosity in the preforms in Examples 1 to 3 was smaller than that in Comparative Examples 1 to 3, and no volume change occurred due to the generation of SiC during impregnation, so that both shape maintenance and a high SiC rate were achieved.

The table below summarizes the types of raw materials, the residual carbon ratios of the preforms, and the SiC area ratios of the SiC-containing molded products for Examples 1 to 3 and Comparative Examples 1 to 3. In Table 1 below, the residual carbon ratios of the preforms for Comparative Examples 1 to 3 indicate the ratio of the mass (D2) of the graphite powder to the total weight (D1) of the SiC raw material powder and graphite powder.

1 SiC high content molded product 2 Graphite crucible 3 Si melt (Si-C solution)
4 SiC seed crystal 5 Seed crystal holder 6 SiC single crystal

Claims (7)

A method for producing a silicon carbide-rich molded product used as a carbon source and a silicon source in the production of silicon carbide single crystals by a solution process, comprising the steps of:
The manufacturing method is a method for manufacturing a molded product with a high silicon carbide content, which includes pressure molding a granule containing silicon carbide powder and a thermosetting resin, and impregnating the carbonized product obtained by a curing reaction with molten silicon to react carbon derived from the thermosetting resin with the molten silicon to produce silicon carbide. The method for producing a silicon carbide-rich molded product according to claim 1, wherein the thermosetting resin contains at least one of the following (a) to (f):
(a) thermosetting phenolic resin, (b) thermosetting epoxy resin, (c) thermosetting urethane resin, (d) thermosetting urea resin, (e) thermosetting acrylic resin,
(f) Thermosetting alkyd resin The method for producing a molded product with a high silicon carbide content according to claim 1 or 2, wherein the thermosetting resin is composed of carbon, hydrogen, and oxygen elements. The method for producing a molded product with a high silicon carbide content according to claim 1 or 2, wherein the content of the silicon carbide powder in the granules is 90 mass% or less. The method for producing a molded product with a high silicon carbide content according to claim 1 or 2, in which the carbonization treatment product has a residual carbon ratio of 15 mass% or more. The method for producing a molded product with a high silicon carbide content according to claim 1 or 2, wherein the molded product with a high silicon carbide content is used as a crucible for containing a solution in the production of silicon carbide single crystals by the solution method. 3. A method for producing a silicon carbide single crystal, comprising using a high silicon carbide content molded product obtained by the method for producing a silicon carbide single crystal by a solution process as described in claim 1 or 2 as a crucible for containing a solution, thereby allowing the crucible to function as a carbon source and a silicon source for the silicon carbide single crystal.

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