JP6802719B2 - Silicon carbide powder - Google Patents

Description

The present invention relates to a silicon carbide powder that is a raw material for producing a single crystal of silicon carbide by a sublimation recrystallization method (improved Rayleigh method).

Silicon carbide powder is used as a material for forming grindstones, ceramic parts, etc. because of its high hardness, high thermal conductivity, and high temperature heat resistance. Further, since a single crystal of silicon carbide has physical properties such that the band gap is about 3 times and the dielectric breakdown electric field strength is about 10 times that of silicon, it is attracting attention as a material for a base for a power semiconductor instead of silicon.

As a method for producing a silicon carbide single crystal, a sublimation recrystallization method in which silicon carbide powder, which is a raw material, is sublimated under a high temperature condition of 2000 ° C. or higher to grow silicon carbide as a single crystal is well known and widely used industrially. Has been done. Various ideas have been made for silicon carbide powder as a raw material in this sublimation recrystallization method.

For example, in Patent Document 1, the ratio of powder having a particle size between the opening size A and the opening size B (where A is a value smaller than B) is 80 in the total amount of silicon carbide powder. A silicon carbide powder is disclosed, which is characterized by a volume% or more and the opening size B being 5 times or less of the opening size A.

Japanese Unexamined Patent Publication No. 2016-84259

Patent Document 1 discloses a technique for providing a silicon carbide powder having a high sublimation rate when used as a raw material for a sublimation recrystallization method by adjusting the particle size of the silicon carbide powder. The shape of is not considered.

Therefore, an object of the present invention is to provide a silicon carbide powder having a higher sublimation rate when used as a raw material for a sublimation recrystallization method by adjusting the shape of the silicon carbide powder.

In order to achieve the above object, the silicon carbide powder of the present invention has a major axis diameter / minor axis diameter ratio of 5 in the silicon carbide powder used as a raw material for producing a single crystal of silicon carbide by the sublimation recrystallization method. When 4 to 26% of plate-like powder having a minor axis diameter / thickness ratio of 2 or more is contained and the particle size range of the silicon carbide powder is d to d × α (μm), α ≦ It is characterized by being 6.

According to the silicon carbide powder of the present invention, by containing plate-like particles in a specific ratio, when a single crystal of silicon carbide is produced by the sublimation recrystallization method, the filling amount varies when the graphite crucible is filled. It is possible to increase the gap between powders while suppressing it. Further, when the particle size range of the silicon carbide powder is d to d × α (μm), the gap between the powders can be increased more effectively by setting α ≦ 6. Therefore, the sublimation speed can be improved.

It is explanatory drawing for demonstrating the shape of the plate-like particle of the silicon carbide powder of this invention. FIG. 5 is a schematic cross-sectional view showing the structure of a crucible used when producing a single crystal of silicon carbide by the sublimation recrystallization method in the examples of the present invention.

Hereinafter, the present invention will be described in more detail with reference to embodiments of the present invention.

First, a method for producing silicon carbide powder will be described. Here, a method using a solid phase reaction will be described, but a method using a liquid phase reaction or the like may be used.

The method for producing silicon carbide powder of the present invention includes a firing step of forming a lump made of silicon carbide by mixing and firing an inorganic siliceous raw material and a carbonaceous raw material, and the carbonization obtained in the firing step. A first crushing step of crushing a mass made of silicon with an impact crusher to obtain a first silicon carbide powder, and a first crushing of the first silicon carbide powder with a shear crusher to obtain a second silicon carbide powder. 2 In the pulverization step, the first silicon carbide powder and the second silicon carbide powder are mixed, and the ratio of major axis diameter / minor axis diameter is 5 or less, and the ratio of minor axis diameter / thickness is 2 or more. It includes a mixing step of obtaining a silicon carbide powder containing 4 to 26% of the plate-like powder.

Here, the definition of the plate-like powder in the present invention will be described with reference to FIG. For example, in the powder having the shape shown in FIG. 1 (but not limited to such a shape), when viewed from the direction A in which the projected area is the widest (in this example, the direction perpendicular to the flat plane). The direction in which the outer diameter is the longest (diagonal direction in this example) is the semimajor axis direction L, and the size in that direction is the semimajor axis diameter. Further, the direction in which the outer diameter is the shortest when viewed from the above direction A (in this example, the short side direction) is defined as the minor axis direction S, and the size in that direction is defined as the minor axis diameter. Then, the maximum size in the direction T orthogonal to the long axis direction L and the short axis direction S is defined as the thickness. In such a case, a powder having a major axis diameter / minor axis diameter ratio of 5 or less and a minor axis diameter / thickness ratio of 2 or more is defined as a plate-like powder in the present invention.

Examples of the inorganic siliceous raw material include crystalline silica such as silica stone, silica fume, and amorphous silica such as silica gel. These may be used individually by 1 type or in combination of 2 or more type. The average particle size of the inorganic siliceous raw material is appropriately selected depending on the environment at the time of firing, the state of the raw material (crystalline or amorphous), the reactivity with the carbonic raw material, and the like. However, it is preferable to use amorphous silica as the inorganic siliceous raw material because the reactivity at the time of firing is good and the control of the furnace is easy.

Examples of the carbonaceous raw material include crystalline carbon such as natural graphite and artificial graphite, and amorphous carbon such as carbon black, coke, and activated carbon. These may be used individually by 1 type or in combination of 2 or more type. The average particle size of the carbonaceous raw material is appropriately selected depending on the environment at the time of firing, the state of the raw material (crystalline or amorphous), the reactivity with the carbonaceous material, and the like.

The inorganic siliceous raw material and the carbonic raw material are mixed to prepare a raw material for silicon carbide powder. The mixing method at this time is arbitrary, and either wet mixing or dry mixing may be used. The optimum mixing molar ratio (C / Si) of the carbonaceous raw material and the inorganic siliceous raw material at the time of mixing is selected in consideration of the environment at the time of firing, the particle size of the raw material for silicon carbide powder, the reactivity, etc. To do. The term "optimal" as used herein means improving the yield of silicon carbide obtained by calcination and reducing the unreacted residual amount of the inorganic siliceous raw material and the carbonaceous raw material.

The obtained mixed powder (raw material for producing silicon carbide) is calcined at 2200 ° C. or higher, preferably 2500 ° C. or higher to obtain massive silicon carbide.

The firing method is not particularly limited, and examples thereof include a method by external heating and a method by energization heating. Examples of the external heating method include a method using a fluidized bed furnace, a batch type furnace, and the like. Examples of the method by energization heating include a method using an Achison furnace. In the present invention, since plate-like crystals can be easily obtained, it is preferable to perform firing using an Athison furnace.

The firing atmosphere is preferably a reducing atmosphere. This is because the yield of silicon carbide decreases when firing in an atmosphere with weak reducing property.

In the present specification, the “Achison furnace” refers to a box-shaped indirect resistance heating furnace with an open upper part. Here, indirect resistance heating is to obtain silicon carbide by a heating element that generates heat by passing an electric current instead of directly passing an electric current through the object to be heated. Further, an example of a specific configuration of such an Achison furnace is described in Japanese Patent Application Laid-Open No. 2013-11254.

By using such a furnace, the reaction represented by the following formula (1) occurs, and a lump product made of silicon carbide (SiC) can be obtained.
SiO ₂ + 3C → SiC + 2CO ... (1)

The type of heating element of the Achison furnace is not particularly limited as long as it can conduct electricity, and examples thereof include graphite powder and carbon rods.

The form of the substance constituting the heating element is not particularly limited, and examples thereof include powder and lumps. The heating element is provided so as to form a rod shape as a whole so as to connect the electrode cores provided at both ends in the energizing direction of the Achison furnace. Examples of the rod-shaped shape here include a columnar shape and a prismatic shape.

After energization, a lump made of silicon carbide is formed in the furnace.

Air cooling is performed by introducing an inert gas such as argon gas until the inside of the furnace reaches room temperature. Then, a lump (ingot) made of the obtained silicon carbide is crushed.

In the present invention, the crushing of the lump is crushed by an impact crusher to obtain a first silicon carbide powder, and the first silicon carbide powder is crushed by a shear crusher to obtain a second silicon carbide. This is performed in the second crushing step of obtaining the powder.

Examples of the impact type crusher used in the first crushing step include a jaw crusher, a ball mill, a hammer mill, etc., which are roughly crushed by a jaw crusher and the coarsely crushed material is crushed by a ball mill until a predetermined particle size is reached. Is preferable. By pulverizing by such a method, it becomes easy to obtain a powder having a shape (non-plate shape) having a major axis diameter, a minor axis diameter, and a thickness similar to those described with reference to FIG.

After that, it is preferable to classify the pulverized product so as to have a desired particle size range. As for the classification, the method using a sieve is the simplest and preferable. However, the classification is not limited to the method using a sieve, and may be either a dry method or a wet method. Further, as the dry classification, for example, a centrifugal classification method using an air flow can be used.

By the above classification, the first silicon carbide powder having a particle size range of a to b in the opening size of the sieve is obtained. In the particle size ranges a to b, it is preferable that a and b are both in the range of 32 to 4000 μm and b ≦ a × 6, and a × 2 ≦ b ≦ a × 6. It is more preferable to do so. The particle size range in the present invention means that 80% by volume or more of the powder falls within the particle size range.

Examples of the shearing type crusher used in the second crushing step include a disc mill, a roller mill, and a top grinder, and a top grinder is particularly preferable. When a top grinder is used, when the particle size range of the first silicon carbide powder obtained in the first pulverization step in the mesh size of the sieve is a to b, the disc spacing I of the top grinder is set to I = a. It is preferable to do so.

By pulverizing the first silicon carbide powder obtained in the first pulverization step with a shearing type pulverizer as described above, the silicon carbide powder is sheared in a predetermined direction to form a plate-like powder according to the above definition. To. In order to further increase the proportion of the plate-like powder in the powder crushed by the shear-type crusher, it is preferable to put the powder on a sieve having a predetermined opening size and recover the powder on the sieve as the second silicon carbide powder. For example, when pulverized using a top grinder, the opening size O of the sieve is preferably O = I with respect to the disc spacing I of the top grinder.

Then, by mixing the first silicon carbide powder obtained in the first pulverization step and the second silicon carbide powder obtained in the second pulverization step in a predetermined ratio, a plate-shaped powder according to the above definition is obtained. A silicon carbide powder containing ~ 26%, preferably 7-23% is obtained.

By containing the plate-shaped powder in this way, when a silicon carbide single crystal is produced by the sublimation recrystallization method, when it is filled in a crucible made of graphite or the like, the gap between the powders is larger than that in the case where the plate-shaped powder is not contained. Can be increased and the sublimation speed can be increased.

If the content of the plate-like powder is less than 4%, the effect of increasing the sublimation rate cannot be sufficiently obtained, and if it exceeds 26%, the variation in the filling amount when filling the crucible becomes large, and the obtained silicon carbide is obtained. The quality of single crystals can vary.

The mixing ratio (volume ratio) of the first silicon carbide powder and the second silicon carbide powder is preferably 1: 0.04 to 0.35. By setting the mixing ratio as described above, the content of the plate-like powder can be easily adjusted within the range specified in the present invention.

The proportion (%) of the plate-like powder can be determined by observing with an optical microscope. Specifically, a field of view (including magnification) in which 20 or more powders are present is selected, and the number of plate-like crystals and other particles is measured. It is possible to measure other such visual fields and perform a total of 5 visual fields or more to obtain the ratio (%) of the plate-like powder as the overall average value.

It is desirable that the silicon carbide powder of the present invention thus obtained has α ≦ 6 when the particle size range is d to d × α μm. As a result, when the crucible is filled, gaps between the powders are well formed and the sublimation rate can be increased. However, when α <2, the yield drops in commercial production, so it is more preferable to set 2 ≦ α ≦ 6. When α> 6, the filling amount in the crucible increases, the gap between the powders decreases, and the sublimation rate tends to decrease.

The method for producing a silicon carbide single crystal by the sublimation recrystallization method using the silicon carbide powder of the present invention may be carried out according to a conventional method, and is not particularly limited, but the outline is as follows.

First, silicon carbide powder, which is a raw material, is filled in a crucible made of graphite, for example, and the crucible is arranged in a heating device. However, the container in which the silicon carbide powder is filled is not limited to the graphite crucible, and may be any container used for producing single crystal silicon carbide by the sublimation recrystallization method.

Then, the crucible is heated so that the raw material in the crucible is 2000 to 2500 ° C. under reduced pressure in an atmosphere of an inert gas such as argon gas. However, the temperature of the portion of the lower surface of the crucible lid on which the silicon carbide single crystal grows is set to be about 100 ° C. lower than this.

This heating is sustained for several hours to several tens of hours. As a result, the silicon carbide powder, which is a raw material, sublimates into a sublimation gas, reaches the lower surface of the lid and becomes a single crystal, and the single crystal grows to obtain a lump of silicon carbide single crystal.

Since the silicon carbide powder of the present invention contains 4 to 26% of plate-like powder, many gaps can be formed between the powders when filled in a crucible, and the sublimation rate of silicon carbide can be increased. In addition, since there is little variation in the filling amount when the crucible is filled, a silicon carbide single crystal of a certain quality can be obtained.

Hereinafter, examples of the present invention will be described with reference to test examples. However, the present invention is not limited to these examples.

(Test Example 1)
Silicon carbide powders having different proportions of plate-like powders were prepared by the following methods, and the variation in specific gravity at the time of filling the crucible and the sublimation rate by the sublimation recrystallization method were determined.

[Ingredients used]
・ Si source: Amorphous silica ・ C source: Carbon black (amorphous carbon)
[Manufacturing of silicon carbide]
The above raw materials were mixed using a twin-screw mixer so that the molar ratio (C / Si) of carbon and silicon was 3.0 to obtain a raw material for producing silicon carbide. 1000 kg of each mixed raw material was calcined in an Achison furnace. The firing was carried out at a center temperature of 2500 ° C. or higher for 16 hours.

[Crushing and adjusting particle size of silicon carbide]
The obtained silicon carbide lump was crushed with a jaw crusher, and then ball mill crushed. Then, the mixture was classified by a sieve to obtain a first silicon carbide powder having a particle size range of 125 to 710 μm in the mesh size of the sieve.

A part of the first silicon carbide powder was extracted, pulverized using a top grinder at a disc interval of 125 μm, classified with a sieve having a mesh size of 125 μm, and the powder on the sieve was recovered as the second silicon carbide powder.

Next, the first silicon carbide powder and the second silicon carbide powder were mixed with a biaxial mixer at different blending ratios, and No. 1 in Table 1 described later. As shown in 1 to 7, silicon carbide powder in which the ratio of the plate-like powder was changed was prepared. The ratio of the plate-like powder was measured by the method described above.

[Variation of specific gravity]
No. in Table 1 Each of the silicon carbide powders 1 to 7 was filled in a carbon crucible, and the variation in specific gravity was measured by the following method.

That is, as shown in FIG. 2, 100 g of each silicon carbide powder 2 was filled in the carbon crucible 1. The crucible 1 filled with silicon carbide powder was tapped, and the volume of the powder was calculated from the inner dimensions of the crucible 1 to obtain the bulk specific gravity. The above operation was repeated 10 times for each sample, the maximum value, the minimum value, and the average value of the bulk specific gravity were obtained, and the variation was calculated from the following formula (2).
Variation = {(maximum value-minimum value) / average value} x 100 (%) ... (2)

[Sublimation speed]
The crucible 1 is covered with the lid 4 (seed crystals are not attached in this test), and the temperature of the lower part of the crucible 1 (around the silicon carbide powder 2) becomes 2200 ° C. under an argon atmosphere and a pressure of 0.5 kPa. By heating the upper part of 1 (around the precipitated single crystal 3) to 2070 ° C., the silicon carbide powder 2 in the crucible 1 is sublimated, and the silicon carbide single crystal 3 is precipitated on the lower surface of the lid 4. I let you. The heating time was 6 hours. Then, the sublimation rate ratio was obtained from the following formula (3).
Sublimation rate ratio = (100 [g] -weight remaining after heat treatment [g]) / 6 [hours] ... (3)

The above results are shown in Table 1 below.

As shown in Table 1, the sample No. 1 in which the ratio of the plate-like powder is within the range specified in the present invention. In Nos. 3 to 6, the sublimation rate ratio can be increased while keeping the variation in bulk specific gravity at the time of filling the crucible relatively low. On the other hand, the sample No. in which the proportion of the plate-like powder is smaller than the range specified in the present invention. In 1 and 2, the sublimation rate ratio did not improve. In addition, the sample No. in which the proportion of the plate-like powder is larger than the range specified in the present invention. In No. 7, it can be seen that the variation in bulk specific gravity at the time of filling the crucible becomes remarkably large.

(Test Example 2)
In Test Example 1, the classification by the sieve when obtaining the first silicon carbide powder was carried out so that the mesh size of the sieve was 125 to 1000 μm, and the sample No. was the same as that of Test Example 1. .. 8 silicon carbide powders were obtained. For this powder, the variation in bulk specific gravity and the sublimation rate ratio at the time of filling the crucible were measured in the same manner as in Test Example 1. This result is presented in Sample No. The results of 4 are shown in Table 2 below.

When the particle size range is d to d × α, the sample No. The α of 4 and 8 are as follows.

No. 4: α = 5.68
No. 8: α = 8

As shown in Table 2, the sample No. having a large particle size range. No. 8 is a sample No. 8 having a small particle size range. It can be seen that the sublimation rate ratio tends to decrease as compared with 4.

Claims (1)

In the method for producing silicon carbide powder used as a raw material when producing a single crystal of silicon carbide by the sublimation recrystallization method.
By crushing a mass made of silicon carbide with an impact crusher and classifying it, the particle size range in the mesh size of the sieve is a to b, and both a and b are in the range of 32 to 4000 μm. The step of obtaining the first silicon carbide powder having b ≦ a × 6 and
By crushing the first silicon carbide powder with a shearing type crusher, a plate-like powder having a major axis diameter / minor axis diameter ratio of 5 or less and a minor axis diameter / thickness ratio of 2 or more is included. The process of obtaining the second silicon carbide powder and
By mixing the first silicon carbide powder and the second silicon carbide powder in a predetermined ratio, the content of the plate-shaped powder is 4 to 26%, and the overall particle size range of the carbon silicon powder is d. A method for producing a silicon carbide powder, which comprises a step of obtaining a silicon carbide powder in which α ≦ 6 when ~ d × α (μm).