JP7518593B2 - Manufacturing method of silicon carbide powder - Google Patents

Description

The present invention relates to a method for producing silicon carbide powder containing boron.

Conventionally, methods for producing silicon carbide powder containing boron are known. For example, in the method described in Patent Document 1, a sublimation recrystallization method using a seed crystal is used to introduce a high concentration of boron atoms as an impurity in the growth atmosphere, resulting in a silicon carbide single crystal containing a specified amount or more of boron atoms. In addition, in the method described in Patent Document 2, a raw material mixture of an inorganic siliceous raw material, a carbonaceous raw material, and a boron compound is fired at 2200°C or higher, and the resulting agglomerates are pulverized to produce silicon carbide powder containing 0.8% to 6% boron throughout the entire particle.

Japanese Patent Application Publication No. 9-157092 JP 2018-158871 A

However, the boron concentration in the silicon carbide in the examples described in the above patent documents is at most about 0.1% by weight, and no examples are described in which the boron concentration is several tens of ppm. Furthermore, even if one attempts to produce silicon carbide powder containing several tens of ppm of boron using the methods described in the patent documents, it is actually not easy to control the boron concentration as calculated.

The present invention was made in consideration of these circumstances, and aims to provide a method for producing silicon carbide powder that can control the boron content to several tens of ppm.

(1) In order to achieve the above object, the method for producing silicon carbide powder of the present invention is characterized by including a first step of mixing raw material powders of amorphous silica, carbon black, and boron carbide and firing the mixture in an inert gas atmosphere to produce a boron-containing silicon carbide precursor, and a second step of mixing the precursor with impurity-free silicon carbide powder so that the boron concentration falls within a desired range, and firing the mixture in an inert gas atmosphere.

By producing silicon carbide powder containing boron through a two-stage firing process in this way, the concentration of boron contained in the silicon carbide can be adjusted to several tens of ppm. In addition, the loss of boron during the firing process can be prevented.

(2) Furthermore, the method for producing silicon carbide powder of the present invention is characterized in that the precursor contains 100 ppm or more of boron. This makes it possible to control the boron concentration of the silicon carbide in the final product to several tens of ppm with high reproducibility.

(3) Furthermore, the method for producing silicon carbide powder of the present invention is characterized in that in the first step, sintering is performed at 1800°C or higher and 2000°C or lower. This makes it possible to suppress the reaction of boron doping while also suppressing the residual boron carbide. As a result, the amount of boron lost in the first or second step can be reduced.

According to the present invention, the boron content in silicon carbide powder can be adjusted to several tens of ppm.

1A to 1C are diagrams illustrating an example of a manufacturing process of the present invention.

As a result of intensive research, the inventors have invented a method for obtaining silicon carbide (SiC) powder containing several tens of ppm of boron (B) through a two-stage firing process. Such silicon carbide powder is suitable as a material for power semiconductors because the electrical resistance of the boron-doped silicon carbide single crystal falls within an appropriate range. The following describes in detail an embodiment of the present invention.

[Method for producing silicon carbide powder]
FIG. 1 is a diagram showing an example of the manufacturing process of the present invention. The manufacturing method shown in FIG. 1 comprises a first step and a second step. In the first step, a precursor of silicon carbide powder containing several hundred to several thousand ppm of boron is first synthesized. The precursor is an intermediate product obtained when the reaction of doping silicon carbide with boron is not yet completed after the reaction has started. In the second step, the obtained precursor is mixed with silicon carbide powder and sintered. In this way, boron-containing silicon carbide powder containing several tens of ppm of boron is obtained. Details of each step are described below.

(1) First step: Predetermined amounts of amorphous silica (SiO ₂ ), carbon black (C) and boron carbide (B ₄ C) are measured. Boron carbide is used as a boron source. Amorphous silica and carbon black are mixed so that C/Si=3 (molar ratio). This allows the reaction shown in formula (1) to proceed without excess or deficiency, producing silicon carbide. By using such a reaction, boron can be easily substituted into the molecular skeleton of silicon carbide.

The amount of boron carbide mixed can be calculated using formula (2). The boron concentration in the feed refers to the boron concentration in the mixed raw materials before the target process is carried out. The measured raw materials are mixed using a mixer such as a Hobart mixer, a Huddle mixer, or a Henschel mixer. The resulting mixed raw materials are placed in a graphite crucible and fired in an inert gas atmosphere using a closed high-temperature furnace. An argon atmosphere is preferably used as the inert gas atmosphere.

The firing is preferably performed at a temperature of 1800° C. to 2000° C. for 3 hours or more. This makes it possible to suppress the boron doping reaction without completely proceeding, while suppressing the remaining boron carbide. As a result, a precursor can be obtained, and the amount of boron lost in the first or second step can be reduced.

A predetermined amount of the precursor and silicon carbide powder obtained in the first step is measured out. The precursor preferably contains 100 ppm or more of boron. This allows the boron concentration of the final silicon carbide product to be controlled to several tens of ppm with high reproducibility. It is more preferable that the boron concentration of the precursor is 300 ppm or more and 500 ppm or less. The concentration of impurities other than boron in the precursor is 100 ppm or less. Silicon carbide powder that does not contain impurities is used. The particle size of the silicon carbide powder used is preferably 500 μm or more and 850 μm or less. The amount of the precursor mixed can be calculated using formula (3).

The measured raw materials are mixed in a bag, container, mixer, etc., and charged into a graphite crucible, and sintered in an inert gas atmosphere using a closed high-temperature furnace. Firing is preferably maintained at 2200°C or higher for 6 hours or more in an argon atmosphere. The resulting product is sintered, so it is pulverized. The particle size after pulverization is preferably 50 μm or more and 1000 μm or less. The pulverized product is washed with an acid such as hydrochloric acid or nitric acid to remove contamination due to pulverization. In this way, silicon carbide powder containing boron can be obtained. The concentration of impurities other than boron in the produced silicon carbide powder is 100 ppm or less, making it preferable as a material for power semiconductors.

By producing silicon carbide powder containing boron through a two-stage firing process in this way, the concentration of boron contained in the silicon carbide can be adjusted to several tens of ppm. In addition, the loss of boron during the firing process can be prevented.

[Example]
(Common to Examples 1 to 5 and Comparative Examples 1 and 2)
The first and second steps were carried out under different conditions according to the above manufacturing method. In the first step, amorphous silica, carbon black, and boron carbide were weighed out and mixed in predetermined amounts. The resulting mixed raw material was placed in a graphite crucible and fired using a Fuji Electric Industrial Co., Ltd. multipurpose high-temperature furnace "Hi-Multi (registered trademark)". The firing was carried out under an argon atmosphere at 2000°C for 3 hours.

When sufficient precursor was produced in the first step, the second step was carried out. In the second step, a predetermined amount of precursor and 500 g of silicon carbide powder were weighed out. Silicon carbide powder containing no impurities and with a particle size of 500 μm to 850 μm was used. The weighed out raw materials were mixed in a bag and placed in a graphite crucible, and sintered using a Fuji Electric Industrial Co., Ltd. multipurpose high-temperature furnace "Hi-Multi (registered trademark)". Firing was carried out under argon atmosphere at 2200°C for 6 hours. The sintered product was pulverized to adjust the particle size to 100 μm to 850 μm. It was also washed with hydrochloric acid to remove contamination due to pulverization.

The boron concentration and impurity concentration in the products obtained in each process were measured by wet analysis. Measurement solutions were prepared by the alkali fusion method or pressurized acid decomposition, and the boron concentration was measured using an ICP optical emission spectrometer "ULTIMA2" manufactured by Horiba, Ltd. Tables 1 and 2 show the experimental data. Note that "feed" and "product" in the tables refer to the feed and product for each process. For example, the product for the first process is an intermediate product, and the product for the second process is the final product.

Example 1
In the first step, 180 g of carbon black, 300 g of amorphous silica, and 0.43 g of boron carbide were weighed and mixed to synthesize a precursor. The boron concentration of the mixed raw materials was 1700 ppm. The mixed raw materials were fired under an argon atmosphere at 2000°C for 3 hours to obtain a precursor. Analysis of the obtained precursor revealed that it contained 1211 ppm of boron.

In the second step, 8.4 g of the obtained precursor was mixed with 500 g of silicon carbide powder and calcined at 2200 ° C. for 6 hours under an argon atmosphere to obtain a silicon carbide powder containing boron. When this was analyzed, the boron concentration of the feed was 20.0 ppm, while the actual boron concentration was 17.8 ppm. In this way, a silicon carbide powder containing boron at a concentration almost equal to the feed concentration could be obtained. The impurity concentration of the produced silicon carbide powder was 100 ppm or less, as shown in Table 3.

(Examples 2 to 4)
As shown in Tables 1 and 2, the boron concentration in the feed was changed, and boron-containing silicon carbide was synthesized in the same manner as in Example 1. Silicon carbide containing boron at a concentration almost equivalent to the feed concentration could be obtained.

Example 5
In the first step, as shown in Table 1, when calcination was performed with a boron concentration of 100 ppm in the feed, the reduction in the boron concentration in the product stopped at 49.1 ppm. If this product is used in the second step, it is expected that silicon carbide with a boron concentration of several tens of ppm can be obtained. However, the reduction in the first step was large, and compared to Examples 1 to 4, it cannot be said that the reproducibility of the control of the boron concentration is sufficiently high.

(Comparative Examples 1 to 2)
In the first step, when calcination was performed with a boron concentration of 20 ppm to 50 ppm as shown in Table 1, the boron concentration in the product was significantly reduced compared to the concentration of the feed. It is presumed that boron is incorporated by reacting a part of the amorphous silica with boron carbide, but when carbon black reacts with amorphous silica, a part of the amorphous silica volatilizes. Therefore, when the amount of boron source charged is small, the effect of volatilization of the amorphous silica that has incorporated boron becomes large, and it is considered that a precursor with a boron concentration equivalent to the concentration of the charge could not be synthesized.

(Comparative Examples 3 and 4)
Silicon carbide powder and boron carbide were mixed and fired in a process conforming to the second process described above. In this process, boron carbide was used instead of the precursor. The amount of boron carbide mixed was calculated using formula (4). At this time, the boron concentration in the product was significantly reduced compared to the concentration in the feed. This is thought to be because boron is less easily incorporated compared to when synthesized from carbon black and amorphous silica.

Claims (2)

a first step of mixing raw material powders of amorphous silica, carbon black, and boron carbide and firing the mixture at 1800° C. or more and 2000° C. or less in an inert gas atmosphere to produce a boron-containing silicon carbide precursor;
a second step of mixing the precursor with silicon carbide powder not containing impurities so that the boron concentration is 10 ppm or more and less than 100 ppm , and firing the mixture at 2200°C or more in an inert gas atmosphere. The method for producing silicon carbide powder according to claim 1, characterized in that the precursor contains 100 ppm or more of boron.

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