JP7620545B2 - Silicon nitride powder and its manufacturing method, and method for manufacturing sintered silicon nitride - Google Patents

Description

The present disclosure relates to silicon nitride powder and a method for producing the same, as well as a method for producing a silicon nitride sintered body.

Silicon nitride is a material with excellent strength, hardness, toughness, heat resistance, corrosion resistance, and thermal shock resistance. For this reason, the use of silicon nitride sintered bodies as insulating substrates for machine parts such as gas turbines, turbo rotors, and valves, as well as power modules for automobiles and machine tools, is being considered. Silicon nitride sintered bodies used for these applications are required to have a dense and homogeneous structure. In order to manufacture such silicon nitride sintered bodies, technologies to provide high-purity and homogeneous silicon nitride powder are being considered.

Known methods for synthesizing silicon nitride powder include a direct nitridation method (direct reaction method) in which metal silicon powder is nitrided in a mixed atmosphere of hydrogen gas or ammonia gas and nitrogen gas, a reduction nitridation method of silica powder, and an imide decomposition method. Patent Document 1 proposes a method for producing silicon nitride powder with a high content of α-type Si ₃ N _4. Patent Document 2 proposes the production of silicon nitride powder with fewer impurities by the imide decomposition method.

Japanese Patent Application Publication No. 7-81910 JP 2000-302421 A

Silicon nitride sintered bodies are sometimes used in high-temperature environments, so they are required to have excellent strength at high temperatures (high-temperature strength). It is believed that in order to improve high-temperature strength, it is necessary to make the silicon nitride sintered body dense. One factor that may hinder the densification of silicon nitride sintered bodies is thought to be the influence of impurities contained in the raw material, i.e., silicon nitride powder.

In this disclosure, therefore, there is provided a silicon nitride powder capable of producing a silicon nitride sintered body having excellent high-temperature strength by reducing impurities. In addition, there is provided a method for producing silicon nitride powder capable of producing silicon nitride powder with sufficiently reduced impurities at low production cost. In addition, there is provided a method for producing silicon nitride sintered body capable of producing a silicon nitride sintered body having excellent high-temperature strength at low production cost.

The silicon nitride powder according to one aspect of the present disclosure has an oxygen content of 3.0 mass% or less and a total fluorine and chlorine content of 25 mass ppm or less. Such silicon nitride powder has a sufficiently low content of oxygen, fluorine, and chlorine, which affect high-temperature strength, and therefore can produce a silicon nitride sintered body having excellent high-temperature strength.

The alpha conversion rate of the silicon nitride powder is preferably 92% by mass or more. By having a high alpha conversion rate, grain growth during sintering of the silicon nitride powder can be promoted. This makes it possible to produce a sufficiently densified silicon nitride sintered body. Therefore, it is possible to obtain a silicon nitride sintered body having even better high-temperature strength.

In the above silicon nitride powder, it is preferable that the proportion of primary particles having a particle size of 2 μm or more is 2% or less. This makes it possible to make the microstructure of the silicon nitride sintered body obtained by sintering the silicon nitride powder more uniform. Therefore, it is possible to obtain a silicon nitride sintered body having even better high-temperature strength.

The method for producing silicon nitride powder according to one aspect of the present disclosure includes a calcination step in which raw powder containing silica powder, carbon powder, and silicon nitride seed crystals is calcined in a nitrogen atmosphere at 1300 to 1550 ° C for 50 hours or more to obtain silicon nitride powder. In this production method, the raw powder is calcined at 1300 to 1550 ° C for a sufficiently long time in the calcination step, so that the reduction-nitridation reaction of the silica powder proceeds sufficiently. Therefore, silicon nitride powder with sufficiently reduced impurities can be produced. This production method can reduce the production cost of silicon nitride powder compared to the imide method. In addition, in the direct nitridation method, crushing is usually required to finely pulverize, but acid treatment is required to reduce impurities derived from crushing. In this case, the chemical components of the acid treatment remain and become impurities. Therefore, according to the method for producing silicon nitride powder of the present disclosure, silicon nitride powder with sufficiently reduced impurities can be produced at low production costs.

It is preferable that the oxygen content of the silicon nitride powder obtained by the above-mentioned firing process is 3.0 mass% or less, and the total content of fluorine and chlorine is 25 mass ppm or less. By using such silicon nitride powder, it is possible to produce a silicon nitride sintered body having excellent high-temperature strength.

It is preferable that the silicon nitride powder obtained by the above-mentioned firing process has a proportion of primary particles having a particle size of 2 μm or more of 2% or less. This makes it possible to make the microstructure of the silicon nitride sintered body obtained by firing the silicon nitride powder more uniform. Therefore, it is possible to obtain a silicon nitride sintered body having even better high-temperature strength. The proportion of primary particles described above in this disclosure is based on the number of particles.

A method for producing a silicon nitride sintered body according to one aspect of the present disclosure includes a sintering step of sintering any of the silicon nitride powders described above, or silicon nitride powder obtained by any of the production methods described above, to obtain a silicon nitride sintered body. In this production method, silicon nitride powder with reduced impurities is sintered, so that a silicon nitride sintered body having excellent high-temperature strength can be produced.

According to the present disclosure, it is possible to provide silicon nitride powder capable of producing silicon nitride sintered bodies having excellent high-temperature strength by reducing impurities. It is also possible to provide a method for producing silicon nitride powder capable of producing silicon nitride powder with sufficiently reduced impurities at low production costs. It is also possible to provide a method for producing silicon nitride sintered bodies capable of producing silicon nitride sintered bodies having excellent high-temperature strength at low production costs.

An embodiment of the present disclosure will be described below. However, the following embodiment is an example for explaining the present disclosure, and is not intended to limit the present disclosure to the following content.

The silicon nitride powder according to one embodiment has an oxygen content of 3.0% by mass or less, and a total content of fluorine and chlorine of 25 ppm by mass or less. The oxygen content may be 2.5% by mass or less, 2.3% by mass or less, or 1.8% by mass or less, from the viewpoint of being able to produce a silicon nitride sintered body having even better high-temperature strength. By reducing the oxygen content of the silicon nitride powder, it is possible to reduce internal defects in the silicon nitride sintered body obtained by sintering the silicon nitride powder. This makes it possible to obtain a silicon nitride sintered body having sufficiently high high-temperature strength. The oxygen content tends to depend on the firing time when producing the silicon nitride powder and the blending ratio of carbon powder in the starting material used when producing the silicon nitride powder.

From the viewpoint of reducing the manufacturing cost of silicon nitride powder, the oxygen content of the silicon nitride powder may be 0.1% by mass or more, or may be 0.5% by mass or more. An example of the oxygen content of the silicon nitride powder may be 0.1 to 3.0% by mass, or may be 0.5 to 2.5% by mass. The oxygen content of the silicon nitride powder can be measured using a commercially available oxygen/nitrogen analyzer. The measurement is performed in an atmosphere of helium gas, by continuously raising the temperature from 20°C to 3000°C. The oxygen content can be quantified from the area of the peak resulting from the desorption of oxygen from the obtained measurement results.

The total content of fluorine and chlorine in the silicon nitride powder may be 22 mass ppm or less, 20 mass ppm or less, or 19 mass ppm or less, from the viewpoint of being able to produce a silicon nitride sintered body having even better high-temperature strength. By reducing the total content of fluorine and chlorine in the silicon nitride powder, the fluorine and chlorine contained in the grain boundary phase of the silicon nitride sintered body obtained by sintering the silicon nitride powder is reduced. Therefore, the grain boundary phase of the silicon nitride sintered body can be suppressed from softening at high temperatures. Therefore, it is possible to obtain a silicon nitride sintered body having sufficiently high high-temperature strength. The total content of fluorine and chlorine tends to depend on the purity of the starting material used in producing the silicon nitride powder, the compounding ratio of the silica powder and the carbon powder (C/SiO ₂ ), the firing time, and the presence or absence of post-treatment (washing) after firing.

For example, if the compounding ratio of silica powder to carbon powder (C/ _SiO2 ) when preparing the raw material powder is too small, the desorption of _SiCl4 due to the reaction of _SiO2 or SiO with _Cl2 tends to be difficult to proceed. The same tendency also occurs when the firing time is too short.

From the viewpoint of reducing the manufacturing cost of silicon nitride powder, the total fluorine and chlorine content of the silicon nitride powder may be 1 ppm by mass or more, or may be 5 ppm by mass or more. An example of the total fluorine and chlorine content of the silicon nitride powder may be 1 to 25 ppm by mass, or may be 5 to 22 ppm by mass. The total fluorine and chlorine content of the silicon nitride powder can be measured by heating the silicon nitride powder and quantifying the fluorine and chlorine contained in the generated gas by ion chromatography.

The alpha-conversion rate of the silicon nitride powder may be 92% by mass or more, 95% by mass or more, or 96% by mass or more. By having a high alpha-conversion rate, grain growth can be promoted during sintering of the silicon nitride powder. This makes it possible to produce a sufficiently densified silicon nitride sintered body. Therefore, it is possible to obtain a silicon nitride sintered body having even better high-temperature strength.

From the viewpoint of reducing the manufacturing cost of silicon nitride powder, the alpha conversion rate of the silicon nitride powder may be 99% by mass or less, or may be 98% by mass or less. An example of the alpha conversion rate of the silicon nitride powder may be 92 to 99% by mass, 95 to 99% by mass, or 96 to 98% by mass. The alpha conversion rate of the silicon nitride powder can be determined based on the diffraction line intensity of X-ray diffraction.

Of the primary particles contained in the silicon nitride powder, the proportion of primary particles having a particle size of 2 μm or more is preferably 2% or less. This proportion may be 1% or less, or may be 0.5% or less. By reducing this proportion, the microstructure of the silicon nitride sintered body obtained by sintering the silicon nitride powder can be made more uniform. Therefore, it is possible to obtain a silicon nitride sintered body with even superior high-temperature strength.

From the viewpoint of reducing the manufacturing cost of silicon nitride powder, the proportion of primary particles having a particle size of 2 μm or more among the primary particles contained in the silicon nitride powder may be 0.1% or more, or may be 0.3% or more. One example of the above proportion may be 0.1 to 2%, or 0.3 to 1%. The above proportion is a proportion based on the number, and can be determined based on the particle size distribution measured by importing an image taken with a scanning electron microscope into image analysis particle size distribution measurement software.

The silicon nitride powder may contain carbon or carbide as a component other than silicon nitride. The total content of carbon and carbide in the silicon nitride powder is preferably 10 mass% or less, more preferably 7 mass% or less, calculated as carbon.

The method for producing silicon nitride powder according to one embodiment includes a blending step of blending silica powder, carbon powder, and silicon nitride seed crystals to prepare a raw material powder, and a firing step of firing the raw material powder in a nitrogen atmosphere at 1300 to 1550°C for 50 hours or more to obtain silicon nitride powder. The firing time may be, for example, 200 hours or less from the viewpoint of production efficiency. Examples of silica powder include fused silica powder, crystalline silica powder, and silicate compounds. Examples of carbon powder include acetylene black, furnace black, channel black, and graphite. The silicon nitride used as the seed crystals preferably has a high alpha conversion rate (for example, an alpha conversion rate of 90% or more) from the viewpoint of increasing sinterability.

The compounding ratio of the carbon powder to the silica powder in the compounding step may be 2.0 to 3.8, or may be 3.0 to 3.7, on a molar basis (C/SiO ₂ ), from the viewpoint of obtaining silicon nitride powder in which the total content of fluorine and chlorine and the oxygen content are sufficiently reduced. The compounding amount of the silicon nitride seed crystals per 100 parts by mass of the silica powder may be 10 to 20 parts by mass, or may be 11 to 18 parts by mass, from the viewpoint of obtaining silicon nitride powder with sufficiently high purity while reducing production costs.

The total content of fluorine and chlorine in the raw material powder is preferably 50 ppm by mass or less, more preferably 40 ppm by mass or less, from the viewpoint of sufficiently reducing the total content of fluorine and chlorine in the silicon nitride powder to be produced. On the other hand, the total content of fluorine and chlorine in the raw material powder may be 10 ppm by mass or more, or may be 20 ppm by mass or more, from the viewpoint of reducing the production cost of the silicon nitride powder.

In the firing step, the raw material powder is fired using, for example, an electric furnace, whereby the following reaction formula (1) proceeds. From the viewpoint of sufficiently proceeding with the reaction of reaction formula (1), the firing temperature may be 1450 to 1550° C. From the viewpoint of sufficiently proceeding with the reduction-nitridation reaction of the silica powder, the firing time may be 150 hours or more, or may be 200 hours or more. On the other hand, from the viewpoint of reducing the production cost of the silicon nitride powder, the firing time may be 500 hours or less, or may be 400 hours or less.
3SiO ₂ +6C+2N ₂ →Si ₃ N ₄ +6CO↑ (1)

The firing process is carried out in a nitrogen atmosphere. The oxygen concentration in the nitrogen atmosphere may be 100 ppm by volume or less, or may be 20 ppm by volume or less. By sufficiently lowering the oxygen concentration in the nitrogen atmosphere, the oxygen content of the silicon nitride powder produced can be further reduced. If the silicon nitride powder contains carbon or carbide, a decarburization process may be carried out. The decarburization process can be carried out, for example, by heating the silicon nitride powder to 650 to 900°C in air. This allows the total carbon and carbide content in the silicon nitride powder to be reduced.

According to the above-mentioned manufacturing method, the silicon nitride powder according to the above-mentioned embodiment can be obtained. Examples of properties such as the oxygen content, the total content of fluorine and chlorine, the alpha conversion rate, and the particle size of the silicon nitride powder are as described above. In the manufacturing method of silicon nitride powder of this embodiment, silicon nitride powder is manufactured by a reduction-nitridation reaction of silica powder. Therefore, the manufacturing cost of silicon nitride powder can be reduced compared to the imide method. In addition, in the direct nitridation method, pulverization is usually required to pulverize the powder, but acid treatment is required to reduce impurities derived from pulverization. In this case, the chemical components of the acid treatment remain and become impurities. Therefore, according to the manufacturing method of silicon nitride powder disclosed herein, silicon nitride powder with sufficiently reduced impurities can be manufactured at a low manufacturing cost.

The method for producing a silicon nitride sintered body according to one embodiment includes a sintering step in which the silicon nitride powder according to the above embodiment is sintered to obtain a silicon nitride sintered body. In this production method, for example, the silicon nitride powder is pressed at a molding pressure of, for example, 3.0 to 10.0 MPa to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP. It may also be sintered while being molded by hot pressing.

The sintering of the molded body may be carried out in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure during sintering may be 0.7 to 0.9 MPa. The sintering temperature may be 1700 to 1900°C. The sintering time at the sintering temperature may be 4 to 20 hours, or 8 to 16 hours. The rate of temperature rise to the sintering temperature may be, for example, 1.0 to 10.0°C/hour.

The silicon nitride sintered body obtained in this manner has a sufficiently reduced oxygen content and a sufficiently reduced total content of fluorine and chlorine. Because the oxygen content is sufficiently reduced, defects occurring inside the silicon nitride sintered body can be suppressed. This results in excellent high-temperature strength as well as insulation and thermal conductivity. Defects contained in the silicon nitride sintered body include lattice defects such as dislocations and pores. In addition, because the total content of fluorine and chlorine is sufficiently reduced, softening of the grain boundary phase of the silicon nitride sintered body at high temperatures can be suppressed. Therefore, a silicon nitride sintered body having sufficiently high high-temperature strength can be obtained.

In this disclosure, high-temperature strength refers to strength at 1300°C. Silicon nitride sintered bodies having high strength in such a temperature range can be particularly suitably used in applications such as gas turbines, automotive power modules, and bearings. The bending strength of the silicon nitride sintered body according to one embodiment at a temperature of 1300°C may be, for example, 700 MPa or more, or 750 MPa or more. This strength is the four-point bending strength (1300°C) measured using a commercially available measuring device.

Although several embodiments have been described above, the present disclosure is not limited to the above-described embodiments. For example, the silicon nitride powder may be used for purposes other than the manufacture of silicon nitride sintered bodies.

The contents of the present disclosure will be explained in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the examples below.

<Preparation of silicon nitride powder>
Example 1
A raw material powder was obtained by blending fused silica powder (particle size: 0.6 μm), acetylene black powder, and silicon nitride powder (seed crystals). The total content of fluorine and chlorine in the raw material powder was 40 ppm by mass. The blending ratio (mass basis) was fused silica powder: acetylene black powder: silicon nitride powder = 55.2%: 38.5%: 6.3%. The molar ratio (C/SiO ₂ ) of acetylene black to fused silica powder was 3.5. 320 g of this raw material powder was filled into a 2 L polyethylene container, silicon nitride balls (φ: 15 mm) were added, and the mixture was mixed in a ball mill for 2 hours.

The raw material powder was fired in an electric furnace in a nitrogen atmosphere at atmospheric pressure at 1500°C for 60 hours to obtain lump nitride. This nitride was crushed in a silicon nitride mortar to obtain nitride powder. This was placed in an alumina crucible and heated in an electric furnace at 800°C for 3 hours to decarburize. The decarburized powder obtained by decarburization was loaded into an alumina pot together with silicon nitride balls and pulverized in a vibrating mill for 3 hours to obtain silicon nitride powder.

Example 2
Silicon nitride powder was obtained in the same manner as in Example 1, except that the time for sintering the raw material powder at 1500° C. was changed from 60 hours to 110 hours.

(Comparative Example 1)
Silicon nitride powder was obtained in the same manner as in Example 1, except that the time for sintering the raw material powder at 1500° C. was changed from 60 hours to 10 hours.

(Comparative Example 2)
A silicon nitride powder was obtained in the same manner as in Example 2, except that the compounding ratio (by mass) of fused silica powder: acetylene black powder: silicon nitride powder was 52.3%: 41.7%: 6.0%. The molar ratio of acetylene black to fused silica powder (C/ _SiO2 ) was 4.0.

(Comparative Example 3)
Silicon nitride powder was obtained in the same manner as in Example 1, except that the time for sintering the raw material powder at 1500° C. was changed from 60 hours to 5 hours.

<Evaluation of Silicon Nitride Powder>
The oxygen content in the silicon nitride powder of each Example and Comparative Example was measured by the following procedure. The crucible containing the graphite powder was degassed using an oxygen/nitrogen analyzer (manufactured by Horiba, Ltd., device name: EMGA-920W). The prepared silicon nitride powder was weighed and mixed with the graphite powder in the crucible. Thereafter, the temperature was raised from 20°C to 2300°C in a helium gas atmosphere, and the oxygen generated with the temperature rise was detected. The amount of oxygen contained in the silicon nitride powder was quantified from the peak area resulting from the desorption of oxygen. The measurement results are shown in Table 1.

The total fluorine and chlorine content in the silicon nitride powder was measured by the following procedure. The silicon nitride powder was heated using an automatic sample combustion device (Mitsubishi Chemical Corporation, device name: AQF-2100H type) and the generated gas was dissolved in water. The fluorine and chlorine dissolved in water were measured using an ion chromatograph (Thermo Fisher Scientific, device name: ICS-2100) in accordance with JIS R 1603:2007. Based on this measured value, the fluorine and chlorine contained in the silicon nitride powder were quantified. The measurement results are shown in Table 1. In Table 1, "halogen content" refers to the total content of fluorine and chlorine.

The alpha-phase ratio of the prepared silicon nitride powder was measured by the following procedure. X-ray diffraction of the silicon nitride powder was performed with CuKα radiation using an X-ray diffractometer (manufactured by Rigaku, device name: Ultima IV). The alpha phase was represented by the diffraction line intensity I _a102 of the (102) plane and the diffraction line intensity I _a210 of the (210) plane, and the beta phase was represented by the diffraction line intensity I _b101 of the (101) plane and the diffraction line intensity I _b210 of the (210) plane. Using these diffraction line intensities, the alpha-phase ratio was calculated by the following formula. The results were as shown in Table 1.
Alpha conversion rate (mass%) =
(I _a102 +I _a210 )/(I _a102 +I _a210 +I _b101 +I _b210 )×100

The proportion of particles with a particle size of 2 μm or more contained in the prepared silicon nitride powder was measured using the following procedure. Using a scanning electron microscope (manufactured by JEOL Ltd., device name: JSM-6301F), the silicon nitride powder was observed at 5000 times magnification and images were taken (field of view: 16 μm x 23 μm). The images taken were imported into image analysis type particle size distribution measurement software (manufactured by Mountec Co., Ltd., product name: Mac View version 4.0) and the particle size distribution was measured. From the measurement results, the proportion of primary particles with a particle size of 2 μm or more was calculated. The results are shown in Table 1.

<Preparation of sintered silicon nitride>
The silicon nitride powders of each Example and Comparative Example were each uniaxially pressed to produce a cylindrical molded body. The molded body was placed in an electric furnace equipped with a carbon heater and heated to 1850° C. in a nitrogen atmosphere. After sintering at 1850° C. for 6 hours, the body was cooled to obtain a silicon nitride sintered body.

<Evaluation of sintered silicon nitride>
The density (20°C) and four-point bending strength at 1300°C of the obtained silicon nitride sintered body were measured. The density was measured by the Archimedes method. The four-point bending strength at 1300°C was measured using an Autograph AG-2000 (product name) manufactured by Shimadzu Corporation. The relative values of the density and four-point bending strength when the measurement results of the silicon nitride sintered body produced using the silicon nitride powder of Example 1 were used as the standard were as shown in Table 1.

Comparative Examples 1 and 3, which had a higher oxygen content than Examples 1 and 2, had lower high-temperature strength than Examples 1 and 2. Comparative Example 2, which had a higher total fluorine and chlorine content than Examples 1 and 2, also had lower high-temperature strength than Examples 1 and 2. As shown in Table 1, it was confirmed that by using silicon nitride powder with a low oxygen content and a low total fluorine and chlorine content, a silicon nitride sintered body with excellent high-temperature strength can be obtained.

According to the present disclosure, it is possible to provide silicon nitride powder capable of producing silicon nitride sintered bodies having excellent high-temperature strength by reducing impurities. It is also possible to provide a method for producing silicon nitride powder capable of producing silicon nitride powder with sufficiently reduced impurities at low production costs. It is also possible to provide a method for producing silicon nitride sintered bodies capable of producing silicon nitride sintered bodies having excellent high-temperature strength at low production costs.

Claims (8)

The oxygen content is 1.2 to 3.0 mass%, the total content of fluorine and chlorine is 22 mass ppm or less, and the proportion of primary particles having a particle size of 2 μm or more is 2% or less,
A silicon nitride powder containing carbon or carbide, the total content of carbon and carbide being 7 mass% or less . The silicon nitride powder according to claim 1, in which the gelatinization rate is 92 mass% or more. The silicon nitride powder according to claim 1 or 2, wherein the total content of fluorine and chlorine is 1 to 20 ppm by mass. A firing step of firing a raw material powder containing silica powder, carbon powder, and silicon nitride seed crystals in a nitrogen atmosphere at 1300 to 1550° C. for 50 hours or more to obtain silicon nitride powder,
A method for producing silicon nitride powder, wherein the silicon nitride powder obtained in the firing step has an oxygen content of 3.0 mass% or less, and a total fluorine and chlorine content of 25 mass ppm or less. The method for producing silicon nitride powder according to claim 4, wherein the silicon nitride powder obtained in the firing step has an oxygen content of 1.2 to 3.0 mass %, and a total content of fluorine and chlorine of 22 mass ppm or less. The method for producing silicon nitride powder according to claim 4 or 5, wherein the silicon nitride powder obtained in the firing step has a ratio of primary particles having a particle size of 2 μm or more of 2% or less. A method for producing a silicon nitride sintered body, comprising a sintering step of sintering the silicon nitride powder according to any one of claims 1 to 3 to obtain a silicon nitride sintered body. A method for producing a silicon nitride sintered body, comprising a sintering step of sintering the silicon nitride powder obtained by the method according to any one of claims 4 to 6 to obtain a silicon nitride sintered body.