TWI906273B - Silicon nitride powder, and method for producing silicon nitride sintered body - Google Patents

Abstract

One aspect of the invention provides a silicon nitride powder containing silicon nitride primary particles, wherein in a distribution curve of the volume-based particle diameter measured using a laser diffraction and scattering method, if the particle diameters when the cumulative value reaches 10% and 90% of the total from the small particle diameter side are deemed D10 and D90 respectively, then the difference between D90 and D10 is 5.5 µm or less.

Description

Method for manufacturing silicon nitride powder and silicon nitride sintered body

This disclosure relates to a method for manufacturing silicon nitride powder and silicon nitride sintered bodies.

Silicon nitride is a material with excellent strength, hardness, toughness, heat resistance, corrosion resistance, and thermal shock resistance, making it widely used in parts for various industries, such as die-casting equipment and melting furnaces, as well as automotive parts. Furthermore, because silicon nitride also exhibits excellent mechanical properties at high temperatures, research has been conducted on its application in gas turbine parts requiring high-temperature strength and high-temperature creep resistance.

Further improvements in thermal conductivity and mechanical properties are required for silicon nitride sintered bodies. For example, Patent 1 discloses a silicon nitride sintered body characterized by a thermal conductivity of 100~300 W/(m·K) at room temperature and a three-point bending strength of 600~1500 MPa at room temperature. [Prior Art Documents] [Patent Documents]

[Patent Document 1] Japanese Patent Application Publication No. 2004-262756

[The problem the invention aims to solve]

One objective of this disclosure is to provide silicon nitride powder capable of producing sintered bodies with excellent flexural strength. Another objective of this disclosure is to provide a method for manufacturing silicon nitride sintered bodies with excellent flexural strength. [Means for Solving the Problem]

One aspect of this disclosure is to provide a silicon nitride powder containing primary silicon nitride particles, wherein the particle size distribution curve of the volume reference determined by laser diffraction is set as D10 and D90 respectively when the cumulative value from the smallest particle size reaches 10% and 90% of the total particle size, and the difference between D90 and D10 is less than 5.5 μm.

The aforementioned silicon nitride powder, by ensuring that the difference between D90 and D10 (D90-D10) is below a predetermined value, can produce a molded body (uncalcined material) with a narrow particle size distribution and a denser microstructure. The silicon nitride sintered body obtained by calcining the aforementioned molded body exhibits suppressed porosity and excellent flexural strength.

The D50 of the aforementioned silicon nitride can be below 1.5 μm. If the upper limit of D50 falls within the above range, the silicon nitride powder will have a suitable particle size distribution, thus further improving the filling density of primary particles.

The D90 of the aforementioned silicon nitride powder can be below 6.0 μm. If the upper limit of D90 falls within the above range, the proportion of coarse particles can be sufficiently reduced, and the density reduction of the sintered body can be more effectively suppressed. Furthermore, the workability will be superior.

The BET specific surface area of the above silicon nitride powder may not reach 9.0 ^m² /g.

One aspect of this disclosure is a method for manufacturing a silicon nitride sintered body, comprising the steps of forming and calcining a sintering raw material containing the aforementioned silicon nitride powder.

The above-described method for manufacturing silicon nitride sintered bodies utilizes sintering raw materials containing the aforementioned silicon nitride powder, resulting in silicon nitride sintered bodies exhibiting excellent flexural strength. [Effects of the Invention]

According to this disclosure, silicon nitride powder capable of producing sintered bodies with excellent flexural strength can be provided. Furthermore, according to this disclosure, a method for manufacturing silicon nitride sintered bodies with excellent flexural strength can be provided.

The following describes embodiments of this disclosure. However, the embodiments described below are merely illustrative examples of this disclosure and do not imply that this disclosure is limited to the following content.

Unless otherwise specified, the materials illustrated in this instruction manual may be used alone or in combination of two or more. The content of each component in a composition, when referring to a plurality of substances equivalent to the presence of each component in the composition, means, unless otherwise specified, the aggregate amount of that plurality of substances present in the composition. The term "step" in this instruction manual may refer to independent steps or steps performed simultaneously.

One embodiment of silicon nitride powder is a primary particle containing silicon nitride, wherein the particle size distribution curve of the volume reference determined by laser diffraction is set as D10 and D90 respectively when the cumulative value from the smallest particle size reaches 10% and 90% of the total particle size, and the difference between D90 and D10 is less than 5.5 μm.

The upper limit of the difference between D90 and D10 (D90-D10) is 5.5 μm or less, for example, it can be 5.4 μm or less, 5.2 μm or less, 5.0 μm or less, 4.8 μm or less, or 4.6 μm or less. If the upper limit of the difference falls within the above range, the molded body prepared by compressing silicon nitride powder can have a denser structure, thus better suppressing the generation of porosity during sintering. That is, the flexural strength of the obtained silicon nitride sintered body can be further improved. The lower limit of the difference between D90 and D10 can be, for example, 3.0 μm or more, 3.2 μm or more, or 3.4 μm or more. If the lower limit of the aforementioned difference falls within the aforementioned range, the primary particle packing density can be further improved because silicon nitride powder will have a suitable particle size distribution. The aforementioned difference can be adjusted within the aforementioned range, for example, to 3.0~5.5μm, 3.2~5.4μm, or 3.4~4.6μm. The aforementioned difference can be controlled by adjusting the pulverization conditions during the manufacturing of the silicon nitride powder.

The upper limit of the D90 of silicon nitride powder can be, for example, 6.0 μm or less, 5.8 μm or less, 5.6 μm or less, 5.4 μm or less, or 5.2 μm or less. If the upper limit of D90 falls within the above range, the proportion of coarse particles can be sufficiently reduced, and the density reduction of the sintered body can be more effectively suppressed. The lower limit of D90 can be, for example, 3.5 μm or more, 3.7 μm or more, 3.9 μm or more, or 4.0 μm or more. D90 can be adjusted within the above range, for example, from 3.5 to 6.0 μm, or from 4.0 to 5.2 μm. The D90 of silicon nitride powder can be controlled, for example, by adjusting the pulverization conditions during the manufacture of silicon nitride powder.

The upper limit of the D50 of silicon nitride powder can be, for example, 1.5 μm or less, or 1.4 μm or less. If the upper limit of D50 falls within the above range, the strength of the silicon nitride sintered body can be further improved. The lower limit of the D50 of silicon nitride powder can be, for example, 1.1 μm or more, or 1.2 μm or more. The D50 of silicon nitride powder can be adjusted within the above range, for example, it can be 1.1~1.5 μm, or 1.2~1.4 μm.

In this manual, D10, D50, and D90 represent the particle sizes at 10%, 50%, and 90% of the cumulative particle size distribution curve of the volumetric standard determined by laser diffraction scattering, respectively. Laser diffraction scattering can be performed according to the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction Scattering". A laser diffraction particle size distribution measuring device (manufactured by Beckman Coulter, trade name: LS-13 320) can be used for measurement. Additionally, D50, also known as the median particle size, refers to the average particle size of silicon nitride powder.

The upper limit of the BET specific surface area of silicon nitride powder can be, for example, less than 9.0 ^m² /g, less than 8.8 ^m² /g, less than 8.6 ^m² /g, or less than 8.5 ^m² /g. The lower limit of the BET specific surface area of silicon nitride powder can be, for example, 5.0 ^m² /g or more, ^{5.1 m²/g or more, 5.2 m² / g} or more, 5.3 ^m² /g or more, 5.4 m²/g or more, 5.5 ^m² /g or more, 6.0 ^m² /g or more, or 7.0 ^m² /g or more. The BET specific surface area of silicon nitride powder can be adjusted within the above ranges, for example, it can be 5.0~9.0 ^m² /g, or 5.5~8.5 ^m² /g, or 7.0~8.5 ^m² /g. The BET specific surface area of silicon nitride powder can be controlled, for example, by adjusting the pulverization conditions during the manufacturing of silicon nitride powder.

The BET specific surface area in this manual is determined using the method described in JIS Z 8830:2013 "Determination of specific surface area of powder (solid) by gas adsorption", using nitrogen and the BET one-point method.

The upper limit of the surface oxygen content of silicon nitride can be, for example, 2.0% by mass or less, 1.5% by mass or less, or 1.3% by mass or less. If the upper limit of the surface oxygen content of silicon nitride falls within the above range, the grain boundary phase during the manufacture of silicon nitride sintered bodies can be reduced more sufficiently. The lower limit of the surface oxygen content of silicon nitride can be, for example, 0.20% by mass or more, 0.30% by mass or more, 0.35% by mass or more, 0.40% by mass or more, 0.45% by mass or more, 0.60% by mass or more, 0.80% by mass or more, or 1.0% by mass or more. If the lower limit of the surface oxygen content of silicon nitride falls within the above range, grain growth during the calcination of silicon nitride can be promoted, and the flexural strength of the silicon nitride sintered body can be further improved. The surface oxygen content of silicon nitride can be adjusted within the above range, for example, it can be 0.20~2.0% by mass, 0.20~1.5% by mass, or 1.0~1.5% by mass. The surface oxygen content of silicon nitride can be controlled, for example, by adjusting the composition of the gas environment, calcination temperature, and calcination time during the calcination step in the manufacturing of silicon nitride powder.

The "surface oxygen content" in this manual refers to the value obtained by the following procedure. The oxygen and nitrogen content of silicon nitride powder is analyzed using an oxygen-nitrogen analyzer. In a helium atmosphere, the sample is heated from 20°C to 2000°C at a heating rate of 8°C/second. Infrared absorption is used to detect the oxygen released during heating. Initially, oxygen bonded to the surface of the silicon nitride powder is released during heating. If further heating is applied, reaching a temperature of approximately 1400°C, silicon nitride begins to decompose. The initiation of silicon nitride decomposition can be confirmed by the detection of nitrogen. If silicon nitride begins to decompose, oxygen within the silicon nitride powder is released. Therefore, the oxygen system that is released at this stage is equivalent to the internal oxygen content, so the oxygen content detected and quantified before nitrogen is detected is taken as the surface oxygen content.

The aforementioned silicon nitride powder can be manufactured, for example, by the following method. One embodiment of the method for manufacturing silicon nitride powder includes the following steps: calcining silicon powder in a gas environment containing nitrogen and at least one gas selected from the group consisting of hydrogen and ammonia to obtain a calcined product (hereinafter also referred to as the calcination step); dry pulverizing the aforementioned calcined product to obtain a pulverized product (hereinafter also referred to as the pulverization step); and dry classifying the aforementioned pulverized product (hereinafter also referred to as the classification step).

Regarding the silicon powder, silicon powder with low oxygen concentration can be used. The upper limit of the oxygen concentration of the silicon powder can be, for example, 0.40% by mass or less, 0.30% by mass or less, or 0.20% by mass or less. If the oxygen concentration of the silicon powder falls within the above range, the amount of oxygen inside the obtained silicon nitride powder can be further reduced. The lower limit of the oxygen concentration of the silicon powder can be, for example, 0.10% by mass or more, or 0.15% by mass or more. The oxygen concentration of the silicon powder can be adjusted within the above range, for example, from 0.10 to 0.40% by mass.

The oxygen concentration of the silicon powder in this manual refers to the value measured using infrared absorption.

Commercially available silicon powder can be used, or it can be prepared separately. When the oxygen concentration of the silicon powder is high, a pretreatment solution containing hydrofluoric acid can be used to reduce the amount of oxygen bonded to the silicon powder. For example, the above-described method for manufacturing silicon nitride powder can also include a pretreatment step of using a pretreatment solution containing hydrofluoric acid to pretreat the silicon powder and obtain silicon powder with an oxygen concentration of less than 0.40% by mass.

The pretreatment solution contains hydrofluoric acid, but it can also be a mixture of acids such as hydrochloric acid, or it can consist solely of hydrofluoric acid. The temperature of the pretreatment solution in the pretreatment step can be, for example, 40~80°C. Furthermore, the contact time between the pretreatment solution and the silicon powder can be, for example, 1~10 hours.

In the calcination step, silicon powder is calcined in a mixed gas environment containing nitrogen and at least one of the groups selected from hydrogen and ammonia to obtain a calcined product containing silicon nitride. Based on the overall mixed gas environment, the total content of hydrogen and ammonia in the mixed gas environment can be, for example, 10-40% by volume. The calcination temperature can be, for example, 1100-1450°C or 1200-1400°C. The calcination time can be, for example, 30-100 hours.

In the pulverization step, the calcined material obtained in the calcination step is pulverized using a dry method to obtain pulverized material. In the method for manufacturing silicon nitride powder of this embodiment, the pulverization step includes two steps: ball milling and vibratory grinding. Pulverizing the calcined material and adjusting the particle size makes it easier to control in the subsequent classification step. The effect of the pulverization step is more significant when the calcined silicon nitride obtained in the calcination step is in the form of blocks, ingots, etc.

The grinding process can also be divided into multiple stages, such as coarse grinding and fine grinding. The grinding step is carried out using a dry grinding process.

The ball milling process can adjust the ball filling rate of the container to match the particle size distribution of the target silicon nitride powder. The lower limit of the ball filling rate is based on the container volume and can be, for example, 40% or more, 45% or more, 50% or more, or 60% or more. The upper limit of the ball filling rate is also based on the container volume and can be, for example, below 70% or below 65% or below.

The lower limit of the grinding time in the ball milling step can be, for example, 5 hours or more, 6 hours or more, 7 hours or more, or 8 hours or more. If the lower limit of the grinding time falls within the above range, the pulverized material can be sufficiently fined, which can further improve the grinding efficiency when further pulverized by dry methods. The upper limit of the grinding time in the ball milling step can be, for example, 15 hours or less, 14 hours or less, 13 hours or less, or 12 hours or less. If the upper limit of the grinding time falls within the above range, the calcined material can be fully pulverized, and over-pulverization can be prevented. The grinding time can be adjusted within the above range, for example, 5 to 15 hours, or 8 to 12 hours.

The pulverization step involves further pulverizing the calcined material using a vibratory grinding step after the ball milling step. The ball bearing capacity of the container in the vibratory grinding step can be adjusted to match the particle size distribution of the target silicon nitride powder. The lower limit for the ball bearing capacity is based on the container volume and can be, for example, 50% or more, 55% or more, or 60% or more. The upper limit for the ball bearing capacity is based on the container volume and can be, for example, below 80% or below, or below 75% or below.

The lower limit of the grinding time in the vibratory grinding step can be, for example, 8 hours or more, 9 hours or more, 10 hours or more, or 12 hours or more. If the lower limit of the grinding time falls within the above range, the pulverized material can be sufficiently fined, and the processing efficiency of the classification step can be further improved. The upper limit of the grinding time in the vibratory grinding step can be, for example, less than 20 hours, less than 19 hours, less than 18 hours, or less than 17 hours. If the upper limit of the grinding time falls within the above range, the calcined material can be sufficiently pulverized, and over-pulverization can be prevented. The grinding time can be adjusted within the above range, for example, from 8 to 20 hours, or from 12 to 17 hours.

In the classification step, the pulverized material obtained from the pulverization step is further classified using a dry method to produce silicon nitride powder with the desired particle size distribution. For example, coarse powder can be removed to adjust the D90 of the silicon nitride powder. Dry classification can be performed, for example, by airflow classification. Airflow classifiers can be used, for example, with swirling airflow. The primary gas pressure (inlet pressure) can be, for example, 0.2~0.8 MPa or 0.3~0.7 MPa.

The silicon nitride powder obtained by the above manufacturing method has excellent sintering properties. That is, the silicon nitride powder can be ideally used as raw material for sintered bodies.

One embodiment of the method for manufacturing silicon nitride sintered bodies includes a step of forming and calcining a sintering raw material containing the aforementioned silicon nitride powder.

In addition to silicon nitride powder _, sintering raw materials may also contain oxide-based sintering aids. Examples of oxide-based sintering aids include _Y₂O₃ , MgO, and _Al₂O₃ . The content of oxide-based sintering aids in the sintering raw materials may be, for example _, 3 to 10% by mass.

In the above steps, the sintering raw material is pressurized at a forming pressure of, for example, 3.0 to 30.0 MPa to obtain a molded body. The molded body can be manufactured by uniaxial pressurization or by CIP (Continuous In-Place Assembly). Alternatively, it can be formed and calcined simultaneously by hot pressing. The calcination of the molded body can be carried out in a passive gas environment such as nitrogen or argon. The calcination pressure can be 0.7 to 1.0 MPa. The calcination temperature can be 1860 to 2100°C or 1880 to 2000°C. The calcination time at this temperature can be 6 to 20 hours or 8 to 16 hours. The heating rate up to the calcination temperature can be, for example, 1.0~10.0℃/hour.

The obtained silicon nitride sintered structure exhibits excellent bending strength due to the reduction of grain boundary phases and the presence of a dense structure.

The flexural strength of silicon nitride sintered bodies can be set to 550 MPa or higher, 600 MPa or higher, or 650 MPa or higher at room temperature. The flexural strength of silicon nitride sintered bodies in this specification refers to the 3-point flexural strength measured at room temperature using a strength test piece prepared in accordance with JIS R 1601:2008.

The above description addresses several embodiments, but this disclosure is not limited to any of these embodiments. Furthermore, the descriptions of the above embodiments are interchangeable. [Examples]

The contents of this disclosure will now be described in more detail with reference to embodiments and comparative examples. However, this disclosure is not limited to the following embodiments.

(Example 1) <Preparation of Silicon Nitride Powder> Commercially available silicon powder (specific surface area: 3.0 ^m² /g) was immersed in a mixed acid containing hydrogen chloride and hydrogen fluoride at a temperature adjusted to 60°C and maintained at 60°C for 2 hours. The mixed acid was prepared by mixing commercially available hydrochloric acid (concentration: 35% by mass) and hydrofluoric acid (concentration: 55% by mass) in a mass ratio of 10:1. Afterward, the silicon powder was removed from the mixed acid and washed with water, and then dried under a nitrogen atmosphere. The oxygen concentration of the dried silicon powder was 0.4% by mass. This oxygen concentration was determined by infrared absorption method.

A molded body (bulk density: 1.4 g/ ^cm³ ) was prepared using dried silicon powder. The resulting molded body was then placed in an electric furnace and calcined at 1400°C for 60 hours to produce a calcined body containing silicon nitride. The gas environment during calcination was a mixture of nitrogen and hydrogen (a mixture of _N₂ and _H₂ at a standard volume ratio of 80:20). The resulting calcined body was coarsely pulverized and then dry-milled using a ball mill. For ball milling, the ball filling rate to the container was set to 60% by volume, and the milling time was set to 8 hours. Then, dry grinding was carried out by vibratory grinding, with the ball filling rate of the container set to 70% by volume and the grinding time set to 15 hours.

Silicon nitride powder obtained by dry grinding is classified under a primary gas pressure of 0.4 MPa to obtain silicon nitride powder.

<Evaluation of Silicon Nitride Powder: Determination of D10, D50, and D90> Following the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction Scattering," the D10, D50, and D90 of silicon nitride powder were determined by laser analytical scattering. The determination was performed using a laser diffraction particle size distribution measuring apparatus (Beckman Coulter, trade name: LS-13 320).

<Evaluation of Silicon Nitride Powder: Determination of BET Specific Surface Area> The BET specific surface area was determined by the BET one-point method using nitrogen, in accordance with JIS Z 8803:2013. The results are shown in Table 1.

<Evaluation of Silicon Nitride Powder: Determination of Surface Oxygen Content> Surface oxygen content was measured using a simultaneous oxygen/nitrogen analyzer (manufactured by Horiba Seisakusho, device name: EMGA-920). Specifically, silicon nitride powder was heated from 20°C to 2000°C at a heating rate of 8°C/second in a helium atmosphere, and the amount of oxygen before nitrogen detection was quantified.

[Manufacturing of Silicon Nitride Sintered Body] Weigh 90 parts by weight of the prepared silicon nitride powder, 5 parts by weight of _{Y₂O₃ powder} with an average particle size of 1.5 μm, and 5 parts by weight of _Yb₂O₃ powder with an average particle size of 1.2 _μm into a container, add methanol, and wet mix for 4 hours. Then, the dried mixed powder (calcined raw material) is molded under a pressure of 10 MPa, and then cold homogenized pressing (CIP) is performed under a pressure of 25 MPa. The obtained molded body and the filler powder composed of a mixture of silicon nitride powder and BN powder are placed together in a carbon crucible and calcined at 1900°C for 12 hours under a nitrogen pressurization gas environment of 1 MPa to produce a silicon nitride sintered body.

<Determination and Evaluation of Flexural Strength of Silicon Nitride Sintered Structures> Following JIS R1601:2008, test pieces for strength testing were prepared from silicon nitride sintered structures, and the flexural strength at three points at room temperature was measured. The test results were evaluated against a reference standard. The results are shown in Table 1. Furthermore, the flexural strength test results in Table 1 are relative values obtained using the silicon nitride sintered structure prepared in Comparative Example 1 (described later) as a reference. A: Flexural strength (relative value) is 1.10 or higher. B: Flexural strength (relative value) is 1.05 or higher but less than 1.10. C: Flexural strength (relative value) is less than 1.05.

(Example 2) The vibratory grinding conditions of the dry pulverizer were changed to the conditions recorded in Table 1, and silicon nitride powder was prepared in the same manner as in Example 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Example 3) The dry grinding ball milling conditions were changed to those recorded in Table 1, and silicon nitride powder was prepared in the same manner as in Example 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1) The conditions for dry grading were changed to those described in Table 1, and silicon nitride powder was prepared in the same manner as in Example 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[Table 1] Implementation Example 1 Implementation Example 2 Implementation Example 3 Comparative example 1 Conditions for dry grinding (ball milling) Ball filling rate [volume %] 60 60 60 60 Grinding time [hours] 8 8 5 8 Conditions for dry grinding (vibratory grinding) Ball filling rate [volume %] 70 70 70 70 Grinding time [hours] 15 10 15 15 Conditions for dry grading Inlet pressure [MPa] 0.4 0.4 0.4 0.1 Silicon nitride powder D90[μm] 5.0 5.4 5.5 6.6 D10[μm] 0.47 0.48 0.48 0.53 D50[μm] 1.34 1.41 1.42 1.52 D90-D10[μm] 4.5 4.9 5.0 6.1 BET specific surface area [ ^m² /g] 7.7 7.5 7.5 6.9 Surface oxygen content [mass %] 1.1 1.0 1.0 0.9 Silicon nitride sinter Bending strength [relative value] 1.11 1.07 1.06 1.00 Evaluation A B B C [Industrial applicability]

According to this disclosure, silicon nitride powder capable of producing sintered bodies with excellent flexural strength can be provided. Furthermore, according to this disclosure, a method for manufacturing silicon nitride sintered bodies with excellent flexural strength can be provided.

Claims (5)

A silicon nitride powder containing primary silicon nitride particles with a surface oxygen content of 1.0% by mass or more, wherein the particle sizes at which the cumulative value from the smallest particle size to the total 10% and 90% of the particle size distribution curve determined by laser diffraction are set as D10 and D90 respectively, the difference between D90 and D10 is less than 5.5 μm, and D90 is more than 4.0 μm. For example, the silicon nitride powder in Request 1, wherein the D50 is less than 1.5 μm. For example, the silicon nitride powder in request item 1 or 2, wherein the D90 is less than 6.0 μm. For example, the silicon nitride powder in claim 1 or 2, wherein the BET specific surface area does not reach 9.0 ^m² /g. A method for manufacturing a silicon nitride sintered body includes the steps of forming and calcining a sintering raw material comprising silicon nitride powder as claimed in any one of claims 1 to 4.