Method for producing silicon nitride powder and silicon nitride sintered body

Abstract

One aspect of the present disclosure provides a silicon nitride powder which contains primary particles of silicon nitride, and wherein if D10 and D90 are particle diameters at which the integrated value from the smallest particle diameter reaches 10% and 90% of the total in the volume-based particle size distribution curve as determined by a laser diffraction / scattering method, the difference between D90 and D10 is 5.5 μm or less.

Description

[Technical field]

[0001] The present disclosure relates to a silicon nitride powder and a method for producing a silicon nitride sintered body. [Background technology]

[0002] Silicon nitride is a material that has excellent strength, hardness, toughness, heat resistance, corrosion resistance, thermal shock resistance, etc., and is therefore used in various industrial parts such as die-casting machines and melting furnaces, as well as automobile parts, etc. In addition, silicon nitride also has excellent mechanical properties at high temperatures, and therefore its application to gas turbine parts, which require high-temperature strength and high-temperature creep properties, is being considered.

[0003] Silicon nitride sintered bodies are required to have further improved thermal conductivity and mechanical properties. For example, Patent Document 1 describes a silicon nitride sintered body having a thermal conductivity of 100 to 300 W / (m K) at room temperature and a three-point bending strength of 600 to 1500 MPa at room temperature. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2004-262756 A Summary of the Invention [Problem to be solved by the invention]

[0005] The present disclosure has an object to provide a silicon nitride powder capable of producing a sintered body having excellent bending strength. The present disclosure also has an object to provide a method for producing a silicon nitride sintered body having excellent bending strength. [Means for solving the problem]

[0006] One aspect of the present disclosure provides a silicon nitride powder comprising primary particles of silicon nitride, in which, in a volume-based particle size distribution curve measured by a laser diffraction / scattering method, when the particle sizes at which integrated values ​​from small particle sizes reach 10% and 90% of the total are defined as D10 and D90, respectively, the difference between D90 and D10 is 5.5 μm or less.

[0007] The silicon nitride powder has a difference between D90 and D10 (D90-D10) of a predetermined value or less, so that it is possible to prepare a molded body (unfired product) having a narrow particle size distribution and a denser structure. The silicon nitride sintered body obtained by firing the molded body has excellent bending strength and is suppressed from generating voids.

[0008] The silicon nitride may have a D50 of 1.5 μm or less. When the upper limit of D50 is within the above range, the silicon nitride powder has a suitable particle size distribution, so that the packing density of the primary particles can be further improved.

[0009] The silicon nitride powder may have a D90 of 6.0 μm or less. If the upper limit of D90 is within the above range, the proportion of coarse particles can be sufficiently reduced, and the density decrease of the sintered body can be more sufficiently suppressed. In addition, the handling property is excellent.

[0010] The silicon nitride powder has a BET specific surface area of ​​9.0 m 2 / g.

[0011] One aspect of the present disclosure provides a method for producing a silicon nitride sintered body, comprising the steps of molding and sintering a sintering raw material containing the above-mentioned silicon nitride powder.

[0012] The method for producing the silicon nitride sintered body uses a sintering raw material containing the silicon nitride powder described above, and therefore the resulting silicon nitride sintered body can exhibit excellent bending strength. Effect of the Invention

[0013] According to the present disclosure, it is possible to provide a silicon nitride powder capable of producing a sintered body having excellent bending strength. According to the present disclosure, it is also possible to provide a method for producing a silicon nitride sintered body having excellent bending strength. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents.

[0015] Unless otherwise specified, the materials exemplified in this specification can be used alone or in combination of two or more. When a plurality of substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified. In this specification, the "steps" may be steps independent of each other or steps performed simultaneously.

[0016] One embodiment of the silicon nitride powder comprises primary particles of silicon nitride, and in a volume-based particle size distribution curve measured by a laser diffraction / scattering method, when the particle sizes at which the integrated values ​​from small particle sizes reach 10% and 90% of the total are defined as D10 and D90, respectively, the difference between D90 and D10 is 5.5 μm or less.

[0017] The upper limit of the difference between D90 and D10 (D90-D10) is 5.5 μm or less, and may be, for example, 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. When the upper limit of the difference is within the above range, the molded body prepared by compression molding or the like of the silicon nitride powder can have a denser structure, so that the generation of voids during sintering can be more suppressed. In other words, the bending strength of the obtained silicon nitride sintered body can be more improved. The lower limit of the difference between D90 and D10 may be, for example, 3.0 μm or more, 3.2 μm or more, or 3.4 μm or more. When the lower limit of the difference is within the above range, the silicon nitride powder has a moderate particle size distribution, so that the packing density of the primary particles can be more improved. The difference can be adjusted within the above range, and may be, for example, 3.0 to 5.5 μm, 3.2 to 5.4 μm, or 3.4 to 4.6 μm. The above difference can be controlled by adjusting the grinding conditions during the production of the silicon nitride powder.

[0018] The upper limit of D90 of the silicon nitride powder may 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. When the upper limit of D90 is within the above range, the ratio of coarse particles can be sufficiently reduced, and the density decrease of the sintered body can be more sufficiently suppressed. The lower limit of D90 may 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, and may be, for example, 3.5 to 6.0 μm, or 4.0 to 5.2 μm. D90 of the silicon nitride powder can be controlled, for example, by adjusting the grinding conditions during the production of the silicon nitride powder.

[0019] The upper limit of D50 of the silicon nitride powder may be, for example, 1.5 μm or less, or 1.4 μm or less. When the upper limit of D50 is within the above range, the strength of the silicon nitride sintered body can be further improved. The lower limit of D50 of the silicon nitride powder may be, for example, 1.1 μm or more, or 1.2 μm or more. The D50 of the silicon nitride powder can be adjusted within the above range, and may be, for example, 1.1 to 1.5 μm, or 1.2 to 1.4 μm.

[0020] In this specification, D10, D50, and D90 refer to the particle diameters at which the cumulative values ​​from the small particle diameters reach 10%, 50%, and 90% of the total in the volume-based particle diameter distribution curve measured by the laser diffraction / scattering method, respectively. The laser analysis / scattering method can be measured in accordance with the method described in JIS Z 8825:2013 "Particle diameter analysis-laser diffraction / scattering method". For the measurement, a laser diffraction / scattering method particle size distribution measuring device (manufactured by Beckman Coulter, Inc., product name: LS-13 320) or the like can be used. Note that D50 is also called the median diameter and refers to the average particle diameter of the silicon nitride powder.

[0021] The upper limit of the BET specific surface area of ​​silicon nitride powder is, for example, 9.0 m 2 / g, less than 8.8m 2 / g or less, 8.6m 2 / g or less, or 8.5m 2 The lower limit of the BET specific surface area of ​​the silicon nitride powder may be, for example, 5.0 m 2 / g or more, 5.1m 2 / g or more, 5.2m 2 / g or more, 5.3m 2 / g or more, 5.4m 2 / g or more, 5.5m 2 / g or more, 6.0m 2 / g or more, or 7.0m 2 The BET specific surface area of ​​the silicon nitride powder can be adjusted within the above range, for example, 5.0 to 9.0 m 2 / g, or 5.5 to 8.5 m 2 / g, or 7.0 to 8.5 m 2 The BET specific surface area of ​​the silicon nitride powder can be controlled, for example, by adjusting the grinding conditions during the production of the silicon nitride powder.

[0022] The BET specific surface area in this specification is a value measured by the BET single-point method using nitrogen gas in accordance with the method described in JIS Z 8830:2013 "Method for measuring specific surface area of ​​powder (solid) by gas adsorption."

[0023] The upper limit of the surface oxygen content of silicon nitride may be, for example, 2.0 mass% or less, 1.5 mass% or less, or 1.3 mass% or less. When the upper limit of the surface oxygen content of silicon nitride is within the above range, the grain boundary phase can be more sufficiently reduced when the silicon nitride sintered body is produced. The lower limit of the surface oxygen content of silicon nitride may be, for example, 0.20 mass% or more, 0.30 mass% or more, 0.35 mass% or more, 0.40 mass% or more, 0.45 mass% or more, 0.60 mass% or more, 0.80 mass% or more, or 1.0 mass% or more. When the lower limit of the surface oxygen content of silicon nitride is within the above range, grain growth can be promoted when the silicon nitride is sintered, and the bending 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, and may be, for example, 0.20 to 2.0 mass%, 0.20 to 1.5 mass%, or 1.0 to 1.5 mass%. The surface oxygen content of silicon nitride can be controlled, for example, by adjusting the components of the atmosphere in the sintering step in the production of silicon nitride powder, as well as the sintering temperature, sintering time, and the like.

[0024] In this specification, the "amount of surface oxygen" means a value obtained by the following procedure. The amount of oxygen and the amount of nitrogen in silicon nitride powder are analyzed using an oxygen / nitrogen analyzer. The measurement sample is heated from 20°C to 2000°C at a heating rate of 8°C / sec in a helium gas atmosphere. The oxygen released with the increase in temperature is detected by infrared absorption. At the beginning of the temperature increase, oxygen bonded to the surface of the silicon nitride powder is released. When the temperature is further increased to about 1400°C, silicon nitride begins to decompose. The start of decomposition of silicon nitride can be grasped by the start of detection of nitrogen. When silicon nitride begins to decompose, oxygen inside the silicon nitride powder is released. Therefore, the oxygen released at this stage corresponds to the amount of internal oxygen, so the amount of oxygen detected and quantified before nitrogen is detected is taken as the amount of surface oxygen.

[0025] The silicon nitride powder can be produced, for example, by the following method. One embodiment of the method for producing silicon nitride powder includes a step of calcining silicon powder in an atmosphere containing nitrogen and at least one selected from the group consisting of hydrogen and ammonia to obtain a calcined product (hereinafter also referred to as the calcination step), a step of dry-pulverizing the calcined product to obtain a pulverized product (hereinafter also referred to as the pulverization step), and a step of dry-classifying the pulverized product (hereinafter also referred to as the classification step).

[0026] As the silicon powder, silicon powder having a low oxygen concentration may be used. The upper limit of the oxygen concentration of the silicon powder may be, for example, 0.40 mass% or less, 0.30 mass% or less, or 0.20 mass% or less. By setting the oxygen concentration of the silicon powder 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 may be, for example, 0.10 mass% or more, or 0.15 mass% or more. The oxygen concentration of the silicon powder can be adjusted within the above range, and may be, for example, 0.10 to 0.40 mass%.

[0027] In this specification, the oxygen concentration of the silicon powder means a value measured by an infrared absorption method.

[0028] The silicon powder may be commercially available or may be prepared separately. When the oxygen concentration of the silicon powder is high, for example, a pretreatment liquid containing hydrofluoric acid may be used to reduce the amount of oxygen bonded to the silicon powder. For example, the method for producing the silicon nitride powder may further include a pretreatment step of pretreating the silicon powder with a pretreatment liquid containing hydrofluoric acid to obtain a silicon powder having an oxygen concentration of 0.40 mass% or more.

[0029] The pretreatment liquid contains hydrofluoric acid, but may be a mixed acid with an acid such as hydrochloric acid, or may consist of hydrofluoric acid alone. The temperature of the pretreatment liquid in the pretreatment step may be, for example, 40 to 80° C. The time for which the pretreatment liquid is contacted with the silicon powder may be, for example, 1 to 10 hours.

[0030] In the firing step, the silicon powder is fired in a mixed atmosphere containing nitrogen and at least one selected from the group consisting of hydrogen and ammonia to obtain a fired material containing silicon nitride. The total content of hydrogen and ammonia in the mixed atmosphere may be, for example, 10 to 40 volume % based on the entire mixed atmosphere. The firing temperature may be, for example, 1100 to 1450°C, or 1200 to 1400°C. The firing time may be, for example, 30 to 100 hours.

[0031] In the pulverization step, the sintered product obtained in the sintering step is pulverized in a dry manner to obtain a pulverized product. In the method for producing silicon nitride powder according to this embodiment, the pulverization step includes two steps, a ball mill pulverization step and a vibration mill pulverization step. By pulverizing the sintered product and adjusting the particle size, it becomes easier to control the subsequent classification step. When the sintered product containing silicon nitride obtained in the sintering step is in the form of a lump, an ingot, or the like, the effect of performing the pulverization step is more pronounced.

[0032] Each pulverization may be carried out in a plurality of stages, such as coarse pulverization and fine pulverization. The pulverization step is carried out in a dry pulverization step.

[0033] The ball filling rate in the container in the ball mill grinding process can be adjusted according to the particle size distribution of the target silicon nitride powder. The lower limit of the ball filling rate in the container may be, for example, 40 volume% or more, 45 volume% or more, 50 volume% or more, or 60 volume% or more based on the volume of the container. The upper limit of the ball filling rate in the container may be, for example, 70 volume% or less, or 65 volume% or less based on the volume of the container.

[0034] The lower limit of the time of the grinding process (grinding time) in the ball mill grinding step may be, for example, 5 hours or more, 6 hours or more, 7 hours or more, or 8 hours or more. By setting the lower limit of the grinding time within the above range, the ground material can be sufficiently finely ground, and the grinding efficiency during further dry grinding can be further improved. The upper limit of the time of the grinding process in the ball mill grinding step may be, for example, 15 hours or less, 14 hours or less, 13 hours or less, or 12 hours or less. By setting the upper limit of the grinding time within the above range, the fired material can be sufficiently ground, and excessive grinding can also be prevented. The grinding time may be adjusted within the above range, and may be, for example, 5 to 15 hours, or 8 to 12 hours.

[0035] In the pulverization step, the above-mentioned fired product is further pulverized by a vibration mill pulverization step after the ball mill pulverization step. The ball filling rate in the container in the vibration mill pulverization step can be adjusted according to the particle size distribution of the target silicon nitride powder. The lower limit of the ball filling rate in the container may be, for example, 50 volume % or more, 55 volume % or more, or 60 volume % or more based on the volume of the container. The upper limit of the ball filling rate in the container may be, for example, 80 volume % or less, or 75 volume % or less based on the volume of the container.

[0036] The lower limit of the time (grinding time) of the grinding process in the vibration mill grinding step may be, for example, 8 hours or more, 9 hours or more, 10 hours or more, or 12 hours or more. By setting the lower limit of the grinding time within the above range, the ground material can be sufficiently finely ground, and the processing efficiency of the classification step can be further improved. The upper limit of the time of the grinding process in the vibration mill grinding step may be, for example, 20 hours or less, 19 hours or less, 18 hours or less, or 17 hours or less. By setting the upper limit of the grinding time within the above range, the fired material can be sufficiently ground, and excessive grinding can also be prevented. The grinding time may be adjusted within the above range, and may be, for example, 8 to 20 hours, or 12 to 17 hours.

[0037] In the classification step, the pulverized product prepared through the pulverization step is further classified in a dry manner to prepare silicon nitride powder having a 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 air classification. The air classifier can classify using, for example, a swirling air current. The primary air pressure (inlet pressure) can be, for example, 0.2 to 0.8 MPa, or 0.3 to 0.7 MPa.

[0038] The silicon nitride powder obtained by the above-mentioned production method has excellent sinterability, and thus can be suitably used as a raw material for a sintered body.

[0039] One embodiment of a method for producing a silicon nitride sintered body includes a step of molding and sintering a sintering raw material containing the above-mentioned silicon nitride powder.

[0040] The sintering raw material may contain an oxide-based sintering aid in addition to the silicon nitride powder. Examples of the oxide-based sintering aid include Y 2 O 3、 MgO and Al 2 O 3 The content of the oxide-based sintering aid in the sintering raw material may be, for example, 3 to 10 mass %.

[0041] In the above process, the above-mentioned sintering raw material is pressed at a molding pressure of, for example, 3.0 to 30.0 MPa to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP. Also, it may be fired while being molded by hot pressing. The molded body may be fired in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure during firing may be 0.7 to 1.0 MPa. The firing temperature may be 1860 to 2100°C, or may be 1880 to 2000°C. The firing time at the firing temperature may be 6 to 20 hours, or may be 8 to 16 hours. The heating rate up to the firing temperature may be, for example, 1.0 to 10.0°C / hour.

[0042] The resulting silicon nitride sintered body has a reduced grain boundary phase and a dense structure, and therefore can exhibit excellent bending strength.

[0043] The bending strength of the silicon nitride sintered body at room temperature can be, for example, 550 MPa or more, 600 MPa or more, or 650 MPa or more. In this specification, the bending strength of the silicon nitride sintered body means a three-point bending strength measured at room temperature using a test piece for strength measurement prepared in accordance with JIS R1601:2008.

[0044] Although several embodiments have been described above, the present disclosure is not limited to the above-described embodiments. In addition, the descriptions of the above-described embodiments can be mutually applied. EXAMPLES

[0045] The present disclosure will be described in more detail below with reference to examples and comparative examples. However, the present disclosure is not limited to the following examples.

[0046] Example 1 <Preparation of silicon nitride powder> Commercially available silicon powder (specific surface area: 3.0 m 2 100g) was immersed in a mixed acid containing hydrogen chloride and hydrogen fluoride, the temperature of which was adjusted to 60°C, and the mixed acid was maintained at 60°C for 2 hours for pretreatment. The mixed acid used was a mixture of commercially available hydrochloric acid (concentration: 35% by mass) and hydrofluoric acid (concentration: 55% by mass) in a mass ratio of 10:1. The silicon powder was then removed from the mixed acid, washed with water, and dried under a nitrogen atmosphere. The oxygen concentration of the dried silicon powder was 0.4% by mass. This oxygen concentration was measured by an infrared absorption method.

[0047] The dried silicon powder was used to produce a compact (bulk density: 1.4 g / cm 3 The obtained molded body was placed in an electric furnace and sintered at 1400°C for 60 hours to produce a sintered body containing silicon nitride. The sintering atmosphere was a mixture of nitrogen and hydrogen gas (N 2 and H 2A gas mixture of 80:20 and 100% benzene was supplied to the sample. The obtained sintered body was coarsely pulverized and then dry-pulverized in a ball mill. The ball milling was carried out with a ball filling rate of 60% by volume in the container and a pulverization time of 8 hours. The sample was then dry-pulverized in a vibration mill with a ball filling rate of 70% by volume in the container and a pulverization time of 15 hours.

[0048] The silicon nitride powder obtained by dry pulverization was classified under the condition of a primary air pressure of 0.4 MPa to obtain silicon nitride powder.

[0049] <Evaluation of silicon nitride powder: Measurement of D10, D50, and D90> The D10, D50, and D90 of the silicon nitride powder were measured by a laser analysis scattering method in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction and scattering method". For the measurements, a laser diffraction scattering particle size distribution analyzer (manufactured by Beckman Coulter, Inc., product name: LS-13 320) was used.

[0050] <Evaluation of silicon nitride powder: Measurement of BET specific surface area> The BET specific surface area was measured by a single-point BET method using nitrogen gas in accordance with JIS Z 8803:2013. The results are shown in Table 1.

[0051] <Evaluation of silicon nitride powder: Measurement of surface oxygen content> The surface oxygen amount was measured using an oxygen / nitrogen simultaneous analyzer (Horiba, Ltd., device name: EMGA-920). Specifically, the silicon nitride powder was heated from 20°C to 2000°C at a temperature increase rate of 8°C / sec in a helium atmosphere, and the amount of oxygen before nitrogen was detected was quantified.

[0052] [Production of sintered silicon nitride] In a container, 90 parts by mass of the prepared silicon nitride powder and 1.5 μm of Y 2 O 3 5 parts by mass of powder and Yb 2 O 3Five parts by mass of the powder were weighed out, methanol was added, and the mixture was wet mixed for 4 hours. The mixture (sintered raw material) obtained by drying was then molded in a die at a pressure of 10 MPa, and then further molded by cold isostatic pressing (CIP) at a pressure of 25 MPa. The obtained molded body was placed in a carbon crucible together with a packing powder consisting of a mixture of silicon nitride powder and BN powder, and sintered at a temperature of 1900°C for 12 hours under a nitrogen pressure of 1 MPa to produce a silicon nitride sintered body.

[0053] <Measurement and evaluation of bending strength of sintered silicon nitride> A test piece for strength measurement was prepared from the silicon nitride sintered body in accordance with JIS R1601:2008, and the three-point bending strength at room temperature was measured. Evaluation was performed based on the measurement results. The results are shown in Table 1. In Table 1, the bending strength measurement results are shown as relative values ​​based on the silicon nitride sintered body prepared in Comparative Example 1 described later. A: Bending strength (relative value) is 1.10 or more. B: The bending strength (relative value) is 1.05 or more and less than 1.10. C: The bending strength (relative value) is less than 1.05.

[0054] Example 2 Silicon nitride powder was prepared in the same manner as in Example 1, except that the vibration mill conditions for dry pulverization were changed to those shown in Table 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0055] Example 3 Silicon nitride powder was prepared in the same manner as in Example 1, except that the ball mill pulverization conditions for the dry pulverization were changed to those shown in Table 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0056] Comparative Example 1 Silicon nitride powder was prepared in the same manner as in Example 1, except that the conditions for dry classification were changed to those shown in Table 1. The obtained silicon nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0057] [Table 1] [Industrial Applicability]

[0058] According to the present disclosure, it is possible to provide a silicon nitride powder capable of producing a sintered body having excellent bending strength. According to the present disclosure, it is also possible to provide a method for producing a silicon nitride sintered body having excellent bending strength.

Claims

1. Contains primary particles of silicon nitride, The surface oxygen content is 1.0 mass% or more, A silicon nitride powder in which, in a volume-based particle size distribution curve measured by a laser diffraction / scattering method, the particle sizes at which the integrated values ​​from small particle sizes reach 10% and 90% of the total are defined as D10 and D90, respectively, and the difference between D90 and D10 is 5.5 μm or less, and D90 is 4.0 μm or more.

2. 2. The silicon nitride powder according to claim 1, having a D50 of 1.5 μm or less.

3. The silicon nitride powder according to claim 1 or 2, having a D90 of 6.0 μm or less.

4. BET specific surface area is 9.0m ２ The silicon nitride powder according to any one of claims 1 to 3, wherein the surface area is less than 100 nm.

5. A method for producing a silicon nitride sintered body, comprising the steps of molding and sintering a sintering raw material containing the silicon nitride powder according to any one of claims 1 to 4.

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