Silicone nitride powder

Abstract

To provide a silicon nitride powder capable of preparing sintered bodies with excellent flexural strength.SOLUTION: The present disclosure provides a silicon nitride powder having a BET specific surface area of 10.3 m2 / g or more, and in a cumulative distribution of particle sizes on a volume basis measured with a particle size distribution measuring device using a laser diffraction / scattering method, when the particle size at which the cumulative value from small particle sizes reaches 10% of the total is defined as D10, the particle size at which the cumulative value from small particle sizes reaches 50% of the total is defined as D50, and the particle size at which the cumulative value from small particle sizes reaches 90% of the total is defined as D90, (D90-D10) / D50 is 1.70 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to silicon nitride powders. [Background technology]

[0002] Silicon nitride is a material with excellent strength, hardness, toughness, heat resistance, corrosion resistance, and thermal shock resistance, and is therefore used in various industrial parts such as die-casting machines and melting furnaces, as well as in automotive parts. Silicon nitride also has excellent mechanical properties at high temperatures, and its use in gas turbine parts, which require high-temperature strength and high-temperature creep properties, is being considered. Patent Document 1 describes a silicon nitride sintered body characterized by 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]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-262756 Summary of the Invention [Problem to be solved by the invention]

[0004] When silicon nitride powder is sintered, the silicon nitride particles dissolve in a sintering aid melt, and then undergo reprecipitation and grain growth, resulting in a phase transformation to the thermally more stable β phase. The grain growth of silicon nitride accompanying this phase transformation sinters the silicon nitride powder, resulting in a silicon nitride sintered body. Further improvements in the mechanical properties of silicon nitride sintered bodies are desired.

[0005] An object of the present disclosure is to provide a silicon nitride powder that can be used to prepare a sintered body having excellent bending strength. [Means for solving the problem]

[0006] One aspect of the present disclosure provides the following silicon nitride powder:

[0007] [1] BET specific surface area is 10.3m 2 / g or more, In the cumulative distribution of particle diameters on a volume basis measured with a particle size distribution analyzer using the laser diffraction / scattering method, the particle diameter when the cumulative value from small particle diameters reaches 10% of the total is defined as D10, the particle diameter when the cumulative value from small particle diameters reaches 50% of the total is defined as D50, and the particle diameter when the cumulative value from small particle diameters reaches 90% of the total is defined as D90. Silicon nitride powder having a (D90-D10) / D50 ratio of 1.70 or less.

[0008] The silicon nitride powder in [1] above has a BET specific surface area of ​​10.3 m 2 / g or more, and (D90-D10) / D50 is 1.70 or less. A large BET specific surface area of ​​a silicon nitride powder corresponds to fine silicon nitride particles constituting the silicon nitride powder. By having a BET specific surface area within the above range and a narrow particle size distribution of the silicon nitride powder such that (D90-D10) / D50 is a predetermined value or less, even if the silicon nitride powder contains silicon nitride having a β phase, the silicon nitride particles having a β phase remaining in the liquid phase of the sintering aid at the start of grain growth during sintering will be appropriately small.

[0009] During the production process of silicon nitride sintered bodies, if silicon nitride powder contains silicon nitride with a relatively stable β-phase, it will not melt but will serve as a nucleus for crystal growth. Grain growth will proceed preferentially from these nuclei, resulting in the generation of coarse particles in the sintered body. The presence of coarse particles in the sintered body will result in stress concentration on the coarse particles, reducing the bending strength of the sintered body. On the other hand, the silicon nitride powder described in [1] above has a controlled particle size and a narrow particle distribution so that the specific surface area is greater than a predetermined value. Therefore, even if silicon nitride particles with a β-phase are present, their size as crystal nuclei is small, resulting in a more uniform environment during sintering. Even when such silicon nitride powder is used as a sintered body raw material, it is possible to reduce the amount of coarse particles in the resulting sintered body, and the resulting sintered body will exhibit excellent bending strength.

[0010] The silicon nitride powder of the above [1] may be any of the following [2] to [4].

[0011] [2] The silicon nitride powder according to [1], wherein the gelatinization rate is 96.0 mass% or less. [3] The silicon nitride powder according to [1] or [2], wherein D97 is the particle diameter when the cumulative value from small particle diameters reaches 97% of the total in the cumulative distribution, and D97 is 2.25 μm or less. [4] The silicon nitride powder according to any one of [1] to [3], which has an oxygen content of 0.65 to 1.60 mass %.

[0012] The silicon nitride powder of [2] above has a gelatinization rate of 96.0 mass % or less. By using silicon nitride powder with a low gelatinization rate, it is possible to reduce costs during production.

[0013] The silicon nitride powder [3] above has a D97 of 2.25 μm or less. Such silicon nitride powder allows for more complete melting of the silicon nitride, and even if the silicon nitride having the β phase does not melt and begins to grow as grains, it is possible to suppress the difference in particle size between the silicon nitride particles and the new silicon nitride particles generated by reprecipitation. This makes it possible to further suppress the generation of coarse particles and further improve the bending strength of the sintered body.

[0014] The silicon nitride powder [4] has an oxygen content of 0.65 to 1.60 mass%. Silicon nitride powder with this oxygen content can improve reactivity during sintering while suppressing the formation of embrittled phases in the sintered body due to oxygen, thereby suppressing embrittlement of the sintered body itself. Therefore, the bending strength of the sintered body can be further improved. [Effects of the Invention]

[0015] According to the present disclosure, it is possible to provide a silicon nitride powder that can be used to prepare a sintered body having excellent bending strength. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Comparative Example 4. [Figure 2] 1 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Example 4. [Figure 3] 1 is a scanning electron microscope photograph showing a cross section of a sintered body produced using the silicon nitride powder of Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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 content. The upper or lower limit of a numerical range specified in this disclosure may be replaced with any value shown in the examples. Furthermore, the upper and lower limits individually stated may be arbitrarily combined. The symbol "~" used in a numerical range indicates a numerical range that includes the upper and lower limit. For example, "X~Y" indicates a numerical range of "greater than or equal to X and less than or equal to Y." Unless otherwise specified, the materials or components exemplified in this disclosure can be used alone or in combination of two or more.

[0018] <Silicon nitride powder> The silicon nitride powder (Si3N4 powder) according to one embodiment has a BET specific surface area of ​​10.3 m 2 / g or more, and in the cumulative distribution of volumetric particle sizes measured with a particle size distribution analyzer using laser diffraction / scattering, the particle size when the cumulative value from small particle sizes reaches 10% of the total is defined as D10, the particle size when the cumulative value from small particle sizes reaches 50% of the total is defined as D50, and the particle size when the cumulative value from small particle sizes reaches 90% of the total is defined as D90, where (D90-D10) / D50 is 1.70 or less. By using such silicon nitride powder, a sintered body with excellent bending strength can be obtained.

[0019] The BET specific surface area of ​​silicon nitride powder is, for example, 10.5 m 2 / g or more, and 2 / g or more, and 2 / g or more. Silicon nitride powders with a BET specific surface area lower limit within this range have finer particle sizes, and even if β-phase silicon nitride particles are present, when used as a raw material for a sintered body, the generation of coarse particles due to these particles can be more sufficiently suppressed. Silicon nitride powders with a BET specific surface area lower limit within this range also have a larger contact surface with the liquid phase of the sintering aid during sintering, and tend to dissolve more easily. Therefore, a large BET specific surface area further increases the uniformity of the growth field of β-phase silicon nitride particles, thereby improving the bending strength of the sintered body.

[0020] The BET specific surface area is, for example, 20.0 m 2 / g or less, and 2 / g or less, and 2 / g or less. There is no technical upper limit to the BET specific surface area, but generally, as the BET specific surface area increases, the oxygen content also increases. Therefore, as the BET specific surface area increases, the oxygen content increases excessively, which leads to the formation of an embrittlement phase during the preparation of a sintered body, resulting in a decrease in strength. In other words, silicon nitride powder with an upper limit for the BET specific surface area in this range can maintain the oxygen content within an appropriate range. Therefore, the bending strength of the resulting sintered body can be further improved. An example of a BET specific surface area is 10.3 to 20.0 m 2 / g.

[0021] The BET specific surface area of ​​the silicon nitride powder may be adjusted, for example, by changing the grinding conditions during production of the silicon nitride powder. The BET specific surface area in the present disclosure is a value measured by the single-point BET method using nitrogen gas in accordance with the method described in JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method."

[0022] In the cumulative particle size distribution measured by a laser diffraction / scattering particle size analyzer, the particle size at which the cumulative value from the smallest particle size reaches 10% of the total is defined as D10, the particle size at which the cumulative value from the smallest particle size reaches 50% of the total is defined as D50, and the particle size at which the cumulative value from the smallest particle size reaches 90% of the total is defined as D90. (D90 - D10) / D50 can be less than 1.70, 1.65 or less, 1.60 or less, or 1.50 or less. The value (D90 - D10) / D50 is called the span value and is an index of the breadth of the particle size distribution. The smaller the span value, the shorter the difference in particle size between D90 and D10, and the sharper the particle size distribution. A sharp particle size distribution results in less particle size variation in silicon nitride powder, resulting in a powder with high particle size uniformity. This allows for more uniform sintering and better suppresses the formation of coarse particles in some parts of the sintered body. Therefore, the bending strength of the sintered body can be further improved.

[0023] Furthermore, when multiple sintered bodies are produced using silicon nitride powder whose (D90-D10) / D50 value falls within the above range, the variation in bending strength of each sintered body can be reduced, thereby increasing the Weibull modulus of the sintered body.

[0024] The value of (D90-D10) / D50 may be, for example, 1.30 or more, or even 1.40 or more. Silicon nitride powders with a lower limit of (D90-D10) / D50 in this range have smaller particle size variations. This further homogenizes the environment in which the silicon nitride powder dissolves in the liquid phase of the sintering aid and reprecipitates during the production of a sintered body, thereby more fully suppressing the particle size variations of growing β-phase particles. This further improves the bending strength of the resulting sintered body. The value of (D90-D10) / D50 of the sintered body may be, for example, 1.30 to 1.70.

[0025] D90 may be, for example, 2.00 μm or less, 1.50 μm or less, or 1.30 μm or less. Silicon nitride powders with an upper D90 value in this range are composed of smaller particles, and when used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with even higher bending strength. Furthermore, D90 may be, for example, 0.60 μm or more, 0.80 μm or more, or 1.20 μm or more. By ensuring that the lower limit of D90 is within this range, excessive increases in oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, classification productivity can be improved, silicon nitride powder can be produced more inexpensively, and manufacturing conditions such as the firing temperature during sintered body production can be stabilized.

[0026] D50 may be, for example, 1.00 μm or less, 0.90 μm or less, 0.80 μm or less, or 0.70 μm or less. Silicon nitride powders with an upper D50 value in this range are composed of smaller particles, and when used as a raw material for a sintered body, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with higher bending strength. Furthermore, D50 may be, for example, 0.30 μm or more, or 0.40 μm or more. By setting the lower D50 value in this range, excessive increases in oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, classification productivity can be improved, silicon nitride powder can be produced more inexpensively, and manufacturing conditions such as the firing temperature during sintered body production can be stabilized.

[0027] D10 may be, for example, 0.60 μm or less, 0.50 μm or less, or 0.40 μm or less. Silicon nitride powders with an upper D10 value in this range are composed of smaller particles, and when used as a sintered body raw material, the generation of coarse particles in the sintered body can be further suppressed, resulting in a sintered body with even higher bending strength. Furthermore, D10 may be, for example, 0.05 μm or more, or 0.10 μm or more. By setting the lower limit of D10 in this range, excessive increases in oxygen content due to smaller particle size can be prevented, the generation of embrittled phases during sintered body production can be further suppressed, and the bending strength of the sintered body can be further improved. Furthermore, classification productivity can be improved, silicon nitride powder can be produced more inexpensively, and manufacturing conditions such as the firing temperature during sintered body production can be stabilized.

[0028] In the cumulative particle size distribution of silicon nitride powder measured by a particle size analyzer using a laser diffraction / scattering method, when the particle size at which the cumulative value from the smallest particle size reaches 97% of the total is defined as D97, D97 may be 2.25 μm or less, 2.00 μm or less, 1.80 μm or less, 1.60 μm or less, or 1.40 μm or less. Silicon nitride powders having D97 in this range have sufficiently small particle sizes. Silicon nitride powders having D97 in this range can further suppress the generation of coarse particles during sintering, even when they contain a large amount of β phase, which easily becomes a crystal nucleus during sintering. Therefore, the generation of coarse particles in the sintered body can be further suppressed. Such sintered bodies have even higher bending strength.

[0029] D97 may be, for example, 0.80 μm or more, 0.90 μm or more, or 1.00 μm or more. The range of D97 may be, for example, 0.80 to 2.25 μm.

[0030] The laser diffraction / scattering method can be used in accordance with the method described in JIS R 1629:1997, "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction / scattering method." A laser diffraction / scattering particle size distribution analyzer (manufactured by Microtrack Bell, product name: MT-3300EX) or the like can be used for the measurement.

[0031] The silicon nitride powder according to the present disclosure has an adjusted BET specific surface area and (D90-D10) / D50 value, so that even if the alpha phase ratio is not particularly high, a sintered body exhibiting excellent bending strength can be prepared. The alpha phase ratio of the silicon nitride powder (the mass ratio of α-Si3N4 to the total amount of Si3N4) may be 96.0 mass% or less, 95.0 mass% or less, or 94.0 mass% or less. By keeping the alpha phase ratio within this range, the proportion of the β phase in the silicon nitride powder is further increased, resulting in excellent productivity. The alpha phase ratio may be, for example, 89.0 mass% or more, or 90.0 mass% or more. By keeping the alpha phase ratio within this range, a sintered body with excellent bending strength can be obtained.

[0032] The alpha-phase ratio of silicon nitride powder can be adjusted by the conditions for calcining the silicon nitride powder. Silicon nitride powder with a high alpha-phase ratio can be obtained by chemical synthesis using imide pyrolysis or by a high-temperature vapor phase method in which euhedral particles are formed by crystal growth at high temperatures. However, these methods require chemical synthesis, which generates a large amount of by-products, and are therefore costly. On the other hand, silicon nitride powder with a low alpha-phase ratio can be obtained by a nitridation reaction in which metal silicon is directly nitrided, which allows for simple production while suppressing cost increases. The alpha-phase ratio of silicon nitride powder can be determined by the diffraction intensity of X-ray diffraction. Specifically, the alpha-phase ratio of silicon nitride powder is determined by the method described in the Examples.

[0033] The oxygen content of the silicon nitride powder may be, for example, 0.65 to 1.60 mass%. The oxygen content in this disclosure refers to the total oxygen content. Silicon nitride powders with such an oxygen content contain sufficient oxygen to promote sintering while further suppressing the formation of embrittled phases due to excess oxygen. Furthermore, when the oxygen content is within the above range, a certain amount of oxygen is present on the surface of the silicon nitride particles, further promoting the sintering reaction and producing sintered bodies with higher bending strength. Therefore, sintered bodies made using silicon nitride powders with such an oxygen content have even higher bending strength. By ensuring that the oxygen content of the silicon nitride powder is within the above range, the sintering reaction can be promoted based on the oxygen content of the silicon nitride particles and the contact area with the liquid phase of the sintering aid, while the formation of embrittled phases can be suppressed by adjusting the oxygen content of the silicon nitride particles. The oxygen content can be measured using a commercially available oxygen / nitrogen analyzer.

[0034] The oxygen content may be, for example, 0.70% by mass or more, or 0.80% by mass or more. Silicon nitride powder with such an oxygen content contains sufficient oxygen, which further promotes the sintering reaction and further improves the bending strength of the sintered body.

[0035] The oxygen content may be, for example, 1.50% by mass or less, or 1.30% by mass or less. Silicon nitride powder with such an oxygen content can further suppress the formation of embrittlement phases due to excess oxygen during sintering, and can further improve the bending strength of the sintered body.

[0036] The method for producing the silicon nitride powder described above is to prepare a raw material silicon nitride powder by pulverizing the powder into a silicon nitride powder having a BET specific surface area of ​​10.3 m. 2The method includes a step of adjusting the particle size so that (D90-D10) / D50 is 1.70 or less, where D10 is the particle size at which the cumulative value from the smallest particle size reaches 10% of the total, D50 is the particle size at which the cumulative value from the smallest particle size reaches 50% of the total, and D90 is the particle size at which the cumulative value from the smallest particle size reaches 90% of the total, in a cumulative volumetric particle size distribution measured using a particle size distribution analyzer using a laser diffraction / scattering method. This manufacturing method uses silicon nitride powder as a raw material and adjusts the particle size, resulting in a relatively simple method of producing the desired silicon nitride powder. Therefore, silicon nitride powder can be produced at a lower cost than the manufacturing cost of silicon nitride powder with a high degree of gelatinization as a raw material for sintered compacts that exhibit excellent bending strength.

[0037] In the above-mentioned method for producing silicon nitride powder, the starting silicon nitride powder may have a gelatinization rate of 96.0% by mass or less. If such starting silicon nitride powder is available, it may be used as is, or a starting silicon nitride powder having a gelatinization rate of 96.0% by mass or less may be separately prepared and used. In other words, the method for producing silicon nitride powder may include a step of preparing the starting silicon nitride powder before the step of adjusting the particle size. The step of preparing the starting silicon nitride powder may include at least one step selected from the group consisting of a mixing step of mixing silicon-containing starting materials to obtain a mixture, a nitriding step of firing the mixture to obtain a nitride, and a post-treatment step of treating the nitride with hydrofluoric acid. The starting silicon nitride powder may be obtained through the mixing step, the nitriding step, and the post-treatment step.

[0038] In the mixing step, raw silicon powder and fluoride are mixed. The fluoride mixed with the silicon powder functions as a nitriding aid to promote nitriding, and examples thereof include fluorides of elements such as Li, Na, K, Mg, Ca, Sr, and Ba. The fluoride content may be 0.5 to 1.5 parts by mass per 100 parts by mass of silicon powder. If fluoride is not added, nitriding is not promoted and unreacted metallic silicon remains, resulting in a decrease in the purity of the raw silicon nitride powder and a decrease in productivity. The purity of the silicon powder mixed with the fluoride may be, for example, 99.0 to 99.9% by mass. Silicon powder with such a purity can be obtained by treating with a mixed acid containing hydrofluoric acid (hydrofluoric acid) and hydrochloric acid.

[0039] In the nitriding step, raw materials containing silicon powder and fluoride are fired in a mixed atmosphere containing nitrogen and at least one selected from the group consisting of hydrogen and ammonia to obtain nitride. The total content of hydrogen and ammonia in the mixed atmosphere may be, for example, 10 to 40% by 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.

[0040] In the post-treatment step, the nitride is mixed with hydrofluoric acid having a hydrogen fluoride concentration of, for example, 1.0 to 4.0 mass % and treated. For example, the nitride may be dispersed in hydrofluoric acid for treatment. The hydrogen fluoride concentration in the hydrofluoric acid may be, for example, 1.3 to 2.3 mass %. The temperature of the hydrofluoric acid in the post-treatment step is, for example, 40 to 80°C. The time for immersing the nitride in hydrofluoric acid is, for example, 1 to 10 hours.

[0041] The raw silicon nitride powder preparation process described above allows the adjustment of the alpha-conversion rate and the preparation of the raw silicon nitride powder. This production method corresponds to the so-called direct nitridation method, and allows the production of silicon nitride powder at a lower production cost than the imide method. Although the raw silicon nitride powder produced by the direct nitridation method has a low alpha-conversion rate and contains a certain amount of impurities, it can be suitably used for the production of silicon nitride powder as a sintering raw material according to the present disclosure, which can provide a silicon nitride sintered body having high bending strength while keeping production costs low.

[0042] The gelatinization rate of the raw silicon nitride powder may be 96.0% by mass or less, 95.0% by mass or less, or 94.0% by mass or less. Silicon nitride powders with a gelatinization rate in the above-mentioned range have low production costs and excellent productivity, and can be easily obtained or prepared. Even when raw material powders with a gelatinization rate in the above-mentioned range are used, silicon nitride powders that can be used to produce sintered bodies with excellent bending strength can be provided. The gelatinization rate may be 89.0% by mass or more, or 90.0% by mass or more.

[0043] The BET specific surface area of ​​the raw material silicon nitride powder is, for example, 5.0 to 20.0 m 2 / g, and 10.3m 2 / g, and may be less than 10.1m 2 Even if the BET specific surface area of ​​the raw silicon nitride powder is within this range, the particle size may be adjusted in the particle size adjusting step so that the BET specific surface area is less than 10.3 m / g. 2 From such a raw silicon nitride powder, a silicon nitride powder that can be used to produce a sintered body having excellent bending strength can be obtained.

[0044] In the particle size adjustment step, the resulting silicon nitride powder was adjusted to have a BET specific surface area of ​​10.3 m 2 / g or more, and the (D90-D10) / D50 is 1.70 or less. Examples of methods for adjusting the particle size include adjusting the particle size by mixing powders of different particle sizes, adjusting the particle size by pulverization, and adjusting the particle size by classification.

[0045] The particle size adjustment by pulverization may be carried out by dry or wet pulverizing the raw silicon nitride powder (hereinafter also referred to as the pulverization step). The pulverization step may be carried out in multiple stages, such as coarse pulverization and fine pulverization. For example, the pulverization step may include two steps: a ball mill pulverization step and a vibration mill pulverization step.

[0046] The ball filling rate in the container in the ball milling step may be, for example, 30 to 70% by volume. The lower limit of the ball filling rate in the container may be, for example, 50% by volume or 60% by volume, based on the volume of the container. The upper limit of the ball filling rate in the container may be, for example, 65% by volume, based on the volume of the container.

[0047] The time for the pulverization treatment (pulverization time) in the ball mill pulverization step may be, for example, 5 to 15 hours, or 8 to 12 hours, which allows the silicon nitride powder to be sufficiently fine.

[0048] The pulverized product obtained in the ball mill pulverization step may be further pulverized in a vibration mill pulverization step. The ball filling rate in the container in the vibration mill pulverization step may be, for example, 50 to 80 volume %, or 60 to 75 volume %. The pulverization time (pulverization time) in the vibration mill pulverization step may be 8 to 20 hours, or 12 to 17 hours. This allows the raw silicon nitride powder to be sufficiently fine, adjusts D97, and more easily obtains the silicon nitride powder specified in the present application.

[0049] The particle size adjustment process involves dry classification to determine the BET specific surface area of ​​the powder. 2It is also possible to employ a method for obtaining silicon nitride powder so that the silicon nitride powder has a D90 / g or more and a (D90-D10) / D50 ratio of 1.70 or less. For example, if the silicon nitride powder contains agglomerated particles (also called secondary particles) formed by agglomeration of multiple silicon nitride primary particles, this increases the particle size of the silicon nitride powder as a whole. Therefore, the particle size distribution of the silicon nitride powder can be adjusted by eliminating at least some of the agglomerated particles, and the particle size distribution of the silicon nitride powder can be adjusted by eliminating some of the silicon nitride primary particles with large particle diameters.

[0050] Dry classification may be performed by sieving or by using an air classifier. The air classifier may be a swirling air classifier that uses primary and secondary air. For example, the MP-150 manufactured by Nippon Pneumatic Mfg. Co., Ltd. may be used as such an air classifier.

[0051] The operating conditions of the air classifier can be adjusted, for example, by adjusting the classification air volume and the louver opening of the air guide vanes to control the swirling air velocity. Here, the classification air volume refers to the amount of air required to generate a swirling airflow within the air classifier. The primary air introduced into the air classifier becomes a swirling flow as it passes through the air guide vanes. By introducing raw silicon nitride powder into the swirling flow, a large centrifugal force can be applied to coarse particles, allowing for easy classification. Furthermore, by introducing air (secondary air) that is more compressed than the primary air into the swirling flow, the accuracy of classification can be further improved. Thus, for example, by adjusting the classification air volume and louver opening of the air classifier and adjusting the swirling air velocity, the BET specific surface area of ​​the raw silicon nitride powder can be increased to 10.3 m. 2 It is possible to produce silicon nitride powder with an adjusted D90-D10) / D50 of 1.70 or less and a D90-D10) / D50 of 1.70 or less. When sintered, such silicon nitride powder has high bending strength.

[0052] An example of the operating conditions for the air classifier is as follows: Classification air volume: 1 to 4 m 3 / min, and 1 to 3 m 3 / min. The louver opening of the air guide vane may be 12 mm or more and 2 mm or less, for example, in the range of 2 to 8 mm. The louver opening refers to the distance between the centers of the louvers. The louver opening can also be expressed as the angle of the louvers. In this case, the angle may be 70°, 80°, or 90°. The louver opening indicates the size of the inlet for compressed air (secondary air). When the distance between the centers of the louvers is small or the louver angle is large, the inlet becomes smaller, increasing the pressure and the airflow rotation speed. The secondary air pressure may be 0.2 to 1.0 MPa or 0.4 to 0.8 MPa. By classifying under these conditions, the swirling airflow speed can be adjusted to 100 to 300 m / s, and agglomerated particles that are not sufficiently pulverized in the pulverization process can be removed as coarse particles with high precision. Furthermore, the desired silicon nitride powder can be produced more easily than by mixing powders of different particle sizes or adjusting the particle size by pulverization.

[0053] From the viewpoint of accelerating the classification of the silicon nitride powder and further reducing D97 to obtain silicon nitride powder that will become a sintered body with a large Weibull coefficient, the swirling air velocity may be 150 m / s or more, or 200 m / s or more. Also, from the viewpoint of improving the yield after classification and increasing productivity, the swirling air velocity may be 300 m / s or less, or 250 m / s or less. The swirling air velocity is expressed as V, where V is the flow velocity (m / s), Q is the flow rate (m 3 / s), and A is the cross-sectional area of ​​the swirling field (m 2 ) and can be calculated from the equation V=Q / A. The swirling air velocity can also be measured using, for example, a thermal airflow transducer (product name: TA10 ZG2d, manufactured by Centronic Co., Ltd.).

[0054] The D97 may be adjusted to, for example, 2.25 μm or less, 2.00 μm or less, 1.80 μm or less, 1.60 μm or less, or 1.40 μm or less. Silicon nitride powder with a D97 adjusted to this range further suppresses the generation of coarse particles after sintering, and can produce sintered bodies with higher bending strength. The D97 may be adjusted to, for example, 0.80 μm or more, 0.90 μm or more, or 1.00 μm or more.

[0055] In the particle size adjustment step, D90 may be adjusted to, for example, 2.00 μm or less, 1.50 μm or less, or 1.30 μm or less. Silicon nitride powders with an upper D90 value in this range have smaller particle sizes, which further suppresses the generation of coarse particles in the sintered body, resulting in a sintered body with even higher bending strength. D90 may also be adjusted to, for example, 0.60 μm or more, 0.80 μm or more, or 1.20 μm or more.

[0056] In the particle size adjustment step, D50 may be adjusted to, for example, 1.00 μm or less, 0.90 μm or less, 0.80 μm or less, or 0.70 μm or less. Silicon nitride powders with an upper D50 value in this range have smaller particle sizes, which further suppresses the generation of coarse particles in the sintered body, resulting in a sintered body with even higher bending strength. D50 may also be adjusted to, for example, 0.30 μm or more, or 0.40 μm or more.

[0057] In the particle size adjustment step, D10 may be adjusted to, for example, 0.60 μm or less, 0.50 μm or less, or 0.40 μm or less. Silicon nitride powders with an upper limit of D10 in this range have smaller particle sizes, which further suppresses the generation of coarse particles in the sintered body, resulting in a sintered body with even higher bending strength. D10 may also be adjusted to, for example, 0.05 μm or more, or 0.10 μm or more.

[0058] In the particle size adjustment step, the value of (D90-D10) / D50 may be adjusted to 1.60 or less, or may be adjusted to 1.50 or less. When multiple sintered bodies are produced using such silicon nitride powder, the variation in bending strength of each sintered body can be reduced. In other words, the Weibull modulus of the sintered body can be increased.

[0059] The (D90-D10) / D50 value may be adjusted to, for example, 1.30 or greater, or 1.40 or greater. Silicon nitride powders with a (D90-D10) / D50 value in this range have smaller particle size variations. This further homogenizes the environment in which the silicon nitride powder dissolves in the liquid phase of the sintering aid and reprecipitates during the production of a sintered body, thereby more effectively suppressing the variation in particle size of growing β-phase particles. This further improves the bending strength of the resulting sintered body. The (D90-D10) / D50 value of the sintered body may be adjusted to, for example, 1.30 to 1.70.

[0060] The silicon nitride powder of this embodiment can be produced by the above steps. However, the above-mentioned production method is only an example and is not limited thereto. Since the silicon nitride powder of this embodiment has reduced coarse particles, it can be suitably used as a raw material for sintered bodies having high bending strength.

[0061] When producing a sintered body using silicon nitride powder as a raw material, the sintering raw material containing the silicon nitride powder is molded and sintered. In addition to the silicon nitride powder, the sintering raw material may also contain an oxide-based sintering aid. Examples of the oxide-based sintering aid include YO. 3、 Examples include MgO and Al2O3. The content of the oxide-based sintering aid in the sintering raw material may be, for example, 3 to 10 mass %.

[0062] The above-mentioned sintering raw materials are pressed at a molding pressure of, for example, 3.0 to 200 MPa to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP. In some cases, the process may include two steps, uniaxial pressing and CIP. Alternatively, the molded body may be fired while being molded by hot pressing. The firing of the molded body may include two steps, a primary firing step and a secondary firing step, which are carried out in an inert gas atmosphere such as nitrogen gas or argon gas. The pressure in the primary firing step may be, for example, 0.7 to 1 MPa. The firing temperature may be, for example, 1700 to 1900°C or 1750 to 1850°C. The firing time at the firing temperature may be, for example, 0.5 to 20 hours or 1.5 to 16 hours. The heating rate to the firing temperature may be, for example, 1.0 to 10.0°C / min.

[0063] The pressure in the secondary firing step in which the sintered body obtained in the primary firing step is fired may be, for example, 70 MPa or more, preferably 100 MPa or more. The firing temperature may be, for example, 1650 to 1850°C, or 1700 to 1800°C. The firing time at the firing temperature may be, for example, 0.5 to 5 hours, or 1 to 2 hours. The rate of temperature rise up to the firing temperature may be, for example, 1.0 to 10.0°C / hour.

[0064] The resulting sintered body has a fine structure with excellent uniformity due to reduced coarse particles. It also has a sufficiently dense structure, resulting in excellent bending strength. Furthermore, the reduced variation in particle size reduces the variation in the properties of the silicon nitride sintered body.

[0065] The bending strength of the sintered body can be 920 MPa or more, 930 MPa or more, or 950 MPa or more. Such silicon nitride sintered bodies have excellent strength and can be suitably used as parts for various industries. The bending strength of the sintered body is a three-point bending strength, which can be measured using a commercially available bending strength tester in accordance with JIS R 1601:2008. The bending strength of the sintered body may be 1100 MPa or less.

[0066] The fracture toughness of the sintered body is 4.5 (MPa / m 2 ) or more, 4.8 (MPa / m 2 ) or more, or 5.2 (MPa / m 2 ) or more. Such silicon nitride sintered bodies have excellent strength and can be suitably used as parts for various industries. The fracture toughness value of the sintered bodies can be measured in accordance with JIS R 1607:2015. The fracture toughness value of the sintered bodies can be 6.0 (MPa / m 2 ) or less.

[0067] The Vickers hardness of the sintered body can be 1.400 HV or more, 1.420 HV or more, or 1.430 HV or more. Such silicon nitride sintered bodies have excellent wear resistance and can be suitably used as parts for various industries. The Vickers hardness of the sintered body can be measured in accordance with JIS R1610:2003. The Vickers hardness of the sintered body may be 1.600 HV or less.

[0068] The silicon nitride sintered body has a highly uniform microstructure, and the above-mentioned distribution of bending strength is sufficiently suppressed. The silicon nitride sintered body has a relatively large Weibull coefficient in a Weibull statistical analysis of its bending strength. The Weibull coefficient of the silicon nitride sintered body for bending strength is, for example, 10.0 or more, 12.0 or more, 13.0 or more, 15.0 or more, 17.0 or more, or 20.0 or more. The Weibull coefficient can also be 25.0 or less.

[0069] Weibull statistics are used to evaluate the distribution of bending strength. When a Weibull plot is created for sintered silicon nitride, with the fracture probability F(σ) on the vertical axis and the bending strength σ (strength at fracture, transverse strength) on the horizontal axis, the slope m is the Weibull coefficient. A large Weibull coefficient means that the distribution of bending strength is narrow and close to a normal distribution. The fracture probability F(σ) in a Weibull plot is given by the following formula (1): F(σ)=1-exp[-(σ / η)m]···(1) In the above formula (1), η is a fitting parameter.

[0070] Although several embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments. [Example]

[0071] The present disclosure will be described in more detail below with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0072] (Comparative Example 1) <Preparation of raw silicon nitride powder> A raw material powder was prepared by blending 1 part by mass of fluorite with 100 parts by mass of silicon powder. That is, the raw material powder contained 1 part by mass of fluoride (fluorite) with 100 parts by mass of silicon powder. An alumina container having a main body with a recess and a lid was prepared. The raw material powder was filled into the recess. The filling shape of the raw material powder was a rectangular parallelepiped, and the filling height was 45 mm. The recess of the main body was covered with the lid, and the raw material powder was placed in the alumina container. The raw material powder placed in the container was fired using the following procedure.

[0073] The container containing the raw material powder was placed in an electric furnace and fired under the following temperature conditions. The temperature was increased from 20°C to 1150°C at a rate of 5°C / min. After holding at 1150°C for 8 hours, the temperature was increased to 1450°C at a rate of 0.15°C / min. After holding at 1450°C for 4 hours, the material was allowed to cool naturally to room temperature. The atmosphere in the electric furnace was nitrogen gas. The time from the start of holding at 1150°C to the end of holding at 1450°C was 45 hours.

[0074] The resulting ingot was coarsely crushed and then wet-pulverized for 8 hours in an attritor mill with a ball filling rate of 70% by volume, followed by drying in a nitrogen atmosphere.

[0075] The pulverized material obtained by wet pulverization was immersed in hydrofluoric acid (hydrogen fluoride concentration: 1.6% by mass) at 70°C for 4 hours for acid treatment. The pulverized material was then removed from the hydrofluoric acid, washed with water, and dried under a nitrogen atmosphere. In this way, raw silicon nitride powder (powder a) was obtained.

[0076] (Comparative Example 2) A raw material silicon nitride powder (powder b) of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.8 mass %.

[0077] (Comparative Example 3) A raw material silicon nitride powder (powder c) of Comparative Example 3 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.9 mass %.

[0078] Comparative Example 4 A raw material silicon nitride powder (powder d) of Comparative Example 4 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.7 mass %.

[0079] (Comparative Example 5) A raw material silicon nitride powder (powder e) of Comparative Example 5 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 1.4 mass %.

[0080] (Comparative Example 6) A raw material silicon nitride powder (powder f) of Comparative Example 6 was prepared in the same manner as in Comparative Example 1, except that the hydrogen fluoride concentration of the hydrofluoric acid was changed to 2.0 mass %.

[0081] Example 1 <Classification of raw silicon nitride powder> The silicon nitride powder (powder a) of Comparative Example 1 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1m 3 / min Louver opening in main air guide vane: 2mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0082] Example 2 The silicon nitride powder (powder b) of Comparative Example 2 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1m 3 / min Louver opening in main air guide vane: 2mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0083] Example 3 The silicon nitride powder (powder c) of Comparative Example 3 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1m 3 / min Louver opening in main air guide vane: 2mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0084] Example 4 The silicon nitride powder (powder d) of Comparative Example 4 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., product name: EVX-1) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 1.1m 3 / min Louver opening in main air guide vane: 2mm Secondary air pressure: 0.4 MPa Swirling air velocity: 230 m / s

[0085] Example 5 The silicon nitride powder (powder b) of Comparative Example 2 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., trade name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3m 3 / min Louver opening in main air guide vane: 3mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160m / s

[0086] Example 6 The silicon nitride powder (powder c) of Comparative Example 3 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., trade name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3m 3 / min Louver opening in main air guide vane: 3mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160m / s

[0087] Example 7 The silicon nitride powder (powder d) of Comparative Example 4 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nippon Pneumatic Mfg. Co., Ltd., trade name: MP-150) to obtain a silicon nitride powder. The classification conditions were as follows: Classification air volume: 2.3m 3 / min Louver opening in main air guide vane: 3mm Secondary air pressure: 0.6 MPa Swirling air velocity: 160m / s

[0088] Example 8 The silicon nitride powder (powder e) of Comparative Example 5 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nisshin Engineering Inc., product name: AC-20) to obtain a silicon nitride powder. The classification conditions were as follows. Classification air volume: 2.0m 3 / min Louver opening angle of main air guide vane: 90° Secondary air pressure: 0.7 MPa Swirling air velocity: 200m / s

[0089] Example 9 The silicon nitride powder (powder e) of Comparative Example 5 was used as a raw material silicon nitride powder and was classified using an air classifier (manufactured by Nisshin Engineering Inc., product name: AC-20) to obtain a silicon nitride powder. The classification conditions were as follows. Classification air volume: 2.4m 3 / min Louver opening angle of main air guide vane: 90° Secondary air pressure: 0.8 MPa Swirling air velocity: 300m / s

[0090] [Measurement of alpha conversion rate] The gelatinization rate of silicon nitride powder was measured as follows. X-ray diffraction of silicon nitride powder was performed using CuKα radiation using an X-ray diffractometer (Rigaku, model Ultima IV). The α phase was represented by the diffraction line intensity Ia102 of the (102) plane and the diffraction line intensity Ia210 of the (210) plane. The β phase was represented by the diffraction line intensity Ib101 of the (101) plane and the diffraction line intensity Ib210 of the (210) plane. Using these diffraction line intensities, the gelatinization rate was calculated according to the following formula. Table 1 shows the measurement results for each example and comparative example. αization rate (mass%) = (Ia102+Ia210) / (Ia102+Ia210+Ib101+Ib210)×100

[0091] [Measurement of particle size distribution] The particle size distribution of silicon nitride powder was measured by laser diffraction and scattering. The measurement was performed in accordance with the method described in JIS R 1629:1997, "Method for measuring particle size distribution of fine ceramic raw materials by laser diffraction and scattering." For particle size distribution measurement, 60 mg of silicon nitride powder was weighed into a 500 mL container. This was mixed with 2 mL of a 20% aqueous solution of sodium hexametaphosphate and 200 g of water as dispersants. This container was placed in a Sharp Corporation ultrasonic disperser so that the entire dispersion was immersed, and ultrasonic dispersion was performed for 1 minute. The particle size distribution measurement described above was performed using the ultrasonically dispersed sample. D10, D50, D90, and D97 were determined for each example and comparative example. Furthermore, (D90-D10) / D50 was calculated from the obtained results. The results are shown in Table 1.

[0092] [BET specific surface area measurement] The BET specific surface area of ​​the silicon nitride powder was measured by the single-point BET method using nitrogen gas in accordance with JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method." The measurement results for each example and comparative example are shown in Table 1.

[0093] [Oxygen content measurement] The oxygen content of the silicon nitride powder was determined as the total amount of oxygen. The oxygen content was measured using an oxygen / nitrogen analyzer (manufactured by Horiba, Ltd., model EMGA-920). Specifically, the silicon nitride powder was heated in a helium atmosphere from 20°C to 2000°C at a heating rate of 8°C / s, and the amount of oxygen released was quantified to determine the oxygen content (mass%) of the entire silicon nitride powder. The measurement results for each example and comparative example are shown in Table 1.

[0094] <Preparation of sintered body> For each example and comparative example, 91 parts by weight of silicon nitride powder, 5 parts by weight of Y2O3 powder with an average particle size of 1.5 μm, and 4 parts by weight of Al2O3 powder with an average particle size of 1.2 μm were blended and wet-mixed in methanol for 4 hours. The dried mixed powder was then molded under a pressure of 10 MPa and then further molded by CIP at a pressure of 100 MPa. The resulting compact was placed in a carbon crucible along with a packed powder consisting of a mixed powder of silicon nitride powder and BN powder. It was then heated to 900°C at 10°C / min under vacuum, nitrogen was introduced, and the mixture was heated to 1400°C at 5°C / min under a nitrogen atmosphere. It was then maintained at 1400°C for 2 hours and then heated to 1800°C at 5°C / min. It was then maintained at 1800°C for 2 hours, cooled to 1000°C at 5°C / min, and naturally cooled to produce a primary silicon nitride sintered body. The obtained sintered body was further pressurized to 100 MPa in a nitrogen atmosphere, heated to 1700°C at a temperature increase rate of 5°C / hour, and then maintained at 1700°C for 1 hour to perform a secondary firing process, producing a silicon nitride sintered body.

[0095] [Measurement of bending strength of sintered body and calculation of Weibull modulus] The bending strength was a three-point bending strength, and was measured in accordance with JIS R 1601:2008 using a commercially available flexural strength tester (manufactured by Shimadzu Corporation, device name: AG-2000). The measurement was performed at room temperature (20°C). The results are shown in Table 2. Twenty sintered bodies were prepared for each silicon nitride powder in each example, and the Weibull coefficient was calculated based on the distribution of their bending strengths. Similar measurements were performed for each example and comparative example. The results are shown in Table 2.

[0096] [Measurement of fracture toughness of sintered body] Fracture toughness (K IC ) is a value measured by the IF method in accordance with JIS R1607:2015 using a commercially available measuring device (manufactured by Matsuzawa Corporation, device name: Via-F). The results are shown in Table 2.

[0097] [Measurement of Vickers hardness of sintered body] The Vickers hardness was measured in accordance with JIS R 1610:2003 using an electric Vickers hardness tester (manufactured by Matsuzawa Co., Ltd., trade name: Via-F). The results are shown in Table 2.

[0098] [Observation of cross section of sintered body] A portion of the cross section of the produced sintered body was observed with a scanning electron microscope. The results are shown in Figures 1, 2, and 3. Figure 1 is a scanning electron microscope photograph showing a portion of the cross section of a sintered body prepared using the silicon nitride powder of Comparative Example 4. Similarly, Figure 2 shows the cross section of the sintered body of Example 4, and Figure 3 shows the cross section of the sintered body of Example 7. Comparing Figures 1, 2, and 3, it was found that the sintered body of Figure 1 had the most coarse particles, while the sintered body of Figure 2 had the finest particles.

[0099] [Table 1]

[0100] [Table 2]

[0101] A comparison of the Examples and Comparative Examples confirmed that the sintered bodies obtained using the silicon nitride powders of the Examples, which have large BET specific surface areas and small (D90-D10) / D50 values, tend to have higher bending strengths than the sintered bodies obtained using the silicon nitride powders of the Comparative Examples. Furthermore, the Weibull coefficient of bending strength was also improved for the sintered bodies obtained using the silicon nitride powders of the Examples, confirming that sintered bodies with small bending strength variations could be consistently obtained. The above results also confirmed that it is possible to prepare sintered bodies with high bending strength. [Industrial Applicability]

[0102] According to the present disclosure, it is possible to provide a silicon nitride powder that can be used to prepare a sintered body having excellent bending strength.

Claims

1. BET specific surface area is 10.3m ２ / g or more, In the cumulative distribution of particle diameters on a volume basis measured with a particle size distribution analyzer using a laser diffraction / scattering method, the particle diameter when the cumulative value from small particle diameters reaches 10% of the total is defined as D10, the particle diameter when the cumulative value from small particle diameters reaches 50% of the total is defined as D50, and the particle diameter when the cumulative value from small particle diameters reaches 90% of the total is defined as D90. Silicon nitride powder, wherein (D90-D10) / D50 is 1.70 or less.

2. 2. The silicon nitride powder according to claim 1, wherein the gelatinization rate is 96.0 mass% or less.

3. 3. The silicon nitride powder according to claim 1, wherein, when D97 is the particle diameter when the integrated value from small particle diameters reaches 97% of the total in the cumulative distribution, D97 is 2.25 μm or less.

4. 3. The silicon nitride powder according to claim 1, wherein the oxygen content is 0.65 to 1.60 mass%.

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