Silicon nitride powder

Abstract

To provide a silicon nitride powder having an appearance that is white in color, with suppressed blackness.SOLUTION: A silicon nitride powder containing silicon nitride and non-nitride silicon, wherein a ratio (RSi / A) of a mass ratio RSi of non-nitride silicon in the silicon nitride powder with respect to a BET specific surface area A of the silicon nitride powder is 4×10-4 g / m2 or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to silicon nitride powder. [Background technology]

[0002] Silicon nitride is a material that has excellent strength, hardness, toughness, heat resistance, corrosion resistance, thermal shock resistance, etc. For this reason, silicon nitride sintered bodies are used in various industrial parts such as die-casting machines and melting furnaces, as well as automobile parts, etc. Furthermore, because silicon nitride sintered bodies also have excellent mechanical properties at high temperatures, they are being considered for use in gas turbine parts, which require high-temperature strength and high-temperature creep properties.

[0003] It is known that the carbon and oxygen contained in silicon nitride powder, a sintering raw material, affect the strength of silicon nitride sintered bodies. Patent Document 1 proposes using silicon nitride powder obtained by imide decomposition to reduce the amounts of oxygen and carbon in order to obtain silicon nitride sintered bodies with high strength. Patent Document 2 also proposes a technology for obtaining silicon nitride sintered bodies with high bending strength using silicon nitride powder with a β fraction of 30 to 100% and an oxygen content of less than 0.5 wt%. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-193914 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-262756 Summary of the Invention [Problem to be solved by the invention]

[0005] Silicon nitride powder, the raw material for silicon nitride sintered bodies, is usually white in appearance, but some production lots have a blackish tinge. The appearance of silicon nitride powder is thought to vary due to slight differences in the raw materials and conditions used in producing the silicon nitride powder, but the exact cause is not yet clear. Differences in the color appearance of silicon nitride powder between production lots are thought to indicate differences in the properties of the silicon nitride powder, which may ultimately affect the properties of the silicon nitride sintered bodies produced from the silicon nitride powder.

[0006] Therefore, an object of the present invention is to provide a silicon nitride powder that has a white appearance with reduced black tinge. [Means for solving the problem]

[0007] The present invention includes, for example, the following [1] to [9]. [1] A silicon nitride powder containing silicon nitride and non-nitrided silicon, The mass ratio R of the unformed silicon nitride in the silicon nitride powder to the BET specific surface area A of the silicon nitride powder Si The ratio (R Si / A) is 4 x 10 -4 g / m 2 Silicon nitride powder, which is: [2] The silicon nitride powder further contains carbon, The mass ratio R to the BET specific surface area A Si and the mass ratio R of the carbon in the silicon nitride powder. C The ratio of the product of (R Si ×R C / A) is 4.5 × 10 -7 g / m 2 The silicon nitride powder according to [1], which is as follows: [3] The silicon nitride powder further contains carbon, The mass ratio R to the BET specific surface area A Si and the mass ratio R of the carbon in the silicon nitride powder. C The ratio of the sum of (R Si +R C) / A) is 5.5 × 10 -4 g / m 2 The silicon nitride powder according to [1] or [2], which is: [4] The silicon nitride powder further contains iron, The mass ratio R to the BET specific surface area A Si and the mass ratio R of the iron in the silicon nitride powder. Fe The ratio of the product of (R Si ×R Fe / A) is 6.5 × 10 -7 g / m 2 The silicon nitride powder according to any one of [1] to [3] below: [5] The silicon nitride powder further contains iron, The mass ratio R to the BET specific surface area A Si and the mass ratio R of the iron in the silicon nitride powder. Fe The ratio of the sum of (R Si +R Fe ) / A) is 6 x 10 -4 g / m 2 The silicon nitride powder according to any one of [1] to [4] below: [6] The silicon nitride powder further contains iron and carbon; The mass ratio R to the BET specific surface area A Si and the mass ratio R of the carbon in the silicon nitride powder. C and the mass ratio R of the iron in the silicon nitride powder. Fe The ratio of the product of (R Si ×R C ×R Fe / A) is 7 x 10 -10 g / m 2 The silicon nitride powder according to any one of [1] to [5] below: [7] The silicon nitride powder further contains iron and carbon; The mass ratio R to the BET specific surface area A Si and the mass ratio R of the carbon in the silicon nitride powder. C and the mass ratio R of the iron in the silicon nitride powder. Fe The ratio of the sum of (R Si +R C +RFe ) / A) is 7.5 x 10 -4 g / m 2 The silicon nitride powder according to any one of [1] to [6] below: [8] The mass ratio R Si The silicon nitride powder according to any one of [1] to [7], wherein is 0.003 or less. [9] The silicon nitride powder according to any one of [1] to [8], wherein the α rate is 91% or more. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide silicon nitride powder that has a white appearance with reduced black tinge. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described. However, the following embodiments are merely examples for explaining the present invention and are not intended to limit the present invention to the following content. Numerical ranges indicated with the symbol "to" include the lower and upper limits. That is, a numerical range indicated as "x to y" means greater than or equal to x and less than or equal to y. Numerical ranges in which the upper or lower limit of each numerical range in each embodiment is replaced with the numerical value of any of the examples are also included in the present invention. In each embodiment, both cases in which one of the multiple materials exemplified in parallel is included alone and cases in which two or more of the multiple materials are included in combination are included. In this specification, the mass ratio of each component in a silicon nitride powder means the mass ratio of each component in the entire silicon nitride powder, unless otherwise specified.

[0010] The silicon nitride powder (Si3N4 powder) according to one embodiment is a silicon nitride powder containing silicon nitride and un-silicon nitride, and the mass ratio R of the un-silicon nitride in the silicon nitride powder to the BET specific surface area A of the silicon nitride powder is Si The ratio (R Si / A) is 4 x 10 -4 g / m 2 The following is the result.

[0011] The inventors have found that the lower the mass ratio of un-silicon nitride in silicon nitride powder, the whitish the appearance of the silicon nitride powder tends to be. Furthermore, the larger the BET specific surface area, the larger the specific surface area per unit mass. Furthermore, the mass ratio R of un-silicon nitride in silicon nitride powder to the BET specific surface area A of the silicon nitride powder is. Si The ratio (R Si When the ratio (A) is small to a certain extent and the amount of unformed silicon nitride present in the silicon nitride powder is sufficiently small, the silicon nitride powder has a white appearance and blackness can be suppressed.

[0012] Mass ratio R of uncompounded silicon nitride in silicon nitride powder Si From the viewpoint of further suppressing the blackening of the silicon nitride powder, the mass ratio R may be 0.003 or less, 0.0025 or less, 0.002 or less, or 0.0015 or less. Si From the viewpoint of production costs, the mass ratio R may be 0.001 or more. The non-nitride silicon in the silicon nitride powder means silicon other than silicon nitride. Si can be measured by the method described in the Examples below.

[0013] Mass ratio R of uncompounded silicon nitride in silicon nitride powder Si For example, the mass ratio of unsilicon nitride can be reduced by increasing the firing temperature in the firing step or by extending the firing time.Furthermore, the mass ratio of unsilicon nitride can be reduced by wet-pulverizing the silicon nitride powder.

[0014] The BET specific surface area A of the silicon nitride powder is set to 15m from the viewpoint of further suppressing the black color of the silicon nitride powder. 2 / g or less, 13m 2 / g or less, 11m 2 / g or less, 9m 2 / g or less, 7m 2 / g or less, or 6.7m 2 / g or less. From the viewpoint of improving the sinterability of the silicon nitride powder, the BET specific surface area A of the silicon nitride powder may be 5 m 2 / g or more, or 6m 2 / g or more. In this specification, the BET specific surface area A 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." In the case of a direct nitriding process, the BET specific surface area A can be adjusted by changing the grinding conditions when grinding the fired product after nitriding.

[0015] The mass ratio R of uncommon silicon nitride in the silicon nitride powder to the BET specific surface area A of the silicon nitride powder Si The ratio (R Si / A) is 4 x 10 -4 g / m 2 The ratio (R Si / A) is 3.5 × 10 -4 g / m 2 Below, 3 x 10 -4 g / m 2 Below, 2.8 x 10 -4 g / m 2 Below, 2.6 x 10 -4 g / m 2 Below, 2.4 x 10 -4 g / m 2 or less, or 2.2 x 10 -4 g / m 2 The ratio (R Si / A) is set to 1×10 from the viewpoint of further suppressing the black color of the silicon nitride powder. -4 g / m 2 That's it, 1.2 x 10 -4 g / m 2 That's it, 1.4 x 10 -4 g / m 2 or more, or 1.6 x 10 -4 g / m 2 It may be more than that.

[0016] The silicon nitride powder may further contain carbon. When the silicon nitride powder further contains carbon, the mass ratio R of carbon in the silicon nitride powder CFrom the viewpoint of further suppressing the black color of the silicon nitride powder, the mass ratio R may be 0.002 or less, 0.0015 or less, or 0.0012 or less. C From the viewpoint of further suppressing the black color of the silicon nitride powder, the mass ratio R may be 0.0005 or more, 0.0007 or more, or 0.0009 or more. C can be measured by the method described in the Examples below.

[0017] Mass ratio of carbon in silicon nitride powder R C The mass ratio of carbon can be reduced by extending the degreasing process time.

[0018] When the silicon nitride powder further contains carbon, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and the carbon mass ratio R C The ratio of the product of (R Si ×R C / A) is 4.5 × 10 -7 g / m 2 Below, 4 x 10 -7 g / m 2 Below, 3.5 x 10 -7 g / m 2 or less, or 3 x 10 -7 g / m 2 The ratio (R Si ×R C / A) is set to 1×10 from the viewpoint of further suppressing the black color of the silicon nitride powder. -8 g / m 2 That's it, 5 x 10 -8 g / m 2 That's it, 1 x 10 -7 g / m 2 or more, or 1.5 x 10 -7 g / m 2 It may be more than that.

[0019] When the silicon nitride powder further contains carbon, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and the carbon mass ratio R C The ratio of the sum of (RSi +R C ) / A) is set to 5.5 × 10 in order to further suppress the black color of the silicon nitride powder. -4 g / m 2 Below, 5 x 10 -4 g / m 2 Below, 4.5 x 10 -4 g / m 2 or less, or 4 x 10 -4 g / m 2 The ratio (R Si +R C ) / A) is set to 1×10 -4 g / m 2 That's it, 2 x 10 -4 g / m 2 That's it, 2.5 x 10 -4 g / m 2 or more, or 3 x 10 -4 g / m 2 It may be more than that.

[0020] The silicon nitride powder may further contain iron. When the silicon nitride powder further contains iron, the mass ratio R of iron in the silicon nitride powder is Fe From the viewpoint of improving the sinterability of the silicon nitride powder, the mass ratio R may be 0.0025 or less, 0.002 or less, or 0.0018 or less. Fe From the viewpoint of manufacturing costs, the mass ratio R may be 0.0001 or more, 0.001 or more, or 0.0012 or more. Fe can be measured by the method described in the Examples below.

[0021] When the silicon nitride powder further contains iron, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and iron mass ratio R Fe The ratio of the product of (R Si ×R Fe / A) is 6.5 × 10 -7 g / m 2 Below, 5.5 x 10 -7 g / m 2 Below, 5 x 10 -7 g / m2 or less, or 4.5 x 10 -7 g / m 2 The ratio (R Si ×R Fe / A) is set to 1×10 from the viewpoint of further suppressing the black color of the silicon nitride powder. -10 g / m 2 That's it, 1 x 10 -9 g / m 2 That's it, 1 x 10 -8 g / m 2 That's it, 1 x 10 -7 g / m 2 or more, or 2 x 10 -7 g / m 2 It may be more than that.

[0022] When the silicon nitride powder further contains iron, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and iron mass ratio R Fe The ratio of the sum of (R Si +R Fe ) / A) is set to 6×10 -4 g / m 2 Below, 5.5 x 10 -4 g / m 2 or less, or 5 x 10 -4 g / m 2 The ratio (R Si +R Fe ) / A) is set to 1×10 -5 g / m 2 That's it, 1 x 10 -4 g / m 2 That's it, 2 x 10 -4 g / m 2 or more, or 3 x 10 -4 g / m 2 It may be more than that.

[0023] Mass ratio of iron in silicon nitride powder R Fe can be adjusted, for example, by adjusting the purity of the silicon raw material.

[0024] When the silicon nitride powder further contains carbon and iron, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and the carbon mass ratio R C and iron mass ratio R Fe The ratio of the product of (R Si ×R C ×R Fe / A) is set to 7 × 10 from the viewpoint of further suppressing the black color of the silicon nitride powder. -10 g / m 2 Below, 6 x 10 -10 g / m 2 or less, or 5 x 10 -10 g / m 2 The ratio (R Si ×R C ×R Fe / A) is set to 1×10 from the viewpoint of further suppressing the black color of the silicon nitride powder. -13 g / m 2 That's it, 1 x 10 -12 g / m 2 That's it, 1 x 10 -11 g / m 2 or more, or 1 x 10 -10 g / m 2 It may be more than that.

[0025] When the silicon nitride powder further contains carbon and iron, the mass ratio R of uncomposed silicon nitride to the BET specific surface area A of the silicon nitride powder is Si and the carbon mass ratio R C and iron mass ratio R Fe The ratio of the sum of (R Si +R C +R Fe ) / A) is set to 7.5 × 10 in order to further suppress the black color of the silicon nitride powder. -4 g / m 2 Below, 7 x 10 -4 g / m 2 or less, or 6.5 x 10 -4 g / m 2 The ratio (R Si +R C +R Fe ) / A) is set to 1×10 -4 g / m 2 That's it, 3 x 10-4 g / m 2 or more, or 5 x 10 -4 g / m 2 It may be more than that.

[0026] The silicon nitride powder may further contain oxygen. When the silicon nitride powder further contains oxygen, the mass ratio R of oxygen in the silicon nitride powder O From the viewpoint of improving the sinterability of the silicon nitride powder, the mass ratio R may be 0.0165 or less, 0.016 or less, 0.0155 or less, or 0.015 or less. O From the viewpoint of further suppressing the black color of the silicon nitride powder, the mass ratio R may be 0.012 or more, 0.0125 or more, or 0.013 or more. O can be measured by the method described in the Examples below.

[0027] Mass ratio of oxygen in silicon nitride powder R O can be adjusted, for example, by subjecting the silicon nitride powder to an acid treatment to remove oxygen from the surface of the silicon nitride powder.

[0028] When the silicon nitride powder further contains oxygen, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen O The mass ratio R of unnitrided silicon to the product of Si The ratio (R Si / A×R O ) is 2.5 × 10 in order to further suppress the black color of the silicon nitride powder. -2 g / m 2 Below, 2.2 x 10 -2 g / m 2 Below, 2 x 10 -2 g / m 2 or less, or 1.8 x 10 -2 g / m 2 The ratio (R Si / A×R O ) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -3 g / m 2 That's it, 5 x 10 -3 g / m 2 or more, or 1 x 10-2 g / m 2 It may be more than that.

[0029] When the silicon nitride powder further contains oxygen and carbon, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen O The mass ratio R of unnitrided silicon to the product of Si and the carbon mass ratio R C The ratio of the product of (R Si ×R C / A×R O ) is 2.5 × 10 in order to further suppress the black color of the silicon nitride powder. -5 g / m 2 Below, 2.2 x 10 -5 g / m 2 or less, or 2 x 10 -5 g / m 2 The ratio (R Si ×R C / A×R O ) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -6 g / m 2 That's it, 5 x 10 -6 g / m 2 or more, or 1 x 10 -5 g / m 2 It may be more than that.

[0030] When the silicon nitride powder further contains oxygen and carbon, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen O The mass ratio R of unnitrided silicon to the product of Si and the carbon mass ratio R C The ratio of the sum of (R Si +R C ) / A×R O ) is 3.5 × 10 in order to further suppress the black color of the silicon nitride powder. -2 g / m 2 Below, 3.2 x 10 -2 g / m 2 or less, or 3 x 10 -2 g / m 2 The ratio (R Si +R C ) / A×R O) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -2 g / m 2 That's it, 1.5 x 10 -2 g / m 2 or more, or 2 x 10 -2 g / m 2 It may be more than that.

[0031] When the silicon nitride powder further contains oxygen and iron, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen O The mass ratio R of unnitrided silicon to the product of Si and iron mass ratio R Fe The ratio of the product of (R Si ×R Fe / A×R O ) is 4 × 10 in order to further suppress the black color of the silicon nitride powder. -5 g / m 2 Below, 3.5 x 10 -5 g / m 2 or less, or 3 x 10 -5 g / m 2 The ratio (R Si ×R Fe / A×R O ) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -6 g / m 2 That's it, 5 x 10 -6 g / m 2 or more, or 1 x 10 -5 g / m 2 It may be more than that.

[0032] When the silicon nitride powder further contains oxygen and iron, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen O The mass ratio R of unnitrided silicon to the product of Si and the carbon mass ratio R Fe The ratio of the sum of (R Si +R Fe ) / A×R O ) is 4 × 10 in order to further suppress the black color of the silicon nitride powder. -2 g / m 2 Below, 3.5 x 10 -2 g / m 2or less, or 3.2 x 10 -2 g / m 2 The ratio (R Si +R Fe ) / A×R O ) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -3 g / m 2 That's it, 1 x 10 -2 g / m 2 or more, or 2 x 10 -2 g / m 2 It may be more than that.

[0033] When the silicon nitride powder further contains oxygen, carbon, and iron, the BET specific surface area A of the silicon nitride powder and the mass ratio R of oxygen are O The mass ratio R of unnitrided silicon to the product of Si and the carbon mass ratio R C and iron mass ratio R Fe The ratio of the product of (R Si ×R C ×R Fe / A×R O ) is 4.5 × 10 in order to further suppress the black color of silicon nitride powder. -8 g / m 2 Below, 4 x 10 -8 g / m 2 or less, or 3.5 x 10 -8 g / m 2 The ratio (R Si ×R C ×R Fe / A×R O ) is 1 × 10 from the viewpoint of further suppressing the black color of silicon nitride powder. -11 g / m 2 That's it, 1 x 10 -10 g / m 2 That's it, 1 x 10 -9 g / m 2 or more, or 1 x 10 -8 g / m 2 It may be more than that.

[0034] The silicon nitride powder may further contain aluminum. When the silicon nitride powder further contains aluminum, the mass ratio R of aluminum in the silicon nitride powder is AlFrom the viewpoint of improving the sinterability of the silicon nitride powder, the mass ratio R may be 0.002 or less, 0.0016 or less, or 0.0012 or less. Al From the viewpoint of manufacturing costs, the mass ratio R may be 0.0005 or more, or 0.0007 or more. Al can be measured by the method described in the Examples below.

[0035] Mass ratio R of aluminum in silicon nitride powder Al can be adjusted, for example, by adjusting the purity of the silicon raw material.

[0036] The silicon nitride powder may further contain calcium. When the silicon nitride powder further contains calcium, the mass ratio R of calcium in the silicon nitride powder is Ca From the viewpoint of improving the sinterability of the silicon nitride powder, the mass ratio R may be 0.0025 or less, or 0.0022 or less. Ca From the viewpoint of manufacturing costs, the mass ratio R may be 0.0008 or more, 0.0015 or more, or 0.002 or more. Ca can be measured by the method described in the Examples below.

[0037] Mass ratio R of calcium in silicon nitride powder Ca can be adjusted, for example, by adjusting the purity of the silicon raw material.

[0038] The α-fraction of the silicon nitride powder may be 91% or more. When such silicon nitride powder is used as a sintering raw material, the β-phase is more likely to form. Therefore, the strength of the silicon nitride sintered body can be sufficiently increased while maintaining a sufficiently low density. From the viewpoint of further reducing production costs, the α-fraction of the silicon nitride powder may be 97% or less, 95% or less, or 94% or less. The α-fraction of the silicon nitride powder can be determined based on the diffraction line intensity of X-ray diffraction. In the case of a direct nitriding process, the α-fraction of the silicon nitride powder can be adjusted by changing the heating conditions during nitriding. For example, increasing the heating temperature tends to generate the β-phase and lower the α-fraction.

[0039] The average particle size (D50, median size) of the silicon nitride powder may be 0.7 to 2 μm, or 1 to 1.6 μm. The particle size distribution in this specification is measured in accordance with the method described in JIS Z 8825:2013, "Particle size analysis - laser diffraction and scattering method." In the particle size distribution (cumulative distribution) shown with the horizontal axis representing particle size [μm] on a logarithmic scale and the vertical axis representing frequency [volume %], the particle size at which the cumulative value from the smallest particle size reaches 50% of the total is the average particle size (D50).

[0040] In the above particle size distribution, the particle size (D10) when the cumulative value from small particle sizes reaches 10% of the total may be 0.3 to 0.7 μm or 0.4 to 0.6 μm. In the above particle size distribution, the particle size (D90) when the cumulative value from small particle sizes reaches 90% of the total may be 3 to 6.5 μm or 3.5 to 6 μm.

[0041] The silicon nitride powder described above can be suitably used as a sintering raw material. The silicon nitride powder may be used for, but is not limited to, a silicon nitride sintered body. The silicon nitride content in the silicon nitride powder may be 95% by mass or more, 98% by mass or more, or 99% by mass or more. The silicon nitride content in the silicon nitride powder can be measured, for example, by X-ray diffraction.

[0042] A method for producing silicon nitride powder according to one embodiment includes a molding step of molding a kneaded product containing silicon powder and an organic binder to obtain a molded body; a degreasing step of heating the molded body at a temperature of 900°C or higher but lower than 1100°C for at least one hour to degrease it; a firing step of firing the molded body in a mixed atmosphere containing at least one selected from the group consisting of nitrogen, hydrogen, and ammonia to obtain a fired product containing silicon nitride; and a crushing step of crushing the fired product.

[0043] The oxygen content of the silicon powder used in the compacting step may be, for example, 0.2 to 0.6% by mass. The carbon content of the silicon powder may be 0.05 to 0.10% by mass. The oxygen content and carbon content of the silicon powder can be measured by infrared absorption. In order to adjust the oxygen concentration before the compacting step, a pretreatment liquid containing hydrofluoric acid may be used to reduce oxygen bound to the silicon powder. The pretreatment liquid may contain hydrofluoric acid, or may be a mixed acid with an acid such as hydrochloric acid.

[0044] Examples of organic binders include urethane resins, vinyl butyral resins, vinyl alcohol resins, vinyl acetal resins, vinyl formal resins, polyimide resins, phenolic resins, melamine resins, epoxy resins, coumarone-indene resins, acrylic resins, aromatic vinyl resins, cellulose and cellulose derivatives, wax, and starch. The amount of organic binder may be, for example, 3 to 30 parts by mass, or 5 to 20 parts by mass, per 100 parts by mass of silicon powder.

[0045] The kneaded product may contain a solvent. Examples of the solvent include water, alcohols, ketones, esters, ethers, aromatic hydrocarbons, and polybasic acids. The kneaded product is molded by a conventional molding method to produce a molded product. Examples of the molding method include extrusion molding and uniaxial press molding. The bulk density of the molded product after drying is 0.5 to 1.5 g / cm. 3 and may be 0.8 to 1.2 g / cm 3 may be.

[0046] In the degreasing step, the compact is heated in a heating furnace to decompose and / or volatilize at least a portion of the organic binder contained in the compact. If a solvent is used, a drying step of heating to a temperature equal to or higher than the boiling point of the solvent may be performed before the degreasing step.

[0047] In the debinding step, the compact is heated, for example, in an atmosphere containing hydrogen gas, at a temperature of 900°C or higher but lower than 1100°C for 1 hour or more. This allows the organic binder contained in the compact to be efficiently reduced. The heating temperature in the debinding step may be 950 to 1050°C. The heating time in the debinding step may be 2 hours or more, or may be 3 hours or more, from the viewpoint of sufficiently reducing the organic binder in the compact. The heating time in the debinding step may be 20 hours or less, or may be 10 hours or less, from the viewpoint of improving the efficiency of the manufacturing process. An example of the range of the heating time in the debinding step is 1 to 20 hours. The atmosphere may be hydrogen gas, or may be a mixed gas with a hydrogen gas concentration of 1.0 to 10.0 volume %. This volume ratio is a value under standard conditions (temperature 0°C, pressure 1 atmosphere).

[0048] In the firing step, the degreased molded body is fired in a mixed atmosphere containing nitrogen and at least one gas selected from the group consisting of hydrogen and ammonia to obtain a fired product containing silicon nitride. Carbon may be contained inside the silicon nitride particles, or may be attached or bonded to the surface of the silicon nitride particles as particles. 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 1500°C or 1200 to 1450°C. The firing time may be, for example, 30 to 100 hours or 50 to 90 hours. The firing temperature and time are preferably 1200 to 1450°C and 50 to 90 hours.

[0049] In the pulverization step, the sintered product obtained in the sintering step is pulverized in a dry state to obtain a pulverized product. The pulverization step may be carried out in multiple stages, including coarse pulverization and fine pulverization, to adjust the particle size distribution of the silicon nitride powder. For example, the pulverization step may include two steps: a ball mill pulverization step and a vibration mill pulverization step. After pulverization of the sintered product, further wet pulverization may be carried out. After pulverization of the sintered product, a classification step may be carried out to adjust the particle size.

[0050] After the pulverization or classification step, a post-treatment step may be carried out to adjust the oxygen concentration. In the post-treatment step, for example, the pulverized fired product may be dispersed in hydrofluoric acid. Then, filtration and drying are carried out to obtain silicon nitride powder.

[0051] Silicon nitride powder can be produced by this method. This production method can produce silicon nitride powder at a lower production cost than the imide decomposition method. The shape, composition, and properties of the silicon nitride powder obtained by this production method are as described in the embodiment of the silicon nitride powder. Therefore, the contents described in the embodiment of the silicon nitride powder also apply to the production method of this embodiment. However, the above-mentioned production method is an example, and the method for producing silicon nitride powder is not limited to the above-mentioned production method.

[0052] A method for producing a silicon nitride sintered body according to one embodiment includes a sintering step of molding and firing a sintering raw material containing the silicon nitride powder described above.

[0053] The sintering raw material may contain an oxide-based sintering aid in addition to silicon nitride powder. Examples of oxide-based sintering aids include Y2O3, MgO, and Al2O3. The content of the oxide-based sintering aid in the sintering raw material may be, for example, 3 to 10 mass%.

[0054] In the sintering step, the sintering raw materials are pressed at a molding pressure of, for example, 3.0 to 30 MPa to obtain a molded body. The molded body may be produced by uniaxial pressing or by CIP. Alternatively, the molded body 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 1.5 MPa. The firing temperature may be 1860 to 2100°C, or 1880 to 2000°C. The firing time at the firing temperature may be 6 to 20 hours, or 8 to 16 hours. The rate of temperature rise to the firing temperature may be, for example, 1.0 to 10.0°C / hour.

[0055] The silicon nitride sintered body obtained in this manner has low density but high strength because excessive densification is suppressed and the β phase is easily generated. The composition and properties of the silicon nitride sintered body obtained by this manufacturing method are as described in the embodiment of the silicon nitride sintered body. Therefore, the contents described in the embodiment of the silicon nitride sintered body also apply to the manufacturing method of this embodiment. However, the above manufacturing method is one example, and the method for manufacturing the silicon nitride sintered body is not limited to the above manufacturing method. [Example]

[0056] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0057] Example 1 <Preparation of silicon nitride powder> A kneaded mixture was obtained by kneading silicon powder (oxygen content: 0.3% by mass, carbon content: 0.10% by mass), an organic binder (vinyl alcohol resin), and a solvent (water). The amount of organic binder blended per 100 parts by mass of silicon powder was 10 parts by mass. This kneaded mixture was then subjected to uniaxial pressing (molding pressure: 8 MPa) to form a compact (bulk density: 1.0 g / cm). 3The obtained compact was dried in a thermostatic chamber at 150°C for 3 hours (drying step). After drying, the compact was placed in an electric furnace and maintained at 1000°C in a hydrogen gas atmosphere for 2 hours (debinding step).

[0058] The degreased compact was placed in another electric furnace and fired at 1400°C for 60 hours to obtain a fired product containing silicon nitride. A mixed gas of nitrogen gas and hydrogen gas (a mixed gas in which N2 and H2 were mixed at a volume ratio of 80%:20% under standard conditions) was supplied as the firing atmosphere. The fired product was coarsely crushed and then dry-pulverized in a ball mill. The silicon nitride powder obtained by dry crushing was classified using a classifier.

[0059] By classification, coarse particles (agglomerated particles) were removed from the silicon nitride powder. The mass ratio of the silicon nitride powder after classification to the total mass of the silicon nitride powder before classification was 80%. The silicon nitride powder after classification refers to the silicon nitride powder obtained by removing the coarse particles. The silicon nitride powder obtained in this manner was evaluated as follows.

[0060] <Measurement of mass ratio of unnitrided silicon> The obtained silicon nitride powder sample was lightly crushed in a mortar, and the amount of remaining silicon was quantified from the diffraction pattern obtained using an X-ray diffractometer (device name: Multiflex manufactured by Rigaku Co., Ltd.), and the mass ratio of unreacted silicon nitride was calculated.

[0061] <Measurement of the mass ratio of iron, aluminum, and calcium> The metal elements of the silicon nitride powder thus obtained were detected by X-ray fluorescence spectroscopy and quantitatively analyzed by an atomic absorption spectrophotometer.

[0062] <Measurement of oxygen mass ratio> The mass ratio of oxygen in the silicon nitride powder and the mass ratio of oxygen contained within were measured using an oxygen / nitrogen analyzer (manufactured by 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 mass ratio of oxygen was determined by quantifying the amount of oxygen before nitrogen was detected. The mass ratio of oxygen was also determined by quantifying the amount of oxygen after nitrogen began to be detected. Furthermore, the mass ratio of oxygen and the mass ratio of oxygen were summed to determine the mass ratio of oxygen in the entire silicon nitride powder, R O The measurement results are shown in Tables 1 to 3.

[0063] <Measurement of carbon mass ratio> The carbon mass ratio R of silicon nitride powder was measured using a commercially available carbon / sulfur analyzer (manufactured by LECO, model IR412). C The measurement procedure was as follows: the sample was heated from 20°C to 2000°C at a heating rate of 10°C / sec in an oxygen atmosphere. The carbon monoxide and carbon dioxide generated as the temperature increased were detected with an infrared detector, and the mass ratio R of carbon contained in the entire silicon nitride powder was calculated. C The measurement results are shown in Tables 1 to 3.

[0064] <Measurement of alpha rate> The α-phase of silicon nitride powder was measured by the following procedure. X-ray diffraction of silicon nitride powder was performed using an X-ray diffractometer (Rigaku, device name: Ultima IV) with CuKα radiation. The α-phase was determined by the diffraction intensity of the (102) plane. a102 and the diffraction intensity of the (210) plane I a210 The β phase is represented by the diffraction intensity I b101 and the diffraction intensity of the (210) plane I b210 Using these diffraction line intensities, the α ratio was calculated by the following formula. The measurement results are shown in Tables 1 to 3. α rate (%)= (I a102 +I a210 ) / (I a102 +I a210 +I b101 +I b210 ) x 100

[0065] <Measurement of BET Specific Surface Area> In accordance with JIS Z 8830:2013, using nitrogen gas, the BET specific surface area A of silicon nitride powder was measured by the BET single-point method. Also, the ratio of each component to the BET specific surface area A was calculated. The measurement results of the BET specific surface area A and the calculation results of the ratios are shown in Tables 1 to 3, respectively.

[0066] <Measurement of Particle Size Distribution> The particle size distribution of silicon nitride powder was measured by the laser diffraction / scattering method. The measurement was carried out in accordance with the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction / Scattering Method". In the particle size distribution (cumulative distribution) shown with the horizontal axis as the logarithmic scale particle size [μm] and the vertical axis as the frequency [volume%], the particle sizes when the integrated value from the small particle size reached 10%, 50%, 90%, and 100% of the whole were determined as D10, D50, and D90, respectively. The measurement results are shown in Tables 1 to 3.

[0067] (Example 2) Silicon nitride powder was obtained in the same manner as in Example 1, except that a different lot of silicon powder was used.

[0068] (Example 9) <Preparation of Silicon Nitride Powder> Silicon powder (oxygen content: 0.3 mass%, carbon content: 0.10 mass%), an organic binder (vinyl alcohol resin), and a solvent (water) were kneaded to obtain a kneaded product. The blending amount of the organic binder with respect to 100 mass parts of silicon powder was 10 mass parts. Using this kneaded product, a molded body (bulk density: 1.0 g / cm 3 ) was produced. The obtained molded body was dried in a constant temperature bath at 150°C for 3 hours (drying step). After drying, the molded body was left standing in an electric furnace and held at 1000°C for 2 hours in an atmosphere of hydrogen gas (debinding step).

[0069] The degreased compact was placed in another electric furnace and fired at 1400°C for 60 hours to obtain a fired product containing silicon nitride. A mixed gas of nitrogen gas and hydrogen gas (a mixed gas in which N2 and H2 were mixed at a volume ratio of 80%:20% under standard conditions) was supplied as the firing atmosphere. The fired product was coarsely crushed and then dry-pulverized in a ball mill.

[0070] The silicon nitride powder obtained by dry milling was wet milled in a ball mill with a ball filling rate of 60% by volume relative to the container, water as the solvent, and a milling time of 9 hours.

[0071] The ground material obtained by wet grinding was immersed in hydrofluoric acid for acid treatment. The ground material was then removed from the hydrofluoric acid, washed with water, and dried under a nitrogen atmosphere. The dried ground material was classified using a classifier.

[0072] By classification, coarse particles (agglomerated particles) were removed from the silicon nitride powder. The mass ratio of the silicon nitride powder after classification to the total mass of the silicon nitride powder before classification was 80%. The silicon nitride powder after classification refers to the silicon nitride powder obtained by removing the coarse particles. The silicon nitride powder obtained in this manner was evaluated as follows.

[0073] [evaluation] <Appearance> The appearance of the obtained silicon nitride powder was evaluated by brightness (0 to 10) of the Munsell color system (based on JIS Z 8721), with brightness of 9 or more being judged as white and brightness of 8 or less being black. The evaluation results are shown in Tables 1 to 3. [Table 1]

[0074] [Table 2]

[0075] [Table 3]

Claims

1. A silicon nitride powder containing silicon nitride and non-nitrided silicon, The mass ratio R of the unnitrided silicon in the silicon nitride powder to the BET specific surface area A of the silicon nitride powder Ｓｉ The ratio (R Ｓｉ / A) is 4 x 10 －４ g / m ２ Silicon nitride powder, which is:

2. the silicon nitride powder further contains carbon; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of carbon in the silicon nitride powder Ｃ The ratio of the product of (R Ｓｉ ×R Ｃ / A) is 4.5 × 10 －７ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

3. the silicon nitride powder further contains carbon; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of carbon in the silicon nitride powder Ｃ The ratio of the total (R Ｓｉ +R Ｃ ) / A) is 5.5 x 10 －４ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

4. the silicon nitride powder further contains iron; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of iron in the silicon nitride powder. Ｆｅ The ratio of the product of (R Ｓｉ ×R Ｆｅ / A) is 6.5 × 10 －７ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

5. the silicon nitride powder further contains iron; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of iron in the silicon nitride powder. Ｆｅ The ratio of the total (R Ｓｉ +R Ｆｅ ) / A) is 6 x 10 －４ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

6. the silicon nitride powder further contains iron and carbon; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of carbon in the silicon nitride powder Ｃ and the mass ratio R of iron in the silicon nitride powder. Ｆｅ The ratio of the product of (R Ｓｉ ×R Ｃ ×R Ｆｅ / A) is 7 x 10 －１０ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

7. the silicon nitride powder further contains iron and carbon; The mass ratio R to the BET specific surface area A Ｓｉ and the mass ratio R of carbon in the silicon nitride powder Ｃ and the mass ratio R of iron in the silicon nitride powder. Ｆｅ The ratio of the total (R Ｓｉ +R Ｃ +R Ｆｅ ) / A) is 7.5 x 10 －４ g / m ２ 2. The silicon nitride powder of claim 1, wherein:

8. The mass ratio R Ｓｉ The silicon nitride powder according to any one of claims 1 to 7, wherein is 0.003 or less.

9. The silicon nitride powder according to any one of claims 1 to 7, wherein the α ratio is 91% or more.

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