Crystalline silicon nitride powder, silicon nitride sintered body containing said powder, method for producing silicon nitride sintered body, article containing said sintered body, and vehicle

Abstract

The present invention, by including a specific amount of impurity carbon, provides a crystalline silicon nitride powder that is easily sintered and makes it possible to produce a silicon nitride sintered body having excellent mechanical strength or thermal conductivity. Also provided by using this silicon nitride powder is a silicon nitride sintered body that is easily sintered and has excellent mechanical strength or thermal conductivity. The present invention also provides a method for producing a silicon nitride sintered body that is easily sintered and has excellent mechanical strength or thermal conductivity. Provided is a crystalline silicon nitride powder containing silicon nitride, carbon, and oxygen, in which the mass ratio Cp of the carbon to the whole powder is 0.200 mass% or less, the mass ratio C1 of a first volatile carbon detected at from 100°C to less than 700°C measured by temperature-programmed morphology analysis is from 0.001 mass% to 0.030 mass%, and the mass ratio C2 of a second volatile carbon detected at from 700°C to 1200°C measured by temperature-programmed morphology analysis from 0.005 mass% to 0.060 mass%.

Description

Crystalline silicon nitride powder, silicon nitride sintered body containing said powder, method for producing silicon nitride sintered body, article containing said sintered body, and vehicle

[0001] The present invention relates to a crystalline silicon nitride powder suitable as a raw material for producing a silicon nitride sintered body useful as a heat-resistant structural material, a wear-resistant material, and a circuit board material, a silicon nitride sintered body containing said powder, a method for producing the same, an article containing said sintered body, and a vehicle containing said article.

[0002] Silicon nitride sintered bodies have attracted attention as heat-resistant structural components for gas turbines, automobile engines, and other applications, as well as wear-resistant components for bearings and other applications, due to their high strength, corrosion resistance, and thermal shock resistance. Silicon nitride sintered substrates are also increasingly being used as insulating substrates for power modules in electric vehicles (EVs), railway vehicles, and other applications. These silicon nitride sintered bodies are typically produced by mixing silicon nitride powder with a sintering aid, forming a compact using press molding, injection molding, extrusion molding, sheet molding, or other methods, and sintering the compact under atmospheric pressure, a pressurized atmosphere, or a pressurized press. In each of these methods, the powder characteristics and impurity content of the silicon nitride powder used as the raw material, particularly the oxygen and carbon contents, are known to significantly affect the sinterability and properties of the sintered body. In other words, silicon nitride powders with high oxygen contents are generally easy to sinter but lack sufficient high-temperature strength. On the other hand, silicon nitride powders with low oxygen contents are difficult to sinter and are difficult to densify sufficiently.

[0003] For example, Patent Document 1 describes a method for calculating the specific surface area of ​​silicon nitride powder to 10 m ２ / g or more and the total oxygen content is 1.5 mass % or less, thereby reducing the proportion of grain boundary phases formed during sintering, maintaining a high melting temperature, and improving high-temperature properties.

[0004] In Patent Document 2, in order to obtain a silicon nitride sintered body having high strength, the amounts of oxygen and carbon are reduced by using silicon nitride powder obtained by an imide decomposition method.

[0005] Patent Document 3 describes Si ３ N ４It has been disclosed that, in a ceramic substrate made of silicon nitride, by setting the concentration of silicon oxide and silicon composite oxide on the substrate surface to 2.7 atom% or less as measured by surface concentration using an electron probe microanalyzer, a power module substrate with sufficient bond strength when bonded to a metal member such as aluminum can be obtained, and bond reliability during thermal cycling can be improved. For such insulating substrate applications, high insulation and heat dissipation properties are required. The oxygen content of silicon nitride powder is generally defined by the oxygen present in the silicon oxide layer and / or silicon oxynitride layer on the particle surface (surface oxygen), the oxygen present in the silicon nitride crystals inside the particles (internal oxygen), and the total oxygen content (the sum of the surface oxygen content and the internal oxygen content). Thus, when the oxygen content of silicon nitride powder is divided into surface oxygen and internal oxygen, it is said that surface oxygen in particular has a significant effect on sinterability.

[0006] Patent Document 4 proposes a technology for obtaining a silicon nitride sintered body having low density and high strength at low production costs, using a powder in which the carbon content in the silicon nitride powder is 0.050 mass% or more, the carbon content in the surface portion is 0.050 mass% or less, and the oxygen content is 0.300 to 1.600 mass%, and the oxygen content in the surface portion is 0.050 mass% or less.

[0007] Japanese Patent Application Laid-Open No. 7-206409 Japanese Patent Application Laid-Open No. 5-193914 Japanese Patent Application Laid-Open No. 2009-231388 International Publication No. 2023 / 189539

[0008] An object of the present invention is to provide a crystalline silicon nitride powder which is easily sinterable and can give a silicon nitride sintered body having sufficient strength or thermal conductivity.

[0009] Another object of the present invention is to provide a silicon nitride sintered body having sufficient strength or thermal conductivity.

[0010] A further object of the present invention is to provide a method for producing a silicon nitride sintered body having sufficient strength or thermal conductivity.

[0011] Another object of the present invention is to provide an article comprising a silicon nitride sintered body having sufficient strength, or an article comprising a silicon nitride sintered body having sufficient thermal conductivity.

[0012] Another object of the present invention is to provide a vehicle comprising an article having sufficient strength or containing a silicon nitride sintered body.

[0013] The present inventors have conducted extensive research into the effects of the BET specific surface area, carbon content and distribution, and oxygen content and distribution on sinterability and sintered body properties, particularly high-temperature strength and thermal conductivity, of silicon nitride powder. As a result, they have found that a silicon nitride powder capable of yielding a silicon nitride sintered body that is easily sinterable and has sufficient strength is a crystalline silicon nitride powder in which the mass ratio Cp of carbon to the entire powder is 0.200 mass% or less, the mass ratio C1 of first volatile carbon present on the powder surface to the entire powder is 0.001 mass% or more and 0.030 mass% or less, and the mass ratio C2 of second volatile carbon present inside the powder surface to the entire powder is 0.005 mass% or more and 0.060 mass% or less.

[0014] The above-mentioned crystalline silicon nitride powder has a mass ratio C1 of the first volatile carbon present on the surface of the powder that is not more than a predetermined value, and a mass ratio C2 of the second volatile carbon present inside the surface of the powder that is not more than a predetermined value. Therefore, when used as a sintering raw material, a molten phase of an appropriate volume is formed during the sintering process through a reaction mediated by a sintering aid, making it easy to produce a high-density silicon nitride sintered body.

[0015] Literature 4 discloses a silicon nitride powder in which the mass ratio Os of oxygen contained in the surface portion of the powder to the entire silicon nitride powder and the mass ratio Cs of carbon contained in the surface portion of the powder to the entire powder satisfy Os / Cs ≧ 10. However, when used as a sintering raw material, the resulting silicon nitride sintered body has a low density (for example, relative density: 95% or less) and a bending strength that is only at a low level of 730 MPa or less, probably because the ratio of oxygen to carbon in the surface portion is high.

[0016] The present invention provides the following embodiments.

[0017] [1] A crystalline silicon nitride powder containing silicon nitride, carbon, and oxygen, characterized in that: the mass ratio Cp of the carbon to the entire powder is 0.200 mass% or less; the mass ratio C1 of the first volatile carbon detected in a temperature range of 100°C or higher and lower than 700°C in a temperature programmed morphological analysis is 0.001 mass% or higher and 0.030 mass% or lower; and the mass ratio C2 of the second volatile carbon detected in a temperature range of 700°C or higher and 1200°C or lower in a temperature programmed morphological analysis is 0.005 mass% or higher and 0.060 mass% or lower.

[0018] [2] The crystalline silicon nitride powder according to [1], characterized in that the mass ratio Op of the oxygen to the entire powder is 0.650 mass% or more and 1.500 mass% or less, and the mass ratio Os of surface oxygen to the entire powder measured by temperature-programmed morphology analysis is 0.050 mass% or more and less than 0.400 mass%.

[0019] [3] The crystalline silicon nitride powder according to [2], characterized in that the mass ratio Os of the surface oxygen and the surface carbon amount Cs, which is the sum of the mass ratio C1 of the first volatile carbon and the mass ratio C2 of the second volatile carbon, satisfy the following formula (1): 1.5≦Os / Cs<10 (1)

[0020] [4] BET specific surface area is 2.0 m ２ / g or more, 12.0m ２ The crystalline silicon nitride powder according to any one of [1] to [3], characterized in that the crystalline silicon nitride powder has a molecular weight of 1000 or less.

[0021] [5] The crystalline silicon nitride powder according to any one of [1] to [4], characterized in that the alpha phase ratio (α / (α+β)) is 95.0% or more, where α / (α+β) is the ratio of the X-ray diffraction intensity of the α phase to the total amount of the α phase (a phase consisting of α-type silicon nitride, hereinafter referred to as "α phase") and the β phase (a phase consisting of β-type silicon nitride, hereinafter referred to as "β phase").

[0022] [6] A silicon nitride sintered body comprising the crystalline silicon nitride powder according to any one of [1] to [5] and a sintering aid.

[0023] [7] A method for producing a silicon nitride sintered body, comprising a step of molding and sintering a sintering raw material containing the crystalline silicon nitride powder according to any one of [1] to [5] and a sintering aid.

[0024] [8] An article comprising the silicon nitride sintered body according to [6].

[0025] [9] The article according to [8], characterized in that the article is a bearing part or an insulating substrate for a power module.

[0026]

[10] A vehicle comprising the article according to [8] or [9].

[0027] Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. The present disclosure may include the following several embodiments, either independently or in combination.

[0028] [A] In one embodiment of the crystalline silicon nitride powder, the ratio (C1 / Ci) of the mass ratio C1 of carbon contained inside the powder to the mass ratio Ci of carbon contained in the entire powder is 0.800 or less.

[0029] [B] In one embodiment of the crystalline silicon nitride powder, the ratio (C2 / Ci) of the mass ratio C2 of carbon contained inside the powder to the mass ratio Ci of carbon contained in the entire powder is 1.000 or less.

[0030] [C] In one embodiment of the crystalline silicon nitride powder, the mass ratio Os of oxygen contained in the surface portion of the powder relative to the entire powder and the mass ratio C2 satisfy the following formula (2): Os / C2≦25.0 (2)

[0031] [D] In one embodiment of the crystalline silicon nitride powder, the mass ratio Os of oxygen contained in the surface portion of the powder relative to the entire powder is 0.400 mass% or less.

[0032] [E] In one embodiment, a method for producing crystalline silicon nitride powder includes the steps of obtaining amorphous silicon nitride powder produced by an imide decomposition method, firing the amorphous silicon nitride powder in a mixed atmosphere containing nitrogen and at least one selected from the group consisting of hydrogen and ammonia to obtain a fired silicon nitride product containing a small amount of carbon, and crushing the fired product.

[0033] [F] In one embodiment of the method for producing crystalline silicon nitride powder, the crystalline silicon nitride powder has a mass ratio Cp of carbon to the entire powder of 0.080 mass% or more, a mass ratio C1 of first volatile carbon adsorbed to the surface portion of the powder to the entire powder of 0.030 mass% or less, and a mass ratio C2 of second volatile carbon contained inside the surface portion of the powder to the entire powder of 0.060 mass% or less.

[0034] [G] In one embodiment of the method for producing crystalline silicon nitride powder, the ratio (C1 / Ci) of the mass ratio C1 to the mass ratio Ci of carbon contained inside the powder relative to the entire crystalline silicon nitride powder is 0.800 or less, and the ratio (C2 / Ci) of the mass ratio C2 to the mass ratio Ci of carbon contained inside the powder relative to the entire crystalline silicon nitride powder is 1.000 or less.

[0035] [H] In one embodiment of the method for producing a crystalline silicon nitride powder, the mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder relative to the entire crystalline silicon nitride powder and the mass ratio C2 satisfy the following formula (3): Os / C2≦25.0 (3).

[0036] [I] In one embodiment of the method for producing a crystalline silicon nitride powder, the mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder relative to the entire crystalline silicon nitride powder is 0.400 mass % or less.

[0037] [J] In one embodiment of the method for producing a crystalline silicon nitride powder, the BET specific surface area of ​​the crystalline silicon nitride powder is 2.0 to 12.0 mm ２ / g.

[0038] [K] In one embodiment of the method for producing a crystalline silicon nitride powder, the crystalline silicon nitride powder has an alpha conversion rate of 95.0% or more.

[0039] [L] A silicon nitride sintered body according to one embodiment comprises the crystalline silicon nitride powder according to any one of the above and a sintering aid.

[0040] [M] One embodiment of a method for producing a silicon nitride sintered body includes a step of obtaining a silicon nitride sintered body using a sintering raw material containing the crystalline silicon nitride powder described in any one of the above, or the crystalline silicon nitride powder obtained by the production method described in any one of the above.

[0041] According to the present invention, it is possible to provide a crystalline silicon nitride powder that can be used to obtain a silicon nitride sintered body having excellent mechanical properties or high thermal conductivity.

[0042] One aspect of the present invention provides a method for producing a silicon nitride sintered body, comprising the step of obtaining a silicon nitride sintered body using a sintering raw material containing the crystalline silicon nitride powder of the present invention. In this production method, since a sintering raw material containing the crystalline silicon nitride powder of the present invention is used, a silicon nitride sintered body having excellent mechanical properties or thermal conductivity can be obtained.

[0043] Sintered bodies produced using the crystalline silicon nitride powder of the present invention typically have a bending strength of 1000 MPa or more, a relative density of 97.5% or more, and a ratio of bending strength to bulk density of 310 MPa cm ３ / g or more. Such silicon nitride sintered bodies have excellent mechanical properties. Therefore, they can be suitably used as articles such as vehicle or structural members, bearing parts, etc., and vehicles including such articles can also be provided.

[0044] Sintered bodies produced using the crystalline silicon nitride powder of the present invention may typically have a thermal conductivity of 100 W / (m·K) or more. Such silicon nitride sintered bodies have excellent thermal conductivity. For this reason, they can be suitably used in products such as circuit boards, such as insulating substrates used in power modules, and vehicles incorporating such products can also be provided.

[0045] 1A and 1B are diagrams showing an example of carbon analysis and oxygen / nitrogen analysis, respectively;

[0046] 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 x or more and y or less. 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 disclosure. 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 the present disclosure.

[0047] <Crystalline Silicon Nitride Powder> A crystalline silicon nitride powder according to one embodiment is a crystalline silicon nitride powder containing silicon nitride, carbon, and oxygen, wherein the mass ratio Cp (total carbon amount) of carbon to the entire powder is 0.200 mass% or less, the mass ratio C1 of the first volatile carbon adsorbed on the surface of the powder to the entire powder is 0.001 mass% or more and 0.030 mass% or less, and the mass ratio C2 of the second volatile component carbon present inside the surface of the powder to the entire powder is 0.005 mass% or more and 0.060 mass% or less. The carbon contained in the crystalline silicon nitride powder may be contained as a simple substance, as an organic compound or an inorganic compound, or in a solid solution. When contained as an organic compound, an inorganic compound, or a solid solution, the mass ratio of carbon is determined by converting the compound into carbon.

[0048] The mass ratio Cp of carbon contained in the crystalline silicon nitride powder relative to the total mass of the crystalline silicon nitride powder is 0.200 mass% or less, and may be less than 0.200 mass%, or may be 0.150 mass% or less, which makes it easier to generate the β phase when used as a sintering raw material, and enables the realization of a silicon nitride sintered body having both a high sintered density and high strength or thermal conductivity.

[0049] The crystalline silicon nitride powder may have a carbon mass ratio Cp (total carbon amount) of 0.080 mass% or more, which means that the allowable impurity level in each manufacturing process can be increased, allowing for low manufacturing costs.

[0050] The mass ratio Cp of carbon contained in the crystalline silicon nitride powder relative to the entire crystalline silicon nitride powder may be greater than 0.080 mass%, may be 0.090 mass% or more, or may be 0.120 mass% or more. This reduces manufacturing costs. Furthermore, when used as a sintering raw material, silicon carbide (SiC) is generated, and silicon nitride and silicon carbide are combined, resulting in a silicon nitride sintered body with high strength. An example of the range of the mass ratio Cp of carbon relative to the entire crystalline silicon nitride powder is 0.080 to 0.200 mass%.

[0051] The mass ratio C1 of carbon adsorbed to the surface of the crystalline silicon nitride powder relative to the total mass of the crystalline silicon nitride powder can be detected as the first volatile carbon volatilized by temperature-programmed morphological analysis of the silicon nitride powder at a temperature range of 100°C to less than 700°C. The mass ratio C2 of carbon present inside the surface of the silicon nitride powder can be detected as the second volatile carbon volatilized by temperature-programmed morphological analysis of the silicon nitride powder at a temperature range of 700°C to 1200°C. If the surface carbon content Cs is the carbon present near the surface of the crystalline silicon nitride powder relative to the total mass of the crystalline silicon nitride powder, Cs can be expressed as the sum of C1 and C2, "Cs = C1 + C2." The mass ratio Ci of carbon present inside the crystalline silicon nitride powder relative to the total mass of the crystalline silicon nitride powder can be calculated by dividing the masses of carbon C1 and C2 present near the surface of the crystalline silicon nitride powder by the total mass of the silicon nitride powder. Therefore, the following equation (4) holds for Cp, C1, C2, and Ci. Cp (%) = C1 (%) + C2 (%) + Ci (%) (4)

[0052] The mass ratio C1 of the first volatile carbon adsorbed to the surface of the powder relative to the total powder is 0.030 mass% or less, or may be 0.020 mass% or less, or may be 0.010 mass% or less. The mass ratio C2 of the second volatile carbon present inside the surface of the powder relative to the total powder is 0.060 mass% or less, or may be 0.050 mass% or less, or may be 0.020 mass% or less. By sufficiently reducing the amount of carbon present near the surface, reaction with oxygen present in the surface of the powder is suppressed, and liquid phase sintering proceeds smoothly. Furthermore, a β-phase is more likely to form during sintering, resulting in a silicon nitride sintered body with both high sintered density and high strength. The lower limits of the mass ratios C1 and C2 of carbon relative to the total powder are 0.001 mass% or less, or may be 0.010 mass%, for C1, in order to sufficiently reduce production costs, and 0.005 mass% or less, or may be 0.010 mass%, for C2. The amounts of C1 and C2 can be reduced by controlling the amount of toluene in the imide raw material, and further by annealing the silicon nitride powder after firing at 700° C. or higher to remove easily volatile carbon from the surface.

[0053] The mass ratio Ci of carbon contained within the powder relative to the total mass of the powder may be 0.030 to 0.180 mass%, or 0.050 to 0.120 mass%, which allows for a sufficiently good balance between the mass ratio Cp and the mass ratios C1 and C2.

[0054] The ratio of mass ratio C1 to mass ratio Ci (C1 / Ci) may be 0.6 or less, 0.3 or less, or even 0.1 or less. Because such crystalline silicon nitride powder has little carbon present in the surface portion of the powder, when used as a sintering raw material, the amount of carbon trapped within the sintered body is small, making it more likely for the β phase to form during liquid-phase sintering. Furthermore, the internal carbon of the powder generates silicon carbide and combines with silicon nitride. These factors enable the production of a silicon nitride sintered body with high strength. The ratio C1 / Ci may be 0.01 or more to reduce manufacturing costs. An example of the numerical range of the ratio C1 / Ci is 0.01 to 0.60.

[0055] The ratio of mass ratio C2 to mass ratio Ci (C2 / Ci) may be 1.4 or less, 1.0 or less, or 0.5 or less. Because such crystalline silicon nitride powder has little carbon present inside the surface of the powder, when used as a sintering raw material, β-phase tends to form during liquid phase sintering. Furthermore, the carbon inside the powder generates silicon carbide and combines with silicon nitride. These factors enable the production of a silicon nitride sintered body with high strength. The ratio C2 / Ci may be 0.15 or more to reduce manufacturing costs. An example of the numerical range of the ratio C2 / Ci is 0.15 to 1.40.

[0056] In the above-mentioned crystalline silicon nitride powder, the ratio (C1 + C2) / Ci, which is the mass ratio of carbon present near the surface of the powder to the mass ratio Ci of carbon contained inside the powder relative to the total powder, may be 1.20 or less. Since such crystalline silicon nitride powder has little carbon present near the surface, when used as a sintering raw material, a molten phase of an appropriate volume is formed during the sintering process through a reaction mediated by a sintering aid. Furthermore, carbon present in the deeper layers of the powder generates silicon carbide and combines with silicon nitride. These factors enable the production of silicon nitride sintered bodies with even higher strength.

[0057] The mass ratios Cp, C1, C2, and Ci in this specification can be determined by the following procedure. The carbon content of crystalline silicon nitride powder can be analyzed using a carbon / sulfur analyzer. A powder sample for measurement is heated in an oxygen atmosphere at a heating rate of 48°C / sec over a temperature range of 100°C to 1200°C. As the temperature rises, carbon desorbed from the powder surface combines with oxygen to form carbon monoxide (CO) or carbon dioxide (CO ２ ) Silicon nitride decomposes at temperatures above 1350°C, so by raising the temperature to above 1350°C, all of the carbon present inside the powder becomes carbon monoxide or carbon dioxide. By detecting the carbon monoxide and carbon dioxide thus produced with an infrared detector, the mass ratio Cp of carbon contained in the entire crystalline silicon nitride powder can be determined.

[0058] The mass ratio C1 of the first volatile carbon adsorbed on the surface of the powder relative to the total powder is measured by heating in an air atmosphere to a temperature at which silicon nitride does not decompose. Specifically, the temperature is raised at a rate of 48°C / sec over a temperature range of 100°C or higher and lower than 700°C. In this case, only the carbon adsorbed on the surface of the silicon nitride powder combines with oxygen to form carbon monoxide or carbon dioxide. The mass ratio C1 of the first volatile carbon adsorbed on the surface of the crystalline silicon nitride powder can be determined by detecting the carbon monoxide and carbon dioxide thus produced with an infrared detector.

[0059] The mass ratio C2 of the second volatile carbon present within the surface of the powder to the total powder is measured by heating in an air atmosphere to a temperature at which silicon nitride does not decompose. Specifically, the temperature is raised at a rate of 48°C / sec over a temperature range of 700°C to 1200°C. In this case, only the carbon present within the surface of the silicon nitride powder combines with oxygen to form carbon monoxide or carbon dioxide. The mass ratio C2 of the second volatile carbon present within the surface of the crystalline silicon nitride powder can be determined by detecting the carbon monoxide and carbon dioxide thus produced with an infrared detector.

[0060] The mass ratio Ci of carbon contained inside the powder relative to the entire powder can be calculated from the above formula (4) using the mass ratios Cp, C1, and C2 determined by the above procedure.

[0061] FIG. 1 is an example of a chart obtained by carbon analysis of silicon nitride. Peak 1 is the peak of carbon (C1) present in the surface portion of the silicon nitride powder, and peaks 2 and 3 are peaks of carbon (C2) present in the interior surface portion of the silicon nitride powder. From the integrated value (area) of the peaks, the mass ratio C1 of the first volatile carbon adsorbed and contained in the surface portion and the mass ratio C2 of the second volatile carbon present in the interior surface can be determined based on the calibration curve. Although not shown, the mass ratio Ci of carbon contained in the interior can be determined by heating to 1350°C or higher. In addition, the sum of the mass ratio Ci of carbon, the mass ratio C1 of the first volatile carbon contained in the surface portion, and the mass ratio C2 of the second volatile carbon present in the interior surface becomes the mass ratio Cp of carbon contained in the entire silicon nitride powder.

[0062] The mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder relative to the total mass may be less than 0.400 mass%, 0.150 mass% or less, 0.140 mass% or less, or 0.100 mass% or less. When used as a sintering raw material, such crystalline silicon nitride powder can suppress excessive densification and prevent the density of the silicon nitride sintered body from becoming too high. The mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder relative to the total mass may be 0.050 mass% or more, 0.100 mass% or more, or 0.200 mass% or more. When used as a sintering raw material, such crystalline silicon nitride powder is sufficiently densified by liquid phase sintering. An example of the range of the mass ratio Os of the concentration contained in the surface portion of the powder is 0.050 mass% or more but less than 0.400 mass%.

[0063] The mass ratio Oi of oxygen contained within the crystalline silicon nitride powder relative to the total mass of the powder may be 1.200 mass% or less, 0.900 mass% or less, or 0.700 mass% or less. When such a crystalline silicon nitride powder is used as a sintering raw material, a silicon nitride sintered body having sufficiently high thermal conductivity can be obtained. The mass ratio Oi of oxygen contained within the crystalline silicon nitride powder relative to the total mass of the powder may be 0.620 mass% or more, 0.800 mass% or more, or 0.850 mass% or more. Such a crystalline silicon nitride powder can be produced relatively easily. An example of the range of the mass ratio Oi of oxygen contained within the powder is 0.620 to 1.200 mass%.

[0064] The mass ratio Op of oxygen (total oxygen amount) to the entire crystalline silicon nitride powder may be 0.650 mass% or more and 1.500 mass% or less, or may be 0.750 to 1.150 mass%. Such crystalline silicon nitride powder has the advantage that the mass ratios Oi and Os of oxygen contained in the interior and surface portions of the crystalline silicon nitride powder can be easily adjusted to the above-mentioned ranges.

[0065] The oxygen in the entire crystalline silicon nitride powder is silicon dioxide (SiO ２) The oxygen content of each oxide can be determined by converting the amount of such oxide into oxygen. The mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder can be adjusted by changing the conditions (concentration, time, etc.) of the surface treatment using hydrogen fluoride. For example, by extending the time of such surface treatment, the mass ratio Os can be reduced. Furthermore, the mass ratio Op of oxygen in the entire crystalline silicon nitride powder can also be reduced accordingly.

[0066] The mass ratio Oi of oxygen contained within the crystalline silicon nitride powder can be adjusted, for example, by adjusting the oxygen content of the amorphous silicon nitride after imide decomposition. For example, by using an amorphous silicon nitride powder with a low oxygen content, the mass ratio Oi of oxygen contained within the crystalline silicon nitride powder can be reduced. Accordingly, the mass ratio Op of oxygen in the entire crystalline silicon nitride powder can also be reduced.

[0067] The oxygen mass ratio Op (total oxygen amount) is determined by dividing the mass of oxygen contained in the entire crystalline silicon nitride powder by the mass of the crystalline silicon nitride powder. The oxygen mass ratio Os is determined by dividing the mass of oxygen contained in the surface portion of the crystalline silicon nitride powder by the mass of the entire crystalline silicon nitride powder. The oxygen mass ratio Oi is determined by dividing the mass ratio of oxygen contained inside the crystalline silicon nitride powder by the mass of the entire crystalline silicon nitride powder. Therefore, the following formula (5) holds for the mass ratios Op, Os, and Oi.

[0068] Op = Os + Oi (5) The oxygen mass ratios Op, Os, and Oi of the crystalline silicon nitride powder of the present invention are determined using an oxygen / nitrogen analyzer according to the following procedure. A measurement sample is heated in a helium gas atmosphere at a rate of 8°C / sec over a temperature range of 20°C to 2000°C. As the temperature increases, the oxygen released is detected. At the beginning of the temperature increase, oxygen bonded to the surface of the crystalline silicon nitride powder is released. The mass ratio Os of oxygen contained in the surface portion is determined by quantifying the amount of oxygen released.

[0069] Then, when the temperature reaches around 1350°C, silicon nitride begins to decompose. The start of silicon nitride decomposition can be detected by the start of nitrogen detection. When silicon nitride begins to decompose, oxygen inside the crystalline silicon nitride powder is released. By quantifying the oxygen released at this stage, the mass ratio Oi of oxygen contained inside can be determined. The mass ratio Op of oxygen can be determined from the above formula (5) and the mass ratios Os, Oi of oxygen determined by the above procedure.

[0070] FIG. 2 shows an example of a chart obtained by oxygen / nitrogen analysis of silicon nitride. Peak 4 is the peak of the oxygen component in the organic compound adsorbed on the particle surface. Peaks 5 and part of Peak 6 are peaks of oxygen contained in the surface portion of the silicon nitride powder, while the remainder of Peak 6 and Peak 7 are peaks of oxygen contained inside the silicon nitride powder. Peak 6 is defined by the temperature at which nitrogen generation begins. This temperature is the temperature at which detection of Peak 8 begins, and is typically between 1350 and 1500°C. The temperature at which detection of Peak 5 begins (the temperature at the left end of Peak 5) is, for example, 600 to 1000°C. The temperature at which detection of Peak 6 ends (the temperature at the right end of Peak 6) is, for example, 1600 to 1800°C. From the integrated values ​​(areas) of Peaks 4 to 7, the mass ratio Oi of oxygen contained inside and the mass ratio Os of oxygen contained in the surface portion can be calculated based on the calibration curve. The sum of the mass ratio Oi and the mass ratio Os of oxygen is the mass ratio Op of oxygen contained in the entire silicon nitride powder.

[0071] The ratio (Os / Cs) of the mass ratio of oxygen Os to the mass ratio of carbon Cs (= C1 + C2) may satisfy the following formula (1). This ensures that such crystalline silicon nitride powder has a sufficient ratio of oxygen to carbon present in the surface region, facilitating liquid phase sintering when used as a sintering raw material. This improves the grain boundary strength of the silicon nitride sintered body. 1.5≦Os / Cs<10 (1)

[0072] From the viewpoint of reducing cracks and pores near the grain boundaries of the silicon nitride sintered body, the Os / Cs ratio may be 1.5 or more, or may be 2.0 or more. On the other hand, from the viewpoint of suppressing excessive densification, the Os / Cs ratio may be less than 10, or may be 7.0 or less, or 6.0 or less. An example of the range of the Os / Cs ratio is 1.5 or more and less than 10.

[0073] The ratio of the mass ratio Os of oxygen to the mass ratio C2 of carbon (Os / C2) may satisfy the following formula (2). This ensures that such crystalline silicon nitride powder has a sufficient ratio of oxygen to carbon present in the surface and interior, facilitating liquid phase sintering when used as a sintering raw material. Therefore, the grain boundary strength of the silicon nitride sintered body is improved. Os / C2≦25.0 (2)

[0074] From the viewpoint of improving the grain boundary strength of the silicon nitride sintered body, the Os / C2 ratio may be 1.5 or more, or may be 4.0 or more. On the other hand, from the viewpoint of suppressing excessive densification, the Os / C2 ratio may be 20.0 or less, 10.0 or less, or 5.0 or less, and an example of the range of the Os / C2 ratio is 1.5 to 25.0.

[0075] The BET specific surface area (SSA) of the crystalline silicon nitride powder is 2.0 to 12.0 m ２ / g. When such a crystalline silicon nitride powder is used as a sintering raw material, sintering proceeds smoothly. Furthermore, the uniformity of the microstructure of the silicon nitride sintered body can be improved. Therefore, the variation in the strength, etc. of the silicon nitride sintered body can be reduced.

[0076] The BET specific surface area in the present invention is a value measured by the BET single-point method under nitrogen gas using a Macsorb manufactured by Mountech Co., Ltd., in accordance with the method described in JIS Z8830:2013 "Method for measuring specific surface area of ​​powder (solid) by gas adsorption." The BET specific surface area can be adjusted by changing the firing conditions (temperature, time, etc.) of the amorphous silicon nitride powder and the crushing conditions after firing.

[0077] The alpha phase ratio of the crystalline silicon nitride powder may be 95.0% or more. When such crystalline silicon nitride powder is used as a sintering raw material, the beta phase is likely to be generated. Therefore, the strength of the silicon nitride sintered body can be sufficiently increased. The alpha phase ratio of the silicon nitride powder can be determined based on the diffraction intensity of X-ray diffraction.

[0078] The α-phase ratio (proportion of α-phase) of the crystalline silicon nitride powder may be 97.0% or more, preferably 98.0% or more. Such crystalline silicon nitride powder is easy to sinter and can promote the formation of an appropriate amount of β-phase with a high aspect ratio that suppresses abnormal grain growth, thereby achieving better strength properties, particularly good fracture toughness.

[0079] In the case of the imide decomposition method, the alpha conversion rate of silicon nitride can be adjusted by changing the maximum holding temperature.

[0080] The average particle size (D50, median size) of the crystalline silicon nitride powder may be 0.10 to 3.00 μm, or 0.50 to 2.00 μm. The particle size distribution in the present invention is measured using a Microtrac MT3000 manufactured by Microtrac-Bell Co., Ltd. in accordance with the method described in JIS Z8825:2013 "Particle size analysis - laser diffraction and scattering method." In a particle size distribution shown with particle size (μm) on the horizontal axis and frequency (volume %) on the vertical axis, the particle size when the cumulative value from the smallest particle size reaches 50% of the total is the average particle size (D50) described above.

[0081] In the particle size distribution, the particle size (D10) when the cumulative value from small particle sizes reaches 10% of the total may be 0.10 to 0.80 μm, and the particle size (D90) when the cumulative value from small particle sizes reaches 90% of the total may be 1.60 to 2.80 μm.

[0082] Furthermore, in the particle size distribution, the half-value width of the average particle size (D50) may be 0.7 to 1.7 μm, and the (D90-D10) / D50 ratio may be 2.8 or less. Such crystalline silicon nitride powder has a sufficiently small particle size and a sufficiently small variation in particle size distribution, so when used as a sintering raw material, it can be easily molded during molding and sintering proceeds smoothly. Therefore, it is easy to improve the uniformity of the microstructure of the silicon nitride sintered body, and therefore, variation in the strength, etc. of the silicon nitride sintered body can be reduced.

[0083] The above-mentioned crystalline silicon nitride powder can be suitably used as a sintering raw material. The application of the crystalline silicon nitride powder may be, but is not limited to, a silicon nitride sintered body. The silicon nitride content in the crystalline silicon nitride powder may be 97.0% by mass or more, or 99.0% by mass or more. The silicon nitride content in the silicon nitride powder can be measured, for example, by X-ray diffraction.

[0084] <Method for producing crystalline silicon nitride powder> There are no particular limitations on the method for producing the crystalline silicon nitride powder of the present invention.

[0085] A method for producing crystalline silicon nitride powder according to one embodiment includes the steps of: obtaining amorphous silicon nitride powder by imide pyrolysis; calcining the amorphous silicon nitride powder in a mixed atmosphere containing nitrogen, hydrogen, and at least one selected from the group consisting of ammonia to obtain a calcined product containing silicon nitride and carbon; and crushing the calcined product.

[0086] [Imide pyrolysis method] The imide pyrolysis method can produce an amorphous Si-N(-H) compound. The obtained amorphous silicon nitride powder can be fired and crushed to produce a crystalline silicon nitride powder.

[0087] In this method, a nitrogen-containing silane compound such as silicon diimide, silicon tetraamide, or silicon chlorimide, in other words, an amorphous Si—N(—H)-based compound, is produced by a known method, for example, a method of reacting a silicon halide with ammonia, specifically a method of reacting a silicon halide such as silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, or silicon tetraiodide with ammonia in the gas phase, or a method of reacting a liquid silicon halide with liquid ammonia.

[0088] If toluene used in producing the nitrogen-containing silane compound remains, it may affect the carbon content of the final crystalline silicon nitride powder. The obtained nitrogen-containing silane compound may be further dried to reduce the amount of remaining toluene. The temperature of the further drying may be adjusted depending on the amount of remaining toluene. For example, the further drying temperature may be higher than 100°C. There is no particular upper limit for the further drying temperature, but an excessively high further drying temperature is not preferred because it increases production costs. To reduce production costs, for example, the further drying temperature may be lower than 230°C.

[0089] [Firing Step] In the firing step, an amorphous Si—N(—H) compound 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 silicon nitride product containing a small amount of carbon. The 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. Examples of mixed gases of hydrogen gas and nitrogen gas include nitrogen gas containing approximately 0.5 to 15% by volume of hydrogen gas, and examples of mixed gases of ammonia gas and nitrogen gas include nitrogen gas containing 0.1 to 4.4% by volume of ammonia gas. The firing temperature may be, for example, 1100 to 1450°C, or 1200 to 1400°C. The firing time may be, for example, 15 to 40 hours.

[0090] In order to obtain a fired powder with a low oxygen content, it is desirable that the atmosphere contains no oxygen at all, but even if it does contain oxygen, the oxygen concentration may be 100 ppm (V) or less, and more preferably 50 ppm (V) or less.

[0091] (CO concentration in furnace) During the firing process, the carbon monoxide (CO) concentration in the furnace tends to increase due to the volatilization of adsorbed moisture from the insulating materials and the like that make up the firing furnace. The CO concentration in the furnace is preferably, for example, 60 ppm (V) or more and 240 ppm (V) or less. A CO concentration in the furnace of 240 ppm (V) or less is preferable because it suppresses the increase in the internal oxygen content and total oxygen content of the resulting silicon nitride powder. In terms of suppressing the oxygen content, a lower CO concentration in the furnace is preferable. However, to reduce the CO concentration in the furnace, it is necessary to supply a nitrogen-containing inert gas for dilution. If the CO concentration in the furnace is 60 ppm (V) or more, it is not necessary to supply a large amount of nitrogen-containing inert gas for dilution, which prevents cost increases. The CO concentration in the furnace may be 120 ppm (V) or more and 190 ppm (V) or less.

[0092] The specific surface area of ​​the silicon nitride obtained by firing is affected by the specific surface area of ​​the amorphous Si-N(-H) compound, the rate of temperature rise during firing, and the maximum holding temperature.

[0093] For this reason, in the firing step, it is preferable that the heating rate be 53°C / h or more and 100°C / h or less, and the maximum holding temperature be 1430°C or more and 1595°C or less. Furthermore, because the proportion of the β phase in the crystalline phase of the silicon nitride obtained by firing is also affected by the heating rate and the maximum holding time, it is more preferable that the heating rate be 59°C / h or more and 100°C / h or less, and the maximum holding temperature be 1480°C or more and 1590°C or less. The holding time at the maximum temperature may be, for example, 1 hour or more and 5 hours or less.

[0094] The oxygen content in the silicon nitride powder obtained by calcination can be reduced by using a carbon container made of a non-oxidizing composition. On the other hand, since calcination is performed in a carbon container, it is thought that solid carbon and vaporized carbon diffuse into the silicon nitride particles upon heating (calcination), allowing the silicon nitride powder obtained by calcination to contain carbon. This production method reduces the amount of carbon originally contained in the sintered material, thereby reducing the amount of carbon remaining in the silicon nitride particles. Furthermore, the amount of easily volatile carbon on the surface of the silicon nitride powder may be reduced by annealing the sintered silicon nitride powder at 700°C or higher. Furthermore, to reduce the variation in the silicon nitride powder, it is preferable to convert the surface of the carbon container to SiC. When the silicon nitride powder obtained by calcination in this way is used as a sintering raw material, the carbon content in the surface portion of the powder is sufficiently reduced, making it easier to produce a high-strength β-phase silicon nitride during sintering.

[0095] [Crushing Treatment] In the crushing step, the fired product obtained in the firing step is crushed in a dry state to obtain a crushed product. The crushing step may be carried out in multiple stages, including coarse crushing and fine crushing, to adjust the particle size distribution of the silicon nitride powder. For example, the crushing step may include two steps: a ball mill crushing step and a vibration mill crushing step. After crushing the fired product, a classification step may be carried out to adjust the particle size.

[0096] The silicon nitride powder obtained by firing can be crushed in an oxygen-containing inert gas atmosphere (e.g., under air flow) to adjust the surface oxygen content Os. Crushing is performed by milling, and common crushing devices such as a bead mill, vibration mill, planetary ball mill, or jet mill can be used. The surface oxygen content Os of the crystalline silicon nitride powder is affected by the moisture concentration in the gas supplied to the mill container, etc., and the organic solvent used as the solvent, such as ethanol. The surface oxygen content Os, particularly derived from the particle surface, can be adjusted by the moisture concentration.

[0097] (Adjustment of particle size distribution) The conditions of the crushing treatment may be adjusted to control the surface oxygen content Os and surface carbon content Cs (= C1 + C2) of the silicon nitride particles, as well as the particle size distribution. For example, when a continuous vibration mill is used, the grinding ball diameter, ball loading amount, powder loading amount, mill amplitude, mill time, etc. are adjusted comprehensively to adjust the specific surface area of ​​the crystalline silicon nitride powder to 2.0 to 12.0 m. ２ Within this range, the surface oxygen content Os and the surface carbon content Cs (= C1 + C2) can be set to appropriate amounts, and a molten phase with an appropriate volume is formed by the reaction with the sintering aid, making it possible to easily produce a high-density silicon nitride sintered body.

[0098] Specific surface area is 2.0m ２ If the sintered body has a molecular weight of less than 12.0 m / g, it is difficult to densify the sintered body when it is produced, and the strength of the sintered body may decrease. ２ When the surface oxygen content Os exceeds 1 / g, the surface oxygen content Os becomes excessive, and therefore the amount of grain boundary phase contained in the sintered body increases during the sintering process, making it easier for abnormal grain growth of β-type columnar crystals and making it difficult to achieve sufficient high-temperature strength. In the above-mentioned production method, the silicon nitride powder may have a mass ratio Cp of carbon to the entire powder of 0.080 mass% or more, a mass ratio C1 of the first volatile carbon present on the surface of the powder to the entire powder of 0.030 mass% or less, and a mass ratio C2 of the second volatile carbon present inside the surface of the powder of 0.060 mass% or less.

[0099] After the crushing or classification step, a post-treatment step may be performed to adjust the oxygen concentration. In the post-treatment step, for example, the crushed fired product may be dispersed in hydrofluoric acid. This is followed by filtration and drying to obtain silicon nitride powder. Furthermore, the fired silicon nitride powder may be annealed in nitrogen or air to adjust the amount of first volatile carbon C1 adsorbed on the particle surfaces.

[0100] Crystalline silicon nitride powder can be produced by this method. The shape, composition, and properties of the crystalline silicon nitride powder obtained by this production method are as described in the embodiment of the crystalline silicon nitride powder. Therefore, the content described in the embodiment of the crystalline silicon nitride powder also applies to the production method of this embodiment. For example, the mass ratio Cp of carbon to the entire powder of the crystalline silicon nitride powder obtained by this production method may be 0.200 mass% or less. Furthermore, the mass ratio C1 of the first volatile carbon adsorbed to the surface portion of the crystalline silicon nitride powder to the entire crystalline silicon nitride powder may be 0.030 mass% or less, and the mass ratio C2 of the second volatile carbon present inside the surface portion of the crystalline silicon nitride powder may be 0.060 mass% or less. However, the above-mentioned production method is an example, and the method for producing crystalline silicon nitride powder is not limited to the above-mentioned production method.

[0101] <Silicon nitride sintered body and its manufacturing method> The silicon nitride sintered body according to the present invention can be produced by the following manufacturing method. The silicon nitride sintered body according to the present invention can be produced by mixing the crystalline silicon nitride powder according to one embodiment of the present invention with a sintering aid, molding the resulting mixed powder, and sintering the resulting molded body. Alternatively, the silicon nitride sintered body according to the present invention can be produced by simultaneously carrying out molding and sintering. The silicon nitride sintered body according to one embodiment has high strength and can be used for high-temperature structural components, etc.

[0102] (Sintering Aids) Because silicon nitride is a difficult-to-sinter material, sintering aids are usually added to promote sintering when producing sintered bodies. When sintering silicon nitride, high-temperature stable β-type columnar crystals precipitate during the sintering process, resulting in the majority of silicon nitride crystal grains in the sintered body being β-type columnar crystals. The microstructure of these β-type columnar crystals, such as their aspect ratio and particle size, is significantly affected not only by the raw silicon nitride powder, but also by the type and amount of sintering aid added, and the sintering conditions. These factors can be appropriately selected depending on the physical properties of the silicon nitride powder and the characteristics desired for the silicon nitride sintered body.

[0103] In order to increase the mechanical strength of silicon nitride sintered bodies, it is desirable to make the structure of the silicon nitride sintered bodies a fine structure with a high aspect ratio of β-type columnar crystals. In this case, sintering aids such as magnesium oxide (MgO) and aluminum oxide (Al ２ O ３ ) and yttrium oxide (Y ２ O ３ ) is used in combination with an appropriate one. In the production of silicon nitride sintered bodies for high-temperature structural components, such as gas turbine components, which require particularly high-temperature strength, rare earth oxides such as ytterbium oxide, which are effective in improving the heat resistance of the grain boundary phase, may be used in combination with the sintering aids described above. The amount of sintering aid, particularly oxide-based sintering aids, added to the raw materials for producing the sintered body may be, for example, 5% by mass or more and 12% by mass or less.

[0104] The method for mixing the crystalline silicon nitride powder and the sintering aid may be any method, wet or dry, that can uniformly mix them, and known methods such as a rotary mill, a barrel mill, a vibrating mill, etc. For example, a method can be used in which the crystalline silicon nitride powder, the sintering aid, the molding binder, and the dispersant are mixed in a ball mill using water or the like as a dispersion medium, and then the mixed powder is granulated by spray drying.

[0105] (Method of Molding Mixed Powder) As a method of molding the mixed powder, known methods such as press molding, slip casting, extrusion molding, injection molding, drain molding, cold isostatic pressing, etc. can be used. For example, CIP (cold isostatic pressing) molding can be used, in which the obtained granular mixed powder is filled into a rubber mold and pressure is applied to obtain a molded body. For example, a molded body can be obtained by pressing at a molding pressure of 0.2 to 10 MPa.

[0106] (Method of Sintering the Molded Body) The method of sintering the molded body may be any method that densifies the resulting sintered body, but preferably, atmospheric sintering in a nitrogen-containing inert gas atmosphere or pressurized atmosphere sintering in which the atmospheric gas pressure is increased to approximately 0.2 to 10 MPa is employed. Nitrogen gas, argon gas, or the like can be used as the inert gas. Preferably, nitrogen gas is used for sintering. In atmospheric sintering, sintering is performed in a temperature range of approximately 1700 to 1800°C, and in pressurized atmosphere sintering, sintering is performed in a temperature range of approximately 1800 to 2000°C. The rate of temperature increase from approximately 1400°C to the maximum temperature may be, for example, 40°C / h or more and 150°C / h or less, and the holding time at the maximum temperature may be, for example, 2 hours or more and 20 hours or less.

[0107] Alternatively, hot pressing can be employed, which is a method of simultaneously performing molding and sintering. Sintering by hot pressing is preferably carried out in a nitrogen atmosphere, with a pressure of 2 to 20 MPa and a sintering temperature in the range of 1750 to 1950°C.

[0108] The silicon nitride sintered body obtained in this manner contains appropriate amounts of carbon, oxygen, etc., and therefore densification proceeds and the β phase is easily generated, resulting in a dense and high strength. The silicon nitride sintered body produced in this manner has excellent mechanical properties. For example, the three-point bending strength at room temperature may be 800 MPa or more, 930 MPa or more, or 1000 MPa or more. The three-point bending strength is measured in accordance with JIS R1601:2008.

[0109] The bulk density of the silicon nitride sintered body is 3.12 g / cm3 to maintain high strength. ３ From the same viewpoint, the relative density of the silicon nitride sintered body may be 97.9% or more. The ratio of bending strength to bulk density may be 310 MPa cm ３ / g or more.

[0110] The bulk density of the silicon nitride sintered body in the present invention is measured by the Archimedes method. The relative density of the silicon nitride sintered body in the present invention is the theoretical density of silicon nitride, 3.186 g / cm. ３ The bulk density is a relative value to the above.

[0111] The silicon nitride sintered body may contain components other than silicon nitride and carbon. Carbon may be contained, for example, in the form of silicon carbide. The silicon nitride sintered body of this embodiment is dense and has high strength.

[0112] The high-strength silicon nitride sintered body as described above can be suitably used for articles such as structural and vehicle members and bearing parts, and vehicles including such articles can also be provided.

[0113] The above-mentioned manufacturing method is one embodiment, and the method for manufacturing the silicon nitride sintered body is not limited to the above-mentioned manufacturing method.

[0114] In another embodiment, the following method for producing a silicon nitride sintered body is provided. The silicon nitride sintered body produced by this embodiment has high thermal conductivity and is suitable for use in circuit boards such as insulating substrates for power modules.

[0115] A raw material mixture is prepared by adding sintering aids, organic binders, etc. to the crystalline silicon nitride powder of the present invention, and then the resulting raw material mixture is subjected to a sheet forming method such as the doctor blade method to obtain a green sheet. The organic binder is then removed by degreasing, and the resulting degreasing green sheet is sintered in a nitrogen-containing inert atmosphere under the above-mentioned sintering conditions (heating rate, maximum temperature, and holding time at maximum temperature) to produce a silicon nitride sintered body. To evaluate the thermal conductivity and mechanical properties of silicon nitride sintered bodies for circuit boards, etc., the thermal and mechanical properties of the sintered bodies obtained by the above-mentioned CIP (cold isostatic pressing) forming method may be measured.

[0116] (Sintering Aid) In this embodiment, yttrium oxide, lanthanoid rare earth oxide, magnesium oxide, magnesium silicon nitride (MgSiN ２ , MgSi ４ N ６ ), rare earth-silicon composite nitride (Y ２ Si ３ N ６In this case, the amount of the sintering aid added to the raw material for producing the sintered body may be, for example, 3% by mass or more and 8% by mass or less.

[0117] Increasing the purity of the β-crystals and enlarging their size is an effective way to increase the thermal conductivity of silicon nitride sintered bodies. However, since there is often a trade-off between the crystalline form (large crystal grain size) of silicon nitride sintered bodies and their mechanical strength, a sintering aid that can form a structure in which β-crystals of appropriate grain size are intricately intertwined and that can be densified with a minimum amount of grain boundary phase is selected appropriately from the above sintering aids, in an appropriate amount.

[0118] The silicon nitride sintered body thus obtained can have both excellent mechanical properties and heat dissipation (thermal conductivity). For example, the three-point bending strength at room temperature may be 660 MPa or more, or even 720 MPa or more. Furthermore, the thermal conductivity at room temperature may be 100 W / (m·K) or more, or even 110 W / (m·K) or more. Therefore, the silicon nitride sintered body of this embodiment can be suitably used in products for circuit board applications such as insulating substrates for power modules, and vehicles including such products can also be provided.

[0119] The present disclosure will be described in more detail below with reference to examples and comparative examples, although the present disclosure is not limited to the following examples.

[0120] Example 1 Production of Crystalline Silicon Nitride Powder The crystalline silicon nitride powder of Example 1 was produced by the imide pyrolysis method using the following procedure. After the air in a 40 cm diameter, 60 cm high, vertical pressure-resistant reactor cooled to 0°C was replaced with nitrogen gas, 40 liters of liquid ammonia and 5 liters of toluene were charged into the reactor. The liquid ammonia and toluene were slowly stirred in the reactor, causing the liquid ammonia to separate into an upper layer and a lower layer, respectively. A previously prepared solution (reaction solution) consisting of 2 liters of silicon tetrachloride and 6 liters of toluene was supplied through a conduit to the lower layer of the slowly stirred reactor. As the solution was supplied, a white reaction product precipitated near the interface between the upper and lower layers. At the initial stage of the reaction, a large excess of liquid ammonia was present; however, as the reaction progressed, ammonia was consumed, and ammonia was also continuously supplied to the reactor. At steady state, the volume ratio of silicon tetrachloride supplied to the reactor to the liquid ammonia in the reactor was 4 / 100. After the reaction was completed, the reaction product and the remaining liquid in the reaction tank were transferred to a filtration tank, and the reaction product was batch washed five times with about 47 liters of liquid ammonia (total of 234 liters), and then filtered.

[0121] The wet cake thus obtained was dried to obtain a nitrogen-containing silane compound powder. In the drying operation, hot water at 90°C was circulated through the jacket of the filtration tank to heat it, and the pressure inside the tank was maintained at 0.6 MPa (gauge pressure) while the internal pressure was appropriately released, and the end point was when the temperature inside the tank reached 60°C. Further, additional drying was performed at 0.1 MPa, and the end point was when the temperature inside the tank reached 170°C and was maintained for 1 hour, and the specific surface area was 800 m ２ Approximately 1 kg of purified silicon diimide was obtained, with a toluene content of 0.38% by mass and a toluene content of 0.38% by mass. Note that the higher the temperature of the additional drying, the easier it is to reduce the amount of residual toluene, which results in a decrease in the amount of carbon remaining and diffusing within the particles, and therefore a decrease in the amount of surface carbon and total carbon.

[0122] The resulting silicon diimide was thermally decomposed at 900°C using a rotary kiln furnace while passing nitrogen gas containing 105 ppm (V) of oxygen to obtain a silicon diimide having the composition formula Si ６ N １０．６２ H ７．８６That is, Si ６ N ２ｘ (NH) １２－３ｘ An amorphous Si—N(—H) compound was obtained in which x in the formula was 1.38. The specific surface area of ​​the obtained amorphous Si—N(—H) compound was 565 m ２ / g, and the oxygen content was 0.43 mass %.

[0123] (Method for Composition Analysis of Amorphous Si—N(—H) Compounds) The composition of amorphous Si—N(—H) compounds, which are used as raw materials for firing crystalline silicon nitride powder, was analyzed using the following method. Silicon (Si) content was measured by a method for quantifying total silicon using a dehydrated gravimetric ICP-AES method in accordance with JIS R1603:2007, "Methods for Chemical Analysis of Silicon Nitride Fine Powders for Fine Ceramics." Nitrogen (N) content was measured by a method for quantifying total nitrogen using a steam distillation separation neutralization titration method in accordance with JIS R1603:2007. Oxygen (O) content was measured using an oxygen / nitrogen simultaneous analyzer using an inert gas fusion-carbon dioxide infrared absorption method in accordance with JIS R1603:2007. In this case, to suppress oxidation of the amorphous Si—N(—H) compounds, samples were handled in a nitrogen-purged glove box. For example, when measuring silicon and nitrogen content by ICP optical emission spectrometry or steam distillation separation neutralization titration, the atmosphere in which the sample was stored until just before sample pretreatment for measurement was a nitrogen atmosphere, and when measuring oxygen content by infrared absorption, the atmosphere during sample storage until just before measurement and when the capsule was inserted was a nitrogen atmosphere. The hydrogen (H) content of the amorphous Si—N(—H) compound was calculated based on the stoichiometric composition as the remainder after subtracting the silicon (Si), nitrogen (N), and oxygen (O) contents from the total amount of the amorphous Si—N(—H) compound. From the above, the ratios of Si, N, and H were calculated to determine the composition formula of the amorphous Si—N(—H) compound.

[0124] The resulting amorphous Si—N(—H) compound was then milled in a vibration mill and formed into almond-shaped pieces measuring approximately 6 mm thick, 8 mm short axis diameter, and 12 mm long axis diameter using a briquette machine. The almond-shaped pieces, some of which contained burr-like fragments, were loaded into a silicon carbide (SiC)-coated carbon container and fed into a pusher furnace. They were heated and fired under a nitrogen gas flow atmosphere under the conditions listed in Table 1 (maximum high-temperature firing temperature), producing a grayish-white crystalline silicon nitride powder. The holding time at the maximum temperature (1520°C) was 1.5 hours. This crystalline silicon nitride powder was then placed in a vibration mill and milled for 26 minutes at an amplitude of 7.8 mm in an air atmosphere with a moisture concentration of 105 ppm (V). The milled silicon nitride powder was then annealed at 700°C or higher.

[0125] <Preparation of sintered silicon nitride> Two types of sintered silicon nitride were prepared using the obtained crystalline silicon nitride powder: one was prepared with particular emphasis on strength, and the other was prepared with particular emphasis on thermal conductivity.

[0126] (Preparation of sintered body focusing on strength) 90 parts by mass of crystalline silicon nitride powder was mixed with yttrium oxide (specific surface area 3 m) as a sintering aid. ２ / g, manufactured by Shin-Etsu Chemical Co., Ltd.) and 6 parts by mass of aluminum oxide (specific surface area 7.4 m ２ The blended powder, to which 4 parts by mass of ethylenediaminetetraacetic acid (E100 / g, manufactured by Sumitomo Chemical Co., Ltd.) had been added, was wet-mixed in a ball mill using ethanol as a medium for 36 hours and then dried under reduced pressure. The resulting mixture was molded into a 6 x 45 x 75 mm shape at a molding pressure of 50 MPa and then CIP-molded at a molding pressure of 150 MPa. The resulting molded body was placed in a silicon nitride crucible and sintered at 1750°C for 3 hours in a nitrogen gas atmosphere.

[0127] (Preparation of sintered body focusing on thermal conductivity) 94.5 parts by mass of crystalline silicon nitride powder was mixed with yttrium oxide (specific surface area 3 m) as a sintering aid. ２ / g, manufactured by Shin-Etsu Chemical Co., Ltd.) 3.5 parts by mass and magnesium oxide (specific surface area 3 m ２The blended powder, to which 2 parts by weight of ethylenediaminetetraacetic acid (E100 / g, manufactured by Kojundo Chemical Laboratory) had been added, was wet-mixed in a ball mill using ethanol as a medium for 24 hours and then dried under reduced pressure. The resulting mixture was molded into a 6 x 45 x 75 mm shape and a 12.3 mm diameter x 1.6 mm thick shape at a molding pressure of 50 MPa, and then CIP molded at a molding pressure of 150 MPa. The resulting compacts were placed in a boron nitride crucible and sintered at 1900°C for 22 hours under a nitrogen gas pressure of 0.8 MPa.

[0128] <Method for evaluating crystalline silicon nitride powder and silicon nitride sintered body> The powder properties (surface oxygen content, internal oxygen content, total oxygen content, carbon content, etc.) of the obtained crystalline silicon nitride powder and the sintered body were measured as follows, and the measurement results are shown in Table 1.

[0129] (Measurement of BET specific surface area (SSA)) The BET specific surface area of ​​the crystalline silicon nitride powder was measured by the BET single-point method using nitrogen gas in accordance with JIS Z8830:2013. The results are shown in Table 1.

[0130] (Measurement of α-phase ratio) The α-phase ratio (proportion of α-phase) of the crystalline silicon nitride powder was measured by the following procedure. X-ray diffraction of the crystalline silicon nitride powder was performed using an X-ray diffractometer (manufactured by Rigaku, device name: Ultima IV) with CuKα radiation. The α-phase was measured by the diffraction intensity Ia １０２ and the diffraction intensity Ia of the (210) plane ２１０ The β phase is represented by the diffraction intensity Ib of the (101) plane. １０１ and the diffraction intensity Ib of the (210) plane ２１０ Using these diffraction line intensities, the gelatinization rate was calculated by the following formula. The results are shown in Table 1. １０２ +Ia ２１０ ) / (Ia １０２ +Ia ２１０ +Ib １０１ +Ib ２１０ ) x 100

[0131] (Measurement of carbon mass ratio) The carbon mass ratio Cp of the crystalline silicon nitride powder was measured using a commercially available carbon / sulfur analyzer (manufactured by Horiba, Ltd., device name: EMIA-Step). The measurement procedure involved raising the temperature from room temperature to 1350°C or higher in an oxygen atmosphere, and detecting the carbon monoxide and carbon dioxide produced during the heat treatment (heating) with an infrared detector to determine the carbon mass ratio Cp contained in the entire crystalline silicon nitride powder.

[0132] Using the same measuring device, the mass ratio C1 of the first volatile carbon adsorbed on the surface of the powder relative to the total powder was measured. The mass ratio Cs of carbon contained in the surface of the crystalline silicon nitride powder was measured. C1 was measured by heating in an air atmosphere to a temperature at which silicon nitride does not decompose. Specifically, the temperature was raised at a rate of 48°C / sec over a temperature range of 100°C or higher and lower than 700°C. In this case, only the carbon adsorbed on the surface of the crystalline silicon nitride powder combines with oxygen to form carbon monoxide or carbon dioxide. The carbon monoxide and carbon dioxide thus produced were detected with an infrared detector to determine the mass ratio C1 of the first volatile carbon adsorbed on the surface of the crystalline silicon nitride powder.

[0133] The mass ratio C2 of the second volatile carbon present within the surface of the powder to the total powder was measured by heating in an air atmosphere to a temperature at which silicon nitride does not decompose. Specifically, the temperature was raised at a rate of 48°C / sec over a temperature range of 700°C to 1200°C. In this case, only the carbon present within the surface of the crystalline silicon nitride powder combines with oxygen to form carbon monoxide or carbon dioxide. The carbon monoxide and carbon dioxide thus produced were detected with an infrared detector to determine the mass ratio C2 of the second volatile carbon present within the surface of the crystalline silicon nitride powder.

[0134] The mass ratio Ci of carbon contained inside the powder relative to the entire powder can be calculated from the formula (Cp (%) = C1 (%) + C2 (%) + Ci (%)) using the mass ratios Cp, C1, and C2 determined by the above-mentioned procedure.

[0135] The mass ratios of carbon Cp, C1, C2, and Ci were as shown in Table 1.

[0136] (Measurement of Oxygen Mass Ratio) The mass ratio Os of oxygen contained in the surface portion of the crystalline silicon nitride powder and the mass ratio Oi of oxygen contained in the interior were measured using a commercially available oxygen / nitrogen analyzer (manufactured by Horiba, Ltd., device name: EMGA-930). Specifically, the crystalline 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 Os of oxygen was determined by quantifying the amount of oxygen before nitrogen was detected. The mass ratio Oi of oxygen was also determined by quantifying the amount of oxygen after nitrogen began to be detected. Furthermore, the mass ratio Os of oxygen and the mass ratio Oi of oxygen were summed to determine the mass ratio Op of oxygen in the entire crystalline silicon nitride powder. The mass ratios Op, Os, and Oi of oxygen were as shown in Table 1. Table 1 also shows the ratio of the mass ratio Os to the mass ratio Cs.

[0137] (Measurement of particle size distribution) The particle size distribution of the crystalline silicon nitride powder was measured by laser diffraction and scattering. The measurement was performed in accordance with the method described in JIS Z8825:2013 "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 sizes at which the cumulative value from the smallest particle size reached 10%, 50%, 90%, and 100% of the total were determined as D10, D50, D90, and D100, respectively. The results are shown in Table 1.

[0138] (Relative Density of Silicon Nitride Sintered Body) The bulk density of the silicon nitride sintered body was measured by Archimedes' method. On the premise that all silicon nitride particles are converted to β-type silicon nitride, the relative density (%) was calculated from the calculated density obtained from the raw material composition and the bulk density.

[0139] (Bending strength of silicon nitride sintered body) The obtained silicon nitride sintered body was cut, milled, and polished to prepare a 3 mm x 4 mm x 40 mm bending test piece in accordance with JIS R1601:2008. The three-point bending strength at room temperature was measured using an Instron universal testing machine in accordance with JIS R1601:2008. The crosshead speed was 0.5 mm / min. The bending strength at room temperature is the average value of 40 pieces. The bulk bending strength is obtained by dividing the bending strength by the bulk density.

[0140] (Thermal conductivity of silicon nitride sintered body) The obtained silicon nitride sintered body was cut, machined, and polished to prepare a disk-shaped test piece with a diameter of 10 mm and a thickness of 1 mm for measuring thermal conductivity in accordance with JIS R1611:2010. The thermal conductivity was measured at room temperature by the flash method in accordance with JIS R1611:2010. The thermal conductivity is the average value of three disk-shaped test pieces.

[0141]

[0142]

[0143] (Examples 2 to 6) When preparing the crystalline silicon nitride powder, crystalline silicon nitride powder and sintered silicon nitride bodies were prepared under the same conditions as in Example 1, except that the temperature for additional drying of the imide and the maximum temperature for high-temperature firing were changed as shown in Table 1. Then, the crystalline silicon nitride powder and sintered silicon nitride bodies were measured using the same methods as in Example 1. The results are shown in Table 1.

[0144] (Comparative Examples 1 to 6) When preparing crystalline silicon nitride powder, the temperature for additional drying of the imide and the maximum temperature for high-temperature firing were changed as shown in Table 1. In addition, the milled silicon nitride powder was not annealed at 700°C or higher. Other than that, crystalline silicon nitride powder and sintered silicon nitride were prepared under the same conditions as in Example 1. Then, measurements of the crystalline silicon nitride powder and sintered silicon nitride were carried out using the same methods as in Example 1. The results are shown in Table 1.

[0145] As shown in Table 1, the silicon nitride sintered body prepared with a focus on strength (those labeled "high strength materials" in Table 1) exhibited higher relative densities and higher bending strengths than the comparative examples, and also had higher bulk bending strengths. Also, the silicon nitride sintered body prepared with a focus on thermal conductivity (those labeled "high thermal conductive materials" in Table 1) exhibited higher relative densities and higher bending strengths than the comparative examples, and also had higher thermal conductivities.

Claims

1. A crystalline silicon nitride powder containing silicon nitride, carbon, and oxygen, characterized in that the mass ratio Cp of the carbon to the entire powder is 0.200 mass% or less, the mass ratio C1 of the first volatile carbon detected in a temperature range of 100°C or higher and lower than 700°C in a temperature programmed morphological analysis is 0.001 mass% or higher and 0.030 mass% or lower, and the mass ratio C2 of the second volatile carbon detected in a temperature range of 700°C or higher and 1200°C or lower in a temperature programmed morphological analysis is 0.005 mass% or higher and 0.060 mass% or lower.

2. The crystalline silicon nitride powder according to claim 1, characterized in that the mass ratio Op of oxygen to the entire powder is 0.650 mass% or more and 1.500 mass% or less, and the mass ratio Os of surface oxygen to the entire powder measured by temperature-programmed morphology analysis is 0.050 mass% or more and less than 0.400 mass%.

3. The crystalline silicon nitride powder according to claim 2, wherein the mass ratio Os of the surface oxygen and the surface carbon amount Cs, which is the sum of the mass ratio C1 of the first volatile carbon and the mass ratio C2 of the second volatile carbon, satisfy the following formula (1): 1.5≦Os / Cs<10 (1) 4. BET specific surface area is 2.0m ２ / g or more, 12.0m ２ 2. The crystalline silicon nitride powder according to claim 1, wherein the crystalline silicon nitride powder has a molecular weight of 1 / g or less.

5. Crystalline silicon nitride powder according to claim 1, characterized in that the alpha phase ratio (proportion of alpha phase) is 95.0% or more.

6. A silicon nitride sintered body comprising the crystalline silicon nitride powder according to any one of claims 1 to 5 and a sintering aid.

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

8. An article comprising the silicon nitride sintered body according to claim 6.

9. The article according to claim 8, characterized in that the article is a bearing part or an insulating substrate for a power module.

10. A vehicle comprising an article according to claim 8.

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