Silicon nitride powder and method for producing silicon nitride sintered body

Abstract

The main problem addressed by the present invention is to provide a crystalline silicon nitride powder capable of obtaining a silicon nitride sintered body having easy sinterability, sufficient mechanical strength characteristics, and excellent thermal conductivity.　The present invention provides a silicon nitride powder characterized by containing primary particles of silicon nitride, having an amorphous layer comprising silicon oxide and / or silicon oxynitride on the surface of the primary particles, having an average thickness of the amorphous layer of 5.0 nm or less, having a coefficient of variation in the amorphous layer thickness represented by a specific formula (1) of 0.60 or less, and having a halogen content of 130 ppm or less. Also provided is a method for producing a silicon nitride sintered body, the method having a step for molding and sintering a sintering raw material containing the silicon nitride powder and a sintering aid.

Description

Silicon nitride powder and method for producing silicon nitride sintered body

[0001] The present invention relates to a crystalline silicon nitride powder suitable as a raw material for producing silicon nitride sintered bodies useful as heat-resistant structural materials, wear-resistant materials and circuit board materials, and to a method for producing silicon nitride sintered bodies.

[0002] Silicon nitride sintered bodies, with their high strength, corrosion resistance, and thermal shock resistance, have attracted attention as heat-resistant structural components for gas turbines, automobile engine parts, and other applications, as well as wear-resistant components for bearings and other applications. Silicon nitride sintered substrates are also increasingly being used as insulating substrates for power modules in electric vehicles (EVs) and railway vehicles. 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 of the silicon nitride powder, particularly its oxygen content, are known to significantly affect the sinterability and properties of the sintered body. In general, silicon nitride powders with a high oxygen content are easily sintered but lack sufficient high-temperature strength. Silicon nitride powders with a low oxygen content are difficult to sinter and are difficult to densify sufficiently.

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

[0004] In addition, Patent Document 2 discloses Si ３ N ４It has been disclosed that, in a ceramic substrate made of a material comprising a silicon oxide and a silicon composite oxide, by setting the concentration of silicon oxide and a 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 having sufficient bonding strength when bonded to a metal member such as aluminum can be obtained, and bonding reliability during thermal cycling can be improved. For such insulating substrate applications, high insulating properties and heat dissipation characteristics are required.

[0005] Japanese Patent Laid-Open No. 7-206409 Japanese Patent Laid-Open No. 2009-231388 Japanese Patent Laid-Open No. 5-70110 International Publication No. 2021 / 200868 Japanese Patent Publication No. 5-87441 Japanese Patent Laid-Open No. 6-329404 International Publication No. 2013 / 146713 International Publication No. 2021 / 200830

[0006] G. P. Gazzara and D. P. Messier, Am. Ceram. Soc. Bull. , 56 [9] pp. 777-80 (1977).

[0007] In general, the oxygen contained in silicon nitride powder is determined by the oxygen present in the silicon oxide layer and / or silicon oxynitride layer on the particle surface (surface oxygen), the oxygen present as a solid solution in the silicon nitride crystals inside the particles (internal oxygen), and the total oxygen amount (the sum of the surface oxygen amount and the internal oxygen amount). Thus, if the oxygen contained in 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.

[0008] Patent Document 3 describes amorphous SiO ２ The thickness of the amorphous film that can be read from the TEM photograph is about 6 nm, but no TEM photographs that allow the distribution of the thickness of the amorphous film to be determined are published. ２ SiO based membrane ２ The amount of oxygen is an excessive value of 1.7 wt% or more, and the ratio of the amount of surface oxygen to the BET specific surface area is 0.75 m ２ / g or more (the excellent thermal and mechanical properties inherent to silicon nitride sintered bodies cannot be expected). Furthermore, because the carbon content in silicon nitride powders A and B is as high as 0.20 wt % or more, voids are likely to occur in the silicon nitride sintered body, which causes problems such as large variations in thermal and mechanical properties.

[0009] Patent Document 4 discloses silicon nitride powder having an oxide film on the surface of primary particles, the oxide film having a thickness of 20.0 nm or less. However, the description "the primary particles have an oxide film on at least a portion of their surface" is vague, and no measurement data is disclosed that would allow for the distribution of oxide film thickness. Furthermore, the oxide film thickness disclosed in the examples is a large value of 6.0 nm or more. As a result, the resulting "bending strength of the silicon nitride sintered body can be, for example, 550 MPa or more, or 610 MPa or more (paragraph 0044)." This result confirms that an excessively thick oxide film cannot be expected to exhibit the excellent thermal and mechanical properties inherent to silicon nitride sintered bodies. Furthermore, the amount of surface oxygen is not disclosed, making it impossible to determine the ratio of the surface oxygen content to the BET specific surface area.

[0010] Patent Document 5 describes a silicon nitride powder having a surface oxygen content of 0.3 to 0.6 wt % and an internal oxygen content of 0.5 to 1.1 wt %. However, although this invention discloses the surface oxygen content and the internal oxygen content, it makes no mention of the influence of the ratio of the surface oxygen content to the specific surface area or the influence of the ratio of the total oxygen content to the specific surface area, and as a result, the high-temperature bending strength of the resulting sintered bodies remains at 550 MPa or less for sintered body A and 640 MPa or less for sintered body B.

[0011] Patent Document 6 discloses a silicon nitride powder characterized in that the amount of oxygen present on the particle surface in the form of hydroxide or oxide is 0.2 to 1.0 wt %, the amount of oxygen present in the form of solid solution is 0.5 to 1.2 wt %, and of the oxygen present in the form of solid solution, the amount of oxygen present within 3 nm directly below the particle surface is 0.1 to 0.5 wt %, and the amount of oxygen present 3 nm or more inside the particle surface is 0.8 wt % or less. In this patent publication, the surface oxygen amount (O) is defined as the sum of the amount of oxygen present on the particle surface in the form of hydroxide or oxide (surface oxygen amount (A)) and the amount of oxygen present in the form of solid solution within 3 nm directly below the particle surface (surface oxygen amount (B)). Paragraph 0010 of the same patent publication mentions that "if the surface oxygen content is less than 0.3 wt%, the wettability between the silicon nitride particles and the liquid phase consisting of the sintering aid during sintering decreases," and explains the importance of having a surface layer of appropriate thickness on the surface of the silicon nitride particles (a layer containing oxygen in the form of hydroxide, oxide, or solid solution). Furthermore, according to Table 2 in paragraph 0020, the surface oxygen content (O) of Examples 1 to 8 was 0.48 to 0.86 wt%, and the specific surface area (S) was 9.5 to 12.2 m. ２ / g, and the ratio S / O of the specific surface area (S) to the surface oxygen content (O) is 11.9 to 20.2. That is, the ratio O / S of the surface oxygen content (O) to the specific surface area (S) is 0.50 to 0.84 mg / m ２ (0.050~0.084% / m ２ g －１ (also written as ".")

[0012] Patent Document 7 discloses that the use of silicon nitride powder with low internal oxygen content and surface oxygen suitable for sintering makes it possible to obtain dense silicon nitride sintered bodies with excellent mechanical strength, particularly silicon nitride sintered bodies with both high thermal conductivity and excellent mechanical strength. However, the method for producing silicon nitride powder disclosed in this patent publication involves sintering an amorphous Si—N(—H) compound at a temperature of 1400 to 1700°C in a nitrogen-containing inert gas atmosphere or a nitrogen-containing reducing gas atmosphere while fluidizing it in a continuous sintering furnace. In this method, the amorphous Si—N(—H) compound is rapidly heated to a high temperature, which increases the degree of supersaturation near the particles and promotes nucleation (increasing the number of nuclei generated), resulting in finer silicon nitride particles with a high specific surface area. In contrast, in a normal firing method (for example, a method in which raw materials are placed in a crucible or the like and fired in a batch furnace, pusher furnace, or the like without fluidizing), if the oxygen content of the raw materials is excessively low, the BET specific surface area will be 8 m ２ / g or more. Moreover, the surface oxygen content of the crystalline silicon nitride powders shown in Examples 1 to 12 is a high concentration of 0.62 mass% or more, which may be a characteristic of the method of firing an amorphous Si-N(-H) compound while fluidizing it in a continuous firing furnace.

[0013] Patent Document 8 describes a method for reducing the ratio of the amount of surface oxygen to the specific surface area to 0.02 to 0.09% / m ２ g －１ (In the present invention, 0.2 to 0.9 mg / m ２ However, the silicon nitride powder is 0.02 to 0.09% / m ２ g －１ The regulation is that the BET specific surface area is 10 m ２In this case, the surface oxygen content is 0.2 to 0.9 wt%, which falls within the range of 0.3 to 0.6 wt% disclosed in Patent Document 5. Patent Document 8 also makes no mention of the internal oxygen content. Furthermore, Patent Document 8 describes acid treatment in which pulverized material with an extremely high oxygen content of 3.6 to 5.1 mass% is immersed in a high-concentration hydrofluoric acid solution for a long period of time. Treating pulverized material with a high-concentration hydrofluoric acid solution of 10 to 40 mass% to remove oxygen from the pulverized material results in residual fluorine remaining in the silicon nitride powder even after washing with water. However, the fluorine content of the resulting silicon nitride powder is not described. Table 1, which shows examples, lists the properties (bending strength and thermal conductivity) of silicon nitride sintered bodies obtained by sintering at 1900°C for 10 hours under a nitrogen pressure of 1 MPa as relative values, making it difficult to assess the effectiveness of the invention.

[0014] The object of the present invention is to solve the above problems and to provide a crystalline silicon nitride powder that can be used to obtain silicon nitride sintered bodies that are easy to sinter and have excellent sintered body properties such as fracture toughness. Another object of the present invention is to provide a crystalline silicon nitride powder that enables the production of silicon nitride sintered bodies that have sufficient mechanical strength and excellent thermal conductivity. A further object of the present invention is to provide a method for producing silicon nitride sintered bodies that have both high strength and excellent thermal conductivity.

[0015] The present inventors have extensively investigated the effects of the specific surface area, halogen content, oxygen content, and oxygen distribution of silicon nitride powder on sinterability and sintered body properties, particularly strength properties (fracture toughness, etc.) and thermal conductivity. As a result, it has been found that silicon nitride powder capable of yielding silicon nitride sintered bodies that are easily sintered and have excellent sintered body properties such as high toughness is silicon nitride powder that has an amorphous film of silicon oxide and / or silicon oxynitride on the surfaces of its primary particles, and the thickness of this amorphous film is not more than a specific value and the coefficient of variation of this film is smaller than a predetermined value (i.e., the distribution of the amorphous film thickness is within a narrow range). That is, one aspect of the present invention provides a crystalline silicon nitride powder comprising primary particles of silicon nitride, the surfaces of which have an amorphous film made of silicon oxide and / or silicon oxynitride, the average thickness of the amorphous film being 5.0 nm or less, and the coefficient of variation of the thickness of the amorphous film, expressed by the following mathematical formula (1), being 0.60 or less:

[0016]

[0017] The silicon nitride powder of one embodiment of the present invention comprises primary particles of silicon nitride, and the primary particles have an amorphous film on at least a portion of their surface. In this specification, the amorphous film refers to the amorphous portion present on the surface of the primary particles. That is, the amorphous film is a crystalline silicon nitride (α-type Si ３ N ４ or β-type Si ３ N ４ The contrast of the amorphous film is different from that of the area where lattice fringes corresponding to a specific lattice plane of crystalline silicon nitride (e.g., α-Si) are observed, and the amount of oxygen atoms present is higher than in the interior of the primary particle. ３ N ４The thickness of the portion where no lattice fringes are observed can be determined by measuring the thickness of the portion where no lattice fringes are observed, using the (1010) plane (the l in the exemplary Miller indices (hklm) represents 1 with a superscript line) as a reference, and the chemical composition (the abundance of oxygen atoms is higher than in the grain interior) can be confirmed by elemental analysis using energy dispersive X-ray spectroscopy (EDX). The amorphous film contains, for example, silicon oxide and / or silicon oxynitride.

[0018] In one embodiment of the present invention, the thickness of the amorphous film is measured at at least two locations on any silicon nitride particle, and at least 15 locations on any silicon nitride particle, for a total of 30 locations or more, and the average value and coefficient of variation are determined. The value of n in the above formula (1) is defined as 30 or more, but a larger value of n is preferred, more preferably 40 or more, and even more preferably 60 or more. If the value of n is less than 30, the variance in the thickness of the amorphous film cannot be accurately reflected. The coefficient of variation is expressed by the above formula (1), but when expressed as a percentage, the value of the above formula (1) is multiplied by 100 to express the percentage.

[0019] The silicon nitride powder has an amorphous film of a predetermined thickness on the surfaces of the primary particles, and its coefficient of variation is small. This makes it suitable as a raw material for producing silicon nitride sintered bodies with sufficient strength and excellent thermal conductivity. Furthermore, it enables the production of silicon nitride sintered bodies with both high strength and excellent thermal conductivity. The reason for this effect is believed to be as follows: During the sintering of silicon nitride molded bodies, the sintering aid reacts with the amorphous film on the surfaces of the primary particles during the heating process to form a low-melting silicate phase. After α-silicon nitride dissolves in this silicate melt phase, it reprecipitates as β-silicon nitride, forming a dense particle structure. The melting point of the silicate phase varies depending on its chemical composition. Therefore, the small coefficient of variation in the thickness of the amorphous film on the surfaces of the primary particles results in the formation of a uniform melt phase in the molded body, facilitating the rearrangement of silicon nitride particles, the dissolution of α-silicon nitride, and smooth reprecipitation as β-silicon nitride. This results in the formation of a favorable fine structure. Furthermore, the small coefficient of variation of the amorphous film thickness allows rapid densification with a minimum amount of liquid phase, resulting in a high-density silicon nitride sintered body with a low proportion of grain boundary phase. Furthermore, densification proceeds with a minimum amount of liquid phase, resulting in the growth of favorable silicon nitride columnar crystals, improving strength characteristics and developing a high-density fine structure with a low proportion of grain boundary phase. This allows for the production of a silicon nitride sintered body with higher thermal conductivity than conventional sintered bodies.

[0020] Unless otherwise specified, the materials exemplified in this specification can be used singly or in combination of two or more. When a plurality of substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified.

[0021] The average thickness of the amorphous film is 5.0 nm or less, and the coefficient of variation of the thickness of the amorphous film, expressed by the above formula (1), is 0.60 or less.

[0022] In one aspect of the present invention, there is provided a crystalline silicon nitride powder in which the amorphous film made of silicon oxide and / or silicon oxynitride has an average thickness of 2.0 nm or less and a coefficient of variation in the thickness of the amorphous film is 0.50 or less.

[0023] In one embodiment of the present invention, the BET specific surface area (SA) is 3.0 m ２ / g or more 13.0m ２ / g or less, a surface oxygen content (FSO) measured by temperature-programmed morphological analysis of 0.15% by mass or more and 0.65% by mass or less, and an internal oxygen content (FIO) measured by temperature-programmed morphological analysis of 0.25% by mass or more and 1.00% by mass or less.

[0024] In one embodiment of the present invention, the BET specific surface area (SA) is 3.0 m ２ / g or more 8.0m ２ / g, the surface oxygen content (FSO) measured by temperature-programmed morphology analysis is 0.15 mass% or more and 0.40 mass% or less, the internal oxygen content (FIO) measured by temperature-programmed morphology analysis is 0.25 mass% or more and 0.90 mass% or less, and the ratio of the surface oxygen content to the BET specific surface area (FSO / SA) is 0.25 mg / m ２ 0.50mg / m or more ２ A crystalline silicon nitride powder is provided, wherein the powder is less than

[0025] In one embodiment of the present invention, the BET specific surface area (SA) is 8.0 m ２ / g or more 13.0m ２ / g or less, the surface oxygen content (FSO) measured by temperature-programmed morphology analysis is 0.20 mass% or more and 0.65 mass% or less, the internal oxygen content (FIO) measured by temperature-programmed morphology analysis is 0.50 mass% or more and 1.00 mass% or less, and the ratio of the surface oxygen content to the BET specific surface area (FSO / SA) is 0.23 mg / m ２ 0.50mg / m or more ２ A crystalline silicon nitride powder is provided, wherein the powder is less than

[0026] The ratio (FSO / SA) of the surface oxygen content (FSO) measured by temperature-programmed morphological analysis to the BET specific surface area (SA) of the silicon nitride powder of one embodiment of the present invention is a measure for evaluating the thickness of the amorphous film consisting of silicon oxide and / or silicon oxynitride present on the surface of the primary particles of silicon nitride. ２ / g or more 13.0m ２ / g or less, the value is 0.23 mg / m ２ 0.50mg / m or more ２ When the sintering temperature is less than 100°C, an amorphous film of appropriate thickness is formed on the surface of the primary particles, and a molten phase of appropriate volume is formed by reaction with a sintering aid during the sintering process, making it easy to produce a high-density silicon nitride sintered body. However, in silicon nitride powder, the primary particles aggregate to form secondary and even tertiary particles, so the thickness of the amorphous film calculated based on a geometric model from the ratio of the surface oxygen content (FSO) to the BET specific surface area (SA) (FSO / SA) differs from the thickness of the amorphous film measured by transmission electron microscopy. Furthermore, since the amorphous film contains silicon oxynitride, and the ratio of silicon oxide to silicon oxynitride varies depending on the manufacturing conditions, it is difficult to calculate the thickness of the amorphous film from FSO / SA based on a geometric model. Even if the thickness of the amorphous film is calculated based on some assumption, it will be significantly different from and inconsistent with the thickness of the amorphous film measured by transmission electron microscopy. Therefore, in one aspect of the present invention, the thickness of the amorphous film measured by transmission electron microscope observation and the ratio (FSO / SA) of the surface oxygen content (FSO) measured by temperature-programmed morphological analysis to the BET specific surface area (SA) are treated as completely separate and independent physical quantities.

[0027] In one aspect of the present invention, there is provided a crystalline silicon nitride powder having a halogen content of 130 ppm or less, wherein the halogen content means the total amount of the fluorine content and the chlorine content.

[0028] In one aspect of the present invention, a crystalline silicon nitride powder is provided having a fluorine content of less than 120 ppm.

[0029] In one aspect of the present invention, there is provided a crystalline silicon nitride powder having a surface fluorine content of 110 ppm or less.

[0030] In one aspect of the present invention, the cumulative 10% diameter D １０ The crystalline silicon nitride powder has a particle size of 0.20 μm or more and 0.50 μm or less.

[0031] In one aspect of the present invention, there is provided a crystalline silicon nitride powder having a β fraction (β / (α+β)) of 1.3% by mass or more and 14.0% by mass or less. The β fraction is the mass ratio β / (α+β) of the β 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").

[0032] In one aspect of the present invention, there is provided a method for producing a silicon nitride sintered body, comprising the steps of: molding and sintering a sintering raw material containing the above-mentioned crystalline silicon nitride powder and a sintering aid. In the following paragraphs of this specification, the surface oxygen content and internal oxygen content measured by thermal morphology analysis will be referred to as the surface oxygen content and internal oxygen content, respectively, and the modifier "measured by thermal morphology analysis" will sometimes be omitted.

[0033] The present invention provides a silicon nitride powder that can be used to obtain a silicon nitride sintered body having both excellent mechanical strength and high thermal conductivity, and a method for producing a silicon nitride sintered body having both excellent mechanical strength and high thermal conductivity.

[0034] 3 is a transmission electron microscope photograph of the silicon nitride powder obtained in Example 1. FIG. 4 is a partial enlarged view of region R identified by the arrow in FIG. 1. FIG. 5 is a transmission electron microscope photograph of the silicon nitride powder obtained in Example 10. FIG. 6 is a partial enlarged view of region R identified by the arrow in FIG. 3. FIG. 7 is a diagram showing an example of an oxygen / nitrogen analysis chart.

[0035] <Silicon nitride powder> (Thickness of amorphous film present on the surface of primary particles) The average thickness of the amorphous film is 5.0 nm or less, and the coefficient of variation of the thickness of the amorphous film, expressed by the above formula (1), is 0.60 or less. The lower limit of the average thickness of the amorphous film may be, for example, 0.4 nm or more, 0.6 nm or more, 0.8 nm or more, or 0.9 nm or more. When the lower limit of the average thickness of the amorphous film is within the above range, sinterability can be further improved. In addition, grain growth during sintering can be promoted, and β-Si ３ N ４ The columnar crystals of β-Si are developed, and the mechanical strength and toughness are improved. The thermal conductivity is also further improved. The upper limit of the average thickness of the amorphous film may be, for example, 4.0 nm or less, 3.0 nm or less, 2.0 nm or less, or less than 1.0 nm. When the upper limit of the average thickness of the amorphous film is within the above range, β-Si ３ N ４ The growth of columnar crystals is not excessively suppressed, and fine columnar crystals develop, resulting in increased mechanical strength. Furthermore, the thickness (or volume fraction) of the grain boundary phase produced does not become excessively large, allowing thermal conductivity to be controlled to an appropriate level. The thickness of the amorphous film present on the primary particle surface does not necessarily need to be uniform. However, to ensure sinterability and uniformity of the sintered body structure and achieve good strength characteristics and thermal conductivity, it is necessary to control the coefficient of variation of the thickness of the amorphous film, expressed by the above mathematical formula (1), within an appropriate range. The upper limit of the coefficient of variation of the thickness may be, for example, 0.50 or less, 0.40 or less, or 0.36 or less. The lower limit of the coefficient of variation of the thickness may be, for example, 0.10 or more, 0.20 or more, or 0.24 or more.

[0036] (Adjusting the thickness of the amorphous film) In the production of silicon nitride powder by the imide pyrolysis method, the average thickness and its coefficient of variation of the amorphous film can be controlled, for example, by adjusting the atmospheric components (oxygen concentration, moisture concentration) in the milling process of the fired powder, the milling conditions (mill strength, processing time), etc. To reduce the coefficient of variation of the thickness, comprehensive conditions are set, for example, by slightly lowering the mill amplitude, moisture concentration, etc., and slightly lengthening the processing time.

[0037] In the production of silicon nitride powder by the direct nitriding method, the average thickness and coefficient of variation of the amorphous film can be controlled, for example, by adjusting the concentration of hydrofluoric acid during the acid treatment of the pulverized powder. To reduce the coefficient of variation of thickness, comprehensive conditions can be set, for example, by slightly lowering the concentration of hydrofluoric acid and slightly extending the treatment time. Note that the coefficient of variation of thickness is affected by the particle size distribution of the primary particles, and therefore, it may be necessary to take measures such as narrowing the particle size distribution of the primary particles.

[0038] (BET Specific Surface Area) The silicon nitride powder in one embodiment of the present invention has a silicon oxide layer and / or a silicon oxynitride layer on the particle surface, and has a specific surface area of ​​3.0 m2 measured by the BET method. ２ / g or more 13.0m ２ The specific surface area is an important powder property that governs densification through the dissolution and precipitation process of silicon nitride particles during sintering, and the BET specific surface area is 8.0 m ２ When the sintering rate is less than 1 / g, the driving force for sintering begins to decrease, and the BET specific surface area becomes 3.0 m ２ If the BET specific surface area is less than 13.0 m / g, the driving force for sintering is significantly reduced, and therefore a high-density silicon nitride sintered body cannot be obtained unless a large amount of sintering aid is added. Addition of a large amount of sintering aid deteriorates the properties of the silicon nitride sintered body. ２ If the specific surface area exceeds 4.5 m / g, not only will the green density decrease and molding become difficult in the usual molding method, but microcracks, voids, etc. will occur after sintering. Furthermore, not only will it have a negative effect on dimensional accuracy, but when sintering products of complex shapes, it will also cause cracks (crack clusters) at the corners of the sintered body, which is not desirable. ２ / g or more 12.7m ２ / g or less is preferable, and 8.0m ２ / g or more 12.3m ２ / g or less is more preferable, and 8.6m ２ / g or more 12.0m ２ It is more preferable that the range is 1 / g or less.

[0039] In the production of silicon nitride powder by the imide pyrolysis method, the specific surface area can be controlled, for example, by adjusting the specific surface area of ​​the amorphous Si—N(—H) compound, the oxygen content, etc. In the production of silicon nitride powder by the direct nitriding method, the specific surface area can be controlled, for example, by adjusting the pulverization treatment conditions during production.

[0040] The specific surface area in one embodiment of the present invention is a value (BET specific surface area) measured by the BET single-point method using nitrogen gas in accordance with the method described in JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption."

[0041] (Surface Oxygen Content (FSO)) In one embodiment of the present invention, the surface oxygen content (FSO) of the silicon nitride powder is in the range of 0.15 mass% or more and 0.65 mass% or less. If the surface oxygen content (FSO) is less than 0.15 mass%, the amount of melt phase generated during the sintering process is small, the densification rate is reduced, and a high-density sintered body cannot be obtained. On the other hand, if the surface oxygen content exceeds 0.65 mass%, although a high-density sintered body can be obtained, the amount of grain boundary phase contained in the sintered body increases, resulting in reduced strength properties (strength, toughness, etc.) and reduced thermal conductivity. The surface oxygen content (FSO) is more preferably 0.20 mass% or more and 0.60 mass% or less, and its lower limit may be 0.25 mass% or more and 0.30 mass% or more. Furthermore, its upper limit may be 0.50 mass% or less and 0.40 mass% or less. In the present disclosure, the surface oxygen content is the ratio of the mass of surface oxygen to the total mass of the silicon nitride powder. A detailed measurement method will be described later.

[0042] (Relationship between specific surface area (SA) and surface oxygen content (FSO)) The surface oxygen content of a silicon nitride powder suitable for realizing good sintered body strength characteristics (strength, toughness, etc.) and high thermal conductivity varies depending on its specific surface area. For example, a silicon nitride powder having a BET specific surface area (SA) of 3.0 m ２ / g or more 8.0m ２ / g, the surface oxygen content (FSO) is preferably 0.15% by mass or more and 0.40% by mass or less. The lower limit may be 0.18% by mass or more and 0.23% by mass or more. The upper limit may be 0.35% by mass or less and 0.30% by mass or less. If the surface oxygen content (FSO) is less than 0.15% by mass, the amount of molten phase generated during the sintering process is small, the densification rate decreases, and a high-density sintered body cannot be obtained. On the other hand, if the surface oxygen content exceeds 0.40% by mass, not only will the strength properties (strength, toughness, etc.) of the resulting sintered body decrease, but the thermal conductivity may also decrease.

[0043] BET specific surface area (SA) is 8.0 m ２ / g or more 13.0m ２ / g or less, the surface oxygen content (FSO) is preferably 0.20 mass% or more and 0.65 mass% or less. The lower limit may be 0.23 mass% or more and 0.28 mass% or more. The upper limit may be 0.60 mass% or less and 0.55 mass% or less. If the surface oxygen content (FSO) is less than 0.20 mass%, the amount of melt phase generated during the sintering process is small, the densification rate is reduced, and a high-density sintered body cannot be obtained. On the other hand, if the surface oxygen content exceeds 0.65 mass%, not only will the strength properties (strength, toughness, etc.) of the resulting sintered body be reduced, but the thermal conductivity will also be reduced. The BET specific surface area (SA) of the silicon nitride powder is preferably selected appropriately depending on the production conditions of the sintered body using it (e.g., the type and amount of sintering aid, sintering conditions, etc.) and the application of the resulting sintered body.

[0044] (Adjustment of Surface Oxygen Content (FSO)) In the production of silicon nitride powder by the imide pyrolysis method, the surface oxygen content (FSO) can be controlled, for example, by adjusting the components of the atmosphere in the firing furnace (oxygen concentration, carbon monoxide concentration) or the components of the atmosphere in the milling process of the fired powder (oxygen concentration, water concentration).

[0045] In producing silicon nitride powder by the direct nitriding method, the surface oxygen content (FSO) can be controlled, for example, by adjusting the components of the atmosphere in the sintering furnace (oxygen concentration, carbon monoxide concentration), the grinding conditions, and the hydrofluoric acid concentration during the acid treatment of the ground powder.

[0046] (Internal Oxygen Content (FIO)) In one embodiment of the present invention, the internal oxygen content (FIO) of the silicon nitride powder is 0.25% by mass or more and 1.00% by mass or less. If the internal oxygen content (FIO) is less than 0.25% by mass, abnormal grain growth of β-type columnar crystals occurs, which is undesirable as it reduces strength characteristics. Furthermore, in a typical sintering method, under conditions where the internal oxygen content (FIO) is less than 0.25% by mass, the particle size distribution of the sintered powder is unstable and the BET specific surface area is 3.0 m. ２ Since it becomes difficult to stably produce a crystalline silicon nitride powder having a crystalline oxygen content of 1 / g or more, the pulverization may be strengthened in a subsequent process. In this disclosure, the internal oxygen content is the ratio of the mass of internal oxygen to the total mass of the silicon nitride powder. The detailed measurement method will be described later.

[0047] On the other hand, if the internal oxygen content (FIO) exceeds 1.00 mass%, when silicon nitride particles dissolve into the grain boundary phase composed of sintering aids, etc. during the sintering process of silicon nitride, the composition of the grain boundary phase changes, inhibiting the precipitation and growth of β-type columnar crystals, making it impossible to fully exhibit the sintered body's properties, particularly high-temperature strength. Furthermore, the thermal conductivity of the sintered body decreases. The internal oxygen content is more preferably 0.35 mass% or more and 0.95 mass% or less, and its lower limit may be, for example, 0.45 mass% or more, 0.55 mass% or more, or 0.65 mass% or more. Furthermore, its upper limit may be 0.90 mass% or less, 0.85 mass% or less, or 0.80 mass% or less.

[0048] The internal oxygen content (FIO) of silicon nitride powder suitable for realizing good sintered body strength characteristics (strength, toughness, etc.) and high thermal conductivity also varies depending on its specific surface area. For example, when the BET specific surface area (SA) is 3.0 m ２ / g or more 8.0m ２ When the FIO is less than 0.25% by mass / g, the FIO is preferably 0.25% by mass or more and 0.90% by mass or less. The lower limit may be 0.35% by mass or more or 0.45% by mass or more. The upper limit may be 0.80% by mass or less or 0.70% by mass or less.

[0049] BET specific surface area (SA) is 8.0 m ２ / g or more 13.0m ２ / g or less, the internal oxygen content (FIO) is preferably 0.50% by mass or more and 1.00% by mass or less. The lower limit may be 0.60% by mass or more or 0.70% by mass or more. The upper limit may be 0.95% by mass or less or 0.90% by mass or less. If the internal oxygen content (FIO) is less than 0.50% by mass, the densification rate in the later stage of the sintering process decreases, and the density achieved by the sintered body tends to decrease. On the other hand, if the internal oxygen content exceeds 1.00% by mass, not only will the strength properties (strength, toughness, etc.) of the resulting sintered body decrease, but the thermal conductivity will also decrease.

[0050] (Ratio (FSO / SA) of Surface Oxygen Content (FSO) to BET Specific Surface Area (SA)) The densification rate of a molded body containing silicon nitride powder and a sintering aid varies depending on the specific surface area of ​​the silicon nitride powder used as a raw material, and therefore the preferred value of the surface oxygen content (FSO) in a silicon nitride powder according to one embodiment of the present invention varies depending on the specific surface area (SA). In one embodiment of the present invention, %×g = g / 100 = 10 mg, and the ratio (FSO / SA) of the surface oxygen content (FSO) to the BET specific surface area (SA) is expressed in mg / m ２ (The numerator m is 10 －３ The denominator m means meters), so % / m ２ g －１ = 10 mg / m ２ The BET specific surface area (SA) is 3.0 m ２ / g or more 13.0m ２ / g or less, the ratio of the surface oxygen content (FSO) to the BET specific surface area (SA) (FSO / SA) is in the range of 0.23 mg / m ２ 0.50mg / m or more ２ is less than (0.023% / m ２ g －１ 0.050% / m or more ２ g －１When the FSO / SA ratio is within the above range, the densification rate is improved, grain growth during sintering is appropriate, and a high-density silicon nitride sintered body is obtained that is composed of β-type columnar crystals having a desired major axis diameter and aspect ratio and has a reduced proportion of grain boundary phase, thereby satisfying both high levels of strength characteristics (strength, toughness, etc.) and thermal conductivity.

[0051] FSO / SA ratio is 0.23 mg / m ２ If the FSO / SA ratio is less than 0.50 mg / m, the amount of the molten phase generated by the reaction with the sintering aid is small, and the molten phase is unable to sufficiently cover the silicon nitride particles, so the densification rate decreases and it becomes difficult to obtain a high-density sintered body. ２ If the FSO / SA ratio is more than 0.25 mg / m, the amount of the molten phase produced will be too large, resulting in an increase in the amount of the grain boundary phase, which will not only reduce the strength characteristics (strength, toughness, etc.) of the resulting sintered body, but also reduce the thermal conductivity. ２ 0.47mg / m or more ２ The lower limit is preferably 0.27 mg / m or less, for example. ２ Above, 0.30mg / m ２ Above, 0.33mg / m ２ The upper limit may be, for example, 0.45 mg / m ２ Below, 0.43mg / m ２ Below, 0.41mg / m ２ It may be the following:

[0052] (Adjustment of FSO / SA ratio) In the production of silicon nitride powder by the imide pyrolysis method, the FSO / SA ratio can be controlled, for example, by adjusting the components of the atmosphere (oxygen concentration, moisture concentration) in the milling process of the fired powder, and is also affected by the components of the atmosphere in the firing furnace (oxygen concentration, carbon monoxide concentration).

[0053] In the production of silicon nitride powder by the direct nitriding method, the FSO / SA ratio can be controlled, for example, by adjusting the acid treatment conditions (such as the concentration of hydrofluoric acid) during production. In addition, it is also affected by, for example, the components of the atmosphere in the firing furnace (oxygen concentration, carbon monoxide concentration) and the milling conditions.

[0054] (Ratio (FIO / SA) of Internal Oxygen Content (FIO) to BET Specific Surface Area (SA)) Furthermore, since the ratio (FIO / SA) of the internal oxygen content (FIO) to the BET specific surface area (SA) is what primarily determines the later stage of densification of a molded body containing silicon nitride powder and a sintering aid, the preferred value of the internal oxygen content (FIO) in the silicon nitride powder of one embodiment of the present invention also varies depending on the specific surface area (SA). That is, the ratio (FIO / SA) of the internal oxygen content (FIO) to the BET specific surface area (SA) is 0.60 mg / m ２ 1.30mg / m or more ２ It is preferable that the ratio is 0.060% / m or less (0.060% / m ２ g －１ 0.130% / m or more ２ g －１ (The FIO / SA ratio can also be expressed as "FIO / SA ratio" or "SA ratio" below.) When the FIO / SA ratio is within the above range, abnormal grain growth during sintering can be suppressed, and the homogeneity of the sintered body structure consisting of β-type columnar crystals can be improved. Furthermore, a high-density silicon nitride sintered body can be obtained in which the composition and abundance of the grain boundary phase are appropriately controlled, thereby improving oxidation resistance and enabling the production of sintered parts with high high-temperature strength and excellent thermal conductivity.

[0055] FIO / SA ratio is 0.60 mg / m ２ If the content is above this level, abnormal grain growth of β-type columnar crystals is unlikely to occur, reducing the risk of a decrease in strength characteristics. In addition, the oxidation resistance of the resulting sintered body is good, making it suitable for use in high-temperature machine parts, etc., and is therefore preferable.

[0056] On the other hand, the FIO / SA ratio was 1.30 mg / m ２ When the FIO / SA ratio is 0.63 mg / m or less, the composition of the grain boundary phase is stabilized when the silicon nitride particles dissolve in the grain boundary phase composed of sintering aids, etc. during the sintering process of silicon nitride, and the precipitation and growth of β-type columnar crystals are not inhibited, and sufficient expression of sintered body properties, particularly fracture toughness, can be expected. In addition to good high-temperature strength, the thermal conductivity is also good. ２ 1.20mg / m or more ２ It is more preferable that the lower limit is 0.65 mg / m or less, for example. ２ Above, 0.70mg / m ２ Above, 0.74mg / m ２The upper limit may be, for example, 1.10 mg / m ２ Below, 1.00mg / m ２ Below, 0.90mg / m ２ It may be the following:

[0057] (Total oxygen content (TO)) Also, the BET specific surface area (SA) is 3.0 m ２ / g or more 13.0m ２ / g or less, the total oxygen content (TO) is preferably in the range of 0.50% by mass or more and 1.60% by mass or less. When the total oxygen content is 0.50% by mass or more, the amount of molten phase generated during the sintering process is sufficient, increasing the densification rate and resulting in a high-density sintered body. On the other hand, when the total oxygen content (TO) is 1.60% by mass or less, a densified sintered body can be obtained, and the oxygen content of the sintered body is not excessive, and the amount of grain boundary phase is not large, so the strength characteristics (room temperature strength, high-temperature strength, fracture toughness, etc.) are good. The sintered body also has high thermal conductivity. The total oxygen content (TO) is more preferably 0.60% by mass or more and 1.50% by mass or less. In the present disclosure, the total oxygen content is the ratio of the mass of the total amount of oxygen to the total mass of the silicon nitride powder. A detailed measurement method will be described later.

[0058] (Halogen Content) In one embodiment of the present invention, the halogen content refers to the total amount of fluorine and chlorine. The silicon nitride powder of one embodiment of the present invention preferably has a halogen content of 130 ppm or less, particularly 10 ppm to 130 ppm. A halogen content within this range facilitates densification during sintering, and provides excellent strength properties, such as high-temperature strength. Confirming that the halogen content is less than 10 ppm makes the analytical method itself difficult in terms of detection sensitivity. A halogen content of 130 ppm or less suppresses an increase in the proportion of halogen contained in the grain boundary phase of the resulting silicon nitride sintered body, preventing softening of the grain boundary phase at high temperatures and a decrease in high-temperature strength. Furthermore, deterioration of creep properties at high temperatures can be prevented. The halogens contained in the silicon nitride powder are primarily fluorine and chlorine, and the total amount of the fluorine and chlorine contents is preferably 10 ppm to 130 ppm. The upper limit of the total amount of fluorine and chlorine may be, for example, 120 ppm or less, 100 ppm or less, or 90 ppm or less, and the lower limit may be, for example, 20 ppm or more, 25 ppm or more, or 30 ppm or more.

[0059] (Fluorine Content) In one embodiment of the silicon nitride powder of the present invention, the total fluorine content is preferably less than 120 ppm, particularly 5 ppm to less than 120 ppm. A total fluorine content within this range facilitates densification during sintering, and provides excellent strength properties, such as high-temperature strength. Confirming that the total fluorine content is less than 5 ppm makes the analytical method itself difficult in terms of detection sensitivity. A total fluorine content of less than 120 ppm suppresses an increase in the proportion of fluorine contained in the grain boundary phase of the resulting silicon nitride sintered body, preventing softening of the grain boundary phase at high temperatures and preventing a decrease in high-temperature strength. It also prevents deterioration of creep properties at high temperatures. The total fluorine content is preferably 5 ppm to 100 ppm, and the lower limit may be, for example, 10 ppm to 20 ppm, or 30 ppm to 30 ppm. The upper limit may be, for example, 90 ppm or less, 80 ppm or less, or 70 ppm or less.

[0060] Because the impurity fluorine in silicon nitride powder affects the high-temperature strength of the resulting sintered body, silicon nitride powder with a low fluorine content has long been sought, but the effects of the combination of fluorine content and other powder properties have not been understood. In one aspect of the present invention, we have discovered the effects on the thermal and mechanical properties of the resulting sintered body not only of the fluorine content alone, but also of the halogen content or fluorine content in combination with the thickness and coefficient of variation of the amorphous film on the particle surface.

[0061] Fluorine is present mainly on the particle surface of the silicon nitride powder. The surface fluorine content of the silicon nitride powder is preferably 5 ppm or more and 110 ppm or less. When the surface fluorine content is within this range, densification during sintering is facilitated, and strength properties such as high-temperature strength are also good. Confirming that the surface fluorine content is less than 5 ppm is difficult using the analytical method itself in terms of detection sensitivity. When the surface fluorine content is 110 ppm or less, a decrease in the high-temperature strength of the resulting silicon nitride sintered body can be prevented, as in the case where the total fluorine content is less than 120 ppm. Furthermore, deterioration of creep properties at high temperatures can also be suppressed.

[0062] The surface fluorine content is preferably 5 ppm or more and 110 ppm or less, and the lower limit may be, for example, 15 ppm or more or 25 ppm or more. The upper limit may be, for example, 90 ppm or less or 70 ppm or less. The ratio of the surface fluorine content to the total fluorine content is preferably 0.7 or more, and more preferably 0.8 or more.

[0063] In the production of silicon nitride powder by the imide pyrolysis method, acid treatment of the calcined powder is not generally required, making it easy to reduce the total fluorine content to less than 120 ppm. On the other hand, in the direct nitriding method, pulverization is generally required to produce fine powder, so acid treatment is required to reduce impurities resulting from pulverization. In this case, chemical components from the acid treatment remain, increasing the content of impurities such as fluorine. For this reason, many attempts have been made to reduce the fluorine content of silicon nitride powder, and these tend to rely on post-treatment (washing) after calcination.

[0064] (Chlorine Content) The total chlorine content of the silicon nitride powder of one embodiment of the present invention is preferably 110 ppm or less, particularly 5 ppm to 110 ppm. A total chlorine content within this range facilitates densification during sintering, and provides excellent strength properties, such as high-temperature strength. Confirming that the total chlorine content is less than 5 ppm makes the analytical method itself difficult in terms of detection sensitivity. A total chlorine content of 110 ppm or less prevents an excessive increase in the proportion of chlorine contained in the grain boundary phase of the resulting silicon nitride sintered body, preventing softening of the grain boundary phase at high temperatures and a decrease in high-temperature strength. Furthermore, deterioration of creep properties at high temperatures can be suppressed. The total chlorine content is preferably 10 ppm to 100 ppm, and the lower limit may be, for example, 20 ppm to 30 ppm. The upper limit may be, for example, 90 ppm or less or 80 ppm or less.

[0065] Chlorine is also present mainly on the particle surface of silicon nitride powder. The surface chlorine content of silicon nitride powder is preferably 100 ppm or less, particularly 5 ppm to 100 ppm. When the surface chlorine content is within this range, densification during sintering is facilitated, and strength properties such as high-temperature strength are also good. Confirming that the surface chlorine content is less than 5 ppm makes the analytical method itself difficult in terms of detection sensitivity. When the surface chlorine content is 100 ppm or less, a decrease in the high-temperature strength of the resulting silicon nitride sintered body can be prevented, as in the case where the total chlorine content is 110 ppm or less. Furthermore, deterioration of creep properties at high temperatures can also be suppressed.

[0066] (Adjustment of Halogen Content) In the production of silicon nitride powder by the imide pyrolysis method, silicon halide is used as a raw material, and therefore, ammonium halide such as ammonium chloride, which is a by-product of the imide synthesis reaction, is thoroughly washed and removed with liquid ammonia to reduce the halogen content of the nitrogen-containing silane compound such as silicon diimide to 250 ppm or less, preferably 180 ppm or less. Alternatively, a large amount of ammonia gas is supplied during the pyrolysis of the resulting imide to significantly reduce the hydrogen halide concentration (e.g., hydrogen chloride concentration) in the atmosphere.

[0067] On the other hand, in the direct nitriding method, pulverization is basically required to produce fine powder, and therefore acid treatment is required to reduce impurities resulting from pulverization. In this case, chemical components from the acid treatment remain, increasing the content of impurities such as chlorine. Therefore, as with the fluorine content, care must be taken with the post-treatment (washing) after firing to reduce the chlorine content.

[0068] (Method for measuring total fluorine content and surface fluorine content) The total fluorine content of silicon nitride powder can be determined in accordance with JIS R 1603:2007 "Method for chemical analysis of silicon nitride fine powder for fine ceramics" by quantifying the amount of fluorine desorbed by burning silicon nitride powder in an oxygen and water vapor stream using ion chromatography. The surface fluorine content can be determined by boiling a dispersion of silicon nitride powder in water to extract the fluorine present on the surface of the silicon nitride powder into the water, and quantifying the extracted fluorine using ion chromatography. The internal fluorine content is determined by subtracting the surface fluorine content from the total fluorine content. In the present disclosure, the total fluorine content refers to the ratio of the total mass of fluorine to the total mass of the silicon nitride powder. Meanwhile, the surface fluorine content refers to the ratio of the mass of surface fluorine to the total mass of the silicon nitride powder. The internal fluorine content refers to the ratio of the mass of internal fluorine to the total mass of the silicon nitride powder.

[0069] (Method for measuring total chlorine content and surface chlorine content) The total chlorine content of silicon nitride powder can also be determined in accordance with JIS R 1603:2007 "Method for chemical analysis of silicon nitride fine powder for fine ceramics" by quantifying the amount of chlorine desorbed by burning silicon nitride powder in oxygen and water vapor stream using ion chromatography. The surface chlorine content can be determined by boiling the dispersion of silicon nitride powder in water to extract the chlorine present on the surface of the silicon nitride powder into water, and quantifying the extracted chlorine using ion chromatography. The total chlorine content is the ratio of the mass of the total chlorine content to the total mass of the silicon nitride powder, and in the present disclosure, the total chlorine content is referred to as the chlorine content.

[0070] (Carbon Content) In one embodiment of the present invention, the silicon nitride powder preferably has a carbon content of 0.01% by mass or more and 0.14% by mass or less relative to the total mass of the silicon nitride powder. During the temperature rise process, the silicon oxide layer and silicon oxynitride layer present on the surface of the silicon nitride particles are reduced by carbon. A carbon content of 0.14% by mass or less is preferable because it ensures the amount of molten phase generated at high temperatures, increases the densification rate, and makes it easier to obtain a high-density sintered body. A carbon content of 0.01% by mass or more and 0.10% by mass or less is more preferable.

[0071] (The mass ratio β / (α+β) of the β phase to the total amount of the α phase and the β phase) In one embodiment of the silicon nitride powder of the present invention, the mass ratio β / (α+β) of the β phase to the total amount of the α phase and the β phase (sometimes referred to as the β fraction) is preferably 1.3 mass% or more and 14.0 mass% or less. When the β / (α+β) ratio is 14.0 mass% or less, the minor axis of the β-type columnar crystals constituting the sintered body is prevented from becoming thicker, the aspect ratio (ratio of major axis to minor axis) is increased, and good strength characteristics can be obtained. In particular, a significant decrease in fracture toughness can be prevented. The β / (α+β) ratio is preferably 3.0 mass% or more and 12.0 mass% or less, and more preferably 4.0 mass% or more and 8.0 mass% or less. When the β / (α+β) ratio is less than 1.3 mass%, abnormal grain growth occurs during sintering, resulting in a non-uniform sintered body structure composed of β-type columnar crystals, which is undesirable.

[0072] (Particle size distribution) Preferably, in the silicon nitride powder of one embodiment of the present invention, the cumulative 10% diameter D １０ is 0.22 μm or more and 0.50 μm or less, and the cumulative 90% diameter D ９０ is 1.90 μm or more and 2.40 μm or less, and the difference [D ９０ -D １０ When the volume-based cumulative particle size distribution is within the above range, the packing properties of the silicon nitride powder are good, and a homogeneous silicon nitride molded body can be obtained with almost no molding defects.

[0073] (Cumulative 10% diameter D １０ ) Cumulative 10% diameter D １０If the diameter is 0.22 μm or more, the proportion of fine particles is not too high, so that not only is it easy to mold by a normal molding method with a sufficient green density, but also microcracks, voids, etc. are less likely to occur after sintering. Furthermore, it is preferable because it does not adversely affect the dimensional accuracy and, when sintering a product of a complex shape, it is possible to prevent breaks (crack clusters) from occurring at the corners of the sintered body. １０ When the cumulative 10% diameter D is 0.50 μm or less, the proportion of fine particles is high, so that a driving force for sintering is obtained, and a high-density silicon nitride sintered body can be obtained without adding a large amount of sintering aid. Addition of a large amount of sintering aid may deteriorate the properties of the silicon nitride sintered body. １０ The upper limit of the cumulative 10% diameter D may be, for example, 0.48 μm or less, 0.46 μm or less, or 0.44 μm or less. １０ The lower limit may be, for example, 0.25 μm or more, 0.29 μm or more, or 0.32 μm or more.

[0074] (Cumulative 90% diameter D ９０ ) Cumulative 90% diameter D in the volume-based cumulative particle size distribution ９０ The cumulative 90% diameter D is preferably 1.90 μm or more and 2.40 μm or less. ９０ When the diameter is 1.90 μm or more, the particle size distribution is not too sharp, and the green density can be ensured by a normal molding method. When this is sintered, a sintered body with a fine structure is obtained, but since the individual silicon nitride particles are not too fine, the fracture toughness is good. In addition, the high-temperature strength is also high. ９０ When the cumulative 90% diameter D is 2.40 μm or less, the proportion of coarse particles is small, so that the particles do not grow into larger particles during sintering, and it is easy to prevent microcracks, voids, etc. from occurring around the coarse particles and becoming a source of fracture. ９０ The upper limit of the cumulative 90% diameter D may be, for example, 2.35 μm or less, 2.25 μm or less, or 2.15 μm or less. ９０ The lower limit may be, for example, 1.95 μm or more, 2.00 μm or more, or 2.05 μm or more.

[0075] (Difference in particle size [D ９０ -D １０]) In this way, the cumulative 90% diameter D ９０ and cumulative 10% diameter D １０ can be adjusted within the above range, and the difference D ９０ -D １０ is preferably 1.60 μm or more and 2.00 μm or less. ９０ -D １０ When [D] is 1.60 μm or more, the particle size distribution does not become too sharp, and the green density can be ensured by a normal molding method. ９０ -D １０ When [D] is 2.00 μm or less, the particle size distribution is not too wide, and the sintered structure can be made uniform. Specifically, it is possible to prevent coarse particles from growing during sintering, which can cause microcracks, voids, etc. to occur around the particles and become the source of fracture. ９０ -D １０ ] may be, for example, 1.65 μm or more and 1.95 μm or less, or 1.70 μm or more and 1.90 μm or less.

[0076] In one embodiment of the present invention, the cumulative 10% diameter D in the volume-based cumulative particle size distribution １０ and cumulative 90% diameter D ９０ is the particle diameter (D) at which the cumulative value in the volume-based cumulative particle size distribution reaches 10% or 90% when the particle size distribution is measured by the laser diffraction / scattering method. １０ and D ９０ The laser diffraction scattering method can be measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - laser diffraction and scattering method."

[0077] <<Method for Producing Silicon Nitride Powder>> <<Imide Pyrolysis Method>> The method for producing silicon nitride powder in one embodiment of the present invention is not particularly limited. For example, in the imide pyrolysis method, an amorphous Si—N(—H) compound is calcined to produce crystalline silicon nitride powder. In this method, a nitrogen-containing silane compound such as silicon diimide, silicon tetraamide, or silicon chlorimide is produced by a known method, such as reacting a silicon halide with ammonia, specifically, reacting a silicon halide such as silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, or silicon tetraiodide with ammonia in the gas phase, or reacting a liquid silicon halide with liquid ammonia.

[0078] <Raw Material: Nitrogen-Containing Silane Compound> (Amorphous Si—N(—H) Compound) These nitrogen-containing silane compounds are represented by the following composition formula (I).

[0079] (In the formula, n = 0 to 12, and although not specified in the composition formula, compounds containing halogen as impurities are included.) The amount of halogen contained as an impurity is 0.05 mass% or less, preferably 0.02 mass% or less. In one aspect of the present invention, for the sake of convenience, nitrogen-containing silane compounds represented by n = 8 to 12 are referred to as silicon diimides.

[0080] The amorphous Si—N(—H) compound is produced by a known method, such as a method of thermally decomposing the obtained nitrogen-containing silane compound at a temperature of 1100° C. or less in a nitrogen or ammonia gas atmosphere, or a method of reacting a silicon halide such as silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, or silicon tetraiodide with ammonia at a high temperature.

[0081] The amorphous Si—N(—H) compound used in one embodiment of the present invention is an amorphous Si—N—H compound containing the elements Si, N, and H obtained by thermal decomposition of a part or all of the nitrogen-containing silane compound, or an amorphous silicon nitride containing Si and N, and is represented by the following composition formula (II). In one embodiment of the present invention, the amorphous Si—N(—H) compound is Si—N(—H) compound represented by the following composition formula (II) where x=1. ６ N ２(NH) ９ Amorphous Si represented by x=4 ３ N ４ The series of compounds up to and including Si where x=3 ６ N ６ (NH) ３ is called silicon nitrogen imide.

[0082] (where x = 0 to 4, and includes compounds containing halogen as impurities, although not specified in the composition formula)

[0083] The value of x is preferably 1.30 or more and 3.60 or less, and more preferably 1.65 or more and 3.10 or less.

[0084] In the composition formula (II), Si represented by x=0 ６ (NH) １２ is a silicon diimide, but silicon diimide is a nitrogen-containing silane compound and is not included in the amorphous Si—N(—H)-based compound in one embodiment of the present invention.

[0085] (Halogen Content) The amount of halogens such as chlorine contained as impurities in the amorphous Si—N(—H) compound is preferably 0.020 mass% (200 ppm) or less, more preferably 0.012 mass% (120 ppm) or less. The amount of halogen can be controlled by the amount of cleaning solution per kg of produced imide. The ratio of cleaning solution amount / imide amount (cleaning solution amount per kg of produced silicon diimide) may be, for example, 50 liters / kg or more to 300 liters / kg or less, and more preferably 60 liters / kg or more to 250 liters / kg or less.

[0086] (Carbon content) The amount of carbon contained as an impurity in the silicon nitride powder depends on the content of hydrocarbons such as toluene contained in the nitrogen-containing silane compound. For example, when the nitrogen-containing silane compound is silicon diimide, the toluene content of the silicon diimide can be controlled to 0.01% by mass or more and 0.40% by mass or less, thereby making it possible to control the carbon content of the silicon nitride powder after high-temperature firing to 0.01% by mass or more and 0.14% by mass or less. It is more preferable that the toluene content of the silicon diimide is 0.01% by mass or more and 0.30% by mass or less.

[0087] (Specific Surface Area) The specific surface area of ​​the amorphous Si—N(—H) compound, which is an intermediate raw material for producing silicon nitride powder according to one embodiment of the present invention, is 300 m ２ / g or more 680m ２ / g or less. ２ / g or more, rapid crystallization occurs in the temperature range of 1000 to 1400°C, and the generation of needle-shaped particles or aggregated particles can be prevented. When a sintered body is produced from such powder, a homogeneous structure is formed, and the strength and thermal conductivity of the resulting sintered body are good. On the other hand, ２ When the specific surface area of ​​the amorphous Si-N(-H) compound is 340 m / g or less, the degree of supersaturation near the particles increases during crystallization, resulting in a large number of nuclei being generated, and the specific surface area of ​​the crystalline silicon nitride powder does not decrease. As a result, the sinterability is good, and the strength and thermal conductivity of the sintered body are good. The specific surface area of ​​the amorphous Si-N(-H) compound is 340 m / g or less. ２ / g or more 610m ２ / g or less, and ２ / g or more 570m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0088] The specific surface area of ​​the amorphous Si—N(—H) compound can be adjusted by the maximum temperature during thermal decomposition of the nitrogen-containing silane compound that serves as the raw material. The lower the maximum temperature during thermal decomposition, the larger the specific surface area of ​​the amorphous Si—N(—H) compound. When the nitrogen-containing silane compound is silicon diimide, the maximum temperature for thermal decomposition may be, for example, 400° C. or higher and 1000° C. or lower, and more preferably 550° C. or higher and 950° C. or lower.

[0089] (Oxygen Content Ratio) The oxygen content ratio of the amorphous Si—N(—H) compound can be controlled by adjusting the amount of oxygen contained in the nitrogen-containing silane compound and the oxygen partial pressure (oxygen concentration) in the atmosphere during thermal decomposition of the nitrogen-containing silane compound. The lower the oxygen content of the nitrogen-containing silane compound and the lower the oxygen partial pressure in the atmosphere during thermal decomposition, the lower the oxygen content ratio of the amorphous Si—N(—H) compound can be. The oxygen concentration in the inert gas supplied to the thermal decomposition furnace may be, for example, 60 ppm (Volume / Volume) (hereinafter, ppm (Volume / Volume) may also be referred to as ppm(V)) or more and 1000 ppm(V) or less, and more preferably 100 ppm(V) or more and 600 ppm(V) or less. The oxygen content of the amorphous Si—N(—H) compound may be, for example, 0.36 mass % or more and 1.20 mass % or less, 0.55 mass % or more and 1.10 mass % or less, and more preferably 0.62 mass % or more and 1.00 mass % or less.

[0090] When the oxygen content of the amorphous Si—N(—H) compound is 0.36 mass% or more, the degree of supersaturation near the particles increases during crystallization, resulting in a large number of nuclei, resulting in a crystalline silicon nitride powder with a good specific surface area. When the oxygen content of the amorphous Si—N(—H) compound is 1.20 mass% or less, an increase in the internal oxygen content (FIO) of the resulting crystalline silicon nitride powder can be suppressed, which is preferable.

[0091] <Firing and Crystallization of Amorphous Si—N(—H) Compound> In one embodiment of the present invention, an amorphous Si—N(—H) compound is converted into a crystalline silicon nitride powder by firing it in a nitrogen-containing inert gas atmosphere or a nitrogen-containing reducing gas atmosphere, preferably at a temperature of 1400 to 1600° C. As the heating furnace used to heat the amorphous Si—N(—H) compound, a general firing furnace such as a batch-type electric furnace or a pusher-type electric furnace can be used.

[0092] The pusher furnace mentioned above is a firing furnace that can fire the material to be fired by sequentially pushing and transporting multiple base plates loaded with crucibles containing the ceramic raw materials to be fired into the furnace using a pusher mechanism, and is equipped with a furnace chamber in which the temperature and atmospheric conditions can be controlled.

[0093] (Nitrogen-containing inert gas atmosphere) The nitrogen-containing inert gas atmosphere refers to a nitrogen atmosphere, an inert gas atmosphere consisting of nitrogen and a rare gas such as argon, etc. To obtain a sintered powder with a low oxygen content, it is desirable that the atmosphere does not contain any oxygen, but even if oxygen is contained, the oxygen concentration may be 100 ppm (V) or less, and preferably 70 ppm (V) or less. The nitrogen-containing reducing gas atmosphere refers to an atmosphere consisting of an inert gas such as nitrogen and a reducing gas such as hydrogen or ammonia.

[0094] (Carbon monoxide (CO) concentration in furnace) During the crystallization 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. A CO concentration in the furnace of 60 ppm (V) or more eliminates the need to supply a large amount of nitrogen-containing inert gas to dilute the generated CO gas, thereby preventing cost increases. The CO concentration in the furnace may be 120 ppm (V) or more and 190 ppm (V) or less. The CO concentration in the furnace also affects the amount of surface oxygen derived from the silicon oxynitride layer on the particle surface.

[0095] (Firing temperature, heating rate) The specific surface area of ​​the crystalline silicon nitride powder is affected by the specific surface area of ​​the amorphous Si—N(—H) compound, the heating rate during firing, and the maximum holding temperature. Therefore, for example, it is preferable to set the heating rate to 40°C / h or more and 95°C / h or less, and the maximum holding temperature to 1470°C or more and 1600°C or less. Furthermore, since the β / (α+β) ratio in the crystalline phase is also affected by the heating rate and the maximum holding temperature, it is more preferable to set the heating rate to 45°C / h or more and 90°C / h or less, and the maximum holding temperature to 1480°C or more and 1570°C or less. The holding time at the maximum temperature may be, for example, 1 hour or more and 5 hours or less.

[0096] <Crushing Treatment> The obtained crystalline silicon nitride powder is crushed under an oxygen-containing inert gas atmosphere (e.g., under air flow) to adjust the surface oxygen content (FSO) and the ratio of the surface oxygen content (FSO) to the BET specific surface area (SA) (FSO / SA). Crushing is performed by milling, and a common milling device such as a bead mill, vibration mill, planetary ball mill, or jet mill can be used. The surface oxygen content of the silicon nitride powder is affected by the moisture concentration in the gas supplied to the mill container, etc. The moisture concentration may be, for example, 40 ppm (V) or more and 750 ppm (V) or less, and is more preferably adjusted to 90 ppm (V) or more and 650 ppm (V) or less. The moisture concentration particularly affects the surface oxygen content derived from the silicon oxide layer on the particle surface.

[0097] <Adjustment of particle size distribution> The particle size distribution of the crystalline silicon nitride powder is adjusted by the post-milling conditions. For example, when a continuous vibration mill is used, the particle size distribution is changed by comprehensively adjusting the milling ball diameter, ball filling amount, powder filling amount, mill amplitude, milling time, etc. The milling ball diameter is, for example, 15 mmΦ, and the ball space filling rate is, for example, 85%. When fine milling is required, the milling ball diameter is reduced, the ball space filling rate is increased, and the powder filling rate is reduced. The desired cumulative 90% diameter (D ９０ ) and cumulative 10% diameter (D １０To obtain this, for example, the mill amplitude may be adjusted to 6.5 mm or more and 8.3 mm or less, and the mill time may be adjusted to 25 minutes or more and 80 minutes or less. It is more preferable to set the mill amplitude to 7.0 mm or more and 8.1 mm or less, and the mill time to 35 minutes or more and 70 minutes or less.

[0098] <Adjusting the Thickness of the Amorphous Film on the Surface of Primary Particles> The average thickness and coefficient of variation of the amorphous film present on the surface of primary particles of crystalline silicon nitride powder are adjusted primarily by the post-milling conditions. For example, this can be controlled by adjusting the atmospheric components (oxygen concentration, moisture concentration) in the post-milling process, the milling conditions (mill amplitude, processing time), etc. To reduce the thickness variation coefficient, comprehensive conditions are set, such as by slightly lowering the milling amplitude, moisture concentration, etc., and slightly extending the processing time. Note that the thickness of the amorphous film is also affected by the atmospheric components (oxygen concentration, carbon monoxide concentration) in the firing furnace, so it is preferable to set the conditions accordingly.

[0099] <<Direct Nitridation Method>> In the production of silicon nitride powder by the direct nitridation method, in which metal silicon powder is heated in a nitrogen-containing non-oxidizing gas atmosphere, the average particle size and oxygen content of the raw material metal silicon powder, as well as the moisture content of the atmosphere, are controlled to obtain the desired silicon nitride powder. Specifically, metal silicon powder whose average particle size and oxygen content have been adjusted to fall within appropriate ranges is used as the raw material, and this is preferably nitrided in a nitrogen gas or nitrogen-containing non-oxidizing gas atmosphere with a moisture concentration of 1000 ppm or less, followed by pulverization, thereby obtaining silicon nitride powder having the specific surface area and oxygen content (total oxygen content, surface oxygen content, and internal oxygen content) as described above.

[0100] <Raw metal silicon powder> (cumulative 50% diameter D ５０ (Average particle diameter) The raw material silicon powder has a cumulative 50% diameter D ５０It is sufficient to use a metal silicon powder having an average particle size of 5.0 μm or less, and it is preferable to set the average particle size to 0.7 μm or more and 5.0 μm or less. When the average particle size of the metal silicon powder is 0.7 μm or more, the internal oxygen content of the silicon nitride powder obtained by nitriding and pulverization can be reduced. When the average particle size of the metal silicon powder is 5.0 μm or less, the nitriding reaction time can be shortened, resulting in excellent productivity. In addition, it is possible to prevent unreacted silicon from remaining. It is more preferable that the average particle size of the metal silicon powder is 1.0 μm or more and 3.0 μm or less.

[0101] (Oxygen Content) The oxygen content of the metal silicon powder can be adjusted by the milling and pretreatment conditions. As an example, the oxygen content of a metal silicon powder having an average particle size of 3 μm is 0.50 mass%, the oxygen content of a metal silicon powder having an average particle size of 6 μm is 0.30 mass%, and the oxygen content of a metal silicon powder having an average particle size of 10 μm is 0.20 mass%. Since the oxygen content of the metal silicon powder affects the internal oxygen content of the resulting crystalline silicon nitride powder, the oxygen content of the metal silicon powder used in the production of the silicon nitride powder of one embodiment of the present invention may be, for example, 0.40 mass% or more and 1.00 mass% or less. The oxygen content is more preferably 0.50 mass% or more and 0.90 mass% or less.

[0102] (Additives) The sintered raw material is prepared by adding a diluent, a nitriding accelerator, and an organic binder for molding to the above metal silicon powder. The diluent is crystalline silicon nitride powder with an average particle size of approximately 0.7 μm. The nitriding accelerator is calcium fluoride powder. The organic binder may be polyvinyl alcohol, carboxymethyl cellulose, wax emulsion, acrylic emulsion, or the like.

[0103] (Diluent) The addition of a diluent results in a fired product (nitrided ingot) that is easy to crush. The amount of crystalline silicon nitride powder added may be, for example, 2.5% by mass or more and 20.0% by mass or less, based on the total mass of the metal silicon powder, diluent, and nitriding promoter. Addition of less than 2.5% by mass is not preferred because not only does the nitriding reaction take a long time, but the ingot after nitriding becomes significantly hard, increasing the load on the crushing process. Addition of more than 20.0% by mass has little effect on the high-temperature firing conditions and the properties of the fired product, but increases costs, which is not preferred.

[0104] (Nitriding promoter: calcium fluoride powder) The amount of calcium fluoride powder added may be, for example, 0.01 mass % or more and 0.10 mass % or less relative to the total mass of the metal silicon powder, diluent, and nitriding promoter. If the amount added exceeds 0.10 mass %, the amount of internal fluorine in the resulting crystalline silicon nitride powder increases, which is not preferable. Note that if residual calcium and fluorine in the nitrided ingot is a concern, calcium fluoride need not be added.

[0105] (Organic Binder) In the case of polyvinyl alcohol, the amount of organic binder added may be, for example, 0.2 parts by mass or more and 1.8 parts by mass or less per 100 parts by mass of the total mass of the metal silicon powder, diluent, and nitriding promoter. If the amount added exceeds 1.8 parts by mass, the carbon content of the resulting crystalline silicon nitride powder increases, which is not preferable. If it is not necessary to increase the amount of firing raw material filled into the firing container, it is not necessary to add an organic binder.

[0106] (Diluent, Nitriding Promoter, and Organic Binder) The amount of the crystalline silicon nitride powder added is preferably 6.0% by mass or more and 18.0% by mass or less. The amount of the calcium fluoride powder added is preferably 0.02% by mass or more and 0.08% by mass or less. The amount of the polyvinyl alcohol added is preferably 0.2 parts by mass or more and 1.6 parts by mass or less.

[0107] <Feed Gas> Nitrogen or a non-oxidizing gas containing nitrogen is used as the feed gas for nitriding the sintered raw material containing metal silicon powder. To obtain a sintered powder with a low oxygen content, a feed gas containing no oxygen is desirable. However, even if oxygen is contained, the oxygen concentration may be 120 ppm (V) or less, and preferably 100 ppm (V) or less. The preferred lower limit of the oxygen concentration is 15 ppm (V) or more, or 25 ppm (V) or more. A nitrogen-containing reducing gas atmosphere is an atmosphere consisting of nitrogen, a nitrogen-containing inert gas, or the like, and a reducing gas such as hydrogen or ammonia. An example of a mixed gas of hydrogen gas and nitrogen gas is nitrogen gas containing approximately 0.5 to 15% by volume of hydrogen gas. An example of a mixed gas of ammonia gas and nitrogen gas is nitrogen gas containing 0.1% by volume or more and 4.4% by volume or less of ammonia gas. The mixed ratio of ammonia gas is preferably 0.6% by volume or more and 3.4% by volume or less. In this case, it is further desirable to set the moisture concentration in the gas atmosphere to 40 ppm or more and 1000 ppm or less, and a moisture concentration of 1000 ppm or less can suppress an increase in the amount of internal oxygen. The moisture concentration is more preferably 100 ppm or more and 800 ppm or less. The concentrations of the components (moisture, oxygen, and carbon monoxide) in the furnace atmosphere also affect the amount of surface oxygen derived from the silicon oxynitride layer on the particle surface.

[0108] <Nitriding step; firing> (nitriding temperature, time) When the firing raw material containing metal silicon powder is nitrided in the gas atmosphere, the nitriding temperature may be, for example, 1150°C or higher and 1565°C or lower, or 1200°C or higher and 1555°C or lower. The nitriding time may be, for example, about 15 to 40 hours, but the firing time including the temperature rise process will be, for example, 40 to 70 hours. Note that the maximum temperature in the high-temperature firing may be increased to around 1600°C, which also serves as a heat treatment for the fired product (nitrided ingot), thereby stabilizing the surface state of the resulting crystalline silicon nitride powder.

[0109] (Heating Rate) In the production of silicon nitride powder according to one embodiment of the present invention, it is preferable to investigate the nitriding reaction rate through preliminary studies and adjust the heating rate to 3.0°C / h or more and 10.0°C / h or less until a nitriding rate of 50% is reached. Setting the heating rate to 3.0°C / h or more until a nitriding rate of 50% shortens the firing time, thereby avoiding cost increases, and is therefore preferable. Setting the heating rate to 10.0°C / h or less until a nitriding rate of 50% is preferable because the proportion of β-type silicon nitride particles in the fired product (nitrided ingot) does not increase too much, and the β / (α + β) ratio can be kept to 40 mass% or less. In the temperature range where the nitriding rate is 50% or more, adjusting the heating rate to 10°C / h or more and 36°C / h or less increases the production rate, thereby limiting the nitriding time to approximately 15 to 40 hours. Setting the heating rate to 10°C / h or more until a nitriding rate of 50% or more shortens the firing time, thereby avoiding cost increases, and is therefore preferable. If the temperature rise rate is 36° C. / h or less at a nitriding rate of 50% or more, the fired product (nitrided ingot) does not become too hard, and the load on the crushing step does not increase, which is preferable.

[0110] It is more preferable to adjust the temperature rise rate up to a nitriding rate of 50% to be 4.5°C / h or more and 8.8°C / h or less, and to adjust the temperature rise rate at a nitriding rate of 50% or more to be 15°C / h or more and 34°C / h or less.

[0111] (Maximum Temperature) The maximum temperature in the high-temperature firing is, for example, 1485°C or higher and 1565°C or lower. If the maximum temperature is 1485°C or higher, the quality of the fired product (nitrided ingot) is stable, and in the crushing step after the completion of nitriding, there is no variation in the crushing degree, and the desired particle size distribution (particularly the cumulative 10% diameter D １０ and cumulative 90% diameter D ９０ ) can be realized. A maximum temperature of 1565°C or less is preferable because it suppresses aggregation of the generated particles and prevents the fired product from becoming hard, which increases the load on the pulverization process. It is also preferable because it prevents the growth of β-type silicon nitride particles and an increase in the β / (α+β) ratio. In order to obtain crystalline silicon nitride powder of stable quality with a controlled particle size distribution, it is more preferable to set the maximum temperature at 1505°C or higher and 1555°C or lower.

[0112] <Pulverization> In the pulverization step after nitriding, the fired product (nitrided ingot) is pulverized by a method such as dry pulverization or wet pulverization, and the particle size is adjusted to obtain silicon nitride powder pulverized to an average particle size of 1.0 μm or less, more preferably 0.6 to 0.9 μm. The specific surface area of ​​the pulverized silicon nitride powder is, for example, 7 to 16 m. ２ / g.

[0113] The milling time may be, for example, 1.0 hour or more and 10 hours or less, but the required milling time varies depending on the type of milling device. The milling conditions vary depending on the high-temperature sintering conditions. As an example, when a dry vibration mill is used to mill an ingot nitrided under the above-mentioned high-temperature sintering conditions, it is preferable to adjust the mill amplitude to, for example, 6.5 mm or more and 9.0 mm or less, and the milling time to, for example, 1.3 hours or more and 4.5 hours or less. An average particle size of 1.0 μm or less is advantageous in terms of sinterability, such as ensuring a densification rate in the sintering process. By setting the mill amplitude and milling time within the above ranges, the cumulative 10% diameter (D １０ ) and cumulative 90% diameter (D ９０ The mill amplitude may be 7.0 mm or more and 8.4 mm or less, and the mill time may be 1.5 hours or more and 4.2 hours or less.

[0114] The lower limit of the time for the pulverization treatment (pulverization time) may be, for example, 1.6 hours or more, 1.8 hours or more, or 2.0 hours or more. By setting the lower limit of the pulverization time within the above range, the pulverized material can be sufficiently finely ground, thereby further improving the acid treatment efficiency in the acid treatment step. The upper limit of the time for the pulverization treatment may be, for example, 4.0 hours or less, 3.8 hours or less, or 3.6 hours or less. By setting the upper limit of the pulverization time within the above range, the fired material can be sufficiently pulverized, and excessive pulverization can be prevented, thereby reducing production costs. Since the particle size distribution of the pulverized material affects the coefficient of variation of the thickness of the amorphous film on the surface of the primary particles after the acid treatment and washing described below, it is also important to set the mill amplitude low and the mill time long to narrow the particle size distribution.

[0115] The oxygen content of the pulverized product is adjusted to, for example, 1.5% by mass or more and 8.0% by mass or less. The upper limit of the oxygen content of the pulverized product may be, for example, 7.5% by mass or less, 7.0% by mass or less, 6.5% by mass or less, or 6.0% by mass or less. By setting the upper limit of the oxygen content of the pulverized product within the above range, the acid treatment time in the subsequent acid treatment step can be reduced. The lower limit of the oxygen content of the pulverized product may be, for example, 2.0% by mass or more, 2.5% by mass or more, or 3.0% by mass or more. Setting the lower limit of the oxygen content of the pulverized product within the above range facilitates adjustment of the thickness of the amorphous film on the primary particle surface of the silicon nitride powder after acid treatment, and the coefficient of variation can also be kept small. The oxygen content of the pulverized product can be adjusted within the above range, for example, 2.0 to 7.5% by mass, 2.5 to 7.0% by mass, or 3.0 to 6.0% by mass or less. The oxygen content of the pulverized material can be controlled by adjusting, for example, the amount of oxygen in the silicon powder, the components of the atmospheric gas in the firing step, and the pulverization time, and affects the thickness of the amorphous film on the surface of the primary particles.

[0116] <Acid treatment and cleaning> After crushing, the powder is washed with hydrofluoric acid (HF), hydrochloric acid (HCl), and sulfuric acid (H ２ SO ４ ), nitric acid (HNO ３ In this method, impurities in the pulverized material are removed and purified by using an inorganic acid such as fluorine or nitric acid in combination with the pulverized material. That is, acid treatment and washing are carried out using a mixed acid of hydrofluoric acid and hydrochloric acid, a mixed acid of hydrofluoric acid and sulfuric acid, a mixed acid of hydrofluoric acid and nitric acid, etc., but a mixed acid of hydrofluoric acid and hydrochloric acid is often used from the viewpoint of the solubility of SUS-based impurities. In this method, chemical components from the mixed acid treatment remain, and the content of impurities such as fluorine tends to be high.

[0117] (Hydrofluoric Acid) It is preferable to use hydrofluoric acid having a concentration of 0.50% by mass or more and 10.00% by mass or less. The upper limit of the hydrofluoric acid concentration may be, for example, 8.00% by mass or less, 7.00% by mass or less, or 6.50% by mass or less. By setting the upper limit of the hydrofluoric acid concentration within the above range, elution of the fine silicon nitride can be suppressed, and excessive reductions in the specific surface area and surface oxygen content can be suppressed. Furthermore, excessive reductions in the thickness of the amorphous film present on the primary particle surface can be suppressed. The lower limit of the hydrofluoric acid concentration may be, for example, 0.70% by mass or more, 1.00% by mass or more, or 1.30% by mass or more. By setting the hydrofluoric acid concentration within the above range and adjusting the treatment temperature and treatment time, it becomes easy to adjust the thickness of the amorphous film on the primary particles of the silicon nitride powder. The concentration of hydrofluoric acid also affects the coefficient of variation of the thickness of the amorphous film on the surface of the primary particles, so it is preferable to set the concentration to a slightly low level.

[0118] (Inorganic Acid Other Than Hydrofluoric Acid) The inorganic acid other than hydrofluoric acid used in combination preferably has a concentration of 1.0 mass% or more and 20.0 mass% or less. The upper limit of the concentration of the inorganic acid other than hydrofluoric acid may be, for example, 18.0 mass% or less, 16.0 mass% or less, or 14.0 mass% or less. The lower limit of the concentration of the inorganic acid other than hydrofluoric acid may be, for example, 2.0 mass% or more, 3.0 mass% or more, or 4.0 mass% or more. By setting the upper and lower limits of the concentration of the inorganic acid other than hydrofluoric acid within the above ranges, it becomes easy to adjust the amount of metal impurities in the silicon nitride powder.

[0119] (Conditions for acid treatment of pulverized material) As a means for contacting the pulverized material with acid, for example, a method of dispersing the pulverized material in an acid solution can be adopted, but in order to reduce the coefficient of variation of the thickness of the amorphous film on the surface of the primary particles, it is preferable to focus on quickly and uniformly dispersing the pulverized material.

[0120] The temperature of the acid (e.g., aqueous solution) in the acid treatment step is preferably set to 45°C or higher and 75°C or lower. The lower limit of the temperature of the acid in the acid treatment step may be, for example, 50°C or higher, or 55°C or higher. The upper limit of the temperature of the acid in the acid treatment step may be 70°C or lower, or 65°C or lower. The temperature of the acid in the acid treatment step may be adjusted within the above-mentioned range, and may be, for example, 50°C or higher and 70°C or higher, or 55°C or higher and 65°C or lower. The time for immersion in the treatment liquid may be, for example, 2 to 10 hours, or 4 to 8 hours.

[0121] (Liquid Temperature in the Cleaning Step) After the acid treatment, the solid matter (silicon nitride powder) in the acid-treated slurry is separated from the acid while the temperature is maintained near the set temperature. Therefore, liquid temperature control in the cleaning step after the acid treatment is particularly important. The temperature of the cleaning liquid (e.g., ion-exchanged water) is preferably set to 45°C or higher and 75°C or lower. The lower limit of the cleaning water temperature may be, for example, 50°C or higher or 55°C or higher. The upper limit of the cleaning water temperature may be 70°C or lower or 65°C or lower. The temperature of the cleaning liquid in the cleaning step may be adjusted within the above-mentioned range, for example, 50°C or higher and 70°C or lower or 55°C or higher and 65°C or lower. If the temperature of the cleaning liquid is lower than 45°C, the solid matter is cooled while in contact with the high-concentration acid, and the chemical liquid components adsorbed on the solid matter surface tend to remain, resulting in a high content of impurities such as fluorine and chlorine. Setting the temperature of the washing liquid to 45 to 70°C means adjusting the temperature of the slurry after the acid treatment to a range of 45°C or higher and 70°C or lower.

[0122] (pH of cleaning solution) The pH of the cleaning solution at the end of cleaning is adjusted to 1.3 or more and 5.0 or less. The pH of the cleaning solution may be 3.2 or more and 4.2 or less, or 3.4 or more and 4.0 or less.

[0123] (Drying Temperature) The drying temperature of the filter cake obtained by washing and filtering the solid matter in the slurry is preferably adjusted to 70° C. or higher and 100° C. or lower. The drying temperature may be 75° C. or higher and 95° C. or lower, or 85° C. or higher and 90° C. If the drying temperature is low, a long drying time is required, and if the drying temperature is too high, the dried product becomes hard and puts a strain on the pulverization process, which are undesirable.

[0124] <Adjusting the Thickness of the Amorphous Film on the Primary Particle Surface> The average thickness and coefficient of variation of the amorphous film present on the primary particle surface of crystalline silicon nitride powder are mainly controlled by adjusting the acid treatment and washing conditions (hydrofluoric acid concentration, acid treatment temperature, and acid treatment time). However, since the coefficient of variation of the amorphous film thickness is affected not only by the acid treatment and washing conditions but also by the milling conditions (mill amplitude, treatment time), the components of the atmospheric gas in the firing process, and the oxygen content of the raw silicon powder, comprehensive conditions must be set. To reduce the thickness coefficient of variation, it is preferable to set conditions such as narrowing the particle size distribution by adjusting the milling conditions (setting the mill amplitude slightly lower and the milling time slightly longer), controlling the components of the atmosphere in the firing furnace (oxygen concentration, carbon monoxide concentration) within a predetermined concentration range, and controlling the oxygen content of the raw silicon powder to an appropriate value.

[0125] (Method for measuring surface oxygen content (FSO), internal oxygen content (FIO), and total oxygen content (TO)) In one embodiment of the present invention, the surface oxygen content (FSO), internal oxygen content (FIO), and total oxygen content (TO) of a silicon nitride powder can be measured by the following procedure. The silicon nitride powder is weighed, and the oxygen and nitrogen contents of the silicon nitride powder are analyzed using an oxygen / nitrogen simultaneous analyzer. The measurement sample is heated from 20°C to 2000°C in a helium gas atmosphere at a heating rate of, for example, 4 to 8°C / sec, and the oxygen desorbed as the temperature rises is detected to perform a temperature-rise morphology analysis as follows. At the beginning of the temperature rise, oxygen bonded to the surface of the silicon nitride powder is desorbed, and the surface oxygen content can be determined by quantifying the amount of desorbed oxygen. In other words, the surface oxygen content derived from the silicon oxide layer on the particle surface is determined. Thereafter, when the temperature reaches approximately 1400°C, the silicon oxynitride near the particle surface begins to decompose. The start of decomposition of silicon oxynitride can be detected by the start of nitrogen detection, and the amount of surface oxygen derived from the silicon oxynitride layer near the particle surface can be determined.

[0126] The amorphous film on the surface of the primary particles defined in one embodiment of the present invention may be composed of silicon oxide or silicon oxynitride. However, based on the results of confirming the elemental distribution of silicon, oxygen, and nitrogen on the observation surface by energy dispersive X-ray analysis (EDX), it was determined that in most silicon nitride powders, the amorphous film was an Si—O—N compound containing both silicon oxide and silicon oxynitride.

[0127] FIG. 5 shows an example of an oxygen and nitrogen concentration chart for silicon nitride powder obtained by simultaneous oxygen and nitrogen analysis. Peak 1 is the surface oxygen peak, and Peak 2 is the internal oxygen peak. Peak 3 is the nitrogen peak. Line 4 shows the temperature rise line. Peaks 1 and 2 correspond to the temperature T ２ The left end of peak 3 where nitrogen begins to be detected (temperature T １ ) and the temperature T ２ In FIG. 5, in the temperature range where the top of Peak 1 is detected, Peak 3 clearly begins to rise. From this, it can be seen that the temperature T １ The rise of Peak 3 in the vicinity also reveals that a silicon oxide layer and a silicon oxynitride layer are formed on the surface of the silicon nitride particles of one embodiment of the present invention.

[0128] Temperature T at which peak 3 rises １ is usually between 1350 and 1450°C, and is the temperature T ２ is usually between 1390 and 1460°C. The temperature at which detection of Peak 1 begins (the temperature at the left end of Peak 1) is, for example, 750 to 1300°C. The temperature at which detection of Peak 2 ends (the temperature at the right end of Peak 2) is, for example, 1900 to 2000°C. From the integrated values ​​(areas) of Peaks 1 and 2, the internal oxygen content (FIO) and surface oxygen content (FSO) can be determined based on the calibration curve.

[0129] Usually, the temperature at which the detection of peak 3 begins, that is, the temperature T １However, the nitrogen concentration peak shows a broadening characteristic of a Gaussian function, and has a wide tail on both the low-temperature side and the high-temperature side. Therefore, the temperature at which nitrogen detection starts (i.e., the temperature at which Peak 3 rises) T １ Therefore, in order to improve the analytical accuracy, in one embodiment of the present invention, a tangent line is drawn to peak 3 at a height that is 20% (i.e., 1 / 5) of the maximum value (height of the peak top) of peak 3, and the point where the tangent line intersects with the time axis (height zero) (time t ２ ) as a reference, time t ２ Temperature T at ２ The concentration peak due to surface oxygen and the concentration peak due to internal oxygen are separated. The surface oxygen content (FSO) in one embodiment of the present invention is the amount of oxygen released from both the silicon oxide layer and the silicon oxynitride layer present on the particle surface. The temperature T at which Peak 3 appears after the start of temperature rise is １ The amount of surface oxygen derived from the silicon oxide layer on the particle surface can be measured by the time t ２ The temperature T ２ The amount of surface oxygen derived from the silicon oxynitride layer on the particle surface (temperature T １ From temperature T ２ This allows the amount of surface oxygen derived from the surface layer (composed of silicon oxide and silicon oxynitride) of the silicon nitride particles to be analyzed. １ From temperature T ２ The amount of surface nitrogen coming from the surface layer can also be measured from the amount of nitrogen released during the temperature rise to 1000°C.

[0130] As mentioned above, the compartment temperature T ２ The surface oxygen content (FSO) can be calculated as the total amount of surface oxygen derived from the silicon oxide layer and the silicon oxynitride layer on the particle surface. The total oxygen content (TO) is the sum of the internal oxygen content (FIO) and the surface oxygen content (FSO).

[0131] Silicon nitride powders such as those shown in Figure 5 have surface oxygen content (FSO) and internal oxygen content (FIO) within appropriate ranges, and when used as a sintering raw material, they can produce silicon nitride sintered bodies with excellent thermal conductivity and mechanical properties. The reason for this is thought to be that a surface oxygen content (FSO) within the appropriate range allows for a fast densification rate and a high-density sintered body, while a internal oxygen content (FIO) within the appropriate range suppresses excessive growth of β-type columnar crystals and reduces internal defects. As shown in Figure 5, the surface oxygen content (FSO) of the silicon nitride powder is preferably equal to or less than the internal oxygen content (FIO). This allows for a silicon nitride sintered body with both high thermal conductivity and excellent mechanical properties to be obtained.

[0132] <<Method for Producing Silicon Nitride Sintered Body>> The silicon nitride sintered body according to one embodiment of the present invention is produced by the following production method. The silicon nitride powder according to one embodiment of the present invention is mixed with a sintering aid, the resulting mixed powder (sometimes referred to as a sintering raw material or raw material mixture) is molded, and the resulting molded body is sintered to produce the silicon nitride sintered body according to one embodiment of the present invention for use in high-temperature structural components. Alternatively, the silicon nitride sintered body according to one embodiment of the present invention for use in high-temperature structural components can be produced by simultaneously performing molding and sintering.

[0133] (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, and most of the silicon nitride crystal particles in the sintered body become β-type columnar crystals. The microstructure of these β-type columnar crystals, such as their aspect ratio and particle size, is greatly influenced 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 are selected appropriately depending on the physical properties of the silicon nitride powder and the characteristics desired for the silicon nitride sintered body.

[0134] 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 (Al２ 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 further used in combination with the sintering aids. The amount of oxide-based sintering aid 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 of the raw materials for producing the sintered body.

[0135] As a method for mixing the silicon nitride powder and sintering aid of one embodiment of the present invention, any method can be used, whether wet or dry, as long as they can be mixed uniformly, and known methods such as a rotary mill, barrel mill, vibrating mill, etc. For example, a method can be used in which the silicon nitride powder, sintering aid, molding binder, and 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.

[0136] (Method of Molding Mixed Powder) As a method of molding the mixed powder, known methods can be used, such as press molding, slip casting, extrusion molding, injection molding, drain molding, cold isostatic pressing, etc. 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.

[0137] (Method of Sintering the Compacted Body) The compacted body may be sintered by 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 about 0.2 to 10 MPa is employed. Preferably, nitrogen gas is used for sintering, and atmospheric sintering is performed at a temperature range of about 1700 to 1800°C, while pressurized atmosphere sintering is performed at a temperature range of about 1800 to 2000°C. The rate of temperature increase from 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.

[0138] 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.

[0139] <<Method for Producing Silicon Nitride Sintered Body for Circuit Boards>> A silicon nitride sintered body for circuit boards according to one embodiment of the present invention is produced by the following method. A raw material mixture is prepared by adding a sintering aid, an organic binder, etc. to the silicon nitride powder according to one embodiment of the present invention. The resulting raw material mixture is then subjected to a sheet forming method such as a doctor blade process 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 sintering conditions (heating rate, maximum temperature, and holding time at maximum temperature) described above to produce the silicon nitride sintered body for circuit boards according to one embodiment of the present invention. To evaluate the thermal conductivity and mechanical properties of the silicon nitride sintered body for circuit boards, the thermal and mechanical properties of the sintered body obtained by cold isostatic pressing (CIP) molding described above may be measured.

[0140] (Sintering Aid) In one embodiment of the present invention, 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 ６ In 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.

[0141] 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.

[0142] <<Mechanical Properties of High-Strength Silicon Nitride Sintered Body for Structural Members>> The high-strength silicon nitride sintered body for structural members produced as described above has excellent mechanical properties. For example, the three-point bending strength at room temperature may be 800 MPa or more, or even 950 MPa or more. Furthermore, the three-point bending strength at high temperature (1250°C) may be 520 MPa or more, or even 610 MPa or more.

[0143] <<Mechanical Properties and Heat Dissipation of High Thermal Conductivity Silicon Nitride Sintered Body for Circuit Boards>> Furthermore, the high thermal conductive silicon nitride sintered body for circuit boards has excellent mechanical properties and heat dissipation. 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.

[0144] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples.

[0145] (Method for Composition Analysis of Amorphous Si—N(—H) Compounds) The silicon (Si) content of amorphous Si—N(—H) compounds was measured by a method for quantifying total silicon using a dehydrated gravimetric ICP-AES combined with atomic emission spectroscopy in accordance with JIS R1603:2007, "Methods for Chemical Analysis of Silicon Nitride Fine Powders for Fine Ceramics." The 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. The oxygen (O) content was measured using an oxygen / nitrogen simultaneous analyzer by a method for quantifying oxygen 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 using ICP-AES or a steam distillation separation neutralization titration method, samples were stored in a nitrogen atmosphere until immediately before sample pretreatment for measurement. When measuring the oxygen content by infrared absorption, the atmosphere during sample storage immediately before measurement and during capsule insertion 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 obtained by subtracting the silicon (Si), nitrogen (N), and oxygen (O) contents from the total amount of the amorphous Si—N(—H) compound.

[0146] From the above, the ratios of Si, N and H were calculated to determine the composition formula of the amorphous Si-N(-H) compound.

[0147] <Evaluation of Silicon Nitride Powder: Measurement of Amorphous Film Thickness> The thickness of the amorphous film on silicon nitride primary particles was measured. First, 0.01 g of silicon nitride powder was ultrasonically dispersed in 5 ml of 2-propanol, dropped onto a microgrid-shaped Cu mesh, and allowed to air dry before being subjected to electron microscope observation. For electron microscope observation, a field emission transmission electron microscope (product name: JEM-2100F) manufactured by JEOL Ltd. and a scanning image observation device (product name: EM24541SIOD) manufactured by JEOL Ltd. were used. For X-ray analysis, an energy dispersive X-ray analyzer (product name: JED-2300) manufactured by JEOL Ltd. was used. The elemental distributions of silicon, oxygen, and nitrogen on the observation surface were confirmed using energy dispersive X-ray spectroscopy (EDX). Since the silicon distribution closely corresponded to the position of the silicon nitride primary particles, it was confirmed that many oxygen atoms were localized in the region corresponding to the surface of the silicon nitride primary particles. The region where oxygen atoms are localized is a crystalline silicon nitride (α-type Si ３ N ４ or β-type Si ３ N ４ Since the thickness of the amorphous film measured in this embodiment of the present invention corresponds well to the area where no lattice fringes are observed, the area where the contrast is different from that of the particle interior and no lattice fringes are observed is regarded as an amorphous film. The thickness of the amorphous film was measured at a total of 30 locations on at least two or more arbitrary silicon nitride particles in the observation field, and on at least 15 arbitrary silicon nitride particles, and the average value and the coefficient of variation defined in the above formula (1) (paragraph 0016) were calculated (n=30).

[0148] (Method for Measuring Oxygen Content) The oxygen (O) content was measured using an oxygen / nitrogen simultaneous analyzer (EMGA-620W, manufactured by Horiba, Ltd.) in accordance with the oxygen quantification method by inert gas fusion-carbon dioxide infrared absorption method specified in JIS R1603:2007. The oxygen content of the silicon nitride powder was measured using the following procedure. 30 mg of silicon nitride powder was weighed and the measurement sample was placed in the oxygen / nitrogen simultaneous analyzer. The sample was heated from 20°C to 2000°C at a heating rate of 4-8°C / sec in a helium gas atmosphere. The oxygen and nitrogen desorbed with the temperature increase were detected to obtain concentration charts of oxygen (by non-dispersive infrared absorption method) and nitrogen (by thermal conductivity method). An example of the measurement results is shown in Figure 5 (heating rate: 6.5°C / sec). As shown in Figure 5, the oxygen and nitrogen concentration charts showed peak 1 derived from surface oxygen, peak 2 derived from internal oxygen, and peak 3 derived from nitrogen. Line 4 indicates temperature.

[0149] As explained above in the section (Methods for measuring surface oxygen content (FSO), internal oxygen content (FIO) and total oxygen content (TO)), oxygen bonded to the surface of the silicon nitride powder is released at the beginning of the temperature rise, and the amount of surface oxygen derived from the silicon oxide layer on the particle surface can be determined by quantifying the amount of released oxygen. １ (for example, 1350 to 1450°C), nitrogen desorption begins and detection of Peak 3 begins. １ Since what is detected at temperature T ２ By quantifying the amount of oxygen desorbed during the temperature rise process up to T, the total amount of surface oxygen derived from the silicon oxide layer near the particle surface and the silicon oxynitride layer can be determined. ２ In the case of the oxygen concentration chart, the surface oxygen amount (FSO) is calculated by adding the surface oxygen amount derived from the silicon oxide layer near the particle surface and the surface oxygen amount derived from the silicon oxynitride layer near the particle surface from the integrated value (area) of the oxygen concentration chart. ２ At temperature T２ In this method, the average chemical composition (both the surface oxygen and surface nitrogen contents) of the surface layer (consisting of silicon oxide and silicon oxynitride) of silicon nitride particles can be measured. From these analytical values, the reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN), FSN / FSO (mass % / mass %) (sometimes referred to as the N / O ratio in the surface layer), can be calculated.

[0150] As mentioned above, the temperature T ２ The amount of oxygen desorbed during the temperature rise process up to T is determined as the surface oxygen amount. ２ The amount of oxygen desorbed at the above temperatures was determined as the internal oxygen amount (FIO). ２ The internal oxygen content (FIO) and surface oxygen content (FSO) were calculated from the integrated values ​​(areas) of peaks 1 and 2 based on the calibration curve described below. The sum of the internal oxygen content (FIO) and surface oxygen content (FSO) was defined as the total oxygen content (TO).

[0151]

[0152] When measuring the oxygen content and nitrogen content, a calibration curve is first prepared using a standard sample. The peak intensity integrated value of oxygen detected with a non-dispersive infrared absorption cell is not necessarily linear with respect to the actual oxygen concentration. Therefore, calibration measurements were separately performed with the amount of desorbed oxygen (which depends on the mass of the standard sample) changed to about five levels, and a calibration curve of the amount of oxygen versus the peak intensity integrated value was prepared, and the oxygen content was calculated based on the calibration curve.

[0153] (Method for measuring fluorine content) The total fluorine content of the silicon nitride powder was measured by a fluorine quantification method using pyrohydrolysis separation-ion chromatography in accordance with JIS R 1603:2007 "Methods for chemical analysis of silicon nitride fine powder for fine ceramics." The surface fluorine content was determined by boiling a dispersion of 0.50 g of silicon nitride powder in water to extract the fluorine present on the surface of the silicon nitride powder into the water, and quantifying the extracted fluorine using ion chromatography. The internal fluorine content was determined by subtracting the surface fluorine content from the total fluorine content. Therefore, the following equation (3) holds:

[0154]

[0155] (Method for measuring chlorine content) The total chlorine content of the silicon nitride powder was also measured by a method for quantifying chlorine using pyrohydrolysis separation-ion chromatography in accordance with JIS R 1603:2007 "Method for chemical analysis of silicon nitride fine powder for fine ceramics." In one embodiment of the present invention, the halogen content means the total amount of the fluorine content and the chlorine content.

[0156] (Method for measuring metal silicon content) The metal silicon content of the silicon nitride powder was measured by the hydrogen gas generation-gas volumetric method in accordance with "9. Method for determining free silicon" of JIS R1616:2007, which applies to a free silicon content of 0.01% by mass or more and 1% by mass or less.

[0157] (Method of Measuring Specific Surface Area) The specific surface areas of the silicon nitride powder and the amorphous Si-N(-H) compounds were determined by measuring with a Macsorb manufactured by Mountech by the BET single-point method using nitrogen gas adsorption.

[0158] (Method for measuring crystallinity) 0.15 g of precisely weighed silicon nitride powder was added to a 0.5 N aqueous NaOH solution and heated to 100° C. NH ３ The gas was absorbed in a 1% aqueous boric acid solution, and the NH ３ The amount was titrated with a 0.1N sulfuric acid standard solution. ３ The degree of crystallinity was calculated from the amount of decomposed nitrogen and the theoretical nitrogen amount of silicon nitride, 39.94 mass%, using the following formula (4):

[0159]

[0160] (Method for measuring α phase, β phase, and β / (α + β) ratio) The X-ray diffraction pattern of the silicon nitride powder was measured using a RINT-TTRIII wide-angle X-ray diffractometer manufactured by Rigaku Corporation. Powder X-ray diffraction measurement was carried out using CuKα radiation, and the crystalline phases (α phase and β phase) of the produced silicon nitride powder were identified by the Gazzara & Messier method described in Non-Patent Document 1, and the β / (α + β) ratio (i.e., β fraction) was calculated.

[0161] (Method for measuring particle size distribution, median diameter, 10% diameter, and 90% diameter of silicon nitride powder) The particle size distribution of a silicon nitride powder according to one embodiment of the present invention was measured as follows. 0.01 g of the powder was placed in 200 ml of a 0.2% by mass aqueous solution of sodium hexametaphosphate, and dispersed for 6 minutes at 300 W using an ultrasonic homogenizer equipped with a 26 mm diameter stainless steel center cone to prepare a dilute solution, which was used as a measurement sample. The particle size distribution of the measurement sample was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3000, manufactured by Microtrac-Bell Co., Ltd.). The frequency (volume %) based on the cumulative undersize distribution was calculated from the volume-based particle size distribution data of the silicon nitride powder according to one embodiment of the present invention, and a cumulative particle size distribution curve was obtained. The median diameter (50% diameter) in the cumulative particle size distribution curve was used as the average particle diameter (D ５０ ), and the particle diameter corresponding to 10% of the cumulative undersize distribution is defined as the cumulative 10% diameter (D １０ ), and the particle diameter corresponding to 90% of the cumulative undersize distribution is called the cumulative 90% diameter (D ９０ )

[0162] (Method for producing and evaluating high-strength sintered bodies for structural members) 93 parts by mass of 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 5 parts by mass of aluminum oxide (specific surface area 7.4 m ２The blended powder, to which 2 parts by mass of sucrose (1000 ppm / 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 1780°C for 2 hours under a nitrogen gas atmosphere at atmospheric pressure. The resulting sintered body was cut, milled, and polished to prepare 3 mm x 4 mm x 40 mm flexural test specimens in accordance with JIS R1601. The bulk density of the sintered body was measured using the Archimedes method. Assuming that all silicon nitride particles were converted to β-type silicon nitride, the relative density (%) (sometimes referred to as the attained density) was calculated from the calculated density determined from the raw material composition and the bulk density of the sintered body. The three-point bending strength at room temperature and 1250°C (sometimes referred to as bending strength (room temperature) and bending strength (1250°C) respectively) was measured using an Instron universal testing machine in accordance with JIS R1601. The crosshead speed was 0.5 mm / min. The bending strength at room temperature is the average value of 40 specimens. For the high-temperature bending test, the test specimens were held at 1250°C in an air atmosphere for 10 minutes, and then strength measurements were performed on eight or more specimens to calculate the average value. In addition, 3 mm x 4 mm x 40 mm test specimens were heat-treated in air at 1300°C for 100 hours, and the three-point bending strength at room temperature after oxidation (sometimes referred to as post-oxidation strength (room temperature)) was measured for each of 10 test specimens, and the average value was calculated.

[0163] (Method for producing and evaluating a highly thermally conductive sintered body for a circuit board) 94.5 parts by mass of 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, containing 2 parts by weight of ethylenediaminetetraacetic acid (E100 / g, manufactured by Kojundo Chemical Laboratory), 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 6 x 45 x 75 mm and 12.3 mm diameter x 1.6 mm thick shapes at a molding pressure of 50 MPa, followed by CIP molding at a molding pressure of 150 MPa. The resulting compacts were placed in a boron nitride crucible and sintered at 1900°C for 10 hours under a nitrogen gas pressure of 0.8 MPa. The resulting sintered bodies were cut, milled, and polished to prepare 3 mm x 4 mm x 40 mm bending test specimens conforming to JIS R1601 and 10 mm diameter x 1 mm thick disc test specimens for thermal conductivity measurement conforming to JIS R1611. The bulk density of the sintered bodies was measured using the Archimedes method. On the assumption that all silicon nitride particles are converted to β-type silicon nitride, the relative density (%) (sometimes referred to as the attained density) was calculated from the calculated density obtained from the raw material composition and the bulk density of the sintered body. The three-point bending strength at room temperature (sometimes referred to as the bending strength (room temperature)) was measured at room temperature using an Instron universal testing machine in accordance with JIS R1601, and the thermal conductivity was measured at room temperature using the flash method in accordance with JIS R1611. The crosshead speed for the bending test was 0.5 mm / min. The bending strength at room temperature is the average value of 40 specimens. The thermal conductivity is the average value of three disc-shaped test specimens.

[0164] Example 1 Silicon nitride powder was produced using the following production method (imide pyrolysis) under the production conditions shown in Table 1. The thickness (average value) and coefficient of variation of the surface amorphous film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (surface nitrogen content / surface oxygen content ratio in the surface layer), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 2 below, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio, and particle size distribution) are shown in Table 3 below.

[0165] [Method for producing silicon nitride powder by imide pyrolysis] 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. A previously prepared solution (reaction solution) consisting of 2 liters of silicon tetrachloride and 6 liters of toluene was fed through a conduit to the lower layer of the slowly stirred reactor. As the solution was fed, 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, but ammonia was consumed as the reaction progressed, so ammonia was continuously fed into the reactor. At steady state, the volume ratio of silicon tetrachloride fed into 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 43 liters of liquid ammonia (total 215 liters), filtered, dried, and purified to a specific surface area of ​​800 m ２ About 1 kg of purified silicon diimide having a toluene content of 0.38% by mass was obtained.

[0166] The resulting silicon diimide was thermally decomposed at 517°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 １０．５８ H ７．７４ That is, Si ６ N ２ｘ (NH) １２－３ｘ An amorphous Si—N(—H) compound was obtained in which x in the formula was 1.42. The specific surface area of ​​the obtained amorphous Si—N(—H) compound was 560 m ２ / g, and the oxygen content was 0.43 mass %.

[0167] 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 diameter, and 12 mm long 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 (furnace carbon monoxide (CO) concentration, heating rate, and maximum temperature) to produce a grayish-white silicon nitride powder. The holding time at the maximum temperature (1585°C) was 1.5 hours. The CO concentration in the furnace was adjusted by the purity (oxygen concentration, dew point) and flow rate of the nitrogen gas flowing through. This crystalline silicon nitride powder was then placed in a vibration mill and milled for 39 minutes at an amplitude of 7.0 mm in an air atmosphere with a moisture concentration of 173 ppm (V). The thickness (average value) and coefficient of variation of the amorphous surface film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (ratio of surface nitrogen content to surface oxygen content in the surface layer), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 2 below, and the impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α+β) ratio, and particle size distribution) are shown in Table 3 below.

[0168] (Examples 2 to 14) The properties of the amorphous Si—N(—H) compound (x value in the composition formula, specific surface area, and oxygen content) were controlled by the imide thermal decomposition conditions (thermal decomposition temperature, supply gas oxygen concentration) listed in Table 1. The chemical composition, properties, and particle size distribution of the crystalline silicon nitride powder were controlled by the production conditions listed in Table 1 (imide synthesis conditions, properties of the amorphous Si—N(—H) compound, high-temperature firing conditions, and conditions for crushing the fired powder). Silicon nitride powder was produced in the same manner as in Example 1, except for the production conditions listed in Table 1.

[0169] (Examples 15 to 18) As in Examples 2 to 14, the properties of the amorphous Si—N(—H)-based compound (the x value in the composition formula, the specific surface area, and the oxygen content) were controlled by the imide pyrolysis conditions (pyrolysis temperature, oxygen concentration in the supply gas) listed in Table 1. The chemical composition, properties, and particle size distribution of the crystalline silicon nitride powder were controlled by the production conditions (imide synthesis conditions, properties of the amorphous Si—N(—H)-based compound, high-temperature firing conditions, and conditions for crushing the fired powder) listed in Table 1. Silicon nitride powder was produced in the same manner as in Example 1, except for the production conditions listed in Table 1. In Examples 15 to 18, the resulting silicon nitride powder was further heated in air for oxidation treatment. The oxidation treatment conditions were: 910°C for 4 hours in Example 15, 1000°C for 4 hours in Example 16, 1050°C for 4 hours in Example 17, and 1065°C for 4 hours in Example 18. The thickness of the amorphous film on the particle surface increased by the oxidation treatment in air.

[0170] The thickness (average value) and coefficient of variation of the amorphous surface film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (ratio of surface nitrogen content to surface oxygen content in the surface layer), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 2, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio, and particle size distribution) are shown in Table 3.

[0171] In Examples 4 and 11, the surface oxygen content after post-milling was slightly high, so the resulting silicon nitride powder was immersed in a dilute aqueous hydrofluoric acid solution to reduce the surface oxygen content. The temperature of the aqueous hydrofluoric acid solution was 30°C, the immersion time was 0.5 hours, and the hydrofluoric acid concentration was 0.02% by mass in Example 4 and 0.10% by mass in Example 11. As a result, the fluorine content of the silicon nitride powder after drying was slightly high.

[0172] In Examples 1 to 3 and 15, the thickness (average) of the amorphous film present on the particle surface was 1.16 nm or more and 1.40 nm or less, in Examples 4 to 14, the thickness (average) of the amorphous film was 0.92 nm or less, and in Examples 16 to 18, the thickness (average) of the amorphous film was 2.00 nm or more. Furthermore, in Examples 1 to 10, 12 and 14, the coefficient of variation of the thickness of the amorphous film was 0.49 or less, and in Examples 11, 13 and 15 to 18, the coefficient of variation of the thickness of the amorphous film was 0.51 or more and 0.58 or less.

[0173] In Examples 1 to 4, 6 to 11, 16, and 17, the surface oxygen content (FSO) measured by temperature-programmed morphology analysis was 0.31 mass% or more and 0.48 mass% or less, in Examples 12 to 14 the surface oxygen content was 0.14 mass% or more and 0.19 mass% or less, and in Examples 5, 15, and 18 the surface oxygen content was 0.53 mass% or more and 0.66 mass% or less.

[0174] In Examples 1 to 3 and 16 to 18, the reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass % / mass %), measured by temperature-programmed morphological analysis, was 0.35 or less, while in Examples 4 to 15, the FSN / FSO (mass % / mass %) was 0.39 or more, indicating that both silicon oxide and silicon oxynitride were present in the surface layer of the silicon nitride particles.

[0175] Examples 1, 12, 13 and 16 to 18 have a specific surface area (SA) of 8.0 m ２ / g, and Examples 2 to 11, 14 and 15 have a specific surface area (SA) of 8.3 m ２ / g to 12.4m ２ / g.

[0176] In Examples 8 to 14, the surface oxygen amount / specific surface area (FSO / SA) was 0.22 to 0.37 mg / m ２ In Examples 1 to 7 and 15 to 18, the surface oxygen amount / specific surface area (FSO / SA) was 0.42 mg / m ２ In particular, in Example 18, the surface oxygen amount / specific surface area (FSO / SA) was 1.00 mg / m ２ That's all.

[0177] In addition, in Example 11, the fluorine content exceeds 100 ppm and the halogen (F+Cl) content exceeds 110 ppm.

[0178] In Examples 1, 4, 5, 7 to 9, 12, 14, 16, and 18, β / (α+β) was 1.8% by mass or more and 3.0% by mass or less, and in Examples 3, 6, 10, 11, 13, 15, and 17, β / (α+β) was 3.2% by mass or more and 5.6% by mass or less.

[0179] In Examples 1 to 12, 14 and 15, the cumulative 10% diameter D １０ In Examples 6 and 11, the cumulative 10% diameter D １０ In Examples 6 and 11, the cumulative 90% diameter D ９０ In Examples 1 to 5, 7 to 10, and 12 to 17, the cumulative 90% diameter D ９０ is 2.03 μm or more and 2.38 μm or less.

[0180] (Comparative Examples 1 to 6) As in Examples 2 to 18, the properties of the amorphous Si—N(—H)-based compound (the x value in the composition formula, the specific surface area, and the oxygen content) were controlled by the imide pyrolysis conditions (pyrolysis temperature, oxygen concentration in the supplied gas) listed in Table 1. The chemical composition, properties, and particle size distribution of the crystalline silicon nitride powder were controlled by the production conditions (imide synthesis conditions, properties of the amorphous Si—N(—H)-based compound, high-temperature firing conditions, and conditions for crushing the fired powder) listed in Table 1. Except for the production conditions listed in Table 1, silicon nitride powder was produced in the same manner as in Example 1. Furthermore, in Comparative Examples 1 and 2, the obtained silicon nitride powder was heated in air and subjected to an oxidation treatment. The oxidation treatment conditions were: 1100°C for 4 hours in Comparative Example 1, and 1025°C for 4 hours in Comparative Example 2. The thickness of the amorphous film on the particle surface increased due to the oxidation treatment in air.

[0181] The thickness (average value) and coefficient of variation of the amorphous surface film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (ratio of surface nitrogen content to surface oxygen content in the surface layer), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 2, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio, and particle size distribution) are shown in Table 3.

[0182] In Comparative Examples 3 to 5, the surface oxygen content after post-milling was slightly high, so the resulting silicon nitride powder was immersed in a dilute aqueous hydrofluoric acid solution to reduce the surface oxygen content. The temperature of the aqueous hydrofluoric acid solution was 30°C, the immersion time was 0.5 hours, and the hydrofluoric acid concentration was 0.10 mass% for Comparative Examples 3 and 5, and 0.06 mass% for Comparative Example 4. As a result, the fluorine content of the silicon nitride powder after drying was slightly high.

[0183] In Comparative Examples 4 to 6, the thickness (average value) of the amorphous film present on the particle surface was 0.60 nm or less, in Comparative Examples 2 and 3, the thickness (average value) of the amorphous film was 1.20 nm or more and 2.40 nm or less, and in Comparative Example 1, the thickness (average value) of the amorphous film exceeded 5.00 nm. Furthermore, in Comparative Examples 1 and 3, the coefficient of variation of the thickness of the amorphous film was 0.55 or less, and in Comparative Examples 2 and 4 to 6, the coefficient of variation of the thickness of the amorphous film was 0.65 or more.

[0184] In Comparative Examples 2, 5 and 6, the surface oxygen content (FSO) is 0.28 mass % or less, and in Comparative Examples 1, 3 and 4, the surface oxygen content (FSO) is 0.52 mass % or more.

[0185] Comparative Example 2 has a specific surface area (SA) of 3.0 m ２ / g, and Comparative Examples 4 and 6 have a specific surface area (SA) of 13.0 m ２ / g.

[0186] In Comparative Examples 5 and 6, the surface oxygen amount / specific surface area (FSO / SA) was 0.20 mg / m ２ In the following Comparative Example 4, the surface oxygen amount / specific surface area (FSO / SA) was 0.39 mg / m ２In Comparative Examples 1 to 3, the surface oxygen amount / specific surface area (FSO / SA) was 0.51 mg / m ２ That's all.

[0187] In Comparative Examples 3 to 5, the fluorine content is 100 ppm or more, and the halogen (F+Cl) content exceeds 130 ppm.

[0188] In all of Examples 1 to 34 and Comparative Examples 1 to 13, the crystallinity of the resulting silicon nitride powder was 99.6% or more.

[0189]

[0190]

[0191]

[0192] Example 19 Silicon nitride powder was produced using the following production method (direct nitridation) under the production conditions shown in Tables 4 and 5. The thickness (average value) and coefficient of variation of the surface amorphous film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (surface nitrogen content / surface oxygen content ratio in the surface layer), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 6, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio, and particle size distribution) are shown in Table 7.

[0193] [Method for producing silicon nitride powder by direct nitriding] Commercially available metal silicon powder (average particle size D ５０ 4.0 μm, oxygen content 0.40 mass %, crystalline silicon nitride powder (average particle size D ５０ The powder was mixed with 3.7 mass% of α, 0.7 μm, oxygen content 0.90 mass%, β / (α+β)=6.0 (mass%), and calcium fluoride at a ratio of 0.03 mass%, and an organic binder (polyvinyl alcohol) was added in a slurry state at a ratio of 1.6 mass parts per 100 mass parts of the mixture, followed by drying to prepare a raw material powder. Using this raw material powder, almond-shaped molded articles measuring approximately 4 mm thick, 5 mm minor axis diameter, and 8 mm major axis diameter were produced using a briquetting machine and packed into a silicon nitride firing container. The packing density was 1.0 g / cm.３ The almond-shaped product was heated to 1540°C in an electric furnace and fired to produce a silicon nitride ingot. The heating program was set so that the heating rate from 1100°C to the temperature at which the nitriding rate reached 50% was 9.2°C / h, and the heating rate from the temperature at which the nitriding rate reached 50% to 1540°C was 33°C / h. The maximum temperature (1540°C) was maintained for 1 hour, and then the temperature was lowered. The firing atmosphere was a mixed atmosphere of nitrogen and ammonia (N ２ :NH ３ The moisture concentration of the supplied gas was 80 ppm (V), and the oxygen concentration was 18 ppm (V).

[0194] After nitriding, the ingot was cooled and removed, and then subjected to coarse and medium crushing using a jaw crusher and a roll crusher. A crushing aid (methyl ethyl ketone) was added, and dry crushing was carried out in a vibration mill (container made of SUS304) loaded with silicon nitride balls to produce silicon nitride powder. The mill amplitude was set to 7.3 mm, and the milling time was set to 140 minutes.

[0195] The silicon nitride powder obtained by dry milling was post-treated by immersing it for 7 hours in a mixed acid of hydrofluoric acid (concentration 2.0% by mass) and hydrochloric acid (concentration 4.4% by mass) heated to 70°C. The mixed acid slurry was then repeatedly washed and filtered with ion-exchanged water heated to 47°C. The washing was completed when the pH of the washing solution reached 4.1, and the filter cake was removed and dried in a dryer at 95°C. In this way, the crystalline silicon nitride powder of Example 19 was obtained.

[0196] Examples 20 to 34 Commercially available metal silicon powder was milled using a vibration mill (the inner wall of the container was coated with polyurethane) loaded with silicon nitride balls to prepare various metal silicon powders with different average particle sizes and oxygen contents. ５０The silicon nitride powder was produced from the silicon nitride ingot by the same method as in Example 19 except for the production conditions (raw materials fired, nitriding conditions by high-temperature firing) listed in Table 4. The silicon nitride powder was produced from the obtained silicon nitride ingot by the same method as in Example 19 except for the post-treatment conditions (pulverization conditions, mixed acid treatment conditions, and washing / drying conditions) listed in Table 5.

[0197] The thickness (average value) and coefficient of variation of the amorphous surface film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (surface nitrogen content / surface oxygen content ratio in the surface layer portion), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 6, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio and particle size distribution) are shown in Table 7.

[0198] In Examples 19 to 27 and 29 to 32, the thickness (average) of the amorphous film present on the particle surface was 0.74 nm or less, in Example 28 the thickness (average) of the amorphous film was 0.94 nm, in Example 33 the thickness (average) of the amorphous film was 1.90 nm, and in Example 34 the thickness (average) of the amorphous film was 2.35 nm. In addition, in Examples 19 to 28, 30 to 32, and 34, the coefficient of variation of the amorphous film thickness was 0.49 or less, and in Examples 29 and 33 the coefficient of variation of the amorphous film thickness was around 0.58.

[0199] In Examples 20, 31, and 32, the surface oxygen content (FSO) measured by temperature-programmed morphology analysis was 0.13 to 0.18 mass%, in Examples 19 and 21 to 30, the surface oxygen content (FSO) was 0.30 to 0.50 mass%, and in Examples 33 and 34, the surface oxygen content (FSO) was 0.80 to 0.86 mass%.

[0200] In Examples 33 and 34, the reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass % / mass %), measured by temperature-programmed morphological analysis, was 0.17 or less. On the other hand, in Examples 19 and 21 to 31, the FSN / FSO (mass % / mass %) was 0.35 to 0.73, indicating that both silicon oxide and silicon oxynitride were present in the surface layer of the silicon nitride particles.

[0201] Examples 19, 20, 30 and 31 have a specific surface area (SA) of 8.0 m ２ / g, and Examples 21 to 29 and 32 to 34 have a specific surface area (SA) of 8.1 to 12.4 m ２ / g.

[0202] In Examples 29, 31, and 32, the surface oxygen amount / specific surface area (FSO / SA) was 0.20 mg / m ２ 0.27mg / m or more ２ In the following Examples 19 to 28 and 30, the surface oxygen content / specific surface area (FSO / SA) was 0.31 mg / m ２ 0.46mg / m or more ２ The following is the result.

[0203] Examples 30 and 31 have a fluorine content of more than 80 ppm, and Examples 29-34 have a halogen (F+Cl) content of more than 100 ppm.

[0204] In Examples 19 to 34, β / (α+β) is 4.2 mass % or more and 13.7 mass % or less.

[0205] Examples 20 and 22 to 30 are cumulative 10% diameter D １０ In Examples 24 and 29, the cumulative 90% diameter D ９０ In Examples 19 to 23, 25 to 28 and 30 to 34, the cumulative 90% diameter D ９０ is 2.02 μm or more and 2.36 μm or less.

[0206] (Comparative Examples 7 to 13) Commercially available metal silicon powder was milled using a vibration mill (the inner wall of the container was coated with polyurethane) loaded with silicon nitride balls to prepare various metal silicon powders with different average particle sizes and oxygen contents. ５０ The silicon nitride powder was produced from the silicon nitride ingot by the same method as in Example 19 except for the production conditions (raw materials fired, nitriding conditions by high-temperature firing) listed in Table 4 (crushing conditions, mixed acid treatment conditions, and washing / drying conditions) listed in Table 5).

[0207] The thickness (average value) and coefficient of variation of the amorphous surface film of the obtained crystalline silicon nitride powder, as well as the chemical composition (surface oxygen content FSO, N / O ratio in the surface layer (surface nitrogen content / surface oxygen content ratio in the surface layer portion), internal oxygen content FIO, total oxygen content TO, etc.) are shown in Table 6, and the halogen impurity concentrations (fluorine content, surface fluorine content, chlorine content, etc.) and powder properties (BET specific surface area SA, crystalline phase β / (α + β) ratio and particle size distribution) are shown in Table 7.

[0208] In Comparative Examples 11 to 13, the thickness (average value) of the amorphous film present on the particle surface was 0.69 nm or less, in Comparative Examples 8 to 10, the thickness (average value) of the amorphous film was 1.00 nm or more and 1.28 nm or less, and in Comparative Example 7, the thickness (average value) of the amorphous film exceeded 5.00 nm. Furthermore, in Comparative Examples 7, 9, and 11, the coefficient of variation of the amorphous film thickness was 0.54 or less, and in Comparative Examples 8, 10, 12, and 13, the coefficient of variation of the amorphous film thickness was 0.63 or more.

[0209] In Comparative Examples 8 and 11 to 13, the surface oxygen content (FSO) was 0.21 mass % or more and 0.26 mass % or less, and in Comparative Example 7, the surface oxygen content (FSO) was 1.20 mass %.

[0210] Comparative Example 8 has a specific surface area (SA) of 3.0 m ２ / g, and Comparative Example 13 has a specific surface area (SA) of 13.0 m ２ / g.

[0211] In Comparative Examples 12 and 13, the surface oxygen amount / specific surface area (FSO / SA) was 0.22 mg / m ２ In the following Comparative Example 11, the surface oxygen amount / specific surface area (FSO / SA) was 0.49 mg / m ２ In Comparative Examples 8 to 10, the surface oxygen amount / specific surface area (FSO / SA) was 0.52 to 0.76 mg / m ２ is.

[0212] The fluorine contents of Comparative Examples 7 to 13 are 89 ppm or more, and in particular, the fluorine contents exceed 120 ppm in Comparative Examples 11 and 13. The halogen (F + Cl) contents of Comparative Examples 8, 9, and 11 to 13 are 140 ppm or more. The β / (α + β) ratio of Comparative Examples 7, 12, and 13 is 18.5 mass% or more.

[0213] In all of Examples 19 to 34 and Comparative Examples 7 to 13, no metallic silicon was detected in the resulting silicon nitride powders, i.e., the metallic silicon content of the resulting silicon nitride powders was less than 0.01 mass%.

[0214]

[0215]

[0216]

[0217]

[0218] Using the silicon nitride powders obtained in Examples 1 to 18 and Comparative Examples 1 to 6 as raw materials, silicon nitride sintered bodies for structural components were produced by the imide pyrolysis method according to the method described above in "Method for producing and evaluating high-strength sintered bodies for structural components," and their strength properties were evaluated. The results of measuring the ultimate density (relative density) and bending strength (bending strength (room temperature), bending strength (1250°C), and strength after oxidation (room temperature)) of the sintered bodies are shown in Table 8 below.

[0219] Similarly, silicon nitride sintered bodies for circuit boards were produced using the silicon nitride powders obtained in Examples 1 to 18 and Comparative Examples 1 to 6 as raw materials according to the method described above in (Method for producing and evaluating high thermal conductive sintered bodies for circuit boards), and the strength properties and thermal conductivity were evaluated. The measurement results for the achieved density (relative density), bending strength (room temperature), and thermal conductivity (room temperature) of the sintered bodies are also shown in Table 8 below.

[0220]

[0221] (Results of sintering test of silicon nitride powders obtained in Examples 1 to 18) The crystalline silicon nitride powders according to Examples 1 to 3 had an amorphous film thickness (average value) on the particle surface of 1.16 to 1.33 nm, a coefficient of variation of 0.23 to 0.48, and a surface oxygen content / specific surface area (FSO / SA) of 0.50 to 0.52 mg / m ２ This is probably because of the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the additive-based sintered body was 1005 to 1080 MPa at room temperature and 693 to 740 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 705 to 785 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 743 to 825 MPa, and the thermal conductivity was 108 to 114 W / (m·K).

[0222] The crystalline silicon nitride powders according to Examples 4 to 9 and 12 had an amorphous film thickness (average value) on the particle surface of 0.60 to 0.92 nm, a coefficient of variation of 0.24 to 0.49, and a surface oxygen content / specific surface area (FSO / SA) of 0.32 to 0.45 mg / m ２ The reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass % / mass %), is 0.39 to 0.62. ２ O ３ -Al ２ O ３The bending strength of the additive-based sintered body was 1130 MPa or more at room temperature, 760 MPa or more at high temperature (1250°C), and the strength after oxidation (room temperature) was 780 MPa or more. ２ O ３ The bending strength (room temperature) of the crystalline silicon nitride powder (based on the crystalline silicon nitride powders of Examples 4 to 9 and 12) was 837 MPa or more, and the thermal conductivity was increased to 116 to 131 W / (m·K). A silicon nitride sintered body having excellent strength characteristics and thermal conductivity was obtained. The cumulative 90% diameter D ９０ and cumulative 10% diameter D １０ The difference between the ratio [D ９０ -D １０ ] was 1.69 to 1.79 μm.

[0223] The crystalline silicon nitride powders according to Examples 10 and 11 have an amorphous film thickness (average value) on the particle surface of 0.42 to 0.56 nm, a coefficient of variation of 0.38 to 0.51, and a surface oxygen content / specific surface area (FSO / SA) of 0.25 to 0.32 mg / m ２ The reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass% / mass%), is about 0.72. ２ O ３ -Al ２ O ３ The bending strength of the additive-based sintered body was 1005 to 1060 MPa at room temperature and 610 to 723 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 660 to 805 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (based on the MgO auxiliary agent) was 750 to 794 MPa, and the thermal conductivity was 108 to 115 W / (m·K).

[0224] Crystalline silicon nitride powders according to Examples 13 and 14 (specific surface areas of 5.9 and 8.7 m, respectively) ２ / g) has an amorphous film thickness (average value) on the particle surface of 0.42 to 0.50 nm, a coefficient of variation of 0.49 to 0.58, and a surface oxygen content / specific surface area (FSO / SA) of 0.24 or 0.22 mg / m ２ This is probably because of the high strength material (Y２ O ３ -Al ２ O ３ The bending strength of the additive-based sintered body was 910-940 MPa at room temperature and 630-665 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 660-740 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 670 to 715 MPa, and the thermal conductivity was 98 W / (m·K). The thermal and mechanical properties of the resulting sintered body tend to be lower.

[0225] The crystalline silicon nitride powder according to Example 15 had an amorphous film thickness (average value) on the particle surface of 1.40 nm, a coefficient of variation of 0.51, and a surface oxygen content / specific surface area (FSO / SA) of 0.57 mg / m ２ This is probably because the value is somewhat high, and the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 1010 MPa at room temperature and 675 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 690 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 770 MPa, and the thermal conductivity was 111 W / (m·K).

[0226] The crystalline silicon nitride powders according to Examples 16 to 18 had an amorphous film thickness (average value) on the particle surface of 2.00 to 3.50 nm, a coefficient of variation of 0.53 to 0.56, and a surface oxygen content / specific surface area (FSO / SA) of 0.64 mg / m ２ In addition, the reciprocal of the mass ratio of the surface oxygen content (FSO) to the surface nitrogen content (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass% / mass%), is about 0.21. ２ O ３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 900-970 MPa at room temperature and 640-660 MPa at high temperature (1250°C), and the strength after oxidation was 650-680 MPa. ２ O３ The bending strength (room temperature) of the resulting sintered body (containing MgO as an auxiliary agent) was 660 to 720 MPa, and the thermal conductivity was 97 to 108 W / (m·K). The thermal and mechanical properties of the resulting sintered body tended to decrease slightly.

[0227] Examples 4 to 9 (the amorphous film thickness (average value) on the particle surface was 0.60 to 0.92 nm, and the coefficient of variation was 0.24 to 0.49) were grouped as Group A, and Examples 2 and 3 (the amorphous film thickness (average value) on the particle surface was about 1.20 nm), Examples 13 and 14 (the amorphous film thickness (average value) on the particle surface was 0.50 nm or less), and Examples 15 to 18 (the coefficient of variation of the amorphous film thickness on the particle surface was 0.51 or more) were grouped as Group B. The fluorine content was 45 ppm or less in both groups, and there was no difference between the groups. Nevertheless, the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength (at high temperature (1250°C)) of the additive-based sintered body is 770 to 830 MPa for Group A, while it is 630 to 705 MPa for Group B, so Group A has a higher strength. ２ O ３ The thermal conductivity of the silicon nitride powder (sintered body containing MgO additive) was also higher in Group A. It has long been known that the impurity fluorine in silicon nitride powder affects the high-temperature strength of the resulting sintered body. However, in one aspect of the present invention, it has been revealed that the high-temperature strength and thermal conductivity of the resulting sintered body are determined not by the fluorine content alone, but by the combination of the halogen content or fluorine content with the thickness of the amorphous film on the particle surface and its coefficient of variation. In particular, when the thickness of the amorphous film on the particle surface was 0.50 nm or less or 1.16 nm or more and the coefficient of variation exceeded 0.43, the thermal and mechanical properties of the resulting sintered body tended to decrease (this tendency was particularly noticeable when the coefficient of variation exceeded 0.53).

[0228] (Results of sintering test of silicon nitride powder obtained in Comparative Examples 1 to 6) Reflecting the powder properties listed in [Table 2] and [Table 3], the crystalline silicon nitride powders according to Comparative Examples 1 to 6 are high strength materials (Y ２ O３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 840 MPa or less at room temperature, 585 MPa or less at high temperature (1250°C), and the strength after oxidation (room temperature) was 655 MPa or less. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 660 MPa or less, and the thermal conductivity was 96 W / (m·K) or less, and the properties of the sintered body obtained were inferior to those of Examples 1 to 18.

[0229] [Sintering test of silicon nitride powder produced by direct nitriding method] Using the silicon nitride powders obtained in Examples 19 to 34 and Comparative Examples 7 to 13 as raw materials, silicon nitride sintered bodies for structural members were produced by the direct nitriding method according to the method described above in (Method for producing and evaluating high-strength sintered bodies for structural members), and their strength properties were evaluated. The results of measuring the ultimate density (relative density) and bending strength (bending strength (room temperature), bending strength (1250°C), and strength after oxidation (room temperature)) of the sintered bodies are shown in Table 9 below.

[0230] Similarly, silicon nitride sintered bodies for circuit boards were produced using the silicon nitride powders obtained in Examples 19 to 34 and Comparative Examples 7 to 13 as raw materials according to the method described above in "Method for producing and evaluating high thermal conductive sintered bodies for circuit boards," and their strength properties and thermal conductivity were evaluated. The measurement results for the achieved density (relative density), bending strength (room temperature), and thermal conductivity (room temperature) of the sintered bodies are also shown in Table 9 below.

[0231]

[0232] (Results of sintering test of silicon nitride powders obtained in Examples 19 to 34) The crystalline silicon nitride powders according to Examples 19 and 21 had an amorphous film thickness (average value) on the particle surface of 0.72 nm, a coefficient of variation of 0.26 to 0.30, a surface oxygen content (FSO) of around 0.32 mass%, and a surface oxygen content / specific surface area (FSO / SA) of 0.41 to 0.43 mg / m ２ , specific surface area is 7.4 to 8.1 m ２ / g, high strength material (Y ２ O ３ -Al ２ O３ The bending strength of the highly thermal conductive material (Y) was about 870 MPa at room temperature and about 600 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was about 610 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (based on the MgO auxiliary agent) was 700 to 715 MPa, and the thermal conductivity was approximately 111 W / (m·K).

[0233] The crystalline silicon nitride powders according to Examples 22 to 27 have an amorphous film thickness (average value) on the particle surface of 0.44 to 0.74 nm, a coefficient of variation of 0.26 to 0.49, and a surface oxygen content / specific surface area (FSO / SA) of 0.31 to 0.43 mg / m ２ The reciprocal of the mass ratio of the surface oxygen amount (FSO) to the surface nitrogen amount (FSN) derived from the silicon oxide layer and silicon oxynitride layer on the particle surface, FSN / FSO (mass % / mass %), is 0.41 to 0.70, and the specific surface area is 9.0 to 12.4 m ２ / g, high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 920 MPa or more at room temperature and 620 MPa or more at high temperature (1250°C), and the strength after oxidation (room temperature) was 620 MPa or more. ２ O ３ The crystalline silicon nitride powders according to Examples 22 to 27 had a 90% cumulative diameter D ９０ and cumulative 10% diameter D １０ The difference between the ratio [D ９０ -D １０ ] was 1.72 to 1.80 μm.

[0234] The crystalline silicon nitride powder of Example 28 had an amorphous film thickness (average value) of 0.94 nm on the particle surface, a coefficient of variation of 0.43, and a surface oxygen content / specific surface area (FSO / SA) of 0.46 mg / m ２ , and the FSN / FSO (mass % / mass %) is 0.35, the high strength material (Y ２ O ３-Al ２ O ３ The bending strength of the additive-based sintered body was 950 MPa at room temperature, but 610 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 642 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 742 MPa, and the thermal conductivity was 109 W / (m·K).

[0235] The crystalline silicon nitride powder according to Example 29 had an amorphous film thickness (average value) of 0.37 nm on the particle surface, a coefficient of variation of 0.58 (slightly high), and a surface oxygen content / specific surface area (FSO / SA) of 0.27 mg / m ２ , and the FSN / FSO (mass % / mass %) is 0.64, it is likely that the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the additive-based sintered body was 860 MPa at room temperature and 595 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 585 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (based on the MgO additive) was 730 MPa, and the thermal conductivity was 107 W / (m·K). １０ The cumulative 90% diameter D ９０ is 1.98 μm, [D ９０ -D １０ ] was 1.70 μm.

[0236] The crystalline silicon nitride powder according to Example 30 had an amorphous film thickness (average value) on the particle surface of 0.69 nm, a coefficient of variation of 0.49, a surface oxygen content (FSO) of 0.30 mass%, and a surface oxygen content / specific surface area (FSO / SA) of 0.44 mg / m ２ and the specific surface area is 6.7 m ２ / g, which is a low value, ２ O ３ -Al ２ O ３The bending strength of the high thermal conductive material (Y) was 805 MPa at room temperature and 540 MPa at high temperature (1250°C), and the strength after oxidation (room temperature) was 535 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (containing MgO as an auxiliary agent) was 710 MPa, and the thermal conductivity was 98 W / (m·K).

[0237] The crystalline silicon nitride powders according to Examples 31 and 32 had an amorphous film thickness (average value) on the particle surface of 0.24 to 0.35 nm, a coefficient of variation of 0.39 or 0.22, a surface oxygen content (FSO) of 0.13 to 0.18 mass%, and a surface oxygen content / specific surface area (FSO / SA) of 0.22 mg / m ２ The oxygen concentration in the surface layer is too low (the surface amorphous film is thin), or the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 800-850 MPa at room temperature and 520-560 MPa at high temperature (1250°C), and the strength after oxidation was 540-630 MPa. ２ O ３ The bending strength (room temperature) of the sintered body (based on the MgO auxiliary agent) was 670 to 700 MPa (tending to be lower than in Examples 22 to 27). The thermal conductivity was 94 to 95 W / (m·K).

[0238] The crystalline silicon nitride powders according to Examples 33 and 34 have an amorphous film thickness (average value) on the particle surface of 1.90 to 2.35 nm, a coefficient of variation of 0.59 or 0.28, a surface oxygen content (FSO) of 0.80 to 0.86 mass%, and a surface oxygen content / specific surface area (FSO / SA) of 0.75 to 1.01 mg / m ２ However, the oxygen concentration in the surface layer is too high (the surface amorphous film is thick), and the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength of the high thermal conductive material (Y) was 835-890 MPa at room temperature and 520-590 MPa at high temperature (1250°C), and the strength after oxidation was 620-650 MPa. ２ O ３The bending strength (room temperature) of the sintered body (MgO additive-based sintered body) was 720 to 740 MPa (tending to be lower than in Examples 22 to 27). The thermal conductivity was 95 to 100 W / (m·K).

[0239] Examples 22 to 28 (the amorphous film thickness on the particle surface was 0.44 to 0.94 nm (average value) and the coefficient of variation was 0.26 to 0.49) were grouped into Group C, and Example 29 (the amorphous film thickness on the particle surface was 0.58 (average value)), Example 32 (the amorphous film thickness on the particle surface was 0.24 (thin)), and Examples 33 and 34 (the amorphous film thickness on the particle surface was 1.90 to 2.35 (thick)) were grouped into Group D. The fluorine content was 21 to 77 ppm in both groups, and the difference between the groups was small. Nevertheless, the high strength material (Y ２ O ３ -Al ２ O ３ The bending strength (at high temperature (1250°C)) of the high thermal conductive material (Y) was 610-640 MPa for Group C, while it was 520-595 MPa for Group D, so Group C had a higher strength. ２ O ３ The thermal conductivity of the sintered bodies (containing sintered bodies containing fluorine-containing additives) was 109 to 114 W / (m K) for Group C, while it was 94 to 107 W / (m K) for Group D, meaning that Group C had higher thermal conductivity. Therefore, as with the comparison between Group A and Group B in Examples 2 to 9 and Examples 13 to 18, it is possible to see the influence, not of the fluorine content alone, but of the combination of the halogen content or fluorine content with the thickness of the amorphous film on the particle surface and its coefficient of variation, on the mechanical properties and thermal conductivity of the sintered bodies obtained.

[0240] (Results of sintering test of silicon nitride powders obtained in Comparative Examples 7 to 13) Reflecting the powder properties listed in [Table 6] and [Table 7], the crystalline silicon nitride powders according to Comparative Examples 7 to 13 are high strength materials (Y ２ O ３ -Al ２ O ３The bending strength of the high thermal conductive material (Y) was 740 MPa or less at room temperature, 470 MPa or less at high temperature (1250°C), and the strength after oxidation (room temperature) was 530 MPa or less. ２ O ３ The bending strength (room temperature) of the sintered body (containing the sintered body of the present invention) was 685 MPa or less, and the thermal conductivity was 93 W / (m·K) or less. Compared with Examples 19 to 34, the properties of the sintered body obtained were significantly inferior.

[0241] As described above, the present invention provides a silicon nitride powder and a method for producing the same, which can produce silicon nitride sintered bodies having both excellent mechanical properties and high thermal conductivity, with the thickness and coefficient of variation of the amorphous film on the particle surface and the halogen content (particularly the fluorine content) falling within appropriate ranges. Furthermore, the mechanical properties and thermal conductivity can be improved by appropriately adjusting the BET specific surface area and the ratio of surface oxygen to specific surface area (FSO / SA). Furthermore, the present invention provides a method for producing silicon nitride sintered bodies that have both excellent mechanical properties and high thermal conductivity, in addition to the mechanical properties of silicon nitride sintered bodies, such as high strength (high room-temperature strength, high-temperature strength, and room-temperature strength after oxidation).

[0242] 1 Peak due to surface oxygen 2 Peak due to internal oxygen 3 Peak due to nitrogen 4 Temperature T １ The temperature at the left end of Peak 3 at which nitrogen detection begins (the temperature at which Peak 3 rises) T ２ A tangent line is drawn to peak 3 at a height of 20% (i.e., 1 / 5) of the maximum value (height of the peak top) of peak 3, and the point where the tangent line intersects with the time axis (height zero) (time t ２ ) at temperature T ２ and the concentration peak due to surface oxygen and the concentration peak due to internal oxygen are partitioned and separated.

Claims

1. A silicon nitride powder comprising primary particles of silicon nitride, the surfaces of which have an amorphous film made of silicon oxide and / or silicon oxynitride, the average thickness of the amorphous film being 5.0 nm or less, the coefficient of variation of the thickness of the amorphous film, represented by the following mathematical formula (1), being 0.60 or less, and the halogen content being 130 ppm or less.

2. BET specific surface area (SA) is 3.0m ２ / g or more 13.0m ２ / g or less, a surface oxygen content (FSO) measured by temperature-programmed morphological analysis is 0.15 mass% or more and 0.65 mass% or less, and an internal oxygen content (FIO) measured by temperature-programmed morphological analysis is 0.25 mass% or more and 1.00 mass% or less.

3. BET specific surface area (SA) is 3.0m ２ / g or more 8.0m ２ / g, the surface oxygen content (FSO) measured by the temperature-programmed morphological analysis is 0.15 mass% or more and 0.40 mass% or less, the internal oxygen content (FIO) measured by the temperature-programmed morphological analysis is 0.25 mass% or more and 0.90 mass% or less, and the ratio of the surface oxygen content to the BET specific surface area (FSO / SA) is 0.25 mg / m ２ 0.50mg / m or more ２ 3. The silicon nitride powder according to claim 2, wherein the powder has a molecular weight of less than 1.

0.

4. BET specific surface area (SA) is 8.0 m ２ / g or more 13.0m ２ / g or less, the surface oxygen content (FSO) measured by the temperature-programmed morphological analysis is 0.20 mass% or more and 0.65 mass% or less, the internal oxygen content (FIO) measured by the temperature-programmed morphological analysis is 0.50 mass% or more and 1.00 mass% or less, and the ratio of the surface oxygen content to the BET specific surface area (FSO / SA) is 0.23 mg / m ２ 0.50mg / m or more ２ 3. The silicon nitride powder according to claim 2, wherein the powder has a molecular weight of less than 1.

0.

5. The silicon nitride powder according to claim 1, characterized in that the fluorine content is less than 120 ppm.

6. Silicon nitride powder according to claim 5, characterized in that the β fraction is 1.3 mass% or more and 14.0 mass% or less.

7. A method for producing a silicon nitride sintered body, comprising the steps of molding and sintering a sintering raw material containing the silicon nitride powder according to any one of claims 1 to 6 and a sintering aid.

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