Crystalline silicon nitride powder, silicon nitride-based sintered body, and method for producing silicon nitride-based sintered body

Abstract

The present invention provides: a crystalline silicon nitride powder which makes it possible to obtain a silicon nitride-based sintered body that has high strength, low variability, and excellent properties as a sintered body; a silicon nitride-based sintered body which has high strength and low variability; and a method for producing the silicon nitride-based sintered body.　Provided are: a crystalline silicon nitride powder characterized in that the amount of metal foreign matter of more than 20 μm present in 1 kg of the crystalline silicon nitride powder is 10-200 μg in a specific measurement method, and that in 1 cm3 of the crystalline silicon nitride powder, when said amount of metal foreign matter is converted as a 40-μm diameter, the total number of particles of the metal foreign matter, coarse particles of 3 to 50 μm, and aggregated particles and / or fused particles is less than 160; a silicon nitride-based sintered body obtained by sintering said crystalline silicon nitride powder; and a method for producing a silicon nitride-based sintered body characterized by comprising a step for molding and sintering a sintering raw material which contains said crystalline silicon nitride powder and a sintering aid.

Description

Crystalline silicon nitride powder, silicon nitride sintered body, 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 wear-resistant materials and circuit board materials, a silicon nitride sintered body, and a method for producing the silicon nitride sintered body.

[0002] According to Patent Document 1, the specific surface area is 6 to 25 m ２ / g, and the silicon nitride powder is ３ The silicon nitride powder has been disclosed in which the number of metallic foreign particles exceeding 20 μm is 3 or less and the number of metallic foreign particles having a size of 10 to 20 μm is 15 or less.

[0003] According to Patent Document 2, the specific surface area is 6 to 25 m ２ / g and no coarse particles of 20 μm or more are present.

[0004] : JP-A-8-104505: JP-A-10-291810

[0005] In Patent Document 1, the amount of metallic impurities is measured by ultrasonically dispersing 250 g of silicon nitride powder, passing it through sieves with 20 μm and 10 μm mesh sizes, and observing the residue on the sieves with an optical microscope. However, because a large amount of silicon nitride powder remains on the sieves, it is not possible to accurately count the number of metallic impurities alone. Furthermore, because the composition of the counted impurities is not confirmed, it is not possible to determine that all of the counted impurities are metallic impurities. Alternatively, the method involves collecting metallic impurities in the residue on the sieves with a magnet, attaching the metallic impurities on the magnet to a tape or sheet with an adhesive surface, and measuring the number of metallic impurities on the adhesive surface by observing with an optical microscope. However, even if the metallic impurities are collected with a magnet, silicon nitride powder remains on the sieve, so not all of the metallic impurities can be collected. In addition, if the magnetic force of the magnet is weak, metallic impurities that are not attracted to the magnet, such as those with weak magnetic properties like stainless steel, cannot be collected, making it impossible to accurately count the number of metallic impurities. Therefore, the amount of metallic foreign matter in silicon nitride cannot be measured accurately, and the Weibull coefficient remains at 30 or less.

[0006] Patent Document 2 does not describe the number of metallic foreign matters, and the Weibull coefficient remains at 30 or less.

[0007] In order to solve the above problems, the present invention has developed a method for measuring the amount of metallic foreign matter with a particle size of more than 20 μm present in 1 kg of crystalline silicon nitride powder, and based on the results, provides a crystalline silicon nitride powder that can be used to obtain silicon nitride sintered bodies with high strength and little variation in sintered body properties. Another object of the present invention is to provide a silicon nitride sintered body with high strength and little variation, and a method for producing the same.

[0008] According to the present invention, means for solving the problems are provided in the following aspects.

[0009] [1] The present invention relates to a method for producing a crystalline silicon nitride powder having a particle size of more than 20 μm, the particle size of which is 10 μg or more and 200 μg or less, as measured by the following method: ３The present invention relates to a crystalline silicon nitride powder characterized in that the total number of metallic particles present in the powder, calculated as a 40 μm diameter, and the total number of coarse particles and agglomerated and / or fused particles of 3 to 50 μm, is less than 160. [Method for measuring the amount of metallic particles greater than 20 μm] 250 g of crystalline silicon nitride powder and 750 g of ion-exchanged water are added to a 1000 ml beaker to prepare a slurry. While stirring the slurry, a bar magnet with a magnetic force of 10,000 gauss is inserted into a stainless steel tube and immersed for 15 minutes to capture the metallic particles. The stainless steel tube with the captured metallic particles is removed from the slurry, the magnet is removed over an empty beaker, and the stainless steel tube is washed with ion-exchanged water to release the metallic particles, which are then dispersed in the water. The resulting dispersion is passed through a stainless steel sieve (mesh size: 20 μm), and metal particles larger than 20 μm are collected on the stainless steel sieve. Over an empty beaker, the metal particles larger than 20 μm on the stainless steel sieve are desorbed with distilled water and dispersed in water. The resulting dispersion is passed through a suction filtration device equipped with a Teflon (registered trademark) filter (mesh size: 1 μm), and metal particles larger than 20 μm are collected on the Teflon (registered trademark) filter. The Teflon (registered trademark) filter from which the metal particles larger than 20 μm were collected is placed in a container containing a mixture of hydrofluoric acid and nitric acid, the container is sealed, and microwave irradiation is performed to heat the container. The resulting decomposition solution is adjusted to a constant volume with ultrapure water to obtain a test solution. Using ICP-MS, the amount of Fe in the test solution is quantified from the detected wavelength and its emission intensity. The amount is converted to a value per kg of silicon nitride powder to determine the amount (mass) of metal particles larger than 20 μm. [Method of converting the number of 40 μm metallic foreign particles] The representative diameter of metallic foreign particles exceeding 20 μm is set to 40 μm, and the average density of the metallic foreign particles is set to 8.0 g / cm ３ The density of the silicon nitride powder was set to 3.186 g / cm ３ and calculate it using the following formula: [Method for measuring the number of coarse particles and agglomerated and / or fused particles with a particle size of 3 to 50 μm] 20 g of silicon nitride powder was placed in a 100 ml polyethylene bottle together with 28 g of ethylene glycol and 480 5 mm diameter silicon nitride balls, and the bottle was sealed. The bottle was then premixed for 5 minutes using a vibration mill with an amplitude of 5 mm. Further mixing was performed using a ball mill at 120 rpm for 48 hours. One or two drops of the resulting silicon nitride powder slurry were dropped onto the slurry drop section of a grind gauge, and the slurry was then swept away with a scraper at a slow speed of 5 cm / sec or less. The number of coarse particles and agglomerated and / or fused particles with a particle size of 3 to 50 μm was read from the linear scratches that appeared. The same measurement was repeated four times, and a total of five measurements were taken per 1 cm of silicon nitride powder. ３ By converting the number of particles into the number per particle, the number of coarse particles and aggregated particles and / or fused particles can be obtained.

[0010] [2] In one embodiment of the present invention, the BET specific surface area (SA) is 7.0 m ２ / g or more 16.0m ２ / g or less, the crystalline silicon nitride powder has a silicon oxide layer and / or a silicon oxynitride layer on the particle surface, and has a surface oxygen content (FSO) measured by temperature-programmed morphological analysis of 0.30 mass% or more and 0.60 mass% or less, and a ratio of the surface oxygen content to the BET specific surface area (FSO / SA) of 0.20 mg / m ２ 0.50mg / m or more ２ A crystalline silicon nitride powder is provided, which is:

[0011] [3] In one embodiment of the present invention, there is provided a crystalline silicon nitride powder having a median diameter D50 of 0.50 μm or more and 1.00 μm or less, D90 of 1.75 μm or more and 2.35 μm or less, and D10 of 0.20 μm or more and 0.50 μm or less in a volume-based cumulative particle size distribution measured by a laser diffraction / scattering method.

[0012] [4] In one aspect of the present invention, there is provided a crystalline silicon nitride powder having a total oxygen content (TO) of 0.70 mass % or more and 1.90 mass % or less.

[0013] The aspects [1] to [4] of the present invention may be combined as desired.

[0014] [5] In one aspect of the present invention, there is provided a silicon nitride sintered body obtained by sintering the crystalline silicon nitride powder.

[0015] [6] 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 sintered body raw material containing the above-mentioned crystalline silicon nitride powder and a sintering aid.

[0016] The present invention provides a crystalline silicon nitride powder that can be used to obtain silicon nitride sintered bodies with high strength and excellent sintered body properties with little variation. It also provides a crystalline silicon nitride powder that enables the production of silicon nitride sintered bodies with sufficient mechanical strength and excellent thermal conductivity. It also provides a silicon nitride sintered body that combines high bending strength and excellent thermal conductivity, and a method for producing the same.

[0017] Typically, high strength refers to a strength that is higher than that of sintering aids such as Y when measured by the room temperature bending strength test method for fine ceramics in JIS R-1601:2008. ２ O ３ -Al ２ O ３ The sintered body is a Y-based sintered body, and the strength is more than 1130 MPa, preferably more than 1150 MPa, more preferably more than 1180 MPa. ２ O ３ The strength of the sintered body may be more than 810 MPa, preferably more than 830 MPa, and more preferably more than 850 MPa for a -MgO-based sintered body. Furthermore, the term "low variation" means that the Weibull coefficient is more than 30, preferably more than 32, as determined by the Weibull statistical analysis of strength data for fine ceramics in JIS R-1625:2010.

[0018] FIG. 1 is a diagram showing an example of an oxygen / nitrogen analysis chart.

[0019] The silicon nitride powder of the present invention is a crystalline silicon nitride powder in which the amount of metallic impurities with particle diameters exceeding 20 μm present per 1 kg of silicon nitride powder is 10 μg to 200 μg. The metallic impurities remain in the sintered body as coarse particles even after sintering, and these become the starting points for fracture. Therefore, if the amount exceeds 200 μg, defects with particularly low strength are generated in the resulting sintered body, resulting in variability in strength tests. On the other hand, if the amount is less than 10 μg, a sintered body with high strength and little variability can be obtained, but the conditions, for example, the number of passes through a magnetic separator and the feed rate, become excessive, resulting in increased production costs.

[0020] The amount of metallic foreign matter can be controlled, for example, by adjusting the number of times the material is passed through a magnetic separator having a magnetic attracting body with a surface magnetic flux density of 10,000 gausses or more.

[0021] The silicon nitride powder in the present invention is silicon nitride powder 1 cm ３ The crystalline silicon nitride powder has a total of less than 160 particles, including the number of metallic foreign particles when the amount of metallic foreign particles is converted to a diameter of 40 μm, and the number of coarse particles and agglomerated particles and / or fused particles having a size of 3 to 50 μm.

[0022] If the total number of these particles is 160 or more, the dissolution rate of silicon nitride particles dissolving into the grain boundary phase consisting of sintering aids, etc., will be delayed during the sintering process of silicon nitride, hindering the progress of densification. Furthermore, grain growth will proceed with coarse particles as nuclei during densification, resulting in a sintered body structure with areas of abnormal grain growth, and microcracks will occur around the areas of abnormal grain growth, reducing the strength of the sintered body.

[0023] In the production of silicon nitride powder by the imide pyrolysis method, the number of particles can be controlled by adjusting the conditions for the classification process of the fired powder.

[0024] In one embodiment of the present invention, the silicon nitride powder has a BET specific surface area (SA) of 7.0 m ２ / g or more 16.0m 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 a BET specific surface area of ​​7.0 m or less may be used. ２If the sintering ratio is less than 16.0 m / g, the driving force for sintering decreases, and therefore a high-density silicon nitride sintered body cannot be obtained unless a large amount of sintering aid is added, which may result in large variations in properties. ２ If the BET specific surface area exceeds 7.0 m / g, not only will the green density decrease and molding become difficult using a normal molding method, but microcracks, voids, etc. will occur in the sintered compact, resulting in increased variation in properties. ２ / g or more 13.0m ２ / g or less is preferable, and 8.4m ２ / g or more 12.5m ２ It is more preferable that the range is 1 / g or less.

[0025] In the production of silicon nitride powder by the imide pyrolysis method, the BET specific surface area (SA) can be controlled by adjusting, for example, the specific surface area and oxygen content of the amorphous Si-N(-H) compound.

[0026] In one embodiment of the present invention, the silicon nitride powder may be a crystalline silicon nitride powder having a silicon oxide layer and / or a silicon oxynitride layer on the particle surface and a surface oxygen content (FSO) of 0.30% by mass or more and 0.60% by mass or less, as measured by temperature-programmed morphology analysis. If the surface oxygen content (FSO) is less than 0.30% by mass, the amount of melt phase generated during the sintering process is small, the densification rate is reduced, and a high-density sintered body may not be obtained. On the other hand, if the surface oxygen content exceeds 0.60% by mass, although a high-density sintered body is obtained, the amount of grain boundary phase contained in the sintered body increases, which may not only reduce the strength properties (strength, toughness, etc.) of the resulting sintered body but also reduce the thermal conductivity. The surface oxygen content (FSO) is more preferably 0.33% by mass or more and 0.57% by mass or less, and the lower limit may be 0.35% by mass or more. The upper limit may be 0.53% by mass or less.

[0027] 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).

[0028] In one embodiment of the present invention, the silicon nitride powder has a ratio of the surface oxygen amount to the BET specific surface area (FSO / SA) of 0.20 mg / m ２ 0.50mg / m or more ２ The powder may be a crystalline silicon nitride powder having an FSO / SA ratio of 0.20 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, which reduces the densification rate and makes it difficult to obtain a high-density sintered body. ２ If the FSO / SA ratio exceeds 0.23 mg / m, the amount of the generated molten phase will be too large, resulting in an increase in the amount of the grain boundary phase, which may result in a decrease in the strength characteristics (strength, toughness, etc.) of the resulting sintered body, as well as a decrease in the thermal conductivity. ２ 0.47mg / m or more ２ The lower limit is preferably 0.28 mg / m or less, for example. ２ Above, 0.30mg / m ２ Above, 0.38mg / m ２ The upper limit may be, for example, 0.44 mg / m ２ Below, 0.42mg / m ２ Below, 0.41mg / m ２ It may be the following:

[0029] 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 (oxygen concentration, water concentration) of the atmosphere in the milling process of the fired powder.

[0030] In one embodiment of the present invention, the silicon nitride powder may be a crystalline silicon nitride powder having a median diameter D50 of 0.50 μm or more and 1.00 μm or less, a D90 of 1.75 μm or more and 2.35 μm or less, and a D10 of 0.20 μm or more and 0.50 μm or less in a volume-based cumulative particle size distribution measured by a laser diffraction / scattering method.

[0031] If the volume-based cumulative particle size distribution is within the above range, the silicon nitride powder packs well, resulting in almost no molding defects and a homogeneous silicon nitride molded body. If the cumulative 10% diameter D10 is less than 0.20 μm, the proportion of fine particles is too high, which not only reduces green density and makes molding difficult using conventional molding methods, but also can lead to the formation of microcracks and voids after sintering. Furthermore, this not only adversely affects dimensional accuracy, but also leads to cracks (crack clusters) at corners of the sintered body when sintering complex-shaped products, which is undesirable. If the cumulative 10% diameter D10 exceeds 0.50 μm, the proportion of fine particles is too low, reducing the driving force for sintering, and a high-density silicon nitride sintered body may not be obtained unless a large amount of sintering aid is added. Adding a large amount of sintering aid can deteriorate the properties of the silicon nitride sintered body. The upper limit of the cumulative 10% diameter D10 may be, for example, 0.47 μm or less, 0.44 μm or less, or 0.40 μm or less. The lower limit of the cumulative 10% diameter D10 may be, for example, 0.26 μm or more, 0.30 μm or more, or 0.34 μm or more.

[0032] The median diameter D50 in the volume-based cumulative particle size distribution may be 0.50 μm or more and 1.00 μm or less. If the median diameter D50 is less than 0.50 μm, the average particle size becomes too fine, and the green density tends to decrease when using a conventional molding method. When this is sintered, a sintered body with a fine structure is obtained, but the individual silicon nitride particles are too fine, and the fracture toughness tends to decrease. In addition, high-temperature strength may also tend to decrease. If the median diameter D50 exceeds 1.00 μm, the proportion of coarse particles increases, the densification rate decreases, and the sintered body structure may easily become non-uniform. Furthermore, aggregates of micropores tend to form within the sintered body, which often become the source of fracture. The upper limit of the median diameter D50 may be, for example, 0.91 μm or less, 0.87 μm or less, or 0.84 μm or less. The lower limit of the median diameter D50 may be, for example, 0.60 μm or more, 0.63 μm or more, or 0.75 μm or more.

[0033] The cumulative 90% diameter D90 in the volume-based cumulative particle size distribution may be 1.75 μm or more and 2.35 μm or less. If the cumulative 90% diameter D90 is less than 1.75 μm, the particle size distribution becomes too sharp, and green density tends to decrease when using conventional molding methods. When this is sintered, a sintered body with a fine structure is obtained, but the individual silicon nitride particles are too fine, and fracture toughness tends to decrease. High-temperature strength may also tend to decrease. If the cumulative 90% diameter D90 exceeds 2.35 μm, the proportion of coarse particles increases, and not only do they grow into even coarser particles during sintering, but microcracks, voids, etc. tend to occur around the coarse particles, which often become sources of fracture. The upper limit of the cumulative 90% diameter D90 may be, for example, 2.30 μm or less, 2.25 μm or less, or 2.19 μm or less. The lower limit of the cumulative 90% diameter D90 may be, for example, 1.93 μm or more, 2.00 μm or more, or 2.05 μm or more.

[0034] The cumulative 10% diameter D10, median diameter D50 and cumulative 90% diameter D90 in the volume-based cumulative particle size distribution of the silicon nitride powder can be controlled, for example, by adjusting the crushing conditions and classification conditions during the production of the silicon nitride powder.

[0035] In one embodiment of the present invention, the silicon nitride powder may be a crystalline silicon nitride powder having a total oxygen content (TO) of 0.70% by mass or more and 1.90% by mass or less. If the total oxygen content is less than 0.70% by mass, the amount of melt phase generated during the sintering process is small, the densification rate is reduced, and a high-density sintered body may not be obtained. On the other hand, if the total oxygen content (TO) exceeds 1.90% by mass, a densified sintered body can be obtained, but the oxygen content of the sintered body increases, the amount of grain boundary phase increases, and strength properties (room-temperature strength, high-temperature strength, fracture toughness, etc.) may decrease. It is more preferable that the total oxygen content (TO) be 0.80% by mass or more and 1.60% by mass or less.

[0036] When producing silicon nitride by the imide pyrolysis method, the total oxygen content (TO) can be controlled, for example, by adjusting the atmospheric components (oxygen concentration, carbon monoxide concentration) in the firing furnace or the atmospheric components (oxygen concentration, water concentration) in the milling process of the fired powder.

[0037] [Method for measuring the amount of metal particles greater than 20 μm] 250 g of crystalline silicon nitride powder and 750 g of ion-exchanged water are added to a 1000 ml beaker to prepare a slurry. While stirring the slurry, a bar magnet with a magnetic force of 10,000 gauss is inserted into a stainless steel tube and immersed for 15 minutes to capture the metal particles. The stainless steel tube with the captured metal particles is removed from the slurry, and the bar magnet is removed from the stainless steel tube over an empty beaker. The stainless steel tube is washed with ion-exchanged water to release the metal particles, which are then dispersed in water. The resulting dispersion is passed through a stainless steel sieve (mesh opening: 20 μm), and the metal particles greater than 20 μm are collected on the stainless steel sieve. Over an empty beaker, the metal particles greater than 20 μm on the stainless steel sieve are released with distilled water and dispersed in water. The resulting dispersion is passed through a suction filtration device equipped with a Teflon (registered trademark) filter (mesh size: 1 μm) to collect metallic foreign particles larger than 20 μm on the Teflon (registered trademark) filter. The Teflon (registered trademark) filter from which the metallic foreign particles larger than 20 μm have been collected is placed in a container containing a mixture of hydrofluoric acid and nitric acid, the container is sealed, and microwaves are irradiated and heated. The resulting decomposed liquid is adjusted to a constant volume with ultrapure water to obtain a test liquid. Using ICP-MS, the amount of Fe in the test liquid is quantified from the detected wavelength and its emission intensity. The amount is converted to a value per kg of silicon nitride powder to determine the amount of metallic foreign particles larger than 20 μm.

[0038] [Method for converting the number of 40 μm metallic foreign particles] The representative diameter of the metallic foreign particles exceeding 20 μm is set to 40 μm, and the average density of the metallic foreign particles is set to 8.0 g / cm ３ The density of the silicon nitride powder was set to 3.186 g / cm ３ and calculate it using the following formula:

[0039] [Method for measuring the number of coarse particles and agglomerated and / or fused particles with a particle size of 3 to 50 μm] 20 g of silicon nitride powder was placed in a 100 ml polyethylene bottle together with 28 g of ethylene glycol and 480 5 mm diameter silicon nitride balls, and the bottle was sealed. The bottle was then premixed for 5 minutes using a vibration mill with an amplitude of 5 mm. Further mixing was performed using a ball mill at 120 rpm for 48 hours. One or two drops of the resulting silicon nitride powder slurry were dropped onto the slurry drop section of a grind gauge, and the slurry was then swept away with a scraper at a slow speed of 5 cm / sec or less. The number of coarse particles and agglomerated and / or fused particles with a particle size of 3 to 50 μm was read from the linear scratches that appeared. The same measurement was repeated four times, and a total of five measurements were taken per 1 cm of silicon nitride powder. ３ By converting the number of particles into the number per particle, the number of coarse particles and aggregated particles and / or fused particles can be obtained.

[0040] [Method for Producing Silicon Nitride Powder] The method for producing silicon nitride powder in 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, for example, a method of reacting a silicon halide with ammonia, specifically, a method of reacting a silicon halide such as silicon tetrafluoride, silicon tetrachloride, silicon tetrabromide, or silicon tetraiodide with ammonia in the gas phase, or a method of reacting a liquid silicon halide with liquid ammonia.

[0041] These nitrogen-containing silane compounds are represented by the following composition formula (1): Si ６ (NH)n(NH ２ ) ２４－２ｎ (where n = 0 to 12, and includes compounds containing halogen as impurities, although not specified in the composition formula)... (1)

[0042] In the present invention, for convenience, nitrogen-containing silane compounds where n=8 to 12 are referred to as silicon diimides.

[0043] 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.

[0044] The amorphous Si—N(—H) compound used in the present invention is an amorphous Si—N—H compound containing the elements Si, N, and H obtained by thermally decomposing 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 (2). In the present invention, the amorphous Si—N(—H) compound is Si—N(—H) compound represented by the following composition formula (2) 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. ６ N ２ｘ (NH) １２－３ｘ (where x = 0 to 4, and includes compounds containing halogen as impurities, although not specified in the composition formula)... (2)

[0045] The value of x is preferably 1.3 or more and 3.6 or less, and more preferably 1.65 or more and 3.10 or less.

[0046] In the composition formula (2), Si represented by x=0 ６ (NH) １２ is a silicon diimide, but silicon diimide is the nitrogen-containing silane compound described above and is not included in the amorphous Si-N(-H) compounds of the present invention.

[0047] 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 carbon content of the silicon nitride powder after high-temperature firing can be made 0.01% by mass to 0.20% by mass by controlling the toluene content of the silicon diimide to 0.01% by mass to 0.54% by mass. It is more preferable that the toluene content of the silicon diimide be 0.01% by mass to 0.40% by mass.

[0048] The specific surface area of ​​the amorphous Si-N(-H) compound, which is an intermediate raw material for producing silicon nitride powder according to the present invention, is 215 m ２ / g or more 590m ２ / g or less. ２ If the specific surface area is less than 590 m / g, rapid crystallization occurs in the temperature range of 1000 to 1400°C, resulting in the formation of needle-shaped particles and agglomerated particles. If a sintered body is produced from such powder, a homogeneous structure will not be formed, and the strength and thermal conductivity of the resulting sintered body will be low. ２ If the specific surface area is greater than 230 m / g, the degree of supersaturation near the particles during crystallization is difficult to increase, and the number of nuclei generated tends to decrease, resulting in a decrease in the specific surface area of ​​the crystalline silicon nitride powder. This results in poor sinterability and reduced strength and thermal conductivity of the sintered body. The specific surface area of ​​an amorphous Si-N(-H) compound is 230 m / g. ２ / g or more 570m ２ / g or less, and ２ / g or more 510m ２ It is more preferable that the SiO2 content is 1 / g or less.

[0049] 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, 500° C. or higher and 1030° C. or lower, and more preferably 580° C. or higher and 950° C. or lower.

[0050] The oxygen content 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 of the amorphous Si—N(—H) compound. The oxygen concentration in the inert gas supplied to the thermal decomposition furnace may be, for example, 100 ppm to 1100 ppm, and more preferably 200 ppm to 600 ppm. The oxygen content of the amorphous Si—N(—H) compound may be, for example, 0.40 mass% to 1.4 mass%, or 0.60 mass% to 1.35 mass%, and more preferably 0.62 mass% to 0.99 mass%.

[0051] If the oxygen content of the amorphous Si—N(—H) compound is less than 0.40 mass%, the degree of supersaturation near the particles during crystallization does not increase, the number of nuclei generated decreases, and the specific surface area of ​​the crystalline silicon nitride powder decreases.If the oxygen content of the amorphous Si—N(—H) compound exceeds 1.4 mass%, the internal oxygen content (FIO) of the resulting crystalline silicon nitride powder increases, which is undesirable.

[0052] In 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 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.

[0053] 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.

[0054] 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 contains no oxygen at all, but even if oxygen is contained, the oxygen concentration may be 100 ppm (V) or less, and preferably 50 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.

[0055] 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 other components of the firing furnace. The CO concentration in the furnace is preferably, for example, 70 ppm (V) or more and 230 ppm (V) or less. A CO concentration in the furnace exceeding 230 ppm (V) is undesirable because it tends to increase the internal oxygen content and total oxygen content of the resulting silicon nitride powder. Reducing the CO concentration in the furnace to less than 70 ppm (V) requires the supply of a large amount of nitrogen-containing inert gas to dilute the generated CO gas, which increases costs. The CO concentration in the furnace may be 120 ppm (V) or more and 190 ppm (V) or less.

[0056] 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 53°C or higher and 110°C or lower, and the maximum holding temperature to 1450°C or higher and 1585°C or lower. It is more preferable to set the heating rate to 56°C or higher and 95°C or lower, and the maximum holding temperature to 1490°C or higher and 1580°C or lower. The holding time at the maximum temperature may be, for example, 1 hour or higher and 5 hours or lower.

[0057] The resulting crystalline silicon nitride powder is subjected to a crushing treatment in 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 common crushing devices 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, from 100 ppm (V) to 2000 ppm (V), and more preferably from 190 ppm (V) to 1400 ppm (V).

[0058] 1cm of crystalline silicon nitride powder ３ The number of coarse particles of 3 to 50 μm and aggregated and / or fused particles present in the mixture can be adjusted by adjusting the classification conditions. When an air classifier is used, the number can be changed by comprehensively adjusting the supply speed, air volume, rotation speed, etc. For example, when the supply speed is 2.0 kg / h and the air volume is 2.2 Nm ３ / min and the rotation speed is 750 rpm, the number can be reduced to less than 160 pieces.

[0059] The amount of metallic foreign matter of over 20 μm present in 1 kg of crystalline silicon nitride powder can be adjusted by adjusting the magnetic separation treatment conditions. For example, when a magnetic separator equipped with a permanent magnet with a surface magnetic flux density of 10,000 gauss is used, the desired amount of metallic foreign matter can be achieved by adjusting, for example, the feed rate and the number of treatments.

[0060] [Silicon nitride sintered body and its manufacturing method] Next, the silicon nitride sintered body according to the present invention and its manufacturing method will be described. The silicon nitride sintered body according to the present invention is a silicon nitride sintered body obtained by sintering the silicon nitride powder according to the present invention. The silicon nitride sintered body according to the present invention is produced by the following manufacturing method. The silicon nitride sintered body according to the present invention, which has high strength and little variation, can be produced by mixing the silicon nitride powder according to the present invention with a sintering aid, molding the resulting mixed powder, and sintering the resulting molded body. Alternatively, the silicon nitride sintered body according to the present invention, which has high strength and little variation, can be produced by simultaneously carrying out molding and sintering.

[0061] Because silicon nitride is a difficult-to-sinter material, sintering aids are usually added to promote sintering when producing sintered bodies. During the sintering process, high-temperature stable β-type columnar crystals precipitate, resulting in the majority of silicon nitride crystal grains in the sintered body being β-type columnar crystals. The microstructure of these β-type columnar crystals, including their aspect ratio and particle size, is significantly affected not only by the raw silicon nitride powder, but also by the type and amount of sintering aids used, as well as the sintering conditions. These factors are selected appropriately based on the physical properties of the silicon nitride powder and the desired characteristics of the silicon nitride sintered body.

[0062] (Sintering aid is Y ２ O ３ -Al ２ O ３ In order to increase the mechanical strength of silicon nitride sintered bodies, it is desirable to make the structure of the silicon nitride sintered body a fine structure with a high aspect ratio of β-type columnar crystals. In this case, the sintering aid is generally aluminum oxide (Al ２ O ３ ) and yttrium oxide (Y ２ O ３) is used in combination with an appropriate one. In the production of silicon nitride sintered bodies for high-temperature structural components, such as gas turbine components, which require particularly high-temperature strength, rare earth oxides such as ytterbium oxide, which are effective in improving the heat resistance of the grain boundary phase, may be 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.

[0063] The method for mixing the silicon nitride powder and sintering aid according to the present invention may be any method, wet or dry, that can uniformly mix them, 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.

[0064] The mixed powder can be molded by known methods 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.

[0065] The sintering method for the compact may be any method that densifies the resulting sintered body, but atmospheric pressure sintering in an inert gas atmosphere such as nitrogen or argon mixed with nitrogen, or gas pressure sintering in an inert gas atmosphere such as nitrogen or argon mixed with nitrogen at a gas pressure of approximately 0.2 to 10 MPa, is employed. Sintering is generally carried out using nitrogen gas at a temperature range of 1700 to 1800°C for atmospheric pressure sintering and 1800 to 2000°C for gas pressure sintering. 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.

[0066] Alternatively, hot pressing, which is a method of simultaneously performing molding and sintering, can be employed. Sintering by hot pressing is usually performed in a nitrogen atmosphere at a pressure of 0.2 to 10 MPa and a sintering temperature of 1950 to 2050°C.

[0067] The strength of the resulting silicon nitride sintered body can be further improved by subjecting it to HIP (hot isostatic pressing), which is typically carried out in a nitrogen atmosphere at a pressure of 30 to 200 MPa and a sintering temperature of 2100 to 2200°C.

[0068] (Sintering aid is Y ２ O ３ (Method for producing MgO-based silicon nitride sintered body) It 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 the present invention, and then the obtained raw material mixture is subjected to a sheet forming method such as the doctor blade method to obtain a green sheet. In this case, the sintering aid is generally magnesium oxide (MgO), yttrium oxide (Y ２ O ３ ) is used in combination. The organic binder is then removed by degreasing, and the resulting degreasing molded body is sintered in a nitrogen-containing inert atmosphere under the above-mentioned sintering conditions (heating rate, maximum temperature, and holding time at maximum temperature) to produce a silicon nitride sintered body. When the thermal conductivity and mechanical properties of the silicon nitride sintered body are to be evaluated, the thermal and mechanical properties of the sintered body obtained by the above-mentioned CIP (cold isostatic pressing) molding may be measured.

[0069] In one embodiment, yttrium oxide, lanthanide rare earth oxide, magnesium oxide, magnesium silicon nitride (MgSiN ２ , MgSiN ６ ), rare earth-silicon composite nitride (Y ２ SiN 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.

[0070] Increasing the purity of the β-crystals and increasing their size tends to be effective in increasing 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.

[0071] The resulting silicon nitride sintered body preferably has a relative density of 96% or more, more preferably 97% or more, or even 98% or more. The relative density of a silicon nitride sintered body is the density of the silicon nitride sintered body relative to the true density of silicon nitride, expressed as a percentage. If the relative density is low, it may be difficult to increase the mechanical strength.

[0072] The present invention will be described in more detail below with reference to specific examples and comparative examples, but the present invention should not be construed as being limited to these examples.

[0073] [Method for Producing Silicon Nitride Powder by Imide Pyrolysis] (Example 1) <Synthesis of Silicon Diimide> 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 as the reaction progressed, ammonia was consumed, so ammonia was also continuously fed to the reactor. At steady state, the volume ratio of silicon tetrachloride fed to the reactor and 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 26 liters of liquid ammonia (total of 130 liters), filtered, dried, and purified to a BET specific surface area of ​​800 m ２ About 1 kg of purified silicon diimide having a toluene content of 0.21% by mass was obtained.

[0074] <Thermal decomposition of silicon diimide> Using a rotary kiln furnace, the obtained silicon diimide was thermally decomposed at 730°C while passing nitrogen gas containing 493 ppm (V) of oxygen through the furnace to obtain a silicon diimide having the composition formula Si ６ N １０．６２ H ７．８６ That is, Si ６ N ２ｘ (NH) １２－３ｘ An amorphous Si—N(—H) compound was obtained in which x in the formula was 2.28. The specific surface area of ​​the obtained amorphous Si—N(—H) compound was 410 m ２ / g, and the oxygen content was 0.93 mass %.

[0075] <High-Temperature Sintering> The resulting amorphous Si—N(—H) compound was then milled in a vibration mill and formed into an almond-shaped product measuring approximately 6 mm thick, 8 mm short axis diameter, and 12 mm long axis diameter using a briquette machine. The almond-shaped product, some of which contained burr-like fragments, was loaded into a silicon carbide (SiC)-coated carbon container and fed into a pusher furnace. The product was heated and fired in a nitrogen gas atmosphere at a carbon monoxide (CO) concentration of 155 ppm, a heating rate of 74°C / h, and a maximum temperature of 1520°C to produce a grayish-white silicon nitride powder. The holding time at the maximum temperature (1520°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 being circulated.

[0076] <Crushing> This crystalline silicon nitride powder was placed in a vibration mill and subjected to milling treatment at an amplitude of 9.0 mm for 38 minutes in an air atmosphere with a moisture concentration of 285 ppm (V).

[0077] <Classification> Then, the mixture was put into a classifier, and the feed rate was 2.0 kg / h and the air volume was 2.2 Nm ３ The classification treatment was carried out at a rotation speed of 750 rpm.

[0078] <Magnetic Separation> Finally, the crystalline silicon nitride powder was passed nine times through a magnetic separator equipped with a permanent magnet having a surface magnetic flux density of 10,000 gauss at a feed rate of 40 kg / h to obtain the crystalline silicon nitride powder of Example 1.

[0079] (Examples 2 to 17, Comparative Examples 1 to 18) For each of Examples 2 to 17 and Comparative Examples 1 to 18, crystalline silicon nitride powders were obtained in the same manner as in Example 1, except that the production conditions shown in Table 1 were used.

[0080]

[0081] The specific surface area, oxygen content, and X value of the composition formula of the amorphous Si—N(—H) compounds in Table 1 were measured by the following methods.

[0082] (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 dehydration 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, the samples were handled in a nitrogen-purged glove box. For example, when measuring silicon and nitrogen content by ICP optical emission spectrometry or steam distillation separation neutralization titration, the atmosphere in which the sample was stored until just before sample pretreatment for measurement was a nitrogen atmosphere, and when measuring oxygen content by infrared absorption, the atmosphere during sample storage until just before measurement and when the capsule was inserted was a nitrogen atmosphere. The hydrogen (H) content of the amorphous Si—N(—H) compound was calculated based on the stoichiometric composition as the remainder after subtracting the silicon (Si), nitrogen (N), and oxygen (O) contents from the total amount of the amorphous Si—N(—H) compound. From the above, the ratios of Si, N, and H were calculated to determine the composition formula of the amorphous Si—N(—H) compound.

[0083] (BET Specific Surface Area) The BET specific surface area of ​​the silicon nitride powder and amorphous Si-N(-H) compound according to the present invention was measured by the BET single-point method using nitrogen gas adsorption using a Macsorb manufactured by Mountech.

[0084] (Methods for measuring oxygen content, surface oxygen content and internal oxygen content) To measure the oxygen content of the silicon nitride powder according to the present invention, an oxygen / nitrogen simultaneous analyzer (EMGA-620W model manufactured by Horiba, Ltd.) was used, and the oxygen content of the silicon nitride powder was measured according to the following procedure in accordance with the oxygen quantification method by inert gas fusion-carbon dioxide infrared absorption method specified in JIS R1603:2007.

[0085] Thirty mg of silicon nitride powder was weighed and the measurement sample was placed in an oxygen / nitrogen simultaneous analyzer. The temperature was increased from 20°C to 2000°C at a rate of 4-8°C / sec in a helium gas atmosphere. The oxygen and nitrogen desorbed with the temperature increase were detected, yielding concentration charts of oxygen (by non-dispersive infrared absorption) and nitrogen (by thermal conductivity). An example of the measurement results is shown in Figure 1 (the heating rate was 6.5°C / sec). As shown in Figure 1, 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. At the beginning of the temperature increase, oxygen bonded to the surface of the silicon nitride powder is desorbed, and the amount of desorbed oxygen can be determined by quantifying the amount of surface oxygen derived from the silicon oxide layer on the particle surface. Then, at temperature T 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 is a point where 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 is zero) (time t ２ ) is the temperature at ２ (or t ２ ) separates and separates the concentration peak due to surface oxygen from the concentration peak due to internal oxygen. ２ In this case, the surface oxygen content (FSO) is calculated by adding the surface oxygen content derived from the silicon oxide layer near the particle surface and the surface oxygen content derived from the silicon oxynitride layer near the particle surface from the integrated value (area) of the oxygen concentration chart. 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). ２The 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).

[0086] When measuring the oxygen content and nitrogen content, a calibration curve was first prepared using a standard sample. Since the peak intensity integrated value of oxygen detected by a non-dispersive infrared absorption cell is not necessarily linear with respect to the actual oxygen concentration, 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 oxygen amount versus the peak intensity integrated value was prepared, and the oxygen content was calculated based on the calibration curve.

[0087] Using the crystalline silicon nitride powders obtained in the examples and comparative examples, the amount of metallic foreign matter of more than 20 μm present in 1 kg of the crystalline silicon nitride powder was measured by the above-mentioned method, and the amount of metallic foreign matter of more than 20 μm present in 1 cm of the crystalline silicon nitride powder was measured. ３ The quantities of coarse particles of 3 to 50 μm and aggregated particles and / or fused particles present in the powder were measured, as well as the BET specific surface area, oxygen content, surface oxygen content, and internal oxygen content. The particle size distribution was also measured by the following method. The results are shown in Table 2. Additionally, various parameters of the crystalline silicon nitride powder were measured by the following method. The results are also shown in Table 2.

[0088] (Method for Measuring Particle Size Distribution) The particle size distribution of the silicon nitride powder according to the present invention was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3000, manufactured by Nikkiso Co., Ltd.) to prepare a dilute solution by dispersing a sample in a 0.2% by mass aqueous solution of sodium hexametaphosphate at 300 W for 6 minutes using an ultrasonic homogenizer equipped with a 26 mm diameter stainless steel center cone. The frequency (volume %) based on the cumulative undersize distribution was calculated from the volumetric particle size distribution data of the obtained silicon nitride powder, and a cumulative particle size distribution curve was obtained. The median diameter (50% diameter) in the cumulative particle size distribution curve was defined as the average particle size (D50), the particle diameter corresponding to 10% of the cumulative undersize distribution was defined as the cumulative 10% diameter (D10), and the particle diameter corresponding to 90% of the cumulative undersize distribution was defined as the cumulative 90% diameter (D90).

[0089] [Silicon nitride sintered body and its manufacturing method] A silicon nitride sintered body was manufactured using the crystalline silicon nitride powders obtained in the above examples and comparative examples.

[0090] (Sintering aid is Y ２ O ３ -Al ２ O ３ Preparation and Evaluation Method of Silicon Nitride-Based Sintered Body (Example 1) To 90 parts by mass of the silicon nitride powder obtained in the Examples and Comparative Examples, yttrium oxide (specific surface area 3 m) was added as a sintering aid. ２ / g, manufactured by Shin-Etsu Chemical Co., Ltd.) and 6 parts by mass of aluminum oxide (specific surface area 7.4 m ２ The blended powder, to which 4 parts by mass of ethylenediaminetetraacetic acid (E100 / g, manufactured by Sumitomo Chemical Co., Ltd.) had been added, was wet-mixed in a ball mill using ethanol as a medium for 36 hours and then dried under reduced pressure. The resulting mixture was molded into a 6 x 45 x 75 mm shape at a molding pressure of 50 MPa and then CIP-molded at a molding pressure of 150 MPa. The resulting molded body was placed in a silicon nitride crucible and sintered at 1780°C for 2 hours in a nitrogen gas atmosphere. 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.

[0091] The relative density of the sintered body was measured by the Archimedes method using the obtained test pieces. The room temperature bending strength (25°C) was measured in accordance with JIS R1601 using an Instron universal testing machine. The crosshead speed was 0.5 mm / min. The bending strength at room temperature was the average value of 40 pieces. The Weibull coefficient was also calculated by fitting the strength distribution of the bending test to a Weibull distribution. The results are shown in Table 2.

[0092] (Sintering aid is Y ２ O ３ - Preparation and evaluation method of MgO-based silicon nitride sintered body) (Example 1) To 94.5 parts by mass of the silicon nitride powder obtained in the examples and comparative examples, yttrium oxide (specific surface area 3 m) was added as a sintering aid. ２ / g, manufactured by Shin-Etsu Chemical Co., Ltd.) 3.5 parts by mass and magnesium oxide (specific surface area 3 m ２The blended powder, to which 2 parts by weight of ethylenediaminetetraacetic acid (E10 / g, manufactured by Kojundo Chemical Laboratory) had been added, was wet-mixed in a ball mill using ethanol as a medium for 24 hours and then dried under reduced pressure. The resulting mixture was molded into 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 compacts were then 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.

[0093] The bulk density of the sintered body was measured using the obtained test specimens by the Archimedes method. Assuming that all silicon nitride particles were converted to β-type silicon nitride, the relative density (%) was calculated from the calculated density obtained from the raw material composition and the bulk density of the sintered body. The three-point bending strength at room temperature was measured 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 for 40 specimens. The thermal conductivity is the average value for three disc-shaped test specimens. The Weibull coefficient was calculated by fitting the strength distribution from the bending test to a Weibull distribution. The results are shown in Table 2.

[0094]

[0095] The sintered body using the crystalline silicon nitride powder according to the embodiment contains Y as a sintering aid. ２ O ３ -Al ２ O ３ The sintered body is a Y-sintered body. ２ O ３ It was confirmed that the Weibull modulus exceeded 810 MPa for the -MgO-based sintered bodies. In addition, the Weibull modulus of all the sintered bodies according to the examples exceeded 30.

[0096] As described above, according to the present invention, it is possible to provide a silicon nitride powder that can be obtained by sintering the silicon nitride powder, in which the amount of metallic foreign matter contained in the silicon nitride powder selected using a magnet, and the total number of the metallic foreign matter, coarse silicon nitride particles, and agglomerated particles and / or fused particles, are within appropriate ranges, and which can produce a silicon nitride sintered body that combines high reliability (with little variation) and high strength; a silicon nitride sintered body obtained by sintering the silicon nitride powder; and a method for producing the same.

[0097] 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 ) which separates and separates the concentration peak due to surface oxygen and the concentration peak due to internal oxygen.

Claims

1. The amount of metal foreign matter larger than 20 μm present in 1 kg of crystalline silicon nitride powder is 10 μg or more and 200 μg or less, as measured by the following method, and ３ A crystalline silicon nitride powder characterized in that the total number of metallic particles present in the powder, calculated as a 40 μm diameter, and the total number of coarse particles of 3 to 50 μm, agglomerated particles, and / or fused particles, is less than 160. [Method for measuring the amount of metallic particles greater than 20 μm] 250 g of crystalline silicon nitride powder and 750 g of ion-exchanged water are added to a 1000 ml beaker to prepare a slurry. While stirring the slurry, a bar magnet with a magnetic force of 10,000 gauss is inserted into a stainless steel tube and immersed for 15 minutes to capture the metallic particles. The stainless steel tube with the captured metallic particles is removed from the slurry, and the bar magnet is removed from the stainless steel tube over an empty beaker. The stainless steel tube is then washed with ion-exchanged water to release the metallic particles, and the metallic particles are dispersed in the water. The resulting dispersion is passed through a stainless steel sieve (mesh size: 20 μm), and metal particles larger than 20 μm are collected on the stainless steel sieve. Over an empty beaker, the metal particles larger than 20 μm on the stainless steel sieve are desorbed with distilled water and dispersed in water. The resulting dispersion is passed through a suction filtration device equipped with a Teflon (registered trademark) filter (mesh size: 1 μm), and metal particles larger than 20 μm are collected on the Teflon (registered trademark) filter. The Teflon (registered trademark) filter from which the metal particles larger than 20 μm were collected is placed in a container containing a mixture of hydrofluoric acid and nitric acid, the container is sealed, and microwave irradiation is applied to heat the container. The resulting decomposed solution is adjusted to a constant volume with ultrapure water to obtain a test solution. Using ICP-MS, the amount of Fe in the test solution is quantified from the detected wavelength and its emission intensity. The amount is converted to a value per kg of silicon nitride powder to determine the amount of metal particles larger than 20 μm. [Method of converting the number of 40 μm metallic foreign particles] The representative diameter of metallic foreign particles exceeding 20 μm is set to 40 μm, and the average density of the metallic foreign particles is set to 8.0 g / cm ３ The density of the silicon nitride powder was set to 3.186 g / cm ３ and calculate it using the following formula: [Method for measuring the number of coarse particles, agglomerates, and / or fused particles with a particle size of 3 to 50 μm] 20 g of crystalline silicon nitride powder was placed in a 100 ml polyethylene bottle together with 28 g of ethylene glycol and 480 5 mm diameter silicon nitride balls, and the bottle was sealed. The bottle was then premixed for 5 minutes using a vibration mill with an amplitude of 5 mm. Further mixing was performed using a ball mill at 120 rpm for 48 hours. One or two drops of the resulting silicon nitride powder slurry were placed on the slurry drip section of a grind gauge, and the slurry was then removed using a scraper at a slow speed of 5 cm / sec or less. The number of coarse particles, agglomerates, and / or fused particles with a particle size of 3 to 50 μm was read from the linear marks that appeared. The same measurement was repeated four times, and the total of five measurements was calculated based on the number of coarse particles per cm of crystalline silicon nitride powder. ３ By converting the number of particles into the number per particle, the number of coarse particles and aggregated particles and / or fused particles can be obtained.

2. BET specific surface area (SA) is 7.0 m ２ / g or more 16.0m ２ / g or less, the crystalline silicon nitride powder has a silicon oxide layer and / or a silicon oxynitride layer on the particle surface, and has a surface oxygen content (FSO) measured by temperature-programmed morphological analysis of 0.30 mass% or more and 0.60 mass% or less, and a ratio of the surface oxygen content to the BET specific surface area (FSO / SA) of 0.20 mg / m ２ 0.50mg / m or more ２ 2. The crystalline silicon nitride powder according to claim 1, wherein:

3. A crystalline silicon nitride powder according to claim 1, characterized in that the median diameter D50 in the volume-based cumulative particle size distribution measured by a laser diffraction / scattering method is 0.50 μm or more and 1.00 μm or less, D90 is 1.75 μm or more and 2.35 μm or less, and D10 is 0.20 μm or more and 0.50 μm or less.

4. Crystalline silicon nitride powder according to claim 1, characterized in that the total oxygen content (TO) is 0.70 mass % or more and 1.90 mass % or less.

5. A silicon nitride sintered body obtained by sintering the crystalline silicon nitride powder according to any one of claims 1 to 4.

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

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