Silicon nitride presintered body, method for producing same, and method for producing silicon nitride powder

Abstract

A method for producing a silicon nitride pre-sintered body, said method comprising a step for obtaining a silicon nitride pre-sintered body by heating a starting material powder containing a metal silicon powder in a nitrogen-containing atmosphere using a continuous furnace. If RTm is the average value of the rate of temperature increase of the raw material powder in the range from the temperature in the furnace exceeding 1100 DEG C to 1200 DEG C, and RTh is the average value of the rate of temperature increase of the raw material powder in the range from the temperature in the furnace exceeding 1200 DEG C to the position at which the temperature in the furnace reaches the maximum temperature, RTm < RTh is satisfied.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a silicon nitride pre-sintered body and a manufacturing method thereof, and a manufacturing method of a silicon nitride powder. BACKGROUND

[0002] A sintered body containing silicon nitride is a material having excellent strength, hardness, toughness, heat resistance, corrosion resistance, and heat shock resistance, and is thus used for various industrial equipment parts such as die casting machines and melting furnaces, and for automobile parts such as an insulating substrate. As a raw material of the sintered body, a silicon nitride powder having a high α-phase conversion rate is generally used in order to obtain a high-quality sintered body. As a manufacturing method of such a silicon nitride powder, a direct nitriding method of metallic silicon is known.

[0003] If the direct nitriding method is performed in a continuous furnace, silicon nitride can be mass-produced smoothly. In Patent Literature 1, a tunnel-type pusher furnace is used to manufacture a silicon nitride powder having a high α-phase conversion rate by the direct nitriding method. In this Patent Literature 1, it is pointed out that there is a large difference between the furnace temperature and the actual temperature of the metallic silicon powder in the tunnel-type pusher furnace, and thus it is proposed to select an operation condition in which the temperature of the nitriding reaction is 1410°C or lower.

[0004] PRIOR ART

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 07-257909 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] In the case of using the direct nitriding method with a continuous furnace, the reason why the raw material temperature is higher than the furnace temperature is that the exothermic reaction caused by nitriding proceeds vigorously. When such an exothermic reaction occurs, β-type silicon nitride is locally generated, resulting in a large fluctuation in the α-phase conversion rate. In order to suppress such an exothermic reaction, it is conceivable to slow down the temperature increase rate. However, if the temperature increase rate is slowed down, the production efficiency will decrease, and the advantage of using a continuous furnace cannot be sufficiently exerted.

[0009] Therefore, in the present disclosure, a manufacturing method of a silicon nitride pre-sintered body is provided, in which the fluctuation in the α-phase conversion rate is reduced while the production efficiency is improved, and the silicon nitride pre-sintered body has excellent crushability. In addition, a silicon nitride pre-sintered body is provided, in which the fluctuation in the α-phase conversion rate is sufficiently reduced, and the silicon nitride pre-sintered body has excellent crushability. Furthermore, by using such a silicon nitride pre-sintered body, a manufacturing method of a silicon nitride powder is provided, in which the fluctuation in the α-phase conversion rate is sufficiently reduced.

[0010] MEANS FOR SOLVING THE PROBLEMS

[0011] One embodiment of the present disclosure provides a method for producing a silicon nitride powder of [1] and [2] below.

[0012] [1] A method for producing a silicon nitride calcined body, comprising a step of heating a raw material powder containing a silicon metal powder in an atmosphere containing nitrogen using a continuous furnace, thereby obtaining a silicon nitride calcined body,

[0013] The average value of the temperature increase rate of the raw material powder in the range from when the temperature inside the furnace exceeds 1100°C to when it reaches 1200°C is set as RT m The average value of the temperature increase rate of the raw material powder in the range from when the temperature inside the furnace exceeds 1200°C to when it reaches the highest temperature inside the furnace is set as RT h When RT m <RT h .

[0014] When the raw material powder containing a silicon metal powder is heated in a continuous furnace, the exothermic reaction is most intense in the range from when the temperature inside the furnace exceeds 1100°C to when it reaches 1200°C. Therefore, if the temperature increase rate in this temperature range is accelerated, β-type silicon nitride is locally generated, resulting in a larger fluctuation in the α-phase conversion rate. Therefore, if the average value of the temperature increase rate of the raw material powder in the range from when the temperature inside the furnace exceeds 1100°C to when it reaches 1200°C is reduced to RT m , the local temperature increase due to the exothermic reaction can be suppressed, thereby suppressing the generation of β-type silicon nitride and excessive grain growth.

[0015] In the temperature range where the temperature inside the furnace exceeds 1200°C, the exothermic reaction is not as active as in the temperature range of 1100 to 1200°C. Therefore, by increasing RT h , the production efficiency can be improved using the advantage of the continuous furnace. That is, by the production method of [1] above, the production efficiency can be improved using the continuous furnace while sufficiently reducing the fluctuation in the α-phase conversion rate of the silicon nitride calcined body. In addition, since the generation of β-type silicon nitride and excessive grain growth can be suppressed, a silicon nitride calcined body having excellent crushability can be obtained.

[0016] [2] A method for producing a silicon nitride calcined body, comprising a step of heating a raw material powder containing a silicon metal powder in an atmosphere containing nitrogen using a continuous furnace, thereby obtaining a silicon nitride calcined body,

[0017] The average value of the temperature increase rate of the raw material powder in the range from when the temperature inside the furnace exceeds 1100°C to when it reaches 1200°C is set as RT m is 5 to 20°C / hour.

[0018] When the raw material powder containing the metal silicon powder is heated in the continuous furnace, the exothermic reaction is most intense in the range from when the temperature in the furnace exceeds 1100°C to when it reaches 1200°C. Therefore, if the heating rate in this temperature range is accelerated, the β-type silicon nitride is locally generated, resulting in a larger fluctuation in the α-phase conversion rate. Therefore, by controlling the average value RT m of the heating rate of the raw material powder in the range from when the temperature in the furnace exceeds 1100°C to when it reaches 1200°C to be less than 20°C / hour, the local temperature rise due to the exothermic reaction can be suppressed, thereby suppressing the generation of the β-type silicon nitride and the excessive grain growth.

[0019] By controlling the average value RT m of the heating rate to be more than 5°C / hour, the production efficiency can be improved using the advantage of the continuous furnace. That is, by the manufacturing method of the above [2], the production efficiency can be improved using the continuous furnace while sufficiently reducing the fluctuation in the α-phase conversion rate of the silicon nitride calcined body. In addition, since the generation of the β-type silicon nitride and the excessive grain growth can be suppressed, a silicon nitride calcined body having excellent pulverizability can be obtained.

[0020] The manufacturing methods of the above [1] and [2] can also be any one of the following [3] to [6].

[0021] [3] The manufacturing method of the silicon nitride calcined body according to [1] or [2], wherein the average value of the heating rate in the range from when the raw material powder is started to be heated in the continuous furnace to when the temperature in the furnace reaches 1100°C is set to RT l . l > RT m .

[0022] [4] The manufacturing method of the silicon nitride calcined body according to any one of [1] to [3], wherein the required time from when the raw material powder is started to be heated in the continuous furnace to when the temperature in the furnace reaches the highest temperature is less than 40 hours.

[0023] [5] The manufacturing method of the silicon nitride calcined body according to any one of [1] to [4], wherein in the continuous furnace, the raw material powder is heated,

[0024] in a single container or a plurality of containers filled with the raw material powder, respectively.

[0025] [6] The manufacturing method of the silicon nitride calcined body according to any one of [1] to [5], wherein the plurality of containers are stacked in two or more layers and introduced into the continuous furnace, and the raw material powder is heated.

[0026] From the range in which the raw material powder is started to be heated in the continuous furnace until the furnace temperature reaches 1100°C, an exothermic reaction due to a nitriding reaction of the metallic silicon component hardly proceeds. Therefore, as described in the above [3], by making the average value RT l greater than RT m The required time for manufacturing the silicon nitride pre-sintered body can be shortened, and thus the production efficiency can be further improved.

[0027] According to the manufacturing method of the above [4], the required time for manufacturing the silicon nitride pre-sintered body can be sufficiently shortened. Therefore, the production efficiency of the silicon nitride pre-sintered body can be further improved.

[0028] The direct nitriding method is a method in which the metallic silicon component reacts with nitrogen contained in a sintering atmosphere. In the manufacturing method of the above [5], since the raw material is in a powder form and the filling height of the raw material powder in the container is 40 mm or less, the nitriding reaction proceeds smoothly. Therefore, a silicon nitride pre-sintered body having a high α-phase conversion rate and in which the fluctuation of the α-phase conversion rate is sufficiently reduced can be obtained. In addition, when a plurality of containers are used, the production efficiency of the silicon nitride pre-sintered body can be further improved.

[0029] According to the manufacturing method of the above [6], the plurality of containers are stacked in two or more layers and introduced into the continuous furnace. Therefore, compared to the case where the containers are not stacked, the production efficiency can be further improved while the quality of the silicon nitride pre-sintered body is maintained.

[0030] One embodiment of the present disclosure provides the following [7] of the manufacturing method of the silicon nitride powder.

[0031] [7] A manufacturing method of a silicon nitride powder, comprising a step of pulverizing a silicon nitride pre-sintered body obtained by the manufacturing method of any one of the above [1] to [6].

[0032] In the manufacturing method of the silicon nitride powder of the above [7], there is a step of pulverizing a silicon nitride pre-sintered body in which the fluctuation of the α-phase conversion rate is sufficiently reduced. Therefore, a silicon nitride powder in which the fluctuation of the α-phase conversion rate is sufficiently reduced can be obtained. If such a silicon nitride powder is used for the preparation of a sintered body, a silicon nitride sintered body having a high degree of compositional uniformity can be obtained.

[0033] One embodiment of the present disclosure provides the following [8] of the silicon nitride pre-sintered body.

[0034] [8] A silicon nitride pre-sintered body, wherein the main component is silicon nitride, the average value of the α-phase conversion rate of the silicon nitride is 90% or more, and the maximum value of the thickness is less than 45 mm.

[0035] The silicon nitride pre-sintered body of the above [8] has not only a high average value of the α-phase conversion rate of silicon nitride, but also a small maximum value of thickness. Since the nitriding reaction of such a silicon nitride pre-sintered body is performed with sufficiently high uniformity, the fluctuation of the α-phase conversion rate is small. In addition, since the content of β-type silicon nitride is small, the silicon nitride pre-sintered body has excellent pulverizability. Therefore, the above silicon nitride pre-sintered body is suitable for use in the production of a silicon nitride powder.

[0036] The silicon nitride pre-sintered body of the above [8] can also be the following [9].

[0037] [9] The silicon nitride pre-sintered body according to [8], wherein the standard deviation of the α-phase conversion rate is 1.0% or less.

[0038] The fluctuation of the α-phase conversion rate of the silicon nitride pre-sintered body of the above [9] is further reduced. If such a silicon nitride pre-sintered body is used, a silicon nitride powder in which the fluctuation of the α-phase conversion rate is further reduced can be obtained.

[0039] One embodiment of the present disclosure provides a production method of a silicon nitride powder of the following

[10] .

[0040]

[10] A production method of a silicon nitride powder, comprising a step of pulverizing the silicon nitride pre-sintered body of the above [8] or [9].

[0041] In the production method of the above

[10] , since the above silicon nitride pre-sintered body is used, a silicon nitride powder in which the fluctuation of the α-phase conversion rate is sufficiently reduced can be smoothly obtained.

[0042] Effects of the Invention

[0043] A production method of a silicon nitride pre-sintered body in which the fluctuation of the α-phase conversion rate is reduced and excellent pulverizability is achieved while improving the production efficiency using a continuous furnace can be provided. In addition, a silicon nitride pre-sintered body in which the fluctuation of the α-phase conversion rate is sufficiently reduced and excellent pulverizability is achieved can also be provided. In addition, by using such a silicon nitride pre-sintered body, a production method of a silicon nitride powder in which the fluctuation of the α-phase conversion rate is sufficiently reduced can also be provided. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 is a perspective view showing a silicon nitride pre-sintered body.

[0045] Figure 2 is an example showing changes over time in the temperature of a continuous furnace (furnace temperature) and the temperature of a raw material powder when the raw material powder is heated in the continuous furnace.

[0046] Figure 3is an example of a temperature change curve of a raw material powder (vessel) in a continuous furnace.

[0047] Figure 4 is an oblique view showing a state in which the vessel 3 having a housing member for housing a raw material powder is stacked in three layers.

[0048] Figure 5 is a cross-sectional view showing a state in which the vessel housing a raw material powder is cut in the height direction after being stacked in three layers. DETAILED DESCRIPTION

[0049] Hereinafter, several embodiments of the present disclosure will be described. However, the following embodiments are examples to illustrate the present disclosure, and are not intended to limit the present disclosure to the following. In addition, in the present embodiment, a numerical range exemplified in the form of "a ~ b" means a numerical range in which the lower limit is a and the upper limit is b, and includes the numerical range of a and b. In the case where the upper limit or the lower limit of each numerical range is replaced with a numerical value in any one of the embodiments, or with the upper limit or the lower limit of another numerical range, it is also included in the present disclosure.

[0050] The silicon nitride preform according to one embodiment contains silicon nitride as a main component (silicon nitride component). The content of silicon nitride in the silicon nitride preform can be 90% by mass or more, 95% by mass or more, 98% by mass or more, or 99% by mass or more. The content of silicon nitride in the silicon nitride preform can be measured, for example, by a commercially available X-ray diffraction device.

[0051] The silicon nitride preform according to one embodiment can contain a metal component of Fe, Cr, Ni, or the like, or a metal compound containing these components as constituent elements, as an auxiliary component. However, a sintering aid (oxide-based sintering aid) can not be contained. Thereby, the pulverizability can be further improved. For example, the total content of Y2O3, MgO, and Al2O3, which are known oxide-based sintering aids, can be 0.1% by mass or less.

[0052] The average of the α-phase conversion rate of silicon nitride contained in the silicon nitride preform is 90% or more. Such a silicon nitride preform has excellent pulverizability because the α-phase conversion rate is sufficiently high compared to a general silicon nitride preform. From the viewpoint of further improving the pulverizability, the average of the α-phase conversion rate of silicon nitride can be 91% or more, 92% or more, or 93% or more. The average of the α-phase conversion rate is calculated by arbitrarily selecting five or more positions in the silicon nitride preform and measuring the α-phase conversion rate. By performing arithmetic averaging of the five or more measurement values thus measured, the average of the α-phase conversion rate can be calculated.

[0053] The standard deviation of the α-phase conversion rate of the silicon nitride contained in the silicon nitride calcined body can be 1.0% or less, 0.9% or less, 0.8% or less, or 0.7% or less. In such a silicon nitride calcined body, the fluctuation of the α-phase conversion rate of the silicon nitride is further reduced. The standard deviation of the α-phase conversion rate can be determined by using the five or more measured values ​​used to calculate the average value of the α-phase conversion rate. From the perspective of ease of manufacture of the silicon nitride calcined body, the standard deviation of the α-phase conversion rate can be 0.2% or more or 0.3% or more. An example range of the standard deviation of the α-phase conversion rate is 0.1 to 1.0%.

[0054] For example, if the silicon nitride calcined body 30 is Figure 1 If the silicon nitride calcined body is in the shape of a flat plate (a rectangular prism), the α-phase conversion rate is measured at five locations on one side, including the four corners E and the center C. In this way, the reproducibility of the average value and standard deviation of the α-phase conversion rate can be sufficiently high. If the silicon nitride calcined body is in the shape of a circular plate, the α-phase conversion rate can be measured at four or more locations arranged at equal intervals in the center and along the periphery. Thus, the measurement position of the α-phase conversion rate can be arbitrarily selected according to the shape of the silicon nitride calcined body. The α-phase conversion rate can be measured by the method described in the examples.

[0055] The bulk density of the silicon nitride calcined body can be 1.4 to 2.0 g / cm 3 Such a silicon nitride calcined body can have both sufficiently high purity and sufficiently excellent crushing properties. From the perspective of further improving the purity of silicon nitride, the lower limit of the bulk density can be 1.5 g / cm 3 or 1.6 g / cm 3 From the perspective of further improving the crushing properties of the silicon nitride calcined body, the upper limit of the bulk density may be 1.9 g / cm 3 or 1.8g / cm 3 .

[0056] The thickness of the silicon nitride calcined body can be less than 45 mm, less than 40 mm, less than 35 mm, or less than 30 mm. By reducing the maximum thickness of the silicon nitride calcined body, crushability is improved. Furthermore, the smaller the thickness of the silicon nitride calcined body, the easier it is for the nitridation reaction to proceed with sufficiently high uniformity. Therefore, the smaller the thickness, the smaller the fluctuation in the α-phase conversion rate. To improve production efficiency, the thickness of the silicon nitride calcined body can be 5 mm or greater, 10 mm or greater, or 15 mm or greater.

[0057] In the manufacturing method of the silicon nitride powder, the silicon nitride powder can be obtained by a pulverization process of the above-described silicon nitride calcined body. The pulverization can be performed using a coarse pulverizer, a wet-type pulverizer, a ball mill, a vibration mill, or the like. Since the silicon nitride, which is the main component in the silicon nitride calcined body, has a high α-phase conversion rate and the β-phase is reduced, the pulverization can be smoothly performed. Thus, the silicon nitride powder, in which the fluctuation of the α-phase conversion rate is further reduced, can be smoothly obtained. By using such a silicon nitride powder in which the fluctuation of the α-phase conversion rate is reduced, a silicon nitride sintered body having high compositional uniformity and excellent reliability can be obtained. The use of the silicon nitride powder is not limited to the use for manufacturing the silicon nitride sintered body, and the silicon nitride powder can be mixed with other kinds of powder (for example, ceramic powder such as boron nitride) to manufacture a composite.

[0058] The silicon nitride powder (Si3N4 powder) thus obtained contains, for example, silicon nitride having an α-phase conversion rate of 90.0% or more. The content of the silicon nitride (purity) in the silicon nitride powder can be 90% by mass or more, 95% by mass or more, 98% by mass or more, or 99% by mass or more. The content of the silicon nitride in the silicon nitride powder can be measured, for example, by a commercially available X-ray diffractometer.

[0059] When the value of the cumulative 50% in the particle size distribution curve on a volume basis is taken as D50 and the value of the cumulative 90% is taken as D90, the D50 of the silicon nitride powder can be 0.5 to 2.0 μm. Such a silicon nitride powder has sufficiently excellent sinterability, and abnormal grain growth at the time of sintering can be sufficiently suppressed. The upper limit of the D50 of the silicon nitride powder can be 1.6 μm, 1.4 μm, 1.2 μm, or 1.0 μm. Thus, the abnormal grain growth at the time of sintering can be further suppressed. The lower limit of the D50 of the silicon nitride powder can be 0.6 μm or 0.7 μm. Thus, the sinterability can be further improved.

[0060] The D90 of the silicon nitride powder can be 1.0 to 3.0 μm. Such a silicon nitride powder has sufficiently excellent sinterability, and abnormal grain growth at the time of sintering can be sufficiently suppressed. The upper limit of the D90 of the silicon nitride powder can be 2.8 μm, 2.6 μm, or 2.4 μm. Thus, the abnormal grain growth at the time of sintering can be further suppressed. In addition, from the viewpoint of improving productivity, the lower limit of the D90 of the silicon nitride powder can be 1.2 μm, 1.4 μm, or 1.6 μm.

[0061] The D50 and D90 of each powder in the present specification are obtained according to the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction / Scattering Method". In the particle size distribution (cumulative distribution) determined according to the above method, with the horizontal axis as the logarithmic scale of the particle size [μm] and the vertical axis as the frequency [vol%], the particle size at which the cumulative value from the small particle size reaches 50% of the total is defined as the D50 (average particle size, or median particle size), and the particle size at which the cumulative value reaches 90% of the total is defined as the D90. The measuring device can use the device described in the examples. The D50 and D90 of the silicon nitride powder can be adjusted by changing the particle size of the raw material powder, the temperature increase rate when the silicon nitride powder is manufactured, the sintering temperature, the sintering time, and the conditions when the silicon nitride pre-sintered body is pulverized.

[0062] The BET specific surface area of the silicon nitride powder can be 5 to 15 m 2 / g. Such a silicon nitride powder has sufficiently excellent sinterability, and can sufficiently suppress abnormal grain growth. The upper limit of the BET specific surface area of the silicon nitride powder can be 14 m 2 / g or 13 m 2 / g. Thereby, abnormal grain growth at the time of sintering can be further suppressed. The lower limit of the BET specific surface area of the silicon nitride powder can be 6 m 2 / g or 7 m 2 / g. Thereby, the sinterability can be further improved.

[0063] The BET specific surface area of each powder in the present specification is a value determined by the BET single-point method using nitrogen gas according to the method described in JIS Z 8830:2013 "Method for Determining Specific Surface Area of Powder (Solid) by Gas Adsorption". The BET specific surface area of the silicon nitride powder can be adjusted by changing the particle size of the raw material powder, the temperature increase rate when the silicon nitride powder is manufactured, the sintering temperature, the sintering time, and the conditions when the silicon nitride pre-sintered body is pulverized.

[0064] The manufacturing method of the silicon nitride pre-sintered body according to one embodiment has a step of heating a raw material powder containing a silicon metal powder in an atmosphere containing nitrogen using a continuous furnace having a temperature gradient, thereby obtaining a silicon nitride pre-sintered body. By this manufacturing method, the above-described silicon nitride pre-sintered body can be manufactured. Therefore, the description of the silicon nitride pre-sintered body also applies to this manufacturing method.

[0065] As the metal silicon powder, a powder obtained by pulverizing metal silicon particles (or metal silicon blocks) can be used. As the pulverizing device, a hammer mill, a pin mill, a ball mill, a vibration mill, a bead mill, a jet mill, or the like can be used. The average particle diameter (D50, median particle diameter) of the metal silicon powder can be 15 to 30 μm. From the viewpoint of smoothly performing nitriding, the average particle diameter of the metal silicon powder can be 28 μm or less, 26 μm or less, or 24 μm or less. When the raw material is subjected to nitriding in a powder state, excessive heat release can sometimes occur. From the viewpoint of suppressing such heat release, the average particle diameter of the metal silicon powder can be 16 μm or more, or 18 μm or more. The average particle diameter of the metal silicon powder can be measured using the same method as that for the average particle diameter of the silicon nitride powder.

[0066] The purity of the metal silicon powder can be 98% by mass or more, or 99% by mass or more. The metal silicon powder can contain impurities other than those derived from the metal silicon particles, in addition to impurities mixed in when pulverization is performed using a pulverizing device.

[0067] The metal silicon powder can be directly used as the raw material powder, or the metal silicon powder and other components can be compounded to prepare the raw material powder. The content of the metal silicon component in the raw material powder can be 95% by mass or more, 97% by mass or more, or 98% by mass or more. The content of the metal silicon component in the raw material powder can be measured, for example, by a commercially available X-ray fluorescence analysis device.

[0068] In order to promote nitriding of the metal silicon, the raw material powder can contain fluorite. The content of fluorite can be 0.2 to 3 parts by mass with respect to 100 parts by mass of the raw material powder. From the viewpoint of sufficiently promoting nitriding of the metal silicon, the content of fluorite can be 0.5 parts by mass or more, or 0.8 parts by mass or more with respect to 100 parts by mass of the metal silicon powder. From the viewpoint of reducing the contents of Ca and F in the obtained silicon nitride powder, the content of fluorite can be 2 parts by mass or less, or 1.5 parts by mass or more with respect to 100 parts by mass of the metal silicon powder.

[0069] The raw material powder can contain components other than the metal silicon component and fluorite. Such a raw material powder can be prepared at low cost, and thus the manufacturing cost of the silicon nitride and the silicon nitride powder can be reduced.

[0070] The raw material powder is heated using a continuous furnace to obtain a silicon nitride pre-sintered body. As the continuous furnace, a container conveying type continuous furnace, such as a tunnel type pusher furnace or a roller hearth kiln, or the like can be exemplified. Such a continuous furnace can have a temperature gradient in the furnace. That is, it can have one heating zone, the temperature of which gradually increases from the furnace inlet to the furnace outlet. The raw material powder (container) introduced into the heating zone of such a continuous furnace is gradually heated while moving in the continuous furnace.

[0071] Figure 2 is an example of the change in the internal temperature of the continuous furnace (furnace internal temperature) and the temperature of the raw material powder in the container over time when the raw material powder (container) is heated in the continuous furnace. In Figure 2 , the solid line 1 is the temperature of the raw material powder, and the dashed line 2 is the internal temperature (furnace internal temperature). As Figure 2 indicated, in a region where the furnace internal temperature is 1100°C to 1200°C, the nitriding reaction becomes active, and the raw material powder significantly releases heat.

[0072] Therefore, in the continuous furnace, it is preferable to heat the raw material powder (container) under conditions that satisfy RT m < RT h . RT m is the average of the rate of temperature increase of the raw material powder (container) in the range from a position exceeding 1100°C (furnace internal temperature) to a position reaching 1200°C (furnace internal temperature). RT h is the average of the rate of temperature increase of the raw material powder (container) in the range from a position exceeding 1200°C (furnace internal temperature) to a position reaching the maximum temperature (furnace internal temperature). RT m may be, for example, 5 to 20°C / hour. By heating under such conditions, excessive heat release of the raw material powder in the temperature region where the heat release reaction is most active can be suppressed. Therefore, not only can the generation of β-type silicon nitride be suppressed, but also excessive grain growth of the silicon nitride particles can be suppressed. As a result, fluctuations in the α-phase conversion rate are sufficiently reduced, and the pulverizability can also be improved. In addition, by taking advantage of the continuous furnace, the production efficiency can be improved.

[0073] RT m may be 7°C / hour or more, or 9°C / hour or more. Thereby, the required time for heating the raw material powder can be shortened, and the production efficiency can be further improved. From the same viewpoint, RT h may be 10°C / hour or more, 13°C / hour or more, 14°C / hour or more, or 18°C / hour or more.

[0074] RT m may be 19°C / hour or less, 17°C / hour or less, 12°C / hour or less, or 10°C / hour or less. Thereby, fluctuations in the α-phase conversion rate can be further reduced, and the pulverizability of the silicon nitride calcined body can be further improved. In the temperature region exceeding 1200°C, although the heat release reaction due to the nitriding reaction is not as active as in the temperature region of 1100 to 1200°C, if RT h is too large, the amount of generation of β-type silicon nitride (β-phase) can increase due to the heat release reaction, and excessive grain growth of the silicon nitride particles can also occur. Therefore, RT hRTmay be 50°C / hour or less, 45°C / hour or less, 40°C / hour or less, or 35°C / hour or less. RT h One example of the temperature range is 10 to 50°C / hour.

[0075] The maximum temperature of the furnace temperature is, for example, 1300 to 1500°C. Thus, the metal silicon contained in the raw material powder can be sufficiently nitrided. From the viewpoint of sufficiently performing the nitriding reaction, the holding time of the maximum temperature of the furnace can be 0.1 hour or more, 0.5 hour or more, or 1 hour or more. From the viewpoint of improving the productivity of the silicon nitride pre-sintered body, the holding time of the maximum temperature of the furnace can be 5 hours or less, 4 hours or less, 3 hours or less, or 2 hours or less. One example of the holding time of the maximum temperature of the furnace is 0.1 to 5 hours.

[0076] In the continuous furnace, the average value of the temperature rising rate in the range from the start of heating the raw material powder to the position at which the furnace temperature reaches 1100°C is set as RT l When RT l > RT m , for example, RT l may be 40°C / hour or more, 60°C / hour or more, or 80°C / hour or more. Thus, the required time for manufacturing the silicon nitride pre-sintered body can be shortened, and the production efficiency can be further improved. From the viewpoint of the facility limitation of the continuous furnace, RT l may be 300°C / hour or less, or 250°C / hour or less. RT l One example of the temperature range of RT l > RT h . Thus, the production efficiency and the crushability of the silicon nitride pre-sintered body can be sufficiently improved, and the fluctuation of the α-phase conversion rate of the silicon nitride pre-sintered body can be further reduced.

[0077] Figure 3 is one example showing the relationship between the heating time of the raw material powder (container) and the furnace temperature. The horizontal axis shows the time elapsed from the start of heating the raw material powder (container) (heating time), and the vertical axis shows the furnace temperature at the position at which the raw material powder (container) has passed in the heating time. In the example shown in FIG. 1, the raw material powder (container) is heated at a constant rate of 50°C / hour until the furnace temperature reaches 1100°C. Figure 3The temperature change curve shown indicates that, from the start of heating (20°C) to the point at which the temperature in the furnace reaches 1100°C, 11 hours are required (region I), from the point at which the temperature in the furnace exceeds 1100°C to the point at which it reaches 1200°C, 10 hours are required (region II), and from the point at which the temperature in the furnace exceeds 1200°C to the point at which the temperature in the furnace reaches the maximum temperature (1400°C), 7 hours are required (region III). In this case, RT l , RT m , and RT h are approximately 98°C / hour, 10°C / hour, and approximately 29°C / hour, respectively. However, the temperature increase rate can not be linear as shown in Figure 3 , for example, it can be a stepwise or curved temperature increase pattern. There can be a time zone in which the temperature is held constant.

[0078] RT l , RT m , and RT h may be in the order of RT m < RT h < RT l as shown in Figure 3 . As a result, fluctuations in the α-phase conversion rate can be sufficiently reduced, and silicon nitride pre-sintered bodies having excellent crushability can be produced with sufficiently high productivity. From the viewpoint of further balancing these effects, RT h / RT m may be 1.1 to 4.0. The lower limit of RT h / RT m may be 1.2, 1.5, 1.8, or 2.0. The upper limit of RT h / RT m may be 3.5 or 3.0. From the same viewpoint, RT l / RT h may be 2.0 to 10.0. The lower limit of RT l / RT h may be 2.5, 3.0, 3.5, or 4.0. The upper limit of RT l / RT h may be 8.0, 7.0, or 5.5.

[0079] The temperature increase rate in the present disclosure is calculated from the temperature in the continuous furnace (furnace temperature) at the position of the raw material powder (container). The temperature increase rate (as well as RT l , RT m , and RT h) can be adjusted by changing the temperature gradient in the continuous furnace. In addition, the temperature increase rate can also be adjusted by changing the moving speed of the container containing the raw material powder in the continuous furnace. The temperature in the furnace can be measured using a thermocouple provided in the furnace.

[0080] The time required from the start of heating the raw material powder (container) in the continuous furnace until the temperature in the furnace reaches the highest temperature can be less than 40 hours, less than 35 hours, or less than 32 hours. Thus, the production efficiency of the silicon nitride pre-sintered body can be further improved by taking advantage of the continuous furnace. From the viewpoint of allowing the nitriding reaction to proceed sufficiently, the time required can be 21 hours or more, or 25 hours or more.

[0081] The container for filling the raw material powder can use a container made of a material that does not deteriorate even at a temperature of about 1500°C in an inert atmosphere. The container can use, for example, a container made of carbon, a container made of alumina, or a container made of boron nitride. The structure of the container is not particularly limited, and for example, a container having a containing member capable of containing the raw material powder can be used. From the viewpoint of facilitating the operation, the container can have a container main body having a recess, and a lid covering the recess of the container main body. In addition, a plurality of containers can be used in a stacked state, and the upper container can be used as the lid of the lower container.

[0082] In Figure 4 , a state in which the containers 10 are stacked in three layers is shown, wherein the containers 10 have containing members 20 for containing the raw material powder. In Figure 4 , the containing member 20 is not shown. As Figure 5 indicated, the containing member 20 of each container 10 is filled with the raw material powder 22 before being introduced into the continuous furnace. Then, the containers 10 are stacked as shown in Figure 4 and Figure 5 , and are introduced into the continuous furnace. As Figure 4 indicated, by introducing the stacked containers 10 side by side into the continuous furnace, the silicon nitride pre-sintered body can be efficiently produced. In the lower and middle containers 10, the air vents 12 are provided. Thus, the metal silicon contained in the raw material powder 22 can sufficiently contact with the nitrogen contained in the atmosphere.

[0083] The filling height H of the raw material powder 22 in each container 10 can be 40 mm or less, 35 mm or less, 30 mm or less, 25 mm or less, or 20 mm or less. By thus reducing the filling height H, the nitriding reaction proceeds smoothly. Therefore, a silicon nitride pre-sintered body having a high α-phase conversion rate and in which the fluctuation in the α-phase conversion rate is sufficiently reduced can be obtained. From the viewpoint of improving the production efficiency, the filling height H can be 5 mm or more, 10 mm or more, or 15 mm or more. When there are irregularities on the surface of the raw material powder 22 filled in the container 10, the maximum value of the filling height of the raw material powder 22 is taken as the filling height H.

[0084] The atmosphere at the time of firing can contain nitrogen and hydrogen. From the viewpoint of promoting the nitriding of metallic silicon, the proportion of nitrogen in the firing atmosphere can be 95.1% by volume or more, or 96.0% by volume or more. From the viewpoint of reducing the oxides such as SiO2 contained in the raw material powder and promoting the nitriding, the proportion of hydrogen in the firing atmosphere can be 2.0% by volume or more, 3.0% by volume or more, or 4.0% by volume or more. One example of the proportion of nitrogen in the firing atmosphere is 95.1 to 98.0% by volume. One example of the proportion of hydrogen in the firing atmosphere is 2.0 to 4.9% by volume. The atmosphere at the time of firing can contain other gases in addition to nitrogen and hydrogen. As the other gas, argon can be cited. The proportions (by volume) of the respective gases in this specification are values under standard conditions (0°C, 1 atm).

[0085] The continuous furnace can be provided with a heating zone for heating the raw material powder (container) and a cooling zone for cooling the product (silicon nitride pre-sintered body) produced by firing. The raw material powder (container) is moved in the heating zone, and after reaching a position in the furnace where the temperature reaches the maximum temperature, can be moved to the cooling zone. The cooling speed is not particularly limited, and for example, in the above atmosphere, cooling can be performed at a temperature decrease rate of 5 to 200°C / hour. When the temperature reaches about 300°C or less, cooling can be performed in air. Thus, a silicon nitride pre-sintered body can be obtained. In this production method, since the raw material powder does not need to be shaped to be fired, the nitriding reaction can be efficiently performed. Thus, a silicon nitride pre-sintered body can be obtained in a short time. The silicon nitride contained in such a silicon nitride pre-sintered body has a high α-phase conversion rate and has excellent crushability. The average value and the standard deviation of the α-phase conversion rate are as described above.

[0086] If a pulverization process is performed on the silicon nitride pre-sintered body obtained by the above firing, a silicon nitride powder can be obtained. The composition, properties, and particle diameter of the silicon nitride powder are as described above. The pulverization can be performed using, for example, a coarse pulverizer, a wet-type grinder, a ball mill, a vibration mill, or the like. Since the silicon nitride, which is the main component in the silicon nitride pre-sintered body, has a high α-phase conversion rate, and the generation of β-phase is suppressed, the silicon nitride pre-sintered body can be easily pulverized.

[0087] After the pulverization, a treatment process can be performed as needed. For example, the silicon nitride powder obtained by the pulverization can be combined with hydrofluoric acid having a hydrogen fluoride concentration of 10 to 40% by mass to reduce impurities. The silicon nitride powder can be treated by being dispersed in the hydrofluoric acid, for example. The hydrogen fluoride concentration in the hydrofluoric acid can be 15 to 30% by mass. The temperature of the hydrofluoric acid in the treatment process is, for example, 40 to 80°C. In addition, the time for which the silicon nitride powder is immersed in the hydrofluoric acid is, for example, 1 to 10 hours.

[0088] The silicon nitride powder can be used for a silicon nitride sintered body. By using a silicon nitride powder having a small fluctuation in α-phase conversion rate as a raw material, the quality fluctuation of the silicon nitride sintered body can be sufficiently reduced. In addition to the silicon nitride powder, a sintering aid can be used as a raw material. As the sintering aid, for example, an oxide-based sintering aid such as Y2O3, MgO, and Al2O3 can be cited. The content of the component derived from the sintering aid in the silicon nitride sintered body is, for example, 3 to 10% by mass.

[0089] The above describes several embodiments, but the present disclosure is not limited to the above-described embodiments. For example, in Figure 4 and Figure 5 In the above, an example in which the containers are stacked in three layers is shown, but the number of layers is not particularly limited. When a plurality of containers are arranged together for firing, the number of columns is also not particularly limited. In addition, the shape of the container is also not particularly limited. The uppermost container can be covered with a lid.

[0090] Example

[0091] Next, the content of the present disclosure is described in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0092] (Example 1)

[0093] Preparation and Evaluation of Metal Silicon Powder

[0094] A metal silicon block having a particle size of 10 to 50 mm was prepared. The metal silicon block was coarsely pulverized using a pulverizing device (device name: jaw crusher, Kabushiki Kaisha Makino Seizo) and then finely pulverized using a pulverizing device (device name: vibration mill, manufactured by Chuo Kogyo Machine Co., Ltd.), thereby obtaining a metal silicon powder having a particle size of 20.3 μm. In the metal silicon powder, fluorite, chromium oxide powder (Cr203), and nickel oxide powder (NiO) were blended and mixed, thereby preparing a raw material powder. The fluorite was blended at 1 mass% based on the metal silicon powder, the chromium oxide powder was blended at 200 mass ppm, and the nickel oxide content was 60 mass ppm.

[0095] <Preparation of silicon nitride powder>

[0096] A plurality of graphite containers as shown in Figure 4 and Figure 5 were prepared. The size of the container was 300 mm x 300 mm x 70 mm in length x width x height. The raw material powder was filled in each of the grooves of the plurality of containers. The raw material powder was filled in each of the containers at 1.4 kg. The filling height H of the raw material powder in the grooves was 30 mm. The containers filled with the raw material powder were stacked in three layers and introduced into a container conveying type continuous furnace. The inlet temperature of the continuous furnace (temperature at the start of heating) was 20°C.

[0097] While supplying a mixed gas containing nitrogen and hydrogen (nitrogen: 96.0 vol%, hydrogen: 4.0 vol%) to the continuous furnace, the containers were introduced into the heating zone of the continuous furnace. The time required for moving from the inlet (20°C) of the heating zone of the continuous furnace to a position where the temperature inside the furnace was 1100°C was 10.8 hours. Therefore, the average value RT l of the temperature rising rate in the range from the start of heating the raw material powder in the continuous furnace to the temperature inside the furnace reaching 1100°C was 100°C / hour.

[0098] The time required for moving from a position where the temperature inside the furnace exceeded 1100°C to a position of 1200°C was 6.1 hours. Therefore, the average value RT m of the temperature rising rate in the range from the temperature inside the furnace exceeding 1100°C to reaching 1200°C was 16.3°C / hour. The time required for moving from a position where the temperature inside the furnace exceeded 1200°C to a position of 1400°C (the maximum temperature) was 9.9 hours. Therefore, the average value RT h of the temperature rising rate in the range from the temperature inside the furnace exceeding 1200°C to reaching a position of 1400°C (the maximum temperature) was 20.3°C / hour. In addition, the time required for the containers to move from the inlet of the continuous furnace to a position of 1400°C (the maximum temperature) was 26.8 hours.

[0099] The container was immediately moved to the cooling zone of the continuous furnace after reaching the position of 1400°C (maximum temperature) and cooling was started in the mixed gas atmosphere described above. After cooling to 200°C over about 12 hours, the plate-like silicon nitride preform whose main component was silicon nitride was taken out of the container while being naturally cooled in air. The silicon nitride preform had a shape as shown in Figure 1 .

[0100] < Evaluation of the Silicon Nitride Preshape >

[0101] The thickness of the plate-like preform was measured using a vernier caliper. The maximum value of the thickness is shown in Table 1. Then, the silicon nitride preform was divided into five parts including four corners on one side and a center part. At this time, the division was performed in such a manner as to ensure that the volumes of the five preforms were the same. Each preform was pulverized using a pulverizing device (device name: jaw crusher, Kabushiki Kaisha Makiyama Seisakusho) and a pulverizing device (device name: roll crusher, Kabushiki Kaisha Makiyama Seisakusho), and then further pulverized using a wet-type pulverizing device (device name: wet-type mill, Nippon Coke & Chemicals Co., Ltd.). In the wet-type pulverization, water was used as a solvent and the pulverization time was set to 10 hours. The above pulverized product obtained by the wet-type pulverization was immersed in hydrofluoric acid (hydrofluoric acid concentration: 30 mass%) at 70°C for 2 hours to perform acid treatment. Then, the pulverized product was taken out of the hydrofluoric acid, washed with water, and dried under a nitrogen atmosphere. In this way, a silicon nitride powder was obtained.

[0102] The α-phase conversion rate of the silicon nitride powder was measured according to the following procedure. The silicon nitride powder obtained from each of the five divided preforms was subjected to X-ray diffraction measurement using an X-ray diffraction device (manufactured by Rigaku Corporation, device name: Ultima IV) with CuKα rays. The α-phase was represented by the diffraction line intensity I a102 of the (102) plane and the diffraction line intensity I a210 of the (210) plane. The β-phase was represented by the diffraction line intensity I b101 of the (101) plane and the diffraction line intensity I b210 of the (210) plane. Using these diffraction line intensities, the α-phase conversion rate was calculated by the following equation. The average value and the standard deviation of the α-phase conversion rates of the five positions are shown in Table 1.

[0103] α-phase conversion rate (%) =

[0104] (I a102 + I a210 ) / (I a102 + I a210 + I b101 + I b210 ) x 100

[0105] The pulverizability of the silicon nitride calcined body was evaluated from the average value of D90 of the silicon nitride powder obtained from each of the five divided calcined bodies. The particle size distribution of the silicon nitride powder was measured by a laser diffraction / scattering method. Specifically, the measurement was performed in accordance with the method described in JIS Z 8825:2013 "Particle Size Analysis - Laser Diffraction / Scattering Method". For the measurement of the particle size distribution, 60 mg of the silicon nitride powder was weighed into a 500 mL container. To this, 20% of a sodium hexametaphosphate aqueous solution (2 mL) and water (200 g) were added as dispersants. The container was placed in an ultrasonic disperser manufactured by Sharp Corporation so that the containing portion of the dispersion liquid was completely immersed, and ultrasonic dispersion was performed for 1 minute. The sample after the ultrasonic dispersion was used to measure the particle size distribution described above. In the measurement of the particle size distribution, LS-13 320 (device name: manufactured by Beckman Coulter Inc.) was used. The value of the cumulative 90% in the volume-based particle size distribution curve obtained was taken as D90. The pulverizability of the silicon nitride calcined body was evaluated by the following criteria. The results are shown in Table 1.

[0106] A: The average value of D90 is 3.0 μm or less

[0107] B: The average value of D90 is more than 3.0 μm

[0108] (Examples 2 to 5)

[0109] In addition to adjusting the temperature gradient in the continuous furnace and changing at least one of the average value RT l , RT m , and RT h as shown in Table 1, heating and cooling were performed by the same steps as in Example 1 to obtain a plate-shaped silicon nitride calcined body. Then, the silicon nitride calcined body obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 1.

[0110] (Examples 6 and 7)

[0111] In addition to changing the ratio of nitrogen and hydrogen in the mixed gas supplied to the continuous furnace and the average value RT m of the temperature rise rate as shown in Table 2, heating and cooling were performed by the same steps as in Example 1 to obtain a plate-shaped silicon nitride calcined body. In addition, the component other than hydrogen in the mixed gas was nitrogen. Then, the silicon nitride calcined body obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 2.

[0112] (Examples 8 and 9)

[0113] The maximum temperature of the continuous furnace was changed as shown in Table 2. In addition, the required time of the container from the inlet of the continuous furnace to the position where the maximum temperature was reached is shown in Table 2. Except for the firing conditions, heating and cooling were performed under the same conditions as in Example 1 to obtain a plate-shaped silicon nitride preform. Then, the silicon nitride preform obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 2. m and the required time of the container from the inlet of the continuous furnace to the position where the maximum temperature was reached is shown in Table 2. Except for the firing conditions, heating and cooling were performed under the same conditions as in Example 1 to obtain a plate-shaped silicon nitride preform. Then, the silicon nitride preform obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 2.

[0114] [Example 10, Comparative Example 1]

[0115] Except that the moving speed of the container moving in the continuous furnace was adjusted, and at least one of the average value of the temperature increasing rate RT l , RT m , and RT h was changed as shown in Table 3, heating and cooling were performed under the same steps as in Example 1 to obtain a plate-shaped silicon nitride preform. Then, the silicon nitride preform obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 3.

[0116] (Examples 11, 12)

[0117] Except that the filling height H of the raw material powder in the groove of the container and the average value of the temperature increasing rate RT m were changed as shown in Table 3, heating and cooling were performed under the same steps as in Example 1 to obtain a plate-shaped silicon nitride preform. Then, the silicon nitride preform obtained by the same steps as in Example 1 was evaluated. The evaluation results are shown in Table 3.

[0118] [Table 1]

[0119]

[0120] [Table 2]

[0121]

[0122] [Table 3]

[0123]

[0124] The silicon nitride preforms of Examples 1 to 12 had sufficiently high average values of the α-phase conversion rate, and the fluctuation of the α-phase conversion rate was also sufficiently small. Furthermore, they also had excellent crushability. In contrast, the silicon nitride preform of Comparative Example 1 had a low average value of the α-phase conversion rate, and the fluctuation of the α-phase conversion rate was also large. Furthermore, it also had poor crushability. This is believed to be due to excessive exothermic reaction during the temperature increasing process, resulting in the generation of a large amount of β-type silicon nitride.

[0125] Industrial applicability

[0126] A manufacturing method of a silicon nitride pre-sintered body in which production efficiency is improved by using a continuous furnace, fluctuation in α-phase conversion rate is reduced, and excellent crushability is provided can be provided. In addition, a silicon nitride pre-sintered body in which fluctuation in α-phase conversion rate is sufficiently reduced and excellent crushability is provided can be provided. In addition, by using such a silicon nitride pre-sintered body, a manufacturing method of a silicon nitride powder in which fluctuation in α-phase conversion rate is sufficiently reduced can be provided.

[0127] Explanation of symbols

[0128] 10 container

[0129] 12 vent

[0130] 20 housing member

[0131] 22 raw material powder

[0132] 30 silicon nitride pre-sintered body

Claims

1. A method for producing a silicon nitride pre-sintered body, comprising a step of heating a raw material powder containing a metal silicon powder in an atmosphere containing nitrogen using a continuous furnace to obtain a silicon nitride pre-sintered body. RT is set to an average value of the temperature increase rate of the raw material powder in a range from when the temperature in the furnace exceeds 1100°C to when it reaches 1200°C m RT is set to an average value of the temperature increase rate of the raw material powder in a range from when the temperature in the furnace exceeds 1200°C to when it reaches the maximum temperature h RT is set to an average value of the temperature increase rate of the raw material powder in a range from when the temperature in the furnace exceeds 1100°C to when it reaches 1200°C m RT is set to an average value of the temperature increase rate of the raw material powder in a range from when the temperature in the furnace exceeds 1100°C to when it reaches 1200°C h .

2. A method for producing a silicon nitride pre-sintered body, comprising a step of heating a raw material powder containing a metal silicon powder in an atmosphere containing nitrogen using a continuous furnace to obtain a silicon nitride pre-sintered body. average value RT of the temperature rising rate of the raw material powder in the range from over 1100°C to up to 1200°C m 5 to 20°C / hour.

3. The method of producing a silicon nitride calcination body according to claim 1 or 2, wherein The average value of the temperature increase rate in the range from the start of heating the raw material powder to the time when the in-furnace temperature reaches 1100°C in the continuous furnace is set as RT l RT l > RT m .

4. The method of producing a silicon nitride calcination body according to claim 1 or 2, wherein The required time from the start of heating the raw material powder to the point at which the temperature in the furnace reaches the maximum temperature in the continuous furnace is less than 40 hours.

5. The method of producing a silicon nitride calcination body according to claim 1 or 2, wherein In the continuous furnace, the raw material powder is heated while being filled in a single container or a plurality of containers. In each of the single container or the plurality of containers, the filling height of the raw material powder is 40 mm or less.

6. The method of producing a silicon nitride biscuit according to claim 5, wherein The plurality of containers are stacked in two or more layers and introduced into the continuous furnace, and the raw material powder is heated.

7. A method for producing a silicon nitride powder, comprising a step of pulverizing the silicon nitride pre-sintered body obtained by the production method according to claim 1 or 2.

8. A silicon nitride pre-sintered body, wherein the main component is silicon nitride, the average value of the α-phase conversion rate of the silicon nitride is 90% or more, and the maximum value of the thickness is less than 45 mm.

9. The silicon nitride preform according to claim 8, wherein The standard deviation of the α-phase conversion rate is 1.0% or less.

10. A method for producing a silicon nitride powder, comprising a step of pulverizing the silicon nitride pre-sintered body according to claim 8 or 9.

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