Silicon nitride powder

Abstract

To provide a β-type silicon nitride powder that can easily produce sintered bodies with high dielectric breakdown voltage. [Solution] A silicon nitride powder having a β conversion rate of 80% or more, and in a volume-based particle size distribution curve measured by laser diffraction scattering, where D10 is the cumulative 10% particle size, D99 is the cumulative 99% particle size, and Dp is the particle size with the highest frequency, the ratio (D99-Dp) / (Dp-D10) is between 10 and 50.

Description

[Technical Field]

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

[0002] Silicon nitride powder is attracting attention as a ceramic raw material for various industrial materials because its sintered body possesses excellent properties such as high thermal conductivity, high insulation, and high strength.

[0003] It is known that silicon nitride powder exists in two crystalline forms: α-type and β-type. The sinterability differs between the α-type and β-type. Specifically, the α-type transforms into the β-type during sintering, resulting in easy grain growth and good sinterability. On the other hand, the β-type does not undergo this transformation, making grain growth difficult and resulting in poor sinterability.

[0004] Because β-type silicon nitride powder has the advantage of relatively low manufacturing costs, studies have been conducted to improve its sinterability, and methods for controlling the particle size distribution are known.

[0005] Patent Document 1 describes an invention relating to silicon nitride powder for sintering, wherein the β-conversion rate is 80% or higher, and as measured by laser diffraction scattering, the average particle size D50 is 0.5 to 1.2 μm, the proportion of particles 0.5 μm or smaller is 20 to 50% by mass, and the proportion of particles 1 μm or larger is 20 to 50% by mass. It is described that by including fine particles (particles with a particle size of 0.5 μm or less) and large particles (particles with a particle size of 1 μm or larger) in such proportions, a phenomenon called Ostwald ripening occurs during sintering, in which the fine particles dissolve and precipitate in the liquid phase surrounding the large particles, resulting in a dense sintered body.

[0006] Patent Document 2 discloses an invention relating to silicon nitride powder, characterized by comprising a mixed powder of silicon nitride powder A having a β-phase content of 50% or more and an average particle size Da of 5 μm or less, and silicon nitride powder B having a β-phase content of 50% or more and an average particle size Db of 1 to 25 μm, with a weight ratio of A / B of 1 to 1000 and a Db / Da of 2 to 50. It is also stated that a silicon nitride sintered body with excellent sinterability, high strength, high toughness, and high reliability can be obtained using this silicon nitride powder. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2019 / 167879 [Patent Document 2] Japanese Patent Application Publication No. 6-64906 [Overview of the project] [Problems that the invention aims to solve]

[0008] The silicon nitride powder described in Patent Document 1 has a particle size distribution suitable for Ostwald growth, with a relatively high proportion of small-particle silicon nitride particles. Furthermore, regarding the silicon nitride powder described in Patent Document 2, referring to the specification and comparative examples, it is stated that using more of the silicon nitride powder B, which has a larger average particle size, reduces the strength of the sintered body. Therefore, it is necessary to use a relatively larger amount of silicon nitride powder A, which has a smaller average particle size.

[0009] While β-type silicon nitride powder containing both large-particle silicon nitride particles (coarse particles) and small-particle silicon nitride particles (fine particles) is known, conventional β-type silicon nitride powders are designed with a relatively high proportion of fine particles. However, our research has shown that a high proportion of fine particles leads to the generation of many voids during sintering, affecting the dielectric breakdown voltage. Furthermore, conventional literature does not show the relationship between the particle size distribution of the powder and the dielectric breakdown voltage, indicating that there is room for improvement from the viewpoint of obtaining a sintered body with high dielectric breakdown voltage.

[0010] Therefore, the object of the present invention is to provide a β-type silicon nitride powder that can easily produce a sintered body having a high dielectric breakdown voltage. [Means for solving the problem]

[0011] The inventors diligently conducted research to achieve the above objective. As a result, they discovered that the above problem could be solved by controlling the particle size distribution of β-type silicon nitride powder, and thus completed the present invention.

[0012] The gist of this invention is as follows [1] to [4]. [1] The β-conversion rate is 80% or higher, Silicon nitride powder in which, in a volume-based particle size distribution curve measured by laser diffraction scattering, the cumulative 10% particle size is D10, the cumulative 99% particle size is D99, and the particle size with the highest frequency is Dp, and the ratio (D99-Dp) / (Dp-D10) is between 10 and 50. [2] The silicon nitride powder according to [1], wherein the Dp is 0.45 to 0.60 μm. [3] BET specific surface area is 5-50m 2 Silicon nitride powder as described in [1] or [2], in a quantity of / g. A method for producing a silicon nitride sintered body, comprising the step of firing a green body containing silicon nitride powder as described in any of [4][1] to [3] to obtain a silicon nitride sintered body. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a β-type silicon nitride powder that can easily produce a sintered body having a high dielectric breakdown voltage. [Modes for carrying out the invention]

[0014] The silicon nitride powder of the present invention has a β-conjugation rate of 80% or more, and in the volume-based particle size distribution curve measured by laser diffraction scattering, when the cumulative 10% particle size is D10, the cumulative 99% particle size is D99, and the particle size with the highest frequency is Dp, the ratio (D99-Dp) / (Dp-D10) is between 10 and 50.

[0015] The silicon nitride powder of the present invention has a β-conversion rate of 80% or more. The β-conversion rate of silicon nitride powder refers to the ratio of the peak intensity of the β phase to the sum of the α and β phases in the silicon nitride powder [100 × (peak intensity of β phase) / (peak intensity of α phase + peak intensity of β phase)], and is determined by powder X-ray diffraction (XRD) measurement using CuKα rays. More specifically, it can be determined by calculating the weight ratio of the α phase and β phase of the silicon nitride powder using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780.

[0016] Since silicon nitride powder can be obtained without setting strict manufacturing conditions as the β conversion rate increases, from the viewpoint of suppressing the overall manufacturing cost of silicon nitride sintered bodies, the β conversion rate of silicon nitride powder is preferably 85% or higher, more preferably 90% or higher, even more preferably 99% or higher, and particularly preferably 100%.

[0017] The silicon nitride powder of the present invention has a value of (D99 - Dp) / (Dp - D10) that is 10 or more and 50 or less. Dp represents the size of the main particles in the silicon nitride powder. As described above, since it is difficult for the β-type silicon nitride powder to grow grains, in the particles constituting the silicon nitride sintered body obtained using this silicon nitride powder as a raw material, Dp becomes the main particle. And, by (D99 - Dp) / (Dp - D10), it becomes possible to represent the relationship between the main particles, the coarse particles, and the fine particles. By setting (D99 - Dp) / (Dp - D10) within the above range, coarse particles and fine particles will exist in an appropriate ratio in the silicon nitride sintered body, and the fine particles will fill the voids between the coarse particles, resulting in a decrease in the void ratio. As a result, it is presumed that when a voltage is applied, the discharge between the silicon nitride particles is reduced, and a high dielectric breakdown voltage is exhibited in the silicon nitride sintered body.

[0018] (D99 - Dp) / (Dp - D10) preferably has a lower limit value of 15 or more, more preferably 18 or more, and an upper limit value preferably of 40 or less.

[0019] The lower limit value of the above Dp is preferably 0.45 μm or more, more preferably 0.50 μm or more, and even more preferably 0.52 μm or more. When Dp is at least the above lower limit value, the proportion of fine particles does not become too large, and it is possible to prevent a decrease in the toughness of the silicon nitride sintered body. Also, the upper limit value of the above Dp is preferably 0.60 μm or less, more preferably 0.58 μm or less, and even more preferably 0.55 μm or less. When Dp is at most the above upper limit value, it has an appropriate amount of fine particles, and it becomes easier to improve the bending strength of the silicon nitride sintered body.

[0020] In the volume-based particle size distribution curve measured by the laser diffraction scattering method, the volume frequency of the above Dp is preferably 3% to 10%, and more preferably 4% to 7%.

[0021] The lower limit of the D10 is preferably 0.10 μm or more, more preferably 0.20 μm or more. By setting D10 to be not less than the lower limit value, the formation of grain boundary phases due to a large amount of fine particles in the silicon nitride sintered body can be prevented, and it becomes easier to improve the thermal conductivity. Also, the upper limit of the D10 is preferably 0.70 μm or less, more preferably 0.50 μm or less. By setting D10 to be not more than the upper limit value, the amount of fine particles becomes an appropriate value, and it becomes easy to impart flexibility to the silicon nitride sintered body and improve the bending strength.

[0022] The lower limit of the D99 is preferably 3.0 μm or more, more preferably 4.0 μm or more, and even more preferably 4.3 μm or more. When D99 is not less than the lower limit value, the proportion of coarse particles increases, and it becomes easier to improve the toughness of the silicon nitride sintered body. Also, the upper limit of the D99 is preferably 10.0 μm or less, more preferably 8.0 μm or less, and even more preferably 6.0 μm or less. When D99 is not more than the upper limit value, the proportion of coarse particles is suppressed, and it becomes easy to increase the bending strength of the silicon nitride sintered body.

[0023] The D10, D99, and Dp are determined from a volume-based particle size distribution curve measured by the laser diffraction scattering method. Specifically, as described in the examples, after performing a pretreatment of firing silicon nitride powder in air at a temperature of about 500 °C for 2 hours, a sample dispersed in an aqueous sodium pyrophosphate solution is used to measure the particle size distribution with a laser diffraction / scattering particle size distribution measuring device. The particle size distribution curve is a volume frequency distribution curve with the horizontal axis being the particle diameter (μm) and the vertical axis being the volume frequency. In this curve, the particle diameter at which the cumulative value reaches 10% is D10, the particle diameter at which the cumulative value reaches 99% is D99, and the particle diameter at which the volume frequency is the highest is Dp. Also, the relationship among D10, D99, and Dp is D10 < Dp < D99. When there are two or more Dp values, the smallest particle size side is taken as Dp.

[0024] In particle size measurement, the reason for pre-treatment by calcining silicon nitride powder is that if the surface oxygen content of the silicon nitride powder is low, or if the particle surface is covered with hydrophobic substances due to grinding aids during grinding, the particles themselves may exhibit hydrophobicity, resulting in insufficient dispersion in water and making reproducible particle size measurement difficult. To prevent this, the silicon nitride powder sample is calcined in air at a temperature of approximately 200°C to 500°C for several hours to impart hydrophilicity to the silicon nitride powder, making it easier to disperse in aqueous solvents and enabling highly reproducible particle size measurement. It has been confirmed that calcination in air has almost no effect on particle size.

[0025] The silicon nitride powder of the present invention has a BET specific surface area of ​​5 to 50 m². 2 It is preferable that the amount be / g, which is 10-40m 2 It is more preferable that the value be / g, and that it is 12-28m 2 It is even more preferable that it be / g, which is 12-15m 2 A value of / g is particularly preferred. Since the particle size distribution is measured by laser diffraction scattering, it is not possible to confirm the presence of nano-order ultrafine particles. On the other hand, information regarding the amount of such ultrafine particles can be supplemented by measuring the BET specific surface area. The larger the BET specific surface area, the greater the proportion of ultrafine particles. In the silicon nitride powder of the present invention, by controlling the BET specific surface area to be within the above range, the amount of ultrafine particles that have high surface free energy and contribute to the progress of sintering is controlled, allowing sintering to proceed appropriately and making it easier to obtain a high-density, high-strength silicon nitride sintered body.

[0026] The silicon nitride powder of the present invention has a density of 0.2 tons / cm³. 2 The bulk density when press-molded under this pressure is 1.7 g / cm³. 3 The above is preferable. By achieving such a high bulk density, the sintering ratio can be suppressed. The above pressurized bulk density is 0.2 ton / cm³ of silicon nitride powder. 2The material is molded into a disc-shaped pellet under pressure, its mass is measured using a precision balance, and its thickness and diameter are measured using a micrometer or similar instrument to calculate the volume of the molded body. Using these measurements, the pressurized bulk density can be calculated by "mass ÷ volume".

[0027] The total oxygen content of the silicon nitride powder of the present invention is preferably 1% by mass or more. The total oxygen content is the sum of the amount of dissolved oxygen (internal oxygen) and the amount of external oxygen. When the total oxygen content is above the lower limit, the sintering process is more easily promoted by silicon oxide on the particle surface. Furthermore, the total oxygen content of the silicon nitride powder is preferably 10% by mass or less.

[0028] The amount of dissolved oxygen in the silicon nitride powder of the present invention is preferably 0.2% by mass or less, and more preferably 0.1% by mass or less. When the amount of dissolved oxygen is within the above range, it becomes easier to increase the thermal conductivity of the silicon nitride sintered body.

[0029] Note that the amount of dissolved oxygen refers to the oxygen dissolved within the particles of silicon nitride powder, and does not include oxygen derived from oxides such as SiO2 that inevitably exist on the particle surface.

[0030] (Silicon nitride powder manufacturing process) The method for producing the silicon nitride powder of the present invention is not particularly limited as long as it is a method that can obtain silicon nitride powder having the above-described properties. For example, the silicon nitride powder of the present invention may be obtained by classifying a known β-type silicon nitride powder until (D99-Dp) / (Dp-D10) is in the range of 10 to 50. The classification method is not particularly limited and may be wet or dry, but dry classification is preferred from the viewpoint of productivity, and airflow classification is particularly preferred. The conditions for airflow classification are not particularly limited, and conditions such as nozzle pressure can be appropriately adjusted to obtain the desired particle size distribution. For example, if the (D99-D10) / (Dp-D10) of the obtained powder is greater than the desired value, the nozzle pressure can be increased, and if it is less than the desired value, the nozzle pressure can be decreased, and airflow classification can be performed while making fine adjustments to obtain the desired powder, thereby obtaining the silicon nitride powder of the present invention.

[0031] As a method for producing β-type silicon nitride powder, for example, a reduction nitriding method can be applied, in which silica powder is used as a raw material and nitrogen gas is passed through it in the presence of carbon powder to produce silicon nitride; a direct nitriding method can be applied, in which silicon powder and nitrogen are reacted at high temperature; and an imide decomposition method can be applied, in which silicon halide is reacted with ammonia. However, from the viewpoint of easily producing silicon nitride powder having the above-mentioned properties, the direct nitriding method is preferred, and among these, the direct nitriding method (combustion synthesis method) utilizing the autocombustion method is more preferred.

[0032] The combustion synthesis method is a method of synthesizing silicon nitride using silicon powder as a raw material, by forcibly igniting a portion of the raw material powder under a nitrogen atmosphere, and generating heat through the self-exothermic reaction of the raw material compounds. The combustion synthesis method is a known method, and for example, refer to Japanese Patent Publication No. 2000-264608, International Publication No. 2019 / 167879, etc.

[0033] The raw material powder used in the above combustion synthesis method contains at least silicon powder, and preferably contains silicon powder and a diluent. By preparing the raw material powder as described below, it becomes easier to obtain the silicon nitride powder of the present invention.

[0034] The average particle size of the silicon powder contained in the raw material powder is preferably 1 to 20 μm, and more preferably 2 to 10 μm.

[0035] It is preferable to use silicon nitride powder as a diluent. The average particle size of the silicon nitride powder used as a diluent is preferably 0.1 to 10 μm, and more preferably 0.5 to 5 μm.

[0036] The average particle size of the silicon powder and silicon nitride powder mentioned above is the 50th percentile value on the volume-based cumulative curve showing the particle size distribution obtained by laser diffraction scattering.

[0037] The content of silicon powder in the raw material powder is preferably 50 to 95% by mass, more preferably 70 to 90% by mass. Also, the content of the diluent in the raw material powder is preferably 5 to 50% by mass, more preferably 10 to 30% by mass. When the content of the diluent is at or above these lower limit values, the heat generation of the raw material powder is reduced, making it easier to control the temperature. When the content of the diluent is at or below these upper limit values, the heat of nitriding combustion of the metal can be easily propagated throughout the raw material powder filled in the reaction vessel.

[0038] The raw material powder may contain components other than silicon powder and the diluent used as required. Examples of other components include chlorides such as sodium chloride and ammonium chloride, and oxides such as calcium oxide, yttrium oxide, and magnesium oxide. Other components are preferably 10% by mass or less, more preferably 5% by mass or less, still more preferably 1% by mass or less, and still more preferably 0% by mass based on the total amount of the raw material powder.

[0039] In the method for producing the silicon nitride powder, by using high-purity raw materials, it becomes easy to keep the amount of dissolved oxygen in the silicon nitride powder low. For example, when producing silicon nitride powder by the direct nitridation method, it is preferable to use silicon powder that has no factor for oxygen to dissolve inside as the raw material to be used. Specifically, it is preferable to use silicon powder derived from semiconductor-grade silicon, for example, silicon powder typified by the cutting powder generated when processing the above silicon by cutting or the like. The above semiconductor-grade silicon is typically polycrystalline silicon obtained by the so-called "Siemens method" in which high-purity trichlorosilane and hydrogen are reacted in a bell-jar type reaction vessel.

[0040] In the combustion synthesis method, the above-described raw material powder is filled into a reaction vessel (setter). The reaction vessel is preferably a heat-resistant reaction vessel made of ceramics, graphite, or the like. The bulk density of the raw material powder layer in the reaction vessel is preferably in the range of 0.1 to 1.0 g / cm 3 and more preferably in the range of 0.3 to 0.7 g / cm 3It is preferable to set it within the range of [this range].

[0041] Since the silicon nitride particles obtained by this combustion synthesis method are obtained as lumps, it is preferable to crush the lumps by rubbing them against each other using a crusher, and then perform mechanical grinding as described below. Examples of grinders used for mechanical grinding include roll crushers, cutter mills, stamp mills, mortars, grinders, pin mills, colloid mills, ball mills, high-pressure dispersion devices, and stone mill grinders. When using a ball mill, the particle size can be adjusted by changing the grinding time. Generally, the longer the grinding time, the more fine particles are generated due to partial grinding of the particles, so D10 decreases and the BET specific surface area increases.

[0042] (Silicon nitride sintered body manufacturing process) The silicon nitride powder of the present invention has no particular applications, but since a sintered body with high dielectric breakdown voltage can be easily obtained, it is preferable to use it as a silicon nitride powder for sintering, as a raw material for obtaining a silicon nitride sintered body. The silicon nitride sintered body has no particular applications, but since a sintered body with high dielectric breakdown voltage can be easily obtained, substrate applications can be cited as a preferred application.

[0043] One method for producing a silicon nitride sintered body is to heat a green body containing the silicon nitride powder and sintering aid of the present invention, as described above, to a temperature of 1500 to 1900°C under an inert gas atmosphere to sinter the silicon nitride.

[0044] In the method for producing the silicon nitride sintered substrate, known sintering aids can be used without particular limitation, and examples include oxides such as yttria, magnesia, ceria, and calcia, as well as oxygen-free compounds such as carbonitride compounds and nitride compounds. Examples of carbonitride compounds include Y2Si4N6C, Yb2Si4N6C, Ce2Si4N6C, and MgSi4N6C. Examples of nitride compounds include MgSiN2. These sintering aids may be used individually or in combination of two or more. The amount of the sintering aid is not particularly limited, but is preferably 3 to 20 parts by mass, and more preferably 7 to 10 parts by mass, per 100 parts by mass of silicon nitride powder.

[0045] The green body may be molded using a binder. The binder is not particularly limited, but examples include polyvinyl alcohol, polyvinyl butyral, methylcellulose, alginic acid, polyethylene glycol, carboxymethylcellulose, ethylcellulose, and acrylic resin. The binder content used in the production of the green body is preferably 1 to 30 parts by mass per 100 parts by mass of silicon nitride powder, and the ratio can be appropriately determined depending on the molding method.

[0046] The method for producing the green body is not particularly limited, and one example is a method of forming a molding composition containing at least silicon nitride powder and a sintering aid using known molding means. Examples of known molding means include press molding, extrusion molding, injection molding, and doctor blade molding, but the doctor blade molding method is particularly preferred when producing sintered bodies for substrate applications.

[0047] Furthermore, the molding composition may contain a solvent for ease of handling and ease of molding. The solvent is not particularly limited and can include alcohols, hydrocarbons and other organic solvents, and water. Using water as a solvent is preferable because it reduces the environmental burden compared to using organic solvents.

[0048] When a solvent is used in the production of the aforementioned green material, a dispersant may be used to improve the dispersibility of the silicon nitride powder and sintering aid. Generally, surfactants can be suitably used as dispersants. Known surfactants can be used without any limitations. Specific examples of surfactants that can be suitably used in the present invention include carboxylated trioxyethylene tridecyl ether, diglycerin monooleate, diglycerin monostearate, carboxylated heptaoxyethylene tridecyl ether, tetraglycerin monooleate, hexaglycerin monooleate, sorbitan laurate, sorbitan oleate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate. These surfactants may be used individually or in combination of two or more. The amount of the dispersant can be appropriately selected, but for example, it can usually be selected from a range of 0.1 to 5 parts by mass per 100 parts by mass of the total amount of silicon nitride powder and sintering aid. Within that range, the upper limit of the amount of dispersant is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less.

[0049] The firing of the green body is carried out under an inert gas atmosphere. An inert gas atmosphere means, for example, a nitrogen atmosphere or an argon atmosphere. The firing pressure is not particularly limited and may be carried out at atmospheric pressure or under pressure. The heating temperature during firing should be 1500 to 1900°C. If the heating temperature is below 1500°C, the sintering of silicon nitride will not proceed easily, and if it exceeds 1900°C, the silicon nitride will decompose easily. From this viewpoint, the heating temperature during firing is preferably 1500 to 1900°C. The firing time is not particularly limited, but it is preferably about 3 to 20 hours.

[0050] Furthermore, when organic components such as binders or dispersants are used in the formation of the green material, it is preferable to include a degreasing step to remove the organic components. The degreasing conditions are not particularly limited, but for example, they may be carried out by heating the green material to 450-650°C in air or in an inert atmosphere such as nitrogen or argon. [Examples]

[0051] The following examples illustrate the present invention in more detail, but the present invention is not limited to these examples.

[0052] [Evaluation Method] The various physical properties in the examples and comparative examples were measured by the following methods.

[0053] (1) Beta conversion rate of silicon nitride powder The β-conversion rate of silicon nitride powder was determined by powder X-ray diffraction (XRD) measurements using CuKα rays. Specifically, the weight ratio of the α-phase to the β-phase of silicon nitride powder was calculated using the method described in CPGazzara and DRMessier: Ceram. Bull., 56 (1977), 777-780, and the β-conversion rate was determined from there.

[0054] (2) Particle size of silicon nitride powder (D10, Dp, D99) (i) Sample pretreatment As a pretreatment for the silicon nitride powder sample, the silicon nitride powder was calcined in air at a temperature of approximately 500°C for 2 hours. (ii) Measurement of particle size distribution In a beaker (60 mm inner diameter, 70 mm height) with a maximum mark of 100 mL, 45 mL of water and 5 mL of 5% by mass sodium pyrophosphate were added and thoroughly mixed. Then, a small amount (about the size of an ear pick) of pre-treated silicon nitride powder was added, and the silicon nitride powder was dispersed using an ultrasonic homogenizer (US-300E, manufactured by Nippon Seiki Seisakusho Co., Ltd., with a tip diameter of 26 mm) at an amplitude of 50% (approximately 2 amperes) for 2 minutes. The tip was inserted until its end reached the 20 mL mark in the beaker for dispersion. Next, the obtained dispersion of silicon nitride powder was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT3300EXII, manufactured by Microtrac Bell Co., Ltd.) to obtain a volume-based particle size distribution curve with particle size (μm) on the x-axis and volume frequency on the y-axis. The measurement conditions were as follows: water (refractive index 1.33) was selected as the solvent, a refractive index of 2.01 was selected for particle properties, permeability was set to permeable, and the particle shape was set to non-spherical. (iii) Measurement of particle size From the obtained volume frequency distribution curve, the particle size at which the cumulative percentage reaches 10% was defined as D10, the particle size at which the cumulative percentage reaches 99% was defined as D99, and the particle size at which the volume frequency is highest was defined as Dp.

[0055] (3) Void fraction 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, 0.5 parts by mass of dispersant, and 22 parts by mass of binder were weighed out and mixed in a ball mill using water as the solvent, with a resin pot and silicon nitride balls for 48 hours. Next, degassing and viscosity adjustment were performed using a vacuum degasser (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. Then, a sheet with a width of 750 mm and a thickness of 420 μm was formed using the raw material slurry by the doctor blade method. The resulting molded body was dried in air at 40°C to vaporize the solvent, and then cut so that the main surface was 200 mm × 268 mm to obtain a green material (green sheet). The aforementioned green material was degreased by holding it at 500°C for 5 hours in an air atmosphere. Subsequently, it was fired at 1830°C or higher for 9 hours or more under a nitrogen atmosphere and a pressure of 0.8 MPa·G to obtain a silicon nitride sintered body. The cross-section of the obtained silicon nitride sintered body was observed using a scanning electron microscope (Hitachi High-Technologies Corporation "TM3030") (10 fields of view at 1000x magnification), and the ratio of the void area to the total area of ​​the cross-section was defined as the void fraction (%).

[0056] (4) Dielectric breakdown withstand voltage For the silicon nitride sintered body obtained in the same manner as the evaluation of the void fraction described above, the breakdown voltage (kV) was measured using a 7473 ultra-high voltage withstand voltage tester (manufactured by Tama Densoku Co., Ltd.), and the dielectric breakdown voltage (kV / mm) was calculated by dividing this by the thickness of the sintered body (mm).

[0057] (5) Specific surface area The specific surface area was measured using a BET method specific surface area analyzer (Macsorb HM model-1201) manufactured by Mountec Co., Ltd., employing the BET single-point method with nitrogen gas adsorption. Prior to the specific surface area measurement described above, the silicon nitride powder to be measured was heat-treated in air at 600°C for 30 minutes to remove organic matter adsorbed on the powder surface.

[0058] <Example 1> Silicon powder (semiconductor grade, average particle size 5 μm) and silicon nitride powder (average particle size 1.5 μm), used as a diluent, were mixed to obtain a raw material powder (Si: 80% by mass, Si3N4: 20% by mass). This raw material powder was packed into a reaction vessel to form a raw material powder layer. Next, the reaction vessel was placed in a pressure-resistant, closed reactor equipped with an ignition device and a gas supply and discharge mechanism. The reactor was degassed by reducing the pressure, and then nitrogen gas was supplied to replace the atmosphere with nitrogen. Subsequently, nitrogen gas was gradually supplied and the pressure was increased to 0.7 MPa. At the point when the predetermined pressure was reached (at the time of ignition), the bulk density of the raw material powder was 0.5 g / cm³. 3 Subsequently, the end of the raw material powder in the reaction vessel was ignited, and a combustion synthesis reaction was carried out to obtain a massive product consisting of silicon nitride. The resulting agglomerated product was crushed by rubbing the agglomerated material against itself using a crusher, and then ground for 6 hours using a vibrating ball mill (manufactured by Chuo Kako Kiki) to obtain unclassified silicon nitride powder. During the grinding process, 0.5 wt% ethanol was added as a grinding aid.

[0059] The silicon nitride powder obtained by the above method was classified using an air-flow classifier (manufactured by Nisshin Engineering Co., Ltd.) to obtain silicon nitride powder. The upper nozzle pressure of the air-flow classifier was set to 0.45 MPa. The evaluation results of the obtained silicon nitride powder are shown in Table 1.

[0060] <Examples 2-7, Comparative Example 2> In Example 1, silicon nitride powder was obtained in the same manner as in Example 1, except that the nozzle pressure during classification and the grinding time of the vibrating ball mill were changed as shown in Table 1. The evaluation results of the obtained silicon nitride powder are shown in Table 1.

[0061] <Comparative Example 1> The unclassified silicon nitride powder obtained in Example 1 was evaluated. The results are shown in Table 1.

[0062] [Table 1]

Claims

1. The β-conversion rate is 80% or higher. In a volume-based particle size distribution curve measured by laser diffraction scattering, where D10 represents the cumulative 10% particle size, D99 represents the cumulative 99% particle size, and Dp represents the particle size with the highest frequency, the ratio (D99 - Dp) / (Dp - D10) is between 10 and 50. Silicon nitride powder.

2. The silicon nitride powder according to claim 1, wherein the Dp is 0.45 to 0.60 μm.

3. BET specific surface area is 5 to 50 m² ２ The silicon nitride powder according to claim 1, wherein the weight is / g.

4. A method for producing a silicon nitride sintered body, comprising the step of firing a green body containing silicon nitride powder according to any one of claims 1 to 3 to obtain a silicon nitride sintered body.

Images