Silicon nitride powder, resin composition, and method for producing silicon nitride powder

Abstract

To provide a silicon nitride powder having improved fluidity. 【Solution means】The silicon nitride powder is a silicon nitride powder having a β fraction of 80% or more, and the equivalent diameter D of the maximum inscribed circle of the particle aggregates is 25 μm or less.

Description

Technical Field

[0001] The present invention relates to silicon nitride powder, a resin composition, and a method for producing silicon nitride powder.

Background Art

[0002] In recent years, with the increasing density and high power of electronic devices and semiconductor devices, the heat generation density of power modules has been increasing. The temperature rise of power modules is a factor that causes malfunction of elements and damage to circuit boards. By mixing silicon nitride powder, which is an excellent heat conductive material, as a filler into resin compositions (for example, substrates, sheets, spacers, etc.) used in such electronic devices and semiconductor devices, and electronic materials such as grease, adhesives, and paints, the heat dissipation performance can be improved. Here, silicon nitride has two types: α-type silicon nitride (α-Si 3 N 4 ) having different crystal phases and β-type silicon nitride (β-Si 3 N 4 ). α-type silicon nitride has the property of irreversibly undergoing a phase transformation to β-type silicon nitride at high temperatures (near the sintering temperature of 1500 to 1700 °C). β-type silicon nitride crystal grains are columnar crystal grains elongated in the c-axis direction, and are known to have higher thermal conductivity than α-type silicon nitride crystal grains. Therefore, when silicon nitride powder is used as a filler for heat dissipation materials, it is more preferable that the ratio of β-type silicon nitride crystal grains is larger. On the other hand, the material properties required for fillers include fillability and kneadability. The higher the fillability of the filler, the more powder can be mixed into materials such as resins at a high concentration. Also, the higher the kneadability of the filler, the easier it is to mix the filler into the material at a higher concentration. And various efforts have been made to improve the material properties of fillers.

[0003] For example, Patent Document 1 discloses a silicon nitride filler added to a resin or the like constituting an insulating member for the purpose of improving heat dissipation performance. According to Patent Document 1, when the silicon nitride filler contains 50% by volume or more of agglomerated particles having a particle diameter of 5 μm or more and 200 μm or less, the ratio of particles having a particle diameter of less than 5 μm or particles larger than 200 μm decreases, so that the dispersibility can be enhanced. Furthermore, it is possible to prevent roughness on the surface of the resin composite obtained by mixing with a resin or the like and a decrease in mechanical strength, and to enhance the thermal conductivity. This silicon nitride filler can be preferably produced by a production method having the following respective steps. The production method includes: (a) a step of filling a heat-resistant container with silicon or a mixture of silicon and silicon nitride; (b) a step of producing an agglomerate of silicon nitride by a self-combustion reaction in a non-oxidizing atmosphere containing nitrogen at 1 atm or more; and (c) a step of pulverizing the agglomerate of silicon nitride. Since the self-combustion reaction of silicon (nitriding combustion reaction) proceeds at a high temperature of 1900°C or more, the growth of silicon nitride particles sufficiently proceeds during the reaction process, and an agglomerate having a structure in which β-phase silicon nitride particles having a columnar shape with developed crystal planes are intertwined can be obtained as a product. Alternatively, instead of the production method of the silicon nitride filler using the self-combustion method, a direct nitriding method can be used to nitride granular bodies or molded bodies of silicon powder at around 1400°C in nitrogen, further develop β-phase columnar particles by heat treatment at a high temperature, and obtain the product by pulverizing the agglomerate. The silicon nitride filler thus obtained can be made into a resin composite including the silicon nitride filler and a resin composition.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the silicon nitride powder synthesized by the conventional combustion synthesis method or direct nitridation method is obtained by pulverizing the agglomerates of silicon nitride formed after the nitridation treatment. The pulverization treatment of such agglomerates increases the coarse aggregates and particles with distorted shapes in the silicon nitride powder, and as a result, the fluidity of the silicon nitride powder decreases. Due to this decrease in the fluidity of the silicon nitride powder, the viscosity of the fluid mixed with the silicon nitride powder increases, and the filling amount and kneadability of the filler into the fluid are restricted. Such restrictions on the fillability and / or kneadability prevent the improvement of the heat dissipation performance of the resin composition kneaded with the silicon nitride powder. Therefore, the inventors of the present invention have made it an object to obtain a silicon nitride powder having better fluidity.

[0006] In order to solve the above problems, an object of the present invention is to provide a silicon nitride powder having better fluidity, a resin composition kneaded with the silicon nitride powder, and a method for producing the silicon nitride powder.

Means for Solving the Problems

[0007] (Item 1) The silicon nitride powder according to one embodiment of the present invention is a silicon nitride powder having a β fraction of 80% or more, and the maximum External connection equivalent circle diameter D of the aggregate of particles is 25 μm or less.

[0008] (Item 2) The silicon nitride powder according to a further embodiment of the present invention is more preferably the silicon nitride powder of Item 1, wherein the maximum External connection equivalent circle diameter D of the aggregate is 10 to 20 μm.

[0009] (Item 3) The silicon nitride powder according to a further embodiment of the present invention is more preferably the silicon nitride powder of Item 1 or 2, wherein the average unevenness of the primary particles is 1.3 or less.

[0010] (Item 4) In a further form of the silicon nitride powder of the present invention, more preferably in the silicon nitride powder according to any one of Items 1 to 3, the β fraction is 98% or more.

[0011] (Item 5) In a further form of the silicon nitride powder of the present invention, more preferably in the silicon nitride powder according to any one of Items 1 to 4, the equivalent circle diameter DA of the average projected area of primary particles is 2 to 15 μm.

[0012] (Item 6) In a further form of the silicon nitride powder of the present invention, more preferably in the silicon nitride powder according to any one of Items 1 to 5, the maximum External connection The ratio D / DA of the equivalent circle diameter D of the aggregate to the equivalent circle diameter DA of the average projected area of primary particles is 4 or less.

[0013] (Item 7) In a further form of the silicon nitride powder of the present invention, more preferably in the silicon nitride powder according to any one of Items 1 to 6, the viscosity at 25 °C of a mixture obtained by mixing 150 parts by weight of the silicon nitride powder and 50 parts by weight of silicone oil having a kinematic viscosity of 30 cSt at 25 °C is 5 Pa· s or less.

[0014] (Item 8) The further form of the silicon nitride powder of the present invention is a silicon nitride powder having a β fraction of 80% or more, and the viscosity at 25 °C of a mixture obtained by mixing 150 parts by weight of the silicon nitride powder and 50 parts by weight of silicone oil having a kinematic viscosity of 30 cSt at 25 °C is 5 Pa· s or less.

[0015] (Item 9) A resin composition according to one form of the present invention is characterized in that the silicon nitride powder according to any one of Items 1 to 8 is dispersed in a thermoplastic resin or a thermosetting resin.

[0016] (Item 10) The manufacturing method of one form of the present invention is a method for manufacturing silicon nitride powder having a β fraction of 80% or more, comprising: a step of charging a raw material containing 90 to 98 mol% of SiO 2 powder and 2 to 10 mol% of Ca compound powder, a predetermined amount of carbon powder, and a solution in which a predetermined amount of a dispersant is dissolved into a container; a step of wet-mixing the materials charged into the container to produce a paste-like mixture; a step of drying the paste-like mixture and then crushing it to produce a mixed powder; a step of nitriding the mixed powder by a reduction nitriding method; and a step of removing the carbon powder from the mixed powder that has been subjected to reduction nitriding treatment.

[0017] (Item 11) The method of a further form of the present invention is more preferably, in the method of item 10, characterized in that the dispersant is a polymer dispersant selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, and hydroxypropyl cellulose.

[0018] (Item 12) The method of a further form of the present invention is more preferably, in the method of item 10 or 11, characterized in that the dispersant is added to pure water in an amount of 0.5 to 1% by weight based on the raw material.

[0019] (Item 13) The method of a further form of the present invention is more preferably, in any one of the methods of items 10 to 12, characterized in that the carbon powder is blended in an amount 2 to 3 times the molar ratio with respect to the SiO 2 powder.

Advantages of the Invention

[0020] The silicon nitride powder of the present invention has a high thermal conductivity as a heat dissipation filler for heat dissipation material applications because the β fraction is 80% or more. Further, the silicon nitride powder of the present invention has a maximum that serves as an index of the size of aggregates of particles External connectionBy controlling the equivalent circle diameter D to 25 μm or less and suppressing the formation of aggregates in the powder, the fluidity of the silicon nitride powder is enhanced, and the filling property and kneading property into fluids such as molten resins and electronic materials are improved.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Modes for Carrying Out the Invention

[0022] The silicon nitride powder according to an embodiment of the present invention is an aggregate of silicon nitride particles, and its β fraction is 80% or more. That is, the silicon nitride powder contains 80% or more, preferably 98% or more, of β-type silicon nitride particles among α-type silicon nitride (α-Si 3 N 4 ) particles and β-type silicon nitride (β-Si 3 N 4 ) particles. By containing 80% or more of β-type silicon nitride particles, the silicon nitride powder can have significantly higher thermal conductivity than α-type silicon nitride powder in which α-type silicon nitride particles are dominant. Further, the silicon nitride powder of the present invention is used as a heat dissipation (or heat conduction) filler for improving heat dissipation by being dispersed in resin compositions (for example, substrates, sheets, spacers, etc.) or electronic materials such as adhesives and paints, although not limited thereto.

[0023] Figure 1 is a typical SEM photograph (magnification: 1000 times) of silicon nitride powder. Here, FIGS. 1(a) and 1(b) illustrate the SEM photographs of the silicon nitride powders of Example 1 and Comparative Example 3, respectively. FIGS. 2 to 4 also illustrate the SEM photographs of the silicon nitride powders of the present invention (Examples 3, 5, and 6), respectively. FIGS. 5(a) and 5(b) illustrate the SEM photographs of the silicon nitride powders of Comparative Examples 1 and 2, respectively. By observing these SEM photographs, it can be seen that the silicon nitride powder of the present invention is produced through a specific manufacturing process, and compared with the silicon nitride powders of the comparative examples that are not, the amount of aggregates formed is small, and the size of the formed aggregates is also small. In other words, the silicon nitride powder of the present invention is characterized in that each particle is well separated and has higher fluidity.

[0024] The silicon nitride powder of the present embodiment has a maximum External connection equivalent circle diameter D (also referred to as the maximum equivalent circle diameter D) of 25 μm or less for the aggregates of particles. It is more preferable that the maximum External connection equivalent circle diameter D of the aggregates is 10 to 20 μm. It is even more preferable that the maximum External connection equivalent circle diameter D of the aggregates is 12 to 19 μm. As shown in FIG. 7, the External connection equivalent circle diameter of the aggregates is the diameter of the circle formed by the projected image of the aggregates, and can be an index for evaluating the size of the aggregates. Among the evaluated aggregates (lumps of particles), the External connection equivalent circle diameter of the largest-sized aggregate is the maximum External connection equivalent circle diameter D. This maximum External connection equivalent circle diameter D is an index for evaluating the size of the aggregates in the powder. That is, when comparing samples of silicon nitride powder, it can be said that the smaller the maximum External connection equivalent circle diameter D, the relatively smaller the size of the aggregates, which is advantageous for the fluidity of the silicon nitride powder. External connection

[0025] Also, as an index for evaluating the silicon nitride powder of the present embodiment, the shape characteristics of 200 or more primary particles constituting the silicon nitride powder were evaluated, and the average value was used as the primary particle characteristics. ​

[0026] The silicon nitride powder of this embodiment preferably has an average projected area equivalent circle diameter DA (also referred to as average circle equivalent diameter DA) of the primary particles in the range of 2 to 15 μm. The projected area equivalent circle diameter is the diameter of a circle having the same area as the projected area of the particle, and can be an index for evaluating the size of the particle. The average projected area equivalent circle diameter DA is the average value of the projected area equivalent circle diameters of 200 or more particles. And, by excluding the maximum External connection circle equivalent diameter D from the average projected area equivalent circle diameter DA, the degree of aggregation in the aggregate can be evaluated by the ratio D / DA. The silicon nitride powder of this embodiment preferably has a ratio D / DA of 4 or less. More preferably, the ratio D / DA is in the range of 1.3 to 3.9.

[0027] The silicon nitride powder of this embodiment preferably has an average unevenness degree of the primary particles of 1.3 or less. More preferably, the average unevenness degree of the primary particles is in the range of 1.10 to 1.26. The unevenness degree is an index indicating the degree of unevenness of the primary particles. The average unevenness degree is the average value of the unevenness degrees of 200 or more particles. The larger the average unevenness degree, the more distorted the shape of the primary particles as a whole powder. That is, the smaller the average unevenness degree, the more advantageous for the fluidity of the silicon nitride powder. Fig. 6(a) shows a method for calculating the unevenness degree of each primary particle in a planar projection image by SEM. As shown in Fig. 6(a), the unevenness degree X is calculated by the following formula using the perimeter length L and the area S. X = L 2 / (S × 4π)

[0028] The silicon nitride powder of this embodiment preferably has an average circularity of the primary particles of 0.7 or more. More preferably, the average circularity of the primary particles is 9 in the range of 0.7 づThe more spherical the particles are, the more advantageous it is for the fluidity of the silicon nitride powder. Fig. 6(b) shows a method for calculating the circularity of each primary particle in a planar projection image by SEM. As shown in Fig. 6(b), the circularity Y is calculated by the following formula using the perimeter length L and the area S. Y = 4πS / L 2

[0029] The silicon nitride powder of this embodiment preferably has an average aspect ratio of primary particles of 1.4 or less. More preferably, the average aspect ratio of the primary particles is 1.14 to 1.26. The aspect ratio is an index indicating the shape of the primary particles. The average aspect ratio is the average value of the aspect ratios of 200 or more particles. The larger this average aspect ratio, the more particles have a hexagonal prism shape with a larger major axis, which is considered disadvantageous for the fluidity of the silicon nitride powder. Fig. 6(c) shows a method for calculating the aspect ratio of each primary particle in a planar projection image by SEM. As shown in Fig. 6(c), the aspect ratio Z is calculated by the following formula using the absolute maximum length l of the particle and the ratio of the width w, which is the shortest distance between two straight lines that sandwich the particle with two straight lines parallel to the absolute maximum length l. Z = l / w

[0030] Furthermore, from an aspect different from the particle characteristics of the silicon nitride powder described above, as an index for evaluating the fluidity of the silicon nitride powder of this embodiment, the viscosity (Pa· s ) at 25°C of a mixture obtained by mixing 150 parts by weight of the silicon nitride powder and 50 parts by weight of silicone oil (silicone resin) having a kinematic viscosity of 30 cSt at 25°C was used. The lower the viscosity, the better the fluidity of the silicon nitride powder, and it can be evaluated that the silicon nitride powder has higher filling properties and kneading properties as a filler. The silicon nitride powder of this embodiment preferably has a viscosity of 5 Pa· s or less. More preferably, the viscosity is 1.8 to 4 .6 Pa· s .

[0031] Therefore, the silicon nitride powder of the present embodiment has higher fluidity and has improved filling properties and kneading properties as a heat dissipation filler. And the silicon nitride powder of the present embodiment provides a resin composition having higher heat dissipation performance by being filled, kneaded, and dispersed in a thermoplastic resin or a thermosetting resin. For example, after the silicon nitride powder with the maximum filling amount is filled, kneaded, and dispersed in the molten resin, the resin composition can be obtained by curing the molten resin. Here, the resin composition is not particularly limited, and examples thereof include thermoplastic resins such as polyethylene and polycarbonate, thermosetting resins such as phenol resin and epoxy resin, silico ー ne resin, rubber, etc. can be selected from the group consisting of. Similarly, the silicon nitride powder of the present embodiment provides an electronic material excellent in heat dissipation by being filled, kneaded, and dispersed in a fluid such as grease, adhesive, paint, etc.

[0032] Subsequently, a method for manufacturing the silicon nitride powder of the present embodiment will be described. The manufacturing method of the present embodiment mainly includes: (1) a step of putting into a container a raw material containing 90 to 98 mol% of silicon dioxide (SiO 2 ) powder (including sol) and 2 to 10 mol% of calcium (Ca) compound powder, a predetermined amount of carbon (C) powder, and a solution in which a predetermined amount of dispersant is dissolved; (2) a step of wet-mixing the raw materials put into the container to produce a paste-like mixture; (3) a step of drying the paste-like mixture and then crushing it to produce a mixed powder; (4) a step of nitriding the mixed powder by a reduction nitriding method; and (5) a step of removing the carbon powder from the mixed powder subjected to the reduction nitriding treatment.

[0033] First, prepare SiO 2 powder and Ca compound powder as raw materials. The SiO 2 powder as the base material preferably contains high-purity fine powder with few metal impurities. The SiO 2 powder may be prepared in the form of a sol (colloidal solution) in which particles are dispersed in a solvent. Also, the Ca compound is CaF 2 , CaCO 3Or it can be selected from these combinations. And the raw material is prepared to consist of 90 - 98 mol% of SiO 2 and 2 - 10 mol% of a Ca compound.

[0034] Prepare an appropriate amount of carbon powder. The amount of the carbon powder is preferably 2 - 3 times the molar ratio with respect to the SiO 2 powder. More preferably, the carbon powder is 2.4 - 2.7 times the molar ratio with respect to the SiO 2 powder. As the carbon powder, fine particles mainly composed of carbon such as furnace black and acetylene black can be used. Also, it is preferable to use carbon having an average particle diameter of 10 - 50 nm and an ash content of 0.1% or less.

[0035] Prepare a solution in which a predetermined amount of a dispersant is dissolved in a solvent. Preferably, the solution is an aqueous solution in which a small amount of a dispersant is dissolved in an appropriate amount of pure water. Preferably, the dispersant is a polymer dispersant selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, and hydroxypropyl cellulose. Preferably, the dispersant is added to pure water in an amount of 0.5 - 1% by weight based on the raw material with respect to the mixture of the raw material and the carbon powder. Preferably, the pure water is adjusted so that the solid content concentration is **~** vol% with respect to the mixture of the raw material and the carbon powder. In this embodiment, the pure water is adjusted so that the solid content concentration is 25 vol% with respect to the mixture of the raw material and the carbon powder. Note that an organic solvent may be used instead of pure water.

[0036] Next, put the prepared materials into a resin container and perform raw material mixing wet. The wet mixing is performed by a vibration mill or a planetary mill to produce a paste-like mixture. After drying the paste-like mixture in an oven or the like, a mixed powder is produced by crushing with a mixer and passing through a sieve (for example, with a mesh size of 300 μm).

[0037] Next, the mixed powder is packed into a carbon sheath having a predetermined dimension. The carbon sheath is placed in a furnace such that the external atmosphere is introduced into the sheath and the CO gas generated inside the sheath during reductive nitridation is exhausted to the outside.

[0038] Subsequently, the carbon sheath filled with the mixed powder is subjected to a reductive nitridation treatment using a pressurized nitrogen atmosphere furnace of a carbon heater. After evacuating the inside of the furnace before heating, nitrogen is preferably filled up to about 0.2 MPa, and heat treatment is performed up to about 1200 °C. Then, the nitrogen pressure inside the furnace is preferably increased up to about 0.4 MPa, and heat treatment for reductive nitridation is performed at 1400 °C to 1550 °C for 1 to 12 hours. If the pressure inside the furnace during the reductive nitridation treatment is lower than 0.2 MPa, SiC is likely to be generated during the reductive nitridation reaction, so the nitrogen pressure is preferably 0.2 MPa or more. Also, if the treatment temperature or time of reductive nitridation is low or short, unreacted SiO 2 may remain. Further, if the temperature of reductive nitridation is high, the grain growth of silicon nitride particles after reductive nitridation progresses too much, and the columnarization of the particle shape becomes remarkable. Therefore, it is necessary to appropriately adjust the treatment temperature and treatment time during reductive nitridation.

[0039] Since the powder after the reductive nitridation treatment contains excess carbon powder, it is heated at a temperature of 600 °C to 700 °C in dry air to remove the residual carbon. Since the heating time varies depending on conditions such as the heating temperature, gas flow rate, and powder amount, it is preferable to appropriately adjust while confirming the weight loss. If the heating time is too long, the oxygen content of the silicon nitride powder increases, so the heating time is preferably about 2 to 12 hours. Through the above steps, the silicon nitride powder of the present embodiment can be obtained.

[0040] Here, the mechanism of reductive nitridation will be described. In the reductive nitridation method of silicon nitride powder, it is necessary to mix three kinds of powders: carbon powder, silicon dioxide powder, and calcium fluoride (or calcium carbonate) powder. SiO 2 and CaF 2 (or CaCo 3When the mixed powder of the raw material powder of 2 and CaF 2 is heated, a liquid phase of SiO

[0041] is formed at around 1250 °C, and spherical particles of the Si-Ca-F-O-N system are generated. In the initial stage of the formation of these spherical particles, they have a uniform Ca distribution inside the grains. However, as the temperature rises, nitrogen dissolution into the liquid phase proceeds, and nuclei of silicon nitride are generated. Along with this, Ca is excluded from inside the silicon nitride grains and volatilizes to the outside of the particles as a gas phase, resulting in the formation of pores inside and on the surface of the particles. On the other hand, if the amount of the liquid phase is sufficient, the liquid phase is refilled into those pores. Then, the nucleation and grain growth of the silicon nitride particles continue until the liquid phase disappears. Here, when the mixing of the raw materials is non-uniform, in this series of mechanisms, particles with a significantly high Ca concentration and particles with a low Ca concentration will locally exist. When the Ca concentration is high, since Ca remains as a Si-Ca-N-based crystal without being volatilized, aggregates in which the particles are fused together are generated. On the other hand, when the Ca concentration is low, there is insufficient liquid phase to fill the pores generated after Ca volatilizes to the outside of the particles as a gas phase. If the amount of the liquid phase is slightly insufficient, only the pores on the particle surface are filled, and the shape of the primary particles is likely to be distorted.

[0042] Among the materials to be wet-mixed, the densities of the carbon powder, silicon dioxide powder, and calcium fluoride (or calcium carbonate) powder are 1.8 g / cm 3 , 2.2 g / cm 3 , 3.2 g / cm 3 (2.7 g / cm 3) That is, if these powders with different specific gravities are not evenly mixed, it can be said that defects are likely to occur in the silicon nitride powder after reduction nitridation. In particular, in wet mixing, when drying the slurry after mixing, the carbon powder with a small specific gravity moves to the vicinity of the surface layer, and the calcium fluoride with a large specific gravity settles, resulting in a non-uniform mixed state in the mixed powder obtained after drying. Also, generally, the fact that carbon powder is hydrophobic and silicon dioxide and calcium fluoride are hydrophilic can also be a factor causing the powder to separate during wet mixing and reducing the mixing uniformity. When reduction nitridation is carried out in a state where the mixing uniformity has decreased, according to the above-described reduction nitridation mechanism, the primary particles become distorted or the formation of aggregates is promoted, which can be a factor in deteriorating the fluidity of the silicon nitride powder. In the present invention, by adding a dispersant (for example, polyvinylpyrrolidone (PVP)) during wet mixing, hydrophilicity can be imparted to the surface of carbon particles, and the adhesion of carbon powder to the particle surfaces of silicon dioxide and calcium fluoride can be enhanced. Furthermore, by suppressing the sedimentation separation of the slurry after mixing due to the thickening property of the dispersant, it becomes possible to improve the uniformity of the material of the paste-like mixture. That is, the inventors have found that by improving the material mixing process, there is an effect of suppressing the formation of coarse aggregates and / or the formation of distorted primary particles in the silicon nitride powder after reduction nitridation. Therefore, the manufacturing method of the present embodiment enables the provision of silicon nitride powder having higher fluidity as a heat dissipation filler with improved filling property and kneadability.

Examples

[0043] Hereinafter, the present invention will be described more specifically based on examples and comparative examples, but the present invention is not to be construed as being limited by the following examples.

[0044] The silicon nitride powders according to Examples 1 to 6 and Comparative Examples 1 to 3 were produced according to the following conditions and procedures.

[0045] Into a resin container, pure water in which 0 to 1.0 wt% of PVP (polyvinylpyrrolidone K30, manufactured by Fujifilm Wako Pure Chemical Corporation) was dissolved in the mixed powder was weighed in, 0 to 10 mol% of Ca compound powder, and 90 to 100 mol% of SiO 2 powder, and SiO 2 and carbon powder in a molar ratio of 2.4 to 2. 8 times were added. In Examples 1 to 5 and Comparative Examples 1 and 3, high-purity powder with an average particle size of 0.4 μm was used as the SiO 2 powder. In Examples 6 and Comparative Example 2, a sol in which SiO 2 powder (average particle size 0.04 μm) was dispersed in a solvent was used. Then, using a resin container and silicon nitride jade, raw material mixing was carried out wet. The pure water was adjusted so that the solid content concentration became 25 vol%. Wet mixing was performed using a vibration mill or a planetary mill to prepare a paste-like mixture. After drying the paste-like mixture, crushing with a mixer and passing through a sieve (opening size 300 μm) were carried out to prepare a mixed powder for synthesis. The mixed powder was filled into a carbon sheath and introduced into the furnace. The carbon sheath was placed in the furnace so as to provide a gap of about 10 mm between the frame and the lid, and the external atmosphere was placed inside the sheath. After evacuating the furnace before heating, nitrogen was filled up to 0.2 MPa and heating was carried out up to 1200 °C. Then, the nitrogen pressure in the furnace was increased up to 0.4 MPa, and reduction nitridation was carried out at 1450 °C or 1500 °C for about 12 hours. And by heating in dry air at a temperature of 600 °C to 700 °C for 2 to 12 hours until the weight of carbon became 0 and removing the remaining carbon, silicon nitride powders of Examples 1 to 6 and Comparative Examples 1 to 3 were prepared.

[0046] For each of the samples of Examples 1 to 6 and Comparative Examples 1 to 3 prepared, powder X-ray diffraction measurement was performed, and by referring to the peak positions of the X-ray diffraction patterns, it was confirmed that silicon nitride powder was produced. By analyzing the X-ray diffraction patterns, the β fraction indicating the mass fraction of β-type silicon nitride in the synthesized powder was derived. Also, SEM photographs (magnification 1000 times) were obtained to analyze the particle shape and particle cross-sectional shape of the silicon nitride powder. By performing image analysis of the obtained SEM photographs with a computer, the size of the aggregates and the shape characteristics of the primary particles were evaluated. Furthermore, each sample of the silicon nitride powder was kneaded with silicone oil, and the fluidity of the silicon nitride powder was evaluated by measuring its viscosity. Various measurements and analyses were carried out under the following conditions.

[0047] · Powder X-ray diffraction measurement and its analysis Using a powder X-ray diffractometer UltimaIV manufactured by Rigaku Corporation, the X-ray diffraction intensity of each sample was measured by the powder X-ray diffraction method using Cu-Kα rays. Using the integrated intensities of the diffraction patterns of the silicon phase, α-type silicon nitride phase, and β-type silicon nitride phase, the mass fraction of the crystal phase was measured by the method of Jovanovic and Kimura represented by the following three known equations.

Equation

[0048] · Observation and analysis of powder shape by SEM The shape of the powder was observed using a scanning electron microscope JSM-IT700HR manufactured by JEOL Ltd. A sample of silicon nitride powder was dispersed on a carbon tape using a dry powder disperser (PD-10 manufactured by AmbiValue), and Pt sputtering treatment was performed on the powder surface as a conductive coating film to obtain an observation sample. The acceleration voltage was set to 5 kV, and an SE image was acquired at an observation magnification of 1000 times. The acquired SE image was imported into WinROOF2021 (Ver5.7.4) of Mitani Shoji Co., Ltd., and image analysis was performed by a computer. Specifically, for the imported image, filter operations, noise removal, binarization processing, etc. were performed, and the shape characteristics of the primary particles were processed to measure the equivalent circle diameter, unevenness degree, circularity, and aspect ratio of the primary particles. The calculation methods for the equivalent circle diameter, unevenness degree, circularity, and aspect ratio of the projected area of the primary particles are as described in Fig. 6. Particles touching the frame set in the ROI were not used for shape measurement, and the shape characteristics of more than 200 other primary particles were measured, and the average value was calculated.

[0049] ·Analysis of the particle cross-sectional shape of the powder by SEM Silicon nitride powder and epoxy resin were mixed and degassed under vacuum using a planetary mixer and then cured. The cross-section of the cured resin composition was processed by ion milling, and Pt sputtering treatment was performed on the observation surface as a conductive coating film to obtain an observation sample. The acceleration voltage was set to 10 kV, and an SE image was acquired at an observation magnification of 1000 times. The acquired SE image was imported into WinROOF2021 (Ver5.7.4) of Mitani Shoji Co., Ltd., and image analysis was performed by a computer. Specifically, for the imported image, filter operations, noise removal, binarization processing, etc. were performed, and from the processing of the shape characteristics of more than 200 particles, a lump (aggregate) of particles with the largest size was detected. Then, as shown in Fig. 7, the maximum External connection equivalent circle diameter D of the aggregate was calculated.

[0050] ·Viscosity of silicon nitride particle - silicone resin mixed paste Using a rotary-revolution mixer, 50 parts by weight of silicone oil having a kinematic viscosity of 30 cSt at 25°C and 150 parts by weight of silicon nitride powder of each sample were kneaded to prepare a silicon nitride particle-silicone resin mixed paste. Using a cone plate viscometer (TV-100E, 1°34’×R24 cone rotor) manufactured by Toki Sangyo Co., Ltd., the viscosity of the silicon nitride particle-silicone resin mixed paste at 25°C was measured.

[0051] The conditions and various measurement results for each sample of Examples 1 to 6 and Comparative Examples 1 to 3 are shown in Tables 1 and 2. Also, FIGS. 1(a) and (b) show SE images at a magnification of 1000 times for Example 1 and Comparative Example 3. FIGS. 2 to 4 show SE images for Examples 2 to 4. FIGS. 5(a) and (b) show SE images for Comparative Examples 1 and 2.

[0052]

Table 1

[0053]

Table 2

[0054] Examples 1 to 6 are samples prepared by adding a dispersant and mixing SiO 2 powder, a Ca compound, and carbon powder. In contrast, Comparative Example 1 is a sample in which the addition of the Ca compound is omitted. Comparative Examples 2 and 3 are samples in which the addition of the dispersant is omitted. By comparing the SEM photographs in FIGS. 1 to 5, it can be observed that in the SE images of Examples 1, 3, 5, and 6 obtained, the separation of each particle is good and the formation of aggregates is significantly suppressed compared to Comparative Examples 1 to 3. In particular, Example 1 and Comparative Example 3 were prepared under the same conditions except for the presence or absence of the dispersant in the mixing step. It can be visually observed that there is more aggregation of particles in Comparative Example 3 (FIG. 1(b)) than in Example 1 (FIG. 1(a)). That is, according to FIG. 1, it was demonstrated that the formation of aggregates was effectively suppressed by the presence or absence of the dispersant in the mixing step.

[0055] Table 1 quantitatively shows the primary particle characteristics of each sample. The silicon nitride powders of Examples 1 to 6 have an average equivalent circle diameter DA of 3.70 to 12.02 μm, an average unevenness of 1.10 to 1. 26 average unevenness degree, an average circularity of 0.79 to 0.91, and an average aspect ratio of 1.14 to 1.26 in terms of primary particle characteristics.

[0056] Table 2 quantitatively shows the β fraction, the maximum equivalent circle diameter D of the aggregates, the degree of aggregation (D / DA), and the viscosity of the silicon nitride particle / silicone resin mixed paste of each sample.

[0057] According to Table 2, the silicon nitride powders of Examples 1 to 6 show a β fraction of 98.1 to 100%, and it can be seen that β-type silicon nitride (β-Si 3 N 4 ) particles are formed at 80% or more. In Comparative Example 1, since no Ca compound was added, the β fraction is 88%.

[0058] According to Table 2, the silicon nitride powders of Examples 1 to 6 show a maximum equivalent circle diameter D of the aggregates of 12.0 to 19.0 μm. On the other hand, the maximum equivalent circle diameter D of Comparative Examples 1 to 3 is 25.6 to 35 .0 μm. Similar to the observation results of the SEM photographs, a significant difference is observed between the examples and the comparative examples in terms of the size of the aggregates. Furthermore, the ratio D / DA considering the size (average equivalent circle diameter DA) of the primary particles obtained in Table 1 is compared. According to Table 2, the ratio D / DA of Examples 1 to 6 is 1.33 to 3.86, while the ratio D / DA of Comparative Examples 1 to 3 is 4.88 to 24.82. A significant difference is also observed between the examples and the comparative examples in terms of the degree of aggregation. That is, the silicon nitride powder of the present invention shows superiority in terms of the size of the aggregates and the degree of particle aggregation.

[0059] According to Table 2, the silicon nitride powders of Examples 1 to 6 exhibit a viscosity of 1.8 to 4.6 Pa·s. In contrast, the viscosities of Comparative Examples 1 to 3 are 8.2 Pa·s or higher. The silicon nitride powders of Examples 1 to 6 exhibit a significantly lower viscosity compared to Comparative Examples 1 to 3. That is, the silicon nitride powder of the present invention shows superiority in the fluidity of the powder.

[0060] Based on the above experimental results, the silicon nitride powders of Examples 1 to 6 have been qualitatively and quantitatively demonstrated to have superior powder characteristics as fillers compared to the comparative examples. That is, the silicon nitride powders of the present embodiment (Examples 1 to 6) have improved fluidity by suppressing the coarsening of aggregates. Therefore, the silicon nitride powder of the present invention exhibits excellent filling properties and kneading properties as a heat dissipation filler compared to the prior art.

[0061] The present invention is not limited to the above-described embodiments, and can be implemented in various modes as long as it belongs to the technical scope of the present invention.

Claims

1. A silicon nitride powder having a β fraction of 80% or more, characterized in that the maximum circumscribed circle equivalent diameter D of particle agglomerates is 25 μm or less, and the average irregularity of primary particles is 1.3 or less.

2. A silicon nitride powder having a β fraction of 80% or more, characterized in that the maximum circumscribed circle equivalent diameter D of the particle agglomerates is 25 μm or less, and the average projected area circle equivalent diameter DA of the primary particles is 2 to 15 μm.

3. 3. The silicon nitride powder according to claim 1, wherein the aggregate has a maximum circumscribed circle equivalent diameter D of 10 to 20 μm.

4. 3. The silicon nitride powder according to claim 1, wherein the β fraction is 98% or more.

5. 3. The silicon nitride powder according to claim 2, wherein the ratio D / DA of the maximum circumscribing circle equivalent diameter D of the agglomerates to the average projected area circle equivalent diameter DA of the primary particles is 4 or less.

6. 3. The silicon nitride powder according to claim 1, characterized in that a mixture of 150 parts by weight of the silicon nitride powder and 50 parts by weight of a silicone oil having a kinetic viscosity of 30 cSt at 25°C has a viscosity at 25°C of 5 Pa·s or less.

7. 3. A resin composition comprising the silicon nitride powder according to claim 1 or 2 dispersed in a thermoplastic resin or a thermosetting resin.

8. A method for producing silicon nitride powder having a β fraction of 80% or more, comprising the steps of: 90 to 98 mol% SiO ２ a step of putting a raw material containing the powder and 2 to 10 mol % of a Ca compound powder, a predetermined amount of carbon powder, and a solution in which polyvinylpyrrolidone is dissolved as a predetermined amount of dispersant into a container; wet-mixing the materials charged in the container to prepare a paste-like mixture; drying the paste-like mixture and then pulverizing it to prepare a mixed powder; nitriding the mixed powder by a reduction-nitridation method; and removing the carbon powder from the reduced-nitrided mixed powder.

9. 9. The method according to claim 8, wherein the dispersant is added to the pure water in an amount of 0.5 to 1% by weight based on the mixture of the raw material and the carbon powder.

10. The carbon powder is ２ The method according to claim 9, characterized in that the powder is mixed in an amount of 2 to 3 times the amount of the powder in terms of molar ratio.

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