β-type silicon nitride powder, resin composition, thermally conductive paste, and method for producing β-type silicon nitride powder

Abstract

The present invention provides a β-type silicon nitride powder having a particle shape in which the particle diameter in the ab-plane direction is larger than that in the c-axis direction. [Solution] The β-type silicon nitride powder is a β-type silicon nitride powder having a β fraction of 80% or more, characterized in that the ratio Iβ(101) / Iβ(210), which is the ratio of the X-ray diffraction peak intensity Iβ(101) of the (101) plane of β-Si3N4 obtained from the X-ray diffraction pattern of the β-type silicon nitride powder to the X-ray diffraction peak intensity Iβ(210) of the (210) plane of β-Si3N4, is 1.1 or more.

Description

Technical Field

[0003]

[0001] The present invention relates to β-type silicon nitride powder, a resin composition, a thermal conductive paste, and a method for producing β-type 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 β-type silicon nitride powder, which is an excellent thermal conductive material, as a filler into resin compositions (for example, substrates, sheets, spacers, etc.) used in such electronic devices and semiconductor devices, and paste-like electronic materials (thermal conductive pastes) such as greases, adhesives, and paints, the heat dissipation performance can be improved.

[0003] Here, there are two types of silicon nitride: α-type silicon nitride (α-Si3N4) and β-type silicon nitride (β-Si3N4), which have different crystalline phases. α-type silicon nitride has the property of irreversibly transforming into β-type silicon nitride at high temperatures (around the sintering temperature of 1500-1700°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. The plane perpendicular to the c-axis in the elongation direction is defined as the ab plane. Generally, a hexagonal crystal structure has six faces: hexagonal upper and lower faces and rectangular faces perpendicular to the opposing upper and lower faces. The hexagonal upper and lower faces are called the base planes, and the six rectangular faces perpendicular to the base planes are also called prism planes. Therefore, when silicon nitride powder is used as a filler for heat dissipation applications, a higher proportion of β-type silicon nitride crystal grains is considered preferable. Furthermore, larger particle sizes of β-type silicon nitride crystals result in longer heat conduction paths, which is advantageous for the heat dissipation properties of the filler. On the other hand, the required material properties for a filler include packing ability and fluidity (or kneadability). The higher the packing ability of the filler, the higher the concentration of powder that can be mixed into materials such as resins. Also, the higher the fluidity of the filler, the easier it is to mix the filler with the material at a higher concentration.

[0004] For example, Patent Document 1 discloses a silicon nitride filler that is added to resins and the like that constituting insulating members for the purpose of improving heat dissipation performance. According to Patent Document 1, when the silicon nitride filler contains 50% or more by volume of condensed particles with a particle diameter of 5 μm or more and 200 μm or less, the proportion of particles with a particle diameter of less than 5 μm and particles with a particle diameter of more than 200 μm decreases, thereby improving dispersibility. Furthermore, it is possible to prevent surface roughness and a decrease in mechanical strength of the resin composite obtained by mixing with resins and the like, and to improve thermal conductivity. This silicon nitride filler can preferably be manufactured by a manufacturing method having the following steps. The manufacturing 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 condensed silicon nitride lumps by a self-combustion reaction in a non-oxidizing atmosphere containing nitrogen at 1 atmosphere or more, and (c) a step of crushing the condensed silicon nitride lumps. The self-combustion reaction of silicon (nitriding combustion reaction) proceeds at high temperatures of 1900°C or higher. During this reaction, silicon nitride particles grow sufficiently, and a condensed mass with an intertwined structure of β-phase silicon nitride particles having well-developed crystalline faces can be obtained as a product. Alternatively, instead of using the self-combustion method to produce silicon nitride filler, a direct nitriding method can be used to nitride silicon powder granules or molded bodies in nitrogen at around 1400°C, further heat treatment at high temperatures to develop β-phase columnar particles, and then the resulting condensed mass can be crushed. The silicon nitride filler thus obtained can be used to make a resin composite containing the silicon nitride filler and a resin composition. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2015-81205 [Overview of the project] [Problems that the invention aims to solve]

[0006] In conventional β-type silicon nitride powders such as those described in Patent Document 1, the β-type silicon nitride crystal grains consist of hexagonal prismatic particles, where the particle diameter (length) is larger in the c-axis direction than in the ab-plane direction. These grains are needle-shaped or elongated prismatic, extending in the long axis (c-axis) direction from the base to the top surface that defines the polygonal ab-plane. It is known that β-type silicon nitride crystal grains produced by conventional methods have a larger particle diameter in the c-axis direction than in the ab-plane direction, and that the thermal conductivity in the c-axis direction is about twice as high as in the ab-plane direction. When such elongated hexagonal prismatic particles in the c-axis direction are embedded in a resin, the ab-plane direction (short axis direction) of the hexagonal prismatic particles is oriented in the direction of gravity (e.g., thickness direction), and the c-axis direction (long axis direction) is oriented in the direction perpendicular to gravity (e.g., in-plane direction), resulting in significant anisotropy in the direction perpendicular to gravity. As a result, in composite products (resin compositions) of silicon nitride filler and resin, the thermal conductivity in the thickness direction was sometimes about half that of the thermal conductivity in the in-plane direction. The inventors aimed to provide a new β-type silicon nitride powder in which the particle size in the ab-plane direction relative to the c-axis direction is relatively large for β-type silicon nitride crystal grains, in order to effectively control such anisotropy of thermal conductivity and enable the design of products with various properties.

[0007] The present invention aims to solve the above problems by providing a β-type silicon nitride powder having a particle shape in which the particle diameter in the ab-plane direction is larger than that in the c-axis direction, a resin composition containing the β-type silicon nitride powder, a thermally conductive paste containing the β-type silicon nitride powder, and a method for producing the β-type silicon nitride powder. [Means for solving the problem]

[0008] (Composition 1) One embodiment of the present invention is a β-type silicon nitride powder having a β-fraction of 80% or more, The present invention is characterized by having a ratio, Iβ(101) / Iβ(210), which is the ratio of the X-ray diffraction peak intensity Iβ(101) of the (101) plane of β-Si3N4 obtained from the X-ray diffraction pattern of β-type silicon nitride powder to the X-ray diffraction peak intensity Iβ(210) of the (210) plane of β-Si3N4, of 1.1 or greater.

[0009] (Configuration 2) A further embodiment of the present invention, the β-type silicon nitride powder, is more preferably characterized in that, in the β-type silicon nitride powder of configuration 1, the average aspect ratio W / L, where L is the length in the c-axis direction of the primary particle and W is the length in the ab-plane direction, is 2.5 or more.

[0010] (Composition 3) One embodiment of the present invention is a β-type silicon nitride powder having a β-fraction of 80% or more, The invention is characterized by having an average aspect ratio W / L of 2.5 or greater, where L is the length in the c-axis direction of the primary particle and W is the length in the ab-plane direction.

[0011] (Composition 4) A further embodiment of the present invention is a β-type silicon nitride powder, more preferably a β-type silicon nitride powder having any of the three configurations 1 to 3, characterized in that it has a thermal conductivity of 0.1 W / mK or more.

[0012] (Composition 5) A further embodiment of the present invention is a β-type silicon nitride powder, more preferably a β-type silicon nitride powder of any of the four configurations 1 to 4, characterized by containing an element Sr.

[0013] (Composition 6) One embodiment of the present invention is a resin composition characterized by containing any of the β-type silicon nitride powders of components 1 to 5.

[0014] (Composition 7) A further embodiment of the present invention is characterized in that the resin composition of the present invention is more preferably a thermoplastic resin or a thermosetting resin in the resin composition of configuration 6.

[0015] (Composition 8) One embodiment of the present invention is a thermally conductive paste characterized by containing any of the β-type silicon nitride powders of components 1 to 5.

[0016] (Composition 9) A manufacturing method according to one embodiment of the present invention is a method for manufacturing β-type silicon nitride powder having a β fraction of 80% or more, comprising: a step of preparing a mixed powder including a raw material containing a predetermined amount of SiO2 powder and a predetermined amount of Sr compound powder, and a predetermined amount of carbon powder; a step of nitriding the mixed powder by a reduction nitridation reaction in a nitrogen atmosphere; and is characterized by including the above steps.

[0017] (Configuration 10) A manufacturing method according to a further embodiment of the present invention is, more preferably, the method according to Configuration 9, wherein the Sr compound is a halogen compound.

[0018] (Configuration 11) A manufacturing method according to a further embodiment of the present invention is, more preferably, the method according to Configuration 9 or 10, wherein the addition amount of the Sr compound is 5 to 10 mol% when the SiO2 powder is 100 mol%.

[0019] (Configuration 12) A manufacturing method according to a further embodiment of the present invention is, more preferably, the method according to any one of Configurations 9 to 11, wherein the mol ratio (C / SiO2) of the carbon powder to the SiO2 powder is 2 to 3.

[0020] (Configuration 13) A manufacturing method according to a further embodiment of the present invention is, more preferably, the method according to any one of Configurations 9 to 12, wherein the reduction nitridation reaction is carried out in a temperature environment of 1450 to 1600 °C.

[0021] (Configuration 14) A manufacturing method according to a further embodiment of the present invention is, more preferably, the method according to any one of Configurations 9 to 13, wherein the reduction nitridation reaction is carried out in a nitrogen atmosphere of 2 atm or more.

Advantages of the Invention

[0022] The β-type silicon nitride powder of the present invention has a particle shape in which the particle diameter in the ab-plane direction is larger than that in the c-axis direction, due to having an Iβ(101) / Iβ(210) ratio of 1.1 or higher and / or an average aspect ratio W / L of 2.5 or higher, and exhibits a particle shape suitable for use as a filler in heat dissipation applications. [Brief explanation of the drawing]

[0023] [Figure 1] The X-ray diffraction patterns of silicon nitride powders from Examples 1-4 and Comparative Examples 1-3 are shown. [Figure 2] SEM image of silicon nitride powder from Example 1. [Figure 3] SEM image of silicon nitride powder from Example 2. [Figure 4] SEM image of silicon nitride powder from Example 3. [Figure 5] SEM image of silicon nitride powder from Example 4. [Figure 6] SEM image of silicon nitride powder of Comparative Example 1. [Figure 7] SEM image of silicon nitride powder of Comparative Example 2. [Figure 8] SEM image of silicon nitride powder from Comparative Example 3. [Figure 9] This example shows how to measure the lengths W and L of a primary particle selected from a planar projection (SEM image) of a β-Si3N4 crystal grain, where the length W in the width direction of the base plane (in the ab-plane) and the length L of the prism plane connecting the base planes (in the c-axis direction) can be determined. [Modes for carrying out the invention]

[0024] The β-type silicon nitride powder of one embodiment of the present invention is an aggregate of silicon nitride particles, with a β-fraction of 80% or more. That is, the silicon nitride powder contains 80% or more β-type silicon nitride particles, preferably 98% or more, among α-type silicon nitride (α-Si3N4) particles and β-type silicon nitride (β-Si3N4) particles. By containing 80% or more β-type silicon nitride particles, the silicon nitride powder can have significantly higher thermal conductivity compared to α-type silicon nitride powder in which α-type silicon nitride particles are dominant. Furthermore, the β-type silicon nitride powder of the present invention can be used as a heat dissipation (or thermal conductivity) filler to improve heat dissipation by being dispersed in resin compositions (e.g., substrates, sheets, spacers, etc.) or paste-like electronic materials such as greases, adhesives, and paints, although this is not limited to the present invention. The β-type silicon nitride powder of the present invention may be used as a main filler on its own, or as a sub-filler added to a main filler.

[0025] The β-type silicon nitride powder of this embodiment is characterized in that the ratio Iβ(101) / Iβ(210), which is the ratio of the X-ray diffraction peak intensity Iβ(101) of the (101) plane of β-Si3N4 obtained from the X-ray diffraction pattern of the β-type silicon nitride powder to the X-ray diffraction peak intensity Iβ(210) of the (210) plane of β-Si3N4, is 1.1 or higher. Figure 1 shows the X-ray diffraction pattern of the β-type silicon nitride powder of this embodiment. Powder X-ray diffraction measurements were performed on a measurement surface that had been smoothed by compressing its surface with a glass slide.

[0026] Generally, according to "01-078-2963" of the "2022 International Centre for Diffraction Data," the 2θ, Miller index (hkl), and diffraction peak intensity I of β-Si3N4 powder in X-ray diffraction using Cu-Kα rays are given by Table 1 below. Note that β-Si3N4 powder has a random orientation. [Table 1] According to Table 1, the intensity ratio Iβ(101) / Iβ(210) of the X-ray diffraction peak intensity Iβ(101) of the (101) plane and the X-ray diffraction peak intensity Iβ(210) of the (210) plane of β-Si3N4 is 1.03 (=999 / 970). In other words, the intensity ratio Iβ(101) / Iβ(210) is approximately 1 in random orientation states.

[0027] In this embodiment, the relative size difference between the particle diameter in the c-axis direction and the particle diameter in the ab-plane direction (perpendicular to the c-axis) of β-Si3N4 was statistically evaluated by comparing the X-ray diffraction peak intensity Iβ(101) of the (101) plane and the X-ray diffraction peak intensity Iβ(210) of the (210) plane. On the measurement surface, which was compressed and smoothed, the larger Iβ(101) value, the greater the proportion of particles whose ab-plane direction is parallel to the measurement surface, while the larger Iβ(210) value, the greater the proportion of particles whose c-axis direction is parallel to the measurement surface. Generally, particles tend to orient themselves so that the larger particle diameter (long axis direction) is parallel to the measurement surface when compressed and smoothed. When Iβ(101) / Iβ(210) is approximately 1, the ab-plane direction and c-axis direction of the particles are randomly oriented, and theoretically, there is no difference in particle diameter in the ab-plane direction and c-axis direction. Furthermore, when the intensity ratio Iβ(10¹) / Iβ(2¹⁰) is significantly greater than 1, particles whose ab-plane direction is parallel to the measurement surface are dominant. From this, it can be concluded that the particle diameter in the ab-plane direction of the β-type silicon nitride crystal grains is larger than the particle diameter in the c-axis direction. In the β-type silicon nitride powder of this embodiment, Iβ(10¹) / Iβ(2¹⁰) is between 1.1 and 1.5, which quantitatively indicates that the particle diameter in the ab-plane direction of the particle shape is significantly larger than the particle diameter in the c-axis direction.

[0028] Furthermore, the β-type silicon nitride powder of this embodiment is characterized in that the average aspect ratio W / L, where L is the length in the c-axis direction of the primary particle and W is the length in the ab-plane direction, is 2.5 or greater. The aspect ratio is an index indicating the shape of the primary particle. The average aspect ratio of the primary particle is obtained by analyzing a planar projection image obtained by SEM. The average aspect ratio is the average value of the aspect ratios of 50 or more particles. If this average aspect ratio is greater than 1, it means that for many particles, the length W in the ab-plane direction of the primary particle is greater than the length L in the c-axis direction. In the β-type silicon nitride powder of this embodiment, preferably, the average length W of 50 or more particles is 1.8 to 3.0 μm, the average length L of 50 or more particles is 0.6 to 1.2 μm, and the average aspect ratio W / L is in the range of 2.5 to 4.0. More preferably, the average aspect ratio W / L is 2.6 to 3.8. This quantitatively demonstrates that the particle diameter in the ab-plane direction is significantly larger than the particle diameter in the c-axis direction. In the SEM image, particles in which both the base plane and the prism plane could be observed were measured. The width W of the base plane, whose planar projection of the β-Si3N4 crystal grain is hexagonal, and the length L of the prism plane connecting the base planes were measured. Figure 9 shows an example of the measurement method for W and L in each primary particle.

[0029] Figures 2 to 5 are actual SEM planar projection images illustrating the β-type silicon nitride powder of the present invention. As shown in Figures 2 to 5, the primary particles of the β-type silicon nitride powder of the present invention form hexagonal prisms with the c-axis direction being relatively shorter than the ab-plane direction. As shown in Figure 6, β-type silicon nitride powder produced by a conventional method has a needle-like hexagonal prism shape that is relatively long in the c-axis direction, whereas the β-type silicon nitride powder of the present invention has a hexagonal flat plate shape. In other words, it can be observed from the SEM images that in the β-type silicon nitride powder of the present invention, the particle diameter in the ab-plane direction is significantly larger than the particle diameter in the c-axis direction. Furthermore, since the β-type silicon nitride powder of the present invention is produced without a grinding process after synthesis, it can be observed that the separation of each particle is good and the formation of aggregates is significantly suppressed.

[0030] Furthermore, the β-type silicon nitride powder of this embodiment preferably has a thermal conductivity of 0.1 W / mK or higher. The β-type silicon nitride powder of the present invention may have a thermal conductivity equivalent to or higher than that of conventional β-type silicon nitride powder produced by adding Mg compounds or Ca compounds. The thermal conductivity of the β-type silicon nitride powder of the present invention is more preferably 0.12 to 0.13 W / mK or higher.

[0031] Furthermore, the β-type silicon nitride powder of this embodiment is characterized by containing Sr element. In the manufacturing process of the β-type silicon nitride powder of this embodiment, an appropriate amount of Sr compound is used as an additive instead of conventional Mg compounds or Ca compounds. Therefore, the β-type silicon nitride powder contains a predetermined amount (or a detectable amount) of Sr element. The β-type silicon nitride powder of this embodiment preferably contains 0.5% by weight or less of Sr. More preferably, the Sr content is 0.05 to 0.5% by weight.

[0032] In other words, the β-type silicon nitride powder of this embodiment is an aggregate of particles in which the particle diameter in the ab-plane direction is (statistically) larger than that in the c-axis direction. Furthermore, since the β-type silicon nitride powder of this embodiment has a thermal conductivity equal to or greater than that of conventional β-type silicon nitride powder, it can have performance equal to or greater than that of conventional heat dissipation fillers. Therefore, in a composite product (resin composition) of a silicon nitride filler and resin, in which β-type silicon nitride powder is embedded in a resin, the anisotropy of the thermal conductivity in the thickness direction can be effectively controlled by appropriately adjusting the amount of β-type silicon nitride powder added, making it possible to design products with various properties.

[0033] Furthermore, the β-type silicon nitride powder of this embodiment provides a resin composition having heat dissipation properties when filled, kneaded, and dispersed in a thermoplastic resin or a thermosetting resin. For example, a resin composition can be obtained by filling, kneading, and dispersing the maximum filling amount of silicon nitride powder in a molten resin, and then curing the molten resin. Here, the resin composition is not particularly limited, but can be selected from the group consisting of, for example, thermoplastic resins such as polyethylene and polycarbonate, thermosetting resins such as phenolic resins and epoxy resins, silicone resins, rubber, etc. Similarly, the β-type silicon nitride powder of this embodiment provides a thermally conductive paste with excellent heat dissipation properties when filled, kneaded, and dispersed in a fluid such as grease, adhesive, or paint. Only the β-type silicon nitride powder of this embodiment may be included in the resin composition or thermally conductive paste. Alternatively, the β-type silicon nitride powder of this embodiment and conventional β-type silicon nitride powder may be included in a predetermined ratio (i.e., in a main filler and sub-filler relationship) in the resin composition or thermally conductive paste.

[0034] Next, a method for producing the β-type silicon nitride powder of this embodiment will be described. The production method of this embodiment includes (1) a step of preparing a mixed powder containing a predetermined amount of SiO2 powder and a predetermined amount of Sr compound powder as raw materials and a predetermined amount of carbon powder; (2) a step of nitriding the mixed powder by a reductive nitridation reaction in a nitrogen atmosphere; and (3) a step of removing the carbon powder from the reductively nitrided mixed powder.

[0035] First, SiO2 powder and Sr compound powder are prepared as raw materials. The SiO2 powder used as the base material preferably contains high-purity fine powder with few metal impurities. The SiO2 powder may also be prepared in the form of a sol (colloidal solution) in which particles are dispersed in a solvent. The Sr compound is preferably SrF2, which is a halogen compound. The raw materials are then prepared so that the amount of Sr compound added is 5 to 10 mol% in molar ratio, with the SiO2 powder being 100 mol%.

[0036] Prepare an appropriate amount of carbon powder. The amount of carbon powder is preferably 2 to 3 times the amount of SiO2 powder in molar ratio (C / SiO2). More preferably, the amount of carbon powder is 2.4 to 2.7 times the amount of SiO2 powder in molar ratio. As the carbon powder, fine particles mainly composed of carbon, such as furnace black or acetylene black, can be used. Furthermore, it is preferable to use carbon with an average particle size of 10 to 50 nm and an ash content of 0.1% or less.

[0037] Next, SiO2 powder, Sr compound powder, and carbon powder are wet-mixed using a planetary mill to prepare a powder slurry. When preparing the slurry, a solution is prepared in which a predetermined amount of dispersant is dissolved in a solvent. Preferably, the solution is an aqueous solution obtained by dissolving a small amount of dispersant in an appropriate amount of pure water. Preferably, the dispersant is a polymeric dispersant selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, and hydroxypropyl cellulose. Preferably, the dispersant is added to the mixture of raw materials and carbon powder in an amount of 0.5 to 1% by weight relative to the raw materials. Wet mixing is performed using a vibrating mill or a planetary mill. After drying the prepared slurry in a constant temperature bath at 120°C, the mixture is passed through a sieve (e.g., with a mesh size of 100 μm) to prepare a mixed powder for synthesis.

[0038] Next, the mixed powder is packed into a carbon sheath of predetermined dimensions. The carbon sheath is placed inside the furnace so that the external atmosphere is introduced into the sheath and the CO gas generated inside the sheath during reductive nitriding is exhausted to the outside.

[0039] Next, the carbon pods filled with the mixed powder are subjected to reductive nitriding treatment in a pressurized nitrogen atmosphere furnace with a carbon heater. The reductive nitriding reaction is preferably carried out at a temperature of 1450 to 1600°C. Furthermore, the reductive nitriding reaction is preferably carried out in a nitrogen atmosphere of 2 atmospheres (0.2 MPa) or higher. The processing time for this reductive nitriding synthesis is preferably 1 to 10 hours. Note that if the furnace pressure during the reductive nitriding treatment is low, SiC is more likely to be generated during the reductive nitriding reaction, so a nitrogen pressure of 2 atmospheres or higher is preferable. Also, if the reductive nitriding treatment temperature is low or the time is short, there is a risk that unreacted SiO2 will remain. On the other hand, if the reductive nitriding temperature is high, the particle growth of silicon nitride particles after reductive nitriding will progress too much, and the columnar shape of the particles will become pronounced, so it is necessary to appropriately adjust the processing temperature and processing time during reductive nitriding.

[0040] The powder after reduction nitriding treatment contains excess carbon powder, so it is heated in dry air at a temperature of 600°C to 700°C to remove the remaining carbon. The heating time varies depending on conditions such as heating temperature, gas flow rate, and powder amount, so it is preferable to adjust it as needed while checking the weight loss. If the heating time is too long, the oxygen content of the silicon nitride powder will increase, so a heating time of about 2 to 12 hours is preferable.

[0041] Through the above steps, the β-type silicon nitride powder of this embodiment can be obtained. The obtained β-type silicon nitride powder can be confirmed by crystal phase analysis by powder X-ray diffraction and SEM image analysis.

[0042] Here, we will explain the role of additives in reductive nitriding. In the reductive nitriding method for silicon nitride powder, it is necessary to mix at least three types of powders: carbon powder, silicon dioxide powder, and additive powder. Conventionally, magnesium compounds (MgF2, MgCO3, etc.) and calcium compounds (CaF2, CaCO3, etc.) have been commonly selected as additives, but the present invention is characterized by the selection of a strontium compound (SrF2) as the additive. Generally, additives have the effect of forming a liquid phase during the synthesis of silicon nitride (Si3N4) powder and promoting the phase transition from α-Si3N4 to β-Si3N4. The generated liquid phase exists between the silicon nitride particles, and after synthesis, it causes the powders to fuse together. However, additives with high vapor pressure gradually decrease during synthesis and are removed from between the particles after synthesis. Among alkaline earth additives, Mg and Ca have the effect of elongating β-Si3N4 crystals in the c-axis direction, while Sr has a weaker effect, resulting in β-Si3N4 crystals with a larger diameter in the ab-plane. The detailed mechanism of such particle growth is not yet clear, but it is presumed that this is because Sr, which has a low ionic electric field strength among alkaline earth elements, has weak adsorption of β-Si3N4 to the prism surface. Similarly, although Ba also has a low ionic electric field strength, it is thought that Ba has a higher vapor pressure than Sr, which causes the additive to volatilize during synthesis, thus preventing grain growth in the ab-plane direction of the crystal.

[0043] The manufacturing method of the present invention makes it possible to obtain β-type silicon nitride powder containing crystalline particles with a larger particle diameter in the ab-plane direction than in the c-axis direction by controlling the amount of Sr compound added as an additive to an appropriate amount, thereby promoting grain growth in the ab-plane direction of the crystal.

[0044] Furthermore, the method for producing β-type silicon nitride powder of the present invention is characterized by not including the pulverization process of silicon nitride aggregates after nitriding, as is done in the conventional method. For example, in the conventional method for producing silicon nitride powder, such as that described in Patent Document 1, silicon nitride aggregates are generated after nitriding, and silicon nitride powder is produced by pulverizing these aggregates. Such pulverization of aggregates increases the amount of coarse aggregates and irregularly shaped particles in the silicon nitride powder, resulting in a significant decrease in the fluidity of the silicon nitride powder. This decrease in the fluidity of the silicon nitride powder increases the viscosity of the fluid into which the silicon nitride powder is mixed, limiting the amount of filler that can be packed into the fluid and its kneadability. Such limitations in packing and / or kneadability hinder the improvement of the heat dissipation performance of the resin composition into which the silicon nitride powder is kneaded. The production method of the present invention, by considering additives, makes it possible to synthesize fine powder with uniform particle size directly without going through a pulverization process after synthesis. [Examples]

[0045] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0046] The β-type silicon nitride powders used in Examples 1-4 and Comparative Examples 1-3 were prepared under the following conditions and procedures.

[0047] Pure water containing 0-1.0% by weight of PVP (polyvinylpyrrolidone K30, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) dissolved in the mixed powder was weighed into a resin container. SiO2 powder, an appropriate amount of additive powder (Sr compound, Mg compound, Ca compound, or Ba compound), and carbon powder in a molar ratio of approximately 2.4-2.7 times the amount of SiO2 were then added. High-purity SiO2 powder with an average particle size of 0.4 μm was used. A sol in which SiO2 powder (for example, with an average particle size of 0.04 μm) was dispersed in a solvent may also be used. The raw materials were then wet-mixed using the resin container and silicon nitride pebbles. Wet mixing was performed using a vibrating mill or planetary mill to produce a mixed slurry. After drying the mixed slurry in a constant-temperature bath at 120°C, it was passed through a sieve (mesh size 100 μm) to produce a mixed powder for synthesis. The mixed powder was packed into a carbon sheath and placed in a furnace. The carbon sheath was placed inside the furnace with a gap of approximately 10 mm between the frame and the lid, allowing the external atmosphere to enter the sheath. After evacuating the furnace before heating, the nitrogen pressure inside the furnace was increased to 0.8 MPa, and reductive nitriding was performed at 1500°C or 1600°C for approximately 10 hours. Then, the silicon nitride powders of Examples 1-4 and Comparative Examples 1-3 were prepared by heating in dry air at 700°C for 3 hours until the weight of the carbon became 0, thereby removing the remaining carbon.

[0048] Powder X-ray diffraction measurements were performed on each of the prepared samples from Examples 1-4 and Comparative Examples 1-3. The synthesis of silicon nitride powder was confirmed by referring to the peak positions of the X-ray diffraction patterns. By analyzing the X-ray diffraction patterns, the β fraction, which indicates the mass fraction of β-type silicon nitride in the synthesized powder, was derived. In each sample, the silicon nitride powder showed a β fraction of 98.1-100%, confirming that more than 80% were β-type silicon nitride (β-Si3N4) particles with high thermal conductivity. Furthermore, SEM images (magnification 5000x, 40000x for Comparative Example 3 only) were acquired to analyze the particle shape and cross-sectional shape of the silicon nitride powder. The shape characteristics of the primary particles were evaluated by performing image analysis of the obtained SEM images using a computer.

[0049] The various measurements and analyses were performed under the following conditions.

[0050] (1) Powder X-ray diffraction measurement The decarburized synthesized powder was passed through a 30 μm sieve to remove aggregates, and the resulting powder was prepared for measurement. The sieved powder was packed into a 2 mm deep aluminum sample holder and compressed with a glass slide to smooth the sample surface. At this time, the sample surface was packed uniformly so that it aligned with the reference surface, and the edge of the glass slide was used to slide over the sample surface. Using this sample surface as the measurement target, the X-ray diffraction intensity of each sample was measured using the Cu-Kα powder X-ray diffraction method with a Rigaku Ultima IV powder X-ray diffractometer.

[0051] (2) Calculation of the beta fraction Based on the X-ray diffraction patterns obtained by powder X-ray diffraction measurements, the mass fractions of the crystalline phases were measured using the integrated intensities of the diffraction patterns of the Si phase, α-type silicon nitride phase, and β-type silicon nitride phase, according to the known Jovanovic and Kimura method, expressed by the following three equations.

number

[0052] (3) Calculation of Iβ(10¹) / Iβ(2¹⁰) From the X-ray diffraction pattern obtained by powder X-ray diffraction measurement, the diffraction peak intensities Iβ(211) of the (210) plane and Iβ(101) plane of β-Si3N4 were measured as integrated intensities using analysis software, and the peak intensity ratio Iβ(101) / Iβ(210) was calculated.

[0053] (4) Observation and analysis of powder shape using SEM The shape of the powder was observed using a scanning electron microscope (JSM-IT700HR) manufactured by JEOL Ltd. A silicon nitride powder sample was dispersed on carbon tape using a dry powder disperser (AmbiValue PD-10). A Pt sputter treatment was applied to the powder surface as a conductive coating film, and this was used as the observation sample. The acceleration voltage was set to 5kV, and SE images were acquired at an observation magnification of 5000x (or 40000x). The acquired SE images were imported into WinROOF2021 (Ver5.7.4) from Mitani Corporation, and computer-based image analysis was performed. Specifically, the acquired images were processed using filtering, noise reduction, and binarization to capture the shape characteristics of the primary particles, and the length (particle diameter) W in the ab-plane direction and the length (particle diameter) L in the c-axis direction of the primary particles were measured. For 50 or more primary particles where both the length W in the ab-plane direction and the length L in the c-axis direction could be determined, W and L were measured (see Figure 9), and their average values ​​were calculated. The average aspect ratio (W / L) was determined using the calculated average values.

[0054] (5) Measurement of thermal conductivity The thermal conductivity of the paste was measured using a thermal conductivity measuring device (TRIDENT, C-Therm Technologies Ltd.). Specifically, 1.8 cc of the synthetic powder, after being passed through a 30 μm sieve, was placed in a Modified Transient Plane Source sensor, and five measurements were taken for each sample while a load of 2 kgf was applied. The average value was used as the measurement result. The thermal conductivity was measured at room temperature (25°C).

[0055] Table 2 shows the conditions and various measurement results for each sample in Examples 1-4 and Comparative Examples 1-3. Figure 1 shows the X-ray diffraction patterns for each sample in Examples 1-4 and Comparative Examples 1-3. Figures 2-5 show the SE images of Examples 1-4 at an observation magnification of 5000x. Figures 6-8 show the SE images of Comparative Examples 1-3 at an observation magnification of 5000x (40000x for Comparative Example 3 only).

[0056] [Table 2]

[0057] Examples 1-4 are samples prepared from a mixed powder consisting of SiO2 powder, Sr compound (SrF2), and carbon powder. In contrast, Comparative Example 1 is a sample prepared from a mixed powder consisting of SiO2 powder, Mg compound (MgF2), and carbon powder. Comparative Example 2 is a sample prepared from a mixed powder consisting of SiO2 powder, Ca compound (CaF2), and carbon powder. Comparative Example 3 is a sample prepared from a mixed powder consisting of SiO2 powder, Ba compound (BaF2), and carbon powder.

[0058] According to the SEM images in Figures 1 to 4, the SEM images obtained for Examples 1 to 4 showed good separation of individual particles and significantly suppressed aggregate formation. Furthermore, while Examples 1 to 4 had a flattened hexagonal plate shape with the c-axis direction being relatively shorter than the ab-plane direction, Comparative Examples 1 and 2, which used conventional additives (Mg compound, Ca compound), were observed to have a hexagonal prism shape with at least the c-axis direction being relatively longer than the ab-plane direction. On the other hand, Comparative Example 3, which used a Ba compound as an additive, showed a particle shape that was smaller in size, had a smaller aspect ratio, and was closer to a spherical shape.

[0059] Table 2 quantitatively shows the characteristics of each sample.

[0060] According to Table 2, in Examples 1-4, the Iβ(101) / Iβ(210) ratio is 1.11-1.53. In contrast, in Comparative Example 1, which used a Mg compound as an additive, the Iβ(101) / Iβ(210) ratio is 0.84, indicating that particles with smaller particle diameters in the ab-plane direction than in the c-axis direction are statistically dominant. In Comparative Example 2, which used a Ca compound as an additive, the Iβ(101) / Iβ(210) ratio is 0.88, and similar to Comparative Example 1, particles with smaller particle diameters in the ab-plane direction than in the c-axis direction are statistically dominant. In Comparative Example 3, which used a Ba compound as an additive, the Iβ(101) / Iβ(210) ratio is 1.05, which is close to random orientation (Iβ(101) / Iβ(210)=1). In other words, in Examples 1-4, Iβ(101) / Iβ(210) was greater than 1 (random orientation) and significantly greater than in Comparative Examples 1-3. This result, similar to the observation results from SEM images, quantitatively indicates that in the β-type silicon nitride powder of the present invention, particles with a larger particle size in the ab-plane direction than in the c-axis direction are statistically dominant.

[0061] Furthermore, in Examples 1 to 4, the average length W of the primary particles in the ab-plane direction was 1.88 to 2.96 μm, the average length L in the c-axis direction was 0.60 to 1.11 μm, and the W / L ratio was 2.6 to 3.8 (>1). In contrast, in Comparative Example 1, which used a Mg compound as an additive, the average length W of the primary particles in the ab-plane direction was 1.02 μm, the average length L in the c-axis direction was 2.37 μm, and the W / L ratio was 0.4 (<1). In Comparative Example 2, which used a Ca compound as an additive, the average length W of the primary particles in the ab-plane direction was 1.11 μm, the average length L in the c-axis direction was 1.28 μm, and the W / L ratio was 0.9 (<1). In Comparative Example 3, which used a Ba compound as an additive, the average length W of the primary particles in the ab-plane direction was 0.60 μm, the average length L in the c-axis direction was 0.54 μm, and the W / L ratio was 1.1 (>1). This result, similar to the observation results from SEM images, quantitatively indicates that the β-type silicon nitride powder of the present invention has a particle shape in which the primary particles have a larger particle diameter in the ab-plane direction than in the c-axis direction.

[0062] Furthermore, the thermal conductivity in Examples 1-4 is 0.124-0.129 W / mK. In contrast, Comparative Example 1, which used a Mg compound as an additive, has a thermal conductivity of 0.110 W / mK. Comparative Example 2, which used a Ca compound as an additive, has a thermal conductivity of 0.098 W / mK. Comparative Example 3, which used a Ba compound as an additive, has a thermal conductivity of 0.084 W / mK. In other words, the thermal conductivity of Examples 1-4 is significantly higher than that of Comparative Examples 1-3. This result indicates that the β-type silicon nitride powder of the present invention has improved thermal conductivity compared to conventional materials.

[0063] Therefore, it has been demonstrated that the present invention yields a β-type silicon nitride powder containing crystalline particles with a larger particle size in the ab-plane direction than in the c-axis direction, while having a thermal conductivity equal to or better than that of the conventional powder.

[0064] The present invention is not limited to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

Claims

1. A β-type silicon nitride powder having a β fraction of 80% or more and containing Sr element, β-Si obtained from the X-ray diffraction pattern of β-type silicon nitride powder ３ N ４ The X-ray diffraction peak intensity Iβ(101) of the (101) plane and β-Si ３ N ４ A β-type silicon nitride powder characterized in that the ratio Iβ(101) / Iβ(210), which is the ratio of the X-ray diffraction peak intensity Iβ(210) of the (210) plane to Iβ(210), is 1.1 or greater.

2. The β-type silicon nitride powder according to claim 1, characterized in that the average aspect ratio W / L of the primary particle, where L is the length in the c-axis direction and W is the length in the ab-plane direction, is 2.5 or more.

3. A β-type silicon nitride powder having a β fraction of 80% or more and containing Sr element, A β-type silicon nitride powder characterized in that the average aspect ratio W / L of the primary particle, where L is the length in the c-axis direction and W is the length in the ab-plane direction, is 2.5 or greater.

4. The β-type silicon nitride powder according to any one of claims 1 to 3, characterized in that its thermal conductivity is 0.12 W / mK or higher.

5. A resin composition characterized by containing the β-type silicon nitride powder described in any one of claims 1 to 3.

6. The resin composition according to claim 5, characterized in that it is a thermoplastic resin or a thermosetting resin.

7. A thermally conductive paste characterized by containing the β-type silicon nitride powder described in any one of claims 1 to 3.

8. A method for producing β-type silicon nitride powder having a β fraction of 80% or more, SiO ２ A step of preparing a mixed powder comprising a raw material containing powder and an Sr compound powder in which the amount of Sr compound added is 5 to 10 mol% when the SiO2 powder is considered to be 100 mol%, and carbon powder in which the mol ratio (C / SiO2) is 2 to 3 times that of the SiO2 powder, A step of nitriding the mixed powder by a reduction-nitridation reaction in a nitrogen atmosphere, A method characterized by including the following.

9. The method according to 8, characterized in that the Sr compound is a halogen compound.

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