Boron nitride powder, and, method for producing boron nitride powder

Abstract

To provide boron nitride powder capable of preparing a resin sheet excellent in packing property into a resin and capable of exhibiting excellent heat dissipation when packed in a resin.SOLUTION: One aspect of the present disclosure provides boron nitride powder comprising aggregated particles in which primary particles of hexagonal boron nitride are aggregated, and primary particles of hexagonal boron nitride, the powder containing a crystalline component having calcium as a constituent element, wherein the powder has a tapped density of 0.80 g / cm3 or more and an orientation index of 50 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to boron nitride powder and methods for making the same. [Background technology]

[0002] In electronic components such as power devices, transistors, thyristors, and CPUs, efficient heat dissipation is a challenge. Therefore, heat dissipation materials with high thermal conductivity are used together with these electronic components. Meanwhile, boron nitride particles, with their high thermal conductivity and high insulating properties, are widely used as a filler in heat dissipation materials.

[0003] Hexagonal boron nitride primary particles have a thermal conductivity of 400 W / (m·K) in the in-plane direction (a-axis direction) but only 2 W / (m·K) in the thickness direction (c-axis direction), resulting in a large anisotropy in thermal conductivity due to their crystalline structure and scale shape. Furthermore, when hexagonal boron nitride powder is filled into resin and molded, the force applied during molding causes multiple primary particles to orient in the same direction within the resin molded body. In this case, for example, the in-plane direction (a-axis direction) of the hexagonal boron nitride primary particles is oriented perpendicular to the thickness direction of the heat dissipation component, resulting in low thermal conductivity in the thickness direction of the heat dissipation component and not being able to fully utilize the high thermal conductivity of the hexagonal boron nitride particles in the in-plane direction (a-axis direction).

[0004] From the viewpoint of reducing the anisotropy due to the shape as described above, a method of forming agglomerated particles by suppressing the orientation of a plurality of primary particles and aggregating them has been investigated. Patent Document 1 discloses boron nitride agglomerated particles formed by aggregating boron nitride primary particles, and describes that by increasing the strength of the agglomerated particles to a degree that prevents the agglomerated particles from collapsing even when a predetermined molding pressure is applied, the boron nitride primary particles are prevented from being oriented in the same direction.

[0005] Patent Document 2 discloses a method for producing boron nitride powder, which includes a step of mixing boron nitride powder A, which is formed by agglomeration of boron nitride primary particles and has a mode diameter in the range of 5 μm or more and less than 30 μm in a volumetric particle size distribution, with boron nitride powder B, which is formed by agglomeration of boron nitride primary particles and has a mode diameter in the range of 50 μm or more and less than 100 μm in a volumetric particle size distribution. Although the boron nitride powder obtained by this production method is excellent in terms of voltage resistance, there is room for improvement in applications requiring higher insulation properties. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-135731 [Patent Document 2] Japanese Patent Publication No. 2020-164365 Summary of the Invention [Problem to be solved by the invention]

[0007] The present disclosure aims to provide a boron nitride powder that has excellent fillability in resin and can be used to prepare a resin sheet that exhibits excellent heat dissipation when filled into resin, and a method for producing the same. The present disclosure also aims to provide a heat dissipation sheet containing the above-mentioned boron nitride powder. [Means for solving the problem]

[0008] The present disclosure provides the following [1] to [7].

[0009] [1] A powder containing agglomerated particles in which a plurality of primary particles of hexagonal boron nitride are aggregated, and primary particles of hexagonal boron nitride, Contains a crystalline component having calcium as a constituent element, The powder has a tap density of 0.80 g / cm 3 or more, and an orientation index of 50 or less. [2] The boron nitride powder according to [1], wherein the orientation index is 10 or more. [3] The boron nitride powder according to [1] or [2], wherein the content of boron nitride in the powder is 80 mass % or more. [4] The boron nitride powder according to any one of [1] to [3], wherein the average particle size of the primary particles is 9 to 20 μm. [5] A heat-dissipating sheet comprising a resin and a heat-dissipating filler, A heat-dissipating sheet, wherein the heat-dissipating filler comprises the boron nitride powder according to any one of [1] to [4]. [6] The heat-dissipating sheet according to [5], wherein the heat-dissipating filler content is 15 to 70% by volume. [7] The method includes a firing step of firing a raw material powder containing at least one of carbon black and acetylene black, a boron-containing compound, and calcium carbonate under a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles in which a plurality of primary particles of hexagonal boron nitride are agglomerated, the blending amount of the calcium carbonate is 4 to 20 parts by mass relative to 100 parts by mass of the boron-containing compound, The method for producing boron nitride powder, wherein the boron-containing compound contains at least one of boric acid and boron oxide. [8] The method according to [7], wherein the amount of calcium carbonate blended is 4 to 20 parts by mass with respect to 100 parts by mass of the boron-containing compound. [Effects of the Invention]

[0010] According to the present disclosure, there are provided a boron nitride powder that has excellent fillability in resin and that can be used to prepare a resin sheet that exhibits excellent heat dissipation when filled into resin, and a method for producing the same. According to the present disclosure, there is also provided a heat dissipation sheet containing the above-mentioned boron nitride powder. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing an example of a heat dissipation sheet. [Figure 2] FIG. 2 is a scanning electron microscope photograph showing the appearance of the boron nitride powder prepared in Example 1. [Figure 3] FIG. 3 is a scanning electron microscope photograph showing the appearance of the boron nitride powder prepared in Comparative Example 1. [Figure 4] FIG. 4 is a scanning electron microscope photograph showing the appearance of the boron nitride powder prepared in Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents.

[0013] Unless otherwise specified, the materials exemplified in this specification can be used singly or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

[0014] One embodiment of the present disclosure relates to agglomerated particles formed by aggregating multiple primary particles of hexagonal boron nitride, and a powder containing the primary particles of hexagonal boron nitride. Here, the term "primary particle" refers to a single particle of hexagonal boron nitride. The term "aggregated particle" refers to a secondary particle formed by aggregating multiple primary particles bound together by a crystalline component containing calcium, such as calcium borate, at least in part. In the agglomerated particle, the multiple primary particles may be aggregated such that the a-axes of the primary particles are oriented in multiple directions, or may be aggregated such that some of the a-axes are parallel to one another.

[0015] The boron nitride powder contains a crystalline component having calcium as a constituent element, which may be at least one selected from the group consisting of calcium metaborate (CaB2O4), calcium pyroborate (Ca2B2O5), and calcium borate (Ca3B2O6).

[0016] The lower limit of the content of the crystalline component having calcium as a constituent element in the boron nitride powder may be, for example, 4% by mass or more, 6% by mass or more, 8% by mass or more, or 10% by mass or more. When the lower limit of the content of the crystalline component having calcium as a constituent element is within the above range, the proportion of agglomerated particles in which primary particles of hexagonal boron nitride are agglomerated via the crystalline component can be further increased, thereby further improving the heat dissipation properties when the resulting boron nitride powder is used as a heat dissipation filler. A high content of the crystalline component can also increase the porosity in the agglomerated particles, preventing excessive increases in crushing strength. The upper limit of the content of the crystalline component having calcium as a constituent element in the boron nitride powder may be, for example, 18% by mass or less, 16% by mass or less, 14% by mass or less, or 12% by mass or less. By ensuring that the upper limit of the content of crystalline components having calcium as a constituent element is within the above range, an excessive increase in porosity in the agglomerated particles can be suppressed, and the tap density can be further improved. In addition, by ensuring a sufficient proportion of hexagonal boron nitride in the boron nitride powder, the filling ability and insulating properties of the boron nitride particles according to the present disclosure can be achieved at a higher level.

[0017] The composition and content of the crystalline component having calcium as a constituent element herein are determined by X-ray diffraction measurement of the boron nitride powder and Rietveld analysis of the obtained X-ray diffraction spectrum. As an X-ray diffraction device, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used. For the Rietveld analysis, for example, software such as "TOPAS" manufactured by BRUKER can be used, which allows qualitative and quantitative analysis.

[0018] The lower limit of the tap density of the boron nitride powder is, for example, 0.80 g / cm 3 More than 0.85g / cm 3 More than 0.86g / cm 3 More than 0.88g / cm 3 or more, or 0.90 g / cm 3 When the lower limit of the tap density is within the above range, the filling ability of the boron nitride powder into the resin can be further improved. The upper limit of the tap density of the boron nitride powder is not particularly limited, but may be, for example, 1.50 g / cm. 3 Below 1.20g / cm 3 Below 1.10g / cm 3 or less than 1.00 g / cm 3 It may be the following:

[0019] The tap density in this specification means a value determined in accordance with the method described in JIS R 1628:1997 "Method for measuring bulk density of fine ceramic powders." 3 The bulk density is measured after tapping under the conditions of a tapping time of 180 seconds, a tapping number of 180 times, and a tap lift of 18 mm, and the obtained value is taken as the tap density. A commercially available device can be used for the measurement, such as "Powder Tester" (trade name) manufactured by Hosokawa Micron Corporation.

[0020] The boron nitride powder contains agglomerated particles formed by agglomeration of multiple hexagonal boron nitride primary particles, and hexagonal boron nitride primary particles, and the content ratio of the agglomerated particles to the non-agglomerated primary particles may be adjusted so that the orientation index is 50 or less. The upper limit of the orientation index of the boron nitride powder may be, for example, 50 or less, 48 ​​or less, 46 or less, 44 or less, or 42 or less. When the upper limit of the orientation index is within the above range, the boron nitride powder has reduced thermal anisotropy due to the shape of the hexagonal boron nitride primary particles, thereby improving the heat dissipation properties when used as a heat dissipation filler. The lower limit of the orientation index of the boron nitride powder may be, for example, 10 or more, 15 or more, 20 or more, 25 or more, 30 or more, or 35 or more. When the lower limit of the orientation index is within the above range, the fillability of the resulting boron nitride powder into resin and the heat dissipation properties of the resulting resin molded product (e.g., a heat dissipation sheet) can be both achieved at a higher level.

[0021] Since the orientation index is measured on boron nitride powder, the orientation index tends to be higher when the powder is composed of primary particles that do not contain aggregated particles (lumped particles) in which the primary particles are substantially not oriented, or when the proportion of such primary particles is high. On the other hand, when aggregated particles are present and their proportion is high, the orientation index tends to approach a value of about 6 to 7. Furthermore, when the average particle size of the primary particles is large or the average thickness of the primary particles is small, the orientation between particles is easier, and the orientation index tends to be higher.

[0022] The orientation index in this specification refers to a value measured according to the following method. An X-ray diffraction spectrum of the boron nitride powder is obtained by performing X-ray diffraction measurement on the boron nitride powder. Then, the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The obtained peak intensities are used to calculate the orientation index [I(002) / I(100)] of the boron nitride powder. An X-ray diffraction device that can be used is, for example, an "ULTIMA-IV" (product name) manufactured by Rigaku Corporation.

[0023] The boron nitride powder has, for example, a tap density of 0.80 g / cm 3 or more, and the orientation index may be 50 or less.

[0024] The lower limit of the boron nitride content in the boron nitride powder may be, for example, 80% by mass or more, 84% by mass or more, 86% by mass or more, 88% by mass or more, or 90% by mass or more. When the lower limit of the boron nitride content is within the above range, the proportion of hexagonal boron nitride in the boron nitride powder can be more sufficiently high, and the filling ability and insulating properties of the boron nitride powder can be both achieved at a higher level. The upper limit of the boron nitride content in the boron nitride powder may be, for example, 98% by mass or less, 97% by mass or less, 96% by mass or less, 95% by mass or less, 94% by mass or less, 93% by mass or less, or 92% by mass or less. When the upper limit of the boron nitride content is within the above range, the proportion of the crystalline component required to form aggregated particles of boron nitride can be more sufficiently high, and the heat dissipation properties of the resulting boron nitride powder when used as a heat dissipation filler can be further improved.

[0025] In this specification, the boron nitride content in the boron nitride powder refers to a value calculated by titration. More specifically, the boron nitride powder is first alkaline decomposed with sodium hydroxide, and ammonia is distilled from the decomposition liquid by steam distillation and collected in an aqueous boric acid solution. This collected liquid is then titrated with a normal sulfuric acid solution. The nitrogen atom (N) content in the boron nitride powder is calculated from the titration results. The boron nitride content in the boron nitride powder is determined from the obtained nitrogen atom content according to the following formula (1), and the boron nitride powder content can be calculated. Note that the formula weight of boron nitride is 24.818 g / mol, and the atomic weight of nitrogen atoms is 14.006 g / mol. Boron nitride content in sample [mass%] = nitrogen atom (N) content [mass%] × 1.772 (1)

[0026] The lower limit of the average particle size of the primary particles in the boron nitride powder may be, for example, 9 μm or more, 10 μm or more, 11 μm or more, or 12 μm or more. When the lower limit of the average particle size of the primary particles is within the above range, the voids between the particles are reduced, thereby increasing the tap density and improving the fillability in resin. The upper limit of the average particle size of the primary particles in the boron nitride powder may be, for example, 20 μm or less, 19 μm or less, 18 μm or less, or 17 μm or less. When the upper limit of the average particle size of the primary particles is within the above range, the aggregate particles composed of the primary particles do not become too large, thereby improving the tap density and improving the fillability in resin. The average particle size of the primary particles in the boron nitride powder may be adjusted within the above range, for example, 9 to 20 μm.

[0027] The term "average particle size" used herein refers to the 50% cumulative diameter (median diameter) in the volume-based cumulative particle size distribution. More specifically, it refers to the particle size (D50) at which the cumulative value reaches 50% in the volume-based cumulative particle size distribution obtained by laser diffraction scattering for a powder. Laser diffraction scattering is measured in accordance with the method described in JIS Z 8825:2013, "Particle Size Analysis - Laser Diffraction and Scattering Method." A laser diffraction scattering particle size distribution analyzer, such as the "LS-13 320" (product name) manufactured by Beckman Coulter, Inc., can be used for the measurement. The measurement is performed after the powder to be measured has been treated with a homogenizer or the like. A homogenizer such as the "VC-505" (product name) manufactured by Ieda Trading Co., Ltd. can be used. The homogenization may be performed, for example, at a frequency of 20 kHz for 2 minutes.

[0028] An example of the boron nitride powder described above is a powder containing agglomerated particles in which a plurality of primary particles of hexagonal boron nitride are aggregated, and primary particles of hexagonal boron nitride, and containing a crystalline component having calcium as a constituent element, and the tap density of the powder is 0.80 g / cm 3or more, and the orientation index of the powder is not more than 50. The boron nitride powder contains agglomerated particles in which primary particles of hexagonal boron nitride are agglomerated together via the predetermined crystalline component, and the orientation index is within the predetermined range, thereby reducing thermal anisotropy based on the shape of the primary particles, and further, the presence of primary particles in addition to agglomerated particles improves tap density and provides excellent fillability in resins.

[0029] The boron nitride powder described above has excellent filling properties for resins and contains agglomerated particles, which also provides excellent heat dissipation, making it suitable for use as a heat dissipation filler. One embodiment of the heat dissipation sheet is a heat dissipation sheet containing a resin and a heat dissipation filler. The heat dissipation filler contains the boron nitride powder described above. The heat dissipation filler may be dispersed in the resin. Figure 1 shows a schematic diagram of an example of a heat dissipation sheet 100.

[0030] The lower limit of the content of the thermally conductive filler in the thermally conductive sheet may be, for example, 15% by volume or more, 30% by volume or more, 40% by volume or more, or 50% by volume or more, based on the total volume of the thermally conductive sheet. Having the lower limit of the content of the thermally conductive filler within the above range can further improve the heat dissipation properties of the thermally conductive sheet. The upper limit of the content of the thermally conductive filler in the thermally conductive sheet may be, for example, 85% by volume or less, 80% by volume or less, 70% by volume or less, or 60% by volume, based on the total volume of the thermally conductive sheet. Having the upper limit of the content of the thermally conductive filler within the above range can further prevent the formation of internal voids during molding of the thermally conductive sheet, and can also prevent a decrease in insulation and mechanical strength. The content of the thermally conductive filler may be adjusted within the above range, for example, 15 to 70% by volume, 30 to 70% by volume, 30 to 60% by volume, or 50 to 70% by volume.

[0031] The content of the thermally conductive filler in the thermally conductive sheet is a value determined by thermogravimetric analysis (TG). In other words, the content is determined by heat-treating the thermally conductive sheet and taking the residue after removing the resin component as the thermally conductive filler content. More specifically, the content of the thermally conductive filler can be calculated from the weight loss rate in thermogravimetric analysis (TG) when the thermally conductive sheet is heated to 800°C at 10°C / min in a nitrogen atmosphere.

[0032] The thermally conductive filler may contain the above-mentioned boron nitride powder or may consist of the above-mentioned boron nitride powder, but may also contain other thermally conductive fillers as long as it does not deviate from the spirit of the present application. Examples of other fillers include alumina, aluminum nitride, silica, and the like. The amount of other fillers may be, for example, 10% by volume or less, 5% by volume or less, or 2% by volume or less, based on the total amount of the thermally conductive filler, in order to more sufficiently suppress the collapse of agglomerated particles due to collisions between the agglomerated particles and other fillers.

[0033] The resin may contain a cured resin or may consist of a cured resin. Examples of the cured resin constituting the resin include silicone resin, epoxy resin, phenol resin, melamine resin, urea resin, polyimide, polyamideimide, polyetherimide, and maleimide-modified resin. From the viewpoint of improving the flexibility of the heat dissipation sheet, the cured resin preferably contains a silicone resin, and more preferably is a silicone resin. The silicone resin may be, for example, a two-component heat-curing liquid silicone resin.

[0034] The lower limit of the cured resin content in the heat dissipation sheet may be, for example, 15% by volume or more, 20% by volume or more, or 30% by volume or more, based on the total volume of the heat dissipation sheet. The upper limit of the cured resin content in the heat dissipation sheet may be, for example, 70% by volume or less, 60% by volume or less, or 50% by volume or less, based on the total volume of the heat dissipation sheet.

[0035] The heat dissipation sheet is prepared, for example, by molding a resin composition containing a boron nitride powder containing agglomerated particles of hexagonal boron nitride primary particles and hexagonal boron nitride primary particles, and a thermosetting resin. The molding process may be, for example, hot-press molding. The resin composition may contain other components, such as a curing agent, as necessary.

[0036] The above-mentioned heat dissipation sheet has excellent heat dissipation properties because it contains the boron nitride powder according to the present disclosure as a filler. The thermal conductivity of the heat dissipation sheet can be, for example, 3.0 W / (m·K) or more, 3.2 W / (m·K) or more, 3.5 W / (m·K) or more, 3.7 W / (m·K) or more, 3.8 W / (m·K) or more, or 3.9 W / (m·K) or more. The thermal conductivity of the heat dissipation sheet can be measured by the method described in the examples.

[0037] One embodiment of a method for producing boron nitride powder includes a firing step of firing a raw material powder containing at least one of carbon black and acetylene black, a boron-containing compound, and calcium carbonate under a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles formed by agglomeration of multiple primary particles of hexagonal boron nitride. The boron-containing compound includes at least one of boric acid and boron oxide. The calcium carbonate is blended in an amount of 4 parts by mass or more per 100 parts by mass of the boron-containing compound.

[0038] The amount of calcium carbonate may be 4 to 20 parts by mass, or 4 to 20 parts by mass, per 100 parts by mass of the boron-containing compound. The lower limit of the amount of calcium carbonate may be, for example, 5 parts by mass or more, 6 parts by mass or more, or 7 parts by mass or more, per 100 parts by mass of the boron-containing compound. When the lower limit of the amount of calcium carbonate is within the above range, a sufficient amount of agglomerated particles, in which primary particles of hexagonal boron nitride are aggregated together via a crystalline phase, is present, thereby reducing thermal anisotropy and further improving heat dissipation. The upper limit of the amount of calcium carbonate may be, for example, 15 parts by mass or less, 12 parts by mass or less, 10 parts by mass or less, or 8 parts by mass or less, per 100 parts by mass of the boron-containing compound. When the upper limit of the amount of calcium carbonate is within the above range, excessive generation of crystalline components such as calcium borate is further suppressed, the boron nitride content is increased, and the resulting boron nitride powder has improved properties such as packing ability and insulating properties.

[0039] The firing step may be carried out in a pressurized environment. The lower limit of the atmospheric pressure in the firing step may be, for example, 0.5 MPaG or more, 0.6 MPaG or more, 0.7 MPaG or more, or 0.8 MPaG or more. By setting the lower limit of the atmospheric pressure within the above range, volatilization of the boron-containing compound is suppressed, and the liquid phase of the boron-containing compound is maintained, thereby further promoting the growth of primary particles of hexagonal boron nitride and suppressing the production of boron carbide as a by-product. The upper limit of the atmospheric pressure in the firing step is not particularly limited, but may be 0.9 MPaG or less in industrial applications. The atmospheric pressure in the firing step may be adjusted within the above range, for example, 0.5 to 0.9 MPaG. In this specification, pressure refers to gauge pressure.

[0040] The upper limit of the firing temperature may be, for example, 2200°C or lower, 2150°C or lower, or 2100°C or lower. By setting the upper limit of the firing temperature within the above range, it is possible to sufficiently suppress the generation of by-products. The lower limit of the firing temperature may be, for example, 1800°C or higher, 1850°C or higher, 1900°C or higher, 1950°C or higher, 2000°C or higher, or 2050°C or higher. By setting the lower limit of the firing temperature within the above range, it is possible to promote the reaction on the carbon-containing compound and further improve the yield of boron nitride obtained. The firing temperature in the firing step may be, for example, 1800 to 2200°C.

[0041] The lower limit of the holding time in the firing step may be, for example, 7 hours or more, or 8 hours or more. By setting the lower limit of the holding time within the above range, a boron nitride powder with a higher tap density can be prepared. The upper limit of the holding time in the firing step is not particularly limited, but from the viewpoint of reducing the production cost of the boron nitride powder, it may be, for example, 20 hours or less, 18 hours or less, 16 hours or less, 14 hours or less, 12 hours or less, 10 hours or less, or 9 hours or less. The holding time in the firing step may be adjusted within the above range, and may be, for example, 7 to 20 hours, or 7 to 10 hours.

[0042] The above-mentioned manufacturing method may include other steps in addition to the firing step. Examples of such other steps include a pulverization step. The pulverization step crushes the fired product obtained in the firing step, and can adjust the orientation index of the boron nitride powder.

[0043] In the crushing step, for example, a crusher such as a Henschel mixer, a pulverizer, or a grinder mill can be used.

[0044] When a Henschel mixer is used, the rotation speed of the crusher may be as follows: The upper limit of the rotation speed of the crusher may be, for example, 1000 rpm or less, 950 rpm or less, or 900 rpm or less. When the upper limit of the rotation speed of the crusher is within the above range, over-crushing of the particles can be prevented. The lower limit of the rotation speed of the crusher may be, for example, 700 rpm or more, or 750 rpm or more. When the lower limit of the rotation speed of the crusher is within the above range, the fired product can be sufficiently crushed and loose agglomerations of the boron nitride primary particles can be released.

[0045] When a pulverizer is used, the rotation speed of the crusher may be as follows: The upper limit of the rotation speed of the crusher may be, for example, 8500 rpm or less, 8000 rpm or less, or 7500 rpm or less. When the upper limit of the rotation speed of the crusher is within the above range, excessive crushing of the particles can be further suppressed. The lower limit of the rotation speed of the crusher may be, for example, 4000 rpm or more, 4500 rpm or more, or 5000 rpm or more. When the lower limit of the rotation speed of the crusher is within the above range, the fired material can be sufficiently crushed, loose agglomerations of boron nitride primary particles can be broken down, and the sieving yield can be further improved.

[0046] The lower limit of the disintegration time in the disintegration step may be, for example, 5 minutes or more, 6 minutes or more, 7 minutes or more, or 8 minutes or more. By setting the lower limit of the disintegration time within the above range, the fired product can be sufficiently disintegrated, loose agglomerations of boron nitride primary particles can be broken down, and the sieving yield can be further improved. The upper limit of the disintegration time in the disintegration step may be, for example, 15 minutes or less, 14 minutes or less, 12 minutes or less, or 10 minutes or less. By setting the upper limit of the disintegration time within the above range, excessive crushing of the particles can be more sufficiently suppressed. The disintegration time may be adjusted within the above range, and may be, for example, 5 to 15 minutes, or 8 to 12 minutes.

[0047] Although several embodiments have been described above, the present disclosure is not limited to the above embodiments. Furthermore, the descriptions of the above embodiments can be applied to each other. [Example]

[0048] The present disclosure will be described in more detail below using examples and comparative examples, but the present disclosure is not limited to the following examples.

[0049] Example 1 A raw material powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 26 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: Li-400), and 7.3 parts by mass of calcium carbonate (manufactured by New Lime Co., Ltd.) using a Henschel mixer. The resulting mixed powder was placed in a dryer at 250°C and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a 100 mm diameter mold of a press molding machine and molded under conditions of a heating temperature of 200°C and a pressing pressure of 30 MPa. The pellets of the raw material powder obtained in this manner were subjected to the subsequent heat treatment.

[0050] First, pellets of the raw material powder were placed in a carbon atmosphere furnace, and the temperature was increased to 1900°C at a rate of 5°C / min in a nitrogen atmosphere pressurized to 0.5 MPaG, and the pellets were heat-treated by holding them at 1900°C for 8 hours to obtain a fired product (firing process).

[0051] The resulting fired product was crushed in a Henschel mixer at a rotation speed of 900 rpm for a crushing time of 10 minutes to prepare boron nitride powder (crushing step). The resulting boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components containing calcium as a constituent element, tap density, orientation index, and purity (boron nitride content). The results are shown in Table 1. For reference, a scanning electron microscope photograph of the appearance of the boron nitride powder obtained in Example 1 is shown in Figure 2. The white areas in Figure 2 are crystalline components of calcium carbonate.

[0052] Example 2 A powder containing boron nitride particles was prepared in the same manner as in Example 1, except that the amount of calcium carbonate in the raw material powder was changed to 15.8 parts by mass. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components containing calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 1.

[0053] Example 3 A powder containing boron nitride particles was prepared in the same manner as in Example 1, except that the amount of calcium carbonate in the raw material powder was changed to 19.9 parts by mass. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components having calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 1.

[0054] Example 4 Boron nitride powder was prepared in the same manner as in Example 1, except that the sintered product obtained in the sintering step was pulverized using a pulverizer (manufactured by Hosokawa Micron Corporation, trade name: AP-1SH) under the following conditions: a rotation speed of 7500 rpm, a screen diameter of φ1 mm. The boron nitride powder obtained was then measured for the presence or absence of agglomerated particles, the proportion of crystalline components containing calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 1.

[0055] (Comparative Example 1) A boron nitride powder was prepared in the same manner as in Example 1, except that calcium carbonate was not used. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components having calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 2. For reference, a scanning electron microscope photograph of the appearance of the boron nitride powder obtained in Comparative Example 1 is shown in Figure 3. Because calcium carbonate was not used in Comparative Example 1, white crystals, which are crystalline components of calcium carbonate, were not observed in Figure 3.

[0056] (Comparative Example 2) A boron nitride powder was prepared in the same manner as in Example 1, except that sodium carbonate (manufactured by Tokuyama Corporation) was used instead of calcium carbonate and the amount of sodium carbonate was 3.3 parts by mass. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components containing calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 2.

[0057] (Comparative Example 3) 100 parts by mass of boric acid (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 82 parts by mass of melamine (manufactured by Fujifilm Wako Pure Chemical Corporation), and 5.4 parts by mass of calcium carbonate (manufactured by Fujifilm Wako Pure Chemical Corporation) were mixed under humidification to obtain a raw material powder. The obtained raw material powder was heat-treated in a tubular furnace at 1000°C under a nitrogen atmosphere at atmospheric pressure (0 MPaG) for 2 hours to obtain a heat-treated product (calcination step). 100 parts by mass of the heat-treated product was heated to 1900°C under a nitrogen atmosphere at atmospheric pressure (0 MPaG) in an electric furnace and fired at 1900°C for 6 hours to obtain a fired product (calcination step). The fired product was acid-treated with 5% by mass of diluted nitric acid, washed with water, suction-filtered, and dried to obtain boron nitride particles. The obtained powder did not exhibit any agglomerated particles, as was the case with the boron nitride powder prepared in Example 1. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components having calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 2.

[0058] Comparative Example 4 Boron carbide powder was loaded into a boron nitride crucible and heated in a resistance heating furnace under a nitrogen gas atmosphere of 0.85 MPaG at 2100°C for 25 hours to obtain boron carbonitride (BCN) powder. The obtained boron carbonitride powder was placed in a muffle furnace and heated in an air atmosphere at 700°C for 5 hours to obtain a heat-treated product. Boric acid, the boron source, was added to 100 parts by mass of the heat-treated product so that the content of boric acid was 40 parts by mass, and the mixture was mixed using a Henschel mixer to obtain a mixture. The mixture was loaded into a boron nitride crucible and decarbonized by heating in a resistance heating furnace to synthesize boron nitride powder having aggregated particles formed by aggregation of primary particles (crystallization process). The crystallization process was carried out under conditions of a nitrogen gas atmosphere at a pressure of 13 kPa, with the temperature increased from room temperature to 2000°C and maintained at 2000°C for 5 hours. The synthesized boron nitride powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with 75 μm mesh, and the powder that passed through the sieve was designated as the boron nitride powder of Comparative Example 4. The obtained boron nitride powder was measured for the presence or absence of agglomerated particles, the proportion of crystalline components containing calcium as a constituent element, tap density, orientation index, and purity. The results are shown in Table 2. For reference, a scanning electron microscope photograph of the appearance of the boron nitride powder obtained in Comparative Example 4 is shown in Figure 4.

[0059] <Evaluation of boron nitride powder as a heat dissipating filler> The boron nitride powders prepared in the examples and comparative examples were evaluated as a heat dissipating filler.

[0060] [Preparation of evaluation sheet] A resin composition was obtained by mixing boron nitride powder at 50% by volume with a two-component heat-curing liquid silicone resin (manufactured by Momentive, product name: TSE3033 A / B). A Thinky Mixer (Awatori Rentaro) manufactured by Thinky Corporation was used to knead the resin. The kneading conditions were 1600 rpm for 3 minutes. The obtained resin composition was applied to a PET film to a thickness of 1 mm. Then, the mixture was kneaded at a temperature of 150°C and 150 kgf / cm. 2Heating and pressure were applied by uniaxial pressing under relatively mild conditions for 45 minutes under the above conditions to prepare a 1 mm resin sheet (evaluation sheet).

[0061] [Measurement of thermal conductivity] The thermal conductivity H (unit: W / (m K)) of the obtained laminated sheet for evaluation in the uniaxial press direction was calculated as a function of the thermal resistance R (unit: K / W), sheet thickness t (unit: m), and sheet heat transfer area A (unit: m 2 ) was used to calculate the thermal diffusivity using the formula H = (1 / R) × t × (1 / A). The thermal diffusivity was measured by using a sample prepared by processing the evaluation sheet into a size of length × width × thickness = 10 mm × 10 mm × 1 mm. The measurement device used was a thermal resistance measurement device (manufactured by Hitachi Technology and Services Co., Ltd., product name: Resin Material Thermal Resistance Measurement Device). The results are shown in Tables 1 and 2.

[0062] [Evaluation of filling properties of boron nitride powder] The resulting boron nitride powder was used as a heat-dissipating filler in resin to evaluate its filling properties. Specifically, the boron nitride powder was blended with silicone oil (Shin-Etsu Chemical Co., Ltd., product name: KF96L) at a volume ratio of 20% by volume based on the total volume of the resin composition, and a slurry was prepared by stirring at 2000 rpm for 2 minutes using a planetary centrifugal mixer (Thinky Corporation, product name: Awatori Rentaro RE-310). The viscosity of the slurry was measured at 25°C using a rheometer (Anton Paar Japan, product name: MCR302, parallel plate (diameter: 25 mmφ), gap: 1 mm) at 0.01 to 100 sec. -1 The shear viscosity was measured within a range of 100°C, and the fillability was evaluated based on the results obtained, according to the following criteria. The results are shown in Tables 1 and 2. Note that if the viscosity of a resin composition containing boron nitride powder is high, it will be difficult to handle and mold, making it necessary to reduce the amount of boron nitride powder added, which tends to make it difficult to increase the amount of powder that can be filled into a molded product. Therefore, it is desirable that the shear viscosity obtained by this evaluation is low. A: The viscosity is 1200 mPa·s or less at a shear rate of 20 rpm. B: The viscosity at a shear rate of 20 rpm is greater than 1200 mPa·s and less than 1800 mPa·s. C: Viscosity exceeds 1800 mPa·s at a shear rate of 20 rpm.

[0063] [Table 1]

[0064] [Table 2] [Industrial Applicability]

[0065] According to the present disclosure, there are provided a boron nitride powder that has excellent fillability in resin and that can be used to prepare a resin sheet that exhibits excellent heat dissipation when filled into resin, and a method for producing the same. According to the present disclosure, there is also provided a heat dissipation sheet containing the above-mentioned boron nitride powder. [Explanation of symbols]

[0066] 100...Heat dissipation sheet.

Claims

1. A powder containing agglomerated particles in which a plurality of primary particles of hexagonal boron nitride are aggregated, and primary particles of hexagonal boron nitride, Contains a crystalline component having calcium as a constituent element, The powder has a tap density of 0.80 g / cm ３ or more and an orientation index of 50 or less.

2. 2. The boron nitride powder according to claim 1, wherein the orientation index is 10 or more.

3. 3. The boron nitride powder according to claim 1, wherein the boron nitride content in the powder is 80 mass% or more.

4. 3. The boron nitride powder according to claim 1, wherein the average particle size of the primary particles is 9 to 20 μm.

5. A heat-dissipating sheet comprising a resin and a heat-dissipating filler, A heat-dissipating sheet, wherein the heat-dissipating filler comprises the boron nitride powder according to claim 1 or 2.

6. The heat-dissipating sheet according to claim 5, wherein the content of the heat-dissipating filler is 15 to 70% by volume.

7. The method includes a firing step of firing a raw material powder containing at least one of carbon black and acetylene black, a boron-containing compound, and calcium carbonate under a pressurized nitrogen atmosphere to obtain a fired product containing agglomerated particles in which a plurality of primary particles of hexagonal boron nitride are agglomerated, the blending amount of the calcium carbonate is 4 to 20 parts by mass relative to 100 parts by mass of the boron-containing compound, The method for producing boron nitride powder, wherein the boron-containing compound contains at least one of boric acid and boron oxide.

8. The method according to claim 7, wherein the amount of calcium carbonate blended is 4 to 20 parts by mass per 100 parts by mass of the boron-containing compound.

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