Boron nitride particle, boron nitride powder, and heat radiation sheet

Abstract

To provide boron nitride particles capable of preparing a heat dissipation sheet excellent in packing property into a resin and capable of exhibiting excellent heat dissipation property when packed in a resin.SOLUTION: One aspect of the present disclosure provides boron nitride particles in which primary particles of hexagonal boron nitride are aggregated, and which have a crystal phase containing at least one selected from the group consisting of CaB2O4,Ca2B2O5,and Ca3B2O6.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to boron nitride particles, boron nitride powder, and a heat dissipation sheet. [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 boron nitride particles that have excellent fillability in resin and can be used to prepare a heat-dissipating sheet that exhibits excellent heat dissipation when filled into resin. The present disclosure also aims to provide a heat-dissipating sheet containing the above-mentioned boron nitride particles. [Means for solving the problem]

[0008] The present disclosure provides the following [1] to [8].

[0009] [1] It is composed of multiple agglomerates of hexagonal boron nitride primary particles, Boron nitride particles having a crystalline phase containing at least one selected from the group consisting of CaB2O4, Ca2B2O5, and Ca3B2O6. [2] The boron nitride particles according to [1], wherein the crystalline phase contains at least Ca2B2O5. [3] Determined by Rietveld analysis of X-ray diffraction spectra The boron nitride particles according to [1] or [2], wherein the ratio of the total amount of CaB2O4, Ca2B2O5, and Ca3B2O6 to the total amount of the hexagonal boron nitride, CaB2O4, Ca2B2O5, and Ca3B2O6 is 3 to 20%. [4] A boron nitride powder comprising the boron nitride particles according to any one of [1] to [3] and primary particles of hexagonal boron nitride. [5] The boron nitride powder according to [4], having an orientation index of 50 or less. [6] The boron nitride powder according to [4] or [5], wherein the average particle size of the primary particles is 9 to 20 μm. [7] 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 particles according to any one of [1] to [3]. [8] The heat-dissipating sheet according to [7], wherein the heat-dissipating filler content is 30 to 70% by volume. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide boron nitride particles that have excellent fillability in resin and can be used to prepare a heat-dissipating sheet that exhibits excellent heat dissipation when filled into resin.The present disclosure also provides a heat-dissipating sheet containing the above-mentioned boron nitride particles. [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 powder X-ray spectrum obtained for the boron nitride particles prepared in Example 1. [Figure 3] FIG. 3 is a powder X-ray spectrum obtained for the boron nitride particles prepared in Comparative Example 1. 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 boron nitride particles is composed of a plurality of agglomerated primary particles of hexagonal boron nitride. The boron nitride particles can also be referred to as agglomerated particles composed of a plurality of the primary particles. The present disclosure can also be said to provide a boron nitride powder containing the above-described agglomerated particles. The boron nitride powder may contain, in addition to the agglomerated particles, unagglomerated primary particles of hexagonal boron nitride. Here, primary particles refer to single particles of hexagonal boron nitride. Furthermore, agglomerated particles are secondary particles formed by agglomeration of a plurality of primary particles bound together by a crystalline component containing calcium, such as calcium borate, at least in part. In the agglomerated particles, the plurality of primary particles may be agglomerated so that the a-axes of the primary particles are oriented in multiple directions, or may be agglomerated so that some of the a-axes are parallel to each other.

[0015] The agglomerated particles are composed of primary particles of hexagonal boron nitride agglomerated together via a crystalline phase containing at least one selected from the group consisting of CaB2O4, Ca2B2O5, and Ca3B2O6, thereby reducing the thermal anisotropy caused by the anisotropy of the shape of the primary particles.When the agglomerated particles are dispersed in a resin and molded into a heat dissipation sheet, the heat dissipation sheet exhibits excellent heat dissipation properties.

[0016] The above crystalline phase preferably contains at least Ca2B2O5, and may further contain two components, at least one of CaB2O4 and Ca3B2O6, or may be a ternary component containing both, but preferably contains Ca3B2O6 and more preferably does not contain CaB2O4.

[0017] The X-ray diffraction spectrum of the agglomerated particles can be obtained by X-ray diffraction, and the components and the proportions of each component in the crystalline phase within the agglomerated particles can be determined by Rietveld analysis of the X-ray diffraction spectrum. The upper limit of the ratio of the total amount of CaB2O4, Ca2B2O5, and Ca3B2O6 to the total amount of hexagonal boron nitride (h-BN), CaB2O4, Ca2B2O5, and Ca3B2O6, as determined by Rietveld analysis of the X-ray diffraction spectrum, may be, for example, 20% or less, 19% or less, or 18% or less. By ensuring that the upper limit of this ratio falls within the above range, the proportion of hexagonal boron nitride within the agglomerated particles can be more sufficient, thereby achieving a higher level of both packing ability and insulating properties of the boron nitride particles according to the present disclosure. The lower limit of this ratio may be, for example, 2% or more, 4% or more, 8% or more, 10% or more, 12% or more, 14% or more, or 16% or more. When the lower limit of the ratio is within the above range, the ratio of aggregates in which primary particles of hexagonal boron nitride are aggregated together via a crystalline phase can be increased, and the heat dissipation properties can be further improved when the obtained boron nitride powder is used as a heat dissipation filler. The ratio of the total amount of CaB2O4, Ca2B2O5, and Ca3B2O6 to the total amount of the hexagonal boron nitride, CaB2O4, Ca2B2O5, and Ca3B2O6, as determined by Riedveld analysis of the X-ray diffraction spectrum, may be adjusted within the above range, and may be, for example, 3 to 20%.

[0018] The present inventors have newly discovered that by adjusting the production conditions of the boron nitride powder so that the proportion of Ca2B2O5 among the CaB2O4, Ca2B2O5, and Ca3B2O6 is high, the volatilization rate is relatively low, the viscosity of the liquid phase is relatively high, and the liquid phase can be more stably maintained during production of the boron nitride powder, which is preferable from the viewpoint of excellent solid solution amount of the nitrogen component. The lower limit of the proportion of Ca2B2O5 relative to the total amount of CaB2O4, Ca2B2O5, and Ca3B2O6, which is determined by obtaining an X-ray diffraction spectrum of the agglomerated particles by X-ray diffraction method and performing Rietveld analysis on the X-ray diffraction spectrum, may be, for example, 30 mass% or more, 40 mass% or more, 45 mass% or more, or 50 mass% or more. The upper limit of the proportion of Ca2B2O5 relative to the total amount of CaB2O4, Ca2B2O5, and Ca3B2O6 determined by performing the Rietveld analysis may be, for example, 99% by mass or less, 95% by mass or less, 93% by mass or less, or 90% by mass or less.

[0019] The total content of CaB2O4, Ca2B2O5, and Ca3B2O6 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, based on the total amount of the boron nitride powder. When the lower limit of the total content is within the above range, the proportion of agglomerated particles in which primary particles of hexagonal boron nitride are aggregated together 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 total content 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. When the upper limit of the total content is within the above range, an excessive increase in the porosity in the agglomerated particles can be suppressed, and the tap density can be further improved. In addition, by making the proportion of hexagonal boron nitride in the boron nitride powder more sufficient, the filling property and insulating property of the boron nitride particles according to the present disclosure can be simultaneously achieved at a higher level.

[0020] The various components and compositions of the crystalline phase herein are determined by X-ray diffraction measurement of the aggregated particles 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.

[0021] The boron nitride powder contains agglomerated particles formed by agglomerating a plurality of primary particles of hexagonal boron nitride, and primary particles of hexagonal boron nitride, 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, 46 or less, 44 or less, 40 or less, 36 or less, 34 or less, 30 or less, 26 or less, 24 or less, 20 or less, or 18 or less. When the upper limit of the orientation index is within the above range, the boron nitride powder exhibits reduced thermal anisotropy due to the shape of the primary particles of hexagonal boron nitride, thereby further 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, 8 or more, 10 or more, or 12 or more. When the lower limit of the orientation index is within the above range, both the fillability of the resulting boron nitride powder into resin and the heat dissipation properties of the resulting resin molded product (for example, a heat dissipation sheet) can be achieved at a higher level.

[0022] Since the orientation index is measured on boron nitride powder, when the powder contains agglomerated particles in which the primary particles are not substantially oriented and the proportion of these particles is high, the orientation index tends to approach a value of about 6 to 7. On the other hand, when the powder is composed of primary particles that do not contain agglomerated particles, or when the proportion of these primary particles is high, the orientation index tends to be high. Furthermore, when the average particle diameter 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 high.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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:

[0027] 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." 3The 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.

[0028] The lower limit of the boron nitride content in the boron nitride powder may be, for example, 80% by mass or more, 82% by mass or more, or 84% 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, 96% by mass or less, 94% by mass or less, 92% by mass or less, 90% by mass or less, 88% by mass or less, or 86% 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 the boron nitride particles according to the present disclosure can be more sufficiently high, and the heat dissipation properties can be further improved when the resulting boron nitride powder is used as a heat dissipation filler.

[0029] 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)

[0030] The boron nitride particles described above have excellent filling properties with resins and are agglomerated particles, so a thermally conductive sheet incorporating the boron nitride particles as a thermally conductive filler exhibits excellent heat dissipation. In other words, the boron nitride particles described above can be suitably used as a thermally conductive filler. One embodiment of the thermally conductive sheet is a thermally conductive sheet comprising a resin and a thermally conductive filler. The thermally conductive filler comprises the boron nitride particles described above. The thermally conductive filler may comprise the boron nitride particles described above and primary particles of hexagonal boron nitride. In this case, the thermally conductive filler can also be said to comprise boron nitride powder. The thermally conductive filler may be dispersed in the resin. FIG. 1 shows a schematic diagram of an example of a thermally conductive sheet 100.

[0031] The lower limit of the content of the thermally conductive filler in the thermally conductive sheet may be, for example, 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 or less, 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 when the thermally conductive sheet is molded, 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, 30 to 70% by volume, 30 to 60% by volume, or 50 to 70% by volume.

[0032] 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.

[0033] The thermally conductive filler may include the above-mentioned boron nitride particles, the above-mentioned boron nitride particles, and primary particles of hexagonal boron nitride, or may consist of the above-mentioned boron nitride particles and primary particles of hexagonal boron nitride, but may also contain other thermally conductive fillers within the scope of the present application. Examples of other fillers include alumina, aluminum nitride, silica, and other thermally conductive fillers. 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.

[0034] 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.

[0035] 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.

[0036] The heat dissipation sheet may be prepared, for example, by molding a resin composition containing the boron nitride particles and a thermosetting resin, or 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.

[0037] The above-mentioned heat dissipation sheet has excellent heat dissipation properties because it contains the boron nitride particles or 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.

[0038] One embodiment of a method for producing a powder containing boron nitride particles (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. In this production method, the amount of calcium carbonate blended is 4 to 20 parts by mass per 100 parts by mass of the boron-containing compound. The boron-containing compound contains at least one of boric acid and boron oxide.

[0039] In the above-described manufacturing method, the calcium carbonate functions as a sintering aid. Compared to sodium carbonate, calcium carbonate is less likely to evaporate during the firing process and is therefore more likely to remain in the system as a growth site for hexagonal boron nitride primary particles. During the firing process, calcium carbonate transforms into a crystalline phase containing at least one selected from the group consisting of CaB2O4, Ca2B2O5, and Ca3B2O6, and the primary particles of hexagonal boron nitride bond together via this crystalline phase to form aggregated particles. In the above-described manufacturing method, the calcium carbonate content is further adjusted to be relatively high. By adjusting the content in this manner, it is possible to increase the proportion of Ca2B2O5, which has a relatively low volatilization rate and a relatively high liquid phase viscosity, thereby more stably maintaining the liquid phase during the production of boron nitride powder and providing a high nitrogen solid solution.

[0040] The amount of calcium carbonate blended is 4 to 20 parts by mass relative to 100 parts by mass of the boron-containing compound. The lower limit of the amount of calcium carbonate blended may be, for example, 5 parts by mass or more, 6 parts by mass or more, or 7 parts by mass or more relative to 100 parts by mass of the boron-containing compound. When the lower limit of the amount of calcium carbonate blended 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 properties, and sufficient particle growth of the primary particles can also improve packing properties. The upper limit of the amount of calcium carbonate blended may be, for example, 20 parts by mass or less, 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 relative to 100 parts by mass of the boron-containing compound. When the upper limit of the blending amount of calcium carbonate is within the above range, excessive generation of crystalline components such as calcium borate can be further suppressed, the content of boron nitride can be increased, and the properties of the obtained boron nitride powder, such as packing property and insulating property, can be further improved.

[0041] The amount of calcium carbonate is 5.0% by mass or more, based on the total amount of the raw material powder. The amount of calcium carbonate may be, for example, 5.2% by mass or more, 5.3% by mass or more, 5.4% by mass or more, or 5.5% by mass or more, based on the total amount of the raw material powder. 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, reducing thermal anisotropy and further improving heat dissipation. Furthermore, sufficient particle growth of the primary particles can improve packing properties. The amount of calcium carbonate may be, for example, 13.0% by mass or less, 12.0% by mass or less, or 11.5% by mass or less, based on the total amount of the raw material powder. 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 can have improved properties such as packing properties and insulating properties.

[0042] 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.

[0043] 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 black and acetylene black and further improve the yield of boron nitride obtained. The firing temperature in the firing step may be, for example, 1800 to 2200°C.

[0044] 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.

[0045] 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.

[0046] In the crushing step, for example, a crusher such as a Henschel mixer, a pulverizer, or a grinder mill can be used.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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]

[0051] 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.

[0052] Example 1 To 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.) were added and mixed using a Henschel mixer to obtain a raw material powder. 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 raw material powder pellets obtained in this manner were subjected to the subsequent heat treatment.

[0053] 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).

[0054] 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 a powder containing boron nitride particles (crushing step). The resulting powder containing boron nitride particles was measured for orientation index and boron nitride content. The results are shown in Table 1. For reference, the X-ray diffraction spectrum of the boron nitride particles obtained in Example 1 is shown in Figure 2.

[0055] [Crystal composition analysis] Boron nitride particles were collected from the boron nitride powder obtained as described above, and an X-ray diffraction spectrum was obtained from the boron nitride particles using X-ray diffraction. Rietveld analysis was then performed to determine the composition of the crystalline phase. The measurement was performed using an "ULTIMA-IV" (product name) manufactured by Rigaku Corporation. The Rietveld analysis was performed using "TOPAS" software manufactured by BRUKER. The results are shown in Table 1.

[0056] 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 particles were subjected to crystal composition analysis in the same manner as in Example 1, and the orientation index and boron nitride content of the powder containing the particles were measured. The results are shown in Table 1.

[0057] 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 particles were subjected to crystal composition analysis in the same manner as in Example 1, and the orientation index and boron nitride content of the powder containing the particles were measured. The results are shown in Table 1.

[0058] Example 4 A powder containing boron nitride particles 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, product name: AP-1SH) under the following conditions: a rotation speed of 7500 rpm, a screen diameter of φ1 mm. Crystal composition analysis was performed on the obtained boron nitride particles in the same manner as in Example 1, and the orientation index and boron nitride content of the powder containing the particles were measured. The results are shown in Table 1.

[0059] (Comparative Example 1) Boron nitride powder was prepared in the same manner as in Example 1, except that calcium carbonate was not used. No agglomerated particles like those found in the boron nitride powder prepared in Example 1 were observed in the resulting powder. The resulting boron nitride powder was subjected to crystal composition analysis in the same manner as in Example 1, and the orientation index and boron nitride content of the powder were measured. The results are shown in Table 2. For reference, the X-ray diffraction spectrum of the boron nitride particles obtained in Comparative Example 1 is shown in Figure 3.

[0060] (Comparative Example 2) 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. No agglomerated particles like those found in the boron nitride powder prepared in Example 1 were observed in the resulting powder. Crystal composition analysis was performed on the resulting boron nitride powder in the same manner as in Example 1, and the orientation index and boron nitride content of the powder were measured. The results are shown in Table 2.

[0061] (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 9.2 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 subjected to crystal composition analysis in the same manner as in Example 1, and the orientation index and boron nitride content of the powder were measured. The results are shown in Table 2.

[0062] 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 5. The obtained boron nitride particles were subjected to crystal composition analysis in the same manner as in Example 1, and the orientation index and boron nitride content of the powder containing these particles were measured. The results are shown in Table 2.

[0063] <Evaluation of powder containing boron nitride particles (boron nitride powder) as a thermally conductive filler> The boron nitride powders prepared in the examples and comparative examples were evaluated as a heat dissipating filler.

[0064] [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. 2 Heating 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).

[0065] [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.

[0066] [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.

[0067] [Table 1]

[0068] [Table 2] [Industrial Applicability]

[0069] According to the present disclosure, it is possible to provide boron nitride particles that have excellent fillability in resin and can be used to prepare a heat-dissipating sheet that exhibits excellent heat dissipation when filled into resin.The present disclosure also provides a heat-dissipating sheet containing the above-mentioned boron nitride particles. [Explanation of symbols]

[0070] 100...Heat dissipation sheet.

Claims

1. It is composed of multiple agglomerates of hexagonal boron nitride primary particles, CaB ２ O ４ , Ca ２ B ２ O ５ , and Ca ３ B ２ O ６ Boron nitride particles having a crystalline phase containing at least one selected from the group consisting of:

2. The crystalline phase contains at least Ca ２ B ２ O ５ The boron nitride particles of claim 1 , comprising:

3. The hexagonal boron nitride, the CaB, as determined by Rietveld analysis of the X-ray diffraction spectrum ２ O ４ , the Ca ２ B ２ O ５ , and the Ca ３ B ２ O ６ The CaB ２ O ４ , the Ca ２ B ２ O ５ , and the Ca ３ B ２ O ６ The boron nitride particles according to claim 1 or 2, wherein the proportion of the total amount of is 3 to 20%.

4. A boron nitride powder comprising the boron nitride particles according to claim 1 or 2 and primary particles of hexagonal boron nitride.

5. 5. The boron nitride powder according to claim 4, having an orientation index of 50 or less.

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

7. The orientation index is 50 or less, 5. The boron nitride powder according to claim 4, wherein the average particle size of the primary particles is 9 to 20 μm.

8. 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 particles according to claim 1 or 2.

9. The heat-dissipating sheet according to claim 8, wherein the content of the heat-dissipating filler is 30 to 70% by volume.

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