Hexagonal boron nitride powder, method for producing same, and resin sheet

Abstract

The present disclosure provides hexagonal boron nitride powder capable of imparting high thermal conductivity and high dielectric withstand voltage with low variation to a resin composition. The hexagonal boron nitride powder according to the present disclosure comprises hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles. The ratio of the boron nitride single particles to 100 parts by mass of the boron nitride aggregated particles is 20-60 parts by mass. In a volume-based particle size distribution curve, the particle diameter (D50) at which the cumulative value from the small particle diameter reaches 50% is in the range of 15-30 μm. The ratio (D90B / D90A) of the particle diameter (D90B) at which the cumulative value reaches 90% after processing with ultrasonic waves at 250 W for one minute using ethanol as a dispersant to the particle diameter (D90A) at which the cumulative value reaches 90% is 0.22-0.45.

Description

Hexagonal boron nitride powder, its manufacturing method, and resin sheet

[0001] The present disclosure relates to a novel boron nitride powder. Specifically, the present disclosure provides a hexagonal boron nitride powder that, when filled into a resin, can give a resin composition that has good thermal conductivity and dielectric strength voltage and little variability in dielectric strength voltage.

[0002] In recent years, devices have become increasingly smaller and more powerful. While the amount of heat generated by these devices has been increasing, their heat dissipation areas and paths have been shrinking, posing a challenge for efficient heat dissipation. To address this issue, a common approach is to add thermally conductive fillers to the insulating layers of mounted and peripheral components to improve heat dissipation. Hexagonal boron nitride powder, which possesses high thermal conductivity, high insulating properties, and low dielectric constant, is often used as the thermally conductive filler. However, hexagonal boron nitride is a plate-like particle with thermal anisotropy. While its thermal conductivity in the a-axis direction is 200–400 W / (m·K), its thermal conductivity in the c-axis direction is only 2 W / (m·K). Therefore, to achieve sufficient thermal properties, it is necessary to control the orientation of hexagonal boron nitride when filling it into a resin. One method for controlling the orientation of hexagonal boron nitride involves agglomerating boron nitride particles and orienting the plate-like particles in multiple directions to eliminate thermal anisotropy (see Patent Document 1). However, the resin composition filled with hexagonal boron nitride agglomerated particles of Patent Document 1 contains many hexagonal boron nitride agglomerated particles with uneven surfaces, which tend to contain bubbles, which inevitably lead to a decrease in dielectric strength voltage. In response to this issue, it has been proposed to produce hexagonal boron nitride powder under specific conditions, resulting in a mixed powder of agglomerated particles and thick single particles with a small aspect ratio (particle major axis / particle thickness) (see Patent Document 2). The hexagonal boron nitride powder, by combining agglomerated particles and single particles and forming densely agglomerated particles, can reduce the entrapment of bubbles when filled into a resin, achieving both extremely high thermal conductivity and dielectric strength voltage. However, when the dielectric strength voltage of an entire sheet made of the resin composition filled with the hexagonal boron nitride powder of Patent Document 2 was measured, a decrease in dielectric strength voltage was occasionally observed in some parts of the sheet, resulting in a problem of variation in dielectric strength voltage within the sheet.

[0003] Japanese Patent Publication No. 11-26661 Japanese Patent No. 6483508

[0004] Therefore, an object of the present disclosure is to provide a hexagonal boron nitride powder that, when filled into a resin, can impart high thermal conductivity and dielectric strength voltage to the resulting resin composition, and further, reduces the number of areas in the resin composition where the dielectric strength voltage is low, thereby achieving the effect of reducing variation in the dielectric strength voltage.

[0005] The inventors' investigations revealed that the cause of the variation in dielectric strength voltage is the density of agglomerated particles that occurs accidentally when the powder is filled into a resin. Specifically, the agglomerated particles contained in the hexagonal boron nitride powder described in Patent Document 2 form strong agglomerations. Therefore, when the powder is filled into a resin, voids form between the particles at the densely packed locations. The resin is depleted in these voids, resulting in the formation of voids. Generally, when voids exist in a resin composition, which is an insulator, corona discharges occur at voltages much lower than the breakdown voltage of the insulator because the dielectric constant of the gas is lower than that of the resin composition. Therefore, the voids present in the densely packed locations of the agglomerated particles cause a partial decrease in dielectric strength voltage, resulting in a large variation in the dielectric strength voltage of the entire resin composition.

[0006] Therefore, the present inventors conducted extensive research to solve the above problems, and discovered that by appropriately reducing the cohesive strength of agglomerated particles in a hexagonal boron nitride powder containing hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles, when the powder is filled into a resin, the agglomerated particles will not be destroyed in sparsely dispersed areas, but will be destroyed at the contact areas in densely packed areas where the agglomerated particles are in contact with each other, filling the voids between the particles; this effectively prevents the generation of voids between the agglomerated particles and makes it possible to suppress variation in the dielectric strength of the resulting resin composition, and led to the completion of the present invention.

[0007] That is, the hexagonal boron nitride powder provided by the present disclosure comprises hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles, and is characterized in that the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 20 parts by mass or more and 60 parts by mass or less, the particle diameter (D50) at 50% of the integrated value from the smallest particle diameter in a volume-based particle size distribution curve measured by a laser diffraction scattering method is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle diameter (D90B) at 90% of the measured particle diameter (D90A) after treatment with 250 W ultrasonic waves using ethanol as a dispersion medium for 1 minute to the particle diameter (D90A) at 90% of the measured particle diameter is 0.22 or more and 0.45 or less.

[0008] The hexagonal boron nitride powder of the present disclosure is a boron oxide (B ) powder having a specific particle size distribution, i.e., an average particle size of 150 μm or more and 300 μm or less, a content of particles with a particle size of 75 μm or less of 15 mass % or less, and a content of particles with a particle size of 500 μm or more of 8 mass % or less. ２ O ３ ) powder, a carbon source, and an oxygen-containing calcium compound are mixed together in such a manner that the ratio of the boron oxide to the carbon source is 0.5 or more and 1.0 or less in terms of B / C (element ratio), and the total amount of the boron oxide and the carbon source (B ２ O ３ The oxygen-containing calcium compound can be obtained by heating a mixture containing the oxygen-containing calcium compound in an amount of 3 to 30 parts by mass, calculated as CaO based on Ca, per 100 parts by mass of the oxygen-containing calcium compound (calculated as C), in a nitrogen atmosphere, and once the heating temperature reaches 1500°C, continuing to circulate nitrogen, the mixture is heated to a temperature of 1700°C or higher.

[0009] The present disclosure also proposes a resin sheet that has a high dielectric breakdown voltage and reduced variation in the dielectric breakdown voltage in the planar direction, which can be achieved by using the hexagonal boron nitride powder. That is, according to the present disclosure, there is provided a resin sheet made of a resin composition containing 80 to 230 volume parts of hexagonal boron nitride powder per 100 volume parts of resin, wherein the sheet is cut into 10 × 10 cm pieces and divided into 16 sections (4 × 4), and the average thermal conductivity measured at measurement points in the centers of each section exceeds 9.5 W / (m·K), the average dielectric breakdown voltage measured at the measurement points is 60 KV / mm or more, and σ, which indicates the variation in the measured dielectric breakdown voltage at all of the measurement points, is 3 KV / mm or less.

[0010] Resin compositions filled with the hexagonal boron nitride powder of the present disclosure as a filler have excellent thermal conductivity and dielectric strength, and because the number of areas where the dielectric strength is reduced is reduced, variations in the dielectric strength can also be suppressed. As a result, for example, by using the hexagonal boron nitride powder of the present disclosure as a filler in insulating resin layers of electronic components such as automobile parts and metal-based substrates, the insulating reliability of the insulating resin layers can be further improved compared to conventional methods, and fatal defects due to dielectric breakdown can be reliably prevented.

[0011] <Hexagonal Boron Nitride Powder> The hexagonal boron nitride powder of the present disclosure contains hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles. The mass ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 20 parts by mass or more, more preferably 30 parts by mass or more, and even more preferably 35 parts by mass or more. Furthermore, the mass ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 60 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 45 parts by mass or less. If the proportion of the hexagonal boron nitride single particles is less than 20 parts by mass, the agglomerated particles, which have a larger specific surface area than the single particles, will occupy most of the powder, which makes it more likely that air bubbles will be entrapped when the powder is mixed with a resin, which is undesirable as it may reduce the dielectric strength voltage of the resin composition. Furthermore, if the proportion of the hexagonal boron nitride single particles exceeds 60 parts by mass, it becomes difficult to form a heat path within the resin, which may result in a decrease in the thermal conductivity of the resin composition.

[0012] Furthermore, the hexagonal boron nitride powder of the present disclosure has a particle size (D50) at 50% of the integrated value from the smallest particle size in a volume-based particle size distribution curve measured by laser diffraction scattering, which is 15 μm or more, preferably 18 μm or more, and more preferably 20 μm or more. Furthermore, the particle size (D50) is 30 μm or less, preferably 27 μm or less, and more preferably 24 μm or less. Having a D50 within these ranges is effective in suppressing an increase in the viscosity of the resin composition when filling it into a resin, and in ensuring good dispersibility in the resin.

[0013] Furthermore, in the hexagonal boron nitride powder of the present disclosure, the average aspect ratio of the hexagonal boron nitride single particles is preferably 3 or more, more preferably 5 or more, and even more preferably 7 or more. The average aspect ratio of the hexagonal boron nitride single particles is preferably 25 or less, more preferably 20 or less, and even more preferably 10 or less. By having an average aspect ratio of 3 or more and 25 or less, an increase in the viscosity of the resin composition when filled into a resin is suppressed, and the entrapment of air bubbles is reduced, thereby preventing a decrease in the dielectric strength voltage. Moreover, the orientation of the hexagonal boron nitride single particles in the resin composition is easily suppressed, which contributes to improving thermal conductivity.

[0014] The average particle size of the hexagonal boron nitride single particles in the hexagonal boron nitride powder is preferably 4 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. Furthermore, the average particle size of the hexagonal boron nitride single particles in the hexagonal boron nitride powder is preferably 15 μm or less, more preferably 12 μm or less, and even more preferably 9 μm or less. By setting the average particle size of the hexagonal boron nitride single particles within the above range, it becomes easier to prevent the hexagonal boron nitride single particles from aligning in a specific orientation within the resin composition, making it easier to form a heat path.

[0015] The greatest feature of the hexagonal boron nitride powder of the present disclosure is that the ratio (D90B / D90A) of the particle diameter (D90A) at which the cumulative value of the frequency percentage from small particle diameters reaches 90% in a volume-based particle size distribution curve measured by laser diffraction scattering, to the particle diameter (D90B) at which the cumulative value reaches 90% in the measurement after treatment with 250 W ultrasonic waves using ethanol as a dispersion medium for 1 minute, is 0.22 or more and 0.45 or less.

[0016] The particle sizes (D90A and D90B) are useful for determining the characteristics of agglomerated particles in hexagonal boron nitride powder because the proportion of hexagonal boron nitride agglomerated particles contained in the hexagonal boron nitride powder is relatively high. Therefore, D90B / D90A can be used as an index for determining the degree to which agglomerated particles contained in the hexagonal boron nitride powder are easily disintegrated by ultrasonic treatment. The closer D90B / D90A is to 0, the more easily the agglomerated particles are disintegrated. The closer D90B / D90A is to 1, the harder the agglomerated particles are and the more difficult they are to disintegrate. The particle size ratio (D90B / D90A) of the hexagonal boron nitride powder of the present disclosure is 0.22 or more, more preferably 0.25 or more, and even more preferably 0.30 or more. The particle size ratio (D90B / D90A) is 0.45 or less, more preferably 0.40 or less, and even more preferably 0.35 or less. If the D90B / D90A ratio is less than 0.22, the agglomerated particles will disintegrate excessively when mixed with a resin, making it difficult for a thermal path to be formed in the resin composition, resulting in a decrease in thermal conductivity. Furthermore, if the D90B / D90A ratio is higher than 0.45, the agglomerated particles will be less likely to disintegrate. Therefore, in areas where the agglomerated particles accidentally gather in the resin, the particles will contact the agglomerated particles without disintegrating. As a result, voids will form in the resin composition, resulting in areas where the dielectric strength voltage is reduced and variations in dielectric strength will occur. Therefore, by keeping the D90B / D90A ratio within the above range, the phenomenon of excessive disintegration of the agglomerated particles when mixed with a resin can be prevented. This prevents the particles from disintegrating and forming voids at the contact points with the agglomerated particles in areas where the agglomerated particles accidentally gather in the resin, thereby reducing areas where the dielectric strength voltage is reduced and effectively preventing variations in dielectric strength.

[0017] In the hexagonal boron nitride powder of the present disclosure, other physical properties are not particularly limited, but the following physical properties are preferred.

[0018] In the hexagonal boron nitride powder of the present disclosure, the BET specific surface area is preferably 0.5 m ２ / g or more, more preferably 0.8m ２ / g or more, and more preferably 1.0m ２The BET specific surface area is preferably 2.0 m / g or more. ２ / g or less, more preferably 1.7m ２ / g or less, and more preferably 1.5m ２ / g or less. BET specific surface area is 0.5 m ２ When the BET specific surface area is 2.0 m / g or more, the ratio of agglomerated particles in the hexagonal boron nitride powder to single particles of hexagonal boron nitride is sufficiently high, and the thermal conductivity of the resin composition tends to be improved. ２ / g or less, it is expected that the proportion of agglomerated particles in the hexagonal boron nitride powder and the unevenness of the agglomerated particle surfaces will be suppressed to a suitably small range, making it difficult for air bubbles to be entrained when mixed with a resin, and therefore the dielectric strength voltage tends to improve.

[0019] Furthermore, the hexagonal boron nitride powder preferably has an oil absorption of 70 g / 100 g or less, more preferably 68 g / 100 g or less, and even more preferably 65 g / 100 g or less, as measured in accordance with JIS K 5101-13-1. When the oil absorption is 70 g / 100 g or less, it is expected that the proportion of agglomerated particles and the surface irregularities of the agglomerated particles are suitably kept to a low range, making it difficult for air bubbles to be entrapped when mixed with a resin, which tends to improve the dielectric strength. The oil absorption of the hexagonal boron nitride powder is preferably 50 g / 100 g or more, more preferably 55 g / 100 g or more, and even more preferably 60 g / 100 g or more. When the oil absorption is 50 g / 100 g or more, the proportion of agglomerated particles is sufficiently high, making it easier to form a thermal path when mixed with a resin, thereby improving the thermal conductivity of the resin composition.

[0020] The hexagonal boron nitride powder of the present disclosure preferably has a loose bulk density of 0.15 g / cm ３ More preferably, 0.16 g / cm ３ More preferably, it is 0.17 g / cm ３ The loose bulk density of the hexagonal boron nitride powder is preferably 0.30 g / cm ３ or less, more preferably 0.27 g / cm ３ More preferably, it is 0.24 g / cm or less. ３The tapped bulk density is preferably 0.50 g / cm or less. ３ More preferably, it is 0.55 g / cm ３ More preferably, it is 0.58 g / cm ３ The tapped bulk density is preferably 0.70 g / cm ３ More preferably, it is 0.65 g / cm or less. ３ More preferably, it is 0.63 g / cm or less. ３ The following is the result.

[0021] The relationship between the loose bulk density and the tapped density can be said to represent the appropriate ease of crumbling of the agglomerated particles in the hexagonal boron nitride powder of the present disclosure. The hexagonal boron nitride powder of the present disclosure preferably has a loose bulk density / tap density ratio of about 0.25 or more and about 0.40 or less.

[0022] The use of the hexagonal boron nitride powder of the present disclosure is not particularly limited, and it can be used in various known uses of hexagonal boron nitride powder. Among these, a particularly suitable use is as a filler for resins to be filled into resins to improve electrical insulation, impart thermal conductivity, etc.

[0023] <Method for producing hexagonal boron nitride powder> The hexagonal boron nitride powder of the present disclosure is a boron oxide (B ) powder having an average particle size of 150 μm or more and 300 μm or less, a content of particles with a particle size of 75 μm or less of 15 mass % or less, and a content of particles with a particle size of 500 μm or more of 8 mass % or less. ２ O ３ ), a carbon source and an oxygen-containing calcium compound are mixed so that the ratio of boron oxide to the carbon source is 0.5 or more and 1.0 or less in terms of B / C (element ratio), and the total amount of the boron oxide and the carbon source (B ２ O ３ The calcium sintered body can be produced by heating a mixture containing an oxygen-containing calcium compound in an amount of 3 to 30 parts by mass, calculated as CaO based on Ca, per 100 parts by mass of the mixture (calculated as C), in a nitrogen atmosphere, and once the heating temperature reaches 1500°C, continuing to circulate nitrogen, the mixture is heated to a temperature of 1700°C or higher.

[0024] (Preparation of Raw Materials) In the manufacturing method of the present disclosure, the boron oxide must have an average particle size of 150 μm or more and 300 μm or less, with the content of particles with a particle size of 75 μm or less being 15 mass% or less and the content of particles with a particle size of 500 μm or more being 8 mass% or less. That is, if the average particle size of boron oxide is 150 μm or less, the resulting hexagonal boron nitride agglomerated particles will be hard and difficult to crumble, making it difficult to achieve a D90B / D90A ratio of 0.45 or less for the resulting hexagonal boron nitride powder. On the other hand, if the average particle size exceeds 300 μm, the resulting hexagonal boron nitride agglomerated particles will be easily crumbled, making it difficult to achieve a D90B / D90A ratio of 0.22 or more. In this specification, the particle size of boron oxide is evaluated based on a volume-based particle size distribution measured by a dry laser diffraction particle size distribution method, and the average particle size refers to the D50 in the particle size distribution.

[0025] Although the mechanism by which the particle size of boron oxide affects the hardness of agglomerated particles is unclear, the inventors speculate as follows. Generally, boron oxide has a melting point of 480°C and is in a glassy state during the nitriding reaction. However, its high viscosity allows it to maintain a certain shape. Therefore, it is expected that the specific surface area in the glassy state at the time of melting varies depending on the particle size before melting. When boron oxide particles used as raw materials are large, the specific surface area of ​​boron oxide in the glassy state decreases, and calcium and carbon tend to segregate on the surface of the boron oxide in the mixed raw material. As a result, the local calcium concentration increases, which likely promotes the growth of boron nitride particles, increases the proportion of hexagonal boron nitride single particles, and makes the agglomerated particles more likely to crumble. Conversely, when boron oxide particles are small in the mixed raw material, the specific surface area in the glassy state increases, and calcium and carbon tend to disperse on the surface of the boron oxide in the mixed raw material. As a result, the local calcium concentration decreases, suppressing the growth of boron nitride particles and making them more susceptible to aggregation, which is expected to increase the proportion of hexagonal boron nitride aggregated particles and make the aggregated particles less likely to break down.

[0026] In the manufacturing method of the present disclosure, the boron oxide has an average particle size of 150 μm or more, more preferably 180 μm or more, and even more preferably 200 μm or more. The boron oxide has an average particle size of 300 μm or less, more preferably 280 μm or less, and even more preferably 250 μm or less.

[0027] Furthermore, even when the boron oxide used contains more than 15% by mass of particles with a particle diameter of 75 μm or less, the agglomerated particles tend to be difficult to disintegrate, as described above, and as described above, variations in insulation resistance are likely to occur. Furthermore, when the content of particles with a particle diameter of 500 μm or more in the boron oxide exceeds 8% by mass, the agglomerated particles tend to disintegrate too easily, which may result in a decrease in thermal conductivity. The content of particles with a particle diameter of 75 μm or less contained in the boron oxide is more preferably 10% by mass or less, and even more preferably 5% by mass or less. The content of particles with a particle diameter of 500 μm or more contained in the boron oxide is more preferably 5% by mass or less, and even more preferably 3% by mass or less.

[0028] As the carbon source, known carbon materials can be used without any particular limitation. Examples include crystalline carbons such as carbon black, activated carbon, nanocarbon, graphite, and carbon fiber, and pyrolytic carbon obtained by pyrolyzing a monomer or polymer. Generally, carbon black is used, which is readily available, relatively inexpensive, and industrially quality-controlled. Examples of carbon black that can be used include acetylene black, furnace black, and thermal black. The average particle size of the carbon source is preferably 0.01 μm or more, more preferably 0.02 μm or more, and particularly preferably 0.05 μm or more. The average particle size of the carbon source is preferably 5 μm or less, more preferably 4 μm or less, and particularly preferably 3 μm or less. By setting the average particle size of the carbon source to 5 μm or less, the reactivity of the carbon source is increased, and by setting the average particle size to 0.01 μm or more, the carbon source is easily handled.

[0029] In the production method of the present disclosure, the ratio of boron oxide to the carbon source, calculated as B / C (element ratio), is preferably 0.5 or more, more preferably 0.6 or more, and even more preferably 0.7 or more. Furthermore, the ratio of boron oxide to the carbon source, calculated as B / C (element ratio), is preferably 1.0 or less, more preferably 0.9 or less, and even more preferably 0.8 or less. That is, when the molar ratio (element ratio) exceeds 1.0, the proportion of boron oxide that volatilizes without being reduced increases, resulting in a decrease in yield and making it easier to produce single particles than agglomerated particles. Therefore, it becomes difficult to achieve a ratio of 20 parts by mass or more and 60 parts by mass or less of hexagonal boron nitride single particles per 100 parts by mass of hexagonal boron nitride agglomerated particles. Furthermore, the volatilized components adversely affect the production line. On the other hand, if the molar ratio is less than 0.5, the proportion of unreacted carbon source increases, which not only causes the production of black foreign matter by-products but also accelerates the production of boron nitride seed crystals because the temperature at which most boron nitride is produced is 1550°C or higher. As a result, boron nitride particles with small particle diameters are produced, and the BET specific surface area is 2.0 m ２ / g, and the oil absorption measured in accordance with JIS K 5101-13-1 exceeds 70 g / 100 g, so it is not preferable for the molar ratio to be less than 0.5.

[0030] In the production method of the present disclosure, the oxygen-containing calcium compound can be any compound containing oxygen and calcium, without any particular limitation. For example, calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, calcium oxalate, etc. can be used, or a mixture of two or more of these can be used. Furthermore, the average particle size of the oxygen-containing calcium compound is preferably 0.01 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more, from the viewpoint of ease of reaction control. The average particle size of the oxygen-containing calcium compound is preferably 500 μm or less, more preferably 400 μm or less, and particularly preferably 300 μm or less, from the viewpoint of ease of reaction control. The amount of the oxygen-containing calcium compound added is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more, calculated as CaO based on Ca, per 100 parts by mass of the total amount of boron oxide and carbon source. The amount of oxygen-containing calcium compound added is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and even more preferably 20 parts by mass or less, calculated as CaO based on Ca, relative to 100 parts by mass of the total amount of boron oxide and the carbon source. By adding more than 3 parts by mass of oxygen-containing calcium, excessive volatilization of the composite oxide composed of boron oxide and calcium oxide required for growing hexagonal boron nitride particles can be suppressed. This facilitates achieving an average single particle size of 4 μm to 15 μm and an average aspect ratio of 3 to 25 in the resulting hexagonal boron nitride powder of the present disclosure, and also enables the production of hexagonal boron nitride powder with higher crystallinity. By adding less than 30 parts by mass of oxygen-containing calcium, the liquid phase formation temperature of the composite oxide composed of boron oxide and calcium oxide can be reduced to less than 2000°C, thereby facilitating the growth of single particles having an average particle size of 4 μm to 15 μm and an average aspect ratio of 3 to 25. In addition, boron oxide and oxygenated calcium form CaB ６ Therefore, the CaB of the present disclosure can suppress the generation of by-products such as ６ It becomes easier to reduce the content.

[0031] (Production Conditions) In the present disclosure, reduction-nitridation can be carried out by heating a mixture containing the boron oxide, a carbon source, and an oxygen-containing calcium compound (hereinafter also referred to as a raw material mixture) in an atmosphere containing nitrogen gas, but it is important to control the heating temperature and the amount of nitrogen passed through. That is, hexagonal boron nitride powder can be produced by generating hexagonal boron nitride at a temperature of 1700°C or higher from the raw material mixture in a nitrogen atmosphere while passing nitrogen through once the heating temperature reaches 1500°C.

[0032] Generally, the reduction reaction of a boron compound with a carbon source begins at 1200°C or higher, and amorphous hexagonal boron nitride begins to be produced. However, by flowing nitrogen from the point when the heating temperature reaches 1500°C, unreacted boron oxide and composite oxides can be volatilized, effectively removing the liquid phase components of the reaction system. As a result, the concentration of hexagonal boron nitride particles in the reaction system can be increased, allowing hexagonal boron nitride agglomerated particles to be produced. The flow rate of the nitrogen can be set to a flow rate that can remove unreacted boron oxide and composite oxides that have volatilized and are present on the surface of the reactants. For example, if the cross-sectional area of ​​the reaction vessel in which the reactants are present is 1 m, the nitrogen flow rate can be set to a flow rate that can remove the unreacted boron oxide and composite oxides that have volatilized and are present on the surface of the reactants. ２ It is preferable that the flow rate of nitrogen is 280 NL / min or more. There is no particular upper limit on the flow rate of nitrogen. That is, as the reaction progresses and the liquid phase components in the reaction system volatilize, they change to a less volatile composition, so there is little possibility of excessive volatilization, and there is no problem even if the flow rate of nitrogen flowing is high. However, in order to suppress a decrease in heating efficiency and the scattering of the generated boron nitride powder, it is preferable that the flow rate of nitrogen is 600 NL / min or less. In addition to the volatilization of the liquid phase components, by setting the nitrogen flow rate to 280 NL / min or more, it is possible to prevent the CaB ６ Heating the agglomerated particles obtained by the reaction at a temperature of 1700°C or higher, in combination with the properties of the raw materials, makes it possible to impart an appropriate hardness to the agglomerated particles, and to obtain a hexagonal boron nitride powder having a particle size ratio (D90B / D90A) of 0.22 or more and 0.45 or less.

[0033] The temperature profile up to 1500°C may include a pattern in which the temperature is maintained constant at a specific temperature, or may include a pattern in which the temperature is increased at a specific gradient. Similarly, the temperature profile in which the temperature is increased from 1500°C to 1700°C or higher may include a pattern in which the temperature is maintained constant at a specific temperature, or may include a pattern in which the temperature is increased at a specific gradient. However, because nitrogen defects and yellowing of hexagonal boron nitride are likely to occur at temperatures above 2100°C, the maximum temperature in the temperature profile in which the temperature is increased from 1500°C to 1700°C or higher is preferably 1700°C or higher and 2000°C or lower.

[0034] The nitride powder obtained by the nitriding reaction is generally obtained as a coagulate, and therefore it is preferable to crush the coagulate to a size of 250 μm or more and 500 μm or less before carrying out the acid washing described below. Crushing makes it easier for the acid to penetrate into the interior of the nitride powder in the acid washing described below, making it easier to wash efficiently. There are no particular restrictions on the crushing method as long as it can crush the coagulate, and examples include a stone mill, a ball mill, a hammer mill, a roll crusher pin mill, a jet mill, and a mortar.

[0035] The nitrided powder obtained by the above-mentioned nitriding reaction is mainly composed of boron nitride, but also contains by-products such as calcium borate, so it is preferable to wash it using acid and pure water. There are no particular restrictions on the washing method or the amount of pure water used for washing, and any known method can be used without restriction. There are no particular restrictions on the method for drying the crushed powder after washing, and any known method can be used without restriction. Examples of drying devices that can be used include a tray dryer, a fluidized bed dryer, a spray dryer, a rotary dryer, and a belt dryer.

[0036] Classification may be performed as a method for removing any aggregates remaining in the crushed powder obtained in the crushing step. The classification method is not particularly limited, and known methods can be applied without limitation. Specific examples include a vibrating sieve, a wet sieve, an air classifier, a cyclone, and a liquid cyclone. The sieve opening is preferably 64 μm or more, more preferably 85 μm or more. The sieve opening is preferably 106 μm or less, more preferably 90 μm or less. By using a sieve opening of 106 μm or less, it is possible to efficiently remove aggregates that are unintentionally left in the crushing step. Furthermore, by using a sieve opening of 64 μm or more, it is possible to easily perform the classification process without biasing the ratio of agglomerated particles to single particles of the hexagonal boron nitride powder.

[0037] The resin composition of the present disclosure contains at least the hexagonal boron nitride powder of the present disclosure and a resin. The resin composition has excellent solder heat resistance and is resistant to swelling and damage due to heat, and is useful as a heat dissipation material such as a printed circuit board resin, a semiconductor encapsulant, and a heat dissipation sheet.

[0038] The resin constituting the resin composition is not particularly limited, and may be, for example, a silicone-based resin or a curable epoxy-based resin. Examples of curable epoxy resins include bisphenol A type epoxy resins, bisphenol S type epoxy resins, bisphenol F type epoxy resins, hydrogenated bisphenol A type epoxy resins, polypropylene glycol type epoxy resins, polytetramethylene glycol type epoxy resins, naphthalene type epoxy resins, phenylmethane type epoxy resins, tetrakisphenolmethane type epoxy resins, biphenyl type epoxy resins, phenol novolac type epoxy resins, tetrafunctional naphthalene type epoxy resins, cresol novolac type epoxy resins, dicyclopentadiene type epoxy resins, trisphenol epoxy resins, naphthol novolac epoxy resins, naphthylene ether type epoxy resins, aromatic glycidylamine type epoxy resins, hydroquinone type epoxy resins, stilbene type epoxy resins, triphenolmethane type epoxy resins, aralkyl type epoxy resins, polypropylene glycol type epoxy resins, polysulfide-modified epoxy resins, epoxy resins having a triazine nucleus in the skeleton, and bisphenol A alkylene oxide adduct type epoxy resins. These curable epoxy resins may be used alone or in combination of two or more. Furthermore, amine-based resins, acid anhydride-based resins, phenol-based resins, imidazoles, active ester-based curing agents, cyanate ester-based curing agents, naphthol-based curing agents, benzoxazine-based curing agents, and the like may be used as curing agents. These curing agents may also be used alone or in combination of two or more. The amount of these curing agents blended relative to the epoxy resin, in terms of equivalent ratio relative to the epoxy resin, is preferably 0.5 or more, more preferably 0.7 or more. Furthermore, the amount of curing agent blended relative to the epoxy resin, in terms of equivalent ratio relative to the epoxy resin, is preferably 1.5 or less, more preferably 1.3 or less. In this specification, these curing agents are also included in the term "resin."

[0039] Furthermore, as the silicone-based resin, known curable silicone resins that are mixtures of addition-reaction-type silicone resins and silicone-based crosslinking agents can be used without limitation. Examples of addition-reaction-type silicone resins include polyorganosiloxanes such as polydimethylsiloxanes that have alkenyl groups such as vinyl groups or hexenyl groups as functional groups in the molecule. Examples of silicone-based crosslinking agents include polyorganosiloxanes having silicon-bonded hydrogen atoms, such as dimethylhydrogensiloxy-group-endblocked dimethylsiloxane-methylhydrogensiloxane copolymers, trimethylsiloxy-group-endblocked dimethylsiloxane-methylhydrogensiloxane copolymers, trimethylsiloxane-group-endblocked poly(methylhydrogensiloxane), and poly(hydrogensilsesquioxane). Furthermore, as the curing catalyst, known platinum-based catalysts used for curing silicone resins can be used without limitation. Examples include fine particle platinum, fine particle platinum supported on carbon powder, chloroplatinic acid, alcohol-modified chloroplatinic acid, olefin complexes of chloroplatinic acid, palladium, and rhodium catalysts.

[0040] Furthermore, as the resin, it is also possible to use liquid crystal polymers, polyesters, polyamides, polyimides, polyphthalamides, polyphenylene sulfides, polycarbonates, polyaryl ether ketones, polyphenolene oxides, fluororesins, cyanate ester compounds, maleimide compounds, and the like.

[0041] Examples of the liquid crystal polymer include thermotropic liquid crystal polymers that exhibit liquid crystallinity in a molten state and lyotropic liquid crystal polymers that exhibit liquid crystallinity in a solution state, and either liquid crystal polymer may be used.

[0042] Examples of thermotropic liquid crystal polymers include polymers synthesized from parahydroxybenzoic acid (PHB), terephthalic acid, and 4,4'-biphenol, polymers synthesized from PHB and 6-hydroxy-2-naphthoic acid, and polymers synthesized from PHB, terephthalic acid, and ethylene glycol.

[0043] Examples of fluororesins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (PFEP), and tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA).

[0044] Examples of cyanate ester compounds include phenol novolac-type cyanate ester compounds, naphthol aralkyl-type cyanate ester compounds, biphenyl aralkyl-type cyanate ester compounds, naphthylene ether-type cyanate ester compounds, xylene resin-type cyanate ester compounds, and adamantane skeleton-type cyanate ester compounds, and phenol novolac-type cyanate ester compounds, biphenyl aralkyl-type cyanate ester compounds, and naphthol aralkyl-type cyanate ester compounds are preferred.

[0045] Examples of the maleimide compound include N-phenylmaleimide, N-hydroxyphenylmaleimide, bis(4-maleimidophenyl)methane, 2,2-bis{4-(4-maleimidophenoxy)-phenyl}propane, bis(3,5-dimethyl-4-maleimidophenyl)methane, bis(3-ethyl-5-methyl-4-maleimidophenyl)methane, bis(3,5-diethyl-4-maleimidophenyl)methane, and other maleimide compounds.

[0046] The content of the hexagonal boron nitride powder of the present disclosure in the resin composition is preferably 80 parts by volume or more, more preferably 120 parts by volume or more, per 100 parts by volume of resin. Furthermore, the content of the hexagonal boron nitride powder is preferably 230 parts by volume or less, more preferably 190 parts by volume or less, per 100 parts by volume of resin. If the content of the hexagonal boron nitride powder is less than 80 parts by volume, the thermal conductivity of the resin composition tends to decrease. If the content of the hexagonal boron nitride powder is more than 230 parts by volume, the amount of voids increases, which may result in a decrease in dielectric strength. It is also possible to mix the hexagonal boron nitride powder of the present disclosure with other hexagonal boron nitride powders and fill them into the resin composition of the present disclosure. In this case, it is preferable that the hexagonal boron nitride powder of the present disclosure accounts for 50% by mass or more, preferably 80% by mass or more, more preferably 100% by mass, of the total hexagonal boron nitride powder.

[0047] The resin composition may also contain components other than the hexagonal boron nitride powder and the resin, such as inorganic fillers other than hexagonal boron nitride, discoloration inhibitors, surfactants, dispersants, coupling agents, etc., as appropriate, provided that the effects of the present disclosure are not affected. Examples of inorganic fillers include aluminum oxide, silicon oxide, zinc oxide, magnesium oxide, titanium oxide, silicon nitride, aluminum nitride, aluminum hydroxide, magnesium hydroxide, silicon carbide, calcium carbonate, barium sulfate, and talc.

[0048] The method for producing the resin composition is not particularly limited. For example, when the resin is a curable epoxy resin, a method for producing the resin composition can be adopted, which includes a mixing step of mixing the curable epoxy resin, the hexagonal boron nitride powder of the present disclosure, and, if necessary, other components (such as a curing agent for the curable epoxy resin) to obtain a curable composition, a molding step of molding the curable composition into a desired shape, and a curing step of curing the curable composition.

[0049] <Resin Sheet> The present disclosure makes it possible to produce a sheet having high dielectric breakdown resistance and extremely little variation in dielectric breakdown resistance by using the resin composition filled with hexagonal boron nitride powder.

[0050] The sheet is made of a resin composition containing 80 to 230 parts by volume of hexagonal boron nitride powder per 100 parts by volume of resin, and the sheet is cut into 10 x 10 cm pieces and divided into 16 sections (4 x 4). The average thermal conductivity measured at the center of each section exceeds 9.5 W / (m·K), the average dielectric breakdown voltage measured at the measurement points is 60 KV / mm or more, and the σ, which indicates the variation in the dielectric breakdown voltage measured at all of the measurement points, is 3 KV / mm or less. This allows for even greater insulation reliability than before when the sheet is used in, for example, electronic components such as automotive parts or insulating resin layers in metal-based substrates, and makes it possible to reliably prevent fatal defects caused by dielectric breakdown.

[0051] <Summary> As can be understood from the above description, the present disclosure includes the following aspects.

[0052] [1] A hexagonal boron nitride powder comprising hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles, wherein the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 20 parts by mass or more and 60 parts by mass or less, the particle size (D50) at 50% of the particle size distribution measured by a laser diffraction scattering method, integrated from the smallest particle size in a volume-based particle size distribution curve, is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle size (D90B) at 90% of the particle size distribution measured after one minute of treatment with 250 W ultrasonic waves using ethanol as a dispersion medium to the particle size (D90A) at 90% of the particle size distribution is 0.22 or more and 0.45 or less.

[0053] [2] The hexagonal boron nitride powder according to [1], wherein the hexagonal boron nitride single particles have an average aspect ratio, expressed as the length of the long axis of the particle divided by the length in the thickness direction, of 3 to 25 and an average particle diameter of 4 μm to 15 μm.

[0054] [3] BET specific surface area is 0.5 m ２ / g or more 2.0m ２ The hexagonal boron nitride powder according to [1] or [2], wherein the hexagonal boron nitride powder has a densitometric value of 1 / g or less.

[0055] [4] The hexagonal boron nitride powder according to any one of [1] to [3], having an oil absorption of 70 g / 100 g or less as measured in accordance with JIS K 5101-13-1.

[0056] [5] Boron oxide (B) having an average particle size of 150 μm or more and 300 μm or less, a content of particles having a particle size of 75 μm or less being 15 mass% or less, and a content of particles having a particle size of 500 μm or more being 8 mass% or less. ２ O ３ ), a carbon source and an oxygen-containing calcium compound are mixed so that the ratio of the boron oxide to the carbon source is 0.5 or more and 1.0 or less in terms of B / C (element ratio), and the total amount of the boron oxide and the carbon source (B ２ O ３a mixture containing the oxygen-containing calcium compound in an amount of 3 to 30 parts by mass, calculated as CaO based on Ca, per 100 parts by mass of the oxygen-containing calcium compound (calculated as C), is heated in a nitrogen atmosphere, and once the heating temperature reaches 1500°C, the mixture is heated to a temperature of 1700°C or higher while circulating nitrogen.

[0057] [6] A sheet made of a resin composition containing 80 to 230 parts by volume of hexagonal boron nitride powder per 100 parts by volume of resin, wherein the sheet is cut into a size of 10 x 10 cm and divided into 16 sections (4 x 4). The average thermal conductivity measured at measurement points in the center of each section exceeds 9.5 W / (m K), the average dielectric breakdown voltage measured at the measurement points is 60 KV / mm or more, and σ, which indicates the variation in the measured dielectric breakdown voltage at all of the measurement points, is 3 KV / mm or less.

[0058] [7] The resin sheet according to [6], wherein the hexagonal boron nitride powder is any one of the hexagonal boron nitride powders [1] to [4].

[0059] Examples will be described below to specifically explain the present invention, but the present invention is not limited to these examples. The measurements of the various items in the examples and comparative examples were performed by the following methods.

[0060] (1) Particle size (D90A) and average particle size (D50) of hexagonal boron nitride powder were measured by laser diffraction scattering using a laser diffraction / scattering particle size analyzer MT3000 (manufactured by Microtrac Bell Co., Ltd.). Specifically, 50 cc of ethanol and 0.1 g of hexagonal boron nitride powder were placed in the measurement tank of the above-mentioned device, and the particle size distribution was measured while operating the ultrasonic dispersion device attached to the device (output 40 W). In the volume-based particle size distribution curve obtained by the above-mentioned measurement, the particle size when the cumulative value of the frequency % from small particle sizes reached 90% was taken as D90A, and the particle size when the cumulative value of the frequency % from small particle sizes reached 50% was taken as D50.

[0061] (2) Particle size (D90B) of hexagonal boron nitride powder After ultrasonic treatment using an ultrasonic homogenizer, Sonifier SFX250 (manufactured by Emerson Japan Co., Ltd.), the particle size distribution was measured by laser diffraction scattering using a laser diffraction / scattering particle size analyzer, MT3000 (manufactured by Microtrac Bell Co., Ltd.). Specifically, 50 cc of ethanol and 0.1 g of hexagonal boron nitride powder were placed in a container, and ultrasonic treatment was performed for 1 minute at an output of 250 W (amplitude 35%) using an ultrasonic homogenizer, Sonifier SFX250 (manufactured by Emerson Japan Co., Ltd.). The treated liquid was transferred to the measurement tank of the device, and the particle size distribution was measured while operating the ultrasonic dispersion device attached to the device (output 40 W). In the volume-based particle size distribution curve obtained by the above measurement, the particle size when the integrated value of the frequency percentage from the small particle size reached 90% was taken as D90B.

[0062] (3) Particle size ratio (D90B / D90A) of hexagonal boron nitride powder This was calculated by dividing D90B obtained by the above-described measurement by D90A.

[0063] (4) Mass parts of hexagonal boron nitride single particles relative to 100 mass parts of hexagonal boron nitride agglomerated particles. A 250 μm × 170 μm square SEM image of a hexagonal boron nitride powder observed at 500x magnification was analyzed using an image analyzer (A-Image-kun, manufactured by Asahi Kasei Engineering Co., Ltd.), and 5,000 different particles were randomly selected and sorted into agglomerated particles and single particles. In the sorting, aggregates of two or more single particles were defined as agglomerated particles. For the sorted particles, the ratio of the total area of ​​the single particles to the total area of ​​the agglomerated particles was calculated by image analysis, and this ratio was expressed as the mass parts of hexagonal boron nitride single particles relative to 100 mass parts of hexagonal boron nitride agglomerated particles.

[0064] (5) Aspect Ratio and Average Particle Diameter of Hexagonal Boron Nitride Single Particles 10 parts by mass of hexagonal boron nitride powder was dispersed in 100 parts by mass of epoxy resin (EA E-30CL, manufactured by Henkel), and the resulting resin composition was degassed under reduced pressure, poured into a 10 mm square, 1 mm thick mold, and cured at a temperature of 70°C.

[0065] Next, the cured sheet-like resin composition was removed from the mold and polished so that both surfaces were parallel. Thereafter, the center of one of the surfaces perpendicular to the thickness direction of the resin composition was cross-sectionally milled, and the processed surface was imaged using an SEM at a magnification of 2500x. One hundred boron nitride single particles were randomly selected from the obtained image, and the long side (major axis) and short side (thickness) of the particles were measured. The average values ​​were taken as the average major axis (μm) and average thickness (μm), respectively. Furthermore, the value obtained by dividing the average major axis by the average thickness was taken as the average aspect ratio. The average major axis was taken as the average particle diameter of the single particles.

[0066] (6) BET specific surface area of ​​hexagonal boron nitride powder A gas adsorption test was carried out using a Flowsorb III 2310 (manufactured by Micromeritics) with nitrogen gas as the adsorbent, and the nitrogen adsorption isotherm was measured. Specifically, the crude hexagonal boron nitride powder was pretreated by vacuum drying and degassing at 200°C for 10 minutes, and the nitrogen adsorption isotherm was measured at a gas flow rate of 15 cm. ３ The adsorption / desorption isotherm of nitrogen gas was measured using a continuous flow method under the condition of 1000 kJ / min, and the specific surface area was calculated by the BET method.

[0067] (7) Loose bulk density and tapped bulk density of hexagonal boron nitride powder Loose bulk density and tapped bulk density (g / cm) were measured using a tap densityr KYT-5000 (manufactured by Seishin Enterprises). ３ ) was measured. Specifically, hexagonal boron nitride powder was dropped from a height of 10 cm through a screen with 500 μm openings into a 100 mL sample cell, and the loose bulk density was calculated from the weight. After that, tapping was performed under the conditions of a tapping speed of 120 times / min, a tapping height of 5 cm, and 500 tapping times, and the mass was measured to calculate the tapped bulk density.

[0068] (8) Oil absorption of hexagonal boron nitride powder Measurement was carried out for hexagonal boron nitride powder in accordance with JIS K 5101-13-1. That is, 2 g of hexagonal boron nitride powder was measured as a sample, and refined linseed oil was gradually added in four drops at a time using a burette, and kneaded with a palette knife. The end point was when the paste reached a smooth consistency. The mass of linseed oil required from the start to the end point was multiplied by 50, and the amount per 100 g of sample was calculated as the oil absorption.

[0069] Example 1 4.0 kg of boron oxide having an average particle size of 240 μm, a content of particles with a particle size of 75 μm or less of 4.3 mass%, and a content of particles with a particle size of 500 μm or more of 2.3 mass% were mixed in a ball mill. 1.9 kg of carbon black and 1.0 kg of calcium oxide were mixed in a ball mill. The raw material mixture was heated to 1500°C at 15°C / min in a nitrogen gas atmosphere using a graphite Tammann furnace. After the temperature was raised to 1500°C, the flow rate of the nitrogen gas was reduced to 1 / m of the cross section of the reactor. ２ The nitriding reaction was carried out by holding the mixture at a flow rate of 350 NL / min for 4 hours, followed by heating to 1800°C at 15°C / min and sintering for 2 hours. The resulting nitride powder was then crushed to a size of 300 μm using a millstone grinder and then washed with acid by mixing and stirring 1.0 kg of nitride powder with 1.0 kg of hydrochloric acid (37% by mass) and 3.0 kg of pure water in a polyethylene container. After washing, the acid slurry was filtered using a Buchner funnel, and the residue was washed with pure water in an amount 10 times or more by mass of the nitride powder to remove the hydrochloric acid. The mixture was then dehydrated by suction filtration and dried using a vacuum dryer. After drying, the mixture was classified using a vibrating sieve with 90 μm openings to obtain a white hexagonal boron nitride powder. The production conditions and the analysis results of the powder physical properties of the resulting hexagonal boron nitride powder are shown in Table 1.

[0070] The obtained boron nitride powder was filled into a resin, and the thermal conductivity and dielectric strength were evaluated as follows.

[0071] A mixture of 100 parts by weight of epoxy resin (JER806 manufactured by Mitsubishi Chemical Corporation) and 28 parts by weight of curing agent (alicyclic polyamine curing agent, JER Cure 113 manufactured by Mitsubishi Chemical Corporation) was prepared as the base resin. Next, 35% by volume of each base resin and 65% by volume of the hexagonal boron nitride powder were mixed using methyl ethyl ketone as a solvent, and the solvent was then dried to obtain a resin composition. This was poured into a mold and cured using a heat press at a temperature of 200°C, a pressure of 10 MPa, and a holding time of 30 minutes to produce a 100 mm x 100 mm, 0.2 mm thick resin sheet. This was then divided into 16 equal sections, each measuring 25 mm x 25 mm and 0.2 mm thick. The thermal conductivity of each sheet was measured using temperature wave thermal analysis, and the average value was calculated. Furthermore, the dielectric strength voltage of each sheet was measured using a voltage resistance tester (manufactured by Tama Densoku Co., Ltd.), and the average value was calculated. Furthermore, the standard deviation σ was calculated from the measured values ​​of the dielectric strength voltage of each sheet, and this was taken as the variation in the dielectric strength voltage. The evaluation results are shown in Table 1.

[0072] Example 2 A raw material mixture was prepared by mixing 4.0 kg of boron oxide having an average particle size of 170 μm, 14.3 mass% of particles with a particle size of 75 μm or less, and 0.8 mass% of particles with a particle size of 500 μm or more, 1.9 kg of carbon black, and 1.0 kg of calcium oxide in a ball mill. A hexagonal boron nitride powder and a resin composition were then prepared in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0073] Example 3 A raw material mixture was prepared by mixing 4.0 kg of boron oxide having an average particle size of 290 μm, 1.2 mass% of particles with a particle size of 75 μm or less, and 4.1 mass% of particles with a particle size of 500 μm or more, 1.9 kg of carbon black, and 1.0 kg of calcium oxide in a ball mill. A hexagonal boron nitride powder and a resin composition were then prepared in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0074] Example 4 In Example 2, the firing temperature was changed from 1800°C to 1850°C, and the cross section of the reactor was ２The nitrogen flow rate per unit area was changed from 350 NL / min to 490 NL / min. Other production conditions were the same as in Example 2, and a hexagonal boron nitride powder and a resin composition were produced. The evaluation results are shown in Table 1.

[0075] Example 5 A raw material mixture was prepared by mixing 4.0 kg of boron oxide having an average particle size of 240 μm, 4.3 mass% of particles with a particle size of 75 μm or less, and 2.3 mass% of particles with a particle size of 500 μm or more, 1.9 kg of carbon black, and 1.79 kg of calcium carbonate in a ball mill. A hexagonal boron nitride powder and a resin composition were produced in the same manner as in Example 1, except that the firing temperature was changed from 1800°C to 1780°C. The evaluation results are shown in Table 2.

[0076] Comparative Example 1 A raw material mixture was prepared by mixing 4.0 kg of boron oxide having an average particle size of 80 μm, 21.9 mass% of particles with a particle size of 75 μm or less, and 0.1 mass% or less of particles with a particle size of 500 μm or more, 1.9 kg of carbon black, and 1.0 kg of calcium oxide in a ball mill. A hexagonal boron nitride powder and a resin composition were then produced in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0077] Comparative Example 2 A raw material mixture was prepared by mixing 4.0 kg of boron oxide having an average particle size of 330 μm, 1.2 mass% of particles with a particle size of 75 μm or less, and 13.8 mass% of particles with a particle size of 500 μm or more, 1.9 kg of carbon black, and 1.0 kg of calcium oxide in a ball mill. A hexagonal boron nitride powder and a resin composition were then prepared in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0078]

[0079]

[0080] As for the evaluation results, the hexagonal boron nitride powders produced in Examples 1 to 5 using boron oxide having an average particle size of 150 μm or more and 300 μm or less, a content of particles with a particle size of 75 μm or less of 15 mass% or less, and a content of particles with a particle size of 500 μm or more of 8 mass% or less had a particle size ratio (D90B / D90A) of 0.29 or more and 0.45 or less, and showed good values ​​for thermal conductivity, dielectric strength voltage, and variation in dielectric strength voltage.

[0081] On the other hand, in Comparative Example 1, which used boron oxide having an average particle size of 90 μm, a content of particles having a particle size of 75 μm or less of 21.9 mass%, and a content of particles having a particle size of 500 μm or more of 0.1 mass% or less, the particle size ratio (D90B / D90A) of the hexagonal boron nitride powder was 0.49, the agglomerated particles were less likely to break down, and the thermal conductivity showed good values; however, a decrease in the dielectric strength voltage was observed in some parts of the sheet, and an increase in the variation in the dielectric strength voltage was confirmed.

[0082] In addition, in Comparative Example 2, which used boron oxide having an average particle size of 330 μm, a content of particles having a particle size of 75 μm or less of 1.2 mass%, and a content of particles having a particle size of 500 μm or more of 13.8 mass%, the particle size ratio (D90B / D90A) of the hexagonal boron nitride powder was 0.21, which made the agglomerated particles excessively prone to crumbling, and although the dielectric strength voltage and the variation in dielectric strength voltage were good, the thermal conductivity was reduced.

Claims

1. A hexagonal boron nitride powder comprising hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles, wherein the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 20 parts by mass or more and 60 parts by mass or less, the particle size (D50) at 50% of the particle size distribution measured by a laser diffraction scattering method, integrated from the smallest particle size on a volume-based particle size distribution curve, is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle size (D90B) at 90% of the particle size distribution measured after one minute of treatment with 250 W ultrasonic waves using ethanol as a dispersion medium to the particle size (D90A) at 90% of the particle size distribution measured by the above method is 0.22 or more and 0.45 or less.

2. The hexagonal boron nitride powder according to claim 1, wherein the hexagonal boron nitride single particles have an average aspect ratio, expressed as the length of the particle's major axis divided by the length in the thickness direction, of 3 to 25 and an average particle diameter of 4 μm to 15 μm.

3. BET specific surface area is 0.5m ２ / g or more 2.0m ２ 2. The hexagonal boron nitride powder according to claim 1, wherein the hexagonal boron nitride powder has a molecular weight of 1 / g or less.

4. The hexagonal boron nitride powder according to claim 1, which has an oil absorption of 70 g / 100 g or less as measured in accordance with JIS K 5101-13-1.

5. Boron oxide (B) having an average particle size of 150 μm or more and 300 μm or less, a particle size of 75 μm or less being 15% by mass or less, and a particle size of 500 μm or more being 8% by mass or less. ２ O ３ ), a carbon source and an oxygen-containing calcium compound are mixed so that the ratio of the boron oxide to the carbon source is 0.5 or more and 1.0 or less in terms of B / C (element ratio), and the total amount of the boron oxide and the carbon source (B ２ O ３ a mixture containing the oxygen-containing calcium compound in an amount of 3 to 30 parts by mass, calculated as CaO based on Ca, per 100 parts by mass of the oxygen-containing calcium compound (calculated as C), is heated in a nitrogen atmosphere, and once the heating temperature reaches 1500°C, the mixture is heated to a temperature of 1700°C or higher while circulating nitrogen.

6. A resin sheet made of a resin composition containing 80 to 230 parts by volume of hexagonal boron nitride powder per 100 parts by volume of resin, wherein the sheet is cut into 10 x 10 cm pieces and divided into 16 sections (4 x 4), and the average thermal conductivity measured at measurement points in the center of each section exceeds 9.5 W / (m·K), the average dielectric breakdown voltage measured at said measurement points is 60 KV / mm or more, and σ, which indicates the variation in the measured dielectric breakdown voltage at all of said measurement points, is 3 KV / mm or less.

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