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

Abstract

The present disclosure provides hexagonal boron nitride powder that can impart high thermal conductivity and high and less variable dielectric breakdown voltage to a resin composition. The hexagonal boron nitride powder according to the present disclosure includes hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles, the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the boron nitride aggregated particles is 20 to 60 parts by mass, and in the integrated value from the small particle size in the particle size distribution curve based on volume, when it reaches 50%, the particle size (D50) is in the range of 15 to 30 μm, and the ratio (D90B / D90A) of the particle size (D90B) when it reaches 90% after being treated with ultrasonic waves of 250 W for 1 minute using ethanol as a dispersion medium to the particle size (D90A) when it reaches 90% is 0.22 to 0.45.

Description

Technical Field

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

Background Art

[0002] In recent years, the miniaturization and high performance of devices have been rapidly progressing. While the heat generated from devices is on the increase, the heat dissipation area and heat dissipation path have been reduced, and how to dissipate heat efficiently has become an issue. As such a heat dissipation measure, a method of adding a thermally conductive filler to the insulating layers of mounting components and surrounding components to improve heat dissipation is generally adopted. As the thermally conductive filler, hexagonal boron nitride powder having high thermal conductivity, high insulation, and low relative dielectric constant characteristics is often used. However, hexagonal boron nitride is plate-like particles with thermal anisotropy. The thermal conductivity in the a-axis direction is 200 to 400 W / (m·K), while the thermal conductivity in the c-axis direction is only 2 W / (m·K). Therefore, it is necessary to control the orientation when filling hexagonal boron nitride into a resin in order to obtain sufficient thermal properties. As one of the methods for controlling the orientation of hexagonal boron nitride, a method of aggregating boron nitride particles and orienting the plate-like particles in multiple directions to eliminate thermal anisotropy can be mentioned (see Patent Document 1). However, the resin composition filled with the hexagonal boron nitride agglomerated particles of Patent Document 1 has many hexagonal boron nitride agglomerated particles with irregularities on the surface, so it is likely to contain bubbles, and it is inevitable that the breakdown voltage is reduced due to these bubbles. On the other hand, by producing the hexagonal boron nitride powder under specific conditions, it has been proposed to obtain a mixed powder of agglomerated particles and single particles with a small aspect ratio (major axis of the particle / thickness of the particle) and a thick wall (see Patent Document 2). The above-mentioned hexagonal boron nitride powder can reduce the entrainment of bubbles when filled into the resin by a combination of agglomerated particles and single particles, and by making the agglomerated particles densely agglomerated, achieving both extremely high thermal conductivity and breakdown voltage. However, when measuring the breakdown voltage of the entire sheet made of the resin composition filled with the hexagonal boron nitride powder of Patent Document 2, a decrease in the breakdown voltage was rarely observed at some parts of the sheet, and as a result, the problem of variation in the breakdown voltage within the sheet was discovered.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Therefore, an object of the present disclosure is to provide a hexagonal boron nitride powder that can impart high thermal conductivity and breakdown voltage to the resulting resin composition when filled into the resin, and moreover, reduces the locations where the breakdown voltage decreases in the entire resin composition and exhibits the effect of little variation in the breakdown voltage.

Means for Solving the Problems

[0005] As a result of investigations by the present inventors, it has been found that the cause of the variation in the breakdown voltage of insulation is the concentration of aggregated particles that occur accidentally when filling the resin. That is, the aggregated particles contained in the hexagonal boron nitride powder described in Patent Document 2 form strong aggregates. Therefore, when filling the resin, voids are generated between the particles at the locations where the aggregated particles are concentrated, and the resin is depleted in such voids, resulting in the occurrence of voids. Generally, when voids are present in a resin composition that is an insulator, since the dielectric constant of the gas is smaller than that of the resin composition, corona discharge occurs at a voltage much lower than the breakdown voltage of the insulator. Therefore, it has been found that the voids present in the portion where the aggregated particles are concentrated cause a partial decrease in the breakdown voltage of insulation, and as a result, the variation in the breakdown voltage of the entire resin composition has increased.

[0006] Therefore, the present inventors have conducted intensive studies to solve the above problems. As a result, in hexagonal boron nitride powder containing hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles, by appropriately reducing the aggregation strength of the aggregated particles, when this is filled into a resin, the aggregated particles are not broken in the sparse portions where the aggregated particles are dispersed, and in the dense portions where the aggregated particles are in contact with each other, the aggregated particles are broken at the contact portions and the voids between the particles can be filled. As a result, it has been found that the generation of voids between the aggregated particles can be effectively prevented, and the variation in the breakdown voltage of the resulting resin composition can be suppressed, leading to the completion of the present invention.

[0007] That is, the hexagonal boron nitride powder provided by the present disclosure includes hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles, and the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride aggregated particles is 20 parts by mass or more and 60 parts by mass or less. In the cumulative value from the small particle size in the volume-based particle size distribution curve measured by the laser diffraction scattering method, when it reaches 50%, the particle size (D50) is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle size (D90B) when it reaches 90% in the above measurement after treating with ultrasonic waves of 250 W for 1 minute using ethanol as the dispersion medium to the particle size (D90A) when it reaches 90% is 0.22 or more and 0.45 or less.

[0008] The hexagonal boron nitride powder of the present disclosure uses boron oxide (B2O3) powder having a specific particle size distribution, that is, 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 of 15% by mass or less, and a content of particles having a particle size of 500 μm or more of 8% by mass or less. This, a carbon source, and an oxygen-containing calcium compound are mixed such 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 oxygen-containing calcium compound is contained in a ratio of 3 parts by mass or more and 30 parts by mass or less in terms of CaO conversion based on Ca with respect to the total amount (B2O3, C conversion value) of 100 parts by mass of the boron oxide and the carbon source. The mixture is heated in a nitrogen atmosphere, and heated to a temperature of 1700 °C or higher while flowing nitrogen from the time when the heating temperature reaches 1500 °C.

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

Effect of the Invention

[0010] The resin composition filled with the hexagonal boron nitride powder of the present disclosure as a filler is excellent in thermal conductivity and breakdown voltage resistance, and the variation in the breakdown voltage resistance can also be suppressed because the locations where the breakdown voltage resistance decreases are reduced. Thereby, for example, by using the hexagonal boron nitride powder of the present disclosure as a filler in an insulating resin layer of an electronic member such as an automotive part or a metal base substrate, the insulation reliability of the insulating resin layer can be further enhanced as compared with the conventional case, and it is possible to surely prevent the occurrence of fatal defects due to dielectric breakdown.

Modes for Carrying Out the Invention

[0011] <Hexagonal Boron Nitride Powder> The hexagonal boron nitride powder of the present disclosure contains hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles. The parts by mass of the hexagonal boron nitride single particles with respect to 100 parts by mass of the hexagonal boron nitride aggregated particles are 20 parts by mass or more, more preferably 30 parts by mass or more, and still more preferably 35 parts by mass or more. Also, the parts by mass of the hexagonal boron nitride single particles with respect to 100 parts by mass of the hexagonal boron nitride aggregated particles are 60 parts by mass or less, more preferably 50 parts by mass or less, and still more preferably 45 parts by mass or less. When the proportion of the hexagonal boron nitride single particles is less than 20 parts by mass, aggregated particles with a higher specific surface area than single particles will occupy most of the powder, and air bubbles are likely to be entrapped when the powder is mixed with a resin, so there is a risk of a decrease in the dielectric breakdown voltage of the resin composition, which is not preferable. Also, when the proportion of the hexagonal boron nitride single particles exceeds 60 parts by mass, it becomes difficult to form a heat path in the resin, so there is a risk of a decrease in the thermal conductivity of the resin composition.

[0012] Further, the hexagonal boron nitride powder of the present disclosure has a particle diameter (D50) of 15 μm or more, preferably 18 μm or more, and more preferably 20 μm or more when reaching 50% in the integrated value from the small particle size in the volume-based particle size distribution curve measured by the laser diffraction scattering method. Also, the particle diameter (D50) is 30 μm or less, preferably 27 μm or less, and more preferably 24 μm or less. The fact that D50 is within these ranges is effective for suppressing an increase in the viscosity of the resin composition and filling the resin with good dispersibility in the filling of the resin.

[0013] Furthermore, in the hexagonal boron nitride powder of the present disclosure, the average aspect ratio of single hexagonal boron nitride particles is preferably 3 or more, more preferably 5 or more, and even more preferably 7 or more. Also, the average aspect ratio of single hexagonal boron nitride particles is preferably 25 or less, more preferably 20 or less, and even more preferably 10 or less. When the average aspect ratio is from 3 to 25, an increase in the viscosity of the resin composition when filled in the resin can be suppressed, and less air bubbles are entrapped, thus preventing a decrease in dielectric breakdown voltage. Moreover, it becomes easier to suppress the orientation of single hexagonal boron nitride particles in the resin composition, contributing to an improvement in thermal conductivity.

[0014] The average particle diameter of single hexagonal boron nitride 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. Also, the average particle diameter of single hexagonal boron nitride 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 diameter of single hexagonal boron nitride particles within the above range, it becomes easier to suppress the alignment of single hexagonal boron nitride particles in a specific orientation within the resin composition, and easier to form a heat path.

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

[0016] The above particle diameters (D90A and D90B) are effective for observing the characteristics of the aggregated particles in the hexagonal boron nitride powder because the proportion of the hexagonal boron nitride aggregated particles contained in the hexagonal boron nitride powder is relatively large. Therefore, D90B / D90A can be used as an index for observing the degree to which the aggregated particles contained in the hexagonal boron nitride powder are likely to break down by ultrasonic treatment. And the closer D90B / D90A is to 0, the easier it means the aggregated particles are to break down, and the closer it is to 1, the harder it means the aggregated particles are and the more difficult they are to break down. The particle diameter 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 still more preferably 0.30 or more. Also, the particle diameter ratio (D90B / D90A) is 0.45 or less, more preferably 0.40 or less, and still more preferably 0.35 or less. When D90B / D90A is less than 0.22, the aggregated particles break down excessively when mixed with the resin, making it difficult to form a heat path in the resin composition and reducing the thermal conductivity. Also, when the value of D90B / D90A is higher than 0.45, the aggregated particles become difficult to break down, so at the places where the aggregated particles accidentally gather densely in the resin, the particles contact without breaking down at the contact part with the aggregated particles. Therefore, voids are formed in the resin composition, and there are places where the dielectric breakdown voltage decreases, resulting in variations in the insulation resistance. Therefore, by setting D90B / D90A within the above range, it is possible to prevent the phenomenon that the aggregated particles break down excessively when mixed with the resin. As a result, at the places where the aggregated particles accidentally gather densely in the resin, it is possible to prevent the particles from breaking down at the contact part with the aggregated particles and forming voids, reduce the places where the dielectric breakdown voltage decreases, and effectively prevent variations in the insulation resistance.

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

[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 0.8 m 2 / g or more, more preferably 1.0 m 2 / g or more. The BET specific surface area is preferably 2.0 m 2 / g or less, more preferably 1.7 m 2 / g or less, still more preferably 1.5 m 2 / g or less. When the BET specific surface area is 0.5 m 2 / g or more, the proportion of agglomerated particles in the hexagonal boron nitride powder relative to the single particles of hexagonal boron nitride is sufficiently large, and the thermal conductivity of the resin composition tends to improve. Also, when the BET specific surface area is 2.0 m 2 / g or less, it is assumed that the proportion of agglomerated particles in the hexagonal boron nitride powder and the unevenness on the surface of the agglomerated particles are preferably suppressed to a small range, and since it becomes difficult to entrap air bubbles in the mixing with the resin, the dielectric breakdown voltage tends to improve.

[0019] Also, the oil absorption amount of the hexagonal boron nitride powder measured based on JIS K 5101-13-1 is preferably 70 g / 100 g or less, more preferably 68 g / 100 g or less, and still more preferably 65 g / 100 g or less. When the oil absorption amount is 70 g / 100 g or less, it is assumed that the proportion of agglomerated particles and the unevenness on the surface of the agglomerated particles are preferably suppressed to a small range, and since it becomes difficult to entrap air bubbles in the mixing with the resin, the dielectric breakdown voltage tends to improve. The oil absorption amount of the hexagonal boron nitride powder is preferably 50 g / 100 g or more, more preferably 55 g / 100 g or more, and still more preferably 60 g / 100 g or more. When the oil absorption amount is 50 g / 100 g or more, the proportion of agglomerated particles is sufficiently large, and it becomes easier to form a heat path when mixed with the resin, so the thermal conductivity of the resin composition improves.

[0020] The loose bulk density of the hexagonal boron nitride powder of the present disclosure is preferably 0.15 g / cm 3 or more, more preferably 0.16 g / cm 3 or more, still more preferably 0.17 g / cm 3 or more. Also, the loose bulk density of the hexagonal boron nitride powder is preferably 0.30 g / cm 3is as follows, more preferably 0.27 g / cm 3 is as follows, still more preferably 0.24 g / cm 3 is as follows. Also, the tapped bulk density is preferably 0.50 g / cm 3 or more, more preferably 0.55 g / cm 3 or more, still more preferably 0.58 g / cm 3 or more. Also, the tapped bulk density is preferably 0.70 g / cm 3 or less, more preferably 0.65 g / cm 3 or less, still more preferably 0.63 g / cm 3 or less.

[0021] It can be said that the relationship between the loose bulk density and the tapped density represents an appropriate degree of ease of breakdown in the agglomerated particles of the hexagonal boron nitride powder of the present disclosure. The hexagonal boron nitride powder of the present disclosure preferably has a loose bulk density / tapped density ratio of about 0.25 or more and 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 for various uses known as uses of hexagonal boron nitride powder. Among them, if particularly suitable uses are exemplified, there is a use as a filler for resins for the purpose of improving electrical insulation and imparting thermal conductivity by filling resins.

[0023] <Manufacturing method of hexagonal boron nitride powder> The hexagonal boron nitride powder of the present disclosure is prepared by heating a mixture containing boron oxide (B2O3), a carbon source, and an oxygen-containing calcium compound, wherein the average particle size is 150 μm or more and 300 μm or less, the content of particles having a particle size of 75 μm or less is 15% by mass or less, and the content of particles having a particle size of 500 μm or more is 8% by mass or less. 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 oxygen-containing calcium compound is contained in an amount of 3 parts by mass or more and 30 parts by mass or less in terms of CaO conversion based on Ca per 100 parts by mass of the total amount (B2O3, C-converted value) of the boron oxide and the carbon source. The heating is carried out in a nitrogen atmosphere, and nitrogen is circulated while heating to a temperature of 1700 °C or higher starting from the time when the heating temperature reaches 1500 °C.

[0024] (Preparation of raw materials) In the production method of the present disclosure, it is necessary that the boron oxide has an average particle size of 150 μm or more and 300 μm or less, the content of particles having a particle size of 75 μm or less is 15% by mass or less, and the content of particles having a particle size of 500 μm or more is 8% by mass or less. That is, when the average particle size of the boron oxide is 150 μm or less, the obtained hexagonal boron nitride aggregated particles become hard and difficult to break, and it becomes difficult to make D90B / D90A of the obtained hexagonal boron nitride powder 0.45 or less. On the other hand, when the average particle size exceeds 300 μm, the obtained hexagonal boron nitride aggregated particles are easily broken, and it becomes difficult to make D90B / D90A 0.22 or more. In this specification, the particle size of the boron oxide is evaluated based on the volume-based particle size distribution measured by the dry laser diffraction particle size distribution method, and the average particle size refers to D50 in the particle size distribution.

[0025] Thus, although the principle by which the hardness of the aggregated particles changes depending on the particle size of boron oxide has not been clearly elucidated, the inventors have made the following speculation. That is, generally, since boron oxide has a melting point of 480°C, it is in a glass state during the nitridation reaction. However, since its viscosity is high and it is easy to maintain a certain shape, it is expected that the specific surface area in the glass state during melting changes depending on the particle size before melting. When the particles of boron oxide used as the raw material are large, the specific surface area of boron oxide in the glass state becomes small, and in the mixed raw material, calcium and carbon are likely to segregate on the surface of boron oxide. As a result, the local calcium concentration becomes high, so the growth of boron nitride particles is likely to be promoted, the proportion of single hexagonal boron nitride particles increases, and it is expected that the aggregated particles are easily broken. On the contrary, when the particles of boron oxide in the mixed raw material are small, the specific surface area in the glass state becomes high, and in the mixed raw material, calcium and carbon are likely to be dispersed on the surface of boron oxide. As a result, the local calcium concentration decreases, so the growth of boron nitride particles is suppressed and aggregation is likely to occur. Therefore, the proportion of hexagonal boron nitride aggregated particles increases, and it is expected that the aggregated particles are difficult to break.

[0026] In the production 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 still 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 still more preferably 250 μm or less.

[0027] Also, even when the boron oxide used contains particles having a particle diameter of 75 μm or less in an amount exceeding 15% by mass, the agglomerated particles are likely to be difficult to break down as in the above case, and as described above, variations in insulation resistance are likely to occur. Further, when the content of particles having a particle diameter of 500 μm or more in boron oxide exceeds 8% by mass, the agglomerated particles are likely to break down too easily, and there is a risk of a decrease in thermal conductivity. The content of particles having a particle diameter of 75 μm or less contained in boron oxide is more preferably 10% by mass or less, and still more preferably 5% by mass or less. The content of particles having a particle diameter of 500 μm or more contained in boron oxide is more preferably 5% by mass or less, and still more preferably 3% by mass or less.

[0028] As the carbon source, known carbon materials can be used without particular limitation. For example, carbon black, activated carbon, nanocarbon, graphite, crystalline carbon such as carbon fiber, and pyrolytic carbon obtained by pyrolyzing monomers or polymers can be used without particular limitation. Generally, carbon black that is easily available, relatively inexpensive, and industrially quality-controlled is used. Further, as the carbon black, acetylene black, furnace black, thermal black, etc. can be used. Also, the average particle diameter of the above 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 diameter of the carbon source is preferably 5 μm or less, more preferably 4 μm or less, and particularly preferably 3 μm or less. That is, by setting the average particle diameter of the carbon source to 5 μm or less, the reactivity of the carbon source is increased, and by setting it to 0.01 μm or more, it becomes easier to handle.

[0029] In the manufacturing method of the present disclosure, the ratio of boron oxide to the carbon source is preferably 0.5 or more, more preferably 0.6 or more, and still more preferably 0.7 or more in terms of B / C (element ratio). Also, the ratio of boron oxide to the carbon source is preferably 1.0 or less, more preferably 0.9 or less, and still more preferably 0.8 or less in terms of B / C (element ratio). That is, when the molar ratio (element ratio) exceeds 1.0, the ratio of boron oxide that volatilizes without being reduced increases, not only reducing the yield but also making it easier to generate single particles compared to aggregated particles. Therefore, it becomes difficult to make the mass part of hexagonal boron nitride single particles 20 mass parts or more and 60 mass parts or less with respect to 100 mass parts of hexagonal boron nitride aggregated particles. Further, the above-mentioned volatile components have an adverse effect on the production line. On the other hand, when the molar ratio is less than 0.5, the proportion of the unreacted carbon source increases, which not only causes black foreign matter by-products but also many boron nitride generation temperatures are 1550 °C or higher, so the generation of boron nitride seed crystals occurs acceleratively. Therefore, boron nitride particles with a small particle size are generated, the BET specific surface area exceeds 2.0 m 2 / g, and the oil absorption measured based on JIS K 5101-13-1 exceeds 70 g / 100 g. Therefore, it is not preferable that the molar ratio is less than 0.5.

[0030] In the manufacturing method of the present disclosure, as the oxygen-containing calcium compound, a compound containing oxygen and calcium can be used without particular limitation. For example, calcium carbonate, calcium hydrogen carbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, calcium oxalate, etc. can be used, and it is also possible to use a mixture of two or more of these. Further, the average particle size of the above 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 controlling the reaction. The average particle size of the above 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 controlling the reaction. As the addition amount of the above oxygen-containing calcium compound, in terms of CaO conversion on a Ca basis with respect to a total amount of 100 parts by mass of boron oxide and the carbon source, it is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, and still more preferably 10 parts by mass or more. Further, as the addition amount of the oxygen-containing calcium compound, in terms of CaO conversion on a Ca basis with respect to a total amount of 100 parts by mass of boron oxide and the carbon source, it is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, and still more preferably 20 parts by mass or less. By making the addition amount of the oxygen-containing calcium more than 3 parts by mass, excessive volatilization of the composite oxide composed of boron oxide and calcium oxide for growing hexagonal boron nitride particles can be suppressed. As a result, it becomes easy to make the average particle size of single particles in the obtained hexagonal boron nitride powder of the present disclosure 4 μm or more and 15 μm or less, and the average aspect ratio 3 or more and 25 or less, and it is also possible to obtain a more crystalline hexagonal boron nitride powder. By making the addition amount of the oxygen-containing calcium less than 30 parts by mass, the liquid phase formation temperature of the composite oxide composed of boron oxide and calcium oxide can be made less than 2000 °C, so it is easy to grow into single particles having an average particle size of 4 μm or more and 15 μm or less and an average aspect ratio of 3 or more and 25 or less. In addition, since the generation of by-products such as CaB6 from boron oxide and oxygen-containing calcium can be suppressed, it becomes easy to reduce the content of CaB6 in the present disclosure.

[0031] (Manufacturing Conditions) In the present disclosure, reductive 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 the raw material mixture) in an atmosphere containing nitrogen gas. However, it is important to control the heating temperature and the amount of nitrogen to be circulated. That is, the hexagonal boron nitride powder can be produced by generating hexagonal boron nitride at a temperature of 1700 °C or higher while circulating nitrogen from the time when the heating temperature of the raw material mixture reaches 1500 °C in a nitrogen atmosphere.

[0032] Generally, the reduction reaction of boron compounds by a carbon source starts at 1200 °C or higher, and amorphous hexagonal boron nitride begins to form. However, by circulating nitrogen from the time when the heating temperature reaches 1500 °C, unreacted boron oxide and complex oxides can be volatilized, and the liquid-phase components in the reaction system can be effectively removed. As a result, the concentration of hexagonal boron nitride particles in the reaction system can be increased, so that hexagonal boron nitride agglomerated particles can be produced. The flow rate of the nitrogen may be set to a flow rate capable of removing unreacted boron oxide and complex oxides that volatilize and exist on the surface of the reactants. For example, per cross-sectional area of 1 m 2 of the reaction vessel in which the reactants are present, the flow rate is preferably 280 NL / min or more. The upper limit of the flow rate of the nitrogen is not particularly limited. That is, since the liquid-phase components in the reaction system change to a composition that is less volatile as the reaction progresses and volatilizes, the possibility of excessive volatilization is low, and there is no problem even if the flow rate of the circulated nitrogen is large. However, in order to suppress a decrease in heating efficiency and scattering of the produced boron nitride powder, the flow rate of nitrogen is preferably 600 NL / min or less. In addition to the volatilization of the liquid-phase components, setting the nitrogen flow rate to 280 NL / min or more also has the advantage of easily suppressing the generation of by-products such as CaB6. By heating the agglomerated particles obtained by the reaction at a temperature of 1700 °C or higher, the agglomerated particles can have an appropriate hardness in combination with the characteristics of the raw materials, and hexagonal boron nitride powder with a particle size ratio (D90B / D90A) of 0.22 or more and 0.45 or less can be obtained.

[0033] The temperature profile up to 1500°C may include a pattern of maintaining a constant temperature at a specific temperature, or a pattern of increasing the temperature with a specific gradient. Also, the temperature profile for increasing the temperature from 1500°C to 1700°C or higher may include a pattern of maintaining a constant temperature at a specific temperature, or a pattern of increasing the temperature with a specific gradient. However, when exceeding 2100°C, nitrogen defects and yellowing of hexagonal boron nitride are likely to occur. Therefore, the maximum temperature in the temperature profile for increasing the temperature from 1500°C to 1700°C or higher is preferably 1700°C or higher and 2000°C or lower.

[0034] Since the nitride powder obtained by the nitridation reaction is generally obtained as a solid mass, it is preferable to crush the solid mass to a size of 250 μm or more and 500 μm or less before performing the washing with the following acids. By crushing, the acid can easily penetrate into the nitride powder, making it easier to wash efficiently. If the solid mass can be crushed, the crushing method is not particularly limited, and examples include a mortar mill, a ball mill, a hammer mill, a roll crusher pin mill, a jet mill, a mortar, and the like.

[0035] The nitride powder obtained by the above-described nitridation reaction contains boron nitride as the main component but also contains by-products such as calcium borate. Therefore, it is preferable to wash using an acid and pure water. The washing method and the amount of pure water or the like used for washing are not particularly limited, and known methods can be used without limitation. The method for drying the crushed powder after washing is not particularly limited, and known methods can be used without limitation. Examples of the drying apparatus that can be used include a shelf dryer, a fluidized bed dryer, a spray dryer, a rotary dryer, a belt dryer, and the like.

[0036] As a method for removing the solidified matter remaining in the crushed powder obtained in the crushing step, classification may be performed. The classification method is not particularly limited, and known methods can be applied without limitation. Specifically, a vibrating sieve, a wet sieve, an air classifier, a cyclone, a liquid cyclone, etc. can be mentioned. The mesh opening of the sieve is preferably 64 μm or more, more preferably 85 μm or more. Further, the mesh opening of the sieve is preferably 106 μm or less, more preferably 90 μm or less. By setting the mesh opening of the sieve to 106 μm or less, the solidified matter remaining unintentionally in the crushing step can be efficiently removed. Also, by setting the mesh opening to 64 μm or more, it becomes easy to perform the classification treatment without bias in the ratio of the aggregated particles and single particles of the hexagonal boron nitride powder.

[0037] <Resin composition> 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 is excellent in solder heat resistance, and is less likely to swell or break 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, for example.

[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 the curable epoxy resin include bisphenol A type epoxy resin, bisphenol S type epoxy resin, bisphenol F type epoxy resin, hydrogenated epoxy resin of bisphenol A type, polypropylene glycol type epoxy resin, polytetramethylene glycol type epoxy resin, naphthalene type epoxy resin, phenylmethane type epoxy resin, tetrakisphenol methane type epoxy resin, biphenyl type epoxy resin, phenol novolak type epoxy resin, tetrafunctional naphthalene type epoxy resin, cresol novolak type epoxy resin, dicyclopentadiene type epoxy resin, tris-phenol epoxy resin, naphthol novolak epoxy resin, naphthylene ether type epoxy resin, aromatic glycidylamine type epoxy resin, hydroquinone type epoxy resin, stilbene type epoxy resin, triphenol methane type epoxy resin, aralkyl type epoxy resin, polypropylene glycol type epoxy resin, polysulfide-modified epoxy resin, epoxy resin having a triazine nucleus as a skeleton, and bisphenol A alkylene oxide adduct type epoxy resin, etc. These curable epoxy resins may be used alone or in combination of two or more. Also, as the curing agent, an amine-based resin, an acid anhydride-based resin, a phenol-based resin, imidazoles, an active ester-based curing agent, a cyanate ester-based curing agent, a naphthol-based curing agent, a benzoxazine-based curing agent, etc. may be used. These curing agents may also be used alone or in combination of two or more. The blending amount of these curing agents with respect to the epoxy resin is preferably 0.5 equivalent ratio or more, more preferably 0.7 equivalent ratio or more, in terms of the equivalent ratio with respect to the epoxy resin. Also, the blending amount of the curing agent with respect to the epoxy resin is preferably 1.5 equivalent ratio or less, more preferably 1.3 equivalent ratio or less, in terms of the equivalent ratio with respect to the epoxy resin. In this specification, these curing agents are also included in the resin.

[0039] In addition, as the silicone resin, a known curable silicone resin which is a mixture of an addition reaction type silicone resin and a silicone crosslinking agent can be used without limitation. Examples of the addition reaction type silicone resin include polyorganosiloxanes such as polydimethylsiloxane having an alkenyl group such as a vinyl group or a hexenyl group as a functional group in the molecule. Examples of the silicone crosslinking agent include polyorganosiloxanes having a silicon atom-bonded hydrogen atom such as dimethylhydrogen siloxy group-terminated dimethylsiloxane-methylhydrogen siloxane copolymer, trimethylsiloxy group-terminated dimethylsiloxane-methylhydrogen siloxane copolymer, trimethylsiloxane group-terminated poly(methylhydrogen siloxane), poly(hydrogensilsesquioxane), etc. Further, as the curing catalyst, a known platinum-based catalyst etc. used for curing the silicone resin can be used without limitation. For example, particulate platinum, particulate platinum supported on carbon powder, chloroplatinic acid, alcohol-modified chloroplatinic acid, olefin complex of chloroplatinic acid, palladium, rhodium catalyst, etc. are mentioned.

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

[0041] Examples of the thermotropic liquid crystal polymer include polymers synthesized from p-hydroxybenzoic acid (PHB), terephthalic acid, and 4,4'-biphenol, polymers synthesized from PHB and 6-hydroxy-2-naphthoic acid, polymers synthesized from PHB, terephthalic acid, and ethylene glycol, etc.

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

[0043] Examples of the fluororesin include, for example, tetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer resin (PFEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA), and the like.

[0044] Examples of the cyanate ester compound include, for example, phenol novolak type cyanate ester compound, naphthol aralkyl type cyanate ester compound, biphenyl aralkyl type cyanate ester compound, naphthylene ether type cyanate ester compound, xylene resin type cyanate ester compound, adamantane skeleton type cyanate ester compound, and among them, phenol novolak type cyanate ester compound, biphenyl aralkyl type cyanate ester compound, and naphthol aralkyl type cyanate ester compound are preferable.

[0045] Examples of the maleimide compound include, for example, 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, maleimide compounds, and the like.

[0046] In the resin composition, the content of the hexagonal boron nitride powder of the present disclosure is preferably 80 parts by volume or more, more preferably 120 parts by volume or more, based on 100 parts by volume of the resin. Also, the content of the hexagonal boron nitride powder is preferably 230 parts by volume or less, more preferably 190 parts by volume or less, based on 100 parts by volume of the resin. When 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. When it exceeds 230 parts by volume, there may be many voids and the dielectric breakdown strength may decrease. It should be noted that it is also possible to mix and fill the resin composition of the present disclosure with the hexagonal boron nitride powder of the present disclosure and other hexagonal boron nitride powders. In that case, it is preferable that 50% by mass or more, preferably 80% by mass or more, more preferably 100% by mass of the total hexagonal boron nitride powder is occupied by the hexagonal boron nitride powder of the present disclosure.

[0047] Further, the resin composition may contain components other than the hexagonal boron nitride powder and the resin. For example, inorganic fillers other than hexagonal boron nitride, discoloration inhibitors, surfactants, dispersants, coupling agents, etc. may be appropriately contained within a range that does not affect the effects of the present disclosure. For example, examples of the inorganic filler 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, talc, etc.

[0048] The method for producing the resin composition is not particularly limited. For example, when the resin is a curable epoxy resin, a mixing step of mixing a curable epoxy resin, the hexagonal boron nitride powder of the present disclosure, and optionally 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 can be adopted as the method for producing the resin composition.

[0049] <Resin sheet> The present disclosure can use the resin composition filled with the hexagonal boron nitride powder to obtain a sheet having high dielectric breakdown resistance and extremely little variation in dielectric breakdown resistance.

[0050] The above sheet is a sheet made of a resin composition containing hexagonal boron nitride powder in a ratio of 80 to 230 parts by volume with respect to 100 parts by volume of the resin. The average value of the thermal conductivity measured at the measurement points at the centers of each section obtained by dividing a sheet cut out to a size of 10×10 cm into 16 sections (4×4) exceeds 9.5 W / (m·K), the average value of the breakdown voltage measured at the measurement points is 60 kV / mm or more, and σ indicating the variation in the measured values of the breakdown voltage at all of the measurement points is 3 kV / mm or less. Thereby, for example, when the above sheet is used for an insulating resin layer of an electronic member such as an automobile part or a metal base substrate, the insulation reliability can be further enhanced as compared with the conventional case, and it becomes possible to surely prevent the occurrence of fatal defects due to dielectric breakdown.

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

[0052] [1] Hexagonal boron nitride agglomerated particles and hexagonal boron nitride single particles are included, the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride agglomerated particles is 20 to 60 parts by mass, and in the cumulative value from the small particle size in the volume-based particle size distribution curve measured by the laser diffraction scattering method, when it reaches 50%, the particle size (D50) is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle size (D90B) when it reaches 90% in the above measurement after treating with ultrasonic waves of 250 W for 1 minute using ethanol as a dispersion medium to the particle size (D90A) when it reaches 90% is 0.22 or more and 0.45 or less. Hexagonal boron nitride powder characterized by the above.

[0053] [2] The hexagonal boron nitride single particles are the hexagonal boron nitride powder of [1] in which the average value of the aspect ratio represented by the length of the major axis of the particle / the length in the thickness direction is 3 or more and 25 or less, and the average particle size is 4 μm or more and 15 μm or less.

[0054] [3] The BET specific surface area is 0.5 m 2 / g or more and 2.0 m 2 / g or less of the hexagonal boron nitride powder of [1] or [2].

[0055] [4] The oil absorption measured based on JIS K 5101-13-1 is 70 g / 100 g or less of the hexagonal boron nitride powder of any one of [1] to [3].

[0056] [5] Boron oxide (B2O3), a carbon source, and an oxygen-containing calcium compound having an average particle diameter of 150 μm or more and 300 μm or less, a content of particles having a particle diameter of 75 μm or less of 15% by mass or less, and a content of particles having a particle diameter of 500 μm or more of 8% by mass or less, wherein 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 oxygen-containing calcium compound is contained in a ratio of 3 parts by mass or more and 30 parts by mass or less in terms of CaO conversion based on Ca with respect to a total amount (B2O3, C conversion value) of 100 parts by mass of the boron oxide and the carbon source. The mixture is heated in a nitrogen atmosphere, and nitrogen is circulated while heating to a temperature of 1700 °C or higher starting from the time when the heating temperature reaches 1500 °C. A method for producing hexagonal boron nitride powder, characterized by this.

[0057] [6] A sheet made of a resin composition containing hexagonal boron nitride powder in a ratio of 80 parts by volume or more and 230 parts by volume or less with respect to 100 parts by volume of the resin. The average value of the thermal conductivity measured at the measurement points at the centers of each section obtained by dividing a sheet cut into a size of 10 × 10 cm into 16 sections (4 × 4) exceeds 9.5 W / (m·K), the average value of the breakdown voltage resistance measured at the measurement points exceeds 60 KV / mm, and σ indicating the variation in the measured values of the breakdown voltage resistance at all of the measurement points is 3 KV / mm or less. A resin sheet, characterized by this.

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

Examples

[0059] Hereinafter, in order to specifically describe the present invention, examples will be described, but the present invention is not limited to these examples. The measurement of each item in the examples and comparative examples was measured by the following method.

[0060] (1) Particle size (D90A) and average particle size (D50) of hexagonal boron nitride powder Using a laser diffraction / scattering particle size analyzer MT3000 (manufactured by Microtrac Bell Co., Ltd.), it was measured by the laser diffraction scattering method. Specifically, 50 cc of ethanol and 0.1 g of hexagonal boron nitride powder were put into the measuring tank of the above 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 cumulative value of the frequency % from the small particle size reached 90% was defined as D90A, and the particle size when the cumulative value of the frequency % from the small particle size reached 50% was defined as D50.

[0061] (2) Particle size (D90B) of hexagonal boron nitride powder After ultrasonic treatment with an ultrasonic homogenizer Sonifier SFX250 (manufactured by Nippon Emason Co., Ltd.), the particle size distribution was measured by the laser diffraction scattering method 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 put into 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 Nippon Emason Co., Ltd.). The treated liquid was transferred to the measuring tank of the above 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 cumulative value of the frequency % from the small particle size reached 90% was defined as D90B.

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

[0063] (4) Parts by mass of single hexagonal boron nitride particles with respect to 100 parts by mass of hexagonal boron nitride agglomerated particles The hexagonal boron nitride powder was observed at a magnification of 500 times, and a 250 μm × 170 μm square SEM observation image was analyzed by an image analyzer (A image kun: manufactured by Asahi Kasei Engineering Corporation). 5000 different particles were randomly selected and sorted into agglomerated particles and single particles. In the above sorting, aggregates of two or more single particles were regarded as agglomerated particles. For the selected 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 shown as the parts by mass of single hexagonal boron nitride particles with respect to 100 parts by mass of hexagonal boron nitride agglomerated particles.

[0064] (5) Aspect ratio and average particle diameter of single hexagonal boron nitride particles 10 parts by mass of hexagonal boron nitride powder was dispersed in 100 parts by mass of an epoxy resin (manufactured by Henkel, EA E - 30CL). After the obtained resin composition was degassed under reduced pressure, it was poured into a mold with a size of 10 mm square and a thickness of 1 mm and cured at a temperature of 70°C.

[0065] Next, the cured sheet - shaped resin composition was taken out of the mold and polished so that both sides were parallel. Then, for one of the surfaces perpendicular to the thickness direction of the resin composition, the center of that surface was subjected to cross - section milling, and an image of the processed surface was taken by SEM under the condition of a magnification of 2500 times. 100 single boron nitride particles were randomly selected from the obtained images, the long side (major axis) and short side (thickness) of the particles were measured, and the respective average values were taken as the average major axis (μm) and average thickness (μm). Further, the value obtained by dividing the average major axis by the average thickness was taken as the average value of the aspect ratio. Also, the above 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 Using a Flow - sorb III 2310 (manufactured by Micromeritics), a gas adsorption test with nitrogen gas as the adsorption species was carried out to measure the nitrogen adsorption isotherm. Specifically, for the as - received hexagonal boron nitride powder that was vacuum - dried and degassed at 200°C for 10 minutes as a pretreatment, with a gas flow rate of 15 cm3 The adsorption - desorption isotherm of nitrogen gas was measured using a continuous - flow method under the condition of / 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 The loose bulk density and tapped bulk density (g / cm 3 ) were measured using a tap densimeter KYT - 5000 (manufactured by Seishin Enterprise Co., Ltd.). Specifically, hexagonal boron nitride powder was filled into a 100 - mL sample cell by dropping it from a height of 10 cm through a 500 - μm sieve, and the loose bulk density was calculated from its weight. Then, after tapping under the conditions of a tapping speed of 120 times / min, a tapping height of 5 cm, and a tapping number of 500 times, the mass was measured, and the tapped bulk density was calculated.

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

[0069] Example 1 4.0 kg of boron oxide with 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%, 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 a rate of 15 °C / min in a nitrogen gas atmosphere using a graphite - made Tamman furnace. After heating to 1500 °C, the flow rate of nitrogen gas was adjusted to the cross - sectional area of the reactor of 1 m 2Around, after maintaining at 350 NL / min for 4 hours to carry out the nitridation reaction, the temperature was raised to 1800 °C at 15 °C / min and maintained for 2 hours for firing. Next, the obtained nitride powder was crushed to a size of 300 μm using a mortar mill, and then washed with acid by mixing and stirring 1.0 kg of hydrochloric acid (37 mass%) and 3.0 kg of pure water with respect to 1.0 kg of the nitride powder in a polyethylene container. After washing, the acid slurry was filtered using a Buchner funnel, and then the residue was washed with pure water more than 10 times the mass of the nitride powder to remove hydrochloric acid, followed by dehydration by suction filtration and drying using a vacuum dryer. After drying, classification was performed using a vibrating sieve with an opening of 90 μm to obtain white hexagonal boron nitride powder. Table 1 shows the production conditions and the analysis results of the powder physical properties of the obtained hexagonal boron nitride powder.

[0070] Also, the evaluation of the thermal conductivity and dielectric breakdown voltage when the obtained boron nitride powder was filled in a resin was carried out as follows.

[0071] As a base resin, a mixture of 100 parts by mass of an epoxy resin (JER806 manufactured by Mitsubishi Chemical Corporation) and 28 parts by mass of a curing agent (alicyclic polyamine-based curing agent, JER Cure 113 manufactured by Mitsubishi Chemical Corporation) was prepared. 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 then the solvent was dried to obtain a resin composition. This was poured into a mold and cured under the conditions of temperature: 200 °C, pressure: 10 MPa, and holding time: 30 minutes using a hot press to produce a resin sheet with dimensions of 100 mm × 100 mm and a thickness of 0.2 mm. This was divided into 16 equal parts, and each section was made into a sheet with dimensions of 25 mm × 25 mm and a thickness of 0.2 mm. Then, the thermal conductivity of each sheet was measured by the temperature wave thermal analysis method, and the average value was calculated. Also, the dielectric breakdown voltage of each sheet was measured using a dielectric breakdown voltage tester (manufactured by Tama Denki Sokki Co., Ltd.), and the average value was calculated. Furthermore, the standard deviation σ was calculated from the measured values of the dielectric breakdown voltage of each sheet as the variation in the dielectric breakdown voltage. The evaluation results are shown in Table 1.

[0072] Example 2 4.0 kg of boron oxide with an average particle diameter of 170 μm, a content of particles with a particle diameter of 75 μm or less of 14.3 mass%, and a content of particles with a particle diameter of 500 μm or more of 0.8 mass%, 1.9 kg of carbon black, and 1.0 kg of calcium oxide were mixed in a ball mill to prepare a raw material mixture. Thereafter, hexagonal boron nitride powder and a resin composition were produced in the same manner as in Example 1. The evaluation results are shown in Table 1.

[0073] Example 3 4.0 kg of boron oxide with an average particle diameter of 290 μm, a content of particles with a particle diameter of 75 μm or less of 1.2 mass%, and a content of particles with a particle diameter of 500 μm or more of 4.1 mass%, 1.9 kg of carbon black, and 1.0 kg of calcium oxide were mixed in a ball mill to prepare a raw material mixture. Thereafter, hexagonal boron nitride powder and a resin composition were produced 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 nitrogen flow rate per cross-sectional area of 1 m of the reactor was changed from 350 NL / min to 490 NL / min. Otherwise, manufacturing conditions were the same as in Example 2, and hexagonal boron nitride powder and a resin composition were produced. The evaluation results are shown in Table 1. 2 Thereafter, hexagonal boron nitride powder and a resin composition were produced in the same manner as in Example 2. The evaluation results are shown in Table 1.

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

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

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

[0078]

Table 1

[0079]

Table 2

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

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

[0082] Also, in Comparative Example 2 using boron oxide with an average particle size of 330 μm, a content of particles with a particle size of 75 μm or less of 1.2% by mass, and a content of particles with a particle size of 500 μm or more of 13.8% by mass, the particle size ratio (D90B / D90A) of the hexagonal boron nitride powder was 0.21, the agglomerated particles were excessively easy to break down, the dielectric breakdown voltage and the variation of dielectric breakdown voltage were good, but the thermal conductivity decreased.

Claims

1. It contains hexagonal boron nitride aggregated particles and hexagonal boron nitride single particles, the ratio of the hexagonal boron nitride single particles to 100 parts by mass of the hexagonal boron nitride aggregated particles is 20 parts by mass or more and 60 parts by mass or less, and in the cumulative value from the small particle size in the volume-based particle size distribution curve measured by the laser diffraction scattering method, when it reaches 50%, the particle size (D50) is in the range of 15 μm or more and 30 μm or less, and the ratio (D90B / D90A) of the particle size (D90B) when it reaches 90% in the above measurement after treating with ultrasonic waves of 250 W for 1 minute using ethanol as the dispersion medium to the particle size (D90A) when it reaches 90% is 0.22 or more and 0.45 or less. A hexagonal boron nitride powder characterized by this.

2. The hexagonal boron nitride single particles according to claim 1, wherein the average value of the aspect ratio represented by the length of the long axis of the particle / the length in the thickness direction is 3 or more and 25 or less, and the average particle size is 4 μm or more and 15 μm or less. Hexagonal boron nitride powder.

3. The BET specific surface area is 0.5 m ２ / g or more and 2.0 m ２ / g or less, the hexagonal boron nitride powder according to claim 1.

4. The hexagonal boron nitride powder according to claim 1, wherein the oil absorption measured based on JIS K 5101-13-1 is 70 g / 100 g or less.

5. Boron oxide (B) having an average particle diameter of 150 μm or more and 300 μm or less, a content of particles having a particle diameter of 75 μm or less of 15% by mass or less, and a content of particles having a particle diameter of 500 μm or more of 8% by mass or less ２ O ３ ), a carbon source, and an oxygen-containing calcium compound, wherein 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 , C conversion value) is heated in a nitrogen atmosphere at a ratio such that the oxygen-containing calcium compound is 3 parts by mass or more and 30 parts by mass or less in terms of CaO conversion based on Ca with respect to 100 parts by mass, and heating is performed up to a temperature of 1700 °C or higher while flowing nitrogen from the time when the heating temperature reaches 1500 °C. A method for producing hexagonal boron nitride powder, characterized in that.

6. A sheet made of a resin composition containing hexagonal boron nitride powder at a ratio of 80 parts by volume or more and 230 parts by volume or less with respect to 100 parts by volume of the resin. The average value of the thermal conductivity measured at the measurement points at the centers of each section obtained by dividing the sheet cut out to a size of 10×10 cm into 16 sections (4×4) exceeds 9.5 W / (m·K), and the average value of the breakdown voltage resistance measured at the measurement points is 60 KV / mm or more, and σ indicating the variation in the measured values of the breakdown voltage resistance at all of the measurement points is 3 KV / mm or less. A resin sheet characterized by this.

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