Hexagonal boron nitride powder, resin sheet, resin composition, and method for producing hexagonal boron nitride powder

Abstract

The present invention realizes a hexagonal boron nitride powder having a good thermal conductivity. This hexagonal boron nitride powder contains aggregated particles of primary particles, has an average particle size of 15-80 µm, and exhibits a ratio (second intensity / first intensity) of not more than 1.0 for a second intensity indicating the maximum emission intensity at a wavelength of 250-440 nm with respect to a first intensity indicating the maximum emission intensity at a wavelength of not more than 235 nm, when being subjected to a cathodoluminescence measurement. The average area circularity of the aggregated particles having an area circle equivalent diameter of 20-40 µm is 0.20-0.50.

Description

Hexagonal boron nitride powder, resin sheet, resin composition, and method for producing hexagonal boron nitride powder

[0001] The present invention relates to a hexagonal boron nitride powder, a resin sheet, a resin composition, and a method for producing the hexagonal boron nitride powder.

[0002] In recent years, as electronic components have become smaller and more powerful, the amount of heat generated by them has become an issue. Therefore, materials with excellent thermal conductivity are being developed to efficiently dissipate heat from electronic components.

[0003] Hexagonal boron nitride powder can be blended with resins used in electronic components to improve the thermal conductivity of the resin. Conventional hexagonal boron nitride powders contain primary particles with a scale-like structure derived from the crystalline structure. These scale-like particles have thermal anisotropy, so when filled into a sheet or the like, there is a direction in which the thermal conductivity is low.

[0004] In order to improve such thermal anisotropy, for example, Patent Document 1 proposes a hexagonal boron nitride powder containing hexagonal boron nitride aggregates.

[0005] Japanese Patent Application Publication No. 2020-84154

[0006] In conventional hexagonal boron nitride powders such as those described in Patent Document 1, heat is transferred through the contact points between the contained agglomerated particles, and therefore, as the density increases during packing, the thermal conductivity tends to improve. Therefore, in order to achieve high thermal conductivity, the amount of hexagonal boron nitride powder packed per unit volume tends to be large. However, since there is a limit to the packing amount of hexagonal boron nitride powder, there is a need for a hexagonal boron nitride powder that will produce a resin sheet with a higher thermal conductivity even with the same packing amount.

[0007] It is also known that the higher the crystallinity of hexagonal boron nitride powder, the higher its thermal conductivity, and there is a need for research and development into highly crystalline hexagonal boron nitride powder.

[0008] An object of one aspect of the present invention is to provide a hexagonal boron nitride powder or the like that has good crystallinity and improves the thermal conductivity of the resin it is filled in. Another object of the present invention is to provide a hexagonal boron nitride powder or the like that not only exhibits higher thermal conductivity at the same filling amount as conventional powders, but also exhibits good thermal conductivity at a smaller filling amount than conventional powders.

[0009] In order to solve the above-mentioned problems, a hexagonal boron nitride powder according to one embodiment of the present invention comprises agglomerated particles of hexagonal boron nitride primary particles, and has an average particle size of 15 μm or more and 80 μm or less, and in cathodoluminescence measurement, a ratio (second intensity / first intensity) of a first intensity indicating the maximum luminescence intensity observed at a wavelength of 235 nm or less to a second intensity indicating the maximum luminescence intensity observed at a wavelength of 250 nm or more and 440 nm or less is 1.0 or less, and the agglomerated particles have an areal equivalent circle diameter of 20 μm or more and 40 μm or less and an average areal circularity of 0.20 or more and 0.50 or less.

[0010] In order to solve the above-mentioned problems, a method for producing hexagonal boron nitride powder according to one embodiment of the present invention includes a reduction-nitridation step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound to a temperature of 1600°C or higher and 1950°C or lower in a nitrogen atmosphere to produce hexagonal boron nitride powder; an acid washing step of acid washing the hexagonal boron nitride powder containing unreacted raw materials; and a method for producing hexagonal boron nitride powder containing unreacted raw materials. The method comprises a crushing step of crushing the hexagonal boron nitride powder so that the volumetric proportion of boron particles having a particle diameter of 10 μm or less is less than 10% and the volumetric proportion of particle diameters greater than 10 μm and less than 50 μm is 20% to 65%; and a re-firing step of heating the hexagonal boron nitride powder to a temperature of 1850° C. to 2050° C., holding the temperature thereat for 1 hour to 15 hours, and then cooling the powder to 1500° C. or less at a cooling rate of 5° C. / min or less.

[0011] According to one aspect of the present invention, it is possible to realize a hexagonal boron nitride powder or the like that has good crystallinity and improves the thermal conductivity of the resin that it is filled in. Furthermore, it is possible to realize a hexagonal boron nitride powder or the like that not only exhibits higher thermal conductivity with the same filling amount as conventional, but also exhibits good thermal conductivity with a filling amount smaller than conventional.

[0012] FIG. 1 shows CL spectra of hexagonal boron nitride powders according to Examples 1, 3, and Comparative Example 1. FIG. 2 shows the distribution of circularity of agglomerated particles having an equivalent circle diameter of 20 to 40 μm for hexagonal boron nitride powders according to Examples 1, 9, and Comparative Example 8. FIG. 3 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 40 μm or less but around 40 μm for hexagonal boron nitride powder according to Example 1. FIG. 4 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 100 μm or more for hexagonal boron nitride powder according to Example 1. FIG. 5 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 40 μm or less but around 40 μm for hexagonal boron nitride powder according to Comparative Example 8. FIG. 6 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 100 μm or more for hexagonal boron nitride powder according to Comparative Example 8.

[0013] Hereinafter, one embodiment of the present invention will be described in detail. In this specification, unless otherwise specified, the expression "A to B" representing a range of numerical values ​​means "A or more and B or less."

[0014] [Hexagonal Boron Nitride Powder] A hexagonal boron nitride powder according to one embodiment of the present invention (hereinafter sometimes referred to as "the present hexagonal boron nitride powder") contains agglomerated particles of hexagonal boron nitride primary particles, and has an average particle diameter of 15 μm to 80 μm. The agglomerated particles have an area-equivalent circle diameter of 20 μm to 40 μm, and the average area circularity of the agglomerated particles is 0.20 to 0.50.

[0015] In one embodiment of the present invention, the maximum value of luminescence intensity observed at a wavelength of 235 nm or less in cathodoluminescence measurement is defined as a first intensity, and the maximum value of luminescence intensity observed at a wavelength of 250 nm to 440 nm is defined as a second intensity, and in this case, the hexagonal boron nitride powder has a ratio of the second intensity to the first intensity (second intensity / first intensity) of 1.0 or less.

[0016] (Crystallinity) The present inventors have conducted extensive detailed studies on hexagonal boron nitride powder and have succeeded in obtaining new findings. Specifically, they have independently discovered that by adopting specific conditions when producing hexagonal boron nitride powder, it is possible to obtain hexagonal boron nitride powder having a second intensity / first intensity ratio of 1.0 or less. The hexagonal boron nitride powder having such a second intensity / first intensity ratio is highly crystalline and close to a single crystal. When filled into a resin, the hexagonal boron nitride powder exhibits high thermal conductivity and is suitable as a heat-dissipating filler.

[0017] Cathodoluminescence (CL) is a method for detecting light emitted when a sample is irradiated with an electron beam. Hereinafter, the emission spectrum obtained by CL is sometimes referred to as a CL spectrum. The emission intensity at each wavelength in the CL spectrum reflects the properties of the crystal (crystal defects, impurities, carrier concentration, stress, etc.).

[0018] In hexagonal boron nitride powder, the CL spectrum observed at wavelengths of 235 nm or less is light emission specific to hexagonal boron nitride crystals and is a spectrum that serves as an indicator of crystal defects. In the CL spectrum of hexagonal boron nitride powder, the first intensity, which indicates the maximum value of the emission intensity observed at wavelengths of 235 nm or less, becomes higher as the hexagonal boron nitride crystal is more perfect and the number of crystal defects is fewer. Crystal defects in hexagonal boron nitride include, for example, point defects such as vacancies, interstitial atoms, and substitutional defects, line defects such as dislocations, planar defects such as stacking faults, and crystal distortions.

[0019] Furthermore, the CL spectrum observed at wavelengths of 250 nm or more and 440 nm or less is light emission derived from impurity atoms, and is a spectrum that serves as an indicator of impurity atoms such as oxygen or carbon in the hexagonal boron nitride crystal. In the CL spectrum of hexagonal boron nitride powder, the second intensity, which indicates the maximum value of the luminescence intensity observed at wavelengths of 250 nm or more and 440 nm or less, becomes smaller as the amount of impurities such as oxygen or carbon in the hexagonal boron nitride crystal decreases.

[0020] Therefore, the smaller the ratio of the second intensity to the first intensity (second intensity / first intensity), the fewer the crystal defects and impurities in the hexagonal boron nitride powder, and the higher the crystallinity. The present hexagonal boron nitride powder has a value of second intensity / first intensity of 1.0 or less. This hexagonal boron nitride powder has high purity, high crystalline perfection, and excellent thermal conductivity.

[0021] As the measuring device for the cathodoluminescence method, a known measuring device can be used.

[0022] From the viewpoint of high crystallinity and further improving the thermal conductivity of the resin, the value of the second intensity / first intensity in the present hexagonal boron nitride powder is 1.0 or less, preferably 0.8 or less, more preferably 0.7 or less, and even more preferably 0.6 or less.

[0023] (Agglomerated Particles of Hexagonal Boron Nitride Primary Particles) Hexagonal boron nitride forms plate-like or scale-like primary particles due to its crystalline structure. When filled into a resin to form a resin sheet, for example, these primary particles are oriented approximately parallel to the surface of the resin sheet. This reduces the thermal conductivity of the resin sheet in the thickness direction. To improve this thermal anisotropy, hexagonal boron nitride powders containing agglomerated particles of hexagonal boron nitride primary particles have been proposed. These agglomerated particles can be made more spherical than primary particles, thereby reducing the problem of thermal anisotropy. In this specification, primary particles of hexagonal boron nitride and agglomerated particles of these primary particles may be collectively referred to as hexagonal boron nitride particles.

[0024] Hexagonal boron nitride powder is used, for example, as a heat dissipation material, as a filler for resins to obtain resin compositions. The thermal conductivity of such resin compositions tends to increase as the packing density of the hexagonal boron nitride powder increases.

[0025] Conventional hexagonal boron nitride powders have improved packing density by, for example, containing two or more types of agglomerated particles with different particle sizes, because the packing density can be easily increased by the inclusion of smaller agglomerated particles in the gaps between the relatively larger agglomerated particles.

[0026] However, increasing the packing density of hexagonal boron nitride powder to obtain a desired thermal conductivity also increases the amount of hexagonal boron nitride powder required, leaving room for improvement in terms of cost. In this regard, the present hexagonal boron nitride powder can achieve the same thermal conductivity as conventional hexagonal boron nitride powders with a smaller packing amount than conventional powders.

[0027] The hexagonal boron nitride powder satisfying the above-mentioned parameters preferably contains, for example, at least two types of agglomerated particles of hexagonal boron nitride primary particles having different equivalent-area circle diameters. The equivalent-area circle diameter indicates the diameter of a circle having the same area as a projected particle image. The equivalent-area circle diameter can be determined using a two-dimensional projected image of the agglomerated particle as the diameter of a circle having the same area as the projected agglomerated particle.

[0028] The present hexagonal boron nitride powder includes, as such agglomerated particles, at least agglomerated particles having an equivalent circle diameter of 20 μm to 40 μm. Hereinafter, agglomerated particles having such an equivalent circle diameter may be referred to as first agglomerated particles. The first agglomerated particles have an average area circularity of 0.20 to 0.50, preferably 0.25 to 0.45.

[0029] The areal circularity is closer to 1.0 as the particle shape approaches a perfect sphere. Therefore, the first aggregated particles having the above-mentioned average areal circularity value can be said to be aggregated particles having a distorted shape that is far from a sphere. The areal circularity is the ratio of the projected area of ​​a two-dimensional projected image of a particle to the square of the perimeter of the two-dimensional projected image. The areal circularity of an aggregated particle can be calculated by using the two-dimensional projected image of the aggregated particle and calculating "4π × projected area / (perimeter)" ２ " can be calculated by

[0030] The first aggregate particles preferably have an average area envelopment ratio of 0.50 or more and 0.80 or less.

[0031] The area envelopment ratio is the ratio of the projected area of ​​a two-dimensional projected image of a particle to the envelope area, which is the area of ​​the region surrounded by the convex hull of the two-dimensional projected image (the region wrapped around the envelope perimeter), and is calculated by the equation: area envelopment ratio = projected area / envelope area. If the shape of the two-dimensional projected image is circular, the area envelopment ratio is 1.0. In other words, the smoother the contour of the agglomerated particles, the closer the area envelopment ratio is to 1.0. The first agglomerated particles having the above-mentioned average area envelopment ratio can be said to be agglomerated particles with a rough contour and a distorted shape.

[0032] The first agglomerate particles having such a distorted shape may have, for example, a branched shape in which a plurality of spherical agglomerate particles are connected. Such first agglomerate particles are likely to come into contact with other agglomerate particles even when the packing density is low. Because heat transfer between agglomerate particles occurs efficiently due to contact between the agglomerate particles, the shape of the first agglomerate particles described above contributes to improving thermal conductivity. As a result, the hexagonal boron nitride powder having the above configuration can exhibit high thermal conductivity, and the amount of the hexagonal boron nitride powder used for packing can be reduced.

[0033] The proportion of agglomerated particles having a particle diameter of 20 μm or more and 40 μm or less, which correspond to the first agglomerated particles, relative to the entire hexagonal boron nitride powder may be 10 vol% or more, 15 vol% or more, or 20 vol% or more. The proportion of agglomerated particles having a particle diameter of 20 μm or more and 40 μm or less may be 55 vol% or less, 50 vol% or less, or 45 vol% or less. In this specification, the term "particle diameter" refers to a particle diameter determined by a laser diffraction scattering method, as opposed to an area-equivalent circle diameter.

[0034] The proportion of agglomerated particles having a particle diameter of 20 μm or more and 40 μm or less is a volume-based proportion based on particle diameters determined by laser diffraction scattering. The volume frequency distribution of area-equivalent circle diameters is known to closely match the particle size distribution determined by laser diffraction scattering. If the present hexagonal boron nitride powder contains first agglomerated particles in such a proportion, it can exhibit high thermal conductivity and, further, can exhibit good thermal conductivity even with a smaller filling amount than conventional powders.

[0035] Furthermore, the present hexagonal boron nitride powder preferably contains agglomerated particles having a larger equivalent circle diameter than the first agglomerated particles. Such agglomerated particles preferably contain, for example, second agglomerated particles having a particle diameter of 60 μm or more and 110 μm or less. By including the second agglomerated particles in addition to the first agglomerated particles, the present hexagonal boron nitride powder can improve thermal conductivity by efficiently positioning the first agglomerated particles in the gaps between the second agglomerated particles and contacting each other.

[0036] The proportion of the second agglomerated particles relative to the entire hexagonal boron nitride powder may be 20 vol% or more, 25 vol% or more, or 30 vol% or more. The proportion of the second agglomerated particles may be 70 vol% or less, 65 vol% or less, or 60 vol% or less. The proportion of the second agglomerated particles is a volume-based proportion based on the particle diameter determined by laser diffraction scattering. If the hexagonal boron nitride powder contains the second agglomerated particles in such a proportion, the thermal conductivity during filling is likely to be improved.

[0037] The hexagonal boron nitride powder may further contain agglomerated particles other than the first agglomerated particles and the second agglomerated particles, and may also contain unagglomerated primary particles. The unagglomerated primary particles contained in the hexagonal boron nitride powder preferably account for 20% by volume or less, based on a volumetric proportion based on a particle diameter determined by a laser diffraction scattering method.

[0038] (Average particle size) The average particle size of the hexagonal boron nitride powder is 15 μm or more, preferably 20 μm or more, and more preferably 25 μm or more. From the viewpoint of improving flowability, the average particle size is 80 μm or less, preferably 70 μm or less, and more preferably 60 μm or less. The average particle size is the particle size (D50) value at which the cumulative value reaches 50% when the volume frequency is accumulated from the smallest particle size in a volume frequency distribution (particle size distribution) based on particle sizes determined by laser diffraction scattering.

[0039] The present hexagonal boron nitride powder has such an average particle size and contains the first agglomerated particles as described above, and therefore, contact between the agglomerated particles is likely to occur, and therefore, good thermal conductivity can be achieved with a smaller filling amount than conventional powders.

[0040] (Specific surface area) The specific surface area of ​​the hexagonal boron nitride powder is 3.5 m ２ / g or less, and ２ / g or less, and ２ / g or less is more preferable, and 2.5m ２ The lower limit of the specific surface area of ​​the hexagonal boron nitride powder is not particularly limited, but for example, it is 1.0 m ２ The specific surface area of ​​the present hexagonal boron nitride powder can be determined by the BET single-point method (nitrogen adsorption single-point method).

[0041] According to this configuration, the present hexagonal boron nitride powder contains a small amount of fine particles, such as aggregated particles or primary particles, whose particle size is smaller than that of the first aggregated particles. A small amount of fine particles can reduce the increase in viscosity of the resulting resin composition when the present hexagonal boron nitride powder is kneaded into a resin, for example. This makes it easier to fill the resin with the present hexagonal boron nitride powder. As a result, a resin sheet made using the present hexagonal boron nitride powder exhibits good thermal conductivity.

[0042] (Summary) As described above, the present hexagonal boron nitride powder exhibits high thermal conductivity due to the excellent crystallinity of the primary particles. Furthermore, even when gaps exist between agglomerated particles, the contact area between the distorted first agglomerated particles and other agglomerated particles, such as the second agglomerated particles, can be increased while maintaining the gaps. Therefore, in fillers such as resin compositions, the desired thermal conductivity can be achieved with a smaller filling amount than conventional fillers, thereby reducing the amount of the present hexagonal boron nitride powder used in the production of resin compositions, etc. These effects also contribute to the achievement of, for example, Goal 12 of the United Nations' Sustainable Development Goals (SDGs), "Ensure sustainable consumption and production patterns."

[0043] [Method for producing hexagonal boron nitride powder] A method for producing hexagonal boron nitride powder according to one embodiment of the present invention (hereinafter sometimes referred to as "the present production method") is a method in which a reduction-nitriding step, an acid washing step, a crushing step, and a re-firing step are carried out in this order. The present production method may further include at least one of a heat treatment step and a recovery step. Each step will be described below.

[0044] (Reduction-nitridation step) In the reduction-nitridation step, a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound is heated in a nitrogen atmosphere to a temperature of 1600° C. to 1950° C. This step produces hexagonal boron nitride.

[0045] The oxygen-containing boron compound may be a compound containing boron and oxygen atoms. Examples of oxygen-containing boron compounds include boric acid, boric acid anhydride (boron oxide), metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. These compounds may be used alone or in combination of two or more. Among these compounds, boric acid or boric acid anhydride, which are easily available, may be preferably used.

[0046] Examples of carbon sources include amorphous carbon such as carbon black, activated carbon, and carbon fiber; crystalline carbon such as diamond, graphite, and nanocarbon; and pyrolytic carbon obtained by pyrolyzing a monomer or polymer. These may be used alone or in combination. Among these, highly reactive amorphous carbon is preferred, and carbon black is particularly preferred because its quality is industrially controlled. Examples of carbon black include acetylene black, furnace black, and thermal black.

[0047] Examples of oxygen-containing calcium compounds include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. These compounds can be used alone or in combination of two or more. Among these, calcium oxide or calcium carbonate is preferably used. By including an oxygen-containing calcium compound in the raw material mixture, a highly crystalline hexagonal boron nitride powder can be obtained when heated in the reduction-nitridation step.

[0048] The oxygen-containing calcium compound forms a complex oxide with the oxygen-containing boron compound, which forms a complex oxide with a high melting point and plays a role in preventing the oxygen-containing boron compound from volatilizing.It has also been confirmed that the oxygen-containing calcium compound plays a role in the direct nitriding reaction of boron carbide.

[0049] The boron-carbon compound may be a compound containing carbon and boron (e.g., boron carbide). The particle size of the boron-carbon compound is preferably 1 μm or more and 500 μm or less, more preferably 10 μm or more and 400 μm or less, and even more preferably 20 μm or more and 300 μm or less.

[0050] The mixture may be obtained by mixing the oxygen-containing boron compound, the carbon source, the oxygen-containing calcium compound, and the carbon-containing boron compound in the ratio shown below. The ratio B / C (element ratio) of the mass of the oxygen-containing boron compound converted into B (boron) to the mass of the carbon source converted into C (carbon) may be 0.75 or more and 1.05 or less. ２ O ３ The oxygen-containing calcium compound may be contained in an amount of 5 parts by mass or more and 20 parts by mass or less in terms of CaO relative to 100 parts by mass of the total of the converted mass of the oxygen-containing boron compound and the converted mass of the carbon source. ２ O ３ The carbon-containing boron compound may be contained in an amount of 5 parts by mass or more and 45 parts by mass relative to 100 parts by mass of the total amount of the converted mass of C and CaO.

[0051] The raw material mixture containing the raw materials may be supplied to the reduction-nitridation reaction in the form of, for example, powder or granules. The raw materials can be mixed using a mixer such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration agitator, or a V-shaped mixer.

[0052] The heating of the raw material mixture in the reduction-nitridation step is preferably carried out in a furnace under a nitrogen atmosphere. Examples of gases introduced into the furnace include nitrogen gas and ammonia gas. That is, the nitrogen atmosphere may be a gas atmosphere containing nitrogen atoms. A gas obtained by mixing nitrogen gas or ammonia gas with a non-oxidizing gas such as hydrogen, argon, or helium may also be used. Nitrogen gas is preferred as the gas introduced into the heating furnace. Furthermore, the nitrogen gas is preferably nitrogen gas with a controlled dew point temperature, and the dew point temperature is preferably −85° C. or lower.

[0053] The maximum temperature to which the raw material mixture is heated in the reduction-nitridation step (maximum reduction-nitridation temperature) is 1600°C or higher and 1950°C or lower, preferably 1650°C or higher and 1900°C or lower, and more preferably 1680°C or higher and 1880°C or lower.

[0054] The raw material mixture in the reduction-nitridation step is preferably heated at a temperature increase rate of 10°C / min or less from 1500°C to the maximum reduction-nitridation temperature. Heating at this temperature increase rate can slow the grain growth rate of the hexagonal boron nitride powder, allowing for the production of highly crystalline hexagonal boron nitride powder. From the perspective of obtaining highly crystalline hexagonal boron nitride powder, the temperature increase rate from 1500°C to the maximum reduction-nitridation temperature is preferably 7°C / min or less, more preferably 5°C / min or less, and even more preferably 3°C / min or less.

[0055] In the reduction-nitridation reaction, a holding time may be provided as needed in the temperature range up to the maximum reduction-nitridation temperature. Furthermore, after the maximum reduction-nitridation temperature is reached, a holding time may be provided as needed in the temperature range within the maximum reduction-nitridation temperature. From the viewpoint of promoting the reduction-nitridation reaction, the holding time after reaching the maximum reduction-nitridation temperature is preferably 1 hour or more, more preferably 2 hours or more. Furthermore, the holding time after reaching the maximum reduction-nitridation temperature is preferably 10 hours or less, more preferably 5 hours or less.

[0056] The reduction-nitriding step can be carried out using a known reaction apparatus capable of adjusting the reaction atmosphere. Examples of the reaction apparatus include an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating, and in addition to a batch furnace, continuous heating furnaces such as a pusher-type tunnel furnace and a vertical reaction furnace can also be used.

[0057] (Acid Washing Step) In the acid washing step, the hexagonal boron nitride powder containing unreacted raw materials obtained in the reduction-nitridation step is subjected to acid washing. The hexagonal boron nitride powder obtained by the reduction-nitridation reaction described above contains unreacted raw materials such as oxides or metal impurities. The acid washing step allows for the production of hexagonal boron nitride powder from which the unreacted raw materials have been removed.

[0058] The method for acid washing the hexagonal boron nitride powder containing unreacted raw materials is not particularly limited, and any known method may be used without limitation. For example, there is a method in which the hexagonal boron nitride powder obtained after the reduction-nitridation reaction is placed in a container, and dilute hydrochloric acid (10 to 20 mass % HCl) is added in an amount 5 to 10 times the amount of the hexagonal boron nitride powder, and the mixture is allowed to come into contact for 4 to 8 hours.

[0059] The acid used in the acid washing may be hydrochloric acid, or may be nitric acid, sulfuric acid, acetic acid, or the like.

[0060] After the acid washing, water washing using pure water may be performed to wash away any remaining acid. As a method of water washing, the acid used in the acid washing may be filtered, and then the acid-washed hexagonal boron nitride powder may be dispersed in pure water in the same amount as the acid used, followed by filtering again.

[0061] After acid washing or water washing, the hydrous aggregates may be dried. Drying conditions are preferably, for example, in the atmosphere at 50° C. or higher and 250° C. or lower, or under reduced pressure. There are no particular limitations on the drying time, but it is preferable to dry until the moisture content of the hexagonal boron nitride powder approaches 0%.

[0062] (Crushing step) The hexagonal boron nitride powder after the acid washing step is subjected to a crushing step to adjust the particle size distribution to a predetermined value. The crushing step is a step of crushing the hexagonal boron nitride powder after the acid washing step so that the volumetric proportion of hexagonal boron nitride particles contained in the hexagonal boron nitride powder having a particle size of 10 μm or less is less than 10% and the volumetric proportion of particles having a particle size of more than 10 μm and less than 50 μm is 20% to 65%. The particle size and volumetric proportion here are those determined by laser diffraction scattering.

[0063] With respect to the hexagonal boron nitride particles after the crushing step, the volumetric proportion of particles having a particle size of 10 μm or less is preferably 7% or less, more preferably 5% or less, and the volumetric proportion of particles having a particle size of more than 10 μm and 50 μm or less is preferably 25% to 60%, more preferably 30% to 60%.

[0064] The crushing step is preferably carried out gently using a jet mill, ball mill, hammer mill, stone mill, or the like. Among these crushing methods, a grinding method using a stone mill, or the like, in which shear force is applied to the hexagonal boron nitride powder to grind it, is preferred. Such a grinding method introduces crystal defects into the surface of the hexagonal boron nitride particles without excessively crushing the agglomerated particles. It is preferable that the hexagonal boron nitride particles after the crushing step have a moderate amount of crystal defects, because this promotes a change in the crystal structure that repairs the crystal defects in the re-firing step described below. The particle size of the powder after crushing may be adjusted appropriately by classification using air classification or sieving.

[0065] In the past, the above-described crushing step was generally performed after the reduction-nitriding step and before the acid washing step. In the present production method, by performing the crushing step after the acid washing step, the effect of introducing crystal defects in the crushing step can be improved, and therefore, a hexagonal boron nitride powder containing the above-described first agglomerated particles can be obtained.

[0066] (Re-firing step) In the re-firing step, the hexagonal boron nitride powder obtained after the crushing step is heated to a temperature of 1850°C or higher and 2050°C or lower, held for 1 hour or higher and 15 hours or lower, and then cooled to a temperature of 1500°C or lower at a rate of 5°C / min or lower.

[0067] The heating in the re-baking step may be performed under a nitrogen atmosphere. The nitrogen source may be supplied to the reaction system in the re-baking step in the same manner as in the reduction-nitridation step. The reaction apparatus may also be the same as that used in the reduction-nitridation step.

[0068] The maximum temperature to which the hexagonal boron nitride powder is heated in the re-firing step (maximum re-firing temperature) is 1850°C or higher and 2050°C or lower, and preferably 1900°C or higher and 2000°C or lower. This heating makes it possible to obtain a hexagonal boron nitride powder with high crystallinity. However, if the hexagonal boron nitride powder is heated to a temperature higher than 2050°C, the hexagonal boron nitride powder may turn yellow due to the formation of defects such as nitrogen atom vacancies, resulting in a hexagonal boron nitride powder with low crystallinity and low thermal conductivity.

[0069] The hexagonal boron nitride powder may be heated in the re-firing step to the maximum re-firing temperature at a temperature increase rate of 5°C / min or less. The temperature at which the temperature increase rate adjustment in the re-firing step starts is not particularly limited, and may be, for example, room temperature (20°C to 25°C).

[0070] From the viewpoint of heating efficiency in the re-firing step, the temperature at which adjustment of the temperature rise rate starts is preferably at least 1450° C. The phrase "the temperature at which adjustment of the temperature rise rate starts is at least 1450° C." includes starting adjustment of the temperature rise rate after the temperature of the hexagonal boron nitride powder reaches 1450° C. or higher.

[0071] From the viewpoint of obtaining a highly crystalline hexagonal boron nitride powder, the heating rate in the re-firing step is preferably 4° C. / min or less, and more preferably 3° C. / min or less.

[0072] The hexagonal boron nitride powder heated to the maximum re-firing temperature is maintained at a temperature within the above-mentioned allowable maximum re-firing temperature range for 1 hour to 15 hours. From the viewpoint of sufficiently re-firing the hexagonal boron nitride powder, the maintenance time is 1 hour or more, preferably 3 hours or more. Furthermore, the maintenance time is 15 hours or less, more preferably 10 hours or less.

[0073] After being maintained at a temperature within the allowable range of the maximum re-firing temperature, the hexagonal boron nitride powder is cooled to a temperature of 1500° C. or less at a rate of 5° C. / min or less. This cooling facilitates the production of hexagonal boron nitride powder in which the value of the second intensity / first intensity in the CL spectrum is 1.0 or less.

[0074] From the viewpoint of improving the crystallinity of the hexagonal boron nitride powder, the temperature decreasing rate is 5°C / min, preferably 4°C / min or less, and more preferably 3°C / min or less.

[0075] (Heat Treatment Step) The hexagonal boron nitride powder after the re-firing step is preferably further subjected to a heat treatment step, in which the hexagonal boron nitride powder after the re-firing step is held at a temperature of 1150°C or higher and 1500°C or lower for 1 hour or higher and 15 hours or lower.

[0076] The holding temperature (heat treatment temperature) at which the hexagonal boron nitride powder is held in the heat treatment step is 1150°C or higher and 1500°C or lower, and preferably 1250°C or higher and 1450°C or lower.

[0077] The hexagonal boron nitride powder that has reached the heat treatment temperature is maintained at a temperature within the above-mentioned allowable range for holding the heat treatment temperature for 1 hour to 15 hours. From the viewpoint of improving the crystallinity of the hexagonal boron nitride powder, the holding time at the heat treatment temperature is 1 hour or more, preferably 3 hours or more. Furthermore, the holding time at the heat treatment temperature is 15 hours or less, more preferably 10 hours or less.

[0078] The inventors have independently discovered that at the maximum re-firing temperature in the re-firing step, the crystal lattice of the hexagonal boron nitride powder is mobile, and impurities such as oxygen and carbon are eliminated from the crystal lattice. However, the maximum re-firing temperature also results in the incorporation of such impurities and lattice distortion.

[0079] Furthermore, the inventors independently discovered that by performing a heat treatment step in which the hexagonal boron nitride powder after the re-firing step is maintained at the heat treatment temperature described above, impurities can be eliminated from the crystal lattice and the crystal lattice can be aligned. That is, by performing the heat treatment step, it is possible to preferably obtain a hexagonal boron nitride powder with high crystallinity. Specifically, by performing the heat treatment step, it is possible to preferably obtain a hexagonal boron nitride powder in which the value of the second intensity / first intensity in the CL spectrum is 1.0 or less.

[0080] To bring the hexagonal boron nitride powder to the heat treatment temperature, the temperature may be further lowered after the temperature drop in the re-firing step, and temperature holding may be started once the desired heat treatment temperature is reached. Alternatively, the hexagonal boron nitride powder may be further lowered below the above-mentioned heat treatment temperature range, and then heated again, and temperature holding may be started once the desired heat treatment temperature is reached.

[0081] (Recovery Step) In the recovery step, after the re-firing step or the heat treatment step, the hexagonal boron nitride powder is cooled to 50°C or less and removed from the furnace. By cooling the hexagonal boron nitride powder recovered from the furnace to 50°C or less, the reaction with moisture in the air can be reduced. By reducing this reaction, the surface oxygen concentration (B ２ O ３ This can reduce the increase in the concentration of hexagonal boron nitride powder and improve the crystallinity of the hexagonal boron nitride powder.

[0082] Furthermore, by lowering the temperature of the hexagonal boron nitride powder recovered from the furnace to 50°C or less, it becomes easy to obtain hexagonal boron nitride powder in which the value of the second intensity / first intensity in the CL spectrum is 1.0 or less.

[0083] From the viewpoint of sufficiently reducing the reaction with moisture in the air and improving the crystallinity of the hexagonal boron nitride powder, the temperature of the hexagonal boron nitride powder recovered from the furnace is preferably 40°C or less, and more preferably 30°C or less.

[0084] [Uses of hexagonal boron nitride powder] (Resin composition) A resin composition according to one embodiment of the present invention (hereinafter sometimes referred to as "the resin composition") contains the hexagonal boron nitride powder. By containing the hexagonal boron nitride powder, the resin composition has high thermal conductivity and is suitable for heat dissipation applications.

[0085] The resin composition can be used in various applications. For example, by mixing it with a resin described below to form a thermally conductive resin composition or a thermally conductive molded product, it can be preferably used for applications such as thermal interface materials such as polymer-based heat-dissipating sheets and phase change sheets, organic heat-dissipating sheets such as heat-dissipating tapes, heat-dissipating greases, heat-dissipating adhesives, and gap fillers, heat-dissipating paints such as heat-dissipating paints and heat-dissipating coats, heat-dissipating resin substrates such as PWB-based resin substrates and CCL-based resin substrates, insulating layers for metal-based substrates such as aluminum-based substrates and copper-based substrates, and encapsulants for power devices.

[0086] Examples of resins contained in the resin composition include thermoplastic resins such as polyolefins, vinyl chloride resins, methyl methacrylate resins, nylons, and fluororesins; thermosetting resins such as epoxy resins, phenolic resins, urethane resins, polyimide resins, urea resins, melamine resins, unsaturated polyester resins, silicone resins, and bismaleimide triazine resins; and synthetic rubbers.

[0087] The resin composition may contain a thermally conductive filler such as aluminum nitride and aluminum oxide, which are known high thermal conductivity insulating fillers.

[0088] Furthermore, the present resin composition may contain, as necessary, as compounding agents for the present resin composition, known polymerization initiators, curing agents, curing accelerators, polymerization inhibitors, polymerization retarders, coupling agents, dispersants, antifoaming agents, leveling agents, plasticizers, ultraviolet absorbers, flame retardants, pigments, dyes, antibacterial agents, organic fillers, organic-inorganic composite fillers, rubber particles, etc. Furthermore, the present resin composition may contain other inorganic fillers or resin components within a range that does not impair the effects of the present invention.

[0089] The content of the hexagonal boron nitride powder in the resin composition is preferably 20% by volume or more and 80% by volume or less, and more preferably 30% by volume or more and 70% by volume or less, in order to obtain high thermal conductivity.

[0090] The content of the resin in the resin composition may correspond to the amount obtained by subtracting the content of the hexagonal boron nitride powder from the total volume of the resin composition, taken as 100% by volume. The content of the resin, including the volume of compounding ingredients, is preferably 20% by volume or more and 80% by volume or less, and more preferably 30% by volume or more and 70% by volume or less.

[0091] (Resin Sheet) A resin sheet according to one embodiment of the present invention (hereinafter sometimes referred to as "the present resin sheet") is made of the present resin composition. The use of the present resin sheet is not particularly limited, but it can be used, for example, for circuit boards and heat dissipation of electronic components such as multilayer printed wiring boards.

[0092] The thermal conductivity of the present resin sheet is preferably 18 W / m K or more, more preferably 20 W / m K or more, and even more preferably 22 W / m K or more. By using the present hexagonal boron nitride powder, the present resin sheet having such good thermal conductivity can be easily produced.

[0093] The method for producing the resin sheet is not particularly limited, and for example, when the resin is a curable epoxy resin, a production method can be used that includes a mixing step of mixing the curable epoxy resin, the hexagonal boron nitride powder, and, if necessary, other components 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.

[0094] (Other Applications) Due to its high crystallinity, the present hexagonal boron nitride powder can also be used as a raw material for boron nitride processed products such as cubic boron nitride or boron nitride molded products, a nucleating agent for engineering plastics, a phase change material, a solid or liquid thermal interface material, a release agent for molten metal or molten glass molds, cosmetics, and a raw material for composite ceramics.

[0095] [Summary] A hexagonal boron nitride powder according to aspect 1 of the present invention comprises agglomerated particles of hexagonal boron nitride primary particles, and has an average particle size of 15 μm or more and 80 μm or less, and in cathodoluminescence measurement, a ratio of a first intensity indicating the maximum luminescence intensity observed at a wavelength of 235 nm or less to a second intensity indicating the maximum luminescence intensity observed at a wavelength of 250 nm or more and 440 nm or less (the second intensity / the first intensity) is 1.0 or less, and the agglomerated particles have an areal equivalent circle diameter of 20 μm or more and 40 μm or less and an average areal circularity of 0.20 or more and 0.50 or less.

[0096] In the hexagonal boron nitride powder according to Aspect 2 of the present invention, in Aspect 1, the agglomerated particles having an equivalent circle diameter of 20 μm or more and 40 μm or less may have an average area envelopment ratio of 0.50 or more and 0.80 or less.

[0097] The hexagonal boron nitride powder according to Aspect 3 of the present invention is the same as that of Aspect 1 or 2, except that the specific surface area is 3.5 m ２ / g or less.

[0098] A resin composition according to a fourth aspect of the present invention contains the hexagonal boron nitride powder according to any one of the first to third aspects.

[0099] A resin sheet according to a fifth aspect of the present invention comprises the resin composition of the fourth aspect.

[0100] A method for producing hexagonal boron nitride powder according to a sixth aspect of the present invention includes a reduction-nitridation step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron carbon-containing compound to a temperature of 1600°C or higher and 1950°C or lower in a nitrogen atmosphere to produce hexagonal boron nitride powder; an acid washing step of acid washing the hexagonal boron nitride powder containing unreacted raw materials; and a method for producing hexagonal boron nitride particles contained in the hexagonal boron nitride powder. the volumetric proportion of particles having a particle size of 10 μm or less being less than 10% and the volumetric proportion of particles having a particle size of more than 10 μm and 50 μm or less being 20% ​​to 65%; and the re-firing step of heating the hexagonal boron nitride powder to a temperature of 1850° C. to 2050° C., holding the temperature therefor for 1 hour to 15 hours, and then cooling the powder to 1500° C. or less at a cooling rate of 5° C. / min or less.

[0101] A seventh aspect of the present invention relates to the method for producing a hexagonal boron nitride powder according to the sixth aspect, and may further include, after the re-firing step, a heat treatment step of holding the hexagonal boron nitride powder at a temperature of 1150°C or higher and 1500°C or lower for 1 hour or higher and 15 hours or lower.

[0102] [Additional Notes] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0103] An embodiment of the present invention will now be described.

[0104] [Method for producing hexagonal boron nitride powder] (Example 1) 259 g of a mixture containing 141 g of boric anhydride (boron oxide), 56 g of carbon black, 32 g of calcium oxide, and 30 g of boron carbide was mixed using a ball mill. The mixture was heated to 1500°C in a graphite Tammann furnace under a nitrogen gas atmosphere, held at 1500°C for 4 hours, and then heated at a rate of 2°C / min to a maximum reduction-nitridation temperature of 1850°C. The mixture was then held at 1850°C for 2 hours to perform a reduction-nitridation treatment (reduction-nitridation step).

[0105] The resulting reduced-nitrided powder (hexagonal boron nitride powder after reduction-nitridation treatment) was placed in a container, and a five-fold amount of hydrochloric acid (7 mass% HCl) was added. The mixture was stirred at 700 rpm for 24 hours to perform pickling (pickling step). After the pickling treatment, the hydrochloric acid was filtered, and the hexagonal boron nitride powder obtained by filtration was dispersed in pure water with the same amount of hydrochloric acid as used, and then filtered again. This procedure was repeated five times, and the resulting mixture was vacuum dried at 200°C for 6 hours.

[0106] The resulting acid-washed powder (hexagonal boron nitride powder obtained after acid washing and drying) was subjected to a crushing treatment (grinding treatment) using a stone mill to adjust the particle size distribution. Specifically, the volumetric proportion of hexagonal boron nitride particles of 10 μm or less was adjusted to 3.5%, and the volumetric proportion of hexagonal boron nitride particles of more than 10 μm and 50 μm or less was adjusted to 42% (crushing step).

[0107] The obtained crushed powder (hexagonal boron nitride powder obtained after crushing) was heated to 1500°C in a graphite Tammann furnace under a nitrogen gas atmosphere, and then heated at a rate of 2°C / min to a maximum re-firing temperature of 1950°C. Then, a re-firing treatment was carried out by holding at 1950°C for 6 hours, and then nitrogen gas was flowed into the furnace and the temperature was lowered to 1350°C at a rate of 1°C / min (re-firing step).

[0108] Subsequently, a heat treatment was carried out by holding the resultant product at 1350° C. for 6 hours in a nitrogen gas atmosphere (heat treatment step).

[0109] Then, nitrogen gas was further passed through the furnace to cool it down to 25° C., and the hexagonal boron nitride powder was then recovered from the furnace.

[0110] The heat-treated powder (hexagonal boron nitride powder obtained after re-firing and heat treatment) was passed through a sieve with 106 μm openings to obtain the hexagonal boron nitride powder according to Example 1.

[0111] Example 2 Hexagonal boron nitride powder according to Example 2 was prepared in the same manner as in Example 1, except that the temperature drop rate after the re-firing treatment in the re-firing step was changed to 2° C. / min.

[0112] Example 3 A hexagonal boron nitride powder according to Example 3 was prepared in the same manner as in Example 1, except that after the re-firing treatment in the re-firing step, the temperature was lowered to 1500°C at a rate of 1°C / min, and the furnace interior was cooled from 1500°C to 25°C by flowing nitrogen gas without going through a heat treatment step.

[0113] Example 4 A hexagonal boron nitride powder according to Example 4 was prepared in the same manner as in Example 1, except that after the re-firing treatment in the re-firing step, the temperature was lowered to 1500°C at a rate of 2°C / min, and the furnace interior was cooled from 1500°C to 25°C by flowing nitrogen gas without going through a heat treatment step.

[0114] Example 5 A hexagonal boron nitride powder according to Example 5 was prepared in the same manner as in Example 1, except that after the re-firing treatment in the re-firing step, the temperature was lowered to 1200°C at 1°C / min, and heat treatment was performed at 1200°C in the heat treatment step.

[0115] Example 6 A hexagonal boron nitride powder according to Example 6 was prepared in the same manner as in Example 1, except that the heat treatment temperature holding time in the heat treatment step was changed to 2 hours.

[0116] Example 7 A hexagonal boron nitride powder according to Example 7 was prepared using the same procedure as in Example 1, except that the conditions for adjusting the particle size distribution in the crushing step were changed and the heat treatment temperature holding time in the heat treatment step was changed to 8 hours. In the crushing step, the volumetric proportion of hexagonal boron nitride particles of 10 μm or less was adjusted to 3.0%, and the volumetric proportion of hexagonal boron nitride particles of more than 10 μm and 50 μm or less was adjusted to 30%.

[0117] Example 8 Hexagonal boron nitride powder according to Example 8 was prepared using the same procedure as in Example 1, except that the maximum reduction-nitridation temperature in the reduction-nitridation step was set to 1750° C., the conditions for adjusting the particle size distribution in the crushing step were changed, and the maximum re-firing temperature and holding temperature in the re-firing step were set to 1900° C. In the crushing step, the volumetric proportion of particles of 10 μm or less was adjusted to 4.1%, and the volumetric proportion of agglomerated particles of more than 10 μm and 50 μm or less was adjusted to 46%.

[0118] Example 9 A hexagonal boron nitride powder according to Example 9 was prepared using the same procedure as in Example 1, except that the maximum reduction-nitridation temperature in the reduction-nitridation step was set to 1750°C, the conditions for adjusting the particle size distribution in the crushing step were changed, the maximum re-firing temperature and holding temperature in the re-firing step were set to 1900°C, and the furnace interior was cooled to 25°C from 1500°C by flowing nitrogen gas without going through a heat treatment step. In the crushing step, the volumetric proportion of particles of 10 μm or less was adjusted to 4.1%, and the volumetric proportion of agglomerated particles of more than 10 μm and 50 μm or less was adjusted to 46%.

[0119] (Comparative Example 1) A hexagonal boron nitride powder according to Comparative Example 1 was prepared using the same procedure as in Example 1, except that the re-firing step and the heat treatment step were not performed. After the crushing step, a sieve treatment with a mesh size of 106 μm was performed. (Comparative Example 2) A hexagonal boron nitride powder according to Comparative Example 2 was prepared using the same procedure as in Example 1, except that the re-firing step and the heat treatment step were not performed and the conditions for adjusting the particle size distribution in the crushing step were changed. In the crushing step, the volumetric proportion of hexagonal boron nitride particles of 10 μm or less was adjusted to 2.8%, and the volumetric proportion of hexagonal boron nitride particles of more than 10 μm and 50 μm or less was adjusted to 30%, and then a sieve treatment with a mesh size of 106 μm was performed.

[0120] Comparative Example 3 A hexagonal boron nitride powder according to Comparative Example 3 was prepared in the same manner as in Example 1, except that the maximum re-firing temperature in the re-firing step was changed to 1800°C.

[0121] Comparative Example 4 A hexagonal boron nitride powder according to Comparative Example 4 was prepared in the same manner as in Example 1, except that the maximum re-firing temperature in the re-firing step was changed to 1600°C.

[0122] Comparative Example 5 Hexagonal boron nitride powder according to Comparative Example 5 was prepared in the same manner as in Example 1, except that the maximum re-firing temperature in the re-firing step was changed to 2060°C.

[0123] Comparative Example 6 A hexagonal boron nitride powder according to Comparative Example 2 was prepared using the same procedure as in Example 1, except that the re-firing step was not performed and the conditions for adjusting the particle size distribution in the crushing step were changed. In the crushing step, the volumetric proportion of hexagonal boron nitride particles of 10 μm or less was adjusted to 2.8%, and the volumetric proportion of hexagonal boron nitride particles of more than 10 μm and 50 μm or less was adjusted to 30%. In the heat treatment step, the temperature was raised to 1350°C in a nitrogen gas atmosphere, and then heat treatment was performed at that temperature in the same manner as in Example 1.

[0124] Comparative Example 7 Hexagonal boron nitride powder according to Comparative Example 7 was prepared using the same procedure as in Example 1, except that the maximum reduction-nitridation temperature in the reduction-nitridation step was set to 1980°C and the conditions for adjusting the particle size distribution in the crushing step were changed. In the crushing step, the volumetric proportion of hexagonal boron nitride particles of 10 μm or less was adjusted to 2.5%, and the volumetric proportion of hexagonal boron nitride particles of more than 10 μm and 50 μm or less was adjusted to 40%.

[0125] (Comparative Example 8) A commercially available agglomerated hexagonal boron nitride powder (average particle size 37 μm, specific surface area 1.7 m) containing agglomerated particles was used. ２ / g) was used.

[0126] [Method for evaluating hexagonal boron nitride powder] (Measurement of average particle size and particle size distribution) The average particle size of the obtained hexagonal boron nitride powder, the volumetric proportion of particles of 10 μm or less in size in the crushing step, and the volumetric proportion of particles of more than 10 μm and 50 μm or less in size were determined by the following particle size distribution measurement method.

[0127] The sample powder was dispersed in ethanol at a concentration of 0.2% by mass, and the particle size distribution of the dispersion was measured using a laser diffraction / scattering particle size distribution analyzer (MICROTRACK (registered trademark)-MT3300EXII, manufactured by Microtrack-Bell Co., Ltd.). The measurement was performed without subjecting the dispersion to any disintegration treatment such as ultrasonic dispersion.

[0128] In the obtained particle size volume frequency distribution (particle size distribution), the volume frequencies were accumulated from the smallest particle size to the particle size distribution, and the particle size (D50) at which the accumulated value reached 50% was defined as the average particle size. In addition, the volume-based percentage (%) of particles having a particle size of 10 μm or less and the volume-based percentage (%) of particles having a particle size of more than 10 μm and 50 μm or less were calculated.

[0129] (Measurement of Specific Surface Area) The specific surface area of ​​the hexagonal boron nitride powder was determined by the BET one-point method (nitrogen adsorption one-point method) using a flow-type automatic specific surface area measuring device (Shimadzu Corporation: Flowsorb II-2300). 2 g of powder sample was used for the measurement, which had been previously dried at 100°C for 1 hour in a nitrogen gas flow.

[0130] (CL Spectrum Measurement) The CL spectrum of the obtained hexagonal boron nitride powder was measured from 190 nm to 450 nm using a cathodoluminescence spectrometer (spectrometer: HR-320 manufactured by Horiba, Ltd., scanning electron microscope: Schottky emission type SEM S-4300SE manufactured by Hitachi High-Tech Corporation). The measurement conditions were: detector: CCD (Jobin Yvon), spectrometer diffraction grating: 300 gr / mm, blaze wavelength: 250 nm, measurement temperature: room temperature, acceleration voltage: 5 kV, W.D.: 10.3 mm, magnification: 2000 times (50 × 50 μm ２ ), and CL spectrum integration time: 240 seconds. In the measured CL spectrum, the intensity of the maximum emission peak observed at a wavelength of 235 nm or less was defined as the first intensity, and the intensity of the maximum emission peak observed in the wavelength range of 250 nm to 440 nm was defined as the second intensity, and the emission peak intensity ratio (second intensity / first intensity) was calculated. Measurement was performed in any five visual fields, and the average value was calculated.

[0131] (Average value of area circularity and average value of area envelopment) The obtained hexagonal boron nitride powder was subjected to particle image analysis using an image particle size distribution analyzer (Morphologi (registered trademark) G3SE manufactured by Malvern Panalytical, Spectris Co., Ltd.). Using a dispersion unit attached to the apparatus, the sample powder was dispersed in 11 mm ３was dispersed on a glass plate under the following conditions: dispersion pressure: 1.0 bar, compressed air application time: 10 ms, and post-dispersion rest time: 300 seconds. Next, binarized particle images were acquired under the following conditions: objective lens: 10x (3.5-210 μm), light source: transmitted light, focus adjustment: 3 times, overlap: 20%, and measurement range: 44 mm x 49 mm. The software attached to the image particle size analyzer calculates the area-equivalent circle diameter, area circularity, and area envelopment ratio for each particle image acquired.

[0132] The volume frequency distribution of the equivalent circle diameter closely matches the particle size distribution obtained by the laser diffraction scattering method. Agglomerated particles having an equivalent circle diameter of 20 μm to 40 μm were extracted, and the volume frequency distribution of the area circularity of these agglomerated particles was obtained, and the average area circularity was calculated. Similarly, agglomerated particles having an equivalent circle diameter of 20 μm to 40 μm were extracted, and the volume frequency distribution of the area envelopment was obtained, and the average area envelopment was calculated.

[0133] (Evaluation of Thermal Conductivity of Resin Composition) The hexagonal boron nitride powder obtained in each of the Examples and Comparative Examples was filled into an epoxy resin to prepare a resin composition, and the thermal conductivity was evaluated. 50 parts by mass of jER® 828 (a bisphenol A-type epoxy resin manufactured by Mitsubishi Chemical Corporation, epoxy equivalent weight 189, viscosity (25°C) 135 poise), 50 parts by mass of EPPN-501HY (a trisphenolmethane-type epoxy resin manufactured by Nippon Kayaku Co., Ltd., epoxy equivalent weight 166, softening point 60°C), 87 parts by mass of GPH-103 (a biphenyl aralkylphenol novolac manufactured by Nippon Kayaku Co., Ltd., hydroxyl equivalent weight 230, softening point 103°C) as a curing agent, and 0.8 parts by mass of 4-dimethylaminopyridine (DMAP, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a curing accelerator were added to 140 parts by mass of methyl ethyl ketone, stirred, and dissolved. 675 parts by mass of hexagonal boron nitride powder and 250 parts by mass of methyl ethyl ketone were then added, and the mixture was stirred for 30 minutes using a homodisper. Further, methyl ethyl ketone was appropriately added to adjust the viscosity, and the mixture was degassed under reduced pressure to prepare a varnish of the resin composition.

[0134] Next, the varnish of the resin composition obtained above was uniformly applied to the release-treated surface of a support so that the thickness after drying would be 80 μm, and then dried at 80°C to produce a B-stage sheet of the resin composition. A release-treated polyimide film (UPILEX (registered trademark)-50S, manufactured by UBE Corporation, thickness 50 μm) was used as the support. The B-stage sheet is a sheet of the resin composition in a semi-cured state.

[0135] Two B-stage sheets of the resin composition were stacked so that the resin composition layers were in contact with each other, and then heat-pressed at 100 ° C. under reduced pressure and a press pressure of 4 MPa for 3 minutes to bond them together. The temperature was then raised to 180 ° C., and the resin composition sheet was cured by heat-pressing for 60 minutes under reduced pressure and a press pressure of 20 MPa. The resin was then completely cured by heat treatment at 190 ° C. for 2 hours. The polyimide films on both sides were then peeled off to obtain a resin sheet.

[0136] The thermal conductivity (W / m K) of the resin sheet is ２ / sec) x density (kg / m ３ ) × specific heat (J / kg K). Thermal diffusivity was measured using a desktop thermal diffusivity / thermal conductivity measuring device (NETZSCH: Xenon Flash Analyzer LFA467 HyperFlash (registered trademark)) by the xenon flash method (laser flash method, ISO22007-4). Both sides of the sample sheet were gold-coated and graphite-sprayed, and the charge voltage was 200 V and the pulse width was 20 μsec. Density was measured by the Archimedes method using an analytical balance (Mettler-Toledo: XS204V). Specific heat was measured using a differential scanning calorimeter (Rigaku Corporation: Thermo Plus Evo DSC8230).

[0137] [Evaluation Results] The manufacturing conditions for each example and comparative example, as well as the evaluation results of the hexagonal boron nitride powder and resin sheet, are shown in Tables 1 and 2. As shown in Table 1, the hexagonal boron nitride powder obtained by a method conforming to the present manufacturing method had properties within the ranges specified in the present invention with respect to the value of second intensity / first intensity and area circularity, and therefore showed good thermal conductivity when made into a resin sheet.

[0138] FIG. 1 shows CL spectra of the hexagonal boron nitride powders of Example 1, Example 3, and Comparative Example 1. FIG. 2 shows the distribution of circularity of agglomerated particles having an equivalent circle diameter of 20 to 40 μm for the hexagonal boron nitride powders of Example 1, Example 9, and Comparative Example 8. FIG. 3 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 40 μm or less but around 40 μm for the hexagonal boron nitride powder of Example 1. FIG. 4 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 100 μm or more for the hexagonal boron nitride powder of Example 1. FIG. 5 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 40 μm or less but around 40 μm for the hexagonal boron nitride powder of Comparative Example 8. FIG. 6 shows a two-dimensional projection image of agglomerated particles having an equivalent circle diameter of 100 μm or more for the hexagonal boron nitride powder of Comparative Example 8. In FIGS. 3 to 6, the numerical values ​​shown alongside the two-dimensional projected images of each agglomerated particle indicate the equivalent circle diameter of the agglomerated particle.

[0139] As shown in Figure 1, the hexagonal boron nitride powder according to an example of the present invention has a smaller second intensity relative to the first intensity, indicating good crystallinity, compared to the hexagonal boron nitride powder according to the comparative example. Also, as shown in Figure 2, the hexagonal boron nitride powder according to an example of the present invention has a smaller areal circularity in agglomerated particles having an areal circle equivalent diameter of 20 μm or more and 40 μm or less, indicating a distorted shape, compared to the hexagonal boron nitride powder according to the comparative example.

[0140] 3 to 6 also show that the hexagonal boron nitride powder according to an example of the present invention has agglomerated particles with an equivalent circle diameter of 40 μm or less that have a branched, distorted shape compared to the hexagonal boron nitride powder according to the comparative example. It is believed that the presence of such agglomerated particles in this hexagonal boron nitride powder results in excellent contact between the hexagonal boron nitride particles when filled into a resin sheet, and thus good thermal conductivity.

[0141]

[0142]

Claims

1. A hexagonal boron nitride powder comprising agglomerated particles of hexagonal boron nitride primary particles, the powder having an average particle size of 15 μm or more and 80 μm or less, wherein, in cathodoluminescence measurement, the ratio of a first intensity indicating the maximum luminescence intensity observed at a wavelength of 235 nm or less to a second intensity indicating the maximum luminescence intensity observed at a wavelength of 250 nm or more and 440 nm or less (the second intensity / the first intensity) is 1.0 or less, and the agglomerated particles have an area-equivalent circle diameter of 20 μm or more and 40 μm or less, and the average area circularity of the agglomerated particles is 0.20 or more and 0.50 or less.

2. The hexagonal boron nitride powder according to claim 1, wherein the agglomerated particles having an equivalent circle diameter of 20 μm or more and 40 μm or less have an average area envelopment ratio of 0.50 or more and 0.80 or less.

3. Specific surface area is 3.5m ２ 2. The hexagonal boron nitride powder according to claim 1, wherein the SiO2 content is 0.1 / g or less.

4. A resin composition containing the hexagonal boron nitride powder according to any one of claims 1 to 3.

5. A resin sheet made of the resin composition according to claim 4.

6. A reduction-nitridation process in which a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound is heated to a temperature of 1600°C or higher and 1950°C or lower in a nitrogen atmosphere to produce hexagonal boron nitride powder; an acid washing process in which the hexagonal boron nitride powder containing unreacted raw materials is acid washed; and a crushing process in which the hexagonal boron nitride powder contains hexagonal boron nitride particles whose volume ratio is 10 μm or less and whose particle diameter is greater than 10 μm and less than 50 μm and whose volume ratio is 20% or higher and 65% or lower. a re-firing step in which the hexagonal boron nitride powder is heated to a temperature of 1850°C or higher and 2050°C or lower, held for 1 hour or higher and 15 hours or lower, and then cooled to a temperature of 1500°C or lower at a rate of 5°C / min or lower.

7. The method for producing hexagonal boron nitride powder according to claim 6, further comprising a heat treatment step of holding the hexagonal boron nitride powder at a temperature of 1150°C or higher and 1500°C or lower for 1 hour or higher and 15 hours or lower after the re-firing step.

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