Hexagonal boron nitride powder and method for producing hexagonal boron nitride powder

Abstract

We will develop hexagonal boron nitride powder to obtain resins that combine excellent thermal conductivity and dielectric strength. [Solution] A hexagonal boron nitride powder in which the ratio (first cumulative pore volume / second cumulative pore volume) of a first cumulative pore volume in the pore diameter range of 0 μm or more and 1.0 μm or less to a second cumulative pore volume in the pore diameter range of more than 1.0 μm and 5.0 μm or less, measured by mercury intrusion porosimetry in accordance with JIS R 1655:2003, is 0.50 or less.

Description

[Technical Field]

[0001] The present invention relates to a hexagonal boron nitride powder and a method for producing the hexagonal boron nitride powder. [Background technology]

[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 have been developed to efficiently dissipate heat from electronic components.

[0003] Hexagonal boron nitride powder can be filled into resins used in electronic components to form resin compositions that improve the thermal conductivity of the resin composition. It is known that the hexagonal boron nitride powder filled into resins can improve the thermal conductivity of resin compositions by including agglomerated particles. Patent Document 1 also discloses that a composite with excellent thermal conductivity can be produced when the proportion of pores inside the agglomerated particles is small. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 079912 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the hexagonal boron nitride powder described in Patent Document 1 may have minute pores inside the aggregated particles, which may result in insufficient dielectric strength of the resin composition.

[0006] An object of one aspect of the present invention is to provide a hexagonal boron nitride powder or the like for obtaining a resin composition that has both excellent thermal conductivity and dielectric strength. [Means for solving the problem]

[0007] In order to solve the above problems, the hexagonal boron nitride powder according to one aspect of the present invention is The ratio (first cumulative pore volume / second cumulative pore volume) of a first cumulative pore volume in a pore diameter range of 0 μm or more and 1.0 μm or less, measured by mercury intrusion porosimetry in accordance with R 1655:2003, to a second cumulative pore volume in a pore diameter range of more than 1.0 μm and 5.0 μm or less, is 0.50 or less.

[0008] In order to achieve the above object, one embodiment of the present invention provides a method for producing a hexagonal boron nitride powder, which comprises a reduction-nitridation step of holding a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron carbon compound in a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or more, then heating the mixture to 1650°C to 1850°C at a heating rate of less than 1°C / min, and then holding the mixture at 1650°C to 1850°C for 2 hours or more, wherein the oxygen-containing boron compound has an average particle size of 150 μm to 300 μm, and particles having particle sizes of 75 μm or less and 425 μm or more account for 1 mass % or less, and the raw material mixture has a ratio of the mass of the oxygen-containing boron compound converted into B to the mass of the carbon source converted into C (the mass converted into B / the mass converted into C) of 0.70 to 0.90. [Effects of the Invention]

[0009] According to one aspect of the present invention, a hexagonal boron nitride powder or the like can be realized to obtain a resin composition that has both excellent thermal conductivity and dielectric strength. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a graph showing the distribution of logarithmic differential pore volume versus pore diameter for the hexagonal boron nitride powder according to Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] One embodiment of the present invention will be described in detail below. In this specification, "A to B" representing a numerical range means "A or more and B or less" unless otherwise specified. Furthermore, the upper and lower limits of a numerical range can be arbitrarily combined to form a numerical range.

[0012] [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") can realize a resin composition that combines excellent thermal conductivity and dielectric strength by reducing the proportion of fine pores.

[0013] Specifically, the cumulative pore volume in the pore diameter range of 0 μm to 1.0 μm is defined as the first cumulative pore volume, and the cumulative pore volume in the pore diameter range of more than 1.0 μm to 5.0 μm is defined as the second cumulative pore volume. In this case, the ratio of the first cumulative pore volume to the second cumulative pore volume (first cumulative pore volume / second cumulative pore volume) of this hexagonal boron nitride powder is 0.50 or less. The cumulative pore volume is measured by mercury intrusion porosimetry in accordance with JIS R 1655:2003.

[0014] (pore size) As a result of extensive research into hexagonal boron nitride powder, the present inventors have succeeded in making a new discovery: they have independently discovered that by adopting specific conditions when producing hexagonal boron nitride powder, it is possible to obtain hexagonal boron nitride powder in which the value of first cumulative pore volume / second cumulative pore volume is 0.50 or less.

[0015] The hexagonal boron nitride powder having such a value of the first cumulative pore volume / second cumulative pore volume has a small proportion of fine pores with a pore diameter of 0 μm or more and 1.0 μm or less within the agglomerated particles, and a large proportion of moderately sized pores with a pore diameter of more than 1.0 μm and less than 5.0 μm. Hereinafter, when simply referred to as "fine pores," unless otherwise specified, this refers to pores with a pore diameter of 0 μm or more and 1.0 μm or less within the agglomerated particles. Furthermore, when simply referred to as "moderately sized pores," this refers to pores with a pore diameter of more than 1.0 μm and less than 5.0 μm within the agglomerated particles, unless otherwise specified.

[0016] The first integrated pore volume indicates the integrated value of the logarithmic differential pore volume in the pore diameter range of 0 μm or more and 1.0 μm or less. The second integrated pore volume indicates the integrated value of the logarithmic differential pore volume in the pore diameter range of more than 1.0 μm and 5.0 μm or less. The first integrated pore volume and the second integrated pore volume may each be calculated as the area of ​​the range surrounded by a line connecting the measured values ​​of the logarithmic differential pore volume in the target pore diameter range in a graph showing the logarithmic differential pore volume distribution against pore diameter.

[0017] 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 thermal anisotropy.

[0018] Pores are formed inside the agglomerated particles as gaps between the primary particles. Conventional techniques such as those described in Patent Document 1 attempt to improve the thermal conductivity of hexagonal boron nitride powder by reducing the size and proportion of these pores. However, if many fine pores exist within the agglomerated particles, when the hexagonal boron nitride powder is filled into a resin, the resin will not easily penetrate the fine pores. If the pores remain as voids in the final product, the thermal conductivity of the resin composition will ultimately decrease.

[0019] Furthermore, voids remaining without resin penetration can cause a decrease in dielectric strength. Furthermore, there is a risk that liquid solvent may seep into the voids and remain in the final product. This residual solvent is unacceptable in manufacturing because it can cause adverse effects such as deterioration of parts placed around the resin composition due to the solvent seeping out of the voids.

[0020] The present inventors have discovered that by adjusting the pore size in hexagonal boron nitride powder to fall within a predetermined range, it is possible to obtain excellent dielectric strength in a resin composition containing the hexagonal boron nitride powder while maintaining high thermal conductivity, and have completed the present hexagonal boron nitride powder.

[0021] Mercury intrusion porosimetry in accordance with JIS R 1655:2003 can be used to measure the diameter of pores formed in hexagonal boron nitride powder. It is common technical knowledge that pores with diameters of 5.0 μm or less measured by mercury intrusion porosimetry are pores formed within the above-mentioned agglomerated particles, while pores with diameters greater than 5.0 μm are pores formed as gaps between a plurality of agglomerated particles. In Patent Document 1, the size of pores with diameters of 5.0 μm or less is made as small as possible, specifically, the pores are minimised so that a peak is obtained in the range of 0 μm to 1.0 μm in the logarithmic differential pore volume distribution for pore diameters of 5.0 μm or less.

[0022] On the other hand, in the present hexagonal boron nitride powder, the second cumulative pore volume in the range of more than 1.0 μm and not more than 5.0 μm is larger than the first cumulative pore volume in the range of 0 μm or more and 1.0 μm or less. In other words, in the present hexagonal boron nitride powder, the proportion of pores of a moderate size with a pore diameter of more than 1.0 μm is adjusted to be sufficiently larger than the proportion of fine pores with a pore diameter of 1.0 μm or less.

[0023] The value of the first cumulative pore volume / second cumulative pore volume is 0.50 or less, preferably 0.40 or less, more preferably 0.30 or less, and even more preferably 0.20 or less. The value of the first cumulative pore volume / second cumulative pore volume may be greater than 0. The smaller the value of the first cumulative pore volume / second cumulative pore volume, the more the proportion of fine pores can be reduced, and the more the dielectric strength of a resin composition containing the present hexagonal boron nitride powder can be improved.

[0024] Within the range satisfying this first cumulative pore volume / second cumulative pore volume value, the first cumulative pore volume is preferably 0.20 mL / g or less, and the second cumulative pore volume is preferably 0.20 mL / g or more and 1.00 mL / g or less. From the viewpoint of reducing the proportion of fine pores, the first cumulative pore volume may be 0.15 mL / g or less or 0.10 mL / g or less. The first cumulative pore volume may be 0 mL / g or more. Furthermore, from the viewpoint of increasing the proportion of pores of an appropriate size, the second cumulative pore volume may be 0.30 mL / g or more, 0.40 mL / g or more, or 0.50 mL / g or more. Furthermore, the second cumulative pore volume may be 1.00 mL / g or less or 0.90 mL / g or less.

[0025] In addition, in a graph showing the distribution of logarithmic differential pore volume versus pore diameter obtained by the above-mentioned mercury intrusion porosimetry, it is preferable that the maximum peak in the pore diameter range of 5.0 μm or less exists in the pore diameter range of more than 1.0 μm and 5.0 μm or less. The maximum peak refers to the peak at which the logarithmic differential pore volume value at the peak top is the largest within the above-mentioned range.

[0026] (specific surface area) The specific surface area of ​​this hexagonal boron nitride powder is 3.5m 2 / g or less, and 2 / g or less, and 2.7m 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 is, for example, 1.0 m 2 The specific surface area of ​​the hexagonal boron nitride powder can be determined by the BET one-point method (nitrogen adsorption one-point method).

[0027] If the hexagonal boron nitride powder has the specific surface area described above, the primary particles constituting the aggregated particles have a particle diameter of about 1 μm or more, and pores of an appropriate size according to one aspect of the present invention can be formed. This reduces thermal anisotropy and simultaneously achieves excellent thermal conductivity and dielectric strength. 2 If it exceeds 1 / g, the primary particles constituting the aggregated particles become finer and the number of fine pores increases, which is not preferable.

[0028] (Hardness) The present hexagonal boron nitride powder has pores of appropriate size formed within the aggregated particles as described above, while also being hard enough to prevent the aggregated particles from breaking, for example, during pressure molding of a resin composition after filling. Specifically, the average particle size of the present hexagonal boron nitride powder is defined as the first average particle size, and the average particle size after dispersing the present hexagonal boron nitride powder in a liquid and subjecting it to ultrasonic treatment is defined as the second average particle size. In this case, the ratio of the first average particle size to the second average particle size of the present hexagonal boron nitride powder (first average particle size / second average particle size) is 1.25 or more and 6.00 or less. The first average particle size is the average particle size of the present hexagonal boron nitride powder before ultrasonic treatment.

[0029] When the ratio of the first average particle size to the second average particle size is within this range, the hexagonal boron nitride powder has a hardness that makes it resistant to breakage due to forces that may act on the resin composition. Therefore, the hexagonal boron nitride powder is resistant to breakage of agglomerated particles in the resin composition, and the final product exhibits good thermal conductivity and dielectric strength. The ratio of the first average particle size to the second average particle size may be 1.50 or more and 5.00 or less, or 2.00 or more and 4.00 or less.

[0030] The first 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 first average particle size is 80 μm or less, preferably 70 μm or less, and more preferably 60 μm or less.

[0031] (Method for measuring average particle size) The first average particle size was measured by the following method: 0.5 g of the hexagonal boron nitride powder was dispersed in 20 g of an ethanol dispersion medium, and the particle size at 50% cumulative volume of the hexagonal boron nitride powder was measured using a particle size distribution analyzer (MT-3000) manufactured by Microtrac-Bell Corporation, and the measured value was taken as the first average particle size.

[0032] The second average particle size was measured by the following method. 0.5 g of the hexagonal boron nitride powder was dispersed in 20 g of ethanol as a dispersion medium using a homogenizer (SONIFER) manufactured by Branson. The powder was subjected to ultrasonic treatment at an amplitude of 35% for 20 minutes using a particle size distribution analyzer (MT-3000) manufactured by Microtrac-Bell Corporation. The particle size at 50% cumulative volume of the hexagonal boron nitride powder after the ultrasonic treatment was measured and used as the second average particle size.

[0033] (summary) As described above, this hexagonal boron nitride powder exhibits excellent thermal conductivity and dielectric strength when used in resin compositions because the pore size formed within the aggregated particles is controlled. Therefore, when used in fillers such as resin compositions, it is possible to achieve the desired thermal conductivity while also improving product life. These effects also contribute to the achievement of Goal 12 of the United Nations' Sustainable Development Goals (SDGs), "Ensure sustainable consumption and production patterns."

[0034] [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") includes a reduction-nitridation step. The present production method may also be a method in which the reduction-nitridation step, the acid washing step, and the re-firing step are carried out in this order. The present production method may also further include at least one of a crushing step and a recovery step. Each step will be described below.

[0035] (reduction nitridation process) In the reduction-nitridation process, a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron carbon compound is held in a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or more. Here, the temperature held for 5 hours or more is sometimes referred to as the first heating temperature. Next, the raw material mixture held at the first heating temperature is heated to a temperature of 1650°C to 1850°C at a heating rate of less than 1°C / min, and then held at 1650°C to 1850°C for 2 hours or more. Here, the temperature held for 2 hours or more is sometimes referred to as the second heating temperature. This process can produce hexagonal boron nitride.

[0036] The oxygen-containing boron compound used has an average particle size of 150 μm or more and 300 μm or less, and particles with particle sizes of 75 μm or less and 425 μm or more account for 1 mass % or less. The average particle size of the oxygen-containing boron compound may be 200 μm or more and 250 μm or less. Furthermore, in the oxygen-containing boron compound, particles with particle sizes of 75 μm or less and 425 μm or more may account for 0.8 mass % or less.

[0037] The average particle size of the oxygen-containing boron compound can be measured by laser diffraction particle size distribution measurement, and the mass proportions of particles with particle sizes of 75 μm or less and 425 μm or more can be measured by a dry sieving test.

[0038] As such an oxygen-containing boron compound, a compound containing boron and oxygen atoms may be used. Examples of the oxygen-containing boron compound include boric acid, Boron oxide ( Boric anhydride ) Examples of suitable boric acid include sodium tetraborate, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. These may be used alone or in combination of two or more. Among these, boric acid or boron oxide, which are readily available, may be preferably used.

[0039] 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.

[0040] 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. Among these, calcium oxide or calcium carbonate is preferred. By including an oxygen-containing calcium compound in the raw material mixture, highly crystalline hexagonal boron nitride powder can be obtained when heated in the reduction-nitridation step.

[0041] 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.

[0042] As the carbon-containing boron compound, a compound containing carbon and boron (for example, boron carbide) may be used. The average particle size of the carbon-containing boron compound is preferably 1 μm or more and 500 μm or less, and more preferably 10 μm or more and 400 μm or less. twist The average particle size of the carbon-containing boron compound can be measured by a laser diffraction particle size distribution analyzer.

[0043] The raw material mixture is obtained by mixing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron-carbon compound in the ratios shown below. The ratio (mass converted to B / mass converted to C, element ratio) of the mass of the oxygen-containing boron compound converted to B (boron) and the mass of the carbon source converted to C (carbon) is 0.70 to 0.90. The oxygen-containing calcium compound may be 5 to 20 parts by mass, converted to CaO on a Ca basis, per 100 parts by mass of the total of the mass of the oxygen-containing boron compound converted to BO on a B basis and the mass of the carbon source converted to C. The boron-carbon compound may be 5 to 45 parts by mass, per 100 parts by mass of the total of the masses of the oxygen-containing boron compound, carbon source, and oxygen-containing calcium compound converted to BO, C, and CaO, respectively.

[0044] The raw material mixture containing each raw material may be supplied to the reduction-nitridation reaction in the form of powder, or may be formed into granules. The raw materials may be mixed using a vibrating mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibrating mixer, or a mixing mill. Stir This can be done using a mixer such as a stirrer or a V-shaped mixer.

[0045] 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.

[0046] In the heating of the raw material mixture in the reduction-nitridation step, the first heating temperature may be 1460°C or higher and 1540°C or lower, and the second heating temperature may be 1670°C or higher and 1800°C or lower. Furthermore, after holding at the first heating temperature, the heating rate during heating to the second heating temperature is preferably less than 1°C / min and 0.8°C / min or lower. By heating at such first and second heating temperatures and heating rates, the growth of primary particles of hexagonal boron nitride in the reduction-nitridation step can be uniformly controlled. The size of the primary particles affects the size of pores formed in the agglomerated particles. The reduction-nitridation step described above can produce hexagonal boron nitride powder containing agglomerated particles with pores of appropriate size formed therein.

[0047] From the viewpoint of promoting the reduction-nitridation reaction, the holding time at the first heating temperature is 5 hours or more, or may be 6 hours or more, or may be 7 hours or more. From the viewpoint of reducing production costs, the holding time at the first heating temperature may be 20 hours or less, or may be 15 hours or less, or may be 10 hours or less.

[0048] From the viewpoint of promoting uniform growth of primary particles of hexagonal boron nitride, the holding time at the second heating temperature is 2 hours or more, or may be 3 hours or more, or may be 4 hours or more. From the viewpoint of reducing production costs, the holding time at the second heating temperature may be 10 hours or less, or may be 8 hours or less, or may be 6 hours or less.

[0049] 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.

[0050] (acid washing process) 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 the production of hexagonal boron nitride powder from which the unreacted raw materials have been removed.

[0051] 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.

[0052] The acid used in the acid washing may be hydrochloric acid, or may be nitric acid, sulfuric acid, acetic acid, or the like.

[0053] 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.

[0054] After acid washing or water washing, the hydrated aggregates may be dried. Drying conditions are preferably, for example, in the atmosphere at a temperature of 50°C to 250°C or less, 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%.

[0055] (Crushing process) The hexagonal boron nitride powder obtained after the acid washing step may be subjected to a crushing step to adjust the particle size distribution to a predetermined value. Crushing in the crushing step is preferably carried out gently using a jet mill, ball mill, hammer mill, stone mill, or the like. The crushing step may be carried out before or after the acid washing step.

[0056] (Re-firing process) The re-firing step is a step in which the hexagonal boron nitride powder obtained after the crushing step is heated to a re-firing temperature of 1850°C or higher and 2050°C or lower.

[0057] 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.

[0058] The re-firing temperature to which the hexagonal boron nitride powder is heated in the re-firing step is 1850° C. or higher and 2050° C. or lower, and preferably 1900° C. or higher and 2000° C. or lower. This heating can improve the crystallinity of the hexagonal boron nitride powder.

[0059] The hexagonal boron nitride powder heated to the re-firing temperature is preferably maintained at a temperature within the above-mentioned 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.

[0060] (Recovery and classification process) In the recovery step, after the re-firing step, the hexagonal boron nitride powder is removed from the furnace after having been cooled to 50°C or below. By reducing the temperature of the hexagonal boron nitride powder recovered from the furnace to 50°C or below, it is possible to reduce reaction with moisture in the air. This reduction in reaction reduces an increase in the surface oxygen concentration (B2O3 concentration) of the hexagonal boron nitride powder, thereby improving the crystallinity of the hexagonal boron nitride powder. From the perspective of sufficiently reducing 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 below, and more preferably 30°C or below.

[0061] The particle size of the recovered hexagonal boron nitride powder may be adjusted using a known classifier, such as a dry air classifier or a dry sieve classifier.

[0062] [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.

[0063] The resin composition can be used in a variety of 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 in 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.

[0064] 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.

[0065] The resin composition may contain a thermally conductive filler such as aluminum nitride and aluminum oxide, which are known high thermal conductivity insulating fillers.

[0066] 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.

[0067] 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.

[0068] 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 agents, if any, 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.

[0069] (resin sheet) A resin sheet according to one embodiment of the present invention (hereinafter sometimes referred to as "the resin sheet") is made of the resin composition. The uses of the resin sheet are 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.

[0070] The thermal conductivity of the resin sheet is preferably 15 W / m K or higher, more preferably 18 W / m K or higher, and even more preferably 20 W / m K or higher. By using the hexagonal boron nitride powder, the resin sheet with such good thermal conductivity can be easily manufactured. The thermal conductivity of the resin sheet can be measured by the method described in the Examples below.

[0071] The dielectric strength of the resin sheet is preferably 60 kV / mm, and more preferably 70 kV / It is more preferable that the dielectric strength of the resin sheet is mm. By using the present hexagonal boron nitride powder, the present resin sheet having such good thermal conductivity can be easily produced. The dielectric strength of the present resin sheet can be measured by the method described in the examples below.

[0072] 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.

[0073] 〔summary〕 The hexagonal boron nitride powder according to aspect 1 of the present invention has a ratio (first cumulative pore volume / second cumulative pore volume) of 0.50 or less between a first cumulative pore volume within a pore diameter range of 0 μm or more and 1.0 μm or less, and a second cumulative pore volume within a pore diameter range of more than 1.0 μm and 5.0 μm or less, as measured by mercury intrusion porosimetry in accordance with JIS R 1655:2003.

[0074] In the hexagonal boron nitride powder according to Aspect 2 of the present invention, in Aspect 1, the first cumulative pore volume may be 0.20 mL / g or less, and the second cumulative pore volume may be 0.20 mL / g or more and 1.00 mL / g or less.

[0075] The hexagonal boron nitride powder according to aspect 3 of the present invention is the powder of aspect 1 or 2, wherein the specific surface area is 3.5 m 2 / g or less.

[0076] The hexagonal boron nitride powder according to Aspect 4 of the present invention is any one of Aspects 1 to 3, wherein the ratio of a first average particle size of the hexagonal boron nitride powder to a second average particle size obtained after dispersing the hexagonal boron nitride powder in a liquid and subjecting it to ultrasonic treatment (the first average particle size / the second average particle size) is 1.25 or more and 6.00 or less.

[0077] A fifth aspect of the present invention provides a method for producing a hexagonal boron nitride powder, comprising: a reduction-nitridation step of holding a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron carbon compound in a furnace under a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or longer, then heating the mixture to 1650°C to 1850°C at a heating rate of less than 1°C / min, and subsequently holding the mixture at 1650°C to 1850°C for 2 hours or longer; wherein the oxygen-containing boron compound has an average particle size of 150 μm to 300 μm, and particles having particle sizes of 75 μm or smaller and 425 μm or larger account for 1 mass % or less; and the raw material mixture has a ratio of the mass of the oxygen-containing boron compound converted into B to the mass of the carbon source converted into C (the mass converted into B / the mass converted into C) of 0.70 to 0.90.

[0078] A sixth aspect of the present invention relates to a method for producing a hexagonal boron nitride powder in the fifth aspect, wherein the raw material mixture contains 5 to 20 parts by mass of the oxygen-containing calcium compound, calculated as CaO on a Ca basis, per 100 parts by mass of the total of the mass of the oxygen-containing boron compound, calculated as BO on a B basis, and the mass of the carbon source, calculated as C, and the raw material mixture may contain 5 to 45 parts by mass of the carbon-containing boron compound, calculated as BO on a Ca basis, per 100 parts by mass of the total of the mass of the oxygen-containing boron compound, calculated as BO, the mass of the carbon source, calculated as C, and the mass of the oxygen-containing calcium compound, calculated as CaO.

[0079] [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. [Example]

[0080] An embodiment of the present invention will now be described.

[0081] [Method for producing hexagonal boron nitride powder] Example 1 3652 g of a mixture containing 1950 g of boron oxide, 761 g of carbon black, 415 g of calcium carbonate, and 526 g of boron carbide was mixed using a ball mill. The boron oxide used had an average particle size of 250 μm, and particles with particle sizes of 75 μm or less and 425 μm or more accounted for 0.1 mass % (1.0 mass % or less). The converted masses of each raw material are shown below. Mass of boron oxide converted to B: 605g -Mass of carbon black converted to C: 761g -Mass of boron oxide converted to B2O3 based on B standard: 1950g Mass of calcium carbonate converted to CaO on a Ca basis: 233g

[0082] The raw material mixture was heated to 1500°C (first heating temperature) in a graphite Tammann furnace under a nitrogen gas atmosphere, held at 1500°C for 6 hours, and then heated at a rate of 0.2°C / min to 1700°C (second heating temperature).The mixture was then held at 1700°C for 2 hours to carry out a reduction-nitriding treatment (reduction-nitriding step).

[0083] The obtained reduced and nitrided powder (hexagonal boron nitride powder after reduction and nitridation treatment) was placed in a container, and 10 times the amount of hydrochloric acid (10 mass % HCl) was added. The mixture was then stirred at a rotation speed of 120 rpm for 6 hours. Stir The mixture was stirred and subjected to an acid washing treatment (acid washing step). After the acid washing treatment, the hydrochloric acid was filtered, and the hexagonal boron nitride powder obtained by the filtration was dispersed in pure water with the same amount of hydrochloric acid as that used, and then filtered again. This procedure was repeated five times, and the mixture was then vacuum dried at 200°C for 18 hours.

[0084] The obtained acid-washed powder (hexagonal boron nitride powder obtained after acid washing and drying) was subjected to a crushing treatment using a stone mill (crushing step).

[0085] The obtained crushed powder (hexagonal boron nitride powder obtained after crushing) was heated to 1900°C at a rate of 5°C / min in a nitrogen gas atmosphere using a graphite Tammann furnace, and then re-fired by holding at 1900°C for 6 hours (re-fired step).

[0086] Then, nitrogen gas was further passed through the furnace to cool it down to 28°C, and the hexagonal boron nitride powder was then recovered from the furnace.

[0087] The re-fired powder (hexagonal boron nitride powder obtained after re-fired) was passed through a sieve with 106 μm openings to obtain the hexagonal boron nitride powder of Example 1. Table 1 shows the raw material composition values ​​and temperature conditions for the hexagonal boron nitride powders of each Example and Comparative Example.

[0088] [Table 1]

[0089] In Table 1, "B equivalent mass / C equivalent mass" indicates the ratio of the mass of boron oxide converted to B to the mass of carbon black converted to C. "Calcium carbonate ratio" indicates the ratio (parts by mass) of the mass of calcium carbide converted to CaO on a Ca basis to a total of 100 parts by mass of the mass of boron oxide converted to B2O3 on a B basis and the mass of carbon black converted to C. "Boron carbide ratio" indicates the ratio (parts by mass) of the mass of boron oxide converted to B2O3, the mass of carbon black converted to C, and the mass of calcium carbonate converted to CaO to a total of 100 parts by mass of charcoal The percentage of boron dioxide (parts by mass) is shown.

[0090] Example 2 Hexagonal boron nitride powder according to Example 2 was prepared in the same manner as in Example 1, except that the proportion of boron oxide particles having particle sizes of 75 μm or less and 425 μm or more was changed to 0.3 mass %.

[0091] Example 3 Hexagonal boron nitride powder according to Example 3 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.6°C / min and the proportion of boron oxide particles with particle sizes of 75 μm or less and 425 μm or more was changed to 0.3 mass%.

[0092] Example 4 Hexagonal boron nitride powder according to Example 4 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.6°C / min and the proportion of boron oxide particles having particle sizes of 75 μm or less and 425 μm or more was changed to 0.6 mass%.

[0093] Example 5 Hexagonal boron nitride powder according to Example 5 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.6°C / min and the proportion of boron oxide particles having particle sizes of 75 μm or less and 425 μm or more was changed to 0.8 mass%.

[0094] Example 6 Hexagonal boron nitride powder according to Example 6 was prepared in the same manner as in Example 1, except that the holding time at 1500°C was changed to 8 hours.

[0095] Example 7 Hexagonal boron nitride powder according to Example 7 was prepared in the same manner as in Example 1, except that the holding time at 1700°C was changed to 6 hours.

[0096] (Comparative Example 1) Hexagonal boron nitride powder according to Comparative Example 1 was prepared using the same procedure as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.8°C / min, the average particle size of the boron oxide was changed to 200 μm, and the proportion of particles with particle sizes of 75 μm or less and 425 μm or more was changed to 6.2 mass%.

[0097] (Comparative Example 2) A hexagonal boron nitride powder according to Comparative Example 2 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.8°C / min, the average particle size of the boron oxide was changed to 400 μm, and the proportion of particles having particle sizes of 75 μm or less and 425 μm or more was changed to 22.8 mass%.

[0098] (Comparative Example 3) Hexagonal boron nitride powder according to Comparative Example 3 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 2.0°C / min. Made did.

[0099] Comparative Example 4 The same procedure as in Example 1 was used to prepare a comparative example, except that the heating rate from 1500°C to 1700°C was changed to 5.0°C / min. 4 Hexagonal boron nitride powder was prepared. Made did.

[0100] (Comparative Example 5) A hexagonal boron nitride powder according to Comparative Example 5 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.8°C / min and the value of "mass converted into B / mass converted into C" was changed to 0.67. Made In Comparative Example 5, the amounts of boron oxide added were 1640 g, carbon black 761 g, calcium carbonate 369 g, and boron carbide 463 g.

[0101] (Comparative Example 6) A hexagonal boron nitride powder according to Comparative Example 6 was prepared in the same manner as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 0.8°C / min and the value of "B equivalent mass / C equivalent mass" was changed to 0.95. Made In Comparative Example 6, the amounts of boron oxide added were 2335 g, carbon black 761 g, calcium carbonate 476 g, and boron carbide 592 g.

[0102] [Method for evaluating hexagonal boron nitride powder] (Measurement of the first and second cumulative pore volumes) The first and second cumulative pore volumes of the obtained hexagonal boron nitride powder were calculated according to the JIS The measurement was carried out by the mercury intrusion method in accordance with R 1655:2003. Specifically, the measurement was carried out by the following method.

[0103] The obtained hexagonal boron nitride powder was pretreated by constant temperature drying at 120°C for 4 hours. The pore size distribution (pore diameters 0.0055 to 500 μm) was determined using mercury intrusion porosimetry. The surface tension of mercury was 480 dynes / cm, and the contact angle between mercury and the sample was 140 degrees. The first and second cumulative pore volumes were calculated from the cumulative pore size distribution, which was plotted with pore diameter on the horizontal axis and cumulative pore volume on the vertical axis. These values ​​were calculated using an Autopore V9620 (Micromeritics).

[0104] (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, and was previously dried at 100°C for 1 hour in a nitrogen gas flow. of Used.

[0105] (Measurement of the first average particle size and the second average particle size) The first average particle size and the second average particle size of the obtained hexagonal boron nitride powder were determined by the following particle size distribution measurement method.

[0106] Disperse 0.5 g of sample powder in ethanol. Medium The particle size distribution of a measurement sample dispersed in 20 g was measured using a laser diffraction / scattering particle size distribution analyzer (MICROTRACK (registered trademark)-MT3000, manufactured by Microtrack-Bell Corporation). The particle size distribution of the first average particle diameter was measured without performing any disintegration treatment such as ultrasonic dispersion treatment. On the other hand, the particle size distribution of the second average particle diameter was measured after subjecting the measurement sample to ultrasonic treatment at an amplitude of 35% for 20 minutes using a homogenizer (SONIFER SFX250) manufactured by Branson.

[0107] In the obtained volume frequency distribution (particle size distribution) of particle diameters, the volume frequencies were accumulated from the smallest particle diameter to obtain a particle diameter (D50) at which the accumulated value reached 50% was defined as the average particle diameter of each measurement sample.

[0108] (Evaluation of thermal conductivity of resin composition) The hexagonal boron nitride powder obtained in each example and comparative example was filled into an epoxy resin to prepare a resin composition, and its thermal conductivity was evaluated. 50 parts by weight of jER® 828 (a bisphenol A epoxy resin manufactured by Mitsubishi Chemical Corporation, with an epoxy equivalent of 189 and a viscosity (25°C) of 135 poise), 50 parts by weight of EPPN-501HY (a trisphenolmethane epoxy resin manufactured by Nippon Kayaku Co., Ltd., with an epoxy equivalent of 166 and a softening point of 60°C), 87 parts by weight of GPH-103 (a biphenyl aralkylphenol novolac manufactured by Nippon Kayaku Co., Ltd., with a hydroxyl equivalent of 230 and a softening point of 103°C) as a curing agent, and 0.8 parts by weight of 4-dimethylaminopyridine (DMAP, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a curing accelerator were added to 140 parts by weight of methyl ethyl ketone and dissolved by stirring. To this was added 675 parts by mass of hexagonal boron nitride powder and 250 parts by mass of methyl ethyl ketone, 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. Made did.

[0109] 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 prepare 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.

[0110] Two B-stage sheets of the resin composition were stacked so that the resin composition layers were in contact with each other and heat-pressed at 100°C under reduced pressure and a 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 pressure of 12 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.

[0111] 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®) by the xenon flash method (laser flash method, ISO 22007-4). Both sides of the sample sheet were gold-coated and graphite-sprayed, and the charging voltage was 200 V with a pulse width of 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: Thermo Plus Evo DSC8230).

[0112] Thermal conductivity was evaluated as "good" when it was 15 W / m·K or higher, and as "unsuitable" when it was less than 15 W / m·K.

[0113] (Evaluation of dielectric strength of resin composition) The dielectric strength was evaluated using the resin sheet used to create the thermal conductivity. The dielectric strength was measured using a withstand voltage tester (manufactured by Tama Densoku Co., Ltd.).

[0114] Dielectric strength was evaluated as "good" if it was 70 kV / mm or more, "compliant" if it was 60 to 69 kV / mm, "poor" if it was 50 to 59 kV / mm, and "unsuitable" if it was less than 50 kV / mm.

[0115] [Evaluation results] The evaluation results of the hexagonal boron nitride powder and resin sheet in each example and comparative example are shown in Table 2 below. As shown in Table 2, the hexagonal boron nitride powder obtained by a method in accordance with the present production method had properties with a first cumulative pore volume / second cumulative pore volume value within the range specified in the present invention, and therefore showed good thermal conductivity and dielectric strength when made into a resin sheet. On the other hand, the comparative examples, in which the value of the first cumulative pore volume / second cumulative pore volume was outside the range specified in the present invention, resulted in poor thermal conductivity and / or dielectric strength.

[0116] [Table 2]

[0117] FIG. 1 shows the distribution of logarithmic differential pore volume versus pore diameter for the hexagonal boron nitride powder of Example 1. As shown in FIG. 1, for the hexagonal boron nitride powder of Example 1, a maximum peak was observed in the pore diameter range of more than 1.0 μm to 5.0 μm when the pore diameter was 5.0 μm or less. Furthermore, no clear peak was observed in the pore diameter range of 0 μm or more to 1.0 μm or less. Similar distributions were observed for each example other than Example 1.

[0118] The above results show that in the hexagonal boron nitride powder according to one embodiment of the present invention, the pores formed inside the aggregated particles are of an appropriate size, and the proportion of fine pores is very small. It has also been demonstrated that such hexagonal boron nitride powder can achieve both excellent thermal conductivity and dielectric strength in a resin composition. [Industrial Applicability]

[0119] The hexagonal boron nitride powder according to one embodiment of the present invention can be used, for example, as a filler for resins used in electronic components.

Claims

1. A hexagonal boron nitride powder, wherein the ratio (first cumulative pore volume / second cumulative pore volume) of a first cumulative pore volume in a pore diameter range of 0 μm or more and 1.0 μm or less, measured by mercury intrusion porosimetry in accordance with JIS R 1655:2003, to a second cumulative pore volume in a pore diameter range of more than 1.0 μm and 5.0 μm or less, is 0.50 or less.

2. 2. The hexagonal boron nitride powder according to claim 1, wherein the first cumulative pore volume is 0.20 mL / g or less, and the second cumulative pore volume is 0.20 mL / g or more and 1.00 mL / g 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. 4. The hexagonal boron nitride powder according to claim 1, wherein a ratio (first average particle size / second average particle size) of a first average particle size of the hexagonal boron nitride powder to a second average particle size obtained after dispersing the hexagonal boron nitride powder in a liquid and subjecting it to ultrasonic treatment is 1.25 or more and 6.00 or less.

5. a reduction-nitridation step of holding a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a boron carbon compound at 1450°C or higher and 1550°C or lower for 5 hours or longer in a nitrogen atmosphere, then heating the mixture to a temperature of 1650°C or higher and 1850°C or lower at a temperature increase rate of less than 1°C / min, and then holding the mixture at 1650°C or higher and 1850°C or lower for 2 hours or longer; the oxygen-containing boron compound has an average particle size of 150 μm or more and 300 μm or less, and particles having particle sizes of 75 μm or less and 425 μm or more account for 1 mass % or less; a ratio of the mass of the oxygen-containing boron compound converted into B to the mass of the carbon source converted into C (mass converted into B / mass converted into C) of 0.70 to 0.90 in the raw material mixture.

6. The raw material mixture is B based on the B standard of the oxygen-containing boron compound ２ O ３ and the carbon source in terms of C, the oxygen-containing calcium compound is contained in an amount of 5 to 20 parts by mass, calculated as CaO on a Ca basis, relative to 100 parts by mass of the total of the oxygen-containing calcium compound in terms of CaO and the carbon source in terms of C, The B of the oxygen-containing boron compound ２ O ３ 6. The method for producing a hexagonal boron nitride powder according to claim 5, wherein the carbon-containing boron compound is contained in an amount of 5 to 45 parts by mass relative to a total of 100 parts by mass of the carbon source converted into C and the oxygen-containing calcium compound converted into CaO.

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