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

Abstract

Provided is a hexagonal boron nitride powder or the like for obtaining a resin having both excellent thermal conductivity and dielectric strength. The ratio (first cumulative pore volume / second cumulative pore volume) of a first cumulative pore volume in the range of pore diameters of 0-1.0 μm to a second cumulative pore volume in the range of pore diameters of more than 1.0 μm and 5.0 μm or less in the hexagonal boron nitride powder is 0.50 or less as measured by a mercury intrusion method in accordance with JIS R 1655:2003.

Description

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

[0001] This invention relates to hexagonal boron nitride powder and a method for producing hexagonal boron nitride powder.

[0002] In recent years, the miniaturization and increased power of electronic components have led to a problem of increased heat generation. Therefore, research is underway to develop materials with excellent thermal conductivity to efficiently dissipate heat from electronic components.

[0003] Hexagonal boron nitride powder can be used to fill resins used in electronic components, thereby improving the thermal conductivity of the resin composition. It is known that hexagonal boron nitride powder, when used to fill resins, can improve the thermal conductivity of the resin composition by containing aggregated particles. Furthermore, Patent Document 1 discloses that a composite material with excellent thermal conductivity can be manufactured if the proportion of pores within the aggregated particles is small.

[0004] International Publication No. 2021 / 079912

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

[0006] One aspect of the present invention aims to realize hexagonal boron nitride powder and the like for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0007] To solve the aforementioned problems, a hexagonal boron nitride powder according to one aspect of the present invention has a ratio (first integrated pore volume / second integrated pore volume) of 0.50 or less between the first integrated pore volume in the range of pore diameters from 0 μm to 1.0 μm and the second integrated pore volume in the range of pore diameters from more than 1.0 μm to 5.0 μm, as measured by the mercury intrusion method in accordance with JIS R 1655:2003.

[0008] To solve the aforementioned problems, a method for producing hexagonal boron nitride powder according to one aspect of the present invention includes a reduction nitriding step 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 held in a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or more, then 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, wherein the average particle diameter of the oxygen-containing boron compound is 150 μm to 300 μm, and particles with a particle diameter of 75 μm or less and 425 μm or more are present at 1% by mass or less, and the ratio of the mass of the oxygen-containing boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) of the raw material mixture is 0.70 to 0.90.

[0009] According to one aspect of the present invention, it is possible to realize hexagonal boron nitride powder and the like for obtaining a resin composition that achieves both excellent thermal conductivity and dielectric strength.

[0010] This figure shows the distribution of logarithmic differential pore volume with respect to pore diameter for the hexagonal boron nitride powder according to Example 1.

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

[0012] [Hexagonal Boron Nitride Powder] The hexagonal boron nitride powder according to one embodiment of the present invention (hereinafter sometimes referred to as "this hexagonal boron nitride powder") can realize a resin composition that achieves both excellent thermal conductivity and dielectric strength by reducing the proportion of fine pores.

[0013] Specifically, the cumulative pore volume in the range of pore diameters from 0 μm to 1.0 μm is defined as the first cumulative pore volume, and the cumulative pore volume in the range of pore diameters from 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 the mercury intrusion method in accordance with JIS R 1655:2003.

[0014] (Pore Size) The inventors have conducted detailed studies on hexagonal boron nitride powder and have succeeded in obtaining novel findings. Specifically, they have independently discovered that by employing specific conditions during the production of hexagonal boron nitride powder, it is possible to obtain hexagonal boron nitride powder in which the value of the first integrated pore volume / second integrated pore volume is 0.50 or less.

[0015] In this hexagonal boron nitride powder having such a first integrated pore volume / second integrated pore volume value, the proportion of fine pores with a pore diameter of 0 μm or more and 1.0 μm or less is small, and the proportion of moderately sized pores with a pore diameter of more than 1.0 μm and 5.0 μm or less is large within the aggregated particles. In this specification, when simply referred to as "fine pores," unless otherwise specified, it refers to pores with a pore diameter of 0 μm or more and 1.0 μm or less within the aggregated particles. Also, when simply referred to as "moderately sized pores," unless otherwise specified, it refers to pores with a pore diameter of more than 1.0 μm and 5.0 μm or less within the aggregated particles.

[0016] The first integrated pore volume represents the cumulative value of the logarithmic differential pore volume in the range of pore diameters from 0 μm to 1.0 μm. The second integrated pore volume represents the cumulative value of the logarithmic differential pore volume in the range of pore diameters from more than 1.0 μm to 5.0 μm. The first and second integrated pore volumes may also be calculated as the area enclosed by the line connecting the measured values ​​of the logarithmic differential pore volume in a graph showing the distribution of logarithmic differential pore volume against pore diameter, within the range of pore diameters in question.

[0017] Hexagonal boron nitride forms plate-like or flaky primary particles due to its crystal structure. When these primary particles are packed into a resin sheet, for example, they are oriented approximately parallel to the surface of the resin sheet. This reduces the thermal conductivity in the thickness direction of the resin sheet. To improve this thermal anisotropy, hexagonal boron nitride powder containing aggregated particles of hexagonal boron nitride primary particles has been proposed. Since these aggregated particles can be made more spherical than the primary particles, thermal anisotropy can be reduced.

[0018] Within the aggregated particles, pores are formed as gaps between primary particles. In the prior art described in Patent Document 1, attempts were made to improve the thermal conductivity of hexagonal boron nitride powder by reducing the size and proportion of these pores. However, if there are many fine pores within the aggregated particles, the resin does not easily penetrate these fine pores when the hexagonal boron nitride powder is filled into a resin. If the pores remain as voids in the final product, the thermal conductivity of the resin composition will decrease as a result.

[0019] Furthermore, voids remaining due to incomplete resin absorption can cause a decrease in dielectric strength. Moreover, there is a risk that liquid solvent may seep into these voids and remain in the final product. Such residual solvent is unacceptable in manufacturing because it can adversely affect components surrounding the resin composition by causing the solvent to leach out of the voids.

[0020] The inventors have discovered that by adjusting the pore size of hexagonal boron nitride powder to a predetermined range, excellent dielectric strength can be obtained for a resin composition containing the hexagonal boron nitride powder while maintaining high thermal conductivity, and have thus completed the hexagonal boron nitride powder.

[0021] According to the mercury intrusion method conforming to JIS R 1655:2003, the pore diameter formed in hexagonal boron nitride powder can be measured. It is common technical knowledge that pores with a diameter of 5.0 μm or less measured by this mercury intrusion method are pores formed within the aggregated particles mentioned above, while pores with a diameter greater than 5.0 μm are pores formed as gaps between multiple aggregated particles. Patent Document 1 describes how to make the size of pores with a diameter of 5.0 μm or less as small as possible, specifically by refining the pores so that a peak is obtained in the range of 0 μm to 1.0 μm in the logarithmic differential pore volume distribution in the range of pore diameters of 5.0 μm or less.

[0022] On the other hand, in this hexagonal boron nitride powder, the second integrated pore volume in the range of over 1.0 μm and up to 5.0 μm is larger than the first integrated pore volume in the range of 0 μm to 1.0 μm. In other words, in this hexagonal boron nitride powder, the proportion of pores of a moderate size with a pore diameter of over 1.0 μm is sufficiently larger than the proportion of fine pores with a pore diameter of 1.0 μm or less.

[0023] The ratio of the first integrated pore volume to the second integrated 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. Furthermore, the ratio of the first integrated pore volume to the second integrated pore volume may be greater than 0. The smaller the ratio of the first integrated pore volume to the second integrated pore volume, the lower the proportion of fine pores can be, and the greater the dielectric strength of the resin composition containing this hexagonal boron nitride powder can be.

[0024] Within the range that satisfies the value of the first integrated pore volume / second integrated pore volume, the first integrated pore volume is preferably 0.20 mL / g or less, and the second integrated 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 integrated pore volume may be 0.15 mL / g or less, or 0.10 mL / g or less. The first integrated 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 integrated pore volume may be 0.30 mL / g or more, or 0.40 mL / g or more, or 0.50 mL / g or more. Furthermore, the second integrated pore volume may be 1.00 mL / g or less, or 0.90 mL / g or less.

[0025] Furthermore, it is preferable that the maximum peak in the graph showing the distribution of logarithmic differential pore volume against pore diameter, obtained by the mercury intrusion method described above, in the range where the pore diameter is 5.0 μm or less, is located in the range where the pore diameter is greater than 1.0 μm and less than or equal to 5.0 μm. The maximum peak is the peak in which the value of the logarithmic differential pore volume at the peak top is the largest value within the above range.

[0026] (Specific surface area) The specific surface area of ​​this hexagonal boron nitride powder is 3.5 m². ２ It may be less than or equal to 3.0 m ２ It is preferable that the amount is less than or equal to 2.7 m ２ It is more preferable that it be less than or equal to 2.5 m ２ It is even more preferable that the amount is less than or equal to / g. The lower limit of the specific surface area of ​​this hexagonal boron nitride powder is not particularly limited, but for example, 1.0 m² ２ It may be greater than or equal to / g. The specific surface area of ​​this hexagonal boron nitride powder is determined by the BET one-point method (nitrogen adsorption one-point method).

[0027] If this hexagonal boron nitride powder has the specific surface area described above, the primary particles constituting the aggregated particles have a particle size of about 1 μm or more, and can form pores of an appropriate size in one aspect of the present invention. This reduces thermal anisotropy and allows for both excellent thermal conductivity and dielectric strength. Specific surface area of ​​3.5 m² ２When the value exceeds / g, the primary particles constituting the aggregated particles become finer, and the number of fine pores increases, which is undesirable.

[0028] (Hardness) This hexagonal boron nitride powder has appropriately sized pores formed within the aggregated particles as described above, while also possessing sufficient hardness to prevent the aggregated particles from breaking during pressure molding of the resin composition after filling. Specifically, the average particle diameter of this hexagonal boron nitride powder is defined as the first average particle diameter, and the average particle diameter after dispersing this hexagonal boron nitride powder in a liquid and performing ultrasonic treatment is defined as the second average particle diameter. In this case, the ratio of the first average particle diameter to the second average particle diameter (first average particle diameter / second average particle diameter) of this hexagonal boron nitride powder is between 1.25 and 6.00. The first average particle diameter is the average particle diameter of this hexagonal boron nitride powder before ultrasonic treatment.

[0029] If the ratio of the first average particle diameter to the second average particle diameter is within this range, the hexagonal boron nitride powder can be said to have hardness that makes it resistant to breakage under forces acting on the resin composition. Therefore, the aggregated particles of the hexagonal boron nitride powder are less likely to break in the resin composition, and good thermal conductivity and dielectric strength can be obtained in the final product. The ratio of the first average particle diameter to the second average particle diameter 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 this hexagonal boron nitride powder is 15 μm or more, preferably 20 μm or more, and more preferably 25 μm or more. Furthermore, from the viewpoint of improving fluidity, 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 diameter) The first average particle diameter was measured by the following method. 0.5 g of the hexagonal boron nitride powder was dispersed in 20 g of ethanol dispersion medium, and the measured particle size at 50% of the cumulative volume of the hexagonal boron nitride powder was taken as the first average particle diameter using a particle size distribution analyzer (MT-3000) from Microtrac-Bell Co., Ltd.

[0032] The second average particle diameter was measured by the following method. 0.5 g of this hexagonal boron nitride powder was subjected to ultrasonic treatment for 20 minutes at an amplitude of 35% using a homogenizer (SONIFER SFX250) manufactured by BRANSON in 20 g of an ethanol dispersion medium. Then, using a particle size distribution measuring device (MT-3000) of Microtrac Bell Co., Ltd., the measured value of the particle diameter at 50% cumulative volume of this hexagonal boron nitride powder after the ultrasonic treatment was taken as the second average particle diameter.

[0033] (Parentheses) As described above, since the size of the pores formed in the aggregated particles of this hexagonal boron nitride powder is adjusted, excellent thermal conductivity and dielectric strength can be exhibited when used in a resin composition. Therefore, in a filler filled with this hexagonal boron nitride powder such as a resin composition, it is possible to achieve an improvement in product life while obtaining a desired thermal conductivity. Such an effect also contributes to the achievement of, for example, Goal 12, "Ensure sustainable production and consumption patterns," of the Sustainable Development Goals (SDGs) proposed by the United Nations.

[0034] [Method for producing hexagonal boron nitride powder] The method for producing hexagonal boron nitride powder according to an embodiment of the present invention (hereinafter may be referred to as "this production method") includes a reduction nitridation step. Further, this production method may be a method of performing a reduction nitridation step, an acid washing step, and a re-firing step in this order. Further, this production method may further include at least one of a crushing step and a recovery step. Hereinafter, each step will be described.

[0035] (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 held at 1450°C or higher and 1550°C or lower for 5 hours or longer in a nitrogen atmosphere. Here, the temperature held for 5 hours or longer may be referred to as the first heating temperature. Next, the raw material mixture held at the first heating temperature is heated at a heating rate of less than 1°C / min to a temperature of 1650°C or higher and 1850°C or lower, and then held at 1650°C or higher and 1850°C or lower for 2 hours or longer. Here, the temperature held for 2 hours or longer may be referred to as the second heating temperature. By this step, hexagonal boron nitride can be produced.

[0036] As the oxygen-containing boron compound, an oxygen-containing boron compound having an average particle diameter of 150 μm or more and 300 μm or less, and in which particles having a particle diameter of 75 μm or less and 425 μm or more are 1 mass% or less is used. The average particle diameter of the oxygen-containing boron compound may be 200 μm or more and 250 μm or less. Further, in the oxygen-containing boron compound, the particles having a particle diameter of 75 μm or less and 425 μm or more may be 0.8 mass% or less.

[0037] The average particle diameter of the oxygen-containing boron compound can be measured by laser diffraction particle size distribution measurement. Also, the particle mass ratio of particles having a particle diameter 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), metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. These can be used alone or in combination of two or more. Among these, boric acid or boron oxide, which is easily available, may be preferably used.

[0039] Examples of the carbon source 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 thermally decomposing a monomer or polymer. These can be used alone or in combination of two or more. Among them, amorphous carbon with high reactivity is preferred, and carbon black is particularly preferably used in terms of industrial quality adjustment. Examples of carbon black include acetylene black, furnace black, and thermal black.

[0040] Examples of the oxygen-containing calcium compound include calcium carbonate, calcium hydrogen carbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. These may be used alone or in combination of two or more. Among them, it is preferable to use calcium oxide or calcium carbonate. 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 high-melting-point composite oxide by forming a composite oxide with the oxygen-containing boron compound, and plays a role in preventing the volatilization of the oxygen-containing boron compound. It has also been confirmed that it plays a role as a catalyst in the reaction of directly nitriding 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 diameter of the carbon-containing boron 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. The average particle diameter 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 carbon-containing boron compound in the following ratios. The (mass converted to B (boron) of the oxygen-containing boron compound / mass converted to C (carbon) of the carbon source, element ratio) is 0.70 or more and 0.90 or less. Also, with respect to 100 parts by mass in total of the mass converted to B in terms of B of the oxygen-containing boron compound and the mass converted to C of the carbon source, the oxygen-containing calcium compound may be 5 to 20 parts by mass in terms of the mass converted to CaO on a Ca basis. With respect to 100 parts by mass in total of the masses of the oxygen-containing boron compound, the carbon source, and the oxygen-containing calcium compound converted to B, O, and CaO, respectively, the carbon-containing boron compound may be 5 to 45 parts by mass. ２ O ３ With respect to 100 parts by mass in total of the mass converted to B in terms of B of the oxygen-containing boron compound and the mass converted to C of the carbon source, the oxygen-containing calcium compound may be 5 to 20 parts by mass in terms of the mass converted to CaO on a Ca basis. With respect to 100 parts by mass in total of the masses of the oxygen-containing boron compound, the carbon source, and the oxygen-containing calcium compound converted to B, ２ O ３ With respect to 100 parts by mass in total of the masses of the oxygen-containing boron compound, the carbon source, and the oxygen-containing calcium compound converted to B, C, and CaO, respectively, the carbon-containing boron compound may be 5 to 45 parts by mass.

[0044] The raw material mixture containing each raw material can be supplied to the reduction nitridation reaction in any form, for example, as a powder or in the form of granules. The raw materials can be mixed using a mixer such as a vibratory mill, bead mill, ball mill, Henschel mixer, drum mixer, vibratory agitator, and V-mixer.

[0045] In the reduction nitriding process, heating of the raw material mixture 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. Alternatively, a gas mixture of nitrogen gas or ammonia gas with a non-oxidizing gas such as hydrogen, argon, or helium may be used. Nitrogen gas is preferred as the gas introduced into the heating furnace. Furthermore, it is preferable that the nitrogen gas has a controlled dew point temperature, preferably -85°C or lower.

[0046] In the heating of the raw material mixture in the reduction nitriding process, 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, the heating rate during the heating from the first heating temperature to the second heating temperature is preferably less than 1°C / min and less than or equal to 0.8°C / min. By heating with such a first heating temperature, second heating temperature, and heating rate, the growth of primary particles of hexagonal boron nitride in the reduction nitriding process can be uniformly adjusted. The size of these primary particles affects the size of the pores formed within the aggregated particles. According to the reduction nitriding process described above, hexagonal boron nitride powder containing aggregated particles with appropriately sized pores can be obtained.

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

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

[0049] The reduction nitriding process can be carried out using known reactors capable of adjusting the reaction atmosphere. Examples of reactors include atmosphere-controlled high-temperature furnaces that perform heat treatment by high-frequency induction heating or heater heating. In addition to batch furnaces, continuous heating furnaces such as pusher-type tunnel furnaces and vertical reactors can also be used.

[0050] (Acid Washing Process) In the acid washing process, the hexagonal boron nitride powder, which contains unreacted raw materials obtained in the reduction nitriding process, is washed with acid. The hexagonal boron nitride powder obtained by the reduction nitriding reaction described above contains unreacted raw materials such as oxides or metal impurities. The acid washing process removes the unreacted raw materials, resulting in hexagonal boron nitride powder.

[0051] The method for acid washing the hexagonal boron nitride powder containing unreacted raw materials is not particularly limited, and known methods may be used without restriction. For example, one method involves placing the hexagonal boron nitride powder obtained after the reductive nitridation reaction into a container, adding 5 to 10 times the amount of dilute hydrochloric acid (10 to 20% by mass HCl) to the hexagonal boron nitride powder, and allowing it to come into contact for 4 to 8 hours.

[0052] In addition to hydrochloric acid, nitric acid, sulfuric acid, or acetic acid can also be used as acids during acid cleaning.

[0053] After acid washing, water washing with pure water may be performed to remove any remaining acid. For water washing, after filtering the acid used during acid washing, the hexagonal boron nitride powder that was acid-washed may be dispersed in the same amount of pure water as the acid used, and then filtered again.

[0054] After acid washing or water washing, the water-containing mass may be dried. Preferred drying conditions are, for example, in air at a temperature of 50°C to 250°C, or under reduced pressure. The drying time is not particularly limited, 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 after the acid washing process may be adjusted to a predetermined particle size distribution by performing a crushing process. Crushing in the crushing process is preferably carried out slowly using a jet mill, ball mill, hammer mill, or stone mill type grinder. The crushing process may be performed before or after the acid washing process.

[0056] (Re-calcination process) In the re-calcination process, the hexagonal boron nitride powder after the crushing process is heated to a re-calcination temperature of 1850°C to 2050°C.

[0057] The heating in the re-firing process may be carried out under a nitrogen atmosphere. The supply of a nitrogen source to the reaction system in the re-firing process may be in the same manner as in the reduction nitriding process. Furthermore, the reaction apparatus may be the same as the one used in the reduction nitriding process.

[0058] The re-calcination temperature for heating the hexagonal boron nitride powder in the re-calcination process is preferably between 1850°C and 2050°C, and more preferably between 1900°C and 2000°C. This heating can improve the crystallinity of the hexagonal boron nitride powder.

[0059] It is preferable that the hexagonal boron nitride powder, heated to the re-calcination temperature, be held at a temperature within the aforementioned re-calcination temperature range for a period of 1 hour to 15 hours. From the viewpoint of ensuring sufficient re-calcination of the hexagonal boron nitride powder, the holding time is preferably 1 hour or more, and preferably 3 hours or more. Furthermore, the holding time is preferably 15 hours or less, and more preferably 10 hours or less.

[0060] (Recovery and Classification Process) In the recovery process, after the re-calcination process, the hexagonal boron nitride powder, which has been cooled to below 50°C, is removed from the furnace. By cooling the hexagonal boron nitride powder recovered from the furnace to below 50°C, the reaction with moisture in the air can be reduced. This reduction in reaction reduces the surface oxygen concentration (B) of the hexagonal boron nitride powder. ２ O ３ This reduces the increase in concentration and improves the crystallinity of the hexagonal boron nitride powder. 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 lower, and more preferably 30°C or lower.

[0061] The recovered hexagonal boron nitride powder may have its particle size adjusted using a known classification apparatus. Known classification apparatuses include dry air flow classifiers and dry sieve classifiers.

[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 "this resin composition") contains this hexagonal boron nitride powder. By containing this hexagonal boron nitride powder, this resin composition has high thermal conductivity and is suitable for heat dissipation applications.

[0063] This resin composition can be used in a variety of applications. For example, by mixing it with the resins described later to form a thermally conductive resin composition or a thermally conductive molded body, it can be preferably used in applications such as: thermal interface materials such as polymer-based heat dissipation sheets and phase change sheets; organic heat dissipation sheets such as heat dissipation tapes, heat dissipation greases, heat dissipation adhesives, and gap fillers; heat dissipation paints such as heat dissipation paints and heat dissipation coatings; heat dissipation 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 encapsulating materials for power devices.

[0064] Examples of resins included in this resin composition include thermoplastic resins such as polyolefins, vinyl chloride resins, methyl methacrylate resins, nylon and fluororesins; thermosetting resins such as epoxy resins, phenolic resins, urethane resins, polyimide resins, urea resins, melamine resins, unsaturated polyester resins, silicon resins and bismaleimidotriazine resins; and synthetic rubbers.

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

[0066] Furthermore, the resin composition may optionally contain known polymerization initiators, curing agents, curing accelerators, polymerization inhibitors, polymerization retarders, coupling agents, dispersants, defoamers, leveling agents, plasticizers, ultraviolet absorbers, flame retardants, pigments, dyes, antibacterial agents, organic fillers, organic-inorganic composite fillers, or rubber particles as compounding agents. In addition, the resin composition may contain other inorganic fillers or resin components to the extent that they do not impair the effects of the present invention.

[0067] In this resin composition, the content of the hexagonal boron nitride powder is preferably 20% to 80% by volume, and more preferably 30% to 70% by volume, in order to obtain high thermal conductivity.

[0068] The resin content in this resin composition may correspond to the amount obtained by subtracting the content of the hexagonal boron nitride powder when the total volume of the resin composition is 100% by volume. The resin content, 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 use of the resin sheet is not particularly limited, but it can be used, for example, for circuit board applications and for heat dissipation applications of electronic components such as multilayer printed wiring boards.

[0070] The thermal conductivity of this 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 this hexagonal boron nitride powder, this resin sheet having such good thermal conductivity can be easily manufactured. The thermal conductivity of this resin sheet can be measured by the method described in the examples below.

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

[0072] The method for manufacturing this resin sheet is not particularly limited. For example, if the resin is a curable epoxy resin, a manufacturing method can be employed that includes a mixing step of mixing the curable epoxy resin, the hexagonal boron nitride powder, and other components as needed to obtain a curable composition; a molding step of shaping 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 embodiment 1 of the present invention has a ratio (first integrated pore volume / second integrated pore volume) of 0.50 or less between the first integrated pore volume in the range of pore diameters from 0 μm to 1.0 μm and the second integrated pore volume in the range of pore diameters from more than 1.0 μm to 5.0 μm, as measured by the mercury intrusion method in accordance with JIS R 1655:2003.

[0074] In the hexagonal boron nitride powder according to embodiment 2 of the present invention, the first integrated pore volume may be 0.20 mL / g or less, and the second integrated 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 embodiment 3 of the present invention has a specific surface area of ​​3.5 m² in embodiment 1 or 2. ２ It may be less than / g.

[0076] In any of embodiments 1 to 3, the hexagonal boron nitride powder according to embodiment 4 of the present invention may have a ratio (first average particle diameter / second average particle diameter) of 1.25 or more and 6.00 or less between the first average particle diameter of the hexagonal boron nitride powder and the second average particle diameter after the hexagonal boron nitride powder is dispersed in a liquid and subjected to ultrasonic treatment.

[0077] A method for producing hexagonal boron nitride powder according to aspect 5 of the present invention includes a reduction nitriding step 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 held in a furnace under a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or more, then 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, wherein the average particle size of the oxygen-containing boron compound is 150 μm to 300 μm, and particles with a particle size of 75 μm or less and 425 μm or more are present at 1% by mass or less, and the ratio of the mass of the oxygen-containing boron compound converted to B to the mass of the carbon source converted to C (mass converted to B / mass converted to C) of the raw material mixture is 0.70 to 0.90.

[0078] A method for producing hexagonal boron nitride powder according to embodiment 6 of the present invention is, in embodiment 5, the raw material mixture is B based on the oxygen-containing boron compound B ２ O ３ The oxygenated calcium compound is contained in 5 to 20 parts by mass in terms of the mass converted to CaO on a Ca basis, with respect to 100 parts by mass of the total of the mass converted to C of the carbon source, and the oxygenated boron compound is B ２ O ３ The carbon-containing boron compound may be present in 5 to 45 parts by mass per 100 parts by mass of the sum of the mass converted to C of the carbon source, the mass converted to CaO of the oxygen-containing calcium compound, and the total mass converted to CaO of the oxygen-containing calcium compound.

[0079] [Additional Notes] The present invention is not limited to the embodiments described above, 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.

[0080] One embodiment of the present invention is described below.

[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 was a raw material with an average particle size of 250 μm, and particles with particle sizes of 75 μm or less and 425 μm or more amounting to 0.1% by mass (1.0% by mass or less). The converted mass of each raw material is shown below. - Mass of boron oxide converted to B: 605 g - Mass of carbon black converted to C: 761 g - Mass of boron oxide converted to B ２ O ３ Mass converted to: 1950g, Mass of calcium carbonate converted to CaO based on Ca: 233g.

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

[0083] The obtained reduced nitrided powder (hexagonal boron nitride powder after reduced nitriding treatment) was placed in a container, 10 times the amount of hydrochloric acid (10% by mass HCl) was added, and the mixture was stirred at 120 rpm for 6 hours to perform an acid pickling treatment (acid pickling step). After this acid pickling treatment, the hydrochloric acid was filtered, and the hexagonal boron nitride powder obtained from the filtration was dispersed in the same amount of pure water as the hydrochloric acid used, and filtered again. This operation was repeated 5 times, and then the mixture was vacuum dried at 200°C for 18 hours.

[0084] The resulting acid-washed powder (hexagonal boron nitride powder obtained after acid washing and drying) was subjected to a crushing process using a millstone grinder (crushing process).

[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 graphite Tamman furnace under a nitrogen gas atmosphere. Then, it was re-fired by holding it at 1900°C for 6 hours (re-fired process).

[0086] Then, nitrogen gas was circulated to cool the furnace to 28°C, after which the hexagonal boron nitride powder was recovered from inside the furnace.

[0087] The re-calcined powder (hexagonal boron nitride powder obtained after re-calcination) was sieved through a 106 μm mesh to obtain the hexagonal boron nitride powder according to Example 1. Table 1 shows the raw material composition values ​​and temperature conditions for the hexagonal boron nitride powders according to each example and comparative example.

[0088]

[0089] In Table 1, "B-equivalent mass / C-equivalent mass" shows the ratio of the mass of boron oxide converted to B to the mass of carbon black converted to C. "Calcium carbonate ratio" is based on the B of boron oxide. ２ O ３ This shows the ratio (parts by mass) of the mass of calcium carbide converted to CaO based on Ca, relative to 100 parts by mass of the total mass converted to C in carbon black. "Boron carbide ratio" refers to the B of boron oxide. ２ O ３ This shows the ratio (parts by mass) of boron carbide to 100 parts by mass, which is the sum of the mass converted to carbon dioxide, the mass converted to carbon dioxide (C) in carbon black, and the mass converted to calcium carbonate (CaO).

[0090] (Example 2) Hexagonal boron nitride powder according to Example 2 was prepared using the same procedure as in Example 1, except that the proportion of boron oxide particles with a particle size of 75 μm or less and 425 μm or more was changed to 0.3% by mass.

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

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

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

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

[0095] (Example 7) The hexagonal boron nitride powder according to Example 7 was prepared using the same procedure 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, and the average particle size of boron oxide was changed to 200 μm, and the proportion of particles with a particle size of 75 μm or less and 425 μm or more was changed to 6.2% by mass.

[0097] (Comparative Example 2) Hexagonal boron nitride powder according to Comparative Example 2 was prepared by 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, and the average particle size of boron oxide was changed to 400 μm, and the proportion of particles with a particle size of 75 μm or less and 425 μm or more was changed to 22.8% by mass.

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

[0099] (Comparative Example 4) Hexagonal boron nitride powder according to Comparative Example 4 was prepared using the same procedure as in Example 1, except that the heating rate from 1500°C to 1700°C was changed to 5.0°C / min.

[0100] (Comparative Example 5) Hexagonal boron nitride powder according to Comparative Example 5 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 and the value of "mass B equivalent / mass C equivalent" was changed to 0.67. In Comparative Example 5, the amount of boron oxide added was 1640 g, carbon black 761 g, calcium carbonate 369 g, and boron carbide 463 g.

[0101] (Comparative Example 6) Hexagonal boron nitride powder according to Comparative Example 6 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 and the value of "mass equivalent to B / mass equivalent to C" was changed to 0.95. 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] [Evaluation Method for Hexagonal Boron Nitride Powder] (Measurement of First and Second Integrated Pore Volumes) The first and second integrated pore volumes of the obtained hexagonal boron nitride powder were measured by the mercury intrusion method in accordance with JIS R 1655:2003. Specifically, the measurements were performed using the method shown below.

[0103] The obtained hexagonal boron nitride powder was pre-treated by constant-temperature drying at 120°C for 4 hours. The pore distribution, ranging from 0.0055 to 500 μm in diameter, was determined by the mercury intrusion method. The surface tension of mercury was set to 480 dynes / cm, and the contact angle between mercury and the sample was set to 140 degrees. The first and second integrated pore volumes were calculated from the integrated pore distribution, plotted with pore diameter on the x-axis and integrated pore volume on the y-axis. These values ​​were calculated using an Autopore V9620 manufactured by Micromeritics.

[0104] (Measurement of specific surface area) The specific surface area of ​​hexagonal boron nitride powder was determined using a flow-type automatic specific surface area analyzer (Shimadzu Corporation: FlowSorb II-2300) by the BET one-point method (nitrogen adsorption one-point method). For measurement, 2 g of powder sample was used, which had been pre-dried at 100°C in a nitrogen gas flow for 1 hour.

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

[0106] For a sample prepared by dispersing 0.5 g of sample powder in 20 g of ethanol dispersion medium, the particle size distribution was measured using a laser diffraction scattering particle size analyzer (MICROTRACK®-MT3000, manufactured by Microtrac Bell Co., Ltd.). The first average particle size was measured without any crushing treatment such as ultrasonic dispersion. On the other hand, the second average particle size was measured after ultrasonic treatment of the sample with a BRANSON homogenizer (SONIFER SFX250) for 20 minutes at an amplitude of 35%.

[0107] In the obtained volume frequency distribution (particle size distribution) of particle sizes, the volume frequencies were accumulated from the smallest particle size, and the particle size at which the accumulated value reached 50% (D50) was taken as the average particle size for each measured sample.

[0108] (Evaluation of thermal conductivity of resin compositions) The hexagonal boron nitride powder obtained in each example and comparative example was filled into epoxy resin to prepare resin compositions, and the thermal conductivity was evaluated. To the epoxy resin, 50 parts by mass of jER® 828 (bisphenol A type epoxy resin manufactured by Mitsubishi Chemical Corporation, epoxy equivalent 189, viscosity (25°C) 135 poise), 50 parts by mass of EPPN-501HY (trisphenolmethane type epoxy resin manufactured by Nippon Kayaku Co., Ltd., epoxy equivalent 166, softening point 60°C), 87 parts by mass of GPH-103 (biphenylaralkylphenol novolac manufactured by Nippon Kayaku Co., Ltd., hydroxyl group equivalent 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, and the mixture was stirred and dissolved. Then, 675 parts by mass of hexagonal boron nitride powder and 250 parts by mass of methyl ethyl ketone were added, and the mixture was stirred for 30 minutes using a homodisperser. Furthermore, the viscosity was adjusted by adding methyl ethyl ketone as needed, and then the resin composition was degassed under reduced pressure to prepare a varnish.

[0109] Next, the varnish of the resin composition obtained above was uniformly applied to the release-treated surface of the support so that the thickness after drying was 80 μm, and dried at 80°C to produce a B-stage sheet of the resin composition. A release-treated polyimide film (UBE Corporation's UPILEX®-50S, 50 μm thick) 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 then heat-pressed at 100°C under reduced pressure with a press pressure of 4 MPa for 3 minutes to bond them together. Subsequently, the temperature was raised to 180°C, and the resin composition sheets were cured by heat-pressing at 12 MPa under reduced pressure for 60 minutes. Furthermore, the resin was completely cured by heat treatment at 190°C for 2 hours. After that, the polyimide films on both sides were peeled off to obtain the resin sheet.

[0111] The thermal conductivity (W / m·K) of a resin sheet is equal to the thermal diffusivity (m ２ / sec) x density (kg / m ３The specific heat was calculated from ( ) × specific heat (J / kg·K). The thermal diffusivity was measured using the xenon flash method (laser flash method, ISO 22007-4) with a benchtop thermal diffusivity / thermal conductivity measuring device (NETZSCH: Xenon Flash Analyzer LFA467 HyperFlash®). Both sides of the sample sheet were treated with gold coating and graphite spray, and the charge voltage was 200V and the pulse width was 20μsec. The density was measured using the Archimedes method with an analytical balance (Mettler Toledo: XS204V). The specific heat was measured using a differential scanning calorimeter (Rigaku Corporation: Thermo Plus Evo DSC8230).

[0112] Thermal conductivity was evaluated as "good" if it was 15 W / m·K or higher, and as "unsuitable" if 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 prepare the thermal conductivity sample. Dielectric strength was measured using a dielectric strength tester (manufactured by Tama Densoku Co., Ltd.).

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

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

[0116]

[0117] Furthermore, Figure 1 shows the distribution of logarithmic differential pore volume as a function of pore diameter for the hexagonal boron nitride powder according to Example 1. As shown in Figure 1, for the hexagonal boron nitride powder according to Example 1, a maximum peak was observed in the range of pore diameters greater than 1.0 μm and greater than 5.0 μm for pore diameters of 5.0 μm or less. Also, no clear peak was observed in the range of pore diameters between 0 μm and 1.0 μm. Similar distributions were observed for each of the other examples.

[0118] From the above results, it was shown 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. Furthermore, it was demonstrated that such hexagonal boron nitride powder can achieve both excellent thermal conductivity and dielectric strength in resin compositions.

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

Claims

1. Hexagonal boron nitride powder, wherein the ratio of the first integrated pore volume in the range of pore diameters from 0 μm to 1.0 μm to the second integrated pore volume in the range of pore diameters from more than 1.0 μm to 5.0 μm (first integrated pore volume / second integrated pore volume), measured by the mercury intrusion method in accordance with JIS R 1655:2003, is 0.50 or less.

2. The hexagonal boron nitride powder according to claim 1, wherein the first integrated pore volume is 0.20 mL / g or less, and the second integrated pore volume is 0.20 mL / g or more and 1.00 mL / g or less.

3. Specific surface area is 3.5 m² ２ The hexagonal boron nitride powder according to claim 1, wherein the amount is less than or equal to / g.

4. The hexagonal boron nitride powder according to any one of claims 1 to 3, wherein the ratio of the first average particle diameter of the hexagonal boron nitride powder to the second average particle diameter after the hexagonal boron nitride powder is dispersed in a liquid and subjected to ultrasonic treatment (first average particle diameter / second average particle diameter) is 1.25 or more and 6.00 or less.

5. A method for producing hexagonal boron nitride powder, comprising a reduction nitridation step 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 held in a nitrogen atmosphere at 1450°C to 1550°C for 5 hours or more, then 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, wherein the average particle size of the oxygen-containing boron compound is 150 μm to 300 μm, and particles with a particle size of 75 μm or less and 425 μm or more constitute 1% by mass or less, and the ratio of the raw material mixture, converted to B, of the mass of the oxygen-containing boron compound to C, converted to C (mass converted to B / mass converted to C), is 0.70 to 0.

90.

6. The raw material mixture is B according to the B standard of the oxygenated boron compound. ２ O ３ The oxygenated calcium compound is contained in 5 to 20 parts by mass in terms of the mass converted to CaO on a Ca basis, with respect to 100 parts by mass of the total of the mass converted to and the mass of the carbon source converted to C, and the oxygenated boron compound is B ２ O ３ A method for producing hexagonal boron nitride powder according to claim 5, comprising 5 to 45 parts by mass of the carbon-containing boron compound with respect to 100 parts by mass of the total of the mass converted to C of the carbon source, the mass converted to CaO of the oxygen-containing calcium compound, and the mass converted to CaO of the oxygen-containing calcium compound.

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