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Definitions

• the present disclosure relates to a boron nitride powder and a method for producing a boron nitride powder.
• Hexagonal boron nitride is excellent in lubricity, high thermal conductivity, insulation, etc. Therefore, hexagonal boron nitride is used in various applications such as fillers for heat dissipation materials, solid lubricants, mold release materials for molten gas and aluminum, raw materials for cosmetics, and raw materials for sintered bodies. There is.
• Patent Document 1 a hexagonal boron nitride powder capable of increasing the thermal conductivity and withstand voltage (dielectric breakdown voltage) of the resin or the like when used as a filler for an insulating heat radiating material such as a resin and the like thereof.
• a manufacturing method has been proposed.
• the components used in these electronic components are also required to have higher performance.
• the heat transfer sheet incorporated in the electronic component is also required to have better insulating properties.
• Boron nitride powder is used together with a resin as a material for forming a heat transfer sheet, but according to the study by the present inventors, when a conventional boron nitride powder which is considered to have sufficiently high purity and excellent performance is used.
• dielectric breakdown of the heat transfer sheet may occur.
• An object of the present disclosure is to provide a boron nitride powder having superior insulation performance when used as a filler, and a method for producing the same, as compared with the conventional boron nitride powder.
• the present inventors conducted a detailed analysis on the conventional high-purity boron nitride powder, and examined the effect on the use in a heat transfer sheet.
• carbon-containing particles which were previously considered to be no problem, are exposed to high voltage, etc., it affects the performance of products such as heat transfer sheets.
• One aspect of the present disclosure is a boron nitride powder containing agglomerated particles composed of agglomerated primary particles of hexagonal boron nitride, having a purity of 98.5% by mass or more and a number of carbon-containing particles. , 10 or less per 10 g of boron nitride powder.
• the boron nitride powder has high purity and the content of carbon-containing particles is reduced, so that it has excellent insulation performance when used as a filler.
• the insulation performance in the present disclosure is a performance evaluated under stricter conditions than before. Specifically, the insulation performance in the present disclosure is that a resin composition prepared of boron nitride powder and a resin is subjected to a DC voltage of 1100 V in an environment of 65 ° C. and 90 RH% until dielectric breakdown occurs. This is the performance evaluated based on the energization conditions.
• the number of carbon-containing particles may be 0.05 to 10 per 10 g of boron nitride powder.
• the above-mentioned boron nitride powder may have an impurity carbon content of 170 ppm or less.
• the above-mentioned boron nitride powder may have a graphitization index of 2.3 or less.
• the graphitization index of the primary particles is within the above range, the boron nitride powder is superior in insulation performance.
• the above-mentioned boron nitride powder may have an average particle size of 7 to 100 ⁇ m and a specific surface area of 0.8 to 8.0 m 2 / g.
• the boron nitride powder can improve the thermal conductivity in addition to the insulating property. Therefore, the boron nitride powder can be more preferably used as a filler for preparing a heat transfer sheet having excellent insulation performance and heat dissipation performance.
• One aspect of the present disclosure is a raw material powder containing agglomerated particles composed of aggregated primary particles and containing hexagonal boron nitride having a purity of 98.0% by mass or more and an oxygen content of 15% by volume or more.
• a method for producing a boron nitride powder which comprises heat-treating at a temperature of 500 ° C. or higher in an atmosphere.
• the above-mentioned boron nitride powder can be produced by further heat-treating a high-purity boron nitride raw material powder under conditions containing oxygen at a certain level or higher.
• the orientation index of the raw material powder may be 30 or less.
• the graphitization index of the raw material powder may be 2.3 or less.
• boron nitride powder having superior insulation performance when used as a filler as compared with the conventional boron nitride powder, and a method for producing the same.
• each component in the composition means, when a plurality of substances corresponding to each component in the composition are present, the total amount of the plurality of substances present in the composition unless otherwise specified. ..
• the "process" in the present specification may be a process independent of each other or a process performed at the same time.
• boron nitride powder comprises agglomerated particles composed of agglomerated primary particles of hexagonal boron nitride.
• the boron nitride powder has a purity of 98.5% by mass or more, and the number of carbon-containing particles is 10 or less per 10 g of the boron nitride powder.
• Hexagonal boron nitride may have a small variation in the particle shape of the primary particles.
• the shape of the primary particles of hexagonal boron nitride may be, for example, scaly or disc-shaped.
• the purity of the boron nitride powder may be higher, for example, 98.7% by mass or more, or 99.0% by mass or more.
• the purity of the boron nitride powder in the present specification means a value calculated by titration. Specifically, titration is performed and determined by the method described in the examples of the present specification.
• Boron nitride powder may generally contain colored particles in addition to the colorless particles of hexagonal boron nitride.
• the colored particles include particles containing carbon, particles having magnetism, and the like.
• the boron nitride powder according to the present embodiment has high purity and a reduced content of carbon-containing particles (hereinafter, also referred to as carbon-containing particles). Since carbon-containing particles (hereinafter, also referred to as carbon-containing particles) often have conductivity and have a relatively large effect on the properties of boron nitride powder, the content of carbon-containing particles should be reduced. Insulation performance can be improved.
• the tint of the above-mentioned colored particles means that the tint of the colored particles is different from that of the hexagonal boron nitride particles, and does not specify the tint.
• the particles containing carbon and the particles having magnetism are generally brown or black, but the color may change depending on the content of carbon and the content of the magnetizing component.
• the number of carbon-containing particles in the boron nitride powder is 10 or less per 10 g of the boron nitride powder, but the upper limit of the number of carbon-containing particles is, for example, 9 or less, 8 or less, 7 per 10 g of the boron nitride powder. It may be 5 or less, or 3 or less.
• the upper limit of the number of carbon-containing particles is within the above range, the influence of the boron nitride powder on the insulation performance and the like can be more sufficiently suppressed.
• the lower limit of the number of carbon-containing particles in the boron nitride powder is not particularly limited and may not be contained, but for example, 0.05 or more or 0.1 or more per 10 g of boron nitride powder. It may be there.
• the number of carbon-containing particles in the boron nitride powder can be adjusted within the above range, and may be, for example, 0.05 to 10 or 0.05 to 5 per 10 g of boron nitride powder.
• the number of carbon-containing particles in the present specification is a number obtained by measuring as follows. First, 10 g of boron nitride powder to be measured and 100 mL of ethanol are measured in a container and stirred with a stirring rod to prepare a mixed solution. Next, the above mixed solution is dispersed using an ultrasonic disperser to prepare a dispersion. The obtained dispersion liquid is put into a sieve having a mesh opening of 63 ⁇ m (JIS Z 8801-1: 2019 “Test Sieve-Metal Net Sieve”), and then 2 L of distilled water is put into the sieve. In addition, continue to run distilled water until no cloudy water comes out from under the sieve and sift.
• JIS Z 8801-1 2019 “Test Sieve-Metal Net Sieve
• the sieved product obtained as described above is dried and the powder is dispersed on the medicine wrapping paper, a permanent magnet is installed under the medicine wrapping paper, and the powder not magnetized with respect to the permanent magnet is dispersed on another medicine wrapping paper. Then, observe with an optical microscope and count the number of colored particles observed. The same operation is performed for 5 or more samples, the arithmetic mean of the number of obtained colored particles is calculated, and this average value is taken as the number of carbon-containing particles per 10 g of boron nitride powder. The fact that it contains carbon can be confirmed by measuring it with an energy dispersive X-ray analyzer (EDX).
• EDX energy dispersive X-ray analyzer
• Boron nitride powder may contain carbon as an impurity. Even a small amount of carbon can affect properties such as insulation performance depending on the situation in which the boron nitride powder is used. It is preferable that the content of carbon (impurity carbon) in the boron nitride powder is reduced.
• the upper limit of the amount of impurity carbon in the boron nitride powder may be, for example, 170 ppm or less, 165 ppm or less, or 160 ppm or less. When the upper limit of the amount of impurity carbon is within the above range, the insulation performance of the boron nitride powder is more excellent.
• the lower limit of the amount of impurity carbon in the boron nitride powder is not particularly limited and may not be contained, but may be, for example, 5 ppm or more, 10 ppm or more, or 15 ppm or more.
• the amount of impurity carbon in the boron nitride powder may be adjusted within the above range, and may be, for example, 5 to 170 ppm.
• the amount of impurity carbon in the present specification means a value measured by a carbon / sulfur simultaneous analyzer.
• the powder obtained by removing the above-mentioned carbon-containing particles (particle size of 63 ⁇ m or more) from the boron nitride powder to be measured shall be the measurement target.
• the carbon / sulfur simultaneous analyzer for example, "IR-412 type" (product name) manufactured by LECO can be used.
• the hexagonal boron nitride contained in the boron nitride powder is preferably highly crystalline.
• a graphitization index (sometimes referred to as Graphitization Index (GI)) can be used as the above-mentioned index of crystallization. That is, the boron nitride powder containing hexagonal boron nitride having a low graphitization index has less impurities and is excellent in insulation performance, and has high crystallinity, so that heat dissipation performance can also be improved.
• GI Graphitization Index
• the upper limit of the graphitization index of the boron nitride powder may be, for example, 2.3 or less, 2.2 or less, 2.1 or less, or 2.0 or less. When the upper limit of the graphitization index of the boron nitride powder is within the above range, the boron nitride powder is more excellent in insulation performance.
• the lower limit of the graphitization index of the boron nitride powder is not particularly limited, but is generally 1.2 or more, or 1.3 or more for heat-dissipating fillers.
• the graphitization index of the boron nitride powder may be adjusted within the above range, and may be, for example, 1.2 to 2.3 or the like.
• the graphitization index in the present specification is an index also known as an index value indicating the degree of crystallinity of graphite (for example, J. Thomas, et. Al, J. Am. Chem. Soc. 84, 4619). (1962) etc.).
• the graphitization index is calculated based on the spectrum measured by the powder X-ray diffractometry of the primary particles of hexagonal boron nitride. First, in the X-ray diffraction spectrum, the integrated intensity (that is, each diffraction peak) of each diffraction peak corresponding to the (100) plane, (101) plane, and (102) plane of the primary particle of hexagonal boron nitride and its baseline.
• the area value (the unit is arbitrary) surrounded by and is calculated and used as S100, S101, and S102, respectively.
• the value of [(S100 + S101) / S102] is calculated to determine the graphitization index. More specifically, it is determined by the method described in the examples of the present specification.
• the lower limit of the average particle size of the boron nitride powder may be, for example, 7 ⁇ m or more, 8 ⁇ m or more, 9 ⁇ m or more, or 10 m or more.
• the upper limit of the average particle size of the boron nitride powder may be, for example, 100 ⁇ m or less, 90 ⁇ m or less, 80 ⁇ m or less, or 75 ⁇ m or less.
• the sheet having a thickness of 500 ⁇ m or less can be suitably filled.
• the average particle size of the boron nitride powder can be adjusted within the above range, and may be, for example, 7 to 100 ⁇ m or 8 to 80 ⁇ m.
• the average particle size of the boron nitride powder can be selected according to the thickness of the sheet.
• the average particle size in the present specification is a value obtained by measuring the boron nitride powder without homogenizer treatment, and is an average particle size including aggregated particles.
• the average particle size in the present specification is also a particle size (median size, d50) at which the cumulative value of the cumulative particle size distribution is 50%.
• the average particle size in the present specification is measured by using a laser diffraction / scattering method particle size distribution measuring device according to the description of ISO 13320: 2009. Specifically, the measurement is carried out by the method described in the examples of the present specification.
• the laser diffraction / scattering method particle size distribution measuring device for example, "LS-13 320" (product name) manufactured by Beckman Coulter can be used.
• the lower limit of the specific surface area of the boron nitride powder may be, for example, 0.8 m 2 / g or more, 1.0 m 2 / g or more, 1.2 m 2 / g or more, or 1.4 m 2 / g or more.
• the upper limit of the specific surface area of the boron nitride powder may be, for example, 8.0 m 2 / g or less, 7.5 m 2 / g or less, 7.0 m 2 / g or less, or 6.5 m 2 / g or less.
• the specific surface area of the boron nitride powder can be adjusted within the above range, and may be, for example, 0.8 to 8.0 m 2 / g or 1.0 to 7.0 m 2 / g.
• the specific surface area in the present specification means a value measured using a specific surface area measuring device in accordance with the description of JIS Z 8830: 2013 "Method for measuring the specific surface area of powder (solid) by gas adsorption", and nitrogen gas. It is a value calculated by applying the BET one-point method using. Specifically, the measurement is carried out by the method described in the examples of the present specification.
• the agglomerated particles have voids because they are composed of agglomeration of a plurality of primary particles of hexagonal boron nitride. Therefore, it is desirable to use not only the value of the average particle size but also the value of the specific surface area as an index for property evaluation.
• the average particle size and specific surface area of the boron nitride powder may be adjusted within the above ranges, and the boron nitride powder has, for example, an average particle size of 7 to 100 ⁇ m and a specific surface area of 0.8 to 8. It may be 0.0 m 2 / g, an average particle size of 8 to 80 ⁇ m, and a specific surface area of 1 to 7 m 2 / g.
• the agglomerated particles are preferably excellent in crushing strength.
• the lower limit of the crushing strength of the aggregated particles may be, for example, 6 MPa or more, 8 MPa or more, 10 MPa or more, or 12 MPa or more.
• the upper limit of the crushing strength of the aggregated particles may be, for example, 20 MPa or less or 15 MPa or less.
• the crushing strength of the aggregated particles may be adjusted within the above range, and may be, for example, 6 to 20 MPa or 8 to 15 MPa.
• the crushing strength in the present specification is measured in accordance with the description of JIS R 1639-5: 2007 "Fine ceramics-Measuring method of (condyle) grain characteristics-Part 5: Single or grain crushing strength". Means the value. Specifically, the measurement is carried out by the method described in the examples of the present specification.
• the upper limit of the orientation index of the boron nitride powder may be, for example, 30 or less, 20 or less, 18 or less, or 15 or less.
• the lower limit of the orientation index of the boron nitride powder is not particularly limited, but may be, for example, 2 or more, 3 or more, or 5 or more.
• a boron nitride powder having better heat dissipation can be provided.
• the orientation index of the boron nitride powder may be adjusted within the above range, and may be, for example, 2 to 30 or the like.
• the orientation index in the present specification means the ratio of the peak intensity of boron nitride on the (002) plane to the peak intensity on the (100) plane measured by an X-ray diffractometer, and is [I (002) / I. (100)] can be calculated. Specifically, the measurement is carried out by the method described in the examples of the present specification.
• the boron nitride powder according to the present embodiment has sufficiently high purity and the content of carbon-containing particles is suppressed to be lower than that of the conventional product, it is subjected to a harsh environment (for example, a high voltage is applied for a long time, etc.). ), High performance (eg, insulation performance, etc.) can be exhibited.
• the boron nitride powder can be suitably used as a filler used by being dispersed in a resin, rubber or the like, for example.
• the boron nitride powder can be suitably used as a constituent material such as a heat transfer sheet.
• the above-mentioned boron nitride powder can be prepared, for example, by the following method.
• One embodiment of the method for producing boron nitride powder includes agglomerated particles formed by aggregating primary particles of hexagonal boron nitride, and heats a raw material powder having a purity of 98.0% by mass or more in an oxygen-containing atmosphere. It includes a step of treating (hereinafter, also referred to as an oxidation treatment step).
• the raw material powder may contain aggregated particles formed by agglomerating primary particles of hexagonal boron nitride and may have a purity of 98.0% by mass or more, and commercially available boron nitride powder may be used separately.
• the prepared one can also be used.
• a method of firing boron carbide in an atmosphere containing nitrogen hereinafter, also referred to as B4C method
• a method of firing in an atmosphere containing nitrogen hereinafter, also referred to as a carbon reduction method. It can be prepared by such as).
• An example of a method for preparing a raw material powder to which the B 4 C method is applied is a calcined product containing boron carbide (B 4 C N 4 ) obtained by calcining boron carbide powder (B 4 C powder) in a nitrogen-pressurized atmosphere.
• a nitriding step and a mixed powder containing the calcined product and a boron-containing compound containing boric acid are heated to generate scaly primary particles of hexagonal boron nitride (hBN).
• hBN hexagonal boron nitride
• It also has a step of obtaining a powder containing agglomerated particles formed by agglomerating primary particles (hereinafter, also referred to as a crystallization step).
• boron carbide powder for example, one prepared by the following procedure can also be used. After mixing boric acid and acetylene black, the mixture is heated at 1800 to 2400 ° C. for 1 to 10 hours in an inert gas atmosphere to obtain a boron carbide mass. Boron carbide powder can be prepared by pulverizing the boron carbide mass, sieving it, washing it, removing impurities, drying it, and the like as appropriate.
• the firing temperature in the nitriding step may be, for example, 1800 to 2400 ° C, 1900 to 2400 ° C, 1800 to 2200 ° C, or 1900 to 2200 ° C.
• the pressure in the nitriding step may be 0.6 to 1.0 MPa, 0.7 to 1.0 MPa, 0.6 to 0.9 MPa, or 0.7 to 0.9 MPa.
• the nitrogen gas concentration in the nitrogen-pressurized atmosphere in the nitriding step may be, for example, 95% by volume or more, or 99% by volume or more.
• the firing time in the nitriding step is not particularly limited as long as the nitriding progresses sufficiently, and may be, for example, 6 to 30 hours or 8 to 20 hours. In the present specification, the firing time means the time (holding time) for maintaining the temperature of the ambient environment of the object to be heated at the predetermined temperature after reaching the predetermined temperature.
• the boron nitride obtained in the nitriding step is decarbonized, and while producing scaly primary particles of a predetermined size, these are aggregated to obtain a boron nitride powder containing lumpy particles.
• Examples of the boron-containing compound include boron oxide and the like in addition to boric acid.
• the mixed powder heated in the crystallization step may contain known additives.
• the mixing ratio with the boron-containing compound can be appropriately set according to the molar ratio.
• the purity of the raw material powder can be improved by setting the content of the boron-containing compound in the mixed powder so that the amount of the boron-containing compound is excessive with respect to the boron nitride.
• the heating temperature for heating the mixed powder in the crystallization step may be, for example, 1800 to 2200 ° C, 2000 to 2200 ° C, or 2000 to 2100 ° C. By setting the heating temperature within the above range, grain growth can be promoted more sufficiently.
• the crystallization step may be heated in an atmosphere of normal pressure (atmospheric pressure), or may be pressurized and heated at a pressure exceeding the atmospheric pressure. When pressurizing, it may be, for example, 0.5 MPa or less, or 0.3 MPa or less.
• the heating time in the crystallization step may be, for example, 0.5 to 40 hours, 0.5 to 35 hours, or 1 to 30 hours. If the heating time is too short, grain growth tends not to proceed sufficiently. On the other hand, if the heating time is too long, it tends to be industrially disadvantageous.
• Hexagonal boron nitride powder can be obtained by the above steps.
• a pulverization step may be performed.
• a general crusher or crusher can be used.
• a ball mill, a vibration mill, a jet mill, or the like can be used.
• "crushing” also includes “crushing”.
• An example of a method for preparing a raw material powder to which the carbon reduction method is applied is a calcined product containing boron nitride, which is obtained by calcining a mixed powder containing a boron-containing compound containing boric acid and a carbon-containing compound in a nitrogen-pressurized atmosphere.
• the fired product is heat-treated at a temperature lower than 2050 ° C. to generate primary particles of hexagonal boron nitride (hBN), and the primary particles are produced.
• hBN hexagonal boron nitride
• It has a step of obtaining a powder containing agglomerated particles formed by agglomerating the particles (hereinafter, also referred to as a firing step).
• the boron-containing compound is a compound having boron as a constituent element.
• a raw material having high purity and relatively inexpensive can be used.
• examples of such a boron-containing compound include boric acid and, for example, boron oxide.
• the boron-containing compound contains boric acid, which is dehydrated by heating to form boron oxide, which can also serve as an auxiliary agent for forming a liquid phase and promoting grain growth during the heat treatment of the raw material powder.
• a carbon-containing compound is a compound having a carbon atom as a constituent element.
• the carbon-containing compound a raw material having high purity and relatively inexpensive can be used. Examples of such carbon-containing compounds include carbon black and acetylene black.
• the boron-containing compound may be blended in an excess amount with respect to the carbon-containing compound.
• the mixed powder may contain other compounds in addition to the carbon-containing compound and the boron-containing compound. Examples of other compounds include boron nitride as a nucleating agent. Since the mixed powder contains boron nitride as a nucleating agent, the average particle size of the hexagonal boron nitride powder to be synthesized can be more easily controlled.
• the mixed powder preferably contains a nucleating agent.
• the mixed powder contains a nucleating agent, it becomes easier to prepare a hexagonal boron nitride powder having a small specific surface area (for example, a hexagonal boron nitride powder having a specific surface area of less than 2.0 m 2 / g).
• the low temperature firing process is performed under pressure.
• the pressure in the low temperature firing step is, for example, 0.25 MPa or more and less than 5.0 MPa, 0.25 to 3.0 MPa, 0.25 to 2.0 MPa, 0.25 to 1.0 MPa, 0.25 MPa or more and less than 1.0 MPa. , 0.30 to 2.0 MPa, or 0.50 to 2.0 MPa.
• the volatilization of raw materials such as boron-containing compounds can be further suppressed, and the formation of boron carbide, which is a by-product, can be suppressed.
• the upper limit of the pressure in the low-temperature firing step within the above range, the growth of the primary particles of boron nitride can be further promoted.
• the heating temperature in the low temperature firing step may be, for example, 1650 ° C or higher and lower than 1800 ° C, 1650 to 1750 ° C, or 1650 to 1700 ° C.
• the reaction can be promoted and the yield of the obtained boron nitride can be improved.
• the upper limit of the heating temperature in the low-temperature firing step within the above range, the formation of by-products can be sufficiently suppressed.
• the heating time in the low temperature firing step may be, for example, 1 to 10 hours, 1 to 5 hours, or 2 to 4 hours.
• a heating time means the time (holding time) that the temperature of the ambient environment of the object to be heated reaches a predetermined temperature and is maintained at the temperature.
• the fired product obtained in the low-temperature firing step is heat-treated at a temperature higher than that in the low-temperature firing step to generate primary particles of hexagonal boron nitride (hBN), and the primary particles are aggregated and configured.
• hBN hexagonal boron nitride
• the heating temperature in the firing step is higher than that in the low temperature firing step and is less than 2050 ° C.
• the heating temperature in the firing step may be 2000 ° C. or lower.
• the heating time in the firing step may be, for example, 3 to 15 hours, 5 to 10 hours, or 6 to 9 hours.
• the pressure in the firing step is, for example, 0.25 MPa or more and less than 5.0 MPa, 0.25 to 3.0 MPa, 0.25 to 2.0 MPa, 0.25 to 1.0 MPa, 0.25 MPa or more and less than 1.0 MPa. It may be 0.30 to 2.0 MPa or 0.50 to 2.0 MPa.
• Hexagonal boron nitride powder can be obtained by the above steps.
• a pulverization step may be performed after the low-temperature firing step or the firing step.
• a general crusher or crusher can be used.
• the carbon content in the raw material powder is converted into carbon dioxide gas by heat-treating the raw material powder in the presence of oxygen, and carbon in the raw material powder is removed from the system. This is a step of reducing the residual amount of the minute. By this step, the content of carbon-containing particles and impurity carbon can be further reduced.
• the lower limit of the heating temperature in the oxidation treatment step may be, for example, 500 ° C. or higher, 600 ° C. or higher, or 700 ° C. or higher. By setting the lower limit of the heating temperature within the above range, the carbon content in the raw material powder can be further reduced.
• the upper limit of the heating temperature in the oxidation treatment step may be, for example, less than 1000 ° C., 900 ° C. or lower, or 800 ° C. or lower. By setting the upper limit of the heating temperature within the above range, it is possible to prevent excessive oxidation of boron nitride while performing the decarburization treatment.
• the heating temperature in the oxidation treatment step may be adjusted within the above range, and may be, for example, 500 ° C. or higher and lower than 1000 ° C., 500 to 900 ° C., or the like.
• the pressure in the oxidation treatment step can be adjusted to be, for example, atmospheric pressure or reduced pressure.
• the upper limit of the pressure in the oxidation treatment step may be, for example, 150 kPa or less, 130 kPa or less, or 120 kPa or less.
• the lower limit of the pressure in the oxidation treatment step is not particularly limited, but may be, for example, 15 kPa or more, 20 kPa or more, or 30 kPa or more.
• the pressure in the oxidation treatment step may be adjusted within the above range, and may be, for example, 15 to 150 kPa.
• the lower limit of the ratio of oxygen to the atmosphere in the oxidation treatment step may be, for example, 15% by volume or more, 18% by volume or more, or 20% by volume or more. By setting the lower limit of the oxygen ratio in the above range, the carbon content in the raw material powder can be further reduced.
• the upper limit of the ratio of oxygen to the atmosphere in the oxidation treatment step may be, for example, 80% by volume or less, 70% by volume or less, or 60% by volume or less.
• the ratio of oxygen means a value determined by volume in a standard state.
• the ratio of oxygen in the atmosphere in the oxidation treatment step may be adjusted within the above range, and may be, for example, 15 to 80% by volume.
• Example 1 [Preparation of boron carbide powder] 100 parts by mass of orthoboric acid manufactured by Nippon Denko Co., Ltd. and 35 parts by mass of acetylene black (trade name: HS100L) manufactured by Denka Co., Ltd. were mixed using a Henschel mixer. The obtained mixture was filled in a graphite crucible and heated by an arc furnace at 2200 ° C. for 6 hours in an argon atmosphere to obtain massive boron carbide (B4C). The obtained lump was coarsely pulverized with a jaw crusher to obtain a coarse powder. The obtained coarse powder was further pulverized by a ball mill having a silicon carbide ball (diameter: 10 mm) to obtain pulverized powder.
• B4C massive boron carbide
• a powder containing agglomerated particles composed of agglomerated primary particles of hexagonal boron nitride was obtained.
• the obtained powder was decomposed and crushed by 20 with a Henschel mixer, and then sieved through 95 ⁇ m to obtain a raw material powder.
• the purity of the raw material powder thus obtained was 99.2% by mass, the orientation index was 7, and the graphitization index was 2.5.
• the obtained raw material powder was subjected to the following oxidation treatment.
• carbon in the raw material powder was subjected to oxidation treatment for 2 hours while stirring the powder in the furnace at 700 ° C. and 1 rpm using a rotary kiln furnace under an atmospheric pressure atmosphere (oxygen ratio 21% by volume) with respect to 500 g of the raw material powder.
• a powder from which components (impurity carbon, etc.) were removed was obtained.
• Example 2 Boron nitride powder was prepared and evaluated in the same manner as in Example 1 except that the heating temperature in the oxidation treatment step was changed to 550 ° C.
• Example 3 Boron nitride powder was prepared and evaluated in the same manner as in Example 1 except that the amount of boric acid in the preparation of the raw material powder was changed to 70 parts by mass and the temperature of firing in the resistance heating furnace was changed to 1950 ° C.
• Example 4 Boron nitride powder was prepared in the same manner as in Example 1 except that the average particle size of the raw material powder was 45 ⁇ m by changing the treatment time of pulverization by a ball mill in the preparation of the boron carbide powder to 60 minutes. evaluated.
• Example 5 The boron nitride powder was prepared in the same manner as in Example 1 except that the condition of pulverization by a ball mill in the preparation of the boron carbide powder was changed to 3 hours at a rotation speed of 50 rpm so that the average particle size of the raw material powder was 10 ⁇ m. Prepared and evaluated.
• Example 6 By changing the amount of boric acid in the preparation of the raw material powder to 55 parts by mass and changing the temperature of firing in the resistance heating furnace to 1890 ° C., the raw material powder was changed to G.I. I. Boron nitride powder was prepared and evaluated in the same manner as in Example 1 except that the value was changed to 2.2.
• Example 7 By changing the temperature of the resistance heating furnace firing in the preparation of the raw material powder to 2100 ° C., the raw material powder G.I. I. Boron nitride powder was prepared and evaluated in the same manner as in Example 1 except that the value was changed to 1.4.
• Example 8 The conditions for pulverization by a ball mill in the preparation of boron carbide powder were set to 60 minutes at a rotation speed of 25 rpm, and then the pulverized powder was changed to be classified using a vibrating sieve with an opening of 63 ⁇ m, and the amount of boric acid in the preparation of the raw material powder was changed.
• the specific surface area of the raw material powder was 2.7, the average particle size was 30 ⁇ m, and G.I. I. Boron nitride powder was prepared and evaluated in the same manner as in Example 1 except that the value was changed to 1.7.
• Boron nitride powder was alkaline-decomposed with sodium hydroxide, and ammonia was distilled from the decomposition solution by a steam distillation method and collected in an aqueous boric acid solution. This collected liquid was titrated with a sulfuric acid specified liquid. The content of nitrogen atom (N) in the boron nitride powder was calculated from the titration result. From the obtained nitrogen atom content, the content of hexagonal boron nitride (hBN) in the boron nitride powder was determined based on the formula (1), and the purity of the hexagonal boron nitride powder was calculated.
• N nitrogen atom
• hBN hexagonal boron nitride
• the formula amount of hexagonal boron nitride was 24.818 g / mol, and the atomic weight of the nitrogen atom was 14.006 g / mol.
• Content of hexagonal boron nitride (hBN) in the sample [mass%] content of nitrogen atom (N) [mass%] ⁇ 1.772 ... Equation (1)
• the graphitization index of the boron nitride powder was calculated from the measurement results by the powder X-ray diffraction method.
• the area value surrounded by the line (the unit is arbitrary) was calculated and used as S100, S101, and S102, respectively. Using the area value calculated in this way, the graphitization index was determined based on the following formula (2).
• GI (S100 + S101) / S102 ... Equation (2)
• the average particle size of the boron nitride powder was measured using a laser diffraction / scattering method particle size distribution measuring device (device name: LS-13 320) manufactured by Beckman Coulter Co., Ltd. in accordance with the description of ISO 13320: 2009. The measurement was performed without performing the homogenizer treatment on the boron nitride powder.
• water was used as the solvent for dispersing the boron nitride powder, and hexametaphosphate was used as the dispersant. At this time, a value of 1.33 was used as the refractive index of water, and a value of 1.80 was used as the refractive index of the boron nitride powder.
• the specific surface area of the boron nitride powder was calculated by applying the BET one-point method using nitrogen gas in accordance with the description of JIS Z 8830: 2013 “Method for measuring the specific surface area of powder (solid) by gas adsorption”.
• a specific surface area measuring device (device name: Cantersorb) manufactured by Yuasa Ionics Co., Ltd. was used. The measurement was carried out after the boron nitride powder was dried and degassed at 300 ° C. for 15 minutes.
• the crushing strength of the agglomerated particles was measured according to the description of JIS R 1639-5: 2007 "Fine Ceramics-Measuring Method of (Condyle) Grain Characteristics-Part 5: Single or Grain Crushing Strength".
• the orientation index of the boron nitride powder was determined from the measurement results by the powder X-ray diffraction method.
• a boron nitride powder is filled in the recess of a glass cell having a recess of 0.2 mm, which is attached to an X-ray diffractometer (manufactured by Rigaku Co., Ltd., trade name: ULTIMA-IV), and a powder sample molding machine ( A measurement sample was prepared by solidifying at a set pressure M using Amena Tech Co., Ltd., trade name: PX700). If the surface of the filling material hardened by the above molding machine was not smooth, the surface was manually smoothed before measurement.
• the peak intensity ratio between the (002) plane and the (100) plane of boron nitride was calculated, and the orientation index [I (002) was calculated based on this value. ) / I (100)] was determined.
• the amount of impurity carbon in the boron nitride powder was measured by a carbon / sulfur simultaneous analyzer (manufactured by LECO, trade name: IR-412 type).
• the number of carbon-containing particles in boron nitride powder was measured as follows. First, 10 g of boron nitride powder to be measured and 100 mL of ethanol were measured in a container and stirred with a stirring rod to prepare a mixed solution. Next, the above mixed solution was dispersed using an ultrasonic disperser to prepare a dispersion. The obtained dispersion is put into a sieve having a mesh opening of 63 ⁇ m (JIS Z 8801-1: 2019 “Test Sieve-Metal Net Sieve”), and then 2 L of distilled water is put into the sieve, and white turbid water is discharged from under the sieve.
• JIS Z 8801-1 2019 “Test Sieve-Metal Net Sieve
• the sieve is dried and the powder is dispersed on the medicine wrapping paper, a permanent magnet is placed under the medicine wrapping paper, and the powder that is not magnetized with respect to the permanent magnet is dispersed on another medicine wrapping paper and observed with an optical microscope. This was done and the number of colored particles observed was counted. The same operation was performed for 5 or more samples, an arithmetic average of the number of obtained colored particles was calculated, and this average value was taken as the number of carbon-containing particles per 10 g of boron nitride powder. It was confirmed by measuring by XRF that it contained carbon.
• a resin sheet containing boron nitride powder was prepared.
• a mixture of 100 parts by mass of a naphthalene type epoxy resin (manufactured by DIC Corporation, trade name HP4032) and 10 parts by mass of imidazoles (manufactured by Shikoku Chemicals Corporation, trade name MAVT) as a curing agent was prepared.
• Boron nitride powder was stirred and mixed with a planetary mixer at a ratio of 55 parts by volume to 100 parts by volume of this mixture for 15 minutes. The obtained mixture was applied onto a PET sheet, and then defoamed under a reduced pressure condition of 500 Pa for 10 minutes.
• the epoxy resin composition is applied onto a film made of polyethylene terephthalate (PET) having a thickness of 0.05 mm so as to have a thickness of 0.10 mm after curing, and is heated and dried at 100 ° C. for 15 minutes by a press machine.
• PET polyethylene terephthalate
• a heat-dissipating sheet having a thickness of 0.1 mm was obtained by heating and curing at 180 ° C. for 180 minutes while applying a surface pressure of 160 kgf / cm 2 .
• the obtained heat dissipation sheet was used as an evaluation target.
• the dielectric strength of the heat radiating sheet was measured according to the method described in JIS C 2110. Specifically, a sheet-shaped heat-dissipating member (heat-dissipating sheet) is processed to a size of 5 cm ⁇ 5 cm, a circular copper layer having a diameter of 25 mm is formed on one surface of the processed heat-dissipating member, and a circular copper layer having a diameter of 25 mm is formed on the other surface. A copper layer was formed on the entire surface to prepare a test sample. The electrodes were arranged so as to sandwich the test sample, and a DC voltage of 1100 V was applied at 65 ° C. and 90 RH%.
• the energization time (called breakdown time) from application to dielectric breakdown was measured and evaluated according to the following criteria. The same evaluation was performed 10 times for each evaluation sample, and the average value was taken as the insulation performance of each evaluation sample.
• ThermoPlusEvo DSC8230 As the specific heat capacity C, a value measured using a differential scanning calorimeter (manufactured by Rigaku Co., Ltd., trade name: ThermoPlusEvo DSC8230) was used. Based on the obtained thermal conductivity H, the heat dissipation performance of the boron nitride powder was evaluated according to the following criteria.
• C The thermal conductivity H is 6 W / mK or more and less than 9 W / mK.
• D Thermal conductivity H is less than 6 W / mK.
• boron nitride powder having superior insulation performance when used as a filler as compared with the conventional boron nitride powder.

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