Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B21/00—Nitrogen; Compounds thereof
  • C01B21/06—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron
  • C01B21/064—Binary compounds of nitrogen with metals, with silicon, or with boron, or with carbon, i.e. nitrides; Compounds of nitrogen with more than one metal, silicon or boron with boron

Definitions

• the present invention relates to a method for producing hexagonal boron nitride powder and to hexagonal boron nitride powder.
• Hexagonal boron nitride has excellent electrical insulation, thermal conductivity, and thermal stability, and is used as a material in a variety of fields.
• hexagonal boron nitride it is desirable to use hexagonal boron nitride powder with minimal foreign matter in order to ensure that the properties of hexagonal boron nitride are utilized at a high level.
• colored foreign matter containing carbon and metal elements can be conductive, so it is necessary to remove them to a high degree from the hexagonal boron nitride powder.
• Patent Document 1 discloses hexagonal boron nitride in which the number of colored particles containing carbon is 50 or less per 10 g.
• the hexagonal boron nitride powder described in Patent Document 1 is examined for the number of colored particles on the order of 10 g.
• foreign matter in an amount that may be overlooked on the order of 10 g can also be a problem. Therefore, there is a demand for hexagonal boron nitride powder with even less foreign matter than that disclosed in Patent Document 1.
• One aspect of the present invention aims to produce hexagonal boron nitride powder that contains less foreign matter than conventional powders.
• a method for producing a hexagonal boron nitride powder comprises the steps of: (1) a magnetic separation step for removing magnetizable foreign matter by magnetic separation treatment; (2) An oxidation treatment step of reducing the content of organic components by oxidation treatment under conditions of 300° C. or higher and 700° C. or lower; (3) an acid washing step for removing acid-soluble impurities by acid washing; and (4) a firing step for firing at a temperature of 1800° C. or higher and 2050° C. or lower.
• This is a method for producing a hexagonal boron nitride powder, characterized in that the treatment by the oxidation treatment step (2) and the treatment by the acid washing step (3) are performed before the treatment by the firing step (4).
• the hexagonal boron nitride powder according to one embodiment of the present invention is a hexagonal boron nitride powder that contains hexagonal boron nitride particles, has a brightness of 110 or less, and has a number of colored foreign matter with a particle size of 50 ⁇ m or more of 30 or less per kg.
• the method for producing hexagonal boron nitride powder according to this embodiment is a method for producing hexagonal boron nitride powder from crude hexagonal boron nitride powder, and includes treatments by magnetic separation, oxidation treatment, acid washing, and firing.
• crude hexagonal boron nitride powder is used as the starting material, but in each of the above steps, the crude hexagonal boron nitride powder treated in the previous step may be further treated. Therefore, the hexagonal boron nitride powder treated in each of the above steps may be referred to as "treated hexagonal boron nitride powder".
• the crude hexagonal boron nitride powder is a powder of hexagonal boron nitride particles from which foreign matter has not been removed.
• the crude hexagonal boron nitride powder is not particularly limited, and one obtained by any method may be used.
• "foreign matter contained in the crude hexagonal boron nitride powder” refers to objects such as particles other than hexagonal boron nitride particles, components other than hexagonal boron nitride particles attached to the surfaces of hexagonal boron nitride particles, or components or particles other than hexagonal boron nitride contained in aggregates of hexagonal boron nitride.
• Examples of foreign matter contained in the crude hexagonal boron nitride powder include colored foreign matter.
• colored foreign matter refers to a colored object with a particle size of 50 ⁇ m or more.
• a colored object refers to an object such as a particle colored with a component other than hexagonal boron nitride particles, or a particle itself other than hexagonal boron nitride particles.
• colored refers to a brightness of 110 or less in 256-level grayscale image data in which black is 0 and white is 255.
• a colored foreign matter refers to an object with a brightness of 110 or less in 256-level grayscale image data that includes crude hexagonal boron nitride powder as the subject.
• Colored foreign matter includes, but is not limited to, magnetic foreign matter, organic components, and acid-soluble impurities. Colored foreign matter may be determined by any method, for example, by a conventional foreign matter inspection machine that inspects for colored foreign matter in powders.
• Magnetic foreign matter refers to foreign matter that has magnetism, and other characteristics are not particularly limited.
• magnetizable foreign matter includes foreign matter containing elements such as Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, Zn, and Al. These elements are often contained in crude hexagonal boron nitride powder or its manufacturing equipment.
• the organic component refers to a foreign matter containing carbon, and other characteristics are not particularly limited.
• the organic component may be a component derived from carbon contained in the raw material of the crude hexagonal boron nitride powder, for example, a component derived from a carbon source or a carbon-containing boron compound.
• Acid-soluble impurities refer to foreign matter that is soluble in acid, and other characteristics are not particularly limited.
• acid-soluble impurities include impurities such as composite oxides consisting of boron oxide and calcium oxide that are generated as a by-product during the production of crude hexagonal boron nitride powder.
• Magnetic foreign matter, organic components, and acid-soluble impurities are colored foreign matter. For this reason, these colored foreign matter are removed by the magnetic separation process, oxidation treatment process, and acid washing process described below. Note that magnetic foreign matter, organic components, and acid-soluble impurities that are not colored foreign matter may also be removed in each process in the same way as colored foreign matter. Colored foreign matter can be conductive, so it is preferable that it is highly reduced, especially when hexagonal boron nitride powder is applied to an insulating sheet.
• the magnetic separation process removes magnetizable foreign matter from the treated hexagonal boron nitride powder by magnetic separation.
• the magnetic separation process is preferably performed at a throughput of 50 kg/h or less, and more preferably at a throughput of 40 kg/h or less.
• the magnetizable foreign matter can be removed to a high degree.
• the smaller the throughput of the magnetic separation process the higher the accuracy of removing the magnetizable foreign matter.
• the throughput of the magnetic separation process may be 0.5 kg/h or more, and more preferably 2 kg/h or more.
• the magnetic separation process may be performed using an electromagnetic sieve device having a magnetic pole portion.
• an electromagnetic sieve device having a magnetic pole portion.
• the magnetic separation process may be carried out before or after any step, but is preferably carried out as the first step.
• the conditions of the magnetic pole part of the electromagnetic sieve device may be as follows. Note that the lower limit values of the magnetic pole part area and the magnetic force of the screen are shown below, but since the larger the magnetic pole part area and magnetic force, the greater the effect of magnetic separation in removing magnetized foreign matter, there is no upper limit for the magnetic force and magnetic pole part area.
• the electromagnetic sieving device is a device that can be used by stacking 20 or more screens at specific intervals.
• the electromagnetic sieving device is, for example, a device with protrusions on the inner side wall of a vertically long cylindrical device to hold the screens, and after all the screens are set, powder is added from the top, and the entire device is vibrated so that the powder falls by gravity and passes through all the screens, thereby performing magnetic separation.
• the oxidation treatment step reduces the content of organic components from the treated hexagonal boron nitride powder by oxidation treatment.
• the oxidation treatment may be performed under conditions of 300°C or higher, and is preferably performed under conditions of 400°C or higher.
• the oxidation treatment step may be performed under conditions of 700°C or lower, and is preferably performed under conditions of 680°C or lower.
• the processing time of the oxidation treatment is preferably 2 hours or more, and more preferably 3 hours or more, from the viewpoint of suitably reducing the content of organic components.
• the upper limit of the processing time of the oxidation treatment is not particularly limited, but taking into consideration the processing efficiency, it is preferably 10 hours or less, and more preferably 8 hours or less.
• the oxidation treatment step is preferably performed after the magnetic separation step, although there is no particular restriction on the order in which it is performed as long as it is performed before the firing step.
• the method of acid washing the hexagonal boron nitride powder is not particularly limited, and any known method may be used without limitation.
• the treated hexagonal boron nitride powder is placed in a container, and diluted hydrochloric acid (5 to 10 mass % HCl) is added in an amount 5 to 10 times the amount of the treated hexagonal boron nitride powder, and the mixture is allowed to come into contact for 4 to 8 hours.
• diluted hydrochloric acid (5 to 10 mass % HCl) is added in an amount 5 to 10 times the amount of the treated hexagonal boron nitride powder, and the mixture is allowed to come into contact for 4 to 8 hours.
• As the acid used in the acid washing in addition to hydrochloric acid, nitric acid, sulfuric acid, acetic acid, etc. can also be used.
• a water wash using pure water may be performed to wash off any remaining acid.
• the water wash method may involve filtering out the acid used in the acid washing, dispersing the pickled hexagonal boron nitride powder in the same amount of pure water as the acid used, and filtering again.
• the hydrous aggregates may be dried. Drying conditions may be, for example, in air at 50 to 250°C, under reduced pressure, or in a vacuum. There is no particular time for drying, but it is preferable to dry until the moisture content approaches 0%.
• the acid washing step is preferably performed after the oxidation treatment step, although there is no particular restriction on the order in which it is performed as long as it is performed before the firing step.
• the firing step is a step of firing the treated hexagonal boron nitride powder.
• the heating temperature in firing may be 1800°C or higher and 2050°C or lower, and is preferably 1900°C or higher and 2000°C or lower. By setting the heating temperature in firing within this range, colored foreign matter with a boiling point of 1900°C or lower can be volatilized. This makes it possible to reduce the amount of colored foreign matter.
• by carrying out the firing step after the oxidation treatment step and the acid washing step it is possible to improve the crystallinity of the hexagonal boron nitride powder whose surface crystallinity has been reduced by oxidation or acid washing.
• the nitrogen-containing gas atmosphere in which the firing is performed may be, for example, an atmosphere containing nitrogen and oxygen.
• the nitrogen source can be supplied to the reaction system during firing by known means.
• the nitrogen source used is not limited to nitrogen gas, and is not particularly limited as long as it is a gas capable of nitriding.
• ammonia gas can be used in addition to nitrogen gas. It is also possible to mix nitrogen gas and ammonia gas with non-oxidizing gases such as hydrogen, argon, and helium.
• the above-described firing can be carried out using a known reaction device capable of controlling the reaction atmosphere.
• a known reaction device capable of controlling the reaction atmosphere.
• an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating can be used.
• continuous heating furnaces such as pusher-type tunnel furnaces and vertical reaction furnaces can also be used.
• the manufacturing method for hexagonal boron nitride powder which includes the magnetic separation process, oxidation treatment process, acid washing process, and firing process, can remove foreign matter with different characteristics from the crude hexagonal boron nitride powder at each process with high precision. Therefore, this manufacturing method can produce hexagonal boron nitride powder with few colored foreign matter on the order of kilograms.
• the suction removal step is a step of determining colored foreign matter contained in the treated hexagonal boron nitride powder based on image data acquired by a camera, and removing the determined colored foreign matter by suction.
• the colored foreign matter may have a brightness of 110 or less and a particle size of 50 ⁇ m or more.
• a camera acquires 256-level grayscale image data for the hexagonal boron nitride powder, with black being 0 and white being 255.
• foreign matter with a brightness of 110 or less and a particle size of 50 ⁇ m or more is identified and determined to be a colored foreign matter.
• the camera that acquires image data containing the hexagonal boron nitride powder as a subject is not particularly limited as long as it has a configuration that can achieve the above function.
• the configuration for determining colored foreign matter from grayscale image data is also not particularly limited as long as it has a configuration that can achieve the above function.
• colored foreign matter may be determined using any image processing device.
• the colored foreign matter is removed from the hexagonal boron nitride powder by suction.
• the configuration for suction is not particularly limited. It is preferable that the hexagonal boron nitride powder subjected to the suction removal process has a small amount of foreign matter.
• the hexagonal boron nitride powder subjected to the removal suction process may have 50 or less colored foreign matter particles with a particle size of 50 ⁇ m or more per kg, and more preferably 30 or less.
• the hexagonal boron nitride powder subjected to the suction removal process preferably has a whiteness W calculated by the following formula (1) of 93 or more, and more preferably 95 or more.
• L, a, and b are the L * value , a * value, and b * value based on the L* a * b * color system.
• the value can be measured by a known method described in JIS Z8722, for example, using a colorimeter/color difference meter.
• hexagonal boron nitride powder with a whiteness W below the lower limit or with a number of colored foreign matter above the upper limit will have a large number of colored foreign matter sucked in in the suction removal process, resulting in a large loss of hexagonal boron nitride powder due to suction removal.
• hexagonal boron nitride powder with a sufficiently high whiteness W or with a sufficiently small number of colored foreign matter, i.e., powder that has been subjected to at least the magnetic separation process, oxidation treatment process, and acid washing process, to the suction removal process, the loss of hexagonal boron nitride powder can be reduced.
• the processing volume in the suction removal process is not particularly limited.
• the processing volume in the suction removal process is 15 kg/h or less, and a processing volume of 10 kg/h or less is particularly preferred.
• the processing volume in the suction removal process may be 0.1 kg/h or more, and a processing volume of 0.5 kg/h or more is particularly preferred.
• the magnetic separation process may be performed using an electromagnetic sieve device having a magnetic pole section.
• the hexagonal boron nitride powder subjected to the suction removal process may be in the form of a uniform layer of any thickness between 50 ⁇ m and 300 ⁇ m.
• the hexagonal boron nitride powder may be smoothed with a roller to achieve a uniform layer.
• the configuration for realizing the suction removal process is not particularly limited.
• the suction removal process may be realized by an apparatus that includes a camera, a configuration for processing image data, a roller, and a configuration for suction removal.
• the suction removal process can remove colored foreign matter that was not completely removed by the magnetic separation process, oxidation treatment process, acid washing process, and firing process. Therefore, a manufacturing method that includes a suction removal process can produce high-purity hexagonal boron nitride powder.
• the crude hexagonal boron nitride powder may be prepared by a reduction-nitridation step.
• a raw material mixture of raw materials for the crude hexagonal boron nitride powder is heated to a temperature of 1600 to 1950°C in a nitrogen atmosphere to carry out a reduction-nitridation reaction.
• the nitrogen source may be supplied to the reaction system in the reduction-nitridation reaction in the same manner as in the firing step.
• the raw material mixture may contain an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound.
• oxygen-containing boron compound a compound containing boron and an oxygen atom may be used.
• oxygen-containing boron compounds include boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. Of these, boric acid or boron oxide, which are easily available, may be preferably used.
• Carbon sources include, for example, amorphous carbon such as carbon black, activated carbon, and carbon fiber, as well as crystalline carbon such as diamond, graphite, and nanocarbon, and pyrolytic carbon obtained by pyrolyzing a monomer or polymer.
• amorphous carbon such as carbon black, activated carbon, and carbon fiber
• crystalline carbon such as diamond, graphite, and nanocarbon
• pyrolytic carbon obtained by pyrolyzing a monomer or polymer.
• highly reactive amorphous carbon is preferred, and carbon black is particularly preferred because its quality is industrially controlled.
• Examples of carbon black include acetylene black, furnace black, and thermal black.
• oxygen-containing calcium compounds include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. These can be used alone or in combination of two or more. Of these, it is preferable to use calcium oxide or calcium carbonate.
• the carbon-containing boron compound a compound containing carbon and boron (e.g., boron carbide) may be used.
• the particle size of the carbon-containing boron compound is preferably 1 to 500 ⁇ m, more preferably 10 to 400 ⁇ m, and even more preferably 20 to 300 ⁇ m.
• the hexagonal boron nitride powder obtained after the reduction-nitridation reaction may be adjusted to a desired particle size distribution by crushing. Crushing is preferably carried out gently using a jet mill, ball mill, hammer mill, stone mill, or the like. The particle size of the powder after crushing may also be adjusted appropriately by classification using air classification or sieving.
• the range of particle size to be adjusted is preferably set so as to remove particles larger than the mesh of the screen of the electromagnetic sieve device used in the magnetic separation process. In other words, if powder larger than the mesh is present, it will not be able to pass through the screen, causing clogging and preventing the electromagnetic sieve device from functioning properly, and in the worst case scenario, this may cause malfunctions such as equipment breakdown.
• hexagonal boron nitride powder A hexagonal boron nitride powder according to another embodiment of the present invention will be described below.
• the hexagonal boron nitride powder of this embodiment contains hexagonal boron nitride particles.
• the number of colored foreign matter having a brightness of 110 or less and a particle size of 50 ⁇ m or more is 30 or less per 1 kg.
• the number of colored foreign matter in the hexagonal boron nitride powder may be 30 or less per kg, preferably 25 or less, more preferably 20 or less, and even more preferably 10 or less. Colored foreign matter may interfere with the properties of the hexagonal boron nitride powder, so the fewer the number of colored foreign matter, the better; for example, there may be no colored foreign matter.
• There are no particular limitations on the method for measuring the number of colored foreign matter so long as it is configured to measure 1 kg of hexagonal boron nitride powder. For example, it may be achieved by any powder foreign matter inspection and removal machine equipped with a camera.
• Hexagonal boron nitride powder with colored inclusions in this range is hexagonal boron nitride powder with extremely low levels of colored inclusions. This allows it to stably demonstrate high voltage resistance when filled into a sheet. Furthermore, even when used for other purposes, the properties of hexagonal boron nitride powder can be stably demonstrated at a high level.
• the hexagonal boron nitride powder preferably has a whiteness W calculated by the following formula (1) of 93 or more, and more preferably 95 or more.
• Such hexagonal boron nitride powder has less foreign matter. Therefore, when filled into a sheet, it can exhibit a higher withstand voltage. Furthermore, even when used for other purposes, the characteristics of the hexagonal boron nitride powder can be stably exhibited at a high level.
• the total content of Fe, Ca, Al, and Si elements contained in the hexagonal boron nitride powder is preferably 3 ppm or less, and more preferably 2.5 ppm or less. By ensuring that the total content of Fe, Ca, Al, and Si elements does not exceed the upper limit, the hexagonal boron nitride powder can have excellent electrical insulation properties.
• the amount of Fe contained in the hexagonal boron nitride powder is preferably 1 ppm or less, and more preferably 0.5 ppm or less.
• the amount of Ca contained in the hexagonal boron nitride powder is preferably 2.5 ppm or less, and more preferably 2 ppm or less.
• the amount of Al contained in the hexagonal boron nitride powder is preferably 0.8 ppm or less, and more preferably 0.5 ppm or less.
• the amount of Si contained in the hexagonal boron nitride powder is preferably 0.8 ppm or less, and more preferably 0.5 ppm or less.
• the amount of C contained in the hexagonal boron nitride powder is preferably 0.05% or less, and more preferably 0.035% or less. By ensuring that the amount of C does not exceed the upper limit, the hexagonal boron nitride powder can have excellent electrical insulation properties.
• the Fe, Ca, Al, and Si elements in the hexagonal boron nitride powder can be quantified based on the conventionally known ICP measurement.
• the amount of C in the hexagonal boron nitride powder can be quantified from the amount of CO gas and CO2 gas generated by burning the hexagonal boron nitride powder in an oxygen stream.
• the average particle size of the hexagonal boron nitride powder may be 2 to 90 ⁇ m, and is preferably 5 to 70 ⁇ m. If the average particle size is less than 2 ⁇ m, the particle size may be too small and difficult to handle, and if it exceeds 90 ⁇ m, the particle size may be too large and difficult to handle.
• the graphite index (GI value) which indicates the crystallinity of the hexagonal boron nitride powder, is preferably 1.0 to 2.5, more preferably 1.3 to 2.0, more preferably 1.4 to 1.8, and even more preferably 1.5 to 1.7.
• the GI value is calculated by dividing the sum of the peak areas originating from the (100) and (101) planes in the X-ray diffraction spectrum by the peak area originating from the (102) plane.
• the above-mentioned hexagonal boron nitride powder can be filled into a resin as a filler.
• a resin sheet When filled into a resin as a filler, it is possible to impart extremely high insulating performance to the resulting resin composition.
• the resin sheet when a resin sheet is produced from a resin filled with the above-mentioned hexagonal boron nitride powder, the resin sheet can be imparted with insulating performance of 100 kv/mm or more.
• 70% or more of the produced resin sheet has a resistance of 100 kv/mm or more, it may be determined that insulating performance has been imparted.
• the use of such a resin sheet is not particularly limited, but it can be used, for example, for circuit board applications and insulating layer applications for multilayer printed wiring boards.
• Hexagonal boron nitride powder can also be used as a raw material for boron nitride processed products such as cubic boron nitride or boron nitride molded products, a nucleating agent for engineering plastics, a phase change material, a solid or liquid thermal interface material, a release agent for molten metal or molten glass molds, cosmetics, and a raw material for composite ceramics.
• Hexagonal boron nitride powder according to one embodiment of the present invention is obtained, for example, by using the manufacturing method described above.
• the hexagonal boron nitride powder obtained by the manufacturing method described above is characterized in that it has a brightness of 110 or less and the number of colored foreign matter having a particle size of 50 ⁇ m or more is 30 or less per kg.
• a resin sheet made using the above-mentioned hexagonal boron nitride powder has high dielectric strength.
• the above-mentioned method for producing hexagonal boron nitride powder is characterized by including a step for removing colored foreign matter, but the final product, hexagonal boron nitride powder, does not contain the removed colored foreign matter, so the removed colored foreign matter cannot be considered as a specific item of the hexagonal boron nitride powder. Therefore, it is impossible to generally identify the hexagonal boron nitride powder according to one aspect of the present invention in words based on anything other than the above-mentioned characteristics.
• the present invention is not limited to the above-mentioned embodiments, and various modifications are possible within the scope of the claims, and embodiments obtained by appropriately combining the technical means disclosed in each of the different embodiments are also included in the technical scope of the present invention.
• the method for producing a hexagonal boron nitride powder according to the first aspect of the present invention includes the steps of: (1) a magnetic separation step for removing magnetizable foreign matter by magnetic separation treatment; (2) An oxidation treatment step of reducing the content of organic components by oxidation treatment under conditions of 300° C. or higher and 700° C. or lower; (3) an acid washing step for removing acid-soluble impurities by acid washing; and (4) a firing step for firing at a temperature of 1800° C. or higher and 2050° C. or lower.
• the oxidation treatment step (2) and the acid washing step (3) are carried out before the firing step (4).
• the method for producing hexagonal boron nitride powder according to aspect 2 of the present invention may further include a suction removal step in which, in the above-mentioned aspect 1, colored foreign matter contained in the hexagonal boron nitride powder that has a brightness of 110 or less and a particle size of 50 ⁇ m or more is determined based on image data acquired by a camera, and the determined colored foreign matter is removed by suction.
• the hexagonal boron nitride powder according to aspect 3 of the present invention contains hexagonal boron nitride particles, has a brightness of 110 or less, and contains 30 or less colored foreign particles with a particle size of 50 ⁇ m or more per kg.
• the hexagonal boron nitride powder according to aspect 4 of the present invention has a whiteness W of 93 or more as calculated by the following formula (1) in the above-mentioned aspect 3.
• hexagonal boron nitride powders according to the examples and comparative examples were each produced by the methods described below.
• Example 1 A crude hexagonal boron nitride powder according to Example 1 was prepared. 259 g of a mixture containing 141 g of boron oxide, 56 g of carbon black, 32 g of calcium oxide, and 30 g of boron carbide was mixed using a ball mill. The mixture was subjected to a reduction-nitridation treatment in a graphite Tammann furnace by holding it in a nitrogen gas atmosphere at a maximum temperature of 1750° C. for 2 hours.
• the obtained crude hexagonal boron nitride was crushed, and the volume ratio of agglomerated particles of 10 ⁇ m or less in the particle size distribution was adjusted to 4%, and the volume ratio of agglomerated particles of more than 10 ⁇ m and less than 50 ⁇ m was adjusted to 48%.
• the crude hexagonal boron nitride powder after particle size distribution adjustment was placed in a container, and five times the amount of hydrochloric acid (7 mass% HCl) was added and stirred at a rotation speed of 700 rpm for 24 hours to perform an acid washing treatment.
• the acid was filtered, and the crude hexagonal boron nitride powder obtained by filtration was dispersed in the same amount of pure acid as used, and filtered again. This operation was repeated five times, and then it was vacuum dried at 200°C for six hours.
• the coarse hexagonal boron nitride powder was subjected to magnetic separation using an electromagnetic sieve device having 40 stacked screens with a diameter of 145 mm, a thickness of 10 mm, mesh size of 5 mm x 8 mm, a magnetic pole area of 54 cm2 , and magnetized to 1.6 T.
• the magnetic separation was performed at a throughput of 15 kg/h (magnetic separation step).
• the crude cubic boron nitride powder after magnetic separation was heated in air at an oxidation temperature of 600°C for 5 hours to oxidize the organic components (oxidation treatment process).
• the hexagonal boron nitride powder after the reduction nitridation process was placed in a container, five times the amount of hydrochloric acid (7% by mass HCl) was added, and the mixture was stirred at 700 rpm for five hours to perform an acid washing process (acid washing process). After the acid washing process, the acid was filtered, and the hexagonal boron nitride powder obtained by filtration was dispersed in the same amount of pure acid as that used, and then filtered again. This process was repeated five times, and the mixture was then vacuum dried at 200°C for six hours.
• the crude hexagonal boron nitride powder obtained after drying was subjected to a sintering process by holding it in a nitrogen gas atmosphere at 1900°C for 6 hours in a graphite Tammann furnace (sintering process). After cooling the furnace, the hexagonal boron nitride powder according to Example 1 was recovered from the furnace.
• Table 1 also shows a list of the conditions under which each process was carried out in Examples 1 to 6 and Comparative Examples 1 to 4.
• Example 2 Magnetic separation throughput: 25 kg/h, oxidation temperature: 500° C., firing temperature: 1850° C.
• Example 3 Magnetic separation throughput: 10 kg/h, oxidation temperature: 650° C., firing temperature: 1950° C.
• Example 4 Magnetic separation throughput: 15 kg/h, oxidation temperature: 550° C.
• Example 5 Magnetic separation throughput: 45 kg/h, oxidation temperature: 450° C., firing temperature: 1850° C.
• Example 6 Magnetic separation throughput: 15 kg/h, oxidation temperature: 600° C.
• PPS-R2 manufactured by Nisshin Chemical Industry Co., Ltd.
• PPS-R2 manufactured by Nisshin Chemical Industry Co., Ltd.
• SEM-EDS inspection Of the detected colored foreign bodies, five were sampled and subjected to elemental analysis by SEM-EDS inspection. The equipment and conditions used for the SEM-EDS inspection are shown below.
• Ultrapure water was added to a 25 ml polypropylene measuring flask to make the volume constant.
• the amount of Fe, Ca, Al, and Si present in the solution was measured using an ICP light emitting device (ICAP6500 manufactured by Thermo Fisher Scientific Co., Ltd.), and the content of each sample was calculated.
• the average value of the 10 samples is shown in Table 2.
• the total carbon content in the hexagonal boron nitride powder was measured using a ceramic carbon analyzer EMIA-110 manufactured by Horiba, Ltd. The powder was combusted in an oxygen stream, and the total carbon content was quantified from the amounts of CO gas and CO2 gas generated.
• the measurement results of the total carbon content for the hexagonal boron nitride powders of Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 2.
• the whiteness W of the hexagonal boron nitride powder was calculated by the following formula (1).
• the average particle size of the hexagonal boron nitride powder was measured using a particle size distribution measuring device MT3000 manufactured by Nikkiso Co., Ltd.
• the measurement sample was prepared by the following method. First, 20 g of ethanol was added as a dispersion medium to a 50 mL screw tube bottle, and 1 g of hexagonal boron nitride powder was dispersed in the ethanol. Then, the particle size distribution of the measurement sample was measured without ultrasonic treatment.
• the average particle sizes of the hexagonal boron nitride powders according to Examples 1 to 6 and Comparative Examples 1 to 4 are shown in Table 2.
• GI value Graphitization Index
• the hexagonal boron nitride powders according to Examples 1 to 6 and Comparative Examples 1 to 4 were mixed with epoxy resin to prepare resin compositions, and the thermal conductivity was evaluated.
• a varnish-like mixture was prepared using 100 parts by mass of epoxy resin (JER828 manufactured by Mitsubishi Chemical Corporation) and 5 parts by mass of a curing agent (imidazole-based curing agent, Curazol 2E4MZ manufactured by Shikoku Kasei Co., Ltd.) and adding 210 parts by mass of methyl ethyl ketone as a solvent.
• the varnish-like mixture and the hexagonal boron nitride powder were mixed in a rotation/revolution mixer (MAZERUSTAR manufactured by Kurashiki Boseki Co., Ltd.) so that the epoxy resin was 65% by volume and the hexagonal boron nitride powder was 35% by volume, to obtain a resin composition.
• a rotation/revolution mixer MAZERUSTAR manufactured by Kurashiki Boseki Co., Ltd.
• the resin composition was applied to a PET film to a thickness of approximately 250 to 300 ⁇ m using a Tester Sangyo Co., Ltd. automatic coater PI-1210, dried, and cured under reduced pressure at a temperature of 200°C, a pressure of 5 MPa, and a holding time of 30 minutes to produce 10 sheets of 200 ⁇ m thickness for each example and comparative example.
• the insulation performance of each sheet was measured using a voltage resistance tester (Tama Densoku Co., Ltd.).
• the hexagonal boron nitride powder of the present invention contains an extremely small amount of colored inclusions, making it suitable for use as a raw material for many applications.

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