Hexagonal boron nitride powder

Abstract

To provide a boron nitride powder, which has a relatively small particle size, and in which the amount of metal impurities present on its surface is remarkably reduced to achiever a high degree of purity, and a method for producing the same.SOLUTION: Hexagonal boron nitride powders having an average particle size (D50) of 2.0 to 6.0 μm and a specific surface area of 4 to 12 m2 / g as measured by the BET method, wherein the concentration of calcium elements on the surface of hexagonal boron nitride particles constituting the above powders is less than 1 ppm or less, the concentration of silicon elements is 5 ppm or less, the concentration of sodium elements is 5 ppm or less, and the concentration of iron elements is 1 ppm or less. Preferably, the average aspect ratio (long diameter / thickness) of the particle is 1 to 7 and the powder is useful as filler for resin.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a novel hexagonal boron nitride powder. Specifically, the present invention provides a high-purity hexagonal boron nitride powder with an extremely low amount of metal element impurities, which could not be achieved by conventional manufacturing methods.

Background Art

[0002] Hexagonal boron nitride is a white powder having a hexagonal layered structure, and has many excellent properties such as thermal conductivity, electrical insulation, lubricity, corrosion resistance, mold release property, high-temperature stability, and chemical stability. Therefore, it is used in many applications such as fillers for thermally conductive insulating heat dissipation sheets, highly flexible thermally conductive silicone rubbers, heat dissipating greases, heat dissipating sealants, semiconductor encapsulating resins, etc., mold release agents for molten metals and molten glass molding dies, solid lubricants, and cosmetic raw materials.

[0003] In addition, typical manufacturing methods of hexagonal boron nitride include: (1) The melamine method in which boron oxides such as boric acid and boron oxide are reduced and nitrided with a nitrogen-containing compound such as melamine, (2) The reduction nitridation method in which boron oxide and a carbon source are reacted with nitrogen at a high temperature for reduction and nitridation etc. are known.

[0004] In the applications of the hexagonal boron nitride, heat dissipation materials such as thermally conductive insulating heat dissipation sheets are used by highly filling hexagonal boron nitride powder in a matrix such as resin, and are used as a heat dissipation layer of a semiconductor device mounting a semiconductor element.

[0005] Under such circumstances, as the high integration of semiconductor devices progresses, higher insulation resistance has been required in the heat dissipation layer. It is said that the insulation resistance of the heat dissipation layer is greatly affected by the impurity concentration on the surface of the hexagonal boron nitride powder filled therein.

[0006] On the one hand, with the progress of high integration, the semiconductor device is also becoming smaller, and the demand for a thermally conductive insulating heat dissipation sheet with a thickness of about 10 μm to 50 μm as a heat dissipation layer is increasing. Therefore, it is considered advantageous to use boron nitride powder obtained by the melamine method, which is advantageous for producing boron nitride powder having a relatively small particle size, to manufacture such an extremely thin sheet.

[0007] However, due to the small particle size of the boron nitride powder obtained by the melamine method, it is difficult to perform advanced cleaning. Currently, there has been no report of a case where high purity has been achieved in the boron nitride powder produced by the melamine method. In addition, there is also known a boron nitride powder in which the aspect ratio (major axis / minor axis) is reduced by crystal growth of the boron nitride powder obtained by the melamine method in a flux containing lithium carbonate. However, the hexagonal boron nitride powder obtained by such a method is also a boron nitride powder composed of small particle size particles and still has the same problems.

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, an object of the present invention is to provide a boron nitride powder having a relatively small particle size, in which the amount of metal impurities present on the surface is significantly reduced and highly purified boron nitride powder is achieved, and a method for producing the same.

Means for Solving the Problems

[0009] As a result of intensive studies, the present inventors have combined a specific treatment with an aqueous acid solution and a specific treatment with pure water for boron nitride powder having a relatively small particle size obtained by a method represented by the melamine method, and have succeeded in obtaining highly purified boron nitride powder that could not be achieved by conventional methods, thus completing the present invention.

[0010] That is, according to the present invention, hexagonal boron nitride powder having an average particle diameter (D50) of 2.0 to 6.0 μm and a specific surface area measured by the BET method of 4 to 12 m 2 / g, wherein the concentration of calcium element on the surface of hexagonal boron nitride particles constituting the powder is 1 ppm or less, the concentration of silicon element is 5 ppm or less, the concentration of sodium element is 5 ppm or less, and the concentration of iron element is 1 ppm or less. There is provided hexagonal boron nitride powder characterized by this.

[0011] Furthermore, the boron nitride powder of the present invention preferably has an average aspect ratio (major axis / minor axis) of 1 to 7.

[0012] The hexagonal boron nitride powder of the present invention is useful as a filler for resins due to the above characteristics.

[0013] Also, the hexagonal boron nitride powder of the present invention is obtained by adding an acid aqueous solution in which hydrochloric acid having an iron element of 1 ppm or less and a strong heat residue of 5 ppm or less and pure water having a conductivity at 25°C of 5 μS / cm or less are mixed to the crude hexagonal boron nitride powder obtained by the melamine method, adjusting the pH to 1 or less to form a slurry, and stirring and washing this slurry for 8 to 15 hours while maintaining the pH within the above range. It can be produced by a method including a pickling step, and a water washing step in which the boron nitride powder obtained in the pickling step is supplied into a filter and filtered while supplying pure water having a conductivity at 25°C of 5 μS / cm or less, and the boron nitride powder and pure water are brought into contact until the pH of the filtrate becomes 6 or more.

Effect of the Invention

[0014] The hexagonal boron nitride powder of the present invention has an average particle diameter (D50) of 2.0 to 6.0 μm and a specific surface area measured by the BET method of 4 to 12 m 2Although it has a small particle size of / g, the concentration of metal impurities on the surface of the hexagonal boron nitride particles constituting the powder is extremely low, and the amount of boron oxide is also kept low. Therefore, when it is highly filled and used in a matrix such as resin as a filler and used as a heat dissipation layer of a semiconductor device mounting a semiconductor element, it can exhibit extremely excellent insulation resistance and the high heat dissipation characteristics originally possessed by hexagonal boron nitride.

[0015] Further, according to the method for producing hexagonal boron nitride powder of the present invention, the above-mentioned high-purity hexagonal boron nitride powder can be reliably produced.

Embodiments for Carrying Out the Invention

[0016] <Hexagonal Boron Nitride Powder> The hexagonal boron nitride powder of the present invention has an average particle diameter (D50) of 2.0 to 6.0 μm and a specific surface area measured by the BET method of 4 to 12 m 2 / g of hexagonal boron nitride powder, characterized in that the concentration of calcium element on the surface of the hexagonal boron nitride particles constituting the powder is 1 ppm or less, the concentration of silicon element is 5 ppm or less, the concentration of sodium element is 5 ppm or less, and the concentration of iron element is 1 ppm or less.

[0017] The hexagonal boron nitride powder has a small particle size due to the production method by the melamine method described later. Further, the particles obtained by using the flux method in combination therewith include plate-shaped hexagonal boron nitride primary particles with a thick wall. As described above, it is extremely difficult to purify the hexagonal boron nitride powder containing such small-particle-size hexagonal boron nitride particles by washing, and there has been no hexagonal boron nitride powder purified to the above level in the past.

[0018] The average particle size (D50) of the hexagonal boron nitride powder of the present invention is 2.0 μm to 6.0 μm. The upper limit of the average particle size (D50) is preferably 5.0 μm or less, more preferably 4.0 μm or less. Also, the lower limit of the average particle size (D50) is preferably 3.0 μm or more. The hexagonal boron nitride powder having the above average particle size is useful as a filler for producing a resin sheet with a sheet thickness as thin as 10 to 50 μm, which has been in increasing demand in recent years, because the filler does not protrude from the sheet surface and maintains the surface smoothness. While having such a size, as will be described later, a resin sheet filled with highly purified hexagonal boron nitride powder exhibits high insulation resistance along with the imparted thermal conductivity by the hexagonal boron nitride powder.

[0019] Incidentally, as will be described in detail in the examples, the above average particle size (D50) is measured by the laser diffraction / scattering method using a dispersion of boron nitride powder in ethanol as a measurement sample, and represents the average value of the particle sizes of hexagonal boron nitride aggregated particles composed of single or aggregated hexagonal boron nitride primary particles.

[0020] The specific surface area of the hexagonal boron nitride powder of the present invention, measured by the BET method, is 4 to 12 m 2 / g, more preferably 4 to 10 m 2 / g, still more preferably 5 to 10 m 2 / g, and even more preferably 6 to 10 m 2 / g. The fact that the specific surface area is within the above range means that the hexagonal boron nitride primary particles constituting the hexagonal boron nitride powder have a small particle size.

[0021] In addition, when the hexagonal boron nitride primary particles constituting the hexagonal boron nitride powder of the present invention are relatively thick and the aggregation is relatively small, the specific surface area of the hexagonal boron nitride powder tends to be within the above range. As a result, when kneading the hexagonal boron nitride powder with a resin, an increase in the viscosity of the resin composition is suppressed, and it becomes easier to fill the resin. As a result, a resin sheet produced using the hexagonal boron nitride powder according to one embodiment of the present invention exhibits good thermal conductivity and good dielectric breakdown voltage.

[0022] The greatest feature of the hexagonal boron nitride powder of the present invention is that, as the concentration of metal impurities on the surface of the hexagonal boron nitride particles constituting the powder, the concentration of calcium element is 1 ppm or less, the concentration of silicon element is 5 ppm or less, the concentration of sodium element is 5 ppm or less, and the concentration of iron element is 1 ppm or less. Furthermore, it is more preferable that the concentration of the calcium element is 0.5 ppm or less, the concentration of the silicon element is 4 ppm or less, the concentration of the sodium element is 3 ppm or less, and the concentration of the iron element is 0.5 ppm or less. When the concentration of metal impurities on the surface of the hexagonal boron nitride particles constituting the hexagonal boron nitride powder of the present invention is within the above range, it is possible to fully exhibit the insulating property inherent in hexagonal boron nitride when producing a resin sheet using the hexagonal boron nitride powder according to one embodiment of the present invention.

[0023] In the present invention, the measurement of the concentration of metal impurities on the surface of the hexagonal boron nitride particles constituting the hexagonal boron nitride powder will be described in detail in the examples. After immersing in a sulfuric acid aqueous solution with a concentration of 0.04 mol / L at 25°C for 120 minutes, the recovered solution is subjected to elemental analysis by ICP emission spectrometry.

[0024] In addition, the hexagonal boron nitride powder of the present invention preferably has an average aspect ratio (major axis / thickness) of 1 to 7, more preferably 1 to 6, still more preferably 1 to 5, and even more preferably 1 to 4. The fact that the average aspect ratio of the hexagonal boron nitride powder is within the above range indicates that, as specified by the specific surface area, the hexagonal boron nitride primary particles are plate-like particles with a relatively thick thickness.

[0025] The aspect ratio of the hexagonal boron nitride primary particles still represents the average value measured by the measurement method described in the examples below.

[0026] Although the oxygen content of the hexagonal boron nitride powder of the present invention is not particularly limited, when used for heat dissipation material applications, since the thermal conductivity tends to decrease when the oxygen content is high, the oxygen content is preferably 1.0% by mass or less. In the case of hexagonal boron nitride powder produced by the melamine method, the oxygen content is generally about 0.1% by mass to 1.0% by mass. The oxygen content of the hexagonal boron nitride powder represents the value measured by the measurement method described in the examples below.

[0027] <Resin composition> A resin composition can be obtained by blending the hexagonal boron nitride powder of the present invention and a resin. The resin composition can be used, for example, as a resin sheet, and a resin sheet exhibiting high thermal conductivity and high dielectric breakdown strength can be obtained by the hexagonal boron nitride powder of the present invention. Since the obtained resin composition has an extremely high purity of the hexagonal boron nitride powder, a resin composition with improved insulation resistance can be formed, and the above effects can be exhibited in its molded body, for example, a resin sheet.

[0028] The resin constituting the resin composition is not particularly limited and may be, for example, a silicone resin or an epoxy resin. Examples of epoxy resins include bisphenol A type epoxy resin, bisphenol S type epoxy resin, bisphenol F type epoxy resin, hydrogenated epoxy resin of bisphenol A type, polypropylene glycol type epoxy resin, polytetramethylene glycol type epoxy resin, naphthalene type epoxy resin, phenylmethane type epoxy resin, tetrakisphenol methane type epoxy resin, biphenyl type epoxy resin, phenol novolac type epoxy resin, tetrafunctional naphthalene type epoxy resin, cresol novolac type epoxy resin, dicyclopentadiene type epoxy resin, trisphenol epoxy resin, naphthol novolac epoxy resin, naphthylene ether type epoxy resin, aromatic glycidylamine type epoxy resin, hydroquinone type epoxy resin, stilbene type epoxy resin, triphenol methane type epoxy resin, aralkyl type epoxy resin, polypropylene glycol type epoxy resin, polysulfide braided epoxy resin, epoxy resin having a triazine nucleus as a skeleton, and bisphenol A alkylene oxide adduct type epoxy resin, etc. These epoxy resins may be used alone or in combination of two or more. Also, as the curing agent, amine resins, acid anhydride resins, phenol resins, imidazoles, active ester type curing agents, cyanate ester type curing agents, naphthol type curing agents, benzoxazine type curing agents, etc. may be used. These curing agents may also be used alone or in combination of two or more. The blending amount of these curing agents with respect to the epoxy resin is an equivalent ratio with respect to the epoxy resin, 0.5 to 1.5 equivalent ratio, preferably 0.7 to 1.3 equivalent ratio. In this specification, these curing agents are also included in the resin.

[0029] In addition, as the silicone-based resin, a known curable silicone resin which is a mixture of an addition reaction type silicone resin and a silicone-based crosslinking agent can be used without limitation. Examples of the addition reaction type silicone resin include polyorganosiloxanes such as polydimethylsiloxane having an alkenyl group such as a vinyl group or a hexenyl group as a functional group in the molecule. Examples of the silicone-based crosslinking agent include polyorganosiloxanes having a silicon atom-bonded hydrogen atom such as dimethylhydrogen siloxy group-terminated dimethylsiloxane-methylhydrogen siloxane copolymer, trimethylsiloxy group-terminated dimethylsiloxane-methylhydrogen siloxane copolymer, trimethylsiloxane group-terminated poly(methylhydrogen siloxane), poly(hydrogensilsesquioxane), etc. Further, as the curing catalyst, a known platinum-based catalyst etc. used for curing the silicone resin can be used without limitation. For example, particulate platinum, particulate platinum supported on carbon powder, chloroplatinic acid, alcohol-modified chloroplatinic acid, olefin complex of chloroplatinic acid, palladium, rhodium catalyst, etc. are mentioned.

[0030] In addition, as the resin, it is also possible to use liquid crystal polymers, polyesters, polyamides, polyimides, polyphthalamides, polyphenylene sulfides, polycarbonates, polyaryl ether ketones, polyphenylene oxides, fluororesins, cyanate ester compounds, maleimide compounds, etc.

[0031] Liquid crystal polymers include thermotropic liquid crystal polymers that exhibit liquid crystallinity in the molten state and lyotropic liquid crystal polymers that exhibit liquid crystallinity in the solution state, and either type of liquid crystal polymer may be used.

[0032] Examples of the thermotropic liquid crystal polymer include polymers synthesized from p-hydroxybenzoic acid (PHB), terephthalic acid, and 4,4'-biphenol, polymers synthesized from PHB and 2,6-hydroxynaphthoic acid, polymers synthesized from PHB, terephthalic acid, and ethylene glycol, etc.

[0033] Examples of the fluororesin include, for example, ethylene tetrafluoride resin (PTFE), ethylene tetrafluoride - hexafluoropropylene copolymer resin (PFEP), ethylene tetrafluoride - perfluoroalkyl vinyl ether copolymer resin (PFA), and the like.

[0034] Examples of the cyanate ester compound include, for example, phenol novolak type cyanate ester compound, naphthol aralkyl type cyanate ester compound, biphenyl aralkyl type cyanate ester compound, naphthylene ether type cyanate ester compound, xylene resin type cyanate ester compound, and adamantane skeleton type cyanate ester compound, with phenol novolak type cyanate ester compound, biphenyl aralkyl type cyanate ester compound, and naphthol aralkyl type cyanate ester compound being preferred.

[0035] Examples of the maleimide compound include, for example, N - phenylmaleimide, N - hydroxyphenylmaleimide, bis(4 - maleimidophenyl)methane, 2,2 - bis{4-(4 - maleimidophenoxy)-phenyl}propane, bis(3,5 - dimethyl - 4 - maleimidophenyl)methane, bis(3 - ethyl - 5 - methyl - 4 - maleimidophenyl)methane, bis(3,5 - diethyl - 4 - maleimidophenyl)methane, the maleimide compound represented by the following formula (1), the maleimide compound represented by the following formula (2), and the like.

[0036]

Chemical formula

[0037] In the above formula (1), R 5 each independently represents a hydrogen atom or a methyl group, preferably a hydrogen atom. Also, n 1 represents an integer of 1 or more, preferably an integer of 10 or less, more preferably an integer of 7 or less.

[0038]

Chemical formula

[0039] In the above formula (2), when there are a plurality of Rs, each R independently represents a hydrogen atom, an alkyl group having 1 to 5 carbon atoms (for example, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, etc.), or a phenyl group. From the viewpoint of further improving the flame retardancy and peel strength, it is preferably a group selected from the group consisting of a hydrogen atom, a methyl group, and a phenyl group, more preferably one of a hydrogen atom and a methyl group, and even more preferably a hydrogen atom.

[0040] The blending ratio of the resin and the hexagonal boron nitride powder may be appropriately determined according to the use. For example, the above-mentioned hexagonal boron nitride powder is preferably blended in the total resin composition in an amount of 30 to 90% by volume, more preferably 40 to 80% by volume, and even more preferably 50 to 70% by volume.

[0041] The resin composition may contain components other than hexagonal boron nitride and the resin. The resin composition may appropriately contain, for example, an inorganic filler, a curing accelerator, a discoloration inhibitor, a surfactant, a dispersant, a coupling agent, a colorant, a plasticizer, a viscosity modifier, an antibacterial agent, etc. within a range that does not affect the effects of the present invention.

[0042] The uses of the resin composition of the present invention include, for example, sheet-like laminated materials (resin sheets) such as adhesive films and prepregs, circuit boards (for laminated boards and multilayer printed wiring boards), solder resists, underfill materials, thermal adhesives, die bonding materials, semiconductor encapsulants, hole-filling resins, component-embedded resins, thermal interface materials (sheets, gels, greases, etc.), substrates for power modules, heat dissipation members for electronic components, etc.

[0043] The resin composition of the present invention can be used, for example, in applications such as circuit boards. In applications for circuit boards, particularly in applications for copper-clad laminates obtained by laminating a resin composition and a copper foil, examples of suitable resins include epoxy resins, polyimide resins, liquid crystal polymers, fluorine resins, cyanate ester compounds, maleimide compounds, and the like. Among these, in applications for copper-clad laminates mounted on electronic communication devices such as satellite broadcast receivers and mobile phones, fluorine resins are particularly preferred as resins because they are excellent in high-frequency characteristics, heat resistance, weather resistance, chemical resistance, and water repellency. Further, when the circuit board is manufactured using lead-free solder, since the reflow temperature of lead-free solder is about 260°C, it is preferable to use a liquid crystal polymer having high heat resistance. Among them, thermotropic liquid crystal polymers are particularly preferred because they are more excellent in heat resistance and flame retardancy.

[0044] Furthermore, in applications for copper-clad laminates, a preferred form can be cited in which an epoxy resin and / or a maleimide compound and a cyanate ester compound are used as the resin. By adopting such a resin composition, it becomes easy to obtain a resin composition excellent in peel strength and moisture absorption heat resistance. In this case, as the epoxy resin, biphenyl aralkyl type epoxy resins, naphthylene ether type epoxy resins, polyfunctional phenol type epoxy resins, and naphthalene type epoxy resins are preferable from the viewpoints of flame retardancy and heat resistance. As the maleimide compound, 2,2'-bis{4-(4-maleimidophenoxy)-phenyl}propane, bis(3-ethyl-5-methyl-4-maleimidophenyl)methane, the maleimide compound represented by the formula (B-1), and the maleimide compound represented by the formula (B-2) are preferable from the viewpoints of thermal expansion coefficient and glass transition temperature. As the cyanate ester compound, phenol novolak type cyanate ester compounds, biphenyl aralkyl type cyanate ester compounds, and naphthol aralkyl type cyanate ester compounds are preferable from the viewpoints of glass transition temperature and plating adhesion.

[0045] In addition, the resin composition of the present invention can also be used as an insulating layer of a multilayer printed wiring board. In this case, as the resin, an epoxy resin is preferably used because it is excellent in heat resistance and adhesiveness to a copper foil circuit. As the epoxy resin, it is preferable to use a combination of a liquid epoxy resin at 20°C and a solid epoxy resin at 20°C, because a resin composition having excellent flexibility can be obtained and the breaking strength of the insulating layer is improved. Preferred liquid epoxy resins at 20°C include bisphenol A type epoxy resin, bisphenol F type epoxy resin, and naphthalene type epoxy resin. Preferred solid epoxy resins at 20°C include tetrafunctional naphthalene type epoxy resin, biphenyl type epoxy resin, and naphthylene ether type epoxy resin. The mixing ratio of the liquid epoxy resin at 20°C and the solid epoxy resin at 20°C is preferably in the range of 1:0.1 to 1:4 by mass ratio, and more preferably 1:0.8 to 1:2.5.

[0046] In addition, when the resin composition of the present invention is used as an underfill material, the resin is preferably an epoxy resin from the viewpoints of heat resistance, bulkiness, mechanical strength, etc., and it is particularly preferable to use a liquid epoxy resin at room temperature.

[0047] In addition, when the resin composition is used as a grease-like thermal interface material, it is preferable to use a silicone resin as the resin. As the silicone resin, it is preferable to use a polyorganosiloxane represented by the following formula (3) as an addition reaction type silicone resin and a polyorganosiloxane having at least a silicon atom-bonded hydrogen atom in one molecule as a silicone-based crosslinking agent.

[0048]

Chemical formula

[0049] In the formula, R 1 is independently an unsubstituted or substituted monovalent hydrocarbon group, and is preferably a monovalent hydrocarbon group having 1 to 3 carbon atoms. R 3is independently an alkyl group having 1 to 4 carbon atoms, an alkoxyalkyl group, an alkenyl group or an acyl group. p is an integer of 5 to 100, preferably 10 to 50. a is an integer of 1 to 3.

[0050] <Method for producing hexagonal boron nitride powder> The method for producing hexagonal boron nitride powder of the present invention is not particularly limited. As a typical production method, an acid aqueous solution obtained by mixing hydrochloric acid having an iron element of 1 ppm or less and a strong heat residue of 5 ppm or less and pure water having a conductivity of 5 μS / cm or less at 25°C is added to the crude hexagonal boron nitride powder obtained by the melamine method, and the slurry is adjusted to a pH of 1 or less. An acid washing step of stirring and washing the slurry for 8 to 15 hours while maintaining the pH within the above range, and the boron nitride powder obtained in the acid washing step is supplied into a filter, and filtration is performed while supplying pure water having a conductivity of 5 μS / cm or less at 25°C. A method including a water washing step of bringing it into contact with pure water until the pH of the filtrate becomes 6 or more can be mentioned.

[0051] <Crude hexagonal boron nitride powder> In the production method of the present invention, the production method of the crude hexagonal boron nitride powder includes a heating step of heating a mixed powder containing a boron oxide and an organic compound containing nitrogen as a method for obtaining the hexagonal boron nitride powder having the above average particle size, that is, the so-called melamine method. The melamine method is easy to obtain a boron nitride powder having a small particle size and is also easy to obtain particles having a low aspect ratio. Therefore, a boron nitride powder having an average particle diameter (D50) of 2.0 to 6.0 μm, a specific surface area of 4 to 12 m 2 / g and an aspect ratio of 1 to 7 can be obtained, which is a more preferable method than the reduction nitridation method.

[0052] In the method for producing the rough hexagonal boron nitride powder, examples of the boron oxide contained in the mixed powder include boron trioxide (boron oxide), diboron dioxide, tetraboron trioxide, tetraboron pentoxide, borax, or anhydrous borax, etc. Among them, it is preferable to use boron trioxide. By using boron trioxide as the boron oxide, inexpensive raw materials are used, which is industrially beneficial. In addition, two or more kinds may be used in combination as the boron oxide.

[0053] Examples of the nitrogen-containing organic compound contained in the mixed powder include melamine, ammeline, ammelide, melam, melon, dicyandiamide, and urea, etc. Among them, it is preferable to use melamine. By using melamine as the nitrogen-containing organic compound, inexpensive raw materials are used, which is industrially beneficial. In addition, two or more kinds may be used in combination as the nitrogen-containing organic compound.

[0054] The weight ratio (B / N) of boron atoms to nitrogen atoms in the mixed powder is preferably 0.2 or more and 0.5 or less, and more preferably 0.25 or more and 0.35 or less. When B / N is 0.2 or more, a B source can be ensured and a sufficient yield can be ensured. Also, when B / N is 0.5 or less, a sufficient N source for nitridation can be ensured. Note that the nitrogen atoms in the mixed powder heated in the heating step are derived from the nitrogen-containing organic compound, and the boron atoms in the mixed powder heated in the overheating step are derived from the boron oxide.

[0055] In addition to the boron oxide and the organic compound containing nitrogen, the mixed powder may contain carbonates or oxides of alkali metals and alkaline earth metals as a fluxing agent. Examples of the carbonates or oxides of the alkali metals and alkaline earth metals include lithium carbonate, sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, strontium carbonate, lithium oxide, sodium oxide, potassium oxide, magnesium oxide, calcium oxide, and strontium oxide. Among them, lithium carbonate serves as a flux that acts as an auxiliary agent for growing hexagonal boron nitride primary particles, promotes the growth in the thickness direction of the primary particles, suppresses the orientation when dispersed in the resin, and can obtain particles with a thickness that can reduce the thermal anisotropy of the resin composition, which is preferable. Note that two or more of the carbonates or oxides of alkali metals and alkaline earth metals may be used in combination.

[0056] Also, the weight ratio of boron atoms to lithium carbonate in the mixed powder (B / Li 2 CO 3 ) is preferably 0.22 or more and 0.98 or less, and more preferably 0.30 or more and 0.80 or less. When B / Li 2 CO 3 is 0.22 or more, the amount of the flux can be appropriately suppressed, so that the hexagonal boron nitride primary particles can be appropriately aggregated. Also, when B / Li 2 CO 3 is 0.98 or less, a sufficient amount of the flux can be formed, so that the hexagonal boron nitride primary particles having the aspect ratio can be obtained uniformly.

[0057] In the method for producing a hexagonal boron nitride powder, in the heating step, it is preferable to heat the mixed powder at a maximum temperature of 1200 °C or higher and 1500 °C or lower. By heating the mixed powder at a temperature of 1200 °C or higher, it is possible to prevent the particle size of the hexagonal boron nitride primary particles from becoming excessively small and to suppress an increase in the aspect ratio. The maximum temperature is more preferably 1250 °C or higher, and even more preferably 1300 °C or higher. Further, by heating the mixed powder at a temperature of 1500 °C or lower, when a fluxing agent is used, volatilization thereof can be prevented, and in particular, when lithium carbonate is used, an increase in the particle size and aspect ratio of the hexagonal boron nitride primary particles can be suppressed. The maximum temperature is more preferably 1450 °C or lower.

[0058] In the heating step, it is preferable to heat the mixed powder in an inert gas atmosphere under normal pressure or reduced pressure. By heating in the above environment, damage to the heating furnace body can be suppressed. In the present specification, the inert gas atmosphere means a state in which an inert gas is introduced into the container for heating the mixed powder and the gas inside the container is replaced with the inert gas. The inflow rate of the inert gas is not particularly limited, but the inflow rate of the inert gas may be 5 L / min or more. The inert gas may be, for example, nitrogen gas, carbon dioxide gas, or argon gas.

[0059] From the heating step, a hexagonal boron nitride powder is obtained. In the present invention, the obtained hexagonal boron nitride powder may be further crystal-grown by supplying it into the fluxing agent and heating it at a temperature of 1500 to 2200 °C for about 1 to 10 hours. The hexagonal boron nitride powder obtained after the above heating is also treated as a hexagonal boron nitride powder.

[0060] <Crushing step> In the method for producing hexagonal boron nitride powder of the present invention, since the as-obtained hexagonal boron nitride powder is aggregated, a crushing step may be provided for the purpose of adjusting the particle size. Such a crushing step is a step of crushing the as-obtained hexagonal boron nitride powder into agglomerated particles composed of hexagonal boron nitride primary particles contained in the as-obtained hexagonal boron nitride powder aggregated at high density. The crushing method is not particularly limited and may be crushing by a roll crusher, a jet mill, a bead mill, a planetary mill, a mortar-type grinder, or the like. Further, these crushing methods may be combined, or may be performed multiple times. Although metal impurities may be mixed into the apparatus or the like during the crushing step, they can be highly removed by the acid washing step and the water washing step described later.

[0061] <Acid washing step> In the method for producing hexagonal boron nitride powder of the present invention, in the acid washing step, an acid aqueous solution obtained by mixing hydrochloric acid in which the iron element is 1 ppm or less and the ignited residue is 5 ppm or less and pure water having a conductivity of 5 μS / cm or less at 25°C is added to the as-obtained hexagonal boron nitride powder, and the slurry is adjusted to a pH of 1 or less. This slurry is stirred and washed for 8 to 15 hours, preferably 10 to 14 hours while maintaining the pH within the above range.

[0062] Generally, in the acid washing step, by washing a fired product containing hexagonal boron nitride powder obtained by a heating step using an acid, lithium carbonate, boron oxide, or a composite oxide of lithium carbonate and boron oxide adhering to the hexagonal boron nitride powder, impurities soluble in the acid contained in the mixed powder, acid-soluble foreign substances mixed in up to the acid washing step, etc. are dissolved and removed. However, in the present invention, it is characteristic that such a step is carried out for a long time under stirring.

[0063] In the above acid washing, it is preferable that the pH of the slurry obtained by mixing the as-obtained hexagonal boron nitride powder with the acid aqueous solution is adjusted to 1 or less throughout the washing time, so as to sufficiently dissolve the impurities on the particle surface of the as-obtained hexagonal boron nitride powder and prevent the phenomenon that undissolved impurities remain in the slurry and adhere to the particles of the as-obtained hexagonal boron nitride powder, resulting in poor acid washing.

[0064] Also, it is necessary to stir the slurry during the pickling cleaning. By such stirring, the contact between the acid-soluble substance and the acid aqueous solution is promoted, the local increase in pH near the surface of the acid-soluble substance due to the dissolution of the acid-soluble substance is prevented, and the dissolution reaction can proceed smoothly to prevent poor cleaning. The degree of the above stirring is sufficient to make the slurry flow, and specifically, a method of stirring with a stirring blade can be mentioned.

[0065] Furthermore, the cleaning time is preferably 8 to 15 hours. If the cleaning time is shorter than 8 hours, the dissolution reaction may not be completed, and acid-soluble substances may remain in the slurry, resulting in poor pickling cleaning. If the cleaning time is 15 hours or more, the efficiency is poor and it is not preferable for industrial production. Also, the cleaning temperature is preferably 20 to 60°C.

[0066] In the pickling cleaning step, the distribution of the coarse hexagonal boron nitride powder and the acid aqueous solution in the slurry is preferably such that the mass of the acid aqueous solution is 2 to 5 times the mass of the coarse hexagonal boron nitride powder. If the amount of the acid aqueous solution is too small relative to the coarse hexagonal boron nitride powder, the viscosity of the slurry becomes too high, the whole slurry cannot be stirred, and there is a possibility of poor pickling cleaning. If the amount of the acid aqueous solution is large, there is no problem with stirring, but the amount of the slurry increases, which is not efficient.

[0067] The acid used in the pickling cleaning step is preferably a dilute acid such as hydrochloric acid. Hydrochloric acid with an iron element content of 1 ppm or less and a strong heat residue of 5 ppm or less is more preferable, and hydrochloric acid with an iron element content of 0.5 ppm or less and a strong heat residue of 3 ppm or less is even more preferable. Also, the pure water used in the pickling cleaning step is preferably pure water with a conductivity at 25°C of 5 μS / cm or less, more preferably a conductivity at 25°C of 1 μS / cm or less, and even more preferably a conductivity at 25°C of 0.5 μS / cm or less. And it is preferable to use an acid aqueous solution obtained by mixing the hydrochloric acid and the pure water as the cleaning liquid.

[0068] <Water washing step> In the present invention, in the water washing step, the boron nitride powder obtained in the pickling step is supplied into a filter, and filtration is performed while supplying pure water having a conductivity of 5 μS / cm or less at 25°C, and the powder is brought into contact with the pure water until the pH of the filtrate becomes 6 or more.

[0069] Generally, the water washing step is a step of bringing hexagonal boron nitride powder into contact with water to remove the acid and acid-soluble substances adhering to the hexagonal boron nitride powder in the acid cleaning step. However, in the present invention, in the contact between the hexagonal boron nitride powder obtained in the acid cleaning step and water, pure water is used, and by adopting a "water passing and filtering" method in which filtration is performed while supplying the pure water, it is possible to highly remove the acid and acid-soluble substances.

[0070] The pure water used for the washing preferably has a conductivity of 5 μS / cm or less at 25°C, more preferably a conductivity of 1 μS / cm or less at 25°C, and even more preferably a conductivity of 0.5 μS / cm or less at 25°C. That is, when the conductivity of the water is greater than the above range, the impurity components adhering to the hexagonal boron nitride powder cannot be sufficiently removed. For example, when water having a conductivity exceeding 5 μS / cm such as tap water is used, it is not appropriate because metal impurities contained in the tap water may remain in the hexagonal boron nitride powder.

[0071] In the water washing step, the slurry that has undergone the acid cleaning step is supplied into a filter to separate the hexagonal boron nitride powder from the cleaning liquid. Pure water is intermittently, preferably continuously supplied to the filter and brought into contact with the hexagonal boron nitride powder, so that the acid and acid-soluble substances adhering to the hexagonal boron nitride powder are transferred to the pure water side, and the hexagonal boron nitride powder and the pure water containing the acid and acid-soluble substances are immediately separated as a filtrate by filtration. By performing the supply of pure water and filtration simultaneously, it is possible to always bring new pure water into contact with the hexagonal boron nitride powder and at the same time remove the contaminated pure water, enabling a high level of cleaning.

[0072] The water washing is preferably continued until the pH of the pure water after contacting with the hexagonal boron nitride powder is 6 or more, preferably 6.5 or more. Further, in the water washing, it is preferable to contact with pure water at 1 to 20 L / min, preferably 2 to 10 L / min, per 1 kg of the hexagonal boron nitride powder. Further, the water washing temperature is preferably 20 to 60°C.

[0073] By performing such washing by flowing water filtration, it is possible to obtain hexagonal boron nitride powder having an extremely low concentration of metal impurities on the surface of the hexagonal boron nitride particles.

[0074] The filter used for the flowing water filtration is not particularly limited, but filters such as vacuum filtration by suction, filtration by pressure, and filtration by centrifugation are preferably used.

[0075] In the method for producing hexagonal boron nitride powder of the present invention, at the end of washing with pure water, filtration is continued in a state where the supply of pure water is stopped, that is, dehydration is performed, whereby purified hexagonal boron nitride powder can be obtained.

[0076] The dehydration is preferably performed so that the water content is 50 Wt% or less, more preferably 45 Wt% or less.

[0077] <Drying step> In the method for producing hexagonal boron nitride powder of the present invention, it is preferable to carry out a drying step following the water washing step. As the drying conditions for the hexagonal boron nitride powder, drying at a temperature of 50 to 250°C under atmospheric pressure, preferably under reduced pressure, is preferable. The drying time is not particularly limited, but it is preferable to dry until the water content becomes 0% as much as possible, and generally, it is recommended to carry out for 1 to 48 hours at the above temperature.

[0078] <Other steps> The method for producing hexagonal boron nitride powder may include steps other than the above. Such steps are referred to as "other steps" in this specification. Examples of the other steps included in the method for producing hexagonal boron nitride powder include a mixing step and a classification step.

[0079] The mixing step is a step of mixing boric oxide, an organic compound containing nitrogen, lithium carbonate, etc. before the heating step. By mixing the mixed powder in advance, the reaction proceeds substantially uniformly, so that variations in the particle diameter, etc. of the produced hexagonal boron nitride primary particles are suppressed.

[0080] The classification step is a step of classifying the hexagonal boron nitride powder according to the particle size and / or particle shape, etc. The classification operation may be sieving, or may be wet classification or air classification.

Examples

[0081] Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited to these examples. Each test method is as follows.

[0082] <Aspect ratio> The aspect ratio of the hexagonal boron nitride primary particles was measured using an analytical scanning electron microscope (manufactured by Hitachi High-Technologies Corporation: S-3400N). 100 different hexagonal boron nitride primary particles were randomly selected from the scanning electron microscope observation image at a magnification of 5000 times, and the length of the major axis and the thickness of the hexagonal boron nitride primary particles were measured to calculate the aspect ratio (length of major axis / length of thickness) of each, and the average value was taken as the aspect ratio.

[0083] <Water content> 10 g of hexagonal boron nitride powder was collected, and the water content was determined using a moisture meter (manufactured by A&D: MX-50).

[0084] <Specific surface area> The specific surface area of the hexagonal boron nitride powder was measured by the BET method using Macsorb HM model-1201 manufactured by Mountech.

[0085] <Average particle diameter (D50)> The particle size distribution 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 method shown below. First, 20 g of ethanol was added as a dispersion medium to a 50 mL screw tube bottle, and 0.3 g of hexagonal boron nitride powder was introduced into the ethanol. The lid of the screw tube bottle was tightened, and the screw tube bottle was held by hand, and the operation of "inverting up and down and then returning" was defined as one time, and this operation was continuously repeated 10 times to complete the adjustment. Then, the particle size distribution of the measurement sample was measured, and the volume-based average particle diameter D50 was calculated from the obtained results.

[0086] <Calcium elution amount, silicon elution amount, sodium elution amount, and iron elution amount> 50 g of a sulfuric acid aqueous solution with a concentration of 0.04 mol / L and 2 g of hexagonal boron nitride powder were introduced into a 150 cc beaker, shaken and stirred, and then left standing for 120 minutes. During that time, the temperature of the liquid was adjusted to 25°C. Then, the boron in the obtained liquid was analyzed by an ICP emission spectrometer (iCAP6500 manufactured by THERMO FISHER), and the calcium elution amount (ppm), silicon elution amount (ppm), sodium elution amount (ppm), and iron elution amount (ppm) were determined and used as the respective element concentrations on the surface of the hexagonal boron nitride particles constituting the hexagonal boron nitride powder.

[0087] <Oxygen concentration measurement> The oxygen concentration of the hexagonal boron nitride powder was measured using an oxygen / nitrogen analyzer EMGA-620 manufactured by Horiba, Ltd.

[0088] <Conductivity of pure water> The conductivity of pure water was measured with a conductivity meter (RG-12 manufactured by Organo Corporation).

[0089] <Iron concentration of hydrochloric acid> For the preparation of the standard sample solution, a total of 10 mL of the standard sample (iron standard solution (10 μg / mL) and hydrochloric acid for precision analysis (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)) was measured into a volumetric flask with a stopper so as to have a predetermined iron content. Then, 1 - 2 drops of 0.02 mol / L potassium permanganate solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 5 mL of ammonium thiocyanate were added thereto. After adding pure water to adjust the liquid volume to 50 mL, the mixture was thoroughly mixed. Next, for the preparation of the measurement sample solution, 10 mL of the measurement sample (hydrochloric acid) was taken into another volumetric flask with a stopper. Then, 1 - 2 drops of 0.02 mol / L potassium permanganate solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 5 mL of ammonium thiocyanate were added, and after adding pure water to adjust the liquid volume to 50 mL, the mixture was thoroughly mixed. Then, the color hues of the measurement sample solution and the standard sample solution were visually compared with white as the background. If the color hues matched, the iron concentration in the hydrochloric acid was calculated assuming it was the iron content of the measurement sample. If the color hues did not match, the volume of the iron standard solution in the standard sample solution was changed and the comparison was made again.

[0090] <Residue on ignition of hydrochloric acid> The residue on ignition of hydrochloric acid was measured by the following method. The weight m 1 (g) of an evaporating dish that had been washed, dried (at 105 - 110 °C for 30 minutes), and cooled to room temperature in a desiccator was measured. Then, 85 mL of hydrochloric acid was measured into the evaporating dish, and a measurement sample was prepared by adding 1 - 2 drops of special grade sulfuric acid (Wako Pure Chemical Industries, Ltd.). Thereafter, the measurement sample together with the evaporating dish was placed on a sand bath set at 200 °C and allowed to stand until white smoke no longer occurred, and then it was placed in an electric furnace set at 650 °C and allowed to stand and ignite for 1 hour. After ignition, it was dried (at 105 - 110 °C for 30 minutes) and cooled to room temperature in a desiccator, and then the weight m 2 (g) of the evaporating dish was measured. From the obtained results, the residue on ignition of hydrochloric acid was determined by the following formula. Residue on ignition of hydrochloric acid (%) = (m 2 - m 1 ) / (85 × ρ) × 100 In the formula, ρ is the density of hydrochloric acid (g / mL).

[0091] <Preparation of resin composition (resin sheet)> 100 parts by mass of hexagonal boron nitride powder, 22.4 parts by mass of a liquid curable epoxy resin (manufactured by Mitsubishi Chemical Corporation, jER828, bisphenol A type epoxy resin, epoxy equivalent 184 - 194 g / eq) as a resin, and 5.6 parts by mass of an epoxy resin curing agent (manufactured by Mitsubishi Chemical Corporation, jER cure WA, modified aromatic amine, amine value 623 - 639) were weighed, and 225 parts by mass of cyclohexanone (manufactured by Wako Pure Chemical Industries, Ltd., special grade) as a solvent was weighed. They were mixed using a planetary mixer (manufactured by Kurashiki Boseki Co., Ltd.: Mazelstar KK - 250S). The viscosity of the obtained mixture was measured at a temperature of 25°C using a Brookfield type rotational viscometer (manufactured by Brookfield Engineering Laboratories, Inc.: HBDV2TCP), and cyclohexanone was additionally blended and stirred and mixed using a planetary mixer until the viscosity at a shear rate of 200 s -1 was in the range of 700 - 800 mPa·s. The obtained composition was coated on a release polyimide film (manufactured by Ube Industries, Ltd.: Upilex - 50S, thickness 50 μm) to a film thickness of 50 μm using an automatic coating device (manufactured by Tester Sangyo Co., Ltd.: PI - 1210) with a Bird applicator. Then, the coated film was air - dried in a draft for 15 minutes and then dried at 130°C for 40 minutes using a vacuum dryer to remove the solvent and obtain an uncured resin sheet. Next, two sheets of the uncured resin sheet were overlapped together with the release polyimide film so that the uncured resin sheets were in contact with each other, and using a vacuum heating press device (manufactured by Imoto Seisakusho Co., Ltd.: manual hydraulic vacuum heating press), under reduced pressure, heat - pressed at 100°C and a press pressure of 4 MPa for 3 minutes to be pressure - bonded. Subsequently, the temperature was raised to 150°C, and under reduced pressure, heat - pressed at a press pressure of 20 MPa for 60 minutes to cure the uncured resin sheet. Then, it was transferred into a box - type oven and heat - treated at 165°C for 2 hours, and further heat - treated at 190°C for 2 hours to completely cure the uncured resin sheet. After that, the release polyimide films on both sides were peeled off to obtain a resin sheet. The thickness of the obtained resin sheet was 50 μm. Also, the volume fraction of the hexagonal boron nitride powder in the resin sheet was 65%.

[0092] 〔Example 1〕 A mixed powder was prepared by mixing 4450 g of anhydrous borax as a boron oxide and 5580 g of melamine as an organic compound containing nitrogen. In the prepared mixed powder, B / N was 0.26.

[0093] Using a batch firing furnace for the prepared mixed powder, in the heating process, it was heated at a maximum temperature of 1300 °C for 1 hour in a nitrogen atmosphere to prepare a crude hexagonal boron nitride powder. The prepared crude hexagonal boron nitride powder was crushed with a mortar grinder (manufactured by Masuda Sangyo Co., Ltd.: Super Mascoloider MKCA6-5J) at a rotational speed of 2200 rpm and a grinding wheel interval of 20 μm.

[0094] Next, an acid washing process was carried out. In the acid washing process, the container was filled with the crushed crude hexagonal boron nitride powder, 35% hydrochloric acid of the purity shown in Table 1, and the same pure water used for the subsequent water washing to form a slurry, and while stirring with a stirrer at a rotational speed of 500 rpm, it was adjusted to the pH shown in Table 1 and treated for the time shown in Table 1. After the above acid washing was completed, a water washing process was carried out. In the water washing process, the total amount of the slurry after the completion of the acid washing process was supplied to a centrifugal filter (rotational speed: 1900 rpm), and pure water having the conductivity shown in Table 1 was continuously supplied to the boron nitride powder after acid washing present on the filtration surface while performing "water passing filtration". After the water washing was completed, the supply of pure water was stopped and centrifugal dehydration was carried out. The hexagonal boron nitride powder after dehydration was dried under reduced pressure to a moisture content of 0.02% in a drying process to obtain a hexagonal boron nitride powder. The results of the above measurement for the obtained hexagonal boron nitride powder are shown in Table 2.

[0095] [Example 2] A mixed powder was prepared by mixing 3980 g of anhydrous borax as a boron oxide, 6580 g of melamine as an organic compound containing nitrogen, and 1480 g of lithium carbonate. In the prepared mixed powder, B / N was 0.28, and B / Li 2 CO 3 was 0.58.

[0096] Using a batch firing furnace for the prepared mixed powder, in the heating process, heating was performed at a maximum temperature of 1300 °C for 1 hour in a nitrogen atmosphere to produce a rough hexagonal boron nitride powder.

[0097] The obtained rough hexagonal boron nitride powder was subjected to each step of crushing, acid washing, water washing, and drying according to the method of Example 1 under the conditions shown in Table 1 to obtain a hexagonal boron nitride powder. Table 2 shows the results of the above measurements for the obtained hexagonal boron nitride powder.

[0098] [Example 3] A mixed powder was prepared by mixing 2850 g of boron oxide as a boron oxide, 5140 g of melamine as an organic compound containing nitrogen, and 2030 g of lithium carbonate. In the prepared mixed powder, B / N was 0.26, and B / Li 2 CO 3 was 0.44.

[0099] Using a batch firing furnace for the prepared mixed powder, in the heating process, heating was performed at a maximum temperature of 1400 °C for 1 hour in a nitrogen atmosphere to produce a rough hexagonal boron nitride powder.

[0100] The obtained rough hexagonal boron nitride powder was subjected to each step of crushing, acid washing, water washing, and drying according to the method of Example 1 under the conditions shown in Table 1 to obtain a hexagonal boron nitride powder. Table 2 shows the results of the above measurements for the obtained hexagonal boron nitride powder.

[0101] [Example 4] A hexagonal boron nitride powder was obtained in the same manner as in Example 1 except that the prepared rough hexagonal boron nitride powder was washed without being crushed. Table 2 shows the results of the above measurements for the obtained hexagonal boron nitride powder.

[0102] [Example 5] The conditions for crushing the prepared hexagonal boron nitride powder with a mortar grinder were the same as in Example 1 except that the rotation speed was 2000 rpm and the grinding wheel interval was 40 μm, and hexagonal boron nitride powder was obtained. The results of the above measurements for the obtained hexagonal boron nitride powder are shown in Table 2.

[0103] 〔Examples 6 - 10〕 Using the hexagonal boron nitride powder prepared in Examples 1 - 5, a resin sheet was prepared by the method for preparing the resin sheet described above. The obtained resin sheet was visually confirmed, and when it was smooth, it was evaluated as "○ (qualified)", and when unevenness occurred, it was evaluated as "× (unqualified)", and the evaluation of sheet formation was performed. The evaluation results are shown in Table 3.

[0104] 〔Comparative Example 1〕 In the pickling process, it was the same as in Example 1 except that the pH of the slurry during pickling was set to the value shown in Table 1. The results of the above measurements for the obtained hexagonal boron nitride powder are shown in Table 2.

[0105] 〔Comparative Example 2〕 In the pickling process, it was the same as in Example 1 except that the slurry was not stirred during pickling. The results of the above measurements for the obtained hexagonal boron nitride powder are shown in Table 2.

[0106] 〔Comparative Example 3〕 In the pickling process, it was the same as in Example 1 except that the pickling time was set to the value shown in Table 1. The results of the above measurements for the obtained hexagonal boron nitride powder are shown in Table 2.

[0107] 〔Comparative Example 4〕 In the water washing process, it was the same as in Example 1 except that the water washing was performed "discontinuously". In Table 1, "discontinuous" in the water washing process adopted a water washing method in which slurry and pure water were added to a container, stirred, and the operation of dehydrating by suction filtration under reduced pressure was repeated 10 times. The results of the above measurements for the obtained hexagonal boron nitride powder are shown in Table 2.

[0108]

Table 1

[0109] [Table 2]

[0110] [Table 3]

Claims

1. Hexagonal boron nitride powder with an average particle diameter (D50) of 2.0 to 6.0 μm and a specific surface area measured by the BET method of 4 to 12 m ２ / g, wherein the concentration of calcium element on the surface of hexagonal boron nitride particles constituting the powder is 0.5 ppm or less, the concentration of silicon element is 5 ppm or less, the concentration of sodium element is 3 ppm or less, and the concentration of iron element is 1 ppm or less. Hexagonal boron nitride powder characterized by being.

2. The hexagonal boron nitride powder according to Claim 1, having an average aspect ratio (major axis / thickness) of 1 to 7.

3. The hexagonal boron nitride powder according to Claim 1 or 2, which is a filler for resins.

4. After crushing the as - synthesized hexagonal boron nitride powder obtained by the melamine method with a mortar - type grinder, an acid aqueous solution is added, which is a mixture of hydrochloric acid with iron element of 1 ppm or less, strong heat residue of 5 ppm or less, and pure water with a conductivity at 25°C of 5 μS / cm or less, to form a slurry adjusted to a pH of 1 or less. The slurry is stirred and washed for 8 to 15 hours while maintaining the pH within the above range. And a water - washing step in which the boron nitride powder obtained in the above acid - washing step is supplied into a filter, and filtration is performed while supplying pure water with a conductivity at 25°C of 5 μS / cm or less, and the boron nitride powder and pure water are brought into contact until the pH of the filtrate becomes 6 or more. A method for producing hexagonal boron nitride powder, characterized by including these steps.

5. A resin composition containing the hexagonal boron nitride powder according to any one of Claims 1 to 3 and a resin.

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