JP2017014064A - Hexagonal boron nitride particle and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide hexagonal boron nitride particles which reduce anisotropy of thermal conduction, and can impart high thermal conductivity and insulating resistance to a resin composition structured so as to fill a resin, and to provide hexagonal boron nitride powder containing the same.SOLUTION: There are provided hexagonal boron nitride particles having such a polyhedral structure that crystals having a crystal form of hexagonal crystal continue in two or more bending portions, and boron nitride powder containing the particles, where the boron nitride powder can be obtained by mixing a composite oxide which contains 33 mol% or more boron in terms of oxides (BO) and is formed of boron and alkaline earth metal, and a carbon source so that the composite oxide and the carbon source is 0.7-1.0 in terms of B/C (element ratio), heating the mixed substance to a temperature of 1,700-2,200°C under nitrogen atmosphere, reduction nitriding a boron oxide in the composite oxide, and then removing the composite oxide existing in a reaction product.SELECTED DRAWING: Figure 1

Description

  The present invention relates to novel hexagonal boron nitride particles, hexagonal boron nitride powder containing the particles, and a method for producing the same. Specifically, it includes hexagonal boron nitride particles having a polyhedral structure in which crystals having a hexagonal crystal form are continuous at two or more bent portions, and imparts high thermal conductivity to a resin composition obtained by filling a resin. The present invention provides a hexagonal boron nitride powder and a method for producing the same.

  Hexagonal boron nitride powder is generally a white powder having a hexagonal layered structure similar to that of graphite, and has high thermal conductivity, high electrical insulation, high lubricity, corrosion resistance, release properties, high temperature stability, It has many properties such as chemical stability. Therefore, the resin composition filled with hexagonal boron nitride powder is suitably used as a heat conductive insulating sheet by molding.

  The hexagonal boron nitride can be produced by (i) a method of directly nitriding boron using nitrogen, ammonia, or the like, (ii) a method of reacting boron halide with ammonia or an ammonium salt, (iii) boric acid, oxidation. A melamine method in which a boron compound such as boron and a nitrogen-containing compound such as melamine are reacted at a temperature of about 800 ° C. to reduce and nitride the boron compound; (iv) a boron compound and a carbon source at a high temperature of 1600 ° C. or higher in a nitrogen atmosphere There is a reductive nitriding method in which a boron compound is reductively nitrided by heating.

  The hexagonal boron nitride powder thus obtained contains primary particles made of scale-like particles derived from the crystal structure, and the scale-like particles have thermal anisotropy. That is, the scale-like particles have a much better thermal conductivity in the thickness direction than in the plane direction. Usually, in the case of a thermally conductive insulating sheet using boron nitride powder containing the above flaky particles as a filler, since the flaky particles are oriented in the surface direction of the thermally conductive insulating sheet, the contact between the flaky particles There are few opportunities, and the thermal conductivity in the thickness direction of the thermally conductive insulating sheet is low.

  In order to improve the thermal anisotropy of hexagonal boron nitride particles having such a flaky structure, boron nitride agglomerated particles in which the flaky particles are agglomerated in multiple directions have been proposed (patents). Reference 1). The advantages of boron nitride aggregated particles include two points: improvement of thermal anisotropy and increase in particle size.

  However, since the boron nitride aggregated particles are disintegrated when handled or kneaded with the resin, the primary particles constituting the aggregated particles are liberated, and the thermal resistance is increased. There was a problem that the thermal conductivity was reduced.

  Furthermore, since boron nitride aggregated particles obtained by agglomerating small-sized boron nitride particles have many fine voids between the particles, bubbles are likely to remain in the resin composition when the resin is filled. There is also a problem that the insulation resistance of a molded product obtained by molding the composition is lowered.

JP 2011-98882 A

  Accordingly, an object of the present invention is to provide a resin composition that exhibits high thermal conductivity when filled in a resin, and in which bubbles do not easily remain and has high insulation resistance, and a molded body obtained by molding the resin composition. The object is to provide a hexagonal boron nitride powder which can be obtained.

  The present inventors have intensively studied to solve the above problems. As a result, by adopting a specific manufacturing method in which a reductive nitriding reaction is performed using a complex oxide of boron and an alkaline earth metal as a boron source, a hexagonal crystal form is obtained at two or more bent portions. We have succeeded in obtaining novel hexagonal boron nitride particles having a continuous polyhedral structure. Then, by using the hexagonal boron nitride powder containing the hexagonal boron nitride particles obtained in this way to form a thermally conductive insulating sheet, the hexagonal boron nitride crystals are oriented in multiple directions. Because it is continuous, the thermal anisotropy in the face direction inherent to hexagonal boron nitride crystals is reduced, and the generation of fine gaps existing between particles is avoided compared to conventional aggregated particles. It has been found that the decrease in the insulation resistance of the resin due to the air remaining in the gap can be greatly reduced, and the present invention has been completed.

  That is, according to the present invention, there is provided hexagonal boron nitride particles characterized by having a polyhedral structure in which crystals having a hexagonal crystal form are continuous at two or more bent portions.

  Hereinafter, the hexagonal boron nitride particles having the polyhedral structure are also referred to as “polycrystalline hexagonal boron nitride particles” or “polycrystalline h-BN particles”.

  The hexagonal boron nitride particles preferably have a bent portion angle of 139 ± 3 degrees.

  The present invention also provides a boron nitride powder containing the polycrystalline h-BN particles.

  Furthermore, the present invention also provides a resin filler made of the boron nitride powder, a resin composition filled with the resin filler, and a heat dissipation material for an electronic component made of the resin composition.

As a method for producing hexagonal boron nitride powder containing polycrystalline h-BN particles of the present invention, a composite of boron and an alkaline earth metal containing boron in an amount of 33 mol% or more in terms of oxide (B ₂ O ₃ ). An oxide and a carbon source are mixed so as to be 0.7 to 1.0 in terms of B / C (element ratio), and heated to a temperature of 1700 to 2200 ° C. in a nitrogen atmosphere. Provided is a method for producing hexagonal boron nitride powder, characterized by removing complex oxide present in a reaction product after reducing and nitriding the boron oxide therein.

In the reduction nitridation of the above production method, the amount of the carbon source used in the presence of excess nitrogen is such that the proportion of the composite oxide present in the reaction product obtained by the reduction nitridation reaction is 45 to 60% by mass. The target hexagonal boron nitride powder is efficiently adjusted by adjusting the ratio of B ₂ O ₃ to 25 to 50 mol% and terminating the reaction when consumption of the carbon source is completed. Preferred for manufacturing.

  As described above, the polycrystalline h-BN particles of the present invention have a polyhedral structure in which crystals having a hexagonal crystal form are continuous in two or more bent portions, so that the hexagonal boron nitride crystal has multiple directions. Since it is fixed facing, the anisotropy of the thermal conductivity is reduced, and when the resin is filled and, for example, a thermally conductive insulating sheet is formed, a high thermal conductivity is exhibited.

  In addition, since the polycrystalline h-BN particles of the present invention are composed of hexagonal boron nitride crystals continuously through tilt grain boundaries, conventional nitrided aggregated particles obtained by agglomerating hexagonal boron nitride crystals. The primary particles are unlikely to be liberated, and the introduction of bubbles into the gaps between the agglomerated particles, which can be said to be the fate of the agglomerated particles, is avoided, which effectively reduces the insulation withstand voltage of the resin. Can be reduced.

  In addition, according to the method for producing polycrystalline h-BN particles of the present invention, a composite oxide of boron and an alkaline earth metal containing boron in a specific ratio is used as a raw material. The hexagonal boron nitride powder containing the polycrystalline h-BN particles of the present invention can be produced with good reproducibility.

  Although the reason why the polycrystalline h-BN particles of the present invention are obtained in the above production method is not clear, the present inventors have shown that the composite oxide particles of boron and alkaline earth metal are subjected to a reduction nitridation reaction. It is presumed that a uniform liquid phase is formed from the time when the temperature is reached, and hexagonal boron nitride particles grow along the surface of the liquid phase.

2 is a SEM (Scanning Electron Microscope) photograph showing a typical particle structure of polycrystalline h-BN particles obtained by the production method of the present invention specifically shown in Example 1.

(Polycrystalline h-BN particles)
The crystal form of the hexagonal boron nitride of the polycrystalline h-BN particles of the present invention is confirmed by X-ray diffraction measurement using a fully automatic horizontal multipurpose X-ray diffractometer SmartLab manufactured by Rigaku as shown in the examples described later. be able to. Such hexagonal boron nitride crystals are generally plate crystals, and the polycrystalline h-BN particles of the present invention can be seen in FIG. 1 showing the polycrystalline h-BN particles obtained in Example 1 described later. The hexagonal boron nitride crystal has a continuous polyhedral structure at two or more, preferably four or more, more preferably eight or more bent portions. The thickness of the plate crystal is not particularly limited, but is preferably about 0.5 to 10 μm.

  As described above, the polycrystalline h-BN particles of the present invention are novel in comparison with conventional hexagonal boron nitride particles in that they have a polyhedral structure in which crystals having a hexagonal crystal form are continuous at two or more bent portions. It has a form.

  In the polycrystalline h-BN particles of the present invention, the angle (θ) of the bent portion is almost 139 ± 3 degrees, and from this, the bent portion is the target tilt grain boundary of the <10-10> plane. Is presumed to be formed.

  In addition, the polycrystalline h-BN particles of the present invention are generally formed into a nearly spherical shape, so-called quasi-spherical shape, and are formed in a form in which a cavity is formed inside, due to a polyhedral structure formed by two or more bent portions. However, the cavity is generally relatively large in a part of the pseudospherical particle, specifically, there are multiple openings having an equivalent diameter of 2 μm or more. It can penetrate into the particles more easily than the gap.

  The polycrystalline h-BN particles of the present invention are not limited to the pseudospherical shape described above, but include those having a polyhedral structure formed by two or more bent portions.

  The average particle size of the polycrystalline boron nitride particles of the present invention is preferably 10 to 150 μm, particularly preferably 20 to 125 μm, and more preferably 30 to 100 μm.

  In the present invention, the angle (θ) of the bent portion of the polycrystalline h-BN particles, the average particle diameter, and the like are values measured by the method shown in the examples.

(Hexagonal boron nitride powder)
Although the polycrystalline h-BN particles of the present invention can be obtained by the production method described later, some hexagonal boron nitride particles other than the polycrystalline h-BN particles, for example, particles having a tabular structure are also produced. Therefore, it is generally obtained as hexagonal boron nitride powder containing the polycrystalline h-BN particles.

  In the above hexagonal boron nitride powder, the content of polycrystalline h-BN particles is preferably as large as possible in order to sufficiently exhibit the effect of the particles, and is preferably 30% by volume or more, particularly preferably 60% by volume or more.

  Of course, by removing coarse particles in the sieving process, fine powder removal by dry classification, etc., the particles other than polycrystalline h-BN particles are removed and separated from the obtained hexagonal boron nitride powder, and the content rate is increased. It is also possible to raise.

(Method for producing boron nitride powder)
The method for producing the hexagonal boron nitride powder of the present invention is not particularly limited. However, if a typical production method is exemplified, boron is contained in an amount of 33 mol% or more in terms of oxide (B ₂ O ₃ ). Boron and alkaline earth metal composite oxide and carbon source are mixed so as to be 0.7 to 1.0 in terms of B / C (element ratio), and are 1700 to 2200 ° C. in a nitrogen atmosphere. A method of producing hexagonal boron nitride powder, characterized in that the boron oxide in the composite oxide is reduced and nitrided by heating to a temperature, and then the composite oxide present in the reaction product is removed. .

(material)
The greatest feature of the production method of the present invention lies in that a boron source and an alkaline earth metal are not used separately as raw materials but are used in the form of a composite oxide. That is, by using these in the form of complex oxides, the time difference of the melting time of each raw material can be reduced at the reduction nitriding temperature described later, and the reduction nitriding reaction and the grain growth of the generated hexagonal boron nitride can be reduced. It can be performed uniformly in the melted liquid phase, and a polyhedral structure composed of continuous crystals can be formed on the surface of the liquid phase in cooperation with the manufacturing conditions described later.

  The complex oxide of boron and alkaline earth metal (hereinafter sometimes simply referred to as complex oxide) used in the present invention is an oxide containing boron and alkaline earth metal, or a complex that is heated. Compounds that can be oxides are used without limitation. For example, natural minerals such as colemanite and urexite, which are mainly composed of boron oxide and calcium oxide, can be preferably used. A composite oxide synthesized from a compound containing boron and a compound containing an alkaline earth metal can also be used. As a synthesis method, a compound containing boron and a compound containing alkaline earth metal are dry-mixed and heated, melted, cooled, pulverized, an aqueous solution of a compound containing water-soluble boron and a compound containing alkaline earth metal, Alternatively, a method of dehydrating and drying the suspension after heating to reflux can be used without any particular limitation.

  As the compound containing boron used for preparing the composite oxide, boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, sodium perborate and the like can be used without particular limitation. Use calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, calcium oxalate, magnesium oxide, magnesium carbonate, magnesium hydroxide, etc. as compounds containing alkaline earth metals I can do it.

  The purity of the composite oxide prepared in this way is preferably as high as possible, but it is also possible to use a natural mineral as described above, and the amount contained therein is generally about 5% by mass or less. Impurities contained in a proportion of can be tolerated.

The composite oxide used for the production of the hexagonal boron nitride powder of the present invention should contain boron containing 33 mol% or more, preferably 40 mol% or more in terms of oxide (B ₂ O ₃ ). is necessary. That is, when a complex oxide having a boron content lower than the above range is used, it becomes difficult to form a sufficient liquid phase when the reduction nitriding temperature is reached, and the polycrystalline h-BN particles of the present invention Is difficult to generate. On the other hand, even if the proportion of boron is excessively large, there is no problem in the formation of polycrystalline h-BN particles. However, since excessive boron is volatilized at the time of temperature rise, it is economically disadvantageous and there is a concern about contamination of the apparatus. The Accordingly, boron in the composite oxide is preferably 70 mol% or less, particularly 60 mol% or less in terms of oxide (B ₂ O ₃ ).

  In the method for producing hexagonal boron nitride powder of the present invention, the average particle size of the composite oxide subjected to the reduction nitriding reaction is preferably 30 to 300 μm from the viewpoint of the particle size and reactivity of the obtained polycrystalline h-BN particles. 50 to 200 μm is more preferable, and 70 to 150 μm is still more preferable. That is, if it is 30 μm or less, the obtained h-BN particles are difficult to be polycrystalline, and if it is 300 μm or less, there is a concern about poor nitriding inside the composite oxide, which is not preferable.

  In the production method of the present invention, a known carbon material is used as the carbon source without any particular limitation. Examples thereof include amorphous carbon such as carbon black, activated carbon, and carbon fiber, crystalline carbon such as diamond, graphite, and nanocarbon, and pyrolytic carbon obtained by pyrolyzing a monomer or a polymer. Of these, highly reactive amorphous carbon is preferable, and carbon black is particularly preferably used in terms of industrial quality control. Moreover, as said carbon black, acetylene black, furnace black, thermal black, etc. can be used. Moreover, 0.01-5 micrometers is preferable, as for the average particle diameter of the said carbon source, 0.02-4 micrometers is more preferable, and 0.05-3 micrometers is especially preferable. That is, when the average particle diameter of the carbon source is 5 μm or less, the reactivity of the carbon source is increased, and when it is 0.01 μm or more, handling is facilitated.

  In the production method of the present invention, the supply form to the reaction of the mixture containing each of the above raw materials is not particularly limited, and may remain in a powder form, but may be performed by forming a granulated body.

  In the production method of the present invention, the mixing method of the composite oxide and the carbon source is not particularly limited, and general mixing such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration stirrer, a V-shaped mixer, etc. The machine is ready for use.

  Moreover, the granulation method in the case of granulating can also be implemented by well-known methods, such as extrusion granulation, rolling granulation, granulation by a compactor, using a binder as needed. In this case, the size of the granulated body is preferably about 5 to 10 mm.

(Preparation of raw materials)
In the present invention, the reductive nitriding reaction is carried out by supplying a carbon source and nitrogen. In order to effectively obtain the target polycrystalline h-BN particles, the ratio of the composite oxide and the carbon source is as follows: It is necessary to set it as 0.7-1.0, preferably 0.75-0.95 in terms of B / C (element ratio). That is, when the molar ratio exceeds 1.0, the ratio of the boron compound that volatilizes without being reduced increases and the yield decreases, and the above-mentioned volatilizing component adversely affects the production line. Further, if the molar ratio is less than 0.7, the amount of unreacted boron oxide is small, and a sufficient liquid phase cannot be formed when the reduction nitriding temperature is reached. It becomes difficult to generate.

In order to obtain the desired polycrystalline h-BN particles more effectively, the use amount of the carbon source is present in the reaction product obtained by the reductive nitridation reaction in the presence of excess nitrogen under the above conditions. The ratio of the composite oxide to be 45 to 60% by mass, particularly 50 to 55% by mass, and the ratio of B ₂ O ₃ in the composite oxide is 25 to 50% by mol, particularly 30 to 40% by mol. Further, it is preferable to adjust the reaction amount to be less than normal conditions and to terminate the reaction when the consumption of the carbon source is completed.

  Here, the completion of the consumption of the carbon source can be confirmed by the elimination of the generation of carbon monoxide from the reaction system.

In the determination of the amount of carbon used, when the ratio of the composite oxide present in the reaction product is less than 45% by mass, the amount of liquid phase is small and the grain growth to form polycrystalline h-BN particles is sufficient. When the amount exceeds 60% by mass, the liquid phase exists excessively with respect to the boron nitride particles, so that the generated hexagonal boron nitride crystal tends to hardly form a polyhedral structure. The yield of polycrystalline h-BN particles may decrease. In addition, when the proportion of B ₂ O ₃ in the composite oxide exceeds 50 mol%, the yield decreases, and when it is less than 25%, it is difficult to maintain the liquid phase until the end of the reduction nitriding. There is a possibility that grain growth for forming polycrystalline h-BN particles is not sufficiently performed.
(Reduction nitriding)
In the production method of the present invention, in order to obtain hexagonal boron nitride powder having high crystallinity, heat treatment is usually performed at 1700 ° C. or higher, preferably 1700-2200 ° C., more preferably 1800-2000 ° C. to obtain hexagonal boron nitride. Get. That is, if the heat treatment temperature is lower than 1700 ° C., highly crystalline hexagonal boron nitride cannot be obtained, and if it is 2200 ° C. or higher, the effect reaches its peak, which is economically disadvantageous.

  In the boron nitride production method of the present invention, the supply of the nitrogen source to the reaction system can be formed by a known means. For example, the most common method is to circulate nitrogen gas in the reaction system of the reaction apparatus exemplified later. Further, the nitrogen source to be used is not limited to the above nitrogen gas, and is not particularly limited as long as it is a gas capable of nitriding in the reductive nitriding reaction. Specifically, ammonia gas can be used in addition to the nitrogen gas. A gas obtained by mixing a non-oxidizing gas such as hydrogen, argon or helium with nitrogen gas or ammonia gas can also be used.

  The method for producing hexagonal boron nitride powder of the present invention can be carried out using a known reaction apparatus capable of controlling the reaction atmosphere. For example, an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating can be used. In addition to a batch furnace, a continuous furnace such as a pusher-type tunnel furnace or a vertical reactor can be used.

(Acid cleaning)
In the production method of the present invention, the reaction product obtained by the above-described reduction nitridation is a composite in which the composition of raw material composite oxide is changed by the reaction in addition to hexagonal boron nitride powder containing polycrystalline h-BN particles. Since impurities such as oxides are present, it is preferable to wash with an acid. The acid washing method is not particularly limited, and a known method is employed without limitation. For example, by-product-containing boron nitride obtained after nitriding treatment is crushed and put into a container, and 5-10 times the amount of dilute hydrochloric acid (10-20 wt% HCl) of hexagonal boron nitride powder containing the impurities And a method of contacting for 4 to 8 hours.

  In addition to hydrochloric acid, nitric acid, sulfuric acid, acetic acid and the like can be used as the acid used for the acid cleaning.

  After the acid cleaning, cleaning is performed using pure water for the purpose of cleaning the remaining acid. As the washing method, after filtering the acid at the time of the acid washing, boron nitride washed with acid is dispersed in the same amount of pure water as the acid used, and filtered again. By performing this operation several times, the purity of the boron nitride powder of the present invention can be achieved.

(Use of boron nitride powder)
The use of the boron nitride powder of the present invention is not particularly limited, and can be applied to known uses without particular limitation. If the use used suitably is illustrated, the use used as a filler for resin for the purpose of electrical insulation improvement, thermal conductivity provision, etc. is mentioned. In the use of the boron nitride powder, the obtained resin composition has high electrical insulation and thermal conductivity.

  Therefore, the boron nitride powder of the present invention can be suitably used as a filler for a solid or liquid thermal interface material typified by a heat dissipation sheet or a heat dissipation gel of an electronic component.

  Examples of the resin include thermoplastic resins such as polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, and fluorine resin, and thermosetting resins such as epoxy resin, phenol resin, urea resin, melamine resin, unsaturated polyester resin, and silicon resin. Examples thereof include resins and synthetic rubbers.

  Further, the boron nitride powder of the present invention is a raw material for processed boron nitride products such as cubic boron nitride and boron nitride molded products, a nucleating agent for engineering plastics, a phase change material, a solid or liquid thermal interface material, It can also be used for applications such as mold release agents for molten metal and molten glass molds, cosmetics, and composite ceramic raw materials.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples at all.

  In the examples, each measurement is a value measured by the following method.

[Identification of hexagonal boron nitride]
Hexagonal boron nitride was identified using X-ray diffraction measurements. X-ray diffraction measurement was performed using a fully automatic horizontal multipurpose X-ray diffractometer SmartLab manufactured by Rigaku. The measurement conditions were a scan speed of 20 degrees / minute, a step width of 0.02 degrees, and a scan range of 10 to 90 degrees.

[Average particle size of hexagonal boron nitride particles]
100 different polycrystalline h-BN particles were randomly selected from an SEM observation image at a magnification of 500 times, the length of the long axis (L: average particle diameter) of the polycrystalline h-BN particles was measured, and the average value Was calculated.

[Number of bent portions of polycrystalline h-BN particles]
100 different polycrystalline h-BN particles were randomly selected from an SEM observation image at a magnification of 500 times, and for each polycrystalline h-BN particle, the number of bends confirmed in the field of view was counted, and this number was doubled. Was the number of bent parts, and the average value of 100 particles was calculated.

[Polycrystalline h-BN particle content ratio (volume%)]
Polycrystalline h-BN particles and non-polycrystalline h-BN particles were selected from 10 levels of SEM observation image at a magnification of 500 times to determine the polycrystalline h-BN particle content volume%.

[Proportion of composite oxide present in reaction product and ratio of B ₂ O ₃ in composite oxide]
The ratio (mass%) of the complex oxide present in the reaction product and the ratio (mol%) of B ₂ O ₃ in the complex oxide were determined by the following method.

X-ray diffraction measurement of reaction product and X-ray diffraction measurement result in reaction product Using Rietveld analysis, the ratio (mol%) of B ₂ O ₃ in the composite oxide, present in the reaction product The ratio (% by mass) of the composite oxide was determined. X-ray diffraction measurement was performed using a fully automatic horizontal multipurpose X-ray diffractometer SmartLab manufactured by Rigaku. The measurement conditions were a scan speed of 20 degrees / minute, a step width of 0.02 degrees, and a scan range of 10 to 90 degrees. Rietveld analysis was determined using the X-ray diffraction measurement result of the reaction product.

Example 1
Using a ball mill, 120.5 g of a mixture containing 100 g of colemanite (average particle size 100 μm) 0.72CaO—B ₂ O ₃ and 20.5 g of carbon black, which is a natural mineral composed of boron oxide and calcium oxide, is used as a composite oxide. And mixed. The (B / C) element ratio conversion of the mixture is 0.84. 100 g of the mixture was subjected to nitriding treatment by holding it at 1950 ° C. for 2 hours in a nitrogen gas atmosphere using a graphite Tamman furnace. Table 1 shows the mass% of the composite oxide present after the completion of the reductive nitriding reaction and the mol% of B ₂ O ₃ in the composite oxide.

  Next, the by-product-containing boron nitride was crushed and charged into a container, and 5 times the amount of hydrochloric acid (7 wt% HCl) of the by-product-containing boron nitride was added, followed by stirring at a rotational speed of 800 rpm for 24 hours. After the acid washing, the acid was filtered, and boron nitride obtained by filtration was dispersed in pure water in the same amount as the acid used, and filtered again. This operation was repeated 5 times and then dried at 150 ° C. for 6 hours.

  The powder obtained after drying was passed through a sieve having an opening of 125 μm to obtain white hexagonal boron nitride powder. The presence or absence and ratio of polycrystalline boron nitride particles in the obtained hexagonal boron nitride powder were confirmed by SEM observation, the number of bent portions, angle, average particle size (L), average particle size of hexagonal boron nitride powder (L) was measured and shown in Table 1.

Example 2
A composite oxide synthesized from boric acid and an aqueous calcium hydroxide solution was set to 0.62CaO—B ₂ O _3, and the same procedure as in Example 1 was performed except that the (B / C) element ratio was changed to 0.79. The conditions and measured values are shown in Table 1.

Example 3
The composite oxide consisting of boron oxide and calcium oxide was changed to 0.43CaO—B ₂ O ₃ and (B / C) element ratio was converted to 0.92, and the same procedure as in Example 1 was performed. The conditions and measured values are shown in Table 1.

Comparative Example 1
Boron oxide, calcium oxide, and carbon black were mixed in the same manner as in Example 1 except that the CaO / B ₂ O ₃ molar ratio was 0.72 and the (B / C) element ratio was 0.84. The conditions and measured values are shown in Table 1.

Examples 4-6
The boron nitride powder obtained in Examples 1 to 3 was filled in a silicone resin and an epoxy resin to prepare a resin composition, and the thermal conductivity was evaluated. The epoxy resin prepared the mixture of 100 weight part (JER806 by Mitsubishi Chemical Corporation) and 28 weight part of hardening | curing agent (alicyclic polyamine type hardening | curing agent, JER cure 113 by Mitsubishi Chemical Corporation). The silicone resin prepared a mixture of 100 parts by weight (KE-109 manufactured by Shin-Etsu Chemical Co., Ltd.) and 10 parts by weight of a curing agent (CAT-RG manufactured by Shin-Etsu Chemical Co., Ltd.). Next, 35% by volume of each base resin and 65% by volume of the specific boron nitride powder were mixed by a rotation / revolution mixer (MAZURUSTAR, Kurashiki Boseki Co., Ltd.) to obtain a resin composition.

  This was cast into a mold body and cured using a hot press under the conditions of temperature: 150 ° C., pressure: 5 MPa, holding time: 30 minutes, and a sheet having a diameter of 10 mm and a thickness of 0.25 mm was produced. The results of measuring the thermal conductivity of the sheet with a temperature wave thermal analyzer are shown in Table 2. All the sheets filled with boron nitride powder prepared in Examples 1 to 3 were 10.0 W / m · K or more, and exhibited high thermal conductivity. Moreover, as a result of measuring the dielectric strength with a withstand voltage tester (manufactured by Tama Denso Co., Ltd.), it was a high dielectric strength of 45 kV / mm or more.

Comparative Example 2
Example 4 was repeated except that the boron nitride powder obtained in Comparative Example 1 was used. Table 2 shows the results of measuring the thermal conductivity with a temperature wave thermal analyzer and the dielectric strength with a withstand voltage tester (manufactured by Tama Denso Co., Ltd.). The sheets filled with boron nitride powder prepared in Comparative Example 1 all had 10.0 W / m · K and 25 kV / mm or less, and exhibited low thermal conductivity and low dielectric strength.

Claims (10)

  A hexagonal boron nitride particle having a polyhedral structure in which crystals having a hexagonal crystal form are continuous at two or more bent portions.   The polycrystalline boron nitride particles according to claim 1, wherein the angle of the bent portion is 139 ± 3 degrees.   A boron nitride powder comprising the hexagonal boron nitride particles according to claim 1.   A filler for resin comprising the boron nitride powder according to claim 3.   A resin composition filled with the filler for resin according to claim 4.   A heat dissipating material for an electronic component comprising the resin composition according to claim 5. A composite oxide of boron and an alkaline earth metal containing 33 mol% or more of boron in terms of oxide (B ₂ O ₃ ) and a carbon source are 0.7 to 1 in terms of B / C (element ratio). The mixture is mixed so as to be 0 and heated to a temperature of 1700 to 2200 ° C. in a nitrogen atmosphere to reduce and nitride the boron oxide in the composite oxide, and then the composite oxide present in the reaction product is reduced. A method for producing hexagonal boron nitride powder, comprising removing the hexagonal boron nitride powder. In the reductive nitridation, the amount of carbon source used in the presence of excess nitrogen is such that the proportion of the composite oxide present in the reaction product obtained by the reductive nitridation reaction is 45-60 mass%, and B in the composite oxide _The method for producing hexagonal boron nitride powder according to claim 7, wherein the ratio of ₂ O ₃ is adjusted to 25 to 50 mol%, and the reaction is terminated when the consumption of the carbon source is completed.   The method for producing hexagonal boron nitride powder according to claim 7 or 8, wherein the alkaline earth metal constituting the composite oxide is calcium.   The method for producing hexagonal boron nitride powder according to claim 9, wherein the complex oxide is colemanite.

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