Method for producing a thermally conductive filler, resin composition, resin sheet, and hexagonal boron nitride powder

Abstract

The purpose of one aspect of the present invention is to realize a hexagonal boron nitride powder having high crystallinity and increasing thermal conductivity of resin.　In order to solve the above-described problem, in the hexagonal boron nitride powder according to one aspect of the present invention, a ratio of spectral intensity at a wavelength of 227 nm to spectral intensity at a wavelength of 330 nm measured by a cathode luminescence (CL) method is 1.0 or more.

Description

Technical Field

[0001] The present invention relates to hexagonal boron nitride powder and a method for producing the same.

Background Art

[0002] In recent years, with the miniaturization and high-powerization of electronic components, an increase in the amount of heat generated by electronic components has become a problem. Therefore, in order to efficiently dissipate heat from electronic components, the development of materials with excellent thermal conductivity has been carried out.

[0003] Hexagonal boron nitride powder is blended with a resin used in electronic components to improve the thermal conductivity of the resin. Therefore, research and development of hexagonal boron nitride powder as a thermal conductive filler have been actively carried out. For example, Patent Document 1 discloses a high-purity hexagonal boron nitride single crystal having an ultraviolet emission band inherent to a wavelength of 235 nm or less.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] While the above high-purity hexagonal boron nitride single crystal is a single crystal that is not affected by impurities, there has been no report on hexagonal boron nitride powder with high crystallinity. Research and development of hexagonal boron nitride powder with high crystallinity that improves the thermal conductivity of the resin are desired.

[0006] One aspect of the present invention aims to realize hexagonal boron nitride powder with high crystallinity that improves the thermal conductivity of the resin.

Means for Solving the Problems

[0007] In order to solve the above problems, the hexagonal boron nitride powder according to one aspect of the present invention is a hexagonal boron nitride powder in which the ratio of the spectral intensity at a wavelength of 227 nm to the spectral intensity at a wavelength of 330 nm, measured by the cathode luminescence (CL) method, is 1.0 or more.

[0008] Further, a method for producing hexagonal boron nitride powder according to one aspect of the present invention is a method for producing hexagonal boron nitride powder including the following steps: (1) A reduction nitridation step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound in a furnace under a nitrogen atmosphere to a temperature of 1500 °C or higher and 1850 °C or lower at a heating rate of 10 °C / min or lower to obtain hexagonal boron nitride; (2) An acid washing step of performing acid washing on the hexagonal boron nitride powder containing unreacted raw materials obtained by the reduction nitridation step; (3) A heating step of heating the hexagonal boron nitride powder after the acid washing step to a temperature exceeding 1850 °C and less than 2050 °C at a heating rate of 5 °C / min or lower; (4) A temperature lowering step of lowering the temperature of the hexagonal boron nitride powder after the heating step to at least 1550 °C at a temperature lowering rate of 5 °C / min or lower; (5) A recovery step of taking out the hexagonal boron nitride powder cooled to 50 °C or lower from the furnace after the temperature lowering step.

Advantages of the Invention

[0009] According to one aspect of the present invention, it is possible to realize hexagonal boron nitride powder having high crystallinity and improving the thermal conductivity of a resin composition filled therewith.

Brief Description of the Drawings

[0010]

Figure 1

Modes for Carrying Out the Invention

[0011] In this specification, "A to B" indicates A or more and B or less unless otherwise specified.

[0012] 〔Hexagonal boron nitride powder〕 The hexagonal boron nitride powder according to one aspect of the present invention has a ratio of the spectral intensity at a wavelength of 227 nm to the spectral intensity at a wavelength of 330 nm (CL spectral intensity at 227 nm / CL spectral intensity at 330 nm) measured by the cathode luminescence method of 1.0 or more.

[0013] As a result of conducting a detailed study on hexagonal boron nitride powder, the present inventors succeeded in obtaining new findings. That is, when producing hexagonal boron nitride powder, by adopting specific conditions, it was uniquely found that hexagonal boron nitride powder with a CL spectral intensity at 227 nm / CL spectral intensity at 330 nm of 1.0 or more can be obtained. And it was uniquely found that the hexagonal boron nitride powder has high crystallinity and is a hexagonal boron nitride powder close to a single crystal. In addition, it was uniquely found that the hexagonal boron nitride powder is a hexagonal boron nitride powder that improves the thermal conductivity of resins and is suitable as a heat dissipation filler.

[0014] The cathode luminescence method is a method of detecting light emitted when an electron beam is irradiated onto a sample. Cathode luminescence reflects properties as a crystal (crystal defects, impurities, carrier concentration, stress, etc.). The CL spectrum at 227 nm is a spectrum indicating high crystallinity. The CL spectrum at 330 nm is a spectrum caused by carbon impurities. The CL spectral intensity may be the peak intensity of the CL spectrum.

[0015] A known measuring device can be used as the measuring device in the cathode luminescence method.

[0016] In terms of higher crystallinity and further improving the thermal conductivity of the resin, the CL spectrum intensity at 227 nm / the CL spectrum intensity at 330 nm is preferably 1.1 or more, more preferably 1.2 or more, and still more preferably 1.3 or more.

[0017] The above-mentioned hexagonal boron nitride powder preferably has an eluted boron amount of 250 ppm or less in terms of B2O3 after being immersed in an aqueous sulfuric acid solution with a concentration of 2 mol / L at 25°C for 120 minutes. The eluted boron amount is correlated with the surface oxygen concentration of the hexagonal boron nitride powder. When the eluted boron amount is 250 ppm or less, the oxygen concentration on the powder surface is low, and the crystallinity of the hexagonal boron nitride powder is high. The eluted boron amount can be measured, for example, by inductively coupled plasma (ICP) optical emission spectrometry. The above-mentioned eluted boron amount may be 200 ppm or less, or may be 180 ppm or less.

[0018] In terms of further improving the thermal conductivity of the resin and being easy to handle, the average particle diameter of the above-mentioned hexagonal boron nitride powder is preferably 2 μm or more, more preferably 5 μm or more, and still more preferably 10 μm or more. Also, in terms of being easily and uniformly dispersed in the resin composition, the average particle diameter is preferably 90 μm or less, more preferably 70 μm or less, and still more preferably 50 μm or less. The average particle diameter is the volume-based average particle diameter (D50).

[0019] In terms of higher crystallinity and further improving the thermal conductivity of the resin, the whiteness of the above-mentioned hexagonal boron nitride powder is preferably 90 or more. The whiteness is measured by a color difference meter.

[0020] [Manufacturing method of hexagonal boron nitride powder] The manufacturing method of the hexagonal boron nitride powder according to one aspect of the present invention (hereinafter, may be referred to as "this manufacturing method") includes a reduction nitridation step, an acid washing step, a heating step, a cooling step, and a recovery step. Hereinafter, each step will be described.

[0021] (Reduction nitridation step) In the reduction nitridation process, a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound is heated in a furnace under a nitrogen atmosphere to a temperature of 1500 °C or higher and 1850 °C or lower at a heating rate of 10 °C / min or lower. By this process, hexagonal boron nitride containing unreacted raw materials can be obtained.

[0022] (Raw materials) As the oxygen-containing boron compound, a compound containing boron and oxygen atoms may be used. Examples of the oxygen-containing boron compound include boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. Among these, boric acid or boron oxide, which is easily available, may be preferably used.

[0023] Examples of the carbon source include amorphous carbon such as carbon black, activated carbon, and carbon fiber, crystalline carbon such as diamond, graphite, and nanocarbon, and pyrolytic carbon obtained by thermal decomposition of a monomer or polymer. Among them, amorphous carbon with high reactivity is preferred, and carbon black is particularly preferably used in terms of industrial quality control. Examples of carbon black include acetylene black, furnace black, and thermal black.

[0024] Examples of the oxygen-containing calcium compound include calcium carbonate, calcium hydrogen carbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. These can be used alone or in combination of two or more. Among them, it is preferable to use calcium oxide or calcium carbonate. By including an oxygen-containing calcium compound in the raw material mixture, highly crystalline hexagonal boron nitride powder can be obtained when heated at a heating rate of 10 °C / min or lower in the reduction nitridation process.

[0025] The oxygen-containing calcium compound forms a high-melting-point composite oxide by forming a composite oxide with the oxygen-containing boron compound, and plays a role in preventing the volatilization of the oxygen-containing boron compound. It has also been confirmed that it plays a role as a catalyst in the reaction of directly nitriding boron carbide.

[0026] As the carbon-containing boron compound, a compound containing carbon and boron (for example, boron carbide) may be used. The particle size of the carbon-containing boron compound is preferably 1 to 500 μm, more preferably 10 to 400 μm, and even more preferably 20 to 300 μm.

[0027] The oxygen-containing boron compound, the carbon source, the oxygen-containing calcium compound, and the carbon-containing boron compound may be mixed, for example, in the following ratios to obtain a mixture. The ratio B / C (element ratio) of the mass of the oxygen-containing boron compound in terms of B to the mass of the carbon source in terms of C is set to 0.75 to 1.05. With respect to 100 parts by mass of the total amount of the mass of the oxygen-containing boron compound in terms of B2O3 and the mass of the carbon source in terms of C, the oxygen-containing calcium compound is 5 to 20 parts by mass in terms of CaO. With respect to 100 parts by mass of the total amount of the masses of the oxygen-containing boron compound, the carbon source, and the oxygen-containing calcium compound in terms of B2O3, C, and CaO, respectively, the carbon-containing boron compound is 5 to 45 parts by mass.

[0028] The supply form of the raw material mixture containing each raw material to the reduction nitridation reaction may be, for example, in a powder form, or may be carried out after forming a granulated body. The mixing of each raw material can be carried out, for example, using a mixer such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration stirrer, and a V-shaped mixer.

[0029] The heating of the raw material mixture in the reduction nitridation process is carried out in a furnace under a nitrogen atmosphere. Examples of the gas introduced into the furnace include nitrogen gas and ammonia gas. A gas in which a non-oxidizing gas such as hydrogen, argon, or helium is mixed with nitrogen gas or ammonia gas may be used. As the gas introduced into the heating furnace, nitrogen gas is preferred. Further, the nitrogen gas is preferably nitrogen gas with the dew point temperature controlled, and the dew point temperature is preferably -85°C or lower.

[0030] The heating of the raw material mixture in the reduction nitridation process is carried out at a heating rate of 10°C / min or less up to a temperature of 1500°C to 1850°C or lower (the maximum reduction nitridation temperature). By heating at such a heating rate, the grain growth rate of the hexagonal boron nitride powder can be slowed down, and a highly crystalline hexagonal boron nitride powder can be obtained. In terms of being able to obtain a more crystalline hexagonal boron nitride powder by slowing down the grain growth rate of the hexagonal boron nitride powder, the heating rate from 1500°C to the maximum reduction nitridation temperature is preferably 7°C / min or less, more preferably 5°C / min or less, and even more preferably 3°C / min or less.

[0031] In the reduction nitridation reaction, a holding time may be provided as appropriate in the temperature range up to 1500°C if necessary. After reaching the maximum reduction nitridation temperature, in terms of promoting the reduction nitridation reaction, the holding time at the maximum reduction nitridation temperature is preferably 1 hour or more, more preferably 2 hours or more. Further, the holding time at the maximum reduction nitridation temperature is preferably 10 hours or less, more preferably 5 hours or less.

[0032] The reduction nitridation process can be carried out using a known reaction apparatus capable of controlling the reaction atmosphere. For example, an atmosphere-controlled high-temperature furnace for heat treatment by high-frequency induction heating or heater heating can be mentioned. In addition to batch furnaces, continuous heating furnaces such as pusher-type tunnel furnaces and vertical reaction furnaces can also be used.

[0033] For the hexagonal boron nitride powder after the reduction nitridation step, it may be adjusted to a predetermined particle size distribution by crushing. The crushing is preferably gently performed using a jet mill, ball mill, hammer mill, or mortar type crusher, etc. Further, the particle size of the powder after crushing may be appropriately adjusted by classification treatment by air classification or sieving treatment.

[0034] (Pickling step) In the pickling step, pickling of the hexagonal boron nitride powder containing unreacted raw materials obtained by the reduction nitridation step is performed. In the hexagonal boron nitride powder obtained by the above reduction nitridation reaction, there are unreacted raw materials such as oxides and metal impurities. By the pickling step, a hexagonal boron nitride powder from which unreacted raw materials have been removed can be obtained.

[0035] The method for pickling the hexagonal boron nitride powder containing unreacted raw materials is not particularly limited, and known methods may be adopted without limitation. For example, the hexagonal boron nitride powder containing unreacted raw materials obtained after the reduction nitridation reaction is put into a container, and dilute hydrochloric acid (10 - 20 mass% HCl) 5 to 10 times the amount of the hexagonal boron nitride powder containing the unreacted raw materials is added and contacted for 4 to 8 hours, and the like.

[0036] As the acid used during pickling, in addition to hydrochloric acid, it is also possible to use nitric acid, sulfuric acid, acetic acid, etc.

[0037] After pickling, for the purpose of washing the remaining acid, water washing using pure water may be performed. As the method of water washing, after filtering the acid during pickling, the hexagonal boron nitride powder pickled in the same amount of pure water as the acid used is dispersed and filtered again.

[0038] After pickling or water washing, drying of the water-containing mass may be carried out. As the drying conditions, for example, in the atmosphere at 50 - 250 °C or under reduced pressure is preferable. The drying time is not particularly specified, but it is preferable to dry until the water content approaches 0% as much as possible.

[0039] (Heating step) In the heating process, the hexagonal boron nitride powder obtained in the pickling cleaning process is heated at a heating rate of 5 °C / min or less to a temperature exceeding 1850 °C and less than 2050 °C.

[0040] The heating process may be carried out under a nitrogen atmosphere. The supply of the nitrogen source to the reaction system in the heating process may be carried out in the same manner as in the reduction nitridation process. Also, the reaction apparatus may be the same as the reaction apparatus used in the reduction nitridation process.

[0041] The heating of the hexagonal boron nitride powder in the heating process is carried out at a heating rate of 5 °C / min or less to a temperature exceeding 1850 °C and less than 2050 °C (maximum recrystallization temperature). By this heating, highly crystalline hexagonal boron nitride powder can be obtained. Note that if heated to 2050 °C or higher, the hexagonal boron nitride powder may turn yellow due to nitrogen defects or the like, resulting in hexagonal boron nitride powder with low crystallinity.

[0042] The starting temperature for adjusting the heating rate in the heating process is not particularly limited. For example, room temperature (20 °C to 25 °C) may be used as the starting temperature for adjusting the heating rate.

[0043] In terms of improving the efficiency of the heating process, etc., it is preferable that the starting temperature for adjusting the heating rate is at least 1450 °C. That the starting temperature for adjusting the heating rate is at least 1450 °C includes starting to adjust the heating rate after the temperature of the hexagonal boron nitride powder reaches 1450 °C.

[0044] In terms of obtaining more highly crystalline hexagonal boron nitride powder, the heating rate in the heating process is preferably 4 °C / min or less, and more preferably 3 °C / min or less.

[0045] In terms of being able to sufficiently carry out the heating (recrystallization) of the hexagonal boron nitride powder after reaching the maximum recrystallization temperature, the holding time at the maximum recrystallization temperature is preferably 1 hour or more, and more preferably 3 hours or more. Also, the holding time at the maximum recrystallization temperature is preferably 15 hours or less, and more preferably 10 hours or less.

[0046] (Cooling Process) In the cooling process, the hexagonal boron nitride powder after the heating process is cooled to a temperature of at least 1550°C at a cooling rate of 5°C / min or less. By this cooling process, hexagonal boron nitride powder with a CL spectrum intensity at 227 nm / CL spectrum intensity at 330 nm of 1.0 or more can be obtained.

[0047] Cooling to a temperature of at least 1550°C includes cooling the temperature of the hexagonal boron nitride powder to 1550°C.

[0048] In terms of obtaining hexagonal boron nitride powder with a higher CL spectrum intensity at 227 nm / CL spectrum intensity at 330 nm, the heating rate in the cooling process is preferably 4°C / min or less, and more preferably 3°C / min or less.

[0049] (Recovery Process) In the recovery process, after the cooling process, the hexagonal boron nitride powder cooled to 50°C or less is taken out of the furnace. By cooling the hexagonal boron nitride powder recovered from the furnace to 50°C or less, the reaction with moisture in the air can be suppressed. By suppressing this reaction, an increase in the surface oxygen concentration (B2O3 concentration) of the hexagonal boron nitride powder can be suppressed, and the crystallinity of the hexagonal boron nitride powder can be improved. Also, by cooling the hexagonal boron nitride powder recovered from the furnace to 50°C or less, hexagonal boron nitride powder with a CL spectrum intensity at 227 nm / CL spectrum intensity at 330 nm of 1.0 or more can be obtained.

[0050] In terms of sufficiently suppressing the reaction with moisture in the air and improving the crystallinity of the hexagonal boron nitride powder, the temperature of the hexagonal boron nitride powder recovered from the furnace is preferably 40°C or less, and more preferably 30°C or less.

[0051] 〔Use of Hexagonal Boron Nitride Powder〕 (Resin Composition) The resin composition according to one aspect of the present invention contains the above-mentioned hexagonal boron nitride powder. By containing the hexagonal boron nitride powder, the resin composition has high thermal conductivity and is suitable for heat dissipation applications.

[0052] The above resin composition can be used in various applications. By mixing with the resins described later to form a thermally conductive resin composition or a thermally conductive molded body, for example, thermal interface materials such as polymer-based heat dissipation sheets and phase change sheets; organic-based heat dissipation sheets such as heat dissipation tapes, heat dissipation greases, heat dissipation adhesives, and gap fillers; heat dissipation paints such as heat dissipation paints and heat dissipation coatings; heat dissipation resin substrates such as PWB-based resin substrates and CCL-based resin substrates; insulating layers of metal-based substrates such as aluminum-based substrates and copper-based substrates; and encapsulants for power devices; etc. It can be preferably used for such applications.

[0053] Examples of the resin contained in the above resin composition include thermoplastic resins such as polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, and fluororesin; thermosetting resins such as epoxy resin, phenol resin, urea resin, melamine resin, unsaturated polyester resin, silicone resin, and bismaleimide triazine resin; and synthetic rubber; etc.

[0054] The above resin composition may contain thermally conductive fillers such as aluminum nitride and aluminum oxide, which are known high thermal conductivity insulating fillers.

[0055] Further, the above resin composition may contain, as a compounding agent of the resin composition as necessary, known polymerization initiators, curing agents, polymerization inhibitors, polymerization retarders, coupling agents, plasticizers, ultraviolet absorbers, pigments, dyes, antibacterial agents, organic fillers, and organic-inorganic composite fillers, etc. Also, the resin composition may contain other inorganic fillers as long as the effects of the present invention are not impaired.

[0056] In the above resin composition, the content of the hexagonal boron nitride powder is preferably 20 to 80% by volume, more preferably 30 to 70% by volume, in terms of obtaining higher thermal conductivity.

[0057] In the above resin composition, the content of the resin corresponds to the total volume of the resin composition being 100% by volume minus the content of the hexagonal boron nitride powder, and includes the volume thereof when a curing agent is included, and is preferably 80 to 20% by volume, more preferably 70 to 30% by volume.

[0058] (Resin sheet) The resin sheet according to one aspect of the present invention is composed of the above resin composition. The use of the resin sheet is not particularly limited, and for example, it can be used for circuit board applications and heat dissipation applications of electronic components such as multilayer printed wiring boards.

[0059] The manufacturing method of the above resin sheet is not particularly limited. For example, when the resin is a curable epoxy resin, a mixing step of mixing a curable epoxy resin, the above hexagonal boron nitride powder, and other components as necessary to obtain a curable composition, a molding step of molding the curable composition into a desired shape, and a curing step of curing the curable composition can be adopted.

[0060] (Other uses) Due to its high crystallinity, the hexagonal boron nitride powder of the present invention can also be used for applications such as raw materials for boron nitride processed products such as cubic boron nitride or boron nitride molded products, nucleating agents for engineering plastics, phase change materials, solid or liquid thermal interface materials, mold release agents for molten metal or molten glass molds, cosmetics, and composite ceramic raw materials.

[0061] 〔Summary〕 The hexagonal boron nitride powder according to Aspect 1 of the present invention has a ratio of the spectral intensity at a wavelength of 227 nm to the spectral intensity at a wavelength of 330 nm, measured by the cathode luminescence method, of 1.0 or more.

[0062] In the hexagonal boron nitride powder according to Embodiment 2 of the present invention, in Embodiment 1 of the present invention, the amount of boron eluted after immersion in an aqueous sulfuric acid solution with a concentration of 2 mol / L at 25°C for 120 minutes may be 250 ppm or less in terms of B2O3.

[0063] In the hexagonal boron nitride powder according to Embodiment 3 of the present invention, in Embodiment 1 or 2 of the present invention, the average particle size may be 2 μm or more and 150 μm or less.

[0064] The resin composition according to Embodiment 4 of the present invention contains the hexagonal boron nitride powder according to any one of Embodiments 1 to 3 of the present invention.

[0065] The resin sheet according to Embodiment 5 of the present invention is a resin sheet made of the resin composition according to Embodiment 4 of the present invention.

[0066] The method for producing hexagonal boron nitride powder according to Embodiment 6 of the present invention includes the following steps: (1) A raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound is heated in a furnace under a nitrogen atmosphere to a temperature of 1500°C or more and 1850°C or less at a heating rate of 10°C / min or less to obtain hexagonal boron nitride in a reduction nitridation step; (2) An acid washing step of performing acid washing on the hexagonal boron nitride powder containing unreacted raw materials obtained in the reduction nitridation step; (3) A heating step of heating the hexagonal boron nitride powder after the acid washing step to a temperature exceeding 1850°C and less than 2050°C at a heating rate of 5°C / min or less; (4) A temperature reduction step of reducing the temperature of the hexagonal boron nitride powder after the heating step to at least 1550°C at a temperature reduction rate of 5°C / min or less; (5) A recovery step of taking out the hexagonal boron nitride powder cooled to 50°C or less from the furnace after the temperature reduction step.

[0067] In the method for producing hexagonal boron nitride powder according to Embodiment 7 of the present invention, in Embodiment 6 of the present invention, the adjustment of the heating rate in the heating step may be performed at a temperature higher than at least 1450°C.

[0068] Examples are shown below to explain the embodiments of the present invention in more detail. Of course, the present invention is not limited to the following examples, and it goes without saying that various aspects are possible in terms of details. Furthermore, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope indicated in the claims. Embodiments obtained by appropriately combining the disclosed technical means are also included in the technical scope of the present invention. Also, all the documents described in this specification are incorporated by reference.

Example

[0069] 〔Preparation Example 1〕Preparation of hexagonal boron nitride powder A mixture of 259 g containing 141 g of boron oxide, 56 g of carbon black, 32 g of calcium oxide, and 30 g of boron carbide was mixed using a ball mill. The mixture was heated to 1500°C in a nitrogen gas atmosphere using a graphite crucible furnace, held at 1500°C for 4 hours, and then heated to a reduction nitridation maximum temperature of 1850°C at a rate of 2°C / min. Then, reduction nitridation treatment was performed by holding at 1850°C for 2 hours (reduction nitridation step).

[0070] The hexagonal boron nitride powder after reduction nitridation treatment was put into a container, 5 times the amount of hydrochloric acid (7 mass% HCl) was added, and pickling treatment was performed by stirring at a rotation speed of 700 rpm for 24 hours (pickling and cleaning step). After the pickling treatment, the acid was filtered, the hexagonal boron nitride powder obtained by filtration was dispersed in the same amount of pure water as the acid used, and filtered again. This operation was repeated 5 times, and then vacuum dried at 200°C for 6 hours.

[0071] The hexagonal boron nitride powder obtained after drying was heated to 1500°C in a nitrogen gas atmosphere using a graphite crucible furnace, and then heated to a re-firing maximum temperature of 1950°C at a rate of 2°C / min. Then, re-firing treatment was performed by holding at 1950°C for 6 hours (heating step).

[0072] Next, nitrogen gas was flowed into the furnace and the temperature was decreased to 1500 °C at a rate of 1 °C / min (temperature decrease step). Then, after further flowing nitrogen gas to cool the furnace to 25 °C, the hexagonal boron nitride powder was recovered from the furnace (recovery step).

[0073] 〔Example 2〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the temperature decrease rate in the temperature decrease step was changed to 2 °C / min.

[0074] 〔Example 3〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the maximum temperature for re-firing in the heating step was set to 2010 °C.

[0075] 〔Example 4〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the temperature decrease rate in the temperature decrease step was changed to 3 °C / min.

[0076] 〔Example 5〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the maximum temperature for re-firing in the heating step was set to 1900 °C.

[0077] 〔Comparative Example 1〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the maximum temperature for re-firing in the heating step was set to 2050 °C.

[0078] 〔Comparative Example 2〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared only through the reduction nitridation step and the acid washing step in Example 1. Note that the temperature increase from 1500 °C to 1850 °C in the reduction nitridation step was carried out at a rate of 1 °C / min. Otherwise, it was carried out under the same conditions as in the reduction nitridation step and the acid washing step of Example 1.

[0079] 〔Comparative Example 3〕Preparation of hexagonal boron nitride powder Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the cooling rate in the cooling process was changed to 10 °C / min.

[0080] [Comparative Example 4] Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the maximum firing temperature in the heating process was set to 1800 °C.

[0081] [Comparative Example 5] Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the temperature increase from 1500 °C to 1850 °C in the reduction nitridation process was carried out at 15 °C / min.

[0082] [Comparative Example 6] Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the hexagonal boron nitride powder was recovered from the furnace after cooling the inside of the furnace to 100 °C in the recovery process.

[0083] [Comparative Example 7] Hexagonal boron nitride powder was prepared in the same procedure as in Example 1, except that the reduction nitridation process was carried out by the melamine method by thermal decomposition of melamine borate at 1950 °C.

[0084] [Evaluation Example 1] Evaluation of hexagonal boron nitride powder The hexagonal boron nitride powders obtained in Examples 1 to 5 and Comparative Examples 1 to 7 were evaluated by the following evaluation methods.

[0085] (Measurement by the cathode luminescence (CL) method) The measurement by the CL method was carried out under the following conditions using a Hitachi Schottky emission type SEM S-4300SE and a spectroscope HR-320 (manufactured by Horiba, Ltd.). · Electron beam acceleration voltage: 5 kV · Sample temperature: 296 K · Measurement area: 50 × 50 μm 2 (Measure at three points by changing the location)

[0086] (Measurement of total oxygen content) The total oxygen content in the hexagonal boron nitride powder was measured using the ceramic oxygen and nitrogen analyzer EMGA-620W manufactured by Horiba, Ltd.

[0087] (Measurement of total carbon content) The total carbon content in the hexagonal boron nitride powder was measured using the ceramic carbon analyzer EMIA-110 manufactured by Horiba, Ltd. The powder was burned in an oxygen stream, and the total carbon content was quantified from the amounts of generated CO gas and CO2 gas.

[0088] (Measurement of B2O3 concentration) 50 g of a 2% sulfuric acid aqueous solution and 2 g of hexagonal boron nitride powder were put into a 50 mL Erlenmeyer flask, and the mixture was shaken and stirred for 1 minute while adjusting the liquid temperature to 25°C. After standing for 120 minutes, the amount of boron in the obtained liquid was analyzed using an ICP emission spectrometer (iCAP6500 manufactured by Thermo Fisher Scientific). The obtained measurement result was divided by the mass of the hexagonal boron nitride powder used in the test to obtain the eluted boron amount per unit mass of the hexagonal boron nitride powder (B2O3 concentration, ppm).

[0089] (Measurement of whiteness) The whiteness of the hexagonal boron nitride powder was measured using a color difference meter ZE7700 (manufactured by Nippon Denshoku Industries Co., Ltd.).

[0090] (Average particle size) The average particle size (volume-based average particle size (D50)) of the hexagonal boron nitride powder was measured by the laser diffraction method using a laser diffraction / scattering particle size analyzer MT3000 (manufactured by Microtrac·BEL).

[0091] 〔Evaluation Example 1〕Evaluation of thermal conductivity of resin composition The hexagonal boron nitride powders obtained in each of the examples and comparative examples were filled into an epoxy resin to prepare a resin composition, and the thermal conductivity was evaluated. The epoxy resin was a mixture of 100 parts by mass of (JER828 manufactured by Mitsubishi Chemical Corporation), 5 parts by mass of a curing agent (imidazole-based curing agent, Curezol 2E4MZ manufactured by Shikoku Kasei Co., Ltd.), and 210 parts by mass of methyl ethyl ketone as a solvent. Next, the varnish-like mixture and the hexagonal boron nitride powder were mixed with a rotation-revolution mixer (MAZERUSTAR manufactured by Kurashiki Boseki Co., Ltd.) so that the volume ratio of the base resin was 35%, 40%, or 45% and the volume ratio of the specific boron nitride powder was 65%, 63%, or 55%, respectively, to obtain a resin composition.

[0092] The above resin composition was applied and dried on a PET film to a thickness of about 180 to 220 μm using an automatic coater PI-1210 manufactured by Tester Sangyo Co., Ltd., and cured under reduced pressure at a temperature of 200 °C, a pressure of 5 MPa, and a holding time of 30 minutes to produce a sheet with a thickness of 150 μm. The sheet was analyzed with a temperature wave thermal analyzer to calculate the thermal conductivity.

[0093] The production conditions of the hexagonal boron nitride powders in each of the examples and comparative examples, and the evaluation results of Evaluation Example 1 are shown in Table 1.

[0094] The measured CL spectra are shown in FIG. 1. In FIG. 1, "1" is the measurement result of the hexagonal boron nitride powder of Comparative Example 2. "2" is the measurement result of the hexagonal boron nitride powder of Example 4. "3" is the measurement result of the hexagonal boron nitride powder of Example 2. "4" is the measurement result of the hexagonal boron nitride powder of Example 1. "5" is the measurement result of the hexagonal boron nitride powder of Comparative Example 1.

[0095]

Table 1

[0096] In Table 1, the "heating rate for reductive nitridation" indicates the heating rate from 1500 °C to the maximum temperature of re-firing in the reductive nitridation process. The "cooling rate" indicates the cooling rate from the maximum temperature of re-firing to 1500 °C in the cooling process. The "withdrawal temperature" indicates the temperature inside the furnace when collecting the re-fired hexagonal boron nitride powder from the furnace. "227 nm / 330 nm" indicates the ratio of (the intensity of the CL peak at a wavelength of 227 nm) / (the intensity of the CL peak at a wavelength of 330 nm).

[0097] As shown in Table 1, the thermal conductivity of the resin composition containing the hexagonal boron nitride powder of the example where the CL peak intensity ratio of 227 nm / 330 nm was 1.0 or more exceeded 15 W / m·K, and it was found that the hexagonal boron nitride powder of the example was suitable as a heat dissipation filler.

[0098] In Comparative Example 1 where the maximum temperature of re-firing was 2050 °C, the CL peak intensity ratio of 227 nm / 330 nm was as low as 0.2, and the powder turned yellow. Nitrogen defects and the like occurred in the yellowed powder, and the crystallinity deteriorated. Also, the thermal conductivity of the resin composition containing the hexagonal boron nitride powder of Comparative Example 4 where the maximum temperature of re-firing was 1850 °C was 15 W / m·K or less, and it is considered that the purification effect by re-firing was not exhibited.

[0099] In Comparative Examples 3 and 5 where the heating rate in the heating process (re-firing) or the cooling rate in the cooling process was high, the CL peak intensity ratio of 227 nm / 330 nm was 0.9, and the thermal conductivity of the resin composition was 15 W / m·K or less. From these results, it was found that the heating rate and the cooling rate during re-firing affected the CL peak intensity ratio of 227 nm / 330 nm and the thermal conductivity of the resin composition.

[0100] The hexagonal boron nitride powder of Comparative Example 6 where the withdrawal temperature of the hexagonal boron nitride powder was high reacted with the moisture in the air, and due to the increase in the surface oxygen concentration, the crystallinity on the surface of the hexagonal boron nitride powder decreased, and it is considered that the CL peak intensity ratio of 227 nm / 330 nm was low.

Industrial Applicability

[0101] The hexagonal boron nitride powder of the present invention has high crystallinity, improves the thermal conductivity of the resin, and can be used as a raw material for materials used in electronic components.

Claims

1. A thermally conductive filler comprising hexagonal boron nitride powder, the ratio of the spectral intensity at a wavelength of 227 nm to the spectral intensity at a wavelength of 330 nm being 1.0 or more as measured by a cathode luminescence method.

2. The amount of eluted boron after immersion in a sulfuric acid aqueous solution with a concentration of 2 mol / L at 25° C. for 120 minutes is B ２ O ３ The thermally conductive filler according to claim 1, wherein the thermally conductive filler has a content of 250 ppm or less in terms of carbon black.

3. The thermally conductive filler according to claim 1, having an average particle size of 2 μm or more and 150 μm or less.

4. A resin composition comprising the thermally conductive filler according to claim 1 .

5. A resin sheet comprising the resin composition according to claim 4.

6. A method for producing a hexagonal boron nitride powder having a ratio of a spectral intensity at a wavelength of 227 nm to a spectral intensity at a wavelength of 330 nm of 1.0 or more, as measured by a cathodoluminescence method, comprising the steps of: (1) a reduction-nitridation step of heating a raw material mixture containing an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound, and a carbon-containing boron compound in a furnace under a nitrogen atmosphere to a temperature of 1500° C. or more and 1850° C. or less at a heating rate of 10° C. / min or less to obtain hexagonal boron nitride; (2) an acid washing step of washing with an acid the hexagonal boron nitride powder containing unreacted raw materials obtained by the reduction-nitridation step; (3) a heating step of heating the hexagonal boron nitride powder after the acid washing step to a temperature of more than 1850° C. and less than 2050° C. at a heating rate of 5° C. / min or less; (4) a cooling step of cooling the hexagonal boron nitride powder after the heating step to a temperature of at least 1550° C. at a cooling rate of 1° C. / min or more and 3° C. / min or less; (5) A recovery step of removing the hexagonal boron nitride powder, whose temperature has been lowered to 50° C. or less, from the furnace after the temperature lowering step.

7. 7. The method for producing hexagonal boron nitride powder according to claim 6, wherein in the heating step, the temperature rise rate is adjusted from a temperature of at least 1450°C.

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