HEXAGONAL BORN NITRIDE POWDER AND METHOD FOR ITS PREPARATION

Abstract

One objective of the present invention is to produce hexagonal boron nitride powder exhibiting high crystallinity and improving the thermal conductivity properties of the resin. To achieve this objective, hexagonal boron nitride powder according to one aspect of the present invention has a ratio of spectral intensity at a wavelength of 227 nm to spectral intensity at a wavelength of 330 nm of not less than 1.0, as measured by a cathodoluminescence (CL) method.

Description

Technical field

[0001] The present invention relates to hexagonal boron nitride powder and a method for its production. background

[0002] With the miniaturization and advanced capabilities of electronic components, the increased amount of heat generated within them has recently become a problem. Therefore, materials with excellent thermal conductivity properties have been developed to efficiently dissipate heat from electronic components.

[0003] Hexagonal boron nitride powder, when mixed with a resin used for electronic components, improves the thermal conductivity of the resin. Therefore, the research and development of hexagonal boron nitride powder as a thermally conductive filler is actively pursued. For example, patent literature 1 discloses a high-purity hexagonal boron nitride single crystal with an intrinsic ultraviolet emission band at wavelengths not exceeding 235 nm. Bibliography [Patent Literature]

[0004] [Patent Literature 1] Japanese patent application Publication Tokukai No. 2005-145788 Summary of the invention; Technical task

[0005] The above high-purity hexagonal boron nitride single crystal is a single crystal unaffected by impurities. However, hexagonal boron nitride powder with high crystallinity has not yet been reported. Research and development is desired for hexagonal boron nitride powder that exhibits high crystallinity and improves the thermal conductivity properties of the resin.

[0006] One objective of the present invention is to realize hexagonal boron nitride powder which has a high crystallinity and improves the thermal conductivity properties of the resin. Solution to the task

[0007] To achieve the objective, hexagonal boron nitride powder according to one aspect of the present invention is 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 not less than 1.0, measured by a cathodoluminescence (CL) method.

[0008] Furthermore, a process for producing hexagonal boron nitride powder according to one aspect of the present invention is a process for producing hexagonal boron nitride powder, wherein the process comprises: (1) a reduction nitridation step for obtaining hexagonal boron nitride by heating a mixture of raw materials in an oven under a nitrogen atmosphere to a temperature of not less than 1500 °C and not more than 1850 °C at a heating rate of not more than 10 °C / min, wherein the mixture of raw materials contains an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound and a carbon-containing boron compound; (2) an acid purification step of the purification of the hexagonal boron nitride powder obtained by the reduction nitridation step and containing unreacted raw material, with acid; (3) a heating step in which the hexagonal boron nitride powder is heated after the acid purification step to a temperature of more than 1850 °C and less than 2050 °C at a heating rate of not more than 5 °C / min; (4) a cooling step in which the hexagonal boron nitride powder is cooled after the heating step to a temperature of at least 1550 °C at a cooling rate of not more than 5 °C / min; and (5) a recovery step in which the hexagonal boron nitride powder, which has been cooled to a temperature of not more than 50 °C, is removed from the oven after the cooling step. Advantageous effects of the invention

[0009] An object of one aspect of the present invention makes it possible to realize hexagonal boron nitride powder which has a high crystallinity and improves the thermal conductivity properties of a resin composition filled with the hexagonal boron nitride powder. Brief description of the drawings Fig. Figure 1 is a view illustrating results of the measurement of a CL spectrum in an embodiment of hexagonal boron nitride powder of the present invention. Description of exemplary implementations

[0010] It should be noted that the expression “A to B” herein means “no less than A and no more than B”, unless otherwise specified in this description. [Hexagonal boron nitride powder]

[0011] Hexagonal boron nitride powder according to one aspect of the present invention has a ratio of a spectral intensity at a wavelength of 227 nm to a spectral intensity at a wavelength of 330 nm, measured by a cathodoluminescence method (a ratio of a CL spectral intensity at 227 nm to a CL spectral intensity at 330 nm) of not less than 1.0.

[0012] The inventors of the present invention conducted a detailed study of hexagonal boron nitride powder and successfully achieved novel results. In particular, the inventors of the present invention made the unique discovery that, by applying specific conditions during the production of hexagonal boron nitride powder, it is possible to obtain hexagonal boron nitride powder with a ratio of CL spectral intensity at 227 nm to CL spectral intensity at 330 nm of at least 1.0. The inventors of the present invention also made the unique discovery that this hexagonal boron nitride powder exhibits high crystallinity and is almost a single crystal. Furthermore, the inventors of the present invention made the unique discovery that this hexagonal boron nitride powder improves the thermal conductivity properties of the resin and is suitable as a heat-dissipating filler.

[0013] Cathodoluminescence is a technique for detecting light emitted when an electron beam is applied to a sample. Cathodoluminescence reflects properties of the sample as a crystal, such as crystal defects, impurities, charge carrier concentration, stress, and others. A cathodoluminescence (CL) spectrum at a wavelength of 227 nm indicates high crystallinity. A CL spectrum at a wavelength of 330 nm is associated with carbon impurities. A CL spectral intensity can refer to the peak intensity of a CL spectrum.

[0014] The measuring device used in the cathodoluminescence method can be a known measuring device.

[0015] With regard to higher crystallinity and a further improvement of the thermal conductivity properties of the resin, the ratio of a CL spectral intensity at 227 nm to a CL spectral intensity at 330 nm is preferably not less than 1.1, more preferably not less than 1.2 and even more preferably not less than 1.3.

[0016] It is preferred that the amount of boron eluted from the hexagonal boron nitride powder not exceed 250 ppm, expressed as B₂O₃, as measured after immersion of the hexagonal boron nitride powder in a 2 mol / L aqueous sulfuric acid solution at 25 °C for 120 minutes. The amount of eluted boron correlates with the surface oxygen concentration of the hexagonal boron nitride powder. In cases where the amount of eluted boron does not exceed 250 ppm, the oxygen concentration on the powder surface is low and the crystallinity of the hexagonal boron nitride powder is increased. The amount of eluted boron can be measured, for example, using inductively coupled plasma (ICP) emission spectrometry. The amount of eluted boron may be no more than 200 ppm or no more than 180 ppm.

[0017] The average particle diameter of the hexagonal boron nitride powder is preferably not less than 2 µm, more preferably not less than 5 µm, and even more preferably not less than 10 µm, with regard to further improving the thermal conductivity properties of the resin and ease of handling. Furthermore, for easy uniform dispersion in a resin composition, the average particle diameter is preferably not more than 90 µm, more preferably not more than 70 µm, and even more preferably not more than 50 µm. The average particle diameter is a volume-based average particle diameter (D50).

[0018] With regard to higher crystallinity and further improvement of the resin's thermal conductivity properties, the hexagonal boron nitride powder exhibits a degree of whiteness of preferably not less than 90. The degree of whiteness is measured using a colorimeter. [Process for the production of hexagonal boron nitride powder]

[0019] A process for producing hexagonal boron nitride powder according to one aspect of the present invention (hereinafter also referred to as "the present production process") comprises a reduction nitridation step, an acid purification step, a heating step, a cooling step, and a recovery step. Each step is described below. (Reduction nitriding step)

[0020] In the reduction nitridation step, 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 an oven under a nitrogen atmosphere to a temperature of not less than 1500 °C and not more than 1850 °C at a heating rate of not more than 10 °C / min. (Raw materials)

[0021] An oxygen-containing boron compound can be any compound containing boron and an oxygen atom. Examples of oxygen-containing boron compounds include boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, and sodium perborate. Of these, boric acid or boron oxide, which are readily available, are particularly suitable.

[0022] Examples of carbon sources include crystalline carbons such as diamond, graphite, and nanocarbon; and pyrolytic carbons obtained by the pyrolysis of monomers or polymers, in addition to amorphous carbons such as carbon black, activated carbon, and carbon fiber. Among these, amorphous carbons, which are highly reactive, are preferred, and carbon black is particularly suitable because its quality is industrially controlled. Examples of carbon black include acetylene carbon black, furnace carbon black, and thermal carbon black. Examples of oxygenated calcium compounds include calcium carbonate, calcium bicarbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, and calcium oxalate. It is also possible to use one type, two, or more of these in combination. Among these, calcium oxide or calcium carbonate is preferred.By including an oxygen-containing calcium compound in the raw material mixture, it is possible to obtain hexagonal boron nitride powder with high crystallinity if the oxygen-containing calcium compound is heated at a heating rate of no more than 10 °C / min in the reduction nitridation step.

[0023] The oxygenated calcium compound plays a role in the formation of a complex oxide with the oxygenated boron compound, creating a complex oxide with a high melting point and thus preventing the oxygenated boron compound from volatilizing. It has also been confirmed to act as a catalyst in a reaction that directly nitrates boron carbide.

[0024] The carbon-containing boron compound can be a compound containing carbon and boron (e.g., boron carbide). The particle diameter of the carbon-containing boron compound is preferably 1 µm to 500 µm, more preferably 10 µm to 400 µm, and even more preferably 20 µm to 300 µm.

[0025] The oxygen-containing boron compound, the carbon source, the oxygen-containing calcium compound, and the carbon-containing boron compound can be mixed, for example, in the ratio described below to obtain a mixture. The B / C ratio (element ratio) between a mass of the oxygen-containing boron compound relative to B and a mass of the carbon source relative to C is adjusted to 0.75 to 1.05. The oxygen-containing calcium compound is adjusted to an amount of 5 to 20 parts by mass relative to CaO in a total amount of 100 parts by mass of the oxygen-containing boron compound relative to B₂O₃ and the carbon source relative to C.The carbon-containing boron compound is adjusted to a quantity of 5 parts by mass to 45 parts by mass to a total quantity of 100 parts by mass of the masses of the oxygen-containing boron compound, the carbon source and the oxygen-containing calcium compound in relation to B2O3, C or CaO.

[0026] The raw material mixture containing the raw materials can be fed to the reduction nitridation reaction, for example, in powder form or in the form of granulated products formed from it. Mixing the raw materials can be carried out, for example, using a mixer such as a vibratory mill, bead mill, ball mill, Henschel mixer, drum mixer, vibratory shaker, or V-shaped mixer.

[0027] The heating of the raw material mixture in the reduction nitriding step is carried out in a furnace under a nitrogen atmosphere. Examples of gases introduced into the furnace include nitrogen gas and ammonia 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. Nitrogen gas is preferred as the gas introduced into the heating furnace. The nitrogen gas is preferably nitrogen gas with a controlled dew point temperature, and the dew point temperature is preferably not higher than -85 °C.

[0028] In the reduction nitriding step, the raw material mixture is heated from 1500 °C to a temperature of no more than 1850 °C (the maximum reduction nitriding temperature) at a heating rate of no more than 10 °C / min. Heating at this rate reduces the grain growth rate of the hexagonal boron nitride powder, thus yielding hexagonal boron nitride powder with high crystallinity. To obtain hexagonal boron nitride powder with even higher crystallinity by further reducing the grain growth rate, the heating rate from 1500 °C to the maximum reduction nitriding temperature is preferably no more than 7 °C / min, more preferably no more than 5 °C / min, and even more preferably no more than 3 °C / min.

[0029] During the reduction nitriding reaction, a holding time can optionally be provided as needed within a temperature range of up to 1500 °C. After reaching the maximum reduction nitriding temperature, the holding time to maintain this temperature is preferably not less than 1 hour, more preferably not less than 2 hours. Furthermore, the holding time to maintain the maximum reduction nitriding temperature is preferably not more than 10 hours, more preferably not more than 5 hours.

[0030] The reduction nitridation step can be carried out using a known reaction apparatus capable of controlling a reaction atmosphere. For example, one possible option is an atmosphere-controlled high-temperature furnace in which heating is performed by high-frequency induction heating or by heating with a heater, and a continuous heating furnace, such as a pusher-type tunnel furnace and a vertical reaction furnace, as well as a batch furnace, can be used.

[0031] The hexagonal boron nitride powder after the reduction nitridation step can be adjusted to exhibit a predetermined particle diameter distribution by decomposition. This decomposition is preferably carried out gently using, for example, a jet mill, a ball mill, a hammer mill, or a stone mill. The particle diameter of the powder after decomposition can be adjusted as required by classification treatment such as air classification or sieving. (Acid purification step)

[0032] In the acid purification step, the hexagonal boron nitride powder obtained by the reduction nitridation step, which contains unreacted raw material, is purified with acid. The hexagonal boron nitride powder obtained by the reduction nitridation reaction contains unreacted raw material, such as an oxide or a metal impurity. The acid purification step yields hexagonal boron nitride powder from which the unreacted raw material has been removed.

[0033] A method for the acid purification of hexagonal boron nitride powder containing unreacted raw material is not particularly restricted, and a known method can be used without restriction. For example, one possible method is as follows. After the reduction nitridation reaction, the resulting hexagonal boron nitride powder containing unreacted raw material is placed in a container, and dilute hydrochloric acid (10 wt% to 20 wt% HCl) in a 5 to 10 times the amount of the unreacted raw material is added to the container to keep it in contact for 4 to 8 hours.

[0034] The acid used in acid cleaning can be, for example, nitric acid, sulfuric acid, acetic acid, or hydrochloric acid.

[0035] After acid cleaning, water purification can be performed using pure water to remove any remaining acid. The water purification process can include filtering out the acid used in the acid cleaning, then dispersing the treated hexagonal boron nitride powder in the same amount of pure water as the acid used, and repeating the filtration process.

[0036] Hydrated agglomerates can be dried after acid or water purification. Drying is preferably carried out under certain conditions, e.g., in an atmosphere between 50 °C and 250 °C or under reduced pressure between 50 °C and 250 °C. The drying time is not specifically specified, but drying is preferably carried out until the water content is as close as possible to 0%. (Heating step)

[0037] In the heating step, the hexagonal boron nitride powder obtained by the acid purification step is heated to a temperature of more than 1850 °C and less than 2050 °C at a heating rate of no more than 5 °C / min.

[0038] The heating step can be carried out under a nitrogen atmosphere. The supply of the nitrogen source to the reaction system in the heating step can be performed by a method similar to that used in the reduction nitriding step. Furthermore, a reaction apparatus similar to that used in the reduction nitriding step can be employed.

[0039] The heating of the hexagonal boron nitride powder in the heating step is carried out at a heating rate of no more than 5 °C / min to a temperature of more than 1850 °C and less than 2050 °C (maximum re-firing temperature). This heating makes it possible to obtain hexagonal boron nitride powder with high crystallinity. However, if the heating is carried out to a temperature of no less than 2050 °C, there is a risk that the hexagonal boron nitride powder will yellow due to nitrogen defects or other factors and exhibit low crystallinity.

[0040] The temperature at which the heating rate is set in the heating step is not particularly restricted and can, for example, be room temperature (20 °C to 25 °C).

[0041] From the perspective of improving efficiency in the heating step, it is preferred, for example, that the temperature at which the heating rate adjustment begins is at least 1450 °C. The expression "the temperature at which the heating rate adjustment begins is at least 1450 °C" covers a case in which the heating rate adjustment begins after the hexagonal boron nitride powder has reached a temperature of 1450 °C.

[0042] To obtain hexagonal boron nitride powder with higher crystallinity, the heating rate in the heating step is preferably no more than 4 °C / min and more preferably no more than 3 °C / min.

[0043] To ensure sufficient heating (re-firing) of the hexagonal boron nitride powder after reaching the maximum re-firing temperature, the holding time to maintain the maximum re-firing temperature is preferably not less than 1 hour and more preferably not less than 3 hours. Furthermore, the holding time to maintain the maximum re-firing temperature is preferably not more than 15 hours and more preferably not more than 10 hours. (Cooling step)

[0044] In the cooling step, the hexagonal boron nitride powder is cooled to a temperature of at least 1550 °C after the heating step described above, at a cooling rate of no more than 5 °C / min. This cooling step makes it possible to obtain hexagonal boron nitride powder with a ratio of CL spectral intensity at 227 nm to CL spectral intensity at 330 nm of at least 1.0.

[0045] Cooling to a temperature of at least 1550 °C includes cooling the hexagonal boron nitride powder to a temperature of 1550 °C.

[0046] To obtain hexagonal boron nitride powder with a higher ratio of CL spectral intensity at 227 nm to CL spectral intensity at 330 nm, the heating rate in the cooling step is preferably no more than 4 °C / min and more preferably no more than 3 °C / min. (Recovery step)

[0047] In the recovery step, after the cooling step described above, the hexagonal boron nitride powder, cooled to a temperature of no more than 50 °C, is removed from the oven. Cooling the recovered hexagonal boron nitride powder to a temperature of no more than 50 °C suppresses the reaction with moisture in the air. This suppression prevents an increase in the surface oxygen concentration (B₂O₃ concentration) of the hexagonal boron nitride powder and improves its crystallinity. Furthermore, cooling the recovered hexagonal boron nitride powder to a temperature of no more than 50 °C yields a hexagonal boron nitride powder with a ratio of the Cl spectral intensity at 227 nm to the Cl spectral intensity at 330 nm of no more than 1.0.

[0048] In order to sufficiently suppress the reaction with moisture in the air and to improve the crystallinity of the hexagonal boron nitride powder, the temperature of the hexagonal boron nitride powder recovered from the oven is preferably not more than 40 °C and more preferably not more than 30 °C. [Use of hexagonal boron nitride powder] (resin composition)

[0049] A resin composition according to one aspect of the present invention contains the hexagonal boron nitride powder described above. Due to the inclusion of the hexagonal boron nitride powder, the resin composition exhibits high thermal conductivity properties and is suitable for heat dissipation applications.

[0050] The resin composition described above can be used for various applications. By mixing the resin composition with a resin described later to form a thermally conductive resin composition or a thermally conductive molded product, it can preferably be used, for example, in applications such as: thermal interface materials such as a polymer-based heat-dissipating film and a phase-change film; organic heat-dissipating films such as a heat-dissipating tape, a heat-dissipating grease, a heat-dissipating adhesive, and a gap filler; heat-dissipating paints such as a heat-dissipating paint and a heat-dissipating coating; heat-dissipating resin substrates such as a PCB-based resin substrate and a CCL-based resin substrate; insulating layers for metal-based substrates such as an aluminum-based substrate and a copper-based substrate; and sealing materials for power devices.

[0051] Examples of the resin included in the resin composition include thermoplastic resins such as polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, and fluorocarbon resin; thermosetting resins such as epoxy resin, phenolic resin, urea resin, melamine resin, unsaturated polyester resin, silicon resin, and bismaleimide triazine resin; and synthetic rubbers.

[0052] The resin composition may contain a thermally conductive filler such as aluminum nitride and aluminum oxide, which are known as highly thermally conductive insulating fillers.

[0053] The resin composition may further include, for example, a polymerization initiator, a curing agent, a polymerization inhibitor, a polymerization retarder, a coupling agent, a plasticizer, an ultraviolet absorber, a pigment, a dye, an antibacterial agent, an organic filler, and an organic / inorganic composite filler, which are known as compounding agents for the resin composition, as required. The resin composition may also contain other inorganic fillers to an extent that does not impair the effects of the present invention.

[0054] In the resin composition, the content of hexagonal boron nitride powder is preferably 20 vol.% to 80 vol.% and more preferably 30 vol.% to 70 vol.%, with regard to achieving higher thermal conductivity properties.

[0055] The resin content in the resin composition corresponds to the remaining volume after subtracting 100 vol% of the hexagonal boron nitride powder content from the total volume of the resin composition. The remaining volume includes a volume of curing agent in a case where the curing agent is present, and is preferably 80 vol% to 20 vol% and more preferably 70 vol% to 30 vol%. (resin film)

[0056] A resin film according to one aspect of the present invention is composed of the resin composition described above. Applications of the resin film are not particularly limited, but the resin film can be used, for example, for applications such as a printed circuit board and for heat dissipation of an electronic component such as a multilayer printed circuit board.

[0057] A method for producing the resin film is not particularly restricted. For example, in a case where the resin is a curable epoxy resin, a resin film manufacturing process may be used that includes a mixing step of obtaining a curable composition by mixing the curable epoxy resin, the above hexagonal boron nitride powder, and other components as required; a forming step of shaping the curable composition into a desired shape; and a curing step of curing the curable composition. (Other applications)

[0058] The hexagonal boron nitride powder according to one aspect of the present invention can also be used for applications such as: raw materials for processed boron nitride 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 molds or molten glass molds; cosmetics; and ceramic composite raw materials.

[0059] Aspects of the present invention can also be expressed as follows: Hexagonal boron nitride powder according to aspect 1 of the present invention has a ratio of a spectral intensity at a wavelength of 227 nm to a spectral intensity at a wavelength of 330 nm of not less than 1.0, measured by a cathodoluminescence method.

[0060] Hexagonal boron nitride powder according to aspect 2 of the present invention can be arranged such that, according to aspect 1 of the present invention, the amount of boron eluted from the hexagonal boron nitride powder is no more than 250 ppm, expressed as B2O3, measured after the hexagonal boron nitride powder has been immersed in a 2 mol / L aqueous sulfuric acid solution at 25 °C for 120 minutes.

[0061] Hexagonal boron nitride powder according to aspect 3 of the present invention can be arranged such that, in aspect 1 or 2 of the present invention, the hexagonal boron nitride powder has an average particle diameter of not less than 2 µm and not more than 150 µm.

[0062] A resin composition according to aspect 4 of the present invention contains the hexagonal boron nitride powder according to one of aspects 1 to 3 of the present invention.

[0063] A resin film according to aspect 5 of the present invention is a resin film composed of the resin composition according to aspect 4 of the present invention.

[0064] A process for producing hexagonal boron nitride powder according to aspect 6 of the present invention comprises: (1) a reduction nitridation step for obtaining hexagonal boron nitride by heating a raw material mixture in an oven under a nitrogen atmosphere to a temperature of not less than 1500 °C and not more than 1850 °C at a heating rate of not more than 10 °C / min, wherein the raw material mixture contains an oxygenated boron compound, a carbon source, an oxygenated calcium compound, and a carbonated boron compound; (2) an acid purification step for purifying, with acid, the hexagonal boron nitride powder obtained by the reduction nitridation step, which contains unreacted raw material;(3) a heating step in which the hexagonal boron nitride powder is heated after the acid purification step to a temperature of more than 1850 °C and less than 2050 °C at a heating rate of not more than 5 °C / min; (4) a cooling step in which the hexagonal boron nitride powder is cooled after the heating step to a temperature of at least 1550 °C at a cooling rate of not more than 5 °C / min; and (5) a recovery step in which the hexagonal boron nitride powder, which has been cooled to a temperature of not more than 50 °C, is removed from the furnace after the cooling step.

[0065] A method for producing hexagonal boron nitride powder according to aspect 7 of the present invention can be arranged such that, in aspect 6 of the present invention, the heating rate is adjusted in the heating step from a temperature of at least 1450 °C.

[0066] Examples are provided below to describe the embodiments of the present invention in more detail. Of course, the present invention is not limited to the following examples, and it is understood that details of the present invention may have different aspects. The present invention is not limited to the description of the above embodiments but can be modified in various ways by a person skilled in the art within the scope of the claims. The present invention, in its technical scope, encompasses every embodiment derived by a suitable combination of technical means disclosed in the different embodiments. All documents cited in this description are incorporated herein by reference. Examples

[0067] [Production Example 1] Production of hexagonal boron nitride powder: 259 g of a mixture containing 141 g boron oxide, 56 g carbon black, 32 g calcium oxide, and 30 g boron carbide were mixed using a ball mill. The mixture was subjected to a reduction nitridation treatment (reduction nitridation step) using a graphite Tamman furnace, such that the mixture was heated to 1500 °C under a nitrogen gas atmosphere, then held at 1500 °C for 4 hours, then heated at a rate of 2 °C / min to a maximum reduction nitridation temperature of 1850 °C, and then held at 1850 °C for 2 hours.

[0068] The hexagonal boron nitride powder, after reduction nitridation, was placed in a container, and hydrochloric acid (7 wt% HCl) in a 5-fold volume of the hexagonal boron nitride powder was added. The mixture was stirred at 700 rpm for 24 hours to perform an acid purification treatment (acid purification step). After the acid purification treatment, the acid was filtered out. The hexagonal boron nitride powder obtained by filtration was dispersed in an equal volume of pure water as used in the acid, and the filtration was repeated. This process was repeated 5 times, and then drying was carried out for 6 hours at 200 °C under vacuum.

[0069] The hexagonal boron nitride powder obtained after drying was subjected to a re-firing treatment (heating step) using a graphite Tamman furnace, so that the hexagonal boron nitride powder was heated to 1500 °C under a nitrogen gas atmosphere, then further heated at a rate of 2 °C / min to a maximum re-firing temperature of 1950 °C and then held at 1950 °C for 6 hours.

[0070] Nitrogen gas was then introduced into the furnace to cool the hexagonal boron nitride powder to 1500 °C at a rate of 1 °C / min (cooling step). More nitrogen gas was then introduced into the furnace to cool the interior to 25 °C, and the hexagonal boron nitride powder was then recovered from the furnace (recovery step). [Example 2] Production of hexagonal boron nitride powder

[0071] Hexagonal boron nitride powder was prepared using the same procedure as in Example 1, except that the cooling rate in the cooling step was changed to 2 °C / min. [Example 3] Production of hexagonal boron nitride powder

[0072] Hexagonal boron nitride powder was produced using the same procedure as in Example 1, except that the maximum re-firing temperature in the heating step was set to 2010 °C. [Example 4] Production of hexagonal boron nitride powder

[0073] Hexagonal boron nitride powder was prepared using the same procedure as in Example 1, except that the cooling rate in the cooling step was changed to 3 °C / min. [Example 5] Production of hexagonal boron nitride powder

[0074] Hexagonal boron nitride powder was produced using the same procedure as in Example 1, except that the maximum re-firing temperature in the heating step was set to 1900 °C. [Comparative example 1] Production of hexagonal boron nitride powder

[0075] Hexagonal boron nitride powder was produced using the same procedure as in Example 1, except that the maximum re-firing temperature in the heating step was set to 2050 °C. [Comparative example 2] Production of hexagonal boron nitride powder

[0076] Hexagonal boron nitride powder was prepared using only the reduction nitridation step and the acid purification step as described in Example 1. It should be noted that the heating from 1500 °C to 1850 °C in the reduction nitridation step was carried out at a rate of 1 °C / min. Apart from this, the conditions for the reduction nitridation step and the acid purification step were the same as in Example 1. [Comparative example 3] Production of hexagonal boron nitride powder

[0077] Hexagonal boron nitride powder was prepared using the same procedure as in Example 1, except that the cooling rate in the cooling step was changed to 10 °C / min. [Comparative example 4]

[0078] Hexagonal boron nitride powder was produced using the same procedure as in Example 1, except that the maximum firing temperature in the heating step was set to 1800 °C. [Comparative example 5]

[0079] Hexagonal boron nitride powder was prepared using the same procedure as in Example 1, except that the heating from 1500 °C to 1850 °C in the reduction nitridation step was carried out at a rate of 15 °C / min. [Comparative example 6]

[0080] Hexagonal boron nitride powder was produced using the same procedure as in Example 1, except that in the recovery step the hexagonal boron nitride powder was recovered from the furnace after the interior of the furnace had been cooled to 100 °C. [Comparative example 7]

[0081] Hexagonal boron nitride powder was prepared using the same method as in Example 1, except that the reduction nitridation step was carried out by a melamine process by thermal decomposition of melamine borate at 1950 °C. [Example of evaluation 1] Evaluation of hexagonal boron nitride powder

[0082] Hexagonal boron nitride powder obtained in each of Examples 1 to 5 and Comparative Examples 1 to 7 was evaluated using the following evaluation procedures. (Measurement using cathodoluminescence (CL) methods)

[0083] The measurement using the CL method was carried out under the following conditions using a Schottky emission type SEM, S-4300SE, manufactured by HITACHI, Ltd., and a spectrometer HR-320 (manufactured by HORIBA, Ltd.). • Electron beam acceleration voltage: 5 kV • Sample temperature: 296 K • Measuring range: 50 × 50 µm 2 (measured at three points by changing positions) (Measurement of the amount of total oxygen contained)

[0084] The total oxygen content in the hexagonal boron nitride powder was measured using an EMGA-620W analyzer for analyzing oxygen and nitrogen in ceramics, manufactured by HORIBA, Ltd. (Measurement of the amount of total carbon contained)

[0085] The total carbon content of hexagonal boron nitride powder was measured using an EMIA-110 analyzer for carbon in ceramics, manufactured by HORIBA, Ltd. The powder was burned in an oxygen stream, and the total carbon content was quantified based on the amounts of CO and CO2 gas produced. (Measurement of B2O3 concentration)

[0086] In 50 ml of IBOY, 50 g of 2% aqueous sulfuric acid solution and 2 g of hexagonal boron nitride powder were introduced and then subjected to vibration stirring for one minute while the solution temperature was maintained at 25 °C. The solution was then allowed to stand for 120 minutes, and the boron content was subsequently analyzed using an optical ICP emission spectrometer (iCAP6500, manufactured by THERMO FISHER Inc.). The resulting measurement was divided by the mass of hexagonal boron nitride powder used in the test to calculate the amount of eluted boron (B₂O₃ concentration, ppm) per unit mass of hexagonal boron nitride powder. (Measurement of whiteness)

[0087] The whiteness of the hexagonal boron nitride powder was measured using a ZE7700 colorimeter (manufactured by NIPPON DENSHOKU INDUSTRIES CO., LTD.). (Average particle diameter)

[0088] The average particle diameter (volume-based average particle diameter (D50)) of the hexagonal boron nitride powder was measured by laser diffractometry using a particle diameter measuring device of the MT3000 laser diffraction / scattering type (manufactured by MicrotracBEL Corp.). [Example of Assessment 1] Assessment of the thermal conductivity of resin compositions

[0089] Epoxy resin was filled with the hexagonal boron nitride powder obtained in each example or comparison example to prepare a resin composition, which was then evaluated for thermal conductivity. 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 CHEMICALS CORPORATION), and 210 parts by mass of methyl ethyl ketone, acting as a solvent. A resin composition was then obtained by mixing the lacquer mixture and the hexagonal boron nitride powder using a planetary centrifugal mixer (MAZERUSTAR, manufactured by Kurabo Industries Ltd.) such that the base resin and the specific boron nitride powder constituted 35 vol% and 65 vol%, respectively, or the base resin and the specific boron nitride powder constituted 40 vol% and 63 vol%, respectively.-% or the base resin and the specific boron nitride powder made up 45 vol% or 55 vol% respectively.

[0090] The resin composition was coated onto a PET film using a PI-1210 automatic coating machine manufactured by Tester Sangyo Co., Ltd., resulting in a thickness of approximately 180 µm to 220 µm, and then dried. The resin composition was subsequently 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 film with a thickness of 150 µm. The film was analyzed using a thermal wave analysis device, and its thermal conductivity was calculated.

[0091] The manufacturing conditions of the hexagonal boron nitride powder in each example or comparison example, as well as the evaluation results of evaluation example 1, are shown in Table 1.

[0092] The measured CL spectra are in Fig. 1 shown. In Fig. Figure 1 represents the measurement results of the hexagonal boron nitride powder from comparison example 2. Figure 2 represents the measurement results of the hexagonal boron nitride powder from example 4. Figure 3 represents the measurement results of the hexagonal boron nitride powder from example 2. Figure 4 represents the measurement results of the hexagonal boron nitride powder from example 1. Figure 5 represents the measurement results of the hexagonal boron nitride powder from comparison example 1. Manufacturing process Hexagonal boron nitride powder Resin composition Reduction nitridation heating rate maximum reignition temperature Cooling rate extraction temperature 227 nm / 330 nm Total oxygen Total carbon B2O3 (surface oxygen) Whiteness average particle diameter Thermal conductivity °C / min °C °C / min °C CL peak intensity ratio % % ppm µm W / m·K Example 1 2 1950 1 25 1.3 0.05 0.03 150 98 30 20 Example 2 2 1950 2 25 1.2 0.05 0.04 100 98 30 18 Example 3 2 2010 1 25 1.4 0.01 0.02 160 96 50 21 Example 4 2 1950 3 25 1.0 0.06 0.05 150 98 30 17 Example 5 2 1900 1 25 1.1 0.05 0.02 100 94 30 18 See Example 1 2 2050 1 25 0.2 0.03 0.03 100 88 30 15 See example 2. 1 - - - 0.6 0.07 0.04 100 92 40 13 See example 3. 2 1950 10 25 0.9 0.05 0.02 100 95 25 15 See example 4. 2 1800 1 25 0.9 0.06 0.02 100 96 25 15 See example 5. 15 1950 1 25 0.8 0.05 0.04 125 90 30 14 See example 6. 2 1950 1 100 0.9 0.05 0.04 300 93 30 15 See example 7. Melamine process 1950 1 25 0.8 0.05 0.04 150 92 30 14 In Table 1, "Reduction Nitriding Heating Rate" specifies a heating rate from 1500 °C to the maximum re-firing temperature in the reduction nitriding step. "Cooling Rate" specifies a cooling rate from the maximum re-firing temperature to 1500 °C in the cooling step. "Retrieval Temperature" specifies the temperature in the furnace when the re-firing hexagonal boron nitride powder is recovered from the furnace. "227 nm / 330 nm" specifies the ratio of a Cl peak intensity at a wavelength of 227 nm to a Cl peak intensity at a wavelength of 330 nm.

[0093] As shown in Table 1, the resin composition containing the hexagonal boron nitride powder from each of the examples, where the ratio of the CL peak intensity at 227 nm to the CL peak intensity at 330 nm was not less than 1.0, has a thermal conductivity of more than 15 W / m·K. This indicates that the hexagonal boron nitride powder from each of the examples is suitable as a heat-dissipating filler.

[0094] In comparative example 1, where the maximum re-firing temperature was 2050 °C, the ratio of the CL peak intensity at 227 nm to the CL peak intensity at 330 nm was as low as 0.2, and the powder exhibited yellowing. Yellowed powder suffers from nitrogen defects and the like, and displays poor crystallinity. The resin composition containing the hexagonal boron nitride powder from comparative example 4, where the maximum re-firing temperature was 1850 °C, had a thermal conductivity of no more than 15 W / m·K. This indicates that no cleaning effect was achieved through re-firing.

[0095] In each of the comparative examples 3 and 5, where the heating rate during the heating step (refiring) or the cooling rate during the cooling step was high, the ratio of the CL peak intensity at 227 nm to the CL peak intensity at 330 nm was 0.9, and the thermal conductivity of the resin composition was no more than 15 W / m·K. These results indicate that the heating rate and the cooling rate during refiring influence the ratio of the CL peak intensity at 227 nm to the CL peak intensity at 330 nm and the thermal conductivity of the resin composition.

[0096] Regarding the hexagonal boron nitride powder from Comparative Example 6, where the extraction temperature was high, the following considerations can be made. The hexagonal boron nitride powder reacted with moisture in the air to exhibit an increase in surface oxygen concentration. Consequently, the hexagonal boron nitride powder showed a decrease in surface crystallinity and a low ratio of the CL peak intensity at 227 nm to the CL peak intensity at 330 nm. Commercial applicability

[0097] The hexagonal boron nitride powder according to an embodiment of the present invention exhibits high crystallinity, improves the thermal conductivity properties of resins and can be used as a raw material for materials used in electronic components. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2005-145788

[0004]

Claims

[1] Hexagonal boron nitride powder with a ratio of spectral intensity at a wavelength of 227 nm to spectral intensity at a wavelength of 330 nm of not less than 1.0, as measured by a cathodoluminescence method. [2] Hexagonal boron nitride powder according to claim 1, wherein the amount of boron eluted from the hexagonal boron nitride powder is not more than 250 ppm, expressed as B2O3, measured after the hexagonal boron nitride powder has been immersed in a 2 mol / L aqueous sulfuric acid solution at 25 °C for 120 minutes. [3] Hexagonal boron nitride powder according to claim 1 having an average particle diameter of not less than 2 µm and not more than 150 µm. [4] Resin composition comprising hexagonal boron nitride powder according to any one of claims 1 to 3. [5] Resin film composed of a resin composition according to claim 4. [6] Method for the production of hexagonal boron nitride powder, the method comprising: (1) a reduction nitridation step for obtaining hexagonal boron nitride by heating a mixture of raw materials in an oven under a nitrogen atmosphere to a temperature of not less than 1500 °C and not more than 1850 °C at a heating rate of not more than 10 °C / min, wherein the mixture of raw materials contains an oxygen-containing boron compound, a carbon source, an oxygen-containing calcium compound and a carbon-containing boron compound; (2) an acid purification step of the purification of the hexagonal boron nitride powder obtained by the reduction nitridation step and containing unreacted raw material, with acid; (3) a heating step in which the hexagonal boron nitride powder is heated after the acid purification step to a temperature of more than 1850 °C and less than 2050 °C at a heating rate of not more than 5 °C / min; (4) a cooling step in which the hexagonal boron nitride powder is cooled after the heating step to a temperature of at least 1550 °C at a cooling rate of not more than 5 °C / min; and (5) a recovery step in which the hexagonal boron nitride powder, which has been cooled to a temperature of not more than 50 °C, is removed from the oven after the cooling step. [7] Method according to claim 6, wherein the adjustment of the heating rate in the heating step is carried out from a temperature of at least 1450 °C.

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