Method for manufacturing a sheet and sheet

Abstract

A method for producing a sheet which comprises a step in which a boron carbide powder is nitrided while being pressed by hot isotactic pressing to obtain a boron carbonitride powder, a step in which the boron carbonitride powder is decarburized to obtain a boron nitride powder, a step in which the boron nitride powder is mixed with a resin to obtain a resin composition, and a step in which the resin composition is formed into a sheet by pressing.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a sheet and a sheet.

Background Art

[0002] In electronic components such as power devices, transistors, thyristors, and CPUs, it is an issue to efficiently dissipate heat generated during use. To address this issue, a heat dissipation member containing ceramic powder with high thermal conductivity is used.

[0003] As the ceramic powder, boron nitride powder having properties such as high thermal conductivity, high insulation, and low relative permittivity has attracted attention. Boron nitride powder is generally composed of agglomerated particles (block-like particles) formed by aggregation of flaky boron nitride particles (primary particles). For example, in Patent Document 1, a hexagonal boron nitride powder is disclosed, which is said to achieve improved insulation and stabilized withstand voltage of a heat transfer sheet filled with the powder by further spheroidizing the shape of the agglomerated particles to enhance the filling property, improving the powder strength, and further purifying.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When obtaining a sheet using boron nitride powder, a resin composition containing boron nitride powder and resin is generally molded into a sheet by applying pressure. In this case, the pressure causes the aggregated boron nitride particles in the resin composition to break down, and the boron nitride particles (primary particles) that made up the aggregated particles tend to orient themselves parallel to the main surface of the sheet (perpendicular to the direction of pressure). When the boron nitride particles are oriented parallel to the main surface of the sheet, the thermal conductivity in the thickness direction of the sheet may decrease, which may make it unsuitable for use as a heat transfer sheet.

[0006] Therefore, the main objective of the present invention is to manufacture a sheet in which the orientation of boron nitride particles in a direction parallel to the main surface of the sheet is suppressed. [Means for solving the problem]

[0007] The inventors have discovered that by forming a sheet using boron nitride powder obtained by a manufacturing method including the hot isostatic pressing method (HIP method), it is possible to manufacture a sheet in which the orientation of boron nitride particles in a direction parallel to the main surface of the sheet is suppressed. In several aspects, the present invention provides the following [1] to [3]. [1] A step of obtaining boron carbonitride powder by nitriding boron carbide powder under hot isostatic pressure, The process involves decarburizing the boron carbonitride powder to obtain boron nitride powder, The process involves mixing the boron nitride powder with a resin to obtain a resin composition. A step of forming the resin composition into a sheet by pressurizing it, A method for manufacturing a sheet, comprising the following features. [2] Contains boron nitride particles and resin, A sheet where the average value X calculated using the following steps (1) to (8) is 0.01 or greater. (1) Observe an image of any cross-section of the sheet using a SEM at a magnification of 1000x. (2) The observed image is binarized into a region consisting of boron nitride particles and a region other than the observed image, and a binarized image is obtained. (3) In the binarized image, the region A, which is 50 μm on the short side and 100 μm on the long side, is divided into n equal parts along the short side and 2n equal parts along the long side to form multiple cells (where n is an integer of 1 or more). (4) In one of the plurality of cells, the area ratio of the region consisting of boron nitride particles is calculated based on the total area of ​​the one cell. If the area ratio is 80% or more, the one cell is determined to be a boron nitride region. If the area ratio is less than 20%, the one cell is determined to be a resin region. (5) The determination in (4) above is made for each of the plurality of cells, and the ratio of the number of cells determined to be in the boron nitride region is calculated based on the total number of the plurality of cells. (6) In the steps (3) to (5) above, the proportion of cells determined to be in the boron nitride region when n is between 1 and 200 is calculated. (7) Calculate the sum of the percentages of cells that are determined to be boron nitride regions for each of the following cases: n = 1 to 5. (8) Calculate the average value X from the total sum calculated in the steps (1) to (7) above for each of the five cross-sections of the sheet. [3] The sheet according to [2], wherein in the five cross-sections of the sheet, the average value Y of the ratio of the number of cells determined to be boron nitride regions in each section when n is 200 is 0.5 or more. [Effects of the Invention]

[0008] According to one aspect of the present invention, it is possible to manufacture a sheet in which the orientation of boron nitride particles in a direction parallel to the main surface of the sheet is suppressed. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a flowchart of steps (1) to (7). [Figure 2] Figure 2(a) is a reference diagram showing how region A in the binarized image is divided into multiple cells. Figure 2(b) is a schematic diagram showing how each of the multiple cells in Figure 2(a) is determined to be either a boron nitride region or a resin region. [Figure 3] It is a SEM image of the cross-section of the sheet of Example 1-1. [Figure 4] It is a SEM image of the cross-section of the sheet of Comparative Example 1. [Figure 5] It is a graph showing the relationship between the length of one side of one cell when the sheets of Example 4-1 and Comparative Example 4 are divided into a plurality of cells, and the ratio of the number of cells determined to be boron nitride regions.

Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described in detail.

[0011] The method for manufacturing a sheet according to an embodiment of the present invention includes a step of nitriding boron carbide powder while performing hot isostatic pressing (also referred to as "hot isostatic pressing") to obtain boron carbonitride powder (nitriding step), a step of decarburizing the boron carbonitride powder to obtain boron nitride powder (decarburizing step), a step of mixing the boron nitride powder with a resin to obtain a resin composition (mixing step), and a step of forming the resin composition into a sheet shape by pressing (forming step).

[0012] Boron carbide powder (boron carbide particles) can be manufactured, for example, by a known manufacturing method. For example, after mixing boric acid and acetylene black, it is heated at 1800 to 2400 °C for 1 to 10 hours in an inert gas atmosphere to obtain boron carbide coarse powder containing massive boron carbide particles. By appropriately performing pulverization, sieving, washing, impurity removal, drying, etc. on the obtained boron carbide coarse powder, boron carbide powder can be obtained. The average particle size of the boron carbide powder may be, for example, 5 μm or more, 10 μm or more, or 15 μm or more, and may be 80 μm or less, 60 μm or less, or 40 μm or less. The average particle size of the boron carbide powder means the particle size (D50) at which the volume cumulative particle size distribution is 50%, and can be measured by the laser diffraction scattering method.

[0013] In the nitriding process, boron carbide powder is nitrided to obtain boron carbonitride powder by heating while applying hot isostatic pressure in a state where the boron carbide powder is filled in a container under an atmosphere that promotes the nitriding reaction. The container may be, for example, a carbon crucible. The hot isostatic pressure can be applied, for example, using a hot isostatic pressing device (e.g., manufactured by Kobe Steel, Ltd.).

[0014] The atmosphere that promotes the nitriding reaction in the nitriding process may be a nitriding gas atmosphere that nitrides the boron carbide powder. The nitriding gas may be nitrogen gas, ammonia gas, etc., and may be nitrogen gas from the viewpoints of facilitating the nitriding of the boron carbide powder and cost. The nitriding gas may be used alone or in combination of two or more, and the proportion of nitrogen gas in the nitriding gas may be 95% by volume or more, 99% by volume or more, or 99.9% by volume or more.

[0015] The pressure in the nitriding process may be 10 MPa or more, 30 MPa or more, or 100 MPa or more. The pressure in the nitriding process may be 200 MPa or less or 150 MPa or less.

[0016] From the viewpoint of sufficiently nitriding the boron carbide powder, the heating temperature in the nitriding process may be 1600 °C or more or 1700 °C or more. The heating temperature in the nitriding process may be 2200 °C or less or 2000 °C or less.

[0017] From the viewpoint of sufficiently nitriding the boron carbide powder, the time for applying pressure and heating in the nitriding process may be 3 hours or more, 5 hours or more, or 8 hours or more. The time for applying pressure and heating in the nitriding process may be 30 hours or less, 20 hours or less, or 10 hours or less.

[0018] In the decarburization process, the boron carbonitride powder obtained in the nitriding process (boron carbonitride particles) is decarburized by heating in a state where a mixture containing the boron carbonitride powder and a boron source is filled in a container. The container may be, for example, a boron nitride crucible.

[0019] Examples of boron sources include boric acid, boron oxide, or mixtures thereof. The mixture may further contain other additives used in the art as needed. The mixing ratio of boron carbonitride powder and boron source is selected as appropriate. When boric acid or boron oxide is used as the boron source, the proportion of boric acid or boron oxide may be, for example, 50 parts by mass or more, or 80 parts by mass or more, and 300 parts by mass or less, or 200 parts by mass or less, per 100 parts by mass of boron carbonitride.

[0020] The atmosphere in the decarburization process may be at normal pressure (atmospheric pressure) or a pressurized atmosphere. The pressure in the decarburization process may be, for example, 0.5 MPa or less or 0.3 MPa or less, or 0.01 MPa or more or 0.03 MPa or more.

[0021] In the decarburization process, for example, the temperature is first raised to a predetermined temperature (the temperature at which decarburization can begin), and then further raised to a predetermined holding temperature. The predetermined temperature (the temperature at which decarburization can begin) may be, for example, 1000°C or higher, 1500°C or lower, or 1200°C or lower. The rate at which the temperature is raised from the predetermined temperature (the temperature at which decarburization can begin) to the holding temperature may be, for example, 5°C / min or lower, 4°C / min or lower, 3°C / min or lower, or 2°C / min or lower.

[0022] The holding temperature may be 1800°C or higher or 2000°C or higher, from the viewpoint of promoting good particle growth. The holding temperature may be 2200°C or lower or 2100°C or lower.

[0023] The holding time at the holding temperature may be, for example, 0.5 hours or more, 1 hour or more, 3 hours or more, or 5 hours or more, from the viewpoint of ensuring good particle growth. The holding time at the holding temperature may be, for example, 40 hours or less, 30 hours or less, or 20 hours or less.

[0024] The boron nitride powder (boron nitride aggregated particles) obtained as described above may be subjected to a classification step (classification step) by sieving to obtain boron nitride powder having a desired particle size.

[0025] Boron nitride aggregate particles are composed of, for example, multiple boron nitride flakes (primary boron nitride particles). The boron nitride flakes are formed from boron nitride and may have, for example, a flaky shape.

[0026] Multiple boron nitride fragments may be in physical contact with each other, or they may be chemically bonded. Chemical bonding between multiple boron nitride fragments can be confirmed using a scanning electron microscope (SEM) by observing that no boundaries between the fragments are visible at the bonding site.

[0027] The average thickness of the boron nitride flakes may be 0.5 μm or more, 1.0 μm or more, or 3.0 μm or more, and may be 10 μm or less. The average length in the longitudinal direction of the boron nitride flakes may be, for example, 1 μm or more, and may be 10 μm or less. The average thickness and average length in the longitudinal direction of the boron nitride flakes are defined as the average values ​​of the thickness and longitudinal direction of 40 boron nitride flakes measured in an SEM image obtained by observing the cross-section of boron nitride aggregate particles at a magnification of 1000x using a SEM, and importing the SEM image into image analysis software (for example, "Mac-view" manufactured by Mountec Co., Ltd.).

[0028] Boron nitride aggregate particles may have a cross-section containing a region where multiple boron nitride flakes are stacked. The stacking of multiple boron nitride flakes can be confirmed by observing the cross-section of the boron nitride aggregate particle using a scanning electron microscope (SEM), where the multiple boron nitride flakes are arranged in a line in the thickness direction of the boron nitride flakes.

[0029] The average particle size of boron nitride powder may be, for example, 10 μm or more, 20 μm or more, or 30 μm or more, and may be 100 μm or less, 80 μm or less, or 60 μm or less. The average particle size of boron nitride powder refers to the particle size (D50) at which the volume cumulative particle size distribution is 50%, and can be measured by laser diffraction scattering.

[0030] The bulk density of the boron nitride powder may be 0.7 g / ml or more, 0.72 g / ml or more, 0.74 g / ml or more, or 0.75 g / ml or more, and may be 0.9 g / ml or less, 0.8 g / ml or less, or 0.75 g / ml or less.

[0031] Boron nitride aggregates may consist substantially of boron nitride alone. The fact that boron nitride aggregates consist substantially of boron nitride can be confirmed by the detection of only peaks originating from boron nitride in X-ray diffraction measurements.

[0032] In the mixing step, the boron nitride powder obtained in the decarburization step is mixed with the resin to obtain the resin composition.

[0033] Examples of resins include epoxy resins, silicone resins, silicone rubbers, acrylic resins, phenolic resins, melamine resins, urea resins, unsaturated polyesters, fluororesins, polyimides, polyamideimides, polyetherimides, polybutylene terephthalate, polyethylene terephthalate, polyphenylene ethers, polyphenylene sulfides, fully aromatic polyesters, polysulfones, liquid crystal polymers, polyethersulfones, polycarbonates, maleimide-modified resins, ABS (acrylonitrile-butadiene-styrene) resins, AAS (acrylonitrile-acrylic rubber-styrene) resins, and AES (acrylonitrile-ethylene-propylene-diene rubber-styrene) resins.

[0034] The content of boron nitride powder may be 50% by volume or more, 55% by volume or more, 60% by volume or more, 65% by volume or more, or 70% by volume or more, based on the total volume of the resin composition, from the viewpoint of improving the thermal conductivity of the resin composition and easily obtaining excellent heat dissipation performance. The content of boron nitride powder may be 85% by volume or less, 80% by volume or less, or 75% by volume or less, based on the total volume of the resin composition, from the viewpoint of suppressing the generation of pores during molding and the reduction of insulation and mechanical strength.

[0035] The resin content may be 15% or more by volume, 20% or more by volume, or 25% or more by volume, based on the total volume of the resin composition, and may be 50% or less by volume, 45% or less by volume, 40% or less by volume, 35% or less by volume, or 30% or less by volume.

[0036] The resin composition may further contain a curing agent for curing the resin. The curing agent is appropriately selected depending on the type of resin. For example, when the resin is an epoxy resin, examples of curing agents include phenol novolac compounds, acid anhydrides, amino compounds, and imidazole compounds. The content of the curing agent may be, for example, 0.5 parts by mass or more, or 1.0 part by mass or more, or 15 parts by mass or less, or 10 parts by mass or less, per 100 parts by mass of the resin.

[0037] The resin composition may further contain other components. These other components may include curing accelerators (curing catalysts), coupling agents, wetting and dispersing agents, surface modifiers, and the like.

[0038] Examples of curing accelerators (curing catalysts) include phosphorus-based curing accelerators such as tetraphenylphosphonium tetraphenylborate and triphenylphosphorate, imidazole-based curing accelerators such as 2-phenyl-4,5-dihydroxymethylimidazole, and amine-based curing accelerators such as boron trifluoride monoethylamine.

[0039] Examples of coupling agents include silane-based coupling agents, titanate-based coupling agents, and aluminate-based coupling agents. Chemical bonding groups contained in these coupling agents include vinyl groups, epoxy groups, amino groups, methacrylic groups, and mercapto groups.

[0040] Examples of wetting and dispersing agents include phosphate ester salts, carboxylic acid esters, polyesters, acrylic copolymers, and block copolymers.

[0041] Examples of surface modifiers include acrylic-based surface modifiers, silicone-based surface modifiers, vinyl-based modifiers, and fluorine-based surface modifiers.

[0042] Boron nitride powder and resin can be mixed by known methods. The resin composition obtained in the mixing step may further contain a solvent (e.g., a solvent for dissolving the resin) as needed, and may further contain the other components mentioned above.

[0043] Examples of solvents include alcohol-based solvents, glycol ether-based solvents, aromatic solvents, and ketone-based solvents. Examples of alcohol-based solvents include isopropyl alcohol and diacetone alcohol. Examples of glycol ether-based solvents include ethyl cellosolve and butyl cellosolve. Examples of aromatic solvents include toluene and xylene. Examples of ketone-based solvents include methyl ethyl ketone and methyl isobutyl ketone.

[0044] In the molding process, the resin composition obtained in the mixing process is molded into a sheet by applying pressure to obtain a sheet. The molding process may be, for example, a process in which the resin composition is coated onto a substrate using a film applicator and molded into a sheet by applying pressure.

[0045] The pressure applied during the molding process may be, for example, 1 MPa or more, 5 MPa or more, or 100 MPa or less, or 50 MPa or less.

[0046] In the molding process, a curing step may be performed simultaneously with or after molding to cure part or all of the resin in the resin composition.

[0047] The method for curing the resin is appropriately selected depending on the type of resin (and the curing agent used if necessary). For example, if the resin is an epoxy resin and the above-mentioned curing agent is used together, the resin can be cured by heating during the curing process. In this case, the resin can be partially cured by adjusting the heating temperature and heating time. If the resin composition contains a solvent, the solvent may be volatilized at the same time as the resin is cured during the curing process.

[0048] The sheet obtained by the above manufacturing method has suppressed orientation of boron nitride particles in a direction parallel to the main surface of the sheet. The inventors speculate on the reason for this as follows: That is, after obtaining boron carbonitride powder by nitriding boron carbide powder under hot isostatic pressure, the boron nitride powder (boron nitride aggregate particles) obtained by decarburizing the boron carbonitride powder is composed of relatively thick boron nitride flakes. Such boron nitride aggregate particles tend to maintain their shape even when external force is applied. Therefore, when a resin composition is made using such boron nitride aggregate particles and a sheet is formed by pressurizing the resin composition, some of the boron nitride aggregate particles in the sheet are less likely to collapse even when pressurized and can maintain their shape to a certain extent. Furthermore, in such boron nitride aggregates, each boron nitride fragment constituting the aggregate is relatively thick, resulting in high density (low porosity) and a higher amount of boron nitride per aggregate compared to conventional boron nitride aggregates. When forming a sheet using such boron nitride aggregates, the amount of boron nitride aggregates (or boron nitride fragments) per unit volume of the sheet is reduced compared to forming a sheet with the same filling amount (mass-based loading amount) using conventional boron nitride aggregates. Therefore, the frequency of compression deformation of the boron nitride aggregates (or boron nitride fragments) during sheet formation is reduced. Because the boron nitride aggregates are less likely to collapse and compression deformation occurs less frequently during sheet formation, it is presumed that the boron nitride fragments (primary boron nitride particles) are less likely to align in a direction parallel to the main surface of the sheet, thereby suppressing the orientation of boron nitride particles within the sheet. However, the mechanism of the present invention is not limited to the reasons described above.

[0049] The sheet obtained by the above manufacturing method has, in its cross-section, multiple regions consisting of boron nitride particles and other regions. Because the boron nitride aggregated particles maintain their shape to some extent within the sheet, the area of ​​the regions consisting of boron nitride particles (each of the multiple regions consisting of boron nitride particles) in the cross-section of the sheet obtained by the above manufacturing method tends to be larger than the area of ​​the regions consisting of boron nitride particles in the cross-section of a sheet made with the same content using conventional boron nitride aggregated particles.

[0050] The relatively large area of ​​the region consisting of boron nitride particles in the cross-section of the sheet can be confirmed by the fact that the average value X calculated by the following steps (1) to (8) is 0.01 or higher. That is, another embodiment of the present invention is a sheet containing boron nitride particles and a resin, wherein the average value X calculated by the following steps (1) to (8) is 0.01 or higher. Figure 1 shows a flowchart of steps (1) to (7). (1) Observe an image of any cross-section of the sheet at a magnification of 1000x using a scanning electron microscope (SEM). (2) The observed image is binarized into regions consisting of boron nitride particles and other regions, and a binarized image is obtained. (3) In the binarized image, the region A, which is 50 μm on the short side and 100 μm on the long side, is divided into n equal parts along the short side and 2n equal parts along the long side to form multiple cells (where n is an integer of 1 or more). (4) In one of the plurality of cells, the area ratio of the region consisting of boron nitride particles is calculated based on the total area of ​​the one cell. If the area ratio is 80% or more, the one cell is determined to be a boron nitride region. If the area ratio is less than 20%, the one cell is determined to be a resin region. (5) The determination in (4) above is made for each of the plurality of cells, and the ratio of the number of cells determined to be in the boron nitride region is calculated based on the total number of the plurality of cells. (6) In the steps (3) to (5) above, the proportion of cells determined to be in the boron nitride region when n is between 1 and 200 is calculated. (7) Calculate the sum of the percentages of cells that are determined to be boron nitride regions for each of the following cases: n = 1 to 5. (8) Calculate the average value X from the total sum calculated in the steps (1) to (7) above for each of the five cross-sections of the sheet.

[0051] In procedure (1), the image format of the observed image to be acquired shall be bmp. The acquired image only needs to be clear enough to be binarized into regions consisting of boron nitride particles and other regions.

[0052] In step (2), the observed image is imported into the image processing software "imageJ" and filtered using a Median filter (3x3 pixels). Then, the region consisting of boron nitride particles and the other region (for example, the region consisting of resin) are binarized using the Otsu method to obtain a binarized image.

[0053] In step (3), the entirety of region A is included in the binarized image. Region A is a region with a short side of 50 μm and a long side of 100 μm. However, due to the limitations of image processing, it is difficult to precisely specify a range of 50 μm x 100 μm as region A. Therefore, a region with a short side of (50 μm ± 5 μm) x a long side of (100 μm ± 10 μm) can be considered as region A, as its influence on the calculation of the average value X is negligible.

[0054] In step (3), region A is divided into multiple cells by dividing the shorter side of region A into n equal parts and the longer side of region A into 2n equal parts, where n is an integer of 1 or more. In other words, region A is divided into 2n 2 The region is divided into n cells, each cell being a square with sides of 50 / n (μm). For example, when n=1, region A is divided into two square cells (50 μm × 50 μm) by dividing its shorter side into one equal part (i.e., not dividing it along its shorter side) and dividing its longer side into two equal parts.

[0055] Figure 2(a) is a reference diagram showing how region A in the binarized image is divided into multiple cells. In Figure 2(a), region A is divided into 4 equal parts along its short side and 8 equal parts along its long side, resulting in 32 square cells.

[0056] In step (4), one of the multiple cells is examined, and the area ratio of the region consisting of boron nitride particles is calculated based on the total area of ​​that one cell. If the area ratio of the region consisting of boron nitride particles is 80% or more in a single cell, it can be determined that the cell is a region occupied by boron nitride particles. If the area ratio of the region consisting of boron nitride particles is less than 20%, it can be determined that the cell is a region occupied by components other than boron nitride particles (a region not occupied by boron nitride particles). In this way, by dividing region A into multiple cells and determining whether each cell is a region consisting of boron nitride particles (boron nitride region) or a region consisting of other components (resin region), it is possible to determine whether the area of ​​each region among the multiple regions consisting of boron nitride particles is relatively large or not in the cross-section of the sheet.

[0057] Figure 2(b) is a schematic diagram showing the results when each of the multiple cells in Figure 2(a) is determined to be either a boron nitride region or a resin region. In Figure 2(b), cells determined to be boron nitride regions are shown as black cells, and cells determined to be resin regions are shown as white cells.

[0058] In step (5), the judgment in step (4) is made for each of the multiple cells, and the proportion of cells determined to be in the boron nitride region is calculated based on the total number of multiple cells. That is, the proportion of cells determined to be in the boron nitride region is calculated by dividing the total number of cells determined to be in the boron nitride region by the total number of multiple cells.

[0059] In step (6), the proportion of cells determined to be boron nitride regions when n is between 1 and 200 is calculated using steps (3) to (5). That is, the proportion of cells determined to be boron nitride regions is calculated when region A is divided into 2 cells, 8 cells, 18 cells, 32 cells, 50 cells, ... 80,000 cells. For example, in Figure 2(b), the total number of cells is 32, and the number of cells determined to be boron nitride regions is 12, so the proportion of cells determined to be boron nitride regions is 0.375 (=12 / 32).

[0060] In step (7), the total value is calculated from the proportion of cells determined to be boron nitride regions for each of the following n values: n = 1 to n = n. When the boron nitride aggregate particles do not easily break down during sheet preparation and the boron nitride aggregate particles maintain their shape to some extent within the sheet, the area of ​​the region consisting of boron nitride particles in the cross-section of the sheet is relatively large. Therefore, even when n = 1 to n = n = n (i.e., when region A is roughly divided), each cell is more likely to be determined to be a boron nitride region, and the proportion of cells determined to be boron nitride regions tends to be larger.

[0061] In step (8), observation images of a total of 5 cross-sections of the sheet are obtained, and the total value is calculated from each observation image using the procedures (1) to (7). The average value X is then calculated from the total values ​​obtained from each of the 5 cross-sections.

[0062] The average value X may be 0.01 or higher, 0.03 or higher, 0.05 or higher, 0.07 or higher, 0.08 or higher, 0.09 or higher, or 0.10 or higher, from the viewpoint of suppressing the orientation of boron nitride particles even when the amount of boron nitride particles packed is large, and making it easier to obtain a sheet with excellent heat dissipation performance. From the viewpoint of suppressing a decrease in insulating properties and mechanical strength, it may be 5 or lower, 3 or lower, 2 or lower, 1.8 or lower, 1.6 or lower, 1.2 or lower, 1 or lower, 0.8 or lower, 0.5 or lower, 0.3 or lower, or 0.2 or lower.

[0063] In the five cross-sections of the sheet, the average value Y of the proportion of each boron nitride region when n is 200 may be 0.5 or higher, 0.52 or higher, or 0.54 or higher, from the viewpoint of improving the thermal conductivity of the sheet and easily obtaining excellent heat dissipation performance. The average value Y may be 0.8 or lower, 0.75 or lower, 0.7 or lower, or 0.65 or lower, from the viewpoint of suppressing a decrease in insulation and mechanical strength.

[0064] The orientation index of the sheet may be 15 or less, 13 or less, or 11.5 or less, from the viewpoint of suppressing the orientation of boron nitride particles in a direction parallel to the main surface of the sheet. The orientation index of the sheet may be 1 or more, 3 or more, or 5 or more.

[0065] The sheet thickness may be 50 μm or more, 80 μm or more, or 100 μm or more, and may be 500 μm or less, 400 μm or less, or 300 μm or less. [Examples]

[0066] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples.

[0067] (Example 1-1) Boron carbide powder with an average particle size (D50) of 26 μm was packed into a carbon crucible and heated and pressurized by the HIP method for 1.5 hours at 1800°C and 196 MPa in a nitrogen gas atmosphere using a hot isostatic pressurizing device (Kobe Steel, Ltd., 02-SYSTEM15×) to nitride the boron carbide powder and obtain boron carbonitride powder (B4CN4). 100 parts by mass of the obtained boron carbonitride powder and 150 parts by mass of boric acid (60% by mass of boric acid) were mixed using a Henschel mixer. The mixture was then packed into a boron nitride crucible and heated in a resistance heating furnace at atmospheric pressure and a nitrogen gas atmosphere at a holding temperature of 2000°C and 0.03 MPa for a holding time of 5 hours to obtain a powder containing coarse particles (coarse powder). The coarse powder was crushed in a mortar for 10 minutes and then classified using a nylon sieve with a mesh size of 175 μm. This resulted in obtaining a particle aggregate (powder). A portion of the obtained powder was collected and subjected to X-ray diffraction measurement using an X-ray diffractometer (Rigaku Corporation, "ULTIMA-IV"). Only peaks originating from boron nitride were detected, confirming that the obtained powder was boron nitride powder (average particle size 42.3 μm, bulk density 0.75 g / ml).

[0068] 100 parts by mass of naphthalene-type epoxy resin (DIC Corporation, HP4032) and 10 parts by mass of imidazole compound (Shikoku Chemicals Co., Ltd., 2E4MZ-CN) as a curing agent were mixed. Then, the resulting boron nitride powder was mixed so that the boron nitride powder filling rate was 70% by volume to obtain a resin composition. This resin composition was degassed under reduced pressure of 500 Pa for 10 minutes and coated onto a PET sheet to a thickness of 1.0 mm. Subsequently, a sheet with a thickness of 0.5 mm was produced by pressing and heating under conditions of 150°C and 30 MPa for 60 minutes.

[0069] (Examples 1-2) A sheet was prepared in the same manner as in Example 1, except that the amount of boric acid was changed to 100 parts by mass (50% by mass) to obtain boron nitride powder (average particle size 44.7 μm, bulk density 0.75 g / ml).

[0070] (Examples 1-3) A sheet was prepared in the same manner as in Example 1, except that the amount of boric acid was changed to 81.8 parts by mass (45% by mass) to obtain boron nitride powder (average particle size 51.5 μm, bulk density 0.74 g / ml).

[0071] (Comparative Example 1) Boron nitride powder (average particle size 42.3 μm, bulk density 0.67 g / ml) was obtained in the same manner as in Example 1-1, except that instead of nitriding boron carbide powder by the HIP method, the boron carbide powder was nitrided using a resistance heating furnace in a nitrogen gas atmosphere at 2000°C and 0.85 MPa for 25 hours to obtain boron carbonitride powder. A sheet was then prepared.

[0072] (Example 2-1) The sheet was prepared in the same manner as in Example 1-1, except that it was prepared by press heating and pressurizing at a temperature of 150°C and a pressure of 15 MPa for 60 minutes.

[0073] (Example 2-2) The sheets were prepared in the same manner as in Example 1-2, except that they were prepared by press heating and pressurizing at a temperature of 150°C and a pressure of 15 MPa for 60 minutes.

[0074] (Comparative Example 2) The sheets were prepared in the same manner as in Comparative Example 1, except that they were prepared by press heating and pressurizing at a temperature of 150°C and a pressure of 15 MPa for 60 minutes.

[0075] (Example 3-1) A sheet was prepared in the same manner as in Example 2-1, except that the filling rate of boron nitride powder was changed to 65 volume% to obtain the resin composition.

[0076] (Example 3-2) A sheet was prepared in the same manner as in Example 2-2, except that the filling rate of boron nitride powder was changed to 65 volume% to obtain the resin composition.

[0077] (Example 3-3) A sheet was prepared in the same manner as in Examples 2-3, except that the filling rate of boron nitride powder was changed to 65 volume% to obtain the resin composition.

[0078] (Comparative Example 3) A sheet was prepared in the same manner as in Comparative Example 2, except that the filling rate of boron nitride powder was changed to 65 volume% to obtain the resin composition.

[0079] (Example 4-1) A sheet was prepared in the same manner as in Example 2-1, except that the filling rate of boron nitride powder was changed to 60 volume% to obtain the resin composition.

[0080] (Example 4-2) A sheet was prepared in the same manner as in Example 2-2, except that the filling rate of boron nitride powder was changed to 60 volume% to obtain the resin composition.

[0081] (Example 4-3) A sheet was prepared in the same manner as in Examples 2-3, except that the filling rate of boron nitride powder was changed to 60 volume% to obtain the resin composition.

[0082] (Comparative Example 4) A sheet was prepared in the same manner as in Comparative Example 2, except that the filling rate of boron nitride powder was changed to 60 volume% to obtain the resin composition.

[0083] [Measurement of average particle size of boron nitride powder] The average particle size of the boron nitride powder obtained in each example and comparative example was measured in accordance with ISO 13320:2009 using a laser diffraction scattering particle size distribution analyzer (Beckman Coulter, Ltd., "LS-13 320"). However, the sample was not homogenized before measurement. For particle size distribution measurement, water was used as the solvent to disperse the boron nitride powder, and hexametaphosphoric acid was used as the dispersant. The refractive index of water was set to 1.33, and the refractive index of the boron nitride particles was set to 1.7.

[0084] [Measurement of Orientation Index] For the sheets prepared in each example and comparative example, X-ray diffraction peaks were measured using an X-ray diffractometer (Ultima IV-N), and the ratio of the peak intensity of the (002) plane to the peak intensity of the (100) plane was measured as the orientation index of boron nitride particles in the sheet. The measurement results of the orientation index are shown in Tables 1 to 4.

[0085] [Calculation of average values ​​X and Y] The mean values ​​X and Y were calculated using the following procedure. The results of calculating the mean values ​​X and Y are shown in Tables 1 to 4. (1) The cross-sections of the sheets prepared in each example and comparative example were observed by SEM at a magnification of 1000x, and observation images in bmp format were obtained. (2) The obtained observation images were imported into the image processing software "imageJ" and filtered using a median filter (3×3 pixels). Next, the region consisting of boron nitride particles and the other region (resin region) were binarized using the Otsu method to obtain a binarized image. (3) In the obtained binarized image, in region A with a short side of 50 μm and a long side of 100 μm, lines were drawn to divide region A into n equal parts in the direction of the short side and 2n equal parts in the direction of the long side, and the region was divided into multiple cells (where n is an integer of 1 or more). (4) First, region A was divided into two 50 μm × 50 μm cells with n=1. In one of the two cells, the total area of ​​the cell (2500 μm) 2 The area percentage of the region consisting of boron nitride particles was calculated based on the reference value. If the calculated area percentage was 80% or more, the cell was judged to be a boron nitride region; if the area percentage was less than 20%, the cell was judged to be a resin region. (5) The judgment described in (4) above was performed on each of the two cells, and the ratio of cells determined to be in the boron nitride region was calculated based on the total number of cells (2). (6) When n is between 2 and 200, the above steps (4) and (5) were performed, and the percentage of cells identified as being in the boron nitride region was calculated. (7) The total value was calculated from the proportion of cells that were determined to be boron nitride regions for each of n values ​​from 1 to 5. (8) The above steps (1) to (7) were performed on a total of 5 cross-sections of the sheet, and the total value was calculated for each cross-section. The average value X was calculated from the total value calculated from the 5 cross-sections. In addition, the average value Y of the proportion of cells determined to be boron nitride regions was calculated from the 5 cross-sections when n is 200 (when the short side direction of region A is divided into 200 equal parts and the long side direction into 400 equal parts). SEM images of the cross-section of the sheet of Example 1-1 are shown in Figure 3, and SEM images of the cross-section of the sheet of Comparative Example 1 are shown in Figure 4. Figure 5 shows a graph showing the relationship between the length of one side of a single cell when the sheet of Example 4-1 and the sheet of Comparative Example 4 are divided into multiple cells, and the proportion of cells determined to be boron nitride regions.

[0086] [Table 1]

[0087] [Table 2]

[0088] [Table 3]

[0089] [Table 4]

[0090] [Measurement of thermal conductivity] From the sheets prepared in Examples 1-2 and Comparative Example 1, 10 mm x 10 mm size samples were cut out, and the thermal diffusivity A(m²) of the sample was determined by the laser flash method using a xenon flash analyzer (NETZSCH, LFA447NanoFlash). 2 The specific gravity B (kg / m³) of the sample was measured. 3The thermal conductivity was measured by the Archimedes method. The specific heat capacity C (J / (kg·K)) of the sample was measured using a differential scanning calorimeter (Rigaku Corporation, ThermoPlusEvoDSC8230). Using these physical properties, the thermal conductivity H (W / (m·K)) was calculated from the formula H = A × B × C. The measured thermal conductivity of Example 1-2 was 24.4 W / (m·K), and the measured thermal conductivity of Comparative Example 1 was 17.2 W / (m·K).

Claims

1. A process to obtain boron carbonitride powder by nitriding boron carbide powder under hot isostatic pressure, The process involves decarburizing the boron carbonitride powder to obtain boron nitride powder, The process involves mixing the boron nitride powder with a resin to obtain a resin composition. A step of forming the resin composition into a sheet by pressurizing it, A method for manufacturing a sheet, comprising the following features.

2. It contains boron nitride particles and resin, A sheet in which the average value X calculated using the following steps (1) to (8) is 0.01 or greater. (1) Observe an image of any cross-section of the sheet at a magnification of 1000x using a scanning electron microscope (SEM). (2) The observed image is binarized into a region consisting of boron nitride particles and a region other than the observed image, and a binarized image is obtained. (3) In the binarized image, the region A, which is 50 μm on the short side and 100 μm on the long side, is divided into n equal parts in the direction of the short side and 2n equal parts in the direction of the long side, and then divided into multiple cells (where n is an integer of 1 or more). (4) In one of the plurality of cells, the area ratio of the region consisting of boron nitride particles is calculated based on the total area of ​​the one cell. If the area ratio is 80% or more, the one cell is determined to be a boron nitride region. If the area ratio is less than 20%, the one cell is determined to be a resin region. (5) The determination in (4) above is made for each of the plurality of cells, and the ratio of the number of cells determined to be in the boron nitride region is calculated based on the total number of the plurality of cells. (6) In the steps of (3) to (5) above, the proportion of cells determined to be in the boron nitride region when n is between 1 and 200 is calculated. (7) Calculate the sum of the proportions of cells that are determined to be boron nitride regions for each of n values ​​from 1 to 5. (8) Calculate the average value X from the total sum calculated in the steps (1) to (7) above for each of the five cross-sections of the sheet.

3. The sheet according to claim 2, wherein in the five cross-sections of the sheet, the average value Y of the ratio of the number of cells determined to be the boron nitride region in each section when n is 200 is 0.5 or more.

Images