JP7505139B2 - Boron nitride powder, heat dissipation sheet, and method for producing boron nitride powder - Google Patents

Description

The present disclosure relates to boron nitride powder, a heat dissipation sheet, and a method for producing boron nitride powder.

Boron nitride powder has lubricity, high thermal conductivity, and insulating properties, and is widely used as a solid lubricant, thermally conductive filler, insulating filler, etc. Boron nitride powder is used as a filler in heat dissipation components, which require particularly high thermal conductivity.

Hexagonal boron nitride primary particles have a relatively thin, scaly shape, and when filled into resin and molded, the primary particles tend to be oriented in a certain direction due to molding pressure, etc. For example, in a resin sheet that is filled with hexagonal boron nitride powder and molded into a sheet by extrusion molding, etc., the major surface of the resin sheet and the major axis of the primary particles of boron nitride are generally oriented so that they are parallel to each other. In addition, the primary particles of hexagonal boron nitride may also have anisotropy in various physical properties due to the anisotropy of their shape. The thermal conductivity of the primary particles of hexagonal boron nitride in the in-plane direction (a-axis direction) is high at about 400 W/(m·K), while the thermal conductivity in the thickness direction (c-axis direction) is only about 2 W/(m·K), and the anisotropy of the physical properties due to the direction is significant.

For the above reasons, a method has been considered in which hexagonal boron nitride powder is used as a filler for resin, and when preparing a heat dissipation sheet, the a-axis direction of the primary particles is adjusted so as to be parallel to the thickness direction of the heat dissipation sheet, thereby taking advantage of the high thermal conductivity in the a-axis direction of the primary particles. For example, a technique is known in which the a-axis direction of the primary particles of hexagonal boron nitride is oriented so as to be parallel to the thickness direction of the heat dissipation sheet (for example, Patent Document 1, etc.).

Also, from the viewpoint of reducing the anisotropy due to the shape as described above, a method of forming an aggregate composed of a plurality of primary particles, which are aggregated and fused together so that the orientation of the a-axis direction of adjacent primary particles differs, has been considered. Patent Document 2 discloses boron nitride aggregate particles formed by agglomerating primary particles of boron nitride, and describes that by increasing the strength of the aggregate particles to a degree that the collapse of the aggregate particles can be suppressed even when a predetermined molding pressure is applied, the primary particles of boron nitride are prevented from being oriented in the same direction.

JP 2000-154265 A JP 2016-135731 A

The manufacturing of a heat dissipation sheet in which the a-axis direction of the primary particles of hexagonal boron nitride is oriented in the thickness direction of the heat dissipation sheet requires complicated condition setting and many work steps, so it would be useful to have a boron nitride powder that can exhibit excellent heat dissipation properties using a simple method similar to that of general heat dissipation fillers. Furthermore, even if the agglomerated particles have relatively excellent strength and resistance to the molding pressure of the resin as described above, when used in combination with other heat dissipation fillers, the agglomerated particles may break down due to collisions with the other heat dissipation fillers during kneading with the resin, and the proportion of primary particles may increase, thereby compromising the effect of using the agglomerated particles. There is a demand for further improvement in the performance of the primary particles of hexagonal boron nitride themselves.

The present disclosure aims to provide a boron nitride powder containing primary particles of hexagonal boron nitride, which has excellent filling properties when filled into resin and can be used to prepare a heat dissipation sheet with excellent heat dissipation properties, and a method for producing the same. The present disclosure also aims to provide a heat dissipation sheet containing the above-mentioned boron nitride powder.

The present disclosure provides the following [1] to [7].
[1] Contains primary particles of hexagonal boron nitride having a scale shape,
A boron nitride powder having an average particle size of 4.0 to 15.0 μm, an orientation index of 25.0 or less, and a tap density of 0.70 g/cm ³ or more.
[2]
The boron nitride powder according to [1], wherein the average thickness of the primary particles is 0.8 to 2.0 μm.
[3]
The boron nitride powder according to [1] or [2], which has a specific surface area of 3.0 m ² /g or less.
[4]
The boron nitride powder according to any one of [1] to [3], having an orientation index of 10.0 or more.
[5]
A heat dissipating sheet comprising a resin and a heat dissipating filler,
A heat dissipating sheet, wherein the heat dissipating filler comprises the boron nitride powder according to any one of [1] to [4].
[6]
The heat dissipating sheet according to [5], wherein the heat dissipating filler further contains at least one of aluminum nitride and aluminum oxide.
[7]
a sintering step of sintering a raw material powder containing a carbon-containing compound, a boron-containing compound, and a sintering aid under a pressurized nitrogen atmosphere of 0.5 to 0.9 MPaG to obtain a sintered product containing primary particles of hexagonal boron nitride having a scale shape;
and a pulverization step of pulverizing the fired product to obtain a powder having an average particle size of 4.0 to 15.0 μm and an orientation index of 25.0 or less.
The method for producing boron nitride powder comprises maintaining the firing temperature in the firing step at 1800 to 2200° C. for 7 hours or more.

One aspect of the present disclosure provides a boron nitride powder comprising primary particles of hexagonal boron nitride having a scale shape, an average particle size of 4.0 to 15.0 μm, an orientation index of 25.0 or less, and a tap density of 0.70 g/ ^cm3 or more.

The boron nitride powder contains primary particles of hexagonal boron nitride having a scale shape, and the average particle size, orientation index, and tap density are within a predetermined range. This allows the powder to have excellent fillability in resin, and the resin molded body (e.g., a resin sheet) obtained by filling the powder in resin and molding the powder can exhibit excellent heat dissipation. When the thickness of the primary particles of hexagonal boron nitride is large relative to the particle size, the orientation between the particles tends to be suppressed, and when the particle size of the primary particles is large, the orientation between the particles tends to be promoted when an external force is applied. In addition, when the thickness of the primary particles of hexagonal boron nitride is large, the tap density increases, and the fillability in resin tends to be improved. Based on these findings, the inventors focused on the average particle size, orientation index, and tap density in boron nitride powder mainly composed of primary particles of boron nitride, and found that a powder prepared so that these values are within a predetermined range can exhibit boron nitride powder that is excellent in both fillability in resin and heat dissipation in a resin molded body obtained by filling the resin.

The average thickness of the primary particles may be 0.8 to 2.0 μm.

The boron nitride powder may have a specific surface area of 3.0 m ² /g or less.

The above boron nitride powder may have an orientation index of 10.0 or more.

One aspect of the present disclosure provides a heat dissipation sheet comprising a resin and a heat dissipation filler, the heat dissipation filler comprising the boron nitride powder described above.

The heat dissipating filler may further contain at least one of aluminum nitride and aluminum oxide.

One aspect of the present disclosure provides a method for producing boron nitride powder, comprising: a firing step of firing a raw material powder containing a carbon-containing compound, a boron-containing compound, and a sintering aid under a pressurized nitrogen atmosphere of 0.5 to 0.9 MPaG to obtain a fired product containing primary particles of hexagonal boron nitride having a scale shape; and a crushing step of crushing the fired product to obtain a powder having an average particle size of 4.0 to 15.0 μm and an orientation index of 25 or less, the firing step including maintaining the firing temperature at 1800 to 2200°C for 7 hours or more.

In the above-mentioned method for producing boron nitride powder, in the sintering step, the time for which the sintering temperature is reached and then maintained at that temperature is relatively long in a pressurized environment, promoting the growth of the primary particles of hexagonal boron nitride in the thickness direction, and in the subsequent crushing step, the loose association between the primary particles is broken down, thereby producing a powder having a predetermined particle size and composed mainly of primary particles of hexagonal boron nitride. Furthermore, the above-mentioned powder has a low orientation index for a powder composed of primary particles of hexagonal boron nitride.

According to the present disclosure, there can be provided a boron nitride powder containing primary particles of hexagonal boron nitride, which can be used to prepare a heat dissipation sheet that has excellent filling properties when filled into a resin and excellent heat dissipation properties, and a method for producing the same. According to the present disclosure, there can also be provided a heat dissipation sheet containing the above-mentioned boron nitride powder.

FIG. 1 is a schematic diagram showing an example of a heat dissipation sheet. FIG. 2 is a cross-sectional view taken along line II-II of FIG.

Below, embodiments of the present disclosure will be described with reference to the drawings where appropriate. However, the following embodiments are merely examples for explaining the present disclosure, and are not intended to limit the present disclosure to the following contents. In the description, the same elements or elements having the same functions will be designated by the same reference numerals, and overlapping descriptions will be omitted where appropriate. In addition, unless otherwise specified, the positional relationships such as up, down, left, right, etc. will be based on the positional relationships shown in the drawings. Furthermore, the dimensional ratios of each element are not limited to the ratios shown in the drawings. In this specification, a numerical range indicated by the symbol "~" includes a lower limit and an upper limit. In other words, a numerical range indicated by "x~y" means a range equal to or greater than x and equal to or less than y.

Unless otherwise specified, the materials exemplified in this specification may be used alone or in combination of two or more. When multiple substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the multiple substances present in the composition, unless otherwise specified.

One embodiment of the boron nitride powder includes primary particles of hexagonal boron nitride having a scale shape. The boron nitride powder has an average particle size of 4.0 to 15.0 μm, an orientation index of 25.0 or less, and a tap density of 0.70 g/ ^cm3 or more.

The upper limit of the average particle size of the boron nitride powder may be, for example, 14.9 μm or less, or 14.8 μm or less. By suppressing the particle size so that the upper limit of the average particle size falls within the above range, the orientation of the primary particles is suppressed, and the increase in the orientation index of the boron nitride powder can be further suppressed. The lower limit of the average particle size of the boron nitride powder may be, for example, 4.5 μm or more, 5.0 μm or more, 5.5 μm or more, 6.0 μm or more, 6.5 μm or more, 7.0 μm or more, 7.5 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. By the lower limit of the average particle size being within the above range, the filling property into the resin and the heat dissipation property of the obtained resin molded body (e.g., heat dissipation sheet) can be achieved at a higher level. The average particle size of the boron nitride powder may be adjusted within the above range, and may be, for example, 5.0 to 15.0 μm, 8.0 to 15.0 μm, or 10.0 to 15.0 μm.

In this specification, the average particle size means the 50% cumulative diameter (median diameter) in the cumulative particle size distribution based on volume. More specifically, it means the particle size (D50) when the cumulative value in the cumulative particle size distribution based on volume obtained by the laser diffraction scattering method for the powder becomes 50%. The laser diffraction scattering method is measured in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - laser diffraction and scattering method". For the measurement, a laser diffraction scattering method particle size distribution measuring device or the like can be used. For the laser diffraction scattering method particle size distribution measuring device, for example, "LS-13 320" (product name) manufactured by Beckman Coulter, Inc. can be used. In addition, if the powder to be measured is such that the primary particles are loosely associated with each other, the measurement is performed after processing with a homogenizer or the like. For the homogenizer device, "VC-505" (product name) manufactured by Ieda Trading Co., Ltd. can be used. The homogenizer processing may be performed, for example, at a frequency of 20 KHz for 2 minutes.

The boron nitride powder in the present disclosure is mainly composed of primary particles of hexagonal boron nitride. The boron nitride powder may contain a small amount of particles in which multiple primary particles are aggregated, or may not contain any particles in which multiple primary particles are aggregated. The boron nitride powder may have one peak in the volume-based particle size distribution measured without ultrasonic dispersion treatment using a homogenizer in the particle size distribution measurement by the laser diffraction scattering method, and the ratio (D50(A)) of the average particle size measured by ultrasonic dispersion treatment under the above-mentioned conditions to the average particle size measured without ultrasonic dispersion treatment (D50(B)) (value of D50(A)/D50(B)) may be 0.85 or more. The value of D50(A)/D50(B) may be, for example, 1.00 or less, or less than 1.00.

The upper limit of the orientation index of the boron nitride powder may be, for example, 24.8 or less, 24.6 or less, 24.4 or less, 24.0 or less, 23.5 or less, 23.0 or less, 22.5 or less, 22.0 or less, 21.5 or less, 21.0 or less, 20.5 or less, or 20.0 or less. When the upper limit of the orientation index is within the above range, the thermal anisotropy of boron nitride can be further reduced and the properties as a heat dissipation filler can be improved. The lower limit of the orientation index of the boron nitride powder may be, for example, 10.0 or more, 10.5 or more, 11.0 or more, 11.5 or more, or 12.0 or more. The orientation index can be an indicator that the proportion of aggregates in the boron nitride powder is small and that the powder contains primary particles having an appropriate thickness. By setting the lower limit of the orientation index within the above range, it is possible to achieve a higher level of both the fillability into the resin and the heat dissipation of the resulting resin molded body (e.g., a heat dissipation sheet). The orientation index of the boron nitride powder may be adjusted within the above range, and may be, for example, 10.0 to 25.0, or 12.0 to 24.8.

The orientation index in this specification refers to a value measured according to the following method. An X-ray diffraction spectrum of the boron nitride powder is obtained by performing X-ray diffraction measurement on the boron nitride powder, and the peak intensities I(002) and I(100) corresponding to the (002) and (100) planes are obtained from the X-ray diffraction spectrum. The orientation index [I(002)/I(100)] of the boron nitride powder is calculated using the obtained peak intensities. As an X-ray diffraction device, for example, "ULTIMA-IV" (product name) manufactured by Rigaku Corporation can be used.

In addition, since the orientation index is measured on boron nitride powder, when the powder contains clumped particles (agglomerated particles) in which the primary particles are not substantially oriented and the proportion of these particles is large, the orientation index value tends to approach a value of about 6 to 7. On the other hand, when the powder is composed of primary particles that do not contain agglomerated particles, or when the proportion of the primary particles is large, the orientation index value tends to be large. Furthermore, when the average particle diameter of the primary particles is large or the average thickness of the primary particles is small, orientation between the particles is easier, and the orientation index value tends to be large.

The lower limit of the tap density of the boron nitride powder may be, for example, 0.72 g/cm ³ or more, 0.75 g/cm ³ or more, 0.80 g/cm ³ or more, or 0.85 g/cm ³ or more. When the lower limit of the tap density is within the above range, the filling property of the boron nitride powder in the resin can be further improved. The upper limit of the tap density of the boron nitride powder is not particularly specified, but may be, for example, 1.00 g/cm ³ or less, 0.98 g/cm ³ or less, 0.96 g/cm ³ or less, or 0.94 g/cm ³ or less. The tap density of the boron nitride powder may be adjusted within the above range, and may be, for example, 0.70 to 1.00 g/cm ³ , or 0.85 to 1.00 g/cm ³ .

The tap density in this specification means a value obtained according to the method described in JIS R 1628:1997 "Method for measuring bulk density of fine ceramic powders". Specifically, boron nitride powder is filled into a ¹⁰⁰ cm3 dedicated container, and tapped under the conditions of a tapping time of 180 seconds, tapping number of 180 times, and tap lift of 18 mm, and the bulk density is measured, and the obtained value is taken as the tap density. For the measurement, a commercially available device can be used, such as "Powder Tester" (product name) manufactured by Hosokawa Micron Co., Ltd.

The primary particles of hexagonal boron nitride having a scale shape contained in the boron nitride powder according to the present disclosure have a relatively small particle diameter and a relatively large thickness, so that the specific surface area of the entire boron nitride powder is also suppressed to be low. The upper limit of the specific surface area of the boron nitride powder may be, for example, 3.0 m ² /g or less, 2.5 m ² /g or less, 2.0 m ² /g or less, or 1.5 m ² /g or less. The boron nitride powder having the upper limit of the specific surface area within the above range can exhibit excellent filling properties in resin. The lower limit of the specific surface area of the boron nitride powder is not particularly limited, but may be, for example, 0.5 m ² /g or more, 0.6 m ² /g or more, 0.7 m ² /g or more, or 0.8 m ² /g or more. The specific surface area of the boron nitride powder may be adjusted within the above range, and may be, for example, 0.5 to 3.0 m ² /g.

In this specification, the specific surface area refers to a value measured using a specific surface area measuring device in accordance with the description of JIS Z 8830:2013 "Method for measuring the specific surface area of powders (solids) by gas adsorption," and is a value calculated by applying the BET single point method using nitrogen gas. As a specific surface area measuring device, for example, "MONOSORB MS-22 type" (product name) manufactured by QUANTACROME Co., Ltd. can be used.

The lower limit of the average thickness of the primary particles of hexagonal boron nitride may be, for example, 0.8 μm or more, 0.9 μm or more, 1.0 μm or more, 1.1 μm or more, or 1.2 μm or more. By having the lower limit of the average thickness within the above range, the orientation of the primary particles when kneaded with the resin is suppressed, and the heat dissipation of the obtained resin molded body (for example, a heat dissipation sheet) can be further improved. The upper limit of the average thickness is not particularly limited, but may be, for example, 3.0 μm or less, 2.5 μm or less, or 2.0 μm or less. The average thickness of the primary particles of hexagonal boron nitride may be adjusted within the above range, and may be, for example, 0.8 to 2.0 μm. By having the thickness of the primary particles of hexagonal boron nitride within the above range, it is possible to further suppress the particles from being oriented in one direction during sheet molding.

The average thickness of the primary particles in this specification means a value measured by the method shown below. Specifically, first, 3 g of powder is molded into a disk shape (diameter: 30 mm) at a pressure of 5 MPa using a press molding machine. Next, the molded body obtained as described above is embedded in resin (manufactured by GATAN Co., Ltd., product name: G2 Epoxy). Then, a sample in which the cross section of the primary particles of hexagonal boron nitride is exposed is prepared by performing cross-sectional milling in a direction parallel to the direction in which pressure was applied during press molding. This cross section is photographed using a scanning electron microscope. The obtained particle image is imported into image analysis software (manufactured by Mountec Co., Ltd., product name: Mac-View), and the short side of the rectangular particle (corresponding to the particle thickness and particle short diameter) is measured from the obtained photograph. The measurement is performed on 100 arbitrarily selected primary particles, and the arithmetic average value is taken as the average thickness of the primary particles. As a press molding machine, for example, "BRE-32" (product name) manufactured by Rigaku Corporation can be used. As the scanning electron microscope, for example, "JSM-6010LA" (product name) manufactured by JEOL Ltd. can be used.

The above-mentioned boron nitride powder can be manufactured, for example, by the following method. One embodiment of the method for manufacturing boron nitride powder includes a sintering step in which a raw material powder containing a carbon-containing compound, a boron-containing compound, and a sintering aid is sintered under a pressurized nitrogen atmosphere of 0.5 to 0.9 MPaG to obtain a sintered product containing primary particles of hexagonal boron nitride having a scale shape, and a crushing step in which the sintered product is crushed to obtain a powder having an average particle size of 4.0 to 15.0 μm and an orientation index of 25.0 or less. The sintering step includes maintaining the sintering temperature at 1800 to 2200° C. for 7 hours or more.

A carbon-containing compound is a compound having carbon atoms as a constituent element. The carbon-containing compound reacts with a boron-containing compound and a compound having nitrogen atoms as a constituent element to form boron nitride. As the carbon-containing compound, a raw material having high purity and relatively low cost can be used. Examples of such carbon-containing compounds include carbon black and acetylene black.

A boron-containing compound is a compound having boron as a constituent element. A boron-containing compound is a compound that reacts with a carbon-containing compound and a compound having a nitrogen atom as a constituent element to form boron nitride. A raw material having high purity and relatively low cost can be used as the boron-containing compound. Examples of such boron-containing compounds include boric acid and boron oxide. The boron-containing compound preferably contains boric acid. In this case, boric acid is dehydrated by heating to become boron oxide, which forms a liquid phase during the heat treatment of the raw material powder and can also act as an auxiliary agent to promote grain growth.

The sintering aid reacts with the boron-containing compound to form a liquid phase, promoting the growth of primary particles of boron nitride. Examples of the sintering aid include oxides and carbonates of alkali metals, and oxides and carbonates of alkaline earth metals. More specifically, examples of the sintering aid include sodium oxide, sodium carbonate, calcium oxide, and calcium carbonate.

In the raw material powder, the boron-containing compound may be blended in an excess amount relative to the carbon-containing compound. The raw material powder may contain other compounds in addition to the carbon-containing compound, the boron-containing compound, and the sintering aid. Examples of other compounds include boron nitride as a nucleating agent. When the raw material powder contains boron nitride as a nucleating agent, the average particle size of the synthesized hexagonal boron nitride powder can be more easily controlled. The raw material powder preferably contains a nucleating agent. When the raw material powder contains a nucleating agent, it becomes easier to prepare a hexagonal boron nitride powder with a small specific surface area.

The firing process is carried out in a pressurized environment. The lower limit of the atmospheric pressure in the firing process may be, for example, 0.5 MPaG or more, 0.6 MPaG or more, 0.7 MPaG or more, or 0.8 MPaG or more. By setting the lower limit of the atmospheric pressure within the above range, the volatilization of the boron-containing compound is suppressed, and the liquid phase of the boron-containing compound is maintained, thereby further promoting the growth of the primary particles of hexagonal boron nitride and suppressing the generation of boron carbide, which is a by-product. The upper limit of the atmospheric pressure in the firing process is not particularly limited, but may be 0.9 MPaG or less industrially. The atmospheric pressure in the firing process may be adjusted within the above range, and may be, for example, 0.5 to 0.9 MPaG. In this specification, pressure means gauge pressure.

The firing temperature in the firing step is, for example, 1800 to 2200°C. The upper limit of the firing temperature may be, for example, 2150°C or less, or 2100°C or less. By setting the upper limit of the firing temperature within the above range, the generation of by-products can be sufficiently suppressed. The lower limit of the firing temperature may be, for example, 1850°C or more, 1900°C or more, 1950°C or more, 2000°C or more, or 2050°C or more. By setting the lower limit of the firing temperature within the above range, the reaction on the carbon-containing compound can be promoted, and the yield of boron nitride obtained can be further improved.

In the manufacturing method according to the present disclosure, in the firing step, the time (holding time) during which the firing temperature is reached and then held at that temperature is relatively long under the pressure environment described above, thereby promoting the growth of the primary particles of hexagonal boron nitride in the thickness direction. The lower limit of the holding time in the firing step is 7 hours or more, but may be, for example, 8 hours or more. By setting the lower limit of the holding time within the above range, the thickness of the primary particles of hexagonal boron nitride can be increased, the orientation between the primary particles can be further reduced, and a boron nitride powder with a larger tap density can be prepared. The upper limit of the holding time in the firing step is not particularly limited, but may be, for example, 20 hours or less, 18 hours or less, 16 hours or less, 14 hours or less, or 12 hours or less, from the viewpoint of reducing the manufacturing cost of the boron nitride powder. The holding time in the firing step may be adjusted within the above range, and may be, for example, 7 to 20 hours, or 7 to 12 hours.

In the grinding process, the fired product obtained in the firing process is crushed to obtain a powder having an average particle size of 4.0 to 15.0 μm and an orientation index of 25.0 or less.

In the crushing process, a crusher such as a Henschel mixer or a grinder mill can be used.

When a Henschel mixer is used, the rotation speed of the crusher may be as follows: The upper limit of the rotation speed of the crusher may be, for example, 950 rpm or less, 900 rpm or less, or 850 rpm or less. By having the upper limit of the rotation speed of the crusher within the above range, over-crushing of the particles can be suppressed. The lower limit of the rotation speed of the crusher may be, for example, 500 rpm or more, or 550 rpm or more. By having the lower limit of the rotation speed of the crusher within the above range, the fired material can be sufficiently crushed and loose agglomerations of the boron nitride primary particles can be released.

The lower limit of the disintegration time in the disintegration step may be, for example, 5 minutes or more, 6 minutes or more, 7 minutes or more, or 8 minutes or more. By setting the lower limit of the disintegration time within the above range, the loose agglomeration of the primary particles of hexagonal boron nitride can be more sufficiently released. The upper limit of the disintegration time in the disintegration step may be, for example, 15 minutes or less, 14 minutes or less, 13 minutes or less, or 12 minutes or less. By setting the upper limit of the disintegration time within the above range, the fired product can be sufficiently disintegrated, and the collapse of the primary particles of hexagonal boron nitride themselves can be more suppressed. The disintegration time may be adjusted within the above range, for example, 5 to 15 minutes, or 8 to 12 minutes.

The above-mentioned boron nitride powder has excellent filling properties for resin and can suppress the orientation of primary particles in a resin molded sheet, so it can be suitably used as a heat dissipating filler. One embodiment of the heat dissipating sheet is a heat dissipating sheet containing a resin and a heat dissipating filler. The heat dissipating filler contains the above-mentioned boron nitride powder.

Figure 1 is a schematic diagram showing an example of a heat dissipation sheet. Figure 2 is a cross-sectional view taken along line II-II in Figure 1. The heat dissipation sheet 100 includes a resin part 10 and a plurality of primary particles 20 of hexagonal boron nitride filled in the resin part 10. In the heat dissipation sheet 100, the primary particles 20 are relatively thick, and therefore the main surface of the heat dissipation sheet 100 and the a-axis of the primary particles are not parallel, but are maintained at a moderate inclination. This allows the heat dissipation sheet 100 to exhibit sufficient heat dissipation properties in the thickness direction as well.

The orientation of the primary particles of hexagonal boron nitride in the heat dissipation sheet can be confirmed by examining the ratio of the peak intensity of the <002> plane to the peak intensity of the <100> plane (a value expressed as [peak intensity of the <002> plane]/[peak intensity of the <100> plane]) in the X-ray diffraction spectrum obtained by irradiating the sheet with X-rays in the thickness direction. The upper limit of the ratio of the peak intensity of the <002> plane to the peak intensity of the <100> plane may be, for example, 60 or less, or 50 or less. By keeping the upper limit of the above value within the above range, the heat dissipation sheet can be provided with high heat dissipation properties. The lower limit of the ratio of the peak intensity of the <002> plane to the peak intensity of the <100> plane may be, for example, 7 or more, 8 or more, or 9 or more.

The peak intensity ratios in this specification can be calculated using values measured by the method described below. A square measurement sample measuring 10 mm in length and 10 mm in width is cut out from the heat dissipation sheet to be measured. An X-ray is irradiated in the thickness direction of the measurement sample using an X-ray diffraction analyzer to obtain an X-ray diffraction spectrum. In the obtained X-ray diffraction spectrum, the peak intensity of the <100> plane and the peak intensity of the <002> plane are measured, and the ratio between them can be calculated to obtain the peak intensity ratio. As an X-ray diffraction analyzer, for example, "Ultima-IV" (product name) manufactured by Rigaku Corporation can be used.

The lower limit of the content of the boron nitride powder in the heat dissipation sheet may be, for example, 30 volume % or more, 40 volume % or more, or 50 volume % or more, based on the total volume of the heat dissipation sheet. When the lower limit of the content of the boron nitride powder is within the above range, the heat dissipation properties of the heat dissipation sheet can be further improved. The upper limit of the content of the boron nitride powder in the heat dissipation sheet may be, for example, 85 volume % or less, 80 volume % or less, or 70 volume % or less, based on the total volume of the heat dissipation sheet. When the upper limit of the content of the boron nitride powder is within the above range, it is possible to further suppress the generation of voids inside the heat dissipation sheet when it is molded, and also to suppress the deterioration of insulation and mechanical strength.

The resin part 2 may contain a cured resin or may be made of a cured resin. Examples of the cured resin constituting the resin part 2 include epoxy resin, phenolic resin, melamine resin, urea resin, polyimide, polyamideimide, polyetherimide, and maleimide-modified resin.

The lower limit of the content of the cured resin in the heat dissipation sheet may be, for example, 15 volume % or more, 20 volume % or more, or 30 volume % or more, based on the total volume of the heat dissipation sheet. The upper limit of the content of the cured resin in the heat dissipation sheet may be, for example, 70 volume % or less, 60 volume % or less, or 50 volume % or less, based on the total volume of the heat dissipation sheet.

The above-mentioned heat dissipation sheet can be prepared by, for example, performing heating and pressure molding of a resin composition containing a boron nitride powder containing primary particles of hexagonal boron nitride having a scale shape and a thermosetting resin. The above-mentioned resin composition may contain other components, for example, a curing agent. The curing agent may be appropriately selected depending on the type of thermosetting resin. For example, when the resin is an epoxy resin, examples of the curing agent include a phenol novolac compound, an acid anhydride, an amino compound, and an imidazole compound. The lower limit of the content of the curing agent may be, for example, 0.5 parts by mass or more, or 1.0 parts by mass or more, per 100 parts by mass of the resin. The upper limit of the content of the curing agent may be, for example, 15 parts by mass or less, or 10 parts by mass or less, per 100 parts by mass of the resin.

Since the above-mentioned boron nitride powder is mainly composed of primary particles, it can also be used in combination with other heat dissipating fillers. In addition to the above-mentioned boron nitride powder, the heat dissipating filler may further contain, for example, at least one of aluminum nitride and aluminum oxide, and preferably contains aluminum nitride. In the case of a powder that is mainly composed of aggregated particles formed by aggregation of primary particles of hexagonal boron nitride, when used in combination with other heat dissipating fillers, the aggregates may collapse due to collisions between fillers during kneading with resin, and the expected performance may not be achieved, so control of kneading conditions, etc. is required.

Although several embodiments have been described above, the present disclosure is in no way limited to the above-described embodiments. Furthermore, the descriptions of the above-described embodiments can be mutually applied.

The present disclosure will be described in more detail below using examples and comparative examples. Note that the present disclosure is not limited to the following examples.

Example 1
A raw material powder was obtained by mixing 100 parts by mass of boric acid (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 26 parts by mass of acetylene black (manufactured by Denka Co., Ltd., grade name: Li-400), and 3.4 parts by mass of sodium carbonate (manufactured by New Lime Co., Ltd.) using a Henschel mixer. The obtained mixed powder was placed in a dryer at 250°C and held for 3 hours to dehydrate the boric acid. The dehydrated mixed powder was placed in a mold with a diameter of 100 mm in a press molding machine and molded under conditions of a heating temperature of 200°C and a pressing pressure of 30 MPa. The pellets of the raw material powder obtained in this manner were subjected to the subsequent heat treatment.

First, pellets of the raw material powder were placed in a carbon atmosphere furnace, and the temperature was increased to 1900°C at a heating rate of 5°C/min in a nitrogen atmosphere pressurized to 0.5 MPaG. The pellets were then heated at 1900°C for 8 hours to obtain a fired product (firing process).

The resulting fired product was crushed in a Henschel mixer at a rotation speed of 900 rpm for a crushing time of 10 minutes to prepare a powder (crushing process). The average particle size, orientation index, tap density, specific surface area, average thickness of primary particles, and D50(A)/D50(B) values of the resulting powder were measured. The results are shown in Table 1.

<Evaluation of filling properties of boron nitride powder>
The filling property of the obtained boron nitride powder when used as a filler for resin was evaluated. Specifically, the boron nitride powder was mixed with silicone oil (manufactured by Shin-Etsu Chemical Co., Ltd., product name: KF96L) so that the amount was 20 volume % based on the total amount of the resin composition, and a slurry was prepared by stirring at 2000 rpm for 2 minutes using a rotation/revolution mixer (manufactured by Thinky Corporation, product name: Awatori Rentaro RE-310). The viscosity of the slurry was measured at 25°C in the range of 0.01 to 100 sec ^-1 using a rheometer (manufactured by Anton Paar Japan, product name: MCR302, parallel plate (diameter: 25 mmφ), gap: 1 mm), and the filling property was evaluated based on the following criteria from the obtained results. The results are shown in Table 1. In addition, if the viscosity of the resin composition containing boron nitride powder is high, it will be difficult to handle and mold, so it will be necessary to reduce the amount of boron nitride powder, and it will tend to be difficult to increase the amount of powder that can be filled into the molded body. Therefore, it is desirable to have a low shear viscosity obtained by this evaluation.
A: The viscosity at a shear rate of 20 rpm is 1000 mPa·s or less.
B: The viscosity at a shear rate of 20 rpm is more than 1000 mPa·s and not more than 2000 mPa·s.
C: Viscosity exceeds 2000 mPa·s at a shear rate of 20 rpm.

<Evaluation of heat dissipation properties of heat dissipation sheets containing boron nitride powder>
The obtained boron nitride powder was used as a filler in resin and its heat dissipation properties were evaluated.

[Preparation of evaluation sheet]
A resin composition was obtained by mixing 100 parts by mass of naphthalene-type epoxy resin (manufactured by DIC Corporation, product name: HP4032) and 10 parts by mass of an imidazole compound (manufactured by Shikoku Chemical Industry Co., Ltd., product name: 2E4MZ-CN) as a curing agent with 60% by volume of boron nitride powder. A Thinky Mixer manufactured by Thinky Corporation was used for kneading with the resin. The kneading conditions were 1600 rpm and 3 minutes. The obtained resin composition was applied to a PET film to a thickness of 0.3 mm. Thereafter, a 0.3 mm resin sheet (evaluation sheet) was produced by heating and pressing under relatively mild conditions of a temperature of 150°C and 50 kgf/ ^cm2 for 60 minutes.

[Measurement of thermal conductivity]
The thermal conductivity H (unit: W/(m·K)) in the uniaxial press direction of the obtained evaluation laminate sheet was calculated by the formula H=T×D×C using the thermal diffusivity T (unit: m ² /sec), density D (unit: kg/m ³ ), and specific heat capacity C (unit: J/(kg·K)). The thermal diffusivity T was measured by the laser flash method using a sample processed into a size of length×width×thickness=10 mm×10 mm×0.3 mm from the evaluation laminate sheet. The measurement device used was a xenon flash analyzer (manufactured by NETZSCH, product name: LFA447NanoFlash). The density D was measured by the Archimedes method. The specific heat capacity C was measured using a differential scanning calorimeter (manufactured by Rigaku, device name: ThermoPlusEvo DSC8230). Based on the obtained results, the evaluation was performed according to the following criteria. The results are shown in Table 1.
A: Thermal conductivity is 7 W/mK or more.
B: Thermal conductivity is 4 W/mK or more and less than 7 W/mK.
C: Thermal conductivity is less than 4 W/mK.

Example 2
A boron nitride powder was prepared in the same manner as in Example 1, except that the pressure in the sintering step was 0.8 MPaG and the sintering temperature was 2050° C. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 3
A boron nitride powder was prepared in the same manner as in Example 1, except that the firing temperature in the firing step was changed to 2000° C. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

Example 4
A boron nitride powder was prepared in the same manner as in Example 1, except that the pressure in the sintering step was 0.9 MPaG, the sintering temperature was 2100° C., and the time for holding at 2100° C. was 12 hours. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 1)
100 parts by mass of boric acid (manufactured by Kojundo Chemical Laboratory Co., Ltd.), 9 parts by mass of melamine (manufactured by Fujifilm Wako Pure Chemical Co., Ltd.), and 13 parts by mass of sodium carbonate (manufactured by Fujifilm Wako Pure Chemical Co., Ltd.) were added and mixed under humidification to obtain a raw material powder. The obtained raw material powder was heat-treated at 1000°C for 2 hours under a nitrogen atmosphere at atmospheric pressure (0 MPaG) using a tubular furnace to obtain a heat-treated product (calcination process). 100 parts by mass of the obtained heat-treated product was heated to 1900°C under a nitrogen atmosphere at atmospheric pressure (0 MPaG) using an electric furnace, and calcined at 1900°C for 6 hours to obtain a boron nitride powder (calcination process). The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 2)
A boron nitride powder was obtained in the same manner as in Comparative Example 1, except that the firing temperature in the firing step was 1800° C. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

(Comparative Example 3)
A boron nitride powder was obtained in the same manner as in Comparative Example 1, except that the firing temperature in the firing step was 1700° C. and the time for holding at 1700° C. was 4 hours. The obtained boron nitride powder was evaluated in the same manner as in Example 1. The results are shown in Table 1.

According to the present disclosure, there can be provided a boron nitride powder containing primary particles of hexagonal boron nitride, which can be used to prepare a heat dissipation sheet that has excellent filling properties when filled into a resin and excellent heat dissipation properties, and a method for producing the same. According to the present disclosure, there can also be provided a heat dissipation sheet containing the above-mentioned boron nitride powder.

10...resin part, 20...primary particles, 100...heat dissipation sheet.

Claims (7)

A boron nitride powder containing primary particles of hexagonal boron nitride having a scale shape,
The average particle size is 4.0 to 15.0 μm, the orientation index is 25.0 or less, and the tap density is 0.70 g/ ^cm3 or more ,
The boron nitride powder has a particle size distribution measured by a laser diffraction scattering method, in which the volume-based particle size distribution measured without performing ultrasonic dispersion treatment using a homogenizer has one peak, and the ratio (D50(A)) of the average particle size measured with ultrasonic dispersion treatment to the average particle size measured without performing ultrasonic dispersion treatment (D50(B)) (the value of D50(A)/D50(B)) is 0.85 or more . The boron nitride powder according to claim 1, wherein the average thickness of the primary particles is 0.8 to 2.0 μm. 3. The boron nitride powder according to claim 1, having a specific surface area of 3.0 m ² /g or less. The boron nitride powder according to claim 1 or 2, having an orientation index of 10.0 or more. A heat dissipating sheet comprising a resin and a heat dissipating filler,
A heat dissipating sheet, wherein the heat dissipating filler comprises the boron nitride powder according to claim 1 or 2. The heat dissipation sheet according to claim 5, wherein the heat dissipation filler further contains at least one of aluminum nitride and aluminum oxide. a firing step of firing a raw material powder containing a carbon-containing compound, a boron-containing compound, and a sintering aid under a pressurized nitrogen atmosphere of 0.5 to 0.9 MPaG to obtain a fired product containing primary particles of scaly hexagonal boron nitride;
and a pulverization step of pulverizing the fired product to obtain a powder having an average particle size of 4.0 to 15.0 μm and an orientation index of 25.0 or less.
The method for producing boron nitride powder comprises maintaining the firing temperature in the firing step at 1800 to 2200° C. for 7 hours or more.

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