JP6905719B2 - Hexagonal boron nitride single crystal, composite material composition containing the hexagonal boron nitride single crystal, and heat-dissipating member formed by molding the composite material composition. - Google Patents

Description

The present invention relates to a hexagonal boron nitride single crystal in which growth in the c-axis direction is promoted and a method for producing the same.

Hexagonal boron nitride (h-BN) is widely used in the field of electrical and electronic materials because it has excellent thermal conductivity, solid lubricity, chemical stability, and heat resistance.
In recent years, especially in the fields of electricity and electronics, heat generation due to high density of integrated circuits has become a big problem, and how to dissipate heat has become an urgent issue. Since h-BN has high thermal conductivity despite its insulating property, it is attracting attention as such a thermally conductive filler for heat-dissipating members.

h-BN is a plate-like crystal, and the inside of the plate surface (inside the ab plane) is strongly bonded by a covalent bond, but the thickness direction (c-axis direction) is weakly bonded by a van der Waals force. Not too much. Therefore, a large thermal conductivity anisotropy occurs in the plate surface (usually, the thermal conductivity is about 250 W / mK) and in the thickness direction (usually, the thermal conductivity is about 2 to 3 W / mK).
Generally, when a composite composition is prepared by blending a plate-shaped crystal as a filler with a resin or the like, the plate surface of the plate-shaped crystal is oriented in a specific direction in a process such as raw material mixing, press molding, or injection molding. The phenomenon occurs. Therefore, when a composite material composition is prepared by blending h-BN plate crystals as a filler with a resin or the like, the obtained composite material composition has high thermal conductivity in a specific direction but in a direction perpendicular to the heat conduction. There arises a problem that the thermal conduction anisotropy of the h-BN plate-like crystal is transferred, such as low thermal conductivity.

Conventionally, in order to prevent such transfer of thermal conduction anisotropy, a method of aggregating h-BN plate crystals in a random direction in advance and then using them as a filler has been studied (Patent Documents 1, 2, 3 and 3). 4). However, such an agglomerated filler has voids in the filler, so that the amount of filling into the composite composition is restricted, and heat is generated by phonon scattering at the interface between the h-BN plate-like crystals constituting the agglomerated filler. There was a problem that conduction was hindered.

If an h-BN single crystal having an aspect ratio close to 1 defined by "maximum thickness in the c-axis direction of crystal / maximum width of crystal ab plane" can be used as a filler, the heat conduction difference as described above can be used. Directional transcription does not occur. Further, since such a filler does not have a large void or a crystal interface inside, restrictions on the filling amount and inhibition of heat conduction are alleviated. However, since the growth rate of the h-BN crystal in the c-axis direction is usually very small compared to the growth rate of the ab plane, there has been a method for growing an h-BN single crystal having an aspect ratio close to 1. I didn't. Further, even if an attempt is made to produce an h-BN single crystal having an aspect ratio close to 1 by crushing a plate-shaped h-BN single crystal, cleavage is likely to occur on the ab plane that is weakly bonded by the Van der Waals force. It is expected that the crushed product produced will eventually become a plate-shaped h-BN crystal.
On the contrary, to produce an h-BN single crystal having an aspect ratio close to 1 from an h-BN single crystal having an aspect ratio greater than 1, ab the h-BN single crystal having an aspect ratio greater than 1. It is expected that it will be easy because it is only necessary to open the surface. Therefore, it has been desired to establish a method for easily producing an h-BN single crystal in which the c-axis direction growth of the h-BN crystal is promoted more than the ab plane growth.

Japanese Unexamined Patent Publication No. 2006-257392 Japanese Patent Application Laid-Open No. 2008-510878 Japanese Unexamined Patent Publication No. 9-202663 WO2013 / 081061 Pamphlet WO2006 / 087982 Pamphlet

Donev, Aleksandar, et al. "Improving the density of jammed disordered packings using ellipsoids." Science 303.5660 (2004): 990-993.

The present invention has been made in view of the above problems, and provides an h-BN single crystal having an aspect ratio close to 1, specifically, an h-BN single crystal having an aspect ratio of 0.3 or more. It is an object of the present invention to provide a production method capable of easily producing such an h-BN single crystal having an aspect ratio of 0.3 or more.

As a result of diligent studies, the present inventors prepare an h-BN single crystal having the above aspect ratio of 0.3 or more by a flux method using a specific boron nitride raw material and a lithium salt as a flux. I found that I could do it. Conventionally, h-BN crystals have been produced using flux, but the obtained h-BN crystals are plate-shaped, and there is an example in which an h-BN single crystal having an aspect ratio close to 1 is produced. It does not exist (see Patent Document 5). The present invention has been achieved based on such findings, and the gist of the present invention is as follows.

[1] A hexagonal boron nitride single crystal having an aspect ratio of 0.3 or more defined by the maximum thickness in the c-axis direction of the crystal / the maximum width of the crystal ab plane.
[2] The hexagonal boron nitride single crystal according to [1], wherein the maximum width of the crystal ab plane is 100 nm or more.
[3] The hexagonal boron nitride single crystal according to [1] or [2], which is an automorphic crystal or a semi-automorphic crystal.
[4] A method for producing a hexagonal boron nitride single crystal, which comprises a step of mixing a boron nitride powder having a peak half width of 0.4 ° or more on the 002 surface and a lithium salt in XRD analysis and heating the mixture.
[5] The production method according to [4], wherein the hexagonal boron nitride single crystal has a maximum width of 100 nm or more on the crystal ab surface.
[6] The production method according to [4] or [5], wherein the hexagonal boron nitride single crystal is an automorphic crystal or a semi-automorphic crystal.
[7] Hexagonal boron nitride single crystal according to any one of [1] to [3], or hexagonal boron nitride single crystal produced by the production method according to any one of [4] to [6]. A composite composition obtained by blending crystals with a matrix.
[8] A heat radiating member obtained by molding the composite composition according to [7].

INDUSTRIAL APPLICABILITY The present invention provides an h-BN single crystal in which the c-axis direction growth of the h-BN crystal is promoted. The composite composition in which the h-BN single crystal of the present invention is blended in a matrix such as a resin does not cause the transfer of thermal conduction anisotropy, which is a problem when the conventional h-BN single crystal is used. Further, since the h-BN single crystal provided by the present invention does not have a large void or crystal interface inside, restrictions on the filling amount and inhibition of heat conduction are alleviated. That is, an h-BN single crystal useful as a heat conductive filler for a heat radiating member is provided.

It is an XRD pattern of Example 1 (h-BN raw material powder A and sample A). It is an electron diffraction pattern of Example 1 (Sample A) (drawing substitute photograph). It is an SEM image of Example 1 (h-BN raw material powder A) (drawing substitute photograph). It is an SEM image of Example 1 (Sample A) (drawing substitute photograph). It is an SEM image of Example 1 (Sample A) (drawing substitute photograph). It is an XRD pattern of Comparative Example 1 (h-BN raw material powder B and sample B). It is an SEM image of Comparative Example 1 (h-BN raw material powder B) (drawing substitute photograph). It is an SEM image of Comparative Example 1 (Sample B) (drawing substitute photograph).

Hereinafter, the present invention will be described in detail, but the scope of the present invention is not limited to specific embodiments.
The hexagonal boron nitride single crystal according to the embodiment of the present invention has an aspect ratio of 0.3 or more defined by "maximum thickness in the c-axis direction of crystal / maximum width of crystal ab plane".
As described above, the conventional h-BN has anisotropy of heat conduction because it grows mainly so as to widen the crystal ab plane. However, the h-BN single crystal according to the present embodiment is a single crystal having the above aspect ratio of 0.3 or more. Therefore, even if a composite material composition is prepared by blending it as a filler in a matrix such as a resin as it is, it is expected that the composite material composition does not have thermal conduction anisotropy because it is difficult to be oriented in the composite material composition. Will be done. Further, when the h-BN single crystal according to the present embodiment is used as the filler, unlike the conventional h-BN agglomerated filler, there are no voids in the filler, so that the h-BN single crystal is more composite than the conventional h-BN agglomerated filler. It is expected that restrictions on the amount of filling in the material composition will be relaxed.

When the h-BN single crystal according to the present embodiment is used as a filler, the closer the aspect ratio is to 1, the more difficult it is to be oriented in the composite composition, but the one slightly deviated from 1 is the maximum filler. The fillable amount increases. Specifically, when the filler is randomly filled without being oriented, the maximum filler chargeable amount is larger than that when the aspect ratio is 1, that the aspect ratio is 0.3 or more and 3.5 or less (excluding 1). ) (See Non-Patent Document 1). The maximum filler chargeable amount increases monotonically when the aspect ratio is 0.3 or more and 0.6 or less, decreases monotonically when the aspect ratio is 0.6 or more and 1 or less, and monotonically increases when the aspect ratio is 1 or more and 1.5 or less, and 1.5 or more and 3 Below .5, it decreases monotonically (see Non-Patent Document 1). Therefore, the preferred aspect ratio of the filler which is difficult to be oriented and has a large maximum filling amount is 0.3 or more, more preferably 0.6 or more, 3.5 or less, and more preferably 1.5 or less. .. For the aspect ratio specified in the present specification, automorphic and / or semi-automorphic particles are selected from 50,000 times images measured using a scanning electron microscope, and the maximum of the particles in the crystal c-axis direction is selected. It can be obtained by actually measuring the thickness / maximum width of the crystal ab plane and averaging them.

The h-BN single crystal according to the present embodiment is a single crystal in which growth in the c-axis direction of the crystal is promoted, and the maximum width of the crystal ab plane determines the effect of thermal resistance between the fillers when used as a filler. In order to keep it low, it is preferably 100 nm or more, more preferably 150 nm or more, and further preferably 200 nm or more. On the other hand, the upper limit is not particularly limited, but is usually 200 μm or less, preferably 100 μm or less, more preferably 50 μm or less, and further preferably 10 μm or less.
The maximum thickness in the c-axis direction of the crystal is preferably 30 nm or more, more preferably 100 nm or more, and further preferably 200 nm or more.
The maximum width of the ab plane of the h-BN single crystal and the maximum thickness in the c-axis direction of the crystal are obtained by enlarging one particle obtained by scanning electron microscope (SEM) measurement to form one particle. It is a value obtained by averaging the maximum lengths of the primary particles that can be observed on the image for the primary particles.

The h-BN crystal according to this embodiment is a single crystal, not a polycrystal. Whether it is a single crystal or a polycrystal can be grasped by, for example, X-ray diffraction measurement or electron beam diffraction measurement by a transmission electron microscope (TEM).

The h-BN single crystal according to this embodiment is preferably an automorphic crystal or a semi-automorphic crystal. Therefore, it is expected that the crystal defect content is lower than that of polymorphic crystals grown while being disturbed by the outside world, and that the decrease in thermal conductivity due to crystal defects is small. Here, an automorphic crystal is a crystal surrounded by a crystal plane that reflects the crystal structure, and a semi-automorphic crystal is a crystal in which a part of the crystal is surrounded by a crystal plane that reflects the crystal structure. An allogeneic crystal is a crystal in which a plane reflecting the crystal structure does not appear in the crystal. For example, it is described in the literature of Flux Crystal Growth Story (Nikkan Kogyo Shimbun).
Whether it is an automorphic crystal, a semi-automorphic crystal or a polymorphic crystal can be grasped by, for example, a scanning electron microscope (SEM) measurement.

The h-BN single crystal according to the present embodiment has, for example, the shapes shown in FIGS. 4 and 5, and can be expressed as a prism or a cylinder having a polygonal shape on the top surface and the bottom surface, and also has a top. It can also be expressed as a shape in which the cross-sectional area changes from the surface to the bottom surface. Further, it can be called a barrel type, and can also be called a bullet type.

Another embodiment of the present invention is a method for producing an h-BN single crystal, and more specifically, in XRD analysis, a boron nitride powder having a peak half-value width of 0.4 ° or more on the 002 surface and a lithium salt are mixed. It is a method for producing an h-BN single crystal, which comprises a step of heating and heating. The h-BN single crystal produced by such a production method has an aspect ratio of 0.3 or more defined by "maximum thickness in the crystal c-axis direction / maximum width of the crystal ab plane".

<Raw material BN powder>
The raw material BN powder used in this embodiment is synthesized from commercially available h-BN, commercially available α and β-BN, BN produced by a reduction nitriding method of a boron compound and ammonia, and a nitrogen-containing compound such as a boron compound and melamine. Any of the above-mentioned BN can be used without limitation, but h-BN is particularly preferably used.
From the viewpoint of h-BN crystal growth, it is preferable to use the raw material BN powder having a peak half width of 0.4 ° or more on the 002 surface in the XRD analysis, and it is preferably 0.5 ° or more. In the XRD analysis, the large peak half-value width of the 002 plane, that is, the peak being relatively broad means that the raw material BN powder contains impurities, and means that the crystallinity is low. In the present embodiment, it is preferable to use such a raw material BN powder having low crystallinity.

As an impurity, it is preferable that oxygen is present in the raw material BN to some extent, and it is preferable to use a raw material BN powder having a total oxygen concentration of 1% by mass or more. Further, it is usually 30% by mass or less. Most of the BN powders having a total oxygen concentration within the above range have undeveloped crystals, so that crystals are likely to grow by heat treatment.
The total oxygen concentration in the raw material BN powder is more preferably 3% by mass or more, and preferably 10% by mass or less, more preferably 9% by mass or less.
If the total oxygen concentration of the raw material BN powder is less than the above lower limit, the crystallites will not grow sufficiently due to the high purity and crystallinity of the BN itself, and conversely, if it exceeds the above upper limit, the oxygen concentration will increase even after heat treatment. It is not preferable because it is too expensive to achieve high thermal conductivity when used as a thermally conductive filler in a composite composition.

<Li salt flux>
In this embodiment, a lithium salt is used as the flux. The lithium salt is not particularly limited, and examples thereof include lithium carbonate, lithium hydroxide, lithium chloride, lithium iodide, lithium fluoride, lithium nitrate, lithium sulfate, lithium borate, lithium molybdate, and mixtures thereof. It is preferably a carbonate, and any carbonate containing lithium such as _{Li 2} CO _{3 can be used without limitation.}
Further, the lithium salt preferably has a melting point of 100 ° C. or higher, and more preferably 400 ° C. or higher.

In the present embodiment, as described above, in the XRD analysis, the raw material BN powder having a peak half width of 0.4 ° or more on the 002 surface is used, and by using a lithium salt as a flux, the aspect ratio of h is 0.3 or more. -The BN single crystal can be produced, and the present inventors consider the reason as follows.
In general, lithium salt flux, particularly lithium carbonate flux, has high solubility, so that a high concentration of h-BN is dissolved in a solvent. On the other hand, it decomposes to generate carbon dioxide in the high temperature region. Therefore, crystal growth started with evaporation as a driving force during the holding process, and h-BN grew in the c-axis direction by pseudo-single-dimensional growth accompanied by the formation of a solute-concentrated layer near the solid-liquid interface between the crystal and the solution. It is considered that a single crystal was produced.

<Mixing and heating steps>
In this embodiment, the raw material BN powder and the lithium salt are mixed. The mixing ratio of the raw material BN powder and the lithium salt is not particularly limited, but the lithium salt is usually contained in an amount of 1 mL% or more, preferably 5 mL% or more, more preferably 10 mL% or more, still more preferably 15 mL%, based on the total amount of the mixture. As described above, it may be particularly preferably contained in an amount of 20 mL or more, and particularly preferably in an amount of 25 mL or more. Further, it is usually contained in an amount of 80 mL or less, preferably 75 mL% or less, more preferably 70 mL or less, still more preferably 65 mL or less, and particularly preferably 60 mL or less.
In addition to the raw material BN powder and the lithium salt, other materials may be added as long as the effects of the present invention are not affected. Examples of other materials include carbonates such as barium carbonate, strontium carbonate, manganese carbonate, calcium carbonate, potassium carbonate, and sodium carbonate.

The mixed raw material BN powder and lithium salt are heated. The heating temperature is not particularly limited, but is usually 1000 ° C. or higher. The heating time is also not particularly limited, but is usually 1 hour or more, preferably 5 hours or more, and usually 10 hours or less.
The heating may be performed in an air atmosphere or an inert gas atmosphere, but it is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere. In addition, the raw materials are usually stored in a crucible and mixed and heated. As the material of the crucible, it is preferable to use a material that does not react with the raw material BN powder, the lithium salt, and the heating temperature.

After heating, the h-BN single crystal is cooled to room temperature, but the cooling rate is not particularly limited, and is usually 300 ° C./hour or less. Moreover, it is usually 5 ° C./hour or more.
The h-BN single crystal according to the present embodiment obtained by the above-mentioned production method is usually 1% by mass or more, preferably 5% by weight or more, more preferably 5% by weight or more, based on the total amount of the obtained product. It is 10% by weight or more, more preferably 20% by weight or more. If this amount is too small, anisotropy of heat conduction tends to occur when the resin is compounded.
Further, a compound such as a normal plate-shaped h-BN or a metal oxide may be mixed with the obtained h-BN.

<Composite composition>
Another embodiment of the present invention is a composite composition in which the above h-BN single crystal is blended in a matrix.
The matrix used preferably has high thermal conductivity, and the thermal conductivity of the matrix is preferably 0.2 W / mK or more, and particularly preferably 0.22 W / mK or more.
As a method for measuring the thermal conductivity of the matrix, the thermal diffusivity, the specific gravity, and the specific heat are measured using the following devices, and the thermal conductivity is obtained by multiplying these three measured values.
(1) Thermal diffusivity: "Eye Phase Mobile 1u" manufactured by Eye Phase
(2) Specific gravity: "Balance XS-204" manufactured by METTLER TOLEDO (using solid density measurement kit)
(3) Specific heat: Seiko Instruments Inc. "DSC320 / 6200"

A resin is usually used as the matrix, and either a curable resin or a thermoplastic resin can be used without limitation. The curable resin may be any crosslinkable resin such as thermosetting, photocurable, and electron beam curable, but a thermosetting resin is preferable in terms of heat resistance, water absorption, dimensional stability, and the like. Used.
As the thermosetting resin and the thermosetting resin, for example, those exemplified in WO2013 / 081061 can be used. Of these, it is preferable to use a thermosetting resin, and it is particularly preferable to use an epoxy resin.
As the epoxy resin, a phenoxy resin having at least one skeleton selected from the group consisting of a naphthalene skeleton, a fluorene skeleton, a biphenyl skeleton, an anthracene skeleton, a pyrene skeleton, a xanthene skeleton, an adamantane skeleton and a dicyclopentadiene skeleton is preferable. Among them, a phenoxy resin having a fluorene skeleton and / or a biphenyl skeleton is particularly preferable because the heat resistance is further enhanced, and in particular, it has at least one or more skeletons of a bilphenol A skeleton, a bisphenol F skeleton and a biphenyl skeleton. It is preferably a phenoxy resin.

The content of the matrix in the composite composition is usually 2 wt% or more, preferably 5 wt% or more, more preferably 7 wt% or more, usually 70 wt% or less, preferably 60 wt% or less, more preferably 40 wt% or less. be.
The content of the h-BN single crystal in the composite composition is usually 30 wt% or more, preferably 40 wt% or more, more preferably 50 wt% or more, usually 99 wt% or less, preferably 98 wt% or less, and more. It is preferably 95 wt% or less.

An organic solvent can be used to prepare the composite composition. Examples of the organic solvent include alcohol solvents, aromatic solvents, amide solvents, alkane solvents, ethylene glycol ethers and ether ester-based easy agents, propylene glycol ethers and ether ester solvents, ketone solvents, and ester solvents. From the above, it can be preferably selected and used in consideration of the solubility of the resin and the like.
As a specific example of the organic solvent, those exemplified in WO2013 / 081061 can be used.
As the organic solvent, one type may be used alone, or two or more types may be used in combination in any combination and ratio.

The composite composition may contain a curing agent, if necessary.
The curing agent refers to a substance that contributes to the cross-linking reaction between the cross-linking groups of the matrix, such as the epoxy group of the epoxy resin.
In the epoxy resin, a curing agent and a curing accelerator for the epoxy resin are used together, if necessary.
Further, for the purpose of further improving the functionality, various additives (other additives) may be contained as long as the effects of the present invention are not impaired. Examples of other additives include functional resins having functionality added to the matrix, such as liquid crystal epoxy resins, nitride particles such as aluminum nitride, silicon nitride, and fibrous boron nitride, alumina, and fibrous alumina. , Insulating metal oxides such as zinc oxide, magnesium oxide, beryllium oxide and titanium oxide, insulating carbon components such as diamond and fullerene, resin curing agents, resin curing accelerators, viscosity modifiers and dispersion stabilizers.

Further, as other additives, as an additive component for improving the adhesiveness between the matrix and the h-BN single crystal, a coupling agent such as a silane coupling agent or a titanate coupling agent, and for improving storage stability. Examples thereof include UV inhibitors, antioxidants, plasticizers, flame retardants, colorants, dispersants, fluidity improvers and the like.

In addition, a surfactant, an emulsifier, a low elasticity agent, a diluent, an antifoaming agent, an ion trapping agent, etc., which improve the dispersibility of each component in the composition, can be added.
One of these may be used alone, or two or more thereof may be mixed and used in any combination and ratio.
As a specific example of the additive, those exemplified in WO2013 / 081061 can be used, and the amount of the additive can also be within the range described in WO2013 / 081061.

The composite composition is prepared by a paint shaker, a bead mill, a planetary mixer, a stirring type disperser, or a self-rotating stirring mixer for the purpose of dispersing and mixing h-BN single crystals, matrices, solvents and other additives. , It is preferable to mix using a general kneading device such as a three-roll, kneader, single-screw or twin-screw kneader.
The mixing order of each compounding component of the composite material composition is also arbitrary as long as there are no particular problems such as reaction or precipitation, and any two components or three or more components of the composition are mixed in advance. Then the remaining ingredients may be mixed or all at once.

The composite material composition can be a heat radiating member by forming a molded body.
As a method for molding this molded product, a method generally used for molding a resin composition can be used.
For example, it can be molded by curing the coating liquid for a heat radiating sheet in a desired shape, for example, in a state of being filled in a mold. As a method for producing such a molded product, an injection molding method, an injection compression molding method, an extrusion molding method, and a compression molding method can be used.
When the composite composition contains a thermosetting resin composition such as an epoxy resin or a silicone resin, the molded product can be molded, that is, cured under curing temperature conditions according to the respective compositions.

When the composite composition contains a thermoplastic resin composition, the molded product can be molded under conditions of a temperature equal to or higher than the melting temperature of the thermoplastic resin and a predetermined molding speed and pressure. Further, a molded product can also be obtained by cutting the composite material composition into a desired shape from a solid material that has been molded and cured.

・酸素濃度
原料ＢＮ粉末の全酸素濃度は、不活性ガス融解−赤外線吸収法により、株式会社堀場製作所製の酸素・窒素分析計を用いて測定することができる。
・ＸＲＤパターン
粉末Ｘ線回折測定は、ＰＡＮａｌｙｔｉｃａｌ社製Ｘ線回折装置「Ｘ‘Ｐｅｒｔ Ｐｒｏ ＭＰＤ」を用いた。ＢＮ原料または生成したＢＮを０．２ｍｍ深さのガラス試料板に表面が平滑になるように充填し、粉末Ｘ線回折測定を行った。
・アスペクト比
アスペクト比は、走査型電子顕微鏡（Ｚｅｉｓｓ Ｕｌｔｒａ５５、加速電圧３ｋＶ）を用いて測定された５万倍の画像から、自形および／または半自形の粒子の結晶ｃ軸方向の最大厚さ／結晶ａｂ面の最大幅を実測することにより求めた。 Hereinafter, the present invention will be described in more detail, but the present invention is not limited to the following examples as long as the gist of the present invention is not exceeded.
<Measurement method>
-Maximum half width of raw material The peak half width of raw material is 2θ = 26.5 of X-ray diffraction measurement using _{X-ray (CuKα 1} ) wavelength (λ) = 1.54056 Å (1 Å = 1 × 10 ^{-10 m).} It is the half-value width of the (002) plane peak that appears in the vicinity, and was calculated by the following formula. The full width at half maximum (βo) is calculated by the profile fitting method (Peason-XII function or Pseud-Voigt function), and further corrected by the device-derived full width at half maximum βi previously obtained by standard Si to obtain the full width at half maximum βi. Asked.

-Oxygen concentration The total oxygen concentration of the raw material BN powder can be measured by the inert gas melting-infrared absorption method using an oxygen / nitrogen analyzer manufactured by Horiba Seisakusho Co., Ltd.
-XRD pattern For powder X-ray diffraction measurement, an X-ray diffractometer "X'Pert Pro MPD" manufactured by PANalytical Co., Ltd. was used. The BN raw material or the produced BN was filled in a glass sample plate having a depth of 0.2 mm so that the surface was smooth, and powder X-ray diffraction measurement was performed.
Aspect ratio The aspect ratio is the maximum thickness of automorphic and / or semi-automorphic particles in the crystal c-axis direction from a 50,000-fold image measured using a scanning electron microscope (Zeiss Ultra55, acceleration voltage 3 kV). It was determined by actually measuring the maximum width of the crystal ab plane.

<Example 1>
As the raw material h-BN powder, commercially available h-BN raw material powder A (in XRD analysis (X-ray source: CuKα), the peak half-value width of the 002 surface is 0.67 ° and the oxygen concentration is 8% by mass) is used as the lithium salt. A commercially available Li ₂ CO ₃ powder (purity 99.0%) was used. The amounts of the raw material h-BN powder and Li ₂ CO ₃ having a melting point of 723 ° C. were set to 50 mol%, respectively. The h-BN raw material powder A and Li ₂ CO ₃ were placed in a crucible and heat-treated at 1000 ° C. for 5 hours under nitrogen flow. Flux was dissolved and removed from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid) to obtain Sample A.
FIG. 1 shows the XRD patterns of the h-BN raw material powder A and the sample A. The h-BN peak of the sample A is sharper than that of the h-BN raw material powder A (half-price width of the peak on the 002 surface of the sample A). Was 0.35 °, which is a 48% decrease from that of the h-BN raw material powder A), and it was found that the crystallinity of the sample A was improved as compared with the h-BN raw material powder A.
The electron diffraction pattern of sample A is shown in FIG. 2, and clear spots were obtained, and it was found that sample A was a highly crystalline h-BN single crystal.
FIG. 3 shows an SEM image of the h-BN raw material powder A.
The SEM images of sample A are shown in FIGS. 4 and 5, and it was found that sample A has a structure different from that of h-BN raw material powder A and contains crystals having an aspect ratio of 0.33 to 1.5. .. The width of the ab surface was 100 to 500 nm. In the SEM image, a hexagonal column structure characteristic of hexagonal crystals can be seen, and the bottom surface of the hexagonal column corresponds to the ab plane of h-BN.

<Comparative example 1>
As the raw material h-BN powder, commercially available h-BN raw material powder B (in XRD analysis, the peak half width of the 002 surface is 0.23 ° and the oxygen concentration is 0.4% by mass) is used, and as Li ₂ CO ₃ , the commercially available Li is used. ₂ CO ₃ powder (purity 99.0%) was used. The amounts of the raw material h-BN powder and Li ₂ CO ₃ were set to 50 mol%, respectively. The h-BN raw material powder B and Li ₂ CO ₃ were placed in a crucible and heat-treated at 1000 ° C. for 5 hours under nitrogen flow. Flux was dissolved and removed from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid) to obtain sample B.
FIG. 6 shows the XRD patterns of the h-BN raw material powder B and the sample B, but no significant difference is observed in the h-BN peak shapes of the two (the peak half width of the 002 surface of the sample B is 0.20 °). , Only 13% decrease from that of h-BN raw material powder B), and it was found that the crystallinity of sample B was not significantly improved from that of h-BN raw material powder B.
FIG. 7 shows an SEM image of the h-BN raw material powder B.
FIG. 8 shows an SEM image of sample B, and it was found that the shape of sample B remained the same as that of h-BN raw material powder B and remained plate-like. The aspect ratio of sample B was 0.2 or less.

<Comparative example 2>
A commercially available h-BN raw material powder A was used as the raw material h-BN powder, and a commercially available BaCO ₃ powder (purity 99.9%, melting point 911 ° C.) was used as the flux. The amounts of the raw material h-BN powder and BaCO ₃ were set to 50 mol%, respectively. Put h-BN raw material powder A and BaCO ₃ in a crucible,
Heat treatment was performed at 1000 ° C. for 5 hours under nitrogen flow. Flux was dissolved and removed from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid) to obtain sample C. From the SEM image, it was found that the shape of the sample C was almost the same as that of the h-BN raw material powder A. Further, it did not have crystallinity capable of calculating the aspect ratio defined by the maximum thickness in the crystal c-axis direction / the maximum width of the crystal ab plane.
<Comparative Examples 3 and 4>
Samples D and E were prepared in the same manner as in Comparative Example 2 except that the BaCO ₃ powder was changed to MnCO ₃ (purity 99.9%, decomposition temperature 100 ° C.) and CaCO _{3 (purity 99.5%, melting point 825 ° C.).} I got each. From the SEM image, it was found that the shapes of the samples D and E were almost the same as those of the h-BN raw material powder A. Further, it did not have crystallinity capable of calculating the aspect ratio defined by the maximum thickness in the crystal c-axis direction / the maximum width of the crystal ab plane.

Therefore, according to this embodiment, the aspect ratio defined by the maximum thickness in the c-axis direction of the crystal / the maximum width of the crystal ab plane is 0.3 or more, the maximum width of the crystal ab plane is 100 nm or more, and the single crystal or self-shaped crystal or It can be said that a semi-automorphic h-BN single crystal could be produced.

Claims (4)

Hexagonal boron nitride single crystal with an aspect ratio of 0.3 or more and 1.5 or less defined by the maximum thickness in the c-axis direction of the crystal / the maximum width of the crystal ab plane, which is an automorphic crystal or a semi-automorphic crystal. .. The hexagonal boron nitride single crystal according to claim 1, wherein the maximum width of the crystal ab plane is 100 nm or more. A composite composition obtained by blending the hexagonal boron nitride single crystal according to claim 1 or 2 into a matrix. A heat radiating member obtained by molding the composite composition according to claim 3.

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