JP7233657B2 - A hexagonal boron nitride single crystal, a composite material composition containing the hexagonal boron nitride single crystal, and a heat dissipating member formed by molding the composite material composition - Google Patents

Description

The present invention relates to a hexagonal boron nitride single crystal with accelerated growth along the c-axis and a method for producing the same.

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

h-BN is a plate crystal, and the plate plane (in the ab plane) is strongly bonded by covalent bonds, but the thickness direction (c-axis direction) is weakly bonded by van der Waals force. Not too much. Therefore, a large anisotropy of thermal conductivity occurs in the plate surface (normally about 250 W/mK as thermal conductivity) and in the thickness direction (normally about 2 to 3 W/mK as thermal conductivity).
In general, when a composite material composition is produced by blending plate-like crystals as a filler into a resin or the like, the plate surface of the plate-like crystals is oriented in a specific direction during the process of raw material mixing, press molding, injection molding, and the like. phenomenon occurs. Therefore, when a composite material composition is produced by blending h-BN plate crystals as a filler in a resin or the like, the resulting composite material composition has high thermal conductivity in a specific direction, but in a direction perpendicular to it A problem arises in that the thermal conductivity anisotropy of h-BN plate crystals is transferred, such as low thermal conductivity.

Conventionally, in order to prevent the transfer of such anisotropic thermal conductivity, a method has been studied in which h-BN plate crystals are previously aggregated in random directions and then used as a filler (Patent Documents 1, 2, 3, 4). However, such agglomerated filler has voids in the filler, which limits the filling amount of the composite material composition. There were problems such as inhibition of conduction.

If an h-BN single crystal having an aspect ratio defined by "maximum thickness in the c-axis direction of the crystal/maximum width of the ab plane of the crystal" close to 1 can be used as a filler, the above-mentioned thermal conductivity difference No directional transfer occurs. In addition, since such a filler does not have large voids or crystal interfaces 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, a method for growing an h-BN single crystal with an aspect ratio close to 1 has existed so far. didn't. In addition, even if an h-BN single crystal having an aspect ratio close to 1 is produced by pulverizing a plate-like h-BN single crystal, cleavage is likely to occur at the ab planes, which are weakly bonded by the van der Waals force, resulting in It is expected that the pulverized product that is obtained is likely to eventually become plate-like h-BN crystals.
On the contrary, producing an h-BN single crystal having an aspect ratio close to 1 from an h-BN single crystal having an aspect ratio larger than 1 is ab It is expected to be easy because it is sufficient to cleave on the plane. Therefore, it has been desired to establish a method for easily producing an h-BN single crystal in which the c-axis growth of the h-BN crystal is promoted more than the ab plane growth.

JP 2006-257392 A Japanese Patent Publication No. 2008-510878 JP-A-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 also 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 extensive studies, the present inventors have found that h-BN single crystals having an aspect ratio of 0.3 or more are produced by a flux method using a specific boron nitride raw material and a lithium salt as a flux. I found that it is possible. Conventionally, h-BN crystals have been produced using flux, but the obtained h-BN crystals are plate-like, and there are no examples of producing h-BN single crystals with an aspect ratio close to 1. It does not exist (see Patent Document 5). The present invention has been achieved based on such findings, and the gist thereof is as follows.

[1] A hexagonal boron nitride single crystal having an aspect ratio defined by the maximum thickness in the crystal c-axis direction/the maximum width of the crystal ab plane of 0.3 or more.
[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 or semi-automorphic crystal.
[4] A method for producing a hexagonal boron nitride single crystal, comprising a step of mixing a boron nitride powder having a peak half-value width of 0.4° or more on the 002 plane in XRD analysis with a lithium salt, followed by heating.
[5] The production method according to [4], wherein the hexagonal boron nitride single crystal has a crystal ab plane with a maximum width of 100 nm or more.
[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] The hexagonal boron nitride single crystal described in any one of [1] to [3], or the hexagonal boron nitride single manufactured by the manufacturing method described in any one of [4] to [6] A composite material composition comprising crystals incorporated in a matrix.
[8] A heat dissipating member obtained by molding the composite material composition according to [7].

The present invention provides an h-BN single crystal in which c-axis growth of the h-BN crystal is promoted. A composite material composition in which the h-BN single crystal of the present invention is incorporated in a matrix such as a resin does not cause transfer of anisotropic thermal conductivity, which is a problem when using a conventional h-BN single crystal. In addition, since the h-BN single crystal provided by the present invention does not have large internal voids or crystal interfaces, restrictions on filling amount and inhibition of heat conduction are alleviated. In other words, an h-BN single crystal useful as a thermally conductive filler for heat dissipating members is provided.

1 is an XRD pattern of Example 1 (h-BN raw material powder A and sample A). 1 is an electron beam diffraction pattern of Example 1 (Sample A) (photograph substituting for drawing). 1 is an SEM image of Example 1 (h-BN raw material powder A) (photograph substituting for drawing). 1 is an SEM image of Example 1 (Sample A) (photograph substituting for drawing). 1 is an SEM image of Example 1 (Sample A) (photograph substituting for drawing). 1 is an XRD pattern of Comparative Example 1 (h-BN raw material powder B and sample B). 1 is an SEM image of Comparative Example 1 (h-BN raw material powder B) (photograph substituting for drawing). Fig. 2 is an SEM image of Comparative Example 1 (Sample B) (photograph substituting for drawing).

Although the present invention will be described in detail below, the scope of the present invention is not limited only 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 as "maximum thickness in crystal c-axis direction/maximum width of crystal ab plane".
As described above, conventional h-BN has anisotropy in thermal conductivity because it grows mainly by expanding the crystal ab plane. However, the h-BN single crystal according to this embodiment is a single crystal having the aspect ratio of 0.3 or more. Therefore, even if a composite material composition is produced by blending it as a filler in a matrix such as a resin, it is difficult to orient it in the composite material composition, so it is expected that thermal conductivity anisotropy will not occur in the composite material composition. be done. In addition, when the h-BN single crystal according to the present embodiment is used as a filler, unlike the conventional h-BN aggregated filler, there are no voids in the filler, so it is more complex than the conventional h-BN aggregated filler. It is expected that the restrictions on the filling amount 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 orient in the composite material composition, but the slightly shifted from 1 is the maximum filler. Larger fill capacity. Specifically, when the filler is randomly filled without being oriented, the maximum filler filling amount is greater than when the aspect ratio is 1 because the aspect ratio is 0.3 or more and 3.5 or less (excluding 1 ) (see Non-Patent Document 1). The maximum fillable amount of filler monotonously increases when the aspect ratio is 0.3 or more and 0.6 or less, monotonously decreases when the aspect ratio is 0.6 or more and 1 or less, monotonically increases when 1 or more and 1.5 or less, and 1.5 or more and 3 It monotonically decreases below 0.5 (see Non-Patent Document 1). Therefore, the aspect ratio of the filler that is difficult to orient and has a large maximum fillable amount is preferably 0.3 or more, more preferably 0.6 or more, 3.5 or less, and more preferably 1.5 or less. . The aspect ratio defined in this specification is determined by selecting idiomorphic and/or semi-emorphic particles from an image of 50,000 times magnification measured using a scanning electron microscope, and determining the maximum It can be obtained by actually measuring the thickness/maximum width of the ab plane of the crystal.

The h-BN single crystal according to the present embodiment is a single crystal whose growth in the crystal c-axis direction is promoted, and the maximum width of the crystal ab plane is the effect of thermal resistance between fillers when used as a filler. In order to keep the thickness low, the thickness is preferably 100 nm or more, more preferably 150 nm or more, and even more 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 still more preferably 10 μm or less.
The maximum thickness in the crystal c-axis direction is preferably 30 nm or more, more preferably 100 nm or more, and even more 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 grain obtained by scanning electron microscope (SEM) measurement, and constituting one grain. It is the maximum length of a primary particle that can be observed on an image.

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 determined by, for example, X-ray diffraction measurement or electron beam diffraction measurement using 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 content of crystal defects is lower than that of polymorphic crystals grown while being disturbed from the outside, and that the reduction in thermal conductivity due to crystal defects is small. Here, automorphic crystals are crystals surrounded by crystal planes that reflect the crystal structure, and semi-automorphic crystals are crystals that are partly surrounded by crystal planes that reflect the crystal structure. A polymorphic crystal is a crystal that does not have faces that reflect the crystal structure. For example, it is described in a document such as the story of flux crystal growth (Nikkan Kogyo Shimbun).
Whether it is an automorphic crystal, a semi-automorphic crystal, or a polymorphic crystal can be grasped, for example, by scanning electron microscope (SEM) measurement.

The h-BN single crystal according to the present embodiment has, for example, the shape shown in FIGS. It can also be expressed as a shape in which the cross-sectional area changes from the surface to the bottom surface. Furthermore, it can also be called a barrel shape, and can also be called a bullet shape.

Another embodiment of the present invention is a method for producing an h-BN single crystal, more specifically, a boron nitride powder having a peak half-value width of 0.4 ° or more on the 002 plane in XRD analysis, and a lithium salt are mixed. and heating. The h-BN single crystal produced by such a production method has an aspect ratio of 0.3 or more defined as "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 the present embodiment includes commercially available h-BN, commercially available α and β-BN, BN prepared by a reduction nitridation method of a boron compound and ammonia, a boron compound and a nitrogen-containing compound such as melamine. Although any BN can be used without limitation, h-BN is particularly preferably used.
From the viewpoint of h-BN crystal growth, it is preferable to use raw material BN powder having a peak half-value width of 002 plane in XRD analysis of 0.4° or more, preferably 0.5° or more. A large peak half width of the 002 plane in the XRD analysis, that is, a relatively broad peak, means that the raw material BN powder contains impurities and that the crystallinity is low. In the present embodiment, it is preferable to use such raw material BN powder with low crystallinity.

As an impurity, oxygen is preferably 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. Moreover, it is usually 30% by mass or less. BN powders having a total oxygen concentration within the above range often have undeveloped crystals, and thus 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, preferably 10% by mass or less, and more preferably 9% by mass or less.
If the total oxygen concentration of the raw material BN powder is less than the lower limit, the crystallites do not grow sufficiently due to the high purity and crystallinity of the BN itself, and if it exceeds the upper limit, the oxygen concentration remains even after the heat treatment. When it is used as a thermally conductive filler for a composite material composition, it is not preferable because high thermal conductivity cannot be achieved.

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

In the present embodiment, as described above, by using a raw material BN powder having a peak half-value width of 0.4 ° or more on the 002 plane in the XRD analysis, and using a lithium salt as a flux, the aspect ratio is 0.3 or more h -BN single crystals can be produced, and the inventors consider the reason for this as follows.
Lithium salt fluxes in general, and lithium carbonate fluxes in particular, have high solubility, so a high concentration of h-BN is dissolved in the solvent. On the other hand, it decomposes in a high temperature range to generate carbon dioxide gas. Therefore, during the holding process, crystal growth started with the driving force of evaporation, and h-BN grew in the c-axis direction by quasi-one-dimensional growth accompanied by the formation of a solute-enriched layer near the solid-liquid interface between the crystal and the solution. Suppose a single crystal is produced.

<Mixing and heating step>
In this embodiment, raw BN powder and 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 mol% or more based on the total amount of the mixture, preferably 5 mol% or more, more preferably 10 mol% or more, and still more preferably 15 mol%. As described above, the content may be particularly preferably 20 mol % or more, and particularly preferably 25 mol % or more. In addition, the content is usually 80 mol % or less, but it may be preferably 75 mol % or less, more preferably 70 mol % or less, still more preferably 65 mol % or less, and particularly preferably 60 mol % or less.
In addition to the raw material BN powder and lithium salt, other materials may be added as long as they do not affect the effects of the present invention. Other materials include carbonates such as barium carbonate, strontium carbonate, manganese carbonate, calcium carbonate, potassium carbonate and sodium carbonate.

The mixed raw 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 is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere. Also, the raw materials are usually placed in a crucible and mixed and heated. The crucible is preferably made of a material that does not react with the raw material BN powder and the lithium salt at 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-described production method is usually 1% by mass or more, preferably 5% by weight or more, more preferably 10% by weight or more, more preferably 20% by weight or more. If this amount is too small, there is a tendency that anisotropy in thermal conductivity tends to occur when compounded with a resin.
In addition, the obtained h-BN may be mixed with a compound such as an ordinary plate-like h-BN or a metal oxide.

<Composite composition>
Another embodiment of the present invention is a composite composition comprising the above h-BN single crystal incorporated in a matrix.
The matrix to be used preferably has high thermal conductivity, and the thermal conductivity of the matrix is preferably 0.2 W/mK or more, particularly preferably 0.22 W/mK or more.
The thermal conductivity of the matrix is measured by measuring the thermal diffusivity, the specific gravity, and the specific heat using the following equipment, and multiplying these three measured values to obtain the thermal conductivity.
(1) Thermal diffusivity: “I-Phase Mobile 1u” manufactured by I-Phase
(2) Specific gravity: “Balance XS-204” manufactured by Mettler Toledo (using a solid specific gravity measurement kit)
(3) Specific heat: “DSC320/6200” manufactured by Seiko Instruments Inc.

Resins are usually used as the matrix, and both curable resins and thermoplastic resins can be used without limitation. The curable resin may be any crosslinkable resin such as thermosetting, photo-curing, and electron beam curing, but thermosetting resins are preferred 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. Among these, it is preferable to use a thermosetting resin, and it is particularly preferable to use an epoxy resin.
The epoxy resin is preferably a phenoxy resin having at least one skeleton selected from the group consisting of naphthalene skeleton, fluorene skeleton, biphenyl skeleton, anthracene skeleton, pyrene skeleton, xanthene skeleton, adamantane skeleton and dicyclopentadiene skeleton. 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 skeleton of a virphenol A skeleton, a bisphenol F skeleton and a biphenyl skeleton. A phenoxy resin is preferred.

The content of the matrix in the composite material composition is usually 2 wt% or more, preferably 5 wt% or more, more preferably 7 wt% or more, and usually 70 wt% or less, preferably 60 wt% or less, more preferably 40 wt% or less. be.
Further, the content of the h-BN single crystal in the composite material 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, more Preferably, it is 95 wt% or less.

An organic solvent can be used to prepare the composite composition. Organic solvents include alcohol-based solvents, aromatic solvents, amide-based solvents, alkane-based solvents, ethylene glycol ethers and ether-ester-based facilitating agents, propylene glycol ethers and ether-ester-based solvents, ketone-based solvents, and ester-based solvents. Considering the solubility of the resin, etc., it can be suitably selected and used.
As specific examples of the organic solvent, those exemplified in WO2013/081061 can be used.
One organic solvent may be used alone, or two or more organic solvents may be used in any combination and ratio.

The composite composition may optionally contain a curing agent.
A 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 an epoxy resin.
In epoxy resins, both curing agents and curing accelerators for epoxy resins are used as necessary.
Moreover, for the purpose of further improving the functionality, various additives (other additives) may be included within a range that does not impair the effects of the present invention. Other additives include, for example, functional resins imparting functionality to the matrix, such as liquid crystalline 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.

Furthermore, other additives include coupling agents such as silane coupling agents and titanate coupling agents as additive components for improving the adhesion between the matrix and the h-BN single crystal, and for improving storage stability. UV inhibitors, antioxidants, plasticizers, flame retardants, colorants, dispersants, fluidity improvers and the like.

In addition, surfactants, emulsifiers, elasticity reducing agents, diluents, antifoaming agents, ion trapping agents, etc., which improve the dispersibility of each component in the composition, can also be added.
One of these may be used alone, or two or more of them may be used in any combination and ratio.
As specific examples of the additive, those exemplified in WO2013/081061 can be used, and the additive amount can also be within the range described in WO2013/081061.

For the preparation of the composite material composition, for the purpose of dispersing and mixing the h-BN single crystal, matrix, solvent and other additives, paint shakers, bead mills, planetary mixers, stirring dispersers, rotation and revolution stirring mixers , a triple roll, a kneader, a single-screw or twin-screw kneader, or the like.
The mixing order of each compounding component of the composite material composition is arbitrary as long as there is no particular problem such as reaction or precipitation, and among the constituent components of the composition, any two or three components are mixed in advance and then the remaining ingredients, or all at once.

The composite material composition can serve as a heat radiating member by forming a molded body.
As a method for molding this molded article, a method generally used for molding a resin composition can be used.
For example, the heat-dissipating sheet coating liquid can be shaped into a desired shape, for example, by curing the liquid while filling it in a mold. Injection molding, injection compression molding, extrusion molding, and compression molding can be used as methods for producing such molded articles.
Moreover, when the composite material composition contains a thermosetting resin composition such as an epoxy resin or a silicone resin, molding of the molded body, that is, curing can be performed under curing temperature conditions according to each composition.

Further, when the composite material composition contains a thermoplastic resin composition, molding of the molded body can be performed at a temperature equal to or higher than the melting temperature of the thermoplastic resin and at a predetermined molding speed and pressure. Alternatively, a molded body can be obtained by cutting out a desired shape from a solid material obtained by molding and curing the composite material composition.

・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。 Although the present invention will be described in more detail below, the present invention is not limited to the following examples as long as the gist thereof is not exceeded.
<Measurement method>
・Peak half width of raw material The peak half width of the raw material is 2θ = 26.5 in X-ray diffraction measurement using X-ray (CuKα ₁ ) wavelength (λ) = 1.54056 Å (1 Å = 1 × 10 ^-10 m). It is the (002) plane peak half width appearing in the vicinity, and was obtained by the following formula. The half-value width (βo) is calculated by the profile fitting method (Peason-XII function or Pseud-Voigt function), and further corrected with the apparatus-derived half-value width βi obtained in advance using standard Si, and the half-value width β asked for

· Oxygen concentration The total oxygen concentration of the raw material BN powder can be measured using an oxygen/nitrogen analyzer manufactured by Horiba, Ltd. by an inert gas fusion-infrared absorption method.
-XRD pattern Powder X-ray diffraction measurement was performed using an X-ray diffractometer "X'Pert Pro MPD" manufactured by PANalytical. The BN raw material or the produced BN was packed into a glass sample plate having a depth of 0.2 mm so as to have a smooth surface, and powder X-ray diffraction measurement was performed.
· Aspect ratio The aspect ratio is the maximum thickness in the crystal c-axis direction of idiomorphic and / or semi-idiomorphic particles from a 50,000-fold image measured using a scanning electron microscope (Zeiss Ultra 55, accelerating voltage 3 kV). It was obtained by actually measuring the height/maximum width of the ab plane of the crystal.

<Example 1>
As the raw material h-BN powder, commercially available h-BN raw material powder A (peak half width of 002 plane in XRD analysis (X-ray source: CuKα) is 0.67 °, oxygen concentration is 8% by mass), and as a lithium salt Commercially available Li ₂ CO ₃ powder (purity 99.0%) was used. The amount of raw material h-BN powder and Li ₂ CO ₃ having a melting point of 723° C. was 50 mol % each. 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. A sample A was obtained by dissolving and removing the flux from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid).
FIG. 1 shows the XRD patterns of h-BN raw material powder A and sample A. The h-BN peak of sample A is sharper than that of h-BN raw material powder A (peak half width of 002 plane of sample A is 0.35°, which is 48% lower than that of the h-BN raw material powder A).
FIG. 2 shows the electron beam diffraction pattern of sample A. Clear spots were obtained, and sample A was found to be an h-BN single crystal with high crystallinity.
FIG. 3 shows an SEM image of h-BN raw material powder A. As shown in FIG.
SEM images of sample A are shown in FIGS. 4 and 5, and it was found that sample A had a different structure from h-BN raw material powder A and contained crystals with an aspect ratio of 0.33 to 1.5. . The width of the ab plane was 100-500 nm. The SEM image shows a hexagonal columnar structure characteristic of hexagonal crystals, 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 (peak half width of 002 plane in XRD analysis: 0.23°, oxygen concentration: 0.4% by mass), and as Li ₂ CO ₃ , commercially available Li ₂ CO ₃ powder (purity 99.0%) was used. The amounts of raw material h-BN powder and Li ₂ CO ₃ were each 50 mol %. 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. A sample B was obtained by dissolving and removing the flux from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid).
FIG. 6 shows the XRD patterns of h-BN raw material powder B and sample B, and no significant difference was observed in the h-BN peak shapes of both (the peak half width of the 002 plane of sample B was 0.20 ° , only 13% less than that of the h-BN raw material powder B), and it was found that the crystallinity of the sample B was not significantly improved from that of the h-BN raw material powder B.
FIG. 7 shows an SEM image of the h-BN raw material powder B. As shown in FIG.
An SEM image of sample B is shown in FIG. 8, and it was found that the shape of sample B was the same as that of h-BN raw material powder B and remained plate-like. Moreover, the aspect ratio of the sample B was 0.2 or less.

<Comparative Example 2>
A commercially available h-BN raw material powder A was used as the starting h-BN powder, and a commercially available BaCO ₃ powder (99.9% purity, melting point 911° C.) was used as the flux. The amounts of raw material h-BN powder and BaCO ₃ were each 50 mol %. Put the h-BN raw material powder A and BaCO ₃ into a crucible,
Heat treatment was performed at 1000° C. for 5 hours under nitrogen flow. A sample C was obtained by dissolving and removing the flux from the heat-treated sample (impurities were dissolved with 1M hydrochloric acid). From the SEM image, it was found that the shape of sample C was almost the same as that of h-BN raw material powder A. In addition, it did not have a crystallinity that enabled calculation of 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). Got each. From the SEM images, it was found that the shapes of samples D and E were almost the same as that of h-BN raw material powder A. In addition, it did not have a crystallinity that enabled calculation of 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 the present embodiment, the aspect ratio defined by the maximum thickness in the crystal c-axis direction/the maximum width of the crystal ab plane is 0.3 or more, and the maximum width of the crystal ab plane is 100 nm or more. It can be said that a semi-automorphic h-BN single crystal could be produced.

Claims (3)

The aspect ratio defined by the maximum thickness in the crystal c-axis direction/maximum width of the crystal ab plane is 0.3 or more and 3.5 or less (excluding 1.5 or less), and the automorphic crystal or semi-crystalline A hexagonal boron nitride single crystal that is an automorphic crystal. 2. The hexagonal boron nitride single crystal according to claim 1 , wherein the maximum thickness in the crystal c-axis direction is 100 nm or more. The hexagonal boron nitride single crystal according to claim 1 or 2 is blended in a matrix,
A composite material composition, wherein the content of the hexagonal boron nitride single crystal is 40 wt % or more and 99 wt % or less.

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