JP2016216271A - Hexagonal crystal boron nitride particle - Google Patents
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本発明は、新規な六方晶窒化硼素粒子及びそれを含む六方晶窒化硼素粉末に関する。詳しくは、双晶構造を有する新規な六方晶窒化硼素粒子、及び該六方晶窒化硼素粒子を含み、樹脂に充填した際、六方晶窒化硼素の板状結晶に由来する熱的異方性が低減され、且つ、凝集粒子に見られる気泡の巻き込みが抑制されて高い絶縁耐力を発揮することが可能な六方晶窒化硼素粉末を提供するものである。 The present invention relates to novel hexagonal boron nitride particles and hexagonal boron nitride powder containing the same. Specifically, the novel hexagonal boron nitride particles having a twin crystal structure and the thermal anisotropy derived from the plate crystals of hexagonal boron nitride are reduced when the resin is filled with the hexagonal boron nitride particles. In addition, the present invention provides a hexagonal boron nitride powder capable of exhibiting a high dielectric strength by suppressing entrainment of bubbles seen in aggregated particles.
六方晶窒化硼素は、一般に黒鉛と同様の六方晶系の層状構造を有する白色粉末であり、高熱伝導性、高電気絶縁性、高潤滑性、耐腐食性、離型性、高温安定性、化学的安定性等の多くの特性を有する。そのため、六方晶窒化硼素粉末を充填した樹脂組成物は、成形加工することで熱伝導性絶縁シートとして好適に使用されている。 Hexagonal boron nitride is generally a white powder with a hexagonal layered structure similar to that of graphite, and has high thermal conductivity, high electrical insulation, high lubricity, corrosion resistance, mold release, high temperature stability, chemical It has many characteristics such as mechanical stability. Therefore, the resin composition filled with hexagonal boron nitride powder is suitably used as a heat conductive insulating sheet by molding.
該窒化硼素の製造方法としては、（ｉ）硼素を窒素、アンモニア等を用いて直接窒化する方法、（ｉｉ）ハロゲン化硼素をアンモニアやアンモニウム塩と反応させる方法、（ｉｉｉ）硼酸、酸化硼素等の硼素化合物とメラミン等の含窒素化合物とを８００℃程度の温度で反応させて硼素化合物を還元窒化するメラミン法、（ｉｖ）窒素雰囲気下、硼素化合物とカーボン源を１６００℃以上の高温に加熱して、硼素化合物を還元窒化する還元窒化法がある。（ｉｖ）の還元窒化法は、低コストの原料を使用でき、窒化硼素の製造に最も好適な方法である。 The method for producing boron nitride includes (i) a method of directly nitriding boron using nitrogen, ammonia or the like, (ii) a method of reacting boron halide with ammonia or an ammonium salt, (iii) boric acid, boron oxide or the like. A melamine method in which a boron compound is reacted with a nitrogen-containing compound such as melamine at a temperature of about 800 ° C. to reduce and nitride the boron compound, (iv) the boron compound and the carbon source are heated to a high temperature of 1600 ° C. or higher in a nitrogen atmosphere There is a reduction nitriding method for reducing and nitriding a boron compound. The reduction nitriding method (iv) can use a low-cost raw material, and is the most suitable method for producing boron nitride.
上記還元窒化法において、得られる窒化硼素の結晶性を向上させて、六方晶窒化硼素を得るためには、通常、原料に結晶化触媒を添加する技術が採用されている。その際使用される結晶化触媒としては金属酸化物が多く用いられる。その中でも含酸素カルシウム化合物が多く用いられる。 In the reduction nitriding method, in order to improve the crystallinity of the obtained boron nitride and obtain hexagonal boron nitride, a technique of adding a crystallization catalyst to the raw material is usually employed. As the crystallization catalyst used at that time, a metal oxide is often used. Of these, oxygen-containing calcium compounds are often used.
そして、このようにして得られる窒化硼素粉末は、結晶に由来する鱗片状粒子よりなる一次粒子を含み、該鱗片状粒子は熱的異方性を有している。即ち、前記鱗片状粒子は面方向よりも厚み方向の熱伝導率の方が格段に優れている。通常、上記鱗片状粒子を含む窒化硼素粉末を充填剤として用いた熱伝導性絶縁シートの場合、該熱伝導性絶縁シートの面方向に鱗片状粒子が配向するため、鱗片状粒子同士の接触の機会が少なく、該熱伝導性絶縁シートの厚さ方向の熱伝導率は低い。このような熱的異方性を改善するために、上記鱗片状粒子が多方向を向いて凝集した窒化硼素凝集粒子が提案されている（特許文献１参照）。この窒化硼素凝集粒子の特徴としては熱的異方性の改善と、大粒径化の２点が挙げられる。熱伝導性絶縁シートの樹脂と窒化硼素粒子の界面は熱抵抗となるため、熱伝達路に存在する窒化硼素粒子は少ない方が好ましく、大粒径の凝集粒子が高い熱伝導率を発現する。 The boron nitride powder thus obtained contains primary particles composed of scaly particles derived from crystals, and the scaly particles have thermal anisotropy. That is, the scale-like particles have a much better thermal conductivity in the thickness direction than in the plane direction. Usually, in the case of a thermally conductive insulating sheet using boron nitride powder containing the above flaky particles as a filler, since the flaky particles are oriented in the surface direction of the thermally conductive insulating sheet, the contact between the flaky particles There are few opportunities, and the thermal conductivity in the thickness direction of the thermally conductive insulating sheet is low. In order to improve such thermal anisotropy, boron nitride aggregated particles in which the scaly particles are aggregated in multiple directions have been proposed (see Patent Document 1). The boron nitride agglomerated particles are characterized by two points: improvement in thermal anisotropy and increase in particle size. Since the interface between the resin of the thermally conductive insulating sheet and the boron nitride particles becomes a thermal resistance, it is preferable that the boron nitride particles present in the heat transfer path be less, and the aggregated particles having a large particle diameter exhibit a high thermal conductivity.
しかしながら、上記窒化硼素凝集粒子は、取扱時、或いは、樹脂との混練時に凝集粒子が崩壊し、凝集粒子を構成する一次粒子が遊離するだけでなく、凝集粒子内部に存在する気泡が絶縁耐力の低下を引き起こすといった問題があった。 However, the boron nitride agglomerated particles are not only disintegrated during handling or kneading with the resin, and the primary particles constituting the agglomerated particles are not only released, but also the bubbles present inside the agglomerated particles have a dielectric strength. There was a problem of causing a drop.
また、熱伝導性絶縁シートは近年薄膜化の傾向にあり、それに伴い充填用フィラーの粒径も小粒径化の流れにあり、凝集させることなく高い熱伝導率を発現する窒化硼素粉末が求められている。 In recent years, thermal conductive insulating sheets have been in the trend of thinning, and the particle size of fillers for filling is also decreasing. Accordingly, boron nitride powder that exhibits high thermal conductivity without agglomeration is desired. It has been.
更に、小粒径の窒化硼素粒子を凝集させてなる窒化硼素凝集粒子は、高い比表面積に起因する、樹脂との相溶性が悪いという欠点があった。 Further, boron nitride agglomerated particles obtained by agglomerating boron nitride particles having a small particle diameter have a drawback of poor compatibility with the resin due to a high specific surface area.
従って、本発明の目的は、凝集粒子を構成しなくとも、樹脂に充填した際、六方晶窒化硼素の板状結晶に由来する熱的異方性が低減され、且つ、凝集粒子に見られる気泡の巻き込みが抑制されて高い絶縁耐力を発揮することが可能な六方晶窒化硼素粒子並びにそれを含む六方晶窒化硼素粉末、更には、前記六方晶窒化硼素粒子を含む六方晶窒化硼素粉末の製造方法を提供することにある。 Accordingly, an object of the present invention is to reduce the thermal anisotropy derived from the hexagonal boron nitride plate-like crystal when the resin is filled without forming the agglomerated particles, and the bubbles observed in the agglomerated particles. Hexagonal boron nitride particles capable of exhibiting high dielectric strength by suppressing entrainment of hexagonal crystals, hexagonal boron nitride powder containing the same, and method for producing hexagonal boron nitride powder containing the hexagonal boron nitride particles Is to provide.
本発明者等は、上記課題を解決するため鋭意検討を行った。その結果、後述する特定の条件下に、硼素化合物を還元窒化することにより、通常平らな板状結晶として得られる六方晶窒化硼素粒子に対して、該板状結晶が１つの屈曲部において連続した双晶構造を有する六方晶窒化硼素粒子を得ることに成功した。そして、かかる双晶構造を有する六方晶窒化硼素粒子は、樹脂に充填して成形した際、屈曲部を有するため一方向に配向し難く、これにより熱的異方性が抑制され、しかも、前記凝集体を構成することなく上記効果を発揮できるため、凝集体の問題点であった、樹脂に充填した際の気泡の巻き込みによる絶縁耐性の低下が無く、更には、小粒化が可能であり、前記課題を全て解消することができることを見出し、本発明を完成するに至った。 The present inventors have intensively studied to solve the above problems. As a result, the hexagonal boron nitride particles that are usually obtained as flat plate crystals by reducing and nitriding a boron compound under specific conditions described later, the plate crystals continued in one bent portion. He succeeded in obtaining hexagonal boron nitride particles with twin structure. And, when the hexagonal boron nitride particles having such twin structure are filled with a resin and molded, since they have a bent portion, it is difficult to orient in one direction, thereby suppressing thermal anisotropy, Since the above effect can be exhibited without constituting an aggregate, there was no decrease in insulation resistance due to entrainment of bubbles when filling the resin, which was a problem of the aggregate, and further, it was possible to reduce the size, The present inventors have found that all the above problems can be solved and have completed the present invention.
即ち、本発明によれば、板状結晶が１つの屈曲部において連続した双晶構造を有することを特徴とする六方晶窒化硼素粒子（以下、双晶ｈ−ＢＮ粒子）が提供される。 That is, according to the present invention, hexagonal boron nitride particles (hereinafter referred to as twin h-BN particles) are provided in which the plate-like crystals have a continuous twin structure at one bent portion.
また、本発明によれば、アスペクト比が３〜１０である双晶ｈ−ＢＮ粒子が提供される。また、本発明の双晶ｈ−ＢＮ粒子において、屈曲部の角度は、１３９±２度であるのが一般的である。 Moreover, according to this invention, the twin h-BN particle | grains whose aspect-ratio is 3-10 are provided. In the twin h-BN particles of the present invention, the angle of the bent portion is generally 139 ± 2 degrees.
更に、本発明によれば、前記双晶ｈ−ＢＮ粒子を含む六方晶窒化硼素粉末、該六方晶窒化硼素粉末よりなる樹脂用フィラー、該樹脂用フィラーを充填した樹脂組成物、該樹脂組成物よりなる電子部品の放熱材が提供される。 Furthermore, according to the present invention, hexagonal boron nitride powder containing the twin h-BN particles, a resin filler comprising the hexagonal boron nitride powder, a resin composition filled with the resin filler, and the resin composition An electronic component heat dissipating material is provided.
前記本発明の双晶ｈ−ＢＮ粒子を含む六方晶窒化硼素粉末は、硼素化合物、カーボン源及び含酸素カルシウム化合物の混合物を窒素雰囲気下にて１７００〜２２００℃の温度に加熱して該硼素化合物を還元窒化するに際し、前記混合物における硼素化合物とカーボン源との割合が、元素比（Ｂ／Ｃ）換算で、１を超え、１．２以下、含酸素カルシウム化合物の割合が、硼素化合物とカーボン源との合計量（但し、硼素化合物はＨ３ＢＯ３換算値、カーボン源はＣ換算値）１００質量部に対して、ＣａＯ換算値で１５〜３０質量部とすることにより製造することができる。 The hexagonal boron nitride powder containing twin h-BN particles of the present invention is obtained by heating a mixture of a boron compound, a carbon source and an oxygen-containing calcium compound to a temperature of 1700 to 2200 ° C. in a nitrogen atmosphere. In the reductive nitridation, the ratio of the boron compound and the carbon source in the mixture is more than 1 and 1.2 or less in terms of element ratio (B / C), and the ratio of the oxygen-containing calcium compound is boron compound and carbon. The total amount with the source (however, the boron compound is H 3 BO 3 converted value, the carbon source is C converted value) is 100 to 30 parts by mass, and the CaO equivalent is 15 to 30 parts by mass. .
本発明の双晶ｈ−ＢＮ粒子は、公知の六方晶窒化硼素粒子の代表的な構造である、板状結晶が単なる平板状を成した構造に対して、六方晶の結晶形態を有する板状結晶が１つの屈曲部において連続して構成された構造を有するため、樹脂に充填して成形する際、平板状の結晶に見られる配向が起こり難く、多方向を向いた状態で樹脂中に存在し易い。それ故、六方晶窒化硼素結晶が元来有する熱的異方性を緩和でき、該成形体の熱伝導性を効果的に解消することができる。 The twin h-BN particles of the present invention are a typical structure of known hexagonal boron nitride particles, a plate-like shape having a hexagonal crystal form compared to a structure in which the plate-like crystal has a simple plate shape. Since the crystal has a continuous structure in one bent part, when it is filled and molded into the resin, the orientation seen in the flat crystal is unlikely to occur, and it exists in the resin in a multi-directional state. Easy to do. Therefore, the thermal anisotropy inherent to the hexagonal boron nitride crystal can be relaxed, and the thermal conductivity of the molded body can be effectively eliminated.
また、上記したとおり、前記凝集粒子を構成しなくても熱的異方性が解消できるため、樹脂に充填した際の空気の巻き込みが低減でき、その結果、高い絶縁耐力を発現することが可能である。 In addition, as described above, since the thermal anisotropy can be eliminated without forming the aggregated particles, the entrainment of air when filled in the resin can be reduced, and as a result, a high dielectric strength can be expressed. It is.
（双晶ｈ−ＢＮ粒子）
粒子の特徴を示す概略図である図１に示すように、本発明の双晶ｈ−ＢＮ粒子１は、六方晶の結晶形態を有する板状結晶２が、１つの屈曲部３において連続した構造を有することを特徴とする。上記粒子の構造は、ＳＥＭによって確認することができる。
(Twinned h-BN particles)
As shown in FIG. 1, which is a schematic diagram showing the characteristics of the particle, the twin h-
本発明の六方晶窒化ホウ素粒子は、従来の六方晶窒化硼素粒子が、鱗片状の板状結晶より構成されているのに対して、該板状結晶が前記屈曲部３において屈曲して連続する屈曲構造をとる点において大きく異なる。 In the hexagonal boron nitride particles of the present invention, the conventional hexagonal boron nitride particles are composed of scale-like plate crystals, whereas the plate crystals are bent and continuous at the bent portion 3. It differs greatly in the point which takes a bending structure.
本発明の双晶ｈ−ＢＮ粒子において、前記屈曲部の角度（θ）は、殆どが１３９±２度を示し、このことから、該屈曲部は、＜１０−１０＞面の対象傾角粒界により形成されているものと推定される。 In the twin h-BN particles of the present invention, the angle (θ) of the bent portion is almost 139 ± 2 degrees, and this indicates that the bent portion is the target tilt grain boundary of the <10-10> plane. Is presumed to be formed.
また、本発明の双晶ｈ−ＢＮ粒子は、粒子の長軸の長さ（Ｌ）と厚み（ｔ）との比で表されるアスペクト比（Ｌ／ｔ）が３〜１０、好ましくは、３〜５であるものが、肉厚であり、樹脂に充填した際に粘度の上昇が抑えられ、しかも、熱伝導性も良好であるため好ましい。 The twin h-BN particles of the present invention have an aspect ratio (L / t) represented by a ratio of the length (L) of the major axis of the particles to the thickness (t) of 3 to 10, preferably What is 3-5 is preferable because it is thick, suppresses an increase in viscosity when filled in a resin, and has good thermal conductivity.
本発明の双晶ｈ−ＢＮ粒子は、平均粒径（長軸の長さ（Ｌ）の平均値）は、５〜５０μｍ、特に、１０〜３０μｍが好ましい。 The twin h-BN particles of the present invention preferably have an average particle size (average value of length (L) of major axis) of 5 to 50 μm, particularly 10 to 30 μm.
また、本発明の双晶ｈ−ＢＮ粒子において、短軸の長さ（Ｗ）は、長軸の長さ（Ｌ）に対して、０．３〜１．５、特に、０．７〜１．３の長さであることが好ましい。 In the twin h-BN particles of the present invention, the length (W) of the minor axis is 0.3 to 1.5, particularly 0.7 to 1, with respect to the length (L) of the major axis. .3 length is preferred.
尚、本発明において、双晶ｈ−ＢＮ粒子の屈曲部３の角度（θ）、長軸の長さ（Ｌ）、短軸の長さ（Ｗ）、アスペクト比（Ｌ／ｔ）などは、実施例に示す方法により測定した値である。 In the present invention, the angle (θ) of the bent part 3 of the twin h-BN particles, the length of the long axis (L), the length of the short axis (W), the aspect ratio (L / t), etc. It is the value measured by the method shown in the Example.
（六方晶窒化硼素粉末）
本発明の双晶ｈ−ＢＮ粒子は、後述の製造方法によって得ることができるが、双晶ｈ−ＢＮ粒子以外の六方晶窒化硼素粒子、例えば、平板状の構造を有する粒子も一部生成するため、一般には、該双晶ｈ−ＢＮ粒子を含む六方晶窒化硼素粉末として得られる。
(Hexagonal boron nitride powder)
The twin h-BN particles of the present invention can be obtained by the production method described later, but some hexagonal boron nitride particles other than twin h-BN particles, for example, particles having a tabular structure are also produced. Therefore, it is generally obtained as a hexagonal boron nitride powder containing the twin h-BN particles.
上記六方晶窒化硼素粉末において、双晶ｈ−ＢＮ粒子の含有率は、該粒子による効果を十分発揮するためには多いほど好ましく、２０容量％以上、特に、３０容量％以上が好ましい。 In the above hexagonal boron nitride powder, the content of twin h-BN particles is preferably as large as possible in order to sufficiently exhibit the effect of the particles, and is preferably 20% by volume or more, and particularly preferably 30% by volume or more.
勿論、篩工程での粗粒業凝集粒子の除去、乾式分級による微粉除去などにより、得られる六方晶窒化硼素粉末から、双晶ｈ−ＢＮ粒子以外の粒子を除去して分離して含有率を上げることも可能である。 Of course, by removing coarse particles in the sieving process, fine powder removal by dry classification, etc., particles other than twin h-BN particles are removed and separated from the obtained hexagonal boron nitride powder to increase the content rate. It is also possible to raise.
（窒化硼素粉末の製造方法）
本発明の窒化硼素粉末の製造方法は、特に制限されるものではないが、代表的な製造方法を例示すれば、以下の方法が挙げられる。
(Method for producing boron nitride powder)
The method for producing the boron nitride powder of the present invention is not particularly limited, but the following methods can be mentioned as typical production methods.
即ち、前記本発明の窒化硼素粉末は、硼素化合物、カーボン源としてカーボンブラック、及び含酸素カルシウム化合物を含有する混合物を、硼素化合物とカーボン源との割合が、元素比（Ｂ／Ｃ）換算で、１を超え、１．２以下、含酸素カルシウム化合物の割合が、硼素化合物とカーボン源との合計量（Ｈ３ＢＯ３、Ｃ換算値）１００質量部に対して、ＣａＯ換算で１５〜３０質量部となる割合で含有する混合物を窒素雰囲気下、１７００℃〜２２００℃で加熱することを特徴とする還元窒化法により再現性良く製造することが可能である。 That is, the boron nitride powder of the present invention is a mixture containing a boron compound, carbon black as a carbon source, and an oxygen-containing calcium compound, and the ratio of the boron compound to the carbon source is calculated in terms of element ratio (B / C). 1 to 1.2 and the proportion of the oxygen-containing calcium compound is 15 to 30 in terms of CaO with respect to 100 parts by mass of the total amount of boron compound and carbon source (H 3 BO 3 , C equivalent). It is possible to manufacture with good reproducibility by a reductive nitriding method characterized by heating a mixture containing a mass part in a nitrogen atmosphere at 1700 ° C. to 2200 ° C.
上記製造方法は、硼素化合物とカーボン源との割合とＣａＯ重量部を上記範囲に調整することが最大の特徴であり、これにより、双晶ｈ−ＢＮ粒子を含む六方晶窒化硼素粉末を得ることができる。 The above production method has the greatest feature that the ratio of the boron compound and the carbon source and the weight part of CaO are adjusted to the above ranges, thereby obtaining hexagonal boron nitride powder containing twinned h-BN particles. Can do.
反応機構は明らかではないが、本発明者らは、上記原料比とすることにより、還元窒化反応が進行し、終了する１７００〜２２００℃付近で、酸化硼素と酸化カルシウムの複合酸化物が充分な量の液相を形成し、かかる液相中で対称傾角粒界の形成を含む結晶成長が起こることによるものと推定している。上記結晶成長を確実に行わせるためには、還元窒化反応が終了後に、上記複合酸化物が、生成した窒化硼素に対して、５５質量％以上、特に、５５〜７５質量％、複合酸化物中のＣａＯ含有割合が６５質量％以下、特に４５〜６０質量％の割合で存在することが好ましい。 Although the reaction mechanism is not clear, the inventors of the present invention have sufficient boron oxide and calcium oxide composite oxide around 1700 to 2200 ° C. when the reduction nitridation reaction proceeds and is completed by setting the above raw material ratio. It is presumed that crystal growth including the formation of symmetrical tilt grain boundaries occurs in the liquid phase. In order to ensure the crystal growth, after the reduction nitridation reaction is completed, the composite oxide is 55% by mass or more, particularly 55 to 75% by mass in the composite oxide with respect to the generated boron nitride. The CaO content is preferably 65% by mass or less, particularly 45 to 60% by mass.
尚、反応終了後に残存する上記複合酸化物は、後述の洗浄操作により、生成した六方晶窒化硼素粉末から除去することができる。 The composite oxide remaining after completion of the reaction can be removed from the produced hexagonal boron nitride powder by a cleaning operation described later.
（原料の調製方法）
上記本発明の製造方法において、原料の硼素化合物としては、硼素原子を含有する化合物が制限なく使用される。例えば、硼酸、無水硼酸、メタ硼酸、過硼酸、次硼酸、四硼酸ナトリウム、過硼酸ナトリウムなどが使用できる。一般的には、入手が容易な硼酸が好適に用いられる。また、使用する硼素化合物の平均粒子径も特に限定されないが、操作性及び還元反応制御の観点から、１〜８００μｍが好ましく、１０〜７００μｍがより好ましく、２０〜６００μｍが更に好ましい。即ち、硼素化合物の平均粒子径が１μｍより大きいものを使用することによって、取扱いが容易となる。しかし、８００μｍを超えると硼素化合物の還元反応が進行し難くなる虞がある。
(Method for preparing raw materials)
In the production method of the present invention, as a raw material boron compound, a compound containing a boron atom is used without limitation. For example, boric acid, boric anhydride, metaboric acid, perboric acid, hypoboric acid, sodium tetraborate, sodium perborate and the like can be used. In general, boric acid, which is easily available, is preferably used. The average particle size of the boron compound to be used is not particularly limited, but is preferably 1 to 800 μm, more preferably 10 to 700 μm, and still more preferably 20 to 600 μm from the viewpoint of operability and reduction reaction control. That is, use of a boron compound having an average particle size larger than 1 μm facilitates handling. However, if it exceeds 800 μm, the reduction reaction of the boron compound may not easily proceed.
本発明の製造方法において、カーボン源としては公知の炭素材料が特に制限無く使用される。例えば、カーボンブラック、活性炭、カーボンファイバー等の非晶質炭素の他、ダイヤモンド、グラファイト、ナノカーボン等の結晶性炭素、モノマーやポリマーを熱分解して得られる熱分解炭素等が挙げられる。そのうち、反応性の高い非晶質炭素が好ましく、更に、工業的に品質制御されている点で、カーボンブラックが特に好適に使用される。また、上記カーボンブラックとしては、アセチレンブラック、ファーネスブラック、サーマルブラック等を使用することができる。また、上記カーボン源の平均粒子径は、０．０１〜５μｍが好ましく、０．０２〜４μｍがより好ましく、０．０５〜３μｍが特に好ましい。即ち、該カーボン源の平均粒子径を５μｍ以下とすることにより、カーボン源の反応性が高くなり、また、０．０１μｍ以上とすることにより、取り扱いが容易となる。 In the production method of the present invention, a known carbon material is used as the carbon source without any particular limitation. Examples thereof include amorphous carbon such as carbon black, activated carbon, and carbon fiber, crystalline carbon such as diamond, graphite, and nanocarbon, and pyrolytic carbon obtained by pyrolyzing a monomer or a polymer. Of these, highly reactive amorphous carbon is preferable, and carbon black is particularly preferably used in terms of industrial quality control. Moreover, as said carbon black, acetylene black, furnace black, thermal black, etc. can be used. Moreover, 0.01-5 micrometers is preferable, as for the average particle diameter of the said carbon source, 0.02-4 micrometers is more preferable, and 0.05-3 micrometers is especially preferable. That is, when the average particle diameter of the carbon source is 5 μm or less, the reactivity of the carbon source is increased, and when it is 0.01 μm or more, handling is facilitated.
本発明の製造方法において、硼素化合物とカーボン源との割合は、元素比（Ｂ／Ｃ）換算で１を超え、１．２以下とすることが必要である。即ち、上記割合が１．０以下であると、還元窒化反応終了後に残存する、酸化硼素の量が低下し、酸化硼素と酸化カルシウムから形成される複合酸化物液相が双晶窒化硼素粒子を生成させる充分な量、具体的には、該複合酸化物液相が生成した窒化硼素に対して、５５質量％となる量を下回るため、本発明の目的とする双晶ｈ−ＢＮ粒子を得ることが困難となる。また、前記割合が、１．２を超えると還元されずに揮散する硼素化合物の割合が増加し、収率が低下するばかりでなく、上記揮散成分により、製造ラインに悪影響を及ぼす。 In the production method of the present invention, the ratio between the boron compound and the carbon source needs to be more than 1 and 1.2 or less in terms of element ratio (B / C). That is, when the ratio is 1.0 or less, the amount of boron oxide remaining after the reduction and nitridation reaction is reduced, and the composite oxide liquid phase formed from boron oxide and calcium oxide contains twinned boron nitride particles. A sufficient amount to be generated, specifically, the amount of 55% by mass with respect to the boron nitride generated by the composite oxide liquid phase is less than 55% by mass, so that the twin h-BN particles targeted by the present invention are obtained. It becomes difficult. Further, when the ratio exceeds 1.2, the ratio of the boron compound that volatilizes without being reduced increases and the yield decreases, and the above-mentioned volatilizing component adversely affects the production line.
本発明の製造方法において、結晶化触媒として使用される含酸素カルシウム化合物としては、酸素とカルシウムが含まれる化合物を特に制限なく使用できる。例えば炭酸カルシウム、炭酸水素カルシウム、水酸化カルシウム、酸化カルシウム、硝酸カルシウム、硫酸カルシウム、リン酸カルシウム、シュウ酸カルシウム等を使用することが出来るし、これら２種類以上を混合して使用することも可能である。その中でも、後述する多孔質バルク体を形成するためには、酸化カルシウム、炭酸カルシウムを使用するのが好ましく、発泡性を有することから炭酸カルシウムが特に好ましい。また、上記炭酸カルシウムの平均粒子径は、二酸化炭素生成反応の制御容易性から、平均粒子径０．０１〜５００μｍが好ましく、０．０５〜４００μｍがより好ましく、０．１〜３００μｍが特に好ましい。 In the production method of the present invention, as the oxygen-containing calcium compound used as a crystallization catalyst, a compound containing oxygen and calcium can be used without particular limitation. For example, calcium carbonate, calcium hydrogen carbonate, calcium hydroxide, calcium oxide, calcium nitrate, calcium sulfate, calcium phosphate, calcium oxalate and the like can be used, or a mixture of two or more of these can be used. . Among them, in order to form a porous bulk body to be described later, it is preferable to use calcium oxide or calcium carbonate, and calcium carbonate is particularly preferable because it has foamability. The average particle size of the calcium carbonate is preferably 0.01 to 500 μm, more preferably 0.05 to 400 μm, and particularly preferably 0.1 to 300 μm, from the viewpoint of easy control of the carbon dioxide production reaction.
本発明の製造方法において、含酸素カルシウム化合物は、硼素化合物とカーボン源との合計量（Ｈ３ＢＯ３、Ｃ換算値）１００質量部に対して、ＣａＯ換算で１５〜３０質量部、特に、２０〜２５質量部の割合で使用することが重要である。即ち、含酸素カルシウム化合物の上記割合が、１５質量部未満である場合、還元窒化反応終了後に残存する、酸化カルシウムの量が低下し、酸化硼素と酸化カルシウムから形成される複合酸化物液相が双晶窒化硼素粒子を生成させる充分な量、具体的には、該複合酸化物液相が生成した窒化硼素に対して、５５質量％となる量を下回るため、本発明の目的とする双晶ｈ−ＢＮ粒子を製造することが困難となる。また、含酸素カルシウム化合物の上記割合が、３０質量部を超えると、液相を構成する複合酸化物中の酸化カルシウム割合が６５質量％を超えることにより、その融点が上昇し、双晶ｈ−ＢＮ粒子を製造することが困難となる。 In the production method of the present invention, the oxygen-containing calcium compound is 15 to 30 parts by mass in terms of CaO, particularly 100 parts by mass of the boron compound and the carbon source (H 3 BO 3 , C equivalent), It is important to use at a ratio of 20 to 25 parts by mass. That is, when the proportion of the oxygen-containing calcium compound is less than 15 parts by mass, the amount of calcium oxide remaining after the completion of the reductive nitridation reaction is reduced, and the composite oxide liquid phase formed from boron oxide and calcium oxide is reduced. A sufficient amount for generating twinned boron nitride particles, specifically, the amount of the composite oxide liquid phase is less than 55% by mass with respect to the generated boron nitride. It becomes difficult to produce h-BN particles. Moreover, when the said ratio of an oxygen-containing calcium compound exceeds 30 mass parts, when the ratio of the calcium oxide in the complex oxide which comprises a liquid phase exceeds 65 mass%, the melting | fusing point will raise, and twin h- It becomes difficult to produce BN particles.
本発明の製造方法において、上記の各原料を含む混合物の形態は特に制限されず、粉末状のままでもよいが、多孔質バルク体、造粒体を形成してもよい。尚、かかる多孔質バルク体は、例えば、硼酸、カーボン源、炭酸カルシウム、ポリオールを含む混合粉末を加熱し、硼酸からメタ硼酸の生成、メタ硼酸の溶融によりバルク体を形成すると共に、メタ硼酸が溶融している状態で、炭酸カルシウムの分解により二酸化炭素ガスを生成せしめて発泡させる方法が挙げられる。 In the production method of the present invention, the form of the mixture containing the respective raw materials is not particularly limited and may remain in a powder form, but a porous bulk body and a granulated body may be formed. The porous bulk body is formed by heating a mixed powder containing, for example, boric acid, a carbon source, calcium carbonate, and a polyol to form a bulk body by generating metaboric acid from boric acid and melting the metaboric acid. In a molten state, carbon dioxide gas is generated by decomposition of calcium carbonate and foamed.
また、上記多孔質バルク体の形状は、混合粉末の加熱に使用する容器等の形状に応じて適宜決定すればよく、特に限定されるものではないが、例えば、四角柱状、円柱状、球状、多角形状、不定形状、針状及び板状等の形状が挙げられるが、ハンドリング性の観点から、四角柱状、円柱状、球状等の形状であることが好ましい。また、その大きさは、径（球状以外は相当径）５〜３００ｍｍ程度が一般的である。 本発明の製造方法において、前記硼素化合物、カーボン源、含酸素金属化合物の混合方法特に制限されず、振動ミル、ビーズミル、ボールミル、ヘンシェルミキサー、ドラムミキサー、振動攪拌機、Ｖ字混合機等の一般的な混合機が使用可能である。
（還元窒化）
本発明の製造方法において、結晶性の高い六方晶窒化硼素粉末を得るために、通常１７００℃以上、好ましくは、１７００〜２２００℃、更に好ましくは１８００〜２０００℃で熱処理を行い、六方晶窒化硼素を得る。即ち、かかる熱処理温度が１７００℃未満では結晶性の高い六方晶窒化硼素は得られず、２２００℃以上では、効果が頭打ちとなり、経済的に不利である。
In addition, the shape of the porous bulk body may be appropriately determined according to the shape of the container or the like used for heating the mixed powder, and is not particularly limited, but for example, a quadrangular prism shape, a cylindrical shape, a spherical shape, Examples of the shape include a polygonal shape, an indefinite shape, a needle shape, and a plate shape. From the viewpoint of handling properties, a shape such as a quadrangular prism shape, a cylindrical shape, or a spherical shape is preferable. Further, the size is generally about 5 to 300 mm in diameter (equivalent diameter other than spherical). In the production method of the present invention, the mixing method of the boron compound, the carbon source, and the oxygen-containing metal compound is not particularly limited, and general methods such as a vibration mill, a bead mill, a ball mill, a Henschel mixer, a drum mixer, a vibration stirrer, and a V-shaped mixer are used. A simple blender can be used.
(Reduction nitriding)
In the production method of the present invention, in order to obtain hexagonal boron nitride powder having high crystallinity, heat treatment is usually performed at 1700 ° C. or higher, preferably 1700-2200 ° C., more preferably 1800-2000 ° C. to obtain hexagonal boron nitride. Get. That is, if the heat treatment temperature is lower than 1700 ° C., highly crystalline hexagonal boron nitride cannot be obtained, and if it is 2200 ° C. or higher, the effect reaches its peak, which is economically disadvantageous.
また、上記温度での反応時間は、酸化硼素と酸化カルシウムから形成される複合酸化物液相中での双晶結晶の成長を確実に行わせるため、１〜１０時間、特に、２〜８時間とすることが好ましい。 The reaction time at the above temperature is 1 to 10 hours, particularly 2 to 8 hours, in order to ensure the growth of twin crystals in the complex oxide liquid phase formed from boron oxide and calcium oxide. It is preferable that
本発明の窒化硼素製造方法において、窒素雰囲気は、公知の手段によって形成することが出来る。使用するガスとしては、上記窒化処理条件で硼素に窒素を与えることが可能なガスであれば特に制限されず、窒素ガス、アンモニアガスを使用することも可能であり、窒素ガス、アンモニアガスに、水素、アルゴン、ヘリウム等の非酸化性ガスを混合したガスも使用可能である。 In the boron nitride manufacturing method of the present invention, the nitrogen atmosphere can be formed by a known means. The gas to be used is not particularly limited as long as it is a gas that can give nitrogen to boron under the above nitriding conditions, and nitrogen gas and ammonia gas can also be used. A gas in which a non-oxidizing gas such as hydrogen, argon, or helium is mixed can also be used.
本発明の窒化硼素製造方法は、反応雰囲気制御の可能な公知の装置を使用して行うことができる。例えば、高周波誘導加熱やヒーター加熱により加熱処理を行う雰囲気制御型高温炉が挙げられ、バッチ炉の他、プッシャー式トンネル炉、縦型反応炉等の連続炉も使用可能である。
（酸洗浄）
本発明の製造方法において、上述の還元窒化処理を施した直後は窒化硼素を主成分とするが、硼酸カルシウム等の副生成物も含まれているため、酸を用いて洗浄することが必要となる。酸洗浄の方法は特に制限されず、公知の方法が制限無く採用されるが、例えば、窒化処理後に得られた副生成物含有窒化硼素を解砕して容器に投入し、該副生成物含有窒化硼素の５〜１０倍量の希塩酸（１０〜２０質量％ＨＣｌ）を加え、４〜８時間接触せしめる方法が挙げられる。
The boron nitride production method of the present invention can be performed using a known apparatus capable of controlling the reaction atmosphere. For example, an atmosphere-controlled high-temperature furnace that performs heat treatment by high-frequency induction heating or heater heating can be used. In addition to a batch furnace, a continuous furnace such as a pusher-type tunnel furnace or a vertical reaction furnace can also be used.
(Acid cleaning)
In the production method of the present invention, boron nitride is the main component immediately after the above-described reduction nitriding treatment, but it contains a by-product such as calcium borate. Become. The method of acid cleaning is not particularly limited, and a known method is adopted without limitation. For example, by-product-containing boron nitride obtained after nitriding is crushed and put into a container, and the by-product containing A method of adding 5 to 10 times the amount of dilute hydrochloric acid (10 to 20% by mass HCl) of boron nitride and bringing it into contact for 4 to 8 hours can be mentioned.
上記酸洗浄時に用いる酸としては、塩酸以外にも、硝酸、硫酸、酢酸等を用いることも可能である。 In addition to hydrochloric acid, nitric acid, sulfuric acid, acetic acid and the like can be used as the acid used for the acid cleaning.
上記酸洗浄の後、残存する酸を洗浄する目的で、純水を用いて洗浄する。上記洗浄の方法としては、上記酸洗浄時の酸をろ過した後、使用した酸と同量の純水に酸洗浄した窒化硼素を分散させ、再度ろ過する。この操作を数回実施することで、本発明の窒化硼素粉末の純度を達成可能となる。 After the acid cleaning, cleaning is performed using pure water for the purpose of cleaning the remaining acid. As the washing method, after filtering the acid at the time of the acid washing, boron nitride washed with acid is dispersed in the same amount of pure water as the acid used, and filtered again. By performing this operation several times, the purity of the boron nitride powder of the present invention can be achieved.
（窒化硼素粉末の用途）
本発明の窒化硼素粉末の用途は、特に限定されず、公知の用途に特に制限無く適用可能である。好適に使用される用途を例示するならば、電気絶縁性向上や熱伝導性付与等の目的で樹脂に充填剤として使用する用途が挙げられる。上記窒化硼素粉末の用途において、得られる樹脂組成物は、高い電気絶縁性や熱伝導性を有する。
(Use of boron nitride powder)
The use of the boron nitride powder of the present invention is not particularly limited, and can be applied to known uses without particular limitation. If the use used suitably is illustrated, the use used as a filler for resin for the purpose of electrical insulation improvement, thermal conductivity provision, etc. is mentioned. In the use of the boron nitride powder, the obtained resin composition has high electrical insulation and thermal conductivity.
従って、本発明の窒化硼素粉末は、電子部品の放熱シートや放熱ゲルに代表される固体状または液体状のサーマルインターフェイスマテリアル用の充填剤として好適に使用することができる。 Therefore, the boron nitride powder of the present invention can be suitably used as a filler for a solid or liquid thermal interface material typified by a heat dissipation sheet or a heat dissipation gel of an electronic component.
前記樹脂としては、ポリオレフィン、塩化ビニル樹脂、メタクリル酸メチル樹脂、ナイロン、フッ素樹脂等の熱可塑性樹脂、エポキシ樹脂、フェノール樹脂、尿素樹脂、メラミン樹脂、不飽和ポリエステル樹脂、ケイ素樹脂等の熱硬化性樹脂、合成ゴムなどが挙げられる。 Examples of the resin include thermoplastic resins such as polyolefin, vinyl chloride resin, methyl methacrylate resin, nylon, and fluorine resin, and thermosetting resins such as epoxy resin, phenol resin, urea resin, melamine resin, unsaturated polyester resin, and silicon resin. Examples thereof include resins and synthetic rubbers.
また、本発明の窒化硼素粉末は、立法晶窒化硼素や窒化硼素成型品等の窒化硼素加工品製品の原料、エンジニアリングプラスチックへの核剤、フェーズチェンジマテリアル、固体状または液体状のサーマルインターフェイスマテリアル、溶融金属や溶融ガラス成形型の離型剤、化粧品、複合セラミックス原料等の用途にも使用することができる。 Further, the boron nitride powder of the present invention is a raw material for processed boron nitride products such as cubic boron nitride and boron nitride molded products, nucleating agents for engineering plastics, phase change materials, solid or liquid thermal interface materials, It can also be used for applications such as mold release agents for molten metal and molten glass molds, cosmetics, and composite ceramic raw materials.
以下、本発明を実施例により更に詳細に説明するが、本発明は、これらの実施例に何ら限定されるものではない。 EXAMPLES Hereinafter, although an Example demonstrates this invention still in detail, this invention is not limited to these Examples at all.
尚、実施例において、各測定値は、以下の方法により測定した値である。 In the examples, each measured value is a value measured by the following method.
［屈曲部の角度］
倍率１００００倍のＳＥＭ観察像から異なる双晶ｈ−ＢＮ粒子１００個を無作為に選び、各粒子の板面と板面の成す角θを測定し、その平均値を屈曲部の角度とした。
[Bend angle]
100 different twin h-BN particles were randomly selected from a SEM observation image at a magnification of 10,000 times, and the angle θ formed by the plate surface of each particle was measured, and the average value was taken as the angle of the bent portion.
［アスペクト比、長軸の長さ（Ｌ）、短軸の長さ（Ｗ）］
倍率２０００倍のＳＥＭ観察像から異なる双晶ｈ−ＢＮ粒子１００個を無作為に選び、双晶ｈ−ＢＮ粒子の長軸の長さ（Ｌ）、厚み（ｔ）を測定してそれぞれのアスペクト比（Ｌ／ｔ）を算出し、その平均値をアスペクト比とした。また、長軸の長さ（Ｌ：平均粒子径）、短軸の長さ（Ｗ）は、測定された値の平均値を算出して求めた。
[Aspect ratio, major axis length (L), minor axis length (W)]
100 different twin h-BN particles were randomly selected from a SEM observation image at a magnification of 2000 times, and the length (L) and thickness (t) of the long axis of the twin h-BN particles were measured to determine the aspect ratio. The ratio (L / t) was calculated, and the average value was taken as the aspect ratio. Further, the length of the major axis (L: average particle diameter) and the length of the minor axis (W) were obtained by calculating the average value of the measured values.
［双晶粒子含有割合（容量％）］
倍率５００倍のＳＥＭ観察像１０水準から双晶ｈ−ＢＮ粒子と非双晶ｈ−ＢＮ粒子を選別して双晶ｈ−ＢＮ含有容量％を求めた。
[Twin grain content ratio (volume%)]
Twin h-BN particles and non-twin h-BN particles were selected from 10 levels of SEM observation image at a magnification of 500 times to determine the twin h-BN content volume%.
実施例１
硼酸、アセチレンブラック、酸化カルシウムを表１に示す割合（元素比）で、混合物として１００ｇをボールミルを使用して混合した。該混合物５０ｇを、黒鉛製タンマン炉を用い、窒素ガス雰囲気下、１５℃／分で１８００℃まで昇温し、１８００℃で２時間保持することで窒化処理した。
Example 1
Boric acid, acetylene black, and calcium oxide were mixed at a ratio (element ratio) shown in Table 1 and 100 g as a mixture was mixed using a ball mill. 50 g of the mixture was subjected to nitriding treatment by heating to 1800 ° C. at 15 ° C./min and holding at 1800 ° C. for 2 hours in a nitrogen gas atmosphere using a graphite Tamman furnace.
次いで、副生成物含有窒化硼素を解砕して容器に投入し、該副生成物含有窒化硼素の５倍量の塩酸（７重量％ＨＣｌ）を加え、回転数８００ｒｐｍで２４時間撹拌した。該酸洗浄の後、酸をろ過し、使用した酸と同量の純水に、ろ過して得られた窒化硼素を分散させ、再度ろ過した。この操作を５回繰り返した後、１５０℃で６時間乾燥させた。 Next, the by-product-containing boron nitride was crushed and charged into a container, and 5 times the amount of hydrochloric acid (7 wt% HCl) of the by-product-containing boron nitride was added, followed by stirring at a rotational speed of 800 rpm for 24 hours. After the acid washing, the acid was filtered, and boron nitride obtained by filtration was dispersed in pure water in the same amount as the acid used, and filtered again. This operation was repeated 5 times and then dried at 150 ° C. for 6 hours.
乾燥後に得られた粉末を目開き９０μｍの篩にかけて、白色の六方晶窒化硼素粉末を得た。得られた六方晶窒化硼素粉末中の双晶窒化硼素粒子の含有割合をＳＥＭ観察にて測定、双晶窒化硼素粒子のアスペクト比、長軸の長さ（Ｌ）、短軸の長さ（Ｗ）、屈曲部の角度を測定して表１に示した。 The powder obtained after drying was passed through a sieve having an opening of 90 μm to obtain white hexagonal boron nitride powder. The content ratio of twin boron nitride particles in the obtained hexagonal boron nitride powder was measured by SEM observation, the aspect ratio of the twin boron nitride particles, the length of the major axis (L), the length of the minor axis (W ), The angle of the bent portion was measured and shown in Table 1.
実施例２
硼酸とアセチレンブラックの割合を、元素比（Ｂ／Ｃ）換算で１．１となるようにし、ＣａＯを２５重量部とした以外は実施例１と同様にした。各測定値を表１に示した。
Example 2
The ratio of boric acid to acetylene black was 1.1 in terms of element ratio (B / C), and the same procedure as in Example 1 was performed except that CaO was 25 parts by weight. The measured values are shown in Table 1.
実施例３
硼酸とアセチレンブラックの割合を、元素比（Ｂ／Ｃ）換算で１．１８となるようにし、ＣａＯを３０重量部とした以外は実施例１と同様にした。各測定値を表１に示した。
Example 3
The ratio of boric acid to acetylene black was set to 1.18 in terms of element ratio (B / C), and the same procedure as in Example 1 except that CaO was 30 parts by weight. The measured values are shown in Table 1.
比較例１
硼酸とアセチレンブラックの割合を、元素比（Ｂ／Ｃ）換算で０．９となるようにし、ＣａＯを１４重量部とした以外は実施例１と同様にした。各測定値を表１に示した。
Comparative Example 1
The ratio of boric acid to acetylene black was set to 0.9 in terms of element ratio (B / C), and was the same as Example 1 except that CaO was 14 parts by weight. The measured values are shown in Table 1.
比較例２
硼酸とアセチレンブラックの割合を、元素比（Ｂ／Ｃ）換算で０．８となるようにし、ＣａＯを３５重量部とした以外は実施例１と同様にした。各測定値を表１に示した。
Comparative Example 2
The ratio of boric acid to acetylene black was set to 0.8 in terms of element ratio (B / C), and was the same as Example 1 except that 35 parts by weight of CaO was used. The measured values are shown in Table 1.
実施例４〜６
実施例１〜３で得られた窒化硼素粉末をシリコーン樹脂及びエポキシ樹脂に充填し樹脂組成物を作製し、熱伝導率の評価を行った。エポキシ樹脂は、（三菱化学株式会社製ＪＥＲ８０６）１００重量部と硬化剤（脂環式ポリアミン系硬化剤、三菱化学株式会社製ＪＥＲキュア１１３）２８重量部との混合物を準備した。シリコーン樹脂は（信越化学工業社製ＫＥ−１０９）１００重量部と硬化剤（信越化学工業社製ＣＡＴ−ＲＧ）１０重量部との混合物を準備した。次に、各基材樹脂４０体積％と、前記特定窒化硼素粉末６０体積％とを自転・公転ミキサー（倉敷紡績株式会社製ＭＡＺＥＲＵＳＴＡＲ）にて混合して樹脂組成物を得た。
Examples 4-6
The boron nitride powder obtained in Examples 1 to 3 was filled in a silicone resin and an epoxy resin to prepare a resin composition, and the thermal conductivity was evaluated. The epoxy resin prepared the mixture of 100 weight part (JER806 by Mitsubishi Chemical Corporation) and 28 weight part of hardening | curing agent (alicyclic polyamine type hardening | curing agent, JER cure 113 by Mitsubishi Chemical Corporation). The silicone resin prepared a mixture of 100 parts by weight (KE-109 manufactured by Shin-Etsu Chemical Co., Ltd.) and 10 parts by weight of a curing agent (CAT-RG manufactured by Shin-Etsu Chemical Co., Ltd.). Next, 40% by volume of each base resin and 60% by volume of the specific boron nitride powder were mixed by a rotation / revolution mixer (MAZURUSTAR, Kurashiki Boseki Co., Ltd.) to obtain a resin composition.
これを金型体に注型し、熱プレスを使用し、温度:１５０℃、圧力：５ＭＰａ、保持時間：１時間の条件で硬化させ、直径１０ｍｍ、厚さ０．２ｍｍのシートを作製した。温度波熱分析装置にて熱伝導率を測定した結果を表２に示した。実施例１〜３で作製した窒化硼素粉末を充填したシートはいずれも８．０Ｗ／ｍ・Ｋ以上、３５ｋＶ／ｍｍ以上であり、高熱伝導率、高絶縁耐力を示した。 This was cast into a mold body and cured using a hot press under the conditions of temperature: 150 ° C., pressure: 5 MPa, holding time: 1 hour, and a sheet having a diameter of 10 mm and a thickness of 0.2 mm was produced. The results of measuring the thermal conductivity with a temperature wave thermal analyzer are shown in Table 2. The sheets filled with boron nitride powder prepared in Examples 1 to 3 were 8.0 W / m · K or more and 35 kV / mm or more, respectively, and exhibited high thermal conductivity and high dielectric strength.
比較例３
比較例４
比較例１、比較例２で得られた窒化硼素粉末を用いた以外は実施例４と同様にした。温度波熱分析装置にて熱伝導率を測定した結果を表２に示した。比較例１で作製した窒化硼素粉末を充填したシートは熱伝導率が８．０Ｗ／ｍ・Ｋ以下であり、低熱伝導率であった。比較例２で作製した窒化硼素粉末を充填したシートは絶縁耐力が３５ｋＶ／ｍｍ以下であり、低絶縁耐力を示した。
Comparative Example 3
Comparative Example 4
The same procedure as in Example 4 was performed except that the boron nitride powder obtained in Comparative Examples 1 and 2 was used. The results of measuring the thermal conductivity with a temperature wave thermal analyzer are shown in Table 2. The sheet filled with boron nitride powder prepared in Comparative Example 1 had a thermal conductivity of 8.0 W / m · K or less and a low thermal conductivity. The sheet filled with boron nitride powder prepared in Comparative Example 2 had a dielectric strength of 35 kV / mm or less, and exhibited a low dielectric strength.
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