JP4237328B2 - Method for producing silicon carbide powder - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、炭化ケイ素粉末の製造方法に関する。さらに詳しくは、紫外線吸収能、光触媒機能等の光反応性に優れる炭化ケイ素粉体の製造方法に関する。
【０００２】
【従来の技術】
炭化ケイ素は、機械的強度、硬度に優れることから、構造材料や抵抗体等のバルク材あるいは研磨剤等の幅広い分野に粉末形態で使用されている。
【０００３】
これらの分野で使用される炭化ケイ素粉体は、原料や製造工程から炭素や鉄、アルミニウム等の金属が不純物として混入したものであり、白色度が低い。ここで、「白色度」とは、分光式色彩計で測定される明度（Ｌ* ）で定義されるものであり、可視光線の反射率が高く、透過率が実質的に高いことを意味する。
【０００４】
また、炭化ケイ素は、ケイ素と炭素の積層多形（以下、結晶形と称する）を有し、例えば、３Ｃ、２Ｈ、４Ｈ、６Ｈ、１５Ｒ等の多くの結晶形が存在し、共存していることが知られている（セラミックス、３１巻、No．７、１９９６）。このような種々の結晶形が混在する場合、吸収される光の波長領域が広くなり光に対する選択的な反応性が低くなるという欠点を有する。
【０００５】
例えば、特開昭45-10413号公報には炭化ケイ素粉体を大気中で酸化処理し白色の炭化ケイ素を顔料として用いる方法が開示されているが、この方法で得られる炭化ケイ素粉体は、白色度が低く、実用上問題がある。特開平4-364116号公報には、炭化ケイ素の透過スペクトルが開示されているが、可視域での透過率が低く、即ち白色度が低く、実用上の問題がある。特開平7-330543号公報にも炭化ケイ素の酸化処理法が開示されているが、錫、コバルト、バリウム等の重金属の含有を必須とするため、重金属の発色により炭化ケイ素の白色度が低下するという問題点がある。
【０００６】
【発明が解決しようとする課題】
本発明の目的は、主成分の結晶形の含有率を高め、実質的に白色度が高く、粒径が小さな炭化ケイ素粉末を得る製造方法を提供することにある。
【０００７】
【課題を解決するための手段】
本発明は、炭化ケイ素を不活性雰囲気下で熱処理し、次いで酸素を含有する雰囲気下で酸化し、粉砕する炭化ケイ素粉末の製造方法、に関する。
【０００８】
【発明の実施の形態】
本発明では、まず、炭化ケイ素を不活性雰囲気下で熱処理を行う。この操作を行うことにより、炭化ケイ素の主成分の結晶形以外の結晶形の混入量を低減できる。
【０００９】
本発明で出発物質として用いる炭化ケイ素の製造方法は、特に限定されない。また、炭化ケイ素の結晶形としては、例えば、β型である３Ｃ、α型である２Ｈ、４Ｈ、６Ｈ、１５Ｒ、３３Ｒ等のいずれでもよい。処理前の炭化ケイ素中における主成分の結晶形の含有率は、特に限定されない。
【００１０】
炭化ケイ素の純度は、９０％以上が好ましく、９５％以上がより好ましい。炭化ケイ素中の遊離炭素は、後に行う酸化処理により除去できるが、不活性雰囲気下での熱処理による凝集防止の観点から、その含有量は０．１〜５重量％であることが好ましく、酸化処理の負荷の低減の観点から、３重量％以下であることが好ましい。
【００１１】
不活性雰囲気に用いられるガスとしては、炭化ケイ素を酸化しないものであれば特に限定されず、炭化ケイ素の分解抑制、ガスの炭化ケイ素への固溶の防止、焼結抑制の観点からアルゴンガスが好ましい。また、アルゴンガス以外の窒素、炭酸ガス等の不活性ガスを少量含むアルゴンガスとの混合ガスであってもよい。熱処理中のガスの圧力は、好ましくは０．０１〜１００ＭＰａ、より好ましくは０．１〜１０ＭＰａの範囲である。
【００１２】
熱処理温度は、１２００〜１９００℃であることが好ましく、１２５０〜１９００℃であることがより好ましく、１３００〜１８００℃であることがさらに好ましい。
【００１３】
熱処理温度の下限は、炭化ケイ素の結晶形の制御や結晶性の向上及び経済性の観点から、１２００℃以上であることが好ましく、また、その上限は、炭化ケイ素の焼結による凝集を抑え、分散処理を容易にする観点から、１９００℃以下であることが好ましい。また、熱処理の時間は、炭化ケイ素の構成、熱処理の温度条件等により一概には限定されないが、０．５〜２４時間程度であることが好ましい。
【００１４】
また、原料の炭化ケイ素に、白色度を低下させない範囲でＣａ，Ｍｇ、Ａｌ、Ｂ、Ｆｅ等の金属あるいはＮ等を複合することにより、主成分以外の結晶形の割合を減少させることができる。
【００１５】
また、本発明に用いる炭化ケイ素を、必要に応じ、塩酸、硫酸、リン酸、硝酸等の酸性溶液又は水酸化ナトリウム、水酸化カリウム等のアルカリ性溶液中に添加して、炭化ケイ素中の金属不純物の除去を行うことができる。炭化ケイ素の添加量は、酸性又はアルカリ性溶液１００重量部に対し、１〜４０重量部が好ましく、５〜３０重量部がより好ましい。なお、この除去工程は、熱処理後、次の酸化処理の前後又は粉砕処理後に行っても良い。
【００１６】
熱処理に用いる方法としては、特に限定はなく、例えば、炭化ケイ素を入れた黒鉛るつぼを、グラファイト炉内で加熱する方法等が挙げられる。
【００１７】
かかる熱処理で得られた炭化ケイ素の白色度（但し、Ｌ* の値、以下同じ）は、特に限定はない。また、炭化ケイ素中における主成分の結晶形の含有率は、７０〜１００％が好ましく、８０〜１００％がより好ましい。
【００１８】
次に、熱処理して得られた炭化ケイ素を酸素を含有する雰囲気下で酸化処理する。
【００１９】
酸素を含有する雰囲気としては、特に限定されないが、経済性の観点から、空気が好ましい。酸素濃度としては、熱処理温度により一概には限定されないが、遊離炭素を除去できる量以上であることが好ましい。特に、酸素を含有する雰囲気として空気を用いる場合、酸化を均一に行う観点から、空気気流中で行うことが好ましい。
【００２０】
酸化処理温度は、酸素濃度と密接に関係があるため一概には限定されないが、４００〜８００℃の範囲、特に５００〜７００℃の範囲が望ましい。酸化処理温度の下限は、酸化効率の観点から、４００℃以上であることが好ましく、また、その上限は、炭化ケイ素の過度の酸化を抑制する観点から、８００℃以下であることが好ましい。また、酸化処理の処理時間は、０．５〜１０時間程度が好ましい。
【００２１】
酸化方法は、炭化ケイ素と雰囲気が互いに接する方式が好ましく、固定式、流動式のどちらでもよい。また、加熱方法も粉体あるいは雰囲気が加熱されるものであれば特に限定されず、例えば、炭化ケイ素を入れたアルミナるつぼを大気炉で加熱する方法等が挙げられる。
【００２２】
酸化処理は、炭化ケイ素を常温まで冷却することにより終了させることができる。
【００２３】
酸化処理で得られた炭化ケイ素の白色度は、特に限定はないが、７５〜１００が好ましく、８０〜１００がより好ましい。また、炭化ケイ素中における主成分の結晶形の含有率は、７０〜１００％が好ましく、８０〜１００％がより好ましい。
【００２４】
次に、酸化処理で得られた炭化ケイ素を、分散性を高めるために、粉砕処理する。
【００２５】
粉砕処理の方法は、特に限定されず、乾式、湿式のいずれでも良いが、着色の原因となる金属等の不純物の混入を抑制する配慮あるいは粉砕による結晶性の低下を抑制する観点から、湿式による粉砕が望ましい。具体的には、アトリッションミル、ボールミル、サンドミル等の粉砕装置を用いることが好ましい。また、粉砕の条件は、特に限定されない。
【００２６】
このような方法で得られた炭化ケイ素粉末中の主成分の結晶形の含有率は、７０〜１００％であることが好ましく、８０〜１００％であることがより好ましく、８５〜１００％であることがさらに好ましい。その白色度は、好ましくは７５〜１００、より好ましくは８０〜１００、さらに好ましくは８２〜１００である。また、その平均粒径は、１μｍ以下、好ましくは０．５μｍ以下であることが望ましい。また、その色彩は白、黄色、緑色が主であるが、他の色彩も取り得る。
【００２７】
また、得られた炭化ケイ素粉末を表面処理したり、疎水化したり、不活性化してもよい。
【００２８】
以上のようにして得られる炭化ケイ素粉末は、主成分の結晶形の含有率が高く、実質的に白色度が高く、粒径が小さいため、紫外線防御剤、光触媒、顔料、フィラーとして有用である。
【００２９】
なお、炭化ケイ素粉末中の各結晶形の含有率及び白色度の測定は、以下の方法により測定した。
【００３０】
〔各結晶形の含有率〕
炭化ケイ素粉末の試料をＸ線回折装置（「ＲＡＤ−Ｃ」、リガク（株）製）に供し、Ｃｕターゲットを用い、管電圧４０ｋＶ、管電流１２０ｍＡ、スキャンスピード１゜／分で炭化ケイ素粉末中の各結晶形の混入量を測定した。測定結果から、Ｒｕｓｋａの方法（Journal of Material Science 、14巻、1979) により、３Ｃ、４Ｈ、６Ｈ、１５Ｒのそれぞれの結晶形の含有率を算出した。
【００３１】
〔白色度の測定〕
石英製のセル（φ３１mm×１５ｍｍ）に炭化ケイ素粉末の試料を５０％充填し、分光式色彩計（「ＳＥ２０００」、日本電色工業（株）製）を用い、反射法によりＬ* を測定した。
【００３２】
【実施例】
実施例１
Ｌ* が３５、３Ｃの含有率が９６％の、高周波プラズマＣＶＤ法により合成された炭化ケイ素粉末（遊離炭素量：１．１重量％）を５ｇを黒鉛るつぼに入れ、グラファイト炉（中外炉工業（株）製）を用い１４００℃で２時間、アルゴン（高圧ガス工業（株）製）を、流量６ｄｍ3 ／分、炉内圧力１２２ｋＰａで熱処理を行った。アルゴン熱処理後のＬ* は４０、３Ｃの含有率が９７％であった。次に、酸化処理はアルミナるつぼに５ｇを大気炉（「スーパーバン」、モトヤマ（株）製）を用い０．１ｄｍ3 ／分の空気気流中、大気圧下、７００℃で行った結果、Ｌ* は７６であった。得られた炭化ケイ素粉末をポリプロピレン製ライニングのアトリッションミル（「６ＴＳＧ−１／４」、五十嵐機械製造所（株）製）に粉砕媒体φ０．５ｍｍのジルコニアボールを用い粉砕した。粉砕後の炭化ケイ素粉末（薄黄色）の３Ｃの含有率は９７％、Ｌ* は８３、平均粒径は０．２６μｍであった。
【００３３】
実施例２
Ｌ* が５６、６Ｈの含有率が６２％の、アチソン法で合成された炭化ケイ素粉末を実施例１と同様の方法で不活性雰囲気中で熱処理を行った結果、Ｌ* が７１、６Ｈの含有率が９２％であり、また、酸化処理後のＬ* は７９であった。さらに、粉砕処理後の炭化ケイ素粉末（薄緑色）の６Ｈの含有率は９２％、Ｌ* は８０、平均粒径は０．２８μｍであった。
【００３４】
比較例１
Ｌ* が３５、３Ｃの含有率が９６％の、高周波プラズマＣＶＤ法により合成された炭化ケイ素粉末５ｇを０．１ｄｍ3 ／分の空気気流中、大気圧下で７００℃で行った結果、得られた炭化ケイ素粉末（黒色）の３Ｃの含有率は９６％、Ｌ* は６８、平均粒径は１．６μｍであった。
【００３５】
比較例２
Ｌ* が５６、６Ｈの含有率が６２％の、アチソン法で合成された炭化ケイ素粉末を比較例１と同様の方法で酸化処理した結果、得られた炭化ケイ素粉末（灰色）の６Ｈの含有率は５６％、Ｌ* は６８、平均粒径は１．１μｍであった。
【００３６】
以上の結果より、実施例１〜２で得られた炭化ケイ素粉末は、比較例１〜２で得られた炭化ケイ素粉末に比べ、いずれも主成分の結晶形の含有率が高く、明度（白色度）が高く、さらに平均粒径が小さなものであることがわかる。
【００３７】
【発明の効果】
本発明により、主成分の結晶形の含有率が高く、実質的に白色度が高く、粒径が小さな炭化ケイ素粉末を得るという効果が奏される。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing silicon carbide powder. More specifically, the present invention relates to a method for producing silicon carbide powder having excellent photoreactivity such as ultraviolet absorption ability and photocatalytic function.
[0002]
[Prior art]
Since silicon carbide is excellent in mechanical strength and hardness, it is used in a powder form in a wide range of fields such as bulk materials such as structural materials and resistors, or abrasives.
[0003]
Silicon carbide powder used in these fields is a mixture of impurities such as carbon, iron, and aluminum from raw materials and manufacturing processes, and has low whiteness. Here, “whiteness” is defined by lightness (L * ) measured with a spectroscopic colorimeter, and means that the reflectance of visible light is high and the transmittance is substantially high. .
[0004]
Silicon carbide has a laminated polymorphism (hereinafter referred to as crystal form) of silicon and carbon, and many crystal forms such as 3C, 2H, 4H, 6H, and 15R exist and coexist. (Ceramics, Vol. 31, No. 7, 1996). When such various crystal forms coexist, there is a disadvantage that the wavelength range of absorbed light is widened and selective reactivity to light is lowered.
[0005]
For example, Japanese Patent Laid-Open No. 45-10413 discloses a method in which silicon carbide powder is oxidized in the atmosphere and white silicon carbide is used as a pigment. The whiteness is low and there is a problem in practical use. Japanese Laid-Open Patent Publication No. 4-364116 discloses a transmission spectrum of silicon carbide, but has a problem in practical use because of low transmittance in the visible region, that is, low whiteness. Japanese Laid-Open Patent Publication No. 7-330543 discloses a method for oxidizing silicon carbide. However, since the inclusion of heavy metals such as tin, cobalt, and barium is essential, the whiteness of silicon carbide decreases due to the coloration of heavy metals. There is a problem.
[0006]
[Problems to be solved by the invention]
An object of the present invention is to provide a production method for obtaining a silicon carbide powder having an increased content of a main component crystal form, a substantially high whiteness, and a small particle size.
[0007]
[Means for Solving the Problems]
The present invention relates to a method for producing silicon carbide powder in which silicon carbide is heat-treated in an inert atmosphere, then oxidized and ground in an oxygen-containing atmosphere.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, first, silicon carbide is heat-treated in an inert atmosphere. By performing this operation, the amount of mixed crystal forms other than the main crystal form of silicon carbide can be reduced.
[0009]
The method for producing silicon carbide used as a starting material in the present invention is not particularly limited. Further, as the crystal form of silicon carbide, for example, any of 3C which is β-type, 2H, 4H, 6H, 15R, 33R which is α-type may be used. The content rate of the crystal form of the main component in the silicon carbide before the treatment is not particularly limited.
[0010]
The purity of silicon carbide is preferably 90% or more, and more preferably 95% or more. Free carbon in silicon carbide can be removed by an oxidation treatment performed later, but from the viewpoint of preventing aggregation by heat treatment under an inert atmosphere, the content is preferably 0.1 to 5% by weight, and oxidation treatment From the viewpoint of reducing the load, it is preferably 3% by weight or less.
[0011]
The gas used in the inert atmosphere is not particularly limited as long as it does not oxidize silicon carbide. Argon gas is used from the viewpoint of suppressing decomposition of silicon carbide, preventing solid solution of gas in silicon carbide, and suppressing sintering. preferable. Further, it may be a mixed gas with argon gas containing a small amount of inert gas such as nitrogen and carbon dioxide other than argon gas. The pressure of the gas during the heat treatment is preferably in the range of 0.01 to 100 MPa, more preferably 0.1 to 10 MPa.
[0012]
The heat treatment temperature is preferably 1200 to 1900 ° C, more preferably 1250 to 1900 ° C, and further preferably 1300 to 1800 ° C.
[0013]
The lower limit of the heat treatment temperature is preferably 1200 ° C. or higher from the viewpoint of controlling the crystal form of silicon carbide, improving crystallinity, and economy, and the upper limit thereof suppresses aggregation due to sintering of silicon carbide, From the viewpoint of facilitating the dispersion treatment, it is preferably 1900 ° C. or lower. Further, the heat treatment time is not generally limited by the structure of silicon carbide, the temperature condition of the heat treatment, etc., but is preferably about 0.5 to 24 hours.
[0014]
Further, by combining the raw material silicon carbide with a metal such as Ca, Mg, Al, B, Fe, or N within a range that does not reduce the whiteness, the proportion of crystal forms other than the main component can be reduced. .
[0015]
Further, if necessary, the silicon carbide used in the present invention is added to an acidic solution such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid or an alkaline solution such as sodium hydroxide, potassium hydroxide, etc. Can be removed. The amount of silicon carbide added is preferably 1 to 40 parts by weight and more preferably 5 to 30 parts by weight with respect to 100 parts by weight of the acidic or alkaline solution. This removal step may be performed after the heat treatment, before or after the next oxidation treatment or after the pulverization treatment.
[0016]
The method used for the heat treatment is not particularly limited, and examples thereof include a method of heating a graphite crucible containing silicon carbide in a graphite furnace.
[0017]
The whiteness (however, the value of L * , the same applies hereinafter) of silicon carbide obtained by such heat treatment is not particularly limited. Moreover, 70-100% is preferable and, as for the content rate of the crystal form of the main component in silicon carbide, 80-100% is more preferable.
[0018]
Next, the silicon carbide obtained by the heat treatment is oxidized in an atmosphere containing oxygen.
[0019]
The atmosphere containing oxygen is not particularly limited, but air is preferable from the viewpoint of economy. The oxygen concentration is not generally limited depending on the heat treatment temperature, but it is preferably at least an amount capable of removing free carbon. In particular, when air is used as the atmosphere containing oxygen, it is preferably performed in an air stream from the viewpoint of uniformly oxidizing.
[0020]
The oxidation treatment temperature is not generally limited because it is closely related to the oxygen concentration, but is preferably in the range of 400 to 800 ° C, particularly in the range of 500 to 700 ° C. The lower limit of the oxidation treatment temperature is preferably 400 ° C. or higher from the viewpoint of oxidation efficiency, and the upper limit thereof is preferably 800 ° C. or lower from the viewpoint of suppressing excessive oxidation of silicon carbide. The treatment time for the oxidation treatment is preferably about 0.5 to 10 hours.
[0021]
The oxidation method is preferably a method in which the silicon carbide and the atmosphere are in contact with each other, and may be either a fixed type or a fluid type. The heating method is not particularly limited as long as the powder or atmosphere is heated, and examples thereof include a method of heating an alumina crucible containing silicon carbide in an atmospheric furnace.
[0022]
The oxidation treatment can be terminated by cooling silicon carbide to room temperature.
[0023]
The whiteness of silicon carbide obtained by the oxidation treatment is not particularly limited, but is preferably 75 to 100, and more preferably 80 to 100. Moreover, 70-100% is preferable and, as for the content rate of the crystal form of the main component in silicon carbide, 80-100% is more preferable.
[0024]
Next, the silicon carbide obtained by the oxidation treatment is pulverized in order to improve dispersibility.
[0025]
The method of the pulverization treatment is not particularly limited and may be either dry or wet. However, from the viewpoint of suppressing the mixing of impurities such as metals that cause coloring, or from the viewpoint of suppressing the decrease in crystallinity due to pulverization, it is possible to use the pulverization method. Grinding is desirable. Specifically, it is preferable to use a pulverizer such as an attrition mill, a ball mill, or a sand mill. Further, the pulverization conditions are not particularly limited.
[0026]
The content of the main component crystal form in the silicon carbide powder obtained by such a method is preferably 70 to 100%, more preferably 80 to 100%, and 85 to 100%. More preferably. The whiteness is preferably 75 to 100, more preferably 80 to 100, and still more preferably 82 to 100. The average particle size is 1 μm or less, preferably 0.5 μm or less. The main colors are white, yellow, and green, but other colors are possible.
[0027]
Further, the obtained silicon carbide powder may be surface-treated, hydrophobized, or inactivated.
[0028]
The silicon carbide powder obtained as described above is useful as an ultraviolet protective agent, a photocatalyst, a pigment, and a filler because of its high content of the main component crystal form, substantially high whiteness, and small particle size. .
[0029]
In addition, the content rate and whiteness of each crystal form in the silicon carbide powder were measured by the following methods.
[0030]
[Content of each crystal form]
A sample of silicon carbide powder was subjected to an X-ray diffractometer (“RAD-C”, manufactured by Rigaku Corporation), and a Cu target was used. In a silicon carbide powder at a tube voltage of 40 kV, a tube current of 120 mA, and a scan speed of 1 ° / min. The amount of each crystal form was measured. From the measurement results, the content of each crystal form of 3C, 4H, 6H, and 15R was calculated by the method of Ruska (Journal of Material Science, Vol. 14, 1979).
[0031]
(Measurement of whiteness)
A quartz cell (φ31 mm × 15 mm) was filled with 50% of a sample of silicon carbide powder, and L * was measured by a reflection method using a spectroscopic colorimeter (“SE2000”, manufactured by Nippon Denshoku Industries Co., Ltd.). .
[0032]
【Example】
Example 1
5 g of silicon carbide powder (free carbon content: 1.1 wt%) synthesized by a high-frequency plasma CVD method with L * of 35, 3C content of 96% is placed in a graphite crucible, (Trade name) was used, and heat treatment was performed at 1400 ° C. for 2 hours with argon (manufactured by High Pressure Gas Industry Co., Ltd.) at a flow rate of 6 dm 3 / min and a furnace pressure of 122 kPa. L * after argon heat treatment was 40%, and the content of 3C was 97%. Next, the oxidation treatment was carried out at 700 ° C. under atmospheric pressure in an air stream of 0.1 dm 3 / min using an atmospheric furnace (“Supervan”, manufactured by Motoyama Co., Ltd.) in an alumina crucible. * Was 76. The obtained silicon carbide powder was pulverized in a polypropylene-lined attrition mill (“6TSG-1 / 4”, manufactured by Igarashi Machinery Co., Ltd.) using zirconia balls having a pulverization medium of φ0.5 mm. The 3C content of the pulverized silicon carbide powder (light yellow) was 97%, L * was 83, and the average particle size was 0.26 μm.
[0033]
Example 2
A silicon carbide powder synthesized by the Atchison method having an L * content of 56 and 6H of 62% was heat-treated in an inert atmosphere in the same manner as in Example 1. As a result, L * was 71 and 6H. The content rate was 92%, and L * after the oxidation treatment was 79. Further, the content of 6H in the silicon carbide powder (light green) after the pulverization treatment was 92%, L * was 80, and the average particle size was 0.28 μm.
[0034]
Comparative Example 1
As a result, 5 g of silicon carbide powder synthesized by a high-frequency plasma CVD method having an L * content of 35 and a content of 3C of 96% was obtained at 700 ° C. in an air stream of 0.1 dm 3 / min at atmospheric pressure. The obtained silicon carbide powder (black) had a 3C content of 96%, L * of 68, and an average particle size of 1.6 μm.
[0035]
Comparative Example 2
The silicon carbide powder synthesized by the Atchison method with L * of 56 and 6H content of 62% was oxidized by the same method as in Comparative Example 1, and as a result, the obtained silicon carbide powder (gray) contained 6H. The rate was 56%, L * was 68, and the average particle size was 1.1 μm.
[0036]
From the above results, the silicon carbide powders obtained in Examples 1 and 2 have a higher content of the main component crystal form and lightness (white color) than the silicon carbide powders obtained in Comparative Examples 1 and 2. It is clear that the average particle size is small.
[0037]
【The invention's effect】
According to the present invention, it is possible to obtain a silicon carbide powder having a high content of the main component crystal form, a substantially high whiteness, and a small particle size.
Claims (2)
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