JP3743464B2 - Silicon oxynitride powder and method for producing the same - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、構造用セラミックスの中で、特に耐食性、耐酸化性に優れた酸窒化ケイ素セラミックスの製造原料として好適な易焼結性の酸窒化ケイ素粉末に関する。
また、本発明は、排ガス触媒などの触媒担体として有用な高比表面積の酸窒化ケイ素粉末に関するものである。
【０００２】
【従来技術及びその問題点】
酸窒化ケイ素セラミックスは、窒化ケイ素セラミックスや炭化ケイ素セラミックスと同様に、高温で使用される非酸化物系セラミックスとして、その用途展開が期待されている。
酸窒化ケイ素セラミックスは、特に高温における耐食性、耐酸化性に優れており、耐食部材、ルツボなどへの適用が検討されている。
しかしながら、これまで、高度に品質の管理された酸窒化ケイ素粉末は製造されておらず、その焼結特性に関する研究報告は非常に少ない。
【０００３】
一方、排ガス触媒などの触媒担体の分野では、シリカ（ＳｉＯ2 ）、アルミナ（Ａｌ2Ｏ3）などに代わる新しい触媒担体が待望されている。
シリカは酸性（等電点３．０付近）、アルミナは中性（等電点９．０付近）であるが、粒子表面の塩基点の多い物質の使用が検討されている。
窒化ケイ素粒子表面には、Ｓｉ＝ＮＨ、Ｓｉ−ＮＨ2 結合が存在し、多数の塩基点が形成されている。しかしながら、窒化ケイ素は耐酸化性が悪く、また、高比表面積の窒化ケイ素粉末は販売されていない。実験室規模では、気相反応などにより高比表面積の非晶質窒化珪素粉末が合成されているが、非晶質窒化ケイ素は不安定であり、空気中に取り出すと、たちまち酸化されてしまうという難点がある。
このような観点から注目されるのが、酸窒化ケイ素であり、粒子表面には、Ｓｉ−Ｎ結合に由来する塩基点が多数存在し、なおかつ、空気中に取り出しても安定である。
【０００４】
従来、酸窒化ケイ素粉末の製法としては、（１）市販の結晶質窒化珪素粉末とシリカ粉末とを混合して、１４００〜１６００℃に焼成する方法、（２）微粒のシリカ粉末をアンモニア雰囲気中１０００〜１２００℃で焼成する方法、（３）四塩化ケイ素とアンモニアとの反応により生成したシリコンジイミド、非晶質窒化ケイ素粉末などの含窒素シラン化合物を、ある程度酸化させた後、１０００〜１２００℃で焼成する方法などが知られている。
しかしながら、（１）の方法で得られた酸窒化ケイ素粉末は、結晶質窒化ケイ素、結晶質酸窒化ケイ素（Ｓｉ2ＯＮ2）及びシリカの混合物であり、粒径も粗くて不均質である。（２）の方法で得られた酸窒化ケイ素粉末には、多量の未反応シリカが残存しており、粒度分布が広くて不均質である。（３）の方法によれば、均質な酸窒化ケイ素粉末が得られる。しかしながら、これまでこの方法による系統的な酸窒化ケイ素粉末の合成に関する報告はない。
【０００５】
酸窒化ケイ素材料自体の用途開発が遅れているため、アルミナや窒化ケイ素セラミックスに比べて、酸窒化ケイ素材料に関する研究報告は極めて少ない。その原因の一つは、高度に品質の管理された酸窒化ケイ素粉末が得られていないということにある。
従来、報告されている酸窒化珪素粉末は、シリカ相、窒化ケイ素相及び酸窒化ケイ素相の混合物であり、組成的にも不均質であり、粒度分布も広い。また、高比表面積のものは、空気中５００℃以上では不安定であった。
このため、酸窒化ケイ素セラミックスを作製しても十分な強度特性が得られない、触媒担体に使用しても十分な触媒活性が出ないなどの問題があった。
酸窒化ケイ素粉末に要求される化学組成、比表面積及び粒度分布については、全く分かっておらず、またこれらの因子の制御方法も知られていなかった。
【０００６】
【発明の目的】
本発明の目的は、上記課題を解決し、化学組成、比表面積及び粒度分布の高度に制御された高品質の酸窒化ケイ素粉末を提供することにある。
また、本発明の目的は、このような高品質の酸窒化ケイ素粉末を再現性良く安定して製造できる製造方法を提供することにある。
【０００７】
【問題点を解決するための手段】
本発明者等は、酸窒化ケイ素粉末の合成方法について、種々検討した結果、シリコンジイミド、シリコンニトロゲンイミド、非晶質窒化ケイ素粉末などの含窒素シラン化合物を酸化処理した後、窒素含有不活性ガス雰囲気下、１２５０℃以上の高温に焼成することにより、化学組成、比表面積及び粒度分布がそれぞれ特定の範囲にある高品質の酸窒化ケイ素粉末を製造することができることを知見した。
本発明は、上記の知見に基づいてなされたものであり、酸素含有量１０〜３０重量％、窒素含有量３２〜１７重量％であって、比表面積が５０ｍ2 ／ｇ以上、粒度分布曲線におけるメジアン平均径が１．０〜５０μｍであることを特徴とする酸窒化ケイ素粉末に関するものである。
【０００８】
本発明の酸窒化ケイ素粉末は、酸素含有量１０〜３０重量％、好ましくは１２〜２５重量％、窒素含有量３２〜１７重量％、好ましくは、３１〜２１重量％であって、比表面積が５０ｍ2 ／ｇ以上、好ましくは７０ｍ2 ／ｇ以上、粒度分布曲線におけるメジアン平均径が１．０〜５０μｍ、好ましくは１．５〜３０μｍである。
【０００９】
酸素含有量が１０重量％よりも少ないか、または、窒素含有量が３２重量％よりも多くなると、結晶質または非晶質の窒化ケイ素相が混在する。結晶質窒化ケイ素が混在すると粉末の比表面積が低下し、非晶質窒化ケイ素が混在すると、空気中で加水分解するなど不安定となって取り扱いが難しいので、共に好ましくない。
また、酸素含有量が３０重量％よりも多いか、または、窒素含有量が１７重量％よりも少なくなると、シリカ相が混在する。シリカ相は粗大化して粒度分布が広がり、また粒子表面の塩基点が極度に減少するので好ましくない。
【００１０】
比表面積が５０ｍ2 ／ｇ未満になると、焼結活性が低下して、高密度な酸窒化ケイ素セラミックスが得られない。また、触媒活性が低下して、所望の触媒性能を満足できないなどの問題が発生するので好ましくない。
さらに、粒度分布曲線におけるメジアン平均径が１．０μｍ以下となると、粉末がフワフワとして嵩高く取り扱いが難しくなる。成形体にした場合には、気孔率が著しく上昇して焼結が難しくなり、また、触媒担体として利用する際も、触媒成分の担持等が難しくなって好ましくない。メジアン平均径が５０μｍを超えて粗大化すると、焼結体製造原料として使用できない。また、触媒担体としても、凝集塊が大き過ぎ、粒子表面を有効に利用することができなくて、触媒性能が低下するので好ましくない。
【００１１】
また、本発明の酸窒化ケイ素粉末は、レーザー回折法により測定した重量基準の粒度分布曲線における粗粒側の９０％径ｄ90と微粒側の１０％径ｄ10との比率ｄ90／ｄ10が２．５〜１０、特に３〜８の範囲であることが好ましい。
ｄ90／ｄ10が２．５よりも小さくなると、充填密度が低下し、酸窒化ケイ素セラミックスの製造においては、成形体密度の著しい低下、触媒への利用においては、体積効率（反応容器の体積当たりの生成物の収量）の低下という問題を引き起こすので好ましくない。
ｄ90／ｄ10が１０よりも大きくなると、粒度分布が極端に広く、成形体が不均質で焼結体の強度特性が低下する。また、触媒担体としても不均一であり、再現性ある安定した触媒性能が発現しないなどの問題が発生するので好ましくない。
【００１２】
さらに、本発明の酸窒化ケイ素粉末は、大気中９５０℃、５時間焼成後の比表面積の変化が１０％以下、特に、８％以下であることが好ましい。比表面積の変化が１０％を超えるというような変化の大きな粉末は、６００〜１０００℃の温度域で安定して使用することができない。
例えば、焼結用原料として使用した場合には、不活性ガス雰囲気中での焼結にもかかわらず、焼結の初期過程で、粒子間の接触点のネック成長が優先的に起こり、バルク体としての緻密化を阻害する要因となる。
また、各種の燃焼ガス中に含有される大気汚染ガスを分解、除去するための排ガス触媒担体として使用した場合には、使用中の担体比表面積の低下により触媒活性が低下し、排ガス触媒としての安定した性能の維持と耐久性の確保が困難となる。
【００１３】
次に、本発明の酸窒化ケイ素粉末を製造する方法について説明する。
本発明の酸窒化ケイ素粉末は、シリコンジイミド、シリコンニトロゲンイミド、非晶質窒化ケイ素粉末等の含窒素シラン化合物を酸化処理することにより、酸素含有量が１０．５〜３２重量％であって、比表面積が２００ｍ2 ／ｇ以上である前駆体粉末とした後、これを充填密度が０．１５ｇ／ｃｍ3 以下の状態で窒素含有不活性ガス雰囲気下、１２５０℃以上の高温で焼成することにより製造される。
【００１４】
含窒素シラン化合物の酸化処理は、粉末を薄く広げて大気に開放、接触させる方法、粉末を充填した容器内に加湿空気を流通させて加水分解する方法、粉末を乾燥空気流通下３００〜７００℃に加熱する方法など、種々の方法により実施することができる。
本発明において重要なことは、このような酸化処理により得られた酸窒化ケイ素前駆体粉末の比表面積を２００ｍ2 ／ｇ以上、好ましくは、２５０ｍ2 ／ｇ以上とすることである。前駆体粉末の比表面積が２００ｍ2 ／ｇよりも小さくなると、焼成過程における粒成長の進行により、１２５０℃以上の高温で焼成した後の比表面積が７０ｍ2 ／ｇ以下、条件によっては５０ｍ2 ／ｇ以下に低下してしまうので好ましくない。
また、前駆体粉末の酸素含有量は、１０．５〜３２重量％とする。これにより、得られる酸窒化ケイ素粉末の酸素含有量を１０〜３０重量％の範囲とすることができる。
【００１５】
次いで、得られた前駆体粉末を、焼成容器に充填密度が０．１５ｇ／ｃｍ3 以下となるように充填し、窒素含有不活性ガス雰囲気下、１２５０℃以上の高温で焼成する。焼成温度を１２５０℃未満に下げれば、得られる酸窒化ケイ素粉末は高比表面積となる。しかしながら、高温焼成を経ていないため、粉末の安定性が悪く、例えば、大気中９５０℃で５時間焼成した後の比表面積の変化が１０％以上となってしまう。したがって、１２５０℃以上の高温焼成は、安定性、耐久性に優れた高比表面積の酸窒化ケイ素粉末製造のための必須要件である。
また、充填密度が０．１５ｇ／ｃｍ3 よりも大きくなると、得られる酸窒化ケイ素粉末のメジアン平均径が５０μｍよりも大きくなってしまう。
【００１６】
１２５０℃以上の高温で焼成して比表面積５０ｍ2 ／ｇ以上の酸窒化ケイ素粉末を安定して得るためには、焼成前の酸窒化ケイ素前駆体の比表面積は２００ｍ2 ／ｇ以上でなければならない。できるだけ高温で焼成した方が、高安定性の酸窒化ケイ素粉末が得られるので、本発明における焼成温度は、１２５０℃以上、好ましくは、１３００〜１６００℃、さらに好ましくは、１４００〜１６００℃である。１４００℃以上の高温焼成で比表面積５０ｍ2 ／ｇ以上の酸窒化ケイ素粉末を得るのに必要な前駆体粉末の比表面積は２５０ｍ2 ／ｇ以上、好ましくは、３００ｍ2 ／ｇ以上である。
【００１７】
本発明の酸窒化ケイ素粉末を用いて製造された酸窒化ケイ素セラミックスは、気孔率が低くて高密度であり、耐食性、耐酸化性並びに強度特性に優れていることから、本発明の酸窒化ケイ素粉末は、耐食部材、高温構造部品として使用される酸窒化ケイ素セラミックスの製造用原料として好適なものである。
また、本発明の酸窒化ケイ素粉末を用いて製造された排ガス触媒は、高活性かつ高寿命であることから、本発明の酸窒化ケイ素粉末は、各種燃焼ガスの浄化に使用される排ガス触媒向けの触媒担体としても好適なものである。
【００１８】
【実施例】
以下に実施例と比較例を挙げて更に具体的に本発明を説明する。
実施例１〜１４及び比較例１〜５
下記の製造方法（イミド分解法）及び表１に示す製造条件により、酸窒化ケイ素粉末を製造した。得られた酸窒化ケイ素粉末の特性を表２に示す。
〔酸窒化ケイ素粉末の製造方法〕
−４０℃に冷却された直径３０ｃｍ、高さ４５ｃｍの縦型反応槽内の空気を窒素ガスで置換した後、液体アンモニア及びトルエンを仕込んだ。反応槽内では、上層の液体アンモニアと下層のトルエンとに分離した。予め調製した四塩化ケイ素２０〜３５重量％、残部トルエンよりなる溶液を、導管を通じて、ゆっくり撹拌されている下層に供給した。トルエン溶液の供給と共に、上下層の界面近傍に白色の反応生成物が析出した。
反応終了後、反応液を濾過槽へ移送し、生成物を濾別して、液体アンモニアで洗浄し、精製シリコンジイミドを得た。
【００１９】
反応の際の四塩化ケイ素と液体アンモニアとの比率（体積基準）を３／２００〜１／２５の範囲で変化させることにより、比表面積４５０〜８８０ｍ2 ／ｇのシリコンジイミドを合成した。なお、前記反応の初期段階では、液体アンモニアは大過剰に存在するが、反応の進行によりアンモニアが消費されるため、液体アンモニアも連続的に反応槽へ供給することになる。そして、定常状態において反応槽内へ供給する四塩化ケイ素と液体アンモニアとの体積比率を３／２００〜１／２５の範囲で変化させることにより、比表面積４５０〜８８０ｍ2 ／ｇのシリコンジイミドを合成した。
【００２０】
生成したシリコンジイミドをバットに薄く広げて、大気に開放し、表１に記載の時間だけ大気と接触させた。
また、生成したシリコンジイミドを丸底容器に入れ、自公転式撹拌機を用いて撹拌しながら、６３℃の飽和水蒸気を含有する空気を２ｌ／ｍｉｎの割合で流通しつつ、表１に記載の時間だけ酸化処理した。
このようにして酸化処理したシリコンジイミドの酸素含有量と比表面積を確認した後、内径φ３８０ｍｍのカーボン容器（内面は炭化ケイ素で被覆されている）に３００ｇ程度を充填し、電気炉にて窒素雰囲気中、表１に記載の温度に１時間保持して焼成した。焼成に際しては、室温から１０００℃まで１時間で昇温した後、表１に記載の最高保持温度まで、１００℃／ｈで昇温した。
得られた酸窒化ケイ素粉末の比表面積、化学組成及び粒度分布を測定した結果を表２に示す。
【００２１】
なお、比表面積は、ＢＥＴ一点法、酸素含有量は、ＪＩＳ Ｒ １６０３記載の不活性ガス融解−赤外線吸収法（ＬＥＣＯ法）、窒素含有量は、ＪＩＳ Ｒ １６０３記載の加圧酸分解−水蒸気蒸留分離−中和滴定法により測定した。
また、粒度分布はレーザー回折法により測定した。
酸窒化ケイ素粉末の比表面積の変化率は、酸窒化ケイ素粉末１０ｇを厚さ５ｍｍ程度に薄く広げ、箱型電気炉を用いて、大気中９５０℃で５時間焼成し、焼成前後の比表面積を測定して求めた。
【００２２】
【表１】
【表２】
実施例１５〜１７及び比較例６〜７
実施例１〜１４と同様にして、シリコンジイミドを合成した。
得られたシリコンジイミド約３００ｇをバットに薄く広げて、電気炉にて５％の酸素を含有する窒素ガスを５ｌ／ｍｉｎの割合で流通させながら、７００℃で表３に記載の時間だけ仮焼して、アモルファス状態の酸窒化ケイ素前駆体粉末を得た。
このようにして得られた前駆体粉末の酸素含有量と比表面積を確認した後、内径φ３８０ｍｍのカーボン容器（内面は炭化ケイ素で被覆されている）に３００ｇ程度を充填し、電気炉にて窒素雰囲気中、表３に記載の温度に１時間保持して焼成した。焼成に際しては、室温から１０００℃まで１時間で昇温した後、表３に記載の最高保持温度まで、１００℃／ｈで昇温した。
得られた酸窒化ケイ素粉末の比表面積、化学組成及び粒度分布を測定した結果を表４に示す。
【００２５】
【表３】
【表４】
［焼結試験］
実施例１〜１７及び比較例１〜７で得られた酸窒化ケイ素粉末について焼結体特性を測定した。
酸窒化ケイ素粉末９２重量部に焼結助剤としてＹ2Ｏ3６重量部とＡｌ2Ｏ3２重量部を添加し、エタノール溶媒にて４８時間ボールミルで湿式混合した後、スラリーを乾燥した。
得られた顆粒をゴム型に充填し、１．５ｔｏｎ／ｃｍ2 の加圧下で静水圧成形を行い、４０×７０×５．５ｍｍ程度の寸法の矩形状の成形体を得た。
これらの成形体を窒化ケイ素製容器に入れ、窒素ガス雰囲気下１６５０℃の温度で３時間焼結して、酸窒化ケイ素質焼結体を得た。
得られた焼結体からテストピースを切り出し、表面を研削、研磨後、嵩密度測定、酸化試験、酸化後強度試験を以下の方法で行った。それらの結果を表５に示す。
（ｌ）嵩密度測定：アルキメデス法により測定し、相対密度で表示した。
（２）酸化試験：箱型電気炉にテストピースを装入し、空気中１３００℃で５０時間加熱した後の重量増加を測定した。
（３）酸化後強度試験：（２）の酸化試験を行った後のテストピースの室温における四点曲げ強度を測定した。
【００２８】
【表５】
［触媒担体としての性能評価試験］
実施例１〜１７及び比較例１〜７で得られた酸窒化ケイ素粉末について触媒担体としての性能評価試験を行った。
酸窒化ケイ素粉末にパラジウムアセチルアセトナート（三津和化学薬品）を添加し、エタノール溶媒にて２４時間ボールミル処理した後、スラリーを乾燥した。
乾燥後の試料を８５０℃で４時間焼成してＰｄを担持した酸窒化ケイ素触媒（Ｐｄ／Ｓｉ−Ｏ−Ｎ）を調製した。Ｐｄの担持量は、０．５重量％とした。得られた触媒試料は直径０．６〜１．２ｍｍの顆粒にして、活性試験に供した。
触媒活性の評価には、常圧固定床流通式反応装置を使用した。内径１０ｍｍの石英ガラス反応管の中央部に触媒試料２ｍｌを充填した。触媒充填層を外熱により６００℃に加熱し、反応管上部よりメタン−空気混合ガス（メタン濃度０．５ｖｏｌ％）を１．６６ｌ／ｍｉｎの流量で供給した。
燃焼ガスの組成をガスクロマトグラフにより測定し、メタンのピーク面積から転化率を求めた。反応は連続して１００時間以上行い、１００時間経過した時点におけるメタンの転化率と燃焼速度定数で活性を評価した。触媒担体としての性能評価結果を表６に示す。
燃焼速度定数はメタンの転化速度を触媒充填層の体積で割って算出した。
【００３０】
【表６】
BACKGROUND OF THE INVENTION
The present invention relates to a readily sinterable silicon oxynitride powder suitable as a raw material for producing silicon oxynitride ceramics having excellent corrosion resistance and oxidation resistance among structural ceramics.
The present invention also relates to a high specific surface area silicon oxynitride powder useful as a catalyst carrier for exhaust gas catalysts and the like.
[0002]
[Prior art and its problems]
Silicon oxynitride ceramics, like silicon nitride ceramics and silicon carbide ceramics, are expected to be used as non-oxide ceramics used at high temperatures.
Silicon oxynitride ceramics are particularly excellent in corrosion resistance and oxidation resistance at high temperatures, and application to corrosion-resistant members, crucibles, and the like has been studied.
However, so far, highly controlled silicon oxynitride powder has not been produced, and there are very few research reports on its sintering characteristics.
[0003]
On the other hand, in the field of catalyst carriers such as exhaust gas catalysts, new catalyst carriers that can replace silica (SiO 2 ), alumina (Al 2 O 3 ), and the like are desired.
Silica is acidic (near isoelectric point 3.0), and alumina is neutral (near isoelectric point 9.0), but the use of substances with many basic points on the particle surface has been studied.
On the surface of the silicon nitride particles, there are Si═NH and Si—NH 2 bonds, and a large number of base points are formed. However, silicon nitride has poor oxidation resistance, and high specific surface area silicon nitride powder is not sold. On the laboratory scale, amorphous silicon nitride powder with a high specific surface area is synthesized by gas phase reaction, etc., but amorphous silicon nitride is unstable and will be oxidized immediately when taken out into the air. There are difficulties.
From this point of view, silicon oxynitride is attracting attention, and there are many basic points derived from Si—N bonds on the particle surface, and it is stable even when taken out in the air.
[0004]
Conventionally, as a method for producing silicon oxynitride powder, (1) a method in which commercially available crystalline silicon nitride powder and silica powder are mixed and fired at 1400 to 1600 ° C., and (2) fine silica powder in an ammonia atmosphere. A method of firing at 1000 to 1200 ° C., (3) Nitrogen-containing silane compounds such as silicon diimide and amorphous silicon nitride powder produced by the reaction of silicon tetrachloride and ammonia are oxidized to some extent, and then 1000 to 1200 ° C. The method of baking with is known.
However, the silicon oxynitride powder obtained by the method (1) is a mixture of crystalline silicon nitride, crystalline silicon oxynitride (Si 2 ON 2 ) and silica, and has a coarse particle size and is inhomogeneous. A large amount of unreacted silica remains in the silicon oxynitride powder obtained by the method (2), and the particle size distribution is wide and heterogeneous. According to the method (3), homogeneous silicon oxynitride powder can be obtained. However, there are no reports on the systematic synthesis of silicon oxynitride powders by this method.
[0005]
Since the development of applications of the silicon oxynitride material itself has been delayed, there are very few reports on silicon oxynitride materials compared to alumina and silicon nitride ceramics. One of the reasons is that highly-controlled silicon oxynitride powder is not obtained.
Conventionally reported silicon oxynitride powders are a mixture of a silica phase, a silicon nitride phase and a silicon oxynitride phase, are not homogeneous in composition, and have a wide particle size distribution. Those having a high specific surface area were unstable at 500 ° C. or higher in air.
For this reason, even if silicon oxynitride ceramics are produced, sufficient strength characteristics cannot be obtained, and there is a problem that sufficient catalytic activity does not appear even when used as a catalyst carrier.
The chemical composition, specific surface area, and particle size distribution required for the silicon oxynitride powder are not known at all, and a method for controlling these factors has not been known.
[0006]
OBJECT OF THE INVENTION
An object of the present invention is to solve the above-mentioned problems and provide a high-quality silicon oxynitride powder having a highly controlled chemical composition, specific surface area and particle size distribution.
Moreover, the objective of this invention is providing the manufacturing method which can manufacture such a high quality silicon oxynitride powder stably with sufficient reproducibility.
[0007]
[Means for solving problems]
As a result of various studies on the method for synthesizing silicon oxynitride powders, the present inventors have oxidized nitrogen-containing silane compounds such as silicon diimide, silicon nitrogen imide, and amorphous silicon nitride powder, followed by nitrogen-containing inertness. It has been found that high-quality silicon oxynitride powder having a chemical composition, a specific surface area, and a particle size distribution in specific ranges can be produced by firing at a high temperature of 1250 ° C. or higher in a gas atmosphere.
The present invention has been made on the basis of the above findings, and has an oxygen content of 10 to 30% by weight, a nitrogen content of 32 to 17% by weight, a specific surface area of 50 m 2 / g or more, and a particle size distribution curve. The present invention relates to a silicon oxynitride powder having a median average diameter of 1.0 to 50 μm.
[0008]
The silicon oxynitride powder of the present invention has an oxygen content of 10 to 30% by weight, preferably 12 to 25% by weight, a nitrogen content of 32 to 17% by weight, preferably 31 to 21% by weight, and a specific surface area. It is 50 m 2 / g or more, preferably 70 m 2 / g or more, and the median average diameter in the particle size distribution curve is 1.0 to 50 μm, preferably 1.5 to 30 μm.
[0009]
When the oxygen content is less than 10% by weight or the nitrogen content is more than 32% by weight, a crystalline or amorphous silicon nitride phase is mixed. When crystalline silicon nitride is mixed, the specific surface area of the powder is decreased, and when amorphous silicon nitride is mixed, it is unstable because it becomes unstable such as hydrolysis in the air and difficult to handle.
Further, when the oxygen content is more than 30% by weight or the nitrogen content is less than 17% by weight, the silica phase is mixed. The silica phase is not preferable because it is coarsened and the particle size distribution is widened, and the base point on the particle surface is extremely reduced.
[0010]
When the specific surface area is less than 50 m 2 / g, the sintering activity is lowered and a high-density silicon oxynitride ceramic cannot be obtained. Moreover, since the catalyst activity is lowered and problems such as failure to satisfy the desired catalyst performance occur, it is not preferable.
Furthermore, when the median average diameter in the particle size distribution curve is 1.0 μm or less, the powder is bulky and difficult to handle. In the case of a molded body, the porosity is remarkably increased and sintering becomes difficult. Also, when used as a catalyst carrier, it becomes difficult to carry catalyst components and the like. When the median average diameter exceeds 50 μm and becomes coarse, it cannot be used as a sintered compact production raw material. Further, the catalyst carrier is not preferable because the aggregate is too large and the particle surface cannot be used effectively and the catalyst performance is lowered.
[0011]
Further, the silicon oxynitride powder of the present invention has a ratio d 90 / d 10 of the 90% diameter d 90 on the coarse grain side to the 10% diameter d 10 on the fine grain side in the weight-based particle size distribution curve measured by the laser diffraction method. Is preferably in the range of 2.5 to 10, particularly 3 to 8.
When d 90 / d 10 is less than 2.5, the packing density decreases. In the production of silicon oxynitride ceramics, the density of the compact is significantly reduced. This is not preferable because it causes a problem of reduction in yield of per product.
When d 90 / d 10 is larger than 10, the particle size distribution is extremely wide, the molded body is inhomogeneous, and the strength characteristics of the sintered body deteriorate. Further, the catalyst carrier is not uniform, and problems such as the occurrence of reproducible and stable catalyst performance are not preferable.
[0012]
Furthermore, the silicon oxynitride powder of the present invention preferably has a change in specific surface area of 10% or less, particularly 8% or less after firing at 950 ° C. for 5 hours in the air. A powder having such a large change that the change in specific surface area exceeds 10% cannot be stably used in a temperature range of 600 to 1000 ° C.
For example, when used as a raw material for sintering, neck growth of contact points between particles occurs preferentially in the initial stage of sintering in spite of sintering in an inert gas atmosphere. It becomes a factor which inhibits densification as.
In addition, when used as an exhaust gas catalyst carrier for decomposing and removing air pollutant gases contained in various combustion gases, the catalytic activity is reduced due to a reduction in the specific surface area of the carrier during use, and as an exhaust gas catalyst, It is difficult to maintain stable performance and ensure durability.
[0013]
Next, a method for producing the silicon oxynitride powder of the present invention will be described.
The silicon oxynitride powder of the present invention has an oxygen content of 10.5 to 32% by weight by oxidizing a nitrogen-containing silane compound such as silicon diimide, silicon nitrogen imide, and amorphous silicon nitride powder. After the precursor powder having a specific surface area of 200 m 2 / g or more is fired at a high temperature of 1250 ° C. or higher in a nitrogen-containing inert gas atmosphere with a packing density of 0.15 g / cm 3 or less. Manufactured by.
[0014]
The oxidation treatment of the nitrogen-containing silane compound is a method in which the powder is spread thinly to be exposed to the atmosphere and contacted, a method in which humidified air is circulated in a container filled with the powder and hydrolyzed, and the powder is 300 to 700 ° C. under a flow of dry air. It can be carried out by various methods such as a method of heating to a low temperature.
What is important in the present invention is that the silicon oxynitride precursor powder obtained by such an oxidation treatment has a specific surface area of 200 m 2 / g or more, preferably 250 m 2 / g or more. If the specific surface area of the precursor powder is less than 200 meters 2 / g, with the progress of the grain growth in the sintering process, the specific surface area after firing at a temperature higher than 1250 ° C. is 70m 2 / g or less, depending on the conditions 50 m 2 / It is not preferable because it decreases to g or less.
The oxygen content of the precursor powder is 10.5 to 32% by weight. Thereby, the oxygen content of the obtained silicon oxynitride powder can be made into the range of 10 to 30 weight%.
[0015]
Next, the obtained precursor powder is filled in a firing container so that the filling density is 0.15 g / cm 3 or less, and fired at a high temperature of 1250 ° C. or higher in a nitrogen-containing inert gas atmosphere. If the firing temperature is lowered below 1250 ° C., the resulting silicon oxynitride powder has a high specific surface area. However, since it has not undergone high-temperature firing, the stability of the powder is poor. For example, the change in the specific surface area after firing at 950 ° C. for 5 hours in the air becomes 10% or more. Therefore, high-temperature firing at 1250 ° C. or higher is an essential requirement for the production of silicon oxynitride powder having a high specific surface area and excellent stability and durability.
On the other hand, when the packing density is larger than 0.15 g / cm 3 , the median average diameter of the obtained silicon oxynitride powder becomes larger than 50 μm.
[0016]
In order to stably obtain a silicon oxynitride powder having a specific surface area of 50 m 2 / g or higher by firing at a high temperature of 1250 ° C. or higher, the specific surface area of the silicon oxynitride precursor before baking must be 200 m 2 / g or higher. Don't be. Since a highly stable silicon oxynitride powder can be obtained by firing at as high a temperature as possible, the firing temperature in the present invention is 1250 ° C. or higher, preferably 1300 to 1600 ° C., more preferably 1400 to 1600 ° C. . The specific surface area of the precursor powder necessary for obtaining a silicon oxynitride powder having a specific surface area of 50 m 2 / g or higher by high-temperature firing at 1400 ° C. or higher is 250 m 2 / g or higher, preferably 300 m 2 / g or higher.
[0017]
The silicon oxynitride ceramic produced using the silicon oxynitride powder of the present invention has a low porosity and a high density, and is excellent in corrosion resistance, oxidation resistance and strength properties. The powder is suitable as a raw material for producing silicon oxynitride ceramics used as corrosion-resistant members and high-temperature structural parts.
In addition, since the exhaust gas catalyst produced using the silicon oxynitride powder of the present invention has high activity and long life, the silicon oxynitride powder of the present invention is suitable for exhaust gas catalysts used for purification of various combustion gases. It is also suitable as a catalyst carrier.
[0018]
【Example】
Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.
Examples 1-14 and Comparative Examples 1-5
Silicon oxynitride powder was produced according to the following production method (imide decomposition method) and production conditions shown in Table 1. Table 2 shows the characteristics of the obtained silicon oxynitride powder.
[Method for producing silicon oxynitride powder]
After replacing the air in a vertical reaction tank having a diameter of 30 cm and a height of 45 cm cooled to −40 ° C. with nitrogen gas, liquid ammonia and toluene were charged. In the reaction tank, it was separated into upper liquid ammonia and lower toluene. A previously prepared solution consisting of 20-35% by weight silicon tetrachloride and the balance toluene was fed through a conduit to the slowly stirred lower layer. With the supply of the toluene solution, a white reaction product was deposited near the interface between the upper and lower layers.
After completion of the reaction, the reaction solution was transferred to a filtration tank, and the product was filtered off and washed with liquid ammonia to obtain purified silicon diimide.
[0019]
Silicon diimide having a specific surface area of 450 to 880 m 2 / g was synthesized by changing the ratio (volume basis) of silicon tetrachloride and liquid ammonia during the reaction in the range of 3/200 to 1/25. In the initial stage of the reaction, liquid ammonia is present in a large excess. However, since ammonia is consumed as the reaction proceeds, liquid ammonia is also continuously supplied to the reaction vessel. Then, silicon diimide having a specific surface area of 450 to 880 m 2 / g is synthesized by changing the volume ratio of silicon tetrachloride to be supplied into the reaction tank in a steady state and liquid ammonia in the range of 3/200 to 1/25. did.
[0020]
The produced silicon diimide was thinly spread on a vat, released to the atmosphere, and contacted with the atmosphere for the time shown in Table 1.
In addition, while the produced silicon diimide was put in a round bottom container and stirred using a self-revolving stirrer, air containing 63 ° C. saturated water vapor was circulated at a rate of 2 l / min, as described in Table 1 Oxidation treatment was performed for only time.
After confirming the oxygen content and specific surface area of the oxidized silicon diimide in this way, a carbon container having an inner diameter of 380 mm (inner surface is coated with silicon carbide) is filled with about 300 g, and a nitrogen atmosphere is obtained in an electric furnace. Inside, it was baked by holding at the temperature shown in Table 1 for 1 hour. In firing, the temperature was raised from room temperature to 1000 ° C. over 1 hour, and then the temperature was raised to the maximum holding temperature shown in Table 1 at 100 ° C./h.
Table 2 shows the results of measuring the specific surface area, chemical composition and particle size distribution of the obtained silicon oxynitride powder.
[0021]
The specific surface area is the BET single point method, the oxygen content is an inert gas melting-infrared absorption method (LECO method) described in JIS R 1603, and the nitrogen content is a pressurized acid decomposition-steam distillation described in JIS R 1603. It was measured by a separation-neutralization titration method.
The particle size distribution was measured by a laser diffraction method.
The rate of change of the specific surface area of the silicon oxynitride powder was as follows: 10 g of silicon oxynitride powder was spread thinly to a thickness of about 5 mm and fired at 950 ° C. in the atmosphere for 5 hours using a box-type electric furnace. Determined by measurement.
[0022]
[Table 1]
[Table 2]
Examples 15-17 and Comparative Examples 6-7
Silicon diimide was synthesized in the same manner as in Examples 1-14.
About 300 g of the obtained silicon diimide was spread thinly on a vat and calcined at 700 ° C. for the time shown in Table 3 while flowing nitrogen gas containing 5% oxygen at a rate of 5 l / min. Thus, an amorphous silicon oxynitride precursor powder was obtained.
After confirming the oxygen content and specific surface area of the precursor powder thus obtained, a carbon container having an inner diameter of 380 mm (inner surface is coated with silicon carbide) is filled with about 300 g, and nitrogen is supplied in an electric furnace. In the atmosphere, firing was performed at the temperature shown in Table 3 for 1 hour. In firing, the temperature was raised from room temperature to 1000 ° C. over 1 hour, and then the temperature was raised to the maximum holding temperature shown in Table 3 at 100 ° C./h.
Table 4 shows the results of measuring the specific surface area, chemical composition and particle size distribution of the obtained silicon oxynitride powder.
[0025]
[Table 3]
[Table 4]
[Sintering test]
The sintered compact characteristics of the silicon oxynitride powders obtained in Examples 1 to 17 and Comparative Examples 1 to 7 were measured.
6 parts by weight of Y 2 O 3 and 2 parts by weight of Al 2 O 3 were added to 92 parts by weight of silicon oxynitride powder as a sintering aid, wet-mixed with an ethanol solvent for 48 hours by a ball mill, and then the slurry was dried.
The obtained granules were filled into a rubber mold and subjected to isostatic pressing under a pressure of 1.5 ton / cm 2 to obtain a rectangular shaped body having a size of about 40 × 70 × 5.5 mm.
These molded bodies were put in a silicon nitride container and sintered at a temperature of 1650 ° C. for 3 hours in a nitrogen gas atmosphere to obtain a silicon oxynitride sintered body.
A test piece was cut out from the obtained sintered body, and after grinding and polishing the surface, a bulk density measurement, an oxidation test, and a post-oxidation strength test were performed by the following methods. The results are shown in Table 5.
(L) Bulk density measurement: Measured by Archimedes method and expressed as a relative density.
(2) Oxidation test: A test piece was placed in a box-type electric furnace, and the weight increase after heating at 1300 ° C. in air for 50 hours was measured.
(3) Strength test after oxidation: The four-point bending strength at room temperature of the test piece after performing the oxidation test of (2) was measured.
[0028]
[Table 5]
[Performance evaluation test as a catalyst carrier]
The silicon oxynitride powders obtained in Examples 1 to 17 and Comparative Examples 1 to 7 were subjected to performance evaluation tests as catalyst carriers.
Palladium acetylacetonate (Mitsuwa Chemical) was added to the silicon oxynitride powder, ball milled with an ethanol solvent for 24 hours, and the slurry was dried.
The dried sample was calcined at 850 ° C. for 4 hours to prepare a silicon oxynitride catalyst (Pd / Si—O—N) supporting Pd. The amount of Pd supported was 0.5% by weight. The obtained catalyst sample was made into granules having a diameter of 0.6 to 1.2 mm and subjected to an activity test.
An atmospheric pressure fixed bed flow reactor was used for evaluation of the catalyst activity. 2 ml of the catalyst sample was filled in the center of a quartz glass reaction tube having an inner diameter of 10 mm. The catalyst packed bed was heated to 600 ° C. by external heat, and a methane-air mixed gas (methane concentration 0.5 vol%) was supplied from the upper part of the reaction tube at a flow rate of 1.66 l / min.
The composition of the combustion gas was measured by a gas chromatograph, and the conversion rate was determined from the peak area of methane. The reaction was continuously performed for 100 hours or more, and the activity was evaluated based on the conversion rate of methane and the combustion rate constant when 100 hours passed. Table 6 shows the results of performance evaluation as a catalyst carrier.
The combustion rate constant was calculated by dividing the conversion rate of methane by the volume of the catalyst packed bed.
[0030]
[Table 6]
Claims (4)
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