JP2004292292A

JP2004292292A - Method for firing ceramic honeycomb structure

Info

Publication number: JP2004292292A
Application number: JP2003090561A
Authority: JP
Inventors: Hirohisa Suwabe; 博久諏訪部; Osamu Tokutome; 修徳留
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2003-03-28
Filing date: 2003-03-28
Publication date: 2004-10-21

Abstract

<P>PROBLEM TO BE SOLVED: To provide a firing method for preventing the crack due to the thermal stress caused by a temperature difference in the central part, outer peripheral part and/or upper and lower parts of a honeycomb structure in a warm-up process in firing the large-sized honeycomb structure of an external diameter ≥150 mm and length ≥150 mm. <P>SOLUTION: The method for firing the ceramic honeycomb structure comprises preparing raw materials for cordierite formation in such a manner that the main crystalline phase turns to cordierite, and extrusion molding the raw materials to the honeycomb structure, then firing the honeycomb structure, in which the warm-up rate of a temperature region causing the thermal shrinkage of the honeycomb structure is specified to <30°C/h and the warm-up rate of the temperature region after the end of the thermal shrinkage is specified to <80°C/h. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明はコージェライト質セラミックハニカム構造体の焼成方法に関し、特にディーゼルエンジンの排気ガスを浄化するための触媒担体や排気ガス中に含まれる微粒子を除去するためのフィルタに使用される、大型のコージェライト質セラミックハニカム構造体を焼成するに好適な焼成方法に関するものである。
【０００２】
【従来技術】
従来、タルク、カオリン、アルミナ、シリカ等のコージェライト化原料に、バインダー、潤滑剤等の成形助剤、造孔剤、及び水を添加した、混合、混練した可塑化した坏土からハニカム成形体を成形し、このハニカム成形体を焼成するセラミックハニカム構造体の焼成方法として、各種方法が知られている。
【０００３】
例えば、特許文献１に記載の発明では、コージェライトハニカム成形体を１１００℃までは２５０℃／ｈを超えない昇温速度で、更に１１００℃以上では３０〜３００℃／ｈの昇温速度で昇温し、１３００〜１４４０℃の温度域で０．５〜２４時間焼成することにより、低い熱膨張係数の得られることが開示されている。
【０００４】
特許文献２に記載の発明では、セラミックハニカム構造体の変形を防止するため、ハニカム構造体が熱収縮する温度領域にて、昇温を所定時間停止する技術が開示されている。本文献において、昇温速度は温度１１００℃より低い温度領域及び１２５０℃より高い温度領域にて昇温速度を１００℃／ｈとし、温度１１００〜１２５０℃の範囲は、昇温速度を６０℃／ｈ以下とする例が記載されている。
【０００５】
特許文献３に記載の発明では、気孔率、熱膨張係数等の特性を最適化するため、熱収縮する温度域（１１００〜１２００℃）で６０℃／ｈ以下、固相反応温度域（１２００〜１３００℃）で８０℃／ｈ以上、液相反応温度域（１３００℃〜保持温度）で６０℃／ｈ以下の昇温速度で焼成する技術が開示されている。
【０００６】
特許文献４に記載の発明では、ハニカム構造体の端面切れや寸法不均一を防ぐため、焼成雰囲気を１．０〜５．０ｍ／ｓの速度で、ハニカム構造体の貫通孔と平行に強制的に通過させることにより、ハニカム構造体全体を均一に加熱する技術が開示されている。
【０００７】
一方、環境規制の強化から、ディーゼルエンジンの排気ガス中に含まれるＮＯｘや微粒子を除去するために、ＮＯｘを浄化するための触媒物質を担持させたセラミックハニカム構造体や、セラミックハニカム構造体の隔壁を多孔質構造とし、このハニカム構造体の流路開口部の両端を交互に目封止することにより、微粒子を含んだ排気ガスを前記隔壁に通過せしめる構造の微粒子捕集用のセラミックハニカムフィルタを採用する検討が進められている。これらのディーゼルエンジン用のハニカム構造体については、その寸法は外径が１５０ｍｍ以上、長さ１５０ｍｍという大型のものが必要になりつつある。更に、セラミックハニカムフィルタについては、その浄化性能を高めるため、ハニカム構造体の多孔質隔壁の気孔率を５０％以上とする要求が強くなっている。
【０００８】
【特許文献１】
特開昭５３−８２８２２号公報
【特許文献２】
特許２５５３１９２号公報
【特許文献３】
特許２９８１０３４号公報
【特許文献４】
特公平７−９３５８号公報
【０００９】
【発明が解決しようとする課題】
外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体を、上述した従来の焼成方法により得ようとすると、寸法が大きいために昇温過程において、ハニカム構造体の中心部、外周部及び／または上下部の温度差が発生し易く、この温度差から生ずる収縮率差により発生する熱応力により、昇温過程で割れが発生しやすいという問題があった。
【００１０】
この焼成過程で発生する割れについて、以下に詳しく説明する。
コージェライト質セラミックハニカム構造体の焼成は、単独炉或いは連続炉内に図１に示すようにハニカム構造の成形体１の流路方向が概略重力方向に沿うように配置され、ガスバーナや電気ヒータの熱エネルギーが供給されることにより、１０００℃以上の温度でコージェライト合成反応が促進されて行われる。ここで、ハニカム構造の成形体は、焼成炉の棚板３の上に、焼成台２を介して配置され、外周壁５が棚板３に対して概略直角、上端面４が棚板３に対して、概略平行となるよう配置される。焼成台２は棚板３とハニカム構造体１との焼成過程における膨張、収縮差を吸収するため配置されており、トチと呼ばれるハニカム構造体の薄板などが使用される。ハニカム構造体は、直交する隔壁で仕切られ、軸方向に伸びた多数の流路を有することから、この流路が断熱層として機能し、焼成時に供給された熱エネルギーがハニカム構造体の中心部まで伝わりにくいため、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体の場合は、焼成時の昇温過程において、ハニカム構造体中心部の温度に比べて、上端面４や外周壁５の温度が高くなるため、ハニカム構造体内での温度差の発生は避けられない。
一方、コージェライト化原料を調合し、ハニカム構造体に押出成形後、該ハニカム構造体を所定の温度、雰囲気で焼成する際には、特許文献３の図５で示されるように、コージェライト合成反応に伴う、ハニカム構造体の寸法変化が発生する。具体的には約１１００〜１２００℃の間で大きな熱収縮が認められ、約１２００〜１３００℃間の固相反応が進む温度域では膨張が認められ、更に約１３００℃以上の液相反応が進む温度域においても膨張が認められる。
【００１１】
この、約１１００〜１２００℃の間の熱収縮が発生する温度域で、上記のようにハニカム構造体内での温度差が発生すると、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体の場合は、上端面や外周壁が収縮しようとするのに対し、中心部が収縮できずに、上端面や外周壁の収縮が拘束されるため、上端面４や、外周壁５に、引張応力が発生する。セラミックスは、圧縮には強いが、引張には弱いため、引張応力が作用すると割れにつながり易く、特に、上端面を構成する隔壁の厚さは、外周壁厚さに比べて薄いことから、図２の上端面の拡大模式図で示すように、上端面を構成する隔壁の交点に割れが発生すると、隣接した隔壁交点に進展し、上端面割れが発生する。
【００１２】
一方、約１２００℃以上の温度域の熱膨張が認められる温度域で、上記のようにハニカム構造体内での温度差が発生すると、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体の場合は、上端面や外周壁が膨張しようとするのに対し、中心部が膨張出来ずに、中心部が上端面や外周壁に引張られる形となり、ハニカム構造体の中心部に引張応力が発生するようになる。このためハニカム構造体中心部を構成する隔壁の交点に引張応力による割れが発生し、隣接した隔壁交点に進展し、中心部割れが発生する。
【００１３】
以上のように、コージェライト質セラミックハニカム構造体焼成過程の約１１００℃以上の温度域では、温度上昇に伴い大きな寸法変化が発生することから、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体の場合は、焼成時の昇温過程においてハニカム構造体内の温度差が発生し易く、ハニカム構造体各部位の収縮率の違いによる熱応力が発生し、割れ発生に至るのである。
【００１４】
特許文献１に記載の発明では、１０００℃以上の温度域を３０〜３００℃／ｈで昇温することから、特に熱収縮する温度域では、昇温速度が３０℃／ｈ以上と早いため、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体ではハニカム構造体内に生じた温度差により割れが発生し易いという問題があった。
【００１５】
また特許文献２に記載の発明では、熱収縮する温度域で昇温を所定時間停止し、ハニカム構造体各部位の温度差が小さくなるようにしているものの、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体では昇温過程においては温度差が発生し、特に実施例に記載があるような１００℃／ｈで昇温すると割れが発生する問題があった。
【００１６】
また、特許文献３に記載の発明では、固相反応が進む温度域（約１２００−１３００℃）の昇温速度を８０℃／ｈ以上としていることから、本文献の図３に示されるように、この温度域においても寸法変化（膨張）が発生するため、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体ではハニカム構造体内に生じた温度差により割れが発生するという問題があった。
【００１７】
また、特許文献４に記載の発明では、焼成雰囲気を１．０〜５．０ｍ／ｓの速度で、ハニカム構造体の貫通孔と平行に強制的に通過させることにより、ハニカム構造体内外に生じる温度差を、小さくできるものの、割れ発生を完全になくすことは困難であるとともに、焼成雰囲気を貫通孔に強制的に通過させるために、特殊な装置や、焼成棚板等の焼成治具が必要となり、設備コストや、ランニングコストが膨大になるという問題点も抱えていた。
【００１８】
また、この昇温過程での割れは、セラミックハニカムフィルタに好適な、気孔率が５０％以上の多孔質隔壁を有するハニカム構造体の場合は、気孔率が大きいことから隔壁自体の強度が小さくなるため、ハニカム構造体内各所の温度差から発生する熱応力が小さい場合でも、割れにつながり易いという問題があった。
【００１９】
本発明の目的は、上述した課題を解消して、ディーゼルエンジンの排気ガス浄化装置に適したセラミックハニカム構造体を通常の単独炉や連続炉を使用して焼成するに当たり、外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体、或いは、セラミックハニカムフィルタに好適な、気孔率が５０％以上の多孔質隔壁を有するセラミックハニカム構造体の場合であっても、焼成時に割れの発生しにくいセラミックハニカム構造体の焼成方法を提供するものである。
【００２０】
【課題を解決するための手段】
本発明のセラミックハニカム構造体の焼成方法は、主結晶相がコージェライトとなるよう、コージェライト化原料を調合し、ハニカム構造体に押出成形後、該ハニカム構造体を焼成するセラミックハニカム構造体の焼成方法において、ハニカム構造体の熱収縮する温度域の昇温速度を３０℃／ｈ未満とし、熱収縮終了後の温度域の昇温速度を８０℃／ｈ未満とすることを特徴とするものである。
【００２０】
本発明では、セラミックハニカム構造体の熱収縮する温度域の昇温速度を３０℃／ｈ未満とし、熱収縮終了後の温度域の昇温速度を８０℃／ｈ未満としていることから、外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型のコージェライト質セラミックハニカム構造体であっても焼成過程で割れの発生が少なくすることができる。
ここで、ハニカム構造体の熱収縮する温度域の昇温速度を３０℃／ｈ未満とするのは、昇温速度を３０℃／ｈ以上とすると、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体の場合は、ハニカム構造体の中心部に比べて上端面や外周壁の温度が高くなり、上端面や外周部が収縮しようとするのを、中心部が拘束し、上端面や、外周部に引張応力が発生するため、上端面や、外周壁に割れが発生し易くなるからである。特に、外周壁に比べ、隔壁厚さの方が薄いことから上端面の隔壁に割れが発生しやすい。ここで熱収縮温度域とは約１０００℃〜約１２００℃の間の温度域のことを言うが、特に約１１００℃〜約１２００℃の間の温度域は、収縮率の急激な変化があることから、この温度域の昇温速度を３０℃／ｈ未満とする必要がある。尚、本熱収縮温度域では、昇温速度は３０℃／ｈ未満にする必要があるが、ハニカム構造体を焼成する上で、極端に低い昇温速度は、焼成時間が長時間となり、経済的でなくなるため、昇温速度は２℃／ｈ以上、３０℃／ｈ未満が好ましい。更に、より好ましい昇温速度の範囲は５℃／ｈ以上、３０℃／ｈ未満である。
【００２１】
また、熱収縮終了後の約１２００℃以上の温度域の昇温速度を８０℃／ｈ未満とするのは、昇温速度を８０℃／ｈ以上とすると、この温度域では、熱収縮温度域とは逆に熱膨張が発生するため、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型ハニカム構造体の場合は、ハニカム構造体の中心部に比べて上端面や外周壁の温度が高くなり、中心部が上端面や外周壁の膨張に引張られ、ハニカム構造体の中心部に引張応力が発生するようになり、中心部の隔壁交点に割れが発生し易いためである。
本熱収縮の終了後の温度域では、昇温速度は８０℃／ｈ未満にする必要があるが、ハニカム構造体を焼成する上で、極端に低い昇温速度の場合は、焼成時間が長時間となり、経済的でなくなるため、昇温速度は２℃／ｈ以上、８０℃／ｈ未満が好ましい。更に、より好ましい昇温速度は５℃／ｈ以上、６０℃／ｈ以下である。
【００２２】
また、主結晶相がコージェライトであり、気孔率５０％以上８０％以下であるハニカム構造体となるよう、コージェライト化原料を調合し、ハニカム構造体に押出成形後、該ハニカム構造体を焼成するセラミックハニカム構造体の焼成方法においては、ハニカム構造体の熱収縮する温度域の昇温速度は２５℃／ｈ以下、熱収縮終了後の温度域の昇温速度は７０℃／ｈ以下が好ましい。
【００２３】
本発明において、気孔率５０％以上８０％以下となるような、セラミックハニカム構造体の場合には、気孔率が大きいために、セラミックハニカム構造体自体の強度が低くなる。しかし、ハニカム構造体の熱収縮する温度域の昇温速度を２５℃／ｈ以下とし、熱収縮終了後の温度域の昇温速度を７０℃／ｈ以下とすることによって、外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型、且つ気孔率が５０％以上８０％以下の多孔性を有するコージェライト質セラミックハニカム構造体の焼成過程で発生する割れを効果的に低減できる。
【００２４】
ここでハニカム構造体の熱収縮する温度域の昇温速度を２５℃／ｈ以下とするのが好ましいのは、昇温速度が２５℃／ｈを超えると、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上、気孔率５０％以上８０％以下の大型、高気孔率セラミックハニカム構造体の場合は、昇温過程のハニカム構造体の中心部と上端面の温度差により発生する熱応力により、上端面の隔壁交点を連鎖的に進展する割れが発生し易くなるからである。この、外径１５０ｍｍ以上、長さ１５０ｍｍ以上、気孔率５０％以上８０％以下の大型、高気孔率セラミックハニカム構造体の場合は、熱収縮温度域では、昇温速度は２５℃／ｈ以下が好ましいが、ハニカム構造体を焼成する上で、極端に低い昇温速度の場合は、焼成時間が長時間となり、経済的でなくなるため、昇温速度は２℃／ｈ以上、２５℃／ｈ以下がより好ましい。更に、より好ましい昇温速度は５℃／ｈ以上、２５℃／ｈ以下である。
【００２５】
また、熱収縮終了後の温度域の昇温速度を７０℃／ｈ以下とするのが好ましいのは、特に外径１５０ｍｍ以上、長さ１５０ｍｍ以上、気孔率５０％以上８０％以下の大型、高気孔率セラミックハニカム構造体の場合は、昇温過程のハニカム構造体の中心部と上端面の温度差により発生する熱応力により、中心部の隔壁交点を連鎖的に進展する割れが発生し易くなるからである。この、外径１５０ｍｍ以上、長さ１５０ｍｍ以上、気孔率５０％以上８０％以下の大型、高気孔率セラミックハニカム構造体の場合は、本熱収縮の終了後の温度域では、昇温速度は７０℃／ｈ以下にすることが好ましいが、ハニカム構造体を焼成する上で、極端に低い昇温速度の場合は、焼成時間が長時間となり、経済的でなくなるため、昇温速度は２℃／ｈ以上、７０℃／ｈ以下が、より好ましい。更に、より好ましい昇温速度は５℃／ｈ以上、５０℃／ｈ以下である
【００２６】
気孔率が５０％以上８０％以下であるコージェライト質セラミックハニカム構造体中の気孔は、主にコージェライト化原料に含まれるタルクや石英、溶融シリカのようなシリカ源成分やグラファイト、小麦粉等の造孔材によって形成される形骸により形成されるものである。中でもシリカ源成分は、他の原料に比べて高温まで安定に存在し、１３００℃以上で溶融拡散し、細孔を形成することが知られていることから、気孔率が５０％以上であるような高気孔率のコージェライト質セラミックハニカム構造体を得るためには、有効な材料である。特に平均粒径５μｍ以上のシリカ源粉末成分を少なくとも１０質量％以上コージェライト化原料に含有させることは、気孔率５０％以上、平均細孔径５μｍ以上の特性を有するコージェライト質セラミックハニカム構造体を得る上で有効である。ところが、平均粒径５μｍ以上の石英及び／又は溶融シリカからなるシリカ源成分の添加量を１０質量％以上含有させると、コージェライト化原料の他の成分、即ち、水酸化アルミニウム、カオリン、タルク等からの比較的低温で起こる結晶水の放出や結晶構造の変化、或いはそのものの分解によって生じる収縮を抑制することができる。一方、１２００℃以上のコージェライト化反応の過程で、特に１３００℃以上の高温では、急激にコージェライト化が進行し、その体積膨張が大きくなるため、焼成時の割れが発生しやすくなる。しかしながら、コージェライト化原料に、平均粒径５μｍ以上の石英及び／又は溶融シリカからなるシリカ源成分の添加量を１０質量％以上含有させた場合であっても、熱収縮温度終了後の約１２００℃以上の温度域で、昇温速度を７０℃／ｈ以下、より好ましくは４０℃／ｈ以下としているため、この温度域で発生する割れを有効に防止することができる。
【００２７】
【発明の実施の形態】
（実施例１）
コージェライト質セラミック焼成体の気孔率３５％を目標として、表１に示す平均粒径、化学組成のカオリン、仮焼カオリン、タルク、水酸化アルミニウム、酸化アルミニウムＡ、シリカＡ（石英）を、表２に示す配合ＮＯ．１の割合で、秤量した。次いで、バインダーとしてメチルセルロース、及びヒドロキシプロピルメチルセルロースを添加し、混合調整した後、水を投入し、混合、混練を加え、可塑化可能な坏土を作製し、この坏土を押出成形機に投入して、成形、乾燥を行い、ハニカム構造を有する試験ＮＯ．１の成形体を準備した。セラミックハニカム構造成形体の形状は全て同じ形状で、直径３００ｍｍ、長さ３５０ｍｍの略円柱形状であり、隔壁の厚さ０．１５ｍｍ、隔壁のピッチ１．３ｍｍであった。次に準備したセラミックハニカム成形体を、焼成用単独炉で表３に示すＡ〜Ｖの２１種類の昇温パターンで昇温し、１４００℃で１０時間保持して焼成を行った後、割れの発生したセラミックハニカム構造体の割合を求めて、焼成割れの評価を行った。セラミックハニカム構造体の割れ発生の確認は、上端面については目視で、中心部の割れについてはＸ線で行い、焼成したハニカム構造体のうちの割れ発生したハニカム構造体の割合が、０％であったものを◎、２％未満であったものを○、５％未満であったものを△、５％以上であったものを×として、各焼成パターンに対するの割れ評価を行った。また、それぞれの昇温パターンで作成したセラミックハニカム構造体の気孔率を水銀圧入法により求めた。表３に２１種類の昇温パターンＡ〜Ｖで焼成したセラミックハニカム構造体の気孔率及び焼成割れの評価結果を示す。
【００２８】
表３の結果から、ハニカム構造体の熱収縮が発生する領域である１０００℃〜１２００℃の間の昇温速度を３０℃／ｈ未満、熱収縮が終了した後の１２００℃〜１４００℃の間の昇温速度を８０℃／ｈ未満とすることにより、焼成割れ評価は、◎、或いは○となり、割れ発生率を２％以下とし、気孔率３４〜３７％のコージェライト質セラミックハニカム構造体の得られることが判る。特に、熱収縮が終了した後の１２００℃〜１４００℃の間の昇温速度を６０℃／ｈ以下とすると、焼成割れ評価は◎となり、割れ発生率はいずれも０％であることもわかった。
【００２９】
（実施例２）
表１に示す平均粒径、化学組成のカオリン、仮焼カオリン、タルク、水酸化アルミニウム、酸化アルミニウムＢ、シリカＢ（溶融シリカ）を、表２に示す配合ＮＯ．２（目標気孔率：６０％）、ＮＯ．３（目標気孔率：６１％）、ＮＯ．４（目標気孔率：６３％）の割合で秤量した。次いで、このコージェライト化原料１００質量部に対して、造孔剤としてグラファイト、及び有機発泡材を表２に示す割合で添加し、さらにバインダーとしてメチルセルロース、及びヒドロキシプロピルメチルセルロースを添加し、混合調整した。その後、水を投入し、混合、混練を加え、可塑化可能な坏土を作製し、この坏土を押出成形機に投入して、成形、乾燥を行い、ハニカム構造を有する試験ＮＯ．２〜４の成形体を準備した。セラミックハニカム構造成形体の形状は全て同じ形状で、直径３００ｍｍ、長さ３５０ｍｍの略円柱形状であり、隔壁の厚さ０．３１ｍｍ、隔壁のピッチ１．５２ｍｍであった。次に準備したセラミックハニカム成形体を、焼成炉で表３に示すＡ〜Ｕの２１種類の昇温パターンで昇温し、１４００℃で１０時間保持して焼成を行った後、実施例１と同様に気孔率の測定及び割れの評価を行った。表４に、試験ＮＯ．２、３、４の３種類のハニカム構造体に対する２１種類の昇温パターンＡ〜Ｖの気孔率及び割れ評価結果を示す。
【００３０】
表４の結果から、ハニカム構造体の熱収縮が発生する領域である１０００℃〜１２００℃の間の昇温速度を３０℃未満、熱収縮が終了した後の１２００℃〜１４００℃の間の昇温速度を８０℃／ｈ未満とすることで、焼成割れ評価は、◎、○、△となり、気孔率５８〜６２％の高気孔率コージェライト質セラミックハニカム構造体においてもが割れ発生率を５％以下として、得られることが判った。但し、試験ＮＯ．２、３、４のセラミックハニカム構造体は、気孔率が５０％以上であることから、試験ＮＯ．１のセラミックハニカム構造体の割れ評価が◎であった昇温パターンＢでは、割れ評価が○、試験ＮＯ．１のセラミックハニカム構造体の割れ評価が○であった、昇温パターンＮ、及びＵでは、割れ評価が△であった。しかし、ハニカム構造体の熱収縮する温度域である１０００℃〜１２００℃の間の昇温速度を２５℃／ｈ以下とし、熱収縮終了後の温度域である１２００℃〜１４００℃の間の昇温速度を７０℃／ｈ以下とすることにより、割れ発生率を少なくすることができた。
また、試験ＮＯ．２のセラミックハニカム構造体は、１２００℃〜１４００℃の間の昇温速度を５０℃／ｈ以下とすると、焼成割れ評価は◎となり、割れ発生率は０％となった。一方、試験ＮＯ．３及び４のセラミックハニカム構造体は、コージェライト化原料中に、溶融シリカを１１．８〜１８．１質量％含有していることから、例えば昇温パターンＪ及びＲの場合、溶融シリカを９％含有している試験ＮＯ．２のセラミックハニカム構造体の割れ評価◎に対して、割れ評価は○となり、割れ発生率が僅かに高くなるものの、実用上問題ない２％以下であったが、１２００℃〜１４００℃の間の昇温速度を４０℃／ｈ以下とした、昇温パターンＨ、Ｉ、Ｐ、Ｑでは、割れ評価が◎となり、割れ発生率は０％となった。
【００３１】
【表１】

【００３２】
【表２】

【００３３】
【表３】

【００３４】
【表４】

【００３５】
【発明の効果】
以上の説明から明らかなように、本発明によれば、外径１５０ｍｍ以上、長さ１５０ｍｍ以上の大型セラミックハニカム構造体を焼成するに当たり、昇温速度を適切な範囲に調整していることから、焼成割れ発生のない、セラミックハニカム構造体を得ることができる。
【００３６】
【図面の簡単な説明】
【図１】セラミックハニカム構造体を焼成する際のセラミックハニカム構造体の配置の一例を示す図である。
【図２】セラミックハニカム構造体を焼成する際に、隔壁交点に発生する割れの一例を示す図である。
【符号の説明】
１：セラミックハニカム構造体
２：焼成台
３：棚板
４：上端面
５：外周壁
１１：隔壁
１２：流路
１３：隔壁交点の割れ[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for firing a cordierite ceramic honeycomb structure, and particularly to a large cordier used for a catalyst carrier for purifying exhaust gas of a diesel engine and a filter for removing fine particles contained in the exhaust gas. The present invention relates to a firing method suitable for firing a light ceramic honeycomb structure.
[0002]
[Prior art]
Conventionally, a honeycomb formed body is obtained by mixing and kneading a plasticized clay obtained by adding a forming aid such as a binder, a lubricant, a pore-forming agent, and water to a cordierite forming raw material such as talc, kaolin, alumina, and silica. Various methods are known as a method for firing a ceramic honeycomb structure in which the formed honeycomb body is fired.
[0003]
For example, in the invention described in Patent Document 1, the cordierite honeycomb formed body is heated at a temperature rising rate not exceeding 250 ° C./h up to 1100 ° C., and further at a temperature rising rate of 30 to 300 ° C./h above 1100 ° C. It is disclosed that a low thermal expansion coefficient can be obtained by heating and baking for 0.5 to 24 hours in a temperature range of 1300 to 1440C.
[0004]
In the invention described in Patent Document 2, in order to prevent deformation of the ceramic honeycomb structure, a technique of stopping the temperature rise for a predetermined time in a temperature region where the honeycomb structure thermally contracts is disclosed. In this document, the heating rate is 100 ° C./h in a temperature range lower than 1100 ° C. and a temperature range higher than 1250 ° C., and in a range of 1100 to 1250 ° C., the heating rate is 60 ° C./h. h is described below.
[0005]
In the invention described in Patent Document 3, in order to optimize characteristics such as porosity and coefficient of thermal expansion, the temperature range of heat contraction (1100 to 1200 ° C) is 60 ° C / h or less, and the solid phase reaction temperature range (1200 to 1200 ° C). A technique is disclosed in which firing is performed at a heating rate of 80 ° C./h or more at 1300 ° C.) and 60 ° C./h or less in a liquid phase reaction temperature range (1300 ° C. to holding temperature).
[0006]
In the invention described in Patent Literature 4, in order to prevent the end faces of the honeycomb structure from being cut off and uneven dimensions, the firing atmosphere is forcibly forced at a speed of 1.0 to 5.0 m / s in parallel with the through holes of the honeycomb structure. To uniformly heat the entire honeycomb structure by passing the honeycomb structure through the honeycomb structure.
[0007]
On the other hand, in order to remove NOx and fine particles contained in the exhaust gas of a diesel engine, a ceramic honeycomb structure carrying a catalytic substance for purifying NOx or a partition wall of the ceramic honeycomb structure in order to remove NOx and fine particles contained in exhaust gas of a diesel engine due to strengthening of environmental regulations Has a porous structure, and by alternately plugging both ends of the flow path opening of the honeycomb structure, a ceramic honeycomb filter for collecting fine particles having a structure in which exhaust gas containing fine particles is passed through the partition wall. The adoption is under consideration. As for these honeycomb structures for diesel engines, large-sized ones having an outer diameter of 150 mm or more and a length of 150 mm are becoming necessary. Further, with respect to ceramic honeycomb filters, in order to enhance the purification performance, there is an increasing demand for the porosity of the porous partition walls of the honeycomb structure to be 50% or more.
[0008]
[Patent Document 1]
JP-A-53-82822 [Patent Document 2]
Japanese Patent No. 2553192 [Patent Document 3]
Japanese Patent No. 2981034 [Patent Document 4]
Japanese Patent Publication No. 7-9358
[Problems to be solved by the invention]
When attempting to obtain a large honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more by the above-described conventional firing method, the central portion, the outer peripheral portion, and / or the honeycomb structure during the heating process because of their large dimensions. There is a problem that a temperature difference between the upper and lower portions is easily generated, and cracks are easily generated in a temperature rising process due to a thermal stress generated due to a difference in shrinkage caused by the temperature difference.
[0010]
The cracks generated during the firing process will be described in detail below.
The firing of the cordierite ceramic honeycomb structure is performed in a single furnace or a continuous furnace such that the flow direction of the formed body 1 having the honeycomb structure is substantially along the direction of gravity as shown in FIG. By supplying the thermal energy, the cordierite synthesis reaction is accelerated and performed at a temperature of 1000 ° C. or higher. Here, the formed body having the honeycomb structure is disposed on the shelf plate 3 of the firing furnace via the firing table 2, the outer peripheral wall 5 is substantially perpendicular to the shelf plate 3, and the upper end surface 4 is formed on the shelf plate 3. On the other hand, they are arranged to be substantially parallel. The firing table 2 is arranged to absorb the difference in expansion and contraction in the firing process of the shelf plate 3 and the honeycomb structure 1, and a thin plate of a honeycomb structure called a tochi is used. Since the honeycomb structure is partitioned by orthogonal partition walls and has a large number of flow passages extending in the axial direction, the flow passages function as a heat insulating layer, and the heat energy supplied at the time of firing is applied to the central portion of the honeycomb structure. In particular, in the case of a large ceramic honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more, the upper end surface 4 and the outer peripheral wall of the honeycomb structure during the heating process during firing are lower than the temperature at the center of the honeycomb structure. Since the temperature of No. 5 increases, the occurrence of a temperature difference in the honeycomb structure is inevitable.
On the other hand, after the cordierite-forming raw material is prepared and extruded into a honeycomb structure, and the honeycomb structure is fired at a predetermined temperature and atmosphere, as shown in FIG. A dimensional change of the honeycomb structure occurs due to the reaction. Specifically, a large thermal contraction is observed between about 1100 and 1200 ° C., expansion is observed in a temperature range where the solid phase reaction proceeds between about 1200 and 1300 ° C., and a liquid phase reaction of about 1300 ° C. or more further proceeds. Expansion is also observed in the temperature range.
[0011]
When the temperature difference in the honeycomb structure occurs as described above in the temperature range in which thermal contraction occurs between about 1100 and 1200 ° C., particularly, a large ceramic honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more. In the case of, the upper end face and the outer peripheral wall try to shrink, but the central portion cannot be contracted, and the contraction of the upper end face and the outer peripheral wall is restrained. Stress occurs. Ceramics are strong in compression but weak in tension, so they tend to crack when tensile stress is applied.Particularly, the thickness of the partition walls that form the upper end surface is thinner than the outer peripheral wall thickness. As shown in the enlarged schematic diagram of the upper end face of No. 2, when a crack occurs at the intersection of the partition walls constituting the upper end face, the crack progresses to the adjacent partition intersection, and the upper end face crack occurs.
[0012]
On the other hand, when a temperature difference occurs in the honeycomb structure as described above in a temperature range where thermal expansion in a temperature range of about 1200 ° C. or more is observed, a large ceramic honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more is obtained. In the case of, the upper end face and the outer peripheral wall are about to expand, but the central part is not expanded, and the central part is pulled to the upper end face and the outer peripheral wall, and the tensile stress is applied to the central part of the honeycomb structure. Will occur. For this reason, a crack due to tensile stress occurs at the intersection of the partition walls constituting the central portion of the honeycomb structure, and the crack progresses to the intersection of the adjacent partition walls, and a central portion crack occurs.
[0013]
As described above, in the temperature range of about 1100 ° C. or more during the firing process of the cordierite-based ceramic honeycomb structure, a large dimensional change occurs as the temperature rises, and therefore, especially a large ceramic having an outer diameter of 150 mm or more and a length of 150 mm or more. In the case of the honeycomb structure, a temperature difference in the honeycomb structure easily occurs in a heating process during firing, and a thermal stress is generated due to a difference in shrinkage rate of each part of the honeycomb structure, thereby causing cracking.
[0014]
In the invention described in Patent Literature 1, the temperature is raised in the temperature range of 1000 ° C. or more at 30 to 300 ° C./h. In particular, a large honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more has a problem that cracks easily occur due to a temperature difference generated in the honeycomb structure.
[0015]
Further, in the invention described in Patent Document 2, although the temperature rise is stopped for a predetermined time in a temperature range in which heat contraction occurs, the temperature difference between the respective portions of the honeycomb structure is reduced, but in particular, the outer diameter is 150 mm or more, and the length is 150 mm. In the above-mentioned large honeycomb structure, there is a problem that a temperature difference occurs during the temperature rising process, and cracks occur particularly when the temperature is raised at 100 ° C./h as described in Examples.
[0016]
Further, in the invention described in Patent Document 3, the temperature rising rate in the temperature range (about 1200-1300 ° C.) where the solid-phase reaction proceeds is set to 80 ° C./h or more, so as shown in FIG. However, since dimensional change (expansion) occurs even in this temperature range, there is a problem that cracks are generated due to a temperature difference generated in the honeycomb structure particularly in a large honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more. .
[0017]
Further, according to the invention described in Patent Document 4, the firing atmosphere is generated inside and outside the honeycomb structure by forcibly passing the firing atmosphere at a speed of 1.0 to 5.0 m / s in parallel with the through holes of the honeycomb structure. Although the temperature difference can be reduced, it is difficult to completely eliminate the occurrence of cracks, and special equipment and firing jigs such as firing shelves are required to force the firing atmosphere through the through holes. Therefore, there was a problem that the equipment cost and the running cost became enormous.
[0018]
In addition, the cracks during the heating process are caused by a honeycomb structure having a porous partition wall having a porosity of 50% or more, which is suitable for a ceramic honeycomb filter. For this reason, there is a problem that even when the thermal stress generated due to the temperature difference at various points in the honeycomb structure is small, it is likely to be cracked.
[0019]
SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems and fire a ceramic honeycomb structure suitable for a diesel engine exhaust gas purifying apparatus using a normal single furnace or a continuous furnace. Even in the case of a large-sized ceramic honeycomb structure having a thickness of 150 mm or more, or a ceramic honeycomb structure having a porous partition wall having a porosity of 50% or more, which is suitable for a ceramic honeycomb filter, a ceramic which is not easily cracked during firing. A method for firing a honeycomb structure is provided.
[0020]
[Means for Solving the Problems]
The firing method of the ceramic honeycomb structure of the present invention is such that a cordierite forming raw material is prepared so that the main crystal phase becomes cordierite, and after extrusion molding into the honeycomb structure, the honeycomb structure is fired. In the sintering method, the rate of temperature rise in the temperature range where the honeycomb structure thermally shrinks is less than 30 ° C./h, and the rate of temperature rise in the temperature range after the heat shrinkage is less than 80 ° C./h. It is.
[0020]
In the present invention, the temperature increase rate in the temperature region where the ceramic honeycomb structure thermally contracts is set to less than 30 ° C./h, and the temperature increase rate in the temperature region after the end of the thermal contraction is set to less than 80 ° C./h. Even in the case of a large cordierite ceramic honeycomb structure having a length of 150 mm or more and a length of 150 mm or more, generation of cracks in the firing process can be reduced.
Here, the reason why the heating rate in the temperature region where the honeycomb structure thermally shrinks is less than 30 ° C./h is that the heating rate is 30 ° C./h or more, especially when the outer diameter is 150 mm or more and the length is 150 mm or more. In the case of a large honeycomb structure, the temperature of the upper end face and the outer peripheral wall becomes higher than that of the central part of the honeycomb structure, and the central part restrains the upper end face and the outer peripheral part from shrinking. This is because a tensile stress is generated in the outer peripheral portion, so that the upper end surface and the outer peripheral wall are easily cracked. In particular, since the thickness of the partition wall is smaller than that of the outer peripheral wall, cracks are likely to occur in the partition wall on the upper end surface. Here, the heat shrinkage temperature range means a temperature range between about 1000 ° C. and about 1200 ° C. In particular, a temperature range between about 1100 ° C. and about 1200 ° C. has a sharp change in the shrinkage rate. Therefore, it is necessary to set the heating rate in this temperature range to less than 30 ° C./h. In the present heat shrinkage temperature range, the heating rate needs to be less than 30 ° C./h. However, when firing the honeycomb structure, an extremely low heating rate requires a long firing time, resulting in an economical problem. Therefore, the rate of temperature rise is preferably 2 ° C./h or more and less than 30 ° C./h. Further, a more preferable range of the temperature rising rate is 5 ° C./h or more and less than 30 ° C./h.
[0021]
Further, the reason why the temperature rising rate in the temperature range of about 1200 ° C. or more after the heat shrinkage is set to less than 80 ° C./h is that the temperature rising rate is set to 80 ° C./h or more. Conversely, since thermal expansion occurs, especially in the case of a large honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more, the temperature of the upper end surface and the outer peripheral wall becomes higher than the center of the honeycomb structure, This is because the central portion is pulled by the expansion of the upper end surface and the outer peripheral wall, and a tensile stress is generated in the central portion of the honeycomb structure, and cracks are easily generated at the partition intersections in the central portion.
In the temperature range after the completion of the thermal contraction, the heating rate needs to be less than 80 ° C./h. However, when the honeycomb structure is fired, if the heating rate is extremely low, the firing time is long. It is time-consuming and not economical, so that the heating rate is preferably 2 ° C./h or more and less than 80 ° C./h. Further, a more preferable heating rate is 5 ° C./h or more and 60 ° C./h or less.
[0022]
Further, a cordierite-forming raw material is prepared so that the main crystal phase is cordierite, and a honeycomb structure having a porosity of 50% or more and 80% or less is formed. After extrusion molding into the honeycomb structure, the honeycomb structure is fired. In the firing method of the ceramic honeycomb structure to be performed, the rate of temperature rise in the temperature range in which the honeycomb structure thermally contracts is preferably 25 ° C./h or less, and the rate of temperature rise in the temperature range after the end of the thermal contraction is preferably 70 ° C./h or less. .
[0023]
In the present invention, in the case of a ceramic honeycomb structure having a porosity of 50% or more and 80% or less, the strength of the ceramic honeycomb structure itself is low because the porosity is large. However, by setting the rate of temperature rise in the temperature range where the honeycomb structure thermally contracts to 25 ° C./h or less and the rate of temperature rise in the temperature range after heat contraction to 70 ° C./h or less, the outer diameter is 150 mm or more. Cracks generated during the firing process of a cordierite ceramic honeycomb structure having a large size of 150 mm or more and a porosity of 50% to 80% can be effectively reduced.
[0024]
Here, it is preferable that the rate of temperature rise in the temperature range where the honeycomb structure thermally contracts is 25 ° C./h or less. When the rate of temperature rise exceeds 25 ° C./h, the outer diameter is particularly 150 mm or more and the length is 150 mm. As described above, in the case of a large-sized, high-porosity ceramic honeycomb structure having a porosity of 50% or more and 80% or less, the thermal stress generated by the temperature difference between the central portion and the upper surface of the honeycomb structure during the temperature rise causes the upper surface of the upper surface. This is because cracks that extend in a chain at the partition intersections are likely to occur. In the case of a large-sized, high-porosity ceramic honeycomb structure having an outer diameter of 150 mm or more, a length of 150 mm or more, and a porosity of 50% or more and 80% or less, the heating rate is 25 ° C./h or less in the heat shrinkage temperature range. It is preferable that, when firing the honeycomb structure, if the heating rate is extremely low, the firing time becomes long and it is not economical. Therefore, the heating rate is 2 ° C / h or more and 25 ° C / h or less. Is more preferred. Further, a more preferable heating rate is 5 ° C./h or more and 25 ° C./h or less.
[0025]
Further, it is preferable that the rate of temperature rise in the temperature range after the completion of the heat shrinkage is 70 ° C./h or less, particularly when the outer diameter is 150 mm or more, the length is 150 mm or more, and the porosity is 50% or more and 80% or less. In the case of a porosity ceramic honeycomb structure, cracks that propagate in a chain at the intersection of partition walls at the center are likely to occur due to thermal stress generated by the temperature difference between the center and the upper end surface of the honeycomb structure during the temperature rise process. Because. In the case of this large-sized, high-porosity ceramic honeycomb structure having an outer diameter of 150 mm or more, a length of 150 mm or more, and a porosity of 50% or more and 80% or less, the temperature rising rate is 70 in the temperature range after the completion of the thermal contraction. It is preferable that the heating rate be 2 ° C./h or less. However, in the case of firing the honeycomb structure, if the heating rate is extremely low, the firing time becomes long and it is not economical. h or more and 70 ° C./h or less is more preferable. Further, a more preferable heating rate is 5 ° C./h or more and 50 ° C./h or less.
Pores in the cordierite-based ceramic honeycomb structure having a porosity of 50% or more and 80% or less are mainly caused by silica source components such as talc, quartz, and fused silica contained in the cordierite-forming raw material, graphite, flour, and the like. It is formed by a shaped body formed by the pore former. Above all, since the silica source component is known to exist more stably at higher temperatures than other raw materials, and is known to melt and diffuse at 1300 ° C. or more and form pores, the porosity seems to be 50% or more. It is an effective material for obtaining a cordierite ceramic honeycomb structure having a high porosity. In particular, including at least 10% by mass or more of a cordierite-forming raw material with a silica source powder component having an average particle diameter of 5 μm or more requires a cordierite ceramic honeycomb structure having a porosity of 50% or more and an average pore diameter of 5 μm or more. It is effective in obtaining. However, when the added amount of the silica source component composed of quartz and / or fused silica having an average particle size of 5 μm or more is included in an amount of 10% by mass or more, other components of the cordierite-forming raw material, ie, aluminum hydroxide, kaolin, talc, etc. It is possible to suppress the release of water of crystallization, the change in crystal structure, or the shrinkage caused by the decomposition of the water, which occurs at a relatively low temperature. On the other hand, in the course of the cordierite-forming reaction at 1200 ° C. or higher, particularly at a high temperature of 1300 ° C. or higher, cordierite rapidly progresses and its volume expansion increases, so that cracks during firing tend to occur. However, even when the cordierite-forming raw material contains 10% by mass or more of a silica source component composed of quartz and / or fused silica having an average particle size of 5 μm or more, about 1200% after the end of the heat shrinkage temperature. Since the heating rate is set to 70 ° C./h or less, more preferably 40 ° C./h or less in a temperature range of not less than 0 ° C., cracks generated in this temperature range can be effectively prevented.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
(Example 1)
With the target of 35% porosity of the cordierite ceramic fired body, kaolin, calcined kaolin, talc, aluminum hydroxide, aluminum oxide A, and silica A (quartz) having the average particle size and chemical composition shown in Table 1 are shown in Table 1. Formulation No. 2 shown in It was weighed at a rate of 1. Next, methylcellulose as a binder, and hydroxypropyl methylcellulose were added, and after mixing and adjustment, water was added, and mixing and kneading were added to produce a plasticizable clay, and the clay was charged into an extrusion molding machine. Then, molding and drying were performed to obtain a test No. 1 having a honeycomb structure. 1 was prepared. The shapes of the formed ceramic honeycomb structures were all the same, were substantially cylindrical, having a diameter of 300 mm and a length of 350 mm, the partition wall thickness was 0.15 mm, and the partition pitch was 1.3 mm. Next, the prepared ceramic honeycomb formed body was heated in a single furnace for firing according to 21 types of heating patterns A to V shown in Table 3, held at 1400 ° C. for 10 hours, fired, and then cracked. The ratio of the generated ceramic honeycomb structure was determined, and firing cracks were evaluated. The occurrence of cracks in the ceramic honeycomb structure was confirmed visually with respect to the upper end face, and X-rays were used for cracks in the center, and the ratio of cracked honeycomb structures in the fired honeycomb structure was 0%. Cracks were evaluated for each fired pattern, with ◎ being less than 2%, ○ being less than 2%, Δ being less than 5%, and X being being 5% or more. In addition, the porosity of the ceramic honeycomb structure prepared in each of the heating patterns was determined by a mercury intrusion method. Table 3 shows the evaluation results of the porosity and the firing crack of the ceramic honeycomb structure fired in 21 types of the temperature rising patterns A to V.
[0028]
From the results in Table 3, the rate of temperature rise between 1000 ° C. and 1200 ° C., which is the region where the thermal shrinkage of the honeycomb structure occurs, is less than 30 ° C./h, and between 1200 ° C. and 1400 ° C. after the heat shrinkage is completed. By setting the temperature rise rate to less than 80 ° C./h, the firing crack evaluation becomes ◎ or ○, the crack occurrence rate is 2% or less, and the cordierite ceramic honeycomb structure having a porosity of 34 to 37%. It turns out that it can be obtained. In particular, when the heating rate between 1200 ° C. and 1400 ° C. after the completion of the heat shrinkage was set to 60 ° C./h or less, the firing crack evaluation was ◎, and it was also found that the crack occurrence rate was 0% in each case. .
[0029]
(Example 2)
A mixture of kaolin, calcined kaolin, talc, aluminum hydroxide, aluminum oxide B, and silica B (fused silica) having an average particle size and a chemical composition shown in Table 1 was mixed with the compound No. shown in Table 2. 2 (target porosity: 60%), NO. 3 (target porosity: 61%), NO. 4 (target porosity: 63%). Next, to 100 parts by mass of this cordierite-forming raw material, graphite and an organic foaming material were added at a ratio shown in Table 2 as a pore-forming agent, and methylcellulose and hydroxypropylmethylcellulose were further added and mixed as a binder. . Thereafter, water was added, and mixing and kneading were added to produce a plasticizable kneaded clay. The kneaded clay was charged into an extruder, formed, dried, and subjected to a test NO. 2 to 4 molded bodies were prepared. The shapes of the formed ceramic honeycomb structures were all the same, were substantially cylindrical, having a diameter of 300 mm and a length of 350 mm, the partition wall thickness was 0.31 mm, and the partition pitch was 1.52 mm. Next, the prepared ceramic honeycomb formed body was heated in a firing furnace in accordance with 21 types of heating patterns A to U shown in Table 3, held at 1400 ° C. for 10 hours, and fired. Similarly, the porosity was measured and the cracks were evaluated. Table 4 shows the test No. The porosity and the crack evaluation result of 21 kinds of heating patterns A to V for three kinds of

honeycomb structures

2, 3, and 4 are shown.
[0030]
From the results in Table 4, it can be seen that the rate of temperature rise between 1000 ° C. and 1200 ° C., which is the region where the thermal shrinkage of the honeycomb structure occurs, is less than 30 ° C., and the temperature rise between 1200 ° C. and 1400 ° C. after the heat shrinkage is completed. By setting the temperature rate to less than 80 ° C./h, the sintering crack evaluation becomes ◎, △, and Δ, and even in the high porosity cordierite ceramic honeycomb structure having a porosity of 58 to 62%, the crack occurrence rate is 5%. % Or less was obtained. However, the test NO. Test Nos. 2, 3, and 4 have the porosity of 50% or more for the ceramic honeycomb structures of Nos. 2, 3, and 4. In the heating pattern B in which the ceramic honeycomb structure of No. 1 had a crack evaluation of ◎, the crack evaluation was 割れ and the test No. The ceramic honeycomb structure of No. 1 had a crack evaluation of ○, and the heating patterns N and U had a crack evaluation of △. However, the rate of temperature rise between 1000 ° C. and 1200 ° C., which is the temperature range in which the honeycomb structure thermally shrinks, is set to 25 ° C./h or less, and the temperature rise between 1200 ° C. and 1400 ° C. which is the temperature range after the end of the heat shrinkage. By setting the temperature rate to 70 ° C./h or less, the rate of occurrence of cracks could be reduced.
In addition, the test NO. When the rate of temperature rise between 1200 ° C. and 1400 ° C. was 50 ° C./h or less, the firing crack evaluation of the ceramic honeycomb structure of No. 2 was ◎ and the crack occurrence rate was 0%. On the other hand, in test NO. The ceramic honeycomb structures of Nos. 3 and 4 contain 11.8 to 18.1% by mass of fused silica in the cordierite-forming raw material. % Test NO. The crack evaluation of the ceramic honeycomb structure of No. 2 was ◎, and the crack evaluation was ○. Although the crack occurrence rate was slightly higher, it was 2% or less, which was not a problem in practical use, but was between 1200 ° C. and 1400 ° C. In the heating patterns H, I, P, and Q in which the heating rate was 40 ° C./h or less, the evaluation of cracks was となり and the crack occurrence rate was 0%.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
[Table 4]

[0035]
【The invention's effect】
As is clear from the above description, according to the present invention, when firing a large-sized ceramic honeycomb structure having an outer diameter of 150 mm or more and a length of 150 mm or more, the heating rate is adjusted to an appropriate range. A ceramic honeycomb structure without firing cracks can be obtained.
[0036]
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of an arrangement of ceramic honeycomb structures when firing the ceramic honeycomb structures.
FIG. 2 is a view showing an example of cracks generated at intersections of partition walls when a ceramic honeycomb structure is fired.
[Explanation of symbols]
1: ceramic honeycomb structure 2: firing table 3: shelf plate 4: upper end surface 5: outer peripheral wall 11: partition wall 12: flow path 13: crack at partition partition intersection

Claims

A cordierite-forming raw material is prepared so that the main crystal phase becomes cordierite, and after extrusion-molding into a honeycomb structure, the honeycomb structure is fired. A method for sintering a ceramic honeycomb structure, wherein the temperature rising rate in the temperature range is less than 30 ° C./h and the temperature rising rate in the temperature range after the heat shrinkage is less than 80 ° C./h.

A cordierite-forming material is prepared such that the main crystal phase is cordierite, and the porosity is 50% or more and 80% or less, and after extrusion molding into a honeycomb structure, the honeycomb structure is fired. In the firing method, the rate of temperature rise in the temperature range where the honeycomb structure thermally shrinks is 25 ° C./h or less, and the rate of temperature rise in the temperature range after the heat shrinkage is 70 ° C./h or less. Item 6. A method for firing a ceramic honeycomb structure according to Item 1.