JP2004223358A - Silicon carbide honeycomb structure and ceramic filter using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous silicon carbide honeycomb structure having satisfactory corrosion resistance and thermal shock resistance and a ceramic filter which is obtained by using this honeycomb structure and is suitable for a DPF (diesel particulate filter). <P>SOLUTION: This honeycomb structure consisting of a porous body of silicon carbide satisfies an inequality E/σ<SP>2</SP>≥2.5 x 10<SP>-2</SP>(wherein σ is the strength (MPa) of the porous body; E is the Young's modulus (GPa) of the porous body). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００１８】
ここでν、αは同じ材料であれば変化はほとんど見られない材料固有の値であり、σ及びＥについてはその材料の微構造などによって大きく変化する値である。従って前記特許文献１で述べられているようにσ／Ｅを耐熱衝撃性の指標として用いる例は多い。
【００１９】
しかし、耐熱衝撃性を表す別の指標として、下記式（２）で示されるような、熱衝撃損傷抵抗係数（Ｒ’）というものが一方で知られている。
【００２０】
【数３】

【００２１】
ここでＥ、σ、νは式（１）と同じである。前述のＲはクラックの発生に対する抵抗性の指標であるのに対し、Ｒ’は発生したクラックの進展に対する抵抗性の指標となる。強度は一般的にはクラックの長さに対応することから、換言すれば、Ｒが大きいことは強度の低下が生じるΔＴが大きいことを示し、一方Ｒ’が大きいことは強度の低下の度合が小さいことを示す。
【００２２】
フィルターとして使用する場合、耐熱衝撃性として問題視するのは、熱衝撃により生じるクラックがＰＭ漏れを生じる程度にまで大きくなってしまう場合である。ところが、ハニカム構造体を形成している炭化珪素多孔体は、平均気孔径で数μｍ〜数十μｍ、気孔率３０％以上の特性を有する多孔体であることから、微細なマイクロクラックが入ることはさほど大きな問題ではない。従って、フィルターの耐熱衝撃性の指標としては、Ｒ’を適用する方がふさわしいと考えられる。
【００２３】
本発明者は、前記の考え方に基づき、いろいろな炭化珪素多孔体に関する強度（σ）とヤング率（Ｅ）との関係について実験的検討を重ねた結果、そのＥ／σ^２値が特定の値以上であるときに本発明の目的を達成できることを見出し、本発明に至ったものである。
【００２４】
即ち、炭化珪素質多孔体の強度をσ（ＭＰａ）、ヤング率をＥ（ＧＰａ）とした場合に、Ｅ／σ^２≧２．５×１０^−２の関係が満足されるときに、熱衝撃を受けても大きなクラックに進展しないハニカム構造体が得られるし、その結果、当該ハニカム構造体をフィルターに用いたときには、ＰＭ漏れを生じることがない、高い耐熱衝撃性を有するフィルターが得られる。
【００２５】
また、本発明のハニカム構造体に於いて、ハニカム構造体を構成する炭化珪素質多孔体の強度σ（ＭＰａ）の範囲が２〜４５であり、ヤング率Ｅ（ＧＰａ）が０．５〜２５であることが好ましい。強度、ヤング率がそれぞれ前記範囲より小さいときには、ハニカム構造体を製造する際や、ハニカム構造体を他の部品と共に組み立ててＤＰＦ等のフィルターに適用しようとする際に変形したり、酷い時には破損したりする問題が生じることがあるからである。
【００２６】
一方、強度、ヤング率がそれぞれ前記範囲より大きな場合には、格別の技術上の不都合はないものの、当該特性を有する炭化珪素多孔体を得るには高温で長時間の焼結操作を必要としたり、炭化珪素多孔体中の気孔径や気孔率等の特性を犠牲にせざるを得なくなることがある。また、本発明の目的を達成する上で、ハニカム構造体を構成する炭化珪素質多孔体の強度σ（ＭＰａ）の２〜１９が、ヤング率Ｅ（ＧＰａ）の０．７〜１０がそれぞれより好ましい範囲である。
【００２７】
本発明のハニカム構造体においては、セル壁に形成される気孔の平均気孔径と気孔率については特に制限はない。しかし、フィルターとしての使用を考える場合には、セル壁の気孔率としては４０％以上、特に５０〜８０％が好ましく、また平均気孔径については５〜５０μｍであることが好ましい。セル壁の気孔率が４０％未満では通気時の圧力損失が高くなり、一方８０％を超える場合には強度が低下し、ハンドリング性が不良となる。また、セル壁の平均気孔径が５μｍ未満ではセル壁内部でのＰＭの目詰まりしやすく、５０μｍを超える場合には逆にＰＭの漏れが発生する可能性が出てくるとともに、強度の保持が困難になってくる。なお、本発明におけるセル壁の平均気孔径とは、水銀圧入法により求めたものをいう。
【００２８】
強度σ、ヤング率Ｅはともに気孔率、気孔径の影響を受けることから、耐熱衝撃性を気孔率や気孔径で制御することも考えられる。しかし強度やヤング率に対しては、気孔率・気孔径の他、組織の微細構造の影響も大きく、またそれらが複雑に絡んでいることから、明確な相関を得ることは困難である。従って強度・ヤング率が上記の関係を満たす、という基準でハニカム構造体を構成する炭化珪素多孔体を選択する方が良い。
【００２９】
次に、本発明のハニカム構造体を製造する方法について説明する。本発明のハニカム構造体は、炭化珪素粉末、あるいは炭化珪素粉末と窒化珪素粉末の混合物に炭素質物質の所定量を加えた混合物をハニカム形状の成形体に成形し、それを非酸化性雰囲気中で加熱し、焼結させることによって製造することができる。また窒化珪素粉末の代わりに金属珪素粉末を用い、窒素雰囲気中で加熱することによっても、同様に製造することが可能である。
【００３０】
まずハニカム形状の成形体の作製にあたっては、炭化珪素粉末、または炭化珪素粉末と窒化珪素粉末の混合粉、あるいは炭化珪素粉末と窒化珪素粉末の混合粉末に、窒化珪素粉末または金属珪素粉末が反応して炭化珪素になるのに必要な分以上の炭素質物質を加えた混合物に、適量の水と有機バインダーを添加し、混合して押出成形用の坏土を得る。混合・混練については、乾式、湿式混合等の均一に混合できる方法であれば何れの方法でも採用することができる。有機バインダーにも特に制限はなく、メチルセルロースやポリビニルアルコール等、あるいはそれらを主成分とする一般的なもので良い。
【００３１】
なお、炭素質物質は、酸化性雰囲気中で熱処理することにより容易に除去することができることから、その添加量及び粒度を調節することによって、ハニカム構造体の気孔率、気孔径等を制御することができる。
【００３２】
ついで、得られた坏土を押出成形法などにより所望のハニカム形状に成形し、乾燥、脱脂工程を経て加熱し焼結する。焼結は、窒素、アルゴン等の非酸化性雰囲気中で行う。この際、焼結方法には特に制限はなく、ヒーター加熱炉、高周波加熱炉等一般的な加熱炉を用いる事ができる。またその他に、窒化珪素粉末を原料に含むなどして炭化珪素中に若干の窒素を固溶させた場合には、導電性が発現することから、公知の通電焼結法（特許文献２参照）を用い、焼結を短時間で行うことも可能である。
【００３３】
【特許文献２】特開平１０−５２６１８号公報
【００３４】
焼結温度は、１８００℃〜２５００℃であることが好ましい。焼結温度が１８００℃未満では、炭化珪素の粒成長や焼結が不十分である他、未反応の窒化珪素及び炭素質物質が残存するなどで耐熱性が低下する可能性がある。一方２５００℃を超えると結晶転移や昇華などが生じ、極端な粒成長により強度が低下する。
【００３５】
また、本発明のハニカム構造体からなるフィルターの製造にあたっては、ハニカム貫通孔をそれぞれの両端面で目封じすることによって製造することができる。その目封じ方法については、特許文献３等に記載された方法等によって行うことができる。
【００３６】
【特許文献３】特開平９−１９６１３号公報
【００３７】
なお、本発明のフィルターは、前記ハニカム構造体を用いて構成されているので、実使用条件下での高温において十分な耐食性を有し、しかも大きな熱衝撃に耐えられるという特徴を有している。
【００３８】
【実施例】
（実施例１〜６、比較例１〜２）
炭化珪素粉末（平均粒径１０μｍ）、窒化珪素粉末（平均粒径５μ）、及び炭素粉末２種（平均粒径２５μｍのものと５０μｍのもの）を表１に示す割合とした混合物１００質量部に対し、水２０質量部、バインダーとしてメチルセルロースを表１に示す質量部配合し、ヘンシェル混合機で１０分間混合して混練物を調整した。
【００３９】
【表１】

【００４０】
ついで、前記混練物を真空押出成形機を用い、成形圧力８ＭＰａの条件で、シート状成形体とハニカム状成形体とを押出成形した。シート状成形体は厚さ２ｍｍで長さ１００ｍｍであり、ハニカム成形体は、外形寸法１００ｍｍ、セル寸法２．０ｍｍ角、壁厚０．４ｍｍで、長さ１００ｍｍとした。得られたシート状成形体及びハニカム成形体を乾燥後、窒素雰囲気中、４５０℃×１ｈｒの脱脂を行ってから、窒素雰囲気中２２００℃で１時間焼成し、焼結体を得た。さらにこの焼結体を大気１１００℃で３時間熱処理し、残存する炭素を焼失させた。
【００４１】
得られたシート状焼結体のそれぞれから適当な大きさの試験片を切出し、水銀ポロシメーターによって平均細孔直径を測定すると共に、アルキメデス法によって気孔率を測定した。また別にシート状焼結体を切断加工し、４０×１０×２ｍｍのテストピースを多数作製して、強度試験より三点曲げ強さ（単に強度という）σを測定し、またその際の荷重と変位量から静的ヤング率計算式を用いてヤング率Ｅを算出した。以上の結果を表２に示す。なお、Ｘ線回折にて結晶相を同定したところ、炭化珪素のみからなっていることが確認された。
【００４２】
【表２】

【００４３】
得られたハニカム状焼結体を切断加工し、３×３セルのハニカムテストピースを多数作製して、その一部について初期強度を測定した。残りの一部については大気中電気炉で所定温度に加熱して２０分保持後、水中に投下することで熱衝撃を加え、その残存強度を測定した。その結果を表２に示す。実施例１〜６では、強度σ（ＭＰａ）、ヤング率Ｅ（ＧＰａ）とがＥ／σ^２≧２．５×１０^−２をみたしており、初期強度に対する残存強度の比が７０％以上であり、比較例１、２がそれぞれ３８％、３２％であるのに対し、格段に耐熱衝撃性に優れることが明らかである。
【００４４】
（実施例７）実施例１と同じ原料配合及び操作で作製した混練物を真空押出成形機を用い、成形圧力８ＭＰａの条件で、外形寸法１００ｍｍ、セル寸法２．０ｍｍ角、壁厚０．４ｍｍのハニカム形状に押出成形してから、長さ１４０ｍｍに切断した。得られたハニカム成形体を乾燥後、ハニカム形状の成形体の貫通孔の入口端面と出口端面を炭化珪素質封止材で市松模様に交互に封止し、窒素雰囲気中、４５０℃×１ｈｒの脱脂を行ってから、窒素雰囲気中２２００℃で１時間焼成し、焼結体を得た。さらにこの焼結体を大気１１００℃で３時間熱処理し、残存する炭素を焼失させて、炭化珪素質ハニカムフィルターを作製した。
【００４５】
得られた炭化珪素質ハニカムフィルターに煤を８ｇ担持し、空気気流中７００℃に加熱して煤を燃焼させた。その後フィルターを観察したところ、クラックは見られなかった。
【００４６】
（比較例３）比較例１と同じ原料配合及び操作で作製した混練物を真空押出成形機を用い、成形圧力８ＭＰａの条件で、外形寸法１００ｍｍ、セル寸法２．０ｍｍ角、壁厚０．４ｍｍのハニカム形状に押出成形してから、長さ１４０ｍｍに切断した。得られたハニカム成形体を乾燥後、ハニカム形状の成形体の貫通孔の入口端面と出口端面を炭化珪素質封止材で市松模様に交互に封止し、窒素雰囲気中、４５０℃×１ｈｒの脱脂を行ってから、窒素雰囲気中２２００℃で１時間焼成し、焼結体を得た。さらにこの焼結体を大気１１００℃で３時間熱処理し、残存する炭素を焼失させて、炭化珪素質ハニカムフィルターを作製した。
【００４７】
得られた炭化珪素質ハニカムフィルターに煤を８ｇ担持し、空気気流中７００℃に加熱して煤を燃焼させた。その後フィルターを観察したところ、円筒縦方向にクラックが発生した。
【００４８】
【発明の効果】
本発明のハニカム構造体は、それを構成する炭化珪素質多孔体について、強度とヤング率とが特定条件を満足し極めて耐熱衝撃性に優れるので、大きな熱衝撃を受けた際にも強度低下が小さい特性を有していることから、また、Ｘ線回折結果からあきらかな通りに炭化珪素以外の成分を含まず耐食性にも優れることから、セラミックスフィルターや触媒担体に好適に用いることができるので、産業上有用である。
【００４９】
また、本発明のセラミックフィルターは、前記特徴のあるハニカム構造体を用いているので、耐熱衝撃性に優れ、実使用時於ける熱衝撃を受けても大きなクラックに進展することがなく、ＰＭ漏れを生じることを防止できる特徴があり、産業上非常に有用である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a ceramic honeycomb structure having excellent thermal shock resistance, and a ceramic filter constituted by the honeycomb structure.
[0002]
[Prior art]
BACKGROUND ART In recent years, the problem of global environmental pollution due to harmful substances contained in various exhaust gases has been increasing in severity, and countermeasures have become an urgent issue. A typical example of a filter that collects harmful substances from exhaust gas is a diesel particulate filter (hereinafter “DPF”) that collects particulate matter (hereinafter “PM”) contained in exhaust gas of a diesel engine. .).
[0003]
As the DPF, a porous ceramic structure having a honeycomb structure, having cordierite or silicon carbide as a main component and having a large number of through holes extending from an inlet end face to an outlet end face, is known. A large number of through holes are separated by a porous wall called a cell wall, and the inlet end face and the outlet end face of the many through holes are alternately sealed in a checkered pattern, and the through hole with the sealed inlet end face is an outlet. The through-hole that is open at the end face and the open end face is sealed at the outlet end face.
[0004]
The DPF is installed as a part of an exhaust gas system of a diesel engine. When exhaust gas flows in through an open through hole at an inlet end face, PM is collected when passing through a porous cell wall, and PM is collected. Exhaust gas is not contained and flows out from the through hole opened at the outlet end face. Therefore, it is necessary that the cell wall has a pore diameter and a porosity so that the exhaust gas containing PM can easily permeate and, at that time, most or all of the PM can be collected.
[0005]
When the PM is collected and deposited on the cell walls of the DPF, the ventilation resistance increases. Therefore, it is necessary to periodically remove the collected PM. The main component of PM in the exhaust gas of a diesel engine is soot, and therefore, a simple and common method for removing it is to burn it in air. However, when the soot burns, a large amount of heat is generated, so that a temperature gradient is generated within the filter in a short time, and a thermal shock corresponding thereto is applied.
[0006]
Cordierite has a small coefficient of thermal expansion and thus is strong against such thermal shock, but has a problem in corrosion resistance at high temperatures. In that respect, silicon carbide has excellent corrosion resistance even at high temperatures, and is useful in cases where high temperatures are expected, such as when used for exhaust gas of automobile diesel engines. However, since silicon carbide has a relatively high coefficient of thermal expansion, there is a concern about thermal shock resistance. That is, there is a problem that a large crack is generated in the filter due to a thermal shock generated when the PM is burned, thereby causing a PM trapping leak.
[0007]
An example of an attempt to improve the thermal shock resistance of a honeycomb structure using a silicon carbide-based porous body is one in which the ratio of strength to Young's modulus is strength (MPa) / Young's modulus (GPa) ≧ 1.1. It is disclosed (see Patent Document 1).
[0008]
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-154876
According to the known technique, in a thermal shock resistance test by a water drop method, the residual strength at a temperature difference ΔT = 500 ° C. is 50 to 70% of the initial strength, and a conventional general recrystallized silicon carbide porous material is used. It is disclosed that the thermal shock resistance is improved as compared with the case of a body (residual strength 30%). However, it still falls below 70% of the initial strength, which is not sufficient. In addition, in the above-mentioned technology, metallic silicon is added to silicon carbide in order to obtain the above-mentioned effects. It is also feared that the corrosion resistance of the sintered body is reduced by the above.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a porous silicon carbide-based honeycomb structure having sufficient corrosion resistance at high temperatures under actual use conditions when used for a ceramic filter or a catalyst carrier and capable of withstanding a large thermal shock, An object of the present invention is to provide a ceramic filter suitable for a DPF or the like, which is constituted by the porous silicon carbide honeycomb structure having high thermal shock resistance.
[0011]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and found that the value of the parameter E / σ ² consisting of the strength σ (MPa) and the Young's modulus (GPa) of the silicon carbide porous body constituting the honeycomb structure. It is found that when the value of is equal to or more than a specific value, the thermal shock resistance of the honeycomb structure is significantly improved, and the present invention has been accomplished.
[0012]
That is, the present invention is a honeycomb structure made of silicon carbide-based porous body, the strength of the silicon carbide based porous material sigma (MPa), when the Young's modulus and E (GPa), E / σ 2 Is 2.5 × 10 ⁻² or more.
[0013]
Further, the present invention is a silicon carbide-based honeycomb structure, wherein the strength (σ) of the silicon carbide-based porous body satisfying the relationship between the strength and the Young's modulus is 2 MPa or more and 45 MPa or less.
[0014]
In addition, the present invention provides a silicon carbide based honeycomb structure characterized by having a Young's modulus (E) of 0.5 GPa or more and 25 GPa or less in a silicon carbide based porous material satisfying the above relationship between strength and Young's modulus. is there.
[0015]
Furthermore, the present invention is a ceramic filter characterized by being configured using the above-mentioned silicon carbide honeycomb structure.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
One of the indices indicating the thermal shock resistance is a thermal shock fracture resistance coefficient (R). This R is expressed by the following equation (1), where σ is the breaking strength, ν is the Poisson's ratio, E is the Young's modulus, and α is the coefficient of thermal expansion.
[0017]
(Equation 2)

[0018]
Here, ν and α are values peculiar to the material that hardly changes when the material is the same, and σ and E are values that greatly change depending on the microstructure of the material. Therefore, as described in Patent Document 1, there are many examples in which σ / E is used as an index of thermal shock resistance.
[0019]
However, as another index representing the thermal shock resistance, a thermal shock damage resistance coefficient (R ′) as shown by the following equation (2) is known.
[0020]
[Equation 3]

[0021]
Here, E, σ, and ν are the same as in equation (1). The aforementioned R is an index of resistance to the occurrence of cracks, whereas R 'is an index of resistance to the development of cracks. Since the strength generally corresponds to the length of the crack, in other words, a large R indicates a large ΔT at which the strength decreases, while a large R ′ indicates a small degree of the strength reduction. Indicates small.
[0022]
When used as a filter, what is regarded as a problem in terms of thermal shock resistance is a case where cracks caused by thermal shock are large enough to cause PM leakage. However, since the silicon carbide porous material forming the honeycomb structure is a porous material having an average pore diameter of several μm to several tens μm and a porosity of 30% or more, fine microcracks may occur. Is not a big problem. Therefore, it is considered that R ′ is more appropriate as an index of the thermal shock resistance of the filter.
[0023]
The present inventor repeatedly conducted experimental studies on the relationship between the strength (σ) and the Young's modulus (E) of various porous silicon carbide bodies based on the above-described concept, and as a result, the E / σ ² value was a specific value. In the above, it has been found that the object of the present invention can be achieved, and the present invention has been accomplished.
[0024]
That is, when the strength of the silicon carbide-based porous body is σ (MPa) and the Young's modulus is E (GPa), when the relationship of E / σ ² ≧ 2.5 × 10 ⁻² is satisfied, thermal shock Thus, a honeycomb structure that does not develop into large cracks even when subjected to heat is obtained. As a result, when the honeycomb structure is used as a filter, a filter that does not leak PM and has high thermal shock resistance can be obtained.
[0025]
Further, in the honeycomb structure of the present invention, the range of the strength σ (MPa) of the silicon carbide-based porous body constituting the honeycomb structure is 2 to 45, and the Young's modulus E (GPa) is 0.5 to 25. It is preferable that When the strength and Young's modulus are respectively smaller than the above ranges, the honeycomb structure may be deformed when manufacturing the honeycomb structure or when assembling the honeycomb structure with other parts to apply to a filter such as a DPF, or may be damaged when severe. This is because a problem may occur.
[0026]
On the other hand, when the strength and Young's modulus are respectively larger than the above ranges, although there is no particular technical disadvantage, a long-time sintering operation at a high temperature is required to obtain a silicon carbide porous body having the characteristics. In some cases, it is necessary to sacrifice characteristics such as pore diameter and porosity in the silicon carbide porous body. In order to achieve the object of the present invention, the strength σ (MPa) of the silicon carbide porous body constituting the honeycomb structure is 2 to 19, and the Young's modulus E (GPa) is 0.7 to 10 respectively. This is a preferred range.
[0027]
In the honeycomb structure of the present invention, the average pore diameter and the porosity of the pores formed in the cell wall are not particularly limited. However, in consideration of use as a filter, the porosity of the cell wall is preferably 40% or more, particularly preferably 50 to 80%, and the average pore diameter is preferably 5 to 50 μm. If the porosity of the cell wall is less than 40%, the pressure loss at the time of ventilation increases, while if it exceeds 80%, the strength decreases and the handling property becomes poor. If the average pore diameter of the cell wall is less than 5 μm, PM is easily clogged inside the cell wall. If the average pore diameter exceeds 50 μm, there is a possibility that PM leakage may occur, while maintaining strength. It becomes difficult. In addition, the average pore diameter of the cell wall in the present invention means a value determined by a mercury intrusion method.
[0028]
Since both the strength σ and the Young's modulus E are affected by the porosity and the pore diameter, it is conceivable to control the thermal shock resistance by the porosity and the pore diameter. However, the strength and Young's modulus are greatly affected by the microstructure of the structure in addition to the porosity and pore diameter, and they are complicatedly entangled. Therefore, it is difficult to obtain a clear correlation. Therefore, it is better to select the silicon carbide porous body constituting the honeycomb structure on the basis that the strength and Young's modulus satisfy the above relationship.
[0029]
Next, a method for manufacturing the honeycomb structure of the present invention will be described. The honeycomb structure of the present invention is obtained by forming a mixture of a silicon carbide powder or a mixture of a silicon carbide powder and a silicon nitride powder with a predetermined amount of a carbonaceous substance into a honeycomb-shaped molded body, and forming the mixture in a non-oxidizing atmosphere. And by sintering. Alternatively, it can be manufactured similarly by using metal silicon powder instead of silicon nitride powder and heating in a nitrogen atmosphere.
[0030]
First, in manufacturing a honeycomb-shaped molded body, silicon nitride powder or metal silicon powder reacts with silicon carbide powder, a mixed powder of silicon carbide powder and silicon nitride powder, or a mixed powder of silicon carbide powder and silicon nitride powder. An appropriate amount of water and an organic binder are added to a mixture containing at least a carbonaceous substance necessary for forming silicon carbide by mixing to obtain a kneaded material for extrusion molding. Regarding mixing and kneading, any method such as dry mixing and wet mixing can be adopted as long as mixing can be performed uniformly. The organic binder is not particularly limited, and may be methylcellulose, polyvinyl alcohol, or the like, or a general material containing them as a main component.
[0031]
Since the carbonaceous substance can be easily removed by heat treatment in an oxidizing atmosphere, it is necessary to control the porosity, the pore diameter, and the like of the honeycomb structure by adjusting the addition amount and the particle size. Can be.
[0032]
Next, the obtained kneaded material is formed into a desired honeycomb shape by an extrusion molding method or the like, and is heated and sintered through a drying and degreasing process. Sintering is performed in a non-oxidizing atmosphere such as nitrogen or argon. At this time, the sintering method is not particularly limited, and a general heating furnace such as a heater heating furnace and a high-frequency heating furnace can be used. In addition, when a slight amount of nitrogen is dissolved in silicon carbide by including silicon nitride powder as a raw material or the like, conductivity is exhibited, and therefore, a known electric current sintering method (see Patent Document 2) And sintering can be performed in a short time.
[0033]
[Patent Document 2] JP-A-10-52618
The sintering temperature is preferably from 1800C to 2500C. When the sintering temperature is lower than 1800 ° C., the grain growth and sintering of silicon carbide are insufficient, and the heat resistance may decrease due to the remaining unreacted silicon nitride and carbonaceous material. On the other hand, when the temperature exceeds 2500 ° C., crystal transition or sublimation occurs, and the strength is reduced due to extreme grain growth.
[0035]
Further, in manufacturing a filter comprising the honeycomb structure of the present invention, the filter can be manufactured by plugging the honeycomb through holes at both end surfaces. The plugging method can be performed by a method described in Patent Document 3 or the like.
[0036]
[Patent Document 3] JP-A-9-19613
In addition, since the filter of the present invention is configured by using the honeycomb structure, it has a characteristic that it has sufficient corrosion resistance at high temperatures under actual use conditions and can withstand a large thermal shock. .
[0038]
【Example】
(Examples 1 to 6, Comparative Examples 1 and 2)
100 parts by mass of a mixture of silicon carbide powder (average particle size: 10 μm), silicon nitride powder (average particle size: 5 μm), and two types of carbon powder (average particle size: 25 μm and 50 μm) in the ratio shown in Table 1. On the other hand, 20 parts by mass of water and methylcellulose as a binder were mixed with parts by mass shown in Table 1 and mixed with a Henschel mixer for 10 minutes to prepare a kneaded material.
[0039]
[Table 1]

[0040]
Then, the kneaded product was extruded into a sheet-shaped molded product and a honeycomb-shaped molded product using a vacuum extruder under a molding pressure of 8 MPa. The sheet-shaped formed body had a thickness of 2 mm and a length of 100 mm, and the honeycomb formed body had an outer size of 100 mm, a cell size of 2.0 mm square, a wall thickness of 0.4 mm, and a length of 100 mm. After drying the obtained sheet-like molded body and honeycomb molded body, they were degreased at 450 ° C. × 1 hr in a nitrogen atmosphere, and then fired at 2200 ° C. for 1 hour in a nitrogen atmosphere to obtain a sintered body. Further, this sintered body was heat-treated at 1100 ° C. for 3 hours in the atmosphere to burn out remaining carbon.
[0041]
A test piece of an appropriate size was cut out from each of the obtained sheet-shaped sintered bodies, and the average pore diameter was measured by a mercury porosimeter, and the porosity was measured by an Archimedes method. Separately, a sheet-like sintered body is cut and processed to prepare a large number of test pieces of 40 × 10 × 2 mm, and a three-point bending strength (simply called strength) σ is measured by a strength test. The Young's modulus E was calculated from the displacement using a static Young's modulus calculation formula. Table 2 shows the above results. When the crystal phase was identified by X-ray diffraction, it was confirmed that the crystal phase was composed of only silicon carbide.
[0042]
[Table 2]

[0043]
The obtained honeycomb-shaped sintered body was cut to prepare a large number of honeycomb test pieces of 3 × 3 cells, and the initial strength was measured for a part thereof. The remaining part was heated to a predetermined temperature in an electric furnace in the atmosphere, held for 20 minutes, and then dropped into water to apply a thermal shock, and the residual strength was measured. Table 2 shows the results. In Examples 1 to 6, the strength σ (MPa) and the Young's modulus E (GPa) satisfy E / σ ² ≧ 2.5 × 10 ⁻² , and the ratio of the residual strength to the initial strength is 70% or more. Comparative Examples 1 and 2 are 38% and 32%, respectively, while it is clear that the thermal shock resistance is remarkably excellent.
[0044]
(Example 7) A kneaded material produced by the same raw material blending and operation as in Example 1 was molded using a vacuum extrusion molding machine under the conditions of a molding pressure of 8 MPa, an outer dimension of 100 mm, a cell dimension of 2.0 mm square, and a wall thickness of 0.4 mm. And then cut to a length of 140 mm. After drying the obtained honeycomb formed body, the inlet end face and the outlet end face of the through-hole of the honeycomb shaped formed body are alternately sealed in a checkered pattern with a silicon carbide sealing material, and heated at 450 ° C. × 1 hr in a nitrogen atmosphere. After performing degreasing, it was baked at 2200 ° C. for 1 hour in a nitrogen atmosphere to obtain a sintered body. Further, this sintered body was heat-treated at 1100 ° C. for 3 hours in the atmosphere to burn off remaining carbon, thereby producing a silicon carbide honeycomb filter.
[0045]
8 g of soot was loaded on the obtained silicon carbide honeycomb filter, and the soot was heated to 700 ° C. in an air stream to burn the soot. Thereafter, when the filter was observed, no crack was observed.
[0046]
(Comparative Example 3) A kneaded product produced by the same raw material blending and operation as in Comparative Example 1 was molded using a vacuum extrusion molding machine under the conditions of a molding pressure of 8 MPa, an outer dimension of 100 mm, a cell dimension of 2.0 mm square, and a wall thickness of 0.4 mm. And then cut to a length of 140 mm. After drying the obtained honeycomb formed body, the inlet end face and the outlet end face of the through-hole of the honeycomb shaped formed body are alternately sealed in a checkered pattern with a silicon carbide sealing material, and heated at 450 ° C. × 1 hr in a nitrogen atmosphere. After performing degreasing, it was baked at 2200 ° C. for 1 hour in a nitrogen atmosphere to obtain a sintered body. Further, this sintered body was heat-treated at 1100 ° C. for 3 hours in the atmosphere to burn off remaining carbon, thereby producing a silicon carbide honeycomb filter.
[0047]
8 g of soot was loaded on the obtained silicon carbide honeycomb filter, and the soot was heated to 700 ° C. in an air stream to burn the soot. Then, when the filter was observed, cracks occurred in the vertical direction of the cylinder.
[0048]
【The invention's effect】
The strength and Young's modulus of the silicon carbide-based porous body constituting the honeycomb structure of the present invention satisfy specific conditions and are extremely excellent in thermal shock resistance, so that the strength decreases even when subjected to a large thermal shock. Because it has small properties, and because it does not contain components other than silicon carbide and is also excellent in corrosion resistance as apparent from X-ray diffraction results, it can be suitably used for ceramic filters and catalyst carriers. Industrially useful.
[0049]
Further, since the ceramic filter of the present invention uses the honeycomb structure having the characteristics described above, it has excellent thermal shock resistance, does not develop into a large crack even when subjected to a thermal shock in actual use, and has a high PM leakage. This is a feature that can prevent the occurrence of, and is very useful industrially.

Claims (4)

A honeycomb structure comprising a silicon carbide-based porous body, wherein the strength (σ; MPa) and the Young's modulus (E; GPa) of the silicon carbide-based porous body satisfy the following relationship.

2. The honeycomb structure according to claim 1, wherein the strength (σ) of the silicon carbide porous body is 2 MPa or more and 45 MPa or less. The honeycomb structure according to claim 1 or 2, wherein the silicon carbide-based porous body has a Young's modulus (E) of 0.5 GPa or more and 25 GPa or less. A ceramic filter comprising the honeycomb structure according to claim 1.