Compositions: ceramic – Ceramic compositions – Pore-forming
Patent
1995-01-06
1997-04-08
Marcantoni, Paul
Compositions: ceramic
Ceramic compositions
Pore-forming
501 97, 501 98, 501152, C04B 3558
Patent
active
056187653
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention generally relates to a ceramics porous body which is useful as a filter material for removing foreign matter from a fluid or as a catalytic carrier, and more specifically, it relates to a silicon nitride ceramics porous body and a method of preparing the same.
BACKGROUND ART
Known porous bodies employed for filter materials or catalytic carriers include those consisting of various materials such as resin, metals or ceramics. Among these, a filter or a catalytic carrier consisting of a ceramics material is generally employed in high temperature or strongly corrosive environments which other materials cannot withstand. A filter or a catalytic carrier consisting of oxide ceramics such as alumina (Al.sub.2 O.sub.3) has already been put into practice.
As to a porous body consisting of nonoxide ceramics, on the other hand, only small examples have been put into practice while Japanese Patent Laying-Open No. 63-291882 discloses a silicon nitride based or silicon carbide based porous body prepared by a heat treatment. Further, Japanese Patent Laying-Open No. 1-188479 discloses a method of compacting mixed powder of silicon powder and silicon nitride powder of relatively coarse particles and thereafter nitriding the same thereby preparing a porous body as a solid target.
As hereinabove described, it is difficult to use a porous body consisting of resin or a metal in a high temperature or corrosive atmosphere. It is inevitably necessary to employ a porous body made of ceramics for a filter for removing foreign matter from a high-temperature exhaust gas or for a carrier serving as a catalyst for decomposing a harmful matter.
As an example of such porous bodies made of ceramics, porous bodies made of alumina have been put into practice. While the porous bodies of alumina are varied in pore size, porosity and bending strength, a porous body having a porosity of 35 to 40% and a mean pore size of 25 to 130 .mu.m has a bending strength of 20 to 35 MPa, whereby the strength of the porous body is insufficient depending on its use.
In the silicon nitride based porous body disclosed in the aforementioned Japanese Patent Laying-Open No. 63-291882, the porosity is less than 30% and fluid permeability is insufficient. In general, the strength of ceramics tends to be reduced following an increase in the porosity, and it has been extremely difficult to attain compatibility between the porosity and the strength.
SUMMARY OF THE INVENTION
In view of the above, the present invention has been proposed in order to solve the aforementioned problems, and an object thereof is to provide a ceramics porous body having high porosity as well as high strength.
The present inventors have deeply studied on the aforementioned subject, and discovered that it is possible to prepare a silicon nitride ceramics porous body that is mainly composed of columnar .beta.-Si.sub.3 N.sub.4 (.beta.-silicon nitride) crystal grains and capable of maintaining high strength also when its porosity is high, by heat treating a compact of mixed powder of silicon nitride (Si.sub.3 N.sub.4) powder and prescribed additive powder at a high temperature.
Namely, a ceramic porous body according to the present invention is generally characterized in that it has a porosity of at least 30% and is mainly composed of columnar ceramics grains having an aspect ratio of at least 3. Specifically, it is a porous body having a mean pore size of at least 0.05 .mu.m and not more than 12 .mu.m. Further, the crystal grains preferably have the shape of hexagonal poles or columns, i.e. a rod-like crystal shape with a hexagonal cross-section.
More specifically, the ceramics porous body according to the present invention is a silicon nitride ceramics porous body that is mainly composed of silicon nitride with a ratio of at least 60%, preferably at least 90%, of .beta.-Si.sub.3 N.sub.4 columnar grains with respect to the entire silicon nitride grains, contains at least one compound of a rare earth element with at least 1 volume % and not more t
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Journal of Ceramic Society of Japan 100(5) 758-762 (1992) no month.
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Ceramic Transactions, ISSN 1042-1122, vol. 31 Porous Materials (1992) no month.
JIS R 1601 (English version) Testing Method for Flexural Strength (Modulus of Rupture) of High Performance Ceramics, Japanese Standard Assoc. (1981) no month.
Journal of Ceramic Society of Japan 100(5) 758-762 (1992) no month.
Microstructure And Electrical-Properties In A Humid Atmosphere by Susumu Nakayama et al.
Ceramic Transactions, ISSN 1042-1122, vol. 31 Porous Materials (1992) no month.
Kawai Chihiro
Matsuura Takahiro
Nakahata Seiji
Takeuchi Hisao
Fasse W. F.
Fasse W. G.
Marcantoni Paul
Sumitomo Electric Industries Ltd.
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