Patent Publication Number: US-2007116908-A1

Title: Honeycomb structure, porous body, pore forming material for the porous body, and methods for manufacturing the pore forming material, the porous body and the honeycomb structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a divisional of U.S. patent application Ser. No. 11/450,460 filed on Jun. 12, 2006, which is a divisional of U.S. patent application Ser. No. 11/033,151, filed on Jan. 12, 2005, which claims the benefit of priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2004-006152, filed Jan. 13, 2004. The entire contents of those applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a honeycomb structure, a porous body and a pore forming material for the porous body. The present invention further relates to methods for manufacturing the pore forming material, the porous body and the honeycomb structure.  
      2. Discussion of the Background  
      Unexamined Japanese Patent Publication (kokai) No. 2003-10617 discloses a method for manufacturing a porous body in which polymer particles (a pore forming material for a porous body) having a pore formation function are blended into a skeleton material comprised of a ceramic, a metal, etc. and the resultant mixture is subjected to molding, drying, degreasing, and calcination to obtain a porous body. In such a manufacturing method, the polymer particles are burnt and removed by decomposition in a degreasing step, and thereby pores are formed in the portion so far occupied by the polymer particles. The contents of Unexamined Japanese Patent Publication (kokai) No. 2003-10617 are incorporated herein by reference in their entirety.  
     SUMMARY OF THE INVENTION  
      According to one aspect of the present invention, a pore forming material for a porous body includes organic polymer particles and inorganic particles.  
      According to another aspect of the present invention, a method for manufacturing a pore forming material for a porous body includes polymerizing an organic monomer in an organic solvent to form organic polymer particles having void spaces inside the organic polymer particles. The organic polymer particles and inorganic particles are mixed. The mixture is degassed to inject the inorganic particles in the void spaces of the inside of the organic polymer particles.  
      According to yet another aspect of the present invention, a method for manufacturing a porous body includes mixing skeleton particles and a pore forming material containing an organic polymer particles and inorganic particles to make a mixture. The mixture is molded to produce a molded product. The molded product is heated.  
      According to yet another aspect of the present invention, a porous body includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The inorganic compound contains a second element different from the first element. The second element exists at least on the inner surface.  
      According to yet another aspect of the present invention, a porous body includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The inorganic compound contains the first element which is scattered on the inner surface.  
      According to yet another aspect of the present invention, a porous body includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The void space has a major axis which is equal to or larger than a major axis of the skeleton particles. The inorganic compound contains a second element different from the first element. The second element exists at least on the inner surface.  
      According to yet another aspect of the present invention, a porous body includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The void space has a major axis which is equal to or larger than a major axis of the skeleton particles. The inorganic compound contains the first element which is scattered on the inner surface.  
      According to yet another aspect of the present invention, a porous body is made by mixing skeleton particles and a pore forming material containing an organic polymer and inorganic particles.  
      According to yet another aspect of the present invention, a honeycomb structure includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The inorganic compound contains a second element different from the first element. The second element exists at least on the inner surface.  
      According to yet another aspect of the present invention, a honeycomb structure includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The inorganic compound contains the first element which is scattered on the inner surface.  
      According to yet another aspect of the present invention, a honeycomb structure includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The void space has a major axis which is equal to or larger than a major axis of the skeleton particles. The inorganic compound contains a second element different from the first element. The second element exists at least on the inner surface.  
      According to yet another aspect of the present invention, a honeycomb structure includes skeleton particles and an inorganic compound. The skeleton particles include a first element and constitute an inner surface to form a void space. The void space has a major axis which is equal to or larger than a major axis of the skeleton particles. The inorganic compound contains the first element which is scattered on the inner surface.  
      According to yet another aspect of the present invention, a method for manufacturing a honeycomb structure includes mixing skeleton particles and a pore forming material containing an organic polymer particles and inorganic particles to make a mixture. The mixture is molded to produce a molded product. The molded product is heated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
       FIG. 1A  and  FIG. 1B  are views showing a pore forming material for a porous body (with a hollow portion) according embodiments of the present invention;  
       FIG. 2A  and  FIG. 2B  are views showing a pore forming material for a porous body (with no hollow portion) according to embodiments of the present invention;  
       FIG. 2C  is a view showing a pore forming material for a porous body (with inorganic particles in a balloon shape) according to an embodiment of the present invention;  
       FIG. 3  is a flow chart for manufacturing a pore forming material for a porous body according to an embodiment of the present invention;  
       FIGS. 4A  to  4 F are schematic diagrams of the steps for manufacturing a pore forming material for a porous body according to an embodiment of the present invention;  
       FIG. 5  is a flow chart for manufacturing a porous body according to an embodiment of the present invention;  
       FIGS. 6A  to  6 D are schematic diagrams of the steps for manufacturing a porous body according to an embodiment of the present invention;  
       FIGS. 7A and 7B  are schematic diagrams of a porous body according to an embodiment of the present invention;  
       FIGS. 8A and 8B  are schematic diagrams of a porous body according to an embodiment of the present invention;  
       FIGS. 9A  to  9 C are schematic diagrams of a porous body according to embodiments of the present invention;  
       FIG. 10  is an enlarged view of a neck portion of a porous body according to and embodiment of the present invention;  
       FIG. 11  is a schematic diagram of a honeycomb structure according to an embodiment of the present invention;  
       FIG. 12A  is a schematic diagram of a porous ceramic member constituting a part of a honeycomb structure according to an embodiment of the present invention;  
       FIG. 12B  is a cross view cut along the line XIIB-XIIB;  
       FIG. 13  is an enlarged view of the circled portions of  FIG. 12B  and the  FIG. 16B ;  
       FIG. 14  is an enlarged view of the circled portions of  FIG. 12B  and  FIG. 16B  in the case where a catalyst is added;  
       FIGS. 15A  to  15 D are illustrations showing the manufacturing method of a honeycomb structure according to an embodiment of the present invention;  
       FIG. 16A  is a perspective view diagrammatically showing an example of a single unit-type honeycomb structure;  
       FIG. 16B  is a cross sectional view cut along the line XVIB-XVIB; and  
       FIGS. 17A-17C  are views showing a pore forming material for a porous body according to embodiments of the present invention. 
    
    
     DESCRIPTION OF THE EMBODIMENTS  
      The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.  
       FIGS. 1A and 1B  show pore forming materials  1 A and  1 B according to an embodiment of the present invention. Referring to  FIG. 1A , the pore forming material  1 A includes organic polymer particles  2 . The organic polymer particle  2  has a hollow portion  3  in the organic polymer particle  2  and includes an inorganic particle  4  in the hollow portion  3 . Referring to  FIG. 1B , the pore forming material  1 B includes organic polymer particles  2 . The organic polymer particle  2  has a hollow portion  3  in the organic polymer particle  2  and includes a plurality of inorganic particles  4  in the hollow portion  3 .  
      One inorganic particle may be contained in one organic polymer particle  2  as shown in  FIG. 1A , or a plurality of inorganic particles may be contained in one organic polymer particle  2  as shown in  FIG. 1B .  
      It is inferred that when a pore material has such a structure, even in a case where the pore forming material for a porous body is formulated with a high content of organic polymer particles with the aim of manufacturing a porous body with a high porosity, since the total amount of organic components is low, local heat shock caused by rapid decomposition and combustion of organic components is unlikely to occur at the time of heating. Thus, cracking in a molded body is considered unlikely to occur.  
      Furthermore, having a structure of containing inorganic particles in the hollow organic polymer, the inorganic particles function as a reinforcing material for the pore forming material for a porous body, imparting excellent mechanical strength to the pore forming material for a porous body. Accordingly, even in the case where a pressure has to be applied in extrusion molding or press molding, etc., to form a shape, no distortion of the pore forming material for a porous body occurs. In this way, since the pore forming material for a porous body having greater strength can be added in a great amount, this specific embodiment is particularly suitable for the manufacture of a porous body with a high porosity of 50% or more.  
       FIGS. 2A and 2B  show embodiments wherein there is no hollow inside of organic polymer particles  2 , and organic polymer particles  2  and inorganic particles  4  are closely contacted. Inorganic particles here also function for the prevention of local heat shock caused by rapid decomposition and combustion of organic components and as a reinforcing material for the pore forming material for a porous body. Thus, the improved effects of the mechanical strength of the pore forming material for a porous body can also be observed in these embodiments.  
      In addition,  FIG. 2C  discloses a specific embodiment wherein a hollow portion  3  is provided inside an organic polymer particle  2 , and a microballoon  400  is used as the inorganic particles. Examples of the above-mentioned microballoon include alumina balloon, glass microballoon, silas balloon, fly ash balloon (FA balloon), and mullite balloon. Among them, alumina balloon is preferable. Note that the above-mentioned balloon refers to a concept including a bubble and a hollow ball, and indicate a hollow particle having a pore inside.  
      Since the inner volume of the inner space of Microballoon  400  can be increased or decreased, it is possible to control total amount of inorganic component.  
      Furthermore, when inorganic particles contained in pore forming materials  1 A,  1 B,  2 A,  2 B, and  2 C have a function as a sintering additive, those pore materials will have not only a pore-formation function, but also a function as a sintering additive, thus making it possible to lower calcination temperature, or to produce a porous body excellent in mechanical properties by promoting sintering.  
       FIGS. 17A-17C  show pore materials according to an embodiment of the present invention. Referring to  FIGS. 17A-17C , the pore material includes organic polymer particles  2  and inorganic particles  4 . Referring to  FIG. 17A , the organic polymer particles  2  and the inorganic particles  4  are mixed together. In  FIG. 17A , the inorganic particles  4  exist among three-dimensional network structures of the polymer particles  2 . Referring to  FIG. 17B , the inorganic particles  4  substantially surround the organic polymer particles  2 . Some inorganic particles  4  are mixed with the organic polymer particles  2  inside the surrounding inorganic particles  4 . Referring to  FIG. 17C , the microballoon  400  is surrounded by the organic polymer particles  2 .  
      Examples of the organic polymer particles  2  include a polymer of monomer mixture compositions including a hydrophilic monomer, a multi-functional monomer, and other monomers.  
      Examples of the above-mentioned hydrophilic monomer include methyl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide, (meth)acrylic acid, glycidyl (meth)acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, vinylpyridine, 2-acryloyloxyethyl phthalate, itaconic acid, fumaric acid, and dimethylaminomethyl methacrylate, preferably methyl methacrylate, (meth)acrylic acid, and 2-hydroxyethyl methacrylate. These may be used alone or as a combination of two or more.  
      Examples of the above-mentioned polyfunctional monomer include di(meth)acrylate, and tri(meth)acrylate.  
      As examples of the above-mentioned di(meth)acrylate, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, and trimethylolpropane di(meth)acrylate can be mentioned.  
      As examples of the above-mentioned tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethylene oxide-modified trimethylolpropane tri(meth)acrylate, and pentaerythritol tri(meth)acrylate can be mentioned.  
      Further, as examples of the above-mentioned polyfunctional monomer, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate; a di- or triallyl compound such as diallyl phthalate, diallyl malate, diallyl fumarate, diallyl succinate, and triallyl isocyanurate; a divinyl compound such as divinylbenzene, and butadiene can be mentioned. These may be used alone or as a combination of two ore more.  
      Examples of the above-mentioned other monomers include alkyl(meth)acrylate such as ethyl(meth)acrylate, propyl(meth)acrylate, butyl(meth)acrylate, cumyl methacrylate, cyclohexyl(meth)acylate, mistyryl(meth)acrylate, palmityl (meth)acrylate, and stearyl(meth)acrylate; an aromatic vinyl monomer such as styrene, α-methylstyrene, p-methylstyrene, and p-chrorostyrene; vinyl ester such as vinyl acetate and vinyl propionate; a halogen-containing monomer such as vinyl chloride and vinylidene chloride; and ethylene, propylene, and butadiene can be mentioned. These may be used alone, or as a combination of two or more.  
      Examples of the above-mentioned inorganic particles include particles comprising ceramics such as nitride ceramics including aluminum nitride, silicon nitride, boron nitride, and titanium nitride; carbide ceramics such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, boron carbide, and tungsten carbide; oxide ceramics such as alumina, zirconia, cordierite, mullite, and silica; and particles comprising a metal such as Si, Fe, and Al, or a metal compound such as oxide of iron. Among these, particles including ceramics are preferable, and particularly preferable are particles including oxide ceramics in order to prevent explosion during handling the inorganic particles.  
      The porosity of the above-mentioned pore forming materials for a porous body  1 A,  1 B, and  2 C, the desirable lower limit is about 10% by volume and the desirable upper limit is about 70% by volume. Thermal shock can be reduced while maintaining the mechanical strength of the pore forming materials for a porous body  1 A and  1 B.  
      The more preferable lower limit of the above-mentioned porosity is about 20% by volume and the more preferable upper limit is about 60% by volume.  
      The porosities of the pore forming materials for a porous body  1 A,  1 B and  2 C are obtained by determining the components of the organic polymer and those of inorganic particles for a pore material using, for example, gas chromatography, fluorescent X-ray analysis, or the like and then calculating the respective densities using a specific gravity bottle. The organic element is then eliminated by combustion and the volume of the organic polymer is calculated assuming that the reduced weight is the amount of the organic element, while the volume of the inorganic particles is calculated assuming that the weight of the remained portion is that of the inorganic particles. The porosity is obtained using the two volume values.  
      Further, in the pore forming materials for a porous body  1 A,  1 B,  2 A,  2 B, and  2 C, the desirable lower limit of the volume ratio of the organic polymer particles  2  based on the inorganic particles  4  (the organic polymer/the inorganic particles) is about 0.1 and the desirable upper limit thereof is about 250, and preferably upper limit thereof is about 10. This is because thermal shock can be reduced in this range without lowering the pore forming performance. The more preferable lower limit of the above-mentioned volume ratio is about 0.2 and the more preferable upper limit is about 5.  
      In addition, the particle sizes of the pore forming materials for a porous body  1 A,  1 B,  2 A,  2 B, and  2 C are not particularly limited, and they may be suitably selected considering the pore size of the porous body to be produced. Usually, a particle size of about 20 to 60 μm is desirable. Further, the particle size is desirably about 0.5 to about 10.0 times of that of the skeleton particles, and preferably about 0.5 to about 5.0 times of that of the skeleton particles. This is because the pore forming effects for the porous body can be obtained and the strength can be enhanced.  
      Next, the method for manufacturing the pore forming materials  1 A,  1 B,  2 A,  2 B, and  2 C for a porous body will be explained.  FIG. 3  shows the flow chart of the steps.  
      Referring to  FIG. 3 , first, a solution to polymerize a polymer is prepared as shown in step A. The polymer is then polymerized as shown in step B. Hollow particles are produced by removing the solvent as shown in step C. Then, the polymer particles are mixed into slurry containing inorganic particles as shown in step. The inorganic particles are injected and transferred into the polymer particles while degassing them as shown in step E. Then, the slurry is removed as shown in step F to finally obtain a pore material.  
      Specifically, for example, as shown in  FIG. 4A , a non-polymeric organic solvent is mixed to the above-mentioned mixed monomers  21  including a hydrophilic monomer, a multi-functional monomer, and other monomers to prepare a monomer solution. After suspending the monomer solution to an organic solvent  5 , the monomer components are polymerized as shown in  FIG. 4B  to obtain organic polymer particles  2  containing the non-polymeric organic solvent. Then, as shown in  FIG. 4C , the organic solvent  5  in the polymer particles  2  is removed to obtain hollow organic polymer particles  2 .  
      Here, the method for polymerizing the mixed monomers is not particularly limited, it is desirable to use a suspension polymerization method due to the advantages that the particle size can be easily controlled and that particles each having an effective void space can be easily produced.  
      Further, as shown in  FIG. 4D , the obtained hollow polymer particles  2  are mixed to the slurry  41  containing inorganic particles of a predetermined concentration. The inorganic particles  4  are then injected and transferred into hollow polymer particles  2 . Then, the slurry is removed and drying is performed to obtain the pore forming materials for a porous body  1 A,  1 B,  2 A,  2 B, and  2 C.  
      Here, instead of the slurry  41 , a sol containing inorganic particles such as alumina sol or silica sol may be used. Adjusting the content of inorganic particles to a high ratio can be achieved by repeating several times the steps of mixing hollow polymer particles  2  to the slurry  41 , immersing and drying. In addition, a pore forming material for a porous body can be produced by mixing inorganic particles such as alumina and silica into the above-mentioned mixed monomers beforehand, and then polymerizing the mixed monomers using a publicly known technique of capsulation.  
      Moreover, a pulverization treatment may be performed to granulate each of the inorganic particles  4 . Further, by subjecting the inorganic particles  4  to a chemical treatment (for example, acid treatment) or a physical treatment (for example, grinding treatment), the weight of each inorganic particle can be reduced so as to adjust the ratio of the organic component and the inorganic component. Therefore it is possible to change strength of pore forming material.  
      Now, a specific embodiment of the manufacturing method of a porous body will be explained using the flow in  FIG. 5 .  
      First, in step G, skeleton particles and a pore forming material (including organic polymer particles and inorganic particles) are mixed. Then, in step H, a molded form is produced. In step I, the molded form is heated so that the organic particles are thermally decomposed and gasified to be removed. In step J, heating is continued to promote the sintering reaction among the skeleton particles.  
      In this specific embodiment, it is possible to lower the heating temperature while maintaining the breaking strength of the porous body. It is inferred that the organic polymer particles  2  constituting the pore forming material  1  are thermally decomposed and gasified and the gas is released to create a void space, while the sintering reaction among the skeleton particles are promoted by the inorganic component having a function as a sintering additive to promote calcination at a lower temperature. These are considered to be the reasons why the resultant porous body has a mechanical strength even with a high porosity and why calcination at a lower temperature is made possible.  
      In the manufacturing method according to the embodiments of the present invention, it is desirable to use, for example, silicon carbide particles as the skeleton particles and to use a pore forming material for a porous body containing alumina as the pore forming material for a porous body. In this case, in the subsequent calcination step, alumina contained in the pore forming material for a porous body functions as a sintering additive to lower the calcination temperature, or to promote calcination to produce a porous body excellent in mechanical strength.  
      Specifically, as shown in  FIG. 6A , the above-mentioned pore forming material for a porous body  1  (including, comprehensively,  1 A,  1 B,  2 A,  2 B, and  2 C) and skeleton material  6  are mixed.  
      Examples of the skeleton particles  6  include, nitride ceramics such as aluminum nitride, silicon nitride, boron nitride, and titanium nitride; carbide ceramics such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, tungsten carbide; and oxide ceramics such as alumina, zirconia, cordierite, mullite, and silica.  
      Although the particle size of the skeleton particles is not particularly limited, those resulting in less contraction in the subsequent heating step is preferable, and, for example, a combination of 100 parts by weight of a powder having an average particle size of about 5.0 to 50 μm and 5 to 65 parts by weight of a powder having about 0.1 to 3.0 μm is preferable.  
      Specifically, to the above-mentioned pore forming material for a porous body  1  is added, if necessary, a binder, a dispersion medium, etc., and the resultant mixture is mixed with an attritor, etc., and then is kneaded well with a kneader, etc. The above-mentioned binder is not particularly limited, and examples include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, polyethylene glycol, a phenol resin, and an epoxy resin. The above-mentioned dispersion medium is not particularly limited, and examples include an organic solvent such as benzene, alcohol such as methanol, and water.  
      Here, the content of the above pore forming material for a porous body  1  is not particularly limited, and it may be suitably selected according to the porosity, etc. of the porous body  10  to be produced. Usually, about 100 to 200 parts by weight based on 100 parts by weight of the skeleton particles is preferable. Particularly, it is desirable to blend the material so as to have about 5 to 50 parts by weight of the organic polymer component based on 100 parts by weight of the skeleton particles  100 . Further, the content of the binder is desirably about 1 to 10 parts by weight based on 100 parts by weight of ceramic powder. The dispersion medium may be suitably blended so that the viscosity of the mixture becomes within a given range.  
      When mixing the pore forming material for a porous body  1  and the skeleton material  6 , a formation auxiliary may be added, if necessary.  
      As a formation auxiliary, there is no particular limitation, and for example, ethylene glycol, dextrin, fatty acid soap, and polyalcohol may be used.  
      Further, in the step above, a pore forming material consisting only of the above-mentioned inorganic balloons, or organic polymers may be added, if necessary.  
      Next, as shown in  FIG. 6B , a molded body  7  including the pore forming material for a porous body  1  and the skeleton particles  6  is produced by molding the mixture. Molding of the mixture can be performed by extrusion molding, etc. Further, as shown in  FIG. 6C , by subjecting the molded mixture to heating, the organic polymer particles are thermally decomposed and gasified as gas  8 . Void space  11  can be formed by eliminating the gas.  
      Heating treatment may be performed by dividing, for example, into the drying step, the degreasing step, and the calcining step. The drying step of the molded body can be performed by using, for example, a microwave dryer, a hot air dryer, a conductive dryer, a decompression dryer, a vacuum dryer, and a freeze dryer. As for the conditions for the degreasing step and the calcining step, the conditions used for producing conventional porous bodies can be applied.  
      Specifically, the degreasing step may be performed with the condition of heating at about 300 to 650° C. under the atmosphere containing oxygen. The calcining step may be performed with the condition of, for example, heating at about 1000 to 2300° C. under the inert gas atmosphere such as nitrogen and argon. In the case where those functioning as a sintering additive are contained in the inorganic particles contained in the pore forming material for a porous body, as shown in  FIG. 6D , an inorganic particle or a product  9  (inorganic compound) which is produced as a result of the reaction between the inorganic particles and the skeleton particles is present on the inner wall surface of the void space  11 .  
      Since the sintering proceeds by the reaction between the inorganic particles and the skeleton particles, the sintering temperature can be lowered compared with the case where the skeleton particles alone are sintered. Further, it is possible to produce a porous body  10  with excellent mechanical properties by proceeding the sintering. For example, when a carbon nitride powder is used as the skeleton material and the inorganic particles are alumina, the sintering temperature (usually about 2200° C.) can be lowered by about 100 to 300° C. Further, the total amount of heat released from the molded body by combustion of the organic component can be reduced, and hence the generation of thermal stress due to the rapid heat generation of the molded body can be prevented.  
      Next, specific embodiments of the porous body will be sequentially explained.  
      Specific embodiments of the porous body are shown in  FIG. 7A . In  FIG. 7A , at least on the surface (inner wall “W”) of the void spaces  11  of the skeleton particles  6 , an inorganic compound  9  containing an element different from those of the skeleton particles  6  is present. Since the inorganic compound  9  containing an element different from those of the skeleton particles  6  firmly fixes the skeleton particles, even if the void spaces  11  grow larger (namely, the inner wall W between the void spaces  11  becomes thinner), the breaking strength of the porous body  107 A can be maintained to a high degree.  
      Another specific embodiment, or porous body  107 B is shown in  FIG. 7B . The porous body  107 B includes the skeleton particles  6  and a void space  11  formed by being encircled by the skeleton particles  6 , wherein the major axis L of the void space is greater than the major axis of the skeleton particles  6 , in which an inorganic compound  9  having an element different from those of the skeleton particles  6  is present at least on the surface of the skeleton particles exposing to the void spaces  11 .  
      That is, in this porous body  107 B, void spaces  11  (pores) larger than the void spaces which are supposed to be formed by the skeleton particles  6  are formed. Thus, when the porous body is used for filtering purpose, the deposit capacity of the filtered substance can be made greater. As a result, it is possible to extend the filter life. Further, the pressure loss can be reduced when fluid such as gas or liquid flows through the filter.  
      Even if such large void spaces  11  are present (namely, the inner walls W of the void spaces  11  are thinner), since the inorganic compound  9  containing an element different from those of the skeleton particles firmly bind the skeleton particles, the breaking strength of the porous body  107 B can be maintained to a high degree.  
      Further, still another specific embodiment, the porous body  108 A is shown in  FIG. 8A . In  FIG. 8A , a plurality of elements  90  which are the same as one of the elements constituting the skeleton particles  6  are locally present at least on the surface (the inner wall W) of the skeleton particles  6  exposed to the void space  11 . Elements  90  which are the same as one of the elements constituting the skeleton particles  6  are locally present includes a state, for example, where Si is locally present when silicon carbide, or the combination of silicon carbide and silicon are employed as the skeleton particles  6 .  
      Since the locally present elements  90  which are the same as one of the elements constituting the skeleton particles  6  firmly bind the skeleton particles  6 , even if the void spaces  11  become larger (namely, the inner wall W of the void spaces  11  becomes thinner), the breaking strength of the porous body  108 A can be maintained to a high degree.  
      Still another specific embodiment, the porous body  108 B is shown in  FIG. 8B . The porous body  108 B includes skeleton particles  6  and a void space  11  formed by being encircled by the skeleton particles  6 , wherein the major axis L of the void space is greater than the major axis of the skeleton particles  6 , in which elements  90  which are the same as one of the elements of the skeleton particles  6  are locally present at least on the surface of the skeleton particles exposed to the void spaces  11 .  
      In other words, void spaces  11  (pores) larger than those supposed to be formed by the skeleton particles  6  are formed in this porous body  108 B. Thus, when this porous body is used for filtering purpose, the deposit capacity of the filtered substance can be made greater. As a result, it is possible to extend the filter life. Further, the pressure loss can be reduced when fluid such as gas or liquid flows through the filter.  
      Still another specific example, the porous body  109 A is shown in  FIG. 9A . At least on the surface (the inner wall W) of the skeleton particles exposed to the void spaces  11 , an inorganic compound  9  containing an element different from those of the skeleton particles  6  are present. The inorganic particles are further bound by another binding substance  60 , and further, since the inorganic compound  9  containing an element different from those of the skeleton particles  6  firmly bind the skeleton particles, even if the void spaces  11  become larger (namely, the inner wall W between the void spaces  11  becomes thinner), the breaking strength and the tenacity value of the porous body  109 A can be maintained to a high degree, and thus its thermal shock resistance can be improved.  
      As for the binding substance  60 , a metal or a semi-conductor is desirable. As a metal, aluminum and iron are preferable, and as a semiconductor, silicone is preferable.  
      Another specific embodiment, the porous body  109 B is shown in  FIG. 9B . At least on the surface (inner wall W) of the void spaces  11  of the skeleton particles  6 , an inorganic compound  9  containing an element different from those of the skeleton particles  6  is present. A catalyst  12  is coated on the inner wall of the void spaces  11 .  
      Since the catalyst  12  is chemically bound to the inorganic compound  9 , neither releasing nor falling of the catalyst will take place. Further, since the catalyst can be coated on the inner wall of the void spaces  11 , the amount of the supported catalyst can be increased.  
      Still another specific example, the porous body  109 C is shown in  FIG. 9C . At least on the surface (the inner wall W) of the skeleton particles exposed to the void spaces  11 , an inorganic compound  9  containing an element different from those of the skeleton particles  6  are present. The skeleton particles  6  are further bound by another binding substance  60  and inorganic compound  9 , even if the void spaces  11  become larger (namely, the inner wall W between the void spaces  11  becomes thinner), the breaking strength and the tenacity value of the porous body  109 C can be maintained to a high degree, and thus its thermal shock resistance can be improved. In this embodiment, binding material  60  is containing inorganic compound  9 .  
      As for the binding substance  60 , a metal or a semi-conductor is desirable. As a metal, aluminum and iron are preferable, and as a semiconductor, silicone is preferable.  
      The porosities of these specific embodiments of the porous body  107 A,  107 B,  108 A,  108 B,  109 A, and  109 B are about 45 to 85%, preferably about 50% to 85%.  
      In the case where the porosities are about 45 to 85%, performance of maintaining its shape is insured, no decline in the strength may occur and when the porous body is used as a filter, the increase in the pressure loss caused by the deposit of the filtered substance can be suppressed, and the life as a filter can be extended.  
      The porosities can be measured by, for example, a conventionally known method such as the mercury press-in method, the Archimedes method, and scanning electron microscope (SEM).  
      Additionally, in the specific embodiments of the porous bodies  107 B and  108 B, the major axes L of the void spaces are desirably 100 to 150% of the major axes I of the skeleton particles. The void spaces  11  constituting the porous body do not necessarily consist only of large ones whose major axes L are same as or larger than the major axis  1  of the skeleton particles, but may include those whose major axes L are smaller than the major axis  1  or the skeleton particles. Even if these small void spaces are mixed, the object of increasing the deposit capacity of the filtered substance can be achieved. Further, the pressure loss can be reduced when fluid such as gas or liquid flows through the filter.  
      The major axes I of the skeleton particles and the major axes L of the void spaces can be obtained, for example, by observing any  10  points of the porous body using a SEM of about 350 magnification and calculating the average.  
      The average pore size of the porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, and  109 B is desirably about 5 to 100 μm. When the average pore size is about 5 to 100 μm, when the porous body of the present invention is used as a filter, it is possible to increase the deposit capacity of the substance to be filtered without lowering the filtering function.  
      In these specific embodiments of the porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, and  109 B, examples of the skeleton particles  6  include ceramics such as nitride ceramics including aluminum nitride, silicon nitride, boron nitride, and titanium nitride; carbide ceramics such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, boron carbide and tungsten carbide; and oxide ceramics such as alumina, zirconia, cordierite, mullite, and silica.  
      Examples of the inorganic particles  6  include ceramics such as nitride ceramics including aluminum nitride, silicon nitride, boron nitride, and titanium nitride; carbide ceramics such as silicon carbide, zirconium carbide, titanium carbide, tantalum carbide, and tungsten carbide; and oxide ceramics such as alumina, zirconia, cordierite, mullite, and silica, and a metal such as Si, Fe, and Al and a metal compound such as oxide of iron.  
      When the skeleton particles  6  comprises carbon nitride particles, or the combination of carbon nitride and a binding substance, it is preferable that the inorganic compound  9  is oxide ceramics, for example, at least one selected from alumina, mullite, silica, titania, and silica-alumina, since catalyst  12  is considered to be easily supported as shown in  FIG. 9B . It is inferred that when a catalyst is added to a porous body, a functional group having a polar OH group, etc is added on the surface layer of the oxide ceramics, an oxide catalyst (perovskite type), a catalyst support (coating layer with a high specific surface area such as alumina, titania, and zirconia), or a catalyst can be easily supported, and that the catalyst once supported is unlikely to be detached.  
      When the inorganic compound  9  is an oxide ceramics, it is considered that corrosion of the porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, and  109 B can be prevented. It is also inferred that when the inorganic compound  9  is a metal compound, for example, when the porous body is used as a filter for cleaning the exhaust gas, the oxide ceramics preferentially react with sulfur, alkali metal, alkaline earth metal, etc. contained in the exhaust gas, competitively preventing the reaction between sulfur, etc. in the exhaust gas and the skeleton particles constituting the porous body, and thus the corrosion of the porous body can be prevented.  
      In the porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, and  109 B, it is desirable that the inorganic compound  9  is present at the neck portion  600  of the skeleton particles  6  as shown in  FIG. 10 . When the inorganic compound  9  is present at the neck portion  600 , the joining force among the skeleton particles  6  are increased by thickening the binding portion (neck portion), improving the strength of the porous bodies.  
      It is desirable that the inorganic compound  9  is a crystalline compound crystallized from a liquid phase. By going through the liquid phase, a melt is once gathered on the neck portion due to the surface tension, and re-crystallization proceeds thereafter, and thus the inorganic compound  9  can be easily formed on the neck portion.  
      The neck portion  600  of the skeleton particles  6  indicates a part of the skeleton particle surface exposed to the void spaces, and an area in the vicinity of the outer circumference of the interface where the skeleton particles contact with each other.  
      When silicon carbide or a combination of silicon carbide and metal silicon is used as a skeleton material  6 , and alumina or aluminum is used as the inorganic particles  4  of the pore forming material, alumina crystallizes from the liquid phase on the neck portions of the silicon carbide, or the neck portions of a complex of silicon carbide and a metal silicon (Examples 1 to 5).  
      Next, the honeycomb structure according to an embodiment of the present invention will be explained. The honeycomb structure includes the porous body according to the embodiment of the present invention. The honeycomb structure will be explained by referring to the drawings.  FIG. 11  is a perspective view schematically showing a honeycomb structure according to an embodiment of the present invention,  
       FIG. 12A  is a perspective view schematically showing an example of a porous ceramic material (porous body) constituting the honeycomb structure according to the embodiment of the present invention, and  FIG. 12B  is a cross sectional view of  FIG. 12A  taken along a the line XII B-XII B.  
      As shown in  FIG. 11 , the honeycomb structure  10  includes a ceramic block  15  constituted by a plurality of porous ceramic members  20  bound with each other via a plurality of sealant layers  14 , and a circumferential sealing material layer  13  formed on the circumference of the ceramic block  15 . In this porous ceramic member  20 , as shown in  FIG. 12B , a plurality of through holes are disposed in parallel in the longitudinal direction, and partitions  23  separating the through holes  21  from each other function as a filter. Here, the porous ceramic member  20  includes the above-mentioned porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, or  109 B.  
      That is, the through holes  21  formed in the porous ceramic member  20  are sealed by a sealing medium  22  either on the inlet side or the outlet side of the exhaust gas, so that exhaust gas flowed into one through hole  21  passes, without fail, through the partitions  23  separating the through holes  21  to be flowed out from another through hole  21 . Additionally, a sealing material layer  13  is provided with the aim of preventing the exhaust gas from leaking from the circumferential part of the ceramic block  15  when the honeycomb structure  10  is disposed in the exhaustion passage of an internal combustion engine.  
      An enlarged cross-sectional view of the partition  23  of the porous ceramic member  20  is shown in  FIG. 13 . For the partition, the previously explained porous bodies  107 A,  107 B,  108 A,  108 B,  109 A, or  109 B can be used. In  FIG. 13 , the partition is comprised of skeleton particles  6 , void spaces  11  formed by the particles  6 , and an inorganic compound  9  present in the inner wall of the void space. Soot  113  is to be trapped in the inside of the void spaces  11 . In this specific embodiment, the void space  11  can be enlarged to increase the amount of soot to be trapped without causing lowering of the strength. Therefore, the life of the filter can be extended when the honeycomb structure is used as an exhaust gas filter.  
      When a honeycomb structure  10  having such a configuration is disposed in an exhaust passage of an internal combustion engine, particulates in the exhaust gas emitted from the internal combustion engine are trapped by the partition  23  when passing through the honeycomb structure  10 . Since such a honeycomb structure  10  is extremely excellent in heat resistance and easily recycled, it is used in various heavy vehicles, diesel-powered vehicles, etc.  
      The materials constituting the sealant layer  14  are not particularly limited. For example, those comprising an inorganic binder, an organic binder, an inorganic fiber, and inorganic particles can be mentioned.  
      Examples of the above-mentioned binder include silica sol, titania sol and alumina sol. These may be used alone or in combination of two or more. Out of the inorganic binders, silica sol is desirable.  
      Examples of the above-mentioned binder include polyvinyl alcohol, methyl cellulose, ethyl cellulose, and carboxymethyl cellulose. These may be used alone or in combination of two or more. Out of the organic binders, carboxymethyl cellulose is desirable.  
      Examples of the above-mentioned inorganic fiber include ceramic fibers such as silica-alumina, mullite, alumina, and silica. These may be used alone or in combination of two or more. Out of the inorganic fibers, silica-alumina fiber is desirable.  
      Examples of the above-mentioned inorganic particles include carbide, and nitride. Specifically, inorganic a powder or a whisker comprising carbon nitride, silicon carbide, and boron nitride, can be mentioned. These may be used alone or in combination of two or more. Out of the inorganic particles, carbon nitride which is excellent in thermal conductivity is desirable.  
      Furthermore, the sealant layer  14  may contain a forming material. The reason is that the thermal expansion coefficient of the sealant layer  14  can be adjusted since the porosity of the sealant layer  14  (containing a foaming material) can be changed.  
      The foaming material is not particularly limited as long as the material is decomposed when heated in use, and, for example, a publicly known foaming material, such as ammonium hydrogencarbonate, ammonium carbonate, amyl acetate, butyl acetate, and diazoaminobenzene.  
      Additionally, the sealant layer  14  may contain a resin such as a thermoplastic resin and a thermosetting resin, balloons of an inorganic substance or an organic substance, etc. This is because the porosity of the sealant layer  14  can be controlled and the thermal expansion coefficient of the sealant layer  14  can be adjusted.  
      The above-mentioned thermoplastic resin is not particularly limited. For example, an acryl resin, a phenoxy resin, polyether sulfone, and polysulfone may be mentioned. The thermosetting resin is not particularly limited. For example, epoxy resin, phenol resin, polyimide resin, polyester resin, bismaleimide resin, polyolefin-based resin, and polyphenylene ether resin may be used.  
      The shapes of these resins are not particularly limited. For example, any shape such as a spherical shape, an elliptic spherical shape, a cubical shape, an indeterminate lump form, a cylindrical shape, and a plate-like shape can be used.  
      Further, when the resin is a spherical shape, the average particle size is desirably about 30 to 300 μm.  
      The above-mentioned “balloon” refers to a concept including bubbles or hollow spheres. The balloon of an organic substance is not particularly limited, and, for example, acryl balloon and polyester balloon may be mentioned. The balloon of an inorganic substance is not particularly limited, and, for example, alumina balloon, glass microballoon, silas balloon, fly ash balloon (FA balloon), and mullite balloon may be mentioned.  
      The shape, the average particle size, etc. of these balloons are preferably the same as the above-mentioned resins.  
      Although, the shape of the ceramic block  15  in the honeycomb structure  10  in  FIG. 11  is cylindrical, the shape of the ceramic block in the honeycomb structure of the present invention is not limited to a cylindrical shape. Any shape such as elliptic cylindrical shape, and prismatic shape can be mentioned.  
      The circumferential sealing material layer  13  formed on the circumference of the ceramic block  15  is not particularly limited. For example, the same material as the sealant layer  14  can be used. Further, in the pores of the honeycomb structure according to the embodiments of the present invention, a catalyst may be supported which can purify CO, HC, NOx, etc. in exhaust gas.  
      An enlarged cross-sectional view of the partition to which the catalyst  12  is added is shown in  FIG. 14 . The partition is comprised of skeleton particles  6 , void spaces  11  formed by the particles  6 , and an inorganic compound  9  present in the inner wall of the void space. Soot  113  is to be trapped in the inside of the void spaces  11 . The inorganic compound  9  and the catalyst  12  are chemically bonded, and hence can prevent detaching and falling of the catalyst.  
      Due to the fact that such a catalyst  12  is provided, the honeycomb structure  10  according to the embodiment of the present invention functions as a filter to collect particulates in exhaust gas, and also as a catalyst support for purifying the previously mentioned CO, HC, NOx, etc. in exhaust gas.  
      As for the catalyst  12 , for example, a precious metal such as platinum, palladium, and rhodium can be mentioned. The honeycomb structure according to the embodiment of the present invention in which a catalyst including one out of the above-mentioned metals is supported functions as a gas purification device in the same manner as a conventionally known DPF (diesel particulate filter) with a catalyst. Therefore, a detailed explanation of the case where the honeycomb structure according to the embodiment of the present invention functions as a catalyst supporter will be omitted here.  
      The catalysts which the honeycomb structure according to the embodiment of the present invention can support are not limited to the above-mentioned metals, and any catalyst as long as it can purify CO, HC, and NOx in exhaust gas can be supported.  
      Although the open area ratio at the end face of the inlet side and that at the end face of the outlet side are identical in the above-explained honeycomb structure  10  by referring to the drawings, in the honeycomb structure according to the embodiment of the present invention, the open area ratios at the end faces of the two sides are not necessarily identical. For example, the open area ratio at the end face of the inlet side may be greater than that at the end face of the outlet side.  
      The open area ratio at the end face here means a ratio of the total area occupied by the group of the through holes which are open at one end face.  
      Next, an example of the manufacturing method of the honeycomb structure according to an embodiment of the present invention will be explained by referring to  FIGS. 15A  to  15 D.  
      The porous ceramic member  20  is produced according to the above-mentioned manufacturing method of the porous body ( FIG. 15A ). In producing the porous ceramic member  20 , after drying a produced body using a microwave dryer or the like, a sealing treatment is conducted to fill a sealing material into predetermined through holes. The drying step using, for example, a microwave dryer or the like is performed again, followed by a degreasing step and a calcining step to produce a porous ceramic member  20 .  
      The sealing material is not particularly limited. For example, the same as the above-mentioned mixture composition may be mentioned.  
      Next, a ceramic laminated body will be produced. In this case, an adhesive paste which will become a sealant layer  14  is coated on the porous ceramic member  20  so as to have a uniform thickness, and a step of sequentially laminating other porous ceramic members  20  on which an adhesive paste is coated will be repeated to prepare a prismatic laminated body of a predetermined size.  
      Then, this ceramic laminated body is heated under the condition at about 50 to 100° C. for about one hour to dry and solidify the above-mentioned adhesive paste layer to make it a sealant layer  14 , and subsequently by cutting the circumferential portion into a shape as shown in  FIG. 1  using, for example, a diamond cutter, to produce a ceramic block  15 .  
      The materials constituting the sealant layer  14  are not particularly limited. For example, an adhesive paste containing an inorganic binder, an organic binder, inorganic fiber and inorganic particles may be used.  
      The above-mentioned adhesive paste may contain a little amount of water, solvent, etc. Usually, such water, solvent, etc. are almost dispersed by heating performed after the adhesive paste is coated.  
      Furthermore, the lower limit of the content of the above-mentioned inorganic binder as a solid is desirably about 1% by weight, and more desirably about 5% by weight. Meanwhile, the upper limit of the content of the inorganic binder as a solid is desirably about 30% by weight, more desirably about 15% by weight, and further desirably about 9% by weight. When the content of the inorganic binder is less than about 1% by weight, lowering of the adhesive strength may be caused, and meanwhile, when it exceeds 30% by weight, lowering of heat conductivity may be caused.  
      The lower limit of the content of the above-mentioned organic binder as a solid is desirably about 0.1% by weight, more desirably about 0.2% by weight, and further desirably about 0.4% by weight. Meanwhile, the upper limit of the content of the organic binder as a solid is desirably about 5.0% by weight, more desirably about 1.0% by weight, and further desirably about 0.6% by weight. When the content of the organic binder is less than about 0.1% by weight, control of the migration of the sealant layer  14  may become difficult. Meanwhile, when it exceeds about 5.0% by weight, when the sealant layer  14  is exposed to a high temperature, the organic binder will be burned down and the adhesive strength may be reduced.  
      The lower limit of the content of the inorganic fiber as a solid is desirably about 10% by weight, more desirably 20% by weight. Meanwhile, the upper limit of the content of the inorganic fiber as a solid is desirably about 70% by weight, more desirably about 40% by weight, further desirably about 30% by weight. When the content of the inorganic fiber is less than about 10% by weight, elasticity and strength may be deteriorated. Meanwhile, when it exceeds about 70% by weight, lowering of heat conductivity may be caused and the effect as an elastic body may be reduced.  
      The lower limit of the content of the inorganic particles as a solid is desirably about 3% by weight, more desirably about 10% by weight, further desirably about 20%. Meanwhile, the upper limit of the content of the inorganic particles as a solid is desirably about 80% by weight, more desirably about 60% by weight, further desirably about 40% by weight. When the content of the inorganic fiber is less than about 3% by weight, lowering of heat conductivity may be caused. Meanwhile, when it exceeds about 80% by weight, the adhesive strength may be reduced when the sealant layer  14  is exposed to a high temperature.  
      Further, the lower limit of the shot content of the inorganic fiber is desirably about 1% by weight, and the upper limit is desirably about 10% by weight, more desirably about 5% by weight, further desirably about 3% by weight. The lower limit of the fiber length is desirably about 1 μm, the upper limit is desirably about 100 mm, more desirably about 1000 μm, further desirably about 500 μm.  
      It is difficult from the manufacturing point of view to set the shot content at less than about 1% by weight, and when the shot content exceeds about 10% by weight, the wall surface of the porous ceramic member  20  may be damaged. When the fiber length is less than about 1 μm, it is difficult to form a honeycomb structure  10  having elasticity, and when it exceeds 100 mm, the fiber is likely to take a pill-like form, causing poor dispersion of the inorganic particles, and it is difficult to reduce the thickness of the sealant layer  14 .  
      The lower limit of the particle size of the inorganic powder is desirably about 0.01 μm, more desirably about 0.1 μm. Meanwhile, the upper limit of the particle size of the inorganic particles is desirably about 100 μm, more desirably 15 μm, further desirably about 10 μm. When the particle size of the inorganic particles is less than about 0.01 μm, the cost may be increased. Meanwhile, when the particle size of the inorganic particles exceeds about 100 μm, the filling rate may be deteriorated, and hence adhesive force and heat conductivity may be lowered.  
      This adhesive paste may contain, in addition to the above-mentioned inorganic fiber, inorganic binder, organic binder and inorganic particles, about 35 to 65% by weight of water, other solvents such as acetone, and alcohol base on the total weight, and the viscosity of the adhesive paste is desirably about 15 to 25 Pa·s (10,000 to 20,000 cps (cP)).  
      Next, the circumference of the thus obtained ceramic block  15  is cut so that the block has a cylindrical form. ( FIG. 15C )  
      Then, the sealing material forming step to form a layer of the sealing material layer is performed ( FIG. 15D ). In the sealing material forming step, first, the ceramic block  15  whose circumference is cut so that the block has a cylindrical form is rotated around the longitudinal axis. Although the rotating speed of the ceramic block  15  is not particularly limited, it is desirably about 2 to 10 revolutions per minute.  
      A sealing paste is subsequently adhered to the circumferential portion of the rotating ceramic block  15 . The sealing paste is not particularly limited, and the same one as the above-mentioned adhesive paste may be mentioned.  
      Then, the thus formed sealing paste layer is dried at a temperature of about 120° C. to cause water to evaporate so as to make it a sealing material layer  13 , which finalizes the production of the honeycomb structure  10  according to the embodiment of the present invention in which the sealing material layer is formed on the circumferential portion of the ceramic block  15 .  
      While the above-described honeycomb structure  10  is one in which a plurality of porous ceramic members  20  are bound together via the sealant layers to constitute a ceramic block  15 , and a sealing material layer  13  is formed on the circumference of the ceramic block, the honeycomb structure according to the embodiment of the present invention may include a single porous ceramic member. That is, there is no sealant layer in this case, and, for example, a single porous ceramic member itself is cylindrical. Hereinafter, the honeycomb structure shown in  FIG. 11  will be referred to as a group-type honeycomb structure, and the honeycomb structure to be described below will be referred to as a single unit-type honeycomb structure.  
       FIG. 16A  is a perspective view diagrammatically showing an example of a single unit-type honeycomb structure, and  FIG. 16B  is a cross sectional view cut along the line XVI B-XVI B.  
      As shown in  FIG. 16A , the honeycomb structure  30  is a cylindrical body including a single porous ceramic member in which a plurality of through holes  31  are arranged in parallel in the longitudinal direction, each of them are separated by a wall portion  33 , wherein the end portion of one side or the other side of each through hole is filled with a sealing medium  32  so that the entire wall portion  33  function as a particle collection filter. That is, the through holes  31  formed in the honeycomb structure  30  are sealed on either the inlet side or the outlet side for the exhaust gas as shown in  FIG. 16B  so that the exhaust flowed into one through hole  31  never fails to pass the wall portion  33  separating the through holes  31  so as to be flowed out from another through hole  31 . The porous body constituting the honeycomb structure  30  includes (one of) the above-described specific examples of the porous body.  
      When a honeycomb structure  30  having such a configuration is disposed in an exhaust passage of an internal combustion engine, particulates in exhaust gas emitted from the internal combustion engine are trapped by the wall portion  33  when passing through the honeycomb structure, and the exhaust gas is purified. Since such a honeycomb structure  10  is extremely excellent in heat resistance and is easily recycled, it is used in various heavy vehicles, diesel-powered vehicles and the like. Although the honeycomb structure  30  shown in  FIG. 16A  has a cylindrical shape, any shape, for example, elliptic cylindrical shape and prismatic shape can be used for the honeycomb structure  30 .  
      Further, a sealing material layer may be formed on the circumference of the honeycomb structure  30  as in the honeycomb structure  10  in  FIGS. 11A and 11B . The sealing material layer is not particularly limited. For example, the same material as the sealing material layer  13  of the honeycomb structure  10  may be mentioned.  
      In the pores of the honeycomb structure  30 , a catalyst may be supported which can purify CO, HC, NOx, etc. in the exhaust gas. As examples of such a catalyst, the same catalysts as those used in the honeycomb structure  10  of  FIGS. 11A and 11B  may be mentioned.  
      Although in the honeycomb structure  30 , the open area ratio at the end face of the inlet side and that at the end face of the outlet side are identical, the open area ratios at the end faces of two sides are not necessarily identical in the honeycomb structure of the present invention. For example, the open area ratio at the end face of the inlet side may be greater than that at the end face of the outlet side.  
      The open area ratio at the end face here means a ratio of the total area occupied by the group of the through holes which are open at one end face.  
      The above-mentioned honeycomb structure can be produced by the manufacturing method of the previously mentioned porous bodies according to the embodiment of the present invention. With regard to this honeycomb structure, there is no step of binding produced porous bodies, and the honeycomb structure can be produced at one time.  
      That is, after drying the produced form using a microwave dryer or the like, a sealing treatment is conducted to fill a sealing material into predetermined through holes, and the drying step using a microwave dryer or the like is performed again, followed by a degreasing step and a calcining step to produce a porous ceramic member  30 . Thereafter, a sealing material layer may be formed on the circumference thereof, if necessary.  
     EXAMPLES  
      The examples are explained. The present invention is not limited to these examples.  
      Preparation of pore forming materials for a porous body A to H is explained.  
      (1) A pore forming material for a porous body F including acryl particles with 80% by volume of acryl and 20% by volume of porosity; a pore forming material for a porous body G including acryl particles with 40% by volume of acryl and 60% by volume of porosity; and a pore forming material for a porous body H including acryl particles with 30% by volume of acryl and 70% by volume of porosity were prepared.  
      (2) The pore forming material H for a porous body was provided in an alumina slurry having an average particle size of 0.1 μm and a concentration of 15% by weight to immerse alumina into the pore forming material under degassing. After dehydrating the slurry, the particles were dried for three hours at 80° C. to obtain a pore forming material for a porous body D.  
      (3) The pore forming material for a porous body D was provided in an alumina slurry having an average particle size of 0.1 μm and a concentration of 15% by weight to alumina in the pore forming material under degassing, and after dehydrating the slurry, the step of drying the particles for three hours at 80° C. was repeated twice (i.e., the step of the above (2) was repeated for three times) to obtain a pore forming material for a porous body A.  
      (4) By repeating the above step (2) for five times, a pore forming material for a porous body C was obtained.  
      (5) Alumina particles with a porous structure, a porosity of 30%, and an average particle size of 40 μm were provided in an acryl slurry having a concentration of 15% by weight to immerse acryl into the particles under gassing, and after dehydrating the slurry, the particles were dried for three hours at 80° C. to obtain a pore forming material for a porous body E.  
      (6) The pore forming material for a porous body E was provided in a slurry having an acryl concentration of 15% by weight to immerse acryl into the pore forming material under degassing, and after dehydrating the slurry, the step of drying the particles for three hours at 80° C. was repeated twice (i.e., the step of the above (5) was repeated for three times) to obtain a pore forming material for a porous body B.  
      By these steps (1) to (6), pore forming materials for a porous body A to H were prepared. The composition of the pore forming materials for a porous body A to H is as shown in Table 1 below. The average particle sizes of the pore forming materials for a porous body A to H are all 40 μm.  
                               TABLE 1                               Organic   Inorganic   Ratio of       Pore forming   Poros-   polymer   particle   volume (organic       material for a   ity (vol-   (Acryl)   (Alumina)   polymer/inorganic       porous body   ume %)   (volume %)   (volume %)   particle)                                                    A   20   30   50   0.6       B   20   10   70   0.14       C   40   30   30   1.0       D   60   30   10   3.0       E   60   10   30   0.33       F   20   80   —   —       G   60   40   —   —       H   70   30   —   —                  
 
     Example 1  
      (1) 5950 parts by weight of α-type silicon carbide powder having an average particle size of 40 μm and 2550 parts by weight of β-type silicon carbide powder having an average particle size of 0.5 μm were wet blended, and 600 parts by weight of an organic binder (methyl cellulose) and 1800 parts by weight of water were further added to the mixture and kneaded to obtain a kneaded substance. Then, to the above kneaded substance, 1450 parts by weight of the pore forming material for a porous body A, 150 parts by weight of a plasticizer (glycerin from NOF Corporation), and 330 parts by weight of a lubricant (Unilube from NOF Corporation) were further kneaded, and subsequently an extrusion was performed to obtain a molded product. Then, the molded product was dried using a microwave dryer, and after a paste having a composition same as the molded product was filled into predetermined through holes, the product was dried again with a dryer, degreased for three hours at 400° C. (programming rate: 5° C./min), and calcined for three hours at 2000° C. in argon atmosphere at a normal pressure to produce a porous ceramic member (porous body), as shown in  FIG. 2 , including a calcined body of silicon carbide (size: 34.3 mm×34.3 mm×150 mm; number of through holes: 31/cm 2 ; and partition thickness: 0.3 mm)  
      (2) Using a heat resistant adhesive paste including 100 parts by weight of a mixture containing 17.6% by weight of alumina fiber with a fiber length of 0.2 mm, 61.0% by weight of silicon carbide particles having a particle size of 0.6 μm, 9.1% by weight of silica sol, 2.3% by weight of calboxy methyl cellulose, and 10% by weight of ammonium hydrogencarbonate as a foamed material, to which 20 parts by weight of water is added, a plurality of the above-mentioned porous ceramic members were bound and then cut using a diamond cutter according to the method already explained in the description of the manufacturing method of the honeycomb structure to produce a cylindrical ceramic block with a diameter of 165 mm as shown in  FIG. 1 .  
      Then, 23.3% by weight of ceramic fiber including alumina silicate as an inorganic fiber (shot content ratio: 3%, fiber length: 0.1 to 100 mm); 30.2% by weight of silicon carbide powder with an average particle size of 0.3 μm as inorganic particles; 7% by weight of silica sol (content ratio of SiO 2  in sol: 30% by weight) as an inorganic binder; 0.5% by weight of carboxymethyl cellulose as an organic binder; and 39% by weight of water were mixed and kneaded to prepare a sealing paste.  
      Then, a sealing paste layer having a thickness of 1.0 mm was formed on the circumferential part of the above-mentioned ceramic block using the above-mentioned sealing paste. The sealing paste layer was dried at 120° C. to produce a cylindrical honeycomb structure as shown in  FIG. 1 .  
     Examples 2 to 5  
      A honeycomb structure was produced in the same manner as Example 1 except that the blended amount of an α-type silicon carbide powder with an average particle size of 40 μm (SiC coarse powder), that of an β-type silicon carbide with an average particle size of 0.5 μm (SiC fine powder), and that of the pore forming material for a porous body, and the kinds of the pore forming material for a porous body were changed as shown in Table 2.  
                                       TABLE 2                                      αtype SiC   βtype SiC       Pore forming material               (particle   (particle       for a porous body                                                     diameter   diameter   Methyl       Amount of   Amount                   40 μm)   0.5 μm)   Cellulose   Type   composition   of acryl   glycerin   water                                                             Example 1   5950   2550   600   A   1450   190   150   1800       Example 2   4550   1950   600   B   3940   135   150   1800       Example 3   4550   1950   600   C   2040   410   150   1800       Example 4   4550   1950   600   D   950   410   150   1800       Example 5   3220   1380   600   E   2600   200   150   1800       Comparative   5950   2550   600   F   500   500   150   1800       example 1       Comparative   4550   1950   600   G   550   550   150   1800       example 2       Comparative   5950   2550   600   H   190   190   150   1800       example 3                 Amounts are described in terms of parts by weight.            All Examples and all Comparative examples contain 330 parts by weight of lubricant whose trade name is Uniloop.             
 
     Example 6  
      Silica was used as inorganic particles in the same manufacturing method as the pore forming material for a porous body A. 6000 parts by weight of an 1-type silicon carbide powder with an average particle size of 40 μm (SiC coarse powder); 1000 parts by weight of Si with an average particle size of 0.5 μm; 1000 parts by weight of silica with an average particle size of 1 μm; and 1500 parts by weight of a pore forming material for a porous body A were mixed and calcined at 1800° C. It is inferred that SiC particles are bound together with Si or Si compound, and that SiO 2  are locally present on the neck portions of the structure, so that a porous structure in which SiO 2  being locally present in the neck portions are exposed to the void spaces can be obtained. Further, it is inferred that no cracking occurs in a honeycomb filter having this porous structure. The grounds for this inference are obtained from the experiment results of Examples 1 to 5.  
     Comparative Examples 1 to 3  
      A honeycomb structure was produced in the same manner as Example 1 except that the blended amount of an α-type silicon carbide powder with an average particle size of 40 μm (SiC coarse powder), that of an β-type silicon carbide with an average particle size of 0.5 μm (SiC fine powder), and that of the pore forming material for a porous body, and the kinds of the pore forming material for a porous body were changed as shown in Table 2, and further that the temperature for calcination was changed from 2000° C. to 2200° C.  
      Regarding the honeycomb structures of Examples 1 to 5 and Comparative Examples 1 to 3, the shapes of the pore forming materials for a porous body after extrusion were observed using an LV-SEM. As a result, the molded bodies in Examples 1 to 5 and Comparative Examples 1 and 2, the pore forming materials for a porous body were not deformed. Meanwhile, the molded body in Comparative Example 3, the pore forming material for a porous body was crushed.  
      Further, the honeycomb structures in Examples 1 to 5 and Comparative Examples 1 to 3 were observed with eyes as to whether cracks were occurred or not when the degreasing step was completed. As a result, as shown in Table 3, no crack was observed in the molded bodies in Examples 1 to 5 and Comparative Example 3, while there were cracks in Comparative Examples 1 and 2. Further, samples of the molded bodies for honeycomb structures of Examples 1 to 5 and Comparative Examples 1 to 3 were each cut into a 1 cm cube, and after making each of them into a powder, they were put into a calorimeter and heated from to 20 to 400° C. at a programming rate of 5° C./min to calculate the total amount of heat generated at the time of degreasing. The results were shown in Table 3.  
                               TABLE 3                                           Total calorific           Occurrence   Porosity   value generated           of Cracking   after firing   while firing           while firing   (volume %)   (cal/cm 3 )                                                            Example 1   NON   50   58.7           Example 2   NON   59   53.8           Example 3   NON   60   81.7           Example 4   NON   61   91.9           Example 5   NON   70   79.0           Comparative   Occur   —   105           example 1           Comparative   Occur   —   114           example 2           Comparative   NON   44   66.1           example 3                      
 
      In addition, with regard to the honeycomb structures in Examples 1 to 5 and Comparative Example 3, the porosities were measured using a porosimeter. The results are shown in Table 3.  
      As is clear from these results, since the pore forming materials for a porous body according to the embodiment of the present invention were used in Examples 1 to 5, it was possible to produce honeycomb structures with a high porosity exceeding 50% by volume without causing cracks. Further, even if the calcination temperature was reduced from 2200° C. to 2000° C., they were sufficiently calcined. Meanwhile, in Comparative Examples 1 and 2, cracks were caused when the degreasing step was completed. The reason is considered that since the pore forming materials for a porous body used in Comparative Examples contain a large amount of organic polymers (the amount of acryl), the temperatures of the molded bodies were rapidly increased to a high temperature since the organic polymers were rapidly burnt at the time of degreasing to cause cracks. (The numerical values of Comparative Examples indicated by the calorimeter were higher than those of Examples.) Further, in Comparative Example 3, since a pore forming material for a porous body with a high porosity consisting only of an organic polymer (acryl resin) was used, the pore forming material for a porous body were crushed at the time of extrusion molding. While the target porosity was 50% or more, the actual porosity of the obtained honeycomb structure was 44%, failing to ensure a sufficient porosity.  
      Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.