Patent Publication Number: US-2009239740-A1

Title: Honeycomb structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to PCT Application No. PCT/JP2008/055457, filed Mar. 24, 2008, the contents of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a honeycomb structure. 
     2. Discussion of the Background 
     In recent years, particulate matter (hereinafter, also simply referred to as particulates or PM) contained in exhaust gases discharged from internal combustion engines of vehicles such as buses and trucks, construction machines and the like has raised serious problems to the environment and the human body. 
     For this reason, various porous ceramic honeycomb structures have been proposed as diesel particulate filters (hereinafter, also simply referred to as DPF) that capture particulates in exhaust gases and purify the exhaust gases. 
     As such a honeycomb structure, for example, there has been proposed a honeycomb structure manufactured as follows: combining a plurality of rectangular pillar-shaped honeycomb fired bodies with one another with an adhesive layer interposed therebetween: and cutting into a predetermined shape (for example, see WO01/23069 A1). 
     Moreover, there has been proposed another honeycomb structure manufactured by combining a plurality of honeycomb fired bodies, respectively manufactured through extrusion molding into predetermined shapes in advance, with one another with an adhesive layer interposed therebetween (for example, see JP-A 2004-154718). 
     The contents of WO01/23069 A1 and JP-A 2004-154718 are incorporated herein by reference in their entirety. 
     SUMMARY OF THE INVENTION 
     A honeycomb structure according to the present invention includes a plurality of honeycomb fired bodies. Each of the plurality of honeycomb fired bodies has a longitudinal direction and cell walls extending along the longitudinal direction to define cells. An adhesive layer is provided between the plurality of honeycomb fired bodies to connect the plurality of honeycomb fired bodies so that each longitudinal direction is substantially in parallel with each other. The plurality of honeycomb fired bodies include at least one center-portion honeycomb fired body located at a center portion of the honeycomb structure and at least one periphery honeycomb fired body surrounding the center-portion honeycomb fired body to form a peripheral face of the honeycomb structure. The periphery honeycomb fired body includes contact faces contacting the adhesive layer. At least one of the contact faces has irregularities. 
    
    
     
       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. 
         FIG. 1  is a perspective view schematically showing a honeycomb structure according to one embodiment of the present invention. 
         FIG. 2A  is a perspective view schematically showing a center-portion honeycomb fired body constituting the honeycomb structure according to one embodiment of the present invention, and  FIG. 2B  is an A-A line cross-sectional view of the center-portion honeycomb fired body shown in  FIG. 2A . 
         FIG. 3A  is a perspective view schematically showing a periphery honeycomb fired body constituting the honeycomb structure according to one embodiment of the present invention, and  FIG. 3B  is another perspective view of the periphery honeycomb fired body shown in  FIG. 3A  observed from a direction different from that in  FIG. 3A . 
         FIG. 4  is a plan view schematically showing a cross section of the honeycomb structure according to one embodiment of the present invention when cut perpendicularly to the longitudinal direction thereof. 
         FIG. 5A  is a perspective view schematically showing a periphery honeycomb fired body constituting the honeycomb structure of the first embodiment of the present invention and  FIG. 5B  is another perspective view of the periphery honeycomb fired body shown in  FIG. 5A  observed from a direction different from that in  FIG. 5A . 
         FIG. 6  is a cross-sectional view schematically showing a cross section of the honeycomb structure of the first embodiment of the present invention when cut perpendicularly to the longitudinal direction thereof. 
         FIG. 7A  is a perspective view schematically showing a periphery honeycomb fired body other than the periphery honeycomb fired body shown in  FIGS. 5A and 5B , and  FIG. 7B  is a perspective view of the periphery honeycomb fired body shown in  FIG. 7A  observed from a direction different from that in  FIG. 7A . 
         FIG. 8  is a cross-sectional view schematically showing a cross section of the honeycomb structure including the periphery honeycomb fired body shown in  FIGS. 7A and 7B . 
         FIGS. 9A and 9B  are cross-sectional views each illustrating another example of the method for manufacturing the honeycomb structure according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. 
     The inventors of the present invention made extensive research efforts so as to prevent damage to the honeycomb structure, displacement of the honeycomb fired body, and a coming off of the honeycomb fired bodies, and consequently, the inventors of the present invention have completed the present invention. 
     A honeycomb structure according to embodiments of the present invention includes: a plurality of honeycomb fired bodies combined with one another with an adhesive layer interposed therebetween, each of the honeycomb fired bodies having a large number of cells longitudinally placed substantially in parallel with one another with a cell wall interposed therebetween, wherein the plurality of the honeycomb fired bodies include a center-portion honeycomb fired body located in a center portion of the honeycomb structure, and a periphery honeycomb fired body that forms a part of a peripheral face of the honeycomb structure, and the periphery honeycomb fired body includes at least a single face with irregularities formed thereon among faces of the periphery honeycomb fired body in contact with the adhesive layer. 
     In the present specification, the center-portion honeycomb fired body refers to a honeycomb fired body that does not constitute the peripheral face of the honeycomb structure on a cross section perpendicular to the longitudinal direction of the honeycomb structure. 
     Here, in the honeycomb structure according to the embodiments of the present invention, the irregularities are formed on a predetermined face of the periphery honeycomb fired body, so that an adhesive strength between the periphery honeycomb fired body and the adhesive layer is made higher. For this reason, a crack oriented from the adhesive layer hardly occurs. Therefore, it may become easier to prevent damage to the honeycomb structure, displacement of the honeycomb fired body, and a coming off of the honeycomb fired body. 
     This structure will be specifically described with reference to drawings in the following. First, the entire configuration of the honeycomb structure according to the embodiments of the present invention will be described. 
       FIG. 1  is a perspective view schematically showing the honeycomb structure according to one embodiment of the present invention. 
     As shown in  FIG. 1 , a honeycomb structure  10  is constituted by four center-portion honeycomb fired bodies  20  positioned in the center portion of the honeycomb structure  10 , eight periphery honeycomb fired bodies  30  each constituting a part of a peripheral face  11  of the honeycomb structure  10  and an adhesive layer  40  existing between these honeycomb fired bodies (the center-portion honeycomb fired body  20  and the periphery honeycomb fired body  30 ) so as to bond the honeycomb fired bodies to each other. 
     A configuration of the center-portion honeycomb fired body  20  will be described with reference to the drawings in the following. 
       FIG. 2A  is a perspective view schematically showing the center-portion honeycomb fired body constituting the honeycomb structure according to one embodiment of the present invention, and  FIG. 2B  is an A-A line cross-sectional view of the center-portion honeycomb fired body shown in  FIG. 2A . 
     As shown in  FIG. 2A , the center-portion honeycomb fired body  20  is an almost rectangular pillar-shaped honeycomb fired body which includes four planes  20   a  having areas mutually equal to each other and has a cross section in an almost square shape. Further, as shown in  FIG. 2B , a large number of cells  21  are longitudinally (indicated by an arrow “B” with two heads in  FIG. 2A ) placed substantially in parallel with one another with a cell wall  22  interposed therebetween, and either one end of each cell  21  is sealed with a plug  23  in the center-portion honeycomb fired body  20 . Therefore, exhaust gases G (see an arrow in  FIG. 2B ) having flowed into the cell  21  with an opening on one of the end faces flow out from another cell  21  with an opening on the other end face, after surely passing through the cell wall  22  that separates the cells  21 . 
     Thus, the cell wall  22  functions as a filter for capturing PM and the like. 
     A configuration of the periphery honeycomb fired body will be described with reference to drawings in the following. 
       FIG. 3A  is a perspective view schematically showing a periphery honeycomb fired body constituting the honeycomb structure according to one embodiment of the present invention, and  FIG. 3B  is another perspective view of the periphery honeycomb fired body shown in  FIG. 3A  observed from a direction different from that in  FIG. 3A . 
     As shown in  FIGS. 3A and 3B , the periphery honeycomb fired body  30  includes a large number of cells  31  longitudinally placed substantially in parallel with one another with a cell wall  32  interposed therebetween and either one end of each cell  31  is sealed with a plug  33 , in the same manner as in the center-portion honeycomb fired body  20  shown in  FIGS. 2A and 2B . For this reason, the cell wall  32  is allowed to function as a filter for capturing PM and the like, in the same manner as in the center-portion honeycomb fired body  20 . 
     In addition, the shape thereof is a polygonal pillar shape including four faces, that is, a face  30   a , faces  30   b  and  30   c  having areas mutually equal to each other, and a face  30   d . More specifically, among the four faces  30   a ,  30   b ,  30   c  and  30   d , the face  30   a  is a curved face. 
     Moreover, the face  30   c  opposing to the curved face  30   a  is provided with the irregularities formed along the cross-sectional shape of the cells. 
     Here, the remaining two faces  30   b  and  30   d  are plane faces. In the following description, the face with the irregularities formed thereon is also referred to as an uneven face (with depressions and projections). 
     A specific configuration of the honeycomb structure according to one embodiment of the present invention will be described with reference to the drawing in the following. 
       FIG. 4  is a plan view schematically showing a cross section of the honeycomb structure according to one embodiment of the present invention when cut perpendicularly to the longitudinal direction thereof. 
     As shown in  FIG. 4 , in the honeycomb structure  10 , the curved face  30   a  of the periphery honeycomb fired body  30  forms a part of the peripheral face of the honeycomb structure  10 . 
     Moreover, the plane  30   b  is bonded to a plane  30   b  of another periphery honeycomb fired body  30  adjacent thereto, with the adhesive layer  40  interposed therebetween. 
     The uneven face  30   c  is bonded to the plane  20   a  of the center-portion honeycomb fired body  20 , with the adhesive layer  40  interposed therebetween. 
     The plane  30   d  is bonded to a plane  30   d  of still another periphery honeycomb fired body  30  adjacent thereto, with the adhesive layer  40  interposed therebetween. 
     Here, a contact area between the uneven face  30   c  of the periphery honeycomb fired body  30  and the adhesive layer  40  is larger than a contact area between the plane with no irregularities formed thereon of the honeycomb fired body and the adhesive layer (a contact area between the plane  20   a  of the center-portion honeycomb fired body  20  and the adhesive layer  40 , a contact area between the plane  30   b  of the periphery honeycomb fired body  30  and the adhesive layer  40 , or a contact area of the plane  30   d  of the periphery honeycomb fired body  30  and the adhesive layer  40 ). 
     For this reason, adhesive strength between the periphery honeycomb fired body  30  and the adhesive layer  40  becomes higher, so that a crack oriented from the adhesive layer hardly occurs. 
     Therefore, the honeycomb structure according to the embodiments of the present invention tends to prevent the damage to the honeycomb structure  10 , the displacement of the periphery honeycomb fired body  30 , and the coming off of the periphery honeycomb fired body  30 , which are caused by a crack occurring from the adhesive layer. 
     In the honeycomb structure according to the embodiments of the present invention, one of the faces with the irregularities formed thereon in the periphery honeycomb fired body is the face having the smallest area among the faces in contact with the adhesive layer. 
     In the honeycomb structure according to the embodiments of the present invention, the irregularities are formed on a face having the smallest area among the faces of the periphery honeycomb fired body. Therefore, compared to a case where no irregularities are formed on the face, a contact area between the face and the adhesive layer becomes larger, so that adhesive strength between the periphery honeycomb structure and the adhesive layer in contact with the face becomes higher. Accordingly, a crack hardly occurs from the adhesive layer that is in contact with the face having the smallest area among the faces of the periphery honeycomb structure. 
     Here, in the present specification, in the case where areas of all the faces of the periphery honeycomb fired body are equal to each other, each of the faces is referred to as a face having the smallest area among the faces that are in contact with the adhesive layer. 
     In the honeycomb structure according to the embodiments of the present invention, one of the faces with the irregularities formed thereon in the periphery honeycomb fired body is the face in contact with an other periphery honeycomb fired body with the adhesive layer interposed therebetween. 
     As described above, in the regenerating process, a stress is generated on the peripheral part of the honeycomb structure. Accordingly, a crack tends to occur particularly from the adhesive layer that bonds periphery honeycomb fired bodies to each other that constitute a part of the peripheral face of the honeycomb structure. 
     In the honeycomb structure according to the embodiments of the present invention, however, among the faces of the periphery honeycomb fired bodies, the irregularities are formed on the face which is in contact with another periphery honeycomb fired body with the adhesive layer interposed therebetween. Therefore, compared to a case where no irregularities are formed on the face, a contact area between the face and the adhesive layer becomes larger, so that adhesive strength between the periphery honeycomb fired body and the adhesive layer that bonds the periphery honeycomb fired bodies to each other becomes higher. For this reason, a crack hardly occurs from the adhesive layer that bonds the periphery honeycomb fired bodies to each other. 
     In the honeycomb structure according to the embodiments of the present invention, an area of a cross section perpendicular to the longitudinal direction of the periphery honeycomb fired body is larger than an area of a cross section perpendicular to the longitudinal direction of the center-portion honeycomb fired body. 
     In a case where a honeycomb structure is used as a DPF, after a predetermined amount of particulates have been captured, regenerating process for burning and removing these particulates is carried out. 
     In this regenerating process, high-temperature exhaust gases discharged from an internal combustion engine are allowed to flow into a cell near a center portion of the honeycomb structure that allows exhaust gases to flow comparatively easily. Moreover, it is considered that, since more particulates are captured in a vicinity of the center portion of the honeycomb structure, the burning of the particulates starts from the vicinity of the center portion of the honeycomb structure. 
     For this reason, the temperature of the vicinity of the center portion of the honeycomb structure tends to become higher than the temperature of a vicinity of the peripheral portion of the honeycomb structure and a temperature difference in the radial direction of the honeycomb structure tends to be occurred. 
     When such a temperature difference occurs, a difference in degrees of thermal expansion occurs between the vicinity of the center portion and the vicinity of the peripheral portion of the honeycomb structure and a stress tends to be generated on the peripheral face of the honeycomb structure. 
     Consequently, there may be a case where a crack occurs from the adhesive layer and the honeycomb structure is damaged. Moreover, there may be another case where the crack develops to rupture the adhesive layer, so that the honeycomb fired body is displaced or further comes off from the honeycomb structure because of application of a pressure by the exhaust gases. 
     The honeycomb structure according to the embodiments of the present invention tends to prevent damage to the honeycomb structure, displacement of the honeycomb fired body, and a coming off of the honeycomb fired body from the honeycomb structure. 
     First Embodiment 
     In a honeycomb structure of a first embodiment that is one embodiment of the present invention, one of the faces of the periphery honeycomb fired body, with irregularities formed thereon, is the face having the smallest area among the faces in contact with the adhesive layer. Further, one of the faces of the periphery honeycomb fired body, with the irregularities formed thereon, is a face in contact with another periphery honeycomb fired body with the adhesive layer interposed therebetween. 
     The honeycomb fired body constituting the honeycomb structure of the first embodiment of the present invention will be described with reference to the drawings in the following. 
     Here, the shape of the center-portion honeycomb fired body is the same as that of the center-portion honeycomb fired body constituting the honeycomb structure according to the embodiments of the present invention; therefore, the description thereof will be omitted. 
     The periphery honeycomb fired bodies will be described with reference to the drawings. 
       FIG. 5A  is a perspective view schematically showing a periphery honeycomb fired body constituting the honeycomb structure of the first embodiment of the present invention, and  FIG. 5B  is another perspective view of the periphery honeycomb fired body shown in  FIG. 5A  observed from a direction different from that in  FIG. 5A . 
     As shown in  FIGS. 5A and 5B , a periphery honeycomb fired body  130  includes a large number of cells  131  longitudinally placed substantially in parallel with one another with a cell wall  132  interposed therebetween, and either one end of each cell  131  is sealed with a plug  133 , in the same manner as in a center-portion honeycomb fired body  120 . For this reason, the cell wall  132  is allowed to function as a filter for capturing PM and the like. 
     In addition, the shape of the periphery honeycomb fired body  130  is a polygonal pillar shape including four faces, that is, a face  130   a , faces  130   b  and  130   c  having areas mutually equal to each other, and a face  130   d . Specifically, among the four faces  130   a ,  130   b ,  130   c  and  130   d , the face  130   a  and the face  130   d  having the smallest area thereamong are formed into uneven faces with the irregularities formed thereon along cross-sectional shapes of cells. The remaining two faces  130   b  and  130   c  are plane faces. 
     A detailed configuration of a honeycomb structure  100  of the present embodiment will be described with reference to  FIG. 6 . 
       FIG. 6  is a cross-sectional view schematically showing a cross section of the honeycomb structure of the first embodiment of the present invention when cut perpendicularly to the longitudinal direction thereof. In this case, the periphery honeycomb fired body  130  shown in  FIGS. 5A and 5B  is used as a periphery honeycomb fired body. 
     As shown in  FIG. 6 , the honeycomb structure  100  of the present embodiment is constituted by four center-portion honeycomb fired bodies  120  positioned in the center portion of the honeycomb structure  100 , eight periphery honeycomb fired bodies  130  each constituting a part of a peripheral face of the honeycomb structure  100  and an adhesive layer  140  existing between these honeycomb fired bodies (the center-portion honeycomb fired bodies  120  and the periphery honeycomb fired bodies  130 ) so as to bond the honeycomb fired bodies to each other, in the same manner as in the honeycomb structure according to claim  1 . 
     As shown in  FIG. 6 , on the uneven face  130   a  of the periphery honeycomb fired body  130 , no adhesive layer (coat layer)  140  is formed, and the uneven face  130   a  forms a part of the peripheral face of the honeycomb structure  100 . 
     The uneven face  130   d  is bonded to an uneven face  130   d  of another periphery honeycomb fired body  130  adjacent thereto, with the adhesive layer  140  interposed therebetween. 
     The plane  130   b  is bonded to a plane  130   b  of still another periphery honeycomb fired body  130  adjacent thereto, with the adhesive layer  140  interposed therebetween. 
     The plane  130   c  is bonded to the plane  120   a  of the center-portion honeycomb fired body  120 , with the adhesive layer  140  interposed therebetween. 
     That is, in the honeycomb structure  100  of the present embodiment, the irregularities are formed on the face  130   d  having the smallest area among the faces  130   b ,  130   c  and  130   d , which are in contact with the adhesive layer  140  of the periphery honeycomb fired body  130 . 
     The following description will discuss a method for manufacturing the honeycomb structure of the present embodiment. 
     (1) A molding process is carried out to manufacture a honeycomb molded body by extrusion-molding a wet mixture containing ceramic powders and a binder. 
     Specifically, first, silicon carbide powders having different average particle diameters as ceramic powders, an organic binder, a plasticizer in liquid form, a lubricant and water are mixed to prepare the wet mixture for manufacturing a honeycomb molded body. 
     Successively, this wet mixture is loaded into an extrusion molding machine. A honeycomb molded body having a predetermined shape is manufactured by loading the wet mixture into the extrusion molding machine and extrusion-molding the wet mixture. 
     At this time, a die for extrusion molding to be used may be selected in accordance with the cross-sectional shape of the honeycomb fired body, in order to manufacture the honeycomb fired bodies having the various shapes. 
     (2) Next, the honeycomb molded body is cut into a predetermined length, and dried by using a drying apparatus, such as a microwave drying apparatus, a hot-air drying apparatus, a dielectric drying apparatus, a reduced-pressure drying apparatus, a vacuum drying apparatus and a freeze drying apparatus, and a sealing process is carried out by filling predetermined cells with a plug material paste to be a plug for sealing the cells. 
     Here, conditions conventionally used upon manufacturing a honeycomb fired body are applicable for carrying out the cutting process, the drying process and the sealing process. 
     (3) Next, a degreasing process is carried out to remove the organic components in the honeycomb molded body by heating the honeycomb molded body in a degreasing furnace. Then, the degreased honeycomb molded body is transported to a firing furnace, and a firing process is carried out to manufacture a honeycomb fired body. 
     Here, conditions conventionally used upon manufacturing a honeycomb fired body are applicable for carrying out the degreasing process and the firing process. 
     By following the above-mentioned processes, the center-portion honeycomb fired body and the periphery honeycomb fired body are manufactured. 
     (4) Moreover, an adhesive paste is applied to a predetermined side face of each of the center-portion honeycomb fired body and the periphery honeycomb fired body, which have cells each sealed at a predetermined end portion, so that an adhesive paste layer is formed, and after the above process, another honeycomb fired body is successively laminated on this adhesive paste layer. By repeating above process, combining process is carried out to manufacture a honeycomb structure having a predetermined shape in which the honeycomb fired bodies are combined with one another. 
     Here, as the adhesive paste, for example, a substance containing an inorganic binder, an organic binder and inorganic particles is used. Moreover, the adhesive paste may further contain at least one of inorganic fibers and whiskers. 
     Moreover, if necessary, a coat layer forming process may be carried out, in which a sealing material paste is applied to the periphery of the honeycomb structure, and dried and solidified thereon to form a coat layer. 
     As the sealing material paste, the same paste as the above adhesive paste is used. Here, as a coat layer paste, a paste having a different composition may be used. 
     By following the above-mentioned process, the honeycomb structure of the present embodiment can be manufactured. 
     The effects of the honeycomb structure of the present embodiment will be listed in the following. 
     (1) In the honeycomb structure of the present embodiment, among faces of the periphery honeycomb fired body, which are in contact with the adhesive layer, the irregularities are formed on a face that is in contact with another periphery honeycomb fired body with the adhesive layer interposed therebetween, so that adhesive strength between the periphery honeycomb fired body and the adhesive layer bonding the periphery honeycomb fired bodies to each other is made higher. For this reason, a crack hardly occurs from the adhesive layer bonding the periphery honeycomb fired bodies to each other. Accordingly, the honeycomb structure of the present embodiment tends to prevent damage to the honeycomb structure, displacement of the periphery honeycomb fired body and a coming off of the periphery honeycomb fired body. 
     (2) In the honeycomb structure of the present embodiment, one of the faces with the irregularities formed thereon in the periphery honeycomb fired body is the face having the smallest area among the faces in contact with the adhesive layer. 
     For this reason, compared to the case where no irregularities are formed on the face, the contact area between the face and the adhesive layer becomes larger, so that adhesive strength between the periphery honeycomb structure and the adhesive layer becomes higher. 
     EXAMPLES 
     Example 1 
     The following description will discuss Examples specifically disclosing the first embodiment of the present invention. Here, the present invention is not intended to be limited only to these Examples. 
     (1) An amount of 52.8% by weight of a silicon carbide coarse powder having an average particle diameter of 22 μm and 22.6% by weight of a silicon carbide fine powder having an average particle diameter of 0.5 μm were mixed. To the resulting mixture, 2.1% by weight of an acrylic resin, 4.6% by weight of an organic binder (methylcellulose), 2.8% by weight of a lubricant (UNILUB, manufactured by NOF Corporation), 1.3% by weight of glycerin, and 13.8% by weight of water were added, and then kneaded to prepare a wet mixture. Then, molding process for extrusion-molding the obtained wet mixture was carried out. 
     In the present process, a raw honeycomb molded body with no cells being sealed, which has almost the same shape as that of the center-portion honeycomb fired body  20  shown in  FIGS. 2A and 2B  and is to be the center-portion honeycomb fired body after firing process, and a raw honeycomb molded body with no cells being sealed, which has almost the same shape as that of the periphery honeycomb fired body  130  shown in  FIGS. 5A and 5B  and is to be the periphery honeycomb fired body after firing process, were manufactured by changing the shape of a die used for extrusion molding. 
     (2) Next, the raw honeycomb molded body was dried by using a microwave drying apparatus so that a dried honeycomb molded body is obtained. Then, using a plug material paste having the same composition as that of the wet mixture, predetermined cells were filled, and the dried honeycomb molded body filled with the plug material paste was again dried by using a drying apparatus. 
     (3) The dried honeycomb molded body was degreased at 400° C., and then fired at 2200° C. under normal pressure and argon atmosphere for three hours. 
     Accordingly, the center-portion honeycomb fired body  120  having the same shape as that of the center-portion honeycomb fired body  20  shown in  FIGS. 2A and 2B  and including a porous silicon carbide sintered body with physical properties of a porosity of 45%, an average pore diameter of 15 μm, a size of 34.3 mm×34.3 mm×150 mm, a cell density of 300 pcs/inch 2  and a thickness of the cell wall of 0.25 mm (10 mil) was manufactured. And the periphery honeycomb fired body  130  having a shape shown in  FIG. 5A  and  FIG. 5B  and having the porosity, the average pore diameter, the cell density and the thickness of the cell wall as same as those of the center-portion honeycomb fired body  120  was manufactured. 
     (4) An adhesive paste was applied to predetermined side faces of the center-portion honeycomb fired body  120  and the periphery honeycomb fired body  130 , and four center-portion honeycomb fired bodies  120  and eight periphery honeycomb fired bodies  130  were bonded to one another with the adhesive paste being interposed therebetween, so as to be disposed as shown in  FIG. 6 . Then, the adhesive paste was heated at 180° C. for 20 minutes so as to be solidified, thereby a cylindrical honeycomb fired body, which has the adhesive layer  140  having a thickness of 1 mm, was manufactured. 
     Here, the adhesive paste including 30.0% by weight of silicon carbide particles having an average particle diameter of 0.6 μm, 21.4% by weight of silica sol, 8.0% by weight of carboxymethylcellulose, and 40.6% by weight of water was used. 
     The cross-sectional shape of the honeycomb structure manufactured in Example 1 is as shown in  FIG. 6 . 
     That is, in the honeycomb structure  100 , the irregularities are formed on the face  130   a  and the face  130   d  of the periphery honeycomb fired body  130 . Moreover, the face (uneven face)  130   d  with the irregularities formed thereon corresponds to the face having the smallest area among the faces in contact with the adhesive layer  140 , and is in contact with the uneven face  130   d  of another periphery honeycomb fired body  130  with the adhesive layer  140  interposed therebetween (see  FIG. 6 ). 
     Comparative Example 1 
     (1) In the same manner as in the process (1) of Example 1, a raw honeycomb molded body, which is to be a center-portion honeycomb fired body after firing process, was manufactured. 
     Moreover, by changing the shape of a die for extrusion molding, a raw honeycomb molded body was manufactured, which is to be the periphery honeycomb fired body after firing process and has the same shape as that of the periphery honeycomb fired body manufactured in Example 1 except that a face corresponding to the uneven face  130   a  of the periphery honeycomb fired body manufactured in Example 1 is formed into a curved face and a face corresponding to the uneven face  130   d  is formed into a plane face. The processes of (2) to (4) in Example 1 were carried out by using these raw honeycomb molded bodies, so that there was manufactured a honeycomb structure having the same shape as that in Example 1, except that the uneven face  130   a  of the periphery honeycomb fired body  130  is formed into a curved face and the face corresponding to the uneven face  130   d  is formed into a plane face in the cross-sectional shape of the honeycomb structure manufactured in Example 1 shown in  FIG. 6 . 
     (Evaluation of Honeycomb Structure) 
     With regard to the honeycomb structures manufactured in Example 1 and in Comparative Example 1, a cycle test in a following method was performed to confirm presence or absence of damage to the adhesive layer, presence or absence of the displacement of the honeycomb fired body, and presence or absence of the coming off of the honeycomb fired body after the cycle test. 
     (Cycle Test) 
     First, each of the honeycomb structures according to Example 1 and Comparative Example 1 was placed in an exhaust passage of an engine, and a commercially available catalyst supporting carrier (diameter: 144 mm, length: 100 mm, cell density: 400 cells/inch 2 , amount of supported platinum: 5 g/L) was placed in the exhaust passage of an engine at a position closer to a gas-inlet side than the honeycomb structure, so that an exhaust gas purifying apparatus was obtained. Particulates were captured for nine hours, while the engine was driven at the number of revolutions of 3000 min −1  and a torque of 50 Nm. The amount of the captured particulates was 10 g/L. 
     Thereafter, the engine was driven at the number of revolutions of 1250 min −1  and a torque of 60 Nm, and when the temperature of the honeycomb structure became constant, the state was kept for one minute. Subsequently, a post injection was performed, and then the temperature of exhaust gases was raised by having the exhaust gases pass through the oxidation catalyst present at the gas-inlet side, so as to burn particulates. 
     The conditions for the post injection were set so that the temperature of the center portion of the honeycomb structure was raised and became almost constant at 600° C. within one minute from the initiation. Then, after the above-mentioned processes were repeated 20 times, observations were performed to determine whether or not any cracks had occurred in the honeycomb structure. 
     As a result, in the honeycomb structure of Example 1, any of the damage to the honeycomb structure, the displacement of the honeycomb fired body and the coming off of the honeycomb fired body had not occurred. 
     On the other hand, in the honeycomb structure of Comparative Example 1, a crack had occurred in the adhesive layer and the honeycomb structure was damaged. Further, a part of the honeycomb fired bodies came off. This is presumably because, in the honeycomb structure of Comparative Example 1, adhesive strength between the honeycomb fired bodies was lower than a stress on the peripheral face of the honeycomb structure generated by performing the regenerating process repeatedly. 
     In addition to the center-portion honeycomb fired body having the shape shown in  FIGS. 2A and 2B  and the periphery honeycomb fired body having the shape shown in  FIGS. 5A and 5B , a periphery honeycomb fired body shown in  FIGS. 7A and 7B  may be used to constitute the honeycomb structure of the first embodiment of the present invention. 
       FIG. 7A  is a perspective view schematically showing a periphery honeycomb fired body other than the periphery honeycomb fired body shown in  FIGS. 5A and 5B , and  FIG. 7B  is a perspective view of the periphery honeycomb fired body shown in  FIG. 7A  observed from a direction different from that in  FIG. 7A . 
     A periphery honeycomb fired body  250  shown in  FIGS. 7A and 7B  has the same configuration as that of the center-portion honeycomb fired body shown in  FIGS. 2A and 2B , except that the irregularities are formed on faces  250   b  and  250   d  opposing to each other and the remaining faces  250   a  and  250   c  are formed into plane faces. 
       FIG. 8  shows a configuration of the honeycomb structure of the first embodiment of the present invention, which includes the honeycomb fired body shown in  FIGS. 7A and 7B . 
       FIG. 8  is a cross-sectional view schematically showing a cross section of a honeycomb structure including the periphery honeycomb fired body shown in  FIGS. 7A and 7B  when cut perpendicularly to the longitudinal direction thereof. 
     As shown in  FIG. 8 , a honeycomb structure  200  is constituted by nine center-portion honeycomb fired bodies  220  positioned in the center portion of the honeycomb structure  200 , twelve periphery honeycomb fired bodies  230  ( 250 ) each constituting a part of a peripheral face of the honeycomb structure  200 , an adhesive layer  240  existing between these honeycomb fired bodies (the center-portion honeycomb fired bodies  220  and the periphery honeycomb fired bodies  230  ( 250 )) so as to bond the honeycomb fired bodies to each other, and a coat layer  260  that forms the peripheral face of the honeycomb structure  200 . Here, the periphery honeycomb fired bodies are constituted by eight periphery honeycomb fired bodies  230  having mutually different shapes and four periphery honeycomb fired bodies  250  shown in  FIGS. 7A and 7B . 
     Also in the honeycomb structure  200  having such a configuration, the irregularities are formed on at least one face  230   d  ( 250   b  and  250   d ) among faces in contact with the adhesive layer  240  of the periphery honeycomb fired bodies  230  ( 250 ). 
     Moreover, the face  230   d  ( 250   b  and  250   d ) with the irregularities formed thereon corresponds to the face having the smallest area among the faces in contact with the adhesive layer  240  of the periphery honeycomb fired bodies  230  ( 250 ). 
     Other Embodiments 
     In the method for manufacturing the honeycomb structure of the first embodiment of the present invention, honeycomb fired bodies molded into predetermined shapes are preliminarily manufactured so as to manufacture a honeycomb structure; however, a honeycomb structure in accordance with an embodiment of the present invention may be manufactured, for example, by using the following method. 
     By exemplifying a case of manufacturing the honeycomb structure of the first embodiment of the present invention, another method for manufacturing the honeycomb structure in accordance with the embodiment of the present invention will be described in the following. 
       FIGS. 9A and 9B  are cross-sectional views each illustrating another example of the method for manufacturing the honeycomb structure in accordance with the embodiment of the present invention. 
     (1) By using the same methods as those of the processes (1) to (3) for manufacturing the honeycomb structure of the first embodiment of the present invention, honeycomb fired body having cells each sealed at ether one end are manufactured. 
     In this case, a center-portion honeycomb fired body  320  having almost the same shape as that of the center-portion honeycomb fired body  20  shown in  FIGS. 2A and 2B , and a periphery honeycomb fired body  330 ′ having a trapezoidal shape in its cross section and with the irregularities formed on a face  330   d ′ as shown in  FIG. 9A  are manufactured. Here, it is to be noted that the irregularities on the face  330   d ′ are not shown in the drawings. 
     (2) Next, in the same manner as in the process (4) of the first embodiment of the present invention, the center-portion honeycomb fired bodies  320  and the periphery honeycomb fired bodies  330 ′ are combined with one another with the adhesive paste layer interposed therebetween so as to be disposed as shown in  FIG. 9A , and by solidifying the adhesive paste layer, an aggregated body  300 ′ of honeycomb fired bodies is manufactured. 
     (3) Next, periphery processing is carried out on the side faces of the aggregated body  300 ′ of honeycomb fired bodies by using a diamond cutter or the like so that the aggregated body  300 ′ of honeycomb fired bodies is processed into a round pillar shape; thus, a honeycomb structure  300  in which the center-portion honeycomb fired bodies  320  and the periphery honeycomb fired bodies  330  are combined with one another with an adhesive layer  340  interposed therebetween (see  FIG. 9B ) is manufactured. 
     On the face  330   d  of the honeycomb structure  300  thus manufactured, the irregularities are to be formed along the cross sectional shape of cells. 
     Here, if necessary, a coat layer (not shown) may be formed on the peripheral face of the honeycomb structure  300 . 
     The honeycomb structure according to the embodiments of the present invention need not have cells each sealed at an end portion. Such a honeycomb structure may be suitably used as a catalyst supporting carrier. 
     Although not particularly limited, the shape of the honeycomb fired body is preferably designed to easily combine the honeycomb fired bodies with one another when forming a honeycomb structure. For example, a square, rectangular, hexagonal, sector shape, or the like may be used as its cross-sectional shape. 
     The shape of the honeycomb structure according to the embodiments of the present invention is not particularly limited to a round pillar shape, and may be a cylindroid shape. 
     Examples of the inorganic binder contained in the adhesive paste include silica sol, alumina sol, and the like. Each of these may be used alone or two or more kinds of these may be used in combination. Among the inorganic binders, silica sol is preferably used. 
     Examples of the inorganic particles contained in the adhesive paste include those including carbides, nitrides, and the like, more specifically, inorganic particles including silicon carbide, silicon nitride, boron nitride or the like. Each of these may be used alone, or two or more kinds of these may be used in combination. Among the inorganic particles, inorganic particles including silicon carbide, which is superior in thermal conductivity, are more preferably used. 
     Examples of the inorganic fibers and/or the whiskers contained in the adhesive paste include, for example, the inorganic fibers and/or the whiskers including silica-alumina, mullite, alumina, silica, and the like. Each of these may be used alone or two or more kinds of these may be used in combination. Among the inorganic fibers, alumina fibers are preferably used. 
     The porosity of the honeycomb fired body is not particularly limited, and desirably at least about 35% and at most about 60%. 
     When the honeycomb structure is used as a filter, the porosity of about 35% or more tends not to cause clogging in the honeycomb structure of the embodiment of the present invention. In contrast, the porosity of about 60% or less tends not to cause a reduction in the strength of the honeycomb fired body, resulting in less possible breakage. 
     The average pore diameter of the honeycomb fired body is desirably at least about 5 μm and at most about 30 μm. 
     When the honeycomb structure is used as a filter, the average pore diameter of about 5 μm or more tends not to cause clogging due to particulates. In contrast, the average pore diameter exceeding of about 30 μm or less tends not to cause particulates to easily pass through the pores. As a result, the honeycomb fired body is more likely to capture the particulates, which more easily allows the honeycomb fired body to certainly function as a filter. 
     Here, the porosity and the pore diameter can be measured through conventionally known methods such as a mercury porosimetry, Archimedes method, and a measuring method using a scanning electronic microscope (SEM). 
     The cell density in the cross-section of the honeycomb fired body is not particularly limited. However, a desirable lower limit thereof is about 31.0 pcs/cm 2  (about 200 pcs/inch 2 ) and a desirable upper limit is about 93.0 pcs/cm 2  (about 600 pcs/inch 2 ). A more desirable lower limit is about 38.8 pcs/cm 2  (about 250 pcs/inch 2 ) and a more desirable upper limit is about 77.5 pcs/cm 2  (about 500 pcs/inch 2 ). 
     Further, the thickness of the cell walls of the honeycomb fired body is not particularly limited, and desirably at least about 0.1 mm and at most about 0.4 mm. 
     The main component of constituent materials of the honeycomb fired body is not limited to silicon carbide. Examples of other ceramic materials may include ceramic powders of, for example, nitride ceramics such as aluminum nitride, silicon nitride, boron nitride and titanium nitride; carbide ceramics such as zirconium carbide, titanium carbide, tantalum carbide and tungsten carbide; oxide ceramics such as alumina, zirconia, cordierite, mullite, and aluminum titanate; and the like. 
     Among these components, non-oxide ceramics are preferable, and silicon carbide is more preferable because this is excellent in thermal resistance properties, mechanical strength, thermal conductivity and the like. Moreover, examples of the constituent material of the honeycomb structure also include silicon-containing ceramics, in which metallic silicon is blended with the above-described ceramics, as well as a ceramic material such as ceramic bound by silicon or silicate compounds. Among these, those ceramics (silicon-containing silicon carbide) in which metallic silicon is blended with silicon carbide are desirably used. 
     Especially, a silicon-containing silicon carbide ceramic containing about 60% by weight or more of silicon carbide is desirable. 
     The particle diameter of the ceramic powder is not particularly limited, and the ceramic powder that tends not to cause the case where the size of the honeycomb fired body manufactured by the following firing treatment becomes smaller than that of the honeycomb molded body after degreased is preferable. 
     The organic binder to be mixed in the wet mixture is not particularly limited, and examples thereof include methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, and the like. Methylcellulose is desirable among these. A blending amount of the organic binder is desirably at least about 1 part by weight and at most about 10 parts by weight with respect to 100 parts by weight of ceramic powder. 
     The plasticizer to be mixed in the wet mixture is not particularly limited, and examples thereof include glycerin and the like. 
     The lubricant to be mixed in the wet mixture is not particularly limited, and examples thereof include polyoxyalkylene-based compounds such as polyoxyethylene alkyl ether and polyoxypropylene alkyl ether, and the like. Specific examples of the lubricant include polyoxyethylene monobutyl ether, polyoxypropylene monobutyl ether, and the like. 
     Moreover, the plasticizer and the lubricant need not be contained in the wet mixture in some cases. 
     In addition, a dispersant solution may be used upon preparing the wet mixture, and examples of the dispersant solution include water, an organic solvent such as benzene, alcohol such as methanol, and the like. 
     Furthermore, a molding auxiliary may be added to the wet mixture. 
     The molding auxiliary is not particularly limited, and examples thereof include ethylene glycol, dextrin, fatty acid, fatty acid soap, polyalcohol and the like. 
     Furthermore, a pore-forming agent such as balloons that are fine hollow spheres including oxide-based ceramics, spherical acrylic particles, graphite and the like may be added to the wet mixture, if necessary. 
     The balloon is not particularly limited, and examples thereof include alumina balloon, glass micro balloon, shirasu balloon, fly ash balloon (FA balloon), mullite balloon and the like. Alumina balloon is desirable among these. 
     The plug material paste for sealing the cells is not particularly limited, a plug to be manufactured through the subsequent processes desirably has a porosity of at least about 30% and at most about 75%, and for example, it is possible to use apaste having the same composition as that of the wet mixture. 
     The catalyst to convert and/or purify exhaust gases may be supported on the honeycomb structure, and desirable examples of the catalyst to be supported include noble metals such as platinum, palladium and rhodium. Among these, platinum is more desirable. Moreover, an alkali metal such as potassium and sodium, and an alkali earth metal such as barium may be used as other catalysts. These catalysts may be used alone, or two or more kinds of these may be used in combination. 
     In addition to a method for applying an adhesive paste to the side faces of each of the honeycomb fired bodies, the combining process in the method for manufacturing a honeycomb structure according to the embodiment of the present invention may be carried out, for example, by using a method in which each of the honeycomb fired bodies is temporarily fixed in a molding frame having almost the same shape as the shape of the ceramic block (or an aggregated body of the honeycomb fired bodies) to be manufactured and an adhesive paste is injected into the each gap between the honeycomb fired bodies. 
     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.