Abstract:
A honeycomb structure includes four honeycomb units each including inorganic particles, an inorganic binder, and cell walls extending from a first end face to a second end face along a longitudinal direction to define a plurality of cells. The cell walls includes a first cell walls extending along a first direction in a cross sectional plane perpendicular to the longitudinal direction and a second cell walls extending along a second direction substantially perpendicular to the first direction in the cross sectional plane. Adhesive layers are provided between the four honeycomb units to connect the four honeycomb units. The adhesive layers extend in a first extending direction and a second extending direction substantially perpendicular to the first extending direction in the cross sectional plane. A minimum angle between the first direction and the first extending direction or the second extending direction is approximately 22.5 degrees to approximately 45 degrees.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to PCT International Application No. PCT/JP2008/055980, filed on Mar. 27, 2008, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a honeycomb structure and an exhaust gas treating apparatus. 
     2. Description of the Related Art 
     Conventionally, a honeycomb structure is used in an exhaust gas treating apparatus which treats HC, CO, NOx, SOx, and the like including in automotive exhaust gases. The honeycomb structure has plural cells (through holes) which extend from one end face to the other end face of the honeycomb structure along the long length direction of the honeycomb structure, and the cells are separated from each other by walls of the cells (cell walls). 
     A catalyst such as platinum is provided on the cell walls of the honeycomb structure. When exhaust gases flow into a catalyst carrier (honeycomb structure) providing the catalyst, the catalyst on the cell walls can treat components of HC, CO, NOx, Sox, and the like in the exhaust gases by catalytic reaction. WO2005/063653A discloses a honeycomb structure which includes a first inorganic material (for example, ceramic particles), and a second inorganic material (for example, inorganic fibers, inorganic particles having a relatively large diameter, and an inorganic binder). 
     In addition, JPA 2001-190916 discloses a honeycomb structure which is used as a DPF (diesel particulate filter). In the honeycomb structure, first, a predetermined number of honeycomb units are bonded by adhering side faces of the honeycomb units having the same pillar shape by interposing adhesive layers, and the peripheral surface of the bonded honeycomb units is cut so that a honeycomb structure having a desirable shape is formed. Further, JPA 2006-223983 discloses a honeycomb structure. 
     In the honeycomb structure, first, honeycomb units having corresponding predetermined shapes are formed, and a honeycomb structure having a desirable shape is formed by combining the honeycomb units by interposing adhesive layers without a cutting process. In JPA 2006-223983, the honeycomb structure is formed by combining 16 honeycomb units of three different shapes. 
     The entire contents of WO2005/063653A, JPA 2001-190916, and JPA 2006-223983 are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     A honeycomb structure includes four honeycomb units each including inorganic particles, an inorganic binder, and cell walls extending from a first end face to a second end face along a longitudinal direction of the honeycomb structure to define a plurality of cells. The cell walls includes a first cell walls extending along a first direction in a cross sectional plane perpendicular to the longitudinal direction and a second cell walls extending along a second direction substantially perpendicular to the first direction in the cross sectional plane. Adhesive layers are provided between the four honeycomb units to connect the four honeycomb units. The adhesive layers extend in a first extending direction and a second extending direction substantially perpendicular to the first extending direction in the cross sectional plane. A minimum angle between the first direction and the first extending direction or the second extending direction is approximately 22.5 degrees to approximately 45 degrees. 
     An exhaust gas treating apparatus includes a honeycomb structure having a longitudinal direction, a metal casing containing the honeycomb structure, and a holding sealing member disposed between the honeycomb structure and the metal casing to hold the honeycomb structure at a predetermined position in the metal casing. The honeycomb structure includes a first end face, a second end face opposite to the first end face along the longitudinal direction, and four honeycomb units. Each of the honeycomb units includes inorganic particles, an inorganic binder, and a plurality of cell walls extending from the first end face to the second end face to define a plurality of cells. The plurality of cell walls include a first cell walls extending along a first direction in a cross sectional plane perpendicular to the longitudinal direction and a second cell walls extending along a second direction substantially perpendicular to the first direction in the cross sectional plane. Adhesive layers are provided between the four honeycomb units to connect the four honeycomb units each other. The adhesive layers extend in a first extending direction and a second extending direction substantially perpendicular to the first extending direction in the cross sectional plane. A minimum angle between the first direction and the first extending direction or the second extending direction is approximately 22.5 degrees to approximately 45 degrees. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. 
         FIG. 1  is a perspective view of a honeycomb structure according to an embodiment of the present invention; 
         FIG. 2  is an enlarged view of a cross section orthogonal to the long length direction of the honeycomb structure shown in  FIG. 1 ; 
         FIG. 3  is an enlarged perspective view of a honeycomb unit shown in  FIG. 1 ; 
         FIGS. 4A and 4B  are diagrams showing the concept of a minimum angle according to the embodiment of the present invention; 
         FIG. 5  is an enlarged view of a cross section orthogonal to the long length direction of a conventional honeycomb structure; 
         FIG. 6  is a schematic diagram showing stresses being applied onto corresponding peripheral surfaces of the conventional honeycomb structure shown in  FIG. 5  and the honeycomb structure according to the embodiment of the present invention; 
         FIG. 7  is a perspective view of a honeycomb unit in another example according to the embodiment of the present invention; 
         FIG. 8  is a schematic diagram showing an exhaust gas treating apparatus using the honeycomb structure according to the embodiment of the present invention; and 
         FIG. 9  is a graph showing a relationship between isostatic strength and minimum angles in the honeycomb structures of examples and comparative examples. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, an embodiment of the present invention is described. 
       FIG. 1  is a perspective view of a honeycomb structure  100  according to an embodiment of the present invention.  FIG. 2  is an enlarged view of a cross section orthogonal to the long length direction of the honeycomb structure  100  shown in  FIG. 1 .  FIG. 3  is an enlarged perspective view of a honeycomb unit  120 A of the honeycomb structure  100  according to the embodiment of the present invention. 
     As shown in  FIG. 1 , the honeycomb structure  100  according to the embodiment of the present invention includes a first end face  105 , a second end face  106 , four honeycomb units  120 A through  120 D, and adhesive layers  180  which adhere the four honeycomb units  120 A through  120 D each other. 
     Each of the four honeycomb units  120 A through  120 D substantially has the same structure and the same shape. Therefore, in the following, in some cases, the honeycomb unit  120 A is described in detail as a representative of the four honeycomb units  120 A through  120 D. 
     A coating layer  190  shown in  FIGS. 1 and 2  is described below in detail. 
     As shown in  FIG. 3 , the honeycomb unit  120 A has a ¼ pillar shape whose cross section is approximately fan-shaped. The honeycomb unit  120 A includes many cells (through holes)  130 A separated by cell walls  140 A disposed along the long length direction of the honeycomb unit  120 A. The cross section of the cell  130 A orthogonal to the long length direction of the honeycomb unit  120 A is approximately square-shaped. 
     As shown in  FIGS. 1 and 2 , the adhesive layers  180  are extended in corresponding two directions orthogonal to each other. Hereinafter the two directions are referred to as a first extending direction D 1  (X axis direction in  FIG. 2 ) and a second extending direction D 2  (Y axis direction in  FIG. 2 ). 
     In  FIG. 2 , when the honeycomb structure  100  is viewed from the first end face  105  side (or the second end face  106  side), the cell walls  140 A are formed to have a lattice configuration so that center lines of the cell walls  140 A are extended in corresponding two directions (first direction L 1  and second direction L 2 ) substantially orthogonal to each other. In this, the names of “first” and “second” are arbitrarily determined, and one of the two directions is the first direction and the other of the two directions is the second direction. In  FIG. 2 , the first direction L 1  is the direction in which the X axis (horizontal axis) is rotated counterclockwise 45 degrees, and the second direction L 2  is the direction in which the X axis (horizontal axis) is rotated clockwise 45 degrees. 
     In addition, as shown in  FIG. 2 , in the honeycomb unit  120 A, the first direction L 1  (or the second direction L 2 ) of the cell walls  140 A has a “minimum angle” of 45 degrees relative to the first and second extending directions of the adhesive layers  180 . 
     In this, the “minimum angle” signifies a minimum angle between one group of straight lines and a third straight line.  FIGS. 4A and 4B  are diagrams showing the concept of the minimum angle. For example, in  FIG. 4A , when a group of straight lines A and B orthogonal to each other and a third straight line C rotated counterclockwise 45 degrees relative to the straight line A exist, the angle between straight lines A and C is 45 degrees or 135 degrees. In this case, the minimum angle between the group of the straight lines A and B and the third straight line C is 45 degrees shown by α or β. Therefore, the minimum angle between the group of the straight lines and the straight line C is 45 degrees. 
     In addition, for example, in  FIG. 4B , when the third straight line C is rotated counterclockwise 22.5 degrees relative to the straight line A, the angle between the straight lines A and C is 22.5 degrees or 157.5 degrees, and the angle between straight lines B and C is 67.5 degrees or 112.5 degrees. In this case, the minimum angle between the group of the straight lines A and B and the third straight line C is 22.5 degrees. Therefore, the minimum angle between the group of the straight lines and the third straight line C is 22.5 degrees. 
     In  FIG. 2 , in the honeycomb structure  100 , the angle between the first direction L 1  (or the second direction L 2 ) of the cell walls  140 A and the first and second extending directions D 1  and D 2  of the adhesive layers  180  is formed to be the minimum angle of 45 degrees. However, the angle between the first direction L 1  (or the second direction L 2 ) of the cell walls  140 A and the first and second extending directions D 1  and D 2  of the adhesive layers  180  is not limited to 45 degrees; it is desirably in a range approximately 22.5 to approximately 45 degrees. 
     Next, effects of the honeycomb structure  100  according to the embodiment of the present invention are described by comparison with a conventional honeycomb structure  200 . 
     However, the honeycomb structure in WO2005/063653A has a problem that it is weak against external stress. Even if the honeycomb structure of WO2005/063653A uses the adhesive layer described in JPA 2001-190916, since the cell walls are formed in parallel with the adhesive layer, the honeycomb structure is easily broken by relatively low stress from the outside. 
     In JPA 2006-223983, the honeycomb structure is formed by bonding the 16 honeycomb units. In order to make the strength high against external stress, as the honeycomb structure is viewed from the end face of the honeycomb structure, in the minimum size honeycomb units at four corners of the honeycomb structure, the cell disposing direction is rotated 45 degrees relative to the cells of the other honeycomb units. 
     However, in the honeycomb structure of JPA 2006-223983, almost all the stresses rotated 45 degrees from the bonding direction of the honeycomb units are directly applied on the minimum size honeycomb units at the four corners, of the stresses applied onto the peripheral surface of the honeycomb structure from the outside. Therefore, the minimum size honeycomb units at the four corners are easily broken even if a relatively low stress is applied onto the peripheral surface of the honeycomb structure. That is, the strength of the honeycomb structure may not be enough. 
       FIG. 5  is an enlarged view of a cross section orthogonal to the long length direction of a conventional honeycomb structure  200 . 
     The conventional honeycomb structure  200  includes four honeycomb units  220 A through  220 D and adhesive layers  280 . The adhesive layers  280  adhere the four honeycomb units  220 A through  220 D each other. The adhesive layers  280  are extended in the corresponding two extending directions D 1  (X axis direction) and D 2  (Y axis direction) orthogonal to each other. In the conventional honeycomb structure  200 , each of the four honeycomb units  220 A through  220 D has substantially the same structure and the same shape. Therefore, the honeycomb unit  220 A is described in detail as a representative of the four honeycomb units  220 A through  220 D. As shown in  FIG. 5 , when the conventional honeycomb structure  200  is viewed from one end face side of the conventional honeycomb structure  200 , a first direction L 3  (or a second direction L 4 ) of the cell walls  240 A of the honeycomb unit  220 A is substantially the same as the first extending direction D 1  (or the second extending direction D 2 ) of the adhesive layer  280 . That is, the first direction L 3  of the cell walls  240 A is substantially the same as the first extending direction D 1  of the adhesive layer  280 , and the second direction L 4  of the cell walls  240 A is substantially the same as the second extending direction D 2  of the adhesive layer  280 . 
     When stress (compressive stress) is applied onto a peripheral surface of the conventional honeycomb structure  200  from the outside, strength characteristics are different among positions on the peripheral surface to which positions the stress is applied. In addition, a coating layer  290  is formed on the peripheral surface of the honeycomb structure  200  so as to increase the strength against stress from the outside. 
       FIG. 6  is a schematic diagram showing stresses being applied onto the corresponding peripheral surfaces of the honeycomb structures  200  and  100 . For example, in  FIG. 6 , when stresses Pa are applied to the conventional honeycomb structure  200  from the first and second extending directions D 1  and D 2  of the adhesive layers  280 , since the adhesive layers  280  function as reinforcing members, the strength against the external stress is high. However, when stresses Pb are applied to the conventional honeycomb structure  200  from directions shifted approximately 45 degrees from the first and second extending directions D 1  and D 2  of the adhesive layers  280 , the strength against the external stress is low. 
     As shown in  FIG. 5 , the first direction L 3  of the cell walls  240 A is substantially parallel to the first extending direction D 1  of the adhesive layer  280 , and the second direction L 4  of the cell walls  240 A is substantially parallel to the second extending direction D 2  of the adhesive layer  280 . Consequently, the adhesive layers  280  can function as reinforcing members against the stresses Pa; however, it is conceivable that the adhesive layers  280  do not function as the reinforcing members against the stresses Pb. 
     When a honeycomb structure has at least one weak portion on the peripheral surface of the honeycomb structure, the honeycomb structure is easily broken at the weak portion. Consequently, it is conceivable that the conventional honeycomb structure  200  is broken when one of the stresses Pb is applied onto the peripheral surface of the honeycomb structure  200 . 
     In order to solve the above problem, the strength of the honeycomb structure must be high against stress from the outside. That is, it is conceivable that the weak portion of the honeycomb structure must be reinforced. 
     In the honeycomb structure  100  according to the embodiment of the present invention, as described above, the angle between the first direction L 1  (or the second direction L 2 ) of the cell walls  140 A and the first and second extending directions D 1  and D 2  of the adhesive layers  180  is formed to be the minimum angle 22.5 to 45 degrees. Therefore, the honeycomb structure  100  has almost uniform strength on the peripheral surface against stress from the outside. 
     Returning to  FIGS. 2 and 6 , when stresses Pa are applied to the honeycomb structure  100  from the first and second extending directions D 1  and D 2  of the adhesive layers  180 , since the adhesive layers  180  function as reinforcing members, the strength against the external stress is high. In addition, when stresses Pb are applied to the honeycomb structure  100  from directions shifted approximately 45 degrees from the first and second extending directions D 1  and D 2  of the adhesive layers  180 , since the cell walls  140 A of the honeycomb structure  120 A function as reinforcing members, the strength against the external stress is high. That is, as shown in  FIG. 2 , the first direction L 1  or the second direction L 2  of the cell walls  140 A through  140 D are almost the same as the direction of the stress Pb. Therefore, the honeycomb structure  100  according to the embodiment of the present invention has substantially uniform strength against stresses from the outside. Consequently, it is conceivable that the honeycomb structure  100  has greater strength than the conventional honeycomb structure  200  against the stresses from the outside. 
     As described above, in the honeycomb structure of JPA 2006-223983, in order to make the strength high against the stresses from the outside, when the honeycomb structure is viewed from one end face side in the long length direction of the honeycomb structure, only in the minimum size honeycomb units at the corresponding four corners, the arranging direction of cells is rotated by 45 degrees relative to the arranging direction of cells in the adjacent honeycomb units. 
     In JPA 2006-223983, when the conventional honeycomb structure is formed and is used in the embodiment of the present invention, the 16 honeycomb units of three different shapes are adhered by interposing adhesive layers by accurately positioning the 16 honeycomb units. This manufacturing process is very difficult. Actually, the positions of the 16 honeycomb units may be slightly shifted and the shape of the manufactured honeycomb structure may be different from a desirable predetermined shape. 
     In addition, in the honeycomb structure having the minimum size honeycomb units at the corresponding four corners, it is conceivable that the flow of the exhaust gasses is not uniform. When the honeycomb structure is used as a DPF, since the exhaust gasses flow through the walls (wall flow type), it is conceivable that the honeycomb structure is effectively used. However, when the honeycomb structure is used as a catalyst carrier, since this honeycomb structure is used as a straight flow type (exhaust gases flow through the cells), this honeycomb structure cannot be effectively used. Consequently efficiency for treating (converting) components of HC, CO, NOx, Sox, and the like from the exhaust gasses may be lowered. 
     In the honeycomb structure  100  according to the embodiment of the present invention, the four honeycomb units  120 A through  120 D are adhered by interposing the adhesive layers  180 . Therefore, the number of the adhering members (the adhesive layers  180 ) is small and the position shift among the four honeycomb units  120 A through  120 D can be likely to be lowered. In addition, after manufacturing the honeycomb structure  100 , the directions of the cell walls  140 A through  140 D of the corresponding honeycomb units  120 A through  120 D are substantially the same as the first direction L 1  or the second direction L 2 . In the honeycomb structure  100  according to the embodiment of the present invention, the flow of the exhaust gasses is hardly non-uniform and the efficiency for treating (converting) the components of HC, CO, NOx, Sox, and the like from the exhaust gasses is hardly lowered. 
     In the above, the honeycomb structure  100  having a desirable peripheral shape is directly formed by adhering the four honeycomb units  120 A through  120 D each of whose cross section is fan-shaped along the long length direction of the honeycomb units  120 A through  120 D by interposing the adhesive layers  180 . However, the shape and the manufacturing method of a honeycomb structure are not limited to the above.  FIG. 7  is a perspective view of a honeycomb unit  121  in another example according to the embodiment of the present invention. That is, when the four honeycomb units  121  having a square pillar shape are adhered by interposing adhesive layers (not shown) and the adhered four honeycomb units  121  are cut to form a desirable peripheral shape, a honeycomb structure having the desirable peripheral shape can be formed. 
     In addition, as shown in  FIG. 1 , the honeycomb structure  100  has a cylindrical shape whose cross section is approximately circle-shaped. However, the shape of the cross section of the honeycomb structure according to the embodiment of the present invention can be an arbitrarily shape, for example, an elliptical shape, an oval shape, or the like. 
     The honeycomb structure  100  is assumed to be used as a catalyst carrier. When the honeycomb structure  100  according to the embodiment of the present invention is used to treat (convert) predetermined components such as HC, CO, NOx, Sox, and the like from automotive exhaust gases, a catalyst, for example, alkali metal, alkali earth metal, or the like can be supported on the cell walls  140 A through  140 D so as to accelerate the treatment (conversion) of the above components. In addition, a catalyst can be contained in a base material of the honeycomb structure  100 . 
     As described above, the honeycomb structure  100  according to the embodiment of the present invention can be used in an automotive exhaust gas treating apparatus. 
       FIG. 8  is a schematic diagram showing an exhaust gas treating apparatus  70  using the honeycomb structure  100  according to the embodiment of the present invention. As shown in  FIG. 8 , the automotive exhaust gas treating apparatus  70  includes the honeycomb structure  100 , a metal casing  71  for containing the honeycomb structure  100 , and a holding sealing member  72  for holding the honeycomb structure  100  at a predetermined position in the metal casing  71 . In addition, an introducing pipe  74  is connected to one end (introducing section) of the automotive exhaust gas treating apparatus  70  to which introducing pipe  74  exhaust gases emitted from an internal combustion device such as an engine are introduced, and an emitting pipe  75  is connected to the other end (emitting section) of the automotive exhaust gas treating apparatus  70  from which emitting pipe  75  the exhaust gases treated by the automotive exhaust gas treating apparatus  70  are emitted. In  FIG. 8 , the arrows show the flow of the exhaust gases. 
     The exhaust gases emitted from the internal combustion device such as the engine are introduced to the metal casing  71  via the introducing pipe  74 , and the introduced exhaust gases flow into the cells of the honeycomb structure  100  via one end face (for example, a first end face) of the honeycomb structure  100  which end face faces the introducing pipe  74 . The exhaust gases flowing into the cells are emitted from the emitting pipe  75  to the outside. With this process, toxic substances in the exhaust gases are treated (converted). 
     In the honeycomb structure and the exhaust gas treating apparatus using the honeycomb structure according to the embodiment of the present invention, the honeycomb structure has higher strength against stress applied onto a peripheral surface of the honeycomb structure from the outside than a conventional honeycomb structure and is hardly provide a crack. 
     In the following, when a suffix is not attached to a reference number of an element, the reference number represents the set of elements. For example, a honeycomb unit  120  represents the honeycomb units  120 A through  120 D. 
     In the embodiment of the present invention, it is preferable that the cell density of the honeycomb unit  120  be approximately 15.5 to approximately 186 pieces per cm 2  (approximately 100 to approximately 1200 cpsi; cells per square inch), more preferably, approximately 46.5 to approximately 170 pieces per cm 2  (approximately 200 to approximately 1000 cpsi), and still more preferably, approximately 62.0 to approximately 155 pieces per cm 2  (approximately 300 to approximately 800 cpsi). 
     The thickness of the cell walls  140  of the honeycomb unit  120  is not particularly limited. However, in order to obtain sufficient strength, in the thickness of the cell walls  140 , the preferable lower limit is approximately 0.1 mm and the preferable upper limit is approximately 0.4 mm. 
     The specific surface area of the honeycomb unit  120  is not particularly limited; however, the specific surface area is preferably in a range of approximately 25000 m 2 /L to approximately 70000 m 2 /L. 
     The material of the honeycomb unit  120  of the honeycomb structure  100  according to the embodiment of the present invention is not particularly limited; however, the material preferably includes inorganic particles and an inorganic binder, and can further include inorganic fibers. 
     The inorganic particles are preferably particles of such as alumina, ceria, zirconia, titania, silica, zeolite, mullite, and the like. However, as the inorganic particles, one of the above particles can be used solely, or the above two or more particles can be used. 
     When zeolite is used as the inorganic particles, ion-exchanged zeolite by using Cu, Fe, Ni, Zn, Mn, or Co can be used. The sole ion-exchanged zeolite can be used. However, two or more ion-exchanged zeolite can be combined, or ion-exchanged zeolite can be used in which metals whose valence is different from each other are combined. 
     The material of the inorganic fibers is preferably alumina, silica, silicon carbide, silica-alumina, glass, potassium titanate, aluminum borate, or the like. As the inorganic fibers, the material can be used solely, or two or more above materials are combined. However, the material of the inorganic fibers is more preferably alumina. 
     In the embodiment of the present invention, the average aspect ratio (length/diameter) of the inorganic fibers is more than approximately 5. The aspect ratio is preferably approximately 10 to approximately 1000. 
     As the inorganic binder, inorganic sol, a clay based binder, or the like can be used. The inorganic sol is, for example, alumina sol, silica sol, titania sol, liquid glass, or the like. The clay based binder is multiple chain type clay, or the like, for example, white clay, kaolin, montmorillonite, meerschaum, attapulgite, or the like. In addition, as the inorganic binder, one of the above material can be used solely, or two or more of the above materials can be combined. 
     The inorganic binder preferably includes one of alumina sol, silica sol, titania sol, liquid glass, meerschaum, and attapulgite. 
     The lower limit of the amount of the inorganic particles contained in the honeycomb unit  120  is preferably approximately 30 wt %, more preferably, approximately 40 wt %, and still more preferably, approximately 50 wt %. The upper limit of the amount of the inorganic particles containing in the honeycomb unit  120  is preferably approximately 90 wt %, more preferably, approximately 80 wt %, and still more preferably, approximately 75 wt %. 
     When the contained amount of the inorganic particles in the honeycomb unit  120  is approximately 30 wt % or more, the amount of the inorganic particles contributing to the catalytic action is hardly small relatively. When the contained amount of the inorganic particles in the honeycomb unit  120  is approximately 90 wt % or less, the amount of the inorganic fibers contributing to increase the strength of the honeycomb unit  120  becomes relatively small, and the strength of the honeycomb unit  120  is hardly lowered. 
     The lower limit of the amount of the inorganic fibers in the honeycomb unit  120  is preferably, approximately 3 wt %, more preferably, approximately 5 wt %, and still more preferably, approximately 8 wt %. The upper limit of the amount of the inorganic fibers in the honeycomb unit  120  is preferably, approximately 50 wt %, more preferably, approximately 40 wt %, and still more preferably, approximately 30 wt %. 
     When the contained amount of the inorganic fibers in the honeycomb unit  120  is approximately 3 wt % or more, the strength of the honeycomb unit  120  is hardly lowered, and when the contained amount of the inorganic fibers in the honeycomb unit  120  is approximately 50 wt % or less, the amount of the inorganic particles contributing to the catalytic action is hardly small relatively. For example, when the contained amount of the inorganic fibers in the honeycomb unit  120  is approximately 50 wt % or less, the specific surface area of the honeycomb structure  100  becomes small, catalyst components can be likely dispersed when the catalyst components are supported, and further, the catalyst amount per unit volume is hardly lowered. 
     The lower limit of the amount of the inorganic binder as solids content containing in the raw materials is preferably, approximately 5 wt % per total amount of the inorganic particles, the inorganic fibers, and the solids content of the inorganic binder, more preferably, approximately 10 wt %, and still more preferably, approximately 15 wt %. The upper limit of the amount of the inorganic binder as solids content is preferably, approximately 50 wt %, more preferably, approximately 40 wt %, and still more preferably, approximately 35 wt %. 
     When the contained amount of the inorganic binder as solids content is approximately 5 wt % or more, the strength of the honeycomb unit  120  is hardly lowered, and when the contained amount of the inorganic binder as solids content is approximately 50 wt % or less, molding ability of the raw materials is hardly lowered. 
     The adhesive layer  180  of the honeycomb structure  100  according to the embodiment of the present invention can be formed of a dense substance or a porous substance. The material of the adhesive layer  180  can include, for example, an inorganic binder, an organic binder, inorganic fibers, and/or inorganic particles. 
     As the substance of the inorganic binder for the adhesive layer  180 , a substance containing silica sol, alumina sol, titania sol, or the like can be used. However, as the inorganic binder, one of the above substances can be used solely, or two or more of the above substances can be combined. 
     As the substance of the organic binder for the adhesive layer  180 , polyvinyl alcohol, methylcellulose, ethylcellulose, carboxymethyl cellulose, or the like can be used. However, as the organic binder, one of the above substances can be used solely, or two or more of the above substances can be combined. 
     As the substance of the inorganic fibers for the adhesive layer  180 , ceramic fibers including, for example, silica-alumina, mullite, alumina, silica, or the like can be used. However, as the inorganic fibers, one of the above substances can be used solely, or two or more of the above substances can be combined. 
     As the inorganic particles for the adhesive layer  180 , the same inorganic particles as that of the material for the honeycomb unit  120  can be used. However, the inorganic particles can be used solely, or two or more of the above inorganic particles for the raw material can be used. 
     In order to form the adhesive layer  180 , first, a paste is prepared by including the above substances, the paste is adhered onto a predetermined position, and the paste is dried. With this, the adhesive layers  180  are formed. Pore forming agents such as balloons (minute hollow balls) formed of oxide based ceramics, spherical acrylic particles, and graphite can be added to the paste as the raw material. 
     The honeycomb structure  100  can further dispose the coating layer  190  (see  FIG. 1 ) on the peripheral surface of the honeycomb structure  100 . When the coating layer  190  is formed, the strength of the peripheral surface of the honeycomb structure  100  can be further increased. 
     The thickness of the coating layer  190  is preferably in a range of approximately 0.2 to approximately 3.0 mm. The material of the coating layer  190  can be the same as that of the adhesive layer  180  or different from that of the adhesive layer  180 . 
     [Manufacturing Method of Honeycomb Structure] 
     Next, a manufacturing method of the honeycomb structure  100  according to the embodiment of the present invention is described. First, a raw material paste whose base components are ceramic particles, inorganic fibers, and an inorganic binder is prepared, and a honeycomb unit molded body is formed of the raw material paste by using extrusion molding or the like. An organic binder, a dispersion medium, and/or a molding aid can be added to the raw material paste so as to obtain high molding ability. 
     The organic binder is not particularly limited, as the organic binder, there are, for example, methylcellulose, carboxymethyl cellulose, hydroxylethyl cellulose, polyethyleneglycole, phenol resin, epoxy resin, and the like. One of more of the above organic binders can be selected. When the organic binder is added into the original raw material paste, the adding ratio of the organic binder to the original raw material paste (total amount of the ceramic particles, the inorganic fibers, and the inorganic binders) is preferable 1 to 10 wt % to 100 wt %. 
     The dispersion medium is not particularly limited. As the dispersion medium, there are, for example, water, organic solvent (for example, benzene), alcohol (for example, methanol), and the like. The molding aid is not particularly limited. As the molding aid, there are, for example, ethylene glycol, dextrin, fatty acid, fatty acid soap, polyalcohol, and the like. 
     The raw material paste is preferably mixed and kneaded. The raw material paste can be sufficiently mixed by a mixer, an attritor (grinding mill), and the like, and can be sufficiently kneaded by a kneader or the like. The forming method of the raw material paste is not particularly limited, and there is, for example, an extrusion method or the like. The honeycomb unit molded body is formed to have cells (see  FIG. 3 ). 
     The honeycomb unit molded body is preferably dried by, for example, a drying apparatus. The drying apparatus is not particularly limited. As the drying apparatus, there are, for example, a microwave drying apparatus, a hot air drying apparatus, a dielectric drying apparatus, a reduced pressure drying apparatus, a vacuum drying apparatus, a freeze drying apparatus, and the like. The dried honeycomb unit molded body is preferably degreased. The degreasing conditions are determined depending on the amount and kind of organic substances contained in the honeycomb unit molded body; however, the honeycomb unit molded body is degreased under the conditions of at approximately 400° C. for approximately 2 hours. Further, the degreased honeycomb unit molded body is preferably subjected to firing. The firing condition is preferably at approximately 600 to approximately 1200° C. for approximately 2 hours, more preferably at approximately 600 to approximately 1000° C. for approximately 2 hours. When the firing temperature is approximately 600° C. or more, the sintering is likely progressed, and the strength of the honeycomb unit  120  is hardly low. When the firing temperature is approximately 1200° C. or less, since the sintering is not too progressed, the specific surface area per unit volume of the honeycomb unit  120  is hardly small. By the above processes, the honeycomb unit  120  having plural cells (through holes) is obtained. 
     Next, an adhesive layer paste which becomes the adhesive layers  180  is applied with a uniform thickness onto surfaces of the honeycomb unit  120  onto which surfaces other honeycomb units  120  are adhered. Then the four honeycomb units  120  are adhered to each other by interposing the corresponding adhesive layers  180 . For example, two of the honeycomb units  120  are adhered in the vertical direction and the adhered two honeycomb units  120  are adhered in the horizontal direction. With this, the honeycomb structure  100  having a predetermined size is formed. As the adhesive layer paste, the raw material paste for the honeycomb unit  120  can be used. 
     The adhesive layer paste is not particularly limited. As the adhesive layer paste, a paste combining an inorganic binder with ceramic particles, a paste combining an inorganic binder with inorganic fibers, a paste combining an inorganic binder and ceramic particles with inorganic fibers, or the like can be used. In addition, an inorganic binder can be added to the above adhesive layer paste. The organic binder is not particularly limited. As the organic binder, there are, for example, polyvinyl alcohol, methylcellulose, ethylcellulose, carboxymethyl cellulose, and the like, and one or more the above organic binders can be selected. 
     The thickness of the adhesive layer  180  is preferably, approximately 0.3 to approximately 2.0 mm. When the thickness is approximately 0.3 mm or more, sufficient adhering strength is likely obtained. When the thickness is approximately 2.0 mm or less, since the adhesive layer  180  does not function as the catalyst carrier, the specific surface area per unit volume of the honeycomb structure  100  is hardly small and high dispersion of catalyst components is likely executed when the honeycomb structure  100  supports the catalyst components, and further, the conversion performance of the honeycomb structure  100  is hardly lowered by hardly decreasing the catalyst amount per unit volume of the honeycomb structure  100 . In addition, when the thickness is approximately 2.0 mm or less, the pressure loss of gasses flowing into the honeycomb structure  100  is hardly increased. 
     Next, heat is applied to the honeycomb structure in which the four honeycomb units  120  are adhered by interposing the adhesive layers  180 ; with this, the adhesive layers  180  (adhesive layer pastes) are dried and solidified. That is, as shown in  FIG. 1 , the cylindrical honeycomb structure  100  having the four honeycomb units  120  shown in  FIG. 3  is manufactured. 
     In addition, the coating layer  190  can be formed on the peripheral surface of the honeycomb structure  100 . In this case, a coating layer paste is applied onto the peripheral surface of the honeycomb structure  100  and the coating layer paste is dried and solidified. With this, the coating layer  190  is formed. The coating layer paste is not particularly limited. As the coating layer paste, the adhesive layer paste or another paste can be used. The compounding ratios of substances of the coating layer paste can be the same as those of the adhesive layer paste or different from those of the adhesive layer paste. In addition, the thickness of the coating layer  190  is not particularly limited. However, the thickness of the coating layer  190  is preferably, approximately 0.2 to approximately 3.0 mm. 
     Heat treatment is preferably applied to the honeycomb structure  100  after the four honeycomb units  120  are adhered by interposing the adhesive layers  180  (and after the coating layer  190  is further formed). When the adhesive layer paste (and the coating layer paste) includes an organic binder, the adhesive layer paste (and the coating layer paste) can be degreased by heat treatment. The degreasing conditions depend on the amount and kind of the organic binder in the paste; however, the degreasing conditions are preferably at approximately 700° C. for approximately 2 hours. 
     The catalyst material to be supported on the cell walls  140  of the honeycomb structure  100  is noble metal, for example, platinum, palladium, rhodium, or the like. In addition, a compound containing alkali metal, alkali earth metal, rare-earth element, transition metal, or the like can be used as the catalyst material. When a platinum catalyst is disposed on the cell walls  140 , the honeycomb unit  120  is impregnated with a dinitrodiammine platinum nitric acid solution ([Pt(NH 3 ) 2 (NO 2 ) 2 ]HNO 3 ), or the like, and heat is applies to the impregnated honeycomb unit  120 . In addition, the catalyst can include one of potassium, magnesium, barium, calcium, or the like. 
     EXAMPLES 
     Next, examples of the embodiment of the present invention are described. 
     Example 1 
     First, a mixture composition was obtained by mixing and kneading 2250 parts by weight of zeolite, alumina fibers of 680 parts by weight (average fiber length is 100 μm and average fiber diameter is 6 μm), 2600 parts by weight of alumina sol (solids content 30 wt %), 320 parts by weight of methylcellulose, platicizer, and a lubricant agent (Unilube). Then a honeycomb unit molded body whose cross section is fan-shaped (¼ circle in cross section; 69 mm radius) was obtained by molding the composition with the use of an extrusion molding machine. 
     In this honeycomb unit  120 , the first and second directions L 1  and L 2  of the cell walls were substantially orthogonal to each other when the honeycomb unit  120  was viewed from the end face, and the angle between the cell walls  140  and the straight lines (radius of the ¼ circle in cross section) of the fan-shaped honeycomb unit  120  was 45 degrees when the honeycomb unit  120  is viewed from the cross section. That is, the first direction L 1  (or the second direction L 2 ) of the cell walls  140  has 45 degrees as the minimum angle relative to the extending directions of the adhesive layers  180 . 
     Next, the honeycomb unit molded body was sufficiently dried and degreased by using a microwave drying apparatus and a hot air drying apparatus at 400° C. for 2 hours. Then firing was applied to the dried and degreased honeycomb unit molded body at 700° C. for 2 hours. With this, the honeycomb unit  120  whose cross section is a fan shape having two sides (69 mm) and one arc was obtained. The thickness of the cell walls  140  was 0.2 mm and the cell density was 93 pieces per cm 2 . 
     Next, an adhesive layer paste was prepared by mixing 26 wt % of aluminum particles (average particle diameter is 2 μm), 37 wt % of alumina fibers, 31.5 wt % of alumina sol (solids content 30 wt %), 0.5 wt % of carboxymethyl cellulose, and 5 wt % of water. The four honeycomb units  120  were adhered to each other by applying the prepared adhesive layer paste onto the sides of the four honeycomb units  120 . With this, a honeycomb unit aggregated body was obtained. The adhesive layer paste was applied to the sides of the honeycomb units so that the thickness of adhesive layers  180  became uniformly 1 mm, then heat was applied at 120° C. and was solidified. 
     Next, a coating layer paste (the same material as the adhesive layer paste) was applied to the peripheral surface of the honeycomb unit aggregated body and the coating layer  190  of 0.5 mm thickness was formed by being solidified with heat. With this, the honeycomb structure  100  was obtained. 
     Example 2 
     A honeycomb structure  100  was manufactured by a method similar to the method of Example 1. However, in Example 2, the honeycomb structure  100  was manufactured so that the first direction L 1  (or the second direction L 2 ) of the cell walls  140  in the honeycomb unit  120  had the minimum angle of 40 degrees relative to the extending directions of the adhesive layers  180 . 
     Example 3 
     A honeycomb structure  100  was manufactured by a method similar to the method of Example 1. However, in Example 3, the honeycomb structure  100  was manufacture so that the first direction L 1  (or the second direction L 2 ) of the cell walls  140  in the honeycomb unit  120  had the minimum angle of 22.5 degrees relative to the extending directions of the adhesive layers  180 . 
     Comparative Example 1 
     A honeycomb structure  100  was manufactured by a method similar to the method of Example 1. However, in Comparison Example 1, the honeycomb structure  100  was manufactured so that the first direction Li (or the second direction L 2 ) of the cell walls  140  in the honeycomb unit  120  had the minimum angle of 10 degrees relative to the extending directions of the adhesive layers  180 . 
     Comparative Example 2 
     A honeycomb structure  100  was manufactured by a method similar to the method of Example 1. However, in Comparison Example 2, the honeycomb structure  100  was manufactured so that the first direction L 1  of the cell walls  140  in the honeycomb unit  120  fitted relative to the extending directions of the adhesive layers  180 . Therefore, the first direction L 1  (or the second direction L 2 ) of the cell walls  140  in the honeycomb unit  120  had the minimum angle of 0 degrees relative to the extending directions of the adhesive layers  180 . 
     [Measurement of Isostatic Strength] 
     Isostatic strength of the honeycomb structures  100  of Example 1 through Example 3, and Comparative Examples 1 and 2 were measured. The isostatic strength is a compression failure load when a honeycomb structure is broken by isotropically applying a hydrostatic load to the honeycomb structure. Society of Automotive Engineers of Japan stipulates the isotropic strength in JASO M505-87. The entire contents of JASO M505-87 are hereby incorporated by reference. 
     The isostatic strength was measured under the following conditions. First, a metal plate (aluminum plate of 15 mm thickness) was disposed to both end faces of the honeycomb structure. Next, the honeycomb structure and the metal plates were wrapped by a urethane rubber sheet (2 mm thickness) and sealed. Then the sealed honeycomb structure was completely soaked in water, water pressure was increased, and the water pressure was measured when the honeycomb structure was broken. 
     The measurement results of each honeycomb structure  100  are shown in Table 1. In addition, in  FIG. 9 , a relationship between the isostatic strength and the minimum angle is shown in the honeycomb structures  100  of Example 1 through Example 3 and Comparative Examples 1 and 2.  FIG. 9  is a graph showing the relationship between the isostatic strength and the minimum angle in the honeycomb structures  100  of Example 1 through Example 3 and Comparative Examples 1 and 2. The minimum angle is the angle between the cell walls  140  and the adhesive layers  180  (the angle between the first direction L 1  of the cell walls  140  in the honeycomb structure  100  and the extending directions of the adhesive layers  180 ). 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Minimum Angle between First 
                   
               
               
                   
                 Direction of Cell Walls and 
                 Isostatic 
               
               
                   
                 Extending Directions of Adhesive 
                 Strength 
               
               
                   
                 layers (degrees) 
                 (MPa) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 45 
                 3.0 
               
               
                 Example 2 
                 40 
                 3.0 
               
               
                 Example 3 
                 22.5 
                 2.4 
               
               
                 Comparative Example 1 
                 10 
                 1.6 
               
               
                 Comparative Example 2 
                 0.0 
                 1.5 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1 and  FIG. 9 , when the minimum angle between the first direction L 1  of the cell walls  140  and the extending directions D 1  and D 2  of the adhesive layers  180  is approximately 22.5 to approximately 45 degrees, the isostatic strength of the honeycomb structures in Example 1 through Example 3 is significantly greater than that of the honeycomb structures in Comparative Examples 1 and 2. 
     Further, the present invention is not limited to the embodiment, but variations and modifications may be made without departing from the scope of the present invention.