BEST MODE FOR CARRYING OUT THE INVENTION

[0053] As shown in FIGS. 1 ( a ) and 1 ( b ), the honeycomb structural body 10 for exhaust gas purification according to the present invention is a honeycomb structural body 10 for exhaust gas purification, which comprises a plurality of cell partition walls (ribs) 2 forming a group of cells 1 adjacent to each other and a honeycomb outer wall 3 surrounding and holding the group of cells 1 and wherein an exhaust gas flowing through the cells 1 is purified by a catalyst layer (not shown) to be loaded on the cell partition walls 2 or by a catalyst (not shown) to be contained in the cell partition walls 2 , characterized in that the cell partition walls 2 and the honeycomb outer wall 3 satisfy a relation shown by the expression formula (1), with respect to the material properties and the cell structure:

σ/ E≧ 0.0161·α·( GSA )/{ H D ·(ρ c ·C·λ c ) 0.5 } (1)

[0054] {{in the expression (1), σ [MPa] is a material strength (which means a bending strength of one rib and specifically means a material strength as measured by four-point bending according to JIS R 1601(except for the height of beam), or a material strength when the test result by another method has been reduced to the present method based on effective volume); E [MPa] is a material Young's modulus (one-rib bending); α [1/K] is a thermal expansion coefficient of honeycomb in a direction perpendicular to the direction of gas flow; GSA [m 2 /m 3 ] is a geographical surface area per volume of honeycomb structure; H D [m] is a hydraulic diameter of the cell of honeycomb structure; ρ c [kg/m 3 ] is a bulk density of honeycomb structure; C [J/kgK] is a material specific heat; and λ c [W/mK] is a thermal conductivity of the cell of honeycomb structure which is λ·b/p (λ is a material thermal conductivity [W/mK], b is a rib thickness [m], and p is a cell pitch (an interval between ribs) [m])}.

[0055] Specific explanation is made below on the above expression (1).

[0056] Inside-honeycomb temperature gradient which causes a thermal stress therein, appears owing to a phenomenon that the amount of heat conduction between gas and honeycomb differs depending upon the position of honeycomb during heating and cooling honeycomb by gas. When the inside-solid heat conduction is sufficient, the degree of the temperature gradient is alleviated by the inside-solid heat flow from the portion of large heat acceptance to the portion of small heat acceptance or from the portion of small heat loss to the portion of large heat loss. The degree of temperature gradient appearing owing to such transitional and local heat conduction to solid from outside is known to be theoretically proportional to the following expression (6):

Bi·F 0 1/2 (6)

[0057] {in the expression (6), Bi (Biot number) and F o (Fourier number) are shown by the following expressions (7) and (8), respectively:

Bi= ( h· 1)/λ (7),

F 0 =(λ·t 0 )/(ρ· c·l 2 ) (8)

[0058] [in the expression (7) and the expression (8), h is a heat conduction coefficient (between solid and gas); l is a representative length; λ is a heat conductivity (solid); ρ is a density (solid); c is a heat capacity per unit volume (solid); and t o is a representative time].

[0059] The degree of temperature gradient is proportional to the product of a representative temperature difference between gas and solid ΔT and the above expression (6). By substituting the above expressions (7) and (8) for the product, the following expression (9) is obtained:

Δ T·Bi·F 0 1/2 =ΔT·h·t 0 1/2 /(ρ· c·λ ) 1/2 (9)

[0060] With respect to the laminar flow heat conduction within through channels, the following expression (10) holds:

h=Nu·λ g /H D (10)

[0061] [in the expression (10), h is a heat conduction coefficient (between cell partition wall and incoming gas); Nu (Nusselt number) is 3.77; H D is a hydraulic diameter of passage; and λ g is a heat conductivity of gas].

[0062] By substituting the expression (10), the expression (9) is rewritten into the following expression (11)

Δ T·t 0 1/2 ·Nu·λ g /[(ρ· c·λ ) 1/2 ·H D ] (11)

[0063] Here, using a product between the degree of temperature gradient and a heat conduction area GSA·L 3 when a representative length L of local heating and cooling region is assumed, inside-solid temperature difference parameter was expressed by the following expression (12):

(inside-solid temperature difference parameter)= C 1· GSA/[ (ρ· c·λ ) 1/2 ·H D ] (12)

[0064] [in the expression (12), Cl is ΔT·t o 1/2 ·Nu·λ g ·L 3 ].

[0065] Further, thermal stress parameter was defined by the following expression (13) as a product between inside-solid temperature difference parameter, thermal expansion coefficient α and Young's modulus E:

(inside-solid temperature difference parameter)=(inside-solid temperature difference parameter)·α· E (13)

[0066] The thus-introduced thermal stress parameter is functions of material properties and cell structure, and corresponds to a thermal stress assumed to generate when the material properties and the cell structure are employed. When the strength of actually used material is not smaller than the thermal stress parameter, it is considered that no breakage occurs.

[0067] This relation is expressed by the following expression (14):

(strength of material)/ E ≧(thermal stress parameter)/ E (14)

[0068] and this is equivalent to the above expression (1).

[0069] In determining the C1, truly theoretical selection is impossible with respect to the representative time t o and the representative length L. As a result of trial and error based on the comparison with trial production and test result thereof, Δ g =0.061 w/mK and Nu=3.77 were assumed and there were selected L=0.04 m, t o =5 sec and ΔT=500 K to obtain C1=1.61×10 −2 . By employing this C1, it was found that there is a good correlation between whether or not the expression (14), i.e. the expression (1) is satisfied and whether or not thermal stress breakage occurs, in wide ranges of the use conditions, material and structure considered by the present invention.

[0070] The honeycomb structural body for exhaust gas purification according to the present invention is also a honeycomb structural body for exhaust gas purification, which comprises a plurality of cell partition walls (ribs) forming a group of cells adjacent to each other and a honeycomb outer wall surrounding and holding the group of cells and wherein an exhaust gas flowing through the cells is purified by a catalyst layer to be loaded on the cell partition walls or by a catalyst to be contained in the cell partition walls, characterized in that the honeycomb structural body is provided with a thermal stress-relieving means for relieving a thermal stress applied to the cell partition walls and the honeycomb outer wall in exhaust gas purification. (Hereinafter, this honeycomb structural body may be referred to as “the second aspect of the present invention”.)

[0071] Hereinafter, explanation is made on specific examples of the thermal stress-relieving means used in the second aspect of the present invention.

[0072] As the first example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification which is the second aspect of the present invention, there can be mentioned at least one slit 4 which is formed from the surface of the honeycomb outer wall 3 toward the central axis (not shown) of honeycomb structural body and at least part of which opens at the surface of the honeycomb outer wall 3 , as shown in FIGS. 2 ( a ) to 2 ( d ).

[0073] Separately from the formation in the flow direction of exhaust gas (the central axis direction) as in FIGS. 2 ( a ) to 2 ( d ), each slit may also be formed in a direction perpendicular to the flow direction of exhaust gas (the central axis direction), which is not shown. Further, formation in the above two directions is possible as long as the honeycomb structural body has reasonable strength.

[0074] In the honeycomb structural body 10 for exhaust gas purification having the above-mentioned thermal stress-relieving means, it is preferred that each slit 4 is formed at least at one end face 5 at least at the edge portion 6 thereof.

[0075] In this case, in a direction parallel to the flow direction of exhaust gas [the central axis direction, that is, an X direction in FIG. 2 ( a )], the length of the opening portion at the surface of the honeycomb outer wall 3 of each slit 4 formed at the edge portion 6 of end face, is preferably 10% or more of the total length of the honeycomb structural body 10 for exhaust gas purification; and the length of the opening portion at the end face 5 of the slit is preferably 10% or more of the diameter of the honeycomb structural body 10 for exhaust gas purification.

[0076] When the honeycomb structural body 10 for exhaust gas purification is used in such an environment that non-uniformity of temperature extends in the whole portion (total length) of the honeycomb structural body 10 for exhaust gas purification, the slit 4 is preferably formed so as to open over the total length of the surface of the honeycomb outer wall 3 in the flow direction of exhaust gas (the central axis direction, i.e. the X direction).

[0077] In the honeycomb structural body 10 for exhaust gas purification shown in FIG. 2 ( a ), four slits 4 are formed at the edge portion 6 of one end face 5 each with the depth being changed in connection with the position in diameter direction so as to have a triangular shape; in the honeycomb structural body 10 for exhaust gas purification shown in FIG. 2 ( b ), four slits 4 are formed with the depth being unchanged so as to have a rectangular shape; in the honeycomb structural body 10 for exhaust gas purification shown in FIG. 2 ( c ), four slits 4 are formed in such a manner that each slit 4 opens over the total length of the surface of honeycomb outer wall 3 along the flow direction of exhaust gas (the central axis direction, i.e. the X direction), the slits 4 having the depth being changed in connection with the position in diameter direction so as to have a triangular shape; and in the honeycomb structural body 10 for exhaust gas purification shown in FIG. 2 ( d ), four slits 4 are formed in such a manner that each slit 4 opens over the total length of the surface of honeycomb outer wall 3 along the flow direction of exhaust gas (the central axis direction, i.e. the X direction) with the depth being unchanged so as to have a rectangular shape.

[0078] By forming the slit 4 as shown in FIGS. 2 ( a ) to 2 ( d ), each individual portion of the honeycomb structural body 10 for exhaust gas purification can make free deformation without restraint by other portions, even when non-uniform temperature distribution such as local higher or lower temperature appears therein; the thermal stress can be reduced; and the generation of cracks caused by thermal shock can be minimized.

[0079] In FIG. 3 ( a ) as in FIG. 2 ( c ), three slits 4 are formed in such a manner that each slit 4 opens over the total length of the surface of honeycomb outer wall 3 along the flow direction of exhaust gas (the central axis direction, i.e. the X direction), the slits 4 having the depth being changed in connection with the position in diameter direction so as to have a triangular shape. In FIG. 3 ( b ) as in FIG. 2 ( d ), three slits 4 are formed in such a manner that each slit 4 opens over the total length of the surface of honeycomb outer wall 3 along the flow direction of exhaust gas. (the central axis direction, i.e. the X direction) with the depth being unchanged so as to have a rectangular shape Such formation of slits is particularly effective when the resulting honeycomb structural body is used in such an environment that non-uniformity of temperature extends in the whole portion (total length) of the honeycomb structural body.

[0080] FIG. 4 ( a ) is a case wherein slits 4 have been formed at one end face 5 a of the surface of honeycomb outer wall 3 so that two points A and B of the edge portion 6 a of the end face are continuously connected to each other and two points C and D of the edge portion 6 a are continuously connected to each other. FIG. 4 ( b ) is a case wherein slits 4 have been formed at two end faces 5 b and 5 c of the surface of honeycomb outer wall 3 so that, for example, two points A and B of the edge portions 6 b and 6 c are continuously connected to each other and two points C and D of the edge portions 6 b and 6 c are continuously connected to each other.

[0081] By employing such a constitution, the freedom of deformation in the vicinity of the end face 5 of the honeycomb structural body 10 for exhaust gas purification increases further. The thermal stress can be reduced. The crack formations caused by thermal shock can be prevented effectively. In this case, the length of the opening portion at the surface of the honeycomb outer wall 3 of each slit 4 , in a direction parallel to the flow direction of exhaust gas [the central axis direction, that is, an X direction in FIG. 2 ( a )], is preferably 10% or more of the total length of the honeycomb structural body 10 for exhaust gas purification; and the length of the opening portion at the end face 5 of the slit is preferably 10% or more of the diameter of the honeycomb structural body 10 for exhaust gas purification.

[0082] As shown in FIGS. 5 ( a ) to 5 ( d ), it is possible to form, at the intersection of slits 4 , a portion (a connecting portion) 7 wherein no slit 4 is formed, so that the connecting portion 7 is located at the center of the honeycomb structural body 10 for exhaust gas purification and does not open at any of the surface, upper end face 5 d and lower end face 5 e of the honeycomb outer wall 3 .

[0083] By employing this constitution, generation of cracks, etc. caused by thermal shock can be effectively prevented even when the honeycomb structural body 10 for exhaust gas purification is used in such an environment that extremely large non-uniformity of temperature extends in the whole portion (total length) of the honeycomb structural body 10 .

[0084] FIG. 5 ( a ) is a case wherein a sectional shape of the connecting portion 7 at a cross section of a slit-including plane, is rectangular; FIG. 5 ( b ) is a case wherein the sectional shape is a circular; FIG. 5 ( c ) is a case wherein the sectional shape is race-track-like; and FIG. 5 ( d ) is a case wherein the sectional shape is rhombic. By employing such constitution, generation of cracks, etc. caused by thermal shock can be effectively prevented even when non-uniformity of temperature is large (for example, higher temperatures or lower temperatures are scattered locally) and this non-uniformity is distributed in the whole portion of honeycomb structural body.

[0085] Meanwhile, FIGS. 6 ( a ) to 6 ( d ) are each a case wherein part of the connecting portion 7 opens at the lower end face 5 f of the surface of the honeycomb outer wall 3 .

[0086] FIG. 7 and FIG. 8 are each other case wherein the connecting portion 7 is formed so as not to open at the surface of the honeycomb outer wall 3 .

[0087] FIG. 7 , similarly to FIG. 5 ( a ), is a case wherein the sectional shape of the connecting portion 7 at a cross section of a slit-including plane, is a rectangle. In this case, the number of slits 4 is larger than that in FIG. 5 ( a ).

[0088] FIG. 8 is case wherein the sectional shape of the connecting portion 7 at a cross section of a slit 4 -including plane, is a circle or an oval.

[0089] In the slits 4 , filler is preferred to be filled. As such filler, there can be mentioned, for example, a ceramic fiber, a ceramic powder and a cement, all having heat resistance. These materials can be used singly or in combination of two or more kinds. As necessary, it is possible to mix an organic binder, an inorganic binder, etc.

[0090] In the honeycomb structural body according to the second aspect of the present invention, and in such a cell section that the length of slit in diameter direction is largest at the cross section perpendicular to the flow direction of gas (the central axis direction), the length of slit in diameter direction is preferably 10% or more, more preferably 30% or more of the distance from honeycomb outer wall to central axis (that is radius).

[0091] Also in the honeycomb structural body according to the second aspect of the present invention, slits are provided preferably in point symmetry in a honeycomb section perpendicular to the flow direction of exhaust gas (the central axis direction, i.e. the X direction) because it hardly gives deflection to the deformation of the whole structure; however, provision of slits is not restricted thereto. For example, slits 4 may be provided as shown in FIGS. 9 ( a ) to 9 ( d ).

[0092] Slits 4 may be formed so as to cut cell partition walls 2 obliquely as shown in FIG. 10 ( b ). However, formation of slits 4 in parallel to cell partition walls 2 as shown in FIG. 10 ( a ) is more preferred because the stress concentration at the front end of slit 4 is small.

[0093] When the cells 1 of the honeycomb structural body 10 have a triangular shape, slits 4 are formed more preferably in a 60° or 120° direction for the same reason as above.

[0094] With respect to the width of slit 4 , there is no particular restriction. However, too large a width invites increases in the number of steps for filling of filler and the amount of filler used and a decrease in the number of cells usable for purification of fluid (e.g. gas); therefore, the width of slit 4 is preferably smaller than the width of one cell.

[0095] Further, it is more preferred to form, at the front end of slit 4 , a branched portion 4 a obtained by branching the slit 4 [see FIG. 11 ( b )] or a stress-relieving portion 4 b having a radius of curvature [see FIG. 11 ( a )], from the standpoint of relief of thermal stress.

[0096] With respect to the form of slit 4 , it may be a slit obtained by partially cutting the cell partition walls 2 of honeycomb structural body 10 , as shown in FIG. 12 ( a ), or a slit obtained by partially removing the cell partition walls 2 , as shown in FIG. 12 ( b ).

[0097] As the second example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned such that the group of cells is divided into two or more first honeycomb segments at a plane parallel to the central axis of the group of cells (this division may be conducted by cutting after honeycomb production, or each segment itself may be produced in a shape same as that of divided segment), that the first segments are bonded to each other as necessary by a bonding layer 14 , and that an aspect ratio [(L1)/(P1)] of each first honeycomb segment 13 between its length (L1) in the flow direction of exhaust gas (the central axis direction) and the diameter (one side) (P1) at the end face of the group of cells [the major diameter (major side) in the case of a sectional shape having deflection] satisfies a relation shown by the following expression (2). Here, the aspect ratio [(L1)/(P1)] of first honeycomb segment 13 between the length (L1) in the flow direction of exhaust gas (the central axis direction) and the diameter (one side) (P1), shown in FIG. 14 is preferred to satisfy a relation shown by the following expression (2):

2≦[( L 1)/( P 1)]≦10 (2)

[0098] The aspect ratio [(L1)/(P1)] is preferably 10 or less from the standpoint of the strength and thermal shock resistance of segment per se. Meanwhile, when the aspect ratio is less than 2, the aspect ratio of the assembly when segments have been assembled is significantly deflected in the diameter direction; therefore, the aspect ratio of segment is preferred to be in the above range. A range of 3≦[(L1)/(P1)]≦6 is more preferred. When there are first honeycomb segments of different shapes in the assembled honeycomb structural body, it is most preferred that all the segments satisfy the above expression (2); and it is necessary that at least the first honeycomb segments around the central axis (where thermal shock is largest) (they contain the central axis or are in contact with the central axis) satisfy the expression (2). The number of segments assembled is preferably 24 or less and, in view of the strength of the total assembly and the cost of production, more preferably 16 or less.

[0099] Hereinafter, more specific explanation is made on bonding of honeycomb segments.

[0100] As shown in FIGS. 13 ( a ) to 13 ( d ), the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention is preferably divided into first honeycomb segments 13 in various division patterns.

[0101] When these honeycomb segments are bonded to each other, the Young's modulus of bonding layer 14 is made preferably 20% or less, more preferably 1% or less of the Young's modulus of the first honeycomb segment 13 . Also, the material strength of bonding layer 14 is made preferably smaller than that of first honeycomb segment 13 . By thus specifying the Young's moduli of bonding layer 14 and first honeycomb segment 13 , it is possible to make small the thermal stress generated during use, effectively prevent the generation of cracks caused by thermal shock and obtain a structural body of superior durability. Further, even when the Young's modulus of bonding layer 14 is more than 20% of the Young's modulus of the first honeycomb segment 13 and yet when the material strength of bonding layer 14 is smaller than the material strength of the first honeycomb segment 13 , cracks appear only in the bonding layer 14 and each first honeycomb segment 13 undergoes no damage.

[0102] Here, the Young's modulus of bonding layer 14 and the Young's modulus of the first honeycomb segment 13 mean the Young's moduli of respective materials per se and indicate the properties inherently possessed by the materials.

[0103] The meaning of the expression “the material strength of bonding layer is smaller than the material strength of the first honeycomb segment” is explained with reference to FIG. 15 and FIG. 16 .

[0104] That is, a test piece 20 such as shown in FIG. 15 is prepared by cutting out from a honeycomb structural body for exhaust gas purification according to the present invention. Incidentally, the test piece 20 is cut so that it has a diameter direction length of 40 mm or more and a bonding layer 14 is located at the center.

[0105] In the present invention, when the test piece 20 is subjected to a four-point bending test (according to JIS R 1601) shown in FIG. 16 and the probability of breakage occurring inside bonding layer 14 or at interface between bonding layer 14 and the first honeycomb segment 13 is 50% or more, it is expressed as “the material strength of bonding layer is smaller than the material strength of the first honeycomb segment”.

[0106] The average surface roughness (Ra) of at least 30% of the surface portion of the first honeycomb segment 13 contacting the bonding layer 14 is preferably more than 0.4 μm, more preferably 0.8 μm or more. By employing such constitution, the bonding between two or more first honeycomb segments 13 is strong and the peeling therebetween during use can be prevented effectively. Further, even when no bonding is made between segments, slippage between them can be prevented.

[0107] The proportion of the total heat capacity of all the bonding layers 14 in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, to the total heat capacity of all the first honeycomb segments 13 constituting the honeycomb structural body 10 for exhaust gas purification is preferably 30% or less, more preferably 15% or less.

[0108] By employing such constitution, it is possible to make short the time needed for temperature elevation, in an allowable range and activate the catalyst component at an early timing.

[0109] In the cross section of the honeycomb segment 13 parallel to its diameter direction, the sectional corner is preferred to be rounded in a radius of curvature of 0.2 mm or more or chamfered by 0.3 mm or more, because it can reduce generation of thermal stress during use, can prevent generation of cracks, and can impart durability.

[0110] In the cross section of the honeycomb structural body for exhaust gas purification parallel to its diameter direction, the proportion of (S s /S h ) of the total sectional area (S s ) of the bonding layers 14 in the sectional area (S h ) of the honeycomb structural body for exhaust gas purification is preferably 17% or less, more preferably 8% or less from the standpoint of reduction in pressure loss of fluid.

[0111] Also in the second aspect of the present invention, in the cross section of the honeycomb structural body 10 for exhaust gas purification parallel to its diameter direction, the proportion (S s /S c ) of the total sectional area (S s ) of the bonding layers to the total sectional area (S c ) of the partition walls of the cells group is preferably 50% or less, more preferably 24% or less from the standpoint of reduction in pressure loss of fluid.

[0112] Further in the cross section of the honeycomb structural body for exhaust gas purification parallel to its diameter direction, it is preferred that the proportion of the sectional area of the bonding layers to the sectional area of the cells group is large at the central portion and small at the honeycomb outer wall side. By employing such constitution, the flow of exhaust gas concentrating in the central portion can be appropriately dispersed toward the vicinity of the outer wall. Consequently, the temperature difference between the central portion and the honeycomb outer wall side can be reduced, and the thermal stress in the honeycomb structural body for exhaust gas purification can be reduced.

[0113] The shape of the cross section of the honeycomb structural body for exhaust gas purification at a plane perpendicular to the flow direction of exhaust gas, i.e. the sectional shape of honeycomb outer wall may be any of a circle, an oval, a race track shape, etc.

[0114] Here, as the material of the bonding layer used for bonding between the first honeycomb segments, there can be mentioned, for example, a ceramic fiber, a ceramic powder and a cement, all having heat resistance. These may be used singly or in combination of two or more kinds. It is possible to mix an organic binder, an inorganic binder, etc. as necessary.

[0115] When the honeycomb segments have a sufficiently high strength, bonding between them can be omitted, for example, by assembling them as necessary via a ceramic fiber, a ceramic powder, a mat or the like and pressing them from around for canning or providing a stopper at least at an exhaust gas outlet side for clamping.

[0116] As the third example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned such as shown in FIG. 17 wherein a group of cells 1 is divided into two or more second honeycomb segments 15 , that is, into multiple portions [in FIG. 17 , there is shown a case of three portions having lengths L2, L2′ and L2″ of exhaust gas flow direction (central axis direction)] at a plane perpendicular to the central axis of the cells 1 group and wherein an aspect ratio [(P2)/(L2)] of each second honeycomb segment 15 between its diameter (one side) [major diameter (major side) in case of sectional shape having deflection] (P2) and its length (L2) in exhaust gas flow direction (central axis direction) satisfies a relation shown by the following expression (3):

0.5≦[( P 2)/( L 2)]≦5 (3)

[0117] Incidentally, the same applies also when the L2 of second honeycomb segment 15 is L2′ or L2″.

[0118] When at least one of the multiple second honeycomb segments satisfies the above expression (3), the effects of the present invention can be obtained. However, it is preferred that at least the second honeycomb segment provided most upstream and undergoing the largest thermal shock satisfies the expression (3), and it is most preferred that all of the second honeycomb segments satisfy the expression (3).

[0119] The aspect ratio [(P2)/(L2)] is preferably 5 or less from the standpoint of the strength and thermal shock resistance of the second honeycomb segments 15 per se. However, when the aspect ratio is less than 0.5 and when the second honeycomb segments 15 have been assembled, the aspect ratio of the resulting assembly is deflected significantly toward the flow direction of exhaust gas, resulting in increased pressure loss. Therefore, the above range is preferred and a range of 1.0≦[(P2)/(L2)]≦3 is more preferred. As to the number of second honeycomb segments, 5 or less is preferred and 3 or less is more preferred, from the same reason of pressure loss. By employing such constitution, a reduction in thermal stress is possible; the divided second honeycomb segments 15 can be provided in any of various modes (they can be provided in contact with each other at their end faces, or at a certain distance apart from each other, or in respective cans); and bonding between segments may not be made.

[0120] As the fourth example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned at least one notch 16 provided in the cell partition walls 2 forming a group of cells 1 , in the flow direction of exhaust gas (the central axis direction of the cells group), as shown in FIG. 18 .

[0121] Examples of provision of the notch 16 are shown in FIG. 18 to FIG. 23 .

[0122] Each notch 16 in the honeycomb structural body for exhaust gas purification according to the second aspect of the present invention is different from the above-mentioned slits open to outside, formed by cutting the honeycomb structural body from the honeycomb outer wall 3 toward the honeycomb diameter, and is provided substantially uniformly by cutting out predetermined portions of cells in the flow direction of exhaust gas (the central axis direction of honeycomb structural body).

[0123] Notches 16 may be provided separately from each other or continuously over plurality of cells, when a section of honeycomb structural body is viewed. The notches 16 relieve thermal stress. Unlike the above-mentioned slits, the notches 16 need not open at the honeycomb outer wall. Preferably, the notches 16 are basically provided so as to avoid continuity in one direction. However, when they are provided continuously in one direction, the number of continuity is preferably 10 cells or less. When they are formed in continuity of more than 10 cells in one direction, the strength of the whole honeycomb structural body may be significantly low. Also, when they are provided selectively in one direction, there may be deflection in the direction of release of thermal stress.

[0124] From the standpoint of strength, the number of notches 16 is preferably controlled at 40% or less of the number of total cell partition walls (the cell partition wall extending from one intersection to next intersection is counted as one cell partition wall). As to the depth of notch 16 in the flow direction of exhaust gas, there is basically no problem as long as the notches 16 are discontinuous at least at a certain section. However, it is preferred that the notches 16 open at the inlet side of honeycomb structural body in the flow direction of exhaust gas (the central axis direction of honeycomb structural body), which is high in thermal shock during actual use. When thermal shock resistance is required in the total length of cells in the flow direction of exhaust gas (the central axis direction), the notches 16 are preferably provided in the total length of cells in the flow direction of exhaust gas, as shown in FIG. 20 .

[0125] The width of notch 16 is preferably not smaller than 10 μmm and not larger than the width of one cell, independently of the thickness of cell partition wall and the cell pitch (distance between ribs). When the width of notch 16 is less than 10 μmm, the effect of thermal stress relief may be insufficient; when the width is more than the width of one cell, the whole honeycomb structural body may be significantly low in strength.

[0126] The density of notches 16 may be changed in the diameter direction or in the flow direction of exhaust gas. Change of the density may be conducted for a single honeycomb structural body or for the above-mentioned division type. As a preferred example of the change of notch density, there can be mentioned concentrated formation of notches 16 in the central portion of diameter direction or inlet side of exhaust gas flow direction, where thermal shock is large during actual use.

[0127] As the fifth example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned one formed by allowing each cell of the group of cells to have a sectional shape of tree or more cornered polygon.

[0128] A polygonal sectional shape having a large number of corners is preferred because it can reduce thermal stress. Specifically, an at least four cornered sectional shape is preferred, and a hexagonal sectional shape is more preferred. Of tetragons, rectangle is preferred to square for the same reason. Also, it is possible to change the cell shape in the diameter direction or in the flow direction of exhaust gas (change in the latter direction is possible only in the multiple portions type). Change in the diameter direction may be conducted for a single honeycomb structural body or for the above-mentioned division type. Change is preferably conducted, for example, by allowing the cell sectional shape to have a polygonal shape concentratedly in the central portion of diameter direction or inlet side of exhaust gas flow direction, where thermal shock is large during actual use.

[0129] In the second aspect of the present invention, it is preferred to change the thickness of cell partition wall in the diameter direction and/or the flow direction of exhaust gas (change in the latter direction is possible only in the multiple portions type), in view of the distribution of level of thermal shock in actual use. Change in the diameter direction may be conducted for a single honeycomb structural body or for the above-mentioned division type. Here, the change of the thickness of cell partition wall is generally conducted by allowing the central portion in the diameter direction or the vicinity of inlet in the flow direction of exhaust gas (at these portions, temperature elevation and cooling speeds are large) to have a larger thickness, because this is effective in prevention of crack generation caused by thermal shock.

[0130] Specifically, as the sixth example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned such that the partition wall thickness (T 10 ) of the cells present in a portion extending from the central axis up to at least 10% of the radius (half of one side) satisfies a relation shown by the following expression (4) in relation to the basic cell partition wall thickness (T c ):

1.2 ≦T 10 /T c (4)

[0131] Also, increase of thickness of cell partition wall in the central portion in diameter direction or the vicinity of inlet in exhaust gas flow direction (central axis direction) not only reduces the temperature elevation and cooling speeds of the above portions but also makes small the temperature difference between these portions and the outer peripheral portion/the vicinity of outlet, which relieves thermal shock doubly.

[0132] As shown in FIG. 24 , when, in the honeycomb structural body for exhaust gas purification according to the second aspect of the present invention, there coexist portions different in the thickness of cell partition wall 2 , in a cross section perpendicular to the central axis, it is preferred to change the thickness of cell partition wall 2 at the boundary between the portions, as shown in FIG. 24 , so that the cell partition wall 2 has an inverse trapezoidal sectional shape [ FIG. 24 ( a )], a spool-like sectional shape [ FIG. 24 ( b )] or a rectangular sectional shape [ FIG. 24 ( c )] and the rib thickness becomes gradually smaller from the thick-rib portion toward the thin-rib portion. By employing such constitution, improvements in pressure loss and ratio in thermal shock resistance can be achieved.

[0133] As the seventh example of the thermal stress-relieving means used in the honeycomb structural body 10 for exhaust gas purification according to the second aspect of the present invention, there can be mentioned one shown in FIG. 25 , wherein an aspect ratio [(L3)/(P3)] between the total length (L3) of the cells group in the flow direction of exhaust gas (the axial direction of honeycomb structural body) and its diameter (one side) [major diameter (major side) in case of sectional shape having deflection] (P3) satisfies a relation shown by the following expression (5). By employing such constitution, improvements in strength and thermal shock resistance can be obtained. In FIG. 25 , there is shown a case wherein a structural body is divided into honeycomb segments 13 and has bonding layers 14 .

0.5≦[( L 3)/( P 3)]≦2 (5)

[0134] The honeycomb structural body for exhaust gas purification according to the second aspect of the present invention preferably has a weight of 1,500 g or less and a volume of 1,500 cm 3 or less.

[0135] The weight of one honeycomb structural body (in the case of single body, its weight and, in the case of division type, the weight of one segment) differs depending upon the material (thermal expansion coefficient and specific gravity) and the porosity; however, it is preferably at least 1,500 g or less from the standpoint of thermal shock resistance. When the weight is more than 1,500 g, there may appear damages such as cracks and the like in actual use even under relatively mild thermal shock of ordinary driving mode. The weight is more preferably 1,200 g or less, and a weight of 1,000 g or less is particularly preferred because it can withstand even a severe thermal shock caused by sharp temperature change.

[0136] The volume of one honeycomb structural body (in the case of single body, its volume and, in the case of division type, the weight of one segment) is preferably at least 1,500 cm 3 or less from the standpoint of thermal shock resistance. When the volume is more than 1,500 cm 3 , there may appear damage in actual use even under relatively mild thermal shock of ordinary driving mode. The volume is more preferably 1,000 cm 3 or less, and a volume of 800 cm 3 or less is particularly preferred because it can withstand even a severe thermal shock caused by sharp temperature change.

[0137] In the second aspect of the present invention, a honeycomb structural body for exhaust gas purification having a higher effect of thermal stress relief can be obtained by appropriately combining the above-mentioned various means.

[0138] The main constituent material of the cell partition walls of the honeycomb structural bodies according to the first aspect of the present invention and the second aspect of the present invention preferably contains, as the main component, a material which is superior in alkali resistance when the honeycomb structural body is used as a carrier for NO x occlusion catalyst and which further has such a strength and thermal resistance as can be applicable to the exhaust gas of automobile. Specifically, the main constituent material preferably contains at least one kind selected from the group consisting of alumina (of various kinds of alumina, α-alumina is preferred because it has the highest alkali resistance), zirconia, titania, zeolite, SiC, SiN, mullite, lithium aluminum silicate (LAS), titanium phosphate, perovskite, spinel, chamotte, non-oriented cordierite (this means a cordierite wherein the crystals are not oriented; this non-oriented cordierite, because the crystals are not oriented, has a high thermal expansion coefficient as compared with oriented cordierite widely used as a carrier for catalyst for automobile exhaust gas) and mixtures or composites thereof. In particular, alumina, SiC, SiN, mullite, non-oriented cordierite, etc. are suitably used for the alkali resistance; among them, oxides are preferred one in view of the cost. While the cell partition walls are preferred to contain these materials as the main constituent material, the honeycomb outer wall as well is preferred to be constituted by the same material as for the cell partition walls.

[0139] The honeycomb structural body of the present invention exhibits its effects effectively when made of a material showing a thermal expansion coefficient of 1.0×10 −6 /° C. or more in a direction perpendicular to the flow direction of exhaust gas when used in an automobile exhaust gas (in this case, the material needs to have a high thermal expansivity). Particularly when the honeycomb structural body is made of a material having a high thermal expansion coefficient of 3.0×10 −6 /° C. or more and is mounted right downstream of manifold (where the temperature change of exhaust gas is large), the present invention is essential; when the honeycomb structural body is made of a material having a thermal expansion coefficient of 5.0×10 −6 /° C. or more, the present invention is necessary even when the honeycomb structural body is mounted below the floor of automobile (where the temperature change of exhaust gas is relatively small). Meanwhile, the present invention is applicable even to a material of having a small thermal expansion coefficient of less than 1.0×10 −6 /° C.; however, the obtained improvement in thermal shock resistance is small because of the small thermal expansivity (high in thermal shock resistance) of the material.

[0140] With respect to the sectional shape of the honeycomb outer wall of the honeycomb structural bodies according to the first and second aspects of the present invention, there is no particular restriction as long as the sectional shape fits the internal shape of the exhaust gas system in which the honeycomb structural body is mounted. As the sectional shape, there can be mentioned, for example, a circle, an oval, an ellipse, a trapezoid, a triangle, a tetragon, a hexagon and a special shape wherein the left and right are unsymmetrical. Of these, a circle, an oval and an ellipse are preferred.

[0141] With respect to the cell structure of the honeycomb structures according to the first and second aspect of the present invention, the cell density is ordinarily 6 to 1,500 cpsi (cell number per square inch), preferably 300 to 1,200 cpsi, more preferably 400 to 900 cpsi. A cell density of more than 1,200 cpsi may result in strikingly high pressure loss when such a honeycomb structure is used for automobile exhaust gas. Meanwhile, with a cell density of less than 300 cpsi, a high GSA is unable to secure in a limited space for mounting of honeycomb structural body, which may result in shortage in efficiency of contact with exhaust gas.

[0142] The thickness of partition wall is ordinarily 20 to 2,000 μm, preferably 2 to 10 mil (mil is 1/1,000 inch), more preferably 2.5 to 8 mil. A partition wall thickness of more than 10 mil may result in striking pressure loss and striking reduction in warm-up property when such a honeycomb structural body as to the first or second aspect of the present invention is used for automobile exhaust gas. Meanwhile, a partition wall thickness of less than 2 mil may result in shortage in strength. A partition wall thickness of less than 20 μm may result in significant shortage in strength and consequent reduction in thermal shock resistance.

[0143] The present invention may be a combination of the first aspect of the present invention and the second aspect of the present invention. That is, the present invention may be a honeycomb structural body obtained by allowing the honeycomb structural body according to the first aspect of the present invention to have the above-mentioned thermal stress-relieving means (one of the first to seventh examples). By employing such constitution, there can be exhibited, in combination, the effect of the first aspect of the present invention and the effect of the second aspect of the present invention (these two aspects may hereinafter be referred simply to as the present invention).

[0144] The catalyst body for exhaust gas purification according to the present invention is characterized in that, in the above-mentioned honeycomb structural body for exhaust gas purification, a catalyst layer is loaded on the cell partition walls or a catalyst is contained in the cell partition walls.

[0145] That is, the present catalyst body for exhaust gas purification may be suitably used for, for example, a NOx occlusion catalyst body wherein its catalyst layer or the catalyst contains an alkali metal and/or an alkaline earth metal. It is used particularly suitably for a NOx occlusion catalyst body containing K, Na, Li and Ca in a total amount of 5 g/L (honeycomb volume).

[0146] In this case, as the main constituent material of the cell partition walls of the honeycomb structural body for exhaust gas purification, there can be mentioned, for example, one containing at least one kind selected from the group consisting of alumina, zirconia, titania, zeolite, SiC, SiN, mullite, lithium aluminum silicate (LAS), titanium phosphate, perovskite, spinel, chamotte, non-oriented cordierite and mixtures or composites thereof.

[0147] Of these, alumina, SiC, mullite, non-oriented cordierite, mixtures or composites thereof, etc. are used suitably for their better alkali resistances.

[0148] As an application example of the catalyst body for exhaust gas purification according to the present invention, there can be mentioned, when the catalyst layer or the catalyst contains an alkali metal and/or an alkaline earth metal, a catalyst body for exhaust gas purification which contains, on the cell partition walls and/or in the cell partition walls, a substance (hereinafter may be referred to as anchor substance) capable of reacting with the alkali metal and/or the alkaline earth metal in preference to the reaction of the main constituent material of the cell partition walls with the alkali metal and/or the alkaline earth metal, in order to more reliably suppress the reaction between the carrier and the alkali metal and/or the alkaline earth metal.

[0149] As the anchor substance, there can be mentioned, for example, a substance containing at least one element selected from the group consisting of B, Al, Si, P, S, Cl, Ti, V, Cr, Mn, Ga, Ge, As, Se, Br, Zr, Mo, Sn, Sb, I and W.

[0150] It is preferred that specifically, a substance which is reactive with an alkali metal and/or an alkaline earth metal (each used as a catalyst component) and reacts with them in preference to the reaction of the main constituent material of honeycomb structural body with them, is allowed to coexist beforehand. By employing such a measure, the alkali metal and the alkaline earth metal in catalyst layer react with the anchor substance preferentially even when the catalyst body has been exposed to high temperatures during the use, and the reaction with the honeycomb structural body (carrier) is suppressed; as a result, the deterioration of the carrier can be suppressed more reliably.

[0151] For example, by loading the anchor substance on the carrier by impregnation, coating or the like prior to loading of a catalyst and then loading the catalyst, the anchor substance may be allowed to be present between the carrier and the catalyst layer, which can suppress the reaction between the carrier and the alkali metal and/or alkaline earth metal in catalyst layer most effectively.

[0152] As another example of the honeycomb catalyst body for exhaust gas purification according to the present invention, there can be mentioned a catalyst body for diesel exhaust gas purification wherein the catalyst layer or the catalyst is a SCR catalyst material having functions of the main catalyst and co-catalyst of SCR reaction or a function of the main catalyst or the co-catalyst.

[0153] In this case, as the SCR catalyst material, there can be mentioned, for example, one containing at least one kind selected from the group consisting of noble metals; V, VI, VII and VIII group transition metals; two or