FILTER AND METHOD FOR ITS MANUFACTURING

Abstract

Filter (10) comprising several columnar honeycomb structure segments (100) made of a porous ceramic, wherein the side surfaces of the segments are connected to each other via a connecting material (107), wherein Each of the columnar honeycomb structure segments (100) comprises an outer circumferential side wall (102) and partition walls (112) that divide several cells (108, 110) extending from a first end face (104) to a second end face (106) and in each of the columnar honeycomb structure segments (100) the average porosity of the outer circumferential side wall (102) is smaller than that of the partition walls (112), wherein the outer circumferential side wall (102) of each of the columnar honeycomb structure segments (100) is designed such that the average porosity of an area with a thickness of 50% of the outer circumferential side wall (102) from an innermost circumferential section of the outer circumferential side wall (102) outwards is higher than that of an area with a thickness of 50% of the outer circumferential side wall (102) from an outermost circumferential section of the outer circumferential side wall (102) inwards.

Description

AREA OF INVENTION

[0001] The present invention relates to a filter with a structure in which the side surfaces of columnar honeycomb structure segments made of a porous ceramic are connected to one another via a bonding material. The invention also relates to a method for manufacturing such a filter. BACKGROUND OF THE INVENTION

[0002] Suspended particulate matter (hereinafter referred to as PM) in exhaust gases emitted by internal combustion engines, such as diesel and gasoline engines, contains soot. Soot is harmful to the human body, and its emission is regulated. To comply with emissions regulations, filters, embodied by diesel particulate filters (DPFs) and gasoline particulate filters (GPFs), are now widely used. These filters direct the exhaust gas through air-permeable, small-pore baffles and filter out PM, such as soot.

[0003] A filter with a columnar honeycomb structure is used for PM capture. This structure comprises an outer circumferential sidewall and partitions for dividing multiple cells, extending from a first end face to a second end face. This design allows for high PM capture efficiency while suppressing pressure drop within an acceptable range. The filter can incorporate various types of catalysts, such as an SCR catalyst, to simultaneously provide exhaust gas purification, such as NOx reduction, in addition to PM capture.In a generally used method for carrying a catalyst on a filter with a columnar honeycomb structure, a catalyst slurry is introduced into the cells by a conventional, publicly known suction method or the like, and attached to the surface or pores of the partitions, and then subjected to a high-temperature treatment to burn the catalyst contained in the catalyst slurry onto the partitions.

[0004] Patent literature 1 describes that, for the purpose of creating a honeycomb catalyst body with low pressure loss and excellent exhaust gas purification performance, the cell shape is hexagonal, whereby the thickness of the partitions, the cell spacing, the porosity of the partitions, the amount of catalyst carried and the average pore diameter are controlled.

[0005] Patent literature 2 describes an invention for creating a honeycomb structure that prevents the catalyst slurry from seeping out at the outer surface of the outer perimeter wall, thereby improving the strength of the outer perimeter wall and consequently improving the isostatic strength of the entire structure. According to this invention, a coating layer is provided on the outer surface of the outer perimeter wall of the honeycomb substrate, a portion of which penetrates the pores of the outer perimeter wall. If the thickness of the portion of the coating layer that penetrates the pores of the outer perimeter wall is between 1% and 90% of the thickness of the outer perimeter wall, the pores in the outer perimeter wall are closed.

[0006] According to the invention described in patent literature 2, the following effects can be achieved. Even when a honeycomb substrate with a high porosity of 50% or more is used, the slurry does not seep out to the outer surface of the outer perimeter wall when a catalyst slurry is introduced into the cells, thus ensuring good processability during the step of carrying the catalyst on the partitions of the honeycomb structure. Furthermore, because the cover layer reinforces the outer perimeter wall, the strength of the outer perimeter wall is improved, and during the step of carrying the catalyst on the honeycomb structure, fracture of the outer perimeter wall can be effectively prevented when a section of the outer perimeter wall of the honeycomb structure is clamped (gripped).Furthermore, as a result of improving the strength of the outer circumferential wall, the isostatic strength of the entire honeycomb structure is also improved, effectively preventing breakage during transport or actual use of the honeycomb structure.

[0007] Further prior art is disclosed in patent literature 3 and 4. LIST OF COUNTERPOINTS Patent Literature [Patent Literature 1] Japanese Patent No. JP 5 649 836 B2 [Patent Literature 2] Japanese patent application - No. JP 2016- 055 282 A [Patent Literature 3] US Patent Application US 2005 / 0214503A1 [Patent Literature 4] US Patent Application US 2009 / 0022942A1 SUMMARY OF THE INVENTION

[0008] In some cases, a filter with a columnar honeycomb structure is used by joining the side surfaces of several columnar honeycomb segments in one piece with a bonding material to improve thermal shock resistance. In this case, by providing a coating layer, as described in patent literature 2, on the outer circumferential side wall of a filter formed by joining several columnar honeycomb segments, it can be expected that the effect described in patent literature 2, i.e., the effect of improving processability when the catalyst slurry is introduced into the cells and the effect of preventing breakage when the honeycomb structure is clamped, can be obtained.

[0009] On the other hand, both patent literature 1 and patent literature 2 lack an investigation into improving the catalytic performance in cases where the side surfaces of several columnar honeycomb structure segments are connected to each other with a bonding material and are used as a single unit.

[0010] In view of the above circumstances, in one embodiment it is an object of the present invention to provide a filter in which the side surfaces of several columnar honeycomb structure segments made of a porous ceramic are connected to one another via a bonding material, and which can contribute to improving the catalytic performance. In a further embodiment it is also an object of the present invention to provide a method for manufacturing such a filter.

[0011] In a filter where the side surfaces of several columnar honeycomb structure segments made of a porous ceramic are connected to each other via a bonding material, the side surfaces correspond to the outer surface of the outer circumferential side wall of each columnar honeycomb structure segment. Therefore, the bonding material is inserted between the outer circumferential side walls of the adjacent columnar honeycomb structure segments.Because the outer circumferential sidewall sections of the columnar honeycomb structure segments, which adjoin each other via the connecting material, are not present on the outer circumferential section of the filter, but are present on its inner side, the effect of improving processability when a catalyst slurry is introduced into the cells and the effect of preventing breakage when the honeycomb structure is clamped cannot be obtained if a coating layer, as taught in patent literature 2, is provided on this outer circumferential sidewall section.

[0012] On the other hand, it has been observed that when the catalyst slurry is introduced into a filter in which the side surfaces of several columnar honeycomb structure segments made of a porous ceramic are connected via a bonding material, the catalyst slurry is drawn through the bonding material into the outer circumferential side wall sections of the adjacent columnar honeycomb structure segments. Furthermore, it has been observed that the catalyst slurry can pass through the outer circumferential side wall and be drawn into the bonding material. The catalyst and the bonding material, as they are mounted on the outer circumferential side wall, cannot exhibit any catalytic performance and contribute little to exhaust gas purification. Therefore, a portion of the introduced catalyst is wasted, thus reducing exhaust gas purification performance.

[0013] The inventors of the present invention have carried out intensive investigations and found that in each columnar honeycomb structure segment, it becomes difficult to support the catalyst on the outer circumferential sidewall and the connecting material by making the average porosity of the outer circumferential sidewall smaller than the average porosity of the partitions and / or by setting the average thickness of the outer circumferential sidewall so that it has a predetermined ratio with respect to the average thickness of the partitions. On the other hand, it has been found that the ratio of catalyst supported on the partitions, which contributes to improving exhaust gas purification performance, increases. The present invention has been developed on the basis of this discovery and is described below by way of example.

[0014] To solve the aforementioned problem, a filter with the features of claim 1 is specified. According to a dependent aspect of the invention, methods for manufacturing a filter with the corresponding features of claims 14 and 19 are specified. Further advantageous embodiments of the invention are defined in the dependent claims.

[0015] According to one embodiment of the filter of the present invention, it is difficult to ensure that the catalyst is supported on the outer circumferential side wall and the connecting material of each segment, while simultaneously increasing the proportion of catalyst supported on the partitions, which contributes to improving the catalytic performance (e.g., exhaust gas purification performance). This allows a significant improvement in catalytic performance to be achieved in a filter in which the side surfaces of several columnar honeycomb structure segments made of a porous ceramic are connected via a connecting material, while maintaining the same amount of catalyst supported on the filter. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1A is a schematic end-face view of a filter according to an embodiment of the present invention. Fig.Figure 1B is a schematic side view of a filter according to an embodiment of the present invention. Fig. Figure 2 is a schematic perspective view of a column-shaped honeycomb structure segment forming a filter according to an embodiment of the present invention. Fig. Figure 3 is a schematic cross-sectional view of a columnar honeycomb structure segment forming a filter according to an embodiment of the present invention, observed from a cross-section parallel to the direction in which the cells extend. DETAILED DESCRIPTION OF THE INVENTION

[0016] The embodiments of the present invention will now be described in detail with respect to the figures. It should be noted that the present invention is not intended to be limited to the following embodiments, and any modification, improvement, or the like of the design can be suitably added based on the common knowledge of those skilled in the field, without departing from the inventive concept of the present invention. Furthermore, the thickness ratio of each section in the figures differs from the actual ratio, with thin sections being depicted in an enlarged manner for the purpose of easy understanding of the structures and not by reflecting the actual thickness ratio as they are found in the actual products. (1. Filter)

[0017] The filter according to the present invention can be used, for example, as a DPF (diesel particulate filter) and a GPF (gasoline particulate filter) for trapping soot that is located in the exhaust pipe of a combustion device, typically an engine installed in a vehicle. The filter according to the present invention can, for example, be installed in an exhaust pipe. A buffer mat can be arranged between the inner surface of the exhaust pipe and the filter to hold the filter in place within the exhaust pipe.

[0018] Fig. 1A shows a schematic end face view and Fig.Figure 1B shows a side view of a filter according to an embodiment of the present invention. The filter 10 comprises a first end face 104, which serves as an exhaust gas inlet, and a second end face 106, which serves as an exhaust gas outlet. The exhaust gas flowing from the first end face 104 is cleaned as it flows through the interior of the filter 10 and is expelled from the second end face 106. The filter 10 has a structure in which the side faces of several columnar honeycomb structure segments 100 are connected to one another via a bonding material 107. By connecting several columnar honeycomb structure segments 100 to create a segmented composite body, the thermal shock resistance can be improved. In addition, the filter 10 can have an outer circumferential wall 103, which can be ground into a desired shape (e.g., by grinding an outer circumferential section).a cylindrical shape), the application of a coating material to an outer circumferential surface and then the drying and heat treatment of the coating material.

[0019] Fig. Figure 2 shows a schematic perspective view of a column-shaped honeycomb structure segment 100, which forms a filter according to an embodiment of the present invention. Fig. Figure 3 shows a schematic cross-sectional view of a columnar honeycomb structure segment 100 forming a filter according to an embodiment of the present invention, as observed from a cross-section parallel to the direction in which the cells extend.

[0020] The columnar honeycomb structure segment 100 comprises a honeycomb structure with an outer circumferential side wall 102 and porous partitions 112 arranged on the inside of the outer circumferential side wall 102, which subdivide several cells 108, 110 extending from the first end face 104 to the second end face 106. Each of the cells 108, 110 can penetrate from the first end face 104 to the second end face 106 by opening the first end face 104 and the second end face 106 together.To improve PM capture performance, it is preferred that the columnar honeycomb structure 100 comprises several first cells 108 extending from the first end face 104 to the second end face 106, wherein the first end face 104 is open and the second end face 106 is sealed, and several second cells 110 extending from the first end face 104 to the second end face 106, wherein the first end face 104 is sealed and the second end face 106 is open. In this case, the first cells 108 and the second cells 110 can be arranged alternately adjacent to one another in the columnar honeycomb structure segment 100, with the partitions 112 positioned between them, such that both end faces have a checkerboard pattern.

[0021] When soot-laden exhaust gas is fed to the first end face 104 on the upstream side of the columnar honeycomb structure segment 100, the exhaust gas is introduced into the first cells 108, continuing downstream within them. In the first cells 108, the second end face 106 on the downstream side is sealed, so that the exhaust gas flows through the porous partitions 112 that divide the first cells 108 and the second cells 110, and into the second cells 110. Because the soot cannot pass through the partitions 112, it is captured and deposited in the first cells 108. After the soot has been removed, the clean exhaust gas flows into the second cells 110, continuing downstream in the second cells 110 and flowing out of the second end face 106 on the downstream side.

[0022] The external shape of the columnar honeycomb structure segment 100 is not particularly restricted, as long as it has a columnar shape. For example, columnar shapes with polygonal end faces can be used. The polygons can be quadrilaterals (rectangles, squares, and the like), hexagons, and the like. In a typical embodiment, the external shape of the columnar honeycomb structure segment 100 can be a quadrilateral prism. Furthermore, the size of the columnar honeycomb structure segment 100 can be such that, for example, it has an end face of 100 to 3,600 mm². 2 and typically 400 to 2,500 mm 2 exhibits the following characteristics. The length (height) of the columnar honeycomb structure segment 100 in the extension direction of the cells can, for example, be 100 to 500 mm and can typically be 120 to 400 mm.

[0023] The shape of the cells in a cross-section orthogonal to the direction in which the first cells 108 and the second cells 110 extend (the vertical direction) is not restricted, but is preferably a quadrilateral, a hexagon, an octagon, or a combination thereof. Among these, squares and hexagons are preferred. By making the cells in this way, the pressure drop when the exhaust gas flows is reduced when the columnar honeycomb structure segment 100 is used as a particulate filter, thereby improving the cleaning performance.

[0024] The material of the columnar honeycomb structure segment 100 is not restricted, although it may be a porous ceramic. Examples of suitable ceramics include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide, a silicon-silicon carbide composite (e.g., Si-bonded SiC), a cordierite-silicon carbide composite, zirconium dioxide, spinel, indialite, sapphire, corundum, titanium oxide, silicon nitride, and the like. Additionally, one type of ceramic may be used exclusively, or two or more types may be used simultaneously. Other examples of suitable materials for the columnar honeycomb structure segment 100 include porous sintered metal comprising an alloying component that contains, as a major component, one or more elements selected from the group consisting of Fe, Cr, Mo, and Ni.

[0025] The average thickness of the partitions 112 in each columnar honeycomb structure segment 100 is not limited, but is preferably 0.1 mm to 0.5 mm. By specifying the average thickness of the partitions 112 to preferably 0.1 mm or greater, more preferably 0.2 mm or greater, the strength of each columnar honeycomb structure segment 100 can be ensured. Furthermore, by specifying the average thickness of the partitions 112 to preferably 0.5 mm or less, more preferably 0.4 mm or less, the pressure drop can be reduced when the exhaust gas flows through each columnar honeycomb structure segment 100.

[0026] In the present invention, the thickness of a partition wall refers to the length of a line segment across the partition wall, where the centroids of adjacent cells are connected by this line segment in a cross-section perpendicular to the direction in which the cells extend. The average thickness of the partition walls refers to the average thickness of all partition walls in each columnar honeycomb structure segment.

[0027] In each columnar honeycomb structure segment, the average porosity of the partitions 112 can be, for example, 30% to 80%, preferably 50% to 70%. By setting the average porosity of the partitions within the above range, there is the advantage that pressure loss can be suppressed while maintaining the strength of the manufactured columnar honeycomb structure segment. On the other hand, if the average porosity is less than 30%, an increase in pressure loss occurs, and if the average porosity exceeds 80%, effects such as a decrease in strength and a decrease in thermal conductivity are caused. Here, the average porosity of the partitions is an average value obtained by taking several partition samples uniformly from the columnar honeycomb structure segment and measuring the porosity of each sample by image analysis.Specifically, for each sample, an SEM observation image is acquired at 300x magnification or higher and divided into three areas: a cavity section, a catalyst section (if present), and a base material section. The area ratio of the cavity section plus the catalyst section (if present) to the total area of ​​the three sections is defined as the cavity percentage = the porosity. That is, the porosity here is the porosity before the catalyst is applied.

[0028] Additionally, the average pore diameter of the partition walls of each columnar honeycomb structure segment is not restricted; rather, from the standpoint of balancing the ease of catalyst support, PM capture efficiency, and strength, it is preferably 7 µm to 40 µm, more preferably 7 µm to 30 µm, and even more preferably 7 µm to 23 µm. Here, the average pore diameter of the partition walls is an average value obtained by taking several partition wall samples uniformly from the columnar honeycomb structure segment and measuring the average pore diameter of each sample using a mercury penetration method.

[0029] According to one embodiment, in each of the columnar honeycomb structure segments 100, the average porosity of the outer circumferential sidewall 102 is smaller than the average porosity of the partitions 112. With this configuration, it becomes difficult for the catalyst to be carried on the outer circumferential sidewall of each segment, whereas, on the other hand, the proportion of catalyst carried on the partitions, which contributes to improving the catalytic performance, can be increased. That is, because the ratio of catalyst effectively used in the filter increases, the catalytic performance is improved if the amount carried on the filter remains the same. Additionally, the compound material conventionally sometimes contains sodium as an impurity. In this case, there is a possibility that, due to the diffusion of sodium into the interior of the segment, the catalyst component, such as...Zeolite absorbs Na, thereby impairing its catalytic performance. However, it is expected that reducing the average porosity of the outer circumferential sidewall 102 will prevent Na from diffusing into the segment. The ratio of the average porosity of the outer circumferential sidewall 102 to the average porosity of the partitions 112 is preferably 50% or less, more preferably 30% or less, and even more preferably 10% or less. The average porosity of the outer circumferential sidewall is determined by taking several samples covering the entire thickness of the outer circumferential sidewall and measuring the porosity of each sample along its entire outer circumferential sidewall in the thickness direction using image analysis.Specifically, for each sample, an SEM observation image is acquired at 300x magnification or higher and divided into three areas: a cavity section, a catalyst section (if present), and a base material section. The area ratio of the cavity section + catalyst section (if present) to the total area of ​​the three sections is defined as the cavity percentage = the porosity. That is, the porosity here is the porosity before the catalyst is applied.

[0030] The average porosity of the outer circumferential sidewall 102 is preferably, for example, 8% to 35%. It is preferred that the average porosity of the outer circumferential sidewall 102 be lower because the proportion of catalyst slurry filling the pores of the outer circumferential sidewall 102 decreases. In this respect, the average porosity of the outer circumferential sidewall 102 is preferably 35% or less, more preferably 20% or less, and even more preferably 10% or less. However, if almost all the pores of the outer circumferential sidewall 102 are filled, the moisture absorption from the bonding material slurry used to join the segments becomes insufficient, so there is a possibility that the bonding material slurry will not dry easily or that the bond between the bonding material 107 and the outer circumferential sidewall 102 will become weak.Therefore, the average porosity of the outer circumferential sidewall 102 is preferably set at 3% or greater, more preferably at 5% or greater, and even more preferably at 8% or greater. Additionally, a water-absorbing underlayer can be provided on the outer circumferential surface of the outer circumferential sidewall 102 to improve water absorbability. The underlayer can be a porous ceramic layer consisting mainly of silicon dioxide, aluminum oxide, or the like.

[0031] In each of the columnar honeycomb structure segments 100, at least a portion of the pores in the outer circumferential sidewall 102 can be filled with a filler in order to make the average porosity of the outer circumferential sidewall 102 smaller than the average porosity of the partitions 112. The filler can comprise aggregate particles. From the standpoint of preventing catalyst degradation, the Na concentration in the filler is preferably 0.02 wt% or less.

[0032] The aggregate particles may, for example, comprise as a major component a grade selected from the group consisting of aluminum oxide, silicon dioxide, zirconium dioxide, silicon carbide, aluminum titanate, silicon nitride, cordierite, mullite, zirconium phosphate, titanium oxide, an Fe-Cr-Al-based metal, a nickel-based metal, and metallic Si, or comprise as a major component a mixture of two or more grades selected from this group. Preferably, the aggregate particles may comprise as a major component a grade selected from the group consisting of aluminum oxide, silicon dioxide, zirconium dioxide, silicon carbide, aluminum titanate, silicon nitride, and cordierite, or comprise as a major component a mixture of two or more grades selected from this group. Among these, silicon carbide, which is a material with a low affinity for Na, is preferred.If the partitions are made of a silicon-silicon carbide composite (e.g., Si-bonded SiC), it is preferred to use metallic Si as a major component of a binder between the aggregates because bonding between the aggregates is possible by firing at a lower temperature, and the material exhibits high toughness after firing, thus preventing fracture. Here, the "major component" of the aggregate particles means a component comprising 50% or more by weight, preferably 70% or more by weight, and more preferably 90% or more by weight of the aggregate particles. Furthermore, the aggregate particles can also include those contained in an interlayer when the catalyst component is carried on the honeycomb structure, such as γ-aluminum oxide, cerium dioxide, zirconium dioxide, a cerium dioxide-based composite oxide, and a zirconium dioxide-based composite oxide.

[0033] The filler preferably comprises a binder. The binder has the function of bonding the aggregate particles to the inner surface of the pores on the outer circumferential sidewall. Examples of suitable binders include colloidal sols, such as silicon dioxide sol and aluminum oxide sol, and layer compounds that swell and exhibit bonding properties. When an SEM observation image is acquired at a magnification of 300x or greater, in this description, particles with a particle diameter (diameter of an equivalent circle) exceeding 0.3 µm are considered to be the aggregate particles, while particles with a particle diameter (diameter of an equivalent circle) of 0.3 µm or less are considered to be a binder.

[0034] To minimize the amount of catalyst carried in the outer circumferential sidewall 102 that cannot be effectively used, the average thickness of the outer circumferential sidewall 102 is preferably 1.8 times or less, more preferably 1.5 times or less, even more preferably 1.2 times or less, and still more preferably 1.0 times or less, as the average thickness of the partitions 112. Additionally, from the standpoint of ensuring the strength of the columnar honeycomb structure segment 100, the average thickness of the outer circumferential sidewall 102 is preferably 0.5 times or more, more preferably 0.8 times or more, as the average thickness of the partitions 112.In particular, in each columnar honeycomb structure segment 100, if the average thickness of the outer circumferential sidewall 102 is 0.5 times or more and 1.2 times or less than the average thickness of the partitions 112, the balance between the effect of reducing the amount of catalyst carried on the outer circumferential sidewall and the connecting material and the strength of the columnar honeycomb structure segment is improved. In the present invention, the average thickness of the outer circumferential sidewall refers to an average value of the thicknesses of any number of sections of the outer circumferential sidewall, measured uniformly in a cross-section perpendicular to the direction in which the cells extend.

[0035] According to one embodiment, in each of the columnar honeycomb structure segments 100, the outer circumferential sidewall 102 is configured such that the average porosity of the region with a thickness of 20% or more of the outer circumferential sidewall 102, extending inwards from the outermost circumferential section of the outer circumferential sidewall 102, is less than the average porosity of the partitions 112. With this configuration, the ratio of the catalyst slurry filled into the pores of the outer circumferential sidewall 102 can be effectively reduced. The ratio of the average porosity of the region of the outer circumferential sidewall 102 to the average porosity of the partitions 112 is preferably 70% or less, more preferably 50% (half) or less, and even more preferably 25% or less.The average porosity of the region with a thickness of 20% or more of the outer circumferential sidewall is an average value obtained by taking multiple samples containing this region and measuring the porosity of each sample through image analysis. Specifically, for each sample, an SEM observational image is acquired at 300x magnification or higher and divided into three regions: a cavity region, a catalyst region (if present), and a base material region. The area ratio of the cavity region + catalyst region (if present) to the total area of ​​the three regions is defined as the cavity percentage = porosity. That is, the porosity here is the porosity before the catalyst is applied.

[0036] According to one embodiment, in each of the columnar honeycomb structure segments 100, the outer circumferential side wall 102 is such that the average porosity of the area with a thickness of 50% of the outer circumferential side wall 102 from the innermost circumferential section of the outer circumferential side wall 102 outwards (hereinafter also referred to as the "average porosity of the inner circumferential side of the outer circumferential side wall") is higher than the average porosity of the area with a thickness of 50% of the outer circumferential side wall 102 from the outermost circumferential section of the outer circumferential side wall 102 inwards (hereinafter also referred to as the "average porosity of the outer circumferential side of the outer circumferential side wall").Because the average porosity of the inner circumferential side of the outer circumferential side wall is higher than the average porosity of the outer circumferential side wall, it is possible to prevent an area with low porosity from reaching the partitions 112 beyond the outer circumferential side wall. If the porosity of the partitions 112 is low, the catalyst will not adhere easily to the partitions 112, resulting in the partitions 112 not being used effectively for exhaust gas cleaning. The ratio of the average porosity of the outer circumferential side of the outer circumferential side wall to the average porosity of the inner circumferential side of the outer circumferential side wall is preferably 35% or less, more preferably 20% or less, and even more preferably 10% or less.In the measurement procedure for the average porosity of the outer circumferential sidewall and the average porosity of the inner circumferential sidewall, several samples containing the corresponding area are taken. The porosity of each sample is measured by image analysis, and the average is calculated. The specific procedure is the same as the procedure for measuring the average porosity of the area with a thickness of 20% or more of the outer circumferential sidewall.

[0037] According to one embodiment, in each of the columnar honeycomb structure segments 100, there is a difference of 5% between the average porosity of the area with a thickness of 10% of the outer circumferential side wall 102 from the innermost circumferential section of the outer circumferential side wall 102 outwards and the average porosity of the partition walls 112. The difference in average porosity is preferably within 3%, more preferably within 1%.The fact that the average porosity of the region near the innermost circumferential section of the outer perimeter side wall 102 is at a similar level to the average porosity of the partitions 112 means that the low-porosity region does not penetrate deeply into the partitions 112 beyond the outer perimeter side wall 102, which preferably means that the low-porosity region does not reach the partitions 112 beyond the outer perimeter side wall 102. Additionally, the average porosity of the region with a thickness of 10% of the outer perimeter side wall 102, extending outwards from the innermost circumferential section of the outer perimeter side wall 102, is typically less than or equal to the average porosity of the partitions 112. (2. Method for manufacturing the filter)

[0038] The first embodiment of the method for manufacturing a filter according to the present invention comprises: a step 1A of producing several columnar honeycomb structure segments from a porous ceramic with an outer circumferential side wall and partitions arranged on an inner side of the outer circumferential side wall, dividing several cells that extend from a first end face to a second end face; a step 2A of impregnating each of the columnar honeycomb structure segments made of a porous ceramic with a slurry comprising aggregate particles and a solvent, from an outermost circumferential section of the outer circumferential sidewall towards the inside, and subsequent firing to obtain several columnar honeycomb structure segments made of a porous ceramic, wherein an average porosity of the outer circumferential sidewall is less than that of the partitions; and a step 3A of joining via a connecting material the side surfaces of the columnar honeycomb structure segments made of a porous ceramic, wherein the average porosity of the outer circumferential side wall is smaller than that of the partition walls.

[0039] In step 1A, several columnar honeycomb structure segments are produced from a porous ceramic with an outer circumferential sidewall and partitions arranged on the inner surface of the outer circumferential sidewall, dividing several cells that extend from a first end face to a second end face. The columnar honeycomb structure segment made of a porous ceramic can be produced according to a publicly known method for producing a honeycomb structure, the method of which is described below by way of example. First, a blank is produced containing a predetermined ceramic starting material, a dispersion medium, a pore-forming agent, and a binder. Next, the blank is subjected to an extrusion process to form a honeycomb shape. At the time of extrusion, an extrusion die with a desired overall shape, cell shape, partition thickness, cell density, and the like can be used.The method for sealing the end face of the honeycomb preform is not particularly restricted; a well-known method for filling the cell openings on the end face, to which a predetermined mask is attached, with a sealing compound can be used. A columnar honeycomb structure segment can then be produced by firing the dried honeycomb preform. The firing conditions can be any publicly known conditions, depending on the material of the segment, and are not particularly restricted.

[0040] In step 2A, for each of the columnar honeycomb structure segments made of a porous ceramic produced in step 1A, a slurry containing aggregate particles and a solvent is impregnated from the outermost circumferential section of the outer sidewall towards the inside and then fired to obtain several columnar honeycomb structure segments made of a porous ceramic in which the average porosity of the outer circumferential sidewall is lower than that of the partition walls. Preferably, the slurry contains a binder.

[0041] The average particle diameter of the aggregate particles before impregnation with the slurry is preferably 2% to 60%, more preferably 10% to 50%, and even more preferably 30% to 40% of the average pore diameter of the outer circumferential sidewall. If the average particle diameter of the aggregate particles is less than 2% of the average pore diameter of the outer circumferential sidewall before impregnation with the slurry, the particles to be filled into the pores of the outer circumferential sidewall are too small with respect to their pore diameter, so there is a possibility that the pores cannot be adequately filled. That is, they cannot be retained in the pores and can pass through.Conversely, if the average particle diameter of the aggregate particles exceeds 60% of the average pore diameter of the outer circumferential sidewall before impregnation with the slurry, the particles to be filled into the pores of the outer circumferential sidewall are too large with respect to their pore diameter, so there is a possibility that the particles cannot be filled into the pores (cannot enter the pores). The average particle diameter of the aggregate particles refers to a median diameter (D50) in a volume-based cumulative particle diameter distribution measured by a laser diffraction method.

[0042] Furthermore, preferred embodiments of the aggregate particles and the binder in the slurry are as described above. It is preferred that the solvent in the slurry be water, alcohol, or a mixture of both as a major component. Here, the "major component" of the solvent refers to a component comprising 50% or more by weight, preferably 70% or more by weight, and more preferably 90% or more by weight of the solvent.

[0043] From the point of view of reducing the average porosity of the outer circumferential side wall, the concentration of the pore-forming agent in the slurry is preferably 0.5 wt% or less, more preferably 0.2 wt% or less and even more preferably 0 wt%.

[0044] Furthermore, the slurry may contain a suitable dispersing agent and / or an antifoaming agent.

[0045] As a method for impregnating the slurry, which contains the aggregate particles, the bonding agent, and the solvent, from the outermost circumferential section of the outer sidewall inwards, one could, for example, be a method for applying the slurry to the outermost circumferential section of the outer sidewall for each of the columnar honeycomb structure segments. Additionally, a method for sealing the two end faces of each of the columnar honeycomb structure segments and then completely impregnating them with the slurry could be used. As a method for sealing both end faces, the same method as the one described below for applying a bonding agent adhesion-prevention film to both end faces could be mentioned.

[0046] During firing after slurry impregnation, it is preferred to perform a heat treatment of the slurry at a temperature and for a period of time such that the aggregate particles can be fixed to the pores in the outer circumferential sidewall. If the slurry contains, for example, aggregate particles with the same composition as the partitions, a heat treatment under conditions similar to the firing conditions of the partitions is necessary to impart bonding properties. Furthermore, if a binder, such as colloidal silicon dioxide, which develops strength at 700 to 800 °C, is combined, a heat treatment at a lower temperature is possible.

[0047] The amount of decrease in the average porosity of the outer circumferential sidewall relative to the average porosity of the partitions, and the amount of decrease in the average pore diameter of the outer circumferential sidewall relative to the average pore diameter of the partitions, can be adjusted by the average particle diameter and content of the aggregate particles contained in the slurry and the number of impregnations with the slurry. Furthermore, it is preferred to control the particles in the slurry, which is filled into the pores of the outer circumferential sidewall by the impregnation operation, such that they remain only in the outer circumferential wall and do not reach the partitions that divide the cells. This control can be achieved by adjusting the viscosity of the slurry, the impregnation time, the amount of slurry applied, and the like.

[0048] In step 3A, the side surfaces of the columnar honeycomb structure segments, made of a porous ceramic where the average porosity of the outer circumferential side wall is lower than that of the partition walls, are joined together using a bonding material to create a segmented composite body. The segmented composite body can be produced, for example, by the following procedure: A bonding material is applied to the connecting surfaces (side surfaces), with bonding material adhesion-prevention films forming on the two end faces of each columnar honeycomb structure segment. Next, these segments are arranged adjacently so that their side surfaces face each other, and the adjacent segments are bonded by pressure and then dried by heating. In this way, a segmented composite body is produced in which the side surfaces of the adjacent segments are joined by the bonding material.

[0049] The material for the bonding agent's adhesion-prevention film is not particularly restricted, although, for example, a synthetic resin such as polypropylene (PP), polyethylene terephthalate (PET), polyimide, or Teflon (registered trademark) can be suitable. Additionally, the film preferably comprises an adhesive layer, wherein the material of the adhesive layer is preferably an acrylic resin, a rubber resin (e.g., a rubber consisting mainly of natural or synthetic rubber), or a silicone resin.

[0050] The bonding material can be, for example, a material produced by mixing a ceramic powder, a dispersion medium (e.g., water, etc.), and, if necessary, additives such as a binder, a flocculant, and a foamed resin. The ceramics may include cordierite, mullite, zirconium phosphate, aluminum titanate, silicon carbide, a silicon-silicon carbide composite (e.g., Si-bonded SiC), a cordierite-silicon carbide composite, zirconium dioxide, spinel, indialite, sapphire, corundum, titanium oxide, silicon nitride, and the like, preferably being the same material as the columnar honeycomb structure. Examples of the binder include polyvinyl alcohol and methylcellulose, and the like.

[0051] The second embodiment of the method for manufacturing a filter according to the present invention comprises: a step 1B of producing several columnar honeycomb structure segments from a porous ceramic with an outer circumferential side wall and partitions arranged on an inner side of the outer circumferential side wall, dividing several cells extending from a first end face to a second end face; a step 2B of impregnating each of the columnar honeycomb structure segments made of a porous ceramic with a metallic Si from an outermost circumferential section of the outer circumferential sidewall towards the inside and subsequent firing to obtain several columnar honeycomb structure segments made of a porous ceramic, wherein an average porosity of the outer circumferential sidewall is smaller than that of the partitions; and a step 3B of joining via a connecting material the side surfaces of the columnar honeycomb structure segments made of a porous ceramic, wherein the average porosity of the outer circumferential side wall is smaller than that of the partition walls.

[0052] Step 1B is the same as step 1A, therefore its detailed description is omitted. In the present embodiment, however, it is preferred that the columnar honeycomb structure segment produced in step 1B consists of a silicon-silicon carbide composite material (e.g., Si-bonded SiC).

[0053] In step 2B, for each of the columnar honeycomb structure segments made of a porous ceramic, metallic silicon is impregnated from an outermost circumferential section of the outer perimeter sidewall towards the inside and then fired to obtain several columnar honeycomb structure segments made of a porous ceramic, where the average porosity of the outer perimeter sidewall is lower than that of the partition walls. For example, a method for impregnating metallic silicon from the outermost circumferential section of the outer perimeter sidewall towards the inside can be...A method is mentioned in which a metallic Si flake is arranged on or wrapped around the outer circumferential surface of a fired porous segment body and the temperature is increased by heating the entire body to a temperature equal to or higher than the melting point of metallic Si in order to exploit the capillary phenomenon by which the molten metallic Si is absorbed into the pores.

[0054] Step 3B is the same as step 3A, except that its detailed description is omitted.

[0055] The third embodiment of the method for manufacturing a filter according to the present invention comprises: a step 1C of producing several unfired columnar honeycomb structure segments with an outer circumferential side wall and partitions arranged on an inner side of the outer circumferential side wall and dividing several cells extending from a first end face to a second end face, by forming and drying a blank; a step 2C of impregnating each of the unfired columnar honeycomb structure segments with a slurry comprising aggregate particles and a solvent, from an outermost circumferential section of the outer circumferential sidewall towards the inside; a step 3C of firing each of the unfired columnar honeycomb structure segments after step 2C to obtain multiple columnar honeycomb structure segments from a porous ceramic, wherein an average porosity of the outer circumferential sidewall is smaller than that of the partition walls; and a step 4C of joining via a connecting material of the side surfaces of the columnar honeycomb structure segments made of a porous ceramic, wherein the average porosity of the outer circumferential side wall is smaller than that of the partition walls.

[0056] In step 1C, several unfired columnar honeycomb structure segments with an outer circumferential sidewall and partitions arranged on an inner surface of the outer circumferential sidewall, dividing several cells extending from a first end face to a second end face, are produced by molding and drying a blank. The unfired columnar honeycomb structure segment can be produced according to a publicly known method for producing a honeycomb structure, the method of which is described below as an example. First, a blank is produced containing a predetermined ceramic starting material, a dispersion medium, a pore-forming agent, and a binder. Next, the blank is subjected to an extrusion process to form a honeycomb mold.At the time of extrusion, an extrusion die with a desired overall shape, cell shape, partition wall thickness, cell density, and the like can be used. The method for sealing the end face of the honeycomb mold is not particularly restricted; a well-known method for filling the cell openings on the end face, to which a predetermined mask is attached, with a sealing compound can be used.

[0057] In step 2C, for each of the unfired columnar honeycomb structure segments produced in step 1C, a slurry comprising aggregate particles and a solvent is impregnated from the outermost circumferential section of the outer circumferential sidewall towards the inner side. Preferably, the slurry contains a binder. Step 2C can be carried out in accordance with step 2A, omitting its detailed description.

[0058] In step 3C, each of the unfired columnar honeycomb structure segments is fired after step 2C to obtain multiple columnar honeycomb structure segments made of a porous ceramic, wherein the average porosity of the outer circumferential sidewall is lower than that of the partitions. In the present embodiment, the partitions, the outer circumferential sidewall, and the slurry impregnated with the outer circumferential sidewall are fired simultaneously. Therefore, the aggregate particles used for the slurry can be the same ceramic starting material as the partitions. The firing conditions can be any publicly known conditions, depending on the segment material, and are not particularly restricted.

[0059] Step 4C is the same as step 3A, therefore its detailed description is omitted.

[0060] The fourth embodiment of the method for manufacturing a filter according to the present invention comprises: a step 1D of producing several unfired columnar honeycomb structure segments with an outer circumferential side wall and partitions arranged on an inside of the outer circumferential side wall and dividing several cells extending from a first end face to a second end face, by forming and drying a blank; a 2D step of impregnating each of the unfired columnar honeycomb structure segments with a metallic Si from an outermost circumferential section of the outer circumferential side wall towards the inside; a 3D step of firing each of the unfired columnar honeycomb structure segments after step 2D to obtain multiple columnar honeycomb structure segments from a porous ceramic, wherein an average porosity of the outer circumferential sidewall is smaller than that of the partitions; and a step 4D of joining via a connecting material of the side surfaces of the columnar honeycomb structure segments made of a porous ceramic, wherein the average porosity of the outer circumferential side wall is smaller than that of the partition walls.

[0061] Step 10 is the same as step 1C, therefore its detailed description is omitted. In the present embodiment, however, it is preferred that the unfired columnar honeycomb structure segments produced in step 1D comprise a forming starting material for obtaining a silicon-silicon carbide composite (e.g., Si-bonded SiC), i.e., metallic Si particles and silicon carbide particles.

[0062] In step 2D, for each of the unfired columnar honeycomb structure segments, metallic silicon is impregnated from the outermost circumferential section of the outer sidewall towards the inside, followed by a drying step. One method for impregnating metallic silicon from the outermost circumferential section of the outer sidewall towards the inside is, for example, a process in which a metallic silicon flake is placed on or wrapped around the outer circumferential surface of a fired porous segment body, and the temperature is increased by heating the entire body to a temperature equal to or higher than the melting point of metallic silicon, in order to exploit the capillary phenomenon by which the molten metallic silicon is absorbed into the pores.

[0063] Step 3D is the same as step 3C and is not described in detail.

[0064] Step 4D is the same as step 3A and is not described in detail.

[0065] Regarding the segmented composite body produced by the above procedure, the outer circumferential section can be ground into a desired shape (e.g., a cylindrical shape), and a coating material can be applied to the outer circumferential surface. Drying and heat treatment then form an outer circumferential wall. The coating material is not particularly restricted; any publicly known coating material can be used for the outer circumference. For example, a slurry can be mentioned as a coating material for the outer circumference. This slurry is prepared by adding an additive, such as an organic binder, a foamed resin, a dispersant, and the like, and water to an inorganic material, such as an inorganic fiber, colloidal silicon dioxide, clay, ceramic particles, and the like, and kneading the mixture.In addition, the procedure for applying the coating material to the outer perimeter is not particularly restricted, and a publicly known procedure may be used.

[0066] Depending on the application, a suitable catalyst can be carried on the filter. One method for carrying the catalyst on the filter involves introducing a catalyst slurry into the cells using a conventional, publicly known suction method or similar technique, attaching it to the surface or pores of the partition walls, and then subjecting it to a high-temperature treatment. This process causes the catalyst contained in the slurry to be calcined and carried on the partition walls.

[0067] The examples of catalysts include, but are not limited to, a DOC oxidation catalyst for oxidizing and burning hydrocarbons (HC) and carbon monoxide (CO) to increase exhaust gas temperature, a PM combustion catalyst to assist the combustion of PM such as soot, SCR and NSR catalysts for removing nitrogen oxides (NOx), and three-way catalysts that can remove hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) simultaneously. The catalysts can be suitable for various applications. B. Precious metals (Pt, Pd, Rh and the like), alkali metals (Li, Na, K, Cs and the like), alkaline earth metals (Ca, Ba, Sr and the like), rare earths (Ce, Sm, Gd, Nd, Y, Zr, Ca, La, Pr and the like) and transition metals (Mn, Fe, Co, Ni, Cu, Zn, Sc, Ti, V, Cr and the like) and the like.

[0068] In particular, in a diesel particulate filter (DPF) for a passenger car, an selective catalytic reduction (SCR) catalyst, such as a copper-substituted zeolite or an iron-substituted zeolite, is mounted on the DPF to simultaneously perform soot capture and NOx purification functions. In this case, it is possible to purify NOx with ammonia obtained by decomposing urea in the vehicle. Because zeolite often contains sodium as a contaminant, the effectiveness of applying the present invention is particularly high when a zeolite-containing catalyst is mounted on a filter.

[0069] In the case of a DPF and a GPF, in addition to the SCR catalyst, there is a case in which a catalyst containing a precious metal, such as platinum, for oxidizing and burning carbon monoxide (CO), hydrocarbons (HC) and soot is carried in the pores of the partition walls and on the surface of the partition walls; additionally, there is a case in which a three-way catalyst containing a precious metal, such as platinum, palladium and rhodium, for reducing CO, HC and NOx is carried in the pores of the partition walls and on the surface of the partition walls.According to one embodiment of the present invention, in these cases the amount of catalyst and the amount of precious metal that are carried on the outer circumferential sidewall of the honeycomb structure segment and cannot be effectively utilized can also be reduced, thus improving the cleaning efficiency and reducing the amount of catalyst and precious metal used, thereby achieving cost savings. Additionally, costs can be reduced by decreasing the amount of material on the outer circumferential sidewall of the honeycomb structure segment. Furthermore, a secondary effect is achieved by reducing the weight and heat capacity of the honeycomb structure segment, thus improving its start-up performance.There is also a secondary effect that the stiffness of the honeycomb structure segment decreases, thus reducing the forced strain exerted on the joining material due to temperature distribution during use, thereby preventing the occurrence of cracks in the joining material. EXAMPLES

[0070] The following are examples to better understand the present invention and its advantages, but the present invention is not limited to these examples. (Example 1)

[0071] SiC powder and metallic Si powder were mixed in a mass ratio of SiC powder : metallic Si powder = 80 : 20, with the addition of a pore-forming agent, an organic binder, a surfactant, and water to obtain a plastic blank. The blank was extruded and dried to produce a columnar honeycomb mold with an outer circumferential sidewall and partitions dividing several cells extending from a first end face to a second end face. Sealing sections were formed at one end of each cell so that both end faces of the columnar honeycomb mold exhibited a checkerboard pattern. That is, the sealing sections were formed such that adjacent cells were sealed at opposite ends. The same material as that of the columnar honeycomb mold was used for the sealing sections.After the sealing sections had been shaped and dried in this manner, the columnar honeycomb mold was degreased at approximately 400 °C in an air atmosphere and further fired at approximately 1450 °C in an Ar atmosphere to bond the SiC particles with the Si in the mold, thereby obtaining a rectangular parallelepiped honeycomb structure segment with the following specifications. (Specification of the honeycomb structure segment)

[0072] External shape: A rectangular parallelepiped with square end faces of 42 mm on each side and a height (length in the direction in which the cells extend) of 152 mm. Average porosity of the partition walls: 63% Average pore diameter of the partition walls: 20 µm Average thickness of the partition walls: 12 mil (305 µm) Cell cross-sectional shape: square Cell density: approximately 46.5 cells / cm² 2 (300 cells / in 2 ) Average thickness of the outer circumferential sidewall: described in Table 1-1 Average pore diameter of the outer circumferential sidewall: 20 µm

[0073] Next, 150 mass parts of colloidal silicon dioxide (water dispersion with a solids content of 40%) with an average particle diameter (defined as a median diameter (D50) in a volume-based cumulative particle diameter distribution measured by laser diffraction) of 300 nm or less, and 200 mass parts of water were added to 150 mass parts of SiC particles with an average particle diameter shown in Table 1-1. The mixture was stirred thoroughly to produce a slurry with the viscosity shown in Table 1. A suitable dispersant and antifoaming agent were added during the preparation of the slurry, but no pore-forming agent was added. The viscosity of the slurry was measured at 20 °C using a Brookfield rotational viscometer.Then, both end faces of the honeycomb structure segment were sealed with a resin film, and the entire assembly was immersed in the resulting slurry for the contact time shown in Table 1-1 (in the case of immersion, from the start of contact with the slurry until the start of blowing off). The excess slurry was then removed by blowing with air. After the slurry impregnated in the outer circumferential sidewall of the honeycomb structure segment had dried, firing at 700 °C in an air atmosphere was carried out to create a low-porosity region in the outer circumferential sidewall of the honeycomb structure segment.

[0074] Subsequently, water was added to a mixture of SiC powder, aluminosilicate fiber, aqueous silicon dioxide sol solution, and clay. The mixture was kneaded for 30 minutes using a mixer to obtain a paste-like bonding material. This bonding material was applied to the outer circumferential surface of the honeycomb structure segment to a thickness of approximately 1 mm to form a bonding layer. Another honeycomb structure segment, produced using the same procedure, was then placed on top. The process was repeated to produce a segmented laminate containing a total of 16 honeycomb structure segments in a 4-segment (vertical direction) × 4-segment (horizontal direction) configuration. The entire assembly was then bonded by applying appropriate external pressure and subsequently dried for 2 hours at 120 °C to obtain a segmented composite body.After grinding the outer circumference of the segmented composite body to create a cylindrical shape, a coating material with the same composition as the bonding material was applied to the machined surface to rebuild the outer circumferential wall. Drying and curing were then carried out for 2 hours at 700 °C in an air atmosphere to produce the filter of Example 1. (Example 2)

[0075] The filter of Example 2 was obtained by carrying out the same procedure as in Example 1, except that instead of immersing the honeycomb structure segment with both sealed end faces into the slurry, a procedure for applying the slurry to the entire outer circumferential side surface of the honeycomb structure segment was used, and the viscosity of the slurry and the contact time of the slurry (in the case of application, the time from the end of coating to the beginning of blowing) were changed to the conditions shown in Table 1-1. (Example 3)

[0076] The same columnar honeycomb molds with sealing sections as in Example 1 were produced. Next, 80 parts by mass of water were added to 20 parts by mass of a mixture containing the SiC powder and metallic Si powder used in the production of the columnar honeycomb molds, in a mass ratio of SiC powder : metallic Si powder = 80 : 20. After thorough stirring, a slurry with the viscosity shown in Table 1-1 was prepared. A suitable dispersant and antifoaming agent were added during the preparation of the slurry, but no pore-forming agent was added. The resulting slurry was applied to the entire outer circumferential surface of the columnar honeycomb molds.After the applied slurry had dried, the mixture was degreased at 400 °C in an air atmosphere and then further fired at 1430 °C in an argon atmosphere to obtain honeycomb structure segments with an outer circumferential sidewall featuring a low porosity zone. Subsequently, using these honeycomb structure segments, the same procedure as in Example 1 was carried out to produce a segmented composite body, resulting in the filter of Example 3. (Example 4)

[0077] The filter of Example 4 was obtained by performing the same procedure as in Example 2, except that the average thickness of the outer circumferential sidewall of the honeycomb structure segment was changed to the value shown in Table 1-1. (Example 5)

[0078] After mixing 20 mass parts of aluminum oxide particles with an average particle diameter shown in Table 1-1 and 80 mass parts of water, the mixture was stirred thoroughly to produce a slurry with the viscosity shown in Table 1-1. During the preparation of the slurry, a suitable dispersant and antifoaming agent were added, but no pore-forming agent was added. Furthermore, the same honeycomb structure segments as in Example 1 were prepared, except that the average thickness of the outer circumferential sidewall of the honeycomb structure segment was changed to a value shown in Table 1-1. Next, the same procedure as in Example 2 was carried out to obtain the filter of Example 5, except that the slurry and the honeycomb structure segments thus obtained were used, and the contact time of the slurry was changed to the conditions shown in Table 1-1. (Example 6)

[0079] The filter of Example 6 was obtained by performing the same procedure as in Example 5, except that the average thickness of the outer circumferential sidewall of the honeycomb structure segment was changed to the value shown in Table 1-1, and the viscosity of the slurry and the contact time of the slurry were changed to the conditions shown in Table 1-1. (Example 7)

[0080] The forming and firing processes were carried out in the same manner as in Example 1 to obtain honeycomb structure segments. A metallic silicon foil with a thickness of 200 µm was bonded to the entire outer circumferential surface of the honeycomb structure segments and fired again at 1430 °C in an argon atmosphere to impregnate the metallic silicon into the pores in the outer circumferential sidewall of the honeycomb structure segments. As a result, honeycomb structure segments with an outer circumferential sidewall exhibiting a region of low porosity were obtained. Subsequently, using the obtained honeycomb structure segments, the same procedure as in Example 1 was carried out to fabricate a segmented composite body, consequently yielding the filter of Example 7. (Examples 8 to 10, comparison examples 1 to 3)

[0081] The filters of Examples 8 to 10 and of the comparison example 1 to 3 were obtained by performing the same procedure as in Example 1, except that the average thickness of the outer circumferential sidewall of the honeycomb structure segment was changed to the value shown in Table 1-1 and the step of forming a low porosity area in the outer circumferential sidewall of the honeycomb structure segment was not performed. (Feature rating)

[0082] The following average porosity of the filters, based on the examples and comparison samples produced by the procedure described above, was measured according to the method described above. The results are shown in Table 1-2. A: Average porosity of the entire outer perimeter sidewall B: Average porosity of an area with a thickness of 20% of the outer perimeter sidewall from the outermost perimeter section of the outer perimeter sidewall inwards C: Average porosity of an area with a thickness of 10% of the outer circumferential sidewall from the innermost circumferential section of the outer circumferential sidewall outwards D: Average porosity of the area with a thickness of 50% of the outer perimeter sidewall from the outermost perimeter section of the outer perimeter sidewall inwards (average porosity of the outer perimeter sidewall) E: Average porosity of the area with a thickness of 50% of the outer circumferential sidewall from the innermost circumferential section of the outer circumferential sidewall outwards (average porosity of the inner circumferential side of the outer circumferential sidewall)

[0083] The ratio (%) of the pore volume in the outer circumferential sidewall of the honeycomb structure segment to the total pore volume of the honeycomb structure segment was measured by SEM image analysis of the filters according to the examples and comparison examples produced by the procedure described above. The results are shown in Table 1-2.

[0084] The catalyst was then carried on the filters according to the examples and comparison samples prepared by the procedure described above, resulting in a catalyst-carrying filter. An SCR catalyst was used. The catalyst-carrying method involved wet-applying the catalyst solution to the filter and then heat-treating it at 550 °C. The percentage of catalyst in the outer circumferential sidewall of the honeycomb segment relative to the total catalyst in the honeycomb segment was then measured by SEM image analysis for the catalyst-carrying filter. The results are shown in Table 1-2.

[0085] The evaluation criteria were as follows. ◯ (good): the ratio is less than 4.0%. △ (passed): the ratio was 4.0% or greater and less than 5.0% × (failed): the ratio is 5.0% or greater

[0086] Regarding the filters according to the examples and the comparison examples produced by the above procedure, the strength assessment was carried out in accordance with a hydrostatic pressure test (enclosing the filter in a rubber container and applying hydrostatic pressure in water).

[0087] The evaluation criteria were as follows. The results are shown in Table 1-2. ◯: Burst pressure of 1.5 MPa or greater ×: Burst pressure less than 1.5 MPa

[0088] Table 1-1 Material of the honeycomb structure segment Additive of the filler Average particle diameter of the aggregate / filler (µm) Binder in the filler slurry viscosity mPa·s Filling process Contact time with the porridge (s) Average thickness of the outer perimeter side wall (mm) Average thickness of the outer perimeter side wall / average thickness of the partition walls Example 1 Si-SiC SiC 2 silicon dioxide 10 Immersion 10 0,5 1,64 Example 2 Si-SiC SiC 2 silicon dioxide 8 Apply to burned body 5 0,5 1,64 Example 3 Si-SiC SiC 20 Si 5 Application to mold no blowing away 0,5 1,64 Example 4 Si-SiC SiC 2 silicon dioxide 8 Apply to burned body 5 0,3 0,98 Example 5 Si-SiC Aluminum oxide 3 no 10 Apply to burned body 10 0,3 0,98 Example 6 Si-SiC Aluminum oxide 3 no 12 Apply to burned body 11 0,15 0,49 Example 7 Si-SiC none Si - Melting and impregnation with silicon metal - 0,5 1,64 Example 8 Si-SiC - - - - - 0,16 0,52 Example 9 Si-SiC - - - - - 0,31 1,02 Example 10 Si-SiC - - - - - 0,36 1,18 Comparative example 1 Si-SiC - - - - 0,6 1,97 Comparative example 2 Si-SiC - - - - 0,14 0,46 Comparative example 3 Si-SiC - - - - - 0,4 1,31 Note: The silicon dioxide used as the bonding material was colloidal silicon dioxide with an average particle diameter of 300 nm or less. Note: After filling with the porridge, the porridge on the surface was blown away by blowing air.

[0089] Table 1-2 Average porosity (%) A: entire outer perimeter side wall B: outer area with 20% thickness of the outer perimeter side wall C: inner area with 10% thickness of the outer perimeter side wall D: outer perimeter side of the outer perimeter side wall E: inner circumferential side of the outer circumferential side wall Percentage of pore volume in the outer perimeter wall to the total pore volume (%) Percentage of catalyst quantity in the outer circumferential side wall relative to the total catalyst (%) strength Example 1 15 5 62 38 58 3 1,9 ◯ ◯ Example 2 20 5 63 35 60 3 1,9 ◯ ◯ Example 3 10 10 63 45 61 7 4,5 △ ◯ Example 4 20 5 62 25 60 2 1,3 ◯ ◯ Example 5 8 5 59 25 57 2 1,3 ◯ ◯ Example 6 35 2 63 2D 60 1 0,6 ◯ ◯ Example 7 11 1 45 3 35 0,5 0,3 ◯ ◯ Example 8 63 62,5 63,2 63,2 63,2 2,9 1,8 ◯ ◯ Example 9 63 62,5 63,2 63,2 63,2 5,7 3,6 ◯ ◯ Example 10 63 62,5 63,2 63,2 63,2 6,6 4,2 △ ◯ Comparative example 1 63 62,5 63,2 63,2 63,2 11 7 × ◯ Comparative example 2 63 62,5 63,2 63,2 63,2 2,6 1,7 ◯ × Comparative example 3 63 62,5 63,2 63,2 63,2 7 5 × ◯ Description of reference symbols 10 filters 100 honeycomb structure segments 102 Outer perimeter side wall 103 Outer perimeter wall 104 First end face 106 Second front face 107 Connecting material 108 First cell 110 Second cell

Claims

[1] Filter (10) comprising several columnar honeycomb structure segments (100) made of a porous ceramic, wherein the side surfaces of the segments are connected to each other via a connecting material (107), wherein Each of the columnar honeycomb structure segments (100) comprises an outer circumferential side wall (102) and partition walls (112) that divide several cells (108, 110) extending from a first end face (104) to a second end face (106) and in each of the columnar honeycomb structure segments (100) the average porosity of the outer circumferential side wall (102) is smaller than that of the partition walls (112), wherein the outer circumferential side wall (102) of each of the columnar honeycomb structure segments (100) is designed such that the average porosity of an area with a thickness of 50% of the outer circumferential side wall (102) from an innermost circumferential section of the outer circumferential side wall (102) outwards is higher than that of an area with a thickness of 50% of the outer circumferential side wall (102) from an outermost circumferential section of the outer circumferential side wall (102) inwards. [2] Filter (10) according to claim 1, wherein in each of the columnar honeycomb structure segments (100) the average thickness of the outer circumferential sidewall (102) is 0.5 times or more and 1.2 times or less as thick as that of the partition walls (112). [3] Filter (10) according to one of claims 1 or 2, wherein in each of the columnar honeycomb structure segments (100) the outer circumferential side wall (102) is such that an average porosity of a region with a thickness of 20% or more of the outer circumferential side wall (102) from the outermost circumferential section of the outer circumferential side wall (102) inwards is smaller than that of the partition walls (112). [4] Filter (10) according to one of claims 1 or 2, wherein in each of the columnar honeycomb structure segments (100) the outer circumferential side wall (102) is such that an average porosity of a region with a thickness of 20% or more of the outer circumferential side wall (102) from the outermost circumferential section of the outer circumferential side wall (102) inwards is not more than half that of the partition walls (112). [5] Filter (10) according to one of claims 1 or 2, wherein in each of the columnar honeycomb structure segments (100) the outer circumferential side wall (102) is such that an average porosity of a region with a thickness of 20% or more of the outer circumferential side wall (102) from the outermost circumferential section of the outer circumferential side wall (102) inwards is 25% or less than that of the partition walls (112). [6] Filter (10) according to any one of claims 1 to 5, wherein the outer circumferential side wall (102) of each of the columnar honeycomb structure segments (100) is designed such that the ratio of the average porosity of the area with a thickness of 50% of the outer circumferential side wall (102) from the outermost circumferential section of the outer circumferential side wall (102) inwards to the average porosity of the area with a thickness of 50% of the outer circumferential side wall (102) from the innermost circumferential section of the outer circumferential side wall (102) outwards is 35% or less. [7] Filter (10) according to any one of claims 1 to 6, wherein in each of the columnar honeycomb structure segments (100) there is a difference between an average porosity of an area with a thickness of 10% of the outer circumferential sidewall (102) from the innermost circumferential section of the outer circumferential sidewall (102) outwards and an average porosity of the partition walls (112) within 5%. [8] Filter (10) according to one of claims 1 to 7, wherein in each of the column-shaped honeycomb structure segments (100) at least a part of the pores of the outer circumferential side wall (102) is filled with a filler. [9] Filter (10) according to claim 8, wherein the filler comprises additive particles and the additive particles comprise as a major component of one or a mixture of two or more selected from the group comprising aluminium oxide, silicon dioxide, zirconium dioxide, silicon carbide, aluminium titanate, silicon nitride and cordierite. [10] Filter (10) according to claim 8, wherein the filler comprises a metallic Si as a main component. [11] Filter (10) according to any one of claims 1 to 10, wherein an SCR catalyst is carried on the partition walls (112) of each of the column-shaped honeycomb structure segments (100). [12] Filter (10) according to any one of claims 1 to 11, wherein the average porosity of the partitions (112) of each of the columnar honeycomb structure segments (100) is 50% to 70%. [13] Filter (10) according to any one of claims 1 to 12, wherein an average pore diameter of the partitions (112) of each of the columnar honeycomb structure segments (100) is 7 µm to 23 µm. [14] Method for manufacturing a filter (10) according to any one of claims 1 to 13, wherein the method comprises: a step 1A of producing several columnar honeycomb structure segments (100) from a porous ceramic with an outer circumferential side wall (102) and partitions (112) arranged on an inner side of the outer circumferential side wall (102) and dividing several cells (108, 110) extending from a first end face (104) to a second end face (106); a step 2A of impregnating each of the columnar honeycomb structure segments (100) of a porous ceramic with a slurry comprising aggregate particles and a solvent, from an outermost circumferential section of the outer circumferential sidewall (102) towards the inside and subsequent firing to obtain several columnar honeycomb structure segments (100) of a porous ceramic, wherein an average porosity of the outer circumferential sidewall (102) is smaller than that of the partition walls (112); and a step 3A of joining via a connecting material the side surfaces of the columnar honeycomb structure segments (100) made of a porous ceramic, wherein the average porosity of the outer circumferential side wall (102) is smaller than that of the partition walls (112). [15] Method for producing a filter (10) according to claim 14, wherein the average particle diameter of the additive particles is 2% to 60% of the average pore diameter of the outer circumferential side wall (102) before impregnation with the slurry. [16] Method for producing a filter (10) according to claim 14 or 15, wherein the slurry comprises a pore-forming agent in a concentration of 0.5 wt% or less. [17] Method for producing a filter (10) according to any one of claims 14 to 16, wherein step 2A comprises applying the slurry to the outermost circumferential section of the outer circumferential side wall (102) for each of the columnar honeycomb structure segments (100) made of a porous ceramic. [18] Method for producing a filter (10) according to any one of claims 14 to 16, wherein step 2A comprises a step of completely immersing each of the column-shaped honeycomb structure segments (100) made of a porous ceramic into the slurry after sealing both end faces. [19] Method for manufacturing a filter (10) according to any one of claims 1 to 13, comprising: a step 1C of producing several unfired columnar honeycomb structure segments (100) with an outer circumferential side wall (102) and partitions (112) arranged on an inner side of the outer circumferential side wall (102) and dividing several cells (108, 110) extending from a first end face (104) to a second end face (106) by forming and drying a blank; a step 2C of impregnating each of the unfired columnar honeycomb structure segments (100) with a slurry comprising aggregate particles and a solvent, from an outermost circumferential section of the outer circumferential sidewall (102) towards the inside; a step 3C of firing each of the unfired columnar honeycomb structure segments (100) after step 2C to obtain several columnar honeycomb structure segments (100) from a porous ceramic, wherein an average porosity of the outer circumferential sidewall (102) is smaller than that of the partition walls (112); and a step 4C of joining via a connecting material the side surfaces of the columnar honeycomb structure segments (100) made of a porous ceramic, wherein the average porosity of the outer circumferential side wall (102) is smaller than that of the partition walls (112). [20] Method for producing a filter (10) according to claim 19, wherein the slurry comprises a pore-forming agent in a concentration of 0.5 wt% or less. [21] Method for producing a filter (10) according to claim 19 or 20, wherein step 2C comprises applying the slurry to the outermost circumferential section of the outer circumferential side wall (102) for each of the several unfired columnar honeycomb structure segments (100). [22] Method for producing a filter (10) according to claim 19 or 20, wherein step 2C comprises a step of fully immersing each of the unfired columnar honeycomb structure segments (100) in the slurry after sealing both end faces.

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