Ceramic honeycomb filter

Abstract

A ceramic honeycomb filter comprising a cordierite-type ceramic honeycomb structure having large numbers of flow paths partitioned by porous cell walls, and plugs formed in end portions of predetermined flow paths of the ceramic honeycomb structure; the cell walls having a thermal expansion coefficient Tw (×10.sup.−7/° C.) of 10 or less in the flow path direction between 40° C. and 800° C.; the plugs comprising at least ceramic particles, and 5-25 parts by mass of an amorphous oxide matrix per 100 parts by mass of the ceramic particles; the ceramic particles comprising at least 42-90% by mass of amorphous silica particles, and 10-58% by mass of cordierite particles; and the amorphous silica particles comprising 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm, and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm.

Claims

1. A ceramic honeycomb filter comprising a cordierite-type ceramic honeycomb structure having large numbers of flow paths partitioned by porous cell walls, and plugs formed in end portions of predetermined flow paths of said ceramic honeycomb structure; said porous cell walls having a thermal expansion coefficient Tw of 10×10.sup.−7/° C. or less in a flow path direction between 40° C. and 800° C.; said plugs comprising at least ceramic particles, and 5-25 parts by mass of an amorphous oxide matrix per 100 parts by mass of said ceramic particles; said ceramic particles comprising at least 42-90% by mass of amorphous silica particles and 10-58% by mass of cordierite particles, based on a total mass of said ceramic particles; and said amorphous silica particles comprising 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm, based on a total mass of said amorphous silica particles.

2. The ceramic honeycomb filter according to claim 1, wherein an amount of said amorphous silica particles in said ceramic particles is 45 parts by mass or more and 75 parts by mass or less based on 100 parts by mass of said ceramic particles.

3. The ceramic honeycomb filter according to claim 1, wherein said cordierite particles have a median particle diameter of 5-30 μm.

4. The ceramic honeycomb filter according to claim 1, wherein said amorphous silica particles comprise 5-25% by mass of said first silica particles and 75-95% by mass of said second silica particles, based on the total mass of said amorphous silica particles.

5. The ceramic honeycomb filter according to claim 1, wherein said amorphous oxide matrix is silica.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) is a front view showing an example of the ceramic honeycomb filters of the present invention.

(2) FIG. 1(b) is a cross-sectional view longitudinally showing an example of the ceramic honeycomb filters of the present invention.

(3) FIG. 2(a) is a schematic view showing a step of forming plugs of the ceramic honeycomb filter of the present invention.

(4) FIG. 2(b) is a schematic view showing another step of forming plugs of the ceramic honeycomb filter of the present invention.

(5) FIG. 2(c) is a schematic view showing a further step of forming plugs of the ceramic honeycomb filter of the present invention.

(6) FIG. 3 is a front view showing cracks generated in the ceramic honeycomb filter by thermal shock.

(7) FIG. 4 is a scanning electron photomicrograph showing a cross section of a plug in the ceramic honeycomb filter produced in Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The embodiments of the present invention will be explained specifically below, without intention of restricting the present invention thereto. Proper modifications, improvements, etc. may be made based on usual knowledge of those skilled in the art, within the scope of the present invention.

(9) [1] Ceramic Honeycomb Filter

(10) The ceramic honeycomb filter of the present invention comprises a cordierite-type ceramic honeycomb structure having large numbers of flow paths partitioned by porous cell walls, and plugs formed in end portions of predetermined flow paths of the ceramic honeycomb structure; the cell walls having a thermal expansion coefficient Tw (×10.sup.−7/° C.) of 10 or less in the flow path direction between 40° C. and 800° C.; the plugs comprising at least ceramic particles, and 5-25 parts by mass of an amorphous oxide matrix per 100 parts by mass of the ceramic particles; the ceramic particles comprising at least 42-90% by mass of amorphous silica particles, and 10-58% by mass of cordierite particles; and the amorphous silica particles comprising 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm, and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm.

(11) Amorphous silica and cordierite are known as low-thermal expansion materials; amorphous silica having a thermal expansion coefficient of 5×10.sup.−7/° C. or less, and cordierite having a thermal expansion coefficient of 16×10.sup.−7/° C. or less. In the ceramic honeycomb filter of the present invention, amorphous silica particles and cordierite particles are used as ceramic particles constituting the plugs, with their mixing ratio and the ratio of the amorphous oxide matrix for binding the ceramic particles optimized. Accordingly, when a filter undergoes thermal shock in burning PM captured by the cell walls for regeneration, cracking unlikely occurs at interconnections of cell walls between the plugs, with high bonding strength between the plugs and the cell walls and uniform plug length.

(12) [1-1] Cordierite-Type Ceramic Honeycomb Structure

(13) (1) Thermal Expansion Coefficient

(14) The cordierite-type ceramic honeycomb structure constituting the ceramic honeycomb filter of the present invention comprises cordierite as a main crystal, with their cell walls having a thermal expansion coefficient Tw (x 10.sup.−7/° C.) of 10 or less in the flow path direction between 40° C. and 800° C. When the thermal expansion coefficient Tw (×10.sup.−7/° C.) of the cell walls in the flow path direction between 40° C. and 800° C. is more than 10, the honeycomb structure has low thermal shock resistance, failing to withstand practical use as a ceramic honeycomb filter for removing particulate matter from an exhaust gas discharged from diesel engines. The thermal expansion coefficient Tw (x 10.sup.−7/° C.) of the cell walls in the flow path direction between 40° C. and 800° C. is preferably 9.5 or less, and further preferably 9 or less, and its lower limit is 3 or more, and preferably 3.5 or more.

(15) (2) Pore Diameter Distribution

(16) The cordierite-type ceramic honeycomb structure constituting the ceramic honeycomb filter of the present invention preferably has porosity of 50-60%, a pore diameter d5, at which a cumulative pore volume is 5% of the total pore volume in a pore diameter distribution measured by mercury porosimetry, being 22 μm or more and less than 55 μm, a pore diameter d10 at 10% being 15-35 μm, a median pore diameter d50 being 10-20 μm, a pore diameter d85 at 85% being 5-9 μm, pore diameter d90 at 90% being 3-8 μm, and a pore diameter d98 at 98% being 2.5 μm or less. With such a pore diameter distribution, the ceramic honeycomb structure can effectively capture nano-sized PM having large influence on the number of particles in an exhaust gas.

(17) [1-2] Plugs

(18) (1) Ceramic Particles

(19) Ceramic particles constituting the plugs in the ceramic honeycomb filter of the present invention comprise at least 42-90% by mass of amorphous silica particles, and 10-58% by mass of cordierite particles. Namely, the amount X of amorphous silica particles in the ceramic particles is 42-90% by mass.

(20) When the amount X of amorphous silica particles is less than 42% by mass, the plugs have a large thermal expansion coefficient in the flow path direction because of a small amount of amorphous silica particles in the plugs, resulting in too large thermal expansion coefficient difference between the cell walls and the plugs. As a result, cracking occurs by thermal shock at cell wall interconnections between plugs when the filter is regenerated, etc., resulting in low thermal shock resistance. On the other hand, when the amount X of amorphous silica particles is more than 90% by mass, the plugs have a small thermal expansion coefficient because of a large amount of amorphous silica particles in the plugs, providing too large thermal expansion coefficient difference in the flow path direction between the cell walls and the plugs. As a result, cracking occurs by thermal shock at cell wall interconnections between plugs when the filter is regenerated, etc., resulting in low thermal shock resistance.

(21) Further, the amount X (% by mass) of amorphous silica particles in the ceramic particles and the thermal expansion coefficient Tw (×10.sup.−7/° C.) of the cell walls between 40° C. and 800° C. preferably meet the formula (1) of
(118.33−8.1367×Tw)≤X≤(150.88−8.1367×Tw).

(22) With the amount X of amorphous silica particles in a range meeting the formula (1), the difference between the thermal expansion coefficient of the cell walls in the flow path direction and the thermal expansion coefficient of the plugs is small, surely suppressing cracking by thermal shock at cell wall interconnections between plugs when the filter is regenerated, etc. The lower limit of the amount X (% by mass) of amorphous silica particles is preferably (122.4−8.1367×Tw), and further preferably (126.47−8.1367×Tw). The upper limit of the amount X (% by mass) is preferably (146.81−8.1367×Tw), and further preferably (142.74−8.1367×Tw).

(23) Though the ceramic particles are composed of amorphous silica particles and cordierite particles, 1% or less by mass of ceramic particles (alumina, mullite, etc.) other than amorphous silica particles and cordierite particles may be contained, because 1% or less by mass of other ceramic particles would have small influence on the thermal expansion coefficient of the plugs.

(24) (i) Amorphous Silica Particles

(25) Amorphous silica particles in the ceramic particles constituting the plugs comprise 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm, and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm. With at least two types of amorphous silica particles having different median particle diameters contained at the above ratios, the shrinkage of a plugging material is suppressed when heat-dried in the step of forming plugs, resulting in less likelihood of unevenness in plug length, and making stronger the plugs obtained by heat-drying the plugging material.

(26) When the second silica particles in the amorphous silica particles are less than 70% by mass (the first silica particles are more than 30% by mass), the plugging material largely shrinks when heat-dried, likely resulting in uneven plug length. On the other hand, when the second silica particles are more than 96% by mass (the first silica particles are less than 4% by mass), the plugs obtained by heat-drying the plugging material are likely to have low strength.

(27) In the amorphous silica particles, the lower limit of the percentage of the second silica particles is preferably 75% by mass, and further preferably 80% by mass (the upper limit of the percentage of the first silica particles is preferably 25% by mass, and further preferably 20% by mass), and the upper limit of the percentage of the second silica particles is preferably 95% by mass, and further preferably 90% by mass (the lower limit of the percentage of the first silica particles is preferably 5% by mass or more, and further preferably 10% by mass or more).

(28) When the first silica particles in the amorphous silica particles have a median particle diameter of less than 10 μm, the plugging material largely shrinks when heated, likely resulting in uneven plug length. On the other hand, when the median particle diameter is more than 40 μm, the plugs obtained by heating the plugging material are likely to have low strength. Preferably, the lower limit is 15 μm or more, and the upper limit is 35 μm or less. In a curve showing the relation between the particle diameters and cumulative volume of the first silica particles, the lower limit of the particle diameter D10 at a cumulative volume corresponding to 10% of the total volume is 1 μm or more, and preferably 5 μm or more, and the upper limit is 20 μm or less, and preferably 15 μm or less. The lower limit of the particle diameter D90 at a cumulative volume corresponding to 90% of the total volume is 30 μm, and preferably 40 μm, and the upper limit is 70 μm, and preferably 55 μm. Incidentally, D10<D50<D90.

(29) When the median particle diameter of the second silica particles is less than 70 μm, the plugging material largely shrinks when heated, likely resulting in uneven plug length. On the other hand, when the median particle diameter is more than 200 μm, the plugs obtained by heating the plugging material are likely to have low strength. Preferably, the lower limit is 80 μm or more, and the upper limit is 180 μm or less. The lower limit of D10 of the second silica particles is 50 μm or more, and preferably 55 μm or more, and the upper limit is 80 μm or less, and preferably 70 μm or less. The lower limit of D90 is 100 μm or more, and preferably 110 μm or more, and the upper limit is 300 μm or less, and preferably 250 μm or less. Incidentally, D10<D50<D90.

(30) The silica particles preferably have sphericity of 0.5 or more. When the sphericity of the silica particles is less than 0.5, the plugging material has high viscosity, exhibiting low fluidity when filled in predetermined flow paths, and thus likely resulting in uneven plug length. The sphericity of the silica particles is further preferably 0.6 or more, and more preferably 0.7 or more. The sphericity of each silica particle is determined by dividing a projected area of the silica particle by the area of a circle having a diameter corresponding to the maximum length of a line segment connecting two points on a periphery of the silica particle, which passes through the center of gravity of the silica particle, and it can be determined by the image analysis of an electron photomicrograph.

(31) The silica particles may contain CaO, Na.sub.2O, K.sub.2O, Fe.sub.2O.sub.3, etc. as impurities. To prevent the plugs from having too large a thermal expansion coefficient, the total amount of the above impurities is preferably 0.1% or less.

(32) (ii) Cordierite Particles

(33) The cordierite particles in the ceramic particles constituting the plugs preferably have a median particle diameter of 5-30 μm. When the median particle diameter is less than 5 μm, the plugging material largely shrinks when heated, likely resulting in uneven plug length. On the other hand, when the median particle diameter is more than 30 μm, the plugs obtained by heating the plugging material are likely to have low strength. The preferred lower and upper limits of the median particle diameter of the cordierite particles are 7 μm and 25 μm, respectively. The lower limit of D10 of the cordierite particles is preferably 0.5 μm, and more preferably 1 μm, and the upper limit is preferably 10 μm, and more preferably 7 μm. The lower limit of D90 is preferably 15 μm, and more preferably 20 μm, and the upper limit is preferably 50 μm, and more preferably 40 μm. Incidentally, D10<D50<D90.

(34) The cordierite particles may contain Na.sub.2O, Fe.sub.2O.sub.3, K.sub.2O, CaO, TiO.sub.2, etc. as impurities in a total amount of 3% or less by mass.

(35) (2) Amorphous Oxide Matrix

(36) The amorphous oxide matrix constituting the plugs is preferably formed by colloidal oxide such as colloidal silica, colloidal alumina, etc., because it has excellent bondability to amorphous silica particles and cordierite particles as well as to the cell walls, providing the plugs with high strength. Further, to provide the plugs with a desired thermal expansion coefficient, the colloidal metal oxide is more preferably colloidal silica.

(37) The amount of the amorphous oxide matrix constituting the plugs in the ceramic honeycomb filter of the present invention is 5-25 parts by mass per 100 parts by mass of the ceramic particles. When the amount of the amorphous oxide matrix is less than 5 parts by mass per 100 parts by mass of the ceramic particles, the amorphous oxide matrix exhibits an insufficient bonding force to the ceramic particles, failing to provide the plugs with necessary strength. On the other hand, when the amount of the amorphous oxide matrix is more than 25 parts by mass, the plugs are occupied by too much an amorphous oxide matrix, resulting in a small percentage of ceramic particles as aggregates, providing the plugs with low strength. The lower limit of the amount of the amorphous oxide matrix is preferably 10 parts by mass, and the upper limit is preferably 20 parts by mass.

(38) Though the plugs constituting the ceramic honeycomb filter of the present invention comprise the ceramic particles and the amorphous oxide matrix, materials forming the plugs are not limited thereto, but organics such as binders, ceramic fibers, cement, unsintered ceramic powder, etc. may be contained if necessary.

(39) [2] Production Method of Ceramic Honeycomb Filter

(40) (1) Production of Ceramic Honeycomb Structure

(41) At least cordierite-forming material powder formulated to form cordierite by sintering, which has a composition comprising 48-52% by mass of SiO.sub.2, 33-37% by mass of Al.sub.2O.sub.3, and 12-15% by mass of MgO, a pore-forming material, and an organic binder are mixed to prepare a starting material mixture. Kaolin, talc, silica, alumina, and aluminum hydroxide can be used for the cordierite-forming material. Kaolin and talc are preferably in a planar shape. It is preferable to use kaolin having a cleavage index of 0.80 or more, and talc having a morphology index of 0.5 or more.

(42) The cleavage index of kaolin particles can be determined, as described in JP 2006-265034 A, by pressing a predetermined amount of kaolin particles into a container, measuring the X-ray diffraction of a pressed surface, and calculating the following formula:
Cleavage index=I.sub.(002)/[I.sub.(200)+I.sub.(020)+I.sub.(002)],
wherein I.sub.(200), I.sub.(020) and I.sub.(002) represent the peak intensities of a (200) plane, a (020) plane and a (002) plane.

(43) The morphology index of talc can be determined, as described in U.S. Pat. No. 5,141,686, by measuring the X-ray diffraction of planar talc particles, and calculating the following formula:
Morphology index=Ix/(Ix+2Iy),
wherein Ix represents the diffraction intensity of a (004) plane, and Iy represents the diffraction intensity of a (020) plane.

(44) Water, a lubricant, etc. are added to the starting material mixture, and kneaded to prepare a moldable material. The moldable material is extruded through a known extrusion die to a honeycomb shape preferably having an outer diameter of 150 mm or more, a wall thickness of 0.05-0.5 mm, and a cell density of 50-500 cells/square inch. The extruded ceramic honeycomb green body is dried in a hot-air furnace, a microwave drying apparatus, etc., sintered at the highest keeping temperature of 1380-1420° C., and then machined in the end surfaces by a diamond grinder to obtain a cordierite-type ceramic honeycomb structure 11 having a predetermined length. By sintering a green body obtained by extruding the cordierite-forming material powder using planar kaolin and talc to a honeycomb shape at a temperature of 1380-1420° C., it is possible to produce a ceramic honeycomb structure having cell walls whose thermal expansion coefficient Tw (×10.sup.−7/° C.) in the flow path direction is 10 or less between 40° C. and 800° C.

(45) (2) Formation of Plugs

(46) (i) Attaching of Plug-Forming Film and Formation of Holes

(47) A plugging material is alternately charged into the end surfaces 12a, 12b of predetermined flow paths 15a, 15b of the ceramic honeycomb structure 11, and dried to form plugs 13a, 13b in a checkerboard pattern. The charging steps of the plugging material are explained referring to FIGS. 2(a) to 2(c). A plug-forming film 21a is first attached to one end surface 12a of the ceramic honeycomb structure 11, and provided with holes 22 in a checkerboard pattern by a laser, etc. at positions corresponding to the flow paths 15a [FIG. 2(a)]. Likewise, a plug-forming film 21b is attached to another end surface 12b, and provided with holes 22 in a checkerboard pattern by a laser, etc. at positions corresponding to the flow paths 15b [FIG. 2(a)]. The holes 22 are arranged such that the flow paths 15b open on the end surface 12a and the flow paths 15a open on the end surface 12b are alternately positioned. The method of forming the holes 22 is not restricted, as long as the plug-forming film can be provided with openings. For example, a method of pushing a metal rod having a sharp tip end or a heated metal rod to the film may be used.

(48) (ii) Preparation of Plugging Material

(49) The plugging material 23 is prepared as follows. First, amorphous silica particles comprising 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm are prepared in a total amount of 100% by mass. The first and second silica particles are preferably spherical particles having sphericity of 0.5 or more. The spherical particles are preferably produced by melting silica stone in a high-temperature flame for spheroidization by surface tension. Next, 100 parts by mass of ceramic particles comprising 42-90% by mass of the above amorphous silica particles and 10-58% by mass of cordierite particles are dry-mixed with 0.5-5.0 parts by mass of a binder such as methylcellulose, etc., and if necessary, ceramic fibers, cement, unsintered ceramic powder, etc. 5-25 parts by mass on a solid basis of colloidal oxide (colloidal silica, colloidal alumina, etc.) is added to 100 parts by mass of the ceramic particles, and wet-mixed and blended with 20-40 parts by mass of water to obtain the plugging material 23. The amount X (% by mass) of amorphous silica particles in the ceramic particles and the thermal expansion coefficient Tw (×10.sup.−7/° C.) of the cell walls of the honeycomb structure between 40° C. and 800° C. preferably meet the formula (1):
(118.33−8.1367×Tw)≤X≤(150.88−8.1367×Tw).

(50) The cordierite particles are obtained, for example, by molding the cordierite-forming powder formulated to form cordierite by sintering to a block-shaped green body, which is sintered in a furnace whose highest temperature is adjusted in a cordierite-forming temperature range, for example, 1350-1450° C., for about 20 hours, and dry-pulverizing the resultant sintered block by a ball mill, etc.

(51) The colloidal oxide is a colloidal substance, in which nano-sized particles are dispersed in a dispersion medium such as water, etc. The colloidal oxide acts to bind amorphous silica particles and cordierite particles constituting the plugs, and bond the plugs to the cell walls. Also, the addition of the colloidal oxide increases the fluidity of the plugging material 23, so that the plugging material 23 charged into the flow paths 15b through holes 22 formed in the plug-forming film 21a attached to the end surface 12a of the ceramic honeycomb structure 11 can easily enter to a predetermined depth as described below. The use of colloidal silica, colloidal alumina, etc. as the colloidal oxide is preferable to further improve the fluidity of the plugging material 23, because the plugging material 23 charged into the flow paths 15b through holes 22 formed in the plug-forming film 21a attached to the end surface 12a of the ceramic honeycomb structure 11 can more easily enter to a predetermined depth as described below. Particularly, it is more preferable to use colloidal silica, because it makes it easy to provide the plugs with a desired thermal expansion coefficient. The solid fraction of the colloidal oxide is preferably 20-40% by mass.

(52) (iii) Charging of Plugging Material into One End Surface

(53) One end surface 12a of the ceramic honeycomb structure 11 is then immersed in the blended plugging material 23 to let the plugging material 23 enter the flow paths 15a to a predetermined depth through the holes 22 of the plug-forming film 21a attached to the end surface 12a [FIG. 2(b)].

(54) (iv) Drying of Plugging Material

(55) The plugging material 23 charged into the flow paths 15b on the side of one end surface 12a is dried to remove water from the plugging material 23 and the dispersion medium from the colloidal oxide, thereby binding ceramic particles and bonding the plugs 13a to the cell walls. The plugging material 23 can be dried by a hot-air furnace, microwave heating, high-frequency induction heating, etc. Among them, the high-frequency induction heating is preferable.

(56) (v) Charging of Plugging Material into Another End Surface

(57) The plugging material 23 is similarly charged into another end surface 12b of the ceramic honeycomb structure 11, so that the plugging material 23 enters the flow paths 15b to a predetermined depth.

(58) (iv) Drying of Plugging Material

(59) The plugging material 23 charged into another end surface 12b is similarly dried to form plugs 13b, thereby obtaining a ceramic honeycomb filter 10 having flow paths 15a, 15b alternately sealed by the plugs 13a, 13b on one end surface 12a and another end surface 12b [FIG. 2(c)].

(60) In the ceramic honeycomb structure of the ceramic honeycomb filter produced by the above method, the thermal expansion coefficient of the cell walls in the flow path direction and the thermal expansion coefficient of the plugs can meet a proper relation. The cell walls of the ceramic honeycomb filter have a thermal expansion coefficient Tw (×10.sup.−7/° C.) of 10 or less in the flow path direction between 40° C. and 800° C. The plugs comprise at least ceramic particles, and 5-25 parts by mass of an amorphous oxide matrix per 100 parts by mass of the ceramic particles, the ceramic particles comprising at least 42-90% by mass of amorphous silica particles and 10-58% by mass of cordierite particles, and the amorphous silica particles comprising 4-30% by mass of first silica particles having a median particle diameter of 10-40 μm and 70-96% by mass of second silica particles having a median particle diameter of 70-200 μm.

EXAMPLES

(61) The present invention will be explained in further detail by Examples, without intention of restricting the present invention thereto.

Examples 1-9

(62) 6.0 parts by mass of kaolin powder (median particle diameter: 4.2 μm, and cleavage index: 0.92), 41.2 parts by mass of talc powder (median particle diameter: 11.8 μm, D10: 5.4 μm, D90: 35.3 μm, and morphology index 0.59), 18.1 parts by mass of silica powder (median particle diameter: 25.7 μm, D10: 15.3 μm, D20: 18.2 μm, D80: 38.7 μm, D90: and 49.2 μm), 23.3 parts by mass of alumina powder (median particle diameter: 4.9 μm, D90: 9.7 μm, and percentage of 25 μm or more: 0.01%, and 11.3 parts by mass of aluminum hydroxide powder (median particle diameter: 8.9 μm) were mixed to prepare a cordierite-forming material powder, to which 5.5 parts by mass of methylcellulose and hydroxypropyl methylcellulose as a binder, and 5.5 parts by mass of a foamed resin (average particle diameter: 41.5 μm) as a pore-forming material were added. After sufficient dry mixing, 25 parts by mass of water was added, and sufficient blending was conducted to prepare a plasticized moldable ceramic material. The moldable material was extruded from an extrusion die, and cut to a green body of 270 mm in diameter and 300 mm in length. The green body was dried, and sintered at the highest keeping temperature of 1400° C. to obtain a cordierite-type ceramic honeycomb structure 11, whose cell walls had a thickness of 0.3 mm, a cell pitch of 1.5 mm, porosity of 58%, an average pore diameter of 13 μm, a pore diameter distribution in which d5=54.4 μm, d10=25.9 μm, d85=7.9 μm, d90=5.1 μm, and d98=1.4 jam, and a thermal expansion coefficient of 9.2×10.sup.−7/° C. in the flow path direction between 40° C. and 800° C.

(63) Plugs were formed in the ceramic honeycomb structure 11 as follows. The end surfaces 12a, 12b of the ceramic honeycomb structure 11 were ground, and a resin film 21a having a thickness of 0.09 mm was attached as a plug-forming film to each end surface. The film was provided with holes 22 in a checkerboard pattern at positions at which the flow paths were to be plugged, by a laser beam [FIG. 2(a)].

(64) Tables 1-3 show ceramic particles and colloidal oxide, which were plugging materials forming the plugs of the ceramic honeycomb filters of Examples. These plugging materials, an organic binder (methylcellulose) and water were mixed with the formulation shown in Table 4, to prepare the plugging materials of Examples 1-9 by the following procedure.

(65) First, the amorphous silica particles shown in Table 1 and the cordierite particles shown in Table 2 were dry-mixed with methylcellulose. After dry mixing, the colloidal silica shown in Table 3 and ion-exchanged water were added to conduct blending for 30 minutes in a vacuum blending mixer, to prepare plugging material slurries. The particle size distribution and median particle diameter of the ceramic particles used were measured by a Microtrack particle size distribution meter (MT3000 available from Nikkiso Co., Ltd.).

(66) The ceramic honeycomb structure 11 on the side of the end surface 12a was then immersed in the plugging material 23, to let the plugging material 23 enter the flow paths 15a to the depth of 10 mm through the holes 22 formed in the film 21a [FIG. 2(b)]. The end surface 12a, on which the plugging material 23 was charged, was heated by high-frequency induction (output: 7 kW) for 30 seconds to dry the plugging material 23, and the film 21a was then peeled from the end surface 12a. The honeycomb structure 11 on the side of another end surface 12b was similarly immersed in the plugging material 23, to let the plugging material 23 enter the flow paths 15b to the depth of 10 mm through the holes 22 formed in the film 21b. The end surface 12b, on which the plugging material 23 was charged, was then heated by high-frequency induction (power: 7 kW) for 30 seconds to dry the plugging material 23, and the film 21b was then peeled from the end surface 12b to obtain the ceramic honeycomb filters of Examples 1-9.

(67) The scanning electron photomicrograph of a cross section of a plug in the ceramic honeycomb filter of Example 1 is shown in FIG. 4. The plugs comprising amorphous silica particles and cordierite particles bound by an amorphous oxide matrix were fixed to the cell walls in end portions of the flow paths of the ceramic honeycomb structure.

Example 10

(68) 6.0 parts by mass of kaolin powder (median particle diameter: 4.2 μm, and cleavage index: 0.92), 41.2 parts by mass of talc powder (median particle diameter: 10.9 μm, D10: 3.5 μm, D90: 31.2 μm, and morphology index 0.59), 18.1 parts by mass of silica powder (median particle diameter: 21.1 μm, D10: 11.1 μm, D20: 14.2 μm, D80: 34.3 μm, and D90: 45.4 μm), 22.8 parts by mass of alumina powder (median particle diameter: 4.4 μm, D90: 8.5 μm, and percentage of 25 μm or more: 0.01%), and 11.3 parts by mass of aluminum hydroxide powder (median particle diameter: 8.9 μm) were mixed to prepare a cordierite-forming material powder, to which 5.5 parts by mass of methylcellulose and hydroxypropyl methylcellulose as a binder, and 5.5 parts by mass of a foamed resin (average particle diameter 38.8 μm) as a pore-forming material were added. After sufficient dry mixing, 25 parts by mass of water was added, and sufficient blending was conducted to prepare a plasticized moldable ceramic material. The moldable material was extruded from an extrusion die, and cut to a green body of 270 mm in diameter and 300 mm in length. The green body was dried, and sintered at the highest keeping temperature of 1400° C. to obtain a cordierite-type ceramic honeycomb structure 11, whose cell walls had a thickness of 0.3 mm, a cell pitch of 1.5 mm, porosity of 58%, an average pore diameter of 13 μm, a pore diameter distribution in which d5=46.9 μm, d10=23.7 μm, d85=7.4 μm, d90=4.7 μm, and d98=1.1 jam, and a thermal expansion coefficient of 7.0×10.sup.−7/° C. in the flow path direction between 40° C. and 800° C.

(69) Plugs were formed in the ceramic honeycomb structure 11 in the same manner as in Example 1 except for using the plugging material of Example 10 shown in Table 4, to produce the ceramic honeycomb filter of Example 10.

Comparative Examples 1-3 and 5-10

(70) The ceramic honeycomb filters of Comparative Examples 1-3 and 5-10 were produced in the same manner as in Example 1, except for using the plugging materials of Comparative Examples 1-3 and 5-10 shown in Table 4.

Comparative Example 4

(71) 6.0 parts by mass of kaolin powder (median particle diameter: 4.5 μm, and cleavage index: 0.95), 41.2 parts by mass of talc powder (median particle diameter: 14.0 μm, and morphology index 0.77), 18.2 parts by mass of silica powder (median particle diameter: 45 μm), 23.3 parts by mass of alumina powder (median particle diameter: 4.0 μm), and 11.3 parts by mass of aluminum hydroxide powder (median particle diameter: 1.8 μm) were mixed to prepare a cordierite-forming material powder, to which 5.5 parts by mass of methylcellulose and hydroxypropyl methylcellulose as a binder, and 8 parts by mass of a foamed resin (average particle diameter: 43.5 μm) as a pore-forming material were added. After sufficient dry mixing, 26 parts by mass of water was added, and sufficient blending was conducted to prepare a plasticized moldable ceramic material. The moldable material was extruded from an extrusion die, and cut to a green body of 270 mm in diameter and 300 mm in length. The green body was dried, and sintered at the highest keeping temperature of 1400° C. to obtain a cordierite-type ceramic honeycomb structure 11, whose cell walls had a thickness of 0.3 mm, a cell pitch of 1.5 mm, porosity of 61%, an average pore diameter of 26.5 μm, a pore diameter distribution in which d5=78.3 μm, d10=54.5 μm, d85=11.5 μm, d90=8.5 μm, and d98=2.0 μm, and a thermal expansion coefficient of 12.9×10.sup.−7/° C. in the flow path direction between 40° C. and 800° C.

(72) Plugs were formed in the ceramic honeycomb structure 11 in the same manner as in Example 1 to produce the ceramic honeycomb filter of Comparative Example 4.

(73) With respect to the ceramic honeycomb filters of Examples 1-10 and Comparative Examples 1-10, the thermal expansion coefficients and thermal shock resistance of the cell walls and plugs, the bonding strength of the plugs to the cell walls, and the unevenness of plug length were evaluated as follows. The results are shown in Table 5.

(74) (a) Thermal Expansion Coefficient of Cell Walls

(75) The thermal expansion coefficient of the cell walls was determined as an average thermal expansion coefficient between 40° C. and 800° C., on a test piece having a cross section shape of 4.5 mm×4.5 mm×50 mm (length) cut out of the cell walls with its longitudinal direction substantially in alignment with the flow path direction, by heating it from room temperature to 800° C. at a temperature-elevating speed of 10° C./minute and measuring its longitudinal length increase under a constant load of 20 g by a thermomechanical analyzer TMA of a compression load/differential expansion type (ThermoPlus available from Rigaku Corp.).

(76) (b) Thermal Expansion Coefficient of Plugs

(77) The blended plugging material remaining after charged into end portions of the flow paths of the ceramic honeycomb structure was dried and sintered under the same conditions as for the ceramic honeycomb structure, and a test piece having a cross section of 4.5 mm×4.5 mm×50 mm (length) was cut out of the resultant sintered body, to measure the thermal expansion coefficient of the plugs by a thermomechanical analyzer in the same manner as for the cell walls.

(78) (c) Thermal Shock Resistance

(79) The evaluation test of thermal shock resistance was conducted by heating the ceramic honeycomb filter at 500° C. for 30 minutes in an electric furnace, rapidly cooling it to room temperature, and then observing both end surfaces with the naked eye to examine whether or not cracks (see FIG. 3) were generated at cell wall interconnections between plugs. When cracks were not observed, the same test was conducted with the heating temperature elevated by 25° C. (second: 525° C., third: 550° C., . . . ), and this test was repeated until cracks were generated. The test was conducted on three samples, and the difference between a temperature at which cracks were generated in at least one honeycomb structure and room temperature (heating temperature−room temperature) was regarded as a thermal shock resistance temperature and evaluated by the following standard. Excellent: The thermal shock resistance temperature was 575° C. or higher. Good: The thermal shock resistance temperature was 550° C. or higher and lower than 575° C. Fair: The thermal shock resistance temperature was 525° C. or higher and lower than 550° C. Poor: The thermal shock resistance temperature was lower than 525° C.

(80) (d) Bonding Strength of Plugs to Cell Walls

(81) To evaluate the bonding strength of the plugs to the cell walls, a pushing rod of stainless steel having a flat tip end surface and a diameter of 0.8 mm was pressed to a plug from the end surface side on which the plug was formed, to measure a load at which the pushing rod broke the plug or detached the plug from the cell walls, and the load was divided by a cross section area (0.50 mm.sup.2) of the pushing rod to determine plug strength (MPa), which was averaged for 10 plugs. The bonding strength was evaluated by the following standard. Excellent: The plug strength was 40 MPa or more. Good: The plug strength was 30 MPa or more and less than 40 MPa. Poor: The plug strength was less than 30 MPa.

(82) (e) Unevenness of Plug Length

(83) The unevenness of plug length was evaluated by measuring the length of plugs formed in one ceramic honeycomb filter. The plug length was determined by inserting a stainless steel rod with a scale into a flow path from an end surface of the ceramic honeycomb structure having no plugs, measuring the length of the flow path when the rod came into contact with a plug on the opposite side (the length of an inserted portion of the stainless steel rod), and calculating its difference from the length of the ceramic honeycomb structure. With respect to one ceramic honeycomb filter, 17 flow paths were measured from each end surface (34 flow paths in total), to calculate the standard deviation of plug length. The unevenness of plug length was evaluated by the following standard. Excellent: The standard deviation of plug length was less than 0.50. Good: The standard deviation of plug length was 0.50 or more and less than 0.60. Poor: The standard deviation of plug length was 0.60 or more.

(84) TABLE-US-00001 TABLE 1 Amorphous CaO + Na.sub.2O + Silica D50.sup.(1) D10 D90 K.sub.2O + Fe.sub.2O.sub.3 Particles (μm) (μm) (μm) Sphericity (% by mass) Silica A 21 11 46 0.93 0.0014 Silica B 12 3 35 0.65 0.0014 Silica C 86 61 134 0.74 0.0016 Silica D 180 54 263 0.79 0.0016 Note: .sup.(1)Median particle diameter.

(85) TABLE-US-00002 TABLE 2 CaO Na.sub.2O K.sub.2O Cordierite D50.sup.(1) D10 D90 (% by (% by (% by Particles (μm) (μm) (μm) mass) mass) mass) Cordierite A 11 3.1 31 0.91 0.18 0.28 Cordierite B 145 85 220 0.80 0.15 0.21 Note: .sup.(1)Median particle diameter.

(86) TABLE-US-00003 TABLE 3 Amount.sup.(1) Percentage ((% by mass) Colloidal Oxide (% by mass) SiO.sub.2 Al.sub.2O.sub.3 MgO Na.sub.2O CaO Colloidal Silica A 40 99.4 ≤0.1 ≤0.1 0.4 ≤0.1 Note: .sup.(1)Amount on a solid basis.

(87) TABLE-US-00004 TABLE 4 Ceramic Particles Amorphous Silica Particles Formulation First Second Cordierite Silica Silica X .sup.(1) Particles Particles Particles (parts Parts % by % by by by No. Type mass Type mass mass) Type Mass .sup.(2) Example 1 Silica A 14.3 Silica C 85.7 70 A 30 Example 2 Silica A 25.0 Silica C 75.0 70 A 30 Example 3 Silica A 5.0 Silica C 95.0 70 A 30 Example 4 Silica A 14.3 Silica C 85.7 45 A 55 Example 5 Silica A 14.3 Silica C 85.7 75 A 25 Example 6 Silica A 14.3 Silica D 85.7 70 A 30 Example 7 Silica B 14.3 Silica C 85.7 70 A 30 Example 8 Silica A 14.3 Silica C 85.7 49 A 51 Example 9 Silica A 14.3 Silica C 85.7 66 A 34 Example 10 Silica A 14.3 Silica C 85.7 74 A 26 Com. Ex. 1 — 0 — 0 0 A 100  Com. Ex. 2 — 0 Silica C 100.0 100 —  0 Com. Ex. 3 Silica A 100.0 — 0 100 —  0 Com. Ex. 4 Silica A 14.3 Silica C 85.7 70 A 30 Com. Ex. 5 Silica A 14.3 silica C 85.7 40 A 60 Com. Ex. 6 Silica A 100.0 — 0 40 A 60 Com. Ex. 7 — 0 Silica C 100.0 40 A 60 Com. Ex. 8 Silica A 3.0 Silica C 97.0 70 A 30 Com. Ex. 9 Silica A 31.0 Silica C 69.0 70 A 30 Com. Ex. 10 — 0 — 0 0 A + B .sup. 100.sup.(3) Binder Ion- (Methyl- Exchanged Colloidal Oxide cellulose) Water Parts by Parts by Parts by No. Type Mass .sup.(4) Mass .sup.(5) Mass .sup.(5) Example 1 Colloidal Silica A 15 2.5 31 Example 2 Colloidal Silica A 15 2.5 31 Example 3 Colloidal Silica A 15 2.5 31 Example 4 Colloidal Silica A 15 2.5 31 Example 5 Colloidal Silica A 15 2.5 31 Example 6 Colloidal Silica A 15 2.5 31 Example 7 Colloidal Silica A 15 2.5 31 Example 8 Colloidal Silica A 15 2.5 31 Example 9 Colloidal Silica A 15 2.5 31 Example 10 Colloidal Silica A 15 2.5 31 Com. Ex. 1 Colloidal Silica A 15 2.5 31 Com. Ex. 2 Colloidal Silica A 15 2.5 31 Com. Ex. 3 Colloidal Silica A 15 2.5 31 Com. Ex. 4 Colloidal Silica A 15 2.5 31 Com. Ex. 5 Colloidal Silica A 15 2.5 31 Com. Ex. 6 Colloidal Silica A 15 2.5 31 Com. Ex. 7 Colloidal Silica A 15 2.5 31 Com. Ex. 8 Colloidal Silica A 15 2.5 31 Com. Ex. 9 Colloidal Silica A 15 2.5 31 Com. Ex. 10 Colloidal Silica A 15 2.5 31 Note: .sup.(1) The amount of amorphous silica particles in the ceramic particles (amorphous silica particles + cordierite particles). .sup.(2) The amount of cordierite particles in the ceramic particles (amorphous silica particles + cordierite particles). .sup.(3)A mixture of 60 parts by mass of cordierite particles A and 40 parts by mass of cordierite particles B. Note: .sup.(4) The amount of colloidal oxide on a solid basis per 100 parts by mass of ceramic particles (amorphous silica particles + cordierite particles). .sup.(5) Amount per 100 parts by mass of ceramic particles (amorphous silica particles + cordierite particles).

(88) TABLE-US-00005 TABLE 5 Thermal Expansion Coefficient (40-800° C.) Evaluation Results Cell Walls Plugs Thermal Uneven- Tw Tp Shock ness (×10.sup.−7/ (×10.sup.−7/ Resis- Plug of Plug No. ° C.) ° C.) tance Strength Length Example 1 9.2 8 Good Excellent Excellent Example 2 9.2 8 Good Excellent Good Example 3 9.2 8 Good Good Excellent Example 4 9.2 11 Fair Excellent Excellent Example 5 9.2 7.1 Fair Excellent Excellent Example 6 9.2 8 Good Good Excellent Example 7 9.2 8 Good Excellent Good Example 8 9.2 10.5 Good Excellent Excellent Example 9 9.2 8.4 Excellent Excellent Excellent Example 10 7 7.7 Excellent Excellent Excellent Com. Ex. 1 9.2 16.5 Poor Excellent Poor Com. Ex. 2 9.2 4.1 Poor Poor Poor Com. Ex. 3 9.2 4.1 Poor Excellent Poor Com. Ex. 4 12.9 8 Poor Excellent Excellent Com. Ex. 5 9.2 11.6 Poor Excellent Excellent Com. Ex. 6 9.2 11.6 Poor Excellent Poor Com. Ex. 7 9.2 11.6 Poor Good Poor Com. Ex. 8 9.2 7.5 Good Excellent Poor Com. Ex. 9 9.2 7.5 Good Excellent Poor Com. Ex. 10 9.2 13.3 Poor Excellent Poor

(89) It is clear from Table 4 that the ceramic honeycomb filters of Examples 1-10 of the present invention have enough bonding strength of plugs to cell walls and uniform plug length while keeping thermal shock resistance. On the other hand, any of the ceramic honeycomb filters of Comparative Examples 1-10 did not meet all of the requirements of thermal shock resistance, bonding strength between plugs and cell walls, and uniformity of plug length.