CERAMIC HONEYCOMB FILTER AND ITS PRODUCTION METHOD

Abstract

A ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cordierite cell walls, and plugs formed in predetermined flow paths of the ceramic honeycomb structure; the plugs being formed by ceramic particles and an amorphous oxide matrix existing between the ceramic particles; in a cross section of the plugs, an area ratio A1 of the amorphous oxide matrix in a longitudinal range of ⅓×t from one end, and an area ratio A2 of the amorphous oxide matrix in a longitudinal range of ⅓×t from the other end meeting the relation of ½≦A1/A2≦2, wherein t represents the length of the plug in a direction perpendicular to the longitudinal direction of the plug.

Claims

1. A ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal, and plugs formed in predetermined flow paths of said ceramic honeycomb structure; said plugs comprising ceramic particles and an amorphous oxide matrix existing between said ceramic particles; said amorphous oxide matrix being 5-20 parts by mass per 100 parts by mass of said ceramic particles; and in a cross section of said plug including the center axis of said flow path, a ratio A1/A2 meeting the relation of ½≦A1/A2≦2, wherein A1 represents an area ratio of said amorphous oxide matrix in a longitudinal range of ⅓×t from one end, A2 represents an area ratio of said amorphous oxide matrix in a longitudinal range of ⅓×t from the other end, and t represents the length of said plug in a direction perpendicular to the longitudinal direction of said plug.

2. The ceramic honeycomb filter according to claim 1, wherein the ratio A1/A2 of area ratios A1 and A2 of said amorphous oxide matrix meets the relation of ⅔≦A1/A2≦1.5.

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

4. A method for producing a ceramic honeycomb filter comprising a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal, and plugs formed in predetermined flow paths of said ceramic honeycomb structure; comprising charging a plugging material comprising at least 100 parts by mass of ceramic particles, 5-20 parts by mass on a solid basis of colloidal oxide and 1.5-4 parts by mass of a binder into the predetermined flow paths of said ceramic honeycomb structure, and drying said plugging material by microwave heating or high-frequency dielectric heating to form said plugs; said ceramic particles having a particle size distribution at least a first peak and a second peak lower than said first peak, said first peak being in a particle size range of 100-200 μm, and said second peak being in a particle size range of 10-30 μm.

5. The method for producing a ceramic honeycomb filter according to claim 4, wherein after said plugging material is charged into the predetermined flow paths of said ceramic honeycomb structure, and before said microwave heating or high-frequency dielectric heating is conducted, an end surface of said ceramic honeycomb structure on the side that said plugging material is charged is preheated at 30-80° C. for 1-10 minutes with a heat-conducting means contacted.

6. The method for producing a ceramic honeycomb filter according to claim 4, wherein said ceramic particles comprise 20-50% by mass of first ceramic particles having an average particle size of 90-200 μm, and 50-80% by mass of second ceramic particles having an average particle size of 5-30 μm.

7. The method for producing a ceramic honeycomb filter according to claim 4, wherein said microwave heating is conducted by irradiating microwave with power of 1-30 W/g per a unit mass of said plugging material for 1-20 minutes.

8. The method for producing a ceramic honeycomb filter according to claim 4, wherein said high-frequency dielectric heating is conducted by applying a high-frequency electric field with power of 1-20 W/g per a unit mass of said plugging material at a distance of 1-15 mm between an end surface of said ceramic honeycomb structure and a high-frequency electrode, for 1-5 minutes.

9. The method for producing a ceramic honeycomb filter according to claim 4, wherein said colloidal oxide is colloidal silica.

10. The method for producing a ceramic honeycomb filter according to claim 4, wherein said ceramic particles are based on cordierite.

11. The method for producing a ceramic honeycomb filter according to claim 4, wherein said plugs are formed without sintering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1(a) is a perspective view schematically showing an example of the ceramic honeycomb filters of the present invention.

[0032] FIG. 1(b) is a view showing a longitudinal cross section of the ceramic honeycomb filter of the present invention shown in FIG. 1(a)

[0033] FIG. 2(a) is a schematic view showing a plugging step in the production method of a ceramic honeycomb filter.

[0034] FIG. 2(b) is a schematic view showing another plugging step in the production method of a ceramic honeycomb filter.

[0035] FIG. 2(c) is a schematic view showing a further plugging step in the production method of a ceramic honeycomb filter.

[0036] FIG. 3 is a schematic view showing the positions of measuring the area ratio of an amorphous oxide matrix in a plug in the ceramic honeycomb filter.

[0037] FIG. 4 is a graph showing the particle size distribution of ceramic material powder used in Example 3 of the present invention.

[0038] FIG. 5 is an electron photomicrograph showing a cross section of a plug in the ceramic honeycomb filter produced in Example 1 of the present invention.

[0039] FIG. 6 is an electron photomicrograph showing a cross section of a plug in the ceramic honeycomb filter produced in Example 1 of the present invention.

[0040] FIG. 7 is an electron photomicrograph showing a cross section of a plug in the ceramic honeycomb filter produced in Example 1 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] The embodiments of the present invention will be specifically explained below without intention of restricting the present invention thereto. It should be noted that proper modifications and improvements can be made based on the usual knowledge of those skilled in the art within the scope of the present invention.

[0042] [1] Ceramic Honeycomb Filter

[0043] The ceramic honeycomb filter of the present invention comprises a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal, and plugs formed in predetermined flow paths of the ceramic honeycomb structure;

[0044] the plugs comprising ceramic particles and an amorphous oxide matrix existing between the ceramic particles;

[0045] the amorphous oxide matrix being 5-20 parts by mass per 100 parts by mass of the ceramic particles; and

[0046] in a cross section of the plug including the center axis of the flow path, the plugs having a ratio of A1/A2 meeting the relation of ½≦A1/A2≦2, wherein A1 represents an area ratio of an amorphous oxide matrix in a longitudinal range of ⅓×t from one end, A2 represents an area ratio of the amorphous oxide matrix in a longitudinal range of ⅓×t from the other end, and t represents the length of the plug in a direction perpendicular to the longitudinal direction of the plug. In the case of a honeycomb having a square or hexagonal lattice shape, “t” corresponds to the distance between opposing cell walls. In the case of a honeycomb having a triangular lattice shape, “t” corresponds to the height of a triangle. “t” may be called the width of plug below.

[0047] When the plugs meet the above requirement, namely when there is small difference between the concentration of an amorphous oxide matrix in a range of ⅓×t from one end of the plug (for example, plug end on the flow path end surface side), and the concentration of an amorphous oxide matrix in a longitudinal range of ⅓×t from the other end of the plug (for example, on the flow path inner side), wherein t represents the length of the plug in a direction perpendicular to the longitudinal direction of the plug, the plugs have good bonding strength to cell walls in their entire length from one end to the other end, so that the plugs are not easily detached during use, resulting in high resistance to particulate-matter-capturing performance decrease. The ratio A1/A2 of the area ratios A1 and A2 of an amorphous oxide matrix meets the relation of preferably ⅔≦A1/A2≦1.5, more preferably 0.8≦A1/A2≦1.3.

[0048] The area ratios A1 and A2 of an amorphous oxide matrix in a flow-path-direction, center-axis-including cross section of a plug can be determined, for example, as follows. Namely, electron photomicrographs (FIGS. 6 and 7) taken on a flow-path-direction, center-axis-including cross section of a plug in the ceramic honeycomb filter are analyzed by an image analyzer (for example, Image-Pro Plus ver. 7.0 available from Media Cybernetics). In the electron photomicrographs of FIGS. 6 and 7 having black portions, high-concentration gray portions, and low-concentration gray portions, it is confirmed by EDX composition analysis that the high-concentration gray portions (shown by the arrow a) are an amorphous oxide matrix (SiO.sub.2), the low-concentration gray portions (shown by the arrow b) are aggregate (cordierite 5SiO.sub.2.2Al.sub.2O.sub.3.2MgO), and the black portions (shown by the arrow c) are voids. The area of the amorphous oxide matrix (for example, portion shown by the arrow) is determined from the photograph, and divided by a field area to obtain the area ratio. As shown in FIG. 3, the area ratio A1 of an amorphous oxide matrix in a range a1 corresponding to ⅓ of the width t of the plug 13a from one end 131a (an end of a plug on the flow path end surface side in the figure), and the area ratio A2 of an amorphous oxide matrix in a range a2 corresponding to ⅓ of the width t of the plug 13a from the other end 132a (an end of a plug inside a flow path in the figure) are determined.

[0049] In the ceramic honeycomb filter of the present invention, the amorphous oxide matrix is preferably silica. When the amorphous oxide matrix is silica, the plugs have such high bonding strength to the cell walls that they are less detachable during use, avoiding decrease in particulate-matter-capturing performance. The amorphous oxide matrix is preferably made of colloidal oxide. The colloidal oxide is preferably colloidal silica.

[0050] [2] Production Method of Ceramic Honeycomb Filter

[0051] The production method of the ceramic honeycomb filter of the present invention will be explained below. The ceramic honeycomb filter for removing particulate matter from an exhaust gas is produced by forming plugs by charging a plugging material into predetermined flow paths of a ceramic honeycomb structure having pluralities of flow paths partitioned by porous cell walls made of a material comprising cordierite as a main crystal. The plugging material comprises at least 100 parts by mass of ceramic particles, 5-20 parts by mass on a solid basis of colloidal oxide, and 1.5-4 parts by mass of a binder. The ceramic particles have a particle size distribution having at least a first peak, and a second peak lower than the first peak, the first peak being in a particle size range of 100-200 μm, and the second peak being in a particle size range of 10-30 μm.

[0052] A method of charging a plugging material into predetermined flow paths of a ceramic honeycomb structure will be explained referring to FIG. 2. Plugging films 21a, 21b are attached to end surfaces 12a, 12b of a ceramic honeycomb structure 10, and provided with penetrating pores 22 at positions corresponding to the flow paths 15a or 15b, for example, by laser irradiation [FIG. 2(a)]. The plugging films 21a, 21b are provided with penetrating pores 22 in a checkerboard pattern, such that the flow paths 15a are provided with plugs 13a on the side of the end surface 12a while being open on the side of the end surface 12b, and that the flow paths 15b are provided with plugs 13b on the side of the end surface 12b while being open on the side of the end surface 12a. The penetrating pores 22 can be formed by piercing the plugging film with a sharp-pointed metal pin, or by pushing a heated metal pin to the plugging film, as long as openings are formed in the plugging film.

[0053] The ceramic honeycomb structure 10 is then immersed in a slurry of the plugging material 23 on the side of the end surface 12a, so that the plugging material 23 is introduced into the flow paths 15a through the penetrating pores 22 formed in the plugging film 21a [FIG. 2(b)]. To have fluidity for easy charging, the plugging material comprising at least ceramic material powder and colloidal oxide contains water.

[0054] The plugging material charged into the predetermined flow paths of the ceramic honeycomb structure is dried by microwave heating or high-frequency dielectric heating, so that the plugging material is bonded to the ceramic honeycomb structure. In the plugging material comprising at least ceramic particles, colloidal oxide, a binder and water, the colloidal oxide irreversibly forms a strong sold, an amorphous oxide matrix, by dehydration, thereby bonding ceramic particles. Microwave heating or high-frequency dielectric heating may be conducted, after the plugging material is charged into one or both ends of the predetermined flow paths of the ceramic honeycomb structure.

[0055] By heating the plugging material by microwave heating or high-frequency dielectric heating, the entire plugs are uniformly heated without temperature gradient. Because a liquid component is evaporated from the plugging material by such uniform heating, not only from one end of plugs (for example, ends of plugs on the flow path end surface side), but also from the other end (for example, ends of plugs inside flow paths), and partially through cell walls, the phenomenon that colloidal oxide is segregated in one-side portions as in the drying of plugs in a hot-air furnace does not occur, resulting in a smaller concentration difference of colloidal oxide longitudinally in plugs. Accordingly, plugs are well bonded to cell walls at any longitudinal position. Thus, drying by microwave heating or high-frequency dielectric heating provides a ceramic honeycomb filter having a ratio A1/A2 meeting the relation of ½≦A1/A2≦2, wherein A1 represents an area ratio of an amorphous oxide matrix in a longitudinal range of ⅓×t from one end, and A2 represents an area ratio of an amorphous oxide matrix in a longitudinal range of ⅓×t from the other end, in a cross section of the plug including the center axis of the flow path; and t represents the length of plug in a direction perpendicular to the longitudinal direction. An area ratio A3 of an amorphous oxide matrix in an intermediate portion between one end and the other end meets ⅓×A1<A3≦A1, and ⅓×A2<A3≦A2.

[0056] Before microwave heating or high-frequency dielectric heating, the charged plugging material is preferably preheated. The preheating is preferably started within 5 minutes after charging, and conducted at 30-80° C. for 1-10 minutes with the charged plugging material in contact with a heat-conducting means. The preheating is conducted, for example, by bringing an end surface of the ceramic honeycomb structure on the plugging-material-charged side into contact with an electric heating plate, etc. heated to a predetermined temperature. The preheating temperature is preferably 35-70° C., more preferably 40-60° C. With the plugging material preheated, the binder in the plugging material is gelled (hardened), lowering the fluidity of a plugging material slurry, thereby preventing the end surface of the plugging material from being dented on the charged side. Thus, the resultant plugs have enough length and high strength. The binder is preferably thermally hardenable by gelation, particularly methylcellulose, etc. Though the preheating of an end surface of the ceramic honeycomb structure may be conducted by direct contact of the end surface with an electric heating plate, etc., it is preferably conducted, for example, with a paper or a cloth arranged between them, because part of the plugging material remains attached to the electric heating plate, failing to have enough plug length.

[0057] The microwave irradiation is preferably conducted at 1-30 W/g per a unit mass of the plugging material for 1-20 minutes. The high-frequency dielectric heating is preferably conducted by high-frequency power of 1-20 W/g per a unit mass of the plugging material for 1-5 minutes, with high-frequency voltage-applying electrodes and ground electrodes alternately arranged with predetermined intervals placed 1-15 mm separate from the end surfaces of the ceramic honeycomb structure. The plugging material is heated to about 80-200° C. by microwave heating or high-frequency dielectric heating under such conditions, resulting in higher bonding strength of the plugs to the cell walls. Accordingly, even when the plugs are formed at 1000° C. or lower, the plugs are well bonded to the cell walls, resistant to detachment during use, resulting in a ceramic honeycomb filter free from decrease in particulate-matter-capturing performance. The gap between an end surface of the ceramic honeycomb structure and each high-frequency electrode may be determined by placing a ceramic plate having a desired thickness on the high-frequency electrode, and placing the ceramic honeycomb structure thereon.

[0058] The microwave heating or the high-frequency dielectric heating is preferably started within 20 minutes after the plugging material is charged into the predetermined flow paths of the ceramic honeycomb structure, when no preheating is conducted. When 20 minutes or more pass without preheating after charging, a liquid component in the plugging material is absorbed into the cell walls by capillary phenomenon. Accordingly, colloidal oxide in the plugging material likely migrates to the cell walls together with water, resulting in lower bonding strength of the plugs to the cell walls on the flow path end surface side, and lower strength of the plugs per se. As a result, the plugs are likely detached during use, resulting in lower particulate-matter-capturing performance. The microwave heating or high-frequency dielectric heating is more preferably conducted within 10 minutes after the plugging material is charged. While the microwave heating takes a long drying time because the entire honeycomb body including plugs is heated in a microwave apparatus, the high-frequency dielectric heating can efficiently heat the plugs only. When preheating is conducted after the plugging material is charged into the predetermined flow paths of the ceramic honeycomb structure, the binder in the plugging material is gelled. Because water is still contained in the plugging material, high bonding strength of the plugs can be obtained when the microwave heating or high-frequency dielectric heating is started within 60 minutes after preheating.

[0059] The ceramic particles have a particle size distribution having at least a first peak, and a second peak lower than the first peak, the first peak being in a particle size range of 100-200 μm, and the second peak being in a particle size range of 10-30 μm. Namely, the particle size distribution of the ceramic particles has at least two peaks, a higher peak being called “first peak,” and a peak lower than the first peak being called “second peak.” Such particle size distribution means that ceramic particles in the plugging material comprise at least two types of powder, powder having larger particle sizes and powder having smaller particle sizes. Using ceramic particles having such a particle size distribution, ceramic particles having smaller particle sizes intrude space between ceramic particles having larger particle sizes, resulting in plugs having a high filling density of ceramic particles. Accordingly, the plugs are then strongly bonded to the cell walls at a low temperature, resulting in a ceramic honeycomb filter suffering no decrease in particulate-matter-capturing performance, with plugs less detachable during use.

[0060] The ceramic particles having such a particle size distribution is preferably obtained by mixing 20-50% by mass of first ceramic particles having an average particle size of 90-200 μm and 50-80% by mass of second ceramic particles having an average particle size of 5-30 μm. The mixing of two types of such ceramic particles can provide ceramic particles having a particle size distribution having at least a first peak, and a second peak lower than the first peak, the first peak being in a particle size range of 100-200 μm, and the second peak being in a particle size range of 10-30 μm.

[0061] When the first ceramic particles have an average particle size of less than 90 μm, there are likely gaps between heat-dried plugs and cell walls, so that the plugs are easily detachable, resulting in low particulate-matter-capturing performance. When the first ceramic particles have an average particle size of more than 200 μm, there is likely a higher percentage of powder having larger particle sizes, resulting in low heat shock resistance. The first ceramic particles preferably have an average particle size of 100-180 μm.

[0062] When the second ceramic particles have an average particle size of less than 5 μm, there is likely a higher percentage of powder having larger particle sizes, resulting in low heat shock resistance. When the second ceramic material powder has an average particle size of more than 30 μm, the heat-dried plugs likely have voids, so that the plugs are easily detachable, having low particulate-matter-capturing performance. The second ceramic particles preferably have an average particle size of 10-25 μm.

[0063] When the amount of the first ceramic particles mixed is less than 20% (when the amount of the second ceramic particles mixed is more than 80%), the heat-dried plugs likely have voids, so that the plugs are easily detachable, having low particulate-matter-capturing performance. On the other hand, when the amount of the first ceramic particles mixed is more than 50% (when the amount of the second ceramic particles mixed is less than 50%), there is likely a higher percentage of powder having larger particle sizes, resulting in low heat shock resistance. The amounts of the first and second ceramic particles are more preferably 25-45% of the first ceramic particles and 55-75% of the second ceramic particles.

[0064] The particle size distribution of ceramic particles can be measured by a particle size distribution meter (Microtrack MT3000 available from Nikkiso Co., Ltd.). In FIG. 4, the axis of abscissa represents a particle size, and the axis of ordinates represents the frequency (%) of each particle size. The second peak lower than the first peak means that the frequency P2 (height) of the second peak is smaller than the frequency P1 (height) of the first peak. When the plugs are formed, ceramic particles having smaller particle sizes intrude gaps between those having larger particle sizes, resulting in a higher filling ratio. To have high bonding strength between the low-temperature-bonded plugs to the cell walls, the height P1 of the first peak is preferably 3 times or less, more preferably 2 times or less, of the height P2 of the second peak.

[0065] The first ceramic particles preferably has sphericity of 0.6 or more. The first ceramic particles having sphericity of 0.6 or more have small surface areas, so that they are easily bonded to the second ceramic particles, preferably resulting in high bonding strength of the plugging material and between the plugs and the cell walls. The sphericity of the first ceramic particles is preferably 0.7 or more, more preferably 0.8 or more. The sphericity is determined by dividing the area of each projected image of 10 particles measured by image analysis on an electron photomicrograph, by the area of a circle having a diameter corresponding to the maximum length between two points at which a straight line passing a center of gravity of each particle crosses a circumference of the particle, and averaging the calculated ratios for 10 particles.

[0066] With the plugs fixated by cordierite-based ceramic particles at 1000° C. or lower in the production of the ceramic honeycomb filter of the present invention, the difference in a thermal expansion coefficient between the ceramic honeycomb structure and the plugs can be made small, resulting in a ceramic honeycomb filter having good heat shock resistance. The first ceramic particles are preferably sintered porous cordierite powder. The porous cordierite powder preferably has porosity of 40-60%.

[0067] The present invention will be explained in more detail by Examples below without intention of restriction.

Example 1

[0068] Kaolin powder, talc powder, silica powder and alumina powder were mixed to prepare cordierite-forming material powder comprising 50% by mass of SiO.sub.2, 35% by mass of Al.sub.2O.sub.3, and 13% by mass of MgO, which was then fully mixed with a binder such as methylcellulose, hydroxypropyl methylcellulose, etc., a lubricant, and hollow resin balloons as a pore-forming material in a dry state. With a predetermined amount of water added, they were sufficient blended to prepare a plasticized moldable ceramic material. The moldable material was extruded, and cut to a honeycomb-structured green body of 270 mm in diameter and 300 mm in length. The green body was dried and sintered to obtain a cordierite-type ceramic honeycomb structure 10 having a cell wall thickness of 0.3 mm, a cell wall pitch of 1.5 mm, porosity of 63%, and an average pore size of 21 μm.

[0069] As shown in FIG. 2, a plugging resin film of 0.09 mm in thickness was attached to each of both ground end surfaces 12a, 12b of the ceramic honeycomb structure 10, and each plugging resin film was provided with penetrating pores at positions corresponding to flow paths to be plugged in a checkerboard pattern by laser beams [FIG. 2(a)]. The penetrating pores 22 of the plugging films 21a, 21b were formed in a checkerboard pattern, such that flow paths 15b were open at the end surface 12a, and flow paths 15a were open at the end surface 12b.

[0070] As shown in Table 1, 100 parts by mass of ceramic material powder obtained by mixing the first ceramic particles and the second ceramic particles (both made of cordierite) was mixed and blended with colloidal oxide (colloidal silica having a solid concentration of 40% by mass) in an amount shown in Table 2, 50 parts by mass of ion-exchanged water, 2.5 parts by mass of methylcellulose as a binder, to prepare a plugging material slurry. The particle size distribution of the ceramic material powder used was measured by a particle size distribution meter (Microtrack MT3000 available from Nikkiso Co., Ltd.), to determine the frequency P1 (height) of the first peak and the frequency P2 (height) of the second peak.

TABLE-US-00001 TABLE 1 First Ceramic Particles .sup.(1) Second Ceramic Particles .sup.(2) Average Amount Average Amount Particle (parts Particle (parts Size by Size by No. (μm) Sphericity mass) (μm) Sphericity mass) Example 1 125 0.8 32 13.5 0.6 68 Example 2 125 0.8 40 13.5 0.6 60 Example 3 125 0.8 23 13.5 0.6 77 Example 4 190 0.7 31 27 0.5 69 Example 5 125 0.8 32 13.5 0.6 68 Example 6 125 0.8 23 13.5 0.6 77 Com. Ex. 1 25 0.8 100 — — — Com. Ex. 2 167 0.6 5 13 0.6 95 Com. Ex. 3 250 0.4 33 36 0.4 67 Com. Ex. 4 25 0.8 100 — — — Mixed Ceramic Material Powder Particle Size at Particle Size at First Peak (P1) Second Peak (P2) Peak Height Ratio No. (μm) (μm) (P1/P2) Example 1 141 19.5 1.3 Example 2 141 19.5 1.6 Example 3 141 19.5 1.2 Example 4 183 27 1.4 Example 5 141 19.5 1.3 Example 6 141 19.5 1.2 Com. Ex. 1 25 — — Com. Ex. 2 15 160 1.2 Com. Ex. 3 235 33 1.3 Com. Ex. 4 25 — — Note: .sup.(1) Cordierite particles. .sup.(2) Cordierite particles.

TABLE-US-00002 TABLE 2 Formulation (parts by mass) Ceramic Material No. Powder .sup.(1) Colloidal Oxide .sup.(2) Example 1 100 40 [16] Example 2 100 40 [16] Example 3 100 40 [16] Example 4 100 37.5 [15].sup.  Example 5 100 40 [16] Example 6 100 40 [16] Com. Ex. 1 100 40 [16] Com. Ex. 2 100 40 [16] Com. Ex. 3 100 37.5 [15].sup.  Com. Ex. 4 100 40 [16] Note: .sup.(1) A mixture of the first ceramic particles and the second ceramic particles. .sup.(2) Colloidal silica having a solid concentration of 40% by mass.

[0071] The end surface 12a of the ceramic honeycomb structure 10 was immersed in a bath of a plugging material 23, which was introduced into the flow paths 15a to the depth of 10 mm through penetrating pores 22 formed in the plugging film 21a [FIG. 2(b)]. Immediately after introducing the plugging material 23, the end surface 12a on the side of which the plugging material 23 was introduced was preheated via four papers on an electric heating plate at 50° C. for 5 minutes. Another end surface 12b of the ceramic honeycomb structure 10 was then immersed in a bath of the plugging material 23, which was similarly introduced into the flow paths 15b to the depth of 10 mm through penetrating pores 22 formed in the plugging film 21b. The end surface 12b was also preheated by a hot plate like the end surface 12a. With the plugging films 21a, 21b peeled, the plugs were heated by microwave of 2450 MHz having power of 12 W/g per a unit mass of the plugging material in a microwave heating apparatus for 4 minutes (see Table 3), to dry the plugging material 23, thereby producing the of ceramic honeycomb filter of Example 1.

TABLE-US-00003 TABLE 3 Heating Conditions Maximum Time Until Heating Was Started After Heating No. Preheating Charged Heating Method Time Example 1 50° C., 13 minutes Microwave 4 minutes 5 minutes (12 W/g) Example 2 50° C., 13 minutes microwave 4 minutes 5 minutes (12 W/g) Example 3 50° C., 13 minutes Microwave 4 minutes 5 minutes (12 W/g) Example 4 50° C., 15 minutes Microwave 4 minutes 5 minutes (12 W/g) Example 5 No 8 minutes Microwave 4 minutes (12 W/g) Example 6 50° C., 13 minutes High-Frequency 1 minute 5 minutes (6.5 W/g)  Com. Ex. 1 50° C., 35 minutes Hot Air Furnace 3 hours 5 minutes (500° C.) Com. Ex. 2 50° C., 35 minutes Hot Air Furnace 3 hours 5 minutes (500° C.) Com. Ex. 3 50° C., 13 minutes Microwave 4 minutes 5 minutes (12 W/g) Com. Ex. 4 50° C., 13 minutes Microwave 4 minutes 5 minutes (12 W/g)

Examples 2-4

[0072] The ceramic honeycomb filters of Examples 2-4 were produced in the same manner as in Example 1, except that the types and amounts of the ceramic material powder and colloidal oxide (colloidal silica having a solid concentration of 40% by mass) were changed as shown in Tables 1 and 2, and that the heating conditions were changed as shown in Table 3.

Example 5

[0073] A ceramic honeycomb structure 10 was produced in the same manner as in Example 1, and the plugging material 23 was introduced into the ceramic honeycomb structure 10 on the side of the end surface 12a. The end surface 12a on the side of which the plugging material 23 was introduced was not preheated. The plugging material 23 was then introduced into another end surface 12b of the ceramic honeycomb structure 10 as in Example 1, with the end surface 12b similarly not preheated. With the plugging films 21a, 21b peeled, microwave heating was conducted in the same manner as in Example 1 to produce the ceramic honeycomb filter of Example 5.

Example 6

[0074] The ceramic honeycomb filter of Example 6 was produced in the same manner as in Example 3, except that the plugs were heated by high-frequency (40 MHz) power of 6.5 W/g per a unit mass of the plugging material at the distance of 3 mm from an end surface of the ceramic honeycomb structure for 1 minute by a high-frequency heating apparatus, in place of the microwave heating (see Table 3).

Comparative Examples 1-4

[0075] The ceramic honeycomb filters of Comparative Examples 1-3 were produced in the same manner as in Example 1, except that the types and amounts of ceramic material powders and the amount of colloidal oxide (colloidal silica having a solid concentration of 40% by mass) were changed as shown in Tables 1 and 2, and that the heating conditions were changed as shown in Table 3. The ceramic honeycomb filter of Comparative Example 1 produced by the method described in JP 2005-125318 A had a particle size distribution having one peak, because ceramic material powder comprising only one type of ceramic particles was used as aggregate (see Table 1).

[0076] With respect to the plugs in the ceramic honeycomb filters of Examples and Comparative Examples, the area ratios of amorphous oxide matrices, the bonding strength of plugs, soot-capturing performance and heat shock resistance were elevated as follows. The results are shown in Table 4.

[0077] (1) Area Ratio of Amorphous Oxide Matrix in Plugs

[0078] An electron photomicrograph of a cross section of a plug including the center axis of a flow path was analyzed by an image analyzer (Image-Pro Plus ver. 6.3 available from Media Cybernetics) to measure the areas of aggregate and an amorphous oxide matrix, from which an area ratio A1 of an amorphous oxide matrix in a range a1 corresponding to ⅓ of the width t of the plug from one end 131a (ends of the plug on the flow path end surface side in the figure), and an area ratio A2 of an amorphous oxide matrix in a range a2 corresponding to ⅓ of the width t of the plug from the other end 132a (ends of the plug inside the flow path in the figure), to calculate a ratio A1/A2, as shown in FIG. 3. Further, an area ratio A3 of an amorphous oxide matrix in an intermediate portion between one end 131a and the other end 132a of plugs 13a (at center of a range a3 between the range a1 and the range a2) was determined.

[0079] (2) Strength of Plugs

[0080] The bonding strength of plugs to cell walls was determined by pushing a flat-tipped push rod having a diameter of 0.8 mm to a plug, measuring a load when the push rod crashed the plug, or when the plug was detached, dividing the load by a cross section area (2.01 mm.sup.2) of the push rod to calculate the strength (MPa) of each plug, and averaging the strength values measured on 10 plugs. The results are shown in Table.

[0081] (3) Soot-Capturing Performance

[0082] With carbon powder having a particle size of 0.042 μm introduced into a ceramic honeycomb filter at a speed of 3 g/h together with air flow of 10 Nm.sup.3/min in a pressure loss test stand, the number Nin of carbon powder particles flowing into the honeycomb filter and the number Nout of carbon powder particles flowing out of the honeycomb filter were counted by Model 3936 of a scanning mobility particle sizer (SMPS) available from TIS, for 1 minute between 3 minutes and 4 minutes after start, to calculate the capturing ratio of soot by the formula of (Nin Nout)/Nin. Soot-capturing performance was evaluated by the following standard: [0083] Excellent: The capturing ratio was 98% or more, [0084] Good: The capturing ratio was 95% or more and less than 98%, [0085] Fair: The capturing ratio was 90% or more and less than 95%, and [0086] Poor: The capturing ratio was less than 90%.

[0087] (4) Heat Shock Resistance

[0088] The evaluation test of heat shock resistance was conducted by heating the ceramic honeycomb filter at 400° C. for 30 minutes in an electric furnace, rapidly cooling it to room temperature, and observing cracks in cell walls near the plugs by the naked eye. When no cracks were observed, the same test was conducted with the temperature of the electric furnace elevated by 25° C., and this operation was repeated until cracking occurred. The test was conducted three times for each sample. The difference between a temperature at which cracking occurred in at least one honeycomb structure and room temperature (heating temperature−room temperature) was regarded as a heat shock resistance temperature, which was evaluated by the following standard: [0089] Excellent: The heat shock resistance temperature was 550° C. or higher, [0090] Good: The heat shock resistance temperature was 500° C. or higher and lower than 550° C., [0091] Fair: The heat shock resistance temperature was 450° C. or higher and lower than 500° C., and [0092] Poor: The heat shock resistance temperature was lower than 450° C.

[0093] (5) Porosity of Plugs

[0094] The porosity of a peripheral wall was determined by analyzing an electron photomicrograph of a cross section of a plug cut out of the ceramic honeycomb filter by an image analyzer (Image-Pro Plus ver. 7.0 of Media Cybernetics).

TABLE-US-00004 TABLE 4 Amorphous Oxide Matrix Evaluation Results in Plug Porosity Bonding Heat Area Ratio A1/ of Plug Strength Capturing Shock No. A1 A2 A3 A2 (%) (MPa) Ratio Resistance Example 1 42 40 17 1.1 28 51 Excellent Excellent Example 2 40 34 15 1.2 31 45 Excellent Good Example 3 41 35 18 1.2 27 34 Good Excellent Example 4 40 32 19 1.3 30 34 Good Excellent Example 5 42 40 13 1.1 29 29 Excellent Excellent Example 6 41 35 15 1.2 28 34 Good Excellent Com. Ex. 1 45 8 9 5.6 38 22 Poor Good Com. Ex. 2 43 15 17 2.9 24 24 Poor Good Com. Ex. 3 42 19 21 2.2 43 23 Fair Fair Com. Ex. 4 43 18 20 2.4 32 24 Poor Good

[0095] FIG. 5 is an electron photomicrograph of a cross section of a plug in the ceramic honeycomb filter of Example 1 of the present invention, indicating that the plugs produced by the method of the present invention were uniform with little segregation of colloidal oxide. It is clear from Table 4 that the ceramic honeycomb filters of Examples 1-6 of the present invention had excellent bonding strength of plugs, soot-capturing performance and heat shock resistance, though the plug-bonding strength was slightly poor in Example 5, in which preheating was not conducted.

[0096] On the other hand, in any of Comparative Example 1 in which ceramic particles having a particle size distribution having only one peak at 25 μm was used for a plugging material which was dried in a hot-air furnace, Comparative Example 4 in which a plugging material comprising the same ceramic particles as in Comparative Example 1 was dried by microwave, and Comparative

[0097] Example 2 in which ceramic particles having two peaks at 15 μm (first peak) and at 160 μm (second peak), the first peak being outside the particle size range of 100-200 μm, and the second peak being outside the particle size range of 10-30 μm was used for a plugging material which was dried in a hot-air furnace, the plug-bonding strength and the soot-capturing performance were extremely poor. In Comparative Example 3 in which ceramic particles having a first peak outside the particle size range of 100-200 μm and a second peak outside the particle size range of 10-30 μm, the plug-bonding strength was extremely poor, and the soot-capturing performance and the heat shock resistance were slightly poor.