JP2003210922A - Ceramic honeycomb filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ceramic honeycomb filter having a high strength, low in pressure loss, increased in trap quantity and excellent in regeneration efficiency. <P>SOLUTION: The ceramic honeycomb filter comprises a porous ceramic sintered body having a large number of cells extending in the filter axial direction. The cell wall 13 for constituting each of the cells has a core layer 21 and coating layers 22 formed on both surfaces of the core layer 21. The coating layers 22 are relatively higher than the core layer 21 in porosity. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic honeycomb filter having a plurality of cells, and more particularly to the structure of cell walls constituting the cells.

[0002]

2. Description of the Related Art The number of automobiles has dramatically increased since the twentieth century, and in proportion to this, the amount of exhaust gas emitted from the internal combustion engine of automobiles has also been rapidly increasing. In particular, various substances contained in the exhaust gas emitted from a diesel engine cause pollution, and are now seriously affecting the global environment. In addition, recently, research results have been reported that particulate matter (diesel particulates) in exhaust gas sometimes causes allergic disorders and a decrease in sperm count. In other words, taking measures to remove fine particles in exhaust gas is considered to be an urgent task for humanity.

Under these circumstances, various types of exhaust gas purifying apparatuses have been proposed in the past. A general exhaust gas purifying device has a structure in which a casing is provided in the middle of an exhaust pipe connected to an exhaust manifold of an engine, and a DPF (diesel particulate filter) made of a honeycomb structure is arranged therein. . The honeycomb structure forming the DPF includes a plurality of cells extending along the axial direction of the filter, and is formed using a porous ceramic sintered body as a material.

This type of DPF is required to have characteristics such as low pressure loss and large soot collection amount. Therefore, in order to meet such requirements, a DPF made of a porous ceramic sintered body having an increased average particle diameter, average pore diameter and porosity has been conventionally proposed. Specifically,
Set the average particle size and average pore size of the porous ceramic sintered body to 20.
The porosity is 40% to 80% by setting it to about μm to 60 μm.
It was proposed to set the degree.

[0005]

However, in such a DPF, while low pressure loss and an increase in the amount of soot collected are achieved, the mechanical strength remarkably decreases due to the increase in porosity. There was a drawback. Therefore, in recent years, in order to eliminate such drawbacks, a cell wall having a multi-layer structure is formed, and the porosity in the surface layer portion is set relatively lower than that in the deep portion of the cell wall to reinforce the cell wall. DPF has been proposed. As a conventional technique similar to this, there is a technique disclosed in, for example, Japanese Patent Laid-Open No. 2000-202220. In the above publication, the average particle diameter of the ceramic particles in the surface layer portion (“coat layer” in the publication) is set to be smaller than that in the deep layer portion (“base material” in the publication) of the cell wall so that the porosity is increased. Making the difference is substantially disclosed.

However, when the above structure is adopted, the mechanical strength is prevented from lowering due to the presence of the layer formed in the surface layer portion, while the soot does not sufficiently reach the deep layer portion of the cell wall, which is short. Over time, soot accumulates on the surface of the cell walls and forms a cake. In this case, the deep layer portion of the cell wall is not effectively used, and as a result, high regeneration efficiency cannot be realized.

The present invention has been made in view of the above problems, and an object thereof is to provide a ceramic honeycomb filter having high strength and low pressure loss, a large trapping amount, and excellent regenerating efficiency. is there.

[0008]

In order to solve the above problems, according to the invention of claim 1, a honeycomb filter made of a porous ceramic sintered body is provided with a plurality of cells extending along the axial direction of the filter. The gist of the ceramic honeycomb filter is that the cell wall constituting the cell has a higher porosity in the surface layer than in the deep layer.

In a second aspect of the present invention, in a honeycomb filter made of a porous ceramic sintered body having a plurality of cells extending in the axial direction of the filter, the cell walls constituting the cells are core layers, and A ceramic honeycomb filter having a coating layer formed on both surfaces of a core layer, wherein the coating layer has a higher porosity than the core layer.

According to a third aspect of the invention, in the second aspect, the ceramic honeycomb filter is a filter for purifying exhaust gas of a diesel engine, and the coating layer carries a soot combustion catalyst. While
The catalyst was not supported on the core layer.

According to a fourth aspect of the present invention, in the second or third aspect, the coat layer has pores formed by extinguishing the pore-forming agent dispersed in the material for forming the coat layer by heat. I was doing.

The invention according to claim 5 is the invention according to claims 2 to 4.
In any one of the above items, it is assumed that a blunt portion is formed by the coating layer at each corner portion of the cell on a plane perpendicular to the filter axis direction.

The invention according to claim 6 is the invention according to claims 2 to 5.
The thickness of the first coating layer that covers both surfaces of the core layer is 1/3 of the thickness of the first core layer.
It was set to be ⅔.

The invention according to claim 7 is the invention according to claims 2 to 6.
In any one of the above items, both the core layer and the coat layer are made of a porous silicon carbide sintered body. Hereinafter, the "action" of the present invention will be described.

According to the first aspect of the present invention, since the surface layer portion has a higher porosity than the deep portion of the cell wall, the collected material does not become clogged in the surface layer portion, and the trapped material is not trapped. The collection can fully reach the deep layer. Therefore, the formation of cake on the cell wall surface can be suppressed, and a relatively large amount of the object to be collected can be collected. Further, since the deep layer portion of the cell wall is effectively utilized, the area where the trapped substance can come into contact with the ceramic particles is increased, and as a result, the regeneration efficiency is improved. Moreover, since the deep portion of the cell wall has a relatively low porosity, a predetermined mechanical strength is maintained.

Here, "high porosity" means that the ratio of voids in the sintered body forming the cell wall portion is high, and specifically, the average particle diameter, the average pore diameter, and the porosity of the sintered body. It means that at least one of them is relatively large.

According to the second aspect of the present invention, since the coating layer has a higher porosity than the core layer forming the cell wall, there is no clogging of the collected matter in the coating layer. The collected matter can reach the core layer sufficiently. Therefore, the formation of cake on the cell wall surface can be suppressed, and a relatively large amount of the object to be collected can be collected. In addition, since the core layer, which is the deep portion of the cell wall, is effectively utilized, the area where the trapped substance can come into contact with the ceramic particles is increased, and as a result, the regeneration efficiency is improved. Moreover, since the core layer on the cell wall has a relatively low porosity, a predetermined mechanical strength is maintained.

According to the third aspect of the present invention, the soot collected in the coat layer is burned and removed by the oxidizing action of the soot combustion catalyst carried in the coat layer, so that efficient regeneration is achieved. . By the way, since the surface layer portion of the cell wall is richer in oxygen than the deep layer portion, the surface layer portion is originally more suitable as the position for supporting the catalyst than the deep layer portion. In the present invention, the catalyst is supported on the coat layer, but the catalyst is not supported on the core layer, so that the amount of wasteful catalyst is reduced and the cost increase can be prevented.

According to the invention described in claim 4, since the porosity of the portion other than the pores in the coat layer (referred to as "matrix" for convenience) can be set low, it is suitable for the matrix itself regardless of the existence of the pores. It is possible to impart various strengths. Therefore, higher mechanical strength is imparted to the filter.

According to the fifth aspect of the invention, the soot is not concentrated only in the vicinity of the corners of the cell, the thickness of the soot becomes substantially uniform, and the catalyst effectively acts. Moreover,
Since each corner portion of the cell is reinforced by the blunt portion, cracks originating from the corner portion are less likely to occur, and higher mechanical strength is given to the filter.

According to the invention of claim 6, by setting the thickness of the coat layer to ⅓ to ⅔ of the thickness of the core layer, it is possible to surely increase the strength and increase the amount of trapping. Can be achieved. If it is smaller than 1/3, a large specific surface area cannot be secured for the filter as a whole, and the amount of soot trapped cannot be increased. On the other hand, if it exceeds ⅔, the core portion becomes relatively small, the strength of the filter as a whole becomes weak, and cracks and the like are likely to occur.

According to the seventh aspect of the present invention, when the core layer and the coat layer made of the porous silicon carbide sintered body are formed, the thermal expansion coefficient of both layers becomes the same, so that thermal stress acts on the cell wall. Hard to do. Moreover, silicon carbide is a high-strength material in the first place. For these reasons, the present invention can impart extremely excellent mechanical strength to the cell wall.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION An exhaust gas purifying apparatus 1 for a diesel engine according to an embodiment of the present invention will be described below.
This will be described in detail with reference to FIGS.

As shown in FIG. 1, the exhaust gas purification device 1 is a device for purifying exhaust gas emitted from a diesel engine 2 as an internal combustion engine. The diesel engine 2 includes a plurality of cylinders (not shown). Each cylinder has an exhaust manifold 3 made of a metal material.
The branching parts 4 are connected to each other. Each branch 4 is 1
Each of them is connected to the main body 5 of the book. Therefore, the exhaust gas discharged from each cylinder is concentrated in one place.

On the downstream side of the exhaust manifold 3, a first exhaust pipe 6 and a second exhaust pipe 7 made of a metal material are arranged. The upstream end of the first exhaust pipe 6 is connected to the manifold body 5
Are linked to. Between the first exhaust pipe 6 and the second exhaust pipe 7, a tubular casing 8 also made of a metal material is arranged. The upstream end of the casing 8 is the first exhaust pipe 6
Is connected to the downstream side end of the casing 8 and the downstream side end of the casing 8 is the second
It is connected to the upstream end of the exhaust pipe 7. It can also be understood that the casing 8 is arranged along the exhaust pipes 6 and 7. As a result, the internal regions of the first exhaust pipe 6, the casing 8 and the second exhaust pipe 7 communicate with each other, and the exhaust gas flows therein.

As shown in FIG. 1, the casing 8 is formed so that its central portion has a larger diameter than the exhaust pipes 6, 7. Therefore, the inner region of the casing 8 is wider than the inner regions of the exhaust pipes 6 and 7. Inside the casing 8, D as a ceramic honeycomb filter is provided.
PF9 is housed.

A heat insulating material 10 is arranged between the outer peripheral surface of the DPF 9 and the inner peripheral surface of the casing 8. Insulation 10
Is a mat-like material formed by including a ceramic fiber, and its thickness is several mm to several tens of mm. Insulation 10
Preferably has a thermal expansion property. The term “thermally expandable” as used herein means that it has a function of releasing thermal stress because it has an elastic structure. The reason for this is to prevent heat from escaping from the outermost peripheral portion of the DPF 9 to minimize energy loss during regeneration. Further, by expanding the ceramic fiber by the heat at the time of regeneration, the DP caused by the pressure of the exhaust gas and the vibration due to running
This is to prevent the displacement of F9.

As shown in FIGS. 2A and 3, the DPF 9 of this embodiment is formed by bundling and integrating a plurality of filter pieces F1. The filter piece F1 located at the center of the DPF 9 has a square columnar shape, and an atypical filter piece F having no square columnar shape around the filter piece F1.
A plurality of 1s are arranged. As a result, a cylindrical DPF 9 is formed as a whole.

As shown in FIG. 3 etc., these filter pieces F1 are made of a so-called honeycomb structure. The reason why the honeycomb structure is used is that there is an advantage that the pressure loss is small even when the amount of collected soot is increased. A large number of cells 12 are regularly formed in each filter piece F1 so as to extend along the filter axis direction. Each cell 12 has a substantially square cross section, and they are separated from each other by a thin cell wall 13 having a thickness of about 0.1 mm to 1.0 mm.

The opening of each cell 12 is sealed by a sealing body 14 on one of the end faces 9a, 9b. Therefore, the end faces 9a and 9b as a whole have a checkered pattern. The density of the cells 12 is set to about 200 cells / inch, the thickness of the cell wall 13 is set to about 0.3 mm, and the cell pitch is set to about 1.8 mm. About half of the many cells 12 are open at the upstream end surface 9a, and the rest are open at the downstream end surface 9b.

As shown in FIGS. 2A and 3, the 16 filter pieces F1 in total have their outer peripheral surfaces bonded to each other via the ceramic adhesive 15. As shown in FIGS. 2B and 2C, the DP of this embodiment is
The cell wall 13 of F9 has not a single structure but a multilayer structure, and more specifically, a three-layer structure including a core layer 21 and a coat layer 22 formed on both surfaces of the core layer 21. Have Further, the coat layer 22 has a relatively higher porosity than the core layer 21, and as a result, the surface layer portion has a higher porosity than the deep portion of the cell wall 13.

For example, in the cell wall 13 schematically shown in FIG. 4A, the average particle size of the ceramic particles 23 in the core layer 21 and the average particle size of the ceramic particles 23 in the coat layer 22 are almost the same. Are equal. However, the core layer 21 has one peak of pore diameter, whereas the coat layer 22 has two peaks of pore diameter or broad. In other words, the core layer 21 has only the small pores 24, while the coat layer 22 has the atmosphere holes 25 in addition to the small pores 24. Therefore, the average pore diameter and porosity of the coat layer 22 are
It is relatively larger than the average pore diameter and porosity of 1. The air holes 25 in FIG. 4A are formed by extinguishing the pore-forming agent dispersed in the sheet layer forming material by heat.

The average pore diameter D50 of the small pores 24 is not particularly limited, but is preferably 5 μm to 40 μm.
Even more preferably, the thickness is in the range of μm to 35 μm. When the average pore diameter D50 of the small pores 24 is less than 5 μm, the small pores 24 are likely to be closed in an early stage due to a small amount of soot, and the pressure loss may rapidly increase in a short time. On the contrary, when the average pore diameter D50 of the small pores 24 exceeds 40 μm, the mechanical strength of the matrix portion (that is, the portion other than the atmospheric pores 25 in the coat layer 22) decreases,
There is a possibility that cracks may easily occur in the cell wall 13.

The average pore diameter of the air holes 25 is preferably 1.5 times or more, more preferably 2.0 times to 10 times, especially 2.0 times as large as the average pore diameter of the small pores 24. Double
It is preferably 6.0 times. Average pore diameter D of the air holes 25
50 is not particularly limited, but is preferably 30 μm to 80 μm, more preferably 40 μm to 65 μm, and most preferably 50 μm to 60 μm. When the average pore diameter D50 of the air holes 25 is less than 30 μm, the effective filtration area does not sufficiently increase, and clogging easily occurs at an early stage. Therefore, reduction of pressure loss cannot be sufficiently achieved. Conversely, the average pore diameter D5 of the air holes 25
When 0 exceeds 80 μm, the porosity increases and the cell wall 1
The mechanical strength of No. 3 decreases. Not only that, but in some cases, it may not work as a filter structure.

The average particle diameter of the ceramic particles 23 is preferably 5 μm to 30 μm, more preferably 8 μm to 1
More preferably, it is about 5 μm. If the average particle diameter of the ceramic particles 23 is too small, the small pores 24 formed in the matrix portion are reduced in size, so that the soot hardly passes through the coat layer 22 and clogging is likely to occur early. On the other hand, if the average particle size of the ceramic particles 23 is too large, the small pores 24 formed in the matrix portion may have a large diameter, and the mechanical strength of the cell wall 13 may decrease.

On the other hand, in the cell wall 13 schematically shown in FIG. 5A, the average particle size of the ceramic particles 23 in the core layer 21 and the average particle size of the ceramic particles 23 in the coat layer 22 are different from each other. The former is smaller than the latter. However, there is only one peak of pore diameter in the core layer 21 and the coat layer 22. That is, here, the core layer 21 has only small pores 24, and the coat layer 22 has only air holes 25. The average pore diameter and porosity of the coat layer 22 are
It is relatively larger than the average pore diameter and porosity of 1.

The core layer 21 and the coat layer 22 constituting the cell wall 13 are made of a porous ceramic sintered body, and specific examples thereof include porous calcination of silicon carbide, silicon nitride, sialon, alumina, cordierite, mullite and the like. An example is a union. In this case, as the porous ceramic sintered body,
A non-oxide type porous ceramic sintered body containing silicon carbide as a main component is preferably selected. The reason is that such a sintered body is superior in mechanical strength, heat resistance, thermal conductivity and the like to other sintered bodies. The term "mainly composed of silicon carbide" means that the silicon carbide content in the composition is 5 by weight.
It is meant to contain 0% or more. In this embodiment, a porous ceramic sintered body (high-purity porous silicon carbide sintered body) containing 99% by weight or more of silicon carbide is selected for both the core layer 21 and the coat layer 22. The reason is that the core layer 21 and the coat layer 22 are not formed by using a material having high strength and a constant thermal expansion coefficient.

Coat layer 22 for covering both sides of core layer 21
Is preferably 1/3 to 2/3 of the thickness of the core layer 21. If this value is smaller than 1/3, a large specific surface area cannot be secured for the entire filter, and the amount of soot trapped cannot be increased. On the other hand, if it exceeds ⅔, the core layer 21 becomes relatively small, the strength of the filter as a whole becomes weak, and cracks and the like are likely to occur. The core layer 21 is preferably thicker than the individual coat layers 22 in order to secure the strength.

On the surface of the ceramic particles 23 constituting the coat layer 22, an oxidation catalyst composed of a noble metal represented by a platinum group element (such as Pt) and its oxide is supported as a soot combustion catalyst. Has been done. In addition to such a catalyst for soot combustion, for example, a catalyst that oxidizes CO gas or NO gas may be supported. On the other hand, the catalyst may be supported on the core layer 21, but it is more preferable not to support it. The reason is that the surface layer portion of the cell wall 13 is richer in oxygen than the deep layer portion, and thus the surface layer portion is originally suitable as a position for supporting the catalyst as compared with the deep layer portion. . Therefore, in the present embodiment in which the catalyst is carried only on the coat layer 22, the soot collected is burned and removed by the oxidizing action of the soot burning catalyst carried on the coat layer 22.

When the ceramic particles 23 are silicon carbide particles, it is desirable that the catalyst be supported via a thin support material layer made of alumina or the like, rather than being directly supported on the surface of silicon carbide.

As shown in FIGS. 2 (b) and 2 (c), the thickness of the coat layer 22 at each corner of the cell 12 on the plane perpendicular to the filter axis direction is the same as the thickness of the coat other than the corner. It is somewhat larger than the thickness of layer 22. As a result, the coat layer 22 forms the blunt portion 26 at each corner. Figure 2
In FIG. 2B, the blunt portion 26 having a smooth shape with no corners is shown, and in FIG. 2C, the blunt portion 26 having an angular shape is shown. Therefore, each corner of the cell 12 has a blunt portion 2
As a result of being reinforced by 6, the generation of cracks originating from the corners is prevented. The former is easier to form than the latter, and the effect of cracking by reinforcement tends to be higher.

Next, a procedure for manufacturing the DPF 9 will be briefly described. First, the ceramic raw material slurry is charged into an extrusion molding machine and continuously extruded through a mold. After drying, by cutting the honeycomb molded body obtained by extrusion molding into equal lengths and drying,
A honeycomb molded body cut piece having a rectangular column shape is used. Further, a predetermined amount of the paste for sealing is filled in one side opening of each cell 12 of the cut piece, and both end surfaces of each cut piece are sealed in a checkered pattern. Next, after degreasing at a temperature of 300 ° C. to 800 ° C. to remove the binder in the molded body, 2150 ° C. to 2300
Firing is performed at a temperature of ℃ to completely sinter the honeycomb molded body cut pieces and the sealed body. As a result, a honeycomb sintered body in which the cell walls 13 are composed only of the core layer 21 is prepared in advance.

Then, the cell wall 13 of this honeycomb sintered body is obtained.
A ceramic raw material slurry (coat layer forming slurry) different from the above core layer forming slurry is supplied to the outer surface of the above by a conventionally known method. In this embodiment, the slurry is attached to the outer surface of the cell wall 13 by performing a dipping method of immersing the honeycomb sintered body in the coating layer forming slurry.

In order to obtain the one as shown in FIG. 4 (a), as the slurry for forming the coat layer, for example, a raw material containing silicon carbide powder as a main component, an organic binder, a pore-forming agent and water are mixed in predetermined amounts. A kneaded slurry is used. The pore-forming agent is preferably made of a substance that disappears due to heat before reaching the sintering temperature of silicon carbide (around 2200 ° C.). The phrase "disappears by heat" specifically means that the sublimation, evaporation, decomposition, reaction sintering, or the like occurs due to heat, and disappears substantially from the sintered compact. The disappearance temperature is preferably low, specifically 1000 ° C. or less, and particularly preferably 500 ° C. or less. This is because the lower the disappearance temperature, the smaller the probability that impurities will remain in the sintered body.

Examples of suitable pore-forming agents include particles made of synthetic resin. Other than this, for example, particles made of an organic polymer such as starch, metal particles, ceramic particles and the like may be used. Here, a spherical particle-shaped pore forming agent made of a synthetic resin is used. The shape of the pore-forming agent is not limited to the spherical shape, and may be, for example, an elongated spherical shape, a cubic shape, an amorphous lump shape, a columnar shape, a plate shape, or the like. The average particle size of the pore-forming agent is preferably 30 μm to 80 μm.
The pore-forming agent is preferably added to the raw material slurry in an amount of about 5% to 25% by weight.

On the other hand, when it is desired to obtain the one as shown in FIG. 5 (a), the slurry for forming the coat layer is, for example, a raw material containing silicon carbide powder as a main component, an organic binder and water are mixed in predetermined amounts and kneaded. What is slurried is used. In this case, a silicon carbide powder having a larger average particle size than the silicon carbide powder contained in the core layer forming slurry,
Add to the same slurry.

After the slurry is supplied as described above, degreasing is performed at a temperature of 300 ° C. to 800 ° C. to remove the binder in the compact. At the time of manufacturing the structure shown in FIG. 4A, the pore-forming agent is also extinguished by heat, and the air holes 25 are formed at the places where the pore-forming agent was present. Atmosphere formed 2
No. 5 basically corresponds to the shape and size of the pore-forming agent. Furthermore, the temperature is raised and firing is performed to completely sinter the coat layer 22, thereby obtaining the filter piece F1.
The finished product of is obtained. 5A, the air holes 25 are not formed even after degreasing, while the main baking completes the sintering of the coat layer 22 to form the air holes 25 therein. In addition, as shown in FIG.
The blunt portion 26 shown in (b) is manufactured by performing perforation processing after the coating layer 22 is sintered.

Next, the coat layer 22 is formed by a conventionally known method.
After applying a catalyst to the ceramic adhesive 15
Using, the plurality of filter pieces F1 are bonded to each other and integrated. Then, an unnecessary portion of the outer peripheral portion is removed by the outer shape cutting step, and the ceramic paste layer 16 is further formed on the outer peripheral surface in order to eliminate unevenness. As a result, the desired D
PF9 can be completed.

Next, examples embodying the present embodiment and comparative examples will be introduced.

[0050]

Examples and Comparative Examples (Example 1) In Example 1, 70% by weight of α-type silicon carbide powder having an average particle size of 10 μm (manufactured by Yakushima Electric Co., Ltd., trade name: C-1000F) was mixed with an organic binder ( 10% by weight of methyl cellulose) and 20% by weight of water and kneaded, and then a small amount of a plasticizer and a lubricant were added to the kneaded product and further kneaded to obtain a core layer forming slurry. This was used for extrusion molding to obtain a honeycomb-shaped molded body. Then, after sealing the end face of the molded body, degreasing and firing are performed under conventionally known conditions to first obtain a honeycomb sintered body made of porous silicon carbide in which the cell wall 13 includes only the core layer 21. It was made.

Next, after 60% by weight of α-type silicon carbide powder having an average particle diameter of 10 μm, 10% by weight of an organic binder (methylcellulose), 20% by weight of water and 10% by weight of a pore-forming agent were added and kneaded. A small amount of a plasticizer and a lubricant was added to the kneaded product, and the mixture was further kneaded to obtain a slurry for forming a coat layer. As the pore-forming agent, spherical synthetic resin particles having an average particle diameter of 50 μm (metal impurity content is 1% by weight or less) were used.

Subsequently, the slurry was sufficiently adhered to the outer surface of each cell wall 13 by a dipping method in which the honeycomb sintered body was completely immersed in the coating layer forming slurry. Then, the honeycomb sintered body is dried at 150 ° C. for 30 minutes to remove water in the coating layer-forming slurry to some extent, and then degreasing is performed at 300 ° C. for 1 hour and firing at 2200 ° C. for 3 hours to obtain a core. A coat layer 22 made of porous silicon carbide was formed on both surfaces of the layer 21. After that, the blunt portion 26 of the coat layer 22 was made thicker by performing a punching process while leaving the corner portion of the cell 12. Then, the catalyst piece Pt was applied only to the coat layer 22 by a conventionally known method to obtain a filter piece F1 of 34 mm square having the cell wall 13 of the three-layer structure. After that, the process of adhering the filter piece F1, the outer shape cutting process, and the unevenness removing process are performed to perform DP.
F9 (diameter 140 mm, length 150 mm) was completed.

In Example 1, the core layer 21 has a thickness of 200.
μm, the porosity was 40%, the average pore diameter was 10 μm, the porosity was 40%, and the average particle diameter of the silicon carbide particles forming the core layer 21 was 10 μm. The thickness of the coat layer 22 after processing the blunt portion 26 is 100 μm, the average pore diameter D50 of the small pores 24 in the coat layer 22 is 10 μm, and the average particle diameter of the silicon carbide particles forming the coat layer 22 is 10 μm.
Met. The average pore diameter D50 of the air holes 25 in the coat layer 22 was 50 μm. (Examples 2 to 6) In Examples 2 to 6, the DPF 9 was manufactured basically according to the procedure of Example 1, and the composition of the slurry for forming the core layer and the slurry for forming the coat layer, the dipping conditions, etc. Each parameter was set as shown in Table 1 by making appropriate changes. (Examples 7 to 9) In Example 7, first, according to the procedure of Example 1, a honeycomb sintered body made of porous silicon carbide in which the cell wall 13 was composed of only the core layer 21 was produced. Next, 10% by weight of an organic binder (methylcellulose) and 20% by weight of water were added to 70% by weight of α-type silicon carbide powder having an average particle size of 50 μm, and the mixture was kneaded. Then, a plasticizer and a lubricant were added to the kneaded product. A small amount was added and further kneaded to obtain a slurry for forming a coat layer. That is, here, instead of adding the pore forming agent, silicon carbide powder having a large average particle diameter was used. Then, after that, the same procedure as in the first embodiment is carried out, and the DPF 9
Was completed.

In Examples 8 and 9, the DPF 9 was completed by changing the average particle diameter of the α-type silicon carbide powder to 40 μm and 60 μm, respectively. Table 1 also shows each parameter in these examples. (Comparative Example 1) In Comparative Example 1, after performing extrusion molding using the slurry for forming a coat layer of Example 7, each step of end face sealing, degreasing and firing was performed to obtain a porous silicon carbide product. First, a honeycomb sintered body was manufactured. Then, after performing dipping using the slurry for forming a core layer of Example 7, each step of drying, degreasing and firing of the honeycomb sintered body is performed to coat both surfaces of the core layer 21 with porous silicon carbide. Layer 22 was formed. That is, here, the DPF 9 having a lower porosity in the surface layer portion than in the deep portion of the cell wall 13 was manufactured. (Comparative Example 2) In Comparative Example 2, according to the procedure of Example 1,
A honeycomb sintered body made of porous silicon carbide in which the cell wall 13 was composed of only the core layer 21 was produced and used as the filter piece F1 without forming the coat layer 22. That is, in Comparative Example 2, a conventional type DPF 9 having the cell wall 13 having a single layer structure was manufactured. (Comparative Example 3) In Comparative Example 3, the blunt portion 2 in Example 1 was used.
6 is not produced, that is, the coat layer 22.
In, a DPF 9 was produced in which the portion corresponding to the corner portion of the cell 12 had the same shape as the corner portion.

[0055]

[Table 1] (Method and Results of Comparative Test) 11 kinds of DPF9 obtained
The exhaust gas purifying apparatus 1 was actually constructed by using the above and was installed on the exhaust path of the diesel engine 2. And
Engine speed 3000 rpm, engine torque 5
Continuous operation was performed by setting it to 0 Nm. At that time, the soot collection amount (g / L) was gradually increased to 1, 3, 5 g / L.
The thickness (μm) of the soot cake 27 was measured. In addition, this measurement was performed using the SEM photograph which image | photographed the cross section of the cell wall 13 when DPF9 was cross-cut.

As a result, when the amount of collected soot is 1 g / L,
In Comparative Examples 1 and 2, the cake 27 having a thickness of about 20 μm was already deposited, and in Comparative Example 3, the cake 27 having a thickness of about 5 μm was deposited.
When the soot collection amount is 3 g / L, in Comparative Examples 1 and 2, about 50
The cake 27 having a thickness of about .mu.m, about 40 .mu.m in Comparative Example 3, is deposited, whereas in Examples 1 to 9, the cake 27 is at most 10-3.
It stopped at about 0 μm. If the amount of soot collected is 5 g / L,
In Comparative Examples 1 and 2, the cake 27 having a thickness of about 70 μm to 80 μm and about 55 μm in Comparative Example 3 was deposited, whereas in Examples 1 to 9, it was about 40 to 50 μm at most.

Further, paying attention to the inside of the cell wall 13 of each DPF 9, in Examples 1 to 9, the soot reached not only the surface layer portion of the cell wall 13 but also the deep layer portion. On the other hand, in Comparative Examples 1 and 2, soot did not reach the deep layer portion of the cell wall 13, and most of the soot stopped at the surface layer portion.

Next, the DPF 9 in which 1,2,3,7 g / L of soot was collected was regenerated by low temperature post-injection (450 ° C., injection started after 1 minute), and the regeneration rate (%) was determined. investigated. In addition,
The survey of the regeneration rate was conducted by examining the weight of DPF after regeneration and the DPF before regeneration.
It was calculated based on the value of the difference from the weight. Some of the results are shown in the graph of FIG. 8 (results of Example 1, Comparative Example 2, and Comparative Example 3).
Indicates.

According to this graph, in Comparative Example 2, the regeneration rate was low at 20% to 30% regardless of the amount of soot trapped, whereas in Example 1, regeneration was performed when the soot trapped amount was small. It turned out that the rate is extremely high. Incidentally, although not shown in this graph, the results of Examples 2 to 9 were close to the results of Example 1, and the results of Comparative Example 1 were close to the results of Comparative Example 2. In addition, the coat layer 22
By providing the blunt portion 26, the reproduction rate was increased. It should be noted that the difference in the reproduction rate due to the difference in the shape of the blunt portion 26 shown in FIGS. 2A and 2B was hardly recognized.

When the DPF 9 after the regeneration was cross-cut and observed by SEM, no cracks were generated in the cell wall 13 in Examples 1 to 9 and Comparative Example 1, but in comparison, In Examples 2 and 3, cracks were generated in places. Therefore, it was found that Examples 1 to 9 and Comparative Example 1 have higher mechanical strength than at least Comparative Examples 2 and 3.

Therefore, according to this embodiment, the following effects can be obtained. (1) In the DPF 9 of this embodiment, the cell wall 13 has a three-layer structure including the core layer 21 and the coat layer 22,
The coat layer 22 has a higher porosity than the core layer 21. Therefore, the soot does not become clogged in the coat layer 22, and the soot can sufficiently reach the core layer 21. Therefore, the formation of cake on the surface of the cell wall 13 is suppressed, and a relatively large amount of soot can be collected. Also, the cell wall 13
Since the core layer 21, which is the deep layer portion, is effectively utilized and the deep layer filtration is achieved, the area where the soot can contact the silicon carbide particles is increased, and as a result, the regeneration efficiency is improved. Moreover, since the core layer 21 in the cell wall 13 has a relatively low porosity, the predetermined mechanical strength required for the DPF 9 is also maintained.

(2) In this DPF 9, while the Pt catalyst for soot combustion is carried on the coat layer 22, the core layer 2
1 adopts a configuration in which no Pt catalyst is supported. Therefore, since the soot collected in the coat layer 22 is burned and removed by the oxidizing action of the soot combustion catalyst carried in the coat layer 22, efficient regeneration is achieved even by low temperature post-injection. Here, since the surface layer portion of the cell wall 13 is richer in oxygen than the deep layer portion, the surface layer portion is originally suitable as a position for supporting the catalyst as compared with the deep layer portion. Coat layer 2
Since the catalyst is supported on No. 2 but the catalyst is not supported on the core layer 21, the amount of useless catalyst is reduced, the material cost is reduced, and the cost increase can be prevented.

(3) In this DPF 9, the coat layer 22
Has atmospheric holes 25 formed by extinguishing the pore forming agent dispersed in the coating layer forming slurry by heat. Therefore, since the porosity of the matrix portion of the coat layer 22 can be set low, the air holes 2
Despite the presence of 5, suitable strength can be imparted to the matrix itself. Therefore, higher mechanical strength is imparted to the DPF 9.

(4) In this DPF 9, at each corner of the cell 12 in the plane perpendicular to the filter axis direction,
An obtuse portion 26 is formed by the coat layer 22. Therefore, the soot is not concentrated only in the vicinity of the corners of the cell, the thickness of the soot becomes substantially uniform, and the catalyst acts effectively. Moreover, as a result of the corners of the cell 12 being reinforced by the blunt portions 26, cracks originating from the corners are less likely to occur, and the DPF 9 is provided with even higher mechanical strength.

(5) In this DPF 9, since the ratio of the thickness of the coat layer 22 to the core layer 21 is set within the above-mentioned preferable range, it is possible to surely achieve the high strength and the increase in the trapping amount. can do.

(6) In this DPF 9, both the core layer 21 and the coat layer 22 are made of a porous silicon carbide sintered body.
In this case, since the core layer 21 and the coat layer 22 have the same coefficient of thermal expansion, thermal stress hardly acts on the cell wall 13, and silicon carbide is a material having high strength in the first place.
From the above, it is possible to impart extremely excellent mechanical strength to the cell wall 13.

The embodiment of the present invention may be modified as follows. The number of combinations of the filter pieces F1 is not limited to the example (16 pieces) of the embodiment, and can be arbitrarily changed. In this case, it is of course possible to appropriately combine and use the filter pieces F1 having different sizes and shapes.

The present invention is not only embodied in a DPF 9 having a configuration in which a plurality of filter pieces F 1 are adhered and integrated as in the embodiment, but is also made of a single porous silicon carbide sintered body. Of course, it may be embodied as a DPF.

In the embodiment, the ceramic honeycomb filter of the present invention is embodied as a filter for an exhaust gas purifying device attached to the diesel engine 2. The ceramic honeycomb filter of the present invention can be embodied as something other than a filter for an exhaust gas purification device. Examples thereof include filtration filters for high temperature fluids and high temperature steam, filters for plating solutions, and the like.

Next, in addition to the technical ideas described in the claims, the technical ideas grasped by the above-described embodiment will be listed below. (1) A large number of cells extending along the axial direction of the filter are provided, and the cell walls forming the cells have a core layer and a coat layer formed on both surfaces of the core layer, and the porosity of the coat layer is the above-mentioned. A method of manufacturing a honeycomb filter made of a porous ceramic sintered body having a relatively higher porosity than the core layer, the step of producing a honeycomb sintered body having a cell wall consisting of the core layer only, and the core A ceramic honeycomb filter characterized by including a step of supplying a coat layer forming slurry to both surfaces of a layer, and a step of heating the honeycomb sintered body to sinter the slurry to form the coat layer. Production method. Therefore, according to the invention described in this technical idea 1, a ceramic honeycomb filter having a high strength, a low pressure loss, a large collection amount, and an excellent regeneration efficiency can be obtained relatively easily and reliably.

(2) In the technical concept 1, the slurry for forming a coat layer has a pore size larger than the average pore size of the core layer and contains a pore-forming agent that disappears by heat. A method for manufacturing a characteristic ceramic filter.

(3) In the technical idea 1, the coating layer forming slurry contains silicon carbide powder having an average particle diameter larger than that of the silicon carbide powder contained in the core layer forming slurry. A method of manufacturing a ceramic filter, comprising:

[0073]

As described in detail above, according to the inventions described in claims 1 and 2, a ceramic honeycomb filter having high strength, low pressure loss, a large amount of traps, and excellent regeneration efficiency is provided. can do.

According to the third aspect of the present invention, the amount of useless catalyst is reduced and the cost can be prevented from increasing. According to the invention described in claims 4, 5, and 7, higher strength can be imparted.

According to the sixth aspect of the present invention, it is possible to surely achieve the high strength and the increase in the trapped amount.

[Brief description of drawings]

FIG. 1 is an overall schematic view of an exhaust gas purifying apparatus according to an embodiment of the present invention.

FIG. 2A is an end view of the DPF according to the embodiment, FIG.
(C) is an enlarged sectional view of a DPF cell.

FIG. 3 is an enlarged cross-sectional view of a main part of the exhaust gas purification device.

FIG. 4A is an enlarged schematic sectional view of a cell wall of the DPF of Examples 1 to 6 before collecting soot, and FIG. 4B is an enlarged schematic sectional view of the cell wall after collecting soot.

5A is an enlarged schematic cross-sectional view of a cell wall of the DPF of Examples 7 to 9 before soot collection, and FIG. 5B is an enlarged schematic cross-sectional view of the cell wall after soot collection.

6A is an enlarged schematic cross-sectional view of a cell wall of the DPF of Comparative Example 1 before soot collection, and FIG. 6B is an enlarged schematic cross-sectional view of the cell wall after soot collection.

7A is an enlarged schematic cross-sectional view of a cell wall of the DPF of Comparative Example 2 before soot collection, and FIG. 7B is an enlarged schematic cross-sectional view of the cell wall after soot collection.

FIG. 8 is a graph showing the relationship between the amount of soot collected and the regeneration efficiency.

[Explanation of symbols]

2 ... Diesel engine, 9 ... DPF as ceramic honeycomb filter, 12 ... Cell, 13 ... Cell wall, 21
... core layer, 22 ... coat layer, 24 ... small pores, 25 ... atmospheric holes, 26 ... blunt portion.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B01J 35/04 B28B 11/04 4G055 B28B 11/04 C04B 38/06 B 4G069 C04B 38/06 41/85 D 41/85 H F01N 3/02 301B F01N 3/02 301 301E 3/28 301Q 3/28 301 B01D 46/00 302 // B01D 46/00 302 46/42 B 46/42 53/36 103C F term (reference) ) 3G090 AA03 BA01 3G091 AA02 AA18 AB02 AB13 BA01 BA16 GA06 GA18 4D019 AA01 BA05 BB06 BC07 CA01 CB04 CB06 4D048 AA14 AB01 BA10Y BB02 BB14 BB16 BA15 BA15 A15B15 BA15 A13B15 A15B13 A08B15 A13A15 BA41 A16B08B15A08B13A08B15A08B13A08B13A08BA15A15 BD05A BD05B CA03 CA07 CA18 DA06 EA19 EA25 EA27 EB15X EB15Y EB18Y EB20 FA01 FA03 FB15 FB36 FC03

Claims (7)

[Claims] 1. A honeycomb filter made of a porous ceramic sintered body, comprising a plurality of cells extending along the axial direction of the filter, wherein the cell walls constituting the cells have a higher porosity in a surface layer portion than in a deep layer portion. A ceramic honeycomb filter characterized by the above. 2. A honeycomb filter made of a porous ceramic sintered body, which comprises a plurality of cells extending in the axial direction of the filter, wherein the cell walls constituting the cells are core layers,
A ceramic honeycomb filter, comprising: a coating layer formed on both sides of the core layer, wherein the coating layer has a relatively higher porosity than the core layer. 3. The ceramic honeycomb filter is a filter for purifying exhaust gas of a diesel engine, wherein the soot combustion catalyst is carried on the coat layer while the catalyst is carried on the core layer. The ceramic honeycomb filter according to claim 2, wherein the ceramic honeycomb filter is not present. 4. The coating layer according to claim 2, wherein the coating layer has pores formed by extinguishing the pore-forming agent dispersed in the coating layer-forming material by heat. The described ceramic honeycomb filter. 5. A blunt portion is formed by the coat layer at each corner of the cell in a plane perpendicular to the filter axis direction, according to any one of claims 2 to 4. The described ceramic honeycomb filter. 6. The thickness of the coat layer covering both surfaces of the core layer is 1/3 to 2/3 of the thickness of the core layer, according to any one of claims 2 to 5. The ceramic honeycomb filter according to item 1. 7. The core layer and the coat layer are both made of a porous silicon carbide sintered body.
7. The ceramic honeycomb filter according to any one of items 6 to 6.