Honeycomb filter and ceramic filter assembly

TECHNICAL FIELD

The present invention relates to:a honeycomb filter and a ceramic filter assembly, and more particularly, to a honeycomb filter formed from a sintered ceramic body and an integral ceramic filter assembly produced by adhering a plurality of honeycomb filters to one another.

BACKGROUND ART

The number of automobiles has increased drastically this century. As a result, the amount of gas discharged from automobile engines has continued to increase proportionally. Various substances suspended in the exhaust gas that is emitted, especially from diesel engines, cause pollution and severely affect the environment. Further, recently reported research results have shown that the fine particles suspended in gas emissions (diesel particulates) may cause allergies or decrease sperm counts. Thus, actions to eliminate the fine particles suspended in gas emissions must immediately be taken for the sake of mankind.

Due to this situation, many exhaust gas purification apparatuses have been proposed in the prior art. A typical exhaust gas purification apparatus includes a casing, which is located in an exhaust pipe connected to an exhaust manifold of an engine, and a filter, which is arranged in the casing and has fine pores. In addition to a metal or an alloy, the filter may be formed from ceramic. A cordierite honeycomb filter is a known example of a ceramic filter. Recent filters are often formed from sintered porous silicon carbide body that is advantageous from the viewpoints of heat resistance and mechanical strength, has a high accumulating efficiency, is chemically stable, and has a small pressure loss.

The pressure loss refers to the difference between the pressure value taken upstream of the filter and the pressure value taken downstream of the filter. A main cause of power loss is the resistance the exhaust gas encounters when passing through a filter.

The honeycomb filter includes a plurality of cells extending along the axial direction of the honeycomb filter. When the exhaust gas passes through the filter, the walls of the cells trap fine particles. This removes fine particles from the exhaust gas.

However, the honeycomb filter, which is made of a sintered porous silicon carbide body, is vulnerable to thermal impacts. Thus, larger filters are liable to crack. Accordingly, a technique for manufacturing a large ceramic filter assembly by integrating a plurality of small filters has recently been proposed to prevent breakage resulting from cracks (see e.g. US-A- 59 14187).

A typical method for manufacturing a ceramic filter assembly will now be discussed. First, ceramic raw material is continuously extruded from a mold of an extruder to form an elongated square honeycomb molded product. After the honeycomb filter is cut into pieces of equal length, the cut pieces are sintered to form a filter. Subsequent to the sintering process, a plurality of the filters are bundled and integrated by adhering the outer surfaces of the filters to each other with a ceramic seal layer having a thickness of 4 to 5mm. This completes the desired ceramic filter assembly.

A mat-like thermal insulative material, made of ceramic fiber or the like, is wrapped about the outer surface of the ceramic filter assembly. In this state, the assembly is arranged in :a casing, which is located in an exhaust pipe.

However, in the prior art, there is a shortcoming in that the fine particles trapped in the ceramic filter assembly do not burn completely and some of the fine particles remain unburned. Accordingly, the efficiency for processing the exhaust gas is low.

Further, the honeycomb filter of the prior art has corners. Thus, there is a tendency of stress concentrating on the corners of the outer surface and chipping the corners. Further, the seal layer may crack and break the ceramic filter assembly from the corners thereby damaging the entire ceramic filter assembly. Even if the assembly does not break, there is a shortcoming in that leakage of the exhaust gas may decrease the processing efficiency.

During usage of the filter assembly, a high temperature difference between the honeycomb filters may cause thermal stress to crack the honeycomb filters and break the entire assembly. Thus, the strength of each honeycomb filter must be increased to increase the strength of the honeycomb filter assembly.

The prior art ceramic filter assembly as a whole has a rectangular :cross-section. Thus, the periphery of the assembly is :cut so that the assembly as a whole has a generally round or oval cross-section.

However, the filter has a plurality of cells. Thus, if the periphery of the assembly is cut, the cell walls are exposed from the peripheral surface subsequent to the cutting. This forms lands and pits on the peripheral surface. Thus, even if the assembly is accommodated in the casing with the thermal insulative material attached to the peripheral surface of the assembly, gaps are formed in the longitudinal direction of the filters. Thus, exhaust gas tends to leak through the gaps. This lowers the processing efficiency of the exhaust gas.

With regard to diesel particulates trapped in the honeycomb filter, it has been confirmed that particulates having a small diameter have a high lung attaching rate and increase the risk to health. Thus, there is great, need to trap small particulates.

However, when the pore diameter and the porosity of the honeycomb filter are small, the honeycomb filter becomes too dense and hinders smooth passage of the exhaust gas, which, in turn, increases the pressure loss. This lowers the driving performance of the vehicle, lowers fuel efficiency, and deteriorates the driving performance.

On the other hand, if the pore diameter and porosity rate are increased, the above problems are solved. However, the number of openings in the honeycomb filter becomes too large. Thus, fine particles cannot be trapped. This decreases the trapping efficiency. Further, the mechanical strength of the honeycomb filter becomes low.

It is a first object to provide a ceramic filter assembly having an improved exhaust gas processing efficiency.

It is a second object of the present invention to provide a ceramic filter assembly having superior strength.

It is a third object of the present invention to provide a ceramic filter assembly that prevents fluid leakage from the peripheral surface.

It is a fourth object of the present invention to provide a honeycomb filter having small pressure loss and superior mechanical strength.

SUMMARY OF THE INVENTION

A perspective of the present invention is an integral ceramic filter assembly produced by adhering with a ceramic seal layer outer surfaces of a plurality of filters, each of which is formed from a sintered porous ceramic body. The ceramic filter assembly includes a ceramic smoothing layer applied to the outer surface of:the assembly, which as a whole has a generally circular cross-section or generally oval cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic view showing an exhaust gas purification apparatus according to the present invention.</li><li>Fig. 2 is a perspective view showing a ceramic filter assembly of the exhaust gas purification apparatus of Fig. 1.</li><li>Fig. 3 is a perspective view showing a honeycomb filter of the ceramic filter assembly of Fig. 2.</li><li>Fig. 4 is an enlarged cross-sectional view showing the main portion of the exhaust gas purification apparatus of Fig. 1.</li><li>Fig. 5 is an enlarged cross-sectional view showing the main portion of the ceramic filter assembly of Fig. 2.</li><li>Fig. 6 is an enlarged cross-sectional view showing the main portion of a ceramic filter assembly of a first modified example.</li><li>Fig. 7 is a side view showing a ceramic filter assembly according to an embodiment of the present invention.</li><li>Figs. 8 (a) to 8 (c) are schematic perspective views illustrating a manufacturing process of the ceramic filter assembly of Fig. 7.</li><li>Fig. 9 is a side view showing a ceramic filter assembly according to a modified example.</li></ul>

BEST MODE FOR CARRYING OUT THE INVENTION

A diesel engine exhaust gas purification apparatus 1 according to the present invention will now be described with reference to Figs. 1 to 5.

Referring to Fig. 1, the exhaust gas purification apparatus 1 is an apparatus for purifying the exhaust gas emitted from a diesel engine 2, which serves an internal combustion engine. The diesel engine 2 has a plurality of cylinders (not shown). Each cylinder is connected to a branch 4 of an exhaust manifold 3, which is made of a metal material. Each branch 4 is connected to a single manifold body 5. Accordingly, the exhaust gas emitted from each cylinder is concentrated at one location.

A first exhaust pipe 6 and a second exhaust pipe 7, which are made of a metal material, are arranged downstream to the exhaust manifold 3. The upstream end of the first exhaust pipe 6 is connected to the manifold body 5. A tubular casing 8 made of a metal material is arranged between the :first exhaust pipe 6 and the second exhaust pipe 7. The upstream end of the casing 8 is connected to the downstream end of the first exhaust pipe 6, and the downstream end of the casing 8 is connected to the upstream end of the second exhaust pipe 7. With this structure, it may be considered that the casing 8 is arranged in the exhaust pipes 6, 7. The first exhaust pipe 6, the casing 8, and the second exhaust pipe 7 are communicated with each other so that exhaust gas flows therethrough.

As shown in Fig. 1, the middle portion of the casing 8 has a diameter larger than that of the exhaust pipes 6, 7. Accordingly, the interior of the casing 8 is larger than that of the exhaust pipes 6, 7. A ceramic filter assembly 9 is accommodated in the casing 8.

A thermal insulative material 10 is arranged between the outer surface of the assembly 9 and the inner surface of the casing 8. The thermal insulative-material 10 is a mat-like material including ceramic fibers and has a thickness of several millimeters to several tens of millimeters. It is preferred that the heat insulative material 10 be thermally expansive. Thermally expansive refers to the release of thermal stress through an elastic structure. This is to minimize energy loss during reproduction by preventing heat from being released from the outermost portion of the assembly 9. Further, the expansion of ceramic fibers using the heat produced during reproduction prevents displacement of the ceramic filter assembly 9, which would result from the pressure of the exhaust gas or vibrations produced by the moving vehicle.

The ceramic filter assembly 9 eliminates diesel particulates and it thus normally referred to as a diesel particulate filter (DPF). As shown in Fig. 2 and Fig. 4, the assembly 9 is formed by bundling and integrating a plurality of filters F. Elongated square filters F1 are arranged at the central portion of the assembly 9, and the outer dimension of the elongated square filter F1 is 33mm×33mm×167mm (refer to Fig. 3). Filters F1 that have forms differing from the elongated square filters F1 are arranged about the elongated square filters F1. This forms the ceramic filter body 9, which as a whole, is cylindrical (diameter being about 135mm).

These filters F1 are made of a sintered porous silicon carbide, which is one type of sintered ceramic. The reason for employing sintered porous silicon carbide is because it is advantageous especially in that it has superior heat resistance and heat conductance. In addition to sintered porous silicon carbide, the sintered material may be silicon nitride, sialon, alumina; cordierite, or mullite.

As shown in Fig. 3 and the other drawings, the filters F1 have a honeycomb structure. The reason for employing the honeycomb structure is in that the pressure loss is small when the trapped amount of fine particles increases. Each filter f1 has a plurality of through holes 12, which have generally square cross-sections and are arranged regularly extending in the axial direction. The through holes 12 are partitioned from each other by thin cell walls 13. The outer surface of the cell wall 13 carries an oxide catalyst formed from a platinum group element (such as Pt) or other metal elements and there oxides. The opening of each through hole 12 on one of the end surfaces 9a, 9b is sealed by a sealing body 14 (sintered porous silicon carbide body). Accordingly, the end surfaces 9a, 9b have a chessboard appearance. Thus, the filters :F1 have a plurality of cells having square cross-sections. The cell density is about 200/inch, the thickness of the cell wall 13 is about 0.3mm, and the cell pitch is about 1.8mm. Among the plurality of cells, about half are opened to the upstream end surface 9a, and the others are opened at the downstream end surface 9b.

The average porous diameter of the filter F1 is about 1 <i>µ</i>m-50<i>µ</i>m, and more particularly, 5 <i>µ</i>m-20 <i>µ</i>m. If the average pore diameter is less than 1 <i>µ</i>m, the deposited fine particles tend to clog the filter F1. If the average pore diameter exceeds 50 <i>µ</i>m, fine particles would not be trapped: and would decrease the trapping efficiency.

It is preferred that the porosity rate be 30% to 70%, and more particularly, 40% to 60%. If the porosity rate is lower than 30%, the filter F1 becomes: too fine and may hinder the circulation of exhaust gas therein. If the porosity rate exceeds 70%, the amount of gaps in the filters F1 becomes too large. This may decrease the strength of the filters f1 and decrease the fine particle trapping efficiency.

When selecting the sintered porous silicon carbide, it is preferred that the heat conductance of the filter F1 be 20W/mK to 80W/mK, and more particularly, 30W/mK to 70W/mK.

Referring to Figs. 4 and 5, the outer surfaces of a total of 16 filters F are adhered to one another by means of a ceramic seal layer 15.

The ceramic seal layer 15 will now be described in detail.

It is preferred that the heat conductance of the seal layer 15 be 0.1W/mK-10W/mK, and more particularly be 0.2W/mK-2W/mK.

If the heat conductance is less than 0.1W/mK, the heat conductance of the seal layer 15 cannot be sufficiently improved. Thus, the seal layer 15 continues to be: a large resistance and hinders heat conduction between filters F1. On the other hand, if the heat conductance exceeds 10W/mK, properties such as adhesion and heat resistance may be degraded and cause manufacturing to be difficult.

It is required that the thickness t1 of the seal layer 15 be 0.3mm-3mm. Further, it is preferred that the thickness be 0.5mm-2mm.

If the thickness t1 exceeds 3mm, the seal layer 15 continues to be a large seal layer 15 even if the heat conductance is high and the heat conductance between the filters F1 is hindered. In addition, the ratio of the assembly 9 occupied by the filters F1 would relatively decrease and lower the filtration capacity. On the other hand, if the thickness t1 of the seal layer 15 is less than 0.3mm, the seal layer 15 would not become a large resistance. However, the force adhering the filters F1 to each other may become too low and cause the assembly 9 to be vulnerable to breakage.

The seal layer 15 is formed from at least an inorganic fiber, an inorganic binder, an organic binder, and inorganic particles. Further, it is preferred that the seal layer 15 be an elastic material formed by binding inorganic fibers and inorganic particles, which three-dimensionally intersect one another, with an inorganic binder and an organic binder.

At least one type of ceramic fiber selected from silica-alumina fiber, mullite fiber, alumina fiber, and silica fiber are selected as the inorganic fiber included in the seal layer 15. Among these fibers, it is most preferred that silica-alumina ceramic fiber be selected. Silica-alumina ceramic fiber has superior elasticity and serves to absorb thermal stress.

In this case, the content of the silica-alumina ceramic fiber in the seal layer 15 is 10wt%-70wt%, preferably 10wt%-40wt%, and more preferably 20wt%-30wt%. If the content is less than 10wt%, the thermal conductivity decreases and the elasticity decreases. If the content exceeds 70%, the thermal conductivity and elasticity decrease.

The shot content of the silica-alumina ceramic fiber is 1wt%-10wt%, preferably 1wt%-5wt%, and more preferably 1wt%-3wt%. If the shot content is less than 1wt%, manufacture is difficulty and if the shot content is 50wt%, the outer surface of the filter F1 may be damaged.

The fiber length of silica-alumina ceramic fiber is 1mm-100mm, preferably 1mm-50mm, and more preferably 1mm-20mm. If the fiber length is 1mm or less, there is a disadvantage in that an elastic structure cannot be formed. If the fiber length exceeds 100mm, there is a disadvantage in that the fiber may produce balls of fibers and decrease the dispersion of inorganic fine particles. Further, if the fiber length exceeds 100mm, it becomes difficult to make the seal layer thinner than 3mm and to improve the heat conductance between the filters F1.

It is preferred that the inorganic binder included in the seal layer 15 be a colloidal sol selected from at least one of silica sol and alumina sol. It is especially preferred that silica sol be selected:. This is because silica sol is optimal for use as an adhesive agent under high temperatures since it is easily obtained easily sintered to SiO₂. In addition, silica sol has a superior insulative characteristic.

In this case, the content of silica sol in the seal layer 15 as a solid is 1wt%-30wt%, preferably 1wt%-15wt%, and more preferably 5wt%-9wt%. If the content is less than 1wt%, the adhesive strength decreases. On the other hand, if the content exceeds 30wt%, the thermal conductivity decreases.

It is preferred that the organic binder included in the seal layer 15 be a hydrophilic organic high polymer and also be preferred that the organic binder be a polysaccharide selected from at least one of poly vinyl alcohol, methyl cellulose, ethyl-cellulose, and carboxymethyl cellulose. It is especially preferred that carboxymethyl cellulose be selected. This is because the seal layer 15 has optimal fluidity due to carboxymethyl cellulose and thus has superior adhesion under normal temperatures.

In this case, the content of carboxymethyl cellulose as a solid is 0.1wt%-5.0wt%, preferably 0.2wt%-1.0wt%, and more preferably 0.4wt%-0.6wt%. If the content is less than 0.1wt%, sufficient inhibition of migration becomes difficult. Migration refers to a phenomenon in which the binder in the seal layer 15 moves as the solvent is removed as it dries when the seal layer 15 charged-between the sealed bodies hardens. If the content exceeds 5.0wt%, high temperature burns and eliminates the organic binder and decreases the strength of the seal layer 15.

It is preferred that the inorganic particles included in the seal layer 15 be an inorganic powder or an elastic material employing a whisker that is selected from at least one of silicon carbide, silicon nitride, and boron nitride. Such carbide and nitrides have an extremely high thermal conductivity and, when included in the surface of a ceramic fiber or in the surface of inside a colloidal sol, contributes to increasing the thermal conductivity.

Among the above carbide and nitrides, it is especially preferred that the silicon carbide powder be selected. This is because the thermal conductivity of silicon carbide is extremely high and easily adapts to ceramic fiber.: In addition, in the first embodiment, the filter F1, which is the sealed body, is made of sintered porous silicon carbide. Thus, it is preferred that the same type of silicon carbide powder be selected.

In this: case, it is preferred that the content of the silicon carbide powder as a solid be 3wt%-80wt%, preferably 10wt%-60wt%, and more particularly, 20wt%-40wt%. If the content is 3wt% or less, the thermal conductivity of the seal layer 15 decreases and results in the seal layer 15 having a large heat resistance. If the content exceeds 80wt%, the adhesion strength decreases when the temperature is high.

The grain diameter is 0.01µm-100µm, preferably 0.1µm-15µm, and more preferably 0.1<i>µ</i>m-10<i>µ</i>m. If the grain diameter exceeds 100<i>µ</i>m, the adhesion and thermal conductivity decrease. If the grain diameter is less than 0.01 <i>µ</i>m, the cost of the seal material 15 increases.

The procedure for manufacturing the ceramic filter assembly 9 will now be discussed.

First, a ceramic raw material slurry used during an extrusion process, a sealing paste used during an end surface sealing process, and a seal layer formation paste used during a filter adhesion process are prepared.

The ceramic raw material slurry is prepared by combining and kneading predetermined amounts of an organic binder and water with silicon carbide particles. The sealing paste is prepared by combining and kneading an organic binder, a lubricative agent, a plastic agent, and water with silicon carbide powder. The seal layer formation paste is prepared by combining and kneading predetermined amounts of an inorganic fiber; an inorganic binder, an organic binder, and inorganic particles, and water.

Next, the ceramic raw material slurry is put into an extruder and continuously extruded from a mold. Afterward, the extruded honeycomb molded product is cut into equivalent lengths to obtain elongated square honeycomb molded product pieces. Further, a predetermined amount of sealing paste is charged into one of the openings of each cell in the cut pieces such that both end surfaces of each cut piece is sealed.

Then, main sintering is performed by setting predetermined conditions, such as the temperature and time, to completely sinter the honeycomb molded pieces and the sealing bodies 14. All of the sintered porous silicon carbide filters F1 obtained in this manner are still square pole-shaped.

The sintering temperature is set to 2,100°C to 2,300°C in the present embodiment to obtain the average pore diameter of 6 <i>µ</i>m-15 <i>µ</i>m and a porosity of 35% to 50%. Further, the sintering time is set to 0.1 hours to 5 hours. Further, the interior of a furnace has an inert atmosphere during sintering, and the pressure in that atmosphere is the normal pressure.

Then, after forming a ceramic bedding layer to the outer surface of the filters F1 as required, the seal layer formation paste is applied thereto. The outer surfaces of sixteen of such filters F1 are adhered to each other and thus integrated.

In the following outer form cutting process, the assembly 9, which has been obtained through the filter adherence process and has a square cross-section, is ground to form the outer shape of the assembly 9 by eliminating unnecessary sections from the peripheral portion of the assembly 9 and form the ceramic filter assembly 9, which cross-section is round.

The fine particle trapping performed by the- ceramic filter assembly 9 will now be described briefly.

The ceramic filter assembly 9 accommodated in the casing 9a is supplied with exhaust gas. The exhaust gas supplied via the first exhaust pipe 6 first enters the cells that are opened at the upstream end surface 9a. The exhaust gas than passes through the cell wall 13 and enters the adjacent cells, or the cells that are opened at the downstream end surface 9b. From the openings of these cells, the exhaust gas flows out of the downstream end surfaces 9b of the filters F1. However, the fine particles included in the exhaust gas do not pass through the cell walls 13 and are trapped by the cell walls 13. As a result, the purified gas is discharged from the downstream end surface 9b of the filters F1. The purified exhaust gas then passes through the second exhaust pipe 7 to be ultimately discharged into the atmosphere. The trapped fine particles are ignited and burned by the catalytic effect that occurs when the internal temperature of the assembly 9 reaches a predetermined temperature.

(Example 1-1)

<ul id="ul0002" list-style="none"><li>(1) 51.5wt% of a silicon carbide powder having an average grain diameter of 10 <i>µ</i>m and 22wt% of α silicon carbide powder having an average grain diameter of 0.5<i>µ</i>m were wet-mixed. Then, 6.5wt% of the organic binder (methyl cellulose) and 20wt% of water were added to the obtained mixture and kneaded. Next, a small amount of the plastic agent and the lubricative agent were added to the kneaded mixture, further kneaded, and extruded to obtain the honeycomb molded product. More specifically, the α silicon carbide powder having an average particle diameter of about 10 <i>µ</i>m was produced by Yakushima Denkou Kabushiki Kaisha under the product name of C-1000F, and the α silicon carbide powder having an average particle diameter of about 0.5 <i>µ</i>m was produced by Yakushima Denkou Kabushiki Kaisha under the product name of GC-15.</li><li>(2) Then, after drying the molded product with a microwave dryer, the through holes 12 of the molded product was sealed by the sealing paste made of sintered porous silicon carbide. Afterward, the sealing paste was dried again with the dryer. After the end surface sealing process, the dried body was degreased at 400°C and then sintered for about three hours at 2,200°C in an argon atmosphere at the normal pressure. This obtained the porous, honeycomb, silicon carbide filters F1.</li><li>(3) 23.3wt% of a ceramic fiber (alumina silicate ceramic fiber, shot content 3%, fiber length 0.1mm-100mm), 30.2wt% of silicon carbide having an average grain diameter of 0.3 <i>µ</i>m, 7wt% of silica sol (the converted amount of SiO₂ of the sol being 30%) serving as the inorganic binder, 0.5wt% of carboxymethyl cellulose serving as the organic binder, and 39wt% of water were mixed and kneaded. The kneaded material was adjusted to an appropriate viscosity to prepare the paste used to form the seal layer 15.</li><li>(4)-Then, the seal layer forming paste was uniformly applied to the outer surface of the filters F1. Further, in a state in which the outer surfaces of the filters F1 were adhered to one another, the filters F1 were dried and hardened under the condition of 50°C to 100°C × 1 hour. As a result, the seal layer 15 adhered the filters F1 to one another. The thickness t1 of the seal layer 15 was set at 0.5mm. The heat conductivity of the seal layer 15 was 0.3W/mK.</li><li>(5) Next, the peripheral portion was cut to shape the peripheral portion and complete the ceramic filter assembly 9, which has a round cross-section.</li></ul>

Then, the thermal insulative material 10 is wound about the assembly 9 obtained in the above manner. In this state, the assembly 9 is accommodated in the casing 8 and actually supplied with exhaust gas. After a predetermined time elapses, the assembly 9 is removed and cut at a plurality of locations. The cut surfaces were observed with the naked eye.

Consequently, residuals of the fine particles were not confirmed at the peripheral portion of the assembly 9 (especially, the peripheral portion near the downstream end surface) where there is a tendency for unburned particles to remain. The fine particles were of course completely burned at other portions. It is considered that such results are obtained because the usage of the seal layer 15 prevents the conductance of heat between the filters F1 from being decreased and the temperature sufficiently increases at the peripheral portion of the assembly 9. Accordingly, in example 1-1, it is apparent that exhaust gas was efficiently processed.

(Examples 1-2, 1-3)

In example 1-2, the ceramic filter assembly 9 was prepared by setting the thickness t1 of the seal layer 15 at 1.0mm. The other conditions were basically set in accordance with example 1-1. In example 3, the ceramic filter assembly - 9 was formed by setting the thickness t1 of the seal layer 15 at 2.5mm. The other conditions were basically set in accordance with example 1-1.

Then, the obtained two types of assemblies 9 were used for a certain period, and the cut surfaces were observed with the naked eye. The same desirable results as example 1-1 were obtained. Thus, it is apparent that the exhaust gas was efficiently processed in examples:1-2 and 1-3.

(Example 1-4)

In example 1-4, the employed seal layer forming paste was prepared by mixing and kneading 25wt% of a ceramic fiber (mullite fiber, shot content rate 5wt%, fiber length 0.1mm-100mm), 30wt% of silicon nitride powder having an average grain diameter of 1.0 <i>µ</i>m, 7wt% of alumina sol (the conversion amount of alumina sol being 20%) serving as an inorganic binder, 0.5wt% of poly vinyl alcohol serving as an organic binder, and 37.5wt% of alcohol. The other portions were formed in accordance with example 1-1 to complete the ceramic filter assembly 9. The thickness t1 of the seal layer 15 was set at 1.0mm. The thermal conductivity of the seal layer 15 was 0.2W/mK.

Then, the obtained assembly 9 was used for a certain period, and the cut surfaces were observed with the naked eye. The same desirable :results as example 1 were obtained. Thus, it is apparent that the exhaust gas was efficiently processed in example 4.

(Example 1-5)

In example 1-5, the employed seal layer forming paste was prepared by mixing and kneading 23wt% of a ceramic fiber (alumina fiber, shot content rate 4wt%, fiber length 0.1mm-100mm), 35wt% of boron nitride powder having an average grain diameter of 1 <i>µ</i>m, 8wt% of alumina sol (the conversion amount of alumina sol being 20%) serving as an inorganic binder, 0.5wt% of ethyl cellulose serving as an organic binder, and 35.5wt% of acetone. The other portions were formed in accordance with example 1 to complete the ceramic filter assembly 9. The thickness t1 of the seal layer 15 was set at 1.0mm. The thermal conductivity of the seal layer 15 was 2W/mK.

Then, the obtained assembly 9 was used for a certain period, and the cut surfaces were observed with the naked eye. The same desirable results as example 1 were obtained. Thus, it is apparent that the exhaust gas was efficiently processed in example 5.

The ceramic filter assembly 9 of the first embodiment has the following advantages: <ol id="ol0001" ol-style=""><li>(1) In each example, the thickness t1 of the seal layer 15 is set in the preferable range of 0.3mm-3mm, and the thermal conductivity of the seal layer 15 is set in the preferable range of 0.1W/mK-10W/mK. This improves the thermal conductivity of the seal layer and prevents the thermal conductivity between the filters F1 from being decreased. Accordingly, heat is uniformly and quickly conducted to the entire assembly 9. This prevents a temperature :difference from being produced in the assembly 9. Accordingly, the thermal uniformity of the assembly 9 is increased and the occurrence of locally unburned particles is avoided. The exhaust gas purification apparatus 1, which uses the assembly 9, has superior exhaust gas processing efficiency. Further, if the thickness t1 and the thermal conductivity is within the above range, basic properties, such as adhesiveness and heat resistance remain the same. This avoids the manufacturing of the seal layer 15 from becoming difficult. Further, since the seal layer 15 serves to adhere the filters F1 to one another, breakage of the assembly 9 is avoided. In other words, the assembly 9 is relatively easy to manufacture and has superior durability. </li><li>(2) The seal layer 15 in each example contains as a solid 10wt%-70wt% of ceramic fibers. This enables the seal layer 15 to have high thermal conductivity and elasticity. Thus, the thermal conductivity between filters F1 is improved, and the thermal conductivity of the assembly 9 is further increased.</li><li>(3) The seal layer 15 in each example contains ceramic fibers, the lengths of which are 100mm or shorter. Accordingly, the thickness t1 of the seal layer 15 may be set to 3mm or less without any difficulties. This increases the heat conductivity between the filters F1, and thus contributes to the thermal uniformity of the assembly 9.</li><li>(4) The seal layer 15 in each example contains as a solid 3wt%-80wt% of inorganic particles. Thus, the seal layer 15 has high thermal conductivity. This increases the heat conductivity between the filters F1 and contributes to the thermal uniformity of the assembly 9.</li><li>(5) The seal layer 15 in the above examples are formed from at least an inorganic fiber, an inorganic binder, an organic binder, and inorganic particles. Further, the seal layer 15 is made of an elastic material formed by joining three-dimensionally intersecting the inorganic fibers with the inorganic particles with an inorganic binder and an organic binder.</li></ol>

Such material has the advantages described below. Sufficient adhesion strength is obtained in a low temperature range and a high temperature range. Further, the material is:elastic. Thus, when thermal stress is applied to the assembly 9, the release of the thermal stress is ensured.

The first embodiment of the present invention may be modified as described below. <ul id="ul0003" list-style="none"><li>(a) The number of the filters F1 is not limited to 16 and may be any number. In this case, filters F1 having different dimensions and shapes may be combined.</li><li>(b) With reference to Fig. 6, in a ceramic filter assembly 21 of a further embodiment, the filters F1 are offset from:one another in a direction perpendicular to the filter axial direction, and the filters F1 are adhered by the seal layer 15. In this case, the filters F1 resists displacement when being accommodated in the casing 8. This improves the breakage strength of the assembly. 21. In the ceramic filter assembly 21 of Fig. 6, the seal layer 15 does not include:cross-like portions. It is considered that this contributes :to improvement of the breakage strength. Further, since the thermal conductivity in the radial direction of the assembly 21 is further improved, the thermal uniformity of the assembly 21 is further enhanced.</li><li>(c) Instead of the honeycomb filters F1, the filters may have a three-dimensional mesh structure, a foam-like structure, a noodle-like structure, or a fiber-like structure.</li><li>(d) Prior to the outer form cutting process, the form of the filter F1 is not limited to the elongated square shape and may have a triangular pole-like, shape or a hexagonal pole-like shape. Further, the assembly 9 does not necessarily have to be formed to have a round cross-section during the outer form cutting process and may be formed to have a, for example, oval cross-section.</li></ul>

Fig. 7 is a schematic cross-sectional view showing a ceramic filter 39 according to an embodiment of the present invention.

Referring to Fig. 7 and Fig. 8 (b), the ceramic filter assembly 39 of the embodiment has an outer surface 39c to which a ceramic smoothing layer 16 is applied. The smoothing layer 16 is made of a ceramic material that includes at least ceramic fibers and a binder. It is preferred that the ceramic material includes inorganic particles, such as silicon carbide, silicon nitride, and boron nitride. It is preferred that an inorganic binder, such as silica sol or alumina sol, or an organic binder, such as a polysaccharide, be used as the binder. It is preferred that the ceramic material be formed by binding three-dimensionally intersecting ceramic fibers and inorganic particles with a binder. It is preferred that the smoothing layer 16 be formed from the same type of material as the seal layer 15, and especially preferred that the smoothing layer 16 be made of exactly: the same material as the seal layer 15.

It is preferred that the thickness of the smoothing layer 16 be 0.1mm to 10mm, further preferred that the thickness be 0.3mm to 2mm, and optimal that the thickness be 0.5mm to 1mm. If the smoothing layer 16 is too thin, pits 17 that are formed in the outer surface 9c of the ceramic filter assembly 9 cannot be completely filled. Thus, gaps tend to remain in such locations. On the other hand, if the smoothing layer 16 is thickened, the formation of the layer may become difficult, and the diameter of the entire assembly 9 may be enlarged.

It is preferred that the seal layer 15 be formed thinner than the smoothing layer 16, and more particularly, be formed in the range of 0.3mm to 3mm. When the seal layer 15 is thinner than the smoothing layer, the filtering capacity and the thermal conductance are prevented from being decreased beforehand.

The procedure for manufacturing the ceramic filter assembly 39 will now be described with reference to Fig. 8.

First, a ceramic raw material slurry used during an extrusion process, a sealing paste used during an :end surface sealing process, a seal layer formation paste used during a filter adhesion process, and a smoothing layer formation paste are prepared. When using the seal layer formation paste to form the smoothing, layer, the smoothing layer formation paste does not have to be prepared.

The ceramic raw material slurry is prepared by combining and kneading predetermined amounts of an organic binder and water with silicon carbide particles. The sealing paste is prepared by combining and kneading an inorganic binder, a lubricative agent, a plastic agent, and water with silicon carbide powder. The seal layer formation paste (smoothing layer formation paste) is prepared by combining and kneading predetermined amounts of an inorganic fiber, an inorganic binder, an organic binder, inorganic particles, and water.

Next, the ceramic raw material slurry is put into an extruder and continuously extruded from a mold. Afterward, the extruded honeycomb molded product: is cut into equivalent lengths to obtain elongated square honeycomb molded product pieces. Further, a predetermined amount of the sealing paste is charged into one of the openings of each cell in the cut pieces to seal both end surfaces of each cut piece.

Then, main sintering is performed by setting predetermined conditions, such as the temperature and time, to completely sinter the honeycomb molded pieces and the sealing bodies 14. All of the sintered porous silicon carbide filters F1 obtained in this manner are still square pole-shaped.

The sintering temperature is set to 2,100° C to 2,300° C in the present embodiment to obtain the average pore diameter of 6 <i>µ</i>m to 15 <i>µ</i>m and a porosity of 3.5% to 50%. Further, the sintering time is set to 0.1 hours to 5 hours. Further, the interior of a furnace has an inert atmosphere during sintering, and the pressure in that atmosphere is the normal pressure.

Then, after forming a ceramic bedding layer to the outer surface of the filters F1 as required, the seal layer formation paste is applied thereto. The outer surfaces of sixteen of such filters F1 are adhered to each other and thus integrated. At this point, the ceramic filter assembly 39A as a whole has a square cross-section, as shown in Fig. 8 (a).

In the following outer form cutting process, the assembly 39A, which has been obtained through the filter adherence process and has a square cross-section, is ground to form the outer shape of the assembly 9 by eliminating unnecessary sections from the peripheral portion of the assembly 39A.

As a result, the ceramic filter assembly 39 having around cross-section is obtained, as shown in Fig. 8 (b) Cell walls 13 are partially exposed from the surface formed during the outer form cutting. Thus, pits 17 are formed in the outer surface 39c. The pits 17 are about 0.5mm to 1mm and are defined by ridges and valleys extending in the axial direction of the assembly 39 (i.e., the longitudinal direction of the filters F1).

In the following smoothing layer: forming process, the seal layer formation paste is used as: the smoothing layer formation paste and uniformly applied to the outer surface 9c of the assembly 39. This completes the ceramic filter assembly 39 shown in Fig. 8 (c).

(Example 3-1)

<ol id="ol0002" ol-style=""><li>(1) 51.5wt% of a silicon carbide powder and 22wt% of β silicon carbide powder were wet-mixed. Then, 6.5wt% of the organic binder (methyl cellulose) and 20 wt% of water were added to the obtained mixture and kneaded. Next, a small amount of the plastic agent and the lubricative agent were added to the kneaded mixture, further kneaded, and extruded to obtain the honeycomb molded product.</li><li>(2) Then, after drying the molded product with a microwave dryer, the through holes 12 of the molded product were sealed by the sealing paste made:of sintered porous silicon carbide. Afterward, the sealing paste was dried again with the dryer. After the end surface sealing process, the dried body was degreased at 400°C and then sintered for about three hours at 2,200°C in an argon atmosphere at normal pressure. This obtained the porous, honeycomb, silicon carbide filters F1.</li><li>(3) 23.3wt% of a ceramic fiber (alumina silicate ceramic fiber, shot content 3%, fiber length 0.1mm-100mm), 30.2wt% of silicon carbide having an average grain diameter of 0.3 µm, 7wt% of silica sol (the converted amount of SiO₂ of the sol being 30%) serving as the inorganic binder, 0.5wt% of carboxymethyl cellulose serving as the organic binder, and 39wt% of water were mixed: and kneaded. The kneaded material was adjusted to an appropriate viscosity to prepare the paste used to form the seal layer 15 and the smoothing layer 16.</li><li>(4) Then, the seal layer forming paste was uniformly applied to the outer surface of the filters F1. Further, in a state in which the outer surfaces of the filters F1 were adhered to one another, the filters F1 were dried :and hardened under the condition of 50°C to 100°C x 1 hour. As a result, the seal layer 15 adhered the filters F1 to one another. The thickness t1 of the seal layer 15 was set at 1.0mm.</li><li>(5) Next, the peripheral portion: was cut to shape the peripheral portion and complete the ceramic filter assembly 39, which has a round cross-section. Then, the seal and smoothing paste was uniformly applied:to the expose outer surface 39c. The smoothing layer 16 having a thickness of 0.6mm was dried and hardened under the condition of 50°C to 100°C × 1 hour to complete the assembly 39.</li></ol>

The assembly 39 obtained in the above manner was observed with the naked eye. The pits 17 in the outer surface 39c were substantially completely filled by the smoothing layer 16, and the outer surface, 39c was smooth. Further, there were no cracks in the boundary portions of the smoothing layer 16 and the seal layer 15. Accordingly, this indicates that the levels of adhesion and seal were high at the boundary portions.

No gaps were formed in the outer surface 9c of the assembly 39 when accommodating the assembly 39 encompassed by the thermal insulative material in the casing 8. Further, when actually supplying exhaust gas, there was no :leakage of the exhaust gas through the gaps in the outer surface 39c from the downstream side. It is thus apparent that exhaust gas is efficiently processed in the third embodiment.

(Example 3-2)

In example 3-2, the seal and smoothing paste was prepared by mixing and kneading 25wt% of a ceramic fiber (mullite fiber, shot content rate 5wt%, fiber length 0.1mm-100mm), 30wt% of silicon nitride powder having an:average grain diameter of 1.0 <i>µ</i>m, 7wt% of alumina sol (the conversion amount of alumina sol being 20%) serving as an inorganic binder, 0.5wt% of poly vinyl alcohol serving as an organic binder, and 37.5wt% of alcohol. The other portions were formed in accordance with example 3-1 to complete the ceramic filter assembly 39.

Then, observations were made by the naked eye in the same manner as example 1. The pits 17 in the outer surface 39c were substantially completely filled by the smoothing layer 16. Further, there were no cracks in the boundary portions of the smoothing layer 16 and the seal layer 15. Accordingly, this indicates that the levels of adhesion and seal were high at the boundary portions.

No gaps were formed in the outer surface 39c of the assembly 39 during usage. In addition, leakage of exhaust gas through :gaps in the outer surface 39c did not occur. It is thus apparent that exhaust gas was:efficiently processed in example 3-2 in the same manner as example 3-1.

(Example 3-3)

In example 3-3, the seal and smoothing paste was prepared by mixing and kneading 23wt%: of a ceramic fiber (alumina fiber, shot content rate 4wt%, fiber length 0.1mm-100mm), 35wt% of boron nitride powder having an average grain diameter of 1µm, 8wt% of alumina sol (the conversion amount of alumina sol being 20%) serving as the inorganic binder, 0.5wt% of ethyl cellulose serving as the organic binder, and 35.5wt% of acetone. The other portions were formed in accordance with example 3-1 to complete the ceramic filter assembly 39.

Then, observations were made by the naked eye in the same manner as example 3-1. The pits 17 in the outer surface 39c were substantially completely filled by the smoothing layer 16. Further, there were no cracks in the boundary portions of the smoothing layer 16 and the seal layer 15. Accordingly, this indicates that the levels of adhesion and seal were high at the boundary portions.

No gaps were formed in the outer surface 39c of the assembly 39 during usage. In addition, leakage of exhaust gas through gaps in the outer surface: 39c did not occur. It is thus apparent that exhaust gas was efficiently processed in example 3-3 in the same manner as example 3-1.

(Comparative Example)

In the comparative example, the smoothing layer 16 was not formed on the outer surface 39c. The other portions were formed in accordance with example 3-1 to complete a ceramic filter assembly.

Then, observations were made by the naked eye in the same manner :as example 3-1. There were pits 17 in the outer surface 39c. Thus, gaps were formed in the outer surface 39 c during usage of the assembly, and gas leakage through the gaps occurred. Accordingly, in comparison with examples 3-1 to 3-3, it is apparent that the exhaust gas processing efficiency was inferior.

Accordingly, the ceramic filter assembly 39 has the advantages described below. <ol id="ol0003" ol-style=""><li>(1) The smoothing layer 16 fills the pits 17 and smoothes the outer surface 9c. Accordingly, gaps are not formed in the outer surface 39c when the assembly 39 is retained. This prevents the leakage of exhaust gas. As a result, the ceramic filter assembly 39 has superior exhaust gas processing efficiency. This, in turn, results in the exhaust gas purification apparatus 1 having superior exhaust gas processing efficiency. The smoothing layer 16 is made of ceramic and thus has superior adhesion with the filter F1, which is made of a sintered porous ceramic, and superior heat resistance. Accordingly, even if the assembly 39 is exposed to a high temperature of several hundred degrees Celsius, the smoothing layer 16 is not burned nor deformed. Thus, the desired adhesion strength is maintained. </li><li>(2) The thickness of the smoothing layer 16 is set in the preferred range of 0.1mm to 10mm. This prevents the leakage of exhaust gas without making the manufacture of the assembly 39 difficult.</li><li>(3) The seal layer 15 is thinner than the smoothing layer 16. This prevents the filtering capability and the thermal conductivity from decreasing.</li><li>(4) The smoothing layer 16 is made from the same material as the seal layer 15. Since the coefficient of thermal expansion of the smoothing layer 16 and that of the seal layer 15 are the same, the boundary portions of the seal and smoothing layer 15, 16 do not crack. In other words, high adhesiveness, sealing, and reliability of the boundary portions are ensured. Further, a smoothing layer formation paste does not have to be prepared in addition to the seal layer formation paste. This facilitates :the manufacture of the assembly 39 and avoids an increase in the manufacturing cost. </li><li>(5) The following may be used as: the material for forming the seal layer 15 and the smoothing layer 16. An elastic material including at least an inorganic fiber, an inorganic binder, an organic binder, and inorganic particles and bound to one another by the inorganic binder and the organic binder may be used.</li></ol>

Such material has the advantage described below. The material has satisfactory adhesion strength in both low temperature and high temperature ranges. Further, the material is an elastic material. Thus, when thermal stress is applied, the thermal stress is relieved. Further, the material has superior thermal conductance. Thus, heat is uniformly and quickly conducted to the entire assembly 39. This enables efficient exhaust gas processing.

The third embodiment of the present invention may be modified as described below. <ul id="ul0004" list-style="none"><li>(a) As shown in Fig. 9 the present invention may be embodied in:a ceramic filter assembly. 321 by offsetting the filters F1 from one another in a direction perpendicular to the filter axial direction.</li><li>(b) The smoothing layer 16 may be formed from a ceramic material that differs from that of the seal layer 15.</li><li>(c) The smoothing layer 16 may have the same thickness as the seal layer 15 or may have a greater thickness than the seal layer 15.</li><li>(d) In addition to forming the smoothing layer 16 by employing an application technique, other methods, such as a printing technique, a staining technique, a dip technique, and a curtain coat technique, may be employed.</li></ul>

INDUSTRIAL APPLICABILITY

The ceramic filter assembly of the present invention may be applied to an exhaust gas purification filter of a diesel engine 2, a heat exchange device member, a filter for high temperature fluid or high temperature vapor, etc.