Honeycomb filter

Abstract

To provide a honeycomb filter which has PM collection effect.SOLUTION: There is provided a honeycomb filter comprising: an exhaust gas introduction cell whose one end face is closed: and an exhaust gas discharge cell. The honeycomb filter includes, through a cell partition wall, first and second cells 31, 32, the first part 13a of the cell partition wall has a hierarchical structure for every 0.5 μm toward a first direction Y, from a surface of the first cell, in each hierarchy, when a porous part continuing in a second direction X is one pore 40, the total number of the one porous is the pore number in each hierarchy, a number obtained by dividing, a product between a distance of the first part of the cell partition wall of the second direction and a porosity of the first part of the cell partition wall, by an average pore diameter of the first part on the cell partition wall is an average pore number, then, from the surface of the first cell side, comparing the pore number in each hierarchy and the average pore number, and when the hierarchy where the pore number of the hierarchy exceeds the average pore number for the first time, is determined, a distance to the hierarchy in which the pore number in each hierarchy exceeds the average pore number, is 10 μm or smaller.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to a honeycomb filter. [Background technology]

[0002] Exhaust gases emitted from internal combustion engines such as diesel engines contain particulate matter (PM), such as soot, and in recent years, the harm that this PM poses to the environment and human health has become a problem. Furthermore, since exhaust gases also contain harmful gas components such as CO, HC, and NOx, there are concerns about the effects of these harmful gas components on the environment and human health.

[0003] Therefore, various honeycomb filters made of porous ceramics such as cordierite or silicon carbide have been proposed as exhaust gas purification devices that, when connected to an internal combustion engine, capture PM in the exhaust gas or purify harmful gas components in the exhaust gas such as CO, HC, or NOx.

[0004] Patent Document 1 describes such a honeycomb filter comprising: a columnar honeycomb structure having porous partitions arranged to surround a plurality of cells that form a fluid flow path extending from the inlet end face to the outlet end face; and a sealing portion disposed at the opening on the inlet end face side or the outlet end face side of each of the cells, wherein the pore diameter at which the cumulative pore volume of the partitions is 10% is defined as the pore diameter D10. A sealed honeycomb structure is disclosed in which the pore diameter at which the cumulative pore volume is 30% is defined as pore diameter D30, the pore diameter at which the cumulative pore volume is 50% is defined as pore diameter D50, the pore diameter at which the cumulative pore volume is 70% is defined as pore diameter D70, the pore diameter at which the cumulative pore volume is 90% is defined as pore diameter D90, the pore diameter D10 is 6 μm or more, the pore diameter D90 is 58 μm or less, and the relationship in the following formula (1) is satisfied. Equation (1): 0.35 ≤ (D70 - D30) / D50 ≤ 1.5 (However, in equation (1), D30 represents the value of pore diameter D30, D50 represents the value of pore diameter D50, and D70 represents the value of pore diameter D70.)

[0005] The honeycomb filter described in Patent Document 1 improves collection performance by controlling the distribution of cumulative pore volume and suppresses variations in pressure loss after the catalyst for exhaust gas purification is supported.

[0006] Here, we will explain the principle by which PM can be collected by using a honeycomb filter as described in Patent Document 1. Figure 12 is a schematic diagram illustrating an example of collecting PM in exhaust gas using a conventional honeycomb filter. As shown in Figure 12, the conventional honeycomb calcined body 510 comprises a porous cell partition wall 513 that divides and forms multiple cells that serve as exhaust gas flow paths, an exhaust gas introduction cell 511 with an open end face 510a on the exhaust gas inlet side and a sealed end face 510b on the exhaust gas outlet side, and an exhaust gas discharge cell 512 with an open end face 510b on the exhaust gas outlet side and a sealed end face 510a on the exhaust gas inlet side. Because the cell partition wall 513 is a porous material, gaseous components of the exhaust gas can pass through, but solid components such as PM cannot. Therefore, when exhaust gas flows into the honeycomb calcined body 510, the exhaust gas G (in Figure 12, exhaust gas is indicated by G and the flow of exhaust gas is indicated by arrows) flows into the exhaust gas introduction cell 511, passes through the cell partition wall 513 separating the exhaust gas discharge cell 512 and the exhaust gas introduction cell 511, and then flows out from the exhaust gas discharge cell 512. As the exhaust gas G passes through the cell partition wall 513, PM and other particles in the exhaust gas are captured, so the cell partition wall 513 functions as a filter. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2018-149510 [Overview of the project] [Problems that the invention aims to solve]

[0008] In recent years, regulations on PM emissions have become stricter in order to reduce the environmental burden. PM, which has a large number of particles relative to its weight and is small in diameter, is difficult to capture and is easily released into the environment. Such PMs have the problem that they cannot be adequately collected simply by controlling the distribution of cumulative pore volume, as disclosed in Patent Document 1.

[0009] This invention was made to solve the above problems, and aims to provide a honeycomb filter with high PM collection efficiency. [Means for solving the problem]

[0010] The inventors of the present invention have discovered that the PM collection efficiency can be improved by prematurely depositing PM on the surface of the cells that collect PM in a honeycomb filter, thereby forming a cake layer, and have completed the present invention.

[0011] The honeycomb filter of the present invention comprises a porous cell partition wall that divides a plurality of cells that form a flow path for exhaust gas, an exhaust gas introduction cell having an open end face on the exhaust gas inlet side and a sealed end face on the exhaust gas outlet side, and an exhaust gas discharge cell having an open end face on the exhaust gas outlet side and a sealed end face on the exhaust gas inlet side, wherein the honeycomb filter is made of silicon carbide, and in a cross section perpendicular to the longitudinal direction of the honeycomb filter, the plurality of cells include a first cell and a second cell facing the first cell via the cell partition wall, the cell partition wall has a first portion of the cell partition wall formed such that the edges constituting the contour of the first cell and the edges constituting the contour of the second cell are parallel, and the first portion of the cell partition wall has a first direction toward the second cell from the first cell and a second direction perpendicular to the first direction, In a cross-sectional photograph of the first portion of the cell partition wall in a direction perpendicular to the longitudinal direction of the honeycomb filter, the first portion of the cell partition wall is layered every 0.5 μm from the surface on the first cell side toward the first direction, in each layer, a pore continuous in the second direction is considered as one pore, the total number of such pores is defined as the number of pores in each layer, the product of the distance of the first portion of the cell partition wall in the second direction and the porosity of the first portion of the cell partition wall is divided by the average pore diameter of the first portion of the cell partition wall to define the average number of pores, and the number of pores in each layer is compared with the average number of pores in each layer sequentially from the surface on the first cell side to determine the layer in which the number of pores in each layer first exceeds the average number of pores, characterized in that the distance from the surface of the first portion of the cell partition wall on the first cell side to the layer in which the number of pores in each layer first exceeds the average number of pores is 10 μm or less.

[0012] When exhaust gas is purified using a honeycomb filter, most of the PM (particulate matter) contained in the exhaust gas cannot pass through the pores of the cell partitions and is collected on the surface of the exhaust gas inlet cell. However, some PM particles with smaller diameters may pass through the pores of the cell partitions and move to the exhaust gas discharge cell. In other words, some PM particles with smaller diameters may leak out. PM collected on the surface of the exhaust gas introduction cell accumulates to form a cake layer. This cake layer serves as resistance when PM with a small particle size passes through the cell partition walls. That is, when the cake layer is formed, it can prevent PM with a small particle size from leaking. Therefore, it is preferable that the cake layer be formed early.

[0013] Also, when manufacturing a honeycomb filter made of silicon carbide, a honeycomb formed body containing silicon carbide as a component is fired. At this time, depending on the shape and firing state of the silicon carbide particles, there may be differences between the surface state and the internal state of the cell partition walls. For example, when the pore diameter of the pores located near the surface of the cell partition walls is large, PM contained in the exhaust gas reaching the cell partition walls may penetrate from the pores located near the surface of the cell partition walls into the interior of the cell partition walls. Then, a long time is required until the above-described cake layer is formed, and it becomes difficult to collect PM with a small particle size at the initial stage of exhaust gas purification.

[0014] In the honeycomb filter of the present invention, the number of pores and the average pore number of each layer have the above relationship. This means that pores with a small pore diameter are formed near the surface of the cell partition walls. Therefore, when purifying exhaust gas using the honeycomb filter of the present invention, PM is less likely to penetrate into the cell partition walls, and the cake layer can be formed early. Therefore, in the honeycomb filter of the present invention, the collection efficiency of PM with a small particle size is high.

[0015] In the honeycomb filter of the present invention, it is preferable that the above average pore number be 10 to 30 per 1 mm of the length in the above second direction of the above first part of the above cell partition wall. With such an average pore number, it is easy to collect PM in the exhaust gas, and the pressure loss also becomes small.

[0016] It is preferable that the honeycomb filter of the present invention has the above porosity of 35 to 55%. By setting the porosity in this way, the cell partitions can effectively capture PM in the exhaust gas, and the increase in pressure loss caused by the cell partitions can be suppressed. If the porosity of the cell partition is less than 35%, the proportion of pores in the cell partition is too small, making it difficult for exhaust gas to pass through the cell partition, resulting in a large pressure loss when the exhaust gas passes through the cell partition. If the porosity of the cell septum exceeds 55%, the PM collection efficiency may decrease.

[0017] In the honeycomb filter of the present invention, it is preferable that the average pore size is 10 to 35 μm. When the average pore size is within the above range, PM can be collected with high collection efficiency while suppressing an increase in pressure loss. If the average pore size is less than 10 μm, the pores are too small, resulting in a large pressure loss when the exhaust gas passes through the cell partition. If the average pore diameter exceeds 35 μm, the pores become too large, reducing the PM collection efficiency.

[0018] In the honeycomb filter of the present invention, it is preferable that the thickness of the cell partitions in the first direction is 0.1 to 0.46 mm. Cell partitions of this thickness possess sufficient mechanical strength while effectively suppressing the increase in pressure loss.

[0019] The honeycomb filter of the present invention is preferably formed by bonding a plurality of honeycomb fired bodies, each having an outer wall on its outer circumference, via an adhesive layer. In this configuration, even if stress occurs in one honeycomb structure, that stress is relieved by the adhesive layer and less likely to be transmitted to other honeycomb structures. In other words, the stress generated in the honeycomb filter can be relieved. As a result, damage to the honeycomb filter can be prevented.

[0020] In the honeycomb filter of the present invention, it is preferable that an outer periphery coating layer is formed on the outer periphery. The outer coating layer mechanically protects the internal cells. Therefore, it results in a honeycomb filter with excellent mechanical properties such as compressive strength. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a schematic perspective view showing an example of a honeycomb filter according to the first embodiment of the present invention. [Figure 2A] Figure 2A is a schematic perspective view showing an example of a honeycomb firing body constituting a honeycomb filter according to the first embodiment of the present invention. [Figure 2B] Figure 2B is an end view of the exhaust gas inlet side of the honeycomb calcined body shown in Figure 2A. [Figure 3] Figure 3 is a schematic enlarged view showing an example of a cross-section perpendicular to the longitudinal direction of the honeycomb filter of the present invention. [Figure 4] Figure 4 is a schematic cross-sectional view showing an example of the hierarchical arrangement of the first portion of the cell partitions of a honeycomb filter according to the first embodiment of the present invention. [Figure 5] Figure 5 is a schematic cross-sectional view showing an example of the first and second layers of a honeycomb filter according to the first embodiment of the present invention. [Figure 6A] Figure 6A is a schematic cross-sectional view showing an example of the initial stage of exhaust gas purification using a conventional honeycomb filter. [Figure 6B] Figure 6B is a schematic cross-sectional view showing an early example of exhaust gas purification using a conventional honeycomb filter. [Figure 7A] Figure 7A is a schematic cross-sectional view showing an example of the start of exhaust gas purification using a honeycomb filter according to the first embodiment of the present invention. [Figure 7B] Figure 7B is a schematic cross-sectional view showing an initial example of exhaust gas purification using a honeycomb filter according to the first embodiment of the present invention. [Figure 8] Figure 8 is a schematic enlarged view of an end face, showing an example of the end face on the exhaust gas inlet side of a honeycomb filter according to the second embodiment of the present invention. [Figure 9]Figure 9 is a schematic enlarged view of an end face, showing an example of the end face on the exhaust gas inlet side of a honeycomb filter according to the third embodiment of the present invention. [Figure 10A] Figure 10A is an SEM image of the first portion of the cell partition of the honeycomb filter according to Example 1. [Figure 10B] Figure 10B is an SEM image of the first portion of the cell partition of the honeycomb filter according to Comparative Example 1. [Figure 11] Figure 11 is a graph showing the results of the PM deposition test. [Figure 12] Figure 12 is a schematic diagram illustrating an example of collecting PM in exhaust gas using a conventional honeycomb filter. [Modes for carrying out the invention]

[0022] (First Embodiment) Hereinafter, an example of a honeycomb filter according to the first embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is a schematic perspective view showing an example of a honeycomb filter according to the first embodiment of the present invention. Figure 2A is a schematic perspective view showing an example of a honeycomb firing body constituting a honeycomb filter according to the first embodiment of the present invention. Figure 2B is an end view of the exhaust gas inlet side of the honeycomb calcined body shown in Figure 2A.

[0023] In the honeycomb filter 20 shown in Figure 1, multiple honeycomb fired bodies 10 are bound together via an adhesive layer 15 to form a ceramic block 18, and an outer periphery coating layer 16 is formed on the outer circumference of this ceramic block 18 to prevent exhaust gas leakage. Note that the outer periphery coating layer 16 may be formed only if necessary.

[0024] In the honeycomb filter 20, multiple honeycomb firing bodies 10 are bound together via an adhesive layer 15. Therefore, even if stress occurs in one honeycomb firing body 10, that stress is relieved by the adhesive layer 15 and is less likely to be transmitted to other honeycomb firing bodies 10. In other words, the stress generated in the honeycomb filter 20 can be relieved. As a result, damage to the honeycomb filter 20 can be prevented. The adhesive layer 15 is formed by applying and drying an adhesive paste containing an inorganic binder and inorganic particles. The adhesive layer 15 may further contain inorganic fibers and / or whiskers. The thickness of the adhesive layer 15 is preferably 0.5 to 2.0 mm.

[0025] The outer coating layer 16 plays a role in mechanically protecting the internal cells. As a result, the honeycomb filter 20 has excellent mechanical properties such as compressive strength. Furthermore, it is preferable that the material of the outer periphery coating layer 16 is the same as the material of the adhesive layer 15. The thickness of the outer periphery coating layer 16 is preferably 0.1 to 3.0 mm.

[0026] Although the honeycomb-fired body 10 has a rectangular prism shape, as shown in Figures 2A and 2B, the corners at the end faces are chamfered to have a curved shape, thereby preventing thermal stress from concentrating at the corners and causing damage such as cracks. The corners may also be chamfered to have a straight shape.

[0027] The honeycomb calcined body 10 shown in Figure 2A comprises a porous cell partition wall 13 that divides a plurality of cells that serve as exhaust gas flow paths, an exhaust gas introduction cell 11 having an open end face 10a on the exhaust gas inlet side and a sealed end face 10b on the exhaust gas outlet side, and an exhaust gas discharge cell 12 having an open end face 10b on the exhaust gas outlet side and a sealed end face 10a on the exhaust gas inlet side.

[0028] The honeycomb-type sintered body 10 is made of silicon carbide. The honeycomb-type sintered body made of silicon carbide has high heat resistance.

[0029] In the honeycomb filter of the present invention, it is preferable that the sealing material that seals the exhaust gas introduction cell and the exhaust gas discharge cell is made of the same material as the honeycomb firing body.

[0030] In the honeycomb filter 20, the cross-sectional shape perpendicular to the longitudinal direction of the exhaust gas introduction cell 11 and the exhaust gas discharge cell 12 is the same in each cell, from the end face 10a on the exhaust gas inlet side to the end face 10b on the exhaust gas outlet side, except for the sealing portion.

[0031] As shown in Figure 2B, in the honeycomb calcined body 10, the cross-sectional shape of the exhaust gas introduction cell 11 and the cross-sectional shape of the exhaust gas discharge cell 12 are both squares, and these are arranged alternately in a checkerboard pattern.

[0032] Here, we will explain the cell partitions 13 in the honeycomb filter 20 in more detail. Figure 3 is a schematic enlarged view showing an example of a cross-section perpendicular to the longitudinal direction of the honeycomb filter of the present invention.

[0033] As shown in Figure 3, the honeycomb filter 20 includes a first cell 31 and a second cell 32 facing the first cell 31 via a cell partition 13. In the honeycomb filter of the present invention, either of the cells facing each other across the cell partition can be considered the first cell. In other words, if one of the cells facing each other across the cell partition is considered the first cell, the other is the second cell. For the sake of clarity, in the following explanation, the exhaust gas introduction cell 11 will be referred to as the first cell 31, and the exhaust gas discharge cell 12 as the second cell 32.

[0034] As shown in Figure 3, the cell partition wall 13 has a first portion 13a of the cell partition wall formed such that the side 31a constituting the outline of the first cell 31 and the side 32a constituting the outline of the second cell 32 are parallel. The first portion 13a of the cell partition wall has a first direction (indicated by arrow Y in Figure 3) from the first cell 31 toward the second cell 32, and a second direction (indicated by arrow X in Figure 3) perpendicular to the first direction Y. In Figure 3, the first portion 13a of the cell partition wall is the entire cell partition wall 13 located between the first cell 31 and the second cell 32. However, in the honeycomb filter of the present invention, the first portion of the cell partition wall refers to any region where the edges constituting the contour of the first cell and the edges constituting the contour of the second cell are parallel, and the width in the second direction is 400 μm or more.

[0035] In the honeycomb filter 20, in a cross-sectional photograph of the first portion 13a of the cell partition wall perpendicular to the longitudinal direction of the honeycomb filter 20, the first portion 13a of the cell partition wall is layered every 0.5 μm from the surface on the first cell 31 side toward the first direction Y, and in each layer, a pore continuous in the second direction X is considered as one pore, and the total number of pores is defined as the number of pores in each layer. The average number of pores is defined as the product of the distance of the first portion 13a of the cell partition wall in the second direction X and the porosity of the first portion 13a of the cell partition wall, divided by the average pore diameter of the first portion 13a of the cell partition wall. The number of pores in each layer and the average number of pores are compared sequentially from the surface on the first cell 31 side, and when the layer in which the number of pores in each layer first exceeds the average number of pores is determined, the distance from the surface of the first portion 13a of the cell partition wall on the first cell 31 side to the layer in which the number of pores in each layer first exceeds the average number of pores is 10 μm or less.

[0036] In the honeycomb filter 20, the "porosity," "average pore diameter," and "number of pores in each layer" are calculated based on a cross-sectional photograph of the first portion 13a of the cell partition wall perpendicular to the longitudinal direction of the honeycomb filter 20. The cross-sectional image is preferably taken using an electron microscope. Electron microscope images can be taken, for example, using an electron microscope (FE-SEM: Hitachi High-Technologies Corporation's High-Resolution Field Emission Scanning Electron Microscope S-4800). Furthermore, the magnification of the electron microscope image must be such that the irregularities of particles and pores on the surface (inner wall) of the cell septa do not hinder the identification of the cell's cross-sectional shape, the measurement of side lengths, septa thickness, and cell cross-sectional area. At the same magnification, it is necessary to adopt a magnification that allows for the identification of the cell's cross-sectional shape, the measurement of side lengths, cell septa thickness, and cell cross-sectional area. Optimally, measurements should be taken using an electron microscope image with a magnification of 200x. When taking cross-sectional images using an electron microscope, the filter is cut perpendicular to the longitudinal direction of the cell, and a 1cm x 1cm x 1cm sample is prepared so that the cut surface is included. The sample is then ultrasonically cleaned or embedded in resin, and the electron microscope image is taken. Resin embedding does not affect the measurement.

[0037] The "porosity" and "average pore diameter" of the first portion 13a of the cell septum can be calculated using the image analysis software "image J". In this specification, "porosity of the first portion of the cell septum" refers to the proportion of pores present in the first portion of the cell septum in a cross-sectional photograph of the first portion of the cell septum. In this specification, "average pore diameter of the first portion of the cell septum" refers to the average value of the pore diameters of the pores present in each layer of the first portion of the cell septum in a cross-sectional photograph of the first portion of the cell septum. Furthermore, in this specification, "average pore count" means the value obtained by dividing the product of the distance of the first portion of the cell partition wall in the second direction in the cross-sectional photograph and the porosity of the first portion 13a of the cell partition wall by the average pore diameter of the first portion 13a of the cell partition wall.

[0038] Next, we will explain in detail, using diagrams, the method for calculating the number of pores at each level. Figure 4 is a schematic cross-sectional view showing an example of the hierarchical arrangement of the first portion of the cell partitions of a honeycomb filter according to the first embodiment of the present invention.

[0039] As shown in Figure 4, numerous pores 40 are present in the first portion 13a of the cell septum. To calculate the number of pores in each layer, first, as shown in Figure 4, in a cross-sectional photograph of the first portion 13a of the cell partition wall, the first portion 13a of the cell partition wall is layered every 0.5 μm in the first direction Y from the surface on the first cell 31 side. As shown in Figure 4, the layer closest to the first cell 31 is the first layer L1, the next layer is the second layer L2, and so on, with the layers being the nth layer L n It continues up to this point. The thickness T in the first direction Y of each layer is 0.5 μm.

[0040] Here, regarding the “method for calculating the number of pores in each layer”, the first layer L1 and the second layer L2 will be taken as examples for explanation. FIG. 5 is a cross-sectional view schematically showing an example of the first layer and the second layer of the honeycomb filter according to the first embodiment of the present invention.

[0041] When calculating the number of pores in the first layer L1, first, the pore portion 41 and the material portion 51 in the first layer L1 are separated. As a separation method, binarization using software or the like can be adopted. As the software, image analysis processing software “image J” can be used. Next, the continuous pore portions 41 are regarded as one pore (the portions indicated by P 1-1 , P 1-2 , P 1-3 , P 1-4 ···P 1-k in FIG. 5). The total number of these “one pore” is the number of pores in the first layer L1. When calculating the number of pores in the second layer L2, first, the pore portion 41 and the material portion 51 in the second layer L2 are separated. Next, the continuous pore portions 41 are regarded as one pore (the portions indicated by P 2-1 , P 2-2 , P 2-3 , ···P 2-l in FIG. 5). The total number of these “one pore” is the number of pores in the second layer L2. As shown in FIG. 5, P 1-2 is connected to P 2-2 , and P 1-3 is connected to P 2-2 . That is, in the entire cell partition 13, P 1-2 , P 1-3 and P 2-2 are a continuous pore but are branched from the first layer L1 to the second layer L2. Even in such a case, when calculating the number of pores in the first layer L1, P 1-2 and P 1-3 are counted as separate “one pore”.

[0042] Next, we compare the number of pores in each layer with the average number of pores, starting from the surface of the first cell 31. In other words, we compare the number of pores in each layer with the average number of pores, starting from the first layer L1. Then, the distance between the layer in which the number of pores in each layer first exceeds the average number of pores and the surface of the first portion 13a of the cell partition wall on the first cell 31 side is measured.

[0043] This operation is performed at the first portion 13a of the 10 cell partitions, and the average distance is the distance in the honeycomb filter from the surface of the first portion of the cell partition on the first cell side to the layer where the number of pores in each layer first exceeds the average number of pores.

[0044] In the honeycomb filter 20, the distance from the surface of the first portion 13a of the cell partition wall on the first cell 31 side to the layer where the number of pores in each layer first exceeds the average number of pores is 10 μm or less. In the honeycomb filter 20, it is preferable that the distance from the surface of the first portion 13a of the cell partition wall on the first cell 31 side to the layer where the number of pores in each layer first exceeds the average number of pores is 3 to 9 μm.

[0045] The fact that the honeycomb filter 20 has these characteristics means that in the cell partitions 13, pores with small pore diameters are formed near the surface of the cell partitions 13.

[0046] Here, we will explain the effect of purifying exhaust gas using the honeycomb filter 20 in comparison to purifying exhaust gas using a conventional honeycomb filter. Figure 6A is a schematic cross-sectional view showing an example of the initial stage of exhaust gas purification using a conventional honeycomb filter. Figure 6B is a schematic cross-sectional view showing an early example of exhaust gas purification using a conventional honeycomb filter. Figure 7A is a schematic cross-sectional view showing an example of the start of exhaust gas purification using a honeycomb filter according to the first embodiment of the present invention. Figure 7B is a schematic cross-sectional view showing an initial example of exhaust gas purification using a honeycomb filter according to the first embodiment of the present invention.

[0047] As shown in Figure 6A, in the conventional honeycomb filter 520, pores 540 with a large pore diameter are formed near the surface of the cell partition wall 513. Therefore, at the start of exhaust gas purification, PM contained in the exhaust gas G that reaches the cell partition wall 513 enters the pores 540 located near the surface of the cell partition wall 513. As a result, as shown in Figure 6B, the time required for the cake layer 560 to form increases, making it more difficult to capture PM with a smaller particle size.

[0048] On the other hand, as shown in Figure 7A, in the honeycomb filter 20, pores with a small pore diameter are formed near the surface of the cell partition wall 13. Therefore, as shown in Figure 7B, the cake layer 60 is quickly formed on the surface of the cell partition 13. Consequently, the honeycomb filter 20 has a high collection efficiency for PM with a small particle size.

[0049] In the honeycomb filter 20, the average number of pores is preferably 10 to 30 per 1 mm of length in the second direction X of the first portion 13a of the cell partition, and more preferably 12 to 28. With such an average pore count, PM in the exhaust gas is easily captured, and pressure loss is also reduced.

[0050] In the honeycomb filter 20, the porosity in the first portion 13a of the cell partition is preferably 35 to 55%, and more preferably 38 to 50%. By setting the porosity in this way, the cell partition 13 can effectively capture PM in the exhaust gas, and the increase in pressure loss caused by the cell partition 13 can be suppressed. If the porosity of the cell partition is less than 35%, the proportion of pores in the cell partition is too small, making it difficult for exhaust gas to pass through the cell partition, resulting in a large pressure loss when the exhaust gas passes through the cell partition. If the porosity of the cell septum exceeds 55%, the PM collection efficiency may decrease.

[0051] In the honeycomb filter 20, the average pore diameter in the first portion 13a of the cell partition is preferably 10 to 35 μm, and more preferably 12 to 30 μm. When the average pore size is within the above range, PM can be collected with high collection efficiency while suppressing an increase in pressure loss. If the average pore size is less than 10 μm, the pores are too small, resulting in a large pressure loss when the exhaust gas passes through the cell partition. If the average pore diameter exceeds 35 μm, the pores become too large, reducing the PM collection efficiency.

[0052] In the honeycomb filter 20, it is preferable that the thickness of the cell partition wall 13 in the first direction Y is 0.1 to 0.46 mm. Cell partitions of this thickness possess sufficient mechanical strength while effectively suppressing the increase in pressure loss.

[0053] In the honeycomb filter 20, the number of cells per unit area in the cross-section of the honeycomb calcined body 10 is 31 to 93 cells / cm². 2 (200~600 pieces / inch 2 ) is preferable.

[0054] Next, a method for manufacturing a honeycomb filter according to the first embodiment of the present invention will be described.

[0055] (1) A molding process is performed to produce a honeycomb molded body by extruding a wet mixture containing silicon carbide powder and a binder. Specifically, a wet mixture for manufacturing honeycomb molded bodies is first prepared by mixing coarse silicon carbide powder with an average particle size of 18-35 μm and fine silicon carbide powder with an average particle size of 0.1-2.0 μm with an organic binder, a liquid plasticizer, a lubricant, and water. In this case, it is preferable to use silicon carbide crude powder with an average particle size of 18 to 35 μm, and more preferably 20 to 30 μm. By using silicon carbide crude powder of this size, in the honeycomb calcined body produced through a later process, when the first portion of the cell partitions is layered every 0.5 μm from the surface of the first cell in the first direction, the distance from the surface of the first portion of the cell partitions on the first cell side to the layer where the number of pores in each layer first exceeds the average number of pores can be reduced to 10 μm or less.

[0056] The above wetted mixture may optionally contain pore-forming agents such as balloons, which are tiny hollow spheres made of oxide ceramics, spherical acrylic particles, or graphite. The balloon is not particularly limited and examples include alumina balloons, glass microballoons, shirasu balloons, fly ash balloons (FA balloons), mullite balloons, etc. Among these, alumina balloons are preferred.

[0057] Next, the wet mixture is fed into an extrusion molding machine and extruded to produce a honeycomb molded body of a predetermined shape. In this process, a honeycomb molded body is produced using a mold that produces a cross-sectional shape having the cell structure (cell shape and cell arrangement) shown in Figure 2B.

[0058] (2) The honeycomb molded body is cut to a predetermined length, dried using a microwave dryer, hot air dryer, dielectric dryer, vacuum dryer, freeze dryer, etc., and then a sealing step is performed in which a sealing paste that will serve as a sealing material is filled into predetermined cells to seal the cells. Here, the above-mentioned wet mixture can be used as the sealing paste.

[0059] (3) The honeycomb molded body is heated in a degreasing furnace to 300-650°C to remove organic matter from the honeycomb molded body in a degreasing process. After that, the degreasing honeycomb molded body is transported to a firing furnace and fired at 1900-2200°C in a firing process. By firing at such firing temperatures, in the honeycomb fired body produced through the subsequent processes, when the first portion of the cell partition wall is layered every 0.5 μm from the surface of the first cell side toward the first direction, the distance from the surface of the first portion of the cell partition wall on the first cell side to the layer where the number of pores in each layer first exceeds the average number of pores can be made to 10 μm or less. By performing the firing in this manner, a honeycomb-shaped fired body like those shown in Figures 2A and 2B is produced. Furthermore, the sealing paste filled into the ends of the cell is fired by heating to become a sealant. Furthermore, the conditions for the cutting, drying, sealing, degreasing, and firing processes can be those that have been conventionally used when manufacturing honeycomb fired bodies.

[0060] (4) A binding process is performed in which multiple honeycomb fired bodies are sequentially stacked and bound together on a support base using an adhesive paste, thereby creating a honeycomb assembly made up of multiple stacked honeycomb fired bodies. As an adhesive paste, for example, one consisting of an inorganic binder, an organic binder, and inorganic particles may be used. Furthermore, the adhesive paste may also contain inorganic fibers and / or whiskers.

[0061] Examples of inorganic particles included in the above adhesive paste include carbide particles and nitride particles. Specifically, examples include silicon carbide particles, silicon nitride particles, and boron nitride particles. These may be used individually or in combination of two or more. Among the inorganic particles, silicon carbide particles, which have excellent thermal conductivity, are preferred.

[0062] Examples of inorganic fibers and / or whiskers included in the above adhesive paste include silica-alumina, mullite, alumina, silica, and the like. These may be used individually or in combination of two or more. Among the inorganic fibers, alumina fibers are preferred. The inorganic fibers may also be biosoluble fibers.

[0063] Furthermore, the adhesive paste may contain, if necessary, balloons which are tiny hollow spheres made of oxide ceramics, spherical acrylic particles, graphite, etc. The balloons are not particularly limited and include, for example, alumina balloons, glass microballoons, shirasu balloons, fly ash balloons (FA balloons), mullite balloons, etc.

[0064] (5) Next, the honeycomb assembly is heated to solidify the adhesive paste, forming an adhesive layer, and a rectangular prism-shaped ceramic block is produced. The conditions for heating and solidifying the adhesive paste can be those conventionally used when manufacturing honeycomb filters.

[0065] (6) A cutting process is performed on the ceramic block. Specifically, a ceramic block with a roughly cylindrical outer surface is produced by cutting its outer surface using a diamond cutter.

[0066] (7) An outer coating layer formation process is performed in which an outer coating paste is applied to the outer surface of a roughly cylindrical ceramic block and dried and solidified to form an outer coating layer. Here, the adhesive paste described above can be used as the outer perimeter coating paste. Alternatively, a paste with a different composition from the adhesive paste may be used as the outer perimeter coating paste. Furthermore, the outer coating layer is not necessarily required and can be added only if necessary. By applying an outer coating layer, the shape of the outer perimeter of the ceramic block can be adjusted to create a cylindrical honeycomb filter. By following the above steps, a honeycomb filter according to the first embodiment of the present invention, including a honeycomb calcined body, can be manufactured.

[0067] In the above process, a honeycomb filter of a predetermined shape was manufactured by a cutting process. However, in the process of manufacturing the honeycomb fired body, multiple shapes of honeycomb fired bodies having an outer wall around their entire circumference may be manufactured, and these multiple shapes of honeycomb fired bodies may be combined via an adhesive layer to form a predetermined shape such as a cylinder. In this case, the cutting process can be omitted.

[0068] (Second Embodiment) Next, a honeycomb filter according to a second embodiment of the present invention will be described. The honeycomb filter according to the second embodiment of the present invention is identical in appearance to the honeycomb filter according to the first embodiment of the present invention, except that the arrangement of the cells is different. Figure 8 is a schematic enlarged view of an end face, showing an example of the end face on the exhaust gas inlet side of a honeycomb filter according to the second embodiment of the present invention.

[0069] In the honeycomb filter 120 shown in Figure 8, the cross-sectional shape of the exhaust gas inlet cell 111 and the exhaust gas outlet cell 112 are both square. Furthermore, the exhaust gas inlet cells 111 and exhaust gas outlet cells 112 are arranged in a grid pattern such that the exhaust gas inlet cells 111 surround the exhaust gas outlet cell 112 via cell partitions 113. In other words, in the honeycomb filter 120, there are parts where exhaust gas introduction cells 111 (for convenience, indicated by reference numerals 111a and 111b) are adjacent to each other.

[0070] In the honeycomb filter 120, the exhaust gas introduction cell 111a may be designated as the first cell and the exhaust gas introduction cell 111b as the second cell. In addition, in the honeycomb filter 120, the exhaust gas introduction cell 111a may be designated as the first cell, and the exhaust gas discharge cell 112 adjacent to the exhaust gas introduction cell 111a may be designated as the second cell.

[0071] (Third embodiment) Next, a honeycomb filter according to a third embodiment of the present invention will be described. The honeycomb filter according to the third embodiment of the present invention is the same as the honeycomb filter according to the first embodiment of the present invention, except that the shape and arrangement of the cells are different. Figure 9 is a schematic enlarged view of an end face, showing an example of the end face on the exhaust gas inlet side of a honeycomb filter according to the third embodiment of the present invention.

[0072] In the honeycomb filter 220 shown in Figure 9, a first exhaust gas introduction cell 211a with a square cross-section and a second exhaust gas introduction cell 211b with an octagonal cross-section are adjacent to the entire perimeter of an exhaust gas discharge cell 212 with an octagonal cross-section, separated by a cell partition wall 213. The first exhaust gas introduction cell 211a and the second exhaust gas introduction cell 211b are arranged alternately around the exhaust gas discharge cell 212, with the cross-sectional area of ​​the second exhaust gas introduction cell 211b being larger than that of the first exhaust gas introduction cell 211a, and the cross-sectional area of ​​the exhaust gas discharge cell 212 being the same as that of the second exhaust gas introduction cell 211b. The cross-sectional shapes of the second exhaust gas introduction cell 211b and the exhaust gas discharge cell 212 are both octagonal and congruent to each other.

[0073] In the honeycomb filter 220, any of the first exhaust gas introduction cell 211a, the second exhaust gas introduction cell 211b, and the exhaust gas discharge cell 212 may be designated as the first cell. In this case, any cell adjacent to the first cell becomes the second cell.

[0074] (Other embodiments) In the honeycomb filter according to the first embodiment of the present invention, a so-called aggregated honeycomb filter was formed by assembling a plurality of honeycomb firing bodies. However, the honeycomb filter of the present invention may be a so-called integrated honeycomb filter consisting of a single honeycomb firing body. [Examples]

[0075] (Example 1) 54.6 parts by weight of coarse silicon carbide powder having an average particle size of 24 μm and 23.4 parts by weight of fine silicon carbide powder having an average particle size of 0.5 μm were mixed. To the resulting mixture, 4.4 parts by weight of an organic binder (methylcellulose), 2.6 parts by weight of a lubricant (Unilube, manufactured by NOF Corporation), 1.2 parts by weight of glycerin, and 13.8 parts by weight of water were added and kneaded to obtain a wet mixture, after which an extrusion molding process was carried out. In this process, a raw honeycomb molded body was produced that had the same shape as the honeycomb fired body 10 shown in Figures 2A and 2B, but without sealing the cell gaps.

[0076] Next, the raw honeycomb molded body was dried using a microwave dryer to produce a dried honeycomb molded body. After that, sealing paste was filled into predetermined cells of the dried honeycomb molded body to seal the cells. Specifically, the cells were sealed so that the end faces on the exhaust gas inlet side and the end faces on the exhaust gas outlet side were sealed at the positions shown in Figure 2B. The above-mentioned wet mixture was used as a sealing paste. After sealing the cells, the dried honeycomb molded body filled with the sealing paste was dried again using a dryer.

[0077] Next, the honeycomb molded body, with its cells sealed, underwent a degreasing treatment at 400°C, followed by a firing treatment at 2150°C for 3 hours under atmospheric pressure and an argon atmosphere. This allowed us to produce the honeycomb calcined body according to Example 1.

[0078] In Example 1, the cell partition thickness of the honeycomb-fired body was 0.40 mm, and the cell density was 31 cells / cm³. 2 The pressure was 200 cpsi. The dimensions of the honeycomb calcined body were 34.3 × 34.3 × 254 mm.

[0079] Multiple honeycomb-fired bodies were bundled together using an adhesive paste consisting of a mixture of SiC particles, silica sol, and alumina fibers. The outer periphery was then processed, and a coating layer made of the same material as the adhesive paste was applied to the outer periphery to create a cylindrical honeycomb filter measuring φ118.4 mm × 254 mm.

[0080] (Example 2) Except for the size of the honeycomb filter to be manufactured being a cylindrical shape with dimensions of φ143.8 mm × 203.2 mm, the honeycomb filter according to Example 2 was manufactured in the same manner as in Example 1.

[0081] (Comparative Example 1) A honeycomb filter according to Comparative Example 1 was prepared in the same manner as in Example 1, except that a coarse silicon carbide powder with an average particle size of 15 μm was used as the material for the honeycomb calcined body, the calcination temperature was set to 2250°C, and the size of the honeycomb filter to be manufactured was a cylindrical shape of φ143.8 mm × 177.8 mm.

[0082] (Measurement of pore count at each layer) The honeycomb-fired bodies according to each example and comparative example were cut perpendicular to the longitudinal direction, and the cross-sections were photographed using a scanning electron microscope (SEM). Next, in the SEM image, the first portion of the cell septum where the edges constituting the contour of the first cell and the edges constituting the contour of the second cell are parallel was selected. In the SEM image, the direction from the first cell to the second cell is the first direction. The width of the first portion of the cell septum in the second direction in the SEM image was 640 μm. Next, the first portion of the cell partition was binarized using the image analysis software "image J" to separate the pore area from the material area. Next, the first portion of the cell partition wall was layered at 0.5 μm intervals from the surface on the first cell side in the first direction. Then, the porosity, average pore diameter, and number of pores in each layer were measured, and the average pore number was calculated. Subsequently, the number of pores in each layer was compared with the average number of pores to determine the layer in which the number of pores in each layer first exceeds the average number of pores. The layer in which the number of pores in each layer first exceeds the average number of pores was then calculated from the surface of the first portion of the cell partition wall on the first cell side. This operation was performed on the first portion of 10 cell partitions, and the average distance was calculated. The results are shown in Table 1. Furthermore, one SEM image each of the first portion of the cell partition of the honeycomb filter according to Example 1 and Comparative Example 1, used in the above measurements, is shown as a representative example in Figures 10A and 10B. Figure 10A is an SEM image of the first portion of the cell partition of the honeycomb filter according to Example 1. Figure 10B is an SEM image of the first portion of the cell partition of the honeycomb filter according to Comparative Example 1. In addition, Figures 10A and 10B also show a graph of the "number of stomata at each layer" to the left of the SEM image.

[0083] [Table 1]

[0084] (PM deposition test) The honeycomb filters according to each example and comparative example were connected to a 2.5L diesel engine with an oxidation catalyst placed before the honeycomb filter. The diesel engine was operated in WHTC mode (cold), and the amount of PM accumulated on the honeycomb filter (amount of PM accumulated per unit volume of the honeycomb filter: g / L) and the number of PM particles that passed through the honeycomb filter (number of PM particles / kWh per unit of work done by the diesel engine) were measured. Figure 11 shows the relationship between the amount of PM accumulated on the honeycomb filter and the number of PM particles that passed through the honeycomb filter. Figure 11 is a graph showing the results of the PM deposition test.

[0085] As shown in Figure 11, in the honeycomb filters of Examples 1 and 2, despite the small amount of PM deposited on the honeycomb filter, the number of PM particles that passed through the honeycomb filter was small. This is thought to be because a cake layer was quickly formed on the surface of the cell partitions of the honeycomb filter, reducing the number of PM particles that passed through the honeycomb filter. [Explanation of Symbols]

[0086] 10, 510 Honeycomb fired body 10a, 510a End face on the exhaust gas inlet side 10b, 510b End face on the exhaust gas outlet side 11, 111 (111a, 111b), 511 Exhaust gas introduction cell 12, 112, 212, 512 exhaust gas emission cells 13, 113, 213, 513 cell partitions 13a First part of the cell partition 15 Adhesive layer 16 Outer coating layer 18 Ceramic Blocks 20, 120, 220, 520 honeycomb filters 31 Cell 1 31a Edges that make up the outline of the first cell 32 Cell 2 32a Edges that make up the outline of the second cell 40, 540 stomata 41 Pores 51 Materials Department 60,560 cake layers 211a First exhaust gas introduction cell 211b Second exhaust gas introduction cell

Claims

1. Multiple cells that serve as the exhaust gas flow path, A honeycomb filter comprising porous cell partitions that divide the cells, The aforementioned honeycomb filter is made of silicon carbide, In a cross-section perpendicular to the longitudinal direction of the honeycomb filter, The plurality of cells include a first cell and a second cell facing the first cell across the cell partition, The cell partition has a first portion of the cell partition formed such that the sides constituting the outline of the first cell and the sides constituting the outline of the second cell are parallel. The first portion of the cell partition wall has a first direction toward the second cell and a second direction perpendicular to the first direction. In a cross-sectional photograph of the first portion of the cell partition wall in a direction perpendicular to the longitudinal direction of the honeycomb filter, the first portion of the cell partition wall is layered every 0.5 μm from the surface on the first cell side toward the first direction, and in each layer, a pore portion continuous in the second direction is considered as one pore, and the total number of such pores is defined as the number of pores in each layer. The average number of pores is obtained by dividing the product of the distance of the first portion of the cell partition wall in the second direction and the porosity of the first portion of the cell partition wall by the average pore diameter of the first portion of the cell partition wall. Starting from the surface of the first cell, the number of pores in each layer is compared with the average number of pores, and when the layer in which the number of pores in each layer first exceeds the average number of pores is determined, The distance from the surface of the first portion of the cell partition wall on the first cell side to the layer in which the number of pores in each layer first exceeds the average number of pores is 10 μm or less. Each of the plurality of first cells and plurality of second cells contained in the cell belongs to either an exhaust gas introduction cell in which the end face on the exhaust gas inlet side is open and the end face on the exhaust gas outlet side is sealed, or an exhaust gas discharge cell in which the end face on the exhaust gas outlet side is open and the end face on the exhaust gas inlet side is sealed. The honeycomb filter is characterized in that the exhaust gas introduction cell faces at least one of the exhaust gas discharge cells via the cell partition.

2. The honeycomb filter according to claim 1, wherein the average number of pores is 10 to 30 per 1 mm of length in the second direction of the first portion of the cell partition wall.

3. The honeycomb filter according to claim 1 or 2, wherein the porosity is 35 to 55%.

4. The honeycomb filter according to any one of claims 1 to 3, wherein the average pore size is 10 to 35 μm.

5. The honeycomb filter according to any one of claims 1 to 4, wherein the thickness of the cell partition wall in the first direction is 0.1 to 0.46 mm.

6. The aforementioned honeycomb filter is A honeycomb filter according to any one of claims 1 to 5, wherein a plurality of honeycomb fired bodies having outer walls on their outer periphery are bonded together via an adhesive layer.

7. A honeycomb filter according to any one of claims 1 to 6, wherein an outer periphery coating layer is formed on the outer periphery.

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