JP2001097776A - Porous silicon carbide sintered product, honeycomb filter, ceramic filter assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a honeycomb filter which has excellent mechanical strength, excellent heat resistance, and excellent heat conductivity and is used for exhausted gas-cleaning devices. SOLUTION: This honeycomb filter 9 comprises a columnar sintered product in which silicon carbide crystal grains 16 constituting porous tissues are bound to each other through neck portions 17. The neck portions 17 are smoothly curved and have radii of curvature of >=3 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic filter assembly having a structure in which a plurality of filters made of a ceramic sintered body are bonded and integrated, a honeycomb filter usable for manufacturing the same, and a porous silicon carbide sintered body. It is about union.

[0002]

2. Description of the Related Art The number of automobiles has increased exponentially since the turn of the century, and the amount of exhaust gas emitted from internal combustion engines of automobiles has been increasing rapidly.
In particular, various substances contained in exhaust gas emitted from a diesel engine cause pollution, and are now seriously affecting the world environment. Also,
Recently, studies have reported that particulate matter (diesel particulates) in exhaust gas sometimes causes allergic disorders and a reduction in sperm count.
In other words, it is considered that taking measures to remove particulates in exhaust gas is an urgent task for humanity.

Under such circumstances, various types of exhaust gas purifying devices have been proposed. A general exhaust gas purifying apparatus has a structure in which a casing is provided on an exhaust pipe connected to an exhaust manifold of an engine, and a filter having fine holes is disposed therein. Materials for forming the filter include ceramics in addition to metals and alloys. As a typical example of a ceramic filter, a cordierite honeycomb filter is known.

[0004]

However, when a honeycomb filter is formed using a cordierite porous body, sufficient mechanical strength cannot be secured, and the honeycomb filter is likely to be broken. was there.

[0005] Further, it is considered that the cause is that in cordierite composed of plate-like crystals, the shape of the bonding portion of the crystal grains is acute, and the contact area between the crystal grains is extremely small. Was.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a porous silicon carbide sintered body, a honeycomb filter, and a ceramic filter assembly having excellent strength.

[0007]

According to the first aspect of the present invention, there is provided a sintered body in which silicon carbide crystal particles constituting a porous structure are connected to each other by a neck portion. The gist of the present invention is a porous silicon carbide sintered body characterized in that the neck portion has a smooth curved shape.

According to a second aspect of the present invention, in the first aspect, the radius of curvature of the neck portion is 3 μm or more. According to the third aspect of the present invention, there is provided a columnar honeycomb filter formed of a sintered body in which silicon carbide crystal particles constituting a porous structure are connected to each other by a neck portion, wherein the neck portion has a smooth curved shape. The gist of the present invention is a honeycomb filter characterized by the following.

According to the present invention, a plurality of prismatic honeycomb filters made of a sintered body in which silicon carbide crystal grains constituting a porous structure are joined by a neck portion are used as constituent members, and the outer periphery thereof is used. A ceramic filter in which the honeycomb filters are integrated by bonding the surfaces to each other via a ceramic sealing material layer, wherein the neck portion has a smooth curved shape. The aggregate is the gist.

Hereinafter, the "action" of the present invention will be described. According to the first to fourth aspects of the present invention, since the neck portion connecting the silicon carbide crystal particles has a smooth curved shape, the contact area between the crystal particles increases. For this reason, the bonding strength between crystal grains is improved, and breakage at grain boundaries is less likely to occur. Therefore, even if it is a porous structure, sufficient mechanical strength can be ensured.

In this case, the radius of curvature of the neck is preferably 3 μm or more. When the radius of curvature is smaller than 3 μm, the neck portion is not so smooth, and the contact area between crystal grains is still small. Therefore, the bonding strength between crystal grains cannot be sufficiently improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An exhaust gas purifying apparatus 1 for a diesel engine according to an embodiment of the present invention will now be described.
This will be described in detail with reference to FIGS.

As shown in FIG. 1, the exhaust gas purifying device 1 is a device for purifying exhaust gas discharged from a diesel engine 2 as an internal combustion engine. The diesel engine 2 includes a plurality of cylinders (not shown). Each cylinder has an exhaust manifold 3 made of metal material.
Are connected to each other. Each branch 4 is 1
It is connected to each of the manifold bodies 5. Therefore, the exhaust gas discharged from each cylinder is concentrated at one place.

A first exhaust pipe 6 and a second exhaust pipe 7 made of a metal material are provided downstream of the exhaust manifold 3. The upstream end of the first exhaust pipe 6 is connected to the manifold body 5. Between the first exhaust pipe 6 and the second exhaust pipe 7, a cylindrical casing 8 also made of a metal material is provided. 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. It can also be grasped that the casing 8 is disposed on the way of the exhaust pipes 6,7. As a result, the first exhaust pipe 6, the casing 8
The internal region of the second exhaust pipe 7 communicates with each other, and the exhaust gas flows therein.

As shown in FIG. 1, the casing 8 is formed so that its central portion is larger in diameter than the exhaust pipes 6 and 7. Therefore, the internal area of the casing 8 is wider than the internal areas of the exhaust pipes 6 and 7. A honeycomb filter 9 is accommodated in the casing 8.

A heat insulating material 10 is provided between the outer peripheral surface of the honeycomb filter 9 and the inner peripheral surface of the casing 8.
The heat insulating material 10 is a mat-like material including a ceramic fiber, and has a thickness of several mm to several tens mm.
The heat insulating material 10 preferably has thermal expansion properties. The term “thermal expansion” as used herein refers to a function of releasing thermal stress due to having an elastic structure. The reason is that by preventing heat from escaping from the outermost peripheral portion of the honeycomb filter 9, energy loss during regeneration is minimized. Further, by expanding the ceramic fiber by the heat at the time of regeneration, it is possible to prevent the displacement of the ceramic filter assembly 9 caused by the pressure of the exhaust gas, vibration due to running, and the like.

Since the honeycomb filter 9 used in the present embodiment removes diesel particulates as described above, it is generally called a diesel particulate filter (DPF). As shown in FIG. 2 and the like, the honeycomb filter 9 of the present embodiment has a columnar shape.

The honeycomb filter 9 is made of a porous silicon carbide sintered body which is a kind of a ceramic sintered body. The reason for adopting the silicon carbide sintered body is that, compared to other ceramics,
In particular, there is an advantage of being excellent in strength, heat resistance and thermal conductivity.

As shown in FIGS. 2, 3, and 4, the honeycomb filter 9 of the present embodiment has a so-called honeycomb structure. The reason for adopting the honeycomb structure is that there is an advantage that the pressure loss is small even when the amount of collected fine particles increases. In the honeycomb filter 9, a plurality of through-holes 12 having a substantially square cross section are formed regularly along the axial direction. Each through hole 12 is separated from each other by a thin cell wall 13. On the outer surface of the cell wall 13, an oxidation catalyst comprising a platinum group element (for example, Pt or the like) or another metal element and an oxide thereof is supported. The opening of each through-hole 12 is connected to one of the end faces 9.
On the sides of a and 9b, sealing is performed by a sealing body 14 (here, a porous silicon carbide sintered body). Therefore, the end face 9
A and 9b have a checkered pattern as a whole. As a result, a large number of cells having a square cross section are formed in the honeycomb filter 9. Cell density is 200 cells /
The thickness is set to about inch, the thickness of the cell wall 13 is set to about 0.3 mm, and the cell pitch is set to about 1.8 mm. Of the large number of cells, about half of the cells are open at the upstream end face 9a, and the remaining cells are open at the downstream end face 9b.

The average pore diameter of the honeycomb filter 9 is 1 μm.
５０50 μm, more preferably 5 μm-20 μm. If the average pore diameter is less than 1 μm, clogging of the honeycomb filter 9 due to accumulation of fine particles becomes remarkable.
On the other hand, if the average pore diameter exceeds 50 μm, fine particles cannot be collected, and the collection efficiency will be reduced.

The porosity of the honeycomb filter 9 is 30% to 7%.
It is preferably 0%, more preferably 40% to 60%.
If the porosity is less than 30%, the honeycomb filter 9 may be too dense, and exhaust gas may not be allowed to flow inside. On the other hand, the porosity is 70%
Exceeding the formula (1), the number of voids in the honeycomb filter 9 becomes too large, so that the strength is weakened and the efficiency of collecting fine particles may be reduced.

When a porous silicon carbide sintered body is selected, the honeycomb filter 9 has a thermal conductivity of 20 W / mK to
75 W / mK, preferably 30 W / mK
It is particularly preferable that it is ７０70 W / mK. If the thermal conductivity is too small, a temperature difference is likely to occur in the honeycomb filter 9, which leads to generation of a large thermal stress which causes cracks. Conversely, if the thermal conductivity is to be increased, manufacturing becomes difficult, and stable material supply becomes difficult.

As shown schematically in FIG.
In the honeycomb filter 9 of the present embodiment, the silicon carbide crystal particles 16 constituting the porous structure are connected by a so-called neck portion 17. Here, neck portion 17 refers to a structure generated at a grain boundary by solid phase sintering of silicon carbide. In the honeycomb filter 9 of the present embodiment, the neck 1
7 has a smooth curved outer surface. In addition,
FIG. 6 shows an SEM photograph of the sintered body for reference.

The radius of curvature of the outer surface of the neck portion 17 is 3 μm.
More preferably, it is 3 μm to 100 μm,
In particular, the thickness is preferably 5 μm to 20 μm. If the radius of curvature is smaller than 3 μm, the contact area between the crystal grains 16 does not increase, and the bonding strength between the crystal grains 16 cannot be sufficiently improved. Specifically, 45MPa
This is because the above bending strength cannot be imparted to the honeycomb filter 9. Conversely, if the radius of curvature is 100 μm or more, the honeycomb filter 9 becomes too dense, and there is a possibility that exhaust gas cannot be circulated inside.

The silicon carbide crystal particles 1 in the sintered body
6 preferably has an average particle diameter of about 5 μm to 15 μm, and the average particle diameter of the crystal particles 16 is 5 μm to 30 μm.
The abundance of m is preferably 30% or more.

Next, a procedure for manufacturing the above honeycomb filter 9 will be described. First, a ceramic raw material slurry used in the extrusion molding step and a sealing paste used in the end face sealing step are prepared in advance.

As the ceramic raw material slurry, a mixture obtained by mixing a predetermined amount of an organic binder and water with silicon carbide powder and kneading the mixture is used. As the sealing paste, a mixture obtained by mixing and kneading an organic binder, a lubricant, a plasticizer, and water with silicon carbide powder is used.

In this case, the ceramic raw material slurry is desirably produced using two types of silicon carbide powders having different average particle diameters. Specifically, the average particle size is 15 μm
It is desirable to use a mixture of large powder before and after and fine powder having an average particle size of about 1 μm. By doing so, neck sintering is promoted, and the radius of curvature of the neck portion 17 is also increased.

Next, the ceramic raw material slurry is charged into an extruder and continuously extruded through a mold. Thereafter, the extruded honeycomb formed body is cut into equal lengths to obtain cut pieces of the honeycomb formed body having a columnar shape.
Further, a predetermined amount of the sealing paste is filled into one opening of each cell of the cut piece, and both end faces of each cut piece are sealed.

Subsequently, the desired honeycomb filter 9 is completed by setting the temperature, time, and the like to predetermined conditions, and performing the main firing to completely sinter the cut pieces of the formed honeycomb body and the sealing body 14. I do.

In the present embodiment, the firing temperature is set between 2100 ° C. and 2
The temperature is set to 300 ° C., and the firing time is set to 0.1 to 5 hours. The atmosphere in the furnace during firing is an inert atmosphere, and the pressure of the atmosphere at that time is normal pressure.
The firing temperature is desirably set as high as possible within the above range. By setting the temperature in this manner, neck sintering is promoted, and the radius of curvature of the neck portion 17 tends to increase.

Next, the trapping action of the above-described honeycomb filter 9 will be briefly described. Exhaust gas is supplied to the honeycomb filter 9 housed in the casing 8 from the upstream end surface 9a side. Exhaust gas supplied via the first exhaust pipe 6 first flows into a cell opened at the upstream end surface 9a. Next, the exhaust gas passes through the cell wall 13 and reaches the inside of the cell adjacent thereto, that is, the cell opened at the downstream end surface 9b.
Then, the exhaust gas flows out of the downstream end face 9b of the honeycomb filter 9 through the opening of the cell. However, the fine particles contained in the exhaust gas cannot pass through the cell wall 13 and are trapped there. As a result, the purified exhaust gas is supplied to the downstream end face 9 b of the honeycomb filter 9.
Is discharged from The purified exhaust gas further passes through the second exhaust pipe 7 and is finally released into the atmosphere. When the internal temperature of the honeycomb filter 9 reaches a predetermined temperature, the trapped fine particles are ignited by the action of the catalyst and burn.

[0033]

Examples and Comparative Examples (Examples) α having an average particle size of 10 μm
Type silicon carbide powder 51.5% by weight and average particle size 0.5 μm
Was mixed with 22% by weight of α-type silicon carbide powder, and 6.5% by weight and 20% by weight of an organic binder (methyl cellulose) and water were added to the resulting mixture, respectively, and kneaded. Next, a small amount of a plasticizer and a lubricant were added to the kneaded material, and the mixture was further kneaded and extruded to obtain a honeycomb-shaped formed body. Specifically, α-type silicon carbide powder having an average particle size of 10 μm (trade name: C-1000F, manufactured by Yakushima Electric Works Co., Ltd.) and an average particle size of 0.5
μm (Yakushima Denko Corporation, trade name: GC-1
5) was used.

Next, after the formed body was dried using a microwave dryer, the through-holes 12 of the formed body were sealed with a sealing paste made of a porous silicon carbide sintered body. Next, the sealing paste was dried again using a dryer.
After the end face sealing step, the dried body was degreased at 400 ° C.
It was baked at 250 ° C. for about 3 hours. As a result, a honeycomb filter 9 made of a porous silicon carbide sintered body was obtained. The diameter of each honeycomb filter 9 was set to 100 mm, and the length was set to 200 mm.

Next, the structure of the honeycomb filter 9 obtained as described above was observed by SEM. According to this investigation, it was confirmed that the neck portion 17 had a smooth and curved shape. When the radius of curvature of the outer surface of the neck portion 17 was measured based on the SEM photograph, the average value was about 10 μm. Therefore, in the embodiment, as a result of the contact area between the crystal particles 16 being large, the crystal particles 1
It was expected that the bonding strength between the particles 6 was improved, and that fracture at the grain boundaries was unlikely to occur.

The bending strength of the honeycomb filter 9 was measured by a conventionally known method, and the measured value was about 50 MPa. Therefore, the honeycomb filter 9 was provided with extremely high mechanical strength. Further, as a result of housing the honeycomb filter 9 in the casing 8 and using it for a certain period, no crack was generated in the honeycomb filter 9. Comparative Example 1 In Comparative Example 1, a honeycomb filter 9 was manufactured using silicon carbide powder basically in the same manner as in Example. The dimensions and the like of the honeycomb filter 9 were equal to those of the embodiment. However, here, only one type of α-type silicon carbide powder having an average particle size of about 10 μm is used, and the firing temperature is set slightly lower, so that the neck portion 17 can be formed.
Has a radius of curvature of about 1 μm.

When SEM observation was performed, it was found that the neck portion 17
Was not as smooth and curved as in the examples. Further, the measured value of the bending strength of the honeycomb filter 9 was about 30 MPa on average, and the mechanical strength was inferior to the examples. After the honeycomb filter 9 was accommodated in the casing 8 and used for a certain period of time, a slight crack was observed in the honeycomb filter 9.

(Comparative Example 2) In Comparative Example 2, a honeycomb filter having the same dimensions as those of the example was manufactured using a cordierite porous sintered body.

As a result of SEM observation, the neck portion 17 as in the case of the porous silicon carbide sintered body did not exist at the bonding portion of the plate-like crystal particles 16. In addition, the shape of the bonding portion of the crystal grains 16 was also acute, and it was difficult to say that the shape was smooth and curved (see FIG. 5B).

Further, the measured value of the bending strength of the honeycomb filter was about 5 MPa on average, which was extremely poor in mechanical strength as compared with the example. After the honeycomb filter was accommodated in the casing 8 and used for a certain period of time, cracks were observed in the honeycomb filter.

Therefore, according to the embodiment of the present embodiment, the following effects can be obtained. (1) In the honeycomb filter 9 of the embodiment, since the neck portion 17 has a smooth curved shape, the contact area between the crystal grains 16 is large. For this reason, the bonding strength between the crystal grains 16 is improved, and breakage at grain boundaries is less likely to occur. Therefore, even if it is a porous structure, sufficient mechanical strength can be secured, and the honeycomb filter 9 which is hard to break can be obtained. The exhaust gas purifying apparatus 1 using such a honeycomb filter 9 has high strength and can be used for a long period of time, so that it has excellent practicality.

(2) In the embodiment, the radius of curvature of the neck portion 17 is set to 3 μm or more. Therefore, the crystal grains 16
45MPa
The above-described sufficient bending strength can be imparted to the honeycomb filter 9.

The embodiment of the present invention may be modified as follows. The shape of the honeycomb filter 9 is not limited to a columnar shape as in the embodiment, and may be changed to a triangular prism, a quadrangular prism, a hexagonal prism, or the like.

As shown in FIG. 7, a single ceramic filter assembly 21 may be manufactured by combining a plurality of (here, 16) honeycomb filters 23. Prismatic honeycomb filter 2 constituting the aggregate 21
Numeral 3 is a sintered body in which silicon carbide crystal particles 16 constituting a porous structure are joined together by a neck portion 17. The outer peripheral surfaces of the honeycomb filters 23 are bonded to each other via a ceramic sealing material layer 22. As a result, the respective honeycomb filters 23 are integrated in a bundled state. With such a configuration, it is possible to prevent cracks from occurring due to stress caused by a temperature gradient due to heating,
Also resistant to thermal shock. Therefore, the size of the filter can be relatively easily increased.

The number of combinations of the honeycomb filters 23 does not have to be 16 as in the above-mentioned another example, but can be any number. In this case, it is of course possible to use honeycomb filters 23 having different sizes and shapes in appropriate combinations.

The honeycomb filters 9 and 23 are not limited to those having a honeycomb-like structure as shown in the above embodiment and other examples. For example, a three-dimensional mesh structure, a foam-like structure, a noodle-like structure, a fiber-like structure And so on.

In the embodiment, the honeycomb filter (or the ceramic filter assembly) of the present invention is embodied as a filter for an exhaust gas purification device attached to the diesel engine 2. Of course, the honeycomb filter (or ceramic filter assembly) of the present invention
It can be embodied as something other than a filter for an exhaust gas purification device. Examples include heat exchanger components,
Filtration filters for high-temperature fluids and high-temperature steams are exemplified. Furthermore, the porous silicon carbide sintered body of the present invention can be applied to uses other than filters.

Next, in addition to the technical ideas described in the claims, the technical ideas grasped by the above-described embodiment will be listed below. (1) The bending strength of the sintered body according to any one of claims 1 to 4, which is 45 MPa or more. Therefore, according to the invention described in the technical idea 1, high mechanical strength is provided.

(2) In any one of the first to fourth aspects and the technical idea 1, the average particle size of the silicon carbide crystal particles in the sintered body is 5 μm to 15 μm, and The existence ratio of particles having a particle size of 5 μm to 30 μm is 30% or more.

(3) The honeycomb filter according to the third aspect or the ceramic filter assembly according to the fourth aspect is accommodated in a casing provided on the exhaust pipe of the internal combustion engine, and the filter or the assembly is accommodated. An exhaust gas purification device in which a gap formed between an outer peripheral surface of a body and an inner peripheral surface of the casing is filled with a heat insulating material. Therefore, according to the invention described in the technical idea 3, since the device has high strength and can be used for a long period of time, it is possible to provide an apparatus having excellent practicality.

[0051]

As described above, according to the first and second aspects of the present invention, a porous silicon carbide sintered body having excellent strength can be provided.

According to the third aspect of the present invention, a honeycomb filter having excellent strength can be provided. According to the fourth aspect of the present invention, a ceramic filter aggregate having excellent strength can be provided.

[Brief description of the drawings]

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

FIG. 2 is a perspective view of the honeycomb filter of the embodiment.

FIG. 3 is a sectional view taken along line AA of the honeycomb filter of the embodiment.

FIG. 4 is an enlarged sectional view of a main part of the exhaust gas purification device.

5A is an enlarged schematic cross-sectional view of a sintered body structure of a honeycomb filter of an example made of porous silicon carbide, and FIG. 5B is a diagram of a sintered body structure of Comparative Example 2 made of porous cordierite. FIG.

FIGS. 6A and 6B show SE of the honeycomb filter of the embodiment.
M photo.

FIG. 7 is a perspective view of another example of a ceramic filter assembly formed by using a plurality of honeycomb filters.

[Explanation of symbols]

9, 23 ... honeycomb filter, 16 ... crystal particles, 17 ...
Neck part, 21: ceramic filter assembly, 22: ceramic sealing material layer.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3G090 AA03 4D019 AA01 BA05 BB06 BC07 BC12 BC20 BD01 CA01 CB03 CB04 CB06 4G001 BA22 BB22 BC01 BC12 BC13 BC17 BC26 BC41 BC52 BC56 BD01 BD14 BD36 BE02 BE31 BE39

Claims (4)

[Claims] 1. A porous carbonized body comprising a sintered body in which silicon carbide crystal grains constituting a porous structure are connected to each other by a neck portion, wherein said neck portion has a smooth curved shape. Silicon sintered body. 2. The porous silicon carbide sintered body according to claim 1, wherein a radius of curvature of said neck portion is 3 μm or more. 3. A columnar honeycomb filter comprising a sintered body in which silicon carbide crystal particles constituting a porous structure are connected by a neck portion, wherein the neck portion has a smooth curved shape. And a honeycomb filter. 4. A plurality of prismatic honeycomb filters made of a sintered body in which silicon carbide crystal particles constituting a porous structure are connected by a neck portion as a constituent member, and their outer peripheral surfaces are formed of a ceramic sealing material. A ceramic filter assembly, wherein the honeycomb filters are integrated by bonding through a layer, wherein the neck portion has a smooth curved shape.