JPH0971466A - Silicone-carbide honeycomb structure and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a honeycomb structure which can inhibit the porous wall from clogging, is capable of collecting a plenty of combustible particulate, combusting the collected particulate and can be reused. SOLUTION: This conductive honeycomb structure is useful for collecting combustible fine particles in exhaust gas of a diesel engine with a porous wall and is capable of reusing by incinerating the collected particles by electric heating. The porous wall of this conductive honeycomb structure comprises silicone-carbide having 10<-1> -10<1> Θ.cm of the specific resistance at room temperature and 20-50vol.% of void volume based on the total void volume having the granular size of 20-50% if the thickness of the porous wall. This honeycomb structure is produced by molding a composition comprising 20-60wt.% of silicone- carbide powder and 80-40wt.% of a mixture consisting silicone-nitride powder and powder of a carbonaceous material and formulating with 10-30vol.% of graphite having predetermined granular size per 100 vol.pts. of the composition, carrying out the primary calcination at 1,600-2,500 deg.C and subjecting to the secondary calcination at 1,600-2,500 deg.C after removing the remaining graphite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive honeycomb structure for collecting combustible fine particles such as soot from exhaust gas and regenerating the collected combustible fine particles by incineration.
In particular, the present invention relates to a honeycomb structure for a diesel particulate filter that collects combustible fine particles discharged from a diesel engine.

[0002]

2. Description of the Related Art Conventionally, a filter that collects combustible fine particles that are considered to be harmful substances such as soot from exhaust gas, for example, a filter that collects combustible fine particles contained in exhaust gas of a diesel engine is a cordierite. A honeycomb structure including a porous wall containing silicon carbide as a main component is applied. The structure has a number of through holes that are continuous in a honeycomb shape in the longitudinal direction between the inlet end face and the outlet end face through a porous wall, and the inlet end face and the outlet end face of the through holes are alternately sealed, The through hole whose end face is opened is sealed at the outlet end face, and the through hole whose inlet end face is sealed is opened at the outlet end face.
Further, this filter is attached as a part of the exhaust gas system of a diesel engine, and the exhaust gas flowing in from the through hole of the opened inlet end face passes through the porous wall and traps the combustible fine particles to generate the combustible gas. Exhaust gas that does not contain fine particulates flows out through a through hole that is open on the outlet end face. Therefore, it is necessary for the exhaust gas containing the combustible particles to easily pass through the porous wall and to have a pore size and a porosity suitable for collecting most or all of the combustible particles.

In Japanese Patent Publication No. 5-77442, therefore, in the porous wall of the honeycomb structure, the volume of open porosity and the average diameter of the pores forming the open porosity are point 1-G-5-2- on the coordinates. 3-4 (However, point 1; open porosity 58.5%, average pore diameter 1 μm, point G; open porosity 46.8%, average pore diameter 12 μm, point 5; open porosity 39.5%, average pore diameter 15 μm, point 2; open Porosity 33.0%, average pore diameter 15 μm, point 3; open porosity 52.5%, average pore diameter 20 μm, point 4; open porosity
90.0%, having an average pore diameter of 1 μm), a particulate filter for a diesel engine exhaust gas system within a zone defined by a boundary line connecting the same is disclosed.

Further, JP-A-61-83689 discloses that in a silicon carbide honeycomb structure in which a large number of through holes are adjacent to each other in the axial direction with thin partition walls, the partition walls have an average aspect ratio.
It is disclosed that the porous body has a three-dimensional network structure mainly composed of plate crystals in the range of 2 to 50.

As described above, the honeycomb structure has a large collection area and has an excellent collection performance by appropriately selecting the pore diameter and the porosity of the porous wall. However, when a certain amount of combustible particles are collected, the porous wall is clogged and the ventilation resistance increases, so it is necessary to incinerate and regenerate the collected combustible particles at regular intervals. For this regeneration, a method of injecting combustion gas of a burner to a filter to incinerate it, a method of incinerating by heating in combination with a heating metal layer such as a nichrome wire heater, etc. are mainly used. However, the method of heating the filter from the outside to regenerate it has a problem that the filter may be melted due to local heat generation due to the combustion of the combustible particles, or thermal stress cracking due to a temperature gradient may occur.

Further, most of the porous walls of the conventional honeycomb structure are composed of fine pores of 50 μm or less, the combustible fine particles are easily clogged, and the ventilation resistance of exhaust gas rises in a short time. Regeneration by incineration must be repeated frequently. Further, since the collection time is lengthened, the filter becomes large and the temperature distribution during regeneration becomes more uneven.

Therefore, as a method for uniformly incinerating the collected combustible fine particles, a self-heating type filter by energization has been studied. Techniques using JP-A-58-119317 and JP-A-2-42112 are disclosed.

However, oxide ceramics such as lanthanum chromite have a low heat resistance and a high coefficient of thermal expansion, so that thermal stress cracking is likely to occur. In addition, non-oxide ceramics such as molybdenum silicide and titanium carbide are easily oxidized and lose conductivity when the porosity and pore diameter increase. In addition, silicon carbide has a Ti, Z content in order to obtain the desired conductivity.
Periodic Table IV _a-group element or V, such as r, carbides V _a group element such as Nb, nitrides, added boride, but to impart conductivity by forming a continuous conductive phase However, the addition of these conductive substances reduces the oxidation resistance.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and suppresses clogging of a porous wall,
Since a large amount of combustible particles can be collected, the filter can be made compact, and since it has excellent conductivity and oxidation resistance, it is possible to uniformly incinerate and regenerate the combustible particles collected by electric heating. It is an object to provide a silicon honeycomb structure.

[0010]

That is, a feature of the present invention is that a conductive honeycomb structure filter for collecting combustible particles by a porous wall and incinerating the collected combustible particles by electric heating to regenerate them And the room temperature specific resistance of the porous wall made of silicon carbide is 10 ^-1 to 10 ¹ Ωcm, and 20 to 50 volume of pores having a diameter corresponding to 20 to 50% of the wall thickness with respect to the total pore volume. %
included. In particular, the average pore diameter is 10-40 μm, the compressive strength of the through-hole in the axial direction is 1 MPa or more, and in the atmosphere
It is preferable that the room temperature resistivity after exposure for 100 hr or more in the atmosphere of 1000 ° C. is 1.5 or less of the room temperature resistivity before exposure. The characteristic of the method for producing silicon carbide of the present invention is that the silicon carbide powder is 20 to 60% by weight, the silicon component in the silicon nitride and the molar ratio (Si / C) of carbon are 0.5 to 1.5 and the carbonaceous material. Mixture of substances 80-40% by weight and formulation with 10
0 For the volume part, the thickness of the porous wall of the honeycomb structure is 20
Add 10 to 30 parts by volume of graphite powder with a particle size equivalent to ~ 50%, shape, and perform primary firing in a non-oxidizing atmosphere at 1600 to 2500 ° C, then heat-treat in an oxidizing atmosphere to decarburize the remaining graphite. And then secondary firing in a non-oxidizing atmosphere at 1600 to 2500 ° C. A feature of the present invention is a diesel particulate filter made of the above silicon carbide honeycomb structure.

The present invention will be described in more detail below.

The honeycomb structure of the present invention is a conductive honeycomb structure which collects combustible fine particles contained in exhaust gas of a diesel engine, incinerates the collected combustible fine particles by electric heating, and regenerates them. In the filter, the room temperature resistivity of the porous wall made of silicon carbide is 10 ^-1 ~
10 ¹ Ω ・ cm, 20 ~ the thickness of the porous wall for the total pore volume
It contains 20 to 50% by volume of pores with a diameter corresponding to 50%.

The reason why the material is silicon carbide is that it has high thermal conductivity and heat resistance, and that its pore structure forms pores in the gaps between crystal grains, and each grain is entangled in a complicated state. Since the number of relative pores is large and the ventilation resistance is small, it can be suitably used as a honeycomb structure filter.

Furthermore, in terms of its pore characteristics, the wall thickness of 20 to
By including 20 to 50% by volume of pores having a diameter corresponding to 50% with respect to the total pore volume, it is possible to suppress clogging of the combustible fine particles and improve the collection amount of the combustible fine particles.
Therefore, pores with a diameter corresponding to less than 20% of the wall thickness have a high efficiency of collecting combustible particles, but easily clogged to increase ventilation resistance. Reduces mechanical strength. Also included in the porous wall,
The proportion of pores is 20 to 50% by volume of the total pore volume, and 20% by volume.
If it is less than 50% by volume, it is insufficient to suppress an increase in ventilation resistance, and if it exceeds 50% by volume, the mechanical strength decreases.

The porosity is preferably 40% or more, more preferably in the range of 50 to 70%. If it is lower than 40%, the ventilation resistance is high, and if it exceeds 70%, the mechanical strength is lowered. Also,
The average pore diameter is preferably 10 to 40 μm, and if it is smaller than 10 μm, the combustible fine particles are easily clogged, and if it exceeds 40 μm, the mechanical strength is lowered. The term "pore size" as used herein refers to that determined by the mercury porosimetry method, the measuring method of which is described in detail in Examples. Further, in order to use as a filter for collecting combustible fine particles, etc., the mechanical strength of the silicon carbide honeycomb structure is the compressive strength in the axial direction of the through hole, and 1.
The pressure is preferably 0 MPa or more, and when the pressure is less than 1.0 MPa, the durability is reduced.

The room temperature specific resistance of the porous wall is 10 ^-1 to 10
If it is higher than 10 Ω · cm in the range of Ω · cm, for example, when conducting electricity heating as a diesel particulate filter, combustible fine particles will normally burn at a battery capacity of about 24 V mounted on diesel vehicles. It becomes difficult to raise the temperature to ℃ or more, and if it is less than 10 ^-1 Ω · cm, the current is high and it becomes difficult to control the system.

When combustible particles are incinerated by electric heating in an oxidizing atmosphere to be regenerated, the temperature may rise to 1000 ° C. or higher due to self-combustion of the combustible particles. Even in an oxidizing atmosphere at 1000 ° C., it is necessary that the room temperature resistivity of the silicon carbide honeycomb structure does not significantly increase from the initial room temperature resistivity due to oxidation or the like. That is, 1000 in the atmosphere
The change in room temperature resistivity with respect to exposure at 100 ° C for 100 hr is within 50% of the initial room temperature resistivity, more preferably within 30%,
It is more preferably within 10%. 1000 ℃ in air, 10
1000 even when the room temperature resistivity after exposure for 0 hr is 50% or more
It can be used at low temperature below ℃.

The above honeycomb structure, as a diesel particulate filter, contains coarse pores at a specific ratio and has an appropriate average pore diameter and porosity.
Suppresses increase in ventilation resistance due to clogging of combustible particles. Further, the combustible fine particles that are conductive and are heated by being directly energized can be incinerated uniformly. Therefore, it is possible to obtain a honeycomb structure for a diesel particulate filter, which has a long capturing time of the combustible particles and is compact and which can be uniformly heated and regenerated.

Next, a method for manufacturing the honeycomb structure of the present invention will be described.

As a manufacturing method for obtaining the above-mentioned silicon carbide honeycomb structure, the average particle diameter of the silicon carbide powder as a starting material is 50 μm or less, preferably 10 to 50 μm, and if it is smaller than 10 μm, the average pore diameter becomes small. When the thickness exceeds 50 μm, the strength decreases. The particle size of the silicon nitride powder is preferably 100 μm or less, more preferably 50 μm or less in order to promote the carbonization reaction. On the other hand, as a carbonaceous substance,
When solid carbon powder such as carbon black or acetylene black is used, in order to promote the carbonization reaction, the average particle size is 10 μm or less, more preferably 1 μm or less.
It is less than or equal to m. Besides, as the carbonaceous substance, an organic resin such as phenol, furan, or polyimide which is thermally decomposed to serve as a carbon source can be used.

The starting materials are compounded in such a manner that the main raw material is 20 to 60% by weight of silicon carbide powder, and the silicon nitride powder and the carbonaceous material in which the molar ratio (Si / C) of silicon component to carbon in silicon nitride is 0.5 to 1.5. 80-80% by weight of the mixture of substances are compounded. Silicon carbide, which is a starting material, is an aggregate in reaction sintering, and if it is less than 20% by weight, the strength of the sintered body decreases,
When it is more than 60% by weight, the amount of silicon carbide produced by reaction is small, so that the desired specific resistance cannot be obtained. That is, since silicon carbide generated by carbonization of silicon nitride becomes the main conductive phase, the molar ratio of the silicon component in silicon nitride and carbon (Si / C
) Is less than 0.5, the crystal growth of the produced silicon carbide is hindered, and the oxidation resistance of the conductive phase is lowered. When it is more than 1.5, the production of silicon carbide is insufficient and the desired specific resistance cannot be obtained.

Next, graphite powder is used as a pore-forming agent in order to form an appropriate amount of coarse pores. The amount of the graphite powder is 100 parts by volume of the mixed powder described above and the amount of the honeycomb structure is Add 10 to 30 parts by volume of graphite powder adjusted to a particle size corresponding to 20 to 50% of the thickness of the porous wall. The reason why it is preferable to use graphite powder as a pore-forming agent is to stably form coarse pores suitable for the present invention. The generated silicon carbide crystals grow and coarse pores disappear.
If the particle size of the pore-forming agent is less than 20% of the thickness of the porous wall of the honeycomb structure, the diameter of coarse pores with respect to the wall thickness is small,
If the particle size exceeds 50% of the wall thickness, the pore size becomes large and the strength decreases. Furthermore, the addition amount of the pore-forming agent is 10
It is in the range of 10 to 30 parts by volume with respect to 0 parts by volume. When it is less than 10 parts by volume, the amount of coarse pores is small, and when it exceeds 30 parts by volume, the amount of coarse pores is large and the strength is lowered.

The raw material powders can be mixed by any method such as dry type and wet type as long as they can be uniformly mixed. In order to extrude the mixed raw material, water and an organic binder such as methylcellulose or polyvinyl alcohol are used. Is added to form a honeycomb structure having a desired shape.

The firing is performed by first firing at 1600 to 2500 ° C. in a non-oxidizing atmosphere such as nitrogen or argon, and then decarbonizing the remaining graphite in the oxidizing atmosphere. Then, secondary firing is performed at 1600 to 2500 ° C., which is higher than the primary firing temperature, in a non-oxidizing atmosphere. The primary firing is to carbonize the raw material powder silicon nitride to generate silicon carbide. If the temperature is lower than 1600 ° C, silicon carbide is not sufficiently generated and the crystal grains are fine, so that it is easily oxidized during decarburization. Collapse. On the other hand, when the temperature exceeds 2500 ° C, the specific resistance becomes high due to the crystal transition of silicon carbide. Next, decarburization is to remove the remaining graphite of the pore-forming agent to form coarse pores, and an oxidizing atmosphere of 600 to 1300 ° C is preferable, at which the graphite can be oxidized and removed. Below 600 ° C, decarburization is not possible. This is sufficient, and if the temperature exceeds 1300 ° C, the oxidation of silicon carbide proceeds. Sintering strength can be obtained at the stage of primary firing, but higher strength can be obtained by performing secondary firing after decarburization. Particularly preferably, the treatment is performed at the same temperature or higher as in the primary firing. This is because the fine particles disappear due to grain growth, and thus excellent oxidation resistance and the target average pore diameter can be obtained, but if decarburization is insufficient, the remaining graphite hinders sintering during firing. , Properties such as strength, oxidation resistance and average pore size are deteriorated. The temperature of the secondary firing is preferably 1600 to 2500 ° C. If the temperature of the secondary firing is less than 1600 ° C, sintering and grain growth are insufficient, and if it exceeds 2500 ° C, the specific resistance increases due to crystal transition.

[0025]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples.

(Examples 1 to 11 and Comparative Examples 1 to 7) As shown in Table 1, starting materials were silicon carbide powder having an average particle size of 10 μm, silicon nitride powder having an average particle size of 25 μm and an average particle size of 80 nm. A predetermined amount of carbon black was mixed to make 100 parts by volume, and graphite and styrene beads as pore-forming agents were adjusted in particle size with a vibrating screener to add the required amount. Then, 20 parts by weight of water and 8.0 parts by weight of methyl cellulose as a binder were blended, mixed for 10 minutes by a Henschel mixer, and kneaded for 30 minutes using a kneader type kneader. The kneaded product is a vacuum extrusion molding machine, and for Examples 1 to 9 and Comparative Examples 1 to 7, a honeycomb structure having an outer diameter dimension of 100 mm, a cell dimension of 2.0 mm, and a wall thickness of 0.4 mm was prepared. For, a honeycomb structure having an outer diameter of 100 mm, a cell size of 1.5 mm, and a wall thickness of 0.3 mm was extruded. The molded body thus obtained was dried and degreased, and then subjected to primary firing in a nitrogen atmosphere for 30 minutes under the conditions shown in Table 2, and subsequently subjected to oxidation treatment in the atmosphere at 1000 ° C. × 1 Hr. Then, secondary firing was performed for 30 minutes in a nitrogen atmosphere. still,
Comparative Examples 1 and 2 show the results without secondary firing.

The following properties of the resulting honeycomb structure were measured, and the results are shown in Tables 3 and 4. (1) Porosity: Archimedes method. (2) Pore diameter: By mercury injection method, the pore size distribution is calculated assuming that the pores are cylindrical, and the average pore size (average pore size) is calculated by dividing the total pore volume by the pore specific surface area. Further, the volume ratio of pores having a diameter corresponding to 20 to 50% of the honeycomb wall thickness in the total pore volume was obtained. (3) Pressure loss and particulate collection performance: Part of the porous wall (□ 20 × 0.4 mm ^t ) was cut out from the honeycomb structure and 2 L / min
Air was passed through and the pressure loss was measured. For the particulate collection performance, a carbon slurry with a solid content of 5% was applied to the porous wall, dried, and 2 L / min of air was passed through to determine the carbon deposition amount until the pressure loss reached 200 mmHg. . (4) Room temperature specific resistance: The honeycomb structure was cut into a size of 10 × 50 mmL, silver paste was applied as an electrode, and the resistance was measured by a four-terminal method. (5) Oxidation resistance: Treated in air at 1000 ° C for 100 hours, and measured the change in resistivity. (6) Mechanical strength: The honeycomb structure was cut into □ 10 × 10 mm ^L , and the compressive strength in the extrusion direction was measured.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

As shown in Tables 1 to 4, the honeycomb structure of the present invention has a large amount of trapped fine particles, a long time of increase in ventilation resistance, and an excellent trapping performance. Further, since it has excellent conductivity and oxidation resistance without adding a conductivity-imparting material, it is possible to incinerate and regenerate the collected combustible fine particles by electric heating, particularly, a honeycomb structure for a diesel particulate filter. It was shown to have excellent characteristics as.

[0033]

EFFECTS OF THE INVENTION The honeycomb structure of the present invention has a large collection amount of combustible fine particles on the porous wall, and the ventilation resistance of exhaust gas hardly rises. Further, since it has excellent conductivity and oxidation resistance, it can be heated by direct energization, and it can be compactly and uniformly heated, so that melting loss of the filter and thermal stress cracking due to heat cycle are less likely to occur. Therefore, it is excellent as a filter for various kinds of exhaust gas, and can be suitably used particularly as a honeycomb structure for a diesel particulate filter. In addition, since it has a large heat generation area and can improve thermal efficiency, it can be used as a heater for hot air generators used for duct heaters, heat sources for large dryers, heating equipment, cooking equipment,
It is also suitable as a heater used in drying equipment, firing furnaces, etc.

Claims (4)

[Claims] 1. A conductive honeycomb structure which can be regenerated by collecting combustible fine particles on a porous wall, incinerating the collected combustible fine particles by electric heating, and having a room temperature resistivity of 10
A silicon carbide honeycomb structure having a pore diameter of ^{-1 to} 10 Ω · cm and a diameter corresponding to 20 to 50% of a wall thickness of 20 to 50% by volume with respect to the total pore volume. 2. The average pore diameter is 10 to 40 μm, the compressive strength of the through hole in the axial direction is 1 MPa or more, and in the atmosphere.
2. The silicon carbide honeycomb structure according to claim 1, wherein the room temperature resistivity after exposure for 100 hours or more in an atmosphere of 1000 ° C. is 1.5 or less of the room temperature resistivity before exposure. 3. A silicon carbide powder of 20 to 60% by weight and a molar ratio (Si / C) of silicon component to carbon in silicon nitride of 0.5 to 1.
5, which is a mixture of silicon nitride powder and carbonaceous material 80 ~ 4
10 to 30 parts by volume of graphite powder having a particle size corresponding to 20 to 50% of the thickness of the porous wall of the honeycomb structure is added to 100 parts by volume of the mixture with 0% by weight, and the mixture is molded at 1600 to 2500 ° C. After the primary firing in a non-oxidizing atmosphere, heat treatment in an oxidizing atmosphere to decarburize the remaining graphite, and then secondary firing in a non-oxidizing atmosphere at 1600 ~ 2500 ° C. Structure manufacturing method. 4. A diesel particulate filter comprising the silicon carbide honeycomb structure according to claim 1 or 2.