JP2672545B2 - Method for manufacturing silicon carbide honeycomb filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a method for manufacturing a silicon carbide honeycomb filter, and more specifically, it has extremely high efficiency of collecting fine particles, heat resistance and acid resistance. The present invention relates to a method for manufacturing a silicon carbide honeycomb filter which is also excellent in chemical conversion property.

(Prior Art) One end face of a honeycomb-shaped molded body having innumerable through-holes connected in a honeycomb shape through thin partition walls is filled with a plug material, for example, every other length and width, and sealed. Ceramic honeycomb filters consisting of porous partition walls filled with plug material on the other end surface of the through hole adjacent to the hole are contained in exhaust gas of various combustion equipment including automobile diesel engine. It is known as an exhaust gas purifying apparatus that collects and purifies fine particles.

Conventionally, such a honeycomb filter is generally manufactured from a ceramic material such as cordierite, alumina, silica, or mullite. However, since the oxides of these oxides have a relatively low melting point of 1300 to 1600 ° C, a honeycomb formed of these materials is used. The filter is deformed when used under high temperature conditions of 1000 ° C or higher, or when the carbon fine particles contained in the exhaust gas of a diesel engine are combusted in the filter, the above-mentioned substances having low thermal conductivity The honeycomb filter having the above-mentioned structure has a drawback that heat is locally stored in the filter and the portion is melted and damaged. Therefore, recently, a honeycomb filter manufactured using silicon carbide as a main component, which has a high melting point of 2300 ° C. or more and has extremely excellent thermal stability, has been developed.

By the way, in any case, the honeycomb filter is usually an exhaust gas that does not pass through the partition wall as it is unless the plug material is closely attached to the partition wall at the end of the through hole of the honeycomb molded body and plugged. It will impair the function as a filter that will be leaked to. Also, because the temperature inside the filter is extremely high as described above,
Not only the honeycomb-shaped molded body but also the plug material is naturally required to have excellent heat resistance.

2. Description of the Related Art Conventionally, as a method for bringing a plug material into close contact with an end portion of a predetermined through-hole of a honeycomb formed body containing silicon carbide as a main component, there is the following method.

First, as a first method, a low melting point substance such as glass frit or metallic silicon or the like is used as a binder to make them adhere to each other. As a second method, a plug material is made of coarse silicon carbide powder and fine particles. After mixing with another silicon carbide powder and molding the mixture, it is fired at a high temperature of 2000 ° C. or higher, or as a third method, it is disclosed in Japanese Patent Application Laid-Open No. 48-39515.
"A theoretical amount of silicon-based powder that reacts with free carbon from the carbon powder and the binder produced during firing is added with a carbon-based binder with or without addition of carbon powder to the silicon carbide powder. A method for producing a homogeneous porous recrystallized silicon carbide body, which comprises adding and forming, and then heating to 1900 to 2400 ° C. in the carbon powder of this molded body to siliconize the carbon content in the molded body. , Etc. are known.

(Problems to be Solved by the Invention) However, in the case of the first method, since the melting point of glass frit and metallic silicon is as low as about 1400 ° C., they melt when the temperature inside the filter becomes higher than that, and the plug material is melted. There is a problem that the adhesion between the filter and the partition walls is impaired, and as a result, the heat resistance of the entire filter is deteriorated and the efficiency of collecting fine particles is reduced.

In the case of the second method, since coarse particles are used,
The amount of shrinkage of the plug material is larger than the amount of shrinkage of the honeycomb formed body during firing, which causes a gap between the plug and the partition wall constituting the through-hole, and the efficiency of collecting fine particles is deteriorated. is there.

In the case of the third method, since a porous recrystallized silicon carbide body is used as a plug for the molded body, a part of the partition walls is siliconized during the siliconization, and the porosity is impaired. As a result, there is a problem that the function as a filter is reduced.

The present invention solves the above-mentioned problems, and by closely adhering the plug material to the end partition walls of the through holes of the honeycomb-shaped molded body, the collection efficiency of the fine particles is extremely high, and further the heat resistance and the oxidation resistance are high. Another object of the present invention is to provide an excellent method for manufacturing a silicon carbide honeycomb filter.

[Structure of the Invention] (Means for Solving the Problems) The inventors of the present invention have conducted extensive studies to achieve the above object, and as a result, simultaneously sinter the honeycomb-shaped molded body and the plug material, and sinter the both. By providing a difference in the amount of expansion or contraction at the time, it is possible to join the two extremely tightly,
Moreover, they have found that a silicon carbide honeycomb filter having excellent heat resistance and oxidation resistance can be obtained, and completed the present invention.

That is, in the method for manufacturing a silicon carbide honeycomb filter of the present invention, the first step of molding a starting material mainly composed of silicon carbide powder having an average particle size of 1 to 200 μm to obtain a honeycomb molded body; the honeycomb molding The end of the predetermined through-hole of the body is made of a plug material composed of a starting material mainly composed of silicon carbide powder having an average particle size of 5 μm or less and containing at least 30% by weight of particles having a particle size of 1 μm or less. The second step of plugging; the third step of sintering the plugged honeycomb-shaped molded body in a non-oxidizing atmosphere;

First, the average particle size of the starting material of the honeycomb-shaped molded body in the first step needs to be 1 to 200 μm. If it is less than 1 μm, a high-strength sintered body can be obtained, but grain growth during sintering is remarkable and it is difficult to control the aperture of the filter to a predetermined value. This is because when the particle size is larger than 200 μm, the number of bonding points between the particles is small and the mechanical strength of the honeycomb-shaped molded body becomes low. Further, it is advantageous that the above-mentioned silicon carbide powder has a particle size distribution such that 60% by weight or more of powder within ± 20% of the average particle size exists.

Note that there are α-type, β-type, and amorphous silicon carbide crystal systems, and any one of them or a mixture thereof may be used. Above all, β-type
It is easy to obtain in the form of a fine powder, in particular, it can be suitably obtained in a size of 5 μm or less, and is a low-temperature stable type crystal synthesized at a relatively low temperature. During sintering, a part thereof is 4H, 6H or 15R. This is advantageous because it easily undergoes a phase transition to a high-temperature stable type α-type crystal such as a type to form a plate-like crystal and has excellent crystal growth properties. In particular, by using a starting material in which 60% by weight or more of β-type silicon carbide is used, the porous body aimed at by the present invention can be suitably produced. Above all, 70
It is particularly advantageous to use a starting material containing more than 3% by weight of beta silicon carbide.

Further, the content rate of Al element, B element and Fe element which are impurities contained in the starting material mainly composed of silicon carbide is such that the total content rate of Al element, B element and Fe element is 1% by weight or less,
The free carbon content is preferably 5% by weight or less. The reason is that each element of Al, B, Fe is
Since it has the property of shrinking the molded body when it reacts with the carbon contained in silicon carbide or the carbon that is present by being decomposed by thermal decomposition, if it is contained in a too large amount, the molded body will contract significantly during sintering. is there. Also, Al, B, F
Since each element of e is easily dissolved in the silicon carbide crystal during sintering, when a large amount of these elements is contained, when the filter is used especially in a high temperature oxygen atmosphere, SiC is
This is because it becomes iC ₂ , and this SiO ₂ and the oxygen compound of the above elements melt to form a low-melting point glass, and as a result, the oxidation resistance of the filter decreases.

However, if the content of each element of Al, B, Fe in the plug material is made smaller than that in the honeycomb-shaped molded body, the expansion amount of the honeycomb-shaped molded body can be adjusted in combination with the expansion action. Can be suppressed more than the amount of expansion of
Since the shrinkage amount of the plug material is smaller than that of the honeycomb-shaped molded body, the plug material can be closely attached to the end portion of the through-hole of the honeycomb-shaped molded body.

Therefore, the content ratio of impurities in the honeycomb-shaped molded body is preferably 0.8% by weight or less for Al, 0.3% by weight or less for B, 0.8% by weight or less for Fe, and their total is 1% by weight or less, and Preferably, Al is 0.5% by weight or less, and B is
It is preferable that the content is 0.2% by weight or less, Fe is 0.6% by weight or less, the total amount thereof is 1% by weight or less, and the content of free carbon is 5% by weight or less.

Then, for example, after adding a crystal growth aid such as aluminum, boron, iron, and carbon as necessary, a molding binder such as methyl cellulose, polyvinyl alcohol, and water glass is added, and extrusion molding, sheet molding, press molding, and the like are performed. To obtain a honeycomb-shaped formed body.

Next, in the second step, the average particle size of the silicon carbide powder used as a starting material for obtaining the plug material is preferably 5 μm or less. The smaller the average particle size of the starting material is, the smaller the average particle size of the starting material is, so that the particle size of the starting material silicon carbide powder is made smaller than that of the honeycomb formed body. At the time of sintering, the expansion amount of the plug material can be made larger than that of the honeycomb-shaped compact, or in turn, the contraction amount of the plug material can be combined with the action of each element of Al, B, Fe. This is because the shrinkage amount can be smaller than the shrinkage amount of the honeycomb formed body.

Further, it is preferable that the silicon carbide powder contains at least 30% by weight of particles of 1 μm or less.
The reason is that the crystal coarsening is indispensable for expanding the compact during sintering, but the crystal coarsening becomes a nucleus of the crystal in which some particles of the crystal become coarse, and the fine particles around it become fine. This is because it is generated by absorbing. Therefore,
If the amount of fine particles is small, the amount of crystal growth will be small. Therefore, it is preferable that the content of the fine particles is at least 30% by weight or more, more preferably 50% by weight or more.

Furthermore, each composition of impurities is such that Al is 0.8% by weight or less, B
Is 0.3% by weight or less, Fe is 0.8% by weight or less, and the total content of them is 1% by weight or less, and it is preferable that the content of Fe is less than these elements contained in the honeycomb formed body. 0.3-0.8% by weight, B is 0.05-0.3% by weight, F
More preferably, e has a difference in the range 0.4 to 0.8% by weight. The content of free carbon is preferably 5% by weight or less. As described above, with such a composition, the expansion amount of the plug material at the time of sintering can be made larger than the expansion amount of the honeycomb formed body, or the contraction amount of the plug material can be made larger than the contraction amount of the honeycomb formed body. This is because it can be made smaller.

In this case, it is preferable to use a starting material composed of β-type silicon carbide as in the case of the above-mentioned honeycomb formed body.

And, this plug material, at the end of a predetermined through hole of the honeycomb formed body, the end of the honeycomb formed body is immersed in a slurry having the composition of the plug material, or the plug material is a plasticizer. After being processed into a sheet-like shape, it is embedded by press fitting or the like.

In addition, it is preferable that both the silicon carbide component and the plug material occupy at least 40% by volume or more at this stage. The reason is that if it is less than 40% by volume, it becomes difficult to obtain a honeycomb-shaped filter excellent in strength. In order to sufficiently bring the honeycomb-shaped molded body and the plug material into close contact with each other, it is preferable to make the proportion of silicon carbide in the plug material larger than that of the honeycomb-shaped molded body.

Next, as a third step, the honeycomb formed body thus plugged with the plug is placed in a heat-resistant container and sintered in a non-oxidizing atmosphere.

Sintering in a non-oxidizing atmosphere in a heat-resistant container can promote evaporation-recondensation and / or surface diffusion transfer of silicon carbide between silicon carbide particles, and, as a result, silicon carbide. This is because the growth of particles is promoted.

As a heat resistant container for sintering the green body, graphite, silicon carbide, zirconium oxygen, tungsten carbide, titanium carbide, magnesium oxide, molybdenum carbide,
A container made of any one selected from molybdenum, tantalum carbide, tantalum, zirconium carbide, and a graphite-silicon carbide composite can be used. This is because these containers do not melt within the sintering temperature range described later, and can suppress the leakage of vapor and / or decomposition products of silicon carbide particles out of the system.

The sintering temperature is preferably from 2000 to 2500C. The reason is that when the temperature is lower than 2000 ° C., the growth of particles is insufficient and sintering is incomplete, and it becomes difficult to obtain a high-strength sintered body. On the other hand, when the temperature exceeds 2500 ° C., the sublimation decomposition of silicon carbide crystal becomes active, and the developed crystal becomes thinner on the contrary, and it is difficult to obtain a high-strength sintered body. Of these, sintering in the range of 2050 to 2300 ° C. is more preferable.

(Example) Example 1 The silicon carbide powder used as a starting material for the honeycomb formed body was 96% by weight of silicon carbide composed of β-type crystals, had an average particle size of 25 µm, and had an average particle size of ± About 70% by weight of silicon carbide powder is contained within 20%, and 0.30% by weight of free carbon, 0.03% by weight of iron, 0.04% by weight of aluminum and 0.01% by weight of boron are contained. This raw material 100
10 parts by weight of methyl cellulose and 15 parts by weight of water were mixed with the parts by weight, and mixed in a kneader for 5 hours. An appropriate amount of this mixture is sampled and φ is obtained by extrusion molding method using a honeycomb die.
150mm, the through hole is a square of 1.5mm × 1.5mm, and the wall thickness of the partition wall is
A honeycomb-shaped molded body of 0.5 mm was obtained. The ratio of silicon carbide for the honeycomb-shaped molded body was 55% by volume.

On the other hand, the plug material is silicon carbide powder consisting of 97% by weight of β-type crystals as a starting material, and has an average particle size of 0.25 μm and 1 μm.
Content of the following particles is 95% by weight, free carbon 0.28% by weight, iron 0.04% by weight, aluminum 0.03% by weight, boron 0.01% by weight 100 parts by weight of silicon carbide powder to 5 parts by weight of methyl cellulose, water 20 parts by weight were blended and mixed in a kneader for 25 hours. The mixture was used to obtain a flat plate with a 1 mm × 150 mm extrusion die, which was buried in a predetermined location on the end face of the honeycomb formed body, and one end of the through hole was sealed. The proportion of silicon carbide in the molded body for the plug material was 56% by volume.

This green compact was placed in a graphite crucible and fired in a Tamman-type firing furnace at 1 atm mainly in an argon gas atmosphere.

In the heating process, the temperature was raised to 1700 ° C. at a heating rate of 400 ° C./hour, then to a maximum temperature of 2250 ° C. at a heating rate of 30 ° C./hour, and maintained at the maximum temperature for 4 hours.

The resulting honeycomb filter contains aluminum
It contained 0.03% by weight, 0.03% by weight of iron, 0.001% by weight or less of boron, and 0.40% by weight of free carbon.

This honeycomb filter is 1400 ℃, air flow 5 / min
When heated in the oxidation furnace of No. 3, the increase in oxidation after 100 hours was 4.0%, the appearance was not changed, and the product had excellent oxidation resistance.

Further, when the sealed end surface was pressurized with air in water, ventilation of the partition wall of the honeycomb-shaped molded body occurred at 0.3 atm, whereas leakage from the joint between the partition wall and the plug material was 0. .
It originated from 50 atm and had excellent airtightness.

Example 2 As a starting material for a honeycomb formed body, 94% by weight was silicon carbide composed of β-type crystals and had an average particle size of 8.5 μm.
And silicon carbide powder within ± 20% of the average particle size
70% by weight is present, 0.29% by weight of free carbon and 0.
A silicon carbide powder containing 04% by weight, 0.03% by weight of aluminum and a trace amount of boron was extruded by a honeycomb die in the same manner as in Example 1 to obtain a honeycomb-shaped molded body having the same shape as in Example 1. The proportion of silicon carbide for the honeycomb formed body was 56% by volume.

On the other hand, the plug material is silicon carbide of which 96% by weight is a β-type crystal as a starting material, the content of particles having an average particle size of 0.25 μm, 1 μm or less is 95% by weight, free carbon is 0.28% by weight,
0.04 wt% iron, 0.03 wt% aluminum, boron
100 parts by weight of silicon carbide powder of 0.01% by weight is mixed with 5 parts by weight of methylcellulose and 20 parts by weight of water.
Mix for hours. A flat plate of this mixture was obtained with an extrusion die of 1 mm × 150 mm, and the flat plate was embedded in a predetermined place on the end face of the honeycomb-shaped green molded body to seal one end of the through hole. The proportion of silicon carbide in the molded body for the plug material was 56% by volume.

Thereafter, this was charged into a graphite crucible and fired in a argon atmosphere at 1 atm using a tanman firing furnace.

In the heating process, the temperature was raised to 1700 ° C. at a heating rate of 400 ° C./hour, then to a maximum temperature of 2250 ° C. at a heating rate of 30 ° C./hour, and maintained at the maximum temperature for 4 hours.

When this honeycomb filter was evaluated in the same manner as in Example 1, the oxidation resistance was 4.2%, the ventilation of the partition walls was 0.35 atm, and the leak at the joint was 0.60 atm.

Examples 3-5, Comparative Examples 1-5 The same as Example 2, except that the starting material of the plug material has an average particle size of 0.32 μm and the content of particles of 1 μm or less is 60% by weight. Has a silicon carbide content of 57% by volume,
When a silicon carbide powder having the contents of aluminum, iron and boron shown in the table is used (Example 3), a honeycomb-shaped molded body having a mean particle size of 8.6 μm is used as a starting material, and When the silicon carbide content was 46% by volume (Example 4), a silicon carbide powder having a content of aluminum, iron, and boron as shown in the table was used as a starting material for the honeycomb formed body and the plug material. In the case (Example 5), the same as in Example 2, except that the starting material of the plug material has an average particle size of 4.3 μm and a content of particles of 1 μm or less is 12
When using silicon carbide powder having a weight%, a silicon carbide content, and the contents of aluminum, iron, boron, and free carbon as shown in the table (Comparative Example 1), a starting material for a honeycomb-shaped molded body As a result, when the silicon carbide content of the compact was further reduced (Comparative Example 2), the sintering temperature was 18
When the temperature was set to 00 ° C. (Comparative Example 3) and the sintering temperature was set to 2550 ° C. (Comparative Example 4), the starting materials for the honeycomb-shaped molded body and the plug material of Example 5 were used for the honeycomb-shaped molded body. As a starting material for the plug material, the properties of the honeycomb filter in the case where the raw material used for the plug material was used by replacing it as the starting material for the honeycomb formed body (Comparative Example 5) are shown in the table.

As is clear from the table, the honeycomb filter of the present invention has a "oxidation weight increase at 1400 ° C" which is at a desired level and satisfies practical oxidation resistance, and changes in appearance such as generation of cracks. Moreover, the "air pressure of the honeycomb formed body partition wall" has a required airtightness because it is smaller than the "air pressure of the joint portion between the partition wall and the plug material".
It is an excellent honeycomb filter.

[Advantages of the Invention] In the honeycomb filter of the present invention, since the expansion amount or the contraction amount at the time of sintering is different between the honeycomb formed body and the plug material, the plug material adheres to the partition walls of the through holes of the honeycomb formed body. It has excellent airtightness. Therefore, the effluent such as exhaust gas that does not pass through the partition does not flow to the outside as it is, and the collection efficiency of the fine particles is extremely high.

Further, since the content of impurities is small, it has excellent oxidation resistance, and since both are joined without using a binder, the heat resistance is also excellent.

Claims (1)

(57) [Claims] 1. A first step of molding a starting material mainly composed of silicon carbide powder having an average particle diameter of 1 to 200 μm to obtain a honeycomb-shaped molded body; an end portion of a predetermined through hole of the honeycomb-shaped molded body, Second step of plugging with a plug material composed of a starting material mainly composed of silicon carbide powder having an average particle size of 5 μm or less and at least 30% by weight of particles having a particle size of 1 μm or less; A third step of sintering the stopped honeycomb-shaped molded body in a non-oxidizing atmosphere; and wherein the honeycomb-shaped molded body and the plug material contain at least 40% by weight of silicon carbide, and A method for manufacturing a silicon carbide honeycomb filter, wherein the sintering in the third step is performed at a temperature of 2000 to 2500 ° C.