JP2922980B2 - Manufacturing method of ceramics sintered body with honeycomb structure - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a ceramic sintered body, and more particularly to a method for producing a ceramic sintered body having a honeycomb structure used as a filter of an exhaust gas purification device.

[Problems to be Solved by the Related Art and the Invention] For example, an exhaust gas purifying apparatus for an internal combustion engine such as a diesel engine filters a carbon soot or the like in the exhaust gas and carries a catalyst for oxidatively decomposing them. A filter is provided. As shown in FIGS. 1 and 2, this filter is formed using a porous material having a large number of fine open pores, and has a cylindrical shape and a through-hole (cell) 2 extending in the axial direction having a square shape. 10 per inch
The honeycomb structure 1 includes about 0 to 200 honeycomb structures.

Conventionally, such a honeycomb structure is formed by mixing a ceramic powder such as mullite, cordierite, and silicon carbide with an organic resin binder and water as a dispersion solvent into a honeycomb-shaped raw material composition, and heating and drying the formed body. Is performed to remove water from the molded body, and then the ceramic powder is sintered.

However, when a molded body in a water-containing state is charged into a dryer and subjected to rapid heating and dehumidification, distortion due to drying shrinkage occurs due to an imbalance in water evaporation rate between the surface and the interior of the molded body, and the molded body is deformed. On the surface, there was a problem that a coarse crack (macrocrack) 3 as shown in FIG. 3 was formed.

On the other hand, the molded body in a water-containing state is placed in a freezer previously cooled to about −20 ° C., the molded body is temporarily frozen, and then subjected to vacuum drying to sublimate and remove the frozen water. Thus, there is known a method of avoiding the occurrence of cracks by avoiding drying shrinkage of a molded article. However, when the honeycomb formed body is dried by this method, although the generation of the macro crack 3 as described above is not observed, as shown in FIG. 4, the cell walls 4 formed in a lattice shape of the honeycomb structure 1 are formed. In addition, there is a problem that a star-shaped fine crack (microcrack) 5 occurs near the lattice point. Even if such a fine crack is formed, it will not be cured by the subsequent sintering, and will significantly reduce the mechanical strength of the sintered body and, consequently, the durability as an exhaust gas purification filter. Had become.

The present invention has been made in view of the above circumstances, and an object of the present invention is to prevent the occurrence of cracks in a drying step of a honeycomb structure in a water-containing state beforehand, thereby improving the mechanical strength of a sintered body after firing. It is an object of the present invention to provide a method for manufacturing a ceramic sintered body having a honeycomb structure, which can obtain a robust sintered body without lowering the performance.

[Means and Actions for Solving the Invention] In order to solve the above problems, in the present invention, a raw material composition obtained by mixing a binder and water with ceramic powder is formed into a honeycomb shape having a plurality of cells, In the method for manufacturing a ceramic sintered body having a honeycomb structure in which the formed body is dried and fired, the atmosphere humidity in the drying step is set to 70 to 99%.

According to this method, since the atmospheric humidity is relatively high, the evaporation of water from the water-containing compact is slowed down, and the water evaporation rate in the surface layer portion and inside of the compact is uniformized. Therefore, when the molded body is dried, no distortion occurs between the surface layer portion and the inside of the molded body, and the occurrence of the macro crack 3 (see FIG. 3) and the micro crack 5 (see FIG. 4) is prevented. Is done. If the atmospheric humidity is less than 70%, macro cracks are easily generated in the molded product, and the object of the present invention cannot be achieved.

In the drying step, it is preferable to perform heating and drying while increasing the ambient temperature to 10 to 80 ° C. while maintaining the atmospheric humidity.

According to this method, even if the evaporation of moisture from the molded body is promoted with an increase in the ambient temperature, there is no sudden difference in the moisture evaporation rate between the surface layer portion and the inside of the molded body, and the entire molded body is formed. Is uniformly heated, and moisture is quickly removed from the molded body.

Here, when the ambient temperature is lower than 10 ° C., it takes a long time to dry the molded body, and the productivity is reduced. On the other hand, when the ambient temperature exceeds 80 ° C., the evaporation rate of water increases, and the molded body rapidly dries and shrinks, which easily causes cracks.

The temperature rise rate of the ambient temperature in the above case is 0.1 to
A range of 1.0 ° C./min. Is preferable. The heating rate is 0.1 ° C / min.
If less than, the productivity in the drying step is reduced, 1.0 ° C. / min.
If the temperature exceeds the range, a temperature difference occurs between the surface layer portion and the inside of the molded body, which causes a crack.

In the drying step, when the moisture content of the molded body is 5% by weight or more, drying is performed while maintaining the atmospheric humidity at 90 to 99%, and the moisture content of the molded body becomes less than 5% by weight. It is desirable to perform drying at an atmosphere humidity of 70 to 90%.

According to this method, when the moisture content of the molded body is relatively high (5% by weight or more) and there is a high possibility that the molded body will be distorted due to shrinkage due to rapid drying, the atmospheric humidity is set to an allowable range. By performing the drying as slowly as possible near the upper limit, the occurrence of cracks is reliably prevented. And
When the moisture content of the molded body is relatively low (less than 5% by weight) and the shrinkage of the molded body due to drying becomes very small,
By setting the atmospheric humidity near the lower limit of the allowable range and promoting drying, the residual moisture is quickly removed from the molded body while avoiding the occurrence of cracks.

In the above case, when the setting of the atmospheric humidity was changed, it was determined whether or not the moisture content in the molded body reached 5% by weight. (Weight: 500 to 3000 g), which may vary depending on the shape, size, etc. of the molded article to which the present invention is applied.

The raw material composition according to the present invention is obtained by blending a binder and water with ceramic powder.

Examples of ceramics to which the present invention can be applied include silicon carbide, boron carbide, silicon nitride, boron nitride, aluminum nitride, aluminum oxide, zirconium oxide, mullite, cordierite, aluminum titanate, zirconium boride, sialon, and the like. Each ceramic is used alone or in combination of two or more in powder form.

In particular, the quality of a molded body before firing, such as silicon carbide sintered in a gas phase, is greatly affected by the quality of a ceramic to which the present invention is applied, such as the mechanical strength of a sintered body after firing. Is very beneficial.

As the binder, for example, a phenol resin,
Various organic substances such as lignin sulfonate, polyvinyl alcohol, methylcellulose, carboxymethylcellulose, propylene glycol, constarch, molasses, coal tar pitch, and alginate can be used, and these can be used alone or in combination of two or more.

The mixing ratio of the binder is generally 10
The range of 5 to 50 parts by weight relative to 0 parts by weight is suitable. If the blending ratio is less than 5 parts by weight, the molded body cannot be formed, and if it exceeds 50 parts by weight, disadvantages such as a decrease in mechanical strength of the sintered body occur.

Generally, the mixing ratio of the water is preferably in the range of 20 to 50 parts by weight based on 100 parts by weight of the ceramic powder. If the mixing ratio is less than 20 parts by weight, the ceramic powder cannot be uniformly dispersed and the raw material composition cannot be kneaded.
If the amount is more than 10 parts by weight, the viscosity of the raw material composition is unnecessarily reduced, which hinders molding.

The raw material composition is adjusted by sufficiently kneading with a Henschel mixer, a kneader, or the like, and is extruded by extrusion or the like.
It is formed into a honeycomb-shaped formed body 1 as shown in FIGS. Such a honeycomb formed body 1 has a cylindrical shape and a large number of through-holes (cells) 2 formed in the axial direction thereof. The size range of the honeycomb formed body 1 to which the present invention is extremely effectively applied is as follows. , Diameter 80-200mm, length 50-200mm, cell wall 4 thickness 0.15-0.5mm, cell pitch 1.15-2.5m
m, the number of cells is 100 to 500 per square inch.

The honeycomb formed body 1 thus obtained is dried under the drying conditions until the water content becomes about 3 to 4% by weight as described above.

In this drying step, it is preferable to flow an air stream through the cells 2 of the honeycomb formed body 1.

The reason is that it is easy to make the temperature and humidity inside and outside the molded body always uniform through the cell 2.

Here, as a method of flowing an airflow through the cells 2 of the honeycomb formed body 1, for example, as shown in FIG. A method is conceivable in which the honeycomb formed body 1 is placed on the stand, and the honeycomb formed body 1 is placed on the stand, and a fan motor (not shown) is driven to flow an air flow.

In drying the honeycomb formed body, it is preferable that the formed body be placed on a hydrophobic cushion material or a dummy formed body having the same honeycomb structure as the formed body and dried.

The hydrophobic cushion material includes a molded article to be dried (a molded article to be dried) and a hydrophobic member that is interposed between a drying container in which the molded article is placed and a wall surface and reduces frictional resistance therebetween. Good examples include a Teflon sheet and a ball formed of a hydrophobic resin.

This cushioning material reduces frictional resistance of the molded body in contact with the cushioning material, so that contact sliding due to drying shrinkage is prevented from being obstructed, and cracks are generated at the site due to drying shrinkage. Is prevented. still,
The reason why the cushion material is made hydrophobic is that if the cushion material is hydrophilic, water may be exchanged with the member, which may hinder uniform drying of the molded body.

The dummy molded body refers to a molded body that is formed in substantially the same shape as the molded body to be dried and is arranged in a drying container and on which the molded body to be dried is placed.

Since this dummy molded body has the same drying shrinkage as the molded body to be dried, during drying, for example, the dummy molded body may cause distortion due to drying shrinkage at a contact portion with the wall surface of the drying container. Also, distortion due to drying shrinkage does not occur at the portion where the molded article contacts the dummy molded article. For this reason, the dried article is dried without cracks in the contact area with the dummy article.

As described above, the honeycomb formed body is dried in an ideal state without cracks.

After drying, the molded body is degreased as necessary,
It is fired according to the firing conditions of the ceramics used.
If the ceramic is silicon carbide, it is preferable that the solvent and the binder remaining in the molded body are sufficiently decomposed and degreased at a temperature of 300 to 700 ° C., and then fired at a temperature of 2000 ° C. or more.

In this way, a ceramic sintered body having no cracks and having excellent mechanical strength can be obtained.

If the present invention is embodied in a honeycomb filter used in an exhaust gas purifying apparatus for an internal combustion engine, a use mode is conceivable as shown in FIG. That is, the honeycomb filter 11 wrapped in the heat insulating material 14 is disposed in the exhaust gas flow path 10, the heater 12 for heating the filter 11 is disposed near one end face thereof, and silica A heat radiation shielding member 13 formed in a mesh shape with a heat-resistant fiber such as alumina is provided. As a result, carbon soot and the like in the exhaust gas flowing from the upstream side of the circulation path 10 are collected by the honeycomb filter 11. Here, the honeycomb filter 11 is heated by the heater 12,
The heat radiation of 12 is filtered by the presence of the shielding member 13.
Since the filter 11 is directed to the side, the filter 11 is efficiently heated.

The shielding member 13 has a thickness of, for example, 0.5 to 1.0.
A heat-resistant thread having a mesh size of 1.0 to 2.0 mm and an opening ratio of 40 to 60% may be used. Also,
The heater 12 and the shielding member 13 may be provided on either the upstream side or the downstream side of the flow path.

[Examples 1 to 3 and Comparative Examples 1 to 3] Hereinafter, Examples 1 to 3 in which the present invention is embodied in a honeycomb filter used for an exhaust gas purification device for an internal combustion engine will be compared with Comparative Examples 1 to 3. explain.

(Example 1) <Preparation of molded article> 5 parts by weight of methylcellulose, 5 parts by weight of propylene glycol and 100 parts by weight of silicon carbide fine powder having an average particle diameter of 0.28 μm and 96.2% of β-type crystal Water 25
Parts by weight were blended and uniformly mixed with a Henschel mixer to prepare a raw material composition. And using a vacuum extruder,
As shown in FIGS. 1 and 2, a honeycomb formed body 1 having a cylindrical shape and having a large number of through holes (cells) 2 formed in an axial direction was formed. This honeycomb formed body 1 has a diameter of 140 mm and a length of 140 m.
m, cell wall 4 thickness 0.45mm, cell pitch 1.95mm, number of cells 1
70 cells per square inch and weighs about 2200
g, water content was about 18-20% by weight.

<Drying Step> Next, the formed body was placed in a drying container set at a temperature of 15 ° C. and a humidity of 90%, and was placed on a Teflon sheet spread in the container. Then, the temperature in the container was raised to 80 ° C. at a rate of 1 ° C./min. While flowing humidified air through the container, and the molded body was dried at this temperature for 30 hours. In addition, about the humidity in the drying container,
90% to 95% humidity on the inlet side of humidified air sent into the container
The humidity of the outlet air discharged from the container was controlled by adjusting the flow rate of the humidified air so that the difference from the humidity of the humidified air fed into the container was within 3 to 10%. .

The moisture content of the dried compact was about 3% by weight, and no macro cracks, micro cracks, etc. were observed on both the surface and the inside.

<Sintering Step> The dried compact was inserted into a tanman-type firing furnace and fired at 2200 ° C. for 4 hours in an argon gas atmosphere to obtain a honeycomb-shaped porous silicon carbide sintered body. This sintered body
Diameter 140mm, length 140mm, cell wall thickness 0.45mm, cell pitch
Maintains honeycomb shape of 1.95mm, 170 cells / square inch, macro crack, surface and inside
Microcracks and the like were not observed. Further, the three-point bending strength of this sintered body was 5.3 kgf / mm ² .

(Example 2) The drying procedure in Example 1 was changed as follows,
Others were the same as in Example 1.

That is, the honeycomb formed body 1 was placed in a drying container set at a temperature of 15 ° C. and a humidity of 90%, and was placed on a Teflon sheet spread in the container. Then, while humidifying air is passed through the container, the temperature in the container is set to 1 ° C / mi.
The temperature was raised to 80 ° C. at the rate of n. and maintained at this temperature for 20 hours. At this time, the moisture content of the molded body was 4% by weight.
Subsequently, while the temperature in the container was maintained at 80 ° C., the humidity in the container was gradually reduced to 75%, and the molded body was dried at this humidity for 10 hours.

The moisture content of the dried compact was about 3% by weight, and no macro cracks, micro cracks, etc. were observed on both the surface and the inside. In addition, a honeycomb-shaped porous silicon carbide sintered body having no cracks and a three-point bending strength of 5.0 kgf / mm ² was obtained by firing.

(Example 3) In Example 1, in place of the Teflon sheet spread in the drying container, as shown in FIG. 5, a dummy molded body 6 molded in exactly the same manner as the molded body to be dried, as shown in FIG.
(Diameter 140 mm, length 50 mm, cell wall thickness 0.45 mm, cell pitch 1.95 mm, number of cells 170 cells / square inch, water content about 18 to 20% by weight) are placed in the drying container 7 and dried. The compact 1 was placed on the dummy compact 6 and dried. Others were the same as in Example 1.

After the drying was completed, the moisture content of the molded body 1 was about 3% by weight. Although macro cracks and the like were observed in the dummy molded body 6, macro cracks, micro cracks and the like were not observed in the molded body 1 both on the surface and inside. Also,
By firing, no cracks, three-point bending strength 5.
A honeycomb-shaped porous silicon carbide sintered body of ² kgf / mm ² was obtained.

(Comparative Example 1) In the drying step of Example 1, the honeycomb formed body was dried while maintaining the humidity in the drying container at 30% throughout.
Others were the same as in Example 1.

After drying, macrocracks were observed on the surface of the molded body, and the molded body collapsed during firing.

From the results and the results of Examples 1 to 3, when the humidity in the drying container becomes extremely low outside the preferred range of the present invention,
It is considered that the drying speed of the surface layer portion is much faster than the drying speed inside the honeycomb formed body, the drying balance is lost, and macro cracks are generated on the surface of the formed body.

(Comparative Example 2) In the drying step of Example 1, after the honeycomb formed body was charged into a drying container set at a temperature of 15 ° C and a humidity of 90%, the container was kept at 90% humidity. The inside temperature was raised to 80 ° C. at a rate of 5 ° C./min., And maintained at this temperature for 30 hours to dry the honeycomb formed body. Others were the same as in Example 1.

After drying, macrocracks were observed on the surface of the molded body, and the molded body collapsed during firing.

From this result and the results of Examples 1 to 3, even if the temperature rise rate during drying is too fast, the drying rate of the surface layer becomes extremely faster than the drying rate inside the honeycomb formed body, and the balance of drying It is considered that the microstructure collapses and macro cracks occur on the surface of the molded body.

(Comparative Example 3) In the drying step of Example 1, the honeycomb formed body was directly placed on the bottom surface of the drying container without laying a Teflon sheet in the drying container. Others were the same as in Example 1. The bottom surface of the drying container is made of acrylic.

After drying, macrocracks were observed at the contact portions of the molded body that had been in contact with the bottom surface of the drying container. In addition, part of the molded body collapsed due to firing.

From this result, it is considered that if the frictional resistance at the contact portion of the molded body is large, distortion occurs at the contacted portion during drying and shrinkage of the molded body, and cracks occur at the site.

[Effects of the Invention] As described above in detail, according to the present invention, the mechanical strength of a sintered body after firing is prevented by preventing cracks from occurring in a drying step of a honeycomb structure formed in a water-containing state. It is possible to obtain an excellent effect that a ceramic sintered body having a robust honeycomb structure can be obtained without deteriorating the characteristics.

When the moisture content of the molded article is relatively high (5% by weight or more), the molded article is likely to be distorted due to shrinkage due to rapid drying. By setting and drying as slowly as possible,
Cracks can be reliably prevented from occurring. And the moisture content of the molded body is relatively low (less than 5% by weight)
In such a case, when the shrinkage of the molded body due to drying has become extremely small, by setting the atmospheric humidity near the lower limit of the allowable range to promote drying, it is possible to avoid the occurrence of cracks, It has an excellent effect that residual moisture can be quickly removed.

[Brief description of the drawings]

FIG. 1 is a front view of a honeycomb structure embodying the present invention,
FIG. 2 is a partial cross-sectional view taken along line AA of FIG. 1, FIG. 3 is a perspective view showing a state in which macrocracks have entered the surface of the honeycomb structure, and FIG. 4 is a microcrack inside the honeycomb structure. Enlarged view of the cell wall showing the state in which
The figure is a perspective view schematically showing a dried state of the formed body in Example 3, FIG. 6 is an explanatory view showing an example of flowing an air flow through the cells of the honeycomb formed body, and FIG. 7 is a use form of the honeycomb structure. FIG. 1 ... honeycomb formed body, 2 ... cell, 6 ... dummy formed body.

Claims (5)

(57) [Claims] A raw material composition comprising a ceramic powder and a binder and water is formed into a honeycomb shape having a plurality of cells (2), and the formed body (1) is dried and fired. In the method for producing a ceramic sintered body, the atmospheric humidity in the drying step is 70 to 99%, and in the drying step, the moisture content of the compact (1) is 5% by weight.
In the above case, drying is performed while maintaining the humidity of the atmosphere at 90 to 99%, and when the moisture content of the molded body (1) becomes less than 5% by weight, the drying is performed at the humidity of 70 to 90%. A method for producing a ceramic sintered body having a honeycomb structure, characterized by comprising: 2. In the drying step, the atmospheric humidity is 70 to
At 99%, the ambient temperature is increased by 10 to 1.0 ° C / min
The method for producing a ceramic sintered body having a honeycomb structure according to claim 1, wherein heating and drying are performed by raising the temperature to about 80 ° C. 3. The method for manufacturing a ceramics sintered body having a honeycomb structure according to claim 1, wherein in the drying step, an airflow is circulated through the cells (2) of the molded body (1). 4. The molded article (1) is placed on a hydrophobic cushion material or a dummy molded article (6) having the same honeycomb structure as the molded article (1) and dried. The method for manufacturing a ceramic sintered body having a honeycomb structure according to any one of claims 1 to 3. 5. The method for manufacturing a ceramics sintered body having a honeycomb structure according to claim 1, wherein the ceramics sintered body is a silicon carbide sintered body.