JP2002193670A - Burning method for silicon carbide compact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon carbide compact burning method by which porous silicon carbide members can be manufactured at small variation of their mean pore diameters and in almost uniform flexural strengths because the silicon carbide compact can be heated at an almost uniform temperature. SOLUTION: The burning method for silicon carbide compact is to burn the silicon carbide compacts by putting them on burning tools after degreasing the columnar silicon carbide compacts including silicon carbide powder, a binder and a dispersion medium liquid. The burning method for the silicon carbide compact has features that a stacked body is formed by stacking the burning tools with the degreased silicon carbide compacts put on them in a plurality of steps, that spaces between the silicon carbide compacts and the upper and lower burning tools are settled, and that the stacked body is heated from the upper and lower parts of it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for firing a silicon carbide molded body when manufacturing a ceramic filter for collecting particulates in exhaust gas.

[0002]

2. Description of the Related Art Recently, it has become a problem that particulates contained in exhaust gas discharged from internal combustion engines such as vehicles such as buses and trucks and construction machines cause harm to the environment and human bodies. Various ceramic filters have been proposed which purify the exhaust gas by collecting the particulates in the exhaust gas by passing the exhaust gas through a porous ceramic.

[0003] In a ceramic filter, a plurality of porous silicon carbide members 40 as shown in FIG. Further, as shown in FIG. 4, the porous silicon carbide member 40 has a large number of through holes 41 arranged in a longitudinal direction, and a partition 43 separating the through holes 41 functions as a filter.

That is, as shown in FIG. 4 (b), the through hole 41 formed in the porous silicon carbide member 40 has a filling material 42 at either the inlet or outlet end of the exhaust gas.
The exhaust gas that has been plugged in and has flowed into one through hole 41 always passes through a partition 43 separating the through hole 41 and then flows out from another through hole 41. When passing through, the particulates are trapped in the partition 43 and the exhaust gas is purified. Such a porous silicon carbide member 40 has extremely excellent heat resistance and is easy to regenerate, so that it is used for various large vehicles and vehicles equipped with a diesel engine.

Conventionally, when producing such a porous silicon carbide member, first, a silicon carbide powder, a binder, and a dispersion medium are mixed to prepare a mixed composition for producing a molded body. Extrusion molding or the like of the mixed composition is performed to produce a silicon carbide molded body.

Next, the obtained silicon carbide molded body is dried using a heater or the like to produce a dried silicon carbide molded body having a certain strength and which can be easily handled.

After the drying step, the silicon carbide compact is heated to 400 to 650 ° C. in an oxygen-containing atmosphere to volatilize the solvent in the organic binder component and perform a degreasing process for decomposing and eliminating the resin component. Further, a porous silicon carbide member is manufactured through a firing step of sintering the silicon carbide powder by heating it to 2000 to 2200 ° C. in an inert gas atmosphere.

FIG. 5A is a longitudinal sectional view schematically showing a conventional firing step of a silicon carbide molded body, and FIG. 5B is a partially enlarged sectional view thereof. As shown in FIGS. 5A and 5B, in the conventional firing process of the silicon carbide molded body, first, the silicon carbide molded body 52 that has undergone the degreasing step is placed in a firing jig 53 via a clog 55. A plurality of silicon carbide compacts 5
A plurality of firing jigs 53 on which 2 is placed are stacked to form a laminate. Then, after placing the stacked body on the support base 57, the stacked body is transferred onto a moving base 54 such as a belt conveyor,
The porous silicon carbide member has been manufactured by heating the silicon carbide molded body 52 with the heater 51 provided in the left and right direction of the moving table 54.

However, in such a conventional firing process of the silicon carbide molded body, since the silicon carbide molded body placed in the firing jig is heated from the left and right directions, the center of the silicon carbide molded body is heated. The temperature in the vicinity was lower than that in the vicinity of both ends, and a temperature distribution occurred in the silicon carbide molded body. In addition, since the support table was placed on a transfer table such as a belt conveyor, the heat from the bottom of the firing furnace was blocked by the transfer table, and the temperature of the silicon carbide molded body stacked below was increased. The temperature was lower than the temperature of the compact, and a temperature difference occurred between the silicon carbide compacts.

In the firing step of the silicon carbide compact,
If there is a temperature distribution in the silicon carbide compact, the silicon carbide particles will not grow uniformly, and the average pore diameter of the porous silicon carbide member to be produced will have large variations depending on the location. As described above, a porous silicon carbide member having a large variation in the average pore diameter also has a variation in bending strength, and a ceramic filter using such a porous silicon carbide member has a large bending strength. And the collection efficiency of particulates was poor. In addition, since the average pore diameter of the silicon carbide molded body stacked above and the silicon carbide molded body stacked below are different, a porous silicon carbide member having a constant average pore diameter and a certain bending strength is manufactured. It was difficult.

Further, in the above-mentioned conventional firing step,
By lowering the moving speed of the moving table and increasing the firing time, it is possible to reduce the temperature distribution generated in the silicon carbide molded body to some extent, but the productivity is extremely reduced, and it is practical. Not.

[0012]

SUMMARY OF THE INVENTION The present invention has been made to solve these problems, and can heat a silicon carbide molded body to a substantially uniform temperature in a firing step of the silicon carbide molded body. Therefore, it is an object of the present invention to provide a method for firing a silicon carbide molded body capable of obtaining a porous silicon carbide member having small variation in average pore diameter of a sintered porous silicon carbide member and having substantially uniform bending strength. Is what you do.

[0013]

According to the method of firing a silicon carbide compact of the present invention, a columnar silicon carbide compact containing silicon carbide powder, a binder and a dispersion medium is degreased and then placed on a firing jig. A method of firing the silicon carbide molded body, which is mounted and fired, comprising stacking a plurality of firing jigs on which the degreased silicon carbide molded body is mounted to form a laminate, and forming the silicon carbide molded body. A space is provided between the body and the upper and lower firing jigs, and the laminate is heated from above and below.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for firing a silicon carbide molded body of the present invention will be described with reference to FIG. 1 as necessary. In the present invention, first, a columnar silicon carbide molded body including a silicon carbide powder, a binder, and a dispersion medium is prepared.

The structure of the silicon carbide compact is not particularly limited. For example, as described in the prior art,
A columnar structure in which a large number of through holes are juxtaposed in the longitudinal direction across a partition wall, a columnar structure having a large number of communicating pores therein, and the like can be given. The shape is not particularly limited, and examples thereof include a columnar shape, an elliptical columnar shape, and a prismatic shape. In the following description, the shape of the silicon carbide molded body is assumed to be a columnar shape in which a large number of through-holes are juxtaposed in the longitudinal direction across partition walls.

The particle size of the silicon carbide powder is not particularly limited, but preferably has a small shrinkage in the subsequent firing step. For example, 100 parts by weight of powder having an average particle size of about 0.3 to 50 μm and 0.1 part by weight. It is preferable to use a combination of 5 to 65 parts by weight of a powder having an average particle diameter of about 1.0 μm.

The binder is not particularly restricted but includes, for example, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenolic resins and epoxy resins. Generally, the amount of the binder is preferably about 1 to 10 parts by weight based on 100 parts by weight of the silicon carbide powder.

The dispersion medium is not particularly restricted but includes, for example, organic solvents such as benzene; alcohols such as methanol, and water. The dispersion medium is mixed in an appropriate amount so that the viscosity of the mixed composition falls within a certain range.

The silicon carbide powder, the binder, the dispersion medium and the like are mixed by an attritor or the like and then sufficiently kneaded by a kneader or the like, and a columnar silicon carbide molded body is manufactured by an extrusion molding method or the like.

Thereafter, the silicon carbide molded body produced by the above steps is degreased. In the degreasing step of the silicon carbide molded body, usually, the silicon carbide molded body is placed on a degreasing jig, and then carried into a degreasing furnace, and is subjected to 400 to 6 under an oxygen-containing atmosphere.
Heat to 50 ° C. As a result, the binder and the like volatilize, as well as decompose and disappear, and almost only the silicon carbide powder remains.

Next, the degreased silicon carbide molded body 12 is placed on a firing jig 13, and the firing jigs 13 are stacked in a plurality of stages to form a laminate. The silicon carbide molded body 12 is fired by heating with the heater 11. In this firing step, the degreased silicon carbide molded body 12 is heated at 2000 to 2200 ° C. in an atmosphere of an inert gas such as nitrogen or argon to sinter the silicon carbide powder to produce a porous silicon carbide member. Since the silicon carbide molded body after degreasing has a low mechanical strength and is easily broken, the firing jig 13 is also used as a degreasing jig, and after the degreasing step,
A stacked body may be formed by stacking a plurality of firing jigs 13 which also serve as a degreasing jig.

In the present invention, the direction in which the laminate is heated is from above and below. By heating the stacked body from above and below, the silicon carbide compact 12 placed in the stacked body can be heated to a substantially uniform temperature. Further, by controlling the amount of heat generated by heaters 11 provided above and below, it is possible to reduce the temperature distribution generated between silicon carbide compacts 12 stacked above and silicon carbide compacts 12 stacked below.

Further, in the present invention, the above-mentioned laminated body is
The laminate is heated by the heater 11 while being placed on the muffler 7 and moving in the muffle 14. At this time, as a means for moving the laminate and the support 17, a roller (not shown) is used. Is preferred. This is because, when the laminate is heated by the heater 11 provided below, the heat radiated from the heater 11 easily reaches the silicon carbide molded body of the laminate because the area covered by the roller is small. However, usually, even when the above-mentioned laminate is moved using the above-mentioned roller, the temperature of the lower silicon carbide molded body is slightly lower than that of the upper silicon carbide molded body. Therefore, in order to prevent a temperature distribution from occurring between the upper and lower silicon carbide molded bodies, the calorific value of the heater provided below the laminated body may be slightly larger than the calorific value of the heater provided above. desirable.

In the present invention, when firing the silicon carbide molded body 12, a space is provided between the silicon carbide molded body 12 and the upper and lower firing jigs 13. This is for the following reasons.

That is, usually, the firing jig is made of carbon. However, the silicon carbide molded body contains about 3% of S in the silicon carbide powder due to the manufacturing conditions.
Contains iO ₂ . Therefore, in the firing step, the above-mentioned SiO ₂ is sublimated and released from the silicon carbide molded body, a part of which is converted into SiO gas, and the following reaction formula (1) of the SiO gas and carbon constituting the firing jig 11 is used. ;

SiO + 2C → SiC + CO (1)

The reaction shown in FIG. As a result, coarse particles made of silicon carbide are formed on the surface of the firing jig, the smoothness of the surface of the firing jig is lost, and sticking between the silicon carbide molded body and the firing jig occurs. Chips and pinholes are generated in the obtained porous silicon carbide member, which causes a reduction in yield. However, by providing a space between the silicon carbide molded body 12 and the upper and lower firing jigs 13, the sticking between the silicon carbide molded body and the firing jig does not occur. Further, since gas and the like easily flow through this space, the generated gas such as SiO is also easily diffused, and sintering proceeds smoothly.

The method for providing a space between the firing jig and the silicon carbide molded body is not particularly limited. For example, as shown in FIG.
And a method of inserting the clogging material 15 between the baking jig 13 and the baking jig 13.

Since the clogs 15 need to have heat resistance to withstand high temperatures during firing, it is preferable that the clogs 15 be a ceramic material having such heat resistance.

As the ceramic material, those having relatively high thermal conductivity are preferable, and examples thereof include carbon, silicon carbide, aluminum nitride, and silicon nitride.

Among these, the silicon carbide does not react at all with the SiO gas, and is preferably made of the same material as the porous silicon carbide member to be produced. Easy to hurt. Therefore, carbon is more preferable when these matters are comprehensively considered. The form of carbon is not particularly limited, but is preferably formed by assembling a felt (cloth) -like or thread-like one that is unlikely to damage the silicon carbide molded body.

The thickness of the clogging material 15 is
Considering heat conduction from below, the range of 1 to 10 mm is preferable. The specific thickness may be appropriately adjusted in consideration of the thermal conductivity of the clog 15 actually used.

The specific shape of the clogging material is not particularly limited, but is preferably a quadrangular prism from the viewpoint of stability when the silicon carbide molded body 12 is placed. The number of clogs disposed under the silicon carbide molded body is not particularly limited as long as a space can be provided between the silicon carbide molded body and the firing jig. And two or more may be placed.

As described above, the firing jig is usually
The one made of carbon is used.
As shown in the above, it is preferably a box type. This is because the silicon carbide molded body placed inside is excellent in heat retention and the internal temperature can be made uniform. FIG. 2 is a perspective view in which a plurality of silicon carbide compacts 12 are placed in a firing jig 13 via clogs (not shown).

By stacking such firing jigs 13, a space is provided between the silicon carbide molded body 12 placed therein and the bottom surface of the sintering jigs 13 stacked thereabove. Can be. The reason for providing the space on the upper surface of silicon carbide molded body 12 is the same as the reason described for clogging material 15 described above.

The space between the silicon carbide compact 12 and the upper firing jig 13 is preferably substantially the same as the space formed by the clogs 15 described above. Further, it is preferable that the upper surface of the uppermost firing jig is covered with a carbon plate. This is to improve the heat retaining efficiency of the silicon carbide molded body and to prevent falling objects such as soot from adhering to the silicon carbide molded body during firing.

The number of such firing jigs 13 to be stacked is not particularly limited, but is preferably 4 to 10 steps. If it is less than 4 steps, the silicon carbide molded body can be sufficiently fired, but productivity will be reduced. On the other hand, if it exceeds 10 steps, it will be difficult to sufficiently fire the silicon carbide molded body near the center, and it will be necessary to increase the size of the apparatus, making efficient production difficult.

In a series of steps from the degreasing step to the firing step, as described above, the silicon carbide molded body is placed on the firing jig via the clogging material, and the degreasing step and the firing step are performed as it is. Is preferred. The degreasing step and the sintering step can be performed efficiently.
This is because the silicon carbide molded body can be prevented from being damaged.

By using the method for firing a silicon carbide molded body of the present invention, the silicon carbide molded body can be heated to a substantially uniform temperature, and the average pore diameter of the sintered porous silicon carbide member can be reduced. It is possible to obtain a porous silicon carbide member having a small variation and a substantially uniform bending strength. Further, even if the silicon carbide molded bodies are stacked in multiple stages and fired, the variation in temperature at each stage can be reduced, so that a porous silicon carbide member having a constant average pore diameter and bending strength can be obtained. It can be manufactured stably.

[0040]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 70 parts by weight of α-type silicon carbide powder having an average particle diameter of 30 μm, 30 parts by weight of β-type silicon carbide powder having an average particle diameter of 0.28 μm,
5 parts by weight of methylcellulose, 4 parts by weight of a dispersant, and 20 parts by weight of water were blended and uniformly mixed to prepare a mixed composition of raw materials. This mixed composition was charged into an extruder, and a silicon carbide molded body having the same shape as the porous silicon carbide member 40 shown in FIG. 4 was produced at an extrusion speed of 2 cm / min. This silicon carbide compact has a size of 33 mm × 33 mm ×
The thickness was 300 mm, the number of through holes was 31 / cm ² , and the thickness of the partition wall was 0.35 mm.

Next, carbon felt (3 mm × 5 mm × 410 mm) was placed in a box-shaped firing jig (45 mm × 350 mm × 430 mm) made of porous carbon (G100 manufactured by Tokai Carbon Co., Ltd.) and opened on one main surface. ,
A clog material having a thermal conductivity of 0.24 W / m · K) is placed, ten dried silicon carbide molded bodies are placed thereon, and then a mixture of air and nitrogen having an oxygen concentration of 5% is mixed. The degreasing step was performed by heating at 450 ° C. in a gas atmosphere.

Next, the firing jig on which the silicon carbide compact having undergone the degreasing process is mounted is stacked in four stages, and a plate made of porous carbon (10 mm × 360 m) is placed on the uppermost firing jig.
m × 450 mm) and placed on the lid to form a laminate. Then, the laminates are arranged in two rows and carried into a baking apparatus. Under a nitrogen gas atmosphere, the laminate is moved by a roller provided thereunder, and the maximum temperature of the baking jig is 2200 from the vertical direction. The silicon carbide molded body was fired by heating so that the heating time at that temperature and the temperature was 3 hours, thereby producing a porous silicon carbide member (see FIG. 1). The physical properties of the obtained porous silicon carbide member were examined by the following methods. The results are shown in Table 1 below.

Evaluation method (1) Difference in average pore diameter Mercury is filled in the pores by a mercury porosimeter, and the vicinity of both ends and the center of the porous silicon carbide member in each stage are obtained by a correlation calibration curve between pressure and filling amount. And the average pore diameter in the vertical direction and the maximum value of the difference in the average pore diameter in the left-right direction were measured from these values. (2) Variation of average pore diameter Standard deviation was calculated from the value of the average pore diameter of each porous silicon carbide member measured in (1) above, and the variation of the average pore diameter was evaluated. (3) Temperature difference The average pore diameter measured in (1) above and the porous silicon carbide member measured in advance in the vertical and horizontal directions of the laminated body when firing the silicon carbide molded body. And the calibration curve for the average pore diameter and temperature. That is, the temperature in each part was obtained from the average pore diameter measured in the above (1), and the temperature difference was calculated based on this data. (4) Bending strength Using a bending strength tester, a three-point bending test is performed on the obtained porous silicon carbide member, an average value of bending strength is measured, and a standard deviation is calculated from each bending strength value. The variation in strength was evaluated.

Example 2 A porous silicon carbide member was manufactured in the same manner as in Example 1, except that five firing jigs were stacked.

The physical properties of the porous silicon carbide member obtained in Example 2 were examined in the same manner as in Example 1. The results are shown in Table 1 below.

Comparative Example 1 A laminate obtained by stacking the firing jigs formed in Example 1 in four stages is directly placed on a belt conveyor and carried in a line in a firing device. Heat with a heater,
A porous silicon carbide member was manufactured in the same manner as in the example except that the firing was performed (see FIG. 5). The physical properties of the porous silicon carbide member obtained in Comparative Example 1 were examined in the same manner as in Example 1. However, in Comparative Example 1, since the laminate was fired in one row, the difference in the average pore diameter in the left-right direction was:
The maximum value of the difference in average pore diameter in the left-right direction of one porous silicon carbide member was measured. The results are shown in Table 1 below.

[0048]

[Table 1]

As is clear from the results shown in Table 1, the porous silicon carbide members according to Example 1 and Example 2 have a small difference in the average pore diameter in the vertical and horizontal directions and a small variation in the average pore diameter. Also, the temperature difference in the vertical and horizontal directions during firing is small. Further, its bending strength was high, the dispersion was small, and it had sufficiently excellent characteristics. In addition, since the laminates can be arranged in two rows and simultaneously fired, the productivity was excellent.

On the other hand, in the porous silicon carbide member according to Comparative Example 1, the difference in the average pore diameter in the vertical direction, the variation in the average pore diameter, and the temperature difference during firing are large, and the variation in the bending strength is also large. Was something. Comparative Example 1
In the porous silicon carbide member according to the above, the difference in the average pore diameter in the left-right direction, the variation in the average pore diameter, and the temperature difference during firing are slightly better than those in Examples 1 and 2. But there was not much difference. Comparative Example 1
The reason that the porous silicon carbide member according to the above is slightly superior in the characteristics in the left-right direction is due to the fact that the laminate is fired in one row. That is, in general, when the laminated body is arranged in one row, the width is reduced to half and the heat capacity is reduced to half compared to the case where the laminated body is arranged in two rows, so that the temperature difference during firing is greatly improved. However, when Comparative Example 1 is compared with Examples 1 and 2, Comparative Example 1 is only slightly superior, and Example 2 and Comparative Example 1 have almost no difference. Therefore, when compared under the same conditions (that is, when the laminated bodies are arranged in two rows), it is considered that the variations (temperature difference) of the average pore diameter are much smaller in Examples 1 and 2. Note that the difference in pore diameter in the left-right direction of the porous silicon carbide members according to Examples 1 and 2 was sufficiently within an allowable range.

As described above, according to the heating methods of Embodiments 1 and 2 in which the laminated body is heated from the vertical direction, the temperature distribution in the vertical direction of the laminated body is greatly improved, and the temperature distribution in the lateral direction of the laminated body is varied. And the variation in pore diameter and the variation in strength of the resulting porous silicon carbide member are reduced.

[0052]

The method for firing a silicon carbide molded body according to the present invention comprises:
As described above, it is possible to obtain a porous silicon carbide member having a small variation in the average pore diameter and having a substantially uniform bending strength.

[Brief description of the drawings]

FIG. 1A is a longitudinal sectional view schematically showing one example of a method for firing a silicon carbide molded body of the present invention, and FIG.
It is the elements on larger scale of (a).

FIG. 2 is a perspective view schematically showing an example of a firing jig used in the method of firing a silicon carbide molded body of the present invention.

FIG. 3 is a perspective view schematically showing a ceramic structure.

FIG. 4A is a perspective view schematically showing a porous silicon carbide member constituting a ceramic structure, and FIG.
FIG. 3 is a cross-sectional view of the porous silicon carbide member shown in FIG.

FIG. 5A is a longitudinal sectional view schematically showing an example of a conventional method for firing a silicon carbide molded body, and FIG.
It is the elements on larger scale of (a).

[Explanation of symbols]

 11, 51 Heater 12, 52 Silicon carbide molded body 13, 53 Firing jig 14 Muffle 15, 55 Geta material 17, 57 Support stand 54 Conveyor belt

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Claims (1)

[Claims] 1. A method for firing a silicon carbide molded body, comprising: degreased a columnar silicon carbide molded body containing a silicon carbide powder, a binder, and a dispersion medium solution, and then placing it on a firing jig and firing. A firing jig on which the degreased silicon carbide molded body is placed is stacked in a plurality of stages to form a laminate, and a space is provided between the silicon carbide molded body and upper and lower firing jigs, A method for firing a silicon carbide molded body, comprising heating the laminate from above and below.