JPH07330462A - Porous silicon carbide sintered compact and its production - Google Patents

Abstract

PURPOSE:To produce a porous SiC sintered compact excellent in pore characteristics and strength characteristics. CONSTITUTION:A powdery mixture of SiC powder with MoSi2 powder is compacted and fired at a temp. between the m.p. of MoSi2 and 2,200 deg.C in a nonoxidizing atmosphere to produce the objective porous SiC sintered compact contg. MoSi2 fused and filled into the bonding grain boundary parts of SiC grains constituting the sintered compact. The average particle diameter of the SiC powder is preferably 0.1-10mum, that of the MoSi2 powder is 5-30mum and the MoSi2 powder is preferably added by 10-30 pts.wt. to 100 pts.wt. of the SiC powder. The powdery mixture is preferably heat-treated at a temp. below the m.p. of MoSi2 before firing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous SiC sintered body having a high porosity and a high strength, which is suitable as a filter for filtering exhaust gas, for example, and a method for producing the same.

[0002]

2. Description of the Related Art Since SiC has excellent heat resistance and corrosion resistance, porous SiC is used as a catalyst carrier and a filter for filtering corrosive solutions. Also, recently
BACKGROUND ART In an exhaust gas purifying apparatus for an internal combustion engine such as a diesel engine, a honeycomb-shaped filter has been studied as a catalyst carrier for filtering carbon soot in exhaust gas and burning and removing the carbon soot.

When a porous SiC body is used as a filter for purifying exhaust gas, it has high strength and, in addition to reducing pressure loss at the time of flow filtration of exhaust gas, and enhancing the collection efficiency of carbon soot, Porosity of about 45% or more, sharp pore distribution and average pore diameter of about 1
Pore characteristics such as 0 μm or more are required.

As a conventional method for producing a porous SiC body, a method of impregnating an organic porous body having a three-dimensional knit structure such as polyurethane foam with a slurry of SiC and drying it, and then heat-treating it to remove it by incineration. Is known (for example, JP-A-58-122016), the SiC porous body obtained by this method has a high porosity of 80% or more, but has a drawback of low strength.

As a method for producing a porous SiC sintered body, there is a method in which fine particles of SiC are fired to grow grains. In JP-A-3-215374, the average grain size is 100 to 150 μm, and the average grain size is 100 to 150 μm. 9 within ± 20% of diameter
The SiC granules having a particle size distribution in which 0 wt% or more is present are compacted so that the surface portions thereof are crushed and connected to each other, and the inside of the SiC granules is compacted so as to remain in the compacted body and then sintered. A method has been proposed. In addition, JP-A-3
In JP-A-215375, there is further disclosed a method in which a carbonaceous substance is mixed with SiC powder, molded and sintered, and then the sintered body is heated in an oxidizing atmosphere to burn off the carbonaceous substance. Japanese Patent Laid-Open No. 187578 proposes a method of suppressing abnormal grain growth of β-type SiC and controlling the pore diameter by blending β-type SiC powder with α-type SiC powder and molding and then firing. However, in these methods, since the bonding of the SiC particles forming the porous body is only due to the particle growth of the SiC fine particles, the mechanical strength is small, and there is a problem that it is not sufficient to achieve both the pore characteristics and the strength characteristics. there were.

[0006]

Particularly when used as a filter of an exhaust gas purifying apparatus, when a crack is generated due to impact or the like, exhaust gas flows through the crack and the filtration function is not fulfilled, so that a high-strength porous material is obtained. A substance is required, and it is also desired that the carbon soot collected by filtration is self-heated by energization when burning and removing carbon soot.

An object of the present invention is porous SiC which is excellent in pore characteristics and strength characteristics and has a good self-heating property upon energization, and which is suitable as a filter for purifying exhaust gas, for example.
It is to provide a sintered body and a method for manufacturing the same.

[0008]

Means for Solving the Problems A porous SiC sintered body according to the present invention for achieving the above object is obtained by welding and filling MoSi ₂ in the bonding grain boundary portion of SiC particles constituting the porous body. This is a structural feature.

The porous SiC sintered body is made into SiC by sintering.
Since fine particles are grown to form a porous body by bonding these particles to each other,
The strength of the bonded grain boundary portion between the SiC particles becomes the smallest. The porous SiC sintered body of the present invention has MoSi in the bonding grain boundary portion.
By filling ₂ by welding, the bond grain boundary between SiC particles is strengthened. MoSi ₂ has excellent strength and heat resistance, and has good wettability with SiC.
By welding and filling the bond grain boundary portion, the bond strength between the SiC particles is increased without deteriorating the pore characteristics and the heat resistance.

The method for producing a porous SiC sintered body according to the present invention comprises molding a mixed powder of SiC powder and MoSi ₂ powder, and heating the mixture at a temperature not lower than the melting point of MoSi ₂ and not higher than 2200 ° C. in a non-oxidizing atmosphere. It is characterized by being fired at.

The SiC powder used as a raw material may be either α-type or β-type regardless of the crystal structure, but it is preferable to use a powder having an average particle size of 0.1 to 10 μm. If the average particle size is less than 0.1 μm, the porosity and the pore size decrease, so that it becomes difficult to impart preferable pore characteristics as a filter. On the other hand, when the average particle size exceeds 10 μm, the sinterability deteriorates and the strength decreases.

The MoSi ₂ powder has an average particle size of 5 to 3
Preference is given to using 0 μm powder. MoSi ₂ is melted at the time of firing and filled in the bonding grain boundary portion of the SiC particles,
The bond grain boundary portion is strengthened by welding during solidification,
The traces where MoSi ₂ powder was present become pores. Therefore, since the average particle diameter of the MoSi ₂ powder has a great influence on the pore characteristics of the SiC sintered body, if the average particle diameter is less than 5 μm, the average pore diameter of the porous SiC sintered body becomes small and the exhaust gas filtering resistance becomes small. This is because the average pore size increases and the function as a filter decreases when the average pore size exceeds 30 μm.

This MoSi ₂ powder is a SiC powder 100
It is blended in a ratio of 10 to 30 parts by weight with respect to parts by weight.
When the blending amount is less than 10 parts by weight, the amount of the SiC particles deposited and welded to the bonding grain boundary part is small, so that the strength cannot be sufficiently improved. Therefore, the function as a filter is deteriorated due to the decrease in the porosity and the pore diameter.

The SiC powder and the MoSi ₂ powder are prepared into a uniform slurry by adding an organic binder such as carboxymethyl cellulose, polyethylene glycol, polyvinyl alcohol, etc. and water, and the slurry is prepared by a known molding means such as casting or extrusion molding. Shaped into a shape.

This molded product had a temperature of at least the melting point of MoSi _{2 in} a non-oxidizing atmosphere such as N ₂ or Ar and a temperature of 2200.
It is fired at a temperature of ℃ or less. MoSi ₂ melts at the time of firing, and since it has a high wettability with SiC, MoSi ₂ is welded at the time of solidification to strengthen the bond grain boundary portion of the SiC particles. However, if the firing temperature exceeds 2200 ° C,
The firing temperature is set to 2200 ° C. or lower because decomposition of SiC and MoSi ₂ occurs.

In this case, heat treatment is carried out at a temperature below the melting point of MoSi ₂ , for example, at a temperature of 1900 to 2000 ° C. for a certain period of time to promote grain growth of SiC to form a porous body and then rise to a predetermined temperature. Baking at a temperature is preferable in order to improve the pore characteristics.

[0017]

In the porous SiC sintered body of the present invention, since the bonded grain boundary portion of the SiC particles constituting the porous body is filled with molten MoSi ₂ , the grain boundary portion having the smallest strength is weld filled. It is reinforced by MoSi ₂ and provides improved strength properties without compromising porosity properties. Furthermore,
Since MoSi ₂ has good conductivity, it also improves the heat generation by energization and is extremely suitable as a filter for purifying exhaust gas and a porous ceramic heater.

Further, according to the manufacturing method of the present invention, SiC
A mixed powder in which MoSi ₂ powder is mixed with the powder is molded, and the mixture has a melting point of MoSi ₂ or more and 220
By firing at a temperature of 0 ° C. or lower, MoSi ₂ melted at the time of firing is preferentially filled in the concave portion of the bonded grain boundary portion of the SiC particles grown due to the good interfacial wettability with SiC, and solidifies. At the time of welding, the grain boundaries are strengthened by welding. In addition, since the traces of the presence of MoSi _{2 in} the mixed powder form voids, the strength characteristics are improved without degrading the pore characteristics.

The SiC powder and MoSi ₂ used
By adjusting the average particle diameter and the mixing ratio of the powder, it becomes possible to control the pore characteristics and improve the strength characteristics of the porous SiC sintered body. Furthermore, when heat treatment is performed for a certain period of time at a temperature equal to or lower than the melting point of MoSi ₂ , the grain growth of SiC particles is promoted to form a porous body, and then firing is performed, the pore characteristics can be further improved.

[0020]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples.

Example 1 100 parts by weight of α-type SiC powder having an average particle size of 5 μm was mixed with 30 parts by weight of MoSi ₂ powder having an average particle size of 20 μm, and further 1 part by weight of polyvinyl alcohol and 22 parts by weight of water were added. Parts were added to prepare a slurry. Then, this slurry is cast by a casting method to have a diameter of 12 mm and a length of 50.
mm columnar shape, dried and then 2100 in N ₂ atmosphere
The temperature was kept at 1 ° C for 1 hour for firing. The porosity, average pore diameter and three-point bending strength of the obtained SiC sintered body were measured and shown in Table 1. The porosity and the average pore diameter were measured by the mercury penetration method, and the three-point bending strength was measured by JIS R1601. Further, Ag paste was applied to both ends of the SiC sintered body to form electrodes, and the specific resistance was measured in an electric furnace set at 100 ° C. and 500 ° C. The results are also shown in Table 1. In addition, the photograph (magnification: 1000 times) which image | photographed the particle structure of this SiC sintered compact with the scanning electron microscope is shown in FIG. As shown in FIG. 1, MoSi _{2 is formed at} the bonding grain boundary part of the SiC particles.
It can be seen that is filled by welding, and the trace of MoSi ₂ existing during firing is a void as it is.

Comparative Example 1 A SiC sintered body was produced by the same method as in Example 1 except that the MoSi ₂ powder was not mixed, and various characteristics were measured by the same method as in Example 1, and the results are shown. It is also listed in Table 1.
A photograph (magnification: 1000 times) of the grain structure of this SiC sintered body taken by a scanning electron microscope is shown in FIG.

Example 2 10 parts by weight of MoSi ₂ powder having an average particle size of 20 μm, 30 parts by weight of methylcellulose, 10 parts by weight of glycerin and 20 parts by weight of water with respect to 100 parts by weight of β-type SiC powder having an average particle size of 0.3 μm. Parts to prepare a slurry, and the slurry is extruded to have a diameter of 12 mm and a length of 50
A cylindrical molded body having a size of mm was produced. The compact was heated in an Ar gas atmosphere at a temperature rising rate of 10 ° C./min.
After heat treatment at a temperature of ℃ for 4 hours, the temperature is raised again to 210
The temperature was kept at 0 ° C. for 1 hour for firing. Obtained SiC
Various properties of the sintered body were measured by the same method as in Example 1, and the results are also shown in Table 1.

Example 3 Except that the compounding amount of MoSi ₂ powder was changed to 20 parts by weight,
A SiC sintered body was manufactured by the same method as in Example 2.
Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Example 4 Except that the compounding amount of MoSi ₂ powder was changed to 30 parts by weight,
A SiC sintered body was manufactured by the same method as in Example 2.
Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Example 5 Except that the average particle size of MoSi ₂ powder was changed to 13 μm,
A SiC sintered body was manufactured by the same method as in Example 2.
Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Comparative Example 2 A SiC sintered body was manufactured by the same method as in Example 2 except that MoSi ₂ powder was not added. Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Comparative Example 3 Instead of MoSi ₂ powder, α-type Si having an average particle size of 20 μm
Si was prepared in the same manner as in Example 2 except that C powder was used.
A C sintered body was manufactured. Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Comparative Example 4 Instead of MoSi ₂ powder, α type Si having an average particle size of 20 μm
A SiC sintered body was manufactured in the same manner as in Example 2 except that C powder was used and fired in an N ₂ gas atmosphere. Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Comparative Example 5 A SiC sintered body was manufactured in the same manner as in Example 2 except that the firing was completed by heat treatment at a temperature of 1950 ° C. for 4 hours. Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

Comparative Example 6 A SiC sintered body was manufactured by the same method as in Example 2 except that the firing temperature was set to 2300 ° C. Various properties of the obtained SiC sintered body were measured in the same manner as in Example 2, and the results are also shown in Table 1.

[0032]

[Table 1]

From the results shown in Table 1, the porosity of the porous SiC sintered body of the example is higher than that of the porous SiC sintered body of the comparative example.
It has been found that not only the average pore diameter is large and the pore characteristics are excellent, but also the strength is high, and the pore characteristics and the strength characteristics can be adjusted by adjusting the average particle diameter and the compounding ratio of the MoSi ₂ powder. I understand. Further, since the specific resistance is small and the temperature change is small, the heat generation by energization is also excellent.

[0034]

As described above, in the porous SiC sintered body of the present invention, since the bonding grain boundary portion of the SiC particles constituting the porous body is filled with MoSi ₂ by welding, the pore characteristics are deteriorated. It also has high strength characteristics and excellent heat generation by energization. Further, according to the manufacturing method of the present invention, it is possible to manufacture the SiC sintered body by sintering under a specific firing condition, and it is possible to use a porous ceramic used for a filter for exhaust gas filtration, a porous ceramic heater, or the like. It is extremely useful as a high quality SiC sintered body and its manufacturing technology.

[Brief description of drawings]

1 is an electron micrograph showing the particle structure of a porous SiC sintered body produced according to Example 1 (magnification: 1000).
Times).

FIG. 2 is an electron micrograph showing the particle structure of a porous SiC sintered body produced in Comparative Example 1 (magnification: 1000).
Times).

Claims (6)

[Claims] 1. A porous SiC sintered body, characterized in that MoSi ₂ is weld-filled in the bonding grain boundary portion of SiC particles constituting the porous body. 2. A porous SiC calcination characterized by molding a mixed powder of SiC powder mixed with MoSi ₂ powder, and calcination in a non-oxidizing atmosphere at a temperature not lower than the melting point of MoSi ₂ and not higher than 2200 ° C. A method for producing a bound body. 3. The SiC powder has an average particle size of 0.1 to 10 μm.
The method for producing a porous SiC sintered body according to claim 2, wherein m is m 2. 4. The average particle size of MoSi ₂ powder is 5 to 30 μm.
The method for producing a porous SiC sintered body according to claim 2 or 3, wherein m is m 2. 5. MoS based on 100 parts by weight of SiC powder
The method for producing a porous SiC sintered body according to claim 2, 3 or 4, wherein i ₂ powder is mixed in a ratio of 10 to 30 parts by weight. 6. The porous S according to claim 2, 3, 4, or 5, which is heat-treated at a temperature below the melting point of MoSi ₂ and then fired.
Method for manufacturing iC sintered body.