JP2001072479A - Silicon carbide porous body and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon carbide porous body that is suitable for filtration and separation of fine particles. SOLUTION: In this silicon carbide porous body, an outer layer of silicon carbide having pores of smaller pore diameter then the average diameter of the base material is formed at least a part of the surface of silicon carbide base material having a prescribed average pore diameter, and the silicon carbide particles on the surface of the base material are bonded via silica to the silicon carbide particles on the outer layer part and the silicon carbide particles on the outer layer are bonded to each other via silica.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous silicon carbide body and a method for producing the same. In particular, the present invention relates to a porous silicon carbide body which can perform filtration such as removal of fine particles without causing a significant decrease in filtration speed, and has sufficient characteristics as a separation filter.

[0002]

2. Description of the Related Art In addition to high hardness, excellent abrasion resistance, and excellent chemical stability, a silicon carbide sintered body has excellent properties as a heat-resistant material such as a small coefficient of thermal expansion and high strength at high temperatures. Therefore, it can be widely used as a refractory material for high-temperature furnaces and a high-temperature structural material such as a heat exchanger. Taking advantage of these excellent properties, a porous silicon carbide sintered body (hereinafter referred to as
Attempts have been made to apply the present invention to a filter for a dust removal device of a diesel engine or a high-temperature filter for separating ash or the like contained in a high-temperature combustion gas using a silicon carbide-based porous body).

[0003] Porous materials used in such filters include:
Not only heat resistance and corrosion resistance, but also low fluid permeation resistance,
It is required that fine foreign particles can be removed with high efficiency.

[0004] Porous materials suitable for such applications include:
A porous silicon carbide sintered body in which an outer layer portion of silicon carbide is bonded to an outer surface of a silicon carbide porous body having a specific pore diameter is known, for example, Japanese Patent Publication No. 7-100633 and Japanese Patent Laid-Open No. No. 58305.

[0005] Japanese Patent Publication No. 7-100633 discloses that a silicon carbide powder layer is formed on the outer surface of a sintered body obtained by firing a silicon carbide powder compact, and then heated to 1500 ° C. in a non-oxidizing atmosphere.
A method for producing a porous silicon carbide body in which an outer layer is formed by heating at 2200 ° C. at a temperature lower than the firing temperature of a molded body is described.

However, in this method, heat treatment is performed at a high temperature of 1500 ° C. or more to enhance the sinterability of silicon carbide, which is inherently difficult to sinter, and silicon carbide powder is used to strengthen the bonding between silicon carbide particles. Aluminum, boron, yttrium, carbon, etc. are added to the layer.
Further, in order to prevent the silicon carbide from being deteriorated during the heat treatment at a high temperature, it is necessary to maintain the atmosphere in an inert atmosphere such as argon, and there is a problem that the usable heat treatment furnace is limited.

Japanese Patent Application Laid-Open No. 1-58305 discloses that a substrate made of a porous silicon carbide material is immersed in a slurry containing ceramic powder having a uniform particle size to generate a pressure difference between the inside and outside of the substrate. A method is described in which a coating layer made of a slurry is formed on the surface on the side where the pressure is high, and after drying, the coating layer is sintered together with the base material and integrated. However, in this method, adjustment of silicon carbide particles in the slurry and consideration of heat treatment conditions are not sufficient,
The reinforcement of the bond between the substrate and the coating layer and the control of the pore size of the coating layer have not been sufficiently performed.

[0008]

SUMMARY OF THE INVENTION According to the present invention, there is provided a carbonized carbonaceous substrate having a predetermined average pore size, wherein at least a portion of the outer surface of the silicon carbide substrate has an average pore size smaller than the average pore size of the substrate. A silicon carbide-based porous body having a silicon-based outer layer portion formed thereon, which does not contain other components for strengthening the bond between silicon carbide particles, the base material and the outer layer portion are firmly bonded, and the outer layer It is an object of the present invention to provide a silicon carbide-based porous body in which silicon carbide particles in each part are strongly bonded.

Another object of the present invention is to provide a method for producing a silicon carbide porous body which can be produced by subjecting the above porous body to heat treatment at a low temperature in an oxidizing atmosphere.

[0010]

According to the present invention, there is provided a silicon carbide substrate having an average pore size smaller than the average pore size of at least a part of the surface of the silicon carbide substrate having a predetermined average pore size. The outer layer portion of the silicon carbide-based porous body is formed, between the silicon carbide particles of the substrate surface and the silicon carbide particles of the outer layer portion, and between the silicon carbide particles of the outer layer portion is bonded by silica. The present invention provides a silicon carbide-based porous body characterized in that:

Further, the present invention provides a method for coating or impregnating a slurry containing a silicon carbide powder having a maximum particle diameter smaller than the average pore diameter of the substrate on the surface of the silicon carbide substrate having a predetermined average pore diameter. 750 to 120 in an oxidizing atmosphere
Provided is a method for producing a silicon carbide-based porous body, which comprises heating at 0 ° C. to form a silicon carbide-based outer layer on the surface of a substrate.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The silicon carbide porous body of the present invention
A silicon carbide-based porous body having a silicon carbide-based outer layer having an average pore diameter smaller than the average pore diameter of at least a portion of the surface of a silicon carbide-based substrate having a predetermined average pore diameter. It is.

In the porous body of the present invention, the average pore diameter of the silicon carbide base material is preferably 10 μm or less, particularly preferably 5 μm or less. When the average pore diameter of the substrate is 10 μm or less, the pores on the surface of the substrate can be sufficiently closed by the silicon carbide particles forming the outer layer, and the outer layer having a uniform thickness is formed on the surface of the substrate. it can.

On the other hand, the average pore diameter of the silicon carbide substrate is preferably at least 0.01 μm, particularly preferably at least 0.1 μn. When the average pore diameter of the substrate is less than 0.01 μm, it is difficult to make the average pore diameter of the outer layer smaller than the average pore diameter of the substrate. The average pore diameter is a value measured by a mercury intrusion method.

The apparent porosity of the silicon carbide substrate is preferably from 10 to 80%, particularly preferably from 20 to 60%. When the apparent porosity exceeds 80%, the strength of the substrate is not sufficient, and the substrate may be deformed or cracked when the outer layer portion is formed on the surface of the substrate. on the other hand,
If it is less than 10%, the air permeability of the base material is often insufficient, and it is difficult to use it as a filter or the like. The apparent porosity is a value measured by the Archimedes method.

On the other hand, the average pore diameter of the silicon carbide outer layer is preferably 1 μm or less, particularly preferably 0.1 μm or less. The case where the average pore diameter of the outer layer portion is 1 μm or less is preferable because the silicon carbide particles of the outer layer portion are firmly bonded by silica. The average pore diameter is a value measured by a mercury intrusion method.

In order to use the porous silicon carbide material of the present invention as a filter or the like, the pore size distribution of the silicon carbide-based outer layer is preferably sharp, and specifically, the maximum pore size Is preferably not more than twice the average pore diameter.

The thickness of the silicon carbide outer layer is 1 to 10
It is preferably 0 μm, particularly preferably 5 to 50 μm. When the thickness of the outer layer portion is in the above range, the silicon carbide porous body has sufficient air permeability and can form a uniform outer layer portion on the surface of the base material.

In the silicon carbide porous body of the present invention, the silicon carbide particles on the surface of the silicon carbide substrate and the silicon carbide particles in the silicon carbide outer layer are bonded by silica,
Further, the silicon carbide particles in the outer layer portion are bonded by silica.

Such a silicon carbide porous body of the present invention contains a silicon carbide powder having a maximum particle diameter smaller than the average pore diameter of the substrate on the surface of the silicon carbide substrate having a predetermined average pore diameter. After the slurry is applied or impregnated, it can be manufactured by heating at 750 to 1200 ° C. in an oxidizing atmosphere to form a silicon carbide outer layer on the surface of the substrate.

The surface of a silicon carbide substrate having a predetermined average pore size is coated or impregnated with a slurry containing silicon carbide powder having a maximum particle size smaller than the average pore size of the substrate, thereby forming a silicon carbide substrate. An outer layer of silicon carbide having an average pore diameter smaller than the average pore diameter of the substrate can be formed on the surface of the material.

In the present invention, the use of a slurry containing silicon carbide powder makes it difficult to coat the surface of the base material without using the slurry, the thickness of the outer layer becomes uneven, and the strongly bonded outer layer is formed. This is because it is difficult to obtain.

The slurry containing the silicon carbide powder having the desired maximum particle size can be obtained by a commonly used classification method. For example, after the silicon carbide powder having the adjusted particle size is dispersed in water to form a slurry, the slurry is allowed to stand for a predetermined time, and the slurry up to a predetermined position determined by the sedimentation speed of the particles may be collected.

In order to form a desired outer layer on the surface of the base material, the operation of immersing the base material in a slurry containing silicon carbide powder, removing the base material and drying the base material may be repeated as necessary. . At this time, if the suction operation is used together,
Since the penetration and adhesion of the particles into the pores exposed on the surface are promoted, the outer layer portion can be formed more effectively. It is also effective to perform a pressure reduction operation after immersing the substrate in the slurry, since penetration of silicon carbide particles into pores is promoted.

In the production method of the present invention, in order to firmly bond between the silicon carbide particles on the surface of the substrate and the silicon carbide particles in the outer layer and between the silicon carbide particles in the outer layer, After forming the outer layer portion, a heat treatment is performed at 750 ° C. to 1200 ° C. in an oxidizing atmosphere. By performing the heat treatment at the above temperature, the silicon carbide particles on the substrate surface, the silicon carbide particles in the outer layer portion, and the silicon carbide particles in the outer layer portion are oxidized to form vitreous silica (SiO ₂ ). The bonding between the silicon carbide particles on the surface and the silicon carbide particles in the outer layer and the bonding between the silicon carbide particles in the outer layer are strengthened.

The amount of silica formed on the surface of the silicon carbide particles can be determined using the weight change rate before and after the heat treatment as an index. That is, the ratio of the total weight of the base material and the outer layer portion after heating to the total weight of the base material and the outer layer portion before heating is preferably 1.01 to 1.05. By setting the above ratio to the above range, the outer layer portion can be maintained in a layer substantially composed of silicon carbide, and between the silicon carbide particles on the substrate surface and the silicon carbide particles in the outer layer portion, and the carbonization of the outer layer portion. It is possible to firmly bond between silicon particles.

If the temperature of the heat treatment is lower than 750 ° C., the silicon carbide particles are not sufficiently oxidized, so that the bonding layer of vitreous silica is not sufficiently formed, and the bonding strength is weak, so that the outer layer portion is liable to peel off. If the temperature exceeds 1200 ° C., crystallization of the vitreous silica is likely to occur, and cristobalite may be formed, and the bonding effect may not be obtained. A more preferred temperature of the heat treatment is 800 to
1100 ° C.

The time of the heat treatment is not particularly limited as long as the strength of the obtained silicon carbide porous body is sufficient, but it is preferably 1 to 24 hours, particularly preferably 1 to 10 hours.

Although the mechanism for forming the silicon carbide porous body of the present invention is not clearly understood, the following may be considered. That is, when silicon carbide is heated in an oxidizing atmosphere, it undergoes about 45% volume expansion when silica is formed by oxidation. For this reason, it is considered that the fine silicon carbide particles in the outer layer expand along with the formation of silica, thereby effectively miniaturizing the pores existing between the particles and enabling strong bonding.

[0030]

The present invention will be further described below with reference to examples and comparative examples. Example 1 is an example of the present invention, and Example 2 is a comparative example of the present invention.

Example 1 300 g of silicon carbide powder having an average particle size of 0.6 μm (product name: A-1) was added to 2 liters of ion-exchanged water, and monoethanolamine was added dropwise with stirring. To obtain a suspension whose pH was adjusted to 8. This suspension was dispersed using an ultrasonic homogenizer, and then allowed to stand at room temperature for classification, to recover a suspension containing only silicon carbide particles having a particle size of 1 μm or less. A silicon carbide substrate (20 mm × 20 mm × 1 mm) having an average pore diameter of 1.5 μm and an apparent porosity of 35% measured by a mercury intrusion method is immersed in this suspension. A vacuum operation was performed to adsorb the silicon carbide particles on the surface of the substrate.

After the substrate was taken out and dried, the operation of adsorbing silicon carbide particles on the surface of the substrate was performed again. The silicon carbide-based porous body obtained by forming a silicon carbide-based outer layer on the surface of the substrate thus obtained was subjected to a heat treatment at 1000 ° C. for 10 hours in the air using an electric furnace.

After heating, the surface of the outer layer was subjected to elemental analysis by X-ray microanalysis (JXA-88).
00M: using a JEOL apparatus), an oxygen peak was confirmed, and it was found that silica had been formed. The ratio of the total weight of the base material and the outer layer after heating to the total weight of the base material and the outer layer before heating was 1.02.

The surface of the outer layer portion of the obtained porous silicon carbide body was observed with a scanning electron microscope (see FIG. 1). When the cross section of the obtained silicon carbide based porous material was observed with a scanning electron microscope, the thickness of the outer layer was about 10 μm (see FIG. 2).

When measured by a mercury intrusion method, the average pore size of the outer layer was 0.08 μm, and the sharp pore size distribution (80% or more of the pores was 0.05 μm or more, and the maximum pore size was 0.15 μm), indicating that the pores were sufficiently fine. The silicon carbide particles in the outer layer were firmly bonded to each other, and no peeling, cracking, detachment of the particles, etc. were observed.

Example 2 A substrate was prepared in the same manner as in Example 1 except that instead of using silicon carbide powder, alumina powder having an average particle diameter of 0.5 μm (product name: AKP-20, manufactured by Sumitomo Chemical Co., Ltd.) was used. To produce a silicon carbide porous body having an alumina outer layer formed on the surface of
Heat treatment was performed at 000 ° C. for 10 hours.

When measured by a mercury intrusion method, the average pore diameter of the outer layer was 0.3 μm. When the cross section of the obtained silicon carbide based porous material was observed with a scanning electron microscope, the thickness of the outer layer was 8 μm, and cracks were observed in a part of the outer layer.

[0038]

The silicon carbide porous body of the present invention does not require the addition of any other auxiliary agent for strengthening the bond between silicon carbide particles, and its surface is formed of an outer layer having a fine pore diameter. ing. Further, the base material and the outer layer portion are firmly bonded, and the silicon carbide particles in the outer layer portion are firmly bonded. Therefore, when used for applications such as microfiltration and separation filters, fine particles can be removed without causing a significant decrease in filtration speed, and the filter has sufficient durability.

[Brief description of the drawings]

FIG. 1 is an enlarged photograph of a surface of an outer layer portion of a silicon carbide porous body of Example 1 taken by a scanning electron microscope.

FIG. 2 is an enlarged photograph of a cross section of the silicon carbide porous body of Example 1 taken by a scanning electron microscope.

Claims (3)

[Claims] 1. A silicon carbide-based outer layer having an average pore diameter smaller than the average pore diameter of the substrate is formed on at least a part of the surface of the silicon carbide-based substrate having a predetermined average pore diameter. A carbonized silicon carbide body, characterized in that silica is bonded between the silicon carbide particles on the substrate surface and the silicon carbide particles in the outer layer, and between the silicon carbide particles in the outer layer by silica. Silicone porous body. 2. The silicon carbide-based porous body according to claim 1, wherein the silica is vitreous. 3. An oxidizing atmosphere after applying or impregnating a slurry containing silicon carbide powder having a maximum particle diameter smaller than the average pore diameter of the substrate on the surface of the silicon carbide substrate having a predetermined average pore diameter. A method for producing a silicon carbide-based porous body, comprising heating at 750 to 1200 ° C. to form a silicon carbide-based outer layer on the surface of a substrate.