JP2001247381A - Porous silicon carbide and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous SiC having high porosity, mechanical strength and thermal conductivity, provide its production process and provide various filters having excellent permeation performance using the porous SiC. SOLUTION: Si powder is mixed with C powder to obtain a mixture having an Si powder content of 30-61 wt.%. The mixture is sintered and the carbon remaining in the sintered material is eliminated by heating to obtain a porous SiC having a porosity of 61-75% and containing a three-dimensional skeleton texture composed of hexagonal α-SiC particles bonded with each other by sintering necking. Since the porous SiC has high porosity and permeation performance as well as high strength and high thermal conductivity, it is suitable as a filter for liquid filtration and a particulate filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic filter material used for filtration in the field of foods and pharmaceuticals and for removal of suspended solids in exhaust gas from automobiles, and particularly has high heat resistance, high strength and high permeability. The present invention relates to a SiC-based porous material having excellent resistance.

[0002]

2. Description of the Related Art Among various ceramics, silicon carbide (SiC) is a structural ceramic material having high strength, high toughness, high thermal shock resistance and chemical resistance, and has recently attracted attention as various filter materials. . Its main applications are in the fields of food and pharmaceuticals, as well as filters for liquid filtration used for semiconductor cleaning liquid recycling and wastewater treatment, etc.
It is directed to a particulate filter for purifying exhaust gas of an automobile diesel engine.

[0003] In the case of a filter for liquid filtration, it is required to improve not only the ability to collect fine particles in the treatment liquid and the permeation treatment ability of the liquid, but also high corrosion resistance to the treatment liquid. Further, in the case of a particulate filter, heat resistance and corrosion resistance under a high-temperature gas are required in addition to efficient collection of fine particulates in a processing gas and improvement of a toxic gas separation ability. For this reason, attempts have been made to use ceramics having excellent corrosion resistance and heat resistance, to increase the porosity, and to improve the average pore diameter and the structure of the pore diameter portion.

In addition, such a porous ceramic body includes:
When used as a filter or the like, it is necessary to have mechanical strength for maintaining the shape. Further, the particulate filter for purifying exhaust gas is also required to have high thermal conductivity in order to suppress deterioration of basic performance due to temperature rise of the filter itself.

[0005]

As the above ceramic filter material, there has been proposed a porous SiC body having a skeleton structure formed by sintering SiC particles. When this SiC porous body is used as a filter, firstly, high permeability is required. The permeation performance of the filter increases as the porosity of the porous SiC body constituting the filter increases. However, when the porosity of the porous SiC body is increased, there is a problem that the mechanical strength is reduced and the porous body is easily broken, and the thermal conductivity is also reduced.

As a method for producing a porous SiC material having a three-dimensional skeletal structure, although it is for an Al-SiC composite material,
There is a method described on page 255 of “Summary of Powder and Powder Metallurgy Association Lectures (Fall 1999 Meeting)”. In this method, a SiC powder is formed together with a binder and fired at a high temperature of 2000 ° C. or more in an inert gas. Due to this firing, a part of the SiC particles sublimates and gasifies, and
When re-folding, SiC particles are sintered together to obtain a SiC porous body having a skeleton structure.

However, a porous SiC body having a three-dimensional skeletal structure manufactured by such a method has an upper limit on the porosity, and can only obtain a porosity of at most 60%.
Therefore, satisfactory permeation performance could not be obtained either as a filter for liquid filtration or as a particulate filter.

In view of the above circumstances, the present invention provides a porous SiC material having a high porosity and excellent mechanical strength and thermal conductivity, a method for producing the same, and a SiC-based porous material.
An object of the present invention is to provide a liquid filtration filter and a particulate filter having excellent permeation performance using a C-based porous body.

[0009]

In order to achieve the above object, a SiC-based porous material provided by the present invention has a hexagonal plate shape α.
The SiC particles have a three-dimensional skeletal structure obtained by sinter necking, and have a porosity of 61 to 75%. Further, the SiC-based porous body of the present invention can include a carbon component.

The SiC-based porous body of the present invention has an average pore diameter of 0.1 to 200 μm, a JIS three-point bending strength of 100 MPa or more, or 2
It has characteristics such as a thermal conductivity at 0 ° C. of 15 W / m · K or more.

In the method for producing a porous SiC material according to the present invention, the Si powder and the carbon powder are mixed with each other so that the total amount of the Si powder is 30 to 6%.
5% by weight and heat-treating the formed body at a temperature of 2000 to 2400 ° C. in an inert gas atmosphere to obtain a sintered body made of SiC and carbon. It is characterized in that all or part of the carbon component is eliminated by heating to 300 ° C. or more in the inside.

Further, the present invention provides a filter for liquid filtration comprising the above-mentioned porous SiC material, and a particulate filter for purifying exhaust gas of an automobile diesel engine.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventor has already disclosed Japanese Patent Application No. 11-3
No. 69086 proposed a porous SiC body having excellent mechanical strength and high thermal conductivity and a method for producing the same.
In this method, instead of firing SiC powder at a high temperature as in the conventional method, Si powder and carbon powder are used as raw material powders, and in particular, the Si powder is mixed so as to be 71 to 73% by weight of the whole raw material powder, 2000 in an inert gas atmosphere
It is fired at a high temperature of ２2400 ° C.

In this method, Si and carbon react at a high temperature at which Si sublimes to form SiC. That is, Si
The reaction starts around 1450 ° C where
And carbon react to form SiC. At this point, 3C-type SiC having a cubic crystal system is generated. However, when the temperature further rises and exceeds 2000 ° C., the generated 3C-type SiC is converted into hexagonal 6H-type SiC (α-type SiC). And a hexagonal plate-like SiC crystal. Simultaneously with the generation of the hexagonal plate crystal of SiC, the SiC particles are sintered and necked to form a three-dimensional skeleton structure.

However, this Japanese Patent Application No. 11-3690 is disclosed.
The method according to No. 86 is also based on the premise that an Al-SiC composite material is mainly used, so that the obtained SiC porous body has excellent mechanical strength and thermal conductivity, but low porosity. Was. That is, by setting the amount of the Si powder in the mixed raw material powder of the Si powder and the carbon powder to 71 to 73% by weight, surplus Si becomes a gas and functions to firmly neck the SiC particles. However, the porosity of the obtained SiC-based porous body is only 30 to 60%.

On the other hand, in the present invention, Si powder and carbon powder are mixed so that the amount of Si powder is 30 to 65% by weight of the whole. As a result, as shown in FIG. 1A, excess carbon is present in the compact, and the excess carbon remains unreacted after the heat treatment as shown in FIG. A sintered body is obtained. This is further heated in an oxygen-containing atmosphere to remove all or part of the carbon component as shown in FIG. 1 (c), whereby relatively large pores appear and a SiC-based porous body having a high porosity is obtained. Obtainable.

The lower the proportion of the Si powder in the mixed raw material powder of the Si powder and the carbon powder, the more unreacted carbon is. Therefore, the porosity of the SiC-based porous body from which the carbon component is almost completely eliminated finally becomes high. Become. However, the ratio of Si powder is 3
If the amount is less than 0% by weight, the amount of SiC generated by the heat treatment is reduced, and the SiC crystals cannot be firmly bonded to each other, so that the strength of the obtained SiC-based porous body decreases. On the other hand, when the proportion of the Si powder exceeds 65% by weight, the carbon component is reduced, and almost no improvement in porosity can be expected.

The porosity of the porous SiC material according to the present invention is:
In addition to the amount of the Si powder in the mixed raw material powder, it varies depending on the control of the porosity of the molded body during the production of the molded body, the degree of the carbon disappearance treatment, and the like. It goes without saying that the higher the porosity of the molded body, the higher the porosity of the final SiC-based porous body. The porosity of the molded body can be controlled by adjusting the pressure at the time of molding. However, if the molding pressure is too small to increase the porosity, caution is required because the molding tends to collapse during the subsequent processing. In the case of the same amount of Si powder, the porosity of the SiC-based porous body increases as the amount of disappeared carbon increases.

Regarding the SiC powder and the carbon powder, which are the raw material powders, the smaller the impurity element contained, the more preferable the higher the thermal conductivity. In particular, A in SiC powder and carbon powder
By lowering l and Fe to 100 ppm or less, respectively, a SiC-based porous body having even higher thermal conductivity can be obtained. Further, as the average particle diameter of the carbon powder increases, the average pore diameter of the obtained SiC-based porous body increases, but on the other hand, the strength decreases. Since the SiC powder is sublimated and gasified, the particle size is not limited.

The mixed raw material powder of the Si powder and the carbon powder is formed into a compact, and then heat-treated in an inert gas atmosphere and sintered. Although the porosity of the molded body can be increased as the molding pressure is reduced, care must be taken because the molded body is easily collapsed. The molding is preferably performed in a vacuum. Sintering temperature is 2
The temperature must be at least 000 ° C, and if it is less than 2000 ° C, sintering does not proceed. When the sintering temperature exceeds 2400 ° C.,
Since the sublimation of SiC becomes intense, the yield decreases. As an inert gas atmosphere during sintering, argon is preferable. This is because, by performing the heat treatment in argon, stacking faults contained in the generated SiC crystal are easily eliminated, and the thermal conductivity of the obtained SiC-based porous body is increased.

Since the SiC-C-based sintered body obtained by this heat treatment contains a carbon component, the SiC-C-based sintered body is then subjected to
Preferably, heating is performed in the atmosphere to eliminate all or part of the carbon component as CO ₂ gas. The heating temperature needs to be 300 ° C. or higher, and a low temperature is preferable when intentionally leaving a carbon component in the SiC-based porous body. The higher the temperature, the more easily the carbon disappears. However, if the temperature exceeds 1500 ° C., the SiC itself is oxidized and the thermal conductivity decreases, which is not preferable. Normally, if heated at a temperature of about 500 ° C. for several hours, all the carbon components disappear.

The porosity of the thus obtained porous SiC material of the present invention is improved to 61 to 75%, and the average pore diameter is in the range of 0.1 to 200 μm. In addition, compared to a conventional SiC porous body formed by molding and sintering SiC powder, the mechanical strength is increased because the SiC particles are entangled randomly and strongly bonded, and the average particle size of the raw material powder is reduced. By controlling small, etc., JIS-compliant 3
A SiC-based porous body having a point bending strength of 100 MPa or more can be obtained. In addition, since the carbon component remaining in the SiC-C-based sintered body is mainly generated on the surface of the SiC crystal, a decrease in strength is small even if the carbon component is eliminated.

Furthermore, the porous SiC material of the present invention has a high thermal conductivity, and the excellent thermal conductivity at 20 ° C. is 15 W / m · K or more by controlling the amount of impurities in the raw material powder. . That is, in general, the thermal conductivity of a hexagonal plate-like α-type SiC crystal changes depending on the crystal axis direction, the thermal conductivity is small in the c-axis direction perpendicular to the plate surface, and a is parallel to the plate surface. High in the axial direction. Empirically, the thermal conductivity in the c-axis direction is about 0.7 times that in the a-axis direction (High Temperatures-High Pressur
es, 1997, vol. 29, pages 73-79). For such a reason, in the SiC-based porous body of the present invention, as shown in FIG. 2, heat is mainly conducted along the plate-shaped surface of the hexagonal plate-shaped α-type SiC crystal, so that a high thermal conductivity is obtained. It is thought that it is possible.

As described above, the SiC-based porous material of the present invention
Hexagonal plate-shaped α-type SiC particles have a three-dimensional skeletal structure with sintered necking, high mechanical strength, heat resistance and corrosion resistance, and high porosity, so they are used as various filters with high permeability performance it can. In particular, it is effective as a filter for liquid filtration or a particulate filter for purifying exhaust gas of an automobile diesel engine.

Moreover, since the filter made of the porous SiC material of the present invention has a three-dimensional skeletal structure in which hexagonal plate-like SiC crystals are tightly bound while being entangled with each other, the apparent value measured by the mercury intrusion method is used. Particles having a particle size smaller than the pore size of can be collected. In addition, because of its high thermal conductivity, the collected soot and the like can be efficiently burned when used as a particulate filter for purifying exhaust gas of diesel engines.

[0026]

【Example】Example 1  Samples 1 to 7 shown in Table 1 below have an average particle size of 0.1 μm.
Or a commercially available graphite powder of 200 μm and an average particle size of 0.2 μm
Or 36 μm of Si powder, the amount of Si powder is 30 to 71
It was mixed so as to have a composition of weight%. Samples for comparison
In Nos. 8 to 12, the raw material powder has an average particle size of 0.1 μm or
200 μm SiC powder was used. In addition, SiC powder and black
Table 1 shows the amounts of impurities Al and Fe contained in the lead powder.
As shown.

[0027]

[Table 1] Si particle size C particle sizeAl content (ppm) Fe content (ppm) Si powder amount Mixed raw materialsample (μm) (μm) Si powder C powder Si powder C powder (wt%) Powder specific gravity  1 * 0.2 0.1 120 120 110 110 30 2.214 2 0.2 0.1 120 120 110 110 30 2.214 3 0.2 0.1 120 120 110 110 50 2.212 4 * 0.2 0.1 120 120 110 110 71 2.208 5 36 200 12 10 20 10 50 2.212 6 36 200 12 10 20 10 60 2.209 7 to 10 * SiC powder with an average particle size of 0.1 μm 11 * SiC powder with an average particle size of 200 μm (Note) Samples marked with * in the table are comparative examples.

Using the mixed raw material powders of Samples 1 to 6 shown in Table 1 above, as shown in Table 2 below, molded bodies were formed at various molding pressures in a vacuum at a temperature of 1100 ° C. A
Sintering was performed by heat treatment at 2200 or 2300 ° C. in an r gas atmosphere. Table 2 below shows the density and porosity of each of the obtained sintered bodies together with the density and porosity of the molded body. Also, for the samples 7 to 11 of the comparative example, molding and sintering were performed in the same manner as described above except for the conditions shown in Table 2 below, to produce a conventional SiC sintered body (as it was to become a SiC porous body) and molding. The body and the sintered body were evaluated in the same manner as described above.

[0029]

[Table 2] Molding pressure Molded body density Molded body Sintering temperature Sintered body density Sintered bodysample (MPa) (g / cm ³⁾  Porosity (%) (℃) (g / cm ³⁾  Porosity (%)  1 * 120 1.35 39 2300 1.20 53 2 550 1.80 19 2300 1.90 25 3 120 1.42 36 2300 1.44 49 4 * 120 1.50 32 2300 1.75 46 5 120 1.42 36 2200 1.44 49 6 120 1.45 34 2200 1.65 47 7 * 30 Collapse during molding 8 * 60 Collapse during molding 9 * 100 1.30 59 2300 1.30 59 10 * 320 1.80 44 2300 1.80 44 11 * 420 1.60 50 2200 1.60 50 Note: Samples marked with * in the table are comparative examples.

Thereafter, each of the sintered bodies of Samples 1 to 6 was heated at 500 ° C. in air to eliminate carbon.
Heating was carried out for hours to obtain SiC porous materials. About each obtained SiC porous body, density and porosity, average pore diameter,
The thermal conductivity at 20 ° C. and the three-point bending strength were measured, and the results are shown in Table 3 below. Moreover, about the SiC porous body of the samples 7-11 (the above-mentioned heat treatment for carbon disappearance),
Table 3 also shows the results of the same measurement. The density and porosity of the porous SiC bodies of Samples 7 to 11 are shown in Table 2 above.
Are the same as those of each sintered body shown in FIG.

[0031]

[Table 3] Density Porosity Average pore diameter Thermal conductivity Bending strengthsample (g / cm ³⁾  (%) (μm) (W / m ・ K) (MPa)  1 * Cannot be measured due to collapse during heating 2 0.80 75 0.11 15 77 3 1.04 68 0.23 21 103 4 * 1.75 45 0.18 13 220 5 1.04 68 186 43 19 6 1.25 61 170 67 27 7 * Cannot be measured due to collapse during molding 8 * Cannot be measured due to collapse during molding 9 * 1.30 59 0.20 14 70 10 * 1.80 44 0.16 2 99 11 * 1.60 50 180 23 10 (Note) Samples marked with * in the table are comparative examples.

As can be seen from Table 3, the SiC porous body according to the embodiment of the present invention has a high porosity and
It has excellent mechanical strength and thermal conductivity. However, when the strength is increased, the thermal conductivity decreases, and when the thermal conductivity is increased, the strength tends to decrease. Further, in the comparative example using the SiC powder, if the molding pressure is reduced in order to obtain a high porosity, the material tends to collapse after molding. Even in the sample which did not collapse, the strength and thermal conductivity of the obtained porous SiC body were low.

[0033]Example 2  Using each SiC porous body of Example 1 described above, an outer diameter of 8 mm,
Cut out 0.2mm in thickness and 5mm in length.
A pipe-shaped filter was produced by sealing with a fat. Each of these
The following experiment was performed using an IP filter, and the results were
Are shown in Table 4 below. However, the sample number of this Example 2
Corresponds to the sample number of each porous body in Example 1.
Samples 1, 7, and 8 collapsed on the way.
No tests have been performed.

(1) From the inside to the outside of the pipe-shaped filter, 100 ml of a suspension of polyethylene particles having a particle size of 0.05 to 200 μm (concentration: 10 ppm) was applied at a pressure of 0.1 MPa.
The mixture was filtered through a, and the collection rate of the particles was measured. (2) Pure water was continuously supplied from the inside to the outside of the pipe-shaped filter at a pressure of 0.1 MPa, and the permeation flow rate was measured. (3) After the particles were collected in the above experiment (1), a power of 100 W was applied to both ends of the filter to generate electricity and heat was generated, and the burning time until the collected polyethylene particles were completely burned was measured. .

[0035]

[Table 4]Particle collection rate (%) Permeation flow rate Combustion timesample 0.05μm 0.1μm 40 μm 200 μm (l / sec / m ² ) (sec)  2 100 100 100 100 0.24 55 3 100 100 100 100 0.22 26 4 * 100 100 100 100 0.17 16 5 0 0 100 100 5450 11 6 0 0 100 100 3200 8 9 * 25 41 100 100 0.06 54 10 * 35 45 100 100 0.05 26 11 * 0 0 21 100 770 19 (Note) Samples marked with * in the table are comparative examples.

It can be seen that the filter made of the porous SiC material of the present invention has better permeability than the conventional filter made of porous SiC particles. Further, since the porous SiC body of the present invention has a high thermal conductivity, the burning time of the collected polyethylene particles is short, and therefore, it is understood that the porous body is effective as a particulate filter for a diesel engine.

[0037]

According to the present invention, from the compact of Si powder and carbon powder in which carbon is excessively mixed, Si
By preparing a CS-based sintered body and subjecting it to a heat treatment to eliminate carbon, a SiC-based porous body having a high porosity can be obtained. In addition, since the SiC-based porous body has a skeleton structure in which hexagonal plate-like SiC crystal particles having high thermal conductivity are firmly bonded, it has both high strength and high thermal conductivity.

Therefore, when the porous SiC material of the present invention is used as a filter, it is possible to collect particles having an excellent permeation performance and smaller than the average pore diameter measured by a mercury intrusion method. In addition, since the thermal conductivity is high, the collected soot can be burned efficiently,
It is suitable as a particulate filter for purifying exhaust gas of diesel engines.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view for explaining a production process of a SiC-based porous body of the present invention.
(C) shows the state of the SiC-based porous body.

FIG. 2 is a schematic diagram illustrating a state where heat is transmitted through a porous SiC body of the present invention.

Claims (9)

[Claims] 1. A three-dimensional skeleton structure in which hexagonal plate-like α-type SiC particles are sintered and necked, and have a porosity of 61 to 75%.
A SiC-based porous body, characterized in that: 2. The SiC-based porous body according to claim 1, further comprising a carbon component. 3. The SiC-based porous body according to claim 1, wherein the average pore diameter is 0.1 to 200 μm. 4. The JIS-compliant three-point bending strength is 100MP.
The SiC-based porous body according to any one of claims 1 to 3, wherein the SiC-based porous body is at least a. 5. The SiC-based porous body according to claim 1, wherein the thermal conductivity at 20 ° C. is 15 W / m · K or more. 6. A filter for liquid filtration, comprising the SiC-based porous body according to claim 1. 7. A particulate filter for purifying exhaust gas of an automobile diesel engine comprising the SiC-based porous body according to claim 1. 8. The method for producing a SiC-based porous body according to claim 1, wherein the Si powder and the carbon powder are mixed so that the amount of the Si powder is 30 to 65% by weight of the whole, and the molded body is formed. 2000 to 2400 ° C in an inert gas atmosphere
And obtaining a sintered body composed of SiC and carbon by heating at a temperature of 300 ° C. or more in an oxygen-containing atmosphere to eliminate all or a part of the carbon component. A method for producing a porous SiC body. 9. Fe or Al in Si powder and carbon powder
The method for producing a SiC-based porous body according to claim 8, wherein the amount of impurities is 100 ppm or less.