JPH11130558A - Porous silicon carbide sintered product and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous silicon carbide sintered product having high porosity and excellent strength characteristics, and to provide a method for producing the same. SOLUTION: This porous silicon carbide sintered product having a porosity of 40-80% comprises a composite tissue structure in which silicon carbide granules and a fibrous silicon carbide obtained by silicifying carbon fibers are homogeneously dispersed. This method for producing the porous silicon carbide sintered product comprises mixing the mixture of silicon carbide powder, carbon powder subjected to an oxidation treatment, and silicon powder with 0.5-10 wt.% of carbon fibers whose surfaces are subjected to an oxidation treatment, molding the mixture, subjecting the molded product to a heating treatment at a temperature of 1,800-2,100 deg.C in a non-oxidizing atmosphere and subsequently removing the unreacted silicon, or mixing the mixture of silicon carbide powder and carbon powder subjected to an oxidizing treatment with 0.5-10 wt.% of carbon fibers subjected to an oxidizing treatment, molding the mixture, impregnating the molded product with melted silicon, thermally treating the impregnated product at a temperature of 1800-2100 deg.C in a non-oxidizing atmosphere, and subsequently removing the unreacted silicon.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous silicon carbide sintered body having a high porosity and excellent strength characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art Silicon carbide sintered bodies have been widely used as various structural members for high temperatures because of their excellent heat resistance and corrosion resistance. Further, a porous silicon carbide sintered body having a porous structure of the sintered body is a filter for molten metal,
It is useful in various application fields such as air-permeable heat insulating materials, catalyst carriers, and exhaust gas filters.

As a method for producing a silicon carbide porous body, a method of impregnating an organic porous body having a three-dimensional network structure such as a polyurethane foam with a slurry of SiC, drying the resultant, and then performing a heat treatment to incinerate and remove the organic body. Are known (for example, JP-A-58-122016). Since the porous silicon carbide sintered body manufactured by this method sinters a skeleton of silicon carbide formed by incineration and removal of an organic substance, it has a high porosity of 80% or more. Can be granted. However, there is a disadvantage that the material strength is extremely low.

Further, as a method of manufacturing a porous silicon carbide sintered body, an organic resin binder is added to silicon carbide powder and mixed, and the mixture is formed into a predetermined shape and then fired to produce silicon carbide powder particles. A method of growing grains is also known. For example, Japanese Patent Application Laid-Open No. Hei 3-215374 discloses that the average grain size is 100 to 150 μm, and that the average grain size is within ± 20% of the average grain size.
After adding and mixing a molding binder and a plasticizer to silicon carbide granules having a particle size distribution such that 0% by weight or more is present, the surface layer portions of the granules are crushed and interconnected, and the inside thereof is not crushed. In such a state, a method has been proposed in which molding is performed at a molding pressure that remains in the molded body, followed by sintering.

Further, Japanese Patent Application Laid-Open No. Hei 3-215375 discloses that a compact is formed from a raw material composition obtained by mixing a carbonaceous substance with silicon carbide powder, and the molded body is fired in a non-oxidizing atmosphere to form a silicon carbide. By sintering the powder, a silicon carbide sintered body containing the carbonaceous material dispersed therein is formed, and then the sintered body is heated in an oxidizing atmosphere to thereby form the carbonaceous material in the sintered body. Is disclosed in Japanese Patent Application Laid-Open No. Hei 4-187578, in which a molded body is formed from raw silicon carbide powder in which α-type silicon carbide powder is blended with β-type silicon carbide powder. And a method of firing the formed body to suppress abnormal grain growth of β-type silicon carbide and control the pore diameter.

However, while the porous silicon carbide sintered body produced by these methods has a porosity of about 50%, the bonding of the silicon carbide particles constituting the porous body is made of silicon carbide fine particles. There is a problem that the mechanical strength is small because the growth is based only on the growth, and it is not enough to achieve both the pore characteristics and the strength characteristics.

In order to solve these problems, silicon carbide whiskers are mixed with silicon carbide powder having an average particle diameter of 30 μm or less at a ratio of 30 to 50% by weight, and after molding, at a temperature of 1900 to 2100 ° C. in a non-oxidizing atmosphere. (Japanese Patent Application Laid-Open No. 7-300364) has been developed by one of the present applicants, and further improved by using α-type silicon carbide powder as silicon carbide powder. An invention has been proposed (JP-A-9-52780).

[0008]

The above-mentioned JP-A-7-30
According to the methods disclosed in JP-A-0364 and JP-A-9-52780, a porous silicon carbide sintered body having high porosity and high strength can be manufactured, but expensive silicon carbide whiskers are used as a silicon carbide raw material. However, there is a disadvantage that it cannot be manufactured at a low cost because of the problem. As a method for increasing the strength of dense reaction sintered silicon carbide, a carbon fiber coated with glassy carbon on its surface is mixed with a SiC material, impregnated with silicon, and then reaction-sintered to obtain a fiber-reinforced silicon carbide material. Is known (JP-A-60-246265).
No.). In the reaction-sintered silicon carbide, carbon is silicified into silicon carbide. Therefore, mixing inexpensive carbon fibers is a preferable method for increasing strength. However, since carbon fibers are easily entangled and have poor wettability with water, dispersion of carbon fibers poses a problem. For this reason, Japanese Patent Application Laid-Open
According to the method disclosed in Japanese Patent Publication No. 0-246265, it is difficult to select a solvent for dispersing the carbon fiber, and it is difficult to form a homogeneous composite structure with insufficient dispersion of the carbon fiber. is there.

The present inventors have conducted research to solve the above-mentioned difficulties, and as a result, have found that high-porosity silicon carbide whiskers are replaced with fibrous silicon carbide obtained by silicifying carbon fibers whose surfaces are oxidized. It has been found that it is possible to achieve both high modulus and high strength characteristics. By oxidizing the surface of the carbon fiber, the wettability with water can be improved, so that a uniform composite structure can be formed. The present invention has been developed based on this finding, and an object of the present invention is to provide a porous silicon carbide sintered body having high porosity and high material strength, and a method for producing the same.

[0010]

In order to achieve the above object, a porous silicon carbide sintered body according to the present invention is characterized in that fibrous silicon carbide obtained by silicifying carbon fibers whose surface is oxidized and silicon carbide particles are uniform. And a porosity of 40 to 80%.

Further, the method for producing a porous silicon carbide sintered body of the present invention is characterized in that a carbon powder having a surface oxidized with respect to a mixed powder of silicon carbide powder, oxidized carbon powder and silicon powder is mixed with 0.5 to 10%. After mixing and adding the mixture at a ratio of 1% by weight, the mixed powder is formed into a predetermined shape, and then heat-treated at a temperature of 1800 to 2100 ° C. in a non-oxidizing atmosphere to silicify and sinter the carbon powder and carbon fibers, Then, unreacted silicon is removed, and another manufacturing method is to use a carbon powder having a surface oxidized in a ratio of 0.5 to 10% by weight to a mixed powder of silicon carbide powder and oxidized carbon powder. The mixed powder is molded into a predetermined shape and brought into contact with molten silicon to impregnate the molded body with molten silicon. Heat treatment to a temperature of ℃ sintered while silicide carbon powder and carbon fiber,
Then, unreacted silicon is removed.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The fibrous silicon carbide constituting the porous silicon carbide sintered body of the present invention is formed by reacting carbon fibers with silicon, and the silicon carbide particles are composed of silicon carbide powder and carbon powder. Shows a dense and strong tissue structure in which silicon carbide formed by reacting with silicon is mutually bonded. The carbon powder used for the porous silicon carbide sintered body of the present invention is oxidized, and the surface of the carbon fiber is also oxidized. The method of oxidizing the carbon powder and the carbon fiber can be performed by, for example, immersing in a nitric acid solution, heating in the air, or contacting with ozone. Oxidation treatment allows good compatibility with water, alcohol, and organic binders as a solvent, and carbon powder is locally hardened, and carbon fibers are uniformly dispersed in a molded body without being entangled, thereby increasing the material strength. It is. Further, pores are generated when the carbon fiber and the carbon powder are silicified into silicon carbide, and pores are generated when unreacted silicon is removed, so that the porosity can be increased. Thus, the porous silicon carbide sintered body of the present invention can be a porous body having a high porosity of 40 to 80% and a large strength characteristic.

As a raw material system for producing the above porous silicon carbide sintered body, silicon carbide powder, oxidized carbon powder, silicon powder and carbon fiber whose surface is oxidized are used. The crystal form of the silicon carbide powder to be used is not limited, and any of α-type and β-type can be used. Fine powder having an average particle diameter of 30 μm or less, preferably 0.5 to 10 μm is used. As the carbon powder, an appropriate carbonaceous powder such as graphite powder, glassy carbon powder, and carbon black is used after being oxidized, and a powder having an average particle diameter of 50 μm or less is preferable. Silicon powder has an average particle size of 5
Metallic silicon powder of 0 μm or less is used.

The carbon fiber used is not particularly limited, and various carbon fibers such as polyacrylonitrile type, rayon type and pitch type are used after oxidizing the surface.
The silicon carbide powder, the oxidized carbon powder and the silicon powder are added and mixed at a ratio of 0.5 to 10% by weight with respect to the mixed powder. The addition ratio of carbon fiber is 0.
If the content is less than 5% by weight, the volume fraction in the composite structure of the fibrous silicon carbide silicified into silicon carbide is small, and sufficient strength characteristics cannot be provided. On the other hand, if it exceeds 10% by weight, it becomes difficult to uniformly entangle in the composite structure, even though the surface is oxidized. It is more preferable that the ratio of the silicon powder in the mixed powder of the raw materials is set to be equal to or more than the chemical equivalent for silicifying the carbon powder and the carbon fiber.

The mixed powder is dispersed in a solvent such as water or alcohol together with an organic binder for molding to form a uniform slurry, and if necessary, the solvent is dehydrated and then molded into a predetermined shape. As a molding binder, polyvinyl alcohol, methylcellulose, carboxymethylcellulose and the like are used. The mixed slurry is molded by a known molding means such as casting, dewatering, extrusion molding, and press molding, dried, and then dried in a non-oxidizing atmosphere such as nitrogen, argon, or vacuum at 1800 to 2100 ° C.
And sintering.

In the heat treatment process, the silicon powder in the mixed powder is melted, reacts with the carbon powder and the carbon fiber to silicide, and is converted into silicon carbide powder and fibrous silicon carbide. The silicon carbide powder converted in this manner forms silicon carbide particles mutually bonded together with the silicon carbide powder used as a raw material in the mixed powder, and furthermore, by the combined effect of the fibrous silicon carbide in which the carbon fibers are converted to silicon carbide. A strong sintered body can be obtained. If the heat treatment temperature is lower than 1800 ° C., a sufficient sintered body cannot be obtained, and if the temperature exceeds 2100 ° C., sublimation of silicon occurs and silicon required for the reaction is lost. The silicon powder added in excess of the chemical equivalent remains in the sintered body, but the unreacted excess silicon is dissolved and removed with molten sodium hydroxide or hydrogen fluoride or at a temperature of 2100 ° C. or more. Heat treatment and evaporation to remove volatiles.

In the porous silicon carbide sintered body thus manufactured, the fibrous silicon carbide is obtained because the oxidized carbon powder and the carbon fibers whose surface is oxidized are uniformly dispersed in the molded body. And silicon carbide particles are firmly bonded to form a composite structure, so that the material strength is improved, and the pores and unreacted silicon generated when the carbon fibers and carbon powder are silicified into silicon carbide are reduced. The porosity is increased by the porosity generated during the removal. Thus, it is possible to manufacture a porous silicon carbide sintered body having a high porosity of 40 to 80% and a large strength characteristic.

According to another method of the present invention for producing a porous silicon carbide sintered body, a carbon powder having a surface oxidized by 0.5 to 10 wt. %, A molded body having a predetermined shape is prepared by using the mixed powder as a raw material, and the molded body is heated to a temperature of 1800 to 2100 ° C. in a non-oxidizing atmosphere such as nitrogen, argon or vacuum. Heat treatment,
The molded body is impregnated with the silicon melt by being brought into contact with the molten silicon melt, and carbon powder and carbon fibers are silicified by the impregnated silicon melt to be converted into silicon carbide powder and fibrous silicon carbide and sintered. Things. The addition ratio of carbon fiber in the mixed powder raw material is 0.5
If the content is less than 10% by weight, the volume fraction in the composite structure of the fibrous silicon carbide silicified to silicon carbide is small, and sufficient strength characteristics are not provided. If the content exceeds 10% by weight, the surface is oxidized. However, it becomes difficult to entangle in the composite structure and uniformly disperse it.

The silicon melt impregnated in the compact converts carbon powder and carbon fibers into silicon carbide particles and fibrous silicon carbide during heat treatment. The converted silicon carbide and the silicon carbide powder in the mixed powder are mutually bonded during the heat treatment to form a strong bond, and are combined with the converted fibrous silicon carbide to produce a strong sintered body. If the heat treatment temperature is lower than 1800 ° C., sintering does not proceed sufficiently,
At a temperature exceeding 00 ° C., sublimation of silicon occurs and silicon required for the reaction volatilizes and disappears.

Unreacted surplus silicon among the impregnated molten silicon is dissolved and removed with molten sodium hydroxide, hydrogen fluoride, or the like, or is evaporated and removed by heat treatment at a temperature of 2100 ° C. or more. In this way, a porous silicon carbide sintered body having excellent strength characteristics and having a composite structure structure in which fibrous silicon carbide and silicon carbide particles are firmly bonded and uniformly dispersed is produced. Further, the porosity is increased by the porosity generated when the carbon fiber and the carbon powder are silicified into silicon carbide and the porosity generated when the unreacted excess silicon is removed, and the large strength and porosity are increased by 40%.
It is possible to manufacture a porous silicon carbide sintered body having a high porosity of up to 80%.

[0021]

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

Example 1 50 parts by weight of silicon carbide powder having an average particle diameter of 2 μm, 12 parts by weight of ozone-oxidized graphite powder having an average particle diameter of 15 μm,
Carbon fiber whose surface was subjected to ozone oxidation treatment was added at a ratio of 0.5% by weight to a mixed powder mixed at a ratio of 38 parts by weight of a silicon powder having an average particle diameter of 20 μm and mixed. This mixed powder was put into a water / ethanol mixed solution containing 8% by weight of methylcellulose (water: ethanol = 8: 2 ratio) and stirred to prepare a mixed slurry. After removing excess water and ethanol from the mixed slurry by a filter press, the mixture was extruded to form a plate having a length and width of 47 mm and a thickness of 1 mm. After the compact was sintered at a temperature of 1900 ° C. for 1 hour in a nitrogen atmosphere, the sintered body was immersed in sodium hydroxide heated and melted at a temperature of 400 ° C. to dissolve and remove unreacted silicon. . The porosity and three-point bending strength of the obtained porous silicon carbide sintered body were measured and are shown in Table 1 together with the amount of carbon fiber added. The porosity was measured by the Archimedes method.

Examples 2 to 4 and Comparative Examples 1 to 3 A porous silicon carbide sintered body was manufactured in the same manner as in Example 1 except that the mixing ratio of graphite powder, carbon fiber and silicon powder was changed. , Porosity and three-point bending strength were measured. The obtained results are shown in Table 1.

Comparative Example 4 A mixed powder was prepared in the same mixing ratio as in Example 2 except that graphite powder not subjected to oxidation treatment was used, and a porous silicon carbide sintered body was manufactured by the same method as in Example 1. Then, the porosity and the three-point bending strength were measured. The obtained results are shown in Table 1.

Comparative Example 5 A mixed powder was prepared in the same mixing ratio as in Example 2 except that carbon fiber not subjected to oxidation treatment was used, and a porous silicon carbide sintered body was manufactured by the same method as in Example 1. Then, the porosity and the three-point bending strength were measured. Table 1 shows the obtained results.
It was also attached to.

Comparative Example 6 A mixed powder was prepared in the same mixing ratio as in Example 2 except that graphite powder and carbon fiber which were not subjected to oxidation treatment were used, and porous silicon carbide was sintered in the same manner as in Example 1. The body was manufactured and its porosity and three-point bending strength were measured. The obtained results are shown in Table 1.

Example 5 The surface was oxidized at a ratio of 0.5% by weight with respect to a mixture of 80 parts by weight of silicon carbide powder having an average particle diameter of 2 μm and 20 parts by weight of ozone-oxidized graphite powder having an average particle diameter of 15 μm. The treated carbon fibers were added. This mixed powder is put into a water / ethanol mixed solution containing 8% by weight of methylcellulose (water: ethanol = 8: 2 ratio) and stirred,
A mixed slurry was prepared. After removing excess water and ethanol from the mixed slurry by a filter press, the mixture was extruded to form a plate having a length and width of 47 mm and a thickness of 1 mm. One end of the molded body was brought into contact with a graphite crucible filled with metallic silicon sufficient for silicidation, kept at a temperature of 2100 ° C. for 1 hour in a nitrogen atmosphere, impregnated and sintered, and then sintered at 400 ° C.
The unreacted silicon was dissolved and removed by immersion in sodium hydroxide heated and melted at a temperature of. The porosity and three-point bending strength of the obtained porous silicon carbide sintered body were measured and are shown in Table 1 together with the amount of carbon fiber added. The porosity was measured by the Archimedes method.

[0028]

[Table 1]

From the results shown in Table 1, it can be seen that the porous silicon carbide sintered body of the example has higher bending strength and higher porosity than the porous silicon carbide sintered body of the comparative example. In particular, according to the porous silicon carbide sintered body of the example, it has a porosity of 40% or more and a bending strength of 50 MPa or more, and achieves both porosity and strength characteristics. Is admitted.

[0030]

As described above, the porous silicon carbide sintered body of the present invention has a strong composite structure in which fibrous silicon carbide in which carbon fibers are silicified and silicon carbide particles are uniformly dispersed. It has both high porosity and excellent strength characteristics. Further, according to the production method, it is possible to easily produce by adding and mixing the oxidized carbon powder and the carbon fiber whose surface is oxidized at a specific ratio to the raw material powder and subjecting to heat treatment. Therefore, the present invention is extremely useful as a porous silicon carbide sintered body used for an exhaust gas filter, a molten metal filter, a gas permeable heat insulating material, a catalyst carrier, and the like, and a method for producing the same.

Claims (3)

[Claims] 1. A porous material having a composite structure structure in which fibrous silicon carbide obtained by silicifying carbon fibers whose surfaces are oxidized and silicon carbide particles are uniformly dispersed, and having a porosity of 40 to 80%. Silicon carbide sintered body. 2. A mixed powder of silicon carbide powder, oxidized carbon powder and silicon powder is mixed with 0.5 to 10% by weight of carbon fiber whose surface is oxidized, and the mixed powder is subjected to a predetermined process. After forming into a shape, carbon powder and carbon fibers are silicified and sintered by heating at a temperature of 1800 to 2100 ° C. in a non-oxidizing atmosphere, and then unreacted silicon is removed. 2. The method for producing a porous silicon carbide sintered body according to 1. 3. A mixed powder of silicon carbide powder and oxidized carbon powder is mixed with carbon fibers whose surface is oxidized at a ratio of 0.5 to 10% by weight, and the mixed powder is formed into a predetermined shape. And contact with the molten silicon to impregnate the molded body with the molten silicon, and then in a non-oxidizing atmosphere
2. A carbon powder and carbon fibers are silicified and sintered by heating at a temperature of 800 to 2100 [deg.] C., and then unreacted silicon is removed.
A method for producing a porous silicon carbide sintered body according to the above.