JPH111372A - Silicon carbide-based porous body and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce high-purity and homogeneous silicon carbide-based porous body in various forms, high yield and relative short arbitrary process days. SOLUTION: A binder comprising a silicon alkoxide, an aluminum alkoxide, water adjusted to pH<=3 and at least one kind of organic solvent soluble in water and having >=120 deg.C boiling point is added and mixed with silicon carbide powder having 1-500 μm average particle diameter to form slurry. The slurry is subjected to gelation and formed. The resultant formed product is dried and baked.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silicon carbide porous body and a method for producing the same, and more particularly to a silicon carbide porous body formed by utilizing gelation and a method for producing the same.

[0002]

2. Description of the Related Art Porous ceramics are porous materials for firing electronic components, lightweight structural materials such as floors and walls, heat insulating materials for refrigerators and buildings, biological materials such as artificial aggregates, and filters for air, water and the like. It is used for various purposes as a material. Conventionally,
When used as a filter material in a semiconductor manufacturing process, there is a problem in that a ceramic porous body is liable to be charged with static electricity and easily adheres to dust and becomes a contamination source of a semiconductor. For this reason, it has been practiced to form a silicon carbide porous body using conductive silicon carbide as a raw material.

[0003]

However, in the conventional method of forming a porous body using silicon carbide powder as a raw material, there has been no study on the purity of the obtained porous silicon carbide. Further, the shape that can be formed has been limited. For example, in extrusion molding, it was difficult to form a large flat plate, even though a tube could be formed. In addition, since a large amount of a polymer binder is used in molding, impurities are contaminated as a result, and post-treatment such as degreasing becomes complicated, thereby constituting a semiconductor contamination source. As described above, there are various problems in actually using the porous body of silicon carbide manufactured by the conventional method. The present invention, in view of the current state of the silicon carbide-based porous body, any shape, particularly if it is a flat silicon carbide-based porous body, regardless of the size and shape, high precision, high purity, and relatively The purpose is to produce at a high yield in a short number of process days.

[0004]

According to the present invention, there is provided a silicon carbide-based porous body characterized in that mullite particles are uniformly dispersed between silicon carbide particles. Is done. Further, the present invention provides (1) an average particle size of 1 to 50.
0 μm silicon carbide powder, silicon alkoxide,
30-100 parts by weight of aluminum alkoxide, water adjusted to pH 3 or less and water-soluble and at least one organic solvent having a boiling point of 120 ° C. or more are added and mixed to form a slurry by mixing and forming a slurry. Process,
(2) a shaping step of gelling and shaping the slurry generated in the slurry generating step; and (3) drying the shaped body obtained in the shaping step, under reduced pressure, in an inert gas atmosphere, and in a reducing gas atmosphere. 1600-2 in any of the following
A method for producing a silicon carbide-based porous body, comprising a firing step of firing at 000 ° C.

[0005] The present invention is constituted as described above, a binder containing a predetermined alkoxide is added to silicon carbide powder to form a slurry, the obtained slurry is formed into a predetermined shape, and the alkoxide in the slurry is removed. Since a porous body is obtained by hydrolyzing and gel-forming to obtain a molded body and then firing, a porous molded body of any shape can be obtained. Further, since a polymer binder is not used, a porous body with high purity can be obtained. Further, the gelation of the slurry is performed by gel-forming while silicon carbide particles and gel molecules of mullite particles obtained by hydrolysis of alkoxide are uniformly dispersed, and by adjusting the pH value of the slurry, the gel is formed. The gelation time can be adjusted by adjusting the aging time, and the gel can be formed in a relatively short process time.Also, since it contains an organic solvent with a higher boiling point than water, it does not crack even during drying and firing, and has a high yield. Thus, moisture in the gel molecules is smoothly evaporated and evaporated, and a uniform porous silicon carbide body can be obtained.

[0006]

Embodiments of the present invention will be described below in detail. The silicon carbide powder as a starting material of the present invention has an average particle size of 1 to 500 μm, preferably 10 to 1 μm.
What is 00 μm is used. If the average particle size is smaller than 1 μm, the pore diameter of the molded product becomes small, so that it is not preferable that bubbles are hardly emitted during calcination. In addition, the average particle size is 5
If it is larger than 00 μm, the homogeneity and fluidity of the slurry decrease, making it difficult to pour the slurry into a mold.

The binder of the present invention uses a mixture obtained by uniformly mixing a silicon alkoxide, an aluminum alkoxide, water adjusted to pH 3 or less, and an organic solvent soluble in water and having a boiling point of 120 ° C. or more. It can have both functions of a binder and a dispersion medium. Further, silicon alkoxide and aluminum alkoxide are not compatible with water, and if necessary, a compatible solvent such as alcohol may be further added and mixed. The silicon alkoxide and the aluminum alkoxide constituting the binder of the present invention are not particularly limited, and can be appropriately selected from readily available ones. Usually, Si (OCH
₃ ) ₄ , Si (OC ₂ H ₅ ) ₄ , Si (i-OC ₃ H ₇ ) ₄ , A
l (OCH ₃ ) ₃ , Al (OC ₂ H ₅ ) ₃ , Al (i-OC
₃ H ₇ ) _{3 and the} like are preferably used. It is preferable that the silicon alkoxide and the aluminum alkoxide in the binder are added in a mullite composition, that is, in a ratio of Si: Al = 1: 3 in atomic ratio. In addition, water as a component of the binder is added and mixed by appropriately adjusting the pH to 3 or less with an acid in order to hydrolyze the silicon alkoxide and the aluminum alkoxide. The acid for pH adjustment is not particularly limited, but preferably an acid such as hydrochloric acid, nitric acid or acetic acid which volatilizes during heating and hardly remains. The amount of water in the binder is preferably equal to or greater than the hydrolysis equivalent of the alkoxide and equal to or less than 30 times the same. If the amount is less than the hydrolysis equivalent of the alkoxide, the hydrolysis does not proceed sufficiently. On the other hand, if it is more than 30 times the equivalent, gelation is slowed down, the strength of the formed gel is reduced, and there is a problem in work.

The organic solvent constituting the binder of the present invention is soluble in water and can be present in the molded article even after the alcohol formed by the hydrolysis of water and alkoxide is evaporated. Selected. This is to prevent cracks due to surface tension when a liquid such as water in the gelled molded body evaporates at the time of elevating the temperature of drying and firing the molded body. The alcohol formed by hydrolysis of the alkoxide of the lower alcohol as described above has a boiling point lower than that of water, and does not evaporate first and remain in the molded body. Therefore, the organic solvent in the binder of the present invention only needs to have a boiling point higher than that of water.
What is necessary is just to have the above boiling point. However, the boiling point of water is 100 ° C
In order to remain in the molded article, it is preferable that the vapor pressure is lower and the boiling point is 120 ° C. or higher. Usually, ethylene glycol, glycerin and the like are used. The organic solvent only needs to satisfy the above conditions, and two or more organic solvents may be mixed and added.

In the present invention, a molding slurry can be prepared by adding and mixing a predetermined amount of the above-mentioned binder and silicon carbide powder. The mixing ratio of the silicon carbide powder and the binder depends on the particle size of the silicon carbide, but the binder is appropriately added to the silicon carbide powder in a range of 30 to 100 parts by weight according to the particle size. Is preferred. If the amount is less than 30 parts by weight, mixing becomes difficult. If the amount is more than 100 parts by weight, the density of the molded article becomes too low, which is inappropriate for operation. Generally, when the particle size of the silicon carbide powder is small, it is preferable to increase the amount of the binder added. This is because the specific surface area increases. A molding slurry can be obtained as described above. In the present invention, the preparation of the slurry for molding may be performed by simultaneously adding and mixing all of the raw materials such as silicon carbide powder, silicon alkoxide, aluminum alkoxide, water having a pH of 3 or less, and an organic solvent. The addition of a predetermined raw material may be completed before the alkoxide gelled.

In the present invention, the pH of the molding slurry prepared as described above is preferably adjusted to 3.5 to 5.5. The time required for gelation varies depending on the pH value of the slurry. For example, the gelation time at pH 3.5 at 20 ° C. is about 30 hours, which is high in workability for producing a porous body. Time. Generally, the higher the pH value, the shorter the time required for gelation.
However, when the pH value is higher than 5.5, for example, it is 1 hour or less, and the gelation time is too short, which is inconvenient in operation. Accordingly, the pH value of the molding slurry is adjusted to 3.5 to 5.
Adjustment to the range of 5 is preferable because a gel time suitable for a porous body production operation can be obtained. The pH of the molding slurry is usually adjusted by adding a basic aqueous solution. In this case, it is preferable to use an aqueous solution of an organic base such as ammonia which evaporates and hardly remains during heating, and sodium hydroxide and potassium hydroxide are not preferred.

The pH-adjusted molding slurry is cast into a molding die and then left in a sealed state to form a gel. The gelation time varies depending on the pH value of the slurry as described above. The material of the molding die is not particularly limited, but a material that hardly absorbs the organic solvent in the slurry,
Further, a material having a large contact angle with an organic solvent is preferable. Further, a material that can be precision finished is more preferable. Specific examples of the material of the mold include a fluororesin, an ultra-high molecular weight polyethylene resin, and a resin or the like coated on the surface of a metal or the like. Since the gelation of the slurry shrinks with time, it is preferable to release the mold after the gelation is completed. It is preferable that at least the mold part in the shrinking direction is released. Immediately after the completion of gelation, the molded body has low strength, and it is desirable to minimize the exposure time of the molded body, except that it is exposed to an open atmosphere for a short time during mold release. It is preferable to increase the strength. In addition, by heating and raising the temperature at the time of standing in a sealed state, the strength development time can be shortened. When heating and raising the temperature, it is necessary to pay attention to the heating temperature and the like so that the organic solvent does not volatilize from the molded body, and the container in which the molded body is allowed to stand still needs to have a pressure resistance that can withstand the vapor pressure under heating. is there.

The molded body obtained as described above, which has been formed into a gel and exhibits strength, is dried by heating at about 100 to 600 ° C. in an oxygen-containing gas atmosphere such as the atmosphere. The rate of temperature rise until the boiling point of the organic solvent in the slurry during heating during drying is preferably 200 ° C./hour or less, more preferably 1 ° C./hour or less.
The temperature is preferably set to 00 ° C./hour or less. If the temperature exceeds 200 ° C./hour, the organic solvent is rapidly volatilized to cause cracks in the molded product. In the present invention, the drying can be prevented and the purity can be maintained by performing the drying by burying the formed body formed by gelation and strength development in a filling powder such as a coarse silicon carbide powder or a carbon powder. . Next, the dried compact is fired under reduced pressure, in an inert gas atmosphere such as argon, or in a reducing gas atmosphere. The firing temperature varies depending on the particle size of the silicon carbide powder, but is about 1600 to 2000.
It is in the range of ° C. The rate of temperature rise in the firing step is preferably 1000 ° C./hour or less, more preferably 600 ° C./hour or less. If the rate of temperature rise exceeds 1000 ° C./hour, closed pores rapidly occur, and the sintered body swells, which is not preferable.

In the present invention, the formed body obtained by the completion of the gelation of the slurry usually has an upper part that has almost no ceramic particles such as silicon carbide particles because silicon carbide particles and mullite particles settle during the gelation. It consists of a gel portion and a lower gel portion where ceramic particles such as precipitated silicon carbide particles contact each other. Therefore, the mold of the present invention is preferably designed in consideration of the formation of the upper gel portion without ceramic particles due to gelation in advance. The upper gel portion without ceramic particles may be cut and removed before the drying step, or when heated and dried and fired as it is, it can be separated and removed during those steps due to the difference in shrinkage ratio between the upper and lower parts.

[0014]

EXAMPLES The present invention will be described in more detail based on examples. However, the present invention is not limited to the following examples. Example 25 parts by weight of ethyl silicate as an alkoxide, 37 parts by weight of aluminum propoxide, 20 parts by weight of a 0.01 N hydrochloric acid aqueous solution as water adjusted to pH 3 or less, and 20 parts by weight of an organic solvent as raw material silicon carbide powder having an average particle diameter of 30 μm 5 parts by weight of ethylene glycol (boiling point: 197.5 ° C.) was added, and the mixture was stirred for 1 hour to obtain a molding slurry. The obtained slurry for molding was poured into a cylindrical mold shown in FIG. 1 made of Teflon, sealed with the lid 3 and kept as it was for 5 days to gel-form the slurry in the mold. The closed container includes an outer mold 1, an inner mold 2 housed inside the outer mold 1, and a lid 3 for sealing the inside. Thereafter, the upper gel portion 5 without ceramic particles was removed, and the gel was further maintained in a closed container for 10 hours to develop the gel strength. The resulting gel compact having increased strength was buried in an 80-mesh silicon carbide powder filling, dried in an air atmosphere in an electric furnace to evaporate the solvent. At this time, the temperature was raised from room temperature to 600 ° C. in 6 hours, kept at 600 ° C. for 3 hours, allowed to cool naturally, and the molded dried body was taken out of the electric furnace. The dried body is placed in a vacuum sintering furnace at a reduced pressure of 0.3 torr for 30 minutes.
The temperature was raised to 1850 ° C. at a rate of 0 ° C./hour, and 1850 ° C.
Sintering was carried out at a temperature of 3 ° C. for 3 hours. As a result, a silicon carbide porous body having an average pore diameter of 10 μm and a porosity of 32% was obtained.

[0015]

The silicon carbide porous body of the present invention has a high purity and does not contain a polymer binder, and is uniformly porous with silicon carbide particles and mullite particles being uniformly dispersed. In addition, the method for producing a silicon carbide porous body of the present invention generates a raw material slurry for molding and cast-molds it, so that it can correspond to various shapes. Further, by adjusting the pH value of the slurry, it is possible to adjust the number of treatment days required for the formation of the gel, and it is possible to produce a desired number of process days and a relatively short number of treatment process days according to each condition. Furthermore, since the slurry contains an organic solvent having a boiling point of 120 ° C. or higher in a binder mixed with the raw material silicon carbide powder, there is no occurrence of defects such as cracks and cracks during drying and firing, and a high yield is produced. can do.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a molding die used in the production method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Outer die 2 Inner die 3 Lid 4 Molded body 5 Gel part

 ──────────────────────────────────────────────────の Continuing from the front page (72) Inventor Masatoshi Onishi 30 Soya, Hadano-shi, Kanagawa Prefecture Toshiba Ceramics Co., Ltd.

Claims (3)

[Claims] 1. A silicon carbide-based porous body characterized in that mullite particles are uniformly dispersed between silicon carbide particles. 2. (1) Silicon alkoxide, aluminum alkoxide, water adjusted to pH 3 or less, and at least one organic compound soluble in water and having a boiling point of 120 ° C. or more with respect to silicon carbide powder having an average particle diameter of 1 to 500 μm. A slurry forming step of forming a slurry by adding and mixing 30 to 100 parts by weight of a binder composed of a solvent, (2) a forming step of gelling and forming the slurry generated in the slurry forming step, and (3) the forming step A silicon carbide-based porous body characterized by comprising a firing step of drying the molded body obtained in the molding step and then firing at 1600 to 2000 ° C. under reduced pressure, under an inert gas atmosphere or under a reducing gas atmosphere. How to make the body. 3. The method for producing a silicon carbide-based porous body according to claim 2, wherein the molded body obtained in the molding step is dried in a heat-resistant packing powder.