JPH0351678B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing silicon carbide whiskers, and more particularly, to a method for producing acicular silicon carbide whisker crystals with high purity and excellent linearity at a high yield. BACKGROUND ART Silicon carbide whiskers are lightweight, have high strength, and high elasticity, and have recently been expected to be used in various ways as reinforcing materials for composite materials. Conventionally, methods for producing such silicon carbide whiskers include a gas phase synthesis method in which one or both of a carbon-containing raw material and a silicon-containing raw material are supplied in a gas phase to a predetermined high-temperature reaction zone in a reactor, and a carbon It is broadly divided into solid-phase synthesis methods that use solids as raw materials containing silicon and silicon-containing raw materials. The gas phase synthesis method is described in, for example, Japanese Patent Publication No. 50-18480, Japanese Patent Publication No. 52-28757, Japanese Patent Publication No. 52-28759, etc., and the solid phase synthesis method is described, for example, in 1978-
Publication No. 20799, Japanese Patent Application Laid-Open No. 1983-45918, Japanese Patent Application Publication No. 1983
-Described in Publication No. 145700, etc. The conventional gas phase synthesis method described above is generally advantageous in obtaining needle-shaped crystals, but on the other hand, the utilization rate of the gas phase raw material is extremely low, and the gas phase raw material decomposes in the reactor, causing the reaction to occur. It is unsuitable for mass production of silicon carbide whiskers because it has problems such as the furnace being contaminated with these decomposed products and furthermore, these decomposed products being mixed into the produced silicon carbide whiskers. On the other hand, according to the conventional solid-phase synthesis method, silicon carbide is directly produced by mixing raw material powder containing silicon and raw material powder containing carbon, heating the mixture, and directly producing silicon carbide mainly through a solid-phase reaction between these powders. so that
Generates a lot of powdered, granular, and bent silicon carbide,
It is difficult to obtain silicon carbide whiskers with excellent linearity, and it is also difficult to separate and remove irregularly shaped silicon carbide as described above from the obtained silicon carbide whiskers. Therefore, the present inventors have already proposed a method in which a carbon powder is used as the carbon-containing raw material and a molded body having a predetermined shape containing silicon dioxide is used as the silicon-containing raw material. According to this method, silicon as a reaction residue of the compact does not mix with the silicon carbide whiskers produced, and therefore, the silicon carbide whiskers produced contain silicon and/or dioxide by-product during the production. Only a trace amount of silicon is mixed in. In addition, carbon as a reaction residue is
After the reaction is completed, the reaction product can be easily removed by burning it in the atmosphere, so that highly pure silicon carbide whiskers can be obtained. The present inventors have further researched the effects of the carbon-containing raw material on the silicon carbide whiskers produced in the above method, and have found that by using carbon black having predetermined properties as the carbon-containing raw material, In particular, it was discovered that acicular silicon carbide whisker crystals with excellent linearity and size uniformity can be obtained in high yield, leading to the present invention. Means for Solving the Problems The method for producing silicon carbide whiskers according to the present invention includes a molded article containing silicon dioxide, a BET specific surface area of 100 m ² /g or more, an average particle diameter of 35 nm or less,
and carbon black having a bulk density of 0.06 to 0.2 g/cm ³ are heated to a temperature of 1400 to 1700° C. in a hydrogen gas atmosphere. In the method of the present invention, the silicon dioxide-containing molded body is defined as a silicon dioxide-containing raw material such as silica powder, powdered silica gel, various amorphous silicas, precipitated silica, clay, etc., by an appropriate means, e.g. A three-dimensional solid that is formed by extrusion molding, press molding, granulation, etc. and given the shape of a plate, rod, tube, particle or sphere, container or box, linear shape, or a combination of these. . When this molded body is a molded body such as a tubular or box-shaped container, it can also serve as a container for filling the solid carbon-containing raw material. In order to obtain silicon carbide whiskers at a high yield, the silicon dioxide-containing molded article preferably contains silicon dioxide in an amount of 30% by weight or more, particularly preferably 40% by weight or more. In the present invention, carbon black having a BET specific surface area of 100 m ² /g or more, an average particle diameter of 35 nm or less, and a bulk density of 0.06 to 0.2 g/cm ³ is used as the carbon-containing raw material in the present invention. In the method of the present invention, silicon carbide whiskers are obtained by heating the silicon dioxide-containing molded body and carbon black to a predetermined temperature in a limited space such as an electric furnace in a hydrogen gas atmosphere. It is. In the method of the present invention, silicon carbide whiskers are thought to be mainly produced by the following reaction. However, the present invention is not limited in any way by the reaction mechanism. C(s)+2H ₂ (g)→CH ₄ (g) (1) SiO ₂ (s)+CH ₂ (g) →SiO(g)+CO(g)+2H ₂ (g) (2) SiO ₂ (g) 2C (s) → SiC (s) + CO (g) (3) That is, first, hydrogen gas and solid carbon raw material react
Methane gas is produced by (1), and silicon monoxide gas is produced by reaction (2) on the surface of the silicon dioxide-containing molded article. Next, silicon carbide is produced by reaction (3) between this silicon monoxide gas and carbon. Therefore, the overall reaction formula is expressed as: SiO ₂ (s) + 3C (s) → SiC (s) + 2CO (g) (4). In the method of the present invention, the silicon dioxide-containing molded body and carbon black are usually filled in a suitable reaction vessel and heated in a hydrogen atmosphere in a reaction furnace. FIG. 1 shows an example of this, in which plate-shaped silicon dioxide-containing molded bodies 2 are placed in parallel at intervals in a reaction vessel 1, and carbon black 3 is placed in the gap between them.
is filled. The reaction vessel may be made of, for example, alumina or high-purity carbon. In the reaction formula (4), hydrogen is not involved in the production of silicon carbide, but since hydrogen is essential for the methane gasification reaction (1) of carbon, carbon has high reactivity with hydrogen gas. This is required. On the other hand, reaction (2) proceeds almost uniformly throughout the carbon-filled space, and as a result, silicon carbide whiskers are produced throughout this space, while hardly any silicon carbide whiskers are produced outside this space. . Therefore,
In the method of the present invention, it is important to maintain the silicon monoxide produced from the compact at a predetermined concentration in the carbon-filled reaction space, as well as reaction conditions such as reaction temperature, atmospheric gas, and catalyst. ,
It is necessary that the carbon has an agglomerated structure with appropriate voids. As described above, in the above reaction, hydrogen gas makes an important contribution to the production of silicon carbide whiskers, and according to the present invention, hydrogen gas in the atmosphere in the reaction zone is always kept at 70% or more. , it is possible to significantly increase the yield of silicon carbide whiskers and to significantly increase their acicularity. To make the atmosphere in the reaction zone always contain 70% or more hydrogen gas, specifically, for example, by circulating a large amount of hydrogen in the reactor, as described above,
Prevents the reduction in hydrogen concentration due to the production of carbon monoxide as a by-product. When the hydrogen gas concentration is less than 70%, not only the yield of silicon carbide whiskers is significantly reduced, but also the length is short, and the amount of silicon carbide whiskers produced in powdery or bent shapes increases. . In addition, when the reaction furnace is composed of a plurality of zones including a reaction zone in which a silicon dioxide molded body and a solid carbon-containing raw material are heated and reacted, according to the present invention, the limited space is defined as the reaction zone The atmosphere in at least this zone of the reactor should be maintained at an atmosphere with a hydrogen concentration of 70% or more. The three physical properties of carbon black, BET non-surface area, particle size, and bulk density, are not completely independent physical properties, but are interrelated and define the reactivity with hydrogen gas and the agglomerated structure. However, among these, the BET non-surface area is an amount that represents the amount of contact between carbon black and hydrogen gas, and is primarily an indicator of the reaction with gas. On the other hand, the average particle diameter and bulk density are mainly indicators of the agglomerated structure of carbon black. According to the method of the present invention, carbon black having a BET specific surface area of 100 m ² /g or more, an average particle diameter of 35 nm or less, and a bulk density of 0.06 to 0.2 g/cm ³ has high reactivity with hydrogen gas. In order to satisfy the above-mentioned conditions, silicon carbide whiskers with high acicularity can be produced without producing irregularly shaped by-products such as powder, granules, and bent shapes. Further, according to the method of the present invention, there is no need to use a reaction accelerator to secure a reaction space, and therefore, silicon carbide whiskers are not deposited and adhered to the reactor wall during the production of silicon carbide whiskers. ,
Since no corrosive gas is generated, the reactor will not be damaged. In the production of silicon carbide whiskers by the method of the invention, preferably a reaction catalyst is used. As a reaction catalyst, iron, nickel, cobalt or compounds thereof, such as oxides, nitrates,
Carbonates, sulfates, etc. are used. These compounds are added to and mixed with the carbon-containing raw material powder in the form of powder, aqueous solution, or other appropriate form. In particular, these catalysts have the effect of accelerating the reaction (3) to accelerate the production rate of linear, highly pure silicon carbide whiskers, and as a result, suppressing undesirable reactions that occur concurrently. In the method of the present invention, the temperature at which the silicon dioxide-containing molded body and the solid carbon raw material are heated in an atmosphere containing hydrogen is preferably 1300°C or higher, and in particular,
The temperature is preferably 1400°C or higher. At temperatures lower than 1300℃, silicon carbide whisker formation is extremely slow;
This is because it is not practical. On the other hand, when the temperature is too high, the reaction conditions are too extreme, the diameter of the whiskers increases, and disturbances such as branching and bending occur in the whiskers. Therefore, the reaction temperature is usually preferably 1700°C or lower. Further, the heating time is not particularly limited, but is usually suitable for 0.5 to 30 hours. If the reaction time is too short, a large amount of unreacted raw materials will remain; on the other hand, if the reaction is too long, the yield of silicon carbide whiskers will increase only slightly, so it is difficult to reduce the productivity and thermal energy costs. , because there is no advantage. As mentioned above, after heating the silicon dioxide-containing molded body and carbon black having predetermined properties to a predetermined temperature in a predetermined hydrogen atmosphere, this is slowly cooled or allowed to cool, and preferably, the reaction product is heated to a predetermined temperature. By oxidizing and incinerating the excess carbon contained, linear silicon carbide whiskers can usually be obtained. Effects of the Invention As described above, according to the method of the present invention, since a silicon dioxide-containing molded article and carbon black having predetermined physical properties are heated in a hydrogen gas atmosphere,
Highly acicular silicon carbide whisker crystals can be produced in high yield. Silicon carbide whiskers obtained by burning the excess carbon are of the β type and usually have a diameter of 0.1 to 2 μm and a length of 20 to 500 μm. Furthermore, according to the method of the present invention, there is no need to use a reaction accelerator made of a metal halide such as sodium chloride, so there is no need to use a reaction accelerator made of metal halides such as sodium chloride, so there is no need to use a reaction promoter made of metal halides such as sodium chloride. do not have. EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples in any way. A molded plate 4 containing 95% by weight of silicon dioxide and having a length of 120 mm, a width of 40 mm, and a height of 80 mm is placed in a box-shaped reaction vessel made of high-purity carbon with an inner dimension of 130 mm in length and width and a depth of 50 mm.
40 g of carbon black having various properties shown in Table 1 and 0.28 g of nickel oxide as a catalyst were placed in the cavity of the reaction vessel at equal intervals as shown in the figure.
A homogeneous mixture of g was charged. A high-purity carbon lid was placed on the upper opening of the reaction vessel, the vessel was inserted into a reactor, and the temperature was raised from room temperature to 1530°C over 6 hours under a hydrogen gas atmosphere, and maintained at this temperature for 4 hours. Thereafter, it was allowed to cool to room temperature. After removing the molded plate as reaction residue from the reaction vessel, the contents were collected and burned in air at a temperature of 600°C for 4 hours, unreacted carbon black was removed by incineration, and in each case, Pale green β-type silicon carbide whiskers were obtained. The yield of the obtained silicon carbide whiskers, the amount of irregularly shaped materials such as powdery ones and bent ones, the diameter and length of the silicon carbide whiskers are
Shown in the table. As is clear from this result, silicon carbide whiskers with high acicularity can be produced in high yield by using carbon black having predetermined properties in accordance with the method of the present invention, regardless of its type. can be obtained. On the other hand, according to the comparative example, formation of irregularly shaped silicon carbide whiskers

[Table] In addition to the large amount of carbon black, the yield was low, and furthermore, it took a long time to incinerate the unreacted carbon black.

[Brief explanation of drawings]

FIG. 1 shows a silicon dioxide-containing molded body and carbon black filled in a reaction vessel. 1...Reaction container, 2...Silicon dioxide-containing molded body, 3...Carbon black.

Claims (1)

[Claims] 1. A method for producing silicon carbide whiskers by heating silicon dioxide and carbon black to a temperature of 1400 to 1700°C in a hydrogen gas atmosphere, in which silicon dioxide is used as a molded product, and BET
Specific surface area 100m ² /g or more, average particle size 35nm or less,
Using carbon black with a bulk density of 0.06 to 0.2 g/cm ³ , hydrogen gas and carbon black are reacted to generate hydrocarbons, and the silicon dioxide is reduced with the hydrocarbons to form silicon monoxide. 1. A method for producing silicon carbide whiskers, which comprises producing silicon carbide whiskers, and reacting the silicon monoxide with carbon black to produce silicon carbide whiskers. 2. The method for producing silicon carbide whiskers according to claim 1, which comprises mixing carbon black with a catalyst. 3. Claim 1, characterized in that the catalyst is iron, nickel, cobalt, or a compound thereof.
2. Method for producing silicon carbide whiskers as described in Section 1.