JPS63156100A - Production of silicon carbide whisker - Google Patents

Abstract

PURPOSE:To produce the title high-purity SiC whisker having an excellent acicular property in good yield, by heating a compact contg. SiO2, specified carbon black, and H2 to a specified temp. CONSTITUTION:Carbon black having >=100m<2>/g BET specific surface, <=35nm mean particle diameter, and 0.06-0.2g/cm<3> bulk density is prepared. A catalyst (Fe, Ni, Co, or their compd.) is incorporated into the carbon black. The carbon black contg. the catalyst and the compact contg. SiO2 are heated at 1,400-1,700 deg.C in an H2 atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing silicon carbide whiskers, and more specifically, a method for producing acicular silicon carbide whisker crystals of high purity and excellent linearity in a high yield. It relates to a method of manufacturing.

BACKGROUND OF THE INVENTION Silicon carbide whiskers are lightweight, have high strength, and have high elasticity, and have recently been expected to be used in a variety of 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; 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, for example, in Japanese Patent Publication No. 50-18480 and Japanese Patent Publication No. 50-18480.
2-28757, Japanese Patent Publication No. 52-28759, etc., and the solid phase synthesis method is described in, for example, JP-A-58-20799, JP-A-58-45918.
JP-A-58-145700, etc.

The conventional gas phase synthesis method as described above is advantageous in obtaining needle-shaped crystals at -1'ffl, but on the other hand, the utilization rate of the gas phase raw material is extremely low, and the gas phase raw material is not used in the reactor. It is unsuitable for the mass production of silicon carbide whiskers because it decomposes, the reactor is contaminated with these decomposed products, and the resulting silicon carbide whiskers are mixed with these decomposed products. However, according to the conventional solid phase synthesis method,
Raw material powder containing silicon and raw material powder containing carbon are mixed and heated, and silicon carbide is directly produced mainly through a solid phase reaction between these powders, so carbonization in powder, granular, and bent shapes is possible. It is difficult to obtain silicon carbide whiskers that produce a large amount of silicon and have excellent linearity, and it is also difficult to separate and remove the above-mentioned irregularly shaped silicon carbide 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 silicon dioxide, which are by-products during the production. Only a small amount is mixed in. Further, carbon as a reaction residue can be easily removed by burning the reaction product in the atmosphere after the reaction is completed, so that highly pure silicon carbide whiskers can be obtained.

The present inventors have further studied the effects of the carbon-containing raw material on the silicon carbide whiskers produced in the above-mentioned method, and found that by using carbon black having predetermined properties as the carbon-containing raw material, in particular,
The present invention was developed based on the discovery that acicular silicon carbide whisker crystals with excellent linearity and uniformity of dimensions can be obtained in high yield.

Means for Solving the Problems The method for producing silicon carbide whiskers according to the present invention comprises a molded body containing silicon dioxide and a BET specific surface area t00m.
”/g or more, average particle size of 35 nm or less, and bulk density of 0.
The method is characterized in that carbon black having a concentration of 0.6 to 0.2 g/cm'J is 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 thereof. 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 addition, in the present invention, particularly as carbon-containing raw materials,
BET specific surface area 100m”/g or more, average particle size 35n
Carbon black having a bulk density of 0.06 to 0.2 g/cam' is used.

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) +21h(g) −CHa(g)
(t)SiOz(s) +CICll4
(-= SiO(g) +GO(g) +211z(g
) (21SiO(g) +2C(s) −= 5i
C(s) +CO(g) (3) That is, first, hydrogen gas and solid carbon raw material react (11) to produce methane gas, which is converted into -it on the surface of the silicon dioxide-containing molded body by reaction (2). Silicon gas is produced.Next, silicon carbide is produced by the reaction (3) of this -silicon oxide gas and carbon.Therefore, the overall reaction formula is 5iOz(s) +3C(s) 5i
C(s) +2CO(g) (It will be expressed as 41.

In the method of the present invention, the silicon dioxide-containing molded body and carbon black are usually filled into a suitable reaction vessel,
It is heated in a reactor under a hydrogen atmosphere. 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 filled in the gaps. The reaction vessel may be made of, for example, alumina or high-purity carbon.

In the reaction formula (4), hydrogen does not participate in the production of silicon carbide, but since hydrogen is essential for the carbon methane gasification reaction (1), carbon has high reactivity with hydrogen gas. This is required. On the other hand, reaction (2) is
Proceeds almost uniformly throughout the carbon-filled space,
As a result, silicon carbide whiskers form throughout this space, while forming very little outside of 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. Therefore, it is necessary for carbon to have an agglomerated structure with appropriate voids.

As described above, in the above reaction, hydrogen gas makes an important contribution to the generation of silicon carbide whiskers, and according to the present invention, the hydrogen gas in the atmosphere in the reaction zone is always reduced by 70%.
By doing so, the yield of silicon carbide whiskers can be significantly increased, and the acicularity thereof can be significantly improved. In order to make the atmosphere in the reaction zone always contain 70% or more hydrogen gas, for example, a large amount of hydrogen is passed through the reaction zone, and as mentioned above, carbon monoxide is produced as a by-product. prevent the reduction in hydrogen concentration associated with 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 irregular shapes such as powder and bent shapes increases. . In addition, when the reactor 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 say,
The atmosphere in at least this zone of the reactor has a hydrogen concentration of 70%.
It is sufficient to maintain the above atmosphere.

The three physical properties of carbon black, BET specific surface area, particle size, and bulk density, are not completely independent physical properties from each other;
It defines the reactivity with hydrogen gas and the agglomerated structure while being related to each other. However, among these, the BET specific surface area is an amount that indicates the amount of contact between carbon black and hydrogen gas, and is mainly an index of the reaction with gas. On the other hand, the average particle diameter and bulk density are mainly indicators of the aggregate structure of carbon black. Here, according to the method of the present invention, the BET specific surface area is 100 m''/g or more, the average particle diameter is 3
Carbon black, which has a diameter of 5 nm or less and a bulk density of 0.06 to 0.2 g/cm3, has high reactivity with hydrogen gas, and in order to meet the above conditions, it can be made into irregular shapes such as powder, granules, and bent shapes. It would be possible to produce highly acicular silicon carbide whiskers without producing by-products.

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, in the production of silicon carbide whiskers, there is no need to deposit and adhere to the walls of the reactor. 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 the 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) and increasing the rate of production 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 1
The temperature is preferably 300°C or higher, particularly preferably 1400°C or higher. This is because, at a temperature lower than 1300° C., silicon carbide whisker formation is extremely slow, which is practically undesirable. 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 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. , because there is no advantage.

As described 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, flocculent silicon carbide whiskers can usually be obtained.

Effects of the Invention As described above, according to the method of the present invention, silicon dioxide-containing molded bodies and carbon black having predetermined physical properties are heated in a hydrogen gas atmosphere, so that highly acicular silicon carbide whisker crystals can be formed. It can be produced with high yield. Silicon carbide whiskers obtained by incinerating excess carbon are
β type, usually 0.1 to 2 pm in diameter and 2 pm in length
It is 0 to 500 μm. Furthermore, according to the method of the present invention,
Since there is no need to use a reaction accelerator made of a metal halide such as sodium chloride, there is no need for these to adhere to the walls of the reactor or to generate corrosive gases that corrode the reactor.

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.

Four molded plates containing 95% by weight of silicon dioxide and having a length of 120 cm, a width of 4 On, and a height of 80 μl are shown in a box-shaped reaction vessel made of high-purity carbon with an internal dimension of 130 cm and a depth of 5011 cm. 40 g of carbon black having various properties shown in Table 1 was placed in the cavity of the reaction vessel at equal intervals as shown in Table 1.
and 0.28 g of nickel oxide as a catalyst.

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 heated in air for 6 hours.
Burning was carried out for 4 hours at a temperature of 00° C., and unreacted carbon black was removed by incineration, yielding pale green β-type silicon carbide whiskers in each case.

Table 1 shows the yield of the silicon carbide whiskers obtained, the amount of irregularly shaped materials such as powdery and bent ones, the diameter, and the length. As is clear from this result, silicon carbide whiskers with high acicularity can be obtained in high yield by using carbon black having predetermined properties according to the method of the present invention, regardless of its type. can. On the other hand, in the comparative example, many irregularly shaped silicon carbide whiskers were produced, the yield was low, and furthermore, it took a long time to burn off the unreacted carbon black.

[Brief explanation of the drawing]

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. The patent applicant is Kobe Steel, Ltd., and the patent attorney is Itsube Makino. 〉・ Figure 1

Claims (3)

[Claims] (1) A molded body containing silicon dioxide and 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 are heated using hydrogen gas. 1400-1700℃ under atmosphere
A method for producing silicon carbide whiskers, the method comprising heating to a temperature of . (2) The method for producing silicon carbide whiskers according to claim 1, which comprises mixing carbon black with a catalyst. (3) The method for producing silicon carbide whiskers according to claim 1, wherein the catalyst is iron, nickel, cobalt, or a compound thereof.