JPH0324440B2 - - 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 high-purity silicon carbide whiskers in which the content of silicon dioxide as an impurity is significantly reduced. BACKGROUND OF THE INVENTION Due to their high strength, silicon carbide whiskers are expected to find a variety of uses, for example as reinforcing materials. Such silicon carbide whiskers are generally produced by combining a silicon-containing raw material and a carbon-containing raw material under a non-oxidizing atmosphere or a reducing atmosphere such as hydrogen gas for 1200 min.
Produced by heating to a temperature of ℃ or above. Silicon carbide whiskers produced by such a method contain unreacted carbon or carbon generated by decomposition of a carbon-containing raw material as impurities. Publication No. 52-28758 and Special Publication No. 53-
As described in Publication No. 113300, carbon is typically removed by heating the reaction product to burn off the carbon. However, in this way, by heating the reaction product,
According to the method of burning and removing unreacted carbon, in addition to unreacted silicon dioxide mixed in the reaction product and silicon dioxide derived from the oxidation of metal silicon that is produced as a by-product during the reaction, when burning and removing unreacted carbon, The silicon carbide whiskers themselves are partially oxidized to produce silicon dioxide, which is contained in the silicon carbide whiskers as a reaction product.
There is a problem that the amount of silicon dioxide as an impurity increases. On the other hand, the present inventors have already molded a powdered silicon-containing raw material into a predetermined shape and heated it together with a powdered carbon-containing raw material in a hydrogen atmosphere to a high temperature, thereby removing silicon from the molded body. The compound selectively vaporizes and reacts with carbon to obtain silicon carbide whiskers that are straight without bending, have a long fiber length, and a high aspect ratio with high yield and high productivity. are finding. According to this method, since a raw material molded into a predetermined shape is used as the silicon-containing raw material as described above, unreacted silicon dioxide does not mix into the silicon carbide whiskers produced. However, as mentioned above, in order to remove carbon mixed in the generated silicon carbide whiskers, the reaction product is heated and unreacted carbon is burned, so the resulting silicon carbide whiskers still contain silicon carbide. A small amount of silicon dioxide is contained as an oxidation-based impurity, and silicon dioxide derived from the oxidation of metallic silicon produced by a side reaction during the formation of silicon carbide whiskers is also contained. Problems to be Solved by the Invention The inventors of the present invention conducted further intensive research on the oxidation of silicon carbide whiskers in the process of burning off unreacted carbon, and found that the oxidation of silicon carbide whiskers in the process of burning off unreacted carbon. teeth,
In particular, it was found that the influence of the amount of unreacted carbon mixed in the silicon carbide whiskers was large. That is,
The oxidation of the silicon carbide whiskers is promoted because the heat generated during combustion of the unreacted carbon generates a localized high temperature in the silicon carbide whiskers in the vicinity thereof. Therefore, as a result of intensive research in order to prevent the oxidation of silicon carbide whiskers due to the combustion of unreacted carbon, the present inventors decided to control the amount of oxygen supplied to the furnace in order to burn off the unreacted carbon, and to By incompletely burning carbon, the heat generated during combustion of unreacted carbon is reduced, effectively suppressing the oxidation of silicon carbide whiskers in the vicinity, and thus significantly reducing the amount of silicon dioxide as an impurity. The present invention was achieved by discovering that it is possible to produce high-purity silicon carbide whiskers. Therefore, an object of the present invention is to provide a method for producing high-purity silicon carbide whiskers in which the amount of silicon dioxide as an impurity is significantly reduced. Means for Solving the Problems The method for producing high-purity silicon carbide whiskers according to the present invention is a method for producing silicon carbide whiskers by heating and reacting a solid silicon-containing raw material and a carbon-containing raw material. A molded body obtained by forming raw materials into a predetermined shape in advance and a powdered carbon raw material are heated to a predetermined reaction temperature in a hydrogen gas atmosphere to generate silicon carbide whiskers containing unreacted carbon, and then the furnace temperature is lowered. It is characterized by burning and removing the unreacted carbon at a temperature of 600°C or higher and 900°C or lower, and at a rate of 30 to 200 ml/min of air supplied into the furnace per gram of unreacted carbon. do. The method according to the invention will be explained in detail below. The silicon-containing molded article used in the present invention is prepared by kneading any material containing silicon dioxide with an organic binder as necessary, and molding it by an appropriate means such as extrusion molding, press molding, granulation, etc. It can be obtained by firing, molding into a molded body having a shape of a plate, rod, tube, tube, sphere, line, or a combination thereof, and firing. Here, silicon-containing materials include, but are not particularly limited to, inexpensive silica, silver sand,
It is advantageous to use waxite, clay, etc. In particular, in the present invention, when the shape of the silicon-containing molded body is a container shape such as a tubular shape or a box shape,
This is preferable because it can be used as a reaction vessel. That is, in general, in a method for producing silicon carbide whiskers by heating and reacting a solid silicon-containing raw material and a powdery carbon-containing raw material, these raw materials are filled into a reaction vessel, and the reaction tube equipped with a heating means is heated. For example, it is inserted into an electric furnace and heated to a predetermined temperature, but if the silicon-containing molded body is in the shape of a container, the molded body is not only a reaction vessel containing the reaction mixture but also a reaction raw material. Therefore, by heating in the reactor, the silicon compound is selectively vaporized from the molded body, and reacts with carbon itself or the carbon compound vaporized from the carbon-containing raw material to generate and precipitate silicon carbide whiskers, resulting in carbonization. Silicon whiskers can be easily separated from the molded body. However, silicon dioxide-containing molded bodies, for example,
It may be plate-shaped or rod-shaped with an arbitrary cross section. When such a molded body is used, the molded body is packed together with other necessary reaction raw materials into an appropriate reaction container, for example, a container made of alumina or graphite.
Heated in a reactor. In the present invention, in order to obtain silicon carbide whiskers with a particularly good yield, the silicon-containing molded article preferably contains 30% by weight or more of silicon dioxide. As described above, according to the method of the present invention, when a silicon-containing raw material is used as a molded body, the silicon compound is selectively vaporized from the molded body and reacts with carbon to produce silicon carbide whiskers.
High purity silicon carbide whiskers containing impurities of several percent or less can be obtained. Moreover, the produced silicon carbide whiskers are very easy to separate from the molded body, resulting in high productivity. As the powdered carbon-containing raw material, carbon black, powdered activated carbon, etc. can be used, but carbon black is particularly preferred since these carbon raw materials are fine powders and the higher the bulk, the higher the reactivity. In the method of the present invention, it is preferred to use a reaction catalyst. As the reaction catalyst, iron, nickel, cobalt, or compounds thereof, such as oxides, nitrates, chlorides, sulfates, carbonates, etc., are used as a powder or an aqueous solution mixed with the carbon raw material. Iron oxide and iron chloride are catalysts that can be particularly preferably used in the present invention. Furthermore, in the present invention, a reaction accelerator is used in order to accelerate the reaction and obtain silicon carbide whiskers with high purity, high aspect ratio, and low bulk density. As such a reaction accelerator, an alkali metal or alkaline earth metal halide, particularly a chloride or a fluoride, can be suitably used. Therefore, specific examples include lithium chloride, sodium chloride, potassium chloride, calcium chloride, magnesium chloride, lithium fluoride, sodium fluoride, potassium fluoride, and the like. Among these, particularly preferred reaction accelerators are sodium chloride, potassium chloride, sodium fluoride, and potassium fluoride. In the method of the present invention, silicon carbide whiskers are obtained by heating the silicon-containing compact and powdered carbon raw material to a predetermined reaction temperature in a hydrogen gas atmosphere. Here, the hydrogen gas atmosphere includes not only hydrogen gas but also an atmosphere containing 20% by volume or more of hydrogen, with the remainder being a non-oxidizing inert gas. In the method of the invention, the reaction temperature is at least
1400℃ or higher, usually preferably 1500-1700
℃ range. When the reaction temperature is lower than 1400°C, silicon carbide whiskers are insufficiently produced and a large amount of unreacted silicon-containing raw material remains. On the other hand, if the temperature is too high, the effects of improving the yield and reducing impurities will be saturated, and the production cost will increase. Therefore, the upper limit of reaction temperature is usually 1700℃
degree is preferred. Although the heating means is arbitrary, electric heating is easy to use. The reaction time is 30 minutes to 10 hours, usually 2 to 10 hours.
About 6 hours is sufficient. 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. , there is no advantage. In the method of the present invention, as described above, after silicon carbide whiskers are generated at a predetermined temperature, heating is stopped, the reaction product is taken out from the reaction tube, and then this reaction product is placed in a Matsufuru furnace. The unreacted carbon is burnt out by setting the furnace temperature to 600°C or higher and 900°C or lower, and by controlling the amount of oxygen and, by extension, the amount of air supplied into the furnace to a predetermined amount or less relative to the amount of unreacted carbon. By doing so, it is possible to obtain silicon carbide whiskers in which the amount of silicon dioxide as an impurity is significantly reduced. The removal of this unreacted carbon will be explained in detail below. First, silicon carbide whiskers from which silicon dioxide has been completely removed by hydrofluoric acid treatment are filled into an electric furnace (the furnace tube is made of mullite with an inner diameter of 43 mm and a length of 1000 mm), and heated under an air atmosphere (blow rate of 300 ml/min). FIG. 1 shows the oxidation rate of silicon carbide whiskers when heated and fired at various temperatures. From this result, the furnace temperature
At temperatures below 700°C, silicon carbide whiskers alone are hardly oxidized in the absence of carbon.
However, when the temperature inside the furnace is about 1000°C or higher, a large amount of silicon dioxide is produced by air oxidation even if no carbon is present. Next, silicon carbide whiskers from which silicon dioxide had been completely removed by hydrofluoric acid treatment were fired under the same conditions as above in the presence of carbon (carbon black). FIG. 2 shows the relationship between the weight ratio of coexisting carbon to silicon carbide whiskers and the oxidation rate of silicon carbide whiskers. From this result, it is clear that when the amount of coexisting carbon is large, the oxidation of silicon carbide whiskers is significantly promoted, especially by high-temperature heating. That is, when the amount of coexisting carbon is large, a local temperature rise occurs due to significant heat generation during combustion, and oxidation of silicon carbide whiskers in the vicinity is promoted. From the above results, in order to produce high-purity silicon carbide whiskers, it is necessary to suppress the heat generated during combustion of unreacted carbon to the lowest possible temperature in the process of burning off unreacted carbon from the reaction product after heating reaction. It is understood that this is desirable. In general, the combustion of unreacted carbon in silicon carbide whiskers is mainly caused by the reaction of equation (1), but also the reaction of equation (2) also contributes. C+O ₂ →CO ₂ +94.1Kcal/mol (1) C+1/2O ₂ →CO+26.4Kcal/mol (2) The combustion of unreacted carbon according to equation (1) above generates a large amount of heat, whereas the above ( According to equation 2), the amount of heat generated is significantly smaller than that of equation (1). Incomplete combustion of unreacted carbon according to equation (2) occurs when the amount of oxygen in the furnace atmosphere is insufficient. In the method of the present invention, when heating and burning unreacted carbon mixed in silicon carbide whiskers in a furnace, the amount of air supplied to the furnace is set to a predetermined amount or less with respect to the amount of unreacted carbon, and the formula (2) above is satisfied. This causes a reaction to occur, thereby reducing the amount of heat generated during combustion of unreacted carbon, preventing the generation of localized high temperatures, and thus suppressing oxidation of silicon carbide whiskers. According to the present invention, in order to suppress the silicon dioxide content in the silicon carbide whiskers after combustion of unreacted carbon to 5% or less, the furnace temperature is set to 600°C or higher and 900°C or lower, and per gram of unreacted carbon. It is necessary to burn unreacted carbon while supplying air into the furnace so that the amount of air supplied into the furnace is 200 ml/min or less. When the furnace temperature is 900° C. or higher, the oxidation rate of the silicon carbide whiskers exceeds 5% even if the silicon carbide whiskers do not contain unreacted carbon. On the other hand, the amount of air supplied into the furnace is 200% per gram of unreacted carbon.
When the reaction rate exceeds ml/min, (1)
The reaction of the formula takes precedence, making it difficult to suppress the oxidation of silicon carbide whiskers. On the other hand, the amount of air supplied into the furnace is reduced to 1g of unreacted carbon.
If the rate is less than 30 ml/min, it will take a significantly longer time to burn the unreacted carbon, especially if the thermal energy cost is taken into account.
There is no advantage to going below ml/min. Effects of the Invention As described above, according to the method of the present invention, in the method of producing silicon carbide whiskers by heating and reacting a solid silicon-containing raw material and a carbon-containing raw material, the silicon-containing raw material is preliminarily shaped into a predetermined shape. After heating the molded body formed by molding and the powdered carbon raw material to a predetermined reaction temperature in a hydrogen gas atmosphere to generate silicon carbide whiskers containing unreacted carbon, the unreacted material mixed in the silicon carbide whiskers is When burning and removing reactive carbon in the furnace, the temperature inside the furnace is
At the same time as keeping the temperature above 600℃ and below 900℃, supplying air within the specified amount to the amount of unreacted carbon in the furnace, incomplete combustion of unreacted carbon, and lowering the amount of heat generated by combustion of unreacted carbon. As a result, it is possible to produce high purity silicon carbide whiskers in which the amount of silicon dioxide as an impurity is significantly reduced. The present invention will be explained below by giving Examples and Comparative Examples, but the present invention is not limited to these Examples in any way. Example Silicon-containing molded body that also serves as a reaction vessel, containing 49% by weight of silicon dioxide, outer diameter 25 mm, inner diameter 20 mm
A mullite tubular reaction vessel with a length of 100 mm was used. 50 parts of carbon black powder, 0.2 parts of finely powdered iron oxide as a reaction catalyst, and 17 parts of sodium chloride powder (grade 1 reagent) as a reaction promoter were stirred in a ball mill for 1 hour to form a homogeneous mixture. 2g was filled into the reaction vessel, and this was inserted into an electric furnace. After introducing nitrogen gas into this electric furnace at a flow rate of 5 ml/min per unit cross-sectional area (cm ² ) of the furnace tube for 1 hour,
The temperature at the center of the furnace reaches 1530℃ at a heating rate of 5℃/min.
heated until. In this heating process, after the furnace temperature reaches 1000℃, the introduction of nitrogen gas into the electric furnace is stopped, and then hydrogen gas is introduced into the electric furnace at a flow rate of 5 ml/min. The temperature was increased to 1530°C and held at this temperature for 4 hours. After this, the introduction of hydrogen gas was stopped while gradually lowering the temperature inside the furnace, and then the atmosphere inside the furnace was switched to nitrogen gas, and the contents were taken out and the black-green, lightweight, bulky mass was reacted. Obtained as product. The silicon carbide whiskers containing unreacted carbon thus obtained were placed in a quartz glass boat.
Electric furnace (furnace tube is made of mullite, inner diameter 43 mm, length
1000mm) per gram of unreacted carbon

[Table] Air was flowed into the furnace while controlling the amount of air supplied to the furnace as shown in the table, and unreacted carbon was burned for a predetermined period of time to obtain silicon carbide whiskers. The silicon carbide whiskers were β-type and had a shape without branches or bends, and the amount of silicon dioxide contained therein was as shown in the table. In addition, the fiber diameter, fiber length, and aspect ratio are approximately 0.5 μm, 50 to 400 μm, and 100 to 800 μm, respectively.
It was hot. Comparative Example Unreacted carbon was burned and removed in the same manner as in the above example except that the amount of air supplied into the furnace was changed. As shown in the table, the amount of carbon dioxide in the silicon carbide whiskers exceeded 5% in all cases because the amount of air supplied into the furnace was excessive.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the heating temperature and the oxidation rate of silicon carbide whiskers that do not contain silicon dioxide when silicon carbide whiskers that do not contain silicon dioxide are heated for 3 hours in an air atmosphere. It is a graph showing the relationship between the amount of carbon and the oxidation rate of silicon carbide whiskers when silicon carbide whiskers containing carbon are heated to 700° C. or 1000° C. in an air atmosphere in the presence of carbon.

Claims (1)

[Claims] 1. In a method for producing silicon carbide whiskers by heat-reacting a solid silicon-containing raw material and a carbon-containing raw material, a molded body obtained by previously molding the silicon-containing raw material into a predetermined shape and a powdery carbon raw material are heated with hydrogen. Heating to a predetermined reaction temperature in a gas atmosphere,
Silicon carbide whiskers containing unreacted carbon are generated, and then the furnace temperature is set to 600°C or higher and 900°C or lower, and the amount of air supplied to the furnace per 1g of unreacted carbon is 30 to 200ml/min. , a method for producing high-purity silicon carbide whiskers, which comprises burning and removing the unreacted carbon.