JPH0353279B2 - - 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 specifically, silicon carbide whiskers that are linear and have a high aspect ratio.
The present invention also relates to a method for producing highly pure silicon carbide whiskers in high yield.

(Prior art) Silicon carbide whiskers are used for various purposes, for example, taking advantage of their high strength,
When used as various reinforcing materials, it is required that the fiber length is long and the aspect ratio defined as the ratio of the fiber length to the fiber cross-sectional diameter is high.

Traditionally, as a typical method for producing silicon carbide whiskers, for example, as described in JP-A-58-145700, powdered silicon-containing raw materials and carbon-containing raw materials are uniformly mixed, A method is known in which the above raw materials are heated to a high temperature and reacted, and in order to intimately mix the above raw materials, for example,
JP-A-58-20799 also proposes a method in which a powdered silicon-containing raw material and a carbon-containing raw material are mixed and molded and then heated.

However, according to these conventional methods, as mentioned above, it is difficult to obtain silicon carbide whiskers with a long fiber length and a high aspect ratio, and the resulting silicon carbide whiskers do not contain by-product silica. It generally has low purity, and requires purification treatment with hydrofluoric acid or the like before use.

Furthermore, according to the former method, it is not easy to separate the generated silicon carbide whiskers from the raw material. Generally, when silicon carbide whiskers are obtained by a heating reaction between a solid silicon-containing raw material and a carbon-containing raw material, impurities contained in the raw materials are uniformly dispersed and contained in the generated whiskers. Therefore, if a large amount of such impurities is contained, it is necessary to separate and remove them, and on the other hand, in order to omit the step of separating and removing the impurities, it is necessary to reduce the impurity content in the raw materials used. It is necessary to regulate it below a certain amount.

In order to solve the above-mentioned problems, the present inventors have already developed a method in which a metal chloride, typified by sodium chloride, coexists when a silicon-containing substance and a carbon-containing substance are subjected to a contact reaction at at least 1000°C. (Japanese Patent Publication No. 52-28758), hydrogen chloride or chlorine is mixed and introduced into a mixture of a finely powdered silicon raw material, a carbon raw material, and a catalyst in a non-oxidizing atmosphere,
A method of reacting at a temperature of 1350°C to 1600°C has been proposed (Japanese Patent Application Laid-Open No. 58-251964), but
The yield, aspect ratio, etc. should not be fully satisfied.

(Purpose of the Invention) Therefore, as a result of further intensive research into the production of silicon carbide whiskers, the present inventors formed a powdered silicon-containing raw material into a predetermined shape in advance, and molded it into a powdered form in the presence of hydrogen gas. By heating to a high temperature together with the carbon-containing raw material, the silicon compound is selectively vaporized from the molded body and reacts with the carbon,
We have discovered that it is possible to obtain silicon carbide whiskers that are linear without bending, have a long fiber length, a high aspect ratio, and are extremely pure with high yield and high productivity. This led to the invention.

Therefore, the present invention provides a method for producing silicon carbide whiskers in a high yield that are linear without bending, have a long fiber length and a high aspect ratio, and have extremely high purity. The purpose is to

(Structure of the Invention) The present invention provides a method for manufacturing silicon carbide whiskers by heating and reacting a solid silicon-containing raw material and a carbon-containing raw material in a reaction vessel, in which the silicon-containing raw material is shaped in advance into a predetermined shape. The method is characterized by heating a molded body made of carbon and a powdered carbon raw material to a predetermined reaction temperature in a hydrogen gas atmosphere.

Examples of the silicon-containing raw material used in the method of the present invention include silicon, silica powder, powdered silica gel, various amorphous silicas, precipitated silica, and clay, but preferably silicon-containing raw materials include silicon oxide. However, in order to obtain silicon carbide whiskers with a particularly good yield, it is preferable to contain silicon oxide in an amount of 30% by weight or more.

In the present invention, a molded body made of such a silicon-containing raw material is defined as a molded body made of such a silicon-containing raw material, which is formed by processing this raw material by appropriate means, for example,
A three-dimensional solid that is formed by extrusion molding, press molding, granulation, etc. and fired to give it a shape such as a plate, rod, tube, tube, sphere, line, or a combination thereof.

In general, in a method for producing silicon carbide whiskers by heating and reacting a solid silicon-containing raw material and a carbon-containing raw material, these raw materials are filled into a reaction vessel, and the raw materials are charged into a reaction tube equipped with a heating means, such as an electric current. In the method of the present invention, the molded body made of the silicon-containing raw material is inserted into a furnace and heated to a predetermined temperature. It can also serve as a reaction vessel for

As described above, according to the method of the present invention, when a solid silicon-containing raw material is used as a molded article,
The silicon compound is selectively vaporized from this molded body,
Since it reacts with carbon to produce silicon carbide whiskers, it is possible to obtain highly pure silicon carbide whiskers with impurities of a few percent or less even if a material with a low silicon or silicon oxide content is used as the solid silicon-containing raw material. I can do it. 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 is a catalyst that can be particularly preferably used in the present invention.

Furthermore, in the method of the present invention, it is preferable to use a reaction accelerator 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.

The present inventors have proposed the above-cited Japanese Patent Application Laid-Open No. 58-251964.
In the publication, a method is proposed in which hydrogen chloride or chlorine is introduced from outside the reaction system instead of a reaction accelerator such as sodium chloride when producing silicon carbide whiskers by heating reaction between a silicon-containing raw material and a carbon-containing raw material. There is. However, according to the method of the present invention, the reaction accelerator is first heated in the presence of a reaction accelerator and a carbon-containing raw material in a hydrogen atmosphere and the molded body made of a silicon-containing raw material is heated. Hydrogen halide is generated from the molded body, and the silicon compound is selectively vaporized from the molded body, and this is reacted with carbon. Therefore, silicon carbide whiskers with high purity, long fiber length, and high aspect ratio can be produced at a high yield. You can get it at a certain rate.
Therefore, according to the present invention, there is no need to use corrosive and difficult-to-handle gases, such as hydrogen chloride and chlorine, as source gases.

Next, when powder is used as the silicon-containing raw material, as is easily understood, there are many contact parts with the carbon-containing raw material, so the silicon carbide whisker production reaction proceeds in a state close to a solid phase reaction. As a result, the whiskers become powder-like or linear with many bends. However, when the molded body is used as a silicon-containing raw material according to the present invention, since the contact area with the carbon-containing raw material is extremely small, silicon monoxide, which is a reduced form of silica, is mainly produced as a gas phase. Since it reacts with gaseous carbon monoxide, it appears to produce straight silicon carbide whiskers with no bends. however,
The present invention is not limited by the above theory.

To explain in more detail the case where sodium chloride is used as a reaction accelerator, sodium chloride has a melting point of 800°C and a boiling point of 1413°C under normal pressure, so it is boiling under the reaction conditions of the method of the present invention. An atmosphere in which sodium chloride vapor has a high vapor pressure is formed in the reaction system. Sodium chloride does not react chemically even when heated to high temperatures, e.g. 1500°C, in hydrogen or other non-oxidizing atmospheres, even in the presence of inert substances such as platinum or graphite. It does not cause any change and simply produces sodium chloride vapor. However, according to the present invention, when sodium chloride is heated to a predetermined temperature together with a silicon-containing raw material molded body in an atmosphere of hydrogen gas as a carrier gas, the sodium chloride is thermally decomposed in the system and hydrogen chloride is continuously produced. occurs in On the other hand, when using a non-oxidizing gas other than hydrogen gas, such as argon, helium, nitrogen, etc., as the carrier gas,
No hydrogen chloride generation was observed. In the present invention, 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.

That is, when the catalyst and reaction accelerator are mixed with a molded body of a silicon-containing raw material and a powdered carbon-containing raw material and heated in a non-oxidizing atmosphere, for example, in a nitrogen atmosphere, even if heated to 1500°C, No generation of hydrogen chloride is observed, but under a hydrogen atmosphere, hydrogen chloride begins to be generated gradually after the sodium chloride melts, and the amount generated gradually increases after 1000℃.
The maximum amount of hydrogen chloride is produced between 1400 and 1600°C.

Therefore, when carrying out the method of the present invention by batch reaction, a non-oxidizing inert gas other than hydrogen, such as argon, helium, nitrogen, etc., is used as a carrier gas in the reaction system until the predetermined reaction temperature is reached. After flowing and reaching the predetermined reaction temperature, the carrier gas can be switched to hydrogen gas. On the other hand, if the method of the invention is carried out in a continuous manner,
Hydrogen gas is always introduced into the reaction system to generate hydrogen halide from the reaction promoter.

In the method of the present invention, the ^amount of the catalyst is usually 5
×10 ⁻⁶ to 5×10 ⁻³ g, preferably 1×10 ⁻⁵ to 1×
It is used in the range of 10 ^-3 g. Catalyst amount is 5×10 ^-6 g per unit surface area (cm ² ) of molded product of silicon-containing raw material.
When the amount is less than , the catalytic action is insufficient, resulting in insufficient generation of silicon carbide whiskers, and a large amount of unreacted silicon-containing raw material remains even when the reaction is carried out under specified conditions. . On the other hand, 5×
When the amount is more than 10 ^-3 g, the catalyst powder tends to remain in the generated whiskers, making the purification process of the generated silicon carbide whiskers complicated.

^The alkali metal or alkaline earth metal halide used as a reaction accelerator is usually 5
10 ⁻⁴ to 5×10 ⁻¹ g, preferably 1×10 ⁻³ to 1×
It is used in the range of 10 ^-1 g. When the unit surface area (cm ² ) of the silicon-containing raw material molded body is less than 5×10 ^-4 g, the amount of hydrogen halide generated by decomposition of the halide is small, so the yield of silicon carbide whiskers can be improved and It has a poor shape improvement effect, and on the other hand, even if it is used in a larger amount than 5 × 10 ^-1 g,
The generation of hydrogen halide due to decomposition becomes locally concentrated, and the gaseous silicon compound generated from the compact tends to be released outside the system without reacting.
This is not preferable because the amount of silicon carbide whiskers produced is rather reduced.

In the method of the present invention, a molded body of a silicon-containing raw material, a powdered carbon raw material, a catalyst, and a reaction promoter are filled into a reaction vessel, and the container is heated to a predetermined temperature in a predetermined atmosphere. Here, as a reaction vessel, when manufacturing silicon carbide whiskers on a small scale, a magnetic tube or a graphite tube made of mullite, alumina, etc. can be used, and when manufacturing on a large scale, a magnetic tube or a graphite tube can be used. It is preferable to use a box-shaped container made of the ceramic material mentioned above. However, as described above, if the molded body of the silicon-containing raw material is shaped like a container, the molded body serves both as the reaction raw material and the container, so that the reaction operation is simplified.

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.

In the method of the present invention, the carrier gas containing hydrogen gas has a gas flow unit cross-sectional area (cm ² ) of the reactor.
It is introduced into the reaction system at a rate of 0.5 to 50 ml/min, preferably 1 to 20 ml/min.

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. Silicon carbide whiskers can be obtained by heating to a temperature of 600 to 1100°C to oxidize and incinerate the unreacted carbon raw material.

(Effects of the Invention) As described above, according to the method of the present invention, silicon carbide whiskers can generally be obtained at a high yield of 90% or more based on silicon vaporized from a silicon-containing raw material, and moreover, This silicon carbide whisker is straight without bending, has a long fiber length, and has a high aspect ratio. Furthermore, silicon carbide whiskers contain silicon dioxide as an impurity, which is usually 10% or less and has high purity, so it can be used for general purposes without any particular purification. However, if necessary, it may be purified by immersing it in hydrofluoric acid, keeping it at room temperature, or heating it at 70 to 80°C to dissolve silicon compounds other than silicon carbide, and washing it with water.

Further, according to the method of the present invention, since a reaction accelerator is used, highly pure silicon carbide whiskers can be obtained in high yield. Also, if we compare the whisker fiber diameter, fiber length, and aspect ratio, each of them will be different if no reaction accelerator is used.
0.5 to 1 μm, 50 to 500 μm, and 100 to 500, whereas according to the method of the present invention, each of 0.1
-0.5 μm, 50-400 μm and 500-800.

(Examples) The present invention will be explained below by giving Examples and Comparative Examples, but the present invention is not limited by 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
And an NC Tamman tube 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. 2 g was filled into the Tammann tube, 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 core 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 temperature raising process, after the internal 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 1550°C and held at this temperature for 2 hours. After this, while gradually lowering the temperature inside the furnace, the introduction of hydrogen gas was stopped, and then the atmosphere inside the furnace was changed to nitrogen gas, and the contents were taken out and the black-green, lightweight, bulky mass was reacted. Obtained as product.

In order to remove excess carbon from this reaction product, it was fired in a Matsufuru furnace at an internal temperature of 800° C. for 4 hours to obtain 1.5 g of silicon carbide whiskers. The weight loss of the Tammann tube at this time was 1.14 g. The yield was therefore 92%, based on the silicon dioxide vaporized from the Tammann tube, and 6% silicon dioxide as an impurity. In addition, the X-ray diffraction pattern of the obtained silicon carbide whiskers is β-SiC
Only the pattern shown was shown.

Furthermore, the obtained silicon carbide whiskers have a shape without branches or bends, and the fiber diameter, fiber length, and aspect ratio are 0.1 to 0.5 μm and 50 μm, respectively.
~400 μm and 500-800 μm.

Comparative example: 10 parts of quartz powder with an average particle size of 5 to 10 μm, 0.2 parts of finely divided iron oxide, carbon black powder (lion oil)
20 parts of Lion Carbon ECP (manufactured by Lion Carbon Co., Ltd.) and 10 parts of sodium chloride powder (grade 1 reagent) were placed in a ball mill and mixed with stirring for 3 hours. This mixed powder
32g as a reaction tube, outer diameter 65mm, inner diameter 60mm, length
The material was evenly distributed and filled in the center of a 1000 mm alumina tube over a length of about 130 mm, and the tube was inserted into an electric furnace.

After introducing nitrogen gas into this reaction tube at a flow rate of 100 ml/min for 1 hour, the tube was heated at a temperature increase rate of 5° C./min until the temperature at the center of the furnace reached 1550° C. After the internal temperature reaches 1000℃, stop introducing nitrogen gas,
Next, while hydrogen gas was introduced into the reaction tube at a flow rate of 100 ml/min, the temperature inside the tube was maintained at 1550° C. for 2 hours. After this, the introduction of hydrogen gas is stopped while the temperature inside the tube is gradually lowered.Then, the atmosphere inside the tube is switched to nitrogen gas, and the contents are taken out. obtained as.

In order to remove excess carbon from this reaction product, it was fired in a Matsufuru furnace at an internal temperature of 800° C. for 4 hours to obtain 4.6 g of silicon carbide whiskers. The yield was 87% based on silica in the quartz powder used and 11% silicon dioxide as an impurity. Moreover, the X-ray diffraction diagram of the obtained silicon carbide whiskers showed a pattern of only β-SiC.

Furthermore, the obtained silicon carbide whiskers have a shape that includes a small amount of powdery and bent materials, and the fiber diameter,
Fiber length and aspect ratio are 0.5 to 1 μm, respectively.
They were 100-500 μm and 100-500 μm.

Claims (1)

[Claims] 1. A molded body containing no carbon material and formed by molding silicon oxide into a predetermined shape and a powdered carbon raw material are heated to a predetermined reaction temperature in a hydrogen gas atmosphere in a reaction vessel. A method for producing silicon carbide whiskers characterized by: 2. The method for producing silicon carbide whiskers according to claim 1, wherein the powdered carbon raw material is carbon black or powdered activated carbon. 3. The method for producing silicon carbide whiskers according to claim 1, which comprises heating in the presence of a catalyst and a reaction promoter. 4. The method for producing silicon carbide whiskers according to claim 3, wherein the catalyst is iron, nickel, cobalt, or a compound thereof. 5. The method for producing silicon carbide whiskers according to claim 3, wherein the reaction accelerator is an alkali metal halide or an alkaline earth metal halide. 6. The method for producing silicon carbide whiskers according to claim 5, wherein the halide is a chloride or a fluoride. 7. The method for producing silicon carbide whiskers according to claim 6, wherein the reaction accelerator is sodium chloride. 8. The method for producing silicon carbide whiskers according to claim 1, wherein the reaction temperature is 1400°C or higher. 9. The method for producing silicon carbide whiskers according to claim 1, wherein a molded body formed by molding silicon oxide into a predetermined shape is formed in the reaction vessel.