JPS62260798A - Production of silicon carbide whisker - Google Patents

Abstract

PURPOSE:To obtain high-purity silicon carbide whisker having long fibers and high aspect ratio, by reacting a silicon-containing molded article containing a specific amount of silicon dioxide with powdery carbon raw material in a hydrogen gas atmosphere under heating. CONSTITUTION:A silicon-containing molded article containing >=90wt% or <=60wt% silicon dioxide is heat-treated with powdery carbon raw material (carbon black) in a hydrogen gas atmosphere. The hydrogen gas atmosphere contains >=about 20vol% hydrogen and the rest of a nonoxidizing inert gas and the heating is carried out at >=1,400 deg.C for about 30min-10hr. The heat treatment is carried out in the presence of both a catalyst (iron, nickel, etc.) and a reaction promoter (sodium chloride, etc.). After silicon carbide whisker is formed by the heat treatment, the unreacted carbon raw material is further burnt at about 600-1,100 deg.C to give straight whisker free from flex is obtained in high yield and in high productivity.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing high-purity silicon carbide whiskers at low cost, and more particularly, to a method for producing silicon carbide whiskers by heating a silicon-containing raw material and a powdered carbon raw material. The present invention relates to a method for inexpensively producing high-purity silicon carbide whiskers containing no unreacted silicon dioxide by repeatedly using a molded article containing a predetermined amount of silicon dioxide as a silicon-containing raw material in the production reaction.

(Prior Art) Silicon carbide whiskers have high strength and are expected to be used in various ways as reinforcing materials by utilizing this property.

The method for manufacturing such silicon carbide whiskers is as follows:
Conventionally, gas phase synthesis method and solid phase synthesis method are typically known. Gas phase synthesis methods include thermally decomposing a mixed gas of silicon halides and hydrocarbons, or organosilicon compound gas containing carbon and halogen, in a hydrogen stream, and adding hydrocarbons to a reactor containing silicon compounds. A method of generating silicon carbide whiskers in a reactor by circulating a gas such as hydrogen chloride or hydrogen chloride (Japanese Patent Publication No. 52-28757), a crucible filled with silicon dioxide, carbon powder, sodium fluoride, etc. is placed in an inert atmosphere. A method of precipitating and growing silicon carbide whiskers by heating the vaporized silicon monoxide and carbon monoxide in a low temperature range (Special Publication No. 1973-
15555, Japanese Unexamined Patent Publication No. 1983-100125)
etc. are known. However, all of these methods
Since silicon carbide whiskers cannot be obtained continuously by a batch method, it is not suitable for mass production of silicon carbide whiskers.

On the other hand, as a solid phase synthesis method, a powdered silicon-containing raw material and a powdered carbon-containing raw material are uniformly mixed and heated in an Acheson-type heating furnace to produce silicon carbide whiskers (Japanese Patent Laid-Open No. 58 -145700 Publication) A method in which a powdered silicon-containing raw material and a carbon-containing raw material are mixed and molded, and the molded body is placed in a sagger and heated in a heating furnace equipped with a tray pusher (Japanese Unexamined Patent Publication No. 58-20799) , Japanese Unexamined Patent Publication No. 58-91010, Japanese Patent Publication No. 50-25907), etc. are known. Among these solid phase synthesis methods,
The former method is not suitable for mass production because it is a batch method. Therefore, the latter continuous production method is currently employed in most cases to produce silicon carbide whiskers on an industrial scale.

In this solid-phase synthesis method, the silicon-containing raw material has conventionally been derived from rice (Japanese Patent Laid-Open Nos. 53-113300 and 57-209813), silica gel (
JP-A No. 58-145700), Chris Flight,
Shirasu (Japanese Unexamined Patent Publication No. 58-20799) and the like are known, but when these are used as silicon-containing raw materials, they require preliminary weight reduction treatment or granulation molding treatment,
Alternatively, it has problems such as being expensive as a raw material, and it is not always easy to separate it from the generated silicon carbide whiskers or to reuse it as a silicon-containing raw material.

Moreover, according to these conventional methods, the fiber length is long;
In addition, it is difficult to obtain silicon carbide whiskers with a high aspect ratio, and the obtained silicon carbide whiskers generally have low purity because they contain silica as a by-product.

(Purpose of the Invention) As a result of intensive research to solve the above-mentioned problems in 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 in a hydrogen gas atmosphere. By heating the molded body to a high temperature together with a powdered carbon-containing raw material, the silicon compound is selectively vaporized from the molded body, reacts with the carbon, and forms a linear shape with no bends. Moreover, silicon carbide whiskers having a long fiber length and a high aspect ratio can be obtained with extremely high purity in high yield and productivity, and in particular, the molded article can contain silicon dioxide within a predetermined range. The present invention was achieved based on the discovery that the molded body does not undergo any deformation during the heating reaction with the powdered carbon raw material, and therefore can be used repeatedly.

Therefore, the present invention uses a molded article containing silicon dioxide in a predetermined range as a silicon-containing raw material, so that it has a straight shape without bending, a long fiber length, and a high aspect ratio. Furthermore, it is an object of the present invention to provide a method by which silicon carbide whiskers of extremely high purity can be produced at low levels.

(Structure of the Invention) A method for producing 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. The method is characterized in that a molded body containing silicon in an amount of 60% by weight or less and a powdered carbon raw material are heated to a predetermined reaction temperature in a hydrogen gas atmosphere.

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. , fired, plates, bars,
It can be obtained by molding into a molded body having a shape of a tube, cylinder, sphere, line, or a combination thereof, and firing the molded body. Here, the silicon-containing material is not particularly limited, but it is advantageous to use low-grade silica, sand, waxite, clay, and the like.

In particular, in the present invention, it is preferable that the shape of the silicon-containing molded body is a container shape such as a tubular shape or a box shape 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 compact and reacts with the vaporized carbon monoxide from the carbon-containing raw material to generate and precipitate silicon carbide whiskers. Can be easily separated from the body.

However, the silicon oxide-containing molded article may be, for example, rod-shaped or plate-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 a suitable reaction vessel, for example, a vessel made of alumina or graphite, and heated in a reactor.

In the present invention, the silicon-containing molded article needs to contain silicon dioxide in a range of 90% by weight or more or 60% by weight or less, and preferably, the remainder mainly consists of alumina.

In particular, it is preferable that the silicon-containing molded article has a composition in which silicon dioxide (and alumina) is the main component, and oxides other than silicon dioxide and alumina are 10% by weight or less.

The reason for this is not necessarily clear, but when a molded body with a silicon dioxide content in the above range is used as a silicon raw material, not only can silicon carbide whiskers be obtained with a particularly good yield, but also the molded body Although its weight decreases due to the heating reaction with the carbon raw material, it does not generate internal bubbles or undergo deformation such as cracking, blistering, or bending, so it can be used repeatedly as a silicon dioxide-containing molded product. can do.

On the other hand, when using a molded body whose silicon dioxide content does not fall within the above range, the molded body undergoes significant deformation such as generation of bubbles, cracking, blistering, and bending within the molded body due to the heating reaction with the carbon raw material. Therefore, it cannot be used repeatedly as a silicon dioxide-containing molded article. Moreover, the obtained silicon carbide whiskers have large variations in diameter and length, or contain bent parts, and are poor in needle-like properties.

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. Furthermore, the silicon carbide whiskers produced are very easy to separate from the molded body, resulting in high productivity.

Carbon black, powdered activated carbon, etc. can be used as powdered carbon-containing raw materials, but these carbon raw materials are
Carbon black is particularly preferred because it is a fine powder and the more bulky the powder, the higher the reactivity.

In the method of the present invention, it is preferred to use a reaction catalyst. As a reaction catalyst, iron, nickel, cobalt, or compounds thereof, such as oxides, nitrates, chlorides,
Sulfates, carbonates, etc. are used as powders or aqueous solutions mixed with carbon raw materials. Iron oxide is a catalyst 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:
These 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 to 1700°
It is in the range of C. 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 diminished, and the production cost will increase. Therefore, the upper limit of the reaction temperature is usually preferably about 1700°C. Although the heating means is arbitrary, electric heating is easy to use.

The reaction time is 30 minutes to 10 hours, and usually about 2 to 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 time is too long, the yield of silicon carbide whiskers will only increase slightly, which will reduce productivity and heat engineering costs. From my point of view, 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. and thermally incinerating the unreacted carbon raw material.

(Effects of the Invention) As described above, according to the method of the present invention, silicon carbide whiskers that are generally straight without bending, have a long fiber length, and have a high aspect ratio can be produced. Moreover, according to the method of the present invention, by using a silicon-containing molded body with a silicon dioxide content within a predetermined range as a silicon raw material, this molded body can be used repeatedly, so that silicon carbide whiskers can be reduced to a low level. can be manufactured.

(Examples) The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples in any way.

Example 1 1.5 g of carbon black powder, 0.005 g of finely divided iron oxide as a reaction catalyst, and 0.5 g of sodium chloride powder as a reaction promoter were stirred in a ball mill for 1 hour to form a homogeneous mixture.

The silicon-containing molded body, which also serves as a reaction vessel, consists of 49% by weight of silicon dioxide, 45% by weight of alumina, and 6% by weight of other oxides such as magnesium oxide, calcium oxide, potassium oxide, and sodium oxide, and has an outer diameter of 25 mm and an inner diameter of 2 mm.
A mullite-based silicon-containing molded body having a length of 0.0 mm and a length of 100 m+g was filled with the above mixture, and this was heated in an electric furnace to a temperature of 1500° C. in a hydrogen atmosphere for 4 hours.

As a result, it was found that the weight of the silicon-containing molded article decreased by 2.1 g, but no deformation was observed and it was possible to use it again.

On the other hand, by the above reaction, a diameter of 0.5 to 1 μm and a length of 50
~100μm, acicular β-type cubic close-packed perfect single crystal 1
．． I was able to obtain 2g.

Example 2 A container made of high-purity silica consisting of 97% by weight of silicon dioxide, 1.5% by weight of alumina, and 1.5% by weight of other oxides was used as a silicon-containing molded body that also served as a reaction container.
Carboblack powder 1.2g, finely powdered iron oxide 0.005g and sodium chloride powder 0.4g prepared in the same manner as above.
A homogeneous mixture consisting of g was charged and reacted by heating under the same conditions as in the example.

As a result, it was found that the weight of the silicon-containing molded product had decreased by 2.7g, but no abnormality or deformation such as cracking, blistering, or bending was observed, and the product could not be used again. Ta.

On the other hand, 1.6 g of silicon carbide whiskers having substantially the same properties and shape as in Example 1 could be obtained by the above reaction.

In addition, when the above reaction was repeated four more times, the weight reduction of the silicon-containing molded product was 1.9 g by the fourth reaction.
Correspondingly, the yield of silicon carbide whiskers was slightly reduced to 1.1 g, but the shape and properties were exactly the same as those in Example 2.

As described above, silicon carbide whiskers were produced using silicon-containing molded bodies having various silicon dioxide contents. The relationship between the silicon dioxide content in a silicon-containing molded body and the deformation of the molded body is shown in the drawing.

Comparative Example 1 Cross section 10IIm square, length 1, consisting of 77% by weight of silicon dioxide, 21% by weight of alumina, and 2% by weight of other oxides.
1.5 g of carbon black powder, 0.005 g of finely powdered iron oxide, and 0.4 g of sodium chloride powder prepared from a rod-shaped silicon-containing molded body of 00++ Ken in the same manner as in Example 1.
A heating reaction was carried out under the same conditions as in Example 2 with a homogeneous mixture consisting of:

As a result, the weight of the silicon-containing molded product decreased by 1.1 g, and it also became locally vitrified. Furthermore, air bubbles were generated inside the molded product, causing it to swell and become significantly deformed. Its reuse was difficult.

On the other hand, by the above reaction, silicon carbide whiskers 0.6
g was successfully obtained, but it was recognized that the silicon carbide whiskers contained bent parts and also had large dimensional variations.

[Brief explanation of drawings]

The drawing is a graph showing the relationship between the silicon dioxide content in a silicon-containing molded body and the deformation of this molded body after reaction. Patent Applicant: Kobe Steel, Ltd., Kanebo Co., Ltd., Agent: Patent Attorney: Ittsu Makino

Claims (9)

[Claims] (1) A silicon-containing molded article containing silicon dioxide in a range of 90% by weight or more and 60% by weight or less in a method for producing silicon carbide whiskers by heating a solid silicon-containing raw material and a carbon-containing raw material to react. and a powdered carbon raw material are heated to a predetermined reaction temperature in a hydrogen gas atmosphere. (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 the silicon-containing compact and the powdered carbon raw material 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 5, 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 the silicon-containing molded body is formed in a reaction vessel.