JPS61295299A - Preparation of silicon carbide whisker - Google Patents

Abstract

PURPOSE:To prepare SiC whisker contg. no powdery or bent SiC and having high linearity by heating directly a molded body comprising siliceous raw material contg. SiO2 and a molded body comprising carbonaceous raw material contg. C at high temp. in specified H2 atmosphere. CONSTITUTION:A molded body molded previously to a specified shape from SiO2-contg. siliceous raw material (e.g. siliceous stone powder, powdery silica gel, clay, etc.) and a molded body molded previously to a specified shape from C-contg. carbonaceous raw material (e.g. carbon black, powdery active carbon, carbon obtd. by the heat treatment of tar or pitch) are heated at >=1,400 deg.C in H2-contg. atmosphere contg. >=20vol% H2. Formation of SiC whisker by the gaseous phase reaction is prompted. On the other hand, reactions in the solid phase between the SiO2-contg. raw material and the C-contg. raw material or direct gas/solid reaction between SiO2 vapor and solid C-contg. raw material, are retarded. Thus, powdery or bent SiC is scarcely formed and SiC whisker having superior linearity is obtd. at high yield.

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 silicon carbide whiskers of high purity and excellent linearity in a high yield. The present invention relates to a method of manufacturing the same.

(Prior Art) Silicon carbide whiskers are lightweight, have high strength, and have high elasticity, and have recently attracted attention not only as reinforcing materials for composite materials, but also because they have excellent corrosion resistance and oxidation resistance against acids. For example, it is attracting attention as a material for phosphoric acid fuel cells.

Conventionally, the manufacturing method of such silicon carbide whiskers is
A gas-phase synthesis method in which one or both of a carbon-containing raw material and a silicon-containing raw material is supplied in a gas phase to a predetermined high-temperature reaction zone in a reactor, and a solid material is used as both a carbon-containing raw material and a silicon-containing raw material. It is broadly classified into the solid phase synthesis method used. Gas phase synthesis methods are described, for example, in Japanese Patent Publications No. 50-18480, Japanese Patent Publication No. 52-28757, Japanese Patent Publication No. 52-28759, etc., and solid phase synthesis methods are described in, for example,
JP-A-58-20799, JP-A-58-4591
It is described in Publication No. 8, Japanese Patent Application Laid-open No. 58-145700, etc.

However, according to the conventionally known gas phase synthesis method described above, the utilization rate of the gas phase raw material is extremely low, the gas phase raw material decomposes in the reactor, and the reactor is contaminated with these decomposed products. Furthermore, there are problems such as the contamination of these decomposed products into the generated silicon carbide whiskers.On the other hand, according to the conventional solid phase synthesis method, the raw material powder containing silicon and the raw material powder containing silicon are mixed together. are mixed and heated to directly generate silicon carbide mainly through solid phase reaction between these powders, so a large amount of powdered or curved silicon carbide is generated,
It is difficult to obtain silicon carbide whiskers with excellent linearity, and it is also difficult to separate and remove the above-mentioned powdery or curved silicon carbide from the obtained silicon carbide whiskers.

(Objective of the Invention) As a result of intensive research to solve the above-mentioned problems in the production of conventional silicon carbide whiskers, the present inventors found that
A silicon raw material containing silicon oxide and a carbon raw material containing carbon are preformed together and heated to a temperature of 1400°C or higher in an atmosphere containing 20% by volume or more of hydrogen, thereby carbonizing powdery or bent shapes. The present invention was achieved by discovering that it is possible to obtain silicon carbide whiskers that do not contain silicon and have excellent linearity.

(Structure of the Invention) The present invention provides a method for producing silicon carbide whiskers by heating a silicon raw material containing silicon oxide and a carbon raw material containing carbon, in which the silicon raw material containing silicon oxide is formed into a predetermined shape in advance. It is characterized in that the molded body and a molded body obtained by previously molding a carbon raw material containing carbon into a predetermined shape are heated to a temperature of 1400° C. or higher in an atmosphere containing 20% by volume or more of hydrogen.

In the method of the present invention, examples of the silicon raw material containing silicon oxide used include silica powder, powdered silica gel, various amorphous silicas, precipitated silica, and clay. In the present invention, molded bodies of these silicon oxide-containing raw materials (hereinafter sometimes referred to as silicon oxide molded bodies) are formed by molding this raw material by appropriate means such as extrusion molding, press molding, granulation, etc. However, it refers to a three-dimensional solid that has the shape of a plate, rod, tube, particle or sphere, container or box, linear shape, or a combination of these.

In the present invention, as described later, this silicon oxide molded body is heated to generate silicon oxide gas, which is reacted with a carbon compound to generate silicon carbide whiskers. Since the amount of silicon gas produced is approximately proportional to the silicon oxide content in the molded body, in order to obtain silicon carbide whiskers in a high yield, the silicon oxide molded body must contain 30% by weight or more of silicon oxide, particularly, The content is preferably 40% by weight or more. However, it is not limited to this. Incidentally, when the molded body is a molded body such as a tubular or box-shaped container, it can also serve as a reaction vessel for filling raw materials.

As the carbon raw material containing carbon, carbon black, powdered activated carbon, carbon obtained by heat treatment of tar or pitch, etc. can be used.

These carbon-containing carbon raw material molded bodies (hereinafter sometimes referred to as carbon molded bodies) also have the same properties as silicon oxide molded bodies.
It refers to a three-dimensional solid formed by molding a carbon raw material by appropriate means, and when the molded body is formed into a container, it can also serve as a reaction vessel for filling the raw material.

In the method of the present invention, these silicon oxide molded bodies and carbon molded bodies are heated in a predetermined hydrogen atmosphere. As a result of the research conducted by the present inventors, hydrogen not only maintains the heating atmosphere in a reducing atmosphere, but also reacts with the carbon molded body,
It was found that hydrocarbon gas, mainly methane, was produced, which significantly promoted the formation of silicon carbide whiskers.

That is, conventionally, when silicon oxide raw material powder and carbon-containing raw material powder are reacted, the following reaction generally takes place:
It is believed that silicon carbide whiskers are produced.

Sing (solid) 10 (solid) → 5in (ki) + CO (
Qi) (l) Sin (Qi) + 3CO (Qi) = 5
iC (solid) + 2CO□ (air) (2) 2COz (air) + 2C (solid) -4GO (air) (3) Therefore,
The overall reaction is said to be represented by: SiO□+3C-SiC+2GO (4).

However, as a result of detailed research on the above reaction, the present inventors found that in addition to the above reaction, C (solid) + 28z → CH4 (air) (5) CH4 (
Qi) + Sin, (solid) -= 5in (Qi)
+ GO (Ki) + 2H1 (Ki) (6) Sin (Ki) + 2CH, (Ki) → 5IC (Solid)
It has been found that reactions such as + CO (air) + 4Hz (air) (7) occur, and that these reactions make an extremely large contribution to the production of silicon carbide whiskers. That is, in an atmosphere that does not contain hydrogen gas, for example, an inert gas atmosphere such as argon, nitrogen, helium, etc., or an atmosphere in which hydrogen gas is less than 2θ% by volume, even if heated to the same temperature, carbonization will not occur. The amount of silicon whiskers produced is small, and the linearity is also poor. However, according to the method of the present invention, by heating the carbon molded body in the presence of hydrogen, the generation of silicon monoxide from the silicon oxide molded body is promoted, and the reaction of formula (2) is promoted. In addition, silicon carbide whiskers with excellent linearity are produced by the gas phase reaction of the silicon oxide with a carbon compound gas.

The above-mentioned points are in agreement with the point that it has been conventionally believed that reaction in the gas phase is generally suitable for obtaining silicon carbide whiskers with good linearity.

On the other hand, according to the conventional solid-phase synthesis method in which silicon oxide-containing raw material powder and carbon-containing raw material powder are heated and reacted, there are extremely many contact points between the silicon-containing raw material and the carbon-containing raw material. direct solid-phase reaction, Si
A direct gas-solid reaction occurs between the SiO gas produced by the reduction of ng and the solid carbon raw material, and as a result of such a reaction, a large amount of powdery or curved silicon carbide is produced. These silicon carbides are difficult to remove because they are uniformly dispersed in the whiskers.

The above-mentioned solid phase reaction and gas-solid reaction may be expressed by the following equation.

Sing (solid) + 3G (solid) → 5iC (solid)
+ 2CO (air) (8) Sin (air) + 2G (solid) →
5iC (solid) + CO (gas) (9) According to the method of the present invention, by using the silicon oxide molded body and the carbon molded body, such solid phase reactions and gas-solid reactions are significantly reduced, and instead Since silicon carbide whiskers are produced by the gas phase reaction as described above, silicon carbide whiskers with excellent linearity can be obtained. Furthermore, even if some of these solid-phase reactions or gas-solid reactions occur, the resulting silicon carbide is limited to the surface of the carbon compact, so these can be removed from the resulting silicon carbide whiskers. Powdery or curved silicon carbide can be easily removed.

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 used in the form of powder, aqueous solution, or other appropriate forms together with silicon oxide molded bodies and carbon molded bodies. For example, an aqueous solution may be sprayed onto the molded body and dried. Among such catalysts, even when compounds are used, they are all reduced to metals under the reaction conditions of the method of the present invention. These catalysts promote the reactions of formulas (2) and (7) above, increasing the rate of production of linear and highly pure silicon carbide whiskers, and as a result, undesirable reactions (8 ) and (9).

In the method of the present invention, the above-mentioned catalyst is preferably used in the range of 1X10-' to 5X10-''g per unit surface area (ad) of the silicon oxide molded body in terms of metal. Unit surface area (cd) of silicon molded body
When the amount is less than lXl0-'g, the catalytic action is insufficient, and as a result, the amount of non-whisker-like silicon carbide produced on the carbon compact due to reactions (8) and (5) increases, and silicon carbide Whisker yield is reduced. On the other hand, when the amount is more than 5 x 10-'g, there is a strong tendency that fine particles of the catalyst aggregate and become coarse during the reaction process, and the diameter of the silicon carbide whiskers produced becomes excessively large.

In the method of the present invention, the temperature at which the silicon oxide molded body and the carbon molded body are heated in a hydrogen atmosphere is preferably 1400°C or higher, particularly preferably 1450°C or higher. 140
This is because, at a temperature lower than 0° C., silicon carbide whisker formation is extremely slow, which is practically undesirable. on the other hand,
If the temperature is too high, the reaction conditions will be too extreme, the diameter of the whiskers will increase, and disturbances such as branching and bending will occur in the whiskers. Therefore, the reaction temperature is usually 1700°C or lower. In addition, the heating time is not particularly limited, but is usually 0.5 to 30
The time is appropriate. 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 oxide molded body and the carbon molded body to a predetermined temperature in a predetermined hydrogen atmosphere, this is slowly cooled or left to cool, preferably to remove excess carbon contained in the reaction product. A flocculent silicon carbide whisker can usually be obtained by oxidative incineration.

(Effects of the Invention) As described above, according to the method of the present invention, since the silicon oxide molded body and the carbon molded body are heated in a hydrogen atmosphere, the formation of silicon carbide whiskers by a gas phase reaction is promoted. , the solid-phase reaction between the silicon oxide-containing raw material and the carbon-containing raw material and the direct gas-solid reaction between SiO gas and the solid carbon raw material are suppressed, making it difficult to generate powdered or bent silicon carbide. As a result, silicon carbide whiskers with excellent linearity can be obtained in high yield. Furthermore, according to the method of the present invention, even if a solid phase reaction or a gas-solid reaction as described above occurs in a portion, the production of silicon carbide generated by such reaction is localized to the surface of the carbon molded body. Therefore, it is easy to remove these powdered or curved silicon carbide from the silicon carbide whiskers.

(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 A commercially available ceramic tube (NC tube manufactured by Nippon Kagaku Co., Ltd., inner diameter 291 m, outer diameter 251 m, length 100 m, Sin, content 49% by weight) was used as a silicon oxide molded body, and nitric acid as a catalyst was contained inside it. 1 g of a 10% by weight aqueous solution of ferric iron was spray dried. Separately, 10 parts by weight of binder pitch was added to 90 parts by weight of carbon black powder, and after granulation, 100 parts by weight of
Spherical carbon molded body fired at 0℃ (diameter 2-3m>
A 10% by weight aqueous solution of ferric nitrate was sprayed onto the tube, dried, and filled into the ceramic tube.

This was charged into an electric furnace with an inner diameter of 30 mm, and 50 ml of nitrogen was added.
The temperature was raised at a rate of 200° C./hour while flowing at a rate of 1000° C./minute, and when the temperature reached 1000° C., nitrogen was replaced with hydrogen. Thereafter, the temperature was raised to 1600° C. at approximately the same temperature increase rate, and after being maintained at this temperature for 2 hours, it was gradually cooled. When the temperature inside the furnace reached 1000° C. during this slow cooling process, hydrogen was switched to nitrogen. After cooling to room temperature, the contents of the furnace were taken out to obtain a cake-like reaction product.

This reaction product was kept in a Matsufuru furnace at an internal temperature of 750° C. for 5 hours to oxidize and remove excess carbon to obtain a gray-green cake-like substance in which spherical shell-like black-edged substances were mixed.

While loosening this with a 100 mesh vibrating sieve,
After sieving, 1.52 g of floc was obtained.

As a result of X-ray diffraction of this flocculent material, all peaks were assigned to β-silicon carbide. Furthermore, as a result of immersion in a mixed acid containing hydrofluoric acid, the amount dissolved in hydrofluoric acid was 1.8% by weight, and it was confirmed that the silicon carbide had extremely high purity.

Further, the weight reduction of the ceramic tube was 2.47 g, and the yield of silicon carbide whiskers based on the silicon in the ceramic tube was 90.7%.

Incidentally, the yield is calculated by the following formula.

Furthermore, as a result of observing its shape with a scanning electron microscope,
It was confirmed that it was a high aspect ratio silicon carbide whisker with excellent linearity.

Example 2 Commercially available silica bricks (Sing content 97% by weight) were cut into widths of 20mm, thickness of 5days, and length of 100mm using a diamond cutter.
A crane was cut into a plate shape and used as a silicon oxide molded body, and 1 g of a 10% by weight aqueous solution of cobalt nitrate was spray-dried on the surface. Separately, a container (inner width 9 days,
A carbon molded body having a length of 95++n, a height of 20m, and a wall thickness of 5mm was spray-dried with 1g of a 10% by weight aqueous solution of cobalt nitrate on its inner surface.

The silicon oxide molded body cut from the silica brick is placed in the container as a carbon molded body in parallel to its longitudinal direction, and both ends are attached to the container wall so that the surface has a distance from the longitudinal wall. This was supported, placed in an electric furnace, heated in the same manner as in Example 1, and heated at a temperature of 1650° C. for 6 hours. After this, the contents of the furnace were left to cool, and when the contents of the furnace were taken out, 1.95 g of a gray-green cake-like substance was found in the gap between the inner wall of the container, which was a carbon molded product, and the surface of the silica brick plate-shaped molded product.
I got it.

The amount of bone dissolved in this cake-like substance in a mixed acid containing hydrofluoric acid is 0.
It was confirmed that the content was 8% by weight, which was extremely high purity silicon carbide. Further, since the weight reduction of the silica brick plate-shaped molded body was 3.05 g, the yield of silicon carbide whiskers was 95.1% according to the same calculation as in Example 1. Furthermore, as a result of observing the shape with a scanning electron microscope, it was found to be silicon carbide whiskers with excellent linearity and a high aspect ratio.

Example 3 Silicon carbide was produced in exactly the same manner as in Example 1, except that the atmosphere was changed to 30% by volume of hydrogen and 0% by volume of albofluoride, and after removing excess carbon, it was sieved and made into cotton. 1.02 g of a similar substance was obtained.

The amount of bone dissolved in mixed acid containing hydrofluoric acid is 4.2% by weight.
The yield of silicon carbide whiskers was 82.0%. Furthermore, as a result of observation using a scanning electron microscope, it was found to be silicon carbide whiskers with excellent linearity.

Comparative Example 1 Commercially available high-purity SiO□ powder (■Tatsumori Listalite A
A) 100 parts by weight, 200 parts by weight of the carbon black powder used in Example 1, and 30 parts by weight of ferric nitrate were mixed in a ball mill for 1 hour to prepare a raw material.

2.05 g of this mixed powder was put into a high-purity alumina Tanmann tube (5SA-S manufactured by Nihon Kagaku Togyo ■, Ah03 content 97% by weight or more, inner diameter 20 tm, outer diameter 25 mm, length 100+
+n), heated and reacted in the same manner as in Example 1, and excess carbon was removed to obtain 0.42 g of flocculent material.

The hydrofluoric acid-dissolved bone in this cotton material is 24.8% by weight, and
The yield was 76.3% based on the silicon in 5i02 used in the reaction. Further, as a result of observation using a scanning electron microscope, it was confirmed that granular and curved substances were present in the sample.

Comparative Example 2 Silicon carbide was produced in exactly the same manner as in Example 2, except that the atmosphere was 10% by volume of hydrogen and the remainder was argon, and excess carbon was removed. In this way, 0.53 g of a product was obtained, but as a result of observation using a scanning electron microscope, it was confirmed that a small amount of granular and bent substances were present.

Comparative Example 3 Silicon carbide was produced in exactly the same manner as in Example 1, except that carbon black powder was used as the carbon-containing raw material, and excess carbon was removed. 1.61 g of product were thus obtained.

The yield was 93%, and the amount of bone dissolved in the mixed acid containing hydrofluoric acid was 6%. Furthermore, as a result of observation using a scanning electron microscope, it was confirmed that a small amount of granular and curved substances were present.

Patent applicant Kobe Steel, Ltd. Kanebo Co., Ltd. Agent Patent attorney Ittsu Makino Department procedural amendment (voluntary) 12. Description of the case 1985 Patent Application No. 136579 2 Name of the invention Method for manufacturing silicon carbide whiskers 3 Person making the amendment Relationship to the case Patent applicant address 1-3-18 Wakihama-cho, Chuo-ku, Kobe City name
Kobe Steel, Ltd. 4゜Agent Address: 5 Shinmachi Shichifuku Building, 1-8-3 Shinmachi, Nishi-ku, Osaka Date of Amendment Order: Month, Day, 1920 (Date of Shipment: Month, Day, 1920) 6. Inventions Increased by Amendment Number 7, subject of correction Contents of correction in the detailed description of the invention in the specification (1) "Inner diameter 29 cage" on page 13, line 11 of the specification is corrected to [inner diameter 20 mJ].

that's all

Claims (3)

[Claims] (1) A method for producing silicon carbide whiskers by heating a silicon raw material containing silicon oxide and a carbon raw material containing carbon, which includes a molded body obtained by pre-forming a silicon raw material containing silicon oxide into a predetermined shape and carbon. A method for producing silicon carbide whiskers, which comprises heating a molded body obtained by previously molding a carbon raw material into a predetermined shape to a temperature of 1400°C or higher in an atmosphere containing 20% by volume or more of hydrogen. (2) A molded body formed by forming a silicon raw material containing silicon oxide into a predetermined shape in advance and a molded body formed by forming a carbon raw material containing carbon into a predetermined shape in advance are heated in the presence of a catalyst. A method for producing silicon carbide whiskers according to claim 1. (3) The method for producing silicon carbide whiskers according to claim 2, wherein the catalyst is iron, cobalt, nickel, or a compound thereof.