JPH066520B2 - Method for producing silicon carbide whisker - Google Patents

Description

TECHNICAL FIELD The present invention relates to an improved method for producing silicon carbide whiskers.

Whiskers mean needle-like crystals, and since there are almost no lattice defects in a single needle-like crystal, their mechanical strength is a material whose strength is close to the ideal maximum expected for the material. Silicon carbide is a compound having excellent heat resistance and corrosion resistance.

Therefore, since silicon carbide whiskers are a new material that has these properties in combination, that is, it has excellent mechanical strength, heat resistance, and corrosion resistance, it can be added to metals and ceramics to enhance their mechanical strength and heat corrosion resistance. It is expected to be used as a reinforcing agent.

[Prior art]

As a conventional technique for manufacturing a silicon carbide whisker,
Although a method of manufacturing using a gas raw material such as CH ₃ SiCl ₃ has been proposed, a method of manufacturing from a solid raw material, that is, a mixture of silicon oxide and elemental carbon is a general industrial manufacturing method.

The method for producing a silicon carbide whisker from such a solid raw material is a degradable silicon compound such as natural silica stone or SiCl ₄ , a so-called obtained by hydrolyzing in a steam atmosphere,
Silicon oxides such as Aerosil and carbons such as graphite, coke, and carbon black are crushed and mixed, and the bulk specific gravity is kept low in a non-oxidizing atmosphere at 1300 to 1800.
It can be obtained by heating to ℃. However, when this method is carried out on an industrial scale, according to our examination, the apparent volume occupied by the reaction product whiskers as the reaction progresses is such that when the mixture of silicon oxide and elemental carbon is charged. It is reduced to less than half of the apparent volume.Because of the contraction of the apparent volume, voids are generated, but a good whisker that grows straight and long in the part near the wall of the reaction vessel can be obtained, but the central part is short. There is a problem that whiskers with many curves and fine-grained crystals are generated.

Such a whisker having a large amount of curvature or a whisker mixed with fine-grained crystals has a drawback in that even if it is added to a metal or ceramic, its reinforcing effect is not satisfactory.

As a method for producing silicon carbide whiskers from a solid raw material, in addition to the above methods, a method of heating rice husks in a carbon monoxide atmosphere, a method of heating rice husks once in an inert gas atmosphere, Known methods include heating carbonized rice husks again in a non-oxidizing atmosphere, and mixing carbon black with the ash obtained by burning rice husks and then heating it in a non-oxidizing atmosphere. .

However, the properties of rice husk, which is an agricultural by-product, vary depending on the weather, the place of origin, the variety, etc., and various impurities and foreign substances are easily mixed in during the collection and transportation process, so the quality of the obtained silicon carbide whiskers is stable. There was a drawback that it was difficult to do. In addition, in the method of heating the rice husks, the silicon carbide crystals in the form of fine particles are heated at a weight ratio almost twice that of the obtained silicon carbide whiskers, and further the carbide of the rice husks and the mixture of the ash of the rice husks and the carbon black are heated. In the method, fine silicon carbide crystals in the same weight ratio are obtained as a mixture of whiskers and fine crystals that are by-produced at the same time, and thus silicon carbide whiskers cannot be obtained in high yield. It is not easy to separate and take out only the silicon carbide whiskers from this mixture, and there is a drawback that even after the separation operation, a small amount of fine-grained crystals remain in the whiskers.

[Purpose of the present invention]

It is an object of the present invention to provide a novel method for producing silicon carbide whiskers in high yield, with little whiskers with short curvature and by-products of silicon carbide crystals in the form of fine particles.

[Means for solving problems]

As a result of intensive studies to solve the above-mentioned problems of the prior art, the present inventors have found that when heating a powder containing a silicon oxide powder and a simple substance carbon powder, a heating element is heated in the presence of an air-permeable structure. By doing so, the voids generated by the contraction of the reaction product are dispersed appropriately inside, and as a result, the difference in the shape of the silicon carbide whiskers generated due to the heating position is extremely small, and whiskers and fine particles with short curvature are often present. The present invention has been completed by discovering a phenomenon in which the number of silicon carbide crystals in the form of crystals is significantly reduced.

That is, in the method for producing a silicon carbide whisker of the present invention, the silicon oxide powder and the simple substance carbon powder are mixed, and the bulk specific gravity is 0.
When heating a powder composition of 2 g / cc or less at 1300 to 1800 ° C., a breathable structure having continuous voids is formed in the heating part, and the powder composition is filled in the void part of the structure. It is characterized in that the above-mentioned heating is carried out in a closed state.

[Detailed Disclosure of the Invention]

 Hereinafter, the present invention will be described in detail.

Examples of the silicon oxide usable in the present invention include silica stone, silica gel, aerosil and the like. On the other hand, simple carbon includes carbon black, graphite, coke and the like. By pulverizing and thoroughly mixing these silicon oxides and simple substance carbon, it is possible to obtain a powder composition containing a silicon oxide powder having a bulk specific gravity of 0.2 g / cc or less and a simple substance carbon powder.

As a device for mixing silicon oxide and elementary carbon powder,
Suitable are those in which the powder of silicon oxide and the powder of elemental carbon are unlikely to be consolidated during the mixing process, for example, each mixer such as double circle type, V type, ribbon type, or air flow type floating mixer.

When producing silicon carbide whiskers from powdered silicon oxide and elemental carbon, it is generally said that the vapor phase is involved in the growth of the whiskers, and the silicon carbide whiskers are grown in a non-oxidizing atmosphere. In the following case (1)
It is considered that the silicon carbide whiskers grow through the processes of formulas (2) and (3).

SiO ₂ (s) ＋ C (s) → SiO ₂ (g) + CO (g) ・ ・ ・ (1) SiO (g) ＋ 3CO (g) → SiO ＋ 2CO ₂ (g) ・ ・ ・ ・ (2) CO ₂ (g ) + C (s) → 2CO (g) ... (3) Therefore, it is important to efficiently and uniformly generate SiO gas and CO gas.

For this purpose, it is desirable that the silicon oxide powder and the elemental carbon powder are mixed as uniformly as possible in a powder composition. As described above, in order to obtain a mixture as uniform as possible, the silicon oxide powder and the simple substance carbon powder are preferably as fine as possible, and particularly preferably those having a particle size of submicron unit.

In the present invention, the specific surface area of the powder composition is used as a simple measure of the average particle size of the powdery solid matter. Originally, each powdery solid has a unique shape and particle size distribution, so it is extremely difficult to accurately measure and display the particle size and particle size distribution of the entire powder. It is convenient to measure the amount of a substance, such as nitrogen gas, that is adsorbed on the, and use this as a scale corresponding to the average particle diameter, and this is also used in the present invention. A large nitrogen adsorption specific surface area means that the average particle diameter is small.

The powder composition containing the silicon oxide powder and the elemental carbon powder obtained in this manner is heated to 1300 to 1800 ° C., preferably 1400 ° C., using a high frequency heating furnace, an electric resistance heating furnace, or the like.
By heating to ˜1600 ° C., it can be made into a silicon carbide whisker.

However, the silicon carbide whiskers thus obtained are, as described above, good whiskers that grow long and straight in the part close to the wall surface of the reaction vessel, but whiskers and fine particles with a short center and many curves. We have already found that crystal-like crystals are formed, which is not preferable.

MEANS TO SOLVE THE PROBLEM The present invention solves this problem by heating a powder composition containing silicon oxide powder and elemental carbon powder to obtain silicon carbide whiskers. A body is formed, and the heating is performed in a state where the powder composition of the silicon oxide powder and the simple substance carbon powder is filled in the voids of the structure.

The breathable structure referred to in the present invention has the same meaning as that described in the specification of Japanese Patent Application No. 59-137091 already proposed by the present inventors, and has a three-dimensional small piece as described below. Is a structural unit, and is a three-dimensional structure formed by stacking a large number of the structural units and forming continuous voids capable of venting. Further, it means a thin carbonaceous material in which the continuous voids are retained even by heating at 1300 to 1800 ° C. As the shape of the three-dimensional small piece as a structural unit, a cylinder, a cylinder, a cone, a sphere, a triangular prism, a triangular pyramid, a cube, a quadrangular prism, a quadrangular pyramid, a tetrapot shape, or a string shape is suitable, and their size is a cylinder. , Cone, triangular prism, etc., the bottom area is 0.1-20c
m ² , preferably 0.5-5 cm ² , height 0.2-1
0 cm, preferably 0.5 to 5 cm is suitable, and in the case of a string, the width is 0.1 to 5 cm, preferably 0.2 to 2
cm, length 0.5 to 20 cm, preferably 1 to 10 c
m is preferred.

FIG. 3 shows an example of the shape of a small piece that is a structural unit for forming the breathable structure as described above as a precaution, but the regular shape as shown in (a) to (f) is shown. Not limited to the horse saddle shape shown in (g),
The terraret shape shown in (h), a long string (ri), and a short string (nu) may be used. By stacking a large number of one type or two or more types thereof in a regular or irregular manner, a breathable structure having continuous void portions can be easily formed, as will be easily understood.

In order to fill the voids thus formed with the powder composition containing the silicon oxide powder and the elemental carbon powder, first, the structural unit pieces are regularly or irregularly filled, and then the silicon is added. A powder composition containing an oxide powder and a simple substance carbon powder may be added and allowed to flow into the voids, or structural unit pieces and the composition may be filled and mixed before forming a filling layer.

When the latter method is adopted, the powder composition containing the silicon oxide powder and the simple substance carbon powder is heated in the coexistence of the breathable structure composed of these small pieces, in other words, the silicon oxide powder. And a powder composition containing a simple substance carbon powder and a large number of small pieces are mixed and filled in a heating furnace, a silicon oxide powder and a simple substance carbon powder are formed in a void generated by stacking a large number of small pieces. A silicon carbide whisker having a uniform shape can be obtained by heating to 1300 to 1800 ° C. in the presence of the powder composition containing a. Here, if the shape of the small piece is, for example, a cylinder and the inside of the small piece is also a continuous void, the fillable volume occupied by the powder composition in the heating furnace can be prevented from being lowered so much. In the step of heating the powder composition containing the silicon oxide powder and the simple substance carbon powder in the coexistence of the breathable structure, when the oxygen is present in the heating atmosphere, the simple substance carbon is burned and removed, so that argon, helium, nitrogen, etc. It is preferable to heat in a non-oxidizing atmosphere. However,
Under normal conditions, carbon monoxide is by-produced in the process of producing silicon carbide from the carbon-containing powder composition, and naturally becomes a non-oxidizing atmosphere.Therefore, in the present invention, it is not necessary to take special measures for deoxidation. Absent.

A powder composition containing a silicon oxide powder and a simple carbon powder,
A substance that mixes a small piece that is a structural unit that constitutes a breathable structure is also a device in which a powder composition containing a silicon carbide powder and a simple substance carbon powder is less likely to be compacted during the mixing process, for example, a double-circle Sui type, V Type, ribbon type mixers, or air flow type floating mixers are suitable. Here, if the weight of the small piece is large, the powder composition containing the silicon oxide powder and the elemental carbon powder is likely to be compacted during this mixing process. Therefore, it is preferable that the small piece has a light weight and thin shape.

The material of the small pieces in the present invention is 1300 to 1800 ° C.
The shape does not change much even when heated, and it is optional as long as it can maintain voids. Graphite, ceramics, porcelain with a thickness of several millimeters or less, or heat-resistant ceramics such as alumina and zirconia can also be used suitably. However, it is more preferable to use a material containing cellulosic as a main component, that is, paper such as so-called western paper or paperboard. The reason is that paper is a thin substance itself, is easy to process into various shapes as compared with other materials, and is cheap in price. These papers are 1300-18
When heated to 00 ° C, it is carbonized to become a thin charcoal, and the size shrinks by 10 to 35% compared to before heating.
The shape remains almost original and continuous voids can be maintained.

The small pieces formed of paper containing cellulosic as a main component can be burned and removed as described below after the whiskers are formed, but can also be reused after being separated from the generated whiskers. In this case, since the paper is carbonized while maintaining its original shape, it will no longer shrink and can be used more preferably for the purpose of the present invention. Of course, the carbonization may be performed by heating in a non-oxidizing atmosphere such as an inert gas from the beginning.

The bulk specific gravity of the powder composition containing the silicon oxide powder coexisting with the breathable structure and the simple substance carbon powder is 0.2 g / cc or less, preferably 0.15 g / cc or less before heating. desirable. Bare bulk density is 0.2 g / cc
This is based on the experimental findings of the present inventors that the tendency that the silicon carbide obtained by heating tends to become granular or curved is rapidly increased.

The ratio of carbon to silicon oxide in the powder composition containing the silicon oxide powder and the elementary carbon powder is C / Si (formula ratio, that is, defined as g-atom C / g-atom Si) of at least 1. 5 or more, preferably 2.5 or more is preferable for the purpose of obtaining a silicon carbide whisker having a uniform shape. If the C / Si formula weight ratio is less than 1.5, the shape of the silicon carbide whiskers tends to be non-uniform, and the silicon oxide remains unreacted in the silicon carbide whiskers, which is not preferable.

The remaining silicon oxide can be easily removed by washing the generated silicon carbide whiskers with hydrofluoric acid or the like, but the operation is complicated and the silicon oxide is lost. It is not necessary to set the upper limit of the C / Si formula weight ratio, but if it is set too large, for example, C / S
Setting i as high as 20 to 30 or more results in mere loss of the carbon compound.

As a result of the practice of the present invention, the obtained silicon carbide whiskers may contain elemental carbon, but this residual carbon causes the whiskers to fall in the presence of oxygen at 500 to 1000 ° C.
It can be easily removed by heating and burning it. Specifically, it can be easily carried out by heating in air or by placing the fuel in a hot gas atmosphere containing oxygen burned with excess air.

〔The invention's effect〕

When a silicon carbide whisker is produced from a powder composition containing a silicon oxide powder and a simple carbon powder, it is preferable that the powder composition be extremely fine in order to obtain a good whisker with a high yield. When it becomes, it is not enough.

The present invention is to obtain a silicon carbide whisker with a uniform shape in a high yield even if the apparatus is upsized by heating the powder composition in a state where the voids of the breathable structure of the present invention are filled. The industrial profitability is extremely high in that it enables an economical manufacturing method of silicon carbide whiskers on an industrial scale.

Hereinafter, embodiments of the present invention will be described more specifically with reference to Examples.

The whiskers obtained as a result of the experiment were evaluated as follows. That is, the length (L) and the width (D) of each of a large number (about 400 to 600) of whiskers are obtained by using an electron microscope image, and L and L / D are obtained.
The arithmetic average value of (aspect ratio) was used as a factor for evaluating the shape. When whiskers are used as a reinforcing agent in addition to metals, ceramics, etc., it is desirable that the whiskers have a linear shape without branching or curving because the dispersed state in the metal or the like tends to be uniform. And that the longer the length (L) is and the larger the ratio of the length to the diameter (D) (aspect ratio: L / D), the greater the effect as a reinforcing agent. Is taken into consideration.

Mixing the Example 1 SiO ₂ powder (specific surface Seto 190m ² / g) 200g of carbon black (specific surface area 110m ² / g) 400g with a V mixer, a powder composition comprising an SiO ₂ powder and elemental carbon powder 600g Got

Powder composition 60 containing this SiO ₂ powder and elementary carbon powder
0g, outer diameter 15mm, length 2 made from kraft paper
A cylinder with a thickness of 0 mm and a thickness of 0.2 mm (Fig. 3 (b)) 150
After mixing again with a V-type mixer,
It was filled in an electric resistance furnace having a heating space of 80 mm and a height of 600 mm. The bulk specific gravity of the powder containing SiO ₂ and elemental carbon filled in the furnace was 0.075 g / cc, and the apparent volume occupied in the furnace in the state of coexisting with the cylinder as the structural unit and the small pieces was 8.0.

This was heated in an argon atmosphere at 1500 ° C. for 2 hours to generate silicon carbide. After heating, the apparent volume had dropped to 6.5, and the cylindrical kraft paper had carbonized and shrank by about 20% on one side, but the cylindrical shape remained substantially. Was there.

After cooling this once, it is heated to 700 ° C. in air to burn and remove the remaining elemental carbon and carbonized kraft paper, and further wash and filter the remaining SiO ₂ and ash of kraft paper with a hydrofluoric acid aqueous solution to carbonize. 129 g of silicon was obtained. As a result of powder X-ray diffraction spectrum analysis, the crystal shape was β type, and as a result of electron microscope image observation, only needle-like whiskers were observed, and variations in length (L) and aspect ratio (L / D) were small. , And their arithmetic mean values were 24 μm and 48, respectively. The electron microscope is shown in FIG.

Comparative Example 1 In exactly the same manner as in Example 1 except that 600 g of the powder composition containing the SiO ₂ powder obtained in Example 1 and the simple substance carbon powder was not mixed with a cylinder that is a structural unit made of kraft paper. Heated to generate silicon carbide. The bulk specific gravity of the powder composition containing the SiO ₂ powder and the simple substance powder in the state filled in the furnace is 0.075 / g / cc, and the apparent volume is 7.9.
However, the apparent volume after heating was 3.1.
Was falling to. From this, in the same manner as in Example 1, the remaining elemental carbon and SiO ₂ were removed to obtain 151 g of silicon carbide.
Got

As a result of powder X-ray diffraction spectrum analysis, the crystal shape was β type, and as a result of electron microscope image observation, it was observed that the shape was different depending on the position where silicon carbide was generated, and the apparent volume was reduced. Although it was confirmed that whiskers that grew straight were generated in a portion of the heating furnace having a thickness of about 10 mm, which was close to the furnace wall, the inside thereof was shown in the electron micrograph of FIG. So short,
It was observed that whiskers with many bends were produced in large quantities, and the arithmetic mean of L and L / D was 9 μm,
It was 24.

Example 2 A silica gel (SiO ₂ 99.7) obtained by decomposing sodium silicate (water glass) with hydrochloric acid and washing the coagulated product with water and drying.
%) Was pulverized to obtain silica gel fine powder having a specific surface area of 42 m ² / g. 270 g of this silica gel fine powder and 560 g of carbon black (specific surface area 94 m ² / g) were mixed by a V-type mixer, and a powder composition 8 containing SiO ₂ powder and a simple substance powder.
30 g was obtained.

830 g of a powder composition containing this SiO ₂ powder and carbon powder
And a roll paper material of 0.3 mm thickness, 5 mm width,
String-shaped small piece 2700 with a length of 7 cm as shown in Fig. 3 (nu)
After mixing again with the V type mixer again, the inner diameter 18
It was filled in an electric resistance furnace having a heating space of 0 mm and a height of 600 mm. The bulk specific gravity of the powder composition containing SiO ₂ powder and carbon powder filled in the furnace was 0.11 g / cc, and the apparent volume occupied in the furnace was 7.5 in the state of coexisting with the cylindrical small piece as the structural unit. It was

This was heated in an argon gas atmosphere at 1600 ° C. for 4 hours to generate silicon carbide. In the state after heating, the apparent volume was reduced to 5.7, and the string-shaped small pieces were carbonized and contracted by about 20% on one side, but the string-shaped shape was substantially stopped. .

After cooling this once, it is heated to 700 ° C. in the air to burn and remove the remaining elemental carbon and the carbonized kraft paper, and further wash and filter the remaining silicon oxide and ash of the kraft paper with an aqueous hydrofluoric acid solution. 203 g of silicon carbide was obtained. As a result of powder X-ray diffraction spectrum analysis, the crystal form was β type,
As a result of electron microscope image observation, only needle-like whiskers were observed, the fluctuations in the length (L) and the aspect ratio (L / D) were small, and the arithmetic average values thereof were 18 μm and 35, respectively.
Met.

As described above, as can be seen from Examples and Comparative Examples, when a silicon carbide whisker is produced from a powder composition containing a silicon oxide powder and a simple carbon powder, the powder composition is present in the voids of the breathable structure of the present invention. When heated in a filled state, whiskers having a uniform shape and a small number of bends, which are uniform in length and aspect ratio, can be obtained even if the apparatus is enlarged as shown in Examples 1 and 2.

On the other hand, when the breathable structure of the present invention is not used,
As shown in Comparative Example 1, it is understood that a large amount of short whiskers with many bends are mixed.

[Brief description of drawings]

1 and 2 are photographs showing silicon carbide whisker crystals taken by using a scanning electron microscope. The magnification is 1000 times in FIG. 1 and 2000 times in FIG. FIG. 3 is a perspective view showing an example of the shape of a small piece which is a structural unit for forming the breathable structure used in the present invention.

Claims (3)

[Claims] 1. A powder composition having a bulk specific gravity of 0.2 g / cc or less, which is obtained by mixing a silicon oxide powder and a simple carbon powder.
When heating at 0 to 1800 ° C., a breathable structure having continuous voids is formed in the heating portion, and the heating is performed in a state where the powder composition is filled in the voids of the structure. A method for producing a silicon carbide whisker, which is characterized. 2. A gas permeable structure comprising a plurality of small pieces each having a shape selected from the group consisting of a cylinder, a cylinder, a triangular prism, and a quadrangular prism, as a structural unit, and stacking them three-dimensionally. the method of. 3. The method according to claim 1 or 2, wherein the main component of the breathable structure is cellulosic.