JPH02111700A - Production of silicon carbide whisker - Google Patents

Abstract

PURPOSE:To obtain SiC whisker of large diameter suitable for whisker- reinforced ceramics by reaction under heating in a H2-contg. air stream of a mixture of not higher than a specified bulk density comprising silica, carbon and catalyst. CONSTITUTION:A mixture <=0.1g/cm<2> in bulk density comprising silica, carbon and catalyst is first prepared. In this case, for the silica, >=99.7% in purity is preferable, the content of impurities except H, N, S and O in the carbon being <=200ppm, and for the catalyst, metallic or alloy powder containing at least one kind of metal selected from Fe, Ni, Co and Cr is used. It is preferable that the amount of the catalyst to be added be 0.3-10 pts.wt. based on 100 pts.wt. of the silica. Said mixture is made to react at >=1300 deg.C in a H2-contg. air stream, i.e., a mixed air stream made up of H2 gas and inert gas except N2 gas. Thereby the objective SiC whisker of large diameter (an average of 1.5-20mum) can be obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for producing silicon carbide whiskers, and more specifically to an economical and industrial production of silicon carbide whiskers with a large diameter suitable for whisker-reinforced ceramics and mixes. Regarding the method.

<Prior Art> Methods for producing silicon carbide whiskers can be broadly divided into gaseous raw material methods and solid raw material methods, and solid raw material methods can be divided into transportation methods and bulk methods.

The gaseous raw material method uses hydrocarbon gas as a carbon source and 5i as a silicon source.
This method uses a silicon-containing gas such as C1a as a raw material, and specifically, it is a method in which the above-mentioned gas is introduced onto a heated substrate to grow silicon carbide whiskers on the substrate. The advantage is that the aspect ratio of the resulting whiskers is large, but harvesting in the evening sun requires a large substrate area, and the disadvantage is that the utilization of introduced gas is low, making it unsuitable for large-scale production. .

Among the solid raw material methods, the transportation method uses solids such as carbon powder as a carbon source and solids such as silica powder as a silicon source.The raw materials are heated and the generated gas is transferred to the surface of the solid raw material packed bed or This is a method in which solid materials are moved to a location away from where they are placed and whiskers are generated there. This method has the advantage that the aspect ratio of the resulting whiskers is large, but has the disadvantage that a large area is required to produce a large amount of acid phase, and this method is also not suitable for large-scale production.

In addition, the back bulk method of the solid raw material method uses a solid such as carbon powder as a carbon source and a solid such as silica powder as a silicon raw material. Sufficient voids are introduced into the raw material in advance to allow whiskers to grow. In this method, whiskers are grown in the voids introduced into the bulk by heat treatment in an inert atmosphere. This method has the advantage of easy mass production because whiskers grow in a solid powder packed bed, but it is difficult to control the size of the voids, so the aspect ratio varies widely and the average aspect ratio is small. It had the following drawback.

In addition, if there are insufficient voids, silicon carbide powder is likely to be produced, so it is difficult to avoid the presence of silicon carbide powder, and there are drawbacks such as the need to separate whiskers and powder in the next step.

However, despite these drawbacks, this method is currently industrialized.

On the other hand, Becher et al.
anics ofCera+5ics, vol, '
L 61-73+ PIenu+* Press (1
986),] or as reported by Yasuda et al. [Developments in the Production of Ceramic Composite Materials J, 34-37. Japan Institute of Metals Symposium (1988)], in silicon carbide whisker reinforced ceramics, whisker The toughness of ceramics improves as the diameter of
It was so thin that it was not possible to make it sufficiently tough.

Conventionally, for the production of silicon carbide whiskers with a diameter larger than 1, Gac et al. (J, Mater, Sci, 20
．． 1160-68 (1985)) or G,
8. Bootsma et al. (J, Cryst, Gr.
owth, 11.297-309 (1971))
As reported in , transport methods have been used, but as mentioned above, although these methods can be produced in the laboratory, they are not industrially advantageous.

<Problem to be solved by the invention> Previously, the inventors developed an economical and industrial method for producing silicon carbide whiskers using a solid raw material method/bulk method, and filed the application as Japanese Patent Application No. 196195-1983. The present invention is an improvement on this, and aims to propose a method for producing silicon carbide whiskers having a diameter of 1n or more.

<Means for Solving the Problems> The present invention comprises silica, carbon, and a catalyst with a bulk density of 0.
A method for producing silicon carbide whiskers, which is characterized by subjecting a mixture of 1 g/c4 or less to a heating reaction in an air flow containing hydrogen, preferably having a purity of silica of 99.7% or more,
Or even more H in carbon.

Impurities other than N, S, and O are 200 or less, or the catalyst is a metal powder or alloy powder containing one or more of Fe, Ni, Co, and Cr, and the average particle size is 1.5
This is a method for producing silicon carbide whiskers having a grain size of ~20μ.

Effect> As a result of intensive research into the growth of silicon carbide whiskers with a large diameter suitable for whisker-reinforced ceramics, the inventors obtained the following knowledge and arrived at the present invention. In other words, VLS (Vaper-Liquid-
5olid)! Silicon carbide whiskers grow according to the a-structure. ■There is a certain relationship between the diameter of the growing whiskers and the diameter of the catalyst. ■ Fine Si using impurities in raw materials as a catalyst
C whiskers grow.

Therefore, in order to synthesize whiskers with a large diameter using the solid raw material bulk method, it is important to control the bulk density of the raw material, the purity of the raw material, the type and size of the catalyst, and the rate of reaction.

In particular, in the method for producing silicon carbide whiskers according to the present invention, silica and carbon are mixed together with a catalyst, the bulk density of the mixture is limited to 0.1 g/cry or less, and this is reacted in a hydrogen-containing gas stream. Fe as a catalyst
+ Coo If a metal powder containing 111 or more of Ni and Cr is used, it is possible to obtain good silicon carbide whiskers. Specifically, iron powder.

Metal particles such as stainless steel copper powder, Ni powder, and Co powder, and oxides and salts of the above-mentioned elements that substantially become the above-mentioned metal particles at the reaction temperature can be used.

The amount of catalyst added is preferably 0.3 parts by weight to 10 parts by weight per 100 parts by weight of silica. If it is above the surface area, although whiskers are generated, they do not grow sufficiently and many short whiskers are generated, which is not preferable. The amount is preferably 0.3 to 5 parts by weight.

Further, the average particle size of the catalyst is preferably 1.5 to 2 opm.

If the diameter is less than 1.5 μm, only whiskers of 0.5 pm or less will be generated; if the diameter is greater than 204, it will be difficult to grow whiskers. Preferably from 3 pm to lO#ffi.

The bulk density of the mixture of these powders is limited to Q, 1 g/c+d or less. When the bulk density of the mixture is greater than 0.1 g/cJ, silicon carbide whiskers are not produced, or even if they are produced, the aspect ratio is small and silicon carbide particles are mixed in, resulting in poor quality whiskers. Preferably, the bulk density of the mixture is 0.07 g/crl or less.

The mixing ratio of the silicon-containing powder and carbonaceous powder is the silicon carbide whisker formation reaction SiO*+3C→SiC(w)→−2CO・−・・・
−・−・−(Determined by the equivalent weight indicated by +1.

If the amount of carbon is less than the equivalent of formula (1), unreacted silica will remain, and if the amount of carbon is more than the equivalent, unreacted carbon will remain. The mixing ratio is preferably around equivalent.

When silica remains, it can be removed, for example, by treatment with hydrofluoric acid, and when carbon remains, it can be oxidized and decarburized, for example, by heating to 700° C. in air. They can be used properly depending on the process.

The gas flow containing hydrogen in the present invention refers to a mixed gas flow of hydrogen gas and an inert gas excluding N2 gas, and the concentration of hydrogen gas is preferably 15% or more, preferably 50% or more. Further, pure hydrogen gas is more preferable; if the hydrogen gas concentration is less than 15%, the reaction will not proceed sufficiently, resulting in the formation of inferior whiskers or stale silicon carbide.

It is necessary that the hydrogen-containing gas is circulating, and it is not enough to simply create a hydrogen-containing atmosphere. The amount of hydrogen-containing gas required to be introduced into the system takes a constant ratio depending on the amount of the charging layer t4. That is, the amount of hydrogen gas introduced into the system V (Nc
Define the ratio r value of W (g) and charging layer j4W (g), and it is desirable that the r value is 5 or more and 400 or less. If the r value is less than 5, the properties of the resulting whisker becomes inferior. If it is larger than 400, it becomes uneconomical. Preferably IO≦r≦300.

Although it is not necessarily clear why the formation of silicon carbide whiskers is promoted in a hydrogen gas-containing air stream, the following reasons may be considered.

The whisker production reaction is considered to be a reaction via the gas phase. That is, when the solid raw material is heated, SiO as a silicon source evaporates, and it reacts with a carbon source in a gas phase to grow whiskers. In the absence of hydrogen, SiO(g) + 2CO→SiC+C
The reaction of Oz ------(2) is considered. On the other hand, when hydrogen is present, the surrounding carbon and hydrogen react to form CI+.

C211□ is generated, SiO(g) + 2C11,→SiC(w) +G
A reaction like O+ 411z·-·····-(3) occurs, and the reaction (3) is estimated to be more efficient than the reaction (2). Furthermore, it is presumed that the reaction continues to occur due to the CO being discharged from the system by the hydrogen gas flow.

In producing silicon carbide whiskers, the above mixture is filled into a crucible or sheath made of carbonaceous material or the like.

It is preferable to have a structure that can ensure ventilation during filling.

It is necessary that the heating reaction temperature of the mixture is 1300'C or higher. At temperatures lower than 1,300°C, whisker formation is hardly observed, and at temperatures above 1,700°C, whisker formation is observed, but there is a lot of loss to the outside of the system due to evaporation, which is not very beneficial. The reaction temperature ranges from 1400°C to 1650''C.

In recovering silicon carbide whiskers from the heat-treated body, known methods can be used depending on the sio, /c ratio of the raw materials as described above. For example, if silicon-containing powder is in excess, hydrofluoric acid treatment By dissolving and removing the remaining SiO□, silicon carbide whiskers can be obtained. Also,
In the case of excess carbon, the remaining carbonaceous powder can be oxidized and decarburized at a temperature of around 700'C in an oxidizing atmosphere, or it can be mixed and dispersed in a water/kerosene mixture, and the whiskers that have migrated to the water side can be collected by filtration. There are many ways to do this.

The purity of the silica used in the method for producing silicon carbide whiskers according to the present invention is preferably 99.7% or higher. If the purity of silica is lower than 99.7%, the whiskers will grow using impurities contained in the silica as a catalyst. minute whiskers are generated.

Examples of silica include silica flower, white carbon,
Silicic anhydride, silica gel, silica sol, etc. can be used.

In addition, as carbon, it is desirable to use highly pure carbon powder with impurities other than II, N, O, and S of 200 F or less. If the above impurities are more than 200 F, whiskers with a fine diameter will grow using the impurities contained in the carbon as a catalyst. do. As the carbon source, powders such as carbon black, acetylene black, incomplete combustion of organic matter, incomplete combustion residual carbon of organic matter, etc. can be used.

The reason why the amount of carbon impurities is more sensitive to the growth of fine whiskers than the amount of silica impurities is not necessarily clear, but tit is determined as follows.

Sunarichi, silica is heated to 1000°, which is close to the processing temperature mentioned above.
Sintering occurs from around C and the surface area of the silica itself decreases, so impurities are confined within the sintered silica grains, whereas in the case of carbon, even a small amount of impurities can be used as a catalyst as it does not cause sintering. considered to be a thing.

<Example> The present invention will be described in more detail below with reference to Examples.

Example 1 Anilosol 1 of 99.8% pure silica as a silica source
00 parts by weight, 60 parts by weight of carbon black containing 50 parts of impurities other than H, N, O, and S as a carbon source, and a particle size of 2.
Two parts by weight of 5-6 pm stainless steel powder were mixed to obtain a mixture having a bulk density of 0.049 g/ca. 3 g of this mixture was placed in a graphite crucible and fired at 1500° C. for 2 hours in an 11□ gas flow of 10 ON cffl/min.

The product was greenish-gray in color, and only a SiC peak was observed in X-ray diffraction analysis. () in air at 650°C.
The remaining carbon was oxidized and decarburized to obtain silicon carbide whiskers. The diameter of the obtained whisker is 1.
0-2.5pffi+ Average diameter 1.7ttm.

Length 20-2004. The silicon carbide whiskers had an aspect ratio of 20 to 100, had uniform diameters, were free of thin whiskers, and had little bending. Furthermore, the yield of whiskers was as high as 92%, calculated from the silicon source of the raw material.

Example 2 Aerosol 1 of 99.8% pure silica as a silica source
0 Offl parts, IT, N, O as carbon sources
, 100 parts by weight of carbon black containing 20 rm of impurities other than S and 1 part by weight of iron powder having a particle size of 4 to 8- were mixed to obtain a mixture having a bulk density of 0.054. 10g of this mixture was placed in a graphite crucible and placed in an air flow of 50ONcj/min.
Fired at 550"C for 2 hours.

The product was greenish-gray in color, and only a SiC peak was observed in X-ray diffraction analysis. The obtained product was fired in air at 700°C, and the remaining carbon was oxidized and decarburized to obtain silicon carbide whiskers. The diameter of the obtained whiskers is 1.9-3
．． 9-+ average diameter 2,8 rrm, length 25-20
The silicon carbide whiskers had a diameter of 0 pll, an aspect ratio of 20 to 100, a uniform diameter, no thin whiskers mixed in, and few bends. The whisker yield was also as high as 90%.

Example 3 Aerosol 1 of 99.8% pure silica as a silica source
00 parts by weight, Il, N, O, S as carbon sources
100 parts by weight of carbon black containing 30 - of impurities other than the above and 1 part by weight of iron powder having a particle size of 2.5 to 6 Ina were mixed to obtain a mixture having a bulk density of 0.044 g/c++I. Put 300g of this mixture into a graphite crucible, 11. It was fired at 1550° C. for 2 hours in a gas flow of 2ONl for 1 minute.

The product was greenish-gray in color, and only a SiC peak was observed in X-ray diffraction analysis. The obtained product was heated at 650°C'' in air.
The remaining carbon was oxidized and decarburized to obtain silicon carbide whiskers. The obtained silicon carbide whiskers have a diameter of 0.9 to 2.6,11111. The average diameter is 1.6n, the length is 25-200μ, and the aspect ratio is 25-100.
The silicon carbide whiskers had a uniform diameter, were not mixed with thin whiskers, and were less likely to bend. The whisker yield was also as high as 92%.

Comparative Example 1 Using the silica and carbon used in Example 1, silica 10
A mixture of the two, containing no catalyst and 60 parts by weight of carbon and having a bulk density of 0.048 B/cJ, was obtained. 3 g of this mixture was placed in a graphite crucible and heat treated under the same conditions as in Example 1.

The product was grayish black, and X-ray diffraction showed it to be a single phase of SiC, but when the product was calcined in air at 650°C and some remaining carbon was oxidized and decarburized, microscopic observation showed that , 0.2 to 0, s μs was observed, and no silicon carbide whiskers were observed.

Example 4 Silica gel powder 100 with a purity of 99.5% as a silica source
Part IItft, 60 parts by weight of the same carbon black as in Example 1 as a carbon source, and 2 parts by weight of stainless steel powder with a particle size of 2.5 to 6- were mixed to obtain a bulk density of 0.053 g/c.
A mixture of d was obtained, and this mixture was heat-treated in the same manner as in Example 1.

The product was greenish-gray and X-ray diffraction showed it to be a single phase of SiC. Decarburization was performed in the same manner as in Example 1 to obtain silicon carbide whiskers. However, although the obtained whiskers had good length and aspect ratio, they had a diameter of 0.2 to 2.5 pII.
It was widely distributed, and whiskers with a small diameter were mixed in.

The yield of whiskers was as high as 90%, but the yield of lp- or higher whiskers was 64%.

Example 5 Aerosol 1 of 99.8% pure silica as silica source
00 parts by weight, 11.0 parts by weight as a carbon source. N, O, S
100 parts by weight of carbon black containing 300 parts of impurities other than 300 parts by weight and iron powder having a particle size of 2.5 to 64 mm were mixed to give a bulk density of 0.
047 g/cl of γn compound was obtained. This mixture was heat treated under the same conditions as in Example 3.

The product was green-gray in color and X-ray diffraction showed it to be a single phase of SiC. In the same manner as in Example 2, released silicon carbide whiskers were obtained.

However, although the length and aspect ratio of the obtained whiskers were good, the diameters were widely distributed from 0.15 to 2.4 μm, and whiskers with small diameters were mixed in. The whisker yield was as high as 91%, but the whisker yield of Ipm or higher was 59%.

Example 6 Aerosol of 99.8% pure silica as a silica source,
1000 impurities other than H, N, O, and S as a carbon source
When SiC whiskers were synthesized using p carbon black under the same conditions as in Example 5, the results were almost the same as in Example 5 (i:). The yield of whiskers was as high as 89%, but the yield of whiskers of 18 m or more was 47%.

Example 7 100 parts by weight of the same silica and 60 parts by weight of the same carbon as in Example 1, and Co with a particle size of 3.2 to 6.3 μm as a catalyst.
Parts by weight of the flour were mixed to obtain a mixture having a bulk density of 0.048 g/cd. Put 3g of this mixture into a graphite crucible and! 11
It was fired at 1500° C. for 12 hours in a gas flow of 100 Ncd/min.

The product was green-black in color, and xvA diffraction analysis showed only the peak of SiC. The obtained product was heated at 650°C in air.
The remaining carbon was oxidized and decarburized to obtain silicon carbide whiskers. The diameter of the obtained whisker is 1.
0-2.8 ttm, average 1.9n, length 25-200
1111. The silicon carbide whiskers had an aspect ratio of 20 to 100, had uniform diameters, were free of thin whiskers, and had little bending. The whisker recovery rate was 93%.

<Effects of the Invention> As described above, according to the method of the present invention, it is possible to uniformly synthesize whiskers with a large diameter that lead to whisker-reinforced ceramics, as well as to synthesize thin whiskers with less contamination of tR and silicon carbide powder. becomes. In addition, it can be carried out efficiently and on an industrial scale, reducing manufacturing costs, and will greatly contribute to the future development of ceramic composite materials.

Claims (1)

[Claims] 1. Bulk density 0.1g consisting of silica, carbon and catalyst
1. A method for producing silicon carbide whiskers, which comprises heating and reacting a mixture with a diameter of /cm^2 or less in an air flow containing hydrogen. 2. A method for producing silicon carbide whiskers, characterized in that the silica according to claim 1 has a purity of 99.7% or more. 3. H, N, S, O in the carbon according to claim 1 or 2
A method for producing silicon carbide whiskers, characterized in that the content of impurities other than 200 ppm or less is 200 ppm or less. 4. The catalyst according to claim 1, 2 or 3 contains Fe, Ni, Co
A method for producing silicon carbide whiskers, characterized in that the metal powder or alloy powder contains one or more of the following: and Cr, and has an average particle size of 1.5 to 20 μm.