JPH0613440B2 - Method for manufacturing silicon nitride whiskers - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an improved process for producing silicon nitride whiskers.

Whiskers mean needle-like crystals, and since there are almost no lattice defects in needle-like crystals, their mechanical strength is a material whose strength is close to the ideal maximum expected for the material. Further, silicon nitrogen is a compound having excellent heat resistance and corrosion resistance, which has been attracting attention in recent years.

Therefore, silicon nitride whiskers combine these properties, that is, they are new materials that excel in mechanical strength and heat / corrosion resistance, so similar to silicon carbide whiskers, in addition to metals and ceramics, these mechanical strength and It is expected to be used as a reinforcing agent that enhances heat and corrosion resistance.

[Prior art]

Conventional techniques for manufacturing silicon nitride whiskers include:
Although a method of producing using a gaseous raw material such as SiCl ₄ has been proposed, a method of producing from a solid raw material, that is, a mixture of silicon oxide and elemental carbon is a general industrial production method. The method for producing silicon nitride whiskers from such a solid raw material is a degradable silicon compound such as natural silica stone or SiCl _4, so-called obtained by hydrolyzing in a steam atmosphere, graphite and silicon oxide such as Aerosil,
Crush and mix carbon such as carbon black,
In a state of low bulk specific gravity in an atmosphere of nitrogen or ammonia,
It can be obtained by heating to 1200 to 1600 ° C. However, when this method was carried out on an industrial scale, according to our investigation, the apparent volume occupied by the reaction product, whiskers, with the progress of the reaction was as large as that when the mixture of the silicon oxide and the elemental carbon was charged. Since it decreases to less than half of the apparent volume, probably because the shrinkage of the apparent volume creates voids, although the portion close to the wall of the reaction vessel is long and straight, good whiskers can be obtained, but the central part is There was a problem that whiskers with short curvature and fine particles were generated.

Such whiskers having a large amount of curvature and whiskers mixed with fine-grained crystals have a drawback in that the reinforcing effect is not satisfactory even if they are added to metals or ceramics.

Further, as a method of producing nitrogen silicon whiskers from a solid raw material, in addition to the above method, a method of heating rice husks under a nitrogen atmosphere, a method of once heating the rice husks, the carbonized rice husks are re-nitrogenated Alternatively, a method of heating in an ammonia atmosphere, a method of mixing carbon black with an ashed product obtained by burning rice husks, and then heating in an atmosphere of nitrogen or ammonia have been proposed.

However, the properties of rice husks, which are agricultural by-products, differ depending on the weather, the place of origin, the variety, etc., and miscellaneous impurities and foreign substances are easily mixed in during the collection and transportation process. However, there was a shortcoming that it was difficult to stabilize. In addition, in the method of heating rice husks, fine-grained silicon nitride crystals are heated at a weight ratio almost twice that of the obtained silicon nitride whiskers, and further, the carbide of rice husks and the mixture of ash and carbon black of rice husks are heated. In the method, the silicon nitride crystals in the form of fine particles are simultaneously by-produced in the same weight ratio and obtained as a mixture of the whiskers and the fine particles, so that the silicon nitride whiskers cannot be obtained in a high yield, and However, it is not easy to separate and take out only the silicon nitride whiskers from this mixture, and there is a disadvantage that a small amount of fine-grained crystals remain in the whiskers after the separation operation.

The inventors of the present invention have made extensive studies to solve such problems.
As a result, a powder group containing silicon oxide powder and simple substance carbon powder
When heating the product under a nitrogen-containing compound atmosphere,
If a breathable structure is made to coexist in the heat part and heated, reaction is generated
The voids created by the contraction of an object are
Shape of the resulting silicon nitride whiskers
The difference due to the heating position of the
The whiskers with many bends and fine-grained silicon nitride crystals are remarkable.
We found a phenomenon in which the silicic acid
Compound powder and elemental carbon powder in a powder composition containing nitrogen or
When heating in a nitrogen-containing compound atmosphere,
Forming a breathable structure having continuous voids, the powder
Heating is performed with the composition filled in the voids of the structure.
I proposed a method.

If the breathable structure has a structural unit of small pieces having a shape such as a cylinder or a cylinder, the powder composition and the small pieces are simply mechanically mixed, and then the mixture is placed in a heating furnace. By adding the powder composition, the powder composition can be heated in a state of being filled in the voids of the breathable structure. In addition, here, the material of the small piece is a material whose main component is cellulose,
So-called paper is industrially extremely advantageous in that it is easy to process, inexpensive, and can be removed by burning.

As described above, when the carbon-containing composition is heated in the coexistence of the breathable structure, when the whisker of the defective product which is short and has many bends is mixed as a by-product, which is a conventional problem, the problem can be remarkably reduced. Most of the generated whiskers have a long straight growth shape. However, it is not completely satisfactory with regard to the distribution of these lengths, and depending on the conditions, there are cases in which ones that reach five times the average length or one third or less are generated, and It was discovered that defective short whiskers with many bends may also form as by-products.

[Purpose of the present invention]

An object of the present invention is to further improve the method proposed by the present invention, and to provide a method for producing silicon nitride whiskers at a high yield anytime without accompanying short and curved whiskers and by-products of fine-grained silicon nitride crystals. It is to be.

[Means for solving problems]

When the cause of the above is paper for the material of the breathable structure, the present inventors have found that when the paper is heated, it is pyrolyzed and carbonized, so that one side of the original dimension is 10-3
Shrink 5%. Due to this shrinkage, in the mixture of the powder composition containing the silicon oxide powder and the simple substance carbon powder, and the small pieces constituting the breathable structure, voids are generated in the inside or the outer peripheral portion thereof, while the small pieces A part of the powder composition is compacted by the compressive force associated with the shrinkage, and thus the air permeability may be non-uniform due to the spatial position of the powder composition in the furnace. Based on the conclusion that it is possible to avoid shrinkage of the breathable structure by using a carbonaceous substance as the material of the breathable structure,
The present invention has been completed.

That is, the method for manufacturing a silicon nitride whisker of the present invention has a bulk specific gravity of 0.2 g / cc including silicon oxide powder and elementary carbon powder.
The following powder composition was prepared under a nitrogen or nitrogen-containing compound atmosphere 12
Upon heating at 00 to 1600 ° C., a breathable structure composed of a thin carbonaceous material having continuous voids is formed in the heating part, and the powder composition is filled in the voids of the breathable structure. It is characterized in that heating is carried out in the state of being kept. Particularly, the bulk specific gravity including the silicon oxide powder and the simple substance carbon powder is 0.
A powder composition of 2 g / cc or less is charged with a decomposable silicon compound and a decomposable carbon compound in a hot gas containing water vapor and decomposed to produce a mixed aerosol dispersoid containing respective aerosols of silicon oxide and elemental carbon. It is more preferable that the powder composition is obtained by collecting the produced dispersoid by a solid-feeling operation.

[Detailed Disclosure of the Invention]

 The present invention will be described in detail below.

Examples of the silicon oxide usable in the present invention include silica stone, silica gel, aerosil and the like. On the other hand, examples of simple carbon include carbon black, graphite, and coke. By pulverizing and thoroughly mixing these silicon oxides and simple substance carbon, 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 can be obtained.

As a device for mixing the silicon oxide powder and the simple substance carbon powder, a device in which the silicon oxide powder and the simple substance carbon powder are less likely to be consolidated during the mixing process, for example, a double circle type, a V type,
Each mixer such as ribbon type or air flow type floating mixer is suitable.

When producing nitrided whiskers from powdered silicon oxide and a simple substance, it is generally said that the vapor phase is involved in the growth of whiskers, and when growing silicon nitride whiskers in an ammonia atmosphere, ( It is considered that the silicon nitride whiskers grow through the processes of formulas (1) and (2).

SiO ₂ (s) ＋ C (s) → SiO (g) ＋ CO (g) ……… (1) 3SiO (g) ＋ 4NH ₃ (g) → Si ₃ N ₄ (s) ＋ 3H ₂ O (g) ＋ 3H ₂ ( g) ··· (2) Therefore, it is important to efficiently and uniformly generate the SiO gas of formula (1).

For this purpose, it is desirable that the silicon oxide powder and the simple substance carbon powder are mixed as uniformly as possible into 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.

However, it is not always easy to mechanically grind silicon oxide such as silica stone, silica gel, aerosil, etc., or carbon as a simple substance such as coke to a particle size of submicron unit.

When a fine powder composition containing such a submicron silicon oxide powder and elemental carbon powder is particularly required, as already proposed by us, the decomposable silicon compound and the decomposed silicon compound are decomposed in a hot gas containing steam. It can be obtained by a method of charging and decomposing a reactive carbon compound to form a mixed aerosol dispersoid containing respective aerosols of silicon oxide and elementary carbon, and collecting the generated dispersoid by a solid-gas separation operation. I can.

The powder itself (hereinafter abbreviated as carbon-containing powder composition) itself has already been disclosed by the present inventors as described in JP-A-59-82.
It can be manufactured according to the method disclosed in detail in No. 922.

The method for producing the disclosed carbon-containing powder composition will be described in detail below as a reminder.

First, an elemental carbon aerosol can be easily obtained by charging a decomposable carbon compound into hot gas. On the other hand, an aerosol of silicon oxide can be obtained by charging a decomposable silicon compound such as SiCl ₄ into a hot gas containing steam to cause hydrolysis or oxidation. As can be easily understood, when a decomposable carbon compound and a decomposable silicon compound are simultaneously charged in a hot gas containing steam, a mixed aerosol containing elemental carbon and silicon oxide is immediately formed.

The decomposable silicon compound that can be used in the present invention is represented by the general formula Si _n X _{2n + 2} (n is an integer of 1 to 4), and X is a hydrogen atom, a halogen atom, an alkyl group or an alkoxyl group. Yes, such specific silicon compounds include SiCl ₄ , HSiCl ₃ , SiH ₄ , Si ₂ H ₆ , and (CH ₃ ) ₄ S.
i, (CH ₃ ) ₂ SiCl ₂ , CH ₃ SiCl ₃ , SiF ₄ , Si (OC ₂ H ₅ ) ₄ and the like.

These are used alone or as a mixture.

The decomposable carbon compound used in the production of the carbon-containing powder composition is a compound that can be easily decomposed to form simple carbon when charged in a hot gas as described later, and is a gas phase as it is. Alternatively, a liquid phase state or one that can easily be brought into a liquid phase state by increasing the temperature can be suitably used. LP
Petroleum products such as G, naphtha, gasoline, fuel oil, kerosene, light oil, heavy oil, lubricating oil, liquid paraffin; methane, ethane, propane, butane, pentane, methanol, ethanol, propanol, ethylene, acetylene, n-paraffin, Butadiene, isoprene, isobutylene, benzene, toluene, xylene, cyclohexane, cyclohexene, dicyclopentadiene, ethylbenzene, styrene, cumene, pseudocumene, mesitylene, alkylbenzene, α-methylstyrene, dicyclododecatriene, diisobutylene, vinyl chloride, chlorobenzene. ,
Petrochemical products such as C ₉ distillate mixture and ethylene bottom; tar products such as tar, pitch, creosote oil, naphthalene, anthracene, carbazole, tar acid, phenol, cresol, xylenol, pyridine, picoline and quinoline; large Examples include soybean oil, coconut oil, linseed oil, cottonseed oil, rapeseed oil, tung oil, castor oil, whale oil, beef tallow, squalane, oleic acid, stearic acid and other fats and oils.

The decomposable carbon compound is intended to supply carbon,
The type can be selected in a wide range. However, from the viewpoint of easy handling and carbon yield, toluene, xylene, benzene, kerosene, light oil, heavy oil, C ₉ distillate mixture, ethylene bottom and the like are preferable.

A furnace is preferably used to decompose these in a hot gas containing steam to obtain a fine carbon-containing powder composition. This furnace is equipped with a heating device and a nozzle for charging silicon compounds and carbon compounds, a gas charging duct, a mixed aerosol, and an exhaust duct. The heating device includes a combustion burner, an electric heating element, etc. A combustion burner is simple and preferable in terms of thermal efficiency. FIG. 3 shows an example of the furnace used for this. There must be a void area of at least 600 ° C. or above in this furnace. Above this temperature, elemental carbon is obtained from the carbon compound, and silicon oxide is obtained from the silicon compound under an atmosphere containing water vapor, and a mixed aerosol state that is a mixture of gas and these solids is generated. .

As a method for obtaining hot gas containing water vapor, it can also be obtained by injecting water vapor into hot gas obtained by an electric heating method, a high frequency heating method, and a discharge method,
A method of combusting a combustible substance such as hydrogen, methane, ethane, propane, etc. or a hydrocarbon as a raw material that combusts to generate water vapor with air is simple on the apparatus and economical in terms of thermal efficiency.

The silicon compound has a property of being converted into a solid substance of silicon oxide by a hydrolysis reaction in a hot gas containing water vapor, and a property of being converted into a silicon oxide by an oxidation or thermal decomposition reaction. Moreover, these reactions are extremely fast and are completed in about 0.1 to 0.5 seconds. Therefore, if the retention time in the reaction zone is set to about 1 to 10 seconds, the silicon compound is not present in an atmosphere where heat and water vapor coexist. Is rarely volatilized out of the reaction system in the unreacted gas state.

The mixed aerosol obtained as described above is discharged and guided to the outside of the furnace, and then the solid dispersoid contained in the aerosol is subjected to solid-gas separation with a trapping device such as a bag filter, a cyclone, and an electronic dust collector. A carbon-containing powder composition that can be used in the present invention can be obtained by performing an operation and collecting it.

The powder composition having a bulk specific gravity of 0.2 g / cc or less containing the silicon oxide powder and the simple substance carbon powder obtained as described above is present under the coexistence of a breathable structure made of a thin carbonaceous substance, The silicon nitride whiskers can be obtained by heating to 1200 to 1600 ° C, preferably 1300 to 1500 ° C in an atmosphere of nitrogen or a nitrogen-containing compound.

The present invention is to obtain a silicon nitride whiskers by heating a powder composition having a bulk specific gravity of 0.2 g / cc or less containing a silicon oxide powder and a simple substance carbon powder obtained as described above. Part to form a breathable structure having continuous voids, and the powder composition containing the silicon oxide powder and simple carbon powder is filled in the breathable structure voids of the invention, It is characterized by performing heating.

In the present invention, the specific surface area of the powder composition containing the silicon oxide powder and the simple substance carbon powder is 25 m ² / g or more, so that the object of the present invention can be better achieved.

In the present invention, the specific surface area is used as a measure for simply indicating the average particle size of the powdery solid matter. Originally, each powdery solid has its own unique shape and particle distribution.
Since it is extremely difficult to accurately measure and display the particle size and particle size distribution of the entire powder, the amount of the substance adsorbed on the surface of the solid substance, for example, nitrogen gas is measured (BE
T method) It is convenient to use this as a scale corresponding to the average particle diameter, and this will be used in the present invention. The large nitrogen adsorption specific surface area means that
That is, it means that the average particle diameter is small.

The powder composition containing the silicon oxide powder and the simple substance carbon powder thus obtained is subjected to high-frequency heating furnace, current resistance heating furnace, etc. in the coexistence of the carbonaceous breathable structure of the present invention, For example, in a nitrogen-containing compound atmosphere such as nitrogen gas or ammonia, about 1200 to 1600 ℃, preferably 1300 to 1500 ℃
A nitrogen silicon whisker can be obtained by heating to a moderate degree.

The breathable structure made of a carbonaceous material as used in the present invention has the same meaning as that described in the specification of Japanese Patent Application No. 59-221985, filed by the present inventors. It is a three-dimensional structure in which a plurality of small pieces each having an objective shape are used as a structural unit, and a plurality of the small pieces are stacked to hold a continuous air-permeable space. Further, it means a thin carbonaceous substance in which the continuous voids are retained even by heating at 1200 to 1600 ° C. The shape of the three-dimensional small piece that is a structural unit is 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 the size thereof is a cylinder, a cone, a triangular prism, or the like.
0 cm ² Preferably range of from 0.5 to 5 cm ^2, the height 0.2~10cm
0.5 to 5 cm is preferable, and in the case of a string,
The width is 0.1 to 5 cm, preferably 0.2 to 2 cm, and the length is 0.1.
5 to 20 cm, preferably 1 to 10 cm is suitable.

FIG. 4 shows an example of the shape of a small piece, which 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 these, but the horse saddle-shaped one shown in (g), the terraret-shaped one shown in (h), and (li) long strings, (nu)
It may be a short string or the like. 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 so as to 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 the structural unit pieces and the composition may be sufficiently mixed before forming the filling layer.

In the case of adopting the latter method, 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. 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 are formed in a void generated by stacking a large number of small pieces. By heating the powder composition containing carbon powder to about 1200 to 1600 ° C., a silicon nitride whisker having a uniform shape can be obtained. 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 does not decrease so much.

A powder composition containing a silicon oxide powder and a simple carbon powder,
A device for mixing a small piece that is a structural unit that constitutes a breathable structure is also a device that does not easily cause consolidation of powder containing silicon oxide powder and elemental carbon powder during the mixing process, for example, double circular Sui type, V type. , A ribbon type mixer or an air flow type floating mixer is 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. Therefore, the small pieces made of ceramic or porcelain have a large weight and are easily consolidated, and are also inferior to the carbonaceous material of the present invention in terms of separation from the product.

A carbonaceous substance is suitable as the material of the small pieces in the present invention. The reason is that the carbonaceous material hardly changes in size and shape even when heated at 1200 to 1600 ° C., and can maintain voids in the state before heating.

As the carbonaceous material, graphite having a thickness of several millimeters or less, charcoal, a carbonized resin molded body or carbonized wood can be preferably used, and more preferably a material containing cellulosic as its main component, It is preferable to use so-called foreign paper, paperboard or the like to form the structural unit small pieces and carbonize them while maintaining their shape. This is because the paper itself is a thin-walled material, is easy to process into various shapes as compared with other materials, and is inexpensive.

These papers (structural unit pieces formed of) are thin-walled charcoal while maintaining their original shape when heated in an inert gas atmosphere at 300 ° C. or higher, preferably 450 ° C. or higher for 5 minutes or longer. In this case, the dimensions are 10-35 compared to before heating.
% Shrinkage occurs, but the shape remains almost the original shape, and once carbonized by such heating, the size and shape can be reheated to 1200 to 1600 ° C, which is the temperature at which silicon carbide whiskers can be obtained again. Is almost unchanged.

The small pieces of the carbonaceous material can be easily burned and removed after the whiskers are formed as described later, but they can be reused after being separated from the generated whiskers. The bulk specific gravity of a powder composition containing a silicon oxide powder coexisting with a breathable structure made of a carbonaceous material and a simple carbon powder is 0.2 g / cc or less, preferably 0.15 g / cc before heating. The following are preferred. However, it is based on the experimental findings of the present inventors that when the bulk specific gravity exceeds 0.2 g / cc, the tendency that the silicon nitride obtained by heating tends to have a granular or curved shape rapidly increases. .

The ratio of carbon to silicon oxide in the powder composition containing the silicon oxide powder and the simple substance carbon powder is C / Si (formula ratio,
That is, g-atom C / g-atom Si is defined as at least 1.0 or more, preferably 2.5 or more for the purpose of obtaining a silicon nitride whisker having a uniform shape. C
When the / Si formula weight ratio is less than 1.0, the shape of the silicon nitride whiskers tends to be non-uniform, and the silicon oxide remains in the silicon nitride whiskers formed as a part of unreacted, which is not preferable.

The remaining silicon oxide can be easily removed by washing the generated silicon nitride 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 it is too large, for example, C / Si.
However, if the condition is as high as 20 to 30 or more, it is merely a loss of the carbon compound.

The silicon nitride whiskers obtained as a result of carrying out the present invention may contain elemental carbon, and this residual carbon burns the whiskers by heating them to 500 to 1000 ° C. in the presence of oxygen. Therefore, it can be easily removed. Specifically, it can be easily carried out by heating in air or placing the fuel in a hot gas atmosphere containing oxygen burned with excess air.

〔The invention's effect〕

When a silicon nitride whisker is produced from a powder composition containing a silicon oxide powder and a simple substance carbon powder, it is preferable that the powder composition is extremely fine in order to obtain good whiskers in high yield, but the size of the device is increased. If this is done, this alone is not enough. The present invention improves the excellent silicon nitride whiskers with uniform length distribution 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. It is possible to obtain it in yield, and the industrial applicability is extremely great in that it enables an economical production method of silicon nitride whiskers on an industrial scale.

[Preferred modes for carrying out the invention]

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 an electron microscope image, and the arithmetic mean value of L and L / D (aspect ratio) and the standard of L are calculated. The deviation 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. It is said that the larger the length (L) and the larger the ratio of the length to the diameter (D) (aspect ratio: L / D as described above), the greater the effect as a reinforcing agent. Being, L
This is because the smaller the standard deviation of, the smaller the variation in the reinforcing effect.

Example 1 SiO ₂ powder (specific surface area 205m ² / g) 160g of carbon black (specific surface area 123m ² / g) 370g were mixed with a V mixer, a powder composition 350g containing SiO ₂ powder and elemental carbon powder Obtained.

530 g of a powder composition containing this SiO ₂ powder and elemental carbon powder
And an outer diameter of 40 mm and a length of 60 mm made from kraft paper,
A carbonized cylindrical small piece having a composition of carbon 99.6% and ash 0.4% obtained by heating a 0.4 mm thick cylinder (Fig. 4 (b)) at 700 ° C for 10 minutes in a nitrogen gas atmosphere. V again with 700 pieces (outer diameter 30 mm, length 40 mm, wall thickness 0.2 mm)
After mixing using a mold mixer, inner diameter 180mm, height 60
It was packed in an electric resistance furnace having a heating space of 0 mm. The bulk specific gravity of the powder composition of the silicon oxide powder and the simple substance carbon powder filled in the furnace is 0.06 g / cc, and the apparent volume occupied in the furnace in the state of coexisting with the cylindrical small piece as the structural unit is 8.8. Atsuta

This was heated in an ammonia atmosphere at 1370 ° C. for 2 hours to generate silicon nitride. The apparent volume before and after heating did not change at all, and the cylindrical small pieces, which are structural units, remained in their original shape.

After cooling this once, it is heated to 700 ° C in air to burn off the remaining elemental carbon and carbonized kraft paper, and then the remaining silicon oxide and ash of kraft paper are washed with an aqueous solution of hydrofluoric acid for nitriding. 145 g of silicon was obtained. As a result of powder X-ray diffraction spectrum analysis, the crystal shape was α type, and as a result of electron microscope image observation, only needle-like whiskers were observed, and the arithmetic mean values of length (L) and aspect ratio (L / D) were The standard deviation of L was 31 μm and 62, respectively, and was 4.6 μm.

The electron micrograph is shown in FIG.

Reference Example 1 530 g of a powder composition containing the silicon oxide powder obtained in Example 1 and a simple substance carbon powder and a cylinder itself made of kraft paper (outer diameter 30 mm, length 40 mm, wall thickness 0.2 mm) 410
Were mixed using a V-type mixer, and heated in the same manner as in Example 1 to produce silicon nitride. The bulk density of the carbon-containing powder composition as loaded in the furnace is 0.06 g / cc
Although the apparent volume was 8.6, the apparent volume decreased to 6.6 in the state after heating, and the cylindrical kraft paper was carbonized and contracted about 20% on one side. However, it remained in a cylindrical shape.

In the same manner as in Example 1, the remaining elemental carbon and silicon oxide were removed to obtain 135 g of silicon nitride. As a result of powder X-ray diffraction spectrum analysis, the crystal form was α type,
As a result of electron microscopic image observation, it was observed that the shape of silicon nitride was different depending on the position where silicon nitride was generated, and the apparent volume was reduced. It was confirmed that the whiskers grown straight have a length of about 3 times the average length. However, as shown in the electron micrograph of FIG. 2, a short curved whisker was observed as a by-product along the inner wall surface of the carbonized and shrunk cylinder. , L / D have an arithmetic mean value of 22 μm and 44, respectively, and a standard deviation of L is 11.
It was 8 μm.

Example 2 Silica gel (SiO ₂ 99.8%) obtained by decomposing sodium silicate (water glass) with hydrochloric acid and washing the coagulated product with water and drying was pulverized to obtain a silica gel fine powder having a specific surface area of 45 m ² / g. .
280 g of this silica gel fine powder and 580 g of carbon black (specific surface area 95 m ² / g) were mixed with a V-type mixer to obtain Si.
860 g of a powder composition containing O ₂ powder and carbon powder was obtained.

860 g of powder containing this SiO ₂ powder and carbon powder and a roll paper 4 mm thick 0.4 mm, width 6 mm, length 7 cm
A string-like piece like the one in the figure (800) is placed in a nitrogen gas atmosphere at 800 ° C.
Using V-type mixer again with 3800 pieces of string-like pieces (wall thickness 0.3 mm, width 4 mm, length 47 mm) of composition 99.4 wt% carbon and 0.6 wt% ash obtained by heating for 20 minutes After mixing, the mixture was charged into an electric resistance furnace having a heating space having an inner diameter of 180 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 is 0.11 g / cc,
The apparent volume occupied in the furnace in the state of coexisting with the string-like small pieces as the structural unit was 8.0.

This was heated in a nitrogen gas atmosphere at 1410 ° C. for 3 hours to generate silicon nitride. The apparent volume before and after heating did not change at all, and the string-shaped small pieces, which are structural units, also kept their original shape.

After cooling this once, it is heated to 700 ° C in air to burn off the remaining elemental carbon and carbonized kraft paper, and the remaining silicon oxide and ash of kraft paper are washed with an aqueous solution of hydrofluoric acid for nitriding. 210 g of silicon was obtained. Powder X
As a result of line diffraction spectrum analysis, the crystal shape was mostly α type, and a very small β type peak was observed. As a result of electron microscopic image observation, only needle-shaped whiskers were observed, and the length
The fluctuation of the (L) aspect ratio (L / D) was small, and their arithmetic mean values were 23 μm and 46, respectively, and the standard deviation of L was 5.2 μm.

Example 3 furnace shown in FIG. 3 (a diameter of 300 mm, length 3m) with the air from the duct 2, the propane as hot air for fuel from the combustion burners 3 of 80Nm ³ /H,1.7Nm ³ / H, respectively A mixture of SiCl ₄ as a silicon compound and A heavy oil as a carbon compound in a weight ratio of 1: 1.6 was charged at a flow rate of 19 kg / H into the furnace at a flow rate of 19 kg / H. The inside of the furnace was maintained at a temperature of 960 ° C. at the position A in FIG.
The aerosol generated in the furnace is extracted from the duct 5 and the dispersoid is collected by a bag filter to obtain the carbon-containing powder composition 4
8 kg / H (dry weight) was obtained. This one has carbon 50.
0% by weight and 23.4% by weight of silicon (converted to simple substance) are included (the rest is 26.5% by weight of binding oxygen and 0.1% by weight or less of others). ESCA spectrum analysis shows that silicon and other elements Only Si-O bond was observed in the bond form.

After mixing 320 g of this fine carbon-containing composition and 1600 cylindrical small pieces obtained by carbonizing kraft paper of the same shape as used in Example 1 using a V-type mixer,
It was filled in an electric resistance furnace having a heating space having an inner diameter of 180 mm and a height of 600 mm. The bulk specific gravity of the carbon-containing powder composition filled in the furnace was 0.023 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 13.9.
It was.

This was heated in an ammonia atmosphere at 1360 ° C. for 2 hours to generate silicon nitride. After heating, the apparent volume did not change at all.

After cooling this once, it is heated to 700 ° C in air to burn off the remaining elemental carbon and carbonized kraft paper, and the remaining silicon oxide and ash of kraft paper are washed with an aqueous solution of hydrofluoric acid for nitriding. 104 g of silicon was obtained. Powder X
As a result of line diffraction spectrum analysis, the crystal shape was α-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. The arithmetic mean values were 48 μm and 78, respectively, and the standard deviation was 3.4 μm.

Examples 4 to 6 In addition to propane, hydrogen and methane were used as the hot air fuel in Example 3, and the decomposable silicon compounds and carbon compounds shown in Table 1 were used at the weight ratios shown in the same table. The mixture is mixed and charged into the furnace through the nozzle 4, and each is collected with a bag filter.
A carbon-containing powder composition indicated by symbols I to C was obtained. As a result of analyzing the bond form of silicon and other elements in the carbon-containing powder composition by ESCA spectrum, only Si—O bond was observed in each case. Each of these carbon-containing powder compositions having the symbols and weights shown in Table 2 was added with the following small pieces for forming a breathable structure and mixed using a V-type mixer, and then used in Example 3. It was filled in the current-carrying resistance furnace and heated in the atmosphere of ammonia or nitrogen for the temperature and time shown in Table 2 to generate silicon nitride. Here, the small piece as the structural unit forming the breathable structure is a cylinder having the same shape as that used in Example 3 obtained by carbonizing the kraft paper as a raw material in Example 4 while maintaining the original shape. 1500 pieces were used respectively, and in Example 5, a string-like small piece as shown in FIG. 4 (nu) having a wall thickness of 0.3 mm, a width of 7 mm and a length of 11 cm was made at 1000 ° C. in a nitrogen gas atmosphere. Carbonized string-like small pieces (wall thickness: 0.2 mm, width: 4 mm; carbon content: 99.7% by weight, ash content: 0.3% by weight) obtained by heating for 20 minutes.
27.5 pieces each having a length of 7.5 cm) are used, and in Example 6, a paperboard is used as a material, a thickness is 0.6 mm, and a bottom area is 1.5 c.
Carbon 9 obtained by heating a conical piece of m ² and a height of 4.5 cm as shown in Fig. 4 (e) in a nitrogen gas atmosphere at 900 ° C.
6100 pieces of carbonized conical pieces (thickness 0.35 mm, bottom area 0.92 cm ² , height 3.3 cm) having a composition of 9.8% by weight and ash content of 0.2% by weight were used. The bulk specific gravity and apparent volume of the carbon-containing powder composition filled in the heating furnace were as shown in Table 2, respectively. After heating these materials, the apparent volume was hardly changed, and the shape and size of the small pieces were hardly changed.

From these, in the same manner as in Example 1, residual carbon, silicon oxide, etc. were removed to obtain silicon nitride in the amounts shown in Table 2. As a result of powder X-ray diffraction spectrum analysis, all crystal shapes were α type, and as a result of electron microscope image observation, length was
(L), the aspect ratio (L / D) has a small variation, and it is observed that the whiskers are needle-like whiskers containing almost no fine particle crystals. The arithmetic mean value of the length and aspect ratio and the standard deviation of L Are the values shown in Table 2, respectively.

As can be seen from the above Examples and Comparative Examples, when a silicon nitride whisker is produced from a powder composition containing a silicon oxide powder and a simple carbon powder, the powder composition is filled in the voids of the breathable structure of the present invention. If heated in a standing state, Example 1
As shown in (2) and (2), whiskers with a large amount of curvature can be obtained even if the device is upsized, and excellent whiskers with a straight-line grown length distribution can be obtained without substantially forming fine-grained crystals as by-products. .

However, it is understood that when the breathable structure is not a carbonaceous material as in the present invention, there is a difference in shape depending on the position where silicon nitride is generated as shown in Reference Example 1, and the length distribution is particularly wide. To be done.

Further, as a powder composition containing a silicon oxide powder and a simple substance carbon powder, the present inventors have proposed JP-A-58-498.
As shown in Examples 3 to 6, whiskers having a longer aspect ratio and a larger aspect ratio were obtained by using a fine powder composition containing silicon oxide powder and elemental carbon powder, which was manufactured according to the method described in No. 28. It is understood that

[Brief description of drawings]

1 and 2 are photographs showing a silicon nitride whisker crystal taken by using a scanning electron microscope. The magnification is 800 times in both FIG. 1 and FIG. FIG. 3 is a sectional view showing an example of a furnace for obtaining a carbon-containing composition used for carrying out the present invention. FIG. 4 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. In the drawing, 1 ... Furnace material, 2 ... Duct, 3 ... Combustion burner 4 ...
Nozzles, 5 ... Ducts are shown.

Claims (4)

[Claims] 1. When heating a powder composition containing silicon oxide powder and elementary carbon powder and having a bulk specific gravity of 0.2 g / cc or less at 1200 to 1600 ° C. in a nitrogen or nitrogen-containing compound atmosphere, the heating part A nitriding method, characterized in that a breathable structure made of a thin carbonaceous material having continuous voids is formed, and the heating is performed in a state where the powder composition is filled in the voids of the breathable structure. Method of manufacturing silicon whiskers. 2. A powder composition containing a decomposable silicon compound and a decomposable carbon compound in a hot gas containing water vapor and decomposed to obtain a mixed aerosol dispersoid containing respective aerosols of silicon oxide and elementary carbon. The method according to claim 1, which is a powder composition obtained by producing and disperse the produced dispersoid by a solid-gas separation operation. 3. 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 in a three-dimensional manner. The method according to item 2. 4. The method according to claim 1, 2, or 3, wherein the main component of the breathable structure is a carbide of a thin material made of cellulosic material.