JPS63248798A - Production of silicon carbide whisker - Google Patents

Abstract

PURPOSE:To efficiently obtain the title whisker with high conversion efficiency on an industrial scale by heat-treating specified SiO2 or SiO-SiO2 mixed powder and carbon and/or the mixture of carbon and a carbon compd. to be used as the carbon source in a nonoxidizing atmosphere. CONSTITUTION:The mixture of SiO2 or an SiO2-contg. silicic compd. and carbon is heated to >=1,300 deg.C, and the generated SiO vapor is cooled to obtain SiO powder or SiO-SiO2 mixed powder (A) wherein SiO is partially reoxidized. The A component and (B) carbon and/or a carbon compd. to be carbonized in heating and used as the carbon source are mixed in such an A/B mixing ratio that the carbon is slightly in excess of the theoretical amt. for forming SiC. The obtained mixture is lightly packed in a crucible, etc., so that the bulk density is controlled to <=0.2g/cm<3>, and the mixture is heat-treated at 1,300-1,700 deg.C in the nonoxidizing atmosphere of He, Ar, H2, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) This invention relates to a novel method for producing silicon carbide whiskers.

(Prior art) A method for producing silicon carbide whiskers is generally (1) a silicon compound such as SiC14 and C3Hs.
A method of causing a gas phase reaction of carbon compounds such as CCt.

(2) An evaporation method in which SiC is evaporated at a high temperature of 2000°C or higher and then condensed.

(3) An in-phase reaction method in which a mixture of a silicon compound and carbon is heat-treated in a non-oxidizing atmosphere is known (JP-A-53-113300; JP-A-56-10'9811).
(see Publication No. 1) is known.

By the way, method (1) requires that a large amount of unreacted silicon compounds volatilize out of the reaction system during the manufacturing process of silicon carbide whiskers, and that a huge area of whisker-forming substrate is required for industrial production. Also (2
) method is slow in production due to the low evaporation pressure of SiC, and also requires a whisker-generating substrate with a huge area when attempting to produce SiC on an industrial scale. There were difficulties in producing bare whiskers. Method (3) specifically involves heat-treating rice husks in a carbon monoxide atmosphere, or heat-treating a mixture of carbonized and ashed rice husks in an inert cloud atmosphere to form silicon carbide whiskers. However, because the raw material is rice husk, which is an agricultural byproduct, the quality of the rice husk varies depending on the weather, variety, etc., and the quality of the resulting silicon carbide whiskers is difficult to stabilize.

In an attempt to solve these problems and achieve production on an industrial scale, Japanese Patent Publication No. 59-45640 describes a method of using cullet or silica sand containing 5102 and Na2O as the main components as a silicon source. 61-2640 discloses a method of utilizing a fine carbon-containing mixture obtained by charging a decomposable silicon compound and a carbon compound into hot gas containing water vapor, and JP-A-60-26049' No. 9 discloses a method using silica recovered from geothermal hot water, and JP-A-58-14'5700 discloses a method using silica gel.

(Problems to be Solved by the Invention) However, a method using cullet or silica sand containing SiO□ and Na2O as the main components (Japanese Patent Publication No. 5915640), a method using a fine carbon-containing mixture (Japanese Patent Publication No. 5915640), 1988-121198), a method using silica recovered from geothermal hot water (Japanese Patent Laid-Open No. 60-16
0499) and the method using silica gel (Japanese Unexamined Patent Publication No. 58-145700), good whiskers can be obtained in small-scale processing, but when large-scale production is attempted, the surface of the bulk The condition of the whiskers generated differs between the inside and outside, and although good whiskers can be obtained on the surface, there is a disadvantage that no whiskers are generated in the center, or only poor quality whiskers are obtained in the center.

An object of the present invention is to propose a method for producing silicon carbide whiskers that has a high conversion rate to silicon carbide whiskers and can be produced on an industrial scale.

(Means for Solving the Problems) When producing silicon carbide whiskers by the in-phase method, the inventors conducted various studies on starting materials, especially the silica S source, and found that silicic acid or silicic acid compounds and carbon It has been found that it is extremely effective to use a SiO or S i O-S I Ch-based mixture obtained from the evaporation product when a mixture of the following is heat-treated. This invention is based on the above knowledge. That is, SIO obtained by cooling the evaporation product generated when silicic acid or a mixture of silicic acid compound and carbon is heat-treated in a non-oxidizing atmosphere.
powder or SiO-3i02 mixed powder, carbon and/or
Alternatively, there is a method for producing silicon carbide whiskers, which comprises heat-treating a mixture of carbon compounds having a bulk density of 0.2 g/am3 or less, which is carbonized during heating and becomes a carbon source, in a non-oxidizing atmosphere.

(Function) SIO powder or 510 as a silicon source used in this invention
-3iOz mixed powder is obtained as follows.

Mix SiO or a silicic acid compound containing SiO
By heating to the above temperature, SiO vapor is generated by a reaction as shown in equation (1).

SiO□(s)+C(s)-3iO(g)+CD(g)
... (1) And the SiO generated at this time
The steam is cooled and solidified to obtain SIO powder. S obtained here
Depending on the processing conditions, the IO powder may be re-oxidized and partially converted to Sigh, or a portion of its surface may react with oxygen in the air and become 3102.

Even if the SIO powder is slightly oxidized to become a SiO-3iCh-based mixed powder, it will not have much effect on the whisker generation effect, but it is preferable that the degree of oxidation be small.

In obtaining the above-mentioned S10 powder or SIO5102-based mixed powder, SiO□ sources include silica alone such as silica sand, colloidal silica, and silica gel, and 310
Examples of silicic acid compounds containing Zr02 include zircon (Zr02
・SIO2), Mullite (A Is S ].03)
, forsterite (Mg2S 104 ) and other double oxides, clay minerals that decompose into silica and double oxides at high temperatures, or glass containing 5102 such as frit can be used. In this case, the smaller the particle size, the better. Further, as the carbon source, specifically carbon monomers such as carbon black, graphite powder, scaly graphite, or coke powder, pitch, which becomes carbon by carbonization after heating, phenol resin, etc. can be used. Using zircon as an example, S
The reaction when obtaining iO powder is shown as follows: ZrO□・SiO, (s) + C(s)−+ Zr02(
s)+SiO(s)+C00<)... (2) It becomes.

Note that the reaction for producing SlO vapor becomes noticeable when the temperature exceeds 1300°C, but the reaction is further accelerated when the pressure is reduced, for example, to 100 torr or less.

In this invention, the silicon source for producing silicon carbide whiskers is SiO powder or Si obtained as described above.
The reason for using the O-3in2 mixed powder is as follows.

In other words, compared to the case where Si, 02 powder is used as the silicon source, the reaction temperature can be lowered by 50 to 100°C, and furthermore, when the reaction is performed at the same temperature, the reaction time can be reduced to about 1/2 or less. Moreover, the conversion rate to SiC whiskers also increases.

Although the cause of this is not clear in detail, it can be estimated as follows. First, it is presumed that the SiO powder or SiO5102-based mixed powder used as the silicon source in this invention is a condensate from the gas phase, and thus has fine primary particles and is highly reactive. In addition, the formation mechanism of silicon carbide whiskers is thought to be a reaction via the gas phase, and in particular, the vapor pressure pst of SiO(g)
. is estimated to make a large contribution. When using SiO powder as a silicon source, SiO (g) from SiO powder
Evaporation to 5in(s) → SiO(g)...
It is considered to be the reaction (3), and psto is given as a function of temperature. That is, PSiO becomes a constant value at a certain temperature. By the way, when 5102 powder is used as a silicon source, the evaporation from SlO□ to SiO is Si02 (s)
It is given by +c(s)→SiO(g)+C○(g). The activities of SiO□ and C are constant (for example, aslo = 1+
ac = 1), the equilibrium constant is K = Psto ・Pco - (4) Therefore
Psto = K / Pco ”・(5) Since K is a function of temperature, PSiO is a function of temperature and PC
It becomes a function of O. In other words, when 8102 is used in the production of silicon carbide whiskers, the reaction proceeds and CO
When gas is generated, the vapor pressure of Co(g) is P. 0 increases, and it is estimated from equation (5) that the vapor pressure of SiO(g) decreases and the SiC whisker production reaction is inhibited. Therefore, the silicon source is SiO powder or Si obtained as described above.
It is believed that SiC whiskers can be synthesized efficiently and in a short time by using the O-SiO2 mixed powder. Furthermore, it is considered that since SiO(g) is stably supplied during the growth of SiC whiskers, the conversion rate to SiC whiskers is improved.

Now, in producing silicon carbide whiskers, first a mixture of SiO powder or SiO-5102 mixed powder obtained in the manner described above and carbon and/or a carbon compound that becomes carbon when heated is obtained. .

The mixing ratio is preferably such that carbon is slightly in excess of the stoichiometric amount of SiC to be produced. The theoretical amount of SiC production is specifically SiO+2C
→ SiC+ C port... (6
) or SiO3+3C-+SiC+'2C[l...
(7) and the amount of Sigh in the SiO powder.

As the carbon and/or the carbon compound that becomes carbon when heated, carbon black, pitch, etc. are suitable.

Next, a crucible is lightly filled with a mixed powder of SiO powder or SiO-SiO2 mixed powder and carbon and/or a carbon compound that carbonizes to become carbon, and heated in a non-oxidizing atmosphere such as Process. The bulk density of the mixture filled in the crucible at this time is 0.2 g/Cm3
Below, it is preferably 0.15 g/cm3 or less. The reason for this is that when the bulk density exceeds 0.2 g/am', more silicon carbide powder bundles are produced and the resulting whiskers are inferior. The bulk density of the mixed powder is 0.2 g.
/cm3 or less, the silicon source used in this invention is a condensate from the gas phase. Therefore, it is characterized by a particularly low bulk density.

Therefore, by using carbon black, which has a particularly low bulk density, as a carbon source, the bulk density of the mixed powder can be easily reduced to 0.
It can be reduced to 2 g/am3 or less.

Next, as the heat treatment temperature, a range of 1300 to 1700°C is selected.

At temperatures lower than 1300° C., whisker formation is hardly observed. Further, at temperatures exceeding 1700° C., whisker formation is observed, but the loss to the outside of the system due to evaporation increases, which is not very beneficial. A preferred heating reaction temperature is in the range of 1400 to 1650°C.

In addition, in recovering SiC whiskers from the heat-treated body, a known method can be used depending on the Si/C ratio of the raw material. For example, in the case of excess S1, the residual SiO 3102 is removed by treatment with butic acid and the Si
You can get C whiskers. In addition, in the case of excess carbon, there is a method of oxidizing and decarburizing the remaining excess carbon at a temperature of around 700°C in an surrounding atmosphere, or by mixing and dispersing it in a water/kerosene mixture to remove the SiC whiskers that have migrated to the water side. Methods such as filtration recovery can be used.

(Example) Example-1 A mixed powder prepared by adding 7 parts by weight of carbon black having an average particle size of 30 nm to 10 parts by weight of zircon flour having an average particle size of 20 μm was filled in a crucible, and the mixture was heated at 1500°C under an atmosphere of 1 torr of Ar. A heat treatment was performed, and the evaporated material produced by this heat treatment was led to a cooling section to obtain an agglomerated powder. The obtained powder was composed of Sl, 0, and extremely small impurities, and its chemical formula was Sin, 3□. It is presumed that this is because part of the SiC layer was reoxidized to become SiO□.

Next, 10 parts by weight of the above 5IO3IO2 mixed powder and 1 part by weight of 10 parts of carbon black were mixed in a dry method.
Lightly fill 8g into a crucible (bulk density 0.11g/am3
) L, heat treatment was performed under the following conditions.

Treatment conditions: Hydrogen-enriched atmosphere IA/min After heat treatment at 1500° C. for 2 hours, the inner surface of the crucible was observed, and it was visually observed that whiskers were generated. Furthermore, when the inside of the crucible filling layer was observed under a microscope, the formation of whiskers was observed.

X-ray diffraction revealed that it was a mixture of SiC and carbon, and raw material SiC powder and SiO7 powder were not observed.

Next, the carbon remaining in the resulting mixture was oxidized and decarburized, and the whiskers were separated, yielding 10.2 g of silicon carbide whiskers. Si of SiC-3i02 mixed powder
It was confirmed that 51% of it was converted into silicon carbide whiskers.

Example-2 10 parts by weight of silica sand with a particle size of 10 μm and 1 part of 4-weight carbon black with a particle size of 30 nm were mixed, and this mixed powder was heated with Ar, 5
3103102 series mixed powder was obtained in the same manner as in Example-1 by heat treatment at 1,450°C. The chemical formula of the obtained powder was 5 lots, 63.

Next, after mixing 10 parts by weight of the above SiO-5i○2 mixed powder and 8 parts by weight of carbon black, 40 parts by weight of this mixed powder
g was lightly filled in a crucible and heat-treated under the following conditions.

Treatment conditions: Bulk density 0.14 g/am3 Hydrogen atmosphere (1β/min) After heat treatment at 1500° C. for 2 hours, the inner surface of the crucible was observed and formation of whiskers was observed. Furthermore, when the inside of the crucible filling layer was observed under a microscope, the formation of whiskers was also observed within the filling layer. As a result of X-ray analysis, the obtained powder contained only SiC and carbon, and SiO,5i0, which was used as a silicon source,
2 was not observed. Next, after decarburizing the residual carbon from the obtained mixed powder, the whiskers were separated and recovered. 5.8
g of silicon carbide whiskers were obtained. It was confirmed that 45% of the Si in the SiO-3+O□ mixed powder was converted into silicon carbide whiskers.

Example 3 50 g of mixed powder of 10 parts by weight of SiO-SiO2 mixed powder similar to Example 1 and 10 parts by weight of carbon black was filled in a crucible so that the bulk density was 0.12 g / cm3, and Ar air flow was applied. Heat treatment was performed at 1600° C. for 2 hours. When the inner surface of the crucible was observed after the treatment, whisker formation was observed. The formation of whiskers was also observed by microscopic observation of the inside of the crucible filling layer. As a result of analysis by X-ray diffraction, only SiC and carbon were observed, and the raw materials Sin and 5in2 were not observed. When the remaining carbon was separated from this mixed powder, 9.5 g of silicon carbide whiskers were obtained.

Comparative Example-1 A crucible was lightly filled with 45 g of mixed powder obtained by mixing 10 parts by weight of silica sand with an average particle size of 10 μm and 10 parts by weight of carbon black, and the same as in Example-1 was placed in a hydrogen atmosphere (1β/min).
Heat treatment was performed at 15.00° C. for 2 hours. The bulk density of the mixed powder at this time was 0.81 g/cm'. When the inner surface of the crucible was observed after the treatment, the formation of whiskers was observed, but no whisker formation was observed when the inside of the crucible filling layer was observed under a microscope. Furthermore, although the formation of SiC was observed in analysis by X-ray diffraction, it was found that the raw material SiO□ remained.

There were 0 silicon carbide whiskers recovered from the resulting mixed powder.
．． It was 6g.

Comparative Example-2 Silica aerosol (“Aerosil 1” manufactured by Nippon Aerosil)
A crucible was lightly filled with 50 g of a mixture obtained by mixing 10 parts by weight of 30") and 10 parts by weight of carbon black, and heated in a hydrogen atmosphere (11/min) at 1500°C for 2 hours in the same manner as in Example-1.
Heat treatment was performed for an hour.゛The bulk density of the mixed powder at this time was 0.16 g/cm3. When the inner surface of the crucible was observed after the treatment, whisker formation was observed.

In addition, the formation of whiskers was observed by microscopic observation up to a depth of 10 mm from the surface of the crucible-filled layer, but the amount of formation was small and no whisker formation was observed at a depth of 20 mm from the surface. The yield of silicon carbide whiskers from the above mixed powder was 16% based on Si in the raw material.

Analysis by X-ray diffraction also confirmed the formation of SiC, but it was confirmed that the raw material 3102 remained.

Next, using the above mixed powder after heat treatment, the residual Si
Further heat treatment (1600
℃, 2 hours). However, even under these conditions, the yield of whiskers reached only 40%.

As another example, 50 g of a mixed powder of silica aerosol and carbon black mixed in the proportions described above was heat-treated at 1500° C. for 10 hours in a hydrogen atmosphere. Residual Si
Although O□ was not observed, even under these conditions the yield of silicon carbide whiskers reached only 34%.

(Effects of the Invention) According to the present invention, silicon carbide whiskers, which are considered promising as raw materials for various composite materials, can be produced efficiently and on an industrial scale, and the effects are extremely large.

Claims (1)

[Claims] 1. SiO powder or SiO-SiO_2-based mixed powder obtained by cooling the evaporation product generated when silicic acid or a mixture of silicic acid compound and carbon is heat-treated in a non-oxidizing atmosphere, and carbon and/or A method for producing silicon carbide whiskers, which comprises heat-treating a mixture of carbon compounds having a bulk density of 0.2 g/cm^3 or less, which is carbonized during heating and becomes a carbon source, in a non-oxidizing atmosphere.