Process for producing a boron-containing silicon carbide powder

Silicon carbide particles are produced by reacting a gaseous silicon compound or granular silicon with a carbon compound at a high temperature. In the reaction, the amount of free carbon content in the resultant silicon carbide particles can be controlled by monitoring the amount of unsaturated hydrocarbon such as acetylene, as a by-product. Moreover, silicon carbide particles can contain boron dispersed uniformly in the particles by a two step process comprising first reacting a silicon source and a boron source without a carbon source in a first reaction zone, to form boron-containing silicon particles, and second, reacting the resultant particles with a carbon source in a second reaction zone. Further, the above-mentioned monitoring of an unsaturated hydrocarbon by-product allows the obtaining of silicon carbide particles containing no free carbon, and the silicon carbide particles containing boron in the particles but no free carbon may be sintered without the addition of free carbon, to give a dense sinter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing silicone carbide 
particles suitable for making a dense silicon carbide sinter. More 
specifically, it relates to a process for producing silicon carbide 
particles in which a free carbn content in the silicon carbide particles 
is controlled by monitoring a by-product such as acetylene. The present 
invention also relates to a process for producing a silicon carbide sinter 
from silicon carbide particles obtained by the above process. 
2. Description of the Related Art 
Silicon carbide has attracted attention as an excellent high temperature 
strength material suitable for use in gas turbines, etc. It is however 
difficult to sinter a silicon carbide body to a density close to the 
theoretical density of 3.21 g/cm.sub.3. Processes for obtaining a dense 
silicon carbide sinter have been proposed, for example, U.S. Pat. 
application Ser. No. 409073 filed on Oct. 24, 1973, in which a 
boron-containing compound in an amount corresponding to 0.1 to 3.0 wt % of 
boron and a carbon source corresponding to 0.1 to 1.0 wt % of elemental 
carbon as densifying agents are dispersed uniformly with submicron 
.beta.-type silicon carbide particles, the resultant uniform dispersion is 
formed into a shape, and the shape is fired to obtain a dense silicon 
carbide sinter. To produce submicron .beta.-type silicon carbide 
particles, a gaseous trichloromethylsilane and hydrogen, or a suitable 
gaseous hydrocarbon such as silicon trichloride or toluene and hydrogen, 
are introduced into an argon plasma generated between two concentric 
electrodes to produce silicon carbide crystallites having a size of 0.1 to 
0.3 .mu.m. It is also disclosed that free carbon may be contained in 
silicon carbide particles by using the carbon source in an amount slightly 
larger than the stoichiometric amount necessary for producing silicon 
carbide. 
It is known that, to obtain an excellent silicon carbide sinter, silicon 
carbide particles containing boron and free carbon, desirably uniformly 
dispersed in the particles, as densifying agents, are preferable to a 
mixture of silicon carbide particles and densifying agents such as a boron 
source and a carbon source. The former allows a uniform structure of a 
sinter and an improvement of the mechanical properties of a sinter. Thus, 
the above U.S. patent application Ser. No. 409,073 discloses that free 
carbon may be contained in silicon carbide particles in a process for 
synthesizing the silicon carbide particles. 
The amount of densifying agent is critical to the characteristics of a 
sinter. If the amount of densifying agent is not appropriate, a good 
quality sinter cannot be obtained. Therefore, when the free carbon 
produced in silicon carbide particles during the synthesis of silicon 
carbide particles is used as a densifying agent, control of the amount of 
the free carbon produced in the silicon carbide particles is very 
important. 
It is, however, impossible to concurrently determine a quantitative amount 
of free carbon produced during the synthesis of silicon carbide particles. 
Thus, the amount of free carbon in silicon carbide particles is analyzed 
after a certain amount of silicon carbide particles have been synthesized 
to determine whether or not the amount of free carbon produced during the 
synthesis is appropriate. Further, to control the amount of free carbon 
produced in silicon carbide particles in accordance with the results of 
the above analysis, it is necessary to preliminarily examine the 
relationships between the reaction conditions and the amount of free 
carbon, by synthesizing silicon carbide particles under various reaction 
conditions. This necessitates a large number of experiments. Here, the 
reaction conditions include temperature, pressure, feed of raw materials, 
shape of a reaction chamber, etc. 
Even if the reaction is conducted in specified conditions, the amount of 
free carbon in the silicon carbide particles may vary with the amount of 
time lapsed, because silicon carbide particles, etc., are deposited on the 
inside wall of a reaction chamber and, as a result, the residence time of 
a raw material in the reaction chamber and other factors are varied. This 
makes it difficult to produce free carbon in a desired amount. 
U.S. patent application Ser. No. 471,303 filed on May 20, 1974 discloses a 
process for producing particles comprising a uniform dispersion of 
.beta.-type silicon carbide, boron and free carbon, in which a gaseous 
mixture essentially consisting of a silicon halide, a boron halide, and a 
hydrocarbon is introduced to a plasma jet reaction zone. This application 
suggests the effectiveness of a concurrent addition of boron halide during 
the synthesis of silicon carbide particles. 
If a silicon compound, a carbon compound, and a boron compound are 
concurrently introduced into a single high temperature reaction zone, as 
in the U.S. patent application Ser. No. 471,303, mainly, silicon carbide 
is grown onto silicon carbide seeds, to give submicron silicon carbide 
crystals, and boron carbide or boron is grown onto boron carbide or boron 
seeds to give submicron boron carbide or boron crystals, respectively. 
Almost all of the boron is not doped in silicon carbide particles. In 
other words, high temperature stable extremely small seed crystals of 
silicon carbide, boron carbide and boron are first produced respectively, 
and then silicon carbide, boron carbide and boron are grown around these 
seed crystals, respectively, maintaining the same srystal structure, 
resulting in the formation of submicron silicon carbide, boron carbide and 
boron particles, respectively. In such a case, boron is not always 
contained in silicon carbide particles but tends to form boron carbide 
particles or boron particles, although a silicon compound, a carbon 
compound and a boron compound are added in a single reaction zone. Thus, 
almost all of the boron is not dispersed uniformly in the silicon carbide 
particles and it is difficult to obtan silicon carbide particles with 
boron uniformly dispersed therein. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for obtaining 
uniform silicon carbide particles containing a desired amount of free 
carbon by monitoring the amount of produced free carbon in silicon carbide 
particles indirectly but in real time during the synthesis thereof and 
controlling the amount of free carbon accordingly. 
Another object of the present invention is to provide a process for 
producing silicon carbide particles containing boron which corresponds to 
almost all of a boron source fed and is dispersed uniformly in the 
particles. 
A further object of the present invention is to provide a process for 
producing a silicon carbide sinter, which process produces a dense sinter 
having uniform and improved characteristics. 
A still further object of the present invention is to provide a process for 
producing a silicon carbide sinter, which process is simple and provides a 
sinter having improved characterstics. 
The above and other objects of the present invention are attained by a 
process for producing silicon carbide particles from a reaction between a 
gaseous silicon compound or particulate silicon and a gaseous carbon 
compound at a high temperature, in which an amount of an unsaturated 
hydrocarbon produced as a by-product during the reaction is monitored and 
the conditions of the reaction are controlled in accordance with the 
monitored amount of the unsaturated hydrocarbon by-product, to regulate 
the amount of the unsaturated hydrocarbon by-product to a certain range, 
whereby the content of free carbon in the silicon carbide particles is 
controlled. 
If an amount of a carbon compound slightly in excess of the stoichiometric 
amount is used in the synthesis of silicon carbide particles from the 
silicon compound and the carbon compound, the carbon compound in an amount 
of the stoichiometric amount is reacted with the silicon compound, and the 
silicon carbide is produced. On the other hand, a portion of the carbon 
compound in excess of the stoichiometric amount produces free carbon and 
the rest of the carbon compound is emitted from the reaction system as a 
gas. 
A gas emitted from the reaction system includes the carbon compound of the 
starting material and another carbon compound resulted from the starting 
carbon compound, for example, an unsaturated hydrocarbon such as 
acetylene. 
The present inventors found that there is a strong relationship between the 
amount of unsaturated hydrocarbon as a by-product and the amount of free 
carbon as a product. If the amount of an unsaturated hydrocarbon 
by-product is larger, the amount of free carbon product is also larger. 
The relationship between these appears repeatedly. 
By an equipment capable of analyzing a gas in an extremely short time 
period, such as a mass spectrograph or a gas chromatograph, the amount of 
an unsaturated hydrocarbon by-product can be measured in real time, which 
allows the detection of the amount of the free carbon product in real 
time. As the amount of the free carbon product can be detected during the 
synthesis of silicon carbide in real time, it is possible to control the 
amount of the free carbon product to an appropriate value or range at all 
times. That is, once the relationship between the amount of an unsaturated 
hydrocarbon by-product and the free carbon product is determined by 
experiments, controlling the reaction to make the amount of the 
unsaturated hydrocarbon by-product to a certain value results in producing 
a certain desired amount of free carbon. In order to control the reaction, 
the feeding rate of a carbon compound of the starting material is 
ordinarily varied, but the variation of other factors such as temperature 
and pressure also may be used. 
Regarding the reasons why such a strong relationship is present between the 
amount of free carbon product and the amount of an unsaturated hydrocarbon 
by-product, we considered the following: That is, because the reaction 
conditions of producing, an unsaturated hydrocarbon from a carbon compound 
of the starting material have strong similarities to those of producing 
the free carbon, or because an unsaturated hydrocarbon is an intermediate 
reaction product of a free carbon producing reaction. The mechanism of the 
production of free carbon, however, has not been made clear at present. 
In a process according to the present invention, silicon carbide particles 
containing substantially no free carbon can be produced. 
In an embodiment of this process, the particulate silicon mentioned-above 
may be prepared by introducing a silicon compound to a first reaction zone 
at a temperature higher than a melting point of silicon to form fused 
spherical silicon particles. The resultant fused spherical silicon 
particles are then reacted with a carbon compound in a second reaction 
zone at a temperature of less than a boiling point of silicon to produce 
silicon carbide particles. This two step process allows the production of 
desirably spherical silicon carbide particles. 
In the above two step process, silicon carbide particles containing a small 
amount of boron uniformly distributed in the particles can be produced. 
Such boron-containing silicon carbide particles can be prepared by 
introducing silicon or a silicon compound containing no carbon together 
with boron or a boron compound containing no carbon into a first reaction 
zone at a temperature higher than the melting point of silicon to form 
fused boron-containing silicon particles. The resultant fused 
boron-containing silicon particles are then reacted with the carbon 
compound in a second reaction zone at a temperature of less than the 
boiling point of silicon to produce silicon carbide particles containing a 
small amount of boron. 
Preferably, the silicon or silicon compound and the boron or boron compound 
are preliminarily mixed together before being introduced to the first 
reaction zone. 
The silicon carbide particles preferably contain boron, as a densifying 
agent for sintering silicon carbide particles, in a range of from 0.1 to 
5.0% by weight. 
Thus obtained boron-containing silicon carbide particles can be sintered 
without the addition of any carbon source even if the silicon carbide 
particles do not contain free carbon, as explained later in detail. 
Boron-containing silicon carbide particles containing substantially no 
free carbon can be produced in a process described before, i.e., in a two 
step process, by monitoring an unsaturated hydrocarbon by-product. 
Thus, according to the present invention, there is provided a process for 
producing a silicon carbide sinter, comprising the steps of preparing 
silicon carbide particles containing 5% or less by weight of boron and 
substantially no free carbon, forming a shape of the boron-containing 
silicon carbide particles without the addition of a carbon source, and 
firing the shape.

EXAMPLE 1 
The apparatus used was the same as that shown in FIG. 1. Argon gas was 
introduced from a gas inlet pipe 3 at a rate of 20 l/min and an electrical 
discharge was generated between a cathode 1 and an anode 2 under the 
conditions of 30 V and 700 A to produce an argon plasma to a reaction zone 
4. Silane gas was introduced from an inlet pipe 5 at a rate of 1 l/min and 
methane gas was introduced from an inlet pipe 6 at a rate of 1 to 13 
l/min. The diameter of the reaction zone 4 was 40 mm and the temperature 
in the reaction zone was about 2000.degree. C. Thus, silicon carbide 
particles were produced in the reaction zone 4. 
The resultant silicon carbide particles were recovered from an outlet 7, 
and an exhaust gas emitted from the reaction zone 4 through the outlet 7 
was analyzed by a quadrupole mass spectrometer and the amount of acetylene 
produced was measured. The amount of acetylene produced was calculated by 
comparing the peak intensity of acetylene obtained by a quadrupole mass 
spectrometry with that of pure methane, with the amount of pure methane 
set at 100. The amounts of the acetylene and the pure methane were 
normalized by the amount of argon introduced into the apparatus. 
The conditions of synthesizing silicon carbide particles were selected so 
that the amount of acetylene produced was from 0.25 to 6.00, with a pitch 
of 0.25, as the above-mentioned peak intensity; the total number being 24. 
The time period for the synthesis were about 3 hours, respectively. In 
each case, the amount of methane introduced was controlled so that a 
constant amount of acetylene was produced and the free carbon content of 
the resultant silicon carbide particles was always constant. 
X-ray diffraction revealed that the resultant particles were .beta.-type 
silicon carbide. The free carbon content of the silicon carbide particles 
was analyzed by infrared absorptiometric after combustion and hydrogen hot 
extraction technique. Further, chemical analysis, etc., was effected on 
the silicon carbide particles. As a result, it was found that only silicon 
carbide and free carbon were detected. The analyses were conducted on the 
particles obtained during the first, intermediate, and final stages, 
respectively, and the free carbon contents of the particles were the same. 
The resultant relationship between the amount of free carbon product and 
the amount of acetylene by-product is shown in FIG. 3. From FIG. 3, it can 
be seen that the relationship between the two is almost constant. 
EXAMPLE 2 
The apparatus used was the same as that shown in FIG. 2. Argon gas was 
introduced from a gas inlet pipe 13 at a rate of 20 l/min and an 
electrical discharge was generated between a cathode 1 and an anode 2 
under conditions of 30 V and 700 A to produce a plasma. Silane was 
introduced to the first reaction zone 14 from an inlet pipe 15 at a rate 
of 1 l/min and disborane was introduced therein from an inlet pipe 16 at a 
rate of 0.015 l/min, with argon gas as the carrier gas. The diameter of 
the first reaction zone 14 was 70 mm and the temperature was about 
2000.degree. C. Fused silicon particles containing boron uniformly therein 
were produced in the first reaction zone 14. 
The fused silicon particles were fed into a second reaction zone 17 and 
methane was introduced from an inlet pipe 18 to the second reaction zone 
at a rate of 1 to 1.3 l/min. The fused silicon particles were carbonized 
in the second reaction zone at a temperature of 1700 to 1800.degree. C., 
to produce silicon carbide particles containing boron uniformly in the 
particles. 
The resultant silicon carbide particles were recovered from an outlet 17. 
The exhaust gas emitted from the outlet 17 was analyzed by a quadrupole 
mass spectrometer. 
The synthesis conditions and the methods of analyzing the free carbon 
content were the same as in Example 1. 
The resultant relationship between the amount of free carbon product and 
the amount of acetylene by-product was the same as in Example 1. 
X-ray diffraction revealed that the resultant particles were .beta.-type 
silicon carbide. By chemical analysis, etc., about 0.8% by weight of boron 
was detected in addition to silicon carbide and free carbon. X-ray 
photoelectron spectroscopy revealed the boron was elemental boron. 
EXAMPLE 3 
The synthesis of silicon carbide particles in this Example is similar to 
that of Example 2. However, the diameter of the first reaction zone was 60 
mm and the temperature in the first reaction zone was about 2500.degree. 
C. Methane was introduced to the second reaction zone at a temperature of 
about 2000.degree. C. and a rate of about 1.1 l/min. The rate of methane 
introduction was controlled by monitoring the amount of acetylene 
by-product so that the resultant silicon carbide particles contained 0.6% 
by weight of free carbon. 
The resultant particles extracted from the outlet had a grain size in a 
range of 0.1 to 0.7 .mu.m and an average specific surface area of 9.6 
m.sup.2 /g, from observation by a transmission type electron microscope. 
X-ray diffraction showed that the particles were .beta.-type silicon 
carbide. By chemical analysis, etc., 0.8% by weight of boron and 0.6% by 
weight of free carbon were detected. Observation X-ray photoelectron 
spectroscopy showed that the boron was elemental boron. 
The resultant silicon carbide particles were then subjected to sintering. 
100 g of the silicon carbide particles were added and mixed with 5 g of 
oleic acid solved in acetone. The mixture was dried to evaporate acetone 
only. The resultsatnt particles were subject to uniaxial forming in a 
metal die having a size of 5 cm at a pressure of 90 kgf/cm.sup.2. The 
resultant shape was dried at 250.degree. C. to evaporate oleic acid. The 
shape then had a density of 2.05 g/cm.sup.2 (by the Archimedean method), 
corresponding to 64% of the theoretical density. The shape was sintered in 
a 1 atom argon atmosphere at 2160.degree. C. for 2 hours. The temperature 
raising time from room temperature to 2160.degree. C. was about 2 hours 
and 30 minutes, and the sinter was allowed to cool to the room temperature 
after the above sintering at 2160.degree. C. 
The resultant sinter had a density of 3.16 g/cm.sup.3, which corresponds to 
98.4% of the theoretical density. The flexural strengths measured 
according to the method of JIS R 1601 were 68 kgf/mm.sup.2 at room 
temperature and 77 kgf/mm.sup.2 at 1500.degree. C. 
For comparison, commercial .beta.-type silicon carbide particles were added 
with boron and carbon black and sintered under the same conditions and 
procedures as the above. As a result, the silicon carbide sinter had 
flexural strengths of 55 kgf/mm.sup.2 at room temperature and 66 
kgf/mm.sup.2 at 1500.degree. C. As can be seen, superior results were 
obtained by using silicon carbide particles produced by a process 
according to the present invention. It is believed that this is because 
the particles obtained by the present invention contain boron as a 
densifying agent extremely uniformly in all particles. 
EXAMPLE 4 
The synthesis of silicon carbide particles in Example 3 was repeated except 
that silane and diborane were preliminarily mixed in a ratio of 1:0.015, 
followed by introducing the mixture through the inlet pipes into the first 
reaction zone. The characteristics of the resultant silicon carbide 
particles containing boron in the particles were similar to those in 
Example 3. 
Further, the sintering in Example 3 was repeated for the resultant silicon 
carbide particles. The resultant silicon carbide sinter had a density of 
3.17 g/cm.sup.3 (by the Archimedean method), which corresponds to 98.8% of 
the theoretical density. The flexural strength of the sinter were 71 
kgf/mm.sup.2 at room temperature and 81 kgf/mm.sup.2 at 1500.degree. C. 
EXAMPLE 5 
The synthesis of silicon carbide particles in Example 2 was repeated except 
that methane was introduced to the second reaction zone at a rate of about 
1 l/min and the rate of methane introduction was controlled so that no 
free carbon was produced, in accordance with the amount of the acetylene 
by-product which was monitored. Further, the concentration of oxygen in 
the first and second reaction zones was kept below 0.1 ppm. 
X-ray diffraction revealed that the particles were .beta.-type silicon 
carbide by chemical analysis, only silicon carbide and boron were detected 
and the boron content was 0.8% by weight of the particles. According to X 
ray photoelectron spectroscopy the boron was elemental boron. Free carbon 
was not detected by the infrared absorptiometric method after combustion 
or hydrogen hot extraction technique, and the absence of free carbon was 
thus confirmed. 
The resultant silicon carbide particles were then subject to sintering. 20 
g of silicon carbide particles produced as above were transferred into a 
glovebox filled with argon gas and containing 0.01 ppm or less of oxygen, 
while the particles were prevented from coming into contact with air. In 
the glovebox, the particles were uniaxially formed at 90 kgf/cm.sup.2 to 
make pellets and the pellets were charged in a rubber bag. The rubber bag 
with the pellets was discharged from the glovebox and the pellets as 
charged in the rubber bag were statically pressed under a pressure of 7000 
kgf/cm.sup.2 to form a shape. The shape was fired in a 1 atm argon 
atmosphere at 2080.degree. C. for 2 hours. The temperature raising time 
from room temperature to 2080.degree. C. was about 2 hours and 30 minutes. 
The sinter was allowed to cool to room temperature after firing at 
2080.degree. C. 
The resultant silicon carbide sinter had a density of 3.18 g/cm.sup.3 (by 
the Archimedean method), which corresponds to 99.1% of the theoretical 
density. 
EXAMPLE 6 
Example 5 was repeated except that shaping was conducted in air without 
using a glovebox. 
The resultant silicon carbide sinter had a density of 3.16 g/cm.sup.3 (by 
the Archimedean method), which corresponds to 98.4% of the theoretical 
density. 
EXAMPLE 7 
A silicon carbide sinter was produced by the same procedures as in Example 
5 except that oleic acid was used as a binder for the shaping. Before 
uniaxial forming, the silicon carbide particles were added and mixed with 
1 g of oleic acid solved in toluene and the mixture was dried to evaporate 
the acetone only. The oleic acid was evaporated after uniaxial forming and 
static pressing was conducted. During the above procedure, a glovebox was 
used and the silicon carbide was prevented from coming into contact with 
air. 
The thus-obtained silicon carbide sinter had a density of 3.18 g/cm.sup.3 
(by the Archimedean method), which corresponds to 99.1% of the theoretical 
density. 
EXAMPLE 8 
Example 7 was repeated except that the shaping was conducted in air without 
using a glovebox. 
The resultant silicon carbide sinter had a density of 3.16 g/cm.sup.3 (by 
the Archimedean method), which corresponds to 98.4% of the theoretical 
density.