JPH0463006B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a fine mixed powder of silicon nitride and silicon carbide suitable as a raw material powder for a composite sintered body of silicon nitride and silicon carbide.

Silicon carbide sintered bodies have high strength and high thermal conductivity, but sintering requires high temperatures of over 2000°C. On the other hand, silicon nitride is sintered at a temperature of 1750°C or lower, and although the strength of the sintered body is high, its thermal conductivity is lower than that of silicon carbide sintered body. The present invention relates to a method for producing uniform fine particles of a mixture of silicon nitride and silicon carbide with a particle size of 1 micron or less, by which a sintered body of silicon nitride and silicon carbide with high strength and high thermal conductivity can be easily obtained. .

The sintered body obtained from this mixed fine powder can be applied to high-temperature heat exchangers and heat-resistant semiconductor substrates.

Prior Art The following method is known as a conventional method for producing a mixed powder of silicon nitride and silicon carbide.

(1) A method of mixing silicon nitride and silicon carbide with a particle size of 1 micron or less.

(2) A method in which a mixture of silica and carbon is heated in an atmosphere of nitrogen and carbon monoxide.

However, the mixed powder obtained by the method (1) above has the drawback that it is difficult to obtain a homogeneous mixture, and therefore, the sinterability is insufficient, and a high-density sintered body cannot be obtained even by hot pressing. . Furthermore, in the method (2) above, it is necessary to add a large amount of carbon to complete the reaction, and therefore a large amount of carbon remains in the mixed powder. As it is, it will not sinter even if a sintering aid is added, so post-treatment is required to remove excess carbon by heating it in air, which has the disadvantage of making operations complicated and increasing costs. .

Purpose of the Invention The present invention was made in order to eliminate the drawbacks of the conventional method, and its purpose is to uniformly mix fine particles with a particle size of 1 micron or less, and silicon nitride and carbide with a residual carbon content of 1.5% by weight or less. It is an object of the present invention to provide a method for easily and inexpensively producing a mixed fine powder of silicon.

Structure of the Invention As a result of intensive research to achieve the above object, the present inventors found that: (1) silicon alkoxide in which carbon powder, preferably fine powder with a particle size of 0.1 micron or less, is dispersed is hydrolyzed to produce silica and carbon; A mixture of silica and carbon with a uniform particle size of 0.1 micron or less can be easily obtained.

That is, silicon alkoxide can be easily obtained with high purity by distillation, and carbon can also be of high purity because fine powder with few impurities can be obtained at low cost. Furthermore, since silica with a particle size of 0.1 micron or less produced by hydrolysis is precipitated on carbon, silica and carbon become fine particles that are uniformly mixed.

(2) When a fine mixed powder of silica and carbon is heated in a nitrogen stream and then in an argon stream, or in an argon stream and then in a nitrogen stream, the weight ratio of silicon nitride to silicon carbide is determined by controlling the atmosphere and heating conditions. can be adjusted to any ratio within a wide range of 19:1 to 1:19.

Since the raw material mixture is a fine and homogeneous mixture, the reaction can be completed without excess carbon. Therefore, the carbon remaining after the reaction is
The amount is 1.5% by weight or less, and no post-treatment is required to remove it.

(3) Since it is a homogeneous mixture of fine particles of 0.1 micron or less, the reaction can be completed at a temperature of 1550°C or less, and the resulting fine powder has a particle size of 1 micron or less, making it a uniform mixture, and has excellent sinterability. It has been found that a sintered body with excellent properties and high density can be obtained. The present invention was completed based on these findings.

The gist of the present invention is to hydrolyze silicon alkoxide in which carbon powder is dispersed, and to prepare the resulting mixture of silica and carbon in a nitrogen stream and then in an argon stream, or in an argon stream and then in a nitrogen stream.
A method for producing a mixed fine powder of silicon nitride and silicon carbide, which is characterized by heating at 1350 to 1550°C for 30 minutes to 30 hours.

Examples of the raw material silicon alkoxide include methyl silicate, ethyl silicate, propyl silicate, and butyl silicate. The carbon powder is of high purity such as carbon black, and the particle size is
Fine particles of 0.1 micron or less are preferable.
If the particle size exceeds 0.1 micron, it becomes difficult to obtain a uniform and fine mixture, and a larger amount of carbon than calculated is required to complete the reaction between silica and carbon. As a result, a large amount of carbon remains in the resulting mixture of silicon nitride and silicon carbide, and it is necessary to remove this residual carbon.
Preferably, the particle size is as described above.

The molar ratio of silicon alkoxide to carbon is suitably in the range of 1:1.7 to 2.8. Silicon alkoxide becomes silica by hydrolysis, and the yield is 99.8% or more, so the ratio of silicon alkoxide to carbon is the ratio of silica to carbon.

The ratio of silicon nitride and silicon carbide in the powder to be synthesized is determined by (1) the ratio of silicon alkoxide and carbon as raw materials, and (2) the flow of a nitrogen gas flow followed by an argon gas flow or an argon gas flow followed by a nitrogen gas flow during heating. The heating temperature and time in each atmosphere can be varied in the case of silicon nitride vs. silicon carbide.
It can be controlled over a wide range of 1:1 to 1:19.

The silica and carbon obtained by hydrolysis react with nitrogen to produce silicon nitride.

3SiO ₂ +6C+2N ₂ →Si ₃ N ₄ +6CO (1) On the other hand, SiC is generated in an argon atmosphere.

SiO ₂ +3C→SiC+2CO (2) In addition, a reaction occurs in parallel that generates SiO gas and scatters silica out of the system.

In the reaction SiO ₂ +C→SiO+CO (3) (3), silica and carbon are consumed and lost in a 1:1 molar ratio.

In the silicon nitride production reaction in equation (1), silica and carbon are consumed in a molar ratio of 1:2, and in the silicon carbide production reaction in equation (2), silica and carbon are consumed in a molar ratio of 1:1. consumed at a ratio of 3 to 3. Therefore,
To reduce the amount of residual carbon to 1.5% by weight or less,
The ratio of silica to carbon may be set to correspond to the ratio of silicon nitride to silicon carbide, and as described above, the molar ratio of silica to carbon is 1:1.7 to 2.8. When increasing the proportion of silicon nitride in the synthetic powder, the amount of carbon should be decreased, and when increasing the proportion of silicon carbide, the amount of carbon should be increased. The molar ratio of carbon to silica is 1:1.7.
If it is less, the reaction will be insufficient and raw material silica will remain; if the molar ratio of silica to carbon is more than 1:2.8, the reaction will proceed but excess carbon will remain.

Disperse carbon in a predetermined proportion in silicon alkoxide, add and mix 3 to 4 times (weight) of distilled water as the silicon alkoxide, and then add a small amount of acid such as hydrochloric acid or nitric acid or alkaline aqueous solution such as aqueous ammonia to this. Hydrolysis is completed by dropping the solution and heating at about 80°C for 5 hours. Depressurize the container to 10-200 Torr,
By heating to 50 to 90°C, the alcohol and water produced are separated, and a dried fine mixed powder of silica nitride and carbon is obtained. This powder is made up of uniformly dispersed fine particles with a particle size of 0.1 microns or less.
It is amorphous in terms of X-rays.

After the resulting mixture is shaped as necessary, it is heated to 1350-1550° C. in a nitrogen stream and then in an argon stream, or in an argon stream and then in a nitrogen stream. As a result, a fine mixed powder of silicon nitride and silicon carbide is obtained.

When nitrogen and argon gas are flowed alternately, heating in nitrogen gas to synthesize silicon nitride, followed by heating in argon gas to synthesize silicon carbide, or vice versa. . In heating in argon gas or nitrogen gas, the higher the heating temperature and the longer the heating time, the higher the proportion of silicon carbide or silicon nitride, respectively.

The heating temperature is preferably 1350 to 1550°C, and the heating time is preferably 30 minutes to 20 hours in a mixed gas, or 1 to 30 hours in total when flowing nitrogen gas and then argon gas, or argon gas and then nitrogen gas. heating temperature
If the temperature is lower than 1350℃, the reaction will not proceed sufficiently;
If it exceeds this value, the reaction of reaction formula (3) becomes active and the amount of scattered silica increases, making it difficult to control residual carbon in the synthetic powder. Residual carbon amount is 1.5% by weight
The following is preferable.

Silicon nitride to silicon carbide weight ratio: 1:19 to 19
If the ratio is 1:1, it is possible to sinter under normal pressure by using a sintering aid, similar to silicon nitride, and if the ratio is higher than that, it is possible to sinter at high temperature and pressure using a hot press. It becomes necessary.

Example 1 12 g of carbon black having an average particle size of 0.07 microns was added to 132 g of propyl silicate and dispersed by applying ultrasonic vibration. 280 c.c. of distilled water and 35 c.c. of aqueous ammonia (50%) were added dropwise to this and mixed for 2 hours. Next, the mixed solution was heated at 80° C. for 5 hours to cause hydrolysis, and a mixture of silica and carbon was precipitated. The container was evacuated to 100 torr and heated to 90°C to remove water and alcohol produced by hydrolysis, and dried at 100°C for 5 hours to obtain a homogeneous mixture of silica and carbon. 1g of the mixture in a mold with an inner diameter of 12mm
It was pressurized to 300 kg/cm ² to form disc-shaped pellets.
After heating this at 1450℃ for 1 hour in argon,
Subsequently, it was heated at 1500° C. for 7 hours in a nitrogen atmosphere. The obtained powder contained 85% by weight of silicon nitride and 15% by weight of silicon carbide, had an average particle size of 0.5 microns, was gray in color, and had a residual carbon content of 0.2% by weight.

5% by weight of Y ₂ O ₃ was added to this powder and mixed for 2 hours in hexane using a silicon carbide ball mill. After drying the mixture, 300Kg/in a mold with an inner diameter of 16mm
Press to a size of cm ² to form a disk shape, and then use a rubber press to form a disc.
Pressure was applied at 2 tons/cm ² . This molded body was placed in a crucible made of BN sintered body, covered with silicon nitride powder,
Sintering was carried out by heating at 1750° C. for 1 hour in a nitrogen atmosphere at atmospheric pressure. The density of the sintered body is 3.15g/cm ³ (porosity 3.2
%), and the thermal conductivity of this sintered body at room temperature measured by the laser flash method is 37 W/m・K.
This was higher than the 32 W/m·K of silicon nitride sintered body.

Example 2 18 g of carbon black was dispersed in 104 g of ethyl silicate, a mixed powder of silica and carbon was obtained in the same manner as in Example 1, and pellets were made. This was heated at 1450°C for 2 hours in a nitrogen atmosphere, and then heated at 1500°C for 6 hours in an argon gas atmosphere to obtain a mixed powder of silicon nitride and silicon carbide. The resulting mixed powder contained 60% by weight of silicon nitride and 40% by weight of silicon carbide, had an average particle size of 0.9 microns, was black, and contained 0.7% by weight of residual carbon.

Example 1: 5% by weight of MgO was added to this powder.
Mixed in the same manner. 1.5 g of this mixture was placed in a graphite mold with a diameter of 15 mm whose inner surface was coated with BN powder, and heated at 1750° C. for 1 hour under a pressure of 200 kg/cm ² in a nitrogen stream to form a sintered body. The density of the sintered body is 3.25g/
cm ³ (porosity: 0.4%), and thermal conductivity at room temperature was 51 W/m·K.

Comparative Example 1 30 g of silica powder with an average particle size of 0.08 microns and 17 g of carbon black with an average particle size of 0.07 microns were mixed in hexane for 2 hours using a silicon carbide ball mill. (The molar ratio of silica and carbon is the same as in Example 1). After drying the mixture, a mixed powder of silicon nitride and silicon carbide was prepared in the same manner as in Example 1. The resulting powder consisted of 60% by weight of silicon nitride and 34% by weight of silicon carbide, had an average particle size of 1.2 microns, was black, and contained 3.6% by weight of residual carbon.
This mixed powder was shaped and sintered in the same manner as in Example 1. The density of the sintered body is 2.84g/cm ³ (porosity 12.1%)
The thermal conductivity at room temperature was 12 W/m·K. Both density and thermal conductivity were lower than that of silicon nitride sintered body, and the properties as a mixed powder could not be obtained.

Comparative Example 2 Silicon nitride powder with an average particle size of 0.7 microns and silicon carbide powder with an average particle size of 0.5 microns in a weight ratio of 60
Mixed to 40%. (The weight ratio of silicon nitride and silicon carbide is the same as that made in Example 2.) Add 5% by weight of MgO to this mixture and mix for 2 hours in hexane using a silicon carbide ball mill. did. After drying the mixture, a sintered body was produced in the same manner as in Example 2. The density of the sintered body is 2.93g/cm ³ (porosity 10.1%), and the thermal conductivity at room temperature is
It was 25W/m・K. It can be seen that both the density and thermal conductivity are lower than those in Example 2.

Effects of the Invention According to the method of the present invention, by hydrolyzing silicon alkoxide in which carbon powder is dispersed, a mixture of silica and carbon in which fine particles having a particle size of 0.1 micron or less are uniformly mixed can be obtained. By heating this mixture to 1350 to 1550°C in a nitrogen stream, then in an argon stream, or in an argon stream and then in a nitrogen stream, a mixed powder of silicon nitride and silicon carbide with residual carbon of 1.5% by weight or less and an average Particles with a particle size of 1 micron can be easily obtained, and the mixing ratio of silicon nitride and silicon carbide is low in weight ratio.
Control is possible over a wide range of 19:1 to 1:19.
Furthermore, the resulting mixture has excellent effects such as being able to form a dense sintered body at low temperatures, and the thermal conductivity of the sintered body being higher than that of silicon nitride sintered bodies.

Claims (1)

[Claims] 1 Hydrolyze silicon alkoxide in which carbon powder is dispersed, and heat the resulting mixture of silica and carbon in a nitrogen stream and then in an argon stream, or in an argon stream and then in a nitrogen stream at 1350 to 1550°C. at 30
A method for producing a fine mixed powder of silicon nitride and silicon carbide, characterized by heating for minutes to 30 hours. 2. Silicon nitride according to claim 1, wherein a mixture of silica and carbon is heated to 1350 to 1550°C for 1 minute to 10 hours in an argon stream, and then heated to 1400 to 1550°C for 1 to 20 hours in a nitrogen stream. A method for producing a fine mixed powder of silicon carbide and silicon carbide. 3 A mixture of silica and carbon in a nitrogen stream
Production of a fine mixed powder of silicon nitride and silicon carbide according to claim 1, which is heated to 1350 to 1550°C for 1 minute to 10 hours, and then heated to 1400 to 1550°C for 1 to 20 hours in an argon stream. Law. 4 The molar ratio of silica and carbon produced by hydrolysis of silicon alkoxide is 1:1.7.
Claims 1 and 2 falling within the scope of ~2.8
A method for producing a mixed fine powder of silicon nitride and silicon carbide according to item 1 or 3.