JPS5915112B2 - Method for manufacturing high-density silicon carbide sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a high-density silicon carbide sintered body.

Silicon carbide has excellent chemical and physical properties that make it particularly suitable for applications such as gas turbine components and high-temperature structures used under harsh conditions such as high-temperature heat exchangers. It is a suitable material.

Conventionally, when producing a high-density silicon carbide sintered body, a pressure sintering method or a reaction sintering method has been used.

The former pressure sintering method has the drawback that it is difficult to produce sintered bodies with complex shapes and has low productivity, while the latter reaction sintering method has the disadvantage that it is difficult to produce sintered bodies with high strength, and It has the disadvantage of containing a large amount of harmful free silicon, and has poor practicality in the above-mentioned applications.

Silicon carbide is a material that is difficult to sinter, so it is molded under pressure at room temperature, which is commonly used in the production of oxide ceramics.
Until now, sintering without pressure has been considered difficult.

However, JP-A-50-78609 ``Method for manufacturing high-density silicon carbide ceramic'',
No. 16 "Silicon Carbide Sintered Body" ζ discloses a method of pressureless sintering of silicon carbide powder.

According to the pressureless sintering method, a mixed powder of submicron (1 micron or less) particle size consisting of silicon carbide, a boron-containing additive, and a carbonaceous additive is molded, and
It is sintered in an inert atmosphere within a temperature range of 0°C.

The reason why carbon is added in the pressureless sintering method is that when sintering the silica film that always coats the surface of silicon carbide particles even at room temperature and obstructs the self-sintering of silicon carbide, carbon is added. (The silica film is removed by reduction at a high temperature. In order to completely remove the silica film, it is necessary to coat (disperse) the carbon particles on the surface of the silicon carbide particles.

However, in order to disperse carbon particles as uniformly as possible, even if carbon fine particles produced by thermally decomposing an organic substance are used as a carbon source, the amount of carbon added is 0.1 to 1. Since the amount of silicon carbide is extremely small at 0% by weight, and the particle size of silicon carbide is submicron and has a huge surface area, the carbon particles are partially dispersed on the surface of the silicon carbide particles, and the silica film is locally dispersed. It can only be reduced and removed.

As a result, silicon carbide is locally exposed during the sintering stage (not too thick, the residual silica film impedes the grain growth of silicon carbide and reduces sinterability, and also forms an intervening phase within the sintered body). This has a detrimental effect on the physical properties of the sintered body, such as its strength.

That is, the conventional pressureless sintering method requires high-temperature sintering of around 2050°C, which is extremely difficult, and is extremely difficult to obtain and expensive. Ultra-fine silicon carbide powder is required, and an intervening phase made of silica is unevenly distributed in the sintered body, and the crystal shape is uneven, making it possible to create a sintered body in which silicon carbide particles are completely self-sintered. There are disadvantages that are difficult to obtain.

The present inventors have previously applied for a patent for ``Method for manufacturing a silicon carbide sintered body'' (Japanese Patent Application No. 15883-1983), which addresses the above-mentioned drawbacks of the previously known pressureless sintering method. We proposed a new pressureless sintering method for silicon carbide that has been removed and improved.

The present invention further improves the conventional pressureless sintering method by selectively using a substance that has never been used before in order to completely remove the silica film. A first step of producing a molded body containing 100 parts by weight of silicon carbide with an average particle size of 3 microns or less and 10 parts by weight of a solid temporary binder for the purpose of providing a sintering method. and a second step of firing the molded body within a temperature range of 1750 to 2100°C to obtain a density of at least 2.4 Y/cc, and the silicon carbide in the molded body is heated in a non-oxidizing atmosphere before firing. A method for manufacturing a high-density silicon carbide sintered body, which is brought into contact with anhydrous hydrofluoric acid in a non-oxidizing atmosphere from then on until firing.

Next, the present invention will be explained in detail.

According to the present invention, in order to obtain a sintered body having a high density and a regular microstructure, silicon carbide as a starting material (
It is necessary to keep the average particle size to 3 microns or less, and in particular, using powder with an average particle size of 1.5 microns or less is especially important in obtaining a high-density, high-strength sintered body. This is more suitable.

Depending on the crystal structure, silicon carbide has an α type belonging to a hexagonal system and a β type belonging to a cubic system, and either one of these types or a mixture thereof can be used.

The silicon carbide can be produced by heating a mixture of metal silicon and carbon powder, by heating a mixture of silica powder and carbon powder, or by reacting a mixed gas such as silicon halide and a hydrocarbon in a gas phase. It can be manufactured by various methods such as.

A method of finely pulverizing and classifying granular d-type silicon carbide obtained by the Acheson method, and a method previously described by the present inventors in JP-A-52-
The method of continuously synthesizing β-type silicon carbide powder disclosed in No. 14269 ζ is advantageous because the silicon carbide can be produced at a relatively low cost.

According to the invention, it is essential to contact the silicon carbide with anhydrous hydrofluoric acid (also called hydrogen fluoride) in a non-oxidizing atmosphere before firing.

The reason why anhydrous hydrofluoric acid is brought into contact with silicon carbide is to vaporize and remove the silica film covering the surface of silicon carbide particles.
The purpose is to expose silicon carbide itself to improve sinterability.

By contacting with the anhydrous hydrofluoric acid, the silica film is removed according to the following reaction formula (1).

S i02+4HF-+5tF4+2H20...
(1) The silicon carbide particles from which the silica film has been removed are extremely easily oxidized and, for example, react with oxygen in the atmosphere or oxygen dissolved in water as shown in the following reaction formula (2).
), it reacts even if it is always left in 7nl, and is easily covered with silica film again (10 2S i C + 302 → 2SiO2 + 2CO...
(2) However, according to the present invention, the silicon carbide from which the silica film has been removed is handled in a non-oxidizing atmosphere from then until the completion of firing, so the silicon carbide is treated with the silica film again. As a result, as shown in FIG. 1, a high-density sintered body in which silicon carbide particles are strongly self-sintered can be stably obtained.

Therefore, removing the silica film using anhydrous hydrofluoric acid of the present invention is a method for removing granular free silica contained in silicon carbide powder for the purpose of purification using hydrofluoric acid. is entirely purposeful.

According to experiments conducted by the present inventors, the microstructure of a sintered body containing silicon carbide as a main component and boron carbide and carbon, and which was not treated with anhydrous hydrofluoric acid Q according to the present invention, is shown in Figure 2. As shown in the figure, the crystal shape was irregular and the grain size was non-uniform compared to the sintered body of the present invention.

Furthermore, according to the conventional pressureless sintering method of the present invention, a sintering temperature approximately 150° C. higher than that of the present invention was required.

These things interfere with self-sintering between silicon carbide particles,
The present inventors also found that this is due to the fact that the silica film, which reduces properties such as the mechanical strength of the sintered body, is removed only locally.

Another advantage of using the anhydrous hydrofluoric acid of the present invention to remove the silica film is that even if anhydrous hydrofluoric acid is used in excess, it is liquid and has a boiling point of about 19°C, so it is easily vaporized. Since it can be removed by hand, it does not have any harmful effect on the physical properties of the sintered body.

Furthermore, according to the conventional method, it is necessary to strictly control the oxygen content of the silicon carbide powder that is the starting material, but according to the present invention, since the post-treatment is performed with anhydrous hydrofluoric acid, such Strict control is not necessary.

In the present invention, the use of anhydrous hydrofluoric acid is limited to the method described in Japanese Patent Application No. 15883/1983 previously proposed by the present inventors (therefore, even if hydrofluoric acid is used, a silica film cannot be formed). However, anhydrous hydrofluoric acid can remove the silica film more stably and uniformly.

This is because in treatment using hydrofluoric acid in a non-oxidizing atmosphere, silicon carbide from which the silica film has been removed is oxidized by the water contained in the hydrofluoric acid according to the following reaction formula (3). This is because there is a possibility that the silica film will be locally regenerated.

S s C+ 3 HzO-+S t 02+CO+
3 H2... (3) On the other hand, when using the anhydrous hydrofluoric acid of the present invention, silicon carbide is oxidized by the trace amount of water produced according to the reaction formula (1)/I. Even if silicon carbide and hydrogen fluoride are consumed according to the amount of oxidation, the following reaction equation (4
)ζ Therefore, since the silica film can be completely removed, a more stable and homogeneous sintered body can be obtained.

SiO□+2S iC+12HF+3S iF4 +Z
CO16H1... (4) Therefore, the anhydrous hydrofluoric acid used in the present invention is preferably in at least one of gaseous and liquid states, and preferably contains as little water as possible. It is preferable that the content is 2.0% by weight or less.

In the present invention, in order to obtain a high-density sintered body, the molded body must contain a solid temporary binder in addition to the silicon carbide.

The temporary binder used in the present invention is, for example, molasses, and this temporary binder has the function of intervening between silicon carbide particles at the initial stage of sintering and temporarily exerting an adhesive action. The final sintered body is strongly sintered by self-sintering.

The sintered body that did not contain a temporary binder showed significant grain growth as shown in Figure 3, but almost no overall firing shrinkage occurred and it became a porous body. It is difficult to obtain a high-density sintered body.

On the other hand, under the same conditions, the compact containing the temporary binder undergoes firing shrinkage as a whole due to grain growth, as shown in FIG. 1, and becomes a high-density sintered compact.

In the conventional pressureless sintering method that uses carbon as a reducing agent, it is generally not necessary to use a temporary binder, and even without a temporary binder, the entire sintering shrinks and a high-density sintered product is obtained. Solids are obtained.

The reason for this is probably that in the initial stage of sintering when necks are formed between silicon carbide particles, the silica film covering the surface of the silicon carbide particles remains because it is not subjected to the reducing action of the carbon. This is probably due to the fact that this residual silica film acts as a viscous binder on the surface of silicon carbide.

From the above, the temporary binder acts to form a neck at each contact point between silicon carbide particles instead of the silica film, and without the temporary binder, there are places where no neck is formed at the contact point. It is thought that because the grains grow locally, there is almost no overall firing shrinkage.

Therefore, the molded article of the present invention must contain 0.65 to 10.0 parts by weight of a solid temporary binder, and 0.5 to 10.0 parts by weight of a solid temporary binder.
If the amount is less than 1 part by weight, the function as the binder will not be sufficiently exhibited and it will be difficult to obtain a high-density sintered body.
If it is more than 0.0 parts by weight, it inhibits the sintering action and forms an intervening phase within the sintered body, reducing the physical properties of the sintered body.
It needs to be within the range of 5 to 10.0 parts by weight.

Inorganic temporary binders such as sodium silicate, colloidal silica, alumina sol or aluminum phosphate can also be used as temporary binders; An organic temporary binder that is not easily attacked by anhydrous hydrofluoric acid is preferred because it is difficult to use.

Organic temporary binders such as polyvinyl alcohol, cornstarch, molasses, coal tar pitch, phenolic resins, lignin sulfonates, stearates,
Alternatively, a wide variety of organic temporary binders such as alginates can be used.

The temporary binder may be added to silicon carbide in the form of a bath liquid and mixed, or may be added to silicon carbide in the form of a powder and then kneaded by adding water or an organic solvent.

The temporary binder can be added to silicon carbide before it is brought into contact with anhydrous hydrofluoric acid;
Alternatively, it can be added to silicon carbide after contacting it with anhydrous hydrofluoric acid.

If a temporary binder is added to the silicon carbide that has been brought into contact with the latter, an organic temporary binder releasable to an organic solvent is selected, or an inorganic temporary binder in the form of a fine powder is selected. Binders may also be used.

In any case, the bath liquid temporary binder is dried to form a solid temporary binder before being fired.

The molded article of the present invention must contain at least silicon carbide and a temporary binder, but a sintering aid such as boron carbide may also be added.

The action of the sintering aid is to increase the driving force for sintering by diffusing into silicon carbide particles and increasing the number of pores during sintering at high temperatures.

As a result, it has been found that a molded article containing boron carbide according to the present invention can be sintered at a lower temperature than a molded article that does not contain boron carbide.

As the sintering aid, for example, boron, aluminum, aluminum carbide, or boron carbide can be used, but boron does not reduce the oxidation resistance of the sintered body much compared to other sintering aids. , boron-containing materials are preferred.

When adding a boron-containing substance, if a boron-containing substance equivalent to more than 3.0 parts by weight is added, the boron remaining in the sintered body will lower the melting point of the silica layer on the surface of the sintered body, resulting in poor oxidation resistance. Therefore, it is preferable to include 3.0 parts by weight or less, and most preferably selected from boron, boron carbide, or a mixture thereof, each having an average particle size of 1 micron or less.

In many cases, silicon carbide contains a small amount of aluminum compounds and other unavoidable impurities, or impurities such as aluminum and boron intentionally added to improve sinterability, and these impurities are Although it functions as a sintering aid, the present invention does not preclude the presence of such a sintering aid.

To carry out the present invention, a molded article containing silicon carbide and a temporary binder that has been brought into contact with anhydrous hydrofluoric acid is produced by contacting silicon carbide with anhydrous hydrofluoric acid, A method of uniformly mixing silicon carbide and a temporary binder and then forming the mixture, uniformly mixing silicon carbide and a temporary binder,
A method in which the mixture is brought into contact with anhydrous hydrofluoric acid and then molded into a molded body, or a method in which silicon carbide and a temporary binder are uniformly mixed, the mixture is molded, and then the molded body is contacted with anhydrous hydrofluoric acid. It is possible to use any one of the following methods: a method in which the pre-fired molded body is brought into contact with anhydrous hydrofluoric acid.

As a method of bringing anhydrous hydrofluoric acid into contact with the molded body, for example, the inside of the container in which the molded body is charged is made to have an atmosphere of anhydrous hydrofluoric acid gas or a mixed gas of anhydrous hydrofluoric acid gas and a non-oxidizing gas. This method is preferable because it is easy to operate and the silica film can be removed uniformly, but it is preferable to impregnate the molded body with liquid hydrofluoric acid anhydride and bring the silicon carbide powder into contact with the anhydrous hydrofluoric acid. It is also possible to do so.

In addition, the method of contacting the powdered product with anhydrous hydrofluoric acid is to treat it with gaseous anhydrous hydrofluoric acid in the same way as the molded product, or
Alternatively, the powder can be brought into contact with anhydrous hydrofluoric acid by immersing it in liquid hydrofluoric anhydride.

The contact treatment with anhydrous hydrofluoric acid is usually carried out at a temperature close to room temperature, or when using liquid hydrofluoric acid anhydride, at a temperature below the boiling point of about 19°C, but below about 700°C. It can also be carried out at a high temperature ζ.

In addition, in the method of contacting anhydrous hydrofluoric acid gas, in order to remove the air contained in the compact or powder and diffuse the anhydrous hydrofluoric acid inside, the inside of the container is depressurized (and then the anhydrous fluoride gas is applied). As soon as the hydrochloric acid gas is charged,
A stream of anhydrous hydrofluoric acid is preferred.

It is preferable to dry the compact or powder as thoroughly as possible before contacting it with the anhydrous hydrofluoric acid.

In the case of molded bodies, the wet mixture containing silicon carbide and the temporary binder may be dried and then uniformly dispersed and then molded, or the wet mixture may be molded and then dried.

In the former drying method, the temporary binder is in a powdered and dispersed state, but it softens and melts during the heating process during firing, and functions as the temporary binder.

A method using hydrofluoric acid previously proposed by the present inventors (
According to Japanese Patent Application No. 53-15883), as mentioned above, there is a possibility that the silica film can be locally regenerated, and it is necessary to dry it in a non-oxidizing atmosphere. The method of using anhydrous hydrofluoric acid of the invention (according to which the silica film can not only be stably removed, but also the wet mixture and the molded body can be dried in the atmosphere, and especially after molding) The method of contacting with anhydrous hydrofluoric acid has the advantage of further simplifying the process.

Various conventionally known molding methods can be used to mold the mixture containing silicon carbide and a temporary binder into a molded body, such as embossing molding, extrusion molding,
It can be molded into any shape using cast molding or isostatic pressing.

When using embossing molding, usually 250 to 2000 Kt
Pressures around i/cni are used, and the density of the compact is approximately 1
．． It becomes 6-1.9V/cc.

In order to obtain a sintered body of high density, it is preferable to make the density of the compact as high as possible, and for this purpose small amounts of lubricants such as stearates can be added to the mixture before shaping.

Isostatic pressing is preferably carried out by placing the mixture in a container made of rubber, for example, and reducing the pressure inside the container. This is advantageous in obtaining a molded body.

In the present invention ζ, in order to prevent re-oxidation of silicon carbide from which the silica film has been removed using anhydrous hydrofluoric acid, the atmosphere from the step of contacting with anhydrous hydrofluoric acid to the firing is made non-oxidizing. It is necessary to do so.

Non-oxidizing atmospheres include argon, helium, neon,
A gas atmosphere consisting of at least one selected from nitrogen, hydrogen, carbon monoxide, and anhydrous hydrofluoric acid, or a vacuum can be used.

Note that since carbon dioxide can decompose and cause oxidation of silicon carbide, it is better to avoid using a gas atmosphere consisting of carbon dioxide as much as possible.

In carrying out the present invention, the molded product containing silicon carbide that has been brought into contact with anhydrous hydrofluoric acid is not exposed to the atmosphere, and is transported from a room with a non-oxidizing atmosphere such as a glove box into a firing furnace. It may be necessary to load and move it.

This method can be carried out by placing the molded body in a container in a non-oxidizing atmosphere and sealing it.

As containers, it is possible to use, for example, bags or containers which can be fired together with the moldings, such as rubber bags, plastic bags or plastic containers, or ceramic containers, which can withstand temperatures of about 2000° C., for example made of graphite. .

According to the present invention, the molded body is fired in a non-oxidizing atmosphere within a temperature range of 1750 to 2100°C and at least 2.4°C. It is necessary to have a density of /cc.

If the firing temperature is lower than 1750°C, sufficient grain growth will not occur, so 2.4? It is difficult to obtain a sintered body with a density of /cc or higher, and on the other hand, if the temperature is higher than 2100°C, coarsening of crystal grains occurs, which causes a decrease in the physical properties of the sintered body, such as mechanical strength, although the density is high. Therefore, the firing temperature is 1750
It is necessary to perform the firing within the temperature range of ~2100°C, and it is more preferable to perform the firing within the temperature range of 1750°C to 2000°C in order to obtain a sintered body that is particularly dense and has a uniform and fine microstructure. .

According to experiments conducted by the present inventors, the molded product of the present invention has a 150% lower
It was found that almost the same density could be achieved at a firing temperature as low as 10°C.

To describe the advantage that the compact of the present invention can be fired at a relatively low temperature, according to the conventional method, when β-type silicon carbide powder is used as a starting material, the transition from β-type to α-type progresses during sintering. Therefore, the crystal structure tends to include coarse and elongated plate-like crystals, which is extremely unfavorable in terms of mechanical properties.However, according to the present invention, even when β-type silicon carbide is used, α-type Q transition is hardly caused. The reason is that a high-density sintered body consisting of uniform and fine crystals can be obtained.

The firing time within the temperature range of 1,750 to 2,100°C is determined mainly by the desired microstructure and density; generally, firing at a low temperature for a long time produces a sintered body with a uniform and fine microstructure. Because it is easy to become, the above 1750
Most preferred is firing within a temperature range of -2100<0>C for at least 10 minutes.

Furthermore, in the temperature raising process, increasing or maintaining the temperature in the temperature range of around 1700°C over a long period of time will result in the formation of sufficient necks between silicon carbide particles, resulting in a high-density sintered body. It is advantageous.

As the firing furnace for firing the molded body, various conventionally known high-temperature firing furnaces capable of controlling the firing temperature and atmosphere are used,
For example, a firing furnace such as a Tammann furnace equipped with a graphite core tube and a heating element can be used.

According to the present invention, in order to obtain a sintered body having a complicated shape or dimensions as accurate as possible, the compact is first prefired in a non-oxidizing atmosphere, and then shaped into the desired shape. After processing, it can be fired.

The purpose of pre-firing is to reduce dimensional changes during main firing to increase dimensional accuracy and to obtain a molded body with strength suitable for machining.
Preferably it is carried out within the temperature range of .degree.

If the pre-firing temperature is lower than 1550°C, the firing shrinkage rate will be small and strength suitable for machining cannot be obtained, while if it is higher than 1800°C, sintering will progress rapidly and the sintered body will be damaged. This is because machining becomes difficult.

Furthermore, in the present invention, the molded body pre-fired in a non-oxidizing atmosphere can also be machined in the atmosphere.

In this case, since the silicon carbide particles forming the molded body are covered with the silica film again, the pre-fired molded body is processed into the desired shape, and then brought into contact with anhydrous hydrofluoric acid. It is preferable to carry out main firing.

As a method of bringing anhydrous hydrofluoric acid into contact with the pre-fired molded body, the method of bringing anhydrous hydrofluoric acid into contact with the molded body described above can be used.

Next, the present invention will be explained with reference to Examples and Comparative Examples.

Example 1 As a starting material, the following Table 1 (chemical composition, particle size and A β-type silicon carbide powder having the crystal structure shown in FIG. 1 was used.

20.0Of the above β-type silicon carbide powder, 1.42g of blackstrap molasses (manufactured by Tokai Sugar Co., Ltd., solid content 71.3% by weight), 0.20f of magnesium stearate, and 2.0Of distilled water.
was placed in an agate mortar and mixed for 15 minutes.

The wet mixture was placed in a box-type dryer and heated and dried while gradually increasing the temperature to 200° C. After cooling, the mixture was mixed again in an agate mortar for 15 minutes.

An appropriate amount was taken from this mixed powder and molded into a disc shape using a graphite mold at a pressure of 700 kg/cd.

The diameter of the molded body is 20.3 m/rn, and the density is 1.
It was found that the density was 69t/cc (relative theoretical density ratio of about 53%).

In a glove box with an argon gas atmosphere, the molded body was placed in a graphite airtight container with a gas inlet and an outlet, and after reducing the pressure inside the container using an aspirator for 30 minutes, the purity was 99.0% by weight. % or more of anhydrous hydrofluoric acid (
Hashimoto Kasei Kogyo Co., Ltd. (manufactured by Hashimoto Kasei Kogyo Co., Ltd., water content: 0.5% by weight or more) was introduced in a gaseous state and left to stand for 30 minutes, which was repeated three times, and then left to stand for 24 hours in an anhydrous hydrofluoric acid gas atmosphere.

Next, the graphite container containing the molded body was sealed in a glove box, and then placed in a Tammann type firing furnace and fired in an argon gas atmosphere.

The temperature raising process during firing is from room temperature to 400℃ for 40 minutes, 4
The temperature was raised from 00 to 1,680°C for 32 minutes, held at 1,680°C for 45 minutes, and then further raised from 1,680 to 1,900°C for 22 minutes, and the maximum temperature was 1,900°C for 60 minutes.
Hold for minutes.

After cooling, the obtained sintered body has a firing shrinkage rate of 15.2%.
and 2.72 r/c c (relative theoretical density rate approximately 8
It was confirmed that the material had a density of 5%).

The flow sheet of the method of the present invention is shown in FIG. 8.

Example 2 β-type silicon carbide powder 19.821 described in Example 1,
Commercially available 200-mesh boron carbide grains (manufactured by Denki Kagaku Kogyo Co., Ltd.) were crushed to have an average particle size of 0.11' and mixed with boron carbide 0.11' in an agate mortar for 30 minutes.

Furthermore, 1.34 f of blackstrap molasses with a solid fraction of 71.3% by weight, 0.2 Or of magnesium stearate, and 2.002 F of distilled water were added to this mixed powder and mixed for 15 minutes.

A dried molded body was created from this wet mixture in the same manner as in Example 1, and after contacting the molded body with gaseous anhydrous hydrofluoric acid, it was charged into a Tammann-type kiln in an argon gas atmosphere. .

Next, the temperature was raised in the same temperature raising process as in Example 1, and the maximum temperature was 19
It was baked at 00°C for 60 minutes.

The density of this sintered body was 3.09 S'/cc (relative theoretical density ratio, about 96%), and the firing shrinkage rate was 18.8.

The molded body containing the boron carbide powder was lowered to a maximum temperature of 1850° C. in the same temperature raising process, held for 60 minutes, and then cooled.

The firing shrinkage rate of the obtained sintered body was 17.7%, and 2.9%.
It has a density of 6 S'/cc (relative theoretical density, about 92%), and as shown in the scanning electron micrograph (5000x magnification) in Figure 1, crystals of about 2 to 5 microns are tightly bound together. It was confirmed that it had a fine structure.

Further, from the powder X-ray diffraction diagram of this sintered body shown in FIG. 6, it was found that there was almost no transition from the β type to the α type during sintering.

Comparative Example 1 A mixture was prepared with a formulation similar to that described in Example 2, but without molasses as a temporary binder.

A molded body was prepared from this mixture in the same manner as in Example 1, brought into contact with anhydrous hydrofluoric acid gas, and then charged into a Tammann-type kiln.

Next, the inside of the furnace was made into an argon gas atmosphere, and then the temperature was raised in the same temperature raising process as in Example 1, and firing was performed at a maximum temperature of 1900° C. for 60 minutes.

This sintered body has a firing shrinkage rate of 5.7% and a 2.01f
/c c (relative theoretical density rate approximately 63%),
As shown in the scanning electron micrograph of FIG. 3, remarkable grain growth was observed, but it was confirmed that each grain was scattered and had a porous microstructure.

Example 3 The β-type silicon carbide powder 19.821 described in Example 1 and the boron carbide powder 0.18F were mixed in an agate mortar for 30 minutes, and then 0.2Of magnesium stearate was added in a water bath liquid state and dried.

Novolac type phenol resin 0.6% was added to the obtained mixed powder.
The entire solution of 0 g in about 8 cc of acetone was added and mixed for an additional 15 minutes.

The wet mixture was left for 30 minutes with occasional stirring to evaporate the acetone.

Place the dried mixed powder in a fluororesin airtight container having a gas inlet and outlet, reduce the pressure in the container with an aspirator for 30 minutes, then introduce the anhydrous hydrofluoric acid gas and leave it for 30 minutes. After repeating this three times, the sample was left in an anhydrous hydrofluoric acid gas atmosphere (for 24 hours).

In a glove box with an argon gas atmosphere, an appropriate amount of the mixed powder was collected from the sealed container, and then heated at 700 K using a press installed in the glove box.
It was molded into a disk-shaped body with a diameter of 20.3 m at a pressure of g/crrt.

This molded body was sealed in a graphite airtight container, and then charged into a Tammann-type firing furnace, and fired in an argon gas atmosphere inside the furnace.

The temperature was raised in the same manner as in Example 1 in the firing process, and the maximum temperature was held at 1900° C. for 60 minutes, followed by cooling.

The firing shrinkage rate of this sintered body is 18.4%, which is 3.05%.
It had a density of f/cc (relative theoretical density fraction, about 95%).

Comparative Example 2 The mixed powder prepared in Example 3 was molded without contacting with anhydrous hydrofluoric acid, and the molded body was placed in a graphite container and charged into a Tammann-type firing furnace. The sample was placed in an argon gas atmosphere, then heated in the same manner as in Example 1, and fired at a maximum temperature of 1850° C. for 60 minutes.

The density of the sintered body is 2.21? /c c (relative theoretical density ratio, about 69 coefficients), and the scanning electron micrograph in Figure 4 (
5,000 times), it was found that grain growth was extremely small.

The sintered body was fired at a maximum temperature of 1930℃ for 60 minutes.
It had a firing shrinkage rate of 15.1% and a density of 2.70 f/cc (relative theoretical density, about a factor of 84).

The microstructure of this sintered body can be seen in the scanning electron micrograph in Figure 2 (
As shown in the image (5000x), the crystal shape was irregular and the grain size was non-uniform.

Furthermore, the sintered body fired at the maximum temperature of 2000°C for 60 minutes has a firing shrinkage rate of 16.9% and a density of 2.89 W/
c c (relative theoretical density ratio of approximately 90%), and from the powder X-ray diffraction diagram of this sintered body shown in Figure 1, β-type silicon carbide is considerably converted to α-type silicon carbide during the firing process. It was confirmed that

Example 4 As a starting material, β-type silicon carbide having the same composition as the β-type silicon carbide described in Example 1 but having an average particle size of 1.4 microns and a maximum particle size of 4.6 microns was used.

The same operation as in Example 2 was carried out using this β-type silicon carbide as the main ingredient (
The density of the molded product made with a trowel and dried is 1.83 f/cc (
It was confirmed that the relative theoretical density ratio was 57 cm).

After contacting this molded body with anhydrous hydrofluoric acid gas,
It was charged into a Tanmann type kiln.

Next, after setting the inside of the furnace to an argon gas atmosphere, the temperature was raised in the same temperature raising process as in Example 1, and firing was performed at a maximum temperature of 1900° C. for 60 minutes.

The firing shrinkage rate of the obtained sintered body was 14.6%, and the density was 2.74 f/cc (relative theoretical density rate 85%).

Example 5 As a starting material, commercially available α-type silicon carbide powder (manufactured by Waha Abrasives Industry Co., Ltd., particle size 3000) was crushed and classified, and then
The purified α-type silicon carbide powder having a purity of 98.3% and an average particle size of 1.3 microns was used.

A mixed powder having the composition described in Example 2I was prepared mainly from this α-type silicon carbide, and 700 kg/kg was prepared using a graphite press mold.
It was molded into a disk-shaped product with a diameter of 20.3 baits at a pressure of /cr/1.

This formed body was brought into contact with anhydrous hydrofluoric acid gas in the same manner as in Example 1, and then charged into a Tammann-type kiln.

Next, the temperature was raised in the same temperature raising process as in Example 1, and the maximum temperature was 19
50 for 60 minutes.

The firing shrinkage rate of the obtained sintered body was 17.4%, and the density was 2.
．． It was 85 f/cc (relative theoretical density ratio, about 89%).

Example 6 A molded product prepared using the same formulation and operation as in Example 2, brought into contact with anhydrous hydrofluoric acid gas, and then charged into a Tammann-type firing furnace was subjected to the same procedure as in Example 1 in an argon gas atmosphere. Preliminary firing was performed at a maximum temperature of 1680° C. for 45 minutes in the temperature raising process described.

The density of the compact obtained by preliminary firing is 2.06 t/c
c (relative theoretical density rate of about 64%).

This pre-fired molded body was cut into 1.
After processing into a 0.0 m square plate, it was placed in a graphite airtight container with a gas inlet and outlet in a hanging box maintained in an argon gas atmosphere, and then dried in the same manner as in Example 1. Contacted with hydrofluoric acid gas.

After that, it was sealed in a graphite airtight container and placed in a Tammann type firing furnace, and after creating an argon gas atmosphere, the temperature was raised to 1680°C at a rate of 40°C/min, and then 1680 to 1900°C.
After raising the temperature over 22 minutes, the maximum temperature was maintained at 1900° C. for 60 minutes.

After cooling, the obtained sintered body has a density of 3.05 f/cc (relative theoretical density rate of about 95%), and the firing shrinkage rate in the main firing is 11.9%. I understand.

Comparative Example 4 A molded body obtained by preliminary firing in the same manner as in Example 6 was processed into a 10.0 non square plate shape in the air, and then fired in a Tamman type without contacting with anhydrous hydrofluoric acid. The material was charged into a furnace and fired after creating an argon gas atmosphere inside the furnace.

The temperature was raised in the same manner as in Example 6 in the firing process, and the maximum temperature was held at 1900° C. for 60 minutes, followed by cooling.

The firing shrinkage rate of this sintered body in the main firing was 6.6%.
The firing shrinkage rate is approximately one-half of the sintered body subjected to the treatment of contacting with anhydrous hydrofluoric acid after preliminary firing as in Example 6, and the density of the sintered body is 2.46 f/c c (Relative theoretical density ratio of about 77%).

Note that the crystalline state was very similar to that of this sintered body.

As mentioned above, until now, pressureless sintering of silicon carbide powder required high-temperature firing at around 2050°C and ultrafine powder of about 0.5 microns or less, which was expensive and difficult to obtain. In contrast, according to the present invention, by performing pressureless sintering in a relatively low temperature range of about 1850 to 1900°C and even using micron-level silicon carbide powder, a sintered body with high density and high density can be obtained. The present invention is industrially extremely useful because it is possible to stably produce a silicon carbide sintered body in which the crystals are firmly bonded and there is very little intervening phase consisting of a silica film that deteriorates the properties of the silicon carbide.

[Brief explanation of the drawing]

Figure 1 is a scanning electron micrograph (5000x) of the sintered body described in Example 2, and Figure 2 is a scanning electron micrograph (5000x) of the sintered body described in Comparative Example 2. 3 is a scanning electron micrograph (5000x magnification) of the sintered body described in Comparative Example 1, and FIG. 4 is a photograph of Comparative Example 2! This is a scanning electron micrograph (5000x) of another sintered body described above, FIG. 5 is a powder X-ray diffraction diagram of β-type silicon carbide used in Example 1, and FIG. 6 is a powder X-ray diffraction diagram of β-type silicon carbide used in Example 1. FIG. 1 is a powder X-ray diffraction diagram of the sintered body according to Comparative Example 2, and FIG. 8 shows an example of the method of the present invention. It is a flow sheet diagram.

Claims (1)

[Scope of Claims] 1. A first step of producing a molded body containing 100 parts by weight of silicon carbide having an average particle size of 3 microns or less and 0.5 to 10.0 parts by weight of a solid temporary binder; 17 pieces of the molded body
Baked within a temperature range of 50 to 2100°C and at least 2.
The silicon carbide in the molded body is brought into contact with anhydrous hydrofluoric acid in a non-oxidizing atmosphere before firing, and thereafter the atmosphere is maintained until firing. A method for producing a non-oxidizing high-density silicon carbide sintered body. 2. The production method according to claim 1, wherein the anhydrous hydrofluoric acid is in at least one of a gaseous and liquid state with a moisture content of 2.0 times or less. 3. The manufacturing method according to claim 1 or 2, which comprises uniformly mixing silicon carbide and a temporary binder, molding the mixture, and then contacting the molded product with anhydrous hydrofluoric acid. 4. The manufacturing method according to claim 1 or 2, wherein silicon carbide and a temporary binder are uniformly mixed, the mixture is brought into contact with anhydrous hydrofluoric acid, and then molded into a molded article. 5. The manufacturing method according to claim 1 or 2, wherein after contacting silicon carbide with anhydrous hydrofluoric acid, a temporary binder is added and mixed uniformly, and then the mixture is shaped. 6. The manufacturing method according to any one of claims 1 to 5, wherein the average particle size of silicon carbide is 1.5 microns or less. T. The temporary binder is made of an organic material. Claim 1
The manufacturing method according to any one of items 1 to 6. 8. The manufacturing method according to any one of claims 1 to T, wherein the molded article contains a boron-containing material corresponding to 3.0 parts by weight or less of boron. 9. The manufacturing method according to claim 8, wherein the boron-containing material is selected from boron, boron carbide, or a mixture thereof having an average particle size of 1 micron or less. 10. The manufacturing method according to any one of claims 1 to 9, wherein the molded body is fired for at least 10 minutes within a temperature range of 1750 to 2000°C. 11. The manufacturing method according to any one of claims 1 to 10, wherein the molded body is first prefired, then processed into a desired shape, and then main fired. 12. The manufacturing method according to any one of claims 1 to 11, wherein the molded body is first prefired, then processed into a desired shape, brought into contact with anhydrous hydrofluoric acid, and then main fired. . 13. The manufacturing method according to claim 11 or 12, wherein the preliminary firing is performed within a temperature range of 1550 to 1800°C. 14. Claims in which the non-oxidizing atmosphere is a gas atmosphere consisting of at least one member selected from argon, helium, neon, nitrogen, hydrogen, carbon monoxide, and anhydrous hydrofluoric acid, or a vacuum. The manufacturing method according to any one of Items 1 to 13.