JPH0135275B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a firing furnace for sintering non-oxide ceramics, particularly a firing furnace for sintering non-oxide ceramics having a built-in heat insulating layer made of carbon fiber mat, and the use of such a firing furnace. The present invention relates to a method for firing a non-oxide ceramic molded body. (Prior art) Silicon nitride, _Si3N4 , is a material that is difficult to sinter, and MgO and other sintering aids are used to promote its sintering _.
It is common to add 5 to 10% of metal oxide (MeO) such as Al ₂ O ₃ , and the Si ₃ N ₄ compact before sintering usually has pores of about 40% by volume. . The mechanism of the strength development of Si ₃ N ₄ when sintering such a Si 3 N ₄ molded body is that _{Si 3 N 4} acicular as a reinforcing agent is added to the glass phase of the metal oxide added as a sintering aid. It is said that excellent strength characteristics are achieved by forming a type of fiber-reinforced ceramics (FRC) made of dispersed crystals. On the other hand, such Si ₃ N ₄ molded bodies are generally heated in a high temperature inert atmosphere, particularly in a nitrogen gas atmosphere at 1700°C to 1900°C.
It is fired at a temperature of A typical firing furnace for stably maintaining such a high temperature under an inert atmosphere consists of a space that houses the compact, a graphite heater placed around the space, and a carbon coating that covers the inner wall surface. It has a built-in heat insulating layer made of fiber mat, and is further equipped with a vacuum exhaust port and an inert gas supply port. The above-mentioned carbon fiber mat has extremely high porosity in order to improve its heat insulation properties, and usually 70% to 95% by volume are pores, and has an average bulk density of about 0.2 g/cc. Further, especially when the size of the firing furnace is relatively small, a carbon tube may be placed inside the carbon fiber mat to cover the graphite heater and the space for accommodating the molded body. (Problem to be solved by the invention) When firing Si ₃ N ₄ in the above-mentioned furnace,
Carbon fiber mats with a bulk density of about 0.2 g/cc are free from trace amounts of oxygen, oxides, or oxynitrides generated from O ₂ , H ₂ O, or Si ₃ N ₄ compacts containing metal oxides remaining in the furnace. When it comes into contact with objects at high temperatures, the carbon fibers on the surface of the pine will slowly but easily fall off. As a result, the heat insulation properties of the pine gradually decrease and the life of the furnace body is shortened.In addition, the carbon fiber dust that has fallen off scatters and floats in the furnace, attaches to the Si ₃ N ₄ , and causes oxidation. The resulting gases, such as CO and CO ₂ , come into contact with Si ₃ N ₄ due to diffusion, which significantly impairs the properties of the sintered body. That is, as described above, if carbon fibers adhere to a Si ₃ N ₄ molded body with high porosity before or during sintering, this carbon fiber dust may be taken into the interior during shrinkage due to sintering. The carbon reacts with the metal oxide, which is a sintering aid, and becomes CO or CO ₂ and escapes into the furnace. At the same time, the metal oxide is reduced and becomes a metal with a low melting point and evaporates. The metal oxides that would form the glass phase matrix are thus lost, especially in the surface layer, leaving behind a skeleton of Si ₃ N ₄ . In the skeleton state, the Si ₃ N ₄ sintered body no longer has properties such as high strength, high thermal shock resistance, and wear resistance. In addition, by touching the Si ₃ N ₄ molded body such as CO and CO ₂ generated in the furnace, the following reactions are repeated,
Metal oxides (MeO) are rapidly lost. Si ₃ N ₄ +MeO+CO →Si ₃ N ₄ +CO ₂ +Me↑ CO ₂ →CO+O C+O→CO This also promotes the formation of the Si ₃ N ₄ skeleton described above. In order to prevent the adverse effects of carbon fiber dust from the carbon fiber mats that form such a heat insulating layer, attempts have been made to place carbon tubes inside the heat insulating layer as described above, but such tubes are usually about 10 mm thick. Placing such a thick cylinder with a large heat capacity in the furnace body will place an excessive power load on the heater, which will increase the wear and tear on the heater. This is economically disadvantageous as it requires a lot of money and time. Furthermore, the denser the carbon material, the less self-depletion it will cause, and the cleaner the atmosphere inside the furnace.However, since such a material has a large coefficient of thermal expansion, it will be weak in thermal shock resistance and repeated thermal stress.
Cracks occur in the cylinder, and carbon fiber dust enters and scatters from the cracks, roughening the surface of the Si ₃ N ₄ as described above. If a cylinder made of a carbon material with a small coefficient of thermal expansion is used to prevent this cracking, the above-mentioned disadvantages caused by the high porosity of the material itself still remain. The present invention solves these various problems,
This was done to obtain a high quality Si ₃ N ₄ sintered body. That is, the main object of the present invention is to provide a high quality Si ₃ N ₄ sintered body that has high strength and excellent wear resistance and thermal shock resistance. Another object is to obtain such a high quality Si ₃ N ₄ sintered body industrially and economically. Still another object is to provide a relatively inexpensive sintering furnace for Si ₃ N ₄ sintering that reduces wear of the heat insulating material and graphite heater and extends the service life of the furnace body. (Means for Solving the Problems) The device of the present invention for achieving the above-mentioned object comprises a space for accommodating a molded body, a graphite heater disposed around the space, and carbon fibers covering an inner wall surface. In a firing furnace for sintering non-oxide ceramics, which has a heat insulating layer made of
A sintering furnace for sintering non-oxide ceramics characterized in that a sheet formed by laminating and molding graphite flakes of 0.3% by weight or less is extended inside the heat insulating layer. The method of the present invention for firing a non-oxide ceramic molded body using such an apparatus involves firing a non-oxide ceramic powder and a sintering aid in a high-temperature inert atmosphere surrounded by a heat insulating layer made of carbon fiber mat. When sintering the molded body, a sheet formed by laminating and molding graphite flakes with an ash content of 0.3% by weight or less is interposed between the heat insulating layer and the molded body to isolate the heat insulating layer from the molded body. It is characterized by A suitable inert atmosphere in the method of the present invention is a nitrogen gas atmosphere, most preferably applied under pressure. The ash content of the graphite flakes is preferably 0.2
It is not more than 0.1% by weight, more preferably not more than 0.1% by weight. Further, it is desirable that the sheet formed by laminating graphite flakes has a thickness of about 0.2 to 0.4 mm. In order to interpose such a sheet between the heat insulating layer and the molded body, it is preferable to attach it to the inner surface of the heat insulating layer, or, if a carbon cylinder is installed, to the inner wall surface of the cylinder. It is. Further, the configuration of the method of the present invention is suitable for a firing furnace for sintering not only nitride ceramics but also non-oxide ceramics such as carbide ceramics, and the method for firing the above-mentioned non-oxide ceramic molded bodies. Hereinafter, the above configuration of the present invention will be further explained in detail based on the embodiments shown in the accompanying drawings. FIG. 1 is a vertical sectional view showing a main body of a silicon nitride sintering furnace which is an embodiment of the apparatus of the present invention. In the figure, a vertical cylinder 1 such as a cylinder or a rectangular cylinder, an upper lid 2 that seals the upper end of the cylinder, and a lower lid 4 that is detachably attached to the bottom with a clamp 3 constitute a furnace body. The cylinder, upper cover and lower cover each include a water jacket and have a cooling water inlet 5 and a cooling water outlet 6 thereto.
A graphite heater 8 supported by a heater support member 7 is disposed surrounding a molded body storage space A at the center of the furnace, and is connected to a power source via a heater terminal 9. In addition, a table 10 is mounted on the rod 1 on the lower cover 4.
1, and a Si ₃ N ₄ molded body 12 is mounted thereon. The inner wall surfaces of the cylinder, upper cover, and lower cover are thermally shielded by a heat insulating layer 13 made of carbon fiber and molded into a mat shape. The exhaust port 14 is connected to an exhaust device such as a vacuum pump (not shown), and the inert gas supply port 15 is connected to an inert gas injection device. Further, the furnace body is usually equipped with a thermocouple 16 and a sight hole 17 for measuring, controlling, and monitoring temperature conditions during operation. In the silicon nitride sintering furnace as described above,
The apparatus applied to the method of the invention is particularly suitable for the molded body 12
A graphite sheet 18 is uniformly interposed between the molded body 12 and the heat insulating layer 13 to block free flow between the atmosphere surrounding the molded body 12 and the atmosphere along the vicinity of the heat insulating layer 13. In the specific example shown, such a sheet is attached all over the inner surface of the heat insulating layer, but a cap-shaped or cylindrical carbon tube containing a graphite heater 8 and a molded body storage space A is inside. (not shown)
In this case, the sheet is preferably attached uniformly all over the inner wall of the cylinder. The above-mentioned graphite sheet is made by laminating and molding high-purity graphite flakes, and in order to minimize impurities generated from the graphite sheet itself at high temperatures,
It is formed using highly purified graphite with an ash content of 0.3% by weight or less, preferably 0.2% by weight or less, more preferably 0.1% by weight or less. Such sheets are heated at temperatures of at least about 2500°C under a nitrogen gas atmosphere.
can withstand temperatures of In addition, the amount of ash contained in this graphite sheet is related to the lifespan of the graphite sheet when repeatedly used at a temperature of about 2000℃, and the ash content is
When the content is 0.3% or less, preferably 0.1% or less, the life of the furnace material becomes very long, which is preferable. The thickness of the sheet is preferably about 0.2 mm to 0.4 mm; if it is too thin, the strength will be insufficient and there is a risk of breakage during attachment or stretching, while if it is too thick, the workability will deteriorate, which is not preferred. To attach the above sheet to the inner surface of the heat insulation layer,
Sewing with carbon fiber threads, adhesion with special carbon-based adhesive, etc. may be used, but heat-resistant locking devices made of carbon separately proposed by the present inventors, that is, large-diameter disc-shaped members with a smooth contact bottom surface, may be used. It is most preferable because it is simple and easy to lock using a locking tool formed integrally with a small-diameter rod-shaped locking member extending perpendicularly from the center portion of the bottom surface with a graphite. FIG. 2 is a vertical sectional view showing a modification of the device shown in FIG. 1, and the same parts as in FIG. 1 are designated by the same reference numerals. In the device shown in the figure, the upper lid 2 is the cylinder 1.
The molded body 12 is detachably attached to the molded body 12.
The table 10 on which the table 10 is mounted has substantially the same structure as the apparatus shown in FIG. 1, except that it is suspended from the top cover 2 by rods 11. In addition to the above, various design changes can be made without changing the basic technical idea, including the mechanism for attaching and detaching the molded body. (Function) Hereinafter, the function of the apparatus and method of the present invention will be explained with respect to the firing furnace shown in FIG. First, the clamp 3 is disengaged, the lower cover 4 is separated from the cylinder 1 together with the table 10 thereon, and the lower cover 4 is lowered by a lifting means such as a lift.
Molded with metal oxide sintering aid by conventional method
After placing the Si ₃ N ₄ molded body 12 on the table 10,
By raising the lower lid again, the molded body is loaded into the furnace, the lid is closed, and the clamp 3 is used to secure the molded body. Next, the vacuum pump is operated to exhaust the air in the furnace through the exhaust port 14, and then an inert gas, preferably nitrogen gas, is introduced through the inert gas supply port 15 to replace the atmosphere in the furnace with nitrogen gas. In this state, a voltage is applied to the graphite heater 8 through the terminal 9, the temperature inside the furnace is raised to about 1700°C to 1900°C, and the temperature is maintained for about 1 hour for firing. During firing, the furnace wall is shielded by a heat insulating layer 13 and further covered with a water jacket, so that it is maintained at a safe temperature of several hundred degrees at most. Furthermore, trace amounts of oxygen, oxides, oxynitrides, etc. liberated from the sintering aid or silicon nitride in the compact during high-temperature firing are blocked by the barrier made by the graphite sheet 18, resulting in a porous structure. Contact with the high-temperature oxidizable carbon fiber insulation layer is prevented. In addition, fiber dust such as carbon fiber fine fibrils that are generated by cutting and collapsing due to the small amount of surface oxygen that the carbon fibers that make up the heat insulating layer originally possess, or the small amount of oxygen that has penetrated through the barrier mentioned above, is It is confined near the furnace wall by the graphite sheet 18, and does not scatter or float inside the sheet and come into contact with the molded body. In addition, since the graphite sheet itself is made of high-purity graphite with a significantly reduced ash content, impurities such as oxygen and metals generated from the sheet remain within a range that does not cause any problems, and the molded body storage space is An atmosphere with very few impurities is maintained. In this way, the consumption of the sintering aid was significantly reduced, and no skeletonization of Si ₃ N ₄ was observed, resulting in a high-quality product with needle-shaped Si ₃ N ₄ crystals uniformly dispersed in the glassy material of the sintering aid. A Si ₃ N ₄ sintered body is obtained. (Example) Examples of the method of the present invention will be described below. All percentages in the examples are by weight. The amount of ash in a graphite sheet is measured by placing the sheet in a platinum crucible and burning it in a furnace at 800°C, then measuring the weight of the remaining ash (JIS
R7223). Example 1 90% Si ₃ N ₄ powder, 1% SrO as a sintering aid,
After adding 4% MgO and 5% CeO ₂ and mixing thoroughly, the mixture was molded into a plate shape of 10 mm x 60 mm x 60 mm using a mold press. This material was charged into a firing furnace having the structure shown in Fig. 1, and the atmosphere inside the furnace was replaced with _N2 gas.
A sintered body was obtained by holding at 1700 degrees for 1 hour under a N ₂ partial pressure of 1 atm. A graphite sheet was evenly attached to the inner surface of a heat insulating layer made of a carbon fiber felt molded body in the furnace. This graphite sheet is made by bonding graphite flakes with graphite adhesive.
It was a sheet with a thickness of about 0.4 mm made by laminating and bonding V58a (manufactured by Gigli, West Germany) and firing at about 600°C in nitrogen gas, and the ash content was 0.1%. Example 2 A graphite tube (bulk density 1.75 g/cm ³ , wall The procedure was the same as in Example 1, except that a Si ₃ N ₄ (thickness: 5 mm) was placed and a graphite sheet was evenly attached to the inner wall.
A sintered body was obtained. Example 3 A Si ₃ N ₄ sintered body was obtained in the same manner as in Example 1 except that the ash content of the graphite sheet was 0.3%. Comparative Example 1 The same equipment and method as in Example 1 were used except that no graphite sheet was attached.
A Si ₃ N ₄ sintered body was obtained. Comparative Example 2 Using the same equipment and using the same method as in Example 2, except that no graphite sheet was attached.
A Si ₃ N ₄ sintered body was obtained. The properties of the sintered bodies obtained in each of the above Examples and Comparative Examples are summarized in the following table.

[Table] As is clear from the results in the above table, the Si ₃ N ₄ sintered body obtained by the method of the present invention has a significantly lower weight loss rate than the conventional product, and the loss of the sintering aid is significantly lower. It shows that there are few. This means that there is almost no Si ₃ N ₄ skeleton generated, and this is also supported by the fact that the bending strength of the fired surface is much higher than that of conventional products. It is also extremely stable and highly pure against high-temperature oxidation, and fluorescent flaw detection has demonstrated that it has a dense structure with few voids, indicating that it has excellent wear resistance and thermal shock resistance. When sintering Si ₃ N ₄ , the molded body can be placed in a SiC crucible, a Si ₃ N ₄ crucible, or a carbon crucible with SiC densely deposited on the surface, and fired. Common. This is because it has the effect of suppressing the effects of carbon fiber waste, which is an insulating material, that exists in the furnace, and the effect of suppressing the effects of gases such as CO and CO ₂ caused by the decomposition of the heater material. It plays the role of efficiently firing the sintered body by assembling the sintered body. It goes without saying that the present invention produces similar effects even when such a crucible is used. Furthermore, it should be noted that when the crucible is made of Si ₃ N ₄ or the like, it is effective in preventing skeletonization of the crucible and extending its life. When the ash content in the graphite sheet is 0.3%, the life of the furnace is short, but a good silicon nitride sintered body can be obtained. Furthermore, when the ash content is as low as about 0.1%, the life of the furnace becomes extremely long and the properties of the silicon nitride sintered body are also better. Furthermore, it has been found that the ash content of the graphite sheet applied to the present invention has a significant effect on the characteristic values of the sintered body, and the effect of using the graphite tube in combination is also excellent. Furthermore, the graphite sheet of the present invention does not necessarily need to be installed throughout the entire furnace, and if the graphite sheet is not extended, the heat insulating material made of carbon fiber mat will be extremely worn out. That is, it goes without saying that simply attaching it to a portion close to the heater will have a considerable effect. (Effects of the Invention) As described above, according to the method and apparatus of the present invention, the atmosphere inside the Si ₃ N ₄ firing furnace, especially in the Si ₃ N ₄ molded body storage space, is free from contamination by impurities such as carbon fiber dust. Because it is kept clean, Si ₃ N ₄ skeletonization due to depletion of the sintering aid on the surface of the sintered body is prevented, resulting in a homogeneous and high-quality Si with high strength, excellent wear resistance, and thermal shock resistance. _{A 3N4} sintered body is obtained. Furthermore, according to the present invention, the purpose can be easily achieved simply by attaching a graphite sheet to the inner surface of the heat insulating layer. A method for firing Si ₃ N ₄ that is industrially easy and economically advantageous, requiring far less equipment and expenses, and no increase in power consumption, compared to methods that were forced to increase costs due to an increase in heater consumption rate. A firing furnace is provided. In order to further improve the quality and properties, if such a graphite cylinder is used in combination, it is possible to achieve excellent effects that could not be achieved conventionally. Furthermore, the present invention prevents carbon fiber dust generated from the insulation material from coming into contact with the Si ₃ N ₄ molded body, thereby reducing the generation of oxidizing gas and extending the life of the insulation material and graphite heater. This also has the effect of extending the useful life of the furnace body.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view showing a specific example of the device of the present invention, and FIG. 2 is a schematic vertical cross-sectional view showing another embodiment of the device of the present invention. 1...Cylinder, 2...Upper lid, 3...Clamp, 4...Lower cover, 5...Cooling water inlet, 6...Cooling water outlet, 7...Heater support member, 8...Graphite heater, 9 ...Heater terminal, 10...
Table, 11... Rod, 12... Molded object, 1
3... Heat insulation layer, 14... Exhaust port, 15... Inert gas supply port, 16... Thermocouple, 17... Sight hole, 18... Graphite sheet.

Claims (1)

[Claims] 1. A non-oxide ceramic that includes a space for accommodating a molded body, a graphite heater disposed around the space, and a heat insulating layer made of carbon fiber mat covering the inner wall surface. In the sintering furnace,
1. A firing furnace for sintering non-oxide ceramics, characterized in that a sheet formed by laminating and molding graphite flakes having an ash content of 0.3% by weight or less is extended inside the heat insulating layer. 2. The firing furnace for sintering non-oxide ceramics according to claim 1, wherein the non-oxide ceramics are silicon nitride. 3. When sintering a compact made of non-oxide ceramic powder and a sintering aid in a high-temperature inert atmosphere surrounded by a heat insulating layer made of carbon fiber mat, graphite flakes with an ash content of 0.3% by weight or less are A method for firing a non-oxide ceramic sintered body, characterized in that a laminated sheet is interposed between the heat insulating layer and the molded body to isolate the heat insulating layer from the molded body. 4. The method for firing a non-oxide ceramic sintered body according to claim 3, wherein the non-oxide ceramic molded body is silicon nitride.