JPS63393B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing non-oxide ceramics. In recent years, new ceramics has attracted attention in various fields. Non-oxide ceramics are attracting particular attention, and among them, non-oxide heat-resistant ceramics such as Si ₃ N ₄ , SiC, and Sialon are most expected to be of industrial importance. However, looking at the technologies that have been achieved in this field to date, it seems that we are running into a contradiction between increasing sinterability and providing sufficient heat-resistant strength in non-oxide ceramics. That is, in the case of Si ₃ N ₄ , for example, the body diffusivity of Si and N is small, so even if grain growth occurs under normal sintering conditions, shrinkage hardly occurs. In order to sinter this material, it is said that it is necessary to hot press it by applying ultra-high pressures such as those required in diamond synthesis. However, such a method results in too high sintering cost and cannot be used as an industrial heat-resistant material. Therefore, the methods currently being researched mainly use sintering aids, such as MgO, Al ₂ O ₃ , Y ₂ O ₃ , etc.
Addition of ~20% by weight has been carried out. These auxiliary agents form low melting point substances at the Si ₃ N ₄ crystal boundaries to promote sintering. However, the use of such auxiliary agents as described above only results in a decrease in heat resistance strength. As a result of various considerations, the inventors of the present invention first focused on the use of amorphous amorphous powder as a result of various considerations as to how to resolve such contradictions. When organometallic polymers are used in SiC sintering, high strength and high elastic SiC can be produced by heat treatment at 1300℃.
It has been reported by Professor Yajima et al. of Tohoku University that this can be obtained. This mechanism is believed to be due to the thermal decomposition of organometallic polymers, which produce extremely fine SiC, that is, amorphous SiC, which sinter together. Ultrafine powders of these heat-resistant ceramics, particularly fine powders, do not show crystals and are amorphous, which can be easily obtained by a gas phase synthesis method using chlorides. For example, when NH ₃ is applied to SiCl ₄ , amorphous Si ₃ N ₄ powder is produced at low temperatures. From this point of view, the present inventors used amorphous Si ₃ N ₄ powder, pressed it, and sintered it by changing the sintering conditions such as vacuum or high-pressure nitrogen gas atmosphere and temperature. However, a good sintered body could not be obtained. This is thought to be due to the oxidation phenomenon of powder caused by air, which was previously reported by one of the inventors. (Akio Hara: Powder and Powder Metallurgy, Vol. 18, No. 8, p. 338) In other words, fine powder has many recesses, and these recesses contain moisture in the atmosphere according to the following equation. is condensed. lnp/po=Vγ/RT×2/r, where p: saturated vapor pressure in the recess, po: saturated vapor pressure on the plane, V: molecular volume, R: gas constant, γ: surface tension, T: temperature, r: concavity radius. Using this formula to determine whether or not it absorbs moisture in relation to the concave radius (r), it becomes r (μ) p/po (%, 25°C, relative humidity) 1 99.96 0.1 98.96 0.01 90.07 0.001 35.12. Amorphous amorphous powder has a size of angstroms, and since such fine powders stick together (entangle) with each other, it is thought that there are many depressions with a radius of about 0.001 μm. That is, moisture absorption is significant in the atmosphere, and if the absorbed moisture is easily released as it is when heated, there will be no problem, but this moisture together with air will cause a chemical reaction similar to the rusting phenomenon of metals. The above cited document describes the case of WC, but water that dissociates only at 1000℃, a type of hydroxide, is produced, and by heating to high temperatures, CO and H ₂ gases are generated. It is separated in the form of It is easy to understand that the generation of such gas inhibits sintering. The present applicant has already proposed countermeasures against such phenomena in Japanese Patent Publication No. 50-4442 and Japanese Patent Publication No. 50-4443, but the present invention also involves converting such carboxylic acids or acid anhydrides into powder in a vaporized state. This is a method of adhesion. The carboxylic acid or acid anhydride used preferably has a low molecular weight and a low boiling point. If the molecular weight is high, it is difficult to penetrate into the fine details of the powder, and if the boiling point is high, it is difficult to use it in a vaporized state. Therefore, it is preferable to use acetic acid, acrylic acid, propionic acid and acetic anhydride. Among carboxylic acid derivatives, acid anhydrides such as acetic anhydride have very similar properties and can be used in the same manner, but those with other substituents that adversely affect sintering cannot be used. When a carboxylic acid or an acid anhydride is attached to powder in a vaporized state, it not only prevents moisture absorption and the accompanying formation of hydroxides as described above, but also prevents the formation of hydroxides, which can be easily removed by heating at relatively low temperatures. Since the powder is separated from the powder, an extremely fresh powder surface can be obtained. This brings about the same phenomenon that allows high-density, high-strength ceramic sintered bodies and ceramic fibers to be obtained at relatively low temperatures using organometallic polymers. The present invention will be explained below with reference to Examples. Example 1 Ammonia gas (purity 99.99%) and nitrogen gas (purity 99.99%) were used as carrier gas and silicon tetrachloride (purity 99.99%) was used at a flow rate of 25% each.
cc/min, 80 c.c./min into a reaction tube heated to 1050°C. As a result, powder was deposited at the bottom of the reaction tube. This powder was vacuumed with nitrogen gas at a vacuum level of 5×10 ^-2 mm.
It was introduced into a container maintained at Hg. Inside this container
Steam of acetic anhydride obtained by heating to 30 to 40°C was introduced in advance. After 30 minutes, the container was opened, and the powder was taken out and kept in a constant temperature and humidity bath for 7 days together with the powder that was not treated with acetic anhydride. The temperature inside the tank is 40℃.
It was maintained at a humidity of 80%, which facilitates oxidation. After holding under the above conditions, the powder was removed from the constant temperature and humidity chamber, pressed at a pressure of 3 tons/cm ² to create a green compact, and sintered at 1600°C in a nitrogen atmosphere for 1 hour. When the obtained sintered body was tested, as shown in Table 1, the density and strength of the sintered body using the powder treated with acetic anhydride were higher than those of the untreated powder, and the sintered body was found to be able to withstand higher temperatures. The results showed that there was almost no decrease in strength. On the other hand, X-ray diffraction of the powder treated with acetic anhydride, which had not been placed in a constant temperature and humidity chamber, revealed that it had an amorphous structure. Further analysis of the silicon and nitrogen contents in the powder confirmed that it was silicon nitride.

[Table] Example 2 A powder obtained in the same manner as in Example 1 was treated with acrylic acid (monomer) vapor. After leaving this treated powder together with the untreated powder indoors for 7 days, it was pressed at a pressure of 3 tons/cm ² to create a green compact, which was then sintered at 1600°C in a nitrogen atmosphere for 1 hour. Summer. The obtained sintered body was tested as shown in Table 2, and the same results as in Example 1 were obtained.

[Table] Example 3 Powder was prepared in the same manner as in Example 1 using acrylic acid (monomer) and other carboxylic acids in place of acetic anhydride, and then kept at constant temperature and humidity under the same conditions as Example 1 together with the untreated powder. After being held in a tank, it was molded and sintered. When the obtained sintered bodies were tested, as shown in Table 3, the sintered bodies made of the treated powder of the present invention had a higher density and higher strength at high temperatures than the sintered bodies made of the untreated powder. was gotten.

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Claims (1)

[Claims] 1. A non-oxide ceramic powder in an amorphous state is produced by a vapor phase synthesis method, and then a carboxylic acid and/or an acid anhydride is attached to the produced powder in a vaporized state, and then this powder is A method for producing non-oxide ceramics, which comprises forming into a desired shape and then sintering in a vacuum and/or a non-oxidizing atmosphere. 2. The method for producing non-oxide ceramics according to claim 1, wherein the non-oxide ceramics are Si ₃ N ₄ . 3. The method for producing non-oxide ceramics according to claim 1, wherein the non-oxide ceramics are SiC.