METHOD OF PRODUCING BETA-SIALON FINE POWDER

Abstract: A method of producing a β-sialon fine powder which comprises using a silane compound represented by the general formula of SiH x Cl 4-x (0 ≦ x ≦ 4) as a Si source, aluminum chloride as an Al source and an ether represented by the general formula of ROR′ (R,R′ = C y H 2y+1 , 1 ≦ y ≦ 5) as an oxygen source respectively, dissolving them in an organic solvent capable of dissolving them to obtain a solution, introducing ammonia gas into the solution to produce a precipitates, and burning the precipitates in a nonoxidative atmosphere. In the β-sialon fine powder obtained by the method of the invention, respective elements of Si, Al, O and N are uniformly distributed in a chemical viewpoint. A high density high strength sintered body can be produced even at a relatively low temperature by using the β-sialon fine powder.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of producing β-sialon fine powder. Description of the Prior Art β-sialon represented by the general formula of Si 6-z Al z O z N 8-z (0 ≦ z ≦ 4.2) is a solid solution of β-Si₃N₄ of which the Si position is substituted with Al and the N position is substituted with O. Sintered bodies of β-sialon are excellent in strength, heat resistance, corrosion resistance and resistance to oxidation, and are utilized for cutting tools, parts of engines, parts of gas turbine and the like. As the synthesis of β-sialon powder, various methods are known, such as pyrolyzing kaolinite occurring in nature in an ammonia gas atmosphere (Journal of Material Sceince, 11 , 1972-1974, 1976), pyrolyzing a coprecipitation product of SiO₂ and Al(OH)₃ in an ammonia gas atmosphere (Am. Ceram. Soc. Bull., 58 , 191-193, 1979), adding aluminum metal to a mineral such as shirasu or quartz sand and reducing the mixture in an inactive atmosphere (Yogyo Kyokai-shi, 88 , 361-367, 1980), adding C to a SiO₂-Al₂O₃ mixture and burning in a nitrogen gas stream (Yogyo Kyokai-shi, 95 , 570-574, 1987), and mixing amorphous Si₃N₄ with Al₂O₃, AlN and burning in a nitrogen gas stream (Japanese Patent KOKAI No. 62-72507). Among the above methods, in the case of using natural materials such as kaolinite, shirasu and quartz sand, the synthetic powder is liable to be contaminated with impurities. While, in the case of burning the coprecipitation product of SiO₂ and Al(OH)₃, the mixture of SiO₂-Al₂O₃ and the mixture of amorphous Si₃N₄ with Al₂O₃, AlN, the reaction time is long, and unreacted materials remain in the reaction product, because the reaction occurs between solid phases. Moreover, the substitution of the Si position with Al and that of the N position with O are insufficient due to the solid phase reaction, and the bending strength of the particles in the sintered body produced from the above conventional powder is also insufficient. They cause to reduce strength and resistance to oxidation of the sintered body. SUMMARY OF THE INVENTION An object of the invention is to provide a method of producing β-sialon fine powder being highly pure wherein Si and Al, O and N are uniformly mixed in chemical view point or bound to each other. Another object of the invention is to provide a method of producing β-sialon fine powder capable of producing a high density sintered body at a relatively low temperature. The inventors have investigated in order to achieve the above objects, and found that a β-sialon powder where Si, Al, O, N are uniformly mixed in atomic level can be obtained by using a silane compound represented by the general formula of SiH x Cl 4-x (0 ≦ c ≦ 4) as a Si source, aluminum chloride as an Al source and an ether represented by the general formula of ROR′ (R,R′ = C y H 2y+1 , 1 ≦ y ≦ 5) as an oxygen source respectively, dissolving them in an organic solvent capable of dissolving them to obtain a solution where Si, Al and O are uniformly mixed in atomic level, introducing ammonia gas into the solution to produce a coprecipitation product of Si imide and Al amide, and burning the coprecipitation product in a nonoxidative atmosphere. DETAILED DESCRIPTION OF THE INVENTION The silane compound is represented by the general formula SiH x Cl 4-x (0 ≦ x ≦ 4), and includes monosilane, dichlorosilane, trichlorosilane, tetrachlorosilane and the like. The ether is represented by the general formula ROR′ where R and R′ are represented by the general formula of C y H 2y+1 (1 ≦ y ≦ 5) and may be identical with or different from each other. The symbol y is preferably 1 to 3. Examples of the ether are dimethyl ether and diethyl ether. The mixing ratio of the silane compound to aluminum chloride is preferably 0.5 to 60 by the elemental ratio of Si/Al. When the ratio is less than 0.5, Al is out of the scope capable of forming a solid solution of sialon. While, when the ratio is greater than 60, uniform mixing of Si with Al is difficult. The quantity of the ether is not less than 1 as a molar ratio to AlCl₃. It is known to form 1:1 adduct between an Al halide and an ether (Ber. disch. Chem. Ges., 75 , 1055, 1942). When the quantity of the ether is less than 1, the addition reaction is insufficient, and unreacted AlCl₃ remains. The organic solvent has an ability to dissolve the silane compound, aluminum chloride and the ether but does not react with them to produce a foreign material. Suitable organic solvents are dimethyl ether, benzene, nitrobenzene, carbon tetrachloroethyrene, chloroform, and the like. By using the above organic solvents, it is possible to form a solution where Al and Si are uniformly dispersed. Gaseous ammonia is introduced into the solution. The reaction with ammonia may be flow system or batch system. The pressure of the ammonia gas may be selected from the range of 0 to 10 kg/cm² as gauge pressure. A suitable reaction temperature is different according to the raw materials employed and the like, and the range of -30°C to 50°C is preferred, because the vapor pressure of the silane compound is usually high. The range of -20°C to 10°C is particularly preferred. For example, when tetrachlorosilane is employed as the silane compound, it has a vapor pressure of 16.0 kPa (120 mmHg) even at 10°C. Then, a temperature higher than that is not preferable, because the silane compound is lost by evaporation during the operation resulting to change the composition of the reaction mixture. In the case of flow system, the silane compound is also lost by evaporation during the reaction. While, when the temperature is lower than -30°C, the reaction rate with NH₃ is too slow. Therefore, the reaction time becomes long, and unreacted materials is liable to remains. On the other hand, AlCl₃ and silane compounds reacts with a Lewis base such as pyridine and picoline to form an adduct being stable even at ordinary temperature (J. Chem. Soc. (A), 1508, 1966, J. Am. Chem. Soc., 110 , 3231-3238, 1988). In the method of the invention, it is effective to prevent the evaporation of the silane compound by forming the above adduct prior to the reaction with NH₃. Besides, AlCl₃ may be allowed to react with the ether to produce a compound where Al is uniformly mixed with O prior to the mixing with the silane compound, as shown in the following formula. The above compound and the silane compound are dissolved in a common solvent, and ammonolysis reaction is carried out similarly. The compound mentioned above may be one of (A), (B) or (C) or a mixture of two or all of them. The solution where the reaction with NH₃ was finished is composed of the coprecipitation product of Si, Al, a by-product of NH₄Cl and the solvent. Then, the solvent, NH₄Cl, unreacted materials are removed from the solution by a known method such as filtration, drying, sublimation or the like. Subsequently, the coprecipitation product is burned in a nonoxidative atmosphere. The burning may be carried out at 1200 to 1600°C in a N₂, NH₃ atmosphere. When the temperature is lower then 1200°C, the pyrolysis of the coprecipitation product is incomplete, and impurities, H and Cl remain. While, when the temperature is beyond 1600°C, the pyrolysis of the product and the production of SiC occur. A part of the atmosphere may be substituted with a nonoxidative gas such as Ar of H₂. The sialon powder thus obtained is fine powder having a particle size of 0.2 to 0.5 µm where Si, Al, N and O are controlled in an atomic level. The C content of the sialon powder is less than 5 wt. %, preferably less than 0.5 wt. %, and the Cl content is less than 1 wt. %, preferably less than 0.1 wt. %. When the C content or the Cl content is more than the above value, they interfere the densification or become the defects resulting to decrease the strength of the sintered body. A high density high strength β-sialon sintered body can be produced from the above powder by molding it through uniaxial press, CIP or the like and then sintering at 1700 to 1800°C in an N₂ gas stream. In the method of the invention, by using the silane compound having the general formula SiH x Cl 4-x (0 ≦ x ≦ 4), aluminum chloride an the ether having the general formula ROR′ (R,R′ = C y H 2y+1 , 1 ≦ y ≦ 5), an uniformly mixed state can be made by dissolving them. The ether may form a 1:1 adduct with aluminum chloride in the solution. When gaseous ammonia is introduced into the solution, the ammoniation of the silane compound and the aluminum chloride-ether adduct immediately occurs to produce the coprecipitation product where silicon imide and aluminum amide are uniformly mixed in molecular level. The β-sialon fine powder can be produced by pyrolyzing the coprecipitation product in a nonoxidative atmosphere. In the β-sialon fine powder obtained by the method of the invention, respective elements of Si, Al, O and N are uniformly distributed in a chemical viewpoint. A high density high strength sintered body can be produced even at a relatively low temperature by using the β-sialon fine powder. EXAMPLES Example 1 14.0 cm³ (cc) of SiCl₄, 8.15 g of AlCl₃ (Si/Al = 2.0) and 100 cc of dimethyl ether were placed in a 1 l flask having an inlet and an outlet for gas, and stirred sufficiently to form a clear solution where all of them were completely dissolved. The flask was kept at -10°C in a thermostat, and NH₃ gas was introduced into the flask for 2 hours at a rate of 2 l/min by opening both of the inlet cock and the outlet cock. White precipitates were produced in the flask. The precipitates were collected by filtration using a filter paper, and placed in a closed vessel having an evacuation line. Then the precipitates were dryed by heating on a hot plate, while the closed vessel was evacuated. All of the above operations were carried out in a glove box kept at a moisture concentration of less than 1 ppm and a oxygen concentration of less then 1 ppm. The above precipitates was placed in a platinum crucible and put into a tube furnace. N₂ gas was flown in the furnace at 1 l/min, and the temperature was elevated at 10°C/min to 350°C. Then, the temperature was kept for 1 hour to remove a by-product of NH₄Cl. The temperature was elevated again from 350°C to 1450°C at a rate of 5°C/min, and kept for 2 hours. The powder thus obtained was white, and found to be composed of equilateral fine particles of 0.2 to 0.5 µm by using a transmission electron microscope (TEM). As a result of X-ray diffraction analysis, the powder was a single phase of β-sialon, the Z value estimated by the values of a-axis and c-axis was about 1.9. It was confirmed by energy-dispersive element analysis (EDS) that Si and Al are uniformly dispersed. The carbon content of the powder measured by wet analysis was 0.2 wt. %, and the Cl content was 0.1 wt. %. Example 2 Similar to Example 1, 13.7 cm³ (cc) of SiCl₄, 8.0 g of AlCl₃ (Si/Al = 2.0) and 100 cc of dimethyl ether were placed in a 1 l flask, and 200 cc of pyridine was added to produce white precipitates immediately after. The flask was kept at 0°C in a thermostat, and the mixture was allowed to react for 2 hours by introducing NH₃ gas at a rate of 2 l/min The after operations were carried out in the same manner as Example 1. The powder obtained was analyzed by X-ray diffraction analysis, TEM and EDS, and found to be composed of β′ phase containing about 15 wt. % of O′ phase. The Z value was about 2.0, and Si and Al were uniformly dispersed. The particle size was 0.3 to 0.6 µm. To 50 g of the above powder, 300 cm³ (cc) of ethanol and 1.5 wt. % of PVA as a molding assistant were added, and the solvent was evaporated in a rotary evaporator with suction to form granules. The granules were pressed by uniaxial press in a mold having an inside diameter of 20 mm, and then subjected to CIP at 3 t/cm² to obtain a molded body. The molded body was dewaxed at 500°C in a nitrogen gas, and sintered in a carbon crucible at 1750°C in a nitrogen gas stream of 1 atm for 2 hours. The density of the sintered body was 3.10 corresponding to 99.7 % of the theoretical density being 3.11. As a result of X-ray diffraction analysis, the sintered body was composed of β phase containing about 10 wt. % of O′ phase, and the Z value was about 2.0 which maintains the powder composition. Example 3 15.0 cm³ (cc) of SiCl₄, 1.59 g of AlCl₃ and 100 cm³ (cc) of dimethyl ether were placed in a pressure vessel made of tantalium having an inlet and an outlet for gas, an evacuation line and a stirrer, and stirred well. The vessel was kept at -5°C in a thermostat, and ammonia gas was introduced for 3 hours into the vessel of which the outlet cock was closed so that the pressure was kept at 2 kg/cm². After the reaction, the atmospheric gas in the vessel was replaced by N₂ gas, and the solvent was removed at 30°C in vacuo. The powder produced was placed in an alumina crucible, and NH₄Cl was removed by heating at 350°C for 1 hour in a horrizontal tube furnace. Then, the powder was placed in an urethane pot, and ball mill by Si₃N₄ balls after acetone was add. After drying, the powder was placed in a platinum crucible, and heated at 1400°C for 3 hours in a NH₃ gas stream of 1 l/min in a tube furnace. The powder thus obtained was β-sialon powder having a particle size of 0.2 to 0.4 µm, and the Z value was about 0.5. The carbon remaining in the powder was 1.2 wt. %. Example 4 8.0 g of AlCl₃ was placed in a beaker, and dissolved by adding 100 cm³ (cc) of dimethyl ether. The solution was heated, and light yellowish porous matter was produced at about 170° C. The porous matter was lightly ground by a mortar, and dissolved in 50 cm³ (cc) of nitromethane in a beaker. When 14.1 cm³ (cc) of SiCl₄ was added, the mixture was separated into two phases. By adding 100 cm³ (cc) of benzene, the mixture was returned to one phase. Then, the solution was placed in a 1 l flask, and allowed to react for 2 hours by introducing NH₃ gas at a rate of 2 l/min in the same manner as Example 1. The produced precipitates were treated similarly to obtain white powder. As a result of X-ray analysis, the powder was composed of β phase containing 5 wt. % of O′ phase, and the Z value was estimated to be 2.1. The particle size was 0.2 to 0.5 µm.

Claims: A method of producing a β-sialon fine powder which comprises using a silane compound represented by the general formula of SiH x Cl 4-x (0 ≦ x ≦ 4) as a Si source, aluminum chloride as an Al source and an ether represented by the general formula of ROR′ (R,R′ = C y H 2y+1 , 1 ≦ y ≦ 5) as an oxygen source respectively, dissolving them in an organic solvent capable of dissolving them to obtain a solution, introducing ammonia gas into the solution to produce a precipitates, and burning the precipitates in a nonoxidative atmosphere. The method of claim 1 wherein the molar ratio of the ether to the aluminum chloride is more than 1. The method of claim 1 wherein the reaction temperature with the ammonia gas in -30 to 50°C. The method of claim 1 wherein the burning temperature is 1200 to 1600°C. The method of claim 1, claim 2, claim 4 or claim 4 wherein the aluminum chloride is allowed to react with the ether to produce AlOCl, or C y H 2y+1 -O-AlCl₂ or a mixture of two or all compounds mentioned above, and the above product and the silane compound are dissolved in the solvent. The method of claim 1, claim 2, claim 3 or claim 4 wherein the mixing ratio of the silane compound to the aluminum chloride is 0.5 to 60 by the element ratio of Si/Al.

Inventors: NISHIO, HIROAKI, SATO, MICHITAKA, WATANABE, KEIJI
Applicants: NKK CORPORATION
Application year: 1989
Main classification: C04B35
IPC codes: C01B21, C04B35