PRODUCTION OF SILICON IMIDES AND OF SILICON NITRIDE THEREOF

Abstract: A silicon halide, preferably SiCl₄, is reacted with an excess of a base, e.g. a Lewis base, secondary amine, tertiary amine, phosphine, arsine, stibine or a derivative of phosphine, arsine or stibine, to produce an adduct. The adduct, which need not be separated, is treated with ammo­ nia gas to produce the imide or amide of silicon. A light organic solvent is used. The temperature used is -78 to 100°C and the reaction is rapid. Byproduct ammonium halide is removed, and the imide or amide is heated at 1000° to 1400 °C in an oven in an atmosphere containing nitrogen, to produce silicon nitride of good sintering characteristics and more than 70wt.% of which is of the alpha crystal form.

Description: Method for producing silicon-imide or silicon-amide This invention relates to a method for producing a siliconimide or silicon-amide, which is useful as a precursor of silicon nitride. Sintered silicon nitride is one of the most important material which is able to use as heat resistant structural material for gas turbine and diesel engine, and as high performance material (which is useful for saving energy or for saving resources) for cutting tool, since it is superior in strength and shock resistance at high temperature, and also in corrosion resistance. So far, the following four methods have been adopted widely for production of silicon nitride. 1. A direct nitridation method of silicon by heating metal silicon at 1300 C ¯ 150000 in nitrogen or ammonia. 1. A silica reducing method in which silicon produced by a reduction of silica reacts with nitrogen - the reduction of silica is carried out by heating silica or materials containing silica with carbon in the atmosphere of nitrogen or by the reduction reaction with urea. 3. Gas phase synthesis in which silicon tetra-chloride reacts with ammonia directly at high temperature. 4. An imide-amide heat decomposition method in which silicon-imide or silicon-amide is obtained by ammonolysis of silicon tetra-chloride and then is heated in a non-oxidative atmosphere to produce silicon nitride. These processes, however, have many defects which are described below. In the case of the above-mentioned method 1., not only is the reaction time long and the heating process complicated, but also the main product is - type silicon nitride which was rough and contained many impurities. In the case of the above mentioned method 2., it was difficult to purify raw materials, and in addition, the reaction time is long and the produced material has a binary system of type silicon nitride and type silicon nitride. In the case of method 3., since the produced silicon nitride is generally amorphous, not only further a process for crystallization to obtain OL - type silicon nitride is necessary but also it is complicated to deal with chlorine which is by-produced at high temperature reaction. In the case of method 4., although there is an advantage that it is able to produce or - type silicon nitride of high purity in a good yield, the tube of the reaction apparatus is apt to be choked up when silicon-imide or silicon-amide is formed; in addition, there is a defect that it is difficult to control the rapid exothermic reaction. Furthermore, a method to obtain sintered silicon nitride by heating a polysilazane at 8000C cm 20000C, which has been prepared by pyrolyzing organo-polysilazane, was recently proposed (Hajime Saito, Seni Gakkai Shi, 38 No. 1., 65 72 (1982). But, this method had a defect of producing silicon carbide and free carbon together with silicon nitride. In the method 4. above, it was a significant problem to prepare easily silicon-imide or silicon-amide with high purity. As the result of investigations, we found that by preparing an adduct which is easily formed from the reaction of a silicon halide (which had been purified easily) with a base, then carrying out ammonolysis quantitatively on said adduct, silicon nitride powder with high purity can be obtained, since not only no catalysts such as Fe and Ca were necessary and no residues such as base remained, but also it was very easy to remove ammonium chloride. Consequently, the object of the present invention is to provide an easy method to prepare silicon-imide or siliconamide, which is useful as precursor of silicon nitride powder. The present invention thus comprises a method of synthesizing silicon-imide or silicon-amide, as a precursor to prepare sintered silicon nitride powder, wherein an adduct is formed by making silicon halide to react with a base and then reacting the adduct with ammonia. The silicon halide used can be arbitrarily selected from compounds of the formula SiHmX4 - m where X = F, Cl, Br or I, and mixtures thereof; silicon tetrachloride is preferably used. The base used in the present invention can be selected from bases which are able to form adducts with silicon halide; bases which do not react other than with silicon halide to form an adduct are preferable. Examples of these bases are Lewis bases, tertiary amines (e.g., trialkyl amine such as trimethyl amine and triethyl amine, pyridine, picoline and derivatives thereof, e.g. a trialkyl derivative), secondary amines which have a steric hindrance group, phosphine, stibine, arsine and derivatives of them (e.g., a trialkyl derivative such as trimethyl-phosphine, dimethylethylphosphine, methyl-diethyl-phosphine, triethyl-phosphine, trimethyl-arsine and trimethyl-stibine). Bases having a low boiling point and smaller basicity than ammonia (e.g., pyridine, picoline, trimethyl-phosphine, dimethylethylphosphine, methyl-diethylphosphine, triethyl-phosphine) are preferable, but pyridine and picoline are the most preferable since they are not only inexpensive but also easy to treat. The quantity of the base need not be large but it should be a larger amount that the stoichiometric amount of silicon halide. Many kinds of solvents can be used for the reaction. Light solvents (e.g., hexane, benzene, pyridine, methanechlorides, ether or aceto-nitrile) are preferable; pyridine and methane chlorides are most preferable. 'The adduct of the silicon halide can be formed by the conventional method; this adduct is formed as a reaction intermediate, which may need not be separated before being reacted. The silicon-imide or silicon-amide of the present invention is synthesized by the reaction of the said adduct with ammonia. e.g. as ammonia gas. The quantity of ammonia need not be high but it should be a larger amount than the stoichiometric amount of silicon. The reaction temperature to form silicon-imide or silicon amide in the present invention is from - 780C to 1000C, preferably from - 400C to 800C. As for the reaction time and reaction pressure, they can be selected over a wide range since the reaction rate is very high. If the reaction temperature is lower than - 780C, the reaction rate becomes small and if the reaction temperature is higher than 1000C, produced silicon-imide or silicon-amide is decomposed again, therefore, these conditions should not be selected. The described reaction to synthesize the silicon-imide or silicon-amide is preferably carried out in an atmosphere of an inert gas, preferably nitrogen or argon. In an optional further step, by carrying out heat treatment in the presence of nitrogen after separating ammonium halide from the silicon-imide or silicon-amide by a suitable method (e.g., liquid ammonia washing or sublimation), the silicon imide or the silicon-amide obtained in the method described above can be converted into silicon nitride containing much ratio of X - type crystal form and high purity - it is possible to make silicon nitride having more than 70% of oC- type crystal form. The phrase "in the presence of nitrogen" herein means an atmosphere of ammonia, nitrogen, a mixed gas of nitrogen and an inert gas such as a mixture of nitrogen and hydrogen, nitrogen and ammonia, nitrogen and argon or an atmosphere of gases produced by the decomposition of a compound containing nitrogen (e.g., ammonia); nitrogen is always necessary. The ratio of the mixed gases is not restricted, but nitrogen rich mixed gas is preferable. Generally, it is preferable that silicon nitride used as raw powder for sintered silicon nitride has an d - type crystal form and has high purity, since s - phase is transferred to # - phase during the sintering, which improves the sintering characteristics and the development of a textile-like structure, resulting in sintered silicon nitride having high strength. To make silicon nitride powder (more than 70% of which is the i- type crystal form and has excellent sintering characteristics) in a high yield using the silicon-imide or silicon-amide obtained in the present invention, the treating temperature range should be from 10000C to 14000C. When the treating temperature is low (e.g., lower than 5000C), the obtained silicon nitride contains chlorine or hydrogen. On the other hand, if the treating temperature is higher than 19000C, the produced silicon nitride should be decomposed. If the treating temperature range is from 7000C to 10000C, main product should be mixture of amorphous silicon nitride and silicon, and if the temperature range is from 14000C to 19O00C, the main product should be R - type silicon nitride. The treating time can be roughly determined by observing the gas which is formed as a by-product when silicon-imide or silicon-amide is heated. Generally, the time is rather shorter at higher temperature and rather longer at lower temperature, but preferably it is from about 8 to about 20 hours and a time from about 10 to about 16 hours is the best. As a material of an oven in which to treat the silicon-imide or silicon-amide in an atmosphere of nitrogen at high temperature, it is preferable to select materials which are but little oxidized, such as silicon nitride, silicon carbide, carbon, tantalum or molybdenum. The above mentioned conditions make it possible to obtain pure silicon nitride having more than 70% of < - type crystal form and containing more than 39% of nitrogen and less than 0.001% of chlorine. However, it may be obtained silicon nitride containing elements such as Mg, Y, Al, Fe or B which are known as elements able to contribute to sintering, if necessary. The present invention is very useful to produce siliconimide or silicon-amide as precursor of silicon nitride, since these products can be produced through the reaction of said adduct and the ammonia adduct produced is very stable in the reaction solvent. Namely, according to the present invention all defects such as loss of silicon halide and choking up of apparatus can be avoided. For these reasons not only the reaction temperature in the method of the present invention can be raised more than under ordinary conditions but also the concentration of silicon halide in the reaction mixture can be increased without accompanying dangers such as poisoning and firing. Therefore, by the present invention silicon-imide or silicon-amide can be produced in good efficiency by using simple apparatus. In the accompanying drawings: Fig. 1 shows the infrared absorption spectrum of a silicon tetrachloride-pyridine adduct obtained in Example 1; Fig. 2 shows the DTA curve (Differential thermal analysis) and the TG curve (Thermogravimetric analysis) of the silicon tetra-chloride pyridine adduct obtained in Example 1; Fig. 3 shows the infrared absorption spectrum of the mixture obtained in Example 2, which contains silicon-imide and ammonium chloride; Fig. 4 shows the infrared absorption spectrum of commercial ammonium chloride, for comparison. Fig. 5 shows the infrared absorption spectrum of the mixture obtained in Comparative Example 2, which contains siliconimide and ammonium chloride; and Fig. 6 shows the DTA curve and TG curve of the mixture obtained in example 2, which contains silicon-imide and ammonium chloride. The following examples are provided to illustrate the invention. EXAMPLE 1 Synthesis of adduct between silicon tetra-chloride and pyridine Three neck flask (200 m/) having mechanical stirrer, condenser and dropping funnel was purged with nitrogen'gas, then 20 mQ of dried pyridine and 80 ml of di-chloromethane were introduced in the flask and maintained at room temperature. Next, by adding 10 ml of silicon tetrachloride from dropping funnel, about 80 ml of white slurry of adduct was obtained. After a removal of residual reactants and solvent under a reduced pressure (55"C and 5mm Hg), 27.9 g of powderlike solid product was obtained which corresponded to 97.6% of product yield based on the silicon tetra-chloride. IR spectrum of the solid product was same to those of adduct between silicon tetra-chloride and pyridine shown in the article by H.J. Champbell-Ferguson et al, J. Chem. Soc. (A), 705 ¯ 712, (1967) as illustrated in Fig 1. Although a peak at 1485 cm'l is not described in the article, it was confirmed as one of the peak of pyridinium salt. Thermal decomposition of the product started at 60 C as shown in Fig. 2, which was a result of thermal analysis of the product. EXAMPLE 2 Synthesis of silicon-imide ammonium-chloride mixture through an adduct between silicon tetra-chloride and pyridine A four-neck flask (200 ml) having a gas inlet tube, mechanical stirrer, Dewar condenser and dropping funnel was purged with nitrogen gas, then 20 ml of dried pyridine and 80 ml of di-chloromethane were introduced into the flask and maintained at ordinary temperature. Next, by adding 10 mQ of silicon tetra-chloride from the dropping funnel, about 80 my of a white slurry was obtained. While agitating the reaction mixture vigorously at about 160C to about 180C, 10.2g of ammonia which was diluted with nitrogen was introduced in the flask over a period of 1.5 hours to 2 hours. When the reaction was finished, then residual reactants and the solvent were removed under a reduced pressure (53 Cand 5mm Hg), and 23.3g of solid product was obtained which corresponded to 98.5% of product yield based on the silicon tetra-chloride. No fume was observed in any reaction, therefore both gas flow tube and ammonia inlet were never choked. The IR spectra of the product and that of an ammonium chloride for comparison are shown in Figures 3 and 4 respectively. COMPARATIVE EXAMPLE 1 Synthesis of silicon-imide ammonium-chloride mixture by introducing ammonia gas into liquid silicon tetrachloride A three-neck flask (300 ml) having a gas inlet tube, mechanical stirrer and Dewar condenser was purged with nitrogen gas, then 10 ml of silicon tetra-chloride and 200 ml of dried carbon tetra-chloride were introduced in the flask and maintained at 150C. Next, while agitating the mixed solution vigorously, 10.2g of dried ammonia diluted with nitrogen was introduced in it over 1.5 hours. Fumes formed until the reaction ceased and deposited on the wall of the gas flow passage. Consequently the ammonia inlet was often choked and was opened by increasing pressure of diluent nitrogen. The product stuck fast to the inner surface of the flask. After completion of the reaction, residual reactants and solvent were removed under a reduced pressure (500C and 5mm Hg) and 10.9g of solid product was obtained, which meant 46.1% of product yield based on the silicon tetra-chloride. From this result, it is confirmed that the reaction yield in the method of comparative example 1 is much lower than that in the method of the present invention (EXAMPLE 2). COMPARATIVE EXAMPLE 2 Synthesis of silicon-imide ammonium-chloride mixture by the direct reaction of silicon tetra-chloride with ammonia A reaction vessel (100 ml) was cooled by a mixture of dry ice-methanol, then 10 ml of silicon tetra-chloride was introduced in it and maintained at a temperature lower than - 340C. Next, 10.5g of dried ammonia mixed with nitrogen was introduced over 1.5 hours. As soon as a reaction was started, solid product deposited on the liquid surface of said silicon tetra-chloride. Moreover the solid product stuck fast around the inlet for ammonia. After introduction of all ammonia, a part of reactants was still remained. After completion of the reaction, residual reactants and solvent were removed under a reduced pressure (500C and 5mm Hg), and l1.3g of solid product was obtained. The Fig. 5 shows IR spectrum of the product which is in good consistent with that of the product obtained in example 2 of this invention. However, fumes had been formed since the reaction was started, and it was confirmed that the fume product was deposited fast on the wall of the exhaust passage so that the product yield was much lower than that of the present invention. COMPARATIVE EXAMPLE 3 Synthesis of silicon-imide- ammonium-chloride mixture by direct reaction of silicon tetra-chloride with ammonia (at room temperature) The same experiment was carried out except that the reaction vessel was cooled by water at 170C. White fumes were formed as soon as the reaction started and were deposited on the wall of exhaust pass. Moreover, the ammonia inlet was choked four times and the solid product had to be removed by a glass bar to continue the reaction. After introduction of all ammonia, the reactants still remained. After completion of the reaction, residual reactants and the solvent were removed under a reduced pressure (500C and 5mm Hg), and 5.6g solid product was obtained, which corresponded to 23.5% of product yield based on the silicon tetra-chloride. This result proves that the method of the present invention is superior to the method of this Comparative Example 3. Thermal analysis of silicon-imide ammonium chloride mixture obtained in the EXAMPLE 2 Thermogravimetric analysis and differential thermal analysis were carried out in the nitrogen gas flow using 9.27 mg of silicon-imide ammonium chloride mixture. The heating rate from room temperature to 13000C was 10 C/minute. As is shown in Fig. 6, ammonium chloride sublimated at around 3000C, and 76.79% of the mass was decreased. The residue at 10000C was 1.65 mg which corresponds to 17.8% of the initial sample. Assuming that a composition of the sample which had been used for analysis was stoichiometric (Si(NH)x 4NH;Cl), the values above described were calculated as 78.6% and 17.2% respectively. Both values coincide very well with each other, considering that the character of the sample was unstable in the air. This result illustrates that the method of the present invention is excellent to produce silicon-imide quantitatively.

Claims: CLAIMS 1. A method of producing an amide or imide of silicon, suitable for use as a precursor of sintered silicon nitride, from a silicon halide and ammonia, characterized by reacting a silicon halide and a base to produce an adduct and reacting this adduct with ammonia. 2. A method according to Claim 1, wherein the base is a Lewis base, secondary amine, tertiary amine, phosphine, arsine, stibine or a derivative of phosphine, arsine or stibine. 3. A method according to Claim 2, wherein the base is pyridine, an alkyl pyridine, a trialkyolamine, tertiary aromatic amine, trialkylphosphine, trialkylarsine or trialkylstibine. 4. A method according to Claim 3, wherein the base is pyridine or picoline. 5. A method according to any of Claims l to 4, wherein the halide is silicon tetrahalide. 6. A method according to any of Claims 1 to 5, wherein the reaction temperature is from -400 to 800C. 7. A method of producing silicon nitride, which comprises heating a silicon. amide or amide produced by the method of any preceding claim at a temperature of 1000 to i4000C in the presence of an atmosphere comprising nitrogen. 8. Silicon nitride produced by the method of Claim 7 and of which more than 70% by weight is of the alpha-type crystal form and which contains more than 39% of nitrogen and less than 0.001% by weight of chlorine.

Inventors: ARAI, MIKIRO, ISODA, TAKESHI
Applicants: TOA NENRYO KOGYO KABUSHIKI KAISHA
Application year: 1985
Main classification: C01B21
IPC codes: C01B21