Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Patent
1990-01-12
1991-07-16
Chaudhuri, Olik
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
264 65, C01B 2106, C01B 3306
Patent
active
050323709
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to preparing refractory inorganic compounds and, more particularly, to methods of preparing silicon nitride with a high .alpha.-phase content.
Due to unique physical and mechanical properties (high hardness, sufficient strength and wear resistance, thermal stability, extremely low thermal expansion coefficient, inactivity in many aggressive media, low friction coefficient, low density, specific semiconducting and dielectric properties), silicon nitride finds application for manufacturing ceramic materials used for various purposes.
These construction high-temperature materials, refractories, antifriction and tool materials, special materials with a high electric strength and stable dielectric properties within a wide range of conditions, with heat-insulating and heat-conducting properties are widely used in engineering, tool industry, metallurgy, rocket engineering, electronics, electrical and radio engineering, and in other branches of industry.
BACKGROUND OF THE INVENTION
To prepare silicon nitride powders on industrial scale, various methods of direct synthesis have found wide application since initial materials are readily available.
Known methods of direct synthesis of silicon nitride powder via the nitration reaction can be divided into traditional methods of furnace synthesis widely used in industry and a promising relatively new plasmochemical method and a method of self-propagating high-temperature synthesis (SHS) in the combustion regime.
The method of furnace synthesis of silicon nitride powders is based on nitration of elemental silicon powder upon heating in electric furnaces in a flow of nitrogen or nitrogen-containing gas. Nitration of silicon dioxide in a mixture with a reducer, mainly, carbon, is one of the versions of the known method.
Methods of silicon nitration in furnaces are predominantly two-stage and their accomplishment requires much time. The first stage of silicon powder nitration is carried out at a temperature by 100.degree.-250.degree. C. below the melting point of silicon and up to the attainment of 30-40% degree of bonding silicon to nitrogen. This stage requires from 3-5 hrs to 10-20 hrs. At the second stage complete nitration is performed at 1500.degree.-1600.degree. C.
Known in the art is the most effective and rather simple method of preparing silicon nitride powder with a high content of .alpha.-phase by the furnace synthesis method. The method resides in nitration of metallic silicon in a flow of a nitrogen-containing gas under a reduced nitrogen partial pressure for 4-5 hrs upon heating in furnaces at 1200.degree.-1400.degree. C., the nitrogen partial pressure being maintained equal to .about.0.5 atm (0.05 MPa) as long as 50-60% by mass of silicon is reacted. The method provides the content of .alpha.-phase in the silicon nitride powder equal to 97% by mass.
The use of the known method for preparing silicon nitride requires a strict control over temperature, nitrogen partial pressure, and a gas flow rate in the course of the whole nitration process in order to maintain the thermal conditions required for the formation of .alpha.-phase and to compensate the exothermal effect of the nitration reaction with the aid of heat removal. Besides, this method demands rather great power consumption for attaining 1200.degree.-1400.degree. C. in the electric furnaces at which the nitration process takes place.
The method of plasmochemical synthesis, namely, nitration of silicon in a low-temperature nitrogen plasma, is of interest for preparing ultrafine silicon nitride powders possessing good caking ability.
Known in the art is a plasmochemical method of preparing silicon nitride by nitration of silicon in a nitrogen plasma produced by a high-frequency generator with the use of 98.9% pure silicon and nitrogen of a high purity (Izd. Akad. Nauk SSSR, ser. Neorganocheskie materialy, Moscow, 1979, Vol. 15, No.4. G. M. Kheidemans, Ya. P.Grabas and T. A. Miller "High-temperature synthesis of finely dispersed silicon nitride", pp.
REFERENCES:
patent: 3726643 (1973-04-01), Merzhanov et al.
Borovinskaya Inna P.
Kustova Lidia V.
Makhonin Nikolai S.
Merzhanov Alexandr G.
Popov Leonid S.
Chaudhuri Olik
Horton Ken
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