Method of making silicon nitride
This invention concerns the manufacture of silicon nitride powder by the vapor phase reaction of a silicon halide with ammonia at an elevated temperature, such as is disclosed in U.S. Pat. Nos. 4,416,863, 4,399,115, 4,145,224, 4,036,653 and 3,959,446. In a usual process, ammonia and silicon tetrachloride, in gaseous form, are introduced at a steady flow rate into one end of a reactor tube made, for example, of quartz. The gases flow through a hot zone, maintained at about 1000° to 1500° C., in the tube where reaction takes place. The reaction product exits the tube and is condensed and collected in powder form. The powder may then be heated to drive off ammonium chloride and crystallize the silicon nitride. The crystalline silicon nitride powder can then be formed into a usable product by pressing and sintering. For sintering purposes, it is desirable to control the oxygen content of the silicon nitride powder within a narrow range. 
    
    
     We have found that the oxygen content of the silicon nitride can be adequately controlled by preventing the entry of room air into the system, both at the entrance end and exit end of the reactor tube, and then introducing moisture into the system under controlled conditions. The moisture is introduced into the system by flowing nitrogen through water and thence into the system. We have found that the water-bearing nitrogen should be introduced into the system at about or after the exit end of the reactor tube, where the reaction product is in the imide form as silicon chloroimide or diimide, but before the collection means. When the water-bearing nitrogen is introduced into the entrance end of the reactor tube, sinterability of the silicon nitride powder is not as well enhanced. 
     For control purposes, we prefer to maintain the water at a constant temperature above room temperature. This is accomplished by keeping the water in an electrically heated container the temperature of which is controlled by a thermostat. If analysis of the crystalline silicon nitride shows too low an oxygen content, the rate of flow of the nitrogen through the water bottle is increased, and vice-versa for too high an oxygen content. In a typical case, the nitrogen flow rate was 1.5 cubic feet per hour and the oxygen content of the silicon nitride powder was controlled to 1.9% plus or minus 0.3% by weight. 
     We have found that in order to ensure that room air is kept out of the system, that the connector which connects the silicon halide and ammonia conduits to the entrance of the reactor tube should be covered with a nitrogen blanket. This is accomplished by surrounding the connector with a spaced apart collar and maintaining a flowing nitrogen atmosphere within the collar. A similar nitrogen blanket is provided at the exit end of the reactor tube.