A new flame process for synthesis of Si 3N 4 powders for advanced ceramics

Abstract

The possibility of employing a gaseous self-sustaining reaction to prepare pure ceramic powders, particularly silicon nitride, for use in advanced ceramic materials has been explored. The thermodynamics of the process were first examined to determine possible operating conditions. Ignition experiments were then carried out in a closed bomb in which the reactant gases were silane and either hydrazine or a mixture of hydrazine and ammonia. Intense fuse wire ignition sources were used to determine the ignition limits over a broad range of initial conditions. These experiments were followed by flame propagation experiments through a tube into which the reactants were constantly fed and the flame repeatedly ignited at one end in order to produce product for preliminary determination of the powder properties. These experiments showed the feasibility of the flame synthesis process. A flat flame burner apparatus was then designed and constructed. A large number of experiments were carried out to demonstrate the regions of flame stability and to prepare product for evaluation. Flames were stable over a broad range of conditions: equivalence ratios from 0.1 to 1.1, unburned gas flow velocities from 20 to 150 cm/s and adiabatic flame temperatures from about 900 to 2000 K. High quality Si3N4 powders have been prepared in this (small) laboratory flame apparatus at the rate of 10 g/min, and at yields of close to 100%.