Control of particle size in Si sub 3 N sub 4 powders prepared by high-pressure carbothermal nitridation

The grain size variation in "unseeded" Si sub 3 N sub 4 powders, prepared by high-pressure carbothermal nitridation of SiO sub 2 (in stoichiometric 1:2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and "Sedigraph" measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO sub 2 area A(SiO sub 2 )approx50 m exp 2 g exp -1 in the reaction mixture, the resulting mean Si sub 3 N sub 4 particle diameter, d(Si sub 3 N sub 4 ), is very sensitive to the carbon surface area, A(C), such that the minimum d(Si sub 3 N sub 4 )approx1 mu m was obtained with A(C)=30m exp 2 g exp -1 and the maximum d(Si sub 3 N sub 4 )approx7 mu m with A(C)=115 m exp 2 g exp -1 , a slight dependence of d(Si sub 3 N sub 4 ) on the furnace heating rate was also observed; larger grains (approx7 mu m) were obtained with 30 deg min exp -1 than with 2 deg min exp -1 (approx5 mu m). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3-6.5 mPa), annealing temperature (1470-1830 deg C) and gas flow rate (2-201 sub (stp) min exp -1 .