High-Pressure Carbothermal Preparation of Si3N4. Influence of Temperature Program and Additives.
High-pressure nitridation of C+SiO2 powder mixtures for the preparation of Si3N4 powder offers some advantages over the ordinary atmospheric pressure process. For one thing, the useful temperature range can be much expanded upwards without formation of unwanted SiC, which starts at about 1500°C at 0.1 MPa (1 bar).
Here we present and discuss the results of a recent series of runs in the pressure range 1–6 MPa, T = 1830°C, with the samples characterized by XRD, SEM and chemical analyses.
Stoichiometry (C/O), α-phase content and particle size are important powder characteristics that we have studied as functions of heating rate, nitrogen pressure, gas flow rate, starting material types and additives.
It was found that large surface area silica (50 m2/g) produced large Si3N4 particles (hexagonal prisms, ~ 5µm in size), while small surface area quartz (1 m2/g) yielded smaller Si3N4 particles (~ 1µm). Control of the powder grain size below 1µm was achieved by “seeding”, i.e. addition of ~5wt% of Si3N4 to the starting mixture. The effect was obtained when large Si3N4 particles (~ 5µm) and submicron powder were used as “seeds”.
A high α-phase content is promoted by reactive starting materials, and by moderately low gas flow rates.
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