A low-temperature and seedless method for producing hydrogen-free Si$_{3}$N$_{4}$

Abstract

A simple, seedless method for the synthesis of Si$_{3}$N$_{4}$ from a hydrogen-free precursor system (Na$_{2}$SiF$_{6(s)}$-N$_{2(g)})$ was developed. From thermodynamic calculations and experimental results it is concluded that the gaseous chemical species SiF$_{x}$ (SiF$_{4}$,$_{ }$SiF$_{3}$, SiF$_{2}$, SiF and Si) formed during the low-temperature dissociation of Na$_{2}$SiF$_{6}$ in a conventional CVD system react in-situ with nitrogen to produce Si$_{3}$N$_{4}$. Whiskers, fibers, coatings and powders were obtained via the Na$_{2}$SiF$_{6}$-N$_{2}$ system at pressures slightly above atmospheric pressure. Not only does the feasibility of the reactions for Na$_{2}$SiF$_{6}$ dissociation and Si$_{3}$N$_{4}$ formation increase with the temperature but also, once the SiF$_{x}$ chemical species are formed by the former, the latter reaction is even more viable. Amorphous Si$_{3}$N$_{4}$ is obtained at temperatures of up to 1173 K while crystalline $\alpha $- and $\beta $-Si$_{3}$N$_{4}$ are formed in the range 1273-1573 K and with processing times as short as 120 minutes. Optimal conditions for maximizing Si$_{3}$N$_{4 }$formation were determined.