In situ synthesis of Si3N4 in the Na2SiF6–N2 system via CVD: Kinetics and mechanism of solid-precursor decomposition

The kinetics of decomposition of Na2SiF6 in nitrogen to in situ synthesize Si3N4 was investigated. First, an optimization of the following parameters for the thermal decomposition was performed: nitrogen precursor (N2 or N2:NH3), processing time and temperature range. According to the analysis of variance (ANOVA), the optimum conditions to maximize the decomposition of Na2SiF6 are: N2, 120 min, and 465–873 K. Based on thermodynamic predictions and evidence on the synthesis of Si3N4 during the thermal decomposition, it is proposed that in nitrogen Na2SiF6 decomposes endothermically into various gaseous species (SiF4, SiF3, SiF2, SiF and Si) denoted as SiFx, through a series of complex reactions of zero-order with respect to the gaseous products, with activation energy of 156 kJ mol− 1 and a rate-determining-step given by the chemical reaction itself. The gaseous species are formed through a set of simultaneous reactions represented by the next general equation:Na2SiF6 → 2 NaF + SiFx(g) + n F2(g)Where x varies from 0 to 4 and n given by (2–0.5x) takes the values 0, 1/2, 1, 3/2 and 2. Silicon nitride is formed according to:3 SiFx(g) + 2 N2 → Si3N4 + m F2m = 1.5x and takes the values 6, 9/2, 3, 3/2 and 0.