In situ synthesis of Si3N4 in the Na2SiF6-N2 system via CVD: Kinetics and mechanism of solid-precursor decomposition: 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 SiF_x, 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 + SiF_x(g) + n F_2(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 SiF_x(g) + 2 N₂ → Si₃N₄ + m F₂m = 1.5x and takes the values 6, 9/2, 3, 3/2 and 0.