Mechanism and Parameters Controlling the Decomposition Kinetics of Na2SiF6 Powder to SiF4

Sodium hexafluorosilicate (Na₂SiF₆) powder has been used as a silicon source for formation of Si₃N₄ coatings by the hybrid precursor system‐chemical vapor deposition (HYSY‐CVD) route. The quantitative effect of processing time, temperature, gas flow rate, and process atmosphere (N₂ and N₂:5% NH₃) upon the fractional weight loss during the decomposition of Na₂SiF₆ was studied using a standard L₉ Taguchi experimental design and analysis of variance. The decomposition kinetics of Na₂SiF₆(s) was studied theoretically and experimentally in the temperature range of 550–650ºC by applying the shrinking core model. It was found that regardless of atmosphere type, the reaction order is n ≈ 0.12 and that a two‐stage mixed mechanism consisting of chemical reaction and boundary layer gas transfer controls the decomposition rate. The determined fractional weight loss during Na₂SiF₆ decomposition in nitrogen atmosphere is about 1.05–1.5 orders of magnitude greater than that in N₂:NH₃. The gas flow rate affects the dissociation activation energy, being of 121, 109, and 94 kJ/mol in N₂ and of 140, 120, and 115 kJ/mol in N₂:NH₃, for the flow rates of 20, 60, and 100 cm³/min, respectively, in both atmosphere types. A good agreement is observed by comparing experimental weight loss data with model predictions.