Kinetics of a non-catalytic gas-solid chemical reaction under SHS-like conditions

Abstract

Nitration of metallic tantalum under self-propagating high-temperature synthesis (SHS) conditions (i.e. rapid heating rates 2000–3000 Ks⁻¹, and short heating periods 2–100 s) has been studied. Phase analysis and microstructural characterization have been performed. A two-phase mixture region of solid solution TaN_x and tantalum subnitride, namely Ta₂N, was observed in the nitrided tantalum at temperatures of 1600–2600 K and a partial nitrogen pressure of 0.1–0.8 MPa. It was also found that tantalum subnitride phase formed before the formation of Ta-N solid solution throughout the sample during nitration. A theoretical study was conducted in order to determine the intrinsic reaction kinetics of a typical combustion synthesis reaction. A new model was developed to explain all experimental observations. The computer simulation results are in good agreement with experimental data. The results indicate that at sufficiently large heating rates the product layer does not necessarily act as a diffusion barrier that prevents further uptake of nitrogen. The results are helpful in developing an understanding of the mechanism of powder-based SHS.