KINETICS OF A NONCATALYTIC GAS-SOLID CHEMICAL-REACTION UNDER SHS-LIKECONDITIONS

Nitration of metallic tantalum under self-propagating high-temperature synthesis (SHS) conditions (i.e. rapid heating rates 2000-3000 K s(-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 TaNx and tantalum subnitride, namely Ta2N, was observed in the nitrided tantalum at temperatures of 1600-2 600 K and a pa rtial nitrogen pressure of 0.1-0.8 MPa. It was also fou nd that tantalum subnitride phase formed before the formation of Ta-N solid solution throughout the sample during nitration. A theoretical s tudy was conducted in order to determine the intrinsic reaction kineti cs of a typical combustion synthesis reaction. A new model was develop ed to explain all experimental observations. The computer simulation r esults are in good agreement with experimental data. The results indic ate that at sufficiently large heating rates the product layer does no t 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.