Combustion synthesis of nitrides: mechanistic studies

Combustion synthesis (CS) is characterized by extremely high heating rates (up to 10⁶ K/s) and temperatures (up to 4000 K), as well as short times of reaction completion (usually less than 1 s, sometimes even 10⁻³–10⁻² s). The above unique conditions lead to materials microstructure formation mechanisms that cannot be observed in the isothermal or quasi-isothermal cases. In turn, evolution of the reaction media microstructure influences characteristics (temperature, velocity) of the combustion wave. In this work, such microstructural effects are demonstrated on the examples of CS in different gas–solid systems (i.e., Si–N₂, Al–N₂, and Nb–N₂). It is shown that dilution of the reaction media by inert high surface area precursors with phase compositions similar to the CS product leads to a significant increase in combustion velocity in the Si–N system, while the temperature remains constant. Also, it is revealed that dispersion of the metal (Nb) particles in the preheating zone might be responsible for constancy of the combustion velocity as a function of the size of the initial reactants. Finally, a complex two-stage mechanism of AlN formation explains the non-monotonic shape of the temperature–time profile observed during CS in this system.