Combustion front dynamics in the combustion synthesis of refractory metal carbides and di-borides using time-resolved X-ray diffraction

A compact diffraction-reaction chamber, using a 2-inch photodiode array detector, has been employed to investigate the chemical dynamics at the combustion front of a selected series of refractory metal carbides and di-borides from their constituent element reactants as well as binary products from B₄C as a reactant. These systems are denoted as (i) M + C → MC; (ii) M + 2B → MB₂; and (iii) 3M + B₄C → 2MB₂ + MC, where M = Ti, Zr, Nb, Hf or Ta. Time-resolved X-ray diffraction using intense synchrotron radiation at frame rates up to 10 frames s⁻¹ (or 100 ms frame⁻¹) was employed. The combustion reactions were found to complete within 200–400 ms. In contrast to the Ta + C → TaC combustion system studied earlier, in which a discernible intermediate sub-carbide phase was first formed, reacted further and disappeared to yield the final TaC product, no intermediate sub-carbide or sub-boride was detected in the current systems. Combustion for the Ti, Zr and Hf systems involved a liquid phase, in which the adiabatic temperatures T_ad are well above the melting points of the respective reactant metals and have a typical combustion front velocity of 5–6 mm s⁻¹. The Nb and Ta systems have lower T_ad, involving no liquid phase. These are truly solid combustion systems and have a lower combustion front velocity of 1–2 mm s⁻¹. The current study opens up a new avenue to chemical dynamics and macrokinetic investigations of high-temperature solid-state reactions.