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 Tad 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 Tad, 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.