Effect of Particle Size on the Reaction Wave Propagation in the Combustion Synthesis of Al2O3-ZrO2-Nb Composites

The effect of reactant particle size on the kinetics of wave propagation in the thermite system Nb₂O₅+ Al₂Zr + nuAl₂O₃ (with alumina being a diluent) was investigated. Reactants in three size ranges were utilized: fine (1–3 µm), medium (<10 µm), and coarse (10–45 µm). Particle size had an effect on the mode and velocity of the self-propagating combustion wave. For fine particles, the combustion wave propagated in a steady-state mode for all values of dilutions investigated (0 ≤ nu ≤ 0.8). For medium particles with nu > 0.1 and for all coarse particles, wave propagation was in the spin mode. A significantly large difference between the calculated adiabatic combustion temperatures and the measured values was observed and is attributed to large heat losses arising from very wide reaction zones. For fine particles, the temperature dependence of the wave velocity showed a discontinuity at a temperature corresponding to the formation of the eutectic in the binary ZrO₂-Al₂O₃. The velocity can be more than a factor of 3 higher at the same temperature, depending on the amount of the eutectic liquid. The dependence of the wave velocity on particle size ( r ) was found to have the form v is proportional to 1/ r^m with 0.5 < m < 0.8. The upper limit approximates that of m = 1.0 for diffusion-controlled reaction kinetics, according to reported modeling studies. In all cases, regardless of the mode of wave propagation the reactions resulted in the desired products, Nb + ZrO₂+ Al₂O₃ with a microstructure exhibiting the eutectic of the binary ZrO₂-Al₂O₃.