Mechanistic studies involving kinetics of rapid high-temperature reactions for materials synthesis.

摘要

Combustion synthesis (CS) is an attractive method for the synthesis of advanced materials such as ceramics, intermetallics and composites. Its main characteristics are high temperatures and heating rates, and short reaction times. In order to control the microstructure and properties of CS materials, it is critical to know the kinetics of chemical reactions taking place under these unique conditions.; Moreover, since CS is characterized by high temperature gradients, it is important to know the chemical reaction kinetics measured under nonisothermal conditions, and to compare them with data obtained isothermally. In this work, the influence of preheating rate on kinetics of high-temperature gas-solid reactions is investigated under isothermal and nonisothermal conditions, using a computer-assisted electrothermography method. The results showed that for both Nb-N₂ and Ti-N₂ systems, the activation energy of diffusion coefficient does not depend on heating rate in isothermal conditions. However, reaction kinetics are enhanced by increasing the preheating rate. It is suggested that nonisothermal preheating influences chemical reactivity of the solid reactant by changing its specific surface area.; It is also shown that under certain conditions, an overshooting phenomenon occurs in Ti-N₂ system, corresponding to self-ignition of the metal in nitrogen. It is found that activation energy measured under nonisothermal conditions is significantly higher than that obtained from isothermal experiments. The conclusion is made that isothermal kinetics may in general not be applicable for the isothermal case.; Further, it is interesting to investigate the influence of heating rate directly on CS. The thermal explosion phenomenon was investigated in the Ni-Al system with various microstructures (Al particles clad by Ni, and Al + Ni mixtures with different particle sizes). The results show that ignition process is related to either phase or microstructural transformations and depends on the reactant medium microstructure. Both the latter and the heating rate influence the process of initial product formation, which in turn affects the interactions during the post-ignition stages.; Finally, preliminary work done in order to develop the time-resolved X-ray diffraction technique is presented, along with the purpose of using this method in CS investigations.