Formation of charged defects during the nitridation of a metal particle

Abstract

A model explaining and predicting generation of a temporal electric potential during nitridation of a single metal pellet has been developed. The model takes into account the kinetics of defects formation and assumes that the rate of the chemical reaction can be described by the shrinking-core process. The model simulations have shown that time scale of the generated electric potential depends on both the initial nitride shell thickness and heat removal from the particle surface. At thin initial shell and low rate of heat removal the maximum of the surface electric potential is attained before the temperature and surface nitrogen concentration have reached their maximums but after the maximum of nitridation has appeared. Quasi-neutral distributions of metal vacancies and electron holes are formed at the maximum temperature. At thick initial shell and/or high rates of heat removal from the particle surface the potential maximum may be observed much later: after the maximum temperature has been achieved. Correspondingly, non-equilibrium concentrations of the charged defects exist till the end of nitridation. In contrast to oxidation the nitrogen adsorption rate constants (the activation energy and pre-exponent) have negligible effect on the surface potential form and amplitude. At the ignition limit the rate of nitridation is proportional to the power of −1/2 for the ambient nitrogen pressure in the proposed scheme of defects formation. Metal vacancies and electron holes are the main charged defects in nitrides during nitrogen combustion. The nitride formation is limited by transfer of the vacancies in nitride.