In many industrial settings, the kinetics of phase transformations is important, because the time scales of the industrial processes might be of the same order as the time scales of phase transformations. The rates of phase transformations are often poorly quantified experimentally. Therefore, the thermodynamics of the materials, as depicted in a phase diagram, does not give the complete picture of the changes to materials under varying temperatures and pressures. Although different solid phases might be quite close in thermodynamic stability, as quantified by their free energies, the kinetics of transformations between phases might be very slow. Our work focuses on computational methods to calculate the mechanisms and rates of solid-solid phase transformations. To this end we have employed a combination of atomic-scale methods. The Generalized Solid-State Nudged Elastic Band (G-SSNEB) method maps out mechanisms and activation energies of phase transformations with the feature of treating considerable change of unit cell symmetry (deformation). Transition Interface Sampling (TIS) can efficiently calculate rates of complex processes in liquids and solids. It does so by following the dynamics of a chemical process in small steps from reactant to product. We have used G-SSNEB to map out mechanisms of phase transformations and used these calculations to guide the setup and running of the TIS calculations. The calculation of energies and forces on atoms were based on both interatomic potentials and Density Functional Theory (DFT). I will present applications of our computational procedure, including transitions in CdSe, silica (SiO2) and silicon nitride (Si3N4). In addition, a previously uncharacterized phase transformation in silicon nitride will be presented.

Modeling mechanisms and kinetics of phase transformations

255th National Meeting (2018)

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