Nanoparticle Size Effects for Computed Phase Equilibria in Molybdenum and Tungsten Carbides

Transition metal carbides (TMC) are important materials for a variety of applications and industrial processes, in part due to their variable crystal structures. However, the nucleation of TMC during synthesis often proceeds through metastable phases, the appearance of which, cannot be described by traditional density functional theory (DFT) computed phase diagrams. Knowledge of the relative thermodynamic stability of non-equilibrium phases during carbide synthesis could be used to guide rational design of synthetic strategies that target specific phases for individual applications. In this thesis, by combining classical nucleation theory and DFT-computed phase diagrams, we construct particle size-dependent phase diagrams for TMC to reveal the relationships between phase stability and TMC nanoparticle size. We compute size-dependent phase diagrams for a range of molybdenum carbide and tungsten carbide stoichiometries, and polymorphs, and validate our models with available experimental data. We use these thermodynamic models to make predictions for phase stability as a function of temperature, composition, and particle size. We provide insights for the influence of nanoparticle size on TMC nucleation and growth during synthesis. The theoretical framework utilized here provides a computationally-guided road map for navigating the rational synthesis of target TMC materials.