Nanostructured materials: Reaction kinetics and stability

Abstract

An important consequence of the expanding study of the nanocrystalline state is the recognition of new behavior that is exposed at the nanometer length scale, but this also requires the recognition of the scaling of conventional behavior. The synthesis pathways further emphasize the importance of reaction kinetics and especially nucleation processes where the nanometer length scale is central to the kinetics. Similarly, the observed phase selection during nanostructure synthesis is often different than that expected from the thermodynamics of bulk phase stability, but can be analyzed in terms of a scaling of the hierarchy of equilibrium and the influence of large characteristic driving free energies. At the same time, the reaction pathways that yield different phase states and microstructures can be described in terms of open or closed system conditions that reflect the manner in which the excess free energy is developed during synthesis. The principles that govern the genesis of nanostructured materials and the key issues concerning the reaction kinetics and stability are illustrated from the observed behavior in specific amorphous alloys, but the treatment also applies in general to materials systems.