CHARGED INTERFACES: EQUILIBRIUM PROPERTIES, PHASE TRANSITIONS, AND MICROSTRUCTURAL EVOLUTION
Surfaces and interfaces in ionic solids play a pivotal role in defining the trans- port properties and microstructural evolution in many of the existing and emerging material applications, including energy-related systems such as fuel cells, recharge- able batteries, as well as advanced electronics such as those found in semiconducting, ferroelectric, and piezotronic applications. Here, a variational framework has been developed to understand the effects of ionic species and point defects on the structural, electrochemical and chemomechanical stability of grain boundaries in polycrystalline ceramics. The theory predicts the equilibrium and phase transition conditions of charged interfaces, and quantifies the properties induced by the broad region of electrochemical and chemomechanical influence in front of a grain boundary capable of spanning anywhere from a few angstroms to entire grains. As an example application, the microstructural mechanisms leading to the onset of the flash during electric field assisted sintering were predicted, where the experimentally observed cascading charge flow, resulting in the onset of a flash event was rationalized. Also, the model was applied to describe the effects of grain boundary drag by the interfacially accumulated ionic species and charged defects during grain growth under electrical, chemical, mechanical, or structural driving forces. Finally, abnormal grain growth in ionic solids with an emphasis on the structural and electrochemical character of the grain boundaries was demonstrated. Here, two moving grain boundary types, highly mobile and immobile interfaces are identified, resulting in three grain size populations.