Natural-like generation of grain boundary models and the combined effects of microstructural elements and lithiation on the plastic behavior of TiO2: A computational study

Abstract

Microstructural features of ceramics have a significant effect on mechanical properties and ion transport in different technologies ranging from optical materials to renewable energy generation and storage. Atomistic modeling is usually done on postulated grain boundaries which may not account for the distributions of different grain boundaries in real materials. We generate microstructural features of TiO2 with molecular dynamics (MD) by simulating a heat treatment; in this way, a natural-like distribution of grain sizes and boundaries is obtained. We used a model system with about 545,000 atoms and obtained structures with average grain sizes ranging about 6 to 12 nm and trending with the annealing temperature. While the model is nano- rather than micro-meter structured, it present key microstructural features. We observe ‘clean’ grain boundaries as well as boundaries with an amorphous interlayer. The clean boundaries feature low-index surfaces. When an amorphous interlayer is present, higher-index surfaces can be observed. The resulting structures show significant softening (reduction of maximum stress) vs. ideal crystalline titania, with isotropy of mechanical properties. Elastic constants, besides significant isotropy, show moderate stiffening due to the presence of grains. When introducing lithium, we observe segregation at grain boundaries, and lithiation of grainy structures leads to further softening.