The Band Structure of Polycrystalline Al 2 O 3 and Its Influence on Transport Phenomena

The electronic (band) structure of polycrystalline Al 2 O 3 , in particular the density of near‐band edge grain‐boundary localized states, plays a significant role in a host of high‐temperature phenomena, including sintering, high‐temperature creep, oxygen permeability in dense “dry” Al 2 O 3 ceramics, and Al 2 O 3 scale formation on Al 2 O 3 scale‐forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must of necessity involve electrons and holes. Thus, the density of states associated with grain boundaries in Al 2 O 3 assume great importance, and has been calculated using DFT for both nominally undoped and Y‐doped Σ7 bi‐crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y 2 O 3 segregation to Al 2 O 3 grain boundaries is so effective in enhancing high‐temperature creep resistance of polycrystalline Al 2 O 3 , and in understanding the reactive element effect in Al 2 O 3 scale‐forming alloys. Finally, a case will be made that grain‐boundary diffusion is mediated by the migration of a class of grain‐boundary ledge defects called disconnections, which are characterized by a step height h and a Burgers vector b.