Grain growth transitions of perovskite ceramics and their relationship to abnormal grain growth and bimodal microstructures

Barium titanate, strontium titanate, and lithium lanthanum titanate (LLTO) were used to study grain growth in perovskite ceramics. In these materials, a grain growth transition was found. In the case of barium titanate, grain growth shows a gradual transition to faster growth with increasing temperature, whereas strontium titanate indicates exponentially decreasing grain growth with increasing temperature. In reducing atmosphere, strontium titanate shows two transitions; the additional second transition is attributed to a reversible wetting transition. In LLTO, a single grain growth transition was found and seems to be caused by a wetting transition as well. In all cases, the grain growth transitions are strongly correlated to abnormal grain growth. This non-Arrhenius behavior of grain growth in perovskites is discussed in relation to abnormal grain growth and bimodal microstructures: the existence and coexistence of two grain boundary types with different grain boundary mobility is proposed. In this framework, a gradual transition of the boundary population from type 1 to type 2 with temperature seems to cause the growth phenomena in perovskites on a macroscopic scale. Most likely, this gradual transition is driven by the anisotropy of the grain boundary energy. Possible microscopic origins of the grain growth transitions are discussed. The consequences of bimodal growth and boundary anisotropy for classical mean field modeling of grain growth are assessed: the grain growth constant k is not capable to appropriately reflect grain growth in perovskites, and boundary anisotropy cannot be included in standard mean field approaches.