Microstructure development in reactive-templated grain growth of Bi1/2Na1/2TiO3-based ceramics:: Template and formulation effects

Reactive-templated grain growth (RTGG) processing of Bi1/2Na1/2TiO3 (BNT)-based ceramics is reported. Molten salt synthesis was used to prepare platelike (similar to0.2 mum x 5 mum x 5 muwm) Ruddlesden-Popper (Sr3Ti2O7 (ST)) and Aurivillius (BaBi2Nb2O9 (BBN)) phases which were used as "templates" in studies of RTGG with BNT-based matrixes. A "citrate-gel" route was designed to produce intimately mixed, fine-grain matrixes for these studies. The analytical techniques used were powder X-ray diffraction and microstructural examination of dry-pressed and fired compacts. For mixtures templated with BBN, single-phase perovskite readily formed, and an initially heterogeneous microstructure evolved toward a dense assemblage of anisometric, micrometer-scale grains. Perovskite formation was more sluggish in the mixtures templated with ST, and the final sintered microstructure featured larger, porous grains in an equiaxed, micrometer-scale matrix. A qualitative model, which examined the excess constituents in the matrix after formation of stoichiometric ABO(3) perovskite, is proposed to explain the observations. The model predicted an excess of Na2O and TiO2 in the matrix in the case of BBN templates and only excess TiO2 in the case of ST templates. The results indicate that careful examination of matrix and template chemistry could be important in the selection of systems for RTGG processing.