Microstructure development in reactive-templated grain growth of Bi_1/2Na_1/2TiO₃-based ceramics: Template and formulation effects

"Reactive-templated grain growth" (RTGG) processing of Bi_1/2Na_1/2TiO₃ (BNT)-based ceramics is reported. Molten salt synthesis was used to prepare platelike (∼0.2 μm × 5 μm × 5 μm) 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 ABO₃ 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.