Studies of 'reactive templated grain growth' in some complex-oxide perovskites.

摘要

This thesis details research on “reactive-templated grain growth” (RTGG) processing of complex-oxide perovskites. Four independent yet interwoven investigations are described. In the first, molten-salt synthesis yielded plate-like (∼0.2 x 5 x 5 μm) Ruddlesden-Popper (Sr₃Ti ₂O₇) and Aurivillius (Bi₄Ti₃O ₁₂ (BiT) and BaBi₂Nb₂O₉) phases which served as “templates” in the subsequent studies.; “Chemical preparation” routes were designed to produce intimately mixed, fine-grain matrix powders for the second and third investigations. A “citrate gel” method was developed for Bi_1/2Na _1/2TiO₃ (BNT)-based powders and a “sequential precipitation” method for PbNi_1/3Nb_2/3O₃ - PbTiO₃ (PNNT)-based powders. Both methods were original to the present investigation and resulted in dense, μm to sub-μm scale microstructures when applied to un-templated compositions.; Microstructure studies of templated, chemically-prepared mixtures indicated that both Aurivillius phases behaved similarly with BNT-based matrices: An initially heterogeneous microstructure evolved towards a dense assemblage of anisometric (∼1 x 5 x 5 μm) grains. Sr₃Ti ₂O₇ exhibited consistent behavior with both BNT and PNNT-based matrices: The microstructure evolved towards an aggregation of equi-axed, μm-scale grains with larger, isolated pores.; A qualitative model was proposed to account for the behavior. It considered the excess matrix constituents after reaction to ABO₃ perovskite and predicted an A-excess matrix in BNT-based mixtures templated with the Aurivillius phases and a B-excess matrix in BNT-based mixtures templated with Sr₃Ti₂O₇. Predictions for PNNT-based matrices were less definite but not contradictory.; In the final phase of the investigation, the solids formulation for BiT-templated, RTGG-processed Bi_1/2Na_17/40K_3/40TiO ₃ (BNKT) was systematically varied. This approach, original to this thesis, involved pre-reacting to well-defined alkali and bismuth titanate precursors. Use of these in different BNKT formulations indicated that the amount of desintering associated with reacting at 600–800°C was a function of formulation.; Formulation affected the microstructure and texture development in the RTGG-processed BNKT. Pre-reacting the alkali carbonates and TiO₂ (to Na₂Ti₃O₇ and K₂Ti₂O ₅) inhibited texture development. No such effect was observed when Bi ₂O₃ and TiO₂ were pre-reacted to Bi₂Ti ₄O₁₁, instead the measured Lotgering factor F ₀₀₁ increased from ∼0.5 to ∼0.7 for samples sintered at 1200°C.