Grain-growth inhibition in Na2O-doped TiO2-excess barium titanate ceramic

Pressureless-sintered TiO2-excess BaTiO3 (BT) ceramic doped with Na2O (as a n acceptor oxide) has been investigated for the inhibition of platelike (11 1) twin grains and the associated microstructure development. Solid-state r eaction between the Na2O dopant and the excess TiO2 of the nonstoichiometri c BaTiO3 powder during sintering has resulted in the formation of orthorhom bic Na4TiO4 (N4T) at temperatures of less than or equal to 1215 degrees C. Enhanced densification is due to a liquid-phase sintering mechanism, where the liquid eutectic is generated by reacting the excess TiO2 and the trace impurities of SiO2 and Al2O3 with the Na2O dopant, Suppression of the abnor mally large, platelike grains occurs at a critical acceptor concentration o f similar to 0.50 mol% Na2O when sintering is conducted at 1215 degrees C. The inhibition mechanism is associated with the secondary phases, whose for mation gradually modifies the initially TiO2-excess powder toward the stoic hiometric composition. The corresponding sintered microstructure is analyze d using both scanning and transmission electron microscopy. The second-phas e precipitates of Na4TiO4, which are located intragranularly and intergranu larry in the tetragonal BaTiO3 matrix grains, exhibit the following crystal lographic orientation relationships: [110](BT) // [100](N4T), (1(1) over ba r 1)(BT) // (010)(N4T), and (112)(BT) // (001)(N4T).