Barrier layer and grain boundary effects in Nd/Zr doped BaTiO3 ceramics

Gradient structures with barrier layer characteristics and core-shell morphology have been developed in BaTiO3 ceramics with Nd2O3 and ZrO2 as co-dopants. Features include reduced Curie temperatures and anisotropic stress gradients, resulting from an oxidized surface layer and reduced interior, developed during air sintering. Co-doping was typically carried out through solution milling of the BaTiO3 powders with nitrate precursors of the dopant oxides, spray drying and sintering of the pressed pellets in air ambient at 1300-1320 degrees C/60-90 min with furnace cooling. Structural characterization, as well as dielectric and d.c. resistance measurements of the pellets, as-sintered and after removing equal amounts of material from both surfaces, revealed the existence of an oxidized surface layer and barrier layer microstructures consisting of graded regions of oxidized insulating surfaces over partially oxidized or conducting grain interiors. In this complex structure, the ZrO2 segregates to the grain boundary region, forming a core-shell structure, with Nd2O3 partitioning between the BaTiO3 and ZrO2 phases. The overall system was modeled in terms of an equivalent circuit and the analysis indicates that the dielectric constant and the loss behavior are strongly impacted by both the surface and grain boundary barrier characteristics, with the surface barrier effects having the more dominant effect on the dielectric properties of the doped compositions. Indications are that fine-tuning of the system to optimize the grain boundary effect could lead to extraordinary dielectric constant effects which could potentially be utilized in high energy storage devices. (C) 2010 Elsevier B.V. All rights reserved.