Tuning of the microstructural and electrical properties of undoped BaTiO3 by spark plasma sintering

Abstract

The distribution of oxygen vacancies in BaTiO3 specimens can greatly affect material electrical properties. Spark plasma sintering followed by annealing in controlled pO2 atmospheres offers the potential to control the oxygen vacancy distribution. Impedance spectroscopy has been used to study the electrical characteristics of undoped BaTiO3 specimens prepared at 1200 °C under a pressing pressure of 5 MPa and 50 MPa using spark plasma sintering (SPS). For both samples, at temperatures greater than 750 °C, the total conductivity is determined by the bulk conductivity (σb), while at temperatures between 550 °C–750 °C the total conductivity is determined by the grain boundary conductivity (σgb). Below 550 °C, the total conductivity is determined by the BaTiO3–Pt interface conductivity (σel). The bulk, grain boundary and electrode interface resistances and activation energies are lower for samples sintered and pressed at 50 MPa, compared to specimens sintered and pressed at 5 MPa. Plots of grain boundary conductivity (σgb) vs. oxygen partial pressure (pO2) (log(σgb) vs. log(pO2)), over the temperature range 517 °C–683 °C, have slopes of approximately 4.0 indicating that doubly charged oxygen vacancies (VO••) are located at the grain boundaries. The grain boundary region and BaTiO3–Pt interface conductivities are highly sensitive to the oxygen partial pressure. Therefore, SPS sintered BaTiO3 specimens that have been subjected to controlled annealing could be used to tailor BaTiO3 dielectric properties, as well as have potential applications in high temperature O2 sensing. Furthermore, analysis of a.c. conductivity using the Jonscher model reveals that the governing charge transport mechanism is via quantum mechanical tunneling (QMT), which operates at temperatures when grain boundaries control the total conductivity. The mechanism switches to the correlated barrier hopping (CBH) model at temperatures when the bulk controls the total conductivity.