Thermal processing-induced domain reconfiguration and property enhancement in BiFeO3–BaTiO3 ceramics for high temperature applications

High temperature ferroelectric ceramics have generated significant interest in recent years, from both a fundamental perspective and for practical applications as temperature-stable dielectrics or piezoelectric transducers. Particularly, BiFeO₃-BaTiO₃ ceramics show great promise as a replacement for lead-based materials in demanding environments. In the present study, as-sintered BiFeO₃-BaTiO₃ ceramics are subjected to different heat treatment conditions (annealing and quenching processes) and poling procedures to evaluate their influence on the domain configuration and functional properties. As a result, the modified 0.7BiFeO₃-0.3BaTiO₃ ceramics were found to exhibit a remanent polarization of 0.44 C m⁻² at 100 °C, a Curie temperature of 500 °C and planar electromechanical coupling factor of 0.6 near the depolarization temperature of 480 °C after quenching at 800 °C in air; these materials show excellent potential for applications in high temperature piezoelectric transducers. In-situ temperature-dependent X-ray diffraction studies revealed unusual enhancement of the rhombohedral distortion on heating to temperatures ∼200 °C, associated with re-entrant relaxor ferroelectric behaviour. The ferroelectric domain configuration evolved from a disordered nanodomain structure in the as-sintered ceramic to a more structured herringbone-type nano-domain structure for the quenched sample, comprising a mixture of 180° and non-180° domain walls. Subsequent direct current poling led to a pronounced increase in domain size for both annealed and quenched samples, forming well-oriented lamellar domains and resulting in enhancement of ferroelectricity. High energy synchrotron X-ray diffraction experiments demonstrated a high degree of domain alignment along the electric field direction, resulting in a domain orientation fraction of ∼93 % for <222>-oriented grains in the heat-treated samples. Significant enhancement of the total electrostrain upon heating from room temperature to 100 °C was attributed to a combination of the increased extrinsic domain switching contribution, due to the enhancement of spontaneous strain, together with a higher intrinsic electrostrictive lattice strain.