Effect of heat-treatment mechanism on structural and electromechanical properties of eco-friendly (Bi, Ba)(Fe, Ti)O₃ piezoceramics

Lead-free piezoelectric ceramics, (Bi_0.70Ba_0.35)(Fe_0.65Ti_0.35)O₃ (BBFT), were fabricated via a solid-state reaction method and then treated using different heat-treatment processes (furnace cooling, air quenching (AQ), and water quenching). In all these ceramics, the X-ray diffraction analysis revealed morphotropic phase boundaries between the rhombohedral and tetragonal phases. Changes in the average grain size, relative density, and electrical properties in the BBFT–AQ composition were observed. For the optimum BBFT–AQ ceramic, significantly enhanced dynamic (d₃₃* ~ 340 pm/V) and static (d₃₃ ~ 165 pC/N) piezoelectric coefficients were obtained. Moreover, the d₃₃* increased to 40% (d₃₃* ~ 475 pm/V) with increasing temperature from 25 °C to 75 °C upon the application of a 4.0 kV/mm electric field. The related defect states were established by the variation in the transition of Fe³⁺ to Fe²⁺ and O_V or VO∙∙ concentration observed using X-ray photoelectron spectroscopy analysis. These factors are directly related to the electromechanical response enhancement in the BBFT piezoceramics. This study provides a paradigm for a deeper analysis of particular scientific heat-treatment mechanisms and the enhancement of functional properties in BBFT ceramics. Graphical abstract: [Figure not available: see fulltext.]