Effects of Na Excess on the Crystal Structure Ferroelectric and Piezoelectric Properties of (bi0.487na0.427k0.06ba0.026) TiO3 Lead-Free Piezoceramics

In this research, (Bi0.487Na0.427K0.06Ba0.026)TiO3 ceramics were prepared by conventional solid-state mixed-oxide method to incorporate Na excess according to the chemical formula (Bi0.487Na0.427+ xK0.06Ba0.026)TiO3, where x = 0.0, 0.005, 0.010, 0.015 and 0.020. The crystal structure, dielectric, ferroelectric and piezoelectric properties were systematically investigated with respect to the amount of Na excess. X-ray diffraction data identified pure perovskite structure for all compositions. The pseudocubic structure was observed for x = 0.0 - 0.005 before it transformed to rhombohedral structure at x >= 0.015. The grain size tended to be larger with increasing Na excess amount. Increasing x also led to a substantial increase in ferroelectric-relaxor transition temperature (TF-R) from 80 degree celsius (x = 0) to 110 degree celsius (x = 0.020). With increasing x from 0 to 0.020, the standard polarization-electric field (P-E) hysteresis measurements revealed a reduction in remanent polarization (P-r) together with an increase in coercive field (E-c), which was further confirmed by the remanent P-E hysteresis measurements. In addition, the piezoelectric constant (d(33)) was also found to steadily decrease as x increased. The observation of increased TF-R and E-c along with a decrease in d(33) and P-r indicated a stabilization of ferroelectric order induced by Na excess. This research highlights the importance of sufficient control over A-site stoichiometry and how it affects the ferroelectric and piezoelectric properties of BNT-based ceramics.