Giant strain, thermally-stable high energy storage properties and structural evolution of Bi-based lead-free piezoceramics

In this study, the mechanism of electric-field-induced phase transition in 0.96[Bi_0.5(Na_0.84K_0.16)_0.5Ti_1-xTa_xO₃]-0.04SrTiO₃, (where 0.00 ≤ x ≤ 0.030, abbreviated as BNKTT-ST) ceramics was discussed based on crystal structure and electromechanical properties. X-ray powder diffraction (XRD) showed a phase transition from mixed rhombohedral-tetragonal to single pseudocubic when x ≥ 0.020. Large electromechanical strain of ∼0.42% with a dynamic piezoelectric constant (S_max/E_max) of ∼700 pm/V at 6 kV/mm was recorded for 2 mol. % Ta content. Interestingly, at lower field of 4 kV/mm, the S_max/E_max attained the highest value of ∼830 pm/V. Poled, unpoled XRD analysis and electrical properties suggest that the relatively high S_max/E_max at x = 0.020 may be attributed to the combined effect of composition and ferroelectric (FE) to ergodic relaxor (ER) phase transition. Furthermore, the energy storage density was studied as a function of both composition and temperature to demonstrate the suitability for capacitor applications. This system also revealed improved energy storage properties. Particularly, for x = 0.020, a nearly temperature-invariant large recoverable energy density (W = 0.65 J/cm³) was achieved over a wide temperature range (75-150 °C). These properties demonstrate that the fabricated system might be a promising lead-free candidate for actuators and high temperature energy storage capacitor applications.