Phase transition, switching characteristics of MPB compositions and large strain in lead-free (Bi0.5Na0.5)TiO3-based piezoceramics

In this study, the switching characteristics of MPB compositions of (1-x-y)(Bi0.5Na0.5)TiO3-xKNbO3-ySrTiO3 (abbreviated as BNT-KN-ST) and temperature- and composition-driven strain behavior, dielectric, ferroelectric (FE), piezoelectric, and pyroelectric properties were systematically explored to achieve lead-free piezoelectric materials with large strain response for practical applications of piezoelectric actuators. The piezoelectric coefficient (d33) declined dramatically and normalized strain Smax/Emax increased markedly as the morphotropic phase boundary (MPB) compositions shifted away from the MPB (I) into the MPB (II) where the ferroelectric rhombohedral and relaxor pseudo-cubic phases coexisted at around critical composition. The critical compositions for the indicated system exhibited enhanced strain response, and at room temperature a high strain of 0.34% with normalized strain Smax/Emax = 486 p.m./V was gained for the critical composition 0.90BNT-0.05KN-0.05ST. The source of the achieved large strain was sought based on the X-ray diffraction (XRD) and Raman-spectra structure analysis, macroscopic properties, thermal depolarization progress, and temperature-dependent correlations of both polarization and strain, and the results indicated that it derived from the phase transformation contribution from ergodic relaxor (ER) to FE phases. Moreover, the present work also established the relationship between the MPB composition and tolerance factor t in BNT-based ceramics, and demonstrated that the t corresponding to the formation of MPB located in a quite narrow range. As a result, we believe that this study will open the door to quickly detect the approximate MPB region for BNT-based solid solutions.