Electric-field-induced phase transition and large strain in lead-free Nb-doped BNKT-BST ceramics

A suite of Nb-based piezoelectric ceramics of 0.99[Bi0.5(Na0.4K0.1)(Ti1−xNbx)]O3–0.01(Ba0.7Sr0.3)TiO3(BNKTN-BST), with x ranging from 0 to 0.030, was prepared by a conventional solid-state reaction method. X-ray diffraction patterns confirmed a single perovskite phase and the tetragonality was found to decrease with increasing Nb ratio. The BNKTN-BST ceramic had a high field-induced normalized strain coefficient of 634 pm/V at 2 mol% Nb content with a relatively small hysteresis compared with existing lead-free Bi-perovskite ceramics. An electric-field-dependent X-ray study was conducted to identify the main source of the high strain and ascertain the effect of electric fields on the crystal structure. The temperature-dependent P − E hysteresis loops of the BNKTN-BST ceramics were measured under an electric field of 60 kV/cm at various temperatures, and the effect of temperature on the ferroelectricity is discussed.