Mechanisms of enhanced thermal stability of polarization in lead-free (Bi1/2Na1/2)0.94Ba0.06TiO3/ZnO ceramic composites

(Bi1/2Na1/2)TiO3-based solid solutions, one of the major systems of lead-free piezoelectric ceramics, exhibit a low thermal depolarization temperature (T-d similar to 100 degrees C). It was reported that by incorporating 30 mol% ZnO particles to form a ceramic composite of (Bi1/2Na1/2)(0.94)Ba0.06TiO3/ZnO, the depolarization process can be shifted up to -250 degrees C. In the present work, a variety of advanced transmission electron microscopy techniques, including in situ heating, annular bright-field, high-angle annular dark-field, geometric phase analysis, energy-dispersive spectrum and electron energy-loss spectroscopy, are employed to investigate the underlying mechanisms for the enhanced thermal stability of polarization in the composite. It is found that the abrupt depolarization in (Bi1/2Na1/2)(0.94)Ba0.06TiO3/ZnO at T-d becomes diffused over a wide temperature window up to the temperature at maximum dielectric constant (T-m) under the combined actions of the incorporation of Zn into the perovskite lattice, the presence of residual stresses, and the pinning effect on micron-sized domains provided by ZnO particles.