ABSTRACT
Tetragonal (1-x)(Bi,Na)TiO3-xBaTiO3 films exhibit enhanced piezoelectric properties due to domain switching over a wide composition range. These properties were observed over a significantly wider composition range than the morphotropic phase boundary (MPB), which typically has a limited composition range of 1-2%. The polarization axis was found to be along the in-plane direction for the tetragonal composition range x = 0.06-1.0, attributed to the tensile thermal strain from the substrate during cooling after the film formation. A "two-step increase" in remanent polarization against an applied maximum electric field was observed at the high-field region due to the domain switching, and a very high piezoelectric response (effective d33 value, denoted as d33,f) over 220 pm/V was achieved for a wide composition range of x = 0.2-0.5 with high tetragonality, exceeding previously reported values for bulk ceramics. Moreover, a transverse piezoelectric coefficient, e31,f, of 19 C/m2 measured using a cantilever structure was obtained for a composition range of at least 10 atom % (for both x = 0.2 and 0.3). This value is the highest reported for Pb-free piezoelectric thin films and is comparable to the best data for Pb-based thin films. Reversible domain switching eliminates the need for conventional MPB compositions, allowing an improvement in the piezoelectric properties over a wider composition range. This strategy could provide a guideline for the development of environmentally acceptable lead-free piezoelectric films with composition-insensitive piezoelectric performance to replace Pb-based materials with MPB composition, such as PZT.
ABSTRACT
Strong electromechanical coupling is observed in tetragonal Pb-free 0.7(Bi0.5Na0.5)TiO3-0.3BaTiO3 films, which is far from the morphotropic phase boundary, prepared by pulsed laser deposition on a Si substrate. The tensile strain induced during cooling causes in-plane polarization in an oriented film on a Si substrate, while an epitaxial film grown on a SrTiO3 substrate exhibits out-of-plane polarization. S-E curve analysis reveals that the obtained piezoelectric coefficient for the film on the Si substrate (d33,f ≈ 275 pm/V) is approximately eight times higher than that for the epitaxial film on the SrTiO3 substrate (d33,f ≈ 34 pm/V). In situ X-ray diffraction analysis confirms the occurrence of domain switching under an electric field from in-plane to out-of-plane polarization. An effective piezoelectric stress coefficient, e31,eff, of â¼19 C/m2 is obtained from a Si cantilever sample, which is the highest among the reported values for Pb-free piezoelectric films and is comparable to those for Pb-based films. The significant piezoelectric response produced by domain switching in the Pb-free materials with the composition far from the morphotropic phase boundary will expand future applications due to their both outstanding properties and environmental sustainability.
