Search

• Domain switching and lattice strain in polycrystalline K 0.5 Bi 0.5 TiO 3 -BiFeO 3 -PbTiO 3 is evaluated by in-situ high energy synchrotron XRD. • The tetragonal {200} grain family exhibited the highest effective lattice strain up to 8.2 × 10−3 at an electric field of 5 kV/mm. • A partial phase transformation from tetragonal to rhombohedral occurred at high electric field levels, with an average strain of −1.54 × 10−3. • Relaxation of intergranular stress gives rise to enhanced domain switching in KBT-BF-PT ceramics. • The observed self-adapting mechanism could also be exploited in other high performance, high temperature piezoelectric ceramics. We report an in-situ synchrotron X-ray diffraction study of K 0.5 Bi 0.5 TiO 3 -BiFeO 3 -PbTiO 3 ceramics, which exhibit a T c of around 450 °C. The electromechanical actuation mechanisms comprise contributions from coexisting tetragonal and rhombohedral phases. The tetragonal {200} grain family exhibited the highest effective lattice strain, up to 8.2 × 10−3 at 5 kV/mm. Strong strain anisotropy in the tetragonal phase and field-induced intergranular stresses facilitate a partial transformation from tetragonal (high strain anisotropy) to rhombohedral (low strain anisotropy) at high electric field levels, with an average linear transformation strain of -1.54 × 10-3. The domain switching behavior was effectively enhanced in both tetragonal and rhombohedral phases after the phase transformation, due to the release of intergranular stress. This observed self-adapting mechanism in tuning intergranular stress through partial phase switching in the morphotropic KBT-BF-PT composition with large lattice distortion could also be exploited in other perovskite systems in order to achieve high performance high temperature piezoelectric ceramics. [ABSTRACT FROM AUTHOR]

The structures and functional properties of Na0.5Bi0.5TiO3- xKNbO3 (NBT-xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high-resolution synchrotron X-ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long-range ordered ferroelectric domains in the poled state. In contrast, the near-cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of −175°C. Upon heating, the field-induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, TST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field-induced rhombohedral phase in the poled specimens, the pseudo-cubic lattice parameter, ap, exhibited an anomalous reduction while the inter-axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature-dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature. [ABSTRACT FROM AUTHOR]

BiFeO3–BaTiO3 (BF–BT) ceramics exhibit great potential for diverse applications in high temperature piezoelectric transducers, temperature-stable dielectrics and pulsed-power capacitors. Further optimization of functional properties for different types of applications can be achieved by modification of processing parameters or chemical composition. In the present work, the influence of pentavalent niobium substitution for trivalent ferric ions on the structure, microstructure and dielectric properties of 0.7BF–0.3BT ceramics was investigated systematically. Doping with niobium led to incremental reductions in grain size (from 7.0 to 1.3 µm) and suppression of long-range ferroelectric ordering. It was found that core-shell type microstructural features became more prominent as the Nb concentration increased, which were correlated with the formation of distinct peaks in the dielectric permittivity–temperature relationship, at ~470 and 600 °C, which were attributed to the BT-rich shell and BF-rich core regions, respectively. Nb-doping of BF–BT ceramics yielded reduced electronic conductivity and dielectric loss, improved electrical breakdown strength and enhanced dielectric energy storage characteristics. These effects are attributed to the charge compensation of pentavalent Nb donor defects by bismuth vacancies, which suppresses the formation of oxygen vacancies and the associated electron hole conduction mechanism. The relatively high recoverable energy density (Wrec = 2.01 J cm−3) and energy storage efficiency (η = 68%) of the 0.7BiFeO3–0.3BaTiO3 binary system were achieved at 75 °C under an electric field of 15 kV mm−1. This material demonstrates the greatest potential for applications in energy storage capacitors and temperature-stable dielectrics. [ABSTRACT FROM AUTHOR]

(1 − x)Na 0.5 Bi 0.5 TiO 3 -xKNbO 3 (NBT-xKN) ceramics, with x in the range 0.01–0.09, were prepared by solid state reaction and conventional sintering. A progressive transformation from a nano-polar to a long-range ordered ferroelectric phase was observed after the application of a cyclic electric field with an amplitude of 5 kV mm −1 . It is shown by analysis of the crushed ceramic pellets, using high resolution synchrotron x-ray diffraction, that the application the cyclic electric field induced an irreversible transformation from a pseudo-cubic nano-polar phase to a rhombohedral ordered ferroelectric phase having R3c symmetry. Furthermore, a systematic variation, from 89.6° to 90°, was observed in the rhombohedral interaxial angle as the KN content increased from 1 to 9%. By comparing the structural evidence with the ferroelectric polarization-electric field hysteresis loops, it is shown that the electric field-induced transformation is partially reversible for NBT-5KN and completely reversible or nonexistent for NBT-9KN. [ABSTRACT FROM AUTHOR]