Matthias C. Ehmke

Phase coexistence and ferroelastic texture in high strain (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 piezoceramics

Dielectric permittivity and x-ray diffraction measurements were used to identify a region of phase coexistence between the rhombohedral and tetragonal phases near the morphotropic phase boundary in (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT-BCT). This phase coexistence prevails over a considerable composition and temperature range and is bounded by single rhombohedral or tetragonal phases. The maximum piezoelectric response measured in terms of maximum strain divided by maximum electric field, Smax/Emax, is extraordinarily high, with the largest value of 1310 pm/V for x = 0.45. Electrical poling induces ferroelastic domain textures in both the rhombohedral and tetragonal phases simultaneously, which increases the piezoelectric performance significantly. The stability of that ferroelastic texture is limited by the phase transition at the morphotropic phase boundary, suggesting coupling between both coexisting phases and limiting potential applications. The results were confirmed using in situ temperature...

Reduction of the piezoelectric performance in lead-free (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 piezoceramics under uniaxial compressive stress

The effect of a uniaxial compressive stress on the properties of BZT-BCT samples across the morphotropic phase boundary (MPB) is investigated using direct piezoelectric coefficient measurements. In contrast to many lead zirconate titanate compositions, the piezoelectric coefficient decreases monotonically with increasing stress and does not show an initial increase or plateau. Electrically softer rhombohedral and MPB compositions are found to be more susceptible to a decrease in piezoelectric coefficient under an increasing pre-stress than tetragonal compositions. Depoling due to ferroelastic domain switching alone, as observed by x-ray diffraction, does not explain this reduction, but instead a decreasing domain wall density is proposed to be responsible for reduced piezoelectric coefficients under increasing compressive stress. The relaxation of the piezoelectric response after complete unloading supports this proposed mechanism.

The ageing and de-ageing behaviour of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free piezoelectric ceramics

Ageing behaviour usually occurs in acceptor-doped piezoelectric materials (e.g., hard lead zirconate titanate) and exhibits the development of a pinched or shifted hysteresis loop over time. Although no pinched hysteresis loop was observed for lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 material, this study showed that the piezoelectric properties change over time in the poled state. The shift of the hysteresis loop along the electric field axis and the development of asymmetry in strain and permittivity hysteresis loop were observed during the ageing process. The origin of this ageing behaviour is proposed to be local defect dipoles and the migration of the charged defects to the grain boundaries. The reorientation of the defect dipole contributes to a fast but unstable ageing mechanism in this material while the migration of the charged defects contributes to a slow but more stable mechanism.

Electric field effects on piezoelectric and ferroelastic strain in Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 piezoceramics

Rietveld analysis of synchrotron diffraction data is used to resolve the macroscopic strain response of tetragonal barium zirconate titanate-barium calcium titanate (BZT-BCT) piezoceramics and determine the underlying strain mechanisms. A lattice strain contribution and a ferroelastic switching contribution are identified and both contribute significantly to the macroscopic strain. A large electrically induced ferroelastic domain texture is observed that undergoes strong relaxation upon removal of the electric field. This suggests that under cyclic conditions each field application leads to a strong ferroelastic domain reorientation that is identified to be responsible for the good piezoelectric performance observed in this class of materials. Moreover, the significance of orientation dependent structural changes is identified, which is crucial to fully understand the macroscopic strain response.