Modification of microstructure and electrical properties due to high A-site diffusion of cations induced by non-stoichiometry during processing.

No abstract

Na 1/2 Bi 1/2 TiO 3 -based (NBT-based) ceramics offer a viable option to replace lead-based materials for highpower applications as they are characterized by a stable mechanical quality factor with increasing vibration velocity in comparison to lead-based piezoceramics. Recently, the minor and stable extrinsic contributions were revealed as the origin for the stability of the mechanical quality factor with increasing vibration velocity. This work identifies the very unusual high poling degree as cause for the small extrinsic contributions. To this end, complete pole figure densities have been quantified and correlated to the piezoelectric coefficient and electromechanical quality factor. This hypothesis is further strengthened by correlating the piezoelectric constant (sum of intrinsic and extrinsic contributions) with the remanent polarization (correlates to remanent texturing degree). In order to assess a full picture of NBT-based piezoceramics, 0.94Na 1/2 Bi 1/2 TiO 3 -0.06BaTiO 3 has been considered with and without Zn doping and with quenching. It is compared to 0.79Na 1/2 Bi 1/2 TiO 3 -0.21 K 1/2 Bi 1/2 TiO 3 with and without Mg doping. Finally, a contrast to soft Pb(Zr 1/2 Ti 1/2 )O 3 (PZT) flushes out the impact of domain wall motion on the piezoelectric coefficient and the electromechanical quality factor. Whereas a PZT-based reference material exhibits a linear increase in the piezoelectric constant with increasing remanent polarization, the NBT-based materials deviate from the linear trend, indicating a decrease in extrinsic contributions.

A series of morphotropic phase boundary (MPB) compositions of (1-x) Na 1/2 Bi 1/2 TiO 3 -xBaTiO 3 (x = 0.05, 0.055, 0.06, 0.065, 0.07), with and without 0.5 mol% Zn-doping was synthesized using the solid-state route. The samples were characterized using X-ray diffraction, dielectric analysis, and electromechanical measurements (piezoelectric d 33 coefficient, coupling factor k p , mechanical quality factor Q m , and internal bias field E bias ). The increase in the ferroelectric-relaxor transition temperature upon Zn-doping was accompanied by a shift of the MPB toward the Na 1/2 Bi 1/2 TiO 3 -rich side of the phase diagram.Higher tetragonal phase fraction and increased tetragonal distortion were noted for Zn-doped (1 -x)Na 1/2 Bi 1/2 TiO 3 -xBaTiO 3 . In addition, ferroelectric hardening and the presence of an internal bias field (E bias ) were observed for all doped compositions. The piezoelectric constant d 33 and the coupling coefficient k p decreased by up to ∼30%, while a 4-to 6-fold increase in Q m was observed for the doped compositions. Apart from establishing a structure-property correlation, these results highlight the chemically induced shift of the phase diagram upon doping, which is a crucial factor in material selection for optimal performance and commercialization.

Mechanically induced changes in electrical properties have gained increasing interest over the last decade. The research field piezotronics, which describes the change in potential barrier height (e.g., at Schottky contacts) by piezoelectric charges, could lead to promising applications such as sensors or strain trigged transistors. In this contribution, the changes of potential barriers by mechanical stress at several grain boundaries of polycrystalline ZnO have been quantified. Rationalization is provided by concurrent measurement of crystal orientations of both neighboring grains. The highest changes in barrier height could be observed at ZnO–ZnO grain boundaries exhibiting varistor properties. In this case, the barrier height can be almost completely reduced by positive piezoelectric charges. Furthermore, an increase in barrier height is observed with negative piezoelectric charges. The developed physical model suggests an equivalent impact of externally applied voltage and induced positive piezoelectric charge on the barrier properties. In turn, not only the barrier height is modified by piezoelectric charges but also the breakdown voltage. This has, nevertheless, been only indirectly verified in experiments. These allow rationalizing the proposed model. Furthermore, a statistical study reveals a change in the distribution of breakdown voltages with changing stress conditions in the varistor ceramic.