Design of Na-based Perovskites Based Dielectric and Piezoelectric Ceramics
| Author | : Lisheng Gao |
| Publisher | : |
| Total Pages | : |
| Release | : 2019 |
| Genre | : |
| ISBN | : |
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Lead-based ceramics have been widely used in piezoelectric and capacitor applications, such as sensors, actuators, acoustic transducers, and multilayer capacitors. In addition, high voltage switching antiferroelectric dielectrics are typically based on PbZrO3-based ceramics. Despite the extraordinary performance of lead-based piezoelectric and high voltage capacitive ceramics, concerns exist owing to the toxicity of lead in electronic devices, particularly in the European Union, which has pushed research and development to seek high-performance, lead-free alternatives. Fundamental studies have investigated structure-property processing relations. Methodologies such as the use of Rayleigh studies were performed on (Na, K)NbO3 (NKN) ceramics, sintered under different temperatures and low oxygen partial pressure (pO2). It was found that the extrinsic contribution from domain wall motions partly contributed to high-field dielectric and piezoelectric responses. By raising the sintering temperature, both reversible and irreversible coefficients increase. However, the sintering atmosphere has relatively limited impact on performance. An effective electrostrictive coefficient Q^* was introduced to couple the dielectric and piezoelectric properties in these polycrystalline ceramics. It was found that the Q^* value was invariant in samples processed under different conditions. Furthermore, in contrasting dielectric loss with electromechanical loss, the ratio of 180 domain wall motion contributions in NKN can be estimated. The percentage of non-180 domain wall motion contribution was independent from the processing of the materials.Another strategy to enhance piezoelectric performance of the NKN was by aligning the grains along a preferred crystallographic orientation, via a process known as texturing. The NKN was found to be easily textured along 001PC (PC: pseudocubic) in the low pO2 atmosphere, within a range compatible for base metal cofiring, such as Cu. The d_33^*was enhanced up to 680 pm/V. It was considered that there were two factors behind the enhancement of piezoelectric properties; (1) the ceramic was well textured along 001PC direction, and (2) the dissolution of NaNbO3 templates resulted in a polymorphic phase boundary shift to room temperature. A device engineering strategy was to fabricate multilayer structures with integrated interdigitated base-metal electrodes. A polypropylene carbonate (PPC) polymer binder system was used and formulated for these Na-based piezoelectrics and dielectrics. Many of the traditional binders such as (PVB) polyvinyl butrayl had issues of residual carbon in low oxygen partial pressure processing and could limit dielectric performance. PPC enabled clean burnout under a low pO2 environment at low temperatures. In a prototype demonstration, a (Na, K)NbO3 multilayer actuator with Cu inner electrodes was then fabricated in low pO2 atmosphere. The prototype multilayer devices showed reasonable piezoelectric properties with normalized strain coefficient (d_33^*) of 220 pm/V. The value was lower than that of the bulk ceramic, due to mechanical clamping from the interdigitating structure. There was no residual carbon, interdiffusion, alloy formation, or oxidation in the vicinity of metal-ceramic interfaces. Such results demonstrate potential application in using Cu in the inner electrodes for lead-free piezoelectrics, providing a route to high strain actuators. The low pO2 sintered ceramics underwent a re-oxidation process to compensate the oxygen vacancies, to improve reliability and dielectric losses. Therefore, it is essential to understand the kinetics of the re-oxidation process to retain the ceramics. To investigate the re-oxidation of NKN, an in -situ impedance spectroscopy was used on a model system with one active layer. Effective conductivities were extrapolated from the impedance spectra for samples annealed between 500C and 700C. The increasing impedance (improved resistance) of NKN during the re-oxidation process demonstrated that the incorporated atmospheric oxygen diffused into the ceramics, backfilling the oxygen vacancies generated in the higher-temperature sintering process. The effective ionic diffusion coefficient D was obtained using the conductivity relaxation technique, with the assumption of a two-dimensional diffusion model. Diffusivity was found on the scale of 10-6 cm2 s-1 between 500C and 700C. The re-oxidation times for devices of differing sizes at higher temperatures were predicted, and it was found that larger-size devices will require exceptionally long annealing time to compensate the oxygen vacancies. In addition, the end member NaNbO3 itself is also of interest for capacitive applications as an antiferroelectric dielectric. NaNbO3 solid solutions were explored and investigated to achieve stabilized antiferroelectric (AFE) P (with space group Pbma) phase over the ferroelectric (FE) Q (with space group P21ma) phase. It turns out that doping and considering trends of the crystal chemical Goldschmidt tolerance factor of the material can stabilize the antiferroelectric P phase in NaNbO3. Two unique modification strategies by adding A2+B4+O3 type CaHfO3 and A3+B3+O3 type BiScO3 into the solid solution were demonstrated. After being modified, it was found that the AFE P phase was stabilized by lowering the tolerance factor. Such stabilization was verified by transmission electron microscopy (TEM), with only {010} type superlattice diffraction patterns and electric field induced double P-E hysteresis loops.
