Mechanical Stability of the Electromechanical Properties and Phase Transitions in Lead-Containing and Lead-Free Ferroelectrics

In this work, novel experimental setups were developed that are capable of automatically and simultaneously measuring the small signal piezoelectric and dielectric properties of ferroelectrics as a function of frequency, temperature, and compressive uniaxial mechanical stress. In addition, ferroelectric and ferroelastic experiments served to characterize the large-signal properties of ferroelectric materials. The phase transition behavior of single crystal and polycrystalline BaTiO3 was investigated between -150 °C and 250 °C as a function of uniaxial mechanical bias stresses up to -30 MPa (single crystal BaTiO3) and -500 MPa (polycrystalline BaTiO3). An increase of the Curie point and a decrease of the Curie-Weiss temperature with increasing mechanical stress were revealed by the experiments. These observations lead to the conclusion that a mechanical load results in an increase in the first order nature of the ferroelectric-paraelectric phase transition in BaTiO3. By introducing stress-dependent coefficients in the phenomenological LGD theory, a prediction of this change in the nature of the phase transition as well as a qualitative reproduction of the stress-induced shift of the Curie point was achieved. The low-temperature phase transitions could be correlated with anomalies in the temperature-dependent ferroelastic properties of BaTiO3. Ferroelectrically soft and hard commercial lead-containing PZT compositions were investigated with regards to their stress- and temperature-induced depolarization and phase transition behavior up to a mechanical bias stress of -300 MPa between room temperature and 350 °C (soft PZT) or 450 °C (hard PZT). The hard PZT showed higher resistance against stress-induced and thermal depolarization due to the pinning of domain walls by polar defects, but with the drawback of lower piezoelectric properties if compared to soft PZT. With increasing mechanical stress, the ferroelectric-paraelectric phase transition of both materials was found to increase. Stress-dependent measurements of the piezoelectric coefficient up to -400 MPa performed on slow cooled and air quenched samples indicated that the polar defects in the hard PZT could be realigned by a compressive uniaxial mechanical stress. In addition, temperature- and stress-dependent measurements of the piezoelectric properties of PIN-PMN-PT single crystals revealed a stabilization of the high-temperature tetragonal phase, experimentally observed by a decreasing rhombohedral-tetragonal phase transition temperature. Lead-free materials based on the NBTxBT system are of current interest for replacing lead-containing materials in electronic devices. In this work, the piezoelectric and dielectric properties NBT-3BT, NBT-6BT, NBT-9BT, and NBT-12BT were characterized as a function of temperature (up to 400 °C) and uniaxial mechanical compression (up to -300 MPa). In general, the properties of all investigated NBT-xBT materials were found to decrease above a sufficiently large uniaxial mechanical compression. The MPB materials NBT-6BT and NBT-9BT possessed mixed rhombohedral and tetragonal phases after electrical poling and showed the highest stress sensitivity. NBT-6BT was investigated in more detail and a stress-induced phase transition from an initially relaxor to a ferroelectric phase was observed. A stress-temperature diagram could be constructed based on stress-dependent permittivity measurements up to -600 MPa at various temperatures between -50 °C and 160 °C. NBT-xBT compositions doped with 1 mol% Fe showed an apparent inhomogeneous coloring after high-temperature electrical poling, most likely due to the migration of oxygen vacancies. The piezoelectric properties of these materials were found to be significantly lower than the properties of the undoped compositions, which was due to polar defects induced by the Fe-doping. In addition, high ionic conductivity dominated the piezoelectric and dielectric response at higher temperatures. The application of a uniaxial mechanical stress decreased the properties of NBT-xBT:Fe even more and resulted in a shift of the depolarization temperatures, similar to the findings in the undoped material