Broadband Dielectric Response in Hard and Soft PZT: Understanding Softening and Hardening Mechanisms

Lead zirconate titanate (Pb(Zr1−xTix)O3 or PZT) ferroelectric ceramics have been widely used in transducers, actuators, and sensors, since they posses high dielectric and piezoelectric properties with a relatively high temperature of operation. Commercially used PZT ceramics are always modified by different dopants and are divided into the “soft” (donor doped) and “hard” (acceptor doped) groups. Compared with the undoped composition, hard PZT often shows lower but more stable properties after ageing. In contrast, soft PZT shows higher properties and insensitivity to ageing. The difference in properties between the soft and hard PZT ceramics is rather large, even though the doping level is limited to a very low value (on the order of 1 mole %). The large difference of the physical properties between them mainly originates from the contributions from domain wall motion rather than properties of the crystal lattice. However, the mechanisms of hardening and softening are not well understood. In order to understand better the hardening and softening mechanisms, in this thesis the different contributions to the dielectric properties of soft and hard PZT ceramics are studied by means of a broadband dielectric spectroscopy from 10mHz to 20GHz. Properties at THz and infrared frequencies where only crystalline lattice contributes to the dielectric response were also investigated in collaboration with another group. In the frequency range below 20GHz, the different contributions to the permittivity by domain wall motion were revealed in hard and soft materials. In order to correlate the properties to the microscopic structure of hard and soft PZT ceramics, the domain structures were also investigated by transmission electron microscopy. Piezoelectric spectroscopy was employed to help separating different contributions at frequencies below 100Hz. The main results of this work are: The microwave dielectric dispersion of all PZT ceramics (including undoped, soft and hard PZT ceramics), which is characterized by a rapid decrease of the permittivity and a loss peak in the GHz frequency range, is contributed by both domain wall motion and piezoelectric grain resonances. These two mechanisms are separated by gradual poling of samples. The dispersion related to the domain wall motion appears at a higher frequency than the one related to grain resonance and constitutes the main contribution to the microwave dielectric properties of unpoled samples. Above the GHz frequency range, the dielectric properties of hard and soft PZT ceramics are rather close and approach the upper limit value of their intrinsic properties, which are identified by dielectric properties determined by THz dielectric spectrum. The contributions by domain wall motion make up more than 50% of the quasistatic dielectric properties (measured at 100 kHz) in all studied samples.