Dispersão dielétrica em materiais ferroelétricos.
Abstract
In the last decades, the high frequency dielectric response has been one of
the most studied issues in the ferroelectricity area. The main dielectric
characteristic is the strong dependence of the dielectric parameters (real and
imaginary component of the dielectric permittivity) with the measuring frequency
(dielectric dispersion). Among them, the perovskite-structured family, such as
barium titanate (BaTiO3), pure and modified lead zirconate titanate (PZT and
PLZT, respectively), and some others similar compounds can be emphasised. In
order to explain such phenomenon some theoretical models have been proposed,
nevertheless a major discussion about the true mechanism responsible for the
microwave dielectric dispersion remains still open. In this work, a pressurecontrolled
sample holder for microwave dielectric measurements is presented. The
experimental set-up involves a coaxial line coupled to a pressure-controlled
sample holder able to obtain the dielectric characterization as a function of the
temperature. On the other hand, the microwave dielectric dispersion process was
investigated in the relaxor and normal ferroelectric composítions of lanthanum
modified lead titanate ceramics. The obtained results were discussed concerning a
damped harmonic oscillator, considering the vibration of the boundaries of the
polar regions, independently of their structure, as the mechanism responsible for
the high frequency dielectric anomalies (dielectric dispersion and resonance) in
ferroelectric systems.