Obtenção, propriedades e fenomenologia de materiais ferroelétricos com estrutura tungstênio bronze
Abstract
The processing of lead metaniobate (PN) ferroelectric ceramics was optimized, therefore, in
this work, the main details to obtain PN ceramics with density higher than 96 % of the ideal density
are presented and discussed.
Taking into account the potential of the lead metaniobate for different applications, mainly for
high temperatures, the phase transition characteristics of this material, in both undoped and doped
compositions, were detaily investigated.
It was verified that the addition of Ti4+ enhances the densification and increase the Curie
temperature. Nevertheless, the ferro-paraelectric phase transition is affected by an electric
conductive process, which appears in temperatures below and above the Curie temperature. The
double ionized oxygen vacancies are proposed to be the charge carriers responsible for this
conductive process. The undoped and Ti4+-doped PN ceramics show low values of the mechanical
quality factor and high piezoelectric anisotropy, which are desired characteristics for the fabrication
of broad band electro-mechanical transducers, mainly to operate in the thickness mode.
The addition of Ba2+ to the PN (PBN) considerably decreases the Curie temperature, but it
induces a phase transition, leading to the formation of a morphotropic phase boundary (MPB)
around the composition with 37 % of Ba2+, in which some properties are enhanced. In this way, the
characteristics of the ferro-paraelectric phase transition, for compositions around the MPB, were
investigated for Ba2+-doped PN ceramics, textured by hot forging. It was found that the MPB
extends towards a wide composition range, in which both tetragonal 4mm and orthorhombic m2m
tungsten bronze (TB) phases coexist. In addition, with the increasing temperature, two phase
transitions were observed for this composition range. First, the material portion with orthorhombic
(m2m) symmetry transforms into the tetragonal (4mm) phase and, second, the 4mm phase (that
represents the material in its totality) transforms into the paraelectric phase with the tetragonal
4/mmm TB symmetry. These results helped to complete the reported PBN phase diagram.
Moreover, it was found that the La3+ addition to the PBN, for the composition with 56 % of Pb2+,
induces the formation of the orthorhombic (m2m) phase.
All the (doped and undoped) PN ceramics showed two dielectric dispersion processes at low
temperatures, i.e. from 30 to 300 K, which seems to be an inherent feature of all materials with TB
structure and ((A1)x(A2)5-xNb10O30) structural formula. The two processes were associated to a
phase transition and the formation of incommensurate structures, respectively. The influence of
such incommensurate structures on the relaxor behavior and diffusivity of the ferro-paraelectric
phase transition was also analyzed.