M. Venet

Investigation of the dielectric response in PbNb2O6 ferroelectric ceramics

High Curie temperature ferroelectrics are promising materials to construct transducers for high-temperature piezoelectric applications. Among them, the lead niobate system is one that displays such characteristic. In this work, the dielectric behavior of lead niobate ferroelectric ceramics was analyzed in the frequency range of 20 Hz−2 GHz. Ceramic samples with nominal formula PbNb2O6 (PN) were prepared by the solid-state reaction method. The phase transition was investigated with the Santos-Eiras’ phenomenological model. The values of the parameter, which determine the phase transition character (ξ), are close to 1, indicating a conventional ferroelectric phase transition. The PN materials showed two different dispersion processes: (1) a typical high-frequency dispersion process, which may be associated with the polar region boundaries dynamics, as observed in ferroelectric materials in the microwave frequency region, and (2) that associated with the charge carrier conduction, observed in the lowest freq...

Characterization of La-Doped PBN Ferroelectric Ceramics

In this work, pure and La3+-doped PBN56 ceramics (La3+ added from 0 wt% to 4 wt%) were fabricated and studied concerning its structure, microstructure and dielectric properties. The analysis of the microstructure of the ceramics showed a change of the grain morphology for ceramics with La content higher than 1.0 wt%. X-ray diffraction and Raman spectroscopy were performed to study the structure of the PBN56 ceramics. The results showed the presence of an orthorhombic phase, whose concentration is increased with the increasing of the La3+ content. The analysis of the results was performed to investigate the presence of the orthorhombic phase and the composition where its formation begins.

Diffuse phase transition and relaxor behaviour of textured Sr0.63Ba0.37Nb2O6 ceramics

Textured Sr0.63Ba0.37Nb2O6 ferroelectric ceramics, with relaxor characteristics, were fabricated by the hot forging process. The analysis of the complex dielectric constant as a function of the temperature revealed the presence of three dielectric anomaly regions. In addition, the relaxor behaviour of this ceramic was also observed at microwave frequencies, evidencing the existence of active polar regions in a broad frequency range. Using the Vogel–Fulcher approximation, physical parameters such as the freezing temperature (Tf) were estimated. It was verified that the mechanisms of the dielectric response, in the microwave frequency range, are also affected by the presence of compositional fluctuations and local electric fields. A phenomenological equation was applied in order to investigate the nature of the polarization mechanisms at low frequencies. Also, a qualitative model was proposed to explain the anisotropic behaviour observed in the phase transition diffuseness between the different ceramic directions.

Improvement of the phase diagram for the pseudobinary PbNb2O6–BaNb2O6 system

Characteristics of phase transitions of lead barium niobate compositions around the morphotropic phase boundary were investigated. Using structural characterizations, it was found that the morphotropic phase boundary extends toward a wide composition range, in which both tetragonal (4mm) and orthorhombic (m2m) tungsten bronze phases coexist. In addition, on heating, two phase transitions were observed for all the studied compositions. First, the fraction with m2m symmetry phase transforms into the 4mm symmetry one and, second, the 4mm symmetry phase, which represents the whole material, transforms into the paraelectric phase (4/mmm symmetry). The phase transition temperatures were determined from dielectric characterizations. These results helped to improve the phase diagram of the pseudobinary PbNb2O6–BaNb2O6 system around the morphotropic phase boundary region.

Dynamics of normal to diffuse and relaxor phase transition in lead metaniobate-based ferroelectric ceramics

Tetragonal tungsten bronze-structured materials based on lead metaniobate (PbNb2O6) were studied in terms of thermal dynamics of dielectric properties, showing ferroelectric-to-paraelectric phase transition of diffuse and relaxor type in some specific cases. These features are normally ascribed to defects-induced structural disorder and compositional fluctuations associated with an arbitrary lattice site occupation between dopant and host ions. Nevertheless, for these lead metaniobate-based materials, the drastic change in the phase transition from normal to diffuse and relaxor is shown to take place when dopants are able to significantly shift the transition toward low temperatures, where these compounds are known to exhibit incommensurate superstructures that naturally present diffuse and relaxor dielectric characteristics.

Dielectric properties of PbNb2O6 ferroelectric ceramics at cryogenic temperatures

Grupo de Cerâmicas Ferroeĺtricas Departamento de Fisica Universidade Federal de Sao Carlos, Sao Carlos, SP 15385000

La3+-induced (micro)structural changes and origin of the relaxor-like phase transition in ferroelectric lead barium niobate electroceramics

Lead barium niobate (Pb1−xBaxNb2O6, PBN) ferroelectric materials have been and are the subject of numerous studies in literature due to their potential for wide-ranging applications in the electronic industry. In this work, La3+-doped Pb0.56Ba0.44Nb2O6 (PBN44) electroceramics were prepared and investigated in terms of X-ray diffraction, scanning electron microscopy, thermal spectra of dielectric permittivity, Curie–Weiss law, and hysteresis loop characteristics. It was noted that La3+ doping favors the formation of orthorhombic mm2 phase in PBN44, which originally shows only the tetragonal 4mm symmetry-related phase. In particular, the PBN44 material with 1 wt% La2O3 displays (micro)structural characteristics and dielectric properties similar to those from PBN compositions lying within their morphotropic phase boundary region. Our results also show that La3+ is able to promote a change of the ferroelectric to paraelectric phase transition appearance of PBN44 from pseudo-normal to really diffuse. However, conversion to a diffuse plus relaxor transition behavior reveals directly linked to incommensurate superstructures present and dielectrically-active in PBN materials toward low temperatures, with an intrinsically frequency-dispersive dielectric response. This statement is also supported by observation of hysteresis loops showing a transformation trend to pseudo-slim-like, even in the normal-like ferroelectrics, when moving into the temperature region of incommensuration manifestation.

Control of powder morphology for texture enhancement of lead barium niobate ferroelectric ceramics

Abstract In this work, the influence of the powder particle morphology on the microstructural morphology and structural texture levels of tetragonal tungsten bronze Pb0·56Ba0·44Nb2O6 ceramics was investigated. Powders were obtained by solid state reaction synthesis with different aspect ratios and densified through biaxial hot forging. A texture diffractometer was used to investigate the preferential orientation of the crystallographic planes and their relation to the microstructural morphology of the hot forged samples. The results showed that the higher the particle aspect ratio before firing, the higher the structural texture level of the sample after hot forging process. The highest anisotropy of the dielectric properties was observed in the sample with enhanced texture. Therefore, control of the particle morphology of the lead barium niobate precursor powder is very useful for obtaining ceramics with high texture levels and desirable anisotropic properties.

Características estruturais, microestruturais, dielétricas e piroelétricas de cerâmicas de SrxBa1-XNb2O6 forjadas a quente

The texturing process and the characterization of textured SrxBa1-xNb2O6 (x = 0.5; x = 0.6) ferroelectric ceramics were investigated. The ceramic bodies were fabricated through two stage sintering. The first and second stage consisted in conventional firing and hot forging of the ceramic bodies, respectively, both at oxygen atmosphere. The forged ceramics achieved high densification level although showing abnormal grain growth. X-ray diffraction, dielectric and pyroelectric analysis showed that is possible to obtain preferential orientation of the [00l] crystallographic planes due to grain arrangement during hot forging. Consequently, the investigated properties for this textured material were strongly dependent on the measurement direction. On the other hand, high room temperature pyroelectric coefficients, of the same order of that obtained for single crystals of same composition, were observed.

Thermally stimulated depolarization current in a modified SBN ferroelectric ceramic system

The Thermally Stimulated Depolarization Current (TSDC) of the modified SBN ferroelectric ceramic system was studied. The TSDC measurements show the pyroelectric peak for all compositions while a second smaller peak at higher temperatures, possibly associated to induced vacancy-impurity dipoles is also observed in all cases. The second peak contribution was experimentally and mathematically eliminated. The activation energies and infinite temperature relaxation time of both processes are determined.