L. G. Vieira

Dielectric characterization of the (1 ? x)La(Mg1/2Ti1/2)O3?xBaTiO3 microwave ceramics

The dielectric properties of the perovskite mixed system La(Mg1/2Ti1/2)O3–BaTiO3 were studied at radio, microwave and far infrared frequency ranges in respect of potential microwave application. The room temperature values of relative permittivity, dielectric loss and temperature coefficient of the resonant frequency were estimated by different methods. The permittivity and the thermo-stability coefficient were found to follow a simple hyperbolic-type dependence on composition. The dielectric response of air- and oxygen-sintered ceramics was compared and analysed. The origin of the dielectric loss and frequency dispersion in BaTiO3-rich compositions of the system is discussed.

Structure refinement, far infrared spectroscopy, and dielectric characterization of (1-x)La(Mg1/2Ti1/2)O3-xLa2/3TiO3 solid solutions

Dielectric properties of (1−x)La(Mg1∕2Ti1∕2)O3−xLa2∕3TiO3 [(1−x)LMT-xLT] ceramics (0⩽x⩽0.52) have been studied at radio, microwave, and far infrared (FIR) frequency ranges. The crystal structure sequence in (1−x)LMT-xLT reported by different authors has been analyzed and revised. FIR spectroscopy was used to characterize the lattice contribution to the dielectric response at microwave frequencies. The complex dielectric function was evaluated from the reflectivity data and extrapolated down to a gigahertz range. Compositional variations of the fundamental microwave dielectric parameters estimated by different methods are compared and discussed. The dependence of the quality factor on the composition in LMT-LT is interpreted in terms of the reduction of spatial phonon correlations originated from the increasing amount of La vacancies. This approach could account for the compositional behavior of the dielectric loss commonly observed in a number of microwave mixed systems.

Landau model for the phase diagrams of the orthorhombic rare-earth manganites RMnO3 (R=Eu, Gd, Tb, Dy, Ho)

The present work aims to describe, within a single phenomenological approach, the specific sequence of phase transitions observed in the rare-earth manganites RMnO3 at zero magnetic field. It is shown that a single integrated description of the temperature versus composition phase diagrams of these compounds and related solid solutions can be obtained within the scope of Landau theory by adopting the so called type-II description of the modulated phases.

Comparison Between Two Hydrodynamic Models for Flooding Simulations at River Lima Basin

According to EU flood risks directive, flood hazard maps should include information on hydraulic characteristics of vulnerable locations, i.e. the inundated areas, water depths and velocities. These features can be assessed by the use of advanced hydraulic modelling tools which are presented in this paper based on a case study in the river Lima basin, Portugal. This river includes several flood-prone areas. Ponte Lima town is one of the places of higher flood risk. The upstream dams can lower the flood risks if part of its storage capacity is allocated for mitigating flood events. However, proper management of dam releases and the evaluation of downstream river flows should be considered for preventing flood damages. A hydrological and a one-dimensional hydrodynamic model were implemented, and at a particular flood-prone town, inundation was assessed using a two-dimensional model. The hydrological model is based on the well known Sacramento model. For this purpose, two different modelling implementations were analysed: a model based on a finite element mesh and a model based on rectangular grids. The computational performance of the two modelling implementations is evaluated. Historical flood events were used for model calibration serving as a basis for the establishment of different potential flood scenarios. Intense precipitation events in the river’s basin and operational dam releases are determinant for the occurrence of floods at vulnerable downstream locations. The inundation model based on the unstructured mesh reveals to be more computationally efficient if high spatial resolution is required. A new combination of software tools for floods simulation is presented including an efficient alternative for simulation of 2-D inundation using a finite element mesh instead of a grid.

Bismuth-induced dielectric relaxation in the (1−x)La(Mg1∕2Ti1∕2)O3–xBi(Mg1∕2Ti1∕2)O3 perovskite system

The temperature variation of the dielectric permittivity and loss of the solid solutions (1−x)La(Mg1∕2Ti1∕2)O3–xBi(Mg1∕2Ti1∕2)O3 [(1−x)LMT–xBMT] (0⩽x⩽0.3) measured at radio, microwave, and far infrared frequency ranges has been analyzed in comparison with that observed in other bismuth-containing and bismuth-free perovskite ceramics based on LMT. It has been found that the low temperature dielectric response of the (1−x)LMT–xBMT compositions with x⩾0 is frequency dependent over a wide range from radio to microwave frequencies. The considerable compositional growth of the dielectric permittivity and loss associated with the amount of bismuth in the system was revealed to be not contributed by the lattice polar phonon modes. The effect was suggested to be related to the low-temperature dielectric relaxation process due to a hopping movement of charge carriers in crystallographic A-sites of the perovskite lattice. Particular role of local lattice distortions caused by the anisotropic chemical bonds involving...

Dielectric and Spectroscopic Studies of Betaine Phosphate-Betaine Arsenate Mixed System

Betaine arsenate (BA) and betaine phosphate (BP) are addition compounds of the amino acid betaine [(CH3)3N+CH2COO−] and arsenic and phosphoric acids, respectively. The mixed system (BP)x(BA)1 − x displays, at low temperatures, dielectric anomalies that have been interpreted as due to additional transitions to re-entrant glass phases originated by frustrated FE and AF interactions. In this work, Raman and infrared spectroscopic data are analysed and compared with results of complex dielectric constant, pyroelectric measurements performed during thermal cycles under different bias electric fields and low frequency polarization hysteresis loops. The results obtained for the x = 0.75 composition do not give any evidence for the stabilisation of a re-entrant glass phase or a non-ergodic regime, and a long-range order seems to prevail in the whole temperature range studied. The inclusion of BA is however capable of destabilising the AF order, giving rise to a slightly polar ferroelectric phase at low temperatures. Moreover spectroscopic results reveal that methyl groups of the betaine molecules act as excellent probes for the detailed study of the phase transition sequence in this system.

Low Temperature Behaviour of Betaine Phosphate-Betaine Arsenate Mixed Crystals

The mixed system of betaine arsenate and betaine phosphate (BP)x(BA)1−x displays, at low temperatures, dielectric anomalies that have been tentatively associated to re-entrant glass phases. In this work, we report results obtained for the composition x = 0.52. The temperature dependence of the complex dielectric constant, polarization hysteresis loops and pyroelectric currents reported prove that an effective ergodicity breaking occurs in the antiferroelectric phase and show that this phase is destabilized at low temperatures. There is also evidence for a high internal bias field, in the whole temperature range studied. Infrared and Raman spectroscopy indicate that the various dielectric anomalies observed reflect structural transformations.

Processing and dielectric properties of La(Mg0.5Ti0.5)O3-BaTiO3 ceramics

(1−x)La(Mg0.5Ti0.5)O3-xBaTiO3 (0 ≤ x ≤ 0.7) perovskite ceramics have been prepared by a chemical method and their permittivity and dielectric loss have been studied over a wide frequency range. The far infrared complex dielectric function has been evaluated from reflectivity data and extrapolated towards the microwave range. For the compositions with x = 0 and 0.1, both the measured at microwave range and extrapolated values were found to be in good agreement, revealing that microwave losses are essentially intrinsic and due to lattice contributions. For solid solutions with higher BaTiO3 content, the results suggest that additional contributions are present. The analysis of the temperature dependence of the low frequency permittivity indicates that a possible additional mechanism may be ionic conduction. The comparison of the dielectric behaviour observed in ceramics sintered in oxygen and air suggests that the oxygen vacancies due to the reducing of Ti4+ seems to play an important role in the onset of the ionic conduction.

FIR Reflectivity, Pyroelectric Effect and Dielectric Constant in 8% Brominated BCCD

Betaine calcium chloride dihydrate is known to exhibit a rich number of commensurate and incommensurate phases (k = δ(T) c*) between a room temperature paraelectric phase (Pnma) and a lower temperature ferroelectric phase (Pn2 1 a). Partial bromination of this compound induces important changes in the phase diagram of the material. This work reports recent results of FIR reflectivity, dielectric dispersion and pyroelectric effect measured on 8% brominated BCCD. In agreement with previous Raman results reported by Schaack and coworkers, the FIR reflectivity (E∥b) spectra (20 cm -1 < ω < 300 cm -1 ) show that the sequence of commensurate and incommensurate phases observed in the pure compound is shifted towards lower temperatures and that the structure remains modulated at lower temperatures (modulation wavenumber δ 1/4 at T = 20 K). The temperature and frequency dependence of the dielectric constants e & and e b , as well as the behaviour of the pyroelectric coefficient λb observed along a sequence of thermal cycles with and without a bias field, indicate a progressive deviation from thermal equilibrium and a subsequent freezing of the structure at low temperatures. The experimental results are discussed within the framework of a phenomenological model based on a Landau-type free energy expansion, previously reported to account for the phase transition sequence observed in the pure crystal under a positive hydrostatic pressure.

The product form for Debye relaxations

From a phenomenological point of view, the dielectric dispersion function of a medium can be defined by the location of its zeros and poles in the complex-frequency plane. This general approach led to the use of a factorized form of the dielectric function to fit the experimental infrared reststraheln data. Recently, such an approach has been generalized to the description of relaxation. At low frequency ranges, the factorized form for relaxation may be expressed as a product of Debye relaxors. This product form corresponds to the factorization of the conventional sum model and provides a way to describe the contribution of the different Debye relaxing units in a poly-dispersive system. It also allows us to estimate the importance of the interaction between the different relaxing units. The present work summarizes the fundamentals of the product model for relaxation and describes its application to the analysis of the dispersion observed in the vicinity of the paraelectric to antiferroelectric phase transition in the mixed crystal (betaine phosphate) 0.75 (betainearsenate) 0.25 and at the structural phase transition of betaine potassium iodide dihydrate.