J. de Los S. Guerra

Microwave dielectric dispersion process in perovskite ferroelectric systems

The microwave dielectric dispersion process was investigated in relaxor and normal ferroelectric compositions of Pb1−xLaxTiO3 ceramics. The results revealed the following remarkable features: (i) the mechanism responsible for the microwave dielectric dispersion process in perovskite ferroelectric systems may be associated to the vibration of the boundaries of polar regions, independently of their size and dynamical properties; (ii) the characteristic frequency, fR, is governed by the ratio between the force constant and the effective mass of such boundaries; (iii) the temperature dependence of fR above the temperature of the maximum of the dielectric permittivity directly reflects the ferroelectric-type phase transition.

Investigation of microwave dielectric relaxation process in the antiferroelectric phase of NaNbO3 ceramics

Abstract This letter reports microwave dielectric measurements performed in the antiferroelectric phase of NaNbO 3 ceramics from 100 to 450 K. Remarkable dielectric relaxation was found within the antiferroelectric phase and in the vicinity of the ferroelectric–antiferroelectric phase transition. Such dielectric relaxation process was associated with relaxations of polar nanoregions with strong relaxor-like characteristic. In addition, the microwave dielectric measurements also revealed an unexpected and unusual anomaly in the relaxation strength, which was related to a disruption of the antiferroelectric order induced by a possible AFE–AFE phase transition.

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...

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.

Dielectric Anomalies in La Modified PbTiO3 Ferroelectric Ceramics in the Microwave Frequency Region

Electrically controlled ferroelectric microwave components require well known dielectric properties, which are difficult to achieve in some ferroelectric materials, such as barium or lead titanate. Dielectric response of La modified lead titanate ((Pb, La)TiO3) ferroelectric ceramics was studied using radio frequency (50 × 106 Hz −2 × 109 Hz) and microwave techniques in the temperature range of 300 K–430 K. A dielectric anomaly was obtained in both ferroelectric and paraelectric phases where the relaxation frequency softens at the Curie temperature (T c = 393 K). Applying an uniaxial stress along the field direction a resonant response of the real and imaginary part of the dielectric permittivity is observed, in contrast to the relaxation behavior obtained in the absence of the stress.

Anomaly in the conductivity relaxation parameters at the phase transition of ferroelectric materials: A time domain study

The influence of the ferroelectric to paraelectric transition on the relaxation parameters of conductive processes in ferroelectric materials is studied in the time domain. Three well-known ferroelectric systems were chosen with transition temperatures in different regions, these are, high-temperature PbNb2O6-based ceramics; nanostructured Pb(Zr0.6Ti0.4)O3 ceramics; and submicron BaTiO3. The thermal evolution of relaxation parameters shows clear anomalies in their typical behavior when conductivity processes arise in the temperature range where the ferroelectric transition takes place. The method here described allows obtaining information about the correlation between charge transport and the motion of the off-center ions at the phase transition.

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

Investigation of nonlinear dielectric properties in Sr0.75Ba0.25Nb2O6 relaxor ferroelectric thin films

The ac and dc driving fields dependence of the dielectric permittivity for the strontium barium niobate relaxor ferroelectric thin films has been investigated. The nonlinear dielectric properties were obtained by using the measurements of the dielectric permittivity of the material as a function of the ac and dc “bias” electric field amplitude in wide frequency 100 Hz‐10 MHz and temperature 50‐450 K intervals. The results hint the existence of a true mesoscopic dielectric relaxor response in the ferroelectric thin film, which is very similar to those observed in bulk relaxor ferroelectrics. An anomalous behavior of the NL dielectric response was observed when submitted to moderate dc electric fields levels, indicating a crossover from paraelectric to a glasslike behavior on cooling the sample toward the freezing transition. The obtained results were analyzed within the framework of the models proposed in the current literature.

Dielectric behavior of Pb0.76La0.16(Zr0.53Ti0.47)O3 with diffuse phase transition

PLZT (8/53/47) ceramics were prepared by conventional oxide mixture method. The dielectric properties showed a frequency dependence. The temperature of the maximum dielectric permittivity shifts to higher temperatures and its values decreases with increasing frequency. A fitting of the l vs T curves with a gaussian function gave for the phase transition a diffuseness factor i , (135.6 - 0.3)°C. This result is compared with those of the other PLZT and is discussed in light of a model which try to explain the character of the diffuse phase transition.

Non-linear dielectric response in the Sr(1-x)Ba(x)Nb2O6 relaxor ferroelectric thin film

In this work, the non-linear (NL) dielectric properties of the Sr 0.75 Ba 0.25 Nb 2 O 6 (SBN) ferroelectric thin film prepared by the chemical method were investigated. The NL dielectric properties were obtained by using the measurements of the dielectric permittivity of the material as a function of the DC bias and the AC driving fields in a wide frequency (100 Hz–10 MHz) and temperature (150 K–350 K) range. An anomalous behavior of the non-linear dielectric response was observed when submitted to high electric fields levels. The obtained results were analyzed concerning one of the models for the dielectric response of relaxors ferroelectrics materials currently discussed in the literature.