J. Portelles

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part I. Structural and thermo-electrical characterization

With the purpose of fabricating ceramics where ferroelectric and magnetic order coexist, ceramics of Pb(Fe1∕2Nb1∕2)O3 have been prepared using the traditional ceramic method following three different routes. The first is a direct via starting from oxide reagents and the other two use different kinds of FeNbO4 precursors with either monoclinic or orthorhombic structures. Crystallographic and surface morphological studies were carried out by the powder x-ray diffraction and scanning microscopy techniques. The presence of Fe2+, detrimental to the ferroelectric and magnetic performance, was evaluated by x-ray photoelectron spectroscopy. The samples showed no structural differences, uniformly distributed grains, a ferro-paraelectric transition temperature at 110°C and a normal diffuse phase transition (nonrelaxor behavior). Differences in the degree of diffuseness, densities and grain size were observed depending on the kind of precursor. Measurements of dc and ac electrical resistivity, dielectric constant an...

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. Part II. Impedance spectroscopy characterization

The dielectric behavior of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained using the traditional ceramic method employing three different precursors was investigated by impedance spectroscopy in the temperature range of 200–300°C. This study was carried out by means of the simultaneous analysis of the complex impedance Z, electric modulus M, and admittance Y functions from the measurements in the frequency range of 20Hz–1MHz. In correspondence to a previous structural, morphological, and temperature response study, appropriate microstructural and equivalent circuit models were established. Based on the brick layer model, three series of interconnected electrically distinct regions are considered: a conductive grain boundary layer, a capacitive grain boundary surface layer, and a resistive-ferroelectric bulk layer. Two conduction mechanisms were identified: a dielectric relaxation process due to localized conduction associated with the presence of oxygen vacancies and the nonlocalized conduction...

Effect of Nb Doping on (Sr,Ba)TiO3 (BST) Ceramic Samples

The effect of doping the Sr0.3Ba0.7Ti(1−5y/4)NbyO3 ceramic with different concentration of Nb is studied by scanning electron microscopy (SEM), X-ray diffraction and thermoelectric analysis. It is observed that the grain size decreases as the Nb concentration increases. The critical temperature Tc has a linear decrease at a rate of 19°C/mol% of Nb. The temperature dependence of the dielectric permittivity presents strongly broadened curves, which suggest a non Curie-Weiss behavior near the transition temperature. The diffuse phase transition coefficient (δ) was also determined and its value leads to the conclusion that the degree of disorder in the system increases with the presence of the Nb cation.

Frequency-temperature response of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors. III. Dielectric relaxation near the transition temperature

Dielectric relaxation processes occurring near the ferroelectric-paraelectric phase transition of ferroelectromagnetic Pb(Fe1∕2Nb1∕2)O3 ceramics obtained by different precursors are discussed using microstructural and equivalent circuit modeling and the impedance spectroscopy technique. The frequency-temperature response was obtained from room temperature to 300°C and from 20Hzto1Mz. In correspondence with a previous structural, morphological, and temperature response study, appropriate microstructural, and equivalent circuit models were established. The frequency response study was carried out by means of the simultaneous analysis of the complex dielectric constant e and admittance Y functions and the dielectric loss, tanδ. A strong absorption near the transition temperature region at a frequency around 1MHz is discussed and is attributed with relaxation processes associated with domain reorientation, domain wall (DW) motion, and the dipolar behavior of ferroelectric materials. Such processes were foun...

Effects of two kinds of FeNbO4 precursors in the obtainment and dielectric properties of PFN ceramics

We report here a study of the obtainment, structural and dielectric characterization of the monoclinic and orthorhombic phases of FeNbO4 precursors and their effects in the processing and the dielectric properties of Pb(Fe1/2Nb1/2)O3 ceramics. Crystallographic and surface morphological studies were carried out by powder X-ray diffraction and scanning electron microscopy techniques. For structural determination, runs of Rietveld refinement were realized employing FULLPROF software. Measurements of electrical resistivity, dielectric constant and dielectric loss as functions of temperature were realized in PFN ceramics. An analysis and a comparative discussion of results are presented.

ac conductivity in Gd doped Pb(Zr0.53Ti0.47)O3 ceramics

This study is focused in the conduction processes taking place in 0.6 wt % Gd doped lead zirconate titanate samples PbZr0.53Ti0.47O3:Gd (PZT53/47:Gd) in the vicinity of the morphotropic phase boundary. Doped samples show very large dielectric permittivity with respect to that of undoped ones near the transition temperature. The frequency dependent ac conductivity of PZT53/47:Gd ceramics was studied in the 30–450 °C temperature range. X-ray diffraction analyses indicate the incorporation of Gd atoms to the structure. The changes in the dielectric properties as functions of temperature of the doped samples are taken as additional evidence of the incorporation of Gd into the crystal structure. Gd acts as donor center promoting extrinsic n-type conduction. The ac conductivity behavior obeys Jonscher universal relation in the 100 Hz–1 MHz frequency range for temperatures between 30 and 300 °C. The measured conductivity values for Gd doped PZT53/47 are higher than those of pure PZT53/47. According to the correl...

Dielectric anomaly and low frequency dispersion in ferroelectric materials at high temperatures

A dielectric anomaly related to the observed departure from the Curie–Weiss law in several Aferroelectric materials has been characterized in this work in ferroelectric BaTiO3. The electrical properties of the material were investigated using the a.c. analysis technique. From the analysis of the permittivity versus frequency curves collected at various temperatures, we discuss the anomalous behaviour as a result of a low frequency dispersion phenomenon due to the characteristics of the conduction processes at the material inhomogeneities.

Evidence of magnetodielectric coupling in multiferroic Pb"Fe0.5Nb0.5…O3 ceramics from ferroelectric measurements and electron paramagnetic resonance

Magnetodielectric coupling effects of single phase multiferroic Pb(Fe0.5Nb0.5)O3 ceramics have been studied using standard ferroelectric measurements and electron paramagnetic resonance (EPR). Changes in the electrical polarization behavior were observed in the paramagnetic to weakly magnetized antiferromagnetic transition near 103 K associated with a rhombohedral to monoclinic symmetry reduction. The analysis of changes in the EPR spectral parameters confirms the transition from paramagnetic to weakly magnetized antiferromagnetic and reveals noticeable anomalies in the high temperature region near the ferroelectric-paraelectric transition (383 K), which are correlated with the tetragonal-to-cubic symmetry change and the characteristic diffuse phase transition of this material.

Dielectric properties of the La3+ doped Sr0.3-3y/2LayBa0.7Nb2O6 ceramic system

Abstract A process for obtaining Sr 0.3− 3y 2 La y Ba 0.7 Nb 2 O 6 (LSBN) ceramics at different concentrations of the La3+ cation and different sintering times is presented. The dielectric properties of the LSBN system where Sr2+ ions are substituted by La3+ cations follow the pattern of a modification by cationic substitution for a constant Sr Ba ratio. This behavior of the dielectric properties indicate that the La3+ cations occupy the A1 as well as the A2 sites which favors the formation of the LSBN system up to y

Effect of La3+ doping on the polarization of the LSBN ceramic system

The effect of the lanthanum cation as a dopant in the polarization of the Sr0.3−3y2LayBa0.7Nb2O6 system where y = 0.01, 0.03 and 0.05 is studied. The behavior of the spontaneous polarization and the pyroelectric coefficient obtained by means of the direct integration technique, indicates us that the lanthanum produces an increment of the polarization in the material for values of the composition up to 3%. As the dopant concentration increases toward 5% these properties start to decrease due essentially to a solubility saturation of the substituted ions in the SBN solid solution. This behavior is corroborated through dielectric hysteresis measurements using a modified Sawyer-Tower circuit.