F. Guerrero

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...

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.

Sintering characteristics of the LSBN ceramics and influence of the lanthanum content

Abstract SBN-30 ceramics doped stoichiometrically with 1, 3 and 5% of La, i.e. the Sr 0.3 − 3y 2 LayBa0.7Nb2O6 system (LSBN) were obtained. The influence of La content and the sintering characteristics were studied by X-ray diffraction (XRD) and thermo-electric analysis (TEA). When the La concentration is 0.05, various reflections XRD shifted to higher angles. The density increases linearly with Ln t, where t is the sintering time and the values of the maximum of permittivity curves rose monotonously with t for 1 and 3%, with a strong dependence on the porosity. The dynamics, diffusion of the porous respond to a mechanism of D4 type with kD = 0.0017 and 0.0016 for 0.01 and 0.03, respectively. For 0.05 a lower decrease of the porosity with sintering time was found, and the maximum values of the permittivity remained constant. The dielectric properties of the ceramic start to deteriorate in this case.

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 La3+ Doped Sr0.3–3y/2Lay Ba0.7Nb2O6 Ceramics Prepared under Different Sintering Conditions*

A study of the dielectric properties of the Lanthanum doped Sr0.3Ba0.7Nb2O6 (SBN30) ceramic according to the stoichiometric formulation Sr0.3–3y/2LayBa0.7Nb2O6 with y = 0.01, 0.03 and 0.05, and the influence of the sintering conditions is reported. The XRD shows single phase compounds for Sr0.285La0.01Ba0.7Nb2O6 (LSBN1) and Sr0.225La0.03Ba0.7Nb2O6 (LSBN3) ceramics, both samples having similar microstructure, densification and dielectric properties. The density increases linearly with ln t, where t is the sintering time, and the values of the maximum ferroelectric peaks of the permittivity increase steadily with t. Using the Bruggeman model to estimate the theoretical permittivity, it is concluded that the magnitude of the experimental permittivity peaks are mainly affected by the volume fraction of porosity of the samples. In this study we also establish that pore diffusion mechanisms behave according to the Ginstling-Brownshtein equation. For the Sr0.225La0.05Ba0.7Nb2O6 (LSBN5) sample, XRD analysis reveals the presence of isostructural compounds of the intermediate phases BaNb2O6 and SrNb2O6, and the dielectric properties start to deteriorate. This fact indicates the existence of a solubility limit of Lanthanum ions in the SBN solid solution.

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

Ferroelectric properties of the La0.03Sr0.255Ba0.7Nb2-yTiyO6-y/2 ceramic system

The study of the dielectric properties of the stoichiometric Titanium doped Sr0.255La0.03Ba0.7Nb2O6 ceramics prepared according to the formula Sr0.225La0.03Ba0.7Nb2–yTiyO6–y/2 is reported. A single phase compound is observed for low Ti content (y<0.1) in the XRD spectra, being isostructural with the SBN phase. For high Ti concentrations (y<0.1), the XRD patterns show, besides the SBN phase reflections, several small peaks associated with an additional phase present in the ceramics. A possible liquid phase sintering is analyzed. Strongly broadened dielectric curves are obtained in dielectric measurements, where the transition temperature decreases with the Titanium content. The diffuse phase transition coefficients are calculated, corroborating that Titanium increases the diffuse character of the transition in the monophasic region.

Evidence of short-range antiferroelectric local states in lanthanum- and titanium-modified Sr0.3Ba0.7Nb2O6 ferroelectric ceramics

Structural and electrical measurements were performed on lanthanum- and titanium-modified Sr0.3Ba0.7Nb2O6 ferroelectric (FE) ceramics. Dielectric properties show a well-defined relaxor anomaly around −50 °C with double-loop hysteresis curves from this point to the transition temperature. The thesis of short-range antiferroelectric (AFE) local states having incommensurate (INC) structure embedded in the FE polar matrix is proposed to explain the observed behavior. Structural lattice parameters show negative expansion coefficients and INC superlattice diffraction spots were observed through the selected-area diffraction patterns in the [110] zone for this minority local states. The titanium modification of this system seems to be the key fact in the coupling of the INC structure to the AFE local states in the FE–AFE coexistence region.