Polar Microregions Research Articles

Frequency and temperature dependent electrical characteristics of NBT based ceramics prepared by Sol-gel method

The preparation of the individual powders for the sodium bismuth titanate-based composites 0.90NBT + 0.10ST and 0.90NBT + 0.10CT (where NBT = Na0.5Bi0.5TiO3, ST = SrTiO3, and CT = CaTiO3) takes place through a solid state reaction process. Where individual powders are prepared with Sol gel chemical route. Studies using XRD and SEM confirm the formation of composites. Microstructure of the composites shows large grains. Temperature-dependent studies of dielectric and PE loops are conducted. The relative density, grain size, and ST concentration influenced the polarization properties of the specimens. The phase transition in NBT is associated with changes in alignment of dipoles into micropolar regions. For NBCT composite, the ferroelectric and polarization properties are degenerated. Activation energy of the composites was calculated using dielectric parameters.

Elastic and dielectric responses of VDF70/Tr30 near ferroelectric phase transition

The infra-low frequency elastic modulus and the complex dielectric permittivity of vinylidene fluoride trifluoroethylene VDF70/Tr30, are studied within the temperature range of 20 ÷ 140 °C. The experimental results are used to determine the temperature dependence of order parameter η near the Curie temperature (T c). It is revealed that spontaneous polarization P s and spontaneous strain xs are linearly coupled. The dielectric dispersion, due to the relaxation dynamics of polar microregions in the vicinity of the T c, is found. The average relaxation time follows the Arrhenius law with activation energy depending on temperature.

Structure, antiferro/ferro-electric transformation and relaxor behaviour in (1-x)Pb(Yb1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 ceramics

The superlattice structure and dielectric relaxor behaviour were simultaneously observed in (1-x)Pb(Yb1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 ((1-x)PYN-xPMW) ceramics composed by highly ordered end members. Orthorhombic perovskite structure was determined by X-ray diffraction for the compositions of x = 0·0–0·2 and 0·8–1·0 with two sets of superlattice. For intermediate compositions, the superlattice peaks were still preserved with weakening in intensity. Strong dielectric relaxor behaviour seemed to be independent with the superlattice and well fitted by Vogel–Fulcher equation for x = 0·3–0·7. It indicated that the polar microregions in (1-x)PYN-xPMW undergo the dipole glass-like and freezing state analogous to the prototypical relaxor Pb(Mg1/3Nb2/3)O3. Slim ferroelectric hysteresis loops, seemingly made up with ferroelectric and antiferroelectric together, were obtained for the relaxor compositions using huge external field. The antiferroelectric compositions related to superlattice were determined through double ferroelectric polarisation loops. A schematic diagram was sketched to describe the structure transform and generation of relaxor behaviour.

Ferroelectric phase transitions studies in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramics

The ferroelectric phase transitions in 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT 50/50) ceramics, fabricated by a solid state reaction, were studied by using X-Ray diffraction, Raman spectroscopy, and measuring electric polarization, dielectric permittivity and pyroelectric current. X-ray diffraction (XRD) confirms the coexistence of tetragonal (T) and rhombohedral (R) phases at room temperature. The temperature dependence of the Raman modes frequency reveals the existence of two phase transitions corresponding to the rhombohedral – tetragonal, and tetragonal - cubic close to 30 and 100 °C, respectively. The temperature dependence of electric polarization, pyroelectric current, and dielectric permittivity further supports the ferroelectric (tetragonal) to paraelectric (cubic) phase transition. Moreover, the dielectric permittivity reveals the diffuseness of the phase transition and is attributed to the compositional fluctuations of different polar micro-regions.

Enhanced dielectric tunability of Ba0.55Sr0.45TiO3–ZnAl2O4 composite ceramic

(1−x)(Ba,Sr)TiO3–xZnAl2O4 (x=0.00, 0.05, 0.10 and 0.20) composites were prepared by the traditional solid state reaction. The dielectric, tunable and Raman spectroscopy properties are studied. The curie temperature of the composites does not show much difference due to low solubility between BST and ZnAl2O4. For the composition with x=0.20, its tunability is up to 46.4% (@2kV/mm), nearly 1.5 times of that of pure BST, and meanwhile its dielectric constant and loss are decreased. The results imply that making non- or little-soluble BST-nonferroelectric composites is a very effective way to keep high tunability and low dielectric constant simultaneously. Raman results confirm that there are some residual polar microregions in (1−x)BST–xZnAl2O4 composite, and their extrinsic contribution to the dielectric tunability is responsible for a deviation from the ε versus E behavior predicted by Johnson׳s equation.

Effect of sintering temperature on dielectric, piezoelectric and ferroelectric properties of BZT–BCT 50/50 ceramics

Lead free 0.5[Ba(Zr0.2Ti0.8)O3]-0.5[(Ba0.7Ca0.3)TiO3]/[BZT–BCT 50/50] ceramics were synthesized by conventional solid state reaction route. The effect of sintering temperature on structural and electrical properties of BZT–BCT 50/50 ceramics were studied in detail. The coexistence of tetragonal-monoclinic structures confirmed the MPB nature of BZT–BCT 50/50 ceramics. Grain size, density and dielectric constant at room temperature of BZT–BCT 50/50 ceramics increased with the increase of sintering temperature. Increase of Tc with sintering temperature confirmed the effect of relieving of internal stress by grain-boundary sliding. Increase of diffuse phase transition indicated that with the increase in sintering temperature the competition between Ti and Zr ions increased to occupy the same crystallographic site B, which leads to increase in composition fluctuations of different polar micro-regions. A max. strain as high as ∼0.36% has been obtained in BZT–BCT 50/50 ceramic samples sintered at 1300°C.

DIELECTRIC PROPERTIES OF COMPLEX PEROVSKITE PZBT-PMNT CERAMIC UNDER COMPRESSIVE STRESS

Effects of compressive stress on the dielectric properties of complex perovskite PZBT-PMNT ceramic were investigated. The dielectric properties measured under stress-free condition showed a composite nature with two distinct temperatures of dielectric maximum associated with PZBT and PMNT end members. The dielectric properties under the compressive stress were observed at stress levels up to 230 MPa using a home-built compressometer. The results clearly showed that the compression load significantly reduced both the dielectric constant and the dielectric loss tangent in every measuring frequency. The change of the dielectric constant with stress was attributed to competing influences of the intrinsic contribution of non-polar matrix and the extrinsic contributions of re-polarization and growth of micro-polar regions, while the clamping of the domain walls contributed to the stress-dependent changes of the dielectric loss tangent. Finally, a large drop of the dielectric constant after a stress cycle was likely caused by the stress induced decrease in switchable part of spontaneous polarization.

Effects of synthesized method on glassy behavior and freezing process of 0.75PFW–0.25PT ceramics

Two different methods, the conventional direct oxide synthesized method and the two-step indirect synthesized (IS) method, were employed to synthesize the 0.75Pb(Fe2/3W1/3)O3–0.25PbTiO3 ceramics. The low-field dielectric responses were investigated using the empirical law and the Curie–Weiss law. The temperature-dependent local ordering parameters were derived from the experimental results using the spin-glass model and the modified-Landau theory separately. According to the experimental data and the fitting results, it is concluded that the temperature-dependent local order parameters are changed from a short-range-ordered relaxor to a long-range-ordered ferroelectric state using the IS method. On the basis of the physical concepts of the spin-glass model, the modified-Landau theory and the ordering models, it is suggested that local polarizations are affected not only by degree of ordering but also by space charge polarization. In contrast, the glassy behavior of local polarization is affected only by degree of ordering but has nothing to do with space charge polarization. It is because local fields induced by space charge polarization have the same effect on local polarizations and are neutralized when calculating the correlation between neighboring polar microregions. On the other hand, the freezing process of local polarizations is affected by both degree of ordering and space charge polarization because local polarization affected by local fields cannot be neutralized when calculating the individual behavior of local polarization.

Structural, Pyroelectric and Electric Properties of Na1-xLixNbO3 Ceramics

Lead-free ceramics of NaNbO3 - LiNbO3 were prepared by conventional ceramic processing technique. The obtained samples show perovskite structure. Structural, microstructural, dielectric and pyroelectric measurements were made. The remanent polarization depends on the concentration of Li-ions, it increases for x = 0–0.02, and it decreases for x > 0.02. The investigation of the pyroeffect reveals the existence of polar microregions in the temperature range above Tm. The obtained materials are expected to be promising candidate for lead-free electronic ceramics.

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba 0.6Sr 0.4TiO 3 ceramics prepared by the citrate method

Ba 0.6Sr 0.4TiO 3 ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200–1300 °C. Adopting fine Ba 0.6Sr 0.4TiO 3 powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230–1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.

Effects of the sintering temperature on the diffused phase transition and the spin-glassy behavior in Pb0.95La0.05(Fe2/3W1/3)0.65Ti0.35O3 ceramics

In this paper, the effect of the sintering temperature on the low-field dielectric behavior of nonstoichiometric Pb0.95La0.05(Fe2/3W1/3)0.65Ti0.35O3 relaxor ferroelectrics is investigated. The x-ray patterns and the scanning electron microscope images are used to detect the pyrochlore phase and the perovskite structure. The electric properties of the resistivity, the space charge polarization, the temperature-dependent dielectric constant and dielectric loss are discussed. The diffused phase transition and the ordering state are fitted and discussed by using the empirical law and two ordering models. Furthermore, the glassy behavior is determined by using the Curie–Weiss law and the spin-glass model. According to the experimental data and fitting results, the dielectric picture is changed from the short range order relaxorlike behavior to the long range order normal ferroelectric state as increasing the sintering temperature and the glassy behavior is weakened at the lowest and highest sintering temperature at which the pyrochlore phase PWO4 is induced. Therefore, it is suggested that the 1:1 ordered domain is enhanced by increasing the sintering temperature and the glassy behavior is related to not only the ordering degree also the polar defect pairs. For more ordering degree and polar defect pairs, the glassy is weakened and the correlation of neighboring polar microregions is enhanced.

Uniaxial pressure influence on dielectric properties of Pb(Zr0.99Ti0.01)O3 single crystals

The effect of uniaxial pressure (0–1000 bars) applied parallel to AC electric field on dielectric properties of Pb(Zr0.99Ti0.01)O3 single crystals has been investigated. It was found that uniaxial pressure significantly influences these properties. With increasing pressure: (i) peak ϵ(T) decreases, becomes diffuse and shifts to higher temperature, (ii) the thermal hysteresis is reduced, (iii) the hump in ϵ(T) profile connected with antiferroelectric–intermediate phase transition vanishes, and (iv) the local anomaly in ϵ(T) profile connected with polar microregions existence above Tm is protruded and slightly shifts towards higher temperature. It was concluded, that applied uniaxial pressure or increasing Ti-ions content in lead zirconate titanate system induces similar effects. The results were discussed in terms of domain switching under pressure.

Temperature Evolution of the Optical Phonons in Pb(Ni1/3Nb2/3)O3 Single Crystals Studied by Raman Scattering

Raman scattering of lead nickel niobate Pb(Ni 1/3 Nb 2/3 )O 3 (PNN) single crystals grown by the flux method has been performed to investigate the temperature dependence of the optical phonons of relaxors. Anomaly of the temperature dependence of the Raman intensity at 780 cm−1 was observed. Polar microregions promote the growth of volume gradually above the Curie temperature (∼ 170 K), and then freeze around 200 K. From the extrapolation of the temperature dependence of the depolarization ratio, we tried to estimate Burns temperature.

Grain-boundary effects on optical properties in SrTiO3 nanoparticles

The size effects of SrTiO3 nanoparticles are investigated by Raman spectroscopy and UV-Visible absorption spectra with changing grain size in the range of 10—80 nm. It is found that the intensity for the first-order TO2 mode increases significantly as the grain size decreases from 80 to 10 nm. Such behavior suggests the presence of the micropolar regions on the grain boundary induced by the enhanced surface-defect dipoles. Moreover, the first-order TO2 and TO3 modes show red-shifts, indicating the increase of the Ti—O bond-length with decreasing grain size of SrTiO3 nanoparticles. The softening of the Raman mode is consistent with the lattice expansion investigated by XRD. In addition, the UV-Visible absorption spectra indicates the enhancement of absorption due to surface distortion states and hence the lattice expansion, which results in the increase of Eg-value.

Influence of processing parameters on the structure and properties of barium strontium titanate ceramics

Barium strontium titanate (BST) with the molar formula (Ba 0.8Sr 0.2TiO 3) has been prepared by two different processing methods: mixed-oxide (BST-MO) and reaction-sintering (BST-RS). X-ray powder diffraction study shows differences in grain size and crystal symmetry for both these ceramics. The former shows a tetragonal symmetry while the latter presents a cubic symmetry. The occurrence of polar micro-regions associated with the higher chemical non-homogeneous distribution of ion defects from the influence of the processing parameters is the main reason for the higher peak dielectric constant ( K m), the higher remanent polarization ( P r), the higher coercive field ( E c), the higher peak current density ( J m), and the lower temperature of peak dielectric constant ( T m) in BST-MO ceramics.

Central peak and acoustic anomalies of the relaxor ferroelectric lead magnesium niobate-lead titanate single crystal studied by the micro-Brillouin scattering

Quasielastic central peak and acoustic anomalies in a [001] lead magnesium niobate-lead titanate single crystal have been investigated by the high-resolution micro-Brillouin scattering. Two acoustic anomalies were observed at temperatures, TC-T∼415K and TT-R∼348K. The damping showed a significant dispersion at T∼550K, associated with the enhanced dynamics of local strain fields created by the polar microregions. The temperature evolution of the quasielastic central peak exhibited anomalies in the overall intensity and relaxation time that were associated with the line shape interferences of the central peak and the acoustic mode.

Second-harmonic generation and x-ray diffraction studies of the pretransitional region and polar phase in relaxorK(1−x)LixTaO3

Optical second-harmonic-generation(SHG) observations and precise X-ray diffraction experiments have been performed on quantum paraelectrics KTaO3 (KTO) and relaxor K(1-x)LixTaO3 with x=3% (KLT-3) and 7% (KLT-7). It is found in KLT-3 and KLT-7 that a pretransitional region exists between two characteristic temperatures Td and Tp (<Td). The average symmetry of the region is tetragonal with a weak lattice-deformation but non-polar in average. The temperature interval between Td and Tp is consistent with the interval on which neutron diffuse scatterings have been previously reported. These facts strongly suggest that polar micro-regions (PMRs) nucleate around Td and grow toward Tp. Below Tp, a larger deformation and a field-induced SH intensity start to develop, while no significant SHG appear in zero-field cooling process. The temperature dependence of the SH intensity below Tp coincides well with that of the tetragonality determined from the lattice deformation. The Landau-Devonshire phenomenological approach suggests that the ferroelectric phase transition at Tp is of first order and that it approaches the second order transition with the decrease of Li concentration. A marked increase of neutron diffraction intensities below Tp indicates that PMRs are transformed to ferroelectric micro-domains at Tp, and the micro-domains change to macroscopic ones under the electric field below Tp.

The effects of iron as a dopant on the dielectric properties of ferroelectric potassium tantalate niobate (KTaxNb1−xO3)

Presented here is a preliminary study on the effects of iron as a dopant in potassium tantalate niobate (KTaxNb1−xO3) (KTN). Crystals of high purity were grown using a top-seeded isothermal method with starting materials that were purified ensuring that the impurity content was controlled down to less than 10ppm. The study included introducing iron to KTN single crystals of high purity both during crystal growth and through thermal diffusion after growth. Dielectric measurements versus temperature were used to reveal the origins of the ferroelectric phase transition in the KTN:Fe system. Large shifts in the Curie temperature have been shown to be a direct consequence of the presence of iron impurities in KTN. The conventional theories of ferroelectricity do not explain this effect; however, a model is proposed to explain the observed behavior. The model is based on the concept of “polar microregions” proposed previously by Bidault and Maglione [J. Phys. I France 7, 543 (1997)] and others to explain ferroelectricity in potassium tantalate lightly doped with niobium.

Dielectric and ferroelectric properties of Pb(Ni 1/3Nb 2/3)O 3–Pb(Zn 1/3Nb 2/3)O 3–PbTiO 3 ternary ceramics

Ternary ceramics with the type of (1 − x)(0.64PNN–0.36PT)– x(0.91PZN–0.09PT) ( x = 0–0.82) were synthesized by the columbite precursor method. X-ray diffraction results demonstrate that all the samples lie within the morphotropic phase boundary (MPB) region in the pure perovskite structure except for the composition with x = 0.82. The dielectric and ferroelectric properties of compositions with x = 0–0.75 were investigated in detail. All the tested compositions show dielectric behaviors of frequency dispersion and diffuse phase transition. The Ni-rich compositions exhibit superior dielectric and ferroelectric properties to those of Ni-poor ones. The composition with x = 0.15 has the maximum peak dielectric constant (23,410) at 1 kHz and the maximum remnant polarization (14.8 μC/cm 2) with the driving electric field at 10 kV/cm. The variation of peak dielectric constant with x can be explained in terms of the product rate of the polar microregion in the materials.

Dielectric Relaxation Study of Solid Solution of Lead Magnesium Niobate and La-Doped Bismuth Titanate

The diffuse phase transition characteristic of PMN is now believed due to the distribution of ordered and disordered phases on microscopic scales, which can coexist over a broad temperature interval. In this investigation La-doped BT is mixed with PMN to reduce the coupling between polar microregions. Phase analysis exhibits the phase transition of BLT from orthorhombic to cubic phase with lattice parameter ∼20.68 Å. Electric field induced polarization demonstrates the decrease in the saturation polarization and small increase in the remanent polarization. Temperature dependence of dielectric constant reveals the presence of relaxor-like permittivity behaviour in 50–120°C temperature range. BLT addition in PMN led into decrease in the dielectric constant from 20,000 (100 Hz) to 800 (100 Hz). The difference in the temperature of ϵ max between 100 Hz and 100 KHz frequencies was found to increase, which is due to increase in the degree of relaxation.