Search

Interfaces are a major issue when designing ferroelectric nanostructured materials with tailored properties. In a context of integration and multifunctionality in the field of electronics, several strategies have been developed to control the microstructure and defect chemistry of interfaces that strongly impact the macroscopic properties. The suitability of the core-shell approaches that allow a subtle tuning of interface phenomena at different scales has been widely demonstrated. We focus here on the flexibility of the core-shell approach devoted to the processing of nanostructured ferroelectric composites. Our strategy relies on the use of advanced synthesis processes to design ferroelectric grains coated with shells of different nature, morphology and crystallinity. Typical examples will be reviewed with a specific attention on their impact on both microstructure and dielectric properties. Our approach, based also on the use of fast sintering technique, provides a guidance to design 3D bulk nanostructured ferroelectrics while controlling and/or exploiting size, interface and defects chemistry. The contribution of specific spectroscopies to probe interfacial chemistry and defects is underlined. The high density of interfaces in core-shell materials is obviously an advantage to target additional functionality such as magneto-electric coupling. This is illustrated in 3D composites and one dimensional nanostructures that coaxially combine electric and magnetic materials. The core-shell approach described here could be transferred to a much broader range of materials covering many functionalities provided a deeper understanding of the interfaces at the atomic scale is achieved and a further development of low temperature processing is reached. [ABSTRACT FROM AUTHOR]

Dielectric response of Ba2NdFeNb4-xTaxO15(x = 0.3, 0.6 and 2) solid solutions with Tetragonal Tungsten Bronze structure has been investigated by means of broadband dielectric spectroscopy in the frequency range from 20 Hz to 37 GHz. Pure Ba2NdFeNb4O15compound displays ferroelectric behavior. Substitution of Nb5+by Ta5+considerably alters dielectric response of this system. Ferroelectric state was observed only for x = 0.3 compound with slightly lowered ferroelectric phase transition temperature. Further increase of x causes formation of relaxor state in these solid solutions and considerably lowers corresponding dielectric permittivity dispersion temperature region. [ABSTRACT FROM AUTHOR]

Lattice structure transformations in nanopowders of ferroelectric BaTiO3and BaTiO3@SiO2core-shell nanostructured ceramics were studied by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) at the temperatures 120–450 K and particle size of 300 and 500 nm. NMR spectra of all studied samples in the paraelectric phase are identical to the spectra in bulk material indicating their perfect perovskite structure without visible influence of particle surface. However, we have found that surface of particles essentially influence the ferroelectric phase transitions detected by both NMR and EPR techniques. The strongest changes as compared to bulk material were observed in BaTiO3@SiO2core-shell ceramics. Thorough analysis of NMR spectra suggests that the orthorhombic-like symmetry phase coexists here with other polar phases up to the Curie temperature. Depending on temperature, its relative volume varies from 25% to 100%. We assume that the orthorhombic-like symmetry phase is stabilized by anisotropic components of surface stresses which increase also global stability of polar state in nanoceramics to the temperature in bulk material. We summarize our results in a phase diagram. [ABSTRACT FROM AUTHOR]

New lead free ceramics derived from the ferroelectric material BaTiO3 by substituting Ti4 + by Nb5 + and Yb3 +. The dielectric and ferroelectric properties were studied. The compositions BaTi1 - x(Nb0.5Yb0.5)xO3 display a classical ferroelectric (0 ≤ × < 0.1) and relaxor ferroelectric behaviour (0.1 ≤ × ≤ 0.2). BaTi0.9(Nb0.5Yb0.5)0.1O3 specimen present a relaxor behaviour at a temperature close to room with high permittivity (ε'r = 9500 at 1 kHz), which opens the way for possible use of it for capacitor applications. The polarisation hysteresis loops were observed of relaxor BaTi0.875(Nb0.5Yb0.5)0.125O3 specimen. The ceramic BaTi0.9(Yb0.5Nb0.5)0.1O3 provides a piezoelectric constant d31 of about 11.17 pC/N near room temperature. [ABSTRACT FROM AUTHOR]

This paper reports the structural and dielectric properties of Ba1 - xCaxTi1 - y (Zn1/3Nb2/3)yO3 ceramics with y is fixed to 0.2 and x = 0.2 and 0.3. Single-phase solid solutions of the simples were determined by X-ray diffraction. For the solid solution, BaTi1 - y(Zn1/3Nb2/3)yO3, the evolution of dielectric behaviour from a sharp ferroelectric peak (for y < 0.075) and to ferroelectric relaxor (for 0.075 < y < 0.2) was observed with increasing of Zn and Nb concentration but in this case the temperature of the maximum of dielectric permittivity Tm and Δ Tm are lowers. To modulate this relaxor character we have substitute the Ba by Ca to increase Tm and Δ Tm. [ABSTRACT FROM AUTHOR]

The dielectric permittivity as a function of temperature at frequencies from 10 -2 to 10 5 Hz is studied and parameterized in the perovskite Ba(Ti 0.675 Zr 0.325 )O 3 ceramics. The features characteristic of relaxor ferroelectrics are found, namely, the Lorenz-type temperature dependence at high-temperature slope of the real permittivity peak, the Curie-Weiss law observed at temperatures much higher than the temperature of the peak, the Vogel–Fulcher scaling of the temperature/frequency dependencies of the permittivity. However, scaling characteristics differ from those in classical complex perovskite relaxors suggesting different origin of dielectric relaxation. [ABSTRACT FROM AUTHOR]

Focuses on the dielectric properties of Na0.5Bi0.5TiO3 – BaTiO3 ceramics. Evaporation of lead oxides during the sintering of such ceramics; Influence of the formation of solid solutions to the improvement and control of the electrical properties of such materials; Identification of a relaxor like behavior upon increased barium content.

The theory of dc-electric field nonlinear susceptibility in the doped incipient ferro- electrics is developed in a random field approach. The calculations of the susceptibil- ity χ(E, T) had shown that the coefficients χnl(k))(T, n) in power serie of susceptibility χ(E, T) = ∑ χnl(k)E2k (k = 0 and k = 1, 2, 3 … correspond respectively to linear and nonlinear susceptibility of the first, second etc. order) diverge at the same temperature T = Tm that depends on the impurity dipoles concentrations. The critical indexes of the divergency for the concentration of the dipoles which correspond to mixed ferroglass phase with coexistence of short- and long-range polar order were shown to be the same as those calculated in mean field approximation. The comparison of the theory with experimental data obtained for KTa1-xNbxO3 (0.0075 ≤ x ≤ 0.05) is performed. The calculations describe the main peculiarities of temperature, electric field and concentrational dependence of observed nonlinear dielectric susceptibility. Since the temperature Tm corresponds to the transition from dipole glass to ferroglass phase the transition is proven to be equilibrium one in the disordered ferroelectrics. [ABSTRACT FROM AUTHOR]

Non-linear dielectrics which display electric-field dependent dielectric properties can be used in microwave devices such as tunable resonators or phase shifters. The required performances are essentially moderate permitivity (100-1000), low dielectric losses (≤1%) in the GHz frequency range, good thermal stability and high tunability of the dielectric susceptibility (>10%). The mixed perovskite Ba 0.6 Sr 0.4 TiO 3 [BST] is an attractive candidate as its ferroelectric transition occurs close to room temperature and it displays high tunability but also strong temperature sensitivity and high frequency dielectric losses. To overcome these two deficiencies, ceramic/ceramic composites based on BST and a non-ferroelectric oxide such as MgO, MgTiO 3 , SnO 2 and SiO 2 were synthesized and compared to BST. For a better control of the losses a new concept based on a core-shell principle is presented. Through soft chemistry routes, a core made of BST grains is coated by a simple binary oxide shell. Modulation of the dielectric properties is expected by changing the core-shell structure. High frequency experiments are underway in order to check the GHz losses of these composites. [ABSTRACT FROM AUTHOR]

This paper examines the validity of self-report of drug use in several important high-risk samples by comparing survey answers with urinalysis results. The analyses include data obtained from face-to-face interviews with 1536 patients in sexually transmitted disease (STD) clinics, 1564 patients in hospital emergency rooms (ERs), and 2034 recent arrestees in jails. Using urinalysis as the criterion measure, considerable underreporting was observed, particularly among STD and ER subjects using cocaine and opiates. A logistic regression analysis on factors related to the underreporting was conducted for both cocaine and opiates. The results showed that a lower degree of underreporting was associated with subjects at jail sites (compared to STD subjects) and those who self-reported lifetime drug dependence. [ABSTRACT FROM AUTHOR]