Dielectric Investigations of 0.945(Bi0.5Na0.5) TiO3-0.055BaTiO3

Dielectric properties of 0.945(Bi_0.5Na_0.5) TiO₃-0.055BaTiO₃(BNBT5.5) (a composition close to the rhombohedral- tetragonal morphotropic phase boundary) ceramics are studied. It is shown that BNBT5.5 is a relaxor with a characteristic relaxation time that follows Vogel-Fulcher's law. The following Vogel-Fulcher parameters of the relaxation time were calculated: pi0 = (2.0 plusmn 2.4)middot10^-14 s, E/^kB = (1620 plusmn 270) K, T₀ = (262 plusmn 9) K.     

Dielectric properties of cubic bismuth based pyrochlores containing lithium and fluorine

In this work dielectric properties of Bi_1.5Zn_1?xLi_xNb_1.5O_7?xF_x with x = 0.25 were investigated in a 20 Hz–12 GHz frequency and 120–500 K temperature range and compared to that of regular cubic BZN (when x = 0). Measurements showed that both ceramics have dipolar glass type dielectric dispersion with wide relaxation time distributions. Mean relaxation time follows Arrhenius law in the investigated frequency range, although Vogel–Fulcher law was anticipated.     

Application of remote sensing in studies of water quality in the Amazon Region

This paper deals with some basic matters concerning the use of remote sensing techniques related to limnological/water quality surveys in the Amazon Region. The main conclusions are: (i) it is important to use these techniques for detecting problems before the construction of new projects involving water resources in the Amazon Region; and (ii) remote sensing focused on water quality should be seriously considered in planning Brazil's space program as a means of providing data for more effective water basin management.     

Conductivity of nanostructured mesoporous MCM-41 molecular sieve materials

MCM-41 molecular sieve materials have been investigated by means of impedance spectroscopy. Dielectric response of water confined in MCM-41 molecular sieve materials showed a power law dispersion, which was assigned to ac conductivity. This process, which occurs at temperatures close to room temperature, was found in all our investigated samples with the pore diameters ranging from 2.0 to 3.7 nm. Obtained values of the imaginary part of the complex dielectric permittivity were much larger than that of the bulk water. Such a phenomenon can be explained by the electrical response characteristic of the microstructural electrical network formed by water and hosting mesoporous silica material. The real part of conductivity has been calculated, and the influence of the pores size to electric conductivity and its mechanisms are discussed.     

Electrical properties of niobium doped barium bismuth-titanate ceramics

BaBi₄Ti_4–5/4xNb_xO₁₅ (BBNTx, x = 0, 0.05, 0.15, 0.30) ceramics have been prepared by solid state method. XRD data indicate the formation of single-phase-layered perovskites for all compositions. SEM micrographs suggest that the grain size decreases with Nb doping. The effect of niobium doping on the dielectric and relaxor behavior of BaBi₄Ti₄O₁₅ ceramics was investigated in a wide range of temperatures (20–777 °C) and frequencies (1.21 kHz to 1 MHz). Nb doping influences T_c decrease as well as the decrease of dielectric permittivity at Curie temperature. At room temperature, undoped BaBi₄Ti₄O₁₅ exhibits dielectric constant of ～204 at 100 kHz, that slightly increases with Nb doping. The conductivity of BBNT5 ceramics is found to be lower than that of other investigated compositions. The value of activation energy of σ_DC was found to be 0.89 eV, 1.01 eV, 0.93 eV and 0.71 eV for BBT, BBNT5, BBNT15 and BBNT30, respectively.     

Piezoelectric and elastic properties of layered materials of Cu(In,Cr)P2(S,Se)6 system

In this contribution, we present new results of ultrasonic and piezoelectric investigation in layered materials of Cu(In,Cr)P₂(S,Se)₆ system including the phase transitions points. It was shown, that in low temperature phase the electromechanical coupling parameters substantially increased after polarization of the material. The large anisotropy of linear and nonlinear elastic properties was observed. The largest critical ultrasonic attenuation anomalies appeared for ultrasonic wave propagating normal to layers: i.e. along polarization direction. From measurements of the amplitude of second longitudinal ultrasonic harmonic we showed that these layered crystals have extremely high elastic nonlinearity in the direction normal to layers. DC bias electric field induced piezoelectricity due to electrostiction was observed in the paraelectric phase of pure materials and their solid solutions. The piezoelectric memory was observed after samples were kept for long time in DC electric field along polar axis at fixed temperature in the paraelectric phase.     

Synergy effects in dielectric and thermal properties of layered ethylene vinyl acetate composites with carbon and Fe3O4 nanoparticles

Properties of layered composites with carbon nanotube (CNT), GnP, Fe₃O₄ nanoinclusions, and hybrid composites with these nanoinclusions were investigated in the wide frequency range (from 20 Hz to 2 THz). All investigated composites (except with single Fe₃O₄ inclusions) are above percolation threshold. The strong enhancement of microwave and terahertz radiation absorption is observed in hybrid composites in comparison with composites with single inclusions. The synergy effect is discussed in terms of the complex impedance and the distribution of relaxation times. At low temperatures (below 200 K), the electrical conductivity of all hybrid composites follows the tunneling law model and the lowest potential barrier was estimated for composites with CNT inclusions. The best thermal properties are observed for composites with CNT inclusions. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48814.     

Broadband dielectric spectroscopy of PbMg1/3Nb2/3O3–PbSc1/2Nb1/2O3 ceramics

Broadband dielectric spectroscopy results of various ordered and disordered (1 ? x)Pb(Mg_1/3Nb_2/3)O₃–(x)Pb(Sc_1/2Nb_1/2)O₃ (PMN–PSN) ceramics are investigated in the temperature range from 80 K to 300 K and frequency range from 20 Hz to 2 THz. Dielectric dispersion is very broad and in the ferroelectrics case (x = 1, 0.95) consists of two parts: low-frequency part caused by ferroelectric domains and higher frequency part caused by soft mode. The relaxational soft mode exhibits pronounced softening close to phase transition temperature, as it is typical for order–disorder phase transitions. By substituting Sc³⁺ by Mg²⁺ in PMN–PSN ceramics relaxation slows down, and for relaxors (x = 0.2) the most probable relaxation frequency decreases on cooling according to Vogel–Fulcher law.     

Ultrasonic investigation of elastic phase transitions in betaine compounds

The results of ultrasonic velocity and attenuation measurements in deuterated betaine phosphite (DBPI) and its compounds near the antiferrodistortive phase transition (PT) are presented for X, Y and Z crystallographic directions. For longitudinal ultrasonic modes propagating along X and Z axes in DBPI the critical slowing down in ultrasonic velocity and attenuation maxima have been observed at PT. Instead, we have not found the critical elastic anomalies for longitudinal Y mode. In DBP crystals the elastic anomalies appeared at PT for all X, Y and Z modes. In solid solution DBP/DBPI the orientation dependencies of elastic anomalies varied from those of DBPI to those of DBP. The phase diagram has been obtained from ultrasonic data. The experimental data in the antiferrodistortive phase have been interpreted in terms of the mean field Landau theory. It is shown that the phase transitions are very close to the tricritical ones.