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Dielectric properties of the BiFe0.5Cr0.5O3 ceramics synthesized under high pressure were investigated in a broad frequency range (20 Hz - 1 GHz) between 200 and 500 K. It was revealed that the ceramics exhibit electrical conductivity above 300 K. Below 300 K, a dielectric dispersion caused by ferroelectric domains was observed. It was found that the conductivity follows the Almond-West law, which allowed us to determine a DC contribution (σDC). From the σDC values obtained at different temperatures, the activation energy (EA = 0.302 ± 0.006 eV) was calculated using the Arrhenius law. [ABSTRACT FROM AUTHOR]

Results of broadband dielectric investigations of 0.94PbMg1/3Ta2/3O3-0.06PbTiO3 (0.94PMT-0.06PT or PMT-PT) crystals in wide temperature range from 100 K to 950 K are presented. Below 300 K the dielectric properties of crystals are governed by polar nanoregions dynamics. However, at higher temperatures (above 600 K) the electrical conductivity effects also become important. The electrical conductivity occurs presumably due the hopping of oxygen vacancies and demonstrates change in the activation energy close to 750 K. The change in the activation energy can be explained by increase of concentration of single ionized vacancies. No anomaly in the temperature dependence of the static dielectric permittivity was observed in wide temperature range from 213 K to 950 K and the dependence was successfully described by spherical random bonds random fields theory. Moreover, two different contributions were successfully separated in the distributions of relaxation times in PMT-PT crystals. These contributions were attributed to different polar nanoregions dynamics. Moreover, the different freezing temperatures values were obtained for most probable and longest relaxation times, which was explained with an idea of continuous distribution of relaxation times broadening on cooling in relaxors. The critical remarks about the Vogel-Fulcher law application for relaxors are addressed in the paper. [ABSTRACT FROM AUTHOR]

The dielectric behavior of the Ba(Ti0.8Ce0.2)O3 ceramics was studied by broadband dielectric spectroscopy in a temperature interval 140–500 K. The broad dielectric relaxation was analyzed by the Havriliak–Negami relationship and the distribution of relaxation times formalism. It was found that the Havriliak–Negami average relaxation time, longest and most probable relaxation times from the distribution of relaxation times follow the Vogel–Fulcher law. The freezing temperature of the longest relaxation time is substantially higher than the freezing temperature of the most probable relaxation time, indicating the distribution of freezing temperatures. The dielectric dispersion has all features inherent to relaxor ferroelectrics and the relaxation is assigned to the dynamics of polar nanoregions. [ABSTRACT FROM AUTHOR]

Results of broadband dielectric/electric investigations of Bi(Mn0.33Nb0.67)O3.1 ceramics are presented in wide frequency range 20 Hz–3 GHz. The electric properties of Bi(Mn0.33Nb0.67)O3.1 ceramics are mainly governed by high conductivity. The relaxation process, related to electrical conductivity, was detected. No polar phase transition was detected in Bi(Mn0.33Nb0.67)O3.1 ceramics by broadband spectroscopy. Bi(Mn0.33Nb0.67)O3.1 ceramics also not exhibit ferroelectric relaxor behavior. [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]

The dielectric behavior of the Ba(Ti0.8Ce0.2)O3 ceramics was studied by broadband dielectric spectroscopy in a temperature interval 140–500 K. The broad dielectric relaxation was analyzed by the Havriliak–Negami relationship and the distribution of relaxation times formalism. It was found that the Havriliak–Negami average relaxation time, longest and most probable relaxation times from the distribution of relaxation times follow the Vogel–Fulcher law. The freezing temperature of the longest relaxation time is substantially higher than the freezing temperature of the most probable relaxation time, indicating the distribution of freezing temperatures. The dielectric dispersion has all features inherent to relaxor ferroelectrics and the relaxation is assigned to the dynamics of polar nanoregions. [ABSTRACT FROM AUTHOR]

Ceramic composites with nanoparticles are intensively investigated due to their unique thermal, mechanic and electromagnetic properties. In this work, dielectric properties of phosphate ceramics with round silver nanoparticles of various sizes were studied in the wide frequency range of 20 Hz-40 GHz for microwave shielding applications. The percolation threshold in ceramics is close to 30 wt.% of Ag nanoparticles content and it is higher for bigger-sized nanoparticles. The microwave complex dielectric permittivity of ceramics above the percolation threshold is rather high (ε' = 10 and ε″ = 10 at 30 GHz for ceramics with 50 wt.% inclusions of 30-50 nm size, it corresponds to almost 61% absorption of 2 mm-thickness plate) therefore these ceramics are suitable for microwave shielding applications. Moreover, the microwave absorption is bigger for ceramics with a larger concentration of fillers. In addition, it was demonstrated that the electrical transport in ceramics is thermally activated above room temperature and the potential barrier is almost independent of the concentration of nanoparticles. At very low temperature, the electrical transport in ceramics can be related to electron tunneling.

Polymer matrix composites filled with carbon nanoparticles are promising materials for many applications, but their properties strongly depend on the particle features, concentration and distribution within the matrix. Here we present a study of the electrical resistivity and the low-frequency voltage fluctuation of composites based on epoxy resin filled with onion-like carbon (OLC) of different sizes (40-250 nm) above the percolation threshold, which should clarify the electrical transport characteristics in these materials. Electrical measurements were performed in the temperature range of 78 to 380 K, and voltage noise analysis was carried out from 10 Hz to 20 kHz. At low temperatures (below 250 K), thermally activated tunneling, variable-range hopping and generation-recombination of charge carriers take place. Above 250 K, the rapid expansion of the matrix with the temperature increases the resistivity, but above ~330 K, the conductivity of the matrix becomes significant. Quasi one-dimensional electrical transport is observed in composites with the smallest particles (40 nm), while in composites with the largest particles (220-250 nm), the dimensionality of the electrical transport is higher. The temperature dependence of the electrical conductivity of composites with smaller particles is more sensitive to matrix expansion.

The dielectric properties of Ba 1- x Ca x TiO 3 ( x = 0.17) ceramics were studied in a wide frequency range of 20 Hz-53 GHz. Diffused ferroelectric phase transition was revealed close to 339 K in the dielectric properties of ceramics. The behaviour of distributions of relaxation times in vicinity of the ferroelectric phase transition temperature is also typical for order-disorder ferroelectric phase transition. However, at lower temperatures (below 200 K), the most probable relaxation increased according to the Arrhenius law. At lower temperatures the maximum of the imaginary part of dielectric permittivity versus temperature strongly shifted to higher temperatures when the frequency increased (from 125 K at 1.21 kHz to 300 K at 33 GHz). This behaviour was attributed to the dynamics of Ti ions. The origin of the crossover from ferroelectric to relaxor behaviour of Ba 1- x Ca x TiO 3 ( x = 0.17) ceramics is discussed in the paper.

The electromagnetic properties of various carbon gels, produced with different bulk densities, were investigated in a wide frequency range (20 Hz-36 GHz). The values of dielectric permittivity and electrical conductivity at 129 Hz were found to be very high, i.e., more than 10 5 and close to 100 S/m, respectively. Both strongly decreased with frequency but remained high in the microwave frequency range (close to 10 and about 0.1 S/m, respectively, at 30 GHz). Moreover, the dielectric permittivity and the electrical conductivity strongly increased with the bulk density of the materials, according to power laws at low frequency. However, the maximum of microwave absorption was observed at lower densities. The DC conductivity slightly decreased on cooling, according to the Arrhenius law. The lower activation energies are typical of carbon gels presenting lower DC electrical conductivities, due to a higher number of defects. High and thermally stable electromagnetic properties of carbon gels, together with other unique properties of these materials, such as lightness and chemical inertness, open possibilities for producing new electromagnetic coatings.