Conductivity Relaxation Frequency Research Articles

Immittance Studies of Bi6Fe2Ti3O18 Ceramics.

Results of studies focusing on the electric behavior of Bi6Fe2Ti3O18 (BFTO) ceramics are reported. BFTO ceramics were fabricated by solid state reaction methods. The simple oxides Bi2O3, TiO2, and Fe2O3 were used as starting materials. Immittance spectroscopy was chosen as a method to characterize electric and dielectric properties of polycrystalline ceramics. The experimental data were measured in the frequency range Δν = (10−1–107) Hz and the temperature range ΔT = (−120–200) °C. Analysis of immittance data was performed in terms of complex impedance, electric modulus function, and conductivity. The activation energy corresponding to a non-Debye type of relaxation was found to be EA = 0.573 eV, whereas the activation energy of conductivity relaxation frequency was found to be EA = 0.570 eV. An assumption of a hopping conductivity mechanism for BFTO ceramics was studied by ‘universal’ Jonscher’s law. A value of the exponents was found to be within the “Jonscher’s range” (0.54 ≤ n ≤ 0.72). The dc-conductivity was extracted from the measurements. Activation energy for dc-conductivity was calculated to be EDC = 0.78 eV, whereas the dc hopping activation energy was found to be EH = 0.63 eV. The obtained results were discussed in terms of the jump relaxation model.

Crystal structure refinement, dielectric and magnetic properties of Sm modified BiFeO3 multiferroic

Bi1−xSmxFeO3 multiferroics (x=0.0, 0.05 and 0.1 having code BF0/SM0, SM5 and SM10 respectively) were synthesized via stage solid-state reaction method and their detailed structural, magnetic and impedance measurements are performed. X-ray diffraction profile and Rietveld analysis indicates that the crystal structure is rhombohedral (R3c). Due to Sm doping, the weak ferromagnetism in BiFeO3 ceramics may be attributed to the improved crystallinity and non presence of secondary phases. Bi0.9Sm0.1FeO3 ceramic exhibited the highest value of remnant magnetization and coercive field. The values of dielectric constant and dielectric loss of SM5 and SM10 samples showed dispersion at low frequency region. The temperature dependence of electric modulus at different frequencies shows the presence of well defined peaks and thus gives the temperature at which the measuring frequency is equal to conductivity relaxation frequency. The typical features of jump relaxation model are obeyed in ac conductivity analysis. Frequency dependence of the conductivity follows the universal power law.

Appearance of a Debye process at the conductivity relaxation frequency of a viscous liquid

The existence of a Debye-type ultraslow process in dielectric spectra of bulk polyalcohols and similar materials has been reported repeatedly in the recent literature. Its loss peak is observed at frequencies that are decades below those of the primary structural relaxation, in a range where the loss signal is usually dominated by dc-conductivity or even electrode polarization. We show that this peak originates from an incomplete filling of the capacitor volume, e.g., as a result of gas bubbles, a situation that gives rise to a Debye process at the conductivity relaxation frequency of the material, where the values of storage and loss components of permittivity are identical. The result implies that these peaks are not endemic to the liquid and can lead to various misinterpretations of the dielectric relaxation spectra. Techniques avoiding the occurrence of such artifacts are discussed.

Electrical conductivity and relaxation in mixed alkali tellurite glasses

The authors have reported the electrical conductivity and the conductivity relaxation in mixed alkali tellurite glasses of compositions of 70TeO2-xNa2O-(30-x)Li2O in the frequency range from 10 Hz to 2 MHz and in the temperature range from room temperature to just below the glass transition temperature. They have analyzed the relaxation data in the framework of different models. They have observed the mixed alkali effect in the dc and ac conductivities, the crossover frequency, and the conductivity relaxation frequency as well as in their respective activation energies in these glasses. They have also observed the mixed alkali effect in the decoupling index. The scaling property of the modulus spectra of these mixed alkali glasses shows that the conductivity relaxation in the mixed alkali tellurite glasses is independent of temperature but depends on the glass compositions.

Relaxation in mixed alkali fluoride glasses

We have investigated the dynamics of alkali cations as well as fluorine anions in non-oxide fluoride glasses with a total alkali fluoride content varying from 16 to 35mol% in the frequency range from 10Hz to 2MHz and in the temperature range from room temperature to just below the glass transition temperature. We have shown that the dc and ac conductivity, crossover frequency and conductivity relaxation frequency exhibit a minimum in mixed alkali fluoride glasses, where anions also participate in the diffusion processes in addition to cations, unlike mixed alkali oxide glasses and crystals. We have observed lower dimensionality of the conduction pathways in mixed alkali fluoride glasses compared to that in the single alkali glasses. We have shown that the relaxation dynamics in mixed alkali fluoride glasses is independent of temperature.

Effect of alkaline-earth ions on the dynamics of alkali ions in bismuthate glasses

The effect of alkaline earth ions on the dynamics of ${\mathrm{Li}}^{+}$ ions in bismuthate glasses has been studied in the temperature range $353--503\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in the frequency range $10\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}--2\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The dc conductivity increases and activation energy decreases with the increase of a particular alkaline earth content for the glasses with a fixed alkali content. The increased modification of the network due to the increase in alkaline earth content in the compositions is responsible for the increasing conductivity. Also the compositions with smaller alkaline earth ions were found to exhibit higher conductivity. Although the conductivity increases with the decrease of ionic radii of alkaline earth ions, the activation energy shows a maximum for the Sr ion. The electric modulus and the conductivity formalisms have been employed to study the relaxation dynamics of charge carriers in these glasses. The alkali ions were observed to change their dynamics with the change of the alkaline earth ions. The same anomalous trend for activation energy for the conductivity relaxation frequency and the hopping frequency was also observed for glasses containing SrO. It was also observed that the mobile lithium ion concentrations are independent of nature of alkaline earth ions in these glasses.

Ion dynamics and mixed mobile ion effect in fluoride glasses

We report the ionic relaxation and mixed mobile ion effect in 50ZrF4−10BaF2−10YF3−(30−x)LiF−xNaF fluoride glass series, where fluorine anions participate in the diffusion process in addition to alkali cations, unlike mixed alkali oxide glasses and crystals. By analyzing the ion dynamics in the framework of a power-law model as well as modulus formalism we have observed mixed mobile ion effect in the dc conductivity and its activation energy, the crossover frequency and its activation energy, the conductivity relaxation frequency and its activation energy, and also in the decoupling index. We have correlated these phenomena with the fractal dimension of the conduction pathways in the mixed alkali fluoride glasses compared to the single alkali glasses. We have shown that the relaxation dynamics in mixed alkali fluoride glasses is independent of temperature but dependent on glass composition.

Mixed mobile ion effect in fluorozincate glasses

The mixed mobile ion effect has been investigated for the first time in zinc fluoride glasseswhere in addition to alkali cations fluorine anions also participate in the diffusion process,unlike mixed alkali oxide glasses. The minimum in the conductivity, conductivity relaxationfrequency, crossover frequency and decoupling index indicates the existence of the mixedmobile ion effect in these fluoride glasses. It has been observed that the non-exponentialparameter and the frequency exponent are independent of temperature. It has beenestablished that alkali ions and fluorine anions exhibit lower dimensionality of theconduction pathways in mixed alkali zinc fluoride glasses than that in the single alkalilithium based zinc fluoride glasses while they are migrating. From the scaling of theconductivity spectra, it has been established that the relaxation dynamics inmixed alkali zinc fluoride glasses is independent of temperature and composition.

Dielectric relaxation and ac conductivity of sodium tungsten phosphate glasses

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77–420 K. Complex relative permitivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (σm(ω)) has been found to be higher than σdc at low temperatures whereas at high temperature σm(ω) becomes equal to σdc at all frequencies. The ac conductivity obeys the relation σac(ω)=AωS over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(EF)) have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 (A)−1. Values of N(EF) have the order 1020 which are well within the range suggested for localized states. Present values of N(EF) are smaller than those for tungsten phosphate glasses.

Prediction of charge mobility and its temperature dependence in neutral poly(3-Hexylthiophene) from dielectric relaxation measurement

For neutral poly(3-hexylthiophene) (P3HT), the electric modulus formalism of dielectric relaxation measurement together with the use of non-exponential decay function can be applied to describe the behavior of conductivity relaxation. The charge mobility of neutral P3HT calculated from conductivity relaxation frequency with the use of the defect-diffusion model is in agreement with the data from field-effect transistor measurement. The temperature dependence of charge mobility exhibits a local maximum right after the end of glass transition region, which can be attributed to the transition of soft conformons in the disordered phase to localized conformons.