Dielectric spectroscopy studies of ZnO single crystal

Dielectric relaxation and charge transport induced by electron hopping in ZnO single crystal are measured by using a novocontrol broadband dielectric spectrometer. Typical Debye-like dielectric relaxation originating from electronic hopping between electronic traps and conductive band in surface Schottky barrier region is observed for ZnO single crystal-Au electrode system. However, after insulation of ZnO single crystal by heat treatment in rich oxygen atmosphere, dielectric relaxation and alternating current conductance are observed simultaneously in the dielectric spectra, implying that dielectric relaxation and charge transport can be induced simultaneously by electronic hopping at high temperature in an ordered system. The intrinsic correlation between local dielectric relaxation and long range charge transport offers us a new method to explore complicated dielectrics.     

Dielectric relaxation and ac conductivity behavior of carboxyl functionalized multiwalled carbon nanotubes/poly (vinyl alcohol) composites

We study the dielectric relaxation and ac conductivity behavior of MWCNT-COOH/Polyvinyl alcohol nanocomposite films in the temperature (T) range 303–423 K and in the frequency (f) range 0.1 Hz–1 MHz. The dielectric constant increases with an increase in temperature and also with an increase in MWCNT-COOH loading into the polymer matrix, as a result of interfacial polarization. The permittivity data were found to fit well with the modified Cole-Cole equation. Temperature dependent values of the relaxation times, free charge carrier conductivity and space charge carrier conductivity were extracted from the equation. An observed increment in the ac conductivity for the nanocomposites was analysed by a Jonscher power law which suggests that the correlated barrier hopping is the dominant charge transport mechanism for the nanocomposite films. The electric modulus study revealed deviations from ideal Debye-type behavior which are explained by considering a generalized susceptibility function. XRD and DSC results show an increase in the degree of crystallinity.     

Effect of the film thickness on the impedance behavior of sol–gel Ba0.6Sr0.4TiO3 thin films

Perovskite Ba_0.6Sr_0.4TiO₃ sol–gel thin films with different thicknesses are fabricated as MFM configuration to study the effect of the film thickness on the dielectric relaxation phenomenon and the ionic transport mechanism. The frequency dependent impedance, electric modulus, permittivity and AC conductivity have been investigated in this context. Z? plane for all the tested samples shows two regions, corresponding to the bulk mechanism and the distribution of the grain boundaries–electrodes process. Electric modulus versus frequency plots reveal non-Debye relaxation peaks. The observed decrease in both the impedance and permittivity with the increase in film thickness is attributed to the grain size effect. The frequency dependent conductivity plots show three regions of conduction processes, i.e. low-frequency region due to DC conduction, mid-frequency region due to translational hopping motion and high-frequency region due to localized hopping and/or reorientational motion.     

Effect of cation distribution on the properties of Mn0.2ZnxNi0.8−xFe2O4

Mn_0.2Zn_xNi_0.8−xFe₂O₄ (x=0.2, 0.3, 0.4, 0.5, 0.6) are synthesized by the citrate precursor method. Effects of zinc substitution on DC resistivity, dielectric relaxation intensity, initial permeability, saturation magnetization and Curie temperature have been investigated. It is observed that resistivity increases with increase in zinc concentration up to x=0.5 and then decreases. The observed behaviour is explained in terms of hopping and site preference of ions in the lattice. The main contribution to dielectric relaxation intensity is observed to be due to space charge polarization. Initial permeability is observed to increase with increase in zinc concentration. Saturation magnetization increases up to x=0.4 and then starts decreasing. Canting effect is observed for higher zinc concentrations.     

DC conductivity and dielectric properties of maize starch/methylcellulose blend films

The transient current, electrical conductivity, dielectric constant (ε′), and dielectric loss factor (ε″) of starch and methylcellulose homopolymers and their blends with various compositions were studied under different conditions. The x-ray diffraction pattern was obtained for individual polymers and 50:50 wt/wt% blend sample to identify both the structure and degree of crystallinity. From transient current, the ionic and electronic transfer number as well as charge carrier density and drift mobility were determined. The values of activation energy in the temperature range 30–90 °C indicate that the conduction mechanism is due to combined electronic and ionic processes, while in the temperature range 100–160 °C, electronic contribution is predominant. The complex dielectric data of the present samples in an extended frequency and temperature range appear as different relaxation processes, which are connected with polymer dynamics.     

Magnetic interactions and electronic structure of $$\hbox {Pt}_{2}\hbox {Mn}_{1-x}\hbox {Y}_{x} \hbox {Ga (Y = Cr and Fe)}$$ system: An ab-initio calculation

Structural, optical, electrical conductivity and dielectric relaxation properties of bulk 4-amino-3-mercapto-6-(2-(2-thienyl)vinyl)-1,2,4-triazin-5(4H)-one donor (AMT) are studied. The structure of AMT in its powder form was analysed by X-ray diffraction (XRD), infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). AC measurements (impedance, capacitance and phase angle) are done over the temperature range 303–373 K and in the frequency range from 42 Hz to 5 MHz. Analytical approaches for the experimental results of the σ _AC(ω, T) and the temperature behaviour of the frequency exponent show that the correlated barrier hopping (CBH) model is a good model to explain the AC electrical conductivity of bulk AMT organic semiconductor material. Application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The activation energy from the DC conductivity and the relaxation time are quite similar suggesting a hopping mechanism for AMT. The optical band gap of AMT is investigated using spectrophotometric measurement of transmittance at normal incidence of light in the wavelength range 300–1100 nm.     

Dielectric relaxation properties of tysonite-type solid solutions La1−xBaxF3−x

Dielectric relaxation properties of solid solutions La_1?xBa_xF_3?x (x ? 0105) have been studied by thermally stimulated depolanzation current (TSDC)- and a c. dielectric loss (DL) techniques.For x < 30 × 10^?3 the dielectric spectra show a relaxation peak which is ascribed to a simple associate of a substitutional dopant ion and a fluoride ion vacancy (Ba_LaV_F)^x in nearest-neighbour position, the vacancy being confined to the B sublattice For x values of about 1.3 × 10^?2 a relaxation peak appears which is tentatively attributed to a similar type of defect associate with the vacancy now confined to the A sublattice of the tysonite anion array One broad relaxation peak dominates the TSDC and DL spectra over the whole concentration range This peak is due to the relaxation of macroscopic space charge, i e ionic conductivity The low-temperature ion conductivity has been determined for several solid solutions, and extrapolates to the high-temperature conductivity determined previously with impedance spectroscopy Below liquid-nitrogen temperature three relaxations are observed, and ascribed to electronic transitions in cenum impurities. A computer programme has been developed to analyse TSDC relaxation peaks, taking dipole-dipole interactions into account Relaxation parameters and dipole concentrations are presented.     

Electrical properties of KTiOPO4 single crystal in the temperature range from −100 °C to 100 °C

The electrical property of a KTiOPO₄ single crystal was studied by means of a dielectric spectroscopy method in the temperature range from −100 to 100 °C. Dielectric dispersion began at a temperature, T_S=−80 °C. It is believed that this dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of K⁺ ions. The activation energy concerned with hopping conduction is E_a∼0.20 eV above T_S. T_S=−80 °C can be the minimum temperature for the hopping K⁺ ion.     

A study of the magnetic and dielectric properties of Y Fe0.5Cr0.5O3

Y Fe_0.5Cr_0.5O₃ ceramics have been synthesized by a conventional solid-state reaction. Powder X-ray diffraction shows that this compound possesses an orthorhombic structure with Pnma space group. It exhibits a high magnetic transition temperature at around 250 K with weak ferromagnetic behavior below this temperature. A dielectric relaxation following the Arrhenius law found in the Y Fe_0.5Cr_0.5O₃ compound can be attributed to the charge carrier hopping conduction.     

Electrical properties and conduction mechanism in the sodium nickel diphosphate

The Na_3.6Ni_2.2(P₂O₇)₂ compound was obtained by the conventional solid-state reaction. The sample was characterized by X-ray powder diffraction and vibrational and impedance spectroscopy. The AC electrical conductivity and the dielectric relaxation properties of this compound have been investigated by means of impedance spectroscopy measurements over a wide range of frequencies and temperatures, 209 kHz–1 MHz and 564–729 K, respectively. Dielectric data were analyzed using complex electrical modulus M* at various temperatures. The peak positions ω _m of M″ spectra shift toward higher frequencies with increase in temperature. The AC conductivity data fulfill the power law. Application of the correlated barrier hopping model revealed that the ionic conduction takes place by single-polaron and bipolaron hopping processes.     

Electrical study and dielectric relaxation behavior in?nanocrystalline Ce0.85Gd0.15O2?δ material at intermediate temperatures

The nanocrystalline material of 15 mol% Gd-doped ceria (Ce_0.85Gd_0.15O_2−δ ) was prepared by citrate auto ignition method. The electrical study and dielectric relaxation technique were applied to investigate the ionic transport process in this nanocrystalline material with an average grain size of 13 nm and the dynamic relaxation parameters are deduced in the temperature range of 300–600°C. The ionic transference number in the material is found to be 0.85 at 500°C at ambient conditions. The oxygen ionic conduction in the nanocrystalline Ce_0.85Gd_0.15O_2−δ material follows the hopping mechanism. The grain boundary relaxation is found to be associated with migration of charge carriers. The frequency spectra of modulus M″ exhibited a dielectric relaxation peak corresponding to defect associates (Gd-V_{o^{\blacksquare \blacksquare}})^{_\blacksquare}(\mathrm{Gd}\mbox{-}\mathrm{V}_{\mathrm{o}}^{_{_{{\blacksquare\,\blacksquare}}}})^{_{_{{\blacksquare}}}}. The material exhibits very low values of migration energy and association energy of the oxygen vacancies in the long-range motion, i.e., 0.84 and 0.07 eV, respectively.     

Relaxor-like dielectric behavior in La2NiMnO6 double perovskite ceramics

The dielectric and conductive characteristics of La₂NiMnO₆ double perovskite ceramics were investigated together with the crystal structure. La₂NiMnO₆ ceramics crystallized in the monoclinic P2₁/n structure in which the Ni²⁺ and Mn⁴⁺ ions ordered periodically. Relaxor-like dielectric behavior combined with a giant dielectric constant step was observed in the present ceramics, and these unique dielectric characteristics should be attributed to the charge ordering of Ni²⁺ and Mn⁴⁺. The dielectric relaxation was well fitted by the modified Debye equation and Arrhenius law with the activation energy of 0.17 eV. The dc conductivity of La₂NiMnO₆ could be well fitted using a variable-range hopping mechanism instead of a band conduction mechanism.     

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, α_Ac relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, α_c relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and α_a relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for α_Ac and α_c relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.     

Tempered relaxation with clustering patterns

This work is motivated by the relaxation data for materials which exhibit a change of the relationship between the fractional power-law exponents when different relaxation peaks in their dielectric susceptibility are observed. Within the proposed framework we derive a frequency-domain relaxation function fitting the whole range of the two-power-law dielectric spectroscopy data with independent low- and high-frequency fractional exponents γ and −α, respectively. We show that this effect results from a contribution of different processes. For high frequencies it is determined by random stops and movement of relaxing components, and the low-frequency slope is caused by clustering in their temporal changes.     

Oxygen-vacancy-related dielectric relaxation and conduction mechanisms in Bi5TiNbWO15 ceramics

This paper reports that the intergrowth ceramics Bi5TiNbWO15 （BW-BTN） have been prepared with the conventional solid-state reaction method. The dielectric and conductivity properties of samples were studied by using the dielectric relaxation and AC impedance spectroscopy in detail. Two distinct relaxation mechanisms were detected both in the plots of dielectric loss （tanδ） and the imaginary part （Z″） versus frequency in the frequency range of 10 Hz-13 MHz. We attribute the higher frequency relaxation process to the hopping process of the oxygen vacancies inside the grains, while the other seems to be associated with the space charges bound at the grain boundary layers. The AC impedance spectroscopy indicates that the conductivities at 625 K for bulk and grain boundary are about 1.12 × 10^-2 S/m and 1.43 × 10^-3 S/m respectively. The accumulation of the space charges in the grain boundary layers induces a space charge potential of 0.52 eV.     

Dielectric parameters in Se70Te30 and Se70Te28Zn2 chalcogenide glasses

Temperature and frequency dependence of dielectric constant (ε′) and dielectric loss (ε″) are studied in glassy Se₇₀Te₃₀ and Se₇₀Te₂₈Zn₂. The measurements have been made in the frequency range (8-500 kHz) and in the temperature range 300 to 350 K. An analysis of the dielectric loss data shows that the Guintini's theory of dielectric dispersion based on two-electron hopping over a potential barrier is applicable in the present case.No dielectric loss peak is observed in glassy Se₇₀Te₃₀. However, such loss peaks exist in the glassy Se₇₀Te₂₈Zn₂ in the above frequency and temperature range. The Cole-Cole diagrams have been used to determine some parameters such as the distribution parameter (α), the macroscopic relaxation time (τ₀), the molecular relaxation time (τ) and the Gibb's free energy for relaxation (ΔF).     

Frequency-dependent spin-lattice relaxation study of transport processes in superionic conductors

The sensitivity of the¹⁹F spin-lattice relaxation dispersion, T₁,(ω), to motional disorder in crystalline superionic conductors of the type La_1?xSr_xF_3?x (x = 0; 0.03) is shown. T₁ times are measured in the frequency range from 90 kHz to 370 MHz using standard techniques in combination with field-cycling. The relaxation dispersion shows qualitative differences from the standard Bloembergen-Purcell-Pound behavior. At low frequencies a relaxation model using a distribution of correlation times for diffusing ions is found to be consistent with the experimental results. At frequencies higher than 50 MHz another process of the Debye type which is not induced by ionic hopping dominates the relaxation.     

Electrical conduction and dielectric properties of vanadium phosphate glasses doped with lithium

AC conductivity and dielectric studies on vanadium phosphate glasses doped with lithium have been carried out in the frequency range 0.2-100 kHz and temperature range 290-493 K. The frequency dependence of the conductivity at higher frequencies in glasses obeys a power relationship, σ_ac=Aω^s. The obtained values of the power s lie in the range 0.5≤s≤1 for both undoped and doped with low lithium content which confirms the electron hopping between V⁴⁺ and V⁵⁺ ions. For doped glasses with high lithium content, the values of s≤0.5 which confirm the domination of ionic conductivity. The study of frequency dependence of both dielectric constant and dielectric loss showed a decrease with increasing frequency while they increase with increasing temperature. The results have been explained on the basis of frequency assistance of electron hopping besides the ionic polarization of the glasses. The bulk conductivity increases with increasing temperature whereas decreases with increasing lithium content which means a reduction of the V⁵⁺.     

Electrical transport properties of consolidated ZnSe quantum dots at and above room temperature

Here we report a comprehensive study on the prevailing conduction mechanism and dielectric relaxation behavior of consolidated Zinc Selenide quantum dots in the frequency range of 1 kHz ≤ f ≤ 1.5 MHz and in the temperature range of 298K ≤ T ≤ 573 K. The ac conductivity increases either with increase in temperature or with increase in frequency, which is explained by the Jonscher Power law. At higher temperatures, correlated barrier hopping is found to be the prevalent charge transport mechanism with a maximum barrier height of 0.88 eV. The dielectric constant of the sample is found to exhibit weak temperature dependence. DC conductivity study reveals the semiconducting nature of the sample and it is discussed in the light of polaron hopping conduction. From the impedance spectroscopic study, role of the grains and grain boundaries in the overall electrical transport properties have been elucidated by considering an electrical equivalent circuit (composed of resistances and constant phase elements). Electric modulus study reveals non-Debye responses of the sample in the experimental range.