Frequency Dependence of the Dielectric Loss Angle in Disordered Semiconductors in the Terahertz Frequency Range

Frequency dependence of the real part of the conductivity σ₁(ω) in the region of the transition from almost linear (s < 1) to quadratic (s ≈ 2) can indicate a change in the conduction mechanism (the transition from the variable-range to the fixed-range hopping with increasing frequency); in this case, the sharpness of the change in the slope of the frequency characteristic is related to the dependence of the preexponential factor of the resonance integral on the intercenter distance in the pair. The frequency dependence of the imaginary part of the conductivity σ₂(ω) has no kink in the vicinity of the transition frequency ωcr, remaining almost linear. A large dielectric loss angle |cotγ| = |σ₂|/σ₁ can indicate that the imaginary part of the conductivity at ω < ω_cr is defined by the larger zero-phonon contribution in σ₂^res the region of weak variation in the loss angle γ(ω), which significantly exceeds the relaxation contribution σ₂^res.     

Electrical properties and dielectric relaxation of thermally evaporated zinc phthalocyanine thin films

The electrical transport properties and dielectric relaxation of Au/zinc phthalocyanine, ZnPC/Au devices have been investigated. The DC thermal activation energy at temperature region 400-500 K is 0.78 eV. The dominant conduction mechanisms in the device are ohmic conduction below 1 V and space charge limited conduction dominated by exponential trap distribution in potentials >1 V. Some parameters, such as concentration of thermally generated holes in valence band, the trap concentration per unit energy range at the valence band edge, the total concentration of traps and the temperature parameter characterizing the exponential trap distribution and their relation with temperatures have been determined. The AC electrical conductivity, σ_ac, as a function of temperature and frequency has been investigated. It showed a frequency and temperature dependence of AC conductivity for films in the temperature range 300-400 K. The films conductivity in the temperature range 400-435 K increased with increasing temperature and it shows no response for frequency change. The dominant conduction mechanism is the correlated barrier hopping. The temperature and frequency dependence of real and imaginary dielectric constants and loss tangent were investigated.     

Study on ionic conductivity and dielectric properties of PEO-based solid nanocomposite polymer electrolytes

The ionic conductivity and dielectric properties of the solid nanocomposite polymer electrolytes formed by dispersing a low particle-sized TiO₂ ceramic filler in a poly (ethylene oxide) (PEO)-AgNO₃ matrix are presented and discussed. The solid nanocomposite polymer electrolytes are prepared by hot press method. The optimum conducting solid polymer electrolyte of polymer PEO and salt AgNO₃ is used as host matrix and TiO₂ as filler. From the filler concentration-dependent conductivity study, the maximum ionic conductivity at room temperature is obtained for 10 wt% of TiO₂. The real part of impedance (Z′) and imaginary part of impedance (Z″) are analyzed using an LCR meter. The dielectric properties of the highest conducting solid polymer electrolyte are analyzed using dielectric permittivity (ε′), dielectric loss (ε″), loss tangent (tan δ), real part of the electric modulus (M′), and imaginary part of the electric modulus (M″). It is observed that the dielectric constant (ε′) increases sharply towards the lower frequencies due to the electrode polarization effect. The maxima of the loss tangent (tan δ) shift towards higher frequencies with increasing temperature. The peaks observed in the imaginary part of the electric modulus (M″) due to conductivity relaxation shows that the material is ionic conductor. The enhancement in ionic conductivity is observed when nanosized TiO₂ is added into the solid polymer electrolyte.     

Effect of gamma irradiation on the dielectric properties and electrical conductivity of the TlInS<Subscript>2</Subscript> single crystal

The effect of gamma irradiation on the dielectric properties and ac conductivity of a TlInS₂ single crystal with a layered structure has been investigated in the frequency range from 5 × 10⁴ to 3.5 × 10⁷Hz. It has been shown that gamma irradiation of the TlInS₂ single crystal with a dose of 10⁴–2.25 × 10⁶ rad leads to a considerable increase in the dielectric loss tangent tanδ, the real part ɛ′ and imaginary part ɛ″ of the complex permittivity, and the ac conductivity σ _ac across the layers. It has been established that, for all gamma irradiation doses, the TlInS₂ single crystal is characterized by the dielectric loss due to electrical conduction up to a frequency of 10⁷ Hz and by the relaxation loss at a higher frequency. Irradiation of the TlInS₂ single crystal results in an increase in the dispersion of tan δ, ɛ′, and ɛ″. It has been demonstrated that, as the gamma irradiation dose is accumulated in the TlInS₂ single crystal, the density of localized states near the Fermi level N _F increases (from 5.2 × 10¹⁸ to 1.9 × 10¹⁹ eV⁻¹ cm⁻³).     

Determining the relaxation polarization parameters in dielectrics with high electrical conductivity

In dielectrics with high steady leakage of conductivity, the frequency–temperature dependences of the dielectric loss tangent generally allow us to determine the relaxation time and activation energy of relaxation process only when they are strong. With weak relaxation processes, there are no extrema in the frequency dependence of the dielectric loss tangent. In such cases, the parameters of the relaxation processes are initially determined from the frequency behavior of the imaginary parts of the electrical module or impedance. However, the frequency dependences of these quantities when there is electrical conduction can contain three extrema. Identifying the maxima associated with relaxation polarization therefore requires additional research.     

AC conductivity and dielectric relaxation in Ba(Sm1/2Nb1/2)O3 ceramic

The complex perovskite oxide a barium samarium niobate (BSN) synthesized by solid-state reaction technique has single phase with cubic structure. The scanning electron micrograph of the sample shows the average grain size of BSN∼1.22 μm. The field dependence of dielectric response and loss tangent were measured in the temperature range from 323 to 463 K and in the frequency range from 50 Hz to 1 MHz. The complex plane impedance plots show the grain boundary contribution for higher value of dielectric constant in the low frequency region. An analysis of the dielectric constant (ε′) and loss tangent (tan δ) with frequency was performed assuming a distribution of relaxation times as confirmed by the scaling behaviour of electric modulus spectra. The low frequency dielectric dispersion corresponds to DC conductivity. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with an activation energy of 0.71 eV. The frequency dependence of electrical data is also analyzed in the framework of conductivity and electric modulus formalisms. Both these formalisms show qualitative similarities in relaxation times. The scaling behaviour of imaginary part of electric modulus M″ and dielectric loss spectra suggest that the relaxation describes the same mechanism at various temperatures in BSN. All the observations indicate the polydispersive relaxation in BSN.     

Small-polaron versus band conduction in some transition-metal oxides

In this paper an attempt is made to establish the nature of free charge carriers and of charge carriers bound to centres in p-type NiO, CoO and MnO and in n-type MnO and α-Fe₂O₃.

For free charge carriers, d.c. conductivity, Seebeck coefficient and Hall effect are considered. Effects arising from inhomogeneous conduction and impurity conduction are discussed. Impurity conduction appears to have a strong influence on transport properties in the case of α-Fe₂O₃, less so in NiO, whereas no influence of this effect has been found in CoO and MnO. It is shown that NiO and CoO do not exhibit the features characteristic of small-polaron conductors but rather can be consistently conceived of as large-polaron band semiconductors. It is suggested that magnetic resistance due to exchange coupling between charge-carrier spin and cation spins plays an important role. The anomalous behaviour of the Hall effect in NiO and α-Fe₂O₃ is extensively discussed. In contradistinction to NiO, CoO and n-type MnO, free charge carriers in p-type MnO seem to have small-polaron character.

For charge carriers bound to centres, dielectric loss, high-frequency conduction and optical absorption are considered. The dielectric loss data relate to Li or Na centres in NiO, CoO and MnO and to Ti, Zr, Sn, Ta, Nb and presumably oxygen vacancy centres in α-Fe₂O₃. It is concluded from the dependence of dielectric loss on frequency and temperature that bound charge carriers are small polarons. It is shown for the cases of NiO and α-Fe₂O₃ that apart from small-polaron effects, disorder due to locally varying electric fields determines the nature of dielectric loss. The small-polaron character of bound charge carriers in NiO is corroborated by the behaviour of high-frequency conduction and optical absorption due to centres and also by the magnitude of impurity conduction.     

Effect of porosity and pore morphology on the low-frequency dielectric response in sintered ZrO<Subscript>2</Subscript>—8 mol% Y<Subscript>2</Subscript>O<Subscript>3</Subscript> ceramic compact

Effect of porosity and pore size distribution on the low-frequency dielectric response, in the range 0.01–100 kHz, in sintered ZrO₂—8 mol% Y₂O₃ ceramic compacts have been investigated. Small-angle neutron scattering (SANS) technique has been employed to obtain the pore characteristics like pore size distribution, specific surface area etc. It has been observed that the real and the imaginary parts of the complex dielectric permittivity, for the specimens, depend not only on the porosity but also on the pore size distribution and pore morphology significantly. Unlike normal Debye relaxation process, where the loss tangent vis-à-vis the imaginary part of the dielectric constant shows a pronounced peak, in the present case the same increases at lower frequency region and an anomalous non-Debye type relaxation process manifests.     

Dielectric behaviour and AC conductivity of LiCF3SO3 doped H-chitosan polymer films

H-chitosan that exhibited solubility in THF was prepared by acyl modification of chitosan. Films of H-chitosan containing LiCF₃SO₃ were prepared by the solution cast technique. The effect of salt concentration on the frequency-dependent dielectric properties of H-chitosan: LiCF₃SO₃ complexes were investigated by impedance spectroscopy, in the temperature range from 243 to 373 K. The dielectric properties and ac conductivity of the samples prepared have been analyzed. The dielectric constant increases sharply with temperature in the low frequency region. At higher frequencies, the effect of temperature on the dielectric constant is negligible. The values of dielectric constant were also found to increase with increasing conductivity of the samples. The imaginary part, M_i of electrical modulus shows the formation of dispersion peak. Relaxation times for the ionic charge carriers were extracted from the loss tangent maximum peak at various temperatures. The plot of relaxation times as a function of temperature shows Arrhenius type behaviour. The ac conductivity was found to obey the universal power law and as the temperature increases, the feature of σ(θ) α θⁿ is more predominant. The temperature dependence of the power law exponent n is reasonably interpreted by the overlapping large polaron tunneling (OLPT) model.     

Impact of MoO3 concentration,frequency and temperature on the dielectric properties of zinc phosphate glasses

Phosphate glasses with the chemical composition of 47P₂O₅–24ZnO-(29-x)Na₂O-xMoO₃, x = 0, 2, 4, 6, 8 and 10, have been prepared using the melt quenching technique. Dielectric properties of these phosphate glasses are carried out in the frequency range from 1 to 100 kHz at different temperatures. Dielectric parameters such as dielectric constant ε′, dielectric loss ε′′ and ac conductivity of the investigated glasses have been evaluated. The dependences of these dielectric parameters on frequency, composition and temperature have been discussed. It is found that dielectric constant decreases with increasing frequency due to the reduction of space-charge polarization and dipole polarization. The dependence of ac conductivity on the MoO₃ content indicates a competition between electronic and ionic conduction. The temperature dependence of the dielectric parameters reveals a rising trend of the dielectric parameters with temperature. This rising trend is indicated due to the increase of the amplitude of the thermal vibration of the charge carriers which facilitates the electron hopping and drifting of the mobile ions. The linear trend of the ln(σ_ac)-1000/T plot indicates that ac conductivity of the investigated glasses is thermally-activated transport process and follows the Arrhenius equation. The activation energy and its composition dependence have been reported.     

Infralow-frequency dielectric response of Pb<Subscript>3</Subscript>O<Subscript>4</Subscript> polycrystalline layers

This paper reports on the results of the investigation into the frequency dispersion of the capacitance and dielectric loss in capacitor structures based on red lead Pb₃O₄. It is established that, in the range of frequencies f = 1.50 × 10^?3?0.25 Hz, the capacitance decreases and the dielectric loss tangent increases with increasing frequency. The frequency dependence of the electrical conductivity in an alternating-current electric field indicates the applicability of the hopping model of charge transfer under normal conditions. The role of a lone electron pair of Pb²⁺ cations in dielectric polarization is discussed.     

Transport and dielectric properties of thin polycrystalline CoO films

Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>10⁵ V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε ₂, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε ₂ frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high frequencies. The activation energy of a dielectric oscillator is 0.15 eV. Work supported by the Office of Naval Research.     

Dielectric loss,conductivity relaxation process and magnetic properties of Mg substituted Ni–Cu ferrites

The dielectric properties, dc and ac electrical resistivities of Mg substituted Ni–Cu ferrites with general formula Ni_0.5Cu_0.5−xMg_xFe₂O₄ (0.0≤x≤0.5) have been investigated as a function of frequency, temperature and composition. ac resistivity of all the samples decreases with increase in the frequency exhibiting normal ferrimagnetic behavior. The frequency dependence of dielectric loss tangent showed a maximum in between 10 Hz and 1 kHz in all the ferrites. The conductivity relaxation of the charge carriers was examined using the electrical modulus formulism, and the results indicate the presence of the non-Debye type of relaxation in the prepared ferrites. Similar values of activation energies for dc conduction and for conductivity relaxation reveal that the mechanism of electrical conduction and dielectric polarization is the same in these ferrites. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the distribution of relaxation time is temperature independent. The saturation magnetization and coercivity as calculated from the hysteresis loop measurement show striking dependence on composition.     

Ac conductivity and dielectric behavior of hydrated homoionic alkali-cations-exchanged montmorillonite

The dielectric properties of a series of homoionic alkali-exchanged montmorillonites were studied at different treatment temperatures and various water loadings by means of complex impedance spectroscopy. To date, however, this method has been underutilized in clay minerals studies. The main objective of the present work is to understand the relaxation mechanisms of water molecules interacting with different hydration centers in clay minerals, with a view to eventually control their interactions with the alkali extra-framework cations. The other part of our study is to study the dielectric properties such as real and imaginary parts of dielectric permittivity, loss tangent, and ac conductivity in the frequency range 10^?2–10⁶?Hz and temperature range 173–333?K of these clay minerals. The obtained results have been discussed in terms of the Jonscher model.     

Frequency and voltage dependence of dielectric properties and electric modulus in Au/PVC + TCNQ/p-Si structure at room temperature

Au/PVC + TCNQ/p-Si structure was fabricated and real and imaginary parts of the dielectric constant (ɛ′, ɛ″), loss tangent (tanδ), and the real and imaginary parts of the electric modulus (M′, M″) and ac conductivity (σ_ac) of this structure have been investigated in wide frequency a range of 1 kHz–5 MHz at room temperature. All of these parameters were found strong function of frequency and voltage especially in the inversion and depletion regions at low frequencies due to interfacial polarization and charges at interface states (N_ss). The decrease in ɛ′ and ɛ″ with increasing frequency indicated that the interfacial dipoles have less time to orient themselves in the direction of the alternate field. While the value of M′ increase with increasing frequency and reach a maximum, M″ shows a peak and the peak position shifts to higher frequency with increasing applied voltage. The ln(σ_ac) vs ln(ω) plot of the structure for 0.5 V has three linear regions (I, II and III) with different slopes which correspond to low, intermediate and high frequency ranges, respectively. Such behavior of ln(σ_ac) vs ln(ω) plot indicated that there are three different conduction mechanisms in the Au/PVC + TCNQ/p-Si structure at room temperature.     

Dielectric relaxation in complex perovskite oxide BaCo1/2W1/2O3

Polycrystalline BaCo_1/2W_1/2O₃ (BCW) is prepared by the solid-state reaction technique. The X-ray diffraction study of the compound at room temperature reveals the monoclinic phase. The field dependence of the dielectric constant and the conductivity are measured in the frequency range from 50 Hz to1 MHz and in the temperature range from 300 to 413 K. An analysis of the real and imaginary parts of the dielectric permittivity with frequency is performed. The frequency-dependent maxima in the imaginary impedance are found to obey an Arrhenius law with an activation energy=0.86 eV. The frequency-dependent electrical data are also analysed in the framework of the conductivity and modulus formalisms.     

温度相关的微波频率下氯化钠水溶液介电特性

针对NaCl水溶液(0.001–0.5 mol/L)介电特性, 实验调查了频率(200–6.25 GHz), 温度(293–353 K)、 浓度相关复介电常数. 结果表明: 频率增大的过程中虚部呈逐渐减小的趋势, 高温使离子扰动增大, 破坏了溶液内部水分子四面体结构和氢键构象而使介电常数实部减小. 与纯水相比, 溶液的损耗角正切在高温353 K低频区下降明显. 同时发现2.45和5.8 GHz的复介电常数随温度变化的温度窗效应, 温度窗效应导致微波加热时耗散功率的振荡变化, 温度分布不均匀现象在实验中得以证实. 关键词： 微波 复介电常数 介电特性 高温探头     

DSC, TGA and dielectric properties of carboxymethyl cellulose/polyvinyl alcohol blends

Films with different compositions of polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC) blends have been prepared using the casting method. Differential scanning calorimetery (DSC), thermogravimetric analysis (TGA) and dielectric spectroscopy of all compositions have been investigated. It was found that PVA and CMC are compatible in the studied range of composition. With increasing CMC content, the thermal stability of PVA increases. Based on DSC and TGA data, the activation energies of all the investigated samples were calculated. The absorption edge (E_a) was also determined from Ultraviolet-visible (UV-vis) spectra.Dielectric permittivity, loss tangent and ac conductivity of all samples were studied as functions of temperature and frequency. The results show that the dielectric dispersion consists of both dipolar and interfacial polarization. The frequency dependence of the ac conductivity indicates that the correlated barrier hopping (CBH) is the most suitable mechanism for conduction. The polaron binding energy (U_M) was determined. Results of the present system are compared with those of similar materials.     

The impedance spectroscopic study and dielectric relaxation in A(Ni1/3Ta2/3)O3 [A=Ba,Ca and Sr]

We present the results of impedance spectroscopic study with its analytical interpretations in the framework of electric modulus formalism for Barium Nickel Tantalate Ba(Ni_1/3Ta_2/3)O₃ (BNT), Calcium Nickel Tantalate Ca(Ni_1/3Ta_2/3)O₃ (CNT) and Strontium Nickel Tantalate Sr(Ni_1/3Ta_2/3)O₃ (SNT) synthesized by the solid-state reaction technique. The results of powder X-ray diffraction study reveal that BNT and SNT crystallize in cubic structure with lattice parameter a=4.07 Å and 3.98 Å respectively, whereas CNT crystallizes in monoclinic structure having lattice parameters, a=5.71 Å, b=13.45 Å and c=5.47 Å with β=118.3°. The logarithmic angular frequency dependence of the real part of complex dielectric permittivity and loss tangent as a function of temperature indicate significant dielectric relaxation in the samples, which have been explained by the Debye theory. The frequency dependence of the loss peak and the imaginary part of electrical modulus are found to obey the Arrhenius law. The relaxation mechanism of these samples is modeled by the Cole–Cole equation. This confirms that the polarization mechanism in BNT, CNT and SNT is due to the bulk effect arising in semiconductive grains. The scaling behavior of imaginary part of electric modulus M″ suggests that the relaxation describes the same mechanism at various temperatures but relaxation frequency is strongly temperature dependent. The normalized peak positions of tan δ/tan δ_m and M″/M″_m versus log ω for BNT, CNT and SNT do not overlap completely and are very close to each other. These indicate the presence of both long-range and localized relaxation. Due to their high dielectric constant and low loss tangent, these materials may find several technological applications such as in capacitors, resonators, filters and integrated circuits.     

Ionic conduction mechanism and relaxation studies of NaNbAlP<Subscript>3</Subscript>O<Subscript>12</Subscript> compound

The Na superionic conductor (NASICON) NaNbAlP₃O₁₂ compound was prepared by the conventional solid-state reaction method. The formation of single-phase material was confirmed by X-ray diffraction studies, and it was found to be a hexagonal phase at room temperature. The electrical conductivity was measured in the frequency range from 200 Hz to 5 MHz and temperatures between 573 and 773 K using impedance spectroscopy technique. The obtained results were analyzed by fitting the experimental data to the equivalent circuit model. The analysis of Nyquist plots has revealed the contribution of three electrically active regions corresponding to the bulk mechanism, distribution of grain boundaries, and electrode processes. Besides, the frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. Temperature dependence of the power law exponent s strongly suggests that the non-overlapping small polaron tunneling (NSPT) model is the dominant transport process. The variation of the imaginary part of the complex modulus as a function of angular frequency at several temperatures shows a double relaxation peak suggesting the presence of grains and grain boundaries in the sample. An analysis of the dielectric constants ε″ and loss tangent tan (δ) with frequency shows a distribution of relaxation times.