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.     

The effect of conductivity on describing the non-debye relaxation processes in dielectrics

The distributions and integrated distribution of Cole-Cole and Dawidson-Cole relaxators are described. Based on the effect of conductance on the frequency dependences tanδ, the processes of relaxation polarization in dielectrics with such relaxators can be divided into two categories: strong and weak. In the development of strong processes, the increase in flow-through conductivity does not lead to the disappearance of extrema in frequency dependences tanδ.     

Investigation of Relaxation Process of Nylon 1212 Using Dielectric Relaxation Spectroscopy

The relaxation processes of α-form nylon 1212 from 50°C up to 160°C were studied by dielectric relaxation spectroscopy (DRS) in a wide frequency range of 63 Hz to 5 MHz. The α relaxation, the electrode relaxation, and the conductivity relaxation of nylon 1212 were observed and analyzed in detail using permittivity and modulus formalism. Electrode polarization and dc conductivity were the origin of high dielectric permittivity values at low frequencies and high temperatures. The strength of the imaginary part of the electric modulus of conductivity relaxation M″ _max was nearly independent of temperature. The distribution of local conductivity and relaxation time became broader with decreasing temperature.     

Dielectric relaxation in Bi12SiO20: Cr crystals

A study is reported of the temperature and frequency dependences of the permittivity and losses in Cr-doped Bi₁₂SiO₂₀ crystals at sonic frequencies and in the range 300–800 K. A number of dielectric anomalies and a close-to-linear Cole-Cole diagram have been observed. The results are discussed by invoking the concepts of electron hopping and screening of the induced polarization through the relaxation of local lattice distortions.     

Charge transport mechanism of vanadium pentoxide xerogel-polyaniline nanocomposite

The nanocomposites of conducting polyaniline are prepared by intercalating into the layers of vanadium pentoxide (V₂O₅) xerogel. The intercalation is confirmed by the observation of lattice expansion of V₂O₅ xerogel. Dc conductivity of the gel follows Arrhenius type temperature dependence while the nanocomposites exhibit three dimensional variable range hopping. The ac conductivity and dielectric properties are extensively studied at low temperature up to the frequency of 10 MHz. Two semicircles in Cole-Cole plot of impedance are found for the nanocomposites. The ac conductivity spectra reveal three frequency regions. The frequency exponent in the lower frequency region is nearer to 2. The dielectric response exhibit broad spectra which are analyzed by Cole-Cole distribution function. The peak frequency of dielectric spectra appears at the first cross over frequency of conductivity spectra.     

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.     

Strong and weak polarization relaxation processes in solid dielectrics

It is established that an increase in the reach-through electrical conductivity of a dielectric can affect the frequency characteristics of the quantity tanδ in different ways when relaxation polarization processes occur: the extrema of the frequency characteristics can be either suppressed or intensified. In the former case, relaxation processes are referred to as weak; in the latter case, they are referred to as strong. Strong processes lead to the emergence of extrema in the frequency dependence of the imaginary part of complex conductivity. The causes underlying the two polarization relaxation processes are identified.     

Dielectric and structural relaxation phenomena in Na0.5Bi0.5TiO3 single crystal

Abstract

Dielectric permittivity studies of Na_0.5Bi_0.5TiO₃ single crystals in a broad range of frequency up to 10 MHz and temperature 300—823 K are reported. In this temperature range dielectric dispersion below 1 MHz has been found. The obtained data were fitted to the Cole-Cole relation. The mean relaxation time τ is strongly temperature dependent (0.04 ? 2.6 × 10^?5 s). A remarkable hysteresis effect in the values of τ on cooling and heating took place. The Δε(T) dependence (the maximal value of Δε ～ 400) is similar to the global ε′(T) response at low frequency. An isothermal structural transformation in Na_0.5Bi_0.5TiO₃ was observed by X-ray measurements. The order of the time in which the transformation takes place (～300 minutes) corresponds to the time in which the strongest time evolution of electric permittivity and time changes of dielectric dispersion were detected.     

Low frequency dielectric relaxation in lightly doped VO2 single crystals

The relative effects of intrinsic and extrinsic defects on the dielectric relaxation of VO₂ crystals have been investigated by measurement of the dielectric parameters of undoped crystals and crystals doped with Ti, Cr and Al. Measurements have been made in the temperature range 77–250 K and the frequency range 50–100 kHz. The dielectric data is described by a Cole-Cole distribution function with a distribution parameter α ? 0.45 which decreases with increasing temperature. However, the distribution of activation energies g(E) derived from α is almost independent of temperature. The overall dielectric relaxation behaviour is determined primarily by the intrinsic defect structure of VO₂, and the effect of impurities is observed only in changes in the low frequency limiting (static) value of the dielectric constant. The same transport mechanism is found to determine the dc conductivity and the dielectric relaxation and evidence is presented that the dielectric relaxation is of dipolar origin.     

巨介电常数氧化物CaCu3Ti4O12的介电和阻抗特性

采用固相烧结法合成了单相巨介电常数氧化物CaCu₃Ti₄O₁₂（CCTO）.用阻抗分析仪分析了10—420 K温度范围内的介电频谱和阻抗谱特性,并结合ZVIEW软件进行了模拟.结果表明：温度高于室温时,频谱出现两个明显的弛豫台阶,低频弛豫介电常数随温度升高而显著增大,表现出热离子极化特点;温度低于室温时,频谱表现出类德拜弛豫,且高、低平台介电常数值基本不随温度变化,表现出界面极化特点和较好的温度稳定性.频谱中依次出现的介电弛豫对应于阻抗谱中 关键词： 3Ti₄O₁₂')" href="#">CaCu₃Ti₄O₁₂ 介电频谱 阻抗谱 Cole-Cole半圆弧     

Ionic conductivity studies on LiSmO<Subscript>2</Subscript> by impedance spectroscopy

Lithium samarium oxide has been prepared by solid-state reaction method and characterized by X-ray diffraction (XRD) and impedance spectroscopy. XRD pattern of the sample reveals the formation of the sample. The conductivity studies, dielectric studies, and modulus analysis of the samples have been carried out for different temperatures. The bulk conductivity of the sample has been found to be 1.21 × 10⁻⁵ Scm⁻¹ at 420 °C. The temperature variation of the direct current conductivity obeys the Arrhenius relation. The modulus analysis of the sample indicates the non-Debye nature of the sample which corresponds to long-time slow polarization and relaxation of hopping charges.     

AC conductivity and dielectric behavior of polyaniline/sodium metavenadate (PANI/NaVO3) composites

The conducting polyaniline/sodium metavenadate (PANI/NaVO₃) composites were synthesized by single step in situ polymerization technique by placing finely grinded powder of NaVO₃ during the polymerization of aniline. The formation of mixed phases of the polymer together with the conducting emeraldine salt phase was confirmed by spectroscopic techniques like FTIR. SEM images indicated a systematic morphological variation of particles aggregated in the composite matrix as compared to the pristine PANI. AC conductivity and dielectric behavior of these composites were investigated in the frequency range 50 Hz to 5 MHz. It is found that AC conductivity obeyed the power law index and the variation of conductivity with wt% of NaVO₃ could be related to conductivity relaxation phenomenon. These composites have shown high dielectric constant, which is related to polarization. It is seen that both dielectric constant and dielectric loss decrease with increase in frequency. Variations in measured parameters of AC response with increasing frequency of these composites are found to follow systematic trends that are similar to those observed with temperature and doping.     

Effect of melt compounding temperature on dielectric relaxation and ionic conduction in PEO–NaClO<Subscript>4</Subscript>–MMT nanocomposite electrolytes

Polymer nanocomposite electrolytes (PNCEs) of poly(ethylene oxide) and sodium perchlorate monohydrate complexes with montmorillonite (MMT) clay up to 20 wt.% MMT concentration of poly(ethylene oxide) (PEO) are synthesized by melt compounding technique at melting temperature of PEO (∼70 °C) and NaClO₄ monohydrate (∼140 °C). Complex dielectric function, electric modulus, alternating current (ac) electrical conductivity, and impedance properties of these PNCEs films are investigated in the frequency range 20 Hz to 1 MHz at ambient temperature. The direct current conductivity of these materials was determined by fitting the frequency-dependent ac conductivity spectra to the Jonscher power law. The PNCEs films synthesized at melting temperature of NaClO₄ monohydrate have conductivity values lower than that of synthesized at PEO melting temperature. The complex impedance plane plots of these PNCEs films have a semicircular arc in upper frequency region corresponding to the bulk material properties and are followed by a spike in the lower frequency range owing to the electrode polarization phenomena. Relaxation times of electrode polarization and ionic conduction relaxation processes are determined from the frequency values corresponding to peaks in loss tangent and electric modulus loss spectra, respectively. A correlation is observed between the ionic conductivity and dielectric relaxation processes in the investigated PNCEs materials of varying MMT clay concentration. The scaled ac conductivity spectra of these PNCEs materials also obey the ac universality law.     

On the theory of the universal dielectric relaxation

Dielectric relaxation has been investigated within the framework of a modified mean field theory, in which the dielectric response of an arbitrary condensed matter system to the applied electric field is assumed to consist of two parts, a collective response and a slowly fluctuating response; the former corresponds to the cooperative response of the crystalline or noncrystalline structures composed of the atoms or molecules held together by normal chemical bonds and the latter represents the slow response of the strongly correlated high-temperature structure precursors or a partially ordered nematic phase. These two dielectric responses are not independent of each other but rather constitute a dynamic hierarchy, in which the slowly fluctuating response is constrained by the collective response. It then becomes clear that the dielectric relaxation of the system is actually a specific characteristic relaxation process modulated by the slow relaxation of the nematic phase and the relationship governing such a process can be defined as the universal dielectric relaxation law. Furthermore, we have shown that seemingly different relaxation relationships, such as the Debye relaxation law, the Cole-Cole equation, the Cole-Davidson equation, the Havriliak-Negami relaxation, the Kohlrausch-Williams-Watts function, Jonscher’s universal dielectric relaxation law, etc. are only the variants of this universal law under different circumstances.     

Dielectric relaxation in vanadium phosphate glasses

An asymmetric distribution of relaxation times has been inferred from an increase in the Cole-Cole distribution parameter α with increasing values of ωτ in 62% v₂O₅–38% P₂O₂ glass. The conventional Debye type relaxation loss peaks in the frequency range 10²–10⁵ Hz are observed in this sample above 85°K. The extrapolated values of dielectric constant and relaxation time below 100°K seem unexpectedly large while the high temperature extrapolated values of ?' are close to ?_∞ as expected. Probably the conventional dielectric loss peaks are observed only above a critical temperature at which the carriers gain sufficient energy to be excited to the conduction band edge. Below this temperature hopping of carriers within kT of the Fermi level may dominate and conventional Debye type dielectric loss peaks may lose their significance as envisaged in the models of frequency dependent ac conductivity.     

Electronic relaxation of deep bulk trap and interface state in ZnO ceramics

This paper investigates the electronic relaxation of deep bulk trap and interface state in ZnO ceramics based on dielectric spectra measured in a wide range of temperature, frequency and bias, in addition to the steady state response. It discusses the nature of net current flowing over the barrier affected by interface state, and then obtains temperature-dependent barrier height by approximate calculation from steady I--V (current--voltage) characteristics. Additional conductance and capacitance arising from deep bulk trap relaxation are calculated based on the displacement of the cross point between deep bulk trap and Fermi level under small AC signal. From the resonances due to deep bulk trap relaxation on dielectric spectra, the activation energies are obtained as 0.22 eV and 0.35 eV, which are consistent with the electronic levels of the main defect interstitial Zn and vacancy oxygen in the depletion layer. Under moderate bias, another resonance due to interface relaxation is shown on the dielectric spectra. The DC-like conductance is also observed in high temperature region on dielectric spectra, and the activation energy is much smaller than the barrier height in steady state condition, which is attributed to the displacement current coming from the shallow bulk trap relaxation or other factors.     

Macroscopic polarization and thermal conductivity of GaN

We theoretically investigated the effect of macroscopic polarization (sum of spontaneous and piezoelectric polarization) on the thermal conductivity of wurtzite GaN. Macroscopic polarization contributes to the effective elastic constant of the GaN and thus modifies the phonon group velocity. We used the revised phonon velocity to estimate the Debye frequency and temperature. Different phonon scattering rates were calculated as functions of the phonon frequency. The thermal conductivity of GaN was estimated using revised parameters such as the phonon velocity and phonon relaxation rate. The revised thermal conductivity at room temperature increased from 250 to 279 W m⁻¹ K⁻¹ due to macroscopic polarization. The method we developed can be used for thermal budget calculations for GaN optoelectronic devices.     

On the calculation of <H 2> molecular integrals

The density matrix equations of motion arising in the triplet mechanism of chemically induced electron spin polarization are solved exactly without the imposition of the Redfield approximation. It is shown that the triplet spin relaxation time occurring in the final expression is not the true relaxation time because the spectral density involved depends both on the rotational correlation time and on the quenching rate. The effective spin relaxation time differs only slightly from the true time. The equations are extended to the case where the initial triplet passes on its polarization to the secondary triplet and exact solutions for the polarizations of the latter's doublets are obtained in the form Π_B = cΠ_A; an explicit expression for c is presented. The consequences of the secondary triplet being able to pass back its polarization to the initial triplet are explored and a ‘coherence effect’ on the polarization on the first triplet's doublets is analysed.     

Dielectric relaxation in PEO-based polymer electrolytes

Dielectric properties of cross-linked poly(ethylene oxide) (PEO) with different mesh sizes, doped with lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), have been studied in frequency region between 0.1 and 10⁷ Hz and in broad temperature range. Results were compared with linear PEO of 1000 g/mol. Dielectric responses of the systems are dependent on frequency and thermally activated. Systems exhibit different responses in semi-crystalline and molten state. Increase of temperature promotes polarization; whereas, increase of frequency lessens it. In other words, polarization is thermally activated and local conductivity reduced. Generally, one observes enhanced dc conductivity in linear PEO as compared to cross-linked PEO at high temperature and the opposite at low temperature. Resonance responses are observed in low-molecular cross-linked PEO and in linear PEO at low temperature. These responses lead to splitting of polarization relaxation at frequencies beyond low-frequency range. Salt-comprising systems display only relaxation-type dielectric response. Imaginary parts of response spectra show distribution of relaxation times. It turns out that this distribution is independent of temperature in the low-frequency range, but depends on concentration of salt in the cross-linked polymer. In both systems, neat cross-linked and linear polymer of low-molecular mass, one observes coexistence of non-local and local motions of charged entities even at very low temperature.     

From strong to fragile glass- forming systems: A temperature modulated differential scanning calorimetry investigation

The capacity of temperature modulated differential scanning calorimetry (TMDSC) technique to determine dynamical parameters is tested on several glass-formers of different fragility. We show that both the fragility index m and the stretching exponent β deduced from TMDSC are consistent with results from other spectroscopic analyses. In particular, we are interested in the coherency and complementarity with the dielectric measurements. When the dielectric analysis is prevented by conductivity effects or when the molecules are non-polar, TMDSC appears as a very powerful tool to study low frequency relaxation.