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Comparison of the logarithm of the real a and imaginary b parts of the dielectric constant obtained by means of the Sawada's model thin continuous lines with Macdonald's model thick lines. The disagreement between the two models is rather large, in particular in the dc limit.
Source publication
We analyze the models that account the ionic contribution to the complex dielectric constant of a nematic liquid crystal. We compare the predictions of the model of [Sawada, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 318, 225 (1998)] based on the assumption that the electric field in the liquid coincides with the applied one, with the model of...
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Context 1
... Fig. 3 we compare log 10 r S with a log 10 r M and log 10 i S with b log 10 i M, and in Fig. 4 we compare r S with r M a, and i S with b i M. From Figs. 3 and 4 it is evident that the formulas obtained with the model of Sawada et al. strongly differ from the ones obtained by taking into account that the actual electric field in the sample ...
Citations
... To have a more meticulous understanding in this regard, we can consider the discussion proposed by Alexe-Ionescu et al. [48]. Following this, the numerical value of e 0 can be bifurcated into two components, one is e 0 i that depends on ionic impurities in LC material and the other one is e 0 s that depends on inherent properties of LC and anchoring energy of interface (between LC and polyimide layers) as represented by Equation (1). ...
We report here the concentration and temperature dependent optical, electro-optical and dielectric studies on bismuth titanate (Bi2Ti2O7/Bi4Ti3O12) nanocomposite (BT2/BT4 NC, ∼42 nm) doped nematic liquid crystal (NLC, 5CB) mixtures using optical polarising microscope and dielectric spectroscopic techniques. The optical textures confirm the uniform dispersion and miscibility of NCs in 5CB for all concentrations (i.e. 0.1, 0.25, 0.5, 1 and 2 wt %) and mixtures appear to be almost agglomeration free. The dielectric studies demonstrate the maximum changes in the dielectric parameters (dielectric permittivity, dielectric loss, loss factor and dielectric anisotropy) of 5CB sample for the 0.1 wt% mixture. Interestingly, the dielectric anisotropy of 5CB in 0.1 wt% mixture is increased by ∼11%. However, the dielectric memory effect (bias voltage ON-OFF) is observed maximum in the case of 1 wt% mixture. Similarly, the optical memory examined by bias voltage dependent (ON-OFF) optical textures is also significantly enhanced for 1 wt% mixture. The significant changes observed in dielectric properties of 5CB sample could be due to plausible interaction among NLC molecules and ionic impurities with BT2/BT4 NCs. Moreover, the enhanced volatile memory in BT2/BT4 NC-NLC mixture could be attributed to the dipole-dipole coupling between individual permanent dipole moment of anisotropic NLC molecules with the dipole moments generated by the ionic impurities agglomerated on the surface of high dielectric BT2/BT4 NCs. We strongly believe that such NCs-NLC mixtures would be certainly useful in the advancement of wearable devices such as plenoptic cameras and smart switchable windows.
... Furthermore, in the working region of frequency, e 0 can be written in the form of Equation (2). [34] ¼ e b 1 þ ...
Recently nanoparticles (NPs) are extensively explored as the dopant in liquid crystal (LC) materials because of their phenomenal properties such as good guest–host interaction and high surface to volume ratio. Keeping this motivation into mind, we have observed the influence of newly synthesized cobalt oxide (CoO) NPs on the dielectric and optical properties of nematic LC (NLC) namely 5 CB. For the observation of dielectric properties, we have performed dielectric spectroscopic measurements in the frequency range from 20 Hz to 1 MHz. The sample cell thickness-dependent studies have been performed to perceive the effect of applied field strength on dielectric parameters in presence of CoO NPs. Along with it, we have observed the alignment of the mesogens through the optical microscopy. So doping of such NPs serves various potential applications in the field of research as well as being proven to be cost-effective for industrial applications.
... To analyze the degradation with time, we fabricated an ion-doped LC cell and measured the haze and impedance. Impedance and dielectric spectroscopic analyses were carried out to extract the impedance parameters, such as the resistance and capacitance of the LCs and physical parameters of the ionic materials in the bulk regions and near the alignment layers [29][30][31][32][33][34][35]. With the increase in time under the applied electric field, the LC cell exhibited nonuniformity with the decrease of the haze, capacitance of the LC cell, and thickness of the diffusion layer, along with the increase of the resistance of the LC cell and surface concentration of the ions. ...
We reported electrical circuit modeling to analyze the optical performance degradation in an ion-doped liquid-crystal (LC) cell, which exhibited advantages, such as excellent optical performance and simple switching process, but suffered from long-term reliability issues. When an electric field was applied to the cell for an extended period of time, the optical performance became nonuniform, and the haze in the opaque state decreased. By measuring the impedance and fitting the measured data by using an equivalent circuit model, we confirmed the changes of the parameters in the electrochemical impedance spectroscopy and electrophysical properties of the ion-doped LC cell with time. According to the measurement of the optical and physical characteristics, the optical performance degradation was caused mainly by the ionic materials.
... The values of the diffusion coefficient of the ions in the liquid crystal 5CB in the range spectra in the framework of the constant field approach (see also discussion in [63]). ...
We present the analysis of the impedance spectra for a binary electrolyte confined between blocking electrodes with dielectric layers. An expression for the impedance is derived from Poisson-Nernst-Planck equations in the linear approximation taking into account the voltage drop on the dielectric layer. The analysis shows, that characteristic features of the frequency dependence of the impedance are determined by the ratio of the Debay length and the effective thickness of the dielectric layer. The impact of the dielectric layer is especially strong in the case of high concentrated electrolytes, where the Debay length is small and thus comparable to the effective thickness of the dielectric layer. To verify the model, measurements of the impedance spectra and transient currents in a liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) confined between polymer-coated electrodes in cells of different thicknesses are performed. The estimates for the diffusion coefficient and ion concentration in 5CB obtained from the analysis of the impedance spectra and the transient currents are consistent and agree with previously reported data. We demonstrate that calculations of the ion parameters from the impedance spectra without taking into account the dielectric layer contribution lead in most cases to incorrect results. Application of the model to analyze violations of the low-frequency impedance scaling and contradictions in the estimates of the ion parameters recently found in some ionic electrolytes are discussed.
... In the case that the mobile ions have a pure dielectric nature, the relative dielectric constant εi (ω) calculated under a homogeneous electric field is expressed as [41] where For ω = 0, we obtain [42] The value of Cb(0) is calculated to be Cb(0) = ε0εi(0)S/d, where S is the electrode area. Assuming that the maximum of the effective dielectric constant is εeff(max), ...
The Poisson-Nernst-Planck (PNP) model has been widely used for analyzing impedance or dielectric spectra observed for dilute electrolytic cells. In the analysis, the behavior of mobile ions in the cell under an external electric field has been explained by a conductive nature regardless of ionic concentrations. However, if the cell has parallel-plate blocking electrodes, the mobile ions may also play a role as a dielectric medium in the cell by the effect of space-charge polarization when the ionic concentration is sufficiently low. Thus the mobile ions confined between the blocking electrodes can have conductive and dielectric natures simultaneously, and their intensities are affected by the ionic concentration and the adsorption of solvent molecules on the electrodes. The balance of the conductive and dielectric natures is quantitatively determined by introducing an effective dielectric constant to the PNP model in the data analysis. The generalized PNP model with the effective dielectric constant successfully explains the anomalous frequency-dependent dielectric behaviors brought about by the mobile ions in dilute electrolytic cells, for which the conventional PNP model fails in interpretation.
... The limit of this assumption was discussed in Refs. [5,6]. In Ref. [3], the subject of this Comment, Sawada, points out that his experimental data cannot be interpreted by means of the standard Poisson-Nernst-Planck (PNP) model using a reasonable value of the diffusion coefficient for the ions in chlorobenzene, expected to be on the order of 10 −9 m 2 /s. ...
... In the limit of small V 1 , the fundamental equations of the model are Eqs. (3)- (5), that are linear with constant coefficients. For a harmonic applied voltage V (t) = V 1 exp(iωt), the solutions we are looking for are, as assumed in Ref. [3], ...
Recently, Sawada [Phys. Rev. E 88, 032406 (2013)] proposed a model to take into account the dielectric dispersion of ionic origin in a weak electrolyte cell. We first show that the model is based on questionable assumptions. Next, we point out an error in the author's calculation of the current in the external circuit. Finally, we demonstrate why some criticism on recent papers is irrelevant.
... The dielectric spectra observed for the electrolytic cells have been analyzed by using the classical theory, and we have obtained an unrealistic result that the number density of ions in the cell becomes more than one order of magnitude larger than that of the doped ions [18]. Alexe-Ionescu and co-workers have criticized the approximation with the homogeneous field for the analysis of the space-charge polarization, and they have insisted that the assumption of the homogeneous field should lead to incorrect values of the ionic constants, such as the diffusion coefficient and the number density of ions [23]. On the other hand, they have analyzed the space-charge polarization just by using the classical theory, not by considering the contribution of the space-charge polarization to the dielectric constant of Poisson's equation. ...
Dilute electrolytic cells filled with chlorobenzene containing small amounts of tetrabutylammonium
tetraphenylborate show anomalous dielectric dispersions in low-frequency regions. We propose a new model
for electrode polarization in order to analyze the dielectric behavior of the dilute electrolytic cells. The model
comprises two capacitive components: One is the space-charge polarization accompanied with a specific ionic
adsorption on electrodes, and the other is the electrode capacitance which is brought about by an electronic
spillover from the electrode surface. This model can primarily explain the anomalous frequency-dependent
dielectric behavior of the electrolytic cells not only with low electrolyte concentrations, but also with high
concentrations and can correctly describe the characteristics of the electrode polarization reflected in the dielectric
spectra.
... Electrical conduction of the liquid crystals has an ionic origin. The ions are due to either chemical decomposition of the molecules forming liquid crystals themselves or to the impurities introduced intentionally or unintentionally during sample preparation or measurement cell ( [9] and the references cited there). ...
... This approach allows us to solving the problem and to obtain exact analytical form for the admittance (impedance) of the considered physical system. This mathematical model was used previously [24] for simple EP case when the mobile positive and negative charge carriers have equal diffusion coefficients, are univalent and do not recombine [9,16,24]. Assumption that the properties of negative charge carriers, i.e. positive, physical quantities are numerically equal although useful simplification calculations is not realistic. For ion diffusion coefficients and mobility are relatively close in certain samples, but in most cases they are different in value. ...
... Further is analyzed the situation specified by the following assumptions [17,18,20] [9,20,24]; b) blocking electrodes with adsorption/desorption process, the electric current densities are equal to the temporal variation for the surface density of adsorbed ions [16,17,22]. Another hypothesis is that the ions are point-like. ...
The influence of mobile ions on the results of impedance spectroscopy (dielectric spectroscopy) measurements performed on a liquid crystal cell using the new mathematical model recently described was investigated. This mathematical model reformulates the fundamental equation system of continuity for mobile charge carriers and the Poisson equation using new variables. One makes the following assumptions: ions have different mobilities and diffusion coefficients, there is no generation-recombination process, the equilibrium carrier concentrations are uniform and equal each other, the electrodes are either completely blocking or blocked with adsorption-desorption processes. The final result is the analytical expression of the equivalent admittance for the system, allowing to have a clearer picture of the mobile ions and of the processes that occur at the electrode interface influencing the dielectric behavior.
... One of the fundamental problems impeding the understanding of ionic conductors is the inability to unambiguously estimate the free-ion diffusivity and number density from the measured dc conductivity. One possible way to determine these quantities is to analyze the electrode polarization (EP) effect [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] observed by dielectric spectroscopy. In a typical dielectric measurement, cations and anions migrate under the applied electric field and accumulate on the surface of electrodes. ...
... One of the current theoretical models of electrode polarization, proposed by Macdonald [1] and Trukhan [2], is based on the Debye-Hückel type of approach, where the Nernst-Planck equation for ionic motion is combined with the Poisson equation and linearized with respect to the electric field. Other models [4,9,10,16,19] are typically based on similar methods. It is not obvious that the EP analysis will hold at high ion concentrations, when the ionic interaction strength exceeds the thermal energy. ...
Electrode polarization analysis is frequently used to determine free-ion diffusivity and number density in ionic conductors. In the present study, this approach is critically examined in a wide variety of electrolytes, including aqueous and nonaqueous solutions, polymer electrolytes, and ionic liquids. It is shown that the electrode polarization analysis based on the Macdonald-Trukhan model [J. Chem. Phys. 124, 144903 (2006); J. Non-Cryst. Solids 357, 3064 (2011)] progressively fails to give reasonable values of free-ion diffusivity and number density with increasing salt concentration. This should be expected because the original model of electrode polarization is designed for dilute electrolytes. An empirical correction method which yields ion diffusivities in reasonable agreement with pulsed-field gradient nuclear magnetic resonance measurements is proposed. However, the analysis of free-ion diffusivity and number density from electrode polarization should still be exercised with great caution because there is no solid theoretical justification for the proposed corrections.
... Theoretical analyses that assume the presence of equal mobilities for positive and negative charges (e.g., [34,39,43]) might seem to be of much less practical value than those that allow arbitrary values of the mobility ratio. But two-mobile ambiguities involving equal mobility values, which apply to type-B (see ambiguity #3) as well as to type-A situations, show that the results of unlikely KDL = 0 theoretical Table 3. Except for ambiguities #s 1-3, an exact data set calculated from a one-mobile ( m = 10 38 ) fully blocking PNP model is fitted using CNLS with proportional weighting by the same model but set for the two-mobile, equal mobility condition ( m = 1), and the changes induced in the relevant parameters listed in column two are shown in column three. ...
Several important ambiguities in immittance spectroscopy (IS) model data-fitting results are identified and illustrated by means of complex-nonlinear-least-squares (CNLS) fits of experimental and synthetic frequency response data. A well-known intrinsic ambiguity, following from Maxwell's electromagnetic equations, arises from the indistinguishability in external measurements of conduction and displacement currents. Usual fit models for either dielectric or conductive-system situations, such as the Davidson-Cole one, only involve a strength parameter, a dielectric constant, a characteristic relaxation time, and a fractional exponent and lead to no additional ambiguities. But the situation is different for more powerful and useful general models, such as ordinary or anomalous diffusion Poisson-Nernst-Planck ones: PNP and PNPA, used here, whose historical background, current status, and applicability are described and discussed herein. They apply to two different kinds of experimental IS situations and involve several additional, potentially free fit parameters, such as the mobilities of positive and negative charge carriers, and generation-recombination parameters that determine the partial or complete dissociation of a neutral entity of concentration N(0) into positive and negative charge carriers of equal concentration, c(0). Then, several additional ambiguities appear that may require information about the material system involved for their adequate resolution.
