Dielectric Dispersion Spectra Research Articles

Fast crystallization of InSe thin films via pulsed laser welding technique and effect of crystallinity on the optical and dielectric properties

In the current study the crystalline phase of indium selenide thin films which were grown by the thermal evaporation technique is achieved via pulsed laser welding technique (PLW) in a second. The films crystallinity is achieved under various welding conditions including the pulse width ( PW ), repetition frequency ( fr ) and pulse diameter ( d ). The optimum parameters for obtaining well crystalline phase are PW=1.0 ms, fr=10 Hz and d= 1.0 mm. PLW induced crystallinity showed preferred structure relating to monoclinic phase of InSe. Compositionally while amorphous films exhibited In2Se3 chemical structure, crystalline ones preferred InSe phase. Associated with this type of crystallinity, direct and indirect energy band gap values of 2.32 eV and 3.12 eV are determined. The crystalline films showed lower dielectric constant value accompanied with higher optical conductivity and higher terahertz cutoff frequency in the infrared range of light. In addition the dielectric dispersion spectra were treated using Drude–Lorentz model to read the optical conductivity parameters for the PLW assisted crystalline InSe terahertz resonators. The treatment showed that the crystallinity of the films resulted in improved free carrier density, longer relaxation times at femtosecond level, larger plasmon frequencies and higher drift mobility values. These features together with the response of terahertz cutoff frequency to IR excitations make crystalline InSe thin films promising for optoelectronic and terahertz technology applications.

Broadband dielectric behaviour and structural characterization of PVDF/PMMA/OMMT polymer nanocomposites for promising performance nanodielectrics in flexible technology advances

Characterization of broadband dielectric behaviour of polymer nanocomposites (PNCs) is vital for the exploration of efficient nanodielectrics as energy storage, flexible dielectric substrates, and insulators in a wide range of advanced electronic device technologies. Accordingly, herein, PNC films based on poly(vinylidene fluoride) (PVDF)/ poly(methyl methacrylate) (PMMA) blend matrix (80/20 wt/wt%) dispersed with 0, 2.5, 5, and 10 wt% organo-modified montmorillonite (OMMT) nanoclay are developed by state-of-the-art homogenized solution casting method. These PVDF/PMMA/OMMT compositions based flexible PNC films are characterized in detail by employing a scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDX) device, x-ray diffractometer (XRD), Fourier transform infrared (FTIR) spectrometer, differential scanning calorimeter (DSC), inductance-capacitance-resistance (LCR) meter, and impedance/material analyzer (IMA). The SEM microimages, XRD traces, and FTIR spectra evidenced appreciable homogeneity and surface morphology, intercalated and exfoliated OMMT structures, and the α, β and γ-phase crystallites of the PVDF in these complex semicrystalline PNCs. The DSC thermograms confirmed a significant alteration in the melting temperature and degree of crystallinity of the PVDF crystallites with the increased amount of OMMT in the 80PVDF/20PMMA blend host matrix. The broadband dielectric dispersion spectra over the frequency range of 20 Hz−1 GHz explained the contribution of interfacial polarization in the complex dielectric permittivity at lower experimental frequencies, whereas at higher frequencies permittivity is ruled by dipolar polarization in these composites at 27 °C. The dielectric loss angle tangent and electric modulus spectra revealed an intense structural dynamics relaxation process in the upper radio frequency region. The influence of OMMT concentration on the dielectric permittivity and electrical conductivity is explored. The detailed dielectric and electrical characterization of these innovative semicrystalline composites with important structural and thermal properties revealed their immense potential as high-performance nanodielectrics for highlighting current applications of broadband frequency range electrical and electronic device technologies.

Growth and characterization of (glass, Ag)/SeO2 thin films

Herein, thin films of SeO2 coated onto glass and Ag thin film substrates are studied and characterized. The films which are prepared by a vacuum evaporation technique, displayed Ag-metal induced crystallization process. The grown films are composed of tetragonal structure of SeO2 as major phase and orthorhombic β−Ag2Se as minor phase. The optical studies showed that the films exhibit an energy band gap of 3.69 eV. In addition, the analyses of the dielectric dispersion spectra indicated the nonlinear character of dielectric response. The SeO2 films response to oscillatory electromagnetic field is accompanied with drift mobility values in the range of 5.02–12.06 cm2/Vs. Moreover, polycrystalline SeO2 films coated onto Ag substrates are found appropriate for use as negative capacitance (NC) sources in the frequency domain of 0.41–1.80 GHz. Accompany with the NC effect, negative conductance (NG) effect was observed near 0.410 GHz and near 1.53 GHz.

Metaphor of molecular dynamics and dielectric dispersion of morpholine with aprotic solvents

ABSTRACT The dielectric dispersion spectra of morpholine with methanol and ethanol binary mixtures have been studied by using Time Domain Reflectometry. Frequency dependent variations of specified mole fraction for the temperature 288 K, 293 K, 298 K, 303 K were determined. The complex permittivity spectra fitted by Debye model. The dielectric parameters of the associating liquids analysed and the ordering of molecules favours two relaxation sites and discussed on the basis of dielectric properties. The swapping bond of H at N–H favours intermolecular interaction. The intra-molecular exists between O and N within the ring. The excess properties of the mixture solutions has effective Kirkwood correlation factor that predicts molecular interaction and orientation among solute and solvents. The thermodynamic parameters ΔG, ΔH and ΔS of the solutions and dielectric loss factor of the mixed solvents bearing alcohols were determined. FTIR spectral assignments of solutions validate the molecular interaction and their shifts.

Origin of the high dielectric constant in Sm2/3Cu3Ti4O12 ceramics

The microstructure, crystalline structure, dielectric and electrical properties of polycrystalline Sm2/3Cu3Ti4O12 ceramics, prepared via conventional solid-state reaction were investigated. The structural evolution of these powders was analyzed by X-ray diffraction (XRD). Crystal structure investigated by Rietveld refinement was found to be cubic with space group Im3. It has been found that Sm2/3Cu3Ti4O12 ceramic shows giant dielectric permittivity values with low frequency (1 kHz) larger than 30,000 at room temperature. In the temperature domain, a new dielectric relaxation was clearly observed beyond 200 K, in addition to the well-investigated dielectric relaxation close to 100 K. This Maxwell–Wagner type of relaxation was found to be originating from the formation of external depletion layers at the electrode-sample interface. The dielectric relaxation at high frequencies in the dielectric dispersion spectra of Sm2/3Cu3Ti4O12 ceramics is caused by an IBLC effect associated with the insulating grain boundaries and the other one at low frequencies originates from an electrode polarisation effect. The activation energy of semi conducting grain is found to be similar to that of CCTO (∼0.06 eV). The observed giant value of the dielectric constant in the Sm2/3Cu3Ti4O12 ceramics originates due to polarisation at the electrode- sample interface and at the insulating grain boundary interface. The results suggest that the IBLC effect mechanism that was formerly proposed for CCTO ceramics is also valid in explaining the high dielectric constant in the compositionally and structurally CCTO-like, Sm2/3Cu3Ti4O12 ceramics.

Crucial role of percolation transition on the formation and electromagnetic properties of BaTiO3/Ni0.5Zn0.47Fe2O4 ceramic composites

BaTiO3/Ni0.5Zn0.47Fe2O4 ceramic composites with various compositions were sintered at different temperatures. It is found both the formation and electromagnetic properties of the composite ceramics are strongly dependent on the percolation transition of NZFO. As NZFO content x is low (x=0.1 and 0.2), the density increases with increasing the sintering temperature T from 1220°C to 1300°C, while it decreases as x is high (x=0.65, 0.75 and 0.85). It is ascribed to the mismatch between the densification of BTO and NZFO. Both the permeability and low-frequency permittivity of the composites exhibit typical percolation behaviors, which are appropriate to the Kirkpatrick׳s effective medium model with the percolation threshold fc ranging from 0.35 to 0.5. As x<fc, the permeability varies little and the low-frequency permittivity decreases with increasing T, owing to the dominant control of nonmagnetic BTO and the decrease in the defects in BTO, respectively. In comparison, as x>fc, both the permeability and permittivity increase considerably with increasing T. By analyzing the magnetic and dielectric dispersion spectra, it is revealed they are attributed respectively to the enhancement of domain-wall motion and the increase in the grain conductivity of NZFO.

Impedance performances of SnO2–Zn2SnO4 composite ceramics

To investigate the formation of grain boundary barriers, composite SnO2–Zn2SnO4 varistor ceramics were prepared by traditional ceramic processing and, the electrical properties of grains and grain boundaries were evaluated by the measurement of complex impedances spectra. Calculated using the low frequency impedance data for each sample, a lower value about 0.09–0.32 eV and a higher value about 0.50–0.83 eV of activation energy Ea was obtained in the temperature range of 50–110 and 140–200 °C, respectively. The simulated results of complex impedances spectra by “Zsimpwin” software indicate that the equivalent circuit for SnO2–Zn2SnO4 composite ceramics is connected in series by one RC and two QR parallels. The relaxations in the dielectric dispersion spectra at low frequency of ∼1000 Hz suggest that one of the QR parallels is corresponding to oxygen vacancy. With increasing sintering temperature, the Ea values related to oxygen vacancy decreased whereas, the values related to grain boundary barriers increase obviously. The results suggest that the oxygen vacancy is the key factor to the semi-conductance of SnO2 grains and formation of grain boundary barriers.

Large decrease of characteristic frequency of dielectric relaxation associated with domain-wall motion in Sb5+-modified (K,Na)NbO3-based ceramics

The (K,Na)NbO3-based ceramics have drawn considerable attention as a type of promising lead-free piezoelectric materials in recent years. However, investigations on the dielectric dispersion spectra in the microwave range have rarely been conducted so far. Dielectric dispersion spectra of several representative (K,Na)NbO3-based ceramics were measured and compared in this study. An interesting physical phenomenon that the Sb5+-modified (K,Na)NbO3-based ceramics differ distinctly from those without Sb5+-modification in the aspect of characteristic frequency fp of dielectric relaxation associated with the domain-wall motion has been found. The former group shows the fp values in several tens of MHz at room temperature, whereas the latter group has generally the fp values of several GHz at least. For the Sb5+-modified (K,Na)NbO3-based ceramics, the change of fp with temperature follows roughly a thermally activated character. In contrast, the ones without Sb5+-modification exhibit a temperature-insensitive character in fp. Analysis showed that the results could be understood from the viewpoint of domain-wall vibration. It is speculated that a large change occurring in the damping constant due to the incorporation of Sb5+ is possibly the origin.

Microwave dielectric spectra and molecular relaxation in formamide–N,N-dimethylformamide binary mixtures

The dielectric dispersion and absorption spectra of formamide (FA), N,N-dimethylformamide (DMF) and their binary mixtures are investigated in the frequency range of 500 MHz to 20 GHz at 30 °C in view of the organic synthesis by microwaves heating using amides solvents. The concentration dependent values of molecular reorientation relaxation times lower than that of the ideal mixing behaviour have been attributed to the cooperative dynamics of H-bonded FA–DMF structures. The molar ratio of stable adduct is 2:1 of FA to the DMF, which is determined from the concentration dependent excess static dielectric constant and the relaxation time plots of these binary mixtures. Electrode polarization effect and ionic conduction in FA and DMF were investigated from their dielectric dispersion spectra in the low frequency region of 20 Hz to 1 MHz.

Giant dielectric-permittivity property and relevant mechanism of Bi 2/3Cu 3Ti 4O 12 ceramics

A series of Bi 2/3Cu 3Ti 4O 12 (BCTO) ceramics were prepared via conventional solid-state reaction under different sintering temperature conditions. Their microstructures, crystalline structures and dielectric and electrical properties were investigated. It has been found that all of the BCTO ceramics show giant dielectric-permittivity values with low frequency ɛ′ larger than 1.5 × 10 5 at room temperature. There exist two relaxations in the dielectric dispersion spectra and three semicircles in the complex impedance plane. In general, BCTO ceramics are qualitatively quite similar to CaCu 3Ti 4O 12 (CCTO) ceramics in the aspect of dielectric and electrical properties. However, unlike CCTO ceramics, BCTO ceramics possess the dense microstructures of small and uniformly distributed grains, which are insensitive to sintering temperature conditions. From analysis, we attribute the dielectric relaxation in the high frequency region to an internal barrier layer capacitance (IBLC) effect associated with insulating grain boundaries and semiconducting grains and the other one in the low frequency region to an electrode polarization effect due to the Schottky barriers formed at electrode–ceramic interfaces.

Analysis of the Relaxor-Like Behavior in a Ferroelectric Copolymer P(VDF-TrFE)

The frequency dependence of real (ϵ′) and imaginary (ϵ″) components of the complex dielectric permittivity (ϵ*) of poly(vinylidene fluoride-triflouroethylene) copolymer has been determined in the temperature range from −40 to 140°C including the transition to glassy state at Tg ≈ −25°C and ferroelectric-to-paraelectric (F-P) transition. During the cooling, the relaxor-like behavior is observed up to TC for enough high frequencies. Analysis of the dielectric dispersion spectra was carried out using the Havriliak-Negami empirical model. The observed peculiarities of the dielectric response are discussed on the basis of the coexistence of both the short range and long range interactions.

Dielectric and impedance performances of giant dielectric constant oxide CaCu3Ti4O12

Pure colossal dielectric constant oxide CaCu 3 Ti 4 O 12 (CCTO) compound was prepared by traditional ceramic processing. Dielectric dispersion and complex impedances spectra were investigated using a impedance analyzer within a temperature range of 10—420 K. The data were simulated by “ZVIEW” software. The result indicates that there are two obvious relaxations in the dielectric dispersion spectra when the temperature is higher than room temperature and the dielectric constant increases remarkably with increasing temperatures at a low frequency, which indicates a thermal ionic polarization. However, the frequency spectra becomes similar to Debye-type relaxation when the temperature is lower than room temperature and the low-and high-frequency relaxation step almost keeps unchanged with temperature, which reveals a feature of interface polarization and considerable temperature stability for CCTO. The relaxation revealed in the frequency spectra corresponds to the three different semicircles revealed by the impedance spectra, which indicated there are three inhomogeneous regions or polarization processes in CCTO ceramics and the colossal dielectric constant mainly comes from the extrinsic polarization of these inhomogenities. The activation energies are found to be respectively 0.05 eV, 0.58 eV and 0.49eV for the three different polarization processes by simulating the impedance semicircles using an equivalent circuit.

Spectral Representation Theory for Dielectric Behavior of Nonspherical Cell Suspensions**The project supported by the Research Grants Council of the Hong Kong SAR Government

Recent experiments revealed that the dielectric dispersion spectrum of fission yeast cells in a suspension was mainly composed of two sub-dispersions. The low-frequency sub-dispersion depended on the cell length, while the high-frequency one was independent of it. The cell shape effect was simulated by an ellipsoidal cell model but the comparison between theory and experiment was far from being satisfactory. Prompted by the discrepancy, we proposed the use of spectral representation to analyze more realistic cell models. We adopted a shell-spheroidal model to analyze the effects of the cell membrane. It is found that the dielectric property of the cell membrane has only a minor effect on the dispersion magnitude ratio and the characteristic frequency ratio. We further included the effect of rotation of dipole induced by an external electric field, and solved the dipole-rotation spheroidal model in the spectral representation. Good agreement between theory and experiment has been obtained.

First-principle approach to dielectric behavior of nonspherical cell suspensions.

We present a theoretical study of the dielectric behavior of cell suspensions by employing the Bergman-Milton spectral representation of the effective dielectric constant. By means of the spectral representation, we derive the dielectric dispersion spectrum in terms of the electrical and structure parameters of the cell models. Our results show that a better agreement with the experimental data can be obtained, provided that we introduce a conductivity contrast t=sigma(2)/(sigma(2)-sigma(1)). We find that the conductivity of the cell cytoplasm sigma(1) can be much larger than that of the suspending medium sigma(2), in contrast to the previous claim that sigma(1) approximately sigma(2).

Dielectric behaviour of non-spherical cell suspensions

Recent experiments revealed that the dielectric dispersion spectrum of fission yeast cells in a suspension was mainly composed of two sub-dispersions. The low-frequency sub-dispersion depended on the cell length, whereas the high-frequency one was independent of it. The cell shape effect was qualitatively simulated by an ellipsoidal-cell model. However, the comparison between theory and experiment was far from being satisfactory. In an attempt to close up the gap between theory and experiment, we considered the more realistic cells of spherocylinders, i.e. circular cylinders with two hemispherical caps at both ends. We have formulated a Green function formalism for calculating the spectralrepresentation of cells of finite length. The Green function can be reduced because of the azimuthal symmetry of the cell. This simplification enables us to calculate the dispersionspectrum and hence assess the effect of cell structure on thedielectric behaviour of cell suspensions.

Molecular Dynamics Simulations of Polar Polymer Brushes

Brushes of end-grafted, polar polymer chains in a good, nonpolar solvent are studied via molecular dynamics simulations of a bead-spring model. The monomers are connected by finitely extensible nonlinear springs. The springs connecting consecutive monomers in a chain are labeled with dipoles parallel to the chain contour. We consider the dipoles as limits of charge pairs, replacing each dipole by a pair of charges positioned at the centers of mass of two consecutive beads. Equilibrium simulations are performed, and the static and dynamic properties of the chains are related to the dipole magnitude. It was found that the electrostatic interactions lead to a shorter brush and the collapse becomes more pronounced with increasing segmental dipole moment. The dielectric relaxation of the tethered chains is studied, and the dielectric permittivity is obtained from equilibrium dipole moment correlation functions. Results on the dielectric relaxation of tethered chains from experiments conducted by Yao et al. exhibited a broader dielectric dispersion spectrum than that of free macromolecules in homogeneous solutions. These authors attributed the results to the thermodynamic confinement (to keep uniform segment density) and the spatial confinement that prohibits a chain to cross the tethering domain boundary. The simulations indicate that neither the spatial confinement nor the thermodynamic confinement lead to a broader dielectric dispersion spectrum for the tethered chains, and we attribute the discrepancies to the morphology differences between the experimental system and the simulated one. Weak electric fields parallel and perpendicular to the grafting surface are applied, and the system response is related to the equilibrium fluctuations. We also examine the structural properties of the brushes at steady state under the influence of external electric fields.

Dielectric dispersion of the dipolar glass K0.989Li0.011TaO3

Abstract Dielectric dispersion spectra measured on K0.989Li0.011TaO3 at frequencies 10−3 ≤ f ≤ 107 Hz and temperatures 5 ≤ T ≤ 100 K are analyzed in terms of Lacroix-Bene type distributions of relaxation times, G(τ) α (τ - τ)−β τ σ is related to the local hopping diffusion of isolated off-center Li+ dipoles. It varies as exp(δE/kBT) with δE/kB = 910 K at T > 40 K and crosses over into the quantum tunneling limit, τmin ∼ 102 s as T → 0. The exponent β decreases with decreasing T. β ∼ 0.05 as T → 30 K, in accordance with the observed asymmetric low-frequency broadening of e″ vs log f. Upon approaching the static freezing temperature, Tg = 33.7 K, power law divergence of τmax with an exponent zv = 15.1 is observed.

Study of the compatibility of plasticizers with poly(vinyl chloride) by dielectric spectroscopy

Dielectric spectroscopy has been applied for studying the compatibility of plasticizers with poly (vinyl chloride) as well as for detecting the phase-inhomogeneities of plasticized systems. Commercial plasticizers having different polarity and compatibility with the polymer have been investigated. The effect of insufficient processing as well as storage and annealing on the physical structure of the compounds are also discussed. For obtaining dielectric spectra, the real and imaginary parts of the dielectric permittivity of liquid plasticizers and plasticized compounds were recorded continuously as a function of temperature in the range from −170 up to +200°C at fixed frequencies and heating-cooling rates. Exudation, the weight loss of the plasticizer, as well as mechanical properties were measured comparatively. It is a generally accepted fact that the compatibility of a plasticizer with a given polymer depends upon their dielectric properties. According to our observations, the temperature dependence of the dielectric permittivity of the plasticizer is of great importance from this point of view, especially in the range of application of the plasticized system. Comparing the dielectric dispersion spectrum of a liquid plasticizer to that of the corresponding plasticized compound, conclusions about the compatibility can be derived. A special polarization is found to be originated from the phase-separation and from the inhomogeneities. Besides the dielectric polarization arising from the polymer chain and plasticizer dipoles, the dielectric spectra show the existence of an interfacial, Maxwell-Wagner type polarization, which is found to be dependent on the inhomogeneities of the texture of the compound.