Dielectric Loss Spectra Research Articles

A first-principles study of the properties of four predicted novel phases of AlN

Structural, elastic, thermodynamic, electronic and optical properties of four predicted novel AlN phases (Pmn21-AlN, Pbam-AlN, Pbca-AlN and Cmcm-AlN) are calculated using first-principles according to density function theory (DFT). These phases were found using the CALYPSO method but have not yet been synthesized experimentally. Here we predict some of their properties. The properties are analyzed by means of GGA-PBE and PBE0 respectively. The more precision results are obtained by PBE0. Cmcm-AlN owns better plasticity and it's Young's modulus has clearer anisotropy than Pmn21-AlN, Pbam-AlN and Pbca-AlN. The Debye temperature, under higher temperature, shows weak temperature dependence and approach to a constant value. The Dulong-Petit limit of all four novel AlN phases and wz-AlN is about 48Jmol−1K−1 and they have almost the same temperature law. The band structures show that the four AlN are the wide direct band gap semiconductors, which band gaps are 5.95 (Pmn21-AlN), 5.99 (Pbam-AlN), 5.88 (Pbca-AlN) and 5.59eV (Cmcm-AlN). The bonding behaviors are the combination of covalent and ionic nature. The dielectric constants, refractive index, reflectivity, absorption, loss spectra, conductivity and Raman spectra are also calculated in detail. All four phases have a lower plasma frequency than of wz-AlN.

Trap-Induced Dispersive Transport and Dielectric Loss in PbS Nanoparticle Films

Abstract In this work, we investigate the electrical and dielectric response of lead sulfide (PbS) nanoparticle (NP) films with impedance spectroscopy. In particular, the influence of the ligand passivation on the surface trap state density of PbS NPs is demonstrated by comparing two different types of ligands: ethane-1,2-dithiol (EDT) and 3-sulfanylpropanoic acid (MPA). We observe that the MPA treatment passivates the PbS surface more efficiently than EDT. By analyzing the dielectric loss spectra, we are able to visualize shallow trap states in the bulk of PbS-EDT films and correlate this with the dispersive response observed in the impedance spectra. Evidence of deep trap states is revealed for both PbS-EDT and PbS-MPA diodes. Under illumination, the PbS-MPA and PbS-EDT films demonstrate almost identical trap profiles, showing solely the deep trap state densities. We conclude that the deep traps are related to the stoichiometry of the PbS NPs.

Hydrogen Bonding and Dielectric Spectra of Ethylene Glycol-Water Mixtures from Molecular Dynamics Simulations.

Mixtures of ethylene glycol with water are a prominent example of media with variable viscosity. Classical molecular dynamics simulations at room temperature were performed for mixtures of ethylene glycol (EG) and water with EG mole fractions of xE = 0.0, 0.1, 0.2, 0.4, 0.6, 0.9, 1.0. The calculated dielectric loss spectra were in qualitative agreement with experiment. We found a slightly overestimated slowdown of the dynamics with increasing EG concentration compared to experimental data. Statistics of the hydrogen bond network and hydrogen bond lifetimes were derived from suitable time correlation functions and also show a slowdown in the dynamics with increasing xE. A similar picture is predicted for the time scales of EG conformer changes and for molecular reorientation. A slight blue shift was obtained for the power spectra of the molecular center of mass motion. The results were used to give a qualitative interpretation of the origin of three different relaxation times that appear in experimental complex dielectric spectra and of their change with xE.

Electronic, elastic, and optical properties of monolayer BC2N

The structural stability, electronic structure, elasticity, and optical properties of four types of monolayer BC2N have been investigated from first principles using calculation based on density functional theory. The results show that the structural stability of BC2N increases with the number of C–C and B–N bonds. By calculating the two-dimensional Young's modulus, shear modulus, Poisson's ratio, and shear anisotropic factors in different directions, four structures present various anisotropies and the most stable structure is almost isotropic. For C-type BC2N, the values of two-dimensional Young's modulus, shear modulus, and bulk modulus (309, 128, 195GPam−1), are smaller than those of graphene (343, 151, 208) but bigger than those of h-BN (286, 185, 116). Furthermore, the dielectric function, refractive index, reflectivity, absorption coefficient, and energy loss spectrum are also calculated to investigate the mechanism underpinning the optical transitions in BC2N, revealing monolayer BC2N as a candidate window material.

Structural, magnetic and microwave absorption behavior of Co-Zr substituted strontium hexaferrites prepared using tartaric acid fuel for electromagnetic interference suppression

Strontium hexaferrites, doped with varying Co-Zr content (x) have been synthesized by sol-gel auto-combustion route using tartaric acid as fuel at 800°C. X-ray diffraction and Fourier transform Infra-red have been carried out to confirm the phase formation, particle size (average 21.9–36.8nm) and the bond formation respectively. Magnetic properties are scrutinized using vibrating sample magnetometer. Techniques like scanning electron microscopy, transmission electron microscopy and energy dispersive scattering have been employed to explore the surface morphology, particle size and composition of the nano-powders. Electromagnetic characterization of the prepared ferrites has been done using Vector Network Anlyzer in 12.4–18GHz frequency range. The effect of calcination temperature (500–1000°C) on the structure, morphology and magnetic properties has also been studied for x=0.2 and 800°C has been found to be the most suitable temperature with the best magnetic properties. Increase in doping has resulted in resonance peaks in dielectric and magnetic loss spectra, leading to microwave absorption peaks. Ferrites with x=0.2, 0.8 and 1.0 have appropriate reflection loss less than −10dB and bandwidth in Ku-band, hence can be used as effective absorbers in suppression of electromagnetic interference (EMI). The governance of impedance matching in deciding the absorption properties has been proved by using input impedance calculations.

Molecular dynamics simulations of the dielectric properties of fructose aqueous solutions

The static dielectric permittivity and dielectric relaxation properties of fructose aqueous solutions of different concentrations ranging from 1.0 to 4.0 mol l−1 are investigated by means of molecular dynamics simulations. The contributions from intra- and interspecies molecular correlations were computed individually for both the static and frequency-dependent dielectric properties, and the results were compared with the available experimental data. Simulation results in the time- and frequency-domains were analyzed and indicate that the presence of fructose has little effect on the position of the fast, high-frequency (>500 cm−1) components of the dielectric response spectrum. The low-frequency (<0.1 cm−1) components, however, are markedly influenced by sugar concentration. Our analysis indicates that fructose–fructose and fructose–water interactions strongly affect the rotational-diffusion regime of molecular motions in the solutions. Increasing fructose concentration not only enhances sugar–sugar and sugar-water low frequency contributions to the dielectric loss spectrum but also slows down the reorientational dynamics of water molecules. These results are consistent with previous computer simulations carried out for other disaccharide aqueous solutions.

Structural and electrical features of rare earth based double perovskite oxide: Pr2NiZrO6

Dielectric relaxation measurements are carried out on Pr2NiZrO6 (PNZ) sample at several frequencies between temperature 313 K and 593 K. The powder was analyzed by X-ray powder diffraction (XRD). Rietveld analysis identified a highly crystalline double perovskite phase. However, a secondary pyrochlore phase, Pr2Zr2O7, was also detected. The dielectric spectrum shows several relaxation processes in increasing order of temperature. Detailed studies of dielectric and impedance parameters as a function of temperature and frequency have shown the significant effect of grains and grain boundaries in the relaxation process. Dielectric loss spectra reveal that dc conduction contribution predominates in the compound. Imaginary part of electric modulus (M″) exhibit two sets of apparent peaks corresponding to the relaxed dipole and to the conductive process. The scaling analyses reveal different temperature dependence of conductivity for PNZ. The temperature dependent conductivity was conformed from the Arrhenius relation and the activation energy is found to be 0.357 eV.

Valley plasmonics in transition metal dichalcogenides

The rich phenomenology of plasmonic excitations in the dichalcogenides is analyzed as a function of doping. The many-body polarization, the dielectric response function and electron energy loss spectra are calculated using an ab initio based model involving material-realistic Coulomb interactions, band structure and spin-orbit coupling. Focusing on the representative case of MoS$_2$, a plethora of plasmon bands are observed, originating from scattering processes within and between the conduction or valence band valleys. We discuss the resulting square-root and linear collective modes, arising from long-range versus short-range screening of the Coulomb potential. We show that the multi-orbital nature of the bands and spin-orbit coupling strongly affects inter-valley scattering processes by gapping certain two-particle modes at large momentum transfer.

So-called ‘relaxation’ mode located at 0.1 cm−1 in the low-frequency Raman spectra of ethylene glycol - acetone mixtures

Low frequency Raman spectra of ethylene glycol(EG) and acetone mixtures are investigated by high-resolution double monochrometer. A broad peak located at 0.1 cm−1 in susceptibility spectra is compared with the dielectric loss spectra. Both Raman relaxation time(τMRT1) and dielectric relaxation time(τD1) are determined by the model fitting method. The ratio τMRT1/τD1 does not depend on static dielectric constant of mixtures and is almost constant, ≈3.2, from 1.0 (pure EG) to 0.3 mole fraction. This result indicates that both τMRT1 and τD1 are determined by the same molecular dynamics.

Role of pH value during material synthesis and grain-grain boundary contribution on the observed semiconductor to metal like conductivity transition in Ni1.5Fe1.5O4 spinel ferrite

Ni1.5Fe1.5O4 ferrite samples were synthesized by maintaining different pH values (1–12) during chemical reaction at 80 °C. The as-prepared samples were annealed at 1000 °C to form a cubic spinel structure. The heat treated samples were used for the study of electrical conductivity and dielectric properties. In this work, we understand the mechanism of unusual metal-like state in ferrite samples, characterized by negative temperature coefficient of conductivity. We have discussed various aspects, e.g., hopping mechanism through superexchange paths (Fe3+-O2−-Fe3+ and Ni2+-O2−-Ni2+), charge delocalized conduction mechanism affected by the magnetic spins order in t2g and eg electronic energy levels of B sites cations, grain size variation, relaxation of charge carriers at grains and grain boundaries of the particles, for outlining the mechanism of thermal activated charge localization (semiconductor state) and delocalization (metal-like state) effect in our samples. We have carried out a detailed analysis of conductivity spectra (Jonscher's power law fit, scaling of conductivity), impedance spectra (Cole-Cole plot by incorporating constant phase element), modulus spectra (Bergman proposed KWW function), and dielectric loss and dielectric constant spectra to extract the conductivity and relaxation contributions from grains, grain boundaries and space charge polarization in the samples.

Dielectric properties of 1-methylimidazole at temperatures 298–336 K

ABSTRACT Dielectric permittivity and loss spectra of 1-methylimidazole (MeIm) have been measured over the frequency range from 100 MHz to 20 GHz at 298–336 K using an open-ended coaxial probe technique. Dielectric parameters are described by the Davidson–Cole model. The shape parameter shows temperature-independence. The Kirkwood correlation factor (g), enthalpy, entropy and the activation energy of molecular reorientation of MeIm also have been calculated.

Interchannel Hopping in Single Crystalline Lithium Triborate Probed by 7Li NMR: Spin Relaxation, Line Shape Analysis, Selective-Inversion Spin Alignment, and Two-Dimensional Exchange Spectra

The ion dynamics of single crystalline lithium triborate, LiB3O5, an important material in nonlinear optics, is studied using various 7Li and 11B nuclear magnetic resonance (NMR) techniques at temperatures from about 480 to 780 K in order to elucidate the apparent discrepancies underlying previous interpretations of NMR line shape analyses and results from dielectric spectroscopy. Rotating frame spin–lattice relaxation as well as line shape measurements are carried out and are combined with selective-inversion spin alignment as well as two-dimensional chemical exchange spectroscopy to track the temperature-dependent Li ion motion. From symmetry considerations the latter is clearly identified as interchannel hopping. By combining the present results with those from the published, yet so far not fully analyzed, dielectric loss spectra, it is shown how seeming differences in energy barriers hindering the ion motion and in the evolution of the distribution of correlation times can be reconciled.

Dielectric Dispersion and Relaxation in Polymer Blend Based Nanodielectric Film

SummaryPolymer nanodielectrics is the exponential growing field of advanced materials sciences. Such flexible materials have recognized their suitability as substrate and insulator in fabrication of microelectronic devices, and also in preparation of ion conducting solid polymer electrolytes. The nanodielectric film consisted of poly(ethylene oxide) and poly(methyl methacrylate) (PEO–PMMA; 75/25 wt/wt%) blend with 5 wt% montmorillonite (MMT) clay as nanofiller has been prepared by solution‐casting method followed by melt‐pressed technique. The X‐ray diffraction study of the film confirms the intercalated MMT structures in the semicrystalline phase of PEO–PMMA blend. The scanning electron microscopic images confirm the significant variation in morphology of the PEO–PMMA blend on addition of MMT nanofiller. The dielectric relaxation spectroscopy has been employed for measurement of complex dielectric function, ac electrical conductivity and electric modulus spectra of the nanodielectric film in the frequency range 20 Hz–1 MHz. The value of real part of dielectric function of the film is found high at low frequencies due to contribution of electrode polarization effect, but it decreases non‐linearly with the increase of frequency and finally approaches the high frequency limiting value of 2.3 at 1 MHz. The relaxation peak in dielectric loss spectra corresponding to polymer segmental motion is found above 10 kHz, and the peak positions have a shift towards higher frequency side with the increase of temperature. The dc conductivity increases and the relaxation time of the polymer segmental motion decreases with increasing temperature of the nanodielectric film according to the Arrhenius behaviour.

Magnetic and optical properties in the 1D TM–O chain compounds Sr2TMO3 (TM = Ni, Co): A first-principle investigation

In this paper, we have calculated the structural, electronic, magnetic and optical properties of Sr2NiO3 and Sr2CoO3 using density functional theory (DFT) within generalized gradient approximation (GGA). The crystal structure of both materials is well described with Immm (No. 71) symmetry which are isostructural with Sr2CuO3 and both are quasi-one-dimensional (1D) rectangular lattice G-type antiferromagnets, in consistent with the experimental data. Due to a distortion, Sr2CoO3 lifts the near-degeneracy [Formula: see text] and [Formula: see text] states of the local Co electronic configuration, which demonstrates a strong coupling between the structural lattice and the electronic configuration. The calculated band structure shows a band gap of 1.376 eV for Sr2NiO3 and a band gap of 1.735 eV for Sr2CoO3. Ni and Co ions are in the high-spin [Formula: see text] and [Formula: see text] configurations with the magnetic moments of 1.585 [Formula: see text] and 2.587 [Formula: see text], respectively. Based on the Heisenberg Hamiltonian model, we conclude that the superexchange intrachain TM–O–TM superexchange interaction is predominant and interaction between the 1D chains is weak. According to the calculated dielectric function, absorption spectrum and electron energy loss spectrum, the optical responses suggest that Sr2NiO3 shows the unique anisotropic structure and interaction of the application in optoelectronics.

Effect of Compatibilization on Interfacial Polarization and Intrinsic Length Scales in Biphasic Polymer Blends of PαMSAN and PMMA: A Combined Experimental and Modeling Dielectric Study

We describe an approach to tailor the dielectric interfacial properties of polymer blends by the interplay of compatibilizer effects on blend morphology and on blocking of charge carriers. A systematic study of the effect of the concentration of the compatibilizer, a random copolymer of poly(styrene-random-methyl methacrylate) (PS-r-PMMA), on the interfacial properties of stacked polymer films and a phase separating blend of poly[(α-methylstyrene)-co-acrylonitrile]/poly(methyl methacrylate) (PαMSAN/PMMA) was performed. From dielectric spectroscopy, “conductivity free” dielectric loss spectra revealed interfacial blocking of charge carriers at low frequencies owing to the conductivity contrast of the blend components, resulting in a dielectric interfacial peak. From the dielectric response of stacked polymer films, the interfacial polarization was significantly suppressed when the thickness of the more conducting film approached the Debye length (LD), on the basis of which LD could be calculated using the ...

The ionic transport mechanism and coupling between the ion conduction and segmental relaxation processes of PEO20-LiCF3SO3 based ion conducting polymer clay composites.

A series of ion conducting polymer-clay composites has been prepared using a solution casting technique. Relaxation dynamics and the ionic transport mechanism are systematically studied employing broadband dielectric spectroscopy over a wide frequency and temperature range. Among different phenomenological and theoretical models for ion conduction in disordered ionic conductors, conductivity isotherm spectra are analysed using the modified Almond-West and random free energy barrier model. Conductivity scaling suggests that the ionic transport mechanism is independent of temperature, and a similar inference is also obtained using scaled electrical modulus spectra. DC conductivity along with conductivity and segmental relaxation time following the Vogel-Tammann-Fulcher relationship suggests coupling between the ionic transport and segmental relaxation processes. Electrical modulus and dielectric formalism are used to understand the conductivity and segmental relaxation processes, respectively. The presence of first and second universality in the ionic transport mechanism is discussed using the real part of conductivity spectra and dielectric loss spectra. The crossover between the first and second universality is explained using the Kramer-Krönig approach. The ion diffusion coefficient is investigated using Ratner's classical approach in combination with the modified Stokes-Einstein relationship to correlate the coupled nature of the ion conduction mechanism and polymer segmental motion.

Ab Initio Calculations on the Structural, Mechanical, Electronic, Dynamic, and Optical Properties of Semiconductor Half-Heusler Compound ZrPdSn

AbstractThe structural, mechanical, electronic, dynamic, and optical properties of the ZrPdSn compound crystallising into the MgAgAs structure are investigated by the ab initio calculations based on the density functional theory. The lattice constant, bulk modulus, and first derivative of bulk modulus were obtained by fitting the calculated total energy-atomic volume results to the Murnaghan equation of state. These results were compared to the previous data. The band structure and corresponding density of states (DOS) were also calculated and discussed. The elastic properties were calculated by using the stress-strain method, which shows that the MgAgAs phase of this compound is mechanically stable. The presented phonon dispersion curves and one-phonon DOS confirms that this compound is dynamically stable. In addition, the heat capacity, entropy, and free energy of ZrPdSn were calculated by using the phonon frequencies. Finally, the optical properties, such as dielectric function, reflectivity function, extinction coefficient, refractive index, and energy loss spectrum, were obtained under different pressures.

Changes in dynamics of the glass-forming pharmaceutical nifedipine in binary mixtures with octaacetylmaltose

Some molecular glass-formers can crystallize in the glassy state, some of which are van der Waals molecules and some are pharmaceuticals. The molecular mechanism responsible for this glass-to-crystal mode of crystallization is of interest to the glass transition research community as well as to the pharmaceutical industry because the effect is detrimental to stability of amorphous form of the drugs stored below the glass transition temperature. Two prominent models have been proposed for the molecular mechanism. In the homogeneous nucleation-based crystallization model, the molecular mechanism is the secondary relaxation, and the other model assumes that the molecular process responsible for crystal growth in the glassy state is from the local molecular motions. Crystal growth requires motion of the entire molecule, and in the glassy state the only such local molecular motion is engendered by the secondary relaxation of the Johari–Goldstein (JG) kind. While the JG secondary relaxation is the crux in the two models of glass-to-crystal growth, it has not been found in the glassy state of the pharmaceuticals studied so far. The examples include 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile (ROY), indomethacin (IMC) and nifedipine (NIF). In the absence of any evidence of the JG secondary relaxation, the conundrum is that the two models of glass-to-crystal growth cannot be validated. It turns out these pharmaceuticals all have structural α-relaxations with narrow frequency dispersion. Empirically, glass-formers with narrow α-dispersion have JG secondary relaxation with weak relaxation strength, not well separated from the α-relaxation, and hence cannot be resolved. Theoretically, the narrow width of the α-dispersion is due to weak intermolecular coupling. In this article we enhance the intermolecular coupling of NIF by mixing with octaacetylmaltose to enhance the intermolecular coupling of NIF. In this way we have successfully resolved the JG secondary relaxation in the dielectric loss spectra of the NIF component in the glassy state, and validated the two models of glass-to-crystal growth.

Исследование диэлектрических свойств и частотных спектров диэлектрических потерь водного раствора NaCl в заисимости от параметров состояния

The frequency spectra of dielectric permittivity, E1(v), and dielectric loss, E2(v), coefficients for the aqueous solution of sodium chloride and their dependences on the concentration, density, and temperature have been studied. Numerical calculations of those parameters are made for a certain choice of the potential interaction energy Φab(r) and the radial distribution function gab(r). The results obtained are in satisfactory agreement with experimental data.

Dielectric relaxation, resonance and scaling behaviors in Sr3Co2Fe24O41 hexaferrite.

The dielectric properties of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) have been investigated as a function of temperature from 153 to 503 K between 1 and 2 GHz. The dielectric responses of SCFO are found to be frequency dependent and thermally activated. The relaxation-type dielectric behavior is observed to be dominating in the low frequency region and resonance-type dielectric behavior is found to be dominating above 108 Hz. This frequency dependence of dielectric behavior is explained by the damped harmonic oscillator model with temperature dependent coefficients. The imaginary part of impedance (Z″) and modulus (M″) spectra show that there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent at low frequencies. The dielectric loss spectra at different temperatures have not shown a scaling behavior above 108 Hz. A comparison between the Z″ and the M″ spectra indicates that the short-range charges motion dominates at low temperatures and the long-range charges motion dominates at high temperatures. The above results indicate that the dielectric dispersion mechanism in SCFO is temperature independent at low frequencies and temperature dependent at high frequencies due to the domination of resonance behavior.