Characteristic Relaxation Frequency Research Articles

On the stability and accuracy of TRT Lattice-Boltzmann method for non-Newtonian Ostwald-de Waele fluid flows

This paper brings a numerical analysis of the TRT modeling for the Lattice-Boltzmann method when solving flow for dilatant and pseudoplastic power-law fluids. Firstly, the method was reviewed to describe the required simulation parameters and the numerical methodology. Secondly, a mathematical procedure was performed to identify the characteristic relaxation frequency as a function of both flow and fluid properties and to work as a guide parameter for LBM operation. Then, a simple shearing Poiseuille flow was employed so its characteristic shear rate could be calculated as a function of fluid properties, given the flow was characterized by the Reynolds and Mach numbers. For this test case, convergence was then explored for a broad range of parameters, and its non-monotonic dependency on the Mach number for a given convergence criterion was shown. Then, stability maps were constructed based on the characteristic relaxation frequency, which showed a strong dependency between consistency and flow index so the simulation could converge. This was explored against the results from the converged tests, which pointed out the usefulness of the characteristic relaxation frequency in predicting stable solutions. Finally, quantitatively, it was shown that for this power-law fluid flow, the ||L2|| relative error depends on the Mach number to the power of 2(2−n) being now a function of the flow index, extending the previously reported dependency of the Mach number to the power of 2 for plane-Poiseuille flow.

Probing the Recombination Process at the Intermixed Interface of a Solution-Processed Organic Light Emitting Diode By Impedance Spectroscopy

Solution-processed organic light emitting diode (s-OLED) has garnered considerable research interest due to its cost-effectiveness, improved energy efficiency, and material-saving advantages over conventional vacuum deposition methods in OLED production. Recent studies have shown that the thermal annealing process of multilayered s-OLEDs induces intermixing at the interfaces of the layers and enhances the device performance [1,2]. However, the physical origin of the enhanced s-OLED performance by intermixing at the interfaces is poorly understood. To address this issue, we utilized electrical impedance spectroscopy (IS) to investigate the thermal-induced modifications of charge carrier dynamics in a bilayer s-OLED comprising an electron transporting layer (ETL) and an emission layer (EML). We separately assessed the conductance of the ETL and the charge carrier accumulation at the EML/ETL interface by the characteristic relaxation frequency and the plateau at a low frequency regime (20~100 Hz) extracted from the frequency-dependent capacitance spectra, respectively. The IS analysis revealed that improved charge transport in the ETL by thermal annealing process was mainly responsible for the performance enhancement, rather than intermixing at the EML/ETL interface. Notably, we found that annealing the s-OLEDs above the glass transition temperature (Tg) of the ETL, at which significant intermixing occurs, increased the magnitude of negative capacitance, indicating the increased rate of nonradiative trap-assisted recombination [3]. Accordingly, our IS analysis clarified that extended intermixing at the EML/ETL interface is rather detrimental to the performance of s-OLEDs. This study demonstrates that IS can serve as a powerful tool for accurately analyzing charge carrier dynamics in s-OLEDs by simultaneously probing charge transport, interfacial charge accumulation, and recombination processes, thereby providing valuable insights for optimizing s-OLED performance.[1] ACS Appl. Mater. Interfaces 2015, 7 (37), 20779–20785. https://doi.org/10.1021/acsami.5b05818.[2] Advanced Materials Interfaces 2019, 6 (4), 1801627. https://doi.org/10.1002/admi.201801627.[3] Phys. Rev. Lett. 2018, 120 (11), 116602. https://doi.org/10.1103/PhysRevLett.120.116602. Figure caption. The capacitance at 20 Hz and current efficiency at 100 nit are plotted against the ratio of annealing temperature (T) to glass transition temperature (Tg) of ETL. Figure 1

Dielectric spectroscopy of a ferroelectric nematic liquid crystal and the effect of the sample thickness

The recently discovered ferroelectric nematic liquid crystals have been reported to exhibit very large dielectric permittivity values. Here, we report a systematic investigation of the dielectric behavior of a prototypical ferroelectric nematogen by varying the thickness of the parallel capacitor measuring cell. While in the non-polar high temperature nematic phase results show only slight differences due to slight variations of the alignment, the measured permittivity values in the ferroelectric nematic phase show a linear dependence on the cell thickness. It is also shown that the characteristic relaxation frequency decreases inversely proportionally to the thickness. The results are discussed in terms of three different available models based on different underlying mechanisms, accounting for cancellation of the probe electric fields by polarization reorientation or by ionic charges, or based on a recently proposed continuous phenomenological model.

Finite-temperature ferromagnetic transition in coherently coupled Bose gases

A paramagnetic-ferromagnetic quantum phase transition is known to occur at zero temperature in a two-dimensional coherently coupled Bose mixture of dilute ultracold atomic gases provided the interspecies interaction strength is large enough. Here we study the fate of such a transition at finite temperature by performing numerical simulations with the stochastic (projected) Gross-Pitaevskii formalism, which includes both thermal and beyond mean-field effects. By extracting the average magnetization, the magnetic fluctuations and characteristic relaxation frequency (or critical slowing down), we identify a finite-temperature critical line for the transition. We find that the critical point shifts linearly with temperature and, in addition, the three quantities used to probe the transition exhibit a temperature power-law scaling. The scaling of the critical slowing down is found to be consistent with thermal critical exponents and is very well approximated by the square of the spin excitation gap at zero temperature.

Pressure and temperature dependence of the relaxation of the electrical double layer in hydrated magnesite rock (leukolite)

The introduction of water into the pore space of naturally occurring magnesite (leukolite) induces an intense relaxation mechanism, which is related to the electric double layer (EDL) formed on opposing sides of the solid – mater interface. The relaxation of the EDL is studied by using Broadband Dielectric Spectroscopy at different conditions of combined temperature and hydrostatic pressure. The temperature evolution of the characteristic relaxation frequency, reveals two successive temperature regions: in the low temperature one, protonic conductivity over the network of water molecules in on the solid surface, couples to the ionic transport of charged defects occurring in the solid. At higher temperatures, near – zero activation energies are found: the phenomenon is discussed theoretically and attributed to decoupling and an exchange of protons with lattice site magnesium cations, the formation of a proton enriched sub-surface layer of magnesite and subsequent release of cations to the liquid. The values of the activation volume and their independence on temperature, support the aforementioned interpretation.

Enhanced Dielectric Relaxation in Self-Organized Layers of Polypeptides Coupled to Platinum Nanoparticles: Temperature Dependence and Effect of Bias Voltage

Using alternative current impedance spectroscopy, we investigate the dynamical conductivity of hybrid nanomaterials composed of helical polypeptide layers containing platinum nanoparticles (PtNP). The electrical characteristics of the self-organized poly(γ-benzyl-l-glutamate) (PBGL) in bidimensional lamellar assembly in the presence of Pt nanoparticles are well modeled and described by a single equivalent circuit of parallel resistance and capacitance. The latter are determined using a comparison between the measured and calculated Nyquist plots, which allows extracting the characteristic relaxation time and frequency of the dipolar relaxation process. We found that the relaxation frequency in the PBLG–PtNP hybrid materials is enhanced by 4 orders of magnitudes compared to pure PBLG, which indicates a much faster dielectric relaxation in PBLG–PtNP due to dipole orientation and dipole–dipole interactions. The temperature dependence of the relaxation time is analyzed using Arrhenius plots, from which the activation energy of the relaxation process is found to be around 0.1 eV. Such a value close to the peptide vibration energy of the PBLG indicates a vibration-assisted relaxation process and a polaronic charge transport mechanism. An advantage of the PBLG–PtNP nanocomposite material is that the activation energy can be finely tuned by the PBLG degree of polymerization. Finally, an important outcome of this work is the investigation of the dielectric relaxation process in PBLG–PtNP under applied DC bias. We found that the activation energy decreases with increasing bias voltage for all degrees of polymerization of the PBLG molecule. This effect is interpreted in terms of electric field-induced alignment of the dipoles and of increased mobility of the polaronic charge carriers. The presence of piezoelectricity in the hybrid material gives the possibility to use the DC bias as a simple mean of monitoring the dynamical conductivity involving polaronic states.

Disentangling proton-transfer and segmental motion relaxations in poly-vinyl-alcohol aqueous solutions by means of ultrasonic relaxation spectroscopy

A detailed temperature- and concentration-dependent ultrasonic relaxation study has been conducted in aqueous polyvinyl alcohol (PVA) solutions as a function of frequency under isobaric conditions. Concentration and temperature have similar effect on acoustic spectra. Two well-resolved relaxation processes were observed in the experimental acoustic spectra that seem to follow a Cole-Cole distribution function. The systematic analysis of the variation of the acoustic parameters with frequency, temperature and concentration revealed that both processes have collective and not local nature. The low-frequency relaxation mechanism is assigned to segmental motions of the polymer chains and the high-frequency relaxation to proton-transfer reaction, respectively. The characteristic relaxation frequencies were found independent of molecular weight, as expected for relative flexible vinyl-polymer chains with molecular weight above ~104. Based on Eyring's theory, the activation enthalpy of the low- and high-frequency relaxation were estimated equal to ΔΗ1* = 4.5 ± 0.2 kcal/mol and ΔΗ2* = 19.7 ± 0.8 kcal/mol, respectively. The contribution of the low-frequency lying normal mode relaxation has been quantitatively estimated in the context of relevant theories of the field and found almost four order of magnitude lower in amplitude compared to segmental motion process indicating that the experimentally detected relaxation is dominated by the latter. The experimental results were complemented with molecular mechanics calculations performed in parallel. The comparison between the experimental data and the outcome of the theoretical calculations evidenced that the length of the relaxing element in a PVA polymer chain is between 5 and 8 monomers in agreement with what is expected for vinyl-polymers and established the local segmental motions of the PVA polymer chains in the solution.

Exploring conformational change profile of n-propyl ester of formic acid by combining ultrasonic relaxation spectroscopy and molecular orbital calculations

We report a detailed investigation of the ultrasonic absorption spectra of n-propyl formate in the liquid state over a wide frequency and temperature range under constant pressure. The ultrasonic absorption measurements were performed by standard acoustic resonator and pulse techniques. A single relaxational phenomenon has been observed and is attributed to a unimolecular reaction due to a conformational change of the n-propyl formate molecule in the liquid state. The characteristic relaxation frequency increases with increasing temperature and vise versa. In an effort to confirm the assignment of the relaxation process and to determine the corresponding thermodynamic properties, relaxation parameters were further investigated as a function of temperature. Activation enthalpy and enthalpy difference between the two isomeric forms have been estimated equal to ΔH∗ = 4.21 ± 0.16 kcal/mol and ΔH0 = 6.21 ± 0.29 kcal/mol, respectively. The corresponding energy barrier was estimated equal to ΔEexperimental = 10.42 ± 0.33 kcal/mol. Molecular orbital calculations have been applied to obtain the standard enthalpy of formation for n-propyl formate at various dihedral angles around C–O bond. The results exhibited two clear minimum standard enthalpies of formation and the energy barrier has been calculated equal to ΔΕ = 11.62 kcal/mol, close to the experimental value considering that the calculation was performed in a vacuum environment. The results confirm the absence of shear viscosity relaxation in this frequency range. The ratio of the volume viscosity to the static shear viscosity ηV/ηS has been determined revealing a clear temperature dependent and ηV>ηS supporting the assignment of the observed relaxation process to the rotational equilibrium. The results are analyzed in view of the current phenomenological status of the field.

Phase transition and dielectric relaxation of a mixed ferroelectric composite from cellulose nanoparticles and triglycine sulfate

The work is devoted to the study on electrophysical properties of a novel ferroelectric composite consisting of triglycine sulfate (TGS) and cellulose nanoparticles at different composition weight ratios from 25 to 120 °C under a weak electric field at frequencies of 102–106 Hz. The obtained results indicated that an increase in cellulose concentration in the composite led to increasing the phase transition temperature by three to a few tens of degrees Celsius higher than those for single crystal TGS (+49 °C) and led to decreasing characteristic relaxation frequency of TGS inclusion in the studied frequency range.

Thermal, electrical, and dielectric properties of reduced graphene oxide–polyaniline nanotubes hybrid nanocomposites synthesized by in situ reduction and varying graphene oxide concentration

ABSTRACTIn this study, reduced graphene oxide (RGO) has been introduced as conductive filler within polyaniline (PAni) nanotubes (PAniNTs) by in situ chemical reduction method to enhance the properties of PAniNTs. The effect of varied concentration of in situ reduced GO on the structural, thermal, electrical, and dielectric properties of RGO–PAniNTs nanocomposites have been investigated by high resolution transmission electron microscope, X‐ray diffraction, Fourier transform infrared, thermogravimetric analysis, I–V characteristics, and impedance analyzer. The enhanced thermal stability of the nanocomposites has been analyzed from the derivative thermogravimetric curves in terms of onset and rapid decomposition temperature. The transport mechanisms have been studied by fitting the nonlinear I–V characteristics to the Kaiser model. The dielectric relaxation phenomena have been investigated by permittivity and modulus formalisms. Characteristic relaxation frequency of RGO–PAniNTs nanocomposites shifts toward higher frequency with increasing RGO concentration indicating a distribution in conductivity relaxation. The distribution of relaxation time has been studied by fitting the imaginary modulus spectra of the nanocomposites to Bergman modified KWW function. The ac conductivity spectra are fitted to the Jonscher's power law equation and enhanced conductivity value of 1.26 × 10−3 S cm−1 is obtained for 40 wt % of RGO compared to 1.22 × 10−4 S cm−1 for PAniNTs. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45883.

Dielectric properties of (1−x)KNbO3−xNiO two-phase composites

We report a detailed study on structural and dielectric properties, Raman scattering, and ac-conductivity of ferroelectric KNbO3 and antiferromagnetic NiO two-phase composites [() KNbO3 + x NiO]. Two major transitions were observed in the temperature dependent relative dielectric permittivity (T)—the first at C () and the second at C (T1)—respectively associated with the structural transitions from tetragonal to cubic and orthorhombic to tetragonal phases. In addition, a new anomaly across T2 ∼ C has been observed in (T), and the loss-tangent (Tanδ(T)), which may be associated with the combined effect of antiferromagnetic ordering of NiO and ferroelectric coupling of KNbO3. The dispersive nature of , and the Vogel–Fulcher temperature dependence of its characteristic relaxation frequency reveals the existence of relaxor-like behavior, which is further supported by Uchino–Nomura criteria (for x ⩽ ) but departs from relaxor behavior for x . Temperature dependent ac-resistivity analysis (T) provides strong evidence for a Mott’s Variable range hopping process of charge carriers between localized states, with average hopping length of nm and hopping energy eV. The variation of , T1 and T2 as a function of x, in consonance with the crystal structure, has been discussed. Raman spectroscopic studies were carried out to study the local structure of the () + x NiO two-phase composites, which shows significant increase of tilt-angle between adjacent octahedra as the divalent Ni occupies octahedral sites of the matrix. All the Raman-active modes of these composites are indexed as lattice translations, specifically with the motion of (190 ), the core vibrations of the octahedra (220 ⩽ ν ⩽ 320 ) and their collective modes (50 ⩽ ν ⩽ 150 ), and Magnon modes (2M ∼ 1500 ) associated with the antiferromagnetic ordering of NiO.

Optical transmission versus ac magnetization measurements for monitoring colloidal Ni nanorod rotational dynamics

Ni nanorods with an average length nm and diameter nm were synthesized using the AAO template method. The magnetization and optical transmission of nanorod colloidal dispersions in alternating magnetic fields were measured and analyzed with the objective of comparing the intrinsic Brownian relaxation times obtained with the two methods. The different physical origin of the measured signal, related to different moments of the orientation distribution function, and the non-linear effects expected for the large magnetic moments of the Ni nanorods at common field amplitudes required a comprehensive modelling. The time-dependent magnetization and optical transmission in ac magnetic fields was derived by numerical solution of the Fokker–Planck equation. The simulated time-dependent magnetization and optical transmission at a given frequency and field amplitude were analyzed analogous to experimental data to determine characteristic relaxation frequencies. Empirical relationships were derived which enabled extraction of the intrinsic Brownian relaxation time from the characteristic frequencies measured in the non-linear regime. Despite large differences in the characteristic frequencies obtained from magnetization and optical transmission measurements, the retrieved intrinsic Brownian relaxation times were found to agree well. The potential of ac magnetic field-dependent optical transmission for biosensing applications was demonstrated by monitoring the adsorption of the protein gelatine on the nanorod labels.

The novel dielectric relaxation behavior around the Curie point of (Ba0.7Sr0.3)0.96Y0.04TiO3 ceramics with giant effective permittivity

The temperature (80 K ∼ 423 K) and frequency (40 Hz ∼ 5 MHz) dependence of the permittivity were studied in (Ba0.7Sr0.3)0.96Y0.04TiO3 ceramics. A giant effective permittivity over 105 with a certain frequency and temperature stability and Debye-type dielectric relaxation behavior was observed. The complex impedance spectrum analysis reveals that the giant permittivity is due to the heterogeneous structure with a semiconducting bulk and high resistance electrode–ceramic interface. Deviating from low temperature (below 200 K) thermal activated behavior, the odd change of the characteristic relaxation frequency around the Curie point is observed and proven to be induced by the competition between the positive temperature coefficient of the resistance effect of the bulk ceramic and the dielectric anomaly of the surface layer capacitance, both of which originate from the ferroelectric–paraelectric phase transition.

Effect of Hf doping on the structural, dielectric and optical properties of CaCu3Ti4O12 ceramic

Ceramic samples of CaCu3(Ti1−xHfx)4O12 (CCTHO) were prepared by solid state reaction method. The influences of Hf+4 ions substitution on the structural, optical and dielectric properties of the prepared samples were investigated using X-ray diffraction, diffuse reflectance spectroscopy and impedance spectroscopy respectively. The solubility limit of Hf in CaCu3Ti4O12 (CCTO) is found to be ~2 %. The optical spectroscopy data reveals that with Hf doping the optical band gap increases. Further we observed that with Hf substitution in CCTO at Ti site improves the dielectric properties, i.e. the value of dielectric constant (e′) increases and that of dielectric loss (tanδ) decreases with respect to parent un-doped CCTO sample. Two dielectric relaxations are observed in Hf doped sample, whereas only one relaxation is observed in CCTO. The characteristic dielectric relaxation frequency shifts towards higher values with Hf doping. Further from the fitting of e′ versus frequency data it appears that the relaxation mechanism shifts from Debye to Cole–Cole type.

Prediction of melt rheological properties from GPC molecular weights

The SCLAIR® solution polymerization platform produces a wide variety of ethylene-α-olefin copolymers and polyethylene homopolymers. Commercial products exhibit density and melt index values ranging from about 0.920 to 0.962 g/cm3 and 0.3–75 g/10 min respectively. Polymer molecular weight distributions can be tailored to meet a broad selection of end-use requirements. In this study, we have used a chemometric analysis approach using The Unscrambler® software to demonstrate statistical correlations between rheological properties and fundamental structural parameters for thirty-three commercial SCLAIR polyethylenes. We demonstrate that molten rheological properties such as melt index, stress exponent, zero-shear viscosity, characteristic relaxation time, cross-over modulus and frequency show good non-linear correlations with molecular weight characteristics of SCLAIR products as determined by gel permeation chromatography (GPC). We also show that, with the use of Partial Least Squares (PLS) regression techniques, most melt rheological properties can be accurately predicted on the basis of GPC data.

Effect of silver nanoparticle embedment on the frequency dispersive conductivity and electrical relaxation dynamics in dodecylbenzenesulfonic acid-doped polyaniline

The present work aims at studying electrical relaxations in silver–polyaniline nanocomposites using dielectric spectroscopy. The nanocomposites of dodecylbenzenesulfonic acid-doped polyaniline (PANI) with different concentrations of silver nanoparticles (~6–12 nm) are synthesized by simple wet-chemical route. The temperature dependence of dc conductivity in all the samples follows three-dimensional variable range-hopping conduction mechanism. The loss factor, after having subtracted the dc contribution, shows a relaxation peak which simultaneously attributes to the frequency dispersion in conductivity spectra. The observed dielectric relaxation is well fitted by the Havriliak–Negami function, and the fitting parameters are determined. The temperature dependence of characteristic relaxation frequency and dc conductivity is in line with each other and bear a resemblance to the origin of dc transport and dielectric relaxation in these systems. The temperature behavior of the relaxation strength confirms that an exclusive hopping conduction of polarons in the disordered PANI matrix can be considered as the origin of the observed electrical properties of the systems. Further, the self-similar behavior of the real part of normalized ac conductivity, within the measured temperature range, also confirms the above inferences. The variation of frequency exponent with temperature suggests that ac conduction is due to the correlated barrier hopping of polarons which strongly affected by the dispersion of silver nanoparticles within the PANI matrix.

Cooperativity length scale in nanocomposites: Interfacial and confinement effects

Interfacial and confinement effects on the evolution of cooperativity on approaching the glass transition have been studied in poly(propylenecoethylene) functionalized with diethylmaleate, polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol and their nanocomposites with montmorillonite. A small increase of the structural dynamic cooperativity, a weak alteration of the temperature dependence of the characteristic relaxation frequency, and no changes in the glass transition temperature observed in poly(propylenecoethylene)-based samples can be rationalized in terms of interfacial interactions between polymer and exfoliated clay. On the other hand, confinement of polymer chains in the galleries of clay (intercalated nanocomposite) produces a strong reduction of cooperativity, of the temperature dependence of the characteristic relaxation frequency, and of the glass transition temperature in polyethylene 1,4-cyclohexylenedimethylene terephthalate glycol samples. Finally, by investigating the temperature dependence of a generalized fragility and of cooperativity, we evidenced that fragility of glass formers is determined not only by cooperativity.

Dielectric study of polyaniline/poly (methylmethacrylate) composite films below the percolation threshold

Abstract We report results of dielectric relaxation studies of polyaniline/poly(methylmethacrylate) composites with polyaniline amount less than the percolation threshold in the frequency range of 0.1 Hz to 1 MHz and temperature range of 10 °C–170 °C. We find a significant dependence of the glass transition temperature Tg on the polyaniline amount in the composite. α and β relaxation processes relative to the PMMA matrix are also affected by the presence of polyaniline inclusion. We identify a relaxation process due to ionic conductivity and another process attributed to residual solvent. The characteristic relaxation frequency of each process and the activation energy depend on the polyaniline amount in the composite. The ac conductivity in the high frequency range is fitted to the universal power law of Jonscher characteristic of disordered materials.

Ascertaining universal features of yielding of soft materials

Many metastable complex fluids, when subjected to oscillatory shear flow of increasing strain amplitude at constant frequency, are known to show a characteristic nonlinear rheological response which consists of a monotonic decrease in the elastic modulus and a nonmonotonic change in the loss modulus. In particular, the loss modulus increases from its low strain value, crosses the elastic modulus, and then decreases with further increase in the strain amplitude. Miyazaki et al. (Europhys Lett 75:915–921, 2006) proposed a qualitative argument to explain the origin of the nonmonotonic nature of the loss modulus and suggested that in fact this response could be universal to all complex fluids if they are probed in a certain frequency window in which the fluid is dominantly elastic in the small strain limit. In this letter, we confirm their hypothesis by showing that a wide variety of complex fluids, irrespective of their thermodynamic state under quiescent conditions, indeed show the aforementioned characteristic nonlinear response. We also show that the maximum relative dissipation during yielding occurs when the imposed frequency resonates with the characteristic beta relaxation frequency of the fluid.

Debye–Falkenhagen dynamics of electric double layer in presence of electrode heterogeneities

A phenomenological theory of electric double layer (EDL) polarization of an electrode (in the absence and presence of electroactive species) is obtained using the Debye–Falkenhagen equation for the potential. The influence of surface heterogeneities on the compact layer causes the distribution in relaxation time, resulting in constant phase element (CPE) response. This contribution of the compact layer is included through the current balance boundary constraint at the outer Helmholtz plane. The results for the impedance and the capacitance are obtained in terms Debye screening length and dynamic polarization length which is dependent on the surface heterogeneity parameter. At frequencies less than the characteristic compact layer relaxation frequency, the EDL is controlled by the compact layer dynamics and shows CPE response. The intermediate frequencies show the emergence of pseudo-Gerischer and pseudo-Warburg like behavior for systems with lower concentration and lower diffusion coefficients of ions. EDL behaves like a resistor at larger frequencies. Theoretical results capture various observations in the experimental capacitance dispersion data.