Characteristic Relaxation Time Research Articles

Electrical Pulse Driven Insulator-Metal Transition of VO2 Devices As Leaky-Integrate-and-Fire Artificial Neurons

Both artificial neurons and synapses are needed to efficiently implement artificial neural networks directly in hardware. Memristors have been actively investigated to implement synaptic functions. In contrast, the implementation of artificial neurons are still limited to Si-based circuit components, while single device based artificial neuron has not been well explored. In this study, we show that VO2 two-terminal devices, driven under electrical pulses, can be used to implement the leaky, integrate and fire function of spiking neurons. Since the thermally driven metal-insulator transition of VO2 is volatile and electrical pulses can produce Joule heating, while the heating and cooling have delay times, VO2 devices exhibit characteristic firing time and relaxation time, corresponding to timing difference of the exciting pulse and the resistance switching. In addition, the integrating effect of several small pulses can also trigger the firing from high resistance to low resistance transition. These characteristics suggest the potential using the volatile phase transition of VO2 for artificial spiking neuron. Experimental measurements are further elaborated by a 2-D heterogeneous resistor network based modelling. The insulator-metal transition in VO2 under electrical bias was simulated to confirm the dynamic formation of current filament between two electrodes, and the formation of this path is a flashing process during a single pulse in the pulse train when the integrate effect of these pulses arrives the threshold.

Effect of bacterial nanocellulose addition on the rheological properties of gluten-free muffin batters

Replacing wheat flour in bakery products is a technological challenge as the absence of gluten affects rheological characteristics of uncooked batter which are directly related with final product quality. Bacterial nanocellulose (BNC) is a novel hydrocolloid which could improve batters rheological behavior. Five levels of BNC (0.06, 0.12, 0.18, 0.24, and 0.30 g BNC/100 g) and a control were employed to study its effect on rheological behavior of gluten-free muffin batters. Specific gravity, back extrusion, steady flow viscosity, and rheological assays (temperature, frequency and stress sweeps) were performed. Higher BNC levels increased firmness, viscosity, and consistency indexes. The lowest specific gravity was obtained for 0.12–0.18 g BNC/100 g indicating that more air was trapped. Flow curves were fitted with a modified Cross model; η0 and the characteristic relaxation time increased when BNC was added. Batters with BNC showed greater G′ and G″ than control during and after thermal treatment. Curves were modeled showing that molecular mobility decreased with BNC level, related to a more structured matrix. Temperature sweeps showed two transition temperatures. T1, dependent on BNC level, ranged from 81.0 ± 0.8 °C for 0.06 g BNC/100 g to 64 ± 2 °C for 0.24 g BNC/100 g or more, it might be related with BNC-starch interactions. T2 was unchanged (86.1 ± 0.6 °C) and consistent with starch pasting temperature. Levels of dry BNC between 0.12 and 0.18 g/100g batter resulted in viscous systems that could entrap more air and may result into higher volume of the baked products.

Radiative Decay of Bound Electron Pairs in Two‐Dimensional Topological Insulators

Bound electron pairs (BEPs) with energy in the bandgap are interesting because they can participate in charge and spin transport in modern topologically nontrivial materials. The problem of their stability is addressed and the radiative decay of the BEPs formed due to the negative reduced effective mass in 2D topological insulators is studied. The decay time is found to be rather large on the scale of the characteristic relaxation times of the electron system and significantly dependent on the topological properties and dispersion of the band states. In the topological phase, the decay time is much longer than in the trivial one, and is estimated as ≈1 ns for the HgTe/CdHgTe heterostructures. However, the longest decay time is in the topological phase with nearly flat dispersion in the band extrema.

Comparison of Coupled Nonlinear Oscillator Models for the Transient Response of Power Generating Stations Connected to Low Inertia Systems

Coupled nonlinear oscillators, e.g., Kuramoto models, are commonly used to analyze electrical power systems. The cage model from statistical mechanics has also been used to describe the dynamics of synchronously connected generation stations. Whereas the Kuramoto model is good for describing high inertia grid systems, the cage one allows both high and low inertia grids to be modelled. This is illustrated by comparing both the synchronization time and relaxation towards synchronization of each model by treating their equations of motion in a common framework rooted in the dynamics of many coupled phase oscillators. A solution of these equations via matrix continued fractions is implemented rendering the characteristic relaxation times of a grid-generator system over a wide range of inertia and damping. Following an abrupt change in the dynamical system, the power output and both generator and grid frequencies all exhibit damped oscillations now depending on the (finite) grid inertia. In practical applications, it appears that for a small inertia system the cage model is preferable.

Relaxation times obtained from the rate equations using path probability method for the spin-1 Ising model

We have presented the time-dependent behaviors of a spin-1 Ising model (with both bilinear and biquadratic interactions) in the neighborhood of the equilibrium states via the path probability method. The rate equations as introduced by Keskin and co-workers (1989) have been linearized to obtain the characteristic relaxation times. The temperature variations of these times were produced in accordance with the Tanaka and Mannari’s work (1976) on the equilibrium properties. Results are compared to one using Onsager’s phenomenological theory and a good agreement is achieved.

Vibrational studies, dielectric, and electrical conductivity in (Ba0.95Ca0.05)0.1(Ti0.8Sn0.2)0.1Na0.9Nb0.9O3 ferroelectric ceramic

(Ba0.95Ca0.05)0.1(Ti0.8Sn0.2)0.1Na 0.9Nb0.9O3 (BCNTSNO3) ferroelectric ceramic was synthesized by traditional solid-state method. Room temperature structural analysis, using X-ray diffraction data, confirmed the prepared ceramic purity with the existence of morphotropic phase boundary (MPB) which appears from tetragonal to orthorhombic symmetry with P4mm and Pmm2 space group, respectively. Temperature dependence of the dielectric measurement exhibited a ferroelectric–paraelectric transition at 540 K. Raman spectra were studied in the range of 50–1000 cm−1 as a function of temperature of 293 K–573 K. A detailed analysis of the frequency and half-width versus temperature introduces huge changes which are associated to the ferroelectric transitions originating from the internal vibrational modes of NbO6 octahedron. Electrical conductivity was carried out, using impedance spectroscopy, in the frequency and temperature range, 1 Hz–1 MHz and 500–600 K, respectively. Both impedance and modulus analysis exhibit the contribution of grain and grain boundary to the sample electrical response. To explain the impedance results, an equivalent circuit was proposed. The temperature dependence of the alternating current conductivity (σac) and characteristic relaxation time (τ) confirmed the observed ferroelectric phase transitions in the dielectric study. Moreover, temperature dependence of frequency exponent (s) is investigated to explain the conduction mechanism in the different regions. It was attributed to the correlated barrier-hopping model (CBH) in region (I) and (II).

Substantially increased electrical conductivity of polyaniline through blending with babassu oil in the presence of dichloromaleic anhydride

The development of new materials to be employed as active layer in organic semiconductor devices is a highly important research field. Alongside high performance, low cost and environmental safety are among the sought device characteristics. Previously the formation of PAni/Babassu polymer blends with interesting characteristics was reported. In the present work we report improvements on the electric conductivity of capacitor like devices employing Polyaniline (PAni) as active layer by the addition of dichloromaleic anhydride (DCM) during the synthesis of the Babassu-oil based polymer. FT-IR spectra confirm the occurrence of the reaction for the formation of the monoacylglyceride – MAG, the formation of the Polyanhydride (Polydichloromaleic anhydride – PDM) as well as the formation of the PAni/PDM blends. AFM images show that the presence of PDM changes the surface of the thin films, which become more uniform and homogeneous. The direct current (DC) conductivity of the thin films increases by five orders of magnitude as a consequence of a 30% increase in PDM concentration, ranging from ≈10−9 S/m for pure PAni to ≈ 10−4 S/m for the blend with 30% PDM. Impedance measurements at low electrical field frequency confirm these results; furthermore, the dependence of the complex impedance with the measurement's temperature indicate that the conduction processes are thermally activated. Two distinct conduction processes are present in the blends, as can be seen from the presence of two semi-circles in the complex-plane (Argand diagrams) representations. By employing a simple Havriliak-Negami model we obtained the characteristic relaxation time constants for both processes as a function of temperature, Arrhenius behavior was observed as a function of temperature for both processes.

Effect of corrugation on incident qP / qSV-waves between two dissimilar nematic elastomers

The problem of the reflection and transmission of elastic waves at a corrugated interface between two dissimilar nematic elastomer half-spaces has been investigated separately for the incident qP and qSV-waves. The expressions of the phase velocities corresponding to qP and qSV-waves are obtained. It is observed that these phase velocities depend on the angle of propagation of the elastic waves. The expression of the amplitude ratios corresponding to the reflected and transmitted waves for the incident qP and qSV-waves is derived by using appropriate boundary conditions. These ratios corresponding to the irregular waves are found to be functions of elastic constants, angle of incidence, coupling constants, characteristic time of rubber relaxation, director rotation times, frequency and corrugation parameter. The amplitude ratios are computed numerically for a particular model at different values of corrugation parameter. The energy distribution of various reflected and transmitted waves is also obtained analytically and numerically.

Strain rate-dependent large deformation inelastic behavior of an epoxy resin

The objective of this paper is to model high strain rate and temperature-dependent response of an epoxy resin (DER 353 and bis( p-aminocyclohexyl) methane (PACM-20)) undergoing large inelastic strains under uniaxial compression. The model is decomposed into two regimes defined by the rate and temperature-dependent yield stress. Prior to yield, the model accounts for viscoelastic behavior. Post yield inelastic response incorporates the effects of strain rate and temperature including thermal softening caused by internal heat generation. The yield stress is dependent on both temperature and strain rate and is described by the Ree–Erying equation. Key experiments over the strain rate range of 0.001–12,000/s are conducted using an Instron testing machine and a split Hopkinson pressure bar. The effects of temperature (25–120 ℃) on yield stress are studied at low strain rates (0.001–0.1/s). Stress-relaxation tests are also carried out under various applied strain rates and temperatures to obtain characteristic relaxation time and equilibrium stress. The model is in excellent agreement over a wide range of strain rates and temperatures including temperature in the range of the glass transition. Case studies for a wide range of monotonic and varying strain rates and large strains are included to illustrate the capabilities of the model.

Determination of modified polyamide 6's foaming windows by bubble growth simulations based on rheological measurements

ABSTRACTReactive extrusion was used to modify virgin polyamides 6 (v‐PA6) and to prepare chain extended PA6 (CE‐PA6) and long‐chain branched PA6 (LCB‐PA6) for the melt foaming process. This was done using a twin‐screw extruder and the following modifiers: a chain extender ADR‐4368 and a branching agent maleic anhydride grafted polypropylene. A reaction mechanism was proposed to explain the chain extension and long‐chain branching reactions and was verified by the Fourier transform infrared spectroscopy data. The analysis of the gel permeation chromatography data showed that LCB‐PA6 presented a strong increase in the molecular weight and in the dispersity index. Moreover, the rheological properties of the v‐PA6 and modified PA6 resins were characterized by a dynamic shear test. The LCB‐PA6 compared with CE‐PA6 showed much higher shear viscosity and longer characteristic relaxation times, indicating the presence of an LCB structure. A uniaxial elongation test showed that the LCB‐PA6 had the highest melt viscosity and melt strength as well as most obvious strain‐hardening behavior. A high‐pressure differential scanning calorimeter under compressed CO2 was used to investigate the PA6's crystallization properties so as to analyze its minimum temperature of foaming windows. The melt foamability of the CE‐PA6 and the LCB‐PA6 was verified by batch melt foaming experiments with CO2 as the blowing agent and maximum temperature of foaming windows was also quantitatively determined by numerical simulation of bubble growth based on the rheological measurements. The results showed that the LCB‐PA6 foams had a smaller cell diameter, a larger cell density, a greater expansion ratio, and wider foaming temperature window than the CE‐PA6. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48138.

Nuclear Spin Relaxation, Conversion, and Polarization of Molecular Hydrogen in Paramagnetic Solvents

Recent NMR experiments have measured the conversion rate of hydrogen molecules dissolved in paramagnetic organic solvents. The registered proton signals display negative peaks during an initial period. The present interpretation describes a polarization effect: the transient behavior of the nuclear spins directed on the average along the magnetic electronic spins. It relies on a nonequilibrium ortho drift and gives access to numerous parameter functions of the sample magnetic concentration. The most important ones are related to the negative transient nuclear magnetization: its building, maximum, and lifetime. Important ratios are deduced: conversion versus relaxation characteristic times and collision versus sticking time intervals. The contact interaction is found to be stronger than the long-range and fluctuating dipolar one for magnetically concentrated samples during the short sticking of the colliding partners but weaker for diluted ones. Some information on the viscosity of the organic solvent is d...

Influence of chemical substitution on broadband dielectric response of barium-lead M-type hexaferrite

We report on the electrodynamic properties of the single crystalline lead-substituted M-type barium hexaferrite, Ba0.3Pb0.7Fe12O19, performed in the broad frequency range including radio-frequency, terahertz and sub-terahertz bands, which are particularly important for the development of microelectronic devices. We demonstrate how changing on a molecular level the chemical characteristics (composition, intermolecular interaction, spin-orbital interaction) of lead-substituted M-type hexaferrite influences its radio-frequency and terahertz electrodynamic response. Our results indicate a critical temperature range, 50 K < T < 70 K, where significant changes of the electrodynamic response occur that are interpreted as freezing of dynamical oscillations of bi-pyramidal Fe(2b) ions. In the range 5–300 K, the heat capacity shows no sign of any phase transition and is solely determined by electron and phonon contributions. An anomalous electrodynamic response is detected at 1–2 THz that features a rich set of absorption resonances which are associated with electronic transitions within the fine-structured Fe2+ ground state and are visualized in the spectra due to magnetostriction and electron–phonon interaction. We show that lead substitution of barium in barium hexaferrite, BaFe12O19, leads to the emergence of a pronounced dielectric and magnetic relaxational dynamics at radio-frequencies and that both dynamics have the same characteristic relaxation times, thus evidencing the bi-relaxor-like nature of Ba0.3Pb0.7Fe12O19. We associate the origin of the relaxations as connected with the motion of magnetic domain walls. In order to unveil crucial influence of chemical substitution on electrodynamic characteristics of the compound, we analyze our results on substituted compound in comparison with the data available for pristine barium (BaFe12O19) and pristine lead (PbFe12O19) hexaferrites. The obtained spectroscopic data on the dielectric properties of Ba0.3Pb0.7Fe12O19 provide insight into fundamental phenomena responsible for the absorption mechanisms of the compound and demonstrates that chemical ionic substitution is an effective tool to tune the dielectric properties of the whole family of hexaferrites.

Analysis of the generalized heat equation applied to the solution of the dynamic problem of thermoelasticity

Based on the approximate solution of the dispersion equation, the paper presents an analysis of the system of dynamic thermoelasticity equations taking into account the generalized heat equation. It is noted that during the wave process of heat transfer, a sufficiently intensive process of energy exchange between thermal and elastic fields is realized, while depending on the relations of the characteristic relaxation times, the direction of energy exchange can change.

Electric field induced coil-stretch transition of DNA molecule

The dynamics of single DNA molecules in a homogeneous planar elongational electric field is investigated in the framework of the Hookean Dumbbell model. For a Gaussian, rapidly changing advecting flow the Fokker-Planck equation for time-dependent probability density function of DNA elongation is obtained. The analytical solution for probability density function of time independent problem is derived. The coil-stretch transition is studied when the elongation of the DNA molecule is very small than the maximum length. The characteristic relaxation time is calculated as a function of the Weissenberg number. It is observed that the coil-stretch transition and characteristic relaxation time of DNA molecule strongly depend on the angle of the principal axis of molecule with respect to axis of elongation.

Discharge currents in dense and porous PZT films

Discharge currents in short-circuit capacitors with dense and porous PZT films subjected by preliminary polarization are studied. It is shown that discharge currents can be characterized by the sum of exponential functions with the three characteristic relaxation times: 300–400 s, 50–80 s, and 10–20 s. On the assumption that discharge current is caused by relaxation of polarization charge (depolarization), it is established that the remanent polarization decreases after 20–30 min by 5–7% for the dense films and by 60–100% for the porous ones. Strong depolarization in the latter case is due to effect of a dead layer and defects.

Micropolar mixed convective flow with Cattaneo-Christov heat flux: Non-fourier heat conduction analysis

The purpose of this study is to investigate the impact of thermal relaxation time on the mixed convection flow of non-Newtonian micropolar fluid over a continuously stretching sheet of variable thickness in the presence of transverse magnetic field. An innovative and modified form of Fourier?s law, namely, Cattaneo-Christov heat flux is employed in the energy equation to study the characteristics of thermal relaxation time. The governing equations are transformed into ODE, using similarity transformations. Fourth order Runge-Kutta numerical method is used to solve these equations. The effects of relevant parameters such as a micro-rotation parameter, magnetic parameter, thermal relaxation parameter, Prandtl number, surface thickness parameter, and mixed convection parameter, on the physical quantities are graphically presented. Results illustrate that fluid temperature enhances with the rise of thermal relaxation parameter, but it reduces with an increase in micro-rotation parameter. The skin friction decreases with a rise in micro-rotation and micro-element parameters. However, variation in the rate of heat transfer is quite significant for small values of thermal relaxation parameter.

Light Scattering Study of Internal Dynamics of Hyperbranched Polymers with Controlled Branching Patterns and Low Polydispersities in Dilute Solutions

Internal dynamics of flexible polymer chains is one of the most important fundamental problems in polymer physics. Although numerous efforts have been devoted to the understanding of internal dynamics for linear polymers, little attention was paid to branched systems. This work aims to elucidate how the branching effect quantitatively influences the internal motions of long-subchain hyperbranched polymers in dilute solution. By light scattering study of four hyperbranched polystyrene samples with controlled branching patterns and low polydispersities (Mw/Mn < 1.40 and Mw ∼ 107 g/mol), we quantified and analyzed the asymptotic behavior of the reduced first cumulant [Γ* = Γ(q)/(q3kBT/η0)], the value of Γ* at high q-regime [the approximate values of Γ*(∞)], the scattering vector (q) dependence of linewidth [Γ(q)], the number of relaxation modes in Γ(q)-distribution curves, the characteristic relaxation time (τ1) of the first internal mode, and the q-dependent relative strength of internal motions. Our results reveal: (i) Γ*(∞) is not sensitive to the branching density, but more related to the fractal dimension of a given system; (ii) the asymptotic power law in the intermediate qRh regime (1.5 < qRh < 3.0), compared to Γ*(∞), is a better indicator, which could reflect the structural details of different branched systems; (iii) the branching effect could lead to the suppression of overall internal motions, but the introduction of moderate branching could enhance the contribution of some energetically favorable internal modes in linewidth distribution; (iv) by comparing the experimentally determined and theoretically calculated τ1, the result provides direct experimental evidence supporting that the classical theory is not applicable to hyperbranched polymer systems. The present work not only helps clarify some long-standing controversial issues existed in previous studies, but also provides useful experimental data for further theoretical calculations of internal dynamics for branched systems.

Translational and rotational diffusion of rod shaped molecules by molecular dynamics simulations

The results of molecular dynamics simulations of the dynamical evolution of assemblies of linear rigid rods of variable aspect ratio, a, and number density, ρ, in the isotropic phase are reported. The rods consist of m equally spaced sites interacting with the Weeks-Chandler-Andersen repulsive pair potential, where 2 < m < 16. With increasing m, features specific to long rods, such as anisotropic self-diffusion, become apparent. There is also an increasing separation between the characteristic relaxation times of the torque, angular velocity, and reorientational time correlation functions with increasing density. The latter is exponential at high densities even for dimers. The isotropic translational diffusion coefficient, Di, and rotational diffusion coefficient, Dr, are reported as a function of m and ρ or volume fraction, ξ. The mDi data scale with ξ throughout much of the simulated range, while the rotational diffusion coefficients scale approximately as m3Dr against ρ at low densities but as ∼m6Dr at high ρ, consistent with theories of colloidal and noncolloidal rod-containing liquids. The crossover density between the two regimes is parameterized in analytic form. The probability distribution functions for displacements and angular jumps in a given time show evidence of non-Gaussian behavior with increasing density. The shear viscosity and Di scale approximately as m and m-1, respectively, in the semidilute regime, which is consistent with a Stokes-Einstein-like relationship. At high concentrations, a frustrated or glassy structure formed in which the rods were randomly oriented.

Transport through a magnetic impurity: A slave-spin approach

We study transport across a magnetic impurity by means of a recently developed slave-spin technique that does not require any constraint. Within a conserving mean-field approximation we find a conductance that displays both the known zero-bias anomaly but also the expected peak at bias of order U. We extend the slave-spin mean-field approximation to study the out of equilibrium transient evolution of a quantum dot. We apply the method to investigate the time-evolution of a quantum dot induced by a time-dependent electrochemical potential applied to the contacts. Similarly to the time-dependent Gutzwiller approximation, the mean-field slave-spin dynamics is able to capture dissipation in the leads, so that a steady-state is reached after a characteristic relaxation time.

Parametric Experimentation on Pantographic Unit Cells Reveals Local Extremum Configuration

Pantographic metamaterials are known for their ability to have large deformation while remaining in the elastic regime. We have performed a set of experiments on 3D printed pantographic unit cells to parametrically investigate their response when undergoing tensile, compression, and shear loading with the aim of i) studying the role of each parameter in the resultant mechanical behavior of the sample, and ii) providing a benchmark for the mathematical models developed to describe pantographic structures. Results show the existence of local extrema in the space of the geometrical parameters, suggesting the use of optimization techniques to find optimal geometrical parameters resulting in desired functionalities. We have also performed tensile relaxation tests on the samples, with the results indicating the complexity of the dynamic behavior and the existence of multiple relaxation characteristic times. Such results can be used to for calibrating mathematical models describing pantographic structures under dynamic loadings.