Characteristic Relaxation Time Research Articles

Accurate in-operando study of molten carbonate fuel cell degradation processes -part I: Physiochemical processes individuation

This work has the difficult task to deeply study the electrochemical processes that occur inside a 100 cm2 of Molten Carbonate Fuel Cells (MCFC) impedance spectra using the high resolution of Distribution of Relaxation Time (DRT) method. Using this method, it is possible to shed light on the different physicochemical processes occurring within these cells, identifying the characteristic relaxation times by means of an appropriate experimental campaign where temperature and gas compositions in anode and cathode were varied one at a time. The quality of the recorded spectra was verified by Kramers-Kronig relation before applying DRT calculations. In this work, five distinct and separated peaks with different time constants ranging from 0.01 to 500 Hz were identified and associated with physiochemical processes of the cell. Three peaks at high frequency represent the charge transfer processes in anode and cathode active sites. The other two, located at low frequency, are associated with the gas diffusion in the electrodes and to the gas conversion process. This study represents the first application of the DRT approach to this technology allowing to understand the physicochemical origin of the individual polarization processes controlling the cell performance and the degradation. The analysis of degradation processes using the DRT method and the physiochemical processes identification presented in this paper will be shown in part II of this work.

Rapidly Reprocessable Cross-Linked Polyhydroxyurethanes Based on Disulfide Exchange.

Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, cross-linked PHU resins that are not derived from isocyanates.

Structural–Temporal Peculiarities of Dynamic Deformation of Nanostructured and Nanoscaled Metals

The evolution of a structural–temporal integral criterion of yielding is reported for the description of the dynamic deformation of metals. The values of characteristic relaxation times, considered as the constants of the material, are shown to be suitable for the description of dynamic effects upon the nanomaterial deformation in a wide range of loading rates. Three various ways of the determination of characteristic relaxation time of nanomaterials are discussed. The behavior of the ultimate stress in the range of pulse duration from one and two points of change in the dominant mechanism of rate sensitivity is interpreted in the context of integral criterion of yielding.

Combined interpretation of NMR, MICP, and SIP measurements on mud-dominated and grain-dominated carbonate rocks

Carbonate formations still present a challenging target with regard to reservoir characterization as they possess a wide range of pore types and sizes as well as interconnected and isolated pore space. In this study a set of micritic and oomoldic carbonate samples was investigated with three measurement techniques (magnetic resonance (NMR), mercury intrusion (MICP), and spectral induced polarization (SIP, in a frequency range from 5mHz to 100 Hz)) which provide information about a rock's pore space indirectly based on different physical principles. The aims of the study were to combine these pieces of information to characterize the pore space of mud-dominated and grain-dominated carbonate rocks and to investigate potential correlations between pore space properties and SIP-derived parameters.Pore body and pore throat size distributions were derived from NMR and MICP data, respectively, and interpreted in terms of dominant pore throat (rt) radius, pore body (rb) radius and the rb/rt ratio. For validation of the NMR-derivedrb distributions, scanning electron microscope images were used for the computation of image-derived pore size distributions. The SIP data were interpreted in terms of characteristic features for oomoldic and micritic samples. Phase shift, real (σ′) and imaginary part (σ′′) of complex conductivity, respectively, were fitted with Cole-Cole models and the characteristic parameters relaxation time τ, chargeability m, and frequency dependence c were derived for each sample. Subsequently, these parameters were interpreted with regard to correlations with rb,rt, and rb/rt and specific surface Spor.The results indicate that grain-dominated carbonates possess comparatively large radii rb and rtand low Spor, which seems to go along with sharp phase peaks (i.e. high c values) and a low absolute phase level (and therefore low chargeability m). The mud-dominated carbonates, in contrast, possess relatively small radii rb and rt and high Spor which is obviously associated with broad phase peaks (i.e. low c values) and a comparatively high phase level resulting in high m values. τ, in contrast, was found to decrease with increasing ratio rb/rt and may therefore have similar values in both micritic and oomoldic samples. Consequently, the results of this study contribute to a better understanding of the petrophysical properties of carbonate rocks as a function of their pore space characteristics.

Magnetization Dynamics and Energy Dissipation of Interacting Magnetic Nanoparticles in Alternating Magnetic Fields with and without a Static Bias Field

The effect of interparticle interactions on the magnetization dynamics and energy dissipation rates of spherical single-domain magnetically blocked nanoparticles in static and alternating magnetic fields (AMFs) was studied using Brownian dynamics simulations. For the case of an applied static magnetic field, simulation results suggest that the effective magnetic diameter of interacting nanoparticles determined by fitting the equilibrium magnetization of the particles to the Langevin function differs from the actual magnetic diameter used in the simulations. Parametrically, magnetorelaxometry was studied in simulations where a static magnetic field was suddenly applied or suppressed for various strengths of magnetic interactions. The results show that strong magnetic interactions result in longer chain-like particle aggregates and eventually longer characteristic relaxation time of the particles. For the case of applied AMF with and without a static bias magnetic field, the magnetic response of interacting nanoparticles was analyzed in terms of the harmonic spectrum of particle magnetization and dynamic hysteresis loops, whereas the energy dissipation of the particles was studied in terms of the calculated specific absorption rate (SAR). Results suggest that the effect of magnetic interactions on the SAR varies significantly depending on the amplitude and frequency of the AMF and the intensity of the bias field. These computational studies provide insight into the role of particle–particle interactions on the performance of magnetic nanoparticles for applications in magnetic hyperthermia and magnetic particle imaging.

Mechanical relaxation and freezing in the room temperature strain glass alloy Ti50(Pd40Cr10)

The alloy Ti50(Pd40Cr10) undergoes a strain glass transition around room temperature evidenced by frequency dispersion of dynamic mechanical properties and lack of average structure change from that of the high symmetry austenite phase. However, since the strain glass transition is not a thermodynamic phase transition but a dynamic freezing process governed by the kinetics, a quantitative characterization of the slowing down of dynamics during the strain glass transition is still lacking. In the present study, the probability distribution function (PDF) of the relaxation time of the strain glass alloy is investigated spanning the whole transition temperature range (253 K–313 K). The slowing down of dynamics of the strain glass is indicated by the rapid increase of the characteristic relaxation time () upon cooling. The , as a function of temperature, shows a transition from Vogel–Fulcher relationship to an Arrhenius relationship. Such a change suggests two fundamentally different states: unfrozen strain glass state and frozen strain glass state. Furthermore, the spread of the PDF is connected to the fraction of quasi-static nanodomains, which helps the understanding of the dynamic freezing process in the strain glass.

Early relaxation time assessment for characterization of breast tissue and diagnosis of breast tumors.

During recent years, many efforts have been made to increase the efficiency of the treatment methods by adjusting the treatment with the specification of related tissue. Non-penetrating indentation test is a non-invasive method for characterizing mechanical behavior of the tissue. In this study, characterization of the viscoelastic behavior of breast tissue is investigated by means of the relaxation time. To this end, an in-vivo study is conducted on the breast tissue of 18 female cases by performing a compressive indentation test with controlled rate of deformation. The test is performed in two subsequent stages; at first stage, the tissue is compressed by a specified strain rate and is called the loading procedure. At the second stage, which is the unloading procedure, the load is removed by the same rate and the tissue behavior is studied while it returns to the unloaded position. A generalized Maxwell model with two Maxwell-arms is used to model the tissue's viscoelastic response. Force vs. compression data during the unloading course, are obtained and interpolated by the transfer function of the viscoelastic model and two relaxation times are obtained. The smaller in value is referred to as the early relaxation time and results show that it has variation with the tissue's mechanical structure, which makes it suitable as a characteristic property. This property has insignificant variation over each individual's breast tissue, meanwhile major variation is observed between different cases and also in the presence of a tumor. Results show that existence of tumor in the breast tissue can increase the early relaxation time up to 6 times more than the corresponding value at the healthy tissue. Based on case-dependent character of the early relaxation time, tumor detection procedure based on the elevation of characteristic early relaxation time should be performed individually for each case.

A Comparative Study of Intramolecular Mobility of Single Siloxane and Carbosilane Dendrimers via Molecular Dynamics Simulations.

A comparative analysis of intramolecular dynamics of four types of isolated dendrimers from the fourth to the seventh generations belonging to the siloxane and carbosilane families, differing in spacer length, core functionality, and the type of chemical bonds, has been performed via atomic molecular dynamics simulations. The average radial and angular positions of all Si branching atoms of various topological layers within the dendrimer interior, as well as their variations, have been calculated, and the distributions of the relaxation times of their radial and angular motions have been found. It has been shown that the dendrons of all the dendrimers elongate from the center and decrease in a solid angle with an increasing generation number. The characteristic relaxation times of both angular and radial motions of Si atoms are of the order of a few nanoseconds, and they increase with an increasing generation number and decrease with temperature, with the angular relaxation times being larger than the radial ones. The relaxation times in the carbosilanes are larger than those in the siloxanes. The rotational angle dynamics of the carbosilane dendrimers show that the chain bending is mainly realized via trans-gauche transitions in the Si branching bonds.

Rheological behaviors of H-shaped polymers incorporated with short branches under shear and elongational flows via FENE-Rouse model

We present a detailed study of the effects of short branches on the rheological behaviors of H-shaped long-chain branched polymers under shear and uniaxial elongational flows using (single “phantom” chain) bead-spring Brownian dynamics simulations. To clarify the fundamental role of short branches in both flow types, the short branches are distributed either along the chain backbone or along the four dangling long arms of the H-polymer. We observe that the fast random motions of the highly mobile short branches (in association with their very short characteristic relaxation time scales) constantly disturb chain conformation, generally leading to a more compact and less deformed chain structure against the applied flow. Accordingly, the structural and dynamical properties of the short-chain branched (SCB) H-polymers in response to the flow are strongly dependent on the location of the short branches along the chain. For instance, in comparison to the original H-polymer, the H-(SCB_backbone) polymer, where the short branches are allocated along the backbone, exhibits considerably less shear-thinning behavior resulting from the lesser degree of chain alignment and structural deformation of the SCB backbone. In contrast, the H-(SCB_arm) polymer, where the short branches are allocated along the four long arms, displays a higher degree of shear-thinning behavior arising from an effective tensile force (created by the tightly coiled “superbead” character of the arms via fast short-branch dynamics) that stretches out the backbone. Importantly, the fundamental role of the short branches in determining rheological characteristics of the SCB H-polymers remains unchanged, regardless of the flow type and flow strength.

Unsteady Cattaneo-Christov double diffusion of Oldroyd-B fluid thin film with relaxation-retardation viscous dissipation and relaxation chemical reaction

This paper investigates upper-convected Oldroyd-B fluid thin film flow over an unsteady stretching sheet considering modified viscous dissipation and chemical reaction. With Cattaneo-Christov double diffusion theory, the governing equations are obtained. Meanwhile, unlike the viscous dissipation of Newtonian fluid, depending on the constitutive relation of Oldroyd-B fluid, the relaxation-retardation viscous dissipation is developed. Furthermore, the chemical reaction with relaxation-time characteristic has been discussed firstly. The governing equations are solved by a new efficient analytical method known as double-parameter transformation expansion method (DPTEM). Some interesting results can be revealed. Increasing unsteadiness parameter causes a rise in film thickness for β2 > β1, but a decline as β2 < β1. Moreover, the temperature enhances with rising values of retardation time when β2 < β1, while the contrary behaviors are observed when β2 > β1. Furthermore, unsteadiness parameter has the opposite influences on the concentration in the cases of β2 > β1 and β2 < β1.

Evaluation of LLDPE/LDPE blend foamability by in situ rheological measurements and bubble growth simulations

Polyethylene (PE) foams, which constitute a large share of the total production of thermoplastic foams worldwide, are mainly derived from low density PE (LDPE). In this study, linear low-density PE (LLDPE), with its good mechanical properties but inferior foamability, was blended with LDPE by twin-screw compounding at varying mass ratios of 0/100, 5/95, 15/85, 25/75, and 100/0. Differential scanning calorimetry and small amplitude oscillation shear tests confirmed the compatibility of the blends. A high-pressure rheometer was used to measure the rheological properties of the blends under different CO2 pressures with the aid of a homemade fixture. Gotsis’ model was applied to correlate the rheological data for the blends and obtain their characteristic relaxation time and rigid modulus, which were then substituted into the bubble growth and stability model for numerical simulation. Referring to Considère criterion, it was found that the LLDPE/LDPE blends with a mass ratio of 5/95 possessed the best foamability, and the characteristic relaxation time had a significant effect on the foamability. Batch melt foaming experiments also confirmed the simulation results.

Autonomous stochastic resonance driven by colored noise.

A one-dimensional linear autonomous system coupled to a generic stationary nonequilibrium fluctuating bath can exhibit resonant response when its damped oscillation period matches some characteristic bath's relaxation time. This condition justifies invoking the stochastic resonance paradigm, even if it can be achieved more easily by tuning the system to the bath and not vice versa, as is usually the case. The simple nature of the mechanism numerically investigated here suggests a number of interesting applications for instance in the context of (1) energy harvesting from random ambient vibrations, (2) activated barrier crossing through a saddle point, or (3) an unstable limit cycle.

Reflection and transmission of elastic waves at an interface with consideration of couple stress and thermal wave effects

The reflection and transmission of the thermo-elastic coupled waves at an interface of two different couple stress elastic solids are studied in this paper. Based on the Green-Lindsay theory, the governing equations and the constitutive equations are derived. Different from the classic elastic solid, the interface conditions include the surface couple, the rotation angle, the heat flux and the temperature change. The interface conditions are used to obtain the linear algebraic equations set from which the amplitude ratios of reflection and transmission waves to the incident wave can be determined. Then, the normal energy flux conservation is used to validate the numerical results. At last, the influences of two characteristic relaxation times and the five kinds of thermally and micromechanically interface conditions are discussed based on the numerical results. It is found that the thermal wave effects affect only the longitudinal wave while the couple stress effects affect only the transverse waves. The thermo-elastic coupling makes the longitudinal wave and the thermal wave not only dispersive but also attenuated.

Static and dynamic signatures of anisotropic electronic phase separation in La2/3Ca1/3MnO3 thin films under anisotropic strain

The electronic phase separation (EPS) of optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films under various degrees of anisotropic strain is investigated by static magnetotransport and dynamic relaxation measurements. Three LCMO films were grown simultaneously on (001) $\mathrm{NdGa}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition, and then postgrowth annealed at $780{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in ${\mathrm{O}}_{2}$ for different durations of time. With increasing annealing time, the films developed significant strains of opposite signs along the two orthogonal in-plane directions. The static temperature-dependent resistivity, $\ensuremath{\rho}(T)$, was measured simultaneously along the two orthogonal directions. With increasing annealing time, both zero-field-cooled and field-cooled $\ensuremath{\rho}(T)$ show significant increases, suggesting strain-triggered EPS and appearance of antiferromagnetic insulating (AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, $\ensuremath{\rho}(T)$ along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100]. The enhanced resistivity anisotropy indicates that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100], possibly due to the cooperative deformation and rotation/tilting of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra under the enhanced anisotropic strain. The anisotropic EPS can also be tuned by an external magnetic field. During a field cycle at several fixed temperatures, the AFI phases are melted at high fields and recovered at low fields, resulting in sharp resistance changes of ratio as high as ${10}^{4}$. Furthermore, the resistivity was found to exhibit glasslike behavior, relaxing logarithmically in the phase-separated states. Fitting the data to a phenomenological model, the resulting resistive viscosity and characteristic relaxation time are found to evolve with temperature, showing a close correlation with the static measurements in the EPS states.

Ultrasonic rheology of visco-elastic materials using shear and longitudinal waves

We present a technique for determining the rheology of visco-elastic materials in the ultrasonic frequency range based on the measurement of the longitudinal and shear waves reflected and transmitted by a slab of material with a controlled thickness sandwiched between two delay lines. The high sensitivity of this Fabry-Pérot resonator-based technique allows one to access characteristic relaxation times associated with visco-elasticity. As a useful application of the technique, we characterize the Olympus SWC-2 shear-wave couplant in the 0.7–5 MHz frequency range.

Genetic noise mechanism for power-law switching in bacterial flagellar motors

Switching of the direction of flagella rotations is the key control mechanism governing the chemotactic activity of E. coli and many other bacteria. Power-law distributions of switching times are most peculiar because their emergence cannot be deduced from simple thermodynamic arguments. Recently, it was suggested that by adding finite-time correlations into Gaussian fluctuations regulating the energy height of the barrier between the two rotation states, it is possible to generate switching statistics with an intermediate power-law asymptotics. By using a simple model of a regulatory pathway, we demonstrate that the required amount of correlated ‘noise’ can be produced by finite number fluctuations of reacting protein molecules, a condition common to the intracellular chemistry. The corresponding power-law exponent appears as a tunable characteristic controlled by parameters of the regulatory pathway network such as the equilibrium number of molecules, sensitivities, and the characteristic relaxation time.

Nonlinear Viscoelastic Closure of Salt Cavities

Time-dependent hole closure is a major problem for the many cavities present in rock salt. We use analytical and numerical methods to study how cylindrical holes close under pressure loads with time. We treat salt as a viscoelastic fluid and we use an incompressible nonlinear Maxwell constitutive law to model its mechanical behavior. The viscosity is described by either a power law or an Ellis model depending on whether dislocation creep is considered alone or in combination with pressure solution. The instantaneous closure rate of a circular hole in a power law-based viscoelastic salt is fully determined analytically. A proxy for the transient closure velocity at the rim is also proposed based on a modified version of the characteristic relaxation time θ proposed by Wang et al. (Rock Mech Rock Eng 48(6):2369–2382, 2015) and it has less than 3% inaccuracy for times smaller than 3θ, irrespective of the load or salt type. We derive an analytical expression describing the instantaneous closure rate in an Ellis-based viscoelastic salt. A load threshold determines whether steady state is approached initially. The time θ is also a characteristic relaxation time for this constitutive law, and a master curve can be used to describe the evolution of the closure velocity with time. Using these characteristic values in a typical application underlines the importance of considering pressure solution, in addition to dislocation creep, when studying hole closure in rock salt.

Stress relaxation in quasi-two-dimensional self-assembled nanoparticle monolayers.

We experimentally probed the stress relaxation of a monolayer of iron oxide nanoparticles at the water-air interface. Upon drop-casting onto a water surface, the nanoparticles self-assembled into islands of two-dimensional hexagonally close packed crystalline domains surrounded by large voids. When compressed laterally, the voids gradually disappeared as the surface pressure increased. After the compression was stopped, the surface pressure (as measured by a Wilhelmy plate) evolved as a function of the film aging time with three distinct timescales. These aging dynamics were intrinsic to the stressed state built up during the non-equilibrium compression of the film. Utilizing x-ray photon correlation spectroscopy, we measured the characteristic relaxation time (τ) of in-plane nanoparticle motion as a function of the aging time through both second-order and two-time autocorrelation analysis. Compressed and stretched exponential fitting of the intermediate scattering function yielded exponents (β) indicating different relaxation mechanisms of the films under different compression stresses. For a monolayer compressed to a lower surface pressure (between 20 mN/m and 30 mN/m), the relaxation time (τ) decreased continuously as a function of the aging time, as did the fitted exponent, which transitioned from being compressed (>1) to stretched (<1), indicating that the monolayer underwent a stress release through crystalline domain reorganization. However, for a monolayer compressed to a higher surface pressure (around 40 mN/m), the relaxation time increased continuously and the compressed exponent varied very little from a value of 1.6, suggesting that the system may have been highly stressed and jammed. Despite the interesting stress relaxation signatures seen in these samples, the structural ordering of the monolayer remained the same over the sample lifetime, as revealed by grazing incidence x-ray diffraction.

Kinetics of Low-Temperature Microphotoluminescence of Exciton-Impurity Complexes in CdZnTe Single Crystals

Low-temperature (6.5 K) microphotoluminescence near the intrinsic absorption edge of CdZnTe alloy single crystals is studied under conditions of non-resonant and resonant excitation by picosecond pulses. Characteristic relaxation times for free excitons and exciton-impurity complexes of various types are determined. A significant (by a factor of 4–8) decrease in the photoluminescence signal decay time of exciton-impurity complexes on neutral donors during the transition to the resonant excitation mode is detected. The detected sharp decrease in the photoluminescence signal decay time can indicate the manifestation of collective effects in this system of emitters.

Magnetic granulometry and Mössbauer spectroscopy of FemOn–SiO2 colloidal nanoparticles

The colloidal nanoparticles FemOn–SiO2 have been obtained using the sol–gel method. Based on the data obtained with transmission electron microscopy and dynamic light scattering, a lognormal size distribution of nanoparticles with an average size of 20–30 nm have been constructed. The shape of Mössbauer spectra implies that they have the characteristic relaxation time of the order of 10–9–10–10 s that corresponds to the grain size of about 10 nm. X-ray diffractometry indicates that the crystalline phase of the particles is magnetite. Hysteresis loops have been traced with a vibration sample magnetometer and the demagnetization curve of anhysteretic remanent magnetization measured with a SQUID magnetometer. Theoretical analysis of magnetization data based on a model of magnetostatically interacting superparamagnetic nanoparticles confirms the presence of both separate superparamagnetic magnetite particles (less than 20 nm in size) and those organized into clusters with an average size of about 30 nm.