Behavior Of Relaxation Time Research Articles

High-Temperature Creep-Fatigue Behavior of Alloy 617

This paper presents the high-temperature creep-fatigue testing of a Ni-based superalloy of Alloy 617 base metal and weldments at 900 °C. Creep-fatigue tests were conducted with fully reversed axial strain control at a total strain range of 0.6%, 1.2%, and 1.5%, and peak tensile hold time of 60, 180, and 300 s. The effects of different constituents on the combined creep-fatigue endurance such as hold time, strain range, and stress relaxation behavior are discussed. Under all creep-fatigue tests, weldments’ creep-fatigue life was less than base metal. In comparison with the low-cycle fatigue condition, the introduction of hold time decreased the cycle number of both base metal and weldments. Creep-fatigue lifetime in the base metal was continually decreased by increasing the tension hold time, except for weldments under longer hold time (>180 s). In all creep-fatigue tests, intergranular brittle cracks near the crack tip and thick oxide scales at the surface were formed, which were linked to the mixed-mode creep and fatigue cracks. Creep-fatigue interaction in the damage-diagram (D-Diagram) (i.e., linear damage summation) was evaluated from the experimental results. The linear damage summation was found to be suitable for the current limited test conditions, and one can enclose all the data points within the proposed scatter band.

Homogeneous isotropization and equilibration of a strongly coupled plasma with a critical point

We use holography to investigate the process of homogeneous isotropization and thermalization in a strongly coupled mathcal{N}=4 Super Yang-Mills plasma charged under a U(1) subgroup of the global SU(4) R-symmetry which features a critical point in its phase diagram. Isotropization dynamics at late times is affected by the critical point in agreement with the behavior of the characteristic relaxation time extracted from the analysis of the lowest non-hydrodynamic quasinormal mode in the SO(3) quintuplet (external scalar) channel of the theory. In particular, the isotropization time may decrease or increase as the chemical potential increases depending on whether one is far or close enough to the critical point, respectively. On the other hand, the thermalization time associated with the equilibration of the scalar condensate, which happens only after the system has relaxed to a (nearly) isotropic state, is found to always increase with chemical potential in agreement with the characteristic relaxation time associated to the lowest non-hydrodynamic quasinormal mode in the SO(3) singlet (dilaton) channel. These conclusions about the late dynamics of the system are robust in the sense that they hold for different initial conditions seeding the time evolution of the far-from-equilibrium plasma.

Phase behavior, rheological characteristics and microstructure of sodium caseinate-Persian gum system

In this study, the phase behavior of sodium caseinate-Persian gum mixtures was investigated. The effect of thermodynamic incompatibility on phase distribution of sodium caseinate fractions as well as the flow behavior and microstructure of the biopolymer mixtures were also studied. The phase diagram clearly demonstrated the dominant effect of Persian gum on the incompatibility of the two biopolymers. SDS-PAGE electrophoresis indicated no selective fractionation of sodium caseinate subunits between equilibrium phases upon de-mixing. The microstructure of mixtures significantly changed depending on their position within the phase diagram. Fitting viscometric data to Cross and Bingham models revealed that the apparent viscosity, relaxation time and shear thinning behavior of the mixtures is greatly influenced by the volume ratio and concentration of the equilibrium phases. There is a strong dependence of the flow behavior of sodium caseinate-Persian gum mixtures on the composition of the equilibrium phases and the corresponding microstructure of the system.

Thermal properties of Zn2(C8H4O4)2\u2022C6H12N2 metal-organic framework compound and mirror symmetry violation of dabco molecules

Thermal properties of Zn2(C8H4O4)2•C6H12N2 metal-organic framework compound at 8–300 K suggest the possibility of subbarrier tunnelling transitions between left-twisted (S) and right-twisted (R) forms of C6H12N2 dabco molecules with D3 point symmetry. The data agree with those obtained for the temperature behavior of nuclear spin-lattice relaxation times. It is shown that there is a temperature range where the transitions are stopped. Therefore, Zn2(C8H4O4)2•C6H12N2 and related compounds are interesting objects to study the effect of spontaneous mirror-symmetry breaking and stabilization of chiral isomeric molecules in solids at low temperatures.

An approach to the interpretation of Cole–Davidson and Cole–Cole dielectric functions

Assuming that a dielectric sample can be described by Debye's model at each frequency, a method based on Cole's treatment is proposed for the direct estimation at experimental frequencies of relaxation times and the corresponding static and infinite-frequency permittivities. These quantities and the link between dielectric strength and mean molecular dipole moment at each frequency could be useful to analyze dielectric relaxation processes. The method is applied to samples that follow a Cole–Cole or a Cole–Davidson dielectric function. A physical interpretation of these dielectric functions is proposed. The behavior of relaxation time with frequency can be distinguished between the two dielectric functions. The proposed method can also be applied to samples following a Navriliak–Negami or any other dielectric function. The dielectric relaxation of a nanofluid consisting of graphene nanoparticles dispersed in the oil squalane is reported and discussed within the novel framework.

50th Anniversary Perspective: Challenges in the Dynamics and Kinetics of Glass-Forming Polymers

The phenomenology of the glass transition and the associated behavior in the near liquid and glassy states are detailed, including the cooling rate dependence of the glass transition, Kovacs’ three signatures of structural recovery, and enthalpy overshoots. Dynamics in the liquid regime just above Tg and the associated temperature dependences are also covered since this behavior is important to understanding the glassy dynamics. The current models of structural recovery and their shortcomings are presented. A number of important unanswered questions are discussed, including how the relaxation time in the glassy state depends on structure, the relationship between the evolution of different properties, the resolution of the Kauzmann paradox, and the behavior of the equilibrium relaxation time below Tg. New experimental approaches are needed to make breakthroughs, such as two that are described: one involving 20 Ma amber to test whether the Vogel temperature dependence continues for the equilibrium state be...

Sub-Tg relaxation times of the α process in metallic glasses

The current view of structural relaxation in metallic glasses assumes the presence of primary and secondary processes with different activation energies. While the faster, secondary process can be well characterized in the out-of-equilibrium state below the glass transition temperature Tg, the experimental direct determination of the primary process in this temperature region is more difficult due to the long relaxation times. In this work, we merge new and literature data to analyze the temperature behavior of the primary relaxation time below Tg as observed by mechanical spectroscopy and stress relaxation of metallic glasses of different fragility. We compare these results with the microscopic structural relaxation times previously measured with X-ray photon correlation spectroscopy. The coincidence between the macroscopic and microscopic relaxation times allows us to discuss the underlying mechanisms responsible of primary relaxation over different length scales, as well as to propose an overall picture of the primary relaxation behavior in the glassy regime near Tg.

Quantum effects in dynamics of water and other liquids of light molecules.

Nuclear quantum effects in atomic motions are well known at low temperatures [Formula: see text] K, but analyses of structural relaxation in liquids and description of the glass transition traditionally neglect quantum effects at higher temperatures, [Formula: see text] K. Recent studies, however, suggested that nuclear quantum effects in systems of light molecules (e.g., water) might play an important role in structural dynamics and provide non-negligible contributions at such temperatures, and even up to ambient temperature. In this article, we discuss experimental evidences of the quantum effects in glass transition in liquids of light molecules and possible theoretical descriptions of these effects. We show that quantum effects may qualitatively change the temperature behavior of the structural relaxation time in supercooled liquids leading to deviations of some well-established properties of the glass transition when it happens at low temperatures. We also demonstrate that unusual behavior of water dynamics at low temperatures, including apparent fragile-to-strong crossover, can be ascribed to nuclear quantum effects.

Detailed analysis of Rouse mode and dynamic scattering function of highly entangled polymer melts in equilibrium

We present large-scale molecular dynamics simulations for a coarse-grained model of polymer melts in equilibrium. From detailed Rouse mode analysis we show that the time-dependent relaxation of the autocorrelation function (ACF) of modes p can be well described by the effective stretched exponential function due to the crossover from Rouse to reptation regime. The ACF is independent of chain sizes N for N/p < Ne (Ne is the entanglement length), and there exists a minimum of the stretching exponent as N/p → Ne. As N/p increases, we verify the crossover scaling behavior of the effective relaxation time τeff,p from the Rouse regime to the reptation regime. We have also provided evidence that the incoherent dynamic scattering function follows the same crossover scaling behavior of the mean square displacement of monomers at the corresponding characteristic time scales. The decay of the coherent dynamic scattering function is slowed down and a plateau develops as chain sizes increase at the intermediate time and wave length scales. The tube diameter extracted from the coherent dynamic scattering function is equivalent to the previous estimate from the mean square displacement of monomers.

Interacting Brownian motion with resetting

We study two Brownian particles in dimension d = 1, diffusing under an interacting resetting mechanism to a fixed position. The particles are subject to a constant drift, which biases the Brownian particles toward each other, and reset when they are adjacent. We derive the steady-state distributions and study the late time relaxation behavior to the stationary state.

Effect of an intermediate bcc phase on the evolution of superfluid inclusions in an hcp3He–4He matrix

Pulsed NMR is used to study the evolution of liquid inclusions formed in an hcp matrix during rapid cooling of a 3He-4He solution containing 1.05% 3He. The diffusion coefficient of 3He in the liquid inclusions as they evolve is measured by a spin echo technique with two probe pulses. The measurements were made at 1.67 K, which corresponds to the region of the bcc phase in the phase diagram, and at 1.38 K, where the bcc phase is absent. It is found that during the evolution in both cases, the liquid inclusions are smaller than the diffusion length and diffusion is restricted. The measured coefficient of restricted diffusion made it possible to determine the characteristic size of the inclusions. In the first case, during the evolution of the liquid inclusions an intermediate bcc phase in the form of dendrites develops and separates the liquid inclusions into a mass of fine droplets. Because of the rapid growth of the bcc phase, the size of the droplets decreases rapidly and the process ends with the disappearance of the bcc phase and the formation of an amorphous state. The results derived from the measured diffusion coefficient correlate with the behavior of the spin-lattice relaxation time in this kind of system. In the second case, at a lower temperature, the bcc phase does not develop and the evolution of the liquid inclusions is accompanied by a very slow reduction in their size until their complete solidification.

Influence of Swift Heavy O&lt;sup&gt;7+&lt;/sup&gt; Ion Irradiation on Conductivity and Relaxation Dynamics of Two Different Blends of PVA:PEO:LiCF&lt;sub&gt;3&lt;/sub&gt;SO&lt;sub&gt;3&lt;/sub&gt;:EC System

In the present work, the variation in conductivity (σdc) and relaxation properties of two different blends i.e. Blend-1: [PVA(47.5):PEO(47.5):(LiCF3SO3)5:(EC)8] and Blend-2: [PVA(47):PEO(47):(LiCF3SO3)5:(EC)6] of PVA:PEO:LiCF3SO3:EC system, with respect to various fluences of swift heavy O7+ ion radiations ranging from 1x1011 to 3x1012 ions/cm2, is investigated in detail. Here, (i) the behavior of relaxation time (τ) calculated from M” vs. log f curves and (ii) the scaling of imaginary part of modulus (M”), is studied thoroughly for various fluences. The investigation thus, provides an insight regarding the effect of O7+ ion radiations on the relaxation dynamics of both the blends.

Dielectric properties of polyfunctional alcohols: 2,3-butanediol

Using a variety theoretical approaches within the Debye, Davidson–Cole, and Forsman models, and an approach based on the Dissado–Hill theory, dielectric spectra of 2,3-butanediol in the temperature range of 298 to 423 K are analyzed. It is shown that the dielectric spectra of 2,3-butanediole are described by the Davidson–Cole equation, and the βDC parameter depends strongly on temperature. The spectrum of dielectric relaxation of 2,3-butanediol within the Debye theory is presented as the sum of two areas of dispersion, and conclusions are drawn regarding possible mechanisms of dispersion responsible for the obtained fields. The relaxation times of 2,3-butanediol, calculated using different equations describing the nonlinear behavior of relaxation times, are compared. The dipole moments of clusters are obtained for the first time using the Dissado–Hill cluster model, and a preliminary analysis of them is performed.

Relaxation dynamics of two-component fluid bilayer membranes.

We theoretically investigate the relaxation dynamics of a nearly flat binary lipid bilayer membrane by taking into account the membrane tension, hydrodynamics of the surrounding fluid, inter-monolayer friction and mutual diffusion. Mutual diffusion is the collective irreversible process that leads to homogenization of the density difference between the two lipid species. We find that two relaxation modes associated with the mutual diffusion appear in addition to the three previously discussed relaxation modes reflecting the bending and compression of the membrane. Because of the symmetry, only one of the two diffusive modes is coupled to the bending mode. The two diffusive modes are much slower than the bending and compression modes in the entire realistic wave number range. This means that the long time relaxation behavior is dominated by the mutual diffusion in binary membranes. The two diffusive modes become even slower in the vicinity of the unstable region towards phase separation, while the other modes are almost unchanged. In short time scales, on the other hand, the lipid composition heterogeneity induces in-plane compression and bending of the bilayer.

Diffusion under time-dependent resetting

We study a Brownian particle diffusing under a time-modulated stochastic resetting mechanism to a fixed position. The rate of resetting r(t) is a function of the time t since the last reset event. We derive a sufficient condition on r(t) for a steady-state probability distribution of the position of the particle to exist. We derive the form of the steady-state distributions under some particular choices of r(t) and also consider the late time relaxation behavior of the probability distribution. We consider first passage time properties for the Brownian particle to reach the origin and derive a formula for the mean first passage time (MFPT). Finally, we study optimal properties of the MFPT and show that a threshold function is at least locally optimal for the problem of minimizing the MFPT.

Influence of precursor molar concentration on the electrical and magnetic properties of synthesized MnS nanocrystals

MnS Nanocrystals have been synthesized with 1:1, 1:2 and 2:1 molar ratio of precursors using wet chemical method. The electrical and magnetic properties of as-synthesized MnS nanocrystals have been investigated using the impedance spectroscopy and vibrating sample magnetometer respectively. An increase in dielectric constant from 68.5 to 87.9 with increasing Mn content and decrease to 54.1 with increase in the sulfur content was observed. Both the dielectric constant and the loss factor increased with temperature due to the Maxwell–Wagner type interfacial space-charge polarization. The Cole–Cole plot confirmed that the conduction in as-synthesized samples are through the grain and grain boundaries. The resistance and capacitance of grain and grain boundaries have been calculated. The grain resistance varies from 248 to 199 Ω whereas the grain boundary resistance varies from 16 to 6.7 KΩ over the temperature ranges of 323–473 K for 1:1 sample. It endorsed the NTCR type behavior in all the samples. The electric modulus representation revealed the well-defined relaxation peaks. The relaxation time versus temperature behavior revealed that the relaxation time decreases with increase in temperature. AC-conductivity (σac) increases with increasing frequency and temperature. σac increases from 1.58 × 10−5 to 1.51 × 10−3 S cm−1 with the increase of Mn content and then decreases to 1.22 × 10−8 S cm−1 with increase of sulfur content. The activation energy is found to be 0.35 eV (1:1), 0.64 eV (1:2) and 0.28 eV (2:1) at 3 kHz. The chemically modified MnS nanocrystals exhibit paramagnetic behavior. A typical saturation magnetization of 0.56, 0.38, and 0.57 emu/g and coercivity of 2.31, 8.42, and 5.57 Oe at room temperature for 1:1, 1:2 and 2:1 sample respectively.

Rheological and morphological properties of thermoplastic olefin blends containing nanosilica

This study aims at understanding the rheological and morphological behavior of polypropylene (PP) and ethylene octene copolymer (EC) blends containing nanosilica. Maleic anhydride grafted species of PP and EC were also used to favor the localization of the silica particles in the matrix or in the dispersed phase, respectively. The morphological stability of the blends under flow was investigated by shearing at a very low shear rate of 0.01s−1. The morphology of the dispersed polymer phases as well as the microstructure of the silica particles inside the nanocomposites were examined using scanning and transmission electron microscopy techniques. Considering the limited time stability of composites with high level of silica loading, the low frequency data of these systems were obtained using dynamic frequency sweep data as well as an extended set of transient creep data reflecting the long time relaxation behavior. The presence of silica particles resulted in an improved morphological stability of all nanocomposites, but the effect was more pronounced when the nanoparticles were localized in the dispersed EC phase. While shearing diminished the viscoelastic properties of the blends containing no silica particle, the sheared nanocomposites with a high concentration of nanosilica revealed enhanced levels of viscoelasticity. This was attributed to the promoted interconnection of solid particles during shearing and the formation of more extended networks as confirmed by microscopic observations.

A hybrid Brownian dynamics/constitutive model for yielding, aging, and rejuvenation in deforming polymeric glasses.

We present a hybrid model for polymeric glasses under deformation that combines a minimal model of segmental dynamics with a beads-and-springs model of a polymer, solved by Brownian dynamics (BD) simulations, whose relaxation is coupled to the segmental dynamics through the drag coefficient of the beads. This coarse-grained model allows simulations that are much faster than molecular dynamics and successfully capture the entire range of mechanical response including yielding, plastic flow, strain-hardening, and incomplete strain recovery. The beads-and-springs model improves upon the dumbbell model for glassy polymers proposed by Fielding et al. (Phys. Rev. Lett., 2012, 108, 048301) by capturing the small elastic recoil seen experimentally without the use of ad hoc adjustments of parameters required in the model of Fielding et al. With appropriate choice of parameters, predictions of creep, recovery, and segmental relaxation are found to be in good agreement with poly(methylmethacrylate) (PMMA) data of Lee et al. (Science, 2009, 323, 231-234). Our model shows dramatic differences in behavior of the segmental relaxation time between extensional creep and steady extension, and between extension and shear. The non-monotonic response of the segmental relaxation time to extensional creep and the small elastic recovery after removal of stress are shown to arise from sub-chains that are trapped between folds, and that become highly oriented and stretched at strains of order unity, connecting the behavior of glassy polymers under creep to that of dilute polymer solutions under fast extensional flows. We are also able to predict the effects of polymer pre-orientation in the parallel or orthogonal direction on the subsequent response to extensional deformation.

Non-diffusive dynamics in a colloidal glass: Aging versus rejuvenation

The microscopic dynamics of spontaneously aged and rejuvenated glassy Laponite is investigated through X-ray photon correlation spectroscopy. Two different behaviours of the intensity autocorrelation functions are observed depending on the history of the sample: stretched for spontaneously aged samples and samples rejuvenated from a Wigner glass and compressed, typical of anomalous dynamics, for samples rejuvenated from a DHOC glass. The relaxation time behaviour in the three cases indicates a non-diffusive dynamics of the particles. The present system offers therefore an overview of various dynamical behaviours previously observed individually in several systems and the possibility to pass from one to the other choosing ad hoc the time parameter.

An evidence of strong electron–phonon interaction in the neutron irradiation induced defects in silicon

Silicon material was irradiated with reactor neutrons, which unavoidably created clusters of defects. Spectral photo current measurements in the range from 0.5eV up to 1.4eV showed accumulation of the signal and long time relaxation behavior in some series of Si (MCZ and FZ) wafers. The proposed differential spectrum analysis method revealed the defects corresponding to strong electron–phonon coupling centers.