Dependence Of Relaxation Time Research Articles

The study of compression relaxation time of silicon rubber using optical method

When rubber is subjected to a load, deformation will occur. Microscopically, the rubber bond structure is transformed from one equilibrium corresponds to the unloaded rubber that corresponds to the loaded rubber. As with any other transformation process, this process will take time. This paper experimentally demonstrates the relaxation time dependence on the mass of load observed at a constant temperature. The experiment was carried out by inserting the fiber sensor was into a silicon rubber bar. As the rubber bar was loaded, the intensity of light coming out of the fiber decreases exponentially with time. It is shown that the relaxation time increases with the increase of mass loaded on the fiber. By knowing this, the results of the fiber sensor can be adjusted according to the time-based review of rubber deformation.

Calculation of the inverse relaxation time and the activation energy as a function of temperature for the Raman modes close to the phase transitions in solid nitrogen

The inverse relaxation time is calculated as a function of temperature for the transitions of α-β (P = 0) and ε-δloc-δ (at constant pressures) in the solid N2. For this calculation, the observed data from the literature are used for the Raman frequency shifts and linewidths (FWHM) of the librational Eg mode (α-β) and the internal modes of ʋ1 and ʋ2 (ε-δloc-δ) in the nitrogen. From the temperature dependence of the inverse relaxation time, the activation energy is extracted for the Raman modes (Eg, ʋ1 and ʋ2) to explain the order-disorder transitions (α-β and ε-δloc-δ) in this molecular solid. Close to the phase transitions, a power-law formula is suggested for the relaxation time in relation to the frequency shifts and linewidths, which can be applied to the molecular solids.

Microstructural Capture of Living Ultrathin Polymer Brush Evolution via Kinetic Simulation Studies.

Performing dynamic off-lattice multicanonical Monte Carlo simulations, we study the statics, dynamics, and scission-recombination kinetics of a self-assembled in situ-polymerized polydisperse living polymer brush (LPB), designed by surface-initiated living polymerization. The living brush is initially grown from a two-dimensional substrate by end-monomer polymerization-depolymerization reactions through seeding of initiator arrays on the grafting plane which come in contact with a solution of nonbonded monomers under good solvent conditions. The polydispersity is shown to significantly deviate from the Flory-Schulz type for low temperatures because of pronounced diffusion limitation effects on the rate of the equilibration reaction. The self-avoiding chains take up fairly compact structures of typical size Rg(N) ∼ Nν in rigorously two-dimensional (d = 2) melt, with ν being the inverse fractal dimension (ν = 1/d). The Kratky description of the intramolecular structure factor F(q), in keeping with the concept of generalized Porod scattering from compact particles with fractal contour, discloses a robust nonmonotonic fashion with qdF(q) ∼ (qRg)-3/4 in the intermediate-q regime. It is found that the kinetics of LPB growth, given by the variation of the mean chain length, follows a power law ⟨N(t)⟩ ∝ t1/3 with elapsed time after the onset of polymerization, whereby the instantaneous molecular weight distribution (MWD) of the chains c(N) retains its functional form. The variation of ⟨N(t)⟩ during quenches of the LPB to different temperatures T can be described by a single master curve in units of dimensionless time t/τ∞, where τ∞ is the typical (final temperature T∞-dependent) relaxation time which is found to scale as τ∞ ∝ ⟨N(t = ∞)⟩5 with the ultimate average length of the chains. The equilibrium monomer density profile ϕ(z) of the LPB varies as ϕ(z) ∝ ϕ-α with the concentration of segments ϕ in the system and the probability distribution c(N) of chain lengths N in the brush layer scales as c(N) ∝ N-τ. The computed exponents α ≈ 0.64 and τ ≈ 1.70 are in good agreement with those predicted within the context of the Diffusion-Limited Aggregation theory, α = 2/3 and τ = 7/4.

EPR study of NO radicals encased in modified open C60 fullerenes.

Using pulsed electron paramagnetic resonance (EPR) techniques, the low-temperature magnetic properties of the NO radical being confined in two different modified open -derived cages are determined. It is found that the smallest principal value , being assigned to the axis of the radical, deviates strongly from the free electron value. This behaviour results from partial compensation of the spin and orbital contributions to the value. The measured values in the range of 0.7 yield information about the deviation of the locking potential for the encaged NO from axial symmetry. The estimated 17 meV asymmetry is quite small compared to the situation found for the same radical in polycrystalline or amorphous matrices ranging from 300 to 500 meV. The analysis of the temperature dependence of spin relaxation times resulted in an activation temperature of about 3 K, assigned to temperature-activated motion of the NO within the modified open -derived cages with coupled rotational and translational degrees of freedom in a complicated three-dimensional locking potential.

Microwave (2ω1 − ω2) frequency mixing by mobile carriers in electron-doped and gapped graphene

We theoretically study the generation of microwave radiation at frequency 2ω1−ω2, produced by two microwaves, of frequencies ω1 and ω2, normally incident on monolayer graphene grown epitaxially on a SiC substrate and doped by electrostatic gating. Two mechanisms responsible for this third-order nonlinear effect are considered: (i) the graphene’s conduction band nonparabolicity arising from the substrate-induced bandgap opening at the Dirac points of the graphene’s Brillouin zone and (ii) the energy dependence of the electron-momentum relaxation time. Both mechanisms are incorporated in our Boltzmann-equation-based treatment of the graphene’s third-order nonlinear response to the applied microwave radiation field. Within the framework of the approach, we evaluate the output power P2ω1−ω2 of the (2ω1−ω2) microwave radiation as a function of the graphene’s carrier density nS controlled by the applied gate voltage. Our formulation predicts an unexpected nonmonotonous behavior of this function: there is a pronounced minimum in the P2ω1−ω2(nS), which is followed by a maximum. This finding may serve to provide a rational explanation for this type of oscillation as observed by Dragoman et al. [Appl. Phys. Lett. 97, 093101 (2010)] in the output power of the microwave harmonic generation in gated graphene, which are not properly construed so far.

Time dependent stress relaxation and recovery in mechanically strained 3D microtissues.

Characterizing the time-dependent mechanical properties of cells is not only necessary to determine how they deform but also to understand how external forces trigger biochemical-signaling cascades to govern their behavior. At present, mechanical properties are largely assessed by applying local shear or compressive forces on single cells grown in isolation on non-physiological 2D surfaces. In comparison, we developed the microfabricated vacuum actuated stretcher to measure tensile loading of 3D multicellular “microtissue” cultures. Using this approach, we here assessed the time-dependent stress relaxation and recovery responses of microtissues and quantified the spatial viscoelastic deformation following step length changes. Unlike previous results, stress relaxation and recovery in microtissues measured over a range of step amplitudes and pharmacological treatments followed an augmented stretched exponential behavior describing a broad distribution of inter-related timescales. Furthermore, despite the variety of experimental conditions, all responses led to a single linear relationship between the residual elastic stress and the degree of stress relaxation, suggesting that these mechanical properties are coupled through interactions between structural elements and the association of cells with their matrix. Finally, although stress relaxation could be quantitatively and spatially linked to recovery, they differed greatly in their dynamics; while stress recovery acted as a linear process, relaxation time constants changed with an inverse power law with the step size. This assessment of microtissues offers insights into how the collective behavior of cells in a 3D collagen matrix generates the dynamic mechanical properties of tissues, which is necessary to understand how cells deform and sense mechanical forces in vivo.

Observation of inter-layer charge transmission resonance at optically excited graphene–TMDC interfaces

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy (EF) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to EF of the graphene layer and outline a new method to estimate the excitonic binding energies (Eb) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

Analysis of linear viscoelasticity of aging soft glasses

Aging soft glassy materials do not follow time-translational invariance and violate the principles of linear viscoelasticity, such as the relation between the dynamic moduli in the frequency domain and the stress relaxation modulus in the time domain. Using an aqueous suspension of hectorite clay, a model aging soft glassy material, we account for time-dependent behavior by transforming the experimentally obtained stress relaxation and dynamic moduli from the real-time domain to the effective-time domain by normalizing the real-time by a time-dependent relaxation time. We find that the two sets of experiments probe different aging and deformation timescales, and simultaneous analysis provides insights into material behavior over a wide range of timescales. Careful analysis of the dynamic moduli reveals contamination of (slow) aging α modes by (fast) nonaging β modes, which is not clearly evident in the stress relaxation modulus. On the other hand, the stress relaxation measurements probe timescales long enough to characterize the terminal relaxation of the suspension. A toy Maxwell model is used to elucidate the interaction of the slow aging and fast nonaging modes, and their effect of measurements. The synergistic relationship between the two sets of experiments offers guidelines for characterizing time-dependent materials by generalizing the principles of linear viscoelasticity.

Investigation of dielectric relaxation phenomena and AC electrical conductivity in graphite/carbon nanotubes/engine oil nanofluids

In this paper, we report an investigation on electrical conduction mechanisms of nanofluids based on commercial engine oil loaded with graphite (Gt) and multiwalled carbon nanotubes, at different concentrations. The impedance spectroscopy technique was used to measure the resistance and capacity characterizing each sample in a frequency range 100 Hz–1 MHz and a temperature range 300–400 K. Two formalisms were used to analyze the data: (a) the electrical conductivity which has found to follow the Jonscher’s law with single and double exponents for carbon nanotube concentrations below and above the percolation threshold, respectively, and (b) the complex impedance that has permitted to identify the relaxation peaks according to the Cole–Cole model. Both the two formalisms showed that when the carbon nanotube concentration is higher than the percolation threshold, a positive temperature coefficient and a remarkable change in conductivity were observed, suggesting that the presence of the carbon nanotube greatly affect the electrical properties of the engine oil as a result of additional polarization effect induced by these nanoparticles. Furthermore, the analysis of the temperature dependence of dc conductivity and relaxation time using the Arrhenius equation indicated the addition of carbon nanotubes into engine oil increase the activation energies.

A coupled multi-scale method for predicting the viscoelastic behavior of resin-based 3D braided composites

A new alternative multi-scale calculation procedure was presented to characterize the time-dependent viscoelastic behaviors of resin-based three dimensional (3D) braided composites. The present investigation was processed based on micro-scale fiber/resin, meso-scale yarn/resin and macro-scale homogeneous composite. Integrating with the asymptotic expansion homogenization (AEH) method, the multi-scale physical fields in the three-scales is established, and the multi-phase finite element (MFE) method is adopted to overcome the difficulty in discretization for the heterogeneous structural models. The time-dependent stress relaxation responses under three-scales were particularly investigated by the coupling MFE-AEH multi-scale method. The time-dependent effective viscoelastic properties in the micro-sclae and meso-scale were also calculated by utilizing the Prony Series fitting to simplify the inverse Laplace transform. The accuracy and reliability are validated by the corresponding experimental and numerical results, the effects of braided angle and relaxation time on the viscoelastic properties of 3D braided composites were studied, respectively.

Dielectric properties of PVA cryogels prepared by freeze-thaw cycling.

Solutions of polyvinyl alcohol (PVA) in water can form gels upon repeated freezing and thawing. These PVA cryogels have applications as biomaterials, including artificial tissue and drug delivery systems. We have studied the dielectric properties of PVA cryogels within the freeze-thaw cycles as a function of both frequency and temperature in order to understand the physical changes that take place during the thermal cycling process. Our results indicate that most of the changes in dielectric properties occur during the cooling phase of the first cycle and suggest that the solution must be cooled below a critical temperature of about 263 K for the formation of a gel that persists after thawing. The material's dielectric spectrum shows the presence of several relaxation processes. We identify one of these with the dielectric relaxation of ice and two others with motions of the PVA polymer chains. The temperature dependence of the polymeric relaxation times suggests that they are both thermally activated, with an activation energy of roughly 300 kJ/mol.

Visco-hyperelastic characterization of human brain white matter micro-level constituents in different strain rates.

In this study, we propose a computational characterization technique for obtaining the material properties of axons and extracellular matrix (ECM) in human brain white matter. To account for the dynamic behavior of the brain tissue, data from time-dependent relaxation tests of human brain white matter in different strain rates are extracted and formulated by a visco-hyperelastic constitutive model consisting of the Ogden hyperelastic model and the Prony series expansion. Through micromechanical finite element simulation, a derivative-free optimization framework designed to minimize the difference between the numerical and experimental data is used to identify the material properties of the axons and ECM. The Prony series expansion parameters of axons and ECM are found to be highly affected by the Prony series expansion coefficients of the brain white matter. The optimal parameters of axons and ECM are verified through micromechanical simulation by comparing the averaged numerical response with that of the experimental data. Moreover, the initial shear modulus and the reduced shear modulus of the axons are found for different strain rates of 0.0001, 0.01, and 1s-1. Consequently, first- and second-order regressions are used to find relations for the prediction of the shear modulus at the intermediate strain rates. Graphical Abstract The applied procedure for characterization of brain white matter micro-level constituents. The macro-level experimental data in different strain rates are used in the context of simulation-based optimization to obtain the properties of axons and extracellular matrix material.

AC conductivity measurements of liquid-solid insulation of power transformers with high water content

• Electron tunneling takes place between neighboring potential wells, water nano-drops. • In materials there are two types of hopping conductivity. • Activation energy of relaxation times at growth stages in σ( f ) plots are similar. • Composition and structure nanoparticles, between which electrons tunnel, are similar. The paper investigated the influence of temperature on the frequency dependence of the conductivity of cellulose-ester insulation of power transformers containing nano-drops of water. It was observed that in the insulation containing nano-drops of water on the σ( f ) dependence two sections of conductivity increase occur. It was found that the state of the frequency dependence of conductivity is affected simultaneously by the temperature dependence of conductivity and the temperature dependence of dipole relaxation time. Activation energies for conductivity and relaxation times were determined, the values of which are the same for both stages of the conductivity increase. This means that the structure of water nanoparticles, between which electrons tunnel, is the same for both stages of σ( f ) growth. The occurrence of the two growth stages on the frequency dependence of conductivity is associated with differences in distances between adjacent nanoparticles and the associated relaxation times.

Impact of Pressure and Temperature on the Compaction Dynamics and Layer Properties of Powder-Pressed Methylammonium Lead Halide Thick Films

While halide perovskite X-ray detectors based on single crystals could achieve extraordinary sensitivities, detectors based on polycrystalline thick films lag behind in efficiency. This is unfortunate since the processing methods for producing polycrystalline thick films, especially by pressure treatment of powders, are suitable for upscaling. Here, we investigate in detail the pressing of readily prepared powders of methylammonium lead halide perovskites MAPbI3 and MAPbBr3 to thick layers. By time-dependent pressure measurements, we monitor the occurring compaction dynamics, identifying two relaxation processes with different timescales. When pressing at elevated temperatures from room temperature (RT) to 100 °C, the pressure relaxations change drastically. While the layer properties such as relative density and surface roughness only improve to a certain degree by increasing the pressure at RT, we observe relative densities >97%, considerable reduction in surface roughness, and a significant increase in grain size with tempered pressing. Analyses regarding time-dependent pressure relaxations of tempered pressing allow attributing the dynamics to a sintering process, where we find the sinter onset to be surprisingly low at about 30 °C, mainly independent of the applied pressure (10–100 MPa). Our results will allow for an improved and more targeted powder processing of halide perovskite thick films as they are promising candidates for efficient X-ray detectors.

Time-dependent relaxation of observables in complex quantum systems

We consider time-dependent relaxation of observables in quantum systems of chaotic and regular type. Using statistical arguments and exact numerical solutions we show that the spread of the initial wave function in the Hilbert space and the main characteristics of evolution of observables have certain generic features. The study compares examples of regular dynamics, a completely chaotic case of the Gaussian orthogonal ensemble, a bosonic system with random interactions, and a fully realistic case of the time evolution of various initial non-stationary states in the nuclear shell model. In the case of the Gaussian orthogonal ensemble we show that the survival probability obtained analytically also fully defines the relaxation timescale of observables. This is not the case in general. Using the realistic nuclear shell model and the quadrupole moment as an observable we demonstrate that the relaxation time is significantly longer than defined by the survival probability of the initial state. The full analysis does not show the presence of an analog of the Lyapunov exponent characteristic for examples of classical chaos.

Creep and relaxation of polyester acrylate composites

It is shown that creep and relaxation are the most important properties for structural materials. This research presents the results of a study of filled polyester acrylate composites’ creep and relaxation. The comparison of mechanical indicators with similar for unfilled materials is shown in this work. Investigation of mechanical properties of the samples was carried out on a universal testing machine Shimadzu Autograph AG-X Series at room temperature. It was revealed that the intensity of stress and creep relaxation in the composite is much weaker than in pure resin. It was found that composites are characterized by a logarithmic time dependence of stress relaxation and creep. Logarithmic dependences for the studied parameters are of the same nature. Changes in indicators are mainly due to the properties of the binder.

In Ukrainian

An experimental study has beencarried out on the spectral dependence of the relaxation time of surface monopolar photoconductivity in the structures of macroporous silicon, with strong surface absorption of light, while the absorption coefficient varied in the range 2.37∙10 3 –3∙10 5 cm –1 , and the absorption depth from 0.03 μm to 4.22 μm. Visible LEDs 0.38–0.62 µm were used for the studies: ultraviolet, violet, blue, green, yellow, orange and red. Structures of macroporous silicon were formed on n-type silicon with orientation [100] and specific resistivity of 4.5 Ω∙cm using photoelectrochemical etching. The macropores had a diameter of D p = 3.5 μm, a depth of h p = 80 μm and a concentration of N p = 3.5∙10 6 cm –2 . In the structures of macroporous silicon, a "slow" relaxation of monopolar photoconductivity was experimentally observed. It has been found that the relaxation time of monopolar photoconductivity is dependent on the wavelength of the illumination and varieds in the range of 15.5–42 s in the visible range of 0.38–0.62 μm. Moreover, the dependence of the photoconductivity relaxation time on the illumination wavelength had a minimum relaxation time of 15.5 s when illuminated by green light with a wavelength of 0.5215 μm. The reason for the dependence of the relaxation time of the monopolar photoconductivity on the wavelength is the competition of the processes of recombination and trapping of the main charge carriers on the so-called "slow" surface levels. The dominance of the recombination or trapping process is influenced by the characteristics of local centers, the surface conditions, the surface bending of the Y s zones, which determines the surface monopolar photoconductivity and its relaxation. In the long-wavelength region of the spectrum, the decrease in the photoconductivity relaxation time from 42 s to 15.5 s is explained by the dominance of the trapping process of the main charge carriers at "slow" surface levels, which are localized within 1 nm of the silicon surface in the SiO x transition layer. In the short-wavelength region of the spectrum, the slow increase in the photoconductivity relaxation time from 15.5 to 23.7 s is explained by the fact that the trapping of the main charge carriers in the "slow" surface states, which are localized in SiO 2 and have a longer photoconductivity relaxation time, begins to influence the photoconductivity relaxation time. The level of sticking gradually turns into the level of slow recombination with a large absorption coefficient, reaching values of 3∙10 5 cm –1 . The photoconductivity relaxation associated with surface levels changes with a change in the wavelength of illumination, which causes a change in the absorption coefficient, absorption depth, and surface bending of the Y s zones. Due to this, a spectral dependence was observed of the relaxation time of surface photoconductivity in the structure of macroporous silicon.

Microwave dielectric relaxation spectroscopy of paracetamol and its aqueous solutions

The complex dielectric function ε∗(ω) = ε ′ (ω) – jε ″ (ω) is studied for aqueous solutions of Paracetamol (PCM) (at four different temperatures 293.15 K, 303.15 K, 313.15 K and 323.15 K) across the frequency range of 200 MHz to 30 GHz and of pure PCM in pellet form (at a constant temperature of 300.15 K) over the frequency ranging from 200 MHz to 20 GHz. Dielectric properties are measured using Anritsu Shockline Vector Network Analyser (VNA) Model No. MS46322A, SPEAG DAK – 1.2 (5–50 GHz) and DAK – 3.5 (200 MHz–20 GHz) probes. Obtained dielectric data are further analysed with Debye relaxation model in order to understand the dipolar relaxation behaviour of these solutions. From obtained dielectric data; other parameters like power reflected (Pr), power transmitted (Pt), penetration depth (dp) have also been determined at 2.45 GHz and analysed in view of its applicability for microwave dielectric heating in pharmaceuticals. Temperature dependence of the dipolar relaxation time is used to evaluate thermodynamical parameters: Gibbs free energy of activation (ΔFe), entropy of activation (ΔSe) and enthalpy of activation (ΔHe).

Fractional heat conduction model with phase lags for a half‐space with thermal conductivity and temperature dependent

In the current investigation, the thermoelasticity with a model of fractional order is used to discuss a problem of thermoelastic half‐space. Such theory depends on the time‐fractional derivative of Caputo of order α. The thermal conductivity is considered to be a variable, and the medium surface is subjected to a free from traction and a thermal shock. Then, the transform of Laplace has been utilized for the solutions of the governing equations. The inverse of the Laplace transform entertained numerically, employing a technique based on methods of Fourier expansion. The obtained numerical solutions for different variables and the obtained results are offered graphically. The effect of the variation of thermal conductivity as well as a fractional‐order parameter on displacement, temperature, and stress are studied. A comparison is made between the results obtained in the current investigation and the previous results by others when the new parameters neglect. The results obtained indicated to the significant role of the heat conduction fractional model, thermal conductivity, temperature dependent, and thermal relaxation time on the thermoelastic half‐space.

Improved distortion prediction in additive manufacturing using an experimental-based stress relaxation model

In additive manufacturing (AM), numerous thermal cycles make stress relaxation a significant factor in affecting the material mechanical response. However, the traditional material constitutive model cannot describe repeated annealing behavior. Here, we propose an improved constitutive model based on a serial of stress relaxation experiments, which can descript the temperature and time-dependent stress relaxation behavior during AM. By using the proposed relaxation model, the prediction accuracy is significantly improved due to the recovery of inelastic strain during multilayer deposition. The results are validated by both in-situ and final distortion measurements. The influence mechanism of the relaxation behavior on material mechanical response is explained by the three-bar model in thermo-elastic-plastic theory. The relaxation behavior during the whole AM process is clarified. The stress behavior is found to have a limited effect when merely depositing several layers; nevertheless, it becomes a prominent impact when depositing multiple layers. The proposed model can enhance modeling accuracy both in AM and in multilayer welding.