Characteristic Relaxation Time Research Articles

Structure and dynamics in suspensions of magnetic platelets.

In this research, we employ Brownian dynamics simulations, density functional theory, and mean-field theory to explore the profound influence of shape anisotropy of magnetic nanoplatelets on suspension magnetic response. Each platelet is modelled as an oblate cylinder with a longitudinal point dipole, with an emphasis on strong dipolar interactions conducive to self-assembly. We investigate static structural and magnetic properties, characterising the system through pair distribution function, static structure factor, and cluster-size distribution. The findings demonstrate that shape-specific interactions and clustering lead to significant changes in reorientational relaxation times. Under zero field, distinctive modes in the dynamic magnetic susceptibility identify individual particles and particle clusters. In the presence of an applied field, the characteristic relaxation time of clusters increases, while that of single particles decreases. This research provides insights into the intricate interplay between shape anisotropy, clustering, and magnetic response in platelet suspensions, offering valuable perspectives for recent experimental observations.

Исследование нестационарного горения заряда в модельной твердотопливной регулируемой двигательной установке

Based on the coupled combustion model of mixed solid fuel, unsteady combustion of fuel was studied when the pressure was changed in the combustion chamber of a model controlled small solid fuel engine. Modeling of engine thrust control system was carried out in two ways: by changing the critical section area of the nozzle over time according to a given law and by changing the area of the burning surface of the fuel according to a given law. It is found that with a sharp increase in pressure, the combustion rate increases above the quasi-stationary value, which leads to an increase in the pressure in the combustion chamber above that calculated from the quasi-stationary rate. With a sharp decrease in pressure, the combustion rate decreases to a value less than the quasi-stationary rate at the corresponding pressure. The decrease in the combustion rate leads to a decrease in the pressure in the combustion chamber below that calculated from the quasi-stationary rate. At small values of the characteristic relaxation time of the combustion chamber, a sharp decrease in pressure in the combustion chamber will lead to the extinction of the charge combustion.

Study on the Relaxation Time Characteristics of Brain Tissue Based on Multi-Parametric Quantitative Magnetic Resonance Imaging

Traditional magnetic resonance imaging (MRI) is qualitative imaging, and doctors need to rely on experience to diagnose diseases, which cannot meet the current needs of precision medicine. As a new quantitative magnetic resonance imaging technology, magnetic resonance fingerprint imaging can obtain a variety of human tissue parameters at the same time through a data acquisition, which greatly improves the imaging speed and improves the impact of noise on image quality. Several pattern matching algorithms are compared, including direct matching method, Bloch response iterative projection method, covering tree and approximate nearest neighbor search method, and improved methods. Absolute error image, mean absolute error (MAE), normalized root means square error (RMSE) and running time are counted in the experimental results. The results show that the improved method is better than the traditional method, which can greatly improve the quality of MR fingerprint multi-parameter images (T1, T2, B0, PD), and make the running time within an acceptable range. In addition, the improved algorithm is insensitive to random additive noise.

Rheology and agglomeration behavior of semi-crystalline polyamide powders for selective laser sintering: A comparative study of PA11 and PA12 formulations

In this study, we investigated the rheological behavior of various polyamide powders as a function of temperature, shear rate, and relative humidity. Near the glass transition temperature (Tg), powders exhibited an abrupt increase in static flow threshold, indicating caking due to particle agglomeration. At higher temperatures, a transition from elastic to plastic flow was observed around 80∘C, accompanied by changes in the plateau value of the shear modulus (G0′) for different powders. In the dense flow regime, a critical temperature value dictated the transition between caking (powder’s tendency to form lumps or masses) and free-flowing behavior, with temperature promoting agglomeration and shear rate causing dispersion. The critical temperature increased with the strain rate, highlighting the delicate balance between agglomeration and dispersion. Also, it is found that Relative Humidity (RH) plays a crucial role in reducing the stress developed within the sample and counteracting the temperature effect. Additionally, the slow compaction dynamics of powders were studied, revealing an inverse logarithmic law with a characteristic relaxation time (τ) that strongly correlated with the angle of repose.

Direct Observation of the Stretching of the Photoinduced Current Relaxation in α-Ga2O3 Schottky Diodes

Relaxation of photoinduced current in α-Ga2O3-based Schottky diodes was measured. Such Schottky diodes were fabricated using α-Ga2O3 films grown by Halide Vapor Phase Epitaxy (HVPE) on basal plane sapphire. Their photosensitivity in UV-C region was very high and several orders of magnitude higher than for visible light. The photoinduced current rise and decay times are found to be quite long, which is attributed to the hole capture by deep acceptors and to the effects of potential fluctuations. Due to these problems the photoinduced current relaxation process consists of several steps and takes more than hundreds of seconds. The characteristic relaxation times were calculated using the logarithmic differential analysis that demonstrated that the build-up and decay processes can be accurately described by stretched exponents with different broadening factors β. It was shown that additional illumination by long-wave light can help to observe the slowing of the relaxation speed. The temperature activation energies for photoinduced current decay and rise times are 0.37 ± 0.03 eV and 0.06 ± 0.005 eV respectively.

Reprocessing polyurethane foam using dynamic covalent chemistry in extrusion

AbstractThermoset polyurethanes (PUs) pose recycling challenges due to their crosslinked structure. This study investigates the possibility to directly reprocess PU foams through (dynamic) carbamate exchange using reactive extrusion. By varying compounding temperature and catalyst (dibutyltin dilaurate, DBTDL) concentration, the extrusion process is examined using torque measurements. We clearly show that it is possible to reprocess the PU foam at temperatures well below 200°C and that DBTDL catalyst greatly enhances bond exchange rates during compounding. Reproducible extrusions at 160°C with 0.3 wt% DBTDL result in a material with a gel fraction of 0.90 displaying typical dynamic covalent network behavior, as confirmed by stress relaxation measurements. The measured characteristic relaxation times display an Arrhenius‐type temperature dependence with an activation energy of 41 kJ/mol. Successful extrusion of fully crosslinked PU foam at milder temperatures with DBTDL catalyst demonstrates potential for PU foam recycling using reactive extrusion, and generally highlights the feasibility of dynamic crosslink reconfiguration for waste reduction and improved sustainability.

Influence of Polyaniline on the Structural, Linear/Nonlinear Optical, and Dielectric Characteristics of Poly(Vinyl Chloride/Poly (Ethylene Oxide) Blends

Polyvinyl chloride (PVC)/polyethylene oxide (PEO)/x wt % polyaniline (PANI) polymer blends were produced using a solvent casting method. X-ray diffraction was used to investigate the formed blends. The effect of adding different amounts of PANI on the light absorbance, transmittance, reflectance, extinction coefficient, refractive index, optical dielectric properties, nonlinear optical parameters and relaxation time characteristics of the PVC/PEO blends were explored. The direct and indirect optical band gap energies of the undoped PVC/PEO were 4.25 and (4.06, 2.76) eV, respectively. The direct and indirect optical band gap energies were reduced to their lowest values (4.02, 3.1) and (3.68, 2.14, 1.12) eV as the amounts of PANI were increased to 0.2 wt %. The effect of alloying between PVC and PEO and adding PANI on the emitted fluorescence spectra under various excitation wavelengths were investigated. All samples displayed varying degrees of blue-violet, blue, or blue-green colors depending on the concentrations of PANI.

Preparation and Characterization of a Biobased Temporary Plugging Material: Self-Healing and Degradable.

Due to the low success rate in high-temperature gas well drilling or workover operations, a new type of biobased temporary sealing material was synthesized using vanillin, succinic anhydride (SA), cellulose, and acetylacetone zinc hydrate (AZH) as catalysts. Infrared analysis proved the synthesis of vanillin-derived epoxy and the formation of a vitrimer with hydroxyl ester. The results of thermodynamic properties tests show that it has excellent thermal stability with a high glass transition temperature (Tg). The dynamic mechanical test results show that the characteristic relaxation time of the material at 120 °C is 3500 s and the self-healing rate can reach 61% within 20 min. The test results of mechanical properties show that under 10 MPa pressure, it has good elastic deformation performance, the rebound rate is more than 36%, and the crushing rate is less than 17%. The degradation performance results show that the decomposition increases to 80% under 120 °C and 1.2% NaOH. The comprehensive performance evaluation results show that the new temporary plugging material has good compatibility, and its plugging performance is better than those of a gel, composite material, and shape memory polymer. The maximum fracture plugging capacity is 5 × 4 mm, with a pressure up to 10 MPa.

Structure and magnetic properties of hydrothermally synthesized CuFe2O4 and CuFe2O4/rGO composites

This paper presents a comparative analysis of the structural and magnetic modifications in ultrafine CuFe2O4 and CuFe2O4/rGO nanocomposites subjected to annealing. Samples were investigated using X-ray diffraction and Mössbauer spectroscopy. The study outlines the impacts of the annealing temperature on key characteristics, including the lattice parameter, inversion degree, and crystallite sizes. Copper ferrite nanoparticles demonstrate temperature stability as a composite element, displaying the simultaneous presence of both cubic and tetragonal ferrite phases at 500 and 600 °C. For heterogeneous copper ferrite, alterations in magnetocrystalline anisotropy values were calculated as a function of the inversion degree. The investigation also explores the influence of both crystallite size and inversion degree on the characteristic time of magnetic moment relaxation in a single domain CuFe2O4 nanoparticle, setting a precondition for the observation of superparamagnetic CuFe2O4 nanoparticles via the Mössbauer method. The study introduces a model for the inception of a copper ferrite phase predicated on the partial charge theory, offering an analysis of protolysis and polycondensation processes during the interaction of iron and copper hydroxocomplexes. The role of negatively charged graphene oxide nanosheets as ferrite phase nuclei localization centers is further substantiated within this context.

Heat transfer by transiently superheated fluids: Decay of unstable water-based mixtures

The present study sets out to elucidate the characteristic features of instantaneous heat transfer accompanying transient superheating of partially miscible mixtures having a lower critical solution temperature (LCST) with respect to the liquid–liquid binodal and the diffusion (liquid–liquid) spinodal. The research has employed the method of controlled pulsed heating of a wire probe, which can be used to directly measure heat flux into the medium at a strictly fixed heater temperature Tst. At a characteristic heating time of 100 ms, values of Tst were gradually increased from 100 to 500 °C, while the corresponding pressures varied from 0.1 to 35 MPa. The objects of study were aqueous solutions of PPG-425 and PPG-2000 polypropylene glycols, as well as 2,6-Lutidine, all having LCST in the range convenient for achieving large superheats. The values of the heat transfer coefficient (whose highest values reached 90 kW/m2 K for the solution with 5 wt% PPG-2000 content at the superheat degree of about 390 °C) and the energy absorbed by the solution were calculated on the basis of the primary data. The calculation results related the individual features of the studied systems to the phase diagram and characteristic system relaxation times depending on the concentration and superheat degree with respect to the spinodal. The applicability of such systems as coolants under conditions of high-power heat release for elements with a short response time is demonstrated. Based on the experimental results, we validated the selection of solutions as coolants in the concentration range from the left branch of the spinodal to the critical concentration of the solution.

The rheology of saltwater taffy

Saltwater taffy, an American confection consisting of the main ingredients sugar, corn syrup, water, and oil, is known for its chewy texture and diverse flavors. We use a small amplitude oscillatory shear test to probe the linear viscoelastic properties of commercial taffy. At low frequencies, self-similar relaxation behavior characteristic of a critical gel is observed. The storage and loss moduli are power-law functions, with the same exponent, of the frequency. Such self-similarity arises from the distribution of air bubbles and oil droplets in the taffy, where air is incorporated and oil is emulsified through an iterative folding process known as “taffy-pulling.” Taffy obeys the time–temperature superposition principle. Horizontally shifting the dynamic moduli obtained at different temperatures yields a master curve at a chosen reference temperature. As a sufficiently high frequency is exceeded, taffy transitions from a critical gel-like state to an elastic solid-like state. The master curve can be described by the fractional Maxwell gel (FMG) model with three parameters: a plateau modulus, a characteristic relaxation time, and a power-law exponent. The master curves for taffy of different flavors can all be described by the FMG model with the same exponent, indicating that minor ingredients like flavorings and colorings do not significantly affect the rheology of taffy. Scaling the master curves with the plateau modulus and relaxation time results in their collapse onto a supermaster curve, hinting at a more fundamental time–temperature–taffy superposition principle. Guided by this principle, we hand-pull lab-made model taffies successfully reproducing the rheology of commercial taffy.

Development of robust and rapid self-healing polymers for the repair of microcracks in energetic composite materials

Adhesives with exceptional toughness and self-healing properties are vital for the practical application of energetic composite materials (ECMs). However, the majority of self-healing polymers used in ECMs lack sufficient toughness. In this study, we address this challenge by incorporating asymmetric alicyclic and bent biphenyl ring structures into the hard domain units of polymers. The synthesized PUDS-3 films exhibit a remarkable toughness of up to 20.93 MJ·m−3, recovering over 90% of this toughness within just 20 min after a complete fracture. This behavior results from the synergistic interaction between dynamic disulfide and hydrogen-bonding mechanisms during the restructuring process. Furthermore, elevated temperatures led to a rapid decrease in the relaxation modulus and characteristic relaxation time of the PUDS samples, effectively accelerating crack healing. Both experimental measurements and molecular dynamics simulations demonstrated that a higher hard segment content in the adhesive corresponded to increased strength in the interfacial interaction with 1,3,5-trinitro-2,4,6-triaminobenzene (TATB). Upon accidental mechanical damage, the cracked ECMs displayed significant and efficient self-healing capabilities within 24 h at 60 °C. The utilization of these self-healing and highly toughness adhesives hold promising prospects for enhancing the safety and longevity of ECMs.

Time-resolved pump–probe spectroscopic ellipsometry of cubic GaN II: Absorption edge shift with gain and temperature effects

We recently published a study concerning femtosecond pump–probe absorption edge spectroscopy of cubic GaN (fundamental bandgap: 3.23 eV), resulting in the transient dielectric function. In the present study, we continue our investigations of those pump–probe measurements by determining the time-dependent transition energy at the Fermi-vector between the conduction and valence bands. The generation of electron–hole pairs by the 266 nm pump-beam (4.66 eV) shifts the absorption edge by ≈500 meV within 1 ps due to many-body effects like band-filling and bandgap renormalization. Modeling this ultra-fast change is achieved by converting the transition energies into free-carrier concentrations, assuming the electron contributions to be dominant. We consider the relaxation, recombination, and diffusion of those free-carriers as well as either an additional gain-recombination or temperature effects. This allows for describing the transition energies on short time scales. Both models yield similar values for the characteristic relaxation time (≈0.21 ps), recombination time (≈25 ps), and diffusion coefficient (≈1 cm2/s).

Heat flow influence on the Cauchy stress tensor in a thermal wave

Rational thermodynamics with relaxing heat fluxes is used in this work to obtain the constitutive equations of thermoelasticity. It is found that it is possible to formulate the governing equations of the medium where the density of internal energy per unit mass depends on the heat flux, while the density of entropy per unit mass may not depend on it. A situation where the density of the entropy per unit mass depends on the heat flux, while the density of internal energy per unit mass does not, is also possible. A model of a thermoelastic material with a relaxing heat flux, where the characteristic relaxation time of the heat flux is a constant of the material, is considered in detail in this paper. It is shown that the Cauchy stress tensor in this model is not only a function of the temperature and deformations of the medium, but also depends on the heat flux. This gives rise to new effects not present in the classical theory of thermoelasticity. The specific heat of the material will also depend on the magnitude of the heat flux.The paper considers the application of the theory to the modelling of thermal processes during ion plasma treatment of a polymeric material.

Impact of multipath interference in Si/SiO2 substrate on ultrafast relaxation processes in two-dimensional transition metal dichalcogenides

At present, two-dimensional transition metal dichalcogenides (2D TMDs) are of great importance due to their unique properties. These materials attracted considerable attention due to the ultrafast dynamics of excitation and relaxation processes in charge carriers. Recently, many researchers have estimated the characteristic relaxation times using standard optical pump-probe methods. The impact of multipath interference in underlying substrates on the determination of the characteristic relaxation times is generally ignored, although it can be significant due to the high transparency of 2D TMDs. In this work, we propose an approach to estimating the impact of a substrate on relaxation processes in 2D TMDs. We experimentally studied the dynamics of ultra-fast relaxation processes in monolayer, bilayer, and bulk MoS2 on Si/SiO2 substrate using time-resolved spectroscopy. It was shown that the multipath interference in substrate has a strong effect on the determination of the slow (several hundred ps) relaxation times which are associated with interband carrier recombination and can lead to the up to 20% distortion of obtained results.

ON THE EFFECT OF VISCOSITY RELAXATION ON ELECTROMAGNETIC RADIATION INTENSITY OF AN OSCILLATING CHARGED DROPLET

Theoretical asymptotic calculations linear with respect to small dimensionless amplitude of oscillations have been employed to study the influence of the viscoelastic properties of a charged droplet of an electrically conducting viscous liquid on the intensity of its electromagnetic radiation. It has been shown that allowance for the effect of viscosity relaxation leads to a decrease in the damping decrement value, which is determined by energy losses for electromagnetic wave emission, and the intensity of high-frequency electromagnetic radiation; a substantial reduction in the viscous damping decrement of small cloud droplets; and an essential dependence of the viscous damping decrement on the characteristic relaxation time. It has been found that the viscosity relaxation of a liquid has no significant effect on damped capillary oscillations and electromagnetic radiation of rain droplets. Viscoelasticity and viscosity of the liquid do not influence the conditions critical for the realization of the electrostatic instability of a droplet with respect to its own charge.

Prediction of Recurrent Cervical Cancer in 2-Year Follow-Up After Treatment Based on Quantitative and Qualitative Magnetic Resonance Imaging Parameters: A Preliminary Study.

This study investigated predictors of cervical cancer (CC) recurrence from native T1 mapping, conventional imaging, and clinicopathologic metrics. In total, 144 patients with histopathologically confirmed CC (90 with and 54 without surgical treatment) were enrolled in this prospective study. Native T1 relaxation time, conventional imaging, and clinicopathologic characteristics were acquired. The association of quantitative and qualitative parameters with post-treatment tumor recurrence was assessed using univariate and multivariate Cox proportional hazard regression analyses. Independent risk factors were combined into a model and individual prognostic index equation for predicting recurrence risk. The receiver operating characteristic (ROC) curve determined the optimal cutoff point. In total, 12 of 90 (13.3%) surgically treated patients experienced tumor recurrence. Native T1 values (X1) [hazard ratio (HR) 1.008; 95% confidence interval (CI) 1.001-1.016], maximum tumor diameter (X2) (HR 1.065; 95% CI 1.020-1.113), and parametrial invasion (X3) (HR 3.930; 95% CI 1.013-15.251) were independent tumor recurrence risk factors. The individual prognostic index (PI) of the established recurrence risk model was PI = 0.008X1 + 0.063X2 + 1.369X3. The area under the ROC curve (AUC) of the Cox regression model was 0.923. A total of 20 of 54 (37.0%) non-surgical patients experienced tumor recurrence. Native T1 values (X1) (HR 1.012; 95% CI 1.007-1.016) and lymph node metastasis (X2) (HR 4.064; 95% CI 1.378-11.990) were independent tumor recurrence risk factors. The corresponding PI was calculated as follows: PI = 0.011X1 + 1.402X2; the Cox regression model AUC was 0.921. Native T1 values combined with conventional imaging and clinicopathologic variables could facilitate the pretreatment prediction of CC recurrence.

On the probable interpretation of anticorrelation between the proton temperature and density in the solar wind

The anticorrelated distributions of temperature and density of protons are a well-known property of the solar wind. Nevertheless, it is unclear till now if they are formed by some kind of the universal physical mechanism? Unfortunately, a straightforward comparison of the characteristic relaxation times for the temperature and density, on the one hand, and pressure, on the other hand, encounters the problem of inapplicability of the hydrodynamical approach in the situation when the free-path length of the protons is considerably greater than the spatial scale of the structures under consideration. To resolve this problem, some kinds of the MHD turbulence—reducing the effective free paths—are usually assumed. In the present paper, we use an alternative approach based on the electrostatic (Langmuir) turbulence, described by the mathematical formalism of the spin-type Hamiltonians, which was actively discussed in the recent time in the literature on statistical physics. As follows from the corresponding calculations, formation of the anticorrelated distributions of temperature and density is a universal property of the strongly nonequilibrium plasmas governed by the spintype Hamiltonians when they gradually approach the thermodynamic equilibrium. So, just this phenomenon could be responsible for the anticorrelations observed in the solar wind.

Inverse Faraday Effect in Superconductors with a Finite Gap in the Excitation Spectrum

The inverse Faraday effect (generation of a time-independent magnetic moment under the action of a circularly polarized electromagnetic wave) in mesoscopic superconducting samples with a finite gap in the excitation spectrum is analytically described. Within the modified time-dependent Ginzburg–Landau theory (Kramer–Watts-Tobin equations) for thin superconducting disks, it is shown that the temperature dependence of the optically induced magnetic moment is nonmonotonic in a wide range of parameters and contains a maximum. This maximum is due to the dephasing between the spatial oscillations of the magnitude and the phase of the order parameter, which arises with a decrease in the temperature and, correspondingly, in the characteristic relaxation time of perturbations in the superconducting condensate.

Construction of novel Cu-α-diimide interactions for enhancing thermal resistance and dimensional stability of polyimide films

The new-generation polyimide films with simultaneously outstanding thermal stability and low coefficient of linear thermal expansion (CTE) are urgently needed. Herein, coordination bonds were employed to improve polyimide films' heat resistance and dimensional stability. Polyimide-Cu complex films were fabricated by incorporating α-diimide units (bidentate ligands) into the BPDA-ODA (the structure of commercially available Upilex-R® polyimide films) backbones. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Energy dispersive X-ray mapping results showed that the coordination interactions occur between α-diimide and copper ions. Wide-angle X-ray diffraction (WAXD) and two-dimensional small-angle X-ray scattering (SAXS) spectra demonstrated that Cu-α-diimide coordination structures introduced in intermolecular chains leading polyimide films transformed from a semi-crystalline state to amorphous state, and inhibited the long-period formation structures. The plateau modulus and longer characteristic relaxation time (τ∗ = 1996.5 s) revealed by dynamic thermomechanical analysis indicated that crosslink networks were successfully formed by constructing coordination bonds. As a result, polyimide films' thermal resistance and dimensional stability were simultaneously enhanced. The typical PI-Cu50 films exhibited elevated glass transition temperatures of 353 °C (pristine PI-Cu0 ∼ 302 °C) and a lower CTE of 47.4 ppm/K (pristine PI-Cu0 films ∼ 64.4 ppm/K) in ranges of 300–400 °C, showing a promising application in the field of high-temperature environments.