Isothermal Relaxation Research Articles

Three-dimensional electroelastic modeling of the nucleation and propagation of the spin domains in spin-crossover materials

A three-dimensional version of the electroelastic model allowing the description of spin-crossover (SCO) materials taking into account for the volume change at the transition between the LS and the HS spin states is developed. The investigations are realized on a rectangular parallelepiped lattice with cubic symmetry. The SCO units are modeled by two-states fictitious spins coupled by springs whose equilibrium distances depend on the spin states. We implemented massive parallel simulations using CUDA (Compute Unified Device Architecture) programming where the spin states are updated using the Monte Carlo Metropolis algorithm, while the mechanical relaxation (lattice position) is performed by molecular dynamics. In this work, we investigated (i) the case of the thermal spin transition showing the macroscopic deformation of the parallelepiped accompanying the propagation of single domains and (ii) the isothermal relaxation of the photoinduced metastable HS fraction at low temperature. In both cases, the interplay between the electronic and the structural aspects of these transformations is analyzed and discussed in relation with the model parameters.

Microbuckling behavior of unidirectional fiber-reinforced shape memory polymer composite undergoing compressive deformation

The microbuckling mechanics of unidirectional fiber-reinforced shape memory polymer composite (SMPC) with low fiber volume fraction were investigated, and the mechanical models were formulated considering the attenuation of shear strain in the resin matrix near the buckled fibers. We deduced the analytical expression of the attenuation function and the key parameters during the microbuckling process of SMPC, including the critical buckling wavelength and strain. The values determined by finite element analysis verified the accuracy of the above theoretical predictions. The nonlinear stress–strain relationship during the post-buckling process was also investigated. Additionally, the classical elastic–viscoelastic correspondence principle was established to determine the dynamic buckling behaviors of SMPC at different temperatures. The viscoelastic parameters of shape memory polymer (SMP) were obtained from the isothermal stress relaxation experiments, and then the variation of buckling wavelength with time at different temperatures was evaluated.

Investigation on Post-Deformation Recrystallization Behavior of 22MnB5 Steel based on Stress Relaxation

With the aim of analyzing the post-deformation Recrystallization (PDRX) of 22MnB5 steel and the grain size evolution after hot forming of the sheet and using the BAEHR 805 A/D thermo-mechanical simulator, several isothermal stress relaxation tests were performed in a temperature range of 800 to 950 °C, a predeformation range of 0.4 to 0.8 and a strain rate range of 0.01 to 0.8 s−1. As a result of isothermal stress relaxation after deformation, PDRX kinetics can be described based on an Avrami equation as a function of temperature, degree of predeformation and strain rate. As the temperature increases, the recrystallization kinetics accelerates. The strain rate also has a positive influence on the fast PDRX. Predeformation has the opposite effect, as the consumption of stored energy occurred in predeformation process of relaxation tests. Furthermore, the microstructure evolution is described as a function of the predeformation parameters. The investigation of the microstructure after isothermal stress relaxation tests showed that the PDRX kinetics corresponds to the calculations. Furthermore, EBSD analysis helped to analyze the effects of certain stress relaxation conditions on the martensite morphology. The results indicate that a lower temperature and higher strain rate lead to fine martensite packages. In contrast, the martensite block width increases when applying higher strain rates.

Reentrant double glassy states and simultaneous presence of short- and long-range ordering in metamagnetic Co-doped Bi0·5La0·5Fe0·5Mn0·5O3 multiferroic

A study of the structural and magnetic properties of Bi0·5La0·5Fe0·45Co0·05Mn0·5O3 is presented in the article. We report the rare observations of two glassy transitions below the long-range ordering temperature along with the metamagnetic transitions at low temperatures. Frequency and DC field dependence of AC susceptibility and isothermal relaxation of remnant magnetization studies have all shown the slow spin dynamics for the compound. Interestingly, in the glass phase of the materials, metamagnetic transitions of the antiferromagnetic spins were found which indicate the coexistence of the long-range ordered phase along with the short-range ordered glassy phase. The double cluster glass phases can be comprehended, from the relaxations of the spin clusters arising from the competing magnetic interactions present in the system. Moreover, the roles of additional exchange interactions which are brought to the system by Co doping are also considered in explaining the observed double cluster glass transitions.

Slow spin dynamics in a CoM2O4 A-site spinel (M=Al, Ga, and Rh)

We report thermomagnetic properties in magnetically frustrated A-site spinel magnets of CoAl2O4, CoRh2O4, and cluster glass CoGa2O4 to comprehensively show the proximity effects of the Néel-to-spin-spiral (NSS) transition, which is predicted theoretically to occur in a magnetically frustrated A-site spinel antiferromagnet. The low-temperature magnetic phase remains controversial since the magnetic state of CoAl2O4 is in the vicinity of the NSS transition derived by the inherent magnetic frustrated interaction for an A-site spinel magnet and is also considerably sensitive to crystallographic disorder. The antiferromagnetic and spin-glass transitions were detected by measuring the direct-current and alternating-current susceptibilities and also thermoremanent magnetization (TRM) developing below the magnetic transitions at the Néel point (T N) and spin-glass transition temperature (T SG). The relaxation rate and the temperature derivative of TRM were both enhanced at T N and T SG, and they decayed rapidly above and below the transitions. We succeeded in extracting the relaxation time τ and other characteristic parameters from the isothermal relaxation of TRM, which was well fitted with a non-exponential relaxation form formulated by Weron based on purely stochastic theory in order to describe the originally dielectric relaxation. For the typical CoRh2O4 normal spinel (inversion-free A site) antiferromagnet and the CoGa2O4 random spinel spin-glass, the temperature variations of these parameters can distinguish the magnetic states. In contrast to the cases for CoRh2O4 and CoGa2O4, an enhancement of the relaxation rate of TRM for CoAl2O4 is indicated at low temperatures, which is probably related to the suppression of long-range antiferromagnetic order revealed by neutron diffraction studies.

Enhanced mechanical properties of a metastable β titanium alloy via optimized thermomechanical processing

An efficient and simple thermomechanical processing is particularly important to obtain homogenous microstructures. In this work, an optimized thermomechanical processing consisting of multiple isothermal compression processing and stress relaxation processing was conducted on a metastable β titanium alloy (Ti–5Al–5Mo–5V–3Cr-0.5Fe). The sample geometry after designed thermomechanical processing maintain the same with the geometry after an unidirectional compression, but with totally different microstructures. The results show that a fully fine equiaxed microstructure with the mean size of 160 nm is obtained after the designed processing. Such fine microstructure has not been reported previously in titanium alloys processed by conventional isothermal compression and annealing. In addition, the yield strength of the sample after designed processing was significantly enhanced. The mobile dislocation slipping and stacking faults induced shear are the main mechanisms behind stress relaxation stage in thermomechanical processing. This work can provide an effectively novel thermomechanical processing to optimize the mechanical properties.

Aging-Induced Structural Evolution of a GeSe2 Glass Network: The Role of Homopolar Bonds

The structural evolution of GeSe2 glass during aging is studied using Raman spectroscopy and density relaxation measurements. The Raman spectra indicate volume- and entropy-driven changes in the relative concentrations of the corner-sharing (CS) and edge-sharing (ES) GeSe4 tetrahedra and in the degree of chemical order of the tetrahedral network during aging at 65 °C below the nominal glass transition temperature. The attendant structural changes involve a progressive increase in the CS:ES ratio and in the chemical order that can be expressed in the form of a reaction Ge-Ge + Se-Se → 2 Ge-Se, which shifts to the right, with lowering of fictive temperature. The isothermal relaxation of both the structure and density during aging displays rather similar stretched exponential kinetics with a stretching exponent β ∼0.54 and an average relaxation time of ∼13.5 h. In situ high-temperature Raman spectroscopic measurements indicate that structural relaxation does not affect the anharmonicity of the vibrational potential wells in the energy landscape of GeSe2 glass.

Optical microscopy imaging of the thermally-induced spin transition and isothermal multi-stepped relaxation in a low-spin stabilized spin-crossover material.

The thermal spin transition and the photo-induced high-spin → low-spin relaxation of the prototypical [Fe(ptz)6](BF4)2 spin-crossover compound (ptz = 1-propyltetrazole) diluted in the isostructural ruthenium host lattice [Ru(ptz)6](BF4)2, which stabilizes the Fe(II) low-spin state, have been investigated. We demonstrate the presence of a crystallographic phase transition around 145 K (i.e. from the high-temperature ordered high-spin phase to a low-temperature disordered low-spin phase) upon slow cooling from room temperature. This crystallographic phase transition is decoupled from the thermal spin transition. A supercooled ordered low-spin phase is observed as in the pure Fe(II) analogue upon fast cooling. A similar order-disorder phase transition is also observed for pure [Ru(ptz)6](BF4)2 but at relatively higher temperature (i.e. at around 150 K) without involving any spin transition. For Ru-diluted [Fe(ptz)6]2+, the crystallographic phase transition as well as strong cooperative effects involving various degrees of elastic frustration are at the origin of stepped sigmoidal high-spin → low-spin relaxation curves, which are modelled in the framework of a classical mean field model, considering both the tunnelling and thermally activated regimes. Optical microscopy studies performed on two different single crystals showed the existence of hysteretic thermal transitions with slight domain formation, hardly visible in the static crystal images. This behavior is attributed to the double effect upon Ru dilution, which decreases the cooperative character of the transition and simultaneously reduces the optical contrast between the LS and HS states. Moreover, the transition temperature revealed to be slightly crystal dependent, highlighting the crucial role of the spatial distribution of Ru from one crystal to another, in addition to the well-known effects of crystal shape and size.

Observation of giant exchange bias effect in Ni–Mn–Ti all-d-metal Heusler alloy

We report a giant exchange bias (EB) field of about 3.68 kOe during field cooled process in all-d-metal Ni40(FeCo)4Mn36Ti20 Heusler alloy. The study of magnetic memory effect and isothermal magnetic relaxation processes suggest that the giant EB field arises due to the possible coexistence of antiferromagnetic (AFM) and ferromagnetic (FM) phase exchange interaction in the studied system at temperatures below 35 K. Furthermore, the temperature and cooling field dependence of EB effect are analyzed which are related to the change in unidirectional anisotropy at FM/AFM interface. The study of a well-established training effect confirms the intrinsic nature of the observed EB behavior. This result will open up a new way toward the development of EB materials considering all-d-metal Heusler alloy systems.

An Electron and Hole Trap Energy Distribution Estimation Model and Its Application in Polypropylene Nanocomposites

To differentiate the electron and hole traps inside polymers, a model based on the pulsed electro-acoustic (PEA) method is proposed. It is premised that under short-term DC electric field polarization at relatively low temperature, the homocharge is injected into the sample to a shallow depth and dominates the space charge distribution. Thus, the homocharge near the ground electrode and the high voltage electrode can be separated. During depolarization, only the homocharge near the ground electrode is analyzed to reduce the influence of ultrasonic wave attenuation, which also results in unipolar space charge decay behavior being obtained. The isothermal relaxation current (IRC) theory is then employed to obtain the unipolar quasi-continuous trap energy distribution based on the space charge decay. Thereby, the electron and hole traps can be differentiated by applying positive and negative DC electric field polarization, respectively. The experimental results of low density polyethylene (LDPE) verify the validity of the proposed model. Based on the model, polypropylene (PP) and PP/MgO nanocomposites are studied to explore the effect of MgO doping on the trap characteristics. The results show that numerous electron deep traps are introduced into the nanocomposites due to the intrinsic properties of nanoparticles. Besides, the heterogeneous nucleation of the nanoparticles increases the crystal/amorphous and crystal/vacuum interfaces, which may be the main reason for the increase of hole trap density of nanocomposites.

Effects of Trehalose Content and Water Activity on the Fracture Properties of Deep-fried Wheat Flour Particles and Freeze-dried Porous Waxy Corn Starch Solids

Wheat flour-based batter containing 0 to 20 % trehalose was deep-fried, dried and held in various water activity (aw) conditions. The effects of trehalose content and aw on oil content, water sorption, isothermal mechanical relaxation, and fracture properties were investigated. For comparison, the fracture properties of freeze-dried porous waxy corn starch solids were also investigated. The 10 % trehalose sample had the lowest oil content, water content, and aw. A force-reduction value (∆F) of the samples was evaluated as a typical mechanical relaxation parameter. ∆F gradually increased with increasing aw and sharply increased above a specific aw presumed to be associated with the glass to rubber transition. Compared to ∆F values among the glassy samples, 10 and 20 % trehalose samples had higher ∆F values (were more rigid) than 0 and 5 % trehalose samples. From the fracture measurements of the glassy samples, the first fracture force increased linearly and the number of fracture peaks decreased linearly with increasing aw. At each aw, 10 % trehalose had the lowest first fracture force and the highest the number of fracture peaks. Freeze-dried porous waxy corn starch solids showed similar fracture properties to deep-fried samples. These findings suggest that around 10 % trehalose content is optimal for producing deep-fried foods with a brittle texture.

Viscoelastic Behavior of an Epoxy Resin Modified with Recycled Waste Particles Analyzed through a Fractional Model

It is well-known that the addition of randomly dispersed particles in polymers influences their linear viscoelastic behavior and dynamic mechanical properties. The aim of this study was to describe the viscoelastic behavior of an epoxy resin modified by waste glass and rubber particles using the linear fractional spring-pot model. Unlike complex classical exponential models, fractional models, being only two-parameter dependent, make it easier to characterize the viscoelastic behavior of materials. Isothermal relaxation and single frequency sweep temperature dynamic tests were carried out in a dynamic mechanical analyzer DMA150 by varying the content of the particles from 0 to 20% by weight. Overall, the results of this study evidence that using waste materials as additives for polymer compounds is a practical and sustainable possibility when it comes to modifying their viscoelastic properties.

Synthesis and molecular interaction study of a diphenolic hidrazinyl-thiazole compound with strong antioxidant and antiradical activity with HSA

In this study we designed, synthesized and analyzed a water-soluble molecule presenting a good antioxidant and antiradical activity, namely Dihydroxy-Phenyl-Thiazol-Hydrazinium chloride (DPTH). This molecule contains 2’,4’-dihydroxyphenyl and the 2-hydrazinyl-4-methyl-thiazole fragments linked through a hydrazone and having very good antiradical scavenging, antioxidant activity and a low chelation activity in the in vitro evaluations. Knowing that, in the organism, the drugs are transported to the tissues generally bound to a plasma protein we, additionally, investigated its interaction with human serum albumin (HSA)-the main soluble protein in plasma capable of making complexes with various molecules, which deliver then, to the tissues. This binding can influence the pharmacokinetic and pharmacodynamic profile of drugs. Consequently, the interaction between DPTH and human serum albumin (HSA) was carefully examined using isothermal titration calorimetry (ITC) and T1 NMR selective relaxation time spectroscopy. According to ITC, DPTH: HSA interaction process was spontaneous and endothermic with an affinity constant Ka = 4.31 × 102 M−1 and the stoichiometry coefficient (n) was equal 1, which was subsequently confirmed by 1H NMR. Relaxation experiments provide quantitative information about the relationship between the binding affinity and structure of DPTH. Thus, association constant was determined as Ka = 9.65 × 102M −1. The results obtained by ITC and NMR were complemented with a molecular docking study.

The Relaxation Effects in the Preisach - Néel Model of Patterned Media

Abstract In this paper, we present the calculation of thermal magnetic viscosity based on numerical simulations of relaxation isotherms curves for a Stoner–Wohlfarth ensemble. The Stoner–Wohlfarth particles is described by two equilibrium states and thermal activation is done by overcoming the energy barrier. For simulations we have used a new Preisach type model for Patterned Media, which takes into account the effects of time and temperature. The magnetization of the system is treated using the Preisach Model for Patterned Media with bimodal interactions field distributions. We have simulated relaxation curves starting from different points on the negative region of the descending branch of the major hysteresis loop and from the obtained data we evaluated the thermal fluctuation field. The results obtained for the linear energy barrier were compared with the non-linear energy barrier.

Entropy-Entropy Compensation between the Protein, Ligand, and Solvent Degrees of Freedom Fine-Tunes Affinity in Ligand Binding to Galectin-3C.

Molecular recognition is fundamental to biological signaling. A central question is how individual interactions between molecular moieties affect the thermodynamics of ligand binding to proteins and how these effects might propagate beyond the immediate neighborhood of the binding site. Here, we investigate this question by introducing minor changes in ligand structure and characterizing the effects of these on ligand affinity to the carbohydrate recognition domain of galectin-3, using a combination of isothermal titration calorimetry, X-ray crystallography, NMR relaxation, and computational approaches including molecular dynamics (MD) simulations and grid inhomogeneous solvation theory (GIST). We studied a congeneric series of ligands with a fluorophenyl-triazole moiety, where the fluorine substituent varies between the ortho, meta, and para positions (denoted O, M, and P). The M and P ligands have similar affinities, whereas the O ligand has 3-fold lower affinity, reflecting differences in binding enthalpy and entropy. The results reveal surprising differences in conformational and solvation entropy among the three complexes. NMR backbone order parameters show that the O-bound protein has reduced conformational entropy compared to the M and P complexes. By contrast, the bound ligand is more flexible in the O complex, as determined by 19F NMR relaxation, ensemble-refined X-ray diffraction data, and MD simulations. Furthermore, GIST calculations indicate that the O-bound complex has less unfavorable solvation entropy compared to the other two complexes. Thus, the results indicate compensatory effects from ligand conformational entropy and water entropy, on the one hand, and protein conformational entropy, on the other hand. Taken together, these different contributions amount to entropy–entropy compensation among the system components involved in ligand binding to a target protein.

Realizing Quantitative Evaluation of Trap Energy Distribution in Polymers with an Electric Field Stimulated Depolarization Current Method

In this paper, we propose a method for characterizing the trap energy distribution of polymers. The method can accurately extract the de-trapping current from the polarization current of the sample, thus overcoming the accuracy problem of the traditional isothermal relaxation current (IRC) method. Further application of this method on crosslinked polyethylene (XLPE) sheet samples revealed the effect of thermal degradation degree on the trap distribution of XLPE. At the recrystallization stage as polyethylene becomes re-crosslinked, the number of traps decreases. At the thermal-oxidative aging stage as the sample is seriously damaged, the number of traps increases. The de-trapping current of the sample thus exhibits a first decreasing and then increasing trend.

Effect of Surface State on DC Breakdown Characteristics of Thermally Aged Double-Layered Polyester Films

The surface state of polymer films has a significant influence on direct current (DC) breakdown characteristics, but its mechanism needs further investigation. In this paper, DC breakdown strength of thermally aged two-layer polyester films is studied. The charge trap and their space distribution characteristics are analyzed by an isothermal relaxation current and the pulsed electro-acoustic method. Furthermore, a bipolar charge transport model suitable for two-layer dielectrics is established, and the correlation between surface state and DC breakdown performance is dynamically simulated. The experimental results show that as the aging time increases, DC breakdown strength first increases and then decreases. The trap depth and its density first increase and then decrease, and the change mainly comes from dielectric surface area. The simulation results show that through modulating the injection and accumulation behaviors of space charges, the change of trap depth, density and extended depth of surface state have an important effect on the space charge and electric field distribution at the pre-breakdown time.

Insulation Performance Analysis of Field Aged and New MV XLPE Cables Using Various Diagnostic Techniques

Cross-linked polyethylene (XLPE) cables are widely used in the distribution and transmission networks of power systems. The insulation materials of these cables are stressed by the over voltages and are also exposed to various environmental conditions. This leads the power cables to degrade during their normal life span. Therefore, it would be advantageous for the power utilities to acquire the cable’s insulation condition frequently during their operation. In this paper, experimental studies were carried out on short sections of field aged as well as un-aged medium voltage (MV) XLPE cables to investigate the insulation condition using non-destructive diagnostic techniques such as Dissipation Factor (DF), Isothermal Relaxation Current (IRC), and Partial Discharge (PD) characteristics at 60Hz. From this set of experiments, different parameters will be used to analyze the dielectric response and the insulation condition. Finally, the results show how the VLF dissipation factor works efficiently to assess cable aging.

Manipulation of Molecular Qubits by Isotope Effect on Spin Dynamics

Controlling spin behavior via external stimuli is a key route to develop molecular spintronics devices. Photons, temperature, pressure, chemicals, and electric field are the possible stimuli. Herei...

Study on Aging Characteristics of XLPE Cable Insulation Based on Quantum Chemical Calculation

In this paper, quantum chemical calculation and isothermal relaxation current (IRC) measurement are performed on thermally aged XLPE cables to investigate the effect of defects on dielectric properties and trap energy distribution of XLPE insulation. Samples of XLPE cables are thermally aged at 135 °C up to 45 days. Microscopic experiments including gel permeation chromatography, differential scanning calorimetry, and Fourier transform infrared spectrometer show that the molecular weight and crystallinity of XLPE decreases with aging time. The IRC results show that both the trap density and the trap depth increase with aging. To quantitatively calculate the trap depth variation due to physical and chemical defects by thermal aging, a quantum chemical calculation is carried out based on Gaussian 09. It is found that the broken cross-linking bonds between molecular chains weaken the stability of the molecular network structure, introducing the shallow physical trap (0.3 eV) in the aged XLPE. The molecular chain is thermally oxidized, the carbonyl group is introduced as a deep chemical trap (1.44 eV) in the XLPE. The chemical defect not only changes the localised energy state, but also significantly decreases the intrinsic breakdown strength of XLPE molecule from 0.62 to 0.51 V/Angstrom in quantum chemistry calculation.