Structural Relaxation Behavior Research Articles

Structural relaxation behavior of nano silicon-graphite composite anode after lithium-ion insertion

Structural relaxation behavior of nano silicon-graphite composite anode after lithium-ion insertion

Long-term relaxation analysis of steel cables based on viscoelastic model

Steel cables, renowned for their lightweight and high-strength properties, stand as ubiquitous building materials in large-span structures. Serving as crucial load-bearing elements, cables endure sustained tension throughout the structure's lifecycle. To further investigate the long-term relaxation characteristics of steel cables and provide more theoretical basis for structural relaxation calculations, this paper introduces a long-term relaxation constitutive model for steel cables, formulated through theoretical derivation, experimental validation, and data fitting techniques to establish time-dependent constitutive relationships. By employing the Bingham and Kelvin viscoelastic models, a relaxation model for steel cables is presented and optimized accordingly. Experimental investigations involve long-term tests on steel cables with three different sizes and detailed fitting analysis on collected internal force relaxation data. The findings emphasize the indispensability of structural relaxation behavior and the significance of altering internal forces, particularly in large-span cable-strut structures. The developed numerical model enables accurate simulation and prediction of long-term time-dependent performance in such structures, serving as a springboard for future studies in this domain.

Structural investigation and thermal properties of Al2O3-PbO-B2O3 glasses

Two series of aluminum lead borate glasses, (Al2O3)y(PbO)35-y(B2O3)65 and (Al2O3)y(PbO)70-y(B2O3)30, were prepared and effect of PbO → Al2O3 substitution on their structure and thermal properties was studied. The semi-quantitative analysis of the Raman spectroscopy data was used to structural interpretation of glass transition behavior in the Al2O3-doped lead borate glasses. The mechanisms of structural reorganization of glassy network under PbO → Al2O3 substitution were found to differ for the studied series although Al3+ ions are incorporated in the glass network, mainly, in form of AlO4 units in the both cases. The formation of AlO4 tetrahedra is accompanied by transformation of various isolated borate anions into finite-sized metaborate chains as well as the conversion of four-fold coordinated [BØ3O−]− species with one non-bridging oxygen atom into fully polymerized [BØ4]− tetrahedra in the (Al2O3)y(PbO)70-y(B2O3)30 glasses. In the series of glasses with high concentration of B2O3, B[4] → B[3] transformation including the shift of [BØ4]− ↔ BØ2O− equilibrium toward the right side is the main process of structural reorganization caused by PbO → Al2O3 substitution. In this case, the change in the network connectivity is determined by competition between increasing the amount of Al-O-B bonds against the background of decreasing the amount of B-O-B bonds between borate structural species. In the B-poor series, the glass transition temperature increased with Al2O3 from 280 to 360 °C, and the crystallization peak onset occurs at 340 and 380 °C for the glasses with the 0 and 3 mol.% Al2O3 contents, respectively (higher aluminum oxide contents already prevent crystallization). In the B-rich series, Tg increases with the Al2O3 content from 430 to 490 °C, and the crystallization occurs at 560 °C only for the Al2O3-free glass. The structural relaxation behavior in the glass transition rage was described in terms of the Tool-Narayanaswamy-Moynihan model. The activation energy for the relaxation movements was found to be non-monotonous in case of both compositional series.

How temperature-induced depolymerization and plasticization affect the process of structural relaxation

The self-plasticization, i.e., the increase in the polymer segmental mobility by the inclusion of its own monomer, has a major impact on the structural, thermal, and mechanical properties of the polymer. Differential scanning calorimetry (DSC) was used to investigate the influence of thermally induced self-plasticization on the structural relaxation of polydioxanone (PDX). Depolymerization (based dominantly on the end-chain scission mechanism) was found to be controlled by the depolymerization temperature Td as well as the actual number of re-melting cycles (while keeping the time spent at Td constant). PDX samples with the glass transition temperature (Tg) ranging from −52 (highly plasticized) to −13 °C (virgin) were prepared. The DSC data were described in terms of the Tool-Narayanaswamy model; a consistent structural relaxation behavior associating the degree of plasticization with Tg was identified. The activation energy first decreased with plasticization from 430 kJ mol−1 to 210 kJ mol−1 in the Tg range of −40 to −13C, which is consistent with the plasticization-caused spacing-apart of the polymer chains resulting in larger free volume and increased freedom for the relaxation movements. For the highly plasticized PDX samples, the activation energy increased from 210 kJ mol−1 to 310 kJ mol−1, which appears to be associated with the possible segregation of the portion of the plasticizer into a discrete phase. The width of the relaxation times distribution increased with plasticization as a consequence of the plasticizer loosening the polymeric chains and enabling a wider variety of the segmental movement. The plasticization also leads to a higher dependence of the segmental relaxation movements on their current physico-chemical and steric surrounding.

How the Presence of Crystalline Phase Affects Structural Relaxation in Molecular Liquids: The Case of Amorphous Indomethacin.

The influence of partial crystallinity on the structural relaxation behavior of low-molecular organic glasses is, contrary to, e.g., polymeric materials, a largely unexplored territory. In the present study, differential scanning calorimetry was used to prepare a series of amorphous indomethacin powders crystallized to various extents. The preparations stemmed from the two distinct particle size fractions: 50-125 µm and 300-500 µm. The structural relaxation data from the cyclic calorimetric measurements were described in terms of the phenomenological Tool-Narayanaswamy-Moynihan model. For the 300-500 µm powder, the crystalline phase forming dominantly on the surface led to a monotonous decrease in the glass transition by ~6 °C in the 0-70% crystallinity range. The activation energy of the relaxation motions and the degree of heterogeneity within the relaxing matrix were not influenced by the increasing crystallinity, while the interconnectivity slightly increased. This behavior was attributed to the release of the quenched-in stresses and to the consequent slight increase in the structural interconnectivity. For the 50-125 µm powder, distinctly different relaxation dynamics were observed. This leads to a conclusion that the crystalline phase grows throughout the bulk glassy matrix along the internal micro-cracks. At higher crystallinity, a sharp increase in Tg, an increase in interconnectivity, and an increase in the variability of structural units engaged in the relaxation motions were observed.

Site-occupation and structure relaxation behavior of C14-type stoichiometric and iron-rich Fe2Nb Laves phases with addition of aluminum or chromium

Powder X-ray diffraction Rietveld refinement for Al- or Cr-added stoichiometric and Fe-rich off-stoichiometric C14-type Fe2Nb Laves phases were examined to elucidate the site-occupation behavior of Al and Cr, and the resulting changes in the lattice parameters and the c/a axial ratio in the Fe2Nb Laves phases. In the case of the stoichiometric composition of the Fe2Nb Laves phase, no anti-site atoms were presented at the Fe and Nb both sites Al or Cr addition to the Fe-33.3at%Nb Laves phase by replacing Fe with Al or Cr did not generate anti-site Fe atoms, and Al and Cr preferentially occupied the Fe sites. Al and Cr atoms can replace Fe atoms at both the 6 h and 2a Fe sites in the case of Fe-rich alloy, and they also can occupy the Nb site (4f) by replacing anti-site Fe atoms. However, Al and Cr had no preferential occupation of any of these sites. The accuracy of Rietveld refinement was confirmed by observation through high-angle annular dark-field scanning transmission electron microscopy. The lattice parameters of the a and c axes increased almost linearly with increasing the added Al or Cr concentrations, but the c/a ratio showed different behavior. Therefore, the change in c/a cannot be explained solely in terms of the concentration of the added element; however, it is proposed that the change in c/a is interpreted by using parameters consistent with site-occupancy values.

Physical aging of lithium disilicate glass

Refractive index measurements were used to probe the structural relaxation behavior during the physical aging of a lithium disilicate glass at temperatures below the glass transition. The structural relaxation process was recorded using a single sample, starting from 5 K below the initial fictive temperature (Tf) of 720 K and gradually lowering it by 5 K increments until reaching an impractical experimental time to obtain complete relaxation at Tf − 35 K. The aging curves exhibited a stretched exponential relaxation behavior characteristic of glassy systems and showed good reproducibility in relaxation kinetics at three temperatures. A long continuous aging experiment conducted at 705 K demonstrated that the interruptions between isothermal treatments and property determinations did not affect the results. Finally, it was impossible to complete all experiments with the same sample, as after several cycles of cumulative relaxation experiments, the glass began to crystallize with the appearance of a small fraction of lithium disilicate crystals. This result indicates a relationship between structural relaxation and crystallization, corroborating recent studies in the literature.

The peak‐shift method for the description of structural relaxation in glassy materials: a critical review

AbstractNowadays, the glass transition kinetics is most commonly described in terms of the Tool–Narayanaswamy–Moynihan (TNM) model. Evaluation of one of the most prominent features of the structural relaxation motions—the relaxation nonlinearity—was traditionally done using the peak‐shift (PS) method. This paper introduces, for the first time, extensive testing of the PS method by means of theoretical simulations for all practically observable types of structural relaxation behavior and all types of glassy/amorphous materials (tested range of the glass transition temperatures: −55 to 1000°C; tested range of the relaxation activation energies: 300–1300 kJ⋅mol−1). For the majority of types of structural relaxation behavior, the PS method tends to slightly overestimate the value of the TNM nonlinearity parameter x (by ∼ 0.05–0.10). In the specific cases of the highly linear behavior (↑ x) combined with a broad distribution of relaxation times, the PS method systematically vastly underestimates the value of x. A new, improved temperature program was proposed for the PS method, eliminating the major intrinsic drawback of the originally proposed version of the PS methodology. In addition, based on the comparison between the theoretically simulated and real‐life experimental data, a new approach (based on the characteristic PS dependence shape) was introduced to estimate the width of the relaxation times distribution.

Experimental and constitutive analyses of the stress relaxation behavior of glassy polymers

AbstractThe physical mechanism underlying the mechanical behavior of glassy polymers has been studied over decades but remains a long‐standing issue. A consensus view achieved is that the yield, flow, and stress relaxation behaviors are due to structural relaxation in the polymer mainly caused by chain conformation transitions. This is the key physical idea behind the many existing elastic–plastic constitutive models for glassy polymers. In this paper, such a constitutive model was employed for predicting and analyzing the stress relaxation of a glassy polymer. It is found that the model works well in predicting the pre‐yield stress relaxation but significantly underestimates the post‐yield stress relaxation. As considering the chain conformation transition alone leads to a dilemma for the model to concurrently represent the yield/flow and stress relaxation behaviors, the model was extended to incorporate an additional structural relaxation mechanism assumed to originate from the dissociation of weak linkages in the chain network. The extended model succeeds in concurrently representing the yield/flow, and stress relaxation behaviors in the whole deformation region, of which the reasons were analyzed. The knowledge revealed in this paper is instructive and may shed new light on understanding the structural relaxation and mechanical behavior of glassy polymers.

Achieving structural rejuvenation in metallic glass by modulating β relaxation intensity via easy-to-operate mechanical cycling

Structural rejuvenation is an effective measure to optimize the mechanical properties of metallic glasses (MGs). Sophisticated solutions to rejuvenation include thermal cycling, laser shocking, and multiaxial stress loading. Here, we propose an easy-to-operate mechanical cycling as an alternative strategy to tailor the mechanical relaxation, deformation, and structural heterogeneity of MGs. Structural rejuvenation in a La-based MG is achieved via mechanical cycling even at very few cycles (102 tension load cycles) and low frequencies (10−3 Hz). The results manifest intuitively the competition between structural relaxation and rejuvenation, which constitutes the structural evolution in MGs. A theoretical model is constructed which reveals a scenario that mechanical cycling wakes up frozen flow defect, accelerating creep and, thus, enhancing the β relaxation in MGs. Therefore, this handy anti-ageing methodology provides an alternative pathway to optimize the mechanical properties of MGs. It also contributes to a more comprehensive understanding of the structure-property relationship in amorphous materials, especially with regard to the correlation between structural rejuvenation and relaxation behavior in such topologically disordered materials.

Tuning structural-response of PLA/PCL based electrospun nanofibrous mats: Role of dielectric-constant and electrical-conductivity of the solvent system

The role of optimum solvent systems on the fabrication of uniform, bead-free electrospun-nanofibrous-mats (ENMs) of polylactic acid (PLA), poly(ε-caprolactone) (PCL), and their blends, is investigated. The solvent systems influenced the fiber-diameters, morphology, crystallinity, thermal stability, hydrophobicity, quasi-static mechanical, and solid-state visco-elastic responses of the ENMs. Defect-free ENMs were obtained by using CF/DMF (80:20 v/v) binary solvent system while showing a relatively higher extent of crystallinity (PLA/PCL blend ∼34%), lower hydrophobicity (PLA ∼1170), higher strength (PLA ∼6 MPa), and moduli (PLA ∼305 MPa) for PLA and PLA/PCL blend systems whereas a higher strain-at-break (∼ 82%) was shown by PCL based ENMs. PLA/PCL blend based ENMs fabricated using DCM/DMF (80:20 v/v) solvent-mixture exhibited comparatively lower crystallinity (∼ 25%) but higher fiber diameter (1.03 ± 0.21 µm), strain-at-break (∼ 155%), and hydrophobicity (∼ 130 0) compared to CF/DMF (80:20 v/v) system. Dynamic mechanical analysis (DMA) revealed the structural relaxation behaviors indicating the intrinsic structural deformability and flexibility of the mats. The study demonstrated the systematic role of solvent characteristics in terms of their volatility, dielectric constant, and solvent-mixture composition on the electro-spinnability and fabrication of high-strength, deformable, hydrophobic, bead-free ENMs with near monodisperse fibrous assemblies for biomedical applications.

Stress relaxation behavior of left ventricular myocardium in mice

BackgroundMyocardial tissues are known to exhibit viscoelastic behavior, i.e., the myocardial wall stress is gradually decaying when stretched at a constant level, referred to as stress relaxation behavior. The study of this behavior becomes important in structural heart diseases, where both passive and active relaxation behaviors of the myocardium may alter. As myocardial tissues display a complex, 3D micro‐architecture, the anisotropy of stress relaxation remains understudied. Our objective in this study was to investigate the anisotropy of (passive) stress relaxation behavior of the left ventricular (LV) myocardium in healthy small animals.MethodsWe used biaxial stress relaxation tests in wild‐type murine specimens (n=2). Full‐thickness myocardium LV free wall (LVFW) specimens were isolated from the mouse hearts (Fig. 1a), with the slab edges being aligned with the longitudinal (apex‐to‐outflow‐tract), circumferential, and radial directions of the LV. The specimens were mounted in a biaxial mechanical testing machine along the circumferential and longitudinal directions. The specimens were stretched in three different loading protocols with circumferential‐to‐longitudinal strain ratios of 30:30, 15:30, and 30:15%. The samples were loaded to the peak strain at a rate of 0.01 s‐1 and held at this stretch for 10 minutes to allow the relaxation. The time‐varying stress was recorded. We used an exponential decay fit to estimate the stress relaxation time constants in both directions.ResultsStress relaxation time constants showed noticeable differences between multiple loading ratios with equibiaxial loading (30:30) having the largest relaxation time constant followed by the circumferential stress in the 30:15 ratio (Fig. 1b). The larger strains led to larger stress relaxation constants (Fig. 1b‐c) indicating a strain‐dependent viscoelastic behavior. The quasi‐static equibiaxial loading of the same specimens resulted in a nearly isotropic behavior (not shown here), exhibiting insignificant contrast between the stresses in the circumferential and longitudinal directions. Interestingly, the contrast in the relaxation behavior between the circumferential and longitudinal directions were similarly insignificant (Fig. 1b). Non‐equibiaxial loadings (15:30 and 13:50) showed significantly faster relaxation for the direction with smaller strains (Fig. 1b).ConclusionsMyocardial tissue viscoelasticity is an important behavior contributing to both diastolic and systolic cardiac function, which remains understudied, especially in case of structural heart diseases associated with impaired relaxation in the LV. Our study suggests that stress relaxation behavior is prominent in the LV myocardium and may exhibit different relaxation behavior depending on strain magnitudes and strain ratios along different directions. The level of anisotropy in stress relaxation was found to be similar to the level of anisotropy in the tissue quasi‐static behavior. These results serve to motivate further studies to fully characterize the viscoelastic behavior of LV myocardium and its contribution to the LV relaxation and filling in murine models of LV diseases.

Structural, shear and volume relaxation in a commercial float glass during aging

Density, Raman spectroscopy and viscosity measurements are utilized to probe, respectively, the volume, structural and shear relaxation behavior of a Na-Ca-Mg silicate glass of standard commercial float composition at ∼80 °C below its calorimetric glass transition. The results, when taken together, indicate that the lowering of fictive temperature of the glass during aging results in a Q-species disproportionation of the type Q2+Q4→2Q3, and all three relaxation processes are characterized by stretched exponential kinetics with similar average timescale (〈τ〉∼ 300 h) and stretching exponent (β ∼ 0.4), implying a close mechanistic relationship. It is hypothesized that the coexistence of strongly and weakly constrained domains or structural moieties in the network of a glass or supercooled liquid can give rise to a temporal decoupling between the structural and shear relaxation processes, as has been reported in some of the recent studies in the literature.

Effects of Annealing on Enthalpy Recovery and Nanomechanical Behaviors of a La-Based Bulk Metallic Glass

Structural relaxation and nanomechanical behaviors of La65Al14Ni5Co5Cu9.2Ag1.8 bulk metallic glass (BMG) with a low glass transition temperature during annealing have been investigated by calorimetry and nanoindentation measurement. The enthalpy release of this metallic glass is deduced by annealing near glass transition. When annealed below glass transition temperature for 5 min, the recovered enthalpy increases with annealing temperature and reaches the maximum value at 403 K. After annealed in supercooled liquid region, the recovered enthalpy obviously decreases. For a given annealing at 393 K, the relaxation behaviors of La-based BMG can be well described by the Kohlrausch-Williams-Watts (KWW) function. The hardness, Young’s modulus, and serrated flow are sensitive to structural relaxation of this metallic glass, which can be well explained by the theory of solid-like region and liquid-like region. The decrease of ductility and the enhancement of homogeneity can be ascribed to the transformation from liquid-like region into solid-like region and the reduction of the shear transition zone (STZ).

Correction: Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand

Correction for ‘Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand’ by Hai-Ye Li et al., New J. Chem., 2021, 45, 7096–7102, DOI: 10.1039/D1NJ00140J.

Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand

Two {Dy6} skeletons consolidated by nitrate (1) and hydroxide (2) have been isolated. HRESI-MS reveals that 1 retains a higher structural integrity and fragment stability. While 2 has a typical slow magnetic relaxation of SMM behavior.

Structure and Rheology of Se-I Glasses and Supercooled Liquids

The structural evolution and shear relaxation behavior of binary Se100-xIx (0 ≤ x ≤ 3) glasses and supercooled liquids are investigated using Raman spectroscopy and parallel plate rheometry, respectively. The spectroscopic results indicate cleavage and termination of Se polymeric chains by I atoms via the formation of Se-I bonds, leading to shortening of the average chain length with progressive addition of I atoms. Moreover, at I contents of ≥ 2%, a fraction of the I atoms forms I2 dimers. The results from rheometry demonstrate that the structural relaxation rates associated with the fast segmental chain motion and the slow bond scission/renewal dynamics in Se100-xIx liquids increase upon addition of I, with the shortening of the chain length. The increased configurational degrees of freedom of shorter selenium chains leads to a decrease in the isothermal viscosity, Tg and the packing efficiency and an increase in fragility in liquids with higher I content.

Rheology of colloidal and metallic glass formers

Colloidal hard-sphere suspensions are convenient experimental models to understand soft matter, and also by analogy the structural-relaxation behavior of atomic or small-molecular fluids. We discuss this analogy for the flow and deformation behavior close to the glass transition. Based on a mapping of temperature to effective hard-sphere packing, the stress–strain curves of typical bulk metallic glass formers can be quantitatively compared with those of hard-sphere suspensions. Experiments on colloids give access to the microscopic structure under deformation on a single-particle level, providing insight into the yielding mechanisms that are likely also relevant for metallic glasses. We discuss the influence of higher-order angular signals in connection with non-affine particle rearrangements close to yielding. The results are qualitatively explained on the basis of the mode-coupling theory. We further illustrate the analogy of pre-strain dependence of the linear-elastic moduli using data on PS-PNiPAM suspensions.

Modulating the structural topologies and magnetic relaxation behaviour of the Mn–Dy compounds by using different auxiliary organic ligands

A MnIII4DyIII complex and a one-dimensional chain containing MnIII2DyIII units have been obtained by using different combinations of organic ligands, and a slow magnetic relaxation behavior was observed for both complexes.

Features of electronic polarizability and approach to unique properties in tellurite glasses

AbstractTellurite glasses showing unique properties such as low phonon energy and high linear and nonlinear optical properties have been considered as promising materials for photonic device applications. The present article reviews the current status of the electronic polarizability in binary and ternary tellurite glasses and approaches their unique properties. Tellurite glasses consisting of basic modifier oxides (eg, Na2O, ZnO) and basic conditional glass forming oxide (TeO2) exhibit the features of high electronic polarizability and weak chemical bond strength, that is, refractive index‐based large oxide ion polarizability αO2‐(no), large optical basicity ∧(no), small interionic interaction parameter A(no), and small single bond strength BM‐O. The values of αO2‐(no), ∧(no), and A(no) in several tellurite glasses containing WO3, Nb2O5, and Bi2O3 were estimated in this article, and a good correlation was confirmed between ∧(no) and A(no). Unique properties of low glass transition temperature, fragile structural relaxation behavior, poor mechanical properties, high third‐order nonlinear optical susceptibilities in tellurite glasses were discussed from the point of electronic polarizability.