Spectrum Of Relaxation Times Research Articles

Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates, more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively, was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.

Payne effect: A Constitutive model based on a dynamic strain amplitude dependent spectrum of relaxation time

Under dynamic loads, the viscoelastic behavior of filled elastomers exhibits a nonlinearity known as the Payne Effect. The material stiffness decreases as the dynamic strain amplitude increases. The aim of this work is to model this feature, by adapting a previously developed constitutive model. First, a complete experimental dynamic campaign is performed to characterize the dynamic properties of a filled elastomer used on silent block. The continuous spectrum of relaxation time is identified as a function of the strain amplitude. This identification follows the standard method of the literature. The spectrum is discretized into a dynamic strain amplitude dependent Prony Series. Finally, a constitutive model using the Prony Series is developed and numerically implemented. Comparisons between experimental results and model predictions are presented.

Spectrum of relaxation times of polarization in thin film ferroelectrics near the phase transition point

The temperature dependences of the spectrum of polarization relaxation times above the temperature of the second-order phase transition of order-disorder type with relaxation Debye mode in a thin ferroelectric film are determined depending on its thickness and the degree of polarization fixation on the film surface. It has been established that the longest of the relaxation times τ1 behaves abnormally, which is unlimitedly increased τ1∼(T−Tf)−1 as the phase transition temperature Tf approaches, causing a critical deceleration of the relaxation processes. Taking into account intrinsic electric fields decreases the relaxation time at other identical film parameters.

Rejuvenation through plastic deformation of a La-based metallic glass measured by fast-scanning calorimetry

We explore the glassy states achievable after a metallic glass is formed on liquid quenching. Samples of La55Al25Ni20 (at.%) metallic glass (rod and ribbon) are studied. The extent of structural relaxation at room temperature is characterized for this low-glass-transition temperature glass. Plastic deformation (uniaxial compression) rejuvenates the glass to states of higher enthalpy characteristic of glass formation at high cooling rate. Deformation increases the heterogeneity of the glass, widening the spectrum of relaxation times. The extent of rejuvenation in samples of low aspect ratio is compared with that under conditions of high constraint in notched samples. The deformation-induced rejuvenation is particularly susceptible to reduction on subsequent ageing. Fast-scanning calorimetry is useful in characterizing the dynamics of structural relaxation. The shadow glass transition is more evident on fast heating, and is observed in this glass for the first time. A new excess exothermic effect is observed before the glass transition.

Fluid Property Discrimination in Dolostone Reservoirs Using Well Logs

AbstractThe Ordovician Majiagou Formation is one of the main gas‐producing strata in the Ordos Basin, China. The identification of hydrocarbon‐bearing intervals via conventional well logs is a challenging task. This study describes the litholog of Ma 5 (Member 5 of Majiagou Formation) dolostones, and then analyzes the responses of various conventional well logs to the presences of natural gas. The lithology of the gas bearing layers is dominantly of the dolomicrite to fine to medium crystalline dolomite. Natural gas can be produced from the low resistivity layers, and the dry layers are characterized by high resistivities. Neutron‐density crossovers are not sensitive to the presences of natural gas. In addition, there are no significant increases in sonic transit times in natural gas bearing layers. NMR (nuclear magnetic resonance) logs, DSI (Dipole Sonic Imager) logs and borehole image logs (XRMI) are introduced to discriminate the fluid property in Majiagou dolostone reservoirs. The gas bearing intervals have broad NMR T2 (transverse relaxation time) spectrum with tail distributions as well as large T2gm (T2 logarithmic mean values) values, and the T2 spectrum commonly display polymodal behaviors. In contrast, the dry layers and water layers have low T2gm values and very narrow T2 spectrum without tails. The gas bearing layers are characterized by low Vp/Vs ratios, low Poisson's ratios and low P‐wave impedances, therefore the fluid property can be discriminated using DSI logs, and the interpretation results show good matches with the gas test data. The apparent formation water resistivity (AFWR) spectrum can be derived from XRMI image logs by using the Archie's formula in the flushed zone. The gas bearing layers have broad apparent formation water resistivity spectrum and tail distributions compared with the dry and water layers, and also the interpretation results from the image logs exhibit good agreement with the gas test data. The fluid property in Majiagou dolostone reservoirs can be discriminated through NMR logs, DSI logs and borehole image logs. This study helps establish a predictable model for fluid property in dolostones, and have implications in dolostone reservoirs with similar geological backgrounds worldwide.

Detonation nanodiamonds fabricated from tetryl: Synthesis, NMR, EPR and XRD study

Fabrication of bulk quantities of detonation nanodiamonds from a new low-cost explosive, tetryl, necessitates the study of the properties and quality of new materials in comparison with those obtained from TNT-RDX mixtures. Here we report on preparation of detonation nanodiamonds from tetryl and their characterization by XRD, NMR and EPR techniques. XRD patterns, EPR spectra and 13C MAS NMR spectra and relaxation rates in tetryl-fabricated samples were measured and compared with those in “conventional” nanodiamonds prepared from the mixture of TNT and RDX explosives. Our data result in conclusion that the size and structure of new nanoparticles are similar to those obtained in the TNT-RDX fabricated nanodiamonds. Magic angle spinning (MAS) of the samples does not affect spin-lattice relaxation but results in the narrowing of the 13C spectrum and elongation of spin-spin relaxation time with increased MAS rate. These data also correlate with those of the “conventional” nanodiamonds. Our study shows that tetryl is a promising material for the nanodiamond production.

Composition dependence of metallic glass plasticity and its prediction from anelastic relaxation – A shear transformation zone analysis

The mechanical relaxation behavior of metallic glasses (MGs) is composition sensitive. For example, in dynamic mechanical analysis, La70Ni15Al15 shows a pronounced secondary peak, termed β relaxation, of the normalized loss modulus at high frequency and/or low temperature, while La70Cu15Al15 only exhibits a shoulder. We have determined relaxation-time spectra at room temperature for these alloys over eleven orders of magnitude of time from quasi-static anelastic relaxation measurements. These are employed to characterize shear transformation zone (STZ) properties in both the α and β regimes. The pronounced β relaxation peak, observed for La70Ni15Al15, is a result of both larger volume fraction of fast and small potential STZs and smaller volume fraction of slow and large potential STZs as compared to La70Cu15Al15. Room-temperature tensile tests show increasing plasticity with decreasing strain rates, characteristic of thermally activated processes. La70Cu15Al15 exhibits greater plasticity than La70Ni15Al15, a negative correlation with the intensity of the β relaxation that is opposite to that previously suggested. The STZ spectra are used to explain the plasticity difference between the two alloys. Plasticity predictions from loss modulus measurements need to be revised, and require absolute measurements.

Modeling of Polymeric Liquid Material Properties Effect on Pressure Transients in the Elastic Pipe

Material properties of polymeric liquids are of great importance for different technological processes. Particularly, such liquids demonstrate viscoelastic behavior in non-stationary transportation regimes, widely used in polymer processing, which influence the operation of the equipment. The paper is devoted to the modeling of pressure transient in a long thin-walled elastic tube with polymeric liquid. As distinct to previous results of the authors, material properties of the liquid are described by generalized Maxwell rheological equation accounting for a spectrum of relaxation times. It is supposed that the pressure pulse is generated at the tube end and propagates along the waveguide with the speed influenced by the tube geometry and wall elasticity, and the liquid compressibility and viscoelasticity. The problem is formulated in a quasi-one-dimensional approximation and solved by the operational method. The resulting relation for the pressure in the wave is inverted numerically. Effect of liquid relaxation time distribution on the pressure pulse propagation is studied. The results are relevant for the dynamic operation of equipment for polymer processing; they can be useful also for material characterization of high-molecular liquids.

Influence of Carbon Dioxide on the Adsorption of Methane by Coal Using Low-Field Nuclear Magnetic Resonance

The adsorption capacity of coal with respect to carbon dioxide (CO2) is greater than that of methane (CH4). Studying the CO2 replacement of CH4 in coal is essential to understanding the mechanism behind CO2-enhanced coalbed methane. In this paper, scanning electron microscopy was used to qualitatively study the influence of CO2 injection pressure on the fracture characteristics of coal. The time effect of the CH4/CO2 adsorption and the effect of the CO2 injection on the CH4 adsorption were quantitatively studied using low-field nuclear magnetic resonance. The results suggest three different CH4 states in the coal samples: CH4 adsorbed on the pore surface, free CH4 in the pore center, and free CH4 between the coal particles. Indeed, the greater the CO2 injection pressure, the more developed the fracturing and network, which, in turn, increases connectivity. As the adsorption time of CH4/CO2 increases, the adsorption rate of CO2/CH4 gradually decreases. In essence, CO2 preferentially replaces the CH4 of the minipores. As the CO2 injection pressure increases, the difference between the CO2 equilibrium pressure and the initial injection pressure increases. Moreover, the CH4 production rate increases, the transverse relaxation time (T2) spectrum of the adsorption-state area decreases, the T2 spectrum of the free-state area increases, and the number of adsorption holes gradually decreases, whereas the number of seepage holes increases.

Pore Structure and Permeability Characterization of High‐rank Coal Reservoirs: A Case of the Bide‐Santang Basin, Western Guizhou, South China

AbstractThe methods of nuclear magnetic resonance (NMR) spectroscopy, mercury injection porosimetry (MIP), and gas‐water relative permeability (GWRP) were used to reveal the pore structure and permeability characteristics of high‐rank coal reservoirs in the Bide‐Santang basin, western Guizhou, South China, to provide guidance for coalbed methane (CBM) exploration and exploitation and obtain direct insights for the development of CBM wells. The results indicate that the coal reservoirs in the study area are characterized by well‐developed adsorption pores and poorly developed seepage pores. The bimodal NMR transverse relaxation time (T2) spectra and the mutation in the fractal characteristic of the MIP pore volume indicate poor connectivity between the adsorption pores and the seepage pores. As a result, the effective porosity is relatively low, with an average of 1.70%. The irreducible water saturation of the coal reservoir is relatively high, with an average of 66%, leading to a low gas relative permeability under irreducible water saturation. This is the main reason for the low recovery of high‐rank CBM reservoirs, and effective enhanced CBM recovery technology urgently is needed. As a nondestructive and less time‐consuming technique, the NMR is a promising method to quantitatively characterize the pores and fractures of coals.

Study on the hydration and moisture transport of white cement containing nanomaterials by using low field nuclear magnetic resonance

In order to characterize the hydration and moisture transport process of white cement containing nano-SiO2 (NS) and nano-Al2O3 (NA), the transverse relaxation time (T2) spectrum signals of the cement paste and mortars were measured by low field nuclear magnetic resonance (LF-NMR). The effect of NS and NA on the T2 spectrum, T2 geometric average values, and porosity of cement paste from 15 min to 72 h of hydration were studied. Moreover, the compressive strength, flexural strength, and pore structure tested by mercury intrusion porosimetry were given. Furthermore, the variation in gel pore and capillary pore of mortar as a function of moisture transport, including drying and moisture absorption (deionized water and 3% NaCl solution) process, was revealed. The results indicate that the induction time of hydration is shorten and the early porosity is reduced by introducing NS and/or NA. In the case of drying, free water evaporates from large pores to small pores, and the water in the gel pore cannot be extracted during drying. With respect to moisture absorption, the incorporation of 1 wt% NS reduces the number of gel pores, the number of capillary pores is decreased by adding 0.5 wt% NS and 0.5 wt% NA, and the mortar containing 0.5 wt% NS and 0.5% NA shows the lowest hygroscopic rate.

Ultraviolet‐induced chain extension of poly(ethylene terephthalate) based on radical reaction with the aid of trimethylolpropane triacrylate and glycidyl methacrylate during extrusion

AbstractIn order to improve the processing properties of poly(ethylene terephthalate) (PET), carbon–carbon double bonds were end‐capped onto the chain end of PET by reacting with glycidyl methacrylate (GMA) and then the product was reacted with trimethylolpropane triacrylate (TMPTA) based on a free radical reaction initiated by ultraviolet light to form long‐chain branching structures. The reaction was demonstrated by intrinsic viscosity measurements, carboxyl content analysis, Fourier transform infrared spectroscopy and size exclusion chromatography. Then the rheological and thermal properties of PET were investigated with various TMPTA and GMA contents. Chain extended PET displayed higher complex viscosity than pristine PET and pronounced shear‐thinning behavior. Moreover, the relaxation time spectrum revealed that the modified PET displayed a longer relaxation time during the relaxation process, which was attributed to the higher degree of entanglements resulting from long‐chain branching. Besides, its crystallization temperature and melt temperature shifted to lower temperatures, and the glass transition temperature shifted to higher temperature, indicating that the thermal properties of the modified PET had also been improved. Thus this method can be used to improve the overall properties of PET. © 2020 Society of Chemical Industry

Relaxation Time Spectrum and Dynamics of Stretched Polymer Chain in Dilute θ Solution: Implicit Solvent Model versus Explicit Solvent Model

AbstractRelaxation dynamics of a single stretched polymer chain after releasing the two fixed ends in dilute solutions are investigated by using implicit solvent and explicit solvent molecular dynamic simulations. The phase diagram of polymer in explicit solvent solution is determined, and the θ point is estimated by the form factor of the polymer chain. The relaxation time spectrum is obtained by the inverse Laplace transform of length–time decay curves of the relaxation process. The dependencies of the longest relaxation time in the spectrum on the chain length are compared with the scaling prediction of the Rouse model and the Zimm model of flexible chains. In this paper, it is confirmed that the dynamical scaling analysis of theoretical models are in good agreement with the simulation models in all range of polymerization degree of chain studied. The coarse‐grained explicit solvent simulation model of polymer chain in dilute solution developed in this paper can be a good numerical model for the future study of the dynamics process of macromolecules with hydrodynamic interactions in solutions.

Long-time structural relaxation of glass-forming liquids: Simple or stretched exponential?

This paper presents data for the physical aging of the density of squalane upon both non-linear and nearly linear temperature jumps from states of thermal equilibrium. Invoking the single-parameter-aging scenario [Hecksher et al., J. Chem. Phys. 142, 241103 (2015); Proc. Natl. Acad. Sci. U. S. A. 116, 16736-16741 (2019)], the linear-response aging relaxation function is extracted from the data. Based on this, it is shown that the relaxation toward equilibrium follows a simple exponential function at long times; a stretched-exponential function provides a poor fit. This demonstrates the existence of a terminal relaxation rate for the physical aging of squalane, corresponding to an effective long-time cutoff in the spectrum of structural relaxation times.

Relaxor-like behavior of mechanically activated ultrafine SrTiO3

Dielectric properties of nanostructured strontium titanate samples sintered from nanoparticles obtained by grinding of previously synthesized SrTiO3 and nanoparticles obtained by chemical deposition were studied in the temperature range 10–300 K. The first sample shows the characteristic features of a relaxor ferroelectric. The dispersion of the dielectric response observed in the vicinity of 40 K is characterized by two relaxation processes, with broad spectra of relaxation times. It is assumed that the “high-temperature” relaxation process is associated with the dynamics of the elastic domain. Possible mechanisms for the interaction of elastic deformation and polarization are discussed.

Insights in the pore structure, fluid mobility and oiliness in oil shales of Paleogene Funing Formation in Subei Basin, China

Abstract Pore structure is an important factor influencing reservoir properties of oil shales. Routine core analysis, thin section, scanning electron microscope (SEM), mercury injection capillary pressure (MICP) tests, Nuclear Magnetic Resonance (NMR), and computed tomography (CT) were used to provide insights into the pore throat distribution in oil shales of member 2 of Paleogene Funing Formation (E1f2) in Subei Basin, China, with the special aim to unravel the effect of pore structure on fluid mobility and oiliness. The relationships between NMR parameters, petrophysical property and capillary parameters are investigated. The results show that rock composition in the oil shales consist of quartz, feldspar, carbonate particles, clay minerals and organic matters. Pore systems consist of large-scale interparticle pores, intragranular dissolution pores, and small-scale micropores within clay minerals and the organic matter pores. Four types of pore structure (Type Ⅰ, Type Ⅱ, Type Ⅲ and Type Ⅳ) are divided according to the patterns of capillary curves, capillary parameters, NMR T2 (transverse relaxation time) spectrum and BVI (bulk volume of immovable fluid) values. From Type Ⅰ to Type Ⅳ pore structure, the maximum mercury saturation (SHgmax) and mercury extrusion efficiency are decreasing, and the SHgmax are less than 50% averagely, indicating the oil shales are characterized by very poor pore connectivity. The T2 spectrum changes from bi-modal behavior to uni-modal and the right peaks become lower or even disappearing from Type Ⅰ to Type Ⅳ pore structure. Fluid mobility is not primarily controlled by pore size, but dependent on the content of short T2 components ( Pore structure controlled fluid mobility and determined the microscopic oiliness and macroscopic oil bearing property or hydrocarbon productivity. The comprehensive study above gains insights into the microscopic pore structure and their controls on fluid mobility and oiliness in shale oil reservoirs, and this may have implications for resource potential evaluation and effective exploitation of oil shales.

Tailoring the toughness of sustainable polymer blends from biodegradable plastics via morphology transition observed by atomic force microscopy

Super-toughened polylactide/polybutylene succinate/polybutylene adipate terephthalate (PLA/PBS/PBAT) ternary blends can be prepared by reactive extrusion in the presence of peroxide. However, the high peroxide contents always lead to lowered biodegradability and decreased flowability of the prepared materials. In this study, a strategy to prepare such super-toughened PLA ternary blends with a small amount of peroxide, by simultaneous phase transition and interfacial compatibilization, was demonstrated. Firstly, based on thermodynamic predictions, a core-shell PBS/PBAT structure was successfully prepared that worked as a toughening agent for PLA. Secondly, the viscosity ratio between PLA and PBS/PBAT was adjusted via the introduction of a small amount of peroxide because of the different reactive priority between peroxide and the polymers. Via transferring the co-continuous PBS/PBAT to droplet structure with optimum size and improving the compatibility between PLA and core-shell PBAT-PBS, two different super-toughened PLA ternary blends with high impact strength and melt strength were successfully prepared. The resulting ternary blends showed super toughness with significantly improved notched impact strength (~530 J/m). The mechanism behind the morphology transformation is discussed based on the phase inversion theory with the help of relaxation time spectrum analysis and atomic force microscopy (AFM). The studies show that both the morphology transition of the different components and improved compatibility make contributions together to the improved mechanical properties. The research provides a new approach to improving the toughness of the polymer blends in reaction extrusion by tailoring the viscosity ratios between different components. The ternary blends developed are expected to be used widely in industrial applications.

Спектр времен релаксации в ансамбле суперпараэлектрических частиц

Спектр времен релаксации в ансамбле суперпараэлектрических частиц

The variation characteristics of micro-pore structures of underground rocks in cold regions subject to freezing and thawing cycles

In order to investigate the evolution characteristics of the microscopic pore structures of the rocks in the underground cavities subject to freezing and thawing cycles, in this article, we conducted several rounds of freezing and thawing cyclic tests and detected the rock samples after each round of freezing and thawing cycles by means of nuclear magnetic resonance (NMR) and CT scanning. Based on the acquired NMR T2 relaxation time spectra, porosity variations, pore distributions, and imaging results of the rock samples at different freezing and thawing stages, the effects of the number of freezing and thawing cycles on the rock’s NMR T2 spectra, porosity, and microscopic pore structures were analyzed. Results show that, for the rock sample collected from the underground cavities, the micro-pore structures were significantly affected by the number of freezing and thawing cycles. More freezing and thawing cycles resulted in larger porosity and higher damage degree of the rock samples. The present study helps to unveil the evolution characteristics of the micro-pore structures of the rock samples in underground cavities after freezing and thawing cycles, and thus provides theoretical basis for the damage mechanisms and stability analysis of underground rocks in cold regions.

Microstructural differences between Viscose and Lyocell revealed by in-situ studies of wet and dry fibers

Regenerated cellulose fibers manufactured using Lyocell and Viscose processes exhibit differences in their structural response to mechanical deformation. Here, we study the effect of stretching and stress relaxation on the orientation of crystal and amorphous phases. Our results show that on stretching, orientation in both crystal and amorphous phases increases linearly with strain, correlating with the increase in stress and with the stretching of the crystalline unit cell along the c-axis. On holding after stretching to a particular strain, the stress relaxes logarithmically in time, correlating with a decrease in the strain along the c-axis of the crystal unit cell. The stress relaxation is also correlated with a logarithmic increase in amorphous orientation, while crystalline orientation stays constant. We attribute the stress development during stretching to deformation of the crystal unit cell, while crystal reorientation in the fiber direction results in increase in the crystalline orientation parameter. On holding the fiber at a fixed total strain, the stress relaxes as strain is transferred from crystal to amorphous regions. Thus, the strain on the unit cell c-axis decreases and amorphous orientation increases. There are quantitative differences between the rate of increase in amorphous phase orientation during stress relaxation for Lyocell and Viscose fibers. For dry fibers, Lyocell shows a slower increase in orientation during stress relaxation. On wetting the fibers, their structural response changes qualitatively. We combine wide angle x ray diffraction and birefringence experiments with our model to infer that that on stretching the wet fiber, the crystalline phase is neither strained nor oriented. However, orientation develops in the amorphous phase. During stress relaxation in wet condition, Lyocell fibers shows a faster increase in amorphous orientation than Viscose fibers, in line with the comparison of relaxation time spectra for wet Viscose and Lyocell fibers.