Volume Relaxation Research Articles

Composite Membranes Based on the Poly(1-trimethylsylyl-1-propine): Influence of the Porous Aromatic Frameworks Produced from the Friedel–Crafts Reaction and Introduced into the Polymer Matrix

The effect of particles of porous aromatic frameworks, synthesized by the Friedel–Crafts reaction (PAF-FC), introduced into matrix of glassy polymer poly(1-trimethylsilyl-1-propine) (PTMSP), on the stabilization of the transport characteristics of composite membranes with a thin selective layer (2.0–2.2 Μm) was studied. As a comparison, samples of composite membranes with a selective layer based on PTMSP and PTMSP filled with PAF-11 particles synthesized by the Suzuki reaction are considered. All samples of composite membranes showed a decrease in the absolute values of gas permeability for all gases over time due to relaxation of the nonequilibrium free volume of PTMSP, therewith the greatest decrease is observed during the first thousand hours since membrane creation. The stabilization of the transport characteristics of composite membranes with PTMSP/PAF-FC occurs after 5000 h since membrane creation, further the carbon dioxide performance remains at the level of 8–9 m3 m–2 h–1 atm–1 for 9200 h. Similar performance values for aged composite membranes based on PTMSP and PTMSP/PAF-11 were about 1.5 m3 m–2h–1 atm–1.

Improving Delineation of True Tumor Volume With Multimodal MRI in a Rat Model of Brain Metastasis

PurposeBrain metastases are almost universally lethal with short median survival times. Despite this, they are often potentially curable, with therapy failing only because of local relapse. One key reason relapse occurs is because treatment planning did not delineate metastasis margins sufficiently or accurately, allowing residual tumor to regrow. The aim of this study was to determine the extent to which multimodal magnetic resonance imaging (MRI), with a simple and automated analysis pipeline, could improve upon current clinical practice of single-modality, independent-observer tumor delineation.Methods and MaterialsWe used a single rat model of brain metastasis (ENU1564 breast carcinoma cells in BD-IX rats), with and without radiation therapy. Multimodal MRI data were acquired using sequences either in current clinical use or in clinical trial and included postgadolinium T1-weighted images and maps of blood flow, blood volume, T1 and T2 relaxation times, and apparent diffusion coefficient.ResultsIn all cases, independent observers underestimated the true size of metastases from single-modality gadolinium-enhanced MRI (85 ± 36 μL vs 131 ± 40 μL histologic measurement), although multimodal MRI more accurately delineated tumor volume (132 ± 41 μL). Multimodal MRI offered increased sensitivity compared with independent observer for detecting metastasis (0.82 vs 0.61, respectively), with only a slight decrease in specificity (0.86 vs 0.98). Blood flow maps conferred the greatest improvements in margin detection for late-stage metastases after radiation therapy. Gadolinium-enhanced T1-weighted images conferred the greatest increase in accuracy of detection for smaller metastases.ConclusionsThese findings suggest that multimodal MRI of brain metastases could significantly improve the visualization of brain metastasis margins, beyond current clinical practice, with the potential to decrease relapse rates and increase patient survival. This finding now needs validation in additional tumor models or clinical cohorts.

Enthalpies and elastic properties of Ni-Co binary system by ab initio calculations and an energy comparison with the CALPHAD approach

Enthalpies and elastic properties of the full composition range of Ni-Co binary system with FCC structure were investigated applying Density Functional Theory (DFT). The DFT results on enthalpy were compared with the enthalpies calculated applying the CALPHAD approach. Moreover, the effect of relaxation method, i.e. volume relaxation vs. energy-volume (E–V) curves, in DFT approach on the accuracy of the results of thermodynamic and mechanical properties were studied. It was shown that the results are very close together for the regions in which FCC is stable, but different in FCC-metastable regions. The results indicate the DFT method along with SQS approach can provide relatively reliable predictions for both energy and mechanical properties in Ni-Co system.

Human Brown Adipose Tissue Estimated With Magnetic Resonance Imaging Undergoes Changes in Composition After Cold Exposure: An in vivo MRI Study in Healthy Volunteers.

Aim: Magnetic resonance imaging (MRI) is increasingly being used to evaluate brown adipose tissue (BAT) function. Reports on the extent and direction of cold-induced changes in MRI fat fraction and estimated BAT volume vary between studies. Here, we aimed to explore the effect of different fat fraction threshold ranges on outcomes measured by MRI. Moreover, we aimed to investigate the effect of cold exposure on estimated BAT mass and energy content.Methods: The effects of cold exposure at different fat fraction thresholding levels were analyzed in the supraclavicular adipose depot of nine adult males. MRI data were reconstructed, co-registered and analyzed in two ways. First, we analyzed cold-induced changes in fat fraction, T2* relaxation time, volume, mass, and energy of the entire supraclavicular adipose depot at different fat fraction threshold levels. As a control, we assessed fat fraction differences of deltoid subcutaneous adipose tissue (SAT). Second, a local analysis was performed to study changes in fat fraction and T2* on a voxel-level. Thermoneutral and post-cooling data were compared using paired-sample t-tests (p < 0.05).Results: Global analysis unveiled that the largest cold-induced change in fat fraction occurred within a thermoneutral fat fraction range of 30–100% (−3.5 ± 1.9%), without changing the estimated BAT volume. However, the largest cold-induced changes in estimated BAT volume were observed when applying a thermoneutral fat fraction range of 70–100% (−3.8 ± 2.6%). No changes were observed for the deltoid SAT fat fractions. Tissue energy content was reduced from 126 ± 33 to 121 ± 30 kcal, when using a 30–100% fat fraction range, and also depended on different fat fraction thresholds. Voxel-wise analysis showed that while cold exposure changed the fat fraction across nearly all thermoneutral fat fractions, decreases were most pronounced at high thermoneutral fat fractions.Conclusion: Cold-induced changes in fat fraction occurred over the entire range of thermoneutral fat fractions, and were especially found in lipid-rich regions of the supraclavicular adipose depot. Due to the variability in response between lipid-rich and lipid-poor regions, care should be taken when applying fat fraction thresholds for MRI BAT analysis.

Evaluation of Regional Left and Right Ventricular Function in Patients with Ostium-Secundum Atrial Septal Defect Using Speckle Tracking Echocardiography

The aim of this study was to provide evidence of abnormalities of right ventricular (RV) and left ventricular (LV) function using speckle tracking echocardiography (STE) in adults with ostium secundum-atrial septal defect (OS-ASD) and pulmonary hypertension (PH). Sixty-three adult patients with OS-ASD were enrolled. Control group (n=10) with mean pulmonary arterial pressure (MPAP) ≤25 mmHg. Three PH groups were divided based on pulmonary artery systolic pressure (PASP): mild (n=18), moderate (n=24) and severe (n=11). RV free wall strain (RVFWS) and LV global longitudinal strain (LVGLS) were measured. RVFWS and RV fractional area change were significantly lower in the severe PH group. No difference was found in LVGLS among groups. The moderate PH group had significantly higher right atrial (RA) volume and RV impaired relaxation. RVFWS had a positive correlation with PASP and MPAP. STE is a valuable non-invasive technique to assess RV and LV systolic function in patients with OS-ASD.

Biologically inspired transport of solid spherical nanoparticles in an electrically-conducting viscoelastic fluid with heat transfer

Bio-inspired pumping systems exploit a variety of mechanisms including peristalsis to achieve more efficient propulsion. Non-conducting, uniformly dispersed, spherical nanosized solid particles suspended in viscoelastic medium forms a complex working matrix. Electromagnetic pumping systems often employ complex working fluids. A simulation of combined electromagnetic bio-inspired propulsion is observed in the present article. Currents formation has increasingly more applications in mechanical and medical industry. A mathematical study is conducted for MHD pumping of a bi-phase nanofluid coupled with heat transfer in a planar channel. Two-phase model is employed to separately identity the effects of solid nanoparticles. Base fluid employs Jeffery?s model to address viscoelastic characteristics. The model is simplified using long wavelength and creeping flow approximations. The formulation is taken to wave frame and non-dimensionalise the equations. The resulting boundary value problem is solved analytically, and exact expressions are derived for the fluid velocity, particulate velocity, fluid-particle temperature, fluid and particulate volumetric flow rates, axial pressure gradient and pressure rise. The influence of volume fraction density, Prandtl number, Hartmann number, Eckert number, and relaxation time on flow and thermal characteristics is evaluated in detail. The axial flow is accelerated with increasing relaxation time and greater volume fraction whereas it is decelerated with greater Hartmann number. Both fluid and particulate temperature are increased with increment in Eckert and Prandtl numbers, whereas it is reduced when the volume fraction density increases. With increasing Hartmann number pressure rise is reduced

Muscle alterations induced by electrostimulation are lower at short quadriceps femoris length

This study aimed at determining through MRI investigations, force and soreness assessments whether the modulation of muscle length is a relevant strategy for minimising neuromuscular electrical stimulation (NMES)-induced muscle damage in young healthy participants. Comparison of 2 NMES sessions (40 isometric electrically-evoked contractions of the knee extensors) was randomly performed on 1 knee flexed at 50° (short muscle length) and the contralateral at 100° (long muscle length) in a single group of healthy participants. Indirect markers of muscle damage including changes in maximal voluntary isometriccontraction (MVC) force, muscle volume and transverse relaxation time (T2) were measured before, 2days (D2), 4days (D4) and 7days (D7) after the NMES sessions in each limb of the ten participants. Although stimulation intensity was similar during the NMES session on both limbs, significantly lower force production was recorded at long muscle length (peak at 30 ± 5% MVC force) as compared to short muscle length (peak at 61 ± 12% MVC force). In the following days, MVC force at long muscle length was decreased from D2 to D7, whereas no significant changeoccurred at short muscle length. Increases in muscle volume and T2 were found at each time point in stimulated muscles at long muscle length, whereas no change was found at short muscle length. For the same stimulation intensity, NMES-induced isometric contractions generate higher knee extension force output and result in lower muscle tissues alterations that could be related to a lower intramuscular shear strain when exercise is performed at short muscle length.

The Conundrum of Relaxation Volumes in First-Principles Calculations of Charged Defects in UO2

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material.

Structural relaxation of nanocrystalline PdAu alloy: Probing the spectrum of potential barriers

A commonality between nanocrystalline metals and metallic glasses is their dependence of structure and properties upon preparation history and postprocessing. Depending on preparation conditions, stored excess enthalpy and volume—relative to the crystalline ground state—can vary significantly. Annealing of material states of elevated enthalpy or volume induces structural relaxation and concomitant depletion of excess energy and volume. We analyzed the kinetics of volume relaxation in nanocrystalline PdAu alloys by partitioning the overall process into a set of independent and parallel reactions for arbitrary time-temperature protocols. The obtained spectra of kinetic parameters imply a complex relaxation behavior that violates time-temperature superposition and time aging-time superposition. The analysis will enable to reconstruct the effective energy landscape underlying the relaxation dynamics.

Longitudinal assessment of 1H-MRS (GABA and Glx) and TMS measures of cortical inhibition and facilitation in the sensorimotor cortex.

The purpose of the present study was to investigate the long-term stability of water-referenced GABA and Glx neurometabolite concentrations in the sensorimotor cortex using MRS and to assess the long-term stability of GABA- and glutamate-related intracortical excitability using transcranial magnetic stimulation (TMS). Healthy individuals underwent two sessions of MRS and TMS at a 3-month interval. A MEGA-PRESS sequence was used at 3T to acquire MRS signals in the sensorimotor cortex. Metabolites were quantified by basis spectra fitting and metabolite concentrations were derived using unsuppressed water reference scans accounting for relaxation and partial volume effects. TMS was performed using published standards. After performing stability and reliability analyses for MRS and TMS, reliable change indexes were computed for all measures with a statistically significant test-retest correlation. No significant effect of time was found for GABA, Glx and TMS measures. There was an excellent ICC and a strong correlation across time for GABA and Glx. Analysis of TMS measure stability revealed an excellent ICC for rMT CSP and %MSO and a fair ICC for 2ms SICI. There was no significant correlation between MRS and TMS measures at any time point. This study shows that MRS-GABA and MRS-Glx of the sensorimotor cortex have good stability over a 3-month period, with variability across time comparable to that reported in other brain areas. While resting motor threshold, %MSO and CSP were found to be stable and reliable, other TMS measures had greater variability and lesser reliability.

Non-glide effects and dislocation core fields in BCC metals

A hallmark of low-temperature plasticity in body-centered cubic (BCC) metals is its departure from Schmid’s law. One aspect is that non-glide stresses, which do not produce any driving force on the dislocations, may affect the yield stress. We show here that this effect is due to a variation of the relaxation volume of the 1/2langle 111rangle screw dislocations during glide. We predict quantitatively non-glide effects by modeling the dislocation core as an Eshelby inclusion, which couples elastically to the applied stress. This model explains the physical origin of the generalized yield criterion classically used to include non-Schmid effects in constitutive models of BCC plasticity. We use first-principles calculations to properly account for dislocation cores and use tungsten as a reference BCC metal. However, the methodology developed here applies to other BCC metals, other energy models and other solids showing non-glide effects.

A constitutive model of the solid propellants considering the interface strength and dewetting

The solid propellant as a typical viscoelastic composite, consists of oxidizer particles, fuel particles, and polymeric binders. After experiencing the service conditions under complex stress states and temperature cycles, solid propellants would be damaged by the interface dissociation between polymeric binders and oxidizer particles, termed dewetting. The dewetting behaviors would cause combustion instability of solid propellants, and might lead to failure of the rocket motor. To evaluate the health of solid propellants, we introduced a parameter of the normalized crack length to describe the interface dissociation, and developed a physics-based constitutive model. To predict the thermal-mechanical response of solid propellants under complex loading conditions, this model considers the interface strength, the volume relaxation of voids, and the viscoelasticity of polymeric binders. We programmed this model into the finite element software, validated the model by comparing with the experiments, and analyzed the responses at various loading conditions. Towards applications, we modeled the dumbbell-shaped samples in tensile tests and the cylindrical grain under impact loading, and analyzed the damage field. Overall, the developed model can be used to predict the thermal-mechanical responses of solid propellants in the structure level, and to evaluate the state of a solid rocket motor under service.

Effect of filler content on the stress relaxation behavior of fly ash/polyurea composites

In this study, fly ash/polyurea (FA/PU) composites of various fly ash volume fractions were fabricated. The time-domain stress relaxation behaviors of pure polyurea and the FA/PU composites were measured by dynamic mechanical analysis (DMA) in tension mode at various temperatures. Both temperature and volume fraction of fly ash can affect the segmental motion of polyurea as well as the interaction between fly ash hollow spheres and polyurea matrix, which constitutes the intrinsic mechanism of stress relaxation. Master curves of relaxation modulus were constructed and compared for PU and FA/PU composites. A model was proposed to relate the relaxation modulus and volume fraction of fillers based on three-parameter fractional derivative model and Mori-Tanaka model. The numerical predictions obtained from the model are found to be in good agreement with the experimental results.

Effect of Moisture Sorption on Free Volume and Relaxation of Spray Dried Dispersions: Relation to Drug Recrystallization

The effect of vapor sorption on the free volume of drug-polymer spray-dried dispersions (SDDs) was investigated, along with the crystallization propensity of drug molecules in SDDs after exposure to humidity. Subsequently, the correlation of free volume change and relaxation time with drug recrystallization was examined. Four polymers, including polyvinylpyrrolidone, polyvinylpyrrolidone vinyl acetate copolymer, hydroxypropyl cellulose, and hydroxypropyl methylcellulose acetate succinate, and 2 drugs (indomethacin and ketoconazole) were selected for preparing SDDs. Free volume data of the exposed SDDs were obtained with positron annihilation lifetime spectroscopy, while the relaxation time was measured using a TA rheometer. Additionally, the crystallization propensity of active pharmaceutical ingredients (APIs) in the exposed SDDs was assessed using both polarized light microscopy and powder X-ray diffraction, followed by relating API crystallization inclination with expansion of holes and relaxation time. Finally, Cohen and Turnbull molecular transport model, along with its extensions by Vrentas and Duda, was qualitatively utilized for interpreting the recrystallization propensity of API molecules. In conclusion, API recrystallization is closely related to free volume change upon moisture sorption and relaxation time, but system dependent; overall, drug-hydroxypropyl methylcellulose acetate succinate SDDs appear physically stable against recrystallization due to less increase in free volume.

Fe-family of superconductors: Influence of Ni dopant on the superconductivity in BaFe2As2 crystal and the relaxation volume

ABSTRACTThe doped iron arsenides present outstanding properties, one of them is an unconventional superconductivity, an unusual coexistence of superconductivity and magnetism. We calculated the electronic structure of the pure and Ni-doped BaF e2As2 by the embedded cluster method at the electron correlation level; the latter is calculated through the second- order Møller Plesset perturbation theory. For the doped clusters, we calculated the relaxation of the first and second neighbors of the impurity by optimizing their positions in the cluster. The total electronic density is analyzed through natural bond orbitals and the population of each atomic orbital (basis function) is determined; the robustness of this determination is tested comparing results obtained for the unrelaxed and relaxed cluster. The orbital population analysis uncovers some properties of magnetism and superconductivity in BaFe2As2. From our results, linear elasticity allows us to estimate the relaxation volume of Ni impurity.

Thermodynamics and kinetics of intrinsic point defects in plutonium dioxides

Thermodynamic and kinetic properties of intrinsic point defects in plutonium dioxides (PuO2), including their formation, migration, and effects on oxygen (O) self-diffusion, are systematically studied using atomic simulations in this work. Coulomb interactions among charged defects and ions are found play a key factor in determining the energetics and structures of defects in PuO2, where point defects are energetically prefer to binding together forming defect pairs and less relaxation volumes are introduced by these bound point defects. Further calculations of defect migration properties reveal different migration mechanisms of O and Pu point defects, where O point defects prefer to migrate along [100] direction with an energy barrier of 0.31 eV by the vacancy mechanism, while Pu point defects prefer to migrate along [100] direction with an energy barrier of 2.57 eV by the interstitial mechanism. This confirms the cation sublattice is much more stable than the anion sublattice in the fluorite structure PuO2. Given the dominance of O defects in PuO2, their effects on O self-diffusion are investigated. Both vacancies and interstitials could significantly enhance O self-diffusivity. Under the Meyer-Neldel rule, activation energy and pre-exponential factors of O diffusion show a dependence of point defect concentrations. Specifically, internal stresses introduced by defects are found responsible for the different dependency of the activation energy on the vacancy and interstitial concentrations respectively. Finally, an empirical equation is derived to connect the point defect concentration, i.e. O/Pu ratio, and activation energy of O self-diffusion in PuO2.

Relaxation volumes of microscopic and mesoscopic irradiation-induced defects in tungsten

The low-energy structures of irradiation-induced defects in materials have been studied extensively over several decades, as these determine the available modes by which a defect can diffuse or relax, and how the microstructure of an irradiated material evolves as a function of temperature and time. Consequently, many studies concern the relative energies of possible defect structures, and empirical potentials are commonly fitted to or evaluated with respect to these. But recently [S. L. Dudarev et al., Nucl. Fusion 58, 126002 (2018)], we have shown that other parameters of defects not directly related to defect energies, namely, their elastic dipole tensors and relaxation volumes, determine the stresses, strains, and swelling of reactor components under irradiation. These elastic properties of defects have received comparatively little attention. In this study, we compute relaxation volumes of irradiation-induced defects in tungsten using empirical potentials and compare to density functional theory results. Different empirical potentials give different results, but some clear potential-independent trends can be identified. We show that the relaxation volume of a small defect cluster can be predicted to within 10% from its point-defect count. For larger defect clusters, we provide empirical fits as a function of defect cluster size. We demonstrate that the relaxation volume associated with a single primary-damage cascade can be estimated from the primary knock-on atom energy. We conclude that while annihilation of defects invariably reduces the total relaxation volume of the cascade debris, there is still no conclusive verdict about whether coalescence of defects reduces or increases the total relaxation volume.

Control of flow and suppression of separation for Couette-Poiseuille hydrodynamics of ferrofluids using tunable magnetic fields

The present study reports unidirectional laminar flow of ferrofluids between parallel plate channels under the action of a homogeneous magnetic field. The applied field is found to uniquely modify the local velocity distribution and to reduce the effect of both adverse and favorable pressure gradients. We find that it is possible to control the flow separation by varying the magnetic field strength within a certain range of adverse pressure gradients. We also discuss the effect of the magnetization relaxation time scale and volume fraction of the solid magnetic particles on the flow phenomenon. Finally, we propose an analytical solution which provides closed form expressions for velocity and spin field that are in good agreement with the numerically obtained results.

Spatial difference can occur between activated and damaged muscle areas following electrically-induced isometric contractions.

T2 mapping combined to image registration and statistical parametric mapping analysis is a suitable methodology to accurately localize and compare the extent of both activated and damaged muscle areas. Activated muscle areas following electrically-induced isometric contractions are superficial, but damaged regions are muscle specific and can be related to the muscle morphology and/or the relative spatial position within a muscle group leading to potential intramuscular muscle shear strain. Tissues other than active skeletal muscle fibres can be altered during unaccustomed neuromuscular electrical stimulation-induced isometric contractions. Skeletal muscle isometric contractions induced by neuromuscular electrical stimulation (NMES) exercise can generate damage within activated muscles. This study aimed at comparing the localization and the extent of NMES-activated muscle areas and induced damage regions using magnetic resonance imaging. Thirteen healthy subjects performed a single bout of NMES-induced isometric contractions known to induce a decrease in maximal voluntary isometric contraction (MVC) and increase in muscle volume and transverse relaxation time (T2 ). All the parameters were measured before, immediately after (POST), 7days (D7), 14days (D14) and 21days (D21) after the NMES session. Spatial normalization of T2 maps were performed to compare the localization of muscle activation areas and damaged muscle regions from statistical mapping analyses. A significant decrease in MVC was found at POST (-26±9%) and in delayed time at D7 (-20±6%) and D14 (-12±5%). Although muscle activation was statistically detected through T2 increase at POST in superficial parts of the two muscles located beneath the stimulation electrodes (i.e. vastus lateralis and vastus medialis), alterations quantified in a delayed time from increased T2 were mainly located in the deep muscle region of the vastus lateralis (+57±24% of mean T2 ) and superficial area of the vastus medialis (+24±16% of mean T2 ) at D7 and were still observed in whole muscle at D21. The discrepancy between activated and damaged areas in the vastus lateralis implies that tissues other than active skeletal muscle fibres were altered during unaccustomed NMES-induced isomeric contractions.

Effect of stress on vacancy formation and migration in body-centered-cubic metals

Vacancy formation and migration control self-diffusion in pure crystalline materials, whereas irradiation produces high concentrations of vacancy and self-interstitial atom defects, exceeding by many orders of magnitude the thermal equilibrium concentrations. The defects themselves, and the extended dislocation microstructure formed under irradiation, generate strongly spatially fluctuating strain and stress fields. These fields alter the local formation and migration enthalpies of defects, and give rise to the anisotropy of diffusion even if in the absence of stress the diffusion tensor is isotropic. We have performed ab initio calculations of formation and migration energies of vacancies in all the commonly occurring body-centered-cubic (bcc) metals, including alkaline, alkaline-earth and transition metals, and computed elastic dipole and relaxation volume tensors of vacancies at the equilibrium lattice positions and along the vacancy migration pathways. We find that in all the bcc metals the dipole tensor of a migrating vacancy at a saddle point exhibits an anticrowdion character. Applied external stresses or the local stresses generated by dislocations may enhance or suppress anisotropic diffusion by altering the energy barriers with respect to the direction of migration of a defect.