Characteristic Relaxation Time Research Articles

Assessing Asphalt Binder Aging with 1H Spin-lattice NMR Relaxometry: A Comparative Study of Temperature and UV Radiation Effects

Objective: Asphalt binder, a byproduct of the crude-oil refining process, is commonly used in road construction due to its favorable response to physical stresses. However, it is also susceptible to environmental aging, including temperature variations and ultraviolet (UV) radiation. Standard laboratory methods for studying asphalt binder aging primarily focus on factors like temperature, pressure, and oxygen, often neglecting the impact of UV radiation. This study employs 1H spin-lattice nuclear magnetic resonance (NMR) relaxometry to investigate the effects of UV radiation on asphalt binder aging compared to temperature-induced aging. Methods: Primary relaxation times and ratios were derived from a bi-exponential fit of the experimental relaxation decay induced by a 200-MHz NMR magnet. NMR relaxometry analysis revealed that 90% (0.9 primary ratio) of the protons in unaged asphalt binders exhibited a characteristic primary spin-lattice relaxation time (T1) of approximately 470ms. Results: Cold exposure did not significantly alter these parameters. Mimicked heat aging, which considered only temperature and duration, resulted in slight increases in both the primary relaxation time and ratio. In contrast, after 72h of UV exposure, binders showed variable T1 values at a significantly decreased primary ratio. This variability was summarized with the new performance parameter, the Herbaw. Further testing showed that standardized heat aging, which considered sample size and pressure, reduced both the primary relaxation time and ratio when compared to the mimicked aging. Conclusion: These results demonstrate that UV radiation induces significant structural changes in asphalt binder, which can be effectively detected using spin-lattice NMR relaxometry. The Herbaw performance parameter was successful in describing the variability of the UV aging which greatly impacted the primary ratio, indicating oxidative stress. The Herbaw Parameter was not as successful in describing heat aging since only slight changes in the primary relaxation time were detected. While the hydrogen environments were not significantly impacted, temperature-based aging may depend on multiple mechanisms.

Diagnosis of vascular lesions in the brain – comparing T_2 and FLAIR sequences

ABSTRACT Introduction: Vascular brain lesions refer to brain damage caused by problems with blood vessels and can include various disorders such as stroke, aneurysms, blood clots, and others. Because different brain tissues have different characteristic relaxation times, and the FLAIR sequence can also be used to deal with different tissue stresses. the sequence provides high intensity while the FLAIR sequence achieves complete saturation of cerebrospinal fluid. The objectives of this study are to compare the visualization of vascular lesions using and the FLAIR sequence and to determine which sequence provides a better depiction and additional information about vascular lesions. Research Methods: The study was designed as a retrospective descriptive study, conducted at the Private Healthcare Institution 'Medical Center' in Travnik using an MRI machine: MRI Siemens Avanto A + Tim + Dot System, with a strength of 1.5 T. The study involved 50 patients diagnosed with vascular lesions. Results: In the age group from 50 to 59 years, 7 patients with FAZEKAS 0 – 1 (46.66%), 4 patients with FAZEKAS 1 (26.66%), 3 patients with FAZEKAS 2 (20%) and one patient with FAZEKAS 3 (6.66%) were diagnosed. In the age group from 60 to 69 years, 6 patients with FAZEKAS 0 – 1 (28.57%), 6 patients with FAZEKAS 1 (28.57%), 6 patients with FAZEKAS 2 and 3 patients with FAZEKAS 3 (14.28%) were diagnosed. In the 70 to 79 age group, 2 patients were diagnosed with FAZEKAS 0 – 1 (14.28%), 5 patients were diagnosed with FAZEKAS 1 (35.71%), 5 patients were diagnosed with FAZEKAS 2 (35.71%) and 2 patients were diagnosed with FAZEKAS 3 (14.28%). Discussion: Several studies have been conducted to evaluate the sensitivity of and FLAIR sequences in the diagnosis of vascular lesions in the brain. In the mentioned studies, it was found that the FLAIR sequence possesses superior capabilities in identifying and better displaying vascular lesions compared to healthy brain tissue. After measuring lesions in all 50 patients enrolled in the study, they were divided by size into three different groups: Lesions smaller than 10 millimeters; Lesions larger than 10 millimeters and smaller than 20 millimeters; Lesions larger than 20 millimeters. After statistical processing of the examined sample within this study, it was found that the FLAIR sequence provides a better representation of vascular lesions in the brain compared to the sequence in all three different groups, and it is concluded that the differences are statistically significant in favor of the FLAIR sequence in all three observed lesion sizes. The prognostic value of FLAIR vascular hyperintensity still needs to be investigated. Future studies will determine in which settings the presence of FLAIR vascular hyperintensity can be used as valuable information for the clinician. FLAIR vascular hyperintensities indicate a risk of persistent vascular stenosis or occlusion, associated with an increased risk of future stroke. They also help identify patients with favorable collateral blood flow, who could benefit from aggressive revascularization therapy. Conclusion: Statistically, there was a difference in the better presentation of vascular lesions on the FLAIR sequence compared to the sequence. In some cases, a sequence may be sufficient but in most cases a FLAIR sequence is preferable. Although the noise level is higher on the FLAIR sequence compared to the standard sequence, the FLAIR sequence is more useful in detecting vascular lesions.

On the Interrelationship of Electrical and Hydraulic Conductivities in Carbonate Rocks

Carbonate reservoir exhibits "low-resistivity and (ultra) low-permeability" anomaly due to complex pore structure. It means that hydraulic conductivity, closely related to permeability (K), is not simply equivalent to electrical conductivity in this case. As a result, the conventionally well-established permeability-resistivity relationship does not apply to carbonate reservoirs any longer, and this poses a big challenge for the interpretation and evaluation of carbonate reservoirs. In this study, we found that the relationship between permeability and formation factor is non-monotonic and scattering through direct-current (DC) resistivity and nuclear magnetic resonance (NMR) experiments, and this means that the hydraulic conductivity is not well correlated with the electrical conductivity in carbonate rocks as one expects. There are the two factors, the volume fraction ratio of macro- and micro-pores, and the number of dolomitization-formed micropores, predominantly control hydraulic conductivity and electric conductivity, respectively. Moreover, the non-uniform distribution of intercrystalline pore networks will significantly reduce hydraulic conductivity. To more accurately estimate the permeability of carbonate rocks, the new permeability model based on NMR spectra rather than formation factor has been proposed. In this model, the characteristic relaxation time () was defined, which is associated with the macropores that contribute most significantly to permeability and can be used to distinguish between macro- and micro- pores. This model demonstrates a minimal average absolute logarithmic deviation () and exhibits generalizability across various carbonate rocks. The study addresses the mechanism analysis of "low-resistivity and (ultra) low-permeability" phenomenon in carbonate reservoir, and has potential to facilitate the interpretation and evaluation of carbonate reservoir.

Unusual spin dynamics in the van der Waals antiferromagnet FeGa2S4

Spin dynamics in the van der Waals antiferromagnet FeGa2S4with triangular lattices are investigated using magnetometry, neutron scattering, and muon spin relaxation measurements. The characteristic spin relaxation time is thoroughly clarified over thirteen orders of magnitude. Although the temperature dependence of DC and AC susceptibilities recalls a conventional spin-glass transition, nonlinear susceptibilities showing no divergences at the anomalous temperature,T∗=16.87(7) K, deny that and instead hint at other mechanisms. Elastic neutron scattering together with previously measured muon results depict a slowly fluctuated (∼10-5 s) spin state aboveT∗. In juxtaposing the underlying simplest structure among frustrated magnets with an intricate hierarchy of time scales, FeGa2S4can be a playground for studying temporal spin correlations in the two-dimensional limit.

A novel mechanism to accelerate stress relaxation of toughened blends: Stress-induced core-shell morphological reconstruction and its application in thermoforming and dimensional stabilization

Improving thermoforming efficiency and dimensional stability in thermoplastic products is a common challenge. This study investigates toughened polyamide 612 (PA612) blends made by adding maleic anhydride-functionalized SEBS (mSEBS) elastomers, along with high-density polyethylene (HDPE), polyphenylene Oxide (PPO), and polyphenylene Sulfide (PPS). Analyses showed that mSEBS forms a core-shell structure with HDPE or PPO and a sea-island structure with PPS in the PA612 matrix. The study uniquely examines how these structures affect stress relaxation. The results, modeled by a steady-state creep model, revealed that the core-shell structures reduced the characteristic relaxation time (λ) by over three orders of magnitude compared to PA612/mSEBS blends. Additionally, PA612/mSEBS/HDPE blends were more temperature-sensitive, reducing λ by six orders of magnitude compared to PA612/mSEBS/PPO blends. Further analysis showed that stress-induced core-shell morphological reconstruction (SCMR) significantly improved stress relaxation by promoting extensive plastic deformation and energy dissipation. These toughened PA612 blends exhibited excellent thermoforming efficiency and dimensional stability. A 3D finite element model confirmed SCMR as an effective strategy for stress relaxation, providing valuable insights for designing toughened blends with superior processing efficiency and stability.

Quantifying relaxation time constants in MoS2xSe2(1-x) alloys: Impact of Stoichiometry and Si/SiO2 interference

Despite a decade of extensive research on two-dimensional transition metal dichalcogenides, there are still some gaps in our understanding of their remarkable properties, limiting their potential applications. One such gap relates to the properties of these alloys, which offer tunable band gaps and can serve as the basis for various optoelectronic and nonlinear optical devices. A comprehensive understanding of the ultrafast charge carrier excitation and relaxation dynamics as well as their characteristic relaxation time constants, is crucial for the further use of these materials in specific applications. In this work, we experimentally investigate the dynamics of ultrafast relaxation processes in monolayer MoS2xSe2(1-x) alloys on a Si/SiO2 substrate using time-resolved spectroscopy. We determine the characteristic relaxation time constants for structures with different stoichiometric compositions and refine these constants by accounting for interference effects arising from the substrate.

Extracting dynamical maps of non-Markovian open quantum systems.

The most general description of quantum evolution up to a time τ is a completely positive tracing preserving map known as a dynamical mapΛ̂(τ). Here, we consider Λ̂(τ) arising from suddenly coupling a system to one or more thermal baths with a strength that is neither weak nor strong. Given no clear separation of characteristic system/bath time scales, Λ̂(τ) is generically expected to be non-Markovian; however, we do assume the ensuing dynamics has a unique steady state, implying the baths possess a finite memory time τm. By combining several techniques within a tensor network framework, we directly and accurately extract Λ̂(τ) for a small number of interacting fermionic modes coupled to infinite non-interacting Fermi baths. First, we use an orthogonal polynomial mapping and thermofield doubling to arrive at a purified chain representation of the baths whose length directly equates to a time over which the dynamics of the infinite baths is faithfully captured. Second, we employ the Choi-Jamiolkowski isomorphism so that Λ̂(τ) can be fully reconstructed from a single pure state calculation of the unitary dynamics of the system, bath and their replica auxiliary modes up to time τ. From Λ̂(τ), we also compute the time local propagator L̂(τ). By examining the convergence with τ of the instantaneous fixed points of these objects, we establish their respective memory times τmΛ and τmL. Beyond these times, the propagator L̂(τ) and dynamical map Λ̂(τ) accurately describe all the subsequent long-time relaxation dynamics up to stationarity. These timescales form a hierarchy τmL≤τmΛ≤τre, where τre is a characteristic relaxation time of the dynamics. Our numerical examples of interacting spinless Fermi chains and the single impurity Anderson model demonstrate regimes where τre ≫ τm, where our approach can offer a significant speedup in determining the stationary state compared to directly simulating the long-time limit. Our results also show that having access to Λ̂(τ) affords a number of insightful analyses of the open system thus far not commonly exploited.

Low-field MRI study of the 1H distribution and migration in porous sediments during natural gas conversion from methane hydrate

Gas hydrates, crystalline compounds formed from water and guest molecules like methane, are gaining attention as a promising clean energy source. While research has extensively focused on the extraction and transport of natural gas hydrates from offshore marine sediments, the dynamic processes in the porous sediments remain under explored. This study employs low-field Magnetic Resonance Imaging (MRI) to investigate the in-situ formation and dissociation of methane hydrate within a partially saturated stack of glass beads. By integrating advanced MRI techniques, including 1D dhk SPRITE, bulk T1 distribution, and 2D spatial T2 imaging, this research provides a comprehensive analysis of fluid distribution and migration during phase transitions in the porous sediments. Three key findings emerged from the study. First, SE-SPI measurements successfully quantified hydrate and residual water saturation, aligning with previous X-ray CT and high-field MRI studies and validating low-field MRI for hydrate research. Secondly, this study introduced a volumetric approach to T1 and T2 measurements, distinguishing between free and bound water, with characteristic relaxation times for each, confirming the technique's capability to differentiate types of residual water. Finally, spatially resolved T2 relaxation time distributions revealed vertical fluid migration within the pore spaces, identifying an equilibrium zone where inflow and outflow rates were balanced. Overall, this study underscores the effectiveness of low-field MRI as a noninvasive tool for analyzing hydrate-bearing sediments. The findings lay the groundwork for further exploration of hydrate in porous sediments, enhancing our understanding of gas production dynamics in hydrate-rich environments.

Investigation of the strain rate and stretch level dependent behavior of elastomeric nanocomposites in complex uniaxial tests under finite strains

The mechanical behavior of elastomeric nanocomposites in experiments with nested stress-strain cycles and in cyclic tests with increasing strain amplitude was considered. In the proposed testing procedures, long time stops at each stage of the change in loading direction were of great importance. This revealed two significant features of the behavior of elastomeric nanocomposites that have received little attention. It was shown that material softening (the Mullins effect) should be considered not only in the elastic part of the Cauchy stress tensor, but also in its dissipative part. The second peculiarity was the difference between the characteristic relaxation time at loading and the characteristic relaxation time at unloading observed in the experiments.This paper focuses on the behavior of highly-filled elastomeric materials based on different matrices (styrene-butadiene rubber (SBR) and nitrile-butadiene rubber (NBR)) and with different concentrations of carbon black (CB) or a combination of two fillers (CB and purified multi-walled carbon nanotubes (MWCNTs)).A mathematical model of the viscoelastic behavior of elastomeric nanocomposites under finite strains is proposed. It takes into account the peculiarities of the behavior of highly filled elastomers observed in the experiments. The specificity of the model consists in a new variant of the form of the free energy potential. It is shown that the new model satisfies the thermodynamic inequality, which is a consequence of the first law of thermodynamics and the second law in the form of the Clausius-Duhem inequality. A good agreement between theoretical calculations and experimental data was obtained.

Chemically Degradable Vitrimers Based on Divanillin Imine Diepoxy Monomer and Aliphatic Diamines for Enhanced Carbon Fiber Composite Applications

This study presents the development of a diglycidyl monomer containing two imine groups that can act as dynamic and reversible bonds. During the curing of the monomer with two different amine hardeners, we confirmed the formation of new imine groups due to the transamination reaction between the imine groups of the diepoxy monomer with the amine groups of the hardener. The effect of this structural change was observed in the stress relaxation behavior, resulting in the overlapping of two different relaxation modes. The analytical modelling was able to extract two distinct characteristic relaxation times using a double-element Maxwell model. A second characterization of the stress relaxation process by frequency sweep experiments was performed to corroborate the results obtained, confirming speedy stress relaxation. Acid-catalyzed hydrolysis was performed on the studied materials, demonstrating the complete degradation of the network. We finally confirmed that the synthesized diepoxy compound is suitable for preparing carbon-fiber-reinforced composite materials, demonstrating easy fiber impregnation, fast reshaping, and especially a total degradation of the polymer matrix that allows for the recovery of the carbon fibers in mild conditions. This epoxy–amine system is an excellent candidate for overcoming the traditional limits of thermosets in preparing fiber-reinforced composites.

Synovial fluid does not retard fluid exudation during stress-relaxation of immature bovine cartilage

Interstitial fluid load support (FLS) is a dominant mechanism of lubrication in cartilage, producing a low friction coefficient while enhancing the tissue’s load bearing capabilities. Due to its viscosity, synovial fluid (SF) may retard loss of FLS by slowing the exudation of interstitial fluid from the cartilage. This study tested this hypothesis by comparing the stress-relaxation (SRL) response of immature bovine articular cartilage immersed either in phosphate buffered saline (PBS) or in healthy mature bovine SF, under unconfined compression (fluid exudation across cut lateral tissue boundary) and indentation testing (fluid exudation across articular surface). To investigate the influence of diffusion of SF molecular constituents into cartilage, the effect of incubation time in SF on SRL was also investigated. The SRL response in unconfined compression was not significantly different in PBS versus SF when compared directly (p = 0.98) and had a slope ofm = 1.00 ± 0.04 (R2 = 0.989 ± 0.007). Samples tested in PBS exhibited characteristic relaxation times, τPBS=42.6 ± 5.3 s andτSF = 40.8 ± 4.7 s, that were not significantly different (p = 0.40). Incubation time of 24 h in SF resulted in no significant difference in the SRL response (p = 0.39, m=1.03 ± 0.12; R2=0.983 ± 0.011, andτPBS = 43.4 ± 10.7 s versusτSF = 41.5 ± 4.8 s, p = 0.59). Indentation testing showed some statistically significant, but functionally insignificant, difference in SRL responses in PBS versus SF with a slope ofm = 0.958 ± 0.060 (R2 = 0.957 ± 0.020, p = 0.029, andτPBS = 16.9 ± 2.6 s versusτSF = 19.4 ± 3.3 s, p = 0.073). Based on these results, we reject the hypothesis that healthy SF can retard the loss of FLS in cartilage due to its viscosity.

T1 Mapping of the Abdomen, From the AJR "How We Do It" Special Series.

By exploiting different tissues' characteristic T1 relaxation times, T1-weighted images help distinguish normal and abnormal tissues, aiding assessment of diffuse and local pathologies. However, such images do not provide quantitative T1 values. Advances in abdominal MRI techniques have enabled measurement of abdominal organs' T1 relaxation times, which can be used to create color-coded quantitative maps. T1 mapping is sensitive to tissue microenvironments including inflammation and fibrosis and has received substantial interest for noninvasive imaging of abdominal organ pathology. In particular, quantitative mapping provides a powerful tool for evaluation of diffuse disease by making apparent changes in T1 occurring across organs that may otherwise be difficult to identify. Quantitative measurement also facilitates sensitive monitoring of longitudinal T1 changes. Increased T1 in liver helps to predict parenchymal fibro-inflammation, in pancreas is associated with reduced exocrine function from chronic or autoimmune pancreatitis, and in kidney is associated with impaired renal function and aids diagnosis of chronic kidney disease. In this review, we describe the acquisition, postprocessing, and analysis of T1 maps in the abdomen and explore applications in liver, spleen, pancreas, and kidney. We highlight practical aspects of implementation and standardization, technical pitfalls and confounding factors, and areas of likely greatest clinical impact.

Thermal Relaxation in Janus Transition Metal Dichalcogenide Bilayers.

In this work, we employ molecular dynamics simulations with semi-empirical interatomic potentials to explore heat dissipation in Janus transition metal dichalcogenides (JTMDs). The middle atomic layer is composed of either molybdenum (Mo) or tungsten (W) atoms, and the top and bottom atomic layers consist of sulfur (S) and selenium (Se) atoms, respectively. Various nanomaterials have been investigated, including both pristine JTMDs and nanostructures incorporating inner triangular regions with a composition distinct from the outer bulk material. At the beginning of our simulations, a temperature gradient across the system is imposed by heating the central region to a high temperature while the surrounding area remains at room temperature. Once a steady state is reached, characterized by a constant energy flux, the temperature control in the central region is switched off. The heat attenuation is investigated by monitoring the characteristic relaxation time (τav) of the local temperature at the central region toward thermal equilibrium. We find that SMoSe JTMDs exhibit thermal attenuation similar to conventional TMDs (τav~10-15 ps). On the contrary, SWSe JTMDs feature relaxation times up to two times as high (τav~14-28 ps). Forming triangular lateral heterostructures in their surfaces leads to a significant slowdown in heat attenuation by up to about an order of magnitude (τav~100 ps).

Small polaron hopping and tunneling mechanisms in Ba0.9La0.1Fe12O19 hexaferrite ceramic at low temperatures

The correlation between dielectric relaxation and conduction mechanisms in the Ba0.9La0.1Fe12O19 ceramic hexaferrite, at temperatures below 100 K was investigated. The sample was prepared using the conventional solid-state reaction method. X-ray diffraction analysis indicated that the sample is composed of a hexagonal BaFe12O19 phase (lattice parameters a = 5.8778(6) Å and c = 23.170(3) Å) and a small proportion of hematite (α-Fe2O3). The average grain size is 1.48 ± 0.4 μm, with irregular hexagonal and agglomerated grains. The magnetic hysteresis curve shows characteristics typical of ferro and ferrimagnetic materials, with an effective magnetic anisotropy coefficient of 0.960×106 emu/cm3 and an anisotropy field of 1.587×104 Oe. A dielectric relaxation process is observed between 50 and 100 K, with an activation energy of 60.4 ± 0.4 meV and a characteristic relaxation time of 2.09 ×10−10 s, mainly attributed to the electrical behavior of the grains. Two conduction mechanisms were identified: small polaron tunneling (high temperatures and low frequencies) and correlated barrier hopping of electrons (low temperatures and high frequencies). The activation energy for free polaron formation, according to the Komine-Iguchi hopping polarization model, was 7.12 ± 0.02 meV. In the dielectric modulus relaxation process between 30 and 54 K, the activation energy was 40.8 ± 0.9 meV, with a characteristic relaxation time of 2.41×10−10 s. The small polaron conductivity in the grain region was 67.52 meV and 54.0 meV in the grain boundary region. The results highlight that the polarization and conduction mechanisms in this ceramic at low temperatures are dominated by small polaron hopping and tunneling, and emphasize the influence of La3+ doping on the structural, magnetic, and electrical properties of barium hexaferrite.

Asphalt-Binder Mixtures Evaluated by T1 NMR Relaxometry

Asphalt pavements make up a majority of the essential transportation systems in the US. Asphalt mixtures age and degrade over time, reducing the pavement performance. Pavement performance critically depends on the aging of asphalt binder. The aging of asphalt binder during construction is traditionally modeled by rolling thin film oven (RTFO) testing, while aging during service life is modeled by pressure aging vessel (PAV) testing. Comparing these models to the aging of binders in actual pavements is limited because, to be used for current testing, binders must be separated from the pavement’s aggregate by solvent extraction. Solvent extraction will, at least in part, compromise the structural integrity of asphalt binder samples. Spin-lattice NMR relaxometry has been shown to nondestructively evaluate asphalt properties in situ through the analysis of hydrogen environments. The molecular mobility of hydrogen environments and with it the stiffness of asphalt binder samples can be determined by characteristic T1 relaxation times, indicating the complexity of asphalt-binder aging. In this study, two laboratory-generated asphalt mixtures, a failed field sample, and several laboratory-aged binder samples are compared by NMR relaxometry. NMR relaxometry was found to be able to differentiate between asphalt samples based on their binder percentage. According to the relaxometry findings, the RTFO binder aging compared favorably to the 6% laboratory-mixed sample. The PAV aging, however, did not compare well to the relaxometry results found for the field-aged sample. The amount of aggregate was found to have an influence on the relaxation times of the binder in the mixed samples and an inverse proportionality of the binder content to the primary NMR relaxation time was detected. It is concluded that molecular water present in the pores of the aggregate material gives rise to such a relationship. The findings of this study lay the foundation for nondestructive asphalt performance evaluation by NMR relaxometry.

Unraveling the Roles of Switching and Relaxation Times in Volatile Electrochemical Memristors to Mimic Neuromorphic Dynamical Features

AbstractComputing through ensembles of interacting dynamical elements is the next frontier of the diverse field of neuromorphic computing. Spiking neural networks are one of the possible examples. Computation through dynamics and through time requires the development of novel technologies for devices with rich dynamics. Among the various candidates, the most promising ones are volatile electrochemical memristive systems that switch from high to low resistance state by voltage application and self‐recover the high resistance state after a tunable relaxation time. Such devices can perform a wide variety of computational primitives. However, a clear comprehensive picture of their possible dynamics and their physical interpretation is still missing. In the present manuscript, prototypical electrochemical silver/silicon oxide/platinum (Ag/SiOx/Pt) memristive devices are characterized to identify dynamical aspects, like integrative effects and stochastic switching. Integrative effects are evidenced both in high and low resistance states, associated to wake‐up phase and cumulative switching. All the dynamical aspects are related to characteristic switching times and relaxation times, and with reference to the electrochemical and physical processes involved in device operation. The various dynamical aspects are linked to short‐term memory effects and basic temporal processing functions based on paired‐pulse effects that are relevant for neuromorphic applications.

Characteristic Vibrational and Rotational Relaxation Times for Air Species from First-Principles Calculations

We present molecular-scale computational rotational-vibrational relaxation studies for N2, O2, and NO. Characteristic relaxation times for diatom-diatom and diatom-atom interactions are calculated using direct molecular simulation (DMS), with ab initio potential energy surfaces (PESs) as the sole model input. Below approximately 8000 K our N2−N2, O2−O2, and O2−N2 vibrational relaxation times agree well with the Millikan–White (M&W) correlation, but gradually diverge at higher temperatures. Park’s high-temperature correction produces a relatively steeper temperature rise compared to our estimates. DMS further shows that, with increasing temperature, the gap between vibrational and rotational relaxation times shrinks for all species. At T>30,000K their magnitudes become comparable and a clear distinction between both energy modes becomes meaningless. For other interactions, our DMS results differ substantially from the M&W correlation, both in magnitude and temperature dependence. Our predicted N2−O2 vibrational relaxation times are noticeably shorter due to vibration-vibration transfer. For O2−O we observe minimal temperature dependence. Our O2−N and N2−N predictions follow the M&W temperature trend at values roughly one order of magnitude smaller. For NO−NO, N2−O, NO−N, and NO−O we generate partial data due to currently incomplete PES sets. These first-principles-derived relaxation times are useful for informing relaxation models in gas-kinetic and fluid-dynamics simulations of high-enthalpy flows.

Reprocessable and repairable carbon fiber reinforced vitrimer composites based on thermoreversible dynamic covalent bonding

The emergence of vitrimers breaks through the limitation of traditional carbon fiber reinforced thermoset composites (CFRP) caused by their permanent crosslinking molecular networks. Herein, the dynamic cross-linked network of vitrimer was synthesized from Bisphenol A diglycidyl ether (DGEBA) and glutaric anhydride (GA) under the catalysis of zinc acetylacetonate (Zn(acac)2), and the corresponding carbon fiber reinforced vitrimer composites (CF/Vx) were prepared via an impregnation and hot pressing process. The dynamic covalent adaptable networks (CANs) of vitrimer endow CF/Vx with stress relaxation and creep characteristics. By adjusting the catalyst content, the dynamic performance of CF/V0.05 can be optimized to achieve the highest viscous flow activation energy (60 kJ/mol) and the lowest characteristic relaxation time (1.0 × 10−3 s). The unique dynamic properties enable CF/Vx with reprocessability at high temperature. Typically, the flexural modulus of CF/V0.05 decreased from 87 ± 1.81 GPa at room temperature to 4.62 ± 0.28 GPa at 220 °C, which confirms the feasibility of thermoplastic forming of CF/V0.05 at high temperatures. Based on this, a CF/V0.05-based cap-shaped component was successfully manufactured through a thermoforming process. Additionally, the CF/V0.05 composites also exhibit repairability for interlaminar fractures. The optimal CF/V0.05 can achieve a repair efficiency of 128 % under the hot press conditions of 180 °C, 10 MPa and 1 h. Hence, the reprocessable and repairable CF/Vx composites hold enormous potential in the engineering field.

Pore structure characterization and reservoir quality prediction in deep and ultra-deep tight sandstones by integrating image and NMR logs

Pore structure and fracture determine reservoir quality in deep and ultra-deep tight sandstones, however, the limited availability of core data makes it challenging to describe the complexity of pore-throat structure and characterize the wide range of fractures. To fill this gap, a comprehensive analysis was conducted to evaluate pore structure and fractures from multiple perspectives. Integration of core, thin sections, scanning electron microscopy (SEM), high pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) tests are used to evaluate pore structure and characterize fracture of deep and ultra-deep tight sandstones in the Cretaceous Bashijiqike Formation in the Kuqa Depression. Pore structure and reservoir quality types can be divided into dominantly three types according to characteristics of NMR transverse relaxation time (T2) distribution and HPMI curves. Correlation analysis among HPMI and NMR parameters with petrophysical parameters indicate that maximum pore throat size (rmax), logarithmic mean of T2 (T2gm), irreducible water saturation (Swi), and reservoir quality index (RQI) are the most sensitive parameters for pore structure characterization. The Type Ⅰ pore structure is composed of intergranular and intragranular pores, and NMR T2 spectrum shows the highest magnitude, with the calculated mobile fluid saturation content also being the highest. The Type II and III pore structures are composed of intergranular pores, intragranular dissolved pores and intercrystalline micropores in clay minerals. T2gm of Type II and III are smaller, with the calculated mobile fluid saturation content being the lower.The fracture features and parameters including fracture density, porosity, aperture, and length are analyzed using image logs and core. The pore structure characteristics are quantificationally and qualitatively analyzed by NMR logs. Integration of pore structure and fractures by image logs and NMR logs are used for prediction of high quality reservoir and hydrocarbon productivity. High hydrocarbon productivity is obtained in well intervals with abundant natural fractures, and favorable pore structure will also contribute to high matrix porosity. Therefore, NMR logs and image logs are crucial for evaluating reservoir quality and predicting productivity when calibrated with core and analysis data. The comprehensive analysis integrating core data with geophysical well logs provide important insights for reservoir quality prediction of deep and ultra-deep tight sandstones.

Rheological behaviors of the multi-wall carbon nanotubes/polyethylene composites prepared by a master batch method

In this work, the rheological properties of four multi-wall carbon nanotubes (MWCNTs)/polyethylene (PE) composites containing 1 wt.% (mPEC1), 3 wt.% (mPEC3), 5 wt.% (mPEC5), and 10 wt.% (mPEC10) MWCNTs prepared by the masterbatch method were investigated and compared with those of the pure PE. The rheological investigation includes strain sweeping, frequency sweeping, stress relaxation behavior, yield stress, characteristic relaxation time, temperature effect on linear viscoelastic behavior, and gel behavior. It is mainly found that (1) the MWCNTs-MWCNTs interaction resulting in MWCNTs network structure in the PE matrix and the MWCNTs-PE interfacial interaction increase the storage and loss moduli of the MWCNTs/PE composite in comparison with those of pure PE; (2) the time-temperature equivalence principle is applicable only for pure PE and mPEC1; (3) for gel behavior of MWCNTs/PE composite at 160–180 °C, the φ g value decreases with the increase of temperature and the highest S g value is obtained at 170 °C.