关键词： Volume fraction  

摘要： Despite decades of exploration of the colloidal glass transition, mechanistic explanation of glassy relaxation processes has remained murky. State-of-the-art theoretical models of the colloidal glass transition such as random first order transition theory, active barrier hopping theory, and non-equilibrium self-consistent generalized Langevin theory assert that relaxation reported at volume fractions above the ideal mode coupling theory prediction phi(g,MCT)requires some sort of activated process, and that cooperative motion plays a central role. However, discrepancies between predicted and measured values of phi(g)and ambiguity in the role of cooperative dynamics persist. Underlying both issues is the challenge of conducting deep concentration quenches without flow and the difficulty in accessing particle-scale dynamics. These two challenges have led to widespread use of fitting methods to identify divergence, but mosta prioriassume divergent behavior;and without access to detailed particle dynamics, it is challenging to produce evidence of collective dynamics. We address these limitations by conducting dynamic simulations accompanied by experiments to quench a colloidal liquid into the putative glass by triggering an increase in particle size, and thus volume fraction, at constant particle number density. Quenches are performed from the liquid to final volume fractions 0.56 <=phi <= 0.63. The glass is allowed to age for long times, and relaxation dynamics are monitored throughout the simulation. Overall, correlated motion acts to release dynamics from the glassy plateau - but only over length scales much smaller than a particle size - allowing self-diffusion to re-emerge;self-diffusion then relaxes the glass into an intransient diffusive state, which persists for phi< 0.60. We observe similar relaxation dynamics up to phi= 0.63 before achieving the intransient state. We find that this long-time self-diffusion is short-ranged: analysis of mean-square displacement r

关键词： Carbon nanotube   Aluminum   Molecular dynamics   Elastic modulus   UTS  

摘要： Carbon nanotube (CNT)-reinforced aluminum (Al) composites are being developed to replace the conventional materials due to the enhanced stiffness for more cost-effective engineering applications. In the present study, a qualitative analysis has been conducted on carbon nanotube aluminum (CNT-Al) composites to predict the effect of CNT volume fraction, diameter, and structure (zigzag, armchair) on elastic modulus, shear modulus, bulk modulus, and ultimate tensile strength (UTS) through molecular dynamics simulation. This study shows that the elastic modulus was improved by 105% compared with pure Al, for the CNT-Al composite reinforced with the (4, 4) armchair SWCNT at a volume fraction of 8.79%. The highest value of Young's modulus of 130.37 GPa occurred at the volume fraction of 11.75% for zigzag SWCNT. The CNT-Al composites showed better UTS for the Al matrix reinforced with larger diameter CNT. An enhancement of 31.65% was observed in the UTS of CNT-Al composite from 4.74 to 6.24 GPa with (6, 6) CNT at 0.51% volume fraction. Elastic and bulk moduli were found to improve with a higher volume fraction of CNTs. The CNT-Al composite reinforced with smaller diameter CNT's have better elastic modulus than those reinforced with larger diameter CNT for the same volume fraction.

关键词： protein folding   protein binding   staphylococcal nuclease   statistical mechanical model   real-time NMR  

摘要： Protein conformational changes associated with ligand binding, especially those involving intrinsically disordered proteins, are mediated by tightly coupled intra-and intermolecular events. Such reactions are often discussed in terms of two limiting kinetic mech-anisms, conformational selection (CS), where folding precedes binding, and induced fit (IF), where binding precedes folding. It has been shown that coupled folding/binding reactions can pro-ceed along both CS and IF pathways with the flux ratio depending on conditions such as ligand concentration. However, the struc-tural and energetic basis of such complex reactions remains poorly understood. Therefore, we used experimental, theoretical, and computational approaches to explore structural and energetic as-pects of the coupled-folding/binding reaction of staphylococcal nuclease in the presence of the substrate analog adenosine-3 ',5 '-diphosphate. Optically monitored equilibrium and kinetic data, combined with a statistical mechanical model, gave deeper insight into the relative importance of specific and Coulombic protein- ligand interactions in governing the reaction mechanism. We also investigated structural aspects of the reaction at the residue level using NMR and all-atom replica-permutation molecular dynamics simulations. Both approaches yielded clear evidence for accumula-tion of a transient protein-ligand encounter complex early in the reaction under IF-dominant conditions. Quantitative analysis of the equilibrium/kinetic folding revealed that the ligand-dependent CS-to-IF shift resulted from stabilization of the compact transition state primarily by weakly ligand-dependent Coulombic interactions with smaller contributions from specific binding energies. At a more macroscopic level, the CS-to-IF shift was represented as a displace-ment of the reaction "route" on the free energy surface, which was consistent with a flux analysis.

关键词： Polymer-based nanocomposites   Metal oxide   Molecular dynamic simulation   Nanoparticles  

摘要： In this work, SnO(2)and carbon black (CB) nanoparticles (NPs) were used to improve the physical and mechanical properties of polycarbonate films (PC) for industrial applications. Raman spectroscopy proved the successful inclusion of the fillers within the polymeric-based nanocomposites, with a suitable dispersion without changes in the structure and morphology of PC as denoted by scanning electron microscopy (SEM) and X-ray diffraction (XRD). After doping of the PC-based nanocomposites with CB, SnO(2)and CB/SnO(2)NPs, both the polymer hardness and the elastic modulus of the composites were observed to increase, with the highest value achieved for PC/CB-doped ones. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DTG) indicated changes in the thermal properties of the polymer composites upon NP doping. Finally, molecular dynamics simulations were used to gain a better understanding about the interactions of PC and NPs in the nanocomposites. Simulation data obtained were in good agreement with experimental ones, which indicates that simulations can be an efficient toll in order to perform a preliminary high-throughput screening of diverse physico-chemical properties of polymeric-based nanocomposites bearing NP fillers before experimental development.

关键词： Chemical grassland conversion   Mechanical grassland conversion   Maize cropping   Mineralization   NO3-NH4+  

摘要： Grassland conversion to cropland bears a risk of increased nitrate (NO3-) leaching and nitrous oxide (N2O) emission due to enhanced nitrogen (N) mineralization. This study investigates the dynamics of mineral N and N2O emissions following chemical and mechanical conversion from permanent grassland to cropland (maize) at two sites with different texture (clayey loam and sandy loam) and fertilization regime (with and without mineral N-fertilization) over a two-year period. Soil mineral N levels increased shortly after conversion and remained elevated in converted plots compared to permanent grassland or long-term cropland in the second year of investigation. Fluxes of N2O were higher from converted plots than permanent grassland or cropland. However, soil mineral N contents and cumulative N2O emissions did not differ between conversion types. Only the distribution of N2O losses over the two years differed: while losses were of similar magnitude in both years in mechanically converted plots, the major part of N2O loss in chemically converted plots occurred in the first year after conversion while emissions approximated grassland level in the second year. N2O fluxes were mainly controlled by water-filled pore space and soil NO3- levels. Despite differences in N levels at the two sites, these key findings are similar on both study sites. They indicate strongly accelerated mineralization after conversion, an effect that still lasted in the converted plots at the end of the two-year investigation irrespective of the conversion type used.

关键词： Dislocation pile-up   Mechanical interface energy   Interfacial gradient plasticity  

摘要： The present article focuses on interpreting mechanical interface energies parameters which have been introduced within gradient plasticity and can capture the ability of grain boundaries to deform plastically. Molecular dynamics simulation compression tests were performed on body-centered cubic Fe bicrystals with tilt and twist grain boundaries of different misorientations. The simulations allowed for the formation of dislocation pile-ups, which give rise to gradients in the plastic strain, and therefore it was possible to employ interfacial gradient plasticity to interpret the stress-strain curves and obtain values for the mechanical interface parameter. The simulations showed that the atomic structure at the grain boundary changed during dislocation absorption, illustrating that the initial grain boundary static energy cannot characterize boundaries after the onset of plastic deformation. It was found that the value of the mechanical interface parameter (i) depended on the GB type and GB-dislocation interactions that led to slip transmission across the GB, and (ii) was positively related to the magnitude of residual Burgers vector, indicating that as the mechanical interface parameter increased the GB strength increased. Repeating the simulations by adding hydrogen atoms at the grain boundaries, showed that segregation increased the mechanical interface energy. Changing the sample size indicated that such mechanical energy terms are size-dependent, however their ratio with the internal length was size independent and depended only on the GB type. A detailed understanding of these mechanical grain boundary energies may pave a new way to engineering materials in the sub-micron scales. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词： Lighting  

摘要： In this study, we report on the development of triboelectric energy generators (TEGs) made of porous polyvinylidene fluoride (PVDF) and bacterial cellulose (BC) layers and their demonstration as a self-powered sensor for remote security applications. PVDF films are fabricated by room temperature solution casting at different relative humidity (RH) with denser, stronger, and transparent PVDF films obtained at lower relative humidity. Among all the PVDF film based TEGs, the triboelectric generator (TEG) made of the film prepared at a low relative humidity of 10% (PVDF10) shows the highest voltage, current and power density of 410 V, 14.8 mu A (current density: 0.57 mu A cm(-2)) and 3.5 mW, respectively and a maximum power density of 0.136 mW cm(-2)at 33 M omega. Enhancement in the value of power density is almost three-fold as compared to that of the PVDF film cast at a high relative humidity of 54% (PVDF54). The high output response of the PVDF10 based TEG is attributed to the rough surface, higher gamma-phase fraction, and high dielectric constant of the PVDF10 film. Finally, the potential of a TEG device for practical application in biomechanical energy harvesting is demonstrated by lighting up more than 100 light-emitting diodes using a low-frequency foot strike as a mechanical stimulus along with lighting up of a 5-bit 7 segment LCD display. The device was also used as a self-powered motion sensor for application in wireless security, which could be practically realistic. Overall, the study demonstrates the possible applications of PVDF thin film based TEG devices, which can be fabricated using an easy, room temperature, and environment-friendly processing method.

关键词： Nanocrystalline aluminium   molecular dynamics   reverse Hall-Petch   elastic modulus   Poisson's ratio  

摘要： Uniaxial deformation was performed using molecular dynamics to estimate the mechanical properties of nanocrystalline aluminium. It was observed that the stacking faults and sliding of the grain boundaries affected the mechanical properties. In addition, accumulation of atoms near grain boundaries during deformation hardened the nanocrystalline material as the grain diameter increased (reverse Hall-Petch relation). Further, the effects of strain rate and temperature were investigated with various mean grain diameters. Investigation showed that mechanical properties were independent of tested strain rates (10(9)-10(10) s(-1)) and that the nanocrystalline material softened with increasing temperature. The elastic modulus was then compared to experimental results from literature at room temperature. The change in crystalline structure was observed with respect to percent strain and various mean grain diameters of nanocrystalline aluminium. It was observed that stacking faults increased with decreased mean grain diameter, which led to reduced mechanical properties.

关键词： Elastic moduli  

摘要： Through coarse-grained molecular dynamics simulations, the effects of nanoparticle properties, polymer-nanoparticle interactions, chain crosslinks and temperature on the stress-strain behavior and mechanical reinforcement of polymer nanocomposites (PNCs) are comprehensively investigated. By regulating the filler-polymer interaction (miscibility) in a wide range, an optimal dispersion state of nanoparticles is found at moderate interaction strength, while the mechanical properties of PNCs are improved monotonically with the increase of the particle-polymer interaction due to the tele-bridge structures of nanoparticlesviapolymer chains. Although smaller-sized fillers more easily build interconnected structures, the elastic moduli of PNCs at the percolation threshold concentration where a three-dimensional filler network forms are almost independent of nanoparticle size. Compared with spherical nanoparticles, anisotropic rod-like ones, especially with larger aspect ratio and rod stiffness, contribute exceptional reinforcement towards polymer materials. In addition, the elastic modulus with the strain, derived from the stress-strain curve, shows an analogous nonlinear behavior to the amplitude-dependence of the storage modulus (Payne effect). Such a behavior originates essentially from the failure/breakup of the microstructures contributing to the mechanical reinforcement, such as bound polymer layers around nanoparticles or nanoparticle networking structures. The Young's modulus as a function of the nanoparticle volume fraction greatly exceeds that predicted by the Einstein-Smallwood model and Guth-Gold model, which arises primarily from the contribution of the local/global filler network. The temperature dependence of the Young's modulus is further examined by mode coupling theory (MCT) and the Vogel-Fulcher-Tammann (VFT) equation, and the results indicate that the time-temperature superposition principle holds modestly above the critical temperature on the short-time