Excess Potential Energy Research Articles

Study of the mechanism of the strength-ductility synergy of α-Ti at cryogenic temperature via experiment and atomistic simulation

Alpha titanium (α-Ti) is a promising material for making high-performance components for applications in aerospace, marine, energy and healthcare fields. The excellent strength-ductility synergy has been observed for α-Ti at cryogenic temperature. Twinning is generally considered a key mechanism of outstanding cryogenic ductility. The dislocation-grain boundaries (GBs) interaction and void nucleation usually play crucial roles in the plastic deformation of polycrystalline materials, but their effects on the cryogenic ductility of α-Ti are rarely considered. To eliminate this confusion and gain an in-depth insight into the mechanism of the cryogenic strength-ductility synergy of α-Ti, in this work, a series of characterization experiments and molecular dynamics (MD) simulations were designed and carried out. 1) From uniaxial tension tests of the coarse-grained α-Ti sheets at the temperature from 25 to -180 °C, the uniform elongation and post-necking elongation were increased by 92 % and 20 %, respectively. The material maintained a larger strain hardening rate within a greater range of strain at cryogenic temperature compared with room temperature. 2) Via microstructure and fractography observations and the analysis of slip and geometrically necessary dislocation (GND) activities, the uniform plastic deformation was mainly accomplished by prismatic slip, whether at room temperature or at cryogenic temperature. The significantly increased uniform elongation is mainly attributed to the more uniform distribution of GND pile-ups at cryogenic temperature. 3) The MD simulations revealed that cryogenic temperatures made the GBs present a stronger barrier effect on dislocation transmission compared with that at room temperature, contributing to the more uniform distribution of GNDs and lower densities of GND pile-ups. The GBs at cryogenic temperature show a greater ability to resist void nucleation due to the decreased accumulation rate of excess potential energy and increased energy required to void nucleation. The larger strains were thus required to increase the densities of GND pile-ups to induce large stress concentrations for driving void nucleation. This made the uniform elongation of α-Ti increase significantly at cryogenic temperature. This study revealed that the enhanced barrier effect of GBs on dislocation transmission and the improved ability of GBs to resist void nucleation are key mechanisms besides twinning governing the cryogenic strength-ductility synergy of α-Ti. The understanding developed in this work can be useful for the development of new high-performance materials and the precise forming of complex components with high quality.

The role of subduction and gravitational collapse in driving extension in the Aegean Sea

The Aegean domain has experienced lithospheric deformation during continuous northward subduction at the Hellenic trench. Early Cenozoic shortening and thickening were followed by late Cenozoic extension and thinning. Thickened lithosphere, in particular its crust, has excess gravitational potential energy, which is thought to promote orogenic collapse (i.e., gravity spreading). In the Aegean domain, release of this potential energy and collapse of the lithosphere has been proposed as the first-order driver for extension of the overriding plate. Here we show, using dynamically self-consistent analogue experiments of subduction and gravity spreading, that the subducting plate significantly influences overriding plate deformation, despite the overriding plate being positively buoyant. With the introduction of a subducting plate, the trench retreat velocity and overriding plate extension rate increase, which is in contrast with a continuous decline during gravity spreading. In addition, ∼E-W trending normal faults emerge, as opposed to arc-parallel faults trending ∼ENE-WSW to ∼ESE-WNW and arc-normal extensional zones that result from gravity spreading only. Similarly, the subducting plate causes overriding plate displacement to be unidirectional (trenchward), in accordance with the present-day geodetic velocity field, whereas gravity spreading causes a radially outward-directed displacement field. As the decline in trench retreat rate, radial spreading, dominantly arc-parallel normal faults, and arc-normal extensional zones are not observed in nature, this implies that gravity spreading cannot be the sole driver of extensional deformation in the overriding plate. Indeed, the models suggest that subduction-induced mantle flow is the dominant driver of overriding plate deformation because it imposes a basal shear stress on the overriding plate, dragging and extending it trenchward in a unidirectional manner. Our experimental results corroborate earlier studies that highlight the important role of subduction-induced mantle flow in driving overriding plate deformation. Additionally, we find that this role is more pronounced in experiments with a thicker brittle layer.

Velocity and energy of periodic travelling interfacial waves between two bounded fluids

For a periodic travelling irrotational wave propagating at the interface between two homogeneous, incompressible and inviscid fluids bounded by horizontal planes, we generalise the Stokes definitions for the velocity of the wave propagation. Under certain conditions imposed on the horizontal velocity of the motion at the interface and supposing that the horizontal components of the velocity in each layer never reach the wave speed, we prove that the mean horizontal velocity of propagation of the wave is greater than the generalised mean horizontal velocity of the mass of the fluid. We show that, for interfacial waves of small amplitude, the excess kinetic and potential energy of the fluid have the same magnitude, but different signs, and for the nonlinear setting, we prove that the excess kinetic energy is negative.

Development of an inline bidirectional micro crossflow turbine for hydropower harvesting from water supply pipelines

Reliable and continuous power supply to the data monitoring and transmitting devices of the water intelligent network (WIN) system of urban water supply pipelines is still a challenge due to the restricted underground space and complex water environment. In this study, a novel inline bidirectional vertical crossflow turbine (IBVCT) integrated with bell-mouth guide blocks was developed to generate hydropower by converting the excess potential energy of the water flow onsite for power supply to the WIN system in Hong Kong. Specifically, the configurations and geometries of the proposed IBVCT were designed for large-size pipelines, and four schemes of water blocks in IBVCT were investigated based on the computational fluid dynamics method to determine the optimal configuration. A hydraulic test rig in the laboratory and a field test system integrated with an electricity storage module were developed. The IBVCT, after numerical optimizations, was fabricated and tested to validate its overall performance of power generation, water head loss, and long-term operating stability. The results showed that the designed centrosymmetric bell-mouth guide blocks significantly contribute to the efficient conversion of water flow energy into mechanical energy by the turbine. The IBVCT prototype had the best efficiency of 2.6 % at the water flow velocity of 1.3 m/s, which was capable of providing stable and sufficient electric power with a daily electricity generation capacity of 470 Wh. Additionally, the water head loss was effectively restricted within 6.1 m at the high water flow velocity of 3 m/s. These technology metrics of the IBVCT exhibited superior overall performance and met the power requirements of the WIN system used in the main urban water pipelines.

Energy considerations for nonlinear equatorial water waves

<p style='text-indent:20px;'>In this article we consider the excess kinetic and potential energies for exact nonlinear equatorial water waves. An investigation of linear waves establishes that the excess kinetic energy density is always negative, whereas the excess potential energy density is always positive, for periodic travelling irrotational water waves in the steady reference frame. For negative wavespeeds, we prove that similar inequalities must also hold for nonlinear wave solutions. Characterisations of the various excess energy densities as integrals along the wave surface profile are also derived.</p>

On the energy of nonlinear water waves.

This article presents results concerning the excess kinetic and potential energies for exact nonlinear water waves. In particular, it is proven, for periodic travelling irrotational water waves, that the excess kinetic energy density is always negative, whereas the excess potential energy density is always positive, in the steady reference frame. A characterization of the total excess energy density as a weighted mean of the kinetic energy along the wave surface profile is also presented.

Molecular dynamic insight into solubility of H2S in ionic liquids [emim][BF4], [emim][OTf] and [emim][Tf2N

Natural gas sweetening is one of the important processes in petroleum industry. Ionic liquids (ILs) as a new class of compounds are an alternative to conventional solvents to achieve this goal. In this study, the solubility of H2S gas in ionic liquids 1-Ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][OTf]) and 1-Ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide was calculated based on the excess chemical potential energy by molecular dynamics free energy perturbation (FEP) technique. In this regard, validation was carried out for force fields of gas and ILs. Two force fields, All-Atom (A-A) and United-Atom (U-A), were compared to calculate the H2S solubility in the ILs. The performance of A-A force field was better for [emim][BF4] and [emim][OTf]; whereas, that of U-A force field for [emim][Tf2N]. The calculated Henry’s constant and chemical potential energy were in a good agreement with experimental data. The results showed that higher negative potential energy resulted in higher molar fraction and the ionic liquid [emim][BF4] had the best performance for H2S removal from a gas stream.

Interface structures and dislocation nucleation of Cu/graphene interface via molecular dynamic simulations

Molecular dynamic simulations have been performed to investigate the effects of Cu {111}/graphene interface structures on the initial dislocation nucleation behavior in Cu/graphene layered composites. The characteristics of interfacial structures, including four possible types of stacking structures, interfacial misfit dislocations and interfacial energy distribution, have been studied based on geometrical analysis Thompson tetrahedron, disregistry analysis, and excess potential energy. In addition, interfacial structures are transformed by rotating the graphene layer with various angles, which further changing the morphology of the interface. After relaxation, the equilibrium state of the interface consists of a hexagon unit region arranging in periodicity, which also depicts the periodic energy distribution both on the Cu and graphene side. Under uniaxial X and Y directions loading, the evolution of interface patterns provides a non-Schmid factor for initial interfacial dislocation nucleation which prefers to locate the sites of higher potential energy area. Moreover, the rotation of the graphene layer changes the interfacial structures by changing the atoms mismatch on the Cu/graphene interface, which consequently influences the nucleation conditions including sites, morphology and slip system as well. This work provides an understanding of the Cu/graphene interface-dominated dislocation nucleation.

Interface Structures and Dislocation Nucleation of Cu/graphene Incoherent Interface Via Molecular Dynamic Simulations

Molecular dynamic simulations have been performed to investigate the effects of Cu {111}/graphene interface structures on the initial dislocation nucleation behavior in Cu/graphene layered composites. The characteristics of interfacial structures, including four possible types of stacking structures, interfacial misfit dislocations and interfacial energy distribution, have been studied based on geometrical analysis Thompson tetrahedron, disregistry analysis and excess potential energy. In addition, interfacial structures are transformed by rotating the graphene layer with various angles, which further changing the morphology of the interface. After relaxation, the equilibrium state of the interface consists of a hexagon unit region arranging in periodicity, which also depicts the periodic energy distribution both on the Cu and graphene side. Under uniaxial X and Y directions loading, the evolution of interface patterns provides a non-Schmid factor for initial interfacial dislocation nucleation which prefer to locate the sites of higher potential energy area. Moreover, the rotation of graphene layer changes the interfacial structures by changing the atoms mismatch on the Cu/graphene interface, which consequently influences the nucleation conditions including sites, morphology and slip system as well. This work provides an understanding on the Cu/graphene interface-dominated dislocation nucleation.

Influence of the principle of minimum potential energy on the distribution of density and gravitational energy of the Earth for the reference model PREM

From the point of view of modeling the internal structure of the Earth, its figure and evolution play an important role, which to one degree or another are associated with gravitational energy and the principle of its minimization. The realization of the minimum principle of potential for models of the distribution of the Earth’s density is the key in studies on the detection of inhomogeneous mass distribution. Achieving the minimum gravitational energy of the Earth is equivalent to the approximation of the internal structure to the hydrostatic state, and this condition is achieved due to variations in density. Therefore, for correct geophysical interpretation of gravimetric data, it is necessary to align the PREM (Preliminary Reference Earth Model) with harmonic coefficients of geopotential and minimum functional condition that determines gravitational energy, and only on this basis to estimate variations in density and tectonosphere. An algorithm for representing a piecewise continuous density distribution function in a spherical PREM model by Legendre polynomials is proposed in the paper to calculate the density, potential and energy distribution in an ellipsoidal planet using an additional condition — the minimum of gravitational energy. The use of such an algorithm made it possible to transform the spherically symmetric PREM model to a hydrostatically balanced state. It turned out that in the model obtained, the excess potential energy is concentrated in the inner and outer core of the Earth, and also insignificantly in the planet’s crust. The total energy E for the PREM reference model, which is subdivided into ellipsoidal layers, is 2.3364∙1024 erg, and in the modified PREM model after its correction for the hydrostatic component, it is 2.2828∙1024 erg. Estimation of density redistribution and identification of areas of their greatest change provide a mechanism for explaining the dynamic processes in the middle of the Earth.

Quantitative modeling of secondary electron emission from slow-ion bombardment on semiconductors

When slow ions incident on a surface are neutralized, the excess potential energy is passed on to an electron inside the surface, leading to emission of secondary electrons. The microscopic description of this process, as well as the calculation of the secondary electron yield, is a challenging problem due to its complexity as well as its sensitivity to surface properties. One of the first quantitative descriptions was articulated in the 1950s by Hagstrum, who based his calculation on a parametrization of the density of states of the material. In this paper, we present a model for calculating the secondary electron yield, derived from Hagstrum's initial approach. We use first-principles density functional theory calculations to acquire the necessary input and introduce the concept of electron cascades to Hagstrum's model in order to improve the calculated spectra, as well as remove its reliance on fitting parameters. We apply our model to ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{+}$ ions incident on $\mathrm{Ge}(111)$ and $\mathrm{Si}(111)$ and obtain yield spectra that match closely to the experimental results of Hagstrum.

Structural and mechanical properties of homogeneous solid-liquid interface of Al modelled with COMB3 potential

In this study, the homogeneous (1 0 0), (1 1 0) and (1 1 1) interface between solid Al and liquid Al are examined using a recently developed third generation of charge-optimized many body (COMB3) potential and molecular-dynamics simulation (MD). The estimated melting point is about 22% higher than the experimental value, but the reproduced radial distribution function of liquid Al agrees pretty well with experimental result. The interfacial widths (δ) of the (1 0 0), (1 1 0) and (1 1 1) interface derived from smoothed density profiles are about 4, 6 and 4 crystal layers, respectively. The stress profiles are highly orientation dependent and the magnitudes of the stress peaks near the homogeneous interfaces are weaker than those near heterogeneous interfaces. Excess interfacial stress and excess interfacial potential energy are also obtained. The information about the homogeneous interfaces obtained in this study can enrich the understanding of homogeneous solid-liquid interface and will serve as a useful reference for future studies of Al-ceramic interfaces using COMB3 potentials.

Plasticity Mechanism for Glassy Polymers: Computer Simulation Picture

This review summarizes the data published over the past two and a half decades on the mechanism of plastic deformation of bulk isotropic linear glassy polymers in uniaxial tension, compression, and shear at low deformation temperatures (Тdef < 0.6Тg) and moderate loading rates. The main attention is focused on studies concerning the numerical computer simulations of plasticity of organic polymer glasses. The plastic behavior of glassy polymers at nano-, micro-, and macrolevels in the range of macroscopic strains up to ≈100% is discussed. Plasticity mechanisms are compared for organic, inorganic, metallic, polymer, and nonpolymer glasses with different chemical structures and types of interparticle interactions. The general common mechanism of plasticity of glassy compounds, the nucleation of plasticity carriers in them, and the structure of such carriers and their dynamics are covered. Within the framework of the common plasticity mechanism, the specific features of deformation in glassy polymers are analyzed. Specifically, the participation of conformational transformations in macromolecules in the deformation response of polymer glasses, change in intensity of the yield peak with the thermomechanical prehistory of the sample, and the role of van der Waals interactions in the accumulation of excess potential energy by the sample under loading are considered. The role of free volume and structural and dynamic heterogeneities in the plasticity of glasses is also discussed.

AACSD: An atomistic analyzer for crystal structure and defects

We have developed an efficient command-line program named AACSD (Atomistic Analyzer for Crystal Structure and Defects) for the post-analysis of atomic configurations generated by various atomistic simulation codes. The program has implemented not only the traditional filter methods like the excess potential energy (EPE), the centrosymmetry parameter (CSP), the common neighbor analysis (CNA), the common neighborhood parameter (CNP), the bond angle analysis (BAA), and the neighbor distance analysis (NDA), but also the newly developed ones including the modified centrosymmetry parameter (m-CSP), the orientation imaging map (OIM) and the local crystallographic orientation (LCO). The newly proposed OIM and LCO methods have been extended for all three crystal structures including face centered cubic, body centered cubic and hexagonal close packed. More specially, AACSD can be easily used for the atomistic analysis of metallic nanocomposite with each phase to be analyzed independently, which provides a unique pathway to capture their dynamic evolution of various defects on the fly. In this paper, we provide not only a throughout overview on various theoretical methods and their implementation into AACSD program, but some critical evaluations, specific testing and applications, demonstrating the capability of the program on each functionality. Program summaryProgram title: AACSDProgram Files doi:http://dx.doi.org/10.17632/3hrmbbgp6s.1Licensing provisions: GNU General Public License 3 (GPL)Programming language: C++Nature of problem: Post-analysis of atomic configurations for LAMMPS [1] dump files, including crystals structure, defect and grain’s orientation.Solution method: Reading a LAMMPS custom dump file or LAMMPS data file from hard disk, get them analyzed and dumped to a new LAMMPS custom dump file together with the generated results, which can be visualized by OVITO [2].

On the composition dependence of thermodynamic, dynamic and dielectric properties of water-dimethyl sulfoxide model mixtures. NPT molecular dynamics simulation results

We have systematically examined available molecular dynamics (MD) simulation results for thermodynamic, dynamic and dielectric properties of water-dimethyl sulfoxide (DMSO) mixtures with the purpose of analyzing their discrepancies with experimental data. To get a more profound insight into the behavior of these mixtures, we have performed constant pressure-constant temperature simulations in the entire composition range at 1bar and 298.15K and compared the obtained results with experimental data and previous findings. The SPC-E water model was used in combination with the P1, P2 and OPLS united atom models for dimethyl sulfoxide. The dependencies of density, of excess mixing volume and potential energy, of the self-diffusion coefficients of species, and of the dielectric constant on the mixture composition are discussed. The evolution of the hydrogen bonding network was analyzed in terms of the average number of hydrogen bonded molecules and of the distribution of bonding states. In conclusions we discuss the capability of the united atom models for DMSO to adequately reproduce an ample set of properties of these mixtures and outline future work.

Relaxation Mechanisms, Structure and Properties of Semi-Coherent Interfaces

In this work, using the Cu–Ni (111) semi-coherent interface as a model system, we combine atomistic simulations and defect theory to reveal the relaxation mechanisms, structure, and properties of semi-coherent interfaces. By calculating the generalized stacking fault energy (GSFE) profile of the interface, two stable structures and a high-energy structure are located. During the relaxation, the regions that possess the stable structures expand and develop into coherent regions; the regions with high-energy structure shrink into the intersection of misfit dislocations (nodes). This process reduces the interface excess potential energy but increases the core energy of the misfit dislocations and nodes. The core width is dependent on the GSFE of the interface. The high-energy structure relaxes by relative rotation and dilatation between the crystals. The relative rotation is responsible for the spiral pattern at nodes. The relative dilatation is responsible for the creation of free volume at nodes, which facilitates the nodes’ structural transformation. Several node structures have been observed and analyzed. The various structures have significant impact on the plastic deformation in terms of lattice dislocation nucleation, as well as the point defect formation energies.

Strong Photon Energy Dependence of the Photocatalytic Dissociation Rate of Methanol on TiO2(110)

Photocatalytic dissociation of methanol (CH3OH) on a TiO2(110) surface has been studied by temperature programmed desorption (TPD) at 355 and 266 nm. Primary dissociation products, CH2O and H atoms, have been detected. The dependence of the reactant and product TPD signals on irradiation time has been measured, allowing the photocatalytic reaction rate of CH3OH at both wavelengths to be directly determined. The initial dissociation rate of CH3OH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm, suggesting that CH3OH photocatalysis is strongly dependent on photon energy. This experimental result raises doubt about the widely accepted photocatalysis model on TiO2, which assumes that the excess potential energy of charge carriers is lost to the lattice via strong coupling with phonon modes by very fast thermalization and the reaction of the adsorbate is thus only dependent on the number of electron-hole pairs created by photoexcitation.

Pnicogen Bonds: A Theoretical Study Based on the Laplacian of Electron Density

Although, most of the authors classify the pnicogen bonds as σ-hole bonding, there are some evidence that show they do not require any positive electrostatic potential around interacting molecules. In this work, the Laplacian of electron density is used to study pnicogen bonds in different dimer and trimer complexes. It is shown that the noncovalent P···P, P···N, and N···N bonds can be categorized as lump-hole interactions; a region of charge depletion and excess kinetics energy (hole) in the valence shell charge concentration (VSCC) of pnicogen atom combines with a region of charge concentration and excess potential energy (lump) in the VSCC of another molecule and form a pnicogen bond. In fact, since the full quantum potential (according to the local statement of virial theorem) has been used in the definition of the Laplacian, the lump-hole concept is more useful than the σ-hole in which the electrostatic part of potential is only considered. It is shown that the existence of hole in the VSCC of pnicogen atom is responsible for formation and (in the absence of other interactions) geometry of pnicogen bonded complexes. Because there is (at least) one hole in their VSCC, the pnicogen atoms in PH3, PH2F, H2C═PH, H2C═PF, and NH2F can engage in direct pnicogen-pnicogen interactions. However, the VSCC of nitrogen atom in the NH3 is devoid of hole and hence cannot act as an electron acceptor in pnicogen-bonded complexes.

Tethyan collision forces and the stress field of the Eurasian Plate

Resistive forces along convergent plate boundaries have a major impact on surface deformation, most visibly at collisional plate boundaries. Although quantification of these forces is key to understanding the evolution and present state of mountain belts, they remain highly uncertain due to the complexity of plate boundary structures and rheologies. In previous studies of the Eurasian Plate, we have analysed the balance of plate boundary forces, tractions resulting from lithosphere–mantle coupling, and intraplate variations in topography and density structure. This yielded a range of acceptable force distributions. In this study, we investigate to which extent the observed present-day stress field provides further constraints on the distribution of forces. We address the dynamics of the Eurasian Plate as a whole. This enables us to base our analysis on mechanical equilibrium of a tectonic plate and to evaluate all forces as part of an internally consistent set of forces driving and deforming Eurasia. We incorporate tractions from convective mantle flow modelling in a lithospheric model in which edge and lithospheric body forces are modelled explicitly and compute resulting stresses in a homogeneous elastic thin shell. Intraplate stress observations used are from the World Stress Map project. Eurasia's stress field turns out to be particularly sensitive to the distribution of collision forces on the plate's southern margin and, to a much lesser extent, to lithospheric density structure and tractions from mantle flow. Stress observations require collision forces on the India–Eurasia boundary of 7.0–10.5 TN m−1 and on the Arabia–Eurasia boundary of 1.3–2.7 TN m−1. Implication of mechanical equilibrium of the plate is that forces on the contacts with the African and Australian plates amount to 1.0–2.5 and 0–1.3 TN m−1, respectively. We use our results to assess the validity of the classical view that the mean elevation of an orogenic plateau can be taken as a measure of the magnitude of the compressive (in this case: collision-related) forces involved. For both the Tibetan and the Iranian plateaus, two plateaus with significantly different average elevations, we find that the horizontal force derived from the excess gravitational potential energy (collapse force) is in balance with the collision force.

Computational Study of H2 and O2 Production from Water Splitting by Small (MO2)n Clusters (M = Ti, Zr, Hf)

Coupled cluster [CCSD(T)] theory and density functional theory (DFT) have been used to study the production of H2 and O2 from hydrolysis products generated from H2O addition to (MO2)n (M = Ti, Zr, Hf, n = 1-3) clusters on both the lowest singlet and triplet potential energy surfaces (PESs). H2 production occurs via the formation of an M-H containing intermediate followed by H-H recombination and H2 desorption from M(n)O(2n)(OH)2 and M(n)O(2n+2). The hydrogen transfer reactions to form the M-H bond are the rate determining steps and can be considered to be proton coupled, electron transfer (PCET) reactions with one or two electrons being transferred. Oxygen is produced by breaking two weak M-O bonds in an atomic oxygen saturated metal oxide from an M(n)O(2n)•O2 intermediate. On the triplet PES, the activation energies for the first and second H transfer to the metal are calculated to be ~10 to 50 kcal/mol and ~75 to 90 kcal/mol depending on the size of the clusters and the metal. The barriers on the singlet surface for the first and the second H transfer are predicted to be 110 to 140 kcal/mol, in general larger than the H-O bond dissociation energy. The activation barriers for the step of H-H recombination are 15 to 50 kcal/mol, and the H2 desorption energies are less than 10 kcal/mol on the singlet and triplet PESs. The oxygen desorption energies follow the order Ti < Zr < Hf for the triplets and Ti < Zr ≈ Hf for the singlets. The oxygen desorption energy is approximately independent of the size of the cluster for the same metal. The water splitting reactions prefer to take place on the triplet surface. A low excess potential energy is needed to generate 2H2 and O2 from 2H2O after the endothermicity of the reaction is overcome on the triplet PES.