Local Density Theory Research Articles

Engineering Robust Metallic Zero-Mode States in Olympicene Graphene Nanoribbons.

Metallic graphene nanoribbons (GNRs) represent a critical component in the toolbox of low-dimensional functional materials technology serving as 1D interconnects capable of both electronic and quantum information transport. The structural constraints imposed by on-surface bottom-up GNR synthesis protocols along with the limited control over orientation and sequence of asymmetric monomer building blocks during the radical step-growth polymerization have plagued the design and assembly of metallic GNRs. Here, we report the regioregular synthesis of GNRs hosting robust metallic states by embedding a symmetric zero-mode (ZM) superlattice along the backbone of a GNR. Tight-binding electronic structure models predict a strong nearest-neighbor electron hopping interaction between adjacent ZM states, resulting in a dispersive metallic band. First-principles density functional theory-local density approximation calculations confirm this prediction, and the robust, metallic ZM band of olympicene GNRs is experimentally corroborated by scanning tunneling spectroscopy.

Elastic Valley Spin Controlled Chiral Coupling in Topological Valley Phononic Crystals.

Distinct from the phononic valley pseudospin, the real physical spin of elastic waves adds a novel tool kit capable of envisaging the valley-spin physics of topological valley phononic crystals from a local viewpoint. Here, we report the observation of local elastic valley spin as well as the hidden elastic spin-valley locking mechanism overlooked before. We demonstrate that the selective one-way routing of valley phonon states along the topological interface can be reversed by imposing the elastic spin metasource at different interface locations with opposite valley-spin correspondence. We unveil the physical mechanism of selective directionality as the elastic spin controlled chiral coupling of valley phonon states, through both analytical theory and experimental measurement of the opposite local elastic spin density at different interface locations for different transport directions. The elastic spin of valley topological edge phonons can be extended to other topological states and offers new tool to explore topological metamaterials.

Reconstructions of supercritical CO2 adsorption isotherms and absolute adsorption estimation in nanoporous coals considering volumetric effects and varying adsorbed phase densities

• Adsorbed phase density and fugacity facilitate identifying the adsorption mechanisms. • SLD-PR model is efficient in obtaining adsorbed phase density at various pore sizes. • Excess adsorption isotherms are effectively rebuilt considering volumetric effects. • Absolute adsorption amounts are calculated accurately and reasonably considering PSD. • Empirical adsorbed densities for engineering practice are proposed through iteration. Understanding supercritical CO 2 (ScCO 2 ) adsorption behavior in nanoporous coals is crucial for long-term CO 2 geological storage. Four coal samples with a broad spectrum in rank were selected to obtain CO 2 excess adsorption isotherms at 35 ℃ and pressure up to 14 MPa using the volumetric method. As conversion from excess adsorption to absolute adsorption requires accurate capture of adsorbed phase density (APD), the simplified local density (SLD) theory and Peng-Robinson (PR) equation of state (EOS) were tailored to calculate the APD considering both pore size distribution (PSD) and pressure. Besides, volumetric effects, including void volume overestimation and coal matrix shrinkage/swelling, were quantificationally calibrated to reconstruct excess adsorption isotherms. Then, we proposed a novel conversion method based on real adsorption space distribution and adsorption mechanism. Results show that micropores in the range of 0.30–1.50 nm dominate the total pore space. APD and fugacity profiles reveal distinct CO 2 adsorption behaviors in micropores and mesopores. Fitted expansion coefficients indicate that coal matrix undergoes a superimposed effect of shrinkage and swelling during adsorption and the reconstructed isotherms could amend the large deviations around 8 MPa caused by the drastic changes of CO 2 bulk density and compressibility factor. Compared with conventional fixed APDs, the converted absolute adsorption isotherms generated by the varied ones exhibit slight fluctuations in the supercritical state and could plateau at high pressures. Continual iterations suggest that “averaged” APD in the range of 33–36 mol/L could be utilized for conversion in engineering practice only when the original excess adsorption isotherms are correctly reconstructed.

Absolute adsorption of light hydrocarbons on organic-rich shale: An efficient determination method

Absolute adsorption has been found being more accurate than directly measured excess adsorption value for estimating the in-situ shale gas-in-place storage. To obtain accurate absolute adsorption, traditional methods determined the adsorbed gas density using complicated molecular simulation methods, which is employed to correct the measured excess value. In this work, excess adsorption, which is deemed to be inaccurate in previous work, of CH4 and C3H8 is first measured on three typical shale rocks. The adsorbed-layer density is then reasonably calculated using the simplified local density theory (SLD) to obtain corresponding absolute adsorption of CH4 and C3H8. It is found that the adsorbed layer density calculated from SLD theory is comparable to that obtained from molecular simulation methods, while it is more efficient for the density computation. Compared to CH4, C3H8 presents stronger adsorption capacity on shale rocks, suggesting that C3H8 is more affinity to organic shale. The CH4′s absolute adsorption is stronger than its excess value and their deviation enlarges as pressure increases, while absolute adsorption for C3H8 is found to be more or less the same with the measured excess value. The importance of this work lies in that we propose a new method for efficiently obtaining the adsorbed-layer density, which is crucial for accurately determining gas and oil storage in shale reservoirs.

Methane adsorption on shale under in situ conditions: Gas-in-place estimation considering in situ stress

Gas-in-place (GIP) is one of the essential indices for the resource evaluation of shale gas storage capacity. In this study, based on the simplified local density (SLD) theory, the shale adsorption capacity was investigated considering the effect of burial depth on the shale pore structure. Shale isotherm adsorption experiments under triaxial conditions were performed with a homemade adsorption test device, and CH4 adsorption isotherms under different shale reservoir conditions representative of different burial depths were obtained. By establishing the relationship between the SLD model regression parameters and burial depth, a shale GIP estimation model considering the influence of in situ stress was established. In situ stress controls the pore structure of shale and consequently affects the gas storage capacity of shale. The conventional method does not consider the influence of in situ stress, resulting in overestimation of the free gas content, underestimation of the adsorbed gas content, and overall overestimation of the GIP by 1.12–1.26 times. Taking into account the influence of the stress state, the contribution of the free gas content to GIP is weakened. In contrast, the adsorbed gas content plays a key role in GIP. The distribution of the shale reservoir GIP concentrates at depths of 2500–4500 m. These findings will be helpful in the future evaluation of shale gas reserves and improvement in shale gas content evaluation methods.

Adsorption Mechanisms of High-Pressure Methane and Carbon Dioxide on Coals

Carbon dioxide (CO2) geological sequestration and coal-bed methane (CBM) recovery in deep coal seams are usually operated with a pressure higher than 10 MPa. The adsorption mechanisms of methane (CH4) and CO2 on coals in such a situation, however, are not yet revealed. With the help of a high-pressure gas adsorption system, CH4 and CO2 adsorption isotherms were first conducted on two coal samples. Simplified local density (SLD) theory was then tailored and applied to describe the adsorption characteristics of specific CH4 and CO2 on coals. Next, the adsorption mechanisms of high-pressure CH4 and CO2 on coal samples were revealed on the basis of adsorbed and bulk density distributions within the matrix pores. The results show that the high-pressure gas adsorption on coals is different from that at low pressure. The excess adsorption capacity first increases and then decreases with pressure, and the maximum value occurs at a specific pressure referring to reverse pressure. The maximum excess adsorption capacity of CH4 is less than that of CO2, while its reverse pressure is greater than that of CO2. The reversal of excess adsorption with pressure depends on the relative increase in adsorption and bulk phases. However, the mechanisms of adsorption reversal vary with gas types. The reversal of CH4 excess adsorption is due to the saturation near the pore wall, while the CO2 excess adsorption reversal is due to the dramatic bulk density changes near the critical point. Advances in the mechanism of high-pressure gas adsorption on coals suggests that coal seams deeper than 1000 m have more recovery potential, and CO2-enhanced coal-bed methane recovery success depends on the optimal management of its injection pressure and the associated coal swelling.

Electronic and lattice properties of nanostructured TiN/MgO and ScN/MgO superlattices

Various nanostructured TMN(001) dot/MgO(001) (TM = Sc, Ti) superlattices have been investigated. Here, we consider single and double layered TM dot structures with rectangular and rectangular parallelepiped shapes. MgO-2 × 2 and −3 × 3 substrates were used. Their electronic states were obtained using a total energy pseudopotential method. The internal atomic coordinates from the repeated slab model in the unit cell were fully relaxed. Various relaxed structures and electronic properties of the TMN(001) dot/MgO-2 × 2 and −3 × 3 superlattices were obtained. We determined their total and partial densities of states. Peaks of density of states (DOS) of the ScN dot/MgO superlattices around the band gap consist of Sc 3d and N 2p states and those of the TiN dot/MgO superlattices consist of mainly Ti 3d states. The largest band gap value in the TiN(001) dot/MgO(001) (ScN(001) dot/MgO(001)) superlattice is 1.22 eV (1.48 eV) from the density-functional theory-local density approximation calculation. The electronic properties (band gap values, DOS curves, and many more) vary with varying dot shape and substrate size. Most band gap values corrected using a generalized density-functional theory were improved.

Insight on Coal Swelling Induced by Monolayer and Multilayer Adsorption

Summary Gas adsorption, desorption, and displacement occur throughout the coalbed methane (CBM) and enhanced coalbed methane (ECBM) recovery process, causing the coal pores to deform and affecting injectivity and productivity. The primary objectives of this study include determining the pressure at which the monolayer adsorption transitions to multilayer adsorption and linking the swelling strain to both classes of adsorption. In this study, the simplified local density (SLD) theory was first modified and applied to describe the characteristics of both types of adsorption and to determine the transition pressure. A strain model coupled with the SLD theory was then developed to describe adsorption-induced deformation. Next, the measured methane (CH4) and carbon dioxide (CO2) adsorption isotherms, and strain data on the same coals, were collected for model validation. Results suggest that gas adsorbed on coal surfaces at the very beginning (monolayer adsorption) and the adsorption on other gas molecules continues once the surface has been filled (multilayer adsorption). Results also suggest that swelling strain is proportional to both types of adsorption, with the multilayer case being larger than the monolayer case. This difference may be due to the additional repulsion between the adsorbate and multilayer liquid film.

Absolute Adsorption of CH4 on Shale with the Simplified Local-Density Theory

Summary Using a thermogravimetric (TGA) method, the excess methane (CH4) adsorption of four organic shale samples is measured at temperatures of 303.15 to 383.15 K and pressures of 0 to 15.0 MPa. Simplified-local-density (SLD) theory is used to calculate the density distribution of CH4 in nanopores, which is then used to obtain the adsorbed CH4 density on four shale samples. Such density is applied to obtain the absolute CH4 adsorption by correcting the measured excess CH4 adsorption. SLD theory shows that the adsorbed CH4 density is strongly affected by temperature and pressure, as well as the pore size, which is in line with the previous findings from molecular simulations. SLD theory captures the density of the adsorbed phase of CH4 in the presence of CH4/pore-wall interactions. However, the SLD theory is more efficient than molecular simulation methods in determining the adsorbed CH4 density considering that only two parameters in the SLD model are adjusted to match the excess adsorption of CH4 on shale. It is observed that the corresponding absolute adsorption of CH4 is higher than the excess adsorption; this suggests that it is not reasonable to use the measured excess adsorption to estimate the storage of CH4 on shale. This study applies the SLD theory to investigate the adsorption behavior of CH4 in organic pores at different pressure/temperature conditions, and, more importantly, it yields a more-efficient approach (i.e., SLD theory) in determining the absolute adsorption than the sophisticated molecular simulations tools.

UAV Sensor Fault Detection Using a Classifier without Negative Samples: A Local Density Regulated Optimization Algorithm.

Fault detection for sensors of unmanned aerial vehicles is essential for ensuring flight security, in which the flight control system conducts real-time control for the vehicles relying on the sensing information from sensors, and erroneous sensor data will lead to false flight control commands, causing undesirable consequences. However, because of the scarcity of faulty instances, it still remains a challenging issue for flight sensor fault detection. The one-class support vector machine approach is a favorable classifier without negative samples, however, it is sensitive to outliers that deviate from the center and lacks a mechanism for coping with them. The compactness of its decision boundary is influenced, leading to the degradation of detection rate. To deal with this issue, an optimized one-class support vector machine approach regulated by local density is proposed in this paper, which regulates the tolerance extents of its decision boundary to the outliers according to their extent of abnormality indicated by their local densities. The application scope of the local density theory is narrowed to keep the internal instances unchanged and a rule for assigning the outliers continuous density coefficients is raised. Simulation results on a real flight control system model have proved its effectiveness and superiority.

(Invited) Multiscale Modeling of Uptake and Transport in Redox-Flow-Battery Separators

Understanding transport phenomena is a critical component of electrochemical engineering in terms of optimizing energy-conversion and -storage devices including fuel cells and flow batteries. In these systems, transport through the ion-conducting electrolytes including both polymer (e.g., perflurosulfonic-acid (PFSA) membranes) and liquid electrolyte control the overall device performance. In this talk, recent work on the multiscale modeling of ionomers will be discussed including thermodynamic and multiscale continuum treatments of multi-ion transport through the ionomer nanodomains and the relevant changes to the chemical potential resulting in selective ion and water uptake. This treatment involves a simple accounting for polymer backbone deformation, donnan equilibrium, solvation, and interactions with external electrolyte. The continuum modeling at the nanodomain uses a mean-field local-density theory that accounts for ion/ion, ion/water, and ion/polymer effects. In addition, upscaling methodologies using resistor-network approaches informed by experiment that explore phenomenon-dependent transport pathways for ions and electrolyte will be introduced and demonstrated to yield nonlinear and nonintuitive results. The overall modeling results will be compared to experimental property and uptake data and used to explain the nature of observed phenomena including electro-osmosis, ion conduction, and ion transference. Acknowledgements This work was funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U.S. Department of Energy as well as by the ARPA-E IONICS program under contract DE-AC02-05CH11231.

On the Local View of Atmospheric Available Potential Energy

Abstract The possibility of constructing Lorenz’s concept of available potential energy (APE) from a local principle has been known for some time, but it has received very little attention so far. Yet the local APE density framework offers the advantage of providing a positive-definite local form of potential energy, which, like kinetic energy, can be transported, converted, and created or dissipated locally. In contrast to Lorenz’s definition, which relies on the exact from of potential energy, the local APE density theory uses the particular form of potential energy appropriate to the approximations considered. In this paper, this idea is illustrated for the dry hydrostatic primitive equations, whose relevant form of potential energy is the specific enthalpy. The local APE density is nonquadratic in general but can nevertheless be partitioned exactly into mean and eddy components regardless of the Reynolds averaging operator used. This paper introduces a new form of the local APE density that is easily computable from atmospheric datasets. The advantages of using the local APE density over the classical Lorenz APE are highlighted. The paper also presents the first calculation of the three-dimensional local APE density in observation-based atmospheric data. Finally, it illustrates how the eddy and mean components of the local APE density can be used to study regional and temporal variability in the large-scale circulation. It is revealed that advection from high latitudes is necessary to supply APE into the storm-track regions, and that Greenland and the Ross Sea, which have suffered from rapid land ice and sea ice loss in recent decades, are particularly susceptible to APE variability.

(Invited) Multiscale Modeling of Transport Phenomena in Ion-Conducting Membranes and Aqueous CO2 Reduction Cells

Transport phenomena is a critical component of electrochemical engineering of energy-conversion devices including fuel cells and solar-fuel generators. In fuel cells, ion-conducting polymers, such as perflurosulfonic-acid (PFSA) membranes, typically nanophase separate into conducting and nonconducting or structural domains that arrange together on the mesoscale and result in macroscopically observable properties. To examine these interactions, a multiscale model has been developed that includes a mean-field local-density theory at the nanoscale, a physically derived resistor-network for the mesoscale that leads up to macroscale. The nanoscale includes cation and water chemical potentials to model transport in the aqueous domains including cation-polymer electrostatic interactions, solvation energy, and a concentrated-solution transport theory to model transport and account for dielectric friction induced by the strong electrostatic fields of charged polymer groups. For the mesoscale, the model accounts for transport along the segments of the mesoscale network of the aqueous domains where mass and charge are conserved at each node (i.e. branching point) of the network. The difference in ionic potential and species chemical potentials between connected network nodes are driving forces for transport. The resistance of each segment of the network is determined from the nanoscale model. The results reveal nonlinear and nonintuitive transport pathways, where the overall properties agree with experimental property measurements. In solar-fuel generators, including CO2 reduction cells, transport phenomena play a critical, yet not fully realized, role in controlling the overall reaction rate. To understand this interplay, multiscale modeling is used to examine the reaction mechanism of CO2 reduction to CO over a planar silver catalyst. Traditional Nernst-Planck expressions for ion and gas transport are coupled to a microkinetics model, where the reaction rates and barriers for the various reaction steps are taken from DFT calculations. The modeling approach helps to resolve the reaction mechanism and demonstrates good agreement with experimental data and trends without fitting parameters. Acknowledgements The fuel-cell work was funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract DE-AC02-05CH11231. The CO2 reduction was funded under the Joint Center for Artificial Photosynthesis (JCAP), a BES Energy Innovation Hub.

Time-frequency analysis of locally stationary Hawkes processes

Locally stationary Hawkes processes have been introduced in order to generalise classical Hawkes processes away from stationarity by allowing for a time-varying second-order structure. This class of self-exciting point processes has recently attracted a lot of interest in applications in the life sciences (seismology, genomics, neuro-science, …), but also in the modeling of high-frequency financial data. In this contribution, we provide a fully developed nonparametric estimation theory of both local mean density and local Bartlett spectra of a locally stationary Hawkes process. In particular, we apply our kernel estimation of the spectrum localised both in time and frequency to two data sets of transaction times revealing pertinent features in the data that had not been made visible by classical non-localised approaches based on models with constant fertility functions over time.

Determination of the absolute CH4 adsorption using simplified local density theory and comparison with the modified Langmuir adsorption model.

Accurately determining the adsorbed amount of CH4 on shale is significant for understanding the mechanisms of shale gas storage and shale methane recovery from shale gas reservoirs. Excess CH4 adsorption is measured using the thermogravimetric method. Simplified local density (SLD) theory is applied to calculate the adsorbed CH4 density to obtain the absolute adsorption. Moreover, the modified Langmuir adsorption model is employed to fit the excess adsorption to describe the absolute adsorption. The adsorbed CH4 density from the SLD model is affected by the system pressure and temperature, while such density obtained from the modified Langmuir model is only a function of temperature. Compared to the modified Langmuir model, the SLD model can better capture the adsorbed CH4 density, which allows accurate determination of the absolute CH4 adsorption.

Impact of Nano- and Mesoscales on Macroscopic Cation Conductivity in Perfluorinated-Sulfonic-Acid Membranes

A mean-field local-density theory is outlined for ion transport in perfluorinated-sulfonic-acid (PFSA) membranes. A theory of molecular-level interactions predict nanodomain and macroscale conductivity. The effects of solvation, dielectric saturation, dispersion forces, image charge, finite size, and confinement are included in a physically consistent 3D-model domain geometry. Probability-distribution profiles of aqueous cation concentration at the domain-scale are in agreement with atomistic simulations using no explicit fitting parameters. Measured conductivities of lithium-, sodium-, and proton-form membranes with equivalent weights of 1100, 1000, and 825 g/mol(SO3) validate the macroscale predictions using a single-value mesoscopic fitting parameter. Cation electrostatic interactions with pendant sulfonate groups are the largest source of migration resistance at the domain-scale. Tortuosity of ionically conductive domains is the largest source of migration resistance at the macroscale. Our proposed tr...

Modeling Competitive Adsorption between Methane and Water on Coals

Natural gas produced from coals, or coalbed methane (CBM), is a significant component of the energy portfolio for many countries. One challenge associated with CBM production is associated water. Specifically, coalbeds in situ contain significant amounts of water, and ideally this water is removed by pumping prior to the primary recovery of CBM to lower pressure and stimulate methane desorption. Such a prior water production can be challenging because desorption depends on the occurrence state of methane and water in situ, e.g., how much of each fluid is adsorbed or otherwise. Accordingly, primary objectives of this analysis include quantifying both the occurrence state of methane and water of different coals for a range of coalbed properties and conditions, and specifically quantifying the impact of coal moisture on methane desorption. Ultimate and proximate analysis and methane adsorption tests were first conducted on several coal samples from different basins. Simplified local density (SLD) theory was ...

Nano- and Mesoscale Ion and Water Transport in Perfluorosulfonic-Acid Membranes

High performance of polymer-electrolyte fuel cells requires rapid cation conduction through perflurosulfonic-acid (PFSA) membranes. Consequently, understanding water and aqueous cation transport in aqueous nanoscale domains (nano length scale) and connectivity and tortuosity of the microscale domains (meso length scales) is critical to optimizing PFSA membrane design. Due to transport coupling, membrane ionic conduction depends not only on electrostatic potential gradients but also on water content gradients (i.e. electro-osmosis). In this work, we model coupled cation and water transport in PFSA membranes at the nano- and mesoscale and predict macroscopic properties. We adopt a mean-field local-density theory for cation and water chemical potentials to predict transport in the nanoscale PFSA aqueous domains. Cation-polymer electrostatic interactions and solvation energy are included. Concentrated solution transport theory (Stefan-Maxwell formulation) models diffusion and specifically accounts for dielectric friction induced by the strong electrostatic fields of charged polymer groups. To model the mesoscale, a resistor network that includes Stefan-Maxwell coupling quantifies transport along the segments of the collection of aqueous domains in PFSA membranes. The nanoscale model parameterizes the properties of each segment of the resistor network. With this methodology, we decouple the influences of nano- and mesoscale resistances on macroscopic water and cation transport and electro-osmosis. At the nanoscale there is coupling between water and cation transport from both diffusional drag and electrostatic-driven convection. Electrostatic and pressure forces on the membrane are countered by immobilization of the polymer matrix. The models show the nature of electro-osmosis depends on water content and the vapor and electrostatic potential boundary conditions of the membrane. This work provides a compelling approach to elucidate the multiscale resistances to water and cation transport in PFSA membranes.

Review of local strain energy density theory for the fracture assessment of V-notches under mixed mode loading

Review of local strain energy density theory for the fracture assessment of V-notches under mixed mode loading

Decoupling the Influences of Molecular- and Mesoscales on Macroscopic Transport Properties in Perfluorosulfonic-Acid Membranes

Performance of polymer-electrolyte-fuel-cells (PEFCs) links strongly to thermodynamic and mass-transport properties of the polymer membrane. The quintessential PEFC-membrane material is perfluorosulfonic-acid (PFSA) polymer, which phase separates into nanoscale hydrophilic water-filled domains through which ions and water transport and into hydrophobic polymer-matrix domains that provide structural integrity and durability. Although considerable progress has been made to reduce reactant mass-transport limitations in PFSA membranes, mass-transport is still a critical area of concern. Aqueous electrolyte transport across a phase-separated membrane is inherently a multiscale problem with aqueous-dissolved species moving through nanoscale domains that are connected to form a transport network at the intermediate mesoscale. To guide the optimization of PEFC material design and operation, we decouple the impact of micro and macro length-scale on mass transport in PEFC membranes. Specifically, molecular-scale interactions between aqueous-electrolyte and polymer in hydrophilic domains of PFSA membranes are modeled with mean-field local-density theory aligned with an experimentally consistent 3D domain geometry. Our molecular-scale description of membrane conductivity accounts for solvation, electrostatics, solvent dielectric saturation, finite size, and confinement effects. The microscale framework is validated against atomistic simulations and, subsequently, up-scaled to predict macroscale conductivity and solvent absorption by accounting for the interconnectedness of the hydrophilic domains. Excellent agreement is found with experimental data, as shown in Figure 1. Our proposed multiscale model for membrane conductivity and solvent sorption provides a new tool to explore potential avenues for improving PFSA membrane performance. Acknowledgements This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231 and by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1106400 Figure 1.3M PFSA membrane conductivity of proton-form membranes as a function of water content as determined from the model (open) and experiments (filled, with lines to guide the eyes) for equivalent weights of 1100 (circles), 1000 (squares), and 825 g/mol (SO3-) (diamonds) Figure 1