Density Slope Research Articles

3D Core–Shell NiFeCr Catalyst on a Cu Nanoarray for Water Oxidation: Synergy between Structural and Electronic Modulation

Low-cost transition metal-based electrocatalysts for water oxidation and understanding their structure–activity relationship are greatly desired for clean and sustainable chemical fuel production. Herein, a core–shell (CS) NiFeCr metal/metal hydroxide catalyst was fabricated on a 3D Cu nanoarray by a simple electrodeposition–activation method. A synergistic promotion effect between electronic structure modulation and nanostructure regulation was presented on a CS-NiFeCr oxygen evolution reaction (OER) catalyst: the 3D nanoarchitecture facilitates the mass transport process, the in situ formed interface metal/metal hydroxide heterojunction accelerates the electron transfer, and the electronic structure modulation by Cr incorporation improves the reaction kinetics. Benefiting from the synergy between structural and electronic modulation, the catalyst shows excellent activity toward water oxidation under alkaline conditions: overpotential of 200 mV at 10 mA/cm2 current density and Tafel slope of 28 mV/dec. T...

Chemically controlled in-situ growth of cobalt oxide microspheres on N,S-co-doped reduced graphene oxide as an efficient electrocatalyst for oxygen reduction reaction

A chemically controlled facile approach is developed for the in-situ growth of Co3O4 microspheres on N,S-co-doped rGO sheets (Co3O4/N,S-rGO) employing thiamine hydrochloride (THCl) (7.41 mM) as the N and S co-doping agent under mild experimental conditions. The THCl concentration plays a vital role in controlling the growth of cobalt crystals anchored on the surface of N,S-rGO. The change in concentration of THCl from 7.41 to 22.20 mM alters the deposition of cobalt crystal on the N,S-rGO surface from Co3O4 to Co9S8 (Co9S8/N,S-rGO). This change is understood due to the formation of different intermediate complex structure driven by supramolecular interactions. The altered cobalt crystal phase leads to tuned morphologies, surface areas and catalytically active centers which are investigated by various characterization techniques. Co3O4/N,S-rGO exhibits superior electrocatalytic activity for oxygen reduction reaction (ORR) compared to Co9S8/N,S-rGO. Moreover, Co3O4/N,S-rGO shows more positive onset potential (0.90 V vs. RHE), half-wave potential (0.74 V vs. RHE), relatively high limiting current density and smaller Tafel slope (52 mV per decade) compared to its separate components owing to synergistic effects. Co3O4/N,S-rGO hybrid also outperforms the electrocatalytic activity of commercial Pt/C in terms of durability and methanol tolerance under alkaline conditions.

Properties of an accretion disc with a power-law stress–pressure relationship

Recent numerical simulations of magnetized accretion discs show that the radial-azimuthal component of the stress tensor due to the magnetorotational instability (MRI) is well represented by a power-law function of the gas pressure rather than a linear relation which has been used in most of the accretion disc studies. The exponent of this power-law function which depends on the net flux of the imposed magnetic field is reported in the range between zero and unity. However, the physical consequences of this power-law stress-pressure relation within the framework of the standard disc model have not been explored so far. In this study, the structure of an accretion disc with a power-law stress-pressure relation is studied using analytical solutions in the steady-state and time-dependent cases. The derived solutions are applicable to different accreting systems, and as an illustrative example, we explore structure of protoplanetary discs using these solutions. We show that the slopes of the radial surface density and temperature distributions become steeper with decreasing the stress exponent. However, if the disc opacity is dominated by icy grains and value of the stress exponent is less than about $0.5$, the surface density and temperature profiles become so steep that make them unreliable. We also obtain analytical solutions for the protoplanetary discs which are irradiated by the host star. Using these solutions, we find that the effect of the irradiation becomes more significant with decreasing the stress exponent.

The Formation of Extremely Diffuse Galaxy Cores by Merging Supermassive Black Holes

Abstract Given its velocity dispersion, the early-type galaxy NGC 1600 has an unusually massive (M • = 1.7 × 1010 M ⊙) central supermassive black hole (SMBH) surrounded by a large core (r b = 0.7 kpc) with a tangentially biased stellar distribution. We present high-resolution equal-mass merger simulations including SMBHs to study the formation of such systems. The structural parameters of the progenitor ellipticals were chosen to produce merger remnants resembling NGC 1600. We test initial stellar density slopes of ρ ∝ r −1 and ρ ∝ r −3/2 and vary the initial SMBH masses from 8.5 × 108 to 8.5 × 109 M ⊙. With increasing SMBH mass, the merger remnants show a systematic decrease in central surface brightness, an increasing core size, and an increasingly tangentially biased central velocity anisotropy. Two-dimensional kinematic maps reveal decoupled, rotating core regions for the most massive SMBHs. The stellar cores form rapidly as the SMBHs become bound, while the velocity anisotropy develops more slowly after the SMBH binaries become hard. The simulated merger remnants follow distinct relations between the core radius and the sphere of influence, and the SMBH mass, similar to observed systems. We find a systematic change in the relations as a function of the progenitor density slope and present a simple scouring model reproducing this behavior. Finally, we find the best agreement with NGC 1600 using SMBH masses totaling the observed value of M • = 1.7 × 1010 M ⊙. In general, density slopes of ρ ∝ r −3/2 for the progenitor galaxies are strongly favored for the equal-mass merger scenario.

Vibronic Superexchange in Double Perovskite Electrocatalyst for Efficient Electrocatalytic Oxygen Evolution.

Perovskites are prototype electrocatalyts benefiting from their high terrestrial availability and high stability. Electronic state regulation plays a key role in promising higher electrocatalytic efficiencies. Herein, we highlighted a vibronic superexchange in double perovskite to synergistically optimize eg electron filling state and increase the formation of active species on the surface of catalysts. Vibronic superexchange of Ni3+-O-Mn3+ in La2NiMnO6 nanoparticles brings the optimal eg electron filling state of Mn and Ni ions toward unity. Moreover, a vibronic superexchange interaction of Ni3+-O-Mn3+ induces strong Jahn-Teller distortion of MnO6 and NiO6 octahedra, elongating metal-O bonds, which helps to form the active species of Mn/Ni hydroxide/oxide on the surface of catalysts. Surprisingly, La2NiMnO6 nanoparticles exhibit superior oxygen evolution reaction (OER) catalytic performance with higher current density and lower Tafel slope than its bulk counterpart. Our finding will be a promising pathway to develop advanced precious-metal-free catalysts.

Perovskite oxide/carbon nanotube hybrid bifunctional electrocatalysts for overall water splitting

Perovskite oxides recently have emerged as efficient electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline solution. However, most perovskites have poor electrical conductivity at room temperature. Here, to simultaneously enhance the activity and stability towards both OER and HER, we in situ-introduce carbon nanotubes (CNTs) on the perovskite (SrTi0.1Fe0.85Ni0.05O3–δ) surface via a simple chemical vapor deposition (CVD) method. First, we evaluate the influence of the CVD temperature on the growth of the CNTs. The optimized electrocatalyst obtained at a temperature of 700 °C (denoted as STFN/CNT-700) exhibits higher OER and HER activities than the bare perovskite, which is reflected by the current density, onset potential, overpotential and Tafel slope, and thus we construct a homemade electrolyzer for overall water splitting using STFN/CNT-700 as both an anode and cathode in alkaline electrolyte. Lastly, the increased performances are mainly attributed to the improved charge transfer process, increased surface area, and synergistic effect between the perovskite phase and the CNTs phase. This work may be inspiring to adjust the performance of metal oxides and prepare highly efficient and low-cost composite electrocatalysts.

Strong-lensing measurement of the total-mass-density profile out to three effective radii for z ∼ 0.5 early-type galaxies

ABSTRACT We measure the total-mass-density profiles out to three effective radii for a sample of 63$z$ ∼ 0.5, massive early-type galaxies (ETGs) acting as strong gravitational lenses through a joint analysis of lensing and stellar dynamics. The compilation is selected from three galaxy-scale strong-lens samples, namely the Baryon Oscillation Spectroscopic Survey (BOSS) Emission-Line Lens Survey (BELLS), the BELLS for GALaxy-Lyα EmitteR sYstems Survey (BELLS GALLERY), and the Strong Lensing Legacy Survey (SL2S). Utilizing the wide source-redshift coverage (0.8–3.5) provided by these three samples, we build a statistically significant ensemble of massive ETGs for which robust mass measurements can be achieved within a broad range of Einstein radii up to three effective radii. Characterizing the three-dimensional total-mass-density distribution by a power-law profile as ρ ∝ r−γ, we find that the average logarithmic density slope for the entire sample is $\langle \gamma \rangle =2.000_{-0.032}^{+0.033}$ (68 per cent CL) with an intrinsic scatter of $\delta =0.180_{-0.028}^{+0.032}$. Further parametrizing 〈γ〉 as a function of redshift $z$ and the ratio of Einstein radius to effective radius Rein/Reff, we find that the average density distributions of these massive ETGs become steeper at later cosmic times and at larger radii, with magnitudes $\mathrm{d} \langle \gamma \rangle / \mathrm{d}z=-0.309_{-0.160}^{+0.166}$ and $\mathrm{d} \langle \gamma \rangle / \mathrm{d} \log _{10} ({R_{\rm ein}}/{R_{\rm eff}})=0.194_{-0.083}^{+0.092}$.

Determination of Watershed Characteristics and Curve Number (CN) for Ponnaniyaru Dam Catchment Area

In SCS-CN method, curve number is significant parameter in estimating runoff from the catchment of the reservoir or inflow to the reservoir. As this curve number is function of several parameters like hydrological soil group, LULC, land treatment, hydrologic conditions and AMC, the selection of CN for prediction of inflow to the lake or reservoir is considered as a crucial in the hydrological studies. LULC, micro-watershed, drainage density, and catchment slope are obtained using spatial analysis and also SCS Curve Number value for Ponnaniyaru dam catchment area is derived from the LULC data. Further, CN value is evaluated from actual rainfall data and runoff volume collected at the reservoir. The study reveals the significant variation of CN value among each event. The present case study highlights the sensitiveness of CN value in the computation of runoff from the watershed. Keywords: Curve number, LULC, AMC, drainage density.

Relevance of analytical Buckley\u2013Leverett solution for immiscible oil displacement by various gases

To generate the valid numerical simulation model, the sufficient amount of gathered data from the oil field is required. However, it is not always possible to acquire such data at the initial stage of project development. Buckley and Leverett (Pet Trans AIME 146(01):107–116, 1942) developed the analytical solution allowing to easily assess the oil displacement efficiency. One of the main assumptions of this model is incompressibility of oil and injected fluid. For slightly compressible water and oil such assumption is rational. However, that is not always the case when the gas is injected. This research aims to identify the conditions at which the usage of the incompressible gas model is appropriate. Likewise, the cases when the model of compressible gas is required are also evaluated. To accomplish the goals of this research, the comparative analysis between the injection of compressible and incompressible gases was undertaken using the numerical solution of the correspondent reservoir engineering problems. The validation of the numerical model was performed showing that it matches the analytical Buckley–Leverett solution. The findings of this research indicate that the relative and absolute density change with the pressure of the injected gas has the profound impact on the convergence between two models under consideration. With the increase in the injection pressure, the discrepancy between the models of compressible and incompressible gas raises for all the considered injection fluids (CO_2, CH_4 and N_2). Due to a steep slope of density–pressure curve for CO_2 at low initial reservoir pressure, the incompressible model cannot accurately predict the oil displacement efficiency by this gas at any reasonable injection pressure. All one-dimensional results are also representative for two-dimensional simulations (2D). However, the mismatch between two models increases considerably for 2D simulation scenarios. This study might be beneficial for those considering or researching the possibility of immiscible gas flooding by various gases at the particular oil field. Knowing some basic reservoir and technological parameters, the presented results might be used as simple screening criteria allowing to estimate the relevance of analytical Buckley–Leverett solution.

SEAGLE – I. A pipeline for simulating and modelling strong lenses from cosmological hydrodynamic simulations

In this paper we introduce the SEAGLE (i.e. Simulating EAGLE LEnses) program, that approaches the study of galaxy formation through strong gravitational lensing, using a suite of high-resolution hydrodynamic simulations, Evolution and Assembly of GaLaxies and their Environments (EAGLE) project. We introduce the simulation and analysis pipeline and present the first set of results from our analysis of early-type galaxies. We identify and extract an ensemble of simulated lens galaxies and use the GLAMER ray-tracing lensing code to create mock lenses similar to those observed in the SLACS and SL2S surveys, using a range of source parameters and galaxy orientations, including observational effects such as the Point-Spread-Function (PSF), pixelization and noise levels, representative of single-orbit observations with the Hubble Space Telescope (HST) using the ACS-F814W filter. We subsequently model these mock lenses using the code LENSED, treating them in the same way as observed lenses. We also estimate the mass model parameters directly from the projected surface mass density of the simulated galaxy, using an identical mass model family. We perform a three-way comparison of all the measured quantities with real lenses. We find the average total density slope of EAGLE lenses, $t=2.26\; (0.25\; \rm{rms})$ to be higher than SL2S, $t=2.16$ or SLACS, $t=2.08$. We find a very strong correlation between the external shear ($\gamma$) and the complex ellipticity ($\epsilon$), with $\gamma \sim \epsilon/4$. This correlation indicates a degeneracy in the lens mass modeling. We also see a dispersion between lens modeling and direct fitting results, indicating systematical biases.

Structure of planar shock waves in gaseous mixtures based on ab initio direct simulation

The structure of planar shock wave propagating through a helium–argon mixture is modeledby the direct simulation Monte Carlo (DSMC) method based on ab initio potentials for a wide range of the Mach number and for various molar fractions. The use of ab initio potentials allows us to carry out the simulation without any adjustable parameter usually extracted from experimental data. As a result, the density, temperature, molar fraction, diffusion velocity, pressure tensor and heat flow profiles inside of the shock wave are calculated. The temperature overshoot phenomenon is discussed in details. The slopes of density are calculated with the numerical error less than 0.5 %. It is pointed out that the slopes for mixture are always smaller than those for a single gas.

Neutron skin thickness of finite nuclei with finite range effective interaction in droplet model

We analyze the relation between the symmetry energy coefficient [Formula: see text] of finite nuclei with mass number [Formula: see text] in the semi-empirical mass formula. The nuclear matter symmetry energy [Formula: see text] at reference density [Formula: see text] in the subsaturation density region can be determined by the symmetry energy [Formula: see text] and its density slope [Formula: see text] at the saturation density [Formula: see text]. From this relation, the neutron skin thickness ‘[Formula: see text]’ in finite nuclei with droplet model are correlated to the various symmetry energy parameters. A prominent role of the bulk symmetry energy [Formula: see text] to the so-called surface stiffness coefficient [Formula: see text] is observed in deriving the size of the neutron skin. Two types of neutron skins are distinguished: the “surface” and the “bulk”. The linear dependence of the neutron skin thickness for different stable nuclei ([Formula: see text]) on the slope [Formula: see text] of the symmetry energy as well as on the relative neutron excess [Formula: see text] is observed. Though the value of the surface width is found to be limited within 0.1[Formula: see text]fm, its contribution should not be neglected to measure neutron skin thickness.

Significant enhancement in the propagation of cosh-Gaussian laser beam in a relativistic–ponderomotive plasma using ramp density profile

AbstractThe paper presents an investigation on self-focusing of cosh-Gaussian (ChG) laser beam in a relativistic–ponderomotive non-uniform plasma. It is observed numerically that the selection of decentered parameter and initial beam radius determines the focusing/defocusing of ChG laser beam. For given value of these parameters, the plasma density ramp of suitable length can avoid defocusing and enhance focusing effect significantly. Focusing length and extent of focusing may also be controlled by varying slope of the ramp density. A comparison with Gaussian beam has also been attempted for optimized set of parameters. The results establish that ChG beam focuses earlier and sharper relative to Gaussian beam. We have setup the non-linear differential equation for the beam width parameter using Wentzel–Kramers–Brillouin and paraxial ray approximation and solved it numerically using Runge–Kutta method.

THE GROWING SPACE UTILIZATION OF MAIN TREE SPECIES IN NORTHERN TURKEY

ABSTRACT Relationships between tree size and density are important to define the growing space utilization in a stand. Although a universal slope for the maximum size-density relationships (MSDRs) has been previously suggested, recent research have highlighted that these relationships are species-specific. Oriental beech (Fagus orientalis Lipsky), Trojan fir (Abies nordmanniana subsp.equi-trojani), black pine (Pinus nigra Arnold) and Scots pine (Pinus sylvestris L.) represent different crown architecture, growth rate and shade tolerance; however, MSDRs have not been developed for these tree species in northern Turkey. In this study, average maximum density (DAM) slopes for these tree species were determined. Results suggested that MSDRs varied among the species, and that their slope differed from the universal slope of -1.605. The MSDRs described in this study are useful for managing stand density in natural stands of the species analyzed.

Structural controls on distribution of dolines on Mount Anamas (Taurus Mountains, Turkey)

Solution dolines are characteristic landforms on the high plateaus of the Taurus Mountains. In this study, the effects of tectonic structures, drainage and slope conditions on the distribution and morphometric properties of dolines in the western part of the Central Taurus are explained. To delimit the dolines, 1/25000 scale topographic maps were used. The uppermost closed contour lines were digitized as polygons and the elevation, long and short axes, elongation ratios, orientation angles of the polygons were calculated. The doline density is compared with drainage density and mean slope values. A total of 111 rose diagrams were created in order to illustrate the superficial distribution of orientation. As a result, a total of 10,652 dolines were detected in a 711 km2 doline area and maximum density reaches 123 dolines/km2. However, according to the doline density, two distinct regions were detected, high density (R1) and low density (R2). Correlations between drainage density, slope angle and doline density are negative in the two regions. According to negative correlations, the drainage density is not a determinant factor in R1, while it is a determinant factor in R2. A 30° slope angle (in km2) limits doline distribution on the plateau surface.The orientation of all doline long axes is NW-SE, which is parallel to the general orographic extent of Mount Anamas. However, different orientations were detected and these orientations were showed in farther sub-regions according to the 111 gridded rose diagrams. The overall result of this study is that the fault and joint systems that developed on the thick-bedded limestone between thrust faults affected the doline density, while lateral strike-slip, normal faults and joint systems in front of the thrust belt were more effective on the geometric shapes of dolines.

Amorphous molybdenum sulphide @ nanoporous gold as catalyst for hydrogen evolution reaction in acidic environment

A novel catalyst for water splitting has been prepared by cyclic voltammetry (CV) deposition of amorphous MoS2 on nanoporous gold (NPG) obtained by electrochemical de-alloying a metallic glass precursor, Au40Cu28Ag7Pd5Si20 (at.%). As a function of number of CV cycles, Au ligaments are progressively encapsulated by a thin layer of amorphous MoS2 catalyst for hydrogen evolution. NPG provides a good substrate for deposition having excellent properties of handability and mechanical resistance in addition to catalytic activity, high conductivity and high surface area. Low onset potential, low Tafel slope and interesting values of exchange current density were obtained in linear sweep voltammetry of deposited samples. The synergistic behaviour of the multi-component material provides a highly efficient catalyst for hydrogen making of these samples a good candidate in some instances comparable to the conventional Pt catalyst for hydrogen evolution in acidic environment.

Constraints on the nuclear symmetry energy and its density slope from the decay process

We study the impact of the nuclear symmetry energy and its density dependence on the α-decay process. Within the framework of the preformed cluster model and the energy density formalism, we use different parameterizations of the Skyrme energy density functionals that yield different equations of state (EOS). Each EOS is characterized by a particular symmetryenergy coefficient (asym) and a corresponding density-slope parameter L. The stepwise trends of the neutron (proton) skin thickness of the involved nuclei with both asym and L do not clarify the oscillating behaviors of the α-decay half-life Tα with these quantities. We find that the change of the skin thickness after α-decay satisfactorily explains these behaviors. The presented results provide constraints on asym centered around an optimum value asym=32 MeV, and on L between 41 and 57 MeV. These values of asym and L, which indicate larger reduction of the proton-skin thickness and less increase in the neutron-skin thickness after an α-decay, yield a minimum calculated half-life with the same extracted value of the α-preformation factor inside the parent nucleus.

Carrier Dynamics of Nanopillar Textured Ultrathin Si Film/PEDOT:PSS Heterojunction Solar Cell

Computational study of nanopillar Si/poly (3,4-ethylene dioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) heterojunction solar cell is performed by using finite element method to solve Poisson equation, drift-diffusion equations, and current continuity equations. The depletion width around the edge of the Si nanopillar is almost the same as that of planar junction, while the depletion width around the middle of the Si nanopillar is much wider than that of planar junction. As a result, besides the enhanced light absorption due to light trapping induced by nanopillar array as in previous studies, larger depletion region, which leads to higher electron-hole separation efficiency under light illumination, is another reason for short-circuit current increase of nanopillar solar cell. The reason for less open-circuit voltage of nanopillar solar cell is also clarified by explaining the reason for large dark current of the nanopillar solar cell compared to its planar structure counterpart as follows. The kink of the potential profile along the lines around the edge nanopillar leads to much sharper potential drop in the following depletion region, and results in greater hole density slope when the solar cells are forwarded biased. Since the Si/PEDOT:PSS behaves like a pn junction rather than a Schottky junction as clarified in the previous study and the PEDOT:PSS is much highly doped, nanopillar heterojunction has larger dark current than planar junction due to the larger hole diffusion current around Si nanopillar edge.

On filtering ionospheric effects in GPS observations using the Matérn covariance family and its impact on orbit determination of Swarm satellites

With three near-polar low earth orbiter satellites, the Swarm mission contributes to bridging the gap between Earth gravity field missions. However, observations from on-board GPS receivers are strongly impacted by ionospheric scintillations, degrading the kinematic orbit solutions of the satellites and thus the gravity field, which is derived from them. Besides the elimination of parts of the GPS observations strongly impacted by scintillations, an alternative is a physically based filtering of these measurements. Based on the previous empirical and theoretical works, a Matern covariance function, whose parameters are to be chosen adequately, is an answer to mitigate ionospheric effects from the GPS time series. An optimal parameter set corresponding to a smoothness of 1 and a correlation factor of 1.5 is physically plausible and provides an adequate filtering of the ionospheric scintillations. The detection of noisy parts is carried out with an easy to use algorithm to obtain filtered time series with a homogeneous standard deviation. The orbit solution computed with filtered observations does not exhibit ionospheric artifacts compared with the non-filtered solution. At the same time, the high-frequency slope of the orbit power spectral density is similar to the one obtained for GPS carrier phase observations with low ionospheric scintillation errors. The proposed methodology is independent of the processing used, whether double differences, Observed Minus Computed or raw carrier phase observations.

Bee diversity in crop fields is influenced by remotely-sensed nesting resources in surrounding permanent grasslands

Landscape heterogeneity is an important driver of biodiversity in agroecosystems. However, the functional heterogeneity of agricultural landscapes, taking into account the different resources that habitat patches can provide to species, has rarely been studied. In this study, we explored the effect of landscape-scale nest availability provided by permanent grasslands on wild bee communities. Wild bees were sampled in 43 cereal fields in south-western France differing in the surrounding proportion of permanent grasslands. Using remote sensing tools, we measured two parameters of grassland structure known to locally influence bee nest density (slope and proportion of sparse vegetation). We found that mean slope of surrounding grasslands was the factor that most positively influenced bee richness, abundance and trait distribution in bee communities. We also found that mean slope of surrounding grasslands had a better predictive power of bee community structure than the proportion of permanent grasslands. Ground-nesting species, species with high dispersal capacities and species with a generalist diet were positively affected by the availability of sloped ground in the surrounding permanent grasslands. Only bee species with specialized flower requirements responded positively to the proportion of sparse vegetation in grasslands. Our results suggest that landscape-scale availability of nesting resources provided by grasslands affects bee communities in agricultural landscapes and can help sustain functionally diverse bee assemblages. Using simple remote sensing tools, this study highlights the importance of considering nesting resources in agricultural landscapes to maintain wild bee diversity in farmlands.