Effect Of Local Heterogeneity Research Articles

An Electrochemical Model of Lithium Plating and Stripping in Lithium Ion Batteries

The deposition of a metallic lithium phase on the surface of graphite anodes in lithium ion batteries is a major degradation process and causes inherent safety risks. Despite its importance for battery applications the detection of this so-called lithium plating process during battery charge is very challenging. Therefore, a mechanistic understanding of the Li plating mechanism and the identification of characteristic features in the charge curve of the battery are extremely important. We present an electrochemical model, which enables the description of the deposition and dissolution of a metallic lithium phase in three-dimensional microstructure resolved simulations of lithium ion batteries. The features of this model are demonstrated by simulating the overcharge of a graphite electrode in a half-cell configuration. Simulation results show the typical features of the “stripping-plateau”, which is often observed during discharge after Li plating occurrs. Moreover, a similar feature is observed at the onset of Li plating, which can serve as an indicator for lithium plating in lithium ion batteries during charging, for example, of electric vehicles. Finally, we investigate the impact of an inhomogeneous solid-electrolyte-interphase on the distribution of plated lithium, which highlights the effect of local structural heterogeneities on degradation phenomena.

Correlating Photoluminescence Heterogeneity with Local Electronic Properties in Methylammonium Lead Tribromide Perovskite Thin Films

We conduct correlated laser scanning confocal photoluminescence (PL) microscopy, scanning kelvin probe microscopy, and conductive atomic force microscopy (c-AFM) to understand the origins and effects of local heterogeneity in films of the hybrid organic–inorganic perovskite semiconductor methylammonium lead tribromide (MAPbBr3). We compare PL between perovskite films deposited on glass and on hole-transporting contacts. In both systems, we observe heterogeneous PL, but this heterogeneity is due to different mechanisms. On glass substrates, we observe that the PL maps are dominated by lateral carrier diffusion, and on hole-transporting contacts, we observe an anticorrelation between PL and local hole injected current as measured by c-AFM. We conclude that the local variations are due to heterogeneous electronic coupling at the perovskite–electrode interface. We also show that correlated PL and AFM studies are expected to play a key role in studying the electronic heterogeneities in the perovskite itself, w...

Normal mode coupling observations with a rotation sensor

We present observations of cross coupled spheroidal modes in the Earth's free oscillation spectrum recorded by the vertical component G-ring laser (Geodetical Station Wettzell) of the 2011 Mw 9.0 Tohoku-Oki earthquake. In an attempt to determine which are the mechanisms responsible for spheroidal energy in a vertical axes rotational spectra, we first rule out instrumental effects as well as the effect of local heterogeneity. Secondly, we carry out simulations of an ideal rotational sensor taking into account the effects of the Earth's rotation, its hydrostatic ellipticity and structural heterogeneity, which results in a good fit to the data. Simulations considering each effect separately are performed in order to evaluate the sensitivity of rotational motions to various phenomena compared to traditional translation measurements.

Generating Within-Plant Spatial Distributions of an Insect Herbivore Based on Aggregation Patterns and Per-Node Infestation Probabilities.

Most predator-prey models extrapolate functional responses from small-scale experiments assuming spatially uniform within-plant predator-prey interactions. However, some predators focus their search in certain plant regions, and herbivores tend to select leaves to balance their nutrient uptake and exposure to plant defenses. Individual-based models that account for heterogeneous within-plant predator-prey interactions can be used to scale-up functional responses, but they would require the generation of explicit prey spatial distributions within-plant architecture models. The silverleaf whitefly, Bemisia tabaci biotype B (Gennadius) (Hemiptera: Aleyrodidae), is a significant pest of tomato crops worldwide that exhibits highly aggregated populations at several spatial scales, including within the plant. As part of an analytical framework to understand predator-silverleaf whitefly interactions, the objective of this research was to develop an algorithm to generate explicit spatial counts of silverleaf whitefly nymphs within tomato plants. The algorithm requires the plant size and the number of silverleaf whitefly individuals to distribute as inputs, and includes models that describe infestation probabilities per leaf nodal position and the aggregation pattern of the silverleaf whitefly within tomato plants and leaves. The output is a simulated number of silverleaf whitefly individuals for each leaf and leaflet on one or more plants. Parameter estimation was performed using nymph counts per leaflet censused from 30 artificially infested tomato plants. Validation revealed a substantial agreement between algorithm outputs and independent data that included the distribution of counts of both eggs and nymphs. This algorithm can be used in simulation models that explore the effect of local heterogeneity on whitefly-predator dynamics.

Low-intensity agricultural landscapes in Transylvania support high butterfly diversity: implications for conservation.

European farmland biodiversity is declining due to land use changes towards agricultural intensification or abandonment. Some Eastern European farming systems have sustained traditional forms of use, resulting in high levels of biodiversity. However, global markets and international policies now imply rapid and major changes to these systems. To effectively protect farmland biodiversity, understanding landscape features which underpin species diversity is crucial. Focusing on butterflies, we addressed this question for a cultural-historic landscape in Southern Transylvania, Romania. Following a natural experiment, we randomly selected 120 survey sites in farmland, 60 each in grassland and arable land. We surveyed butterfly species richness and abundance by walking transects with four repeats in summer 2012. We analysed species composition using Detrended Correspondence Analysis. We modelled species richness, richness of functional groups, and abundance of selected species in response to topography, woody vegetation cover and heterogeneity at three spatial scales, using generalised linear mixed effects models. Species composition widely overlapped in grassland and arable land. Composition changed along gradients of heterogeneity at local and context scales, and of woody vegetation cover at context and landscape scales. The effect of local heterogeneity on species richness was positive in arable land, but negative in grassland. Plant species richness, and structural and topographic conditions at multiple scales explained species richness, richness of functional groups and species abundances. Our study revealed high conservation value of both grassland and arable land in low-intensity Eastern European farmland. Besides grassland, also heterogeneous arable land provides important habitat for butterflies. While butterfly diversity in arable land benefits from heterogeneity by small-scale structures, grasslands should be protected from fragmentation to provide sufficiently large areas for butterflies. These findings have important implications for EU agricultural and conservation policy. Most importantly, conservation management needs to consider entire landscapes, and implement appropriate measures at multiple spatial scales.

Neutral Dynamics and Cluster Statistics in a Tropical Forest

The neutral theory of biodiversity attributes community structure to the effects of chance alone, assuming that all species and individuals are demographically equivalent. Here we present a spatially explicit version of the neutral theory and test it against the Barro Colorado Island (BCI) data. Monitoring the dynamics of clusters, we show that the effect of local heterogeneities (e.g., microtopography) is weak, making a spatially homogenous model plausible. We then compare the cluster statistics of the three most frequent species with the patterns obtained from neutral dynamics, examining two families of recruitment kernels: one that interpolates between a limited distance and panmictic dispersal (local-global) and one that assumes a scale-free Cauchy kernel. The results rule out the local-global dispersal model and show that the spatial patterns fit very nicely those obtained from the fat-tailed kernel. Our work emphasizes the importance of spatiotemporal cluster dynamics as an instrument for detecting the factors that govern community assembly.

Parameterizing the Leaching Surface by Combining Curve-Fitting for Solute Breakthrough and for Spatial Solute Distribution

Multi-compartment samplers (MCSs) measure unsaturated solute transport in space and time at a given depth. Sorting the breakthrough curves (BTCs) for individual compartments in descending order of total solute amount and plotting in 3D produces the leaching surface. The leaching surface is a useful tool to organize, present, and analyze MCS data. We present a novel method to quantitatively characterize leaching surfaces. We fitted a mean pore-water velocity and a dispersion coefficient to each BTC, and then approximated their values by functions of the rank order of the BTCs. By combining the parameters of these functions with those of the Beta distribution fitted to the spatial distribution of solutes, we described an entire leaching surface by four to eight parameters. This direct characterization method allows trends to be subtracted from the observations, and incorporates the effects of local heterogeneity. The parametric fit creates the possibility to quantify concisely the leaching behavior of a soil in a given climate under given land use, and eases the quantitative comparison of spatio-temporal leaching behavior in different soils and climates.

HeteroFoaMs: Electrode Modeling in Nanostructured Heterogeneous Materials for Energy Systems

Heterogeneous functional materials, e.g., “HeteroFoaMs” are at the heart of countless energy systems, including heat storage materials, batteries, solid oxide fuel cells, and polymer electrolyte fuel cells. HeteroFoaMs are generally nanostructured and porous to accommodate transport of gasses or fluids, and must be multifunctional (i.e., active operators on mass, momentum, energy, or charge, in combinations). This paper will discuss several aspects of modeling the relationships between the constituents and microstructure of these material systems and their device functionalities. Technical advances based on these relationships will also be identified and discussed. Three major elements of the general problem of how to model HeteroFoaM electrodes will be addressed. Modeling approaches for ionic charge transfer with electrochemistry in the nanostructured porosity of the electrode will be discussed. Second, the effect of morphology and space charge on conduction through porous doped ceria particle assemblies, and their role in electrode processes will be modeled and described. And third, the effect of local heterogeneity and morphology on charge distributions and polarization in porous dielectric electrode materials will be analyzed using multiphysics field equations set on the details of local morphology. Several new analysis methods and results, as well as experimental data relating to these approaches will be presented. The value, capabilities, and limitations of the approaches will be evaluated.

Experimental Investigation of Through Air Drying of Tissue and Towel under Commercial Conditions

An experimental setup was developed to study the through air–drying characteristics of permeable grades such as tissue and towel under commercially relevant conditions of basis weight, airflow rate, temperature, and humidity conditions. The experimental setup is capable of evaluating the transient fluid flow, heat, and mass transfer characteristics of relatively larger samples (TAPPI standard hand sheets; 0.1524 m) and is capable of studying the effect of local heterogeneity and structure on convective heat and mass transfer. The system is capable of airflow rates of 0.5–10 m/s with corresponding high-speed data collection and acquisition for measuring important variables such as exhaust air humidity. To study the effect of nonuniformity, local temperature and velocity profiles can also be measured using grid of thermocouples and hot wire anemometers. The instantaneous drying rate and airflow characteristics during through air drying was measured and dry permeability, wet permeability, and convective heat and mass transfer characteristics were then calculated. The experimental results were verified by comparing with the results from literature. Typical experimental results were presented to show the effect of sheet basis weight, initial moisture content, and airflow rates on the drying characteristics for two different types of paper samples.

Effect of Local Heterogeneity on Dielectric Relaxation Spectra in Concentrated Solutions of Poly(vinyl acetate) and Poly(vinyl octanoate)

The effect of local heterogeneity on the distribution of dielectric relaxation times was studied for concentrated solutions of poly(vinyl acetate) (PVAc) in 1-methylnaphthalene (MN) and those of poly(vinyl octanoate) (PVOc) in toluene (Tol). The half-widths A of the dielectric loss curves for the primary processes of those systems are compared with those of the PVAc/Tol system reported previously. The data indicate that A decreases in order of PVAc/Tol, PVAc/MN, and PVOc/Tol systems if compared at the same temperature T/Tg where Tg denotes the glass transition temperature of solutions. The half-width of each solution increases with decreasing temperature. The loss curve is calculated by assuming a Gaussian distribution of the local concentration Φ which results in the distribution of relaxation times g(τ). The calculated loss curves agree fairly well with the observed ones. The broadening behavior is also explained by assuming that A is proportional to the amplitude of the local concentration fluctuation ΔΦ times the slope of the Φ dependence curve of the average relaxation time r. This assumption leads to a linear relationship between A and ΔΦ/(T - T 0 ) 2 where To is the Vogel critical temperature. Using this relation, we have attempted to estimate ΔΦ. Small-angle X-ray scattering was also measured on PVAc/ MN solutions and found that the scattering intensity was lower than that for PVAc/Tol. This is consistent with the fact that A of PVAc/MN solutions is narrower than PVAc/Tol solutions.

Using the continuum approach to model unsaturated flow in fractured rock

The appropriateness of using the continuum approach for simulations of unsaturated flow through fractured rock is examined in a numerical study of water seepage into an underground opening. A continuum model is calibrated against data generated with a high‐resolution model that created discrete flow and seepage behavior. Probabilistic predictions of injection data and seepage rates are then compared to the values from the discrete model. The study shows that reasonable results can be obtained with a calibrated continuum model even for fractured systems in which the underlying flow processes are discrete. Calibration is a crucial step in this approach because it yields effective, model‐related, and process‐specific parameters. The impact of discretization on the estimated parameters is discussed. Prediction uncertainty is evaluated by means of Monte Carlo simulations, including both the impact of parameter uncertainties and the effect of local heterogeneity. In this paper, the advantages and limitations of the continuum approach are discussed from a practical perspective, which includes a critical examination of project objectives, data needs, robustness of assumptions, and prediction uncertainty.

Effect of Local Heterogeneity on Dielectric Segmental Relaxation of Poly(vinyl acetate) in Concentrated Solution

Dielectric segmental mode relaxation (the α-relaxation) was investigated for toluene solutions of poly(vinyl acetate) (PVAc) and for undiluted PVAc. The relaxation spectra in concentrated solutions were found to be much broad compared with those in the undiluted state and in dilute solution. It was also found that the relaxation spectra broadened with decreasing temperature near the glass transition temperature. For concentrated solutions, the correlation length of local heterogeneity (due to concentration fluctuation) was determined to be 1.0 nm by using small-angle X-ray scattering. On the basis of the assumption that the distribution of relaxation times in logarithmic scale is proportional to the magnitude of the heterogeneity, the temperature dependence of the broadness of loss curves can be explained. Thus, the broad dielectric relaxation spectra in concentrated solutions can be attributed to a local configurational irregularity of the polymer segments and the solvent molecules.

Chromatophore heterogeneity explains phenomena seen in Rhodobacter sphaeroides previously attributed to supercomplexes

It is generally considered that metabolic reactions are well described by homogeneous kinetic models in which the reaction phase is statistically uniform. In membranes, especially in photosynthetic systems where the protein complement is high, it has recently been recognized that effects of local heterogeneity might contribute additional factors that perturb the kinetic behavior, and require more extensive treatment. We show in this paper that statistical heterogeneity in vesicular systems can also contribute to quite marked discrepancies from the behavior expected from standard kinetic and thermodynamic models, for reactions involving free diffusion in the aqueous phase. We explain the kinetic and thermodynamic effects observed in studies of photosynthetic electron transfer in cells and chromatophores from Rhodobacter sphaeroides previously attributed to supercomplexes, in terms of a model based on heterogeneity in distribution of electron transfer components among the chromatophore population. We discuss examples of data inconsistent with the supercomplex model, but well explained by the heterogeneity model.

Experimental studies of mass transport in porous media with local heterogeneities

The effect of local heterogeneities in a porous medium, which are randomly distributed, upon longitudinal spreading in groundwater transport problems is described and related to the permeability ratio between the heterogeneities and the surrounding porous medium. Numerous experimental runs using simplified model aquifers provide a large data base. The measured breakthrough curves indicate the transport behavior of the system. They are compared with the solution of the classical Fickian advection-dispersion equation for the given boundary conditions as well as with a numerical approach of a modified transport formulation following the dual porosity concept of Coats and Smith. A significant difference in the transport behavior depending on the permeability of the heterogeneities is observed: while mass transport in the model aquifers with local heterogeneities of higher relative permeability remained Fickian, local heterogeneities of lower relative permeability caused a marked tailing and a significant spreading of the tracer. In the dual porosity formulation of transport the transfer coefficient had to be fitted, while the remaining transport parameter could be determined directly.