Characterization Of Transport Properties Research Articles

Saturation Dependence of Non‐Fickian Transport in Porous Media

AbstractIn two‐phase flow through porous media, the percolating pathways can be hydrodynamically split into the flowing and stagnant regions. The highly variable velocity field in the pore space filled by the carrier fluid leads to significant differences in the transport time scales in the two regions that cannot be explained by the Fickian (Gaussian) advection‐dispersion equation. In contrast with the Darcy‐scale studies, up to now, relatively limited pore‐scale studies have been devoted to the characterization of transport properties in two‐phase flow. In this paper, we report on the results of computer simulation of advection‐dispersion transport in steady state two‐phase flow through porous media using a pore network model, employed as an upscaling tool. The simulation results are upscaled to directly estimate the Darcy‐scale transport coefficients and properties, namely, stagnant saturation, the mass transfer coefficient between the flowing and stagnant regions, and the longitudinal dispersion in the flowing regions. The mobile‐immobile model, one of the most commonly used models for simulating non‐Fickian transport in porous media, is used to estimate the transport properties using the inverse modeling of effluent concentration profiles. The disagreement between the directly estimated parameters and those obtained by the mobile‐immobile‐based inverse modeling implies fundamental shortcomings of the latter for describing transport in two‐phase flow. The simulation results indicate that the relative permeabilities may be used to obtain accurate estimates of the stagnant saturation, which link two‐phase Darcy's law and transport.

Effects of energetics with {001} facet-dominant anatase TiO2 scaffold on electron transport in CH3NH3PbI3 perovskite solar cells

The anatase titania with large fraction of {001} facet on the mesoscopic anatase titania scaffold in the hybrid perovskite solar cells exhibited higher photocurrent and open-circuit voltage. The higher performance with {001}-dominant anatase titania with the fraction of 74% were discussed with the characteristic electron transport properties in addition to the energy levels of trap states and conduction band as compared with those properties of conventional anatase titania scaffold. The resulted higher photocurrent and open-circuit voltage were attributed to the enhancement of the electron injection and suppression of the carrier recombination, respectively, at the titania/perovskite semiconductor interface.

Structure and Doping Determined Thermoelectric Properties of Bi2Se3Thin Films Deposited by Vapour–Solid Technique

In this work, a simple catalyst-free vapour-solid deposition method was applied for controlled deposition of two types (planar and disordered) of continuous Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanostructured thin films on different (fused quartz/glass, mica, graphene) substrates. Characterisation of electron transport (type, concentration and mobility of the main charge carriers) and thermoelectric properties (Seebeck coefficient and power factor) showed that proposed in this work deposition method allows to obtain Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin films with power factor comparable and even higher than reported for the Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thin films grown by molecular beam epitaxy technique. Power factor of the best obtained thin films can be significantly improved by introduction Sb or Fe dopants in low concentrations.

WITHDRAWN: Compressive strength, pore structure and chloride transport properties of alkali-activated slag/fly ash mortars

Abstract The effects of silicate modulus, alkali dosage, fly ash content and steam curing on compressive strength, pore structure and chloride transport properties of alkali-activated slag/fly ash (AASF) mortars have been investigated. The results showed that steam curing exhibited little influences on compressive strength and chloride transport properties of AASF mortars. The compressive strengths of AASF mortars increased with the increase of silicate modulus, alkali dosage, and decreased with the increase of the fly ash content due to the changes of pore structure within the range of 10–104 nm. The decrease of both chloride migration coefficient from the rapid chloride migration (RCM) test and passed charge from the rapid chloride permeability (RCP) test of AASF mortars were observed with the increase of silicate modulus and decrease of fly ash content due to the improvement of their pore structure in the aforementioned range. However, inconsistent results between RCM and RCP tests were observed with the increase of alkali dosage. The immersion of the AASF mortars in water decreased their passed charge due to the ion leaching out from their pore solution. The increase of water-to-specimen ratio increased the amount of leached substance and decreased the passed charge, but showed no effect on their chloride migration coefficient. Thus, RCM test is more suitable for characterization of chloride transport properties of alkali-activated materials than RCP test.

Z-scanning laser photoreflectance as a tool for characterization of electronic transport properties

The physical principles motivating the Z-scanning laser photoreflectance technique are discussed. The technique is shown to provide a powerful non-contact means to unambiguously characterize electronic transport properties in semiconductors. The technique does not require modeling of charge transport in the sample or a detailed theoretical model for the sample physics. Rather, the measurement protocol follows directly from the simple relation describing the radial diffusion of carriers injected by a laser source. The use of a probe laser beam permits an analytic parametrization for the Z dependence of the photoreflectance signal which depends solely on the focal parameters and the carrier diffusion length. This allows electronic transport properties to be determined with high precision using a nonlinear least squares fit procedure. The practical use of the technique is illustrated by the characterization of carrier transport properties in semiconducting p-n junctions.

On the dynamics of metal filled CNOs fusion: the case of sp3-rich Cu filled carbon foam

In the last decade, important research efforts have been posed towards the fabrication of novel carbon polymorphs with controlled sp3/sp2 properties. Nucleation of novel carbon polymorphs has been reported with the use of (1) cold compression experiments and (2) high temperature annealing. In the latter, the formation of a novel foam-like carbon (CFM) material has been reported as the result of an unusual spontaneous process of high temperature fusion of Fe3C-filled carbon nano-onions (Fe3C@CNOs). Being interested in investigating further such CNOs-fusion mechanism we have considered the use of Cu crystals as catalyst for the formation of the filled CNOs to be used as precursor for the CFM nucleation. The choice of these CNOs as precursors relies on their characteristic mass transport properties. This kind of effect is of important interest also for electrochemical battery-related processes. Specifically, we find that a complete conversion of the filled CNOs-precursors into CFM can be achieved within very short timescales of 6 h of annealing. This implies the presence of fast diffusion and mass transport dynamics. In addition, through the use of multiple characterization techniques, such as TEM, HRTEM, XRD, Raman Spectroscopy and XPS we investigate the variation of structural and surface characteristics of the Cu@CNOs and we demonstrate the formation of a sp3-rich mm-scale CFM material completely filled with crystalline copper after the annealing stage. Further investigations are then performed at high pressures of 5 GPa. These measurements reveal a substantial change in the CFM network-morphology and a comprehensive increase in the sp3-content.

Characterization of electron charging and transport properties of Si-QDs with phosphorus doped Ge core

We formed high-density Si quantum dots (Si-QDs) with undoped and P-doped Ge cores on thermally-grown SiO2 by controlling low pressure chemical vapor depositions of Si and Ge. Doping into the Ge core was carried out during selective-growth of Ge on pre-grown Si-QDs. From hard x-ray photoelectron spectroscopy measurements, we confirmed phosphorus incorporation into the Ge core, where phosphorus donor concentration in the Ge core was roughly estimated to be ∼0.09 at%. When the surfaces of Si-QDs with undoped and P-doped Ge cores were scanned with an Rh-coated tip biased at 0 V, the surface potential of Si-QDs with P-doped Ge cores were increased by ∼30 mV, while that of Si-QDs with undoped Ge cores remained unchanged. These results indicate that an electron was extracted from the conduction band of the Ge core due to a phosphorus donor. In the current images measured with the Rh-coated tip, the current level of Si-QDs with P-doped Ge cores on n-Si(100) substrates was higher than that of Si-QDs with undoped Ge cores. This result indicates that phosphorus donor can contribute to an increase in the electron injection rate from the Si-substrate.

A general and simple method for evaluating the electrical transport performance of graphene by the van der Pauw–Hall measurement

Expected for many promising applications in the field of electronics and optoelectronics, a reliable method for the characterization of graphene electrical transport properties is desired to predict its device performance or provide feedback for its synthesis. However, the commonly used methods of extracting carrier mobility from graphene field effect transistor or Hall-bar is time consuming, expensive, and significantly affected by the device fabrication process other than graphene itself. Here we reported a general and simple method to evaluate the electrical transport performance of graphene by the van der Pauw–Hall measurement. By annealing graphene in vacuum to remove the adsorbed dopants and then exposing it in ambient surroundings, carrier mobility as a function of density can be measured with the increase of carrier density due to the dopant re-adsorption from the surroundings. Further, the relationship between the carrier mobility and density can be simply fitted with a power equation to the first level approximation, with which any pair of measured carrier mobility and density can be normalized to an arbitrary carrier density for comparison. We experimentally demonstrated the reliability of the method, which is much simpler than making devices and may promote the standard making for graphene characterization.

Meyer-Neldel rule in the conductivity of manganite single crystals

The Meyer-Neldel behavior of conductivity of low-doped manganite ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{\mathit{x}}{\mathrm{MnO}}_{3}$ single crystals has been investigated. The evolution of the isokinetic temperature of conductivity, modified by Ca-doping, hydrostatic pressure, and current bias has been determined. In addition, the evolution of isokinetic temperature with ageing has also been studied. The Meyer-Neldel behavior of the manganite system stems from the multiexcitation entropy mechanism. The isokinetic temperature turned out to be a sensitive parameter characterizing changes in the transport properties of mixed valence manganites, which in the presence of a detailed theoretical model of the excitations coupling in manganites could become a powerful tool for the characterization and investigation of transport properties of manganites.

Limitations of sorptivity and water permeability for the estimation of the chloride penetration rate in concrete regarding the accomplishment of prescriptive design for durability in the marine environment

This paper presents an analysis of experimental data from conventional concrete regarding sorptivity and penetrability under pressure comparing these parameters to chloride diffusion rate determined in the laboratory and in actual marine environment. Prescriptions for durability assurance of reinforced concrete structures is based on the qualitative characterization of transport properties. For the specific case of the marine environment, it is required to assess the resistance of concrete to chloride ingress. The results show the limitations of both parameters as prescriptive indexes, with capillary absorption rate showing some advantages over water penetration under pressure.

Tunable transmittance in anisotropic two-dimensional materials

A uniaxial strain applied to graphene-like materials moves the Dirac nodes along the boundary of the Brillouin zone. An extreme case is the merging of the Dirac node positions to a single degenerate spectral node which gives rise to a new topological phase. Then isotropic Dirac nodes are replaced by a node with a linear behavior in one and a parabolic behavior in the other direction. This anisotropy influences substantially the optical properties. We propose a method to determine characteristic spectral and transport properties in black phosphorus layers which were recently studied by several groups with angle-resolved photoemission spectroscopy, and discuss how the transmittance, the reflectance and the optical absorption of this material can be tuned. In particular, we demonstrate that the transmittance of linearly polarized incident light varies from nearly 0\% to almost 100\% in the microwave and far-infrared regime.

Advanced approaches for quantitative characterization of thermal transport properties in soft materials using thin, conformable resistive sensors

Noninvasive methods for precise characterization of the thermal properties of soft biological tissues such as the skin can yield vital details about physiological health status including at critical intervals during recovery following skin injury. Here, we introduce quantitative measurement and characterization methods that allow rapid, accurate determination of the thermal conductivity of soft materials using thin, skin-like resistive sensor platforms. Systematic evaluations of skin at eight different locations and of six different synthetic skin-mimicking materials across sensor sizes, measurement times, and surface geometries (planar, highly curvilinear) validate simple scaling laws for data interpretation and parameter extraction. As an example of the possibilities, changes in the thermal properties of skin (volar forearm) can be monitored during recovery from exposure to ultraviolet radiation (sunburn) and to stressors associated with localized heating and cooling. More generally, the results described here facilitate rapid, non-invasive thermal measurements on broad classes of biological and non-biological soft materials.

Simultaneous determination of effective carrier lifetime and resistivity of Si wafers using the nonlinear nature of photocarrier radiometric signals

A rigorous treatment of the nonlinear behavior of photocarrier radiometric (PCR) signals is presented theoretically and experimentally for the quantitative characterization of semiconductor photocarrier recombination and transport properties. A frequency-domain model based on the carrier rate equation and the classical carrier radiative recombination theory was developed. The derived concise expression reveals different functionalities of the PCR amplitude and phase channels: the phase bears direct quantitative correlation with the carrier effective lifetime, while the amplitude versus the estimated photocarrier density dependence can be used to extract the equilibrium majority carrier density and thus, resistivity. An experimental ‘ripple’ optical excitation mode (small modulation depth compared to the dc level) was introduced to bypass the complicated ‘modulated lifetime’ problem so as to simplify theoretical interpretation and guarantee measurement self-consistency and reliability. Two Si wafers with known resistivity values were tested to validate the method.

Characterization of thermal transport and laser absorption properties of an individual graphitized carbon fiber by applying Raman thermography

Graphitized carbon fibers have potential applications owing to their excellent electrical, mechanical, thermal and optical properties. Thermal transport and laser absorption properties are the fundamental parameters for the thermal design, but difficult for determining due to their very small characteristic sizes. In this study, we systematically investigated the temperature (T)-dependent apparent thermal conductivity (λa), thermal conductivity (λ), thermal contact resistance (Rc, between sample and heat sink) and laser absorptivity (α) of an individual graphitized carbon fiber using a non-contact Raman method. This method and the experimental system were verified by comparing the measured thermal conductivity of a 10.0 μm diameter platinum wire with the standard data. The measured λ of this graphitized carbon fiber decreases from 372.4 to 330.1 W/(m·K) as T increases from 338 to 496 K, indicating the three-phonon Umklapp scattering rate increases with temperature. Rc increases with T from 3.44 × 103 to 6.35 × 103 K/W in this experimental temperature range, and the laser (488 nm wavelength) absorptivity is determined to be 0.90 ± 0.02.

Advective Displacement Method for the Characterisation of Pore Water Chemistry and Transport Properties in Claystone

The advective displacement method applies large hydraulic gradients to a confined rock core sample to yield small aliquots of the preserved in situ pore water, applicable to aquitard rocks with hydraulic conductivities as low as 10−14 m/s. Examples from argillaceous rocks indicate that only minor artefacts are induced and that analytical methods optimized for small aliquots provide a comprehensive chemical and isotopic characterisation. Multicomponent transport properties are derived from extending experimental time and using a traced artificial pore water for injection. Examples include quantification of the anion exclusion effect that can even resolve a small difference between transport properties for chloride and bromide in claystone. Controls by mineral saturation and cation exchange processes are also constrained by data from this approach. Technical details are provided for construction, material selection, components, sensors, and analytical issues.

An Observatory of Groundwater in Crystalline Rock Aquifers Exposed to a Changing Environment: Hyderabad, India

Core Ideas Long‐term observatories allow the study of global changes to water resources in India. Crystalline rock aquifers are highly heterogeneous. Management of crystalline aquifers necessitates solving several scientific questions. A multidisciplinary approach is necessary for improving water management in India. Multiscale and long‐term work is needed to tackle the scientific challenges found in areas vulnerable to climate change and anthropic pressure. This is the case in the semiarid and drought‐prone regions of southern India where freshwater is scarce and agriculture near fast‐growing cities is triggering high water demand. The Indo‐French Center for Groundwater Research (IFCGR) was established in 1999 between the Indian National Geophysical Research Institute (NGRI) and the French Geological Survey (BRGM) at the NGRI campus in Hyderabad, India. For almost 20 yr, the IFCGR has studied the hydrodynamic properties and associated hydrological processes in crystalline aquifers. To that end, the Center set up two sites for observing groundwater in crystalline rock aquifers: (i) the Maheshwaram basin for the study of groundwater management at catchment scale, and (ii) the Choutuppal experimental site for the detailed study of hydrogeological processes at local scale (between wells). Multiscale approaches allow the characterization of hydrodynamic and transport properties of the shallow weathered part of such crystalline aquifers and the implications for groundwater management under overexploitation conditions. The objective is to provide suitable characterization of aquifer properties for developing modeling and management tools applicable to such heterogeneous aquifers.

Characterization and modelling of structure and transport properties of porous ceramics

Characterization and modelling of structure and transport properties of porous ceramics

The Effect of Membrane Properties on Performance and Transports inside Polymer Electrolyte Membrane Fuel Cells

The primary focus of this work is evaluation, modeling, and comparison of fuel cell properties for a sulfonated aromatic hydrocarbon polymer membrane under fuel cell operations against the standard commercial Nafion membrane. Membrane characterization of transport properties were reported. The fuel cell performance, current distribution mapping, and water balance measurements of both membrane were also examined at different humidity conditions. The studies show that at high RH, the hydrocarbon membrane gives better performance with less water flooding when compared with standard Nafion. However at low RH, where the membrane needs more hydration and conducting of the proton, the standard Nafion membrane shows higher performance than hydrocarbon membrane. Local distributions of current density, water content and liquid water in hydrocarbon membranes were simulated and compared with a Nafion 212 membrane. Both experimental data and model predictions show that the hydrocarbon membrane gives more uniform current distributions than that of the Nafion membrane. The computational model can predict water transport across the membrane and can be used to observe flooding in PEMFC.

Computational and experimental characterization of a pyrrolidinium-based ionic liquid for electrolyte applications.

The development of Li-ion batteries for energy storage has received significant attention. The synthesis and characterization of electrolytes in these batteries are an important component of this development. Ionic liquids (ILs) have been proposed as possible electrolytes in these devices. Thus, the accurate determination of thermophysical properties for these solvents becomes important for determining their applicability as electrolytes. In this contribution, we present the synthesis and experimental/computational characterization of thermodynamic and transport properties of a pyrrolidinium based ionic liquid as a first step to investigate the possible applicability of this class of ILs for Li-ion batteries. A quantum mechanical-based force field with many-body polarizable interactions has been developed for the simulation of spirocyclic pyrrolidinium, [sPyr+], with BF4- and Li+. Molecular dynamics calculations employing intra-molecular polarization predicted larger heat of vaporization and self-diffusion coefficients and smaller densities in comparison with the model without intra-molecular polarization, indicating that the inclusion of this term can significantly effect the inter-ionic interactions. The calculated properties are in good agreement with available experimental data for similar IL pairs and isothermal titration calorimetry data for [sPyr+][BF4-].

Influence of Resolution of Rasterized Geometries on Porosity and Specific Surface Area Exemplified for Model Geometries of Porous Media

Rasterized representations of geometrical structures are commonplace in science and engineering. They are used in analysis and design of complex geometrical structures; however, the introduced errors for volume and surface estimation are often not considered in detail. To provide insight and information on these effects, in this study model geometries of porous media (simple cubic, body-centered cubic, face-centered cubic) are used to investigate the influence of resolution (voxels per length) on volume and surface approximation. The numerically obtained results are compared with analytical solutions for porosity and specific surface area. Small deviations from the real volume are found for the rasterized geometry at reasonable resolution. For the estimated surface area, in contrast, when using marching cubes considerable deviations from the analytically calculated surface area are found even at relatively fine resolutions. These findings are especially important for the use of rasterized voxel data as input for engineering correlations to estimate characteristic physical transport properties such as pressure drop or effective heat transport.