Measurements Of Transport Coefficients Research Articles

Non-parametric inference of impurity transport coefficients in the ASDEX Upgrade tokamak

We present a non-parametric inference of impurity transport coefficients by using charge exchange recombination spectroscopy measurements of Ne X, Ne VIII, O VIII, and C VI lines. Due to their close atomic numbers, neon, oxygen and carbon impurity ions are assumed to have the same diffusion coefficient D and convection velocity v. Unlike conventional techniques that modulate or perturb the impurity contents, we employ a quasi-stationary plasma with static impurity profiles. Since the ratio of v to D only describes the equilibrated profile of the sum of all impurity charge states, steady-state measurements can still decouple D and v if different charge states are simultaneously observed. We have formulated a non-parametric analysis framework based on the Bayesian probability theory and conducted transport coefficient measurements for a Type III ELMy H-mode plasma at ASDEX Upgrade. The charge exchange reactions with the background neutrals, which are known to affect the impurity charge state balance, are taken into account by introducing additional free parameters. While D at the pedestal is close to the neoclassical level (1 m s−2), a large diffusion coefficient and a strong outward convection are inferred right inside the pedestal top.

Transference Number in Polymer Electrolytes: Mind the Reference-Frame Gap.

The transport coefficients, in particular the transference number, of electrolyte solutions are important design parameters for electrochemical energy storage devices. The recent observation of negative transference numbers in PEO–LiTFSI under certain conditions has generated much discussion about its molecular origins, by both experimental and theoretical means. However, one overlooked factor in these efforts is the importance of the reference frame (RF). This creates a non-negligible gap when comparing experiment and simulation because the fluxes in the experimental measurements of transport coefficients and in the linear response theory used in the molecular dynamics simulation are defined in different RFs. In this work, we show that, by applying a proper RF transformation, a much improved agreement between experimental and simulation results can be achieved. Moreover, it is revealed that the anion mass and the anion–anion correlation, rather than ion aggregates, play a crucial role for the reported negative transference numbers.

Interfacial Effects on Transport Coefficient Measurements in Li-ion Battery Electrolytes

Development of Li+-containing electrolytes with improved transport properties requires reliable, reproducible, and ideally low volume techniques to rigorously understand ion-transport with varying composition. Precisely measuring the complete set of transport coefficients in liquid electrolytes under battery-relevant operating conditions is difficult and the reliability of these methods are sparsely described in electrolyte transport literature. In this work, we apply a potentiostatic polarization-based transport characterization approach typically used for polymer electrolytes to liquid electrolyte systems in an attempt to fully measure all transport coefficients (conductivity, total salt diffusion coefficient, thermodynamic factor and transference number) for the model system of LiPF6 in an ethylene carbonate—ethyl methyl carbonate (EC:EMC) mixture. Using systematic timescale and statistical analyses, we find that transport coefficients measured using potentiostatic polarization of Li-Li symmetric cells exhibit strong correlation to Li electrode interfacial resistance, indicating that such methods are probing both bulk and interfacial phenomena. This reveals a major roadblock in characterizing electrolyte systems where the interfacial resistance is significantly larger than ohmic electrolyte resistance. As a result, we find that methods that rely on potentiostatic Li metal stripping/plating do not readily result in reliable liquid electrolyte transport coefficients, unlike similar methods for solid polymer electrolytes, where interfacial resistances are typically smaller than electrolyte resistances at the elevated temperatures typically of interest for such electrolytes.

Characterizing Ion Transport in Non-Aqueous Electrolyte Solutions for Li-Ion and Li- Metal Batteries

A majority of ionic conductivity in current state-of-the-art liquid electrolytes results from the movement of the anion rather than the electrochemically active Li+ ion. Passing current through such an electrolyte can cause severe ionic concentration gradients to extend into porous electrodes, which at high currents can reduce active material utilization and result in high overpotentials that cause deleterious side reactions to occur (e.g., Li plating on graphite electrodes). The severity of these concentration gradients is directly influenced by the transference number (t+), which describes the ratio of current carried by the electrochemically active Li+ ion to the total current passed. Non-aqueous polyelectrolyte solutions (PESs) are a promising, but underexplored, route to high transference number electrolytes that demonstrate potential for both high ionic conductivity and t+. This combination is achieved by anchoring the anion to a polymer backbone – allowing for high t+ by slowing down the motion of the electrochemically inactive anion – while maintaining high ion conductivity through improved ion dissociation and solvent-mediated Li+ transport. New measurement techniques are necessary to rigorously understand ion-transport and structure-property relationships in this new class of electrolytes.Rigorously measuring the complete set of transport properties of concentrated liquid electrolytes under battery-relevant operating conditions is difficult and sparsely explored in experimental liquid electrolyte literature. Complete transport characterization is particularly challenging for novel electrolytes and additives where cost or synthetic complexity limit the volume of available electrolyte. In particular, small volume characterization is necessary for polyelectrolyte solutions, in order to study many different polymer concentrations and molecular weights. Major challenges limiting full ion transport characterization in liquid systems include significant corrosion of Li metal in the presence of electrolyte solvents, large and unstable interfacial impedance, and difficulty in establishing sufficiently small concentration gradients to ensure measurement validity while maintaining high signal to noise ratio data.In this presentation, we will discuss efforts to understand and address these challenges. Using techniques amenable to small electrolyte volumes, we rigorously measured liquid electrolyte transport properties, including the ideal solution Bruce-Vincent transference number, total salt diffusion coefficients, activity coefficients, and the true transference number under battery-relevant conditions using Newman’s concentrated solution theory framework. While our long-term objective is to fully characterize Li-triflimide-appended polyelectrolyte solutions, we initially attempt to develop the methodology on the well-studied liquid electrolyte system of LiPF6 in an ethylene carbonate:ethyl methyl carbonate blend. Using rigorous statistical analysis, we found that transport coefficients obtained via these measurements were consistently not in agreement with those reported in the literature and prone to significant variation depending on data fitting methodology. We believe the discrepancy can largely be attributed to parasitic corrosion reactions and large interfacial resistances at the lithium electrodes, which appear to dominate at the low current densities necessary to maintain sufficiently small concentration gradients. These experimental findings are supported by COMSOL Multiphysics modeling studies. Additional experimental studies incorporating stabilizing additives indicate that simply improving Li stripping/plating coulombic efficiency does not allow for accurate transport coefficient measurements. This reveals a major roadblock in characterizing liquid electrolyte systems, as methods that rely on Li metal stripping/plating do not readily result in reliable liquid electrolyte transport coefficients, unlike similar methods for solid polymer electrolytes. These results have important implications across the electrolyte engineering field that often relies on similar tests to screen electrolyte candidates.

Benchmarking of Monte Carlo flux simulations of electrons in CO2

Electron velocity distribution functions (EVDFs) in CO2 obtained by means of the Monte Carlo flux (MCF) method are compared with results from two-term and multi-term Boltzmann solvers. The MCF method provides detailed calculations of the EVDF through a highly efficient variance reduction technique. Benchmark calculations of Legendre polynomial coefficients of the EVDF expansion are reported for a wide range of reduced electric fields (E/N), showing excellent agreement with multi-term solutions. Rate coefficients of inelastic processes calculated from two-term Boltzmann solvers differ significantly, up to 70%, from MCF and multi-term solutions, due to the anisotropy of the EVDF. An extension of the method to consider the thermal distribution of the background gas is also presented. This extension, together with an accurate description of the population of rotationally and vibrationally excited states, provides excellent agreement with measured transport coefficients at low E/N. A good agreement is obtained at moderate E/N between experimental values of dissociation rate coefficients and MCF calculations after careful consideration and analysis of several cross sections data sets.

Direct Measurements of Electroviscous Phenomena in Nafion Membranes.

Investigation of electroviscous effects is of interest to technologies that exploit transport of ions through ion exchange membranes, charged capillaries, and porous media. When ions move through such media due to a hydrostatic pressure difference, they interact with the fixed charges, leading to an increased hydraulic resistance. Experimentally this is observed as an apparent increase in the viscosity of the solution. Electroviscous effects are present in all electrochemical membrane-based processes ranging from nanofiltration to fuel-cells and redox flow batteries. Direct measurements of electroviscous effects varying the applied ionic current through Nafion membranes have, to the best of the authors’ knowledge, not yet been reported in literature. In the current study, electroviscous phenomena in different Nafion ion exchange membranes are measured directly with a method where the volume permeation is measured under constant trans-membrane pressure difference while varying the ion current density in the membrane. The direct measurement of the electroviscous effect is compared to the one calculated from the phenomenological transport equations and measured transport coefficients. Within the experimental uncertainty, there is a good agreement between the two values for all membranes tested. We report here an electroviscous effect for all Nafion membranes tested to be .

Self-consistent electron–THF cross sections derived using data-driven swarm analysis with a neural network model

We present a set of self-consistent cross sections for electron transport in gaseous tetrahydrofuran (THF), that refines the set published in our previous study [] by proposing modifications to the quasielastic momentum transfer, neutral dissociation, ionisation and electron attachment cross sections. These adjustments are made through the analysis of pulsed-Townsend swarm transport coefficients, for electron transport in pure THF and in mixtures of THF with argon. To automate this analysis, we employ a neural network model that is trained to solve this inverse swarm problem for realistic cross sections from the LXCat project. The accuracy, completeness and self-consistency of the proposed refined THF cross section set is assessed by comparing the analyzed swarm transport coefficient measurements to those simulated via the numerical solution of Boltzmann’s equation.

Mobility-viscosity decoupling and cation transport in water-in-salt lithium electrolytes

Two super-concentrated aqueous electrolyes, or water-in-salt electrolytes, comprising the lithium trifluoromethanesulfonate (LiTf-H2O) binary system, and the ternary system with bis(trifluoromethanesulfonyl)-imide (LiTFSI-LiTf-H2O), were analyzed in relation to their conductivity, viscosity, diffusion of all the species and cationic transport number. The conductivity–viscosity and the ionic diffusion-viscosity decoupling were analyzed by means of the Walden law and the Stokes–Einstein relationship, respectively. The results, including those already reported for LiTFSI-H2O, reveal the existence of a significant decoupling of the Li+ mobility from the solution viscosity, while the corresponding anions follow the classical hydrodynamic behavior. The Li+ apparent transport number, derived from the self-diffusion coefficients measured by NMR, increases with concentration, due to the decoupling. These facts strongly support the formation of two types of nano-domains in the superconcentrated salt solutions with different mobility characteristics: a region formed by a net of anions with restricted mobility, percolated by clusters of reduced local viscosity that boost the mobility of Li+ ions. Finally, the difference between the experimental conductivity and that calculated using the Nernst–Einstein equation was rationalized in terms of the velocity correlation and resistance coefficients calculated using the measured transport coefficients.

Assessment of the self-consistency of electron-THF cross sections using electron swarm techniques: Mixtures of THF–Ar and THF–N2

The pulsed Townsend technique has been used to measure transport coefficients in mixtures of tetrahydrofuran (THF) with N2 and argon. These measurements are the first investigations of swarm transport in gas mixtures with THF, with the drift velocity and effective Townsend ionization coefficient reported for a range of reduced electric fields between 0.23 and 800 Td (1 Td = 10−21 V m2). These transport coefficients are compared with those calculated using a multiterm kinetic theory, using the cross section set developed in our previous studies [N. A. Garland et al., Phys. Rev. A 88, 062712 (2013) and M. J. E. Casey et al., J. Chem. Phys. 147, 195103 (2017)]. The swarm technique of iteratively adjusting cross sections to reproduce experimental transport measurements is subsequently utilized in this study to address the deficiencies in the earlier cross section sets, exposed by the gas mixture measurements. Refinement of the low-energy extrapolation of the quasielastic database cross section and the low-energy extrapolation and magnitude of the dissociative electron attachment cross section are detailed, as well as the adjustments to the two previously proposed neutral dissociation cross sections. These refinements were necessary in order to minimize differences between our measured and calculated transport coefficients.

Simultaneous transport coefficient measurements for an individual bismuth wire embedded in a quartz template applying nano-fabrication

An individual bismuth wire (1.90 μm in diameter and 1554 μm in length) embedded in a quartz template was used to demonstrate simultaneous transport coefficient measurements for resistivity, magneto-resistivity, Hall coefficient, and Seebeck coefficient at 300 K. To obtain these measurements, we performed nano-fabrication using a dual-beam focused ion beam apparatus and fabricated local electrodes at the six-side and two-edge positions of the wire. Furthermore, using the measured values, we estimated carrier density, mobility, and Fermi energy. The obtained physical properties were similar to those of bulk bismuth, as expected, because of its large diameter.

Determination of external mass transfer coefficients in dynamic sorption (DVS) measurements

ABSTRACTA sorption balance is an instrument used to measure vapor uptake in a sample at controlled temperature and relative humidity. It is most commonly used to determine equilibrium values (sorption isotherms), but is also used for kinetic measurements of transport coefficients. Such measurements can be affected by the external mass transfer resistance in the gas phase around the sample. This paper presents a method to determine the external mass transfer coefficient for a given flow geometry using a water saturated sample, including corrections for temperature changes from evaporative cooling, which is found to have considerable effect on the calculated constant.

Interrelation between Redox Molecule Transport and Li+ Ion Transport across a Model Solid Electrolyte Interphase Grown on a Glassy Carbon Electrode

The solid electrolyte interphase (SEI) on the graphite anode of lithium ion batteries plays a crucial role for the battery performance. It is believed that the SEI blocks electrons and solvent molecules, while Li+ can easily migrate across the SEI. However, quantitative measurements of transport coefficients for these species in the SEI are problematic due to the complex structure of graphite composite anodes. Here we have grown model SEIs on glassy carbon electrodes and have characterized them by a combination of scanning electron microscopy, AFM-based scratching experiments, impedance spectroscopy and redox probe experiments. SEM and AFM experiments reveal a dual-layer structure of the SEI. The redox probe experiments with ferrocene molecules provide strong indication that the diffusion of the redox molecules across pores in the inner SEI layer is faster than electron transport across the SEI. Remarkably, the effective diffusion coefficient of ferrocene in the SEI is virtually identical to the effective diffusion coefficient of Li+ obtained from the SEI semicircle in the impedance spectra. Moreover, both diffusion coefficients show the same temporal evolution after SEI formation. This suggests that in our model SEIs, Li+ is primarily transported in the liquid electrolyte phase inside the pores of the inner layer.

Scanning drift tube measurements of electron transport parameters in different gases: argon, synthetic air, methane and deuterium

Measurements of transport coefficients of electrons in a scanning drift tube apparatus are reported for different gases: argon, synthetic air, methane and deuterium. The experimental system allows the spatio-temporal development of the electron swarms (‘swarm maps’) to be recorded and this information, when compared with the profiles predicted by theory, makes it possible to determine the ‘time-of-flight’ transport coefficients: the bulk drift velocity, the longitudinal diffusion coefficient and the effective ionization coefficient, in a well-defined way. From these data, the effective Townsend ionization coefficient is determined as well. The swarm maps provide, additionally, direct, unambiguous information about the hydrodynamic/non-hydrodynamic regimes of the swarms, aiding the selection of the proper regions applicable for the determination of the transport coefficients.

Finite Size Effect on Drift Mobility and Diffusion Coefficient in Thin Organic Layers: Monte-Carlo and Analytic Modeling

Hopping transport of charge carriers in thin (< 100nm) organic layers is modeled, using Gaussian disorder model. Monte-Carlo simulations yield considerable decrease of drift mobility with thickness of the layer, which is in good agreement with the developed analytic model. The reason is that filling of rare deep states is statistically improbable in course of transient across a thin sample. Quasi-equilibrium initial distribution is considered to omit effects of dispersive transport. Results of the work can explain disagreement in results of non-stationary and stationary measurements of transport coefficients in thin films.

Internal Transport Coefficient Measurements in Random Fiber Matrix Heat Exchangers

Experimental determination of transport coefficients, in particular internal heat transfer coefficients, in heterogeneous and hierarchical heat transfer devices such as compact regenerative heat exchangers has posed a persistent challenge for designers. The goal of this study is to (1) present a new general treatment of the experimental determination of such design data, to (2) provide simple correlations for high porosity random fiber matrices for broad design applications, and to (3) illustrate how such measurements close the formidable integro-differential volume averaging theory (VAT) equations governing transport phenomena in porous media. The combined experimental and computational method employed here for determining the internal heat transfer coefficient in the porous structure is based on the VAT model and combines with simple pressure drop measurements to yield the relevant design data for eight different high porosity random fiber samples. The design data are correlated based on a porous media length scale derived from the VAT model governing equations and the transport coefficient correlations obtained are valid for gas flows over a Reynolds number range between 5 and 70. Finally, the correlations are related to explicit, rigorously derived, lower-scale expressions arising from the VAT model. With the illustration of a new experimental tool, and the production of new simple design correlations for high porosity random fiber matrices for regenerative heat transfer applications, within the context of the hierarchical VAT model, future VAT-based simulation studies of such devices may be pursued. Moreover, the nonlocal modeling provided by VAT paves the way to meaningful optimization studies due to its singular ability to provide rigorous modeling and fast numerical solutions for transport phenomena in regenerative compact heat exchangers.

Scattering cross sections for electrons in C2 H2F4 and its mixtures with Ar from measured transport coefficients

Previous measurements of the drift velocities, W, and the density-normalized effective ionization (multiplication) coefficients (ionization minus attachment), (α − η)/N, measured in C2H2F4 (1,1,1,2 tetrafluoroethane) and in C2H2F4–Ar mixtures have been analysed with a standard swarm procedure. As a result of this analysis a set of electron collision cross sections for the C2H2F4 molecule has been obtained. This set has been further used to calculate other transport parameters such as the characteristic energies and rate coefficients for individual processes.

Universal Out-of-Equilibrium Transport in Kondo-Correlated Quantum Dots: Renormalized Dual Fermions on the Keldysh Contour

The nonlinear conductance of semiconductor heterostructures and single molecule devices exhibiting Kondo physics has recently attracted attention. We address the observed sample dependence of the measured steady state transport coefficients by considering additional electronic contributions in the effective low-energy model underlying these experiments that are absent in particle-hole symmetric setups. A novel version of the superperturbation theory of Hafermann et al. in terms of dual fermions is developed, which correctly captures the low-temperature behavior. We compare our results with the measured transport coefficients.

Analysis of the application of optical two-wavelength techniques to measurement of the Soret coefficients in ternary mixtures

We discuss the application of two-wavelength optical methods for the determination of Soret coefficient in ternary mixtures. Though the principle of such an approach was formulated in the literature earlier, we have found a significant obstacle to its implementation and suggested a way to circumvent that obstacle. The main target of our approach is the analysis of the compatibility of the matrices of contrast factors (the derivatives of refractive indices with respect to concentrations) with available light sources used for measuring the Soret effect in ternary mixtures. We report on the development of a simple mathematical approach to verify the applicability of the chosen wavelengths for laser diodes to measure transport coefficients for a certain class of ternary mixtures. The approach has been applied to a number of aqueous ternary mixtures: water–ethanol–k-ethyleneglycol, k = mono, di, tri, and water–t-butanol–DMSO) and light sources in the visible λ1 = 670 nm and infrared λ2 = 925 nm spectrum. The regions of feasibility/infeasibility of the methods have been revealed for these mixtures.

Shear spectral sum rule in a nonconformal gravity dual

A sum rule which relates a stress-energy tensor correlator to thermodynamic functions is examined within the context of a simple nonconformal gravity dual. Such a sum rule was previously derived using AdS/CFT for conformal $\mathcal{N}=4$ supersymmetric Yang-Mills theory, but we show that it does not generalize to the nonconformal theory under consideration. We provide a generalized sum rule and numerically verify its validity. A useful by-product of the calculation is the computation of the spectral density in a strongly coupled nonconformal theory. Qualitative features of the spectral densities and implications for lattice measurements of transport coefficients are discussed.

A Novel Attempt to Calculate the Velocity Correlation Coefficients in Ternary Electrolyte Solution

This work gives estimated values of the velocity correlation coefficients VCCs for ternary electrolyte solutions (the system may have a tracer ion as one of the components), utilizing available measured transport coefficients. The VCCs originate from linear response theory and give a deeper insight into the microdynamic structure of complex ionic solutions. By assuming Onsager’s relation to be valid, ten sets of velocity correlation coefficients were calculated for a ternary system and were used to calculate the VCCs for 134Cs+ ion (present in trace amount) transport in aqueous solutions of CsCl and KCl at 25 ○C.