Homogeneous Concentration Research Articles

Formation and Characterization of Concentration Inhomogeneities in Melt-Grown Crystals

Formation and Characterization of Concentration Inhomogeneities in Melt-Grown Crystals

Structural and Dynamic Insights into the Conduction of Lithium-Ionic-Liquid Mixtures in Nanoporous Metal-Organic Frameworks as Solid-State Electrolytes.

Metal-organic framework (MOF)-based separators in Li-ion batteries (LIBs) have the potential to improve the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials acting as (quasi) solid-state electrolytes are crucial for the battery power output. Here, we investigate the mobility of a Li-based IL in MOF nanopores and unveil the details of the conduction mechanism by molecular dynamics (MD) simulations. A complex conductivity depending on the Li-IL loading and on the IL composition is observed. Most importantly, the presence of Li prevents the collapse of the conductivity at high IL loadings. The fully atomistic MD simulations including guest-guest and guest-host interactions elucidate the competing mechanisms: Li follows a Grotthuss-like conduction mechanism with large mobility. While at small pore fillings, the Li conduction is limited by the large distance between the anions facilitating the Grotthuss-like conduction; the conduction at high pore fillings is governed by field-induced concentration inhomogeneities. Because of the small MOF pore windows, which hinders the simultaneous passage of the large IL cations and anions in opposite directions, the IL shows field-induced MOF pore blocking and ion bunching. The regions of low anion concentration and high cation concentration represent barriers for Li, decreasing its mobility. In comparison to Li-free IL, the IL bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, resulting in a tremendously increased conductivity at maximum pore filling. The exploitation of this mechanism may enhance the development of advanced batteries based on IL and nanoporous separators.

Molecular dynamics of shear banding flow in associative polymers solutions: effect of sticker groups location

Non-equilibrium molecular dynamics are used in a coarse-grained scale to simulate the simple shear flow of telechelic and non-telechelic associative polymer solutions. The work includes the study of inception of flow and stress relaxation in transient flows and the constitutive curve at steady state. The instabilities (plateau stress, stress maxima) and the formation of bands with different velocity gradient and concentration depend on the location of the hydrophobic sites (sticker sites, SS) in the chain. Results allow to shed light on the origin of banded flow within the transient state, related to depletion of bridges between aggregates. This situation induces instabilities in the velocity profiles and concentration inhomogeneities along the velocity gradient during the stress overshoot. As the steady state is reached, instabilities disappear for systems where the SS are in the middle of the chain. The conformation relaxation occurs during a time scale larger than that of the stress, but this is not sufficient condition for the presence of banded flow in telechelic polymer solutions, as it holds in polymer melts. The difference in concentration among the bands suggests the existence of a shear-gradient concentration coupling mechanism under banded flow conditions, such as spatial non-homogeneities in the network density, induces chain diffusivity in the gradient direction and increasing normal stresses, especially in telechelic chains. The role of normal stresses on the flow-concentration coupling instability is analyzed in detail.

Detection of Concentration Inhomogeneity of Carbon Nanotubes in Polyethylene by Spectral-Correlation Method

It is shown that the concentration of single-walled carbon nanotubes in polyethylene can be estimated using a spectral-correlation method, which consists of statistical processing of random intensity distributions (speckles) formed by scattering on optical inhomogeneity of the material. The optimal conditions for performing measurements are established, possible causes of errors are determined, and the statistical errors of measurements are estimated. Based on experimental data, expressions are obtained for approximating the dependence of the nanotube concentration on the half-width of the normalized cross-correlation function of the intensity distributions of backscattered radiation at different wavelengths. The possibility of using extrapolation of calibration dependencies for two different physical models (linear and exponential) is considered and the expediency of using a linear model at nanotube concentration above 2% is shown. The concentration of nanotubes in one of the elements of the high-voltage coupling design is evaluated and the possibility of detecting inhomogeneity of the filler concentration in the material is demonstrated.

Digital technologies in detecting inhomogeneous concentration zones in heat-resistant nickel alloys structure, including those obtained by selective laser melting

This work presents the experimental data obtained using an integrated approach in the study of the chemical, crystallographic and morphological homogeneity of the structure of a heat-resistant material on a nickel base with carbide-intermetallic hardening and an increased content of the γ-phase, synthesized on single-crystal substrates of various compositions with CHO <001> in Z-direction. Using the proposed method for studying the capabilities and certification of different-level structural structures of samples in the initial state and after thermal study of the impact. The analysis was carried out using the system for EBSD analysis integrated into the scanning electron microscope and the software package for the analysis of electron microscopic images.

Coupling electro-mechanical behaviors in the interdigital electrode device of ferroelectrics

The electro-mechanical coupling behaviors of ferroelectric devices with interdigital electrodes may become complicated due to the material inhomogeneity and local field concentration under the complex working conditions. In this paper, a ferroelectric model, drawn from the typical interdigital electrode structure of a ferroelectric sensor, is established based on phase field theory, to study the polarization evolution and explore the evolution laws in ferroelectrics. Numerical results show that there appears ferroelectric creep even under an applied electric field below the coercive field value. Also, the configurational force theory is introduced to investigate the mechanical behaviors related to polarization switching in the ferroelectric samples with interdigital electrodes. It is found that configurational force and polarization have similar evolution laws in both time evolving and space distribution. And considering the configurational force as the driving force, it is possible to predict the potential direction of polarization evolution and explore its evolution mechanism in ferroelectrics, demonstrating the configurational force as a useful parameter for describing mechanical behavior during the polarization evolution and a powerful tool for investigating the evolution mechanism of microstructure with coupling effects in ferroelectric materials.

H-phase precipitation and its effects on martensitic transformation in NiTi-Hf high-temperature shape memory alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

ANALYTICAL METHOD FOR DETERMINING THE INTERVAL OF SPATIAL CORRELATION OF FADING IN A SINGLE-BEAM DECAMETER RADIO LINE

An analytical method for determining the interval of spatial correlation of fades has been developed based on the analysis of fluctuations in the amplitude-phase front of a decameter wave during its propagation through the ionosphere with small-scale inhomogeneities of the electron concentration. Its analytical dependence on the characteristic scales of inhomogeneities and the standard deviation of the fluctuations of the wave phase front at the outlet of the inhomogeneous ionosphere has been obtained.

Numerical Analysis of Phosphorus Concentration Distribution in a Silicon Crystal during Directional Solidification Process

For bulk doping, boron and phosphorus are usually used as p-type and n-type dopants, respectively. The distribution of these dopant concentrations in a silicon crystal along the vertical direction is governed by the segregation phenomena. As the segregation coefficient of phosphorus is small, phosphorus concentration distribution in a silicon crystal becomes inhomogeneous; inhomogeneous phosphorus concentration distribution affects the distribution of resistivity in the crystal. Therefore, it is important to control the phosphorus concentration distribution in a silicon crystal and make it uniform. In this study, by numerical analysis, we investigated the effect of the evaporation flux at the melt surface on phosphorus concentration distribution during the directional solidification process. To obtain a homogeneous phosphorus concentration distribution in the silicon crystal, we had to control the phosphorous evaporation flux at the melt surface and maintain approximately the same phosphorus concentration in the melt during the entire solidification process even though the growth rate was always changing.

INTERACTION OF SHORT-WAVE ELECTROMAGNETIC WAVES
 WITH SMALL-SCALE IONOSPHERIC INHOMOGENEITIES 
 OF THE POLAR IONOSPHERE 
 (part II)

The results of a comparative analysis of the data of computational and field experiments reflecting the peculiarities of the impact of small-scale inhomogeneities of electron concentration in the Earth's ionosphere on the characteristics of radio wave propagation during vertical ground-based and satellite sounding of the ionosphere are presented. It has been proved that these inhomogeneities are the cause of noticeable distortions of traces in ionograms. The simulation results confirm the possibility of determining the degree of concentration change in a small-scale inhomogeneous structure from experimental data.

Investigation of Mass Transport Properties of Fibrous Electrodes in Vanadium Redox Flow Batteries By Lattice Boltzmann Simulation

Redox flow batteries (RFBs) are gathering much attention as one of the promising candidates for a large-scale electrical energy storage device that is vital to utilize intermittent renewable energy in the grid [1]. In RFBs, fibrous electrodes have been widely applied presumably due to their large specific surface area and better electric conductance. It has been well recognized that the electrode structure profoundly affects cell performance that is attributed to the reaction and transport properties of the electrode [2,3].In this study, we developed a numerical simulation method to analyze pore-scale inhomogeneous behaviors of fluid and electrochemical reaction in fibrous electrodes using Lattice Boltzmann method (LBM). We focused our attention on porous structure affecting mass transport properties and overpotential in the electrodes for the vanadium redox flow battery.We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1(a). As the calculation procedure first, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. All calculation was carried out by the Lattice Boltzmann code we developed. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 50mA/cm2 was performed with different fiber diameter and porosity. As the pressure loss was fixed, the change in total overvoltage was evaluated as the change in energy loss.Figure 1 shows the calculation results of the electrochemical reaction and transport in the fiber electrode with a diameter of 3.75 μm and a porosity of 0.70. A path through which the electrolyte selectively flows is formed in the sparse area of the fibrous electrode and the inhomogeneous vanadium ion concentration and local reaction current density are identical in the electrode. This indicates that the electrolyte flow is locally biased due to the non-uniformity of the electrode. The reactive species concentration decreases due to the electrochemical reaction in the dense fiber region and causes local reaction current density.Based on a series of simulation with different diameter and porosity of the fibrous electrode, we obtained an equation for the electrolyte velocity and pressure drop in the electrode. We also evaluated averaged mass transport coefficients. We confirmed that the mass transfer characteristics are improved when the fiber diameter is large and the porosity is high. This is attributed to the larger the voids, the higher the permeability and the transport by advection was promoted. However, it is noteworthy that the larger diameter and the higher porosity leads to less electrode surface area. In our previous study [3], an optimized electrode architecture with a diameter of 2.4 μm and a porosity of 0.89 was presented by macro-scale simulations.In this fiber-scale simulations, we evaluated overpotential depending on the fiber diameter and the electrode porosity, and demonstrated the LBM simulation applied for optimization of fiber diameter and the electrode porosity in the RFB applications. Acknowledgements This research was supported by a Precursory Research for Embryonic Science and Technology (PRESTO) grant (grant number JPMJPR12C6) from the Japan Science and Technology Agency(JST).

Characterization of the Hydrogen Migration in a VT1-0 Titanium Alloy by Thermo-EMF and Eddy Current Nondestructive Control Methods

Abstract—The processes of hydrogen migration in a titanium plate and inhomogeneous hydrogen saturation are studied by measuring the thermopower and eddy currents. The processes of inhomogeneous hydrogen saturation of commercial-purity VT1-0 titanium are developed, tested, and investigated using these methods of nondestructive testing. To achieve an inhomogeneous hydrogen concentration in a titanium plate, a TiN film 1–2.6 μm thick is deposited onto it by magnetron sputtering, and the parts of the plate free of the film are hydrogenated. The thermo-emf and eddy currents are measured at various probe coordinates before and after hydrogen saturation of the plate. Eddy current testing demonstrates that, after the plate is hydrogen saturated for 1.5 h, the thermo-emf on the side of hydrogen sorption is significantly lower than on the coated side. After 144 h, a uniform thermo-emf ​​ distribution is obtained, as follows from eddy current measurements. The thermo-emf study is performed upon heating at a constant rate. The thermo-emf of each part of the titanium alloy surface (including the uncoated part) is approximately equal to the value obtained before hydrogenation. The electrical resistivity of the coated titanium plate at room temperature is approximately 0.315 mΩ cm. After hydrogenation, the hydrogen content in the uncoated part decreases slowly. The thermo-emf also decreases gradually. A hydrogen content distribution can be formed via a thermo-emf distribution ​​in the titanium plate. After holding for a sufficiently long time, a uniform thermo-emf distribution forms in the plate.

Effect of concentration inhomogeneity on the pulsating instability of hydrogen–oxygen detonations

One-dimensional, unsteady gaseous detonation propagation in a non-homogeneous medium is investigated numerically using the reactive, compressible Navier–Stokes equations with detailed chemistry. The effect of concentration inhomogeneity on the pulsating mode is modeled by a sinusoidal distribution of H2 mole fraction in a H2–O2 mixture. The mixture inhomogeneity, varied by changing the disturbance frequency and amplitude, has significant effects on the pulsating behavior of the detonation due to the interaction of the leading shock with the local gradient. Initially exhibiting a four-period pulsation, the detonation wave entering a non-homogeneous medium can adapt and re-establish into a new propagation mode. For a fixed, large-amplitude sinusoidal disturbance, the period-doubling limit cycle is prone to be destroyed, resulting in a chaotic mode for the propagating detonation front in the non-homogeneous mixture; lowering the disturbance frequency also favors a transition from a periodic pulsation to a chaotic one. When the disturbance amplitude decreases, the propagating detonation can transit quickly to a new pulsating behavior, which tends to be more regular. For a very small amplitude of inhomogeneous variation, it is found that the frequency corresponding to the wavelength close to that of the intrinsic pulsation in the uniform mixture makes the original four-period mode become a double-period mode; for a frequency less than this value, the double-period mode is prone to become more unstable. Consequently, this demonstrates that inhomogeneity could have a positive effect on stabilizing a pulsating detonation.

Surface Evolution of Aluminum Electrodes in Non-Aqueous Aluminum Batteries

Aluminum batteries are considered as novel devices for safe energy storage. However, there is still a lack of understanding of the operation mechanism and evolution behaviors inside the cells of the current aluminum batteries, which limits the rational design and fabrication of high-performance aluminum batteries. To address the issue, systematical studies were applied to understand the surface evolution of the aluminum electrode in the aluminum batteries. Using in situ optical observation and simulation methods, the results suggest that dendrite growth and deposition on the aluminum electrode surface is critical to the aluminum deposition/corrosion evolution during following cycles, which leads to uneven current distribution on the electrode and inhomogeneous ion concentration interface. Additionally, the activity of the aluminum dendrites suggests that the nanosized aggregation would present less activity than the pristine aluminum, which in turn impacts the uniform evolution of the electrode surface. The aluminum-graphite full cells also exhibit a similar trend that is found in the in situ symmetric cells. The combined results from in situ optical observation, simulation, and full cell evaluation provide a deep insight into the aluminum batteries, which would be meaningful to guide research on their life cycle and evolution.

H-phase Precipitation and its Effects on Martensitic Transformation in NiTi-Hf High-Temperature Shape Memory Alloys

Precipitate microstructure in the B2 parent phase is known to have profound impacts on the properties of NiTi-based high temperature shape memory alloys (HTSMAs), including the martensitic transformation (MT) start temperature Ms, temperature- and stress-hysteresis, work output, dimensional stability and functional fatigue resistance. In order to understand the underlying mechanisms and hence to optimize aging heat treatments to achieve desired properties, we systematically investigate both the mechanical and chemical effects associated with nanoscale coherent precipitates on the behavior of MT. Using NiTi-Hf HTSMA as an example, we first study the equilibrium shape and stress and strain fields of an H-phase precipitate as a function of its size. We then determine quantitatively the elastic interaction energy between a precipitate and a nucleating martensitic particle consisting of either a single variant or multiple self-accommodating variants. In the meantime, we calculate the variation of concentration field around an H-phase precipitate during its growth. Finally, we quantify and compare the effects of the spatially inhomogeneous stress and concentration fields around an H-phase precipitate on Ms. The results indicate that the former is the dominant factor for long aging times while latter is the dominant factor for short aging times. Since the model predicts Ms as a function of aging temperature and time, it can aid the design of aging heat treatment schedule to achieve desired Ms.

Inhomogeneous swelling behavior of a bi-layered spherical hydrogel containing a hard core

Hydrogels can absorb a significant amount of water and swell greatly. When the swelling is constrained, hydrogels exhibit inhomogeneous deformation, stress, and water concentration fields. This paper studies the swelling behavior of a bi-layered spherical hydrogel containing a hard core after imbibing sufficient water. By adopting a continuum field theory of hydrogels, the governing differential equation of the hydrogel radial deformation is derived, which is further solved numerically at proper mixed boundary conditions and inner field continuity conditions across the hydrogel interface. Effects of material properties and the presence of the hard core on the deformation, stress, and water concentration fields of the hydrogels are examined. It shows that the inner hard core can remarkably reduce the water concentration near the core surface and simultaneously induce large stresses. In addition, the water concentration in the bi-layered hydrogel heavily depends upon the stiffness of the individual hydrogel. The study offers a rational route to design and regulate hydrogels with tailorable swelling behavior for practical applications in drug delivery, leakage blocking, etc.

Orientation Dynamics of Sedimenting Anisotropic Particles in Turbulence.

We examine the dynamics of small anisotropic particles (spheroids) sedimenting through homogeneous isotropic turbulence using direct numerical simulations and theory. The gravity-induced inertial torque acting on sub-Kolmogorov spheroids leads to pronouncedly non-Gaussian orientation distributions localized about the broadside-on (to gravity) orientation. Orientation distributions and average settling velocities are obtained over a wide range of spheroid aspect ratios, Stokes, and Froude numbers. Orientational moments from the simulations compare well with analytical predictions in the inertialess rapid-settling limit, with both exhibiting a nonmonotonic dependence on spheroid aspect ratio. Deviations arise at Stokes numbers of order unity due to a spatially inhomogeneous particle concentration field resulting from a preferential sweeping effect; as a consequence, the time-averaged particle settling velocities exceed the orientationally averaged estimates.

Formation of low-temperature deformation-induced segregations of nickel in Fe–Ni-based austenitic alloys

ABSTRACT The authors present the results of an investigation in Fe–Ni-Cr austenitic alloys of the low-temperature deformation-induced segregations of nickel that form in the micro regions being (i) located close to grain- and subgrain boundaries and (ii) characteristic of the concentration and magnetic inhomogeneities indicated by the appearance of a dark diffraction contrast at the electron diffraction patterns taken from these regions typical (at the same time) of an enhanced value of Curie temperature. The observed effects were connected with the micro distortions caused by the local change of lattice parameter because of an increase in nickel concentration, as well as in the result of a magnetostriction dilatation. Using methods of the X-ray energy dispersive spectroscopy (XEDS) and atomic-probe body-section radiography (tomography – APT) has made it possible to determine the borders of those regions of austenite that were characteristic of an enhanced concentration of nickel in the fields of the localisation of a deformation-induced segregation of nickel in the vicinity of grain (subgrain) boundaries of austenitic alloys of the types Fe–13Cr–30Ni and Fe–37Ni–3Ti.

Spatially Resolved Effects of Protein Freeze-Thawing in a Small-Scale Model Using Monoclonal Antibodies.

Protein freeze-thawing is frequently used to stabilize and store recombinantly produced proteins after different unit operations in upstream and downstream processing. However, freeze-thawing is often accompanied by product damage and, hence, loss of product. Different effects are responsible, including cold denaturation, aggregation effects, which are caused by inhomogeneities in protein concentration, as well as pH and buffer ingredients, especially during the freeze cycle. In this study, we tested a commercially available small-scale protein freezing unit using immunoglobin G (IgG) as monoclonal antibody in a typical formulation buffer containing sodium phosphate, sodium chloride, and Tween 80. Different freezing rates were used respectively, and the product quality was tested in the frozen sample. Spatially resolved tests for protein concentration, pH, conductivity, and aggregation revealed high spatial differences in the frozen sample. Usage of slow freezing rates revealed high inhomogeneities in terms of buffer salt and protein distribution, while fast rates led to far lower spatial differences. These protein and buffer salt inhomogeneities can be reliably monitored using straight forward analytics, like conductivity and photometric total protein concentration measurements, reducing the need for HPLC analytics in screening experiments. Summarizing, fast freezing using steep rates shows promising results concerning homogeneity of the final frozen product and inhibits increased product aggregation.

Gas-Discharge High-Frequency Generators for Materials Processing

A new approach to the selection of high-frequency oscillations generated in high-voltage glow discharge plasma is developed. It is based on the appearance of pressure pulsations arising during collisions of the inflowing working gas stream with an obstacle. A spatial inhomogeneity in concentration of high-voltage glow discharge plasma (14 kV and 30 mA) caused by pressure pulsations results in discharge combustion instability. The difference between the ignition and combustion voltages of the discharge provides the ionization processes as well as the high-frequency (HF) oscillations at a frequency of 5 MHz selected by an oscillatory circuit. The fraction of power transformed into HF-oscillations reaches 40%. Testing of this oscillator in etching of a lithium niobate crystal demonstrates that the surface charges are removed from the substrate not only by the plasma, but also by displacement and recharge currents. An etching rate of 5 μm/h has been achieved.