Surface Diffusion Layer Research Articles

Nucleation and Growth during Electrochemically Induced Deposition of Li3PO4

The field of electrochemically induced depositions has been gradually gaining a lot of attention in the last few decades due to the unique abilities it offers, namely the thin film deposition of insulating materials on a conductive surface [1]. While direct electrochemical methods can be used to deposit redox active species, their use in depositing insulating materials is limited as some insulating materials are redox inactive. One of these redox inactive insulating materials is Li3PO4. In the synthesis of Li3PO4 thin films a local pH increase near the electrode surface, by the electrochemical reduction of water, triggers the precipitation. Due to the surface confined nature of the diffusion layer, the OH- concentration near the surface will be significantly higher than in the bulk and phosphate near the surface will react with the hydroxides to form the deprotonated species, which in the presence of lithium ions will precipitate. This process can be visualized in Figure 1.In this work, we have studied the nucleation and growth of electrochemically induced Li3PO4 deposition combining experiment and model. We developed a model to calculate the concentration profiles of the different species in the diffusion layer, which is then used to predict the point at which the solubility product is surpassed at the surface and in the near surface diffusion layer. This allows the prediction of the influence of the reaction parameters such as current density, bulk pH, lithium concentration, and initial phosphate concentration on the supersaturation. To further improve the accuracy of the model, turbidity experiments were performed to determine the effective solubility product of the system. The model was combined with the experiment to link supersaturation with seed density and understand the effect of current density and time on island density, size and precipitation rate. It is shown that, as for the case of direct electrodeposition where island density is function of overpotential (supersaturation), a direct correlation can be made between supersaturation and island density, supporting the classical nucleation and growth theories also for electrochemically induced deposition. Figure 1 : Schematic representation of the electrochemically induced deposition of Li3PO4 with water as a probase.

Structural-phase transformations during nitro oxidation of steels and surface properties

Structural and phase changes in the surface diffusion layer during gas nitriding of steel were studied, with the formation of a crystallographic structure of the nitride layer for subsequent oxidation in water vapor. The conditions for forming carbides and nitrocarbides with simultaneous diffusion of nitrogen and decarburization of the steel matrix are determined depending on the nitrogen potential of the saturating medium. The structural-phase states of the surface diffusion layer with a stepwise change in the nitrogen potential during nitriding at the first stage and followed by oxidation at the second stage, the formation of a surface oxide layer, as well as the conditions for participation in mutual diffusion processes of carbon in the nitride layer, decarburized from the steel matrix with the formation compositions of modified nitride phases. The nitride-oxide layer’s corrosion properties and gradient properties in various structural-phase states of nitride phase compositions have been studied.

Study the effects of dry-wet cycles and cadmium pollution on the mechanical properties and microstructure of red clay

In order to study the effect of cadmium ions on the mechanical properties and micro-structure characteristics of the red clay in Guilin, we have conducted triaxial test and the scanning electron microscope tests to analyze the effects of cadmium ion concentration and the number of dry and wet cycles on the mechanical properties and micro-structure changes of the red clay. The results showed the effects of cadmium ions and dry-wet cycles destroy the structure of red clay. The cohesive force of red clay decreases with the increase of cadmium ion concentration, and the internal friction angle first increases and then decreases. With the rise in the number of dry and wet cycles, the cohesive force of cadmium-contaminated red clay first increases and then decreases, and the angle of internal friction rises gradually. Under the action of different cadmium ion concentrations, the stress-strain curve is strain hardening. With the concentration of cadmium ions increases, the strain hardening becomes more apparent; the peak value reached faster. and the axial strain corresponding to the peak value of the line decreases. With the increase in the number of wet and dry cycles, the volume of cadmium-contaminated red clay shrinks and its compactness increases; it gets the peak shear strength faster during the shearing process, and its peak value becomes larger and larger. The main reason for the phenomenon is that cadmium ions destroy the cementation between the particles. The soil particles are mainly in point contact which loosens the structure of the soil; on the other hand, the thickness of the surface diffusion layer of the clay particles increases through chemical action, The exchange of cations increases the porosity of the soil and weakens its strength. The dry-wet cycle shrinks the volume of the red clay, and the soil particles are mainly in surface contact; as the number of dry-wet cycles increases, the soil particles connection is closer, the soil porosity decreases and the strength increases.

Performance evaluation of μDMFCs based on porous-silicon electrodes and methanol modification

This study aims to develop micro direct methanol fuel cells (μDMFCs) incorporating flow-field plates with porous-silicon diffusion layers to form two types of bipolar electrodes, one with a hill-like structure (HLS) and the other with a through-hole silicon (THS) structure. Carbon nanotubes are grown on the surface diffusion layers to serve as catalyst supports. Furthermore, methanol fuel is modified by adding SDSS surfactant with the intent of enhancing its wettability and ability of CO2 removal for preventing CO2 coverage of catalyst layer. The experimental results indicate that the maximum power density of this HLS-THS cell (0.186 mW/cm2) is 4.4 times higher than that of the cell with conventional CP-CP (carbon paper) electrodes. The μDMFCs without SDSS has the highest output voltage at 0.66 V, yet the value linearly decreases to 0 V in only 5.5 h. The μDMFCs with 0.1% SDSS can maintain an average output voltage of 0.45 V for 8 h before the value decreases to approximately 0 V as a result of fuel depletion. Although the output voltage of the μDMFCs with 0.5% SDSS remains steadily at 0.425 V, the voltage decreases to negative values after 7 h because of fuel depletion and crossover.

Origins of Heterogeneous Li Insertion at the Porous Electrode, Single Particle, and Atomic Length Scales

The insertion of a guest ion to a host crystal is the fundamental reaction underpinning all Li-ion batteries. Understanding the mechanisms of how the ions enter the host, and how the host crystal responds, are keys to rational design of battery materials with higher energy density, power density, and cycle life. Our work directly visualizes the ion insertion mechanism in the model phase-separating material LiFePO4 at multiple length scales: porous electrode, single particles, and atomic. We show LiFePO4 is highly heterogeneous in terms of Li insertion, does not necessarily phase separate, and is not a one-dimensional conductor during charge and discharge. We begin by investigating lithiation at the porous electrode length scale (i.e., microns). Using ptychography (Shapiro et al. Nat. Photon. 8, 765, 2014), a synchrotron X-ray imaging technique with a spatial resolution of 10 nm, we study which particles in an electrode lithiates first, we show that the connectivity between the particles and the carbon black network controls the sequence of lithiation. When electronic transport is uniform (achieved via high carbon black loading), smaller particles preferentially lithiate. This further shows that the rate-limiting process in a porous electrode is electronic transport (Li et al. Adv. Mater.27, 6591, 2015). Next, we decrease the length scale to that of single particles (i.e., hundreds of nm), and investigate the fraction of particles actively intercalating. Our work shows that the active particle fraction increases with rates of lithiation, indicating that higher cycling rates improve the uniformity of Li insertion (Li et al. Nat. Mater.13, 1149, 2014). This rate-dependent heterogeneity arises as a result of the unique thermodynamics of phase-separating battery electrodes, whereby the local current density on each particle is mostly invariant with the global C-rate. To understand the role of ion insertion within individual particles, we zoom in further to study heterogeneity at the sub-particle length scale (i.e., tens of nm), We developed operando liquid X-ray imaging platforms to track the Li migration within individual particles (Li et al. Adv Funct. Mater. 25, 3677, 2015; Nelson Weker et al. ChemElectroChem 2, 1765, 2015; Lim, Li, et al. Submitted). By tracking the same particles in real time, we quantified the local current density. Our work shows that phase separation is suppressed at higher rates of discharge, yielding a uniform solid solution pathway. On charge, however, heterogneeous phase separation is much more prevalent, a result of the composition-dependent exchange current density.5 Finally, we study ion insertion at the atomic length scale. We combine experimental and computational tools to study how a solid-solution particle separates. Our results demonstrate that, in the presence of a liquid electrolyte, LiFePO4 rapidly phase separates within individual particles, rather than creating a mosaic of lithiated and delithiated particles. This indicates the presence of a surface diffusion layer perpendicular to the fast bulk diffusion direction that is activated by the electrolyte, and that LiFePO4is not strictly a one-dimensional conductor under dynamic conditions. By directly visualizing Li insertion across diverse length scales, we obtain a comprehensive understanding of ion insertion in LiFePO4. These insights are likely applicable to a broad range of phase-separating materials for lithium-ion batteries and beyond.

Tuning of Microstructure and Magnetic Properties of Nanocrystalline NdFeB Permanent Magnets Prepared by Spark Plasma Sintering

Structure and magnetic properties were studied for bulk nanocrystalline isotropic and anisotropic Nd-Fe-B permanent magnets prepared by spark plasma sintering (SPS). The neck formation among the Nd-Fe-B powders, which was governed by sintering conditions in the SPS process, has a substantial influence on the microstructure and subsequent magnetic properties of the magnets. By tuning the current pulse ratio of power-on to power-off time during sintering from 12:2 to 2:2, we successfully reduced the thickness of the surface diffusion layer of the Nd-Fe-B particles in both hotpressed and hot-deformed magnets. As a result, the maximum energy product (BH) max of the isotropic and anisotropic magnets increases by 27% and 9%, respectively.

Application of active powders at fluidised bed heat treatment technologies

The paper describes the principles and conditions of carrying out of a new type of fluidised bed thermo–chemical treatment in chemically active powders with different ways of fluidisation. These new fluidised bed thermo–chemical treatments in chemically active powders are primarily for the formation of surface diffusion layers on parts made of constructional and tool steel. This method can be used for sherardising, alitising, carbonitriding, nitrocarburising, carburising or boronising. Characteristics of chemically active powders are compared with the chemically inert powders, used in conventional fluidised bed treatments. In addition, the paper presents the feasible ways to use fluidisation by gas flow or by mechanical vibrations for different processes and their main stages, i.e., heating up to treatment temperature, soaking at treatment temperature and cooling down after the soaking. Advantages and disadvantages of these processes, in comparison with the conventional methods of fluidised bed thermo–chemical treatment, are given.

Diffusion interaction of growth steps in vacuum deposition

The interaction of two straight parallel growth steps under saturation conditions was studied using a modified Langmuir adsorption model. It is shown that as the growth steps approach each other, the rate of their motion decreases, which explains the effect of hindered coalescence of condensate islands during vacuum deposition with layer-by-layer filling of atomic layers. The cause of this phenomenon is found to be the overlap of surface diffusion layers which are a source of material for the growth steps. Features of surface diffusion and nucleation under conditions close to the saturation conditions are determined. It is shown that the nucleation of islands of a new atomic layer at supersaturation and the nucleation of islands of vacancies in the underlying layer at unsaturation are asymmetric. The results can be used to develop a technology to produce thin films by vacuum deposition.

Modelling of Phase Evolution during Aluminizing Processes

Aluminizing processes are a well-known set of techniques industrially adopted to enrich in aluminum the surface layers of Ni-based alloys, thus improving their resistance to environmental interactions at high temperature. The results of aluminizing are described in terms of the presence, compositions and thickness of the sequence of the resulting surface diffusion layers. A combination of difficulties arising both from the mathematical and the material side restricted the number of available user-friendly models and their applicability to specific alloys or process conditions. The aim of the research work here presented is to overcome part of these difficulties. A synthesis of some well-established models was implemented in a robust numerical algorithm, that automatically prevents instabilities and convergence problems. Such numerical algorithm has been experimentally validated by comparing the results to the experimentally measured composition of profiles obtained for a set of vapor-phase aluminized samples of commercially pure Ni. The model was then applied to predict the effects of the process temperature and of the chemical composition of the surface.

Modeling bubbles and dissolved gases in the ocean

[1] We report on the development of a bubble concentration model and a dissolved gas concentration model for the oceanic boundary layer. The bubble model solves a set of concentration equations for multiple gases in bubbles of different sizes, and the dissolved gas concentration model simulates the evolution of dissolved gases and dissolved inorganic carbon. The models include the effects of advection, diffusion, bubble buoyant rising, bubble size changes, gas exchange between bubbles and ambient water, and chemical reactions associated with the dissolution of CO2. The formulation consistency and the numerical accuracy are shown by the good agreement with a model describing individual bubble behavior in a test simulating the evolution of a bubble cloud released in the water. To study the bubble and dissolved gas evolution after a single wave-breaking event, the models are coupled with a fluid dynamical Direct Numerical Simulation model with spatially and temporally distributed momentum and bubble injection for a typical breaking wave. The modeled bubble size spectrum compares well with laboratory measurements. The breaker-induced vortex not only advects the bubble-induced dissolved gas anomalies downstream but also entrains the surface diffusion layer to greater depth. Due to the hydrostatic pressure and surface tension exerted on bubbles, gases inside bubbles are able to dissolve in slightly supersaturated water. When the water is highly supersaturated, bubbles add to the venting of dissolved gases.

The Effect of Plastic Deformation on Nitrogen Diffusion in α-Fe

The surface diffusion layers formed in preliminary deformed (350 %) -Fe after nitriding at 853 K in ammonia medium were studied by means of metallography, electron microscopy, microhardness test and X-ray powder diffractometry. The preliminary plastic deformation (PPD) effects non-monotonously on the structure, microhardness and thickness of nitride - and -phases layers formed in -Fe. The narrow intervals of deformations of 3-8 % and 20-30 % were found in which the accelerated formation of nitride - and -phases occurs.

Lipoteichoic Acid Is a Major Component of the Bacillus subtilis Periplasm

Cryo-electron microscopy (cryo-EM) of frozen-hydrated specimens allows high-resolution observation of structures in optimally preserved samples. In gram-positive bacteria, this method reveals the presence of a periplasmic space between the plasma membrane and an often differentiated cell wall matrix. Since virtually nothing is known about the composition of its constituent matter (i.e., the periplasm), it is still unclear what structures (or mechanism) sustain a gram-positive periplasmic space. Here we have used cryo-EM of frozen-hydrated sections in combination with various labels to probe the model gram-positive organism Bacillus subtilis for major periplasmic components. Incubation of cells with positively charged gold nanoparticles showed almost similar levels of gold binding to the periplasm and the cell wall. On cells whose cell walls were enzymatically hydrolyzed (i.e., on protoplasts), a surface diffuse layer extending approximately 30 nm from the membrane was revealed. The thickness and density of this layer were not significantly altered after treatment with a nonspecific protease, whereas it was labeled with anti-lipoteichoic acid (LTA) antibodies conjugated to nanogold. Further, the LTA layer spans most of the thickness of the periplasmic space, which strongly suggests that LTA is a major component of the B. subtilis periplasm.

Manganese enriched diffusion layers on NiAl alloy

Materials based on Ni–Al intermetallics are prospective high temperature materials. Properties of these intermetallics can often be improved by surface diffusion layers enriched by various metals. In this contribution results of a study are presented about microstructure and composition of Mn enriched diffusion layers on a NiAl alloy.A pure Mn plate was annealed at 900°C for 4 h in contact with a prism of the NiAl phase. The ordered B2-NiAl (β′) phase can absorb only a few per cent of manganese without structural changes. When absorbing more Mn, the ordrerd B2-NiAl phase transforms into the disordered bcc β′ NiMnAl phase forming the first structurally different (narrow) layer on NiAl. Its Mn content increases from 25 wt-% (in contact with B2-NiAl phase) up to 35 wt-% across this homogeneous layer. Higher average Mn content in the next layer manifests in precipitation of β-Mn particles (containing several per cent of Ni and Al) from the β″ matrix. Owing to diffusion of Ni and Al into the original Mn plate the β″ NiMnAl precipitates are formed in the Mn matrix containing several per cent of Ni and Al (when studying microstructure and composition at room temperature after annealing).

Diffusion induced recrystallization in alumina

Abstract When Fe2O3 was substantially dissolved into an (0001) alumina single crystal through a vapor phase at 1500 °C, recrystallization of the single crystal occurred. TEM observation showed that the nucleation of the recrystallized grains occurred through the formation of very small angle wall boundaries via a rearrangement of the misfit dislocations in the surface diffusion layer and their subsequent transformation into high angle grain boundaries. The dislocation wall mechanism appears, therefore, to be the nucleation mechanism of diffusion-induced recrystallization in single crystal alumina. However, an abrupt change in Fe2O3 concentration was observed across the boundary between the single crystal and the growing recrystallized grains, indicating that the growth of the recrystallized grains occurred by diffusion-induced grain-boundary migration. During extended annealing, the division of large recrystallized grains into small ones (repeated recrystallization) often occurred. The observed repeated recrystallization was attributed to the redistribution of dislocations formed in the grains where the solute concentration was non-uniform.

The Effect of Deformation on Structure and Properties of Surface Diffusion Layers Formed in Iron Alloys under Alloying by Nitrogen and Carbon

The Effect of Deformation on Structure and Properties of Surface Diffusion Layers Formed in Iron Alloys under Alloying by Nitrogen and Carbon

Structure and properties of potassium iodide nanoparticles. A molecular dynamics study

Properties of solid and liquid clusters ranging in size from 108 to 500KI pairs have been investigated in computer simulations. Melting points, densities or, for the liquid, a density profile, coefficients of thermal expansion, heat capacities, heats of fusion and diffusion coefficients were determined, for the most part as functions of particle size. Coefficients of thermal expansion showed the greatest differences from the bulk, particularly for the liquid where the coefficient was twice as large. Effects of the Laplace pressure could be discerned in both the liquid and solid densities. Although the solid particles retained their well-developed facets and exhibited little surface diffuseness until very warm, the molten clusters had very diffuse liquid–vapor interfaces. Unlike the case of liquid argon clusters, the surface diffuse layer was considerably thicker than could be accounted for by classical capillary waves. As had been found in the liquid state of bulk salts, the closest cation–anion internuclear distances were appreciably shorter than in the solid, a consequence of the lower coordination numbers. The solid clusters possessed what is nominally the face centered cubic rock salt structure but deviated from it in that the cations tended not to be midway between the anions. This tendency resembles that in the liquid, though to a smaller extent. It also can be attributed to the diminished coordination number of those ions at the surface. This asymmetry causes alternations in K–I distances to be propagated into the interior.

Raman spectroscopic study of surface layer in fluorine-implanted Si

Raman scattering application was introduced to directly probe the depth profile of structural changes in a very thin surface layer of F+-implanted Si by use of a single Ar+ laser (488 nm) excitation. The results of Raman scattering and sheet resistance measurement showed an unusual annealing behavior of the F+-implanted Si:In the range of annealing temperature Ta from 200 °C to 400 °C, disordering was observed to increase with increasing Ta but a stronger trend of ordering with Ta increasing further above 400 °C. This abnormal behavior could be explained as due to competition between the ordering effect of thermal annealing with increasing Ta and the disordering effect of the implanted fluorine ions randomly breaking the Si–Si crystal bonds in the surface diffusion layer.

On the effect of organo-silicon block copolymer—solvent—precipitant system composition on asymmetric membrane surface layer thickness

The dependence of dense surface diffusion layer thickness of asymmetric membranes prepared by phase inversion from an organo-silicon block copolymer—solvent—precipitant system on the system composition has been demonstrated.

A technique for determination of microhardness profiles in thin surface diffusion layers

La technique consiste a monter l'echantillon de telle facon que la surface et non la section transversale soit exposee. Les mesures de microdurete sont effectuees sur la surface. L'echantillon est ensuite poli, et les mesures sont effectuees a differentes profondeurs sur la surface polie. Application de cette technique a un superalliage Fe−Ni implante par des ions azote, et a une couche de diffusion dans un acier au carbone

Interdependency of composition and T c, of δ- VN1− x and the influence and determination of its nitrogen surface diffusion layers

Compact δ- VN 1− x samples (0.707⩽(1 − x)⩽0.996) were prepared by nitriding vanadium wire in nitrogen at various pressures and temperatures. The samples were fully characterized by chemical analysis and the dependence of the lattice parameter upon composition was established. An investigation of the superconductive properties of β- VN 1− x by the inductive method showed that thin surface layers with higher nitrogen content were formed during the cooling down period and consequently influence the T c . Measurements of powdered samples yielded the relation T c ( K) = 25.3(1− x)-16.35. Susceptibility measurements of T c can be used to quantitatively determine nitrogen diffusion profiles in compact β- VN 1− x samples.