Near-surface Region Research Articles

Mitigating water-induced surface degradation in water-based Ni-rich Li-ion battery electrodes

Replacement of the toxic solvent used during manufacturing of cathode electrodes for Li-ion batteries with water is an essential step on the road towards eco-friendly battery production. This study aims to improve the performance of pilot-scale water-based LiNi0.83Co0.12Mn0.05O2 cathodes. The importance of proper electrode treatment before cell assembly to achieve maximum cell performance is demonstrated. Temperatures ranging from room temperature to 170 °C were applied to the electrodes to investigate the impact of residual moisture in pouch cells. Electrochemical cycling showed that despite a much higher amount of residual moisture a low drying temperature was beneficial. X-ray photoelectron spectroscopy indicated that with increasing temperature a phase reconstruction in the near-surface region of the cathode material particles is the cause for the difference in performance. This was further supported by impedance spectroscopy showing growing charge-transfer resistance arcs with higher temperature, which split into two contributions with ongoing cycling. Already temperatures above 100 °C resulted in more pronounced surface reconstruction and stronger impedance increase lowering the long-term cycling stability, while the residual moisture seemed to have only a minor impact. By optimizing the drying temperature, the long-term cycling stability could be improved from 1000 to 1700 cycles before reaching 80% capacity retention.

Near-surface damage and mixing in Si-Cl2-Ar atomic layer etching processes: Insights from molecular dynamics simulations

Silicon-chlorine-argon (Si-Cl2-Ar) atomic layer etching (ALE) is simulated using classical molecular dynamics (MD). The simulations provide a detailed view into the near-surface region during ALE processing. Bombardment of Ar+ ions creates a mixed amorphous region that significantly differs from the picture of ideal ALE. There is also a significant change in the Si etch yield and the etch product distribution as a function of Ar+ ion fluence. The Si etch yield is the highest at the beginning of the bombardment step but eventually decays to the physical sputtering yield. Atomic Cl and silicon chlorides are major etch products at the start of an ion bombardment step, but quickly decay. Atomic Si yields remain relatively constant as a function of Ar+ ion fluence. A new schematic of Si-Cl2-Ar ALE is presented in order to emphasize the complex behavior observed in MD simulations.

Increasing the Efficiency of Silicon Solar Elements by Doping with Nickel

It is shown that in the near-surface region of the solar cells (SCs) the concentration of nickel atoms is higher than in the bulk by 2–3 orders of magnitude, therefore, the gettering rate in the near-surface region is higher. Optimal modes of gettering by nickel clusters (i.e., nickel diffusion – Т = 800–850 °С, additional thermal annealing – Т = 750–800 °С) and the structure of a silicon SC were experimentally determined, which makes it possible to increase the efficiency of silicon SCs by 25–30% relative to the control. The physical mechanisms of the influence of the processes of diffusion of impurity nickel atoms and additional thermal annealing on the state of nickel atoms in the near-surface region and the SCs base and, accordingly, on the parameters of the SC are revealed. Physical models of the structure of a cluster of nickel atoms in silicon and the process of fast diffusing impurities gettering by clusters of nickel atoms are created. The binding energy of fast dissusing impurities atoms with a nickel cluster is estimated to be ~ 1.39 eV. The calculation shows that doping with nickel can increase the lifetime of minority charge carriers by 2–4 times, and the collection coefficient by 1.4–2 times. The experiment showed an increase in the minority charge carriers lifetime up to 2 times and an increase in efficiency by 25–30%.

Revealing the effects of microstructural changes of graphite anodes during cycling on their lithium intercalation kinetics utilizing operando XRD

Concerning cell ageing and lithium plating, electrode kinetics – in particular the one of the anode - plays an important role. In our study, we applied a holistic physicochemical post-mortem analysis including operando XRD and SEM-studies as well as electrochemical analysis to get deeper insights into the changes of the microstructure and phase composition of graphite anodes from LiNi0.6Co0.2Mn0.2O2//Graphite pouch cells during cyclic ageing and their influence on the lithium intercalation kinetics. The key findings are as follows: Li plating and loss of cyclable lithium is the main source of capacity fade of the investigated cells. Cycling causes pore clogging of the anode leading to an increased transport resistance for Li-ions and slower charge transfer reactions. Visual inspections and operando XRD measurements in turn indicate an inhomogeneous Li intercalation over the thickness of the electrode resulting in a Li gradient. In the cycled electrode LiC6 forms in surface-near regions, whereas in deeper regions LiC12 formation has not finished yet. As a result, Li plating occurs as the surface-near graphite is already fully intercalated and Li-ion transport to deeper regions is hindered due to pore clogging. By combining complementary methods we could consistently prove the hypothesis of pore clogging, deterioration of anode kinetics and subsequent inhomogeneous lithiation which is supposed to trigger the observed Li-plating.

Ion Beam as an External and Dynamic Metal Reservoir to Induce Nanoparticle Exsolution in Oxides

Nanoparticle exsolution is a recently developed method to produce oxide-supported metal nanoparticles via phase precipitation. It has attracted significant attention in the field of solid oxide fuel cells and electrolyzers (SOFC/SOEC) due to its potential for producing active and nanostructured electrodes. However, conventional exsolution methods face challenges in reaching the solubility limit of metal ions in the oxide matrix. To overcome this issue, here we propose a new approach to induce exsolution by using ion beams as an external and dynamic metal reservoir. We demonstrate the effectiveness of this method by using thin-film Zr0.5Ce0.5O2 (CZO) as a model system. Through ion irradiation experiments, 10 keV Ni ions were implanted into the near-surface regions of CZO films, and then exsolved as nanoparticles on the surface. This research offers a promising new avenue for developing next-generation SOFC/SOEC electrode materials using irradiation-tailored exsolution.

Molecular dynamics study of primary damage in the near-surface region in nickel

We carried out a large-scale molecular dynamics (MD) simulation to study displacement cascades in the near-surface region in pure nickel. These MD simulations were performed both in the bulk and near-surface regions with primary knock-on atom (PKA) energies of EPKA=1,5, or 10keV and at temperatures T=300,425, or 525K. In our study, the primary knock-on atom was taken to be created by a neutron knocking off a lattice atom. Accordingly, cascades were initiated by giving a lattice atom the appropriate velocity corresponding to the PKA energy. Assuming isotropic neutron fluence, the PKA was initiated in random directions (including toward and parallel to the free surface) in both bulk and near-surface simulations. This contrasts with previous studies in which the cascades were initiated by the projectiles directed toward the free surface to mimic ion irradiation. Therefore, for every (T,EPKA), near-surface cascade simulations were performed also as a function of PKA's initiation depth. Present results provide quantitative information about defect production and clustering in the primary damage state for near-surface and bulk displacement cascades. The effect of EPKA and T on the defect production (averaged over all PKA directions) is similar for both the near-surface and bulk cascades. In both cases, the defect production increases with EPKA, but the temperature has minimal effect. However, the production and clustering of vacancies are higher for near-surface cascades. In contrast, the production and clustering of self-interstitial atoms are lower and increase monotonically with depth. The decrease in the average number of vacancies and average vacancy cluster size with increasing depth is not monotonic.

Spatial imaging of stratified heterogeneous microstructures: determination of the hardness penetration depth in thermally treated steel parts by laser ultrasound

We introduce an improved non-destructive and non-contact measurement technique for the hardness penetration depth of thermally hardened steel parts based on all-optical laser ultrasound. The hardening of the near-surface region results in a stratified system with layers of different microstructural characteristics caused by changes in grain size. We employ spatially resolved ultrasonic backscattering signals to provide subsurface imaging of the axial and lateral hardness profile. Spatial scans over a range of 30 mm with a lateral step size of 100 μm were obtained within 30 s. We investigated three common steel grades with different microstructural characteristics and hardened layer thicknesses ranging from about 3 to 10 mm. The hardened surface layer and the unhardened core material can be identified. The data evaluation is assisted by a supervised machine learning approach that takes advantage of the spatio-temporal ultrasonic backscattering information. This method also provides an alternative route to define the hardness penetration depth using spatio-temporal scattering characteristics.

Finite element analysis of material deformation behaviour during DRECE: the sheet metal SPD process

The material deformation behaviour during the innovative SPD process called DRECE (Dual Rolls Equal Channel Extrusion) has been analysed by FEM simulations. In the process, a workpiece in the form of a strip is subjected to plastic deformation by passing through the angular channel; however, the workpiece dimensions remain the same after a pass is finished. Performing consecutive passes allow for increasing the effective strain in the material to a required level. In the conducted simulations two various channel angles (108° and 113°) have been taken into consideration, as well as two processing routes, A and C (without and with turning the strip upside-down between consecutive passes, respectively). The analysis of simulation results has revealed that significant strain and stress inhomogeneities across the strip thickness are generated in a single DRECE pass. The die design (the inner and outer corner radius) and friction conditions affect the material flow, reducing significantly the shear strain in the near-surface regions of the strip. The strain inhomogeneity can be effectively reduced by choosing the processing route C. The strain distributions and the corresponding tensile test results have confirmed that the smaller channel die angle allows to generate larger strain and higher strength of the strip but also reduces its ductility more than the die setup with the larger channel die angle.

Charge Relaxation in Chalcogenide Films under Electron Beam Irradiation

We studied processes of charge relaxation after point irradiation of AsxSe100-x and GexSe100-x chalcogenide films with an electron beam. Two relaxation processes were found in the dose range from 2,05 * 102 μCcm−2 to 9,3 * 107 μCcm−2. The relaxation times of these processes differ by more than 103 times. It is shown that the first relaxation process is associated with the relaxation of the positive charge in the near-surface region of irradiated film. The second, slower relaxation process is due to the process of the formation of a negative charge layer inside the film and electron-induced mass transport under the action of an internal electric field.

Microstructural characterization of near-surface microstructures on rail wheels in service – an insight into “stratified surface layers”

Background: To decrease maintenance costs and improve safety in rail transportation, the understanding of rail and wheel defects is vital. Studies on “white etching layers” (WEL) on rails and wheels, prone to fatigue crack initiation, have been extensively studied. Recently, a relative named “brown etching layer” (BEL) and its combination, the so-called “stratified surface layer” (SSL), are observed in the field. This study presents an investigation on a rail wheel affected by mechanical and thermal loadings from service with focus on the different evolved layers in the near-surface region. Methods: Optical microscopy is performed on etched cross-sectional cuts to identify different evolved microstructures (WEL, BEL, SSL), further, specific regions are investigated in detail by scanning electron microscopy to evaluate the microstructural characteristics. To analyze the change in mechanical properties, low-load Vickers hardness investigations are executed in distinctive zones. Results: This study highlights the broad variety of evolved microstructures, however, a rough classification of WEL (fine mesh-like microstructure, 900 – 1200 HV0.0.1) and BEL (globular cementite particles, 400 – 600 HV0.01) is given. Further, results indicate that the BEL is commonly accompanied by a WEL, representing an SSL. Conclusions: The complex loading situation in a wheel-rail contact can lead to the formation of WEL, BEL and SSL. The observation of numerous initiated fatigue cracks within these regions demonstrates the relevance of in-depth studies on evolved microstructures in wheel-rail contacts.

Cell voltage of mixed conductors under partially frozen conditions

Partially frozen-in states are rather the rule than the exception. Coexistence between equilibrium states and frozen-in states is relevant in view of the diversity and complexity of charge carriers, or sublattices, especially in multinary compounds, but also with respect to differently equilibrated spatial regions. This contribution deals with the open circuit potential of samples where only surface-near regions feel the outer partial pressure, or more generally, the component chemical potentials, established by the electrodes. In view of the significance of such measurements for separating ionic and electronic conductivity contributions, and the kinetic difficulties in getting full equilibration near room-temperature, the value of these considerations is obvious. The necessary relations are derived, or their derivations are sketched within the framework of linear force–flux laws. An account is made of recent emf measurements of hybrid halide perovskites, and a refinement of their standard defect diagram is recommended.

Pivotal Role of Ni/ZrO2 Phase Boundaries for Coke-Resistant Methane Dry Reforming Catalysts

To identify the synergistic action of differently prepared Ni-ZrO2 phase boundaries in methane dry reforming, we compared an “inverse” near-surface intermetallic NiZr catalyst precursor with the respective bulk-intermetallic NixZry material and a supported Ni-ZrO2 catalyst. In all three cases, stable and high methane dry reforming activity with enhanced anticoking properties can be assigned to the presence of extended Ni-ZrO2 phase boundaries, which result from in situ activation of the intermetallic Ni-Zr model catalyst systems under DRM conditions. All three catalysts operate bifunctionally; methane is essentially decomposed to carbon at the metallic Ni0 surface sites, whereas CO2 reacts to CO at reduced Zr centers induced by a spillover of carbon to the phase boundaries. On pure bulk Ni0, dissolved carbon accumulates in surface-near regions, leading to a sufficiently supersaturated state for completely surface-blocking graphitic carbon segregation. In strong contrast, surface-ZrO2 modified bulk Ni0 exhibits virtually the best decoking and carbon conversion conditions due to the presence of highly dispersed ZrO2 islands with a particularly large contribution of interfacial Ni0-ZrO2 sites and short C-diffusion pathways to the latter.

Effect of Si on the hydrogen-based direct reduction of Fe2O3 studied by XPS of sputter-deposited thin-film model systems

Understanding the effect of gangue elements is of critical importance to optimize the efficiency of hydrogen-based direct reduction (HyDR) of iron ore, as one of the key steps towards climate-neutral steel production. Here, we demonstrate on the example of Si-doped Fe2O3, how thin films can be effectively utilized as a model system to facilitate systematic investigation of the solid-state reduction behavior. In-vacuo X-ray photoelectron spectroscopy (XPS) is used to probe the reduction kinetics by analyzing the chemical state of iron oxide thin films before and after annealing at 700 °C in an Ar+5%H2 atmosphere. It is demonstrated that even low Si concentrations of 3.7 at.% inhibit the HyDR of Fe2O3 by the formation of a SiOx-enriched reduction barrier in the surface-near region.

Hybrid Silver-Containing Materials Based on Various Forms of Bacterial Cellulose: Synthesis, Structure, and Biological Activity.

Sustained interest in the use of renewable resources for the production of medical materials has stimulated research on bacterial cellulose (BC) and nanocomposites based on it. New Ag-containing nanocomposites were obtained by modifying various forms of BC with Ag nanoparticles prepared by metal-vapor synthesis (MVS). Bacterial cellulose was obtained in the form of films (BCF) and spherical BC beads (SBCB) by the Gluconacetobacter hansenii GH-1/2008 strain under static and dynamic conditions. The Ag nanoparticles synthesized in 2-propanol were incorporated into the polymer matrix using metal-containing organosol. MVS is based on the interaction of extremely reactive atomic metals formed by evaporation in vacuum at a pressure of 10-2 Pa with organic substances during their co-condensation on the cooled walls of a reaction vessel. The composition, structure, and electronic state of the metal in the materials were characterized by transmission and scanning electron microscopy (TEM, SEM), powder X-ray diffraction (XRD), small-angle X-ray scattering (SAXS) and X-ray photoelectron spectroscopy (XPS). Since antimicrobial activity is largely determined by the surface composition, much attention was paid to studying its properties by XPS, a surface-sensitive method, at a sampling depth about 10 nm. C 1s and O 1s spectra were analyzed self-consistently. XPS C 1s spectra of the original and Ag-containing celluloses showed an increase in the intensity of the C-C/C-H groups in the latter, which are associated with carbon shell surrounding metal in Ag nanoparticles (Ag NPs). The size effect observed in Ag 3d spectra evidenced on a large proportion of silver nanoparticles with a size of less than 3 nm in the near-surface region. Ag NPs in the BC films and spherical beads were mainly in the zerovalent state. BC-based nanocomposites with Ag nanoparticles exhibited antimicrobial activity against Bacillus subtilis, Staphylococcus aureus, Escherichia coli bacteria and Candida albicans and Aspergillus niger fungi. It was found that AgNPs/SBCB nanocomposites are more active than Ag NPs/BCF samples, especially against Candida albicans and Aspergillus niger fungi. These results increase the possibility of their medical application.

Y-TZP nanostructures with non-homogeneous yttria distribution for improved hydrothermal degradation resistance and mechanical properties

Dense and homogeneous nanostructured Y-TZP ceramics were prepared by colloidal processing and 2-step sintering using commercial powders. The influence of non-homogeneous yttria distributions on mechanical properties and Low-Temperature Degradation (LTD) resistance were assessed. Physical, structural, microstructural, and mechanical characterizations of all groups were evaluated and compared before and after artificial aging. Nanostructured Y-TZP samples with non-homogeneous yttria distributions presented the highest fracture toughness and elevated fracture strength. These samples were susceptible to LTD, but with the transformed monoclinic phase fraction restricted to the near-surface region. The characterization data of nanostructured samples were compared to those of conventional 3Y-TZP. The results of this work indicate that it may be possible to prepare Y-TZP ceramics with improved mechanical properties and resistance to LTD by careful and systematic design and engineering of nanostructures.

Effect of Supersonic Nitrogen Flow on Ceramic Material Ta4HfC5–SiC

The behavior of the ceramic material Ta4HfC5-30 vol % SiC has been studied under the effect of supersonic flow of dissociated nitrogen, which is necessary to assess the potential application of these materials in oxygen-free gas environments at temperatures 1800°C. It has been found that as a result of heating the surface to ~2020°C in a few minutes there is a decrease to ~1915°C followed by a slow decrease to 188°C. This is probably due to the chemical processes occurring on the surface and the formation of an extremely rough microstructure. The ablation rate has been determined; it has been shown that neither at introduction of the sample into a high enthalpy nitrogen flow nor at sharp cooling (temperature drop to ~880°C in 9–10 s) cracking of the sample or detachment of the near-surface region has been observed. X-ray powder diffraction and Raman spectroscopy data allow us to conclude the complete removal of silicon carbide from the surface layer and the transformation of complex tantalum-hafnium carbide into the nitride.

Investigation of the Porosity Gradient in Thickness Direction Formed by Cold Rolling in Porous Aluminum

To adapt porous material for its application as low noise trailing edge, a special rolling process, using a time-varied rolling gap, was used in previous research to produce a porosity gradient in the direction of rolling. Investigations suggest that a gradient in porosity may also be produced in the thickness direction of the material, i.e., in the rolling gap direction, without using a specialized rolling mill. Such a gradient may help to further increase acoustic efficiency of porous materials. The aim of this study was to analyze the dependency of such a gradient on the rolling parameters, and to clarify which stress components are significantly responsible for an increased near-surface compaction. Experiments using different relative compressed lengths were performed to analyze shear-dominated and friction-dominated rolling. The material was characterized using compression tests, computed tomography and flow resistance measurements. It is shown that the compressed length is an important parameter for adjusting a porosity gradient. Rolling with small values of compressed length during all rolling passes leads to increased compaction of near-surface regions, compared to interior ones. The difference in porosity achieved was up to 15%. Furthermore, the results suggested that a gradient in hydrostatic stress is responsible for the porosity gradient. Validation of the results by FE simulation is forthcoming, but not part of this publication.

Appraising Forgeability and Surface Cracking in New Generation Cast and Wrought Superalloys

Surface cracking poses a major problem in industrial forging, but the scientific understanding of the phenomenon is hampered by the difficulty of replicating it in a laboratory setting. In this work, a novel laboratory-scale experimental method is presented to investigate forgeability in new generation cast and wrought superalloys. This new approach makes possible appraising the prevalence and severity of surface cracking by mimicking the die chilling effects characteristic of hot die forging. Two high γ′-reinforced alloys are used to explore this methodology. A Gleeble thermo-mechanical simulator is used to conduct hot compression tests following a non-isothermal cycle, with the aim to simulate the cooling of the near-surface regions during the forging process. FEA simulations, sample geometry design, and heat-treatments are used to ensure the correspondence between laboratory and real-scale forging. A wide range of surface cracking results are obtained for different forging temperatures and cooling rates—proving the soundness of the method. Surprisingly, samples heated up to higher initial temperatures typically show more extensive surface cracking. These findings indicate that—along with the local mechanical conditions of the forging—die-chilling effects and forging temperatures are paramount in controlling surface cracking, as they dictate the key variables governing the distribution and kinetics of γ′ formation.

Photoaging of polystyrene microspheres causes oxidative alterations to surface physicochemistry and enhances airway epithelial toxicity.

Microplastics represent an emerging environmental contaminant, with large gaps in our understanding of human health impacts. Furthermore, environmental factors may modify the plastic chemistry, further altering the toxic potency. Ultraviolet (UV) light is one such unavoidable factor for airborne microplastic particulates and a known modifier of polystyrene surface chemistry. As an experimental model, we aged commercially available polystyrene microspheres for 5 weeks with UV radiation, then compared the cellular responses in A549 lung cells with both pristine and irradiated particulates. Photoaging altered the surface morphology of irradiated microspheres and increased the intensities of polar groups on the near-surface region of the particles as indicated by scanning electron microscopy and by fitting of high-resolution X-ray photoelectron spectroscopy C 1s spectra, respectively. Even at low concentrations (1-30 µg/ml), photoaged microspheres at 1 and 5 µm in diameter exerted more pronounced biological responses in the A549 cells than was caused by pristine microspheres. High-content imaging analysis revealed S and G2 cell cycle accumulation and morphological changes, which were also more pronounced in A549 cells treated with photoaged microspheres, and further influenced by the size, dose, and time of exposures. Polystyrene microspheres reducedmonolayer barrier integrity and slowed regrowth in a wound healing assay in a manner dependent on dose, photoaging, and size of the microsphere. UV-photoaging generally enhanced the toxicity of polystyrene microspheres in A549 cells. Understanding the influence of weathering and environmental aging, along with size, shape, and chemistry, on microplastics biocompatibility may be an essential consideration for incorporation of different plastics in products.

Laser-treatment-induced surface integrity modifications of stainless steel

Scanning of a high-power laser beam on the surface of martensitic stainless steel (SS420) has been studied, addressing the effect of scanning rate V on integrity modifications in the near-surface regions. Structural, compositional, and crystallographic characterizations revealed the presence of ablations, surface melting/resolidification, surface oxidations, and austenite (γ-phase) precipitations when V ≤ 20 mm s−1. Melt pool (MP), heat affected zone (HAZ), and base material have been clearly distinguished at the cross-section of the slow-scanned samples. Adjacent MPs partially overlapped when V = 5 mm s−1. The γ-phase precipitations solely occurred in the MPs, i.e., of ∼ 400 μm deep for V = 5 mm s−1, while oxidations dominantly occurred in the surface regions of shallower than ∼30 μm within the MPs. Compositional analysis revealed increased Cr-, Mn-, and Si-to-Fe ratios at the laser-scanned surface but without variations along the surface normal direction. The enhanced surface hardness has been achieved up to 805 HV, and the hardness monotonically decreased when moving deeper (i.e., ∼1000 μm) into the base material. These observations shed new light on surface engineering of metallic alloys via laser-based direct energy treatments.