Compositional Redistribution Research Articles

Consequences of kelp forest ecosystem shifts and predictors of persistence through multiple stressors.

Ecological communities can be stable over multiple generations, or rapidly shift into structurally and functionally different configurations. In kelp forest ecosystems, overgrazing by sea urchins can abruptly shift forests into alternative states that are void of macroalgae and primarily dominated by actively grazing sea urchins. Beginning in 2014, a sea urchin outbreak along the central coast of California resulted in a patchy mosaic of remnant forests interspersed with sea urchin barrens. In this study, we used a 14-year subtidal monitoring dataset of invertebrates, algae, and fishes to explore changes in community structure associated with the loss of forests. We found that the spatial mosaic of barrens and forests resulted in a region-wide shift in community structure. However, the magnitude of kelp forest loss and taxonomic-level consequences were spatially heterogeneous. Taxonomic diversity declined across the region, but there were no declines in richness for any group, suggesting compositional redistribution. Baseline ecological and environmental conditions, and sea urchin behaviour, explained the persistence of forests through multiple stressors. These results indicate that spatial heterogeneity in preexisting ecological and environmental conditions can explain patterns of community change.

Additive Manufacturing of SS316L/IN718 Bimetallic Structure via Laser Powder Bed Fusion.

Laser powder bed fusion (LPBF) is a popular additive manufacturing (AM) technique that has demonstrated the capability to produce sophisticated engineering components. This work reports the crack-free fabrication of an SS316L/IN718 bimetallic structure via LPBF, along with compositional redistribution, phase transformations and microstructural development, and nanohardness variations. Constituent intermixing after LPBF was quantitatively estimated using thermo-kinetic coefficients of mass transport and compared with the diffusivity of Ni in the austenitic Fe-Ni system.

Effect of concurrent grain growth on radiation-induced segregation in nanocrystalline Fe–Cr–Ni alloys

Irradiation of crystalline materials modifies their microchemistry and microstructure. This includes solute segregation toward defect sinks such as grain boundaries (GBs), a phenomenon commonly known as radiation-induced segregation (RIS). Unlike in coarse-grained alloys where GBs are nearly static, RIS is usually accompanied and affected by either thermal or irradiation-induced grain growth in nanocrystalline materials. This work presents a modeling study of concurrent grain growth and RIS in austenitic Fe–Cr–Ni adopting realistic 2D grain structures. RIS can be significantly affected by concurrent grain growth due to (i) increasing grain size, (ii) motion of GBs as defect sinks, and (iii) their combined effect. Consequently, RIS is enhanced by grain growth due to increased grain size and sink motion. More notably, RIS in nanocrystalline materials were found to induce grain-level compositional redistribution in addition to RIS at defect sinks, resulting in grain-size-dependent compositions in individual grains. Without concurrent grain growth, elements depleted at the sinks due to RIS, such as Cr in austenitic steels, will have lower concentrations in smaller grains than in larger grains. The opposite trend becomes true when concurrent grain growth takes place. The compositional difference in individual grains can be significant enough to affect local phase stability. These findings are not discernible with the classical 1D bicrystal model, and they highlight the different effects of RIS in nanocrystalline alloys compared to their coarse-grained counterparts.

On the compositional and structural redistribution during partial recrystallisation: a case of σ-phase precipitation in a Mo-doped NiCoCr medium-entropy alloy

In this work, we discussed the atomic-scale mass transport occurring in a Mo-doped NiCoCr medium-entropy alloy to form σ-precipitates during partial recrystallization at 700°C. We reveal the segregation of Cr/Mo to the fault planes of nano-twins/stacking faults in non-recrystallized zones. Upon annealing, the migration of recrystallized grain boundaries (GBs) redistributes the structure and composition, leading to the annihilation of faults and leaving behind the Mo-rich σ-precursors for precipitation. The segregated Cr/Mo solutes can also diffuse to the moving GBs and create additional σ-nucleation sites. The present results highlight the role of Mo in overcoming strength-ductility trade-off of the alloy.

Machine learning materials physics: Multi-resolution neural networks learn the free energy and nonlinear elastic response of evolving microstructures

Many important multi-component crystalline solids undergo mechanochemical spinodal decomposition: a phase transformation in which the compositional redistribution is coupled with structural changes of the crystal, resulting in dynamically evolving microstructures. The ability to rapidly compute the macroscopic behavior based on these detailed microstructures is of paramount importance for accelerating material discovery and design. Here, our focus is on the macroscopic, nonlinear elastic response of materials harboring microstructure. Because of the diversity of microstructural patterns that can form, there is interest in taking a purely computational approach to predicting their macroscopic response. However, the evaluation of macroscopic, nonlinear elastic properties purely based on direct numerical simulations (DNS) is computationally very expensive, and hence impractical for material design when a large number of microstructures need to be tested. A further complexity of a hierarchical nature arises if the elastic free energy and its variation with strain is a small-scale fluctuation on the dominant trajectory of the total free energy driven by microstructural dynamics. To address these challenges, we present a data-driven approach, which combines advanced neural network (NN) models with DNS to predict the homogenized, macroscopic, mechanical free energy and stress fields arising in a family of multi-component crystalline solids that develop microstructure. The microstructures are numerically generated by solving a coupled, mechanochemical spinodal decomposition problem governed by nonlinear strain gradient elasticity and the Cahn–Hilliard phase field equation. The hierarchical structure of the free energy’s evolution induces a multi-resolution character to the machine learning paradigm: We construct knowledge-based neural networks (KBNNs) with either pre-trained fully connected deep neural networks (DNNs), or pre-trained convolutional neural networks (CNNs) that describe the dominant characteristic of the data to fully represent the hierarchically evolving free energy. We demonstrate multi-resolution learning of the materials physics; specifically of the nonlinear elastic response for both fixed and evolving microstructures.

Bridging the local configurations and crystalline counterparts of bulk metallic glass by nanocalorimetry

The structural understanding of crystallization in bulk metallic glasses (BMGs) has attracted much attention while rapid crystallization occurring under controllable conditions is less involved. In this study, a Ce68Al10Cu20Co2 (at.%) BMG was thermally devitrified by differential scanning calorimetry (DSC) and nanocalorimetry. At a heating rate of 10K/min by DSC, AlCe3 and Ce are the major crystalline phases after devitrification while Al13Co4 quasicrystals and Ce are the dominant phases in the crystallization products at a heating rate of 5000K/s by nanocalorimetry. Attributing to the covalent-like bond in Al–Co atom pairs, Al13Co4 quasicrystals precipitate in the primary crystallization and work as the precursors associating local atomic configurations in the glassy state with crystalline phases after crystallization. Attributing to the enhanced mobility of Cu atoms, compositional redistribution occurs in the as-cast sample. On nanocalorimetry heating, an unambiguous discrepancy in the nucleation and growth of the nano-sized Al13Co4 quasicrystals is thus triggered, contributing to an obvious difference in the crystal size. This research unveils the distinct crystallization behaviors of Ce-based BMG on rapid heating. The formation of quasicrystals demonstrates the multi-stage crystallization on rapid heating and bridges the structural gap between local atomic configurations of metallic glasses and crystalline phases.

Compositional redistribution in CaO-Al2O3-SiO2 glass induced by the migration of a steel microsphere due to continuous-wave-laser irradiation.

A high-power continuous-wave (CW) laser was used to move a steel microsphere through a CaO-Al2O3-SiO2 glass block at room temperature along a trajectory toward the laser source. A compositional analysis revealed that the CaO concentration in the glass decreased at the center of the microsphere's trajectory but increased in the area adjacent to it; the SiO2 concentration showed an opposite trend while the Al2O3 concentration did not change. Further, the compositional difference between the center and the area adjacent to the microsphere trajectory depends on the velocity of the microsphere, which is controllable by tuning the laser power.

Temperature induced compositional redistribution in blended insertion electrodes

Abstract Blending insertion compounds is a novel and promising approach to design advanced electrodes for future lithium-ion batteries. In spite of the considerable improvements regarding safety issues and power density, the understanding of basic interactions between the constituents of the blend and differences towards common single compound insertion electrodes is still ongoing. Herein we explore and verify the effect of temperature induced compositional redistribution of lithium-ions between the constituents of a blended insertion electrode for the first time. A model-like blend electrode and a special experimental setup is used to measure the compositional redistribution current between the constituents when subjected to a temperature change. The amount of lithium exchanged between the constituents of the blend is also derived theoretically based on the thermodynamic properties of the pure constituents, showing excellent agreement to the experimental results. Theoretical and experimental results proof that significant amounts of lithium are exchanged between the constituents without any cycling of the battery, suggesting that this effect may intrinsically reduce the cycle life of batteries with blended insertion electrodes.

Stress-directed compositional patterning of SiGe substrates for lateral quantum barrier manipulation

While vertical stacking of quantum well and dot structures is well established in heteroepitaxial semiconductor materials, manipulation of quantum barriers in the lateral directions poses a significant engineering challenge. Here, we demonstrate lateral quantum barrier manipulation in a crystalline SiGe alloy using structured mechanical fields to drive compositional redistribution. To apply stress, we make use of a nano-indenter array that is pressed against a Si0.8Ge0.2 wafer in a custom-made mechanical press. The entire assembly is then annealed at high temperatures, during which the larger Ge atoms are selectively driven away from areas of compressive stress. Compositional analysis of the SiGe substrates reveals that this approach leads to a transfer of the indenter array pattern to the near-surface elemental composition, resulting in near 100% Si regions underneath each indenter that are separated from each other by the surrounding Si0.8Ge0.2 bulk. The “stress transfer” process is studied in detail using multiscale computer simulations that demonstrate its robustness across a wide range of applied stresses and annealing temperatures. While the “Si nanodot” structures formed here are not intrinsically useful as quantum structures, it is anticipated that the stress transfer process may be modified by judicious control of the SiGe film thickness and indenter array pattern to form more technologically useful structures.

Compositional redistribution and dynamic heterogeneity in mixed lipid membrane induced by polyelectrolyte adsorption: effects of chain rigidity.

Monte Carlo simulation is employed to investigate the interaction between a polyelectrolyte and a fluid mixed membrane containing neutral (phosphatidyl-choline, PC), monovalent anionic (phosphatidylserine, PS), and multivalent anionic (phosphatidylinositol, PIP2) lipids. The effects of the intrinsic polyelectrolyte rigidity and solution ionic strength on the lateral rearrangement and dynamics of different anionic lipid species are systematically studied. Our results show that, the increase of polyelectrolyte chain rigidity reduces the loss of polyelectrolyte conformational entropy and the energy gains in electrostatic interaction, but raises the demixing entropy loss of the segregated anionic lipids. Therefore, the polyelectrolyte/membrane adsorption strength exhibits a non-monotonic dependence on the polyelectrolyte rigid parameter k ang, and there exists a certain optimal k ang for which the adsorption strength is maximal. Because the less loss of chain conformational entropy dominates the increase of the demixing entropy loss of the segregated anionic lipids and the decreases of the electrostatic energy gains, the semiflexible polyelectrolyte adsorbs onto the membrane more firmly than the flexible one. Whereas, for the adsorption of rigid polyelectrolyte, larger anionic lipid demixing entropy loss and less energy gain in the electrostatic interaction dominate over the decrease of the polyelectrolyte conformation entropy loss, leading to the desorption of the chain from the membrane. By decreasing the ionic concentration of the salt solution, the certain optimal k ang shifts to larger values. The cooperative effects of the adsorbing polyelectrolyte beads determine the concentration gradients and hierarchical mobility of the bound anionic lipids, as well as the polyelectrolyte dynamics.

Exploring the drying behaviour and particle formation of high solids milk protein concentrate

Milk protein concentrate (MPC) powders are valuable dairy products, but the understanding of their drying behaviour and particle formation during convective dehydration especially at high concentrations is currently limited. This study aims to improve the understanding of the drying behaviour of MPC, particularly the influence of concentration on compositional redistribution, and particle formation from droplets. We utilised the glass-filament single droplet drying technique to monitor the thermal-moisture drying histories of MPC droplets and used the Reaction Engineering Approach (REA) to model their drying kinetics. Across the range of solids content tested, the rate of moisture removal and final moisture content of dried MPC particles achieved were relatively similar. However, increase in solids content of MPC droplets led to reduced shrinkage and increased rate of temperature rise during drying. The in situ rehydration study indicated slower rehydration with the increase of concentration. Surface composition of dried MPC particles was analysed using X-ray photoelectron spectroscopy (XPS) and the findings indicated the importance of bulk and surface compositions in determining droplet drying behaviour and particle formation during drying.

Compositional and structural characterization of alloyed carbide by 3D atom probe and high‐resolution TEM

ABSTRACTDuring tempering of solute supersaturated ferrous martensite, the face‐centered cubic MC‐type carbides (M is alloy elements) such as VC and NbC phases usually co‐precipitate on crystal defects such as dislocation and take on plate‐like morphology. Over‐tempering makes the plate‐like shape change to spherical shape because of Ostwald coarsening. The coarsening process strongly correlates to the diffusion behaviors of the carbon and carbide‐forming elements, and consequently inhomogeneous compositional and structural distribution in the carbides is formed. Three‐dimensional atom probe and high‐resolution transmission electron microscopy have been proved useful methods to characterize the composition, morphology and nanostructure of the carbides that precipitate in a quench‐tempered micro‐alloyed steel. Depending on the actual affinity with C and the diffusion behavior, Si and Al are rejected from the alloy carbide, whereas Mn, V and Nb are inhomogeneously enriched in it. The morphology and structure change with the compositional redistribution. During the coarsening process of the pre‐existing plate‐like carbide, transition carbide that is semi‐coherent with ferritic matrix is formed because of the disparity in diffusion ratio of different solutes. A core–shell complex nanostructure is consequently formed in the coarsening carbide, and the core and shell are identified as V8C7 enriched in Mn, Mo and Mo2C, respectively. Copyright © 2013 John Wiley & Sons, Ltd.

Stable isotope fractionation by thermal diffusion through partially molten wet and dry silicate rocks

Water plays a fundamental role in affecting geochemical transport and physical properties of magmas. Here we show the previously undocumented behavior of water within partially molten silicate resting in a temperature gradient, producing O, Li and H isotope redistribution by thermal diffusion leading to enrichment of light isotopes at the hot end of the gradient. After weeks to months, fully molten as well as mostly crystalline portions of water-bearing experiments develop remarkably large isotope and chemical redistributions: up to 28‰ for δ18O, 144‰ for δD, and 18‰ for δ7Li. In contrast, long-term dry experiments develop smaller (∼5‰ δ18O) isotopic fractionations only in the hotter end where it is molten or partially molten. Isotope fractionation of oxygen is linearly related to temperature, and the magnitude of isotopic separation per °C is ∼2× larger for wet experiments than dry ones. We explain this by water de-polymerizing the silicate structure leading to a smaller size of diffusing SiOx fragments. The magnitude of isotope separation between the hot and cold ends for Li, Mg, Fe, O, and H isotopes increases linearly with ΔMheavy–light/Mlight. These relationships provide predictive tests for natural rocks and highlight the role of water in isotope and compositional redistribution during temperature gradient mediated processes. We discuss the implications to natural environments in which the lightest stable isotopes (H, Li, O) with the greatest ΔMheavy–light/Mlight and fastest diffusion coefficients are capable of achieving mass-dependent redistribution in a transient temperature gradient. These experiments underscore the importance of solution–reprecipitation in wet subsolidus systems and demonstrate that isotopic redistribution can be established ∼6 orders of magnitude quicker than by diffusion through a traditional silicate melt at higher temperature. This has important implications for timescales of natural isotope and chemical redistribution by thermal diffusion.

One-pot Chemical Synthesis of Zinc Antimonide Nanoparticles as Building Blocks for Nanostructured Thermoelectric Materials

Abstract We report a novel one-pot chemical synthesis of zinc antimonide (Zn–Sb) nanoparticles (NPs) via the sequential reduction of metal precursors and subsequent compositional redistribution. Chemically synthesized Zn–Sb NPs are expected to be promising building blocks for nanostructured thermoelectric materials.

The geometry, topology and structure of amorphous solids

Clusters of atoms can be divided into three categories depending on their topology. One of the categories provides the basis for development of a model of a perfectly random structure (ideal amorphous solid) using the non-equilateral triangle topology in the coordination shell. Metallic glasses solidify as amorphous solids with random arrangement of atoms. A model of Zr-based metallic glass has been constructed and described in terms of cluster topology, and compared with a recently published dynamic molecular model of the same alloy. It is shown that the pair distribution function for the ideal amorphous model relates to the pair correlation function obtained from the dynamic model. Debye X-ray scattering computations reveal the presence of vacancies and other flaws relative to the ideal amorphous solid. A shift in the peak position can be predicted using the Erhenfest formula. Two atomic displacement mechanisms involving a five-atom sub-cluster are identified as the fundamental means of compositional redistribution between clusters in the alloy.

Temperature effect on low-k dielectric thin films studied by ERDA

Low-k dielectric materials are becoming increasingly interesting as alternative to SiO2 with device geometries shrinking beyond the 65 nm technology node. At elevated temperatures hydrogen migration becomes an important degradation mechanism for conductivity breakdown in semiconductor devices. The possibility of hydrogen release during the fabrication process is, therefore, of great interest in the understanding of device reliability. In this study, various low-k dielectric films were subjected to thermal annealing at temperatures that are generally used for device fabrication. Elastic recoil detection analysis (ERDA) was used to investigate compositional changes and hydrogen redistribution in thin films of plasma-enhanced tetraethylortho-silicate (PETEOS), phosphorus doped silicon glass (PSG), silicon nitride (SiN) and silicon oxynitride (SiON). Except for an initial hydrogen release from the surface region in films of PETEOS and PSG, the results indicate that the elemental composition of the films was stable for at least 2 hours at 450°C.

Nano-scale Stress and Compositional Analysis of Epitaxial Si1-xGex/Si (100) Undulated Films

ABSTRACTSpontaneous surface roughening, stress relaxation and composition re-distribution are coupled fundamental processes during the growth of strained epitaxial Si1-xGex / Si (100) alloy layers. In this work, we will present both modeling and experimental approaches to investigate the inter-relationships among these mechanisms. Stress distributions in undulated layers were calculated via the Finite Element Method by assuming a sinusoidal surface geometry. They were investigated as functions of undulation wavelength and amplitude between the trough and peak regions. For example, with a 50 nm layer of × = 0.3 having undulations of 250 nm wavelength and 40 nm amplitude, the compressive stress is twice as high in the trough regions than in the peak regions even without compositional redistribution. To explore compositional redistribution in response to these laterally varying strain fields, an experimental approach has been developed to determine local germanium concentrations for such undulated alloy layers. An etchant consisting of HNO3 (70 wt %): H2O: HF (0.5 wt %), 25: 35: 5, at 28 °C etches Si1-xGex / Si (100) alloy layers at a rate of several nanometers per minute. The etching rate increases linearly with increasing Ge concentration in the alloy layer. Such etching experiments can thus be applied to etch alloy layers with compositionally varying undulations on the free surface and utilized to quantify the local germanium concentrations. For an alloy layer with × = 0.3, the Ge concentration from this analysis is estimated to be about 20 % enhanced / depleted in the peak / trough regions, respectively.

Analysis of constituent redistribution in the γ (bcc) U–Pu–Zr alloys under gradients of temperature and concentrations

Rods of a ternary alloy (71U–19Pu–10Zr by weight percent) were annealed under a temperature gradient of 220°C/cm for 41 days and examined for micro-structural development and compositional redistribution. An enrichment of Zr with concurrent depletion of U was observed within the γ (bcc) phase region on the hot-end side ( T≅740°C). The experimental redistribution of the elements in the γ (bcc) phase was analyzed in the framework of multicomponent mass transport with due consideration of thermotransport and ternary diffusional interactions. Based on a new analysis involving an integration of interdiffusion fluxes in the diffusion zone, kinetic parameters related to the thermotransport and ternary interdiffusion were calculated for each component i over selected ranges of composition. The thermotransport coefficients of U, Pu, and Zr were in the approximate ratio of 1:2:−4.5 in the hot-end region. In addition, the interdiffusion flux contributions arising from the gradients of temperature and concentrations of U and Zr were estimated.

Radio-induced structural membrane modifications: a potential bioindicator of ionizing radiation exposure?

Purpose: This study focused on radio-induced membrane alterations in order to assess some related parameters as potential biological indicators of ionizing radiation effects in cases of accidental overexposure. Materials and methods: Radio-induced membrane alterations were assessed after gamma-irradiation of human blood. Biophysical techniques based on fluorescent probe incorporation into isolated living lymphocytes and erythrocytes membranes were applied. Results: Using the technique of fluorescence polarization, the lipophilic phase of the membrane was shown to be more fluid whereas the lipid-protein interface of the membrane was shown to be more rigid after gamma-irradiation. Fluorescent anisotropy modifications showed dose-time effect relationships after radiation exposure. Ionizing radiation induced a decrease in steadystate anisotropy values but did not affect the probe's lifetime as assessed by fluorescence lifetime distribution technique. These data suggest that the anisotropy variations are representative of the local properties of the fluorescent probe's micro-environment. However, the distribution width showed a decrease pointing towards radiation-induced changes of membrane domain organization, probably due not only to membrane water penetration related to lipoperoxidation, but also to compositional changes and redistribution of membrane components. In contrast, the lack of radiation effect observed using the lateral diffusion index technique may be related to the integrated overview of the radio-induced modifications of the membrane provided by this technique, which pointed out radio-induced damage to the membrane in micro-domains. Conclusion: These findings suggest the utility of structural membrane modification measurements as an early bio-indicator of ionizing radiation exposure.

Characteristics and uniformity of group V implanted and annealed HgCdTe heterostructure

This work presents characterization of implanted and annealed double layer planar heterostructure HgCdTe for p-on-n photovoltaic devices. Our observation is that compositional redistribution in the structure during implantation/ annealing process differs from that expected from classical composition gradient driven interdiffusion and impacts the placement of the electrical junction with respect to the metallurgical heterointerface, which in turn affects quantum efficiency and RoA. The observed anomalous interdiffusion results in much wider cap layers with reduced composition difference between base and cap layer composition. The compositional redistribution can, however, be controlled by varying the material structure parameters and the implant/anneal conditions. Examples are presented for dose and implanted species variation. A model is proposed based on the fast diffusion in the irradiation induced damage region of the ion implantation. In addition, we demonstrate spatial uniformity obtained on molecular beam epitaxy (MBE) material of the compositional and implanted species profile. This reflects spatial uniformity of the ion implantation/annealing Processes and of the MBE material characteristics.