Formation Of Dendritic Structures Research Articles

On the formation of dendritic iron from alkaline electrochemical reduction of iron oxide prepared for metal fuel applications

Low-temperature electrochemical reduction (electroreduction) of iron oxides is a promising alternative to the conventional methods for iron production due to its CO2-free operation and relatively low energy consumption. In this work, we demonstrate a novel approach for electrochemical iron production by promoting the formation of dendritic structures during iron electrodeposition, which facilitates the easy harvesting of deposits in powder form. Experiments were conducted using a single pair of parallel plate electrodes, immersed in a mixture of hematite (Fe2O3) powder and aqueous alkaline (NaOH) slurry. The effects of current density, Fe2O3 mass fraction, temperature, and powder size on current efficiency and deposit morphology are investigated. A large quantity of dendritic iron structures is observed when experiments are carried out without stirring and/or applying heat from a heating plate. This condition suggests temperature and (ion/species) concentration gradients in the system. The dendrites are mainly deposited on the cathode's sides, corners, and edges. Different deposits and dendritic structures (compact layer deposit, moss-like deposit, deposit with whisker-like dendrites, and deposit with crystal-like dendrites) are observed as operating conditions change. Overall, a cathodic deposition of metallic iron with a high Faradaic efficiency (≥90 %) is successfully accomplished. The present findings provide new insights into the production of electrolytic iron powder and its future use as a carbon neutral and sustainable fuel/energy carrier.

The effect of cooling plate, mechanical vibration, and grain refinement on the microstructure and hardness of A380 produced by sand mold

ABSTRACT The mechanical properties of aluminium alloys can be increased by controlling the grain size and morphology of the alloy. In these studies, mechanical vibration, cooling slope plate (CSP), and grain refiner were applied on sand mould casting using A380 alloys. The hardness, and microstructure of the cast samples are investigated. These processes lead to the refinement of grain structures and a decrease in the tendency for dendritic structure formation. The Effect of the solidification time (modulus) on the microstructure is investigated. It was determined that the solidification time varies depending on the section thickness, which affects the SDAS values. It was observed that the lowest SDAS values were in CSP and the highest values were in grain refiners added casting. The lowest hardness value was recorded at CSP casting. Meanwhile, the grain refiner added vibration casting exhibits the highest hardness.

Evidence for localized melting during dynamic hot compression of Ti-6Al-4V alloy

Heat-treated Ti-6Al-4V alloy with three different microstructures was dynamically deformed at a varied temperature range (up to 450 °C). The specimens deformed with 1 and 102 s−1 strain rates at 300 °C show extensive shear banding. Formation of dendritic structure has been observed inside highly strained shear bands. Furthermore, the fracture surface of sheared samples exhibits spherical particles sticking to the surface. Compositional analysis using EDS depicts these structures as close to the overall alloy composition with considerable solute segregation in spherical particles. The reason behind such observation has been described as localized melting of the alloy due to a sudden burst of strain energy during fracture and interface friction between delaminated grains. The temperature rise calculated within the ASB, based on its fraction, predicts a temperature beyond the alloy's melting point.

Toward efficient and industrially compatible fully textured perovskite silicon tandem solar cells: Controlled process parameters for reliable perovskite formation

AbstractCapitalizing on the existing silicon industry, fully textured perovskite‐silicon tandem solar cells have a great potential to penetrate the electricity market. While the use of textured silicon with large pyramid size (> 1 μm) enhances the power conversion efficiency (PCE), it also presents process complications. To achieve high performance, meticulous control of deposition parameters on textured silicon is required. This study provides a guideline for the use of the hybrid evaporation/spin‐coating route to form high‐quality perovskite absorbers. Using various characterization techniques, we highlight intrinsic differences between perovskite growth on flat versus textured substrates. Furthermore, we provide pathways to ensure a high perovskite phase purity, reveal mitigation strategies to avoid the formation of undesired dendritic perovskite structures, give guidelines to ensure photostability, and discuss the “misleading” effect of residual PbI2 on the perovskite photoluminescence response. A good understanding of the perovskite growth on textured silicon enables the fabrication of a tandem device with a PCE > 26% (without employing additives or surface treatments) and a good operational stability. The comprehensive guidelines in this study provide a better understanding of perovskite formation on textured silicon and can be transferred when upscaling the hybrid route perovskite deposition.

On the formation of gradient-distributed dendrites in a single crystal nickel-based superalloy directionally solidified under transverse static magnetic field

Directional solidification (DS) of alloys under static magnetic field has attracted extensive attention due to the development of thermoelectric magnetohydrodynamics (TEMHD). Assessing the influence of static magnetic field on solidification microstructures in single crystal (SX) superalloys is of great significance for both the development of SX superalloys and the understanding of external field-assisted solidification. In this contribution, under different transverse static magnetic fields, the evolution of dendritic structure in a SX nickel-based superalloy directionally solidified over a range of solidification velocities is systematically investigated. The results show that applying a transverse static magnetic field (TSMF) during DS does not disrupt the crystal orientation of SX superalloys. The distribution of dendrites or local primary dendrite arm spacings within the cross section of SX superalloys shows a gradient variation in the direction perpendicular to the TSMF. Additionally, the formation of gradient-distributed dendritic structure is closely related to the applied TSMF intensity and solidification velocity. It is easier to form a gradient-distributed dendritic microstructure at low solidification velocities and high magnetic fields. Based on the experiments and simulations, the formation of gradient-distributed dendritic microstructure is supposed to attribute to the variation of interdendritic constitutional undercooling created by the change of solute concentration in interdendritic region, which is induced by the TEMHD. The SX superalloy with a gradient distribution of dendrites may have potential applications due to its gradient properties. Meanwhile, these findings also provide a new method for preparing alloys with gradient microstructures.

Highly selective recovery of gold from strong acidic wastewater with high-purity by self-driven capture

Traditional gold recovery has problems arising from excessive use of chemical reagents, weak gold selectivity, and difficulty in separating solid adsorbents and gold. This work reports a self-driven capture agent, a magnetic-driven MXene/GO hydrogel (MMGH), that is prepared by self-crosslinking encapsulation of Ti3C2 (MXene), GO and NdFeB. Here, we provide a recovery method for physically peeling the gold, which makes the separation process easy without the use of toxic and hazardous chemicals. Isotherm model fitting proves that the MMGH exhibits excellent gold recovery performance (2821.8 mg/g) in a highly acidic environment, and the purity of the recovered gold can reach 99.9 % (24 K). The reason for high gold loading by the MMGH is the synergistic three-drive system (magnetism/adsorption/reduction) of the designed material. The self-driven magnetic field enhances mass transfer, enabling the rapid enrichment of Au(III) by the MMGH, and the selective reducing ability of the MXene drives the reduction of Au(III) on the outer surface. The physical separation relies on the formation of cypress leaf dendritic gold structures due to kinetic regulation by the MMGH. This study pioneered a full-chain separation and recovery method for gold by using selective reduction combined with physical peeling, which meets the demand for low-carbon recovery of gold from industrial waste liquids.

Intranasal administration of induced pluripotent stem cell-derived cortical neural stem cell-secretome as a treatment option for Alzheimer’s disease

BackgroundAlzheimer’s disease (AD) is the most common neurodegenerative disorder in the elderly, resulting in gradual destruction of cognitive abilities. Research on the development of various AD treatments is underway; however, no definitive treatment has been developed yet. Herein, we present induced pluripotent stem cell (iPSC)-derived cortical neural stem cell secretome (CNSC-SE) as a new treatment candidate for AD and explore its efficacy.MethodsWe first assessed the effects of CNSC-SE treatment on neural maturation and electromagnetic signal during cortical nerve cell differentiation. Then to confirm the efficacy in vivo, CNSC-SE was administered to the 5×FAD mouse model through the nasal cavity (5 μg/g, once a week, 4 weeks). The cell-mediated effects on nerve recovery, amyloid beta (Aβ) plaque aggregation, microglial and astrocyte detection in the brain, and neuroinflammatory responses were investigated. Metabolomics analysis of iPSC-derived CNSC-SE revealed that it contained components that could exert neuro-protective effects or amplify cognitive restorative effects.ResultsHuman iPSC-derived CNSC-SE increased neuronal proliferation and dendritic structure formation in vitro. Furthermore, CNSC-SE-treated iPSC-derived cortical neurons acquired electrical network activity and action potential bursts. The 5×FAD mice treated with CNSC-SE showed memory restoration and reduced Aβ plaque accumulation.ConclusionsOur findings suggest that the iPSC-derived CNSC-SE may serve as a potential, non-invasive therapeutic option for AD in reducing amyloid infiltration and restoring memory.

Effect of Si and Al on structural and physical properties of ternary and quaternary Co2Fe-based Heusler alloys

The Z-element related structural and physical properties of the Co2FeZ Heusler alloys prepared by arc melting were subjected to detail bulk and surface studies. The several experimental methods as scanning and transmission electron microscopy, X-ray diffraction, positron annihilation, Mössbauer spectroscopy, completed by magnetic and hardness measurements were applied. The main attention was devoted to differences in all possible properties as morphology, defects, micro- and macro-magnetic behaviour and microhardness depending upon a choice of Si, Al, or Si + Al as the Z element. The best structural homogeneity, smallest grain size, and no grain boundary segregation were observed at Co2FeSi0.5Al0.5, while about ten-times larger grains were obtained at Co2FeSi, the grain boundaries of which were covered by a thin interfacial phase, and at Co2FeAl with formation of dendritic structure. Nearly all physical parameters obtained for the Co2FeSi0.5Al0.5 alloy were found to be influenced by either Si or Al.

Dendrite structure-induced tunable plastic deformation behavior in (Ti-V-Cr)100-xWx refractory high entropy alloys

Inducing multiple deformation mechanisms to improve the plasticity of refractory high entropy alloys (RHEAs) is crucial for their engineering applications. In this work, the dendrite structure-induced deformation behavior in a series of novel (TiVCr)100-xWx (x = 5, 7.5, 12.5, 15 at.%) RHEAs was investigated. The formation of dendrite structures was confirmed by both theoretical (CALPHAD) and experimental methods, and the relationship between dendrite structure and mechanical properties was established. The findings indicated that the volume of interdendrite was conducive to enhance the fracture strain while the finer dendrite benefited the increase of yield strength. With the change of dendrite structure, the plastic deformation mode of the RHEAs also changed accordingly. The slip bands tended to concentrate in the interdendrite, and propagate along the softer interdendrite regions while being hindered by the harder dendrite. The dendrite refinement strengthening, as well as the solid solution strengthening contributed to the strength enhancement. Overall, the present (TiVCr)100-xWx RHEAs demonstrated high specific yield strength and good plasticity when compared with the similar W-containing HEAs. The present findings not only give more insight into the deformation behavior in RHEAs, but also provide a potential strategy to tune the mechanical properties of RHEAs with dendrite structures.

Effects of Mn addition on mechanical properties, fracture surface, and electrochemical corrosion resistance of CoCrFeNiAl high-entropy alloys

High-entropy alloys consisting of CoCrFeNiAl as the major elements and 2–5 at% Mn as the minor element were prepared using a vacuum arc melting method. The crystalline structures of the prepared alloys were identified by x-ray diffraction. Moreover, the mechanical properties of the alloys were examined under quasi-static (10−1, 10−2 and 10−3 s−1) and dynamic (3000, 4000, and 5000 s−1) loading conditions using a universal testing machine and split-Hopkinson pressure bar system, respectively. The experimental results showed that, for all of the HEA alloys, the flow stress and strain rate sensitivity coefficient increased with increasing strain rate. Among all the alloys, that with 3 at% Mn exhibited the best mechanical properties. A significant loss in plasticity was observed as the Mn content increased to 5 at%. The scanning electron microscope observations showed that the favorable mechanical properties of the alloy with 3 at% Mn were the result of a compact dimple structure, which enhanced the toughness. The HEA with 5 at% Mn showed the best electrochemical corrosion resistance among all the alloys due to the formation of dendritic structures at the grain boundaries.

TiO2–SiO2–Ag electrospun fibers for oxytetracycline detection by SERS

The presence of antibiotic residues such as oxytetracycline (OTC) in wastewater effluents has generated the need to monitor trace concentrations due to the development of bacterial resistance. Surface Enhanced Raman Spectroscopy (SERS) is a promising technique in detection and identification a low concentrations (ng/L) with no sample preparation. A fibrillar support Titania-Silica doped with silver particles was developed in the SERS technique focused on the detection of OTC. The ceramic fibers TiO2SiO2 obtained presented phases of Rutile, Anatase and Amorphous Silica with diameters of 335 ± 76 nm. The fibers were doped with Ag by electrodeposition, showed Ag nanoparticles with sizes from 30 nm to the formation of dendritic structures with sizes greater than 1 μm, depending on the time and voltage applied during synthesis. The support doped with Ag for 30 s at 19 V presented the highest amplification factor (AF) in SERS of 44 for the band at 1278 cm−1.

A novel dendritic polyaniline/phytate@Cu organic-metal composite flame retardant for improving thermal protection of epoxy coatings

Polyaniline (PANI), a polymer with a high nitrogen content, has been shown to have good flame retardant properties. Here, we significantly improved the self-carbon formation ability of PANI at high temperature by doping with phytic acid (PA). Meanwhile, a new organic-metal ion flame retardant (DPANI/PA@Cu) was obtained by introducing Cu2+ with catalytic carbon formation property into PANI/PA using the complexation between transition metal ions and PA. Surprisingly, the introduction of metal ions was also able to trigger the formation of dendritic structure of PANI (DPANI), which is important for its dispersion in resin matrix and strengthening effect on the coating. The test results demonstrated that the composite coating with 5% content of DPANI/PA@Cu had the lowest back-side temperature (174.9 ℃), the highest expansion height (18.5 mm) and the lowest smoke density class (45.3%). In addition, the DPANI/PA@Cu5%/EP composite coating retained the maximum amount of residual carbon (29.7%), which was mainly related to the synergistic catalytic effect of PA and Cu2+. Ultimately, the retained PANI residual carbon and CuO nanoparticles diffused in the char layer acted as interceptors for the external flame and heat.

Mechanism of Stoichiometrically Governed Titanium Oxide Brownian Tree Formation on Stepped Au(111)

Previously observed formation of substoichiometric titanium oxide dendritic structures across terraces of Au(111) is computationally studied and shown to follow the classical fractal formation mechanism of diffusion-limited aggregation (DLA). Globally optimized gas-phase oxide cluster structures are sampled in a variety of landing formations on gold surfaces and shown to favor isomers driving polymerization to Brownian tree fractal structures. Mobility of Ti3O5 monomers is shown to be extremely high, with diffusion barriers of 0.21 eV or less. Through bonding stabilization, polymerization of these monomers is energetically favorable and irreversible on the 111 terrace but geometrically impossible to propagate along the step edge. Simulated scanning tunneling microscopy (STM) images show strong similarity to experiment. By contrast, observation of Ti3O6 aggregating as wires along step edges is explained by the affinity of oxygen to step edges and statistical arguments for aggregation entropy at the step, in addition to low barriers for monomer diffusion and polymerization.

Directionally solidified structure of peritectic steel

ABSTRACT The solidification structure of peritectic steels (15CrMo and 20CrMo) was studied using directional solidification experiments. An island dendrite structure was observed in the hypoperitectic steel (15CrMo) at growth velocities of 15, 50, and 80 μm·s−1. While a traditional peritectic dendritic structure was observed in the hyperperitectic steel (20CrMo) at growth velocities of 15 and 50 μm·s−1, an island dendrite structure was observed at a growth velocity of 80 μm·s−1. An analysis of the solute distributions near the liquid-solid interface during solidification revealed the role of the steel grade and cooling velocity on the formation of island dendrite structures. Higher cooling velocities caused higher carbon concentration gradients in the liquid phase near the liquid-solid interface, leading to a stronger driving force for the peritectic reaction and, consequently, a more favourable formation of island dendrite structures. Hypoperitectic steel is more prone to the formation of island dendrite structures than hyperperitectic steel.

Investigation on the formation of self-assembled CdSe dendrite structures and their photocatalytic efficiency

Investigation on the formation of self-assembled CdSe dendrite structures and their photocatalytic efficiency

Multiphase numerical modeling and investigation of heat transfer for quenching of spherical particles in liquid pool

Quenching is a widely applied heat treatment process for metallic materials in order to adjust the microstructure and material properties by means of intensive cooling. Depending on the cooling rate attained with respect to the operating conditions as well as the cooling medium, specific material properties such as hardness can be adapted to engineering applications. One of the major constraints for intensive cooling remains with the selection of suitable process parameters. An imprudent selection will end up with sample distortion and cracks. The process being highly complex involving solidification, liquid pool immersion phenomena, rotation, the Leidenfrost effect and vapor formation emerging from liquid for these “millimeter” ranged particles, which make the complete process difficult to analyze by experimental investigations alone. In order to tackle this challenge, a multiphase numerical modeling based on a Eulerian framework is developed and experimentally validated in this work for analyzing the cooling rate of high temperature semi-solid spheres quenched in water. This model simulates particle quenching in liquids, where the source and sink terms of the conservation equations are modified to accommodate the phase transfer effects. To validate modeling results, a high temperature molten metal droplet generator is used to generate the droplets from Al 4.5-wt.% Cu with a diameter of about 1 mm which are quenched in water during the semi-solid state. The formation of dendritic structure within the solidifying droplet is highly sensitive to its cooling rate. This allows to calculate the cooling rate corresponding to the operating condition from SDAS (Secondary Dendrite Arm Spacing) and compare with numerical simulation. Finally, we investigate the influence of the particle Reynolds number (flow velocity) and pool temperature on heat transfer with this validated model.

Nanostructured, Alkaline Titanate‐Converted, and Heat‐Treated Ti6Al4V Microspheres via Wet‐Chemical Alkaline Modification and their ORR Electrocatalytic Response

AbstractThis study describes the chemical conversion and heat treatment of Ti6Al4V microspheres (Ti6_MS), and the resulting effects on their electrocatalytic properties. The wet‐chemical conversion (5.0 m NaOH, 60 °C, 24 h; Sample label: Ti6_TC) converts the top surface of the Ti6_MS powder into an ≈820 nm thick sodium titanate surface. Heat‐treatment (Ti6_TC_HT) at 450 °C increases the stability of the surface, through partial titanate crystallization, while mitigating excess rutile formation. All samples are analyzed chemically (XPS, EDX, Raman, EELS), structurally (XRD and TEM), and morphologically (SEM, TEM), demonstrating the characteristic formation of sodium titanate dendritic structures, with minimal chemical, structural, and morphological differences due to the 450 °C heat‐treatment. The effect of the preparation methodology on oxygen reduction reaction (ORR) electrocatalytic performance is also tested. The introduction of the sodium titanate layer changes the mechanism of the ORR, from a mixed 4 electron/2 electron pathway to a predominantly 2‐electron pathway. By maintaining the microspherical nature of the material while also tuning the surface of the material toward different reaction mechanisms, a design strategy for new electrocatalyst materials is explored.

Dendritic Self-assembled Structures from Therapeutic Charged Pentapeptides.

CRENKA [Cys-Arg-(NMe)Glu-Lys-Ala, where (NMe)Glu refers to N-methyl-Glu], an anti-cancer pentapeptide that induces prostate tumor necrosis and significant reduction in tumor growth, was engineered to increase the resistance to endogenous proteases of its parent peptide, CREKA (Cys-Arg-Glu-Lys-Ala). Considering their high tendency to aggregate, the self-assembly of CRENKA and CREKA into well-defined and ordered structures has been examined as a function of peptide concentration and pH. Spectroscopic studies and atomistic molecular dynamics simulations reveal significant differences between the secondary structures of CREKA and CRENKA. Thus, the restrictions imposed by the (NMe)Glu residue reduce the conformational variability of CRENKA with respect to CREKA, which significantly affects the formation of well-defined and ordered self-assembly morphologies. Aggregates with poorly defined morphology are obtained from solutions with low and moderate CREKA concentrations at pH 4, whereas well-defined dendritic microstructures with fractal geometry are obtained from CRENKA solutions with similar peptide concentrations at pH 4 and 7. The formation of dendritic structures is proposed to follow a two-step mechanism: (1) pseudo-spherical particles are pre-nucleated through a diffusion-limited aggregation process, pre-defining the dendritic geometry, and (2) such pre-nucleated structures coalesce by incorporating conformationally restrained CRENKA molecules from the solution to their surfaces, forming a continuous dendritic structure. Instead, no regular assembly is obtained from solutions with high peptide concentrations, as their dynamics is dominated by strong repulsive peptide-peptide electrostatic interactions, and from solutions at pH 10, in which the total peptide charge is zero. Overall, results demonstrate that dendritic structures are only obtained when the molecular charge of CRENKA, which is controlled through the pH, favors kinetics over thermodynamics during the self-assembly process.

Enhanced removal of cadmium from wastewater by electro-assisted cementation process: A peculiar Cd reduction on Zn anode

Cadmium recovery from wastewater is important to prevent water pollution and add value to the circular economy. Cementation is a widely applied method for Cd removal but this process is found less efficient. Herein, a novel electro-assisted cementation process is proposed, which enables effective Cd removal and recovery from Zn processing wastewater. Under optimal conditions, 98.5 % of dissolved Cd was removed and recovered having 94.1 % purity. More importantly, a peculiar Cd reduction on the Zn anode was observed and the mechanism is explained based on the electrochemical test and experimental results. By applying a cell potential of 0.5 V, the negative surface potential of Zn and its localized dissolution facilitated Cd deposition on the Zn anode by suppressing H2 generation. Moreover, in contrast to the conventional cementation process, the formation of Cd dendritic structures on the Zn anode avoided full surface coverage, thus resulting in high cementation efficiency. In summary, this process presents great potential for the efficient removal of Cd from wastewaters and the findings offer new insights into the enhancement mechanism of electro-assisted cementation.

Effect of annealing on chemically deposited Cu2Se thin films

Cu2Se is a promising candidate in the field of photovoltaic and thermoelectric devices. The pristine Cu2Se thin films are observed to be unstable under ambient conditions. One of the instabilities reported in this material is the formation of dendritic structures on the surface. The present work discusses the effect of post-deposition annealing on the dendritic growth formation. The pristine Cu2Se films were vacuum annealed at 60 °C, 80 °C, 100 °C, 150 °C, and 180 °C and the properties of the samples were investigated. The XRD peak profile analysis was done to evaluate the microstructural parameters like crystallite size, microstrain, stress and energy density. The various techniques used for this study include Scherrer method, Modified Scherrer Method, Williamson-Hall Plots, and Size Strain Plot. The modified variants of W–H Plots were employed which included Uniform Deformation Model, Uniform Stress Deformation Model and Uniform Deformation Energy Density Model. In all these cases, the crystallite size was found to increase on annealing. This is attributed to the reduction in defects and dislocations due to annealing of the sample. Microstrain is also observed to increase with the increase in annealing temperature. The calculation of crystallite size and lattice strain using UDM model deviates more from other models. This may be due to the incompatibility of its basic assumption of isotropic strain in UDM model. The W–H plots also suggest the presence of tensile strain in the lattice. The Cu ions in pristine Cu2Se thin films are in a disordered state due to their high mobility and diffusion coefficient. These mobile Cu ions form the centres of anisotropy and participate in dendritic growth formation with suitable stimulants. With an increase in annealing temperature, these ions get ordered due to strain hardening which in turn stabilizes the Cu2Se structure. The minimum temperature required to resist dendritic growth formation was found to be 150 °C. Thus annealing of pristine sample is suggested as a simple but effective method towards stabilization of the matrix. During the annealing process in the temperature range 60 °C–180 °C, it is observed that the optical band gap decreased from 2.2 eV to 1.9 eV.