Formation Of Dendritic Structures Research Articles

Phase field simulation of dendrite growth with boundary heat flux

AbstractBoundary heat flux has a significant effect on solidification behavior and microstructure formation, for it can directly affect the interfacial heat flux and cooling rate during phase transition. In this study, a phase field model for non-isothermal solidification in binary alloys is employed to simulate the free dendrite growth in undercooled melts with induced boundary heat flux, and an anti-trapping current is introduced to suppress the solute trapping due to the larger interface width used in simulations than a real solidifying material. The effect of heat flux input/extraction from different boundaries was studied first. With heat input from boundaries, the temperature can be raised and the dendritic morphology changed with gradient temperature distribution caused by the heat flux input coupling with latent heat release during the liquid-solid phase transition. Also, the concentration distribution can be also influenced by this irregular temperature distribution. Heat flux extraction from the boundaries can decrease the temperature, which results in rapid solidification with small solute segregation and concentration changes in the dendrite structures. Also, dendrite growth manner changes caused by undercooling variation, the result of competition between heat flux and latent heat release from phase transition, are also studied. Results indicate that heat flux in the simulation zone significantly reduces the undercooling, thus slowing down the dendrite formation and enhancing the solute segregation, while large heat extraction can enlarge the undercooling and lead to rapid solidification with large dendrite tip speed and small secondary dendrite arm spacing, while solute segregation tends to be steady. Therefore, the boundary heat flux coupling with the latent heat release from the solidification has an effective influence on the temperature gradient distribution within the simulation zone, which leads to the morphology and concentration changes in the dendritic structure formation.

Zinc Nucleation and Growth in Secondary Alkaline Batteries

Zinc morphology strongly affects the performance of secondary alkaline batteries. The formation of mossy and dendritic structures is detrimental to anode performance and cycle life. The initial stages of zinc deposition on carbon structures are investigated using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and electron-diffraction. Mossy zinc is formed by the aggregation of zinc clusters in a nucleation-controlled process. Dendritic zinc is formed by the unstable growth of zinc along preferred growth axes. An attempt is made to develop a free-energy based phase-field model of electrodeposition to simulate dendritic growth. Preliminary results using a solidification model are able to qualitatively simulate zinc deposition. Understanding the nucleation and growth processes is key to developing innovations to improve battery performance in alkaline zinc anodes.

The role of spartin and its novel ubiquitin binding region in DALIS occurrence

Troyer syndrome is an autosomal recessive hereditary spastic paraplegia (HSP) caused by frameshift mutations in the SPG20 gene that results in a lack of expression of the truncated protein. Spartin is a multifunctional protein, yet only two conserved domains--a microtubule-interacting and trafficking domain and a plant-related senescence domain involved in cytokinesis and mitochondrial physiology, respectively--have been defined. We have shown that overexpressed spartin binds to the Ile44 hydrophobic pocket of ubiquitin, suggesting spartin might contain a ubiquitin-binding domain. In the present study, we demonstrate that spartin contributes to the formation of dendritic aggresome-like induced structures (DALIS) through a unique ubiquitin-binding region (UBR). Using short hairpin RNA, we knocked down spartin in RAW264.7 cells and found that DALIS frequency decreased; conversely, overexpression of spartin increased the percentage of cells containing DALIS. Using nuclear magnetic resonance spectroscopy, we characterized spartin's UBR and defined the UBR's amino acids that are key for ubiquitin binding. We also found that spartin, via the UBR, binds Lys-63-linked ubiquitin chains but does not bind Lys-48-linked ubiquitin chains. Finally, we demonstrate that spartin's role in DALIS formation depends on key residues within its UBR.

Development of an ammonia sensor based on silver nanoparticles in a poly-methacrylic acid matrix

Samples of Ag nanoparticles (AgNPs) in a poly-methacrylic acid (PMA) matrix have been prepared by a photo-induced reduction process. They have been widely characterized by SEM, EDX-mapping, TEM, XRD and XPS measurements. The results have shown that the morphological and microstructural characteristics of AgNPs formed in a PMA matrix depend on the Ag/PMA molar ratio. Thick films of these samples deposited on ceramic substrates have also been prepared. The formation of Ag dendritic structures has been evidenced in samples with a low Ag/PMA ratio, whereas at a higher Ag/PMA ratio the film surface was found to be highly heterogeneous, along with an increase in Ag species in the ionic form. Loading AgNPs/PMA samples with multi-walled carbon nanotubes (MWCNTs) allowed production of conductive films suitable for sensing applications. The samples synthesized were investigated as sensing materials for fabricating ammonia (NH3) gas resistive sensors. The novel sensors developed are active at low temperature, exhibiting fast response/recovery times in a wide range of detection levels from ppm to v/v (%).

Formation of hierarchical structures of Fe2O3 by the liquid–liquid interface technique

We report the formation of dendritic hierarchical structures of α-Fe2O3 and nanostructures of Fe2O3 by the simple liquid–liquid interface method.

Effect of GMAW-CMT Heat Input on Weld Bead Profile Geometry for Freeform Fabrication of Aluminium Parts

In developing a new method for weld based freeform fabrication, parameter affecting the geometry of single-pass need to be determined as it has great influence on dimensional accuracy and mechanical property of metallic part. In this paper, profile geometry and microstructure of single pass weld bead developed using Gas Metal Arc Welding Cold Metal Transfer (GMAW-CMT) was investigated. Observation on cross sectional weld bead indicates GMAW-CMT has capability to produce free spatter and crack defect weld bead. Profile geometry measurement shows weld bead develop at higher heat input has width size larger than the weld bead develop at lower heat input. Microstructure examination in the substrate reveals formation of columnar dendritic, cellular and planar structure while at buildup layer exhibit equiaxed dendritic structure

Formation of dendritic crystal structures in thin silicon films on silicon dioxide by carbon ion implantation and high intensity large area flash lamp irradiation

In this paper, we use large area light pulse induced melting of deposited thin silicon films on oxidised silicon wafers to prepare coarse grained dendritic crystal structures. The results show that the addition of carbon prevents the agglomeration of the molten silicon films and largely influences the crystallisation process. The low solubility of carbon in liquid silicon and its effect on the silicon melting temperature induces a distinctive lateral dendritic grain growth. XTEM, SEM, AFM and ToF-SIMS investigations have been performed to study the crystallisation process and to characterise the resulting film structure.

Solidification of Lead Tin and Lead Telluride Eutectic Alloys in Microgravity

The Brazilian Microgravity Program is mainly based on experiments carried out on sounding rockets. A solidification furnace, capable of producing temperatures up to 900 °C, was developed to process metal and semiconductor alloys in microgravity environment. This paper describes a solidification experiment made in this furnace during a parabolic flight, with two eutectic alloys. The behavior of the eutectic alloys PbSn with 26.1 Pb at. % composition, and PbTe with 10.9 Pb at. % composition were presented and compared with laboratory solidifications carried out in the same furnace and thermal cycle. It was concluded that the formation of dendritic structures in PbSn alloy is related to the presence of sedimentation and convective flow during solidification, and the size of these structures is connected to the solidification time. Thus, in the microgravity alloy, there was no formation of dendritic structures and the profile of solute distribution remained constant throughout the sample. For the PbTe eutectic alloy the microgravity conditions have not caused significant changes compared to the earth solidified sample.

A two-grid adaptive volume of fluid approach for dendritic solidification

A two-grid approach to simulating dendritic growth in the solidification of pure metals is presented. The approach is based on using a uniform Cartesian grid to solve the energy equation and an adaptive Cartesian grid of higher-resolution to solve the interface evolution, providing a more accurate representation of the interface. The energy equation is solved using a diffuse-interface method, and the advection equation of a discretized solid fraction function is solved using an unsplit VOF (volume of fluid) method, with PLIC (piecewise linear interface calculation) reconstruction of the interface. The thermal gradients at both sides of the interface, which are needed to obtain the front velocity, are calculated with the aid of the distance function to the reconstructed interface, which is obtained using an efficient method described in detail in this work. The influence of the grid resolution used to solve the advection equation on the accuracy of the method is analyzed. For the dendritic growth cases considered in this work, particularly, it was found that using a grid resolution for the advection equation two times higher than that used for the energy equation considerably improves the results, while keeping the CPU time consumed at an acceptable level. To underline the importance of an accurate estimation of the interface curvature, the results obtained using three techniques commonly used in VOF methods of different degrees of accuracy (a convolved VOF technique, a distance function technique and an improved height function technique, which provides second-order accuracy) are compared. The proposed methodology is assessed by comparing the numerical results with analytical solutions and with results obtained by different authors for the formation of complex dendritic structures in two and three dimensions.

Preparation and Characterization of Copper Dendrite Like Structure by Chemical Method

Abstract Copper particles are prepared at room temperature using a simple chemical method. The method used is capable of giving high yields. In this method sodium borohydride is used as a reducing agent and poly vinly pyrrolidone (PVP) as a capping reagent. The prepared copper particles are characterized by X-ray diffraction method and Energy dispersive x-ray analysis (EDX). The scanning electron microscopy (SEM) images show the formation of dendrite like structures. The transmission electron microscope (TEM) studies also conform the formation of dendrite structures.

Precise drop dispensation on superhydrophobic surfaces using acoustic nebulization

The adhesion forces of liquid drops on superhydrophobic surfaces are typically in the nano-Newton range which presents problems in their dispensation from pipettes. Furthermore, since the liquid adheres more strongly to the pipette tip, some portion of the liquid will tend to remain on the tip, causing inaccuracy in the volume dispensed. We advance a novel approach here, in which the spray from an acoustic nebulizer is sent to a superhydrophobic receptacle and the volume ascertained precisely using a weighing scale. The superhydrophobic surface was identified to develop via a galvanic displacement mechanism in an electroless deposition process. A time dependent morphology change from granular to dendritic with longer immersion into the silver nitrate solution was found which indicated that granular growth beyond a certain size was not feasible, although granular structures were more preferentially formed just after nucleation. The dendritic structure formation was likely due to the natural tendency of the process to maintain or increase the surface area to volume ratio in order not to limit the rate of deposition. An immersion for at least 7 seconds into the silver nitrate solution, when the granular structures were predominant, was all that was needed to ensure superhydrophobicity of the surfaces. Also, the superhydrophobic state required not just significant numbers of the granular structures to be present but also interrupted coverage on the surface. On using the technique, a single drop was created by subsequently covering the receptacle with a lid and shaking it gently. The volume dispensed was found to vary linearly with the operation time of the nebulizer. We elucidated the observed increased ability of drops to reside on inclines using wetting mechanics and presented an elementary mathematical description of the extent of aerosol coverage on the surface, which has implications for the mechanics of aerosol growth into drops. The structural changes in enhanced green fluorescent protein (EGFP) observed after acoustic dispensation necessitated all samples in a fluorimetric assay to involve equal nebulized volumes of the fluorescent protein marker for measurement consistency.

2p or not 2p: tuppence-based SERS for the detection of illicit materials

Deposition of silver onto British 2p coins has been demonstrated as an efficient and cost effective approach to producing substrates capable of promoting surface enhanced Raman scattering (SERS). Silver application to the copper coins is undemanding taking just 20 s, and results in the formation of multiple hierarchial dendritic structures. To demonstrate that the silver deposition sites were capable of SERS the highly fluorescent Rhodamine 6G (R6G) probe was used. Analyses indicated that Raman enhancement only occurs at the silver deposition sites and not from the roughened copper surface. The robustness of the substrate in the identification and discrimination of illegal and legal drugs of abuse was then explored. Application of the drugs to the substrates was carried out using spotting and soaking methodologies. Whilst little or no SERS spectra of the drugs were generated upon spotting, soaking of the substrate in a methanolic solution of the drugs yielded a vast amount of spectral information. Excellent reproducibility of the SERS method and classification of three of the drugs, 4-methylmethcathinone (mephedrone), 5,6-methylenedioxy-2-aminoindane (MDAI) and 3,4-methylenedioxy-N-methylamphetamine (MDMA) were demonstrated using principal components analysis and partial least squares.

Cube-octameric silsesquioxane-mediated cargo peptide delivery into living cancer cells

Cube octameric silsesquioxanes (COSS) are among the smallest nanoparticles known to date with a diameter of only 0.7 nm. We describe a COSS-based delivery system which allows for the drug targeting in human cells. It comprises a siloxane core with seven pendant aminopropyl groups and a fluorescently labeled peptidic ligand attached to one cage corner via a reversible disulfide bond to ensure its intracellular release. Bimodal amplitude-modulated atomic force microscopy (AFM) experiments revealed the formation of dendritic COSS structures by a self-assembly of single particles on negatively charged surfaces. Nuclear targeting was demonstrated in HeLa cells by selective binding of released p21(Cip1/Waf1)-derived cargo peptide to PCNA, a protein involved in DNA replication and repair.

Mechanical constraint and release generates long, ordered horizontal pores in anodic alumina templates

We describe the formation of long, highly ordered arrays of planar oriented anodic aluminum oxide (AAO) pores during plane parallel anodization of thin aluminum ‘finger’ microstructures fabricated on thermally oxidized silicon substrates and capped with a silicon oxide layer. The pore morphology was found to be strongly influenced by mechanical constraint imposed by the oxide layers surrounding the Al fingers. Tractions induced by the SiO2 substrate and capping layer led to frustrated volume expansion and restricted oxide flow along the interface, with extrusion of oxide into the primary pore volume, leading to the formation of dendritic pore structures and meandering pore growth. However, partial relief of the constraint by a delaminating interfacial fracture, with its tip closely following the anodization front, led to pore growth that was highly ordered with regular, hexagonally packed arrays of straight horizontal pores up to 3 µm long. Detailed characterization of both straight and dendritic planar pores over a range of formation conditions using advanced microscopy techniques is reported, including volume reconstruction, enabling high quality 3D visualization of pore formation.

Reactable ionic liquid synthesis and visible-light photocatalytic activity of dendritic ferric oxide hierarchical structures

Dendritic α-Fe2O3 micropines and octahedral Fe3O4 microcrystals with uniform structures have been successfully synthesised through a reactable ionic liquid process in the presence of 1-butyl-3-methylimidazolium ferricyanide [Bmim]3[Fe(CN)6] ionic liquid. During the reactive process, ionic liquid [Bmim]3[Fe(CN)6] acts as solvents, templates and direct reactants for the fabrication of dendritic α-Fe2O3 micropines and Fe3O4 octahedral microcrystals. It is also found that pH value is vital for the formation of dendritic α-Fe2O3 micropines and Fe3O4 octahedral microcrystals structures. Moreover, both α-Fe2O3 micropines and Fe3O4 octahedral microcrystals showed efficient photocatalytic degradation activity for decolourisation of MB pollutant.

Tunable electrochemical preparation of cobalt micro/nanostructures and their morphology-dependent wettability property

A versatile cobalt micro/nanostructure preparation method via a facile electrochemical growth approach is reported in this study. Based on the existing theoretical analysis, the various crystal growth parameters, including the potential, concentration and temperature, act as parameters to impact the metallic crystal growth rate and the resultant morphology. These parameters were varied to elucidate the growth rate and morphology changes. Electron microscopy technique was applied to reveal the corresponding crystal morphology, which demonstrates that the potential can consequently impact the crystal shapes formed, from bump to flower and dendrite shapes. However, in the current experiments, the morphology achieved by changing the concentration and temperature was a dendritic structure, which suggests that the growth conditions promoted a dendritic structure formation regime. Based on the structures grown at various potentials, without further surface modifications, a morphology-dependent wettability trend exhibited from the superhydrophobic to superhydrophilic structures. The superhydrophobicity stems from the small contact areas of the tips of the flower-like structure. Meanwhile, the superhydrophilicity is due to the nanometer-scale channels existing in the hierarchical structures of the dendritic crystals, in which the capillary effect draws water droplets to spread on the surface rapidly.

Ultrafast Preparation of Three-Dimensional Dendritic Gold Nanostructures in Aqueous Solution and Their Applications in Catalysis and SERS

Three-dimensional dendritic gold nanostructures were prepared via an ultrafast one-step homogeneous solution method. Cationic surfactants, decane-1,10-bis(methylpyrrolidinium bromide) ([mpy-C10-mpy]Br2), were used as the capping agent and l-ascorbic acid (AA) was used as the reducing agent. The as-prepared gold dendrites present a [111] plane and grow along the ⟨211⟩ direction. The AA plays an important role in the formation of the dendritic nanostructures. The methylpyrrolidinium quaternary ammonium headgroups and bolaform nanostructures of the surfactants are the key factors for the formation of dendritic structures. Further investigation revealed that the reaction time and concentration of HAuCl4 weakly affect the formation of the dendrites. Moreover, the obtained gold dendrites exhibit excellent activity toward the catalytic reduction of p-nitraniline and higher enhancement of surface-enhanced Raman scattering when using Rhodamine 6G as the model molecules.

The facile formation of silver dendritic structures in the absence of surfactants and their electrochemical and SERS properties

In this work a simple approach to the creation of highly dispersed electrocatalytically active silver microstructured dendrites on indium tin oxide in the absence of any surface modification or surfactant is presented. It is found that the addition of low concentrations of supporting electrolyte to the AgNO 3 solution dramatically influences the morphology of electrodeposited silver which is independent of both the anion and the cation employed. The silver dendrites are characterized by SEM, XRD, XPS as well as by cyclic voltammetry under alkaline conditions. It is found that the surface oxide formation and removal processes are significantly influenced by the microstructured morphology of the silver electrodeposits compared to a smooth macrosized silver electrode. The facile formation of dendritic silver microstructures is also shown to be beneficial for the electrocatalytic oxidation of both formaldehyde and hydrazine and oxygen reduction. The formation of a continuous film of dendritic silver is also investigated for its SERS activity where the connectivity between the individual dendrites is found to be particularly important.

Gas Tungsten Arc Cladding of Titanium-Nickel Overlays onto the Carbon Steel and Stainless Steel for Cavitation Erosion Resistance

Gas tungsten arc welding with the TiNi intermetallic filler material was used to weld overlays onto the SUS 304 stainless steel and AISI 1045 medium carbon steel. The composition, hardness and cavitation erosion resistance of overlays on both kinds of steel was compared. The microstructure analysis results show that the elements from substrate materials diluted the overlay and caused the formation of dendrite structures in overlays. The crystalline phases in overlays on SUS 304 are TiNi-B2, TiNi-B19’, TiNi3 and Ti3Ni4, while those in overlay on AISI 1045 are TiNi-B2 and TiNi3. The multiple phase structures, precipitation of hard carbides and oxides formed in the overlays increases the hardness of the overlays 3-fold over the TiNi filler rod and steel substrate. This significantly increases the cavitation erosion resistance of substrate steels. The corrosion resistant overlays also enhance the cavitation erosion corrosion resistance of substrates in corrosive 3.5 wt% NaCl solution.

Preparation of dendritic-like Ag crystals using monocrystalline silicon as template

Symmetric dendritic silver structures with controlled morphology were successfully synthesized by a solvothermal method with the assistance of monocrystalline silicon. The morphology and structure of the dendritic silver were characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). It was found that the architecture of silver crystals could be controlled via simply adjusting the experiment parameters: AgNO 3 concentration, reaction time and temperature. Moreover, structural characterizations suggested that the dendritic silver structures preferentially grew along (1 1 1) and (2 0 0) directions, leading to the formation of dendritic structures with 1–2 μm in stem diameter and 10–50 μm in length. Additionally, the formation process of the dendritic silver structures was studied, and a possible formation mechanism was proposed based on the experimental results.