Formation Of Dendritic Structures Research Articles

Self-Assembled Dendritic Pt Nanostructure with High-Index Facets as Highly Active and Durable Electrocatalyst for Oxygen Reduction.

The durability issues of Pt catalyst should be resolved for the commercialization of proton exchange membrane fuel cells. Nanocrystal structures with high-index facets have been recently explored to solve the critical durability problem of fuel cell catalysts as Pt catalysts with high-index facets can preserve the ordered surfaces without change of the original structures. However, it is very difficult to develop effective and practical synthetic methods for Pt-based nanostructures with high-index facets. The current study describes a simple one-pot synthesis of self-assembled dendritic Pt nanostructures with electrochemically active and stable high-index facets. Pt nanodendrites exhibited 2 times higher ORR activity and superior durability (only 3.0 % activity loss after 10 000 potential cycles) than a commercial Pt/C. The enhanced catalytic performance was elucidated by the formation of well-organized dendritic structures with plenty of reactive interfaces among 5 nm-sized Pt particles and the coexistence of low- and high-index facets on the particles.

Successive range expansion promotes diversity and accelerates evolution in spatially structured microbial populations.

Successive range expansions occur within all domains of life, where one population expands first (primary expansion) and one or more secondary populations then follow (secondary expansion). In general, genetic drift reduces diversity during range expansion. However, it is not clear whether the same effect applies during successive range expansion, mainly because the secondary population must expand into space occupied by the primary population. Here we used an experimental microbial model system to show that, in contrast to primary range expansion, successive range expansion promotes local population diversity. Because of mechanical constraints imposed by the presence of the primary population, the secondary population forms fractal-like dendritic structures. This divides the advancing secondary population into many small sub-populations and promotes intermixing between the primary and secondary populations. We further developed a mathematical model to simulate the formation of dendritic structures in the secondary population during succession. By introducing mutations in the primary or dendritic secondary populations, we found that mutations are more likely to accumulate in the dendritic secondary populations. Our results thus show that successive range expansion can promote intermixing over the short term and increase genetic diversity over the long term. Our results therefore have potentially important implications for predicting the ecological processes and evolutionary trajectories of microbial communities.

Anisotropic nanocrystalline SmCo4.8Cr0.12C0.08 permanent magnets fabricated using melt-spinning method

Anisotropic CaCu5-type SmCo4.8Cr0.12C0.08 alloys with a dendritic nanostructure and an out-of-plane <0001> texture were fabricated using the melt-spinning method at a speed of 50 m/s. The microstructure changed from randomly orientated equiaxial grains for the ribbon melt-spun at 30 m/s to dendritic ones for that spun at 50 m/s. High-angle annular dark-field scanning transmission electron microscopy and X-ray energy dispersive spectroscopy analysis reveal that the distribution of carbon in SmCo5 grains obtained at higher speeds is highly inhomogeneous and leads to the formation of C-depleted and C-rich dendrite structure with a [0001]C−richSmCo5∥[0001]C−depletedSmCo5 orientation relationship. Moreover, the incorporation of C into the SmCo5 phase helps to increase the Curie temperature to about 845 °C. A high coercivity of 40.3 kOe and a relatively high remanence of 42.0 emu/g were measured in the direction normal to the ribbon plane.

SIMULATION OF TEMPERATURE FIELD AT THE MOLD SURFACE AND INSIDE THE CASTING AT HIGH-GRADIENT DIRECTIONAL SOLIDIFICATION PROCESS

In order to determine the thermal gradient in a single crystal ingots obtained using the directional solidification method on the UVNS-6 apparatus (Russian Scientific Research Institute of Aviation Materials, Moscow), the single crystal ingots of nickel-based superalloy VJM3 were made. The ingots were made using the liquid metal cooling (LMC) directional solidification method (DS) and without it (Bridgman–Stockbarger technique). The liquid Sn was used for a liquid metal cooling technique. To record the temperature during the ingots obtaining process we use a thermocouple placed on the ceramic mold surface. The directional solidification process of the nickel-based superalloy VJM3 ingots on the UVNS-6 apparatus was simulated using the ProCast software. The thermal properties of the VJM3 alloy, ceramic mold and the DS apparatus parts and the boundary conditions (interface heat transfer coefficient) were found in the literature. There is a good agreement between the calculated and experimental values of the temperature distribution in a mold using the LMC and the Bridgman–Stockbarger technique. A simulation of the directional solidification in the ProCast software is suitable for predicting the thermal gradient on a solidification profile location, and the mushy zone width (a site of the dendritic structure formation). The calculated thermal gradient value in the ingot obtained using the Bridgman–Stockbarger technique is 36 °С/cm. The thermal gradient using the LMC method is 204 °С/cm, is six time higher, than if using the Bridgman–Stockbarger technique. Used thermal properties and boundary conditions can be applied for simulation of Ni-based superalloys blades casting process.

Exploiting Zinc Dendrite Formation and Bromine Transport Properties to Build a Safer, More Reliable Zn-Br2 Battery

In order to address inefficiencies that exist in our electrical grid, low cost grid scale electrochemical energy storage (EES) systems, such as zinc bromine batteries, are being researched extensively. Zinc bromine batteries, however, have issues at both the anode and the cathode that affect the reliability of the cell as well as present hazardous conditions of operation. At the anode, current density localization from the inhomogeneity of the zinc surface leads to the formation of dendritic structures. These dendritic structures can grow to the point of touching the other electrode, causing shorting of the cell. At the cathode, bromine contamination into the electrolyte can lead to issues of bromine corrosion at the anode as well as safety concerns of corrosive, vapor phase bromine escaping from the system.Standard Zn-Br2 flow-cell designs alleviate these limitations with bromine-complexing agents to improve Br2(l) solubility, separation membranes to prevent crossover and shorting, and flowing electrolyte to remove bromine and to minimize dendrite formation. These approaches are implemented at the expense of cell resistance, performance efficiency, system size, and especially capital costs. However, if constructed properly, the cell design could be used to address the issues at the anode and cathode without the added cost and complexity of complexing agents, membranes, or flow systems. Here, we show a simple and scalable, low-cost, membrane-free, non-flowing single-chamber zinc-bromine (SC-Zn-Br2) secondary battery design that utilizes the physical properties of liquid bromine and a highly-porous carbon foam electrode, and also allows zinc dendrites to form freely. We demonstrate the local containment of Br2 in the carbon foam electrode, and discuss a color tracking and feedback monitoring scheme to actively control the reactive species transport and improve reliability.To take advantage of the density and low miscibility of Br2, an anodic carbon current collector is placed on top of the cell. During charging, metallic Zn gets plated onto the carbon cloth electrode, while Br2(l) is generated at a carbon foam cathode. If dendrites form and creep towards this carbon foam electrode (CFE), Zn(s) will react with Br2(l) on the surface of the foam and dissolve back into the electrolyte as Zn2+ and Br- ions. This spontaneous process can prevent short circuiting, as the Zn(s) + Br2(l) reaction occurs faster than zinc deposition at low charging currents. Thus, the Br2(l) loosely held on the CFE surface acts as a natural protection from shorting, and can be exploited for ‘self-maintenance’, removing the need for separation membranes or protective coatings.We also take advantage of the distinct colors of the ZnBr2(aq) electrolyte, Br2(l), and dissolved Br2(aq): clear, red, and yellow, respectively. By calibrating the solution color with Br2(l,aq) concentration, we can track the generation, consumption, and transport of bromine in real time to improve battery performance and prevent unwanted processes (i.e., crossover). In addition, we can use this color tracking technique to prevent bromine escape from the system and thus alleivate safety concerns of operating with voliltite bromine. To optimize performance, a feedback loop is written into the cycling control algorithm to stop the charge step at this point. This optical visualization, tracking, and in operando feedback technique has never been used for any bromine-based electrochemical cells, to the best of our knowledge.

Bacteria Delay the Jamming of Particles at Microchannel Bottlenecks.

Clogging of channels by complex systems such as mixtures of colloidal and biological particles is commonly encountered in different applications. In this work, we analyze and compare the clogging mechanisms and dynamics by pure and mixture suspensions of polystyrene latex particles and Escherichia coli by coupling fluorescent microscopic observation and dynamic permeability measurements in microfluidic filters. Pure particles filtration leads to arches and deposit formation in the upstream side of the microfilter while pure bacteria form streamers in the downstream zone. When mixing particle and bacteria, an unexpected phenomenon occurs: the clogging dynamics is significantly delayed. This phenomenon is related to apparent “slippery” interactions between the particles and the bacteria. These interactions limit the arches formation at the channels entrances and favour the formation of dendritic structures on the pillars between the channels. When these dendrites are eroded by the flow, fragments of the deposit are dragged towards the channels entrances. However, these bacteria/particles clusters being lubricated by the slippery interactions are deformed and stretched by the shear thus facilitating their passage through the microchannels.

Determination of precipitation regularities for nanoparticles of hydrated metal oxides in the anion exchange resin

Organic and inorganic ion-exchange materials containing incorporated inorganic particles of ion exchangers are characterized by enhanced selectivity for ions of toxic metals and resistant to poisoning by organic substances. An important task for achieve the necessary kinetic parameters of ion exchange is to reduce the size of the incorporated particles that is particularly important for hydrated oxides containing functional groups, mainly, on the outer surface of the particles. Regularities of forming Zr(IV), Fe(III), Al(III) oxide particles in a polymeric anion exchange matrix are considered in the article. The aim of research is establishing and experimental confirmation of deposition mechanism for inorganic ionite in the polymer for directional control of size and state of the particles. Using computer analysis of TEM images it was revealed that the fractal dimension of the aggregate particles is 2,38-2,72. Nature of modifier also determines the state of the polymer particles. The effect of these factors was analyzed using the Ostwald-Freundlich equation. It was established that the primary nanoparticle size is determined by reprecipitation. Accordingly, the formation mechanism of aggregates is association of nanoparticles with a small cluster, and the limiting step is the diffusion of the polymer nanoparticles. The equation for the diffusion flux of nanoparticles was obtained and it was shown that flux slowing leads to formation of dendritic multilevel structures. The results can be used to develop technologies for ionite synthesis, ion-exchange membranes and materials for baromembrane separation. The proposed approach allows to obtain materials with a given particle size and, consequently, with certain functional properties.

Electro-deposition as a repair method for embedded metal grids

A method is presented to self-repair cracks in embedded silver grid structures used in large area organic electronics. The repair procedure is based on electro-deposition, incited by the application of a moderate DC voltage across the crack. During this process the organic anode that is in direct electrical contact with the silver grid, functions as an appropriate medium for ion migration. Restoration of conductivity is achieved by the formation of dendritic metal structures that connect the cathodic to the anodic side of the crack. The metal dendrites decrease the gap resistance by one order of magnitude. Subsequently, another three orders of magnitude are gained upon sintering the dendrites using a high voltage pulse, yielding restored conductance levels nearly within one order of magnitude difference from native track conductance.

Failure analysis of 316L austenitic stainless steel bellows used under dynamic sodium in a creep testing chamber

Failure analysis has been carried out on 316L austenitic stainless steel seamless bellows served as mechanical and environmental isolator in a creep chamber for carrying out creep test under dynamic sodium. A set of 4 seamless bellows welded with a supporting ring by TIG welding process has been used. Visual examination and liquid penetration test revealed that the failure reveal as holes occurring at the bellow convolution which was welded with the supporting ring. Severe loss of material at the failed region due to reaction of leakage liquid sodium with residual oxygen and moisture in argon cover gas was observed. Further, deposition of chromium- and iron-rich oxides was observed in and around the failure location. Microstructure studies revealed the presence of interdendritic porosity in the weld fusion zone and evidences of sensitized grain boundary with extensive precipitation of chromium-rich carbides in the coarse heat affected zone in the bellow. The leakage of sodium through the interdendritic pores and micro-cracks might have facilitated the reaction of sodium with bellows. Improper method for welding of the thin-walled bellow convolution with a supporting ring is considered as the root cause of the failure. Welding technique and method facilitating the formation of equi-axed microstructure with least formation of dendritic structure in the fusion zone and the use of proper cooling arrangement during welding to avoid sensitization have been recommended to avoid the bellow failure during service.

Manipulating silver dendritic structures via diffusion and reaction

A dendrite is one of the most esthetically pleasing structures, with the best-known example being snowflakes. Although the formation of dendrites has been studied for centuries, the way to control their growth is limited. Here we report an attempt to manipulate the growth of dendritic structures via controlling chemical diffusion and reaction. The silver dendritic structures are synthesized by reducing silver ions in solution. The diffusion of chemical agents is regulated by changing the viscosity of the solution while the reaction is regulated by adding acids. The decrease of reaction rate leads to the disappearance of side branch of dendrites while the reduction of diffusion rate shortens the main trunk of dendrites. The silver dendrites are finely tuned by chemical diffusion and reaction. Following studies suggest that diffusion and reaction determines the chemical concentration around the growth fronts, which influences the growth of crystal structures, leading to the formation of diverse silver dendritic structures. The findings in this study indicate that chemical diffusion and reaction are two effective tools to regulate dendritic structures, promising a different way to promote or prevent the growth of dendritic structures in industries.

Nectin-1 spots regulate the branching of olfactory mitral cell dendrites.

Olfactory mitral cells extend lateral secondary dendrites that contact the lateral secondary and apical primary dendrites of other mitral cells in the external plexiform layer (EPL) of the olfactory bulb. The lateral dendrites further contact granule cell dendrites, forming dendrodendritic reciprocal synapses in the EPL. These dendritic structures are critical for odor information processing, but it remains unknown how they are formed. We recently showed that the immunoglobulin-like cell adhesion molecule nectin-1 constitutes a novel adhesion apparatus at the contacts between mitral cell lateral dendrites, between mitral cell lateral and apical dendrites, and between mitral cell lateral dendrites and granule cell dendritic spine necks in the deep sub-lamina of the EPL of the developing mouse olfactory bulb and named them nectin-1 spots. We investigated here the role of the nectin-1 spots in the formation of dendritic structures in the EPL of the mouse olfactory bulb. We showed that in cultured nectin-1-knockout mitral cells, the number of branching points of mitral cell dendrites was reduced compared to that in the control cells. In the deep sub-lamina of the EPL in the nectin-1-knockout olfactory bulb, the number of branching points of mitral cell lateral dendrites and the number of dendrodendritic reciprocal synapses were reduced compared to those in the control olfactory bulb. These results indicate that the nectin-1 spots regulate the branching of mitral cell dendrites in the deep sub-lamina of the EPL and suggest that the nectin-1 spots are required for odor information processing in the olfactory bulb.

Dendrite Growth during Freezing of Millimeter-Scale Eicosane Droplets

The freezing characteristics of small diameter eicosane (Tmelt = 37°C) droplets are studied here for their use in novel dry-cooling strategies based on spray freezing of recirculating phase change materials (PCM). PCM can be used to store thermal energy with relatively small changes in temperature (due to latent heat), as well as volume (due to small density changes). 4.2 mm diameter eicosane droplets are superheated to 40°C, placed on a cold stage at 10°C, and imaged during freezing (a). Similarly, liquid eicosane is enclosed within a custom-built experimental package creating a 5 mm diameter, 100 μm thick disc geometry with a temperature controlled boundary that is rapidly dropped from 40°C to 10°C (b). In both cases the liquid-solid interface is tracked, as well as the formation and growth of long dendrite structures which have been observed to play a critical role in the freezing process. (c) and (d) show the vertical position normalized by the droplet height , y/H, and the radial position (measured inward) normalized by the disc radius, r/R, of both the interface location and the average dendrite tip location. The total freezing time is observed visually, resulting in characteristic Fourier numbers of Fo = 0.55 ± 0.15 (droplet) and Fo = 3.5 ±0.15 (disc) at identical Stefan numbers of St = 0.3 ± 0.03, where the characteristic lengths are taken as the ratio of the eicosane volume to the cooled surface area.

Growth of giant silver dendrites on layer-by-layer assembled films

Ag dendrites were previously synthesized, with limited success, via the chemical reduction of Ag+ ions with a soluble reducing agent in homogeneous solutions. Here Ag+ ions were first loaded in hydrogen-bonded layer-by-layer (LBL) films fabricated from poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA), then reduced chemically using p-hydroquinone (HQ) as a reducing agent to afford Ag dendritic structures. The dendrites were characterized by SEM, TEM, XRD and optical microscope. Compared with the Ag dendrites synthesized in homogenous solutions, the dendrites obtained here are much larger. The fully developed ones can reach a size over 600 μm, ca. 2 orders of magnitude larger than those synthesized in homogenous solutions (∼1–5 μm). In addition, the dendrites obtained here are 2 dimensional which grow along the surface of the LBL film, instead of 3 dimensional as those obtained in homogenous solutions. A possible 3-step growth mechanism, which involves the rapid reduction of the Ag+ ions in the film, formation of Ag seed particles on the film surface, and fractal dendritic structure formation as described by the diffusion-limited aggregation (DLA) model, was proposed.

Preparation and characterization of bismuth nanostructures deposited by pulsed laser ablation

Bismuth nanostructures, from nanoparticles to quasi-percolated films, were deposited by pulsed laser ablation (PLA) on different substrates using the 355 nm line of a Nd:YAG laser. The morphology and size distribution of the obtained nanostructures were investigated, as a function of the number of ablation pulses, by high resolution electron microscopy (HRTEM) and atomic force microscopy (AFM). Deposits with a small number of pulses, 50, are formed of separated isolated particles with diameters in the range from 5 to 20 nm. Further increase in the number of pulses (>100) results in coalescence of individual particles with the formation of dendritic structures and finally, for 500 pulses, quasi-percolated Bi films are obtained. Additionally, the nanostructures formed were characterized by XPS, and Raman spectroscopy in order to determine the physical and chemical properties of the deposited material.

Investigation on microstructure and mechanical properties of the dissimilar weld between mild steel and stainless steel-316 formed using microwave energy

Dissimilar weld between a mild steel and a stainless steel-316 was formed by exposing the candidate materials to electromagnetic radiation in the microwave band. Characterisations of the joints were carried out with respect to some aspects of microstructural and mechanical properties of the fusion joints. The joining trials were carried out in an industrial microwave applicator at a fixed frequency of 2.45 GHz and 1.2 kW power while exposed for a duration of 600 s in atmospheric condition. Stainless steel-316 powder was used as the filler material for joining. Principles of microwave hybrid heating were utilised for heating and subsequently melting the metal-based materials in the joint zone. Characterisation of the microwave hybrid heating–induced dissimilar welds were carried out using X-ray diffraction, field emission scanning electron microscopy, microhardness tester and universal testing machine. The presence of carbides and intermetallic in the joint zone were evidenced in the X-ray diffraction results. The microstructures observed through scanning electron microscope show the metallurgical bonding between the substrates to be joined through complete melting of the powder particles and fusion of the base materials. The Vicker’s microhardness in the core of the joint was observed to be 380 HV, which was significantly higher as compared to the base materials due to the formation of dendritic structure and various carbides in the joint zone during microwave hybrid heating process. The average ultimate tensile strength of the joints was measured to be 420 MPa with an elongation of 6.67%. The average flexural strength of the joints was observed to be 787.5 MPa with an elongation of 5.14%. The optimum temperature required for joining was measured using an in-built non-contact infrared pyrometer and was found to be 1360 °C.

Fatigue fracture of cutter blade made of high-speed steel

The quality of the surface of cyclically loaded components is very important. Many observations confirm that the root cause of the micro cracks (causing the fatigue fracture) are primarily a surface's defects appearing during production process. These surface defects can be also caused by engraving processes used to perform identification marks. This paper presents the failure analysis of broken blade of the cutter Ku 500VX. The blade was subject of standard metallographic examination, hardness measurements, fractography analysis and metallographic studies using stereoscopic, light and scanning electron microscopes. The damage of the blade was caused by changes of the structure (formation of the brittle micro dendritic structure) that occurred during manual electric engraving process when the material was heated till its melting point. As a result the stresses occurred in surface what provided to micro cracking and to propagate the fatigue fracture. The origin of this fatigue fracture was in the place where the inscription was made.

Mechanical and Microstructural Evaluation of DMAG Welding of Structural Steel

Double channel torch, which allows concentric flow of two different shielding gases, was designed and manufactured in order to pursue double channel torch gas metal arc welding of unalloyed structural steel S235JR (EN 10025-2) with fourteen passes. Tensile and Charpy V-notch tests were realized and the results were compared with those of conventional gas metal arc welding. In order to evaluate mechanical testing results, microstructural analyses were conducted. It was found that the increase with double channel gas metal arc welding process in yield and tensile strengths as well as in toughness tests, especially in subzero temperatures, compared with conventional gas metal arc welding was due to longer columnar grains and finer tempered zone grain structure between passes and due to solidification and less dendritic structure formation in all-weld metal in double channel gas metal arc welding.

Laser scanning microscopy as a means to assess the augmentation of tissue repair by exposition of wounds to tissue tolerable plasma

Confocal laser scan microscopy (CLSM) has emerged as a tool for in vivo assessment of cutaneous conditions. In particular, its use in wound healing assessment has increasingly moved into focus. In this context, the application of tissue tolerable plasma (TTP) for wound treatment has recently become one of the most innovative therapeutic modalities.We analyzed wound healing parameters such as area decline and histomorphological characteristics of tissue repair in six subjects with vacuum-generated wounds on the forearm with a four-armed design: (A) no treatment, (B) treatment with TTP, (C) treatment with octenidine, and (D) sequential treatment with TTP and octenidine. Assessment of the wounds was conducted during six visits over the course of two weeks. The wounds were analyzed by photography and CLSM.TTP treatment led to a more rapid area decline that was statistically significant in comparison to other treatment groups. Besides mild pain, it was well tolerated. Morphologically, wound healing was found to initiate from the edges with the formation of dendritic structures consisting of keratinocytes.CLSM is a valuable tool for assessing the dynamics of wound healing. TTP, for reasons that still need to be investigated, can accelerate wound repair.

Effects of elastic strain and diffusion-limited aggregation on morphological instabilities in sputtered nitride thin films

Abstract

Phase-Field Simulation of Concentration and Temperature Distribution During Dendritic Growth in a Forced Liquid Metal Flow

A phase-field model with convection is employed to investigate the effect of liquid flow on the dendritic structure formation of a Ni-Cu alloy during rapid solidification. Temperature and solute diffusion are significantly changed with induced liquid metal flow, and distribution changes of concentration and temperature are also analyzed and discussed. The solute segregation is affected due to the concentration diffusion layer thickness change caused by the liquid flow. The flow reduces the solute segregation in the upstream and leads to a fast dendrite growing, while solidifying in the downstream gets constrained with the large solute diffusion layer. Increasing flow velocity increases the asymmetry of dendrite morphology with much more suppressed growth in the downstream. The temperature distribution is also asymmetrical due to the non-uniform latent heat released during solidification coupling with heat diffusion changed by the liquid flow. Therefore, the forced liquid flow significantly affects the dendrite morphology, concentration, and temperature distributions in the solidifying microstructure.