Dendritic Particles Research Articles

Improved Microstructure and Mechanical Properties of AZ31–Mg2Si in Situ Composite Fabricated by Cyclic Closed-die Forging

AZ3111.6 wt.% Mg2Si in situ composite was fabricated by conventional casting method. Then cyclic closed-die forging (CCDF) were applied to severely deform the as-cast billets at 450 °C for one, three, and five passes. During multi-pass CCDF processing, both dendritic and eutectic Mg2Si particles are broken up into much smaller ones due to the shear stress inflicted by the matrix. With the increasing of CCDF passes number, finer grain size and more homogeneous distribution of Mg2Si phase are obtained along with notable improvement in both strength and ductility. AZ3111.6 wt.% Mg2Si composite presents tensile strength of 171 MPa and elongation of 2.6% after five CCDF passes as compared to initial 99 MPa and 1.0%.

Rheological properties of dimorphic magnetorheological gels mixed dendritic carbonyl iron powder

Magnetorheological gels typically are colloids of spherical micrometer magnetic particles dispersed in a high-viscosity polymer matrix. However, some major parameters of this kind of mono-disperse magnetorheological gel, such as the shear storage modulus and yield stress, cannot meet the needs of practical engineering application. In this study, a systematic experiment was investigated on the use of dendritic carbonyl iron particles to enhance the comprehensive performance in conventional (microsphere-based) magnetorheological gels formulated in polyurethane matrix. Two kinds of carbonyl iron particles that have very similar magnetic properties but very different morphologies (one is spherical and the other is dendritic) are employed. The dimorphic magnetorheological gels were prepared by adding dendritic carbonyl iron particles into the conventional spherical carbonyl iron particle–based magnetorheological gel and a series of dimorphic magnetorheological gel samples were prepared. The magnetorheological properties of those samples, both under oscillatory and rotational shear rheometry, were systematically tested. It was found that the dendritic carbonyl iron particle additives can significantly improve the shear storage modulus and response time of the dimorphic magnetorheological gels compared with conventional magnetorheological gels. Meanwhile, when the mass ratio of dendritic particles to carbonyl iron particles is 25% and 50%, the shear stress, static shear yield stress, and dynamic shear yield stress of dimorphic magnetorheological gels can also be greatly enhanced.

The split of dendritic precipitates with interfacial anisotropy in solid transformations in alloys

The dendritic precipitates may be obtained during isothermal treatment at the MSC temperature for a long time in solid transformations in the modeled alloy. The split of dendrite may occur while the temperature declines successively from the MSC point during quenching. The dendritic morphology is determined by the intensity of the interfacial anisotropy, and the split is related to the competition between the interfacial energy and the misfit strain energy. The doublet and octocube are possible while the dendritic particle is small, but if dendritic particle is large, the octodendrite may be obtained. The split of dendritic particles during aging is induced by the MSC effect which may be expressed by the MSC function which is determined by three parameters, the MSC rate β, the MSC delay t0 and the initial value of the MSC function α. The phase transformations during isothermal treatment at MSC point are spontaneous and equilibrium processes, however, phase transformations from MSC to the lower temperature during quenching are the spontaneous but non-equilibrium processes.

Effects of particle shape and temperature on compaction of copper powder at micro scale

This study investigated the effects of particle shape and temperature on the compaction of copper powder at micro scale. Copper powder particles were compressed inside a cylindrical die cavity with 2 mm diameter to form compacts with about 3 mm height. Two kinds of particle shapes, spherical and dendritic, and two forming temperatures, room temperature and 400 °C, were considered in the experiments. Some of the produced compacts were further sintered at 600 °C. The study also used simple upsetting tests to investigate the characteristics of the deformation of the compacts under compressive stresses. The results showed that the compacts produced at room temperature demonstrated brittle deformations. However, by increasing the forming temperature to 400 °C, ductile deformations have been observed on the compacts of dendritic particles. Furthermore, the sintering treatment resulted in increases in dimensions, decreases in relative density and hardness, and an increase in ductility. It also led to pore growths which have been seen on scanning-electron microscope images. These phenomena were most significant in the dendritic powder compacts which were produced at 400 °C and treated by the sintering process.

Decomposition of thin Au films on flat and structured Si substrate by annealing

Thin metallic films are unstable under thermal treatment and disintegrate into particles in a process known as solid state dewetting. This process was studied by annealing 30 nm thin gold films, deposited on flat and structured silicon substrates, by means of electron scanning microscopy and electron backscattered diffraction. Two different modes of decomposition were observed; formation of doughnut-shaped islands which coalesced into dendritic particles of gold with a network of gold particles in between, and patterns which start as circular holes with pronounced rims and subsequently develop in a typical pattern. In the case of structured substrates confinement of particles within ridges was observed for deep scratches, while shallower scratches allowed some spill over of the material.

NaF-assisted combustion synthesis of MoSi2 nanoparticles and their densification behavior

The exothermic reduction of oxides mixture (MoO3+2SiO2) by magnesium in NaF melt enables the synthesis of nanocrystalline MoSi2 powders in near-quantitative yields. The combustion wave with temperature of about 1000–1200°C was recorded in highly diluted by NaF starting mixtures. The by-products of combustion reaction (NaF and MgO) were subsequently removed by leaching with acid and washing with water. The as-prepared MoSi2 nanopowder composed of spherical and dendritic shape particles was consolidated using the spark plasma sintering method at 1200–1500°C and 50MPa for 10min. The result was dense compacts (98.6% theoretical density) possessing submicron grains and exhibiting hardness of 8.74–12.92GPa.

Meso-mechanical constitutive model of bulk metallic glass matrix composites

Based on the existing experimental observations and extended Mori-Tanaka (M-T) method, a new meso-mechanical constitutive model is proposed to predict the monotonic tensile or compressive deformation of bulk metallic glass (BMG) matrix composites with toughening phases. In the proposed model, the deformation behaviors of the BMG matrix and toughening phase are described by using the extended free volume model and unified visco-plastic model, respectively, and corresponding algorithmic tangent operators are also derived. The proposed model is verified by predicting the monotonic tensile or compressive stress-strain responses of the BMG composites with an in-situ dendrite and different ex-situ toughening particles, respectively. Comparison of predicted results and experimental ones demonstrates that the proposed meso-mechanical constitutive model describes both the strain hardening and softening features of the BMG composites reasonably. The simulations agree with the experimentswell.

Controllable Synthesis of Silver Dendrites via an Interplay of Chemical Diffusion and Reaction

Here we report a study on the role of chemical diffusion and reaction on the morphology evolution of silver particles. Electrochemical reaction is employed to synthesize silver particles. The reaction rate is regulated by current density while the diffusion rate of silver ions is regulated by the viscosity of solution. Silver dendritic particles are largely synthesized at various diffusion and reaction conditions. At a high reaction rate, silver dendrites are formed at the medium diffusion rates of silver ions while the dendritic structures are only formed at an extremely low diffusion rate of silver ions when the reaction proceeds slowly. Therefore, the interplay of diffusion and reaction plays a key role on the structure development of dendrites because the interplay determines the chemical distribution on the growth front of crystals. Once a chemical concentration gradient is established at the growth front, the dendritic structures is triggered.

Barium Sulfate Crystallization from Synthetic Seawater

Barium sulfate was crystallized in a synthetic seawater mixture that was chosen to better reflect ocean conditions. The synthetic seawater contained monovalent ions, magnesium, strontium, and calcium as well as bicarbonate and boric acid. The natural pH of the synthetic seawater is 8.1, and this seawater was used to determine the impact on morphology, nucleation rates, and incorporation of foreign ions. It was found that dendritic and diamond-shaped particles are both formed. The main parameters influencing the formation of dendritic particles were the saturation index and the ion ratio, but there was also a significant synergistic effect with the other ions present. The diamond-shaped particles formed later at a lower saturation index. The nucleation rate in synthetic seawater was found to be higher than expected based on an ion ratio basis. This is most probably because the divalent ions induce a higher nucleation rate by lowering the surface free energy. Strontium was found to be the dominant ion subst...

Thermal characteristic of a tube fitted with porous media inserts in the single phase flow

In this paper, the heat transfer rate and the pressure drop of metal porous media inserts are analyzed in the single phase. The testing range of the Reynolds number is between 3654 and 14617. Tap water is used as the working fluid. The effect of particle size and particle shape on the heat transfer performance is studied. The results show that an optimal value existed among the testing samples for dendritic particle. The best heat transfer rate is resulted by the sample D-125 instead of by the sample D-25 with the small particle size, which is approximately 2.2–3.0 times as that of the smooth tube. This is mainly because the velocity at the inlet of inserts increases greatly and the fluid mixing intensity at the annular is also high. Two factors work together to bring about a best heat transfer coefficient. The friction factor increases with the decrease in the particle size. The best Performance Evaluation Criterion (PEC) value is resulted by sample D-125, which is about 1.26–1.71. The particle shape also has an impact on the heat transfer rate. The heat transfer coefficient of the spherical particle is higher than that of dendritic particle. However, the higher heat transfer rate is paid at the cost of high flow resistance. This high flow resistance finally deteriorates the heat transfer performance. As a result, the best heat transfer performance is achieved by the dendrictic particle, which brings a high heat transfer rate as well as a low flow resistance. Therefore, the PEC values have to be taken into consideration in the design of heat exchanger.

Micro-Nanostructured Polyaniline Assembled in Cellulose Matrix via Interfacial Polymerization for Applications in Nerve Regeneration.

Conducting polymers have emerged as frontrunners to be alternatives for nerve regeneration, showing a possibility of the application of polyaniline (PANI) as the nerve guidance conduit. In the present work, the cellulose hydrogel was used as template to in situ synthesize PANI via the limited interfacial polymerization method, leading to one conductive side in the polymer. PANI sub-micrometer dendritic particles with mean diameter of ∼300 nm consisting of the PANI nanofibers and nanoparticles were uniformly assembled into the cellulose matrix. The hydrophobic PANI nanoparticles were immobilized in the hydrophilic cellulose via the phytic acid as "bridge" at presence of water through hydrogen bonding interaction. The PANI/cellulose composite hydrogels exhibited good mechanical properties and biocompatibility as well as excellent guiding capacity for the sciatic nerve regeneration of adult Sprague-Dawley rats without any extra treatment. On the basis of the fact that the pure cellulose hydrogel was an inert material for the neural repair, PANI played an indispensable role on the peripheral nerve regeneration. The hierarchical micro-nanostructure and electrical conductivity of PANI could remarkably induce the adhesion and guiding extension of neurons, showing its great potential in biomedical materials.

Morphological study of branched Sn structure formed under selected electrochemical conditions

A controlled electrodeposition process of branched micrometric and nanometric metallic tin structures was developed. Selected potentiostatic and galvanostatic techniques were explored with the aim of forming hierarchical shaped Sn on carbon porous electrodes by a simple template-free synthesis. We have studied the influence of continuous potential steps ranging from −0.9 to −4 V versus Ag/AgCl which show a classical nucleation growth mechanism. Under high overpotentials above −1.5 V, the reaction is governed by mass transport, which enables the development of vertically aligned dendrites. Upon reaching a dendrite particle size of 2–5 µm, Sn2+ reduction is facilitated on branches extending at an angle of about 45° from the main stem due to enhanced spherical diffusion to these newly evolving sites. A competing reaction of hydrogen evolution plays a significant role during initial nucleation stages and throughout particle evolution by reducing the overall columbic efficiency. Further study of means to affect the mass transport and morphology has led us to investigate the influence of pulse deposition duty cycle as well as use of anionic (SDS—sodium dodecyl sulfate) and cationic (HDTAB—hexadecyltrimethylammonium bromide) surfactants. While short pulses and long rest time promote the formation of high surface density of small nuclei, surfactants directly influence the tin ions (SDS) or adsorbed on the negatively charged electrode (HDTBA). Finally, the study of an electrodeposition method using strong acid additives was developed. It is shown from SEM and EQCM studies that careful selection of the acid type and concentration gives rise to the formation of a much more advanced network structure promoted by selective etching and co-reduction of dissolved ions. Highly interesting two-dimensional tin films formed in this process are also reported.

WC Particles Reinforced Co-Based Alloy Coatings Produced by Laser Cladding

To extend the service life of hot die steel (H13), WC particles reinforced Co-based alloy composite coatings were produced by laser cladding. The microstructure evolution and performance of the cladding layer were investigated. The experiment results showed that there existed fine dendritic crystals and dispersive WC particles in the cladding layer. The coatings were mainly composed of γ-Co, WC and Co3W3C phases. The microhardness of the cladding layer was higher than that of the arc surfacing layer, and the microhardness gradually increased with WC content. Compared to the substrate, the friction coefficient of the cladding layer reduced greatly. The coatings with 15wt.% WC possess the best wear resistance.

Prefrontal white matter pathology in air pollution exposed Mexico City young urbanites and their potential impact on neurovascular unit dysfunction and the development of Alzheimer's disease

Millions of urban children are chronically exposed to high concentrations of air pollutants, i.e., fine particulate matter (PM2.5) and ozone, associated with increased risk for Alzheimer's disease. Compared with children living with clear air those in Mexico City (MC) exhibit systemic, brain and intrathecal inflammation, low CSF Aβ42, breakdown of the BBB, attention and short-term memory deficits, prefrontal white matter hyperintensities, damage to epithelial and endothelial barriers, tight junction and neural autoantibodies, and Alzheimer and Parkinson's hallmarks. The prefrontal white matter is a target of air pollution. We examined by light and electron microscopy the prefrontal white matter of MC dogs (n: 15, age 3.17±0.74 years), children and teens (n: 34, age: 12.64±4.2 years) versus controls. Major findings in MC residents included leaking capillaries and small arterioles with extravascular lipids and erythrocytes, lipofuscin in pericytes, smooth muscle and endothelial cells (EC), thickening of cerebrovascular basement membranes with small deposits of amyloid, patchy absence of the perivascular glial sheet, enlarged Virchow–Robin spaces and nanosize particles (20–48nm) in EC, basement membranes, axons and dendrites. Tight junctions, a key component of the neurovascular unit (NVU) were abnormal in MC versus control dogs (χ2<0.0001), and white matter perivascular damage was significantly worse in MC dogs (p=0.002). The integrity of the NVU, an interactive network of vascular, glial and neuronal cells is compromised in MC young residents. Characterizing the early NVU damage and identifying biomarkers of neurovascular dysfunction may provide a fresh insight into Alzheimer pathogenesis and open opportunities for pediatric neuroprotection.

Effect of Impurity Te on the Morphology of Alumina Particles in Molten Iron

Surface active elements in liquid steel may affect the morphology of non-metallic particles during deoxidation. The effect of Te on the morphology of alumina particles was therefore studied by adding Te to molten iron before Al deoxidation at 1873 K. Dendritic, spherical, faceted, plate-like and clustered particles were identified in all samples. Te, however, considerably changes the relative frequencies of the morphologies, decreasing the amount of dendritic and spherical particles whereas increasing the amount of faceted and plate-like particles. This effect is closely related with the supersaturation degree, stirring, and Te content. The way Te influences the morphology of the alumina particles and the factors influencing Te effectiveness are discussed.

Broadband antireflective superhydrophobic self-cleaning coatings based on novel dendritic porous particles

Novel dendritic porous particles were employed to fabricate broadband antireflective superhydrophobic self-cleaning coatings by facile dip-coating method.

Studying the dynamic mechanism of transporting a single drug carrier-polyamidoamine dendrimer through cell membranes by force tracing.

Although drug delivery based on nanomaterials has shown great potential in practical applications, the trans-membrane mechanism of the drug carrier is still unclear due to technical limitations. Here, we report the dynamic transporting process of a single dendritic polyamidoamine particle via cell membranes in real time by the force tracing technique.

Microstructure and Magnetic Properties of Electrodeposited Ni85.8Fe10.6W1.4Cu2.2 Alloy Powder

Nanostructured 85.8 wt%Ni–10.6 wt%Fe–1.4 wt%W–2.2 wt%Cu alloy powder was deposited from an ammonium citrate bath at a current density of 600 mAcm−2. A cathodic polarization curve was recorded, and the current efficiency of the alloy was determined as a function of current density. The deposition of the alloy at current densities up to 400 mAcm−2 is an activation-controlled process. At higher densities, diffusion control is attained. At potentials more positive than −0.90 V, hydrogen evolution from NH4+ and (HCit)3− ions occurs in parallel with the alloy deposition. XRD analysis revealed an amorphous matrix embedded with nanocrystals of the FCC phase of the solid solution of iron, tungsten and copper in nickel. SEM micrographs showed the formation of two shapes of particles: large cauliflower-like particles and small dendritic particles with a large number of secondary and higher-order branches. The annealing of the as-deposited powder causes structural changes which consequently affect its magnetic properties. In the temperature range of 150°C–460°C, structural relaxation takes place, involving short-range ordering and an increase in the relative magnetic permeability of the cooled sample. At temperatures above 460°C, amorphous matrix crystallization and crystalline grain growth of the FCC solid solution occur, resulting in a decrease in the magnetic permeability of the powder.

3D Morphology and Formation Process of the Icosahedral Quasicrystalline Phase in Rapidly Solidified Al–Mn Alloy

Three-dimensional morphology and formation process of icosahedral quasicrystal phase have been investigated in a melt-spun Al–18Mn alloy (in wt%). Three distinct layers corresponding to varying temperature gradient have been observed on the cross section of the ribbons. 3D morphologies of cellular and dendritic icosahedral phase have been obtained through electro-etching. A model has been proposed to describe the formation process of the icosahedral phase and α-Al during the rapid solidification. The icosahedral phases are primarily precipitated from the melt into fine cellular and dendritic particles, and subsequently engulfed by the α-Al which propagates in a planar morphology.

Multifunctional silver film with superhydrophobic and antibacterial properties

Material properties are strongly dependent on material structure. The large diversity and complexity of material structures provide significant opportunities to improve the properties of the materials, expanding their applications. Here, we discuss the fabrication of a multifunctional silver film prepared by controlling the nucleation and growth of silver particles. Silver films with high hydrophobicity and antibacterial activity were fabricated by adopting an electrochemical approach. The dependence of the hydrophobic and antibacterial properties on the size and shape of the silver particles was first investigated. Small-sized silver particles exhibited a high antibacterial rate, while a porous silver film composed of dendritic particles showed a significant hydrophobic activity. By regulating the reaction time, current density, and silver salt concentration, a silver film with a contact angle of 150.9° and an antibacterial rate of 54.7% was synthesized. This study demonstrates that finding a compromise between different material structures is a suitable way to fabricate multifunctional devices.