Alloy Droplets Research Articles

Pluronic P123 assisted synthesis of hollow InSn/In2O3 hybrid nanoparticles by solution dispersion

Hollow InSn/In2O3 hybrid nanoparticles were fabricated via stirring InSn alloy in polyethylene glycol-400 (PEG 400) solution containing P123 at 180°C. The hollow hybrid nanoparticles were characterized by TEM, XRD, XPS and other techniques. The hybrid nanoparticles have an average particle diameter of 20 nm, and they have a core. XRD studies show the presence of an equilibrium phase In3Sn and InSn4. XPS results indicate that the surface layer of the shell is mainly composed of indium oxides. Based on the time-dependent experiments, the evolution behavior of alloy droplets is proposed to explain the formation mechanism. The hollow nanoparticles display stronger UV emission than that of In2O3 nanoparticles.

Reactive wetting behaviors of Sn/Cu systems: A molecular dynamics study

Influences of temperature and Sn-Cu droplet’s composition on reactive wettings of Cu(100), Cu(110), and Cu(111) surfaces were analyzed, by using molecular dynamics (MD) calculations. As a result, the spreading on Cu(110)(Cu(100)) has the fastest (slowest) wetting kinetics. A higher temperature or a diluter Cu content in the Sn-Cu alloy droplet results in a higher wettability. Moreover, this work has addressed a theory for positioning the interface separating the liquidus and solidus alloys in the spreading film to confirm the hypothesis that the reactive wetting will come to the end when the interface saturates with the temperature-dependent solidus weight fraction of Cu.

Preparation and Tribological Behavior of BiIn/In 2O 3 Composite Dendritic Nanocrystals

BiIn/In 2 O 3 composite dendritic nanocrystals were prepared by dispersing alloy droplets in paraffin oil. The morphology and structure of the alloy nanocrystals were characterized by transmission electron micros (TEM), X-ray photoelectron spectra (XPS), and X-ray powder diffraction (XRD). The composite nanocrystals have dendritic morphology, with an average diameter of 10 nm and lengths up to tens and even hund of nanometers. Tribological results indicate that the composite nanocrystals as oil additives can improve antiwear ability of base oil, which is better than that of the corresponding monometallic particles. In addition the lubricating mechanism of the composite nanocrystals was also discussed.

Modelling and Experiments Concerning Dendrite Re-Melting and Its Role in Microstructural Evolution in Spray Formed Ni Superalloys

This paper presents a combined experimental and modelling approach to understand dendrite fragmentation of atomised metal alloy droplets during deposition in spray forming, and to study quantitatively the relationship between this dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Ni superalloy IN718 atomised powders, which simulated the environment of droplet deposition during the twin-atomiser spray forming of large diameter IN718 alloy billets at BIAM. The experiments were complemented by phase field modelling studies at Oxford. Experiment and modelling supported the hypothesis that the characteristic equiaxed spray formed microstructure depends critically upon the rapid remelting and thermal shock of fine-scale dendrites in solid particles in the spray to provide a high density of embryonic grains.

Reactive wetting behaviors of Sn/Cu systems: A molecular dynamics study

Abstract Influences of temperature and Sn-Cu droplet’s composition on reactive wettings of Cu(100), Cu(110), and Cu(111) surfaces were analyzed, by using molecular dynamics (MD) calculations. As a result, the spreading on Cu(110)(Cu(100)) has the fastest (slowest) wetting kinetics. A higher temperature or a diluter Cu content in the Sn-Cu alloy droplet results in a higher wettability. Moreover, this work has addressed a theory for positioning the interface separating the liquidus and solidus alloys in the spreading film to confirm the hypothesis that the reactive wetting will come to the end when the interface saturates with the temperature-dependent solidus weight fraction of Cu.

Production of Rapidly Solidified Composite Deposits Based on Iron with Vanadium Carbide Particles by Plasma Spraying

In the thermal spraying process, spray material is heated, melted, and accelerated by a high temperature flame. Thermal spraying can produce thick materials that rapidly solidify, because the alloy droplets accumulate successively on the substrate and solidify at a cooling rate in the range of 105-108Ks-1. Depending on the cooling conditions of the substrate and on the alloy composition, deposits are produced with metastable phases or extremely fine crystalline phases. Thermal spraying is an attractive method for the production of composite deposits with fine particles formed in-situ. In particular, iron based alloy with vanadium carbide, is useful in metal molds and also in pump parts due to its high wear resistance and high corrosion resistance. In the present work, low-pressure plasma spraying of Fe-C-V/Ni-Mg and Fe-C-V-Cr-Ni/Ni-Mg blend powders were iron based composite deposits with finely dispersed vanadium carbide particles. The as-sprayed deposit produced from Fe-C-V/Ni-Mg blend powder is composed of αFe and V8C7. The as-sprayed deposit produced from Fe-C-V-Cr-Ni/Ni-Mg blend powder is made up of γFe, αFe, V8C7 and Cr7C3. The fine precipitates of approximately 0.3μm in the as-sprayed deposit are carbide. With increasing the heat-treatment temperature up to 1273K, the carbide particles coarsen. The hardness of as-sprayed deposit produced from the Fe-C-V-Cr-Ni/Ni-Mg, which has many fine carbide precipitates, is the hardest of the deposits.

Mg-Al系合金とTiの高温濡れ現象

Magnesium alloy is expected as one of the environmentally benign materials for significant weight reduction of the components due to its high specific strength. It is, however, necessary to improve its Young's modulus when Mg alloys are applied to structural components. Ceramics hard particles are generally used as reinforcements to improve the stiffness of Mg matrix composite. However, they also cause the decrease of ductility of the composite materials. In the present study, pure Ti particles were used as reinforcement because they had enough ductility and high Young's modulus compared to Mg alloys. For the materials design of Mg matrix composites reinforced with Ti particles, the wetting behavior of Ti by the molten Mg alloy should be investigated to fabricate a good coherence at the interface between the matrix and dispersoids. The sessile drop test results showed an extremely good wettability between pure Mg and pure Ti due to a thermite reaction at the interface between Mg and TiO2 surface films of the Ti substrate. The wettability between Mg-Al alloy (AZ80) and Ti was investigated in high-purity argon gas atmosphere at 1073K. In particular, the effect of Al element contained in AZ80 on the wetting behavior was examined. When AZ80 was dropped on the pure Ti plate (t=0s), the contact angle between the droplet and the Ti plate was 10°. The contact angle and the droplet height were gradually reduced with increasing time. Ti plate was completely covered with molten AZ80 in 60s. The result indicated the wettability of pure Ti by molten Mg-Al alloy was extremely good compared to the combination of pure Mg and pure Ti. When the specimen was kept at 1073K for 180s, Al was enriched on interface between AZ80 and Ti, but Al-Ti intermetallic compounds were not formed. It was concluded the wetting phenomenon of Ti by Mg alloy droplet reached termination before formation of Al-Ti intermetallics and Al element of AZ80 had no harmful effect on the wetting behavior between Mg alloy and Ti.

Growth of Stacking-Faults-Free Zinc Blende GaAs Nanowires on Si Substrate by Using AlGaAs/GaAs Buffer Layers

Vertical GaAs nanowires on Si (111) substrate were grown by metal organic chemical vapor deposition via Au-catalyst vapor-liquid-solid mechanism. Stacking-faults-free zinc blende nanowires were realized by using AlGaAs/GaAs buffer layers and growing under the optimized conditions, that the alloy droplet act as a catalyst rather than an adatom collector and its size and composition would keep stable during growth. The stable droplet contributes to the growth of stacking-faults-free nanowires. Moreover, by using the buffer layers, epitaxial growth of well-aligned NWs was not limited by the misfit strain induced critical diameter, and the unintentional doping of the GaAs nanowires with Si was reduced.

Research on accurate droplet generation for micro-droplet deposition manufacture

Micro-droplet deposition manufacture is a novel direct metal rapid prototyping for building the miniaturized parts. In this paper, the relationship between the droplet diameter and the operating parameters is investigated to obtain the optimal condition for the accurate droplet generation. The droplet generator is designed to adjust the parameters such as the infinitesimal disturbance and the jet velocity. Experiments with the different materials have been conducted to study the feasibility of the accurate droplet generation for the manufacturing. Spherical and uniform water micro-droplets within 2.4% variation are generated using an orifice diameter of 150 μm, and tin–lead alloy droplets are produced with an orifice diameter of 100 μm in the argon environment. The results show that the generator can produce accurate micro-droplets which meet the requirements of fabricating the miniaturized parts.

Heat Transfer During Deposition of Molten Aluminum Alloy Droplets to Build Vertical Columns

To create functional metal parts by depositing molten metal droplets on top of each other, we have to obtain good metallurgical bonding between droplets. To investigate conditions under which such bonds are achieved, experiments were conducted in which vertical columns were formed by depositing molten aluminum alloy (A380) droplets on top of each other. A pneumatic droplet generator was used to create uniform, 0.8 mm diameter, molten aluminum droplets. The droplet generator was mounted on a stepper motor and moved constantly so as to maintain a fixed distance between the generator nozzle and the tip of the column being formed. The primary parameters varied in experiments were those found to have the strongest effect on bonding between droplets: substrate temperature (250–450°C) and deposition rate (1–8 Hz). Droplet temperature was constant at 620°C. To achieve metallurgical bonding between droplets, the tip temperature of the column should be maintained slightly below the melting temperature of the alloy to ensure remelting under an impacting drop and good bonding. The temperature cannot exceed the melting point of the metal; otherwise the column tip melts down. The temperature at the bottom of a column was measured while droplets were being deposited. An analytical one-dimensional heat conduction model was developed to obtain the transient temperature profile of the column, assuming the column and the substrate to be a semi-infinite body exposed to a periodic heat flux. From the model, the droplet deposition frequency required to maintain the tip temperature at the melting point of the metal was calculated.

Growth of In xGa 1−xN quantum dots by nitridation of nano-alloyed droplet method using MOCVD

In x Ga 1− x N quantum dots (QDs) were grown on GaN/sapphire (0 0 0 1) substrates by employing nitridation of nano-alloyed droplet (NNAD) method using metal-organic chemical vapor deposition (MOCVD). In+Ga alloy droplets were initially formed by flowing the precursors TMIn and TMGa. Density of the In+Ga alloy droplets was increased with increasing precursors flow rate; however, the droplet size was scarcely changed in the range of about 100–200 nm. Two cases of In x Ga 1− x N QDs growth were investigated by varying the nitridation time and the growth temperature. It was observed that the In x Ga 1− x N QDs size can be easily changed by controlling the nitridation process at the temperature between 680 and 700 °C for the time of 5–30 min. Self-assembled In x Ga 1− x N QDs were successfully grown by employing NNAD method.

Fabrication and characterization of novel bicrystalline ZnO nanowires

Two types of novel bicrystalline ZnO nanowires have been synthesized by a thermal evaporation method. The morphology and microstructure of the nanowires have been extensively investigated. One type of the nanowires has agg twin boundary extending down its entire length with twinning plane and the zone axis. The other type is those nanowires with twin crystal-single crystal junction. The twin defects in the Sn-Zn alloy droplets in the initial growth process are proposed for interpreting the growth of these two kinds of bicrystalline nanowires.

Rapid solidification of Al-Cu-Ag ternary alloy under the free fall condition

The rapid solidification of Al-30%Cu-18%Ag ternary alloy is investigated by using the free fall method. Its solidified microstructure is composed of θ(Al2Cu), α(Al) and ξ(Ag2Al) phases. The liquidus temperature and solidus temperature are determined as 778 and 827 K, respectively. The alloy melt undercooled amounts up to ΔT Max=171 K (0.20T L). Its microstructural evolution is investigated based on the theoretical analysis of undercooling behavior and nucleation mechanics. It is found that the undercooling increases with the decrease of the diameter of the alloy droplet. When ΔT<78 K, the primary θ(Al2Cu) phase of the alloy grows into coarse dendrite. When 78 K⩽ΔT⩽171 K, its refined θ(Al2Cu) phase grows alternatively with α(Al) phase. Once ΔT⩾171 K, its microstructure is characterized by the anomalous (θ+α+ξ) ternary eutectic.

Core-shell microstructure formed in the ternary Fe-Co-Cu peritectic alloy droplet

The metastable liquid phase separation occurs in the ternary Cu50Fe37.5Co12.5 peritectic alloy droplets during free fall. The separated alloy melt rapidly solidifies and evolves core-shell microstructure composed of L1(Cu) and L2(Fe,Co) phases. Based on the determination of the phase transition temperature, the core-shell microstructure evolution, the interfacial energy, the temperature gradient and the Marangoni migration are analyzed. The interfacial energy of the separated liquid phase increases with the decrease of the temperature. The temperature gradient changes from large to small along the radius direction from inside to outside in the alloy droplet. The Marangoni force (FM) acting on the micro-droplet of L2(Fe,Co) phase increases with the increase of the size of the L2(Fe,Co) phase, and decreases with the increase of undercooling. Driven by FM, the micro-droplet of L2(Fe,Co) phase migrates from outside to inside in the alloy droplet, collides and coagulates each other during migration, and then forms different types of core-shell microstructures.

EFFECT OF CAPPING LAYER ON InxGa1-xN QUANTUM DOTS GROWN USING NNAD METHOD BY MOCVD

In x Ga 1-x N quantum dots (QDs) were grown on GaN epitaxy using nitridation of nano-alloyed droplet (NNAD) method by metal-organic chemical vapor deposition (MOCVD) system. Before the In x Ga 1-x N QDs formation, In + Ga droplets were initially formed by the flow of TMI and TMG, which acts as a nucleation seed for the QDs growth. Density of the alloy droplets was increased with the increasing flow rate; however, droplet size was scarcely changed about 100–200 nm by flow rate. And In x Ga 1-x N QDs size can be easily changed by controlling the nitridation time or various factors. Also, the influence of GaN capping layer on the properties of In x Ga 1-x N QDs was discussed.

Catalyst-free nanowires with axialInxGa1−xAs/GaAs heterostructures

Self-catalyzed growth of axial InxGa1−xAs/GaAs heterostructures has been realized by molecular beam epitaxy. The growth of the wires isachieved from gallium/indium alloy droplets that are nucleated in situ. By variation of theIn/Ga beam flux during the growth it was possible to vary the effective indium content up tox = 5%, as deduced from photoluminescence measurements. We have analyzed thedependence of the alloy concentration on the growth conditions and present a simplemodel for the growth. The heterostructures grown with the method presentedwere spatially mapped along the wires with confocal microphotoluminescenceand cathodoluminescence. It was found as expected that the emission ofGaAs/InxGa1−xAs/GaAs heterostructures is localized. This work is important for the use of an external catalyst-freegrowth of complex axial heterostructures and related opto-electronic devices thatfacilitates its possible integration in the device or system fabrication processes.

Modeling of the in-flight solidification of droplets produced by the uniform-droplet spray process

A mathematical model to describe the solidification of the droplets produced by the uniform-droplet spray process is developed. The model addresses the motion, heat transfer, nucleation and solidification of traveling droplets. A dimensionless formulation for the interface migration scheme during recalescence is developed. The simulation results of Sn–5wt.%Pb alloy droplets subjected to electrical charging are compared with those in uncharged cases. The effects of charging voltage on scattering and subsequent changes in droplet velocities and heat transfer coefficients are analyzed. The computations show substantial increases in heat transfer coefficient upon charging and notable decreases in droplet velocities and cooling rates.

Transient Kinetics of Slag Metal Reactions

The kinetics of slag metal reactions are complex and often transient, in the sense that interfacial area, the equilibrium driving force, temperature gradients, and fluid properties are changing with time. This highly transient behavior is challenging to model using simple ordinary differential equations, and new theoretical approaches must be developed to deal with the complexity associated with these systems. Three examples from recent studies are described to illustrate methods of analyzing transient behavior. The first is desulfurization of steel in ladle metallurgy where the equilibrium driving force is changing with time, and the second is the case of reacting droplets in oxygen steelmaking where “bloating” of the droplet has a dramatic effect on the kinetics. The third is in the case of reactions between an alloy droplet and slag that result in large changes in interfacial area due to surface tension driven flows.

Defect studies of ZnSe nanowires

During the synthesis of ZnSe nanowires various point and extended defects can form,leading to observed stacking faults and twinning defects, and strong defect related emissionin photoluminescence spectra. In this paper, we report on the development of a simplethermodynamic model for estimating the defect concentration in ZnSe nanowires grownunder varying Se vapour pressure and for explaining the results of our experimentalfindings. Positron annihilation spectroscopy was used successfully for the firsttime for nanowires and the results support predictions from the defect model aswell as agreeing well with our structural and optical characterization results.Under very high Se vapour pressure, Se nodules were observed to form on thesidewalls of the nanowire, indicating that beyond a limit, excess Se will begin toprecipitate out of the liquid alloy droplet in the vapour–liquid–solid growth ofnanowires.

Fabrication and Characterization of InxGa1-xN Quantum Dots Using Nitridation of Nano-alloyed Droplet Growth Technique

We have studied the InGaN quantum dots (QDs) grown on a GaN pseudo template on a sapphire substrate (0001) using metal organic chemical vapor deposition (MOCVD). Self assembled InGaN QDs was successfully fabricated using nitridation of nano alloyed droplet (NNAD) method. The density, average diameter, and height of the InGaN QDs were estimated to be 1.0×108 cm-2, 50 nm, and 13 nm, respectively. The structural and optical properties of InGaN QDs were investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL). From the SEM image, it is clearly seen that In+Ga alloy droplets are uniformly distributed on GaN template. Further, InGaN QDs uniform distribution on the GaN/sapphire surface was observed by AFM. Broad emission from the QDs was observed around 580 nm which indicate the grown QDs have relatively high In composition due to low growth temperature. The broad and separated PL peak of QDs induced by the fluctuation in size and composition. Our result suggests that self-assembled InGaN QDs are fabricated well by using NNAD technique.