Alloy Droplets Research Articles

DC electric field-driven heartbeat phenomenon of gallium-based liquid metal on a floating electrode.

The heart beating phenomenon of room temperature liquid metal (LM) mercury has attracted much attention in the past years, but its research and application are limited because of the low vapor pressure and high toxicity. Here, a fundamental scientific finding is reported that the non-toxic eutectic gallium indium (EGaIn) alloy droplets beat periodically at a certain frequency based on a floating electrode under the stimulation of the direct current (DC) field. The essential characteristics of heart beating are the displacement and the projected area change of the LM droplet. The mechanism of this phenomenon is the self-regulation of interfacial tension caused by chemical oxidation, chemical corrosion, and continuous electrowetting. In this article, a series of experiments are also carried out to examine the effects of different factors on the heartbeat, such as voltage, the volume of the droplet, the droplet immersion depth, the electrolyte solution concentration, the distance of electrodes, and the type of floating electrode. Finally, the heartbeat state and application boundary of the LM droplet under different conditions are summarized by imitating the human life process. The periodic changes of the LM droplet under an external DC electric field provide a new method to simulate the beating of the heart artificially, and can be applied to the research of organ chip fluid pumping in the future.

Correlation of radius of cathode spot of vacuum arc of metals on the size of generated droplets

The simple relation to estimate the cathode spot radius of a vacuum arc of pure metals is obtained. On its basis, is established between the cathode spot radius and the size of droplets generated by the cathode spot a correlation. This enables to find ways to reduce droplets in the plasma flow, which forms coatings by the vacuum electric arc method. The paper presents the results of experimental study of the droplet sizes depending on the vacuum arc current iд. The size and amount of the droplets on an area of 1 mm2 of the coating surface are determined using the ImageSP program. As the initial data, the microstructures of the coatings are used with an increase of: ç100, ç200, ç500, ç1000, ç1500. The droplets have been generated by a cathode spot of a vacuum arc for the alloy of the composition, at.%: 68Al–8Cr–4Nb–20Si. It is established that the number of droplets with a diameter of < 2 μm is generated most of all, and the number of droplets with a diameter > 10 μm is generated least of all. The number of generated droplets with a diameter from 2 to 10 μm slightly depends on the arc current iд. It is noted that the diameter of the alloy droplet is smaller than the diameter of the droplets generated by the cathode spot on its components due to the fact that the radius of the cathode spot on the alloy is smaller than the radius of the cathode spot on its pure components.

Verification of Possibility of Molten Steels Decopperization with ZnAl2O4

In the previous research works, ZnAl2O4 material was considered as one of the solutions for the decopperization process of molten steels; up to 33% of decopperization efficiency was reported by utilising the ZnAl2O4 filter. In order to verify the decopperization possibility of ZnAl2O4 materials, iron-based alloys with various copper and carbon contents were interacted with ZnAl2O4 substrates in a heating microscope under an argon gas atmosphere at 1600 °C. Fe-Cu alloys were found to react with the ZnAl2O4 substrate during the interaction process, and a reaction layer with a complex composition around the alloy droplet was formed; however, Cu was not detected in the reaction layer. Cu was later found infiltrated inside of the ZnAl2O4 substrates. Furthermore, the Cu-Zn compounds were detected when the copper content in Fe-Cu alloys was 10 wt% Cu. After interaction experiments, copper was decreased in all cases. Thereby, the copper evaporation and infiltration into the ZnAl2O4 substrate were considered as the reasons for copper loss. Moreover, oxygen dissolved in melt was found to have a great effect on the copper evaporation process.

Efficient recycling of silicon cutting waste by Al[sbnd]Si alloying with the assistance of cryolite

Silicon cutting waste (SCW) generated during Si wafers producing process can be recycled by AlSi alloying process. However, the presence of O in SCW has a detrimental impact on recycling process. In this study, cryolite was introduced to eliminate the hindrance of O. The influences of smelting temperature and the amount of cryolite additive on the yield of the blocky AlSi alloys and the Si recovery ratio of the SCW have been investigated and the alloying conditions were optimized to a smelting temperature of 1000 °C and a cryolite/SCW mass ratio of 0.8, achieving a AlSi alloys yield of 95.99% and a Si recovery ratio of 84.77%, which were far greater than those without cryolite additive. The results showed that the addition of cryolite additive can effectively improve the smelting effect and reduce the alloying temperature. Furthermore, the action mechanism of cryolite in Al-SCW system was analyzed, and the results revealed that the molten cryolite can dissolve the generated Al2O3 existing on the surface of AlSi alloy droplets and finally contributes to the aggregation of these droplets. This method has advantages including high Si recovery ratio of SCW, low alloying temperature and simple technological process.

Liquid-Metal-Assisted Deposition and Patterning of Molybdenum Dioxide at Low Temperature.

Molybdenum dioxide (MoO2), considering its near-metallic conductivity and surface plasmonic properties, is a great material for electronics, energy storage devices and biosensing. Yet to this day, room-temperature synthesis of large area MoO2, which allows deposition on arbitrary substrates, has remained a challenge. Due to their reactive interfaces and specific solubility conditions, gallium-based liquid metal alloys offer unique opportunities for synthesizing materials that can meet these challenges. Herein, a substrate-independent liquid metal-based method for the room temperature deposition and patterning of MoO2 is presented. By introducing a molybdate precursor to the surrounding of a eutectic gallium-indium alloy droplet, a uniform layer of hydrated molybdenum oxide (H2MoO3) is formed at the interface. This layer is then exfoliated and transferred onto a desired substrate. Utilizing the transferred H2MoO3 layer, a laser-writing technique is developed which selectively transforms this H2MoO3 into crystalline MoO2 and produces electrically conductive MoO2 patterns at room temperature. The electrical conductivity and plasmonic properties of the MoO2 are analyzed and demonstrated. The presented metal oxide room-temperature deposition and patterning method can find many applications in optoelectronics, sensing, and energy industries.

Detailed characterization of supported eutectic droplets using photoemission electron microscopy

Pt-Ge eutectic alloy droplets are scrutinized by microscopic analysis of electrons excited by 4.9-eV photons. Using only the work function and contact angle as fitting parameters, we determine the exact droplet shape that reproduces the experimental electron emission profile. The local inclination of the droplets' surface plays a decisive role in the generated emission profile. Intensity variations at the illuminated side of the eutectic droplets are a consequence of standing waves in the electromagnetic field responsible for excitation. Intensity variations at the nonilluminated side are ascribed to a diffraction pattern of the photons after their interaction with the droplet.

Design, analysis, and experimental performance of a novel micro-nozzle based liquid metal direct writing system

The use of liquid alloy micro-printing methods for stretchable conductors and sensors has become a hot topic of micromachining research because of its attractive properties. Due to the high surface tension and easy oxidation characteristics of liquid metal, it is difficult to be accurately processed. In this paper, a direct writing method based on micro-nozzles for Galinstan alloy is presented. In comparison with the existing liquid alloy printing methods, the size of metal drops is reduced to 83 by utilizing its wettability with the substrate and self-made micro-nozzles with the accuracy of micro order in a range from 13 to 87 inner diameter. This micro direct writing system was used to produce an array of alloy droplets, micro droplets and wires patterns, and a pile of alloy on account of the formation of the gallium-oxide layer which preserves their shapes. The diameter of droplets in an array is less than 83 and the line width is as small as 83 . This first demonstration of direct writing based on micro-nozzles provides a variety of liquid metal distribution strategies and improves the accuracy of alloy electronics. Also, attempts have been made to produce a vertical pile of droplets, by the shape protection of gallium-oxide skin. This method makes a good compromise between resolution, process simplicity, design flexibility, and fast delivery automaticity.

Growth, phase transition, and island motion of Au on Ge(111).

Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski-Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au-Ge alloy droplets. Near 600°C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2°C and weakly depends on coverage, varying from 640°C at 1 ML down to 580°C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250°C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.

Live record of vortex-shaped flow by microgravity solidification

A unique type of vortex-shaped microstructure reflecting the flow pattern during the microgravity solidification formed in Al70Ag20Ge10 alloy droplets using drop tube technique. A 2D axisymmetric model coupling heat transfer, fluid flow with phase change was established to simulate the fluid flow variation of alloy droplets during microgravity solidification. The Stokes motion was suppressed and the Marangoni motion was prominent in driving the vortex-shaped flow. The melt flow affected the phase selection, consequently the Ag2Al phase nucleated primarily instead of (Al) phase under near-equilibrium solidification, and the flow pattern was preserved during the rapid growth of the metastable Ag2Al dendrites. The Marangoni effect was weakened with the decrease of droplet size, and anomalous (Al) + Ag2Al and intergranular (Al) + (Ge) eutectics formed owing to the enhanced nucleation and growth competition among phases. In comparison, Al70Ag20Ge10 alloy was solidified under gravity condition by DSC and vacuum arc melting methods respectively. No vortex-shaped microstructure formed in DSC sample ascribed to the suppressed natural convection at small cooling rate. During arc melting, the electromagnetic force and enhanced natural convection caused strong turbulent flow, the metastable Ag2Al dendrites rapidly formed with increased vortex density.

Effects of gold film thickness and annealing time on diameter of silicon nanowires grown by VLS process

In this paper, silicon nanowires were prepared by chemical vapor deposition (CVD) in the presence of gold catalyst. We discussed the effects of gold film thickness and annealing time on droplet diameter and silicon nanowire diameter. The study revealed that the diameter of silicon nanowires increases with the increase of the thickness of the gold film. With the increase of annealing time, the number of alloy droplets decreases, and the diameter increases. The average diameter of silicon nanowires decreases first and then increases. The results provide useful information for long-time annealing of silicon nanowires.

Anisotropic Nd-Fe ultrafine particles with stable and metastable phases prepared by induction thermal plasma

Both stable Th2Zn17-type and metastable TbCu7-type phases of anisotropic Nd-Fe ultrafine particles are prepared selectively by controlling the cooling rate using the low-oxygen induction thermal plasma (LO-ITP) process. The cooling rate is controlled by introducing the additional Ar gas at the end of the plasma flame. Moreover, numerical calculations based on a binary aerosol formation-growth model clarifies the alloying mechanism during rapid cooling wherein Nd-Fe vapor solidifies. The metastable Nd-Fe alloy ultrafine particle is formed in the cooling rate of 5 × 105 K/sec via the alloy droplet which is formed after the Fe and Nd vapors convert simultaneously into the liquid phase. The mean particle sizes of obtained ultrafine alloy powders without and with quenching are 76.3 ± 24.2 nm and 37.0 ± 12.5 nm, respectively. Electron microscopy observation reveals that the obtained particles are single crystalline. The Nd concentration of Nd-Fe particles shows a distribution in the range of 10.1–12.6 at%. X-ray diffraction and magnetic measurement demonstrate that the obtained particles are anisotropic and can be aligned by external magnetic field. Therefore, this process paves the way to achieve a new-generation anisotropic permanent magnet.

Eutectic growth mechanism and heat transfer dynamics of ternary Cr-Ni-Fe alloy during triple containerless processing

The rapid solidification of ternary Cr49.5Ni40.5Fe10 alloy was realized by triple containerless processing techniques including free fall, falling quench and direct quench in order to systematically investigate its eutectic growth mechanism under different heat transfer conditions. Based upon Newtonian cooling model and classical heat transfer equation, the thermal information inside alloy droplets was obtained to derive the cooling rate, temperature difference and temperature gradient. After containerless solidification, the macroscopic segregation of primary δ phase was remarkably reduced owing to the microgravity effect and the spherically symmetrical temperature field within alloy droplets. Three typical eutectic structures were observed in the alloy droplets. With the increases of cooling rate, temperature difference and temperature gradient, the microstructures of alloy droplets evolved from coarse lamellar eutectic to refined anomalous eutectic and finally transformed to coarse anomalous eutectic because of the independent growth of δ and γ phases. Especially, under falling quench condition, a non-uniform eutectic microstructure appeared, which was refined gradually from the surface to the center of alloy droplets. This was attributed to the sudden changes of these three thermodynamic parameters once the alloy droplets were rapidly quenched into liquid coolant.

High-temperature wetting behavior of molten Ni-based superalloy on Y2O3 ceramics with different surface microstructures

To clarify the reaction between molten alloy and ceramic shell in the process of investment casting and control the metallurgical quality of alloy, high-temperature wetting experiments of Ni-based superalloy melt on Y 2 O 3 ceramics with different surface microstructures were carried out. The high-temperature wettability and interfacial interactions at 1873 K between Ni-based superalloy melt and Y 2 O 3 ceramics were studied in the present paper. The results showed that the apparent equilibrium contact angle of alloy melt on the substrate firstly decreases from 90.8° to 75.9° and then increases to 87.7°, with the surface roughness of Y 2 O 3 ceramic substrate changed from 0.859 μm to 2.253 μm. The contact conditions between melts and substrates are in accordance with the Wenzel equation, with the surface roughness of ceramic substrate varied between 0.859 μm and 1.311 μm. When that of the substrate continuously increased from 1.311 μm to 2.253 μm, the contact conditions between alloy melts and substrates were in accordance with the Cassie Baxter equation. Based on component analysis, it was found Al aggregated at the interface between Y 2 O 3 ceramics and alloy melts, resulting in the formation of Y 4 Al 2 O 9 and the change of surface tension of the alloy droplet in the process of high temperature wetting. Proper surface microstructure of the ceramic shell can ensure that the melt has enough filling ability and reduce the melt pollution caused by the reaction between melt and ceramic.

GaN Nanowire Growth Promoted by In-Ga-Au Alloy Catalyst with Emphasis on Agglomeration Temperature and In Composition.

The crystallographic orientation control of GaN nanowires (NWs) has been widely investigated by varying the V–III ratio. Here, we report the tuning of crystallographic orientation of GaN NWs by varying the composition of indium (In) in gallium–gold (Ga–Au) alloy catalyst using metal–organic chemical vapor deposition (MOCVD). The c-plane GaN thin film and sapphire substrate are used as growth templates. We found that the substrates of same orientation have a negligible influence on the orientation of the GaN NWs. The catalyst composition and the dimensions of alloy droplets determine the morphology of the NWs. The density of the NWs was controlled by tuning the droplet size of the alloy catalysts. With the constant V/III ratio, the crystallographic orientation of the GaN NWs was tuned from m- to c-axis by increasing the In composition inside alloy catalyst.

High thermal conductivity and electrical insulation of liquid alloy/ceramic/epoxy composites through the construction of mutually independent structures

The formation of a continuous network for thermal transport is crucial for obtaining materials with high thermal conductivity. Herein, we report a novel epoxy-based composite with unique independent structures of liquid alloy (LQA) droplets and high thermal conductivity ceramic fillers. The ceramic fillers only aggregated in the spaces between the LQA droplets due to the low compatibility of the ceramic fillers and the LQA droplets, so that the LQA droplets and the ceramic fillers eventually form independent structures. This creates a continuous network in epoxy that is advantageous for thermal transport. Thermal conductivity measurements showed that the thermal conductivity of the Epoxy/LQA/ceramic composites can reach 2.46 W m−1K−1, for an enhancement of 1055.87% relative to pure epoxy. More importantly, the independent structures endowed the composites with excellent electrical safety. The electrical resistivity of the Epoxy/LQA composites was 8.53 × 107 Ω cm, while the electrical resistivities of the three Epoxy/LQA/ceramic composites were 3.11 × 1011 Ω cm (Epoxy/LQA/Al2O3), 2.41 × 1011 Ω cm (Epoxy/LQA/Si3N4), 4.45 × 1011 Ω cm (Epoxy/LQA/h-BN), respectively. This work provides a new perspective on both the fundamental science and practical applications of thermal transport in polymer composites.

Statistical learning for evaluation of crystal growth in low-melting alloy droplets with application to quasicrystal-forming Ti–Zr–Ni alloys

The evaluation of the crystal growth in undercooled alloy melts is essential for investigations of their solidification behavior. Low-melting alloy systems usually exhibit little self-illumination and cinematography is not often feasible. A time-temperature data based statistical learning approach is presented to evaluate the crystal growth in an undercooled droplet, where a spatial geometry-informed, errors-in-variables probabilistic model is developed based on the prediction of the dendrite growth paths. The preferable growth mechanism and growth velocities can be evaluated from statistical model selection and parameter estimation. Based only on the pyrometry data of two Ti–Zr–Ni alloys, a bulk growth mechanism is justified and the growth velocities in both the stable C14 phase and metastable icosahedral phase are able to be measured and statistically interpreted.

Anisotropic Nd-Fe Ultrafine Particles with Stable and Metastable Phases Prepared by Induction Thermal Plasma

Both stable Th2Zn17-type and metastable TbCu7-type phases of anisotropic Nd-Fe ultrafine particles are prepared selectively by controlling the cooling rate using the low-oxygen induction thermal plasma (LO-ITP) process. The cooling rate is controlled by introducing the additional Ar gas at the end of the plasma flame. Moreover, numerical calculations based on a binary aerosol formation-growth model clarifies the alloying mechanism during rapid cooling wherein Nd-Fe vapor solidifies. The metastable Nd-Fe alloy ultrafine particle is formed in the cooling rate of 5×105 K/sec via the alloy droplet which is formed after the Fe and Nd vapors convert simultaneously into the liquid phase. The mean particle sizes of obtained ultrafine alloy powders without and with quenching are 76.3 ± 24.2 nm and 37.0 ± 12.5 nm, respectively. Electron microscopy observation reveals that the obtained particles are single crystalline. The Nd concentration of Nd-Fe particles shows a distribution in the range of 10.1–12.6 at%. X-ray diffraction and magnetic measurement demonstrate that the obtained particles are anisotropic and can be aligned by external magnetic field. Therefore, this process paves the way to achieve a new-generation anisotropic permanent magnet.

Motion and Solidification Behavior Analysis of Fe-Based Alloy Droplets During Gas Atomization

Gas atomization uses a high-speed gas flow to impact alloy melt and crush it into small droplets. During this process, the kinetic energy of the high-speed gas flow is transformed into the surface energy of small droplets, and the droplets are dragged and cooled by the gas. In this study, a FeNiCrBSiC alloy was used to investigate the motion and solidification behavior of alloy droplets during gas atomization. A mathematical model of the cooling and solidification process of droplets is modified and used. The initial velocity of the droplets attributed to the breakup by the high-speed gas flow is considered a factor. The results suggest that the velocity difference between the droplet and the gas is appearing to be smaller, indicating that droplets are more likely to collide with each other; as the velocity difference between the droplet and the gas becomes smaller, the heat transfer coefficient decreases. Thus, the solidification process of the droplet needs a longer flight distance; the relation between the cooling rate and the particle size was determined from measured data and empirical formula and model validation was performed.

Oxidation induced restructuring of Rh-Ga SCALMS model catalyst systems.

Supported catalytically active liquid metal solutions have been receiving increasing attention recently. We investigated the oxidation behavior of macroscopic Rh-Ga alloy droplets and Rh-Ga model catalyst nanoparticles supported on SiO2/Si(100) with low Rh content (<2.5 at. %) by x-ray photoelectron spectroscopy in ultra-high vacuum and under near-ambient pressure conditions using different photon energies and also using transmission electron microscopy. The experiments are accompanied by computational studies on the Ga oxide/Rh-Ga interface and Rh-Ga intermetallic compounds. For both Rh-Ga alloy droplets and Rh-Ga model catalyst nanoparticles, exposure to molecular oxygen leads to the formation of an oxide shell in which Rh is enriched. Transmission electron microscopy on the Rh-Ga nanoparticles confirms the formation of an ∼4 nm thick gallium oxide film containing Rh. Based on ab initio molecular dynamics and computational studies on the Ga2O3/Ga interface, it is concluded that Rh incorporation into the Ga2O3 film occurs by substituting octahedrally coordinated Ga.

Nanoscale Barrier Layers to Enable the Use of Gallium-Based Thermal Interface Materials with Aluminum

Performance of thermal interface materials (TIMs), such as thermal pastes and mats, hinders the advance of integrated circuit (IC) devices. Current state-of-the-art TIMs suffer from low thermal conductivity, thick cross sections, and poor long-term performance. Gallium (Ga) and gallium-based alloys and amalgamations, in liquid and solid form, have demonstrated up to three times greater thermal conductivity than conventional TIMs, but rapidly alloy with and destroy aluminum (Al) components, which are commonly found in IC devices. In this work, we investigate the use of thin-film barrier layers on Al to prevent Ga alloying and characterize their performance through accelerated Ga exposure experiments and scanning electron microscopy. It is found that 100-nm-thick layers of the common passivation materials niobium and 304 stainless steel do not sufficiently prohibit Ga migration, but a 100 nm layer of titanium (Ti) does. No alloying is evident in Ti-coated Al samples after exposure to a liquid Ga alloy droplet at 300 °C for 168 h, 250 thermal cycles from room temperature to 150 °C with 30-min dwell, or 50 thermal cycles from room temperature to 300 °C with 2-min dwell. The results present a clear and direct path to the use of Ga and Ga alloys as TIMs through the addition of a thin inexpensive barrier layer on Al components and may enable future IC device technologies.