Formation of immiscible alloy powders with egg-type microstructure

Understanding and Harnessing Nanoscale Immiscibility in Ru-In Alloys for Selective CO2 Hydrogenation

Bimetallic alloys made from immiscible elements are characterized by their tendency to segregate on the macroscopic scale, but their behavior is known to change at the nanoscale. Here, we demonstrate that in the Ru-In system, In atoms preferentially decorate the surface of 6 nm Ru nanoparticles, forming Ru-In superficial immiscible alloys. This surface decoration dramatically affects the catalytic performance of the system, even at small atomic fractions of In added to Ru. The interfaces between Ru and In enabled unexplored methanol productivity from CO2 hydrogenation, which outperformed not only the individual constituents but also ordered RuIn3 intermetallic alloys. Our work highlights that the formation of superficial immiscible alloys could offer new insights into the understanding and design of heterogeneous catalysts.

Effect of Fe on the Hydrogen Production Properties of Al-Bi-Sn Composite Powders

Fe additives may play an important role in the preparation of aluminum-based hydrolysis hydrogen powder, with high hydrogen yield, low cost, and good oxidation resistance. Therefore, it is necessary to ascertain the effect of Fe on the hydrogen production performance of Al-Bi-Sn composite powders. According to the calculated vertical cross-section of the Al-10Bi-7Sn-(0~6)Fe (wt.%) quasi-binary system, Al-10Bi-7Sn-xFe (x = 0, 0.5, 1.5, 3) wt.% composite powders for hydrogen production were prepared by the gas-atomization method. The results showed that the Al-10Bi-7Sn-1.5Fe (wt.%) powder exhibited an extremely fast hydrogen generation rate at 50 °C, which reached 1105 mL·g^-1 in 27 min in distilled water, 1086 mL·g^-1 in 15 min in 0.1 mol·L^-1 NaCl solution, and 1086 mL·g^-1 in 15 min in 0.1 mol·L^-1 CaCl₂ solution. In addition, the antioxidant properties of these powders were also investigated. The results showed that the hydrogen production performance of the Al-10Bi-7Sn-1.5Fe (wt.%) powder could retain 91% of its hydrogen production activity, even though the powder was exposed to 25 °C and 60 RH% for 72 h. The addition of Fe not only promoted the hydrogen generation rate of the Al-Bi-Sn composite powders, but also improved their oxidation resistance. The Al-10Bi-7Sn-1.5Fe (wt.%) composite powder shows great potential for mobile hydrogen source scenarios with rapid hydrogen production.

Pore elimination mechanisms during 3D printing of metals

Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.

3D printing pore-free complex metal parts remains a challenge. Here, the authors combine in-situ imaging and simulations to show thermocapillary force can eliminate pores from the melt pool during a laser powder bed fusion process.

Evidence for Fe-Si-O liquid immiscibility at deep Earth pressures

Seismic observations suggest that the uppermost region of Earth's liquid outer core is buoyant, with slower velocities than the bulk outer core. One possible mechanism for the formation of a stably stratified layer is immiscibility in molten iron alloy systems, which has yet to be demonstrated at core pressures. We find immiscibility between liquid Fe-Si and Fe-Si-O persisting to at least 140 GPa through a combination of laser-heated diamond-anvil cell experiments and first-principles molecular dynamics simulations. High-pressure immiscibility in the Fe-Si-O system may explain a stratified layer atop the outer core, complicate differentiation and evolution of the deep Earth, and affect the structure and intensity of Earth's magnetic field. Our results support silicon and oxygen as coexisting light elements in the core and suggest that [Formula: see text] does not crystallize out of molten Fe-Si-O at the core-mantle boundary.

Liquid phase separation in undercooled Cu-Co alloys under the influence of static magnetic fields

Undercooling of Cu-based alloys often induces metastable liquid phase separation followed by rapid solidification of separated liquids. The rapid solidification can help freeze in the morphology of a higher-melting liquid and eases difficulties in studies of liquid phase separation kinetics. In the present work, the influence of static magnetic fields on liquid phase separation in bulk Cu₈₄Co₁₆ composition was investigated. Inductively melted samples were glass-fluxed, undercooled and solidified under uniform and non-uniform magnetic fields generated by a superconducting magnet. Solidification microstructure of the phase-separated samples was examined using an optical microscope. The imposition of the magnetic fields, both uniform and non-uniform, altered the morphology, segregation pattern and size distribution of Co-rich droplets due to liquid phase separation. The imposition of the non-uniform magnetic fields with positive and negative gradients brought about segregation of the Co-rich droplets at the top and the bottom side of the samples, respectively. Such influence of the static magnetic fields is interpreted by assuming intensification of convective flow and Kelvin force-controlled migration of the Co-rich droplets. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Liquid-liquid hierarchical separation and metal recycling of waste printed circuit boards

Waste electrical and electronic equipment is rapidly increasing worldwide, resulting in a large quantity of waste printed circuit boards (WPCBs). There is a great challenge on how to efficiently separate mixed metals in WPCBs, which consists of more than 10 elements including hazardous Cr, Pb and Cd. In this work, based on atomic interactions, a method of liquid-liquid hierarchical separation is developed to separate the mixed metals dissociated from the pyrolyzed WPCBs of mobile phones. The hierarchical separation of L→L_Fe-rich + L_Cu,Pb-rich, L_Cu,Pb-rich→L_Cu-rich + L_Pb-rich and L_Pb-rich→S_{Cu-dendritical} + L^'_Pb-rich produces four immiscible Fe-rich, Cu-rich, Cu-dendritical and Pb-rich substances. The separation rate between these substances can reach more than 96% in a super-gravity field of G = 1000g. Other metals selectively distribute in the four substances. The Fe-rich substance collects Cr, Co, Ni and Si. Almost all of Au and Ag are trapped in the Cu-rich and Cu-dendritical substances. The low-melting-point metals, i.e. Bi, Cd, In and Sn, are located in the Pb-rich substance. This work provides a green shortcut for efficiently separating and recycling overall metals in WPCBs.

Size-Dependent Phase Transformation during Gas Atomization Process of Cu⁻Sn Alloy Powders

For binary element atomization, it is essential to investigate the phase transformation from liquid to solid as a functions of the droplet sizes, as well as the reaction competitiveness, during gas atomizing solidification of their nuclei. In the present work, a series of phase transformations of undercooled Cu (60.9 wt.%)/Sn droplets were analyzed when atomized by pressure gas. The results indicated that the microstructures of the obtained powders and their morphologies were highly relevant to the droplet size. According to the phase characteristics analyzed by the microstructural observations in combination with the transient nucleation theory, powders with sizes from 10 to 100 μm were divided into three categories, exhibiting lotus-leaf, island, and stripe morphologies. The competitive formation of Cu₆Sn₅ or Cu₃Sn was also controlled by the droplet sizes, and a diameter of approximately 45 μm was identified as the threshold size. After heat treatment at 300 °C for 4 h, the powders consisted of a single η’ Cu₆Sn₅ phase. The obtained Cu₆Sn₅ phase powders can be used in the field of high-temperature applications as intermetallic balls for integrated chip interconnects.

Molecular dynamics study on the formation of self-organized core/shell structures in the Pb alloy at the nanoscale

An abnormal self-organized core/shell structure is formed in the liquid Al–Pb alloy, which can be controlled by confined conditions.

Coalescence of Immiscible Liquid Metal Drop on Graphene

Molecular dynamics simulations were performed to investigate the wetting and coalescence of liquid Al and Pb drops on four carbon-based substrates. We highlight the importance of the microstructure and surface topography of substrates in the coalescence process. Our results show that the effect of substrate on coalescence is achieved by changing the wettability of the Pb metal. Additionally, we determine the critical distance between nonadjacent Al and Pb films required for coalescence. These findings improve our understanding of the coalescence of immiscible liquid metals at the atomistic level.

Three-Dimensional Self-Assembly of Core/Shell-Like Nanostructures for High-Performance Nanocomposite Permanent Magnets

Core/shell nanostructures are fascinating for many advanced applications including strong permanent magnets, magnetic recording, and biotechnology. They are generally achieved via chemical approaches, but these techniques limit them to nanoparticles. Here, we describe a three-dimensional (3D) self-assembly of core/shell-like nanocomposite magnets, with hard-magnetic Nd2Fe14B core of ∼45 nm and soft-magnetic α-Fe shell of ∼13 nm, through a physical route. The resulting Nd2Fe14B/α-Fe core/shell-like nanostructure allows both large remanent magnetization and high coercivity, leading to a record-high energy product of 25 MGOe which reaches the theoretical limit for isotropic Nd2Fe14B/α-Fe nanocomposite magnets. Our approach is based on a sequential growth of the core and shell nanocrystals in an alloy melt. These results make an important step toward fabricating core/shell-like nanostructure in 3D materials.

A Novel Self-Assembling Al-based Composite Powder with High Hydrogen Generation Efficiency

In this study, a novel self-assembling hydrogen generation powder comprised of 80Al-10Bi-10Sn wt.% was prepared using the gas atomization method and then collected in an air environment. The morphological and hydrolysis properties of the powders were investigated. The results indicated that the powders formed unique core/shell microstructures with cracked surfaces and (Bi, Sn)-rich phases distributed on the Al grain boundaries. The powders exhibited good oxidation resistance and reacted violently with distilled water at temperatures as low as 0 °C. Furthermore, at 30 °C, the powders exhibited a hydrogen conversion yield of 91.30% within 16 minutes. The hydrogen produced by this powder could be directly used in proton exchange membrane fuel cells. The mechanisms of the hydrolysis reactions were also analyzed.

Rapid control of phase growth by nanoparticles

Effective control of phase growth under harsh conditions (such as high temperature, highly conductive liquids or high growth rate), where surfactants are unstable or ineffective, is still a long-standing challenge. Here we show a general approach for rapid control of diffusional growth through nanoparticle self-assembly on the fast-growing phase during cooling. After phase nucleation, the nanoparticles spontaneously assemble, within a few milliseconds, as a thin coating on the growing phase to block/limit diffusion, resulting in a uniformly dispersed phase orders of magnitude smaller than samples without nanoparticles. The effectiveness of this approach is demonstrated in both inorganic (immiscible alloy and eutectic alloy) and organic materials. Our approach overcomes the microstructure refinement limit set by the fast phase growth during cooling and breaks the inherent limitations of surfactants for growth control. Considering the growing availability of numerous types and sizes of nanoparticles, the nanoparticle-enabled growth control will find broad applications.

Refining microstructure is an important goal in many material systems. Here, the authors report an approach for microstructure refinement based on nanoparticle self-assembling on a growing phase, which is shown to be effective for both metallic and organic systems.

Magnetic field driving gradient effects on the microstructure in amorphous-nanocrystalline cobalt alloy ribbons

Field effects on the early crystallization of Co-rich amorphous ribbons (Co(68.15)Fe(4.35)Si(12.5)B(15), atomic%) performed at 450 °C for 30 minutes in an applied 10 Oe longitudinal or transverse external magnetic field are investigated by comparing with the as-spun and annealed samples in zero applied magnetic field. Results indicate that the crystallization on the surface skin of the ribbon differs from that of the middle section due to the combined effects from the field direction and the defects or stress characteristics across the ribbons. In particular, the ribbons annealed under a transverse field exhibit an extremely distinct graded microstructure across the ribbon, which features amorphous-phased skin layers (top and bottom) of about 4-5 μm thickness and a middle amorphous-nanocrystalline composite layer about 10-12 μm thick, with the nanograins of 434 ± 99 nm dispersing in the amorphous matrix.

On the Temperature Gradient Induced Interfacial Gradient Force, Acting on Precipitated Liquid Droplets in Monotectic Liquid Alloys

The final morphology of liquid metallic emulsions, produced in a temperature gradient, depends on the interfacial gradient force acting on small metallic droplets. This force is proportional to the temperature coefficient of the interfacial energy between the two immiscible liquid phases. In the present paper first a widely used equation of Young, Goldstein and Block for the steady-state velocity of liquid droplets under the influence of the temperature gradient is discussed. Then a new equation is proposed for the temperature dependence of the interfacial energy.