Magnetorheological and Viscoelastic Behaviors in an Fe-Based Amorphous Magnetic Fluid.

A novel magnetic fluid was obtained using a colloidal dispersion of amorphous magnetic Fe-Ni-B nanoparticles into water. Its magnetorheological and viscoelastic behaviors were all investigated. Results showed that the generated particles were spherical amorphous particles 12-15 nm in diameter. The saturation magnetization of Fe-based amorphous magnetic particles could reach 49.3 emu/g. The amorphous magnetic fluid exhibited shear shinning behavior under magnetic fields and showed strong magnetic responsiveness. The yield stress increased with the rising magnetic field strength. A crossover phenomenon was observed from the modulus strain curves due to the phase transition under applied magnetic fields. The storage modulus G' was higher than the loss modulus G″ at low strains, while G' was lower than G″ at high strains. The crossover points shifted to higher strains with increasing magnetic field. Furthermore, G' decreased and fell off in a power law relationship when the strain exceeded a critical value. However, G″ showed a distinct maximum at a critical strain, and then decreased in a power law fashion. The magnetorheological and viscoelastic behaviors were found to be related to the structural formation and destruction in the magnetic fluids, which is a joint effect of magnetic fields and shear flows.

Uniform Deposition of Li-Metal Anodes Guided by 3D Current Collectors with In Situ Modification of the Lithiophilic Matrix.

The lithium (Li)-metal anode is deemed as the "holy gray" of the next-generation Li-metal system because of its high theoretical specific capacity, minimal energy density, and lowest standard electrode potential. Nevertheless, its commercial application has been limited by the large volume variation during charge and discharge, the unstable interface between the Li metal and electrolyte, and uneven deposition of Li. Herein, we present a 3D host (Cu) with lithiophilic matrix (CuO and SnO2) in situ modification via a facile ammonia oxidation method to serve as a current collector for the Li-metal anode. The 3D Cu host embellished by CuO and SnO2 is abbreviated as 3D CSCC. By increasing interfacial activity, lowering the nucleation barrier, and accommodating changes in volume of the Li metal, the 3D CSCC electrode effectively demonstrates a homogeneous and dendrite-free deposition morphology with an excellent cycling performance up to 3000 h at a 1.0 mA cm-2 current density. Additionally, the full cells paired with Li@3D CSCC anodes and LiCoO2 cathodes show good capacity retention performance at 0.2 C.

Nanoporous Composites of CoO x Quantum Dots and ZIF-Derived Carbon as High-Performance Anodes for Lithium-Ion Batteries.

Transition-metal oxides are attracting considerable attention as anodes for lithium-ion batteries because of their high reversible capacities. However, the drastic volume change and inferior electrical conductivity greatly retard their widespread applications in lithium-ion batteries. Herein, three-dimensional nanoporous composites of CoO x (CoO and Co3O4) quantum dots and zeolitic imidazolate framework-67-derived carbon are fabricated by a precipitation method. The carbon prepared by carbonization of zeolitic imidazolate framework-67 can greatly enhance the electrical conductivity of the composite anodes. CoO x quantum dots anchored firmly on zeolitic imidazolate framework-67-derived carbon can effectively inhibit the aggregation and volume change of CoO x quantum dots during lithiation/delithiation processes. The nanoporous structure can shorten the ion diffusion paths and maintain the structural integrity upon cycling. Meanwhile, kinetics analysis reveals that a capacitance mechanism dominates the lithium storage capacity, which can greatly enhance the electrochemical performance. The composite anodes show a high discharge capacity of 1873 mAh g-1 after 200 cycles at 200 mA g-1, ultralong cycle life (1246 mAh g-1 after 900 cycles at 1000 mA g-1), and improved rate performance. This work may provide guidelines for preparing cobalt oxide-based anodes for LIBs.

3D Hollow Porous Spherical Architecture Packed by Iron-Borate Amorphous Nanoparticles as High-Performance Anode for Lithium-Ion Batteries.

Three-dimensional hollow porous spherical architecture packed by iron-borate amorphous nanoparticles as an anode for lithium-ion batteries is first prepared through a simple method. The anode exhibits a high Coulombic efficiency and an ultralong cycle life under high rate, delivering outstanding reversible capacity of 1170 mAh g-1 after 360 cycles at 100 mA g-1 and 1160 mAh g-1 after 750 cycles at 200 mA g-1. The iron-borate anode has a prominent ultralong cycle life. The reversible capacity can still remain at about 600 mAh g-1 even after 3500 cycles at 2000 mA g-1, which maintains an outstanding capacity and delivers a much longer cycle life than that of the reported iron-based oxide anodes measured at same current density only within 1000 cycles. The hollow porous structure offers efficient electron-transport and Li+-diffusion paths and buffers the structural strains to alleviate excessive pulverization of the anode materials. Large specific surface area of the hollow porous structure increases the contact area between the anode and electrolyte, providing more reaction sites. More importantly, the amorphous characteristics of the iron-borate anode possess higher density of active sites and improved faster reaction kinetics. This work demonstrates that the hollow porous iron-borate particle anode allows mass production and is one of the most attractive anodes in energy-storage applications.

Magnetoviscous Property and Hyperthermia Effect of Amorphous Nanoparticle Aqueous Ferrofluids.

Magnetic Fe-B, Fe-Ni-B, and Co-B nanoparticles were successfully synthesized and introduced to water to prepare aqueous ferrofluids. The Fe-B, Fe-Ni-B, and Co-B particles are homogeneous amorphous nanoparticles with an average particle size 15 nm. The shape of the amorphous nanoparticles is regular. The Fe-B, Fe-Ni-B, and Co-B amorphous nanoparticles are superparamagnetic. Moreover, the saturation magnetizations of Fe-B and Fe-Ni-B amorphous nanoparticles are 75 emu/g and 51 emu/g. These are approximately 2.8 and 1.9-fold larger than Co-B nanoparticles, respectively. The viscosity of the amorphous ferrofluids has a strong response to external magnetic field. The yield stress increases with increasing magnetic field. The hyperthermia research of amorphous ferrofluids was firstly investigated. The experimental results indicate that the heating temperature of Fe-B ferrofluid and Fe-Ni-B ferrofluid could increase to 42 °C in 750 s and 960 s, respectively, when the output current is 300 A. The temperature could reach 61.6 °C for a Fe-B ferrofluid. The heating efficiencies of the amorphous ferrofluids demonstrate that the Fe-B ferrofluid and Fe-Ni-B ferrofluid may have great potential for biomedical applications.

Nanoporous Red Phosphorus on Reduced Graphene Oxide as Superior Anode for Sodium-Ion Batteries.

As a potential alternative to lithium-ion batteries, sodium-ion batteries (SIBs) have attracted more and more attention due to the lower cost of sodium than lithium. Red phosphorus (RP) is an especially promising anode for SIBs with the highest theoretical capacity of 2596 mAh g-1, which faces the challenges of large volume change and low conductivity. Herein, we develop a nanoporous RP on reduced graphene oxide (NPRP@RGO) as a high-performance anode for SIBs through boiling. Its nanoporous structure could accommodate the volume change and minimize the ion diffusion length, and the high electronic conductive network built on RGO sheets facilitates the fast electron and ion transportation. As a result, NPRP@RGO exhibits a superhigh capacity (1249.7 mAh gcomposite-1 after 150 cycles at 173.26 mA gcomposite-1), superior rate capability (656.9 mAh gcomposite-1 at 3465.28 mA gcomposite-1), and ultralong cycle life at 5.12 A gRP-1 for RP-based electrodes (775.3 mAh gRP-1 after 1500 cycles). The successful synthesis of NPRP@RGO marks a significant enhanced performance for RP-based SIB anodes, providing a scalable synthesis route for nanoporous structures.

Morphology- and Porosity-Tunable Synthesis of 3D Nanoporous SiGe Alloy as a High-Performance Lithium-Ion Battery Anode.

The lithium storage performance of silicon (Si) can be enhanced by being alloyed with germanium (Ge) because of its good electronic and ionic conductivity. Here, we synthesized a three-dimensional nanoporous (3D-NP) SiGe alloy as a high-performance lithium-ion battery (LIB) anode using a dealloying method with a ternary AlSiGe ribbon serving as the precursor. The morphology and porosity of the as-synthesized SiGe alloy can be controlled effectively by adjusting the sacrificial Al content of the precursor. With an Al content of 80%, the 3D-NP SiGe presents uniformly coral-like structure with continuous ligaments and hierarchical micropores and mesopores, which leads to a high reversible capacity of 1158 mA h g-1 after 150 cycles at a current density of 1000 mA g-1 with excellent rate capacity. The strategy might provide guidelines for nanostructure optimization and mass production of energy storage materials.

Metal-based magnetic fluids with core-shell structure FeB@SiO2 amorphous particles.

FeB@SiO2 amorphous particles were firstly introduced into Ga85.8In14.2 alloys to prepare metal-based magnetic fluids. The morphology of the FeB amorphous particles is spherical with an average particle size of about 190 nm. The shape of the particles is regular and the particle size is homogeneous. Stable core-shell structure SiO2 modified FeB amorphous particles are obtained and the thickness of the SiO2 coatings is observed to be about 40 nm. The results of VSM confirm that the saturation magnetization of the FeB amorphous particles is 131.5 emu g-1, which is almost two times higher than that of the Fe3O4 particles. The saturation magnetization of the FeB@SiO2 amorphous particles is 106.9 emu g-1, an approximate decrease of 18.7% due to the non-magnetic SiO2 coatings. The results from the torsional oscillation viscometer show that the metal-based magnetic fluids with FeB amorphous particles exhibit a desirable high temperature performance and are ideal candidates for high temperature use.

An equation describing diffusivity of liquid atoms by magnetic confinement.

In this work, we report an obvious low field-induced magnetic confinement effect on the diffusivity in binary metallic melts under a weak magnetic field. A quantitative description of this nontrivial dynamic behavior is given by a physical analytical model based on the Hall effect, which is in agreement with our experimental results. Meanwhile, a quadratic B dependence of the dynamic viscosity obtained in the same confined environment is observed. Our results show that one can effectively control the atomic diffusion process of metallic melts by the application of magnetic field. Meanwhile, this magnetic confinement effect at atomic scale should provide an important new ingredient to deeply understand the condensed matter physics under the external magnetic field.

Layering transition in confined silicon.

The structure of quasi-2D liquid silicon confined to slit nanopores has been investigated using molecular dynamics (MD) simulations. An obvious structural change from a low-density low-coordinated liquid to a high-density highly coordinated liquid has been found in the confined silicon with the increase of the slit size. This kind of structural transition results from layering in the confined silicon, which disappears with the increase of temperature. In the process of layering transition, the coordination distribution of quasi-2D liquid undergoes an evolutionary process from the initial non-uniform distribution to the final uniform distribution. In addition, our results also indicate that the increase of pressure will also induce a layering transition in the confined silicon.

Liquid-liquid phase transition and structure inheritance in carbon films.

Molecular dynamics simulations are performed to study the cooling process of quasi-2D liquid carbon. Our results show an obvious liquid-liquid phase transition (LLPT) from the twofold coordinated liquid to the threefold coordinated liquid with the decrease of temperature, followed by a liquid-solid phase transition (LSPT). The LLPT can be regarded as the preparation stage of LSPT. During the cooling process, the chain structures firstly self-assemble into some ring structures and then aggregate into some stable islands which can further connect together to form a complete polycrystalline film. The threefold coordinated structures play an important role in the formation of atomic rings. The inheritance of the threefold coordinated structures provides essential condition to form rings and islands.

Atomic insight into copper nanostructures nucleation on bending graphene.

Some findings in heterogeneous nucleation that the structural features of a growing crystal are usually inherited from the heterogeneous nucleus, although attracting more and more attention, are not yet well understood. Here we report numerical simulations of copper nucleation on bending graphene (BG) to explore the microscopic details of how the curved surface influences the freezing structure of the liquid metal. The simulation result clearly shows that copper atoms become layered at the solid-liquid interface in a "C"-shaped pattern resembling the BG. This kind of shape control decays with increasing distance from the wall and the outmost layers transform into twin crystal composed of two fcc wedges. It is found that the final structures have striking correlations with the curvature radius, central angle and arc length of the BG. Our study would provide an opportunity for comprehensive and satisfactory understanding of the heterogeneous nucleation on curved surfaces.

The complex structure of liquid Cu(6)Sn(5) alloy.

By applying ab initio molecular dynamics simulation to liquid Cu(6)Sn(5) alloy, the hetero-coordination tendency is discovered by Bathia-Thornton partial correlation functions and a chemical short-range parameter. However the local structural environment of Sn in l-Cu(6)Sn(5) alloy resembles that of liquid Sn by Voronoi analysis. A new feature, i.e. a subpeak in between the first and second peaks, is discovered by the present method which implies that topologically disordered ß-Sn-type structural units may exist in l-Cu(6)Sn(5) alloy. The local density states of electrons show that both Cu-Sn and Sn-Sn bonding exist in l-Cu(6)Sn(5) alloy. This work suggests that chemical short-range order between unlike atoms and self-coordination between Sn atoms coexists in l-Cu(6)Sn(5) alloy.

Thermodynamic basis for cluster kinetics: Prediction of the fragility of marginal metallic glass-forming liquids.

Due to the inaccessibility of the supercooled region of marginal metallic glasses (MMGs) within the experimental time window, we study the cluster kinetics above the liquidus temperature, Tl, to acquire information on the fragility of the MMG systems. The thermodynamic basis for the stability of locally ordered structure in the MMG liquids is discussed in terms of the two-order-parameter model. It is found that the Arrhenius activation energy of clusters, Deltah, is proportional to the chemical mixing enthalpy of alloys, DeltaH(chem). Fragility of the MMG forming liquids can be described by the ratio of the absolute DeltaH(chem) value to the glass transition temperature, Tg. The manner of vitrification during rapid solidification is an important factor for the discrepancy between the data presented in this paper and the prediction of the two-order-parameter model concerning the relation between Delta h and the liquid fragility.

Structure change of liquid GaSb under pressure: an ab initio molecular-dynamics simulation.

We have performed ab initio molecular-dynamics simulation of liquid GaSb (l-GaSb) up to 20.0 GPa. The calculated structure factors are consistent with the recent experimental results, and the partial structure parameters show that the structure of l-GaSb under pressure contracts nonuniformly. In the whole calculated pressure region, the contraction of l-GaSb can be divided into three substages: 1.8-5.4, 5.4-10.0, and 10.0-20.0 GPa. It is further confirmed by analyzing the bond-angle distributions of Ga-Ga-Ga and Sb-Sb-Sb that the rearrangement of Sb atoms under pressure plays a crucial role in the structure change of l-GaSb.

Structure and vibrations of Ga(n)N(n) (n = 3-10) clusters.

The structure and harmonic vibrations of Ga(n)N(n) (n = 3-10) clusters have been investigated using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out of plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1029 cm(-1) for n = 10. Comparisons with C2n, B(n)N(n), and Al(n)N(n) clusters, the structure and bonding type for the Ga(n)N(n) (n=3-10) clusters are consistent with those of the C2n (n = 3, 5, 7, ...) clusters, the B(n)N(n) (n = 3-10), and Al(n)N(n) (n = 3-9) clusters.

Theoretical study of aluminum arsenide clusters: equilibrium geometries and electronic structures of Al(n)As(n) (n=1-4).

The geometry, electronic configurations, harmonic vibrational frequencies and stability of the structural isomers of Al(n)As(n) clusters (n=1-4) have been investigated using density functional theory. For dimers and trimers, the lowest energy structures are planar cumulenic rings (IIs, VIs) with D(nh) symmetry. The caged structure with T(d) symmetry (IXs) lie lowest in energy among the tetramers. The AlAs bond dominates the structures for many isomers so that one preferred dissociation channel is loss of the AlAs monomer. The atomic charges, hybridization and chemical bonding in the different structures are also discussed. Comparisons with valence-isoelectronic Si(2n), Al(n)P(n) and Ga(n)As(n) clusters of same size, the properties of the aluminum arsenide clusters are analogous to those of their corresponding Al(n)P(n), Si(2n) counterparts. The results can explain the modification and refinement of Si phase in AlSi alloy in the molecular level.

Structure and vibrations of AlnNn (n = 3-9) clusters.

The structure and harmonic vibrations of Al(n)N(n) (n = 3-9) clusters have been investigated using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out-of-plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1217 cm(-1) for n = 9. Comparisons with C2n clusters and B(n)N(n) clusters, the structure and bonding type for the Al(n)N(n) clusters are consistent with those of the C2n (n = 3, 5, 7, ...) clusters and the B(n)N(n) clusters.

Thermodynamics and dynamics of metallic glass formers: their correlation for the investigation on potential energy landscape.

Great progress has been made in basic features of the potential energy landscape (PEL) theoretically. The present work, however, attempts to cast new light on it from experimental aspects. By a survey of experimental data related to thermodynamics or dynamics of metallic glass-forming liquids, it is found that the increased rate of excitation of vibrational entropy at glass transition tends to increase the rate of generation of configurational part. Although for the type of metallic materials a generally positive relationship exists between the density of the energy minima at glass transition and the liquid fragility strength, just as expected, our main attention is paid to the phenomenon of the scattering of the slopes. Analysis shows that the phenomenon results from the different average height of energy barriers between minima near glass transition. Investigation on the PdNiP metallic system indicates that the mismatch entropy is a dominant factor in the barrier height: a large value of it results in low energy barriers. Our previous work on the AlNiCe system gives the support to this finding.

Electronic structure and stability of AlnPn (n = 2-4) clusters.

The geometry, electronic configurations, harmonic vibrational frequencies, and stability of the structural isomers of aluminum phosphide clusters have been investigated using the density functional theory. For dimers and trimers, the lowest energy structures are cyclic (IIs, IIIs) with D(nh) symmetry. The caged structure with Td symmetry (Xs) lie lowest in energy among the tetramers. The Al--P bond dominates the structures for many isomers so that one preferred dissociation channel is loss of the AlP monomer. The hybridization and chemical bonding in the different structures are also discussed. Comparisons with silicon and boron nitride clusters, the ground state structures of Al(n)P(n) clusters are analogous to those of their corresponding Si(2n) counterparts. This similarity follows the isoelectronic principle.