Random Packing Of Atoms Research Articles

Copper Nanocomposites In Situ Formed from Metallic Glasses for an Efficient Catalytic Performance.

Metallic glasses (MGs) with the unique long-range disordered and short-range ordered atomic structure have attracted extensive attention in the field of environmental catalysis due to their advanced catalytic capability. Herein, CuZr-based MGs are first proven to exhibit superior catalytic performance toward the degradation of organic pollutants compared to the annealed ribbons with different crystal structures; many Cu nanocomposites are gradually in situ precipitated on the surface of the ribbons. The enhanced catalytic behavior is mainly attributed to the random atomic packing structure accelerating electron transport and providing sufficient active sites. On the other hand, the active species, for example, ·OH, ·O2-, and Cu(III), are generated through an activation reaction between Cu/Cu2O nanocomposites and H2O2 molecules for the catalytic degradation process. Additionally, further investigation indicated that CuZr-based MGs also present superior stability and durability along with an approximate 96% degradation efficiency within 10 min at the 10th run. This research can successfully explain why MGs have a little higher catalytic reactivity than their crystalline counterparts, and more importantly, it will provide a new strategy for the preparation of catalytic materials for wastewater treatment.

Compelling Rejuvenated Catalytic Performance in Metallic Glasses.

Metallic glasses (MGs) with the metastable nature and random atomic packing structure have attracted large attention in the catalytic family due to their superior catalytic performance. In contrast, their crystalline counterparts are restricted by the highly ordered packing structure, fewer surface active sites, and crystallographic defects for catalytic activity. The uncertainty of the different catalytic mechanisms and the intrinsic characteristics correlated to MGs and their crystalline counterparts become a major impediment to promote their catalytic efficiencies and widespread applications. Herein, it is reported that the excellent catalytic behavior in Fe-based MGs goes through a detrimental effect with the partial crystallization, but receives a compelling rejuvenation in the full crystallization. Further investigation reveals that multiphase intermetallics with electric potential differences in fully crystallized alloys facilitate the formation of galvanic cells. More importantly, extensively reduced grain boundaries due to grain growth greatly weaken electron trapping and promote inner electron transportation. The relatively homogenous grain-boundary corrosion in the intermetallics contributes to well-separated phases after reaction, leading to refreshment of the surface active sites, thereby quickly activating hydrogen peroxide and rapidly degrading organic pollutants. The exploration of catalytic mechanisms in the crystalline counterparts of MGs provides significant insights into revolutionize novel catalysts.

Characterization of atomic-level structure in Fe-based amorphous and nanocrystalline alloy by experimental and modeling methods

The atomic structure of Fe70Nb10B20 alloy in “as-cast” state and after annealing was investigated using high-energy X-ray diffraction (XRD), Mössbauer spectroscopy (MS) and high resolution transmission electron microscopy (HRTEM). The HRTEM observations allowed to indicate some medium-range order (MRO) regions about 2nm in size and formation of some kinds of short-range order (SRO) structures represented by atomic clusters with diameter ca. 0.5nm. The Reverse Monte Carlo (RMC) method basing on the results of XRD measurements was used in modeling the atomic structure of Fe-based alloy. The structural model was described by peak values of partial pair correlation functions and coordination numbers determined by Mössbauer spectroscopy investigations. The three-dimensional configuration box of atoms was obtained from the RMC simulation and the representative Fe-centered clusters were taken from the calculated structure. According to the Gonser et al. approach, the measured spectra of alloy studied were decomposed into 5 subspectra representing average Fe–Fe coordination numbers. Basing on the results of disaccommodation of magnetic permeability, which is sensitive to the short order of the random packing of atoms, it was stated that an occurrence of free volume is not detected after nanocrystallization process.

Nonlinear tensile deformation behavior of melt-extracted Co69.5Fe4.5Cr1Si8B17 amorphous wires

Metallic glasses often show excellent mechanical and physical properties due to their random atomic packing. But as a new kind of potential engineering material, metallic glasses have challenged the traditional processing methods. In this letter, continuous Co69.5Fe4.5Cr1Si8B17 metallic glassy wires at micrometer scales with a smooth surface and negligible fluctuation in diameter were prepared by the melt-extraction method. Their mechanical properties were evaluated by carrying out tensile tests. The Co69.5Fe4.5Cr1Si8B17 metallic glassy wires show high tensile fracture strength of the order of 2.8GPa and perform nonlinear deformation behavior prior to fracture. The tensile failure of the glassy wires was controlled by both the normal stress and shear stress.

Micro forming and deformation behaviors of Zr 50.5Cu 27.45Ni 13.05Al 9 amorphous wires

Metallic glasses often show excellent mechanical and physical properties due to their random atomic packing. But as a new kind of potential engineering material, metallic glasses have challenged the traditional processing methods. In this paper, the Zr 50.5Cu 27.45Ni 13.05Al 9 metallic glassy wires at micro scales with a smooth surface and negligible fluctuation in diameter were prepared by the melt-extraction method, and the challenges associated with fabrication of metallic glassy wires were critically assessed. Their mechanical properties were evaluated by carrying out tensile and bending tests, and their strength reliability was estimated by using Weibull analysis. The tensile failure of the wires was controlled by both the normal stress and shear stress. A model of the shear bands evolution was developed based on the bending test.

Simulation of silicon nanoparticles stabilized by hydrogen at high temperatures

The stability of different silicon nanoparticles are investigated at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a “coat” from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen “coat” has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si60 cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si60 cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen “coat” and containing 60 hydrogen atoms in the interior space has a higher stability. Such cluster has smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270-280 K.

Molecular dynamics study of hydrogenated silicon clusters at high temperatures

This paper reports on a study of the stability of silicon clusters of intermediate size at a high temperature. The temperature dependence of the physicochemical properties of 60- and 73-atom silicon nanoparticles are investigated using the molecular dynamics method. The 73-atom particles have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene. They are surrounded by a ‘coat’ from 60 atoms of hydrogen. The nanoassembled particle at the presence of a hydrogen ‘coat’ has the most stable number (close to four) of Si–Si bonds per atom. The structure and kinetic properties of a hollow single-layer fullerene-structured Si60 cluster are considered in the temperature range 10 K ≤ T ≤ 1760 K. Five series of calculations are conducted, with a simulation of several media inside and outside the Si60 cluster, specifically, the vacuum and interior spaces filled with 30 and 60 hydrogen atoms with and without the exterior hydrogen environment of 60 atoms. Fullerene surrounded by a hydrogen ‘coat’ and containing 60 hydrogen atoms in the interior space has a higher stability. Such clusters have smaller self-diffusion coefficients at high temperatures. The fullerene stabilized with hydrogen is stable to the formation of linear atomic chains up to the temperatures 270–280 K.

Computer investigation of the structure of Si73 clusters surrounded by hydrogen

The stabilization of the structure of Si73 clusters that are surrounded by 60 hydrogen atoms and subjected to seventeenfold stepwise heating from 35 to 1560 K (in steps of ∼90 K) is investigated using the molecular dynamics method. The analysis is performed for clusters of three types, i.e., a particle assembled from an icosahedron and a fullerene, a nanocrystal, and a particle with a random atomic packing. In all cases, an increase in the temperature in the course of heating is accompanied by evaporation of a Si atom from the clusters and an increase in the size of silicon particles. The temperature of detachment of Si atoms from clusters is lowest for the cluster with a random atomic packing and highest for the nanocrystal. The nanoassembled particle has the most stable number (close to four) of Si-Si bonds per atom over the entire temperature range 35 ≤ T ≤ 1560 K. For each type of Si73 clusters, the mean length of the Si-Si bond decreases with an increase in the temperature. According to the radial distribution functions, the Si73 clusters have different structures even at the temperature T = 1560 K. The distributions of bond angles reflect the presence of fourfold symmetry elements in the nanoassembled cluster and the nanocrystal. The relative depth of “penetration” of hydrogen atoms into the cluster is largest for the nanocrystal and smallest for the nanoassembled nanoparticle. The largest number of hydrogen atoms is “adsorbed” on the particle with a random atomic packing.

Molecular dynamics simulation of the physicochemical properties of silicon nanoparticles containing 73 atoms

The physicochemical properties of 73-atom silicon nanoparticles that have a crystal structure, a random atomic packing, and a packing formed by inserting a 13-atom icosahedron into a 60-atom fullerene are investigated using the molecular dynamics method. Analysis of the behavior of the internal energy, the radial distribution function, the distribution of bond angles, and the specific heat at a constant pressure C p in the temperature range 10–1710 K indicates that a crystalline nanoparticle undergoes melting at a temperature of 710 K and that the structural transformations occurring in particles with an irregular atomic packing exhibit specific features. It is demonstrated that the temperature dependence of the self-diffusion coefficient follows a linear behavior. Local deviations from the linear behavior are most pronounced for the crystalline nanoparticle.

Magnetic properties of amorphous Fe–Cr–B nanoparticles embedded in an alumina matrix

Amorphous nanoparticles of Fe 80− x Cr x B 20 (5⩽ x⩽25) embedded in an alumina matrix have been studied. The samples have been prepared by the cosputtering technique and by using a mixed target consisting of amorphous ribbons and alumina. Several area ratios have been used leading to different average diameters of particles. The samples have been characterized by means of an electron microprobe and transmission electron microscopy with ETX analysis. Thermal variation of the magnetic properties have been determined mainly from zero-field-cooled magnetization, magnetization versus applied field, AC susceptibility and Mössbauer spectroscopy measurements. The average diameters of the particles obtained range between 1.9 and 5.3 nm. The same kind of random packing of atoms occurs in particles as in bulk (ribbons) though the preparation process and the size scale are different. The hyperfine parameters deduced from Mössbauer spectroscopy are similar to those obtained from the ribbons, but show a size dependence which is only appreciable for the smallest particles. The magnetic structure and the magnetic properties are similar to those of the corresponding ribbons except for x above 20–25 where a canted spin structure probably appears, favoured by the magnetic defects existing in the particle surface. The dynamical properties of particles are more intricate. Interparticle interactions are present, but the model [Dormann et al., Adv. Chem. Phys. 98 (1997) 283] accounting for the dipolar interactions is not sufficient for explaining the properties. It is suggested that, due to the magnetic disorder coming from the Cr substitution, the effective anisotropy acting on the reversal process of the magnetic moments of particles is temperature dependent. A good agreement is obtained for the variation of the blocking temperature with the measuring time.

Structure of Liquid Aluminum Oxide

The total structure factor, S(Q), and the corresponding radial distribution function, G(r), for supercooled and stable liquid aluminum oxide have been measured with x-ray synchrotron radiation. The specimens were levitated in a conical nozzle and melted with a laser, achieving temperatures in the range of 2200{endash}2700K. The first two peaks in S(Q) reveal intermediate-range order and dense random packing of atoms, similar to that observed in many network liquids. The first two peaks in G(r) are consistent with AlO{sub 4}{sup 5-} structural units and show that Al{sub 2}O{sub 3} undergoes a major structural rearrangement on melting with an Al coordination change from octahedral to tetrahedral. The structure does not change appreciably with temperature in the stable and supercooled liquid. Implications of these results are discussed in connection with the solidification of aluminum oxide. {copyright} {ital 1997} {ital The American Physical Society}

Structural and electronic properties of the liquid polyvalent elements. IV. The pentavalent semimetals and trends across the periodic table.

We present Iab initioP calculations of the atomic structure, the electronic density of states, and the photoemission intensities of the molten pentavalent semimetals As, Sb, and Bi. The investigations are based on pseudopotential-based interatomic forces, molecular-dynamics simulations for the liquid structure, and self-consistent linear-muffin-tin-orbital calculations of the electronic structure and photoemission intensities. We show that the anomalous structures of liquid As, Sb, and Bi arise from the modulation of the random packing of atoms by the Friedel oscillations in the effective interatomic potential. These modulations are damped by relativistic effects, which also lead to the formation of an s-p gap in the electronic density of states. The position and width of the gap are also shown to be related to the liquid structure. Trends in the atomic and electronic structures of the polyvalent elements across the Periodic Table are discussed.

Nuclear quadrupole interaction in glassy Hf1-xCux alloys and their local structures.

The electric field gradient (EFG) at Hf sites in amorphous ${\mathrm{Hf}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cu}}_{\mathrm{x}}$ alloys was measured for x=0.33, 0.44, 0.50, and 0.59 with use of the time-differential perturbed angular correlation technique. The results show no evidence of crystallinelike local order in these alloys, supporting a description of their local structure in terms of a dense random packing of atoms. The local and lattice contributions to the EFG, as well as their dependence on composition, are discussed.

Study on the Short-Range Order Structures of Fe-Ge Amorphous Thin Films by EXAFS

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Structure of liquid arsenic: Peierls distortion versus Friedel modulation.

A molecular-dynamics simulation of liquid arsenic shows good agreement with recent neutron-diffraction data. The coordination number and the average bond length and bond angle are very close to their values in rhombohedral As. This shows that the short-range order in liquid As is very close to that in the crystal. The experiment has been interpreted as representing the first evidence for the existence of a Peierls distortion in a liquid. Our results suggest that the structure could also be interpreted as arising from a modulation of the random packing of atoms by the Friedel oscillations in the effective interatomic interactions.

Time-differential perturbed-angular-correlation study of the electric field gradient in amorphous Zr68Hf

The electric field gradient at Zr sites has been measured by time-differential perturbed angular correlations in amorphous and crystalline ${\mathrm{Zr}}_{68}$${\mathrm{Hf}}_{2}$${\mathrm{Cu}}_{30}$ alloys. The results support a dense random packing of atoms and discard any kind of regular atomic arrangements in the amorphous alloy.

Structural model of amorphous As 2S 3

The structural model for the chalcogenide glass has been constructed by using a three-step procedure, which consists of the random packing of atoms, the structural relaxation and the reformation of bonds. At the final stage of this procedure, every atom is located at a position of minimum local energy. The radial distribution function calculated for the resulting model shows fairly good agreement with the observed one. It is shown that the amorphous As 2S 3 system consists of two-dimensional networks which contain 4 or 5 interconnected closed chains.

Similarities in amorphous and crystalline transition metal–metalloid alloy structures

In many amorphous and glassy systems the local structure seems to be essentially the same in amorphous and crystalline phases. It thus becomes possible to define a ‘local structural unit’, such as the SiO4 group in silicates. Determination of the structure then reduces to the problem of describing how the local units are connected and packed in the medium-range structure. There is mounting evidence that alloys of transition metals with metalloids are more adequately described by arrangements of stereo-chemically-defined structural units than by alternative descriptions in terms of random packing of atoms. There are three phases, Fe3C, Ti3P and Fe3P, which may serve as models of local structural units. These lattices differ to a small degree in the symmetry of the metalloid nearest-neighbour shell but there are major differences in the arrangement of local structural units. Although the experimental evidence is limited, it seems that the distinctive features of the local and medium-range topologies of the crystalline phases are represented in the structures of each of the corresponding glasses.

Structure of Be films quenched from the vapour

The crystallite boundary mismatch in crystalline films leads to tensile stresses which act on the substrate. Due to these stresses the substrate is bended. In the dense random packing of atoms in an amorphous film local tensile- and compressive stresses compensate and thus the films seem to be free of stresses with regard to the substrate. From this point of view the measurement of mechanical stresses produced by vapour quenched films can be used for structural analysis. Thus an apparatus is described which allows in situ measurements of mechanical stresses and electrical resistivity of vapour quenched films from 1.2 K to 400 K.

On the structure of simple inorganic amorphous solids

Presents an account of recent studies on the structure of amorphous semiconductors, oxide and chalcogenide glasses and amorphous metals. Emphasis is placed on simple solids which are representative of each class of amorphous material and also on so-called direct structural techniques-for example, X-ray and neutron scattering EXAFS and electron microscopy. For most glasses the arrangement of near-neighbours is similar to that found in at least one crystalline form of the material. Local structure in the so-called transition metal-metalloid glasses can be described in similar terms or by models with random atomic packing and the arguments for an against each hypothesis are presented. Definition of the medium range (8-15 AA) structure is necessarily more vague.