Large Atomic Size Research Articles

Dynamics and Geometry of Icosahedral Order in Liquid and Glassy Phases of Metallic Glasses

The geometrical properties of the icosahedral ordered structure formed in liquid and glassy phases of metallic glasses are investigated by using molecular dynamics simulations. We investigate the Zr-Cu alloy system as well as a simple model for binary alloys, in which we can change the atomic size ratio between alloying components. In both cases, we found the same nature of icosahedral order in liquid and glassy phases. The icosahedral clusters are observed in liquid phases as well as in glassy phases. As the temperature approaches to the glass transition point Tg, the density of the clusters rapidly grows and the icosahedral clusters begin to connect to each other and form a medium-range network structure. By investigating the geometry of connection between clusters in the icosahedral network, we found that the dominant connecting pattern is the one sharing seven atoms which forms a pentagonal bicap with five-fold symmetry. From a geometrical point of view, we can understand the mechanism of the formation and growth of the icosahedral order by using the Regge calculus, which is originally employed to formulate a theory of gravity. The Regge calculus tells us that the distortion energy of the pentagonal bicap could be decreased by introducing an atomic size difference between alloying elements and that the icosahedral network would be stabilized by a considerably large atomic size difference.

Monte Carlo simulations of strain-driven elemental depletion or enrichment in Cu95Al5 and Cu90Al10 alloys

An interatomic potential for the Cu–Al system based on the modified analytic embedded-atom method was proposed, which provides a reasonable description of the phase stability across the phase diagram. Combining with this potential, Monte Carlo simulations were performed to study strain-induced element segregation and intermixing in bulk and nanostructured Cu95Al5 and Cu90Al10 alloys. Al with a large atomic size is found to segregate to the tensile region while Cu to the compressive region. The elemental depletion or enrichment in dilute or nanostructured alloy is more notable than that in concentrated or bulk one, due to alloy effects or change of local stress. In particular, phase separation in nanostructured alloys leads to a single pure Cu inclusion in the matrix, which will induce strain hardening. The element segregation also affects the chemical ordering that will provide solid-solution strengthening.

Heterogeneous Zr solute segregation and Al3Zr dispersoid distributions in Al–Cu–Li alloys

During the homogenisation treatment of Al alloys the Al3Zr phase is known to form heterogeneously in interdendritic areas where the Zr supersaturation is low. Several types of clusters were observed in the present Al–Cu–Li alloy. Although some clusters resemble the shape of the θ′-Al2Cu lath-shaped particles, it is explained here that there is no direct nucleation on these particles and neither is Zr contained in them at amounts detectable via TEM-EDX. These planar arrays of Zr dispersoids were established to form via repeated precipitation on dislocations. Nucleation on dislocations was the dominant mechanism for individual Al3Zr dispersoids also in the dendrite centre. This was explained on the grounds of the large atomic size misfit between Zr and the Al matrix which leads to segregation of the former atoms to dislocations and was verified experimentally by EDX. It is noteworthy that although Zr did not interact with the θ′ phase, it did so with the equilibrium θ phase and produced two different types of particles, one containing only Zr, and another having both Zr and Mn. It was also seen to be contained within Al20Cu2Mn3 dispersoids in agreement with previous findings.

ChemInform Abstract: Poly(3,4‐alkylenedioxyselenophene)s: Past, Present, and Future

AbstractReview: [82 refs.

Poly(3,4-alkylenedioxyselenophene)s: Past, Present, and Future

Because of their optical and electronic properties, polythiophenes and their derivatives, especially poly(3,4-alkylenedioxythiophene)s, are among the most promising materials in the field of electrochromic polymers. The properties of these polymers can be tuned by replacing the sulfur atom of the thiophene moiety with a selenium atom, because selenium has a lower electronegativity, a higher metallic character, a larger atomic size, and greater polarizability than sulfur. This approach has opened the door to a new and vibrant class of conjugated polymers. Today 3,4-alkylenedioxyselenophenes are thought as powerful competitors to 3,4-alkylenedioxythiophenes. Here, an overview is presented of poly(3,4-alkylenedioxyselenophene)s, as one of the most attractive groups of conjugated polymers. The design, synthesis, and applications of poly(3,4-alkylenedioxyselenophene)s and their derivatives are discussed in detail and compared with the corresponding features of their thiophene analogues. The electrochemical and electrochromic properties and band gaps of poly(3,4-alkylenedioxyselenophene)s are also discussed in relation to their chemical structures. 1 Introduction 2 Polyselenophenes 3 Poly(3,4-ethylenedioxyselenophene)s 4 Poly(3,4-propylenedioxyselenophene)s 5 Conclusions

Effects of dopants on the growth of oxidation-induced stacking faults in heavily doped n-type Czochralaki silicon

Through comparative investigation on the growth of oxidation-induced stacking faults (OSFs) in heavily antimony (Sb)-doped and phosphorus (P)-doped Czochralaki (Cz) silicon wafers with almost the same resistivity, effects of dopants on the growth of OSF in heavily doped n-type Cz silicon are studied experimentally. Moreover, the influences of Sb and P atoms on the recombination of self-interstitials and vacancies are also explored on the basis of the first-principles calculations. It is shown experimentally that all the OSF lengths are almost identical regardless of the type and density of OSF nucleation centers, such as copper precipitates and mechanical scratches etc.. However, it is found that the OSF length of heavily Sb-doped Cz silicon wafer is larger than that of heavily P-doped Cz silicon wafer under the same oxidation condition. Essentially, the OSFs are formed by the aggregation of silicon self-interstitials refleased at the Si/SiO2interface during the oxidation. Therefore, a longer OSF implies that a higher quantity of silicon self-interstitials remains after the recombination of vacancies and silicon self-interstitials in the heavily Sb-doped Cz silicon wafer. The first-principles calculations based on density functional theory (DFT) indicate that Sb atoms combine with vacancies more readily than P atoms. This is actually due to the fact that Sb has a much larger atomic size than P. In other words, as compared with P atoms, the Sb atoms are the moreflefficient vacancy-trapping centers, thus retarding the recombination of vacancies and silicon self-interstitials. Consequently, the silicon self-interstitials remain after recombination with the vacancies that are much more in heavily Sb-doped Cz silicon wafer than in heavily P-doped counterpart when undergoing the same oxidation. In turn, the OSFs in heavily Sb-doped silicon wafers are relatively longer.

Trapping of He clusters by inert-gas impurities in tungsten: First-principles predictions and experimental validation

Properties of point defects resulting from the incorporation of inert-gas atoms in bcc tungsten are investigated systematically using first-principles density functional theory (DFT) calculations. The most stable configuration for the interstitial neon, argon, krypton and xenon atoms is the tetrahedral site, similarly to what was found earlier for helium in W. The calculated formation energies for single inert-gas atoms at interstitial sites as well as at substitutional sites are much larger for Ne, Ar, Kr and Xe than for He. While the variation of the energy of insertion of inert-gas defects into interstitial configurations can be explained by a strong effect of their large atomic size, the trend exhibited by their substitutional energies is more likely related to the covalent interaction between the noble gas impurity atoms and the tungsten atoms. There is a remarkable variation exhibited by the energy of interaction between inert-gas impurities and vacancies, where a pronounced size effect is observed when going from He to Ne, Ar, Kr, Xe. The origin of this trend is explained by electronic structure calculations showing that p-orbitals play an important part in the formation of chemical bonds between a vacancy and an atom of any of the four inert-gas elements in comparison with helium, where the latter contains only 1s2 electrons in the outer shell. The binding energies of a helium atom trapped by five different defects (He-v, Ne-v, Ar-v, Kr-v, Xe-v, where v denotes a vacancy in bcc-W) are all in excellent agreement with experimental data derived from thermal desorption spectroscopy. Attachment of He clusters to inert gas impurity atom traps in tungsten is analysed as a function of the number of successive trapping helium atoms. Variation of the Young modulus due to inert-gas impurities is analysed on the basis of data derived from DFT calculations.

Ion conduction and selectivity in acid-sensing ion channel 1.

The ability of acid-sensing ion channels (ASICs) to discriminate among cations was assessed based on changes in conductance and reversal potential with ion substitution. Human ASIC1a was expressed in Xenopus laevis oocytes, and acid-induced currents were measured using two-electrode voltage clamp. Replacement of extracellular Na(+) with Li(+), K(+), Rb(+), or Cs(+) altered inward conductance and shifted the reversal potentials consistent with a selectivity sequence of Li ∼ Na > K > Rb > Cs. Permeability decreased more rapidly than conductance as a function of atomic size, with P(K)/P(Na) = 0.1 and G(K)/G(Na) = 0.7 and P(Rb)/P(Na) = 0.03 and G(Rb)/G(Na) = 0.3. Stimulation of Cl(-) currents when Na(+) was replaced with Ca(2+), Sr(2+), or Ba(2+) indicated a finite permeability to divalent cations. Inward conductance increased with extracellular Na(+) in a hyperbolic manner, consistent with an apparent affinity (K(m)) for Na(+) conduction of 25 mM. Nitrogen-containing cations, including NH4(+), NH3OH(+), and guanidinium, were also permeant. In addition to passing through the channels, guanidinium blocked Na(+) currents, implying competition for a site within the pore. The role of negative charges in an external vestibule of the pore was evaluated using the point mutation D434N. The mutant channel had a decreased single-channel conductance, measured in excised outside-out patches, and a macroscopic slope conductance that increased with hyperpolarization. It had a weakened interaction with Na(+) (K(m) = 72 mM) and a selectivity that was shifted toward larger atomic sizes. We conclude that the selectivity of ASIC1 is based at least in part on interactions with binding sites both within and internal to the outer vestibule.

Identification of the partitioning characteristics of refractory elements in σ and γ phases of Ni-based single crystal superalloys based on first principles

The impurity formation energies of the σ and γ phases of Ni-based single crystal superalloys doped with W, Cr and Co in different sublattices have been investigated using first-principles based on the density functional theory. The bonding characteristics of the doped σ phase were analyzed with the valence charge densities and the density of the states. The results of the calculations indicated that the typical refractory element W, which has a large atomic size, preferentially partitions into the σ phase due to the nature of the bonding and the unique crystal structure with close-packed planes and large interstitial spaces. In addition, the site preference of refractory elements in γ phase was in the order of W, Cr and Co.

Atomic Structure of Luminescent Centers in High-Efficiency Ce-doped w-AlN Single Crystal

Rare-earth doped wurtzite-type aluminum nitride (w-AlN) has great potential for high-efficiency electroluminescent applications over a wide wavelength range. However, because of their large atomic size, it has been difficult to stably dope individual rare-earth atoms into the w-AlN host lattice. Here we use a reactive flux method under high pressure and high temperature to obtain cerium (Ce) doped w-AlN single crystals with pink-colored luminescence. In order to elucidate the atomic structure of the luminescent centers, we directly observe individual Ce dopants in w-AlN using annular dark-field scanning transmission electron microscopy. We find that Ce is incorporated as single, isolated atoms inside the w-AlN lattice occupying Al substitutional sites. This new synthesis method represents a new alternative strategy for doping size-mismatched functional atoms into wide band-gap materials.

Effects of density difference of constituent elements on glass formation in TiCu-based bulk metallic glasses

Glass formation is generally favored by a large atomic size mismatch among constituent elements, which usually leads to large density differences among them as well. During melting, elemental segregation occurs due to Stokes’ law and then inevitably affects glass formation. In this paper, such effects on glass-forming ability in a TiCu-based alloy system have been demonstrated. In the bulk glass-forming composition Ti43Cu42Hf14Si1, macroscopic segregation of Si was observed in the as-melted ingots and silicon was completely depleted in the as-cast rods. In another Ti33Cu47Ni8Zr11Si1 alloy, nevertheless, the effects of density differences among the constituent elements were less severe. It was also confirmed that using proper pre-alloys could be an effective way in alleviating the side effects of the elemental segregation.

Effect of Rare-Earth Additions on the Texture of Wrought Magnesium Alloys: The Role of Grain Boundary Segregation

Magnesium alloys that contain certain rare-earth (RE) additions are known to have improved formability and this can be partly attributed to the different texture they display after recrystallization. Previous experimental work has identified segregation of RE to grain boundaries and dislocations as being potentially important in producing this change in behavior. In the present paper, two classical models (Langmuir–McClean and Cahn–Lucke–Stuwe) are used to explore the likely effect of RE additions on grain boundary solute concentration and drag. It is demonstrated that a wide range of RE elements are predicted to segregate strongly to grain boundaries due to the large atomic size misfit with magnesium. The maximum level of segregation is produced for elements such as Y or Gd that combine a high misfit and high bulk solubility. Segregated Y is predicted to produce a solute drag pressure on migrating boundaries several orders of magnitude greater than that obtained by Al or Zn additions. It is demonstrated that while this drag is predicted to be insufficient to strongly retard static recrystallization under typical annealing conditions, it is expected to suppress dynamic recrystallization by any mechanism requiring boundary migration.

The formation of FecoreAlshell and FeshellAlcore nanoparticles, a molecular dynamics simulation

According to the surface segregation theory, bimetallic nanoparticles can easily form core–shell configuration. An atom with low surface energy and large atomic size distributes on the surface of the nanoparticle, and another atom occupies the internal position. In the present work, the diffusion and growth configuration of Fe–Al nanoparticles are examined. The calculated energy barriers for the interfacet diffusion via the hopping or the exchange mechanisms are larger than that for the intrafacet diffusion. The result suggests that both FecoreAlshell and FeshellAlcore nanoparticles can be obtained at proper temperatures. The growths of the Fe and Al adatoms on the icosahedron of Al and the rhombohedron of Fe are simulated by molecular dynamics along with embedded atom methods in the temperature range of 100–300K. FeshellAlcore, a reversed core–shell configuration, is obtained at low temperatures.

Atomic scale interfacial and compositional characteristics of the σ and γ phases of Ni-based single-crystal superalloys

High-resolution transmission electron microscopy and high-angle annular dark-field imaging were used to study differences in the interfacial and compositional characteristics between the plate-shaped σ and γ phases in a Ni-based single-crystal superalloy. An atomic structural model of the interface between the σ and γ phases that includes a step was proposed. The σ phase exhibits the following relationship with the matrix: [001]γ//[112¯]σ, (2¯20)γ//(1¯10)σ, (2¯2¯0)γ//(111)σ, [011]γ//[110]σ, (11¯1)γ//(001¯)σ. The compositional characteristics of the σ phase indicate that it is rich in high-Z elements (Z is the atomic number), especially the alloying element rhenium (Re). The impurity formation energies for the σ and γ phases doped with Re in different sublattices were investigated using the first-principles method based on the density functional theory. The bonding characteristics of the undoped and doped σ and γ phases were analyzed with the valence charge densities and the density of states. The results indicate that the alloying element Re, with a large atomic size, substitutes the W atom preferentially, owing to the nature of the bonding and the interstitial spaces in the crystal structure.

Enhancement of Thermopower of TAGS‐85 High‐Performance Thermoelectric Material by Doping with the Rare Earth Dy

AbstractEnhancement of thermopower is achieved by doping the narrow‐band semiconductor Ag6.52Sb6.52Ge36.96Te50(acronym TAGS‐85), one of the bestp‐type thermoelectric materials, with 1 or 2% of the rare earth dysprosium (Dy). Evidence for the incorporation of Dy into the lattice is provided by X‐ray diffraction and increased orientation‐dependent local fields detected by125Te NMR spectroscopy. Since Dy has a stable electronic configuration, the enhancement cannot be attributed to 4f‐electron states formed near the Fermi level. It is likely that the enhancement is due to a small reduction in the carrier concentration, detected by125Te NMR spectroscopy, but mostly due to energy filtering of the carriers by potential barriers formed in the lattice by Dy, which has large both atomic size and localized magnetic moment. The interplay between the thermopower, the electrical resistivity, and the thermal conductivity of TAGS‐85 doped with Dy results in an enhancement of the power factor (PF) and the thermoelectric figure of merit (ZT) at 730 K, fromPF= 28 μW cm−1K−2andZT≤ 1.3 in TAGS‐85 toPF= 35 μW cm−1K−2andZT≥ 1.5 in TAGS‐85 doped with 1 or 2% Dy for Ge. This makes TAGS‐85 doped with Dy a promising material for thermoelectric power generation.

Metal-free selenium doped carbon nanotube/graphene networks as a synergistically improved cathode catalyst for oxygen reduction reaction

The ongoing search for new non-precious-metal catalysts (NPMCs) with excellent electrocatalytic performance to replace Pt-based catalysts has been viewed as an important strategy to promote the development of fuel cells. Recent studies have proven that carbon materials doped with atoms which have a relatively small atomic size (e.g. N, B, P or S), have also shown pronounced catalytic activity. Herein, we demonstrate the successful fabrication of CNT/graphene doped with Se atoms, which has a relatively large atomic size, by a simple, economical, and scalable approach. The electrocatalytic performance of the resulting Se-doped CNT-graphene catalyst exhibits excellent catalytic activity, long-term stability, and a high methanol tolerance compared to commercial Pt/C catalysts. Our results confirmed that combining CNTs with graphene is an effective strategy to synergistically improve ORR activity. More importantly, it is also suggested that the development of graphite materials doped with Se or other heteroatoms of large size will open up a new route to obtain ideal NPMCs with realistic value for fuel cell applications.

Icosahedral symmetry, fragility and stability of supercooled liquid state of metallic glasses

The fragility of materials should be related to a local structure formed in supercooled liquid states. Especially, in the cases of metallic glasses, the icosahedral order formation is considered to play an important role. To clarify the relation between the local structure formation and the relaxation behavior (or fragility) in the supercooled liquid states of metallic glasses, molecular dynamics simulations were performed for a simple model of binary alloys, in which we can change the atomic size ratio. From the simulation results, we found that the icosahedral clusters exist in supercooled liquid states and form a medium-range network structure in strongly supercooled regime near the glass transition as well as in the glassy states. We also found that the number density of the clusters increases as the atomic size difference increases. Consequently, the calculated fragility parameter takes a lower value in a system with larger atomic size difference due to the richness of the icosahedral clusters.

Effects of cooling rates on microstructural evolution during solidification process of liquid Ca50Zn50 alloy

A simulation study is performed on the effects of six different cooling rates on microstructural evolution during solidification process of liquid Ca50Zn50 alloy with larger atomic size difference by using the molecular dynamics method. The pair distribution function, Honeycutt-Andersen (HA) bond-type index method, cluster-type index method (CTIM-2) and three-dimensional visualization method are adopted to deeply analyze the microstructural evolution. The results show that there is a critical cooling rate (in a range of 11012 and 51011 K/s) for forming amorphous or crystal structure. When the cooling rate, such as 11014 K/s, 11013 K/s, 11012 K/s and 51011 K/s, is above the critical cooling rate, the amorphous structures are formed mainly to be the 1551, 1541 and 1431 bond-types or the icosahedron basic clustr (12 0 12 0 0 0); while the cooling rate is under the critical cooling rate, such as at 11012 K/s, the partial crystal structures are formed mainly to be the 1441 and 1661 bond-types or the bcc clusters (14 6 0 8 0 0) (containing part of hcp (12 0 0 0 6 6) and fcc (12 0 0 0 12 0) basic crystal clusters) in the system. In the cooling rate range of forming amorphous structure, the first peak of the pair distribution function g(r) is split obviously into three secondary peaks corresponding to the nearest neighbor as Zn-Zn, Ca-Zn and Ca-Ca, respectively, and with the decrease of cooling rate, the secondary peak formed by the like atoms is inereased and the secondary peak formed by unlike atoms is reduced. With the decrease of cooling rate, the Zn atoms can be easily segregated to form the larger clusters; the lower the cooling rate, the bigger the number of basic icosahedrons formed in the system, and the amorphous system is more stable. In the cooling rate range of forming crystal structure, a great number of Zn atoms are segregated to form the bulk bcc crystal structures and part of Ca atoms are segregated to form some hcp and fcc crystal clusters.

Role of rare-earth elements in glass formation of Al–Ca–Ni amorphous alloys

Effects of different rare-earth (RE) elements (i.e., Y, Ce, La and Yb) on the glass forming-ability (GFA) of Al–Ca–Ni alloy system were systematically investigated. The optimum addition content of all the four RE is 1 at.% and ytterbium is the most effective as far as the GFA is considered. Our analysis confirmed that proper additions of Yb could not only impede the formation of the competing α-Al clusters, but also enhance the liquid stability (i.e., lower the liquidus temperature). Although additions of Y, Ce and La in the Al 88Ca 7Ni 5 can also retard the α-Al precipitation, nevertheless, it seemed that their large atomic size difference with Ca could destabilize the original liquid stability (i.e., increase the liquidus temperature).

Development of the high performance magnesium based hydrogen storage alloy

Series of MgNi type alloys with Ti, Al, Zr, Pd and Co additive elements were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. Systematical alloy designing indicated that Mg 0.80Ti 0.15Al 0.05Zr 0.05Ni 0.95 alloy has the best electrode performance. The atomic fractions in this alloy were believed to be optimum to get the reasonable amount of hydrogen storage with the improved cyclic stability. Titanium was estimated to enter into Mg(OH) 2 layer during the discharging process and make this barrier layer more penetrable by elemental hydrogen. Al and/or Al-oxides were predicted to dissolve selectively throughout the barrier hydroxide layer and thus reduce the stability of this layer. The main contribution of Zr was estimated to arise from its large atomic size that Zr atoms can create extra sites for the elemental hydrogen in the alloy structure. As the alloy charge transfer resistances decreased, the alloy retention rates increased. Improvement in the alloy capacity retaining rate was also closely related with the hydrogen diffusion coefficients in the alloy.