C. Z. Wang

Iowa State University, Ames, Iowa, United States

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Publications (260)732.35 Total impact

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    ABSTRACT: The structures and dynamics of liquid Ag74Ge26 alloy at different temperatures were investigated by high-energy X-ray diffraction and ab initio molecular dynamics simulation. The calculated structure factors agree well with the experimental data. Local atomic structure orders have been characterized by angular distribution functions, Honeycutt-Andersen index, Voronoi tessellation and the atomic cluster alignment. We reveal that (i) 〈0, 3, 6, 4, 0〉, 〈0, 2, 8, 4, 0〉, 〈0, 3, 6, 5, 0〉, and 〈1, 3, 4, 5, 1〉 polyhedral dominate in the undercooled Ag74Ge26 liquid. (ii) The icosahedral-like short-range order is enhanced as the temperature decreases while 〈0, 3, 6, 4, 0〉, 〈0, 2, 8, 4, 0〉, 〈0, 3, 6, 5, 0〉, and 〈0, 1, 10, 2, 0〉 polyhedral become the major short-range orders at low temperature. (iii) The medium-range order which was formed for amorphous phase at ambient temperature persists in the undercooled liquid. In addition, it is found that the diffusion constant of Ag is lower than that of Ge over the studied temperature range. Our findings elucidate that highly dominated short-range order associated with nearest-neighbors shell could empower the development of medium-range order and the formation of metallic glasses.
    Acta Materialia 06/2015; 92. DOI:10.1016/j.actamat.2015.03.047 · 3.94 Impact Factor
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    ABSTRACT: Liquid phase diffusion plays a critical role in phase transformations (e.g. glass transformation and devitrification) observed in marginal glass forming systems such as Al-Sm. Controlling transformation pathways in such cases requires a comprehensive description of diffusivity, including the associated composition and temperature dependencies. In the computational study reported here, we examine atomic diffusion in Al-Sm liquids using ab initio molecular dynamics (AIMD) and determine the diffusivities of Al and Sm for selected alloy compositions. Non-Arrhenius diffusion behavior is observed in the undercooled liquids with an enhanced local structural ordering. Through assessment of our AIMD result, we construct a general formulation for Al-Sm liquid, involving a diffusion mobility database that includes composition and temperature dependence. A Volmer-Fulcher-Tammann (VFT) equation is adopted for describing the non-Arrhenius behavior observed in the undercooled liquid. The composition dependence of diffusivity is found quite strong, even for the Al-rich region contrary to the sole previous report on this binary system. The model is used in combination with the available thermodynamic database to predict specific diffusivities and compares well with reported experimental data for 0.6 at.% and 5.6 at.% Sm in Al-Sm alloys.
    Scientific Reports 06/2015; 5:10956. DOI:10.1038/srep10956 · 5.58 Impact Factor
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    ABSTRACT: A semi-empirical potential for the Al 90 Sm 10 alloy is presented. The potential provides satisfactory reproduction of pure Al properties, the formation energies of a set of Al–Sm crystal phases with Sm content about 10%, and the structure of the liquid Al 90 Sm 10 alloy. During molecular dynamics simulation in which the liquid alloy is cooled at a rate of 10 10 K s −1 , the developed potential produces a glass structure with lower ab initio energy than that produced by ab initio molecular dynamics (AIMD) itself using a typical AIMD cooling rate of 8·10 13 K s −1 . Based on these facts the developed potential should be suitable for simulations of phase transformations in the Al 90 Sm 10 alloy.
    Modelling and Simulation in Materials Science and Engineering 06/2015; 23(4-4):045013. DOI:10.1088/0965-0393/23/4/045013 · 1.49 Impact Factor
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    Physical Review B 05/2015; 91(18). DOI:10.1103/PhysRevB.91.180201 · 3.66 Impact Factor
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    ABSTRACT: Phase transitions in amorphous alloys under pressure are an important fundamental problem. Here we report on a first-principles study that reproduces the recently discovered pressure-induced crystallization of amorphous Ce100-xAlx (x<25) alloys and reveals an atomistic transformation mechanism. Contrary to common belief, pressure-induced devitrification of an amorphous Ce-Al alloy is not due to the Ce 4f delocalization that is expected to bring the size of Ce atoms closer to that of Al atoms. Our theoretical results show that the Bader volume of Ce is always larger than that of Al. A continuous increase in the relative Ce/Al Bader volume leads to favourable conditions for forming a close-packed structure at high pressure. The results also show that the Hume-Rothery rules are not applicable to describe the structures of substitutional alloys at high pressure. This study provides a new perspective on the electron distribution in lanthanide alloys under the application of pressure.
    Nature Communications 03/2015; 6:6493. DOI:10.1038/ncomms7493 · 10.74 Impact Factor
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    ABSTRACT: We present an efficient genetic algorithm, integrated with experimental diffraction data, to solve a nanoscale metastable Al 20Sm4 phase that evolves during crystallization of an amorphous magnetron sputtered Al 90Sm10 alloy. The excellent match between calculated and experimental X-ray diffraction patterns confirms an accurate description of this metastable phase. Molecular dynamic simulations of crystal growth from the liquid phase predict the formation of disordered defects in the devitrified crystal.
    Applied Physics Letters 03/2015; 106(10):101903. DOI:10.1063/1.4914399 · 3.52 Impact Factor
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    ABSTRACT: The short-range order (SRO) in Pd78Cu6Si16 liquid was studied by high energy x-ray diffraction and ab initio molecular dynamics (MD) simulations. The calculated pair correlation functions at different temperatures agree well with the experimental results. The partial pair correlation functions from ab intio MD simulations indicate that Si atoms prefer to be uniformly distributed while Cu atoms tend to aggregate. By performing structure analysis using Honeycutt-Andersen index, Voronoi tessellation, and atomic cluster alignment method, we show that the icosahedron and face-centered cubic SRO increase upon cooling. The dominant SRO is the Pd-centered Pd9Si2 motif, namely the structure of which motif is similar to the structure of Pd-centered clusters in the Pd9Si2 crystal. The study further confirms the existence of trigonal prism capped with three half-octahedra that is reported as a structural unit in Pd-based amorphous alloys. The majority of Cu-centered clusters are icosahedra, suggesting that the presence of Cu is benefit to promote the glass forming ability.
    Scientific Reports 02/2015; 5:8277. DOI:10.1038/srep08277 · 5.58 Impact Factor
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    Physical Review B 02/2015; 91(6). DOI:10.1103/PhysRevB.91.064105 · 3.66 Impact Factor
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    ABSTRACT: Nanostructure arrays such as nanowire, nanopillar, and nanocone arrays have been proposed to be promising antireflection structures for photovoltaic applications due to their great light trapping ability. In this paper, the optical properties of Si nanopillar and nanocone arrays in visible and infrared region were studied by both theoretical calculations and experiments. The results show that the Mie resonance can be continuously tuned across a wide range of wavelength by varying the diameter of the nanopillars. However, Si nanopillar array with uniform diameter exhibits only discrete resonance mode, thus can't achieve a high broadband absorption. On the other hand, the Mie resonance wavelength in a Si nanocone array can vary continuously as the diameters of the cross sections increase from the apex to the base. Therefore Si nanocone arrays can strongly interact with the incident light in the broadband spectrum and the absorbance by Si nanocone arrays is higher than 95% over the wavelength from 300 to 2000 nm. In addition to the Mie resonance, the broadband optical absorption of Si nanocone arrays is also affected by Wood-Rayleigh anomaly effect and metal impurities introduced in the fabrication process.
    Scientific Reports 01/2015; 5:7810. DOI:10.1038/srep07810 · 5.58 Impact Factor
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    ABSTRACT: Classically, the onset of nucleation is defined in terms of a critical cluster of the condensed phase, which forms from the gradual aggregation of randomly diffusing adatoms. Experiments in Pb/Si(111) at low temperature have discovered a dramatically different type of nucleation, with perfect crystalline islands emerging "explosively" out of the compressed wetting layer after a critical coverage Θ_{c}=1.22 ML is reached. The unexpectedly high island growth rates, the directional correlations in the growth of neighboring islands and the persistence in time of where mass is added in individual islands, suggest that nucleation is a result of the highly coherent motion of the wetting layer, over mesoscopic distances.
    Physical Review Letters 12/2014; 113(23):236101. DOI:10.1103/PhysRevLett.113.236101 · 7.73 Impact Factor
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    ABSTRACT: The structure of liquid and rapidly quenched amorphous Ag0.74Ge0.26 alloy at the eutectic composition was studied by ab initio molecular dynamics (MD) simulations. The local structural properties were systematically investigated from the liquid at 1123 K to amorphous solid at 300 K. The pair-correlation function at 976 K from the MD simulations agrees well with the experimental data. The local structures were also analyzed using Honeycutt-Andersen (HA) indices, Voronoi tessellation and the atomic cluster alignment (ACA) method. The HA indices analysis reveals that there is a high population of pentagonal bipyramid structure which become more predominant upon solidification. Voronoi tessellation analysis indicates strong icosahedral short-range order (SRO) in the liquid and amorphous samples generated by the MD simulations. Using the ACA method, the development of icosahedral SRO upon cooling is further confirmed and a Bergman medium-range order is also observed. The analysis of structural properties and chemical short-range order suggests that Ag atoms tend to have a Bergman-like packing, while the dispersed Ge atoms prevent the alloy from forming a long-range order upon cooling. The Bergman medium-range order is also confirmed by the constrained reversed Monte Carlo results.
    Acta Materialia 11/2014; 80:498-504. DOI:10.1016/j.actamat.2014.06.049 · 3.94 Impact Factor
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    Physical Review B 11/2014; 90(17). DOI:10.1103/PhysRevB.90.174101 · 3.66 Impact Factor
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    ABSTRACT: Structural and dynamical heterogeneities in metallic glasses, while intensely studied, remain an enigma. For instance, whether and how the dynamical and structural heterogeneities are correlated is still an outstanding question. Meanwhile, the nature of the impact of medium-range order (MRO) on the dynamical heterogeneity remains elusive. In this paper, we analyzed the structural and dynamical heterogeneities in both as-quenched and relaxed Cu64.5Zr35.5 metallic glasses based on the atomistic trajectories collected from molecular dynamics simulations. We found that the majority of the mobile atoms are not involved in icosahedral clusters or Bergman superclusters, indicating that dynamical heterogeneities are strongly correlated with structural heterogeneities. The Bergman-type MRO has an even stronger correlation with the dynamical heterogeneity than the icosahedral short range order. Moreover, we found that the localized soft vibration modes below 1.0 THz are mostly concentrated on the mobile atoms. These results suggest that the vibrational properties can be conveniently utilized to predict the atomic mobility in metallic glasses, which can bridge the studies of dynamical heterogeneity by experiments and simulations. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 10/2014; 105(15). DOI:10.1063/1.4898779 · 3.52 Impact Factor
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    ABSTRACT: The structure of the orthorhombic Zr2Co11 phase was revisited in the present work. Selected-area electron diffraction (SAED) and high-resolution electron microscopy (HREM) techniques were used to investigate the structure. They show the orthorhombic Zr2Co11 phase has a 1-D incommensurate modulated structure. The structure can be approximately described as a B-centered orthorhombic lattice. The lattice parameters of the orthorhombic Zr2Co11 phase have been determined by a tilt series of SAED patterns. A hexagonal network with a modulation wave has been observed in the HREM image and the hexagonal motif is considered as the basic structural unit.
    Journal of Alloys and Compounds 10/2014; 611:167–170. DOI:10.1016/j.jallcom.2014.05.072 · 2.73 Impact Factor
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    ABSTRACT: The role of silicon oxide shell in oxide-assisted SiNWs growth is studied by performing ab initio molecular dynamics simulations on the structural and dynamical properties of the interface between crystalline Si(111) surface and disorder SiO thin film. Si atoms in the SiO film tends to aggregate into the vicinity of the Si(111)/SiO interface. In addition, the diffusion of Si atoms at the interface is anisotropic - the diffusion along the interface is several times faster than that perpendicular to the interface. The segregation and anisotropic diffusion of Si atoms at the Si(111)/SiO interface shed interesting light into the mechanism of oxide-assisted silicon nanowire growth.
    Materials Chemistry and Physics 10/2014; 148(3):1145-1148. DOI:10.1016/j.matchemphys.2014.09.036 · 2.13 Impact Factor
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    ABSTRACT: We present an efficient \textit{ab initio} method for calculating the electronic structure and total energy of strongly correlated electron systems. The method extends the traditional Gutzwiller approximation for one-particle operators to the evaluation of the expectation values of two particle operators in a full many-electron Hamiltonian. The method is free of adjustable Coulomb parameters, and has no double counting issues in the calculation of total energy, and has the correct atomic limit. We demonstrate that the method describes well the bonding and dissociation behaviors of the hydrogen and nitrogen clusters. We also show that the method can satisfactorily tackle great challenging problems faced by the density functional theory recently discussed in the literature. The computational workload of our method is similar to the Hartree-Fock approach while the results are comparable to high-level quantum chemistry calculations.
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    ABSTRACT: The local atomic structure evolution in Al2Au alloy during solidification from 2000 K to 400 K was studied by ab initio molecular dynamics simulations and analyzed using the structure factor, pair correlation functions, bond angle distributions, the Honeycutt–Anderson (HA) index and Voronoi tessellation methods. It was found that the icosahedral-like clusters are negligible in the Al2Au stable liquid and supercooled liquid states, and the most abundant clusters are those having HA indices of 131 and 120 or Voronoi indices of 〈0, 4, 4, 0〉, 〈0, 3, 6, 0〉 and 〈0, 4, 4, 2〉 with coordination numbers of 8, 9 and 10, respectively. These clusters are similar to the local atomic structures in the CaF2-type Al2Au crystal, revealing the existence of structure heredity between liquid and crystalline phase in Al2Au alloy.
    Acta Materialia 04/2014; 68(15):1–8. DOI:10.1016/j.actamat.2014.01.004 · 3.94 Impact Factor
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    ABSTRACT: The temperature dependent optical properties of tin film from solid to liquid were studied by spectroscopic ellipsometry and ab initio molecular dynamics simulations. The dielectric function of liquid Sn was different from solid, and an interband transition near 1.5 eV was easily observed in solid while it apparently disappeared upon melting. From the evolution of optical properties with temperature, an optical measurement to acquire the melting point by ellipsometry was presented. From first principles calculation, we show that the local structure difference in solid and liquid is responsible for this difference in the optical properties observed in experiment.
    Applied Physics Letters 03/2014; 104(12):121907-121907-5. DOI:10.1063/1.4869722 · 3.52 Impact Factor
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    ABSTRACT: Reverse Monte Carlo simulations (RMC) have been widely used to generate three dimensional (3D) atomistic models for glass systems. To examine the reliability of the method for metallic glass, we use RMC to predict the atomic configurations of a "known" structure from molecular dynamics (MD) simulations, and then compare the structure obtained from the RMC with the target structure from MD. We show that when the structure factors and partial pair correlation functions from the MD simulations are used as inputs for RMC simulations, the 3D atomistic structure of the glass obtained from the RMC gives the short- and medium-range order in good agreement with those from the target structure by the MD simulation. These results suggest that 3D atomistic structure model of the metallic glass alloys can be reasonably well reproduced by RMC method with a proper choice of input constraints.
    Journal of Applied Physics 01/2014; 115(5). DOI:10.1063/1.4865164 · 2.19 Impact Factor
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    ABSTRACT: Solving the crystal structures of novel phases with nanoscale dimensions resulting from rapid quenching is difficult due to disorder and competing polymorphic phases. Advances in computer speed and algorithm sophistication have now made it feasible to predict the crystal structure of an unknown phase without any assumptions on the Bravais lattice type, atom basis, or unit cell dimensions, providing a novel approach to aid experiments in exploring complex materials with nanoscale grains. This approach is demonstrated by solving a long-standing puzzle in the complex crystal structures of the orthorhombic, rhombohedral, and hexagonal polymorphs close to the Zr2Co11 intermetallic compound. From our calculations, we identified the hard magnetic phase and the origin of high coercivity in this compound, thus guiding further development of these materials for use as high performance permanent magnets without rare-earth elements.
    Physical Review Letters 01/2014; 112(4):045502. DOI:10.1103/PhysRevLett.112.045502 · 7.73 Impact Factor

Publication Stats

5k Citations
732.35 Total Impact Points

Institutions

  • 1990–2015
    • Iowa State University
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      • • Department of Chemistry
      Ames, Iowa, United States
  • 2011
    • U.S. Department of Energy
      • Ames Lab
      Washington, Washington, D.C., United States
  • 2010
    • Qingdao University
      • College of Physics
      Tsingtao, Shandong Sheng, China
  • 2007–2008
    • Jilin University
      • State Key Lab of Theoretical and Computational Chemistry
      Jilin, Jilin Sheng, China
  • 2006
    • Northeast Normal University
      Hsin-ching, Jilin Sheng, China
  • 1994
    • Wuhan University
      • Department of Physics
      Wu-han-shih, Hubei, China