Li-li Zhou

Bio: Li-li Zhou is an academic researcher from Hunan University. The author has contributed to research in topics: Materials science & Crystallization. The author has an hindex of 10, co-authored 24 publications receiving 236 citations.

The effect of pressure on the crystallization of rapidly supercooled zirconium melts.

Abstract: Molecular dynamics simulations have been performed to explore the effect of pressure (P) on the crystallization of zirconium (Zr) under rapid cooling. The structural evolutions have been analysed in terms of the system energy, the pair distribution function and the largest standard cluster analysis. It was found that at the cooling rate of 1.0 × 1011 K s−1, which can crystallize Zr melts into hcp crystals via the bcc intermediate state under zero pressure, the critical pressure (Pc) for vitrification is about 28.75 GPa, and the larger the pressure, the higher the glass transition temperature Tg. At P < Pc the Ostwald's step rule is applied to Zr melts. Crystallization of rapidly super-cooled Zr melts under pressure always begins with the bcc phase and ends in the hcp crystal; the higher the pressure, the lower the onset temperature (Tc) of crystallization. Unlike the single-intermediate-state crystallization (SisC) under zero pressure, multiple-intermediate-state crystallization (MisC) is usually observed under pressure. Structural analysis reveals that if nucleation is essentially completed at the end of the first crystalline (bcc-dominated) stage, MisC will occur; otherwise, SisC occurs. The origin of such an observation is also discussed from the effect of pressure upon the thermodynamics and kinetics factors. These findings are useful for comprehensively understanding the solidification of metals under pressure.

The short-range order in liquid and A15 crystal of zirconium

Abstract: Molecular dynamic simulations have been carried out to investigate the crystallization mechanism during rapid cooling and isothermal relaxation for Zirconium (Zr) under a pressure of 100 GPa. The properties of the simulated system are analysed in terms of the system energy, the pair distribution function (PDF), and the largest standard cluster analysis. It is found that the so called topologically close-packed (TCP) local structures dominate in number in liquid, supercooled liquid and the final solid. Both solidification and relaxation crystallize into an A15 phase rather than the simple bcc or hcp crystals, the metastable and stable crystal of Zr under the ambient condition. Interestingly on PDF curves for liquids and super-liquids, left to the first major peak (at around r = 3.0 A) there is a mini-peak (at around r = 1.87 A) that decreases with the decrease of temperature and vanishes when crystallization ends. Further analysis reveals that the mini-peak originates from dynamical compact atomic pairs that are however almost nothing to do with TCP local structures. In addition, the multi characteristic lengths of the A15 unit (composed of Z12 and Z14 local structures with ratio of 1:3) result in the distinct split of the first and second peaks on the PDF curves.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

The Structure of Non-Crystalline Materials

Abstract: : Fourier inversion of EXAFS data is a powerful new technique for structural investigation of non-crystalline materials with the unique capability to determine the near neighbor environment of each type of atom in a complex material. The radial structure function so obtained contains information on the number, distance to, and distribution of atoms surrounding the absorbing atom. The data on Ge and Ge02 generally agree with previous findings but with added detail. The environment of both Ge and Se in amorphous GeSe show a significant change from the crystalline material with evidence for local satisfaction of the directed bonding tendencies of each atomic species. (Author)

Water under pressure

Abstract: Water under pressure is investigated by first principles molecular dynamics, with a focus on the changes in hydrogen bonding and the oxygen network in the nondissociative regime. At a pressure of 10 GPa and a temperature of 600 K, which is close to the freezing point, no appreciable molecular dissociation is observed in the simulations. However, the structure of water is substantially altered from that at ambient conditions. The liquid exhibits a much larger coordination of oxygen atoms, an essential weakening of hydrogen bonding, and sizable changes in the density of electronic states close to the Fermi level. Our results provide new structural data for direct comparison with future experiments. (c) 2000 The American Physical Society.

Real Space Imaging of Nucleation and Growth in Colloidal Crystallization

Abstract: Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

Molecular dynamics study of atomic-level structure in monatomic metallic glass

Abstract: Molecular dynamics simulations have been performed to study the vitrification process and the local atomic structures in Ni monatomic metallic glasses (MG). The interatomic interactions are described by embedded atom method (EAM) potentials. Short-range order (SRO) and medium range order (MRO) in nickel monatomic MG is studied using several structural techniques such as the Radial distribution function (RDF); the Common neighbor analysis method (CNA) and the Voronoi tessellation. As a result, we found that the atomic packing in metallic glasses can be described globally as a superposition of the spherical-periodic order (SPO) and the local translational symmetry (LTS). From CNA method, we found that the majority of icosahedral clusters in Ni monatomic MG are connected together by vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS) rather than isolated clusters. These typical cluster connections constitute to the partial RDF of icosahedral atoms except the edge sharing which is hidden between the second and the third subpeaks of RDF. Furthermore, the visualization technologies are applied to follow the formation of clusters during rapid quenching. It is found that the clusters ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩ are more dominant than the full icosahedral polyhedron ⟨0,0,12,0⟩ in our monatomic system. Furthermore, the splitting of the second peak in RDF curve in Ni glass is not only caused by the icosahedral clusters, but also by the Voronoi polyhedrons ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩.