Biomedical subjects

Shifang Xiao

Publications and source records attributed to Shifang Xiao.

4 recordsLinked to original sources

PubMed2006

Melting temperature: from nanocrystalline to amorphous phase.

By extrapolating the mean grain size of nanocrystal to an infinitesimal value, an amorphous phase has been obtained from the Voronoi construction. The molecular dynamics simulations indicated that for nanocrystal, the grain size variation of melting temperature exhibits two characteristic regions. As mean grain size above about 4 nm for Ag, the melting temperatures decrease with decreasing grain size. However, with grain size further shrinking, the melting temperatures almost keep a constant. This is because the dominant factor on the melting temperature of nanocrystal shifts from grain phase to grain boundary. As a result of fundamental difference in structure, the amorphous phase has a much lower solid-to-liquid transformation temperature than that of nanocrystal.

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PubMed2006

Comparative study of microstructural evolution during melting and crystallization.

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs (555), (433), and (544), existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.

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PubMed2005

Melting behaviors of nanocrystalline Ag.

In the atomic scale, the melting behaviors of nanocrystalline Ag with mean grain size ranging from 3.03 to 12.12 nm have been investigated with molecular dynamics simulations, and a method to determine the melting temperatures of the infinite polycrystalline nanostructured materials is presented. It is found that the melting in nanostructured polycrystals starts from their grain boundaries, and the relative numbers of the three typical bonded pairs, (1551), (1431), and (1541), existing in the liquid phase, increase rapidly with the evolvement of melting. The melting temperatures of nanocrystalline Ag decrease with decreasing mean grain size, and it can be estimated from the size-dependent melting temperature of the corresponding nanoparticles.

Journal Article↗

PubMed2005

Shell and subshell periodic structures of icosahedral nickel nanoclusters.

Using the modified analytic embedded atom method and molecular dynamics, the binding energies and their second order finite differences (stability functions) of icosahedral Ni clusters with shell and subshell periodicity are studied in detail via atomic evolution. The results exhibit shell and subshell structures of the clusters with atoms from 147 to 250,000, and the atomic numbers corresponding to shell or subshell structures are in good agreement with the experimental magic numbers obtained in time-of-flight mass spectra of threshold photoionization, and Martin's theoretical proposition of progressive formation of atomic umbrellas. Clusters with size from 147 to 561 atoms are energetically investigated via one-by-one atomic evolution and their magic numbers are theoretically proved. For medium-size Ni clusters with 561 to 2057 atoms, the prediction of magic numbers with atomic numbers is performed on the basis of umbrella-like subshell growth in near face-edge-vertex order. The similarity of the energy curves makes it possible to extend the prediction to even larger Ni nanoclusters in hierarchical Mackay icosahedral configurations.

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