National University of HochiMinh City

Ho Chi Minh City, Vietnam

National University of HochiMinh City

Ho Chi Minh City, Vietnam
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Van Sang L.,Vietnam National University, Ho Chi Minh City | Huong T.T.T.,National University of Hochiminh City | Minh L.N.T.,National University of Hochiminh City
European Physical Journal D | Year: 2014

Molecular dynamics (MD) simulations are used to investigate the thermodynamic properties and structural changes of KCl spherical nanoparticles at various sizes (1064, 1736, 2800, 3648, 4224 and 5832 ions) upon heating. The melting temperature is dependent on both the size and shape of KCl models, and the behaviour of the first order phase transition is also found in the present work. The surface melting found here is different from the melting phenomena of KCl models or other alkali halides studied in the past. In the premelting stage, a mixed phase containing liquid and solid ions covers the surface of nanoparticles. The only peak of heat capacity spreads out a significant segment of temperature, probably exhibiting both heterogeneous melting on the surface and homogeneous melting in the core. The coexistence of two melting mechanisms, homogeneous and heterogeneous ones, in our model is unlike those considered previously. We also found that the critical Lindemann ratio of the KCl nanoparticle becomes much more stable when the size of the nanoparticle is of the order of thousands of ions. A picture of the structural evolution upon heating is studied in more detail via the radial distribution function (RDF) and coordination numbers. Our results are in a good agreement with previous MD simulations and experimental observations. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag.


Sang L.V.,Vietnam National University, Ho Chi Minh City | Hoang V.V.,National University of Hochiminh City | Tranh D.T.N.,National University of Hochiminh City
European Physical Journal D | Year: 2015

In the present work, we use molecular dynamics (MD) simulations to investigate melting of the crystalline Si nanoparticle. Atoms in the nanoparticle interact with each other via the Stillinger-Weber potential. Two heating rates are used. We find that melting of the nanoparticle occurs via propagation of quasi-liquid layer from the surface into the core of the nanoparticle until this layer reaches the critical thickness. We find heating rate affects on mechanism of melting of Si nanoparticle, i.e. coexistence of the two melting mechanisms (homogeneous and heterogeneous ones) occurs if low heating rate is used and it is unlike that proposed in the past. Size affects on melting of Si nanoparticle are found and discussed. In addition, we find that the global bond order parameters Q l can be used to detect melting of Si system unlike some calculations presented in the past. Graphical abstract: [Figure not available: see fulltext.] © 2015 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.© EDP Sciences, Societ'a Italiana di Fisica, Springer-Verlag 2015.


Hoang V.V.,National University of HochiMinh City | Odagaki T.,Tokyo Denki University
Solid State Communications | Year: 2010

Atomic mechanism of glass formation in supercooled monatomic liquids is monitored via analyzing the spatial arrangement of solid-like atoms. The supercooled states are obtained by cooling from the melt using molecular dynamics (MD) simulation. Solid-like atoms, detected via Lindemann-like freezing criterion, are found throughout the liquid. Their number increases with decreasing temperature and they form clusters. In the deeply supercooled region, all solid-like atoms form a single percolation cluster which spans throughout the system. The number of atoms in this cluster increases steeply with further cooling. Glass formation in supercooled liquids occurs when a single percolation cluster of solid-like atoms involves the majority of atoms in the system to form a relatively rigid glassy solid. By analyzing the temperature dependence of static and dynamic properties, we identify three characteristic temperatures of glass formation in supercooled liquids including the VogelFulcher temperature. © 2010 Elsevier Ltd. All rights reserved.


Hoang V.V.,National University of HochiMinh City | Odagaki T.,Tokyo Denki University
Journal of Physical Chemistry B | Year: 2011

Atomic mechanism of glass formation of a supercooled simple monatomic liquid with Lennard-Jones-Gauss (LJG) interatomic potential is studied by molecular dynamics (MD) simulation. Supercooled and glassy states are obtained by cooling from the melt. Glassy state obtained at low temperatures is annealed for very long time, on the order of microsecond, and we find that glassy state remains unchanged and that the long-lived glassy state of a simple monatomic system in three dimensions is realized. We analyze the spatiotemporal properties of solid-like and liquid-like atoms that are defined by the Lindemann-like freezing criterion. The number of solid-like atoms, distributed throughout the liquid, increases with decreasing temperature toward glass transition and they form clusters. In the deeply supercooled region, almost all solid-like atoms form a single percolation cluster and its characteristic size increases sharply on further cooling. Glass formation in supercooled liquid occurs when a single percolation cluster of solid-like atoms involves a majority of atoms to form a relatively rigid solid phase. We also obtain several physical quantities of the system, including temperature dependence of mass density, Lindemann ratio, incoherent intermediate scattering function, α-relaxation time, evolution of radial distribution function, and local bond-pair orders detected by Honeycutt-Andersen analysis. We identify three characteristic temperatures related to the vitrification: a temperature at which crossover from liquid-like to solid-like dynamics occurs on cooling, the glass transition temperature, and the Vogel-Fulcher-Tammann temperature. Behavior of liquid-like atoms in glassy state has been analyzed and discussed. © 2011 American Chemical Society.


Van Sang L.,National University of Hochiminh City | Van Hoang V.,National University of Hochiminh City | Thuy Hang N.T.,National University of Hochiminh City
European Physical Journal D | Year: 2013

Melting of fcc Lennard-Jones (LJ) nanoparticles is studied by heating up models from low temperature toward liquid phase using molecular dynamics (MD) simulation. Atomic mechanism of melting is analyzed via temperature dependence of potential energy, heat capacity, analysis of the spatio-temporal arrangements of liquidlike atoms occurred during the heating process. Moreover, radial distribution function (RDF), mean-squared displacement (MSD) of atoms and radial density profile are also used for deeper analyzing melting. Surface melting is under much attention. We also analyze the evolution of structure of nanoparticles upon heating via the global order parameter Q 6 and Honeycutt-Andersen (HA) analysis. We find previously unreported information as follows. At temperature far below a melting point, a quasi-liquid layer containing both liquidlike and solidlike atoms occurs in the surface shell of nanoparticles unlike that thought in the past. Further heating leads to the formation of a purely liquid layer at the surface and homogeneous occurrence/growth of liquidlike atoms throughout the interior of nanoparticles. Melting proceeds further via two different mechanisms: homogeneous one in the interior and propagation of liquid front from the surface into the core leading to fast collapse of crystalline matrix. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Van Hoang V.,National University of HochiMinh City
Philosophical Magazine | Year: 2011

The atomic mechanism of a glass-to-liquid transition in a monatomic Lennard-Jones (LJ) glass was studied using the molecular dynamics (MD) method. Glassy models were heated up from low temperature at two different heating rates and a glass-to-liquid transition found to occur at the higher heating rate. The temperature dependence of the potential energy, mean-squared-displacements (MSD) of the atoms and the self-intermediate scattering function indicate clearly that a glass transition occurs in the system. The atomic mechanism of the glass-to-liquid transition was investigated by analyzing the spatio-temporal arrangement of liquid-like atoms in the system upon heating. Liquid-like atoms were detected using the Lindemann-melting-like criterion. Upon heating, liquid-like atoms occur at temperatures far below the glass transition temperature (Tg) due to local instabilities. Their number increases upon further heating and they form clusters. Subsequently, a single percolation cluster of liquid-like atoms appears, which spans the system at Tg. The percentage of liquid-like atoms aggregated into this single percolation cluster reaches more than 83% at the melting point (Tm) to form a liquid phase. These results show previously unreported aspects of the glass-to-liquid transition and share some trends observed for homogeneous melting of crystalline solids without a free surface. © 2011 Taylor & Francis.


Hoang V.V.,National University of HochiMinh City
European Physical Journal D | Year: 2011

Glass formation in simple monatomic nanoparticles has been studied by molecular dynamics simulations in spherical model with a free surface. Models have been obtained by cooling from the melt toward glassy state. Atomic mechanism of glass formation was monitored via spatio-temporal arrangement of solid-like and liquid-like atoms in nanoparticles. We use Lindemann freezing-like criterion for identification of solid-like atoms which occur randomly in supercooled region. Their number grows intensively with decreasing temperature and they form clusters. Subsequently, single percolation solid-like cluster occurs at temperature above the glass transition. Glass transition occurs when atoms aggregated into this single percolation cluster are in majority in the system to form relatively rigid glassy state. Solid-like domain is forming in the center of nanoparticles and grows outward to the surface. We found temperature dependence of potential energy, mean-squared displacement (MSD) of atoms, diffusion constant, incoherent intermediate scattering function, radial distribution function (RDF), local bond-pair orders detected by Honeycutt-Andersen analysis, radial density profile and radial atomic displacement distributions in nanoparticles. We found that liquid-like atoms in models obtained below glass transition have a tendency to concentrate in the surface layer of nanoparticles. However, they do not form a purely liquid-like surface layer coated nanoparticles. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Hoang V.V.,National University of HochiMinh City
Journal of Physical Chemistry C | Year: 2012

Melting of the simple monatomic amorphous spherical nanoparticles has been studied via molecular dynamics (MD) simulation. Initial amorphous nanoparticles have been heated toward a normal liquid state to study a melting process with Lennard-Jones-Gauss interatomic potential [Engel, M.; Trebin, H.-R. Phys. Rev. Lett. 2007, 98, 225505]. Temperature dependence of various thermodynamic quantities of the system is found and discussed. Atomic mechanism of melting is monitored via analysis of the appearance/growth of the liquid-like atoms upon heating. Liquid-like atoms are determined by using the Lindemann melting criterion. In the premelting stage (i.e., below a glass transition temperature, T g), liquid-like atoms occur first in the surface shell to form a quasi-liquid surface layer. Further heating leads to the formation of a purely liquid skin at the surface of nanoparticles together with a simultaneous occurrence/growth of liquid-like atoms in the remaining glassy matrix. Melting process proceeds further via propagation/growth of liquid-like configuration into the solid core. Liquid-like configuration includes a purely liquid skin, and liquid-like atoms occurred in the liquid-solid interfacial shell of nanoparticles. Total melting occurs at temperature much higher than T g. Heat capacity of the system exhibits a single peak at around a total melting point. We find a strong thermal hysteresis between amorphous nanoparticles obtained by heating/cooling. © 2012 American Chemical Society.


Thu H.T.T.,National University of Hochiminh City | Hoang V.V.,National University of Hochiminh City
Computational Materials Science | Year: 2010

Diffusion of Ga and As ions in simulated liquid gallium arsenide, GaAs, have been studied in a model containing 3000 ions under periodic boundary conditions via molecular dynamics simulation (MD). Diffusion constant D in system has been calculated over temperatures ranged from 5000 K down to 1400 K. Calculations of liquid GaAs model with a real density at 5.3176 g cm -3 show that the temperature dependence of the diffusion constant D show an Arrhenius law at relatively low temperatures above the melting point and show a power law, D ~ (T - T c) gamma;, at higher temperatures. And upon cooling the system from relatively high temperatures to low temperatures, we found across over from non-Arrhenian to Arrhenian dynamics in the liquids, i.e. corresponding to a transition from fragile to strong liquid behaviours in the system. Furthermore, we also found the glass phase transition temperature T g for the GaAs system is anywhere around 1050 K. © 2009 Elsevier B.V. All rights reserved.


Hoang V.V.,National University of HochiMinh City
Physica B: Condensed Matter | Year: 2011

Atomic mechanism of the heating-induced phase transitions of the monatomic Lennard-Jones (LJ) glass has been studied via molecular dynamics (MD) simulations. Monatomic LJ glass was heated up at two different heating rates, crystallization occurs at the lowest one and further heating leads to the melting of LJ crystal. Thermodynamics of the phase transitions and corresponding evolution of structural properties upon heating have been analyzed in details. Atomic mechanism of a crystallization of the glassy state was monitored via spatio-temporal arrangements of the atoms involved in the 1421 bond-pair of the fcc crystalline structure. The 1421 bond-pair was detected via the HoneycuttAndersen analysis [J.D. Honeycutt, H.C. Andersen, J. Phys. Chem. 91 (1987) 4950]. We found that crystallization of the monatomic LJ glass occurs via homogeneous local rearrangements of atoms in the glassy matrix and we found an important role of the liquid-like atoms (existed in the glassy state) in crystallization of the system. In addition, spatio-temporal arrangements of the liquid-like atoms in the system upon further heating were shown in order to clarify the atomic mechanism of a melting of the obtained LJ crystal. Liquid-like atoms were defined by the Lindemann melting criterion. Our results provide previously un-reported data and give deeper understanding of the heating-induced phase transitions in the less stable metallic glasses, which have been observed in practice. © 2011 Elsevier B.V.

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