P.K. Hung

Diffusion and structure in silica liquid: a molecular dynamics simulation

Diffusion and structure in liquid silica under pressure have been investigated by a molecular dynamics model of 999 atoms with the inter-atomic potentials of van Beest, Kramer and van Santen. The simulation reveals that silica liquid is composed of the species SiO4, SiO5 and SiO6 with a fraction dependent on pressure. The density as well as volume of voids can be expressed as a linear function of the fraction of those species. Low-density liquid is mainly constructed of SiO4 and has a large number of O- and Si-voids and a large void tube. This tube contains most O-voids and is spread over the whole system. The anomalous diffusion behavior is observed and discussed.

Computer simulation of local structure and magnetic properties of amorphous Co–B alloys

Abstract Static relaxation simulation and a systematic analysis of the local atomic structure have been performed to investigate the microstructure of amorphous Co–B alloys with pair Pak–Doyama-like potentials for all the bonds in systems. Calculations show the existence of large vacancy-like cavities in the amorphous state. The Monte Carlo method and the Ising model have been used to simulate the magnetic properties of fcc Co, amorphous Co and Co–B alloys. The temperature dependence of magnetization, local magnetization distribution and critical boron concentration for the onset of spontaneous magnetic order have been investigated and presented.

Molecular dynamics study on local structure of amorphous and liquid Al2O3

Microscopic features of liquid and amorphous alumina (Al2O3) were simulated by molecular dynamics calculations. The simulations were performed in an orthorhombic cell with 3000 particles using the Born-Mayer potential at temperatures of 0, 2500, 2700, and 3000 K under constant pressure. It was found that a large cluster of pores contained several thousand spherical pores, which were formed with radii larger than 0.73 Å. The observed variations of Al2O3 structures with atomic arrangements of AlOx (x=3, 4, 5, and 6) are discussed.

Computer simulation of liquid Al2O3

Molecular dynamic simulation has been done to determine the dynamic and local structure of liquid alumina at 3000 K. Fourteen different systems at densities ranging from 2.5 to 4.5 g cm−3 was prepared by compressing the low-density melt. Two kinds of pore aggregation, pore cluster and pore tube, were examined. Clear evidence was found of structural transformation from a tetrahedral to an octahedral network. For a low-density system there was a large pore tube, which involved 93% of oxygen-vacancy-like pores and spread over the whole simulation cell. Conversely, in a high-density system the largest pore tube contained less than 1% of all oxygen-vacancy-like pores. A similar trend also was observed for other pore kinds such as aluminium-vacancy-like pores and large pore clusters. The diffusion constants significantly decreased in the region of the structural transformation. The diffusion mechanism in low- and high-density systems was examined and is discussed here.

Evidence of ‘microscopic bubbles’ and a new diffusion mechanism for amorphous alloys

Simulation of the diffusion mechanism in amorphous alloys is carried out using three statistical relaxation models Fe(80)B(20) with 10(5) atoms. We found for the first time that the simulated models contain a large number of vacancy bubbles, which could be a diffusion vehicle such as a vacancy, as in the case of diffusion in a crystal. The numbers of these vacancy bubbles vary from 2.8 × 10(-3) to 1.245 × 10(-2) per atom depending on the relaxation degree. Diffusion coefficients have been calculated via the vacancy bubbles and they are very consistent with experiment. The relaxation effect is also studied and interpreted as a result of vacancy-bubble annihilation during thermal annealing.

Molecular dynamic simulation of liquid Al2O3 under densification

We show first that when the densification of liquid Al2O3 occurs, the structure consists of identical units AlOx and blocks OAly. Here x = 4, 5 and 6; y = 2, 3 and 4. Al2O3 liquids with different densities and the same chemical composition differ from each other only in the relative number of these units and blocks. Taking this into consideration, we examine seven MD models with the density ranging from 2.7 to 3.3 g cm−3. The simulation shows that the volume of all constructed models is a linear function of the fraction of AlOx units. Furthermore, we observe the first sharp diffraction peak (FSDP) in the scattering structure factor S(q) located at q1 = 1.85–2.15 A−1. The microscopic mechanism of densification and the origin of FSDP have been studied and discussed here.

A study of pressure-induced polymorphism in liquid GeO2

Pressure-induced polymorphism in liquid GeO2 has been investigated by a molecular dynamic model with 1998 atoms and Oeffner–Elliot potential. The simulation reveals that liquid GeO2 is made up of the species GeOx with fraction depending on density. Here x = 4, 5 and 6. This viewpoint is supported by the fact that the density, the oxygen-connectivity and the volume of void could be expressed as a linear function of the GeOx fractions. The structural change that occurs upon compression is analysed through both atom- and void-species and discussed here.

Polymorphs in GeO2 Liquid

Local structure in liquid GeO2 with density ranging from 3.56 to 5.89 g/cm3 has been investigated in MD model containing 1998 atoms. The simulation revealed that the germania liquid is composed of three species: GeO4, GeO5 and GeO6 with fraction varying with density. The density as well as volume fraction of voids can be expressed as a linear function of the fraction of those species. Low-density liquid is mainly consisted of GeO4 and has a large number of O- and Ge-voids (OV,GV) and very large void tube (LVT). This LVT contained most OVs and was spread over whole system. The low-density liquid differs from high-density liquid also in the oxygen linkage characteristics.

The Study of Pores and Free Volume in Amorphous Models

We presented the results of computing simulation of microspores and free volumes surrounding an atom in the amorphous model which constructed by statistic relaxation method on parallel computers. On purpose to accurately determine the amount of vacancy-like pores, several models containing from 104 to 4.105 atoms with boundary periodic condition have been constructed corresponding to the density of 85.56 atoms/nm 3 or 83.90 atoms/nm 3. The calculations showed that amorphous models have about 0.0009–0.0075 vacancy-like pores per atom depending on the atomic density and local metstable states.