A molecular dynamics simulation has been carried out to investigate the dynamics heterogeneity SiO2 liquid at 2600 K and ambient pressure. We indicate that the diffusion in the liquid is realized by the rate of effective reaction, SiOx®SiOx’ and OSiy®OSiy’. Moreover, the reactions are non-uniform happened, but they are spatially clustered. In addition, we found the clustering from different sets of atoms specified by the mobility of atom or frequency of reactions. We found that the clustering become more pronounced at ambient pressure. This evidences the dynamics heterogeneity in the SiO2 liquid.
Annealing study of amorphous bulk and nanoparticle iron at temperatures from 500 K to 1000 K has been carried out using molecular dynamics (MD) simulations. The simulation is performed for models containing 104 particles Fe at both crystalline and amorphous states. We determine changes of the potential energy, pair radial distribution function (PRDF) and distribution of coordination number (DCN) as a function of annealing time. The calculation shows that the aging slightly reduces the potential energy of system. This result evidences that the amorphous sample undergoes different quasi-equilibrated states during annealing. Similar trend is observed for nanoparticles sample. When the samples are annealed at high temperatures we observe the crystallization in both bulk and nanoparticle. In particular, the system undergoes three stages. At first stage the relaxation proceeds slowly so that the energy of system slightly decreases and the samples structure remains amorphous. Within second stage a structural transformation occurs which significantly changes PRDF and DCN for the relatively short time. The energy of the system is dropped considerably and the amorphous structure transforms into the crystalline. Finally, the crystalline sample undergoes the slow relaxation which reduces the energy of system and eliminates structural defects in crystal lattices.
The domain structural transition and structural heterogeneity (SH) in GeO2 glass at 300 K and pressures up to 100 GPa are studied by means of molecular dynamics (MD) simulation. The results demonstrate that the structure of GeO2 glass comprises domain D4, domain D5, or domain D6, which depends strongly on pressure, where domain Dx (x = 4, 5, or 6) is a cluster of connected GeO x units, in which all Ge atoms possess the same coordination number of x. In the range of 9–18 GPa, GeO2 glass undergoes a structural transformation from domain D4 to domain D6 via domain D5. Under densification, structural evolution occurs along with the O xx → O xy atom variation, which comprises the processes of both merging and splitting of domain Dx and the exchange of domain-boundary (DB) atoms. The densification leads to a decrease of the Voronoi polygon (VP) volume of atoms. We found that the coexistence of separate domain structures is the origin of spatial SH in GeO2 glass. Pressure-dependent structural heterogeneity in GeO2 glass is also discussed in detail.
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.
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