Along with the development of scientific calculation, computer simulation has become an important scientific research technique, computer simulation had many advantages. In this work, the mechanical, optical and structural properties of materials are discussed using computer simulation method at the atomistic and electronic level. The main results are as follows:
1. Intrinsic strengthening of coherent twin boundaries in copper
The effect of twin densities on the strength and ductility of Cu is investigated by molecular dynamic (MD) method. By adjusting the layer thickness of the bicrystals constructed in the model, the densities of twin boundaries can be changed. The stress-strain curve shows that the strength and ductility of Cu are enhanced as the twin densities increase. Moreover, the models with large amount of twin boundaries structure possess higher strength compared to that of perfect crystal. Analysis on stress distribution in deformation process demonstrates that stress concentration and consequent dislocation nucleation are apt to form interior of the crystal rather than areas adjacent to twin boundaries. Therefore, the existence of high density of twin boundaries can not only depress the dislocation nucleation, but also be obstacles to the motion of dislocation gliding along the planes with high Schmid factor.
2. Band structure investigation of B, C, N, S-doped SrTiO3
A semiconductor photocatalyst SrTiO3 sample with visible-light absorption and visible-light photodegradation was synthesized after B doping by means of ball-milling. Plane-wave pseudo-potential method based on density function theory investigates the electronic modification of B-SrTiO3 with B situating three different doped sites. In the model of B substituting O, B 2p states are located at the band gap below the conduction band. The presence of band gap states reduces the energy needed for exciting the valence band electrons to the conduction band then brings the visible-light absorption of SrTiO3; in addition, the band structures and densities of states of C, N, S-doped SrTiO3 are computed by first-principles simulation. It is suggested that the impurity states locating at the top of the valence band or at the bottom of the conduction band can redshift the absorption edge of SrTiO3 and bring the visible-light absorption, on the other hand, the deep level at band gap has negative effect on photocatalytic ability due to the formation of the recombination center.
3. MD simulation of atomic clusters evolution of CuNi alloy under rapid solidification
The MD simulations have been performed to study the atomic cluster evolution during various cooling process of Cu80Ni20 alloy with embedding-atom-method (EAM) to describe atomic interactions. Under high cooling rates, the radial distribution function (RDF) shows the typical characteristics of amorphous state. In contrast, the RDF exhibit typical traits of crystalline state for models solidified under lower cooling rates. The basic structure units of the Cu80Ni20 alloys are investigated. The results indicate that the system quenching at high rates can be considered as a network made up of the icosahedra and defect icosahedra and their diffraction graphs at room temperature exhibit amorphous rings, on the other hand, the basic units of systems quenching at low rates are dominated by clusters characteristic of crystalline state and their diffraction graphs show the diffraction spots which are the typical characteristic of crystal.
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