No abstract

The phase transformation and the structure change of the silicon surface are investigated by molecular dynamics simulation of the incidence and recoil of a nanoparticle at the monocrystlline silicon surface. The simulation shows that during the collision process, the impacted region on the silicon surface transforms from diamond structure to a molten state, then goes through the state of supercooled liquid, and finally solidifies into an amorphous phase. Furthermore, the temperature of solidification transformation calculated from the simulation is very close to the glass transition temperature of silicon. The structure changes taking place during the nanoparticles recoiling process are also revealed. Beginning with an instantaneous, highly disordered, and badly depressed supercooled state, the impacted region evolves along the direction to a more ordered and lessdepressed state. These evolutional tendencies are determined by the cooling andunloading process the impacted region undergoes. The amorphous silicon formed after the collision has the average coordination of 5.27, and the fivefold and sixfold coordinated atoms accounts for 61.5% of the whole atoms in the impacted region.

Molecular dynamics simulation of nanoparticle colliding with monocrystalline silicon surface has been carried out in order to investigate the repulsion behavior of nanoparticles. The dynamical behavior of the nanoparticle, the atomistic structure of elastic and plastic deformation of the substrate, and the transformati on of energy during the collision process are analyzed. A hemispherical crater i s formed on the Si(001) substrate, and there exists an amorphous layer on the wa ll of the crater. In the collision process, the atoms of the substrate that lie near the nanoparticles transform to amorphoustate immediately. And the elastic d eformation which is characterized by the reversible (111)［110］ glide structure is produced outside the amorphous layer. During the incidence stage, the substr ate undergoes compressed elastic deformation. And during the repulsion stage, th e potential energy of the substrate declines oscillatory with compressed elastic deformation and tensed elastic deformation formed alternately. The compressed e lastic deformation energy stored in the substrate is transferred to the transnat ional kinetic energy of the nanoparticle, which forces it back from the surface.

The influence of electric field on near-interface 4-pentyl-4'-cyanobiphenyl(5CB) liquid crystal (LC) is investigated with quartz crystal microbalance (QCM). The results of QCM show that the process of frequency shifting with electric field, which reflects the viscoelasticity change of 5CB, can be divided into two parts. Then the two-layer model of 5CB is proposed to illuminate the results of QCM, thereby indicating that the effect of electric field on near-interface layer is different from on bulk layer. Quantitative analysis is carried out with two-layer model of QCM, which indicates that there is a near-interface layer of about 100nm, adsorbed on the upper electrode of quartz crystal. The complex shear viscosity of the near-interface layer decreases with electric field strength increasing, which is opposite to the rule of bulk viscosity of 5CB.

Slip at the interface of solidified electrorheological （ER） fluid and electrodes is harmful for ER applications. Compression tests using four kinds of electrodes, namely the smooth, hole array patterned by laser pulse, nylon net covered, and acid etched column patterned electrodes, have been done, respectively. Results show that laser patterned and nylon net covered electrodes significantly enhanced the compressive stress of the ER fluid. The enhancement is ascribed to the increase of local electric field near electrodes after patterning, which increased the interfacial strength between ER fluid and electrodes, and effectively suppressed the slip of solidified ER fluid from electrode. The tests of current density during compression and the finite element analysis of the electric field distribution of patterned electrodes supported this slip suppression effect. The results raised a fundamental question of what is the real strength of ER fluids, since the slip of highly solidified ER fluid are usually not considered during various tests. This investigation also shows that patterning electrodes is a good way to improve mechanical performances of ER fluids.