Ruling Chen

Extrusion formation mechanism on silicon surface under the silica cluster impact studied by molecular dynamics simulation

Molecular dynamic simulation is applied in analyzing the deformation of silicon surface under the impact of large silica cluster. The mechanism of such a deformation is largely different from the cases of ion bombardment and indentation. With the impact of large silica cluster, the silicon surface is extruded due to the combinational effects of thermal spread, phase transformation, and crystallographic slip. It is found that thermal spread is the most significant one among these three effects. The extrusions on silicon surface will be in embryo during the impact unloading stage and will grow up during the cluster rebounding stage. Furthermore, the critical impact velocity to induce the formation of extrusions on silicon surface is associated with the incidence angle of the cluster, while it is independent from the size of the cluster. The findings are instructive in optimizing the process parameters for ultraprecision machining of silicon wafer.

Raman study on the effects of sintering temperature on the Jc(H) performance of MgB2 superconductor

The influence of sintering temperature on the critical transition temperature Tc and critical current density Jc for the MgB2 superconductor was investigated systematically with the observation of Raman scattering measurement and flux pinning force Fp analysis. The enhanced E2g mode in Raman spectra with increasing in situ sintering temperature shows gradual strengthening of the electron-phonon coupling in MgB2, which means that the crystals become more harmonic after higher temperature sintering. However, the crystal harmonicity is degraded for samples sintered at even higher temperature due to Mg deficiency. A possible explanation for the Jc(H) performance, which is in accordance with the Raman spectroscopy observation and Fp analysis, is the cooperation between the electron-phonon coupling in the E2g mode and the flux pinning centers, mainly originating from the lattice distortion due to the different sintering temperatures.

Dynamic phase transformation of crystalline silicon under the dry and wet impact studied by molecular dynamics simulation

Molecular dynamics simulation is applied in analyzing quantitatively the dynamic phase transformation of crystalline silicon under the dry and wet impact, respectively. At the impact loading stage, the phase transformation between fourfold silicon atoms and fivefold or threefold silicon atoms is affected only by the local pressure. The influence of the local temperature or the contact area on the phase transformation will emerge from the impact unloading stage. And the dynamic process of phase transformation between fourfold atoms and fivefold or threefold atoms will obey the Boltzmann distribution law by stages. The variance of the number of fivefold or fourfold atoms at impact loading stage is almost the same as the impact unloading stage. Furthermore, the dynamic residence time of fivefold or threefold atoms formed during the impact process will be about 250 fs. The half-life of these atoms is about 50 fs.

Benzoic Acid Doping to Enhance Electromagnetic Properties of

The effect of benzoic acid doping on lattice parameters, microstructure, critical temperature (T_c), critical current density (J_c) and flux pinning force of MgB₂ has been studied. In this work we used benzoic acid as an example of aromatic acids as an additive to MgB₂. For different sintering process, actual carbon (C) substitution for boron (B) was estimated to be from 3.5 at% to 5.0 at% of B while T_c dropped about 2-4 K. The advantages of aromatic acid doping include homogeneous mixing of precursor powders, avoidance of expansive nano-additives, production of highly reactive C and significant enhancement in J_c of MgB₂, compared to undoped samples. High level C substitution of B can induce the MgB₂ crystals grow into bar shape microstructure. The J_c value of 1.5 times 10⁴ Acm^-2 at 5 K and 8 T for preheated 10 wt% C₇H₆O₂ doped MgB₂ sample is higher than that of the undoped MgB₂ by a factor of 13. As there are numerous aromatic acids readily available this finding has significant ramifications not only for the fabrication of MgB₂ but also for many C based compounds and composites.

{\rm MgB}_{2}

Surface damages on silicon surface under the large silica cluster impacts have been observed and analyzed by molecular dynamics simulation. The results show that extrusion formation on silicon is affected by temperature spread, phase transformation and crystallographic slip. And the effect of temperature spread is the strongest among all of these effects. The extrusions on silicon surface will be in embryo during the impact unloading stage and wall grow up during the cluster rebound stage. Moreover, the critical velocity of the extrusion formation is affected by incidence angle and crystallographic orientation not by cluster size. These results will be helpful for the selection of process parameters in elastic emission machining.