The anisotropic mechanical behavior of γ-TiAl alloys has been observed and repeatedly reported, but the effect of crystallographic orientations on the crystal instability of γ-TiAl is still unclear. In this paper, the orientation-dependent crystal instability of γ-TiAl single crystals was investigated by performing nanoindentation on different crystal surfaces. All the nanoindentations are simulated using an interatomic potential finite-element model (IPFEM). Simulation results show that the load–displacement curves, critical indentation depth and critical load for crystal instability as well as indentation modulus, are all associated with surface orientations. The active slip systems and the location of crystal instability in five typical nanoindentations are analyzed in detail, i.e. the (0 0 1), (1 0 0), (1 0 1), (1 1 0) and (1 1 1) crystal surfaces. The predicted crystal instability sites and the activated slipping systems in the IPFEM simulations are in good agreement with the dislocation nucleation in molecular dynamics simulations.
The edges and notches of silicon wafers are usually machined by diamond grinding, and the grinding-induced subsurface damage causes wafer breakage and particle contamination problems. However, the edge and notch surfaces have large curvature and sharp corners, thus it is difficult to be finished by chemo-mechanical polishing. In this study, a nanosecond pulsed Nd:YAG laser was used to irradiate the edge and notch of a boron-doped single-crystal silicon wafer to recover the grinding-induced subsurface damage. The reflection loss and the change of laser fluence when irradiating a curved surface were considered, and the damage recovery behavior was investigated. The surface roughness, crystallinity, and hardness of the laser recovered region were measured by using white light interferometry, laser micro-Raman spectroscopy, and nanoindentation, respectively. The results showed that after laser irradiation the damaged region was recovered to a single-crystal structure with nanometric surface roughness, and the surface hardness was also improved. This study demonstrates that laser recovery is a promising post-grinding process for improving the surface integrity of the edge and notch of silicon wafers.
Photonic crystals made of macroporous silicon and liquid crystals are optical materials with a tunable dispersion relation in the infrared frequency range. Three-dimensional structures can be realized by manufacturing a two-dimensional array of pores that show a spatially periodic variation of the pore diameter. The director fields of cholesteric liquid crystals confined to such modulated pores show different topologies that depend essentially on the ratio between the helix pitch of the liquid crystal and the pore size.
In the crystal growth lab of South Dakota University, we are growing high purity germanium (HPGe) crystals and using the grown crystals to make radiation detectors. As the detector grade HPGe crystals, they have to meet two critical requirements: an impurity level of ~109 to 10 atoms /cm3 and a dislocation density in the range of ~102 to 104 / cm3. In the present work, we have used the following four characterization techniques to investigate the properties of the grown crystals. First of all, an x-ray diffraction method was used to determine crystal orientation. Secondly, the van der Pauw Hall effect measurement was used to measure the electrical properties. Thirdly, a photo-thermal ionization spectroscopy (PTIS) was used to identify what the impurity atoms are in the crystal. Lastly, an optical microscope observation was used to measure dislocation density in the crystal. All of these characterization techniques have provided great helps to our crystal activities