Biomimetic Strategy for Antifouling Materials Developed from Mussel Adhesive Protein Mimetic Polymers
- PDF / 194,196 Bytes
- 2 Pages / 612 x 792 pts (letter) Page_size
- 42 Downloads / 211 Views
RESEARCH/RESEARCHERS
Hardness Measurements of Silicon Nanospheres Yield Values Four Times that for Bulk Silicon Recently, a team of researchers from the University of Minnesota and Los Alamos National Laboratory directly determined the mechanical response of single nanospheres of silicon with hardness up to 50 GPa, which is 4× greater than that normally expected for bulk silicon (12 GPa). As reported in the June issue of the Journal of Mechanics and Physics in Solids, W.W. Gerberich, M.I. Baskes, R. Mukherjee and colleagues first synthesized these monodispersed silicon particles by injecting vapor-phase silicon tetrachloride into an argon hydrogen plasma using a modified hypersonic plasma particle deposition technique. These particles were then transported to an aerodynamic lens system in a streamline under the influence of fluid drag and particle inertia. Lines of nanoparticles were deposited across a sapphire wafer mounted on a computer-controlled substrate translation system. The researchers began their study by performing an atomistic simulation of a 12-nm-diameter silicon nanosphere and nanoindentation measurements of a 38.6-nm-diameter silicon nanosphere. The atomistic simulation of the 12-nm-diameter silicon nanosphere, which used a modified embedded atom method (MEAM) potential, showed that no apparent dislocation nucleation occurred when two stiff platens compressed it. The unloaded nanosphere had only a ~5% change in size in the direction of the surface contact, due to phase transformation and flow of Si atoms toward the outer regions of the nanosphere, in agreement with a geometric (perfectly plastic) contact. In contrast, the nanoindentation measurements on the larger silicon nanosphere showed staircase yielding events, which were attributed to the nucleation of contact loops at the contact edges that then move down or up a glide cylinder. The
408
researchers indicated that the differences between the simulation of the 12 nm nanosphere and the measurements on the 38.6 nm nanosphere may be due to the inability of dislocation loops to nucleate in small enough volumes. Subsequently, spheres from 40 nm to 100 nm in diameter were repeatedly compressed. For these larger nanospheres, the researchers believe that plastic deformation is accomplished by dislocation nucleation. For these, it is shown that reverse plastic strain increases as the particle size decreases. For 38.6-nm-diameter silicon spheres, these particles showed as much as 48.6% strain reversal after they were loaded up to 70 µN and then unloaded. From these observations, the researchers proposed that this strain reversal is due to high dislocation densities within the particles and that these high densities also effectively work to harden the particle. For future studies, the researchers plan to verify this finding by transmission electron microscopy to confirm particle shape, size, and examine the residual dislocation array after indentation. The researchers indicated that superhard nanospheres may have potential applications in chemical–mech
Data Loading...
