Elastic Effects in Diverse Material Systems: Phase Separations of Coherent Binary Alloys and Carbon Nanotube Systems

Abstract

The role of elastic strain is found to have dominant effects in twodisparate topics in materials physics. Specifically, the first partdiscusses large-scale three dimensional simulations of the phaseseparation process for elastically-coherent binary alloys withand without external strain. The second part focuses on the elasticeffects and electrical properties of addimer-induced defects on carbonnanotubes.

For both two- and three-dimensional binary alloys,the effects of long-range elastic fields on the phase separationprocess with and without external strain were investigated withlarge-scale Langevin simulations. The elastic effects incorporated inthe model are the result of anisotropy and dilational misfitsintroduced via inhomogeneities in the elastic constants of theconstituents. To understand the domain morphology, a developedselection criterion indicates their shape and/or orientationbased on the system's shear moduli. Subjected to external stresses, precipitates and theirorientation can be altered continuously intolamellar configurations. For moderate external strain, late-time,large-scale splitting of domains is observed. Other aspects ofthe coarsening process---dynamic scaling of the correlationfunctions and local inverse coarsening---are also discussed.

With tension in carbon nanotubes, the presence of addimers are shownthrough classical molecular dynamics and tight binding simulations toform defects that wrap themselves about thenanotube, which are short segments of a tube with a changed helicity.Such formations can lead to nanotube-based quantum dots. Theseheterojunctions are most favorable for (n,0) zigzagtubes, where addimers induce plastic transformationson these otherwise brittle tubes.

These defects and heterojunctions are also investigated through STMimages and conductance signatures. Prominent "ring-like'' features are observed in the STM images, whose positions correlate with the underlying geometry of the defect. By contrast, most of the defectshave only a relatively modest effect on the transport properties.However, the defects do induce localized-states either above or below the Fermi level. The STM images and conductances ofnanotube-heterojunctions are also explored.