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Size-Dependent Elastic Properties of Ultra-Thin Objects Containing a Nano-Inclusion or a Nano-Inhomogeneity

Pahlevani, Ladan | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41448 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nano Science and Technology
  6. Advisor(s): Mohammadi Shoja, Hossein
  7. Abstract:
  8. The broad range of applicability of nano devices particularly in electronics, optoelectronics, and micro/ nano-electro- mechanical systems has drawn the attentions of the industrial and scientific communities of various disciplines. This work is devoted to study the effect of surface and interface elasticity in the analysis of the mechanical behavior of ultra-thin objects in the presence of some statical or dynamical loadings. Based on the fact that the surface-volume ratio increases in nano-scale, description of the behavior of such a small structure via usual classical theories, which generally neglect the surface/interface effect, ceases to hold. In the present study, first, a theory/method pertaining to the presence of nano-inclusions/ nano-inhomogeneities of various geometries within ultra thin films having fcc structure is developed. The interaction between atoms is modeled via Sutton and Chen potential or Rafii-Tabar and Sutton potential. Some cases as the effect of distance of the inclusion/ inhomogeneity from the free surface, temperature variations, and the interaction between two inhomogeneities are investigated. For comparison with the result from the continuum theory of elasticity, the problem of dislocation loop in an infinite domain is considered. Moreover, the behavior of the thin film in the presence of inclusion/inhomogeneity is studied using three-dimensional (3D) molecular dynamics (MD) simulation, and the result is compared with that obtained from the proposed theory. Next, in order to employ surface and interface elasticity, several key properties such as surface energy, surface stresses, and surface elastic constants of several fcc materials as well as interface properties of the noncoherent fcc bicrystals are derived in terms of Rafii-Tabar and Sutton interatomic potential functions. For determination of the surface/interface parameters a molecular dynamics program which uses the above mentioned potential function is developed. The calculated surface and interface properties are in reasonable agreement with the corresponding results in the literature. Furthermore, size-dependent elastic properties of nanosized structural elements such as nano-plates and nano-beams in presence of tension, bending and torsion are studied. Then, the effect of surface and interface elasticity in the analysis of the Saint-Venant torsion problem of an eccentrically two-phase fcc circular nanorod is considered. The problem is formulated in the context of the surface/interface elasticity. For a rigorous solution of the proposed problem, conformal mapping with a Laurent series expansion are employed together. The numerical results well illustrate that the torsional rigidity and stress distribution corresponding to such nanosized structural elements are significantly affected by the size. Finally, the scattering of anti-plane shear waves in an infinite matrix reinforced by multi-coated nanofibers/ nanotubes is studied. To analysis the problem the wave-function expansion method is coupled with the surface/ interface elasticity theory. In numerical calculations, the essential role of surfaces/ interfaces in nano-scale is well approved. Some applications of the given results can be contemplated in the design of micro/nano electro-mechanical (MEMS/NEMS) systems
  9. Keywords:
  10. Mechanical Behavior ; Eigenstrain ; Nanorod ; Surface/Interface Elastic Properties ; Nano-Inclusion/Nano-Inhomogeneity ; Ultra Thin Film ; Molecular Dynamic Simulation

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