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Title:Computational analysis of thermodynamic and mechanical properties of nano-materials
Author(s):Zhao, Huijuan
Director of Research:Aluru, Narayana R.
Doctoral Committee Chair(s):Aluru, Narayana R.
Doctoral Committee Member(s):Johnson, Harley T.; Lyding, Joseph W.; Geubelle, Philippe H.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):multi-scale method
finite temperature
fracture behavior
thermodynamic properties
mechanical properties
Abstract:Current research in nano-technology has led to advances in design and fabrication of nano-electro-mechanical systems (NEMS). The design, optimization and fabrication of NEMS for various applications can be accelerated by developing accurate physical theories, and computational design tools to describe the function of nano-devices. When the characteristic length of NEMS scales down to tens of nanometers, nano-scale effects, such as quantum effects, surface effects, material defects become significant. Classical theories and bulk material properties based on the continuum assumption may not be directly applicable for nanoscale devices. Accurate and efficient computational models and systematic study of material properties at nanoscale are among the many challenges currently facing the nanotechnology community. In this work, we extend the top-down quasi-continuum (QC) approach for multi-scale analysis of silicon nanostructures at finite temperature. The quasi-continuum method employs the classical continuum mechanics framework and the constitutive relations are extracted from the atomistic description. For finite temperature solid systems under isothermal conditions, the constitutive relation is determined by using the Helmholtz free energy density. While the static part of the Helmholtz free energy density is obtained directly from the interatomic potential, the vibrational part is calculated by using the theory of quantum mechanical lattice dynamics. Specifically, we investigate five quasi-harmonic models, namely the real space quasi-harmonic (QHM) model, the local quasi-harmonic (LQHM) model, the reciprocal space quasi-harmonic (QHMK) model, the quasi-harmonic (QHMG) model with the local phonon density of states, and the semi-local QHMG model. With these models, we compute the vibrational part of Helmholtz free energy density to further calculate the thermodynamic and mechanical properties of silicon nanostructures. Furthermore, we also investigate the variation of material properties of silicon with temperature, strain, and surface conditions. In order to check the accuracy of these models, we compute the thermodynamic properties of silicon at various temperature and strain conditions by employing the molecular dynamics (MD) simulation technique. In addition, we also employ the MD simulation to investigate the thermal expansion coefficient variation of silicon nano-slabs with different thicknesses and propose a theoretical expression for thermal expansion coefficient as a function of slab thickness and surface chirality. Graphene, which has a two-dimensional lattice structure, is a promising material in nano-technology because of its excellent electrical, thermal, optical and mechanical properties. In this work, we employ the classical MD simulation to systematically investigate the strength and stiffness variation of graphene with different chiralities, sizes, temperatures, loading conditions, and defects. We also propose a theoretical expression for fracture strength of graphene as a function of temperature, strain rate, and defect size. The accuracy of the theoretical model is checked by using MD simulation results.
Issue Date:2010-06-29
Rights Information:copyright 2010 Huijuan Zhao
Date Available in IDEALS:2012-06-29
Date Deposited:May 2010

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