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Title:Computational modeling of intrinsic dissipation in nano-structure
Author(s):Kunal, Kumar
Director of Research:Aluru, Narayana R.
Doctoral Committee Chair(s):Aluru, Narayana R.
Doctoral Committee Member(s):Saif, M. Taher; Sinha, Sanjiv; Ertekin, Elif
Department / Program:Mechanical Sci & Engineering
Discipline:Theoretical & Applied Mechans
Degree Granting Institution:University of Illinois at Urbana-Champaign
Computational Modeling
Molecular Dynamics
Abstract:In this work, using computational modeling, we study the different mechanisms of intrinsic dissipation in nano-electro mechanical systems (NEMS). We, first, use molecular dynamics (MD) simulation and gain an understanding of the underlying loss mechanisms. Using insights from the MD simulation, a multi-scale method to model intrinsic damping is developed. The high frequency vibration in NEMS have important applications. A few examples include the sensing of atomic mass, detection of biological molecules and observation of quantum effects in macroscopic objects. For all these potential applications, dissipation plays a limiting role. While a number of experimental and theoretical studies have been performed, the individual role of different mechanisms remains unclear. In this work, we attempt to isolate and understand the surface and size effect on some of the intrinsic mechanisms. We, first, consider the case of the Akhiezer damping. The Akhiezer dynamics is expected to play an important role in nano-resonators with frequencies in the GHz range. Using a judiciously devised MD set-up, we isolate Akhiezer dynamics. We show that the surfaces aid in reducing the dissipation rate through increasing the rate of thermalization of the phonons. We, next, study damping under the flexure mode of operation. A comparative analysis with the stretching mode shows that the flexure mode is less dissipative. A reduced order model is considered to understand this novel behavior. We, also, investigate the role of tension on the Q factor, a measure of the inverse of dissipation rate. From these studies, we conclude that Akhiezer dynamics plays a dominant role in nano-resonators. We, then, develop a quasi-harmonic based multi-scale method to model Akhiezer damping. A stress component, that characterizes the non-equilibrium phonon population, is derived. We obtain constitutive relation that governs the time evolution of the non-equilibrium stress. Different methods to parametrize the constitutive relation are discussed. Using the proposed formulation, we compute the dissipation rate for different cases. The results are compared with those obtained using MD. Next, we use the Boltzmann transport equation and investigate the Q factor due to the thermo-elastic dissipation (TED). The Q factor obtained shows deviations from the classical theory of TED. Correction to the classical formula, for the case of longitudinal modes, is provided. We, then, study damping is low dimensional structure. We first consider the case of two dimensional graphene sheet and under in-plane stretching. We show that the coupling between the in-plane and the out-of-plane motions plays an important role in the loss of mechanical energy. Further, a hysteresis behavior in the out-of-plane dynamics is observed. Next, we investigate the stretching motion of graphene nano-ribbon. A normal mode Langevin dynamics is devised to understand the results from the MD simulation.
Issue Date:2016-01-04
Rights Information:Copyright 2016 Kumar Kunal
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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