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Title:Modeling of mechanical energy dissipation of low-dimensional resonators
Author(s):De, Subhadeep
Director of Research:Aluru, Narayana R.
Doctoral Committee Chair(s):Aluru, Narayana R.
Doctoral Committee Member(s):Sinha, Sanjiv; van der Zande, Arend; Zhang, Yang
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
relaxation time
mode coupling
molecular dynamics
Abstract:Nanoelectromechanical systems (NEMS) made from low-dimensional materials based on carbon, transition metal dichalcogenides, and their combinations have opened up new possibilities in high-precision sensing, signal processing, and studies of fundamental physical phenomena. In the heart of the NEMS is a vibrating mechanical element, known as the resonator. The performance of the NEMS critically depends on the mechanical energy dissipated by the resonator. Studies on dissipation are important because an understanding of the loss mechanisms can suggest ways to mitigate it. In most practical scenarios, the resonators suffer from intrinsic dissipation mediated by its inherent atomic thermal motions or phonons and extrinsic dissipation due to a fluid environment. In this context, low-dimensional resonators need special attention because the dissipation cannot be explained using the existing continuum theories. Due to atomic thickness, sub-micron dimension, and mega- to gigahertz frequencies of these resonators, nano-scale physical processes start becoming important. Most macroscale models do not account for these physical processes, warranting the current line of research. In this thesis, we use atomistic simulations and statistical-mechanical theories to understand and formulate the nanoscale physical processes, and integrate them to develop a multiscale model for dissipation. In the first part of the thesis, we explore fluid coupled resonator systems with an objective to understand different dissipative processes such as phonon-mediated intrinsic dissipation, viscous damping by the fluid, and the cross-interaction between each source of dissipation, i.e., phonons and fluid at a regime of gigahertz frequency, and nanometer length scale. First, we consider a single-walled carbon nanotube (SWCNT) resonator with confined Argon and driven under axial mode. The intrinsic dissipation in the SWCNT at gigahertz frequencies could be explained by Akhiezer theory. We show that intrinsic dissipation, which is conventionally treated as an independent process, can be modified by fluid interactions due to the phonon- fluid coupling. We show that an important consequence of this phonon-fluid coupling is the counter-intuitive inverse scaling of net dissipation with fluid density at low excitation frequencies. Next, we consider flexural vibration of the SWCNT with interior and exterior Argon. When compared with the fluid exterior case, the SWCNT with confined fluid shows a low and anomalous scaling of dissipation with fluid density. We systematically analyzed the sources of dissipation and found that the fluid contributed to the anomalous scaling. A formulation of the fluid response during the flexural motion revealed a viscoelastic nature of the fluid under nano-confinement, which explains the anomalous scaling. Further, we use the framework for dissipation analysis to examine the effect of thermal motion of the resonator atoms on fluid dissipation, demonstrate a frequency dependent dissipation scaling with density, and comment on the mechanism of intrinsic dissipation during flexural resonance of an SWCNT. In the second part, we develop a multiscale framework to model intrinsic dissipation in two-dimensional (2D) microresonators. The work aims to reveal the fundamental limit of dissipation and enable looking at the isolated effect of various parameters over a wide range, both of which are inaccessible in experiments. The damping of the flexural mode of a 2D microresonator takes place due to the nonlinear coupling with other thermally excited elastic modes. A particular flexural mode can couple with another flexural mode with a wavelength ranging from the size of the resonator to that of the lattice spacing. However, the coupling at these disparate length scales needs different modeling approaches. In the multiscale framework, we model the continuum-scale modes as Langevin oscillators (LOs) with nonlinear coupling terms. The parameters of the LOs are computed using continuum mechanical analysis and atomistic simulations. Using this framework, we study the effect of various parameters of interest such as vibration amplitude, resonator size, temperature, and pre-strain in the case of graphene resonators and draw some important conclusions towards engineering high-quality 2D resonators.
Issue Date:2019-02-15
Rights Information:Copyright 2019 by Subhadeep De
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05

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