Browsing by Author "Dr. P. A. Gremaud, Committee Member"

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    Absorbing Boundary Conditions for Molecular Dynamics
    (2009-03-05) Thirunavukkarasu, Senganal; Dr. M. N. Guddati, Committee Chair; Dr. P. A. Gremaud, Committee Member; Dr. M. S. Rahman, Committee Member
    With the goal of minimizing the domain size for Molecular dynamics (MD) simulations, we develop a new class of absorbing boundary conditions (MD-ABC's) that can mimic the phonon absorption of an unbounded exterior. The proposed MD-ABC's are extensions of perfectly matched discrete layers (PMDL), originally developed as an absorbing boundary condition for continuous wave propagation problems. Called MD-PMDL, this extension carefully targets the absorption of phonons, the high frequency waves, whose propagation properties are completely different from continuous waves. This thesis presents the derivation of MD-PMDL for general lattice systems, followed by explicit application to one-dimensional and two-dimensional square lattice systems.The accuracy of MD-PMDL for phonon absorption is proven by analyzing reflection coefficients, and demonstrated through numerical experiments. Unlike existing ABC's that are effective for high frequency phonon absorption, MD-PMDL is local in both space and time and is thus more efficient. Based on their favorable properties, it is concluded that MD-PMDL could provide a more effective alternative to existing MD-ABC's.
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    Electro-Thermo-Mechanical Modeling of Metal to Metal Contact RF MEMS Switches
    (2008-12-05) Shanthraj, Pratheek; Dr. M. A. Zikry, Committee Chair; Dr. K. Peters, Committee Member; Dr. P. A. Gremaud, Committee Member
    SHANTHRAJ, PRATHEEK. Electro-Thermo-Mechanical Modeling of Metal to Metal Contact RF MEMS Switches (Under the supervision of Professor Mohammed A. Zikry.) Three-dimensional fractal representations of surface roughness are incorporated into a finite-element framework to obtain the electro-thermo-mechanical behavior of asperities, and to relate them to the behavior of metal to metal contact RF MEMS switches. Fractal surfaces are generated from the Weierstrass-Mandelbrot function that is representative of AFM surface roughness in RF MEMS devices. A specialized finite element scheme is developed, which couples the thermo-mechanical asperity deformation with the electro-mechanical contact characteristics to obtain accurate predictions of contact evolution as a function of time and loading. Using this approach, simulations are then used to investigate how surface roughness, different material choices, initial residual strains, applied voltages, and operating temperatures affect contact parameters. Based on these detailed predictions, tribological design guidelines are obtained for predictions that can be used to significantly increase the lifetime of low contact resistance RF MEMS switches by limiting stiction, friction, and adhesion.