Browsing by Subject "Friction."
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Item Open Access Frictional and vibrational properties of nanostructures(Bilkent University, 2012) Cahangirov, SeymurFrictional and vibrational properties of low-dimensional nanostructures have been investigated using the state-of-the-art ab-initio calculations. Stringent test of stability based on calculation of phonon dispersions have been performed for various materials having important potential applications in nanoscience and nanotechnology. Silicene, a counterpart of graphene composed of silicon atoms, is one of such materials with its suitability to well established silicon technology together with eccentric electronic structure due to its honeycomb symmetry. Vibrational spectrum of silicene is found to be exempt from imaginary frequencies upon the puckering of atoms in adjacent sublattices while preserving the symmetry necessary for occurrence of massless Dirac Fermions. Analyses of vibrational properties of silicene nanoribbons and carbon atomic chains revealed new interesting physics like fourth acoustical mode and long-ranged interactions due to Friedel oscillations. Basic concepts of friction science like dissipation phenomena, adiabatic and sudden processes together with several simple models of friction have been summarized. A new method for calculation of corrugation potential between layered lubricants under constant loading pressure is introduced. Transition from stickslip to continuous sliding regime is quantified through definition of frictional figure of merit for layered lubricants. Using this measure tungsten oxide is proposed as an oxidation resistant material which can outperform molybdenum disulfide as a superlubricant. It was found that, the corrugation strengths of graphene layers sandwiched between Ni slabs decrease as the number of layers increase.Item Open Access Observer based friction cancellation in mechanical systems(Bilkent University, 2014) Odabaş, CanerIn real life feedback control applications of mechanical systems, friction and time delays are two important issues that might have direct effects on the performance of systems. Hence, an adaptive nonlinear observer based friction compensation for a special time delayed system is presented in this thesis. Considering existing delay, an available Coulomb observer is modified and closed loop system is formed by using a Smith predictor based controller as if the process is delay free. Implemented hierarchical feedback system structure provides two-degree of freedom and controls both velocity and position separately. For this purpose, controller parametrization method is used to extend Smith predictor structure to the position control loop for different types of inputs and disturbance attenuation. Simulation results demonstrate that without requiring much information about friction force, the method can significantly improve the performance of a control system in which it is applied.Item Open Access Theory of atomic scale friction(Bilkent University, 1998) Buldum, AlperFriction is an old and important but at the same time very complex physical event. This thesis aims to develop an atomic scale theory of friction. VVe investigate various atomic processes and stick-slip motion by using simple models and by using simulation of realistic systems based on the stateof-the art molecular dynamics and ab-initio electronic structure and force calculations. Theoretical studies of dry sliding friction, which has a close l)earing· on the experiments done by using the atomic and friction force microscope were performed. First, a simple model is used to investigate the basic mechanisms of friction and stick-slip motion, whereby the effect of material parameters and local elastic deformation of the substrate were also examined. Then, atomic scale study of contact, indentation, subsequent |)ulling and dry sliding of a sharp and blunt metal tips on a metal surface were studied. In order to understand the atomic-scale aspects of boundary lubrication such as interesting covera.ge and load dependent behavior and structural transformations, molecular dynamics simulations were performed on a model system that has two .\'i(110) surfaces and a. xenon layer confined between these two surfaces. Finally, in view of the atomic processes revealed from computer simulations an energy dissipation mechanism and quantum heat conduction were studied.