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Mechanics of nanoscale beams in liquid electrolytes: beam deflections, pull-in instability, and stictionLee, Jae Sang 15 May 2009 (has links)
The pressure between two parallel planar surfaces at equal electric potentials is derived
using the modified Poisson-Boltzmann (MPB) equation to account for finite ion size.
The effects of finite ion size are presented for a z:z symmetric electrolyte and compared
with the pressure derived by the classical Poisson-Boltzmann (PB) equation. The
pressures predicted by the two models differ more as the bulk ion concentration, surface
potential, and ion size increase. The ratio of the pressures predicted by the two models is
presented by varying the ion concentration, surface potential, ion size and distance of
separation. The ratio of pressures is relatively independent of the distance of separation
between the two surfaces.
An elastic beam suspended horizontally over a substrate in liquid electrolyte is
subjected to electric, osmotic, and van der Waals forces. The continuous beam structure,
not a discrete spring, which is governed by four nondimensional parameters, is solved
using the finite element method. The effects of ion concentration and electric potentials
to the pull-in instability are especially focused by parametric studies with a carbon nanotube cantilever beam. The pull-in voltage of a double-wall carbon nanotube
suspended over a graphite substrate in liquid can be less than or greater than the pull-in
voltage in air, depending on the bulk ion concentration. The critical separation between
the double-walled carbon nanotube (DWCNT) and the substrate increases with the bulk
ion concentration. However, for a given bulk ion concentration, the critical separation is
independent of the electric potentials. Furthermore, the critical separation is
approximately equal in liquid and air.
Stiction, the most common failure mode of the cantilever-based devices, is
studied in a liquid environment, including elastic energy, electrochemical work done,
van der Waals work done and surface adhesion energy. We extend the classical energy
method of the beam peeling for micro-electro-mechanical systems (MEMS) in the air to
an energy method for nano-electro-mechanical systems (NEMS) in liquid electrolyte.
We demonstrate a useful numerical processing method to find the parameters to free the
stiction of the beams and to obtain the detachment length of the beams.
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Nanoscale electrostatic actuators in liquid electrolytes: analysis and experimentKim, Doyoung 12 April 2006 (has links)
The objective of this dissertation is to analytically model a parallel plate
electrostatic actuator operating in a liquid electrolyte and experimentally verify the
analysis.
The model assumes the system remains in thermodynamic equilibrium during
actuation, which enables the ion mass balance equations and Guass Law to be combined
into the Poisson-Boltzmann equation. The governing equations also include the linear
momentum equation including the following forces: the electric force, the osmotic force,
the spring force, the viscous damping force, and the van der Waals force. Equations are
also derived for the energy stored in the actuator. The analytical results emphasize the
stored energy at mechanical equilibrium and the voltage versus electrode separation
behavior including the instability. The analytical results predict that the system may not
be a good actuator because the displacement has a very limited stable range, although the
actuator would be suitable for bistable applications.
The experiment consisted of a fixed flat gold electrode and a movable gold
electrode consisting of a gold sphere several micrometers in diameter mounted on the end of an Atomic Force Microscope (AFM) cantilever, which serves as the spring. The
electrodes were separated by approximately 100nm of 1mM NaCl aqueous solution.
The analytical results were not verified by the experiment. Relative to the analysis,
the experiments did not show distinct critical points, and the experiments showed less
electrode separation for a given applied electric potential. The experiments did show
points at which the electrode separation versus electric potential rapidly changed slope,
which may be instability points.
It is suggested that this phenomenon may be due to coalesced gas bubbles on
hydrophobic regions of the electrode surfaces, which are not included in the model.
Although clean gold surfaces are hydrophilic, gold surfaces may become hydrophobic
due to impurities.
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Analysis of Instabilities in Microelectromechanical Systems, and of Local Water SlammingDas, Kaushik 09 December 2009 (has links)
Arch-shaped microelectromechanical systems (MEMS) have been used as mechanical memories, micro-sensors, micro-actuators, and micro-valves. A bi-stable structure, such as an arch, is characterized by a multivalued load deflection curve. Here we study the symmetry breaking, the snap-through instability, and the pull-in instability of bi-stable arch shaped MEMS under steady and transient electric loads. We analyze transient finite electroelastodynamic deformations of perfect electrically conducting clamped-clamped beams and arches suspended over a flat rigid semi-infinite perfect conductor. The coupled nonlinear partial differential equations (PDEs) for mechanical deformations are solved numerically by the finite element method (FEM) and those for the electrical problem by the boundary element method.
The coupled nonlinear PDE governing transient deformations of the arch based on the Euler-Bernoulli beam theory is solved numerically using the Galerkin method, mode shapes for a beam as basis functions, and integrated numerically with respect to time. For the static problem, the displacement control and the pseudo-arc length continuation (PALC) methods are used to obtain the bifurcation curve of arch's deflection versus the electric potential. The displacement control method fails to compute arch's asymmetric deformations that are found by the PALC method.
For the dynamic problem, two distinct mechanisms of the snap-through instability are found. It is shown that critical loads and geometric parameters for instabilities of an arch with and without the consideration of mechanical inertia effects are quite different. A phase diagram between a critical load parameter and the arch height is constructed to delineate different regions of instabilities.
The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation in the commercial FE software LS-DYNA. The Lagrangian formulation is used to describe planestrain deformations of the wedge and the Eulerian description of motion for deformations of the water. A penalty contact algorithm couples the wedge with the water surface. Damage and delamination induced, respectively, in a fiber reinforced composite panel and a sandwich composite panel and due to hydroelastic pressure are studied. / Ph. D.
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