"This thesis presents fundamental and applied research studies designed to enable smart material-based advanced sensing technologies including the use of vanadium dioxide (VO2) thin films in resonant frequency tuning methods, and the self-powering/energy harvesting capabilities of polypropylene ferroelectret (PPFE) polymers. The large compressive stress generated from VO2 thin films during its insulator-to-metal transition (IMT) has been investigated in recent years for thermally actuated MEMS actuators. This same mechanism can be used to generate axial stress that produces large shifts in resonant frequencies. Nevertheless, taking full advantage of all benefits of this technique for tunable devices requires a fundamental understanding of the mechanisms involved and the influences of different parameters; such as structural aspect ratios, boundary conditions, buckling status, and actuation methods. In this work, VO2-based MEMS bridge and cantilever resonators were developed, and their resonant frequency shifts were characterized with respect to these parameters. It is found that residual thermal stress during the fabrication process is responsible for different buckling states in bridge structures. Bi-directional tuning for a monotonic input is observed in pre-buckled structures, which is related to bending moments and actuation methods. A ferroelectret nanogenerator is also introduced in this work as a new tuning technique to provide a programming current that allows fast switching between different resonant frequency states. This demonstrates the potential use of self-powered tuning actuation of MEMS resonators. Studies of VO2-based resonators on the power consumption and the device time constant also pave the way for integrating MEMS devices with piezoelectric energy harvesters as impact sensors.With the goal of enabling self-powered sensing technologies, a series of studies designed to understand the parameters that determine the electromechanical coupling in ferroelectret nanogenerators are presented. The electromechanical response of the active material is analyzed based on fundamental working principles of dipole moments. A lumped model is proposed, which is developed from constitutive equations and validated with experiments. The robustness of the device is verified through a series of tests including mechanical repeatability, thermal stability, and humidity resistance. The energy conversion efficiency and maximum power transfer condition are determined under periodic mechanical input, and a complete energy harvesting system with a fully integrated power management circuit is proposed for providing DC power output to effectively charge lithium-ion batteries or power small electronics."--Pages ii-iii.
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