Date of Award

1-1-2013

Document Type

Dissertation

Degree Name

Ph.D.

Department

Engineering

First Advisor

Yun-bo Yi

Keywords

MEMS, Multi-physical, Oscillator, Piezoresistive effect, Resonator, Thermoelastic damping

Abstract

First, this research presents experimental and theoretical investigation of the response of micro scale dual-plate thermal-piezoresistive resonators (TPR) and self-sustained oscillators (TPO) to different gases and pressures. It is demonstrated that the resonant frequency of such devices follows particular trends in response to the changes in the surrounding gas and its pressure. A mathematical model has been derived to explain the damping dependent frequency shift characteristic of TPO. The solution of the model indicates that the stiffness of the actuator beam decreases as the value of damping coefficient drops at lower gas density caused by the change in the gas molecular mass or pressure. When operated in the TPR mode of the same device, however, the frequency shift of the same silicon structure is mainly a function of gas thermal conductivity. The two different sensing mechanisms are confirmed by the measurement results showing opposite frequency shift for the TPR and TPO in helium-nitrogen mixtures. In pressure tests, frequency shifts as high as -2300ppm were measured for a TPO by changing the air pressure from 84kPa to 43kPa.

Second, the effect of geometry on thermoelastic damping (TED) in micro beam resonators is evaluated using an eigenvalue finite element formulation and its corresponding customized MATLAB program. The vented clamped-clamped (CC) and clamped-free (CF) beams with square-shaped vents along their center lines, are both analyzed. The quality factor and resonant frequency are obtained as functions of various geometrical parameters including the location, number and size of the vents. The numerical results reveal that the addition of vent sections in the clamped end region can significantly enhance the quality factor under TED. The maximum improved quality factor as high as 3,801 and 2,257 times as those of the solid CC and CF beams are realized. The methodology presented in this work provides a useful tool in the design optimization of micro beam resonators against TED.

Third, a new method to compensate the TED by taking the advantage of piezoresistive effect is proposed. Such method is implemented by applying an electrostatic field through the MEMS beam resonator with negative piezoresistive coefficient. In the case of vibration, the stretched part of the beam generates higher electrical power thus higher temperature and vice versa. Such temperature distribution can compensate the opposite thermoelastic temperature to suppress TED. The work principle is described by a set of coupled differential equations and then solved by an eigenvalue finite element method. The numerical result indicates that the TED in beam resonators can be completely suppressed when the strength of electrical field reaches a critical value, namely CEF. The value of the CEF is further analyzed by parametric studies on various material properties and geometric factors.

Provenance

Recieved from ProQuest

Rights holder

Xiaobo Guo

File size

131 p.

File format

application/pdf

Language

en

Discipline

Mechanical engineering

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