Browsing by Author "Yong Zhu, Committee Member"

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    Carbon Nanotubes and Carbon Nanotube Fiber Sensors: Growth, Processing and Characterization.
    (2010-09-15) Zhao, Haibo; Fuh-Gwo Yuan, Committee Chair; Yuntian Zhu, Committee Chair; Yong Zhu, Committee Member; Melur Ramasubramanian, Committee Member
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    Design, Fabrication and Experimental Characterization of PZT Membranes for Passive Low Frequency Vibration Sensing
    (2008-12-16) Zohni, Omar Shariff; Andre Mazzoleni, Committee Member; Richard Siergiej, Committee Member; Richard Keltie, Committee Member; Gregory Buckner, Committee Chair; Yong Zhu, Committee Member
    Low frequency vibration sensing is being used increasingly to monitor the health of machinery and civil structures, enabling “need-based†maintenance scheduling and reduced operating costs. Passive sensors are of particular interest because they don’t require input energy to monitor vibration. Modern vibration sensors are often micro electromechanical systems (MEMS), and are usually very basic in design consisting of a cantilevered beam with some type of deflection sensing circuit. Under the influence of acceleration the beam deflects from its nominal position and its deflection is measured using optical, capacitive or piezoelectric techniques. MEMS sensors tend to exhibit very large stiffness to mass ratios, making them best suited to high frequency vibration sensing. Sensors utilizing the piezoelectric effect can achieve direct energy conversion from the mechanical domain (strain) to the electrical domain (charge) via piezoelectric coupling coefficients. To maximize the electrical output, lead zirconate titanate (PZT) is an excellent piezoelectric material due to its high coupling coefficients. However, the introduction of PZT into standard MEMS processes is problematic because lead is considered a contaminant in most silicon based fabrication facilities. Additional complications with stresses and delamination in thin film stacks have hindered the development of robust fabrication processes for these devices. This dissertation investigates candidate MEMS sensor geometries and fabrication processes for passive low frequency vibration sensing. The addition of silicon nitride (Si3N4) thin films into sol-gel deposited PZT stacks is studied, and the effects of various adhesion layers on delamination and ferroelectric characteristics are quantified. A fabrication process is developed allowing for both front and back side contact for electrical measurements. The effects of thin film stresses on the frequency response of PZT membranes are investigated using experimental, analytical, and computational techniques. Results indicate that thin film stresses in silicon dioxide (SiO2) and Si3N4 can shift the natural frequencies of sensor membranes by as much as 20%. Optimization of sensor membranes is conducted using available numerical methods, particularly finite element analysis (FEA). Coupled electromechanical measurements of fabricated membranes are conducted and experimental results are compared with numerical and analytical solutions. The research outlined in this dissertation represents the first known investigation of passive MEMS vibration sensors specifically targeting such a low frequency range. Also, the integration of PZT into a standard MEMS process requiring low pressure chemical vapor deposition (LPCVD) Si3N4 has not been reported previously. A robust integrated PZT fabrication process is developed which can be used for future work in this field. This process includes a reliable adhesion layer which can be used when deep wet etching of silicon is required. Recommendations for future work and for incorporating these results into packaged sensors are presented.
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    Development of a High Throughput Nano-Positioning System with Applications in Micro-Manufacturing.
    (2010-04-07) Polit, Sebastian; Jingyan Dong, Committee Chair; Yong Zhu, Committee Member; Yuan-Shin Lee, Committee Member
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    Transient Waves from Acoustic Emission Sources in Isotropic Plates Using a Higher Order Extensional and Bending Theory.
    (2010-03-15) Bogert, Philip B.; Fuh-Gwo Yuan, Committee Chair; Eric Klang, Committee Member; Kara Peters, Committee Member; Yong Zhu, Committee Member
    This dissertation presents a derivation for the transient wave response of an infinite isotropic plate to a general acoustic emission (AE) point source discontinuity loading, based on third-order plate theory. The calculation of the wave response is facilitated by employing the concept of a seismic moment tensor (or derived “equivalent†body-forces) to describe the loading from highly localized displacement discontinuities on a fracture surface. Further, the body forces from 3-D elasticity are converted to plate loadings for use in the plate theory wave equations of motion. The transient wave response can be detected as AE signals using piezoelectric sensors. In particular, time-dependent surface strains can be readily obtained experimentally. Therefore the results emphasize the calculation of the surface strains for potential comparison with future experiments. The calculated transient response, which represents waves propagating from a general AE point source in the plate, is expressed in an explicit integral form. It is shown that the transient response, which is given by double inverse Fourier transforms, can be simplified into a finite series involving inverse Hankel transforms which only require one-dimensional inversions for an isotropic plate. Thus numerical evaluation of the transient wave is more robust and accurate than that generated using two-dimensional inverse transforms and also, asymptotic solutions can be readily obtained. Nine types of AE sources representing different micro-damage mechanisms and their corresponding plate loads are discussed. Numerical results for four types of AE point sources with a Heaviside time history loading are presented. The long-term goal of the development, having established a relationship between disturbance and response, is to monitor responses in a structure and be able to determine the source, i.e. damage, type and location by solving the inverse problem in real time. What is new and different from previous work upon which this is building is that the extensional formulation is evaluated for general AE loading, and a higher order bending theory is developed and evaluated. Additionally, the polar conversion reduction to a single variable spatial integration is implemented for both theories.