Mechanical Engineering
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Item Microfabricated Bulk Piezoelectric Transformers(2017) Barham, Oliver M.; DeVoe, Don L; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Piezoelectric voltage transformers (PTs) can be used to transform an input voltage into a different, required output voltage needed in electronic and electro- mechanical systems, among other varied uses. On the macro scale, they have been commercialized in electronics powering consumer laptop liquid crystal displays, and compete with an older, more prevalent technology, inductive electromagnetic volt- age transformers (EMTs). The present work investigates PTs on smaller size scales that are currently in the academic research sphere, with an eye towards applications including micro-robotics and other small-scale electronic and electromechanical sys- tems. PTs and EMTs are compared on the basis of power and energy density, with PTs trending towards higher values of power and energy density, comparatively, indicating their suitability for small-scale systems. Among PT topologies, bulk disc-type PTs, operating in their fundamental radial extension mode, and free-free beam PTs, operating in their fundamental length extensional mode, are good can- didates for microfabrication and are considered here. Analytical modeling based on the Extended Hamilton Method is used to predict device performance and integrate mechanical tethering as a boundary condition. This model differs from previous PT models in that the electric enthalpy is used to derive constituent equations of motion with Hamilton’s Method, and therefore this approach is also more generally applica- ble to other piezoelectric systems outside of the present work. Prototype devices are microfabricated using a two mask process consisting of traditional photolithography combined with micropowder blasting, and are tested with various output electri- cal loads. 4mm diameter tethered disc PTs on the order of .002cm^3 , two orders smaller than the bulk PT literature, had the following performance: a prototype with electrode area ratio (input area / output area) = 1 had peak gain of 2.3 (± 0.1), efficiency of 33 (± 0.1)% and output power density of 51.3 (± 4.0)W cm^-3 (for output power of 80 (± 6)mW) at 1MΩ load, for an input voltage range of 3V-6V (± one standard deviation). The gain results are similar to those of several much larger bulk devices in the literature, but the efficiencies of the present devices are lower. Rectangular topology, free-free beam devices were also microfabricated across 3 or- ders of scale by volume, with the smallest device on the order of .00002cm^3 . These devices exhibited higher quality factors and efficiencies, in some cases, compared to circular devices, but lower peak gain (by roughly 1/2 ). Limitations of the microfab- rication process are determined, and future work is proposed. Overall, the devices fabricated in the present work show promise for integration into small-scale engi- neered systems, but improvements can be made in efficiency, and potentially voltage gain, depending on the applicationItem Miniaturized Thin-Film Piezoelectric Traveling Wave Ultrasonic Motor(2014) Rudy, Ryan Quenon; DeVoe, Don L; Polcawich, Ronald G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For many small-scale systems, compact rotary actuators are highly attractive. Many current millimeter-scale motor technologies, such as electrostatic motors and electromagnetic motors, operate at high speeds (on the order of 105 RPM) but low torque, usually pico- or nano-newton-meters. In order to drive large loads at speeds closer to 100 to 1000 RPM, gearing would be required, which drastically increases system complexity and size. Electromagnetic motors, which are effective at the macro-scale, become less practical at the millimeter-scale due to unfavorable scaling of energy density and complex fabrication. Electrostatic micro-motors require approximately 100 V for operation and produce limited torque. Traveling wave ultrasonic motors (TWUM) can provide micro- to milli-newton-meters of torque at low speeds and fill a necessary place within the millimeter-scale rotary motor landscape. Using recent developments in high quality piezoelectric film deposition and microfabrication techniques, TWUM can be made an order of magnitude smaller than currently possible. The fabrication process for the TWUM is described within, with a focus on stator fabrication and the enabling fabrication methods developed for the manufacture of TWUM, including backside vapor-HF release, deep reactive ion etch footing release, and photoresist deep-trench refill. Design and characterization of the traveling wave stator component, both disc and ring are described. Disc stators, 1 to 3 mm in diameter, exhibited traveling waves up to 1 μm in out-of-plane amplitude with quality factors in air of 95. The design process for ring stators with mechanical impedance transformer tethers is presented. The tethers are designed to allow large motion at the stator perimeter, while tethering the stator to the anchored substrate. This mechanical impedance transformer tether allowed for an in increase in standing wave amplitude by over 100% compared to straight tethers. TWUM were demonstrated and characterized, and represent the smallest TWUM currently reported, at 2 to 3 mm in diameter and less than 1 mm thick. Motor performance characteristics are presented, with speeds exceeding 2000 RPM while consuming 4 mW of power at 10 V. These millimeter-scale motors have potential applications in fields such as fuzing, medical imaging, micro-robotics, and sensor steering and calibration.Item Thermoacoustic-Piezoelectric Systems with Dynamic Magnifiers(2013) Nouh, Mostafa Akram; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermoacoustic energy conversion is an emergent technology with considerable potential for research, development, and innovation. In thermoacoustic resonators, self-excited acoustic oscillations are induced in a working gas by means of a temperature gradient across a porous body and vice versa with no need of moving parts. In the first part of this dissertation, thermoacoustic resonators are integrated with piezoelectric membranes to create a new class of energy harvesters. The incident acoustic waves impinge on a piezo-diaphragm located at one end of the thermoacoustic-piezoelectric (TAP) resonator to generate an electrical power output. The TAP design is enhanced by appending the resonator with an elastic structure aimed at enhancing the strain experienced by the piezo-element to magnify the electric energy produced for the same input acoustic power. An analytical approach to model the thermal, acoustical, mechanical and electrical domains of the developed harvester is introduced and optimized. The performance of the harvesters is compared with experimental data obtained from an in-house built prototype with similar dimensions. In an attempt to further understand the dynamics and transient behavior of the excited waves in the presence of piezoelectric coupling, a novel approach to compute and accurately predict critical temperature gradients that onset the acoustic waves is discussed. The developed model encompasses tools from electric circuit analogy of the lumped acoustical and mechanical components to unify the modeling domain. In the second part of the dissertation, piezo-driven thermoacoustic refrigerators (PDTARs) are presented. The PDTARs rely on the inverse thermoacoustic effect for their operation. A high amplitude pressure wave in a working medium is used to create a temperature gradient across the ends of a porous body located in an acoustic resonator. Finally, PDTARs with dynamic magnifiers are introduced. The developed design is shown, theoretically and experimentally, as capable of potentially enhancing the cooling effect of PDTARs by increasing the temperature gradient created across the porous body.Item Piezoelectrically Driven Thermoacoustic Refrigerator(2010) Chinn, Daniel George; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermoacoustic refrigeration is an emerging refrigeration technology which does not require any moving parts or harmful refrigerants in its operation. This technology uses acoustic waves to pump heat across a temperature gradient. The vast majority of thermoacoustic refrigerators to date have used electromagnetic loudspeakers to generate the acoustic input. In this thesis, the design, construction, operation, and modeling of a piezoelectrically-driven thermoacoustic refrigerator are detailed. This refrigerator demonstrates the effectiveness of piezoelectric actuation in moving 0.3 W of heat across an 18 degree C temperature difference with an input power of 7.6 W. The performance characteristics of this class of thermoacoustic-piezoelectric refrigerators are modeled by using DeltaEC software and the predictions are experimentally validated. The obtained results confirm the validity of the developed model. Furthermore, the potential of piezoelectric actuation as effective means for driving thermoacoustic refrigerators is demonstrated as compared to the conventional electromagnetic loudspeakers which are heavy and require high actuation energy. The developed theoretical and experimental tools can serve as invaluable means for the design and testing of other piezoelectrically-driven thermoacoustic refrigerator configurations.Item Piezoelectric Microbeam Resonators Based on Epitaxial Al0.3Ga0.7As Films(2005-11-22) Li, Lihua; DeVoe, Don; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this work, piezoelectric resonators based on single crystal Al0.3Ga0.7As films are implemented. The combination of Si doped Al0.3Ga0.7As as electrode layers and moderate piezoelectric properties of updoped Al0.3Ga0.7As film leads to lattice matched single crystal resonators with high attainable quality factors and capability of integration with high speed circuits. To validate the fabrication process, simple cantilever beam structures are developed and characterized by laser Doppler vibrometry. In order to achieve higher center frequencies, a clamped-clamped (c-c) beam design is explored. Important resonator parameters including resonance frequency, quality factor, and power handling ability are investigated. Measured quality factors of c-c beams were found to be limited by anchor losses to the substrate. A free-free (f-f) beam design is proposed in order to alleviate the energy dissipation due to anchor losses. Fabricated f-f beam devices show increased quality factors compared to the c-c beam design. Another improvement is the adoption of bimorph configuration instead of unimorph configuration. Compared to unimorph cantilever beam design, bimorph cantilevers showed 80% to 120% of increase in displacement with the same driving voltage without significant change in quality factors. The quality factors of flexural mode resonators in atmospheric pressure are low due to the effect of air damping. For this reason, proper working of flexural mode resonators requires a vacuum package which imposes unwanted complexity in packaging. To solve this problem, length-extensional mode resonators (bar resonators) are proposed to take advantage of low air shear damping. Bar resonators with lengths ranging from 1000 micro-m to 100 mico-m have been fabricated and tested. Measured resonant frequencies range from 2.5 MHz to 72 MHz with good matching to theoretical predictions. The quality factors of bar resonators at their first resonant frequency are measured in air and in high vacuum, showing values between 4,300 - 8,900 and 8,000 - 17,000, respectively, with corresponding measured motional resistances of 7.3 kohm - 10.5 kohm and 4.0 kohm - 7.8 kohm, respectively. The developed bar resonators showed excellent power handling ability up to -10 dBm which is much higher than equivalent electrostatic resonators.