Date of Award

Spring 2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Long Que

Abstract

In this dissertation, a hybrid energy harvesting system based on a lead zirconate titanate (PZT) and carbon nanotube film (CNF) cantilever structure has been designed, fabricated and studied. It has the ability to harvest light and thermal radiation energy from ambient energy and convert them to electricity.

The proposed micro-scale energy harvesting device consists of a composite cantilever beam (SU-8/CNF/Pt/PZT/Pt) which is fixed on a silicon based anchor and two electrode pads for wire bonding. The CNF acts as an antenna to receive radiation energy and convert it to heat energy and then transfer to the whole cantilever structure. The CNF will also convert the radiation energy to a non-uniform distributed static charge. These are two major reasons that cause the cantilever to bend and give the ability of cyclic bending back and forth of the cantilever. The PZT layer, in turn, converts the mechanical energy of repeated deformation of the cantilever to electricity by the piezoelectric effect.

First, the cyclic bending capability of the composite cantilever when receiving radiation energy, named self-reciprocation, has been evaluated by copper-CNF cantilever structures and the proposed mechanisms have been discussed. Based on this idea, a prototype macro-scale device with PZT and CNF integrated has been used to verify the possibility of harvesting energy from light and thermal sources by the self-reciprocation phenomenon. Open circuit voltage (OCV) output recorded from the prototype device showed continuous oscillation while a constant radiation source was presented.

The proposed micro-scale energy harvesting device was then designed and the fabrication process flow has been developed using surface and bulk micromachining techniques. The fabricated device was polarized in a strong electric field at raised temperature to boost the piezoelectric coefficient. A validation step is designed to pick out the working devices before testing. The functioned device was then tested and successfully demonstrated to harvest energy from light and thermal sources. The result showed the power density of the micro-scale device is 4,445 times higher than the macro-scale prototype device calculated from the maximum power transfer theorem.

It was found that the electric output of the micro-scale device contains not only the AC component as the prototype device but also a DC bias shift added to the AC component. An equivalent structure model of the micro-scale device was established to study the electric output characteristic. It was realized that the DC bias shift is generated from the thermoelectric effect (Seebeck effect) by controlled experiments and analysis.

The performance of the micro device was studied under different levels of light and thermal radiation conditions. The relationship between output (both DC and AC components of open circuit voltage and short circuit current) and input (light and thermal energy) were analyzed by the least square regression method.

The device was taken out of the laboratory to demonstrate its ability to harvest energy in ambient conditions. Both the DC and AC components of the open circuit voltage (electricity) were able to be generated from the solar and wind energy. The power density generated from a single device was about 4 µW/cm2. Further enhancement of the power density was proved by concentrating solar energy on the device with a magnifier and operating an arrayed device.

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