Date of Award

Fall 9-2016

Embargo Period

5-23-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Advisor(s)

Gary K. Fedder

Abstract

As we enter the age of the internet of things (IoT), more embedded devices are appearing in our everyday items, such as electrical appliances, watches, mobile phones, and even clothes. These are devices that are able to communicate with one another and collect sensing data about their environment. An emerging area of interest in this field is the wearable devices, such as smart devices for healthcare and wellness implantables. These devices require power and batteries will need to be constantly recharged, adding to the users’ inconvenience. Energy harvesters are devices that are able to harness ambient energy such as movement and convert it into useful electrical energy. As such, energy harvesters are expected to play an important role in powering such devices in order to save space and more importantly to increase the comfort and convenience of the users. This work presents a piezoelectric energy harvester that captures energy from stretchable surfaces such as the human skin, exterior of organs and even garments. The main feature of the harvester is the inclusion of a ribbon structure encased in a flexible elastomer, Ecoflex. This allows the device to stretch to tens of percents, while maintaining strain levels of the piezoelectric material within its mechanical limits, which is required since stretchable surfaces can strain to tens of percent. This device provides an efficient method of converting overall device stretch to bending stress within the piezoelectric material, and can strain both horizontally and vertically. Polyvinylidene fluoride (PVDF) and its copolymer, polyvinylidene fluoride trifluoroethylene (PVDF-TrFE) are the material of choice due to their flexibility and magnitude of piezoelectric coefficients. A thickness of 10-28 μm is chosen for the PVDF film and the total device thickness including the Ecoflex ranges from 1-3 mm. Bimorph structure and alternately-connected electrodes ensure that charge cancellation is minimized. Static and transient finite element modeling are carried out to characterize the devices and obtain trends for design parameters. The trends obtained will allow the user to select device parameters given certain constraints such as film thickness and device effective Young’s modulus. Two fabrication approaches are used to fabricate planar PVDF, PVDF-TrFE films. The first approach involves fabricating the film starting from PVDF-TrFE powder. The powder is dissolved in a solvent and cast using an in-house stainless steel structure onto a wafer that has spincast gelatin. Gelatin acts as a release layer. Aluminum and chromium are sputtered and patterned on the wafer before a second layer of PVDF-TrFE is cast again. A top metal is then patterned and a method to etch the PVDF-TrFE is developed in order to access the bottom metal. Each PVDF-TrFE layer is 10 μm thick. To ensure that the electrodes are flexible, mesh designs are incorporated. Various measurements such as mechanical, ferroelectric and piezoresponse measurements are carried out to verify the performance of the film. The second approach uses commercial PVDF film of 28 μm, with 6 μm of silver ink as the electrodes. These silver ink allows for the metal to stretch to 10% and the mesh design is not needed. Two types of patterning the metal are devised. One method involves using a laser cutter to define the shape of a label, which then acts as a mask. Another lithographic method has been devised in order to accurately define the patterns. This method involves temporarily bonding the film to a wafer and using dry film photoresist. Top and bottom side metal alignment is carried out using backside alignment option of the photolithography tool. With the planar films, a molding method is then developed in order to mold the film into the required ribbon shape. This method involves 3D printing various assembly rigs in order to achieve the desired final shapes. A snap-in locking mechanism has also been devised to enable self-alignment of the film to the mold, which is difficult to achieve on device patterns smaller than 500 μm radius. Ecoflex is used as the elastomer since this material can stretch to 400%. Three device sizes, namely 350, 500 and 750 μm radius devices are fabricated in order to have different devices for comparison with respect to scaling of the design. These devices are able to stretch up to 40%, while still maintaining structural integrity. The 350 μm device is able to generate 292 nW of power for an active volume of 0.387 cm3 when stretched 20%. This energy density is similar to other devices, albeit being able to strain at a much higher overall level. Finally, various extensions of the harvester are explored to provide an overview of the possible future work for the harvester.

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