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«MICRO ELECTRET POWER GENERATORS Thesis by Justin Boland In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA ...»

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MICRO ELECTRET POWER GENERATORS

Thesis by

Justin Boland

In Partial Fulfillment of the Requirements

for the Degree of

Doctor of Philosophy

CALIFORNIA INSTITUTE OF TECHNOLOGY

Pasadena, California

(Defended May 24, 2005)

ii

© 2005

Justin Boland

All Rights Reserved

iii

ACKNOWLEDGEMENTS

Yu-Chong Tai, Trevor Roper, Tanya Owen, Wen Hsieh, Ellis Meng, Tom Tsao, Mattieu

Liger, Qing He, Chi-Yuan (Victor) Shih, Scott Miserendino, Po-Jui (PJ) Chen, Nick Lo, Jayson Messenger, Svanhild (Swan) Simonson, Yuan-Heng (Denny) Chao, JR Heberle, the rest of the Caltech Micromachining Group, my candidacy committee (Yu-Chong Tai, Ken Pickar, David Rutledge, Axel Scherer, Kaushik Bhattacharya), my thesis committee (Yu-Chong Tai, Ken Pickar, David Rutledge, Melany Hunt, Changhuei Yang), funding sources (DARPA, AMRDEC, NSF), and my wife Stacey Boland.

I’ve been fortunate enough to be at the right place at the right time with enough preparation to make more accomplishments than I have time for writing this thesis. The experience has been invaluable to me, and I gained more than I thought possible. I am eternally grateful to Yu-Chong Tai, the Caltech Micromachining Laboratory and the California Institute of Technology for my time here.

iv

Dedication:

For my wife, the first Dr. Boland. So few of our family and friends really understand why we like this stuff and what it is we actually do. No, I am not in school to be an electrician and my wife is not a mechanic!

v ABSTRACT Micro Electret Power Generators The taming of electricity and its widespread use allows people to see in the dark, to speak to one another instantaneously across the earth, and it allows retrieval of data from instruments sent out of the solar system. It is right to expect that the uses and demand for electricity will continue to grow, and to extend the ability to generate electricity; here two new micromachined devices for converting mechanical energy into electrical energy are presented. Aided by the wealth of micromachining process technology, generators that use an oscillatory motion to modify the physical structure of a capacitor with a built-in electric field provided by a permanent electret have been designed, built, and tested. The electret creates an electric field inside the capacitor structure, which induces mirror charge at some potential. The modification of the capacitor then generates an alternating displacement current through an external circuit, which provides useful electrical power.

The electret microphone is a similar well known device for converting pressure waves into electrical signals by varying the distance between two charged capacitive plates.

This work explores and proves feasible the ability to use mechanical forces to change the overlapping area of a charged capacitor structure and using mechanical forces to move a liquid into the gap of a charged capacitor structure, changing its permittivity to produce electricity. This work demonstrates 2.5mW of power from a 2cm diameter rotary

–  –  –

I. Introduction

I.1. Scope of thesis

I.1.a. Organization

I.1.b. Problem Statement

I.2. Electricity

I.2.a. Generating Electricity

I.2.a.i. Triboelectricity

I.2.a.ii. Photovoltaic Generation

I.2.a.iii. Chemical Generation

I.2.a.iv. Electromagnetic Generation

I.2.a.v. Electrostatic Generation

I.2.b. Alternative Energy

I.2.b.i. Portable Alternative Energy

I.3. Energy Harvesting

I.3.a. Energy Harvesting Methods

I.3.b. Survey of Kinetic Energy Harvesting Devices

I.3.b.i. Alternative Definition: Power Scavenging

I.3.b.ii. Figures of Merit

I.3.b.ii.1. Linear Energy Harvesters

I.3.b.ii.2. Rotational Energy Harvesters

–  –  –

I.3.b.iv. Linear Electromagnetic Power Generators

I.3.b.v. Piezoelectric Power Generators

I.3.b.vi. Charge Shuttle

I.3.b.vii. Electrostatic Power Generators

I.4. Displacement Current Power Generators

I.4.a. Origin of Displacement Current

I.4.a.i. Displacement Current in a Capacitor

I.4.b. Displacement Current For Power Generation

I.4.b.i. Variable Distance Electret Power Generators

I.4.b.ii. Variable Area Electret Power Generators

I.4.b.iii. Variable Permittivity Electret Power Generators

I.5. Physical Scaling

I.5.a. Physics-Based Definition of MEMS

I.5.b. Process-Based Definition of MEMS

I.5.c. Applying MEMS

I.6. Funding

II. Electrets

II.1. Electret Classification

II.1.a. Heterocharge Electrets

II.1.b. Homocharge Electrets

II.2. Charging Methods

II.2.a. Triboelectric

–  –  –

II.3. Measurement Techniques

II.3.a. Charge Density

II.3.a.i. Error in Depth of Charge

II.3.a.ii. Lateral Resolution of Charge

II.3.b. Depth Sounding Techniques

II.4. Uniformity

II.4.a. Floating Metal Layer Electret

II.4.a.i. Floating Metal Layer Process

II.5. Conclusions





III. Variable Area Rotational Electret Power Generator

III.1. Introduction

III.1.a. Related Works

III.1.b. Micromachining Electrets

III.2. Theoretical Development

III.3. Design and Fabrication

III.3.a. Design Optimization

III.3.a.i. Charge Density

III.3.a.ii. Dielectric Constant

III.3.a.iii. Gap Spacing

III.3.a.iv. Number of Poles

III.3.b. Fabrication Considerations

III.3.b.i. Teflon Processing

–  –  –

III.3.d. REPG Version 2.0

III.3.e. REPG Version 3.0

III.3.f. REPG Version 4.0, 5.0 / Prototype Version 1.0, 2.0

III.4. Experimental Results

III.4.a. Charge

III.4.b. Testbeds

III.4.b.i. Rotational Speed

III.4.b.ii. Rotational Angular Misalignment

III.4.b.ii.1. Testbed Version 1

III.4.b.ii.2. Testbed Version 2

III.4.b.ii.3. Testbed Version 3

III.4.c. Power Generation Tests

III.4.c.i. REPG V1.0 on Testbed Version 1

III.4.c.ii. 32 Pole REPG V2.0

III.4.c.iii. 64 Pole REPG V2.0

III.5. Conclusions

IV. Liquid Rotor Electret Power Generator

IV.1. Introduction

IV.2. Theory

IV.2.a. Using Liquid Metal Instead of Liquid Dielectric

IV.3. Design and Fabrication

IV.3.a. General Considerations

–  –  –

IV.3.a.ii. Cavity Material

IV.3.a.iii. Electret

IV.3.b. Fabrication

IV.3.b.i. LEPG V1.0: Quick and Dirty

IV.3.b.ii. LEPG V2.0: PDMS Mold and Process Refinements

IV.3.c. LEPG V3.0: Multiple Channels on Single Chip

IV.4. Experimental Details

IV.4.a. Data

IV.4.a.i. Replacing Mercury with Steel Beads

IV.4.a.ii. Parallel Arrays

IV.4.a.iii. Serial Arrays

IV.4.a.iv. Non-Obvious Electrical Connections

IV.5. Conclusions

V. Conclusions and Future Work

V.1. Rotary Electret Power Generator

V.2. Liquid Rotor Electret Power Generator

–  –  –

Figure I-1 Measured vibrations from a microwave oven

Figure I-2 Electrical power delivered over time from various sources [5]

Figure I-3 Generator architecture comparison

Figure I-4 Diagram of Seiko’s Kinetic line of energy harvesting watches

Figure I-5 Exploded Diagram of Seiko’s Kinetic line of energy harvesting watches.

Figure I-6 Lab test of Seiko’s Kinetic generator. Speed corresponds to the relative rotation of the magnet to the coil.

Figure I-7 Linear electromagnetic power generator developed by Perpetuum [15].

Figure I-8 Perpetuum’s 2-terminal power generator package [16]

Figure I-9 PMG0100 Evaluation Model [17]

Figure I-10 Piezoelectric cantilever with proof mass for converting vibrations to electricity

Figure I-11 Schematic for piezoelectric windmill power generator

Figure I-12 Cross section of a charge shuttle

Figure I-13 Drive circuitry for the charge shuttle

Figure I-14 In-plane variable gap capacitance micromachined power generator...........32 Figure I-15 Out-of-plane variable gap capacitance micromachined power generator

Figure I-16 Pulsed Chemical-Electret Generator system concept

Figure I-17 Pulsed combustion of a prototype symmetric PCTR in natural aspirating mode at three different phase angles

Figure I-18 Semi-ceramic combustion chambers

Figure I-19 Two interconnected fluidic microcavities

Figure I-20 Coupon

Figure I-21 CFD simulation of a turbine

Figure I-22 Proposed new design for pulsed combustor thermal resonator and shaker generator system.

Figure II-1 Contour plot of magnetic field from a bar electret

xii Figure II-2 Contour plot of electric potential from a sheet electret

Figure II-3 Streamline plot of electric field from a sheet electret

Figure II-4 Heterocharge by polarization

Figure II-5 Homocharge electret with implanted electrons

Figure II-6.Triboelectrically charged Teflon chip

Figure II-7 Charge density of implanted Teflon using the back lighted thyratron.........61 Figure II-8 Mean charge depth for corona charged FEP Teflon

Figure II-9 Isoprobe mounted on X-Y micropositioner

Figure II-10 Charge density measurement used to determine minimum distance between data points.

Figure II-11 Average charge density and standard deviation when dropping data points from dataset used to produce Figure II-10.

By statistically comparing neighboring data points from the scan of Figure II-10, it was determined that a measurement spacing of 1mm would allow both average measurement of charge and high contrast as shown in Figure II-12

Figure II-13 Polarization map of a 11 µm thick PVDF film poled with a Tshaped electrode. At z=1μm (top graph), the polarization is significantly lower than in the bulk. The arrow indicates the direction of the high-resolution scan [ibid].

Figure II-14 Charge implanted in a chip with floating metal layer.

Figure III-1 First electret power generator. Tada (1992)

Figure III-2. Schematic of electret generator (cross-section view).

Figure III-3. Perspective view of electret generator showing a 4-pole rotor and stator.

Figure III-4 Equivalent circuit for variable area electret power generator.

Figure III-5 Used to find the critical width w from gap distance

Figure III-6 Table of different Fluorinert solvents, which are used to dilute Teflon AF 1601-S

Figure III-7 Process flow for first version of REPG

Figure III-8 REPG V1.0 mounted on testbed version 1. Photo taken before rotor and stator are aligned

Figure III-9 REPG V1.0 mounted on testbed version 1. Photo taken after rotor and stator are aligned.

xiii Figure III-10 Process flow for bulk-etched electrets

Figure III-11 Bulk-etched Teflon with anchors before reflow step.

Figure III-12 Bulk-etched Teflon with anchors after reflow step.

Figure III-13 REPG version 4.0. Cutaway view of final assembled device including bearings.

Figure III-14 Stator for REPG version 5.0. This design incorporated the use of bulk-etched cavities

Figure III-15 Charge density measurements of a 4-pole floating metal layer electret that is triboelectrically charged

Figure III-16 Proceedure for measuring angular misalignment

Figure III-17 Law of reflections on laser trajectory used to find angular misalignment

Figure III-18 Testbed with rotor and stator mounted

Figure III-19 Side view of ball joint

Figure III-20 Inside View of ball joint

Figure III-21 Side view of testbed with rotor and stator mounted

Figure III-22 Newest testbed for REPG

Figure III-23 Power output from 3 experimental trials using different load resistances and theoretical power of a continuously load matched system

Figure III-24 Power measured and theoretical vs. rotation for the 32-pole power generator with a 600kΩ load, -5x10-4C/m2 charge implanted, 2cm diameter rotor-stator pair, and 4.25μm thick Teflon electret

Figure III-25 Load matching test of a 64 pole generator on testbed version 3 at

2.5kRPM

Figure III-26 Power vs. rotation for the 64-pole power generator with a 50.3kΩ load

Figure III-27 Comparison of power measured from an actual Seiko watch to rotational electret power generators.

Figure IV-1 Femlab modeling of spatial potential from an electret that is modified by a sphere of mercury

Figure IV-2. LEPG conceptual image

Figure IV-3. Equivalent circuit for each half of the channel.

Figure IV-4 Normalized function to describe oscillations of liquid in a channel.........122 xiv Figure IV-5 Mathematically defined capacitances over one cycle. No allowance has been made for stray capacitance.

Figure IV-6 LEPG with a small droplet of water in the channel.

Figure IV-7 Process Flow a. deposit metal on glass substrate b. pattern metal c.

spin-on Teflon AF d. mask design used.

Figure IV-8 Mold Master for Sylgard 184 and peeled PDMS

Figure IV-9 Assembled LPG Device. Clear epoxy binds the top plate to the bottom plate and prevents the mercury from leaking

Figure IV-10 Electrode pattern for 6x3 cavities with 2 top and 2 bottom electrodes per cavity.

Figure IV-11 Assembled LEPG device with cutaway to reveal bottom electrodes

Figure IV-12 Test setup for LEPG mounted on shaker

Figure IV-13 Power generated in LPG V2.1 with 100μm Teflon PTFE

Figure IV-14 Still-frame position 1 taken at 2000fps while shaking at 60Hz and 1 mm peak to peak.

Figure IV-15 Still-frames position 2 taken at 2000fps while shaking at 60Hz and 1 mm peak to peak.

Figure IV-16 Experimental values for parallel channels shaking of 2.58 mm peak-to-peak at 60Hz.

Figure IV-17 Experimental values for serial columns shaking at 1 mm peak-topeak at 60Hz

Figure IV-18 Experimental values for shaking at 2.58 mm peak-to-peak at 60 Hz and Rl of 4 MOhm

Figure IV-19 Diagram showing all connections across LEPG



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