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MEMS Piezoelectric Cantilever Beam for Energy Harvesting Applications

Introduction to piezoelectric material, derivation of the constitutive model, example application and conclusion.
by

Tim Pollock

on 9 July 2014

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Transcript of MEMS Piezoelectric Cantilever Beam for Energy Harvesting Applications

By: Tim Pollock
MEMS Piezoelectric Cantilever Beams - Energy Harvesting
What does Piezoelectric mean?
The piezoelectric effect is described as the linear coupling between mechanical and electrical state variables.
Piezoelectric Relationships
Direct Piezoelectric Effect: Creation of polarization caused by mechanical stress on a dielectric material.
Agenda:
Introduction EH and to piezoelectric material
Material Characteristics
Mechanical Relationship
Electrical Relationship
Constitutive Relationships
Piezoelectric Material Types
A look at current research
Example Fabrication
Conclusion/Questions
Piezoelectric material is a type of "smart" material that is capable of performing both sensor and actuator functions by exhibiting coupling between multiple physical domains.
Mechanical Relationship
Electrical Relationship
Dielectric materials (insulators) can be defined similarly to a linear elastic material in a Cartesian coordinate system as follows.
Typically described using a linear elastic model where behaviour can be modeled using Hooks Law.
Converse Piezoelectric Effect: Strain on a material caused by application of an external electric field.
Consitutive Model - Linear Approxamation
Total Electric Displacement
Total Mechanical Strain
Linear Elastic:
Linear Piezoelectric:
Linear Dielectric:
Linear Piezoelectric:
Example Fabrication - PMN-PT Cantilever with Tip Mass
Conclusion:
Questions
Piezoelectric Material Types
4 Catagories
Polycrystalline Ceramics
PZT (Lead Zincronate Titanate)
Very brittle
Widely used for Energy harvesting
Good coupling coef.

Piezoelectric Single Crystal
PZN-PT (Lead zinc niobate-lead titanate)
PMN-PT (Lead Magnesium Niobate / Lead Titanate)
Much higher coupling coef.
Higher costs
Difficult to fabricate
Thin/Thick Films
A1N (Aluminum Nitride)
BaTiO3 (Barium titanate)
A1N used for energy harvesting.
MEMS Compatible
Widely used for sensor applications.
Reduced coupling coef.
Pieozelectric/electrostrictive Polymers
PVDF(Polyvinylidene fluoride)
Very compliant
Widely used for sensor applications
Reduced coupling coef.
Why Vibration Energy Harvest?
Electronics have become increasingly efficient requiring less power.
Wireless sensors are becoming ubiquitous.
Green energy solutions are more important than ever.
Thermal and Solar energy is not always available or practical.
Ambient vibration found everywhere.
Why Piezoelectricity?
Effective vibration transduction method
Piezoelectric materials are becoming cheaper and more available
Can be more efficient than electromagnetic or electrostatic transduction.
Easily integrated into MEMS.
Current Research Suggests...
Multiple fabrication techniques used for various piezoelectric materials.
Bulk Micromachining
Bonding
Sol-gel process
Thin/thick film deposition (CVD)
Process is always subject to defects during fabrication which reduces effective coupling.

Piezoelectric coupling coef. (d31 or d33)
Harvesting in the transvers direction using the d31 coef. is generally easier to fabricate.
Bimorph or unimorph configuration
Series or parallel connections for bimorph.
Reduced coupling coef.
Harvesting in the longitudinal d33 direction requires more fabrication steps.
Inter-digitated electrodes needed.
Increased coupling coef.
Design and fabrication consideration should be made for cantilever length, thickness, and tip mass.
Resonant frequency (design freq.) depends greatly on these factors.
Length, thickness and size of tip mass should be easily controlled during fabrication.
Material selection, preparation and fabrication is highly important.
Reducing resonance at MEMS scale is still a great challenge.
Effective harvesting at design frequency is hard to achieve.
Wideband harvesting techniques may prove more effective.
Continued research is needed to advance this field.
Resonant frequency of 237.4 Hz
1g acceleration input
2.08V peak output voltage
2.704µW output power.
0.6mm^3
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Sources
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O. B. S. O. S. H. SALEM BIN SAADON, "Vibration-based MEMS piezoelectric energy harvesters using cantilever beams," OPTOELECTRONICS AND ADVANCED MATERIALS – RAPID COMMUNICATIONS, vol. 4, no. 8, pp. 1219-1224, 2010. Page 21
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