Abstract
Research of electrostrictive polymers has generated new opportunities for harvesting energy from the surrounding environment and converting it into usable electrical energy. Electroactive polymer (EAP) research is one of the new opportunities for harvesting energy from the natural environment and converting it into usable electrical energy. Piezoelectric ceramic based energy harvesting devices tend to be unsuitable for low-frequency mechanical excitations such as human movement. Organic polymers are typically softer and more flexible therefore translated electrical energy output is considerably higher under the same mechanical force. In addition, cantilever geometry is one of the most used structures in piezoelectric energy harvesters, especially for mechanical energy harvesting from vibrations. In order to further lower the resonance frequency of the cantilever microstructure, a proof mass can be attached to the free end of the cantilever. Mechanical analysis of an experimental bimorph structure was provided and led to key design rules for post-processing steps to control the performance of the energy harvester. In this work, methods of materials processing and the mechanical to electrical conversion of vibrational energy into usable energy were investigated. Materials such as polyvinyledenedifluoridetetra-fluoroethylene P(VDF-TrFE) copolymer films (1um thick or less) were evaluated and presented a large relative permittivity and greater piezoelectric β-phase without stretching. Further investigations will be used to identify suitable micro-electromechanical systems (MEMs) structures given specific types of low-frequency mechanical excitations (10-100Hz).
Highlights
Natural energy sources are attracting a rising amount of interest due to increasing environmental concerns
The characteristic equations of piezoelectric materials are ܦଷ ൌ ߝଷଷܧଷ ݀ଷଵܶଵ and ܵଵ ൌ ݀ଷଵܧଷ ݏଵଵܶଵ where ܦଷ is the electric displacement in the polarization direction, ܵଵ is the strain in the axial direction, ߝଷଷ is the dielectric permittivity of the piezoelectric material in the polarization direction at constant stress condition, ܧଷ is the electric field in the polarization direction, ܶଵ is the stress in the axial direction of the cantilever, ݀ଷଵ is the piezoelectric coefficient, and ݏଵଵ is the compliance of piezoelectric material under constant electric field condition [11]
A noticeable decrease in the initial deflection of the bimorph structures was noticed with the addition of the poly(vinylidene fluoride) (PVDF) layer due to the increase structural rigidity of the arms increasing with the additional thickness
Summary
Natural energy sources are attracting a rising amount of interest due to increasing environmental concerns. The fluorine atom from TrFE stabilizes the ȕcrystalline phase and discourages Į-crystalline phase formation [1] This property permits P(VDF-TrFE) copolymer to be produced in the form of thin-films by spin coating, and allows a suitable control of sample thickness which is ideal for the production of energy harvesting microstructures. Energy harvester performance can be predicted based on the dimensions, mass of the cantilevers, and proof mass In this structure, a thin layer of P(VDFTrFE) will be deposited and patterned into a cantilever and bonded with a top and bottom electrodes serving as conductors for the generated charge. Since PolyMUMPs uses a predefined process and materials, post-processing was required for deposition and patterning P(VDF-TrFE) polymer for energy harvesting application. This release step removed the untrapped sacrificial oxide layers (1st Oxide and 2nd Oxide) freeing the first and second mechanical layers of polysilicon (Poly and Poly2)
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