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Zhang X.,Tongji University | Zhang X.,Institute for Telecommunications Technology | Wu L.,Tongji University | Sessler G.M.,Institute for Telecommunications Technology
AIP Advances | Year: 2015

Piezoelectret films are prepared by modification of the microstructure of polypropylene foam sheets cross-linked by electronic irradiation (IXPP), followed by proper corona charging. Young's modulus, relative permittivity, and electromechanical coupling coefficient of the fabricated films, determined by dielectric resonance spectra, are about 0.7 MPa, 1.6, and 0.08, respectively. Dynamic piezoelectric d33 coefficients up to 650 pC/N at 200 Hz are achieved. The figure of merit (FOM, d33 g33) for a more typical d33 value of 400 pC/N is about 11.2 GPa-1. Vibration-based energy harvesting with one-layer and two-layer stacks of these films is investigated at various frequencies and load resistances. At an optimum load resistance of 9 MΩ and a resonance frequency of 800 Hz, a maximum output power of 120 μW, referred to the acceleration g due to gravity, is obtained for an energy harvester consisting of a one-layer IXPP film with an area of 3.14 cm2 and a seismic mass of 33.7 g. The output power can be further improved by using two-layer stacks of IXPP films in electric series. IXPP energy harvesters could be used to energize low-power electronic devices, such as wireless sensors and LED lights. © 2015 Author(s).

Zhang X.,Tongji University | Zhang X.,Institute for Telecommunications Technology | Sessler G.M.,Institute for Telecommunications Technology | Xue Y.,Tongji University | Ma X.,Tongji University
Journal of Physics D: Applied Physics | Year: 2016

Laminated fluoropolymer films with a regular microstructure were made from compact fluoroethylenepropylene (FEP) and porous polytetrafluoroethylene (PTFE) using a process consisting of patterning and fusion bonding steps. The fabricated films were rendered piezoelectric via the contact charging or corona charging methods. The piezoelectric responses of such piezoelectret films were measured in the frequency range 100 Hz-100 kHz. The results show that the acoustic impedance of the FEP/PTFE films is around 0.014-0.030 MRayl. Dynamic piezoelectric d 33 coefficients of up to 500 pC N-1 were achieved at 100 Hz for these films. Microphones built with such films exhibit flat response curves in a broad frequency range if the diffraction effects are eliminated. Bonded films with all positive charges deposited in the porous PTFE layers show the best thermal stability: after annealing for 1100 min at 125 °C, the remaining d 33 at 1020 Hz is about 30% of the initial value, corresponding to 105 pC N-1, and it remains relatively stable at this temperature. This remarkable thermal stability has to be attributed to the fact that positive charges are more permanent in porous PTFE than in FEP. The entire charge distribution exhibits much better thermal stability than is achievable for customary polypropylene piezoelectrets. © 2016 IOP Publishing Ltd.

Zhang X.,Institute for Telecommunications Technology | Hillenbrand J.,Institute for Telecommunications Technology | Sessler G.M.,Institute for Telecommunications Technology | Haberzettl S.,Institute for Telecommunications Technology | Lou K.,Tongji University
Applied Physics A: Materials Science and Processing | Year: 2012

Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5 μm thick FEP layers and a typical void of 60 μm height, the critical voltage necessary for the built-up of the "macrodipoles" in the inner voids is approximately 800 V. At room temperature, Young'smodulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85 μm, is about 0.21 MPa. Initial quasistatic piezoelectric d 33 coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000-3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) d 33 coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results. © 2012 Springer-Verlag.

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