Dome-like PVDF actuators: Preparation, phase transformation and piezoelectric properties
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Dome-like PVDF actuators: Preparation, phase transformation and piezoelectric properties Michael Wegener1, Robert Schwerdtner2, Martin Schueller3, and Andreas Morschhauser2 1
Functional Materials and Devices, Fraunhofer Institute for Applied Polymer Research (IAP), Geiselbergstraße 69, 14476 Potsdam, Germany 2 ZfM – Center for Microtechnologies, Chemnitz University of Technology, Reichenhainer Straße 70, 09126 Chemnitz, Germany. 3 Multi Device Integration, Fraunhofer Research Institution for Electronic Nano Systems (ENAS), Reichenhainer Straße 88, 09126 Chemnitz, Germany ABSTRACT In this work, dome structures in PVDF films were prepared as ultrasonic transducer. The domes are realized by a deep drawing process. The dome-forming process leads to a phase transformation from the non-polar α into the polar β phase within dome wall and roof of the PVDF films. A ferroelectric polarization is obtained in these dome areas after suitable electrical poling which yields a piezoelectric activity. Because of the piezoelectric activity within the film plane, a dome-roof up-and-down actuation is observed with resonance frequencies in the range between 65 and 93 kHz. INTRODUCTION Piezoelectric materials transform mechanical stresses into electrical signals (direct effect, sensor function) or an electrical stimulus into a form change of the original geometry (inverse effect, actuator function). Piezoelectric sensors are used in various applications for measuring direct or air-transferred impacts from low up to ultrasonic frequencies. As actuators, piezoelectric materials deliver motions also at high frequencies and are often embedded as active materials in ultrasonic systems. The actuation amplitude of piezoelectric polymers (as well as of ceramics) are relatively small in comparison e.g. to that of other electro-active polymers (EAP) such as dielectric elastomer actuators (DEA). However, piezoelectric polymers have also advantages in comparison to other EAP’s, because they don’t need high operating voltages, special environments and frame constructions. Frequently studied piezoelectric polymers are ferroelectric systems such as polyvinylidene fluoride (PVDF) and its copolymers [1, 2] as well as ferroelectrets such as voided space-charge systems made of polyester and polyolefines [3-5]. Ferroelectric PVDF and P(VDF-TrFE) polymers exhibit piezoelectric 33- and 31coefficients of about ten to some ten pC/N or pm/V. This piezoelectric activity leads to thickness changes of some nm if thin film transducers were operated in the piezoelectric thicknessextension actuator mode. In order to overcome the relatively low actuation in the thicknessextension mode other transducer geometries e.g. using the piezoelectric 31-effect in dome-like or rainbow structures are necessary [6, 7]. The usage of the piezoelectric 31 effect as actuator mode in such polymer domes has two advantages: (i) the expected larger elongation as well as (ii) the adjustment of piezoelectric resonances in the range of 20 to 100 kHz due to the much lower freque
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