Experimental Determination of Bending Resonances of Millimeter Size PVF2 Cantilevers

Niloy Mukherjee,Amit Shukla,David F Thompson,Rodney D Roseman

doi:10.3390/s30700263

Abstract

The polymer piezoelectric polvinylidene fluoride has found widespread use in sensors and actuators. The bending mode of piezoelectricity offers very high sensitivities and low mechanical input impedance, but has not been studied in as much detail for sensor applications. We report the dynamic electromechanical properties of millimeter size cantilevers made from electroded films of PVF2. All devices tested had a single polymer layer. Several resonances are found below 1 kHz and the experimentally observed resonance frequency dependence on cantilever thickness and length are seen to agree well with published models which take the properties of the electrodes into account. It is found that bending resonances are also modulated by the width of the cantilever. Therefore, though the length and thickness control the resonance frequency most strongly, the actual realized value can be fine-tuned by changing cantilever width and the electrode material and its thickness. Further, all resonances display high piezoelectric coupling coefficients (keff), ranging between 0.2 - 0.35. The data presented here will be extremely useful in the design of sensors and actuators for a number of applications, since the combination of millimeter size scales and high piezoelectric sensitivities in the low audio range can be realized with this marriage of polymeric materials and cantilever geometries. Such an array of sensors can be used in cochlear implant applications, and when integrated with a resonance interrogation circuit can be used for the detection of low frequency vibrations of large structures. If appropriate mass/elasticity sensitive layers are coated on the electrodes, such a sensor can be used for the detection of a wide range of chemicals and biochemicals.

Highlights

Since the discovery of piezoelectricity in the polymer polyvinylidene fluoride (PVF2) [1], this material and its copolymers have made possible a large number of new transducer applications [2], including widespread use in structural active-passive damping applications
Devices tested were of cantilever geometry, electroded with Nickel-Copper on both sides, of length 2 - 25 mm and width 0.75 – 2 mm
The experimental data confirms, as predicted by theory, the existence of several resonances below 1 kHz, and the modal frequency dependence on cantilever length is similar to that predicted by theoretical equations

Summary

Introduction

Since the discovery of piezoelectricity in the polymer polyvinylidene fluoride (PVF2) [1], this material and its copolymers have made possible a large number of new transducer applications [2], including widespread use in structural active-passive damping applications. Polymer piezoelectrics offer a number of advantages over their ceramic counterparts, chief among them being ease of fabrication into films, flexibility, high voltage sensitivity (piezoelectric g-coefficient), high dielectric breakdown strength (enabling high power devices) and better acoustic impedance matching with air and water. They have been used in novel configurations to extract larger displacements. Bending mode piezoelectricity [4,6,7] offers very high sensitivity and the effect is described by the following equation [4,7]: D3

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