High Frequency Length Mode PVDF Behavior over Temperature
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High Frequency Length Mode PVDF Behavior over Temperature Mitch Thompson1, Minoru Toda1, Melina Ciccarone2 1 Measurement Specialties, Inc, 460 E. Swedesford Rd., Wayne, PA 19087 U.S.A. 2 Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 U.S.A. ABSTRACT In an extension of earlier work, the temperature dependent parameters of PVDF operating in the length mode have been measured at frequencies useful in air ultrasound (20kHz to 100kHz) over a -45ºC to +65ºC temperature range. The length mode resonance of PVDF strips of different lengths was excited by mechanically clamping samples at the mid point during dielectric impedance testing conducted in a desiccated thermal chamber. Material properties were extracted from the impedance magnitude and phase angle data at temperature, and the various sample lengths allowed a range of resonant frequencies to be studied. Overall results generally confirm the visco-elastic behavior of PVDF. Testing was conducted on samples with both thin sputtered metal electrodes (~60nm) and thicker elastomeric silver ink electrodes (~8µm) to assess the performance difference. Silver ink is preferred in production as sputtered metal has current density an other limitations, but it causes a serious loss in performance at high frequencies. INTRODUCTION Piezoelectric PVDF has been successfully commercialized in a wide variety of applications, including: accelerometers, acoustic pickups, contact microphones, flexible switches, hydrophones, impact/shock sensors, medical vital signs, motion sensors, pyroelectric or thermal sensors, piezo cable, shock gages, dynamic strain gages, speakers, traffic sensors, and ultrasonic transducers. Because it is produced as a thin film, and because of its high uniaxial strain sensitivity in plane, PVDF is used in a length extensional mode in the vast majority of these applications. In all but a few of these length mode applications, PVDF is used as a sensor to convert elastic strain into a measurement voltage or charge over frequencies ranging from 0.1Hz to a few kHz and temperatures ranging from -40ºC to +125ºC. As a result, piezo film performance has been reasonably well studied in this lower frequency bandwidth[1]. In addition, the thickness resonance mode behavior common for many ferroelectric materials has led to investigations into the behavior of PVDF in the MHz region[25]. Material parameters for PVDF in the range of 20kHz-100 kHz, however, have not been widely reported. Several air ultrasound applications operating in this general frequency range have become commercially successful in the last several years: digitizers and parametric speakers. The digitizer products use PVDF transducers for both the transmission and reception of ultrasound pulses. When combined with an infrared LED timing signal, a pen or whiteboard marker having a PVDF transducer at its tip allows a module with multiple receivers to triangulate the position of the tip using the time of flight of an ultrasound pulse through the air. The typical operating ba
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