Broadband ultrasonic detector elements each approximately 2 mm in diameter and multielement linear arrays have been constructed using commercially available polyvinylidene fluoride sheet 25 μm thick. Electrode and electrical lead patterns were deposited on both surfaces by vacuum evaporation. Poling was achieved by applying ∼2000 V across the electroded parts of the sheet while the temperature was cycled from room temperature to ∼120°C over a period of ∼1 h. The open-circuit responsivity of the samples to ultrasound was measured in the frequency range of 300 kHz to 1 MHz with the samples freely immersed in water, with the array backed by a steel ref1ector plate, and with the array a distance away from the reflector plate. The measurements were performed with the elements connected to a preamplifier by means of a 50 Ω coaxial tube about 30 cm long. Since the capacitance of an element is about 8 pF, the measured responsivity of about −230 dB re 1 V/μPa was degraded considerably by the cable capacitance. The results are consistent with a piezoelectric response mechanism where ultrasonic pressure fluctuations induce thickness compression of the polymer. Reflector plates provide up to 6 dB boost in the pressure sensed and a corresponding increase in response. Ease of fabrication, low cross-talk between adjacent elements of an array, flat response over a wide range of frequency and low cost suggest usefulness in ultrasonic imaging systems.
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DeReggi et al. (1977) studied this question.