Ferroelectric nylon materials and their feasibility for ultrasound transducers

Measurements of the high frequency electrical and acoustic properties of oriented ferroelectric nylon-7 and nylon-11 are reported for the frequency range of 10 to 50 MHz and temperature range of 20 to 160 degrees C. The dielectric properties of nylon films were measured over a broadband frequency range including the fundamental halfwave resonance frequency for each sample, and a curve-fitting resonance technique was used to determine the properties of interest for ultrasound transducer design. Despite the high remanent polarization measured in the nylon films (125 mC/cm(2)), the measured electromechanical coupling constant (k(t) = 0.11), and mechanical quality factor (Qm = 8) were lower than typically seen for PVDF and P(VDF-TrFE) copolymer. Poor thickness uniformity of the nylon film samples is suspected of dampening the features of the dielectric spectra near resonance, resulting in lower measured values for kt and Qm. Comparisons of both simulated and actual measured pulse-echo performance showed the nylon materials to be competitive with PVDF for ultrasonic transducer performance. Actual pulse-echo tests with nylon-7 yielded a -6 dB fractional bandwidth of 144%. Since the ferroelectric nylons are known to be hydrophilic, tests were conducted to quantitatively determine the effects of water absorption on the ultrasonic performance of nylon-7 transducers. The results showed the water exposure slightly reduces the pulse-echo amplitude response and resonant frequency of a nylon-7 transducer. The water absorption effects were shown to be fully reversible upon drying the nylon film.