Acoustic sensors using 0-3 nanocomposites

Abstract

This thesis describes the optimum conditions for poling 0-3 nanocomposite thin films to obtain high piezoelectric and low pyroelectric activities. The design and the performance of piezoelectric hydrophones with such materials as the sensing elements have been evaluated. Piezoceramic/piezopolymer 0-3 composites with nanosized lead titanate (PT) ceramic powders imbedded in a vinylidene fluoride-trifluoroethylene (P(VDF-TrFE) 70/30 mole%) copolymer have been fabricated. The ceramic powder was prepared using the sol-gel method, and the average crystallite diameter was about 60 nm. The powder was blended into a solution containing dissolved P(VDF-TrFE) to form a composite suspension, which was then fabricated into thin films of thickness 5 um using the spin-coating technique. The two phases of the composite film were poled either in opposite directions or in the same direction using a two-step procedure. The ceramic phase was first poled under a dc electric field at a temperature above the ferroelectric-to-paraelectric phase transition temperature of the copolymer. The copolymer phase was then poled under an ac field at 70 C. By controlling the field direction in the last half-cycle of the ac field, the copolymer phase was poled either in opposite directions or in the same direction to the ceramic phase. Hence, composites of different polarization states were then prepared. The relative permittivity, piezoelectric and pyroelectric properties of the composite films were studied. Composite with two phases poled in opposite directions has enhanced piezoelectric response and reduced pyroelectric response. It is because the piezoelectric coefficients of the ceramic and the copolymer phase have opposite signs, while their pyroelectric coefficients are of the same sign. As its piezoelectric response is about 30 % larger than that of the copolymer, such composite material can be fabricated into hydrophones with good receiving sensitivity. The composite and the copolymer films were used to fabricate single element needle-type hydrophones. Besides, an 8-element hydrophone array was also fabricated using the composite with two phases poled in opposite directions as sensing element. The receiving sensitivity and the angular response of the needle-type hydrophones and the hydrophone array were evaluated in water. It is found that the sensitivity of the hydrophone is approximately proportional to the piezoelectric coefficient d33 of the sensing element. Composite hydrophone with two phases poled in opposite directions shows better receiving response than the copolymer hydrophone. The hydrophone array also has good sensitivity with low inter-element coupling. Both the needle-type hydrophones and the hydrophone array have good angular responses that can be predicted accurately using the unbaffled piston model.