Composite piezoelectric transducer designs for hydrophone applications

Passive sonar transducers were designed, fabricated, and characterized in this work. Two different designs were studied. The first was a piezoelectric ceramic/epoxy composite suitable for fabrication in large quantities. The ceramic topology of these composites mimics the reticulated structure of sacrificial precursor foam. Before coating with ceramic slurry, the foam can be distorted by stretching or compressing while at a temperature above its T_g. It has been shown in previous work that distorting the foam provides a means to tailor the structure of the ceramic, ultimately improving sensitivity of the completed hydrophone. This work concentrated on distorting the foam by compression, which creates a structure with negative Poisson's ratio. Once the ceramic structure was created, an epoxy passive phase was used to backfill the reticulated ceramic, providing the necessary strength to withstand the considerable hydrostatic pressure of an ocean environment. Varying degrees of compression were evaluated, with the greatest piezoelectric sensitivities arising from samples prepared from foam substrates that were compressed to 1/2 their original volume. The piezoelectric figure of merit for these samples, d_hg_h, is 1700 × 10−15 Pa−1 at pressures ranging from 0 to 7 MPa.; The second transducer design is based on the architectural concept of tensegrity. This is a stable structure consisting of rigid compressional elements arranged in tandem with flexible tensional cables. In devices fabricated for this work, six piezoelectric bars acting as compressional elements in the tensegrity structure were coupled with tensional bands of polyaramid or carbon fiber. This structure was wrapped with an outer layer of polyaramid or carbon fiber and rubber film, forming a sealed device, referred to here as a piezotensegritive device. Devices were tested in a hydrostatic environment to determine the relevant piezoelectric coefficients. For devices wrapped with carbon fiber, d_h peaked at ∼6000 pC/N and g_h at ∼275 mVm/N. For devices wrapped with polyaramid, d_h peaked at ∼2000 pC/N and g_h at ∼100 mVm/N.