We have been investigating ultrasonic transducers using a polyurea piezoelectric material, which is fabricated by vapor deposition. To enhance the transducer performance, a multilayered configuration is studied in this work. First, the fabrication and transducer design of the multilayered structure are described. A special twin-vacuum chamber is used for laminating the polyurea layers and aluminum electrodes alternately without breaking vacuum. We fabricate two- and four-layered transducers with 1.5 µm polyurea films. The calculation results show that the force factor and electromechanical coupling coefficient increase as the number of layers increases. Second, to evaluate the transducer performance, we measure the electromechanical coupling factors and electric admittances. The coupling coefficients also increase as the number of layers increases at the resonant frequencies of about 30, 65, and 100 MHz. The pulse/echo measurements are conducted to determine the transmission and receiving characteristics using a reflector. The results of the experiment show that the voltage amplitudes of the received signal increase because of multilayer lamination.

Focusing on the possibilities of polyurea as an acceleration sensor with characteristics such as flexibility, durability against large deformation, good linearity, and capability of deposition in dryprocess and insolubility in organic solvent, we first fabricated a cantilever acceleration sensor and a three-axis acceleration sensor. The output voltage in the large-strain region is measured using a polyurea cantilever sensor attached to a beryllium copper substrate. The results show that output voltages have good linearity for large strains up to 1%. This value is much larger than the breakdown limit of lead zirconate titanate. Second, we fabricated a polyurea three-axis acceleration sensor. The sensor consists of an insulation layer of polyurea on the cross beam substrate of phosphor bronze, a bottom aluminum electrode, a polyurea active layer, and four top aluminum electrodes. The experimental results for harmonic acceleration show that the sensor works as expected. The cross-axis sensitivity of the polyurea sensor was less than 8%, which is close to that of conventional microelectromechanical system sensors.

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In this paper, we present a polyurea transducer that works at 100 MHz under water. The transducer was designed using an equivalent circuit model so that an aluminum (top)-polyurea-aluminum (bottom)-polyimide layer had a resonant frequency of 100 MHz and output sound pressure became maximum at that frequency. The thicknesses of the top aluminum electrode, polyurea, and bottom aluminum electrode were determined to be 3.3, 3.5, and 1.7 μm, respectively. A 100-MHz polyurea transducer with the designed thickness was fabricated using deposition equipment. To evaluate the performance of the designed and fabricated polyurea transducer, transmission-reception experiments with pulsed and burst waves were carried out. The results show that transmitting and receiving ultrasounds at a frequency of 100 MHz are possible as expected with the thickness design. To evaluate actual use, B-mode imaging of an onion was also performed using the transducer, which was formed into a line-focused shape. The result shows that the outer layer of the onion, of 0.1 to 0.2 mm thickness, was successfully imaged.

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