PIEZOELECTRIC COMPOSITE MATERIALS FOR ULTRASONIC TRANSDUCER APPLICATIONS

The aim of the present work was to extend the understanding and usefulness of the piezoelectric composite materials developed previously for low frequency hydrophone applications to higher frequency (low megahertz) applications such as biomedical diagnosis. Among all the various composites, those with PZT rods embedded in Spurrs epoxy with a regular periodicity (1-3 connectivity) appeared to be especially favorable. An extensive investigation of the PZT rod-Spurrs epoxy composites was carried out to understand their high frequency dynamic behavior in terms of the various resonance modes. The acoustic impedance of the composite material was characterized in the frequency range from 0.3 to 3.5 MHz.;The composites were initially characterized by the pulse-echo method. The pulse-echo figure of merit of some of the composites compared favorably with that of commercial transducers. For a complete understanding of the performance, the transmitting and receiving voltage response of the composite transducers were characterized separately. The figure of merit in the receiving mode of a 20 percent PZT composite operating at 0.3 MHz was three times that of the single phase PZT.;The performance of the composite transducer was further improved by the use of quarter wavelength thickness matching layer of lucite mounted between the transducer and load. The figure of merit of the composite transducer in the receiving mode was increased by approximately 50 percent when the matching layer was used. The Q of the matched composite transducer was less than two which is advantageous in achieving a good axial resolution. The composite transducer was focused by a relatively simple technique to narrow the beam width in order to improve the lateral resolution. These results demonstrate that the 1-3 composite materials are excellent candidates for medical diagnostic transducer applications. However, it should be noted that the improved performance of the composite as a transducer demonstrated below to 0.5 MHz should be extended to higher frequencies (2-5 MHz) typical for ultrasonic diagnostic applications. Several suggestions of practical techniques to increase the operating frequency of the composite transducers are made.