Design and fabrication of volume fraction gradient piezoelectric composites for medical imaging applications

In ultrasonic medical imaging it is desirable to have the maximum beam sensitivity along the transmission axis. In a conventional transducer, the presence of side lobes leads to off-axis sensitivity and greatly affects the image quality. One of the approaches to suppress the side lobes is to introduce a ceramic volume fraction gradient (VFG) along the width of the piezoelectric composite. However, it is difficult to fabricate piezocomposites of complex designs using traditional processing routes. Solid freeform fabrication (SFF) techniques provide high flexibility in the design and fabrication of VFG piezocomposites. In this work, one of the SFF techniques, namely, Sanders prototyping (SP) was used to fabricate novel piezoelectric ceramic/polymer composite transducers. A variety of concentric polygon and 2-2 PZT-5H/Spurr epoxy composites were fabricated with and without VFG's. Several mathematical functions, including linear, gaussian and exponential gradients were used to design VFG's using CAD software. After sintering and embedding in epoxy, the different composites had a ceramic volume fraction varying from 60 vol. % at the center to 20 vol. % at the edges. The electromechanical properties of the transducers including PZFLEX modeled beam patterns, and vibration profiles are discussed in this paper. Based on the modeling results, the linear 2-2 gradient showed the lowest off-axis sensitivity.