Novel volume fraction gradient and oriented piezoelectric composites for medical imaging applications

In ultrasonic medical imaging it is desirable to have the maximum beam sensitivity along the transmission axis. However, the presence of grating and side lobes greatly affects the transducer performance. It is known that the grating lobes can be reduced by non-uniform spacing of elements in the composite. In spite of this knowledge, it has been found to be difficult to fabricate piezocomposites with complex designs using traditional processing routes. The ceramic element spacing can be varied easily using solid freeform fabrication (SFF) techniques. In this work SFF techniques, including Sanders Prototyping (SP) and Fused Deposition of Ceramics (FDC) were used to make many novel piezoelectric ceramic / polymer composite transducers. A variety of 2-2 PZT-5H / spurr epoxy volume fraction gradient (VFG) samples have been fabricated. Many mathematical functions, including linear, gaussian and exponential gradients were designed using Pro Engineer software. Novel oriented composites have also been fabricated where the ceramic elements are at an angle to the thickness direction. The piezoelectric properties are found to change with the orientation of piezoelectric rods. The optimum properties have been observed at an orientation of 30 degrees to the vertical where the total contribution from the d(33), d(31) and d(15) components could be the highest. These oriented composites may be used to focus the acoustic beam at a point by varying the orientation angle of the rods within the same composite (angle of orientation gradually increasing from zero at the center to about 65 degrees to the vertical at the edges). The design, fabrication and electromechanical properties of these composites are discussed in this paper.