Tailoring High-Frequency Ultrasonic Transducers Fabricated by the Soft Mold Process

The paper reports on our latest developments on fabrication of fine scale 1-3 piezocomposites for application as high frequency ultrasonic transducers using soft mold process. In previous work we showed that operating frequencies can be increased to frequencies of at least 40 MHz by modifying piezoceramic pillar size and layout within 1-3 piezocomposites. Especially circular pillars with hexagonal arrangement turned out to be beneficial to reduce influence of spurious modes. The current investigation focuses on the improvement of piezoelectric properties of 20 MHz ultrasonic transducers to enhance ultrasonic performance. During experimental verification process parameters like PZT powder as well as sintering and poling conditions were systematically varied to optimize dielectric and electromechanical properties, in particular thickness mode coupling factor k(t). The results show that sintering temperature and polarization field strength are the most crucial parameters to influence coupling factor. By modifying fabrication process coupling factor could be increased to values of up to k(t) = 0.6. Best results were achieved by using calcined and pre-sintered PZT powder in combination with relatively low sintering temperatures theta s = 1060-1110 degrees C and high electric fields during poling E-pol >= 9 V/mu m.