Fundamental performance characterisation of high frequency piezocomposites made with net-shape viscous polymer processing for medical ultrasound transducers

High frequency ultrasound (HFUS) transducers are in demand for medical diagnoses requiring high spatial resolution. Compared with bulk piezoceramics, conventional crystals, and piezopolymers, piezoceramic-polymer composites have highly desirable properties such as improved piezoelectric coupling and acoustic matching to tissue. However, for 30 MHz operation, a typical 1-3 piezocomposite is approximately only 50 mum thick, requiring ceramic pillar widths of around 15 mum or less. Fabrication is thus challenging with dice-and-fill technology. This may be overcome using micromoulding of ceramic paste produced by viscous polymer processing (VPP). This brings a need for material characterisation to support the new approach but conventional techniques cannot be used due to the small structures involved. This paper therefore reports characterisation using electrical impedance spectroscopy followed by data fitting to a theoretical model based on the 1D piezoelectric wave equation and homogenized piezocomposite model of Smith and Auld. Results are presented from multiple VPP and HFUS piezocomposites spanning a development program of several years. The piezocomposites investigated showed effective piezoelectric properties in the following ranges: c₃₃: 1.76-6.77 (times 10¹⁰ Nm², e₃₃: 2.04-8.50 (Cm^-1), e_R ^S: 70.6-460, d₃₃: 7.82-12.7 (times 10^-11 mV^-1), h₃₃: 2.01-2.09 (times 10⁹ Vm^-1) and k_T: 0.45-0.51. These data confirm that the VPP fabrication method has good potential for HFUS piezocomposites.