Generating characteristic acoustic impedances with hydrogel based phononic crystals for use in ultrasonic transducer matching layers

Abstract

Quarter wavelength acoustic impedance matching layers (IMLs) are often attached to an ultrasonic transducer (UT) to enhance energy transmission into a load medium. Natural materials are rarely suitable as an IML but even composites with a suitable impedance can be excessively attenuating, or unsuitable for UT mounting. It would be beneficial then, if a non-absorptive IML with the specified impedance could be manufactured using common fabrication tools. Recent publications have shown that phononic crystal (PC) structures, made using modern printing and curing technology, can operate as IMLs and may satisfy these criteria. It is important to continue this research because it has the potential to improve UT performance, and simplify or surpass existing solutions. Here, finite element analysis (FEA) is used to validate a hydrogel-matrix / steel-inclusion PC IML that separates PZT and water domains. The PZT boundary is excited over a frequency range and the resultant in water pressure is averaged. Results are compared with an ideal, bulk IML and common, in practice materials. Comparison of spectra demonstrate that the PC increases in water pressure at the expected frequency, is an effective medium in the long wavelength regime, and is comparable to the ideal IML.