Implementation of multilayer ultrasonic transducer structures with optimized non-uniform layer thicknesses

Most ultrasonic transducer structures have significantly reduced sensitivity at the even harmonic frequencies of the fundamental resonance. However, by appropriate choice of layer thickness in a multilayer structure, it is possible to design a transducer with much increased even harmonic sensitivity. Using this approach, it has been demonstrated theoretically that structures can be configured to provide very unusual characteristics, such as uniform sensitivity at the fundamental and second and third harmonic frequencies. In this paper, we report investigation of the practical implementation of this concept. The differences between the ideal and the practical are highlighted and the route to implementation is described, based on the use of low-viscosity epoxy resins and lead metaniobate piezoelectric ceramic plates, with devices operating at frequencies from 500 kHz upwards. A comparison is made between experimental results in terms of electrical impedance and time-domain characteristics, and theoretical results generated with one-dimensional modelling and with finite element analysis. Our results indicate that the multilayer approach is promising, but that minimisation of bondline effects is crucial.