Design and prototyping of dual layer linear arrays

This paper studies the designs and prototyping of dual layer linear arrays in a multiplexing configuration where each array element has one piezocomposite layer on top of the other. The arrays have 32 elements with 300 μm pitch and 60 μm kerf width. A composite of PZT and Spurr's epoxy in 2-2 connectivity is used as active layers. Two acoustic matching strategies were tested: A low frequency (LF) version to match for the two active layers electrically coupled in parallel, working at the center frequency 2.5 MHz, and a high frequency (HF) version to match for only the back layer working, at center frequency 5 MHz. Electrical impedances show fair agreement with the simulations, according to the center frequency and bandwidths. Pulse-echo measurement on the LF version showed that the width of the fundamental passband is in accordance with Mason model simulations, though the measured sensitivity is about 5 dB below the simulated values. The measured sensitivity of the third harmonic band is about 8 dB higher than that predicted by the Mason model. The differences can be attributed to imperfections in the fabrication process, especially to the bondline between the piezoelectric layers, which influenced the array performance.