Lateral modes and diffracted field behaviour in non-periodical 1-3 piezocomposite transducers

Due to the periodic nature of 1-3 piezocomposites transducers, there are lateral modes associated with the pitch size. These lateral modes can couple to the thickness mode affecting transducers operation. Such coupling effectively limits the pitch size of the components preventing them to be used in high frequencies. Conventional transducer design will limit the operating frequency of the composite to be half of the pitch resonance in order to avoid mode-coupling effect. For high frequency transducers (>20 MHz) such fine pitch is very difficult to manipulate. Other design techniques reduce the lateral modes by introducing randomness into the structure or varying the elastic properties of the used polymer. In addition earlier works showed that when the wavelength is smaller than the grid periodicity the non-zero diffraction orders produce a spreading in the central field lobe. The main foal of present work was the simultaneous analysis of mode-coupling effects and diffraction field behaviour for non-periodical composite structures. The transducers we designed for this purpose have the first pitch resonance very close to the thickness resonance frequency. The thickness frequency of these composites is at around 0.600 MHz and the lateral mode resonances are at 0.770 MHz and 1.584 MHz with a volume fraction of 25%. Periodicity is broken by removing a certain number of the ceramic rods in different configurations. The pitch resonance reduction obtained depends is function of the periodical structure defects. The pattern of diffraction field is presented showing the central lobe perturbations caused by the nonperiodic grid configurations. In conclusion, optimal nonperiodic configurations are identified.