Coupling coefficient of segmented stack piezoelectric transducers using high-coupling materials

High-power piezoelectric transducers normally use a segmented stack of piezoelectric pieces. The pieces are arranged with alternating polarization and are driven by electrodes of alternating voltage polarity. This allows long stacks to be built from easily manufactured pieces and reduces the voltage required for full power operation. The common understanding among transducer designers is that the mechanical properties of the stack of n pieces are equivalent to those of a single piezoelectric piece of equal total length, while the electrical impedance is reduced by the factor n2. For conventional designs with operating bandwidth less than a half octave, this is quite accurate. However, as new materials with high-electromechanical coupling make it possible to design high-power transducers with bandwidths greater than an octave, the use of segmented stacks must be reexamined. With high coupling materials, there is a significant reduction in effective coupling coefficient caused by stack segmentation. The effect is small for materials such as hard PZT, but is larger in at least some transducers built from materials with coupling coefficient above 80%. This paper will address the effects of stack design based on modeled and measured results. [Work supported by the Office of Naval Research.]