Can a coupling coefficient of a piezoelectric device be higher than those of its active material?

An electromechanical coupling coefficient is a measure of the effectiveness with which a piezoelectric material (or a device employing such a material) converts the energy in an imposed electrical signal to mechanical energy, or vice versa. There are different kinds of material and device coupling coefficients, corresponding to different modes of excitation and response. Device coupling coefficients are properties of the device and, although related to the material coupling coefficients, are generally different from them. It is commonly held that a device coupling coefficient cannot be greater than some corresponding coupling coefficient of the active material used in the device. A class of devices was recently identified in which the apparent coupling coefficient can, in principle, approach 1.0, which corresponds to the limit of perfect electromechanical energy conversion. The key feature of this class of devices is the use of destabilizing mechanical pre-loads to counter inherent stiffness. The approach is illustrated for a symmetric piezoelectric bimorph device: theory predicts a smooth increase of the apparent coupling coefficient with pre-load, approaching 1.0 at the buckling load. An experiment verified the trend of increasing coupling with pre-load: a load corresponding to 50% of the buckling load increased the bimorph coupling coefficient by more than 40%. This approach provides a way to simultaneously increase both the operating displacement and force of a piezoelectric device, distinguishing it from alternatives such as motion amplification, and may allow transducer designers to achieve substantial performance gains for some actuator and sensor devices.