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 effectiv eness with which a piezoelectric material (or a device employing such a material) converts the energy in an imposed electrical signal to mec hanical energy, or vice versa. There are different kinds of material a nd device coupling coefficients; corresponding to different modes of e xcitation and response. Device coupling coefficients are properties of the device and, although related to the material coupling coefficient s, are generally different from them. It is commonly held that a devic e coupling coefficient cannot be greater than some corresponding coupl ing coefficient of the active material used in the device. A class of devices was recently identified in which the apparent coupling coeffic ient can, in principle, approach 1.0, which corresponds to the limit o f 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 symmetri c piezoelectric bimorph device: theory predicts a smooth increase of t he apparent coupling coefficient with pre-load, approaching 1.0 at the buckling load. An experiment verified the trend of increasing couplin g with pre-load: a load corresponding to 50% of the buckling load incr eased the bimorph coupling coefficient by more than 40%. This approach provides a way to simultaneously increase both the operating displace ment and force of a piezoelectric device, distinguishing it from alter natives such as motion amplification, and may allow transducer designe rs to achieve substantial performance gains for some actuator and sens or devices.