Effect of Material Constants and Mechanical Damping on Piezoelectric Power Generation

Vibration-to-electricity conversion using piezoelectric transduction has been studied by several researchers over the last decade. PZT (lead zirconate titanate) - based piezoelectric ceramics such as PZT-5A and PZT-5H have been very frequently employed in design of piezoelectric energy harvester beams. Recently, the single-crystal piezoceramics PMN-PT (lead magnesium niobate – lead titanate) and PMN-PZT (lead magnesium niobate – lead zirconate titanate) have also been investigated for electrical power generation due to their large piezoelectric constants (particularly the d₃₁ constant for the bending mode). Piezoelectric, elastic and dielectric properties of these piezoceramics differ from each other considerably. Even though the d₃₁ constants of two piezoceramics might differ by an order of magnitude (e.g. PZT-5A and PMN-PZT), this large difference is not necessarily the case for their power outputs. It is theoretically discussed and experimentally demonstrated in this paper that the d₃₁ piezoelectric constant alone is an insufficient parameter for selecting the best piezoelectric material to design a power generator for vibration-based energy harvesting. Elastic compliance of a piezoceramic has a strong effect on its electrical power output. In addition, since these devices are usually designed for resonance excitation, mechanical damping constitutes another parameter that might change the entire picture regarding the power generation performance. The last one is particularly critical considering the fact that it is difficult to control mechanical damping due to clamped interfaces and adhesive layers in practice. Theoretical comparisons are given for geometrically identical bimorphs with PZT-5A, PZT-5H, PMN-PT (with 30% PT), PMN-PT (with 33% PT) and PMN-PZT layers using an experimentally validated distributed-parameter electromechanical model. Two experimental demonstrations are presented. The first case compares two geometrically identical bimorphs (using PZT-5A and PZT-5H piezoceramics) and shows that the bimorph with PZT-5A can generate larger power than the one with PZT-5H in spite of the larger d₃₁ constant of the latter. The second experimental case compares the power generation performances of a PZT-5H unimorph and a PMN-PZT unimorph. In agreement with the theory, considerably large damping identified for the PMN-PZT unimorph results in much lower power output compared to that of the PZT-5H unimorph.