Auxetic power amplification mechanisms for low frequency vibration energy harvesting

Abstract

Energy harvesting from locally available small amplitude vibrations can struggle to generate sufficient power for wireless sensor nodes, which thereby constrains their use for structural health monitoring. This work discusses a selection of two-dimensional auxetic substrate designs used to increase a piezoelectric harvester’s power output by 2.18-14.5 times by concentrating the ambient strain energy into the piezoelectric material. The harvesters were modelled and their auxetic designs optimised in COMSOL before empirical testing under sinusoidal or dynamic strain oscillations. The investigated auxetic designs included re-entrant honeycombs, rotating squares, triangles and hexagrams, and 𝙸-hole structures; the most effective of which was found to be the honeycomb design, with a gain of 5.66 and a raw output of 570 μW at 10 Hz, 100 με. This work also compared PZT (Lead Zirconate Titanate), LN (Lithium Niobate), and MFC (Macro-Fibre Composite) as materials for the active piezoelectric layer. The former was found to be detrimentally brittle but delivered the greatest output, while the LN was stronger but with a significantly lower output. The MFC was more flexible, with only a modest reduction in output compared to PZT, and was found to be the most viable of these materials for future research. A crucial issue during the design stages was appropriately modelling the mechanical losses associated with the bonding between substrate and piezoelectric material; this adhesion was modelled using thin elastic layers (TELs) to emulate each sample by comparing to its output. The value of the stiffness constant per unit area in these TELs was found to be consistent for each sample across a range of input excitations. These kinds of energy harvesters open up many new avenues for wireless self-powered structural health monitoring sensor nodes in infrastructure, buildings, and vehicles, where the ambient vibration energy would otherwise be too diffuse to harvest from.