Energy Harvesting Using Flexible Lead-free Piezoelectric Composites

More and more embedded wireless sensor systems are employed into our daily lives (e.g. medical implants, structural health sensors), normally powered by batteries. Batteries contain limited energy, pollute the environment and... [ view full abstract ]

More and more embedded wireless sensor systems are employed into our daily lives (e.g. medical implants, structural health sensors), normally powered by batteries. Batteries contain limited energy, pollute the environment and are the volumetric biggest component of a sensor system. Using energy harvesting, batteries could be eliminated, which could be achieved using piezoelectrics which can harvest energy from vibrations. For energy harvesting applications, the stress energy density, d₃₃g₃₃, is considered to be the appropriated figure of merit, expressed in volume per joule.

The most popular piezoelectric material is lead zirconate titanate (PZT) due to the high piezoelectric properties (d₃₃g₃₃ = 10 – 17 pm³ J^-1) and high Curie temperatures (T_C = 250 – 360 °C). However, they are brittle, making them unsuitable to handle cyclic strains, while also containing a high content of the toxic substance lead. Another popular material is poly(vinylidine fluoride) (PVDF) which is a flexible polymer, having the drawback of having only limited energy harvesting potential (d₃₃g₃₃ = ~5 pm³ J^-1), lower Curie temperature (T_C = ~100 °C) and a high coercive field (60 MV m^-1).

In this work, piezoelectric composites with aligned high aspect ratio K_0.485Na_0.485Li_0.03NbO₃ (KNLN) fillers in a polydimethylsiloxane (PDMS) matrix are fabricated. These lead-free, flexible composites are especially designed to be good candidates for energy harvesting applications. Through a process called dielectrophoresis, KNLN short fibres are aligned in the PDMS matrix during curing, obtaining a d₃₃g₃₃ of 18.0 pm³ J^-1 using 6 Vol.% filler. The extractable electrical energy in the composites excited with 10 N static load and 3 N dynamic load at a frequency of 1 Hz was found to be 34 nW cm^-3, which is higher than that of monolithic PZT ceramics, which can obtain a value in the range of 20 – 30 nW cm^-3.