This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.

Piezoelectric energy harvesting from ambient vibrations offers an environmentally friendly approach to powering distributed sensors for the Internet of Things. This paper gives a direct comparison of Pb(Zr,Ti)O 3 (PZT)-and (K,Na)NbO 3 (KNN)-based harvesters using a compliant mechanism harvester design for resonant frequencies of 20, 40, and 70 Hz. At 70 Hz, the measured power densities for PZT-and KNNbased devices are 1139 and 31 μW/mm 3 , respectively, for unimorph structures on nickel foils of 25 and 50 μm in thickness. The power density ratios scale proportionally to the material energy harvesting figures of merit. Energy harvesting with the compliant mechanism design is twice as efficient when compared to harvesting with a simple cantilever beam.

Zn0.64Mg0.36O (ZMO) is a newly discovered ferroelectric oxide with the wurtzite structure. Epitaxial Zn0.64Mg0.36O films from 0.036 to 0.5 μm in thickness are grown on Pt/sapphire with the crystallographic c-axis out of plane. At room temperature, the remanent polarization is ∼80 μC/cm2 and the coercive field is ∼3 MV/cm. The coercive field is strongly temperature dependent up to 240 °C with a pseudo-activation energy of 23 ± 0.3 meV, suggesting that polarization reversal occurs through an extrinsic process such as domain wall motion. ZMO films can be woken up in 20 electric field cycles on driving near the coercive field; they wake up in a single loop at fields in excess of 4 MV/cm. A thermally activated fluid imprint process, with a pseudo-activation energy of 67 ± 8 meV, enlarges the coercive field by several hundred kV cm−1 after switching the polarization. Additionally, ZMO films exhibit excellent retention characteristics; no reduction in the polarization is observed up to 1000 h from room temperature to 200 °C bakes. This current early generation of ZMO films can survive several thousand switching cycles before dielectric breakdown occurs.