Microstructures, piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Abstract

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K0.5Na0.5)NbO3 were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d33 ∼122 pC/N, kp ∼36 % and Qm ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.