Lead-free KNbO₃ferroelectric nanorod based flexible nanogenerators and capacitors

In spite of high piezoelectricity, only a few one-dimensional ferroelectric nano-materials with perovskite structure have been used for piezoelectric nanogenerator applications. In this paper, we report high output electrical signals, i.e. an open-circuit voltage of 3.2 V and a closed-circuit current of 67.5 nA (current density 9.3 nA cm−2) at 0.38% strain and 15.2% s−1 strain rate, using randomly aligned lead-free KNbO3 ferroelectric nanorods (∼1 μm length) with piezoelectric coefficient (d33 ∼ 55 pm V −1). A flexible piezoelectric nanogenerator is mainly composed of KNbO3–poly(dimethylsiloxane) (PDMS) composite sandwiched by Au/Cr-coated polymer substrates. We deposit a thin poly(methyl methacrylate) (PMMA) layer between the KNbO3–PDMS composite and the Au/Cr electrode to completely prevent dielectric breakdown during electrical poling and to significantly reduce leakage current during excessive straining. The flexible KNbO3–PDMS composite device shows a nearly frequency-independent dielectric constant (∼3.2) and low dielectric loss (<0.006) for the frequency range of 102–105 Hz. These results imply that short and randomly aligned ferroelectric nanorods can be used for a flexible high output nanogenerator as well as high-k capacitor applications by performing electrical poling and further optimizing the device structure.