Characterization of hybrid piezoelectric nanogenerators through asymptotic homogenization

Abstract

In the framework of energy scavenging for applications in flexible/strechable electronics, hybrid piezoelectric nanogenerators, made up with Zinc oxyde nanorods, embedded in a polymeric matrix, and growth on a flexible polymeric support, are investigated. The ZnO nanorods are arranged in clusters, forming nearly regular distributions, so that periodic topologies can be realistically assumed. Focus is on a dynamic multi-field asymptotic homogenization approach, proposed to grasp the overall constitutive behaviour of such complex microstrutcures. A set of applications, both in static and dynamic regime, is proposed to explore different design paradigms, related to nanogenerators based on three working principles. Both extension and bending nanogenerators are, indeed, analysed, considering either extension along the nanorods axis, or orthogonally to it. The study of the wave propagation is, also, exploited to comprehend the main features of such piezoelectric devices in the dynamic regime. 2017;Liu et al., 2018;Askari et al., 2019). Relevant examples concern either the use of piezoelectric zinc oxide (ZnO) nanowire arrays grown on conductive rigid supports (Yi et al., 2005;Wang and Song, 2006), or the adoption of lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) and barium titanate (BT). An important improvement in the design of ZnO nanorods-based piezoelectric generators has been achieved by adopting flexible substrates made of electro-active polymeric materials. The main advantage is, indeed, the possibility of exploiting relevant flexural mechanisms, besides the standard direct compression of the device. In Fan et al. (2016) different typologies of flexible nanogenerators are presented and critically commented. Also patterned growth of ZnO nanowires can be exploited to enhance the performances of flexible nanogenerators, as discussed in Yang et al. (2017). Further benefits can be obtained by resorting to so-called hybrid nanogenerators, made up by embedding the ZnO nanorods within a polymeric matrix. More specifically, Stassi et al. (2015) propose highly oriented ZnO nanotubes in a porous polycarbonate (PC) matrix. The result is an efficient nanogenerator based on such a highly flexible ZnOPC composite. Moreover, in Choi et al. ( 2017) a hybrid piezoelectric structure made of ZnO nanowires and a matrix of PVDF polymer is investigated in order to obtain a power enhancement. The authors, indeed, find that the ZnO nanowires are able to deliver internal strain to the PVDF, which increase the electrical power output of the hybrid nanogenerator. Based on the aforementioned considerations, with the aim of energy harvesting from green and sustainable energy resources, our focus is on hybrid flexible nanogenerators, made up with clusters of ZnO nanorods embedded into a polymeric matrix and growth on a flexible support. The choice of ZnO nanorods, is motivated by their relatively simple forming processes using low temperature. In particular, by exploiting innovative growth techniques, it is possible to synthesi...

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