Mechanics of quasi-1D ZnO nanostructures for energy harvesting
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Mechanics of quasi-1D ZnO nanostructures for energy harvesting Antonio Rinaldi1,2, Rodolfo Araneo3, Marialilia Pea4, Andrea Notargiacomo4 1
University of L'Aquila, International Research Center for Mathematics & Mechanics of Complex System (MEMOCS), Via S. Pasquale, 04012, Cisterna di Latina (LT), Italy 2 ENEA ,C.R. Casaccia, Via Anguillarese 301, Santa Maria di Galeria, 00123, Rome, Italy 3 Sapienza University of Rome, Via Eudossiana 18, 00184, Rome, Italy 4 Institute of Photonics and Nanotechnology – CNR, Via Cineto Romano 42, 00156, Rome, Italy
ABSTRACT The mechanical properties of ZnO nanowires are the “enabling factor” for piezotronic nanogenerators. Examining the size effects entail the determination of both elastic (i.e. the Young’s Modulus, E) and failure strength (e.g. fracture, fatigue, buckling, etc.) properties of ZnO nanostructures for nanogenerators. An investigation directed to both types of effects is presented here for the first time. On one hand the strength size effects are pointed out and discussed in the framework of a generalized Weibull framework that is set forward for ZnO NWs. On the other hand, the implications of the size effects on elasticity properties are discussed and quantified using numerical simulations. The results demonstrate that the stiffening of smaller NWs can adversely affect the performance in a non-negligible manner, suggesting that both mechanical size-effects have to be considered for design purposes.
INTRODUCTION The mechanical properties of volume-confined materials differ dramatically from those of conventional bulk samples as the nanoscale is approached. In the context of piezotronic nanogenerators, the unique mechanical properties of ZnO 1-D nanostructures represent indeed the truly “enabling factor” of this new innovative technology [1-3]. Due to their ability to withstand large elastic deformations (up to 15% vs. < 1% in bulk form) without breaking, ZnO nanowires (NWs) are proving decisive - in terms of energetic efficacy and durability - for the viability of nanogenerators. Accordingly, the accurate accounting of elastic (i.e. Young’s Modulus) and failure (e.g. fracture, fatigue, buckling, etc.) properties of ZnO NWs as a function of size are crucial for design purposes. A recent overview by Espinoza et al. [3] renders a rather accurate scenario of the current state-of-the-art, unmistakably showing the presence of a strengthening effects, as well as stiffening effects associated to a size reduction in ZnO NW diameter, in spite of some uncertainties and disagreement between current reported data. In this paper we offer additional insight about the nature and significance of mechanical properties on the piezoelectrical performance of the NWs. The first and main part is devoted to strength effects, emphasizing the underlying physics and a Weibull description leading to a power-law. The second part turns also to elastic effects, to point out relevant implications on functional performance of ZnO NWs, and ultimately of the piezotronics nanogenerators, in the light o
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