Ultra-strength Nanomaterials: from in-situ Nanomechanics to Wearable Electronics
Abstract: Recent advance in nanotechnology has brought forth a host of nanostructures that exhibit ultrahigh strength. These nanostructures are not only important building blocks for a broad spectrum of nanotechnology applications, but also an ideal platform to study fundamental mechanical properties. In this talk, I will discuss mechanical behavior that is derived from the dramatic dimensional reduction to the nanometer length scale. I will present two studies that reveal important roles of defects in mechanics of crystalline nanowires (NWs) using in-situ scanning and transmission electron microscopy (SEM/TEM) mechanical testing. For metal NWs containing internal twin boundaries along the NW length, we observed a time-dependent and fully reversible plastic behavior, and concluded that vacancies reduce dislocation nucleation barrier, facilitating stress relaxation, while the twin boundaries and their intrinsic stress field promote retraction of partial dislocations, resulting in full strain recovery. For single- crystalline nanowires containing point defects, we discovered giant anelastic behaviour that is up to four orders of magnitude larger than the largest anelasticity observed in bulk materials, with a timescale on the order of minutes. This behavior is due to point defect diffusion under a high strain gradient and short diffusion distance. Next I will present the interface mechanics between monolayer graphene and polymer substrate. Interfacial failure mechanisms, interfacial sliding and buckling, are identified using in-situ Raman spectroscopy and atomic force microscopy, and are supported by nonlinear shear-lag and cohesive-shear-lag models. This talk will conclude with examples and perspectives on nanomaterial-enabled stretchable/wearable electronics for health monitoring, rehabilitation and robotics. Biosketch: Yong Zhu received his B.S. degree in Mechanics and Mechanical Engineering from the University of Science and Technology of China in 1999, and his M.S. and Ph.D. degrees in Mechanical Engineering from Northwestern University in 2001 and 2005, respectively. He was a postdoctoral fellow at the University of Texas at Austin before he joined NCSU in 2007, where he is currently an Associate Professor in Department of Mechanical and Aerospace Engineering ( affiliated in Materials Science and Engineering and Biomedical Engineering). Zhu’s group conducts research at the intersection of solid mechanics and micro/nano-technology, including nanomechanics, micro/nano-electromechanical systems, and stretchable/wearable devices. He has received several awards including Sigma Xi Faculty Research Award, MAE Department Outstanding Research Award, College of Engineering Alcoa Foundation Research Achievement Award, Society of Experimental Mechanics Young Investigator Award, and ASME Sia Nemat-Nasser Early Career Award. He is a University Faculty Scholar at NCSU and the current iMechanica journal club editor.