题目：Programming Mechanical Function at Micro-Nanoscales

 

报告人：Prof. K. Jimmy Hsia，Departments of Mechanical Engineering & Biomedical Engineering，Carnegie Mellon University, Pittsburgh, USA

 

摘要：Micro-nanotechnologies such as micro-patterning are enabling tools to study self-assembly of shapes at small scales. These self-assembly processes, often driven by mechanical interactions between different parts in a material, can lead to formation of tubes, structures, or devices with unique properties. Here I present a few case studies of how micro-nanotechnologies are used to control shape formation at micro- and nanoscales. The first case involves producing microscale photovoltaic devices with patterned Si thin films. A mechanics analysis reveals the fundamental scaling law for such shape formation. Other cases demonstrate mismatch strain-driven curvilinear shape formation by folding of polymer films, or rolling of semiconductor thin films. Experiments with combined top-down and bottom-up approach demonstrate capabilities to form various curvilinear shapes. Finite element modeling of these systems is used to guide the fabrication and manufacturing of nanoscale components. These structures and devices are basic building blocks in electronic and optical applications. Based on the mechanics understanding of these folding mechanisms, one can design different folded shapes including origami.

 

 

题目：Rethink Wood-Its Unconventional Applications in Advanced Materials Design and Energy Systems

 

报告人：Teng Li ，Associate Professor, Department of Mechanical Engineering，Keystone Professor, Clark School of Engineering， Maryland NanoCenter，University of Maryland Energy Research Center，University of Maryland, College Park, Maryland, USA

 

摘要：There exists surging societal needs for products made from renewable and sustainable resources that are biodegradable, carbon neutral and non-petroleum based. Wood cellulose fibers, the major components of paper, are obtained from plants and represent one of the most abundant and renewable materials on earth. Wood cellulose fibers have an intrinsically hierarchical structure, which holds promises to enable an array of highly desirable properties and thus could enable unconventional applications beyond their traditional use. In this talk, I will show case two unconventional applications of wood cellulose fibers: (1) An anomalous scaling law of strength and toughness of cellulose nanopaper. The quest for both strength and toughness is perpetual in advanced material design; unfortunately, these two mechanical properties are generally mutually exclusive. A general mechanism to address the conflict between strength and toughness still remains elusive. We report a first-of-its-kind study of the dependence of strength and toughness of cellulose nanopaper on the size of the constituent cellulose fibers. Surprisingly, we find that both the strength and toughness of cellulose nanopaper increase simultaneously (40 and 130 times, respectively) as the size of the constituent cellulose fibers decreases (from a mean diameter of 27 μm to 11 nm), revealing an anomalous but highly desirable scaling law of the mechanical properties of cellulose nanopaper: the smaller, the stronger and the tougher.  (2) Wood-based sodium ion battery for grid-scale energy storage. Advanced energy storage technology is a crucial component in integrating renewable energy sources, whose success holds promise to revolutionize facets of our daily life through high performance batteries for personal devices and electric cars. Development of next-generation high performance batteries relies on the new choice of electrode materials and structures. We demonstrate tin film anodes coated on a natural wood nanofiber can successfully accommodate the huge volume change during charging/discharging cycling without fracture and enable fast ion transport, offering a potential solution to sodium-ion batteries for low-cost and green grid-scale energy storage.