Mechanics of Energy Storage Materials
Developing lithium ion batteries with higher energy density is considered to be an important challenge of our times in order to enable electric vehicles that can compete and hopefully replace internal combustion engine driven vehicles. Developing the next generation of Li-ion batteries requires designing new anodes, cathodes and electrolytes that can survive thousands of charge-discharge cycles with minimal capacity fading. Stresses in the electrode materials not only determine the mechanical integrity & cycle life, but also influence the electrochemical performance. It is essential to understand the mechanics of damage evolution in the electrode materials in order to develop predictive models to help battery designers in arriving at optimal material designs.
Motivated by such practical applications, we pursue a number of exciting problems at the interface between solid mechanics and chemistry. For example, we recently demonstrated the role of plastic deformation on the energy recovery efficiency of silicon based anodes; coupling between stress and electric potential in silicon; and the evolution of mechanical properties of silicon as a function of Li concentration. A number of other related phenomena are under current investigation. Our effort in this area is currently funded by NSF (MRSEC), NASA and the State of Rhode Island.
Mechanics of Biologically Inspired Adhesion, Friction and Engineered Surfaces:
The ability of small animals such as insects, flies and geckos to climb up vertical walls and to walk up side down on ceilings has been a subject of active research in biology for many centuries. There are a variety of mechanisms employed by these animals, including tiny claws, adhesive secretions, smooth and hairy adhesive pads, etc. Following the accumulation of a large body of anatomical and functional data on various natural adhesion systems, in the last few years biologists and engineers have been working together to develop a quantitative understanding of various natural adhesion and friction systems. This is a growing field of research with a rich set of challenging problems at the interface between biology, applied mechanics and micro/nano-fabrication, with potentially significant benefits if we can understand and mimic some of nature's optimized solutions to develop useful technologies.
The focus of this research is on engineering the topography of surfaces at micron and nano scale in order to understand the mechanics of biological adhesion and friction systems; and to develop biomimetic strategies which implement nature's mechanisms for adhesion and friction. Such a study naturally leads to several basic mechanics problems in rough surface adhesion and friction of soft materials, which are currently being investigated in detail. Some of the specific problems being addressed are: Mechanics of direction dependent friction (Friction Anisotropy) exhibited by biological surfaces and implementing such strategies on laboratory surfaces; Mechanics of roughness induced instabilities at nano and micron scales; Optimal design and fabrication of nano-hairy surfaces. The common features that underlie this class of problems are fabrication of nano-structuree surfaces, coupling between adhesion and deformation of the surface nano-structures, and mechanical instabilities at nano and micron scale to dissipate energy and influence the macroscopic behavior. The key idea is to manipulate surface topography at nano and micron scale to tailor macroscopic adhesion and friction properties; not surface chemistry. This work is supported by the Air Force Office of Scientific Research and the National Science Foundation.
Nanofabrication Strategies by means of Guided Assembly:
Fabricating ordered patterns of controlled shape, size and spacing at nanoscale has been an important goal of nano-scale science and engineering during the past decade. Applications envisaged for such processes include quantum dot devices, nanocomposites, high density data storage devices, etc. Most of the existing nanofabrication techniques can be loosely described as either top-down type or bottom-up type. The top-down approach consists of examples such as photolithography, X-ray lithography, scanning probe/e-beam lithography etc. On the other hand, in the bottom-up approach, one tries to exploit certain configurational forces acting at nanoscale to drive a self-assembly process. However, there are a number of issues associated with the "self-assembly" route of fabricating devices of practical utility. For example, in case of strain driven quantum dot growth in semiconductor thin films, the resulting nanostructures usually do not possess any spatial order and also end up with a non-uniform size distribution. The objective of this project is to develop nanofabrication techniqes that combine the driving configurational forces that underlie self-assembly processes and the spatial control that can be achieved in top-down processes, in order to realize any desired spatial pattern and size distribution of nanostructures. Currently, we are exploring the use of "very strong" electric and magnetic fields at solid surfaces to induce diffusion and patterning. This work is supported by the National Science Foundation.
Mechanics of Carbon Nanotubes:
Research on the mechanics of carbon nanotubes has been dominated by modeling and computational simulations, primarily due to the extreme difficulty involved in performing controlled experiments on such small structures. The goal of this project is to develop new nanoscale experimental techniques to apply controlled force on individual nanotubes, measure the deflection at nanometer resolution and develop mechanics models to describe the experimental observations. We have recently developed an experimental technique to study shell buckling in individual multiwalled carbon nanotubes and showed that the measured buckling force is substantially higher than that predicted by the existing models. Motivated by these experimental observations, improved shell theories are being developed. Funding source: AFOSR.