At the extremely small length-scales, the conventional mechanics of materials cease to be effective and new deformation mechanisms emerge. For example, dislocation based mechanisms in metals give way for grain boundary based mechanisms at the nanoscale. The focus of this research is on how mechanical breakdown at the extreme length-scales influence other physical properties. For example, grain boundaries impede current and heat carriers several orders of magnitude higher than dislocations do. As a result, when the mechanics change from dislocation (bulk) to grain boundary (nano) dominated one, very small change in strain can cause large changes in electrical or thermal conductivity. The core concept of this proposal is that at the extremely small length-scales, the mechanical deformation mechanics is strongly coupled with other physical (thermal and electrical) properties. The research will provide fundamental insights in the mechanics of materials at the extremely small length-scales, which will allow ?tuning? of thermal or electrical properties with strain for enhanced energy transport or conversion efficiency.
The objective of this proposal is to study the role of specimen size, microstructure and defects on the coupling between mechanical deformation and electrical/thermal transport at the nanoscale. The PI will nanofabricate novel experimental tools with less than 3mm x 3mm size footprint to measure mechanical stress, strain, thermal conductivity and electrical conductivity of nanoscale thin films. From the data, the PI will construct the strain-temperature-transport (thermal and electrical) map to validate the proposed coupling concept. Seeing the defects and deformation as they evolve while measuring the thermo-physical properties will herald a paradigm shift in materials characterization and reduce the gap between theory and experiments. The proposed research will potentially impact the mechanical reliability and thermal management issues in future micro-electronics, flexible electronics, opto-electronics and laser devices, to name a few.
The core objective of this project is to investigate how different are the microstructure and properties of materials at the extremely small length scale (<200 nm) and to integrate that insight to the mechanics of materials. To experimentally investigate this, a unique nanofabricated testbed that can perform mechanistic study of nanoscale materials inside electron microscope, which can visualize microstructures and defects in real time with nanometer resolution. The next major goals were (i) to measure and model mechanical properties of nanoscale materials under static and dynamic loading (ii) to investigate tunability of microstructure, that is not possible at micro or meso scales, to enhanceor extract novel properties. We found that nanoscale materials exhibit pronounced sensitivity to external stimuli compared to the bulk or even microscale materials. This is seen in cases (a) metals showing remarkable grain growth at stress and temperature of only 20% of their yield strength or melting temperature (not predicted by classical mechanics) (b) metals showing strong viscoelastic response at room temperature, whereas it is know that this is not seen for bulk. We also found that nanoscale metals exhibit excellent resistance to fracture and fatigue. One unique finding was the absence of stress concentration at the nanoscale,The key outcome of this project is the observation that nanoscale materials show orders of magnitude higher responsitivity to force, temperature, current etc stimuli. We found that such sensitivity can be exploited to tune the microstructure and hence the properties. Using these concepts, it is possible to develop novel materials, that do not fail by fracture or fatigue. Three graduate and three undergraduate students were involved. The students published nine journal papers and made six conference presentations. One provisional patent was obtained. We also involved the next generation work force by performing Middle and Elementary School outreach.
