Atomic Resolution Microscopy and Spectroscopy of Advanced Materials : Oxide nanowires for thermoelectric applications and core/shell precipitates in Al-Li-Sc alloys

Orateur : Prof. Velimir RADMILOVIC (Université de Belgrade & NCEM Berkeley)

Advances in nanoscience and nanotechnology require detailed structural and chemical characterization of nanomaterials with high spatial and energy resolution. Because of the scale and nature of nanostructures, aberration corrected atomic resolution electron microscopy and spectroscopy in tandem with first-principles calculations are essential tools in their characterization. In this context, my presentation will discuss the use of these tools for characterization of advanced materials at nanoscale. Two topics will be discussed : Control of phonon scattering in thermoelectric nanowires at the atomic scale Some complex superlattices in nanostructures contain periodic compositional and structural features, typically on the nanometer scale, making them promising materials for thermoelectric applications. Recently we discovered a novel method to produce M2O3(ZnO)n polytypoid nanowires (M=In, Ga, Fe,..) by a facile solid state diffusion process, in agreement with the theoretical prediction that it is possible to increase the material-dependent figure of merit, zT, by using low dimensional materials, attributed to electronic band structure changes and enhanced interface phonon scattering. Using atomic resolution Z-contrast STEM imaging we performed a detailed structural analysis on the M2O3(ZnO)n nanowires, unambiguously determined the location of indium within the structure and to evaluate lattice strain and the presence of defects. Based on this analysis we propose that the superlattice structure is generated through a defect-assisted process. One of the greatest advantages of this novel synthesis is the ability to tune the nanoscale features of the polytypoid wires by simply adjusting the amount of metal precursor. This will enable future studies on structure-dependent thermoelectric properties and possibly lead to further enhancements in thermoelectric efficiency. Nanostructures embedded in solids This part of my presentation will illustrate the importance of understanding the fundamental features that underlie the behavior of nanoscale phases embedded in a solids and their role in the evolution of microstructure in materials. Al-Li-Sc alloys are of great interest for aerospace and cryogenic applications due to their low density and high strength-to-weight ratio. These alloys were found to contain a fine distribution of remarkably monodisperse core/shell precipitates consisting of an Al3ScLi core surrounded by a shell of pure Al3Li. These core/shell particles can be created via a two-stage heat treatment. Conventional high-resolution phase contrast imaging reveals the fully ordered L12 structure of the shell. Atom-probe tomography reveals that Sc is present in the core while Li is present in both, the core and the shell. The phase of the exit wave distinguished clearly Al columns from Li columns in the Li rich L12 shell. A detailed analysis of these precipitates has pr ovided important insights into their atomic structure and composition.

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