Experimental studies of solid-state reactions in nanoscale systems
Date
2016
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Publisher
University of Delaware
Abstract
This work focuses on the size-dependence of select properties in several nanoscale metallic and oxide systems, including lattice parameter and grain size evolution during diffusion and sintering processes in platinum nanoparticles and nanoscale platinum/nickel systems as well as the activation energies of the oxidation/reduction reactions required to produce nanoscale Sm-Co alloys from Sm-Co-O precursors.
The activation energies of the reduction/oxidation reactions required to produce metallic Sm-Co alloys from Sm-Co-O have been examined via thermogravimetric analysis (TGA). Precursor oxides were synthesized via an autocombustion process and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). TGA measurements determined the activation energy of a SmCoO3 to Sm2O3+CoO nanoscale mixture was 1.29eV/atom, and the activation energy of the subsequent conversion of CoO to metallic Co was 2.11eV/atom, several times larger than the bulk reported value. The conversion of Co3O4 to CoO was found to be 1.04eV/atom, and though the activation energy of the subsequent CoO to metallic Co in the absence of Sm2O3 could not be quantified, it was determined to be much smaller than 1.04eV/atom, and by extension much smaller than the 2.11eV/atom measured for the same transition in the presence of Sm2O 3.
In addition, an in-situ optical pump/x-ray probe technique was used to study the evolution in the size and lattice parameter of aggregated single crystal Pt nanoparticles prepared by a seed-growth method. The as-prepared Pt particles were characterized by XRD and transmission electron microscopy (TEM). Photo-excitation of the as-prepared particles resulted in a monotonic particle growth and a rapid increase in lattice parameter that transitioned from monotonic at low incident laser fluence to non-monotonic at high incident laser fluence. As a result, the lattice parameter of the Pt nanoparticles did not evolve with a simple inverse grain size dependence. This result was compared to conventional sintering measurements in which a more typical size and lattice parameter evolution was observed.