Temperature-dependent extended electron energy loss fine structure measurements from K, L23, and M45 edges in metals, intermetallic alloys, and nanocrystalline materials
This dissertation developed the extended energy loss fine structure (EXELFS) technique. EXELFS experiments using the Al K, Fe L23 and Pd M45 edges in the elemental metals gave nearest-neighbor distances which were accurate to within ± 0.1 A. In addition, vibrational mean-square relative displacement...
| Summary: | This dissertation developed the extended energy loss fine structure (EXELFS) technique. EXELFS experiments using the Al K, Fe L23 and Pd M45 edges in the elemental metals gave nearest-neighbor distances which were accurate to within ± 0.1 A. In addition, vibrational mean-square relative displacements (MSRD) derived from the temperature dependence of the EXELFS compared favorably with predictions from published force constant models derived from inelastic neutron scattering data. Thus, information about "local" atomic environments can be obtained not only from K edges, but from L23 and M45 edges as well. This opens up most of the periodic table to possible EXELFS experiments.
The EXELFS technique was used to study the local atomic structure and vibrations in intermetallic alloys and nanocrystalline materials. EXELFS measurements were performed on Fe3Al and Ni3Al alloys which were chemically disordered by piston-anvil quenching and high-vacuum evaporation, respectively. Chemical short-range order was observed to increase as the as-quenched Fe3Al and as-evaporated Ni3Al samples were annealed in-situ at 300 C and 150 C respectively. Temperature-dependent measurements indicated that local Einstein temperatures of ordered samples of Fe3Al and Ni3Al were higher than those of the corresponding disordered samples. Within a "pair" approximation, these increases in local Einstein temperatures for the ordered alloys corresponded to decreases in vibrational entropy per atom of 0.48 ± 0.25 kB for Fe3Al and 0.71 ± 0.38 kB for Ni3Al. In comparison, the decrease in configurational entropy per atom between perfectly disordered and ordered A3B alloys is 0.56 kB in the mean-field approximation. These results suggest that including vibrational entropy in theoretical treatments of phase transformations would lower significantly the critical temperature of ordering for these alloys.
EXELFS investigations were also performed on nanocrystalline Pd and TiO2. At 105 K, the MSRD in nanocrystalline Pd and TiO2 were found to be greater than that in the corresponding large-grained materials by 1.8 ± 0.3 x 10(-3) A2 and 1.8 ± 0.4 x 10(-3) A2, respectively. Temperature-dependent measurements were inconclusive in measuring differences in local atomic vibrations between the nanocrystalline and large-grained materials. |
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