| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 85 ===
Nanocrystalline materials are materials with grain sizes in about 1-100 nm. Because of the extremely small dimensions, nanocrystalline materials have a large volume fraction of atomes located at the grain boundaries. The properties of nanocrystalline materials are different from those of conventional coarse-grain materials due to their large surface area. In this study, hydrogen was used as a probe to examine the structure and properties of Pd-Ag alloy nanoparticles. Two compositions Pd77Ag23 and Pd60Ag40 were studied. The former one was chosen on purpose because the Pd-Ag alloys do not have lattice parameter difference between the α and β phases when the silver content is above 23%.
The nano particles were prepared by wet chemical method, and the sizese of the particles were about 6nm, which identified by transmission electron microscopy. The broadness of the x-ray diffraction peaks also showed that the particles were rather small. The surface compositions obtained by ESCA indicated that the particles were not homogeneous. It was Pd-rich on the surface, and serious oxidation took palce on the surface of the as-prepared samples. In order to remove the contamination from the particle suface, the samples were heated in vacuum at various temperatures for three hours before the hydrogenation test. Increasing the temperature increased the amount of Ag on the surface dur to its smaller surface energy than palladium. The P-C-T curves of the preheated samples showed very different behaviors, and they were different from those of the bulk materials, indicating that the samples were not homogeneous. The hydrogenation kinetics were also examined. They were more related to the surface composition and
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