Thermal behavior of indium nanoclusters in ion-implanted silica

Fused silica substrates were implanted with 2x10(17) In(2+)/cm(2) ions at 320 keV. Indium crystalline nanoclusters with an average size of about 15-20 nm were found in the as-implanted samples. The thermal behavior of the nanoclusters was studied by performing heating-cooling cycles in vacuum and by using in-situ techniques based on glancing-incidence x-ray diffraction and transmission electron microscopy. The precipitates showed both superheating and supercooling. Moreover, no evidence of clusters growth or reorientation during the thermal cycle was found. A detailed study of the heating sequence showed that the melting temperature of the Indium precipitates depended on their size, i.e., the smallest particles melt first and at a temperature which is about 7 K below the bulk melting point, while the largest ones were superheated until about 13 K above it. Moreover, a remarkable stability of the In cluster well above their melting temperature (up to about 980 K) was evidenced by in-situ transmission electron microscopy analysis. From a thermodynamic point of view, the experimental results were explained by considering two effects acting on the clusters: the thermodynamic size effect and the pressure of the silica matrix

Microstructure and hydrogen desorption in nanostructured MgH2-Fe

Mg-based nanostructured hydrides have been synthesized by ball milling using two alternative approaches. The first is based on the reactive milling of Mg powders in H-2 atmosphere, while the second on the milling of commercial MgH2 powders under inert atmosphere. In both cases 10 wt.% of Fe was added to the powder mixture, with the aim of introducing a catalyst agent. The microstructural characterization was carried out by X-ray diffraction, and both scanning and transmission electron microscopy. Hydrogen desorption behavior was evaluated by differential scanning calorimetry. Almost full hydrogenation of pure Mg powders can be achieved by reactive milling. Catalyst addition strongly accelerates the hydride formation. Both reactive milling of Mg powder and inert gas milling of MgH2 induce a nanosized microstructure with similar H-desorption behavior. The role played by Fe becomes particularly evident in H-desorption. In fact, a temperature decrease of about 100degreesC was found in samples having the same crystallite size and similar powder morphology.Progress in Advanced Materials and Processes, 5th Conference of the Yugoslav-Materials-Research-Society (Yu-MRS 2003), Sep 15-19, 2003, Herceg Novi, Yugoslavi