Mohsen Danaie

The influence of SWCNT?metallic nanoparticle mixtures on the desorption properties of milled MgH2 powders

We have examined the effect of single-walled carbon nanotube (SWCNT)-metallic nanoparticle additions on the hydrogen desorption behavior of MgH(2) after high-energy co-milling. The metallic nanoparticles were the catalysts used for the SWCNT growth. The co-milling consisted of high-energy planetary milling in an inert argon environment of the hydride powder mixed with the SWCNTs. Identically milled pure MgH(2) powders were used as a baseline. The composites were tested using a combined differential scanning calorimeter and thermogravimetric analyzer, while the microstructures were examined using a variety of techniques including x-ray diffraction and transmission electron microscopy (TEM). We found that the SWCNT-nanoparticle additions do have an influence on the desorption kinetics. However, the degree to which they are effective depends on the composites final state. The optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This microstructure is optimum, presumably because of the dense and uniform coverage of the defective SWCNTs on the MgH(2) surface. Prolonged co-milling of 7 h destroys the SWCNT structure and reduces the enhancement. Even after 72 h of co-milling, when the SWCNTs are completely destroyed, the metallic nanoparticles remain dispersed on the hydride surfaces. This indicates that the metallic nanoparticles alone are not responsible for the enhanced sorption and that there is indeed something catalytically unique about a defective SWCNT-metal combination. Cryo-stage TEM analysis of the hydride powders revealed that they are nanocrystalline and in some cases multiply twinned. To our knowledge this is the first study where the structure of milled alpha- MgH(2) has been directly imaged. Since defects are an integral component of hydride-to-metal phase transformations, such analysis sheds new insight regarding the fundamental microstructural origins of the sorption enhancement due to mechanical milling.

Physical Characterization of Cathodically-Activated Corrosion Filaments on Magnesium Alloy AZ31B

In this study, the filiform-like corrosion behavior of Mg alloy AZ31B was characterized using analytical microanalysis. Recent electrochemical measurements have shown that the corrosion filaments on AZ31B support enhanced cathodic reaction kinetics, but there has been little strong physical evidence published to explain this behavior. Hence, the specific aim of the investigation was to contribute to the understanding of the physical origin of the “cathodic activation” of the corrosion filaments. Highlights of this investigation include the presence of through-thickness cracks within the corrosion filaments, Al-Mn particles (identified as Al11Mn4 with energy-dispersive x-ray spectroscopy (EDS) quantification) in the preexisting films and filament corrosion products, and a significant enrichment of Zn at the corrosion filament/metal interface after aging of the filament. Diffraction patterns of the corrosion filaments indicated they were composed of nanocrystalline magnesium oxide (MgO), which was suggested...

Bimetallic Fe–V catalyzed magnesium films exhibiting rapid and cycleable hydrogenation at 200 °C

We examined hydrogen sorption in 1.5 μm thick Mg–Fe–V films, using the binary alloys as baselines. At 200 °C both Mg–V and Mg–Fe–V absorb in tens of seconds, and desorb in tens of minutes. The ternary alloys show minimal kinetic or capacity degradation even after 105 absorption/desorption cycles. Pressure—composition isotherms yield the well-known enthalpies of α-MgH2 formation (decomposition), agreeing with x-ray diffraction results. The x-ray spectrum also shows a broad hump centered near (011) reflection of CsCl-type Fe–V phase. Our hypothesis is that a densely distributed nanoscale Fe–V acts both as a potent hydrogen dissociation catalyst and a heterogeneous nucleation site.

THE SYNTHESIS OF A PLATY CHABAZITE ANALOG FROM DELAMINATED METAKAOLIN WITH THE ABILITY TO SURFACE TEMPLATE NANOSILVER PARTICULATES

Mineral chabazite has shown the unusual ability to surface template nanometal particles, especially Ag. A chabazite analog was synthesized from delaminated metakaolin. The chabazite formed retained the platy morphology of the base clay. This morphology is ideal for displaying surface-supported nanometal particles. The synthetic chabazite analog demonstrated the ability to form and support large concentrations of Ag nanoparticles, as observed in the related natural mineral. Due to greater Al content, the synthetic chabazite manifests significantly improved capacity for the formation of such Ag nanoparticles. As in the case of the mineral chabazite, surface Ag nanoparticles of high uniformity were observed in the range of 5–6 nm.