Magnesium-Titanium nanoparticles by gas phase synthesis for hydrogen storage purposes: A Transmission Electron Microscopy study

Magnesium is a possible candidate for solid state hydrogen storage due to its light weight and low material costs. However, its hydrogen sorption cycling performance is limited by the high thermodynamic stability of magnesium hydride and its poor kinetic properties. These properties can be altered by downscaling from bulk to nanostructured magnesium. Gas phase synthesized magnesium nanoparticles offer a possible storage system, but suffer from oxidation and magnesium evaporation owing to the high reactivity of magnesium. Alloying with titanium not only prevents the latter problems, but also gives the opportunity to improve hydrogen sorption properties. This bimetallic system shows the strength of nanotechnology where out-of-equilibrium materials can be produced, as magnesium and titanium are immiscible in bulk but turn out to be miscible in nanoparticles. In this thesis magnesium titanium nanoparticles and their performance as a solid state hydrogen storage medium are characterized by transmission electron microscopy. The nanoparticles are synthesized with a high pressure magnetron sputtering system, which gives control over the nucleation and growth conditions. A stable nucleation rate could only be sustained by introducing hydrogen or methane gas in the system. Consequently, titanium reacts readily with the elements in the gas which impacts the nanoparticle growth. Small nanoparticles below 25 nm are greatly affected by magnesium oxidation, leading to void development which imposes a bottom limit to the nanoparticle’s size. Hydrogen absorption and magnesium evaporation are competing processes of which the latter can be suppressed by quick absorption within two hours at 250 °C. The crystal structure of the hydride can be tuned from a rutile to a fluorite structure by altering the composition. However, regardless of composition, size, crystal structure, structural motif and shape, no hydrogen is desorbed even at 400 °C in high vacuum conditions. As such gas phase synthesized magnesium titanium bimetallic nanoparticles are in their present forms not a suitable candidate for solid state hydrogen storage.