Hydrogen Storage and Electronic Characterization of Magnesium-Based Intermetallics

The impetus of this research was the synthesis and characterization of new Mg-based hydrogen storage materials. Synthetic methods included metal fluxes, distillations and traditional stoichiometric melts. Pure Mg or Mg-X (X=element other than Mg) eutectic fluxes were useful in synthesizing intermetallic single crystals. Distillations of Mg containing materials were used to modify the structure and properties of a material and to grow large dendrites of nearly pure Mg crystals. Synthesized materials were characterized by X-ray diffraction, electron microscopy and thermogravimetric methods. Analyses for magnetic, transport and hydrogen storage properties were conducted. To address the need for a method of screening newly synthesized products for hydrogen storage properties, a Sievert's-type volumetric system was built to activate and hydrogenate samples. The computer controlled system can hydrogenate samples at 0-2500 PSIG and 25-700 °C while collecting temperature and pressure data at 0.5 second intervals. Automatic absorption and desorption programs were also designed in LabView to allow for volumetric measurements. Samples activated and hydrogenated in the system could also be analyzed for subsequent desorption in a TGA. Stoichiometric melts of Mg-Ca-Si led to the formation of composites containing the controversial phase, Ca2Mg3Si; these were analyzed for phase composition and dehydrogenation properties. Exploration of transition metal doping led to composites of Ca2Mg3Si/Mg2Si/CaMgSi, Ca2Mg3Si/Mg2Co and Ca2Mg3Si/Fe3Si with reversible hydrogen storage capacities up to 4 weight percent. Slight doping of Si on the Mg site in a Mg2Ca matrix produced a single phase material, Ca2Mg4-xSix (x = ~0.3), which displayed a maximum hydrogen capacity of 1.9 weight percent. A second phase in the Mg-Ca-Si system, CaMgSi, was synthesized in the form of large single crystals from a molten Mg-Al flux mixture. CaMgSi undergoes an electronic phase transition from metallic to semiconductor at 50 K. This metal to insulator transition was evident from both resistivity and magnetic susceptibility data. It is accompanied by a structural phase transition observable by low temperature powder X-ray diffraction. The hydrogen storage capacity of CaMgSi was expectedly negligible. Additional reactions in the Mg-Al eutectic flux indicate it is a useful tool for exploratory synthesis of Mg-based or Al-based materials. Rare earth metals readily dissolve and react to form complex silicide phases such as Eu8Mg18Si13 and Yb6Mg11Fe2Si5. Mixed fluxes of Mg-Al-X (X = Ga, In, Sn) improved the reactivity of the flux and created optimal crystal growth conditions for the compounds CaMgSi, CaMgGe and CaMgSn. The mixed fluxes also improve the solubility and reactivity of the lightweight elements Li, B, C and Si; these are desirable elements for incorporation into potential hydrogen storage materials. Further reactions have been conducted and stockpiled to create an organized crystal bank which can be extensively characterized in future work.