Papers by Author: Sebastiano Garroni

Abstract: Samples of a 2Mg-Fe (at.%) mixture were produced by high energy ball-milling (HEBM) with ball to powder ratio = 20:1, in an argon gas atmosphere, in 190 ml vials (sample-1) to produce powders and in 300 ml vials (sample-2) to produce plates. Both samples were cold-pressed into preforms. The preforms were then extruded at 300°C at a ram speed of 1mm/min., with the following extrusion ratios: sample-1 at 3/1 to ensure porosity and sample-2 at 5/1 to increase the adhesion of the plates. The resulting bulks from samples 1 and 2 were hydrogenated for 24h in a reactor under 15 bar of H₂ to produce the Mg₂FeH₆ complex hydride, and at 11 bar of H₂ to produce both the complex hydride and MgH₂ hydride. In addition, sample-1 was severely temperature-hydrogen cycled to verify its microstructural stability and the influence of grain size on the sorption properties. XRD patterns showed Mg(hc), Fe(ccc) and Mg₂FeH₆ in both samples, and sample-2 also contained MgH₂ and MgO (attributed to processing contamination). DSC results demonstrated that the initial desorption temperature of sample-1 was lower than that of sample-2. However, sample-2 showed faster desorption kinetics, presenting a desorption peak about 73°C below that of sample-1. This could be attributed to the activation/catalyst effect of the MgH₂ hydride. The improvement in sorption properties was attributed mainly to porosity and to the type of employed catalysts.

Abstract: Hydrogen storage in the solid state has shown increasing research and development, and recently an approach in mixing two hydride systems together by ball milling (reactive hydride composites) has been investigated in more detail, e.g. NaBH4 plus MgH2. Thermodynamic destabilization may occur by new compounds formation during dehydrogenation, e.g. MgB2. A study of the the role of O2/H2O contamination for the reaction 2NaBH4 + MgH2 ↔ 2NaH + MgB2 + 4H2 was conducted using in-situ X-ray powder diffraction. Desorption reaction is observed to begin by a competition of MgH2 and NaBH4 decomposition due to higher reactivity promoted by ball milling processing summed to O2/H2O contamination. Oxidation of NaBH4 into NaBO2 is observed to happen in higher degree than MgH2/Mg into MgO for the Na-Mg-B-H system.