Neves, Andre

In this work, 1.25 t of AB₂-commercially available hydride-forming alloy is taken as a case study for material selection for large-scale systems. Systematic experimental characterizations, modeling, and life cycle-cost assessment at this industrial scale are performed. Based on the thermodynamic characterization, the equilibrium pressure is calculated by applying the most used Nishizaki and novel 3D representation with 2D-bilinear interpolation approaches, giving accurate values. The kinetic model is comprehensively and successfully developed in a wide range of temperatures and pressures by applying the separable variable method. Life cycle assessment shows that the CO₂ emissions of these kinds of systems can be minimized by increasing the share of recycled material and by using waste heat sources for dehydrogenation. The economic analysis clarifies the influence of the components on the economic viability of large hydride-based systems for emergency power supply. Finally, guidelines are proposed for the development of hydride-based integrated renewable energy systems.

Constrained Friction Processing (CFP) is a novel solid-state processing technique suitable for lightweight materials, such Mg- and Al-alloys. The technique enables grain size refinement to fine or even ultrafine scale. In this study, the effect of CFP on the microstructural refinement of AM50 rods is investigated in terms of particle size and morphology of the eutectic and secondary phases originally present in the base material, in particular the eutectic ß-Mg17Al12 and Al-Mn phases. For that purpose, as-cast and solution heat-treated base material and processed samples were analyzed. The AlMn intermetallic phase was identified as the main secondary phase present in all samples before and after the processing. A notorious refinement of these particles was observed, starting from particles with an average equivalent length of a few micrometers to around 560 nm after the processing. The refinement of the secondary phase refinement is attributed to a mechanism analogous to the attrition comminution, where the combination of temperature increase and shearing of the material enables the continuous breaking of the brittle intermetallic particles into smaller pieces. As for the eutectic phase, the results indicate the presence of the partially divorced ß-Mg17Al12 particles exclusively in the as-cast base material, indicating that no further phase transformations regarding the eutectic phase, such as dynamic precipitation, occurred after the CFP. In the case of the processed as-cast material analyzed after the CFP, the thermal energy generated during the processing led to temperature values above the solvus limit of the eutectic phase, which associated with the mechanical breakage of the particles, enabled the complete dissolution of this phase. Therefore, CFP was successfully demonstrated to promote an extensive microstructure refinement in multiple aspects, in terms of grain sizes of the -Mg phase and presence and morphology of the Al-Mn and eutectic ß-Mg17Al12 .

Among some promising candidates for high-capacity energy and hydrogen storage is the Lithium-Boron Reactive Hydride Composite System (Li-RHC: 2 LiH + MgB₂/2 LiBH₄ + MgH₂). This system desorbs hydrogen only at relatively high temperatures and presents a two-step series of reactions occurring in different time scales: first, MgH₂ desorbs, followed by LiBH₄. Hitherto, the dehydrogenation kinetic behavior of such a system has been described for different temperatures at specific values of operative pressure. However, a comprehensive model representing its dehydrogenation kinetic behavior under different operative conditions has not yet been developed. Herein, the separable variable method is applied to develop a comprehensive kinetic model, including the two-step dehydrogenation series reaction. The MgH₂ decomposition is described with the one-dimensional interface-controlled reaction rate Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) with a (Pequilibrium/Poperative) pressure functionality and an Arrhenius temperature dependence activation energy of 63 ± 3 kJ/mol H₂. The LiBH₄ decomposition is modeled applying the autocatalytic Prout-Tompkins model. A novel approach to describe the Prout-Tompkins t₀ parameter as a function of the operative temperature and pressure model is proposed. This second reaction step presented a (Pequilibrium – Poperative/Pequilibrium)² pressure dependence and an Arrhenius temperature dependence with activation energy 94 ± 13 kJ/mol H₂. The proposed approach is experimentally and computationally validated, successfully describing the decomposition kinetic behavior of MgH₂ and LiBH₄ under three-phase gas, liquid and solid environment and shows good agreement between experimental and modeled curves.