Papers by Author: Fabrizio Cleri

Abstract: Magnesium is one of the most promising materials for hydrogen storage due to its high capacity and low cost. Unfortunately, practical applications are for the moment limited by the slow kinetics and the high operating temperature. Nanostructuring magnesium hydride MgH2, generally by ball milling, introduces plastic deformations and catalysts that highly enhances the H2 absorption and desorption. However a fundamental understanding of the role played by catalysts and interfaces in MgH2 is still lacking. Microscopic characterization of MgH2-Mg system with and without heavy metal catalysts, is achieved by combining accurate SEM observations of samples after partial desorption process and atomic level ab-initio molecular dynamics simulations of MgH2-Mg interfaces. The experimental method is based on low voltage SEM observations of cross sectional powder samples, prepared by a new specific metallographic process. Identification of nucleation sites of the sorption reaction and their correlation with the presence of catalyst particles is achieved by suitable experimental conditions. Moreover ab-initio molecular dynamics clarifies the interplay of interfaces and the deformations induced during desorption by the presence of catalysts that are able to lower binding energies and free hydrogen atoms toward interfaces. Both approaches confirm and characterize the nucleation step in the catalysts driven phase transformation.

Abstract: We studied the atomic-level structure of a model Mg-MgH2 interface by means of the Car-Parrinello molecular dynamics method (CPMD). The interface was characterized in terms of total energy calculations, and an estimate of the work of adhesion was given, in good agreement with experimental results on similar systems. Furthermore, the interface was studied in a range of temperatures of interest for the desorption of hydrogen. We determined the diffusivity of atomic hydrogen as a function of the temperature, and give an estimate of the desorption temperature.

Abstract: Extensive electronic structure calculations, based on the density-functional theory, are used to characterize the equilibrium properties and the behaviour under pressure of MgH2. Moreover, substitutional solid solutions for several 3d metals (Fe, Ni, Ti, Al, Pd, Co, etc.) in MgH2 are characterized by computing both relaxed structures and solution energies. Theoretical results are combined with experimental results on samples synthesized by ball milling. MgH2 was milled with 10 wt.% of 3d metals followed by microstructural characterization and hydrogen desorption behaviour. Good correlation between theoretical solution energies and experimental desorption temperatures reveals a general trend in these systems. A few notable exceptions can be detected, which can be explained in terms of peculiarities in the experimental composite microstructure.

Abstract: A continuum model for composite materials made of short, stiff and tough fibres embedded in a more deformable matrix with distributed microflaws is proposed. Based on the kinematics of a lattice system made of fibres, perceived as rigid inclusions, and of microflaws, represented by slit microcracks, the stress-strain relations of an equivalent multifield continuum is obtained. These relations account for the shape and the orientation of the internal phases and include internal scale parameters, which allow taking into account size effects. Some numerical analyses effected on a sample fibre-reinforced composite pointed out the influence of the size and orientation of the fibres on the gross behaviour of the material.