Sabrina Sartori

Experimental studies of α-AlD3 and α′-AlD3versus first-principles modelling of the alane isomorphs

The thermal phase behaviour of cryomilled α′-AlD3 and α-AlD3 was investigated by in situsynchrotron powder X-ray diffraction (SR-PXD), differential scanning calorimetry and first principles atomic modelling. In situ measurements showed that α′-AlD3 decomposes directly into Al and D2 at around 80 °C during heating at 1 °C min−1. At higher temperatures the transformation of α′-AlD3 to α-AlD3 was observed by DSC measurements at 5 °C min−1, and tentatively by in situSR-PXD at 1 °C min−1. Atomic modelling was carried out to investigate possible structural relationships and transformation pathways between the α- and α′-phase. Group–subgroup relation analyses and direct method lattice dynamics were used to rule out a possible displacive transformation pathway between the α′- and α-phases. The likelihood of a reconstructive transformation was demonstrated by partial transformation of an interface between α′ and α domains during elevated temperature molecular dynamics. Such an α′- to α-phase transformation may be possible when kinetic barriers can be overcome at elevated temperatures or during long time periods. These insights are also relevant to the transformation mechanisms of the β-AlD3 and γ-AlD3 isomorphs to the α-phase.

Small-angle scattering investigations of Mg-borohydride infiltrated in activated carbon

One of the main challenges for introduction of a hydrogen-based economy is storage of hydrogen. Hydrogen storage in solid materials is considered among the most attractive methods. During recent years much emphasis has been placed on the synthesis of nanosized metals and alloys. In the present study Mg(BH4)(2) and Mg((11)BD(4))(2) are infiltrated in pre-treated activated carbon and investigated with small-angle neutron scattering (SANS). The infiltration method is shown to be successful in modifying the size of the Mg-borohydride particles, as confirmed by scanning electron microscopy and x-ray diffraction data. The size of the particles for the infiltrated samples is estimated by SANS measurements to be mainly in the range <4 nm. The results suggest that the smallest pores of the scaffold are partially or fully filled and that this type of scaffold acts as an effective dispersing agent for Mg-borohydride.

Influence of nanoconfinement on morphology and dehydrogenation of the Li11BD4–Mg(11BD4)2 system

The decomposition of a nanoconfined mixture of lithium-magnesium borohydride, Li(11)BD(4)-Mg((11)BD(4))(2), has been investigated and compared to the corresponding mixture in the bulk form. The systems were investigated by thermal analysis, small-angle neutron scattering, (11)B nuclear magnetic resonance and transmission electron microscopy. The dehydrogenation temperatures decreased by up to 60 °C in the nanoconfined system, with gas evolution following different steps, compared to the behaviour of the bulk material under the same conditions. Most importantly, desorption from the nanoconfined hydride proceeds without formation of diborane, B(2)D(6), which evolves from the bulk mixture. From small-angle neutron scattering, differences in morphology between the bulk and the nanoconfined systems are also demonstrated. Evidence of a complete decomposition has been found in the nanoconfined system, after heating up to 460 °C. Furthermore, (11)B NMR data show that nanoconfinement inhibits the formation of dodecaborane, [B(12)D(12)](2-), during decomposition, a result which is important for practical applications of borohydrides.

Small Angle Neutron Scattering

Recent progress on materials for hydrogen storage have pointed out that kinetics and thermodynamics can be modified by nano-confinement of hydrides in porous scaffolds. The investigation of the structural features of these particular systems is challenging with conventional methods, for instance due to the lack of peaks in the diffraction pattern. Small angle neutron scattering is suitable for studying porous systems containing hydrogen and it has been invaluable for the characterisation of the new hydride systems. This chapter presents the basic principles of the technique and gives examples of its application to the field of nano-confined materials for hydrogen storage.