P. Arneodo Larochette

Stability and stabilization of 2H martensite in Cu–Zn–Al single crystals

The stabilization of the 2H martensitic phase in Cu–Zn–Al single crystals with an electron concentration e/a = 1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic β phase by the direct β → 2H or the indirect β → 18R → 2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted σT1. It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300 K, is only obtained if an ageing is performed at a stress above σT1. Additionally, the stresses of the transformation to another martensitic phase, called 18R2, were found to be constant when the value of σT1 is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.

Grain-size dependence of the two-way shape memory effect obtained by stabilisation in Cu-Zn-Al crystals

Abstract Instead of cycling stress or temperature repeatedly through the transformation temperature, it is possible to obtain the two-way shape memory effect (TWME) by pre-ageing the martensite. Results are reported on the influence of the grain size on the magnitude of the TWME and on the work that can be done against an opposing force. The properties of aged Cu–Zn–Al crystals with three different grain sizes are compared, namely single crystals, fine-grained polycrystals obtained by adding grain refiners, and coarse-grained polycrystals without grain refiners as an intermediate case. The usefulness for technological applications of the TWME induced by ageing is discussed.

A novel catalytic route for hydrogenation–dehydrogenation of 2LiH + MgB2via in situ formed core–shell LixTiO2 nanoparticles

Aiming to improve the hydrogen storage properties of 2LiH + MgB2 (Li-RHC), the effect of TiO2 addition to Li-RHC is investigated. The presence of TiO2 leads to the in situ formation of core–shell LixTiO2 nanoparticles during milling and upon heating. These nanoparticles markedly enhance the hydrogen storage properties of Li-RHC. Throughout hydrogenation–dehydrogenation cycling at 400 °C a 1 mol% TiO2 doped Li-RHC material shows sustainable hydrogen capacity of ∼10 wt% and short hydrogenation and dehydrogenation times of just 25 and 50 minutes, respectively. The in situ formed core–shell LixTiO2 nanoparticles confer proper microstructural refinement to the Li-RHC, thus preventing the materials agglomeration upon cycling. An analysis of the kinetic mechanisms shows that the presence of the core–shell LixTiO2 nanoparticles accelerates the one-dimensional interface-controlled mechanism during hydrogenation owing to the high Li+ mobility through the LixTiO2 lattice. Upon dehydrogenation, the in situ formed core–shell LixTiO2 nanoparticles do not modify the dehydrogenation thermodynamic properties of the Li-RHC itself. A new approach by the combination of two kinetic models evidences that the activation energy of both MgH2 decomposition and MgB2 formation is reduced. These improvements are due to a novel catalytic mechanism via Li+ source/sink reversible reactions.

Equipment for hydrogen absorption-desorption cycling characterization of hydride forming materials.

Hydrogen storage materials suffer different degradation processes when they are cycled, forming and decomposing their associated hydride phases. In order to study these processes, we have designed and built an automated apparatus specifically developed for cycling samples of hydride forming materials by changing the external hydrogen pressure. Instead of the standard open configuration involving a high-pressure, high-quality gas bottle and a vacuum pump, the equipment uses another hydride forming material (in our case LaNi5) as a source and sink of hydrogen. The resulting closed-loop configuration eliminates hydrogen waste and ensures that extremely high purity gas is used during the whole experiment, thanks to the purifying properties of the selected hydride as source/sink. Hydrogen pressure is set by changing the source/sink temperature. Cycles can be performed as fast as one cycle every 5 min, a period comparable with typical good hydride forming material kinetics. An example of application of the apparatus is given for 1000 absorption/ desorption cycles on a Mm0.8Ca0.2Ni5 sample.

Stabilization and the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals

Abstract The influence of diffusion on the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals has been studied. The excess vacancy concentration which controls the diffusion velocity has been monitored by different heat treatments: (a) a slow air cooling and aging in the austenite prior to the transformation, leading to a low vacancy concentration; (b) a quench from 800°C; and (c) a quench from 300°C with an increase in the excess vacancies. The TWME has also been studied at low temperatures where diffusion is absent. All these results show that in Cu–Zn–Al polycrystals, diffusion has little or no influence on the TWME, and that instead a spectrum of martensite plate configurations is created by the stress, which then serve as nuclei for the subsequent transformation without an applied stress. The TWME obtained in the polycrystals is in all cases lower than that in single crystals after the stabilization of the stress induced martensite variant.

CNT addition to the LiBH4–MgH2 composite: the effect of milling sequence on the hydrogen cycling properties

The composite 2LiBH4 : MgH2 has recently received attention as a potential hydrogen storage material. This is mainly due to its high storage capacity. However, the temperatures needed to obtain adequate reaction kinetics are still too high for practical applications. In the present work we study the effect of Ni and carbon nanotube addition as catalysers. We found that different synthesis methods of the composite lead to different hydrogen absorption/desorption kinetic behaviours. These changes can be attributed to morphological and microstructural differences caused by the dissimilar milling stages at which the nanotubes were introduced during the sample synthesis. An induction time during the hydrogen desorption appeared as a consequence of the different dispersions of the carbon nanotubes observed in the samples prepared with both synthesis methods. It was also found that equilibrium pressure increased when the temperature decreased below 375 °C, this effect was kinetic and it was possible to conclude that the addition of nanotubes had no effect on the thermodynamics of the system.