Synthesis of a single phase of high-entropy Laves intermetallics in the Ti–Zr–V–Cr–Ni equiatomic alloy

<p>The high-entropy Ti–Zr–V–Cr–Ni (20 at% each) alloy consisting of all five hydride-forming elements was successfully synthesised by the conventional melting and casting as well as by the melt-spinning technique. The as-cast alloy consists entirely of the micron size hexagonal Laves Phase of C14 type; whereas, the melt-spun ribbon exhibits the evolution of nanocrystalline Laves phase. There was no evidence of any amorphous or any other metastable phases in the present processing condition. This is the first report of synthesising a single phase of high-entropy complex intermetallic compound in the equiatomic quinary alloy system. The detailed characterisation by X-ray diffraction, scanning and transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed the existence of a single-phase multi-component hexagonal C14-type Laves phase in all the as-cast, melt-spun and annealed alloys. The lattice parameter <i>a</i> = 5.08 Å and <i>c</i> = 8.41 Å was determined from the annealed material (annealing at 1173 K). The thermodynamic calculations following the Miedema’s approach support the stability of the high-entropy multi-component Laves phase compared to that of the solid solution or glassy phases. The high hardness value (8.92 GPa at 25 g load) has been observed in nanocrystalline high-entropy alloy ribbon without any cracking. It implies that high-yield strength (~3.00 GPa) and the reasonable fracture toughness can be achieved in this high-entropy material.</p

Curious Catalytic Characteristics of Al–Cu–Fe Quasicrystal for De/Rehydrogenation of MgH₂

The present study reports the curious catalytic action of a new class of catalyst, quasicrystal of Al₆₅Cu₂₀Fe₁₅ on de/rehydrogenation properties of magnesium hydride (MgH₂). Catalyzed through this catalyst, the onset desorption temperature of MgH₂ gets reduced significantly from ∼345 °C (for ball-milled MgH₂) to ∼215 °C. A more dramatic effect of the above catalyst has been observed on rehydrogenation. Here, 6.00 wt % of hydrogen storage capacity is observed in just 30 s at 250 °C. Improved rehydrogenation kinetics has been found even at lower temperatures of 200 and 150 °C by absorbing ∼5.50 and ∼5.40 wt % of H₂, respectively, within 1 min and ∼5.00 wt % at 100 °C in 30 min. These are some of the lowest desorption temperatures and rehydrogenation kinetics obtained for MgH₂ through any other known catalyst. The storage capacity of MgH₂ catalyzed with the leached version of Al₆₅Cu₂₀Fe₁₅ quasicrystalline alloy degrades negligibly even after 51 cycles of de/rehydrogenation. The feasible reason for catalytic action has been described and discussed on the basis of structural, microstructural, Fourier transform infrared, and X-ray photoelectron spectroscopic studies

Excellent Catalytic Effects of Graphene Nanofibers on Hydrogen Release of Sodium alanate

One of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled devices and vehicles remains hydrogen storage. More environmentally friendly and effective nonmetal catalysts are required to improve hydrogen sorption. In this paper, through a combination of experiment and theory, we evaluate and explore the catalytic effects of layered graphene nanofibers toward hydrogen release of light metal hydrides such as sodium alanate. Graphene nanofibers, especially the helical kind, are found to considerably improve hydrogen release from NaAlH₄, which is of significance for the further enhancement of this practical material for environmentally friendly and effective hydrogen storage applications. Using density functional theory, we find that carbon sheet edges, regardless of whether they are of zigzag or armchair type, can weaken Al–H bonds in sodium alanate, which is believed to be due to a combination of NaAlH₄ destabilization and dissociation product stabilization. The helical form of graphene nanofibers, with larger surface area and curved configuration, appears to benefit the functionalization of carbon sheet edges. We believe that our combined experimental and theoretical study will stimulate more explorations of other microporous or mesoporous nanomaterials with an abundance of exposed carbon edges in the application of practical complex light metal hydride systems