Mg-MOF-74 Derived Defective Framework for Hydrogen Storage at Above-Ambient Temperature Assisted by Pt Catalyst.

Metal-organic frameworks (MOFs) are promising candidates for room-temperature hydrogen storage materials after modification, thanks to their ability to chemisorb hydrogen. However, the hydrogen adsorption strength of these modified MOFs remains insufficient to meet the capacity and safety requirements of hydrogen storage systems. To address this challenge, a highly defective framework material known as de-MgMOF is prepared by gently annealing Mg-MOF-74. This material retains some of the crystal properties of the original Mg-MOF-74 and exhibits exceptional hydrogen storage capacity at above-ambient temperatures. The MgO5 knots around linker vacancies in de-MgMOF can adsorb a significant amount of dissociated and nondissociated hydrogen, with adsorption enthalpies ranging from -22.7 to -43.6 kJ mol-1, indicating a strong chemisorption interaction. By leveraging a spillover catalyst of Pt, the material achieves a reversible hydrogen storage capacity of 2.55 wt.% at 160 °C and 81 bar. Additionally, this material offers rapid hydrogen uptake/release, stable cycling, and convenient storage capabilities. A comprehensive techno-economic analysis demonstrates that this material outperforms many other hydrogen storage materials at the system level for on-board applications.

Twin and dislocation mechanisms in tensile W single crystal with temperature change: a molecular dynamics study.

Molecular dynamics simulations are performed to investigate the orientation and temperature dependence of tensile response in single crystal W. It is found that W single crystal exhibits distinct temperature-dependent deformation behaviors along different orientations. With increasing temperature, the yield strain in the [001] orientation increases, while those in [110] and [111] orientations first increase and then decrease. The tensile deformations along orientations close to [001] are found to be dominated by twinning; the nucleation and growth of twins are accomplished through the nucleation and glide of ⅙111 partial dislocations on {112} planes. In contrast, the deformations along orientations close to [110] and [111] are found to be dominated by the slip of ½111 full dislocations, which move in a stay-and-go fashion. Moreover, intermediate deformation behaviors, which may become unstable at high temperatures, are observed for some intervening orientations. The distinct deformation behaviors of W along different orientations are rationalized based on the twinning-antitwinning asymmetry of ⅙111 partial dislocations on {112} planes.

Oxidation of the two-phase Nb/Nb5Si3 composite: the role of energetics, thermodynamics, segregation, and interfaces.

Oxidation behavior of the two-phase Nb/Nb(5)Si(3) composite is of significant importance for the potential applications of the composite at high-temperature conditions. We investigate the atomic-scale oxidation mechanism of the Nb/Nb(5)Si(3) composite with first-principles density-functional theory and thermodynamics analysis. In particular, the effects of energetics, thermodynamics, segregation, and interfaces are identified. The clean composite surface is found to be composed of both Nb(110) and Si-terminated Nb(5)Si(3)(001). Energetics and thermodynamics calculations show that, during the oxidation process, the Nb(110) surface is oxidized first, followed by the segregation of niobium of the Nb(5)Si(3)(001) surface and subsequent oxidation of the Nb element of Nb(5)Si(3). High coverage of oxygen results in dissolved oxygen in bulk Nb through the diffusion of oxygen in the surface and at the interface. The theoretical investigation also provides an explanation, at the atomic-scale, for the experimental observation that the oxidation layer is essentially composed of niobium oxide and almost free of silicon. Furthermore, the methodology of this work can be applied to investigations of the oxidation behavior of other two-phase and multi-phase composites.

First-principles study on the incipient oxidization of Nb(110).

The incipient oxidization of Nb(110) has been investigated using the density functional theory method. We rationalize the well-known puzzle, i.e., Nb(110) is difficult to clean, by calculating the O dissolution, and the on-surface and subsurface adsorption at low concentration. It is found that the structure of on-surface O adsorption at 0.50 monolayer (ML) coverage has the largest binding energy and minimum work function, in agreement with experimental results. At 1.00 ML coverage, the inward diffusion of O atoms is promoted by O adatoms, attributed to the formation of a local electric field. Our theoretical results improve the understanding of the experiments showing that NbO(x) oxides on the surface can be formed and decomposed by treating samples at 1500-2000 K in vacuum. Furthermore, the thermodynamic analysis of the O/Nb(110) systems shows that bulk NbO is stable in vacuum, in agreement with the observed formation of NbO nanostructures on Nb(110).

Unexpected relationship between interlayer distances and surface/cleavage energies in γ-TiAl: density functional study.

Density functional calculations were performed to study the γ-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stable under Ti-rich conditions, while the Al-termination (110) surface becomes the most stable with the increase of Al chemical potential. We calculate that in γ-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energy and cleavage energy. This is different from the situation in a pure metal. This phenomenon can be explained by the analysis of the bonding characteristics in γ-TiAl. In particular there are both metallic and covalent bonds in γ-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti-Al-Ti atoms. It is the covalent bonds that affect greatly the cleavage energy, the surface energy and the surface stability.

Atomic structure and adhesion of the Nb(001)/α-Nb5Si3(001) interface: a first-principles study.

The density functional calculations have been performed to study the Nb(001) and α-Nb5Si3(001) surfaces as well as the interface properties of Nb(001)/α-Nb5Si3(001). The surface energy of the Nb(001) surface is about 2.25 J m (- 2). The calculated cleavage energies of bulk Nb5Si3 are 5.103 J m (- 2) and 5.787 J m (- 2) along (001) planes with the breaking of Nb-Si and Nb-NbSi bonds, respectively. For the Nb(001)/α-Nb5Si3(001) models, the Nb atoms in the interface region initially belonging to body centered cubic metal Nb are twisted to the position of the Nb atom layer in Nb5Si3 and the interlayer distance is similar to that of bulk Nb5Si3 after being fully relaxed. The ideal work of adhesion of the Nb(001)/Nb5Si3(001) interface is calculated and compared to those of bulk Nb and Nb5Si3. The results show that the bulk Nb5Si3 has the largest work of adhesion, the bcc Nb ranks second and the interface ranks last. Moreover, the Nb-Si bond is weaker than Nb-NbSi and Nb-Nb bonds in the interface, which means that the Nb-Si bond in the interface is the most possible site for the micro-crack generation when the stress is applied quasi-statically along the [001] direction. The densities of states, Mulliken population and overlap population of the Nb(001)/α-Nb5Si3(001) interface are also analyzed.

Surface segregation of Si and its effect on oxygen adsorption on a γ-TiAl(111) surface from first principles.

We perform first-principles calculations based on the density-functional theory to study the surface segregation of Si and its effect on the oxygen adsorption on a γ-TiAl(111) surface for a range of oxygen coverage 0<Θ≤1.0 monolayer (ML). The calculated results show that the alloying Si atoms prefer occupying surface Ti sites to the sites in the bulk of γ-TiAl, which suggests the occurrence of Si surface segregation. When oxygen atoms adsorb on a pure γ-TiAl(111) surface, the most favorable sites are the adsorption sites with more Ti atoms as their nearest neighbors in the surface layer at all the calculated coverages and the interactions between adsorbed oxygen atoms are repulsive. However, when oxygen atoms adsorb on an Si-alloyed γ-TiAl(111) surface, the interactions between the adsorbed oxygen atoms are attractive at oxygen coverage 0<Θ≤1.0 ML. Meanwhile, the interactions between O and Al atoms become stronger whereas those between O and Ti atoms become weaker relative to oxygen adsorbed on a pure γ-TiAl(111) surface. The atomic geometry and density of state are analyzed. The results show that the surface ripple of the top metal layer for oxygen on a pure γ-TiAl(111) surface is Ti upwards, while that for oxygen on an Si-alloyed γ-TiAl(111) surface is Al upwards at high oxygen coverage (Θ≥0.50 ML). This effect of Si is of benefit to the nucleation of alumina, which is attributed to Si surface segregation and an increase of the surface Al:Ti ratio. This can help to explain why alloying the γ-TiAl(111) surface with Si could favor the formation of the Al(2)O(3) scale at the first stage and result in good oxidation resistance in experiments.

The effects of alloying elements Al and In on Ni-Mn-Ga shape memory alloys, from first principles.

The electronic structures and formation energies of the Ni(9)Mn(4)Ga(3-x)Al(x) and Ni(9)Mn(4)Ga(3-x)In(x) alloys have been investigated using the first-principles pseudopotential plane-wave method based on density functional theory. The results show that both the austenite and martensite phases of Ni(9)Mn(4)Ga(3) alloy are stabilized by Al alloying, while they become unstable with In alloying. According to the partial density of states and structural energy analysis, different effects of Al and In alloying on the phase stability are mainly attributed to their chemical effects. The formation energy difference between the austenite and martensite phases decreases with Al or In alloying, correlating with the experimentally reported changes in martensitic transformation temperature. The shape factor plays an important role in the decrease of the formation energy difference.