Reactive Bimetallic Nanostructures Based on Triply Periodic Minimal Surfaces: A Molecular Dynamics Study toward the Limits of Performance.

A variety of intermetallic compounds possesses high enthalpies of formation. These compounds may be formed from reactive compacts or nanostructures comprised of unreacted precursor metals. These precursor structures support self-propagating high temperature synthesis (SHS) reactions which afford very high specific energy densities and rates, with excellent spatial control and a variety of useful applications. The present work compares the reactivity of notional bimetallic nanostructures based on well-known triply periodic minimal surfaces (TPMSes) with the popular reactive nanolaminate (RNL) modality for the Ni/Al system, using a molecular dynamics approach. TPMS-derived nanostructures were found to have lower ignition energies and faster reaction rates than RNLs of comparable periodicity, while the maximum achievable temperature of ignitions was found to be modulated by a complex interplay of factors including reaction rate and specific metal/metal interface density. Nanostructure reactivity and thermochemistry is also affected by effective diffusion dimensionality and recalescent precipitation of intermetallic crystallites. The TPMS-derived reactive nanostructures presented herein anticipate plausible advances in nanofabrication technology.

The role of interlayer gases and surface asperities in compression-induced intermetallic formation in Ni/Al nanocomposites.

Reactive composites comprising alternating nano- or microscale layers of Ni and Al are known to undergo self-sustaining alloying reactions under compression loading, however the effect of infiltrated gas within the microstructure of such reactive nanolaminates-as well as the presence of asperities on the free surfaces of such composites-is not well understood. This work presents atomistic molecular dynamics simulation and analysis of the mechanical dynamics and thermal evolution of planar Ni/Al nanolaminates under a variety of scenarios of layer dimensions, surface asperity shape and orientation, and interlayer gas identity and concentration. These simulations indicate that the rate of the alloying reaction is inversely correlated with the layer width of the nanolaminate, recapitulating experimental results. The presence of surface asperities of comparable scale to the nanolaminate layer thickness enhances short-term intermetallic mixing but has a marginal accelerant effect on the reaction. Interlayer argon gas acts as a mechanical interferent to reaction, whilst interlayer nitrogen gas-modelled here with a novel interatomic potential-is shown to enhance heat production. These calculations also characterise compression wave dynamics in compression-loaded Ni/Al nanolaminates in greater detail than prior studies and illustrate significant qualitative and quantitative differences between extant embedded atom model (EAM) parameterisations of Ni/Al. Speeds of sound for each metal and EAM are also reported. These differences have implications for the interpretation and comparability of EAM-based modelling of Ni/Al reactions moving forward, and may also have wider implications for EAM modelling of intermetallic systems in general.

Electronic structure modelling of the edge-functionalisation of graphene by MnxOy particles.

The use of graphenic carbon is attractive as a basal or intermediate support for catalytic particles in advanced catalytic electrodes. This popularity is motivated by its excellent electrical properties and ability to form foliated conformal coatings of exceptional surface area and flexibility. Surface- and edge-functionalisation of graphene sheets affords diverse routes to the covalent attachment of candidate catalytic species. Of particular interest to advanced water oxidation is the possibility of covalent attachment of MnxOy species partially recapitulating the chemistry of the Mn4O5Ca active site of Photosystem II (PSII), which achieves the four-electron oxidation of water under physiological conditions. Here, we report aperiodic density functional theory (DFT) investigations of candidate attachment geometries for a variety of manganese oxide particles to graphene sheets. We find that the flexibility of graphene sheets as well as the conformational degrees of freedom of candidate edge functionalisation permits a large variety of realistic attachment geometries that can act as attachment sites for molecular manganese-oxide species or nuclei for the growth of periodic manganese oxides. We find that substantially simplified models of graphene attachment afford an excellent compromise between computational efficiency, tractability, and accuracy, and characterise the accuracy of these models in detail.

Dinitrogen activation by Fryzuk's [Nb(P(2)N(2))] complex and comparison with the Laplaza-Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems.

The reaction profile of N(2) with Fryzuk's [Nb(P(2)N(2))] (P(2)N(2)=PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) complex is explored by density functional calculations on the model [Nb(PH(3))(2)(NH(2))(2)] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N(2) with the three-coordinate Laplaza-Cummins [Mo{N(R)Ar}(3)] and four-coordinate Schrock [Mo(N(3)N)] (N(3)N=[(RNCH(2)CH(2))(3)N](3-)) systems. When the model system is constrained to reflect the geometry of the P(2)N(2) macrocycle, the N--N bond cleavage step, via a N(2)-bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol(-1). In comparison, formation of the single-N-bridged species is calculated to be exothermic by 119 kJ mol(-1), and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N--N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol(-1), but the N(2) cleavage step remains endothermic by 35 kJ mol(-1). Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol(-1) and that of the N--N bond cleavage step by 85 kJ mol(-1), making it a favourable process. Replacing Nb(II) with Mo(III) has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol(-1). The reaction profile for the model [Nb(P(2)N(2))] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}(3)] and Schrock [Mo(N(3)N)] systems. For both [Mo(N(3)N)] and [Nb(P(2)N(2))], the intermediate dimer is calculated to lie lower in energy than the products, although the final N-N cleavage step is much less endothermic for [Mo(N(3)N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}(3)], and therefore this system is predicted to be most suitable for dinitrogen cleavage.