Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction.

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with firstprinciples calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA)produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials

Modelling transition metal cations in zeolites: how do they interact with the framework?

International audienc

Modelling transition metal cations in zeolites: how do they interact with the framework?

International audienc

A Modeling Tentative of Operation of Gas Sensor through Atomic Scale Insights

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Selective Catalytic Reduction of NO by NH3 on Cu-Faujasite Catalysts: An Experimental and Quantum Chemical Approach

International audienceThe selective catalytic reduction (SCR) of NO by NH3 in the presence of O2 on Cu-faujasite (Cu-FAU) has been studied. Substitution of some Cu2_ with H_ and Na_ cations, compensating for the negative charge of the zeolite framework, forms the various CuHNa-FAU studied. The amount of Cu was held constant and the proportion of H_ and Na_ varied in the sample. The substitution of Na_ for H_ increases sharply the SCR rate by lowering the temperature of reaction by about 150 K. It is proposed that the rate increase mainly comes from an unhindered migration of Cu from hidden to active sites and a modification of the redox properties of Cu species. The former was demonstrated by diffuse reflectance IR spectroscopy of adsorbed CO. The change in redox properties was demonstrated by a faster oxidation of Cu_ to Cu2_ (rate-determining step). Quantum chemical calculations on model clusters of CuHNa-FAU indicate that the faster rate of oxidation can be explained by a higher lability of protons in the absence of Na, which can be then removed from the catalyst more easily to yield H2O during the oxidation process

Modélisation de la pression généré dans des thermites à base d'aluminium.

International audienceThe paper proposes a new theoretical model based on local thermodynamic equilibrium enabling the prediction of gas generation during the reaction of aluminum-based thermites. We demonstrate that the model has the capability to predict the total pressure and its partial pressure components as a function of the reaction extent and compaction. Al/CuO, Al/Bi2O3, Al/Sb2O3, Al/MoO3 and Al/WO3 thermites are modeled and their capability to generate pressure compared. Simulation results are also validatedthrough dedicated experiments and showgeneral agreement beyond the state of the art. Mechanisms underlying pressure generation are detailed. A two-stage process for the pressure increase in Al/CuO reaction,also observed experimentally,is shown to be driven by oxygen generation as produced byCuO and Cu2O vaporizationthrough different kinetics. Comparison with experiment stresses the issue of the understanding of the complex chemical processes taking place during vaporization and subsequent gas phase reactions and the need to determine their thermodynamicconstants

Modélisation de la pression généré dans des thermites à base d'aluminium.

International audienceThe paper proposes a new theoretical model based on local thermodynamic equilibrium enabling the prediction of gas generation during the reaction of aluminum-based thermites. We demonstrate that the model has the capability to predict the total pressure and its partial pressure components as a function of the reaction extent and compaction. Al/CuO, Al/Bi2O3, Al/Sb2O3, Al/MoO3 and Al/WO3 thermites are modeled and their capability to generate pressure compared. Simulation results are also validatedthrough dedicated experiments and showgeneral agreement beyond the state of the art. Mechanisms underlying pressure generation are detailed. A two-stage process for the pressure increase in Al/CuO reaction,also observed experimentally,is shown to be driven by oxygen generation as produced byCuO and Cu2O vaporizationthrough different kinetics. Comparison with experiment stresses the issue of the understanding of the complex chemical processes taking place during vaporization and subsequent gas phase reactions and the need to determine their thermodynamicconstants

On the early stage of aluminum oxidation: An extraction mechanism via oxygen cooperation

International audienceWe propose a barrierless mechanism for describing the oxidation of Al(111) in which oxygen atoms located on the outer surface extract aluminum atoms of the surface layers through local cooperation of other pre-adsorbed oxygen atoms. We show the details of this complex chemical process that kinetically competes with the non-destructive formation of an oxygen monolayer onto the Al surface, thus elucidating the initial aluminum oxidation regime. We demonstrate that further stripping of the complete surface Al layer is consistent with both (i) the formation of a defective alumina structure and (ii) an oxide capping layer preventing further oxidation at low temperature

Bottom-up modeling of Al/Ni multilayer combustion: Effect of intermixing and role of vacancy defects on the ignition process

International audienceVapor deposited multilayered aluminum/oxide and bimetallics are promising materials for Micro Electro Mechanical System technologies as energy carriers, for instance, microinitiators or heat microsources in biological or chemical applications. Among these materials, the Al/Ni couple has received much attention both experimentally and theoretically. However, the detailed relation between the chemical composition of the intermixed interfacial regions and its impact on the ignition capabilities remains elusive. In this contribution, we propose a twofold strategy combining atomistic density functional theory (DFT) calculations and a macroscopic 1D model of chemical kinetics. The DFT calculations allow the description of the elementary chemical processes (involving Al, Ni atoms and vacancies basic ingredients) and to parameterize the macroscopic model, in which the system is described as a stack of infinite layers. This gives the temporal evolution of the system composition and temperature. We demonstrate that the amount of vacancies, originating from the deposition process and the Al and Ni lattice mismatch, plays a critical role on both the ignition time and the temperature. The presence of vacancies enhances the migration of atoms between layers and so dramatically speeds up the atomic mixing at low temperatures far below ignition temperature, also pointing to the relation between experimental deposition procedures and ageing of the nanolaminates. V C 2013 AIP Publishing LLC