Pierre Alphonse

Nanoenergetic Materials for MEMS: A Review

New energetic materials (EMs) are the key to great advances in microscale energy-demanding systems as actuation part, igniter, propulsion unit, and power. Nanoscale EMs (nEMs) particularly offer the promise of much higher energy densities, faster rate of energy release, greater stability, and more security (sensitivity to unwanted initiation). nEMs could therefore give response to microenergetics challenges. This paper provides a comprehensive review of current research activities in nEMs for microenergetics application. While thermodynamic calculations of flame temperature and reaction enthalpies are tools to choose desirable EMs, they are not sufficient for the choice of good material for microscale application where thermal losses are very penalizing. A strategy to select nEM is therefore proposed based on an analysis of the material diffusivity and heat of reaction. Finally, after a description of the different nEMs synthesis approaches, some guidelines for future investigations are provided.

Synthesis of large-area and aligned copper oxide nanowires from copper thin film on silicon substrate

Large-area and aligned copper oxide nanowires have been synthesized by thermal annealing of copper thin films deposited onto silicon substrate. The effects of the film deposition method, annealing temperature, film thickness, annealing gas, and patterning by photolithography are systematically investigated. Long and aligned nanowires can only be formed within a narrow temperature range from 400 to 500°C. Electroplated copper film is favourable for the nanowire growth, compared to that deposited by thermal evaporation. Annealing copper thin film in static air produces large-area, uniform, but not well vertically aligned nanowires along the thin film surface. Annealing copper thin film under a N2/O2 gas flow generates vertically aligned, but not very uniform nanowires on large areas. Patterning copper thin film by photolithography helps to synthesize large-area, uniform, and vertically aligned nanowires along the film surface. The copper thin film is converted into bicrystal CuO nanowires, Cu2O film, and also perhaps some CuO film after the thermal treatment in static air. Only CuO in the form of bicrystal nanowires and thin film is observed after the copper thin film is annealed under a N2/O2 gas flow.

Development of a nano-Al∕CuO based energetic material on silicon substrate

Nanoenergetic materials (nEMs) have improved performances compared to their bulk counterpart or microcounterpart. The authors propose an approach to synthesize an Al∕CuO based nEM that has several advantages over previous investigations such as enhanced contact, reduced impurities and Al oxidation, tailored dimensions, and easier integration into microsystem. CuO nanowires are synthesized by thermally annealing Cu film deposited onto silicon. Nano-Al is integrated with the nanowires to realize an Al∕CuO based nEM. The synthesized nEM is characterized by scanning electron microscopy, high resolution transmission electron microscopy, x-ray diffraction, differential thermal analysis, and differential scanning calorimetry.

Multilayered Al/CuO thermite formation by reactive magnetron sputtering: Nano versus micro

Multilayered Al/CuO thermite was deposited by a dc reactive magnetron sputtering method. Pure Al and Cu targets were used in argon–oxygen gas mixture plasma and with an oxygen partial pressure of 0.13 Pa. The process was designed to produce low stress (<50 MPa) multilayered nanoenergetic material, each layer being in the range of tens nanometer to one micron. The reaction temperature and heat of reaction were measured using differential scanning calorimetry and thermal analysis to compare nanostructured layered materials to microstructured materials. For the nanostructured multilayers, all the energy is released before the Al melting point. In the case of the microstructured samples at least 2/3 of the energy is released at higher temperatures, between 1036 and 1356 K.

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 first-principles 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.

Effect of PEG on rheology and stability of nanocrystalline titania hydrosols.

Very stable titania hydrosols were prepared by fast hydrolysis of titanium isopropoxide in a large excess of water. XRD patterns show that these sols contain nanocrystals (5-6 nm) of anatase (70%) and brookite (30%). TEM images indicate that these primary particles form aggregates whose mean hydrodynamic diameter, determined by photon correlation spectroscopy, is in the range of 80-90 nm. The flow curves of these colloids, recorded for several volume fractions of nanoparticles, can be perfectly fitted, in the range 0-100 s(-1), with a power-law model. In this range the behavior is Newtonian but for larger shear rates a shear thinning is observed. The viscosity dependence on particle concentration can be predicted by a Batchelor-type model were the volume fraction of particles is replaced by an effective volume fraction of aggregates, taking into account their fractal dimension. Addition of polyethylene glycol (PEG 2000) induced a marked decrease (more than 50%) of the sol viscosity down to a minimum. This is explained by assuming that PEG adsorbs on the surface of TiO(2) particles producing stabilization by steric effects and leading to formation of more compact aggregates. Without PEG the sol viscosity strongly decreases on aging. This effect is not caused by the growth of primary particles. It is rather interpreted as a progressive reorganization of the aggregates toward a more compact packing.

An efficient route to aqueous phase synthesis of nanocrystalline γ-Al2O3 with high porosity: from stable boehmite colloids to large pore mesoporous alumina.

In this paper we emphasise the important role of Pluronic F127 on the porosity of mesoporous alumina prepared from boehmite colloids. By focusing on the F127/boehmite interactions we show how the concepts of interface science may help to predict and improve the textural characteristics of mesoporous alumina. By varying the synthetic parameters, in particular the copolymer content, we show that the porosity of γ-Al(2)O(3) can be enhanced by 400% and the average pore diameter can be expanded from 5 to 14 nm. These results are discussed in terms of interactions between the Pluronic F127 and boehmite colloids, and are correlated to the critical micelle concentration (CMC) of the copolymer. The textural characteristics of the mesoporous alumina can be further improved either by introducing hydrocarbons in the preformed boehmite/copolymer sols or by concentrating the sols. In comparison with as-synthesised alumina, those prepared with F127 showed improved thermal stability. Furthermore, boehmite/copolymer sols were stable for all surfactant concentrations investigated and can give high quality coatings suitable for catalytic applications.

Synthesis and characterization of non-stoichiometric nickel–copper manganites

Non-stoichiometric nickel–copper manganites Ni Cu Mn h O were synthesized by thermal decomposition of x y 32x2y 3d / 4 41d mixed Ni Cu Mn C O , nH O oxalates in air at low temperature (623–673 K). X-ray diffraction showed that, x / 3 y / 3 (32x2y) / 3 2 4 2 for a nickel content x

Synthesis and characterization of nonstoichiometric nickel manganite spinels NixMn3 − x□3δ4O4 + δ

0.1, the oxalates precipitated presented a mixed crystal structure up to a limit value of copper Ni extent, whereas the oxalates obtained with x ,0.1 were not mixed. This could be explained by the intermediate structure Ni of nickel oxalate (b orthorhombic form) between those of copper and manganese (a monoclinic form) oxalates. The structure (a or b) of the mixed oxalates obtained was also investigated and their lattice parameters are given. The Ni Cu Mn h O oxides crystallize in the spinel structure in a wide range of composition and a stabilizing effect x y 32x2y 3d / 4 41d 2 21 of copper was evidenced. They are highly divided (Sw.100 m g ) however Sw tends to decrease with increasing y . Cu The non-stoichiometry d of such nickel–copper manganites was for the first time determined by selective titration (gas chromatography) of the oxygen released during TPR experiments in argon. The technique is presented and the results, along with those obtained with manganese oxide Mn O and nickel manganites synthesized in the same conditions, showed that d 5 8 depended both on the decomposition temperature of the oxalate and on the chemical composition of the oxide. Such results should provide interesting data concerning the cationic distributions of these non-stoichiometric nickel–copper manganites.

X-ray photoelectron spectroscopic study of non-stoichiometric nickel and nickel-copper spinel manganites

Abstract Nickel manganites have been prepared by thermal decomposition of mixed oxalates Ni x 3 Mn (3 − x 3 C2O4 · nH2O in air at 623 K (0 ≤ x ≤1). These oxides are finely divided materials because their BET specific surface areas are close to 100 m2/g and the average diameter of their crystallites is about 10 nm. Electron diffraction studies of these compounds show that they crystallize in a spinel-like cubic structure. Thermogravimetric analyses show that these oxides are nonstoichiometric cation deficient spinels NixMn3 − x□ 3δ 4 O4 + δ. δ depends on nickel content and reaches a maximum of 0.9 for x = 0.4.