ICMCTF2012 Session TS3-1: Energetic Materials and Micro-Structures for Nanomanufacturing

Since more than a decade, nanostructured thermite materials, which classically consist of a mixture of oxidizer and fuel nanoparticles, have drawn considerable interest due to enhanced energy release rates in comparison to their macro-scale counterparts which arises from an increased fuel and oxidizer interfacial contact area. In comparison with other energy sources such as batteries, capacitors, fuel cells, nanothermites energy and power density is orders of magnitude higher, making them ideal candidates to provide high energy density local source of heat or pressure for mechanical and/or electrical power. Since 2005, progress in nanotechnologies paved the way to engineering nanothermites on a chip. Typically, magnetron sputtering method permit to stack alternatively metal and metal oxide nanofoils, typically Al and CuO, each foil being accurately controlled in thickness (from 25nm‐100nm with a precision of 5nm) and high purity. This deposition technique places the reactants in intimate contact reducing notably the diffusion distance between reactants compared to the same material prepared by powder mixing.

Al-CuO nanolaminates are magnetron sputter deposited from Al and Cu targets using DC power supplied, on oxidized silicon wafer. Copper Oxide thin films are deposited by dc reactive magnetron sputtering method under argon and oxygen plasma from a Cu target.

We compared the ignition characteristics and combustion rate of Al/CuO nanolaminate for different Fuel (Al) to Oxidizer (CuO) ratio (stoichiometric, fuel rich and fuel poor) and for different Al and CuO individual reactive layer thickness. Results show that Al/CuO nanolaminates provide unparalleled selectivity to tailor their energetic response to satisfy desired application based on material composition, structure and stochiometry.

Second point discussed in this paper is the formation and role of the intermixing zone at the interface of Al and CuO reactive layers. We find that an interface layer formed during sputter deposition of alumimum on CuO is composed of a mixture of Cu, O and Al through Al penetration into CuO, and constitutes a poor diffusion barrier (low ignition temperature); and in contrast atomic layer deposition (ALD) of Al₂O₃ using trimethylaluminum (TMA) produces a conformal coating that effectively prevents Al diffusion (higher ignition temperature) even for ultra-thin layer thicknesses (~ 0.5 nm). These findings evidence that interface layers plays a crucial role in the characteristics and performances of energetic nanolaminates. We have combined in-situ FTIR and ex-situ X-ray, DSC and HRTEM to identify the stable configurations that can occur at the interface.

Water-repellent nanocoatings for submerged combustion of nanoenergetic composite materials were developed. Some of the possible applications of submerged combustion are - oceanic power generation, underwater ordnance, propulsion, metal cutting, and torch technologies. Nanocoatings were deposited on thermite pellets by a vapor-phase technique. Two types of deposition techniques, namely chemical vapor deposition (CVD) and atomic layer deposition (ALD) were studied. A total of six types of nanocoatings were applied on the thermite pellets. Various process parameters to produce the coatings were explored. Characterization of the nanocoatings was carried out using FTIR, SEM, AFM and contact angle goniometry. Submerged combustion tests of the nanocoated thermite pellets were performed as a function of submerged time. The pellets were submerged in deionized water for 3, 5 and 10 days. The bubble energy produced was analyzed and compared to other types of nanocoated pellets. Initial results were analyzed using a fluorocarbon (FSAM) based monolayer coating (type 1 – thickness ~1.8nm) and compared with a. nanoparticle+FSAM coating (type 2 – thickness ~300nm).

Results suggest that with increasing submerged time, there is a decrease in the ratio of bubble energy to total energy of combustion (K_c=K_bubble/K_combustion). The bubble energy of the pellets with type 1 and type 2 coating was 532.6 and 528.6 (KJ/Kg) respectively, with immediate ignition after submersion and 27.6 and 48.2 (KJ/Kg) respectively, after 10 days. The value of K_c for Type 1 coating decreased by a factor of 19.3 whereas the Type 2 coating decreased by a factor of 11.0. The hydrophobic coating is critical for energy generation because without it, the pellets do not ignite, resulting in 100% loss of energy. Other coatings are being explored to improve durability of submerged pellets.

Vapor-deposited, metal-metal multilayers are an ideal class of materials for systematic, detailed investigations of reactive material properties. Created in a pristine vacuum environment by sputter deposition, these high purity materials have well-defined reactant layer thicknesses between 1 and 1000 nm, minimal void density and intimate contact between layers. With this presentation, we describe the ignition characteristics, reaction behaviors and final phase formation of multiple exothermic metal-metal pairs. This includes equiatomic Al/Pt, Co/Al and Ni/Ti. Regarding initiation, we show how pulsed laser ignition thresholds vary with material system. For a given reactive material system, the thresholds for ignition are also shown to depend on (i) pulse duration (evaluated from decisecond to femtosecond time scales) and (ii) nanolaminate periodicity. With regards to steady-state propagation, we show that some nanolaminate systems (Al/Pt) exhibit stable propagation modes characterized by rapid reaction rates and microscopically-smooth reaction front morphologies. Other kinetically-constrained systems (Co/Al, Ni/Ti) exhibit an in-plane spin-like (unstable) propagation mode characterized by transverse propagation bands and colliding wavefronts. Regarding phase formation, we describe how reaction environment can affect final phase.

Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Energetic materials based on equiatomic Ru/Al multilayers have been recently introduced by our group. The unusual combination of high temperature as well as room temperature properties makes the equiatomic Ru/Al system interesting as a new energetic material. So far, self-propagating reactions in Ru/Al multilayers have been studied with a focus on the early stages of reaction. Parameters such as front velocity and reaction temperature as well as the reaction mechanism have been experimentally determined and compared to other systems. These results clearly demonstrate that the Ru/Al system expands the reaction parameter range of previously used systems in terms of velocity and temperature. However, for a more detailed understanding of the reactions, numerical simulation which focuses on the early reaction stages as well as microstructural analysis to reveal the processes during later stages of cooling was performed. Experimental studies were carried out to investigate the effects of the interaction with surrounding air as a function of multilayer period. For this, pyrometric measurements to follow the temperature evolution during the reaction and cross sectional transmission electron microscopy analysis of reacted foils were performed.

One of the possibilities to produce nanostructured materials is the combination of two non-equilibrium processing techniques, namely mechanical activation (MA) and self-propagating high-temperature synthesis (SHS). Mechanical activation provides the possibility of both modificating the conditions of the chemical reaction run and changing the thermal parameters of the synthesis (temperature, combustion velocity, heating rate, and others) thus leading to the different structures and properties of the final product.

Our investigation aimed at establishing the influence of MA on SHS. The Ni─Al, Ti─BN and Ti─SiC─C systems were studied. Green mixtures were prepared by dry mixing of the initial components in china crucibles at the stoichiometric ratios corresponding to the following reactions : Ni+Al = NiAl, 3Ti+2BN = 2TiN+TiB₂ and 3Ti+SiC+C = Ti₃SiC₂.

Preliminary MA was preformed in a water cooled planetary ball mill (AGO-2) type at room temperature under argon. The milling procedure was carried out at the ball/mill ratio of 20: 1. The milling time varied from 0.20 s to 30 min.

The resultant mechanically activated particles were composed of layers of the initial components alternating with each other at the nano level [1, 2]..

During mechanical activation of Ni+Al mixtures up to 7 min, specific contact area between reagents Ni and Al increased approximately 15-20 times for each fraction.

Using high-resolution SEM and TEM methods for microstructural investigation of Ni─Al activated samples, nanoscale structural components were observed. Their average atomic weight was intermediate between the Ni and Al. The main influence on the reactivity of heterogeneous systems Ni ─ Al during milling is formation of nanoscale X-ray amorphous phases and solid solutions.

Ignition temperature was measured for small amounts of activated mixtures in the form of activated particles, pressed pellets, and rolled foil. The dependences of the ignition temperature on the MA duration and the way of the sample preparation were studied. It was found that after MA for 0 − 30 min the ignition temperature lowered by approximately 600°C for Ti─BN and Ti─SiC─C systems; after MA for 0– 7 min it could be diminished by 300–400 °C for N─Al system.

For the determination of the internal microstructure, the particle size and microstrains of mechanically activated powders 3Ti+2BN and 3Ti+SiC+C and of the combustion products were analyzed by X-ray powder diffraction on a Huber Guinier diffractometer at the Institute of Crystallography and Structural Physics, in Erlangen, Germany.

Analysis showed that an increase in the MA duration of the 3Ti+2BN and 3Ti+SiC+C mixtures led to a decrease in the peak intensities and broadening of the Ti peaks. With increasing activation time, the BN reflexes in the 3Ti+2BN reaction mixtures were instantly broadened and in three minutes their intensity became comparable to that of the background. Similar behavior was also observed in case of graphite in the activated 3Ti+SiC+C mixtures. This evidences the destruction of the crystal structure (amorphization) of boron nitride and graphite in the MA process [3].

The effect of milling time on the crystalline size and strain in the powder mixtures and SHS products we determined from line broadening analysis of the XRD peaks. The Rietveld full profile refinements were carried out with the Fullprof Suite. As a result of up to 30 min MA, the average size of the Ti crystallites was found to reduced down to 25 and 50 nm in the Ti + BN and Ti─SiC─C systems, respectively. Mechanical activation was found to affect the size of product crystallites. The value of the micro stains increased with the MA duration.

Formation of nanocomposites at the stage of mechanical activation provides proper conditions for synthesizing nanostructured SHS materials with a complete inherence of the precursors’ structural morphology.

References

[1] N.F. Shkodich, N.A. Kochetov, A.S. Rogachev, D.Yu. Kovalev, N.V. Sachkova, Izv. Vyssh. Uchebn. Zaved. Tsvetn. Metall., 5, (2006) 44-50.

[2] M.A. Korchagin, M.R. Sharafutdinov, N.F. Shkodich, B.P. Tolochko, P.A. Tsygankov, I.Yu. Yagubova. Nuclear Instruments and Methods in Physics Research A, 575, (2007) 149-151.

[3] Shkodich N. F, Rogachev A. S, Neder R. B., Magerl A., S.G. Vadchenko, and O. Boyarchenko, High-Temperature Ceram. Mater. Composites, 911 (2010) 881-887.

Acknowledgement

The present work was supported by the Russian Foundation for Basic Research (Project no. 10-03-00217-а) and the Council of the RF President Grants for Support of Leading Scientific Schools (Grant no. NSh-6497.2010.3).