Design of High-Energetic Materials at the Nanoscale
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Design of High-Energetic Materials at the Nanoscale Bijan K. Rao and Purusottam Jena Physics Department, Virginia Commonwealth University, Richmond, VA 23284-2000, U.S.A. ABSTRACT The amount of energy storage and its release in controllable pathways are two of the fundamental requirements of a high-energy material. The novel chemistry brought about by large surface-to-volume ratio of nanomaterials provides an attractive way to design and synthesize materials that optimize these two requirements. First principles calculations based on density functional theory and generalized gradient approximation have been used to study the potential of AlxLiyOz and Al(MnO4)x clusters as candidates for high-energetic materials. The equilibrium geometries and total energies of these clusters and their fragments are obtained to study the energy stored in these clusters and its release along various pathways. Interesting results include the substantial increase in binding energy by either adding an Al to MnO4 or adding a MnO4 to Al(MnO4)x unit indicating that Al(MnO4)3 may be a potential candidate for energetic materials as well as super-oxidizers. Similar calculations also show that during the combustion of Al addition of small amounts of Li to Al nano-powder helps to reduce the amount of non-combustible Al. INTRODUCTION A high-energy material is often defined as a material, which upon fragmentation can release substantial amount of energy. Such materials are normally metastable and protected from their products by an energy barrier. This class of material [1] exemplified by the traditional, TNT (trinitrotoluene), contain both fuel (namely carbon) and oxidizer (O2) in the same formula unit. Upon fragmentation, it produces highly stable products. Note that TNT is not the most effective compound with respect to releasing chemical energy as oxidation of the “fuel” atoms is incomplete. One could also view energetic materials as those capable of yielding large amounts of energy in an exothermic reaction. In this case both reactants and reaction products are stable as long as the reactants are held separately, or no extra energy is provided for the reaction to ensue. The reaction product is far more stable than the reactants, and the energy is released mostly in the form of heat. A good example in this category is aluminum, particularly in the nanophase form. In this case, due to a large number of surface atoms, nanoscale Al is very reactive and can interact with CO2 and water to release heat. Reactions of CO and H2 with bulk aluminum yield 196 kcal/mole and 226 kcal/mole energy respectively. It is for this reason that aluminum is an important ingredient in explosives and propellant compositions [2,3]. There are, however, drawbacks in the use of nano-aluminum. Because of its high reactivity, its safe handling and storage become an important concern. Further, as the aluminum particle interacts with the oxidizer, a dense aluminum oxide layer is formed that blocks diffusion of oxygen, preventing further oxidation of the inner aluminu
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