Heterogeneous processes leading to metal ignition in reactive nanocomposite materials

The main advantage of metallic ingredients in energetic formulation is their high combustion enthalpy and temperature. However, in many cases this advantage is outweighed by relatively low burn rates which do not allow the heat release to occur in a short time frame available in a specific application. Recent research indicates that the major reaction rate bottleneck is often associated with fairly slow heterogeneous processes that lead to ignition. Thus, identification of the mechanisms of such processes is important and will allow developing modified metallic ingredients for energetic materials with optimized performance. Recently, detailed thermal analysis measurements were performed and analyzed to identify the ignition mechanism for aluminum. The validation of this mechanism is possible when low heating rate scanning calorimetry measurements are combined with similar measurements performed at much higher heating rates. In both cases, the heating rates need to be clearly documented so that a quantitative kinetic description of the ignition processes is obtained. This approach is applied here for description of ignition kinetics of recently developed fully dense reactive nanocomposite powders. Specifically, compositions of 2Al+MoO₃ and 2Al+3CuO are addressed. The materials comprise micron-sized powders in which each particle is a pore-free nanocomposite of the starting components. The components are not bonded chemically and thus a rapid exothermic reduction-oxidation reaction is initiated upon heating. (Similar materials taking advantage of an exothermic metal-metalloid reaction, e.g., B-Ti, have also been produced.) For these materials, the kinetics of such heterogeneous reactions determine the kinetics of their ignition. The reaction typically involves multiple steps, e.g., decomposition of MoO₃ into MoO₂ and O ions, diffusion of O ions, and their reaction with Al forming different Al₂O₃ polymorphs. This project develops a technical approach and methodology to identify and quantify these kinetics. Furthermore, it is aimed to validate the kinetics in high heating rate experiments. Once the kinetics of such a reaction are established, it can be used to describe ignition of a wide range of related materials with varied degrees of structural refinement and varied component ratios. For example, materials with excess Al can be prepared so that metallic fuel is available to react with external oxidizer, as desired for many applications in propellants and explosives. This paper will present the experimental approach, methodology of data processing and initial results for the two thermite systems mentioned above.