(701c) Mechanisms of Multistep Reactions in Nanocomposite Metal-Oxide Systems

Nanocomposite thermites received substantial attention due to their increased specific reactive interface and, therefore, higher reactivity compared to regular thermite powders. Such materials with reactive components mixed on the nanometer scale are expected to increase performance characteristics of the current metalized energetic formulations in pyrotechnics, explosives and propellants. A great deal of experimental research has been conducted on preparation of different materials systems and characterizing their combustion performance. In particular, many compositions (e.g., Al-MoO3, Al-CuO, Al-Bi2O3 and others) were produced using arrested reaction milling (ARM). The structure, morphology, and reactivity of these materials were characterized using electron microscopy, X-ray diffraction analysis, Differential Scanning Calorimetry and Thermo-Gravimetric Analysis (DSC/TGA), as well as customized ignition and combustion tests. Data on ignition of nanocomposite thermites at various heating rates and in different environments, and measurements describing their combustion dynamics have been collected. The experiments showed that these materials are useful as rapidly igniting, high energy density additives to various energetic compositions, generating hot spots to promote the reaction of the entire formulation. Thus, detailed description of their ignition kinetics is of significant interest. However, current theoretical models of reaction kinetics in thermites are quite limited. In most cases, the model is reduced to a single Arrhenius-like term. Models including several independent kinetic terms have also been proposed, but are unlikely to accurately describe the multistep reactions over a wide range of reaction rates, occurring when a material is being thermally activated. An alternative approach consists of a mechanistic description of various processes occurring in the thermally activated materials, and accounting for the effect of these processes on the material temperature history leading to ignition. Recent research on ignition of aluminum, a common component of many nanocomposite thermites, showed that the rates of diffusion through different Al2O3 polymorphs and rates of polymorphic phase transitions control aluminum ignition kinetics. In nanocomposite thermites these processes in the alumina, which is the reaction product, are also expected to be important. The situation is further complicated by structural and compositional changes occurring in the oxidizer, the metal oxide component of the thermite. These changes affect primarily the concentration of oxygen ions available to diffuse through the aluminum oxide to react with aluminum. Recently, a modeling approach incorporating many of the above mentioned factors was discussed for the Al-MoO3 system. The model considered reduction of MoO3 to form various MoOx oxides (x<3) and diffusion of oxygen ions to aluminum through different, evolving layers of aluminum oxide polymorphs. Similar processes, i.e., diffusion through various alumina polymorphs and step-wise reduction of the oxidizer, such as CuO, Bi2O3, Fe2O3, and others, control reaction rates in respective Al-metal oxide systems. The current work develops the modeling effort describing these two types of processes as defining the rates of heterogeneous thermite reactions. Kinetic parameters, available from the literature as well as from recent ignition and DSC/TGA measurements, are analyzed and selected for use in the model. Two types of nanocomposite structures are explored: planar alternate layers of aluminum and molybdenum oxides, and spherical inclusions of molybdenum oxide domains in aluminum matrix. Effect of size distribution of the planar layers and spherical domains on the ignition kinetics is considered. The results are compared with DSC/TGA measurements performed at different heating rates.