Swati M. Umbrajkar

Aluminum-Rich Al-MoO3 Nanocomposite Powders Prepared by Arrested Reactive Milling

Fuel-rich Al-MoO 3 nanocomposites were prepared using arrested reactive milling. Powder composition was varied from 4Al + MoO 3 to 16Al + MoO 3 . Powders were evaluated using electron microscopy, thermal analysis, x-ray diffraction, heated-filament-ignition experiments, and constant-volume-explosion experiments. Uniform mixing of MoO 3 nanodomains in the aluminum matrix was achieved for all prepared powders. Multiple and overlapping exothermic processes were observed to start when the nanocomposite powders were heated to only about 350 K. In heated-filament experiments, all nanocomposite powders ignited at temperatures well below the aluminum melting point. Ignition temperatures for these powders were estimated for the higher heating rates that are typical of fuel-air explosions. Constant-volume-explosion experiments indicated that flame propagation in aerosols of nanocomposite thermite powders in air is much faster than that in pure aluminum aerosols. The energy release, normalized per unit mass of aluminum, was higher for the nanocomposite materials with bulk compositions 4Al + MoO 3 and 8Al + MoO 3 and lower for pure aluminum and for the 16Al + MoO 3 nanocomposite sample. The reaction rate was the highest for the 8Al + MoO 3 nanocomposite powder. The combustion efficiency inferred from the measured pressure traces correlated well with the phase compositions of the analyzed condensed combustion products.

Kinetic Analysis of Thermite Reactions in Al-MoO3 Nanocomposites

Reactions in energetic Al-MoO 3 nanocomposites prepared by arrested reactive milling were investigated by scanning calorimetry and heated filament ignition experiments. The calorimetry data were processed to obtain kinetic parameters describing the reaction between Al and MoO 3 . The reaction was treated as a combination of four subreactions, which were described by a combination of a diffusion-controlled reaction model and first-order reactions. The activation energies determined in this study allowed the comparison to reference values for the decomposition of MOO 3 and the diffusion of oxygen through an Al 2 O 3 product layer. The kinetic model was extrapolated to high heating rates in the 10 3 -10 6 K/s range and compared to ignition data. It was concluded that ignition of Al-MoO 3 nanocomposites prepared by arrested reactive milling is primarily controlled by oxygen diffusion in Al 2 O 3 .

Structural Refinement in Al-MoO3 Nanocomposites Prepared by Arrested Reactive Milling

Al-MoO 3 nanocomposites were synthesized based on arrested reactive milling. Altering the milling parameters leads to different scales of refinement in the structure of the reactive nanocomposites. The objective of this work is to determine the range in which the degree of structural refinement can be changed in a controlled manner. The milling intensity was controlled by using different milling media along with varying amounts of process control agent (PCA). XRD, SEM, DSC and wire-ignition tests were performed to analyze the Al-MoO 3 nanocomposites. Results indicate that there is a decrease in the crystallite size with increase in the milling intensity. However increase in milling intensity also stimulates the undesired reaction between Al and MoO 3 . Milling conditions resulting in the highest structural refinement and lowest ignition temperatures were identified.

Heterogeneous Processes Leading To Metal Ignition In Reactive Nanocomposite Materials

The main advantage of metallic ingredients in energetic f ormulation 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. Re cent research indicates that the major reaction rate bottleneck is often associated with fair ly slow heterogeneous processes that lead to ignition. Thus, identification of the mechanisms o f such processes is important and will allow developing modified metallic ingredients for ener getic 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 calor imetry 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. T his approach is applied here for description of ignition kinetics of recently develope d fully dense reactive nanocomposite powders. Specifically, compositions of 2Al+MoO 3 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 r eaction, 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 typicall y involves multiple steps, e.g., decomposition of MoO3 into MoO 2 and O ions, diffusion of O ions, and their reaction with Al forming different Al 2O3 polymorphs. This project develops a technical approach and methodology to identify and quantify these kinetics. Fur thermore, it is aimed to validate the kinetics in high heating rate experiments. Once the k inetics of such a reaction are established, it can be used to describe ignition of a w ide range of related materials with varied degrees of structural refinement and varied comp onent ratios. For example, materials with excess Al can be prepared so that metall ic fuel is available to react with external oxidizer, as desired for many applications in p ropellants and explosives. This paper will present the experimental approach, methodology of data processing and initial results for the two thermite systems mentioned above.