Demitrios Stamatis

A multi-step reaction model for ignition of fully-dense Al-CuO nanocomposite powders

A multi-step reaction model is developed to describe heterogeneous processes occurring upon heating of an Al-CuO nanocomposite material prepared by arrested reactive milling. The reaction model couples a previously derived Cabrera-Mott oxidation mechanism describing initial, low temperature processes and an aluminium oxidation model including formation of different alumina polymorphs at increased film thicknesses and higher temperatures. The reaction model is tuned using traces measured by differential scanning calorimetry. Ignition is studied for thin powder layers and individual particles using respectively the heated filament (heating rates of 103–104 K s−1) and laser ignition (heating rate ∼106 K s−1) experiments. The developed heterogeneous reaction model predicts a sharp temperature increase, which can be associated with ignition when the laser power approaches the experimental ignition threshold. In experiments, particles ignited by the laser beam are observed to explode, indicating a substantial gas release accompanying ignition. For the heated filament experiments, the model predicts exothermic reactions at the temperatures, at which ignition is observed experimentally; however, strong thermal contact between the metal filament and powder prevents the model from predicting the thermal runaway. It is suggested that oxygen gas release from decomposing CuO, as observed from particles exploding upon ignition in the laser beam, disrupts the thermal contact of the powder and filament; this phenomenon must be included in the filament ignition model to enable prediction of the temperature runaway.

Thermal Initiation of Al-MoO3 Nanocomposite Materials Prepared by Different Methods

Two types of nanocomposite reactive materials with the same bulk compositions 8Al MoO3 were prepared and compared with each other. One of the materials was manufactured by arrested reactive milling and the other so-called metastable interstitial composite was manufactured by mixing of nanoscaled individual powders. The materials were characterized using differential scanning calorimetry, thermogravimetric, as well as ignition, experiments using an electrically heated filament and laser as heat sources. The experiments were interpreted using simplified models describing heat transfer in the heated material samples in different experimental configurations. Clear differences in the low-temperature redox reactions, well detectable by differential scanning calorimetry, were established between metastable interstitial composite and arrested-reactive-milling-prepared materials. However, the materials did not differ significantly from each other in the ignition experiments. In the heated filament ignition tests, their ignition temperatures were nearly identical to each other and were in the range of 750–800 K. These ignition temperatures coincided with the temperatures at which main exothermic processes were detected in differential scanning calorimetry experiments. In laser ignition experiments performed with consolidated pellets of bothmaterials, metastable interstitial composite pellets produced consistently stronger pressure pulses. The ignition delays were similar for the pellets of both materials prepared with the same porosity. Analysis of the heat transfer in the pellets heated by the laser suggested that the laser-exposed pellet surfaces were heated to approximately the same temperature before ignition for both materials. This temperature was estimated to be close to 500 K, neglecting the exothermic reactions preceding ignition and possible fragmentation of the heated pellets. Taking into account both phenomena is expected to result in a higher surface temperature, which would better represent the experimental situation. It is proposed that the ignition of both metastable interstitial composite and arrested-reactive-millingprepared materials at the same temperature can be explained by a thermodynamically driven transformation of a protective amorphous alumina into a crystalline polymorph.

Consolidation of Reactive Nanocomposite Powders

There is interest in replacing energetically inert structural components with reactive structures capable of highly exothermic reactions. Consolidated reactive materials are also desired for other applications, including reactive fragments, high density additives to explosives, insensitive pyrotechnic components, etc. Unlike nano-energetic compositions based on mixed nanopowders, reactive nanocomposite powders prepared by Arrested Reactive Milling (ARM) can be readily consolidated to achieve combined characteristics of high reactivity, low porosity, and structural strength. Different consolidation methods can be applied, and as a first step, a simple uniaxial pressing of nanocomposite powders is used in this project. A set of reactive nanocomposite powders with several Al-based thermite compositions prepared by ARM was used to prepare pellet-like consolidated samples. For various mechanical tests, both cylindrical and rectangular pellets were prepared with varied dimensions and varied degrees of compaction. Pellet compaction densities exceeding 90% of theoretical maximum density, were achieved. Despite the presence of Al and oxidizers, including MoO3, CuO, Bi2O3 and others, mixed on the nanoscale in different samples, no reaction was observed to be triggered by the powder compaction at pressures reaching 500 MPa. An experimental technique has been developed to study the thermal ignition initiation of the consolidated samples as a function of their physical and mechanical properties. The experimental technique will be used to develop a theoretical model to describe the ignition behavior of the consolidated materials.

Fully Dense Al-CuO Nanocomposite Powders for Energetic Formulations

The thermite reaction between Al and CuO is well known and highly exothermic. For a conventional thermite mixture comprising mixed metal and oxide powders, this reaction is rate limited by the slow heterogeneous mass transfer at the metal and oxide interface. The relatively low reaction rate and a difficult ignition have restricted practical applications for this reaction. For newly developed, nano-composed thermites, the interface area can be substantially increased resulting in a much higher reaction rate and a new range of possible applications. Recently, magnetron sputtering was used to create Al-CuO nanofoils for applications in joining. Nanocomposite Al-CuO compositions for pyrotechnics were also prepared using mixture or a self-assembling array of respective nanopowders. Such techniques realize the bottom-up approach, when the nanostructures or nanoparticles are built from individual atoms or molecules. Respective materials are generally expensive and difficult to handle. An alternative, top-down approach is discussed in this project. Nanocomposite Al-CuO materials are produced using a technique referred to as arrested reactive milling. Regular metal and oxide powders are blended and ball milled at room temperature resulting in a fully dense and reactive nanocomposite powder. The milling is stopped (or arrested) before a self-sustaining exothermic reaction is triggered. The powder particles are the 10-100 μm size range. Each particle has an aluminum matrix with copper oxide inclusions in the 20-200 nm size range, depending on milling parameters. The produced Al-CuO nanocomposite powders have been considered for applications in propellants, explosives, pyrotechnics, as well as for joining small parts. In accordance to the application requirements, the powder composition and morphology can be modified to optimize performance. Aluminum-rich compositions are of particular interest for novel energetic components. Synthesis methodology, material properties as a function of composition and morphology, and performance tests will be discussed in this paper.