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

Two types of nanocomposite reactive materials with the same bulk compositions 8Al center dot 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 both materials, 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-milling-prepared materials at the same temperature can be explained by a thermodynamically driven transformation of a protective amorphous alumina into a crystalline polymorph.