No abstract

In this work, heterogeneous nanocomposite reactions of Al/CuO, Al/Fe2O3 and Al/ZnO systems were characterized using a recently developed T-Jump/time-of-flight mass spectrometer. Flash-heating experiments with time-resolved mass spectrometry were performed at heating rates in the range of ∼105 K/s. We find that molecular oxygen liberated during reaction is an active ingredient in the reaction. Experiments also conducted for neat Al, CuO, Fe2O3, and ZnO powders show that the oxygen are produced by decomposition of oxidizer particles. Mass spectrometric analysis indicates that metal oxide particles behave as an oxygen storage device in the thermite mixture and release oxygen very fast to initiate the reaction. A clear correlation is observed between the capability of oxygen release from oxidizing particles and the overall reactivity of the nanocomposite. The high reactivity of the Al/CuO mixture can be attributed to the strong oxygen release from CuO, while Fe2O3 liberates much less oxygen and leads to moderate reactivity, and ZnO’s poor oxygen release capability caused the Al/ZnO mixture to be completely not reacting, even though the reaction is overall exothermic. It is likely that the role of the oxygen species is not only as a strong oxidizer but also an energy propagation medium that carries heat to neighboring particles.

Aluminum nanoparticles (Al-NPs) and nano-Al/CuO thermite were investigated in a rapid heating environment by temperature jump time-of-flight mass spectrometry. Upon rapid heating (105 to 106 K/s), Al-containing vapor species (Al and Al2O) are observed to slowly increase with increasing temperature, followed by a rapid increase in concentration at ∼2030 K. The temporal evolution of Al, Al2O species observed in time-resolved mass spectra of rapid heated Al-NPs supports the hypothesis that Al containing species diffuse outward through the oxide shell under high heating rate conditions. The rapid rise in Al-containing species above 2030 K, which is below the bulk melting point of Al2O3, implies that the penetration of Al into the shell probably decreases its melting point. The measurements lead to an effective overall diffusion coefficient of ∼10–10 cm2/s. Time-resolved mass spectra of nano-Al/CuO thermite show for the first time the existence of Al, Al2O, AlO, and Al2O2 intermediate reaction products, with Al2O the main intermediate oxidation product, in agreement with thermochemical calculations.

We describe a new T-Jump/time-of-flight (TOF) mass spectrometer for the time-resolved analysis of rapid pyrolysis chemistry of solids and liquids, with a focus on energetic materials. The instrument employs a thin wire substrate which can be coated with the material of interest, and can be rapidly heated (10(5) K/s). The T-Jump probe is inserted within the extraction region of a linear TOF mass spectrometer, which enables multiple spectra to be obtained during a single reaction event. By monitoring the electrical characteristics of the heated wire, the temperature could also be obtained and correlated to the mass spectra. As examples, we present time-resolved spectra for the ignition of nitrocellulose and RDX. The fidelity of the instrument is demonstrated in the spectra presented which show the temporal formation and decay of several species in both systems. The simultaneous measurement of temperature enables us to extract the ignition temperature and the characteristic reaction time. The time-resolved mass spectra obtained show that these solid energetic material reactions, under a rapid heating rate, can occur on a time scale of milliseconds or less. While the data sampling rate of 10,000 Hz was used in the present experiments, the instrument is capable of a maximum scanning rate of up to approximately 30 kHz. The capability of high-speed time-resolved measurements offers an additional analytical tool for the characterization of the decomposition, ignition, and combustion of energetic materials.

Solid−solid reactions at the nanoscale between a metal passivated with a nascent oxide and another metal oxide can result in a very violent reaction. This begs the question as to what mechanism is responsible for such a rapid reaction. The ignition of nanoscale Al/CuO thermites with different aluminum oxide shell thicknesses were investigated on a fast heated (∼105 K/s) platinum wire. Ramping the wire temperature to ∼1250 K and then shutting off the voltage pulse result in ignition well after the pulse is turned off; i.e., an ignition delay is observed. The delay is used as a probe to extract the effective diffusion coefficient of the diffusing species, which is confirmed by fast time-resolved mass spectrometry. The results of this study are consistent with a diffusion controlled ignition mechanism.