Nanocalorimetry-Coupled Time-of-Flight Mass Spectrometry: Identifying Evolved Species during High-Rate Thermal Measurements

We report on measurements integrating a nanocalorimeter sensor into a time-of-flight mass spectrometer (TOFMS) for simultaneous thermal and speciation measurements at high heating rates. The nanocalorimeter sensor was incorporated into the extraction region of the TOFMS system to provide sample heating and thermal information essentially simultaneously with the evolved species identification. This approach can be used to measure chemical reactions and evolved species for a variety of materials. Furthermore, since the calorimetry is conducted within the same proximal volume as ionization and ion extraction, evolved species detected are in a collision-free environment, and thus, the possibility exists to interrogate intermediate and radical species. We present measurements showing the decomposition of ammonium perchlorate, copper oxide nanoparticles, and sodium azotetrazolate. The rapid, controlled, and quantifiable heating rate capabilities of the nanocalorimeter coupled with the 0.1 ms temporal resolution of the TOFMS provides a new measurement capability and insight into high-rate reactions, such as those seen with reactive and energetic materials, and adsorption\desorption measurements, critical for understanding surface chemistry and accelerating catalyst selection

High Heating Rate Reaction Dynamics of Al/CuO Nanolaminates by Nanocalorimetry-Coupled Time-of-Flight Mass Spectrometry

Highly tunable reactive nanolaminates have been of recent interest for various “on chip” energetic applications. The reaction dynamics of Al/CuO nanolaminates were investigated by nanocalorimetry-coupled time-of-flight mass spectrometry, capable of simultaneous measurement of temporal thermal dynamics and detection of evolved gas phase species at heating rates up to ∼10⁶ K/s. The nanolaminates were synthesized by alternately sputtering Al and CuO onto the heater of nanocalorimeter sensors. For thin films of 80 nm with one bilayer, the stoichiometric ratio of fuel to oxidizer significantly affected the reaction mechanism: initial reactions occurred between 300 and 400 °C, and main reactions varied based on stoichiometry. For thicker films of 199 and 266 nm, a series of samples with varying bilayer numbers were analyzed to determine the effect of diffusion distance and interfacial area. Only one reaction step was observed for a sample with a bilayer thickness of 33 nm. A two-step reaction mechanism is observed as the bilayer thickness was increased to 66 nm and beyond: solid-state reaction occurring at the interfaces of Al and CuO before the melting of Al and a much faster liquid–solid reaction right after the melting of Al. At the same time, interfacial premixed distance during the deposition was also estimated from parallel experiments. Furthermore, the power data from nanocalorimetry provides a more direct method, compared to optical emission and mass spectrometry based methods, in determining the ignition temperature in addition to being able to measure actual energy output for films with nanoscale thicknesses

Mechanistic Studies of [AlCp*]4 Combustion

The article of record as published may be found at https://doi.org/10.1021/acs.inorgchem.8b00589The combustion mechanism of [AlCp*]4 (Cp* = pentamethylcyclopentadienyl), a ligated aluminum(I) cluster, was studied by a combination of experimental and theoretical methods. Two complementary experimental methods, temperature-programmed reaction and T-jump time-of-flight mass spectrometry, were used to investigate the decomposition behaviors of [AlCp*]4 in both anaerobic and oxidative environments, revealing AlCp* and Al2OCp* to be the major decomposition products. The observed product distribution and reaction pathways are consistent with the prediction from molecular dynamics simulations and static density functional theory calculations. These studies demonstrated that experiment and theory can indeed serve as complementary and predictive means to study the combustion behaviors of ligated aluminum clusters and may help in engineering stable compounds as candidates for rocket propellants.Defense Threat Reduction Agency (DTRA)Air Force Office of Scientific Research (AFOSR)Grant No. HDTRA1-15-1-0031Grant No. FA9550-15-1-025

Electrospray Formation of Gelled Nano-Aluminum Microspheres with Superior Reactivity

Nanometallic fuels with high combustion enthalpy, such as aluminum, have been proposed as a potential fuel replacement for conventional metallic fuel to improve propellant performance in a variety of propulsive systems. Nevertheless, nanometallic fuels suffer from the processing challenges in polymer formulations such as increased viscosity and large agglomeration, which hinder their implementation. In this letter, we employ electrospray as a means to create a gel within a droplet, via a rapid, solvent evaporation-induced aggregation of aluminum nanoparticles, containing a small mass fraction of an energetic binder. The gelled aluminum microspheres were characterized and tested for their burning behavior by rapid wire heating ignition experiments. The gelled aluminum microspheres show enhanced combustion behavior compared to nanoaluminum, which possibly benefits from the nitrocellulose coating and the gelled microstructure, and is far superior to the corresponding dense micrometer-sized aluminum

Probing the Reaction Mechanism of Aluminum/Poly(vinylidene fluoride) Composites

Energetic thin films with high mass loadings of nanosized components have been recently fabricated using electrospray deposition. These films are composed of aluminum nanoparticles (nAl) homogeneously dispersed in an energetic fluoropolymer binder, poly­(vinylidene fluoride) (PVDF). The nascent oxide shell of the nAl has been previously shown to undergo a preignition reaction (PIR) with fluoropolymers such as polytetra­fluoro­ethylene (PTFE). This work examines the PIR between alumina and PVDF to further explain the reaction mechanism of the Al/PVDF system. Temperature jump (T-jump) ignition experiments in air, argon, and vacuum environments showed that the nAl is fluorinated by gas phase species due to a decrease in reactivity in a vacuum. Thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) was used to confirm the occurrence of a PIR, and gas phase products during the PIR and fluorination of nAl were investigated with temperature jump time-of-flight mass spectrometry (T-jump TOFMS). Results show a direct correlation between the amount of alumina in the PVDF film and the relative signal intensity of hydrogen fluoride release (HF). Although the PIR between alumina and PVDF plays an important role in the Al/PVDF reaction mechanism, burn speeds of Al/PVDF films containing additional pure alumina particles showed no burn speed enhancement

Mechanistic Studies of [AlCp*]₄ Combustion

The combustion mechanism of [AlCp*]₄ (Cp* = pentamethylcyclopentadienyl), a ligated aluminum­(I) cluster, was studied by a combination of experimental and theoretical methods. Two complementary experimental methods, temperature-programmed reaction and T-jump time-of-flight mass spectrometry, were used to investigate the decomposition behaviors of [AlCp*]₄ in both anaerobic and oxidative environments, revealing AlCp* and Al₂OCp* to be the major decomposition products. The observed product distribution and reaction pathways are consistent with the prediction from molecular dynamics simulations and static density functional theory calculations. These studies demonstrated that experiment and theory can indeed serve as complementary and predictive means to study the combustion behaviors of ligated aluminum clusters and may help in engineering stable compounds as candidates for rocket propellants