Combustion Of Aluminum Nanoparticles Research Articles

Revealing Al–O/Al–F reaction dynamic effects on the combustion of aluminum nanoparticles in oxygen/fluorine containing environments: A reactive molecular dynamics study meshing together experimental validation

Improving the energy conversion efficiency in metallic fuel (e.g., Al) combustion is always desirable but challenging, which often involves redox reactions of aluminum (Al) with various mixed oxidizing environments. For instance, Al–O reaction is the most common pathway to release limited energy while Al–F reaction has received much attentions to enhance Al combustion efficiency. However, microscopic understanding of the Al–O/Al–F reaction dynamics remains unsolved, which is fundamentally necessary to further improve Al combustion efficiency. In this work, for the first time, Al–O/Al–F reaction dynamic effects on the combustion of aluminum nanoparticles (n-Al) in oxygen/fluorine containing environments have been revealed via reactive molecular dynamics (RMD) simulations meshing together combustion experiments. Three RMD simulation systems of Al core/O2/HF, n-Al/O2/HF, and n-Al/O2/CF4 with oxygen percentage ranging from 0% to 100% have been performed. The n-Al combustion in mixed O2/CF4 environments have been conducted by constant volume combustion experiments. RMD results show that Al–O reaction exhibits kinetic benefits while Al–F reaction owns thermodynamic benefits for n-Al combustion. In n-Al/O2/HF, Al–O reaction gives faster energy release rate than Al–F reaction (1.1 times). The optimal energy release efficiency can be achieved with suitable oxygen percentage of 10% and 50% for n-Al/O2/HF and n-Al/O2/CF4, respectively. In combustion experiments, 90% of oxygen percentage can optimally enhance the peak pressure, pressurization rate and combustion heat. Importantly, Al–O reaction prefers to occur on the surface regions while Al–F reaction prefers to proceed in the interior regions of n-Al, confirming the kinetic/thermodynamic benefits of Al–O/Al–F reactions. The synergistic effect of Al–O/Al–F reaction for greatly enhancing n-Al combustion efficiency is demonstrated at atomic-scale, which is beneficial for optimizing the combustion performance of metallic fuel.

Effects of oxidizers on the ignition and combustion characteristics of aluminum nanoparticles

Ignition and combustion of Aluminum nanoparticles (ANPs) under complex environments are of great significance for various propulsion systems. In this study, we investigated the oxidation process of ANPs in O2, CO2, H2O, and their mixture atmospheres using reactive molecular dynamics. In the mixture atmospheres, ANPs experience a shorter ignition delay and parts of C and H atoms are involved in a new reaction named re-oxidation, where the nonbonded C and H inside ANPs detach from particle and re-bond with free O in the environment to produce extra CO and H2O. It reduces C and H in unreactive diffusion and increases O in reactive diffusion. Additionally, it is found that the ignition delay time and reaction heat release can be adjusted to achieve the optimal performance of ANPs combustion by tailoring the contents of O2, CO2, and H2O in the environment. Our works provide a theoretical basis for the precise regulation of ANPs energy release.

Fast reaction of aluminum nanoparticles promoted by oxide shell

Combustion of aluminum nanoparticles (AlNPs) has long been investigated experimentally because of their use in various energetic formulations for propellants and explosives. But the limited spatiotemporal resolution in experiments, in particular, makes it challenging to explore the microstructural evolution of AlNP oxidation and associated mechanisms. Here, we perform large-scale reactive molecular dynamics simulations to study the structural evolution of AlNPs with a 2–4 nm thick oxide shell in an oxygen environment. We find the temporal hollowing processes of AlNPs for both symmetrical and asymmetrical oxidations, in which the morphological evolution can be understood by a discrepant electric field and temperature distributions for different systems. In the early time, core aluminum atoms experience a fast reaction with an oxide shell. Environmental oxygen does not react with AlNPs until the surface O/Al ratio decreases to ∼1.2. Moreover, based on our simulation results, previous experimental data agree well with the proposed model, which can well describe the relationship between combustion efficiency and oxide shell thickness, confirming that the oxide shell promotes rather than hinders the combustion of AlNPs. The molecular insights obtained here would be significant for understanding the underlying mechanism and further modeling of AlNP combustion.

Activation of aluminum nanopowder combustion in water vapor by O2 additions

The paper reports on the effect of molecular oxygen on the combustion of aluminum nanoparticles in water vapor. Model analysis has shown that small additions of oxygen (molar fraction 1% to 3 %) accelerate the aluminum burning in steam by about an order of magnitude.

Combustion of aluminum nanoparticles and exfoliated 2D molybdenum trioxide composites

Exfoliated two-dimensional (2D) molybdenum trioxide (MoO3) of approximately 3-4 monolayers in thickness was produced from sonicating bulk MoO3 powder, and then mixed with 80 nm diameter Al nanoparticles to prepare nanoenergetic composites with high interfacial contacts between the fuel and oxidizer. Combustion measurements demonstrated peak pressures as high as 42.05 ± 1.86 MPa, pressurization rates up to 3.49 ± 0.31 MPa/µs, and linear combustion rates up to 1,730 ± 98.1 m/s, the highest values reported to date for Al/MoO3 composites. TGA/DSC measurements indicate energetic reactions between the Al and 2D MoO3 sheets occur prior to the melting temperature of Al. SEM and TEM analysis of the composites prior to combustion suggests high interfacial contact area between the Al and MoO3. After reaction, we observe that the 2D MoO3 sheets are converted to extended alumina flakes during reaction in a process attributed to Al adsorption and diffusion processes. These alumina features act as a physical barrier against Al NP sintering while also provide separation for reaction gases to flow and preheat unreacted materials.

Modeling of Aluminum Nano-Particles Through Counterflow Combustion in Fuel-Lean Mixture

The combustion of aluminum nano-particles under fuel-lean conditions is studied in the counterflow configuration by means of analytical approach. The flame is assumed to consist of three zones: preheat, flame, and post flame regimes. By extraction and non-dimensionalizing of energy equations and then solving them in preheat zone and using perturbation method in the flame regime, analytical formulas for particles and gas temperature profile are presented. Then dimensionless ignition and ultimate flame temperatures, place of ignition point and flame thickness as a function of equivalence ratio in different strain rates are obtained. In addition, dimensionless ignition temperature, place of ignition point and flame thickness in terms of strain rate for different equivalence ratios are demonstrated. Reasonable agreement between the analytical solution of aluminum nano-particles counterflow combustion and experimental data is obtained in terms of flame temperature.

Mathematical modeling of combustion of a frozen suspension of nanosized aluminum

A mathematical model of combustion of a composite solid propellant called ALICE (frozen suspension of nanosized aluminum in water) is presented. The model takes into account the combustion of aluminum nanoparticles in water vapor, the motion of combustion products, and the smaller velocity of particles as compared to the gas. The calculated burning rate is consistent with available experimental data on the burning rate of ALICE as a function of pressure.

Microstructural Behavior of the Alumina Shell and Aluminum Core Before and After Melting of Aluminum Nanoparticles

The oxidation mechanism of nanoaluminum particles, nominally employed as fuel component, is still an unsettled problem, because of the complex nature of thermomechanical properties of the oxide shell surrounding the elemental core. Although mechanical breakage of the alumina shell upon or after melting of aluminum core has been thought to play a key role in the combustion of aluminum nanoparticles, there has been little direct evidence. In this study, the microstructural behaviors of Al core and alumina shell lattices were investigated with increasing temperatures. Three in situ techniques, high-temperature X-ray diffraction analysis, hot-stage transmission electron microscopy, and high-resolution transmission electron microscopy for heat-treated samples, were employed to probe the thermal behaviors of aluminum and alumina lattices before and after melting of the aluminum core. High-temperature X-ray diffraction analysis revealed that nano aluminum lattice was initially expanded under tension at room temp...

Point model of combustion of aluminum nanoparticles in the reflected shock wave

A point mathematical model of combustion of aluminum nanoparticles in an oxygen-nitrogen mixture is proposed. The phenomenon is modeled within the framework of a single-phase Stefan problem. The effect of gas pressure and ambient medium composition on the burning time of particles is studied. Verification of the model against available experimental dependences of the burning time on temperature and pressure of the ambient medium is performed.

Gas-Phase Reaction in Nanoaluminum Combustion

The presence or absence of gas phase species during combustion of aluminum nanoparticles (n-Al) is a crucial observable in evaluating competing theories such as a diffusive oxidation mechanism and the melt dispersion mechanism. Absorption spectroscopy was used to probe the ground state of aluminum monoxide (AlO) and Al vapor in order to quantify the amount of Al and AlO present under conditions where these species were not observed in emission previously. Absorption measurements were made during combustion of nanoaluminum and micron-sized aluminum in a heterogeneous shock tube. AlO was detected in absorption at temperatures as low as 2000 K in n-Al combustion, slightly below the limit seen in micro-Al combustion. Al vapor was detected during n-Al combustion at temperatures as low as 1500 K, significantly lower than in micro-Al combustion, suggestive of a gas phase component. The detection limit for Al vapor was 1 × 1012 cm−3. The gas phase component was much weaker than that seen in 10 μm Al combustion. A comparison with n-Al in an inert environment did not show Al vapor at temperatures below 2300 K, even though the equilibrium concentration of Al from particles at that temperature were several orders of magnitude higher than the detection limit. This suggests a nearly pristine oxide coat that inhibits the production of Al vapor in appreciable quantities without reaction. These results are contrary to predictions of the melt dispersion mechanism, which should result in the generation of aluminum vapor from high-energy Al clusters produced from n-Al particles that spallate from mechanical stresses under rapid heating. This should further be independent of the bath gas.