Effect of polymorphic phase transformations in Al2O3 film on oxidation kinetics of aluminum powders

Aluminium nanoparticles have gained importance in the last decade because of their increased reactivity as compared with traditional micron-sized particle. The physics of burning of aluminium nanoparticle is expected to be different than that of micron-sized particles, and the current article is motivated by these differences. We have previously measured the size resolved reactivity of nanoaluminium by single-particle mass spectrometry, to which we now add transmission electron microscope (TEM) and an on-line density measurement. The latter two studies revealed the presence of hollow particles following oxidation of nanoaluminium and indicating the significance of diffusion of aluminium in the overall process. Based on experimental evidence, we believe that aluminium nanoparticle oxidation occurs in two regimes. Prior to melting of aluminium slow oxidation occurs through the diffusion of oxygen through the aluminium oxide shell. Above the melting point, we transition to a fast oxidation regime whereby bot...

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Understanding the mechanism of aluminium nanoparticle oxidation

Effect of particle size on combustion of aluminum particle dust in air

Combustion behavior of energetic composite materials was experimentally examined for the purpose of evaluating the unique properties of nano-scale compared with traditional micron-scale particulate media. Behavior of composite systems composed of aluminum (Al) and molybdenum trioxide (MoO3) were studied as a function of Al particle size, equivalence ratio and bulk density. Samples were prepared by mechanically mixing individual fuel and oxidizer particles and combustion experiments included measurements of ignition and flame propagation behavior. Ignition was achieved using a 50-W CO2 laser and combustion velocities were measured from photographic data. Reaction kinetics were studied with differential scanning calorimetry (DSC). Results indicate that the incorporation of nano-Al particles (1) significantly reduces ignition temperatures and (2) produces unique reaction behavior that can be attributed to a different chemical kinetic mechanism than observed with micron-Al particles.

Combustion Behavior of Highly Energetic Thermites: Nano versus Micron Composites

Characteristics of aluminum combustion are summarized in an overview of the subject, focusing on the burning time of individual particles. Combustion data from over ten different sources with almost 400 datum points have been cataloged and correlated. Available models have also been used to evaluate combustion trends with key environmental parameters. The fundamental concepts that control aluminum combustion are discussed, starting from a discussion of the D

 n
 law. The exponent in the D

 n
 law is shown to be lower than two, with nominal values of ≈1.5 to 1.8 being typical. The effect of the ambient medium on the burning time is considered, oxygen as an oxidizer being twice as effective as water and about five times more effective than carbon dioxide. The effect of pressure and initial temperature is minor.

Correlating Aluminum Burning Times

A study has been performed modeling both steady and unsteady combustion of aluminum. Law's steadystate aluminum combustion model has been expanded to include the effects of multiple oxidizers and their products, oxide accumulation on the surface of the burning aluminum particle, and convection. Both transport and thermodynamic properties are calculated internally for varying temperatures, relaxing the normal assumption of unity Lewis number. The aluminum combustion model has been compared to experimental data from burners, laser-ignited particles, and propellant under a variety of conditions, showing a reasonable degree of agreement. Calculations with the model show that O2 is a stronger oxidizer than H2O, which in turn is stronger than CO2. The aluminum combustion model was incorporated into a computer model for predicting acoustic effects in a Rijke burner. Calculations have shown that a significant part of the increase in acousticgrowth due to the addition of aluminum is due strictly to the change in the gas temperature profile. The change in temperature profile apparently causes the location of the velocity antinode to shift relative to the Rijke burner flame and thereby cause an increase in the flame response. The acoustic model agrees reasonably well with available acoustic growth rates for data where aluminum particles have been added to a propane Rijke burner. Nomenclature ak = sum of the mole fractions of the oxidizers in Eq. (1) Cp = heat capacity, J/kg D = diffusivity, irr/s d = particle diameter, m F = ratio of total mass flux to mass flux aluminum H = total flux of energy in aluminum combustion model, W/m2; heat of reaction, J/mol j = mass flux due to diffusion, kg/m2/s k =. arbitrary constant in Eq. (1) M = nondimensional mass flux used in Law's aluminum combustion model /?2 = mass flux, kg/m2/s Nu = Nusselt number p = pressure, N/m2 Qr = heat production due to a reaction, W/m3; heat caused by radiation, W/m2 <92 = enthalpy in the region from the flame to infinity q = heat flux, W/m2 Re = Reynolds number r = radial distance, m; reaction rate, kg/mVs T = temperature, K t = time, s v — velocity, m/s; volume, m3 \v = transport property weighting factor A- = mole fraction 77 = fraction of vaporized oxide that moves toward the particle surface 0 = fraction of metal oxide that vaporizes at the flame A = fraction of metal that reacts with the particular oxidizing species v = stoichiometric mass ratio of oxide or oxidizer to metal f = fraction of condensed products with move inward or outward p = density, kg/m3 T = characteristic time lag, s

Kristen P. Brooks

Papers

Dynamics of aluminum combustion