Oxidation and Melting of Aluminum Nanopowders

Abstract

Recently, nanometer-sized aluminum powders became available commercially, and their use as potential additives to propellants, explosives, and pyrotechnics has attracted significant interest. It has been suggested that very low melting temperatures are expected for nanosized aluminum powders and that such low melting temperatures could accelerate oxidation and trigger ignition much earlier than for regular, micron-sized aluminum powders. The objective of this work was to investigate experimentally the melting and oxidation behavior of nanosized aluminum powders. Powder samples with three different nominal sizes of 44, 80, and 121 nm were provided by Nanotechnologies Inc. The particle size distributions were measured using small-angle X-ray scattering. Melting was studied by differential scanning calorimetry where the powders were heated from room temperature to 750 degrees C in an argon environment. Thermogravimetric analysis was used to measure the mass increase indicative of oxidation while the powders were heated in an oxygen-argon gas mixture. The measured melting curves were compared to those computed using the experimental particle size distributions and thermodynamic models describing the melting temperature and enthalpy as functions of the particle size. The melting behavior predicted by different models correlated with the experimental observations only qualitatively. Characteristic stepwise oxidation was observed for all studied nanopowders. The observed oxidation behavior was well interpreted considering the recently established kinetics of oxidation of micron-sized aluminum powders. No correlation was found between the melting and oxidation of aluminum nanopowders.

Oxidation-assisted ductility of aluminium nanowires

“…Therefore, superplastic oxides become easier to observe at a higher temperature and oxygen rich environment, as _ e c increases with increasing T and P O 2 . The oxide thickness, x 0 was reported to remain at a constant value of B3.8 nm for Al nanopowders with diameters larger than 20 nm 41,42 . This means, equation (3) is expected to be size independent only when the contribution of the thin oxide layer to the total deformation of Al is small and x 0 remains a constant in large-sized samples 22 .…”

“…This means, equation (3) is expected to be size independent only when the contribution of the thin oxide layer to the total deformation of Al is small and x 0 remains a constant in large-sized samples 22 . However, at the nanoscale, when x 0 decreases with the size of the Al 41,42 , without considering any other size-dependent properties, such as oxidation rates, _ e c will increase with decreasing size, so that it will be easier to deform superplasticity in smaller Al NWs.…”

Improving aluminum particle reactivity by annealing and quenching treatments: Synchrotron X-ray diffraction analysis of strain

“…Various explanations for Al oxidation mechanisms have also been proposed, each strongly tied to the ignition mechanism and heating rate [3]- [11]. All theories share a common theme for mass transport of fuel and oxidizer, but differ in how that diffusion is achieved (i.e., via (a) dispersion [3], [4], (b) phase changes in the polymorphous passivation shell [5]- [7] , (c) reactive sintering [8], (d) pressure gradient driven processes [9], [10], and (e) induced electric field influences [11]). This article will not directly deal with any particular reaction mechanism, but rather investigate the influence of a new parameter, mechanical strain, which has only recently been considered in the study of Al oxidation [3], [4], [12], [13].…”

Structure, Thermodynamics, and Energy Content of Aluminum–Cyclopentadienyl Clusters

“…These powders are generally of micrometer size, but in recent years there has been significant study of nanometer scale Al as a route to increasing the oxidation rates and potentially lowering the ignition threshold. 1,2 The outer surface of the aluminum grains consists of an oxidized alumina layer ∼2À6 nm thick, which can limit rapid reactivity of the internal Al. 3 This has led several authors to pursue surface passivation of aluminum nanoparticles with organic functional groups that prevent formation of the oxide.…”