Melting and Oxidation of Nanometer Size Aluminum Powders
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Melting and Oxidation of Nanometer Size Aluminum Powders Mikhaylo A. Trunov, Swati Umbrajkar, Mirko Schoenitz, Joseph T. Mang1, and Edward L. Dreizin New Jersey Institute of Technology, Newark, NJ 07102, USA 1 Los Alamos National Laboratory, Los Alamos, NM 87545, USA 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 nano-sized 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 nano-sized 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 °C in 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 step-wise 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. INTRODUCTION Nanosized aluminum powders have been produced and evaluated as reactive additives to propellants and explosives [1 - 3] as well as components of nanocomposite energetic materials [4, 5]. An accelerated reaction rate is generally due to an increase in the specific surface of the powder. It has also been suggested that the melting point depression reported to occur for metal nanoparticles [6 - 11] could result in an earlier aluminum ignition (triggered by accelerated oxidation) and therefore, in shorter ignition delays [12, 13]. However, experimental data on melting of aluminum nanoparticles are scarce and it was recently shown that no correlation exists between melting and ignition or oxidation for micron-sized aluminum powders [14, 15]. The objectives of this work are to characterize experimentally melting and oxidation of the nanosized aluminum powders. It was also of interest to compare the experimental results with predictions of the existing theoretical models. EXPERIMENTAL Three aluminum nanopowders with the respective BET particle sizes of 44, 80
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