Size-Resolved Kinetic Measurements of Aluminum Nanoparticle Oxidation with Single Particle Mass Spectrometry

Abstract

Aluminum nanoparticles are being considered as a possible fuel in advanced energetic materials application. Of considerable interest therefore is a knowledge of just how reactive these materials are, and what the effect of size on reactivity is. In this paper we describe results of size resolved oxidation rate using a recently developed quantitative single particle mass spectrometer (SPMS). Aluminum nanoparticles used were either generated by DC Arc discharge or laser ablation, or by use of commercial aluminum nanopowders. These particles were oxidized in an aerosol flow reactor in air for specified various temperatures (25-1100 degrees C), and subsequently sampled by the SPMS. The mass spectra obtained were used to quantitatively determine the elemental composition of individual particles and their size. We found that the reactivity of aluminum nanoparticles is enhanced with decreasing primary particle size. Aluminum nanoparticles produced from the DC Arc, which produced the smallest primary particle size (approximately 19 nm), were found to be the most reactive (approximately 68% aluminum nanoparticles completely oxidized to aluminum oxide at 900 degrees C). In contrast, nanopowders with primary particle size greater than approximately 50 nm were not fully oxidized even at 1100 degrees C (approximately 4%). The absolute rates observed were found to be consistent with an oxide diffusion controlled rate-limiting step. We also determined the size-dependent diffusion-limited rate constants and Arrehenius parameters (activation energy and pre-exponential factor). We found that as the particle size decreases, the rate constant increases and the activation energy decreases. This work provides a quantification of the known pyrophoric nature of fine metal particles.

Effects of solvation shells and cluster size on the reaction of aluminum clusters with water

“…We consider two atoms to be bonded when their distance is less than a cutoff distance R c during a prescribed bond lifetime of 12 fs, where R c for Al-O, Al-H, and H-H are 2.4, 2.0, and 1.0 Å, respectively. The number of H-H bonds shows that, within 20 ps, three H 2 molecules are produced each in the Al 16 and Al 17 systems and that six H 2 molecules are produced in the Al 18 system. In contrast, no H 2 molecule is produced in the pure water system.…”

“…8 Nanotechnology has opened new avenues toward solving this problem. For example, the surfacearea to volume ratio is drastically increased, 11 quantum effects on the atomic scale often lead to novel catalytic activities 12 and enhanced reaction rates, 1,7,[13][14][15][16] and surfaces can be tailored at the nanometer scale to prevent oxidation. 17 Metal clusters have been found to react chemically in unusual ways.…”

“…Here, size-selectivity is studied by comparing highly reactive Al 17 with less reactive Al 16 and Al 18 , as observed in gas-phase reaction. 20,21 The observed fast reaction rates and calculated low reaction energy barriers are explained by the same reaction mechanisms as in our previous study.…”

Oxidation of nanocrystalline aluminum by variable charge molecular dynamics

“…238kJ.mol -1 for the oxidation of bulk Al or nano-particles (~50nm) [7][8][9]. This difference might be explained by the very high reactivity of our samples induced by their small nanocrystalline microstructure (5nm grains), their total purity and the absence of a protective alumina scale at the surface of the sample.…”

“…[1][2][3]. At the same time, significant progress has been made to understand the combustion of nano-aluminum powders and the highly exothermic reaction of fine-grained aluminum particles (size < 100 nm) with oxygen because of their use as potential additives to propellants for rockets [4][5][6][7][8][9]. For all these applications, it is crucial to understand the physico-chemical processes which control the reactivity of the metallic substrate and the thickness, morphology, chemical composition and microstructure of the oxide film [1].…”