Abstract
The size effect in the oxidation of aluminum nanoparticles (Al-NPs) has been observed experimentally; however, the mechano-chemistry and the atomistic mechanism of the oxidation dynamics remain elusive. We have performed multimillion atom reactive molecular dynamics simulations to investigate the oxidation dynamics of Al-NPs (diameters, D = 26, 36, and 46 nm) with the same shell thickness (3 nm). Analysis of alumina shell structure reveals that the shell of Al-NPs does not break or shatter, but only deforms during the oxidation process. The deformation depends slightly on the size of Al-NP. This reaction from the oxidation heats the Al-NP to a temperature of T > 5000 K. Ejection of Al atoms from shell starts earlier in small Al-NPs—at t0 = 0.18, 0.28 and 0.42 ns for D = 26, 36 and 46 nm, when they all have the same shell temperature of 2900 K. As the oxidation dynamics proceeds, the total system temperature (including the environmental oxygen) increases monotonically; however, the time derivative of the total temperature, (dTsystem/dt), reaches a maximum at t1 = 0.20, 0.32 and 0.51 ns for D = 26, 36 and 46 nm. At this peak value of (dTsystem/dt), the shell temperature for the three Al-NPs are 3100 K, 3300 K, and 3500 K, respectively. The time lag between t1 and t0 is 0.02, 0.04 and 0.09 ns for D = 26, 36 and 46 nm clearly indicates the size effect.
Highlights
Combustion of aluminum is a subject of great interest in science, engineering, and technology
This paper focuses on the size dependence of oxidation dynamics rate and that of reaction temperature, as well as the role of oxide shell in aluminum nanoparticles (Al-NPs) oxidation
Reactive molecular dynamics (MD) simulations results have provided quantitative information on the mechano-chemistry and the atomistic mechanism of oxidation for Al-NPs of different sizes, D 1⁄4 26, 36 and 46 nm in which the Al core diameter is varied from 20 to 40 nm and the alumina shell thickness is held constant at 3 nm
Summary
Combustion of aluminum is a subject of great interest in science, engineering, and technology. Combustion of micron size Al particles was studied by Dreizin and Jordan et al. using different experimental approaches. By adding Al-NPs into micron size particles, numerous experiments have observed the enhanced oxidation reactivity in terms of burning rate, flame speed, activation energy, ignition sensitivity, combustion velocity, and agglomeration.. Gan and Qiao have conducted experiments on fuel droplets with nano ($80 nm) and micron ($5 mm and $25 mm) size Al particles They found that the combustion is longer and less complete for large agglomerate of nanosuspensions due to the formation of oxide shell on the Al-NPs surface. Puri and Yang performed MD simulation to study the thermo-mechanical behavior of nano-Al particles coated with crystalline and amorphous oxide layers.
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