Valery Levitas demonstrates value of a new melt-dispersion mechanism
Aluminum-fueled composites are known to have high-energy densities, especially in comparison to other fuel. That’s why researchers are interested in the reaction of aluminum particles with various oxidizers.
Valery Levitas, Schafer Professor and faculty member of aerospace engineering and mechanical engineering, says the main direction being taken to increase the reactivity of aluminum particles for energetic applications is to reduce their size to nanoscale.
To characterize how efficient a reaction is during burning, researchers often use the flame propagation rate that occurs when aluminum releases energy in the form of heat.
“The main direction in increasing reactivity of aluminum particles for energetic applications is reduction in their size down to nanoscale. This increased flame propagation rate by two orders of magnitude in comparison with 20-100 micron particles.,” he explained. However, he adds that nanoparticles are 30-50 times more expensive than micron-scale particles and possess safety and environmental issues.
Because of these factors, Levitas and his experimental collaborators from Texas Tech University wanted to see if they could find a way to improve the energetic performance of aluminum micron-scale particles.
Levitas originally started thinking in terms of micron-scale after he developed a melt-dispersion mechanism (MDM) that suggested the aluminum micron-scale particles under a high-heating rate could reach half of the flame propagation rate of the best aluminum nanoparticles. Levitas then predicted the flame rate could be further increased in micro-scale particles if the particle’s core is prestressed before synthesis.
These predictions were both confirmed during an experiment using 3-4.5 micrometer aluminum and copper-oxide particles in a combustion tube. Compressive stresses in the alumina passivation shell of aluminum particles were produced by a special theoretically predicted heat treatment. This technique increased the micron-scale particle flame propagation rate by 36%, which was 68% of the flame speed of the best aluminum nanoparticles.
“We found that for the optimal treatment, as well as for untreated particles, experimental results are in quantitative agreement with the theoretical predictions based on the MDM,” Levitas said. “The research strongly supports the MDM and demonstrates its applied potential.”
Additionally, Levitas says this approach could create new ways to control particle reactivity through mechanochemistry.
The research was published in the Scientific Reports of Nature Publishing Group (Levitas, V.I., McCollum, J. and Pantoya, M. Pre-Stressing Micron-Scale Aluminum Core-Shell Particles to Improve Reactivity. Scientific Reports, 2015, 5, 7879).