277536 Protecting Aluminum Nanoparticles by Hydrophobic Carbon and Fluorine-Based Coating
Due to the high reactivity and high releasing heat of metal oxidation, metal particles are added to explosives and propellants to increase the energy output. Aluminum particles are of special interest because of their high reactivity and amount of heat released. However, the rate of release is lower compared to other energetic compounds such as TNT, HMX, and RDX. One promising solution is to improve the kinetics of the combustion process and increase the rate of energy release by decreasing the particle size to nanometer scale. The drawback is that these particles oxidize readily to produce a passivating oxide layer 2-5 nm in thickness. For nanoparticles, this means that a large fraction of the particle is already oxidized. For 10 nm particles, for example, 70% of the mass is in the form if the oxide. This leads to the need for passivation of the aluminum surface via thin coatings that have the ability to inhibit degradation of the metal under storage conditions without interfering with the oxidation process during combustion.
Here, we discuss a novel plasma deposition process for coating nano-sized aluminum particles. Three different precursors, isopropyl alcohol (IPA), toluene, and perfluorodecalin (PFD) have been deposited through a plasma enhanced chemical vapor deposition process on aluminum nanoparticles to protect the active content of Al. Deposited coatings are hydrophobic and we have studies their hydrophobic properties by measuring a sessile water droplet contact angle with coated particles. Hydrophobic coatings protect particles against contamination and oxidation. Transmission electron micrographs prepared from the surface of coated particles qualitatively show that no oxide is formed on the surface of coated nanoparticles. Quantitatively, we measure the active content of coated and uncoated aluminum particles using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that coatings inhibit oxidation during storage, with the PFD coatings showing the highest degree of protection and IPA the lowest. Although the films are porous and permeable by small molecules such as water, increasing the hydrophobicity of the coating improves substantially resistance against oxidation in the presence of moisture.