Fast reaction of nano-aluminum: A study on fluorination versus oxidation



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The use of fluorine as an oxidizing agent in thermite reactions yields higher heats of combustion and an increase in gas production. Thus fluorination reactions have the potential to excel in situations that require high pressures, temperatures, and flame speeds. This study compares the propagation behaviors of Al/Teflon, Al/MoO3/Teflon, and Al/ MoO3 in an effort to determine the effects that the replacement of oxygen with fluorine (Teflon is 75% by weight fluorine) has upon the reaction characteristics in both open and confined configurations. Data was collected from pressure sensors and high speed recording of the reactions. The mass percent of Al was varied from 10% – 90% for each composite to study the effects of composition. The composites were then further tested at the optimum stoichiometry using either 50 nanometer or 1-3 micrometer Al as the fuel to examine the affect of Al particle size on the reactions. It was found that the addition of Teflon in an open burn configuration hinders the reaction due to a loss of liberated fluorine gas to the surroundings resulting in less energy to propagate the reaction and a higher rate of incomplete combustion. Nanoscale Al produced faster flame speeds as a result of the increased sensitivity and homogeneity associated with the smaller particles. The most significant flame speeds were found in the Al/MoO3 composites in which less energy is lost in the form of escaping gas. Confining the reactions and the intermediate and product gases promotes enhanced convection yielding increased flame speeds. The reactions containing Teflon exhibit much higher pressures which have a dual effect. Initially the increasing pressures result in increasing flame speeds. However, there exists a threshold beyond which an increase in pressure suppresses the reaction and reduces the flame speed.