Combustion Performance Improvement of Energetic Thin Films Using Carbon Nanotubes

Abstract

nergetic systems comprising of metal fuels and oxides, are referred to here as thermites. They have been studied extensively due to their properties such as high energy density, heat of combustion, and reaction rate. 1-4 Since these energetic systems are mixtures of constituent reactant powders, they can be tailored relatively easily to cater to different applications. Thus, they find versatile applications in industry and ordnance, alike. Thermite reactions are self-propagating and are typically localized energy generation and high temperatures. Typical fuels for such energetic systems include aluminum (Al), boron (B), and magnesium (Mg). Whereas the experimental study focuses on fuel and oxidizer mixtures alone, for several in-field applications, the mixtures are combined with binders to consolidate them into desirable shapes. Thermite reaction rates are dependent on a number of factors such as particle diameter, equivalence ratio, binder concentration, reactant temperatures, compaction density (usually referred to as theoretical mean density or TMD), and additives. Additives are reactants added to thermite systems that do not radically change their chemistry, but influence targeted properties. One additive gaining popularity in the recent years for use in thermite systems is carbon nanotubes (CNTs), owing to their exceptional properties. Previous work showed the influence of CNTs on impact sensitivity of Al/polytetrafluoroethylene mixtures. Kim et al. used CNTs as optical igniters to initiate mixtures of Al and copper oxide (CuO). Guo et al. replaced copper thin film microbridge electropyrotechnics with CNTs combined with potassium nitrate and found that CNTs initiated the ceramic substrate using lower input energy, making it more sensitive. Collins et al. showed decreased ESD sensitivity in Al/CuO with the addition of CNTs. Poper et al. researched into this phenomena further and discovered that CNTs increase the flame speed of the Al/CuO along with influencing their electrical properties. Thermites reaction rates have been shown to be influenced significantly by the equivalence ratio of the thermites. Effective and complete burning of the energetic systems has been associated with slightly fuel rich compositions. Thermites have variegated energy transfer mechanisms. High gas generating thermites transport heat convectively; changing stoichiometry during reaction, thus, influences how energy propagates. On the other hand, low gas generating thermites transfer heat through hot particle advection and conduction. Activation energy of thermites has been shown to influence the flame speeds of the reaction, due to the nature of energy propagation in the porous thermites. Despite the increasing interest in CNTs as additives, the exact mechanism through which they influence energy transfer during thermite combustion is not effectively understood so far and is being investigated in the current work. The current work demonstrates the influence of CNTs on the ignition delay and combustion performance of energetic thin films synthesized by doctor blade casting. In order to identify the primary factors that influence thermite combustion in the presence of CNTs, energetic thin films of Mg and manganese oxide (MnO2) with polyvinylidene fluoride (PVDF) binder. This particular energetic system has been chosen as the vehicular matrix to hold the CNT additives in varying concentrations since it has been studied in detail recently. 15 Armstrong modeled flame speeds through random particulate media with the assumption of no gas generation and demonstrated that thermophysical properties of the matrix are key parameters controlling flame speed in the second order reaction dynamics of the global combustion reaction. Interestingly, Meeks at al. showed that Mg/MnO2 samples propagate energy conductively in open test configurations. Thus, by eliminating the influence of convective heat transfer, the