Keerti Kappagantula

Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives

The inclusion of graphene into composite energetic materials to enhance their performance is a new area of interest. Studies have shown that the addition of graphene significantly enhances the thermal transport properties of an energetic composite, but how graphene influences the composite’s ignition sensitivity has not been studied. The objective of this study is to examine the influence of carbon additives in composite energetic material composed of aluminum and polytetrafluoroethylene (Teflon™) on ignition sensitivity due to low velocity, drop weight impact. Specifically, three forms of carbon additives were investigated and selected based on different physical and structural properties: spherically shaped amorphous nano particles of carbon, cylindrically shaped multi walled carbon nanotubes, and sheet like graphene flakes. Results show an interesting trend: composites consisting of carbon nanotubes are significantly more sensitive to impact ignition and require the lowest ignition energy. In contrast,...

Fabrication, characterization, and energetic properties of metallized fibers.

Polystyrene fibers loaded with an energetic blend of nanoaluminum (n-Al) and perfluoropolyether (PFPE) were successfully fabricated via electrospinning producing nanothermite fabrics. Fibers were generated with loadings up to 17 wt % n-Al/PFPE incorporated into the fiber. Microscopy analysis by SEM and TEM confirm a uniform dispersion of PFPE treated n-Al on the outside and inside of the fibers. Metallized fibers were thermally active upon immediate ignition from a controlled flame source. Thermal analysis by differential scanning calorimetry (DSC) found no change in glass transition temperature when comparing pure polystyrene fibers with fibers loaded up to 17 wt % n-Al/PFPE. Thermal gravimetric analysis (TGA) revealed a shift in decomposition temperatures to lower onsets upon increased loadings of n-Al/PFPE blends, consistent with previous studies. Flame propagation studies confirmed that the metallized fibers are pryolants. These metallized fibers are a recent development in metastable intermolecular composites (MICs) and details of their synthesis, characterization, and thermal properties are presented.

Determination of the spatial temperature distribution from combustion products: A diagnostic study

Temperature measurements within the highly complex reaction field of energetic materials are complicated but existing technology enables point source measurements that identify a maximum temperature at a single location. This study presents a method to extend point source measurements to thermally map the spatial distribution of temperature over a large field of interest. The method couples point source temperature measurements from a multi-wavelength pyrometer with irradiance measurements from an infrared camera to produce a highly discretized thermal map that includes the reaction and surrounding field. This technique enables analysis of temperature gradients within the field of interest and an understanding of energy propagation beyond the point of reaction. Point source measurements of maximum temperature are within 10% of reported values. The method was illustrated for the aluminum and polytetrafluoroethylene reaction and the thermal distribution of temperature produced 30,720 temperature measurements over a field of interest corresponding to 3.5 cm × 8 cm.

Combustion Performance Improvement of Energetic Thin Films Using Carbon Nanotubes

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

Fast Reacting Nano Composite Energetic Materials: Synthesis and Combustion Characterization

Abstract Energetic composites are mixtures of solid fuel and oxidizer particles that when combined offer higher calorific output than monomolecular explosives. The composites traditionally deliver energy as diffusion-limited reactions, and thus their power available from reaction is much smaller than powerful explosive. Yet, technology has developed advanced particle synthesis, and nanoparticles have become more readily available. The advent of nanoparticle fuels and oxidizers enables traditionally diffusive controlled reactions to transition toward kinetically dominant reactions. This transition results in faster reacting formulations that show promise of harnessing the power equivalent to a monomolecular explosive but packaged as discretely separate fuel and oxidizer composites. This chapter will focus on developing an understanding of fundamental reaction dynamics associated with particulate media, in general. Once this foundational understanding is established, new strategies for designing aluminum fuel particles toward greater reactivity and thus faster reacting formulations will be presented. In addition to synthesis, several combustion characterization techniques will be examined to quantify combustion performance. All of this information may provide a basis for future research and applications involving aluminum-based fuels in any energetic system (i.e., as an additive to liquid propellants or even explosive formulations). Composite energetic materials with nanoscale aluminum particles play a significant role in nearly every sector of our energy-generating economy from industrial to ordnance technologies. Nanoscale aluminum fuel particles hold numerous advantages over their micron-scale counterparts. Fluoropolymers have been gaining popularity over the last decade as a favored oxidizer in these composite systems because of their unique ability to react with the passivating alumina shell present over aluminum particles. This chapter investigates the tailorability of energetic composites made of nano-aluminum (Al) combined with different fluoropolymers, by incorporating different additives into the reactive material. Diffusion-controlled reactions are limited by the proximity (i.e., diffusion distance) of reactant particles. The effect of the proximity of the oxidizer has been investigated by performing flame propagation experiments on molybdenum trioxide (MoO 3 ) combined with aluminum particles with and without surface functionalized perfluorotetradecanoic (PFTD) acid. Results showed that the surface functionalization enhanced the burn rate twice that of nonfunctionalized energetic composite. In order to control the burn velocity by altering their surface functionalizations, three different energetic composites consisting of aluminum particles with and without surface functionalization, combined with molybdenum trioxide, were used. Perfluorotetradecanoic and perfluorosebacic (PFS) acids were used to form an organic corona around the aluminum nanoparticles. Flame propagation studies revealed that energetic composites made of Al functionalized with PFTD (Al-PFTD) displayed burn velocity higher than Al/MoO 3 whereas Al with PFS/MoO 3 are almost half of Al/MoO 3 . Results showed that the fluorine content in the acids and their structural differences contribute to difference in burn velocity. The mechanisms controlling reactivity is discussed such that new approaches to particle synthesis can be developed to further advance energetic composites for the next generation.