Study of combustion performance of Al/PTFE B/PTFE and B/Al/PTFE composites

Field: Chemical Engineering (Physical Sciences)

Metal particles like aluminum and boron are of great interest as additives to energetic materials due to their high gravimetric and volumetric heat of combustion. However, the ignition temperatures of metals are quite high and their burning rates which depend upon the surface area of the burning metal agglomerates are quite slow. The naturally formed oxide layer on the surface of metal particles can further inhibit their ignition and combustion by limiting fuel/oxidizer transport. The boron oxide layer on boron particles is of particular interest due to the strong diffusion barrier formed by its presence. However, the ignition temperature and burning time of micron-sized metal particles can be reduced through development of additional interfacial area (equivalently, reduction of fuel/oxidizer diffusion distance) through the process of ball milling. Additionally, ball milling of inclusion materials such as polytetrafluoroethylene (PTFE) that mitigate through reaction the formation of metal oxide features on metal particles can reduce ignition delay and further expedite metal particle combustion energy release. Ball milling is of particular interest in fabrication of energetics and energetic ingredients due to its top-down, scalable nature. Both the kinetics of metal fluorocarbon reactions and the ignition/combustion of mixtures of nanoscale metal fuels with nanoscale fluorocarbon oxidizers have received much attention in the past decade. However, ball milling-fabricated (mechanically activated) energetic materials possess complex internal structure and length scales spanning from the nano- to the micro-scale. For these reasons as well as others, their ignition and heterogeneous reactions can be far different than those of either similar nanoscale mixtures or premixed kinetics. The heterogeneity of metal/fluorocarbon energetic material reactions dictates that their reactivity and ignition are dependent upon both pressure and heating rate. While a number of studies have explored ignition at high heating rates alone, the combined effects of both controlled high heating rate and high pressure have not been explored. Such information is of importance to application of nanoscale energetics in energetic systems in areas of rocket propulsion and detonation enhancement.\nThis thesis explores the reactivity, ignition, and combustion efficiency of nanoscale metal/fluorocarbon energetic materials comprised of aluminum and boron fuels with PTFE oxidizer. Differential scanning calorimetry/thermogravimetric analysis with hyphenated, time-resolved (temperature-resolved) evolved gas analysis via both mass spectrometry and Fourier transform infrared absorption are used to explore the anaerobic and aerobic reaction of metal/fluorocarbon energetic materials. Ignition is explored at linear heating rates of 104 °C/s to 105 °C/s and at pressures ranging from one to 70 atm pressure using a custom fabricated, electrically heated filament apparatus fitted within a windowed, high pressure vessel. Combustion rate at high pressure is further inferred by high-speed pressure measurements of small volume combustion bomb experiments and combustion efficiency of nanoscale energetics is measured using oxygen calorimetry. Findings from combustion experiments and interpretation are further supported by material and product characterization via scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), and combustion chemical equilibrium simulation.\nChapter one discusses relevant literature in the areas of metal combustion (specifically aluminum and boron), effects of the addition of fluorocarbons, effects of nanostructure on heterogeneous energetic material reactivity and ignition as well as nanostructuring techniques, and the effects of heating rate on ignition as well as techniques for achieving high heating rate ignition. Chapter two discusses the measurement of aerobic and anaerobic reactivity of mechanically activated Al/PTFE energetic materials and their ignition sensitivity to pressure and heating rate. Chapter three discusses similar trends in B/PTFE nanoscale energetics with a nanoscale boron that fabricated from a unique laser pyrolysis method, and chapter four discusses development of ternary B/Al/PTFE mechanically activated energetic materials, with specific emphasis on the effects of staging of the ball milling process in order to affect reactivity.