Cole D. Yarrington

Combustion of Silicon/Teflon/Viton and Aluminum/Teflon/Viton Energetic Composites

DOI: 10.2514/1.46182 The combustion of Si- and Al-based systems using polytetrafluoroethylene (PTFE) as the oxidizer and Fluorel FC 2175 (a copolymer of hexafluoropropylene and vinylidene fluoride) as a binder has been studied. Experimental data were obtained using two methods: 1) instrumented tube burns and 2) pressed pellets inside a windowed pressure vessel. Loose-powder burning rates were seen to optimize at slightly-fuel-rich mixture ratios for Si/PTFE/FC-2175 (SiTV). Al/PTFE/FC-2175 (AlTV) burning rates optimized near a stoichiometric ratio. Pressures calculated by assuming constant-volume combustion equilibrium were seen to match experimental values from burn-tube experiments when burning rates were at or near peak values. The pressure dependence of SiTV and AlTV pellet burning rates was also characterized and compared with reported Mg/PTFE/Viton (MTV) results. SiTV showed power-law dependence with a constant-pressure exponent over the experimental range of pressures. AlTV was showntoexhibitnonconstant-pressureexponentbehavior.SiTVburningratesoptimizedatmixtureratiossimilarto that of the tube burns. AlTV burning rates increased well past a stoichiometric ratio and decreased at a fuel-rich ratio, which is a similar trend to MTV burning rates.

The dynamics of Al/Pt reactive multilayer ignition via pulsed-laser irradiation

Reactive multilayers consisting of alternating layers of Al and Pt were irradiated by single laser pulses ranging from 100 μs to 100 ms in duration, resulting in the initiation of rapid, self-propagating reactions. The threshold intensities for ignition vary with the focused laser beam diameter, bilayer thickness, and pulse length and are affected by solid state reactions and conduction of heat away from the irradiated regions. High-speed photography was used to observe ignition dynamics during irradiation and elucidate the effects of heat transfer into a multilayer foil. For an increasing laser pulse length, the ignition process transitioned from a more uniform to a less uniform temperature profile within the laser-heated zone. A more uniform temperature profile is attributed to rapid heating rates and heat localization for shorter laser pulses, and a less uniform temperature profile is due to slower heating of reactants and conduction during irradiation by longer laser pulses. Finite element simulations of laser heating using measured threshold intensities indicate that micron-scale ignition of Al/Pt occurs at low temperatures, below the melting point of both reactants.

Instrumented Burn Tube: Experimental Observations and Analysis of Data

experimental techniques to measure the various properties of nanoenergetic systems. Among these techniques, the instrumented burn tube is a simple experiment that is capable of obtaining data related to the combustion of these materials. The purpose of this paper is to review the current state of the conned burn tube experiment, including the drawbacks of the technique and possible remedies. As this report is intended to focus on the specic experimental technique, data from many dierent energetic materials and experimental congurations will be presented. The qualitative and quantitative data that can be gathered using conned burn tube experiments include burning rate, total impulse, and pressurization rate. All of these measurements lend insight into the combustion properties and mechanisms of specic nanoenergetic powders.

Combustion Properties of Silicon/Teflon/Viton and Aluminum/Teflon/Viton Composites

The combustion of aluminum-based nano energetic materials has received much attention in recent years. Silicon-based nano composites have not been studied so extensively, even though the predicted thermochemistry properties show promise and silicon reactives may have other advantages. Also, almost no attention has been given to silicon reactives using fluorinated oxidizers. A more complete body of experimental knowledge with respect to the combustion of nano silicon-based reactives with fluorinated oxidizers is needed to better understand the mechanisms of combustion, as well as to develop the possible applications. Combustion equilibrium calculations were performed on silicon and aluminum-based systems using polytetrafluoroethylene (PTFE or Teflon) as the oxidizer. It was found that silicon and aluminum with fluorinated oxidizers have similar predicted equilibrium temperatures. Basic combustion properties were obtained using stoichiometries chosen based on the thermochemical equilibrium calculations for both aluminum and silicon. A range of stoichiometries were also tested for the silicon systems. Fluorel FC 2175 binder (chemically equivalent to Viton R ©) was added to prevent sample brittleness and allow for the characterization of burning rate of pressed pellets at various pressures. Instrumented tube burns were also performed in order to determine the loose powder burning rate. Silicon reactive loose powder burning rates were found to optimize at slightly fuel rich stoichiometries, similar to aluminum-based reactives. It was found that there exists different combustion modes for aluminum reactives in tube burns. Depending on ignition method, either a fast burning steady state burn would occur, or a slow burn which transitioned to the other mode at a later time. A strong link to pressure is likely key to the presence of these two combustion modes and the transition between them. Aluminum reactive peak pressures and burning rates were 9,000 PSI and 1,070 m/s, and silicon reactive peak pressure and average burning rate were 7660 PSI and 424 m/s. Reactive silicon pellets were also found to optimize at fuel-rich compositions and the burning rates were found to follow a power law dependence on pressure, however, this dependence (characterized by the power law exponent) was not significantly affected by stoichiometry, in contrast to magnesium/Teflon/viton (MTV) systems. Aluminum reactive pressed pellet burning rates were better fit by a second order polynomial. A comparison between silicon and aluminum-based reactive pellets show comparable burning rates for mixture ratios chosen at the calculated maximum temperature. Also, the burning rate did not increase continuously with added wt% fuel in the mixture as MTV systems are reported to do, but reached a peak and then declined, indicating more of a temperature dependence.