Abstract
Fuel-rich composite powders combining elemental Si with the metal fluoride oxidizers BiF3 and CoF2 were prepared by arrested reactive milling. Reactivity of the composite powders was assessed using thermoanalytical measurements in both inert (Ar) and oxidizing (Ar/O2) environments. Powders were ignited using an electrically heated filament; particle combustion experiments were performed in room air using a CO2 laser as an ignition source. Both composites showed accelerated oxidation of Si when heated in oxidizing environments and ignited readily using the heated filament. Elemental Si, used as a reference, did not exhibit appreciable oxidation when heated under the same conditions and could not be ignited using either a heated filament or laser. Lower-temperature Si fluoride formation and oxidation were observed for the composites with BiF3; respectively, the ignition temperature for these composite powders was also lower. Particle combustion experiments were successful with the Si/BiF3 composite. The statistical distribution of the measured particle burn times was correlated with the measured particle size distribution to establish the effect of particle sizes on their burn times. The measured burn times were close to those measured for similar composites with Al and B serving as fuels.
Highlights
Silicon serves as a fuel in several pyrotechnic compositions [1]
The optical emission of the ignited particles was captured by filtered photomultiplier tubes (PMT) to obtain burn times
The oxidation rate of Si increased initially and became relatively stable over a range of temperatures. For both composites the low-temperature rates of heat release associated with the observed initial rate of oxidation of Si could be added to the rates of heat release caused by the Si fluorination occurring simultaneously to describe the exothermic reactions leading to ignition of such composite materials
Summary
Silicon serves as a fuel in several pyrotechnic compositions [1]. Its high energy density, abundance in the lithosphere [2] and cheap manufacturing make it an attractive fuel for a broad range of applications, including propellants and explosives. Transition metal oxides [4,8,9,10,11], alkali metal nitrates [4,9,10,11], perchlorates [9] and manganates [9] were explored as oxidizers for micron-sized and nanometric silicon powders These compositions, though favorably reactive, do not fully exploit the potential of silicon as they yield largely condensed phase combustion products. Respective silicon-condensed oxidizer composites have reduced energy content compared to elemental silicon reacting with oxygen gas; the reactions have reduced adiabatic flame temperatures. Experiments with aluminum and boron-based reactive composites containing metal fluorides as oxidizers show consistently significant improvements in kinetics of reactions leading to ignition [24,25]. Both BiF3 and CoF2 were incorporated into silicon-metal fluoride composites prepared by arrested reactive milling [26]
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