(487e) The Influence of Heat Feedback and Thermal Conductivity on the Burn Rate of Thermite Composites

In this talk we clarify some concepts related to heat transfer to the reaction front in composite burning. In here, 2.5 wt% carbon fiber (CF) is added to various thermite compositions to assess the effect of thermal conductivity on burn rate. We observe that in some cases the burn rate is enhanced and in others decreased. Upon further analysis we find that this behavior depends on the phase of the reaction product (dispersed vs. continuous). Adding carbon fiber increases the burn rate of composites with a dispersed phase product, which includes Al/MnO₂, Ti/MnO₂ and B/Bi₂O₃. However, CF added to composites with continuous phase product like B/MnO₂, B/Fe₂O₃ and B/Fe₃O₄, the burn rate is reduced. A simple thermal analysis shows that for solid-composites, increase in thermal conductivity should actually lead to a decrease in burn rate, as any mechanism that bleads energy away for the reaction front will decrease overall reaction velocity. In contrast, for cases where the products are dispersed aerosol droplets, the fibers serve to capture the droplets as they lift off the surface, and thus offers an additional heat feedback mechanism that enhances the burn rate. This study shows that micro-engineering methods to couple heat to the reaction front with minimal loss of heat to the pre-heat zone will result in an increase in power.