Measuring the pre-heating zone thermal profile: Effect of thermal diffusivity on propagation in nanothermite composites

The combustion behavior of nanothermite composites is governed by the coupled effects of chemical kinetics and transport (heat and mass). In this study, we investigated the role of thermal properties on the propagation of six nanothermite systems with distinct product morphologies. In particular, we manipulate the intrinsic thermal diffusivity by incorporating carbon fiber (CF). High-speed three-color imaging pyrometry combined with infrared thermography was employed to obtain complete temperature profiles during combustion extending from the pre-heating through the reaction zone. These data enabled us to extract in-operando thermal diffusivity via solution to the heat-equation. We find that while the CF, which is inert, did not alter the measured peak combustion temperature, it could be used to alter both the thermal diffusivity and propagation velocity. The nanothermite composites were classified based on the combustion product morphology observed by real-time imaging during combustion. Composites exhibiting dispersed phase products including Al/MnO2, Ti/MnO2, and B/Bi2O3, where CF intercepts ejected agglomerates and increases heat feedback to the pre-heating zone, thereby enhancing the burn rate. In contrast, for the composites with continuous phase products including B/MnO2, B/Fe2O3, and B/Fe3O4, enhanced thermal diffusivity by CF increases energy dissipation from reaction zone, leading to reduced burn rate. Simulations reproduce the observed burn rate trends with CF addition, confirming that heat feedback plays a key role in the propagation of nanothermite composites. An in-operando methodology is developed to extract effective thermal diffusivity directly from experimentally measured combustion temperature profiles, enabling quantitative characterization of heat flux during combustion propagation.