## 1 Introduction  
Thermites in the form of mixed particles composed of a fuel, most commonly aluminum, and an oxidizer, such as iron, copper oxide, or molybdenum oxide [1; 2; 3; 4; 5; 6; 7; 8; 9; 10], have attracted considerable attention due to their controllable versatility and high reaction enthalpies, making them good candidates for a number of applications including actuation [11; 12], micropropulsion [13; 14; 15; 16; 17], heat sources for welding or joining [18; 19], and, more recently, micro-initiation and environmentally clean primers [20; 21; 22; 23]. In the last two decades, nano-sized particles have been widely studied due to their advances in reactivity, e.g., lower initiation temperature, lower initiation delay, and shortened kinetics of heat release, which was a necessary development to further improve the performances of the aforementioned applications, with the potential to defhrone other types of energetic compositions [24; 25; 26; 27]. While nanosized particles were found to outperform their micron counterparts, the performance enhancements were below expectation, and, thus, the exact mechanisms by which these augmentations are accomplished remains debated [28; 29; 30; 31; 32; 33; 34; 35]. In theory, mixed nanoparticles possess higher interfacial contact and a decrease in mass-transport lengths resulting in much wider reaction fronts. However, the loss of their nanostructures during the early stage of initiation due to sintering was experimentally observed, providing an explanation for the longer than expected burn times [5; 36; 37]. Prior to this, a debate in the community of thermite materials was focused on whether condensed phase rather than gas phase mechanisms were responsible for the different combustion regimes, e.g., initiation and combustion. Evidently, both mechanisms are expected to be present when considering the level of heating and temperatures reached upon initiation and subsequent reaction propagation; yet, the balance between the two mechanisms, and the potential domination of one over the other in either stage of combustion is complex and difficult to ascertain. Indeed, this balance is shown to depend heavily on the overall nanostructure (dense vs. low compaction, open vs. closed architecture), as well as the method of initiation (very high vs. low heating rate, laser heating, ESD, etc. ), and obviously, the chemical nature of the thermite ingredients [38; 39]. In this respect, modelling, ranging from atomic to macroscale methodologies, is a useful tool to probe specific, if not the overall, aspects of the complex heterogeneous combustion that are still not fully understood. At the atomic scale, simulations that are governed by molecular dynamics and/or density functional theory (DFT) are mostly limited to a scope of basic mechanisms and early stages of initiation of the reaction due to limitations in system size and the long calculation time required for these types of simulated experiments [40; 41; 42; 43; 44]. At the other extreme, analytical models of macroscale systems are generally using rudimentary approximations of the chemical fundamentals governing the reaction of thermite materials [45; 46; 47; 48]. Therefore, mesoscale models offer a good compromise through the inclusion of physico-chemical insights into the material properties and mechanisms (diffusion/reactions), making them more versatile and predictive for a wider range of materials and experimental conditions. Along this line, several models have been proposed to elucidate the importance of gas phase versus condensed phase mediated thermite reaction processes [39; 49; 50]. A recent publication proposed a pure condensed-phase model based on a spherical formulation of the Deal and Grove diffusion-reaction scheme [51] coupled to the heat equation. This approach included a wetting component allowing the exploration of the effect of a reactive sintering process as defined in References [5; 52].