Metal such as Al, Mg, Ti, Zr and B have very high volumetric (30 - 140 kJ.cm3) compared with other chemical fuels and main explosives. Metal fuels are also chemically stable and solid, simplifying storage and transport. Unlike traditional CHNO energetic materials, metal based reactive materials such as thermites, undergo a rapid deflagration driven by a carbon-free oxidation–reduction reaction, which forms stable reaction products (metal oxides). The reaction fronts propagate sub-sonically and rely on atomic diffusion or other physical transport mechanisms, which means that thermites can be engineered to react over a wide range of timescales depending on the reactant length scale and shape. These characteristics, which are entirely complementary to those of propellants and explosives combined with the safety, tunability, and relatively benign nature of thermites make them promising for a myriad of applications including material synthesis, heat sources for welding and joining, smart initiators, on-chip actuation and thermal neutralization. However, a major challenge in integrating fast-reacting thermite materials into practical applications lies in engineering them to achieve the precise combustion behavior required for specific uses. It requires an in-depth understanding of reaction phenomena and the key variables governing them in order to establish quantitative structure–property relationships (QSRPs). Beyond understanding the mechanisms driving thermite reactions, advancing nanoengineering methods is equally crucial with the aim to explore new metal/oxide combination and process to produce safer, greener while highly reactive materials. At the core of PyroSafe project, two material shaping processes, PVD and 3D printing (direct ink jetting), have been prioritized to structure the reactants (metal and oxide) and control their intimacy to control the exothermic reactions.
In parallel, the development of specialized diagnostic tools and advanced techniques, such as scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS), as well as high-speed and infrared videography, has enabled unprecedented advancements in understanding thermite ignition and combustion mechanisms. For example, in-operando flame characterizations provided a comprehensive analysis of thermal front microstructures subject to heterogeneous reactions which gave inputs to construct numerical combustion models. To complement these advanced experimental techniques, a multi-scale computational approach was followed to facilitate the establishment of QSPRs. DFT and classical MD were used for the elucidation of still unknown reaction mechanisms of Al/CuO reaction (powder and reactive films) as well as for the quantification of thermokinetic data related to mass transport, chemical reactions, and gas-surface interactions. Higher-level numerical models, based on the CFD approach were developed to depict the pressure development of a thermite in close-vessel and self-propagating combustion behavior. These models represent the most advanced approach currently available, providing a detailed and accurate depiction of combustion dynamics for any Al/CuO powder configuration, including variations in density, stoichiometry, and other parameters. Finally, we also explored machine learning (ML) which offers promising transformative approach to conventional physical models toward the development of QSPRs. Gaussian processes can be trained on data from detailed simulations to create surrogate models that approximate complex physical behaviors. These surrogate models provide rapid predictions of key properties, such as burn rate, peak pressure, and combustion temperature, based on material composition and characteristics, enabling quick evaluations without the need for full simulations. ML models were also applied for the analysis of large datasets to identify critical features that influence performance. We demonstrated how variables like particle size distribution, porosity, and metal-to-oxidizer ratios impact combustion efficiency and stability.
