Metallized reactive materials – a road to clean and sustainable pyrotechnics

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Since the introduction of nitroglycerine as a blasting explosive by Nobel in 1867, enormous progress has been made in improving the performance and reducing the sensitivity of energetic materials. The introduction of nitrogen rich (N>60 %) oxidizers such as 5,5′-hydrazinebistetrazole (HBT) was one very important step in increasing the detonation performance and thermal stability of high explosives. Concurrently, the introduction of metal fuels which can react at high temperatures attracted significant interest, improving combustion performance while also reducing the sensitivity of propellants and pyrotechnics. This is largely because metals such as aluminum, magnesium, titanium, zirconium and boron have very high volumetric (30–140 kJ cm−3) and gravimetric energy densities (10–50 kJ g−1) compared with other chemical fuels. These fuels are also chemically stable solids, which simplifies storage and transport. Furthermore, they can be ignited and burned with oxidizers (air, metal nitrate, metal oxides) or water, to produce large amounts of heat, or hydrogen and heat. The study of metal fuel combustion dates back to the 1960s, and is driven by the potential applications of these fuels as additives in propellants, explosives and pyrotechnics to increase energy density and burn rates. Since then, significant research efforts have been devoted to thermites and nanothermites using different metal/metal oxide combinations and custom nanostructured architectures. Unlike explosives, these materials undergo a rapid deflagration driven by a carbon free oxidation-reduction reaction, which forms stable reaction products (metal oxides or hydroxides) that can be recycled with zero-carbon reduction processes. Moreover, these products have stronger mechanical strength and heat resistance than explosives. They are versatile: different combustion effects can be obtained by manipulating the reactive system (metal and oxide powders) and their microscopic and mesoscopic morphology. They can be printed, which enables unique geometries and control over their energetic behavior – such as ignitability and mechanical characteristics – which is not easily possible with explosives manufactured by slurry loading, melt-casting, cast-curing or powder-pressing. Finally, thermites and nanothermites can burn in high vacuum, underwater and in other harsh environments. For this reason, from the year 2000 onwards, metallized reactive materials and related processes have attracted considerable attention from industry as extremely useful energetic substances and technologies for: i) initiation, ii) clean propulsion and power generation (thermal batteries), iii) self-destructing microchips and infrastructure protection, iv) energy sources or supplies for outer space use, v) long ceramic-lined pipes welding in geothermal power plants, and vi) agent deactivation. Many other applications will likely be addressed in the future. Yet, the effective deployment of such promising energetic materials into these various applications faces a major hurdle. The “Edisonian” approach based on trial-and-error processes used in most research laboratories is not suited to these complex energetic systems, in which not only the chemistry but also the microscopic (particle scale) and mesoscopic properties (systems of hundreds of particles, binder) influence macroscopic energetic performance. A requirement-driven design approach relying on both mechanistic and statistical models should be preferred for a cost-effective customization of the best metal/oxide configurations to each function, considering all application requirements such as cost, technological compatibility and other constraints. To achieve these ambitious objectives research is needed on several fronts, specifically: i) developing mechanistic models for metal combustion in various oxidizing environments; ii) quantifying the key condensed and gas phase mechanisms that govern metal oxidation at elevated temperatures, particularly above the critical point where oxide solubility could change significantly and improve reaction rates and yields; iii) enhanced understanding of the physics of turbulent and “discrete” flames. These requirements necessitate the use of advanced diagnostics (laser-based optical diagnostics, time-resolved mass spectrometry) and state-of-the-art experimental methods to enable determination of reaction rates, energies, and rate-limiting mechanisms of these complex reactive systems. Collaboration with data science researchers is also urgently needed to develop not only surrogates for physical models but also inverse models which will enable the discovery of new metal fuel/oxidizer configurations that cannot be imagined by the user because of the quantity of variables impacting metal fuel combustion, e. g., shape and size of the grain of each powder, compaction, stoichiometry, environment, etc. In conclusion, the fusion of physics simulation and machine learning approaches will enable us to fully realize the benefits of metallized reactive materials, and may consequently lead to a paradigm shift in pyrotechnics.