## 3 Results and Discussion
### Benchmark Study
For Al/Fe\({}_{2}\)O\({}_{3}\), the flame extends over \(\sim\)800 \(\upmu\)m while it is only of 200 \(\upmu\)m for Al/CuO (defined as the spatial extension between temperature at its maximum and its upfront thermalization, at a given time of the steady state propagation). To quantify the role of this later process, we artificially reduce the conduction of iron oxide-based thermite to the value of Al/CuO. The calculated burn rate of this Al/Fe\({}_{2}\)O\({}_{3}\) thermite with \(\lambda_{\text{AlCuO}}\) is 19.7 m\(\cdot\)s\({}^{-1}\), roughly one-third of the Al/CuO thermite burn-rate, and with minimal change to the initiation delay. Importantly, this indicates that conductivity, rather than the flame temperature, allows a reasonably high burn rate in Al/Fe\({}_{2}\)O\({}_{3}\). The Al/WO\({}_{3}\) couple presents another relatively low-gas production nanothermite with only 0.14 mol of gas produced per 100 g of thermite mixture [58]. The initiation temperature is calculated at 718 K, the lowest value of all of the considered Al-based thermites, just below that of the Al/CuO couple. WO\({}_{3}\) exhibits a higher inner diffusion rate for oxygen atoms to diffuse towards aluminum and its oxide. As a consequence, this thermite falls within the same category as Al/CuO, i.e., thermites in which initiation is governed by the mass transport across the aluminum oxide barrier layer. This allows an initiation delay as low as 63 ms, the lowest of all considered thermites, lower than that of Al/CuO, which is due to the fairly larger amount of elementary exothermic reactions in this delay, as more oxygen is provided by the oxidizer at equivalent temperature. In addition, the slightly lower conductivity (1.6 vs. 2.9 for stoichiometric Al/CuO) is a mean to locally concentrate the energy into hot points in place of the flame front. The burn rate shows a well-established steady state (see Figure 3c) at the value of 17 cm\(\cdot\)s\({}^{-1}\), lower than that of both Al/Fe\({}_{2}\)O\({}_{3}\) and Al/CuO cases, principally because of a poor thermal conductivity. With the enthalpy of reaction being in the range of Al/Fe\({}_{2}\)O\({}_{3}\), 277 kJ per oxygen atom, a higher burn rate should be expected, but the much lower disruption temperature, 1973 K, 100 K lower than Cu\({}_{2}\)O, is penalizing. The Al/Pb\({}_{3}\)O\({}_{4}\) is an uncommon nanothermite mixture as it contains lead; however, some sectors such as mining continue to use this oxide [46; 63]. The heat of reaction is remarkably high, but the vaporization temperature that limits the overall system temperature during combustion is the lowest of all considered aluminothermites, 1808 K, corresponding to the vaporization of PbO. Interestingly, this thermite offers a sort of compromise between the Al/Fe\({}_{2}\)O\({}_{3}\) and Al/CuO cases. The enthalpy of reaction and conductivity are more in the range of Al/CuO (see Table 1 and Supplementary Information file S2) while the oxygen supply is limited by the oxidizer, as in Al/ Fe\({}_{2}\)O\({}_{3}\). The oxygen diffusivity in the reduced oxide, PbO, is much lower than that of Al\({}_{2}\)O\({}_{3}\), in the range of 10\({}^{-9}\) m\({}^{2}\cdot\)s\({}^{-1}\) (versus 10\({}^{-4}\) m\({}^{2}\cdot\)s\({}^{-1}\) for alumina) even though it is thermally activated at lower temperature with an activation energy of 93.7 kJ\(\cdot\)mol\({}^{-1}\) (versus 140 kJ\(\cdot\)mol\({}^{-1}\) for alumina, see Supplementary Information file S1). This dual effect, low activation, but limited oxygen flux leads to an initiation time of 127 ms, higher than that of the Al/CuO thermite, but still much lower than for Al/Fe\({}_{2}\)O\({}_{3}\). The limited burn rate value, 2 cm\(\cdot\)s\({}^{-1}\), is associated with a poor thermal conductivity. Note that in all Al-based thermites tested, the steady state is always achieved. In addition, they all reach their disruption temperature.