## 3 Results and Discussion
### Benchmark Study
As Al/Pb\({}_{3}\)O\({}_{4}\) possesses the lowest disruption temperature, corresponding to the vaporization of PbO (1808 K), the burning of the remaining thermite materials is operated in the gas phase, which is consistent with the high level of gas production reported for this thermite (0.84 g of gas per g of thermite), almost at the level of Al/I\({}_{2}\)O\({}_{5}\), which is one of the best-known thermites for gas generation. Boron thermites have attracted less attention despite high gravimetric as well as volumetric energy densities, nearly equivalent to those of aluminum [58; 64]. The amount of gas produced using B is generally slightly lower than that of Al based thermites; the B/Fe\({}_{2}\)O\({}_{3}\) couple is actually found to produce negligible gas, making it a high contender for gasless thermite applications [58; 64]. Boron is known to be difficult to initiate for combustion, which is expected due to its native oxide shell that exhibits low oxygen diffusivity compared to alumina (see Supplementary Information file S1). In addition, boron thermites also have the overall lowest elemental heat of reaction of all the thermite reactions considered in this study, (104-244 kJ/mol of atomic O depending on the couple) compared with aluminum-based thermites (277-397 kJ/mol of atomic O). The consequence is that the overall performances are decreased by an order of magnitude compared with aluminothermites; initiation delays all exceeded 150 ms (the highest value being 360 ms for B/Fe\({}_{2}\)O\({}_{3}\)), and burn rates decreased down to the cm\(\cdot\)s\({}^{-1}\) range. This may be the main reason for primarily restricting the use of boron to use as an additive, motivated by its volumetric energy density [65]. B/CuO offers one of the weakest performances (lowest burn rate of all boron-based thermites). The calculated initiation time is 192 ms, which is more than twice that of Al/CuO, with a consistently higher initiation temperature (1285 K for B/CuO). This value corresponds well to experimental values found by [PERSON] et al. with initiation delays of 125 ms for a stoichiometric mixture of particles being 500 nm and 50 nm in diameter for boron and CuO, respectively, as a compressed pellet in a constant-volume vessel [64]. The calculated burn rate is also low: 4.3 cm\(\cdot\)s\({}^{-1}\). Both the low initiation time and burn rate values are caused by the low level of oxygen flux supplied to the fuel, as the thermal conductivity value is not very different from that of Al/CuO. The low reaction enthalpy is also contributing to the weak performance. With iron oxide, the initiation time is multiplied by a factor of two compared to B/CuO, with an ensuing rise in the initiation temperature to 1577 K. The value of 360 ms, which qualitatively corresponds well with the increase seen experimentally, makes it the slowest in reactivity of all tested thermite couples [64]. Once self-sustained reaction is reached, this couple has the highest flame velocity for this fuel at 17 cm.s\({}^{-1}\), which corroborates the fact that thermal conductivity is a crucial factor in the burn rate property, allowing the preheating of the upfront reaction zone. With nearly equivalent thermal conductivity, tungsten and lead oxides show similar initiation delays (186 and 165 ms, respectively), slightly lower than that of B/CuO (\(\sim\)192 ms), with comparable initiation temperatures (\(\sim\)1200K). The burn rate of B/WO\({}_{3}\), 14.5 cm.s\({}^{-1}\), is almost twice that of B/Pb\({}_{3}\)O\({}_{4}\), 8 cm.s\({}^{-1}\), despite B/Pb\({}_{3}\)O\({}_{4}\) having the highest enthalpy of reaction of all tested cases with Boron as a fuel. This is due to the extremely low diffusivity of oxygen within the reduced PbO oxide, which likely becomes the limiting step of the reaction. Similar to aluminothermites, all boron-based couples show a clear steady state propagation, reaching their disruption temperature (see Figure 4), once the steady state regime is established.