Surface engineering boron/graphite fluoride composite with enhanced ignition and combustion performances

Magnesium-Enhanced Reactivity of Boron Particles: Role of Mg/B2O3 Exothermic Surface Reactions

“…As mentioned earlier, the oxidation and energy release kinetics for boron are limited and greatly affected by the transport processes occurring through its oxide shell (B 2 O 3 ). Earlier studies have proposed various species (e.g., gas-phase fluorinated species and H 2 O vapor) that can react with and breach the molten B 2 O 3 shell, resulting in enhanced combustion of B particles. ,,− As discussed earlier, the reaction between Mg and the B 2 O 3 shell of boron is exothermic and might be involved in the combustion of Mg/B-based composites. Table lists the potential reactions involved in the combustion of B-, Mg-, and mixed B-/Mg-based reactive systems.…”

“…Nanoscale metals and metalloids, such as Al, Ti, Mg, B, and Si, have been explored as high-energy fuels in nanoenergetic composites for propellant and pyrotechnic applications. − Among these fuels, boron has always been regarded as the premier candidate fuel as a result of its higher gravimetric and volumetric reaction enthalpies, as shown in Figure . Despite its thermodynamic advantages over other fuels, boron suffers from sluggish oxidation and energy release kinetics as a result of its low-melting oxide shell (B 2 O 3 , with a melting point of ∼450 °C). − Post-melting, the non-volatile liquid oxide layer (boiling point of ∼1860 °C) acts as a diffusion barrier to the oxidizing species and restricts their access to the B core, thereby significantly inhibiting B oxidation and energy release. , Several surface modification strategies, such as oxide removal by solvent washing, surface functionalization with fluorine-based organic, polymeric, and graphitic moieties, and incorporation of fluoride salts, , have been explored to alter or remove the oxide surface of boron to promote its ignition and combustion characteristics.…”

“…Additionally, as a result of the highly negative formation energies of MgO compared to B 2 O 3 (Mg lies lower than B in the Ellingham diagram), the reaction between Mg and B 2 O 3 is thermodynamically feasible with a reaction enthalpy (Δ H r ) of ∼ –420 kJ/mol or −2.9 kJ/g. Therefore, Mg NPs, as an additive fuel, can offer a thermodynamically and kinetically viable source of highly reactive gas-phase Mg that can potentially act as an etchant for the oxide shell of boron according to the following reaction: 3 Mg + normalB 2 normalO 3 → 3 MgO + normalB , goodbreak0em1em⁣ normalΔ H normalr = prefix− 420 nobreak0em.25em⁡ kJ / mol reaction Previous studies have reported that coating boron with materials (e.g., fluorographene and fluoroalkylsilanes) that can release gas-phase fluorine species to etch the B 2 O 3 shell promotes its ignition and combustion. , Similarly, nanoscale Mg can provide these advantages by generating reactive gas-phase species (Mg vapor) to corrode the oxide shell while maintaining a high energy density of the composites. Although prior studies have explored the role of Mg as an additive in B-based powders, ,, these reports have been limited to micrometer-scale Mg powders that, as a result of their low surface area, are expected to have slow vaporization and release kinetics of gas-phase Mg species.…”

“…6−9 Post-melting, the non-volatile liquid oxide layer (boiling point of ∼1860 °C) acts as a diffusion barrier to the oxidizing species and restricts their access to the B core, thereby significantly inhibiting B oxidation and energy release. 6,9 Several surface modification strategies, such as oxide removal by solvent washing, 10 surface functionalization with fluorine-based organic, 11 polymeric, and graphitic moieties, 12 and incorporation of fluoride salts, 5,13 have been explored to alter or remove the oxide surface of boron to promote its ignition and combustion characteristics.…”

“…Previous studies have reported that coating boron with materials (e.g., fluorographene and fluoroalkylsilanes) that can release gas-phase fluorine species to etch the B 2 O 3 shell promotes its ignition and combustion. 11,12 Similarly, nanoscale Mg can provide these advantages by generating reactive gasphase species (Mg vapor) to corrode the oxide shell while maintaining a high energy density of the composites. Although prior studies have explored the role of Mg as an additive in Bbased powders, 16,17,21 these reports have been limited to micrometer-scale Mg powders that, as a result of their low surface area, are expected to have slow vaporization and release kinetics of gas-phase Mg species.…”