Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion

Abstract

Boron has been regarded as a promising high-energy fuel due to its high volumetric and gravimetric heating values. However, it remains challenging for boron to attain its theoretical heat of combustion because of the existence of its native boron oxide layer and its high melting and boiling temperatures that delay ignition and inhibit complete combustion. Boron combustion is known to be enhanced by physically adding fluorine-containing chemicals, such as fluoropolymer or metal fluorides, to remove surface boron oxides. Herein, we chemically functionalize the surface of boron particles with three different fluoroalkylsilanes: FPTS-B (F3-B), FOTS-B (F13-B), and FDTS-B (F17-B). We evaluated the ignition and combustion properties of those three functionalized boron particles as well as pristine ones. The boron particles functionalized with the longest fluorocarbon chain (F17) exhibit the most powerful energetic performance, the highest heat of combustion, and the strongest BO2 emission among all samples. These results suggest that the surface functionalization with fluoroalkylsilanes is an efficient strategy to enhance boron ignition and combustion.

Enhanced Energetic Performance of Aluminum Nanoparticles by Plasma Deposition of Perfluorinated Nanofilms

“…The untreated sample shows representative peaks of Al 2 O 3 (product of oxidation) and unreacted Al, whereas the plasma-coated sample indicates the presence of AlF 3 (product of fluorination) in addition to Al 2 O 3 and Al. The formation of the AlF 3 shell with the release of energy at the interface leads to the formation of channels due to nanoexplosions that facilitate contact between molten Al and oxidizer and lead to more efficient oxidation. ,,− This mechanistic interpretation is supported by comparing the SEM and STEM micrographs of the untreated and plasma-coated particles before and after oxidation (Figures and ). Plasma-coated particles imaged after TGA show a collapsed but compact structure with a surface morphology that suggests the presence of channels leading into the core of the particle (Figure ).…”

“…The effect may or may not be related to combustion in which high heating rates are used for the complete oxidation of Al. However, recently, Zheng et al 56 demonstrated the direct relationship between low heating TGA/DSC and high heating laser combustion. The enhancement effect of the fluorine-based coatings on boron can be observed in similar proportions in low heating as well as high heating oxidation.…”

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

“…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.…”

Nanoenergetic Materials: Enhanced Energy Release from Boron by Aluminum Nanoparticle Addition

“… 5 − 8 The melting of the oxide shell before the solid core clogs the pores leads to particle agglomeration and acts as a diffusion barrier to the incoming oxidizer, thus delaying the boron (B) oxidation. 1 , 5 Attempts to overcome these limitations include surface functionalization of B by organic compounds, 9 − 14 reduction of the oxide, followed by surface passivation using nonthermal plasma processing, 15 and coating with metals to form composites and metal borides by ball milling and high-temperature sintering methods. 16 − 23 Functionalization with organic compounds results in the reduction of the amount of energy released per unit mass due to the presence of less energetic materials on the B surface.…”

“…Boron has shown high promise as a fuel additive for propulsion and energetic applications due to its high gravimetric (58 kJ/g) and volumetric (140 kJ/mL) enthalpies of oxidation. − Its ignition performance, however, is hindered by the presence of a native oxide on the surface, which melts at relatively low temperatures (450 °C at atmospheric pressure). − The melting of the oxide shell before the solid core clogs the pores leads to particle agglomeration and acts as a diffusion barrier to the incoming oxidizer, thus delaying the boron (B) oxidation. , Attempts to overcome these limitations include surface functionalization of B by organic compounds, − reduction of the oxide, followed by surface passivation using nonthermal plasma processing, and coating with metals to form composites and metal borides by ball milling and high-temperature sintering methods. − Functionalization with organic compounds results in the reduction of the amount of energy released per unit mass due to the presence of less energetic materials on the B surface. Nonthermal plasma processing has been shown to be successful in enhancing the energetic performance over untreated boron but requires low-pressure equipment that is harder to scale-up.…”