Enhancing Mechanical and Combustion Performance of Boron/Polymer Composites via Boron Particle Functionalization

Abstract

High-speed air-breathing propulsion systems, such as solid fuel ramjets (SFRJ), are important for space exploration and national security. The development of SFRJ requires high-performance solid fuels with excellent mechanical and combustion properties. One of the current solid fuel candidates is composed of high-energy particles (e.g., boron (B)) and polymeric binder (e.g., hydroxyl-terminated polybutadiene (HTPB)). However, the opposite polarities of the boron surface and HTPB lead to poor B particle dispersion and distribution within HTPB. Herein, we demonstrate that the surface functionalization of B particles with nonpolar oleoyl chloride greatly improves the dispersion and distribution of B particles within HTPB. The improved particle dispersion is quantitatively visualized through X-ray computed tomography imaging, and the particle/matrix interaction is evaluated by dynamic mechanical analysis. The surface-functionalized B particles can be uniformly dispersed up to 40 wt % in HTPB, the highest mass loading reported to date. The surface-functionalized B (40 wt %)/HTPB composite exhibits a 63.3% higher Young’s modulus, 87.5% higher tensile strength, 16.2% higher toughness, and 16.8% higher heat of combustion than pristine B (40 wt %)/HTPB. The surface functionalization of B particles provides an effective strategy for improving the efficacy and safety of B/HTPB solid fuels for future high-speed air-breathing vehicles.

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

“… 1 − 4 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). 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.…”

“…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.…”

Synthesis and Characterization of Magnesium/Boron Solid Solutions for Energetic Applications

“…Metals and metalloids are highly energetic materials commonly added to rocket fuels and propellants to extract more energy and produce more thrust in volume-limited propulsion systems. − They possess higher gravimetric and volumetric energy density than hydrocarbon fuels, and as additives to liquid fuels, they offer the benefits of enhanced combustion efficiency and reduced exhaust emissions. , Magnesium (Mg) and aluminum (Al), whose gravimetric energy densities are 25 kJ/g and 31 kJ/g, respectively, are among the most widely studied metals. − , Boron (B) ranks even higher. It is a prime candidate as a solid fuel and fuel additive with gravimetric and volumetric energy densities of 58 kJ/g and 140 kJ/mL. − A problem limiting the performance of B as an energetic material is the presence of a native oxide layer on its surface. While it provides temporary passivation during storage, the oxide layer acts as a diffusion barrier that hinders direct reaction between the oxidizer and the metal, inhibits ignition kinetics, and leads to incomplete combustion. − Reducing the dimensions of B particles to the nano- or sub-micrometer size leads to higher heat release and improved combustion kinetics. − The downside of B nanoparticles is that the surface oxide layer represents a more significant fraction of the mass and volume, which reduces their gravimetric and volumetric energy density. − Moreover, the tendency of small particles to aggregate into large assemblies is detrimental to the combustion performance of the fuel and causes issues associated with processing, including precipitation, deposition on pipe walls, and pump erosion during fuel transportation. − ,, …”

“…It is a prime candidate as a solid fuel and fuel additive with gravimetric and volumetric energy densities of 58 kJ/g and 140 kJ/mL. − A problem limiting the performance of B as an energetic material is the presence of a native oxide layer on its surface. While it provides temporary passivation during storage, the oxide layer acts as a diffusion barrier that hinders direct reaction between the oxidizer and the metal, inhibits ignition kinetics, and leads to incomplete combustion. − Reducing the dimensions of B particles to the nano- or sub-micrometer size leads to higher heat release and improved combustion kinetics. − The downside of B nanoparticles is that the surface oxide layer represents a more significant fraction of the mass and volume, which reduces their gravimetric and volumetric energy density. − Moreover, the tendency of small particles to aggregate into large assemblies is detrimental to the combustion performance of the fuel and causes issues associated with processing, including precipitation, deposition on pipe walls, and pump erosion during fuel transportation. − ,, …”

Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion

“…However, B suffers from delayed ignition and a slow and incomplete combustion process due to the existence of boron oxide at the surface and its high melting and boiling temperatures that limit B oxidation to occur at the solid phase. , One common method to promote B ignition and combustion is to incorporate additives, such as combustible metals (Mg, Ti, or Fe), − metal oxides (NiO, CuO, or Bi 2 O 3 ), − fluoropolymer (PTFE), , and functionalized graphene . The other method modifies B by surface coating of hydrocarbons, − metal fluorides, and fluorocarbons . For both methods, because fluorination of B has a higher heat of reaction than B oxidation and fluorine can react with the surface boron oxide to form more volatile compounds, , fluorine-containing species were shown to be more efficient in facilitating the ignition and combustion of B. ,, Compared to physically mixing B with fluorine-containing additives, surface modification has a higher contact area and a shorter diffusion distance between B and the introduced compounds.…”