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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.

[a] 1Introduction BPN is aw ell-knownp yrotechnic composition for ignition purposes. It has ah igh energy content and long shelf life. BPN containsb oron as fuel,p otassium nitrate as oxidizer, and polyester resina sb inder.H igh energy of the composition is due to boron, which has ah igh gravimetric (59 kJ kg À1 )a nd volumetric energy content (140 kJ cm À3 ) compared with the other fuels [1,2].B ut boron has ignition and combustion difficulty caused by the oxide layer formation (B 2 O 3 )o ni ts outers urface.T he melting point of the oxide layer is lower than the melting point of the core boron particle, whichi s450 8Ca nd 2077 8Ca t1 .013 10 5 Pa, respectively [3].T he oxide shell melts much before the solid core duringp article heating and ad iffusion-controlled process is initiated through the molten shell [4].I n order to increase ignition and combustion ability of boron, alternative solutionss uch as using nanometer-sized boron particles [5] or coating with materials having lower ignition temperature [6,7] were studied by different researchers.Young et al.[5] investigated the combustionc haracteristics of single boronp articles in nanometer size. He compared results with those of micrometer-sized particles studied in the past. Ignition and burning time characteristics of nanometer-sized boron particles were studied in the post flame region of af lat flame burner.T he first-stage and the second-stagec ombustion intervalswere analyzed and compared with the past studies of Yeha nd Kuo [8] performed on micrometer-sizedb oron particles. At lower temperatures, the second-stageb urning time was longer.Adirect linear relation between particle size and burning time for kinetic controlledc ombustion was observed. But the benefit in the first-stage burning time was not as high as expected when the particle sizew as reduced from 2-3 mmt o 200 nanometers range.Yetter et al.[9] reviewedt he classifications of metal combustion on the basis of thermodynamic considerations and different types of combustion regimeso fm etal particles. They thought that nanometer-sized energeticp articles have numerous characteristics thata re attractive for fuels and energeticm aterials. The majority of the highly desirable features of nanometer-sized metalp owders in combustion systemsw ere attributed to the high specific surface area and potential energy-storagea bility of such powders. The combustion rates of materials in nanometer size were found to increases ignificantly compared with those of similar materials in micrometer size.One of the recents tudies conductedb yY ang et al.[10] revealed the effect of particle size and pressure on the reactivity and kinetics of boron oxidation. The results indicat- Abstract:T he effect of micro-and nanometer-sized boron particles on boron-potassium nitrate (BPN) ignition composition was investigated in this paper.A sastartingp oint, thermochemicalc alculations were madet od eterminet he most promisingi gnition compositions. Both stoichiometric and fuel-richf ormulations of BPN were produced t...

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