Effect of Silane Capping on the Dispersion and Combustion Characteristics of Sub-micrometer Boron Particles Loaded in Jet A-1

The present investigation deals with the surface modification of sub-micrometer boron particles with octadecyltrimethoxysilane (OTMS, a silane compound) to improve the dispersion stability of boron particles in liquid fuel. Characterizations of as-received and silane-coated boron particles in terms of the particle size, morphology, surface chemistry, ignition temperature, and oxidation profile have been conducted using typical material characterization methods, such as scanning electron microscopy, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis (TGA). The results show that the surfaces of as-received boron particles have been successfully functionalized via a condensation reaction of hydroxyl function groups (−OH) with OTMS molecules. The capping of OTMS on the boron surface makes the particle stable against air oxidation. The dispersion stabilities of OTMS-capped boron in Jet A-1 at particle loadings of 1, 5, and 10% are found out to be 20, 18, and 2 h, respectively. Ignition and combustion characteristics of as-received and silane-coated boron particles loaded in Jet A-1 at desired concentrations have been analyzed to understand the effect of silane coating. TGA, true color flame images, and spectroscopic results show that the burning process of OTMS-capped boron is slightly delayed in comparison to as-received boron. The droplet diameter regression profiles show smooth regression up to 70–80% of the droplet lifetime with some intermittent puffing with disruptions at a later stage in both of the particle cases. However, the intensity of disruption is stronger in the case of OTMS-capped boron because of the formation of a more compact shell inside the droplet as a result of the melting of the OTMS layer (particularly toward the end of the droplet lifetime). The micrographs of the combustion residue reveal that some tiny holes are present on the residue surface in the case of as-received boron, whereas multiple blow holes are there in the case of OTMS-capped boron. A blanket of silicon seems to cover the particle surface, which makes them stick together.